Effects of H2 on microstructures of Cr2O3 scales grown in water vapour and consequences for breakaway

Model alloy, Fe-20Cr (wt%), was exposed in Ar-(5, 20)H2O, Ar-(5, 20)H2O-5H2 and Ar-20O2 (vol%) at 750 and 850 °C. The alloy formed Cr2O3 scales plus Fe-rich oxide nodules in H2O-containing gases, but a Cr2O3 scale in Ar-20O2 at 750 °C. The alloy formed an outer (Cr,Fe)-rich oxide layer and an inner, porous Cr2O3 layer in Ar-5H2O-5H2, whereas a dense Cr2O3 scale grew in Ar-5H2O at 850 °C. Application of Wagner's theory successfully differentiated effects of oxide grain sizes and oxygen partial pressures of gases on scaling rates. Chemically inert SiO2 markers showed that Cr2O3 scales grew by outward metal diffusion.