Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

CORROSION

RESISTANCE OF

NICKEL-CONTAINING

ALLOYS IN ORGANIC

ACIDS AND RELATED

COMPOUNDS (CEB-6)

A PRACTICAL GUIDE TO THE USE OF NICKEL-CONTAINING ALLOYS N O 1285

Distributed by NICKEL INSTITUTE

Producedby INCO

CORROSIONRESISTANCEOF

NICKEL-CONTAINING ALLOYS

IN ORGANIC ACIDS AND RELATED

COMPOUNDS (CEB-6)

APRACTICALGUIDETOTHEUSE OF NICKEL-CONTAINING ALLOYS N O 1285

Originally, this handbook was published in 1979 by INCO, The International Nickel Company, Inc. Today this company is part of Vale S.A. The Nickel Institute republished the handbook in 2020. Despite the age of this publication the information herein is considered to be generally valid. Material presented in the handbook has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice. The Nickel Institute, the American Iron and Steel Institute, their members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein.

Nickel Institute

communications@nickelinstitute.org www.nickelinstitute.org

Table of Contents

PART I. INTRODUCTION A. The Organic Acids B. Scope C. Corrosion Testing in Organic Acid Media

PART II. ACETIC ACID A. General

B. Austenitic Stainless Steels 1. General

2. Effect of Alloy Composition 3. Effect of Contaminants 4. Effect of Temperature 5. Effect of Microstructure 6. Quality Control C. Martensitic & Ferritic Stainless Steels D. Duplex Austenitic-Ferritic and Precipitation Hardening Stainless Steels

E. Iron-Base Nickel-Chromium-Copper Molybdenum Alloys F. Nickel-Base Chromium-Iron-Molybdenum- Copper Alloys G. Iron-Base Nickel-Chromium-Molybdenum Alloys H. Nickel-Base Molybdenum-Chromium-Iron Alloys I. Nickel-Copper Alloys J. Copper-Nickel Alloys K. Nickel-Chromium Alloys L. Iron-Nickel-Chromium Alloys M. Nickel-Base Molybdenum Alloys N. Nickel O. Process and Plant Corrosion Data 1. Acetic Acid Production a. Oxidation of Acetaldehyde

b. Liquid Phase Oxidation of Straight-Chain Hydrocarbons c. Methanol-Carbon Monoxide Synthesis

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

P. Acetic Anhydride

PART III. OTHER ORGANIC ACIDS A. Formic Acid B. Acrylic Acid

C. C3 Through C8 Acids (Propionic, Butyric and Higher Acids) D. Fatty Acids (Lauric, Oleic, Linoleic, Stearic, Tall Oil Acids) E. Di and Tricarboxylic Acids (Oxalic, Maleic, Phthalic, Terephthalic, Adipic, Glutaric and Pimelic Acids) F. Naphthenic Acids G. Organic Acids with Other Functional Groups 1. Glycolic Acid

2. Lactic Acid 3. Tartaric Acid 4. Citric Acid 5. Chloroacetic Acids 6. Amino Acids 7. Sulfoacetic Acid

PART III. OTHER ORGANIC ACIDS A. Acetic Esters B. Phthalate Esters C. Esterification of Fatty Acids D. Acrylate Esters

References Trademarks

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

Composition, %

Alloys

Ni

Fe

Cr

Mo

Cu

C

Si

Mn

Other

WROUGHT ALLOYS

Stainless Steels—Austenitic AISI Type 201 AISI Type202 AISI Type 204 AISI Type204L AISI Type 216 AISI Type216L AISI Type 304 AISI Type304L AISI Type309 AISI Type310 AISI Type 316 AISI Type316L AISI Type 317 AISI Type317L AISI Type318 AISI Type321 AISI Type330 AISI Type347

4.5 5.0 5.0 6.0 6.0 6.0 9.5

Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance

17.0 18.0 18.0 18.0 19.5 19.5 18.5 18.5 23.0 25.0 17.0 17.0 19.0 19.0 18.0 18.0 15.0 18.0

