Stepwise corrosion mechanism of 37Mn5 oil tubing orchestrated by bacteria under a water injection well

After 8 months of service, 37Mn5 oil tubing in an oilfield suffered severe external corrosion and rapid failure, which poses a critical threat to oilfield production safety. To clarify the failure mechanism, this study systematically characterised the morphology, composition and phase constitution of corrosion products using scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction and conducted immersion verification tests with oilfield produced water combined with thermodynamic calculations. It is found that corrosion propagates as pitting along with the pearlite structure into the matrix, and the main phase of corrosion products is β-FeOOH, accompanied by minor phases of γ-FeOOH, FeS and graphite C. The corrosion failure is confirmed to be microbiologically influenced corrosion (MIC), and its development may be closely related to the oilfield water injection cycle: oxygen-rich injected water initially activates aerobic bacteria, while sulphate-reducing bacteria (SRB) become dominant after dissolved oxygen depletion, leading to alternating dominant corrosion mechanisms. It is revealed that SRB-produced H ₂ S reacts with Fe to form FeS and can also convert Fe ₃ C into FeS, graphite and H ₂ via thermodynamic reactions. This study discloses the alternating MIC mechanism driven by the water injection cycle, clarifies the synergistic effects of microorganisms in regulating corrosion product formation and matrix damage and identifies the specific role of Fe ₃ C corrosion, providing new insights into the MIC mechanism of 37Mn5 steel under cyclic water injection conditions. The findings may offer valuable reference for optimising oilfield water injection strategies, developing targeted MIC prevention and control technologies and improving the service life of 37Mn5 oil tubing in oilfield environments characterised by cyclic water injection.