Diffusion and growth mechanism of Al2O3 scales on ferritic Fe-Cr-Al alloys

In order to determine the growth mechanism of alpha alumina scales, cationic and anionic diffusion studies have been performed on scales developed on three alumina former Fe-Cr-Al alloys. Two of them were classically elaborated and differed by the presence or not of implanted yttrium. The third one is an ODS alloy, the MA 956. First? the oxidation behaviour in oxygen was characterised in a temperature range 1000-1200 degrees C, with morphological studies and analyses of the scales. Then, anionic self diffusion coefficients were determined by isotopic exchange O-16-O-18, cationic self diffusion was simulated by Cr diffusion and the penetration profiles were determined by SIMS. These profiles were analysed considering two domains: the first one relative to effective diffusion and the second to grain boundary diffusion. The f values, fractions of sites associated with the grain boundaries were modified by considering the roughness of the scales. The anionic and cationic self-diffusion coefficients are compared with those extrapolated from massive aluminas and the experimental parabolic oxidation constants are compared with those calculated with diffusionnal data. The results obtained are discussed by taking into account the differences in the scale morphology which looks like an important parameter. Then, the effect of active elements on the morphology and on the transport and mechanical properties is commented. (C) 1998 Elsevier Science S.A. All rights reserved.