Use of electrochemical impedance spectroscopy for corrosion testing in supercritical water

• In-situ electrochemical impedance spectroscopy was successfully applied in supercritical water. • Instantaneous corrosion rates of 310S steel were integrated and compared with gravimetric data. • Electrochemical results showed excellent agreement with weight loss measurements (deviation < 6% at 500°C). • The study confirms that pressure fluctuations near the critical point significantly impact corrosion rates. • The method allows for real-time monitoring of corrosion kinetics in SCWR environments. In this study, in-situ electrochemical impedance spectroscopy (EIS) was employed to assess charge transfer and corrosion properties of candidate materials for fuel cladding in supercritical water (SCW). Experiments utilized 310S steel samples in a supercritical autoclave setup with a recirculation water loop. Exposures for 1000h at 380°C or 500°C (25 MPa) allowed for real-time corrosion rate monitoring. From the impedance spectra, the faradaic and diffusion resistances were evaluated and used to calculate polarization resistance, which is a fundamental parameter determining the instantaneous corrosion rate. By converting to corrosion current and integrating the time dependence, integral corrosion data can be obtained, which are directly comparable to the results of weight gain/loss measurements. Our findings demonstrate the feasibility of EIS measurements in SCW in a broad temperature range and highlight its utility for understanding corrosion process in SCW environments. The in-situ integrated electrochemical data were directly and quantitatively validated against standard ex-situ mass change measurements obtained under identical conditions. Quantitative comparison revealed sufficient agreement between the integral electrochemical data and the weight loss method, showing a difference of only -6% at 500°C (37 vs. 39 mg·dm -2 ) and +36% at 380°C (28 vs. 18 mg·dm -2 ). The difference between the integral electrochemical data and the weight gain method was +63% at 380°C and +21% at 500°C, which can be attributed to oxide layer delamination at high temperatures causing weight gain measurements to underestimate the true oxidation. Our findings demonstrate the feasibility of EIS measurements in SCW in a broad temperature range and highlight its utility for understanding corrosion process in SCW environments.