Nickel-Containing Alloys in Organic Acids and Related ... · Corrosion Resistance of...

Corrosion Resistance of

Nickel-Containing Alloys

in Organic Acids

and Related Compounds

Table of Contents

PART I. INTRODUCTION A. The Organic Acids 4 B. Scope 4 C. Corrosion Testing in Organic Acid Media 4 PART II. ACETIC ACID A. General 5

B. Austenitic Stainless Steels 5 1. General 5 2. Effect of Alloy Composition 6 3. Effect of Contaminants 10 4. Effect of Temperature 12 5. Effect of Microstructure 14

6. Quality Control 15 C. Martensitic & Ferritic Stainless Steels 15 D. Duplex Austenitic-Ferritic and Precipitation

Hardening Stainless Steels 15 E. Iron-Base Nickel-Chromium-Copper

Molybdenum Alloys 16 F. Nickel-Base Chromium-Iron-Molybdenum-

Copper Alloys 17 G. Iron-Base Nickel-Chromium-Molybdenum Alloys18 H. Nickel-Base Molybdenum-Chromium-Iron Alloys 18

I. Nickel-Copper Alloys 20 J. Copper-Nickel Alloys 21 K. Nickel-Chromium Alloys 23 L. Iron-Nickel-Chromium Alloys 23 M. Nickel-Base Molybdenum Alloys 24 N. Nickel 24 O. Process and Plant Corrosion Data 25

l. Acetic Acid Production 25 a. Oxidation of Acetaldehyde 25 b. Liquid Phase Oxidation of

Straight-Chain Hydrocarbons 26 c. Methanol-Carbon Monoxide Synthesis 28

2. Acetic Acid Storage and Shipping 28 3. Vinegar Production and Storage 29

P. Acetic Anhydride 29 PART III. OTHER ORGANIC ACIDS

A. Formic Acid 31 B. Acrylic Acid 36 C. C3 Through C8 Acids 38

(Propionic, Butyric and Higher Acids) D. Fatty Acids 44

(Lauric, Oleic, Linoleic, Stearic, Tall Oil Acids) E. Di and Tricarboxylic Acids 46

(Oxalic, Maleic, Phthalic, Terephthalic, Adipic, Glutaric and Pimelic Acids)

F. Naphthenic Acids 52 G. Organic Acids with Other Functional Groups 53

1. Glycolic Acid 53 2. Lactic Acid 53 3. Tartaric Acid 54 4. Citric Acid 54 5. Chloroacetic Acids 56 6. Amino Acids 57 7. Sulfoacetic Acid 57

PART IV ESTER PREPARATIONS A. Acetic Esters 58 B. Phthalate Esters 60 C. Esterification of Fatty Acids 60 D. Acrylate Esters 60

References 64 Trademarks Inside Back Cover

Nominal Composition of Nickel-Containing Alloys in Use or Corrosion Tested in Organic Acids and Related Compounds

Composition, %

Alloys Ni Fe Cr Mo Cu C Si Mn Other

WROUGHT ALLOYS

Stainless Steels—Austenitic AISI Type 201 4.5 Balance 17.0 – – 0.15 Max 1.0 Max 6.5 N 0.25 Max AISI Type 202 5.0 Balance 18.0 – – 0.15 Max 1.0 Max 8.0 N 0.25 Max AISI Type 204 5.0 Balance 18.0 – – 0.08 Max 1.0 Max 8.0 N 0.25 Max AISI Type 204L 6.0 Balance 18.0 – 0.03 Max 1.0 Max 8.0 N 0.25 Max AISI Type 216 6.0 Balance 19.5 – – 0.08 Max 1.0 Max 8.0 N 0.25-0.50 AISI Type 216L 6.0 Balance 19.5 – – 0.03 Max 1.0 Max 8.0 N 0.25-0.50 AISI Type 304 9.5 Balance 18.5 – – 0.08 Max 1.0 Max 1.5 AISI Type 304L 10.0 Balance 18.5 – – 0.03 Max 1.0 Max 1.3 AISI Type 309 13.5 Balance 23.0 – – 0.20 Max 1.0 Max 2.0 Max AISI Type 310 20.0 Balance 25.0 – – 0.25 Max 1.0 Max 2.0 Max AISI Type 316 13.0 Balance 17.0 2.25 – 0.08 Max 1.0 Max 1.7 AISI Type 316L 13.0 Balance 17.0 2.25 – 0.03 Max 1.0 Max 1.8 AISI Type 317 14.0 Balance 19.0 3.25 – 0.08 Max 1.0 Max 2.0 Max AISI Type 317L 14.0 Balance 19.0 3.25 – 0.03 Max 1.0 Max 2.0 Max AISI Type 318 14.0 Balance 18.0 3.25 – 0.08 Max 1.0 Max 2.5 Max Cb + Ta 10XC Min AISI Type 321 11.0 Balance 18.0 – – 0.08 Max 1.0 Max 2.0 Max Ti 5XC Min AISI Type 330 35.0 Balance 15.0 – – 0.25 Max 1.0 Max 2.0 Max AISI Type 347 11.0 Balance 18.0 – – 0.08 Max 1.0 Max 2.0 Max Cb + Ta 10XC Min

NITRONIC alloy 50 12.5 Balance 22.0 1.5–3.0 – 0.06 Max 1.0 Max 5.0 N 0.2-0.4, Cb 0.1-0.3

Stainless Steels—Duplex and Precipitation Hardening

AISI Type 326 6.5 Balance 26.0 – – 0.06 Max 0.40 0.40 AISI Type 329 4.5 Balance 27.5 1.0–2.0 – 0.10 Max 1.0 Max 2.0 Max CRUCIBLE alloy 223 - Balance 16.0 0.4 1.0 0.03 Max 1.0 Max 12.0 N 0.3 17-4PH 4.0 Balance 16.5 – 4.0 0.07 Max 1.0 Max 1.0 Max Cb + Ta 0.3 17-7PH 7.0 Balance 17.0 – – 0.09 Max 1.0 Max 1.0 Max Al 1.1 PH15-7Mo 7.0 Balance 15.0 2.5 – 0.09 Max 1.0 Max 1.0 Max Al 1.1

Iron-Base Nickel-Chromium Copper-Molybdenum Alloys

CARPENTER alloy 20(1) 29.0 43.0 20.0 2.0 Min 3.0 Min 0.07 Max 1.0 0.8

CARPENTER alloy 20Cb-3 34.0 39.0 20.0 2.5 3.3 0.07 Max 0.6 0.8 Cb + Ta 0.6

Nickel-Base Chromium-Iron Molybdenum-Copper Alloys

INCOLOY alloy 825 41.8 30.0 21.5 3.0 1.8 0.03 0.35 0.65 AI 0.15, Ti 0.9 HASTELLOY alloy G 45.0 19.5 22.2 6.5 2.0 0.03 0.35 1.3 W 0.5. Cb + Ta 2.12

Iron-Base Nickel-Chromium Molybdenum Alloys

ALLEGHENY alloy AL-6X 24.0 46.0 20.0 6.5 – 0.025 Max 0.5 Max 1.5 Max HAYNES alloy 20 Mod 26.0 42.0 22.0 5.0 – 0.05 Max 1.0 Max 2.5 Max Ti 4XC Min JESSOP alloy JS-700 25.0 46.0 21.0 4.5 – 0.03 0.5 1.7 Cb 0.30 MULTIMET alloy 20.0 29.0 21.0 3.0 – 0.12 1.0 Max 1.5 Co 20.0, W 2.5, N

0.15,Cb + Ta1.0

Nickel-Base Molybdenum Chromium-Iron Alloys

HASTELLOY alloy C(2) 54.0 5.0 15.5 16.0 – 0.08 Max 1.0 Max 1.0 Max Co2.5Max,W4.0, V 0.4 Ma) HASTELLOY alloy C-276 54.0 5.0 15.5 16.0 – 0.02 Max 0.05 Max 1.0 Max Co2.5Max,W4.0, V 0.4 Ma) HASTELLOY alloy C-4 61 ..0 3.0 Max 16.0 15.5 – 0.015 Max 0.08 Max 1.0 Max Co 2.0 Max, Ti 0.7 Max HASTELLOY alloy N 69.0 5.0 7.0 16.5 – 0.06 0.3 0.3 AI 0.5 INCONEL alloy 625 60.0 5.0 Max 21.5 9.0 – 0.1 Max 0.5 Max 0.5 Max Cb + Ta 3.65

Nickel-Copper Alloys MONEL alloy 4OO 66.0 1.35 – – 31.5 0.12 0.15 0.9 MONEL alloy K-500 65.0 1.0 – – 29.5 0.15 0.15 0.6 AI 2.8, Ti 0.5

Copper-Nickel Alloys

Copper-Nickel alloy C70600 10.0 1.25 – – 88.0 – – 0.3 Pb 0.05 Max, Zn 1.0 Max Copper-Nickel alloy C71000 20.0 0.75 – – 78.0 – – 0.4 Pb 0.05 Max, Zn 1.0 Max Copper-Nickel alloy C71500 30.0 0.55 – – 67.0 – – 0.5 Pb 0.05 Max, Zn 1.0 Max

Nickel-Chromium Alloys

INCONEL alloy 600 76.0 7.2 15.8 – 0.1 0.04 0.2 0.2

NICHROME V 80.0 – 20.0 – – – – –

Nominal Composition of Nickel-Containing Alloys in Use or Corrosion Tested in Organic Acids and Related Compounds

Composition, %

Alloys Ni Fe Cr Mo Cu C Si Mn Other

WROUGHT ALLOYS Iron-Nickel-Chromium Alloys

INCOLOV alloy 800 32.0 46.0 20.5 – 0.3 0.04 0.35 0.75 INCOLOY alloy 804 41.0 25.4 29.5 – 0.25 0.05 0,38 0,75 AI 0.3, Ti 0.6

Nickel-Base Molybdenum Alloys HASTELLOY alloy B* 61.0 5.0 1.0 Max 28.0 – 0.05 Max 1.0 Max 1.0 Max Co 2.5 Max, V 0.3, P 0.025 Max, S 0.03 MaxHASTELLOY alloy B-2 67.0 2.0 Max 1.0 Max 28.0 – 0.02 Max 0.1 Max 1.0 Max Co 1.0 Max, P 0.04 Max, S 0.03 Max

Other Nickel and Cobalt-Base Alloys

IN-102 68.0 7.0 15.0 3.0 – 0.06 – – Ti 0.5, Cb 2.9. A1 0.5, W 3.0MP-35N 35.0 – 20,0 10.0 – – – – Co 35.0 ELGILOY 15.0 15.0 20A 7.0 – 0.15 – 2.0 Co 40.0, Be 0.05 HAYNES alloy No. 25 10.0 3.0 Max 20.0 – – 0.10 1.0 Max 1.5 Co. 49.0, W 15.0

