Exploring electrochemical molten salt-metal interactions: Insights from ab initio molecular dynamics

Molten salts are promising for various engineering applications including energy storage, batteries and fuel cells, and advanced nuclear reactors. However, their promise is impaired by the corrosion of structural alloys such as stainless steels and Ni-based alloys. While corrosion is often attributed to impurities, understanding the electrochemical interactions between pure salts and metals is foundational for revealing the corrosion mechanisms. Using ab initio molecular dynamics, this work studies the interaction between prototypical austenitic $\mathrm{FeCr}$ alloy and molten $\mathrm{NaCl}$ and $\mathrm{NaF}$ salts, to elucidate the interaction between molten salts and metals. We observe the formation of a thin electric double layer and preferential segregation of anions nearby $\mathrm{Cr}$. The electronic density of states and electron density contour reveal weak, covalent interactions between anions and metal atoms. Bader charge analysis indicates that charge neutrality is maintained in both metal and salts without charge transfer, suggesting that the preferential anion segregation is likely caused by the different charge states of $\mathrm{Cr}$ and $\mathrm{Fe}$ instead of electrochemical reactions. The findings confirm the widely adopted assumption that pure salts do not corrode metals and emphasizes the importance of improving salt impurity for mitigating corrosion.