Microbiologically influenced corrosion (MIC) and chloride ion attack represent principal factors contributing to the premature failure of marine metallic infrastructures. This study systematically investigated the MIC behavior of X80 steel induced by sulfate-reducing bacteria (SRB) in simulated marine environments with varying chloride concentrations. Under sterile anaerobic conditions, elevated chloride concentrations slightly accelerated the corrosion of X80 steel. However, the biotic system (30 g/L Cl⁻) exhibited an order-of-magnitude increase in the corrosion rate of X80 steel compared to sterile controls. The relative corrosion severity in SRB-inoculated environments followed the descending order: 30 g/L > 10 g/L > 20 g/L > 5 g/L Cl⁻. Chloride concentration critically modulated SRB metabolic activity. Sulfate reduction rates at 5 g/L Cl⁻ demonstrated a significant 85% reduction relative to optimal conditions at 20 g/L Cl⁻, correlating with near-complete bacterial growth inhibition. Intriguingly, suboptimal chloride environments (10 and 30 g/L Cl⁻) triggered substantial extracellular polymeric substance (EPS) production, serving as a protective barrier and ionic exchange medium. EPS exacerbated steel corrosion by accelerating anodic dissolution through complexation with ferrous ions. Fourier-transform infrared spectroscopy results confirmed that EPS contains redox-active functional groups. Injecting 1 g/L EPS into SRB broth increased the corrosion current density from 9.2 ± 0.8 μA/cm² to 14.3 ± 1.2 μA/cm². These findings provide new insights into the dual role of EPS in MIC processes, emphasizing its critical function in extracellular electron transfer-MIC mechanisms.

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure. There is substantial debate over their corrosion mechanisms. We investigated Fe⁰ corrosion with Desulfovibrio vulgaris, the sulfate reducer most often employed in corrosion studies. Cultures were grown with both lactate and Fe⁰ as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes. Fe⁰ was corroded in cultures of a D. vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe⁰ loss and H₂ accumulation expected for Fe⁰ oxidation coupled to H⁺ reduction to H₂. This result and the extent of sulfate reduction indicated that D. vulgaris was not capable of direct Fe⁰-to-microbe electron transfer even though it was provided with a supplementary energy source in the presence of abundant ferrous sulfide. Corrosion in the hydrogenase-deficient mutant cultures was greater than in sterile controls, demonstrating that H₂ removal was not necessary for the enhanced corrosion observed in the presence of microbes. The parental H₂-consuming strain corroded more Fe⁰ than the mutant strain, which could be attributed to H₂ oxidation coupled to sulfate reduction, producing sulfide that further stimulated Fe⁰ oxidation. The results suggest that H₂ consumption is not necessary for microbially enhanced corrosion, but H₂ oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction. The finding that D. vulgaris was incapable of direct electron uptake from Fe⁰ reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes.