IMPROVEMENT OF THE DURABILITY OF ZINC-COATED STEEL EPOXY BONDED JOINTS

The durability properties of bonded lap shear joints made from an epox y/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electropla ted-phosphated (EZ2) steel have been investigated. The degradation mec hanisms have been studied after three accelerated ageing tests: the '' cataplasme humide'' (''C.H.T.''), immersion (''I.T.''), and salt spray (''S.S.T.'') tests. X-ray photoelectron spectroscopy (XPS) analysis o f fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that i ntergranular corrosion phenomena account for poorer performances of th e hot-dipped galvanized substrate during ''C.H.T. '' and ''I.T.'' Sila ne coupling agents were successfully used as primers on both substrate s to increase the hydrolytic stability of the metal/adhesive interface . XPS results indicate that both the interfacial dissolution of the ph osphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear t o depend on the nature of the silane coupling agents. Alkylsilanes hav e been found to perform as well as silanes having a group capable of r eacting with the epoxy/dicyandiamide system. Additional tests were car ried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/si lane mixture prior to bonding. When corrosion was the controlling degr adation mechanism as is the case during the salt spray test, silane tr eated specimens did not generally perform better than control specimen s.