Corrosion processes and microbial activity of carbon steel in the context of geological repository in clay environment

Carbon steel (C-steel) is studied to be the reference material for the metallic components in the high level waste (HLW) repository concepts of several European countries such as France, Switzerland, Belgium. Electrochemical impedance spectroscopy (EIS) was performed over a period of 7 years, to determine the instantaneous corrosion rate (CR) of carbon steel (C-steel) in contact with clay porewater in diffusive regime. The study was conducted at the Mont Terri underground research laboratory (URL) located in Switzerland. The test chamber was at a depth of 8 m under anoxic conditions at 90°C in a vertical and descending borehole drilled in Opalinus clay (OPA). Microbial and chemical investigations were conducted on porewater in contact with C-steel as well as directly on C-steel surface further to dismantling. The results showed clearly a decrease of the CR over time followed by a steady state below 1 µm/year. Sulphate and thiosulphate reducing bacteria were observed in porewater and at the metal surface, with a higher concentration of mesophilic and thermophilic bacteria respectively. The metal surface characterizations revealed the presence of magnetite, mackinawite, hydroxychloride and siderite with local traces of oxidizing species such as goethite.