Inspection of HDG - 1

7/29/2019 Inspection of HDG - 1

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Inspection of Hot-DipGalvanized Steel


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Inspection of Hot-Dip Galvanized Steel

This course is intended to train individuals on the proper inspection techniques and requirements for

hot-dip galvanized steel products. There are four sections in this course:

Hot-Dip Galvanizing Process Galvanizing Standards Types of Inspection Repair

Upon completion of this course, you should be able to recognize specification requirements and

perform all inspection steps to ensure conformance with the requirements. Additionally, any

inspector who completes the course, and passes the test (80% or better) will receive a printable

Certificate of Completion and will be listed on the AGA website as an inspector. Please make sure

to fill out all contact information, including your country, in order to accurately be included in the

Inspector Listing once the course is successfully passed.

Disclaimer

The information contained in this course has been compiled by the American Galvanizers

Association (AGA), a not-for-profit trade association whose members represent the after-fabrication

hot-dip galvanizing industry throughout the United States, Canada, and Mexico. The AGA makes no

endorsement and offers no evaluation of any vendors products, whether listed here or not.


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Contents

1. GalvanizingProcess...................................................................................................................................... 4

2. SurfacePreparation..................................................................................................................................... 5

2.1. Degreasing/CausticCleaning............................................................................................................... 5

2.2. Pickling................................................................................................................................................. 5

2.3. Fluxing.................................................................................................................................................. 6

3.

Galvanizing...................................................................................................................................................

7

3.1. PostTreatment.................................................................................................................................... 8

4. TimetoFirstMaintenance........................................................................................................................... 9

5. OtherCorrosionProtectionSystems......................................................................................................... 10

5.1. Metallizing.......................................................................................................................................... 10

5.2. ZincRichPaint.................................................................................................................................... 10

5.3. ContinuousGalvanizing..................................................................................................................... 11

5.4. Electroplating..................................................................................................................................... 12

6. ASTMSpecifications................................................................................................................................... 13

7. ASTMA123forStructuralSteelProducts................................................................................................. 14

8. ASTMA153forHardware......................................................................................................................... 16

9. ASTMA767forReinforcingSteel.............................................................................................................. 18

10. OtherGalvanizingStandards................................................................................................................. 20

10.1. CAN/CSAG164HotDipGalvanizingofIrregularlyShapedArticles.............................................. 20

10.2. ISO1461HotDipGalvanizedCoatingsonFabricatedIronandSteelArticles...............................20

11. TypesofInspection................................................................................................................................ 21

11.1. CoatingThickness.......................................................................................................................... 21

11.2. CoatingWeight.............................................................................................................................. 23

11.3. Finish&Appearance...................................................................................................................... 24

Appearance................................................................................................................................................ 24

Finish.......................................................................................................................................................... 24

11.3.1. DifferentAppearances............................................................................................................... 24

SteelChemistry.......................................................................................................................................... 25

CoolingRate............................................................................................................................................... 27

SteelProcessing......................................................................................................................................... 27

11.3.2. Finish:VisualDefects................................................................................................................. 28

11.4. AdditionalTests............................................................................................................................. 44

11.4.1. AdherenceTest.......................................................................................................................... 44

11.4.2. BendingTest.............................................................................................................................. 44

11.4.3. ChromatingTest......................................................................................................................... 45

11.4.4. EmbrittlementTest.................................................................................................................... 45


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11.5. Sampling......................................................................................................................................... 45

12. Repair..................................................................................................................................................... 48

12.1. MaximumSizeofRepairableArea................................................................................................. 48

12.2. RepairMethods............................................................................................................................. 48

12.2.1. ZincBasedSolder....................................................................................................................... 49

12.2.2. ZincRichPaint............................................................................................................................ 50

12.2.3. ZincSprayMetallizing................................................................................................................ 51


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1.Galvanizing Process

The term hot-dip galvanizing is defined as the process of immersing iron or steel in a bath of liquid

zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. The

coating is produced as the result of a metallurgical reaction between the liquid zinc and the iron in

the steel. The coating forms an equal thickness on all surfaces immersed in the galvanizing kettle.

This process, similar to the one seen in Figure 1, has been in use since 1742 and has provided long-

lasting, maintenance-free corrosion protection at a reasonable cost for many years. The three main

steps in the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment,

each of which will be discussed in detail.

Figure1:ModeloftheHotDipGalvanizingProcess

Steel structures with visible evidence of corrosion are pictured in the series of photos in Figure 2.

Rust and corrosion can be expensive for business owners and taxpayers because buildings, roads,

and bridges, without sufficient corrosion protection, may need to be repaired often or even rebuilt.

The process is described in more detail later in this section. It is inherently simple, and this

simplicity is a distinct advantage over other corrosion protection methods.

Figure2:CorrodingSteelStructures


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2.Surface Preparation

Figure3:HangingofSteelProducts

The first step in the hot-dip galvanizing process is intended to obtain the cleanest possible steel

surface by removing all of the oxides and other contaminating residues. This is achieved by first

hanging the steel using chains, wires, or specially designed dipping racks, as seen in Figure 3, tomove the parts through the process. There are three cleaning steps to prepare the steel for

galvanizing.

2.1. Degreasing/Caustic CleaningFirst the steel is immersed in an acid degreasing bath or caustic solution in order to remove the dirt,

oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water.

2.2. PicklingNext the steel is immersed in an acid tank filled with either hydrochloric or sulfuric acid, as seen inFigure 4, which removes oxides and mill scale in a process called pickling. Once all oxidation has

been removed from the steel, it is again rinsed with water and sent to the final stage of the surface

preparation.

Figure4:ThePicklingTank


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2.3. FluxingThe purpose of the flux is to clean the steel of all oxidation developed since the pickling of the steel

and to create a protective coating to prevent the steel from any oxidizing before entering the

galvanizing kettle. One type of flux is contained in a separate tank, is slightly acidic, and contains a

combination of zinc chloride and ammonium chloride. Another type of flux, top flux, floats on top of

the liquid zinc in the galvanizing kettle, but serves the same purpose.

After being immersed in the degreasing, pickling, and fluxing tanks, the surface of the steel is

completely free of any oxides or any other contaminants that might inhibit the reaction of the ironand liquid zinc in the galvanizing kettle.


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3.Galvanizing

Figure5:

Hot

Dip

Galvanizing

Kettle

Once the steel has been completely cleaned, it is ready for immersion in the liquid zinc. The

galvanizing kettle contains zinc specified to ASTM B 6, a document that specifies any one of three

different grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the

zinc melt in order to promote certain desirable properties in the galvanized coating.

The galvanizing kettle, like the one seen in Figure 5, is typically operated at a temperature ranging

from 820-860 F (438-460 C), at which point the zinc is in its liquid state. The steel products are

immersed into the galvanizing kettle and remain in the kettle until the temperature of the steel has

reached the temperature required to form a hot-dip galvanized coating. Once the interdiffusion

reaction of iron and zinc is completed, the steel product is withdrawn from the zinc kettle. The entiredip usually lasts less than ten minutes, depending upon the thickness of the steel.

The coating, as seen in the micrograph in Figure 6, is typical for low silicon steels with silicon

impurities less than 0.04% and where the thickness of the coating is limited by the interdiffusion of

iron and zinc.

Figure6:Photomicrographofthegalvanizedcoating


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3.1. Post-Treatment

FilingZincDrips

When the steel is removed from the galvanizing kettle, it may receive a post-treatment to enhance

the galvanized coating. One of the most commonly used treatments is quenching. The quench tank

contains mostly water but may also have chemicals added to create a passivation layer that protects

the galvanized steel during storage and transportation. Other finishing steps include removal of zinc

drips, or icicles, by grinding them off.


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4.Time to First Maintenance

The estimated time to first maintenance for a hot-dip galvanized coating that experiences common

atmospheric exposure can be seen in Figure 7. Time to first maintenance is defined as the time to

5% rusting of the substrate steel. The time to first maintenance of hot-dip galvanized steel is directly

proportional to the zinc coating thickness.

