Characterization of Internal Oxidation in Alloy 690

The oxidation behavior of Alloy 690 was investigated after exposure in hydrogenated steam at 480 °C. The alloy underwent internal oxidation intragranularly, resulting in the expulsion of Ni to the surface. Protective external Cr-rich oxides formed around the grain boundaries and hindered oxygen penetration and intergranular oxidation. A compact Cr-rich healing layer was able to develop over time at the internal oxidation front initiating at the grain boundary surface oxide, thus effectively stopping internal oxidation from penetrating further into the bulk alloy. Additional exposures performed on Alloy 690TT, Ni-30Cr-10Fe and Ni-35Cr-10Fe model alloys found that increased outward Cr flux due to intergranular carbides and increased Cr concentration greatly reduce the depth of internal oxidation. Short circuit diffusion along grain boundaries and diffusion induced grain boundary migration are found to significantly increase the diffusion of the reactive components and promote the formation of the protective oxide layer.M.A.S