(560hv) Initial Stages of Chloride Enhanced Depassivation of Chromium Protected Layer, a Density Functional Theory Study

Corrosion is one of the major problems to the service life of concrete structures and energy infrastructure. Stainless steels are widely used as primary piping material because of their high corrosion resistance largely due to the formation of a protective passive film of metal oxides, such as chromium (e.g. Cr₂O₃). 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 environment surrounding stainless steel, but these ions can lead to a breakdown of the oxide film resulting in the loss of protection at 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 the breakdown mechanism of the passive film. Previous studies have suggested a mixed hydroxyls and adsorbed water termination of the Cr₂O₃ passive film in aqueous solution, so these are included in our study. The initial step of Chloride interactions with the partially hydroxylated surface was modeled by the Cl substitution with hydroxyls/water. The effect of Cl surface concentration on substitution and insertion was also calculated from 1/6ML to a monolayer coverage. The Cl insertion via exchange with subsurface oxygen (O) and Cl induced chromium (Cr) vacancy formation on the surface were examined at various coverages of Cl. These have been proposed to be the initial steps of Cl induced breakdown of the passive film. Competitive adsorption of Cl by substitution with OH and H₂O is more favorable at 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. Cr vacancy formation was found to be favorable at full coverage where Cr atoms on the surface are saturated by three Cl atoms. The result of these calculations implies that the initial stages of Cl induced breakdown of stainless steel passive film is concentration dependent in agreement with experimental observations.