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 degree s C by determining diffusivities, both in bulk and along the gain boun dary, of O-18 and Cr-54 in Cr2O3 scales formed on a Ni70Cr30 alloy wit h 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 obta in a direct comparison of self-diffusion. Ail the concentration profil es were established by SIMS. Without yttrium in Cr2O3 scales, the oxyg en bulk diffusion coefficient is close lo that of chromium, but the ch romium grain boundary diffusion coefficient is greater than that of ox ygen. The presence of yttrium in Cr2O3 scales slightly decreases the g rain 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 conclud ed that the Cr2O3 scale growth is controlled by counter-current diffus ion of oxygen and chromium for the unimplanted and implanted alloys, m ainly by grain boundary diffusion. Yttrium does not change the growth mechanism.