Plating, Finishing, and Coating: State-of-the-Art Assessment · 2018. 6. 13. · use energy beams,...

Electric Power ' Research Institute

Topics: Coating processes Electrotechnology End use Technology assessment Technology utilization Metals-finishing

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EPRl EM-4569 Project 2478-1 Final Report August 1986

Plating, Finishing, and Coating: - State-of-t hemArt Assessment

Prepared by Battelle Columbus Division Columbus, Ohio

R E P O R T S U M M A R Y SUBJECT Industrial electric technologies

TOPICS Coating processes Technology assessment Electrotechnology Technology utilization End use Metals-finishing

Customer service engineers / Marketing managers

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- AUDIENCE

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Plating, Finishing, and Coating: State-of-the-Art Assessment Though metal-finishing techniques use relatively little energy, they involve an important industrial process. Using a helpful classification scheme, this report evaluates the current and future roles of metal pretreatment, surface-coating, and posttreatment methods.

BACKGROUND Many materials, components, and products are not suitable for consumer use directly after fabrication. Their surfaces may be too rough’or unattrac- tive. They may be too soft or poorly resistant to corrosion or wear. Further- more, they may have too high a friction coefficient or poor absorptivity and emissivity. Thus, some form of surface treatment or coating is generally necessary to finish the materials. Manufacturers generally use the term surface finishing to describe treatment of both metal and other materials and the term metal finishing to describe treatment of metal only.

OBJECTIVE

APPROACH

RESULTS

To assess the state of the art of metal-finishing techniques, particularly those for applying metallic and nonmetallic coatings.

Project investigators searched the technical and trade literature for metal- finishing applications and economic data. They supplemented this information with interviews of equipment suppliers and users. Because metal finishing encompasses a broad range of processes, the project team divided the finishing techniques into three categories-pretreatments, surface modifications or coatings, and posttreatments-to aid in their comparisons. Within these classifications, investigators grouped the techniques according to their use of mechanical, chemical, electrical, electrochemical, thermal, or physical processes. They developed a process description for each technique and estimated past, present, and future energy consumption by standard industrial classification (SIC) categories.

The report lists and compares more than 30 surface preparation techniques, about 50 coating and surface modification techniques, and about 20 posttreatment techniques. In 1981, SIC 34 (fabricated metal products) was the sixth-largest energy user among the major industrial groups, consuming 10.3 x 1010 kWh (0.35 x 1015 Btu). Of this total, 20% was purchased electric energy. Though this percentage has increased slowly in the past two decades, overall electricity use has declined since 1979 partly because

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EPRl EM-456%

of national economic conditions and partly because of conservation efforts. Of the total energy purchased for SIC 34, the primary metal- finishing classifications, SIC 3471 and SIC 3479, accounted for 11%. However, because these SIC categories include neither captive plating and painting shops in the steel and automotive industries nor some enamel coatings, Bureau of the Census data do not give a complete picture of energy use in metal finishing. The report provides an estimate of the total and gives examples of energy use by specific technologies.

10 kWh/m*, while electroplating and anodizing require about 30 kWh/m2. The report projects relatively small growth in industrial energy demand, which includes manufacturing-an average of 2 4 % annually through the year 2000.

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For example, painting and hot dipping (galvanizing) require about ~-

EPRl PERSPECTIVE

Despite their relatively small energy demand, increasingly sophisticated metal-finishing activities probably will continue to play an important role in manufacturing processes. Some of the newer coating developments use energy beams, vacuum systems, or posttreatment diffusion by heat. New curing methods for organic coatings require radiant energy such as infrared, ultraviolet: or electron beams. As such techniques find more widespread application, the energy-fuel mix will change, with electricity accounting for a greater share of overall energy demand. Similarly, new materials and products will call for innovative or modified metal-finishing methods. More-stringent property requirements, new safety and waste treatment regulations, and pressures to cut costs will all influence these developments.

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PROJECT RP2478-1 EPRl Project Manager: I. Leslie Harry Energy Management and Utilization Division Contractor: Battelle Columbus Division

For further information on EPRl research programs, call EPRl Technical Information Specialists (415) 855-2411.

Plating, Finishing and Coating: State-of-the-Art Assessment

EM-4569 Research Project 2478-1

Final Report, August 1986

Prepared by

BATTELLE COLUMBUS DIVISION 505 King Avenue

Columbus, Ohio 43201

Principal Investigator E. W. Brooman

Prepared for

Electric Power Research Institute 3412 Hillview Avenue

Palo Alto, California 94304

EPRl Project Manager I. L. Harry

Industrial Program Energy Management and Utilization Division

1

0 R D E R I N G I N FORM AT1 0 N

Requests for copies of this report should be directed to Research Reports Center (RRC), Box 50490, Palo Alto, CA 94303, (415) 965-4081. There is no charge for reports requested by EPRl member utilities and affiliates, US. utility associations, US. government agencies (federal, state, and local), media, and foreign organizations with which EPRl has an information exchange agreement. On request, RRC will send a catalog of EPRl reports.

Electric Power Research Institute and EPRl are registered service marks of Electric Power Research Institute, Inc.

Copyright 0 1986 Electric Power Research Institute, Inc. All rights reserved

NOTICE This report was prepared by the organization(s) named below as an account of work sponsored by the Electric Power Research Institute, Inc. (EPRI). Neither EPRI, members of EPRI, the organization($ named below, nor any person acting on behalf of any of them: (a) makes any warranty, express or implied, with respect to the use of any information, apparatus, method, or process disclosed in this report or that such use may not infringe privately owned rights; or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this report.

Prepared by Battelle Columbus Division Columbus, Ohio

ABSTRACT

This report focuses on the surface finishing of metals or metal finishing as it is usually called. Because of the wide range of techniques used and the similarity between some of the techniques, a classification system was devised to organize the presentation of the material. Metal finishing activities have been divided into (1) pretreatments, which include cleaning and surface preparation, (2 ) surface modification or coating, and (3) post-treatments, which include thermal diffusion for alloying and curing of paint films. grouped together according to whether mechanical , chemical , electrical , electrochemical , thermal, or physical phenomena are involved. Some techniques require a combination of these phenomena to be effective. discussion is given of past, present, and projected future energy consumption in manufacturing industries and in particular with respect to SIC 34 "Fabricated Metals", and SIC 347 "Coating, Engraving and Allied Services". growth in energy consumption of 2 to 3% is projected through the year 2000. Electrical energy will form a larger part of the energy/fuel mix. Developments in metal finishing that will influence energy consumption and the energy/fuel mix are briefly discussed.

Under each category the techniques are

Following process descriptions, a

A modest annual

A bibliography of selected references has been supplied.

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ACKNOWLEDGMENTS

The author wishes to thank Dr. David G. Vutetakis for his help in preparing the section on energy use in metal finishing, and Mr. Tom G. Byrer and Mr. Lee Semiatin for their encouragement and careful review of the draft report. Thanks also must be extended to Miss Diane E. Slawson for her assistance in preparing the manuscript for this report, and t o Dr. Mohamed F. El-Shazly for providing technical comments on the electrolytic processes discussed.

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CONTENTS

Sect i on

1 INTRODUCTION

Page

1-1

2 METAL FINISHING ACTIVITIES C1 assi f i cati on Scheme

Surface Preparation Coating Removal Coating Deposition Finishing Post-Treatments

Metal Finishing Techniques Surface Preparation Techniques

Mechanical Methods Chemical Methods Electrochemical Methods Mechanical-Chemical Methods Mechanical-Electrochemical Methods

Coating Removal Techniques Chemical Methods Electrochemical Methods Thermal Methods

Coating Deposition Techniques Mechanical Methods Chemical Methods Electrical Methods Electrochemical Methods Thermal Methods Physical Methods Mechanical-Chemical Methods Mechanical -Electrical Methods Mechanical-Thermal Methods Chemical-Mechanical Methods Chemical -Electrochemical Methods Chemical -Thermal Methods Chemical -Physi cal Methods

2-1 2-1 2-3 2-6 2-6 2-9 2-9 2-11 2-14 2-20 2-27 2-35 2-35 2-36 2-36 2-38 2-38 2-40 2-40 2-42 2-45 2-48 2-54 2-57 2-61 2-64 2-64 2-65 2-67 2-72 2-76

v i i

Section

Electr ical-Thermal Methods E lec t r i ca l -Phys ica l Methods Electrochemical-Physical Methods Thermal -Chemical Methods Physical-Chemical Methods Phys ica l -E lec t r i ca l Methods Physical -Electrochemical Methods Physical-Thermal Methods '

Post-Treatment Techniques Inorganic Coatings Organic Coati ngs

Technique C1 assi f i c a t i o n Summary

3 ENERGY USE I N METAL FINISHING Present Energy Use Future Energy Use

4 CONCLUSIONS AND FUTURE DEVELOPMENTS Surface Preparation Coati ng Removal Coating App l ica t ion Post-Treatments

Page 2-77 ~~~~

2-77 2-78 2-78 2-81 2-82 2-86 2-88 2-88 2-88 2-91 2-94

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3- 1 3-3 3-17

4- 1 4-2 4-4 4-5 4-7

5 REFERENCES 5-1

6 SELECTED BIBLIOGRAPHY 6- 1

v i i i

ILLUSTRATIONS

Fiqure

2-1

2-2

2-3

2-4

2-5

2-6

2-7

2-8

2-9

2-10

2-11

2-12

2-13

2- 14

2-15

2- 16

Schematic o f P r inc ipa l Metal F in ish ing A c t i v i t i e s Leading t o a Finished Product

C l a s s i f i c a t i o n o f Types o f Surface Preparation and Coating Removal Techniques

C l a s s i f i c a t i o n o f Types o f Coating and Surface Mod i f i ca t i on Techniques

C l a s s i f i c a t i o n o f Types o f Metal F in ish ing Post- Treatment Techniques

Types o f Surface Preparation Techniques Class i f ied by Method

Types o f Coating Deposit ion Techniques C lass i f i ed by Method

Metal F in i sh ing Post-Treatment Techniques C lass i f i ed by Method

Diagram o f a Typical Chemical P i c k l i n g Line f o r Continuous S t e e l S t r i p

Diagrams Showing Pr inc ip les o f Conventional and Electro- s t a t i c F lu id i zed Bed Powder Appl icat ion Techniques

Example o f the Tank and A u x i l i a r y Equipment Needed f o r Hard Chromium E lec t rop la t i ng

Typical Layout Arrangement f o r the Automatic Cadmium P l a t i n g o f Small Parts

Examples o f the D i f f e r e n t Types o f Furnaces Used i n L iqu id Carbur iz ing

D i f f e r e n t Physical Vapor Deposit ion Techniques Depicted Diagrammatically

Diagram Showing the D i f f e r e n t Methods f o r Color ing Anodized Coatings

Equipment Lay-out f o r Spraying Parts w i t h a Cover Coat, Enamel S l i p

Representation o f a Simple Ion-Plat ing Apparatus Using a D i r e c t Current Gas Discharge and Evaporator-Type Fi lament

Page

2-4

2-5

2-7

2-8

2- 10

2- 12

2-13

2-26

2-47

2-51

2-53

2- 58

2-60

2-70

2-80

2-84

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Figure 2-17 Diagram Showing the Principles of Operation of the

Spray Equipment Used for Plasma Arc and Transferred Plasma Arc Spraying

Page

2-87

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TABLES

Tab1 e

2- 1

2-2

2-3

2- 4

2-5

2-6

3-1

3-2

3-3

3-4

3-5

3-6

3-7

3-8

3-9

3-10

Summary of Features of Mechanical Finishing Techniques for Surface Preparation

Summary of Features of Chemical Finishing Techniques for Surface Preparation

Summary of Features of Electrochemical Finishing Techniques for Surface Preparation

Some Commonly Used Chemical Stripping Solutions for Selected Coatings

Summary of Some Electrochemical Stripping Methods Used Most Frequently in Industry

Classification Summary and Index of Coating Deposition, Removal and Surface Modification Techniques

Trends in Electricity Use for All Manufacturing Industries Since 1967

Purchased Fuels and Electricity Used in the Fabricated Metal Products Industry (SIC 34) in 1981

Energy Consumption in the Industrial Sector for 1980 to 1983

Major Fuel Use in the Fabricated Metal Products Industries (SIC 34) in 1983

Specific Electrical Energy Requirements for Electrodepositing Selected Metals

Specific Electrical Energy Requirements for Anodizing Aluminum

Specific Energy Consumption for Selected Fi ni shi ng Operations

Electrical Energy Consumption in Typical Metal Finishing Plants

Process Heat Energy Consumption in Typical Metal Finishing Plants

Process Energy Usage for an Automatic Paint Spraying Line

Page

2-21

2-28

2-34

2-37

2-39

2-95

3-4

3- 5

3-6

3-6

3-8

3-10

3-10

3-11

3-12

3- 14

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Paqe Tab1 e

3-11

3-12

3-13

3-14

3-15

3-16

3-17

3- 18

3- 19

Energy Requirements for Depositing Various Types of Organic Coatings

Relative Energy Requirements and Costs for Spray Painting and Electropainting

Cost of Coating Application by Painting and El ectropl at i ng Techniques

Comparison of Several Methods o f Paint Curing

Production and Energy Use Data for Selected Metal Finishing Operations (1972)

Projected Increase in Industrial Energy Consumption by Type for the Year 2000

Project ions for Electricity Consumption According to Sector for the Year 1990

Historical and Projected Data for GNP Growth and Corresponding Growth in Industrial Electricity Demand

Electricity Consumption Projections for Metal Fabrication Technologies

3-14

3-15

3-16

3-18

3-19

3-20

3-22

3-23

3-25

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SUMMARY

This report is divided into three main sections. In the first of these, metal firi- ishing activities are described after first establishing a classification scheme for the mariy techniques available and in use. The techniques are presented in a logical sequence, and in alphabetical order under each classification category. First are presented the cleaning, surface preparation and other pretreatments. Then comes the surface modification or coating deposition methods. Finally post- treatments are discussed. that depend on (1) mechanical, (2) chemical, (3) electrical, (4) electrochemical , (5) thermal, or (6) physical phenomena to be effective. require a combination of these phenomena, and these are described also. result, over 30 surface preparation techniques; about 50 coating and surface modification techniques; and some 20 or so post-treatment techniques are listed and compared where appropriate.

Each category is organized to group together techniques

Of course, some techniques As a

In the second main section of this report, trends in energy use are discussed for manufacturing industries in general, and the metal finishing industries in particular. Data are provided for historical energy and fuel use, including electrical energy, and projections through the year 2000 are compared. In 1981 SIC 34 (Fabri- cated Metal Products) was the sixth largest energy user among the major industrial groups, consuming some 10.3 x 1010 kWh (0.35 x 1015 Btu). Twenty percent of this total was for purchased electrical energy. increasing over the last 15 to 20 years, but the quantity of electricity used has declined since 1979, partly a reflection of the national economy, and partly because of increased conservation efforts. Of the total energy purchased in SIC 34, about 11% of this total was purchased by SIC 3471 and SIC 3479, which covers most metal finishing activities. Captive plating and painting shops in the steel and automotive industries are not included in these SIC categories, nor are some porcelain enamel coatings. As a result the energy use in metal finishing is greater than the Bureau of Census data indicate. and some specific metal finishing techniques have been chosen to demonstrate specific energy use per unit area and thickness coated. hot dipping (galvanizing) require about 10 kWh/mZ, while electroplating and anodizing require about 30 kWh/m2. the coating process and the overall process energy use is also discussed.

This percentage has been slowly

Estimates are made for the total,

For example, painting and

The relationship between electric energy use in

s- 1

Growth in industrial energy demand, which includes manufacturing, is projected to be relatively small, about 2 to 3% an average each year through the year 2000. This growth rate parallels the modest projections made for GNP growth over this same time period.

In the final section of this report conclusions and future development are summarized. turing processes, even though the energy demand is small compared with some of the other industrial sectors. age of the demand being for electrical energy as some of the newer, more sophisticated techniques are refined and become more widely applied. As new materials and products are developed, the need for modified or new metal finishing techniques wi 1 1 increase. concerning safety and waste treatment; and the desire to contain costs will be factors influencing the new developments. coating removal, coating application and post-treatments are described briefly.

Metal finishing will remain an important and integral part of manufac-

The energy/fuel mix will change, with a greater percent-

More stringent and demanding property requirements; new regulations

Present trends in surface preparation,

s-2

Section 1

INTRODUCTION

Many materials or components or products after fabrication are not suitable for consumer use. enough, not aesthetically pleasing in appearance, have too high a coefficient of friction, have poor absorptivity or emissivity, be too soft, and so on. of desirable surface properties is considerable. erties some form of surface modification or coating is required. as "finishing" steps. finishing" has been used. finishing" is used. of metallic and nonmetallic coatings, in particular. briefly mentioned because, in such a document as this, it is not possible to cover a1 1 aspects of the technology.

The surfaces may be too rough, not corrosion or wear resistant

The l.ist To achieve these desirable prop-

These are known When metals and nonmetals are involved, the term "surface

When only metals are being considered the term "metal This report focuses on metal finishing, and on the application

Some topics will be only

Because the topic area of metal finishing is so broad it is necessary to have some sort of classification scheme. dissimilar processes or products can be easily recognized and categorized and viewed as part of the overall scheme of things. this report lists many books on metal or surface finishing.

. "guidebooks" cover a wide range of topics, other books are limited to more specialized areas.

Similar processes then may be grouped together, and

The bibliography at the end of Some "handbooks" or

In general, all finishing processes can be divided into three steps:

0 Cleaning, preparation and/or pretreatment

0

0 Post- treatment . Surface modification or coating deposition

This report covers all three steps, and uses them as the basis for a classification scheme. quality and effectiveness of a coating or surface treatment is only as good as the quality of the substrate. In general, smooth coatings cannot be obtained on rough surfaces, and good coverage and adhesion will not occur if there is soil on the surface t o be coated.

The first step is important because it is a well known axiom that the

Surface modification in the second step refers to changing

1-1

the structure of composition of the surfaces of interest; the buildup of one or more coatings; and the controlled removal of metal or coatings or finishes applied previously (for temporary protection, or because they are damaged or faulty). Post-treatment, the third step, may be necessary to protect the applied finish or coating in storage, shipment, or use or to provide another desirable feature such as lubricity (by applying a solid film lubricant), corrosion inhibition (by applying an inhibitor), or an identifying mark or color (by painting), for example.

To compound the situation further, different substrate materials will require different pre-treatments. receive a decorative electrodeposited coating of chromium, will not be the same as that needed to prepare a refractory metal surface, such as molybdenum, to receive a barrier coating of chromium for high temperature applications.

What might be satisfactory to prepare a brass surface to

The need for a classification scheme and guidelines for selecting the correct combination of procedures immediately becomes apparent, and prior experience plays an important role. specifications to ensure reproducible results are obtained by different manufacturers or suppliers, ment of Defense contracts for critical applications. Similarly, in the Department of Energy, and in the electric utilities, there is a need for reliable surface finishing treatments that will perform to expectations or specifications.

In some instances it is even necessary to publish standards or

This is especially true for hardware supplied under Depart-

1-2

Section 2

METAL FINISHING ACTIVITIES

In this section a brief descript activities is given. However, f tion scheme that will be used to ent processes and techniques.

CLASS IF I CAT ION SCHEME

on of some of the important metal finishing rst it is important to establish the classif provide a cohesive link between the widely d

ca- ffer-

The topic of surface finishing, of which metal finishing is a part, is very broad and as a result, difficult to organize into components and classify. a recent book (1) - published by the American Society of Metals and Finishing Publi- cations, Ltd., in the UK does provide a rationale and system for categorizing surface finishing processes.

Nevertheless,

In this system the main headings are:

1. Metal Surface Preparation,

2. Coating Deposition.

Under the first category come processes which are further categorized as either "with substrate dissolving" or "without intentionally changed substrate". cal, chemical, and electrochemical processes are included. gory there are three subcategories, namely "inorganic coatings", "organic coatings", and "stripping". The inorganic coating processes are further broken down into "metal 1 ic", "nonmetal 1 ic", and "metal 1 ic and nonmetal 1 ic" coatings. with the surface preparation heading, the subcategories are further broken down into processes that intentionally change or do not change the substrate. electrochemical, mechanical , and gaseous deposition techniques are covered for specific substrates.

Mechani- Under the second cate-

As

Chemical,

Several other systematic classification schemes have been proposed in the 1 itera- ture, each one based on the particular vantage point taken by the authors. Thus, entirely different schemes have been devised from the chemists' viewpoint, the physicists' viewpoint, and the material scientists' viewpoint. presented a variety of these classification schemes. Chapman and Anderson (3) grouped deposition processes into the following categories:

Bunshah et al. (2)

2-1

1.

2.

3.

4.

Buns hah depos i t

Conduction and Diffusion Processes,

Chemical Processes,

Wetting Processes,

Spraying Processes.

and Mattox (4) - proposed a class fication based on the dimens ng specie, e.g., whether atoms/molecules, liquid drop ets or

ties were involved. Their first-level categories were as follows:

1. Atomistic Deposition,

2. Particulate Deposition,

3. Bulk Coatings,

4. Surface Modification.

ons of the bulk quanti-

Other publications such as the Metals Handbook (3) and the Finishing Handbook and Directory (6) - are less systematic in their approach. Handbook, which deals with surface cleaning, finishing, and coating, and is also published by the American Society for Metals, does not have a formal classification system, but its principal chapters deal with:

Volume 5 o f the Metals

1. Metal Cleaning

2. Mechanical Finishing

3. Plating and Electropolishing

4.

5. Nonmetallic Coating Processes.

Metallic Coating Processes Other than Plating

.Other chapters deal with the finishing of specific ferrous and nonferrous materials. In contrast, the Finishing Handbook and Directory does have three broad classification systems dealing with metal, wood, and plastic finishing techniques. Under the category of "metal finishing", first comes the pretreatment steps. These are followed by categories that are a mixture of applications and techniques. The principal metal finishing techniques listed are:

1. Enamelling 5. Paint Coatings

2. Finish Polishing 6. Chemical Coloring

3. Electroplating 7. Plastic Coating.

4. Anodizing

2-2

Under each are listed the various approaches for obtaining the desired surface finish. Under paint coatings, a variety of post-treatments are listed such as drying and curing.

For the purposes of this report, the best features of existing systems have been used to derive a classification scheme for metal finishing systems. a block diagram o f this system, which covers the three principal steps outlined earlier, namely:

Figure 2-1 is

a Pretreatments (Surf ace Preparation)

a

a Post-Treatmen ts . Coating Deposition or Surface Modification

The coating removal step is included as a separate category in Figure 2-1, although it could be considered as a type of surface preparation step in some cases. Figure 2-1, numbers have been assigned to the various steps. For example, "surface preparation" is assigned the number 1.0; "coating removal" is 2.0, and "finishing post-treatments" is assigned the number 4.0. the various techniques that comprise the major steps shown in Figure 2-1, these and other numbers will be used to show their interrelationships. At the end of this section all the techniques will be cross-referenced by classification number and page number on which they are discussed.

In

In the development and discussion of

Surface Preparation

Surface preparation techniques (1.0) are most often used in conjunction with subsequent coating deposition steps (3.0). heavy soi 1s--particularly oi 1s and greases, waxes, and the 1 ike--before coating removal steps (2.0). (sometimes hot, sometimes with impressed direct current) are used to enable the coatings to be removed satisfactorily. surface preparation techniques used prior to coating deposition.

Occasionally it may be necessary to remove

In these situations, organic solvents or alkaline cleaners

Figure 2-2 is a classification of various

Surface preparation may be accomplished by mechanical (l,l), chemical (1.2), and electrochemical (1.3) treatments. In addition, combinations of these three principal approaches may be used to provide faster action, more specific action or for providing a unique action not possible by simpler techniques. Examples are combined mechanical and chemical treatments (1.4) such as vibratory milling in a liquid phase, or combined mechanical and electrochemical treatments (1.5) such as e 1 ec trodeburr i ng .

2-3

L Metal Finishing System

Pretreatment (Surface Preparation)

Techniques 2.0

Coating Removal Techniques

3.0 + I i

Coating Deposition or

Techniques

I I

Post-Treatment Techniques

F i g u r e 2-1. Schematic o f p r i n c i p a l meta l f i n i s h i n g a c t i v i t i e s l e a d i n g t o a f i n i s h e d product .

2-4

System

-+

Techniques

C he mica I Methods

I 1 "

Mechanical

- 1.2

--.+ Chemical - Methods

- 1.3

Elect roc hem i ca I Methods

Chemical

1.5

I Mecha n i ca I - Electrochemical

Note: Specific techniques are listed by method in Figure 2-5and described in the text.

Methods

Methods

Figure 2-2. coating removal techniques.

Classif icat ion o f types o f surface preparation and

2-5

Usually chemical treatments, such as pickling to remove scale from ferrous

ever, the chemical treatment is the final treatment or a treatment given to parts to protect them during prolonged storage.

corrosion during storage. phosphates and chromates that are discussed later under coating methods.

surfaces, are used to prepare the surface for subsequent coating. Sometimes, how- ~~~

Conversion coatings are an example of this type of surface treatment, where inhibitors or oils may be used to prevent -~

Most conversion coatings, however, are those such as __

Not shown in Figure 2-2 (and 2-3, 2-4 also) are the various rinsing and drying steps that form part of an integrated metal finishing system. ing processes, parts proceed directly from the surface preparation step to the coating step. materials, it is important to keep surfaces wet (or otherwise protected) so that air-formed oxide films do not interfere with coating adhesion and integrity.

In many manufactur-

In some finishing activities, for example electroplating on reactive

Coati ng Remova 1

Some parts may have already been finished and are to be refurbished; other parts may be rejects from coating deposition processes; other parts may have been in storage or supplied by the manufacturer with temporary protective films or coatings intact. coatings must be removed. (2.0) in Figure 2-1 and is included also in Figure 2-2. electrochemical (2.2), or thermal (2.3) techniques are used to strip coatings from substrates to prepare them for coating or recoating.

As mentioned earlier, any soi 1, prior coatings, imperfect and temporary

Usually chemical (2.1), Thus, coatings removal is shown as a separate activity

Coating Deposition

This category forms the largest category of metal finishing operations and is often considered to be the most important of the three steps. But as explained earlier, pretreatments are just as important to ensure good coating quality. treatments are important to help the coating retain its desirable properties under some circumstances and in certain environments. Figure 2-3 is a classification of the techniques which can be used to obtain different types of surface finishes on metals.

Also, post-

The wide range of methods shown in Figure 2-3 can be used to apply many different types of coatings or modify surfaces. anodizing, where thin oxide films are built up electrochemically on the surface of

Surface modification techniques include

2-6

System

2.0 1

I Coating Removal Techniques

- - - - - - - - - - - A

Electro- Thermal Physical Methods Methods

Chemical Methods

3.1.1

Chemical

3.1.2

Electrical

3.1.3

Thermal

4 . 2 . 1 , 1 . 3 . 1 , QL Electro-

chemi ca I - Physical

Chemical- Electrical- M ec ha n i ca I Thermal

Chemical- Electrical-

chemical Ph ys ica 1

3.2.3

Thermal

3.2.4

Physical

3.5.1

Thermal-

Note: Specific techniques are listed by method in Figure 2-6 and described in the text.

F igure 2-3. sur face m o d i f i c a t i o n techniques.

C l a s s i f i c a t i o n of types o f coa t i ng and

i 3.6.1

Chemical

3.6.2

3.6.3

Electro- chemical

3.6.4

2-7

Surface Preparation Techniques -

Techniques

2.0

Coating Removal Techniques

4.1.1

Methods

4.1.2

Coating Deposition or Surface . Modification Techniques

I Chemical Methods

I I

_ _ _ _ _ _ _ _ _ _ _ _ _ A '

4.1.3

c hem i ca I Methods

4.1.4

Methods

Note: Specific techniques are listed by method in Figure 2-7 and described in the text.

Coatings

4.2.1

Methods

4.2.2

Methods

4.2.3

Methods

F i g u r e 2-4. t echn iques . C l a s s i f i c a t i o n o f t ypes o f me ta l f i n i s h i n g pos t - t rea tmen t

2 -8

a metal to provide a degree of corrosion protection or aesthetic appeal, and ion implantation where the insertion of atoms of a selected metal or gas are incorporated into the outermost surface layers to change a property, such as corrosion resistance. Inorganic coatings cover those circumstances where metal 1 ic, metal - loid, ceramic or other inorganic materials are deposited onto the surface of the substrate to be coated. Such coatings may be applied by a variety of techniques including electroplating, peen plating, electroless plating, and sintering. The objective usually is to provide adherent, defect-free coatings that act as a barrier between the substrate and the environment and perform one or more useful functions, such as prevent oxidation of the substrate or provide wear resistance. On the other hand, as the title implies, organic coatings can provide relatively inexpensive coatings based on organic compounds. Examples are paints, varnishes, and lacquers. aesthetic appeal, or to give a modest amount of corrosion protection. The films or coatings tend to be flexible, allowing for post-coating fabrication (e.g., painted steel strip used for appliances). They are applied by relatively fewer techniques, such as brushing or dipping, electrostatic spraying, or electrophoretic deposition.

Usually such coatings are applied for identification purposes,

Fi n i s hi ng Pos t-Treatmen ts

The type of finishing post-treatment (4.0) can vary according to the type of coating applied, therefore, this classification has been broken down into two subcategories for convenience in Figure 2-4. These categories are inorganic coatings (4.1) and organic coatings (4.2). Inorganic coatings may be subjected to annealing treatments (4.1.4), mechanical treatments to provide luster (4.1.1), and other treatments to provide improved or desired features. Organic coatings typically are dried (4.2.2) or cured (4.3.3) as a minimum to provide good adhesion and integrity.

METAL FINISHING TECHNIQUES With a classification scheme established, it is now possible to group together methods that are used to produce similar results with respect to metal finishing. Subsequent sections of this report will provide details about most of these techniques and energy consumption trends for some.

Figure 2-5 provides a listing of the most common types of surface preparation techniques according to whether mechanical (l.l), chemical (1.2), electrochemical (1.3) or combined mechanical and chemical (1.4), mechanical and electrochemical (1.5) treatments are used. The information in this, and the following two figures, is

2-9

MECHANICAL

CHEMICAL

iLECTROCHEMICAL

*Or brightening

MECHANICAL

1.1 Blasting Honing Brushing Lapping Buffing Mass Burnishing Finishing Deburring Polishing Grinding Sanding

CHEMICAL

~~

I .4 De burri ng Ultrasonic Cleaning Vibratory Finishing

I .2 Cleaning Etching Conversion Milling Degreasing Pickling Descaling Polishing* Dipping

~~ ~

ELECTROCHEMICAL

I .5 Deburring Grinding Honing Shaping

~~

I .3 Activation Cleaning Deburring Descaling Etching Pickling Pol is hi ng *

Note: The numbers in the boxes correspond t o the methods shown in Figure 2-2.

Figure 2-5. Types o f surface preparation techniques c l a s s i f i ed by method.

2-10

presented in the form of a matrix for convenience. niques comprise a large proportion of the techniques available. electrochemical techniques are used in some cases where a very rapid, but controlled action is needed, especially when the substrate should not be changed or deformed by mec han i cal act i on.

Chemical and mechanical tech- Nevertheless,

The most common coating deposition techniques are listed in Figure 2-6 and range from mechanical (3.1) through chemical (3.2), electrical (3.3), and electrochemical (3.4) to thermal (3.5) and physical (3.6) treatments. Combined action deposition techniques also are included in Figure 2-6. The majority of established techniques are chemical, electrochemical, or thermal in nature. The latest techniques being developed fall under the physical treatment category and involve the use of vacuum systems, lasers, and other high energy beams.

