EFFECT OF ION-IMPLANTED CERIUM ON THE GROWTH-RATE OF CHROMIA SCALES ON NI-CR ALLOYS

The oxidation kinetics of Ni-30 Cr and Ce-implanted Ni-30 Cr alloys are studied at low oxygen partial pressures in the temperature range 900-1100-degrees-C. The growth rate of Cr2O3 scales on the Ce-implanted alloys is slower by at least an order of magnitude. The Cr2O3 growth rate decreases with increasing Ce concentration but is independent of the ambient oxygen partial pressure. Oxide scales on the Ce-implanted alloys grow predominantly by inward oxygen diffusion. Microstructural characterization of the oxide scale shows segregation of Ce to oxide grain boundaries and the presence of CeCrO3 second-phase particles. These observations have led to a model in which Ce dopes into Cr2O3, in both the grain boundary and the lattice regions, consuming Cr interstitials through an equilibrium between CeCrO3 and Cr interstitials. The consumption of Cr interstials retards chromium transport to such an extent that inward oxygen diffusion via doubly ionized oxygen vacancies becomes the dominant mode of transport during oxide scale growth.