Monitoring of the AC-DC-AC Degradation of Organic Coatings Using Embedded Electrodes

The application of embedded sensors is presently being developed to serve as a reliable corrosion sensing technique used for structures protected by organic coatings. Results are presented where embedded sensors were used to monitor the changes associated with a urethane topcoat-epoxy primer coating and an alkyd topcoat-alkyd primer coating on steel substrates. The results were used to demonstrate the ability of the sensors to detect changes at the primer/substrate level that would otherwise be undetected from measurements made from exterior to the coating. The coating systems were subjected to an alternating current-direct current-alternating current (AC-DC-AC) accelerated testing condition under constant immersion in sodium chloride (NaCl) media. Comparison of results from the AC-DC-AC experiments and results of coating systems subjected to constant immersion in NaCl media is presented to demonstrate the degradation associated with the accelerated testing condition. The data obtained from electrochemical impedance spectroscopy and electrochemical noise method demonstrated that a high-resistant topcoat could mask primer degradation and corrosion at the primer/substrate interface. Analysis of the impedance data using an equivalent circuit separated the resistance associated with the different layers of the primer/substrate systems.