Mechanistic Insights into the Initial Stages of Chloride Enhanced Depassivation of Chromium Oxide Film, a Density Functional Theory Study

Abstract

The formation of a passive film of metal oxides, such as chromium (e.g. Cr2O3) has been shown to play a key role in the corrosion resistance of stainless steels. Understanding the surface chemistry of these oxide surfaces, in different conditions, provides an essential insight into the stability and functionality of the protective layers. Aggressive ions, such as chlorides, are often part of the solution surrounding stainless steel, but these ions can lead to a breakdown of the oxide film and loss of protection for the metal surface. Here, we used density functional theory to investigate the interactions of Cl and adsorbed water with α-Cr2O3 (0001) passive film to better understand these breakdown mechanisms. Previous studies have suggested the Cr passive film (Cr2O3) is terminated by a mixture of hydroxyls and adsorbed water in aqueous solution, so mixed hydroxylated/water terminated surfaces are included in our study. The chloride (Cl) interaction was modeled by Cl substitution with hydroxyls and subsurface insertion of Cl via exchange with oxygen (O). These have been proposed to be the initial steps of Cl induced breakdown of the passive film. The effect of Cl surface concentration on substitution and insertion was also calculated from 1/6ML to a monolayer coverage. Competitive adsorption of Cl by substitution with OH and H2O is more favorable at the low Cl coverages but becomes less favorable at higher coverages while the insertion of Cl is unfavorable at lower coverages but becomes favorable at monolayer coverage. This implies that Cl can penetrate into the film at high Cl concentrations suggesting a Cl concentration dependent depassivation.