Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

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

TABLE X

Corrosion of Alloys in Contaminated Acetic Acid

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

Corrosion Rate

Test

Type 316

CARPENTER

HASTELLOY

No.

Test Medium

Stainless Steel alloy 20Cb-3

alloy C

mm/y mpy

mm/y mpy mm/y mpy

1 2

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

.08 .94

3 37

<.03 .84

<1 33

Nil .03

Nil 1

3

1.32

52

1.07

42

.03

1

4

1.73

68

1.47

58

.03

1

5

(4) +1% Water

.03

1

.03

1

.03

1

TABLE XI

Corrosion of Type 316 Stainless Steel in Acetic Acid Solutions

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

Specimen Wall Temperature

Acid Tested

Exposure Period

Corrosion Rate

Addition

º C

º F

hr

mm/y

mpy

Plant A

(None)

48 96 68 92 48 48 68 96 48

136 146 132 131 137 141 149 152 140

277 295 270 268 278 286 300 306 284

.23 .10 .03 .03 .03

9 4 1 1 1 <1

Plant A

1%water

Plant A Plant A Plant B

0.5% formic 1.0% formic (None)

<.03 7.80 12.55 .03

307 494 1

Plant B

1 % water

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

TABLE XII

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

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

190 190

374 374

.69 .56

27 22

– –

– –

240

464

.61

24

.89

35

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

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