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 with an applied cathodic potential. The model predictions are compared to a variety of experimental observations reported in the literature. It is shown that the predicted pH is the result of competing anodic dissolution and cathodic reactions. The pH in the crevice increases with more negative external cathodic potential. The pH increases with a decrease in crevice gap, but the effect is less pronounced at more negative potentials. The presence of CO₂ results in lower pH at short time periods, but after a few thousand hours the pH is determined by the cathodic potential. The agreement between model and experiments is generally better for simple boundary conditions and highly conductive solutions. Further improvements in model development are identified.