APPLICATION OF X-RAY PHOTOELECTRON-SPECTROSCOPY TO THE STUDY OF DEGRADATION MECHANISMS OF EPOXY-BONDED JOINTS OF ZINC COATED STEEL

The degradation mechanisms of bonded joints made from an electroplated -phosphated steel (EZ2) or a hot-dipped galvanized steel (G2F), and a hot-curing epoxy adhesive (1K-A) were investigated using X-ray photoel ectron spectroscopy. In a first stage, the cured adhesive, the metal s ubstrates, and the interfacial adhesive and metal sides of the fractur e surfaces of unaged bonded joints were analyzed to establish the locu s of failure: Adhesive for EZ2/1K-A, and cohesive for G2F/1K-A. The bo nded joints were then aged following two accelerated ageing tests: The 'cataplasme humide' and the 'water immersion' tests. The Zn 2p(3/2) a nd O 1s high resolution spectrum, as well as the Zn L(3)M(45)M(45) Aug er peaks enabled to determine the corrosion products formed in the var ious zones of the fracture surfaces. The ingress of water was responsi ble for the total dissolution of the phosphate coating near the edges of the bonded joints for EZ2 whereas intergranular corrosion phenomena occurred on the whole metal interfacial side of G2F/1K-A bonded joint s.