Corrosion Current Density of API 5L X65 Carbon Steel in Contact with Natural Callovian-Oxfordian Clay Pore Water, Assessed by Various Electrochemical Methods over 180 Days

Near-neutral pH and a low redox potential are considered favorable conditions for immobilizing radionuclides in deep repository systems within clay formations. Cigeo is the future French Industrial Center for Geological Disposal for high- and intermediate-level long-lived radioactive waste, to be built at a depth of 500 m within the Callovian-Oxfordian clay. In-depth knowledge of the mechanisms and kinetics of corrosion occurring on the surface of API 5L X65 (X65) carbon steel tubing is essential for the reversible nuclear waste management of the Cigeo site. By using all-solid and robust handmade electrodes in addition to electrochemical and gravimetric techniques, we determined the corrosion phenomenology and kinetics of X65 in contact with natural Cox pore water in equilibrium with its rock gases, flowing continuously through a multi-parameter probe device and placed at a depth of 500 m at the Bure Underground Research Laboratory, for over 180 days. Two iron oxidants were encountered, namely, depleted dioxygen (O-2) and proton H-(I), accompanied by hydrogen sulfide. Corrosion mechanisms and kinetics were well established for the two X65 electrodes, whether electrochemically perturbed or not. The corrosion thickness loss rates, determined by both electrochemical and gravimetric techniques, were between 0.016 and 0.032 mm/year. This study demonstrates, on site, the reliability of a developed methodology for continuous monitoring of the corrosion kinetics of the API 5L X65 carbon steel at the same time as the temporal variation of the key geochemical parameters of the fluid was assessed.