Stability of Salt and Corrosion Resistance of Circuit Materials in the Themis Power Plant

Summary

Studies on salt chemistry and materials behaviour have been carried out since 1978 at EDF in the framework of the THEMIS programme. The study of nitrate-nitrite mixtures chemistry has allowed the main thermodynamic equilibria to be identified and a method has been defined to stabilize the salt by fixing the rate of oxoacidity. Corrosion tests on the steels used for the circuit in a eutectic mixture have confirmed the good corrosion resistance of 316 L type stainless steel at its nominal operating temperature (500°C). By contrast, low alloy ferritic steels at their operating temperature (450°C) show different behaviours, depending on the oxoacidity of the salt : linear corrosion for steel without chromium in oxoacid salt, parabolic corrosion in all other cases. A deleterious effect of increasing the chloride content of the salt has also been observed on these steels.