Corrosion resistance of MCrAlX coatings in a molten chloride for thermal storage in concentrating solar power applications

Corrosion evaluations of Incoloy 800H (In800H) and stainless steel AISI 310 (310SS), in bare and coated conditions, were performed in 34.42wt% NaCl - 55.47wt% KCl at 700 degrees C in a nitrogen atmosphere. This NaCl-KCl composition has a melting point of 657 degrees C, which makes it suitable for latent-heat thermal energy storage in concentrating solar power applications. Several nickel-based MCrAlX coatings were tested, where M=Ni and/or Co and X=Y, Ta, Hf, and/or Si. Electrochemical testing was carried out to determine corrosion rates. The bare In800H and 310SS alloys corroded rapidly (similar to 2500 and 4500 mu m/yr, respectively, assuming uniform corrosion). Concentrating solar power plants need containment materials with a lifetime of at least 30 years; thus, these corrosion rates are excessive. Corrosion mitigation approaches are being investigated to obtain degradation on the order of 20 mu m/yr or lower. The lowest corrosion rate of 190 mu m/yr was obtained for atmospheric plasma spray NiCoCrAlY coatings pre-oxidized in air at 900 degrees C for 24h with a heating/cooling rate of 0.5 degrees C/min. Metallographic characterization of the corroded surfaces showed that the formation of a uniform thin alumina scale before exposure to the molten chloride system considerably reduced the corrosion of the alloy. However, the rates of corrosion determined herein are considerable, highlighting the relevance of testing materials durability in solar power applications. Solar power: alloys wear protective coatsSpraying materials used in solar thermal power stations with protective coatings could minimize corrosion and extend their lifetimes. Chloride salts are stable at temperatures above 600 degrees C and make excellent candidates for fluids that can deliver heat energy captured from the sun to turbines that generate electricity. Unfortunately, these molten salts are prone to corroding the alloys from which power plant components are typically made. Judith Gomez-Vidal at the National Renewable Energy Laboratory in the US found that spraying and oxidizing NiCoCrAlYTa coatings on inexpensive alloys caused evenly-spread thin alumina films to form, protecting them from corrosion upon exposure to high-temperature salts. Electrochemical measurements revealed that the thicknesses of the coated alloys decreased at rates of 190 microns per year, ever closer to the 20 micron per year rate desired for a 30 year lifespan.