Page 1

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

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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 – – – – –

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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.

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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.

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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

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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

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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 <.03 <1 < .03 <1 .05 2 < .03 <1 .15 6 Type 304 stainless steel .30 12 .05 2 Type 309 stainless steel .89 35 Type 329 stainless steel .03 1 Type 216 stainless steel <.03 <1 Type 410 stainless steel >12.7 >500 Type 430 stainless steel >12.7 >500 CARPENTER1 alloy 20Cb-3 <.03 <1 <.03 <1 JESSOP2 alloy JS-700 <.03 <1 INCOLOY3 alloy 825 <.03 <1 .13 5 <.03 <1 .05 2 HASTELLOY4 alloy C* <.03 <1 .03 1 <.03 <1 CHLORIMET5 alloy 2 .05 2 HASTELLOY alloy B .13 5 INCONEL3 alloy 600 .15 6 .25 10 MONEL3 alloy 400 .08 3 28 11 C 10300 (Copper) .15 6 Nickel 200 .08 3 .08 3 41 16

Corrosion Rate

ACI CF-8M

Annealed Sensitized

Temperature 5%* 10%* 5%* 10%*

Wrought Type 316 Stainless

Steel Annealed

ACI

CD-4MCu

WORTHITE**

ALLOYCO 20***

Type 329 Stainless

Steel 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 Glacial Acetic Acid 117 242 .05 2 .05 2 .03 1 .03 1 .05 2 Nil Nil 03 1 <.03 <1 <.03 <1 50% Acetic Acid 102 216 .03 1 .03 1 .03 1 03 1 Nil Nil Nil Nil Nil Nil .03 1 <.03 <1

10% Acetic Acid 100.5 213 <.03 <1 <.03 <1 .03 1 01 0.5 Nil Nil Nil Nil Nil Nil <.03 <1 – –

85% Acetic-15% Formic 109 228 .15 6 .18 7 .13 5 .15 6 .08 3 <.03 <1 <.03 <1 <.03 <1 – –

50% Acetic-15% Formic 106.5 224 .30 1 2 33 13 .18 7 .20 8 .25 10 08 3 <.03 <1 <.03 <1 .64 25

85% Formic-15% Acetic 104.5 220 .84 33 .89 35 .23 9 .25 10 .13 5 15 6 03 1 .03 1 1.35 53

88% Formic Acid 104.5 220 48 19 43 17 .28 11 .28 11 .33 13 18 7 05 2 .03 1 1.65 65

50% Formic Acid 102 216 .64 25 76 30 .61 24 .66 26 .51 20 .15 6 .15 6 .10 4 8.38 330

10% Formic Acid 100 212 .38 1 5 36 14 .46 18 .46 18 .43 17 <.03 <1 08 3 .10 4 – –

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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

Type 316 Stainless Steel

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 <0.1 .08 3 Acetic acid, 25% 100 212 7.11 280 <.008 0.3 Nil Nil Acetic acid 99%; Acetic anhydride 1% 117 242 .33 13 2.26 89 .22 8.5 Acetic acid 90%; Formic acid 10% 109 228 .23 9 .08 3.1 .17 6.5

Formic acid, 20% 102 216 1.75 69 4.75 187 .56 22 2-Ethyl butyric

acid, 100% 185 365 .53 21 .04 1.5 .04 1.4 Esterification

mixture1 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.

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.

TABLE V

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% β -hydroxy acids, 20% manganese salts and residues. Nitrogen blanket on system.

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

Corrosion Rate

124 ºC (255 ºF) 140 ºC (284 ºF)

Alloy mm/y mpy mm/y mpy

Type 304 Stainless Steel .01 0.4 1.12 44 Type 316 Stainless Steel

(annealed) Nil <0.1 .09 3.7 Type 316 Stainless Steel

(sensitized) .01 0.3 .11 4.2 Type 216 Stainless Steel Nil <0.1 .05 2.0 Type 317 Stainless Steel Nil <0.1 .08 3.2 Type 326 Stainless

Steel (IN-744) Nil <0.1 2.84 112 CARPENTER alloy 20Cb-3 .00 0.1 .05 1.8 INCOLOY alloy 825 .01 0.2 .03 1.2 JESSOP alloy JS-700 Nil <0.1 .01 0.3 HASTELLOY alloy G Nil <0.1 .01 0.4

Page 9

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 <.03 <1 – – – – – – – – .10 4 – –

6% Acetic Acid 10% Formic Acid 3% Water 81% Organics

81 178 55 15 6 .13 5 .08 3 .08 3 <.03 <1 – – – – – – – – .10 4 – –

12% Acetic Acid 3% Formic Acid 85% Water

121 250 355 05 2 – – – – – – – – <.03 <1 – – – – .05 2 .08 3 .03 1

40% Acetic Acid 6% Formic Acid 5% Water 49% Organics

106 223 99 .51 20 .28 11 38 15 – – .18 7 – – – – – – .51 20 .03 1 – –

* Intergranular attack noted

NOTE: AC = Air-Cooled

TABLE VII

Corrosion of Alloys in Acetic-Formic Acid Process Mixtures

Reference 11

*Trademark of Anaconda American Brass Co.

Page 10

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

Page 11

Corrosion Rate

TestNo. Test Medium

Type 316 Stainless Steel

CARPENTER alloy 20Cb-3

HASTELLOYalloy C

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

1 Glacial acetic acid .08 3 <.03 <1 Nil Nil 2 .94 37 .84 33 .03 1

(1) + 0.1 % Acetic Anhydride

3 1.32 52 1.07 42 .03 1

(2) + 0.1% SodiumChloride

4 1.73 68 1.47 58 .03 1

(1) + 0.1% SodiumChloride

5 (4) +1% Water .03 1 .03 1 .03 1

Acid Tested

Addition Exposure

Period

Specimen Wall

Temperature

Corrosion Rate

hr ºC ºF mm/y mpy

Plant A (None) 48 136 277 .23 9 96 146 295 .10 4 Plant A 1%water 68 132 270 .03 1 92 131 268 .03 1 Plant A 0.5% formic 48 137 278 .03 1 Plant A 1.0% formic 48 141 286 <.03 <1 Plant B (None) 68 149 300 7.80 307 96 152 306 12.55 494 Plant B 1 % water 48 140 284 .03 1

Corrosion Rate

Temperature Annealed Sensitized*Additive ºC ºF mm/y mpy mm/y mpy None 190 374 .20 8 – – 1500 ppm hydrogen peroxide 190 374 .23 9 – – 3000 ppm hydrogen peroxide 190 374 .08 3 – – 3000 ppm H

2O

2 +

1500 ppm Fe+++ (a) 190 374 .69 27 – – 1500 ppm Fe+++ (a) 190 374 .56 22 – – 1500 ppm hydrogen peroxide 240 464 .61 24 .89 35

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.

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.

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.

*650 ºC ( 1202 ºF) for one hour a = Added as FeOH(C2H3O2)2

Page 12

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 <.03 < 1

Type 316 Stainless Steel 153 306 55 .05 2

*Incipient pitting

TABLE XIV

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

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

Page 13

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

<.03 <.03 <.03 <.03 3.30 5.33 5.59 6.35 1.75 6.60 8.64

51

<1 <1 <1 <1

130 210 220 250

69 260 340

20

<.03 <.03 <.03 <.03 <.03 <.03 <.03 <.03 <.03 <.03

33 .25

<1 <1 <1 <1 <1 <1 <1 <1 <1 <1 13 10

<.03 <.03 <.03 <.03 <.03

.05

.08 <.03

.18

.13

.05 2.54

<1 <1 <1 <1 <1

2 3

<1 7 5 2

100

.08

.18

.15

.10

.13

.13

.05

.05 <.03

.18

.18

.08

3 7 6 4 5 5 2 2

<1 7 7 3

.51

.71

.69

.20 1.24 1.12

.79 36

.56

.91 1.14

.36

20 28 27

8 49 44 31 14 22 36 45 14

1.3014.73

>25.40>25.40

1.933.053.683.30

.033.051.735.59

51580

>1000>1000

76120145130

1120

68220

*Metal temperature Reference 10. See that publication for apparatus and technique used.

TABLE XVI

Corrosion with Heat Exchange in Aqueous Acetic Acid Containing Additives

Test Conditions: Apparatus and procedure same as de-scribed in Reference 10. Metal tempera-ture 110 ºC (230 ºF) with bulk liquid tem-perature of 100 ºC (212 ºF). Test periods of 4 to 96 hours used. All results represent duplicate specimens.

Corrosion Rate

Test Medium Acetic Acid Additive

Type 304 Stainless

Steel

Type 310 Stainless

Steel

Type 316 Stainless

Steel

Type 329 Stainless

Steel

CARPENTERalloy 20 Cb-3

HASTELLOY alloy C-276

AMBRALOY* 901

MONEL alloy 400

mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy 56% 1% H

2SO

4 36.07 1420 Nil Nil 64 25 – – – – .23 9

5.84- 15.24

230- 600

Nil- 80

Nil- 30

56% 5% H2SO

4 22.35 880 76.63 3017 5.72 225 17.93 706 – – 36.58 1440 .91 36

Nil-

61.57 Nil-

24 24 25% 4% Formic 28.83 1135 50.8 200 .71 28 Nil Nil Nil Nil Nil Nil – – 1.17 46

Acid

*Trademark of Anaconda American Brass Co. Reference 43

Page 14

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

Page 15

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

Page 16

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* – – <.03 < 1 .91 36 5 99 210 254 10,000 57.15 2250 – – – – – –

10 25 77 – – – – – – <.03 < 1 .53 21 20 25 77 – – .20* 8* – – <.03 < 1 – – 20 100 212 – – – – – – 3.05 120 – – 20 116 241 – – – – – – 4.27 168 – – 25 25 77 – – – – – – – – .58 23 25 104 219 – – – – – – .38 15 – – 25 116 241 – – – – – – .25 10 – – 30 116 241 – – 127 5000 – – <.03 < 1 – – 40 116 241 – – – – – – <.03 < 1 – – 50 25 77 – – .20* 8* – – <.03 < 1 1.96 77 50 116 241 – – – – – – 1.02–7.62 40–300 – – 60 110 230 27.69 1,090 – – – – – – – – 75 25 77 – – – – – – – – 1.68 66 75 65 149 – – 7.62 300 – – 1.02 40 – – 75 116 241 – – – – – – 1.27 50 – – 95 25 77 – – 1.02 40 – – – – – – 95 116 241 – – 16.51 650 – – – – – – 99.9 25 77 – – .76 30 – – – – .53 21 99.9 116 241 – – 12.7 500 1.27–4.86 50–585 6.86 270 – – 99.9 90 194 – – 6.86 270 – – – – – –

100 25 77 – – 1.65 65 – – Nil Nil – – 100 35 95 2.03 80 1.70 67 – – – – – – 100 50 122 – – 1.78–11.18 70–440 .01 0.3 – – – – 100 100 212 – – – – – – 1.27 50 – – 100 116 241 20.07 790 – – – – .64–5.08 25–200 – –

TABLE XVIII

Corrosion of Alloys in Acetic Acid

Data combined from various published articles and private communications. *Rates obtained under quiescent conditions. Removal of the corrosion film will greatly increase the rate of attack.

alloy is particularly critical and must be thoroughly under-stood when appraising the alloy for acid services.

Duplex stainless steels can also exhibit good corrosion resistance in acetic and other organic acid environments. Type 329 stainless steel and cast ACI CD-4MCu are examples. Tables II, III, VIII, XVI and XXVIII show the excellent corrosion resistance evidenced by these alloys in certain specific exposures. These alloys are also more sensitive to changes in environment than are the aus-tenitic stainless steels. However, in the proper application, the alloy can exhibit good stability while providing resis-tance to stress-corrosion cracking. It is for this latter reason that the duplex alloys are sometimes appraised for organic acid use.

are some rare cases where the corrosion resistance of these alloys is no better than Type 316 stainless steel, but usually they provide a higher plateau of corrosion resistance to hot organic acids. The higher cost of these materials requires that their area of use in a process be pinpointed and justified by longer service life.

The cast and wrought alloys of this category are essen-tially the same in chemical resistance although some small difference may be noticed in a specific environment. The cast alloys are exemplified by ACI CN-7M. There are many proprietary alloys of this general type which bear trade names. Quite often the designation ends with the number “20,” and indeed this group of alloys is known to many as the “type 20” alloys. Alloys included in this category are: wrought CARPENTER* alloy 20Cb-3 and cast DURIMET** 20, ALLOYCO*** 20, WORTH-ITE**** and others. E. Iron-Base Nickel-Chromium-Copper-

Molybdenum Alloys When an acetic acid environment is too corrosive for utilization of Types 316 or Type 317 stainless steels, the next group of materials usually considered are the iron-based alloys containing higher percentages of nickel and chromium with molybdenum and copper added. There

* Trademark of Carpenter Technology Corporation ** Trademark of The Duriron Company, Inc. *** Trademark of Aloyco Inc. **** Trademark of Worthington Corporation

Page 17

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 <.03 < 1 <.03 <1

PH15-7Mo (as received)b <.03 < 1 .05 2 03 1 <.03 <1 PH15-7Mo (Al 750) 08 3 <.03 < 1 <.03 < 1 <.03 <1 PH15-7Mo (TH1050) 08 3 30 1 2 71 28 76 30 PH15-7Mo (RH950) 08 3 18 7 56 22 51 20 17-7PH* (as received) 30 12 46 18 28 11 <.03 <1 17-7PH (A1750) 38 15 10 4 28 11 <.03 <1 17-7PH (TH1050) 28 11 03 1 <.03 <1 <.03 <1 17-7PH (RH950) 25 10 05 2 08 3 .05 2 17-4PH* (as received) 25 10 15 6 <.03 <1 <.03 <1 17-4PH (H900) 28 11 03 1 <.03 <1 <.03 <1 17-4PH (H1025) 33 13 25 10 <.03 <1 <.03 <1 17-4PH (H1150) 23 9 05 2 <.03 <1 <.03 <1

a. Average of duplicate specimens for three 48-hour exposure periods in boiling acid. b. Heat Treatment– A = Annealed

T = Transformation near 760 ºC (1400 ºF) H = Hardening between 482-593 ºC (900-1100 ºF) of T or R material R = Refrigerate treated to –73 ºC (–100 ºF)

*Trademark of Armco Steel Corporation

Page 18

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

Page 19

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 <.03 < 1 9.12 359 .23 9 .17 6.5 – – – – Stainless Steel INCOLOY alloy 825 1.70 67 20.24 797 – – .36 14 3.66 144 – – – – – – – – IN alloy 102 (A)2 <.03 < 1 .69 27 – – .15 6 .18 7 – – – – – – – – IN alloy 102 (HT)3 .03 1.1 .71 28 – – .10 4 .18 7 – – – – – – – – INCONEL <.03 < 1 .08 3 – – .03 1 .28 11 – – – – – – – – alloy 625 (A)2

INCONEL <.03 < 1 .08 3 – – .03 1 .28 11 – – – – – – – – alloy 625 (HT)3 HASTELLOY <.03 < 1 .38 15 – – .04 1.5 .28 11 – – – – – – – – alloy C-276 Titanium <.03 < 1 1.83 72 – – – – 6.12 241 <.03 < 1 <.03 < 1 – – – – HASTELLOY alloy D <.03 < 1 – – – – – – – – – – – – .05 2 – – WAUKESHA* No. 23 .76 30 .69 27 – – – – .71 28 – – .15 6 – – 1.45 57 WAUKESHA No. 54 .64 25 1.19 47 – – – – 1.14 45 – – .76 30 – – .61 24 WAUKESHA No. 88 1.12 44 .91 36 – – – – .05 2 – – .28 11 – – .56 22 KROMARC** 55 .18 7 2.46 97 – – – – 4.09 161 .41 1 6 .08 3 – – – – JESSOP JS-700 – – .03 1 – – – – .66 26 – – – – – – – – CARPENTER alloy 204 – – .03 1 – – – – .36 14 – – – – – – – – Multiphase .05 2 <.03 < 1 – – – – – – – – – – – – – – MP35N*** CROLOY***** 16-1 1.47 58 .43 17 5.84 230 – – 10.67 420 – – – – – – – – Chromium Carbide – – .69 27 – – – – 1.50 59 – – – – – – – –

with 12% nickel binder

Page 20

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

Page 21

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

Page 22

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

Page 23

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

Page 24

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** – – <.03 < 1 – – – – – – – – – – – – – – – –

Page 25

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 ket- tle, 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 <.03 <1 Type 317 Stainless Steel – – – – – – – – – – – – – – .03 1 Type 329 Stainless Steel .05 2 <.03 <1 .05 2 – – – – – – – – – – – – CARPENTER alloy 20Cb-3 <.03 <1 <.03 < 1 <.03 <1 <.03 < 1 .05 2 <.03 <1 <.03 <1 – – – – HASTELLOY alloy G – – – – – – <.03 <1 – – – – – – – – – – HASTELLOY alloy C .10 4 <.03 < 1 .10 4 .05 2 <.03 < 1 – – Nil Nil Nil Nil <.03 < 1 HASTELLOY alloy B – – – – – – – – – – – – – – – – 5.59 220 INCONEL alloy 600 .13 5 – – – – – – – – – – – – – – – – IN alloy 102 .05 2 – – – – – – – – – – – – – – – – MONEL alloy 400 – – – – – – – – <.03 <1 – – – – – – 3.56 140 STELLITE alloy No. 3** – – – – – – .10 4 – – – – – – – – – – STELLITE alloy No. 4 – – – – – – .18 7 – – – – – – – – – – STELLITE alloy No. 6 – – – – – – .46 18 – – – – – – – – – – HAYNES alloy No. 93** – – – – – – >2.54 >100 – – – – – – – – – – HAYNES alloy 25 – – – – – – .03 1 – – – – – – – – – – ILLIUM B*** – – – – – – <.03 < 1 – – – – – – – – – – ILLIUM P – – – – – – <.03 <1 – – – – – – – – – ILLIUM PD – – – – – – <.03 <1 – – – – – – – – – ILLIUM 98 – – – – – – .10 4 – – – – – – – – – – DURICHLOR**** – – .08 3 – – – – – – – – – – – – – – Titanium <.03 <1 Nil Nil Nil Nil – – .56 22 – – .69 27 – – – – Zirconium – – Nil Nil – – – – – – – – – – – – – – Copper – – – – – – – – – – .05 2 .10 4 .08 3 – – C70600

(90-10Cupro-Nickel) – – – – – – – – .05 2 .05 2 – – .10 4 – –

* See process diagram Figure 4. ** Trademarks of Cabot Corporation *** Trademark or Stainless Foundry & Engineering, Inc. ****Trademark of The Duriron Company, Inc.

Page 26

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 <.03 <1 – – 260 500 – – .36 14 230 446 – – 71 28 Nickel 200 260 500 – – 5.84 230 Silver 230 446 – – 3.05 120 DURIRON 260 500 – – 2.67 105 Titanium 260 500 – – <.03 < 1 Zirconium 260 500 – – <.03 < 1 Tantalum 260 500 – – <.03 < 1

*Pitting Reference 17

hydrate and 7 grams potassium iodide per100 grams of acetic acid). Carbon monoxideatmosphere.

Page 27

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 Catalyst

Alloy mm/y mpy mm/y, mpy

Type 304 Stainless Steel 41 16 15* 6* Type 321 Stainless Steel .51 20 .25* 10* Type 347 Stainless Steel 1.91 75 .33* 13*

Type 316 Stainless Steel .15 6 .08* 3* 24Cr-20Ni-Mo-Cu** <.03 <1 .03* 1*

CARPENTER alloy 20 – – .23* 9* INCOLOY alloy 825 – – .05 2

INCOLOY alloy 800 – – .13* 5* HASTELLOY alloy C – – .10* 4*

HASTELLOY alloy B – – .20 8 INCONEL alloy 600 – – .38 15

Nickel 200 15 6 .33 13 MONEL alloy 400 <.03 <1 .41 16

C71500 (70-30 Cupro-Nickel) .48 19 .94 37 C70600 (90-10 Cupro-Nickel) .53 21 1.14 45

Aluminum Bronze .18 7 2.34 92 Titanium Nil Nil Nil Nil DURIRON Nil Nil Nil Nil

Page 28

By-product formic acid adds acidity to the system, but does not greatly increase corrosion of the stainless steels. Table VII shows corrosion rates of various alloys in acetic-formic acid mixtures typical of those existing in a hydro-carbon oxidation unit, but without peroxides present. As in other processes for acetic acid production, the peroxides react with other components of the stream or are decom-posed with sufficient time at the higher temperatures. Thus, the cupro-nickels and Alloy 400 can be used after the process stream passes the separation column of such a system, if desired.

Actual corrosion data obtained in the various parts of a hydrocarbon oxidation unit are shown in Table XXVIII. Contamination of the process stream with chlorides, metal salts, or other inorganic materials introduced in the feed streams or by leakage into the system can have disastrous effects as noted under the discussion on the effect of contaminants.

c. Methanol-Carbon Monoxide Synthesis The methanol-carbon monoxide process is one of the newer and economically attractive routes for the produc-tion of acetic acid. In this process, all factors contributing to higher corrosion rates are encountered—a 50-75% acid concentration, higher temperatures, higher pressures and the use of halide salts as catalysts. The use of boron trifluoride as a catalyst did not become popular because of the exceedingly high pressures involved, but the use of iodides in combination with other metallic salts has increased in popularity throughout the world.

