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 degrees 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 degrees 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 degrees 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.