Carbon Dioxide, Hydrogen Sulfide, and Bacteria Pitting Corrosion Model

The main purpose of this paper is to develop a comprehensive model that can predict the pitting corrosion rate of oil and gas pipelines under CO₂, H₂S, and Bacteria corrosive conditions. Pitting corrosion under mass transfer control has been described using anodic and cathodic models.               The model considers 22 ionic species and the potential value inside the pit. The diffusion coefficient values of these ions, which strongly affect the corrosion rate, were found in the literature. This model predicts the maximum corrosion rate after the formation of a corrosion product scale.              In the presence of bacteria, the concentration of sulfates at the bottom of the pit must go to zero when convergence occurs. The sulfate reducing bacteria reduces sulfate to sulfide and produces acetic acid at the bottom of the pit, which accelerates corrosion. These acetates also form complexes with iron, which increases the acidity of the pit.                The presence of H₂S allows for the formation of FeS film, which has a suppressive effect on the CO₂ corrosion rate. This suppressive effect on CO₂ corrosion rate is more dramatic with increasing temperature and concentration of H₂S.
              This comprehensive model provides the corrosion rate in mils per year, if the system is CO₂ or H₂S dominated, and if the system is in a pitting or non-pitting condition. It was observed that the primary parameters that affect the model predictions are temperature, bulk pH, concentration of acetates, concentration of sodium chloride, concentration of hydrogen sulfide, concentration of sulfates, mass transfer film thickness, and metal wall thickness.