Electrochemical impedance spectroscopy (EIS) has frequently been used to provide a means to nondestructively evaluate coatings. Equivalent circuit modeling can then be used to interpret the data. Several equivalent circuit models were used to analyze EIS data obtained from a commercial aircraft coating system exposed to laboratory environmental stressors to mimic in-service environments. Equivalent circuit parameter values were linked to outcomes of physics-based models of the resistivities and dielectric constants of the organic polymers making up the coating system. The fits to the measured EIS data from the equivalent circuit models were compared in how closely they matched the measured data. The physics-based input to the equivalent circuit modeling provided insight into the deterioration of the coating system in response to in-service environmental exposures.
Coating systems are used in a wide variety of environments, including seawater immersion and exposure to the atmosphere, as a method for preventing corrosion. A coating system refers to the layers of coatings that are applied over one another to form a structure that performs multiple functions that cannot be provided by a single coating1,2. For example, the initial layer of the system may consist of a surface pretreatment that thickens the oxide film of the aircraft skin material and aids in adhesion of the base coating to the substrate3,4. The second layer may consist of a primer coating that provides corrosion protection, binds to the oxide, and provides an adherent surface for a topcoat5. It may also contain inhibitor additives. The outermost layer usually consists of a thick topcoat layer that protects the primer and provides coloring using pigments and other filler materials6.
When properly applied, organic coatings exhibit excellent durability. However, exposure to ultraviolet radiation from sunlight, temperature and relative humidity changes, and deposition of pollutants and other corrosive ions on the coating surface, along with water from rain and condensation, eventually degrade the coating system7. The degradation can be evidenced in the color change and loss of gloss, in addition to the formation of flaws, e.g., pits or fissures that can aid in the transport of oxygen and electrolyte to the coating-metal interface, with the subsequent loss of mechanical properties. This degradation eventually leads to surface corrosion of the substrate.
