The effect of reactive element additions on the selective oxidation, growth and adhesion of chromia scales

The addition of minor amounts of reactive elements to Cr2O3 or Al2O3 forming alloys has been known to produce a number of beneficial effects in improving their oxidation resistance. The mechanism by which these elements influence oxidation is, however unclear. In this paper, the effect of reactive element addition, whether as surface or alloying additives, on the development, growth and adhesion of Cr2O3 scales is discussed. It is pointed out that the development of a continuous external Cr2O3 layer and the elimination of base metal oxidation are not coupled processes; reactive element additions can eliminate the latter, but not always affect the former. The incorporation of a reactive element in oxide scales is essential for modifying scale growth. However, a slower oxidation rate or a change in the overall oxide growth direction are not critical factors in improving scale adhesion. In light of these and other experimental results, current theories concerning the reactive element effect are critically evaluated.