Effects of chromic acid anodizing of aluminium on adhesion and durability of bonded joints

Anodic film growth on aluminium in chromic acid under different conditions has been examined, with various routes explored to identify precisely the f ilm parameters, including overall film thickness, barrier layer thickness, pore and cell diameters. The selected anodizing conditions allow formation of films with either normally or inversely funnelled pore morphologies. The anodic film with normally funnelled type pores is well penetrated by adhes ive, unlike the film with inversely funnelled type pores. The former shows improved initial bond strength and durability. Transition regions, revealin g variation in electron transparency compared with the bulk adhesive, evide ntly exist between the anodic film and the bulk adhesive. After immersion i n water, bonded samples supporting anodic films with normally funnelled typ e pores reveal a locus of failure within the adhesive i.e. cohesive mode. F or samples supporting anodic films with inversely funnelled type pores, the locus of failure passes close to the alumina film/adhesive interface. Obse rvation of primed-only samples, after immersion in water, reveals aluminum species within the primer, resulting from their outward transport during th e film hydration process. Overall the data reveal the crucial influence of surface treatment of aluminium and its alloys prior to adhesive bonding or painting on the initial bond strength and durability.