Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

In the vast majority of exposures, there is no difference in corrosion resistance between the wrought and cast alloys of similar analysis provided that both are in proper metallurgi- cal condition (annealed). The presence of small amounts of delta ferrite (2-10%) normally found in the austenitic matrix of the cast alloys does not lessen the corrosion resistance of the metal as illustrated by Table II. Even greater amounts of ferrite will show no deleterious effects in most pure acid media. Flowers, et al. 2 investigated ferrite contents in the CF-8 and CF-BM alloys up to 38 per cent and claim anodic polarization of the ferrite in such a dual phase alloy reduces overall attack on the metal. However, such passivity is not to be expected under all conditions of organic acid exposure and thorough testing of specific alloy compositions is advised. Other comparative data for the cast alloys may be found The addition of proper chromium-nickel ratios in a ferrous base to provide an austenitic stainless steel affords a limited resistance to organic acid exposures. Lower concentrations of pure acetic acid may be handled to the boiling point or the higher concentrations may be used to some 90 ºC (194 ºF) with Fe-Cr-Ni alloys such as Type 304 stainless steel. Adding greater amounts of chromium and nickel (Types 309 and 310 stainless steels) does not change the corrosion resistance of the alloys basically (see Table III). Using graphical multiple correlation techniques, Dillon has shown that chromium and nickel variations of the commercial alloys have little effect on the resistance to acetic acid. 3 At this time, there is no reason to believe that obtaining an austenitic matrix by the use of combinations of nickel, manganese and nitrogen imparts any change in the organic acid resistance of the alloy. 4 That is, a Type 204 stainless steel is equivalent to a Type 304 stainless steel and Type 216 is as resistant to acid attack as Type 316. See data in Tables III through V for the corrosion of the high manganese and nitrogen-containing stainless steels. in Table XXVII and Figure 1. 2. Effect of Alloy Composition

Cast ACI CF-8M valves and pumps in finished acetic acid storage service. Piping and tanks are constructed of Type 316L stainless steel. Courtesy Walworth Company-Aloyco Valves.

wrought Type 316 stainless steel, or Type 316L stainless steel if weld fabrication is to be employed. Forgings of these alloys are found as valve parts, perhaps as heat exchanger tube sheets, and for certain other structural parts. The pumps and many valves are constructed of the cast counterpart of the Type 316L stainless steel analysis known as ACI CF-3M. The ACI CF-8M (0.08 max carbon) is equally acceptable if in the solution annealed condition but has the disadvantage that weld repairs have to be followed by solution annealing to restore corrosion resistance.

FIG 2– Effect of Molybdenum Content on Corrosion of Austenitic Stainless Steels in Condensate from Boiling Acetic Acid Solutions

FIG 1– Corrosion of Cast Stainless Steels in Glacial Acetic Acid

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