Figure7:TimetoFirstMaintenanceChartforHotDipGalvanizedCoatings


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5.Other Corrosion Protection Systems

There are many other types of corrosion protection, such as coating steel with oil, grease, tar,

asphalt, polymer coatings or paints, or corrosion protection materials such as stainless and

weathering steel, sacrificial anodes, plating systems and impressed current systems. These are some

of the most commonly used corrosion protection materials and systems and are sometimes used

together with hot-dip galvanized steel. Most of these materials rely on barrier protection, while some

of them rely on cathodic properties to prevent corrosion of the steel. The most effective type of

corrosion protection that provides both barrier and cathodic protection is hot-dip galvanizing.

There are also a wide variety of zinc coatings used for corrosion protection. Many people use

galvanizing to describe all of these coatings, but each has its own unique characteristics and

performance. These coatings have several applications based on their properties and respective

thicknesses. The corrosion protection offered by a zinc coating is linearly related to its coating

thickness. The most commonly used coatings are hot-dip galvanized, metallized, zinc-rich paint,

galvannealed or galvanized sheet, and electroplated. The relative thickness for each of these zinc

coatings can be seen in the photomicrograph (Figure 8). Below is a brief explanation of each type of

zinc coating.

Figure8:PhotomicrogrpahofZincCoatingsThicknesses

5.1. MetallizingMetallizing is the general name for the technique of spraying a metal coating on the surface of non-

metallic or metallic objects. This process is accomplished by feeding zinc in either wire or powder

form into a heated gun, where it is melted and sprayed onto the surface to be coated using

combustion gases and/or auxiliary compressed air to provide the necessary velocity. The limitations

of this process include a difficulty in reaching recesses, cavities, and hollow spaces, even coating

thickness and cost.

5.2. Zinc-Rich PaintZinc-rich paint is applied to a clean, dry steel surface by either a brush or spray and usually contains

an organic binder pre-mix. Paints containing zinc dust are classified as organic or inorganic,

depending on the binder that they contain, and are discussed in more detail later in this course.


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5.3. Continuous Galvanizing

Figure9:ContinuousGalvanizingPlant

The continuous galvanizing process is a hot-dip process where a steel sheet, strip, or wire is cleaned,

pickled, and fluxed on a processing line approximately 500 feet (154 m) in length, and running atspeeds between 100 to 600 feet per minute (30 to 185 m per minute). In the coating of a steel sheet

or strip, the galvanizing kettle contains a small amount of aluminum, which suppresses the formation

of the zinc-iron alloys, resulting in a coating that is mostly pure zinc. A post-galvanizing, in-line heat

treatment process known as galvannealing can also be used to produce a fully alloyed coating.

Galvannealing is usually ordered by those wanting to paint over the zinc surface because the

presence of alloy layers on the steel surface promotes paint adhesion. A photo of a continuous

galvanizing plant is seen in Figure 9 and the common plant setup is shown in Figure 10.

Figure10:ExampleofaContinuousProcess


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5.4. ElectroplatingThe electroplating process, or zinc-plated coating, has a dull gray color, a matte finish, and a thin

coating that ranges up to one mil (25 m) thick. This very thin coating restricts the use of zinc-plated

products to indoor exposures. The specification ASTM B 633 lists the classes of zinc-plated steel

coatings as Fe/Zn 5, Fe/Zn 8, Fe/Zn 12, and Fe/Zn 25, where Fe represents iron and Zn represents

zinc, while the number indicates the coating thickness in microns. The main uses for this type of

coating include screws, light switch plates, and other small products or fasteners.


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6.ASTM Specifications

There are certain specifications that have been developed for hot-dip galvanizing in order to produce

a high-quality coating. The most commonly used specifications design engineers and fabricators

should become familiar with in order to promote a high-quality coating and ensure their steel design

is suitable for hot-dip galvanizing are:

ASTM A 123/A 123M:Standard Specification for Zinc (Hot-Dip Galvanized) Coatings onIron and Steel Products

Single pieces of steel or fabrications with different types of steel products ASTM A 153/A 153M:Standard Specification for Zinc Coating (Hot-Dip) on Iron and

Hardware

Fasteners and small products that are centrifuged after galvanizing to remove excess zinc

ASTM A 767/A 767M:Standard Specification for Zinc-Coated (Galvanized) Steel Bars forConcrete Reinforcement

Reinforcing steel or rebar

ASTM A 780:Standard Practice for Repair of Damaged and Uncoated Areas of Hot-DipGalvanized Coatings

Touch-up procedures for coating bare spots on an existing hot-dip galvanized product

Other commonly used specifications in the hot-dip galvanizing industry include:

ASTM A 143/A 143M:Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement

ASTM A 384/A 384M:Standard Practice for Safeguarding Against Warpage andDistortion During Hot-Dip Galvanizing of Steel Assemblies

ASTM A 385/A 385M:Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)

ASTM B 6:Standard Specification for Zinc ASTM D 6386:Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated

Iron and Steel Product and Hardware Surfaces for Paint

ASTM E 376:Standard Practice for Measuring Coating Thickness by Magnetic-Field orEddy-Current (Electromagnetic) Examination Methods

CAN/CSA G 164:Hot-Dip Galvanizing of Irregularly Shaped Articles ISO 1461Hot-Dip Galvanized Coatings on Fabricated Iron and Steel Assemblies

Specifications and Test Methods

Lets examine a few of these specifications in more detail.


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7.ASTM A 123 for Structural Steel Products

Figure 11: Single Fabrication with Multiple Material Categories

The ASTM A 123/A 123M specification covers individual steel pieces as well as assemblies of

various classes of material. The four material categories covered in ASTM A 123/A 123M includestructural steel and plates, strips and bars, pipes and tubing, and wires. A fabrication can have more

than one material category such as a frame assembly. Any combination of these products can be

assembled into a single fabrication and then can be hot-dip galvanized, as seen in Figure 11.

It is the responsibility of the designer and fabricator to ensure the product has been properly

designed and built before the hot-dip galvanizing process. The galvanizer should be made aware of

any necessary special instructions or requests in advance of shipping the materials to the galvanizing

plant. These requests should be stated on the purchase order for the hot-dip galvanizing.

It is the responsibility of the galvanizer to ensure compliance with the specifications as long as the

product has been designed and fabricated in accordance with the referenced specifications. However,

if the galvanizer has to perform additional work in order to prepare the product for hot-dipgalvanizing, such as drilling holes to facilitate drainage or venting, it must be approved by the

customer. Once the material has been hot-dip galvanized, it can be fully inspected at the galvanizing

plant prior to shipment.

Any materials rejected by the inspectors for reasons other than embrittlement may be stripped,

regalvanized, and resubmitted for inspection. The ASTM specifications A 143/A 143M, ASTM A

384/A 384M, and ASTM A 385 provide guidelines for preparing products for hot-dip galvanizing.

The requirements listed in ASTM A 123/A 123M include coating thickness, finish, appearance, and

adherence. These are each defined below and discussed in more detail later in this course.

ASTMA123/A123MRequirements

Coating Thickness / Weight dependent upon material category and steel thickness Finish continuous, smooth, uniform Appearance free from uncoated areas, blisters, flux deposits and gross dross inclusions as

well as having no heavy zinc deposits that interfere with intended use

Adherence the entire coating should have a strong adherence throughout the service life ofgalvanized steel

The hot-dip galvanized coating is intended for products fabricated into their final shape that will be

exposed to corrosive environmental conditions. Once a product has been hot-dip galvanized, any


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further fabrication, which very rarely occurs, may have negative effects on the corrosion protection

of the coating. The coating grade is defined as the required thickness of the coating and is given in

microns. All coating thickness requirements in specification ASTM A 123/A 123M, as seen in

Tables 1 & 2, are minimums; there are no maximum coating thickness requirements in either

specification.

Table 1: Minimum Average Coating Thickness Grade by Material Category (From ASTM A123)

Table 2: Coating Thickness Grade (From ASTM A 123)

The time to first maintenance of hot-dip galvanized steel is directly proportional to the thickness of

the hot-dip galvanized coating. With all other variables held constant, the thicker the zinc coating,

the longer the life of the steel. The aim of the finish and appearance requirements is to ensure no

coatings have problem areas that are deficient of zinc or have surface defects that would interfere

with the intended use of the product. In addition, the coating should have a strong adherence

throughout the service of the hot-dip galvanized steel.


