Corrosion Behavior of FeCoNiCrAl High-Entropy Alloy in Molten NaNO3-KNO3

Molten nitrates are the main heat transfer fluid (HTF) for concentrated solar power (CSP) systems. However, due to the instability of molten nitrates at high temperatures, the corrosiveness of the molten nitrates poses high requirements for the structural material used in CSP. Therefore, it is urgent to develop highly corrosion-resistant materials. In this study, the corrosion behavior of FeCoNiCrAl high entropy alloys (HEA) in molten NaNO3-KNO3 (60 wt% to 40 wt%) under argon at 600°C is investigated by mass loss and electrochemical methods. The results show that the FeCoNiCrAl HEA experienced severe mass loss during the 100 h immersion due to the high oxygen partial pressure and the galvanic corrosion effect. The corrosion products of FeCoNiCrAl HEA in the melt consist of Fe2O3, Cr2O3, FeCr2O4, and NaFeO2. After immersion for 100 h, an outer layer dominated by porous iron oxides and an inner more compact Cr-rich layer are formed. Furthermore, both of the oxide layers are gradually thickened with the extension of the corrosion time, and the process is manifested by the increased value of the oxide layer resistance Rox and charge transfer resistance Rt in the electrochemical impedance spectra. At the same time, compared with the Rt of 316L stainless steel, it can be seen that with the extension of corrosion time, the Rt of FeCoNiCrAl HEA is larger and shows better corrosion resistance in the same corrosive environment. In addition, FeCoNiCrAl HEA shows a higher corrosion potential and a lower corrosion current density than 316L in molten nitrates at 600°C.