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A long-term in situ corrosion experiment is ongoing in the Mont Terri Underground Research Laboratory in Switzerland to (i) measure the in situ corrosion behaviour of carbon steel in compacted bentonite under simulated repository conditions, (ii) study the effect of the bentonite buffer density on microbial activity and microbially influenced corrosion and (iii) study the effect of welding on the corrosion rate. Carbon steel corrosion coupons, with and without welds, were surrounded by compacted bentonite with a range of dry densities and mounted in modules allowing free exchange with the local anoxic groundwater. After about 20 months of exposure, corrosion coupons and bentonite were sampled. A complex corrosion product was identified, consisting predominantly of magnetite. The bentonite adjacent to the metal was finer grained, more dispersed and enriched in iron. Aerobic, anaerobic and sulphate-reducing bacteria were identified both in the porewater surrounding the modules and in the bentonite.

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International audienceThe synergism between mechanical friction and corrosion may lead to an acceleration of the degradation of materials in sliding contacts exposed to the environment. Electrochemical measurements and sliding tests are proposed as a protocol suitable to decouple the material losses originating from the active material and the passive film, and to identify their electrochemical or mechanical nature. The corrosion resistance of a material, its ability to react onto chemical or mechanical attack, its susceptibility to tribocorrosion and the effect of a passive surface film on the coefficient of friction are evaluated. The proposed tribo-electrochemical protocol allows to evaluate the potential and risks of applying newly developed materials in sliding contacts, and to support the selection and implementation of materials in industry in applications where corrosion and wear are potential degradation processes. This protocol extends the existing Standard Guide ASTM G 119-04 'Determining Synergism between wear and Corrosion' for passivating metallic materials

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Carbon steel is a candidate disposal canister material for high-level radioactive waste and spent fuel in the Swiss deep geological repository concept and so a long-term laboratory-based corrosion experiment was undertaken to measure the anaerobic corrosion behaviour and corrosion rate of carbon steel in compacted bentonite under simulated anoxic repository conditions. The corrosion rate was measured using barometric gas cells over a period of more than 5 years, until quasisteady-state conditions were reached. The measured corrosion rates are in agreement with weight loss measurements and with previous tests under similar conditions. The carbon steel and surrounding bentonite removed from one cell after 3 years were analysed with a range of analytical techniques, weight loss measurements and mineralogical characterisation. The results provide an insight into the prevailing corrosion mechanisms under repository conditions. ARTICLE HISTORY

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The reactivity of zinc in sodium hydroxide electrolytes containing sodium silicate and sodium tetraborate was studied by electrochemical and microstructural examinations. Impedance measurements indicate that a porous surface layer is formed on zinc immersed in sodium hydroxide at its open circuit potential and on polarization in the prepassive region. This surface layer becomes compact at potentials in the passive region. The anodic reactions of a zinc electrode in a sodium hydroxide containing electrolyte are controlled by the mass transport of zincate ions away from the electrode. Silicate and tetraborate ions suppress the anodic dissolution of zinc. In the presence of silicate ions, a compact silicate containing film is formed upon anodic polarization. The electrode rotation affects the microstructure of the surface layer both in silicate-free and silicate-containing sodium hydroxide solutions. On addition of tetraborate ions, the polarization resistance increases and the formation of a surface layer is achieved at increased applied potential.