No data directly derived from the field exposure of alloys in operating equipment of the methanol-carbon monoxide process are available. However, Togano and others have delineated the problem facing the corrosion engineer when materials selection must be made for these processes.16-18 The reaction vessel must be made of the most resistant alloys available. In all probability, the process stream must be carried through the first two still columns before the halogen is reduced to a level sufficient to allow the use of the austenitic stainless steels. Even at this point, care must be exercised in selecting a stainless steel because the acid is not derived from an oxygenated reaction. Thus, no peroxides or oxidizing gases from their decomposition will be available to aid in passivity of the stainless steel.

Certain of the higher nickel alloys do appear to have promise in this process. Tables XXIX and XXX taken from the Tagano reports show HASTELLOY alloys B and B-2 to be worthy of thorough testing along with titanium, zirconium and tantalum for the high pressure, high temperature reaction area. Once the temperature is re-duced to the normal recovery conditions, the use of nickel-molybdenum and nickel-copper alloys appears at-tractive even with the catalyst salts present. It also appears that INCOLOY alloy 825 should be evaluated with close attention to make sure that the resistance to pitting shown in Table XXX is consistent. As indicated, once the halide salts are removed, the conventional materials used for the separation and recovery of the acid can be employed.

Certain other interesting observations can be found in references 17 and 18. The catalyst system requires very high concentrations of catalyst. When half of the catalyst is a halide salt, the potential for corrosion is greatly increased. (See Effect of Contaminants.) By comparison with various other tests, particularly when appraising the austenitic stainless steels, it is apparent that the iodide ion is not as aggressive as is the chloride ion.

The authors found no adverse effect of carbon monox-ide on the nickel-base alloys at the temperatures and pressures explored. Indeed, the presence of CO was reported to reduce corrosion, particularly pitting of iron- base alloys.

2. Acetic Acid Storage and Shipping Stainless steels are used for the construction of storage vessels for acetic acid to maintain the highest quality of the acid. Vessels of the dished-head type, API variety, or external support construction have been used for this purpose. The latter has been used extensively in the wine fields for the storage of wine in the past and provides the most economical method of fabricating field tanks for acetic acid storage if the size is not excessive.

When choosing the stainless steel grade, consideration should be given to the temperature of storage proposed and to the grade of acid required. In the northern latitudes, it will be necessary to provide a heating coil to assure fluid

Of the 15 miles of pipe used in this storage terminal for handling organics, 3 miles are of Type 316 stainless steel. This material protects the purity of formaldehyde, acetic acid and propionic acid.

Page 29

conditions for the acid (m.p. 16 ºC or 61 ºF). For this service, the coil is usually constructed of Type 316L stainless steel. Heat transfer on the surface of Type 304 stainless steel can produce excessive corrosion at normal steam temperatures.

The vessel proper can be constructed of Types 304, 304L, 321, 347, 316, or 316L stainless steels. If a truly meticulous grade of acetic acid is required, such as USP grade, it will be necessary to use the molybdenum-containing grade of stainless steel if temperatures are to exceed 50-60 ºC (122-140 ºF). In this range, a minute amount of metallic contamination of acetic acid can occur in contact with the Type 304 analysis or other grades not containing molybdenum. The use of a molybdenum-containing stainless steel moves this point of initial contamination to some 70-80 ºC (158-176 ºF) before any detectable metallic ion is picked up by the acids.5

When using the stainless steels for tankage equipment in a meticulous service, it is advisable to clean (pickle, passivate) the interior of the vessel to remove all traces of iron contamination that might have been embedded in the stainless steel at the mill. Sulfamic acid, nitric acid, oxalic acid, or other acids as suggested by ASTM A 380 can re-move the embedded iron and at the same time provide a uniform clean surface for the stainless steel. Since the more aggressive acids can cause intergranular corrosion of sensitized stainless steels, it appears prudent to utilize a low carbon or stabilized grade if fabrication by welding is anticipated.

Shipping containers constructed of stainless steel provide the greatest durability combined with the best preservation of the refined acetic acid of all materials available today. Tankers have stainless steel-lined com-partments for shipment of the organic acids and other corrosive products. If entire compartments are not justified on tankers for conveying the acetic acid, deck tanks can be added which are durable and free of harmful corrosion in the severe exposures of marine transportation. Complete barges have been constructed using stainless steel for meticulous care of the product during shipment. Tank cars constructed of stainless steel have been used for 40 years on the rails for acid shipments as well as to provide the versatility required for the shipments of other aggressive commodities.

In the smaller containers, the austenitic stainless steels remain unparalleled as the material of construction for drums, cans and other items used for the shipment of acetic acid. With the higher area-to-volume ratios existing in the small containers, it is imperative that no corrosion occur on the container walls to contaminate the acid. For this reason, the stainless steels constitute standard con-struction identified as ES and ESM (DOT designation for Types 304 and 316 stainless steels) drums and cans for acetic acid shipments. These not only provide good protection of the acid during long periods of shipment and storage, but are durable and reusable for many years because of the good strength and external corrosion resistance of the stainless steel container. Stainless steel containers are readily cleaned on the inside to provide a spotless, uncontaminated surface for reuse.

The forward tanks in this double-skinned barge are clad with Type 316 stainless steel, capable of transporting organic acids as well as other liquid cargo.

3. Vinegar Production and Storage All vinegars contain acetic acid in addition to variable amounts of nonvolatile organic acids such as malic and citric acids and smaller amounts of succinic and lactic acids. The term “grain strength” is used to express the acetic acid concentration, which is ten times the acetic acid content. Protection of the vinegar from metal contamination, particularly iron and copper, has led to the use of the austenitic stainless steels for both production and storage. Table XXXI shows that both Types 304 and 316 stainless steels are unaffected in 40-320 grain vinegar at the -17 to 35 ºC (2 to 95 ºF) process temperatures involved. Because of the low temperatures, intergranular corrosion of sensitized stainless steel is not a problem. Although 120 grain (12 per cent acetic acid) is a common strength, the particular plant at which this test was run produces up to 300 grain vinegar in Type 304 stainless steel equipment and piping without corrosion problems or product contamination.

P. Acetic Anhydride

Acetic anhydride has long been made as a co-product in the “dual” oxidation of acetaldehyde to acetic acid. With the many streams in this process containing both acetic acid and the anhydride, it is important to understand the effect of small amounts of anhydride residual in hot acid streams. However, the anhydride itself is only mildly corrosive. In the absence of acetic acid, distillation in Type 304 stainless steel equipment is acceptable.

In the newer process where acetic acid or acetone is cracked to ketene, which is then reacted with acetic acid to form the anhydride, it is reported that Type 316 stainless steel is fully satisfactory for the construction of distillation columns and other process equipment. However, Type 316L stainless steel is often selected so that the equipment is more versatile and can be used for other organic acid services where more stringent conditions might exist.

In the process utilizing acetic acid as the starting material, the cracking tubes are of prime interest.23 Once

Page 30

TABLE XXXI

Corrosion of Stainless Steels in Vinegar Production

Corrosion Rate

Test 1 Test 2 Test 3 Test 4

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

Type 304 Stainless Steel Nil* Nil Nil Nil Nil Nil Nil Nil Type 316 Stainless Steel Nil Nil Nil Nil Nil Nil Nil Nil Type 304 Stainless Steel Nil Nil Nil Nil Nil Nil Nil Nil

sensitized for 1 hour at 677 ºC (1250 ºF)

Location

Test 1 Storage

Tank

Test 2

Accumulator

Test 3 Freezer

Test 4 Storage

Tank

Vinegar Concentration 40-66 80-122 50-300 80-320 (Grain)

Temperature Range ºC 21-35 27-34 –17 to –2 0-16 ºF 70-95 80-93 2-28 32-60

Test Duration all tests 150 days

Average Penetration

Initial Exposure

200 hours Second Exposure

100 hours

Metal mm/y mpy mm/y mpy

Type 430 Stainless Steel .48 19 1.22 48

Type 446 Stainless Steel – – .91 36

ACI HK Alloy .18 7 .41 16

the acetic acid is fully vaporized, the stream is essentially innocuous from a corrosion standpoint. However, proper-ties of the cracking tube alloy are significant. The catalytic properties of nickel can cause breakdown of hydrocarbons at high temperatures. For this reason, various nickel-free alloys have been developed (Fe-Cr, Fe-Cr-Al, Fe-Cr-Al-Si) for use in such services. However, the operating temperatures of 700-750 ºC (1292-1382 ºF) can develop sigma and other adverse metallurgical conditions in the iron-chromium alloys. To make furnace operations less critical and to obtain improved fabricability, the use of cast austenitic alloys was explored. It was found that the inner surface of the tubing was rapidly coated with a deposit of carbon which sealed the process stream from catalytic effects conferred by the metal surface. Thus, advantage can be taken of the better ductility and fabricability of the austenitic alloys for such service. Rates of degradation of the various alloys caused by the oxidation reactions that occur in the environment are shown in Table XXXII.

Handling of the acetic anhydride and its dilution with acetic acid presents no problem other than that described under the processing of acetic acid. Type 304 stainless steel is eminently satisfactory for the distillation, storage, or shipment of the anhydride. Nickel plating showed a nil rate of attack in acetic anhydride at ambient temperature during a 121-day test.

Type 304L stainless steel equipment and piping and ACI CF-8M valves for metering acetic anhydride to process kettles. Courtesy Walworth Company-Aloyco Valves.

TABLE XXXII

Deterioration of Alloys in Glacial Acetic Acid Vapors at 750 C (1382 F)

*No detectable attack in the form of general corrosion, pitting or crevice corrosion.

Page 31

PART III. OTHER ORGANIC ACIDS

A. Formic Acid

As with other one-carbon homologues of an organic family, formic acid exhibits unique properties. The acid is more highly ionized than are most other members of the group and reacts readily with many oxidizing and reducing compounds. This potent reactivity is apparent also in the reaction with metals. Formic acid is the most aggressive of all organic acids containing only one carboxyl group. This fact and the singular properties of the molecule require that thorough testing of materials be conducted in any medium known to contain the acid.

Comments regarding corrosion by formic acid were introduced in the section on acetic acid, inasmuch as many commerical processes today for producing acetic acid also contain formic acid. A review of Tables 11, IV and XXI, among others, will show the more aggressive character of process streams containing formic acid. In general, the same materials of construction suitable for handling acetic acid can be used for the higher concentrations of formic acid. The corrosion of a specific alloy will be slightly greater when exposed to formic acid at the same tempera-ture.

The major area for concern relates to concentrations of aqueous formic acid between 50 and 90 per cent. In this zone, the corrosion rate for Type 316 stainless steel varies greatly and can be higher than desirable for commercial applications. The variable test data reported probably relate to the period of passivity of the stainless steel during the test, because the presence of the water would tend to extend the life of passive films on the alloy surface. Some of the more consistent laboratory data, which agree well with field experience, are shown in Table XXXIII. Note the aggressive attack on the Type 316 stainless steel until formic acid concentrations of about 90 per cent are encountered. High rates of attack are experienced where

drippage of a formic acid-water azeotrope impinges on the metal surface.

Table XXXIV compares the corrosion of Type 316 stainless steel with a number of other alloys in a closely controlled laboratory test. Some anomalies are apparent, but in general the data reflect corrosion rates to be expected in equipment handling boiling formic acid of the concentrations shown. As in the case of acetic acid, copper and the cupro-nickel alloys are useful for such service in the absence of oxygen or other oxidants. The addition of nickel to the copper makes the resulting alloy somewhat less sensitive to the presence of oxidants.

Data for a much wider range of alloys in aerated and unaerated acid are provided in Table XXXV The presence of air in the test medium has the effect anticipated by decreasing the rate of attack on those alloys forming protective oxide films and increasing the corrosion of copper alloys, nickel and MONEL alloy 400. Note that alloying a stainless steel with higher amounts of chromium and nickel does not improve the resistance of the alloy (Type 310 vs. Type 304 stainless steels), but the addition of molybdenum produces a much more corrosion resistant alloy (Types 316 and 317 stainless steels).

The data of Table XXXV also provide an interesting illustration of the importance of testing techniques in providing meaningful information. For example, the ma-jority of corrosion rates for specimens of Types 316 and 317 stainless steels and CARPENTER alloy 20 show greater attack in the vapor exposure than when the specimens were fully immersed. This phenomenon would be unrecognized if only the usual immersion test were used. Yet a distillation column will have both liquid and vapor exposures which must be analyzed before selecting a material of construction, and the data obtained from the vapor exposures in these tests suggest further avenues of exploration before making a final decision.

TABLE XXXIII

Corrosion of Type 316 Stainless Steel in Boiling Formic Acid Solutions

Test Conditions: Specimens exposed in liquid of boiling, aqueous formic acid solutions under an-aerobic conditions for 72 hours.

Corrosion Rate

Liquid Vapor Condensate* Concentration of Formic Acid, % mm/y mpy mm/y mpy mm/y mpy

50 .38 15 .41 16 .46 18 70 .33 13 .48 19 .89 35 78 .36 14 .51 20 .38 15 90 .15 6 .46 18 .61 24 97 .15 6 .13 5 .25 10 100 .11 4 .08 3 .25 10

*Condesate falling on one side of vapor area specimen.

Page 32

TABLE XXXIV

Corrosion of Five Alloys in Boiling Aqueous Formic Acid Solutions

Test Conditions: Average rate of duplicate specimens ex-posed in boiling 100-107 ºC (212-223 ºF) solutions for 96 hours except as noted. No aeration or deaeration.

*Test solution changed each 24 hours of the 96-hour test. **Test discontinued after 48 hours because of concentration of corrosion salts in solution.

Corrosion Rate Formic Acid

Type 304 Stainless Steel

Test A Test B*

Type 316 Stainless Steel

Test A Test B*

Copper

C70600 (90-10

Cupro-Nickel)

Titanium

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

1 5

10 20 40 50 60 70 80 90

.18

.79 1.34 1.93 3.45 4.24** 3.45** 4.04** 4.29** 3.28**

7 31 53 76

136 167** 136** 159** 169** 129**

.36 1.07 1.52 1.75 2.39 2.11 2.11 2.31 2.13 2.11

14 42 60 69 94 83 83 91 84 83

.08

.05 25

.28

.10

.51**

.46**

.48**

.48**

.28**

3 2

10 11

4 20** 18** 19** 19** 11**

<.03 .20 .20 .20 .25 .28 .23 .25 .25 .28

< 1 8 8 8

10 11

9 10 10 11

.03

.03

.03

.20

.12

.25

.05

.76

.20

.23

1 1 1 8 5

10 2

30 8 9

.03 03

.03 41

.33

.53

.03

.71 13

.18

1 1 1

16 13 21

1 28

5 7

– – 13

2.41 –

3.05 – – –

<.03

– – 5

95 –

120 – – –

<1

TABLE XXXV

Corrosion of Alloys in Boiling Formic Acid Solutions

Test Conditions: Laboratory test results averaged from three separate 48-hour test periods in most cases. Tests conducted with and without aeration in acid concentrations noted.

Corrosion Rate

10% 50% 90% 99%

Unaerated* Unaerated* Aerated Unaerated* Aerated Unaerated

Liquid Vapor Liquid Vapor Liquid Vapor Liquid Vapor Liquid Vapor Liquid Vapor

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 mm/y mpy mm/y mpy

Mild steel – – – – – – – – – – – – 24.13 950 – – – – – – – – – –

Type 430 stainless steel

– – – – – – – – – – – – 11.26 444 .89 35 – – – – – – – –

Type 304 stainless steel

– – – – – – – – – – – – 10.41 410 1.52 60 – – – – – – – –

Type 310 stainless steel

– – – – – – – – – – – – 10.21 402 .61 24 – – – – – – – –

Type 316 stainless steel

.18 7 .23 9 .23 9 .53 21 <.03 <1 74 29 .36 14 .18 7 < 03 < 1 .79 31 .13 5 .10 4

Type 317 stainless steel

– – – – – – – – – – – – .13 5 .13 – – – – – – – – –

CARPENTER alloy 20

– – – – – – – – <.03 <1 30 12 .05 2 – – 03 <1 .66 26 – – – –

HASTELLOY alloy C

– – – – – – – – .10 4 03 1 .05 2 – – 10 4 <.03 <1 – – – –

HASTELLOY alloy B

– – – – – – – – .51 20 10 4 .08 3 – – 08 3 .03 1 – – – –

INCONEL alloy 600

– – – – – – – – – – – – .76 30 .64 25 1.24 45 .20 8 – – – –

MONEL alloy 400

<.03 <.1 .08 3 .23 9 .08 3 – – – – .03 1 .03 1 7.62 300 .23 9 – – – –

Nickel 200 .15 6 .15 6 .36 14 .43 17 .84 33 2.24 88 .61 24 .28 11 69 27 .41 16 – – – –

Copper .08 3 .18 7 .13 5 .28 11 – – – – .25 10 .15 6 14.30 563 37.80 93 – – – –

EVERDUR 1010 – – – – – – – – – – – – .18 7 .23 9 3.30 130 .69 27 – – – –

Aluminum 3003 31.09 1224 21.89 862 – – – – 31.70 1248 10.16 400 7.62 300 – – 10 4 <.03 <1 – – – –

Titanium .13 5 – – 2.92 115 – – – – – – <.03 <1 – – – – – – – – – –

.51 20 – – – – – – – – – – – – – – – – – – – – – – Chromium carbide with 12% nickel binder

*Boiling solution without sparging of any gas.

Page 33

Table XXXVI demonstrates the excellent corrosion resistance of HASTELLOY alloy C-276 in formic acid solutions. Throughout the entire range of temperatures and concentrations of formic acid, the nickel-base molyb-denum-chromium-iron alloy exhibits good stability. However, in formic acid exposures, more than in acetic acid exposures, the HASTELLOY alloys B and B-2 materials must be given consideration as materials of construction. Other nickel and cobalt-base alloys can be useful for specific field applications when the metallurgi-cal properties of these alloys are required.

Applications involving heating are more demanding than isothermal exposures for an alloy. (See comments in the section on Acetic Acid—Effect of Temperature.) Calandria or vaporizer tubes require construction with a corrosion-resistant alloy. Table XXXVII provides data for six alloys tested under heat transfer conditions. The rate of attack on the austenitic stainless steel alloys increases sharply with a higher metal temperature under heat transfer conditions. CARPENTER alloy 20Cb-3 and HASTELLOY alloy B show rates of attack sufficiently low to warrant their selection under most of these conditions. Unfortunately, data for HASTELLOY alloy C-276 and INCONEL alloy 625 are not included, but they would be expected to be as good or better than that shown for the “B” alloy.

Many commercial applications require the use of the

alloys at temperatures above those obtained at one at-mosphere of pressure. Consequently, the effect of increasing the temperature on the corrosion rate of the common alloys must be determined. Table XXXVIII presents a composite of the data contained in the report of Miller and Wachter on corrosion by acids at high temperatures.24 Of greatest interest is the information for the Type 316 stainless steel. The rates are higher for this alloy than would be expected for a test of longer duration. The important inference to be made is that the rate of attack approximately doubles for each 15°C (27°F) increase in temperature. (It should be recognized that this is a very rough approximation that does not always hold true.) Corrosion tests in many other media show a similar relationship rather than one conforming to the ideal Arrhenius equation.

Figure 5 and isocorrosion charts (Figures 6 through 11), originally published by the NACE,25 indicate the corrosion behavior of several alloys in formic acid. Isocorrosion charts are intended only as guides; there are conditions where higher or lower rates can prevail. In fact, Figure 5 shows much lower rates for Type 316 stainless steel in boiling formic acid than is shown in the isocorrosion chart, Figure 6. It is believed that Figure 5 is more representative of pure formic acid and that the higher rates shown by the isocorrosion chart must reflect the presence of unidentified impurities.

TABLE XXXVI

Corrosion of HASTELLOYS and Related Alloys in Formic Acid Solutions

Laboratory data obtained without aeration or deaeration using five 24-hour test periods. (Courtesy of the Cabot Corporation, Stellite Division.)

