Internal oxidation is observed when a less noble alloying element is added to a comparatively more noble base. The internal oxidation front moves inward at a rate determined by the concentration of oxygen at the scale/metal interface, the diffusion rate of oxygen and the solute element in the metal matrix, and the size and distribution of the oxide particles, the last factor contributing the most in case of concentrated alloys. According to Wagner,1 for the transition from internal or internal plus external oxidation, to exclusively external oxidation, it is necessary that the solute concentration should be such that it forms a critical volume fraction of internal oxide precipitate which can then block further reaction between oxygen and solute atoms. This indicates that the transition is related to the morphology of precipitate as it changes with increasing concentrations of alloying elements.
Studies carried out in this laboratory2 on Cu-Mn...
