Underlying structure of bulky oxide nodule on alumina-forming austenitic stainless steel

The bulky oxide nodule and its underlying structure were investigated in an alumina-forming austenitic steel exposed to dry air at 1053 K for 336 h. Some bulky oxide nodules were found to be attached on globular phases in the matrix. By combined application of electron backscattering diffraction and energy dispersive X-ray spectroscopy, the bulky nodule was identified as Cr-rich M3O4 underlying which was Nb-rich MO2. Thermodynamic calculation suggested that the MO2 was transformed from the primary NbC near surface.