Improvement of the application of an electrochemical method for the determination of transport properties of an alpha-alumina scale .2. Influence of yttrium and palladium on alumina scales developed on a beta-NiAl alloy

An electrochemical method bused on analysis of potential-current curves at various applied P-O2 for an alumina scale developed at 1100 degrees C on either undoped or yttrium-palladium eloped beta-NiAl alloys was used to determine transport parameters in the scale. In the case of scales for med on undoped or Y-doped NiAl alloys, the t(i) variations, at least in the P-O2 range experimentally studied, are very close: the scale consists of an outer domain characterized by t(i) similar to 0.1, then t(i) increases with decreasing P-O2. The ionic conductivity in both cases shows a V-shape and decreases in the presence of Y. The presence of palladium notably modifies the shape of the t(i) variation and induces an inversion of the shape of sigma(i) variation with P-O2. The analysis of these results suggests that at low P-O2 yttrium, localized in the inner part of the scale, decreases t(i), creates defect complexes and intergranular yttrogarnet precipitates, and prevents the outer part of the scale from contamination by nickel, which is thought also to give defect complexes when present. The variation of the oxygen chemical potential inside the scale shows a progressive variation of mu(o). The calculated oxidation constants are close to the experimental ones, which indicates that the alumina scale growth is ensured by grain-boundary diffusion of ionic species.