– – – – – – – – – –

– – – – – – – – – – – – – – – – –

0.15 Max 0.15 Max 0.08 Max 0.03 Max 0.08 Max 0.03 Max 0.08 Max 0.03 Max 0.20 Max 0.25 Max 0.08 Max 0.03 Max 0.08 Max 0.03 Max 0.08 Max 0.08 Max 0.25 Max 0.08 Max

1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max

6.5 8.0 8.0 8.0 8.0 8.0 1.5 1.3

N 0.25Max N 0.25Max N 0.25Max N 0.25Max N 0.25-0.50 N 0.25-0.50

10.0 13.5 20.0 13.0 13.0 14.0 14.0 14.0 11.0 35.0 11.0

2.0 Max 2.0 Max

2.25 2.25 3.25 3.25 3.25

1.7 1.8

2.0Max 2.0Max 2.5Max 2.0Max 2.0Max 2.0 Max

Cb + Ta 10XC Min Ti 5XCMin

– – –

Cb + Ta 10XC Min

Balance

NITRONIC alloy 50

12.5

22.0

0.06 Max

1.0Max

N 0.2-0.4, Cb 0.1-0.3

1.5–3.0

5.0

Stainless Steels—Duplex and Precipitation Hardening

AISI Type326 AISI Type329 CRUCIBLE alloy 223

6.5 4.5 - 4.0 7.0 7.0

Balance Balance Balance Balance Balance Balance

26.0 27.5 16.0 16.5 17.0 15.0

– 1.0– 2.0 0.4

– –

0.06 Max 0.10 Max 0.03 Max 0.07 Max 0.09 Max 0.09 Max

0.40 1.0Max 1.0Max 1.0Max 1.0Max 1.0Max

0.40 2.0 Max 12.0 1.0Max 1.0Max 1.0Max

1.0 4.0

N 0.3 Cb + Ta0.3

– –

17-4PH 17-7PH PH15-7Mo

– –

Al 1.1 Al 1.1

2.5

Iron-Base Nickel-Chromium Copper-Molybdenum Alloys CARPENTER alloy 20 (1) CARPENTER alloy 20Cb-3 Nickel-Base Chromium-Iron Molybdenum-Copper Alloys

29.0 34.0

43.0 39.0

20.0 20.0

2.0 Min 2.5

3.0 Min 3.3

0.07 Max 0.07 Max

1.0 0.6

0.8 0.8

Cb + Ta 0.6

41.8 45.0

30.0 19.5

21.5 22.2

3.0 6.5

1.8 2.0

0.03 0.03

0.35 0.35

0.65 1.3

AI 0.15, Ti 0.9 W 0.5. Cb + Ta 2.12

INCOLOY alloy 825 HASTELLOY alloy G

Iron-Base Nickel-Chromium Molybdenum Alloys ALLEGHENY alloy AL-6X

24.0 26.0 25.0 20.0

46.0 42.0 46.0 29.0

20.0 22.0 21.0 21.0

6.5 5.0 4.5 3.0

– – – –

0.025 Max 0.05 Max

0.5 Max 1.0 Max 0.5 1.0 Max

1.5 Max 2.5 Max

Ti 4XC Min Cb 0.30 Co 20.0, W 2.5, N 0.15,Cb + Ta1.0

HAYNES alloy 20 Mod JESSOP alloy JS-700 MULTIMET alloy

0.03 0.12

1.7 1.5

Nickel-Base Molybdenum Chromium-Iron Alloys HASTELLOY alloy C (2) HASTELLOY alloy C-276

54.0 54.0 61 ..0

5.0 5.0 3.0 Max 5.0 5.0 Max

15.5 15.5 16.0 7.0 21.5

16.0 16.0 15.5 16.5 9.0

– – – – –

0.08 Max 0.02 Max 0.015 Max 0.06 0.1 Max

1.0 Max 0.05 Max 0.08 Max 0.3 0.5 Max

1.0Max 1.0Max 1.0 Max 0.3 0.5 Max

Co2.5Max,W4.0, V 0.4 Ma) Co2.5Max,W4.0, V 0.4 Ma) Co 2.0 Max, Ti 0.7 Max AI 0.5 Cb + Ta 3.65

