Transient numerical model for crevice corrosion of pipelines under disbonded coating with cathodic protection

Cathodic protection can strongly affect the chemical and electrochemical characteristics of the solution under a disbonded coating. In this work, a transient mathematical model was developed to evaluate the effectiveness of cathodic protection on the pipeline steel under a disbonded coating, and the mechanism of cathodic protection for preventing crevice corrosion was discussed. The model includes the temporal evolution of the potential of the steel, pH, oxygen concentration, and conductivity of the solution within a crevice. The implicit finite difference method was used to solve the complex set of mass-transfer equations that describe the system. The chemical and electrochemical processes inside the crevice were numerically analyzed, and the results were compared with those obtained from the experimental data. The results showed that the dissolved oxygen inside the crevice is rapidly consumed, thereby inhibiting oxygen corrosion; the alkaline environment, which is generated by oxygen reduction, results in the formation of a passivation environment. The cathodic current gradually flows into the crevice by overcoming the solution resistance, and the steel is completely electrochemically protected. As the local-solution environment changes as a function of time and distance, various mechanisms may synergistically act to protect the metal inside the crevice from corrosion. (C) 2015 Elsevier Ltd. All rights reserved.