Mechanism of Intergranular Corrosion and Pitting Corrosion of Austenitic and Duplex 308 Stainless Steel

Abstract

The influence of microstructure on the intergranular corrosion and pitting corrosion resistance of 308 stainless steel was studied as a function of the volume percent of ferrite which was varied from 0 to 15%. The all‐austenitic matrix was highly susceptible to intergranular corrosion resulting from the formation of nearly continuous chromium‐depleted grain boundary paths caused by the intergranular precipitation of chromium‐rich . The presence of a critical amount and distribution of ferrite phase drastically reduced the susceptibility to intergranular corrosion. In addition to carbide precipitation, aging at 600°–700°C resulted in the decomposition of the ferrite phase into a variety of products. First the ferrite phase transformed to cellular, then into single phase γ, single phase σ, and finally into a duplexα+γ structure. At 480°C the chromium possessed insufficient mobility to form . Instead, an extremely small and finely distributed chromium‐rich precipitate, termed α', formed throughout the ferrite. From phase diagram measurements it was possible to estimate the chemical compositions and, in particular, the chromium contents of each of the phases formed during the various aging treatments. The changes in the sites of initiation of corrosion attack which accompanied the various phase changes could be satisfactorily explained on the basis of the compositions of the new phases. The phase transformations were followed by transmission electron microscopy and the corrosion attack was viewed with a scanning electron microscope.