Transport through chromia films

Chromium oxide surface films that form in situ on alloy surfaces are the basis for providing high-temperature corrosion resistance when such alloys are used in high-temperature service. While the slow growth kinetics of chromium oxide is integral to its acting as a corrosion barrier, its periodic growth and spallation finally render alloys unprotective when the chromium concentration in the alloy gradually decreases from about ∼25–30% to about 10–15%. At these latter concentrations the ability of the alloy surface to form a continuous chromium oxide film becomes severely compromised. The ability to decrease the growth kinetics of chromium oxide films can thus prolong the service life of such alloys. Certain rare earth elements such as Ce and Y, whether they are introduced into the alloy as a dispersion of oxides or ion-implanted on the surface, have the ability to significantly reduce the growth rate of chromium oxide. Concomitantly, the major migrating species in the oxide film changes from chromium to oxygen. There is controversy in the literature on the mechanisms leading to these effects. The present study provides further advances in our understanding of this important effect.