Cerium conversion coating and sol-gel multilayer system for corrosion protection of AA6060

This work evaluates the behavior of a cerium based conversion coating applied on AA6060. In order to improvethe protection provided by the ceriumconversion coating an additional ZrO2 sol-gel coating was applied as a toplayer.The conversion bath was formulated in order to obtain a suitable surface morphology for the deposition of thevery thin sol-gel layer and in order to be free of chlorides, to avoid any possible contamination of the substrate.The ceriumconversion layer is not clearly identifiable by the SEMobservation, while its presence is confirmed byGD-OES composition profiles and XPS. Impedance spectroscopywas used for determining the corrosion behaviorof both protective systems. Samples pretreated only with the cerium conversion coating results show ananometric but protective film for 1 min of immersion in the conversion bath, while longer contact times (e.g.30 min or 120 min) result in a lower quality of the protection provided by the conversion coating. A mechanismfor the deposition of the cerium conversion layer with the employed formulation, relying on the nature of theconversion solution is also proposed in order to explain this behavior. EIS results also show that the depositionof an additional sol-gel layer provides a further improvement of the corrosion protection exhibiting a synergisticeffect with the CeCC obtained for 1 min of immersion in the conversion bath.Moreover the system consisting ofthe ceriumconversion underlayer plus the sol-gel top-layer can provide better corrosion protection than the sol-gel layer alone.