The Role of Carbon, Nitrogen, and Heat Treatment in the Dissolution of Iron-Chromium Alloys in Acids Available to Purchase

Iron-chromium alloys of varying chromium content were used to investigate the effect of chromium on general corrosion in oxidizing and reducing sulfuric acid solutions. The results were applied to the interpretation of the effect of composition and heat treatments on the structure and corrosion performance (rate and type of localized attack) of two commercial heats each of AISI Type 430 (16% Cr) and 446 (25% Cr) stainless steels. Several laboratory heats with controlled amounts of carbon and nitrogen were also investigated to separate the role of these two elements in generating changes in composition which make iron-chromium alloys subject to rapid, localized attack in acids. Corrosion in three oxidizing, boiling acid solutions was investigated to determine their sensitivity in detecting microstructural changes in composition resulting from various heat treatments: (1) 65% nitric acid, (2) ferric sulfate-50% sulfuric acid, and (3) cupric sulfate-50% sulfuric acid with metallic copper. The action of reducing acids was investigated with a boiling 5% sulfuric acid solution. The ferric sulfate-50% sulfuric acid solution was found to be the simplest, most rapid, quantitative method for detecting susceptibility to rapid, localized attack in iron-chromium alloys. It was found that comprehensive characterization of the structures produced by heat treatments requires both metallographic examination of etch structures and determination of corrosion rates and types of preferential localized attack by hot acids. These data were supplemented by chemical and X-ray diffraction analysis of bulk specimens and isolated particles and by electron microscopy. The chromium depletion mechanism was found to provide a generally applicable theoretical basis for comprehension of the many complex phenomena observed. A number of these results provide new support for this theory.