Comprehensive insights into corrosion processes of potential canister materials in bentonite suspension at elevated temperature

Within the multi-barrier system for deep geological repositories for high-level nuclear waste, the stability of potential canister and geotechnical barrier materials is crucial for long-term safety. Over time, it is expected that the near field of the canister will become saturated with groundwater and the geochemical environment consequently changes from oxic – warm – unsaturated to anoxic – cool – saturated conditions. This change in the geochemical environment impacts the material behavior of canister and buffer materials over long time. Corrosion of the canister materials might occur inducing a change in the properties of the canister and/or buffer material.

The aim of the study presented is to enlighten fundamental corrosion processes of potential canister materials in bentonite suspension under anaerobic conditions at elevated temperature. Four metals were studied: wrought copper cold rolled, oxygen-free phosphorous-doped wrought copper, cast iron with spheroidal graphite (EN-GJS-400-15), and a stainless steel (grade: 1.4021). All metal coupons were ground with 600/800/1200-grit SiC paper and subsequently cleaned with acetone to guarantee a defined surface state before exposure experiments. Grinding removes any oxide layers, ensuring a homogeneous and even surface. Metallographic pre-characterization of the coupons was performed by light optical (LOM) and scanning electron microscopy (SEM). Two different bentonites (1.) commercially available bentonite (Sigma Aldrich) and 2.) granulated Wyoming-type (Bara Kada)) were used for the exposure experiments. Bentonite slurries were prepared by mixing 5 g of bentonite with 100 ml of synthetic Opalinus clay porewater. Two distinct compositions of synthetic Opalinus clay porewater were used. The prescribed bentonite suspension and one coupon each were placed into steel autoclaves equipped with a Teflon liner. To ensure anaerobic conditions, the autoclaves were assembled within a glovebox. After assembling and airtight sealing, the autoclaves were kept at 60 °C for 60 and 90 days. Subsequent to the end of the experiments, the autoclaves were transferred into a glovebox, where they were dismantled and the coupons prepared for further analysis. The following characterization techniques are used: (µ)-X-ray diffraction (XRD), µ-X-ray-fluorescence analysis (µXRF), SEM, LOM and Raman spectroscopy.

The results indicate that localized corrosion is the predominant corrosion mechanism on the iron based materials. The attached digital microscopy image shows the localized corrosion of steel 1_4021 after 60 days at 60 °C. Complementary SEM-EDX and µXRF analysis revealed the formation of a Cr-oxide as the main corrosion product. The impact of the observed corrosion characteristics for the long term safety will be discussed.