Deep geological repositories contain metallic materials, either used to construct disposal canisters or as waste. The safety relevance of corrosion is linked to canister lifetime in the former case and to gas generation in the latter. Various repository concepts worldwide consider the use of cement as backfill material, e.g. the Belgian Supercontainer concept and the Swiss low- and intermediate-level repository.
The corrosion behaviours of mild steel and stainless steel in anoxic environments have been studied through the analysis of the hydrogen end-product. Test cell environments were matched against the anticipated Belgian HLW and Swiss L/ILW repository environments, and also against experiments that have been conducted by other researchers for comparative purposes. For this, different kinds of cement were used, and the samples were exposed in synthetic porewaters or in humid atmospheres representing early stages of repository evolution. Gas generation from different cement types was compared.
Hydrogen analysis was conducted using the purge and collect methodology, using a solid state hydrogen sensor, as well as direct monitoring of the cell pressure. Whilst this method is limited to providing only uniform corrosion rates averaged over periods of time ranging from weeks to months, it provides good resolution.
In general terms, it was found that the corrosion behaviour of mild steel that had been pickled in non-inhibited hydrochloric acid was similar to freshly abraded material. Material that had been pre-corroded in anoxic young cement water (YCW), pH 13.7 at 80 °C for 1500 hours, coated in a black uniform magnetite, behaved similarly to as-abraded material. In YCW, uniform corrosion rates ranged from approximately 500 nm/year at 50 °C to 5 µm/year at 80 °C and were initially invariant. Immersion in saturated portlandite, in contrast, provided higher initial corrosion rates (up to 6 µm/year at 50 °C) followed by passivation over several weeks and a reduction in corrosion rate to less than 200 nm/year.
Stainless steel, encased in CEM (I) cement at 50 °C (in water vapour or immersed in YCW) was found to initially corrode at less than 10 nm/year.
Carbon steel, stainless steel, anoxic, corrosion, cement, hydrogen