Bentonite compaction shapes microbial communities but abiotic processes dominate carbon steel corrosion in repository conditions

AIMS: The long-term safety of deep geological repositories (DGRs) for high-level radioactive waste relies on the corrosion resistance of metal canisters embedded in bentonite buffers. Microbiologically influenced corrosion (MIC) is considered a potential threat to metal canisters in deep geological repositories, yet the effect of bentonite compaction on corrosion processes remains poorly understood. This study evaluated microbial and abiotic contributions to carbon steel corrosion and corrosion localization in bentonite under repository-relevant conditions. METHODS AND RESULTS: Carbon steel corrosion was investigated in bentonite Černý Vrch (BCV) under slurry and compacted conditions (1200, 1400, and 1600 kg·m⁻³) during 3-11 months of anaerobic incubation. Corrosion was assessed by weight-loss measurements, X-ray diffraction, and microscopy, while microbial communities were analysed using 16S rRNA gene sequencing and digital PCR. Sterile and low-biomass systems often exhibited higher corrosion rates than biotic treatments, indicating that abiotic processes predominated in bentonite systems. Bentonite prevented localized corrosion and promoted uniform surface alteration. Corrosion was most pronounced in compacted bentonite (1.6 g·cm⁻³), despite reduced microbial activity. Bentonite dry density was the primary driver of microbial community composition in compacted systems, suppressing many taxa associated with steel colonization in slurries, although sulfate-reducing Desulfosporosinus remained abundant. CONCLUSIONS: Under the investigated conditions, bentonite compaction restricted microbial activity and suppressed many candidate MIC taxa but did not suppress abiotic corrosion. Corrosion rates were often lower in biotic than in sterile treatments, indicating that microbial activity may, under certain conditions, mitigate corrosion and should be considered alongside corrosion-promoting processes when evaluating repository-relevant bentonite systems.