A Study of Microbiologically Induced Corrosion by Sulfate-Reducing Bacteria on Carbon Steel Using Hydrogen Permeation

The mechanism of microbiologically influenced corrosion (MIC) on carbon steel (CS) by the bacteria Desulfovibrio desulfuricans subs. desulfuricans was studied using hydrogen permeation, open-circuit potential, and cathodic polarization techniques, in a concentrated culture medium containing bacteria cells (10⁷ cell/mL) and ferrous ions (300 mg/L) designed to simulate a condition common in systems for the secondary recovery of crude oil, characterized by highly contaminated microenvironments that severely corrode iron alloys in a short time period. This research project was carried out using several 24-h experiments to define initial stages of the corrosive process under the conditions indicated. The results evidenced a hydrogen permeation current peak of about 12 μA correlated with a minimum open-circuit potential of –780 mV vs saturated calomel electrode (SCE), 400 min after inoculation. Next, the permeation current decreased abruptly to its base line and the potential increased, stabilizing at –585 mV_SCE at 24 h, a condition that is associated with high, similar bacterial activity both with and without cathodic polarization (10⁸ CFU/mL and 10⁹ CFU/mL), typical hydrogen sulfide (H₂S) attack morphology, and a weak iron sulfide film. These results using CS as the corrodible material, together with those obtained using a palladium strip as previously reported, show definitely that the cathodic depolarization theory does not represent the chief mechanism used by D. desulfuricans in the MIC process, whereas sulfide corrosion together with iron sulfide products seem to better explain the mechanism of this severe bacterial corrosion problem.