Buried steel pipelines can be effectively protected against external corrosion using high resistivity coatings together with cathodic protection (CP). Consequently, corrosion on exposed steel surface at possible coating defects can be prevented by providing protective currents. Moreover, early detection and appropriate repair of the coating defects can minimize the corrosion risk. The authors have developed a technique to detect the occurrence of a coating defect using a change in the AC impedance of a pipeline. In the present study, field tests were carried out on an existing pipeline to measure the change in the AC impedance when a simulated coating defect was connected to the tested pipeline. It has been then revealed that the AC impedance varies depending on the resistance of a coating defect to earth and the distance from AC signal input point. According to the results of the field tests, an on-line monitoring technique to detect the occurrence of a coating defect and specify its location using the change in the AC impedance of a pipeline has been established. In addition, the behavior of the AC impedance obtained in the field tests has been confirmed by a theoretical study using a distributed constant model as an equivalent circuit for the tested pipeline.
