Ab-initio investigation of the effect of adsorbed salts on segregation of Cr at the Ni (100) surface

Abstract

The dissolution and depletion of chromium (Cr) in salt facing nickel (Ni) alloy surfaces is one of the predominant degradation mechanisms of structural components in molten salt technology. In this work, we use density functional theory to investigate the role of electronic level interactions that may underlie the depletion phenomenon of Cr in a Ni 100 surface exposed to various adsorbed salt species. Our results show that, under vacuum, Ni preferentially segregates to the surface layer. Conversely, in the presence of adsorbed anionic salt species (e.g., chlorine (Cl), fluorine (F) or the impurity oxygen (O)) Cr segregation becomes more favorable. In these cases, Cl has the weakest effect on segregation, while O has the strongest effect. Our analysis reveals the strong correlation between Cr segregation and the amount of valence charge transferred between the Cr atom and surface adsorbate: the greater the charge transfer, the lower the segregation energy. We also show that, when considered, secondary cations screen Cr-anion interactions, which in turn reduce the magnitude of the anions effect on segregation. These results shed light on the role of salt impurities likely play in the overall corrosion phenomena in molten salt environments. This work provides insights into the atomic level interactions fundamental to molten salt corrosion and on the importance of maintaining salt purity.