Corrosion resistance of protective coatings against molten nitrate salts for thermal energy storage and their environmental impact in CSP technology

Sol-gel ZrO2-3%molY(2)O(3) coating deposited by means of a dip-coating application on P91 steel was statically tested at 500 degrees C in contact with Solar Salt (60% wt.% NaNO3/40 wt.% of KNO3) for 1000 h. This work assessed the behaviour of the coated P91 steel both from a technical and environmental point of view. Both studies were compared to those obtained with AISI 304 steel, which is currently used in commercial CSP plants. In terms of corrosion evaluation, the behaviour of the coated P91 was directly comparable to that of the uncoated AISI 304, SEM micrographs revealing the better behaviour of coated samples and the maintenance of a compact coating layer, with a thickness ranging between 1 and 1.4 mu m. Furthermore, environmental analyses revealed the environmental benefits obtained by using lower Cr-Ni content steel coated with ZrO2-Y2O3 compared to AISI 304 alloy, since this coating had a negligible environmental impact (its influence is below 0.03%). Thus, the proposed scenario seems to be workable in CSP high-temperature applications both from technical and environmental points of view. The linked technical-environmental quantification provided in this paper highlights the importance of considering the whole assessment when conducting material selection for CSP applications.