Influence of pretreatments and aging on the adhesion performance of epoxy-coated aluminum

In this work, we investigate how the adhesion of epoxy coatings to aluminum surfaces is influenced by surface pretreatments and by aging. First, aluminum substrates were electropolished or grinded as a preliminary step; then different pretreatments (acid, alkaline and immersion in boiling water respectively) were applied in order to create variations in the surface properties of aluminum substrate. Differently pretreated surfaces were then characterized by means of contact angle measurements. The surface energy for each pretreated surface was estimated from the contact angle values obtained with 3 different liquids. Pseudoboehmite oxide, created after immersion in boiling deionized water, with the highest hydroxyl fraction and oxide thickness exhibited the lowest contact angle, so as the highest surface energy, compared to the other surfaces (i.e., reference, acid pretreated, alkaline pretreated). It was observed that the polar component of the surface energy was surface dependent and determined the total surface energy. A direct correlation between polar energy and hydroxyl fraction was then shown for the aged electropolished samples. This study was followed by the application of an epoxy-based coating on the pretreated surfaces. Epoxy/aluminum joints were characterized by means of lap shear testing and adhesive failure was observed for all cases, except the pseudoboehmite surfaces. It was shown that all differently pretreated surfaces along with the reference surface were influenced by aging, i.e., 4h after pretreatment no considerable pretreatment effect on contact angle nor on shear stress was observed, indicating the importance of limiting the time between pretreatment and application of an organic coating. Furthermore, it was observed that surface pretreatments influenced the adhesion performance. The pseudoboehmite surface exhibited a lower adhesion performance, despite having the highest surface energy. This was explained by cohesive failure of the pseudoboehmite oxide layer. This research presents a relation between surface pretreatments and adhesion performance of the aluminum substrates.