Investigation of the Galvanic Mechanism for Localized Carbon Dioxide Corrosion Propagation Using the Artificial Pit Technique Available to Purchase

Localized carbon dioxide (CO₂) corrosion is the most dangerous type of internal corrosion to mild steel pipelines in the oil and gas industry since the penetration rate of localized corrosion can be one or more magnitudes higher than that of uniform corrosion. In this study, the focus is on propagation of localized CO₂ corrosion on mild steel that occurs by a galvanic mechanism. A galvanic cell is established by the coupling of two distinct areas in a conductive CO₂ solution: a bare steel surface and an iron carbonate (FeCO₃) layer-covered steel surface. It was found that localized CO₂ corrosion propagates when a stable difference in corrosion potential is established between the anode (bare steel surface) and the cathode (FeCO₃-covered surface). Stable propagation will occur only when the conditions are in the “gray zone,” i.e., close to saturation with respect to FeCO₃, when no significant FeCO₃ dissolution nor precipitation is expected. Practically, this corresponds to when FeCO₃ supersaturation $(SSFeCO3)$ is in the range from 0.5 to 2. The key environmental factors that affect propagation of localized CO₂ corrosion of mild steel are temperature, pH, partial pressure of CO₂, salt concentration, and flow velocity. A protective FeCO₃ layer forms at high temperature (>50°C); therefore, the galvanic mechanism of localized corrosion is valid only in this range. pH needs to be such that moderately protective FeCO₃ layers form, typically at pH 5.5 to 6.5. Critical partial pressures of CO₂ is around 0.1 bar to 2 bar, above this very protective FeCO₃ films form at high temperature, giving a very low likelihood of localized attack. The solubility of FeCO₃ increases with increasing salt concentration, making it more difficult to form protective FeCO₃ layers and more likely to get localized corrosion propagation. Turbulent flow assists localized corrosion propagation by sweeping away corrosion products from the rapidly corroding steel surface and thereby preventing reformation of the protective FeCO₃ layer.