INFLUENCE OF SURFACE TREATMENTS OF HOT-DIP GALVANIZED STEELS ON THEIRACID-BASE PROPERTIES AND ON THEIR REACTIONS WITH SOME ORGANIC-COMPOUNDS

The surface of hot-dip galvanized steels is a complex system highly de pendent upon the composition of the galvanization bath and upon surfac e treatments such as degreasing, alkaline etching and chromate convers ion. In this study, industrial zinc-coated samples are subjected to de greasing, alkaline etching and chromating and then characterized by XP S and Fourier transform infrared spectroscopy (FTIR). Their surface re activity is studied by vapour-phase adsorption of probe molecules, eit her basic (pyridine) or acidic (phenol), and FTIR analysis. The reacti vity enhancement following the alkaline etching is clearly evidenced b y this technique. in order to gain information upon the reactivity of the hot-dip galvanized steel surface towards some organic components o f epoxy resins, dicyandiamide adsorption was performed at 180 degrees C on the chemically treated substrates. As suggested by the results ob tained with probe molecules, alkaline etching of galvanized steels all ows a dicyandiamide-zinc reaction leading to the formation of a define d complex, In addition, the study of the interaction of model substrat es (pure zinc and aluminum-contaminated pure zinc) with dicyandiamide clearly shows that surface contamination of galvanized steels by alumi nium is responsible for the inhibition of the dicyandiamide-zinc react ion.