Morphology of oxide scales formed on chromium-silicon alloys at high temperatures

[Display omitted] •Addition of Si to Cr significantly changed the oxide scale morphology.•Si additions led to SiO2 growth underneath the chromia scale.•The initially amorphous SiO2 crystallized to cristobalite at 1200°C.•Fine-grained chromia scale contained abnormally large single crystals.•Single crystals are most-likely grown via a gas phase transport mechanism. Oxide scales formed on chromium-silicon model alloys at 1200 °C in synthetic air were characterized using electron microscopy and diffraction methods. Additions of Si to chromium led to SiO2 growth underneath the chromia scale. The chromia scale was mostly fine-grained, but also contained numerous abnormally large single crystals growing out of the scale surface. Evaluation of mass transport rates shows that growth of the large grains can be supported by gas phase transport of CrO3, but not by lattice diffusion.