Corrosion Rate

% Temperature HASTELLOY HASTELLOY HAYNES MULTIMET

Formic Acid ºC ºF alloy B alloy C-276 alloy No. 25 alloy

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

10 26 78.8 .03 1 <.03 <1 Nil Nil Nil Nil 66 150 .23 9 <.03 <1 Nil Nil Nil Nil Boiling .08 3 .13 5 .20 8 .10 4

20 26 78.8 .05 2 < .03 <1 <.03 <1 <.03 <1 66 150 .25 10 <.03 <1 <.03 <1 <.03 <1 Boiling .10 4 .18 7 .25 10 .15 6

30 26 78.8 08 3 <.03 <1 – – – – 66 150 .30 1 2 <.03 <1 – – – – Boiling .08 3 .20 8 – – – –

40 26 78.8 .08 3 <.03 <1 <.03 <1 <.03 <1 66 150 .28 11 <.03 <1 Nil Nil Nil Nil Boiling .05 2 .13 5 .38 15 .20 8

60 26 78.8 .05 2 < .03 <1 <.03 <1 <.03 <1 66 150 .25 10 <.03 <1 Nil Nil <.03 <1 Boiling .03 1 .18 7 .51 20 .15 6

90 26 78.8 <.03 <1 <.03 <1 <.03 <1 Nil Nil 66 150 .03 1 <.03 <1 <.03 <1 Nil Nil Boiling <.03 <1 .05 2 .15 6 .08 3

Page 34

Temperature Corrosion Rate

Formic Acid Test Medium

Without Heat Transfer

With Heat Transfer*

Type 304 Stainless Steel

Type 316 Stainless Steel

CARPENTER alloy 20Cb-3

HASTELLOY alloy B

INCONEL alloy 600

MONEL alloy 400

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

10% aqueous 101 214 – – 18.85 742 .25 10 <.03 <1 .25 10 .89 35 3.38 133 – – 110 230 >25.4 >1000 .41 16 .05 2 1.27 50 1.85 73 16.26 640 – – 125 257 14.48 570 1.57 62 .20 8 1.42 56 1.68 66 >25.4 >1000 – – 140 284 14.48 570 1.85 73 .69 27 2.03 80 1.52 60 >25.4 >1000

50% aqueous 103 217 – – >25.4 >1000 .99 39 .15 6 .28 11 1.55 61 2.21 87 – – 110 230 >25.4 >1000 1.47 58 .23 9 .23 9 1.93 76 1.93 76 – – 125 257 >25.4 >1000 1.52 60 .33 13 .13 5 3.30 130 1.22 48 – – 140 284 >25.4 >1000 2.13 84 .31 12 113 5 2.92 115 2.54 100

89% aqueous 103 217 – – 18.19 716 .25 10 .13 5 <.03 <1 1.02 40 .03 1 – – 110 230 13.72 540 1.22 48 .10 4 .13 5 1.27 50 .56 22 – – 125 257 12.7 500 1.02 40 .15 6 .08 3 1.93 76 .84 33 – – 140 284 13.21 520 1.22 48 .25 10 .18 7 1.42 56 1.27 50

% Formic Acid

Test Period, days 1 1

2* 27

4.6 1

24 1

Test Temperature Corrosion Rate

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

Type 410 stainless steel 170 338 – – – – 16.26 640 – –Type 430 stainless steel 170 338 – – – – 10.41 410 – – Type 446 stainless steel 170 338 – – – – .66 26 – – Type 304 stainless steel 150 302 – – .46 18 – – – – 170 338 – – – – 3.56 140 – – Type 310 stainless steel 170 338 – – – – 2.36 93 – – Type 316 stainless steel 100 212 – – – – .58 23 1.04 41 150 302 .13 5 .10 4 1.52 60 3.05 120 170 338 .20 8 – – 1.83 72 – – 200 392 .89 35 – – 1.68 66 3.30 130 Type 317 stainless steel 150 302 – – .05 2 – – – – 170 338 – – – – 1.60 63 – – HASTELLOY alloy C 170 338 – – – – .10 4 – – HASTELLOY alloy B 170 338 – – – – .23 9 – – INCONEL alloy 600 150 302 – – .08 3 – – – – 170 338 – – – – 3.30 130 – – Nickel 200 150 302 – – .03 1 – – – – 170 338 – – – – 2.34 92 – – MONEL alloy 400 150 302 – – .03 1 – – – – 170 338 – – – – .89 35 – – C71500 (70-30 Cupro-nickel) 150 302 – – .03 1 – – – – Copper 150 302 – – .03 1 – – – – 170 338 – – – – .20 8 – – Silver 170 338 – – – – .15 6 – – Aluminum 1100 170 338 – – – – 10.16 400 – – DURIRON 170 338 – – – – 7.37 290 – –

Process facilities for handling formic acid are normally constructed of Type 316L stainless steel, copper, or the cupro-nickels. Data obtained by the exposure of alloys in a formic acid distillation column are shown in Table XXVIII. Other data generated by the testing of alloys in a 90% formic acid still are contained in the reference NACE report.25 Depending on the acid concentration, the type of

contaminants present in the acid, the temperature of the system and the type of cooling water used, it is not unique to find a distillation column and accessories to be con-structed of a combination of Type 316L stainless steel, C70600 (90-10 cupro-nickel), ACI CN-7M castings and HASTELLOY alloy C-276. Other materials combinations are obviously possible from a perusual of the data, but

TABLE XXXVII

Corrosion by Formic Acid Under Heat Transfer Conditions

*Metal temperature. Reference 10. See that publication for apparatus and technique used

TABLE XXXVIII

Corrosion of Alloys in Formic Acid at High Temperatures

(Tests conducted in sealed pressure tubes)

*Also contained 1.5% formaldehyde. Reference 24

Page 35

extensive testing of the candidate alloys must be conducted beforehand to assure an adequate economic life of the equipment.

When appraising stream compositions for corrosion testing or the designation of materials of construction, it is important to understand the unstable nature of the one-carbon compounds. Formaldehyde reacts readily with oxygen to produce the acid, and it is difficult to handle and store the aldehyde without generating sufficient formic acid to make a corrosive agent out of what would otherwise be a rather innocuous compound. It is for this reason that Type 304 stainless steel is often selected as the material of construction for formaldehyde storage tanks. Not only does the use of the stainless steel provide a trouble-free material of construction, but the lack of contamination of the aldehyde maintains good color in the solution and reduces the rate of oxidation of the product to additional acid. A brief summary of proper formaldehyde storage is provided by Teeple in reference 26.

Formate esters are also most unstable. The methyl ester is often encountered in process streams, and, when any water is present, must be considered as contributing to a significant acidity in the medium.

FIG 5–Comparison of Types 304 and 316 Stainless Steels in Various Concentrations of Boiling Formic Acid

FIG 9–Isocorrosion Chart for HASTELLOY alloy B in Formic Acid

FIG 10–Isocorrosion Chart for HASTELLOY alloy C (C-276) in Formic AcidFIG 6–Isocorrosion Chart for Type 316 Stainless Steel in Formic

Acid

FIG 8–Isocorrosion Chart for Wrought “20 Type” Alloy in Formic Acid

FIG 7–Isocorrosion Chart for Type 304 Stainless Steel in Formic Acid

Page 36

B. Acrylic Acid

Acrylic acid is the most common 3-carbon acid encoun-tered in industry. The great reactivity of this unsaturated acid makes the material and its esters useful in the preparation of a wide variety of resinous products used in manufacturing plastics, paints, textiles, paper and pol-ishes. There are probably some one billion pounds of the esters produced in the USA today, of which 75% is ethyl acrylate.

The significant fact about handling acrylic acid is that temperatures are maintained as low as possible to prevent homopolymerization of the acid. Distillation in vacuum stills, dilution with innocuous solvents, storage of the product at the lowest convenient temperatures and reaction of the acid in polymerization processes at low temperatures are common process conditions. Consequently, exposure conditions in most acrylic acid applications are less severe than in the saturated acid processes.

As with propionic acid, the acrylic acid can be consid-ered as equivalent to acetic acid in aggressiveness at a given temperature. However, the contaminants in acrylic acid process streams can be different from those found in

acetic acid streams and the contaminant may control the corrosion rate.

Acrylic acid per se is not required as an end product in large quantities as is the ester. Consequently, many of the commercial processes are designed to prepare the ester from a basic organic molecule without isolating the acrylic acid. Regardless of the route to the final product, however, all processes produce the acid as an intermediate with subsequent esterification.

Acrylic aid, or the acid-ester in one sequence, has been produced by at least nine different processes. Three basic reactions have been used predominantly.27 These are the acetylene-carbon monoxide, the nitrile and the propylene oxidation processes. Today, the direct oxidation of pro-pylene to acrolein and finally to the acid in a one or two-step process is the most popular.

The acetylene-carbon monoxide process relies on the catalytic activity of nickel carbonyl in the presence of a strong acid (hydrochloric) to prepare the acid. Obviously, the presence of the HCl controls the corrosive conditions existing in the process. Even at the low reaction tempera-tures, 30-52 ºC (86-126 ºF), the reaction step is most corrosive and is conducted in glass, ceramics and TEFLON* equipment. Following this step, the nickel- base molybdenum alloys can be used, and as the mineral acid is removed, the alloy content of the materials of construction can be reduced until Type 316 stainless steel is acceptable to handle the acrylic acid.

The nitrile procedure for production of the acid suffers the same drawback from a materials standpoint with sulfuric acid used to produce an organic sulfate which can be released to the acid or directly reacted to form the ester. Ammonium acid sulfate is formed as a by-product, and the higher process temperatures, 150 ºC (300 ºF) region, generate SO2 and SO3 which must be contended with in equipment design. The problems encountered are essen-tially the same whether using acrylonitrile or ethylene oxide and hydrogen cyanide as the starting materials.

Table XXXIX provides typical corrosion data for a nitrile type process operation through the distillation of the crude acid. Sets of data are given for two exposures in the same equipment (top head of reactor condenser) to show the wide variation in corrosion rates experienced during different periods of operation. Such large changes in the corrosive environment may be found wherever a mineral acid is mixed with an organic acid. When using metallic materials of construction in such processes, the operation must be conducted with particular care to maintain favorable conditions for a maximum life of the equipment. A few hours of adverse operating conditions can severely damage equipment under such circum-stances. Monitoring of the corrosion by continuous or sequential testing is also advised to detect periods of unusual corrosion. This process is obviously most corro-sive until the decomposition products of the inorganic acid and salts are removed. The usual materials, primarily the austenitic stainless steels, are then used to process the acrylic acid.

This HASTELLOY alloy C-276 tube bundle is used in the reboiler in the manufacture of acrylic monomers. It was found to be the most economical material of construction for this severely corrosive service. Courtesy Stellite Division, Cabot Corporation.

*Trademark of E.I. duPont de Nemours & Co.

FIG 11–Isocorrosion Chart for MONEL alloy 400 in Formic Acid

Page 37

TABLE XXXIX

Corrosion of Alloys in a Nitrile-Type Acrylic Acid Process

*1 –Reactor liquid for 1600 hours at ca. 125 ºC (257 ºF). 2 –Reactor vapor line to condenser for 190 hours at ca. 130 ºC (266 ºF). 3A –Reactor vapor condenser top head for 1200 hours at ca. 130 ºC (266 ºF). 3B –Same as 3A during different period for 2600 hours. 4 –Condensate from exposure No. 3 at ca. 30 ºC (86 ºF) for 190 hours. 5 –Bottom of extractor for 1600 hours at ca. 30 ºC (86 ºF). 6 –Top of extractor for 1600 hours at ca. 30 ºC (86 ºF). 7 –Base of crude acid stripping still for 4000 hours at 110 ºC (230 ºF). 8 –Overhead liquid-vapor from exposure No. 7 (feed to acrylic acid refining still) for 3 days at 88 ºC (190 ºF) (air present).

TABLE XL

Corrosion of Alloys in Propylene Oxidation Process for Acrylic Acid

*1–Propylene oxidation converter at 400 ºC (752 ºF) for 180 days. 2–Quench line from converter at 110 ºC (230 ºF) for 300 days. 3–Scrubber circulating line at 85 ºC (185 ºF) for 31 days. 4–Acrolein stripper base at 140 ºC (284 ºF) for 300 days. 5–Crude acrolein separator pot at 25 ºC (77 ºF) for 180 days. 6–Acrolein refining still base at 105 ºC (221 ºF) for 20 days.

7–Acrolein oxidation converter at 220 ºC (428 ºF) for 220 days. 8–Oxidation reactor quench line at 80 ºC (175 ºF) for 220 days. 9–Solvent extraction column overhead at 42 ºC (108 ºF) for 8 days. 10–Water layer from solvent column separator at 35 ºC (95 ºF) for 330 days. 11–Solvent recovery column base at 105 ºC (221 ºF) for 110 days. 12–Recovery column for No. 10 above at 95 ºC (203 ºF) for 360 days.

Corrosion Rate

Exposure* 1 2 3 4 5 6 7 8 9 10 11 12 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 mm/y mpy mm/y mpyType 304 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 Stainless Steel Type 316 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 Stainless Steel CARPENTER <.03 <1 – – – – – – – – <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 alloy 20 HASTELLOY – – – – – <.03 <1 – – <.03 <1 – – – – <.03 <1 <.03 <1 alloy C INCONEL – – – – – – – – – – <.03 <1 – – <.03 <1 – – – – <.03 <1 – – alloy 625 INCONEL – – <.03 <1 <.03 <1 <.03 <1 – – <.03 <1 – – <.03 <1 – – – – <.03 <1 – – alloy 600 MONEL – – .25 10 .25 10 .03 1 – – – – – – – – – – – – .13 5 .48 19 alloy 400 Copper – – .97 38 .81 32 .20 8 .05 2 – – – – – – .13 5 .08 3 .53 21 .91 36 Nickel 200 – – .28 11 <.03 <1 .03 1 .05 2 – – – – – – – – – – – – – –

Corrosion Rate

Exposure* 1 2 3A 38 4 5 6 7 8

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

>7.62 >300 2.24 88 10.16 > 400 – – .03 1 <.03 <1 08 3 03 1 <.03 <1

Type 316 Stainless Steel

>7.62 >300 2.11 83 10.16 400 – – Nil Nil <.03 <1 <.03 <1 <.03 <1 <.03 <1

Type 201 Stainless Steel

– – 3.00 118 – – – – Nil Nil Nil Nil 13 5 – – <.03 <1

CARPENTER alloy 20

>7.62 >300 64 25 4.98 196 – – Nil Nil <.03 <1 Nil Nil – – <.03 <1

INCONEL alloy 600

>7.62 > 300 – – 4.57 180 – – – – 03 1 20 8 <.03 <1 – –

INCONEL alloy 625

– – – – – – .89 35 – – – – – – <.03 <1 – –

HASTELLOY alloy C

7.11 280 – – 2.95 116 74 29 – – – – – – <.03 <1 – –

HASTELLOY alloy B

1.35 53 – – 74 29 25 10 – – – – – – – – – –

MONEL alloy 400

>7.62 >300 2.54 100 2.64 104 – – .05 2 03 1 15 6 .20 8 > 7.62 > 300

C70600 (90-10 Cupro-nickel)

– – – – – – 1.14 45 – – – – – – – – – –

Copper 6.09 240 6.35 250 2.95 116 1.17 46 .05 2 05 2 23 9 76 30 >7.62 > 300 Lead, chemical

>7.62 >300 – – 5.28 208 – – – – 25 1 0 30 12 > 5.08 > 200 – –

Aluminum 3003 >7.62 >300 – – 10. 16 > 400 – – – – 38 1 5 33 13 > 7.62 > 300 – – Titanium .86 34 – – 1.35 53 13 5 .03 1 – – <.03 <1 – – <.03 <1 Zirconium .03 1 – – <.03 <1 <.03 <1 – – – – – – – – – – Tantalum – – – – – – <.03 <1 – – – – – – – – – –

Page 38

TABLE XLI

Corrosion of Stainless Steels During the Preparation of β-Methacrylic Acid

Corrosion Rate

Exposure Type 304

Stainless Steel Type 316

Stainless Steel CARPENTER

alloy 20 HASTELLOY

alloy C HASTELLOY

alloy B Silicon Bronze DURIRON

Aluminum 3003

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

Oxidation of the acid by .03 1 .03 1 <.03 <1 <.03 < 1 – – – – – – – – air blowing of the

aldehyde at 40 ºC (104 ºF) Distillation of the crude <.03 <1 <.03 <1 <.03 <1 <.03 < 1 .03 1 .05 2 Nil Nil .51 20

oxidation product at ca. 55 ºC (131 ºF) (liquid

exposure) As above (vapor exposure) <.03 <1 <.03 <1 <.03 <1 <.03 <1 .38 15 1.88 74 <.03 <1 .23 9

TABLE XLII

Corrosion of Alloys in Dimethyl Acrylic (Seneceoic) Acid

Field test obtained by exposure of alloys in the overhead stream of a refining column at 145ºC (293ºF) for three days.

Corrosion Rate

Alloy mm/y mpy

Type 304 Stainless Steel 1.83 72 Type 316 Stainless Steel (annealed) .18 7

Type 316 Stainless Steel (sensitized) .18 7 HASTELLOY alloy C <.03 < 1

HASTELLOY alloy B .05 2 MONEL alloy 400 .46 18

Copper 1.45 57

pylene-acrolein process. The oxidation product of the ß -methacrylic aldehyde would contain some oxidized de-composition compounds such as formic and acetic acids. However, the reasonable low temperatures at which the product must be handled, combined with the low con-centration of such contaminants, does not produce an aggressive medium for the stainless steels. The data of Table XLI show results obtained in oxidation and primary distillation steps of the process.

Further information relating to the acrylic acids is contained in Table XLII. In processing the more stable dimethyl acrylic acid at the higher temperatures, it is apparent that a Type 316 stainless steel is required. The temperature of the operation exceeds the point where the Type 304 stainless steel is adequately resistant.

C. C3 Through C8 Acids

The first of the remaining higher acids, the 3-carbon propionic acid, is produced in considerable quantity. The acid and its unsaturated counterpart, acrylic acid, are very similar to acetic acid in reactivity with metals. The corrosion rate of the common materials of construction is essentially the same in propionic and acrylic acid as in acetic acid at the same temperature. Certainly, all factors described as influencing the corrosion of alloys in acetic acid are applicable to corrosion mechanisms in the 3-carbon acids.

Corrosion rates of various alloys in boiling propionic acid solutions are shown in Figure 12.5 Elder points out the anomalous results that can result from the short test period used for these tests and the effect of dissolved oxygen on the results. The beneficial effect of added oxygen on austenitic stainless steels is not restricted to laboratory tests but was also attained in the field as shown in Table XLIII. It is interesting to note that a maximum rate of attack on the stainless steels appears to occur at approximately the same concentrations (60-80 per cent) as found for acetic acid in boiling solutions. For welded construction, the low carbon stainless steel grades should be employed unless it has been definitely established that

The major commercial approach to acrylic acid produc-tion today is the direct oxidation of propylene to acrolein with subsequent oxidation to acrylic acid or a one-step oxidation with only the acid recovered. One advantage of the process is the milder corrosive conditions existing throughout the unit. Steel and the austenitic stainless steels may be used for all equipment except where chloride stress-corrosion cracking of the stainless steels requires the use of Alloy 600, Alloy 400, or other crack-resistant alloys. Table XL provides data regarding the corrosion of a number of alloys in significant portions of a propylene oxidation process. Although the austenitic stainless steels are resistant to the primary corrosive agents throughout the process, the use of INCONEL alloy 600 and other high alloys have been used in the process for the reason cited.

Similar data were obtained by the exposure of alloys in a plant preparing ß-methyl acrylic (crotonic, 2-buteneoic) acid. Types 304L and 316L stainless steels and CARPEN-TER alloy 20Cb-3 were unattacked in process handling of the acid up to 90 ºC (194 ºF) in a process similar to that described for the production of acrylic acid by the pro-

Page 39

welded regular carbon grades are free from intergranular corrosion in the heat-affected zones of welds.

To supplement the curves of Figure 12, the data of Table XLIV summarize the resistance of several alloys to propionic acid solutions below the boiling point.

When the temperature is raised appreciably and pro-pionic anhydride is added to the acid, the stainless steels, including the iron-base nickel-chromium-copper-molyb-denum alloys, are no longer useful as a material of construction. Table XLV shows data derived from a test conducted at 260 ºC (500 ºF).

For all alloys considered for a specific service involving propionic acid, the data presented for acetic acid may be used as a general guide. It is important to use data acquired at the proper temperature, keeping in mind that the boiling point of propionic acid is much higher than that of acetic acid and that tests conducted below the boiling point are not the same as those made in a boiling solution.

Organic acids of greater chain length than the 3-carbon acids are produced in smaller quantity, but constitute an important group of products, primarily as intermediates in the preparation of pharmaceuticals, agricultural chemicals, food products, plasticizers and other end-use chemicals.

The chemical characteristics of the longer monocarbox-ylic acids are important in interpreting the corrosive potential of the products. Complete miscibility in water of the two three-carbon acids (propionic and acrylic) is achieved, but the solubility of the remaining acids de-creases rapidly with increasing chain length. The extent of dissociation of the dissolved acid remains essentially the same as acetic acid. However, in the pure form, or in organic dilutions of the acids, the higher acids are in-

HASTELLOY alloy C-276 replaced silver in this primary cooler for propionic acid. The alloy was found to have better resistance to thermal cycling than the precious metal. Courtesy of Stellite Division, Cabot Corporation.

creasingly stable with increasing chain length. Numerous tests conducted in butyric and higher acids indicate that the exposure is innocuous until some specific temperature is reached, at which point sufficient dissociation is achieved to initiate and sustain corrosion. This critical temperature is higher for each succeeding higher homo-logue in the series. Thus, for a specific acid, a temperature of 190 ºC (374 ºF) may produce essentially no corrosion on a Type 304 stainless steel, but a temperature of 210 ºC (410 ºF) may produce exceedingly high corrosion rates.

