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.