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8.ASTM A 153 for Hardware

The specification ASTM A 153/A 153M applies to hardware products such as castings, fasteners,

rolled, pressed and forged products, and miscellaneous threaded objects that will be centrifuged,

spun, or otherwise handled to remove the zinc, as seen in Figure 12.

Figure 12: Galvanized Fasteners

The requirements for ASTM A 153/A 153M are very similar to those reported earlier for ASTM A123/A 123M, except for the addition of threaded products and embrittlement requirements.


Coating Thickness/Weight depends on the material category and steel thickness, valuesare listed in Table 3

Threaded Products areas with threads are not subject to the coating thickness requirement Finish continuous, smooth, uniform Embrittlement high tensile strength fasteners (>150ksi) and castings can be subject to

embrittlement

Appearance free from uncoated areas, blisters, flux deposits and gross dross inclusions aswell as having no heavy zinc deposits that interfere with intended use

Adherence the entire coating should have a strong adherence throughout the service life ofhot-dip galvanized steel

There are fabrication steps that may impair the corrosion protection of the hot-dip galvanized

coating, however, flaking or damage to the coating because of this is not case for rejection. In all

cases, good steel selection results in the formation of a higher quality coating and finish on the

product. The corrosion protection coating for threaded products is applied after the product has been

fabricated and further fabrication may compromise the corrosion protection system. The one

exception to this rule is the internal threads of a nut that should be over-tapped after the coating is

applied in order to accommodate the coating thickness change on the thread of the bolts. In this case,

the zinc on the bolt threads provides the corrosion protection to the uncoated threads in the nut.

There are certain fabrication techniques that can induce stresses into the steel and lead to brittle

failure. There are precautions given in ASTM A 143/A 143M that should be taken in order to

prevent embrittlement. In addition, selecting steels with appropriate chemistries can help prevent

embrittlement of malleable castings. A reproduction and summary of the table given in ASTM A

153/A 153M, which is seen in Table 3, gives the different classes of products and the minimum

coating thickness required by the specification.


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Table 3: Minimum Average Coating Thickness by Material Class (From ASTM A 153)


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9.ASTM A 767 for Reinforcing Steel

The specification ASTM A 767/A 767M is applicable exclusively to the hot-dip galvanizing of

reinforcing steel, otherwise known as rebar, as seen in Figure 13, and is applicable to all types of

rebar, both smooth and deformed. However, wire is not included.

Figure 13: Hot-Dip Galvanized Rebar

The requirements in ASTM A 767/A 767M are also intended to produce a high quality zinc coating

for corrosion protection.


Identity the galvanizer is responsible for consistent material tracking if necessary Coating Thickness/Weight material category and steel thickness Chromating to prevent reaction between cement and recently galvanized material Finish continuous, smooth, and uniform Appearance free from uncoated areas, blisters, flux deposits and gross dross inclusions as

well as having no heavy zinc deposits that interfere with intended use

Adherence should be tightly adherent throughout intended use of the product Bend Diameters flaking and cracking due to fabrication after the hot-dip galvanizing

process are not rejectable

Once rebar is delivered to be hot-dip galvanized, it is the galvanizers responsibility to track and

maintain the identity of the product throughout the hot-dip galvanizing process until shipment of the

finished product. Again, the analogous coating requirements in the areas of coating thickness, finish,

and adherence are present in ASTM A 767/A 767M. However, this single product specification

introduces a few new requirements that apply solely to hot-dip galvanized rebar. In ASTM A 767/A

767M, the coating requirement is given in mass of the zinc coating per surface area. A summary of

the table given in ASTM A 767/A 767M and the minimum required coating thickness / weight of the

classes is seen in Table 4.

Table 4: Mass of Zinc Coating (From ASTM A 767)


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This specification also introduces a new requirement to the galvanized coating known as chromating.

Newly galvanized steel can react with wet cement and potentially form hydrogen gas as a product.

As this evolved hydrogen gas travels through the concrete matrix toward the surface, voids can be

created which weaken the bonding with the concrete or disturb the smoothness of the concrete

surface. In order to help prevent and suppress this reaction, hot-dip galvanized rebar is dipped into a

weak chromate quench solution after being removed from the galvanizing kettle.

The finish requirement for rebar is along the same lines as the finish requirements given in

specifications ASTM A 123/A 123M and A 153/A 153M. The coating is intended for corrosion

protection, so deficiencies that affect the coatings corrosion performance are grounds for rejection.

In addition, since rebar is handled frequently during its installation, any tears or sharp spikes thatmake the material dangerous to handle are grounds for rejection.

Rebar is commonly bent prior to the hot-dip galvanizing process. The table below gives

recommendations for bend diameters based upon the bare steel bar diameter before coating. Steel

reinforcing bars that are bent cold prior to hot-dip galvanizing should be fabricated to a bend

diameter equal to or greater than the specified values. However, steel reinforcing bars can be bent to

diameters tighter than specified in Table 5 providing they are stress relieved at a temperature of 900

to 1050 F (480 to 560 C) for one hour per inch (25 mm) of diameter.

Table 5: Minimum Finished Bend Diameters (From ASTM A 767)


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10.Other Galvanizing StandardsThere are Canadian and international specifications that could be used to specify hot-dip galvanizing

on a project. The differences in these specifications and the ASTM specifications are minimal, and

for the most part, only differ slightly in the minimum coating thickness/weight required for each type

and thickness of product being hot-dip galvanized.

OtherSpecificationsforHotDipGalvanizing(TakenfromCAN/CSAandISO

Standards)

10.1. CAN/CSA-G164 Hot Dip Galvanizing of IrregularlyShaped Articles

Scope

1. Thisstandardspecifiestherequirementsforzinccoating(galvanizing)bythehotdippingprocessonironandsteelproductsmadefromrolled,pressed,orforgedshapessuchasstructuralsections,

plates,bars,pipes,orsheets1mmthickorthicker.

2. Appliestobothunfabricatedandfabricatedproductssuchasassembledsteelproducts,structuralsteel

fabrications,

large

hollow

sections

bent

or

welded

before

galvanizing,

and

wire

work

fabricated

fromuncoatedsteelwire.

3. Appliestosteelforgingsandironcastingsthataretobegalvanizedseparatelyorinbatches.4. Doesnotapplytocontinuousgalvanizingofchainlinkfencefabric,wire,sheet,andstrip.5. Doesnotapplytopipeandconduitthatarenormallyhotdipgalvanizedbyacontinuousor

semicontinuousautomaticprocess.

6. ThevaluesstatedinSIunitsaretoberegardedasthestandard.Thevaluesinparenthesesareimperialunitsandareincludedforinformationonly.

10.2. ISO 1461 Hot Dip Galvanized Coatings on FabricatedIron and Steel Articles

Scope: This Standard specifies the general properties of and methods of test for coatings applied by

hot dipping in zinc (containing not more than 2% of other metals) on fabricated iron and steel

articles.

It does not apply to:

1. Sheetandwirecontinuouslyhotdipgalvanized;2. Tubeandpipehotdipgalvanizedinautomaticprocess;3. Hotdipgalvanizingproductsforwhichspecificstandardsexistandwhichmayincludeadditional

requirementsorrequirementsdifferentfromthoseofthisEuropeanStandard.

4. Aftertreatment/Overcoatingofhotdipgalvanizedarticlesisnotcoveredbythisstandard.NOTE Individual product standards can incorporate this standard for the coating by quoting its

number, or may incorporate it with modifications specific to the product.


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11.Types of InspectionIn this section, the type of inspections performed on hot-dip galvanized steel will be discussed. The

various inspections are used to verify the necessary specifications for the galvanized product are

met. These techniques for each test method are specified in ASTM A 123/A 123M, A 153/A 153M,

or A 767/A 767M, depending upon the type of product being inspected. The most common

inspections, listed below, range from a simple visual inspection to more sophisticated tests to

determine embrittlement or adhesion.

