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Protective multilayer coatings are designed, with each layer providing distinctive features to reduce the transport of water, ions, and oxygen from the environment to the substrate. The top coat provides the barrier, whereas the base coat/primer provides adhesion to the substrate as well as galvanic or passivation protection. The change in the coating capacitance during the ingress or egress of water is associated with the change in the dielectric property of the coating and can be monitored using the electrochemical impedance spectroscopy response of a high-frequency signal applied to the coating. The wetting and drying behaviors of epoxy and urethane single-layer coatings and epoxy primer/urethane top-coat two-layer coatings were experimentally measured. A hydrophilic room-temperature ionic liquid was used to simulate drying while maintaining a nonaqueous electrical contact to the coating. Embedded electrodes were used to access the top-coat-primer interface in the two-layer coating. Mathematical models were developed based on Fick's second law for water transport in single-layer and two-layer coatings. Comparison of measured and simulated water uptake demonstrated that the assumptions used in the single-layer model were appropriate while the assumptions used for transport across the interface between the layers in the two-layer coating were inadequate.

The application of Mg-rich primers ͑MRPs͒ for the protection of aluminum structures represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used. The protective modes of MRPs are similar to those of Zn-rich primers ͑ZRPs͒ on steel and include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment. Interpretation of ZRP electrochemical impedance spectroscopy ͑EIS͒ data has been accomplished using a transmission-line model that accounted for the contact impedance between the zinc particles, the impedance associated with the zinc dissolution, and the percolation resistance of the coating. EIS experiments results are presented here for an MRP on a gold substrate under immersion in dilute Harrison's solution. The data were analyzed using the measurement model technique to determine the consistency with respect to Kramer-Kronig relationships. The transmission-line model was used to analyze the data to demonstrate their applicability for analyzing the protection afforded by the MRP. Gold was used as a noble substrate and to promote that the electromotive degradation of the MRP as the mixed potential of an MPR/gold system was more positive than that of an MRP/Al system. Available commercial, nonchromate inhibitor pigments have been shown to be much less effective in corrosion protection as compared to the industrial standard of SrCrO 4 . A nonchromate pigment has been reported to perform as effectively as SrCrO 4 on Alodine 1200-treated AA 2024-T3. 1 The use of Alodine does expose the substrate to chromates and as such the influence of chromate cannot be discounted. The application of Mg-rich primers ͑MRPs͒ for the protection of aluminum structures has been demonstrated by Bierwagen and co-workers and represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used for such protection. 2-6 This system does not require an Alodine pretreatment and represents a Cr-free system.The protective modes of MRPs include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment by-products of the hydroxide and hydroxy carbonate species of Mg that fill in pores and voids. 3,7,8 The behavior of the MRPs on Al substrate has been characterized using opencircuit potential evolution, potentiodynamic polarization, electrochemical impedance spectroscopy ͑EIS͒, scanning vibrating electrode technique, and scanning electrochemical microscopy. [2][3][4][5][6] Interpretation of the EIS data has been limited to two and three time-constant equivalent circuit models where parameters attributed to coating resistance and charge-transfer resistance were observed to reduce with time immersed in 0.1 wt % NaCl. 3 The protective modes of MRPs are similar to those that have been associated with Zn-rich primers ͑ZRPs͒. 3,4 The c...

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Nonaqueous room-temperature ionic liquids ͑RTILs͒ offer a method of using electrochemical impedance spectroscopy ͑EIS͒ to analyze coatings without the confounding effects, such as plasticization, associated with an aqueous working fluid. Thermal cycling has been used as an accelerated testing method for ranking the corrosion protection of coatings. The influence of thermal cycling on the degradation of a coating was investigated using a hydrophilic RTIL as the electrolyte for the drying cycle of EIS experiments. The change in diffusion coefficient, saturated water volume fraction, and activation energy for the absorption and desorption of water by the coating was obtained.

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.

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