Chromium-free surface pre-treatment of aluminum for adhesive bonding

Adhesive bonding offers many advantages, such as light weight, ease of manufacturing, corrosion resistance and more uniform stress distribution at joint. In order to achieve good initial adhesion and durability of adhesive bonded aluminum joint, pre-treatment of the aluminum substrate is essential. Some of the surface pre-treatment procedures commonly used are chromium-based chemical and electro-chemical methods. However, due to the environmental and health concerns, and legislation to ban the use of hexavalent chromium, there is a need to omit the use of chromium-based pre-treatment methods in the surface preparation process. Therefore, this work aims to study the effect of the developed hybrid chromium-free surface pre-treatment method on the bonding strength of adhesive bonded aluminum joint. The study on the effects of solution concentration level and processing time of the hybrid treatment on the bonding strength suggests that bonding strength can be improved through enhancement of mechanical interlocking and increase of bonding area between the adhesive and adherend. Even though comparable or rougher surface morphologies were obtained from hybrid treatments as compared to the chromiumbased treatment, lower bonding strengths were observed. This further suggests that the nano-scale level interaction between the adhesive and adherend has vital effect on the bonding performance.