Microstructure and microsegregation effects in the intergranular corrosion of austenitic stainless steel

Abstract

Intergranular corrosion in austenitic stainless steel was studied using transmission electron microscopy of corroded thin foils and electron probe microanalysis of bulk specimens. In the sensitized material, since carbides remained unattacked in corroded grain boundaries after exposure to a boiling copper sulfate-sulfuric acid solution, the location and severity of corrosion could be directly observed in relation to individual carbide particles for various precipitate morphologies. After the sensitized material was exposed to a potassium dichromate nitric acid solution, carbides were consistently absent from corroded grain boundaries as the particles themselves apparently became susceptible to attack in this environment. Chemical composition inhomogeneities were measured for nickel and chromium in the commercially annealed material and found to become more pronounced upon sensitization heat treatments. Such inhomogeneities can result in chemical composition differences across grain boundaries, which in turn can lead to electrochemical action that may adversely affect intergranular corrosion behavior.