Application of Impedance Spectroscopy for in-situ Corrosion Tests in Supercritical Water

To increase the efficiency of heat to work transformation in power cycles, admission steam enthalpy has to be increased. This trend peaks currently in advanced ultra-supercritical technology, which offers efficiency as high as 50% at steam pressure of 32MPa and temperature of 650°C. The knowledge gained in the fossil technology can partly be used in the super-critical-water-cooled reactor (SCWR) concept, one of the six designs selected by the Generation IV International Forum (GIF). Most of the SCWR concepts employs temperatures between 280°C and 530-625°C at the reactor core inlet and outlet respectively. Consequently, the SCWR concept will contend with enormous changes in physical and chemical properties of the cooling water around the critical point (374°C).Most of the chemistry-material challenges are still to be resolved. The majority of the corrosion studies is based on an exposure of the studied material in scw and a subsequent analysis. In the presented paper, we aimed at getting in-situ information on charge transfer / corrosion properties of supercritical water (SCW) from electrochemical measurements. In-situ electrochemical impedance spectroscopy (EIS) measurements were performed with 310S steel and 800H nickel alloy samples. The experimental set-up included a supercritical autoclave connected to a recirculation water loop. The experiment consisted of gradual heating up to 380°C or 500°C (at the constant pressure of 25 MPa), than constant temperature was maintained for over 1000 hours. The series of EIS measurements was performed sequentially with the aim to get impedance parameters characterizing the instant corrosion rate.The experiment confirmed feasibility of EIS measurements in a broad temperature range and applicability of EIS data for in-situ determination of corrosion rate. Corrosion rates of the two materials were compared and influence of the 150 ppb of oxygen on corrosion rate was evaluated.