Electrochemical Screening Test for Corrosion Protective Aerospace Coatings for AA2024-T3

The objective of the present study was to develop a sensitive electrochemical test method to rapidly and quantitatively screen the corrosion performance of aerospace coating systems. The present investigation compared electrochemical results obtained using an expanded electrochemical impedance spectroscopy (EIS) protocol (using EIS changes following physical breaching of the coating) to results obtained via the standard ASTM B117 salt spray test. Thirty-two different coating systems comprising various combinations of surface pretreatments, primers, and topcoats on AA2024-T3 (UNS A92024) were applied in a commercial setting. EIS tests were performed in a solution of 3.5 wt% ammonium sulfate ([NH4]SO4) + 0.5 wt% sodium chloride (NaCl). Samples were immersed for 8 days with a pinhole-sized defect made after 4 days of immersion. Barrier properties and changes in Rcorr as a function of time following defect production were used in an empirical formula to generate an overall electrochemical metric, Z*. Salt spray tests were done on a separate series of these coating systems for 2,500 h to 3,000 h, according to ASTM B117, and the results were ranked from 10 (excellent performance) to 1 (very poor performance). Comparison between Z* and the salt spray rankings of the samples suggests a promising correlation between the impedance data and salt spray performance, although additional information is needed to confirm this hypothesis. The magnitude of the interfacial impedance at early time periods does not predict salt spray performance. However, kinetic information that examines the rate of change of the interfacial impedance as a function of time following the introduction of a physical defect in the coating is essential in predicting performance.