Integrity of 3LPE Pipeline Coatings: Residual Stresses and Adhesion Degradation

Both fusion bonded epoxy (FBE) and 3-layer polyolefin (3LPO) coating systems have been widely applied in the world as new-construction external pipeline coatings. 3LPO is regarded as a more damage resistant coating system while FBE is considered to be more compatible with the CP system. In the past few years, several incidences of coating disbondment at the FBE-steel interface of 3LPO have been reported in the literature. The coating disbondment issue has raised concerns over the use of 3LPO. The US Department of Transportation (DOT) initiated a research program to address these technical concerns. Our research program is funded by this DOT initiative with a team of pipeline coating experts participating in this research project. The integrity of the 3LPO pipeline coating is impacted by the residual stresses and adhesion degradation by water penetration. This paper examines the effect of coating thickness on residual stress, degradation of adhesion due to immersion in hot water and their combined affect on disbondment. The integrity of 3LPO is good when the coating is applied properly. Under the following conditions, the 3LPO may delaminate: (1) inadequate FBE coating formulation, (2) inadequate FBE coating application procedure and surface preparation, and (3) induced stress due to high volume shrinkage of PO layer.