Electrochemical properties of pipeline steel X-65 in dilute HCO3-/CO32- solutions

Corrosion and near-neutral pH SCC under shielding coating, i.e., PE tape coating, continue to be integrity issues for underground oil/gas transmission pipelines. The electrical shielding nature of the coating makes it difficult to identify locations of concern based on above-ground methods. It has been aimed to develop a predictive model to determine the consequences for corrosion and near-neutral pH SCC under disbonded shielding coating. This study is intended to examine the electrochemical properties of pipeline steel X-65 in dilute HCO₃^-/CO₃^2- solutions, and to generate essential inputs, i.e., E_corr, Tafel slopes, and reaction rate constants for the anodic and cathodic reactions occurring on the pipeline surface, for the predictive model. In this study, three dilute HCO₃^-/CO₃^2- solutions, i.e., 10 mM, 20 mM, and 50 mM, have been used to simulate the trapped water composition, and the pH of the solution was adjusted by purging N₂ gases containing 0 ~ 100% CO₂ to achieve the typical pH of the trapped water under a disbonded shielding coating, i.e., pH 5.5 to 7.5.   The results from the experiments show that the corrosion process of X65 carbon steel in dilute HCO₃^- solutions involves Fe – 2e^- à Fe²⁺ as the anodic half reaction coupled with H⁺ + e^- à H₂ as the cathodic half reaction. The anodic half reaction is under mixed diffusion-electrochemical control, while the cathodic half reaction is electrochemically controlled. The Tafel slopes for both anodic and cathodic reactions vary with HCO₃^-/CO₃^2- concentration. Additionally, a current-peak has been observed on the cathodic polarization curve at the cathodic overpotential range of -0.95 V to -0.75 V.