Coating Thickness magnetic gauges, optical microscopy Coating Weight weigh-galvanize-weigh, and weigh-strip-weigh Finish and Appearance visual inspection Additional Tests

o Adherence stout knifeo Embrittlement similar bend radius, sharp blow, and steel angleo Chromating spot testo Bending minimum finished bend diameter table

Sampling11.1. Coating Thickness

The term coating thickness refers to the thickness of zinc applied to steel, while coating weight

refers to the amount of zinc applied to steel for a given surface area. Two different methods are used

in order to measure the coating thickness of hot-dip galvanized steel.

Figure 14: Pencil-Style Gauge

The first method to measure coating thickness involves using magnetic thickness gauges. There are

three different types of magnetic thickness gauges and all can be used quite easily in the galvanizing

plant or in the field.

The first type of magnetic thickness gauge is very small and utilizes a spring-loaded magnet encased

in a pencil-like container, as seen in Figure 14. The tip of the gauge is placed on the surface of the

steel and is slowly pulled off in a continuous motion. When the tip of the gauge is pulled away from

the surface of the steel, the magnet, near the tip, is attracted to the steel. A graduated scale indicates

the coating thickness at the instant immediately prior to pulling the magnet off the surface of the

steel. The accuracy of this gauge requires it to be used in the true vertical plane because, due to

gravity, there is more error associated with measurements taken in the horizontal plane or overhead

positions. The measurement should be made multiple times because the absolute accuracy of this


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type of gauge is below average and it is difficult to determine the true coating thickness when only

one reading is taken.

Figure 15: Banana Gauge

A banana gauge, as seen in Figure 15 is the second type of thickness gauge. With this gauge, coating

thickness measurements are taken by placing the rubber magnet housing on the surface of the

product with the gauge held parallel to the surface. A scale ring is rotated clockwise to bring the tip

of the instrument in contact with the coated surface and rotated counter-clockwise until a break in

contact can be heard and felt. The position of the scale ring when the magnetic tip breaks from the

coated surface displays the coating thickness. This type of gauge has the advantage of being able to

measure coating thickness in any position, without recalibration or interference from gravity.

Figure 16: Electronic/Digital Thickness Gauge

The electronic or digital thickness gauge, as seen in Figure 16is the most accurate and arguably, the

easiest thickness gauge to operate. The electronic thickness gauge is operated by simply placing the

magnetic probe onto the coated surface and then a digital readout displays the coating thickness.

Electronic gauges have the advantage of not requiring recalibration with probe orientation, but do

require calibration with shims of different thicknesses in order to verify the accuracy of the gauge at

the time it is being used. These shims are measured and the gauge is calibrated according to the

thickness of the shim, and then this process is repeated for shims of different thicknesses until the

gauge is producing an accurate reading in all ranges of thickness.

ASTM E 376

The specification ASTM E 376 contains information for measuring coating thickness using magnet

or electromagnetic current. It also provides some tips for obtaining measurements with the greatest

accuracy, as well as describing how the physical properties, the structure, and the coating can

interfere with the measurement methods. The requirements for ASTM E 376, as seen below, are

intended to make the coating thickness measurements using magnet or electromagnetic current as

accurate as possible.


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ASTM E 376 Requirements

Measurements on large products should be made at least four inches from the edge to avoidedge effects

Measurement readings should be as widely dispersed as possibleThere are some general guidelines, as seen below, for reducing error and ensuring the most accurate

readings are being collected when using magnetic thickness gauge instruments.

Guidelines for Reducing Error

Recalibrate frequently, using non-magnetic film standards or shims above and below theexpected thickness value

Readings should not be taken near an edge, a hole, or inside corner Readings taken on curved surfaces should be avoided if possible Test points should be on regular areas of the coating Take at least five readings to obtain a good, true value which is representative of the whole

sample

Figure 17: Optical Microscopy

The second method used to measure the coating thickness involves optical microscopy, as seen inFigure 17. This is a destructive technique and is typically only used for inspection of the coating of

single specimen samples that have failed magnetic thickness readings or for research studies. Since it

is not a common method, the accuracy is highly dependent on the expertise of the operator.

11.2. Coating WeightThe term coating weight refers to the amount of zinc applied to a product for a given surface area.

Two different methods can be used to measure the coating weight of hot-dip galvanized steel.


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The first method to measure the coating weight involves using a process called weigh-galvanize-

weigh, and is only appropriate for single specimen samples. The zinc coating weight from this

technique is underestimated because the actual coating is made up of both iron and zinc and this

method will only measure the added zinc weight in the coating. In addition, it can be very difficult to

measure and calculate the surface area of a complex steel fabrication, and this makes coating weight

values even less accurate.

Weigh-strip-weigh is the second method used to measure coating weight, and again is only

appropriate for single specimen samples. This method is destructive since it removes the hot-dip

galvanized coating during the measurement. This process involves first weighing the specimen,

stripping it of all zinc coating that was added, and then weighing it again. The difference in theweights is then equal to the amount of coating added during the galvanizing process. However, this

method is usually only used on very small products like nails, and can be inaccurate because when

the coating is stripped there may be some base metal stripped along with the coating. This means

that there may be extra iron included in the weight measurement, making for a higher than actual

zinc coating weight.

11.3. Finish & Appearance

The inspection of finish and appearance is done with an unmagnified visual inspection. This

inspection is performed by fully observing all parts and pieces of a hot-dip galvanized product to

ensure all necessary components and specifications have been met. It is done in order to observesurface conditions, both inside and out, and check all contact points, as well as welds, junctions, and

bend areas.

Appearance

The appearance of the hot-dip galvanized coating can vary from piece to piece, and even section to

section of the same piece. There are a number of reasons for the non-uniform appearance, but it is

important to note appearance has no bearing on the corrosion protection of the galvanized piece.

This section will overview the reasons for differences in appearance.

FinishThis section will review a number of possible surface defects visible on the galvanized coating.

Some of these surface defects are rejectable, as they will seriously lower the corrosion protection,

while others have little or no effect on the corrosion performance and are acceptable.

11.3.1. Different AppearancesThe appearance of hot-dip galvanized steel immediately after galvanizing can be bright and shiny,

spangled, matte gray, or a combination of these. There are a number of reasons for the difference in

appearance, as explored here, but regardless if the piece is shiny or dull, the appearance has no effect


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on the corrosion performance. And in time after exposure to the environment, all galvanized coatings

will take on a uniform matte gray appearance.

11.3.1.1.Reasons for Different AppearancesSteel Chemistry

The most common reason for galvanized steel to have different appearances is the chemistry of the

steel pieces. There are two elements of steel chemistry which most strongly influence the final

appearance; silicon and phosphorous. Both silicon and phosphorous promote coating growth, and

this thicker coating is responsible for the differing appearance.

The amount of silicon added during the steel making process to deoxidize the steel can create

differences in appearance of galvanized products. The recommended silicon composition is either

less than 0.04% or between 0.15% and 0.25%. Any steels not within these ranges are considered

reactive steels and are expected to form zinc coatings that tend to be thicker.

In addition to producing thicker coatings, highly reactive steels tend to have a matte gray or mottled

appearance instead of the typical bright coating. This difference in appearance is a result of the rapid

zinc-iron intermetallic growth that consumes all of the bright, pure zinc. This growth of the

intermetallic layer is generally out of the galvanizers control, because they usually do not have prior

knowledge of the steels composition. However, this increased coating thickness can be beneficial insome respects because time to firrst maintenance is directly proportional to coating thickness.

In Figure 18, the micrograph on the left shows a regular zinc-iron alloy, while the micrograph on the

right shows an irregular zinc-iron alloy. These clearly show the microscopic level differences that

can occur due to the amount of silicon in the steel being hot-dip galvanized.

