Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure

The chemical conversion treatment for aluminium alloys based on the immersion in cerium chloride/hydrogen peroxide solutions is one of the possible alternatives to the chromate conversion process for the corrosion protection of aluminium alloys. The nucleation and growth of the cerium-based conversion coating on AA2024 was studied using complementary surface analysis techniques, as atomic force microscopy in the Kelvin probe mode (SKPFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of both the intermetallic particles and the copper in solid solution was considered. According to this aim, different surface preparation procedures prior to the conversion process were investigated. Acid pickling and Ce-based deoxidising, as well, were found to cause the formation of copper or copper oxide deposits, i.e. copper smut, on the AA2024 surface. While the intermetallics do not act as preferential nucleation sites, the deposition of copper on the surface is a necessary condition in order to form a thick cerium oxide film. On the other hand, the copper smut strongly decreases the adherence of the conversion layer to the aluminium substrate.