FIG 12—Corrosion of Alloys in Propionic Acid at the Boiling Temperature

Page 40

Corrosion Rate

Type of Test

Additive

Temperature

Exposure

Type 316 Stainless Steel

Type 304 Stainless Steel

Type 202 Stainless Steel

ºC ºF mm/y mpy mm/y mpy mm/y mpy

Laboratory None 122 252 Liquid .15 6 .30 12 – – to to Vapor .20 8 .28 11 – – 135 275 Condensate .05 2 .36 14 – – Laboratory Air Sparged 122 252 Liquid Nil Nil 1.63 64 – – to to Vapor Nil Nil .15 6 – – 135 275 Condensate Nil Nil Nil 01 – – Laboratory 9 ppm H

20

2 122 252 Liquid 01 0.3 .04 1.5 – –

to to Vapor 01 0.2 .02 0.6 – – 135 275 Condensate Nil 0.1 Nil 0.1 – – Laboratory 1 ppm H

2O

2+ 122 252 Liquid Nil Nil Nil Nil – –

air sparged to to Vapor Nil Nil Nil Nil – – 135 275 Condensate Nil Nil Nil Nil – – Laboratory 200 PPM CuSO

4 122 252 Liquid Nil <0.1 Nil <0.1 – –

to to Vapor Nil <0.1 Nil <0.1 – – 135 275 Condensate Nil <0.1 Nil <0.1 – – Field Column Air and H

2O

2 110 230 Kettle Nil <0.1 Nil <0.1 Nil <0.1

processing the acid* injected to to Based on column Nil <0.1-0.1 Nil-.03 <0.1-1 Nil <0.1-0.1 in feed stream 137 279 Feed line Nil 0.1 Nil 0.1 Nil 0.1 Middle of column Nil 0.1 Nil 0.1 Nii <0.1 Top of column Nil <0.1 01 0.3 Nil <0.1

TABLE XLIII

Effect of Oxygen on Corrosion of Stainless Steel in Propionic Acid

Conditions: 95% propionic acid containing 2% water, alcohol, ketone and higher acids used in laboratory tests and processed in field.

*Three separate field exposures made of 168-254 hours.

Table XLVI shows data generated by the laboratory immersion test of five alloys in C2 through C10 acids. The difficulty with such laboratory tests relates to the exposure of the copper alloys and the stainless steels. Organic acids are excellent retainers of air in solution. Heating of the acid at temperatures below the boiling point does not expel all the oxygen, and corrosion rates on the copper alloys will be higher than would be experienced in a closed system devoid of oxygen. On the other hand, the stainless steels retain passivity for a longer time in such media before corrosion is initiated. Longer test periods, dynamic test apparatus and a close control of the entire environ- ment are important when attempting to identify specific materials of construction for a proposed application. However, the data of this table are consistent with field experience. As indicated by the laboratory tests, Types 316 and 316L stainless steels have excellent resistance to the acids to temperatures approaching the boiling point at atmospheric pressure. For this reason, the approximate boiling point temperature of each acid is listed in the table.

The more extensive listing of alloys exposed in four, six and eight-carbon acids is given in Tables XLVII and XLVIII. The essential resistance of Type 316 stainless steel in organic acids is maintained in these higher acids

whether the acid is refined or contaminated with the lower acids (crude). The higher iron or nickel-base alloys containing chromium and molybdenum exhibit the same excellent stability in the higher acids as noted in the one and two-carbon compounds.

Corrosion to be anticipated in a more modern process for the preparation of the longer acids is indicated in Table XLIX. Here, the catalyzed oxidation of a straight-chain hydrocarbon to an eight-carbon acid produced no signifi-cant corrosion of the stainless steels. Although only .13 mm/y (5 mpy) corrosion of Type 304 stainless steel was obtained in this instance, the choice of Types 316 or 316L stainless steels for such a reactor would be advisable to assure adequate resistance to variations in process condi-tions that might occur.

When working with the higher organic acids, it is difficult to provide test conditions and a length of ex-posure sufficient to produce intergranular attack on sen-sitized stainless steel. The higher acids will produce selective attack on a structure containing carbide pre-cipitation, however, and the use of the L-grade or sta-bilized stainless steels at temperatures above 100 ºC (212 ºF) is suggested as a safeguard, regardless of the test data obtained.

Page 41

Corrosion Rate

% Propionic Acid

Type 420 Stainless Steel

Type 304 Stainless Steel

Type 316 Stainless Steel

75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF)

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

99 2.54 100 .03 1 Nil Nil Nil Nil Nil Nil Nil Nil 80 52.32 2060 6.09 240 .05 2 .05 2 <.03 <1 .03 1 67 90.93 3580 9.27 365 .03 1 .05 2 Nil Nil Nil Nil

50 79.76 3140 1.65 65 <.03 <1 <.03 < 1 Nil Nil <.03 <1 33 39.88 1570 4.83 190 <.03 <1 Nil Nil <.03 < 1 <.03 <1

20 42.67 1680 1.57 62 <.03 <1 Nil Nil <.03 < 1 Nil Nil

Corrosion Rate

% Proponic Acid

Type 318 Stainless Steel

HASTELLOY alloy C

HASTELLOY alloy B

75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF)

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

99 <.03 <1 Nil Nil Nil Nil Nil Nil .64 25 .15 6 80 <.03 <1 .03 1 Nil Nil Nil Nil .30 12 .61 24

67 Nil Nil Nil Nil .03 1 Nil Nil .28 11 .30 12 50 Nil Nil Nil Nil <.03 <1 Nil Nil .10 4 .38 15

33 <.03 <1 Nil Nil Nil Nil Nil Nil .08 3 .20 8 20 <.03 <1 Nil Nil Nil Nil Nil Nil .05 2 <.03 < 1

Corrosion Rate

% Propionic Acid

INCONEL alloy 600

MONEL alloy 400

Copper

75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF) 75 ºC (167 ºF) 50 ºC (122 ºF)

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

99 .38 1 5 <.03 <1 1.19 47 .48 19 >1.27 >50 1.02 40 80 .48 19 .41 16 .15 6 .41 16 .23 9 1.09 43 67 .18 7 .36 14 .13 5 .23 9 .28 11 .41 16

50 .10 4 .20 8 .13 5 .28 11 .25 10 .38 15 33 .10 4 .18 7 .10 4 .05 2 .28 11 <.03 <1

20 .13 5 .13 5 .13 5 .18 7 .05 2 .13 5

Combinations of sulfuric acid and the organic acids are often found in the process industry. The mineral acid is added to catalyze certain reactions with the organic acid or to react with unwanted impurities in the acid. The effect of adding one or two per cent of strong sulfuric acid to an eight-carbon organic acid is shown in Table L. Under these conditions, Type 304 stainless steel was as resistant as the Type 316 alloy in the temperature regions explored. The anamolous data of test 4 suggest that the stability of the stainless steels may be borderline under these conditions, although even higher temperatures failed to destroy the passivity of the alloys. The strong oxidizing capacity of the concentrated sulfuric acid probably aids in maintaining

passivity of the stainless steels in this medium. As discussed in the section on acetic acid, the use of a heating coil in such an environment would pose a different problem. Under heat-flux conditions, it is unlikely that the 300 series stainless steels would show adequate resistance to such a mixture. Also, if a water wash of the organic acid is to be made to remove the sulfuric acid, the aqueous phase containing a dilute mineral acid could be extremely aggressive to the 300 series alloys. Higher alloys of the “20” type for wrought materials or the CN-7M castings would be minimal for resistance to the diluted mineral acid in the presence of the organic acid.

TABLE XLIV

Corrosion of Stainless Steels in Propionic Acid Below the Boiling Temperature

Conditions: Duplicate tests in various concentrations of propionic acid at 75 and 50 ºC (167 and 122 ºF) without aeration or deaeration.

Page 42

TABLE XLVI

Corrosion of Alloys in Higher Organic Acids

Conditions: Laboratory tests for 48 hours at temperatures shown in refined (99.9+) acids.

Corrosion Rate

Acid (Approximate boiling point)

Test Temperature Carbon Steel Copper

Silicon Bronze

Type 304 Stainless Steel

Type 316 Stainless Steel

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

Acetic (116 ºC or 240 ºF) 26 78.8 2.79 110 .28 11 .41 16 <.03 <1 <.03 <1 116 240 5.33 210 .08 3 .08 3 .25 10 .08 3 Propionic (140 ºC or 284 ºF) 26 78.8 .71 28 .05 3 .15 6 <.03 <1 <.03 <1 110 240 1.98 78 2.13 84 3.40 134 .13 5 <.03 <1 Butyric (163 ºC or 325 ºF) 26 78.8 .15 6 Nil Nil .05 2 <.03 <1 <.03 <1 163 325 – – – – – – 1.14 45 .10 4 Valeric (185 ºC or 365 ºF) 26 78.8 .05 2 .05 2 .05 2 <.03 <1 <.03 <1 140 284 1.37 54 2.06 27 .13 5 <.03 <1 <.03 <1 2-Ethylbutyric (190 ºC or 374 ºF) 26 78.8 .18 7 .03 1 .03 1 <.03 <1 <.03 <1 150 302 .86 34 .41 16 .23 9 .53 21 <.03 <1 2-Methylpentanoic

(195 ºC or 383 ºF) 26 78.8 .03 1 .08 3 .10 4 <.03 <1 <.03 <1 150 302 .53 21 .30 12 .08 3 <.03 <1 <.03 <1 2-Ethylhexanoic

(220 ºC or 428 ºF) 26 78.8 .03 1 <.03 <1 <.03 <1 <.03 <1 <.03 <1 190 374 1.27 50 <.03 <1 <.03 <1 .20 8 <.03 <1 Iso-octanoic (240 ºC or 464 ºF) 26 78.8 .03 1 05 2 .03 1 <.03 <1 <.03 <1 190 374 .89 35 <.03 <1 <.03 <1 .43 17 <.03 <1 Iso-decanoic (265 ºC or 509 ºF) 26 78.8 .03 <1 <.03 <1 <.03 <1 <.03 <1 <.03 <1

190 374 .84 33 <.03 <1 <.03 <1 .20 8 <.03 <1

Corrosion Rate

Alloy Liquid Vapor

mm/y mpy mm/y mpy Type 430 Stainless Steel 129.41 5095 80,01 3150 Type 304 Stainless Steel 141.86 5585 78.10 3075 Type 347 Stainless Steel 3.63 143 116.84 4600 Type 316 Stainless Steel 9.91 390 7,09 279 Type 316 Stainless Steel (Sen.) 12.01 475 4.01 158 Type 317 Stainless Steel 4.55 179 2.44 96 Type 318 Stainless Steel 7.95 313 7.65 301 CARPENTER alloy 20 7.26 286 8.36 329

Corrosion Rate

Alloy mm/y mpy

Type 316 Stainless Steel .03 1 CRUCIBLE alloy 223 .05 2 Titanium <.03 <1 WAUKESHA 23 .89 35 WAUKESHA 54 .18 7 WAUKESHA 88 .28 11 KROMARC 55 .03 1 E-BRITE 26-1 <.03 <1 CRUCIBLE 26-1 .03 1 CROLOY 16-1 5.33 210 CARPENTER alloy 20Cb-3 <.03 <1

TABLE XLV

Corrosion of Stainless Steel in a Propionic Acid-Anhydride Mixture

Conditions: Specimens exposed in pressure equipment at 260 ºC (500 ºF) and 300 atmospheres pressure from 4 to 7-hour periods in a 65% propionic acid/35%propionic anhydride mixture with continuous feed of two liters/ hour.

TABLE XLVII

Corrosion of Proprietary Alloys in 2-Ethyl Butyric Acid

Duplicate specimens exposed for 48 hours or longer in refined boiling acid without aeration or deaeration.

Reference 5

Page 43

TABLE XLVIII

Corrosion of Alloys in Higher Organic Acids (Laboratory Tests)

TABLE XLIX

Corrosion of Alloys During Preparation of n-Octanoic Acid

TABLE L

Corrosion of Alloys in 2-Ethyl Hexanoic Acid

aTest 1–1% of 98% H2SO

4 added to commercial grade 2-ethyl hexanoic acid. Solution at 90 ºC (194 ºF) at atmospheric pressure for

7 days with agitation. Test 2–1% of 98% H

2SO

4 added as before. Solution at 120 ºC (248 ºF) at atmospheric pressure for 7 days with agitation.

Test 3–1% of 98% H2SO

4 added as before. Solution averaged 143 ºC (290 ºF) at 200 mm pressure for 3 days with agitation.

Test 4–Same as Test 1 except 2% H2SO

4 added.

Test 5–Same as Test 2 except 2% H2SO

4 added.

Test 6–2% H2SO

4 added; average temperature of 142 ºC (288 ºF) at 300 mm pressure for 3 days.

Corrosion Rate

Test No.a 1 2 3 4 5 6

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

Type 304 Stainless Steel 08 3 05 2 08 3 36 14 05 2 .08 3 Type 316 Stainless Steel .15 6 13 5 08 3 46 18 15 6 13 5

CARPENTER alloy 20 .10 4 .05 2 – – .08 3 .08 3 – – HASTELLOY alloy C .03 1 <.03 <1 – – .05 2 .03 1 – –

HASTELLOY alloy B <.03 <1 <.03 <1 – – <.03 <1 .03 1 – – Silicon Bronze 05 2 05 2 .20 8 .18 7 08 3 56 22

Copper 15 6 .15 6 1.12 44 .18 7 .23 9 .66 26

Corrosion Rate

Temp. Time

Type 304 Stainless

Steel

Type 316 Stainless

Steel

CARPENTER

alloy 20

INCOLOY alloy 825

HASTELLOY

alloy C

Copper

Steel

Acid ºC ºF days mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy mm/y mpy

2- Ethyl butyric acid Crude acid (60%) 26 78.8 10 Nil Nil Nil Nil – – – – – – – – – – Crude acid 115 239 10 .08 3 .08 3 – – – – – – – – – –

Refined acid 125 257 1 Nil Nil Nil Nil – – – – – – – – – – 2- Ethyl hexanoic acid

Refined acid 125 257 1 Nil Nil – – – – – – – – – – – – Refined acid 150 302 5 61 24 <.03 <1 – – – – – – <.03 <1 .89 35

n- Butyric acid 130 266 60 – – 05 2 .03 1 .03 1 .01 0.3 1.52 60 – – +5% Acetic Acid

agitated

Corrosion Rate

Temp. Time

Type 304 Stainless

Steel

Type 316 Stainless

Steel

CARPENTER

alloy 20

HASTELLOY

alloy C

Exposure ºC ºF days mm/y mpy mm/y mpy mm/y mpy mm/y mpy

Preparation of the acid by carbonylation of the appropriate olefin 175 347 7 13 5 <.03 <1 <.03 < 1 <.03 < 1

Distillation of the octanoic acid from above preparation 230 446 5 2.54 100 <.03 < 1 <.03 < 1 <.03 < 1

Page 44

Corrosion Rate

Conditionsa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Temperature ºC (ºF) Time, days Exposure

100(212) 30

Vapor

275(527) 260(500) 195

Liquid Vapor

300(572) 3

Liquid

25(77) 19

Liquid

265(509) 54

Liquid

265(509) 134

Liquid

265(509) 100

Liquid

220(128) 242

Vapor

250(482) 66

Liq–Vap.

240(464) 50

Liq–Vap.

265(509) 73

Liquid

247(477) 66

Vapor

247(477) 66

Liq–Vap.

220(428) 66

Vapor

260(500) 63

Vapor

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

Steel .09 3.4 Consumed Consumed – – – – Consumed Consumed – – – – – – – – – – – – – – – – – –Cast Iron .06 2.5 >6.35 >250 >6.35 >250 11.1 440 – – – – – – – – – – – – – – – – – – – – – – – –Ni–Resist Type 2 .02 0.9 .77 30 .25 10 – – – – – – – – – – – – – – – – – – – – – – – – – –Nickel 200 .02 07 .38 15 .08 31 .13 5 0.9 3.5 MONEL alloy 400 .03 1 .20 8 .09 3.6 .20 8 0.1 0.5 .17 6.6 .23 9 – – .15 6 – – – – .91 36 – – – – – – – –INCONEL alloy 600 Nil 0.1 .25 10 .09 3.7 .05 2 0.1 0.2 – – – – .43 17 .28 11 .43 17 .79 31 1.60 83 .43* 17 .61* 24* .15 6 .79 31Type 304 Stainless Steel Nil* 01 >.76 >30 >7.6 >30 1.57 62 Nil 0.1 4.70 185 Consumed – – – – – – – – – – – – – – – – – –Type 316 Stainless Steel Nil 0.1 .36 14 <.03 <1 .05 2 – – Nil 0.1 .86 34 .84 33 .20 8 .08 3I .05 2 1.78* 70 08* 3* .03* 1 .10 4 Nil NilType 31 7 Stainless Steel – – – – – – – – – – Nil NII .23 9 .08 3 – – Nil Nil Nil 0.1 .99* 39 Nil Nil Nil Nil .02 0.8 Nil NilCG–8M Casting – – – – – – – – – – – – – – .10 4 .03 1 Nil 0.1 – – – – – – – – – – – –Type 310 Stainless Steel Nil 0.1 >.76 >30 >7.6 >30 .15 6 – – – – – – – – – – – – – – – – – – – – – – – –Type 330 Stainless Steel Nil 0.1 >.76 >30 .46 18 – – – – – – – – – – – – – – – – – – – – – – – – – –INCOLOY alloy 825 – – – – – – – – – – – – – – – – .05 2 Nil NII .02 0.7 .61* 24* Nil NII – – – – – –INCOLOY alloy 800 – – – – – – – – – – – – – – – – – – – – 2.67 105 >2.13 >84 – – – – – – – –INCOLOY alloy 804 – – – – – – – – – – – – – – – – – – – – 86 34 >2.29 >90 – – – – – – – –HASTELLOY alloy C Nil 0.1 <.03 <1 <.03 < 1 .03 1 – – – – – – Nil Nil Nil Nil Nil Nil .01 .05 Nil Nil – – Nil Nil Nil Nil Nil NilHASTELLOY alloy B – – – – – – – – – – – – – – Nil Nil – – – – – – .01 0.3 – – – – – – – –Copper .08 3 >7.6 >30 >7.6 > 30 .25 10 _ CN–7M Casting – – – – – – – – – – Nil Nil .71 28 – – .03 1 Nil Nil .02 .06 1.04 41 Nil Nil Nil* NII .08 3 Nil NilTitanium – – – – – – – – – – – – – – – – Nil Nil Nil Nil – – – – Nil Nil – – – – – –

The fatty acids comprise those organic acids exceeding four carbons in length according to some chemical text definitions. However, the term as used industrially and in this text refers to the higher acids of six or more carbons. These are characterized by lauric, oleic, linoleic, stearic, tall oil and rosin acids as produced for commerical use from products of the meat, agricultural and paper industry. The large volume product of industry is not a pure compound, but a mixture of two or more of the com-pounds meeting certain chemical specifications.

At the lower temperatures, the acids may be considered as harmless polar “oils.” However, when the products are heated to the high temperatures necessary for processing and production, significant corrosion of steel can result. Fortunately, there is a wide variety of alloys which have excellent resistance to the conditions of production and subsequent use of the acids. A proper economic analysis of the use of the alternative materials is necessary to achieve an optimum selection.

D. Fatty Acids

Data reported for the corrosion of metals in the fatty acids are not explicit regarding stream compositons. As a consequence, a comparison of the results obtained in a number of industrial exposures at various temperatures is necessary to gain a proper view of corrosion to be expected in these media. A number of factors can influence the corrosion rates observed: • Light ends (lower acids), if allowed to remain in the

mixed fatty acids, can result in a more aggressive environment.

• The ratio of fatty to rosin acids affects the corrosion rate. • The presence or absence of water will have an effect,

particularly on the corrosion of the stainless steels. • Decomposition products generated by overheating the

acids will add to the corrosiveness of the solution. • Pretreatment of the acids may leave traces of ions in the

acids that increase corrosivity. • The temperature of the processing operation is a major

variable of concern.

TABLE LI

Corrosion of Alloys in Tall Oil Fractions

*Pitted References 30, 31, 49. 52

aConditions of the exposures:

1– Field test in vapor of light-odor tall oil fraction during distillation in vacuum column. Water present. 2– Field test in bottom and top of tall oil vacuum distillation column. Oil (presumably crude) from southern kraft pulp mill. 3– Laboratory test in crude tall oil acids from kraft pulp mill distilled under vacuum with agitation of 300 rpm in kettle.52 4– Laboratory test in crude and semi-refined oil with velocity of 0.3 fps provided in liquid. 5– Field test in base of tall oil distillation column (20% oleic acids, 60% rosin acids and 20% pitch).30 6– Field test in base of tall oil distillation column (65% fatty acids and 35% rosin acids).30 7– Field distillation of 65% fatty acid–35 % rosin acids. 31

8– Field distillation of 93% fatty acids–5% rosin acids.31 9– Field distillation of 90-93% oleic acids with <1% rosin acids with steam injected. Velocity of 62 fps.31 10– Field test six inches above the outlet of a reboiler on 97% fatty acids,1.5% rosin acids and 1.7% residues with high velocity.31 11– Field test in heat exchanger head handling 85% fatty acids and 15% rosin acids with steam present. 12– Field test in reboiler nozzle at base of distillation column handling 90-93% oleic acids and 1% rosin acids with steam injected. 13– Field test on distillation column tray near bottom while processing 90% oleic acids, 2% stearic acid, 0.4% rosin acids, 0.5 light ends and 6.4% higher

acids with steam present. 14– Field test in top of distillation column handling analysis of No. 13 above. 15– Field test near bottom of distillation column handling 30-32% rosin acids, 8-20% oleic acids and 62-48% higher boiling acids with steam present.

