Application of a General Reactive Transport Model to Predict Environment Under Disbonded Coatings

Understanding the evolution of the chemical environment and potential inside a disbonded region is essential to a quantitative risk assessment of corrosion and stress corrosion cracking of pipelines. A general reactive transport model is presented in this paper that enables calculation of the time evolution of chemistry and potential under disbonded coating exposed to a variety of solutions under an applied potential. Model predictions are compared to a number of experimental observations reported in the literature. It is shown that the predicted pH and potential gradients in the disbonded region are the results of competing anodic dissolution and cathodic reactions. The pH and potential gradients are influenced by exposure time, applied potential, solution conductivity, and crevice geometry. The presence of carbon dioxide (CO2) results in a lower pH at short time periods, but after a few thousand hours, the pH is determined by the cathodic potential. Further improvements to the model are identified.