Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

welded regular carbon grades are free from intergranular corrosion in the heat-affected zones of welds. To supplement the curves of Figure 12, the data of Table XLIV summarize the resistance of several alloys to propionic acid solutions below the boiling point. When the temperature is raised appreciably and pro- pionic anhydride is added to the acid, the stainless steels, including the iron-base nickel-chromium-copper-molyb- denum alloys, are no longer useful as a material of construction. Table XLV shows data derived from a test conducted at 260 ºC (500 ºF). For all alloys considered for a specific service involving propionic acid, the data presented for acetic acid may be used as a general guide. It is important to use data acquired at the proper temperature, keeping in mind that the boiling point of propionic acid is much higher than that of acetic acid and that tests conducted below the boiling point are not the same as those made in a boiling solution. Organic acids of greater chain length than the 3-carbon acids are produced in smaller quantity, but constitute an important group of products, primarily as intermediates in the preparation of pharmaceuticals, agricultural chemicals, food products, plasticizers and other end-use chemicals. The chemical characteristics of the longer monocarbox- ylic acids are important in interpreting the corrosive potential of the products. Complete miscibility in water of the two three-carbon acids (propionic and acrylic) is achieved, but the solubility of the remaining acids de- creases rapidly with increasing chain length. The extent of dissociation of the dissolved acid remains essentially the same as acetic acid. However, in the pure form, or in organic dilutions of the acids, the higher acids are in-

HASTELLOY alloy C-276 replaced silver in this primary cooler for propionic acid. The alloy was found to have better resistance to thermal cycling than the precious metal. Courtesy of Stellite Division, Cabot Corporation.

creasingly stable with increasing chain length. Numerous tests conducted in butyric and higher acids indicate that the exposure is innocuous until some specific temperature is reached, at which point sufficient dissociation is achieved to initiate and sustain corrosion. This critical temperature is higher for each succeeding higher homo- logue in the series. Thus, for a specific acid, a temperature of 190 ºC (374 ºF) may produce essentially no corrosion on a Type 304 stainless steel, but a temperature of 210 ºC (410 ºF) may produce exceedingly high corrosion rates.

FIG 12—Corrosion of Alloys in Propionic Acid at the Boiling Temperature

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