DIFFUSION OF O-18 IN MASSIVE CR2O3 AND IN CR2O3 SCALES AT 900-DEGREES-C AND ITS RELATION TO THE OXIDATION-KINETICS OF CHROMIA FORMING ALLOYS

The lattice and grain-boundary diffusion coefficients of O-18 at P-O2=0.1 atm and at 900 degrees C were determined in massive Cr2O3 and in Cr2O3 scales which were grown on a Ni-30Cr alloy. The diffusion profiles were established by SIMS and analyzed considering two domains in the case of polycrystalline Cr2O3 (massive or scales), the first one relative to apparent diffusion and the second to grain-boundary diffusion. A ridge model is proposed for Cr2O3 scales to modify the f value, fraction of sites associated with the grain boundary. With such a model, f is equal to 0.0006 and 0.0005 for the scales formed during 15 hr and 165 hr; respectively. The oxygen-lattice diffusion coefficients determined in Cr2O3 scales are in very good agreement with those in massive Cr2O3 . With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant equal to the experimental one. Scale growth occurs by countercurrent diffusion of oxygen and chromium, mainly by grain-boundary diffusion.