Oxygen-accelerated corrosion of Alloy 800 in a molten LiCl-KCl-MgCl2 eutectic mixture

This study investigates the corrosion behavior of Alloy 800 (Fe-32Ni-21Cr) in a ternary eutectic molten salt mixture (LiCl-KCl-MgCl₂) at 350 °C and 750 °C under flowing argon (Ar) and Ar+ 1 % O₂ atmospheres. The objective is to understand the effect of minor oxygen ingress on corrosion in a relatively pure salt where corrosion had ceased. At lower temperature (350 °C), increasing exposure time in pure Ar conditions led to minimal changes in corrosion depth after 48 h, suggesting that corrosion had stopped after this time. The addition of 1 % oxygen to the gas flow resulted in only slight increases in corrosion, with larger ligaments observed on the surface and minor roughening in the cross-sectional images. Similarly, 750°C testing under pure Ar showed only grain boundary attack at 12 h, and increasing exposure time did not significantly increase the corrosion depth. However, in contrast to 350 °C testing, the addition of 1 % oxygen resulted in increased grain boundary attack and dealloying, specifically at and adjacent to grain boundaries. These findings suggest that minor oxygen ingress into a molten halide salt environment can significantly increase the corrosion rate in a short period of time at high homologous temperatures, which is relevant to practical applications of molten salts where minor oxygen ingress is a likely upset condition.