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

The non-conventional accelerated ac-d-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 EIS, a d.c. step in which a cathodic potential is applied for a given time period, and a rest 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 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 Army vehicle epoxy primer on a steel substrate is reported here. The influences of applied d.c. potential and temperature were determined by subjecting the coating to potentials in the range of –2 V to –8 V relative to the E_OC at temperatures ranging from 10 ^oC to 50 ^oC. 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 Arrehenius behavior was determined.