Thuan Dinh Nguyen

Microstructures of chromia scales grown in CO2

Abstract 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.

Effects of Si, Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO2 and Ar–20CO2–20H2O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr2O3 layer and an inner SiO2 layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn3O4 and MnCr2O4 above alloy grain boundaries and Mn3O4, Cr2O3 and MnCr2O4 on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.