HASTELLOY alloy C-4 HASTELLOY alloy N INCONEL alloy 625

69.0 60.0

Nickel-Copper Alloys MONEL alloy 4OO MONEL alloy K-500

66.0 65.0

1.35 1.0

– –

– –

31.5 29.5

0.12 0.15

0.15 0.15

0.9 0.6

AI 2.8, Ti 0.5

Copper-Nickel Alloys Copper-Nickel alloy C70600 Copper-Nickel alloy C71000 Copper-Nickel alloy C71500 Nickel-Chromium Alloys INCONEL alloy 600 NICHROME V

10.0 20.0 30.0

1.25 0.75 0.55

– – –

– – –

88.0 78.0 67.0

– – –

– – –

0.3 0.4 0.5

Pb 0.05 Max, Zn 1.0 Max Pb 0.05 Max, Zn 1.0 Max Pb 0.05 Max, Zn 1.0 Max

76.0 80.0

7.2 –

15.8 20.0

– –

0.1 –

0.04 –

0.2

0.2

Page 2

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-ChromiumAlloys

32.0 41.0

46.0 25.4

20.5 29.5

– –

0.3 0.25

0.04 0.05

0.35 0,38

0.75 0,75 AI 0.3, Ti 0.6

INCOLOV alloy 800 INCOLOY alloy804

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 Max Co 1.0 Max, P 0.04 Max, S 0.03 Max

67.0

2.0 Max

1.0 Max

28.0

0.02 Max

0.1 Max

1.0 Max

HASTELLOY alloy B-2

Other Nickel and Cobalt-Base Alloys IN-102 MP-35N ELGILOY HAYNES alloy No. 25

68.0 35.0 15.0 10.0

7.0 – 15.0 3.0 Max

15.0 20,0 20A 20.0

3.0 10.0 7.0 –

– – – –

0.06 – 0.15 0.10

– – – 1.0 Max

– –

Ti 0.5, Cb 2.9. A1 0.5, W 3.0 Co 35.0

2.0 1.5

Co 40.0, Be 0.05 Co. 49.0, W15.0

CAST ALLOYS Stainless Steels ACI CD-4MCu ACI CF-3 ACI CF-3M ACI CF-8

5.5 10.0 11.0 9.0 10.0 11.0 20.0

61.0 66.0 63.0 67.0 64.0 62.0 49.0

26.0 19.0 19.0 19.0 19.0 19.0 26.0

2.0 – 2.5 – 2.5 3.5 –

3.0 – – – – – –

0.04 Max 0.03 Max 0.03 Max 0.08 Max 0.08 Max 0.08 Max 0.4

1.0 Max 2.0 Max 1.5 Max 2.0 Max 2.0 Max 1.5 Max 2.0 Max

1.0 Max 1.5 Max 1.5 Max 1.5 Max 1.5 Max 1.5 Max 2.0 Max

ACI CF-8M ACI CG-8M ACI HK

Iron-Base Nickel-Chromium- Copper-Molybdenum Alloys

ACI CN-7M 3 ) WORTHITE

29.0 24.0

44.0 48.0

20.0 20.0

2.0 Min 3.0

3.0 Min 1.75

0.07 Max 0.07 Max

1.0 3.3

1.5 Max 0.6

Iron-Base Chromium-Nickel- Copper-Molybdenum Alloy ILLIUM alloy P Iron-Base Nickel-Chromium- Molybdenum Alloys IN-862 KROMARC 55 Iron-Base Chromium-Nickel- Iron Alloy ILLIUM alloy PD Nickel-Base Chromium- Molybdenum-Copper-Iron Alloy ILLIUM alloy G

8.0

58.0

28.0

2.0

3.0

0.20

0.75

0.75

24.0 20.0

44.0 50.0

21.0 16.0

5.0 2.0

– –

0.07 Max 0.04

0.8 2.0 Max

0.5 9.5

5.0

57.0

26.0

2.0

0.5 Max 0.08

1.0 Max 1.0 Max Co 7.0

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) ACI CW-12M-2 (5)