Figure 2-7 lists the various techniques commonly used for post-treatments. classification is a 1 ittle different out of necessity, being divided between inorganic and organic applied coatings. identified as being based on mechanical, chemical, electrochemical, thermal, or physical phenomena. Most post-treatments involve drying, baking, or curing, hence involve thermal techniques (4.1.4, 4.2.2). To a lesser extent, thin protective or decorative films may be applied to coatings by chemical or electrochemical means. Only occasionally are mechanical techniques used because of the time and expense involved. must be provided or a given texture obtained. then be used.

The

However, the techniques have still been

However, some consumer products dictate that a superior surface finish Mechanical treatment (4.1.1) may

SURFACE PREPARATION TECHNIQUES

Strip and sheet, extruded and rolled sections, castings, stamped, drilled and cut parts, components and assemblies are examples of metal products that cannot be used in the condition in which they are received. The surfaces may be contaminated with lubricating oils, greases and waxes, drawing compounds, as well as metal particles, scale, casting sand, or other foundry materials. and handling compounds the problem. and other surface defects present, used based upon what is to be accomplished. to remove burrs, flash, contour (radius) sharp corners or control dimensions. Mechanical methods also are used to provide a desired surface finish (polish) or texture before subsequent finishing steps.

General soil from the workplace In addition, there may be burrs, laps, flash,

A variety of surface preparation techniques are Generally, mechanical methods are used

Chemical methods are most often used to

2-11

N I w N

3.3.2 Sputtering

M ECH A NI CAI

CHEMICAL

ELECTRICAL

ELECTRO- CHEMICAL

THERMAL

PHYSICAL

3.4.1 Plasma Anodic Oxidation

MECHANICAL

3.1 Cladding Peen Plating

3.1.1 Air Spraying Airless Spraying Pain ti ng

3.1.2 Electric Arc Spray Ion Bombardment

3.1.3 Flame Spraying

CHEMICAL

3.2.1 Detonation Platins Explosive Bonding Flocking Gilding

3.2 Autophoretic Painting

Coloring Electroless Plating

3.2.2 Anodizing Coloring Electropainting Electropoly- merization

3.2.3 Chemical Vapor Deposition

Galvanizing Hot Dipping

3.2.4 Plasma Oxidation

ELECTRICAL

3.3 Electrostatic

Ion Implantation Spraying

3.3.1 Hardfacing Induction Hardening

ELECTROCHEMICAL THERMAL

3.5.1 Coloring Enameling Thermal Oxidation

3.4 Electroforming Electroplating (Tank, Barrel. Brush, Jet)

~

1.5 Cementation Diffusion (Carburizing, Nitriding)

Flame Hardening

Note: The numbers in the boxes correspond to the methods shown in Figure 2-3.

F i g u r e 2-6. Types o f c o a t i n g d e p o s i t i o n techn iques c l a s s i f i e d b y method.

PHYSICAL I

3.6.1 Photochemical- Assisted CVD

Photolytic Plating Reactive

Ion Plating Plasma Arc Spraying

Plasma Polymerization

3.6.3 Laser-Assisted Plating

3.6.4 Electron Beam

Laser Hardening Hardening

1.6 Laser Glazing Physical Vapor Deposition

Vacuum Evaporation

MECHANICAL

CH EM1 CAL

ELECTROCHEMICAL

THERMAL

PH Y SlCAL

1.1 norganic Coatings

1.1 .I Buffing Grinding Polishing Texturing

1.1.2 Chromating Dry Film Lubricants Passivation Phosphating Sealing

1.1.3 Plating

1.1.4 Baking Heat Treatment

1.2 3rganic Coatings

1.2.1 Inhibitors Lacquering Marking Selective Painting Varnishing

1.2.2 Baking Curing (Drying) Laminating

1.2.3 Cross-Linking Curing

Note: The numbers in the boxes correspond t o the methods shown in Figure 2-4.

Figure 2-7. Metal f inishing post-treatment techniques c lass i f ied by method.

2-13

remove soil, but they can also brighten a surface, or activate a surface prior to electroplating so that an adherent coating is obtained. provide a combination of cleaning and metal removal actions, and are useful for certain metals that are difficult to finish by conventional means. of which methods to use for various metal substrates is covered quite comprehen- sively in Reference (5) - and will not be repeated here.

Electrochemical methods

The selection

Mechanical Methods (1.1)

These surface preparation techniques involve the use of abrasives and cutting tools to remove unwanted metal, such as burrs and flash, or to provide a surface quality amenable to the coating processes that follow. For example, a roughened surface to provide better adhesion of organic coatings, or a textured surface for steel strip to be electroplated and used for consumer products.

It is well known that components having a good surface finish and smooth contours will have greater resistance to fatigue failure and corrosion in use. quality finish specified at this stage of the overall metal finishing operations, there is less chance that the final product will have an inferior finish, contain defects, and cause increased costs for assembly, inspection, and reject reclaim

Also , with a

(1,

Some hand finishing is done on parts with complex geometry, or which are delicate and require special hand1 ing. machines, either individually while being held in a fixture, or in bulk, where they are free to move in the finishing media. The most appropriate technique to use depends on many factors, including:

Most parts are "mechanically" finished on automatic

The latter is known as "mass" finishing.

0 Type of finish required

0 Dimensional tolerances needed

0 Amount of detail/contour to be retained

0 Size and shape of the part

0 Number of parts to be processed.

Most mechanical methods use rotating or oscillating equipment driven by electric motors. buffing wheels. system with a means for adjusting speed and tension.

One motor may drive a shaft that operates several grinding, polishing, or

For finishing large parts or The rotating equipment usually is driven by a belt and pulley

2-14

continuously fed materials such as strip, rod, and tube, the mechanical finishing unit may be directly driven by the motor through a gearbox. provide a description of the types of commercially available equipment.

References (l) and (8)

the former category fall and broaching, which are Grinding precedes polish less aggressive than the describe these and other

Blasting. Metal removal

Mechanical methods can be divided also into precision and nonprecision methods. In polishing and buffing; while grinding, lapping, reaming, done with machine tools, fall into the latter category. ng, which in turn precedes buffing. Each technique is former and removes less metal. The following paragraphs techniques listed in Figure 2-5 in alphabetical order.

is accomplished by the mechanical impact of an abrasive The technique is used for the removal of particle suspended in air or in a liquid.

dry soil, such as mold sand, scale, rust, paint, and carbon deposits. also for roughening surfaces prior to the application of organic finishes, such as paint, adhesives, or other coatings. Depending on the type of abrasive and carrier used, other applications can include deburring and matte or satin finishing. Blasting or shot peening also can be employed to modify surfaces to improve fatigue and stress-corrosion cracking resistance, or to overcome distortion.

It is used

Dry blasting/peening is accomplished by using centrifugal force to direct the abrasive from a rotating, bladed wheel to the part, or by air pressure. is best used for small volume or intermittent processing. more economical for large production lots. suspended in water to form a slurry. allowing more control over metal removal and surface modification. used for general cleaning and peening, especially where metal 1 ic contamination is not desired. Metallic abrasives, such as shot and grit, are used for rough cleaning and peening. Fine abrasives, such as alumina, are used for cleaning and finishing.

The latter Mechanical blasting is

In wet blasting the abrasive is Usually the abrasive particles are smaller

Glass beads are

Coarse abrasives, such as sand, are used for general cleaning.

Brushing. scratch brushing ("frosting"). The design of the wheel and the metal wire used depends on the application. scale, paint, foundry sand, and other encrustations from coatings and sheet metal. They are also used for scratch brushing, which is the final mechanical cleaning step before electroplating. various precious metals and chromium and to nickel prior to chromium plating to

Rotating wire brushes are used for cleaning, satin finishing, and

Coarse, heavy duty wires are used to remove rust,

Finer wires are used to provide a satin finish to

2-15

give a special appearance. lubricants or abrasives at surface speeds of 1285 to 1985 meters (4200 to 6500 feet) per minute.

Brushing operations are usually carried out without

It is a dry technique.

Buffing. Abrasive compounds are loosely held on a flexible cloth or sisal (hemp fiber) backing fashioned into a wheel to produce a fine quality surface contours are followed. may be referred to as satin fin buffing (gives a preliminary sm

wheel. finish. Not much metal is removed, but conversely, Depending on the resulting surface texture, buffing shing (brushed type markings), "cutdown" or "hard" othness), "cut and color" buffing (for an inter-

The part is then held against the rotating

mediate finish), and "color1' buffing (for producing a high-gloss, mirror-like finish). are used to obtain these finishes. Wheel speeds of 915 to 3050 meters 13000 to 10,000 (surface) feet] per minute are used, depending on the substrate and whether or not greaseless compounds are used. Greaseless abrasive compounds are used for satin finishing at 1525 to 1835 meters E5000 to 6000 (surface) feet] per minute. The compounds may be solid or liquid. Automatic machines use 1 iquid spray buffing compounds. contact a small area of the part and this is referred to as "contact" buffing. Softer, less densely packed wheels that rotate at a slower speed and envelope the part are suitable for small contoured parts, and this is known as ''mush'' buffing.

Different types of wheel backing material and *abrasive (buffing compound)

Low speeds are used for soft metals.

Densely packed wheels only

Burnishing. a smoother finish to the inside surfaces of holes or grooves, to generate closer tolerances on diameters, and provide a better wearing surface (10). - Broaching tools may be adapted for burnishing or incorporate burnishing "buttons'l, which

This technique is often used in conjunction with broaching to provide

provide no cutting

Deburring. This i sharp corners left techniques such as substrate material Abrasive belts, gr and other casting

action but smooth and cold work the surface.

a general term that refers to the removal of burrs, flash, and A number of after casting, machining, and pressing operations.

polishing, buffing, brushing, can be used depending on the and shape. nders, and hand f i 1 ing are used to remove heavy flash, gates, r molding defects.

Gears are often deburred using wire brushes,

For small cast or machined components, mass finishing techniques are used.

Grinding. excess weld metal or splatter, burrs, and other unwanted metal or to restore

This technique is used before polishing to remove flash on forgings,

2-16

flatness to damaged surfaces. Abrasive wheels are used to remove nodules and excessive growths from parts plated or electroformed in nickel electroplating baths. They are also used to remove worn or defective plated chromium coatings on printing rolls/cylinders. of stainless steel parts of relatively simple shape. Automated belt grinding uses longer belts and as these dissipate heat better, there is no need for a coolant to be used. ing steps. and the resu t has been that there is less distinction now between some ground surfaces and polished surfaces. typically fall in the range of 1525 to 1835 meters (5000 to 6000 feet) per minute.

Abrasive belts are used to grind and polish the surfaces

Th s eliminates the need to remove the coolant film in subsequent finish- he trend has been to use finer and finer abrasives on grinding wheels,

Surface speeds used in wheel and belt grinding

Honing. surface is slowly moved with a rotating and reciprocating action against the part. Rotation speeds are in the range of 18 to 55 meters [60 to 180 (surface) feet] per minute, while reciprocating speeds range from 18 to 24 meters (60 to 80 feet) per minute. The shearing action between the abrasive and the surface removes a small amount of metal; thus honing is a precision finishing technique usually applied to inside diameters of cast, bored, drilled holes and cavities or cylindrical objects. Some outer surfaces are also amenable to honing. bearing races and valve components. this technique, but the most common are cast iron and steels. Honing often replaces grinding and lapping where dimensional tolerances must be held. production runs, unique parts or special applications, honing tools may be used in drill presses or lathes. For production parts, honing machines are available. Tool design, type of machine, size and composition of abrasive and choice of working fluid depend on nature of part and material from which it has been fabricated.

In this technique a honing stone or stick having an abrasive bonded to its

These include gear teeth, ball A wide range of metals may be finished with

For small

Lapping. Lapping is another low-speed finishing technique in which loose or bonded abrasives are used. differs from honing in that it is a single motion process and the tooling needed is different. rubbed against the surface to be finished by a special tool called a lap. The tools are made from a metal softer than the substrate. very hard, typically silicon carbide or fused alumina. produce very flat surfaces. surfaces ("ring" lapping) and spherical surfaces, such as ball bearings. Matched parts also may be lapped to provide mating surfaces.

When the latter are used, lapping resembles grinding but

Lapping may be used for individual parts, whereby the abrasive is

The abrasives usually are The technique is used to

However, it can be modified to finish cylindrical

In this case each part serves

2-17

as the "lap" for the other part to move the abrasive and provide the smoothing action. Sometimes parts are lapped individually then matched for final mating. Applications include cylinder heads and blocks, valves and seats, pistons and cy1 inders . Mass Finishinq. that would be too difficult or expensive to fixture and finish individually. two principal techniques are barrel finishing and vibratory finishing. is used for improving general appearance and finish where dimensional tolerances are not a concern. The latter can be used for deburring, providing decorative finishes and improving parts where tolerances matter. move freely in a medium containing an abrasive. rate the parts while the machine is rotated or oscillated. action produces the surface finishing effect, which can range from a coarse action [e.g., scouring (grinding), descaling, deburring] to a finer action (e.g., polishing, burnishing) depending on the type of machine, abrasive and duty cycle used. References (5), - ( I ) , (E), and (9) - provide detailed information about the techniques and equipment used.

This is a generic term used for the bulk finishing of small parts

The former

__

The

The parts are allowed to The medium is necessary to sepa-

The sliding/tumbling

Barrel finishing is a low-cost operation because of the relatively simple equipment requirements. An octagonal , horizontal barrel is frequently used. The inside surface is typically rubber or polyurethane lined to reduce noise and protect the parts being finished. The parts, abrasive, and media are loaded and the barrel is rotated at 15 to 40 revolutions per minute 115 to 61 (surface) meters per minute] depending on the type of finish required. The outermost parts and media are carried up the side of the barrel as it rotates then slide or tumble down over the bulk due to gravity. controlled to limit the abrasive action to the desired level. Sliding action is .preferred to minimize surface damage due to the finishing action itself.

Most barrelling is done wet.

The sliding/tumbling action is

Vibratory finishing also uses relatively simple equipment. (round, spiral, or toroidal) is used and this is mounted on springs. motion is produced by a shaft or shafts with eccentric loads driven by an electric motor or series of electromagnets, or a motor attached directly to the bottom of the container. The parts, abrasive, and media are loaded into the container; then the frequency and amplitude of the vibrations are set t o provide the desired finishing action. Most equipment operates in the range of 1200 to 1800 cycles per minute and 3 to 6 mm amplitude.

Either a tub or bowl The vibratory

The higher the frequency the faster the cutting

2-18

action but the rougher the surface produced. that they give a gentler finishing action and enable parts separation during processing .

Bowl vibrators have the advantages

Mass finishing action can be increased in severity, thereby greatly reducing processing time, by superimposing centrifugal force on the basic action. type of equipment the finishing containers are mounted on a turret, which rotates at high speed in one direction. At the same time the containers rotate slowly in the other direction. Good sliding action is obtained in a compact mass of parts, media, and abrasive, thus, the process can be used to produce good tolerances, even with fragile materials.

In one

Polishing. wheels or belts loaded with an abrasive. that the abrasives are firmly attached to a flexible backing material. mentioned earlier, polishing is used after grinding, but before buffing. removes a considerable amount of metal while improving surface finish. used to provide radiusing on sharp corners and edges and better follows the contours of a part to give a uniform, high quality finish. Equipment for polishing and buffing is similar, but has to be more rigid and precise for the former when it is being used to change shapes or produce given tolerances. Fixturing and setup is more rigorous for polishing than buffing. unwanted protrusions from castings and forgings, including flash and parting lines, and unwanted markings such as scratches, pits, and tool marks. are used for parts with a complex shape or where special effects or localized finishing is required. replaced by polishing belts, precoated with abrasives because these are more economical to maintain and operate. have imparted improved flexibility to the belt polishers and some contoured surfaces can now be finished. second [3500 to 7500 (surface) feet per minute]. In contrast, polishing wheels operate at 31 to 46 meters per second [SO00 to 9000 (surface) feet per minute], depending on the material being finished, the shape of the part and the desired finish.

Like grinding and buffing, polishing is usually accomplished by using The difference between the techniques is

As

Buffing is Polishing

Polishing is frequently used to remove

Polishing wheels

For mass production applications, wheels largely have been

Resin bonding the abrasives to plastic belts

Belt speeds typically range from 18 to 38 meters per

Sanding. the table but has been replaced by grinding, polishing, and buffing techniques. sanding, the part was held against a soft wheel (or "bob") in a series of short

This technique was used to finish nickel silver cutlery and flatware for In

2-19

movements while fine sand, lightly loaded with oil, was allowed to fall between the part and the bob to provide the cutting action.

-

Table 2-1 brings together the important features of the principal mechanical

parts, one dealing with individual parts processing, the other with mass finishing. The techniques are listed in order o f decreasing severity of cutting action.

finishing techniques discussed in this report. The table is divided into two -~

-

Chemical Methods (1.2)

Chemical methods are especially suited for removing soils and organic contaminants or coatings to provide clean surfaces. advantage that the processes occur at or near room temperature, and because fric- tional forces from a cutting action are not involved, parts do not become hot and distort or exhibit changed mechanical , physical , and chemical (corrosion) properties. Bright surface finishes can be obtained with some metals. Thus, although chemical methods may be divided into cleaning or finishing techniques, the following paragraphs will describe the various processes in alphabetical order, pointing out their applications. The equipment used in most cases is relatively simple and inexpensive. waters to temperature; parts movement; providing HVAC faci 1 ities, and providing adequate illumination. Waste treatment facilities may have to be supplied to treat effluents and metal sludges. References (l), - (5), - (8), - and (9) - contain information on chemical methods and equipment.

Chemical finishing techniques have the

Energy is consumed in bringing the chemical solutions and rinse

Cleaning. solutions and chemical vapors. The choice of method depends on the type of soil to be removed, the composition and condition of the part surface, the cleanliness of the surface required, effluent treatment requirements and faci 1 i ties available, and the need for subsequent metal finishing steps such as electroplating or painting. Oils and grease from prior fabrication processes can be removed by solvent cleaning. removed by acid pickling. surface, followed by alkaline soaking, emulsion soaking, (solvent) vapor degreasing and finally solvent cleaning, which gives the least cleaning action of these methods. cleaning) can provide even cleaner surfaces. later.

There are a number of techniques used incorporating liquid chemical

Other soils require other types of chemicals and oxide scales are usually Typically alkaline/acid cleaning gives the cleanest

Other combined action techniques (e.g., electrocleaning and ultrasonic These latter techniques are discussed

2-20

Table 2-1

SUMMARY OF FEATURES OF MECHANICAL FIN ISH I NG TECHNIQUES FOR SURFACE PREPARATION

Technique Application Cutt ing Action Type o f Abrasivea Type o f F in ish

A. Ind iv idual Parts

Grinding

Brushing

Blast ing

Polishing

Sanding

Honing

Lapping

Buf f ing

Burnishing

Excess metal removal. e.g., welds, deburring, f lash and par t ing l i n e removal, shaping

Deburring, rus t and scale removed, inorganic so i l , pa int removal

Cleaning; rus t scale removal; pa int dry so i l , carbon removal ; roughening and co ld working o f surfaces; straightening

Surface smoothing, simple shapes, sa t i n f in ish ing (some metal removal)

Cutlery and tableware f in ish ing

Finishing surfaces t o close tolerances, especial ly ins ide cy l indr ica l surface, gear teeth, valves ( l i t t l e metal removal)

F in ish surfaces t o close tolerances, removing small surface imperfec- t ions ( l i t t l e metal removal)

Improve appearance. conplex shapes, ( l i t t l e metal removal)

Inprove tolerances o f holes and grooves; improve mechanical propert ies (no metal removal)

Abrasive bonded t o r i g i d wheel o r b e l t

Impact o f wires mounted on a wheel w i th or without loose abrasive

Stream o f abrasive par t i c les i n a i r o r s lu r r ied i n a l i q u i d

Abrasive bonded t o f l e x i b l e wheel (imp) o r b e l t

Abrasive dropped onto s o f t wheel (bob)

Abrasive bonded t o a r i g i d too l (stone o r s t i ck ) , ro ta t ing as well as reciprocating action

Low speed wvement o f abrasive par t i c les against lap o r mating .surface

Abrasive held i n f l e x i b l e wheel (mop)

None: too l provides smoothing act ion by mechanical force

Coarse alumina, s i l i c a . s i l i c o n carbide and z i rconi a-a1 umi na par t i c les

Metal (wire) brushes, plain, crimped o r knotted, emery paste

Minerals such as alumina, sand, f l i n t , garnet, also s i l i c o n carbide, glass and metal shot

Medium alumina, s i l i c o n carbide. emery par t i c les

Sand, l i g h t l y o i led

Mediun t o f i n e alumina. s i l i c o n carbide, diamond par t i c les

Fine s i l i c o n carbide, alumina, boron carbide par t i c les

Fine alumina, s i l i ca , s i l i c o n carbide. emery par t i c les

None used

Rough w i th grinding marks depending on coarseness of abrasive

Rough, s o i l knocked only burrs. o f f b f lash,

Rough, uniform texture suitable f o r pa int ing or subsequent f in ish ing

Smooth o r textured

Smooth, f i ne texture or br ight

Smooth, f i ne cross- hatched abrasive marks

Smooth, f l a t surfaces; lus te r on hard metals

Bright, lustrous, very smooth o r special textures

Smooth, cold-worked

B. Mass F in ish ing

Barrel Scale m v a l ; de- Impact (s l id ing, Punice, alumina, sand, Smooth, b r igh t o r special burring; edge and comer tumbling action) o f par ts shot, com cobs. wood, textures suitable f o r radiusing, shaping (some mixed w i th abrasive etc, i n wet media paint ing o r p la t ing metal removal) medium

f in ish ing t o close o f parts mixed w i th shot. corn cobs, wood, f o r paint ing o r p la t ing tolerances ( l i t t l e metal abrasive mediun etc, i n wet media removal)

Vibratory Edge and ton ie r removal, Inpact ( s l i d i n g action) Pumice, alumina, sand, Smooth, br ight , su i tab le

aNatural and synthetic A1 and S i oxides are used.

bFine wires o f s o f t metals can be used t o produce a "scratch" o r "satin" f in ish on s o f t and precious metals.

2-21

Solvent cleaning involves immersing the part into a liquid, usually an organic solvent, and often a chlorinated organic solvent such as trichloroethylene, per- chl oroethylene, 1,l , 1- tri chl oroethane, methylene chloride , to1 uene , and benzene. ____

Much attention has been paid recently to the toxic nature of some of the solvents and where possible the most innocuous solvent is used to provide the desired cleaning action. When toxic solvents are used, they must be well contained. can be simply immersed or swabbed, or the liquid can be sprayed or flushed over the surfaces to rinse away loosened soil. Heating the solvent accelerates the cleaning action, but petroleum-based solvents should be used at room temperature because of the fire hazard. Particularly effective from this point of view is vapor degreasing where the part is held above a heated organic solvent. condense on the parts and flush off the soil. bottom of the tank while only pure (condensed) vapor contacts the parts. The main disadvantage with solvent cleaning is that some solvent residues may remain on the surfaces of the parts after removing them from the equipment.

~~

~-~

The parts __

The solvent vapors Contaminated solvent collects in the

Emulsion soak cleaning uses cleaners which emulsify the soils so that they may be removed from the parts surfaces. used and additives are used to stabilize the emulsions that are formed. Soluble soils dissolve while insoluble soils wet then emulsify when they come into contact with water.

Two-phase, water-based emulsifiers are most often

The latter may be removed by rinsing.

Alkaline soak cleaners are the most widely used in industry, and as the name implies, are based on soluble alkaline salts such as sodium hydroxide, silicate, carbonate, and orthophosphate. In addition, they contain wetting agents, disper- sants, sequestering agents, and stabilizers as well as other minor ingredients to facilitate the cleaning action. the parts to be cleaned may be soaked or sprayed with the solution. Alkaline cleaners are best used for removing oils, smut, and light scale, but they are not very efficient for removing buffing compounds. Sometimes they are used after detergent soaking, sometimes they are used in conjunction with acid cleaners, which dissolve oxides (see pickling).

The cleaners are usually used hot (up to 93OC) and

Care must be exercised in the selection of cleaning methods because some metals are susceptible to chemical attack in alkaline or acid solutions. Aluminum and zinc, for example, are attacked by both acid and alkaline solutions. other hand, are resistant to alkaline solutions but attacked by acidic solutions, Titanium is resistant to acids and alkalis, but is severely corroded in the

Steels, on the

2-22

presence of fluorides. References (5) - and (8) - contain useful information on the selection of cleaners for different metals and types of soil.

Degreasing. several techniques for removing greases and other organic soils from surfaces. Chlorinated solvents are most widely used and vapor degreasing is a common technique that has already been described. Alkaline cleaners may also be used for degreasing and removing oils from surfaces. emulsifying the organic material, or both, which is then rinsed away with water.

This is a general term, like cleaning, that is used to describe

They function by saponifying or

Descaling. Descaling is a general term applied to ferrous materials and some nonferrous materials rather than a specific technique. The chemical methods that may be used include acid cleaning, acid pickling, salt bath descaling, and alkaline descaling. The two latter techniques will be briefly described here because acid cleaning and pickling are described elsewhere in this section.

Alkaline descaling or derusting is used to remove light scale, rust, and carbon smut from steels and heat resistant alloys. ing critical parts for jet engines, turbines and other equipment where surface attack and hydrogen embrittlement cannot be allowed. costly process than acid pickling for ferrous materials. fact that the alkali compound used will not attack the bare metal once the scale or rust has been removed. occur as a result of this technique. ment must be considered as a possible undesirable side effect. is a cost effective approach for cleaning aluminum alloys.

Principal applications are for clean-

It is a slower and more Offsetting this is the

Also, there is no chance that hydrogen embrittlement may With acid-based systems, hydrogen embrittle-

Alkaline descaling

The salt bath descaling technique utilizes mixtures of molten salts operating at 400 to 525°C to quickly remove scale from steels, heat resistant alloys, copper alloys, nickel alloys, titanium, and refractory metals. Some baths oxidize the scale, others reduce it. neutral bath to produce an oxidizing or reducing condition at the metal surface. Reducing baths operate at lower temperatures than oxidizing baths. With all these techniques, however, the final step must be an acid dip or acid pickling to ensure complete removal of the scale. Because of the types of salts used, the elevated temperatures of operation, and the explosive reactions that can occur with any trapped moisture in castings, or surface water on forgings, wire, and other types of parts, salt bath descaling is done in totally enclosed equipment that is well

In others, an electric current is superimposed on a

2-23

ventilated.

The main advantages of salt bath descaling are that different metals can be finished in the same bath, some stress relief may occur, and hydrogen embrittlement cannot occur. The main disadvantages are that the technique is not economical for intermittent use, parts have to be quenched and cleaned with acid after descaling, and the elevated temperature can cause microstructural changes.

The immersion heaters for the baths can be heated by gas or electric, and the choice often depends on local availability and cost of the energy source. ~~

___

-~

__

Dipping. surfaces that have received prior surface finishing steps such as alkaline soaking or descaling. Acid dipping uses diluted solutions and is not as aggressive as pickling or bright dipping (polishing), where some metal is removed from the surfaces of the part. As in pickling, inhibitors may be added to acid dipping solutions to passivate the surface and prevent attack on the metal. solutions are used for polishing (brightening) and milling, both of which are briefly described below. Dipping solutions usually contain hydrochloric, sulfuric, nitric, or organic acids, or mixtures of these at temperatures up to 65OC. choice depends on the surface composition and condition of the part being finished.

Acid dipping is a technique for removing residual alkali compounds from

Stronger acid

The

Etching. rous materials to describe the technique of removing surface layers to provide a clean, roughened substrate prior to lacquering, electroplating, painting or anodizing. Surface layers may contain embedded buffing compounds and oxides, particularly if they are rough or porous. remove metal from active sites or inclusions or second phases, thus homogenizing the surface composition. Both these factors are important if the metal is to be plated subsequently. The etching solution compositions used depend on the material of the part, and for aluminum alloys for example, could be alkaline or acidic or a combination of both used sequentially. Aluminum alloy coatings containing silicon are especially difficult to treat and require strong etchants, such as nitric and hydrofluoric acid mixtures, to attack the silicon. remove or hide small surface defects such as scratches and nicks and extrusion die marks by providing a matte appearance. surface to promote adhesion of coatings, both organic and inorganic types.

This is a term applied to aluminum and aluminum alloys and other nonfer-

Etching may remove these or preferentially

Etching can also be used to

Heavy etching can be used to roughen a

Milling. Chemical milling refers to the removal of substantial amounts of metal to shape or size components, usually which would not be amenable to mechanical finish- ings methods because of their brittleness or sensitivity to heating and distortion.

2-24

Strong acids are typically used to mill surfaces. sprayed with these solutions, protected from the acids.

The parts may be immersed in or Areas where metal is not to be removed must be

Pickling. Scale and rust removal from ferrous materials are usually associated with this technique; however, aluminum, copper and its a1 loys, magnesium, nickel and its alloys, zinc and cadmium also can be pickled to remove oxide scales. solutions are used and these often contain an inhibitor to prevent attack on the substrate metal. solutions depends on the metal being treated. Iron and steel are usually pickled with sulfuric or hydrochloric acid solutions. The former is preferred because of the wider choice and lower cost of materials of construction for the equipment. With sulfuric acid, lead sheathed gas immersion heaters, steam coils, or electric immersion heaters can be used to raise the solution temperature, thus speeding up the pickling process. steel strip. much more corrosive.

Acid

The type of acid and other constituents used in the pickling

Figure 2-8 depicts the layout of a typical pickling line for Hydrochloric acid solutions work well at room temperature but are

Polishing. Chemical polishing is defined by the ASTM as the improvement in surface smoothness of a metal by simple immersion in a suitable solution, whereas if a bright surface is produced, the technique is called brightening or referred to as a brightening dip. Normally with polishing, some brightening action occurs as a result of the smoothing process because it is being accomplished on a microscopic scale. The parts may be immersed in a polishing solution or sprayed with one. latter is also known as "jet" polishing. Although metal is removed, it must be done uniformly over the surface of the part. etching) must not occur; otherwise smoothness/brightness is not obtained. obtain the preferred action, the solution chemistry is adjusted so that during the chemical polishing action, films are formed on the surface. thicker in recesses and thinner on protrusions, and the metal removal rate is faster where the films are thinner. provide a desirable self-regulation feature with respect to metal removal. Chemi- cal polishing is used where mechanical techniques would cause undesirable features to be formed on surfaces (e.g., cold worked layers, smeared metal) or would affect magnetic, electrical, and electronic properties at the surface. multicomponent alloys or metals containing inclusions is not as successful as to pure metals and homogeneous materials. are often also used for electropolishing, which is discussed later.

The

Pitting or localized attack (as in To

These films are

The dissolution reaction product films thus

Its application to

The solutions used for chemical polishing

2-25

N I N 0-l

Water rinse

Dryer tanks

u/ Delivery end

accumulator system

Figure 2-8. Diagram of a

Source:

I I

typical chemical

Metals Handbook

pickling

Vol ume 5

Entry end I"- - accumuldtor system

ne for continuous s tee l s t r i p .

Reference (5).

The selection and use of chemical metal finishing techniques for specific metals and alloys are discussed in References (l), - ( 5 ) , - (E), and (2). Table 2-2 summarizes the important features of the chemical finishing techniques discussed in the preceding paragraphs. As in Table 2-1, the techniques are listed roughly in order of decreasing severity of the "cutting" (metal removal) action.

Conversion Coating. Sometimes thin coatings are applied by this method to prepare surfaces for subsequent metal finishing operations, such as coloring and painting, or to protect them during handling and storage. surface is made to react with a chemical, usually in solution, to convert it to the desired compound, typically an oxide, chromate, manganate, or phosphate. Metals treated this way include aluminum, some steels, copper, zinc, silver, and cadmium. References ( 5 ) , - (11), - (12) - , and (23) - contain further information.

