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

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 degrees 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.