CAST ALLOYS Stainless Steels

ACI CD-4MCu 5.5 61.0 26.0 2.0 3.0 0.04 Max 1.0 Max 1.0 Max ACI CF-3 10.0 66.0 19.0 – – 0.03 Max 2.0 Max 1.5 Max ACI CF-3M 11.0 63.0 19.0 2.5 – 0.03 Max 1.5 Max 1.5 Max ACI CF-8 9.0 67.0 19.0 – – 0.08 Max 2.0 Max 1.5 Max ACI CF-8M 10.0 64.0 19.0 2.5 – 0.08 Max 2.0 Max 1.5 Max ACI CG-8M 11.0 62.0 19.0 3.5 – 0.08 Max 1.5 Max 1.5 Max ACI HK 20.0 49.0 26.0 – – 0.4 2.0 Max 2.0 Max

Iron-Base Nickel-Chromium- Copper-Molybdenum Alloys

ACI CN-7M3) 29.0 44.0 20.0 2.0 Min 3.0 Min 0.07 Max 1.0 1.5 Max WORTHITE 24.0 48.0 20.0 3.0 1.75 0.07 Max 3.3 0.6

Iron-Base Chromium-Nickel- Copper-Molybdenum Alloy

ILLIUM alloy P 8.0 58.0 28.0 2.0 3.0 0.20 0.75 0.75 Iron-Base Nickel-Chromium- Molybdenum Alloys

IN-862 24.0 44.0 21.0 5.0 – 0.07 Max 0.8 0.5 KROMARC 55 20.0 50.0 16.0 2.0 – 0.04 2.0 Max 9.5

Iron-Base Chromium-Nickel- Iron Alloy

ILLIUM alloy PD 5.0 57.0 26.0 2.0 0.5 Max 0.08 1.0 Max 1.0 Max Co 7.0

Nickel-Base Chromium- Molybdenum-Copper-Iron Alloy

ILLIUM alloy G 58.0 5.0 22.0 6.0 6.0 0.2 0.2 1.25 Max

Nickel-Base Molybdenum- Chromium-Iron Alloys

ACI CW-12M-1(4) 58.0 6.0 16.5 17.0 – 0.12 Max 1.0 Max 1.0 Max ACI CW-12M-2(5) 57.0 3.0 Max 18.5 18.5 – 0.07 Max 1.0 Max 1.0 Max

Nickel-Base Molybdenum Alloys ACI N-12M-1(6) 60.0 5.0 1.0 Max 28.0 – 0.12 Max 1.0 Max 1.0 Max V 0.2–0.6, Co 2.5 MaxACI N-12M-2(7) 62.0 3.0 Max 1.0 Max 31.5 – 0.07 Max 1.0 Max 1.0 Max

Other Nickel and Cobalt-Base Alloys

WAUKESHA alloy 23 80.0 – – – – – – – Sn 8.0, Zn 7.5, Pb 4.0WAUKESHA alloy 54 75.0 0.4 – – – – – 2.5 Sn 8.0, Zn 7.0, Ag 6.0 WAUKESHA alloy 88 70.0 5.0 12.5 3.0 – 0.05 Max – – Sn 4.0, Bi 3.75 ILLIUM alloy 98 55.0 1.0 28.0 8.0 5.0 0.05 0.7 Max 1.25 Max ILLIUM alloy B 49.0 3.0 28.0 8.0 5.0 0.05 4.5 1.25 Max B 0.05-0.55 STELLITE alloy No. 3(8) 3.0 Max 3.0 Max 31.0 – – 2.35 1.0 Max 1.0 Max W 12.5, Others 1.0 Max,

Bal Co STELLITE alloy No. 4(8) 3.0 Max 3.0 Max 30.0 1.5 Max – 1.0 Max 1.5 Max 1.0 Max W 14.0, Bal Co STELLITE alloy No. 6 3.0 Max 3.0 Max 29.0 1.5 Max – 1.1 1.5 Max 1.0 Max W 4.5, Bal Co

Nickel Alloyed Cast Irons Ni-Resist Type 2 20.0 70.0 2.2 – 0.5 Max 3.0 Max 1.9 1.2 Ni-Resist Type 4 30.5 55.0 5.0 – 0.5 Max 2.6 Max 5.5 0.6

(1) An improved version of this alloy, CARPENTER alloy 20 Cb-3, has replaced CARPENTER alloy 20.

(2) Improved versions of this alloy, HASTELLOY alloys C-276 or C-4, have replaced HASTELLO alloy C.

(3) Cast “type20” alloys such as DURIMET alloy 20, ALOYCO alloy 20, etc.

(4) Includes alloys such as cast HASTELLOY alloy C, ALOYCO alloy N-3, ILLIUM alloy W1, etc.

(5) Includes alloys such as CHLORIMET alloy 3, ILLIUM alloy W2, etc.

(6) Includes alloys such as cast HASTELLOY alloy B, ILLIUM alloy M1,etc.

(7) Includes alloys such as CHLORIMET alloy 2, ILLIUM alloy M2, etc.

(8) STELLITE alloys 3 and 4 are cast wear resistant alloys that are no longer produced by Cabot Corporation.

* An improved version of this alloy, HASTELLOY alloy B-2 has replaced HASTELLOY B.

PART I. INTRODUCTION

The organic acids constitute a group of the most important reactive chemicals of industry today. Billions of pounds of acetic acid are produced in the United States every year to provide the precursor for numerous products from aspirin to the recovery of zaratite minerals. Acetic acid is best known as the astringent compound in vinegar, but the acid and its anhydride are used in the manufacture of cellulosic fibers, commercial plastics, agricultural chemicals, dyes, plas-ticizers, certain explosives, ester solvents, metal salts; pharmaceuticals such as aspirin, sulfa drugs, vitamins, and as a precursor for a host of other organic compounds used in the preparation of drugs.

Other organic acids are produced in much smaller volume, but constitute important chemicals for the prepara-tion of compounds used daily in our lives. The reactive acid (carboxyl) group present in these organic molecules is responsible for their wide use as ready building blocks for many commercial compounds.

Research efforts to provide these chemicals in greater quantity at less cost has paralleled their increasing impor-tance. A multitude of processes have been commercialized for the production of acetic, acrylic, adipic, lactic and the higher acids. The volume and use of corrosive by-product formic acid has continually increased. In all of these processes, nickel-containing alloys are standard materials of construction to withstand the corrosive environment and maintain product purity.

A. The Organic Acids

B. Scope

This bulletin attempts to characterize the corrosion resis-tance of alloys in the wide range of exposure conditions employed today in the production and handling of the organic acids. Space does not allow the complete coverage of alloy use in all organic acid processes, or even full treatment of such a large subject as acetic acid production. However, once the basic properties of the alloys in such media are established, along with adequate warning of problems to be avoided, the judicious choice of an alloy for a similar application can usually be made. The major pitfall in such use of data is assurance that the recorded conditions of exposure are indeed the same as those existing in the proposed application. Only parts per million of certain contaminants in an organic acid process stream can have a profound effect on the corrosion rate of an alloy. Thus, it is critical to learn the details of proposed operating conditions, as well as the possibilities for inadvertent changes in stream composition.

Corrosion data reported throughout this bulletin must be interpreted as providing valuable information regarding the relative corrosion resistance of the various alloys in specific environments and modes of testing. Retesting of the alloys, particularly those containing chromium, under the same apparent conditions may provide variations in corrosion rates of two to three times. However. the relative resistance of the various alloys normally remains the same.

Corrosion data for alloys in all of the many organic acids are reported when they are available. Extensive data for the more common acids encountered are reported. In addition, data for representative homologues of the various types of organic acids are reported. With this information as a guide, the interested party should be able to select candidate materials for an organic acid exposure of any type.

The nominal composition of alloys cited in the tables and text are shown in the table on pages 2 and 3. An attempt has been made to provide as comprehensive a listing of alloys as possible to achieve the maximum utility from these data. Some of the proprietary alloys have been improved by compositional modifications. Where data exist for the newer modification they are included; however, some data on the obsolete alloys are included. Corrosion rates on the newer, improved alloys may be assumed to be approximately equivalent. Trademarks of proprietary alloys have been used in the text and are listed on the inside back cover. All materials are assumed to be in the mill annealed condition unless notations to the contrary are shown.

Some of the techniques used for determining corrosion rates and changes in environment in aqueous systems are difficult to apply in organic acid media. The specific conduc- tance of the higher acid concentrations is low for elec-trochemical studies and the low dissociation constant of the common organic acids requires major dilution of the com-pounds before reliable electrochemical data can be obtained.

C. Corrosion Testing in Organic Acid Media

Type 316L stainless steel tanks and piping and cast ACI CF-8M pumps and valves are utilized in this plant handling organic acids. Courtesy Walworth Company-Aloyco Valves.

Attempts to make potentiometric measurements are most successful in the dilute solutions; ten per cent acetic acid is often used as an investigative medium. Also, the addition of sodium salts or chloride salts is reported to allow measure-ment of potential changes with current variations.1 How-ever, many electrochemical investigators have reported data obtained in strong acetic acid, acetic acid-anhydride and formic acid solutions. These tests showed an active-passive behavior for most alloys, which is consistent with field experience.

The influence of even tenths of a per cent of water in an organic acid can have considerable influence on corrosion. Anomalous results obtained in “glacial” acetic acid are often attributable to small differences in water content in the two different media. In any event, proper testing of alloys in anhydrous organic acid environments is restricted to grav-imetric techniques, mechanical measurements or by the use of changes in electrical resistance of metal cross sections as corrosion occurs.

Data are often obtained by immersion testing in the laboratory. Such tests must be assumed to be without control of the atmosphere unless aeration, nitrogen sparg-ing, or other gaseous injections are identified. Without control of the atmosphere, a test environment above ambient temperature will have two periods of differing exposures. Initially the solution will be air-saturated, while

in the second period little if any air will be present in boiling solutions and a loss of oxygen will occur in solutions held at the lower temperatures. Thus, short test periods can provide results totally different from those obtained by longer exposure times. Unless specifically stated to the contrary in the tests reported, it must be assumed that air was present, at least initially, in a laboratory test and was probably absent in a field test.

In addition, corrosion products form in the test medium and can exert a controlling influence on the corrosion rates in long-term laboratory tests. Aggressive, highly-ionic media, such as the mineral acids, may attack a metal surface almost immediately on contact, and even on those metals and alloys having protective oxide films the passive period may be very short. However, when evaluating materials in acids such as acetic, a considerable variation in rate of corrosion can be obtained depending on the length of the test period and the incubation period required to initiate corrosion. With these and other factors operative, it is not surprising that considerable discrepancy in corrosion data exists for the exposure of alloys in organic acids.

