Role of defect trapping in the enhanced corrosion resistance of Ni-Mo alloys with yttrium and yttrium oxide additions
Corrosion of structural alloys remains a critical challenge for advanced molten salt reactors, where aggressive fluoride salts promote continuous dissolution at the salt-metal interface. While most mitigation strategies emphasize thermodynamic stability, kinetic control of atomic transport has received comparatively less attention. Here, we propose a corrosion mitigation strategy based on diffusion control and vacancy trapping by dilute yttrium (Y) solutes and yttrium oxide (Y$_2$O$_3$) dispersions in Ni alloys. A combined computational and experimental approach integrates density functional theory (DFT), self-consistent mean-field (SCMF) diffusion modeling, and an impurity trapping model to quantify the impact of defect trapping on effective vacancy mobility. Strong first-nearest-neighbor vacancy-Y binding, off-lattice vacancy configurations, and elevated rotational and dissociation barriers result in pronounced suppression of vacancy diffusivity, while Y$_2$O$_3$-associated sites treated as fixed traps in the model further reduce the mobile vacancy population. Ni-Mo (NM), Y-modified (NMY), and Y$_2$O$_3$-containing (NMYO) alloys were tested in molten fluoride salt at 750°C. Corrosion penetration decreases systematically from NM to NMY to NMYO, consistent with the predicted reduction in vacancy-mediated transport. The same trend is observed in mass change, with net weight loss decreasing from 0.52% for NM to 0.23% for NMY and 0.09% for NMYO. These results are consistent with an alloy-side transport contribution from defect trapping and reduced atomic transport. This study therefore suggests point defect engineering as a promising alloy design strategy for extreme nuclear environments.