The Comparative Effect of Carbon and Nitrogen on Inter-Granular Corrosion of 18∕8 Stainless Steel

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Abstract
The effect of high nitrogen in 18/8 stainless steel on susceptibility to intergranular corrosion is of an order of magnitude less than that of carbon, as determined by corrosion in boiling solution. A definite propensity to intergranular attack nevertheless occurs in a 0.2% N, 0.007% C 18/8 as proved by exposure to hot 15% by vol. by vol. pickle. This reagent is sufficiently severe compared with solution to produce slight but unmistakable intergranular corrosion in a columbium‐stabilized 18/8, heat treated for a number of hours at 600° C (1,100° F). Maximum magnetic induction of 0.2% N 18/8 results from heat treatment at 700° C (1,300° F), parallel in this respect to the behavior of 18/8 containing carbon. This temperature of heat treatment also produces maximum susceptibility of 0.2% N 18/8 to corrosion weight loss in pickle, with intergranular effects, however, more pronounced after treatment at 500° C (930°F) or 600° C (1,100° F). These results, although possibly explained by nitride precipitation, call for a more comprehensive theory of intergranular corrosion than any based on precipitation of a chromium nitride or chromium carbide as an a priori factor in the mechanism. This is indicated by definite intergranular corrosion of an austenitic 18% Cr, 24% Ni, balance Fe alloy, heat treated at 500° C (930° F) for 169 hours and exposed to solution, despite a carbon and nitrogen content of 0.003% and 0.006% respectively. The mechanism of corrosion in this alloy and in 18/8 appears to involve grain boundary precipitation of a metallic phase.

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