A kinetic and morphological study of the oxidation of Ti-Cr alloys in CO 2 at 1000°C

Abstract

The oxidation of Ti-5 and Ti-15 wt.% Cr alloys has been studied in CO 2 at 1000 °C. The kinetics of oxidation are parabolic, there being a change in the rate constant after 2–3 h oxidation. The oxide films are duplex consisting of an inner porous layer and an outer dense layer. Electron probe microanalysis of the scales indicates that Cr takes part in the oxidation reaction. The Cr content of the scales on the 5% Cr alloy is 0·15·5%, whereas with the 15% Cr alloy the inner porous oxide contains up to 2% Cr whilst the outer region contains only ~ 0·3% Cr. The oxide scales thicken as a linear function of time and comparison of the results for the alloys with those for unalloyed Ti under similar exposure conditions indicates that both alloys oxidize more readily than Ti. A greater proportion of the total oxygen uptake goes into solid solution in the alloy substrates than is the case with unalloyed titanium. Oxygen diffusion in the substrate is believed to be the rate controlling process in the oxidation of the alloys.