As the name implies, the metal

Electrochemical Methods (1.3)

As the name implies, an electric field is used in these techniques to control or augment chemical action. the rate of metal or compound removal from surfaces when they are made the anode (positive electrode). At higher voltages gas (oxygen) evolution occurs, and this may be used to enhance a cleaning action. When part surfaces are made cathodic, deposition can occur at low applied (negative) voltages or gas evolution (hydrogen) can occur at higher voltages. cleaning action. Depending on the amount of metal removal or gas evolution, the various electrochemical techniques are given different names. These techniques are described below in alphabetical order.

Usually low voltage, direct current is used to accelerate

Again these gas bubbles can be used to enhance a

Activation. denum, aluminum, and tantalum have thin, air-formed oxide films on their surfaces. These passivating films prevent the satisfactorj deposition of coatings using electrochemical techniques. Good adhesion is not obtained. Therefore, prior to electrodeposition the surfaces are "activated", that is stripped of their passivating films. the metal being treated), but often it is done electrochemically in a suitable aqueous solution with the surface made the cathode in a two-electrode system. part is then moved quickly to the next processing step without the activated surfaces being allowed to dry and repassivate. As an example, the ASTM B254 standard for preparing stainless steel to receive a copper electrodeposit includes an activation step prior to the copper strike to ensure good adhesion.

Some metals and alloys such as stainless steels and nickel, molyb-

These may be done chemically (usually with acid solutions specific to

The

The

2-27

Tab le 2-2

Technique

M i l l i n g

Etching

Polishing

Dipping

Pick l ing

Descal i ng

Degreasing

C1 eani ng

SUMMARY OF FEATURES OF CHEMICAL FINISHING TECHNIQUES FOR SURFACE PREPARATION

Application

Shaping o f metals and a l loys tha t cannot be mechanically t reated

Preparing non-ferrous surfaces f o r coatings w i th good adhesion

Improving surface f in ish; removing some surface markings and small defects

Removing a1 ka l i ne contamination

Preliminary step i n f in ish ing sequence f o r cast, forged, heat- treated parts

Prel iminary step i n f in ish ing process f o r cast, forged, heat- t reated parts

Preliminary step i n f in ish ing sequence f o r ro l led, stamped, extruded and drawn parts

Prel iminary step i n f in ish ing sequence f o r ro l led, stamped. extruded and drawn parts

r h t e r i a l Removal Action

General chemical attack (much material removal)

General and local ized chemical attack (some metal removal)

General chemical attack (some metal removal)

General chemical attack ( l i t t l e or no metal removal)

Preferenti a1 chemical attack on scales and oxides ( l i t t l e metal removal)

Preferent ia l chemical a t tack on scales ( l i t t l e metal attack)

Chemical d isso lut ion (no metal attack)

Chemical d isso lut ion (no metal attack)

Type o f Chemical

Acid solut ions (concentrated)



Acid solut ions (d i l u te ) may contain inh ib i to rs

Acid solut ions (d i l u te ) containing inh ib i to rs

Alka l ine solut ions, molten s a l t mixtures

Organic solvents-- l iou ids and vaDors:

Type o f F in ish

Sl ight ly rough t o smooth

Rough but f ree from oxides, scale, etc.: uniform, matte appearance

Smooth and br igh t

Smooth, matte. metal not usually changed

Clean from scales, oxides, other so i l s

Clean from scales, oxides, other so i l s

Clean from organic so i ls , metal not changed

conc. alkal ine'solut ions, emul s i f i e r s

Organic solvents-- l iqu ids and vapors: emul s i f i e r s : conc . a lka l ine solut ions

Clean f r o m organic so i ls , metal not changed

2-28

activation step is carried out for 1 to 5 minutes at room temperature in an electrolyte solution containing 5 to 15 volume percent sulfuric acid. ity of 54 A h 2 is used, which corresponds to a cell voltage of just a few volts. Rinsing with slightly acidic, cold water follows prior to the plating step.

A current dens-

Cleaning. first the surface layers are dissolved electrochemically, typically at a control led potential. As metal is removed and goes into solution, then impurities associated with or embedded in the surface are also dissolved or released and collect as a sludge in the bottom of the tank. Thus, unlike the activation technique, some metal is removed. This cleaning technique has been used successfully to remove layers of low level , radioactive contamination from surfaces exposed in a nuclear reactor facility. In the second approach, the parts are usually immersed for 1 to 3 minutes in a hot alkaline cleaning solution (see chemical cleaning) then made anodic or cathodic with a current density of 100 to 1500 A h ? . brought to the correct level ( 3 to 12 volts) and held there, the surfaces gas vigorously and the bubbles exert a scrubbing action helping to lift soils. the cleansing action is faster than ordinary soak cleaning. level in the cleaner is low, then a higher current density is needed to provide the same degree of c 1 ean i ng .

Electrochemical cleaning methods rely on one of two phenomena. In the

If the voltage is

Thus, If the surfactant

For most metals, the part is made cathodic and hydrogen is evolved. However, for parts that are or may be susceptible to hydrogen cracking, e.g., steels, the part should be made anodic. cathodic electrocleaning because only half the volume of oxygen is evolved compared with hydrogen for the passage of unit charge. that any metal dissolved in the cleaning solution could be electrodeposited on the surfaces being cleaned. consequence rises, there is the chance that some surface oxidation could occur in anodic electrocleaning. In cathodic electrocleaning, particularly for copper and copper alloys, it is usual (7) to reverse the polarity briefly at the end of the cleaning cycle to anodically remove any traces of redeposited metals. For some applications, such a combination of cathodic and anodic cleaning steps is specified as "periodic reverse" electrocleaning. as the cathodic portion. ferrous metals with the anodic and cathodic portion each being 10 seconds long. The cleaning solutions typically contain sodium hydroxide and metal chelating

Anodic electrolytic cleaning action is not as effective as

However, there is no possibility

If the current density is too high and the voltage as a

A subsequent acid dip will remove such oxides.

The anodic portion of the cycle should be at least of the same duration Reference (5) - cites a three-cycle technique for cleaning

2-29

agents. smut formation.

In many operations, electrocleaning is preceded by a hot alkaline soak cleaner to loosen soil. needed. with loose parts in rotating barrels immersed in cleaner tanks. needed is similar to that used for conventional tank or barrel electroplating and electrocleaning is most often used in conjunction with this type of coating technique. Occasionally, sulfuric acid solutions are used for electrocleaning. If they are the materials of construction of the tanks, filters and pumps and electrodes will be different (3) than if alkaline solutions are used because of the more corrosive condi ti ons.

If operated properly, rust and oxides can be removed without etching or ~~~

-

If cathodic cleaning is used, a subsequent activation step is not Electrolytic cleaning may be carried out with parts on racks, in tanks, or

The equipment

Deburrinq. sions, and metal removal is fastest where the current density is greatest. special application of electropolishing is electrolytic deburring (or electrodeburring) where most of the applied direct current is focused onto burrs at cut, stamped, and machined edges. Large burrs and attached chips from milling operations are not good candidates for electrodeburring. On the flat surfaces of the parts the anodic current density is relatively low and little metal removal occurs. The types of solutions and operating conditions used depend on the metal in question and many patented solutions can be found in the early literature on this topic. In principle the solutions are modified versions of electropolishing solutions. For ferrous materials, a sulfuric acid/phosphoric acid solution or an orthophosphoric/chromic/sulfuric acid solution may be used. solution also may be used for aluminum, while the former is also useful for nickel- based alloys. can be used. ferrous materials the temperatures may be as high as 120°C. range from 1000 A/m2 for aluminum to as high as 3000 A/m2 for ferrous materials, copper, and its alloys. The corresponding voltages will be in the range o f 12 to 18 volts. Electrodeburring is used for a wide variety of parts including gears, hydraulic pump cy1 inders, aerospace components, office equipment parts, and hypo- dermic needles.

Electropol ishing works because current is focused at edges and protru- Thus, a

The latter type of

For copper and its alloys, an orthophosphoric/chromic acid solution Temperatures are typically in the range of 45 to 80"C, although for

Current densities

Descal ing. dizing metals, especially alloy, mild, and stainless steels. In cases where scale

Methods have already been described for chemically descal ing and deoxi-

2-30

is particularly heavy, an electrolytic treatment may be used to augment the chemical action. cleaners for descaling such materials. Solution temperature should be in the range of 77 to 99°C and a current density in the range of 200 to 2000 Ah2 (4 to 6 volts) is appropriate for many formulations (l). Those which contain cyanides should be operated at a lower temperature (z), i.e., below 54°C to prevent its excessive decomposition. be a problem with this technique. an acid dip to neutralize any residual alkali on the part surfaces.

For example, parts may be made anodic in proprietary alkaline

No hydrogen is evolved; therefore, hydrogen embri ttlement will not After electrolytic descaling it is common to use

Besides alkaline solutions, electrolytic descaling also is conducted in molten salt baths and acidic solutions. As mentioned earlier, when neutral salt baths are used for descal ing, an impressed direct current can electrochemically produce oxidizing or reducing conditions at the metal surface by supplying or removing electrons. Alternatively the baths can contain sodium hydroxide and additions such as sodium chloride and sodium carbonate, which produce reducing agents at the cathode and oxidizing agents at the anode when the current is applied. techniques are used mainly for the continuous treatment of strip metal and the removal of sand from the surfaces of castings. they also provide a descal ing and deoxidizing function.

Electrolytic descaling

For the latter example, however,

Etching. Anodic etching is used as a final step before electroplating some types of steel to produce a controlled amount of surface roughening to promote adhesion. Simple acid dipping or etching may not provide the degree of etching required for maximum adhesion. Chromic acid or sulfuric acid solutions are frequently used for electrolytic etching. 4500 Ah2 at 4 to 6 volts and treatment times are from 30 to 60 seconds. sulfuric acid is used, the temperature should be held below 30°C and the parts thoroughly rinsed afterwards to prevent carryover of the acid to the electroplating tanks or prevent rusting if left in air.

Current densities typically used are in the range of 1500 to If

Pick1 ing. Descal ing and pick1 ing are similar types of techniques for removing surface scale and oxides or other compounds. rapid than chemical pickling; thus, it can be economic when processing time is limited. the black oxide of iron, and the removal of chromium containing oxides on stainless steels, which are only removed very slowly by chemical pickling. often contain hydrochloric acid or sulfuric acid at concentrations of 30 or

Electrolytic pickling is much more

Also, electrolytic pickling is especially effective for the removal of

Solutions used

2-31

10 weight percent, respectively. The former are operated at about 55°C and the latter at about 80"C, and the voltage is held in the 3 t o 5 volts range. chloric acid electrolytic pickling is used for removing light scale and oxide from steel parts, for example. The parts are racked then made cathodic in the processing tank for a residence time of 2 to 3 minutes. sulfuric acid solution is the removal of light scale from spot welded parts. Immersion time need only be 5 to 20 seconds. thin oxide films from nickel alloys. parts in a solution at room temperature, which also contains sodium fluoride. current density should be in the range of 500 to 1000 A/m2.

Hydro-

An example for the use of a

Another example is the removal o f Here an anodic current is applied to the

The

Polishing. Electropol ishing is an electrochemical technique for producing very smooth, lustrous surfaces. When luster is the main objective, the technique is also known as electrobrightening. Electropolishing is applied to many metals and single-phase alloys, but is especially suited for finishing very hard and brittle materials that would be difficult to polish by conventional (mechanical) techniques. low alloy and stainless steels, nickel and cobalt alloys and zinc die castings.

Metals electropolished in industry include aluminum alloys, copper alloys,

The principle used in electropolishing is simple. When parts are made anodic in a suitable electropolishing bath, general dissolution occurs and some gassing occurs. As a result, a film of reaction products builds up on the surfaces on a macroscopic scale. Recesses contain a relatively thick coating of these products while asperities/protrusions are coated only thinly and offer less resistance to the passage of electric current. areas where dissolution should occur to generate a smoother surface. current density is highest in these regions, the rate of dissolution is the greatest.) dissolved, the current density on them falls and the applied current is shifted to other more prominent regions of the surface. described briefly for electrodeburring. have been cleaned first to obtain maximum effectiveness. Applications include improving the optical (reflective) properties of substrates; removing cold worked or damaged surfaces to provide a clean, stress-free surface; and improving surface electronic and magnetic properties (1) - that cannot be done by other surface finishing techniques. The other main advantage of this electrochemical technique is that the amount of metal removal can be controlled by controlling the applied current and specifying the other processing parameters and keeping them constant.

As a consequence, the current is focused on the very (As the

The process is self-regulating in that as the irregularities are

This is the same principle as was Surfaces to be electropol ished usually

____

~

~

2- 32

Temperature will depend on the solution used, as will time of treatment. However, the latter is also a function of current density, which typically ranges from 1000 to 4000 A/m2 ( 8 to 24 volts). num and its alloys and the higher values for stainless steels and decorative quality brass finishes. 1500 amperes to 18 to 24 volts, 1000 to 4000 amperes. Equipment requirements are described in Reference (5). - Agitation of the solution or parts or both improves the efficiency of the technique. The cathode material chosen will depend on the type of solution being used (1). Acid-resistant steels can be used in perchloric, phosphoric, nitric, sulfuric acids, and some mixtures of these, such as sulfuric/ chromic/phosphoric or sulfuric/phosphoric acids. acid-based solutions. acid solutions and so on. Lead, silver, or zinc cathodes can be used in neutral solutions. Where cost is not a concern, platinum cathodes can be used in most solutions. the surface area of the cathode should be twice that of the part being electropol i shed (anode).

The lower current densities would be used for alumi-

Power supplies needed range from 9 to 12 volts, 500 to

Lead can be used except in nitric Graphite can be used in phosphoric acid and hydrofluoric

In alkaline solutions carbon steel is quite adequate.

However, in general

Chemical polishing (brightening) methods have been improved and now can provide a good surface finish at a lower cost than electropolishing because of reduced capital equipment needs and the fact that electrical power is not needed. for the highest quality surfaces, for example, mirror finishes on high-purity aluminum, electropolishing is necessary and is specified. sequently electroplated, then it is convenient to use electropolishing as a pretreatment because the same fixtures can be used, thereby reducing handling costs.

However,

If parts are to be sub-

Table 2-3 summarizes some key features of the various electrochemical surface preparation techniques. As with Tables 2 - 1 and 2-2, the techniques are presented in approximate order of the decreasing amount of metal removal. The power supplies (usually transformers and rectifiers to give a low voltage, regulated direct current) can be of several types, such as silicon- controlled or saturable reactor designs. Often they are used in the constant current mode, but sometimes in the constant voltage mode. Air-cooled or liquid- cooled rectifiers can be used, but the latter, although more expensive, are preferred because they are totally enclosed and sealed. Consequently, corrosion of interior components is less likely to occur in the corrosive environments that can occur in poorly ventilated, electrochemical treatment areas and plating shops. Reference (5) - states that a power supply should be operated above 50 percent of its

All of these have low power requirements.

2-33

Table 2-3

N I w P

Technique

De b u r r i ng

P o l i s h i n g

E t c h i n g

P i c k l i n g

A c t i v a t i o n

Descal i n g

C 1 eani n g

SUMMARY OF FEATURES OF ELECTROCHEMICAL FINISHING TECHNIQUES FOR SURFACE PREPARATION

A p p l i c a t i o n

Removal o f bu r rs , r a d i u s i n g o f sharp co rne rs

Improve appearance ; removing damaged o r work hardened metal ; shaping and r a d i u s i n g ; removing r a d i o a c t i v e l aye rs ; c o n t r o l s i z e

C o n d i t i o n su r faces f o r subsequent t rea tmen ts by g i v i n g a c o n t r o l l e d roughness (e.g., s t e e l su r faces p r i o r t o C r e l e c t r o p l a t i n g )

Rust and s c a l e removal, p a r t i c u l a r l y f o r s c a l e r e s i s t a n t t o cbemical a t t a c k (e.g., b l a c k ox ide " on f e r r o u s mater i a 1 s )

A c t i v a t i o n o f su r face p r i o r t o e l e c t r o p l a t i n g

Removal o f r u s t and s c a l e

S o i l removal, e.g., grease, o i l , drawing compounds ( u s u a l l y f o l l o w s a l k a l i n e soak) p r i o r t o convers ion coa t ing , p a i n t i n g o r e l e c t r o p l a t i n g

M a t e r i a l Removal A c t i o n

E l e c t r o c hemi c a l ( 1 oca1 - i z e d anodic d i s s o l u t i o n o f me ta l )

E lect rochemical (anodic d i s s o l u t i o n o f me ta l )

E lect rochemical (anodic d i s s o l u t i o n o f me ta l )

Chemical p l u s sc rubb ing a c t i o n o f H2 bubbles (cathode) o r 02 bubbles (anode) ; i n h i b i t o r s p reven t excess ive a t t a c k o f meta l

Chemical p l u s sc rubb ing a c t i o n o f H2 bubbles (cathode) (No meta l removal)

Chemical p l u s sc rubb ing a c t i o n o f H2 bubbles (cathode) (No metal removal )

Chemical p l u s sc rubb ing a c t i o n o f H2 bubbles (cathode) o r 02 bubbles (anode); i n h i b i t o r s p reven t a t t a c k o f metal s u b s t r a t e

Power Requi rementsa Type o f F i n i s h

5-200 A/dm2 (5-25 v o l t s , 5-15 minutes)

5-50 A/dm2 (5-12 v o l t s , 1-15 minutes) l u s t e r p o s s i b l e

L i t t l e chanae i n o r i o i n a l meta l su r face b u t bu r rs , f l a s h , e t c , removed

Very smooth; m i r r o r - l i k e

15-45 A / d d (4-6 v o l t s , c l minute) e n t i r e s u r f a c e

Clean and roughened ove r

5-10 A/dm2 (3-5 v o l t s , Clean from o x i d e and c2 minutes) sca le, perhaps smal l

amount o f aenera l e t c h i n p t o meta l su r face

1-15 A / d d (3-12 v o l t s , <2 minutes) formed ox ide ( o r o t h e r

Clean f rom s o i l and a i r -

p a s s i v a t i n g ) f i l m s , meta l su r face n o t chanoed

sca le , o r i a i n a l meta l 3-6 v o l t s (1-3 minutes, Clean f rom o x i d e and l o n g e r f o r s a l t b a t h descal i n g ) su r face n o t chanqed

1-15 A/dm2 (3-12 v o l t s , Clean f rom s o i l s , o r i g i n a l meta l su r face 2 minutes) n o t chanqed

a1 dm = 0.1 m.

i l i i I '

rated capacity to minimize the ripple in the direct current output. rectifiers are preferred for electrolytic processes (1) to minimize ripple, and phase multiplication (e.g., six-phase) can be used to give less than 5% ripple that will meet all commercial specifications.

Three-phase

Mechanical-Chemical Methods (1.4)

Mass finishing techniques, such as vibratory finishing and barrel finishing, can be modified t o increase the rate of metal removal to accomplish the desired finish, as indicated in Figure 2-5. cal dissolution along with the mechanical action. A combined mechanical-chemical technique can provide deburring with radiusing and contouring. the use of acoustic energy in ultrasonic cleaning to help dislodge soil from the surfaces of small or precision parts to reduce the processing time and increase the efficiency of the cleaning technique. where hand cleaning would be required otherwise.

Chemicals can be added to liquid media to provide chemi-

Another example is

Although an expensive method, it is used

Combined techniques are more expensive than individual finishing techniques, but for some special applications the combined action is necessary and desirable. For example, some small, complex ferrous parts that need to be finished with high precision are subjected to a chemically accelerateq, barrel finishing technique (5). - Watch and camera parts are cleaned ultrasonically.

Mechanical-Electrochemical Methods (1.5)

Electrochemical techniques can a1 so be used to accelerate mechanical surface preparation techniques or provide greater control over metal removal and surface finish. Figure 2-5 lists deburring, grinding, honing, and shaping (contouring) as techniques that can be improved for certain applications by impressing a direct current on the workpiece, which is made the anode in the electrochemical cell setup. Some of these combined techniques are applied to small , precision parts that are processed in mass finishing equipment (5 ) . the individual parts are fixtured in conventional ways.

However, for most applications

With electrolytic (or electrochemical) shaping or grinding an electrically conductive abrasive wheel or tool is made the cathode and the part being processed the anode (positive electrode). An electrolyte solution is supplied to the surface being finished while the current is applied. chemical machining and a considerable amount of metal is removed locally.

The technique resembles electro- The

2-35

electrolytes contain mixtures of inorganic metal salts (e.g., sodium nitrite,

agents) but are differently formulated to a1 low surface passivation where grinding is not occurring. surface areas being treated are smaller. Current in the range of 50 to 3000 amperes, corresponding to voltages in the range of 4 to 10 volts, are typi- tally used. The tool rotates at 4000 to 6000 (surface) feet per minute and the solutions are used at room temperature. Reference electolytic grinding that are used in industry.

sulfate, borate, carbonate; potassium nitrite, nitrate; and surface active/wetting -

____

Less power is needed for grinding and shaping because the

-

lists the various types of

In electrolytic (or electrochemical) honing, the resulting finish is similar to that obtained by conventional honing, but the surface is essentially stress free, and has not been subjected to as much heat damage. The rate of meta removal is faster, but the amount of metal removed is less than with electrolyt c grinding. Solutions used for electrochemical machining can be used for electrolytic honing. Sodium chloride, for example, is inexpensive and suitable for many applications. Most of the metal removed by this technique is removed by electrochemical action, at high-current densities, rather than mechanical action, in contrast to the technique described above. such as some steels.

COATING REMOVAL TECHNIQUES

Thus, the technique is especially suited for hard materials

!

The necessity to strip coatings prior to refurbishing parts occurs from time to time. ing method to ensure satisfactory results are obtained when the parts are recoated. Coating removal activities form a very small part of the total metal finishing activities. For this reason only a brief discussion of coating removal (stripping) techniques will be given in this section of the report.

When it is necessary, as much care must be taken as with the original coat-


A chemical formulation is chosen to provide a solution that will remove the coating, but not affect the substrate material in an adverse way. The solution selected will depend, therefore, on the composition of the coating material and that of the substrate material. solution and temperature o f operation should be selected to provide the optimum stripping conditions for the coating/substrate combination. common chemical stripping solutions for metals and conditions of use taken from

The concentration of the active ingredient(s) in

Table 2-4 1 ists some

2-36

Table 2-4

SOME COMMONLY USED CHEMICAL STRIPPING SOLUTIONS FOR SELECTED COATINGS

Coating Mater i a1

Brass

Brass

Brass

Cadmium

Chromium

Chromium

Copper

Copper

Copper

Gold

Gold

Lead

Nickel

Nickel

Si 1 ver

Si 1 ver

Sol der

Tin

Zinc

Substrate Mater i a1

Aluminum

Steel

Zinc

Steel

Brass or Copper

Steel

A1 uminum

Steel

Zinc

Brass

Nickel

Steel

Steel

Various

Brass or Copper

Nickel

Var i ous

A1 uminum

Steel

Sol uti on Sol uti on Composition

50% HNO3

Solution of CrO3 + H2SO4

50% H2SO4

Solution of 15 g Sb2O3 in 1 liter HC1

Dilute HC1 or H2SO4

5-20% HC1

50% HNO3

Solution of 500 g CrO3 + 50 g H2SO4 per liter

Solution of CrO3 + 0.5% H2SO4

Solution of H202 (1 vol., 20%) + NaCN (8 vol., 1WA)

(Same as for Brass)

20% HNO3

Solution of H3P04 (50 vol.) +HNO3 (20 vol .) + H2SO4 (20 vol .)

Solution of H PO4 (15 vol.) + H2SO (5 ~ 0 1 . 3 + HNO3 (5 vol.) + 5 g h thiosalicycl ic acid

Solution of H2SO4 (19 vol., conc. ) + HNO3 (1 vol . , conc.) (Same as for Brass)

95% acetic acid + 5% H202 (30% solution)

50% HN03

Dilute HC1

Opera t i ng Condi ti ons

Room Temperature --

Room Temperature --

20 to 50 C

20 to 60 C

Room Temperature --

100 c Keep Solution Cool

Keep Solution Cool

65 C --

60 C

50 to 60 C

50 to 60 C

Room Temperature

--

Warm Solution

2-37

References (9) - and (11). - Reference (8) - provides details for specific metals and alloys and gives some data on useful life of the common stripping solutions.

El ectrochemi cal Met hods (2.2)

When chemical stripping is not very effective, the superimposition of a direct current with the part made anodic in the stripping solution, can often facilitate coating removal. A low-voltage source of direct current is required as it is for electrolytic etching and electropolishing, which have already been described. Table 2-5 summarizes some of the conditions used to strip metals of interest in industry. The data were taken from References (E), (!3), and (g). As for chemical methods, the solution and operating conditions selected will depend on the coating and substrate materials. The coating must be removed in a reasonable time, without appreciable attack occurring on the substrate. If the anodic stripping rate is to be increased, then it is better to do so by raising the temperature of the solution or provide more agitation, than to increase the voltage or current (8). - Excessive polarization of the anode may occur and the stripping rate decrease or pitting attack on the substrate increase, both of which are detrimental. (especially old electrodeposited coatings) is resistant to dissolution, then light surface abrasion to bFeak down oxide or other films or a brief cathodic pretreatment evolving hydrogen on the surfaces may be helpful. Equipment requirements will be similar to those used for electroplating or electropol ishing, which are discussed elsewhere in this report. Detailed procedures for specific metals and alloys are given in Reference (E).

If the coating

Thermal Methods (2.3)

Organic coatings may be removed by direct heating (e.g., torches, heat guns) then mechanical treatment, or by immersing the parts in a molten salt bath containing oxidizing agents such as sodium nitrate (15). of stripping action. The molten salt stripping baths are usually alkaline, very stable, easy to use, and easy to dispose of when spent. typically fall in the range of 450 to 48OOC. organics, including rubbers, occurs in seconds, accompanied by gassing. parts are stripped, they are drained, cooled, quenched then any remaining salt is removed by a hot water soak. stripping is used are given in Reference (15).

Catalysts greatly increase the rate

Operating temperatures Oxidation of resins and other

After the

Safety precautions necessary when molten salt

2-38

Coating Mater i a 1

Brass

Cadmium

Chromium

Copper

Gold

Gold

Lead

Nickel

Nickel

Nickel

Si 1 ver

Tin

Tin

Zinc

Zinc

Table 2-5

SUMMARY OF SOME ELECTROCHEMICAL STRIPPING METHODS USED MOST FREQUENTLY IN INDUSTRY

Substrate Mater i a1

Steel

S tee 1

Steel

Steel

A1 umi num

Copper and Brass

Steel

Copper

Steel

Zinc

Steel

Brass

Steel

Brass

Steel

Sol ution Composition

Solution o f NaCN + NaOH

Solution of NaCN (100 to 200 g/l) + NaOH (30 g/l)

Solution o f NaOH

Solution of NaCN (100 g/l) + NaOH (50 g/l)

Solution o f conc. H2SO4 (6 vol.) + H20 (1 vol.)

Solution of NaCN (75 g/l) + NaOH (12 g/l)

Solution o f NaOH (120 g/l)

Solution o f H2SO4 (56 Be)

(Same as for Copper)

(Same as for Copper)

Solution o f NaCN (100 g/l) + NaOH (16 g/l)

10% NaOH

10% NaOH

10% NaOH

10% NaOH

Operating Conditions

Room Temperature, 2-6 V

2-5 A/cm2; 2-6 V

Room Temperature, 6 V

25 or 70 C; 2-6 V

6 V

6 V

80 to 85 C; 2-4 V

Room Temperature , 6 V




6 V

6 V

2-39

COATING DEPOSIT ION TECHNIQUES ~~ ~

Once metal surfaces have been cleaned and prepared to specification, they are often coated to preserve their appearance, to provide aesthetic qualities, or to provide specific functional behavior, such as corrosion resistance or abrasion resistance. As Figure 2-6 shows, there are about 30 fairly conventional methods of applying or depositing organic and inorganic coatings. techniques that are being investigated in the laboratory or developed in industry and finding only limited application at this time. trical, electrochemical , thermal , and physical techniques are discussed first, individually, fol lowed by a brief description of the numerous combined techniques.

-

-~

- In addition, there are another 10 or so

The mechanical, chemical, elec-

Mechanical Methods (3.1)

In both cladding and peen plating a metal coating is usually applied to a metal surface. A mechanical action, such as rolling or localized impact with an inert medium, is used to bond the coating to the surface of the substrate. Under most circumstances the bond (adhesion) is not very good, but the cladding process can be modified to form a good metallurgical bond. Mechanically assisted coating techniques find only limited use, even though the equipment needed is relatively simple and inexpensive. Examples of cladding metal combinations are aluminum on stainless steel, zinc or cadmium on steel , copper on steel , aluminum on aluminum alloy,

appl ications (13), - especially where certain properties are required on one surface and entirely different properties on the other, or where the environments on each side of the cladding will be different (e.g., heat exchangers).

copper and nickel on steel. Other combinations have been developed for specific ~~

Cladding. method is applied to coating semifinished products such as wire, tube, sheet, and strip that are later fabricated into the final products. precleaned metals or alloys are brought into contact and pressure applied to form a sandwich structure. The type o f equipment used to apply the pressure and produce the clad material depends on the form of the substrate. metal are bonded by cold or hot rolling. Better bonds are obtained with the latter, but care must be exercised to prevent one or both surfaces being placed in contact from becoming oxidized. For superior metallurgical bonds, hot rolling may be done in a vacuum using ultraclean materials. bonding of aluminum t o iron at about 100 to 200OC. desired thickness then heat-treated at 535 to 550°C to ensure a good bond (E).

This technique is also known as clad bonding or clad plating (1) and the

In cladding, two or more

Sheet, strip, or plate

An example of roll cladding i s the The product is rolled to the

2-40

Sometimes powdered metals (as a slurry or mixed with a binder) are coated onto a carefully prepared metal substrate using a roll ing technique (1). Additional steps, such as drying and compaction or sintering of the powdered coating, are necessary, as in the production of aluminum coated steel (12). - When heat treatments are necessary, care must be taken not to degrade the mechanical properties of the substrate.

Another variation of the roll cladding process is a technique in which a metal (such as a high-strength aluminum alloy) is placed into a mold as a liner. Steel is then cast into the lined mold. The resulting clad ingot can be reduced to the desired thickness by rolling.

Tubes, rod, wire, and some shaped sections may be clad using an extrusion technique. Tubes, as well as plates, cylinders, pipes, some fabricated components and structures, may be clad using an explosive bonding technique. As the name implies, in explosive bonding a shaped charge is placed on top of the cladding metal that in turn has already been placed on the substrate. A buffer material is used between the explosive and the cladding to prevent damage to the latter when the charge is set off. As the shock wave travels over the cladding, it is forced into intimate contact and a metallurgical bond is formed. and is still being developed.

Explosive cladding is a new technique

Peen Plating. adherent metal 1 ic coating by mechanical means involving compacting of finely divided particles of such metal to a coherent coating" according to the American Society for Testing and Materials. or centrifugal forces to apply adherent, compacted coatings from a suspension of particles of the material to be deposited (1). - Thus, the latter definition also encompasses nonmetal 1 ic substrates and nonmetal 1 ic coating materials.

Mechanical or peen plating is defined as "the application of an

A more general description is the use of impact

Typically a metal powder (particles or small flakes) is suspended in a rotating barrel, tumbler, or centrifuge, along with the parts to be coated. usually small, ranging in size from rivets, nuts, bolts, and washers to fasteners and springs, and not weighing more than 0.25 kg. The impact media are most often beads of glass or other ceramic materials. The suspension or slurry also contains a chemical additive ("promoter") that provides nonoxidizing conditions, which also helps modify the solution viscosity to control the amount of damage one part might cause another.

