Effects of Sulfides on the Corrosion Behavior of Inconel 600 and Incoloy 825 in Supercritical Water

Corrosion tests of Inconel 600 and Incoloy 825 were performed in supercritical water containing sulfides at 520 °C and 25 MPa. The scales formed on Inconel 600 consisted of a thick outer layer of almost pure NiS and an inner layer predominantly of Cr-rich spinels with a NiS network inside. The scales of Incoloy 825 also had a multilayered structure. The outer layer primarily consisted of NiS and (Ni,Fe)S followed by a thicker intermediate mixture layer of chromium-rich oxides, NiS and (Ni,Fe)S, an inner layer of predominantly CrO and a small amount of NiS toward the substrate. Incoloy 825 showed slightly better corrosion resistance than Inconel 600 due to its higher Cr content. By means of superimposed isothermal phase diagrams and evaluation for the partial pressures of oxygen and sulfur in the designated atmosphere, the corresponding corrosion mechanisms were discussed, based on the present investigation.