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Subsea pipelines buried in marine sediments are highly susceptible to microbiologically influenced corrosion (MIC). Although magnetic flux leakage (MFL) inspection is widely employed for pipeline integrity assessment, the effect of residual magnetic fields (MF) on MIC remains poorly understood. In this study, the effect of MFs on the MIC behavior of X80 pipeline steel induced by Desulfovibrio desulfuricans was systematically investigated. The results show that MF slightly accelerate the abiotic corrosion of X80 steel, while MF enhances mass transfer at the metal–electrolyte interface via Lorentz force. While MFs have little influence on the growth of D. desulfuricans utilizing organic carbon sources, they significantly promote extracellular electron transfer by activating magnetite (Fe3O4). This enhancement enables D. desulfuricans to more efficiently acquire electrons from the metal surface. These findings provide critical guidance for MFL inspection practices in offshore oil and gas pipelines, suggesting that residual MF will accelerate MIC. Therefore, the potential risk of MIC should be carefully considered when applying magnetic inspection technologies in microbially active environments.