All percentages expressed in the data are in weight per cent unless another basis is specifically stated. Corrosion rates are reported in millimeters per year (mm/y) followed by the corrosion rate in mils per year (mpy) (one mil = 0.001 inch.)

A. General

PART II. ACETIC ACID

Acetic acid and its derivatives are produced in large quan-tities as commercial products. Perhaps of even greater interest from a corrosion standpoint is the fact that in industries processing many other organic chemicals, acetic acid is a common impurity in process streams as a result of the oxidation of lower compounds or the degradation of larger molecules. Consequently, a knowledge of the corro-sive potential of the acid is necessary to assure the economic life of equipment or to prevent contamination of process streams with metallic corrosion products.

Although acetic acid has a low ionization constant com-pared with many other acids, the effective acidity of aqueous streams contaminated with the acid increases rapidly with concentration. Table I shows change of pH with concentration of acetic acid.

A wide range of alloys can be used in acetic acid exposures. Those alloys renowned for resistance to oxidiz-ing conditions are often a first choice for a specific exposure while in a remarkably similar application the wisest choice will be alloys used to combat reducing conditions. In some process areas, both can be equally resistant and an economic comparison is necessary before making a choice. However, a thorough appraisal of each exposure must be made to identify the optimum material of construction.

B. Austenitic Stainless Steels 1. GeneralThe wrought and cast austenitic stainless steels serve as the workhorse of industries handling acetic acid. The addition of sufficient nickel to iron-base alloys containing chromium is necessary to provide the optimum alloy for ease of fabrication and adequate resistance to attack by the acid.

In a typical acetic acid production facility, such as exempli-fied by the direct oxidation of hydrocarbons to the acid, the reactors, distillation columns, heat exchangers, separators, decanters and much of the tankage are constructed of

TABLE I

Concentration of Acetic Acid Versus pH in Aqueous Solution

Concentration g/I pH

0.0006 5.2 0.006 4.4 0.06 3.9 0.6 3.4 6.0 2.7

60.0 (6%) 2.4

Reference 43

In the vast majority of exposures, there is no difference in corrosion resistance between the wrought and cast alloys of similar analysis provided that both are in proper metallurgi-cal condition (annealed). The presence of small amounts of delta ferrite (2-10%) normally found in the austenitic matrix of the cast alloys does not lessen the corrosion resistance of the metal as illustrated by Table II. Even greater amounts of ferrite will show no deleterious effects in most pure acid media. Flowers, et al.2 investigated ferrite contents in the CF-8 and CF-BM alloys up to 38 per cent and claim anodic polarization of the ferrite in such a dual phase alloy reduces overall attack on the metal. However, such passivity is not to be expected under all conditions of organic acid exposure and thorough testing of specific alloy compositions is advised.

Other comparative data for the cast alloys may be found in Table XXVII and Figure 1.

2. Effect of Alloy Composition The addition of proper chromium-nickel ratios in a ferrous base to provide an austenitic stainless steel affords a limited resistance to organic acid exposures. Lower concentrations of pure acetic acid may be handled to the boiling point or the higher concentrations may be used to some 90 ºC (194 ºF) with Fe-Cr-Ni alloys such as Type 304 stainless steel. Adding greater amounts of chromium and nickel (Types 309 and 310 stainless steels) does not change the corrosion resistance of the alloys basically (see Table III). Using graphical multiple correlation techniques, Dillon has shown that chromium and nickel variations of the commercial alloys have little effect on the resistance to acetic acid.3

At this time, there is no reason to believe that obtaining an austenitic matrix by the use of combinations of nickel, manganese and nitrogen imparts any change in the organic acid resistance of the alloy.4 That is, a Type 204 stainless steel is equivalent to a Type 304 stainless steel and Type 216 is as resistant to acid attack as Type 316. See data in Tables III through V for the corrosion of the high manganese and nitrogen-containing stainless steels.

Cast ACI CF-8M valves and pumps in finished acetic acid storage service. Piping and tanks are constructed of Type 316L stainless steel. Courtesy Walworth Company-Aloyco Valves.

wrought Type 316 stainless steel, or Type 316L stainless steel if weld fabrication is to be employed. Forgings of these alloys are found as valve parts, perhaps as heat exchanger tube sheets, and for certain other structural parts. The pumps and many valves are constructed of the cast counterpart of the Type 316L stainless steel analysis known as ACI CF-3M. The ACI CF-8M (0.08 max carbon) is equally acceptable if in the solution annealed condition but has the disadvantage that weld repairs have to be followed by solution annealing to restore corrosion resistance.

FIG 2– Effect of Molybdenum Content on Corrosion of Austenitic Stainless Steels in Condensate from Boiling Acetic Acid Solutions FIG 1– Corrosion of Cast Stainless Steels in Glacial Acetic Acid

TABLE II

Comparison of Cast Stainless Steels with Wrought Type 316 Stainless Steel in Organic Acid Media

Test Conditions: All tests at boiling temperature for approximately 150 hours in laboratory. Each result shown represents duplicate specimens.

*% Ferrite in alloy **Trademark of Worthington Corp. ***Trademark of Aloyco, Inc.

TABLE III

Field Tests in Acetic Acid Distillation Columns

(1) Trademark of Carpenter Technology Corporation (2) Trademark of Jessop Steel Company (3) Trademark of the Inco family of companies (4) Trademark of Cabot Corporation (5) Trademark of The Duriron Company, Inc.

*An improved version of this alloy, HASTELLOY alloy C-276, has replaced HASTELLOY alloy C.

Location in Column Test Duration (Days) Temperature ºC (ºF) Per Cent Acetic Acid

Top 11

120 (248) 99.5+

Top 40

106 (223) 99.9+

Mid 375

100 (212) 20

Bottom 62

121 (250) 99.9+

Bottom 30

119 (246) 90

Corrosion Rate Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpyType 316 stainless steel

TABLE IV

Comparison of Nickel and Manganese Austenitic Steels in Organic Acid Exposures

Conditions: Duplicate specimens tested in the boiling solution (temperatures shown) for 48 hours or longer. Air not excluded or added.

Corrosion Rate

Temperature

Type 304 Stainless Steel

CRUCIBLE* alloy 223


Test Medium ºC ºF mm/y mpy mm/y mpy mm/y mpy Acetic acid, 100% 117 242 .46 .18 .18 7 .01 0.4 Acetic acid, 75% 104 219 4.06 160 .05 2 .01 0.3 Acetic acid, 50% 102 216 6.98 275 Nil

Corrosion Rate Alloy Condition of Specimen mm/y mpy

Type 316L Stainless Steel Annealed .06 2.5 1 hr 677 ºC (1250 ºF) AC .06 2.5 4 hr 871 ºC (1600 ºF) AC, 1 hr 677 ºC (1250 ºF) AC .04* 1.4* As-welded (316L rod) .08 3.2 Welded, 1 hr 704 ºC (1300 ºF) AC .08 3.0 Welded, 1 hr 871 ºC (1600 ºF) AC .07 2.6 As-welded (310 Mo rod) .06 2.3 Welded (310 rod) 1 hr 871 ºC (1600 ºF) AC .06 2.4 Type 316 Stainless Steel Annealed .39 15.5 2 hr 621 ºC (1150 ºF) AC .39 15.3 1 hr 677 ºC (1250 ºF) AC .65 25.5 As-welded (316 rod) .40 15.9 Welded, 1 hr 871 ºC (1600 ºF) AC .71 27.7 Type 317 Stainless Steel Annealed .05 2.0 4 hr 593 ºC (1100 ºF) AC .16* 6.3* 1 hr 677 ºC (1250 ºF) AC .68* 26.9* As-welded (317 rod) .04 1.73 Welded, 1 hr 704 ºC (1300 ºF) AC .55* 21.5* Type 318 Stainless Steel Annealed .07 2.6

1 hr 677 ºC (1250 ºF) AC .07 2.6 1 hr 1316 ºC (2400 ºF) AC + 1 hr 677 ºC (1250 ºF) AC .64* 25.1* As-welded (318 rod) .06 2.4 Welded + 1 hr 704 ºC (1300 ºF) AC .07 2.6 Welded + 1 hr 871 ºC (1600 ºF) AC .30 12.0

TABLE VI Effect of Thermal Treatments on Molybdenum-Containing

Stainless Steels

Corrosive medium: Acetic acid 35%, formic acid 1.0%, water 64%. Conditions : Process liquid at 131 ºC (268 ºF) (boiling) for 84 days, air free.

Corrosion Rate

Stream

Composition

Temperature

Test

Period

Type 316 Stainless

Steel

Type 317 Stainless

Steel

CARPENTER

alloy 20

INCOLOY alloy 825

HASTELLOY

alloy C

HASTELLOY

alloy B

INCONEL alloy 600

Nickel

200

MONEL

alloy 400

Arsenical Admiralty

EVERDUR*

1010

C F days mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

17% Acetic Acid 1 % Formic Acid

82% Water 100 212 452 03 1 .03 1 – – – – – – 05 2 .61 24 .25 10 – – 08 3 .05 2

18% Acetic Acid 40% Formic Acid 2% Water 40% Organics

91 196 55 .08 3 .05 2 .05 2 .03 1

3. Effect of Contaminants Although pure acetic acid can be handled readily in many alloys, the presence of only parts per million of other chemical agents can render an alloy useless as a material of construction.

Acetic anhydride is produced as a co-product in the older acetaldehyde oxidation process for acetic acid, and the anhydride can often be found in other acetic acid process streams. When small quantities of the anhydride exist in a glacial acid, a greatly accelerated attack on the stainless steels can be anticipated. Tables IV, IX and X incorporate data substantiating the adverse effect of anhydride in acetic acid as reported by Elder5 and others. The difference in the two commercial, glacial acids shown in Table XI can probably be attributed to the presence of anhydride in the product of Plant B. As the amount of anhydride in the acid is increased, the rate of attack rapidly drops to an acceptable level, and high concentrations of anhydride are innocuous. (See section on Acetic Anhydride.) However, the presence of small amounts of anhydride sufficient to dehydrate the acid produces in-creased attack on all alloys.6

Oxygen may influence corrosion rates in acetic acid, and other organic acids as well. Even though process streams have been stripped of gaseous components in distillation systems, the possibility of oxygen pickup from air leaks into the system is present. The use of stainless steels as materials of construction assures that no accelerated attack will occur under such circumstances. Indeed, when corro-sion of the stainless steels in a process system is higher than desired, the rate of attack can often be reduced by introducing oxygen into the system. Table XLIII shows the effect of adding oxygen to a distillation column during the processing of propionic acid. A hundred-fold reduc-tion in the corrosion rate is evident as the oxygen provided

sufficient oxidation capacity in the system to maintain a passive oxide film on the stainless steels. Similar data obtained in a mixed acid column were presented in reference 7. Field experience with the equipment confirmed the validity of the laboratory data. The effect on other types of alloys of adding oxygen to an acetic acid medium can be seen in Tables XXII, XXIII and XXV.

