Characterizing the Relaxation of the Open-Circuit Potential During an AC-DC-AC Accelerated Test

The non-conventional accelerated alternating current-direct current-alternating current (AC-DC-AC) testing method for organic coatings induces degradation by promoting cathodic reactions at the metal coating interface and ionic transport in the bulk of the coating. This cyclic method involves a testing step in which the coating properties are determined using electrochemical impedance spectroscopy (EIS), a DC step in which a cathodic potential is applied for a given time period, and a relaxation step where the coating is allowed to relax back to a stable open-circuit potential (E_OC) condition. Analysis of the relaxation E_OC profile has been reported where characteristic times were attributed to the transport of ions through the coating, dielectric relaxation of the coating, and charge transfer at the metal/coating interface. The AC-DC-AC degradation of a standard United States Army vehicle epoxy primer on a steel substrate is reported here. The influences of applied DC potential and temperature were determined by subjecting the coating to potentials of −2 V and −8 V relative to the stable E_OC at temperatures ranging from 25°C to 65°C. The relaxation E_OC profiles were analyzed with a mathematical model to yield the characteristic times associated with the processes involved and the consistency of these processes with Arrhenius behavior was determined.