Page 45

Corrosion Rate

285 ºC (545 ºF) 300 ºC (572 ºF) 315 ºC (599 ºF) 330 ºC (626 ºF)

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

Type 302 4.57 180 12.7 500 20.32 800 – –

Stainless Steel

Type 316 .10 4 .10 4 1.35 53 12.7 500

Stainless Steel

Type 317 .03 1 .03 1 .53 21 – –

Stainless Steel

HASTELLOY alloy C .10 4 .13 5 .10 4 – –

INCONEL alloy 600 .25 10 .25 1 0 .33 13 .28 11

Pitting and crevice corrosion can occur on essentially all alloys in these environments and must be appraised before a material is selected. Extensive comments on the processing of the fatty acids and the selection of materials of construction are contained in references 28-34.

One of the most important sources of fatty acids today is the pulp and paper industry where tall oil fractions are recovered and refined. These are composed of the straight-chain fatty acids and mixed rosin acids. Table LI shows results compiled from various sources. Whenever possible, the stream compositions have been defined.

Note that Types 316 and 316L stainless steels are useful for many tall oil processing requirements but, in some instances, either an excess of light ends or an excep-tionally high temperature causes high rates with this alloy. In these cases, the use of Types 317 or 317L stainless steels or alloys with a higher molybdenum content should be investigated in the search for an economical material of construction. If these alloys are inadequate, the use of more highly alloyed materials can be considered. The nickel-base molybdenum-chromium-iron alloys show es-sentially a nil corrosion rate in all such exposures. Alloy 600 is a contender for use in a number of applica-tions and should not be overlooked. The use of nickel- copper alloys or copper-nickel alloys varies depending on the oxidizing capacity of the solution, as would be expected. In the absence of oxidants, the rate of attack on these alloys is acceptably low.

It has been stated that streams containing a higher proportion of the straight chain fatty acid produced more corrosion than those containing a higher ratio of rosin (cyclic) acids. This does not appear to be invariably true.

The presence or absence of steam has a significant effect on the corrosion to be expected, particularly as observed for the stainless steels. The oxidizing capacity of the water reduces corrosion rates on the stainless steels appreciably while accentuating attack on the nickel-copper and/or copper-base alloys.

As with any other corrosive environment, the effect of temperature must be carefully defined. Table LII shows data for five alloys exposed to the same refined tall oil

TABLE Llll

Corrosion of Metals in Vegetable Fatty Acids

aConditions:

1–Field test in closed autoclave converting castor oil to drying oil. 2–Field test in top of kettle while refining high purity linseed oil. 3–Field test in receiving tank for dirty palm used in tin-plate line. 4–Field test in distillation column handling crude vegetable oils plus palmitic and stearic acid (acid value of 85-95). 5–Field test in distillation column for cottonseed oil acids. 6–Field test in top of distillation column handling palmitic and stearic acids. 7–Field test in top of distillation column deodorizing crude cottonseed fatty acids by steam distillation.

TABLE LII

Effect of Temperature on Corrosion in Refined Tall Oil

Conditions: Laboratory tests conducted in liquid of same oil at various temperatures.

Conditionsa Temp. ºC (ºF) Time, days Exposure

1 370 (698)

45 Liq-vap.

2 370 (698)

3 Vapor

3 190 (374)

30 Liquid

4 190 (374)

23 Liq-vap.

5 277 (530)

50 Vapor

6 116(240)

32 Vapor

7 255(491)

42 Vapor

Corrosion Rate

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

Carbon Steel – – – – 38 1 5 1.04 41 3.05 120 25 10 – – Cast Iron – – – – 48 19 – – 10.92 430 – – 12.45 49

0Ni-Resist Type 2 – – – 03 1 20 8 43 17 01 05 86 34

Type 304 Stainless Steel 01 0.2 – – Nil Nil Nil 0.1 97 38 Nil 0.1 25 10 Type 309 Stainless Steel – – – – Nil Nil – – – – – – 05 2

Type 316 Stainless Steel Nil 0.1 – – Nil Nil Nil 0.1 Nil 0.1 Nil 0.1 Nil Nil Type 317 Stainless Steel – – – – Nil Nil – – – – – – – –

INCONEL alloy 600 Nil 0.1 Nil 0.1 Nil Nil Nil 0.1 0.3 1 Nil 0.1 Nil 0.1 Nickel 200 08 3 25 10 10 4 10 4 30 12 03 1 20 8

MONEL alloy 400 .08 3 .18 7 .05 2 .13 5 .25 10 .02 0.9 .20 8 C71500 (70-30 Cupro-nickel) – – – – 01 0.4 – – – – – – – –

HASTELLOY alloy C – – – – – – – – – – – – Nil Nil_

Page 46

TABLE LIV

Corrosion of Alloys in Animal Fatty Acids

Conditionsa Temp. ºC (ºF) Time, days Exposure

1 100 (212)

130 Liquid

2 250 (482)

147 Liquid

3 250 (482)

210 Vapor

4 250 (482)

84 Vapor

Corrosion Rate

Alloy mm/y mpy mm/y mpy mm/y mpy mm/y mpy Mild Steel .18 7 – – Consumed Consumed – – Cast Iron 1.63 64 – – >3.46 >140 Consumed Consumed

Ni-Resist Type 2 .23 9 – – .33 13 .79 31 Type 304 Stainless Steel Nil Nil .36* 14* .05* 2* .20 8 Type 316 Stainless Steel Nil Nil Nil 0.1 .01* 0.2* Nil Nil INCONEL alloy 600 Nil Nil .01 0.3 .01 0.3 .05 2 Nickel 200 .08 3 .41 16 .13 5 .08 3 MONEL alloy 400 .05 2 .58 23 .15 6 .10 4 Copper – – – – – – .13 5 *Pitted aConditions: 1–Field test in storage tank for mixed acids from fish oils. 2–Field test in outlet of preheater to distillation column processing animal tatty acids. 3–Field test in overhead vapor of column distilling acids from fish oils. 4–Field test on feed tray of distillation column handling crude fatty acids from tallow.

References 28,49

fraction at various temperatures. It is obvious that at some place above 300 ºC (572 ºF) the use of the 300 series stainless steels is questionable in such a mixture. At this point, the use of the more highly alloyed materials should be investigated.

The vegetable oils, characterized by stearic and palmitic acids among others, appear to have somewhat less aggres-sive characteristics than the tall oil acids. Table LIII shows data for the exposure of alloys in a diverse group of field exposures. It will be noted that INCONEL alloy 600 and Type 316 stainless steel are resistant to all of the processing conditions. Indeed, INCONEL alloy 600 vessels have been used for over 30 years with good success in the processing of vegetable oil acids. Other aspects of the handling of these vegetable oil fatty acids would be the same as described for the tall oil acids.

Those acids derived from animal fats appear to be somewhat more aggressive. Table LIV shows data ob-tained while processing acids derived from fish oils and beef tallow. Again, the INCONEL alloy 600 and Type 316 stainless steel appear to be the most attractive materials for construction of such equipment.

E. Di and Tricarboxylic Acids

Although the di and tricarboxylic acids are produced in less quantity than the monobasic acids, the products constitute a most important industrial commodity. Many of the acids and corresponding anhydrides are used in the synthesis of drugs, food products, plasticizers and resins. Citric, oxalic and certain other of the acids are used extensively as metal cleaning agents. However, the most important of the products are maleic and phthalic anhydrides used to produce alkyd and polyester resins, the para-phthalic acid used in the preparation of polyester fibers and adipic acid required for nylon synthesis.

The lower acids of this series are more aggressive in aqueous solution than are the monobasic acids at the same temperature and concentration. Dissociation of these acids in water is greater than for acetic or formic acid. In addition, the multiple acid grouping has the capacity to solubilize cations by chelation. Thus, protective, insoluble corrosion products are normally not found on the surface of a metal attacked by this group of acids. This allows continuous attack on the clean metal surface. Since the rate of attack is not as severe as when using mineral acids, oxalic, citric and certain other of the dibasic acids are used to clean metal surfaces.

The elemental dibasic acid is oxalic (ethanedioic) acid; sublimes at 150 ºC (302 ºF). As with other first homologues of a series, oxalic acid is extremely aggressive in its attack on most metals. Rates of corrosion are significantly higher on alloys than with acetic acid at the same concentration and temperature (Table LV). However, the relative corrosion resistance of the alloys remains essentially the same. Higher alloying is required to provide an alloy with useful resistance to attack. For instance, Type 304 stainless steel is attacked excessively in most concentrations of the acid at temperatures above ambient, and Type 316 stainless steel, although significantly more resistant, has severe limitations of use.

Table LVI shows the corrosion to be expected by exposure of a variety of alloys to oxalic acid. More corrosion data are available for the 10 per cent concentra-tion of the acid at the boiling temperature than far other combinations, because: (a) 10 percent represents satura-tion in cold, 25 ºC (77 ºF), water, (b) the mixture is an aggressive cleaning solution for metals and (c) the mix-ture is often used as a corrosion test medium for the evaluation of alloys.

It is obvious that higher amounts of nickel in an austenitic base are beneficial in combatting attack by

Page 47

TABLE LV

Corrosion of Annealed and Heat Treated Alloys in Dicarboxylic Acids

Laboratory test in 10% boiling dibasic acid stated for 5 days without aeration or deaeration. Acetic acid added for comparison.

*650 ºC (1200 ºF) for one hour, water-quenched.

oxalic acid. As with corrosion in the monobasic acids, the addition of molybdenum is very beneficial. Nickel-base alloys containing molybdenum exhibit the best resistance of all alloys in hot, aqueous oxalic acid (Table LVI). Less costly alloys, such as Type 316 stainless steel, can be used for specific applications at temperatures somewhat higher than ambient in aqueous solutions of the acid. Streicher has shown that the rate of attack on Type 304 stainless

steel can be reduced essentially to zero in even boiling 10 per cent acid by the addition of approximately 50 ppm of iron as ferric oxalate.35

As noted in Table LV, the dibasic acids above oxalic in the series are much less corrosive. Maleic acid, m.p. 130 ºC (266 ºF), can be considered as the next homologue, and the acid is innocuous in aqueous solution when compared to oxalic acid.

TABLE LVI

Corrosion of Alloys by Oxalic Acid

Laboratory tests without aeration or deaeration except as noted

(1) Type 304L material heat treated at 675 ºC (1250 ºF) for 1 hour (2) Aerated.

*Trademark of Allegheny Ludlum Steel Corporation. **Trademark of Elgiloy Co.

% Oxalic Acid Temp. ºC ºF Test Period (days)

1 Boiling Boiling

1.5

10 25 77

7

10 35 95

6

10 50

122 6

10 80

176 0.1-10

10 Boiling Boiling 2-10

30 60

140 11

Corrosion Rate

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

Type 430 stainless steel – – – – – – – – – – 63.5 2500 – – Type 304 stainless steel .81 32 .03(1) 1(1) – – .81(1) 32(1) 1.52(1) 60(1) 2–16–14.48(1) 85–570(1) – – Type 316 stainless steel – – – – – – – – – – .18– 2.44 7–96 – – Type 216 stainless steel – – – – – – – – – – 1.52 60 – – ALLEGHENY alloy AL-6X* – – – – – – – – – – .28 11 – – Act CN-7M – – – – – – – – – – .18 7 – – HASTELLOY alloy C-276 – – – – – – – – – – .25 10 – – HASTELLOY alloy B – – – – – – – – – – .13 5 – – ELGILOY** – – – – – – – – – – .10 4 – – Titanium – – .03 1 .03 1 11.68 460 – – 24.1–73.7 950– – – Vanadium – – – – .41(2) 4(2) .25(2) 10(2) – – 5.46(2) 215(2) – – C71500 (70-30 Cupro-nickel) – – – – – – – – – – – – .20 8 WAUKESHA No. 23 – – – – – – – – – – .63 25 – – WAUKESHA No. 54 – – – – – – – – – – .48 19 – – WAUKESHA No. 88 – – – – – – – – – – .05 2 – – KROMARC 55 – – – – – – – – – – .23 9 – – Multiphase MP35N – – – – – – – – – – .10 4 – –

Corrosion Rate

Oxalic Acid

Maleic Acid

Phthalic Acid

Acetic Acid

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

Type 316L Stainless Steel (annealed) .94 37 .01 0.2 .01 0.2 .01 0.3

Type 316L Stainless Steel .66 26 .01 0.2 .01 0.3 Nil <0.1

(Heat treated)*

CARPENTER alloy 20Cb-3 (annealed) .58 23 .01 0.2 Nil <0.1 Nil <0.1

CARPENTER alloy 20Cb-3 .23 9 Nil <0.1 Nil 0.1 Nil 0.1

(Heat treated)*

INCOLOY alloy 825 (annealed) .51 20 Nil 0.1 Nil <0.1 Nil <0.1

INCOLOY alloy 825 .38 15 .02 0.7 Nil 0.1 .05 1.8

(Heat treated)

Page 48

TABLE LVII

Corrosion of Alloys in Aqueous Maleic Acid Solutions

*At 50 ºC (122 ºF) for 4 days with agitation by aeration. **Boiling for 6 days without aeration or deaeration.

*Pitting

Actually, there is little industry interest in maleic acid. The acid is a contaminant in processes used to produce maleic anhydride and phthalic anhydride. These anhydrides are important basic building blocks for the preparation of polyester and alkyd resins, plasticizers and agricultural chemicals. The isomer of maleic acid, termed fumaric acid, has commercial applications in the prepara-

tion of paper sizing and other resinous products as well as the synthesis of food additives.

The presence of maleic acid in process streams of the anhydrides does create corrosion problems. The anhydrides are essentially innocuous, but the presence of malefic acid at the high temperatures used in the various processes means attack on lower alloys by streams con-

TABLE LVllI

Field Exposure of Alloys in a Phthalic Anhydride Plant

FIXED BED NAPHTHALENE OXIDATION UNITa1- Mixture of phthalic and malefic anhydride vapors near exit throat of a converter at 200-380ºC (329-716ºF) for 71 days. High temperature

excursion to cause melting of 3003 aluminum. 2- Vapors of phthalic and maleic anhydride near bottom of distillation column at 204ºC (396ºF) for 16 days. 3- Same column as No. 2, but exposed at top at temperature of 195ºC (383ºF). 4- Liquid and vapor of phthalic anhydride in the lights heater at 177ºC (351ºF) for 157 days. 5- Overhead of distillation column for phthalic acid dehydration to phthalic anhydride and resulting distillation at 107-143ºC (225-289ºF) for

14 days. 6- Near top of batch still column with phthalic and malefic acids present. Distillation Involved 1 :10 ratio of total reflux versus distillation at

100-143ºC (212-289ºF) for 95 days. 7- On top tray of phthalic anhydride purification still with small amount of malefic acid and water present at 96-140ºC (205-284ºF) for 45

days. Vapor velocity of 7 ft per sec. 8- Top of distillation column for phthalic and maleic acids at 70ºC (158ºF) for 22 days. 9- Vapor space of column distilling 7% phthalic acid in water at 180ºC (356ºF) for 40 days. 10- Immersed in maleic acid recovery holding tank (10-18% maleic acid plus little phthalic acid and a-naphthoquinine) at 35ºC (95ºF) for 27

days. 11- Crude phthalic anhydride vapor in treater tank at 160-285ºC (320-545ºF) for 59 days. Liquid and vapor exposures essentially the same. 12- On reflux distributor plate of batch still handling crude phthalic anhydride containing phthalic acid, malefic acid, benzoic acid and maleic

anhydride at 165-260ºC (329-500ºF) for 56 days. 13- Same as No. 12, but in another plant using temperatures of 225-290ºC (437-554ºF) for 85 days. 14- Same as No. 11, but in plant of No. 12. Essentially same temperatures for 25 days.

Corrosion Rate

2%* 5%* 10%* 30%** 40%* 59%**

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

Type 304 Stainless Steel .03 1 .03 1 4.06 160 4.06 160 3.71 146 5.33 210

Type 316 Stainless Steel Nil Nil Nil Nil Nil Nil <.03 <1 Nil Nil <.03 <1

CARPENTER alloy 20 – – – – – – <.03 <1 – – <.03 <1

HASTELLOY alloy C – – – – – – – – – – <.03 <1

HASTELLOY alloy B – – – – – – .08 3 – – – –

Nickel 200 – – – – – – – – – – .91 36

Copper (C10200) – – – – – – .10 4 – – .03 2

Silver (fine) – – – – – – – – – – Nil Nil

Corrosion Rate

Exposurea 1 2 3 4 5 6 7 8 9 10 11 12 13 14

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

Mild Steel Cast lron (gray) Ni-Resist Type IV Type 304 Stainless Steel Type 309 Stainless Steel Type 316 Stainless Steel Type311 Stainless Steel CARPENTER alloy 20 ACI CN-7M INCOLOY alloy 825 HASTELLOY alloy C HASTELLOY alloy B INCONEL alloy 600 Nickel 200 MONEL alloy 400 Copper Titanium Aluminum 3003

.03

.05

.03 Nil Nil Nil – – – – – – Nil Nil .05 .18 – Mel

1 2 1

<0.1

<0.1

<0.1

– – – – – –

<0.1

0.1 0.2

7

.05

.05

.03

.01

.01

.01 Nil Nil – Nil – –

.01 Nil .01 .13 –

.02

2 2 1

0.2 0.4 0.2 <0.

1 <0.

1 –

<0.1

– –

0.4 0.1 0.2

5

1.22 .56 .10 .03 .03 .03 – – – – – –

.03

.08

.08 1.14

– .03

48 22

4 1 1 1

– – – – – – 1 3 3

45 – 1

.01*

.01

.01 Nil Nil Nil – – – – – – Nil Nil Nil .69 –

.05*

0.4* 0.3 0.2

<0.1 <0.1 <0.1

– – – – – –

<0.1 <0.1 <0.1

27 –

2

– – –

.99

.48

.01NilNil– Nil– –

.841.881.12

– – –

– – – 3919

0.30.1<0.

1–

<0.1

– – 337444– –

– – –

.41 –

.02 – – – – – – – – – – – –

– – –

16 –

0.6– – – – – – – – – – – –

– – –

.991.22.02Nil– Nil– Nil.02.56

1.55.56– – –

– – – 3948

0.70.1–

<0.1

– 0.10.8225122– – –

– – –

.56–

.01– NilNil– – – – – – – – –

– – – 22–

3.3– NilNil– – – – – – – – –

– – –

5.08– Nil–

.02Nil– – – – – – – – –

– – –

200–

0.1–

0.70.1– – – – – – – – –

– – – Nil

– Nil

– – Nil

– 01

>5.08

01>5.0

84.06– Nil

– – –

<0.1–

<0.1– –

<0.1– 0.2

>2000.4

>200160–

< 0.1–

1.93– –

.01–

.01NilNil–

.01Nil.01Nil.01.01– – –

76 – –

0.2 –

0.2 0.1 0.1 –

0.2 Nil 0.3 0.1

3 2

– – –

>7.62– – –

2.21.99.28.86– .51Nil

– 1.22– 51

– – –

>300– – – 87391134

– 20

<0.1– 48

– 20

– – –

3.10 3.02

.97

.28 –

.02 Nil – – – – –

.15

.69

.13

.46 –

.56

122119

3811–

0.6<0.1

– – – – – 6

275

18– 22

– – – Nil– NilNil.01– Nil– – 0.1.18.20– – –

– – – 0.1–

<0.1<0.1

0.2– 0.1– – 0.3

78

– – –

Page 49

Corrosion Rate

Phthalic Anhydride

1:1 Phthalic Anhydride:

Phthalic Acid Mixture

Phthalic Acid

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

Mild Steel <.03 <1 <.03 <1 .03 1 Type 304 Stainless Steel <.03 <1 <.03 <1 <.03 <1 Type 316 Stainless Steel <.03 <1 <.03 <1 <.03 <1

taining the molten acid or in scrubber waters rich with the water-soluble acid. As an example, the majority of equip-ment used in the butane oxidation process for maleic anhydride was originally of Type 304L stainless steel construction. However, unforeseen accumulations of mal-eic acid in portions of the equipment dictated a shift to the use of the more resistant Type 316L stainless steel. The benzene process to produce the anhydride is even more corrosive, and Type 316L stainless steel is used exten-sively throughout the process chain.

Corrosion to be expected from exposure of alloys in various aqueous concentrations of maleic acid at the boiling point is summarized in Table LVIL These data relate to the corrosion found in process scrubber systems where water is used as the scrubbing medium. Note the loss of Type 304 stainless steel as a usable material of construction at concentrations of 10 per cent or more acid. Higher iron-base stainless steel alloys, such as Type 316L and above, show acceptable resistance in all aqueous concentrations.

The determination of corrosion rates for the stainless steels in both aqueous and molten maleic acid composi-tions is difficult. The maleic acid in the absence of more aggressive anions is slow in penetrating the oxide film on the stainless steels to initiate corrosion. Consequently, multiple tests of sufficient duration must be conducted to provide meaningful “rate” data for the corrosion process. Also, during the test period, a conversion of a portion of the maleic acid to insoluble fumaric acid will occur, which must be taken into account if the data are to be precise.

Pure maleic acid in the molten form is not encountered normally in industry, but does exist in certain of the anhydride process streams. See Table LVIII for field corrosion data obtained in streams containing the molten acid as a contaminant.

