The failure of organic coatings during cathodic protection

• Main Author: Tsay, K. C.
• Published: University of Manchester 1988
• Subjects: 667.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594383

Cathodic disbandment is a major mode of failure of organic coatings during cathodic protection. In this thesis factors influencing the rate of cathodic disbandment and the failure mode of cathodic disbandment of two polymer systems have been investigated. The polymer systems chosen were a chlorinated rubber and a polyamine cured epoxy on mild steel substrates. The interfacial chemistry of disbandment has been studied by X-ray photoelectron spectroscopy (XPS). The application of acoustic emission to monitor the progress of cathodic disbandment was also studied. The disbandment process of the chlorinated rubber and the epoxy polyamine coatings during cathodic polarisation in alkali metal chloride solutions has been shown to be electrochemical in nature. The disbandment of the coatings is primarily caused by attack of the coating/substrate interfacial bonds by the hydroxyl ions generated on the substrate surface under the disbanded coating. The disbondment rate of both coatings tested is controlled by the following factors: coating thickness, applied cathodic potential, electrolyte, electrolyte concentration of the test solution, surface treatment of the substrate, and the nature of the coating. The surface treatment of the substrate has a substantial effect on the disbandment rate; it not only changes the actual area of the interface where cathodic disbandment occurs but also changes the resistance to cathodic disbandment. The initiation of the cathodic disbandment is found to be mainly controlled by a process involving water permeation through the coating. However, the initiation is also influenced by other factors controlling the disbandment rate, such as surface treatment of the substrate, the electrolyte in the test solution.