58.0 57.0

6.0 3.0 Max

16.5 18.5

17.0 18.5

– –

0.12 Max 0.07 Max

1.0 Max 1.0 Max

1.0 Max 1.0 Max

Nickel-Base MolybdenumAlloys ACI N-12M-1 (6) ACI N-12M-2 (7) Other Nickel and Cobalt-Base Alloys

60.0 62.0

5.0 3.0 Max

1.0 Max 1.0 Max

28.0 31.5

– –

0.12 Max 0.07 Max

1.0 Max 1.0 Max

1.0 Max 1.0 Max

V 0.2–0.6, Co 2.5 Max

80.0 75.0 70.0 55.0 49.0 3.0 Max

– 0.4 5.0 1.0 3.0 3.0 Max

– –

– –

– – –

– – 0.05 Max

– – – 0.7 Max 4.5 1.0 Max

– 2.5 –

Sn 8.0, Zn 7.5, Pb4.0 Sn 8.0, Zn 7.0, Ag6.0 Sn 4.0, Bi 3.75

WAUKESHA alloy 23 WAUKESHA alloy 54 WAUKESHA alloy 88 ILLIUM alloy 98 ILLIUM alloy B STELLITE alloy No. 3 (8) STELLITE alloy No. 4 (8) STELLITE alloy No. 6 Nickel Alloyed Cast Irons Ni-Resist Type 2 Ni-Resist Type 4

3.0 8.0 8.0 –

12.5 28.0 28.0 31.0

0.05 0.05 2.35

5.0 5.0 –

1.25 Max 1.25 Max 1.0 Max

B 0.05-0.55 W 12.5, Others 1.0 Max, Bal Co

3.0 Max 3.0 Max

3.0 Max 3.0 Max

30.0 29.0

1.5 Max 1.5 Max

– –

1.0 Max 1.1

1.5 Max 1.5 Max

1.0 Max 1.0 Max

W 14.0, Bal Co W 4.5, Bal Co

20.0 30.5

70.0 55.0

2.2 5.0

– –

0.5 Max 0.5 Max

3.0 Max 2.6 Max

1.9 5.5

1.2 0.6

(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.

(1) An improved version of this alloy, CARPENTER alloy 20 Cb-3, has replaced CARPENTER alloy 20. Improved versions of this alloy, HASTELLOY alloys C-276 or C-4, have replaced HASTELLO alloy C. Cast “type20” alloys such as DURIMET alloy 20, ALOYCO alloy 20, etc. Includes alloys such as cast HASTELLOY alloy C, ALOYCO alloy N-3, ILLIUM alloy W1, etc. (2) (3) (4)

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

Page 3

PART I. INTRODUCTION

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.

A. The Organic Acids

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 maintain product purity.

the corrosive environment and

C. Corrosion Testing in Organic Acid Media 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.

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.

Page 4

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 A. General 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.

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.)

PART II. ACETIC ACID

B. Austenitic Stainless Steels 1. General

The 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 0.006 0.06

5.2 4.4 3.9 3.4 2.7 2.4

0.6 6.0 60.0 (6%)

Reference 43

Page 5

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 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. in Table XXVII and Figure 1. 2. Effect of Alloy Composition

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

Page 6

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.

