In the context of the French repository for high-level wastes, carbon steel (C-steel) materials are currently considered among the most cost effective and reliable materials for the prediction of long-term corrosion behaviour. An in situ experiment was conducted at the Meuse/Haute Marne underground research laboratory to investigate the corrosion behaviour of different grades of C-steel up to a maximum temperature of 85°C. C-steel coupons were placed in a two-phase test chamber containing liquid and gas emanating from the Callovo-Oxfordian host rock, at given times and for specific durations, under anoxic conditions. After their exposure in the test chamber, various characterisations were carried out. Overall, after 36 months, the results highlight a potential acidic transient. This could promote high corrosion rates of C-steel. Therefore, an alternative concept has been considered since 2014, which involves the addition of a cement–bentonite-based grout in order to buffer the possible acidic transient. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.