Blisters are one of the most frequently observed failure modes in FBE coatings of buried pipelines. Initial small disbondment defects at the coating-steel interface, due to embedded abrasives and voids are potential sites for blisters to form and grow. Two types of blisters are observed in FBE coatings - non-osmotic and osmotic blisters. Non-osmotic blisters are associated with water absorption in FBE coatings. Subsequent coating swelling induces compressive stresses in the coating and eventually buckling of the coating at the disbondment locations in the FBE-steel interface to form blisters. Our previous investigation has shown that an initially small disbondment must grow to sufficiently large size before non-osmotic blisters can be formed. Osmotic blistering is a result of an extra amount of water diffusion into the disbondment area due to salt contamination and causing osmotic pressure buildup at the disbonded locations. The osmotic pressure needed to blister FBE coatings is dependent on the size of disbondment. For an initially small disbondment, the pressure required to blister the FBE coating could be very high and the size of disbondment will need to increase before a blister can form. The driving force to enlarge the disbondment is the osmotic pressure generated from the soluble salts. The purpose of this work is to study the blister initiation mechanism. With a clear understanding of the blister initiation mechanism, steps can be taken to mitigate blister formation and improve the integrity of external FBE pipeline coatings.
Two types of blisters are observed in fusion bonded epoxy (FBE) pipeline coatings. These blisters are (1) osmotic blister associated with water diffusion through coating due to the presence of soluble salt on substrates or interlayer, and (2) non-osmotic blister due to swelling and buckling of FBE films associated with water absorption in the coating.
In 2012 Guo, Lo, and Chang published a paper on the non-osmotic blister mechanism of FBE pipeline coatings1. In the paper, the authors demonstrated that an initially small disbondment must grow to a certain size before the swelling stress in the coating can cause buckling of the FBE film.