The Role of the Surface Pretreatment in the Durability of Aluminium-Alloy Structural Adhesive Joints: Mechanisms of Failure

Abstract

The durability of adhesively bonded aluminium-alloy joints have been investigated by undertaking cyclic-fatigue tests in liquid water and in 55% RH, to establish the threshold adhesive fracture energy, Gth, below which crack growth will not occur. Three surface pretreatments were employed: grit-blast and degreasing (GBD), phosphoric-acid anodising (PAA), and PAA followed by the application of an anti-corrosion primer (PAAP). The ranking of the durability for the six systems, as assessed by the value of Gth, is as follows: X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-filtered transmission-electron microscopy have been used to assess the nature of the locus of failure and the mechanisms of failure. For the GBD pretreated joints, tested in both the liquid water and in 55% RH environments, failure occurs predominantly by the thermodynamic displacement of the adhesive from the metal substrate, according to the well-established principles of interfacial thermodynamics. In the case of the PAA and PAAP surface pretreatments, the anodising process provides additional adhesion forces and stability to the interface, which involves the formation of a microcomposite interphase region, and failure is shown to occur in a cohesive manner entirely within the adhesive for the tests undertaken in the 55% RH environment. For the specimens tested in liquid water, in the threshold region, the joints show loci of failure which are predominantly within the adhesive but with small areas of interfacial failure and oxide failure. The differences in durability for the PAAP and PAA when exposed to 55% RH or liquid water is ascribed to the kinetics of aggregation of water in the environs of the metal/adhesive interface, this phenomenon occurring much more rapidly for exposure to liquid water during the cyclic-fatigue process than for the exposure to water vapour at 55% RH. A method based on the XPS analysis of failure surfaces has been used to assess the extent of interfacial failure of the joint prepared from anodised stock. This is shown to be up to 95% depending on treatment and exposure conditions.