In Sweden, spent nuclear fuel will be encapsulated in sealed cylindrical canisters, consisting of a ferrous insert and a copper outer container, for disposal in a geologic repository. Ferrous support structures will also be used in the repository, which will be backfilled with bentonite clay. Once any residual oxygen has been consumed, any ferrous material exposed to anoxic groundwater will undergo anaerobic corrosion, liberating hydrogen, forming a magnetite film, and releasing iron ions into the surrounding matrix. In order to characterise these processes the rate of hydrogen generation of steel in bentonite was measured using a barometric gas cell technique. The initial corrosion rates were found to be higher than measured previously in comparable aqueous solutions, but the long-term corrosion rates were similar. Analysis of the bentonite matrix showed that iron produced by corrosion had penetrated into the bentonite matrix, suggesting that ferrous ion exchange had occurred.