Scaling Kinetics and Scale Microstructure of Chromia Scales Formed on Ni-25%Cr Model Alloy during Oxidation in H2o-Containing High and Low pO2 Test Gas at 1000°C
At high pO2 test gas containing O2 and H2O, the pO2 is independent of the H2O content in the test gas. However both types of oxygen species O2 as well as H2O contributing to the chromia scaling reaction. The effect of specimen thickness on chromia scaling allows to change the concentration of the intrinsic defects and their diffusivities in thermally grown chromia scales independently from the test gas applied. Chromia scales are under pressure during scale growth. For thermodynamic reasons the concentration and diffusivity of the intrinsic, native defects in oxides under hydro-static pressure varies with the amount of pressure applied. The chromia growth kinetics was found to be depending on the thickness of the specimens used for the oxidation experiments. In dry Ar/N2-O2 gas the effect was very clear with the 0.25 mm thick specimen producing a twice as thick chromia scale compared to the 1.0 mm thick specimen. The addition of water vapour into the test gas levelled the differences in chromia scaling kinetics and the chromia scale thickness decreased with increasing water vapour content in the gas.
The experimental findings will be discussed with strong focus on the oxide growth mechanism, the chromia microstructure of the scales formed under the various conditions and the concentration and mobility of the native defects in the chromia scales and its interactions with H-containing species originating from water vapour in the test gas.