Corrosion processes and microbial activity of carbon steel in the context of geological repository in clay environment
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Corrosion processes and microbial activity of carbon steel in the context of geological repository in clay environment Sophia Necib1, Christian Bataillon2, Sylvie Daumas3, Michel L. Schlegel4, Didier Crusset1 1
Andra (French National Agency for Radioactive Waste Management) CMHM, RD960 55290 Bure, France. 2 CEA Saclay /DEN/DANS/DPC/SCCME/LECA, Bât 458, 91 191 Gif-sur-Yvette, France. 3 CFG Services – GROUPE BRGM, 117 avenue de Luminy, 13009 Marseille, France. 4 CEA Saclay/ DEN/DANS/DPC/SEARS/LISL, Bat 391 - PC 33, 91 191 Gif-sur-Yvette, France. ABSTRACT 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. INTRODUCTION C-steel materials are foreseen to be used as structural materials in many repositories that will be implemented in clay soils (Nagra, Andra…) (1, 2). According to the Pourbaix diagram of Fe in Fe – H2O – CO2 system, C-steel should be in the passive region at neutral and alkaline pH (3). In addition, recent results on the characterizations of archeological artifacts seem to confirm the passive state of C-steel materials in clay soils under anoxic conditions (4). In the present study, the instantaneous in situ corrosion rate of C-steel was monitored, taking into consideration, the hygrometry, gases produced by the host rock, natural microbial activity ... Firstly, the challenge was to perform electrochemical measurements, and secondly dismantle the equipment in order to carry out the characterizations from microbial and chemical point of views. In order to achieve these, an in situ experiment was conducted at the Mont Terri underground research laboratory in Switzerland. A vertical and descending borehole was drilled down to 8 m in Opalinus clay (OPA), inside which an equipment with a test chamber and a packer was
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