These are

Initially, the surfaces of the parts are cleaned (as in mass

2-41

finishing) then the rotation causes the metal to be peened onto them by the impact medium and other parts. particular part will ensure that all surfaces, including recesses, key ways, slots, ~

holes and so on, are covered. Coating thickness increases with time up to a practical limit of 50 m. The technique is useful for small, somewhat complex parts that would be difficult to electroplate or would be susceptible to hydrogen cracking if electroplated. oxidize in aqueous solutions and are more soft and ductile than the part material. Examples are zinc and zinc alloys, aluminum, tin, and copper or some of its alloys.

The correct size of the medium for the design of the

___

Best results are obtained with metals that do not readily


There are two types o f chemical coatings methods. reaction occurs and a constituent of a solution in contact with the surface, usually a metal, is deposited on the surface. plating. When an organic material is deposited, it is called autophoretic painting. In the second, a direct chemical reaction occurs with a constituent of a solution or gas and a compound is formed on the surface. approach is coloring. This, and other examples of chemical coating methods, are described briefly below.

In the first, a displacement

This is the principle of electroless

An example of this

Autophoretic Painting. Certain water-reducible paints can be deposited on metal surfaces through the catalytic action of the substrate metal on the materials in the paint ( 5 ) . substrates and the technique is used to apply coatings to tubular frames used in the manufacture of automobiles. be applied to inside and outside surfaces provided they are in contact with the paint bath. low-voltage direct current is not needed and coating costs are lower.

~

At the present time, commercial formulations only exist for ferrous

The advantage of this method is that coatings can

In some respects, the technique is similar to electroplating; however, ~

w. desired color, without necessarily changing any other surface property, is known as coloring. the surfaces of a part that is itself colored, or which can be colored by saturat- ing it with a dye. Chemical coloring is accomplished by producing chromates, phosphates or sulfides on a wide range of metals, including aluminum, cadmium, copper, gold, iron, nickel, silver, and zinc (1). - Surfaces to be colored must be thoroughly cleaned and homogeneous in structure, composition, texture, and so on, if uniform coloring is desired. The chemical solution may be applied by brushing

Any technique that is used to change the color of the substrate to the

Chemical or electrochemical methods can be used to form a compound on

2-42

or wiping or spraying. Alternative y, the parts may be immersed (dipped) into the coloring solution. and sealing if the compound formed s not itself of the desired color. films that are to be dyed should be porous so that the dye is distributed through- out the thickness. If wear occurs, the color then will not be lost. Both solid colors and special effects (e.g. , "antiquing") can be obtained when the chemical method is combined with other finishing techniques.

Post-treatments include rinsing, drying, and sealing, or drying Surface

As examples of the results that might be obtained ( 5 , z ) , the following techniques illustrate procedures that are used. For a glossy black coating on brass, the part is immersed in a solution of cupric carbonate and ammonia at 80 to 90°C for up to 1 minute; whereas to obtain a black coating on copper, a potassium sulfide solution may be used for up to 20 seconds at 15 to 35°C. A yellow coating can be obtained on magnesium by placing the part in a boiling solution of ammonium sulfate, sodium dichromate and ammonia for 30 minutes. using a solution containing cobalt and ammonium chloride if it is held at 60°C and the part immersed for 3 to 5 minutes. information on the coloring, bronzing, and patination o f metal surfaces.

A blue coating can be obtained on zinc

Reference (14) - is a further source o f

Electroless Plating. coatings as final finishes and sometimes as intermediate finishes. coated on steel wire as a lubricant for drawing; nickel is coated onto steel prior to enamelling; tin and zinc are used to prepare aluminum surfaces for electroplating; a gold "wash" is used to decorate costume jewelry; mirrors are "silvered". A number of different techniques are used to obtain these and other electroless coatings .

A wide variety of industries use chemical or electroless Copper is

Reference (1) - points but the confusion in the use of the terms "electroless" or "chemical" plating, which have been used to cover the wide range of coating techniques from solutions that do not require an impressed current to produce the desired effect. according to the driving force for the deposition reaction is a logical approach, as is shown below.

Subdivision of electroless (or chemical) plating techniques

2-43

Electroless P1 at ing 1

I Noncatalytic Processes

I ' 1 I' Cat a 1 y t i c I' "Di spl acemen t" 'I Contact 'I

Coatings Coatings Coatings Coatings "Aut oc a t a 1 y t i c I'

In autocatalytic plating the surface of the part catalyzes the reduction of a metallic ion in solution and the part becomes coated. catalyzes the deposition reaction. be covered with a thin film containing a suitable catalyst that will initiate the reduction reaction. The surface is said to have been sensitized and activated. The latter approach is commonly used for metals and nonmetals because only a few metals are able to autocatalyze metal reduction (1). - Alternatively, a nonauto- catalytic surface could be covered with a thin film of an autocatalytic material, such as nickel or cobalt, to facilitate deposition. Catalytic processes consist of a pretreatment, deposition, and post-treatment steps. aqueous or nonaqueous and contain a chemical compound of the metal(s) to be plated out; a reducing agent (reaction products, e.g., P, B, are usually incorporated into the coating); a complexing agent for the metal(s); a buffering agent to control pH; a stabilizing agent; and other additives to increase deposition rate, modify coating properties, and so on. adhesion. silver, plus nickel a1 loys and nickel with occluded particles.

The deposited metal further In catalytic plating, the substrate must first

The solutions used may be

Post-treatments include rinsing and baking to improve Metals commonly deposited are nickel , cobalt, gold, palladium, copper,

Displacement (or "immersion") coatings are obtained by placing the parts to be coated in contact with a solution containing a soluble salt of a more noble metal. The latter displaces the less noble metal from the surface and gives a coating. Thus, for this technique to work, the substrate has to dissolve and the ions going into solution are replaced stoichiometrically with the more noble ions. surface is covered, the reaction stops; hence only very thin films (10 to 20 m) are deposited. this technique, which also is known as "cementation". context is not a diffusion process.

Once the

Many noble, precious, and refractory metals have been deposited by However, cementation in this

Contact plating involves the electroless deposition of a metal on a substrate that is in contact with another metal. The same principle involved in dissimilar metal (galvanic) corrosion is responsible for the driving force in contact plating. An

2-44

example (A) is the deposition of tin from an aqueous solution on a zinc substrate suspended by an aluminum wire (contact). metal anodically dissolves, not the substrate metal. However, both substrate and contact metals become coated. The rate of deposition is faster, and coatings can be thicker with contact plating than the displacement coating technique, provided some bare contact metal is available. tin, gold, and zinc.

Unlike displacement coating, the contact

Metals coated most frequently are copper,

With all the electroless methods, the main advantage is that all surfaces available to the solution, whether by immersion or spraying, will be uniformly coated. ever, the coatings tend to be thin and not very resistant to wear. of the coatings will be different from those of similar electrodeposited metals and alloys or mixtures. phorous (if an oxy-phosphorus compound is used as a reducer). are heat treated they precipitation harden giving a hard, wear resistant coating with good corrosion resistance, Such coatings are used in dies and molds, valve and pump bodies, on impellers and the like.

How- The properties

As an example, electroless nickel coatings contain some phos- When these coatings

Electrical Methods (3.3)

Electrical methods, for the purposes of this report, refer to coatings deposited with the assistance of an electrical field (voltage gradient). use electrical energy to generate photons or produce plasmas, for example, are discussed under physical methods or combined physical methods (Figure 2-6). both the techniques 1 isted under electrical methods, large voltage gradients (thousands of volts) are used to accelerate ionized particles towards the metal substrate, which is made the anode (in ion implantation) or the ground electrode (electrostatic spraying) in the electrical circuit. As will be discussed below, the other- main difference is that ion implantation is carried out in a vacuum system while electrostatic painting is done at atmospheric pressure.

Other methods that

In

Electrostatic Spraying. liquid and powder, and porcelain enamels in powder form. The technique is attractive because it is not line-of-sight limited. The equipment used for paint spraying consists of a paint supply system; an atomizer (or gun); a power source and a spray booth with an adequate ventilation system. Parts to be coated are passed through the booth on fixtures hung from a conveyor system. The paint i s supplied to the atomizers by air pressure or displacement pumps. compressed air or by a combination of mechanical and electrostatic forces in an

Electrostatic methods are used to apply paints, both

It is atomized using

2-45

"airless" gun (5). - the body and the nozzle and needle are charged to a high negative potential. atomized paint particles acquire this charge and are attracted to the grounded workpieces. An airless gun, in contrast, relies on mechanical and hydraulic forces to propel the paint particles towards the workpiece (8) - and on a negatively charged metal screen placed near the parts to transfer the charge to the paint particles before they are discharged. Airless guns are particularly well suited for applying high solids paints. high voltages (typically 90,000 to 120,000 volts at 5 mA, 60 Hz) and can operate from a 110-, 220-, or 440-volt line supply. One power supply unit can service several atomizers. As may be expected, there is some overspray with this technique and hence the need for spray booths with air velocities in the range of 0.25 to 0.5 m/s. However, with good grounding, control of the voltage, and close spacing of workpieces, the overspray may be minimized. For parts with complex shapes, the fixtures can be rotated or several strategically placed guns may be used. For mass producing coated parts, automated electrostatic spray systems are available, using guns mounted on robot arms. Reference (15) - provides further information.

In using an air gun the nozzle is electrically insulated from The

The power supplies required must be capable of putting out

Powdered paints may be applied successfully by the electrostatic spraying technique, but if the substrate is not heated to above the melting point of the powder, a post-treatment is needed to fuse the paint to the surfaces of the workpieces. Both air and airless guns may be used, as described above. In addition, powdered paints may be applied by passing the parts through an electrostatically generated cloud of paint particles (produced with guns) or an electrostatic fluidized bed of paint particles suspended by a high-voltage applied field, as shown in Figure 2-9. The electrostatic fluidized bed technique (5) - is attractive because preheating is 'not necessary and coating thickness can be controlled more easily. porcelain enamels are applied in a manner similar to the airless electrostatic technique described above. Cloud systems require an explosion control system. Post-treatments are necessary to produce the desired enamel finish.

Powdered

Ion Implantation. As the name implies, this technique involves the acceleration of ions towards an amorphous or crystalline solid and their incorporation into the surface layers to modify the properties of these surfaces. of several KeV to many MeV are used in a vacuum chamber containing the parts to be implanted. Thus, the technique, because of its sophistication, is costly and only small surface areas may be treated in a batch process. Applications are primarily

Accelerating voltages

2-46

/ Fluidized powder

Part preheated above fusion temperature of powder

Input air (less than 10 cfm)

( a ) Conventional F1 u i di zed Bed Technique

Fluidized powder

Charging electrodes

Porous ceramic

( b ) E lec t ros ta t ic Fluidized Bed Technique

Figure 2-9. e l ec t ros ta t

Source

Diagrams showing principles of conventional and c f luidized bed powder application techniques.

Metals Handbook, Volume 5, Reference (5).

2-47

in the electronics industry and include the modification of the properties of semiconducting materials ("doping"). Metallurgical applications primarily involve the formation of alloys on the surfaces of pure metals or the modification of the surface properties of alloys to improve wear or corrosion resistance, for example, the implantation of carbon into stainless steel to case harden it. Sometimes heat treatment is used as a post-treatment to produce a chemical or metallurgical reaction. by a heat treatment to produce their carbides, nitrides, or borides (1). -

An example is the implementation of refractory metals into steels followed

The number of ions implanted and the depth to which they are implanted depends on the composition and structure of the substrate, the nature of the ion, the accelerating voltage and the beam intensity. depth of penetration. implantation voltages and when lower atomic number elements are being implanted. Unlike diffusion coating techniques, most of the elements in the periodic table can be used to implant the workpiece and implantation may be done at low temperature. Surfaces may be implanted with different ions sequentially to form alloys. Metallic (electrically insulated) masks may be used to selectively implant surfaces, as may ceramic masks, metals, and photoresists.

In general, the lower the voltage the less Also, surface damage or sputtering is reduced at lower

Inert gases also may be used for ion implantation either to modify surface properties or to purposely sputter surfaces. nitrogen or argon into steels to improve corrosion resistance. nique can be used to clean surfaces and is more of a scientific or research tool.

An example of the former is the use of The latter tech-

Electrochemical Methods ( 3 . 4 )

Electroplating (also known as electrodeposition, e,lectrocrystal lization, or simply as plating) and electroforming were the first commercial applications of electric 'power, well suited because of their use of low-voltage, direct current that could be supplied by batteries and later motor generators, then most recently by rectifier/transformer units. alternative energy sources such as photovoltaic or wind generator units with associated energy storage devices.

In the future the direct current could be supplied by

Electroforming. deposits are built up to provide net shapes or specific structures rather than coatings. phonograph records or ice cream lollipops, or the flexible, perforated "combs" for

Electroforming is an adaptation of electroplating in which heavy

Examples include making molds for foamed plastic parts, glassware,

2-48

electric razors. formed. Also, it is possible to directly electroform. Many metals may be electroformed, but the most common is nickel because of its combination of strength, good surface finish, corrosion resistance, and ability to reproduce mandrel surfaces exactly. Copper is another metal commonly electroformed. Stainless-steel or chromium-plated mandrels are used to provide a conductive but electrochemically passive surface upon which to build up the electroform. Occasionally base metals or alloys, such as copper and Wood's Metal,* are used and these are removed after electroforming by chemical dissolution or heating to above their me1 ting point. Similarly, organic and sometimes inorganic materials are used, but these have to be metallized before they can be used as mandrels and the metallization has to be removed from the surface of the part after electroforming. Reference (E) provides a thorough review of electroforming technology.

The copper foil used in the electronics industry is electro-

Electroforming is a manufacturing technique rather than a metal finishing technique. However, it is possible to deposit a thin layer of metal or alloy on the mandrel or in a mold, than to build up a thick deposit of another metal on top of it. duces the mold or mandrel surface (whether smooth or textured). ing metal underneath acts as a support for the "coating" and provides mechanical integrity.

The thin layer acts as a coating, provides the desired properties, and repro- The cheaper back-

Electroplating. such as the electrochemical technique for depositing a layer of metal onto a surface made the cathode in an electrolytic bath containing the metal ion. metal ion is reduced by the flow of electrons in the circuit caused by the impressed direct current and, depending on the materials in question and the chemistry o f the substrate surface, coatings ranging from poorly adherent to those exhibiting a metallurgical bond can be obtained. inclusive enough because compounds, alloys, and mixtures of metals and nonmetallic materials can be electrodeposited. compounds can be deposited only when the surface is made the anode in an electrolytic cell.

The traditional definitions of electroplating include statements

The

This type of definition is not

Also, under some circumstances, metals and

An example of the latter is the deposition of lead dioxide coatings

*Wood's Metal is a low melting point Bi-Pb-Sn-Cd alloy.

2-49

from a bath containing lead cations. Most engineering metals, such as iron, copper, chromium, lead, tin, zinc, nickel, and noble metals, such as gold, silver, palladium, are plated out from aqueous solutions on cathodic surfaces. Refractory or very reactive metals, such as aluminum or titanium, cannot be deposited from aqueous solutions. Organic electrolytes or fused salt electrolytes must be used for these metals. When nonmetallic parts, such as plastic items, are to be plated, the surfaces have to be metallized to make them electrically conductive and a good electrical contact has to be made to permit the flow of the desired current ( 9 ) .

Usually the parts to be plated are metallic.

In conventional electroplating processes, the equipment needed is relatively simple: materials containing heating and/or cooling elements, agitators or airspargers, and the anode system, as shown in Figure 2-10. prevent the "slime" generated from falling to the bottom of the tank and contaminating it. Copper busbars or cables supply the low-voltage, direct current needed. Filters may be placed in the plating tanks or adjacent to them. sensitive to trace metal impurities, a small "dummying" tank is used in conjunction with the plating tank proper. able metals, such as iron, which would interfere with nickel plating. plating tanks, soluble or insoluble anodes may be used depending on the metal being plated. dissolve. With insoluble anodes, the composition of the bath has to be maintained by adding metal salts. Bath stability and good chemistry control is critical in obtaining quality deposits reproducibly. Different metal baths have different control requirements because of the variation of cathode deposition efficiency among the metals plated. Nickel and copper, for example, have an efficiency of close to 100%. The situation is further compounded when alloys are being deposited using metals with widely different efficiencies, e.g., iron-chromium alloys.

plating tanks of suitable size and made of suitable corrosion resistant

Sometimes anode bags are used to

For baths very

Dummy electrodes are used to plate out the undesir- In the

With soluble anodes, these have to be replaced as they anodically

Chromium on the other hand has a cathode efficiency of 10 to 20%.

Parts may be placed in the tanks manually (typically in small job shops for special coatings or for complicated shapes needing complex fixtures) or automatically using programmed overhead conveyers carrying fixtures or barrels. are known as tank or rack plating and barrel plating, respectively. method is best suited for small parts that would involve too much labor to rack on conventional fixtures, e.g., fasteners, jewelry items. A variation of tank plating is "jet" plating where a stream of the plating solution (electrolyte) is directed

These two techniques The latter

2-50

Low-pressure air for agitation. 4 &3 To exhaust fan and

I Insulator

fume scrubber

Exhaust hood

Steel support

Reinforcing angle Insulator

A anode rods; B: lead or Iead-tin anodes; C: cathode rod

Figure 2-10. Example of the tank and auxiliary equipment needed for hard chromium electroplating.

Source: Metals Handbook, Vol ume 5, Reference (5).

I ' I '

at the part, and the anode is placed in the stream. may be held over the tank (reservoir) or immersed in the tank. to build up thick coatings quickly on simple shapes and small surface area parts. Brush plating is a technique wherein the anode is covered by a porous material ("brush") saturated in the plating solution. The brush is then moved slowly over small areas at a time of very large parts to be plated or restored. The part is made cathodic and stationed over a reservoir so that the plating solution can be collected and recirculated. used to coat rolls and shafts with hard metals such as chromium or to build up worn areas with nickel prior to finish machining.

Parts to be plated in this way Jet plating is used

-

Brush plating is another fast plating technique often

To a much smaller extent, high-speed plating techniques are used for making foil, coating wire and strip, for reel-to-reel plating in the electronics industry, and for plating piston rings with chromium. Plating speeds are typically one or two orders of magnitude higher than conventional plating speeds. For example, it is possible to build up thickness of some metals at rates of 150 m per minute. Often these high-speed plating processes are integrated into manufacturing lines to produce a coated product. sophisticated, fully automatic, and designed especially for each application. The capital cost is higher but reduced labor, materials, and energy costs usually more than offset equipment and installation costs. Currents used in high-speed plating

in the 5 to 20 volts range depending on the application. ing, where much lower current densities are used, such high currents are only used when the surface area o f the parts being plated is exceptionally high. References (3) - through (9) - and (18) - provide details of processes and equipment along with details for electroplating specific metals and alloys. The scope of this topic is so wide that further discussion in this report is not possible. be pointed out that electroplating is just part of an overall metal finishing scheme that includes pretreatments as we1 1 as post-treatments and incorporates rinsing steps between each processing step. cadmium plating. A maxim that is often used is that the quality of an electrodeposited part is only as good as the quality of the substrate. obtain an adherent, coherent coating with the desired properties, the substrate must be clean, active, and have a suitable surface finish, whether that be smooth or textured.

The plating cells and ancillary equipment tend to be more

are in the thousands and tens of thousands amperes range, with voltages typically ~~

With conventional plat-

~

However, it should

Figure 2-11 shows a typical layout for

In other words, to

2-52

Q 0 - 0 % a .c

0, 32 c .a 0 zu

C 0, > 0 Q, C .- E: n

ml v

a V S a, L W rt a, e

LD

a,

n

E, 7 0 >

Y 0 0

-a S la I wl

a c, a, z

n

n

7

.. W V L S 0 m

2-53

A few words about electrical equipment requirements may be in order. rectifier sets are most commonly used to supply the low-voltage direct current needed. below. is typical for some barrel plating operations. operated at around 16 volts. Three-phase rectifiers are preferred by many electroplat ng facilities, a1 though some ( 7 ) - have modified their supply to provide six-phase rectification. latter the ripple in the direct current is less than 5%. A discussion of the types of rectifier used has been given earlier in this report. Control of the current and voltage to the plating tanks can be done from the power supplies using separate regulating transformers, or by changing the input waveform for thyristor-control led types. need special controls, Occasionally the plating current is reversed during plating or pulse plating techniques are used. Current interrupters and periodic reverse units then must be used to provide the necessary control of the plating process,

Transformer/

Many metals can be deposited at cell voltages of 8 volts or a little -

Hard chromium deposits require a cell voltage of 12 volts and this voltage Fully immersed barrels are usually -

Exact values depend on the metal being plated. ~

With the

the latter can provide constant current or voltage output, but the former

Thermal Methods (3.5)

This category incorporates one type of diffusion coating, namely those obtained by heating a part in a coating medium (solid or gas) and allowing the surface to react to provide a desirable alloy or compound. cementation coatings. Other coating methods such as galvanizing, tinning, and terne plating also involve diffusion, but of alloying elements from a liquid (mol ten metal) phase. These types of coating are discussed under chemical-thermal methods (see Figure 2-6). Other types of diffusion coatings may be produced by controlled heat treatment in solid, liquid and gaseous media. These surface modification techniques include those referred to as case hardening to provide hardness, wear resistance and antigal 1 ing properties to surfaces ("cases") while main- taining desirable properties for the substrate ("core") material s. usually are classified as heat treatments, and will only be briefly mentioned in the following paragraphs. ing, liquid nitriding, gas carburizing, gas nitriding, pack carburizing and carbonitriding, all of which are techniques to form carbide or nitride compounds in the surf ace 1 ayers of s tee1 s.

Diffusion coatings are also known as

Such techniques

Reference ( 4 l ) contains details about 1 iquid carburiz-

The most common diffusion coating methods are aluminizing or calorizing, chromizing, and sherardizing (zinc coating). such as silicon and boron. The purpose of diffusion and cementation reactions is

Other metals and metalloids may be coated

2-54

to provide an alloy surface with desirable properties on a lower cost base metal with less desirable surface characteristics, but good bulk properties. force is cost reduction. Reference (1).

The driving The general principles of diffusion coatings are given in

Cementation. This technique refers to the diffusion coating of parts packed into a suitable solid medium held in a closed reactor. Hence the alternative name of pack cementation is often used. The parts should be clean and preferably have all burrs and sharp edges and corners removed so that the coating when formed does not crack. The parts are placed in the packing material in the reactor and the lid put in place. temperature cycle designed to give the desired type of coating with respect to morphology and composition. The higher the temperature for refractory metals being coated with ceramic materials, for example, the more uniform the coating thickness and density. Molybdenum and tantalum alloys being coated with silicide coatings for example, are held at 1040 to 1150°C for between 4 and 16 hours, depending on the coating thickness required. 100 m. combined form); an activator or carrier gas producer; and an inert filler material such as alumina. metal chloride salt (or other halide) and small alpha-alumina particles (5). -

The reactor is then placed in a furnace and heated and cooled through d

Coating thicknesses typically range from 25 to The packing materials contains the coating material (in elemental or

For siliconizing, silicon powder is used along with an alkali

Sherardizing is the name given to a zinc pack cementation process that applies a zinc coating to steel parts for the purpose of providing corrosion resistance. is especially well suited for coating small parts such as fasteners (nuts, bolts, screws) and chains. Cementation gives a uniformly thick coating on all exposed surfaces. The coating material used is zinc powder containing 5 to 8% zinc oxide and the inert material is alumina or sand. Because of the volatility of zinc, an activator is not needed. After air and water vapor have been excluded, the reactor is brought up to 360 to 530°C and rotated for small parts (12). 3 hours, coatings up to 25 m thick may be formed, depending on the substrate material. The coating consists of a hard, matte grey Fe-Zn intermetallic compound containing about 8 to 9% iron. It provides good surfaces for subsequent painting.

it

After 2 to

Chromizing refers to the diffusion of chromium into steel or iron. done by the pack cementation process or by gas phase diffusion. also done by pack cementation or gas phase diffusion. calorizing. The coating material is powdered aluminum, the activator is ammonium

This may be Aluminizing is

The former is known as

2-55

chloride, and the inert packing material i s alumina (12). - Ferrous substrates and occasionally copper-based substrates, are placed in the reactor and rotated while at a temperature of 850 to 950°C and 700 t o 800"C, respectively, under an atmosphere of hydrogen. Thick coatings of the FeA13 compound are formed on iron. However, these are porous and brittle and a post-treatment is necessary.

Pack carburizing is a process for increasing the wear resistance of steel surfaces such as gears, wheels, pulleys, rolls and brake drums, and relies on generating carbon monoxide from a solid compound such as carbonates bound to wood charcoal and coke by oil, tar or molasses (41). - The carbon monoxide decomposes as follows:

2co-c~ + co2

and the carbon is deposited onto the parts to be coated held at a temperature of 815 to 1095°C. Diffusion into the surface occurs and a programmed cooling/ quenching cycle produces the desired physical properties. similar to gas carburizing, which is now favored for large-scale production.

Pack carburizing is

Diffusion Coatings. diffusion of the coating material into the substrate. As mentioned above, chromizing and aluminizing rely on this method. in a reactor with a mixture of chromium and alumina (or kaolin) powders and an ammonium halide. atmosphere to form a gaseous chromium halide. This decomposes in contact with the steel surface and the chromium diffuses in to form a chrome-rich coating on top of a region of intermetallic formation. Aluminizing is accomplished in an atmosphere containing dry aluminum chloride held at 700 to 1100°C. The aluminum reacts with the ferrous substrate material to form an aluminum-rich surface. .hardened by heating it in contact with silicon carbide powder in an atmosphere of Sic14 vapor. One to 3 hours at 1000°C produces thick, hard layers containing 14% silicon at the surface, but decreasing, of course, with depth.

Diffusion techniques rely on the gas phase transport and

In the former the steel parts are placed ~~

The reactor is then heated to 1000 to 1100°C under a hydrogen

Steel may be case

Gas carburizing and gas nitriding (and also carbonitriding) are techniques for diffusing carbon and nitrogen into the surfaces of steel parts to case harden or otherwise modify surface properties. resistance and strength, whereas nitriding is used to increase wear and antigalling properties. Steels to be nitrided must be hardened and tempered first. carburizing uses gaseous sources of carbon-carbon monoxide, such as natural gas,

__

__

Carburizing is used to harden, increase wear ~

Gas

2-56

Town's gas and certain propanes, or volatile liquid sources such as terpenes, dipentene, benzene, alcohols, glycols and octanes (41). - droplets onto a heated target plate in the reactor whereupon they immediately vaporize. Thermal dissociation at the operating temperatures of 495 to 565°C produces mixtures of COY CO2, CH4 and H20 vapor, depending on the liquid used. The carbon monoxide then reacts on the parts surfaces to produce the elemental carbon, which diffuses into the substrate. The amount of time at a given temperature determines the depth of the case. Continuous or batch furnaces may be used for carburizing or nitriding and forced-gas circulation gives a more uniform coating. Often carrier gases are used, and the ratio of carrier to reactant gas varies depending on furnace size, condition, reactant, amount of circulation and the parts. used after the coating has been formed. A one- or two-stage process is used, depending on the steel composition, to avoid forming a brittle layer known as "white nitride". vapor as the reactant gas.

The liquids are fed as

For gas nitriding the reactant gas is usually ammonia, and quenching is not

Boronizing is a similar process to nitriding and uses diborane

If molten salt mixtures are used to supply carbon and nitrogen for surface modification, the techniques are known as liquid carburizing and liquid nitriding. Both use mixtures containing cyanides, carbonates and chlorides, but carburizing is done at a higher temperature (845 to 955"C), and the nitriding bath contains a controlled amount of cyanate to provide thr? nitrogen. Furnaces may be externally or internally heated as shown in Figure 2-12. used for external heating, while partially or fully submerged electrodes are used for internal heating.

Oil, gas or resistance heating is

Flame Hardening. under the category of heat treatment (41). temperature flame is moved over the surface of the ferrous part to be hardened to cause localized heating above the transformation temperature. cooled in a controlled manner to develop the hardness or other properties desired. The technique is best suited for large parts that cannot be treated in furnaces for practical or economical reasons, or for which only a small area is to be hardened.

This is another surface modification technique usually falling As the name suggests, a high-

The part i s then

Physical Methods (3.6)

In this section methods are considered which rely heavily on some form of physical process or physically induced environment. performed under a vacuum or depend on the generation of energy beams such as

Thus, techniques that can only be

2-57

Alloy or steel pot

r 2 or more Thermocouple tangential

burners

Thermocouple

lnsulotlng materkal

Gas-fired or oil-fired Resistance h e a t e d

( a ) External l y Heated

Thermocouple Contactor Alloy electrodes,

Work-supporl angle

rThermocouple

Power supply -,.

Contactor Electrodes -

V

S u b m e r q e d e l e c t r o d e s

Transformer

( b ) Internal l y Heated

Figure 2-12. Examples of the d i f fe ren t types of furnaces used i n l iqu id carburizing.

Source: Metal s Handbook, Volume 2 , Reference (41).

2-58

photons are covered. tion of plasmas, these techniques are covered under combined physical methods, as shown in Figure 2-6 in the right-hand column.

Where electrical fields are also employed, as in the produc-

Laser Glazing. causes the surface of the workpiece to melt locally. A beam energy input of 15 kw/cm2 applied for less than 1 second to a nickel alloy substrate, for example, will cause melting to a depth of about 250 m. As the beam is rastered to the next location, the molten metal recrystallizes quickly forming a surface layer or coating with modified properties more desirable than those of the original metal. Laser glazing is a relatively new development and is still being studied in the laboratory. the laser and the size of the focused beam spot at the surface; therefore, raster- ing across the surface is necessary to modify large areas. with a refined grain size or amorphous/glass-like properties that can exhibit improved hardness, corrosion resistance or fatigue resistance, for example.

This is a technique in which the energy of a focused laser beam

Only small areas can be molten at one time depending on the power of

The result is a surface

Lasers are also used experimentally as a means of enhancing chemical vapor deposition methods. The laser heats the substrate locally, in one technique, to thermally decompose a coating precursor and intricate patterns of coating materials are obtained without the use of masks. Laser-assisted CVD is of interest, as a result of this feature, in the electronics industry. In another variation, the laser is used to activate coating precursors so that temperature sensitive substrates, such as plastics, glass, high-speed steel , are coated. glazing techniques, laser-assisted CVD is usually carried out in a reduced pressure system. potassium bromide window while the active gases needed to form the coating are bled into the "vacuum". Examples of metals coated are nickel from nickel carbonyl and aluminum from triisobutylaluminum. Carbon coatings may be deposited from acetylene.

Unlike laser

The laser beam, typically from a C02 laser source, is focused through a

Physical Vapor Deposition. which include vacuum evaporation, sputtering, and ion plating. The differences between these techniques is shown diagrammatically in Figure 2-13. ceramic, and semiconducting coatings may be deposited by physical vapor deposition, and in recent years combinations of these techniques have been used to obtain new materials or improved coatings. ion plating and reactive gases have been introduced to allow the deposition o f

This is a term loosely applied to a group of techniques

Metallic,

For example, evaporation has been combined with

2-59

1 2

'3

EVAPORATED ATOMS

- VACUUM CHAMBER

VACUUM EVAPORATION

1. HEATED EVAPORANT 2. UNHEATED SUBSTRATE 3. DEPOSIT

I L - _

I1 I ' l l I I I l l I I I I I I I I I I

(+)

2

'3

' 4 GLOW DISCHARGE

,1

UERT OR E ACT I V E AS - =t

I I L J

VACUUM CHAMBER VACUUMCHAMBER

SPUTTERING ION PLATING

1. UNHEATED EVAPORANT (TARGET) 1. HEATED EVAPORANT CONNECTED TO CONNECTED TO A COOLED CATHODE AN ANODE POSITIVELY CHARGED HV PLATE CHARGED NEGATIVELY HV 2. UNHEATED SUBSTRATE BEING A UP TO -5KV CATHODE NEGATIVELY CHARGED

2. HEATED GROUNDED SUBSTRATE 3. DEPOSIT 4, ANODES I + )

HV (0.5 TO 5KV) 3. DEPOSIT

Figure 2-13. Different physical vapor deposition techniques depicted diagrammatically.

