Effect of CO2 on the SRB-induced corrosion behavior of X65 steel in oilfield-produced water.

X65 steel is widely used in oil and gas pipelines. However, in complex oilfield environments, X65 steel is highly susceptible to CO2 corrosion and microbiologically influenced corrosion (MIC). The corrosion behavior of X65 steel was systematically investigated in the absence and presence of sulfate-reducing bacteria (SRB) under CO2 conditions, using weight loss measurements, electrochemical techniques, and surface characterization. Corrosion products were analyzed to elucidate the underlying mechanisms. Results showed that CO2 initially accelerated abiotic corrosion by forming carbonic acid, and the subsequent corrosion product scales (FeCO3) on X65 steel reduced the corrosion rate. Conversely, in the sterile system, the corrosion product scales provided less protection. On day 17, SRB-CO2 exhibited ∼10-fold lower corrosion resistance (Rct + Rf = 4600 Ω·cm2) than sterile-CO2 (43,763 Ω·cm2), and by day 21, the maximum pit depth was ∼2.8 times greater, highlighting the synergistic effect of SRB and CO2 on corrosion. Mechanistic analysis indicated that more ferrous sulfides formed in the SRB-CO2 system than in the SRB system. In the SRB system, significant localized and uniform corrosion occurred during the middle stage. Additionally, surface passivation decreased the corrosion rate compared to the control, although localized corrosion increased.