Mechanistic study of internal oxidation of alloy 690 in hydrogenated steam

Abstract

The oxidation behavior of Alloy 690 exposed to 480 °C hydrogenated steam, in the nickel metal stability region, was studied using a variety of techniques, including time-resolved SEM, TEM, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and Electron Energy Loss Spectroscopy (EELS). The alloy underwent internal oxidation intragranularly, resulting in the expulsion of Ni metal nodules to the surface. A compact Cr-rich “healing” layer seemed to develop over time at the intragranular internal oxidation front near grain boundaries, and may significantly retard further internal oxidation. Protective external Cr-rich oxide formed on and adjacent to the grain boundaries and hindered intergranular oxidation. In view of the fact that the protective oxide forms microns away from the grain boundaries, one cannot simply say that Cr diffuses up the boundary and forms an oxide – internal stress relief must play a role, just as it does within the grains, only with different detailed mechanisms at play. Measurements of the growth of nickel metal nodules near the grain boundaries gives some insight into this topic.