Lessons learned from corrosion of materials with molten salts during molten salt tank preheating

One of the biggest challenges of concentrating solar power (CSP) is achieving cost-reduction in the thermal energy storage (TES) system. Operating conditions of molten salt CSP plants includes intermittent exposure of the TES tanks materials (i.e., carbon steel, stainless-steel) in contact with molten salt (NaNO₃–KNO₃). This manuscript evaluates the lessons learnt in the performance of such tank materials with molten salts under continuous and intermittent exposure in a demo plant for indirect thermal storage at 400 °C. This study shows that the hot tank upper section developed breakaway corrosion phenomena generating oxides layers, which easily delaminates from the base metal. This damage is produced by the tank preheating system, which introduced a non-controlled amount of CO₂ inside the tanks. Then, the carbon steel upper section is exposed in the hot tank to a mixture composed by CO₂, water vapour, and chlorides at high temperature, producing this type of attack. The materials tested are carbon and low alloyed steels types A516Gr70, A387Gr11 and A387Gr5, Cr–Mo steel A387Gr9, stainless steels types A304L, A316L and A347. The lessons learnt have been implemented in the commissioning of new commercial plants successfully.