Conductive cementite aggravates SRB-induced pitting of carbon steel via enhanced extracellular electron transfer

Microbiologically influenced corrosion (MIC) of carbon steel by sulfate-reducing bacteria (SRB) is a major challenge in oil and gas systems, particularly because of its strong tendency to cause localized attack. Cementite is a common conductive microstructural phase in carbon steels, yet its role in SRB-induced MIC has not been well clarified. In this work, pure iron, 10# steel, and 45# steel with different cementite contents were investigated in sterile and SRB-containing media using surface characterization, corrosion product analysis, pitting quantification, and electrochemical measurements. The results showed that increasing cementite content was associated with more severe localized corrosion, as evidenced by the increased maximum pit depth and broader pit size distribution, while the average corrosion rate did not exhibit a clear monotonic trend. Etched specimens with greater cementite exposure showed further aggravation of pitting, indicating that exposed cementite plays an important role in the corrosion process. Electrochemical results and surface observations suggest that conductive cementite facilitates interfacial electron transfer between sessile SRB and the steel substrate, thereby promoting localized MIC. These findings provide evidence that steel microstructure, especially cementite-containing phases, is an important factor governing SRB-induced pitting and should be considered in the microstructural design of MIC-resistant carbon steels.