Phase-field model of oxidation: Equilibrium.

A phase-field model of an oxide relevant to corrosion resistant alloys for film thicknesses below the Debye length L_{D}, where charge neutrality in the oxide does not occur, is formulated. The phase-field model is validated in the Wagner limit using a sharp interface Gouy-Chapman model for the electrostatic double layer. The phase-field simulations show that equilibrium oxide films below the Wagner limit are charged throughout due to their inability to electrostatically screen charge over the length of the film, L. The character of the defect and charge distribution profiles in the oxide vary depending on whether reduced oxygen adatoms are present on the gas-oxide interface. The Fermi level in the oxide increases for thinner films, approaching the Fermi level of the metal in the limit L/L_{D}→0, which increases the driving force for adsorbed oxygen reduction at the gas-oxide interface.