Investigation of the corrosion behavior of NiVAl multilayer coatings in hot salt melts

Molten nitrate salts applied as thermal storage and heat transfer media are known to cause severe corrosion problems in concentrated solar power plants. In this context, a metallic multilayer PVD coating (NiV-Al) served as a model system in the investigation of corrosion protection for low-alloyed steel in molten salt melts. The corrosion behavior of coated and uncoated P91 steel samples was investigated in salt melt immersion tests. The tests were designed to mimic typical operation conditions ranging from continuous operation to cyclic variations that are typical for day and night service of the solar power plants. In conjunction with SEM analysis of sample cross-sections and surfaces and EDX analysis, the isothermal immersion tests revealed the corrosion promoting effect of high immersion temperatures and the chloride contaminants in the solar salt melt. Moreover, a significant reduction of corrosion rates was obtained for low-alloy steel coated with NiV-Al multilayer coatings in all test scenarios. Thermal cycling immersion tests proved the durability of the developed coatings and the subjacent steel substrate under cyclic thermal stress.