INTERFACIAL CHEMISTRY OF SPONTANEOUS DISBONDING IN STRESS DURABILITY TESTING OF ADHESIVELY-BONDED GALVANIZED STEEL

Epoxy adhesive/galvanized steel bonds subjected to corrosion testing s how a gradual loss of strength. Bonds subjected simultaneously to a st atic mechanical load and corrosion testing rupture spontaneously at re latively short exposure times. The differences in interfacial chemistr y that accompany these exposure conditions were studied using an XPS e lemental mapping technique that allowed the interfacial composition to be resolved spatially over the entire bond failure surface. An interf acial anodic process reminiscent of crevice corrosion dominated the in terfacial 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 applicati on 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 sit es within the growing crack accelerate the corrosion process. The resu lt is rapid expansion of the interfacial failure region and spontaneou s rupture of the specimen.