Pre-treatment of AA6060 aluminium alloy for adhesive bonding

AA6060-T6 aluminium extrusions have been subjected to various surface treatments before bonding with an epoxy based adhesive and subsequent exposure to chloride and humid atmosphere (82% RH, 40°C) for 50 days. Single lap joints given an alkaline etch pre-treatment before bonding suffered a 7% reduction in strength as a result of environmental exposure. Desmutting in nitric acid after a similar alkaline etch pre-treatment to remove the Mg rich hydroxide film resulted in still higher loss in strength. Application of an experimental phosphate–permanganate conversion coating to the alkaline etched substrate also had no beneficial effect. Examination of the bonded surfaces after lap shear testing revealed the presence of filiform corrosion filaments, providing paths for rapid ingress of moisture to the substrate/adhesive interface. The best results were obtained by AC anodising in hot sulphuric acid to a film thickness of about 0.2 μm.