Source: Surface F i n i s h i n g Systems, Reference (1).

compounds not formerly obtainable. In this section only vacuum evaporation is discussed. The other two fa1 1 under combined electrical/physical methods (see Figure 2-6) and are discussed under those headings. line-of-sight limited, so that complex parts sometimes cannot be coated even when multiple sources are used. PVD techniques are best suited for coating temperature sensitive substrates, applying thin films. The equipment needed is similar for all three techniques and consists of a vacuum chamber, a vapor source, a workpiece holder, a shutter to control coating thickness, a power supply to evaporate or ionize or sputter the coating material , and the appropriate controls.

In general, PVD techniques are

Vacuum Evaporation. tion, although recently the technique has been applied to nonmetallic materials. The term vacuum evaporation is more descriptive and is preferred. formed by evaporating the coating material from a source reservoir in a vacuum system. electron beams also may be used to form the vapor depending on the coating material. ing. substrate materials but the equipment is very expensive. sight limited and is a batch process. nickel , aluminum, cadmium sulfide, gallium arsenide, and as such the technique finds application in the electronics industry. Multiple sources are needed to deposit coatings of compounds and each is control led separately.

This technique is sometimes referred to as vacuum metalliza-

A vapor is

A resistive heating element is used often, but inductive coupling or

The vapor condenses on the cold surfaces of the workpiece and forms a coat- As discussed under PVD above, the coating may be applied to a wide range of

Coating materials applied include chromium, The technique is line-of-

References (5) - and (9) - discuss applications in detail and equipment requirements. For example, for a vacuum metallizer measuring 1.8 m in diameter and 2.0 m long, a three-stage vacuum pumping system would be needed to provide an environment of below 0.5 m pressure that is necessary to obtain good coatings. If the metallizer contained 22 evaporation sources for coating many parts with aluminum in a single batch operation, then a transformer capable of an intermittent peak load of 30 kVA at 17 volts would be needed. The output in the form of dc is fed to the resistive heating elements and best results are obtained if the evaporation can be done in 20 seconds or less (9).

Mechanical -Chemi cal Methods ( 3.1.1)

These methods refer to certain types of painting techniques such as spraying, brushing, and roll ing. by means of a mechanical application method.

An organic-based, chemical coating formulation is applied Electrostatic paint spraying has

2-61

already been discussed under electrical methods. paint is atomized and given an electrostatic charge so that it becomes attracted to the workpiece. In electropainting, which will be discussed in a later section, the ~

paint is given an electrical charge. In the methods discussed in this section, the paint is not chemically or electrically modified.

In this latter technique the

Air Spraying. the paint is atomized in a gun then propelled against the part to be coated by means of compressed air. Air spraying and the other paint spraying methods are used for large and small volume production and where a uniform coating with good appearance is desired (5 ) . The techniques are especially suited for large, flat areas such as automobile panels, appliance cabinets, architectural and building products, and the like. Air spraying consumes more paint than electrostatic spraying methods because of the overspray that occurs, but is cheaper to operate. the paint is preheated to 60 to 70°C, as in hot spraying, the viscosity is lessened and less energy is needed for atomization. higher solids coatings can be applied.

Air spraying is the conventional paint spraying technique in which

If

Also, deposition rate increases and

In air spraying, a small amount of the air supply is used to atomize the paint; the balance is used to propel the paint, control the pattern of the spray and the paint droplet size. for specific applications or with interchangeable nozzles ("air caps") to meet changing requirements. In either case, the capac ty must be large enough that fluctuations in pressure do not occur in use because this would affect the quality of the coating. tors and filters are used in conjunction with the compressors. used to contain the overspray and provide some environmental control o f the workplace when proper ventilation is used. trap and remove overspray in the booths so that the air can be exhausted. the booths exhaust large volumes of air, air replacement units are needed and installed next to the booths. These also serve to filter the air and deliver it at the required temperature, both of which help ensure that quality paint coatings are

A wide var ety of commercial equipment is available, often designed ~

A r compressors may supply one paint station or several.

Air regula- Spray booths are

Filters or water wash equipment are used to Because

ob ta i ned.

Airless Spraying. As the name implies, air is not used to atomize and prope paint, as described above. Instead, in this technique hydraulic pressure is to force the paint through an atomizing nozzle then towards the parts to be painted. A Less overspray is achieved with airless than with air spraying.

the used

2-62

thicker films can be built up without runs and sags when viscous paints are sprayed. as stringent ( 5 ) . However, the main disadvantage is that more skill is needed to properly use airless equipment because the paint flow cannot be throttled in use.

The equipment is simpler in construction and exhaust requirements are not

Painting. roller coating, dabbing, dipping, knife coating, curtain coating, flow coating, tumbling, which may be used with and without the use of mechanical masks, stopoffs, or silk screens. These techniques are described in detail in References (5) - and (z), and briefly mentioned below.

There are many techniques used to apply paints, including brushing,

Dabbing or padding is a very old technique little used in metal finishing and consists of dabbing the paint onto the surface by means of a paint impregnated pad or cloth glove. with a brush. The shape of the brush and the type of materials used for the bristles depend on the application and the type of paint being applied. applications, foam pads have replaced brushes. often used to apply decorative and maintenance coatings. apply good finishes. and brushing into one operation and is also used for decorative and maintenance coatings. Automated roller coating is a widely used industrial process and can be subdivided into direct, reverse, and high-speed roller coating. known as coil coating and is used to apply thin coatings of paint to strip steel moving at high speed through the equipment. product is siding and panels for building construction, automobiles and appliances. In knife coating, a doctor blade is used to control the thickness of the paint as well as spreading it over the part to be coated. In dipping the rate of withdrawal from the paint tank controls the film thickness. This is an inexpensive method but requires a large inventory o f paint. Curtain coating is the reverse of dipping. A thin film of paint falls from a slot in the bottom of a paint reservoir onto the parts to be coated as they pass underneath. sively on large, flat parts by this technique. placed in a closed chamber and jets of paints are directed onto them. paint drains off and is recycled. technique. Tumbling also incorporates a closed chamber. In this technique, however, the parts are tumbled into a metered amount of paint as the chamber is

- Brushing is another traditional method whereby the paint is applied

For some Brush painting is inexpensive and

Little skill is needed to Hand roller coating incorporates the principles of padding

Little skill is required, but more paint is used than in brush painting.

The latter is more commonly

The principal application for this

Thick films can be built up inexpen- In flow coating the parts are

The excess Complex shapes can be cheaply painted by this

2-63

rotated. or components .

Tumbling is very cost effective for painting large numbers of small parts

Mechanical-Electrical Methods (3.1.2)

Electric Arc Spraying. This technique is a variation of thermal (flame) spraying. The metal to be applied as a coating is fed into the spray gun and an electric arc is formed between it and another wire electrode of the same metal, but opposite electrical charge. propels it towards the surface of the part to be coated. deposition rates and good adhesion and the substrate temperature is low because o f the very localized heating (arcing) region. also less expensive than other thermal spraying techniques ( 5 ) . metals are used as the electrodes, alloy-like deposits may be obtained. for successful application of this technique the metal(s) must be ductile and have good electrical conductivity.

The arc melts the metal and compressed air atomizes it and This technique gives high

The electric arc spraying technique is If dissimilar

However,

Ion Bombardment. differs in that an inert gas, such as argon, is used at low energy to sputter the surface in order to clean it. vacuum coating techniques, including ion implantation.

This technique is similar in principle to ion implantation but

Often this technique is used as a precursor to other

Powder Compaction. steel strip by a mechanical-electrostatic method (2). given a thin electrodeposited coating of iron from a ferrous chloride-si1 icate solution. trostatic technique. erature, followed by a sintering step at 450°C for 16 hours. be a coated steel strip having good corrosion resistance and good mechanical properties .

In one proprietary technique aluminum coatings are applied to The steel strip is first

Aluminum powder is then applied to this surface uniformly using an elec-

The result is said to The powder layer is then compacted by rolling at room temp-

Mechanical -Thermal Methods (3.1.3)

Flame Spraying. In this thermal spraying technique combustible gases are used to melt the coating material, which may be in wire, rod, or powder form. Compressed air atomizes the material and propels it towards the part to be coated. Commercial equipment is available with which the air cap/nozzle may be changed and different gases used to permit different materials to be melted and different applications to

2-64

be met. While flame spraying uses relatively unsophisticated, inexpensive equipment, and thick coatings may be applied to a range of substrates, there are some disadvantages. More heat is transmitted to the substrate than with plasma arc or electric arc methods, hence some deformation or changes can occur. for building up worn parts that have to be subsequently machined to size.

The coatings tend to be porous and have a low-bond strength.

However, the technique is well suited

A modification to the flame spraying method is known as the flame spray and fuse technique. As the name implies, the materials deposited as the coating are fusible and a subsequent heat treatment at 1000 to 2000°C fuses the coating. applied by this method are mostly nickel- and cobalt-based alloys containing boron, phosphorus, and silicon (or combinations of these) as melting point depressants and fluxing agents. including induction heating or furnace heating in an inert or reducing atmosphere. These types of coatings are used where excessive wear is a problem and are used in the oil industry and for agricultural equipment. The surfaces may be ground after fusing to improve smoothness or dimensional tolerances. contain further information.

The coatings

Heat treatment may be accomplished by one of several methods,

References (5) and (E)

Chemi cal -Mechanical Met hods ( 3.2.1)

There are a few mechanical coating methods where the action of a chemical agent is needed to provide a satisfactory coating not otherwise obtainable. agent may be an adhesive, as in flocking or metallizing, or an explosive mixture, as in explosive bonding.

The chemical

These techniques are briefly described below.

Detonation Plating. energy to powder particles that are directed at the part to be coated (1). - explosion is obtained by the electric spark detonation or an explosive mixture of gases such as oxygen with acetylene, or oxygen (air) with methane or propane. inorganic powder is stored in a separate container and is fed into the reactor (gun) via an inert carrier gas. the gases accelerates the particles to speeds some 4 to 7 times faster than conventional spraying techniques and as a result, when they hit the surface of the part to be coated, an adherent layer is formed. parts and a wide range of materials deposited for wear resistance, corrosion resistance, antifriction and high-temperature lubrication uses. temperature reached is very high, the reactors are water cooled and substrate temperatures do not exceed 200°C.

In this technique an explosion is used to impart high-kinetic The

The

The shock wave resulting from the detonation of

Thick coatings may be applied to large

Even though the gas

This feature is an advantage compared with the

2-65

plasma spraying technique. ties are needed.

As with explosive bonding, soundproof coating facil i-

Explosive Bonding. This technique is similar to the cladding technique already described. The substrate and the coating material, in the form of a flat sheet normally, are placed in contact and an explosive charge is placed on top of the sandwich on top of the coating material. When the explosive charge is detonated, the shock-wave front that travels across the sandwiched structure generates gases that in turn physically bring the two materials together at the interface with such force that a metallurgical bond is formed (1). - For the successful application of this technique, the substrate metal and the coating metal must have sufficient ductility to allow the surfaces at the interface to flow together ("jetting" effect). can range from a few square centimeters to several square meters. A buffer layer is often used between the explosive layer and the cladding to prevent damage to the coating surface. mechanical damage to the substrate or coating materials. niques are limited to parts having relatively simple geometry, e.g., plates and tubes. Special facilities are required for safety and noise abatement reasons. Examples o f applications are stainless steel clad with copper, low-carbon steel clad with stainless steel.

One or more layers of cladding material can be used and areas to be clad

Also, the detonation velocity must be controlled to prevent Explosive bonding tech-

Flocking. plastic or me a1 substrates to provide a decorative appearance. material that can be made of various natural or man-made short fibers, such as cotton, wool, rayon, nylon or acrylic. with an adhes ve, then the flock is applied to the adhesive. Either mechanical or electrostatic processes are used for the flocking operation. Mechanical processes .blow, suck or push the flock onto the adhesive layer. propel the fibers by means of an imposed electric field. i s more expensive, but produces a denser flocking, wearing apparel ; home furnishings such as floor coverings, draperies and uphol- stery; and for automotive parts such as floor coverings and window channel linings. Flocked wallpapers at one time were very popular and are still used to some extent in commercial buildings such as offices and hotels. used on toys also. To a lesser extent, flocks have been attached to metallic substrates to provide unique textures.

This technique is used to impart a texture or pattern to paper, cloth, Flock is a fibrous

Materials to be flocked are first coated

Electrostatic processes

Flocked materials are used for The electrostatic process

Flocked plastic surfaces are

2-66

Gilding. metal, etc., with gold in leaf or powder form. The term also includes the similar application of silver, palladium, aluminum and copper alloys. is drawn on the surface to be coated, and an adhesive is applied within the boundaries of the design. is then placed over the adhesive. excess trimmed to give a decorative coating. stamping, as on leather book bindings. applied to the electrolytic deposition of a thin flash of gold to metals to provide aesthetic or decorative appeal. Used in this context, gilding should be more properly classified as an electrodeposition technique.

This is the technique of decorating objects made of wood, glass, plaster,

In gilding, a design

A very thin sheet of metal foil (e.g., gold leaf) or powder The gild is smoothed and burnished, and the

Gold foil also may be applied by hot In addition, the term gilding is generally

Chemical -El ec troc hemi cal Methods (3.2.2)

These methods differ from the electrochemical methods in that a chemical change occurs during the coating process or subsequent to the coating process, but as an integral part of the overall process. on the surface of the metal rather than a metal or alloy being deposited. tropolymerization monomers are deposited on the surface and polymerized electrochemically. and cure the coating. paragraphs.

Thus, with anodizing an oxide film is formed In elec-

In electropainting the deposit must be heated to drive off the binder These and related techniques are discussed in the following

Anodizing. as anodizing. cell. dc is applied between the anode and an inert counter electrode (cathode). oxide formed is an integral part of the surface and can be compact and coherent (e.g., lead dioxide) or porous (e.g., aluminum oxide). films are built up on compact "barrier" films. because o f their tendency to anodically dissolve, either as metals, or sometimes as oxides (1). - Metals commonly anodized are aluminum, magnesium, lead, titanium, and tantalum. With the latter the coatings tend to be very thin (hundreds of Angstroms) and interference colors result (see "coloring" below). produce decorative finishes (the porous coatings can be impregnated with pigments or dyes) or to improve corrosion and/or abrasion resistance. cation of this technique is the decorative finishing of aluminum and its alloys; thus, the comments given below pertain largely to this application.

The anodic or electrochemical oxidation of surfaces is commonly known The surface to be oxidized is made the anode in an electrochemical

A relatively low-voltage direct or alternating current or ac superimposed on The

However, even the porous Not all metals can be anodized

In addition, some alloys may be anodized, such as stainless steels.

Anodizing is most often done to

The principal appli-

2-67

When aluminum is made anodic in a suitable electrolyte a thin, nonporous barrier film first forms, then on top of this a thick, porous coating develops. ness of this outer coating will depend on the applied voltage, the electrolyte composition, and temperature and the time of treatment. based on fairly common acids such as sulfuric, chromic, phosphoric, boric or oxalic. other than aluminum.) aluminum. oxygen ions migrate in the opposite direction to form Al2O3. sulfuric acid electrolyte operated at 20°C and a current density of 1.5 A/dm2, the coating would build up to a thickness of 25 m in just over an hour (12). - coating ratio is the weight of the unsealed oxide divided by the weight of the metal consumed. If the anode efficiency is loo%, the ratio would be theoretically 1.89; however, lower values are obtained due to some chemical dissolution of the oxide coating as it is forming.

The thick-

Electrolytes are typically

(Occasionally alkaline or fused salt electrolytes are used, but for metals

-

-

The anodizing voltage is in the range of 10 to 25 volts for At this voltage aluminum ions migrate through the barrier film while

In a typical 10%

The

The type of electrolyte and the bath temperature determine the hardness o f the coating. For example, very hard coating are obtained at -5 to +5"C, 2 to 10 A/dm2, and much higher corresponding voltages (80 volts). metal can influence the color of the coating. Silicon gives grey films, manganese gives brown films, and chromium gives green/yellow films. tions, chromic acid electrolytes are widely used because any electrolyte trapped in the pores after anodizing can be detected as a yellow stain. lyte is contained in a steel tank and stainless steel cathodes are used. Lead- lined steel tanks are used for the sulfuric acid-based electrolytes. Chromic acid electrolytes give thinner coatings with good corrosion resistance and high ductility, but only moderate hardness (12). - equivalent sulfuric acid electrolyte techniques. A1 ternating current processes have been developed for specific applications. thick, corrosion-resistant films are formed in a reasonable time at 50 to 60 volts, 28 to 30°C. With sulfuric acid electrolytes, some sulfides are incorporated into the coating and these react with specific chemicals, if later treated, to provide distinctive colors. solution due to the formation of cadmium sulfide. Anodized coatings must be sealed as deposited, or after coloring when this is desired.

Lower temperatures give anodized coatings with better wear resistance.

The a1 loying elements in the substrate

For aerospace applica-

The latter electro-

Anodizing voltages are higher than for

With oxalic acid electrolytes

An example is the bright yellow formed with cadmium acetate

2-68

Coloring. hundred Angstroms thick, interference colors result. As mentioned earlier, it is possible to color some stainless steels this way and colors ranging from golden yellow to a blue-black can be produced by this process. A proprietary process exists, which has been licensed to color architectural hardware, and in one instance in Japan, color roofing shingles for a temple. Thin films other than oxides may be developed to produce colored surfaces, for example sulfides. natively, thick oxide films (anodized surfaces) may be colored by one of several methods. The three common techniques shown in Figure 2-14 are (19):

Thin oxide films may be built up on metals and when these are only a few

Alter-

0 Absorption of a dye,

0 Inclusion of small particles,

0 Electrolytic deposition of small particles.

In the first method, an organic dye or inorganic pigment is incorporated into the pores of the anodized layer by dipping the part into an appropriate solution. The pores are then sealed in boiling water to fix the color in place. Many colors may be produced by this method but organic dyes especially may be subject to bleaching or discoloration when exposed for a long time to sunlight or elevated temperatures. Although equipment requirements are minimal, dipping is a lengthy process. In the second method, sometimes referred to as integral coloring, finely divided particles are incorporated into the coating as it is being deposited. These particles scatter light and produce a color effect ranging from light bronze to black. These types of color coating are more durable than those described above but more sophisticated process equipment is needed and anodizing voltages and currents are higher. The third method depends on electrodepositing small metal particles in the pores of the anodized coating. Commercial processes use an ac that deposits metals such as tin, nickel, or cobalt during the cathodic portion o f the cycle. These metal particles scatter light and produce coloring as in the second method; however, the color is dependent on the amount of metal deposited, not the oxide film thickness. After sealing, these coatings are very durable. plish than in the second method. energy use is less (19). With all three processes, reproducible coloring can only be obtained with carefully controlled processing and identical surface finishes generated during the pretreatments .

The coloring is easier to accom- Equipment requirements are less demanding and

Coloring can be obtained in a relatively short time.

2-69

inclusion of dye

anodically produced oxide coating

barrier layer 1

codeposited metal particles


aluminium\

electrolytically deposited metal particles


I t barrier layer I L

a I u m i n ium\

Figure 2-14. methods for coloring anodized coatings.

Diagram showing the different

Source: T i n and I t s Uses, Number 146, Reference (19). -

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Electropainting. of the principal of electrophoretic deposition, in which charged "molecules" in solution are moved along a voltage gradient towards the substrate to be coated, discharged, and incorporated into a coating. particles may be deposited in this manner, but the largest commercial application is the deposition of primers and paints, especially for the automotive industry where the body frames are coated with a corrosion-resisting primer (20).

Electropainting or electrocoating is the most common application-

Organic and inorganic or ceramic

An electropainting bath consists of the solvent, typically water; acidic or basic organic groups that dissolve to give micelles by adding either an acid or base; inorganic solids such as copper chromate, titanium dioxide, or carbon black; and an organic pigment to supply the desired color. The total solids content is about 10%. The coatings systems may be designed so that the charged micelles are discharged at the workpiece made anodic or cathodic. In anodic electropaint corrosion of the substrate may occur, which in the automobile industry is course. With cathodic electropainting, which is not as widely practiced, popular in the USA, no corrosion of the substrate occurs and the throwing (ability to cover all surfaces) is excellent; however, the paint formulat

ng some iron, of but most power ons are

more complex and require the inclusion of an organic solvent. Also, the adhesion obtained is not as good as with the anodic process. ing obtained is uniform in thickness, contains very little water (40% because of an electroosmotic effect during deposition) and cures rapidly when baked. It can be handled, if necessary, directly after coating because of the low-solvent con tent .

With both processes the coat-

The equipment requirements for anodic electropainting are simple. moved through rectangular steel tanks automatically with an overhead conveyor system. The tank serves as the cathode. Energy consumption is low. Voltages that range from as low as 80 volts to as high as 300 volts are typical, depending on the application, and the current during the deposition stage will be equivalent to about 5 mA/cm2. ( A power supply of 50 to 400 volts, 50 to 4000 amperes is suitable.) A coating up to 25 m thick can be deposited in 5 minutes (residence time) or less, and requires about 1 to 3 kWh per 10 m2. the electrolytic cells are of a more complex construction and separate steel or graphite anodes are used, separated from the paint solution by a membrane. details about equipment requirements are given in References ( 5 ) and (€3).

The parts are

With the cathodic process,

Further

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Electropolymerization. In some respects this technique is similar to electroplating or electropainting. What makes it different is that the coating bath contains organic monomers that are discharged at the surface of the workpiece made an electrode in an electrolytic cell. and a coherent, adherent coating forms. been electropolymerized according to the literature, the technique is still essentially at the laboratory development stage. For example, steel, brass, and bronze have been coated with polymethylmethacrylate from a bath containing the methylmethacrylate monomer. Polybutylmethacrylate has been deposited from an isobutyl- methacrylate bath. Depending on the monomer being polymerized, the workpiece is made the anode or cathode. As an example, for the case of methylmethacrylate the polymerization reaction is initiated at the cathode. The same type of equipment used for cathodic electropainting is 1 ikely to be used if the electropolymerization technique is ever commercialized.

~

During the discharge process, polymerization occurs While some 30 monomers and copolymers have

-

C hemi cal -Thermal Met hods ( 3.2.3)

This category includes those techniques where the use of a heated substrate or the application of a heated coating medium results in a chemical modification or alloying of the surface to be coated. When heat is used to affect a chemical change directly (such as coloring or thermal oxidation) or indirectly (such as porcelain enamel 1 ing) the techniques are classified as thermal -chemical methods and described in a later section.

-~

Chemical Vapor Deposition. This is a general term, like physical vapor deposition (PVD), which includes techniques sometimes referred to as vapor plating, vapor phase growth, gas plating, and pyrolitic plating. The preferred term is chemical vapor deposition (CVD) and refers to techniques where metallic and some metalloid and nonmetallic coatings are deposited onto heated substrates by passing over them vapors of the coating material in a sealed chamber, usually operated at atmospheric pressure, but occasionally at a slightly reduced pressure when coating quality is improved. Very many materials may be deposited on a wide range of substrates, with the limitations that the latter must be heated, typically in the range o f 150 to 1000°C and suitable vapors of the coating material must exist. he process is said (l2) to give a higher deposition rate (m per minute) than vacuum metallization and relatively thick deposits can be formed. isting gives an idea o f the range of materials that may be deposited ( 5 ) .

The following partial

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Metal Carbides, Borides, Si1 icides Oxides, Nitrides

BqC, Sic, Tic, ZrC, VC, WC Al2O3, BeO, Si02 Al, Be, Bi, Cr

Co, Cu, Fe, Ge, Ir AlB2, TiBp, ThB, FeB, Ni3B2 Cr2O3, Sn02, BN

Ni, Pb, Ru, Ta, W NizSi, WSi, FezSi, Co2Si ZrN, Si3N4, TIN

Most reactions require a temperature of 800°C, although a few occur below 200°C. The reactant vapors include chlorides, fluorides, bromides, iodides, carbonyls, hydrides, and some organometall ic compounds and hydrocarbons. Hydrogen is often added as a reducing agent. vapor. erature and widen the range of substrates that can be coated to include materials that were previously too susceptible to heat damage. the plasma-assisted technique are orders of magnitude lower than for conventional CVD (5). - Another variant, laser-assisted CVD, has already been described under "Physical Methods" for applying coatings.

Some reactants have to be heated to produce the desired More recently plasma-assisted CVD has been used to lower the reaction temp-

However, deposition rates for

In chemical vapor deposition the parts to be coated are placed in the reaction chamber and brought up to temperature. When it reaches the hot substrate material, the vapor decomposes and reacts with the surface to form the coating. reactant vapor supply is replenished. chamber to prevent oxidation of the substrate. thickness and the technique is not 1 ine-of-sight 1 imi ted. alloys may form during the reactors that occur. ment is used to diffuse the coating material into the substrate material (1). - types of reactions which can occur include: decomposition, disproportionation, reduction in the vapor phase, displacement and polymerization. Procedures for applying the more common materials are given in Reference ( 5 ) along with the effect of varying different process parameters. Applications include devices in the electronics industry; wear-resistant coatings on tools, dies and molds; coating of nuclear materials; and coatings for surgical prostheses.

The stable reactant vapor is then admitted.

Gaseous reactants products are swept away as the

The coatings formed are uniform in Air must be excluded from the reaction

Chemical compounds or Sometimes a subsequent heat treat-

The

Galvanizing. Galvanizing is one variation of the hot dipping technique and the term is used exclusively for applying a zinc coating to ferrous parts. nique, therefore, is known also as hot dip galvanizing and in recent years this term has been used to differentiate the technique (and the product) from electrogalvanizing.

This tech-

In galvanizing, the part is immersed in a bath of molten zinc metal.

2-73

Adherent coatings are formed which are comprised of layers of zinc alloyed with the substrate material and zinc-rich outer layers. corrosion function in corrosive environments. The corrosion resistance and mechan- ~

ical properties depend on the complexity and types of layers formed, while surface texture controls paintabi 1 i ty. When galvanized surfaces are not painted, they may receive a bright chromate dip and a protective lacquer coating to preserve their bright surface. Otherwise with time the zinc will dull to a matte grey color. Individual parts may be galvanized or some steel mill products can be galvanized continuously. tric resistance welded tubing, for example. uous, automatic, manually operated or semiautomatic ( 5 ) . - include coating structural steel for power generating plants; petrochemical plants; electric transmission towers and poles; structural bridge members; corrugated steel pipe and culverts; reinforcing bars for concrete cooling towers, precast concrete, highway and bridge decks; railroad and highway structure, poles and guard rails; marine pilings and rails; and fasteners. Coating thickness depends on application and depends on resistance time in the zinc bath. There is an upper limit above which sagging occurs and uniform coatings are not obtained, especially for rods, tubes, and wire. to improve drainage and increases brightness of the coating.

The zinc provides a sacrificial

-

Some companies have in-line galvanizing facilities for coating elec- The processes may be batch or contin-

Major applications

The addition of a small amount of aluminum to the zinc bath helps

Hot dip galvanizing is an inexpensive way to apply relatively thick coatings, but air knives or wipers may be used to remove excess metal or control coating thickness. weight on one side and a very thin or no coating on the other because it has been wiped off or electrolytically removed. sides to controlled thicknesses easily, but is only cost effective for relatively thin coatings (420 m) and can increase the susceptibility to hydrogen cracking of some substrates. duced into the steel during acid pickling can be driven off at the zinc bath temperature (about 460°C).

For example, some galvanized steel strip is supplied with a regular coating

Electrogalvanizing can coat one or two

Hot dip galvanizing has the advantage that any hydrogen intro-

There are two types of hot dip galvanizing. In Europe the "dry" process is used, while in the USA the "wet" process is used. The difference between the two con- cerns the type of flux used and when it is applied. applied to the parts, which are then dried before galvanizing. No prefluxing is used in the wet process, but a layer of flux is maintained on top of the molten zinc. The dry process is said to be less energy intensive ( 5 ) , - but is more

~

~~

In the dry process the flux is ~

2-74

affected by improper preparation methods used to clean the surfaces. heat used to heat the zinc kettles depends on the availability and cost of local supplies. is high, temperature can be easily controlled, and uniform heating of the kettle are all possible.

The source of

Oil, gas, electricity may all be used satisfactorily provided efficiency

Hot Dipping. coated in a bath of molten metal to form a coating. molten metal to form an alloy layer or region of intermetallic compounds. outermost layer consists of the unalloyed or unreacted coating metal. is an inexpensive way of applying thick coatings to a wide variety of parts. The process may be operated manually, automatically, or semiautomatically. Parts may be processed in a batch or continuously (e.g., tube, wire, strip). be coated in a barrel fixture, provided that agitation is used to separate the parts. Any metal (or alloy) that can be melted to give a bath, the processing temperature of which would not adversely affect the substrate, can be used in this coating technique. Coating (or residence) times typically will be in the range of a few seconds to a few minutes to provide coatings up to 75 m thick. Coating time wi 11 determine overall coating thickness, but more importantly wi 1 1 also determine the thickness o f the alloyed layer. As most intermetallic compounds are brittle, the thinner this layer the better.

The hot dipping technique refers to the immersion of the part to be The substrate reacts with the

The Hot dipping

Small parts may

The most commonly applied metals are zinc (see "galvanizing"), tin, aluminum, and lead (2). product exhi bi ti ng good, high- temperature (ox idat i on) corrosion resistance, and low-temperature corrosion resistance coupled with good mechanical properties. Several commercial processes exist, such as the Sendzimir method for steel sheet, each one with its own technique for overcoming the problem of wetting the substrate surfaces, and preventing oxidation of the molten aluminum. Fluxes or special pretreatments may be used to condition surfaces. atmospheres are used to protect the molten aluminum and the cleaned substrate surfaces. quenching stage is needed after coating application to harden the coating suffi- ciently to prevent handling marks.

Aluminum is applied to steel in batch or continuous processes to give a

Reducing (or nonoxidizing)

The molten aluminum bath is operated at around 700OC. A cooling or

Thin coatings of tin are applied t o steel for the food industry to provide a bright, nontoxic, protective finish; for the electronics/electrical industries to provide a solderable surface; and in the automotive industry for coating bearing

2-75

shells to facilitate bonding with lead or tin-base alloy coatings. assure good wetting during hot dip tinning also, and the methods used to introduce the flux are similar to those used for galvanizing. called "single-pot" and "two-pot" respectively, are used depending on the application. The single-stage technique is used for applying bonding or solderable coatings that do not have to have a high quality finish. The tin bath is operated at 280 to 325OC, and the parts must be air cooled or quenched afterwards, then rinsed to remove flux, as with applying aluminum coatings. The two-stage process gives a very good surface quality (smooth and bright). The second bath is covered with a layer of oil or molten grease, and is operated at a slightly lower temperature of 235 to 27OOC. After coating, the parts are cooled in air, quenched in oil or allowed to drain over the bath or centrifuged to remove excess metal. needs are similar to other hot dip techniques and as with galvanizing, a good ventilation system is required to remove the fumes. lead-tin alloy containing up to 25% tin. itself will not alloy with ferrous substrates. terne plate and is applied for a number of reasons including corrosion resistance (automobile fuel tanks, radiator parts), lubricity, and solderability (radio, TV chassis, etc.). Terne coatings are usually applied to steel strip, but occasionally to fabricated products. Post-treatments are similar t o those necessary for the other hot dipping techniques. tion heating) by oil or by gas combustion. The choice depends on local availability and price and meeting certain operating criteria, as described for galvanizing. Equipment and specific details of these techniques for different metals are given in Reference (5). -

Fluxing helps

One- or two-stage tinning,

Equipment

Lead is usually applied as a The addition is necessary because lead

The lead-tin alloy is known as

The tin alloy bath is operated at 325 to 390OC.

