Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

• Segregation of Mn occurred during early stage oxidation of modified 9Cr–1Mo steel.
• Initial oxide scale is composed of multiple oxides including Fe–Cr and Mn–Cr spinels.
• Kinetics of oxidation do not follow a parabolic law in initial stages of oxidation.
• Heterogeneous chemistry of initial oxide scale exhibits delineated interface.

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr2O4 along with FeCr2O4 and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.