The long-term safety of deep geological repositories (DGRs) for handling radioactive waste depends critically on understanding the corrosion behaviour of steel canisters buried within bentonite clay. Although Fe-corrosion in compacted bentonite has been widely studied, the mechanisms governing corrosion under various environmental conditions, such as bentonite dilution due to water ingress, salinity variations, oxygen release, and defect formation, remain insufficiently understood. This thesis develops a systematic, stepwise mechanistic framework to disentangle the electrochemical reactions, transport processes, and interfacial transformations that occur when pure iron, aged iron with magnetite scales, and compacted bentonite interact under disturbed conditions.

First, the work focuses on the electrochemistry of iron with bentonite slurries. Electrochemical experiments with Fe reveal that bentonite slurries promote the formation of an acidic Fe-bentonite gel, driven by Fe(2+)/H(+) ion exchange at montmorillonite sites, producing local pH values as low as 5 and suppressing passivation. Using in operando X-ray radiography, the thesis quantifies, for the first time, the threshold current required for gel nucleation and demonstrates that gel growth proceeds linearly with accumulated charge, confirming that the process is governed by interfacial reaction kinetics rather than bulk transport limitations. Variation in salinity shows that increasing NaCl concentrations suppress gel formation, induce or promote passivation, and ultimately yield pitting corrosion, whereas low-salinity environments favour uniform, gel-mediated dissolution.

Thereafter, to simulate long-term repository ageing conditions, oxide-magnetite films (1-5 µm) were fabricated as controlled aged-iron samples. These scales were electrochemically tested with bentonite slurries. The scales exhibit high corrosion resistance when intact, but localised defects - at macro or micro-levels act as potent anodic sites, triggering enhanced dissolution due to galvanic coupling and localised acidic gel development.

Finally, in situ investigations at the Mont-Terri Underground Rock Laboratory demonstrate that the long-term Fe-halo thicknesses observed in Fe-embedded bentonite are consistent with gel-front kinetics, thereby establishing a coherent mechanistic link between field observations and the slurry-based laboratory-scale processes.

Overall, this thesis integrates laboratory electrochemistry, operando imaging, engineered oxide scales, and long-term field data to build a coherent, mechanistic understanding of iron corrosion in reactive bentonite environments. The findings highlight the critical roles of ion-exchange chemistry, environmental transients, and oxide-scale integrity in determining corrosion modes. These insights refine conceptual models for DGR performance and provide an integrated framework for assessing canister durability under relevant repository evolution scenarios.