The pots or kettles may be heated electrically (resistance or induc-

Chemi cal -Physical Methods (3.2.4)

Plasma Oxidation. This coating technique is conducted in a vacuum system in which an electric field (1 to 5 kilovolts) produces ionization of the residual gas and a glow discharge results. two electrodes. The part to be coated with an oxide film is placed in the vacuum system near the plasma region before establishing the plasma. formed during the few seconds the field is applied. plasma anodic oxidation because no additional dc potential is applied to the part (1). Both techniques, however, can be used to produce oxide films on metals that cannot be anodized electrochemical ly.

Simultaneously an oxygen plasma is established between the

A thin oxide film is This technique differs from

2-76.

Electrical -Thermal Methods (3.3.1)

Hardfacing. hard coating to a substrate and at one time encompassed "surfacing" or "hardfacing" accomplished by welding methods (1). variety of techniques, as is apparent from some of the coating methods described in this section of the report; however, here only the welding techniques will be briefly described.

Hardfacing is a general, descriptive term that refers to applying a

Hard coatings may be applied by a wide

In principal, any of the many different welding techniques can be used for hardfacing, namely oxyacetylene, gas tungsten arc, gas metal arc, submerged arc, plasma arc, or electric arc. Metal or alloy coatings may be applied in the same manner welds are made. Thick, uneven coatings result, and the technique is used to build up worn parts (axles, shafts, rollers) which are then machined to final finish and tolerances. Metal and alloys that have been applied by the hardfacing technique include titanium, iron, cobalt, aluminum, nickel, copper, molybdenum, palladium, gold, platinum, tungsten, and their alloys.

Induction Hardening. heat treatment, as is flame hardening. In induction hardening, electromagnetic induction is used to heat the cast iron or steel part to the desired temperature. The distribution of the induced resistive heating and the rate of heating is governed by the strength of the magnetic field, the applied alternating current, the shape and location of the induction coil(s), the number of turns in the coil, and the operating frequency (41). layers, high-power densities and short heating cycles are used, followed by quenching in water or oil. improved fatigue 1 ife.

This surface modification technique is often classified as a

In order to restrict heating to the surface

The result is to provide increased wear resistance or

Electrical -Physi cal Methods (3 .3 .21

Sputtering. which the ionized coating material is transferred to the workpiece and discharged on the substrate. substrates because of the relatively low deposition rate. widely used in the electronics industry, but is used also to coat jewelry with decorative coatings, or to apply corrosion-resistant, organic coatings to metals.

I

This is a form of physical vapor deposition, as explained earlier, in

Sputtering is best suited for applying thin coatings to flat The technique is most

2-77

The coating material is placed on an unheated target in a vacuum chamber containing the parts to be coated. The target is made a cathode (negatively charged) and positive ions generated from an inert gas at separate anodes or from a plasma discharge bombard the target. The coating material is evaporated (sputtered off) and the atoms collide with the substrate material in electrical contact with an anode to form a coating. method in a vacuum chamber backfilled with argon. 5 kilovolts are needed to sustain the plasma. Many elements, metals, and alloys may be used as the coating material. metallic materials, while a RF discharge is necessary to sputter nonconducting materials ( 5 ) . Under carefully control led coating conditions, a1 loys, mixtures, and compounds may be sputtered and deposited on substrates. The deposition rate will depend on the target sputtering rate, chamber geometry and workpiece placement and the working gas pressure. Recent developments in sputtering include the use of magnetic plasma confinement ("magnetrons") to increase sputtering rates. Discharge voltages of only 200 to 800 volts are needed to maintain plasmas with magnetron sources. 25OOC).

~

The most common sputtering method is the plasma discharge -

Voltages in the range of 0.5 to

Direct current discharges are used to sputter

Also, the plasmas produced are stable at much lower temperatures (50 to

Electrochemical-Physical Methods (3.4.1)

Plasma Anodic Oxidation. nique already described. The part to be coated, however, is subjected to a low- voltage dc potential. The part then acquires a positive charge and this facili- tates reaction with the oxygen plasma to build up thicker coatings than if the voltage was not applied. As an example, the glow discharge plasma is established with a field of about 1000 volts, while aluminum parts are biased with a voltage of up to 20 volts. I f microwave energy is used to excite the plasma, then a greater bias, e.g., 50 volts, can be used (1). This latter technique has been called high- frequency plasma excited oxidation, while the plasma anodic oxidation technique itself has also been called "gaseous anodization". Both techniques require sophisticated vacuum and electrical equipment, are only suitable for small parts, processed in a batch mode. As a consequence, this coating technique is very expensive.

This technique is similar to the plasma oxidation tech-

Thermal -Chemical Methods (3.5.1) This category includes techniques where heat is applied to cause a chemical change, as opposed to chemical-thermal methods where a heated substrate i s used or heated coating medium is applied to a part and alloying or chemical changes occur,

2-78

Coloring. thin layers of oxides in air. Angstroms thick, interference colors are generated, ranging from a pale golden yellow to an intense blue-black. ing other gases, such as hydrogen sulfide, colored sulfide layers can be formed. Alternatively, placing parts into heated media such as boiling water or molten metals or salts can induce coloring of surfaces. alloys may be treated by the latter techniques (1). - reproducibly obtain the same colors on parts made from the same material. With thin films such as these, surface finish of the parts also is important because slight variations in color can occur when different surfaces are viewed, or large flat surfaces are viewed from different angles. most suited for decorative purposes because abrasion would quickly remove the coatings.

Heating metals such as copper, iron, steels can cause the formation of When these are of the order of a few hundreds of

If some metals are heated in atmospheres contain-

Nickel and copper and their Close controls are needed to

This type of coloring technique is

Enamelling. avoid any confusion with "enamel" organic paint systems. is sometimes known as vitreous enamelling. and sodium silicate to form a paste, which is applied to the metal surface to be coated. on the coating material, to fuse (or vitrify) the glass and form a chemical bond to the substrate metal. References (5) - and (E) describe how frits are formulated and made. Reference (1) differentiates between the different types of coating that may be applied, such as ceramic, enamel, and glass, although the techniques used for applying each are very similar. ing, electrostatic spraying, dipping, or flow coating. spray system is set up. glass formation, the metal surface oxidizes and the interdiffusion of ions between the metal and the coating causes the bonding to take place. Continuous, intermittent, or batch furnaces are used, and these may be heated by electricity, oil, gas, or propane gas, depending on local availability and cost. however, a muffle furnace is necessary to prevent combustion products from contaminating the coatings.

This technique is more correctly known as porcelain enamel ling to Porcelain enamelling also

A glass frit is mixed with water, clay,

The part is then heated, typically in the range of 500 to 900°C depending

The frits are applied to the substrate by spray- Figure 2-15 shows how a

Frits must be dried before fusing in a furnace. During

With oil heating,

2-79

h

U)I v

a, V S a, L a, cc W e

Lo

W

n

E, 7

0 >

Y 0 0

-0 S ru I VI

ru c, a, E

n

n

- .. a, V L 3 0 w

2-80

The resulting coating is hard and very smooth, abrasion and corrosion resistant, but brittle. Additions to the fr ts can provide opacity or color to the coatings. on ferrous substrates, especially those high in carbon content, it is necessary to use a two-step process wherein a hin frit coating ("ground coat") is first applied to provide good bonding, then the second frit ("cover coat") provides the decorative feature needed. especially if they are first coated with a thin electrodeposited nickel layer, a one-step process has been developed. oped to allow coating of aluminum and magnesium. coatings on jewelry and functional coatings on cookware, hot water tanks, bath tubs, architectural panels, and equipment in the pharmaceutical , agricultural, and food industries, for example.

Recently, on low-carbon steels and unkilled steels,

Low-melting-point frits also have been devel- Applications include decorative

Thermal Oxidation. This technique has been mentioned briefly under "coloring". Heating some metals in air can produce thin films of oxides that produce interference colors depending on their thickness. may be heated at relatively low temperatures (so as not to affect metallurgical properties), for times of the order of minutes to produce a range of colors ranging from pale golden yellow for thin films (short times) through reddish hues (medium thickness times) to dark blue (thick films, long times). thin (measured in Angstroms), the coatings do not have good wear or abrasion resistance. and the substrate surface prepared reproducibly. in industry.

As an example, some stainless steels

As the films are very

Also, for good color matching, the technique must be carefully controlled The technique is not widely used

P hys i ca 1 -C hemi cal Methods ( 3.6.1)

In this section, techniques are described that are variations of physical methods in which chemical additives or reactions are necessary to produce the type of coating desired. while photolytic plating and photochemical-assisted CVD are modifications of electroless plating and conventional CVD, respectively. plasma-assisted CVD and laser-assisted CVD have already been described under "Physical Methods" or "Chemical -Physical Methods" for convenience.

Thus, reactive evaporation is an extension of vacuum evaporation,

Other techniques, such as

P.hotochemica1-Assisted Chemical Vapor Deposition. of the conventional chemical vapor deposition techniques. Light energy, for example, ultraviolet rays, activates a gaseous photosensitizer, such as mercury vapor, which in turn activates the vapor coating reactants.

This technique is a modification

As in plasma-assisted CVD, by

2-81

augmenting the thermal energy, the deposition reaction can occur at lower temperatures and the range of substrates that may be coated is broadened. technique, along with the laser-assisted and plasma-assisted CVD techniques are more sophisticated, resulting in additional equipment and processing costs. Also, with the additional step in the overall reaction process, the chances of obtaining a contaminated coating are larger. Nevertheless, the technique has been used for -

applying silicon carbide or silicon dioxide coatings to electronic components.

Of course, this ____

Photolytical Plating. This technique is a variation of an electroless (displacement) coating technique. Laboratory experiments have shown that a photon beam, such as a laser beam, focused into a solution of a metal salt, such as silver or copper, can cause the metal to be reduced and plated out locally at a high rate because of localized heating in solution. in laser-assisted plating (3).

The result is similar to that obtained Neither technique has been commercialized.

Reactive Evaporation. metals as coatings. this can participate in the coating reaction and produce a compound coating. latter technique is known as reactive evaporation and an example is the use of methane with evaporated titanium metal to provide a titanium carbide coating. Carbides and nitrides are produced on high-speed steel tools by this method to give superior wear resistance.

Vacuum evaporation typically i s used to deposit single If a gaseous reactant is backfilled into the vacuum chamber,

This

If an electron beam is used to activate (ionize) a component of the vaporized reactants in order to facilitate deposition, the technique is called activated reactive evaporation. This modified technique provides better adhesion to the substrate material and is used for applying wear-resistant coatings on high-speed steels, as described above. In addition, activated reactive evaporation is used for deposit- .ing optically transparent, conducting films such as indium oxide and indium tin oxide.

Ion plating also may be modified by admitting a reactive gas to the vacuum chamber during deposition. deposited such as some silicides, nitrides, carbides, and oxides ( 5 ) .

This modification permits compound or alloy coatings to be

Physical -Electrical Methods ( 3.6.2)

Techniques which use a glow discharge/plasma to establish the conditions for deposit.ion are covered under this category. Vacuum systems are necessary along

2-82

,

with the necessary electrical equipment to generate the glow discharge/plasma. Batch processing of small parts is usual and coating costs are relatively high, except ion plating is being used commercially to coat components for the aerospace industry with protective layers, e.g., aluminum coating of fasteners. tion volumes of parts to be coated helps to keep costs low for this application.

Mass produc-

Ion Plating. Ion plating is the third of the three techniques previously loosely collected under the heating of physical vapor deposition. niques, namely sputtering and vacuum evaporation have already been discussed. relationship between these three and the chemical vapor deposition technique is most easily seen from the following diagram.

The two other tech- The

Deposition from Vapor Phase I

With Vacuum I

Ph sical Va or De osition Y Vacuum

Evaporation Sputtering Ion P1 at i.ng

No Vacuum

T Chemical Vapor Deposition

Ion plating combines features of vacuum evaporation and sputtering. the coating material is connected to the anode (positive charge) in the vacuum chamber, as shown in Figure 2-16. moves along the electric field created by a glow discharge established near the cathode (negative charge). The part to be coated is placed in contact with the cathode and becomes coated as the evaporated material is discharged and condenses on its unheated surfaces. The vacuum chamber is backfilled with an inert gas to prevent oxidation of the surfaces of the parts being coated. A direct current power supply or a radio-frequency alternating current power supply is used to generate the glow discharge plasma and establish the voltage gradient of between 0.5 and 5 kilovolts (I.). The anode and cathode are insulated from ground. Less than 5% o f the coating material is ionized. The neutral atoms deposit on any cold surfaces in the chamber, including the part being coated. Deposition rates are, therefore, slow and vary from as little as 10 m to 300 m an hour unless a

In ion plating

When heated, the coating material evaporates and

2-83

C Gas I

Movable shutter

Cathode dark space

Evaporator filament

Glass chamber

Filament supply

Figure 2-16. d i rec t current gas discharge and evaporator-type filament.

Representation of a simple ion-plating apparatus using a

Source: Metals Handbook, Volume 5 , Reference ( 5 ) . -

2-84

magnetron source is used. Deposition rates up to about 1200 m an hour may then be obtained depending on the coating material. Metallic, metalloid, and nonmetallic coatings may be applied to conducting and insulating substrates by this technique. Reference (5) - provides data concerning equipment and process controls and briefly mentions reactive ion plating, a modification discussed briefly under physical- chemical methods. Applications to date for ion plating are limited and include aluminum (as a replacement for cadmium) on fasteners; aluminum on spacecraft components (as a replacement for electroplated cadmium also); and indium tin oxide coatings on glass.

Plasma Arc Spraying. Electric arc spraying, flame spraying, and plasma arc spraying are variations of a technique to propel heated or molten particles towards the substrate to be coated. In plasma arc spraying a plasma is formed in the nozzle chamber of the water-cooled spray gun. Argon or nitrogen is used as the arc gas. When the plasma has been established by applying a direct current at 30 to 80 volts (5), - the powdered coating material is fed into it where it is heated or molten, depending on the plasma temperature (direct current controlled and by addition of secondary arc gases such as helium or hydrogen) and the coating material itself. Thick, compact coatings can be applied. not as porous as coatings obtained by other spraying methods because much higher particle velocities are attained with plasma arc spraying and higher temperatures (>5000°C) can be achieved (1). - Standard spray guns are rated at 40 kW while high energy guns are rated at 80 kW. This technique is used to apply corrosion resistant (zinc) coatings to steel and yttria-stabilized zirconia thermal barriers to engine parts.

Although these tend to be porous, they are

Transferred Plasma Arc Spraying is a variation of plasma arc spraying in which the substrate surface may be heated and the coating melted during the deposition process. This feature provides for better bonding, improves coating density, and increases the deposition rate over the conventional plasma arc spraying technique. A separate direct current is established between the plasma arc and the substrate material. Reference (5) - cites coating rates of up to 6.35 nun in a single pass and coating widths up to 32 mm in a single pass at powder feed rates up to 9 kg an hour. As an example, when depositing a coating containing 88% tungsten carbide, balance a cobalt-based material, plasma arc spraying required 24 passes at 40 to 60 kW to give a satisfactory deposit 0.3 mm thick. In contrast, the transferred plasma arc technique needed only one pass at about 3 kW. technique are that the substrate is heated, must be electrically conductive, and

Disadvantages with this

2-85

must be able to withstand some deformation. "hardfacing" agricultural equipment, valve seats, oil-field equipment, and mining equipment. transferred plasma arc spraying.

Applications primarily involve

Figure 2-17 compares the two types of equipment used for plasma arc and -

Another modification is known as inert atmosphere and low pressure plasma arc spraying and this is widely used to coat components for aircraft engines with MCrAlY-type alloys to provide ox dation resistance. plasma arc spraying, but instead of being conducted in the open, it is carried out in a closed chamber at atmospher c or reduced pressure, in the presence of an inert gas (argon). same time prevents or minimizes the formation of impurities in the coating. impurities might be oxides or nitrides. and costly than for plasma arc spraying.

-

The technique is the same as

The chamber contains any toxic or pyrophoric materials while at the Such

Equipment requirements are more complex

Plasma Polymerization, the precursors (monomers) are bled into a vacuum chamber. The part to be coated is attached to a water-cooled holder (4 to lO"C), then a RF discharge/plasma is generated near the part, rather than an electric field, to initiate the polymerization reaction. This technique has been developed in the laboratory and has been shown to be able to apply thin, pin-hole free, abrasive-resistant coatings on soft polymers, and a range of colored, polymeric coatings to glass, for example, with good adhesion. tion, typically in the range of 0.5 to 3 watts per square centimeter. spraying, in contrast, deposit materials such as epoxies, nylon, polyethylene, polyimide, polyvinyl chloride and polytetrafluoethylene by using these materials as feed powders to the spray gun.

This technique is similar to electropolymerization in that

Plasma polymerization is characterized by a very low-power consump- Plasma arc

Physical -El ec trochemi cal Methods ( 3.6.31

Laser-Assisted Plating. restricted by kinetic or mass transfer limitations. discharging ion could be increased, the deposition rate would increase. Agitation, gas sparging, and flowing electrolytes are all used to improve mass transport over and above natural convection effects. In laser-assisted plating, another approach is taken. A laser beam is focused at the interface between the metallic substrate and the electrolyte. Localized heating of the substrate improves both mass transport and deposition kinetics and metal ions are discharged very rapidly at that location (11).

Conventional electrodeposition processes are often If the mass transport of the

This technique has been developed in the laboratory for selectively

2-86

Copper electrode

holder \

Tungsten - electrode

1 Arcgas I .;.'// Negative positive I electrical

connection and water out

electrical ' connection Powder and

and water in powder gas

( a ) Typical Plasma Arc Spray Gun

Water c o o l e d copper High velocity nozzle m o l t e n particles

\ M o l t e n P o w d e r inlet

\ \

Substrate'

A r c p o w e r Fusion p o w e r SUPPlY SUPPlV

( b ) Schematic o f T r a n s f e r r e d Plasma Arc Spray Gun

F i g u r e 2-17. o f the s p r a y equipment used f o r plasma a r c and t r a n s f e r r e d plasma a r c s p r a y i n g .

Diagram showing the p r i n c i p l e s o f o p e r a t i o n

Source: Metals Handbook, Volume 5 , Reference (5).

2-87

depositing gold, nickel, and copper. way over the substrate, a patterned deposit may be obtained. application for this technique is the maskless electrodeposition of copper circuit elements for electrical/electronic devices. technique is just a laboratory curiosity at this time and commercialization is not imminent.

If the laser beam is rastered in a controlled Thus, one potential

___

Like photolytic plating, however, this

-

Physical-Thermal Methods (3.6.4)

Hardening. Electron or laser beams also may be used to modify surfaces to improve their hardness or wear resistance, while not affecting the properties of the bulk material. Whereas, traditionally diffusion of elements such as carbon or nitrogen into the surface was used, these new developments, incorporating energy beams, do not change the composition of the material, but rely on imposing controlled heating and cooling cycles on a microscopic scale. As such, the techniques are best suited for small parts, with high intrinsic value, and which typical modification on small-surface areas or in selected regions.

POST-TREATMENT TECHNIQUES

As discussed in the introduction, some treatments are sometim

y require surface

s necessary after coating deposition to stabilize the coating, improve adhesion, improve quality, provide an additional feature such as color or luster, or to protect it during hand1 ing, storage, shipment, or subsequent fabrication. The various techniques used will be briefly discussed in this section. Figures 2-4 and 2-7 will be used as a guideline for the discussions. should be noted that many techniques have already been mentioned as being an integral part of various coating methods, or are similar to coating deposition methods already described.

The classification system given in However, it

Inorganic Coatings (4.1)

Mechanical Methods (4.1.1). matte surface which is not aesthetically pleasing for some applications, particularly where consumer products are concerned. used to improve luster or to provide a mirror-like surface finish. The same sort of techniques as those described in the section on mechanical pretreatments can be used. Conversely, sometimes a textured surface is desirable to reduce glare or to hide nicks and dents introduced during handling and fabrication. satin or otherwise textured surface is produced.

Some pretreatment methods and coating methods leave a

Thus buffing and polishing may be

In this case a Techniques such as honing,

2-88

buffing, and blasting may be used. plating, electroforming, detonation plating) the deposit can be very rough and nodular or not close to desired tolerances. When either of these situations occurs, grinding or sandblasting may be used to improve the surface finish and to provide the required dimensions.

With a few coating techniques (e.g., electro-

Chemical Methods (4.1.2). As indicated in Figure 2-7 , there are a number of chemical post-treatments that include chromating or phosphating, passivation or use of inhibitors, sealing and the use of lubricant films. ings are used in the electrical/electronics industry, and for coating cutlery and jewelry but as-plated these have a tendency to tarnish rapidly. only is unsightly but on electrical/electronic components can affect performance. Several techniques have been used to protect silver coatings. In storage the parts, components, or items can be wrapped in such a way as to incorporate an inhibitor, typically a "vapor-phase" inhibitor. On decorative finishes for jewelry and tableware lacquering can be used to prevent tarnishing from handling. But this is just a temporary solution. sion of the plated part in a suitable solution or electrolytically (11). - chromate, conversion-type coatings are much more durable. applied also to anodized aluminum and hot-dip galvanized steel mi 1 1 products, parts and components to provide some wear and corrosion resistance. On zinc coatings, a bright chromate is applied from an acid solution and care must be taken to prevent excessive attack on the coating before conversion. times added to ferrous and zinc substrates and coatings to improve paint adhesion or to provide inexpensive, temporary corrosion protection. Porous, anodized surfaces also can be protected by filling the pores with a wax or by chemically sealing them in a suitable solution. One simple procedure for anodized aluminum is to seal the coating by immersion in boiling water, for example. Other post-treatments for porous, anodized coatings can include a coloring step before sealing; the addition o f dry-film lubricants such as graphite or molybdenum disulfide; the inclusion of corrosion inhibitors; or impregnation with soaps or oils to facilitate extrusion or further fabrication (1, - - 5). Conversion coatings should not be used if adhesive bonding is specified. Anodized coatings are stronger than conversion coatings and provide a better bond. Reference (5) - gives examples of different types of conversion coatings (oxides, phosphates, chromates) applied t o aluminum for different applications. Subsequent coatings such as varnishes, lacquers, paint and silicone resins also are listed.

Electrodeposited silver coat-

Tarnishing not

Bright chromate coatings can be deposited by immer- These

Chromate coatings are

Phosphate coatings are some-

2-89

Electrochemical Methods (4.1.3). chemically inert metal may be electroplated on top of one or more functional coatings. One example is the deposition of chromium over nickel and copper on steel or zinc die castings for the automotive or appliance industries. The copper provides good adhesion (and sometimes a leveling effect); the nickel provides corrosion resistance; and the thin chromium top coatings provides aesthetic appeal (luster, -

color) and ease of cleaning. coated electrical connectors to provide corrosion resistance, arcing resistance, and low-electrical contact resistance. gold and rhodium also may be used on precious metal-plated objects to provide tarnish resistance. transparent.

For some applications very thin films of a

~

Another example is the flash of gold applied to

Very thin films of noble metals such as

Sometimes these films are so thin they are optically

Thermal Methods (4.1.4). Baking is used with some types of coating (e.g., electrodeposited, electropainted) to drive off occluded gases such as hydrogen, or organic binder materials, precursors or other occluded materials. During electrodeposition of some metals, because the (cathodic) deposition efficiency is less than 100%, some hydrogen gas is generated and this may be incorporated into the coating. Later this hydrogen can cause problems such as hydrogen cracking (ferrous substrates) or can cause the coating itself to crack as the hydrogen is slowly released (hard chromium deposits). this hydrogen. hours up to 1 day depending on the complexity of the part/coating, the substrate material and the thickness of the coating itself and its composition. chromium, zinc, and cadmium usually are baked after electrodeposition.

For these reasons it is important to drive off Baking is usually done at temperatures of about 300°C for several

Coatings o f

Heat treatments are used to modify the properties of coatings or to cause diffusion to occur between the coating material and the substrate material to produce a p-ietallurgical bond. of electroless nickel deposits. reducing agent was phosphorous acid or a hypophosphite. by weight of phosphorus can be occluded with the nickel from acid solution. When these coatings are heat-treated at 400°C for 0.5 to 1 hour, nickel phosphide precipitates and increases the hardness of the coating from 500 Vickers Hardness Number (VHN) as deposited to about 1000 VHN (12). Heat treatment also improves wear resistance and corrosion resistance (E), but can affect electrical and magnetic properties (5, - - 22) of electroless nickel deposits. use o f heat treatments is the post-deposition "annealing" o f vacuum deposited

An example of a heat treatment is the precipitation hardening

Typically, up to 7 to 12% These deposits contain some phosphorus if the

Another example of the

2-90

metals, for example, nickel-chromium alloy thin-film resistors. These are heat- treated at 125OC for 24 hours (5) - before final adjustments are made to provide the needed tolerances. Hot-dipped galvanized coatings can be annealed ("galvanneal- ing") to develop the intermetallic layer further and provide excel lent paint adhesion. Vacuum deposited metals such as bismuth, copper, cerium, manganese, and titanium can be converted to oxide films by baking in air. These thin oxide films have optical and dielectric properties not easily obtained by direct-coating methods (5 ) . Plasma sprayed coatings are porous and laser fusion has been used to decrease porosity. Examples are aluminum oxide or Ni-Cr-A1 alloy on nickel alloys.

'

Organic Coatings (4.22

Chemical Methods (4.2.1). A number of chemical post-treatments also may be necessary for organic coatings. Inhibitors may be incorporated into top coats of paint to improve corrosion resistance at holidays or damage sites (nicks, gouges, cuts, etc.). Lacquers and varnishes may be applied as top coats to improve appearance, especially gloss, and to provide greater wear and corrosion resistance. tougher polymeric materials, such as vinyl-based coatings, can be applied to paint finishes to improve resistance to mechanical damage. For example, these are used on lower body automobile panels to protect against "chipping" from stones thrown up by tires. In the automotive industry metal loorganic petroleum-based waxes are used on parts of the frame and wax coatings are used around wheel arches, windows, and the bottom of doors to improve corrosion resistance. mentioned are used also as sealants, while inhibitor-containing coats are used as sound deadening materials. Decorative finishes, selective painting and marks are added to finished products for the purposes of identification in storage or use, or as labeling. deposit on the surface of the part. clear lacquer is applied to seal and protect the coating. coatings also must be sealed and a clear lacquer can be used to provide better wear properties.

Other

The vinyl-based coatings

Conversion coating techniques can sometimes leave a loosely adherent This must be scratch-brushed off before a

Organically colored

Thermal Methods (4.2.2). Curing, drying, and baking are used to modify and improve the physical form of the coating and its properties. electrically, by gas or oil depending on which fuel is available and most cost- effective. Occasionally a plastic film may be bonded to a coated substrate to provide a laminate.

Two types of technique are given in Figure 2-7.

Conventional ovens are used and these may be heated

The laminating film provides protection from damage in use.

2-91

Powder coatings deposited on cold substrates are not coherent and only loosely adherent. After deposition, the coated parts must be transferred to an oven. At the elevated temperature the powder particles fuse and flow together to give a coherent coating. Powder coatings deposited onto heated substrates even may have to be baked to complete the fusion and flowing process or to polymerize or cure the coating to develop the desired characteristics (15).

___

- -

Paint films must be dried ("cured") before use. which is sui table for solvent-based paints known as thermoplastic coatings. include a wide range of decorative and maintenance coatings applied -- in situ, on site. Other types of paint coating (thermosetting) require baking in an oven. Baking is sometimes referred to as stoving, particularly for enamels. The heat generated in the oven evaporates the solvent and cures the film. Convection and radiation are the principal forms of heat transfer. Convection ovens are widely used in the automotive industry. They are primarily heated by gas, although oil, gas, and steam have been used (5 ) . Infrared (IR) radiation (from surfaces heated by gas or electricity) is directed onto the painted parts to be cured by means of special reflectors and is a fast process, especially for low temperature (450°C) curing. types of oven are used according to whether or not processing is done in a batch or continuous manner. Direct-fired batch ovens are efficient and often used because the fuel is burned in the curing chamber. efficient because the fuel is burned outside the chamber and heat must be conducted through the chamber walls. However, with the latter type of oven the paint film is not exposed to combustion products and cannot be contaminated by them. Continuous ovens usually are of the IR radiation type because less insulation is required and ventilation/exhaust systems are less complicated (5). - struction consists of a drying tunnel containing rows of IR lamps. Other methods may be used to generate the IR radiation, for example ceramic-coated resistance wire filaments and electrically heated quartz tubes. ovens are used, care must be taken to control the temperature and rate of heating of the substrate/coating to ensure that blistering does not occur or film disrup- tion due to the solvent being lost too rapidly. tion i s better because it heats both the substrate and coating surface and curing can occur from the "inside out" and the "outside in".

The simplest method is air drying, These

Recently, most IR radiation ovens are heated by electricity. Different

Indirect-fired batch ovens are less

The simplest type of con-

Whether convection or IR

From this point of view, IR radia-

Physical Methods (4.2.3). The previous section discussed IR radiation heating for curing and polymerizing (cross-1 inking) organic coatings. These processes may be

2-92

accomplished by using other sources of radiation such as electron and photon beams. Because they are so fast, they are usually only used in conjunction with fast coating methods such as the curtain coating of paint.

Ultraviolet (UV) radiation is used to cross-link and polymerize organic films to convert them into useful coatings. polymerize unsaturated ethylene groupings in coatings (8). - polymerization reaction, there must be chemical initiators present in the deposited organic coating. sition. If curing is thought likely to be too rapid, then inhibitors also may be added to the coating formulation. UV curing is best suited to film thickness of 25 m or less, especially if the films contain pigments that could disperse the UV radiation. The presence of oxygen also causes a problem with the application of this technique. Many UV light sources exist, these may be carbon arcs; mercury vapor and tungsten halide lamps; zenon arcs and the common "sun lamp".

For example, actinic light can be used to To initiate the photo-

The type of initiator selected will depend on the coating compo-

Electron Beam (EB) heating is used for some commercial applications (s), for example to cure an acrylic-based monomer/prepolymer system coating 30 to 40 m thick, even though it is a relatively new development. investigated include vinyls and polyesters. field of 100 to 500 kilovolts is directed onto the organic coating to be cured or cross-linked. obtained in 1 second (8) - at room temperature. Thus, this technique is especially suited to substrate/coating combinations where the substrate is heat sensitive, for example wood or plastic. If the beam is rastered across the surfaces of the coated part, curing of large areas can be accomplished quickly. However, it should be pointed out that this technique is line-of-sight limited and cannot be used for complex parts or parts with deep recesses and holes. Continuous coating of flat strip at 60 to 70 meters a minute is possible using EB radiation and because the substrate is not heated, the coated material can be handled immediately. advantage of EB curing is that it operates at room temperature and only consumes 10 to 15% o f the energy used if convection heating were to be used (42). _. The handling equipment is not heated, nor is the substrate. be reheated either. because high-solids coatings can be used. emitted, and the apparatus must be shielded from the operators. Another disadvantage is that an inert gas must be used in the curing chamber, otherwise oxygen would react with the coatings to produce peroxy radicals that inhibit cross-linking

Other polymer systems being A beam of electrons, accelerated by a

Polymerization occurs very quickly and a cured coating can be

The main

Fresh-air makeup does not have to Another advantage is the much reduced emission of solvents

The main disadvantage is that X-rays are

2-93

(42) . plastics; thus, the use of EB curing must be evaluated for each coating/substrate comb i na t i on,

Occasionally, the EB radiation will alter the physical properties of some

TECHNIQUE CLASS IF I CATION SUMMARY

A large number of techniques have been described in this section of the report, covering surface preparation/pretreatments, coating deposition and surface modification, coating removal, and post-treatments. Table 2-6 brings these techniques together and provides a cross-reference by classification numbers and page numbers. Focus is on the methods used to apply or remove the coatings. Table 2-1 summarizes the features of mechanical finishing techniques for surface preparation. Table 2-2 is a summary of chemical finishing techniques for surface preparation, while Table 2-3 summarizes the electrochemical techniques used. In Table 2-4 some commonly used chemical stripping solutions are listed for various metals, and electrochemical stripping methods are summarized in Table 2-5. group the various surface preparation, coating, surface modification and post- treatment techniques together by method. conjunction with each other provide an overview of the broad range o f metal finishing techniques available to industry.