Corrosion Rate

Wrought Type 316 Stainless Steel Annealed

ACI CF-8M

Annealed

Sensitized

ACI

Type 329 Stainless Steel

ALLOYCO 20***

5%*

10%*

5%*

10%*

CD-4MCu WORTHITE**

Temperature

Solution

ºC ºF 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

.05 .03

2 1

.05 .03

2 1

.03 .03

1 1

.03

1 1

.05

2

Nil Nil

Nil Nil

03 Nil

1

<.03

<1

<.03 <.03

<1 <1

Glacial Acetic Acid

117 102

242 216

03

Nil

Nil

Nil

.03

1

50% Acetic Acid

<.03

<1

<.03

<1

.03

1

01

0.5

Nil

Nil

Nil

Nil

Nil

Nil

<.03

<1

10% Acetic Acid

100.5

213

.15

6

.18

7

.13

5

.15

6

.08

3

<.03

<1

<.03

<1

<.03

<1

85% Acetic-15%Formic

109

228

.30

1 2

33

13

.18

7

.20

8

.25

10

08

3

<.03

<1

<.03

<1

.64

25

50% Acetic-15%Formic 106.5

224

.84

33

.89

35

.23

9

.25

10

.13

5

15

6

03

1

.03

1

1.35

53

85% Formic-15%Acetic 104.5

220

48

19

43

17

.28

11

.28

11

.33

13

18

7

05

2

.03

1

1.65

65

88% Formic Acid

104.5

220

.64

25

76

30

.61

24

.66

26

.51

20

.15

6

.15

6

.10

4

8.38

330

50% Formic Acid

102

216

.38

1 5

36

14

.46

18

.46

18

.43

17

<.03

<1

08

3

.10

4

10% Formic Acid

100

212

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

TABLE III

Field Tests in Acetic Acid Distillation Columns

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

mpy

<.03

<1 12 35

< .03

<1

.05

2

< .03

<1

.15

6

Type 316 stainless steel Type 304 stainless steel Type 309 stainless steel Type 329 stainless steel Type 216 stainless steel Type 410 stainless steel Type 430 stainless steel CARPENTER 1 alloy 20Cb-3 JESSOP 2 alloy JS-700

.30 .89 .03

.05

2

1

<.03

<1

>12.7 >12.7

>500 >500

<.03

<1

<.03 <.03 <.03

<1 <1 <1

INCOLOY 3 alloy 825 HASTELLOY 4 alloy C* CHLORIMET 5 alloy 2 HASTELLOY alloy B INCONEL 3 alloy 600 MONEL 3 alloy 400 C 10300 (Copper) Nickel 200

<.03 <.03

<1 <1

.13 .03

5 1

.05

2

<.03

<1

.05 .13 .25

2 5

.15 .08 .15 .08

6 3 6 3

10 11

28

.08

3

41

16

(1) Trademark of Carpenter TechnologyCorporation (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.

Page 7

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

Type 304

CRUCIBLE*

Type 316

Temperature

Stainless Steel

alloy 223

Stainless Steel

Test Medium

ºC

ºF

mm/y

mpy

mm/y

mpy

mm/y

mpy

117 104

242 219

.46

.18

.18 .05

7 2

.01 .01

0.4 0.3

Acetic acid, 100% Acetic acid, 75% Acetic acid, 50% Acetic acid, 25% Acetic acid 99%;

4.06

160

102

216

6.98

275

Nil

<0.1

.08

3

100

212

7.11

280

<.008

0.3

Nil

Nil

117

242

.33

13

2.26

89

.22

8.5

Acetic anhydride 1%

Acetic acid 90%;

Formic acid 10%

109

228

.23

9

.08

3.1

.17

6.5

Formic acid, 20% 2-Ethyl butyric acid, 100% Esterification mixture 1

102

216

1.75

69

4.75

187

.56

22

185

365

.53

21

.04

1.5

.04

1.4

86

187

.41

16

2.79

110

.02

0.7

(1) Synthetic mixture of 75% butyl acetate, 11% butanol,10% acetic acid, 4% water, 0.3% sulfuric acid.

*Trademark of Colt Industries, Inc.

TABLE V

When molybdenum is added to produce such alloys as Types 316 and 317 stainless steels, and other alloys, a remarkable increase in resistance to hot organic acids occurs. The startling efficacy of molybdenum is best shown by curves from Uhlig (Figure 2). Note that in the two ex- posures defined for these curves, the effect of molybdenum is fully realized at approximately 2.2 per cent. In the vast majority of organic acid environments, this approximate amount of molybdenum provides satisfactory corrosion resistance. For this reason, Types 316 and 316L stainless steels are utilized for the overwhelming majority of hot organic acid applications. Relative values of corrosion resistance for three common alloys in hot process acid are shown in Table IV to supplement the data for the Type 316 stainless steel shown in Table II. Data generated by all major acetic acid producers confirm that for a pure, uncontaminated acetic acid of any concentration, Type 316 stainless steel or its low carbon counterpart Type 316L is usable as a material of construction to temperatures beyond the boiling point. (See Effect of Temperature.) These alloys are used extensively in the fabrication of distillation columns, heat exchangers, decanters, piping and other apparatus employed in the production or processing of acetic acid. Under certain conditions of exposure, it has been found that additional amounts of molybdenum in the alloy are beneficial. Types 317 and 317L stainless steels are available for such applications when required. Tables V through VIII show process corrosion data where the superiority of the Type 317 stainless steel can be observed.