TABLE VIII

Corrosion of Metals in Acetic Acid Residue Still

Test Conditions: Test assembly installed in liquid and in vapor space of still at temperatures of 80 to 100 ºC (176 to 212 ºF) for 2000 hours. Residues contain acetic acid, anhydride, acetates, tar.

Corrosion Rate

Liquid Vapor

Alloy mm/y mpy mm/y mpy

Cast iron Ni-Resist Type 11 Mild steel Type 501 chrome steel Type 430 stainless steel INCONEL alloy 600 HASTELLOY alloy C DURIMET* 20 Type 329 stainless steel Type 304 stainless steel Type 316 stainless steel Type 317 stainless steel

2.13 .97

2.01 2.01 1.22 .18 Nil .05 .18 .76 .03 .03

84 38 79 79 48 7

Nil 2 7

30 1 1

1.32 .30

2.51 1.47 .36 .13 Nil .13 .30 .36 .18 .05

52 12 99 58 14 5

Nil 5

12 14 7 2

*Trademark of The Duriron Company, Inc.

TABLE IX

Corrosion of Type 316 Stainless Steel in Acetic Acid Solutions Containing Chlorides

Conditions: Duplicate 48-hour tests conducted at the boiling temperature in glacial acetic acid with additions made as shown.

Corrosion Rate Chloride Ion Added, ppm*

0 18 36 61 Diluent addition to acid mm/y mpy mm/y mpy mm/y mpy mm/y mpy

None – – .05 2 .43 17 2.10 81 0.2% Acetic Anhydride 1.98 78 1.27** 50** 1.22** 48** 1.19** 75* 0.1 % Water .03 1 – – – – – – 0.3% Water .03 1 – – – – – – 0.33% Water – – .08 3 .33 13 .71 28 0.50% Water .03 1 – – – – – – 0.67% Water – – .03 1 .66 26 .38 15 1.0% Water – – .18 7 .41 16 .36 14

* Added as sodium chloride ** Minute, profuse pitting

Corrosion Rate

TestNo. Test Medium


CARPENTER alloy 20Cb-3

HASTELLOYalloy C

mm/y mpy mm/y mpy mm/y mpy

1 Glacial acetic acid .08 3

Processes employing halide catalysts in the reaction system to produce acetic acid must be assessed thoroughly to determine where the less costly stainless steels can be used in the process train. Type 316 stainless steel usually cannot be used in the reaction area or in the first separation steps. More highly alloyed materials are required. Once the halide ion is removed, the overhead acid stream from the distillation train can be processed safely in stainless steel. (See section on Process and Plant Corrosion Data.)

Stress-corrosion cracking of the 300 series stainless steels may occur readily in aqueous acidic media containing chlorides. Presumably the cracking will not occur in a completely anhydrous medium, but such a water-free sys-tem is obtained rarely and some water must be assumed to be present. Where the chloride-containing acid solution can concentrate on the surface of stainless steel under stress, cracking of the metal can occur. Such areas as gasket joints, crevices and liquid-vapor interfaces in the equipment are examples of zones where such cracking (and pitting) often occurs in chloride-containing acetic acid. Cracking may also occur beneath deposits or at the base of pits on the surface of the stainless steels. Where the metal surface is washed continually with fresh liquid, there is little likelihood of stress-corrosion cracking. If the process temperature is less than 80-90 ºC (176-194 ºF) the cracking process may be sufficiently slow to allow a respectable service life for the equipment before failure occurs. At temperatures below 50-60 ºC (122-140 ºF), stress-corrosion cracking usually does not occur. Stress-corrosion cracking may be avoided by the use of higher nickel alloys or duplex stainless steels.

With the exception of formic acid, (see Section on Formic Acid), other contaminants found in the usual acetic acid process stream only serve to dilute the acid and reduce the rate of attack. Aldehydes, ketones, esters and higher acids are in this category.

until excessive rates of attack are obtained. However, CF-8M resists the effect of increased temperature quite well and has potential for use at the 200 ºC (392 ºF) temperature. Field applications utilizing CF-8M pumps in acid near this temperature confirm the utility of the alloy for handling hot acid when oxidizing conditions exist.

Table XII shows other data obtained in the upper temperature region of Figure 1. Note the lower corrosion rate for a Type 316 stainless steel at 190 ºC (374 ºF), although the test period is longer. Sufficient peroxide appears to be effective in reducing corrosion, even at these high temperatures. The presence of ferric ion was detri-mental at these temperatures as opposed to the beneficial effect noted at lower temperatures.

Vapors of the acid at higher temperatures are not aggressive in the absence of condensation (Tables VIII and XIV). However, condensation or drippage of liquid on a hot metal surface can produce excessive attack. In addition, pitting of the austenitic stainless steels in acetic acid exposures at the higher temperatures is possible.

It is obvious that careful assessment of the stability of the 300 series stainless steels in an acetic acid environment must be made before discounting their use at even the higher temperatures.

TABLE XIII

Corrosion of Nickel-Containing Alloys in Buffered Acetic Acid at High Temperature

Test Conditions: Specimens exposed in a high pressure autoclave at temperature of 200 ºC (392 ºF) for 8 days to the following solution without aeration or agitation: 15% acetic acid plus 19% ammonium acetate aqueous solution at 250 psi.

4. Effect of Temperature It has been shown that Types 316 and 316L stainless steels are satisfactorily resistant to attack by all concentrations of acetic acid to the boiling point and that Type 304 stainless steel is acceptable for use in all concentrations of acid less than approximately 90 per cent to the boiling point. As the temperature is increased beyond these points, the rate of attack on the stainless steels in the liquid acid increases, but certainly not as rapidly as the Arrhenius equation would indicate.

Laboratory and field data presented in Tables V and XI through XIII show that for both wrought and cast alloys the stainless steels remain useful at temperatures well above the atmospheric boiling point. Various techniques of testing can produce significantly different results and ingenuity is required to establish stable conditions for the desired test environment.

Figure 1 condenses considerable data generated by Ohio State University personnel when exploring the corrosion resistance of the cast alloys in acetic acid up to 200 ºC (392 ºF).9 The cast CF-8 alloy corrodes at in-creasingly greater rates as the temperature is increased

Corrosion Rate

Alloy mm/y mpy HASTELLOY alloy C-276 02 0.6

INCONEL alloy, 625 02 0.7

INCOLOY alloy 825 02 0.8 HASTELLOY alloy G 03 1.0

Nickel 200 04 1.5

IN-862 Cast Alloy 05 1.8

Type 315 Stainless Steel (sensitized) 13* 5.2*

Type 316 Stainless Steel (annealed) 04* 1.5*

Temperature System Pressure Corrosion

Rate

Alloy ºC ºF psig mm/y mpy Type 304 Stainless Steel 142 288 35 .03 1

Type 304 Stainless Steel 153 308 55 .10 4

Type 316 Stainless Steel 142 288 35

Previous comments regarding temperature were in reference to the bulk temperature of a liquid or vapor in contact with a metal surface at essentially the same temperature. These conditions do not exist in heat ex-changers, calandrias and interchangers of an acetic acid process. When a metal surface at a higher temperature is used to evaporate the acid, higher corrosion rates occur than obtained isothermally. One explanation is that the constant heating and cooling of a heat exchanger surface cracks the protective oxide film on a stainless steel to expose active metal. Also, ebulition of the liquid at the surface supplies a mechanical force to dislodge the film.

Decomposition products of organic compounds can form on the hot surface. Lastly, any corrosive heavy ends in the liquid can concentrate at the surface to attack the metal, or tars can form over the metal to produce crevice corrosion in a random configuration. For these reasons, an actual heat exchange test should be conducted in any questionable mixture.

Groves, et al.10 have described a simple apparatus for conducting heat exchange “hot wall” tests. Their data are reproduced in Table XV and illustrate the significant increase in attack which occurs on an alloy when using the surface as a heat exchange medium. Further use of this

TABLE XV

Corrosion by Acetic Acid Under Heat Transfer Conditions

Temperature Corrosion Rate

Without

Heat Transfer

With Heat* Transfer

Type 304 Stainless

Steel

Type 316 Stainless

Steel

CARPENTERalloy 20 Cb-3

HASTELLOY

alloy B

INCONEL alloy 600

MONEL

alloy 400

Test Medium Acetic Acid

ºC ºF ºC ºF mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

10%

50%

99.6%

101 – – –

102 – – –

118 – – –

214 – – –

216 – – –

244 – – –

– 110 125 140

– 110 125 140

– 110 125 140

– 230 257 284

– 230 257 284

– 230 257 284

same technique provided the data of Tables XI and XVI. Table XI illustrates the important point that all glacial acetic acid is not necessarily the same. This fact is particularly noticeable when comparing two different acids by means of the “hot wall” test. Also note that again a small amount of water in the acid is most helpful in reducing attack on the stainless steels. The water is most effective in this respect regardless of the mode of testing, and field work verifies this inhibitory effect. The effect of adding sulfuric or formic acid to the acetic acid is shown in Table XVI. Notice the accelerating effect of only a small amount of formic acid added to the acetic. Such an addition would produce no increase in corrosion of Type 316 stainless steel in an immersion test conducted at 110 ºC (230 ºF). The effect of adding the even more aggressive, higher boiling sulfuric acid, such as used in an esterification reaction, may be catastrophic as can be observed from the data.

5. Effect of Microstructure The austenitic stainless steels are subject to specific types of attack when exposed to hot organic acids in the same manner as that observed in the mineral acids. Adverse mill treatments, fabrication heating cycles, post-fabrica-tion heat treatment and welding can produce changes in the alloy structure which greatly reduce the corrosion resistance in hot acetic acid.

Chromium depletion associated with carbide precipita-tion along the grain boundaries (sensitization) on heating an unstabilized, regular carbon (0.08 C max) stainless steel within the range of 425-760 ºC (800-1400 ºF) gives rise to intergranular attack when the alloy is exposed to hot, concentrated acetic acid. Severe intergranular attack can result in the phenomenon known as “sugaring” or “grain dropping.” The attacked, heat-affected surfaces are left in a very rough condition with a bright, (sugary) faceted surface. If the alloy is sensitized throughout its thickness, such attack may proceed until the entire thick-ness of the metal is penetrated.