Figures 2-5 and 2-7

These tables and figures, when used in

2-94

Table 2-6

CLASSIFICATION SUMMARY AND INDEX OF COATING DEPOSITION, REMOVAL AND SURFACE MODIFICATION TECHNIQUES

Classification Technique Numbers

Air Spraying Airless Spraying Anodic Stripping Anodizing Au tophore t i c Pa i n t i ng Cement at i on Chemical Vapor Deposition C 1 adding Col or i ng Col or i ng Col or i ng Detonation P1 at ing Di f f usi on Electric Arc Spraying Electroforming Electroless Plating Electropai n t i ng Electroplating Electropolymerization Electrostatic Spraying Enamel 1 ing Explosive Bonding Flame Hardening F1 ame Spraying Flocking Galvanizing Gilding Hardening Hardf aci ng Hot Dipping Induct i on Harden i ng Ion Bombardment Ion Implantation Ion Plating Laser-Assi sted P1 at ing Laser Glazing Painting Peen Plating Phot ochemi ca 1 -Ass i s ted CVD Photolytic Plating Physical Vapor Deposition Plasma Anodic Oxidation Plasma Arc Spraying Plasma Oxidation P1 asma Polymerization Powder Composition Reactive Evaporation Sputtering S tr i ppi ng S tr i pp i ng Thermal Oxi dat i on Vacuum Evaporation

3.1.1 3.1.1 2.2 3.2.2 3.2 3.5 3.2.3 3.1 3.2 3.2.2 3.5.1 3.2.1 3.5 3.1.2 3.4 3.2 3.2.2 3.4 3.2.2 3.3 3.5.1 3.2.1 3.5 3.1.3 3.2.1 3.2.3 3.2.1 3.6.4 3.3.1 3.2.3 3.3.1 3.1.2 3.3 3.6.2 3.6.3 3.6 3.1.1 3.1 3.6.1 3.6.1 3.6 3.4.1 3.6.2 3.2.4 3.6.2 3.1.2 3.6.1 3.3.2 2;l 2.3 3.5.1 3.6

2-95

Met hod Mec han i ca 1 - C hemi ca 1 Mec han i ca 1 -C hemi ca 1 Electrochemical Chemical -Electrochemical Chemical Thermal Chemical -Thermal Mec han i ca 1 Chemical Chemical -E 1 ectrochemi ca 1 Thermal -Chemical Chemical -Mechanical Thermal Mechanical -Electrical Electrochemical Chemical Chemical -E 1 ectrochemi cal Electrochemical Chemical -Electrochemical Electrical Therma 1 -C hemi ca 1 Chemical -Mechanical Therma 1 Mechanical -Thermal Chemical -Mechanical Chemical -Thermal Chemi ca 1 -Mechanical Phys i ca 1 -Therma 1 Electrical -Thermal C hemi ca 1 - Therma 1 El ec tr i cal -Thermal Mechanical -Electrical Electrical P hy s i ca 1 -E 1 ec t r i ca 1 Physical -Electrochemical Physical Mechanical -Chemical Mechan i ca 1 P hys i cal -C hemi ca 1 P hy s i ca 1 -C hemi ca 1 Physical E 1 ec trochemi cal -Phys i ca 7 Physical -Electrical C hemi ca 1 - P hy s i ca 1 Physical -Electrical Mechan i ca 1 -E 1 ec tr i ca 1 P hy s i ca 1 -C hemi ca 1 E 1 ec tr i cal -P hys i ca 1 C hemi ca 1 Thermal Thermal -C hemi cal P hy s i ca 1

Page Numbers - 2-62 2-62 2-38 2-67 2-42 2-55 2-72 2-40 2-42 2-69 2-79 2-65 2-56 2-64 2-48 2-43 2-71 2-49 2-72 2-45 2-79 2-66 2-57 2-64 2-66 2-73 2-67 2-88 2-77 2-75 2-77 2-64 2-46 2-83 2-86 2-59 2-63 2-41 2-81 2-82 2-59 2-78 2-85 2-76 2-86 2-64 2-82 2-77 2-36 2-38 2-81 2-61

Section 3

ENERGY USE IN METAL FINISHING

In the discussion of the various metal pretreatment, coating, and post-treatment methods that have been developed, where appropriate, or where data were available, energy requirements were given and sometimes compared with other similar methods. Not only is energy required during specific pretreatment, coating, or post- treatment steps, however, but also for heating or cooling process solutions, rinse waters, the parts themselves; for providing adequate illumination, ventilation, heating and air conditioning; for effluent and solid waste treatment; for pumping, stirring, agitation, filtration, and so on. In this section of the report estimates are given for the total energy usage in the metal finishing industries and what projections have been made for changing energy use patterns. plex topic because in any one segment of the industries, or even for an individual technique, multiple energy or fuel sources may be used. the different types of energy/fuel used can vary with seasons or fluctuations in availability on a local and global level. And, as with any statistical information, the data are always biased by the fact that the sample may not be truly representative; the respondents to a census may have omitted some information; or information was not given or lumped under a catch-all category such as "other fuels'' or "fuels not specified". in Table 3-4 where the "other fuels" category comprises a significant proportion of the total use. Fuel use information sometimes is not documented according to federal disclosure rules because it might reveal sources.

This is a com-

Also, the ratios between

This is especially true for the data given later

In obtaining energy use data, and in particular electrical energy usage, it is convenient to use the SIC classification codes in conjunction with the U.S. Bureau of Census data on fuels and electricity consumption. 34, "Fabricated Metal Products" and 347, "Metal Services". This classification is further subdivided into 3471 and 3479, and the activities covered under each are as follows:

The pertinent SIC codes are

3- 1

SIC 3471: Electroplating, Polishing, Anodizing and Coloring

Anodizing of metals and formed products Buffing C1 ean i ng Descal i ng Coloring and finishing of aluminum Chromium plating of metals and formed products Decontamination and cleaning o f missile and satellite parts Decorative plating and finishing Depolishing metals E 1 ec trol yzi ng s tee 1 Electroplating of metals and formed products Finishing metal products and formed products Gold plating Plating of metals and formed products Polishing of metals and formed products Sandblasting of metal parts Tumbling of machine parts.

SIC 3479: Coating, Engraving and Allied Services, Not Elsewhere C 1 ass1 fled*

Bonderizing of metal and metal products Coating and wrapping of steel pipe Coating (hot dipping) of metals and formed products (with lead,

zinc, aluminum and other metals) Coating (plastic and resinous) of metals Coating of metals with si 1 icon Coating wi th rust preventatives Dipping metal in plastic solution (as a preservative) Enamel ling (including porcelain) of metal products Etching on metals Galvanizing of iron and steel and formed products Japanning of metal Jewel ry enamel 1 i ng Lacquering of metal products Painting (enamelling, lacquering, varnishing) of metal products Pan glazing Parkerizing Retinning of cans and utensils (not done in rolling mills) Rust proofing (hot dipping) of metals and formed products Sherardizing of metals and metal products Varnishing of metal products.

tin,

Some porcelain-enameled products are produced by companies fa1 1 ing under SIC 3461. Also, some of the most recently developed coating techniques, e.g., incorporating plasmas or lasers, are not covered by the above listing. Nevertheless, the energy use data reported are useful for comparison purposes and identifying trends.

*Not incl uding engravi ng-re1 ated activities.

3-2

PRESENT ENERGY USE

According to the 1982 Census of Manufacturers (24), - all industries consumed about 3.40 x 1012 kWh (11.6 x 1015 BTU*) of total energy in 1981. the major industry groups, consuming 0.10 x 1012 kWh (0.35 x 1015 BTU) in 1981, down a little from the 0.11 x 1012 kWh (0.36 x 1015 BTU) consumed in 1980. total energy used by all manufacturing industries, some 20% was in the form of electricity, as Table 3-1 shows. the last 15 years, although the quantity of electricity has declined since 1979, partly due to a decline in the economy and partly as a reflection of increased conservation efforts. Electricity expenditures in industry were $25.5 billion in 1981, up from $21.9 billion in 1980. In comparison, natural gas costs were $17.4 billion and $14.7 billion, respectively, in current dollars. Electricity was the most expensive form of purchased energy in 1981, being equivalent to $11.23 for a million BTUs. purchased energy costs, but only 20% of the total energy consumed by industry (24). -

SIC 34 was sixth among

Of the

This proportion has been slowly increasing over

This price made electrical energy accountable for 46% of total

electr factor finish by all

A more

Table 3-2 shows the distribution of 1981 energy/fuel use in SIC 34--Fabricated Metal Products, and in particular for the two subclassifications pertaining directly to metal finishing, 3471 and 3479. about 3.0% of the energy used by all manufacturing industries in 1981. SIC 34 electricity use represents about 3.8% of the total purchased electricity used by industry. (electroplating and painting) is carried out in ''captive shops" for the appliance and automotive industries and in steel mills, which fall under different SIC classifications. Thus, the total energy use will be higher, and the amount of

city used greater than the numbers in Table 3-2 indicate, perhaps by a of 4 or 5 (25). - Using a factor of 5, the electricity used in the metal ng industry would amount to about 1.5% of the total electrical energy used industries in 1981, rather than the 0.3% computed from Tables 3-1 and 3-2.

Total energy use in SIC 34 represents Similarly,

It should be pointed out, however, that much coating

recent data source, the FY1986-FY1990 Energy Conservation Mu1 tiyear Plan (g), indicates that the breakdown of energy use has not changed dramatically in the 1980-1983 period, as Table 3-3 shows. electrical energy used shown in Table 3-1 continues in Table 3-3.

The small increases in percentage The absolute

*Energy units w

con sump t i on 1 1 be used

is now reported in BTU's rather than kWh's, consequently dual n this report where appropriate.

3-3

Table 3-1

Year ' 1967 1971 -

TRENDS IN ELECTRICITY USE FOR ALL MANUFACTURING INDUSTRIES SINCE 1967

1974 1975 1976 1977 1978 1979 1980 1981

Elec ricity Usea, 10 1 2 kWh 0.43 0.51 0.62 0.60 0.64 0.66 0.68 0.69 0.66 0.67

Percent to Total

Elec ricity Cost,

I fr Energy Use 12 13 15 17 17 17 18 18 19 29

10 4 Dollars 3.72 5.07 8.47 10.3 12.2 14.5 17.0 19.3. 21.9 25.5

Percent to Total Energy Cost 48 49 43 44 43 44 45 45 45 46

Source:

aPurchased electric energy:

U.S. Bureau of Census, 1983 (Reference - 24).

1 kWh is equivalent to 3,412 BTU.

' I i I I I

Table 3-2

PURCHASED FUELS AND ELECTRICITY USED IN THE FABRICATED METAL PRODUCTS INDUSTRY (SIC 34) IN 1981

Category

SIC 34 Total Energy El ec tri c i ty Fuel Oil Natural Gas LPG Coal and Coke Unspecified

Total Energy Electricity Fuel Oil Natural Gas LPG Unspecified

SIC 3479 Total Energy El ec tri c i ty Natural Gas Unspecified

SIC 3471 and 3479 Total Energy El ectri c i ty Fuel Oil Natural Gas Unspec i f i ed

SIC 3471

Ouanti t v (kWH x 109) (BTU x 1012)

103.1 351.9 25.5 87.1 6.36 21.7 5.10 17.4 0.935 3.19 2.96 10.1

-- --

5.98 20.4 1.31 4.46 0.352 1.20 2.69 9.18 0.026 0.09

-- --

5.54 18.9 0.712 2.43 2.60 8.87

-- --

11.5 39.3 2.02 6.89 0.422 1.44 5.29 18.05

-- --

Percent of Total

100% 24.8 6.2 49.4 0.91 2.9

--

100% 21.9 5.9 48.6 0.4

--

100% 12.9 46.9 --

100% 17.5 3.7 45.9 --

cost Percent ($ x 106) of Total

2236.0 100% 1261.0 56.4 127.0 5.7 603.0 27.0 17.2 0.8 22.6 1.0 205.0 9.2

129.0 lOox 71.1 55.1 9.4 7.3 28.7 22.2 0.5 0.4 19.0 14.7

84.0 100% 36.5 43.5 20.4 24.3 (w)a (w)

213.0 100% 108.0 50.7 12.3 5.7 49.1 23.1

( w ) (w)

=e:

a(w) means data were withheld.

U.S. Bureau o f Census, 1983 (Reference 24).

3-5

values differ because of differences in accounting. purchased energy consumption, while Table 3-3 considers total electricity usage including losses. In 1983, SIC 34 category had become the ninth of the most energy intensive manufacturing industries. for 1983 for this SIC code according to Reference (3). Electrical energy consumption in SIC 34 was about 2.9% of the total for all the industrial sectors, compared with 3.8% calculated for 1981 from the Bureau of Census data base.

Table 3-1 only considers

Table 3-4 gives the energy/fuel use breakdown

Table 3-3

ENERGY CONSUMPTION IN THE INDUSTRIAL SECTOR FOR 1980 TO 1983

Percentage of Total Consumed Total Energy Use Natural

Year (kWhg;2i012) (BTU3;.:01!J) Electrici 3o t y Petroleum 3o Gas Coal Other 1980 27- T3 1982 8.00 27.3 32 29 25 10 4 1983 7.97 27.2 33 29 25 9 4

=e: U.S. Department of Energy (Reference 26).

Table 3-4

MAJOR FUEL USE IN THE FABRICATED METAL PRODUCTS INDUSTRIES (SIC 34) IN 1983

Total Energy Consumption (KWh x 109) Energy Us tlectrical L' iquid ' Coal

Fuels Gases Solids Other ---- SIC CODE (kWh X 10 6) Electrici tx Losses

34 142.8 22.7 53.2 5 .8 39.4 2 . 3 19.4 Totala 7,963.0 785.2 1,866.0 2,267.0 1,977.0 705.2 362.8

m e : U.S. Department o f Energy (Reference 26). aIncludes a1 1 manufacturing and nonmanufacturing industries.

3-6

The above figures serve to give a picture of the overall energy consumption in the area of metal finishing, placed in the perspective of overall energy use for the total industrial sector. It is difficult to relate trillions and quadrillions of any unit to everyday activities, hence a few examples of energy consumption in metal finishing processes will be given. These examples will serve to show how energy consumption can vary according to the technique used for any method and how different energy/fuel sources are uti 1 ized for any single technique or method. Examples will be restricted to painting, electroplating, and anodizing because it is difficult to find quantitative data for most of the other techniques.

Before proceeding, two comments need to be made. First, in most metal finishing plants energy costs are low compared to other operating costs such as labor and materials. Energy efficiency improvement and conservation, as a consequence, can have little impact on finishing costs, and there has been little incentive to document energy usage, particularly in a systematic way. Second, the total energy used for a given metal finishing operation includes contributions from many sources. Each contribution can vary considerably depending on the operating conditions chosen. Thus, when evaluating energy use data, care must be taken to determine which fuel/energy sources have been used and included, and what processing conditions were employed.

For electroplating, a base1 ine electrical energy requirement can be calculated and expressed in terms of watt-hours needed per square meter to deposit 1 micron of coating thickness. available, the electrical energy needed to deposit a given thickness on a specific area for a selected metal can be determined. Typical values are listed in Table 3-5, and it should be pointed out that these values are directly proportional to the electrodeposition (cell) voltage, which varies with operating conditions. The cell operating voltage will depend on current density, temperature and electrode spacing, for example. than the equivalent rack plating in tanks.

With values for specific electrical energy (in Wh/mz/m)

Barrel plating requires higher operating voltages

Chromium plating is the most energy intensive electroplating process, consuming about 1000 Wh/mZ/m, depending on the type of coating obtained. a hard chromium coating, 130 m thick, is applied for refurbishing roll surfaces, 130 kWh would be required for each square meter o f surface area coated, not count- ing energy used for agitation, pumping, filtering, heating the plating solution, ventilating the plating tank, and so on. Most other metals have specific energy requirements in the range of 5-100 Wh/mz/m, according to Table 3-5.

As an example, if

3-7

Table 3-5

SPEC IF IC ELECTRICAL ENERGY REQUIREMENTS FOR ELECTRODEPOSITING SELECTED METALS

Reporteda Cal cul ated Specific Deposi tiona Operating Electrical Energy Efficiency Voltage Requirementb

Metal Bath Type (Percent) (Volts) (Wh/m2/micron Thickness)

Cadmium Cyanide (Cd2+) 80-95 1-16 4.7 - 75.3 Chromium Bright or Hard (Crb') 8-24 4-12 556 - 1667 Copper Cyanide (Cu+) 30-100 2-6 11.6 - 34.8 Copper Acid (Cu2') 95-100 2-12 15.5 - 92.8 Iron Sulfate (Fez+) 95-100 6 46.5

Lead F1 uoroborate (Pb2+) 100 1.5-6 4.4 - 17.6 Nickel All Types (Ni2+) 90- 100 6-12 51.4 - 103 Tin Acid (Sn2') 100 1-12 3.3 - 39.5 Tin Cyanide (Sn4+) 70-85 3-12 25.5 - 102 Zinc Acid (Zn2') 100 2-10 11.7 - 58.5 Zinc Cyan i de (Zn2+) 65-95 2-20 16.7 - 167

aEfficiencies and voltages taken from: 4th edition (1984) and The Canning Handbook of Surface tinishing Technology (1982).

Electroplating Engineering Handbook,

bEnergy values represent the electrical requirments for plating only, i .e., just

what is required to drive the electrochemical reactions. when n = valence, F - Faraday's Constant, V = Voltage, d = Density of Metal,

Calculated from nF*Vd RE- M = Molecular Weight of Metal, and E Average Efficiency,

3-8

Similar calculations can be made for anodizing aluminum, and Table 3-6 lists specific energy requirements for several anodizing techniques. higher voltages used in anodizing, the specific energy values are higher than for metal deposition, with the exception o f chromium. As mentioned above for electroplating, the values in Table 3-6 only represent a fraction of the total energy requirement for anodizing. Additional energy is needed for heating the anodizing baths, heating rinse waters, for drying, coloring, sealing, chromating, and other post-treatments. Furthermore, electrical energy losses in the equipment, such as rectifiers, increase the overall energy requirement.

Because of the

Table 3-7 summarizes some practical (measured) energy use data from a midwest plating shop averaged over several summer and winter months in the mid-1970s. The data were obtained by installing meters for electrical and fuel energy consumption in each plating department. energy consumption per unit area could be calculated. ings were not monitored, except as indicated. The energy consumption values given in Table 3-7 are about an order of magnitude greater than the values given in Tables 3-5 and 3-6 just for the electrolytic reduction processes involved in metal deposition, as oxidation processes in the case of anodizing. In Table 3-7, barrel zinc plating is seen as the most energy intensive electroplating technique. This was because of the higher operating voltage and increased drag-out causing increased rinsing requirements.

A record of the area coated also was kept, so that The thickness of the coat-

In general, the total energy consumed in an electrochemical process is approximately three to five times the electrical energy used in the electrolytic processes which comprise a part of the overall technique. process tanks, pump solutions, provide ventilation and 1 ighting, clean parts, move parts, and so on. Additional electrical energy is used in some processes to clean, electropolish, electroetch, electrochemically color, anodize, and the like. anodizing, the electrical energy used to form the oxide film typically comprises only 20% of the total energy requirement. Tables 3-8 and 3-9, taken from Reference ( 2 7 ) , show how electrical energy and process heat consumption is divided among various metal finishing operations. The data are the average of audits conducted for a group of 20 selected metal finishing plants over a period of a year. These plants coated a wide range o f products, including screws, nuts, bolts, locks, staples, scissors, furniture hardware, aircraft and automotive parts, electronic parts, and appliance hardware. goes to space heating, ventilation, lighting, and process tank heating. As

Other fuels are used to heat

In

Notice the high proportion of energy/fuel use that

3-9

Table 3-6

SPECIFIC ELECTRICAL ENERGY REQUIREMENTS FOR ANODIZING ALUMINUM

Cal cul ated Electrical Type of Voltage Energy Requiremen ta

Anodizing Bath (volts) (Wh/m2/micron Thickness)

Sulfuric Acid

- Conventional 12-22 70-130 - Hard 25-80 150-480

Chromic Acid 30-50 240-400

aEnergy values represent the electrical requirements for film formation only. Calculated from current density and film formation rate data, as reported in The Cannin Handbook of Surface Finishing T e c h n o l o g y , - d

Table 3-7

SPECIFIC ENERGY CONSUMPTION FOR SELECTED FIN ISH ING OPERATIONS

Energy Consumption Operation (Thickness) (kWh/mZL ( W h/m2/m)

Rack zinc plating (7.5 m) 15 2000

Rack nickel (12.5 m) + chromium plating 25 >zoo0 - Barrel zinc plating (15 m) 35 2333

Anodizing aluminum (5 m) 32 6400

Painting (various methods) 5-10 --

Source: Midwest Plating Shop (References - 25 and 40).

3-10

Table 3-8

ELECTRICAL ENERGY CONSUMPTION IN TYPICAL METAL FINISHING PLANTS

Operation

E 1 ec trop 1 a t i ng and/or Anodi z i ng

Grinding, Polishing, Buffing

E l ectrocl eani ng

Tank Heating

F i l t e r Pumps

Other Pumps

Air Sparging

Exhaust Fans

Hoists and Drives

Oven Heating

Waste Treatment

Light i ng

Chi 1 l e rs

Air Conditioning

M i scel 1 aneous Other Uses

Percentage of Total E l e c t r i c i t y Used

23.9

2.4

2.4

2.9

3.0

2.6

2.7

24.6

5.0

4.5

2.7

11.7

2.6

2.3

6.7 100;0%a

Source: AES Research Project 46 (Reference 27).

aAverage Electr ical energ consumption for 20 plants was 2.65 x 10 kWh (9.04 x 10 8 BTU) per annum.

3-11

Table 3-9

PROCESS HEAT ENERGY CONSUMPTION IN TYPICAL METAL FINISHING PLANTS

Percentage of Total Operation Heat Used

Process Tank Heating 23.1

C1 eaner Tank Heat i ng 12.0

Rinse Tank Heating 10.0

Vapor Degreaser (Heaters, e t c .) 4.0

Waste Recovery Evaporators

Boi ler Losses

3.8

18.2

Ovens and Dryers 7.0

Space Heating 18.2

Mi scel 1 aneous Other Uses 3.7 m O % a

Source: AES Research Project 46 (Reference - 27).

aAverage heat ener y consumption f o r 20 p l an t s was 15.8 x 106 kWh (53.8 x 10 !I BTU) per annum.

3-12

mentioned above, only about 25% of the electrical energy is used for electrochemical reactions leading to coating deposition.

Some similar data exist for painting techniques as a result of other baseline data collection studies prior to designing energy conservation strategies. of an automatic, solvent-based, spray painting line it was found that if the spray booth energy usage was excluded, 45% of the remaining energy use went to cleaning and phosphating steps; 20% to drying; 30% to baking (curing); and 5% to stripping the hangers (fixtures) after use. Table 3-10 summarizes the energy use figures given in quantitative terms. Table 3-7 for painting operations done in the midwest plating shop, taking into account that large coating facilities (more parts processed) should be more efficient in energy use. As with electroplating, anodizing, and electropainting, a considerable amount of energy in painting operations is used to heat process tanks, provide ventilation and makeup air, 1 ighting and space heating. (cleaners, rinses, phosphating solutions) are heated, heat is transferred to the metal parts being coated, heat is required to drive off the water and cure the coating .

In one study

These are in agreement with the figures given in

Process solutions

A comparison of the energy requirements for the application and curing of a variety of organic coatings is given in Table 3-11. for cleaning and phosphating, which, as shown above, typically account for about 60-75% of overall coating process energy use.

These data do not include energy used

Electrocoating (electropainting) also is listed in Table 3-11, and the energy requirement for this technique is seen to be less than one-half of that for the more conventional spraying techniques 1 isted. Tables 3-12 and 3-13, in contrast, compare the relative energy requirements and costs for electropainting and different types of spray coating techniques. for materials, labor, capital equipment, and power. For electropainting, the cost of rinsing and its energy requirement is included. costs given reflect only the cost o f the materials supplied as the respective coating. What is of interest in these last three tables is that electropainting is a relatively low-cost, energy-efficient coating application technique. However, it should be pointed out that these two criteria are not the only criteria to be used in selecting the best finishing process for a given application. Coating properties, thickness, number of parts to be coated, surface area of each part, availability of equipment, availability of fueldenergy sources, and so on, all play an important part in the selection process, which is beyond the scope of this report.

In Table 3-12, the data include costs

In Table 3-13, however, the

3-13

Table 3-10

PROCESS ENERGY USAGE FOR AN AUTOMATIC PAINT SPRAYING LINE

Energy Use

Process Description (kWh/m2) (BTU/m2)

1.58- 1.89 5380-6460

0-3.16 0- 10,765

Washi ng/Phosphat i ng (66OC)

Spray Booth (Air Heatinga)

Drying (Oven, 175OC) 0.63-0.95 2150-3230

Curing (Oven Baking, 175OC) 0.95-1.26 3230-4305

Hanger Stripping 0.00 538 Totals 3.32-1.42' 11,298-23,298'

Source:

aEnergy consumption depends on temperature of outside air. -8 C, then value given might be used. at about 175°C was assumed.

Finishing High1 ights, May/June, 1976, pp 24-27.

If this is below For the curing step, 20 to 30 minutes

Table 3-11

ENERGY REQUIREMENTS FOR DEPOSITING VARIOUS TYPES OF ORGANIC COATINGS

Approximate Baking Time Organ i c Thickness Ene-rgy Requi re-men ta and Temperature

Coating Type 0 kWh/mL) (loo B T U h rl ("C)

El ectrocoating 25 0.36 3 20 min. at 135 Urethane Powder 30 1.07 9 20 min. at 163 High Solids 25 1.07 9 20 min. at 121 Medium Sol ids 25 1.19 10 20 min. at 135 Waterborne 25 1.43 12 20 min. at 177 Sol vent Based 25 1.43 12 20 min. at 163

25 0.95 8 20 min. at 82

Source:

aBasis:

Products Finishing, September, 1977, pp. 56-57

41 m2 of 19-gage sheet metal processed per minute.

3-14

Table 3-12

RELATIVE ENERGY REQUIREMENTS AND COSTS FOR SPRAY PAINTING AND ELECTROPAINTING

Relative Energy Consumpt i ona Relative Costa

Coating Technique (%) (%)

Sol vent Spray 100 100

Powder Spray 98 108

81 Electrostatic Spray -- El ectropai nt i ng 87 51-64

Source: Finishing Highlights, January/February, 1977, pp. 8-9.

aSolvent spraying used as basis for comparison: into account type of coating material; capital cost; aux i 1 i ary mater i a1 s ; power requ i remen t ; 1 abor .

data took

3-15

Table 3-13

COST OF COATING APPLICATION BY PAINTING AND ELECTROPLATING TECHNIQUES

Application Efficiency

Method of Application (%)

Air Spraying

Conventional Enamel 50

Water Reduced 50

High Solids 50

Electrostatic Spraying

Conventional Enamel 93

Water Reduced 87

High Solids 93

Powder

El ec tropai nt i ng

98

99

cost of Finish ($/m2)

0.159

0.161

0.159

0.085

0.093

0.086

0.137

0.080

Source: Automotive Industries, October 1, 1976, p. 22.

Basis: thick coating.

Note: relative costs.

4660 m2 (50,000 ft2) per 8 hour day, 25 m (0.001 inch)

Cost not coverted to 1985 dollars but given as cited t o show

3-16

The type of equipment used in each process step can have a marked effect on energy consumption and cost. Data taken from Reference (15), for example, compare energy requirements for different ways to cure a powder coating. A summary of these data is given in Table 3-14, and it shows that the energy requirement to cure a powder is a little less than that for a liquid paint using a convection oven. If a combined infrared and convection oven is used to cure a similar powder paint, an energy savings of about 40% results.

In summary, a comparison of selected metal finishing techniques is given in Table 3-15. Although these data are quite old, they do provide an idea of relative specific energy use and total energy consumption. pendently of the data given in Table 3-7 and Reference (E) and show good agreement considering the limited nature of the available data base. face area of parts coated, Table 3-15 shows that painting far exceeds anodizing, electroplating, and hot dipping. anodizing and electroplating rank much higher than hot dipping and painting. If both surface area and energy consumption are considered as a ranking criterion, electroplating and painting processes consume more energy than anodizing, which in turn uses more energy than hot-dipping techniques (25). -

These data were estimated inde-

In terms of total sur-

However, when specific energy use is considered,

FUTURE ENERGY USE

One electric utility sales forecast (28) - for residential, commercial, and industrial energy usage predicts an increase in electricity demand of about 3% per year (average) through the year 2000. In a report (26) - prepared for the Department of Energy, it was stated that the industrial sector consumed more energy than any other in 1983, namely 37% of the total and that this would increase to 42% by the year 2000. This is equivalent to approximately a 2% increase (average) per year, Of the total industrial usage in 1983, about one-third was attributed to electricity consumption and losses, which was more than any other single fuel/energy source. increase to almost 40% at the expense of natural gas and petroleum fuels, while the usage of coal increases to about 12% of the total, as shown in Table 3-16. Fabricated Metals, SIC 34, is projected to account for a total of 180 x 109 kWh (0.614 x 1015 BTU) of the total industrial energy consumption in the year 2000. Of this total, 30.8 x 109 kWh (0.105 x 1015 BTU) are attributed to electricity and 72.7 x 109 kWh (0.248 x 1015 BTU) to electrical losses due to inefficiency. Reference (26) - states that slow growth is expected for the Primary Metals (SIC 33) and Fabricated Metals industries.

By the year 2000 this level of electricity consumption is predicted to

3-17

Table 3-14

COMPARISON OF SEVERAL METHODS OF PAINT CURING

Process Variable Convection Ovena Ir/Convection Ovenb

Type of Paint

Heat Demandc, kW BTU/h

Liquid Paint Powder Powder (Enamel )

422 422 284 1 , 440,000 1,440,000 970 , 000

Accountable Lossesd, kW 443 309 218 BTU/h 1 , 510,000 1 , 055,000 745,000

Total Heat Requirement, kw 865 731 502 BTU/h 2,950,000 2 , 495,000 1,715,000

Source :

Basis:

a 900 k

Chapter J, Reference (2). 745 m2 20-gage steel panels baked at 205OC.

Ih (3 x 106 BTU) oven for liquid; 700 kWh (2.5 x 106 BTU) oven for powcer.

b 500 kWh (1.75 x 106 BTU) oven, radiation sheilding o f conveyor cuts losses.

CConvection heating efficiency - 33%; IR efficiency - 49%.

dIncludes losses from oven wal Is, conveyor (hand1 ing equipment) and ventilation.

3-18

Table 3-15

Opera t i on

PRODUCTION AND ENERGY USE DATA FOR SELECTED METAL FINISHING OPERATIONS (1972)

Estimated Surface Area Procgsged (10 m )

Painting 210

E 1 ectropl at i ng 70

Hot Dippingb 60

Anodizing 20

Energy Usea (kWh/m2)

10

40

10

40

Total Energy Use 106 kWh

2100

2800

600

800

m e : Reference (25). - aValues vary widely so averages given for energy use, which includes electricity and other fuel s/energy sources.

bGalvanizing (zinc coating).

