A NOVEL EXPLANATION OF THE REACTIVE ELEMENT EFFECT IN ALLOY OXIDATION

The precise measurements of oxidation kinetics for an undoped Fe-25Cr alloy by Hussey et al. [1] have been analyzed to determine values for both for parabolic rate constant for diffusion and the linear rate constant for the interfacial step. The greatly reduced scaling kinetics for Fe-25Cr in response to a CeO2 surface doping are consistent with a blocking of the interfacial step for creation of cation interstitials at the metal/scale interface. For sufficient doping (Ce ion segregation) at the metal/scale interface, cation transport is replaced by anion diffusion over vacancies. The poisoning of the interfacial cationic reaction step results from pinning of the misfit edge dislocations whose climb action otherwise creates interstitial cations. But the creation of the anion vacancies at the metal/scale interface is not blocked by the segregated reactive element ions. This model provides a consistent interpretation for the commonly observed REE consequences in the growth of chromia scales on alloys, especially the changes in growth direction and scale growth kinetics.