Figure18:Regularvs.IrregularZincIronAlloyLayers

The Sandelin curve, as seen in Figure 19, compares the zinc coating thickness to the mass

percentage of silicon in the steel. The area on the graph labeled I is called the Sandelin area and

the coatings tend to be thick and dull gray as a direct result of the percentage of silicon present in the

base steel. This area is known as the Sandelin range since Dr. Sandelin, a metallurgist, performed theexperimental work to show this behavior of galvanized steel. The Sandelin area is roughly between

0.05% and 0.15% silicon. The area on the graph labeled II, which represents a steel content of

greater than 0.25% silicon, shows the coating thickness increases with increased silicon content and

then starts to level off at around 0.4% silicon.


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Figure19:SandelinCurve

Figure20:CoatingDuetoPhosphorous

In addition to silicon, the presence of phosphorus influences the reaction between the liquid zinc and

the steel, as seen in Figure 20. Phosphorus is generally considered an impurity in steel except where

its beneficial effects on machinability and resistance to atmospheric corrosion are desired. Somesteels such as ASTM A 242 Type 1 present problems because they may contain both a high level of

phosphorus and a high level of silicon. The presence of phosphorus generally produces smooth dull

coating areas and ridges of a thicker coating where there is increased intermetallic growth. The end-

result is a rough surface with ridges appearance.

Figure 21 is an example of products with separate galvanized pieces that have very different

appearances due to the difference in steel chemistry. However, all of these products still have an

equal amount of corrosion resistance throughout and are acceptable.

Figure21:Shinyvs.Dull


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Cooling Rate

Figure22:CoatingAppearanceDuetoCoolingRateDifference

A visually dull or shiny coating on a product can be caused by the different rate of cooling of a

product. In Figure 22, the outer edges were cooled rapidly, which allowed free zinc or an eta layer to

form on top of the intermetallic layers. The zinc in the center of the product that would have formedthe eta layer was consumed in the reaction with the iron after the part was removed from the

galvanizing kettle and formed an intermetallic layer that gives the dull gray look. Eventually as the

product weathers, the differences in appearance will disappear and it will become a dull gray color

throughout.

Steel Processing

Figure23:CoatingAppearanceDuetoSteelProcessing

In addition to temperature and chemistry of the steel, the processing of the steel can also create a

bright or dull appearance in galvanized products. The top rail in Figure 23 has a winding pattern of

dull gray areas corresponding to processing during the tube making. The stresses in the steel affect

the intermetallic formation and can create this striped look. The corrosion protection is not affected

and these parts are acceptable.


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11.3.2. Finish: Visual DefectsAs stated before, the hot-dip galvanized coating could have any number of surface defects. This

section will review the various defects and discuss whether or not they are cause for rejection

according to the specification. The surface defects reviewed are:

A C

Bare Spots Blasting Damage Chain and Wire Marks Clogged Holes Clogged Threads

D E

Delamination Distortion Drainage Spikes Dross Inclusions Excess Aluminum in

Galvanizing Bath

F O

Fish Boning Flaking Flux Inclusions Oxide Lines

P R

Products in Contact Rough Surface Condition Runs Rust Bleeding

S T

Sand Embedded in Casting Striations Steel Surface Condition Surface Contaminant Touch Marks

U Z

Weeping Weld Welding Blowouts Welding Spatter Wet Storage Stain Zinc Skimmings Zinc Splatter


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11.3.2.1.Visual Defects: A-CBareSpotsBare spots, defined as uncoated areas on the steel surface, are the most common surface defect and

occur because of inadequate surface preparation, welding slag, sand embedded in castings, excess

aluminum in the galvanizing kettle, or lifting aids that prevent the coating from forming in a small

area. Only very small areas, less than 1 inch in the narrowest dimension with a total of no more than

0.5%of the accessible surface area, may be renovated using ASTM A 780. This means narrow, bare

areas may be repaired; however, if they are greater than one inch-square areas, the product must beregalvanized. In order to avoid bare spots, like those seen in Figure 24, the galvanizer must ensure

the surfaces are clean and no contaminants are present after pretreatment. If the size of the bare spot

or total surface area causes rejection, the parts may be stripped, regalvanized, and then re-inspected

for compliance to the standards and specifications.

Figure24:BareSpots

BlastingDamageBlasting damage creates blistered or flaking areas on the surface of the galvanized product. Blasting

damage follows abrasive blasting prior to painting of the galvanized steel. It is caused by incorrect

blasting procedures creating shattering and delamination of the alloy layers in the zinc coating.

Blasting damage, as seen in Figure 25, can be avoided when careful attention is paid to preparation

of the product for painting. In addition, blast pressure should be greatly reduced according to ASTM

D 6386. Since blasting damage is induced by a post-galvanizing process, the galvanizer is not

responsible for the damage.

Figure25:BlastingDamage


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ChainandWireMarksAnother type of surface defect occurs when steel is lifted and transported around the galvanizing

plant using a chain or wire. These lifting aids can leave uncoated areas on the finished product that

will need to be repaired. The superficial marks, like those seen in Figure 26, left on the galvanized

coating from the lifting attachments are not grounds for rejection as long as marks can be repaired.

ASTM specifications do not allow any bare spots on the finished galvanized part.

Figure26:ChainandWireMarks

CloggedHolesClogged holes are holes partially or completely clogged with zinc metal. A good example is the

screen shown in Figure 27. The zinc was trapped because liquid zinc will not drain easily from holes

less than 3/10 (8mm) in diameter due to its high surface tension. Clogged holes can be minimized

by making all holes as large as possible. The trapped zinc can be removed by using active fettlingwhen the part is in the galvanizing kettle, vibrating the cranes to jostle the parts, or blowing

compressed air onto the galvanized products. This condition is not a cause for rejection, unless it

prevents the part from being used for its intended purpose.

Figure27:CloggedHoles


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CloggedThreadsClogged threads are caused by poor drainage of a threaded section after the product is withdrawn

from the galvanizing kettle. These clogged threads, as seen in Figure 28, can be cleaned by using

post-galvanizing cleaning operations such as a centrifuge or by heating them with a torch to about

500 F (260 C) and then brushing them off with a wire brush to remove the excess zinc. Clogged

threads must be cleaned before the part can be accepted.

Figure28:CloggedThreads


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11.3.2.2.Visual Defects: D-EDelaminationDelamination or peeling creates a rough coating on the steel where the zinc has peeled off. There are

a number of causes for zinc peeling. Many large galvanized parts take a long time to cool in the air

and form zinc-iron layers after they have been removed from the galvanizing kettle. This continued

coating formation leaves behind a void between the top two layers of the galvanized coating. If there

are many voids formed, the top layer of zinc can separate from the rest of the coating and peel off

the part. If the remaining coating still meets the minimum specification requirements, then the part isstill acceptable. If the coating does not meet the minimum specification requirements then the part

must be rejected and regalvanized. If delamination, as seen in Figure 29, occurs as a result of

fabrication after galvanizing, such as blasting before painting, then the galvanizer is not responsible

for the defect.

Figure29:Delamination

DistortionDistortion, as seen in Figure 30, is defined as the buckling of a thin, flat steel plate or other flatmaterial such as wire mesh. The cause of this is differential thermal expansion and contraction rates

for the thin, flat plate and mesh than the thicker steel of the surrounding frame. In order to avoid

distortion, use a thicker plate, ribs, or corrugations to stiffen flat sections or make the entire

assembly out of the same thickness steel. Distortion is acceptable, unless distortion changes the part

so that it is no longer suitable for its intended use.

Figure30:Distortion


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DrainageSpikesDrainage spikes or drips are spikes or tear drops of zinc along the bottom edges of the product.

These result when the surfaces of the product are processed horizontal to the galvanizing kettle,

preventing proper drainage of the zinc from the surface as the product is withdrawn from the kettle.

Drainage spikes, as seen in Figure 31, are typically removed during the inspection stage by a buffing

or grinding process. Drainage spikes or drips are excess zinc and will not affect corrosion protection,

but are potentially dangerous for anyone who handles the parts. These defects must be removed

before the part can be accepted.

Figure31:DrainageSpikes

DrossInclusionsDross inclusions are a distinct zinc-iron intermetallic alloy that becomes entrapped or entrained in

the zinc coating. This is caused by picking up zinc-iron particles from the bottom of the kettle.

