Influence of surface treatments of hot‐dip galvanized steels on their acid–base properties and on their reactions with some organic compounds

The surface of hot‐dip galvanized steels is a complex system highly dependent upon the composition of the galvanization bath and upon surface treatments such as degreasing, alkaline etching and chromate conversion. In this study, industrial zinc‐coated samples are subjected to degreasing, alkaline etching and chromating and then characterized by XPS and Fourier transform infrared spectroscopy (FTIR). Their surface reactivity is studied by vapourphase adsorption of probe molecules, either basic (pyridine) or acidic (phenol), and FTIR analysis. The reactivity enhancement following the alkaline etching is clearly evidenced by this technique. In order to gain information upon the reactivity of the hot‐dip galvanized steel surface towards some organic components of epoxy resins, dicyandiamide adsorption was performed at 180°C on the chemically treated substrates. As suggested by the results obtained with probe molecules, alkaline etching of galvanized steels allows a dicyandiamide–zinc reaction leading to the formation of a defined complex. In addition, the study of the interaction of model substrates (pure zinc and aluminum‐contaminated pure zinc) with dicyandiamide clearly shows that surface contamination of galvanized steels by aluminium is responsible for the inhibition of the dicyandiamide–zinc reaction.