LAUSR

The reactive-element (RE) effect, which involves the “doping” of high-temperature Fe-base and Ni-base alloys with the so-called REs (Y, Zr, and Hf being the most common) to improve oxidation resistance, was discovered more than 80 years ago. (See Ref. [1] for a review of the early developments.) A substantial literature exists concerning the specific mechanism(s) underlying RE effectiveness, but general agreement on the mechanism is still lacking. (A recent review can be found in Ref. [2].) In the specific case of an Al2O3 scale, its growth can occur by outward diffusion of Al along scale grain boundaries and new oxide forming at the scale/gas interface, or by inward diffusion of oxygen, also along scale grain boundaries, and new oxide forming at the scale/metal interface [2]. In RE-free alloys, these processes can occur simultaneously over a wide temperature range. This is itself curious, as assuming that the oxygen and Al grain boundary diffusivities show Arrhenius behavior, there should only be a narrow temperature range where the diffusivities are comparable, unless the Arrhenius parameters are very similar. (Lattice diffusion of both oxygen and Al is so sluggish that any contributions to scaling are essentially nil [3].) There is general agreement in the high-temperature corrosion community that the improved oxidation resistance due to RE doping is manifested by greater scale adherence and frequently observed reduced scale growth kinetics. Specifically, judicious RE doping serves to getter S and C trace impurities that would otherwise adversely affect scale adhesion. Moreover, REs tend to segregate to scale grain boundaries where they can apparently interact with diffusing species to cause