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

    Year: 2013

    Journal: Surface & Coatings Technology, 215, pp 260-265, 20130926

    Authors: Ö. Özkanat, F.M. de Wit, J.H.W. de Wit, H. Terryn, J.M.C. Mol

    Organizations: Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands, Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands, Delft University of Technology, Adhesion Institute, Kluyverweg 1, 2629 HS Delft, The Netherlands, Vrije Universiteit Brussel, Research Group Electrochemical and Surface Engineering, Pleinlaan 2, B 1050 Brussels, Belgium

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