Phthalic acid, decomp. ca. 200 ºC (392 ºF), is found in many of the same process streams containing the maleic acid. However, the contribution of phthalic acid to corro-sion of the equipment is minimal. Table LIX shows corrosion data for steel and Types 304 and 316 stainless steels exposed to hot phthalic acid, phthalic anhydride and a mixture of the two. These chemicals are not aggressive. However, the austenitic stainless steels are often used to process these chemicals to prevent contamination of the product and to provide a surface that can be cleaned readily.

TABLE LX

Corrosion of Alloys in Terephthalic Acid Media

TABLE LIX

Corrosion of Alloys in Phthalic Acid and Phthalic Anhydride

Conditions: Laboratory test of duplicate specimens at 150 ºC (302 ºF) for 13 days without aeration ordeaeration.

Laboratory Test 6% Terephthalic

Acid in Water

Laboratory Test 6% Terephthalic Acid

84.6% Acetic Acid 9.4% Water

Field Test TPA Leach Feed

Slurry (TPA + Acetic Acid)

Field Test Leach Crystallizer

Liquid (14.1 % TPA, 82.7% Acetic Acid,

2.7 % water)

Temperature, ºC Temperature, ºF Test Period, days

232 450 24

232 450 24

260 500 14

177 351 523

Corrosion Rate

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

Type 304 Stainless Steel .01 0.4 .06 2.4 – – – – Type 316 Stainless Steel .01 0.3 .02 0.8 – – .03 1.0 Type 216 Stainless Steel – – – – .19 7.3 – – Type 317 Stainless Steel – – – – – – .01 0.2 CARPENTER alloy 20Cb-3 – – – – – – .02 0.7 INCOLOY alloy 800 – – – – – – .35 13.8 HASTELLOY alloy C- 276 Nil 0.1 Nil 0.1 .04 1.5 .03 1.1 Titanium Nil <0.1 Nil 0.1 Nil 0.1 Nil <0.1

Page 50

TABLE LXI

Corrosion of Alloys in Adipic Acid Process

Low temperature, 100 ºC (212 ºF), reaction with am-monium vanadate and cupric ion, process involved oxidation of cyclohexanone and cyclohexanol (KA oil) in strong nitric acid. In addition to the desired adipic acid, succinic, glutaric and lighter acids were formed in the process.

Corrosion Rate

Reactor Scrubber Absorber Separation Still

Crystallizer Lower Acids

Stripper Crystallizer Centrifuge

Product Washing

and Drying Equip.

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 .23 9 .30 12 .23 9 .91 36 .18* 7* .41 16 .03 1 .13 5 Nil NilStainless Steel Type 316 .36 14 .05 2 .05 2 .30 12 .08* 3* .05 2 .03 1 .05 2 Nil NilStainless Steel CARPENTER .10 4 .05 2 .03 1 .13 5 Nil Nil .03 1 Nil Nil .03 1 Nil Nilalloy 20Cb-3 HASTELLOY Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nilalloy C-276 Titanium Nil Nil Nil Nil Nil Nil Nil Nil Nil* Nil* Nil Nil Nil Nil Nil Nil Nil Nil

*Pitting indicated

Data derived from corrosion tests in a large phthalic anhydride plant are presented in Table LVIII. These show that, in addition to the higher stainless steels, the use of Alloys 400, 600 and other nickel-base alloys is permissible in many areas of the process. For equipment handling the brominated anhydride, the use of HASTELLOY alloy C-276 has proven to be attractive.

Terephthalic acid (para-phthalic acid) is produced in large quantity, primarily for the preparation of polyester resins used in the textile industry. A number of processes have been investigated to produce the acid in as pure form as economically as possible.

Initially, the process required the oxidation of xylene using a bromide catalyst. Inasmuch as acetic acid is used as a dilulent in the process, the reaction mixture of a halogen and acetic acid was extremely corrosive. HASTELLOY alloy C-276 was the only contender for use in these areas. Once the bromide ion was removed, Type 316 stainless steel was found to be useful for the vast majority of the remainder of the equipment.

With the research interest to produce a simpler and more economical mode of preparation, a number of new methods have evolved. One of the more common pro-cedures is the use of oxygen along with a cobaltic ion catalyst to effect the reaction. Again, acetic acid is used as the medium for the reaction. Thus, acetic acid is the primary corrosive to be considered. Contamination of the acetic acid by the terephthalic acid (TPA) adds little to the corrosion produced. The major problem is one of han-dling an acetic acid medium at high temperatures. There are a number of steps in the process where the tempera-tures are well above those required for producing acetic acid itself. In these areas, the use of materials suitable for exposure in acetic acid at high temperatures under oxidiz- ing conditions are acceptable. (See discussion of acetic acid.)

Table LX shows data generated by both laboratory and field tests designed to explore corrosion within the area of the leaching step of the process. At this point, the acetic acid medium is taken to a very high temperature to allow the rather insoluble TPA to precipitate from solution before taking the material to a crystallizer. It will be noted that Type 316 stainless steel is a borderline material for this specific area because contamination of this stream is undesirable. Titanium is favored for this most aggressive area in the process. For all of the remainder of the process, the use of Type 316 stainless steel has been found to be most satisfactory. It is necessary to avoid the presence of crevices or other areas where differential corrosion cells can be created in either titanium or the Type 316 stainless steel equipment.

TABLE LXII

Laboratory Tests for Corrosion of Alloys in Molten Adipic Acid at 170 ºC (338 ºF)

Corrosion Rate

Alloy mm/y mpy

Type 304 Stainless Steel 1.30 51

Type 321 Stainless Steel .43 17

Page 51

Adipic acid is an essential ingredient in the production of nylon resin. The process to produce this dibasic acid is quite lengthy and corrosive in the latter stages.

Cyclohexane, produced as a hydrogenated benzene, is oxidized to cyclohexanone and cyclohexanol in a conven-tional oxidation process. The conditions of this prepara-tion of the “KA oil” are not exceptionally corrosive, and steel is used for large portions of the process equipment with Type 304 stainless steel used where moisture, organic acids, or other corrosive agents tend to accumul-ate. The KA oil is then oxidized with strong nitric acid at approximately 100 ºC (212 ºF) to produce adipic acid and other degradation products from the oxidation step. These include succinic acid, glutaric acid and all of the lower monobasic organic acids. The ammonium vanadate and cupric ion catalyst contributes little to the corrosion afforded by the strong nitric acid. Inasmuch as problems relating to this portion of the process are engendered by the nitric acid, Type 304L stainless steel is used exten-sively in the equipment. Where dilution occurs or corro-sion by the organic acids becomes predominant, Type 316L stainless steel is used. This is particularly true in the scrubber, absorber and the first centrifuge of the latter process. The temperatures are maintained as low as possible by vacuum equipment for economy and to reduce corrosion throughout the process. Table LXI shows typical data for common alloys in the latter steps of the adipic process operation. Note the excellent resistance of the more highly alloyed stainless steels to conditions existing through the unit. Care must be exercised in the choice of Type 304 or Type 316 stainless steels for specific uses. However, with a judicious choice of material, the stainless steels can be used extensively throughout the process.

Pure molten adipic acid is corrosive to an austenitic stainless steel without molybdenum. Table LXII shows rates of .43 mm/y or more (17 mils per year or more) in a molten adipic acid at 170 ºC (338 ºF). Type 316 stainless steel should show adequate resistance to such an ex-posure.

The higher dibasic acids present unique problems when appraising the potential for corrosion of the common alloys. In general, aqueous solutions of the acids are only mildly corrosive up to 100 ºC (212 ºF). For instance, water saturated wth succinic (butanedioic) acid at 95 ºC (203 ºF) produced no corrosion of Type 304 stainless steel during a test period of one week.

The molten acids can vary in aggressiveness depending on the residual contaminants from the process. These contaminants may be lower organic acids or inorganic compounds which control to a great extent the rate of penetration of passive films on the stainless steels and the subsequent corrosion rate observed. Corrosion data re-ported for these higher acids seldom if ever identify the purity of the acid tested.

Table LXIII provides information regarding the corro-sion of alloys during the synthesis of a glutaric (pen-tanedioic) acid-anhydride mixture. The oxidation step, conducted at relatively low temperatures, was not corro-sive to the stainless steels, as would be expected. How-ever, when the reaction mixture was heated to higher temperatures with the attendant loss of the oxidizing species, corrosion of the stainless steels became much more pronounced. The more highly alloyed materials retained good resistance to the more rigorous conditions of the high temperature distillation.

Similar data representing the distillation of pimelic

TABLE LXIII

Corrosion of Alloys in Glutaric Acid— Glutaric Anhydride Mixtures

Conditions: Temperature, ºC Temperature, ºF Exposure, days

40-90

104-194 (oxidation)1

7

210 410

(distillation)1 9

2602 500

2

2103 410

13.5

Corrosion Rate

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

Type 304 Stainless Steel Type 316 Stainless Steel

CARPENTER alloy 20

Type 201 Stainless Steel

Type 202 Stainless Steel

HASTELLOY alloy B

HASTELLOY alloy C

INCONEL alloy 600

Copper

MONEL alloy 400

<.03

Nil

–

03

–

–

–

–

3.56

–

<1

Nil

–

1

–

–

–

–

140

–

1.58

.28

.15

.56

66

<.03

Nil

–

30

–

62

11

6

22

26

<1

Nil

–

12

–

.94

.20

–

.86

–

<.03

Nil

–

Nil

–

37

8

–

34

–

<1

Nil

–

1

–

.69

41

–

–

–

–

–

23

18

.20

27

16

–

–

–

–

–

9

7

8

(1) Pilot unit operations. Acetic acid present in mixture. (2) Laboratory kettle test designed to represent mixture for field distillation (no acetic acid present); 3 parts acid: 1 part anhydride. (3) Actual field distillation.

Page 52

F. Naphthenic Acids

The naphthenic acids have received much attention during the past 20 years as a corrosive in process streams of the oil refineries. A considerable volume of data has been generated relating to the operation of equipment handling streams contaminated with these acids.29, 36-40

The term “naphthenic acids” describes a group of aro-matic compounds containing one or more carboxyl groups and does not refer to a specific structure. The term em-braces acids from benzoic through those of the true naph-thenic structure, all of which can contribute to corrosion at the very high temperatures of oil refining. The corrosive potential for streams containing these acids is defined by “neutralization number” rather than acid content. Thus, all acidic materials in the stream are categorized by the term naphthenic acid.

Providing materials of construction to resist naphthenic acid corrosion is not difficult, although the economics of selection are critical. When the neutralization number exceeds 0.5, the streams are considered to be corrosive to steel. The use of an austenitic stainless steel will provide resistance to corrosion during processing of the streams. However, the economics of providing materials of con-struction for such large process equipment requires that the optimum material be found. Thus, the various alloy materials between steel and the 300 series austenitic

(heptanediocic) acid are summarized in Table LXIV The greater corrosive activity of the acid in these tests is probably attributable to a process contaminant. Although the higher iron-base and nickel-base alloys were not tested, it is probable that these alloys would be satisfac-torily resistant under such conditions, particularly the nickel-base molybdenum-chromium-iron alloys.

The tricarboxylic acids without other functional groups are found in nature (e.g., tricarballylic acid in beets), but are produced by industry only as a development chemical. No corrosion data are known to have been published concerning those compounds. It is possible that such a structure would generate corrosion comparable to that observed for citric acid. (See Section G-4, Part III.)

TABLE LXIV

Corrosion of Stainless Steels in Molten Pimelic (Heptanedioic) Acid

Conditions: Metal specimens completely immersed in molten acid under quiescent conditions at 225 ºC (437 ºF). Unreported contaminant suspected to be present. Results shown are averages of duplicate tests.

A vacuum distillation column at a major petroleum company. This photo shows the crossover piping loops in foreground and 1,500 mm (60-inch) transfer line entering column tangentially. The transfer line and the column are lined with Type 316 stainless steel to resist naphthenic acid and sulfidic corrosion.

Fig 13– Corrosion Isotherms for Various Steels and MONEL alloy 400 in White Oil/Naphthenic Acid Blends at 235 ºC (455 ºF) Tempera-ture

Corrosion Rate

Initial 117 hr Second 73 hr

Alloy mm/y mpy mm/y mpy

Type 304 Stainless Steel 9.36 369 12.89 508

Type 347 Stainless Steel 5.89 232 7.26 286

Type 316 Stainless Steel 1.65 65 2.77 109

Page 53

stainless steels are explored as potential candidates. The ease of fabrication of the Type 304 stainless steel, com-bined with its more than satisfactory corrosion resistance, makes the material a prime candidate for such service.

No corrosion of Type 304 stainless steel in the most basic of the aromatic acids (benzoic) is apparent. Tests using a two per cent aqueous solution at 100 ºC (212 ºF) or of 10 per cent in an anhydrous octanol solution at 130 ºC (266 ºF) produced no attack on the alloy.

In many instances, the corrosion attributable to organic acids in such systems is compounded by the presence of sulfur compounds, the lower aliphatic acids, chlorides and other contaminants. Thus, in making a choice of materials for such service, the possible effect of chloride ion, sulfur ions, or other contaminants that may accumulate at times in the equipment must be considered. Stress-corrosion cracking of the austenitic stainless steels can be experi-enced under certain circumstances and must be evaluated thoroughly before the choice of such an alloy is made.

Gutzeit has pointed out that the corrosion in such systems is directly related to the neutralization number. Curves showing the corrosion rate for various alloys as related to the neutralization number are provided in his paper.40 One of those is reproduced here as Figure 13. Corrosion occurs in the liquid phase with only mild corrosion experienced in the vapor areas. Thus, hot condensate is always a potential corrosive in such a system. The use of Alloys 400 or 600 and 800 in such systems has merit. If stress-corrosion cracking of the stainless steels are experienced, the use of these alloys should be considered.

G. Organic Acids with Other Functional Groups

There are a large number of organic acids of complex structure which have found extensive use in industry and home. The corrosion characteristics of this group of compounds varies widely, as do the organic structures. Organic acids with other functional groups describe the

normal carboxylic acid terminus to the molecule, and, in addition, the incorporation of a halogen, an amino, a hydroxy addition, or other active ion added to the mole-cule which brings unique characteristics to the product.

1. Glycolic Acid The simplest of the organic acids in this category is glycolic (hydroxyacetic) acid. As an acid in aqueous solution, the material does not appear to be excessively corrosive at the lower temperatures. For instance, Type 304 stainless steel will show only .003 mm/y (0.1 mpy) corrosion rate or less in a 6% solution of glycolic acid at ambient temperature. At 50 ºC (122 ºF) during tests of eight days, both Types 304 and 316 stainless steels showed no attack in a 50% aqueous solution. Thus, the acid could not be described as exceedingly corrosive at conditions normally encountered. However, it has been found to be corrosive when heated to higher temperatures when contained in process streams as a contaminant. Again, the stainless steels resist attack at the high temperatures, but areas where steel would normally be acceptable become impractical with contamination of the streams by glycolic acid. Type 304 stainless steel is then required.

2. Lactic Acid Lactic acid (hydroxypropionic acid) is familiar to most persons as the corrosive agent in milk. To maintain the purity of the milk, tanks and tank trucks of Types 302 and 304 stainless steels have been constructed for many years for the handling of this precious commodity.

As indicated by Tables LXV and LXVI, Type 304 stainless steel and its cast counterpart CF-8 is most satisfactory for the handling of aqueous lactic acid solu-tions at the lower and intermediate temperatures. At some point between 2 and 10%, aqueous solutions of lactic acid begin to attack Type 304 and CF-8 stainless steels exces-sively. One can then use Type 316 stainless steel and its cast counterpart CF-8M which shows good resistance throughout the range of concentrations and temperatures explored (Table LXV11). Thorough testing of Type 316

TABLE LXV

Corrosion of Alloys in Various Concentrations of Aqueous Lactic Acid

Conditions: % Lactic Acid Temperature, ºC Temperature, ºF Test Period, daysOther

0.5 100 212

1 –

1

65 149

1 –

2

100 212

1 –

5

26 79 21

–

45 26 79 14

Aerated; agitated

10-50

54 129

15 Field Test in vacuum

evap.

Corrosion Rate

0.5% 1% 2% 5% 45% 10-50%

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

Type 304 Stainless Steel .03 1 Nil Nil .03 1 – – – – – – INCONEL alloy 600 – – – – – – – – Nil 0.1 .20 8 C71000

(Cupro- nickel 80- 20) – – – – – – .02 0.9 – – – – C71500

(Cupro-nickel 70-30) – – – – – – – – – – 1.4 57

References 19, 49

Page 54

service in specific applications. It will be noted that some rates exceeding .25 mm/y (10 mpy) have been obtained for the Type 316 alloy in lactic acid at boiling temperatures.

At the higher temperatures, it is considered to be good practice to use an L-grade stainless steel if welding is to be performed on the alloy. Table LXVI shows appreciable differences for annealed and sensitized conditions for cast stainless steels exposed to the acid at elevated tempera- tures and pressures. This admonition is true for the use of the stainless steels in all of the organic acids when exposed at the higher temperatures.

The use of Alloy 400 and the copper-nickel alloys is dependent on the aeration to be encountered in the acid stream. Nickel-copper alloy 400 has excellent resistance to all concentrations of the hot lactic acid solutions in the absence of air. However, corrosion becomes excessive if aeration is provided as a condition of the exposure.

TABLE LXVI

High Temperature Exposure of Cast Stainless Steels in

Aqueous 50% Lactic Acid

(Laboratory tests in autoclaves for 18-22 hours at temperatures shown)

4200-Gallon Uniframe Transport Container. One of a fleet of five Uniframe Type 304 stainless steel transport tanks with 6″ of foam in-place insulation being lifted aboard ship at Seattle with a load of milk for Alaska. These tanks make the long trip by flatbed truck trailer, ship, and rail flatcar between Seattle and Alaska.

3. Tartaric AcidTartaric acid (dihydroxybutanedioic acid) is one of the more innocuous acids produced in large quantity. As indicated by Table LXVIII, the product is not aggressive in aqueous solution up to the boiling point. Any of the austenitic stainless steels maintain purity in the product and prevent undesired contamination when storing or processing the tartaric acid solutions. Higher alloys are not indicated to be required for such service.

4. Citric Acid Citric acid (hydroxypropane tricarboxylic acid) is a more aggressive compound. This tart-tasting constituent of citrus products can be handled well by the austenitic stainless steels. Data for many of the other alloys are shown in Table LXIX. Here it will be noted that Alloy 400 is a candidate for use in many of the food product services. Others have described the use of MONEL alloy 400 and other nickel-base alloys for such use.14, 41, 42 As with the other organic acids, the presence of air will determine the rate of corrosion on Alloy 400 in these solutions. Alloy 600 and other alloys of chromium and nickel have good resistance to the acid and can be used when desired.

TABLE LXVII

Corrosion of Alloys in Boiling Aqueous Lactic Acid Solutions During Five-Day Laboratory Tests

*650 ºC (1202 ºF) for one hour, water quenched. Reference 9

Corrosion Rate

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

Type 304 .08–54.61 3–2150 56.64 2230 7.21 284

Stainless Steel

Type 309 3.30 130 – – – –

Stainless Steel

Type 316 <.03–.38 <1–15 .03 1 .08–.33 3–13

Stainless Steel

CARPENTER <.03 <1 .03 1 .18 7

alloy 20Cb-3

INCOLOY <.03 <1 – – – –

alloy 825

HASTELLOY .05 2 .03 1 .03 1

alloy C-276

HASTELLOY – – .10 4 .05 2

alloy B

INCONEL – – .43 17 .38 15

alloy 600

MONEL .13–.33 5–13 .15 6 .15 6

alloy 400

Copper .33 13 .05 2 .08 3

Zirconium Nil Nil Nil Nil Nil Nil

Titanium .03 1 .03 1 <.03 <1

Tantalum Nil Nil Nil Nil Nil Nil

Columbium <.03 <1 – – <.03 <1

Corrosion Rate

107 ºC (225 ºF) 151 ºC (304 ºF) 157 ºC (315 ºF) 162 ºC (324 ºF)

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

CF-8 Nil Nil – – 27.94 1100 – – CF-8* Nil Nil 44.20 1740 39.88 1570 – – CF-8M Nil Nil Nil Nil – – – – CF-8M* Nil Nil 2.03 80 – – .99 39 Copper 2.29 90 – – – – 1.65 65

References 49, 51

Page 55

Chloride is a commonly encountered contaminant in citric acid solutions. The effect on the austenitic stainless steels of this contaminant in 20% aqueous citric acid is shown in Table LXX. At levels up to 500 ppm of chloride, no significant general corrosion occurs on stainless steel

with such solutions. However, the presence of chlorides in combination with an acid create potential problems of pitting and stress-corrosion cracking particularly in cre-vices and other stagnant areas in the equipment.

TABLE LXVIII

Corrosion of Metals by Aqueous Tartaric Acid Solutions

Conditions: Laboratory tests without aeration or deaeration except as noted.

TABLE LXIX

Corrosion of Metals by Citric Acid

References 19, 46, 50

*Field test in evaporator during concentration of the acid. References 19, 49, 51

Corrosion Rate

2 5 10 25 30 50 57 %Tartaric Acid Temperature ºC (ºF) Test Period, days Other

26 (79) 21 –

26(79) 21 –

35(95) 6

Aerated

60(140) 6

Aerated

100(212) 6

Aerated

103(217) 2 –

35(95) 6

Aerated

60(140) 6

Aerated

100 (212) 6

Aerated

26(79) 11 –

60(140) 11 –

35(95) 6

Aerated

60(140) 6

Aerated

100(212) 6

Aerated

54(129) 10

Field lead.yacuum

yap.

