The Effect of Surface Defects on Chloride-Induced Depassivation of Iron—A Density Functional Theory Study

Chloride-induced depassivation is a large contributor to the degradation of metals, but defects are likely to play a key role in that process. Here density functional theory calculations are used to investigate the mechanism of the initial stages of chloride-induced depassivation of iron by studying the Cl interactions with stepped α-Fe₂O₃ (0001) surfaces and how that can lead to degradation of the passive oxide film. The low coordinated Fe sites near the step edge and O vacancies facilitate high local coverages of adsorbed Cl, which enhance surface Fe vacancy formation significantly. The step edge also lowers the Cl insertion energy, relative to the flat surface, but insertion by exchange with O is still endothermic. This study illustrates the importance of surface defects, step edges, and O vacancies in the depassivation mechanism, but the findings generally support the point defect model as a description of the depassivation mechanism.