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. © 2007 The Electrochemical Society. DOI: 10.1149/1.2764235 All rights reserved. The protection of metallic structures by organic coatings is primarily due to the isolation of the structure from the environment by the coating. The barrier protection provided by a coating is influenced by the absorption of water and ionic species from the environment. Literature associated with the measurement of water absorption of a polymer film has been reported using gravimetry. 1-7 A parameter that significantly influences the transport properties of a coating is the diffusion coefficient of water into the coating Din .A n idealized model that relates the mass changes associated with absorption to Din for an organic coating is based on the series solution of Fick’s second law. 3,4,8 An approximation to the series solution that is applicable for short times is employed in literature to calculate Din based on gravimetric measurements. 3,5,6 The series solution to Fick’s second law and the approximate solutions are also applicable to mass changes associated with desorption of water. The different dielectric properties of water and an organic coating result in changes in the capacitance of the coating that accompanies water absorption or desorption. 1,2 The monitoring of the capacitance by electrochemical impedance spectroscopy EIS has provided a convenient method for the volumetric evaluation of water absorption in organic coatings. 3,4,7,9-15 The capacitance evolution of an organic coating can be used to calculate Din by relating the volumetric changes associated with water absorption with the mass changes.