Epoxy coatings cathodic disbond

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

When the epoxy coated panel containing artificial holidays was allowed to cathodically disbond, areas of greater than one square centimetre became detached in less than 7 days. Once again at no time were realistic A.E. transients observed. 

Although the term was originated to describe the failure of organic coatings, it is clear that cathodic disbondment can lead to the accelerated failure of adhesive joints as well. The work of Davis and Watts illustrates this phenomenon for steel substrates bonded with an epoxy adhesive and shows how a combination of XPS and Secondary Ion mass spectroscopy (SIMS) are able to provide a definitive picture of the locus of failure. Such a combined approach enables a detailed mechanism of failure to be postulated. 

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