STUDY BY SIMS OF THE CR-54 AND O-18 DIFFUSION IN CR2O3 AND IN CR2O3 SCALES

The diffusivities, in lattice and along grain boundary, of Cr-54 and O -18 were determined at 900 degrees C and pO(2)=10(4) Pa in Cr2O3 (sing le and polycrystals) and in Cr,O, scales formed on a Ni70Cr30 alloy by oxidation. All the concentration profiles were established by SIMS wh ich allows an accurate determination of the diffusion profiles in mate rials such as oxides which are characterized by a very low diffusivity , even with an important surface roughness as in oxide scales. With ou r analysis for the concentration profiles and the f value, fraction of atomic sites associated with grain boundaries modified by a ridge mod el, it appears that the lattice diffusion coefficients of Cr and O det ermined in Cr2O3 scales are close to those determined in Cr2O3 single or polycrystals. However, the grain boundary diffusion coefficients of Cr and O determined in polycrystals are smaller than those determined in scales. The lattice diffusion coefficients of Cr and O are close t o each other. But, the chromium grain boundary diffusion coefficient i s slightly greater than that of oxygen.