Adhesive bonding of hot-dipped galvanized steel: use of ToF-SIMS for forensic analysis of failed joints

Abstract

The interfacial chemistry of environmental failure of adhesively bonded hot‐dipped galvanized steel joints, fabricated using a structural epoxy in a lap configuration, has been investigated by time‐of‐flight (ToF) SIMS. The failed lap shear joints show areas of apparent interfacial failure but these regions are 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 a full understanding of the mechanism of such an adhesively bonded system. A method has been developed, using ToF‐SIMS, for mapping the initiation zone of the adhesive joint. The images obtained support earlier evidence of electrochemical activity at the initiation zone showing the presence of cations (Mg2+), indicating that cathodic behaviour played a role in the formation of the initiation zone. The ToF‐SIMS line scans indicate residual polymer in the initiation zone, which supports the hypothesis of a dual effect of electrochemical behaviour and ingress of water being responsible for the formation of the initiation zone. This suggests weakening rather than the clear separation, prior to mechanical testing, observed in the case of classical cathodic delamination. The ToF‐SIMS images extend 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 localized electrochemical activity), and enables cathode size to be estimated. Copyright © 1999 John Wiley & Sons, Ltd.