Preferential intergranular oxidation of Alloy X-750 spacers in CANDU off-chemistry environments

Abstract

Preferential intergranular oxidation (PIO) is often suggested as a potential mechanism for primary water stress corrosion cracking (PWSCC) of Alloy 600 (Ni-16Cr-9Fe). Alloy X-750 is a precipitation-hardened variant of Alloy 600 with additional Al and Ti leading to formation of a γ' phase. Alloy X-750 is used for garter spring spacers in CANDU reactor fuel channels, and has experienced irradiation embrittlement in-core. Recognition of the irradiation embrittlement combined with X-750 being a material with known susceptibly to stress corrosion cracking (SCC) in reactor coolant environments, raised questions regarding any plausible risks of SCC to garter springs. While the spacers normally reside in a dry oxidizing mixture of CO₂ and O₂, extreme, reducing off-chemistry conditions could develop, potentially leading to PIO similar to observations in primary water. Aged and annealed samples of Alloy X-750 were exposed to different CO-CO2 mixtures with the oxygen partial pressure maintained below the NiO dissociation pressure. Internal and intergranular oxidation was observed under these conditions, similar to Alloy 600 exposed to 480 °C hydrogenated steam. Further nanoscale analysis revealed that the presence of intergranular Cr carbides hindered intergranular oxidation. Also, intergranular oxidation of Al and Ti occurred prior to Cr oxidation, suggesting some influence of minor alloying elements in PIO.