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 photoelectron spectroscopy. In a first stage, the cured adhesive, the metal substrates, and the interfacial adhesive and metal sides of the fracture surfaces of unaged bonded joints were analyzed to establish the locus of failure: Adhesive for EZ2/1K-A, and cohesive for G2F/1K-A. The bonded joints were then aged following two accelerated ageing tests: The 'cataplasme humide' and the 'water immersion' tests. The Zn 2p(3/2) and O 1s high resolution spectrum, as well as the Zn L(3)M(45)M(45) Auger peaks enabled to determine the corrosion products formed in the various zones of the fracture surfaces. The ingress of water was responsible 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 joints.