Improvement of the application of an electrochemical method for the determination of transport properties of an?-alumina scale. Part II: Influence of yttrium and palladium on alumina scales developed on a?-NiAl alloy

Abstract

An electrochemical method based on analysis of potential-current curves at various appliedPO2 for an alumina scale developed at 1100°C on either undoped or yttrium-palladium doped β-NiAl alloys was used to determine transport parameters in the scale. In the case of scales formed on undoped or Y-doped NiAl alloys, theti variations, at least in thePo2 range experimentally studied, are very close: the scale consists of an outer domain characterized byti≈0.1, thenti increases with decreasingPo2. 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 theti variation and induces an inversion of the shape of σ1 variation withPo2. The analysis of these results suggests that at lowPO2 yttrium, localized in the inner part of the scale, decreasesti, 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 μo. 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.