The passivity of iron in alkaline media enables the use of carbon steel as reinforcement in concrete, which makes up the majority of modern infrastructure. However, chlorides, mainly from deicing chemicals or marine salts, can break down the iron passive film and cause active corrosion. Despite recent advances in nanoscale characterization of iron passivity, significant gaps exist in our understanding of the dynamic processes that lead to the chloride-induced breakdown of passive films. In this study, chlorideinduced depassivation of iron in pH 13.5 NaOH solution is studied using reactive force field molecular dynamics. The depassivation process initiates by local acidification of the electrolyte near the film surface, followed by iron dissolution into the electrolyte, and iron vacancy formation in the passive film. Chlorides do not penetrate the passive film, but mainly act as a catalyst for the formation of iron vacancies, which diffuse toward the metal/oxide interface, suggesting a depassivation mechanism consistent with the pointdefect model.

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Hematite (α-Fe2O3) has been identified as the dominant structure of the outermost layer of the passive film formed on iron surfaces in alkaline environment but chloride has been suggested to play a major role in the depassivation of this passive film under the same conditions. Here we study the effects of OH and Cl adsorption as well as co-adsorption at different coverages on the structure of α-Fe2O3 Fe-terminated (0001) surface using DFT+U. The adsorption of both OH and Cl alters the structure of the surface, by effecting mostly the positions of the top two layers. The structural change increases with coverage for two adsorbates on a (1x1) surface and the study on a larger (2x2) surface shows that the effects are localized. The large effect of the OH and coadsorption of OH and Cl on the surface suggest that OH could play an important role in the Clinduced depassivation process.