Steel-bentonite interaction at elevated temperatures

The long-term durability of carbon steel, proposed as the outer layer of waste disposal packages in the Czech deep geological repository concept, is influenced by anaerobic conditions, elevated temperatures, irradiation, bentonite composition and water content. These factors were examined in a small-scale laboratory experiment within the European Joint Programme on Radioactive Waste Management, project CONtainer CORrosion under Disposal conditions (EURAD CONCORD). Carbon steel coupons were embedded in compacted (1600 kg⋅m ^-3) bentonite of Ca/Mg-type (BCV) and reference bentonite of Na-type (MX-80), subjected to water saturation under a pressure of 5 MPa, heated to 90 °C and 150 °C, and irradiated with a ⁶⁰Co source at 0.4 Gy·h ⁻¹ for 6 to 18 months. Corrosion rates were primarily affected by temperature and saturation level. Similar rates were observed for both bentonites at 150 °C over 18 months, while lower rates were recorded for MX-80 at 90 °C and at ambient temperature. Irradiation had a negligible effect. Corrosion product composition strongly depended on temperature. At 90 °C, Fe-Ca‑carbonates formed in both bentonites, along with Fe-Ca(Mn-Mg)‑carbonates in BCV. At 150 °C, Fe-Si-O-rich layers and Fe-oxides developed, probably linked to the dissolution of amorphous silica and its incorporation into alteration products. Elevated temperatures altered the mineralogy and chemistry of the bentonite. Changes included shifts in smectite basal spacing, enrichment with Fe ^x+ and Mn ²⁺, and modifications to exchangeable cation composition, including reduced Mg ²⁺ content. Increased SO₄ ²⁻ and Cl ⁻ concentrations in bentonite water leachates, particularly in BCV at 90 °C and MX-80 at 150 °C, partially correlated with higher corrosion rates. These results highlight the interconnected influence of temperature, bentonite type, and chemical interactions on carbon steel corrosion in repository-relevant environments.