Evaluation of Al additions to high-entropy alloys as structural material for the next generation of CSP plants

Molten salt technology using nitrate salts in tubular external receivers is the current state-of-the-art in concentrated solar power (CSP) technology, but new molten salts chemistry, using carbonates and chloride salts, has been proposed in order to increase the TES temperature and improve the turbine block efficiency. The selection of a high-temperature molten salt chemistry is necessary, alongside the need to understand its impact on new containment materials, to achieve acceptable strength, durability, and cost targets at these high temperatures. In this direction, the aim of this work is to explore different high-entropy alloys (HEAs) to evaluate their corrosion resistance in contact with carbonate and chloride molten salts at 650 and 720ºC, respectively. HEAs exhibit a unique combination of properties attributed to four core effects: high mixing entropy, lattice distortion, sluggish diffusion, and cocktail effect. In particular, the equiatomic, single-phase, face-centered cubic (FCC) CrMnFeCoNi Cantor alloy has garnered significant attention for its exceptional thermodynamic stability and remarkable resistance to corrosive environments.

In this study, three cantor alloys with different Al additions and a Nickel base alloy (In702) were exposed to the eutectic ternary Li₂CO₃–K₂CO₃–Na₂CO₃ (32.1–34.5–33.4 wt%) salt mixture at 650 °C for 500 hours, as well as to ternary chloride salt, composed of 20.4 KCl + 55.1 MgCl₂ + 24.5 NaCl at 720ºC under inert atmosphere (N₂). A special setup was designed in order to integrate electrochemical electrodes into the corrosion reactor to carry out electrochemical impedance spectroscopy (EIS) tests and linear polarization resistance (LPR) during the isothermal immersion.

Lower corrosion rates (0.28mm/year) were obtained for Cantor C alloy with a higher addition of Al, improving the corrosion resistance of In702 in the molten salt selected. In this case, the addition of Al to CrMnFeCoNi alloy improves the corrosion resistance significantly due to the protective layers formed as NiAl₂O₄, MgFeAlO₄.