Cathodic Disbondment Test: What Are We Testing?

The cathodic disbondment (CD) test has been a well known and generally accepted laboratory test for pipeline coatings for a long time. Good test results (low delamination radii) indicate that the coating is expected not to delaminate to a large extent in the presence of cathodic protection and a small coating damage with the steel surface exposed to the soil. This makes the CD test essential for quality control of pipeline coatings. Potential, temperature, test duration, electrolyte composition and diameter of the drill are the main parameters of the CD test. Due to numerous variations of the test parameters an extraordinary variety of test conditions and related acceptance criteria has been established. Inconsistent test conditions and conflictive opinions about the mechanism make interpretation of the test results difficult, which raises the fundamental question: What are we testing and what is the significance of the test? By means of basic investigations factors influencing cathodic delamination have been identified and their extent on the delamination radius has been assessed. The relevance of these factors with regard to transferability of laboratory tests to field conditions has also been considered. The valuable insights obtained on CD testing proved helpful for an improved interpretation of the test results.