High temperature corrosion in molten solar salt: The role of chloride impurities

The role of chloride impurities in molten NaNO3-KNO3 (solar salt) mixture on corrosion behavior of low-chromium ferritic-martensitic X20CrMoV11-1 steel (X20) and stainless steel 316 (SS316) was studied at 600 degrees C. Gravimetric and metallographic methods were employed to characterize the kinetics of oxidation and the resulting corrosion products. Steel X20 showed non-protective character in both low- (up to 0.02 wt% Cl-) and high- (up to 0.25 wt% Cl-) chloride salts by forming a thick and non-compact oxide scale. A significant increase in weight gain was observed when X20 steel was immersed in the high-chloride-containing salt. Furthermore, the scale underwent severe deformation. SS316 showed superior corrosion resistance in both low- and high-chloride salts. Oxide scales formed on both steels included two zones: an outer Na-rich oxide and an inner mixed oxide based on Fe2O3 and Fe3O4 structures. The morphology and composition of these zones were significantly different on X20 and SS316 steels. A passive Cr-rich oxide layer at the metal/oxide interface was characterized as a protective layer. In the case of stainless steel 316 this layer showed even higher continuity when tested in the high-chloride salt resulting in better protection during the isothermal test.