Failure mechanism of phosphated adhesively bonded hot-dipped galvanized steel: a small-area XPS study

The failure mechanism of an aged, phosphated, hot-dipped galvanized steel (HDGS) sealant adhesive lap joint system has been investigated using XPS, In a previous study, imaging time-of-flight (ToF) SIMS identified electrochemical activity in the form of local cathodic cells set up within an initiation zone: thin strips of visually interfacial failure at the ends of the overlaps. Complementary polymer maps also identified areas of cohesive failure, suggesting a dual effect of electrochemical behaviour and ingress of water being responsible for the formation of the initiation zone. The role of electrochemical activity in the initiation zone and its effect on the substrate are considered in this paper. Small-area XPS (at a spatial resolution of 20 mu m) has been used to characterize areas within the initiation zone that are deficient in pretreatment and of similar size to the local cathodes observed in the ToF-SIMS study. Quantified XPS linescan data identified areas of high carbon content, once again inferring that water ingress plays a part in the failure mechanism. Scanning electron microscopy also identified denuded areas of the phosphate pretreatment that are a result of exposure to a hostile environment, because a uniform pretreatment exists prior to bonding. Dissolution of phosphate crystals may have occurred within the alkaline environment produced by the cathodic half-reaction. Copyright (C) 2000 John Wiley & Sons, Ltd.