Assessment of corrosive attack of Fe9Cr1Mo alloys in pressurised CO2 for prediction of breakaway oxidation

To provide clarity on the poorly-understood mechanism of breakaway oxidation, corrosion of Fe9Cr1Mo steel in pressurised CO2 is quantified and modelled. Due to practical application in nuclear power plants, the temperature range of 400 to 640 ◦C is emphasised. Attack is in the form of combined oxidation and carburisation, as confirmed by scales of magnetite/spinel and extensive precipitation of carbides in the metal. The activity of carbon in the metal close to the metal/scale interface – despite the isothermal nature of the tests performed – exhibits a strong time dependence consistent with the kinetically-limited transport of carbon due to the retardation of the Boudouard reaction. Breakaway is associated with saturation of the underlying substrate with respect to carbon and therefore a carbon activity at the metal/scale interface which approaches unity; a Robin-type boundary condition is used to treat the rate-dependency. The model is tested against extensive quantification of microstructure including carbide precipitation; it forms a physically faithful approach for the prediction of degradation behaviour. It will be of considerable use for the prediction of the remanent life of critical components in nuclear power plants.