The failure mode of adhesively bonded aluminium following aqueous exposure

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Abstract
Adhesively bonded aluminium sheets pretreated by phosphoric acid anodizing (PAA) or chromic acid anodizing (CAA) were fabricated into 180° T‐peel test pieces. These were then exposed to cycles of aqueous exposure at elevated temperature, followed by a drying procedure at ambient or elevated temperatures. In the case of the CAA‐pretreated substrate, the aqueous exposure brings about a drastic reduction in joint strength and the locus of failure (as assessed by XPS) moves from the centre of the glue line to very close to the oxide/adhesive interface. The drying procedure increases the peel strength, and the failure path once again moves towards the bulk of the adhesive. This behaviour is ascribed to the agregation and to the subsequent removal of water molecules close to the interface. The behaviour of the PAA‐pretreated joints is completely different. The short water immersion times used in this work have little effect on peel strength but subsequent drying at elevated temperature does reduce the joint strength considerably. It is postulated that this may be the result of leaching and subsequent deposition of a component of the adhesive, perhaps some unreacted curing agent. This behaviour is also reversible and the peel strength can be increased by re‐exposing the joint to water. Thus, for both CAA and PAA pretreatments the strength can be affected greatly by environmental exposure, but both phenomena show a degree of reversibility.