The reactive element effect (REE) in high temperature oxidation is discussed, for chromia-forming alloys, in terms of a mechanistic model based on postulations that (a) reactive elementloxide additions promote nucleation of Cr203 and (b) reactive element-oxides react with Cr203 to form perovskite-type compounds, such as YCr03, along scale boundaries which promote inward oxygen diffusion. The standard Gibbs energy of formation and concentrations of ionic defects for YCr03 are calculated. For silicon-containing chromia-forming alloys, the degradation of the scale adherence is shown to be the result of the a-/3 phase transformation in cristobalite. Morphologies of alumina scales formed over yttrium-containing Fe-20Cr-4Al alloys indicated that yttrium increased the stability of metastable alumina phases at 1173 K and decreased the amount of wrinkling in the a-alumina scale at 1273 K.

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