Protective properties of multilayer organic coatings on Al alloys

The protective properties of eight commercial multilayer organic coatings for aeronautical use (based on polyurethane, epoxy, and polyurethane-compatible epoxy resins) on pretreated Al alloys (2024T3 bare, 6013T4 bare, and 6013T4 cladded with pure aluminium) were evaluated in neutral aerated 3.5% NaCl aqueous solution using Electrochemical Impedance Spectroscopy (EIS) technique. The investigation was performed on specimens supplied and prepared in accordance with a industrial specifications. All coating systems exhibit excellent protective properties even after prolonged immersion in the test solution for one year. Although the dielectric properties of all multilayer coatings are quite similar, EIS discriminates among them, making possible a rank of these protective systems. Capacitance measurements allow forecasting coating behaviour. Results obtained with EIS were in good agreement with findings obtained with the salt spray test.