Molten Salt Corrosion of Stainless Steels and Low-Cr Steel in CSP Plants

The corrosive effects of 60 % NaNO3/40 % NaNO3 have been tested at 390 and 550 °C, in order to simulate the working conditions in two principal concentrated solar power (CSP) plants, on stainless steel (AISI 304, 430) and on a low-Cr alloy steel (T22) containing 2.25 % Cr. The corrosion rates were determined by gravimetric tests, measuring the weight gain during 2,000 h, identifying the corrosion products via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thereby, Fe2O3 and Fe3O4 were the most prominent products formed from the tests performed at 390 °C, being observed also the formation of some stable compounds due to the impurities of the salt, as magnesium ferrite (MgFe2O4) and NaFeO2. The study at 550 °C of stainless steels revealed a better behavior under corrosive environments than T22 steel, identifying the formation of FeCr2O4 protective spinels mainly. To complete the corrosion tests, thermo-physical studies were carried out in binary mixture, using DSC and TGA. Also the most important parameters of the salt before and after corrosion test were tested.