Atom probe study of impurity segregation at grain boundaries in chromia scales grown in CO2 gas

Model alloys Fe-20Cr and Fe-20Cr-0.5Si (wt.%) were exposed to Ar-20CO2 and Ar-20CO2-20H2O gas mixtures at 650 and 818 °C, forming Cr2O3 scales. Impurities segregated to chromia grain boundaries: carbon at sub-monolayer level, iron as spinel in narrow (10-20 nm) bands alongside the boundaries and silicon as dispersed glassy SiO2 nanoparticles. Thermodynamic driving forces for development of these nanostructures and enabling transport processes are identified. The consequences for grain boundary diffusion supporting scale growth and alloy carburisation are discussed.