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

Type 304 Stainless Steel – – – – – – – – – – Nil Nil – – – – – – – – – – – – – – – – – – Type 316 Stainless Steel – – – – – – – – – – .03 11 – – – – – – – – – – – – – – – – – – CARPENTER alloy 20 – – – – Nil Nil .01 .4 .01 0.2 – – Nil Nil .01 .02 Nil 0.1 – – – – Nil Nil .01 0.4 .05 2.1 – – INCONEL alloy 600 – – – – – – – – – – – – – – – – – – – – – – – – – – – – .06 2.4 ELGILOY – – – – – – – – – – Nil Nil – – – – – – – – – – – – – – – – – – Titanium – – – – Nil Nil Nil .1 Nil 0.1 – – Nil Nil Nil 0.1 Nil Nil – – – – Nil Nil Nil Nil .01 0.5 – – Zirconium – – – – Nil Nil Nil Nil Nil <0.1 – – Nil Nil Nil Nil Nil <0.1 – – – – Nil Nil Nil Nil Nil Nil – – Vanadium – – – – .01 0.4 .04 1.5 .48 19 – – – – – – – – – – – – – – – – – – – – C71000 (80-20 Cupro- nickel) – – .02 0.8 – – – – – – – – – – – – – – – – – – – – – – – – – – C71500 (70-30 Cupro- nickel) .04 1.6 – – – – – – – – – – – – – – – – .03 1.2 .05 1.8 – – – – – – – –

Corrosion Rate

% Citric Acid

Temperature

Test

Period

Type 316 Stainless

Steel

INCOLOY alloy 825

INCONEL alloy 600

Nickel 200

MONEL

alloy 400

C71500 (70-30 Cupro-nickel)

Copper

CARPENTER

alloy 20Cb-3

ºC ºF 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

1 26 78.8 44 – – – – – – .01 2 Nil Nil – – – – – –

2 26 78.8 7 – – – – – – .06 2.2 – – – – – – – – 26 78.8 21 – – – – – – .16 6.2 – – .06 2.5 .07 2.8 – – 5 16 60.8 30 – – – – – – .02 .9 .03 1.1 – – – – – – 30 86 7 – – – – – – .12 4.9 – – .02 .8 – – – – 60 140 7 – – – – – – .13 5 – – – – – – – – 7 102 216 3 – – – – 09 3.5 – – – – – – – – – – 10 100 212 6 – – – – – – – – – – – – – – .01 .2 15 66 150.8 3 – – – – – – .10 4.1 .07 2.7 – – – – – – 100 212 2 – – – – – – .11 4.2 – – – – – – – – 20 Boiling 45 Nil Nil – – – – – – – – – – – – – – 25 100 212 6 – – – – – – – – – – – – – – Nil .1 30 26 78.8 11 – – – – – – – – .04 1.5 – – – – – – 60 140 11 – – – – – – – – .19 7.4 – – – – – – Boiling 7 – – – – – – .22 8.8 .21 8.4 – – – – – – 50 20 68 1 – – – – – – – – .53 2.1 – – – – – – 100 212 6 – – – – – – – – – – – – – – Nil Nil Boiling 6 – – – – – – – – – – – – – – .14 5.5 58 26 78.8 7 – – – – – – Nil .1 Nil .1 – – – – – – 90 194 2 – – – – 53 21 – – – – – – – – – – Boiling 1 – – – – – – .43 16.8 .16 6.2 – – – – – – 61 60 140 30 – – – – 02 0.6 .01 .5 .02 .9 – – – – – – 60-78* 42-64 37 – – – – 06 2.4 – – – – – – – – – –

107.6- 147.2

65 Boiling 30 .21 8.1 .12 4.8 .79 31 .19 7.3 .11 4.2 – – – – – –

Page 56

TABLE LXX

Corrosion of Stainless Steels by Citric Acid Containing Chlorides

5. Chloroacetic Acids The chloroacetic acids are a most important product for the preparation of drugs, dyes, agricultural chemicals and as intermediates for the preparation of other organic compounds. Monochloroacetic acid and dichloroacetic acid are normally produced simultaneously and separated as desired. Trichloroacetic acid may be produced by an additional process step.

Corrosion data for a wide range of alloys exposed to various monochloroacetic acid solutions are contained in Table LXXI. It will be noted that Type 316 stainless steel appears to be attractive in a number of these exposures. Such an inducement for use of the austenitic stainless steels should be approached carefully. Pitting and stress-corrosion cracking in such a medium could be disastrous. A better choice for handling the product in aqueous

TABLE LXXI

Corrosion of Alloys in Monochloroacetic Acid (MCA)

a1–Laboratory test in 60% monochloroacetic acid liquor from a process containing 1.5% acetyl chloride, 0.5% hydrogen chloride and the remainder acetic acid. Light agitation.

2–Same as No. 1 with high agitation. 3–Field test in MCA liquor comparable to that of Test No. 1 4–Field test in tank containing 78% MCA in water with moderate aeration

5–Field test in same solution as Test No. 3, but with no aeration. 6–Field test in refined MCA in storage tank.

*Trademark of Driver-Harris Company

Testa Temperature ºC (ºF) Test Period, days

1 25 (77)

–

2 25 (77)

–

3 60 (140)

4 18 (64)

28

5 55 (131)

1 7

6 170 (338)

22

Corrosion Rate

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

MONEL alloy 400 .18 7 .28 11 .18 7 .43 17 .05 2 .10 4 Nickel 200 .51 20 .56 22 .18 7 .69 27 .03 1 .08 3 INCONEL alloy 600 – – – – – – .61 24 .03 1 3.56 140 Copper – – – – – – .48 19 .08 3 – – HASTELLOY alloy B .03 1 – – .51 20 .15 6 .03 1 .18 7 HASTELLOY alloy C .10 4 – – .94 37 <.03 <1 <.03 <1 .36 14 HASTELLOY alloy D .03 1 .33 13 .79 31 – – – – – – CARPENTER alloy 20 – – – – – – <.03 <1 .05 2 .70 28 Type 316 Stainless .20 8 .97 38 2.16 85 <.03 <1 .08 3 20.32 800 Type 317 Stainless – – – – – – <.03 <1 .05 2 – – DURICHLOR .66 26 2.03 80 2.21 87 <.03 <1 <.03 <1 1.27 50 Lead – – .94 37 – – – – .33 13 – – Silver – – – – – – .05 2 .03 1 – – 30% Nickel Cast Iron – – – – – – – – – – .53 21 NICHROME V* – – – – – – – – – – 1.50 59

Corrosion Rate

Solution 20 wt. per cent Citric Acid 20 wt. per cent Citric Acid

No Chloride 500 ppm NaCl No Chloride 500 ppm Chloride

Temperature, ºC (ºF) 85 (185) 85 (185) 100 (Boiling) (212) 100 (Boiling) (212)

Metal Specimen 1 Specimen 2 Specimen 1 Specimen 2 Specimen 1 Specimen 2 Specimen 1 Specimen 2

Type 304 <.03 <1 <.03 <1 .10 4 .08 3 <.03 <1 <.03 <1 .03 1 .03 1 Stainless Steel Type 316 <.03 <1 <.03 <1 Nil Nil Nil Nil <.03 <1 <.03 <1 <.03 <1 <.03 <1 Stainless Steel

Page 57

solution would be nickel-copper Alloy 400 or the nickel-based molybdenum-chromium-iron alloy. The use of Alloy 400 is contingent again on the removal of oxidizing species from the aqueous systems; residual chlorine, air or other oxidants, can greatly increase the rate of attack.

For the reaction area in chloroacetic acid production equipment, where chlorine is reacted with acetic acid, glass-lined steels, TEFLON-lined, or other fluorocarbon plastic-lined equipment is often used. HASTELLOY alloy C-276 appears to be acceptable for many of these ex-posures, but the conventional process utilizes lined equip-ment for the reaction area. Other metals such as tantalum or titanium may also be used if available.

Trichloroacetic acid is perhaps even more corrosive than monochloroacetic acid. Glass-lined equipment, titanium, HASTELLOY alloy B-2, DURICHLOR and certain other specific alloys selected after extensive testing may be used for handling the material at lower temperatures. None of the chloroacetic acids should be stored or processed in any quantity without a thorough understanding of the corrosive nature of these materials and the judicious choice of the materials of construction for tankage or process equipment. Although the nickel-based alloys are prime candidates for use in these solutions once the free chlorine is removed, all alloys may show evidence of pitting or crevice corrosion in the halogenated acids, and a thorough exploration of corrosion resistance of the various alloys in a specific stream should be conducted.

6. Amino Acids The aminocarboxylic acids are an important group of chemicals used for the preparation of drugs, agricultural chemicals and as precursors for numerous other organic compounds. As a group, the compounds are not exces-sively corrosive. The basic material glycine (aminoacetic acid) provides essentially the same corrosive charac-teristics as acetic acid at the lower temperatures and is less corrosive than its counterpart at the higher temperatures. As the molecule is lengthened, the amino acids become less corrosive, and those above approximately four car-bons in length can be considered as inhibitors in aqueous

systems at moderate temperatures. The decomposition products of such acids at the higher

temperatures can present unique corrosion problems that should be avoided. This is particularly true when nickel or copper-containing alloys are used. Discoloration of the amino acid can occur when using nickel or copper alloys at temperatures above ambient temperatures.

The austenitic stainless steels are most satisfactory for handling the amino acids. No problem with their use is usually anticipated until temperatures above the boiling point of the aqueous systems are encountered. Some of the acids not normally encountered, such as cyanuric acid, can be corrosive in streams and should be identified as a potential corrodent when choosing materials of construc-tion for applications involving amine solutions.

7. Sulfoacetic Acid Sulfoacetic acid characterizes one of those organic acids containing a sulfur atom. The material is not particularly corrosive once it is prepared and has the general charac-teristics of acetic acid itself. If the preparation is made by the addition of a strong sulfuric acid solution to acetic anhydride, the process conditions are too severe for use of the austenitic stainless steels. HASTELLOY alloys B-2 and C-276 are apparently acceptable for this step based upon service experience. Once the product is prepared, the austenitic stainless steels are almost always excellent for handling the acid up to 100 ºC (212 ºF).

INCOLOY alloy 825 tanks for the storage of monochloracetic acid resin solution. This alloy was required to maintain product purity.

Page 58

A. Acetic Esters

One major use of acetic acid is as a precursor of the various esters that become important solvents for paints and other chemical products. In the production of acetic esters, the acid is combined with other organic compounds contain-ing a hydroxyl group. The more common esters are ethyl acetate, butyl acetate, isopropyl acetate and Cellosolve acetate.

Corrosion to be expected in the preparation of these esters can vary greatly depending on the operation. If acetic acid were the only corrosive contaminant present, the data provided previously for acetic acid could be used as a guide. Unfortunately, a catalyst is necessary to improve the efficiency of the process, and in most instances, the presence of this catalyst determines the corrosion to be expected. Temperatures required for the production of these esters will range from 60 to 150 ºC (140-302 ºF), depending on the boiling point of the ester.

Sulfuric acid has long been used as the catalyst for synthesis of the esters. This is added as concentrated sulfuric acid in small quantities of only 0.5 to 2.0% of the total charge. In anhydrous medium, this would not be excessively corrosive. However, water is produced by the reaction between the alcohol and acid which can serve as a temporary diluent for the sulfuric acid. The water formed

PART IV. ESTER PREPARATIONS is continually removed as the process continues. The residual sulfuric acid concentration continually increases and can then create very aggressive conditions toward latter stages of a batch process run.

The kettle used for this process is of major concern. The heating coils, calandria, or other heating device sustains the major corrosion in the process. On the tubes of such a heater, severe pitting, grooving and general attack develop by concentration of the acid on the hot surface, by the formation of tars on the metal and, in some instances, by the accumulation of corrosive salts from the solution. As a consequence, it is exceptionally difficult to provide defini-tive data for the corrosion of a specific alloy in the preparation of these esters. Only empirical data obtained over a lengthy period of time will provide proper guidance for the final selection of the material of construction for the coils, kettle, vapor lines, condensers and a primary column for the process.

In Table LXXII, field data obtained by the exposure of numerous alloys in five different ester preparations are provided. It will be seen here that considerable variation exists in the data obtained for any one alloy. Because of the great turbulence existing and the factors enumerated above, the corrosion of an alloy in the same process during two different exposures can be greatly different.

Although the data would indicate that Type 304 stain-

TABLE LXXII

Corrosion of Alloys in Batch Acetic Ester Preparations

Conditions: Exposure of racks in same kettle during the preparation of esters using sulfuric acid catalyst. Temperature varies with ester prepared. Cupric ion present. Liquid (L) and vapor (V) exposures.

Test 1–Ethyl and isopropyl acetate alternately for 50 days @ 110 ºC (230 ºF). Test 2–Isopropyl acetate for 14 days @ 110 ºC (230 ºF). Test 3–Amyl acetate for 11 days @ 115 ºC (239 ºF). Test 4–Ethyl and isopropyl acetate alternately for 81 days @ 110 ºC (230 ºF). Test 5–Butyl and methyl Cellosolve acetate alternately for 29 days @ 115 º C(239 ºF) and 150 ºC (302 ºF).

Corrosion Rate

1 2 3 4 5

L V L V L V L V L V

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

– – – – .05 2 .56 22 .15 6 .28 11 .05 2 .43 17 .05 2 .30 12 Type 304 Stainless Steel

– – – – <.03 <1 – – .15 6 – – .28 11 – – .03 1 .03 1 Type 329 Stainless Steel

.18 7 .10 4 <.03 <1 .23 9 .18 7 .23 9 .08 3 .10 4 .03 1 <.03 <1 Type 316 Stainless Steel

.23 9 .41 16 – – – – – – – – .23 9 .08 3 – – – – Type 216 Stainless Steel

.13 5 .13 5 – – – – – – – – – – – – .03 1 CARPENTER alloy 20 Cb-3

– – – – – – – – – – – – – – – – – – .03 1 JESSOP alloy JS-700

– – – – .03 1 .03 1 .08 3 .08 3 .05 2 .05 2 – – – – HASTELLOY alloy G

.86 34 .15 6 – – .13 5 – – .08 3 – – .03 1 – – – – MONEL alloy 400 Copper 1.65 65 .10 4 4.57 180 .05 2 1.42 56 .08 3 2.72 107 .43 17 .13 5 .03 1

Page 59

less steel would be satisfactory for immersion conditions in an esterification kettle, this is most unlikely. The use of Types 316 or 316L stainless steels is borderline for these applications. As the alloy content is increased, reduced rates of attack are obtained, but the economics of the selection require detailed analysis before committing one to a final decision.

It will be noted that those materials high in nickel have good promise for use in the process. It has been reported that nickel-copper Alloy 400 has been used extensively for pumps, reactors, heating coils, piping and agitators for such acetic acid services in unaerated solutions where sulfuric acid is present.22 The data and literature show that in numerous instances nickel-chromium Alloy 600 has exhibited excellent resistance to esterification environ-ments. For instance, INCONEL alloy 600 exposed in an amyl acetate preparation at 149 ºC (300 ºF) during a 28-day exposure showed corrosion of only .15 mm/y (6 mpy) while MONEL alloy 400 showed a rate of .69 mm/y (27 mpy). Some combinations of Type 316 stainless steel, Alloys 400 and 600 and the copper alloys are indicated to be the basic choices for this service.

When using an austenitic stainless steel, such as the Type 316L, it can be shown that a considerable reduction of the corrosion rate can be achieved by the addition of oxidizing ions to the solution. Cupric and ferric ions are both effective for this purpose. One way of providing such an environment is the use of a copper alloy kettle with stainless steel heating coils. The stainless steel can have an adequate life in such service, whereas an all Type 316L stainless steel system would not be acceptable.

The effect of the concentration of sulfuric acid in such a batch process can be noted by reference to Table LXXIII. At the temperatures of the esterification reaction, the corrosion rate of Types 316 and 316L stainless steels

increases rapidly as the sulfuric acid is concentrated in the kettle. Also, there can be some small amount of degrada-tion of the acid to provide corrosive sulfur compounds in the vapor.

The severe effect of the acid conditions on a heating surface is apparent by reviewing the data of Table LXXIV. Laboratory “hot wall” tests of various alloys show the corrosion to be much higher for the materials than would be experienced in a simple boiling solution. Certainly, experience in the field has confirmed the severe corrosion to be expected on such heating surfaces in the process. For this reason, graphite calandrias are sometimes used to assure adequate resistance of the heating element.

There are other, less corrosive acids available for cata-lyzing the esterification reaction. Toluene sulfonic acid (TSA) has often been used for this purpose and, in

TABLE LXXIII

Effect of Sulfuric Acid Concentration on Corrosion of Type 316 Stainless Steel in an Esterification Reaction

Conditions: Solution of 25% acetic acid, 59% butyl acetate, 10% water and 6% butanol prepared and sulfuric acid added as indicated. Tests conducted at the boiling point.

Corrosion Rate % Sulfuric Acid

Added (as 95% H2SO4) mm/y mpy

0.0 Nil Nil 0.1 .48 19 0.5 5.92 233

1.0 17.53 690

TABLE LXXIV

“Hot Wall” Tests of Alloys in a Synthetic Esterification Mixture

Conditions: Laboratory tests using “hot wall” apparatus for three days (<3 days for alloys showing high corrosion rates) in a mixture of 83% acetic acid–9.3% formic acid–3.8% H2S04– 3.9% water. Comparison with conventional immersion test at boiling temperature 112 ºC (234 ºF) provided.

Hot Wall Solution

Temperature

Hot Wall Specimen

Temperature

Hot Wall Corrosion

Rate

Immersion Test Corrosion

Rate

Alloy ºC ºF ºC ºF mm/y mpy mm/y mpy

E-BRITS 26-1 120 248 150 302 3.94 155 1.88 74 HASTELLOY alloy G 112 234 155 311 .99 39 .20 8 HASTELLOY alloy C-276 118 244 140 284 .48 19 – – MONEL alloy 400 118 244 137 277 .41 16 – – Copper (C10200) 118 244 135 275 1.42 56 – – Zirconium 118 244 142 288 .03 1 – –

Type 316 Stainless Steel – – – – – – 3.63 143

Page 60

general, affords less corrosion of austenitic stainless steel surfaces than does the sulfuric acid. Table LXXV shows data obtained by field exposure of various alloys in a TSA catalyzed reaction during two different runs. Other mate-rials that can be used are benzene sulfonic acid and acetylsulfoacetic acid (ASA). Of these, the ASA is the least corrosive to the austenitic stainless steels but in-creases the rate of attack on copper alloys significantly.

Tables IV and XXI show other data relating to the preparation of these esters. Table XXI particularly lists a wide range of alloys evaluated in a synthetic butyl acetate reaction mixture.

One of the newer catalysts for use under certain circumstances for esterification reactions is boron tri-fluoride. Table LXXVI shows data generated by condi-tions required for such a reaction. Type 316 or Type 316L stainless steels appear to be adequate for this reaction. However, extensive testing should be conducted before committing an austenitic stainless steel to such a fluoride environment.

As indicated, essentially all the corrosion to be experi-enced in the esterification process occurs in the reaction kettle and appurtenant equipment. Distillation of the esters from the kettle is normally conducted in a Type 316L stainless steel still to assure low corrosion rates in this equipment. However, further refining of the ester, or other techniques required for improving the quality of the product, can be conducted in Type 304 stainless steel

equipment. The ester itself is innocuous and can be processed or handled in steel equipment if contamination of the product is not objectionable. Thus, the concern with corrosion in such a process is centered totally in the reaction area of the equipment.

B. Phthalate Esters The phthalate esters are prepared directly from the anhy-dride in a manner analogous to the preparation of the acetic esters. The temperatures are higher, but a drier medium is maintained than during acetic ester prepara-tions. Table LXXVII shows typical data generated by three exposures of numerous alloys in phthalic ester preparations. The same general statements as provided for the acetic esters relate to this type of exposure.

Phthalate esters prepared from octyl, decyl and other alcohols are important as plasticizers for various plastics. They also have excellent heat stability and can be used for heating mediums for a number of processes.

C. Esterification of Fatty Acids

Esterification of the fatty acids to produce soap is not exceptionally corrosive.28, 32-34 Data shown in Table LXXVIII reveal moderate corrosion of the stainless steels, and very low corrosion rates of the more highly alloyed materials in three different field exposures. As with other esterifications, once the esterification itself is completed, processing of the product becomes much less difficult; Type 304 stainless steel is satisfactory for such a purpose.TABLE LXXV

Corrosion of Alloys in a Typical Acetic Esters Reaction

Conditions: Batch reactions producing butyl acetate with kettle exposure of alloys in 25-45% acetic acid, 30% acetates, 20% alcohol, 5-8% water and 0.75% toluene sulfonic acid. Test 1 conducted at 107 ºC (225 ºF) for 34 days and Test 2 at 121 ºC (250 ºF) for 29 days.

D. Acrylate Esters

The acrylate esters comprise one of the newer, more reactive group of chemicals available for the synthesis of a

TABLE LXXVI

Comparison of Esterification Catalysts on Corrosion of Alloys

Conditions: Preparation of a higher acetic ester in semi-works equipment using 1.5 per cent sulfuric acid at 75-110 ºC (167-230 ºF) during 32 days for Test 1 and 0.32 per cent boron trifluoride at 75-85 ºC (167-185 ºF) for 5 days in Test 2.

