Effect of H3PO4 anodising variations on Al-epoxy adhesion strength

High adhesion strength of polymeric coatings on Al alloys requires special Al surface pretreatment. The formation of a thin layer of anodic Al oxide (AAO) leads to a strong polymer/Al bond. In this work, a thermosetting epoxy coating was applied on anodised Al surfaces. The effects of the electrolyte temperature, anodising voltage and time on the adhesion strength of coated samples were examined using a pull-off method. Surface morphology and topography of AAO layer were studied using field emission scanning electron microscopy and atomic force microscopy, respectively. Hydrophilic behaviour of the nanostructure anodised layer was investigated by a contact angle test. The results showed that the morphology of AAO layer has a remarkable impact on the adhesion strength. Morphological changes in the AAO layer from customary pore- to whisker-like structure led to a noticeable improvement in the strength of epoxy coatings; the latter structure was highly hydrophilic.