Thermodynamic modelling of the destruction of the surface Cr_2O_3 on Alloy 230 in the impure helium atmosphere of a Gas Cooled Reactor

摘要

Above a given temperature called T_A, the chromium rich oxide which has been developed on the surface of Haynes 230~® and model NiCrWC alloys at a lower temperature becomes unstable in impure helium: carbon monoxide is released. Actually, oxide is reduced by carbon from the alloy. A thermodynamic model is developed to rationalize the variation of T_A as a function of the partial pressure of CO in the gas phase. It was found that, at the early stages of the scale reduction, the relevant reaction occurs at the oxide/metal interface between chromia and carbon from the alloy. The interfacial activity of carbon in the alloy can be calculated based on measurements of the interfacial weight percentage of chromium and using ThermoCalc~® software. Excellent agreement is observed between experimental values of T_A and theoretical predictions.