3-19

Table 3-16

PROJECTED INCREASE IN INDUSTRIAL ENERGY CONSUMPTION BY TYPE FOR THE YEAR 2000

1983 2000 Usage r -, Usage r

Fuel/Energy Type TkWh x l (BTU x 10152 (%I 1 kWh x lo=) (BTU x 1013) (%I

Electricity

Electrical Loses w I

785 2.68 33.0 1,377 4.7 39.9

1,867 6.37 (a) 3,253 11.1 (a)

Coal Solids 703 2.40 9.0 1,348 4.6 11.6

Gases 1.978 6.75 25.0 2.521 8.6 21.7

Petroleum Liquids 2,266 7.73 29.0 2,403 8.2 20.7 - -

Totals 7,963 27.17 100.0 11,610 39.6 100.0

Source:

aIncluded in "Electricity" usage.

National Energy Policy Plan, Scenario B (1983), Reference (26). -

' I ~I

Other sources also contain projections for energy consumption. Reference (30) suggested that total energy usage would increase at about the same rate as the GNP for the period 1981 through 1990, i.e., about 2 to 3% per annum, as Table 3-17 shows. about 50% greater than that of the GNP. corresponding industrial electricity demand are compared with historical data in Table 3-18. Another source (31) - reported that gas consumption should grow at about the same rate as the rate of growth of the GNP, that is at about 3% per annum. However, if the "low" estimates are taken from a number of sources, gas consumption is projected to level off at about 8 trillion cubic feet by the year 2000.

However, for the period 1990 through 2000 the rate of increase would be Projections for the growth in GNP and the

It also was pointed out in Reference (30) - that an increase in fuel prices since the oil embargo has occurred, but that the price of delivered electricity has not risen at the same rate as other fuel/energy supplies. was about 370% higher than in 1960, compared with an increase of only about 160% for electricity on a delivered BTU basis.

In 1982 the price of natural gas

A complication in projecting energy/fuel use is the fact that energy consumption per unit of production is falling at a rate of about 2% each year due to increased process efficiency , increased use of conservation measures, and trends towards manufacturing less energy intensive products (26). Also, production growth has been limited due to the state of the present economy. combined, it is immediately apparent that the increase in industrial (manufacturing) energy demand will be small. Energy conservation will remain a concern, although incentives have moderated due to the leveling off in fuel prices and the greater availability of some fuels. Significant increases in energy demand, as a result, will only occur if production increases and the U.S. economy moves out of the recessional mode. used will be different to what it has been in the past.

When these two factors are

As discussed earlier, when this occurs, the mix of fuels

Opportunities for energy conservation are discussed in References (s), (30), and (32) and more specifically for the metal finishing industry in References (25) and (27). Values in the range of 20 to 40% have been quoted and 20 to 25% reductions in energy consumption have already been achieved in some metal finishing operations since concern was first expressed during the oil embargo. conservation methods on the energy demand data projections discussed in this report cannot be ascertained at this time, but conservation could have a significant impact.

The effect of energy

For example, Reference (26) - states that of the 1.38 x 1012 kWh

3-21

Table 3-17

PROJECTIONS FOR ELECTRIC I TY CONSUMPTION ACCORDING TO SECTOR FOR THE YEAR 1990

Projection A (1981) Projection B (1982) Projection C (1982) 1980 Annual Annual Annual U age Demand Growth Rate Demand Growth Rate Demand Growth Rate

Sector (105 kWh) (109 kWh) (%I (109 kWh) (%I (109 kWh) (%I w N N

Res i den ti a1 703 879 2.2 879 2.2 908 2.6

Commerc i a 1 586 703 1.8 733 2.2 791 3.0

I ndus tr i a 1 820 1,143 3.3 1.231 4.1 996 1.9

Total /Average 2,110 2,726 2.6 2,872 3.1 2,667 2.3

Source: Various (Reference - 30).

Table 3-18

HISTORICAL AND PROJECTED DATA FOR GNP GROWTH AND CORRESPONDING GROWTH IN

INDUSTRIAL ELECTRICITY DEMAND

Time Period Parameter 1% 1 - 19 / 3 19 /3- 1980 1980-1990 1990-2006

Average Annual GNP Growth Rate, % 3.51 2.36 2.8 2.2

Average annual i ndus- trial electricity 6.45 2.46 2 .O-2.8 3.3-3.7

Ratio of electricity demand to GNP growth 1.84 1.04 0.7-1.0 1.5-1.7

Source: Various (Reference 3). Note: are mid-range estimates. boundaries and interpolation is not appropriate.

The values given for the projections were derived from different models and They should not be taken to indicate the upper/lower

3-23

( 4 . 7 x 1015 BTU) of total industrial/electrical energy demand projected for the year 2000, it would theoretically be possible to conserve 0.563 x 1012 kWh (1.92 x 1015 BTU), and practically to save 0.26 x 1012 kWh (0.90 x 1015 BTU) through waste heat recovery, improving combustion efficiency, waste uti 1 ization, improving electrolytic processes, better materials processing, more use of sensors and controls and more efficient use of coatings and adhesives.

In terms of specific methoddtechniques in metal finishing, in a report (29) - prepared for EPRI, a discussion of the growth of electricity using processes includes data on radiation curing and electrical discharge/electrochemical machining. The installed capacity for the former is projected to grow from 10 MW in 1980 to 200 MW in 1990 and 500 MW in the year 2000. from 50 to 130 and to 338 MW over the same time span. Process heating and lighting applications that use electricity will grow, while the use of electricity for electrolysis and machine drives will decrease. In another EPRI report (33) the rate of growth for electrical discharge and electrochemical machining i s not as large, growing from 300,000 MWh in 1980 to 500,000 MWh in 1990 and 1,400,000 MWh in the year 2000. Laser processing techniques for metals fabrication will increase from 28,000 MWh in 1980 to 50,000 MWh in 1990, and 135,000 MWh in 2000. percentage growth in advanced drying and curing methods will be similar, as shown in Table 3-19, but the amount of electricity consumed will be large. all plasma applications--not just spraying--will be large, but by far the largest growth and demand will be for adjustable speed, ac drives, which includes motors for pumps, fans, blowers, compressors, conveyors, and so on.

Similarly, the latter is projected to grow

The

The growth in

3-24

Table 3-19

ELECTRIC I TY CONSUMPTION PROJECT IONS FOR METAL FABRICATION TECHNOLOGIES

Projected Electrici t Consumption (MWh x 10 3 ,

Process Equipment '1980 1990 2000

Electric Discharge and Electrochemical Machining 300 400 750

Infrared Drying and Curing 1 , 170 1,190 3,110

Ultraviolet and Electron Beam Curing 810 1 , 545 2 , 700

Laser Processing 28 50 135

Plasma Melting, Cutting, and Spraying 10 75 195

Adjustable Speed ac Drives 20,700 175,300 350 , 000

=e: Electrotechnology Reference Guide (Reference - 33).

3-25

Section 4

CONCLUSIONS AND FUTURE DEVELOPMENTS

Metal finishing is an important and integral part of manufacturing processes, even though the energy demand is relatively small compared with other technologies in the industrial sector. The energy/fuel mix is changing for metal finishing methods, with a greater percentage of the demand being for electrical energy. at the time electricity demand is growing, energy conservation is having a larger effect, and the economic recession has limited production and moved the USA more toward a service-based economy. turing should shift back to the USA and the demand for metal finishing increase. As a result of the above factors, growth in industrial energy demand, which includes manufacturing, will be relatively small, perhaps 2 to 3% average each year through the year 2000. GNP growth over this same time period.

However,

As the value of the dollar declines, some manufac-

This growth rate parallels the modest projections made for

As new materials and products are developed, the need for modified or new metal finishing methods will increase. Historically, existing methods have been modified to provide surface treatments for new materials. However, with such a wide array of materials being developed for today's sophisticated new applications; much more stringent requirements for specific properties; along with new regulations concerning safety and waste treatment; and a desire to constrain or reduce costs, new methods become necessary.

Some of the more recent coating developments have been described in this report. They typically incorporate energy beams, require vacuum systems, and sometimes require post-treatments such as diffusion by heat treatment. New curing methods for organic coatings require radiant energy such as IR, UV, or electron beam to be effective. recent developments in metal finishing technology will be given later.

This general trend will continue in the future, and a few examples of

It was not the purpose of this report to review and discuss trends in coatings, but of course developments in coating requirements will require the corresponding development of coating techniques. In a recent review (34) - of the paint industry,

4-1

the need for meeting a changing business climate was recognized and metal finishers surveyed indicated that they would be increasing their equipment budgets by 35% in 1986. equipment were most often mentioned. an interest in purchasing color computers, color matching equipment, high-speed dispersing equipment , horizontal mi 1 1 s and solvent recovery sti 1 1 s, without planning to increase their budgets.

Spray equipment (air, airless, and air-assisted airless) and flow coating ___

On the other hand, paint manufacturers stated

-

An opinion of some is that the metal finishing industry is driven by materials requirements rather than developments in equipment. Others argue that there are occasions when the capabilities and features of equipment are recognized as providing opportunities to modify existing techniques or develop new techniques. Synergism becomes important. Whichever opinion is held, however, it is widely recognized that the successful development of new finishing operations results through the close cooperation of (1) the materials supplier, (2) the metal finishing companies, and (3) the equipment suppliers. As government regulations and industrial needs create incentives for developing new materialdcoatings, the suppliers and equipment manufacturers will respond. techniques will depend, however, on how the materials/coatings perform, the relia- bility of the equipment, and the cost. As a result, the metals finishing industry may become more spec i a1 i zed.

Acceptance of the new methods and

SURFACE PREPARATION

Degreasing and cleaning solutions are being formulated to operate at lower temperatures (20 to 65OC) to conserve energy. Some cleaners can remove rust and provide protection, others provide protection during handling by incorporating inhibitors, as are used in pickling, descaling, and etching solutions. been used to augment the cleaning of small parts and will be used more often in the electronics industry. Methods are being developed to clean metallized ceramic and plastic materials prior to electroplating, again principally for the electronics industry. A method has been developed to replace abrasive blasting of plastics by a chemical coating prior to zinc arc spraying. Patents are appearing for the cleaning of composite materials. In mechanical methods such as blasting, grinding and honing, the trend is toward more automatic, preprogrammed and fully integrated systems. Abrasive blasting is being used by some companies as an alternative to chemical cleaning. New equipment designs are being patented as well as blasting media.

Ultrasonic energy has

For example, solid carbon dioxide has been patented as a medium and

4-2

suggested as a more environmentally acceptable method. be separated before reuse because it vaporizes.

The medium does not have to

In the automotive industry where quality coatings must be obtained routinely, the trend is away from spray cleaners to immersion cleaners. A precleaning step is sometimes used to remove most of the heavy soils so that the immersion cleaning tank does not become contaminated and ineffective too quickly. Similarly, cleaning steps are not being used as widely for zinc phosphate grain refinement. A separate step is used before painting and the alkaline cleaning solutions can be optimized for providing the cleanest surfaces. As a consequence, however, the zinc phosphating step has had to be modified to provide a smaller, as-deposited crystal size. Also, the technique has had to be modified to permit sound deposits to be obtained on galvanized steel rather than bare steel. In the laboratory it has been found possible to accelerate phosphating techniques electrochemically by anodically polarizing the steel surfaces. Offsetting this trend toward specialized zinc phosphate coatings is a move towards greater use of iron phosphate coatings because of lower cost, easier waste management and because the paints now being used can cover iron-phosphated surfaces as we1 1 as zinc-phosphated surfaces. Trivalent chromium rinses are being used to seal zinc-phosphate coating porosity. A1 though more expensive than the traditional hexavalent chromium rinse, no waste treatment costs are incurred and a net savings results. Chromium-free rinses still are being developed, however, while combined trivalent/hexavalent rinses currently give the best protection.

Impregnation to seal defects and porosity in castings (32) - may become more widespread as a means of avoiding subsequent problems with electrodeposited and painted coatings. Materials (soil, solvents, process solutions, etc.) can be trapped in such defects and bleed out later, causing blistering; can prevent complete coverage; or can react with the coating material; all of which are undesirable.

Ultrafiltration is being more widely used to extend the life of cleaning solutions by removing oils and greases and other soils. emphasis is placed on increasing line speeds to improve production rates. Special cleaning solutions and techniques are being developed to be compatible with lines running at up to 250 meters a minute. For example, a two-stage alkaline cleaning system is designed so that in the first step surfactants are high but emulsifiers and saponfier concentrations are low to free the surfaces of heavy oils. second step the concentration of emulsifier is increased and smut removers are

For the coil coating industry,

In the

4-3

added. Powder cleaning compounds are being replaced by liquid cleaners as they are safer to handle and easier to use in automatic feed equipment. ing, there will be more widespread use of programmable handling equipment to remove the operator away from any chance exposure to fumes from the chlorinated solvents. Heat pumps are being incorporated into some systems to reduce heating requirements and improve energy efficiency.

In vapor degreas-

COAT I NG REMOVAL

Methods are necessary to quickly strip organic coatings from surfaces without affecting their integrity or properties. on military aircraft requires that the stripping technique can be used on specified areas to quickly remove paint. genic blastings are being evaluated. for stripping the heavy buildup of paint on hangers and other fixtures as a replacement for incineration techniques. nique depends on the shrinking and embrittlement of the coating at subzero temperatures. pellets. The process works well for alkyd, acrylic, polyester, vinyl, and lacquer coatings thicker than 250 m but not as well for epoxy and methane coatings (36). The parts do not have to be cleaned or rinsed after stripping. A cryogenic blasting method also has been patented for cleaning clogged abrasive belts.

For example, the maintenance of coatings

Approaches such as water jet impingement and cryo- Cryogenic stripping also is being evaluated

The effectiveness of the cryogenic tech-

The brittle material can then be blasted off with nonabrasive plastic

In order to control smoke emission, pyrolysis of organic coatings will be carried out at lower temperatures with lower oxygen levels. Integral water spray systems may be used to control the smoke generation in the afterburners when temperatures rise to unacceptable levels.

In other applications it is anticipated that the use of electrochemical methods for stripping metal1 ic coatings selectively, or electropol ishing to remove contaminated metallic coatings or alloyed layers, will increase but will still not represent a sign i f i cant energy use.

In the future, methods will have to be developed to strip or decontaminate coatings subjected to chemical and biological warfare agents. Areas to be treated may range from mi 1 i tary equipment and vehicles to buildings. not endanger the personnel using the stripping equipment.

The techniques developed must

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COATING APPLICATION

Electroplating and electroforming techniques are becoming more sophisticated as new coatings and new applications are being developed. Automation, real-time process control via sensors, and a trend toward higher deposition rates has been documented and these are becoming more widespread as computer/microprocessor technology improves and a wider range of sensors becomes available. control wi 11 become more common for mu1 tistep operations and where reproducible coating quality is desired.

Computer-aided process

More products will be fabricated by applying coatings as an integral part of the forming process rather than subsequent to metalworking, casting, or molding steps. Organic and inorganic coatings and claddings will be placed in the mold before casting or injection molding, for example, to provide coated parts, both metallic and nonmetal 1 i c.

Powder painting of aluminum architectural extrusions will compete with the more traditional anodizing techniques. Chrome-free conversion coatings for anodized aluminum parts will be developed but find only limited use in the near future. The application of techniques to provide streaked and textured anodized surfaces may increase.

Electrodeposition and vapor deposition methods will provide a wider range of a1 loy, composite, and multilayer coatings. lubricants, wear-resistant materials, and the 1 ike. to operate at low temperatures so that substrate properties are not adversely affected by heating. to meet new demands for high-quality coatings on larger parts. PVD system has been built with a vacuum chamber diameter of about 0.8 m and a height of about 3 .3 m to coat broaches with titanium nitride. A total of 18 cathodic arc evaporators are used to provide a uniform coating. sold by Multi-arc USA, costs a little over $1 million.

Inclusions in the coatings may be inhibitors, These methods can be designed

Equipment for vapor deposition methods is increasing in size As an example, a

The equipment,

Computer control, automation and robotics will be more widely used for the application of organic coatings, particularly spray deposition techniques. More fully automated finishing lines will be designed and built and operated continuously where production rates justify this.

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Because of the implementation of EPA regulations on waste water discharges, substi- tutes for wear-resistant, hard-chromium coatings will continue to be attractive. Electroless nickel-phosphorus and nickel-boron coatings have been used and wi 1 1 continue to be used. In addition, an electrolytic method has been developed for nickel-phosphorus alloy coatings. For thick coatings, plasma arc sprayed chromium can circumvent the waste treatment problem. intensive a technique as electroplating and a 40 kw system can be purchased for less than $60,000. These include A1203 + Ti02; Mo + Ni + Cr + Fe + B + Si; Fe + Mo; WC + Co; Cr2O3 + Si02 and Ni + Cr + B + Si/A1 + Mo. 3% and are usually rough and do not provide corrosion resistance. is comparable to or better than that of electrodeposited chromium. necessary after deposition to provide the same degree of surface smoothness as electroplated chromium.

~

Plasma spraying is not as capital -

Other plasma sprayed coatings can provide wear resistance (37). -

The coatings have porosities of up to Their hardness Grinding is

Research on ion implantation continues and materials being studied include optical waveguides, magnetic bubble memories, ferroelectric ceramics, high-temperature engineering ceramics and polymers with conductive surfaces (38). - Ion implantation can increase the conductivity of some polymers to that of graphite, thus implanted

shielding. Another appl cation, which may receive commercial interest, is the modified technique known as ion beam mixing. A thin layer of material is deposited on the substrate materia from a vapor. Simultaneously, or subsequently, this thin film is exposed to a ion beam. takes place and an alloy layer is formed at the interface between the film and the substrate. Good bonding results and the method is less expensive than alloying by direct ion implantation.

surfaces could act as electromagnetic or radio-frequency interference (EMI/RFI) ~~

As a result o f the collisions that occur, mixing

Interest also will continue in developing coating techniques incorporating laser beams. Laser-enhanced electroless plating of gold, platinum, or nickel-pal ladium alloy has been reported (36) - on semiconductor substrates such as silicon, indium phosphide, and gal 1 ium arsenide to provide conductive paths (circuits) or ohmic and Schottky barrier contacts. Because good resolution can be obtained and no masks are required, high-speed selective plating can be accomplished by programming the path of the laser beam. Similar techniques can be used for laser-assisted electrodeposition on metals and electroetching and chemical etching of metallic and nonmetallic (ceramic, polymer) substrates. the electronics, electro-optics and photovoltaics industries. Laser-assisted CVD

Such techniques might find application in

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and photochemical ly-assisted CVD, two other new techniques, have been described briefly in this report.

POST-TREATMENTS

Nonconvective, infrared drying and curing ovens are now available with emitters of closely control led wavelength and integral reflectors to minimize convection losses. Depending on the application, the IR output can be set in the medium- and short-wavelength ranges to rapidly dry and bake coatings. Increase in the use of this equipment should be seen in the future. Additional developments in the use of electron beam and pulsed xenon light ("flash") curing are expected because of lower heat input, good penetration, and fast curing speed possible.

Because of the more stringent EPA regulations on effluent discharges, there wil a trend toward using more chrome-free conversion coatings to passivate surfaces during extended periods of storage. At least one state no longer permits the d charge of trivalent chromium in rinse waters or other effluent, so alternatives needed where chromated surfaces are not specified.

be

S'

are

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Section 5

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

G. J. Rudzki. Surface Finishing Systems (Metal and Non-Metal Finishing Handbook-Guide) . Finishing Publ ications, Ltd. , Teddington, UK, 1983.

R. F. Bunshah. "Deposition Technologies: An Overview". Chapter 1 in Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, New Jersey, 1982.

B. N. Chapman and J. C. Anderson, Editors. Coating.

Science and Technology of Surface Academic Press, New York, New York, 19/4.

R. F. Bunshah and D. M. Mattox. "Applications of Metallurgical Coatings". Physics Today, 33(5), 50, 1980.

W. G. Wood, Editor. Metals Handbook. Vol. 5: Surface Cleaning, Finishing, and Coating, 9th Edition, American Society for Metals, Metals Park, Ohio, 1982.

R. S. Capp, Editor. Finishing Handbook and Directory. 1984 Edition, Sawell Publ ications Ltd. , London, UK, 1984. The Canning Handbook. Birmingham, UK, 1982.

(Surface Finishing Technology) , W . Canning plc,

M. Murphy, Editor. Metal Finishing Guidebook and Directory. 1983 Edition, Metals and Plastics Publ ications, Inc. , Hackensack, New Jersey, 1985.

J. Ward, Editor. International Finishing Industries Manual. 3rd Wheatland Journals Ltd. , Watford, UK, 1974. T. Lyman, Editor. Metals Handbook. Vol. 3: Machining, 8th Edit Society for Metals, Metals Park, Ohio, 1967.

M. A. Brimi and J. R. Luck. Electrofinishing. American Elsevier Company, Inc., New York, New Pork, 1965.

J. A. vonfraunhofer. Basic Metal Finishing. Chemical Publishing Inc., New York, New York, 1976.

Edition,

on, American

Publ i shi ng

Company,

"Clad It". (Clad Metal: Its Manufacture and Use). Texas Instruments, Inc., Metal Systems Publications No. 852, 1978.

R. Hughes and M. Rowe. The Colouring, Bronzing and Patination of Metals. Van Nostrand Reinhold Company, New York, New York, 1983.

E. P. Miller and D. D. Taft. Fundamentals of Powder Coatinq. Society of Manufacturing Engineers , Dearborn , Michigan , 19/4.

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16. E. W. Brooman and G. R. Schaer. "A New Manufacturing Technique for Flexible

17. P. Sprio. Electroforming. Robert Draper, Ltd. , Teddington, UK, 1971.

Circuitry". Electronic Packaging and Production, 21 August, 1981, p. 147.

18. L. J. Durney, Editor. Electroplating Engineering Handbook. 4th Edition, Van Nostrand Reinhold Company, New York, New York, 1984.

19. S. Karpel . "Electrolytic Bronzing of Aluminum Using Tin Electrolytes". - Tin and Its Uses, No. 146, 1985, p. 10.

20. D. Pletcher. Industrial Electrochemistry. Chapman and Hal 1, Ltd., London, UK, 1982.

21. L. T. Romankiw. "Laser-Enhanced Metal Deposition". Paper B6-8, Extended Abstracts, Proc. 35th Mtg., Int. SOC. of Electrochem. , Berkeley, California, August 5-10, 1985.

Metal Finishing Suppliers' Association, Birmingham, Michigan, 1983. 22. Quality Metal Finishing Guide, Vol. I, No. 1, "Electroless Nickel Plating".

23. T. Biestek and J. Weber. Conversion Coatings. Portcullis Press Ltd., Redhill, UK, 1976.

24. U.S. Department o f Commerce. Bureau of Census. 1982 Census of Manufacturers, Industry Groups and Industries". "Fuels and Electric Energy Consumed, Part 1:

MC 825-4 (Part l), June, 1983,

25. E. W. Brooman and W . H. Safranek. "Energy Consumption and Conservation in the Paper presented at the First Joint Industry Energy Metal Finishing Industry".

Conservation Seminar, Chicago, Illinois, October 12-13, 1976.

Mu1 ti-Year Plan FY1986-FY1990". Washington, D.C. , July, 1984. 26. U.S. Department of Energy. Office of Conservation. "Energy Conservation

27. D. A. Mazzeo and W. D. Holcombe. "Energy Conservation Study of the Plating and Surface Finishing Industry". American Electroplaters' Society Research Project 46, Final Report to Department of Energy, Office of Industrial Conservation, October, 1978.

28. D. E. Hall and E. Spore. "Report o f the Electrolytic Industries for the Year 1984". J. Electrochem. SOC., 132, 252C, 1985.

29. A. Faruqui and P. C. Gupta, et al. "Industrial Structural Changes and the

30. U.S. Department o f Energy. Office of Policy, Planning, and Analysis. "The

Future Demand for Electricity". Proc. Amer. Power Conf., 46, 563, 1984.

Future o f Electric Power in America: Report No. DOE/PE-0045, Washington, D.C., June, 1983.

Arlington, Virginia, 1984.

Supply for Economic Growth".

31. "The Gas Energy Demand Outlook: 1984-2000". American Gas Association,

32. M. Ross. "Energy Consumption by Industry". Ann. Review Energy, 6, 379, 1981.

33. "Electrotechnology Reference Guide". EPRI Report No. EM-4527, Electric Power Research Institute, Palo Alto, CA, 1985.

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35.

34. S. Suslik. "Finishing/Formulator Forecast 1986 and Beyond". Industrial Finishing, 62 (l), 44, 1986.

P. 1. E. Gebhard. "Impregnation Cuts Casting Painting Problems". Industrial Finishing, 61 (l), 33, 1985.

36.

37.

38.

39.

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"Freezing Paint for Removal".

F. N. Longo. Plating and Surface Finishing, 72 (7), 28, 1985.

S. T. Picraux and P. S. Peercy. "Ion Implantation of Surfaces". Scientific American, 252 (3), 84, 1985.

3 . Zahavi and M. Halliwell Metal Finishing, 83 (7), 61, 1985.

S. John, N. Shanmugam and B. A. Shenoi. in Electroplating Industries". National Conf., Karaikudi, India, 1983, p. 217.

Industrial Finishing, 60 (5), 30, 1984.

"Plasma Sprayed Coatings: An A1 ternative to Hard Chromium?"

"Selective Gold Plating Using Laser Beams".

"Conservation of Materials and Energy Electroplating and Metal Finishing, Proc. 2nd

T. Lyman, Editor. Metals Handbook, Vol. 2: Heat Treating, Cleaning and Finishing, 8th Edition, American Society for Metals, Metals Park, Ohio, 1964.

R. Ziegler. "Electron Beam Curing: A Viable A1 ternative". Industrial Finishing, 59 (8), 22, 1983.

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Section 6

SELECTED BIBLIOGRAPHY

Andrews, A. I. Enamels. 2nd Edition, Gerrard Press, Inc., Champaign, Illinois, 1961.

Porcelain Enamels; The Preparation, Application and Properties of

Ballard, W. E. Griffin, London, UK, 1963.

Metal Spraying and the Flame Deposition of Ceramics and Plastics.

Biestek, T., and J. Weber. Conversion Coatings. Portcullis Press Ltd., Redhill, UK, 1976.

Brimi, M. A., and J. R. Luck. Company, Inc., New York, New York, 1965.

Electrofinishing. American Elsevier Publishing

Bunshah, R. F. et al. Pub1 ications, Park Ridge, New Jersey, 1982.

Deposition Technologies for Films and Coatings. Noyes

The Canning Handbook. (Surface Finishing Technology), W. Canning plc, Birmingham, K, 1982.

Capp, R. S. Finishing Handbook and Directory. 1984 Edition, Sawell Publications Ltd., London, UK, 1984.

Durney, L. J. Electroplating Engineering Handbook. 4th Edition, Van Nostrand Reinhold Company, New York, New York, 1984.

Eisen, F. H., and L. T. Chadderton (Eds.). Ion Implantation. Gordon 81 Breach Science Publications, New York, New York, 1911.

Gabe, D. R. Pergamon Press, Oxford, UK, 1978.

Principles of Metal Surface Treatment and Protection. 2nd Edition,

Galvanizing Manual. Australia, 1982.

2nd Edition, Zinc and Lead Asian Service, Melbourne,

Hawkins, D. T. (Ed.). Chemical Vapor Deposition, 1960-1980: A Bib1 iography. IFI/Plenum Press, New York, New York, 1981.

Henley, V. F. Oxford, UK, 1982.

Anodic Oxidation of Aluminum and Its Alloys. Pergamon Press,

Holland, L. Vacuum Deposition o f Thin Films. John Wiley and Sons, Inc., New York, New York, 1956.

Hughes, R., and M. Rowe. The Colouring, Bronzing and Patination of Metals Nostrand Reinhold Company, New York, New York, 1983.

Van

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Huminik, Jr., J. High Temperature Inorganic Coatings. Reinhold Publishing Corporation, New York, New York, 1963.

Ingham, H. S., and A. P. Shephard. Flame Spray Handbook. Vols. 1-111, 7th Edition, Metco Inc., Westbury, New York, 1959.

Ion Implantations as a New Surface Treatment Technology. Final Report, National Materials Advisory Board, Washington, D.C., 1919.

Lehman, E. Y. Plasma and Flame Spraying. (A bibliography with abstracts, 1964- 1974) , National Technical Information Service, Springfield, Virginia, 1975.

Maissel, L. J., and R. Gland (Eds.). Handbook of Thin Film Technology. McGraw- Hill, New York, New York, 1970.

Meyers, R. R., and J. R. Long. Treatise on Coatings. Vols. 1-5, Marcel Dekker, Inc., New York, New York, 1967-1975.

Miller, E. P., and D. D. Taft. Manufacturing Engineers, Dearborn, Michigan, 19/4.

Fundamentals of Powder Coating. Society of

Mockey, B. J., and R. W. Rice. The Science of Ceramic Machining and Surface Finishing: U.S. Department of Commerce, 19i9.

Mulcahy, E. W. The Pickling of Steels. Industrial Newspapers Ltd., London, UK, 1973.

Proceedings of a Symposium at the National Bureau of Standards.

Murphy, J. A. Surface Preparation and Finishes for Metals. McGraw-Hill Book Company, New York, New York, 1971.

Murphy, M. Metal Finishing Guidebook and Directory. 1985 Edition, Metals and Plastics Publications, Inc. , Hackensack, New Jersey, 1985.

Murr, L. E. Dekker, Inc., New York, New York, 1984.

Industrial Materials Science and Engineering. Chapters 11-13, Marcel

Paint and Coatings Dictionary. Philadelphia, Pennsylvania, 1978.

Plaster, H. J. Blast Cleaning and Allied Processes. Vols. I and 11, Portcullis Press Ltd., London, UK, 19/4.

Federation of Societies for Coating Technology,

Quality Metal Finishing Guide. Vol. I, No. 1, "Electroless Nickel Plating", Metal b inishing Suppl iers I Association, Birmingham, Michigan, 1983.

Reed, W . E. Plasma and Flame Sprayed Coatings. Vol. 2 (Bibliography 1974-1977), National Technical Information Service, Springfield, Virginia, 1978.

Rudzki, G. J. Guide).

Surface Finishing Systems (Metal and Non-Metal Finishing Handbook- Finishing Publications, Ltd. , Teddington, UK, 1983.

Silman, H., G. Isserlis, and A. F. Averill. Protective & Decorative Coatings for Metals. Finishing Publications Ltd., Teddington, UK, 1918.

Smart, R. F., and J. A. Catherall. Plasma Spraying. Mills and Boon Ltd., London, UK, 1972.

6-2

Sputtering and Ion Plating. Aeronautics and Space Administration, Washington, D.C. , 1972.

Tatton, W. H., and E. W. Drew. Industrial Paint Application. Butterworths Ltd., London, UK, 1971.

VonFraunhofer, J. A. Instrumentation in Metal Finishing. Elek Science Ltd., London, UK, 1975.

VonFraunhofer, J. A. New York, New York, 1976.

NASA Special Pub1 ication No. NASA SP-5111, National

Basic Metal Finishing. Chemical Publishing Company, Inc. ,

Weiner, R. Electroplating of Plastics. Finishing Publications Ltd. , Teddington, UK, 1977.

Wernick, A., and R. Pinner. A1 loys.

Surface Treatment and Finishing of Aluminum and Its Robert Draper Ltd. , Teddington, UK, 19/2.

Wiederholt, W. Teddington, UK, '1965.

The Chemical Surface Treatment of Metals. Robert Draper Ltd.,

Wood, W. G. Metals Handbook. Vol. 5: Surface Cleaning, Finishing, and Coating, 9th Edition, American Society for Metals, Metals Park, Ohio, 1982.

Yeates, R. L. E l e c t r o p a i n t i n g . Robert Draper Ltd. , Teddington, UK, 1970.

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