Anoxic Corrosion of Carbon Steel in Bentonite in Relation to the Swiss Deep Geological Repository Concept

摘要

The safety assessment of deep geological repositories for the disposal of radioactive waste involves the evaluation of a large number of relevant parameters for timescales of up to a million years. An important parameter is the lifetime of disposal canisters which depends mainly on the corrosion rate of the canister material. In order to obtain relevant data on which to base such assessment, long-term corrosion rate measurements under conditions which are representative of those expected in a deep geological repository are required. In this work, the corrosion of carbon steel, a candidate canister material, embedded in saturated bentonite under anoxic conditions was studied. The long-term evolution of the corrosion rate was monitored based on H_2 evolution using barometric gas cells. The measured corrosion rates are in agreement with weight loss measurements and with previous tests under similar conditions. Dismantling one of the gas cells and characterizing the surface of the corrosion coupons and the steel/bentonite interface provided an insight into the prevailing mechanisms under relevant repository conditions. The main corrosion product was identified as magnetite and other iron oxy-hydroxides. The thickness of the corrosion product layer was characterized by metallographic cross-sections. Surface microstructural analysis and profilometry indicated that some roughening of the surface and localized phenomena occurred.