Effects of Chloride on Copper Corrosion in Extremely Low-Oxygen Concentration Conditions

Abstract Copper (Cu) is one of the candidate waste container materials for high-level radioactive waste geologic disposal systems located in the saturated zone, because of its thermodynamic stability in anoxic water. However, species in the groundwater could change Cu stability and electrochemical properties, especially corrosion resistance. This study investigated the role of chloride (Cl−) in Cu corrosion in solutions containing sulfate (SO42−) and Cl− with residual O2 concentrations of about 0.1–0.2 ppb at 50 °C. SO42− concentration was kept constant at 2 × 103 ppm, while Cl− concentration was varied from 0 to 1 × 103, 1 × 104, and 1 × 105 ppm. Electrochemical methods, including corrosion potential (Ecorr) monitoring, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS), were used to study the electrochemical properties of Cu affecting corrosion resistance. All the results obtained from different methods consistently demonstrate that Cl− and temperature play significant roles in enhancing corrosion of copper when the O2 concentration is extremely low. Ecorr decreased with increasing Cl− concentration and was lower at 50 °C than at 20 °C. During both forward and reverse potentiodynamic polarization scans, Ecorr consistently decreased and the current density increased with increasing Cl− concentration. Tafel slopes from the forward scan regions decreased and the exchange current density increased with increasing Cl− concentration. The EIS data differed in solutions with different Cl− concentration levels and they became more complex at elevated temperature. A layered film structure was inferred from the EIS data. Polarization resistance derived by fitting to the EIS data was the lowest at the highest Cl− concentration, which is consistent with increasing corrosion rates with increasing Cl− concentration. Uncertainty remains with regard to the composition and structure of the films formed from the corrosion process, the cathodic reactions that support the corrosion process, and the implications of the films for long-term performance under repository conditions.