Microstructures of chromia scales grown in CO₂

Scales grown on chromia forming alloys in CO2 are less protective than those developed during exposure to oxygen or air. Reaction with CO2 leads to faster chromia scale growth, the more rapid onset of breakaway and internal carburisation of the alloy. Conventional and laser Raman microscopy studies of Fe–Cr alloys show that local fluctuations in scale thickness are associated with varying degrees of chromium depletion. Local conversion of Cr2O3 to spinel leads to rapid outward iron diffusion and nucleation of Fe-rich oxide nodules. A TEM investigation reveals that reaction in CO2 produces finer grained Cr2O3, inward scale growth and more rapid scaling, as well as internal carburisation. Appropriate silicon additions to the alloys lead to the formation of a thin, glassy silica layer beneath the chromia, greatly slowed chromia thickening rates and the prevention of carburisation. Atom probe tomography is used to locate carbon within the chromia.