New Model for Estimating Water Absorption by Coating using Electrochemical Impedance Spectroscopic Evaluation

A new model for estimating the water absorption of coatings has been developed using electrochemical impedance spectroscopy data and based on an effective medium theory. It is assumed that the volume fraction of the electrolyte varies along the cross section of coating thickness according to a power-law which is defined based on coating thickness and its capacitance capacity and the coating resistivity and permittivity profiles of immersed coating can be calculated through an effective medium theory (EMT) formula corresponding to a parallel combination of the two media (electrolyte and coating material). The total volume water uptake of proposed model was compared with the results calculated based on the Brusher-Kingsbury theory and gravimetric data for a commercial epoxy coating sample immersed in 3.5% sodium chloride solution for 120 days. The results showed that the estimated water absorption values according to this model, gravimetric method and Brusher-Kingsbury model is 7.5, 7.1 and 10.3%, respectively, and the results of this model are in good agreement with the gravimetric results.. In addition, this method made it possible to estimate the water absorption profile of the coating, changes in specific strength and permeability of the coating during immersion time.