Anion adsorption-induced charge separation at the alumina/aluminum interface

We investigated the interaction between the $\alpha$-Al${}_2$O${}_3$(0001)/Al(111) interface and corrosion-related adsorbates using density functional theory calculations. The adsorption energies of Cl, OH, and H${}_2$O on the outermost oxide surface are endothermic, whereas O adsorption is unfavorable. Charge analysis revealed that the Cl, OH, and O adsorbates have a single negative charge due to charge transfer between the substrate and adsorbates, whereas the adsorbed H${}_2$O molecule remains nearly neutral. The spatial distribution of atomic charge in the interface system with Cl, OH, and O adatoms indicates charge separation between the $\alpha$-Al${}_2$O${}_3$(0001)/Al(111) interface region and the adsorbate. The intensity of the induced charge depends on adatom coverage. Local density of states revealed that charge separation generates an electric field within the oxide film. Additionally, calculations of oxygen vacancy formation and Cl insertion energies indicate that this induced electric field suppresses the energy barrier for oxygen vacancy and Cl migration within the passivation layer. Thus, charge separation induced by anion adsorbates promotes Cl atom migration through the passivation layer. These findings provide valuable insights into the mechanisms behind pitting corrosion and corrosion protection.