DIFFUSION OF O-18 IN CR2O3 - BULK AND SCALES AND RELATION WITH OXIDATION-KINETICS

The lattice and grain boundary diffusion coefficients of O-18(at 0.1 a rm) were determined at 900 degrees C in bulk Cr2O3, and in Cr2O3 scale s developed on a Ni70Cr30 alloy. The diffusion profiles were establish ed by SIMS and analyzed considering two domains, the first one relativ e to apparent diffusion and the second to grain boundary diffusion. Wi th such an analysis and taking a f value = 0.0006, modified from a rid ge model, the oxygen lattice diffusion coefficients determined in Cr2O 3 scales are then in very good agreement with those in bulk Cr2O3. Wit h some assumptions, our diffusion data lead to a parabolic oxidation c onstant equal to the experimental one. The scale growth is ensured by counter-current diffusion of oxygen and chromium, mainly by grain boun dary diffusion.