Non-substrate EIS monitoring of organic coatings with embedded electrodes

Electrochemical impedance spectroscopy provides a quantitative evaluation of the protection afforded by coatings on metals. Two constraints are that the coating is under immersion and that the substrate acts as the working electrode with the counter and reference electrodes located in the electrolyte. The use of embedded electrodes placed between a topcoat and primer can relax these constraints and make EIS monitoring more applicable to coatings in the field. A two-electrode, non-substrate configuration involves two embedded electrodes on a coated panel acting as the working and counter/reference electrodes. This configuration has been used to characterize the interlayer between a topcoat and primer under the assumption that the current passed through the interlayer. Simulated results have been presented where current passage for a non-substrate configuration was through the metal substrate. The results associated with a urethane topcoat/epoxy primer system and an alkyd topcoat/alkyd primer system are presented to demonstrate the feasibility of monitoring the substrate where the substrate is not an electrode. The degradations of the coatings were induced using the ac–dc–ac accelerated test where the immersed coatings were subjected to cycles that involved a dc cathodic potential condition that promoted the cathodic reactions at the metal/coating interface.