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.

INTRODUCTION

The application of a strong and resilient coating in combination with sacrificial anodes on steel pipelines is the preferred method for preventing or mitigating corrosion during offshore service. Currently, the installation of such pipelines consists of welding the coated pipeline sections together and then applying a field joint coating over the bare metal-weld areas prior to lowering them to the seabed. Cathodic protection (CP), in the form of sacrificial anodes, is also normally installed to help protect small defects in the coating that may be created during pipeline installation. Nevertheless, the application of the joint coating is time consuming and a financial burden for the oil and gas companies. Consequently, there is an economic incentive to redesign the CP system to prevent corrosion of the large resulting exposed bare metal and weld areas created by not applying the field coating.1,2 However, at the increased cathodic currents required to maintain CP on the bare metal, there is the risk that coating disbondment would initiate at the cut-back coating edge adjacent to the bare metal. This is problematic as the delamination creates additional bare surface exposed to a very thin layer of seawater in a crevice-like feature. The seawater composition between the bare metal and disbonded coating is expected to differ significantly from that of the overlying sea, which may affect hydrogen embrittlement (HE) susceptibility and thus long-term reliability of the pipeline.

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