Chromate conversion coating on aluminum alloys. III. Corrosion protection

Electrochemical impedance spectroscopy (EIS) was used to investigate the corrosion resistance of the chromate conversion coating (CCC) formed on two different substrates, namely bare and clad 2024 aluminum alloys, and after different surface preparation procedures, namely acid pickling with or without desmutting. The impedance data were fitted using previously published equivalent circuits, which were developed on the basis of a physical model of the CCC. The evolution of the network parameters during exposure to the chloride solution enabled the correlation of the differences in the film morphology, which are caused by the variation in the substrate microstructure, with the corrosion protection provided by the conversion film. Specifically, the CCC corrosion resistance is strongly decreased by the presence of both intermetallic particles and copper smut on the Al surface, since the former are locations of defects in the conversion layer and the latter inhibits the film growth and lowers its adherence to the substrate. The corrosion resistance of the CCC formed on the different substrates and after the different surface preparation treatments can be clearly differentiated using the EIS technique and fitting the data with the proposed equivalent circuits, which represent the conversion film as a nonhomogeneous system containing large defects where the corrosion attack principally occurs.