Mechanism of cathodic disbonding of three-layer polyethylene-coated steel pipe

The mechanism of cathodic disbonding of three-layer polyethylene (PE)-coated steel pipe composed of liquid epoxy primer, adhesive PE, and a PE protective layer at an elevated temperature was investigated in sodium chloride (NaCl) solution. Cathodic disbonding tests were carried out to determine the route by which the substances reached the disbonding front under various conditions. Tested parameters included coating thickness, dissolved oxygen, NaCl concentration, and cathodic potential. The cathodic disbonding radius (r) decreased with increased PE thickness. Both oxygen and water migrating through the coating strongly affected cathodic disbonding. Oxygen migrated not from the interface between coating and steel at the holiday (defect) but through the PE coating to the disbonding front. Sodium did not migrate through the PE coating but through the interface between the coating and steel. Therefore, the decrease of r with increased coating thickness was caused by retardation of water and oxygen migration. The cathodic disbonding test, carried out at an elevated temperature with vigorous hydrogen evolution, such as in an environment of < -1,500 mV(SCE), did not accelerate but instead actually inhibited the disbonding.