Morphology and Buckling of the Oxide Scale after Fe–9Cr Steel Oxidation in Water Vapor Environment

Abstract

Under water vapor exposure at 550–560 °C, Fe–9Cr ferritic–martensitic steels form a triplex oxide scale made of an outer magnetite layer surrounded by a hematite layer and an inner Fe–Cr spinel layer. Long-time oxidation tests have been performed to study scale degradation with time. It revealed that buckling and spallation of the oxide scale always occurred during isothermal oxidation or during cooling down to room temperature. The interfacial zone of delamination has been proved to be located inside the magnetite layer, where a voids belt is formed and grows. It is assumed that voids are the preponderant factor initiating delamination of the magnetite layer under compressive stresses during the oxide scale growth. A mechanism of accumulation of vacancies leading to voids formation and then to the spallation of the outer oxide scale is proposed.