Electrochemically characterizing the ac–dc–ac accelerated test method using embedded electrodes

Embedded sensors were used as an in situ corrosion-sensing device for aircraft and vehicular structures protected by organic coatings. Results are presented where changes associated with standard air force aircraft and army vehicle coatings were monitored by embedded sensors. These coatings consisted of a polyurethane topcoat and an epoxy primer, however are formulated to provide different characteristics. The ac–dc–ac testing method was used to accelerate the degradation of these coatings while being immersed in a NaCl medium. Electrochemical impedance spectroscopy and electrochemical noise measurement experiments were used to monitor the induced changes. A comparison of the results between coatings subjected to the ac–dc–ac exposure and coatings subjected to only constant immersion in the NaCl medium is presented. The results were used to demonstrate the effectiveness of the ac–dc–ac method at accelerating the degradation of an organic coating without observably changing the normal mechanism of degradation. The data highlights the different features of the coating systems and tracks them while the coating is being degraded. The aircraft coating was characterized by a high-resistant topcoat that can mask corrosion/primer degradation at the primer/substrate interface whereas the vehicle coating was characterized by a low-resistant topcoat with an effective corrosion inhibiting primer. Details of the ac–dc–ac degradation were evaluated by using an equivalent circuit to help interpret the electrochemical impedance data.