Combining the physics of metal/oxide heterostructure, interface dipole, band bending, crystallography, and surface state to understand heterogeneity contrast in oxidation and corrosion

Numerous effective medium models of metal oxidation and corrosion have been proposed over the years. These models are based on the macroscopic descriptions, and the driving forces for mass transport are conventionally expressed in terms of the electrochemical potential drops at the metal/oxide, oxide/environment interfaces, and the electric field in the oxide. Implicitly they average in some sense over microstructure, composition, and crystallography. An important issue with any effective medium approach is the degree of heterogeneity contrast, that is how much relevant properties or parameters vary spatially. Here the existing literature and, with additional density functional theory calculations, the magnitude of the heterogeneity contrast are analyzed. The physical phenomena in metal/oxide heterostructure, p/n semiconductor junction, and oxide surface such as the presence of interfacial dipole, band bending, doping effect, crystallography variation, and surface reconstruction is found, as well as surface state, lead to large heterogeneity contrasts. This implies that the simple, linear, effective, medium approaches may fail to describe the behavior properly.