Modeling of Chloride Effect on Localized Corrosion Initiation at Grain Boundary Sites of Passive Oxide Surfaces

To understand the chloride (Cl)-induced initiation mechanism of localized corrosion on passive oxide films of structural alloys, we applied density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to investigate the interactions between Cl and hydroxylated <i>α</i>–Al₂O₃ / <i>α</i>–Cr₂O₃ surfaces, mainly (0001) orientation, under aqueous electrochemical conditions. Hydroxylated oxide surfaces thermodynamically stable in aqueous environments were constructed based on DFT calculations for both the single-crystal and bicrystal configurations. AIMD simulations suggest a Cl anion can only be stabilized on these surfaces by substituting a surface hydroxyl (OH) group. This substitution is thermodynamically favorable at sites on surface terminations of grain boundaries (GBs) in bicrystal configurations but not favorable at sites on single-crystal surfaces. Electronic structure analyses show that the different adsorption behaviors originate from the higher sensitivity of the cation–OH bond strength to the local coordination than its counterpart of the cation–Cl bond. The adsorbed Cl significantly increases the thermodynamic driving force for cation dissolution from passive oxide surfaces into the aqueous electrolyte, which can initiate localized corrosion. These analyses suggest the ideal alloying elements to increase the resistance of passive oxides to localized corrosion should be thermodynamically stable in these oxides and reduce the difference between Cl-related and OH-related interatomic bond strengths due to local atomistic structure variations.