Corrosion of Alloys in Acetic-Hydroxy Acid Solution

Conditions: Exposure of approximately 50 days in strip- ping of acetic acid at temperatures shown from a 70% acetic acid containing ca. 8%  -hydroxy acids, 20% manganese salts and residues. Nitrogen blanket on system.

Corrosion Rate

124 ºC (255 ºF) 140 ºC (284 ºF)

Alloy

mm/y

mpy mm/y mpy

Type 304 Stainless Steel Type 316 Stainless Steel (annealed) Type 316 Stainless Steel (sensitized) Type 216 Stainless Steel Type 317 Stainless Steel Type 326 Stainless Steel (IN-744) CARPENTER alloy 20Cb-3 INCOLOY alloy 825 JESSOP alloy JS-700

.01

0.4

1.12

44

Nil

<0.1

.09

3.7

.01 Nil Nil Nil .00 .01 Nil

0.3 <0.1 <0.1 <0.1 0.1 0.2 <0.1 <0.1

.11 .05 .08

4.2 2.0 3.2

2.84 .05 .03 .01

112 1.8 1.2 0.3

HASTELLOY alloy G

Nil

.01

0.4

The effect of further alloying on the corrosion resistance of commercial alloys is indicated in succeeding sections.

Page 8

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

Alloy

Condition of Specimen

mm/y

mpy

Annealed 1 hr 677 ºC (1250 ºF) AC 4 hr 871 ºC (1600 ºF) AC, 1 hr 677 ºC (1250 ºF) AC As-welded (316L rod) Welded, 1 hr 704 ºC (1300 ºF) AC Welded, 1 hr 871 ºC (1600 ºF) AC As-welded (310 Mo rod) Welded (310 rod) 1 hr 871 ºC (1600 ºF) AC Annealed

.06 .06 .04*

2.5 2.5 1.4*

Type 316L Stainless Steel

.08 .08 .07 .06 .06 .39 .39 .65 .40 .71 .05

3.2 3.0 2.6 2.3 2.4

15.5 15.3 25.5 15.9 27 . 7 2.0

Type 316 Stainless Steel

2 hr 621 ºC (1150 ºF) AC 1 hr 677 ºC (1250 ºF) AC As-welded (316 rod) Welded, 1 hr 871 ºC (1600 ºF) AC Annealed

Type 317 Stainless Steel

6.3* 26.9* 1.73 21.5* 2.6 2.6 25.1* 2.4 2.6 12.0

4 hr 593 ºC (1100 ºF) AC 1 hr 677 ºC (1250 ºF) AC As-welded (317 rod) Welded, 1 hr 704 ºC (1300 ºF) AC Annealed 1 hr 677 ºC (1250 ºF) AC 1 hr 1316 ºC (2400 ºF) AC + 1 hr 677 ºC (1250 ºF) AC As-welded (318 rod)

.16* .68* .04 .55* .07 .07 .64*

Type 318 Stainless Steel

.06 .07 .30

Welded + 1 hr 704 ºC (1300 ºF) AC Welded + 1 hr 871 ºC (1600 ºF) AC

Reference 11

* Intergranular attack noted

NOTE: AC = Air-Cooled

TABLE VII

Corrosion of Alloys in Acetic-Formic Acid Process Mixtures

Corrosion Rate

Type 316 Stainless Steel

Type 317 Stainless Steel

Stream

Test

CARPENTER alloy 20

INCOLOY alloy 825

HASTELLOY alloy C

HASTELLOY alloy B

INCONEL alloy 600

Nickel 200

MONEL alloy 400

Arsenical Admiralty

EVERDUR* 1010

Composition

Temperature Period

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 18% Acetic Acid 40% Formic Acid 2% Water 40% Organics 6% Acetic Acid 10% Formic Acid

