  Pitting corrosion is a much-studied and technologically relevant subject.
However, the fundamental mechanisms responsible for the breakdown of the
passivating oxide layer are still subjects of debate. Chloride anions are known
to accelerate corrosion; relevant hypotheses include Cl insertion into
positively charged oxygen vacancies in the oxide film, and Cl adsorption on
passivating oxide surfaces, substituting for surface hydroxyl groups. In this
work, we conduct large-scale first principles modeling of explicit
metal/Al(2)O(3) interfaces to investigate the energetics and electronic
structures associated with these hypotheses. The explicit interface models
allow electron transfer that mimics electrochemical events, and the
establishment of the relation between atomic structures at different interfaces
and the electronic band alignment. For multiple model interfaces, we find that
doubly charged oxygen vacancies, which are key ingredients of the point defect
model (PDM) often used to analyze corrosion data, can only occur in the
presence of a potential gradient that raises the voltage. Cl-insertion into
oxide films can be energetically favorable in some oxygen vacancy sites,
depending on the voltage. We also discuss the challenges associated with
explicit DFT modeling of these complex interfaces.
