Sir,
I would like to draw the readers’ attention to the ‘Review of the cathodic disbondment resistance of pipeline coatings’ on page 14 of the September/October issue, in which it is claimed that ‘the main cause of coating degradation on pipelines with CP is cathodic disbondment’.
There are many papers from established pipeline experts that roundly dispute this. Correctly deigned and applied pipeline coatings do not fail when there is a properly designed and operated CP system. For example, the paper presented at the 2007 NACE Corrosion Expo from David Norman and Colin Argent, both well-known and respected pipeline specialists, who described and defined the failure modes for pipe and field joint coatings. There are many more.
I appreciate that, for example FBE coatings can blister in service; perhaps the world expert in FBE who wrote the book on same (ISBN-13 : 978-1575901480), Alan Kehr, presented clear data in his book and in subsequent work, that such blisters are not significant and do not lead to corrosion if CP is properly operated. They are largely caused by water intake. 3LPE and 3LPP coatings are so high in dielectric strength that cathodic disbondment is inconceivable except, possibly, at coating damage down to steel. Also, Section 9 of Macaw’s Encyclopaedia of Pipeline Defects lists some 40 causes of pipe coating failures, cathodic disbondment is not one of them.
Brian Wyatt, Corrosion Control Ltd.
