Assessing Electrolyte Transport through an Aircraft Coating System Using EIS

Abstract

Electrochemical impedance spectroscopy (EIS) is frequently employed to nondestructively evaluate coating systems. However, there are several challenges associated with this approach. One of these challenges is associated with isolating the physical processes that are contributing to the measured impedance. We attempted to address this concern by measuring several of the coating system components before starting the exposures and attempted in-situ measurements of some properties during the course of the exposures.In this work, we analyze impedance data obtained from a two-layer coating system that was applied to AA 2024 substrates that were subjected to either immersion and simulated outdoor exposure conditions. The impedance data from the full coating system is compared with data obtained from coating systems in which gold electrodes were inserted between the layers of the coating system that could be used to monitor changes in each coating layer. Several equivalent circuit models were evaluated before settling on a modified coatings equivalent model. Using this approach, we were able to estimate several coating system parameters, including electrolyte penetration depth as a function of temperature, and solution concentration from the values obtained from the equivalent circuit model. Acknowledgments The Department of Defense Strategic Environmental Research and Development Program (SERDP) sponsored this project under work unit 5399. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.