100 212 452

03

1

.03

1

05

2

.61

24

.25

10

08

3

.05

2

91 196

55

.08

3

.05

2

.05

2

.03

1

<.03 <1

.10

4

81 178

55

15

6

.13

5

.08

3

.08

3

<.03 <1

.10

4

3% Water 81% Organics

12% Acetic Acid 3% Formic Acid

121 250 355

05

2

<.03 <1

.05

2

.08

3

.03

1

85% Water

40% Acetic Acid 6% Formic Acid

106 223

99

.51

20

.28

11

38

15

.18

7

.51

20

.03

1

5% Water 49% Organics

*Trademark of Anaconda American Brass Co.

Page 9

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 Elder 5 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

2.13 .97 2.01 2.01 1.22 .18 Nil

84 38 79 79 48 7 Nil 2 7 30

1.32 .30 2.51 1.47

52 12 99 58 14 5 Nil 5 12 14

Type 501 chrome steel Type 430 stainless steel

.36 .13 Nil .13 .30 .36 .18 .05

INCONEL alloy 600 HASTELLOY alloy C DURIMET * 20

.05 .18 .76 .03 .03

Type 329 stainless steel Type 304 stainless steel Type 316 stainless steel Type 317 stainless steel

1 1

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 2 50**

mm/y .43 1.22**

mpy 17 48**

mm/y 2.10 1.19**

mpy

– 1.98

– 78

.05 1.27**

81 75*

None 0.2% Acetic Anhydride

.03 .03 – .03

1 1

– – .08 – .03 .18

– – 3 – 1 7

– – .33 – .66 .41

– – 13 – 26 16

– – .71 – .38 .36

– – 28 – 15 14

0.1 % Water 0.3% Water 0.33% Water 0.50% Water 0.67% Water 1.0% Water

1

– –

– –

* Added as sodium chloride ** Minute, profuse pitting

Page 10

Peracids or other per compounds are often formed in the reaction step of most oxidation processes designed to produce acetic acid. Peracetic acid is the common, strongly oxidizing compound formed although various other per compounds can be produced. The per compounds act similarly to oxygen in the system. Thus, the stainless steels again provide good stability in such media and can often be stabilized by the addition of such compounds. The effect of adding a peroxide to acetic acid can be noted in Tables XII and XLIII. Iron, copper, manganese and similar salts present in an operating system can serve as powerful oxidizing agents if in the higher valence state. Such salts quite often ac- cumulate in portions of a system from corrosion products or as carry-over from the reaction catalyst system. As long as the anion of the salt is an acetate, such as in ferric acetate, the presence of these compounds is normally beneficial to the stainless steels. However, the data of Table XII would suggest that a thorough investigation should be made if ferric ion is present at high tempera- tures. The presence of the ion in these tests actually accelerated the attack. Cupric ion is particularly effective as an oxidizing ion, and occasions arise in the processing of acetic acid solutions in stainless steel equipment where the addition of cupric acetate is advantageous in reducing attack on the stainless steel and maintaining passivity of the surface. Rabald cites the efficacy of mercuric salts in eliminating attack on a Type 304 stainless steel in glacial acid. 8 The presence of the reduced (ous) state of these cations shows no effect on the corrosion rate of the stainless steels or other metals and alloys. Chlorides can be considered as the major hazard when processing acetic acid in stainless steels. Acid contami- nated with chlorides can produce pitting and rapid stress- corrosion cracking of the 300 series stainless steels in specific areas of the equipment. Greatly accelerated, general corrosion can also ensue if the chloride content is sufficiently high. Tables IX and X reveal the effect of chloride ion added as sodium chloride. It will be seen that a concentration of less than 20 ppm can be allowed before the rate of attack on Type 316 stainless steel is intolerable. These data correlate well with the data of reference 7 that no more than 25 ppm of chloride is permissible before excessive attack occurs at the boiling temperature. It is assumed that increasing amounts of hydrochloric acid are formed as the weak acid is heated over prolonged periods with the strong acid salt. Where small quantities of chloride salt in a process steam are allowed to accumulate and concentrate in process equipment, the effect can be disastrous for the stainless steels. Both pitting and exces- sive overall attack on the stainless steels may occur. The last line of data of Table X is suspect in that the Type 316 stainless steel maintained passivity throughout the test period. This result is in conflict with the data of the last line in Table IX. It is believed that Table IX provides a more accurate description of the effect of chlorides in the presence of water. Pitting of the stainless steel would ensue also if the test period were extended.

