Sigma Phase Intergranular Corrosion Effects in Austenitic Welds Containing Ferrite

It has been suggested that the sigma phase and its preliminary phases are responsible for intergranular corrosion of molybdenum-containing austenitic stainless steels in such environments as 65 wt% boiling nitric acid when the material is heat treated or when ferrite is introduced by welding. In this paper, data obtained for joints welded by the tungsten inert gas (TIG) technique, annealed at 1100 C, and sensitized at 600 to 750 C for different times are discussed. The factors affecting intergranular corrosion included not only a third phase, but also (1) the morphology of the precipitates and (2) the thermal history of the material.