Persons evaluating the possible effects of sensitization of an alloy in a specific environment should be aware that a comparison of weight loss measurements between sensi-tized and annealed specimens of the metal are not always an adequate procedure after organic acid exposures. Little difference in weight loss may be noted between the two. In fact, many data indicate that the mass of the austenite grain in a sensitized metal becomes cathodic to the grain boundary which results in a tower overall loss in weight than for the annealed structure (Table XVII is typical). Unless obvious “sugaring” or the dropping of grains from the metal has occurred, the welded or sensitized corrosion test specimen should be evaluated by bending to open and expose the attack, by “ringing” to determine if the metal has lost the characteristic metallic tone, by conducting magnetic permeability tests, or preferably by a metal-lographic examination of a cross section of the metal to observe the type and extent of any selective attack on the structure.

Susceptibility of the austenitic stainless steels to this type of attack may be avoided by utilizing a low carbon grade (.03 C max) or restricting the use of regular carbon grades (.08 C max) to the annealed condition, without any subsequent heating into the sensitizing temperature range. With low carbon grades, there is little likelihood of sensitization developing in the alloy during welding or heat treatments. A stabilized counterpart to Type 316 stainless steel known as Type 318 stainless steel is now obsolete because present melting technology can readily attain low carbon levels on a routine basis.

The exposure of the chromium-nickel-molybdenum stainless steels after various thermal treatments to a process stream containing acetic acid has been reported by the Welding Research Council.11 (Table VI.) The corrosion rates obtained were high for such an exposure for reasons not detailed in the stream analysis. Also, the higher corrosion rates exhibited by the Type 316 stainless steel are in conflict with the usual data obtained when comparing the alloy with the Type 316L alloy. However, the data are emphatic in pointing out the effect of adverse heat treatments on susceptible materials. Note particularly the adverse effect of solution annealing followed by a sensitization treatment on the columbium-stabilized Type 318 alloy. This type of treatment can occur during multi-ple-pass welding and may result in “knife-line attack” on stabilized alloys.

Although carbide precipitation is the best known and most common cause of intergranular attack on the stain-less steels, certain other metallurgical phenomena must be recognized as presenting potential problems as a result of fabrication procedures. The formation of sigma phase or chi phase in the alloy can be as devastating as carbide precipitation under certain conditions of acetic acid ex-posure. Welding alloys such as Types 316L and 317L stainless steels presents no problems when using solid construction. However, as the process pressure increases and the use of clad construction is indicated to be economically desirable, problems can be encountered if adequate precautions in the fabrication of the vessel are

TABLE XVII

Corrosion in Acetic Acid Vaporizer

Field Test: 312 hr, 140 C (284 F) mass temperature. Chlorides present

Corrosion Rate

Alloy mm/y mpy

Type 316 stainless steel, annealed 8.13 320 sensitized 6.86 270

Type 304 stainless steel 33.02 min* 1300 min* CARPENTER alloy 20Cb-3 6.35 250 INCONEL alloy 600 6.60 260 Titanium .08** 3** HASTELLOY alloy C-276 .08 3

*Dissolved **Pitting

not observed. Type 316L stainless steel, when heated for prolonged periods in certain temperature regions above 500 ºC (932 ºF), can produce sigma or chi phase in the alloy. Type 317L stainless steel with higher molybdenum con-tent is slightly more prone to formation of these phases. These phases are rich in chromium (chromium and molybdenum in the case of chi phase) and can have much the same effect as the more commonly known M23C6 and M6C carbide precipitation in the alloy. Such a metallurgi-cal phase change can occur in the fabrication of the clad vessel when it becomes necessary to stress relieve the steel backing. At the 500-650 ºC (932-1202 ºF) stress relief desired, sigma or chi phase can be produced to create severe corrosion of the clad material on the interior during process operations. Lower stress relieving temperatures are required to avoid such an undesirable metallurgical condition if these grades of stainless steel are to be used. 6. Quality Control Qualification tests are often used to assure that the initial material is of proper quality and that any heat treatment of the equipment has not produced undesirable effects.

Clippings from sheet and plate, small sections of tubes and other small sections removed from pieces of equip-ment are sent to the laboratory for validation of the existing condition of the material and its ability to maintain appropriate corrosion resistance. These qualification tests have been standardized by the American Society for Testing and Materials (ASTM) and are divided into practices A through E of Recommended Practice A 262. Each of these is designed to detect specific types of phase formation in the alloy. Of these, Practice A, the electrolytic oxalic acid etch (EOAE) test, is the most sensitive. Normally, if a heat of stainless steel fails to pass the EOAE test, samples are tested in accordance with one of the other practices before rejection of the heat is allowed. However, because of the sensitivity of the EOAE test, some workers have advocated that acceptance or rejection be based upon this test alone to assure maximum corrosion resistance in the alloy. Major losses in equipment and even more expensive, extended periods of downtime may be avoided by these simple procedures.

Castings to be used for pumps, valves and other critical parts of the equipment can be tested in the same manner. Solution annealing of castings is mandatory to assure the optimum corrosion resistance desired. Small amounts of ferrite provided in the matrix to assure crack-free castings of the best strength and quality are not harmful. However, carbides and other constituents which might be isolated along the dendrites of a casting should be in solution to prevent selective attack of such areas.

The quality control program for assuring that the stainless steels used in acetic acid manufacture meet specification requirements is sometimes extended to qualitative chemical analysis by means of spot testing of all material received by the fabricator of the equipment and by those in the field responsible for installing piping, heat exchangers, vessels and all other equipment to be exposed to the hot acid to help assure the proper grade of

stainless steel has been supplied. The molybdenum spot test is most often utilized in this regard. The cost of such a procedure is appreciable, but becomes insignificant in comparison with the failure of a piece of equipment once the unit is in operation. Simple items such as the drain plug in a pump, a welding elbow in a hot acid line, a few incorrect tubes in the heat exchanger and many other small items can create disastrous problems if an inadvertent substitution of a lower grade of stainless steel has been made for the Types 316 or 316L analysis identified for the use. A materials identification procedure on the site to provide assurance of proper alloy installation is very easily justified economically. Kits are commercially available with complete instructions for doing such work on the site very quickly and easily. One person assigned to this work throughout the life of a project may pay for the services many times over. C. Martensitic and Ferritic Stainless Steels

The standard AISI grades of martensitic and ferritic stainless steels generally do not possess sufficient corro-sion resistance for use in acetic acid service, except possibly at low concentrations and temperatures. Table XVIII shows typical corrosion data for the martensitic Type 410 stainless steel. Included for comparison are steel, cast iron and a nickel alloyed cast iron. When evaluating these materials for an application, it is important to assure that the service conditions are reproduced as closely as possible. Laboratory tests can show a considerable disparity in results because of the possibility of forming a fragile protective film on the alloy in a short time. After a high initial rate of attack, the rate will subside to a low value if the film is undisturbed by flow or other mechanical effects.

D. Duplex Austenitic-Ferritic and Precipitation Hardening Stainless Steels

Duplex structured austenitic-ferritic stainless steels and certain precipitation hardening stainless steels can show remarkable resistance to organic acids depending on the ratio of nickel to chromium and other minor alloying constituents. Table XIX illustrates the resistance of several precipitation hardening stainless steels in acetic acid at various temperatures. It is important to understand that the selection of such alloys for a specific application is more critical than when appraising an austenitic stainless steel. Prior processing of the alloy can have a significant effect on the corrosion resistance. The influence of heat treatment on the corrosion resistance of three precipitation hardening stainless steels in acetic acid is shown in Table XIX. It is obvious that the metallurgical condition of the alloy must be known when considering these alloys for acid service. Certain treatments of the alloys can greatly reduce their corrosion resistance. The data also reveal the borderline passivity of these alloys in such service, par-ticularly in the intermediate concentration of acid. The effect of heat treatment on the molybdenum-containing

Corrosion Rate Per Cent Temperature Type 410 Type 430 Ni-Resist

Acetic Acid ºC ºF Cast Iron Carbon Steel Stainless Steel Stainless Steel Type 2

mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

5 25 77 – – .25* 10* – –

The superiority of this class of alloy may be noted by reference to Tables II, V, XXVII and XXIX. Particularly when the acid is contaminated with agents inimical to the use of Type 316 stainless steel, these alloys usually provide significant improvement in resistance. For hot acid pumps, the CN-7M composition shows greater resistance to erosion-corrosion than CF-8M castings and is often used in installations that are otherwise entirely of Type 316L stainless steel construction.

The higher nickel content of the “type 20” alloys provides a fully austenitic structure, imparts good strength with ductility, is in optimum ratio with the chromium for maximum corrosion resistance in the iron-base alloys and increases the resistance of the alloy to chloride stress-corrosion cracking considerably. The wrought or cast “type 20” alloys will not crack in many environments which produce stress-corrosion cracking in Type 316 stainless steel. The “type 20” alloys are susceptible to sensitization as described for the 300 series stainless steels unless stabilized or solution annealed. Low carbon con-tents or the addition of columbium is used to combat the problem. “Knife-line attack” may sometimes occur along beads of multiple-pass welds in the metal-stabilized alloys. Castings should be used in the solution annealed condition.

Black, Sivalls and Bryson Inc. utilize a number of different alloys to resist various corrosives in its extensive line of rupture disks. Included are Alloys 400, 600, and HASTELLOY alloy C-276 as well as Type 316 stainless steel and other high nickel alloys to insure reliability.

F. Nickel-Base Chromium-Iron-Molybdenum-Copper Alloys

The nickel-base Cr-Fe-Mo-Cu alloys such as HASTEL-LOY* alloy G and INCOLOY** alloy 825 are generally equivalent to 316L stainless steel in “mild” acetic acid

environments and far superior to Type 316L stainless steel in the hotter, more aggressive organic acid environments. This is shown in Tables V, VII, XII1, XXVII, XXVII and XXX. Their superiority is also indicated in later sections of this bulletin. (See Tables LI, LVIII, LXVII, LXXIV and LXXVIII.)

*Trademark of Cabot Corporation ** Trademark of the Inco family of companies

Averagea Corrosion Rates of Precipitation Hardening Stainless Steels in Acetic Acid

TABLE XIX

Acetic Acid Concentration

100% 75% 50% 25%

Corrosion Rate

Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy Type 430 4.90 193 1.32 52 7.67 302 4.27 168 Type 304 .43 17 2.21 87

TABLE XX

Corrosion of the HASTELLOY and Associated Alloys in Acetic Acid

Tests of 120 hours’ duration at the temperature shown.

