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

The ability of water vapour to alter chromia scaling rates has long been recognised, but is poorly understood because of the diversity of scale-water vapour interactions possible. One obstacle is the complex defect structure of thermally growing chromia scales. For technological reasons a protective oxide scale needs to be essentially gas-tight to ensure that its growth is governed by diffusion. A gas-tight thermally growing chromia scale is commonly assumed to be in equilibrium with the atmosphere the scale is growing in, as well as the metal the scale is formed on. Consequently the type of defects may change throughout a thermally growing chromia scale. Another difficulty lies in the fact that the concentration and the mobilities of the intrinsic defects depend strongly on the pO₂ and pH₂O in the test gas used for the experiments.

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.