Internal oxidation of alloy 690 in hydrogenated steam

Highlights: • An external Cr-rich oxide formed on grain boundaries limiting intergranular oxygen penetration in Alloy 690. • Intragranular internal oxidation of Alloy 690 resulted in expulsion of Ni to the surface. • Negligible lattice diffusion at 480 °C allowed for intragranular internal oxidation, even with high Cr content. • External surface oxidation at grain boundaries likely justifies the SCC resistance observed in Alloy 690. - Abstract: Alloy 690 was exposed to a hydrogenated steam environment at 480 °C and 1 bar, 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