The influence of water when bonding with heat-curing epoxy adhesives on sealed sulphuric acid-anodised aluminium

In this paper, it is demonstrated how the formation of blisters or foaming in a heat-curing epoxy adhesive can be avoided during bonding to sealed sulphuric acid-anodised aluminium. The crucial points to control are the release of water from the anodic film and the total amount of water in the joint. The cure cycle will affect the rate of water release, while the total amount to a large degree depends on the relative humidity in the area of bonding before joint assembly. Besides the adhesive, also the anodic film absorbs water within a short time scale. This behaviour is caused by aluminium sulphate, which is always present in the anodic film after sulphuric acid anodisation. At room temperature the aluminium sulphate is in equilibrium with water vapour at about 30% r.h. It has been experimentally demonstrated that the amount of water stored in the anodic film has a dramatic step change between 30% and 40% r.h. at room temperature. Outside this range up to condensing conditions differences are relatively small. The release of water during the curing of the adhesive can in addition to blistering or gassing also cause a boundary layer with changed properties in between the adhesive and the anodic film.