Investigation of the corrosion of electro-less nickel-plated alloys in molten salt and its effect on phase change properties for energy storage applications

Interactions between electroless nickel plated Inconel 601, Monel 400, and stainless steel 316L alloys with two eutectic salts used as phase change materials, K2CO3 + Na2CO3 and NaCl + Na2CO3, were investigated at 720 °C (isothermal). The results of this study show that at these high temperatures, which is the target temperatures of the next generation of concentrated solar thermal power plants, these salt phase change materials are highly corrosive to stainless steel, whereas the Inconel 601, a high temperature nickel alloy, was resistant. Monel 400 experienced the most corrosion damage (500 µm thick corrosion layer) and resulted in the salts experiencing the greatest latent heat decrease by 18.1%, as confirmed by Differential Scanning Calorimetry (DSC). IN601 performed the best with an observed corrosion layer thickness of 107 µm, and resulted in one of the salts only decreasing in latent heat by 5.9%. IN601 with the nickel plating has shown improved corrosion resistance over uncoated samples. The thermophysical properties of the salt was measured before and after the exposure tests to demonstrate the significant effect corrosion has on the performance of the salts as a phase change material. Scanning Electron Microscopy (SEM) analysis showed that constituent elements have diffused from the alloy specimens, through the nickel plated layer and leached into the molten sat, which was characterized by Inductively Coupled Plasma- Optical Emission Spectroscopy (ICP-OES). A correlation was found between the corrosion of the specimens, the solubility of the alloying elements and a large decrease in the salt's latent heat properties.