Corrosion Rate 25 ºC (77 ºF) 66 ºC (151 ºF) Boiling

Medium mm/y mpy mm/y mpy mm/y mpy

10% Acetic Acid HASTELLoy alloy B .01 0.5 .15 6 .02 0.7 HASTELLoy alloy C .01 0.2 .01 0.2 .01 0.4 HASTELLoy alloy D .02 0.6 .23 9 .05 2 HASTELLoy alloy N .03 1 .07 2.7 .03 1.2 HAYNES* alloy No. 25 Nil Nil Nil Nil .00 0.1 MULTIMET* alloy Nil Nil Nil Nil .00 0.1

50% Acetic Acid HASTELLOY alloy B .03 1 .10 4 .01 0.4 HASTELLOY alloy C .00 0.1 .00 0.1 .00 0.1 HASTELLOY alloy D .08 3 .46 18 .08 3 HASTELLOY alloy N .03 1 .06 2.5 .04 1.7 HAYNES alloy No. 25 Nil Nil Nil Nil .00 0.1 MULTIMET alloy Nil Nil Nil Nil .00 0.1

99% Acetic Acid (Glacial) HASTELLOY alloy B .00 0.1 .01 0.5 .01 0.2 HASTELLOY alloy C .01 0.2 .00 0.1 .00 0.1 HASTELLOY alloy D .01 0.5 .13 5 .02 0.9 HASTELLOY alloy N 02 0.7 .02 0.7 .02 0.8 HAYNES alloy No. 25 Nil Nil Nil Nil Nil Nil MULTIMET alloy Nil Nil Nil Nil .00 0.1

G. Iron-Base Nickel-Chromium-Molybdenum Alloys

H. Nickel-Base Molybdenum- Chromium Iron Alloys

There are several proprietary alloys of approximately 25Ni-21Cr and 4 to 6.5 per cent molybdenum that were developed mainly for resistance to localized attack such as pitting and crevice corrosion in chloride environments. Included among these alloys are wrought JESSOP* alloy JS-700, HAYNES** alloy 20 Mod, ALLEGHENY-LUDLUM*** alloy AL-6X and cast IN-862. Judging by their composition, their corrosion resistance in acetic acid and organic acids generally should be superior to Type 316 stainless steel in many halide contaminated environ-ments. Unfortunately, data on these alloys are sparse although some data exist as shown in Tables III, V, XIII, LXXII and LXXVIII. Note the superiority of alloy JS-700 in the acetic-hydroxy acid solution in Table V and the freedom from pitting exhibited by cast IN-862 in the buffered acetic acid solution at 200 C (392 F) shown in Table XIII. This type of alloy should certainly be evaluated for aggressive acetic acid environments. Welded samples of comparable thickness to the equipment under consideration are suggested for test evaluations because of the possible formation of sigma or chi phases.

Increases in temperature, increases in pressure and a more complex chemistry in the acetic acid process stream are characteristics of the more modern processes for produc-ing the acid. In many of these process streams, the presence of formic acid, higher acids, or halides requires that the ultimate material of construction in acid resis-tance, resistance to pitting and resistance to chloride stress-corrosion cracking be used. The nickel-base alloys containing molybdenum, iron and chromium are those materials. The alloys are exemplified by wrought HASTELLOY alloys C-276 and C-4, INCONEL* alloy 625, cast CHLORIMET** alloy 3 and ILLIUM*** alloys W1 and W2, among others.

The data in Tables III, VII, VIII, XIII, XV1, XVII, XX, XXI, XXVII and XXVIII through XXX show the excel-lent resistance of these alloys to corrosion by hot acetic acids. In pure aqueous acid streams, or in uncontaminated glacial acids, the use of these alloys in preference to Type 316 stainless steel is usually not economically justifiable. However, when impurities are present, they often offer the most economical choice.

* Trademark of Jessop Steel Company ** Trademark of Cabot Corporation *** Trademark of Allegheny Ludlum Steel Corporation

* Trademark of the Inco family of companies ** Trademark of The Duriron Company, Inc. *** Trademark of Stainless Foundry & Engineering, Inc.

*Trademark of Cabot Corporation Reference 45

As shown previously, the presence of anhydride in the acetic acid can render the use of Type 316 stainless steel unsuitable. Titanium is also attacked by the acid-anhydride mixtures (Table XXI). These nickel-base higher alloys retain immunity to attack in all mixtures of the acid and anhydride. For this reason, parts of the distillation columns of the acetaldehyde-to-acetic acid process were constructed of these high alloy wrought materials and many of the required pumps were of the cast counterparts.

When formic acid is a co-product of the oxidation reaction to produce acetic acid, the process stream can again be overly aggressive to Type 316 stainless steel and more highly alloyed corrosion resistant alloys must be considered for use. If air or other contaminants are present, the nickel-base molybdenum-chromium-iron al-loys are prime candidates as materials of construction.

When the process conditions or operating problems

contaminate an acetic acid stream with halide ions, the use of the nickel-base, high alloy materials offers the greatest certainty of economical operation. As discussed under the effects of contaminants, the presence of chlorides in an acetic acid stream may produce disastrous results with the stainless steels. Titanium is also severely attacked when sufficient chloride ion is present. The copper alloys may be useful depending on the corrosion allowable in the system and depending on what other contaminants are in the stream (e.g., oxygen, heavy metal cations, peroxides, etc.). The nickel-base alloys containing molybdenum, chromium and iron are essentially unaffected by such contaminants. As an example, the data of Table XVII show the results of a test conducted in an acetic acid vaporizer using acid contaminated with a small amount of chloride. The effect on other alloys was severe while the HASTELLOY alloy C-276 material maintained adequate stability.

TABLE XXI

Comparison of Corrosion of Various Proprietary Alloys in Acetic Acid Solutions

Conditions: Duplicate specimens tested in the boiling solution for 48 hours or longer. Air not excluded or added.

1. Synthetic mixture of 75% butyl acetate, 11% butanol, 10% acetic acid, 4% water, 0.3% sulfuric acid 2. Annealed 3. 840 ºC (1544 ºF) for one-half hour and furnace cooled 4. CARPENTER alloy 20 has been superseded by an improved alloy CARPENTER alloy 20Cb-3 * Trademark of Waukesha Foundry Company ** Trademark of Westinghouse Electric Corporation *** Trademark of Standard Pressed Steel Co. ***** Trademark of Babcock & Wilcox Co.

Corrosion Rate

Acetic Acid,

glacial

50% Acetic Acid 50% Acetic Anhydride

30% Aqueous

Acetic Acid

10% Acetic Acid

2% Formic Acid

Esterifi- cation

Mixture1

99% Acetic Acid1% Acetic Anhydride

90% Acetic Acid

10% Formic Acid

Acetic Acid70%

90% Acetic Acid10% AceticAnhydride

Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

Type 316 .01 0.4 1.07 42 .03 1

Corrosion Rate

Temperature % Acetic Acid ºC ºF

MONEL alloy 400

Nickel 200

INCONEL alloy 600

mm/y mpy mm/y mpy mm/y mpy

2 30 86 .03B 1B .05 2

2 70 158 .10 4

2 116 241 .01 0.2 5 116 241 .03 1 .28 11 .08 3 6 26-30 79-86 .30A,.05B 12A,2B 1.19A,.10B 47A, 4B

10 26-30 79-86 .33A,.08B 13A,3B .10B 4B .02 0.8

10 70 158 1.37A 54A 10 116 241 .33 13 20 70 158 1.30A 51A

25 26-30 79-86 .41A,.08B 16A,3B 30 26-30 79-86 3.30A 130A

30 60 140 .46B 18B

50 26-30 79-86 .74A,.10B 29A,4B 4.32A,.25B 170A, 10B 50 80 176 1.68B 66B 50 116 241 .05 2 .48 19

70 116 241 .36 14

75 26-30 76-86 .36A,.05B 14A,2B

99.9 26-30 79-86 .23A,.08B 9A,3B .13B 5B

99.9 80 176 .61B 24B

99.9 116 241 .15 6 .36 14

100 26-30 79-86 .10 4

100 116 241 .30 12 .99 39 3.05 120

TABLE XXII

Corrosion of High Nickel Alloys in Acetic Acid

A = Aerated B = Unaerated

Reference 46 primarily.

There are process conditions which require that essen-tially no corrosion of the material of construction occur. Critical items of equipment required to operate with close tolerances such as orifice plates or control valve trim are examples. Another possibility is that the catalyst system used in the reactor of the process will not tolerate contamination with foreign metallic ions. In these cases, the maximum in corrosion resistance is demanded of an alloy, and only the nickel-base Mo-Cr-Fe, the nickel-base molybdenum, zirconium, titanium and tantalum alloys are potential candidates as solid or clad materials of construction.

Although the organic acids are less aggressive than mineral acids in detecting sensitization of this class of alloy, prolonged exposure of the sensitized alloy in hot acetic acid can produce intergranular attack. The newer wrought materials, such as HASTELLOY alloys C-276,12 C-4 and INCONEL alloy 625 are stabilized to forestall such attack on fabricated items of equipment. Castings of this type of alloy should be purchased in the fully solution-annealed condition. A test for susceptibility to intergranular attack is defined in reference 13.

These alloys usually provide the ultimate in corrosion resistance to hot organic acid streams. If the environmental

conditions are such that general attack or pitting of this type of alloys is excessive, the use of tantalum, zirconium, graphite and brick-lined construction may be explored.

I. Nickel-Copper Alloys

Alloy 400 and other nickel-copper alloys have very good resistance to pure acetic acid solutions in the absence of air or other oxidants. Tables XXII and XXV, among others, show the low rate of corrosion of MONEL* alloy 400 when the exposure is free of oxidants. As with other alloys, the maximum corrosion appears to occur in the 50-70 per cent acid range. The data agree well with the curve (Figure 3) published by Uhlig for corrosion of the alloy in acetic acid at 30 ºC (86 ºF).

MONEL alloy 400 withstands the effects of oxidants added to acetic acid better than do either nickel or copper alone, as shown by Table XXIII. However, the presence of air or an oxidizing agent such as ferric or cupric ion in solution is cause for concern and may lead to excessive attack. Corrosion tests should be run to ascertain the behavior of these alloys under operating conditions if oxidants are suspected to be present.

* Trademark of the INCO family of companies

FIG 3—Corrosion of MONEL alloy 400 in Acetic Acid

The presence of oxidizing agents in an organic acid stream completely changes the corrosive characteristics of the medium. Parts per million of oxygen, cupric or ferric salts, or peracid compounds in the stream will react stoi-chiometrically with alloys which do not produce protective oxide films. For instance, copper is essentially immune to attack by pure, uncontaminated acetic acid. Yet a small ingress of air at a circulating pump can drive the corrosion rate in a copper column to > 2.5 mm/y (hundreds of mils per year). Indeed, copper can be used as a scavenger of oxidizing species in an organic acid medium and has been so used.

The addition of nickel to the copper moderates the effect of oxidants. In general, the greater the amount of nickel in the alloy, the less the effect of oxygen on the corrosion rate. This is illustrated by the data of Table XXV. The addition of nickel to copper appears to have little influence on the rate of attack in acid contaminated with heavy metal ions. The accelerating effect of these ions produces higher rates of attack which remain excessive regardless of the alloy composition. It is interesting to note the effect of dilution on the corrosive properties of the various mixtures. As would be anticipated, the corrosion rate is greatly accelerated when adding water to an air sparged solution or one containing ferric ions. However,

The effect of liquid velocity on the corrosion of MONEL alloy 400 is shown in Table XXIV. No acceleration of the corrosion rate occurred up to 12.5 ft/sec velocity at a temperature of 30 ºC (86 ºF). It is believed that velocities of this magnitude would not increase the attack on MONEL alloy 400 up to temperatures of 100 ºC (212 ºF).

