Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

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.

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

Similar data representing the

distillation of pimelic

stainless process.

steels can be used extensively throughout the

TABLE LXIII

Corrosion of Alloys in Glutaric Acid— Glutaric Anhydride Mixtures

Conditions:

260 2 500

210 3 410

40-90 104-194 (oxidation) 1 7

210 410 (distillation) 1 9

Temperature, ºC Temperature, ºF Exposure, days

2

13.5

Corrosion Rate

Alloy

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

1.58

62

.94

37

.69

27

Type 304 Stainless Steel

<.03

<1

.28

11

.20

8

41

16

Type 316 Stainless Steel

Nil

Nil

.15

6

–

–

–

–

CARPENTER alloy 20

–

–

.56

22

.86

34

–

–

03

1

Type 201 Stainless Steel

–

–

66

26

–

–

–

–

Type 202 Stainless Steel

<.03

<1

<.03

<1

–

–

–

–

HASTELLOY alloy B

–

–

Nil

Nil

Nil

Nil

–

–

HASTELLOY alloy C

–

–

–

–

–

–

23

9

INCONEL alloy 600

3.56

140

30

12

Nil

1

18

7

Copper

–

–

–

–

–

–

.20

8

MONEL alloy 400

(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.

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