The barrier protection of an organic coating for a metallic substrate is compromised by the transport of water, oxygen and ions from the environment to the metal/coating interface. Metallic outdoor structures are exposed to the weather and undergo cyclic wetting and drying conditions. In the absence of holidays, the transport of water is diffusion limited. The diffusion coefficients associated with water ingress Din and egress Dout are indicators of the wetting and drying rates, respectively, of the coating. Gravimetric and capacitance methods are used for the measurement of Din whereas Dout measurement has been limited to the gravimetric method. The objective of this paper is to demonstrate the application of a recently reported method that employed a hydrophilic room temperature ionic liquid to monitor the capacitance changes associated with the egress of water from which Dout was calculated. Experimental results associated with cyclic dilute NaCl wetting and ionic liquid drying are presented for an epoxy coating on an AA 2024-T3 substrate that was subjected to 50 cycles. The Din and Dout values were calculated using a short-time approximate solution and a series solution to Fick's second law. Presented electrochemical impedance spectra associated with the ends of wet and dry stages of the cycles were analyzed using an equivalent circuit model that separated the bulk coating and metal/coating characteristics.
Bibliographical noteFunding Information:
The Army Research Laboratory and the Air Force Office of Scientific Research supported this work under Contract No. W911NF-04-2-0029 and Grant No. FA9550-04-1-0368, respectively.
- Diffusion coefficient
- Ionic liquid
- Single-frequency impedance
- Wet-dry cycling