Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium

Oxidation of the Ni-based superalloy RR1000 has been undertaken in air over the temperature range 600-900 °C for times up to 5000 h. The surface oxide consisted of a protective Ti-doped chromia layer but with rutile forming on its outer surface. Sub-surface oxidation of Al and Ti also occurred. The thickening kinetics of the chromia layer were sub-parabolic with initial rates around two orders of magnitude higher than expected for Ti-free chromia. This enhancement and the sub-parabolic kinetics are accounted for by Ti-doping of the chromia layer. Over time the enhancement reduced because of Ti-depletion in the alloy.