Interfacial chemistry of spontaneous disbonding in stress durability testing of adhesively-bonded galvanized steel

Epoxy adhesive/galvanized steel bonds subjected to corrosion testing show a gradual loss of strength. Bonds subjected simultaneously to a static mechanical load and corrosion testing rupture spontaneously at relatively short exposure times. The differences in interfacial chemistry that accompany these exposure conditions were studied using an XPS elemental mapping technique that allowed the interfacial composition to be resolved spatially over the entire bond failure surface. An interfacial anodic process reminiscent of crevice corrosion dominated the interfacial chemistry of specimens exposed to corrosion testing without application of a static load. Bonds exposed under high loads exhibited both anodic and cathodic corrosion sites within the bond failure area. The changes in interfacial chemistry and failure mode upon application of a load are attributed to the opening of an interfacial crack at the locus of the initial corrosive attack. The ingress of electrolyte and the formation of cathodic sites adjacent to interfacial anodic sites within the growing crack accelerate the corrosion process. The result is rapid expansion of the interfacial failure region and spontaneous rupture of the specimen.