In this paper, failure analysis methodology will be applied to the principal mechanisms by which FBE coatings fail during long term service; with specific application to case studies involving blistering. The case studies apply standard failure analysis techniques to determine the primary causes and modes of failures.
Shielding of cathodic protection (CP) by disbonded coatings is considered a “worst case scenario” for external corrosion in the pipeline industry. It has been hypothesized that if coatings were partially permeable to CP, the imbalance between cathodic and anodic reactions would induce a high pH environment under disbonded coatings and that would impede corrosion. However, direct experimental evidence of this process has been limited. Moreover, this line of reasoning is often inversed incorrectly assuming that the only reason for the presence of a high pH environment under disbonded coatings is their partial permeability to CP current.
The recent development of an electrochemical self-validating technique to measure the relatively small ionic currents that could permeate through defect free coatings has shed a new light on the topic. It has been found that most of the commercial pipeline coatings tested in unaged conditions present a CP shielding behavior. Accelerated hydrothermal ageing of fusion-bonded epoxy (FBE) coatings was performed to study the effect of water absorption. Ageing resulted in major plasticization, which increased the conduction of CP current through FBE coatings. However, FBE seems to be partially permeable to H+ and/or OH-, which reduces its ability to form and retain a high pH environment under disbonded coatings. In conjunction, these results thus indicate the extremely limited capability of commercial pipeline coatings to prevent corrosion in case of disbondment.
The purpose of this NACE International technical report is to provide basic information regarding the mechanisms involved with cathodic protection (CP) shielding for external coatings applied to carbon steel pipelines, and the manner in which the breakdown of coating systems may or may not contribute to CP shielding. It is intended to be a basic reference from which the tendency of a coating system to fail and shield CP may be understood on a conceptual basis. Non-corrosion-related pipeline protection systems, including concrete weight coating, polyethylene encasement, insulation, and mechanical pipeline protection systems, are outside the scope of this report. End users of this report may include pipeline designers, corrosion engineers, integrity management specialists, regulators, CP service providers, coating suppliers, coating service providers, equipment suppliers, and pipeline/facility owners.