The durability of bonded aluminium joints: a comparison of AC and DC anodising pretreatments

The environmental durability of anodised aluminium alloy AA6060-T6 bonded with a one-component epoxy adhesive has been investigated using the wedge test at 40°C/96% RH. DC and AC anodising in phosphoric and sulphuric acid solutions have been compared. Substrates that were AC anodised in hot phosphoric and sulphuric acid solutions performed very well, almost as good as the well-established Forest Product Laboratory+DC anodising in phosphoric acid pretreatment. The anodic oxide films were studied by reflection–absorption FT-IR spectroscopy. It was shown that the anodic films released water during curing of the adhesive at 180°C. The observations suggested that a transformation from the hydroxide, Al(OH)x, to the oxide, Al2O3, state took place. Hot water sealing of the oxides produced in sulphuric acid significantly reduced the durability.