Failure Analysis of Disbondment of Three-Layer Polyethylene Coatings from the Surface of Buried Steel Pipelines

A failure analysis was conducted to analyze the mode and causes of the disbondment of the three-layer polyethylene coatings from the surface of buried steel pipelines. The failure interface was analyzed through visual and microscopy investigations. Analysis of the coating at the disbonded interface revealed that the fusion bonded epoxy (FBE) layer had a fragile integrity which may have resulted from the in-service conditions of the pipeline. According to failure analysis of the disbonded interface, it can be concluded that degradation of the molecular structure of FBE layer was the most probable cause of the coating failure. It is believed that degradation of the molecular structure of FBE layer and subsequent disbondment of the coatings was caused by a mechanical-assisted cathodic delamination process which was due to the simultaneous action of chemical degradation and residual stresses at the coating-substrate interface. It is believed that surface pretreatment of the steel substrate in addition to coating process parameters of the powder coating are the main parameters in determining the long-term performance of the coating-substrate interface and should be carefully controlled.