Environmental effects on the hydrolytic ageing of epibond 1590-aluminium joints - a dielectric study

Dielectric measurements are reported on Epibond 1590-aluminium joints exposed to water at 50 and 75°C. Differences in the ageing behaviour are observed which are not simply explained in terms of the expected Arrhenius dependence of moisture ingress into the joints. Ageing is the result of a combination of the effects of water entering the adhesive, leading to plasticisation and changes in the surface layer of oxide on the aluminium substrate. Surprisingly, the initial rate of corrosion at 50°C is higher than that at 75°C, although the rate of moisture uptake into the resin is higher at the higher temperature. This apparently anomalous behaviour can be attributed to filiform corrosion assisting in the opening of the bondline and aiding subsequent moisture and oxygen ingress. Comparison of joints that were periodically withdrawn for dielectric and gravimetric measurements with those aged continuously without withdrawal indicated that atmospheric oxygen plays a significant role in aiding the corrosion process. The dielectric measurements provide a non-destructive method of probing the processes taking place within the joint as it is aged