Growth mechanism of Cr2O3 scales: Oxygen and chromium diffusion, oxidation kinetics and effect of yttrium

The growth mechanism of Cr2O3 scales was studied at 900 and 800 degrees C by determining diffusivities, both in bulk and along the gain boundary, of O-18 and Cr-54 in Cr2O3 scales formed on a Ni70Cr30 alloy with and without yttrium implants (10(16) ions cm(-2)). The diffusion of O-18 and Cr-54 was carried out simultaneously which allowed us to obtain a direct comparison of self-diffusion. Ail the concentration profiles were established by SIMS. Without yttrium in Cr2O3 scales, the oxygen bulk diffusion coefficient is close lo that of chromium, but the chromium grain boundary diffusion coefficient is greater than that of oxygen. The presence of yttrium in Cr2O3 scales slightly decreases the grain boundary diffusivities of O-18 and Cr-54, but enhances the oxygen lattice diffusion. The parabolic oxidation constant rates calculated from our diffusion data according to Wagner theory, were close to the experimental ones determined by oxidation kinetic tests. It is concluded that the Cr2O3 scale growth is controlled by counter-current diffusion of oxygen and chromium for the unimplanted and implanted alloys, mainly by grain boundary diffusion. Yttrium does not change the growth mechanism.