Three steels were exposed in carburizing sodium at 600 and 650°C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases.

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In the framework of studies on the control rods lifetime for the Sodium Fast Reactor prototype development, three commercial steels were exposed to B 4 C powder in sodium at 600 °C for durations up to 3000 h. Analyses by optical and secondary electron microscopy, electron microprobe and glow discharge optical emission spectrometry revealed the formation of borides layers on the steel surface and slight carburization deeper in the steels. The growth of the boride layers followed parabolic kinetics. The natures of the formed boride layers were in good agreement with thermodynamic equilibrium predicted by Thermo-Calc software. The carburization depths were much lower than the ones obtained in pure carburizing sodium at 600 °C. It did not grow with time revealing possible protective character of the boride layers against carbon penetration.

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