Microbial Influence on Corrosive Processes at the Cast Iron-Bentonite Interface during 3.5 Years of Incubation at 37 and 70 °C

Bentonite-based geotechnical barriers constitute currently an integral part of international disposal concepts of high-level radioactive waste in claystone and crystalline rock. Elevated temperature, canister corrosion, and metabolic activity of microorganisms can influence their performance. To study these effects on processes at the cast iron-bentonite interface, microcosms were incubated with two different bentonites (B27 and GMZ-001), synthetic Opalinus Clay pore water with a high water-to-bentonite ratio and cast iron plates at 37 and 70 °C over a period of 3.5 years. No microorganisms or nutrients were added to those naturally occurring in the applied materials. Cast iron surfaces in contact with GMZ-001 bentonite appeared strongly corroded, and secondary layer-like structures formed at both temperatures. Furthermore, an accelerated release of iron into the pore water, H2 generation, and gas pressure build-up were observed. In contrast, incubation with B27 bentonite at 37 °C resulted in less corroded cast iron surfaces covered with iron sulfides, which seemed to passivate the surface and led to lower iron release, H2 generation, and gas pressure build-up. Respective B27 microcosms were dominated by metabolically active sulfate-reducing bacteria that induced iron sulfide formation. This study shows that the corrosion behavior of cast iron and changes in geochemical parameters like pH and pore water composition depend on the applied bentonite, naturally occurring microorganisms, and temperature.