Density Functional Theory Study of the Interactions of Cl and α-Fe2O3 Surfaces: The Role of Cl in the Depassivation Process

Abstract

Under the alkaline environment inside reinforced concrete, a protective passive film is formed on the iron surface. Chloride ions have been shown to be one of the aggressive ions that can cause depassivation of this passive film under the same condition but the atomistic mechanism of depassivation is not fully understood. Several hypotheses have been proposed for the role of Cl in the depassivation, such as ion exchange model and point defect model. Here we use density functional theory (DFT+U) calculations of Cl interactions with hematite (α-Fe2O3) to test these different hypotheses of Cl induced depassivation of iron passive film. The different hematite surfaces are used to represent the Fe (III) oxides which are the dominant structure in the outer layer of the passive film. On the flat (0001) surface, both pristine surfaces and surfaces with point defects are studied. Three point defects on the surface are considered, Fe vacancy, O vacancy and Fe-O pair vacancies. We found that the O vacancies enhanced the adsorption of Cl while the Fe vacancy weakens the adsorption. The insertion of the Cl into the sub surface was studied to investigate the stability of Cl staying in the bulk of oxide proposed in ion exchange model and was found to be endothermic for all four surfaces but surface defects have positive effects on the insertion of Cl, making it less endothermic with the smallest reaction energy of about 0.3 eV on the O vacancy surface. However, it is more difficult for Cl to stay in the sub surface lattice further away from the surface. The vacancy diffusion direction was studied by calculating the energy difference of vacancies in the different layers to investigate the point defect model. Fe vacancy is energetically more favorable in the deeper sub surface while O vacancy prefers to stay in the layers closer to the surface. The adsorbed Cl can affect the two diffusion process by making the two diffusion process more energetically favorable. These results indicate that the point defect model is the more promising model to explain the Cl-induced depassivation process.