The influence of certain reactive elements on the oxidation behaviour of chromia- and alumina-forming alloys

Abstract

The influence of implanted yttrium and lanthanum on the oxidation behaviour of Co25Cr1Al chromia former as well as of β-NiAl alumina former has been studied under isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr 2O 3 scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results, on the one hand, from promotion of fine-grained scale formation and on the other, from changing the mechanism of scale growth from the outward diffusion of cations to the inward diffusion of anions. The influence of implanted yttrium (but not lanthanum) on the growth mechanism and adherence of alumina scale is analogous: the scale adherence is greatly improved but its growth rate is only slightly reduced. Lanthanum, on the other hand, greatly improves the protective properties of the scale but does not, in practice, affect its adherence to the substrate. As in the case of chromia formers, it has been found that the presence of reactive element atoms in the surface layer of the substrate considerably influences the mechanism of alumina scale growth by changing it from predominant inward diffusion of anions to prevailing outward diffusion of cations. It is believed that the influence of an implanted reactive element on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and in various interfacial phenomena.