Chemical interaction of austenitic and ferritic steels with B₄C powder in liquid sodium at 600°C

In the framework of studies on the control rods' lifetime for Sodium Fast Reactor, three commercial steels were exposed to B₄C powder in liquid sodium at 600°C for durations up to 3,000 h. Analyses by optical and secondary electron microscopy, x-ray diffraction, electron microprobe, and glow discharge optical emission spectrometry revealed the formation of boride layers at the surface of the steels and slight carburization underneath. The growth of the boride layers followed parabolic kinetics. The nature of the formed boride layers was in good agreement with thermodynamic equilibrium predicted by software. The carburization depths were much lower than the ones obtained in carburizing liquid sodium at 600°C. Finally, the carbon penetration depth did not grow with time revealing a transient carburization phenomenon and possible protective character of the boride layers against carbon penetration.