Dross, as seen in Figure 32, may be avoided by changing the lifting orientation or redesigning the

product to allow for proper drainage. If the dross particles are small and completely covered by zinc

metal, they will not affect the corrosion protection and are acceptable. If the dross particles are large,

then the dross must be removed and the area repaired.

Figure32:DrossInclusions


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ExcessAluminuminGalvanizingBathAnother type of surface defect, shown in Figure 33, is caused by an excess amount of aluminum in

the galvanizing bath. This creates bare spots and black marks on the surface of the steel. The excess

aluminum can be avoided by ensuring proper control of the aluminum level in the galvanizing bath

by means of regular sampling and analysis, and by adjusting the levels in a regular and controlled

manner. For small areas of bare spots, the part may be repaired as detailed in the specification. If this

condition occurs over the entire part, then it must be rejected and regalvanized.

Figure33:ExcessAluminuminGalvanizingBath


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11.3.2.3.Visual Defects: F-OFishBoningFish boning is an irregular pattern over the entire surface of the steel part. This is caused by

differences in the surface chemistry of a large diameter steel piece and variations in the reaction rate

between the steel and zinc. These reaction differences cause the thickness of the galvanized coating

to vary in sharply defined zones across the surface. Fish boning, as seen in Figure 34, has no effect

on the corrosion protection provided by the zinc coating and is not cause for rejection of the hot-dip

galvanized part.

Figure34:FishBoning

FlakingFlaking results when heavy coatings develop in the galvanizing process, usually 12 mils or greater.

This generates high stresses at the interface of the steel and the galvanized coating and causes the

zinc to become flaky and separate from the surface of the steel. Flaking can be avoided byminimizing the immersion time in the galvanizing kettle and cooling of the galvanized steel parts as

quickly as possible. Figure 35 shows a micrograph of flaking. In addition, using a different steel

grade, if possible, may also help avoid flaking. If the area of flaking is small, it can be repaired and

the part can be accepted; however, if the area of flaking is larger than allowed by the specifications,

the part must be rejected and regalvanized.

Figure35:MicrographofFlaking


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FluxInclusionsFlux inclusion can be created by the failure of the flux to release during the hot-dip galvanizing

process. If this occurs, the galvanized coating will not form under this flux spot. If the area is small

enough, it must be cleaned and repaired; otherwise, the part must be rejected. Flux spots can increase

if the flux is applied using the wet galvanizing method, which is when the flux floats on the zinc bath

surface. Flux deposits on the interior of a hollow part, such as a pipe or tube, as seen in Figure 36,

cannot be repaired, thus the part must be rejected. Any flux spots or deposits, picked up during

withdrawal from the galvanizing kettle do not warrant rejection if the underlying coating is not

harmed, and the flux is properly removed.

Figure36:FluxInclusion

OxideLinesOxide lines are light colored oxide film lines on the galvanized steel surface. Oxide lines are caused

when the product is not removed from the galvanizing kettle at a constant rate. This may be due to

the shape of the product or the drainage conditions. Oxide lines, as seen in Figure 37, will fade over

time as the entire zinc surface oxidizes. They will have no effect on the corrosion performance; only

the initial appearance will be affected. This condition is not a cause for rejection of the hot-dip

galvanized parts.

Figure37:OxideLines


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11.3.2.4.Visual Defects: P-RProductsinContactAnother type of surface defect is caused by products that come in contact with each other or are

stuck together. This usually occurs when many small products are hung on the same fixture, which

creates the chance products may become connected or overlapped during the galvanizing process, as

seen in Figure 38. The galvanizer is responsible for proper handling of all products in order to avoid

this defect. In addition, if the surface of a product has a larger bare area than the specified repair

requirement allows, then that product must be rejected and regalvanized.

Figure38:ProductsinContact

RoughSurfaceConditionRough surface condition or appearance is a uniformly rough coating with a textured appearance over

the entire product. The cause for this rough surface condition is hot-rolled steel with a high level of

silicon content. This can be avoided by purchasing steel with a silicon content less than 0.03% of the

steel by weight. Rough surface condition, as seen in Figure 39, can actually have a positive effect on

corrosion performance because of the thicker zinc coating produced. One of the few situations where

rough coating is cause for rejection is if it occurs on handrails. The corrosion performance of

galvanized steel with rough coatings is not affected by the surface roughness.

Figure39:RoughSurfaceCondition


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RunsRuns are localized thick areas of zinc on the surface. Runs occur when zinc freezes on the surface of

the product during removal from the zinc bath. This is more likely to occur on thinner sections with

large surface areas that cool quickly. In order to avoid runs, as seen in Figure 40, adjustments of the

dipping angles can be made, if possible, to alter the drainage pattern to a more acceptable mode. If

runs are unavoidable and will interfere with the intended application, they can be buffed. Runs are

not cause for rejection.

Figure40:Runs

RustBleedingRust bleeding appears as a brown or red stain that leaks from unsealed joints after the product has

been hot-dip galvanized. It is caused by pre-treatment chemicals that penetrate an unsealed joint.

During galvanizing of the product, moisture boils off the trapped treatment chemicals leaving

anhydrous crystal residues in the joint. Over time, these crystal residues absorb water from the

atmosphere and attack the steel on both surfaces of the joint, creating rust that seeps out of the joint.

Rust bleeding, as seen in Figure 41, can be avoided by seal welding the joint where possible or by

leaving a gap greater than 3/32 (2.4mm) wide in order to allow solutions to escape and zinc to

penetrate during hot-dip galvanizing. If bleeding occurs, it can be cleaned up by washing the joint

after the crystals are hydrolyzed. Bleeding from unsealed joints is not the responsibility of the

galvanizers and is not cause for rejection.

Figure41:RustBleeding


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11.3.2.5.Visual Defects: S-TSandEmbeddedinCastingAnother type of surface defect occurs when sand becomes embedded in the castings and creates

rough or bare spots on the surface of the galvanized steel. Sand inclusions are not removed by

conventional acid pickling, so abrasive cleaning should be done at the foundry before the products

are sent to the galvanizer. This type of defect also leaves bare spots and must be cleaned and

repaired or the part must be rejected, stripped, and regalvanized. Sand embedded in a casting can be

seen in Figure 42.

Figure42:SandEmbeddedinCasting

StriationsStriations are characterized by raised parallel ridges in the galvanized coating, mostly in the

longitudinal direction. This can be caused when sections of the steel surface are more highly reactive

then the areas around them. These sections are usually associated with segregation of steel

impurities, especially phosphorous, created during the rolling process in steel making. Striations, as

seen in Figure 43, are related to the type of steel galvanized and while the appearance is affected, the

performance of the corrosion protection is not. Striations are acceptable on most parts; however, if

the striations happen to occur on handrails, then the parts must be rejected and regalvanized.

Sometimes regalvanizing does not improve the striations and the handrail must be refabricated out of

better quality steel.

Figure43:Striations


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SurfaceContaminantWhen surface contaminants create an ungalvanized area where the contaminant was originally

applied, a surface defect may occur. This is caused by paint, oil, wax, or lacquer not removed during

the pretreatment cleaning steps. Surface contaminants, as seen in Figure 44, should be mechanically

removed prior to the galvanizing process. If they result in bare areas, then the repair requirements

apply and small areas may be repaired, but a large area is grounds for rejection and the entire part

must be regalvanized.

Figure44:SurfaceContaminant

TouchMarksAnother type of surface defect is known as touch marks, which are damaged or uncoated areas on

the surface of the product. Touch marks are caused by galvanized products resting on each other or

by the material handling equipment used during the galvanizing operation. Touch marks, as seen inFigure 45, are not cause for rejection if they meet the size criteria for repairable areas. They must be

repaired before the part is accepted.

Figure45:TouchMarks


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11.3.2.6.Visual Defects: U-ZWeepingWeldWeeping welds stain the zinc surface at the welded connections on the steel. They are caused by

entrapped cleaning solutions that penetrate the incomplete weld. In order to avoid weeping welds for

small overlapping surfaces, completely seal weld the edges of the overlapping area. For larger

overlapping areas, the area cannot be seal welded since the volume expansion of air in the trapped

area can cause explosions in the galvanizing kettle. To avoid weeping welds in large overlapping

areas, the best plan is to provide a 3/32 (2.4mm) or larger gap between the two pieces whenwelding them and let the zinc fill the gap between the pieces. This will actually make a stronger joint

when the process is complete. Weeping welds, as seen in Figure 46, are not the responsibility of the

galvanizer and are not cause for rejection.

