The enhancement of adhesive joint strength by extending the surface of anodized aluminium

The initial joint strengths and durabilities of adhesively bonded 5251 aluminium alloy lap shear joints were determined after the surface of anodized aluminium was extended by etching. The post-anodizing etching solutions investigated were sodium hydroxide and, for comparison purposes, phosphoric acid. Durability was assessed by measuring joint strength after immersion in water at 50°C for 1500 hours. Improvement in initial joint strength and durability occurred when wetting of the surface by the adhesive was good. The improvement was ascribed to an increased contact area between oxide and adhesive. Good wetting was obtained with acrylic adhesives. The difficulty of wetting the surface with epoxy adhesives was eased by using a primer or by preheating the surface before bonding. Etching conditions were critical: temperature, immersion time and dissolved aluminium content of the sodium hydroxide solution all affected the rate of dissolution. The strongest joints were obtained after approximately 8 μm of the anodic oxide film thickness had been dissolved and left about 4–6 μm of unetched inner oxide beneath 5–6 μm of etched outer oxide.