Application of embedded sensors in the thermal cycling of organic coatings

A standard Air Force polyurethane topcoat/epoxy chromate pigmented primer system was characterized using the thermal cycling method. This method is an accelerated test method for organic coatings that involves exposing the coating to alternating room temperature and elevated temperature cycles under constant immersion. Electrochemical properties were monitored using electrochemical impedance spectroscope for the total topcoat/primer system, the inner layer of primer, and the outer layer of the topcoat. The individual layers were monitored using platinum sensors embedded between the layers. Characterization of a system comprising of two layers of the epoxy chromate pigmented primer was also performed using EIS monitoring, and embedded sensors. The systems were cycled between room temperature, and set temperatures up to 80 °C. The barrier property of the topcoat and the bulk resistance of the primer were consistent with Arrehenius behavior. The results obtained were consistent with the intended design of the polyurethane topcoat and chromate primer with the former providing the significant barrier protection, whereas the primer providing the passivating chromate to the metal/coating interface. Analysis of the EIS data obtained from the sensor with an equivalent circuit decoupled the bulk primer properties from the metal/coating interface properties.