Growth mechanism of Cr 2O 3 scales: oxygen and chromium diffusion, oxidation kinetics and effect of yttrium

Abstract

The growth mechanism of Cr 2O 3 scales was studied at 900 and 800 °C by determining diffusivities, both in bulk and along the grain boundary, of 18O and 54Cr in Cr 2O 3 scales formed on a Ni 70Cr 30 alloy with and without yttrium implants (10 16 ions cm −2). The diffusion of 18O and 54Cr was carried out simultaneously which allowed us to obtain a direct comparison of self-diffusion. All the concentration profiles were established by SIMS. Without yttrium in Cr 2O 3 scales, the oxygen bulk diffusion coefficient is close to that of chromium, but the chromium grain boundary diffusion coefficient is greater than that of oxygen. The presence of yttrium in Cr 2O 3 scales slightly decreases the grain boundary diffusivities of 18O and 54Cr, 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 Cr 2O 3 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.