CHROMIA SCALE GROWTH IN ALLOY OXIDATION AND THE REACTIVE ELEMENT EFFECT

An analysis for the kinetics of scale growth involving simultaneous cation and anion diffusion, and for anion diffusion with blocked cation diffusion, is presented and compared to experimental data in the literature. The significant reduction in scaling kinetics for the growth of chromia on pure Cr and on Fe-, Ni-. or Co-base alloys is attributed to the elimination of cation diffusion by the blocking of the cationic reaction step at the metal/scale interface. Large highly charged reactive element (RE) ions segregate at the metal/scale interface and pin the misfit dislocations whose climb otherwise serves to create interstitial cations (or annihilate vacancies). Then scale growth must proceed by oxygen diffusion over anion vacancies, corresponding to the lateral climb of misorientation dislocations (reaction at monoatomic steps) at the metal/scale interface. This poisoned interface model provides an interpretation for the reactive element effect (REE) which is consistent with the four well-known REE characteristics.