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

Abstract

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 Fe 2.3 Cr 0.7 O 4 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 FeCr 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 8C 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.

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

“…1 Comparison between the simulated total oxide thicknesses and the observed oxide thicknesses formed after corrosion tests on ground polished T91 at 1 atm and 250 bars magnetite layer was removed, the kinetics of oxide growth was not accelerated. From this result, they concluded that it was the iron diffusion through the inner FeCr spinel oxide lattice which was the rate limiting process at 470°C in lead bismuth [7]. Surman et al [15] proposed the same rate limiting oxide layer for mild steel and low alloy steel oxidation in CO 2 without demonstrating it clearly.…”

“…Martinelli et al [7] have reported an extensive study of the expression of the iron diffusion coefficient in magnetite and Fe 2.3 Cr 0.7 O 4 from literature data [8][9][10]. They concluded that the expression of the iron diffusion coefficient in magnetite D Fe M at intermediate temperature can be written as:…”

“…As a consequence, the oxide growth depends on iron diffusion in both layers. As in [6,7], a simulation of the oxide growth on 9Cr-1Mo steel surface in CO 2 is proposed with the following assumptions:…”

“…Then, the methodology of the simulation is the one exposed in detail in [6] and summarized in [7]. Since the iron flux which goes through the Fe-Cr rich spinel layer is equal the iron flux going through the magnetite layer, the magnetite layer growth kinetics can be calculated by two different ways: either by considering the iron flux through magnetite or by considering the iron flux through the Fe-Cr spinel layer; this methodology leads to the following expressions of the magnetite thickness:…”

Analysis of resistance of T91 steel under load in liquid PbBi

“…Magnetitová vrstva roste difúzí železa až do rozhraní PbBi/oxid, zatímco spinelová vrstva Fe-Cr roste na rozhraní kov/oxid uvnitř prostoru. Mezním mechanismem procesu tvorby ochranné vrstvy je difúze, která umožňuje vytvoření prostoru pro růst spinelu Fe-Cr [6,7,9]. Podle těchto předpokladů závisí tloušťka oxidické vrstvy spinelu Fe-Cr od schopnosti tvorby magnetitu, která je také závislá na obsahu kyslíku v prostředí a na mikrostruktuře povrchové vrstvy.…”