High temperature oxidation of vanadium-chromium alloys by low pressure oxygen

Oxidation behaviour of vanadium-chromium alloys containing 5 and 10at% chromium has been investigated at temperatures ranging from 800 to 993 K and at oxygen pressures of 1.33 and 133 Pa by means of a thermo-electrobalance, an X-ray diffractometer, an electron probe microanalyzer, a micro-hardness tester as well as metallographic techniques. Whereas mass gain due to oxidation initially followed the parabolic rate law, it then turned into very slow linear kinetics. Consequently, the oxidation resistance of vanadium was highly improved, especially at lower temperatures, lower oxygen pressures and higher chromium concentrations. Both alloys exhibited similar oxidation kinetics and scale structures. The oxide scale consisted mostly of a monolayer of VO2 of low chromium content (<=0.65 mol% Cr2O3) except for the oxidation at 993 K and 133 Pa. A very thin chromium-concentrated layer (<=0.2μm) was formed at the oxide scale-alloy substrate interface, to which could be attributed the retarded diffusion of vanadium into the oxide scale as well as that of oxygen into the substrate. (author)