Electrochemical and surface analytical study of vinyl-triethoxy silane films on iron after exposure to air

One of the applications of silanes is to improve adhesion of organic coatings to metals. In this work, films of vinyl-triethoxy silane (VTES) on iron (Fe/VTES system) were used to study the effect of exposure to air on protective properties and on chemical transformations of silane films. After exposure to air for up to 12 days, impedance measurements were made in 0.01 M Na2HPO4, and surface analysis was carried out with AES, XPS and FTIR. Exposure to air resulted in an increase of charge transfer resistance of Fe/VTES, and led to chemical changes involving growth of iron oxides at the Fe/VTES interface, formation of SiO2 and related species, formation of polysiloxanes and polymerisation of vinyl groups. These transformations include hydrolysis and probably also photochemical reactions with the participation of free radicals (formation of SiO2 and polymerisation). Identified products can contribute to the improved protectiveness by enhancing the barrier properties and by inhibition of corrosion. It is suggested that apart from improving adhesion, silanes play an important role also by protecting against corrosion.