Figure46:WeepingWeld

WeldingBlowoutsWelding blowout is a bare spot around a weld or overlapping surface hole. These are caused by pre-treatment liquids penetrating the sealed and overlapped areas that boil out during immersion in the

liquid zinc. This causes localized surface contamination and prevents the galvanized coating from

forming. In order to avoid welding blowouts, as seen in Figure 47, check weld areas for complete

welds to insure there is no fluid penetration. In addition, products can be preheated prior to

immersion into the galvanizing kettle in order to dry out overlap areas as much as possible. Welding

blowouts cause bare areas that must be repaired before the part is acceptable.

Figure47:WeldingBlowouts


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WeldingSpatterWelding spatter appears as lumps in the galvanized coating adjacent to weld areas. It is created when

welding spatter is left on the surface of the part before it is hot-dip galvanized. In order to avoid

welding spatter, welding residues should be removed prior to hot-dip galvanizing. Welding spatter,

as seen in Figure 48, appears to be covered by the zinc coating, but the coating does not adhere well

and can be easily removed. This type of defect can leave an uncoated area or bare spot if the zinc

coating is damaged and must be cleaned and properly repaired.

Figure48:WeldingSpatter

WetStorageStainWet storage stain is a white, powdery surface deposit on freshly galvanized surfaces. It is caused by

newly galvanized surfaces being exposed to fresh water, such as rain, dew, or condensation that react

with the zinc metal on the surface to form zinc oxide and zinc hydroxide. It is found most often on

tightly stacked and bundled items, such as galvanized sheets, plates, angles, bars, and pipes. Wet

storage stain can have the appearance of light, medium, or heavy white powder on the galvanized

steel product. Each of these appearances can be seen from right to left in Figure 49.

One method to avoid wet storage stains is to passivate the product after galvanizing by using a

chromate quench solution. Another precaution is to avoid stacking products in poorly ventilated,

damp conditions. Light or medium wet storage stain will weather over time in service and is

acceptable. In most cases, wet storage stain does not indicate serious degradation of the zinc coating,

nor does it necessarily imply any likely reduction in the expected life of the product. However,

heavy wet storage stain should be removed mechanically or with appropriate chemical treatments

before the galvanized part is put into service. Heavy storage stain must be removed or the part must

be rejected and regalvanized.

Figure49:WetStorageStain


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ZincSkimmingSkimming deposits are usually caused when there is no access to remove the skimmings during the

withdrawal of the steel from the galvanizing kettle. The skimmings on the liquid zinc surface are

trapped on the zinc coating. In order to remove zinc skimmings without harming the soft zinc

coating underneath, lightly brush them off the surface of the galvanized steel during the in-house

inspection stage with a nylon-bristle brush. Zinc skimmings, as seen in Figure 50, are not grounds

for rejection. The zinc coating underneath is not harmed during their removal and it meets the

necessary specifications.

Figure50:ZincSkimmingInclusions

ZincSplatterZinc splatter is defined as splashes and flakes of zinc that loosely adhere to the galvanized coating

surface. Zinc splatter is created when moisture on the surface of the galvanizing kettle causes liquid

zinc to pop and splash droplets onto the product. These splashes create flakes of zinc looselyadherent to the galvanized surface. Zinc splatter, as seen in Figure 51, will not affect the corrosion

performance of the zinc coating and is not cause for rejection. The splatter does not need to be

cleaned off the zinc coating surface, but can be if a consistent, smooth coating is required.

Figure51:ZincSplatter


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11.4. Additional Tests11.4.1. Adherence Test

Testing of the zinc coating adherence to the steel is achieved using a stout knife. The steps used in

this test are listed below and a photo of the test being performed can be seen in Figure 52. The

coating shall be deemed not adherent if it flakes off and exposes the base metal in advance of the

knifepoint. The test is not an attempt to pare or whittle the zinc coating. If the coating is adherent the

knife should put a slight mark in the zinc metal surface, but should not cause any delamination of the

coating layers.

Figure 52: Stout Knife Test

AdhesionTestwithaStoutKnife

Push down point of stout knife Coating must not flake off exposing the base metal Do not perform at edges or corners of the product No paring or whittling with knife is acceptable

11.4.2. Bending TestThe hot-dip galvanized coating on a steel bar must withstand bending without flaking or peeling

when the bending test is preformed in accordance with the specifications in ASTM A 143. There are

various tests used to assess the ductility of steel when subjected to bending. One test may include the

determination of the minimum radius or diameter required to make a satisfactory bend. Another test

may include the number of repeated bends that the material can withstand without failure when it is

bent through a given angle and over a definite radius.

Rebar is commonly bent prior to the hot-dip galvanizing process. Steel reinforcing bars bent cold

prior to hot-dip galvanizing should be fabricated to a bend diameter equal to or greater than the

specified value in ASTM A 767/A 767M. However, steel reinforcing bars can be bent to diameters

tighter than the specified values if they are stress relieved at a temperature of 900 to 1050 F (480 to

560 C) for one hour per inch (25mm) of diameter.


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11.4.3. Chromating TestThe specification to determine the presence of chromate on zinc surfaces is ASTM B 201. This test

involves placing drops of a lead acetate solution on the surface of the product, waiting 5 seconds,

and then blotting it gently. If this solution creates a dark deposit or black stain, then there is

unpassivated zinc present. A clear result indicates the presence of a chromate passivation coating.

11.4.4. Embrittlement TestWhen there is suspicion of potential embrittlement of a product, it may be necessary to test a small

group of the products to measure the ductility. These tests are usually destructive to the zinc coating

and possibly to the product as well. Products suspected of embrittlement shall be tested according to

the specification ASTM A 143. Depending on the service conditions the product will be exposed to,

one of three embrittlement tests may need to be performed. These embrittlement tests include the

similar bend radius test, sharp blow test, and steel angle test. The embrittlement test uses a known

force to provide a stress that should be lower than the yield stress of the part. If there is a fracture or

permanent damage created during the testing process, the parts must be rejected.

11.5. SamplingA sampling protocol has been developed by ASTM to ensure high quality products because the

inspection of the coating thickness for every piece of material galvanized in a project would not be

practical. ASTM A123/A123M states for a unit of products whose surface area is equal to or less

than 160 in (1032 cm), the entire surface of each test product constitutes a specimen. In the case of

a product containing more than one material category or steel thickness range, that product will

contain more than one specimen. In addition, products with surface areas greater than 160 in (1032

cm) are multi-specimen products. There are four important terms used in the ASTM specifications

and each is defined below.

SamplingTerms

Lot unit of production or shipment from which a sample is taken for testing Sample a collection of individual units of product from a single lot Specimen the surface of an individual test product or a portion of a test product which is a

member of a lot or a member of a sample representing that lot

Test Product an individual unit of product that is a member of the sampleFor single specimen products, each randomly selected product is a specimen. In thickness

measurement tests, five measurements are taken widely dispersed over the surface area of the

specimen in order to represent the total coating thickness. The mean value of the five coating

thicknesses for one specimen must have a minimum average coating thickness grade of not less thanone grade below the minimum average coating thickness for the material category. In Figure 53, the

separation of a lot into a sample and individual specimen is shown.


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Figure 53: Single Specimen Product Sampling

A multi-specimen product is defined as having a surface area that may be larger than 160 in (1032

cm), have multiple steel thicknesses, or contain more than one coating category. In order to test

coating thickness of products whose surface area is greater than 160 in (1032 cm), they are

subdivided into three continuous local sections with equivalent surface areas, each of which

constitutes a unique specimen. In the case of any such local section containing more than one

material category or steel thickness range, that section will contain more than one specimen. In

Figure 54, the separation of a lot into a sample and individual specimen is shown.

