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In order to understand the corrosion processes of low-alloyed carbon steel (C-steel) materials in contact with porewater and/or gas in clay environments under reducing conditions at 85 °C, an in situ experiment called “MCO-Gravimetry” has been carried out in the Meuse/Haute–Marne Underground Research Laboratory (Bure, France). In a test chamber drilled in clay rock, samples of C-steel were exposed either to “clay porewater” (actually a liquid phase initially made of synthetic clay porewater gradually replaced by water seepage) or to an atmosphere of gases released by the host rock and by corrosion processes. Samples series were removed from the test chamber at given times (between 3 and 30 months), and the metal loss and corrosion rate ( r ) were quantified by gravimetric measurements. Characterisations of certain coupons were performed by using micro-Raman and X-Ray Diffraction analyses. Chemical and microbial analyses of the water seepage were also performed. Gravimetric measurements showed that r was lower in the gas phase (about 5 μm/yr) than in the liquid phase (up to 200 μm/yr). Two trends were observed in the liquid phase, one with an active behaviour (high r of about 200 μm/yr) and one with a passive behaviour (low r <30 μm/yr). The high r could be explained by the presence of an acidic pH transient that occurred 100 days after the experiment had started and lasted for 200 days. The rods with very low r may have been shielded in the setup, which could explain the comparatively limited damage, even under acidic conditions. Corrosion products (CPs) characterisation mainly showed the presence of siderite. In addition, Fe hydroxychloride was detected at the metal/corrosion product interface on coupons with high r . Finally, results from microbial analysis revealed the presence of sulphate- and thiosulphate-reducing bacteria (SRB and ThRB) in both media. The SRB/ThRB metabolism may explain the presence of sulphide in the CPs. Overall, this study highlights the potential risk of an acidic pH transient that would promote high corrosion rates for the C-steel structural barriers in a high-level nuclear waste repository. Therefore, HLW French repository concept has been modified by adding a cement-bentonite based grout between the host rock and casing. This is expected to neutralise the potential acidity and bring the C-steel surface to the passivity domain. [ABSTRACT FROM AUTHOR]