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

The interfacial chemistry of environmental failure of adhesively bonded hot -dipped galvanized steel joints, fabricated using a structural epoxy in a l ap configuration, has been investigated by time-of-flight (ToF) SIMS. The f ailed lap shear joints show areas of apparent interfacial failure but these regions are limited to thin strips at the ends of the overlap, termed init iation zones. An initial study highlighted the importance of small area sur face analysis, using XPS, to demonstrate that electrochemical activity was responsible for the initial bond degradation and the formation of the initi ation zones at the ends of the overlap. Despite having been employed succes sfully 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 mech anism of such an adhesively bonded system. A method has been developed, usi ng ToF-SIMS, for mapping the initiation zone of the adhesive joint. The ima ges obtained support earlier evidence of electrochemical activity at the in itiation zone showing the presence of cations (Mg2+), indicating that catho dic behaviour played a role in the formation of the initiation zone. The to F-SIMS line scans indicate residual polymer in the initiation zone, which s upports the hypothesis of a dual effect of electrochemical behaviour and in gress of water being responsible for the formation of the initiation zone. This suggests weakening rather than the clear separation, prior to mechanic al testing, observed in the case of classical cathodic delamination, The To F-SIMS images extend this model by showing corresponding cation-rich and ad hesive-rich areas within the initiation zone, possibly demonstrating that t he different processes dominate in different regions (a result of localized electrochemical activity), and enables cathode size to be estimated. Copyr ight (C) 1999 John Wiley & Sons, Ltd.