Cut-back edge cathodic disbondment susceptibility of exposed bare field joints in offshore pipelines

The cathodic disbondment susceptibility of coated offshore grade pipeline steel samples with a single holiday or a cut-back edge was investigated. Several accelerated conditions were used, including elevated temperature of 60°C or mechanical stress in the form of an elastic strain of 0.15%, and an overprotection potential of -1.5 VSCE. The resulting extent of disbondment on the coated samples was determined by lifting the coating with a sharp thin-bladed knife. Also, the delamination of unstressed cut-back edge samples was tracked by local electrochemical impedance spectroscopy (LEIS). Mechanical lifting of the FBE (fusion bonded epoxy) coatings on cathodically-protected samples demonstrated that the geometry of the defect does not affect the rate of coating disbondment during accelerated exposure conditions and time. However, the delaminated area on cut-back edge samples was larger than those with holidays due to the larger amount of exposed metal-coating interface on the former. Thus, a much higher current supply would be required to protect the large surface area of uncoated field joints and the associated large circumferential metal-coating interface area susceptible to cathodic disbondment in offshore applications.