Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

3. Effect of Contaminants Although pure acetic acid can be handled readily in many alloys, the presence of only parts per million of other chemical agents can render an alloy useless as a material of construction. Acetic anhydride is produced as a co-product in the older acetaldehyde oxidation process for acetic acid, and the anhydride can often be found in other acetic acid process streams. When small quantities of the anhydride exist in a glacial acid, a greatly accelerated attack on the stainless steels can be anticipated. Tables IV, IX and X incorporate data substantiating the adverse effect of anhydride in acetic acid as reported by Elder 5 and others. The difference in the two commercial, glacial acids shown in Table XI can probably be attributed to the presence of anhydride in the product of Plant B. As the amount of anhydride in the acid is increased, the rate of attack rapidly drops to an acceptable level, and high concentrations of anhydride are innocuous. (See section on Acetic Anhydride.) However, the presence of small amounts of anhydride sufficient to dehydrate the acid produces in- creased attack on all alloys. 6 Oxygen may influence corrosion rates in acetic acid, and other organic acids as well. Even though process streams have been stripped of gaseous components in distillation systems, the possibility of oxygen pickup from air leaks into the system is present. The use of stainless steels as materials of construction assures that no accelerated attack will occur under such circumstances. Indeed, when corro- sion of the stainless steels in a process system is higher than desired, the rate of attack can often be reduced by introducing oxygen into the system. Table XLIII shows the effect of adding oxygen to a distillation column during the processing of propionic acid. A hundred-fold reduc- tion in the corrosion rate is evident as the oxygen provided

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

TABLE VIII

Corrosion of Metals in Acetic Acid Residue Still

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

Corrosion Rate

Liquid

Vapor

Alloy

mm/y

mpy mm/y

mpy

Cast iron Ni-Resist Type 11 Mild steel

2.13 .97 2.01 2.01 1.22 .18 Nil

84 38 79 79 48 7 Nil 2 7 30

1.32 .30 2.51 1.47

52 12 99 58 14 5 Nil 5 12 14

Type 501 chrome steel Type 430 stainless steel

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

INCONEL alloy 600 HASTELLOY alloy C DURIMET * 20

.05 .18 .76 .03 .03

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

1 1

7 2

*Trademark of The Duriron Company, Inc.

TABLE IX Corrosion of Type 316 Stainless Steel in Acetic Acid Solutions Containing Chlorides Conditions: Duplicate 48-hour tests conducted at the boiling temperature in glacial acetic acid with additions made as shown.

Corrosion Rate Chloride Ion Added, ppm*

0

18

36

61

Diluent addition to acid

mm/y

mpy

mm/y

mpy 2 50**

mm/y .43 1.22**

mpy 17 48**

mm/y 2.10 1.19**

mpy

– 1.98

– 78

.05 1.27**

81 75*

None 0.2% Acetic Anhydride

.03 .03 – .03

1 1

– – .08 – .03 .18

– – 3 – 1 7

– – .33 – .66 .41

– – 13 – 26 16

– – .71 – .38 .36

– – 28 – 15 14

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

–

1

– –

– –

* Added as sodium chloride ** Minute, profuse pitting

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