Alloy 400 and the cast counterpart of Alloy 400, ACI M-35 alloy, have found useful service for many years in some dilute acetic acid solutions handled in the food industry at the lower temperatures. Alloy 400 is attractive because contamination of the food products with ferric or cupric ions is undesirable. Small amounts of iron can contaminate the products if ferrous alloys are used and excessive copper pickup can be experienced if the copper content of the alloy is higher than that of Alloy 400. Corrosion rates for MONEL alloy 400 in a typical dilute acetic acid solution of this type are shown in Table XXII. Mason has covered the subject of the alloy’s use in food products very well.14

J. Copper-Nickel Alloys

All of the copper alloys excepting those with high (> 15%) zinc are resistant to acetic acid in the absence of air and other oxidants. Until the advent of the stainless steels, copper was used almost exclusively for the handling of acetic acid.

TABLE XXIII

Effect of Aeration on Corrosion of Nickel, Copper and Their Alloys in Acetic Acid

Conditions: Laboratory tests in 6% acetic acid at 30 ºC (86 ºF)

Corrosion Rate

Without Aeration With Aeration

Alloy mm/y mpy mm/y mpy

Nickel 200 MONEL alloy 400 C 71500 (70-30 Cupro-nickel) Copper C 10300

.08

.05

.08

.08

3 2 3 3

.28

.20 .81 .48

11 8

32 19

Temperature Corrosion Rate

Medium ºC ºF Test

Period, hr Aeration Velocity

ft/sec mm/y mpy

50% aqueous Acetic Acid 30 86 48 100 cc/min 0 .38 15

1.8 .41 16 3.8 .43 17

8.7 .41 16

12.5 .46 18

TABLE XXIV

Effect of Velocity on Corrosion of MONEL Alloy 400 in Acetic Acid

Reference 47

dilution markedly decreases the attack in a solution containing cupric acetate. This is probably attributable to the formation of a protective film on the surface, such as a basic cupric acetate.

Note that the addition of ferric ion as the chloride produced significantly higher corrosion rates than when ferric acetate was used as an additive in glacial acetic acid. A comparison of the effect of the same additives in the 50 per cent acid suggests that the chloride was not mainly responsible for the greater attack in the 100 per cent acid, but that the small amount of water added as the ferric chloride hydrate produced the greater corrosion.

Further evidence that chloride ion does not greatly affect the corrosion of copper-nickel alloys in organic acids is shown in Table XXVI. Adding 0.05 to 2.0 per cent sodium chloride to a synthetic mixture of various organic acids produced a ten-fold change in the corrosion rate on copper and the cupro-nickel alloys. However, the rates remained low enough that copper and copper-nickel alloys

could still be used as materials of construction without a practical limitation. Increasing nickel content in the alloy provided no change in the corrosion resistance. Data for Type 316 stainless steel are provided in this table for comparison.

The excellent corrosion resistance of the cupro-nickel alloys in hot acetic acid and the retention of that resistance in chloride-contaminated acid has significant commercial implications. The chemical industry around the world has constructed seashore installations predominantly during the past 20 years. For such plants, the least costly cooling water system is the direct use of filtered seawater. The cupro-nickel alloys are essentially a standard for handling clean saline cooling water in condensers and other heat exchange surfaces if compatible with the process stream. Consequently, in organic acid plants using unpolluted salt- water cooling of condensers, the C70600 alloy (90-10 cupro-nickel) is widely used, and C71500 alloy (70-30 cupro-nickel) and Alloy 400 are used for certain special

TABLE XXV

Corrosion of Copper-Nickel Alloys in Acetic Acid Solutions

Conditions: Quadruplicate specimens exposed in pure aqueous acid solutions for 120 hours at the boiling temperature except tests without air sparging were extended to 336 hours. Additives added as shown.

Corrosion Rate

Per Cent Acetic Acid

Per Cent Nickel in Alloy

No Air Sparge

Air Sparged

3200 ppm Cu++

Added as Cu(OAc)2

2900 ppm Fe+++

Added as Fe(OH)(OAc)2

2100 ppm Fe+++

Added as FeCl3•6H20

mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

100 0 .01 0.4 .08 3 51 20 .25 10 .76 30 10 .02 0.7 .08 3 1.32 52 .30 12 .76 30 20 .01 0.3 .08 3 2.87 113 .28 11 .74 29 30 .01 0.2 .08 3 6.15 242 .25 10 .74 29 67 Nil 0.1 .05 2 2.97 117 .18 7 1.30 51

100 .04 1.4 .03 1 81 32 .13 5 5.21 205

75 0 .03 1 10 .03 1 20 .03 1 30 .03 1 67 .05 2

100 .01 0.4

50 0 .03 1 7.87 310 .48 19 3.28 129 3.00 118 10 .03 1 5.41 213 .79 31 2.64 104 2.59 102 20 .03 1 4.95 195 .86 34 2.69 106 2.06 81 30 .03 1 4.78 188 .84 33 2.36 93 2.46 97 67 .03 1 2.13 84 .91 36 1.83 72 2.82 111

100 .08 3 1.60 63 .71 28 1.98 78 4.39 173

25 0 .05 2 10 .03 1 20 .03 1 30 .03 1 67 .03 1

100 .15 6

Portion of Data from Reference 48

exposures. For a complete description of the excellent properties of these alloys in seawater, see “Guidelines for Selection of Marine Materials.”15 In addition, if mechan-ical problems arise which allow seawater contamination of the process stream, such as a leaking condenser tube, the cupro-nickels and Alloy 400 are not excessively corroded by the contaminated acid.

K. Nickel-Chromium Alloys

The nickel-chromium alloys represented by Alloy 600 and ACI CY-40 are little used in the production and handling of acetic acid. In general, the iron-base alloys with chromium, nickel and molybdenum exhibit superior corrosion resistance in the acid streams and economic considerations dictate no better choice. For certain specific appurtenances on the major equipment, INCONEL alloy 600 has been used when required because of availability or to take advantage of certain mechanical properties of the alloy. However, these uses have been minimal. The more corrosion-resistant iron-base nickel-chromium-molyb-denum-copper alloys are used to combat stress-corrosion cracking when the stainless steels are not useful and forestall any consideration of the nickel-chromium alloys for the new construction of major items of equipment. When existing equipment of the versatile nickel-chromium alloy is available, the processing of various acetic acid mixtures is permissible if the corrosion characteristics of the medium have been properly defined. In general, the lower concentrations of acetic acid (< 60%) in aqueous solution can be handled without excessive corrosion. If oxygen is present in the solution, the nickel-chromium alloy is superior to the nickel-copper or cupro-nickel alloys in corrosion resistance.

Data showing the resistance of the basic nickel-chro-mium alloys to corrosion by acetic acid are presented in Tables VII, VIII, XV XVII, XXII, XXVII, XXVIII and XXX.

This 15,000 pound capacity reactor kettle of INCONEL alloy 600 was used for over 27 years for the dehydration or polymerization of castor, linseed and soybean oils. Alloy 600 was chosen to withstand the corrosive effects of vegetable oil acids and C18 fatty acids at a temperature of 600 ºF.

L. Iron-Nickel-Chromium Alloys

Alloy 800 has fair resistance to hot acetic acid solutions. The iron and chromium of the alloy dictate that conditions should be slightly oxidizing to realize the best resistance from the alloy. However, the alloy cannot compete with Alloy 825 or other metals containing molybdenum as a prime candidate for process use.

The good chloride stress-corrosion cracking resistance of the alloy makes use of the material attractive for small, specialty applications, but the corrosion rate must be determined closely to assure that adequate life will be obtained. As a general statement, the better solution to a problem involving acetic acid corrosion and chloride stress-corrosion cracking is the use of the “type 20” alloys, or the nickel-base iron-chromium-molybdenum-copper alloys.

TABLE XXVI

Effect of Sodium Chloride in a Mixed Acid Medium on the Corrosion of Copper-Nickel Alloys

Conditions: Duplicate specimens immersed in a boiling 116 ºC (241 ºF) solution of 60% acetic acid, 10% formic acid, 10% heavy organic acids and 20% water for 100 hours.

Corrosion Rate

Per Cent NaCI Added to Acid

Copper

C70600 (90-10

Cupro-Nickel)

C71500 (70-30

Cupro-Nickel)

Type 316

Stainless Steel

mm/y mpy mm/y mpy mm/y mpy mm/y mpy

0.05 .01 0.4 .01 0.3 .01 0.3 .38 15

0.10 .01 0.3 .01 0.3 .01 0.5 .56 22

1.0 .08 3 .05 2 .08 3 12.27 483

2.0 .10 4 .08 3 .10 4 22.66 892

M. Nickel-Base Molybdenum Alloys

Greater attention has been given to this class of alloy for acetic acid exposures in recent years. For most acetic acid applications, the nickel-base iron-chromium-molybdenum-copper alloys are superior to the nickel-base molybdenum alloys without chromium. However, HASTELLOY alloys B and B-2 have good organic acid resistance and have sometimes been used for the distillation of acetic acid mixtures. The cast alloys in this family of alloys include ASTM A 494 grades N-12M-1 and N-12M-2. Trade names associated with these cast grades include CHLORIMET alloy 2 and ILLIUM alloys M1 and M2.

These alloys offer excellent corrosion resistance in certain of the newer acetic acid processes utilizing chloride catalysts under reducing conditions at high temperatures. Under these conditions, only zirconium, titanium, and the nickel-base molybdenum alloys appear to be attractive.16, 17, 18 For the high pressures employed for the reaction area, the use of clad construction is very attractive.

Corrosion data for this type of alloy are given in Tables III, VII, XV, XX, XXVII and XXVIII through XXX.

N. Nickel Commercial nickel is less resistant to attack by acetic acid at any temperature than are the nickel-copper alloys, the cupro-nickel alloys, or the austenitic stainless steels. Consequently, nickel as a basic material of construction is not generally used. The material is used as the underbead in the welding of copper-clad steel, being compatible with both the copper and the steel backing.

Data showing the resistance of wrought Nickel 200 to acetic acid under varying conditions are contained in Tables III, VII, XIII, XXII, XXIIL XXV, XXVII, XXIX and XXX.

