Effect of Active Elements on the Oxidation Behaviour of Cr2O3-Formers

Abstract

Active elements with a high affinity for oxygen, such as yttrium, cerium, lanthanum and other rare earths, have been known for half a century to improve the oxidation behaviour of high temperature alloys whose protection derives from the formation of Cr2O3 scales. It is now clear that the active elements can promote the formation of the protective scale, can reduce the rate of continuing scale growth and can inhibit scale failure processes. Various mechanisms, involving modifications to transport properties and mechanical behaviour of scales, have been proposed to explain the observed beneficial effects. These mechanisms will be reviewed in the light of available experimental evidence, including the dependence of protection upon the individual active element, its concentration and initial distribution and source (i.e. whether an alloying addition, oxide dispersion, coating or surface implant).