EIS characterization of thick flawed organic coatings aged under cathodic protection in seawater

Electrochemical impedance spectroscopy (EIS) was used to monitor, up to 20 months, the degradation kinetics of four thick organic coatings under cathodic protection in seawater. EIS experiments were realized onto a flawless zone, which surrounds an artificial hole of the coating using the Luo's cell or a removable cell. EIS spectra were analyzed first with classic equivalent circuits and then with an approach involving the dipolar relaxation of polymer layer. These two models were unable to explain the experimental data, especially for a long immersion experiments. Taking into account the presence of vacuoles, namely pre-existing air pockets that will be then filled by electrolyte, modified equivalent circuits were used and a good representation of experimental data was obtained. The parameters extracted allowed the coating degradation to be followed. It was found that a solvent-free coating exhibited the best compatibility with cathodic protection.