Corrosion mechanism of Cr-rich nickel-based alloys in a ternary molten salt: morphology analysis and first-principles study of Cl adsorption

Abstract

The corrosion of metal materials by chloride molten salts has limited their high-temperature application as heat transfer/storage media, while the composition of alloys strongly affects the corrosiveness of molten salts. This work combines experiments and first-principles calculations to study the long-term corrosion mechanism at the interface between nickel-based alloys and NaCl–CaCl₂–MgCl₂ chloride molten salts. The findings indicate the considerable depletion of chromium within the metallic stratum situated beneath the corrosion oxide layer of chromium-enriched alloys. The corrosion morphology shows that the Cr-rich alloys Hastelloy C-276 and Hastelloy X produce many holes in their cross-section, with MgCr₂O₄ formed on their surface, while Hastelloy B-2 has fewer stable oxidation products on its surface and no MgCr₂O₄ is generated. First-principles calculations focusing on Cl atom adsorption suggest that the reaction process of Cl atoms on the surface of doped Cr (Fe or Mo)–Ni (111) crystals is the key to Cr loss, and the results show that the doped Cr–Ni (111) crystal surface exhibited a larger adsorption energy for Cl, higher charge transfer amount, and lower desorption energy for the corrosion product CrCl₄. The segregation energy difference of transferring a doped Cr atom from the second layer to the top surface layer is only 0.04 eV, while the adsorbed Cl is located around Cr on the Cr–Ni (111) crystal surface, indicating continuous Cr loss during the corrosion process. Therefore, if Cl atoms in molten salts are prevented from forming an adsorption layer on the alloy surface, the corrosion of Cr-rich nickel alloys in chloride molten salts can be reduced.