- Carbon steel, which contains mostly iron, has been widely used as reinforcement in reinforced concrete. In the alkaline environment in reinforced concrete, a thin iron oxide film, called the passive film, forms on the surface of the steel, which protects the surface from active corrosion. The exact atomic structure of the passive film is complex but experimental studies suggest that the passive film is made of two layers, an FeII-rich oxide/oxyhydroxides inner layer and an FeIII-rich oxide/oxyhydroxides outer layer or three layers with an intermediate layer between the two layers. Hematite (α-Fe2O3) has been suggested as one of the dominant oxides in the outer layer, which is used to represent the outer layer of the iron passive film in this study.
Aggressive ions such as chloride, from deicing agents and marine salts, cause the breakdown of the passive film (depassivation) in the alkaline environments, leading to active corrosion. The mechanism of the Cl-induced depassivation is still debated but revealing the atomistic depassivation mechanism is the main focus of this dissertation. Several hypotheses have been proposed to explain the mechanism, including the ion exchange model and the point defect model. The ion exchange model proposes Cl penetration into the oxide lattice via O vacancies or exchange with bulk O while the role of Cl proposed in the point defect model is in enhancing the Fe vacancy formation on the surface, which then diffuses to the metal/oxide interface. This study uses density functional theory (DFT) to investigate the feasibility of the ion exchange model and the point defect model in explaining the Cl-induced depassivation mechanism of the iron passive film.
First, we investigate the OH and Cl adsorption and co-adsorption on Fe-terminated α-Fe2O3 (0001) surface and the structural effects of these adsorptions. The adsorption of both OH and Cl alters the structure of the surface, but the effect is localized and it mostly affects the positions of the top two layers. The adsorption of Cl and OH at low coverage (1/3 ML) has comparable effects on the surface structure, pulling the outermost Fe atom out of the surface by up to 0.4 Å. The structural change increases with coverage and the largest change is caused by OH adsorption at 3/3 ML, which pulls the outermost Fe atom out by 0.64 Å. This suggests that although the adsorption does affect the surface structure, the adsorption on the pristine surface is not strong enough to lead to the breakdown of the surface nor the ingress of Cl into the oxide film.
Second, the effect of surface vacancies is investigated by comparing the Cl interactions with pristine and vacancy surfaces. On both pristine and vacancy surfaces, subsurface Cl is less stable than adsorbed Cl which contradicts the ion exchange model that assumes Cl ingress into the passive film. The vacancy formation on the surface is found to be enhanced by Cl, which is consistent with the point defect model. Additionally, the diffusion direction of vacancies proposed in the point defect model is supported by the energy difference of these vacancies in different oxide layers. Also, O vacancy has a positive effect on the Fe vacancy formation and Cl insertion, suggesting that O vacancy may play a role in the depassivation process.
Finally, the investigation of the feasibility of the two models in explaining Cl-induced depassivation of iron passive film is extended to using the stepped surfaces to represent the complexity of the passive film. Iron surface atoms at the step edge and near an O vacancy have lower charge than the bulk iron atoms but these lower charged iron atoms facilitate higher local coverage of Cl, where more than one Cl interact with the same iron atom. The Fe-Clx (x = 2 or 3) interactions pull the Fe atom out of the surface by up to 2.07 Å, which favors the Fe vacancy formation with the lowest formation energy of 0.18 eV. The Cl insertion by exchange with bulk O is still endothermic and the effect of the step edge on the Cl insertion is comparable to that of a Fe vacancy on the flat surface. The initial stages of the Cl-induced depassivation shown in a reactive force field molecular dynamics study are confirmed here with DFT.
Overall, the findings support the point defect model in explaining the depassivation of the iron passive film, and the step edges and O vacancies play important roles in the Cl-induced depassivation process.