Internal oxidation of alloy 690 in hydrogenated steam

Alloy 690 was exposed to a hydrogenated steam environment at 480°C and 1bar, considered to simulate primary water in a pressurized water reactor. The possibility of internal oxidation in Alloy 690 was evaluated and the likely relevance to SCC in primary water is discussed. Initial surface imaging revealed intragranular metallic Ni nodules, indicating that internal oxidation had occurred. Grain boundaries provided a short circuit for outward Cr diffusion and an external surface oxide was observed at grain boundaries. FIB trenching was used to study oxidation phenomena occurring beneath the surface and EDX elemental maps were generated.