Effects of post-treatment of anodic aluminium oxides on adhesive bonding strength and durability

摘要

Anodizing is one of the best methods to create anodic oxide films for adhesive bonding of aerospace aluminium alloys. The produced layer is typically a few micrometre thick and nano-porous, providing extra corrosion resistance and a large surface area for oxide-adhesive adsorptive interactions and mechanical interlocking. Different types of electrolytes are typically applied for this purpose. These include: chromic acid-, phosphoric acid-, sulphuric acid and phosphoric-sulphuric acid anodizing (CAA, PAA, SAA, PSA respectively). Each electrolyte has its own optimal operation regime that results in specific oxide composition and pore- and film structures [1, 2]. However, it is possible to modify the chemical and morphological oxide film properties even further by applying a post-treatment. In this paper we discuss different types of post-treatments and their effect on adhesive bond strength and durability, as evaluated by peel and bondline corrosion tests. Results show that dipping the freshly prepared oxide in a sodium hydroxide bath will open the pores, but also alternate the surface composition, in turn affecting the adsorptive interaction with an amine molecule [3]. On the other hand, the presence of fluorides in this post-treatment bath leads to fluoride adsorption that consequentially caused significant reduction in the subsequent adhesion of the joint. The application of a silane layer on different types of oxides was found to improve the interface stability under water ingress, even in the absence of porosity, i.e. mechanical interlocking.