Figure 54: Mutli-Specimen Product Sampling

For products hot-dip galvanized to either ASTM A123/A123M or A153/A153M, Table 6is used to

determine the minimum number of specimens for sampling from a given lot size.

No. of Pieces in Lot No. of Specimens

3 or less All

4 to 500 3

501 to 1200 5

1201 to 3200 8

3201 to 10,000 13

10,001+ 20

Table 6: Minimum Number of Specimensfor ASTM A123 and A153


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For rebar hot-dip galvanized according to ASTM A767, the information below is used to determine

the minimum number of samples per lot, measurements per sample, and the total number of

measurements required for each of the different coating thickness measurement techniques.

Magnetic Thickness:o 3 samples per loto 5 or more measurements per sampleo 15 measurements, at the minimum, comprise the average

Microscopy Method:o 5 samples per loto 4 measurements per sampleo 20 measurements, at minimum, comprise the average

Stripping and Weighing:o 3 samples per lot

The minimum average coating thickness for a lot is the average of the specimen values and must

meet the minimum for the material category. The minimum for an individual specimen is one grade

below the minimum for the material category. An individual measurement has no minimum, but bare

areas are not allowed on the part. The final inspection of a part shall include thickness measurements

and visual inspection. All parts that do not meet the requirement must be resorted and reinspected or

rejected and then regalvanized.


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12.RepairIf the galvanized product does not meet all of the requirements of the specification, it must be

repaired or rejected along with the lot it represents. When repair of the product is allowed by the

specification or bare spots are present, the galvanizer is responsible for the repair unless directed

otherwise by the purchaser. The specifications allow for some retesting of products that represent

lots or retesting after the lot has been sorted for non-conformance. The coating thickness of the

repaired area must match the coating thickness of the surrounding area. However, if zinc-rich paint

is used for repair, the coating thickness must be 50% higher than the surrounding area, but not

greater than 4.0 mils because mud cracking tends to result when the paint coating is too thick. Themaximum sizes for allowable areas that can be repaired during in-plant production are defined in the

specifications as summarized below.

12.1. Maximum Size of Repairable Area ASTM A 123/A 123M:

o One inch or less in narrowest dimensiono Total area can be no more than 0.5% of the accessible surface area to be coated or 36

square inches per piece, whichever is less

ASTM A 153/A 153M:o The bare spots shall have an area totaling no more than 1% of the total surface area tobe coated, excluding threaded areas of the piece

ASTM A 767/A 767M:o No area giveno If the coating fails to meet the requirement for finish and adherence, the bar may be

stripped, regalvanized, and resubmitted

o Damage done to the coating due to fabrication or handling shall be repaired with azinc-rich formulation

o Sheared ends shall be coated with a zinc-rich formulation

12.2. Repair MethodsAny repairs made to galvanized products must follow the requirements of ASTM A 780, which

defines the acceptable materials and the required procedures. Repairs are normally completed by the

galvanizer before the products are delivered, but under certain circumstances, the purchaser may

perform the repairs on their own. The touch-up and repair materials are formulated to deliver an

excellent color that matches either brightly coated, newly galvanized products or matte gray, aged

galvanized products. Materials used to repair hot-dip galvanized products include zinc-based solder,

zinc-rich paint, and zinc spray metallizing, and are explained in the following sections.


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12.2.1. Zinc-Based Solder

Figure55:ZincBasedSolder

Soldering with zinc-based alloys is achieved by applying zinc alloy in either a stick or powder form.

The area being repaired needs to be preheated to approximately 600 F (315 C). The most commonly

used solders for repair, as seen in Figure 55, include zinc-tin-lead, zinc-cadmium, and zinc-tin-

copper alloys.

SurfacePreparation

According to ASTM A 780, the surface to be reconditioned shall be wire brushed, lightly ground, or

mildly blast cleaned. In addition, if wire brushing or light blasting is inadequate, all weld flux and

spatter must be removed by mechanical methods. The cleaned area also needs be preheated to 600 F

(315 C) and wire brushed while heated. Pre-flux may also be necessary to provide chemical cleaning

of the bare spot. Finally, special care should be given to insure that the surrounding galvanizedcoating is not overheated and burned by the preheating.

Application

The soldering method is the most difficult of the three repair methods to complete. A high level of

caution must be taken while heating the bare spot to prevent oxidizing the exposed steel or damaging

the surrounding galvanized coating. Solders are typically not economically suited for touch-up of

large areas because of the time involved in the process and because heating of a large surface area to

the same temperature is very difficult. When the repair has been completed, the flux residue needs to

be removed by rinsing the surface with water or wiping with a damp cloth.

FinalRepairedProduct

The final coating thickness for this repair shall be agreed upon between the galvanizer and the

purchaser, and is generally in the 1 to 2 mil range. The thickness shall be measured by any of the

methods in ASTM A 123/A 123M that are non-destructive. Zinc-based solder products closely

match the surrounding zinc and blend in well with the existing coating appearance.


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12.2.2. Zinc-Rich Paint

Figure56:ZincRichPaint

Zinc-rich paint is applied to a clean, dry steel surface by either a brush or spray as seen in Figure 56,and usually contains an organic binder pre-mix. Zinc-rich paints must contain either between 65% to

69% metallic zinc by weight or greater than 92% metallic zinc by weight in dry film. Paints

containing zinc dust are classified as organic or inorganic, depending on the binder they contain.

Inorganic binders are particularly suitable for paints applied in touch-up applications around and

over undamaged hot-dip galvanized areas.

SurfacePreparation

According to ASTM A 780, the surface to be repaired shall be blast cleaned to SSPC-SP10/NACE

No.2 near white metal for immersion applications and SSPC-SP11 near bare metal for less

aggressive field conditions. When blasting or power tool cleaning is not practical, hand tools may beused to clean areas to be reconditioned. The blast cleaning must extend into the surrounding,

undamaged, galvanized coating.

Application

This method of repairing galvanized surfaces must take place as soon as possible after preparation is

completed and prior to the development of any visible oxides. The spraying or brushing should be in

an application of multiple passes and must follow the paint manufacturers specific written

instructions. In addition, proper curing of the repaired area must occur before the product is put

through the final inspection process. This repair can be done either in the galvanizing plant or on the

job site and is the easiest repair method to apply because limited equipment is required. Zinc-rich

painting should be avoided if high humidity and/or low temperature conditions exist becauseadhesion may be adversely affected.


The coating thickness for the paint must be 50% higher than the surrounding coating thickness, but

not greater than 4.0 mils, and measurements should be taken with either a magnetic, electromagnetic

or eddy current gauge. Finally, the surface of the painted coating on the repaired area should be free

of lumps, coarse areas, and loose particles.


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12.2.3. Zinc Spray Metallizing

Figure57:ZincSprayMetallizing

Zinc spray, which is also referred to as metallizing, is done by melting zinc powder or zinc wire in a

flame or electric arc and projecting the liquid zinc droplets by air or gas onto the surface to be

coated, as seen in Figure 57. The zinc used is nominally 99.5% pure or better and the corrosion

resistance of the wire or powder is approximately equal.

SurfacePreparation

According to ASTM A 780, the surface to be reconditioned shall be blast cleaned to SSPC-

SP5/NACE No.1 near white metal and must be free of oil, grease, weld flux residue, weld spatter

and corrosion products. The blast cleaning must extend into the surrounding, undamaged, galvanized

coating.

Application

Zinc spraying of the clean, dry surface must be completed by skilled workers and should take place

within four hours after preparation or prior to development of visible oxides. Spraying should also be

done in horizontal overlapping lines, which yield a uniform thickness more consistent than the

crosshatch technique. The zinc coating can be sealed with a thin coating of low viscosity

polyurethane, epoxy-phenolic, epoxy, or vinyl resin. The details of the application sequence and

procedures can be found in ANSI/AWS C2.18-93. The application of zinc spray can be done either

in the galvanizers plant or at the job site. In addition, if high humidity conditions exist during

spraying, adhesion may be degraded.


The renovated area shall have a zinc coating thickness at least as thick as that specified in ASTM A

Inspection of HDG - 1

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