The presence of air accentuates the corrosion of nickel. For example, Uhlig reports a rate of attack of .02 mm/y (0.9 mpy) for nickel in a 6% acetic acid solution charged with nitrogen at room temperature, but a rate of .28 mm/y (11 mpy) when air is introduced.19

TABLE XXVII

Corrosion of Metals and Alloys in Acetaldehyde Oxidation Process for Acetic Acid

*Exposure 1-Product flash kettle base liquid at 95-100 ºC (203-212 ºF) for 737 days. Approx. 58% acetic acid, 40% anhydride, 2% residue with peroxides present. 2-Stripping still kettle liquid at 148-150 ºC (298-302 ºF) for 56 days. Approx. 65% acetic acid, 36% anhydride, residues, peroxides and catalyst salts. 3-Liquid of stripping still base section at 120 ºC (248 ºF). 4-Vapor of stripping still base section at 120 ºC (248 ºF). 5-Liquid of stripping still mid-section. 6-Anhydride still kettle liquid at 145 ºC (293 ºF). Essentially anhydride. 7-Anhydride still kettle vapor at 145 ºC (293 ºF). 8-Acetic acid refining still base liquid at 145 ºC (293 ºF). Mostly anhydride. 9-Acetic acid refining still base vapor at 145 ºC (293 ºF). 10-Acetic acid refining still overhead at 120 ºC (248 ºF).

**Trademark of The Duriron Company, Inc.

Corrosion Rate

Exposure* 1 2 3 4 5 6 7 8 9 10

Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

ACI CF-8 – – – – – – – – – – – – – – 0.38 15 – – – –

ACI CF-8M – – – – – – – – – – – – – – .03 1 – – – –

ACI CN-7M Nil 0.1 – – – – – – .13 5 Nil 0.1 .02 0.6 .01 0.4 .01 0.5 – –

Type 446 Stainless Steel .19 7.5 – – – – – – – – – – – – – – – – – –

Type 204 Stainless Steel .06 2.5 – – 2.54 100 1.70 67 – – – – – – – – – – – –

Type 304 Stainless Steel .09 3.5 – – 1.78 70 1.22 48 2.16 85 .18 7 .13 5 .08 3 .03 1 .25 10

Type 316 Stainless Steel .06 2.4 2.34 92 .33 13 .43 17 .05 2 .03 1 .03 1 Nil Nil Nil Nil .01 0.4

Type 317 Stainless Steel .02 0.7 – – .01 0.5 .01 0.5 .03 1 – – – – Nil Nil Nil Nil Nil 0.2

CARPENTER alloy 20 .01 0.5 .89 35 .18 7 .20 8 .05 2 Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil

INCOLOY alloy 825 – – – – – – – – – – .03 1 Nil Nil Nil Nil Nil Nil Nil Nil

HASTELLOY alloy C Nil Nil .03 1 .03 1 .03 1 .03 1 Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil

HASTELLOY alloy B 01 0.5 .28 11 – – – – .23 9 .03 1 .05 2 .18 7 .25 10 – –

HASTELLOY alloy D – – .1 5 6 – – – – – – – – – – – – – – – –

INCONEL alloy 600 .12 4.6 – – .36 14 .23 9 – – – – – – – – – – – –

Nickel 200 .10 4.1 – – .86 34 .81 32 – – – – – – – – – – – –

MONEL alloy 400 .11 4.4 – – 1.12 44 1.07 42 .94 37 .01 0.3 .03 1 Nil Nil Nil Nil – –

EVERDUR 1010 Silicon Bronze .10 3.9 – – – – – – – – – – – – – – – – – –

Copper .28 11 – – – – – – – – – – –

DURIRON** – –

Nickel plating appears to have essentially the same corrosion resistance in acetic acid solutions as the wrought metal. An increase in corrosion resistance is reported for electroless nickel which is properly heat treated. Volokhova, et al. report rates of .10 and .05 mm/y (4 and 2 mpy) for untreated electroless nickel plate in 5% and glacial acid, respectively, at room temperature while specimens of heat-treated plating showed only .01 and nil mm/y (0.3 and 0.09 mpy) in the same acids.20

tion. Such a situation demands more detailed testing and economic evaluation of alloys, taking into account not only first cost but maintenance costs and reliability as well.

a. Oxidation of Acetaldehyde The oldest of the current processes used for any

significant production of acetic acid is the oxidation of acetaldehyde. In this process, acetaldehyde is air-blown in a small tubular converter with distillation of the product and recycling of unreacted acetaldehyde to the reactor.21,22

The primary converter product contains, in addition to acetic acid and unreacted acetaldehyde, varying quantities of acetic anhydride, ester, peracetic acid and catalyst salts from the converter. As pointed out previously, the pre-sence of the anhydride increases the corrosive nature of the stream. (See Tables IV, IX and X.) Until the anhydride and catalyst salts are separated from the acid, a close evaluation of the corrosion to be expected in all sections of the equipment is necessary. For instance, the still used to separate the acid and anhydride may require a nickel- base molybdenum-chromium-iron alloy for the base kettle, the calandria and a few lower sections of the column.

O. Process and Plant Corrosion Data 1. Acetic Acid Production The modern industrial chemical plant has changed radi-cally during the past few decades. Efficient, economical production requires large single-train units that put greater emphasis on the reliability of components. If a failure does occur, it causes a shutdown of the entire process. When this happens, production losses will often far overshadow any differences in cost between alloys of marginal corro-sion resistance and more durable materials of construc-

TABLE XXVIII

Corrosion of Alloys in a Hydrocarbon Oxidation Unit for Acetic Acid

Corrosion Rate

Location * 1 2 3 4 5 6 7 8 9

Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy Type 304 Stainless Steel – – – – – – – – – – – – – – – – 28 11 Type 202 Stainless Steel – – – – – – – – – – – – – – – – .30 12 Type 316 Stainless Steel .05 2 .05 2 .05 2 .03 1 .25 10 .03 1 .03 1 .05 2

The middle column sections may require somewhat less highly alloyed materials, such as the iron-base nickel-chromium-copper-molybdenum alloys, while the top por-tion of the column, the condenser and all associated piping may be made of Type 316L stainless steel. Returning to the higher temperatures of the base area, the calandria circulating pump and other cast appurtenances must be of a Type CN-7M casting as a minimum, and the use of more corrosion-resistant alloys or graphite may be necessary. The remainder of all operating facilities of an acetaldehyde-based acetic acid unit can normally be con-structed of Type 316L stainless steel with Types CF-8M or CF-3M cast valves and pumps. The anhydride refining still normally presents no exceptional corrosion problems for Type 316L stainless steel. Copper, cupro-nickel alloys and Alloy 400 nickel-copper alloy can be used for any required applications once the peracetic acid is destroyed in the system by high temperatures in holding tanks or column bases and the equipment is sealed from the ingress of air. Corrosion data obtained in an acetaldehyde oxidation process unit are tabulated in Table XXVII. Additional data for a wide range of allays exposed in an acetic acid residue still of the same process are given in Table VIII.

b. Liquid Phase Oxidation of Straight- Chain Hydrocarbons

Among the important processes of today for acetic acid production are those based on the direct oxidation of straight-chain hydrocarbons, such as propane, propylene, butane, butene and higher aliphatics. The oxidation can be achieved using air or oxygen. Reaction conditions are much more severe than for the simple oxidation of an aldehyde with temperatures near 200 ºC (392 ºF) at pressures of more than 700 psi. Breaking up a hydrocarbon by such a severe oxidation obviously produces many by-products in addition to acetic acid. Among these are formic, propionic, butyric and higher acids, ketones, esters and peroxide compounds. The reaction conditions of the converter can be varied to increase or decrease the ratio of the by-products. This mix of products and by-products creates two problems not present in an aldehyde oxidation process: (1) much more separation equipment is required to recover the products and (2) the corrosion medium is more complex. Added to this is the large size of the equipment required for the large volume output of a modern single-train unit.

A simplified flow diagram for a typical hydrocarbon oxidation unit is shown in Figure 4. Essentially the entire

TABLE XXIX

Corrosion of Allays in Laboratory Equivalents of the Methanol-Carbon Monoxide Reaction Medium

Conditions: Small autoclave tests for 48 hours using 50% acetic acid at autogenous pressure without

and with catalyst (7 grams cobalt acetate

Corrosion Rate

Temperature

Without Catalyst

With Catalyst

Alloy ºC ºF mm/y mpy mm/y mpy

Type 304 Stainless Steel 250 482 – – 2.03 80 Type 310 Stainless Steel 300 572 >25.4* >1000* – – Type 321 Stainless Steel 250 482 – – 10.16 400 Type 347 Stainless Steel 300 572 >25.4 > 1000 – – Type 316 Stainless Steel 300 572 9.14* 360* – – 250 482 5.08 200 – – 260 500 – – 22.35 880 CARPENTER alloy 20 300 572 1.63 64 – – 250 482 3.81 150 – – INCOLOY alloy 825 260 500 – – 5.08 200 HASTELLOY alloy C 280 536 .36 14 – – 260 500 – – 1.78 70 230 446 – – 5.08 200 HASTELLOY alloy B 280 536

facility can be constructed of Type 316L stainless steel. There are certain precautions, however:

•The high temperature of the reactor requires that the Type 316L stainless steel be fully qualified and in its most corrosion-resistant condition. (See Effect of Microstruc-ture.) Type 316L stainless steel clad construction over a steel substrate offers the most economy for the high pressure reactor but the fabrication techniques must assure that the maximum corrosion resistance of the stainless steel is retained.

•Proper operation of the plant is essential. Although Type 316L stainless steel is resistant to the normal conditions of operation existing in the reactor, if the temperature reaches much more than the nominal 185 ºC (365 ºF), the corrosion rate for the alloy increases rapidly.

•There have been instances in which weld deposits have been less corrosion resistant than the base metal, perhaps because of compositional differences. For this reason, many welds are made using a more highly alloyed weld rod or filter wire.

•Circulating pumps for the hot process liquid are usually of a solution treated CN-7M alloy casting. If temperatures are maintained at lower levels for the reaction, the CF-8M or CF-3M alloy castings will exhibit a satisfactory service life.

The acids longer than acetic (propionic, butyric, etc.) produced in the reaction add little if any to the corrosivity of the stream, because temperatures of the process following the reaction are lower than those required to promote corrosion of the stainless steels by these acids. (See section on Higher Organic Acids.)

TABLE XXX

Corrosion of Alloys in Synthetic Reactor Product from Methanol-Carbon Monoxide Process for Acetic Acid

Conditions: Aqueous 70% acetic acid at the boiling tem-perature 107 ºC (243 ºF) without and with catalyst (ca. 6% cobalt acetate hydrate and 6% potassium iodide). Purged with CO.

* Pitting occurred. Authors correctly reported only observations and weight loss of coupons. For comparison, the weight loss was convert-ed to corrosion rate on basis of data given.

** Contained 2.3% Mo and 2.0%Cu Reference 18

Corrosion Rate

Without Catalyst

With Cata

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