Simulation of wet-dry cycling of organic coatings using ionic liquids

The capacitance evolution of an organic coating undergoing cyclic wetting and drying conditions was monitored by single frequency electrochemical impedance spectroscopy. Monitoring of the drying condition was possible with the use of a hydrophilic room temperature ionic liquid and the methodology employed is presented. Experimental results associated with cyclic dilute NaCl wetting and ionic liquid drying are presented for an epoxy coating on an AA 2024-T3 substrate. The calculated capacitance evolutions associated with wetting and drying were generally consistent with Fick's second law. The calculation of the water ingress and egress diffusion coefficients using a short-time approximate solution and a series solution to Fick's second law are presented. The latter solution is shown to address the capacitance evolution better than the former with the ingress coefficient larger than the egress coefficient for a given exposed coating surface. There was agreement between the calculated diffusion coefficient ingress values for coating areas exposed to cyclic NaCl wetting-ionic liquid drying and cyclic NaCl wetting-natural drying conditions. Comparison of the impedance spectra for test areas indicated that the use of the ionic liquid as a drying medium influenced the electrochemical properties of the coating only after a number of cycles.