Interpretation of EIS data from accelerated exposure of coated metals based on modeling of coating physical properties

Electrochemical studies on the accelerated weathering of corrosion protective coatings systems in this laboratory have yielded time series data in the low frequency portion of the spectrum that can be fit by a simple exponential decay function in time. A description of how this can be empirically used for lifetime prediction for coatings has already been presented. Attempts have been made to use other parts of the EIS spectrum for analyzing coating performance in accelerated exposure, but these have not yet yielded a predictive model for the effects of exposure on the structure and performance of organic coatings. First, a simple model which considers only loss of coating thickness by UV-induced ablation is applied to the interpretation of EIS time series results from coatings systems undergoing accelerated weathering. Next, more complete models which include a diffusion front of electrolyte that moves with either Fickian or Type II diffusion kinetics are applied to the same problem. Incorporating these models for degradation as a simple RC equivalent circuit representation of a coating gives predictions of EIS performance that have reasonable agreement with experimental data. Further, using a two layer representation of coating changes due to water intrusion and known effective medium theories for dielectric and resistivity changes due to water intrusion allow prediction of the changes in EIS spectra seen in immersion or other accelerated testing of coating films. These results provide further evidence of the power of modeling physical processes in interpreting EIS data and predicting coating protective lifetimes.