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

Abstract

Localized carbon dioxide (CO 2) 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 CO 2 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 CO 2 solution: a bare steel surface and an iron carbonate (FeCO 3) layer-covered steel surface. It was found that localized CO 2 corrosion propagates when a stable difference in corrosion potential is established between the anode (bare steel surface) and the cathode (FeCO 3-covered surface). Stable propagation will occur only when the conditions are in the "gray zone," i.e., close to saturation with respect to FeCO 3 , when no significant FeCO 3 dissolution nor precipitation is expected. Practically, this corresponds to when FeCO 3 supersaturation (SS FeCO 3) is in the range from 0.5 to 2. The key environmental factors that affect propagation of localized CO 2 corrosion of mild steel are temperature, pH, partial pressure of CO 2 , salt concentration, and flow velocity. A protective FeCO 3 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 FeCO 3 layers form, typically at pH 5.5 to 6.5. Critical partial pressures of CO 2 is around 0.1 bar to 2 bar, above this very protective FeCO 3 films form at high temperature, giving a very low likelihood of localized attack. The solubility of FeCO 3 increases with increasing salt concentration, making it more difficult to form protective FeCO 3 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 FeCO 3 layer.

Localized CO2 corrosion propagation at moderate FeCO3 supersaturation initiated by mechanical removal of corrosion scale

“…The corrosion potential started to increase after 10 h as the iron carbonate scale continued to form. This phenomenon agrees with previous reports, possibly due to the formation of a pseudo-passive film [5,13]. The scale was scratched after 1 day of immersion and scale formation after a stable corrosion potential was achieved.…”

“…3) indicating that the potential on the scale-covered surface was higher than that on the newly scratched surface. This rapid change in potential is consistent with the establishment of a galvanic cell between the two surfaces [5,13].…”

“…This created a fresh steel surface which acted as an anode with respect to the scale-covered surface which served as a cathode. This geometry simulated a ''mesa'' type of localized corrosion typically observed in oil and gas pipelines [5].…”

“…In these articles, a worst-case scenario was investigated assuming localized corrosion proceeded at a rate corresponding to a freshly developed, bare pit anode where the galvanic cell held maximum galvanic potential difference. Steady localized corrosion continued at small degrees of FeCO 3 supersaturation of 0.5-2 [5,13]. In this study, we investigate localized corrosion at moderate levels of FeCO 3 supersaturation and hypothesize that a porous corrosion scale forms in the pitted surface, resulting in a reduced rate of localized corrosion, but that does not stop localized corrosion.…”

“…These processes are affected by interrelated factors including flow (causing flow-induced localized corrosion) [2,3], FeCO 3 supersaturation [4,5], pH [5,6], anions/cations [7,8], temperature [4], scale properties [9,10], and metallurgy [11,12]. Recent research by Han et al [5,13] clarified the galvanic mechanism of localized CO 2 corrosion. Two surfaces with and without scale were studied; a different open circuit potential measured across the surfaces established the existence of a galvanic cell.…”