Growth of Cr2O3 blades during alloy scaling in wet CO2 gas

A model alloy Fe-20Cr-0.5Si (wt.%) exposed to Ar-20CO2 and Ar-20CO2-20H2O at 818 °C formed a duplex scale of an outer Cr2O3 layer and an inner SiO2 layer in both gases. Water vapour promoted formation of fine Cr2O3 grains and additional Cr2O3 blades. The blades were discovered by high resolution electron microscopy to be bicrystals, containing an internal interface, available as a fast diffusion path for chromium. Different blade cross-section shapes are accounted for by the competition between blade lengthening and thickening, the latter supported by vapour phase deposition.