(732h) Density Functional Theory Study of the Effect of Step Edges on ?-Fe2O3 Surfaces on Cl-Surface Interactions and the Cl-Induced Depassivation Process

In a highly alkaline environment, such as inside reinforced concrete, a passive film is formed on the surface of iron which protects the surface from active corrosion. Experimentally, chloride in sufficient concentration has been suggested to be one of the aggressive ions that can cause depassivation of this passive film under the same condition. Several hypotheses have been proposed for the role of chloride in the depassivation, such as ion exchange model and point defect model, but the atomistic mechanism of depassivation is not fully understood. Here we use density functional theory to investigate the role of chloride in the depassivation process.

Fe (III) rich oxides are the dominant oxides in the outer layer of the iron passive film. Here hematite (α-Fe₂O₃) and different step edges of that surface are used to represent the Fe(III) rich outer layer of the passive film. The adsorption of Cl is mostly affected by the step edge site and the Fe site closest to the step edge, but the effect mostly dies out beyond two Fe atoms from the step edge. The energetics of two different depassivation models, ion exchange and point defect models are compared. The ion exchange model assumes a diffusion of Cl into the bulk but no exothermic insertion of Cl into the bulk is identified. The point defect model on the other hand proposes the diffusion of Fe and O vacancies. Previous studies on flat surface have suggested thermodynamic favorable diffusion of the two vacancies. It also assumes that Cl can assist with the surface Fe vacancy formation and a Cl enhanced Fe vacancy formation has been confirmed on the flat surface. Here the effect of steps and defects on these processes is discussed.