Corrosion Rate

Test 1 Test 2

Alloy mm/y mpy mm/y mpy

Type 304 Stainless Steel 11.18 440 8.38 330

Type 329 Stainless Steel .25 10 – –

Type 316 Stainless Steel .33 13 1.14 45

Type 317 Stainless Steel .43 17 – –

CARPENTER alloy 20 .36 14 – –

INCOLOY alloy 825 1.27 50 – –

ILLIUM alloy G .58 23 – –

HASTELLOY alloy C .20 8 – –

HASTELLOY alloy B .58 23 – –

INCONEL alloy 600 .23 9 – –

MONEL alloy 400 .51 20 .18 7

Nickel 200 .99 39 .43 17

Copper .51 20 .18 7

Corrosion Rate

Test 1 Test 2

Alloy mm/y mpy mm/y mpy

Type 304 Stainless Steel .03 1 .43 17

Type 316 Stainless Steel .03 1 .05 2

E-BRITE 26-1 .05 2 2.49 98

CARPENTER alloy 20Cb-3 .03 1 .03 1

HASTELLOY alloy C-276 Nil Nil Nil Nil

Copper (C10200) .18 7 .30 12

Page 61

wide range of resinous products. The esters are best known as the starting material for the preparation of latex paints.

Previous comments given in the sections on acrylic acid and the acetic esters are pertinent to the production of the acrylates. It was pointed out in the discussion of the acrylic acid that a simultaneous production of the ester can be achieved starting with propylene. If the process produces only acrylic acid, the acid is reacted in a manner analogous to that used for the acetic esters.27

Ethyl acrylate is produced in a continuous system by the addition of sulfuric acid, or a similar catalyst, to the acid in alcohol. As in the case of the acetic esters, the conditions in the reactor are most aggressive. Type 316L stainless steel can usually be used for all equipment following the reaction step, and Type 304 stainless steel can be used for many of the recovery areas. On the other hand, the conditions in the kettle can be so severe that alloy materials higher than the austenitic stainless steels are required.

Table LXXIX shows typical data from the exposure of coupons in an ethyl acrylate synthesis. Note the extreme corrosion of Type 316 stainless steel which occurred. As in the case of acetic esters, combinations of nickel-copper Alloy 400, nickel-chromium Alloy 600, copper alloys and the nickel-base molybdenum-chromium-iron alloys may

be used to reduce corrosion in the reaction area. In the data shown, conditions in the vapor line from the reactor are even more corrosive than those encountered in the kettle liquid. This situation may or may not occur in a similar unit, depending on the mode of operation.

The coils, or other heating apparatus used on the kettle, will again experience the greatest corrosion. For this reason, a major effort should be made to identify the optimum material of construction for this service. Graph-ite construction is sometimes used for this specific area.

As for the other ester preparations, Type 304 stainless steel is adequate for many of the recovery stages following the reaction. If wash waters are used in the process, the possibility of stress-corrosion cracking from chlorides in the water should be considered. Otherwise, the stainless steels will provide product of a good quality at a minimum cost. Duplex structured stainless steel such as Type 329 or alloys containing higher nickel contents such as Alloys 600 and 800 are resistant to chloride stress-corrosion cracking in this service.

The higher acrylate esters (four carbon and higher) are produced in a manner comparable to the ethyl acrylate process. However, the temperatures are higher and the attendant corrosion is increased. Here the reaction condi-tions are extremely severe, as noted in Table LXXX. Extensive corrosion testing should be conducted to iden-tify the desired materials of construction for the reactor

TABLE LXXVII

Corrosion Generated in Phthalic Anhydride Esterifications

Exposure 1–Octyl phthalate batch preparation using 0.15% H2SO4 with trace chloride present in some batches. Exposure of 83 days on 84 rpm agitator shaft in kettle liquid at average of 149 ºC (300 ºF).

Exposure 2–Higher alcohols and phthalic anhydride plus 0.5% toluene sulfonic acid and 0.25% H2SO4 at 140 ºC (284 ºF) average for 135 days in kettle liquid.

Exposure 3–Toluene sulfonic acid catalyzed reaction of phthalic anhydride and higher alcohols at 174 ºC (345 ºF) for 10 days in glass laboratory kettle.

Corrosion Rate

1 2 3

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

Type 304 Stainless Steel .25 10 >1.27 >50 .05 2 Type 202 Stainless Steel .30 12 – – – – Type 316 Stainless Steel .15 6 1.60 63 .05 2 Type 317 Stainless Steel .08 3 1.04 41 – – CARPENTER alloy 20 .03 1 .03 1 – – ACI CN-7M Casting <.03 < 1 – – – – HASTELLOY alloy C .03* 1* – – .03 1 HASTELLLOY alloy B .08* 3* <.03 <1 – – INCOLOY alloy 825 .03 1 .03 1 – – INCONEL alloy 600 .10 4 .33 13 – – MONEL alloy 400 – – .08 3 .13 5 Nickel 200 .18 7 .20 8 – – Ni-Resist Type2 – – .15 6 – – Copper – – – – .18 7 Titanium .08* 3* – – – –

*No Pitting. All other alloys pitted to some extent.

Page 62

and accompanying equipment. In general, the vapor from the reactor is no worse than that described for the ethyl acrylate process. However, the first distillation column in the recovery chain can experience severe corrosion in the base, and the use of nickel-base molybdenum-chromium-iron alloys and other highly corrosion-resistant materials should be evaluated for use in this area.

As described before, conditions in the recovery system are not severe. The austenitic stainless steels are used for the vast majority of the equipment. Again, adequate attention should be given to the possible detrimental introduction of chlorides or other foreign species into the streams.

One other method of preparing ethyl acrylate is of

interest from a corrosion standpoint. The Reppe process prepares the ester from acetylene, carbon monoxide and alcohol. This reaction is conducted in an acid medium with nickel chloride present. As a consequence, corrosion in the reaction area can be very high.

Table LXXXI shows data obtained in a reaction to prepare ethyl acrylate by this procedure. Note that the liquid contains over two per cent hydrochloric acid along with free acrylic acid. Among the alloys tested, only HASTELLOY alloys B and C-276 appear to offer good corrosion resistance in this environment. Once the ester is removed from the reaction medium, the conventional materials of construction for recovery of the ester can be employed.

TABLE LXXVIII

Field Exposure of Alloys in Fatty Acid Esterifications

Exposure 1–On agitator shaft in liquid of kettle during esterification of C12-C18 fatty acids with alcohols + 0.25% H2SO4 at 100 ºC (212 ºF) for 33 days.

Exposure 2–In liquid of kettle near head during esterification of fatty acids (myristic present) with alcohols (isopropanol present) with sulfuric acid at approximately 110 ºC (230 ºF) for 82 days.

Exposure 3–Liquid and vapor phase of a kettle (liquid velocity ca. 16 ft/sec) for 18 days during glyceryl esterification, amidation and sulfurization of tall oil.

Corrosion Rate

1 2 3*

Liquid Liquid Liquid Vapor

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

Type 304 Stainless Steel .51 20 .16 6.2 .23 9 <.03 <1 Type 216 Stainless Steel – – .03 1.1 – – – – Type 316 Stainless Steel .51 20 .15 5.8 .05 2 <.03 <1 Type 317Stainless Steel – – .08 3.1 – – – – CARPENTER alloy 20Cb-3 .10 4 .01 0.4 – – – – ACI CN-7M Cast Alloy – – – – <.03 <1 <.03 <1 NITRONIC** 50 – – .02 0.9 – – – – INCOLOY alloy 825 – – .01 0.4 – – – – INCONEL alloy 600 .25 10 – – .13 5 <.03 <1 INCONEL alloy 625 – – .02 0.7 – – – – HASTELLOY alloy G – – .01 0.4 – – – – HASTELLOY alloy C-276 – – .01 0.5 <.03 <1 <.03 <1 HASTELLOY alloy B – – .09 3.7 .18 7 .15 6 JESSOP JS-700 – – .05 2.0 – – – – MONEL alloy 400 .10 4 – – .56 22 .30 12 Nickel 200 – – – – .97 38 .36 14 Copper .15 6 – _ – – – – Titanium – – .01 0.3 – – – – 30% Nickel Cast Iron – – – – .33 13 .15 16 Ni-Resist Type 1 – – – – .15 6 .30 12

*Reference 30 **Trademark of Armco Steel Corporation

Page 63

TABLE LXXIX

Corrosion of Alloys in Ethyl Acrylate Synthesis

Conditions: Specimens exposed in an ethyl acrylate reactor for 74 days at a temperature of 110 ºC (230 ºF) with sulfuric acid catalyst.

TABLE LXXXI

Corrosion of Alloys in Ethyl Acrylate Preparation from Acetylene

Conditions: Specimens exposed in the reaction kettle where acetylene, carbon monoxide, ethanol and nickel chloride were agitated heavily for 17 days. Acids formed were approximately 5% acrylic and 2.5% hydrochloric acid.

Corrosion Rate

Base of Reactor

Vapor Line from Reactor

Alloy mm/y mpy mm/y mpy

Type 304 Stainless Steel .89 35 5.08 200 Type 316 Stainless Steel .25 10 >10.16 >400 CARPENTER alloy 20 .13 5 .33 13 HASTELLOY alloy G .13 5 – – HASTELLOY alloy C .08 3 .20 8 HASTELLOY alloy B .03 1 .51 20 HASTELLOY alloy D .05 2 – – INCONEL alloy 600 .08 3 .91 36 MONEL alloy 400 .03 1 .58 23 Nickel 200 – – .63 25 Copper .03 1 1.14 45 Titanium .53 21 – – Zirconium <.03 <1 – –

Corrosion Rate

Alloy mm/y mpy

Type 316 Stainless Steel 1.27 50 CARPENTER alloy 20 .33 13 HASTELLOY alloy C-276 .08 3 HASTELLOY alloy B .05 2 Nickel 200 .25 10

MONEL alloy 400 2.03 80

Copper 3.30 130

TABLE LXXX

Corrosion of Alloys in Higher Acrylate Esters Production

Conditions: Exposure in base of reactor processing higher acrylates at 110-160 ºC (230-320 ºF) for times shown. Sulfuric acid catalyst used.

Butyl

67 days Octyl

11 days Decyl

11 days

Corrosion Rate

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

Type 316 Stainless Steel 3.63-5.49 143-216 10.16 400 >38.1 >1500 CARPENTER alloy 20 .13-.58 5-23 1.78 70 1.27 50 INCOLOY alloy 825 .61 24 – – – – HASTELLOY alloy G .10 4 – – – – HASTELLOY alloy C .08-.61 3-24 1.02 40 .76 30 HASTELLOY alloy B .15-.28 6-11 – – – – HASTELLOY alloy D .08 3 – – – – INCONEL alloy 600 .66-1.24 26-49 2.79 110 1.90 75 MONEL alloy 400 .18-.25 7-10 – – – – DURIRON .03 1 – – – – Titanium .08-.13 3- 5 – – – – Zirconium .03-.05 1- 2 – – <.03 <1

Page 64

l. Tsinman, A. L, et al., “Corrosion-Electrochemical Behavior of Metals in Organic Media,” Elecktrokhimiya, 11, No. 1, 1975, p. 127.

2. Flowers, J. W, Beck, E H. and Fontana, M. G., “Corrosion and Age Hardening Studies of Some Cast Stainless Alloys Containing Ferrite,” Corrosion, 19, No. 5. 1963, p. 186t.

3. Dillon, C. P, “Compositional Effects in the Corrosion of Type 347 and 316 Stainless Steel in Chemical Environments,” Corrosion, 16, No. 9, 1960, p. 433t.

4. Hamstead, A. C. and VanDelinder, L. S., “Comparative Corrosion Resistance of 200 and 300 Series Stainless Steels in Chemical Manufacturing Processes,” Corrosion, 15, No. 3, 1959, p. 147t.

5. Elder, G. B., “Corrosion by Organic Acid,” Process Industries Corrosion, Published by the National Association of Corrosion Engineers, 1975, p. 247.

6. Shimose, T, et al., “Corrosion Behavior of Various Metals and Alloys in Acetic Acid Environments,” Boshoku Gijutsu, 15, 2, 1966, pp. 49-55.

7. NACE Technical Committee Report, “Corrosion by Acetic Acid,” Corrosion, 13, No. 11, 1957, pp. 757t-766t.

8. Rabald, E., “The Influence of Oxygen on the Corrosion of Metals in Organic Acids,” International Symposium on Stainless Steels and Alloys with Special Regard to the Chemical Industry, Prague, No. 16, 11-13, September 1961, 10 pp.

9. Ohio State University Research Foundation, “Corrosion at Elevated Temperatures and Pressures,” Report to the Alloy Casting Institute, May 1, 1951.

10. Groves, N. D., et al., “Corrosion of Metals by Weak Acids Under Heat Transfer Conditions.” Corrosion, 17, No. 4, 1961, p. 173t.

11. “Intergranular Corrosion of Chromium-Nickel Stainless Steel,” Progress Report No. 1, Welding Research Council Bulletin No. 93, January, 1964.

12. Leonard, R. B., “Thermal Stability of Hastelloy Alloy C-276,” Corrosion, 25, No. 5, 1969, pp. 222-228.

13. ASTM Standards, “Detecting Susceptibility to Intergranular Attack in Wrought Nickel-Rich, Chromium-Bearing Alloys,” Part 10, Standard G 28-72.

14. Mason, J. E, Jr., “The Resistance of Alloys to Corrosion During the Processing of Some Foods,” Corrosion, 4, No. 7, 1948, p. 305.

15. “Guidelines for Selection of Marine Materials,” Publication A-404, The International Nickel Company, Inc., New York, 1971.

16. Togano, H., et al., “Corrosion Tests on Materials Used in the Synthesis of Acetic Acid from Methanol and Carbon Dioxide. I. Examination in Acetic Acid Solutions at Increased Temperatures,” Tokyo Kogyo Shikenso Hokoku, 57, 1962, pp. 342-50.

17. Togano, H., et al., “Corrosion Test on the Construction Materials for Acetic Acid Synthesis from Methanol and Carbon Monoxide (1) Corrosion Tests by Acetic Acid Solutions at Elevated Temperatures,” Boshoku Gijutsu, 10, 1961, pp.529-33.

18. Togano, H., et al., “Corrosion Tests on the Materials of Construction for Acetic Acid Synthesis from Carbon Monoxide and Methanol (II) Tests in Acetic Acid Solutions at Temperatures Below Boiling Points,” Tokyo Kogyo Shikensho Hokoku, 60, 6, 1965, pp. 221-31.

19. Uhlig, H. H., Corrosion Handbook, John Wiley and Sons, New York, 1958, pp. 89, 154, 156, 260, 274, 275, 289, 290, 298, 760.

20. Volokhova, V. 1., et al., “Study of the Corrosion Resistance of Electroless Nickel-Phosphorus Coatings,” Prot. Met., 11, No. 3, 1975, p. 370.

21. Anon., “Acetic Acid,” Chemical and Process Engineering, 47, No. 10, 1966, p. 51.

22. Kirk-Othmer, Encyclopedia of Chemical Technology; 2nd Edition, Vol. 8, John Wiley and Sons, New York, 1965, p. 386.

23. Loveless, L. W. J., “The Catalytic Cracking of Acetic Acid for the Production of Acetic Anhydride,” Trans. of the Am. Inst. of Chem. Engrs., 27, 1949, p. 7.

24. Miller, R. E, et al., “Corrosion by Acids at High Temperatures,” Corrosion, 10, No. 1, 1954, p. 7.

25. NACE Technical Committee T5A, “Formic Acid Corrosion of Common Materials of Construction,” Materials Performance, 13, No. 7, 1974, p. 13.

26. Teeple, H. O., “Corrosion by Some Organic Acids and Related Compounds,” Corrosion, 8, No. 1, 1952, p. 14.

27. Sittig, M., “Acrylic Acid and Esters,” Noyes Development Corp., 1965.

28. Friend. W Z. and Mason, J. E, “Corrosion Tests in the Processing of Soap and Fatty Acids,” Corrosion, 5, No. 11, 1949, p. 355.

29. Friend, W Z. and LaQue, F L., “Corrosion-Resisting Nickel Alloys and Chemical Progress,” Ind. and Eng. Chem., 44, No. 5, 1952, p. 965.

30. Templeton, H. C., “Corrosion Testing and Corrosion Problems in Processing of Tall Oil,” J. Am. Oil Chemists' Society, 36, March, 1959, p.127.

31. Templeton, II. C. “Corrosion of Molybdenum-Bearing Austenitic Stainless Steels and Other Alloys in Tall Oil Distillation,” TAPPI, 44, No. 10. 1961, p. 702.

32. Anon., “Fatty Acids,” Chem. Eng., 55, November, 1948, p. 103. 33. Potts, R. H. and McBride, G. W, “Armour's Star,” Chem. Eng.,

57, February, 1950, p. 124. 34. McBride, G. W, “Continuous Process for Soap,” Chem. Eng., 54,

April, 1947, p. 94. 35. Streicher, M. A., “Corrosion-Inhibition of Oxalic Acid,” USA

Patent No.2, 793, 190, May 21, 1957. 36. NACE Technical Committee Report, “Corrosion of Refinery

Equipment by Naphthenic Acid,” Materials Prot., 2, No. 9, September, 1963, pp. 90-96.

37. Witten, H. M., “Why Steels Fail in Refinery Service,” Hydrocarbon Process and Petroleum Refiner, 42, No. 2, 1963, pp. 151-156.

38. Derungs, W. A., “Naphthenic Acid Corrosion—An Old Enemy of the Petroleum Industry,” Corrosion, 12, No. 12, 1956, p. 617t.

39. NACE Technical Committee Report 8B163, “Corrosion of Refinery Equipment by Naphthenic Acid,” 1963.

40. Gutzeit, J., “Naphthenic Acid Corrosion,” Materials Performance, 16, 10, 1977, pp. 24-35.

41. Various, “Materials of Construction vs. Citric Acid,” Chem. Eng., 58, No. 3,1951, p. 211.

42. Cash, D. B., “Effect of Corrosion Products on the Flavor of Proc-essed Food,” Materials Performance, 14, No. 4, April, 1975, p. 28.

43. Pourbaix, M., Lectures on Electrochemical Corrosion, Plenum Press, New York, 1973, p. 50.

44. Eisenbrown, C. M. and Barbis, P R., “Corrosion of Metals by Acetic Acid,” Chem. Eng., 70, No. 9, 1963, p. 148.

45. “Corrosion Resistance of Hastelloy Alloys,” Stellite Division, Cabot Corporation.

46. Friend, W. Z., “Acetic Acid vs. Materials of Chemical Plant Construction: Nickel, Nickel Alloys,” Chem. Eng., 53, December, 1946, p. 218.

47. Labutin, A. L., “Corrosion and Methods of Protection of Equipment in the Manufacture of Organic Acids and Their Derivatives. The Corrosion of Metals and Nonmetallic Materials in Organic Acids,” Korroziya v Khim. Proizvodstv. i Sposoby Zashchity, 13, 1959, pp. 12-26.

48. Dillon, C. P, “The Role of Contaminants in Acetic Acid Corrosion,” Materials Protection, 21, No. 9, 1965, p. 4.

49. Private communication from L. S. VanDelinder. 50. MacLeary, D. L., “Testing of Columbium and Columbium

Alloys,” Corrosion, 18, No. 2, 1962, p. 671. 51. Lane, I. R., Jr., et al., “Corrosion Resistance of Titanium, Zir-

conium, and Stainless Steel in Organic Compounds,” Ind. and Eng. Chem., 45, 5, 1953, p. 1067.

52. Ward, L. E., Jr., “Design Characteristics for By-Product Fatty Acid Recovery,” Thesis, Virginia Polytechnic Institute, 1937.

REFERENCES

TRADEMARKS PRODUCT OF ALLEGHENY Allegheny Ludlum Steel Corporation ALOYCO Aloyco Inc. AMBRALOY Anaconda American Brass Co. CARPENTER Carpenter Technology Corporation CHLORIMET The Duriron Company, Inc. CROLOY Babcock & Wilcox Co. CRUCIBLE Colt Industries Inc. DURALOY The Duraloy Co. DURICHLOR The Duriron Company, Inc. DURIMET The Duriron Company, Inc. DURIRON The Duriron Company, Inc. E-BRITE Allegheny Ludlum Steel Corporation ELGILOY Elgiloy Co. EVERDUR Anaconda American Brass Co. HASTELLOY Cabot Corporation HAYNES Cabot Corporation ILLIUM Stainless Foundry & Engineering, Inc. INCOLOY INCO family of companies INCONEL INCO family of companies JESSOP Jessup Steel Company KROMARC Westinghouse Electric Corporation MONEL INCO family of companies MULTIMET Cabot Corporation MP35N Standard Pressed Steel Co. NICHROME Driver-Harris Company NITRONIC Armco Steel Corporation STELLITE Cabot Corporation TEFLON E. I. duPont de Nemours & Co. WAUKESHA Waukesha Foundry Company WORTHITE Worthington Corporation PH 15-7Mo Armco Steel Corporation 17-7PH Armco Steel Corporation 17-4PH Armco Steel Corporation