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

Abstract

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.