The influence of sulfate and iron reducing bacteria on carbon steel corrosion in marine environments

The current study highlights the corrosion consequences observed in API 5L X65 carbon steel when exposed to a marine-like environment containing carbon dioxide (CO₂), in the presence of sulfate- reducing bacteria consortium (C-SRB), iron-reducing bacteria consortium (C-IRB), and a combination of both (C-SRB+C-IRB). The corrosion behaviour was assessed through the utilisation of weight loss and surface analysis techniques. The biofilms and corrosion products were characterised using field emission scanning electron microscopy (FESEM) and infinite focus microscope (IFM), respectively. The results obtained from the weight loss technique provided confirmation that in the absence of bacteria, uniform corrosion was the prevailing mechanism in the medium. The uniform corrosion rate was found to be 0.95 mm/year, which was higher than the pit penetration rate of 0.28 mm/year. The combined actions of the C-SRB+C-IRB consortium result in a synergistic impact, leading to a significantly higher rate of pit penetration compared to the individual activities of SRB. Specifically, the pit penetration rate value of 2.13 mm/year is observed, which is notably higher than the uniform corrosion rate of 0.59 mm/year. The utilisation of surface analysis techniques has shown evidence of the existence of sulfur on carbon steel samples that were subjected to exposure from the C-SRB+C-IRB consortium in a CO₂ environment. This sulphur presence is believed to potentially play a role in the development of the iron sulphide (FeS) layer. FeS presence is a contributing factor to the occurrence of pit corrosion due to the SRB metabolite activities. The activities of these metabolites readily dissolve in the medium, hence altering the chemical interface properties of both the metal and the biofilm. For specimens exposed to C-SRB only, the results obtained were almost identical to exposure to the C- SRB+C-IRB consortium. However, the corrosion rate value measured for the C-SRB+C-IRB consortium was higher due to the synergistic effect of the two bacteria. In conclusion, the presence of C-SRB alone and the consortium of C-SRB+C-IRB in a CO₂ environment induces the formation of pitting corrosion. The findings of this study are presumed to contribute a deep understanding of the microbiology-influenced corrosion (MIC) mechanism in CO₂ environments troubling the oil and gas industry. With a better understanding of MIC, new and improved corrosion prevention strategies can be put into action.