Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

PART III. OTHER ORGANIC ACIDS

A. Formic Acid

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.

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

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 70 78 90 97 100

.38 .33 .36 .15 .15

15 13 14

.41 .48 .51 .46 .13

16 19 20

.46 .89 .38 .61 .25

18 35 15 24 10

6 6

18

5

.11

4

.08

3

.25

10

*Condesate falling on one side of vapor area specimen.

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