Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low PO2-environments at 1000 and 1050°C

The oxidation behaviour of pure Cr at 1000 and 1050 degrees C was studied in Ar-O-2 and Ar-H-2-H2O mixtures. It was found that in the low-pO(2) gases the oxide scales exhibited higher growth rates than in the high-pO(2) gases. The scales formed in the low-pO(2) gases showed substantially better adherence during cooling, than scales formed in the high-pO(2) gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O-2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O-2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H-2-molecules or due to OH- ions, cannot be derived with certainty from the present results.