TABLE X

Corrosion of Alloys in Contaminated Acetic Acid

Condition: Duplicate tests of 120 hours conducted at the boiling temperature with additions made as shown.

Corrosion Rate

Test

Type 316

CARPENTER

HASTELLOY

No.

Test Medium

Stainless Steel alloy 20Cb-3

alloy C

mm/y mpy

mm/y mpy mm/y mpy

1 2

Glacial acetic acid (1) + 0.1 % Acetic Anhydride (2) + 0.1% Sodium Chloride (1) + 0.1% Sodium Chloride

.08 .94

3 37

<.03 .84

<1 33

Nil .03

Nil 1

3

1.32

52

1.07

42

.03

1

4

1.73

68

1.47

58

.03

1

5

(4) +1% Water

.03

1

.03

1

.03

1

TABLE XI

Corrosion of Type 316 Stainless Steel in Acetic Acid Solutions

Conditions: Coupons exposed in hot wall tester to glacial acetic acid from Plant A and Plant B with the additions shown.

Specimen Wall Temperature

Acid Tested

Exposure Period

Corrosion Rate

Addition

º C

º F

hr

mm/y

mpy

Plant A

(None)

48 96 68 92 48 48 68 96 48

136 146 132 131 137 141 149 152 140

277 295 270 268 278 286 300 306 284

.23 .10 .03 .03 .03

9 4 1 1 1 <1

Plant A

1%water

Plant A Plant A Plant B

0.5% formic 1.0% formic (None)

<.03 7.80 12.55 .03

307 494 1

Plant B

1 % water

Note: All coupons pitted to some extent under all conditions.

TABLE XII

Corrosion of Type 316 Stainless Steel in Acetic Acid with Additives at Higher Temperatures Conditions: Laboratory tests in glacial acetic acid con- tained in pressure autoclaves at temperature shown for multiple runs of 48 hours each. Data averaged. Additions to the acetic acid made as shown. Corrosion Rate Temperature Annealed Sensitized* Additive º C º F mm/y mpy mm/y mpy None 1500 ppm hydrogen peroxide 3000 ppm hydrogen peroxide 3000 ppm H O + 2 2 190 374 .20 8 – – 190 374 .23 9 – – 190 374 .08 3 – –

1500 ppm Fe +++ (a) 1500 ppm Fe +++ (a) 1500 ppm hydrogen peroxide

190 190

374 374

.69 .56

27 22

– –

– –

240

464

.61

24

.89

35

*650 ºC ( 1202 ºF) for one hour a = Added as FeOH(C 2 H 3 O 2 ) 2

Page 11

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. 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

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.

Corrosion Rate

Alloy

mm/y

mpy

HASTELLOY alloy C-276 INCONEL alloy, 625 INCOLOY alloy 825 HASTELLOY alloy G Nickel 200 IN-862 Cast Alloy Type 315 Stainless Steel (sensitized)

02 02 02 03 04 05 13*

0.6 0.7 0.8 1.0 1.5 1.8 5.2*

Type 316 Stainless Steel (annealed)

04*

1.5*

*Incipient pitting

TABLE XIV Corrosion of Stainless Steels in Vapors Over 52 Per Cent Aqueous Acetic Acid at High Temperature

System Pressure

Corrosion Rate

Temperature

Alloy

ºC

ºF

psig

mm/y

mpy

142

288

35

.03

1

Type 304 Stainless Steel

153

308

55

.10

4

Type 304 Stainless Steel

Type 316 Stainless Steel

142

288

35

<.03

< 1

Type 316 Stainless Steel

153

306

55

.05

2

Each datum is average of eight tests conducted in closed pressure vessel.

Page 12

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