Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

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 A. General 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.

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.)

PART II. ACETIC ACID

B. Austenitic Stainless Steels 1. General

The 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 0.006 0.06

5.2 4.4 3.9 3.4 2.7 2.4

0.6 6.0 60.0 (6%)

Reference 43

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