Corrosion and Electrochemical Conditions of Pipeline Steel under Tape Coating Disbondments: Effect of Disbondment Gap Size and Morphology

Abstract

In this study, the effect of coating disbondment gap size on corrosion and electrochemical conditions of the X-52 pipeline under simulated tape coating disbondment protected by cathodic protection (CP) was investigated. In this regard, corrosion cells simulating coating disbondment with gap sizes of 5, 3, and 1 mm and also a steep disbondment with a slope ratio of 5/1 [5 mm at the open mouth (OM) and 1 mm at the far end from the OM] were used. Different levels of CP were applied to the OM of the simulated coating dibondment using a three-electrode system, whereas pH and potential were recorded under the simulated coating disbondment at different distances and, also, time intervals. Results showed that depending on the CP potential level at the OM of the coating disbondment and disbondment gap size, there could be variations in corrosion and electrochemical conditions on the pipeline surface under the disbonded region. In fact, with a decrease in coating disbondment gap size, lower pH and higher levels of CP were observed. However, under narrow disbondment (gap sizes less than 1.6 mm), despite the better CP, significantly higher corrosion rates (more than 500%) were observed. This was explained by a crevice corrosion mechanism.