Chromate conversion coating on aluminum alloys - II: Effect of the microstructure

By means of the combined use of scanning electron microscopy, energy dispersive spectroscopy, and atomic force microscopy in the Kelvin probe mode the mechanism of the chromate conversion coating (CCC) formation, discussed in the first part of the paper, was extended to the case of AA2024, taking into account the effect of the intermetallics (IMCs). It was found that the cathodic IMCs play a fundamental role during the nucleation of the CCC, since they act as preferential nucleation sites. Besides, the influence of the surface preparation procedure was also considered. The use of an acid pickling treatment prior to the chromate process was shown to strongly affect the growth of the CCC, since it results in the formation on the AA2024 surface of a copper-rich smut, which inhibits the chromate reduction reaction. Both IMCs and copper deposits negatively affect the CCC morphology. Indeed, the former are locations of defects and the latter decreases the thickness and adherence of the film. (C) 2004 The Electrochemical Society.