A local equilibrium model for internal oxidation Original Research Article

Alloys that are internally oxidized under conditions of local equilibrium have concentration profiles that must satisfy certain conditions. These conditions are outlined in general and then illustrated with an error function model that assumes a constant effective diffusivity in the oxidized region and a linear variation of oxide solubility with concentration. The error function model is different from previous error function models in that it satisfies all of the local equilibrium conditions. A numerical example is given to illustrate predictions of the model. The model predictions include that the average concentration of the solute (oxidized solute) in the internal oxidation region will always be less than the initial concentration of solute in the alloy, that the average concentration of solute decreases with increasing solute diffusivity, and that the concentration of solute at the moving boundary between the internal oxidation region and unoxidized region approaches the initial alloy concentration as the solubility product and solute diffusivity approach zero.