Mechanistic study of moisture corrosion of FeCr alloys in molten salts by ab-initio molecular dynamics simulations

Molten salts are promising for various energy applications including fuel and solar cells and nuclear energy. These applications face a common challenge: corrosion of structural materials by impurities such as H₂O. This work employs ab-initio molecular dynamics simulations to study H₂O induced corrosion of FeCr alloys in molten NaF and NaCl salts. H₂O is found highly stable in both salts, with infrequent, reversible dissociation into OH⁻ and H⁺ along with HF or HCl formation. The dissociation tendency correlates positively with the electronegativity and negatively with the size of halogen atoms. Accordingly, H₂O reaches the salt/metal interface as a molecule before reacting with metal. Reduction of H⁺ is found to occur without simultaneous oxidation of specific metal atoms such as Cr, suggesting sequential instead of the commonly proposed concurrent reduction and oxidation. The reduced H atoms prefer to stay at the interface and may re-enter NaF but not NaCl, highlighting the influence of salt chemistry.