Modelling of the oxide scale formation on Fe-Cr steel during exposure in liquid lead-bismuth eutectic in the 450-600 C temperature range

Previous studies showed that the oxidation of T91 (Fe-9Cr martensitic steel) in liquid Pb-Bi eutectic leads to the formation of a duplex oxide layer containing an inner Fe2.3Cr0.7O4 spinel layer and an upper magnetite layer. The magnetite layer is easily removed by the Pb-Bi flow when the oxygen concentration is low and the flow velocity is high. This phenomenon is not currently understood. The magnetite layer growth rate is limited by the iron diffusion in the oxide layer lattice. The Fe-Cr spinel layer grows in the nanometric cavities formed at the Fe-Cr spinel /T91 interface by the outwards diffusion of iron. Due to this mechanism the growth rate of the Fe-Cr spinel layer is linked to that of magnetite. A modelling of this mechanism is presented. The modelling is in agreement with the experimental data in the case of a high oxygen concentration. However, the calculated oxide scale thicknesses are systematically lower than the experimental values in the case of a low oxygen concentration when the iron diffusion only occurs via interstitials in the oxide scale. Consequently, the estimation of the iron diffusion coefficient, when diffusion occurs via interstitials, is not reliable. To have a better estimation of this iron diffusion coefficient in the Fe-Cr spinel, when diffusion occurs via interstitials, a fit is done using experimental data coming from the European DEMETRA project. Although this evaluation is only based on a fit on the experimental data, it permits to estimate the oxide layer growth kinetics in case of the formation of the described duplex oxide layer, for each oxygen concentration (leading to a vacancy and/or an interstitial diffusion), each temperature between 450 and 620 C and each hydrodynamic flow. This model shows that the hydrodynamic flow affects the corrosion rate only by the removal of the upper magnetite layer leading to an increase of the oxygen concentration at the spinel/magnetite interface. The oxidation mechanism is thus neither changed by the Pb-Bi flow nor by the oxygen concentration. However, the oxygen concentration modifies the iron diffusion process in the oxide lattice. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)