High-Temperature Electrochemical Corrosion Behavior of Ni20Cr Alloy in Molten Nitrate Salts up to 500 °C

The time-dependent electrochemical behavior of a nominal Ni20Cr alloy exposed to molten Solar Salt (60 wt.% NaNO3–40 wt.% KNO3) was investigated at 300, 400, and 500 °C for isothermal exposure times of up to 100 h. Open-circuit potential (OCP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) were used to monitor the evolution of the alloy/molten-salt interface. OCP measurements revealed time-dependent potential changes during exposure. Because a Pt pseudo-reference electrode was used without independent calibration against a thermodynamic reference or internal redox couple, the OCP data were interpreted primarily in terms of their temporal evolution within each isothermal experiment rather than through direct comparison of absolute potential values between temperatures. LPR measurements showed a strong temperature dependence of the polarization resistance, with a pronounced decrease at 500 °C. The impedance response was consistent with two interfacial contributions, represented using a two-time-constant equivalent circuit and tentatively associated with the surface corrosion-product layer and the underlying alloy/corrosion-product interface. At 500 °C, both resistance contributions were markedly lower than those measured at 300 and 400 °C, consistent with a substantial loss of interfacial barrier effectiveness. The persistence of a surface-related impedance response at 500 °C was therefore not considered evidence of a highly protective layer. Rather, the electrochemical results suggest that a surface-related interfacial contribution remained distinguishable at 500 °C, although its associated resistance was substantially lower than at 300 and 400 °C. These assignments represent electrochemically based interpretations and should not be regarded as unique identification of the underlying microscopic processes. Overall, the results demonstrate that temperature strongly influences the electrochemical behavior of Ni20Cr in molten nitrate salts and that the electrochemical resistance to corrosion-related processes decreases markedly at 500 °C. The 100 h electrochemical exposures investigated here characterize short-term interfacial behavior and do not establish the long-term structural suitability of Ni20Cr for concentrated solar power service.