Alloy oxidation studies related to PWSCC

摘要

The intergranular oxidation model of PWSCC has been tested by exposing several Ni based materials to water and steam environments containing hydrogen, at temperatures up to 480°C, and analyzing the resultant oxidized surfaces. If the surfaces were mechanically prepared, short-circuit outward diffusion of Cr occurred, and the differences between alloys or alloy conditions (600, 690, 600TT) were minimal. Only on deformation-free surfaces were the differences in oxidation response of the materials displayed clearly. For hydrogen partial pressures greater than that corresponding to the Ni/NiO equilibrium, Alloy 600 showed intergranular and internal oxidation with the growth of nodules of pure Ni within the grains. The oxidation morphology was strongly dependent on grain orientation. Nodule-free zones appeared near grain boundaries, which were also enriched in metallic Ni. Thermal treatment to precipitate grain boundary carbides eliminated or strongly modified the intergranular oxidation and the presence of nodules. No signs of internal oxidation were observed on Alloy 690, which underwent some thermal etching but formed a nearly continuous external oxide. The role of surface diffusion in nodule formation is discussed.