Abstract
The interfacial chemistry and environmental degradation of adhesively bonded hot dipped galvanised steel (HDGS) has been investigated. The principle aim of the work has been to determine the failure mechanisms responsible for the poor durability of adhesively bonded HDGS. Surface analysis techniques have been used to characterise components of the respective adhesive systems and interrogate the surface of failed joints with a view to establishing the exact locus of failure. Durability studies have been carried out by exposing joints to 98% relative humidity at 35°C for up to 12 months. Alkali cleaned HDGS joints failed cohesively through a zinc corrosion product acting as a weak boundary layer. Joints manufactured from phosphated substrates, however, display better durability. The failed lap shear joints show areas of apparent interfacial failure limited to thin strips at the ends of the overlap, termed initiation zones. An initial study highlighted the importance of small area surface analysis, using XPS, to demonstrate that electrochemical activity was responsible for the initial bond degradation and the formation of the initiation zones at the ends of the overlap. Despite having been employed successfully in a number of adhesion studies, XPS is unable to give the molecular level of specificity that is required for the full understanding of the mechanism of such an adhesively bonded system. ToF-SIMS has the potential to overcome the limitations of XPS and a rigorous investigation has been carried out culminating in the ToF-SIMS imaging of regions within the initiation zone. The images obtained support earlier evidence of electrochemical activity at the initiation zone showing the presence of cations (Mg2+), indicating cathodic behaviour played a role in the formation of the initiation zone. ToF-SIMS line scans indicate residual polymer in the initiation zone, which indicates a dual effect of electrochemical behaviour and ingress of water together being responsible for the formation of the initiation zone. This suggests weakening rather than the clear separation, prior to mechanical testing, which is observed in the case of classical cathodic delamination. ToF-SIMS images have provided analytical data that extends this model by showing corresponding cation rich and adhesive rich areas within the initiation zone, possibly demonstrating that the different processes dominate in different regions (a result of localised electrochemical activity), and enables cathode size to be estimated. Small area XPS (at a spatial resolution of 20 mum) has been used to characterise areas within the initiation zone deficient in pretreatment that are similar in size to the local cathodes observed in the ToF-SIMS study. Quantified XPS line scans identified areas of high carbon content, once again inferring that water ingress plays a part in the failure mechanism. SEM also identified areas denuded of the phosphate pretreatment that are a result of exposure to a hostile environment, as a uniform pretreatment exists prior to bonding. Dissolution of phosphate crystals may have occurred within the alkaline environment produced by the cathodic half reaction. Mechanical data obtained throughout the durability study has reinforced the importance of the initiation zone with its size being directly related to joint performance (load to failure). Electrochemical activity has been found to be responsible for initial bond degradation. By using an established model, it is proposed that cathodic activity is a result of local corrosion cells set up at the substrate/adhesive interface.