A Permeable Coating Model for Predicting the Environment at the Pipe Surface Under CP-Compatible Coatings

Some underground pipeline coatings, such as asphalt, coal tar enamel and fusion-bonded epoxy, are said to be CP-compatible. When these coatings degrade and groundwater contacts the pipe, the surface is still protected from corrosion and stress corrosion cracking (SCC) as the CP current can pass through the permeable coating. The electrochemical reactions on the pipe surface generate a local environment under the coating that is quite different from that in the surrounding soil. In general, the pH of the trapped water increases due to the cathodic reduction of water and oxygen by the CP current. A mathematical model has been developed to predict the generation and evolution of the environment under a disbonded permeable coating as a consequence of the action of CP. The model couples the electrochemical reactions on the surface of the pipe to the transport of species to and from the pipe surface through the permeable coating and the surrounding soil. The model is structured to use available field data (such as soil and ground water data and information from CP surveys) to predict conditions on the pipe surface. The model can be used to predict CP and environmental conditions under which the pipe may be susceptible to corrosion or SCC.