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

• A cerium conversion coating (CeCC) was evaluated as protective system for AA6060.
• An additional sol–gel top-layer was applied in order to enhance corrosion protection.
• Morphology, composition and corrosion resistance of coated samples were investigated.
• Low contact times with the CeCC solution provide better corrosion protection.
• The sol–gel layer is capable of improving the protection provided by the CeCC.

This work evaluates the behavior of a cerium based conversion coating applied on AA6060. In order to improve the protection provided by the cerium conversion coating an additional ZrO2 sol–gel coating was applied as a top-layer.The conversion bath was formulated in order to obtain a suitable surface morphology for the deposition of the very thin sol–gel layer and in order to be free of chlorides, to avoid any possible contamination of the substrate. The cerium conversion layer is not clearly identifiable by the SEM observation, while its presence is confirmed by GD-OES composition profiles and XPS. Impedance spectroscopy was used for determining the corrosion behavior of both protective systems. Samples pretreated only with the cerium conversion coating results show a nanometric 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 mechanism for the deposition of the cerium conversion layer with the employed formulation, relying on the nature of the conversion solution is also proposed in order to explain this behavior. EIS results also show that the deposition of an additional sol–gel layer provides a further improvement of the corrosion protection exhibiting a synergistic effect with the CeCC obtained for 1 min of immersion in the conversion bath. Moreover the system consisting of the cerium conversion underlayer plus the sol–gel top-layer can provide better corrosion protection than the sol–gel layer alone.