Effect of the amine functional group on corrosion rate of iron coated with films of organofunctional silanes

Silane pretreatments were investigated as environmentally friendly replacements for the existing chromating processes. The objective was to compare the corrosion protection properties of various silanes. The corrosion inhibiting action of silane films on iron was examined in 3% sodium chloride (NaCl) solutions. Coatings made with two silanes, one with a functional group (gamma-aminopropyltriethoxysilane [gamma-APS]) and one without a functional group (bis-triethoxysilyl ethane, BTSE), were studied Characterization of the silane films was performed by ellipsometry, reflection-absorption infrared spectroscopy (RA-IR), atom force microscopy (AFM), energy dispersive x-ray analysis (EDX), and x-ray photoelectron spectroscopy (XPS). Corrosion rates were determined from polarization studies. It was found that gamma-APS films had no effect on the corrosion rate of iron. However BTSE films deposited from solutions of pH 4 to 6 reduced the corrosion rate by a factor of 15. The greater effect of a BTSE film was explained by formation of more stable Fe-O-Si bonds and the higher degree of cross-linking of the BTSE films compared to the gamma-APS films.