It is 'cathodic disbondment' and is the degradation of coating adhesion caused by the chemical reactions associated with the application of cathodic protection.
Cathodic disbondment Somewhere in my basement files I have a test specification issued by British Gas Co for testing a coating or lining. A small hole is drilled through the lining into the steel surface - -measurements are taken of the rate at which corrosion slowly radiates out from the hole. This is called the Cathodic Disbondment test I never came across it anywhere else. Let me know if you want more details.
Cheers
CMan.
What SJones said, here is the more detailed technical explanation you could get from a text book.
Cathodic disbondment or cathodic delamination is the loss of adhesion between an organic coating and a cathodically protected metal substrate typically initiating from a defect site such as a pinhole, scratch, or crack which exposes the underlying cathodically protected substrate to an electrolyte. An aqueous environment supplies oxygen and moisture, which permeate through the coating leading to the formation of hydroxyl anions as a result of the oxygen reduction reaction:
(1)
2H2O + O2 + 4e? ? 4OH?
As the reaction ensues, cations from the electrolyte such as Na+ dissociate to charge balance the OH? build-up at the metal/coating interface and migrate along the interface during the radial disbondment growth from the defect site. The generation of hydroxyl can raise the local pH to approach 14, creating an extremely tortuous alkaline environment. While the oxygen reduction reaction and subsequent hydroxyl formation and cation migration are the cause of CD, the actual degradation and delamination have been reported as one or any combination of: (a) displacement of the coating by a thin layer of high pH water, and, (b) dissolution of the underlying oxide layer, (c) cohesive failure within the polymer at the metal/polymer interface as a result of hydrolysis of bonding groups, (d) cation ingression/migration along the metal/polymer interface, (e) mechanical loss due to plasticization of coating, (f) microcracking and micro-crazing, and (g) critical crack propagation as a result of corrosion stress.
