Bonding and corrosion protection mechanisms of γ-APS and BTSE silane films on aluminum substrates

Films of γ-aminopropyltriethoxysilane (γ-APS), 1,2-bis[triethoxysilyl] ethane (BTSE) and their mixtures adsorbed onto pure aluminum from aqueous solutions were characterized by means of ellipsometry, infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). It was found that after hydrolysis in water the silanes were readily adsorbed onto aluminum oxide surfaces initially forming hydrogen bonds. Upon curing, such bonds are replaced by metallosiloxane bonds, Si - O - Al. The remaining silanol groups in the film condense and form Si - O - Si bonds. As the Si - O - Al bonds are known to hydrolyze, the corrosion protection is related to the hydrophobicity of the siloxane films formed on the metal substrate. BTSE films are acidic as they contain free silanol groups, therefore these are compatible with some paints but not with others. Electrochemical impedance spectroscopy (EIS) results, salt spray test results and filiform corrosion test results showed that some silane treatments, such as two-step γ-APS/BTSE and BTSE only, provided better corrosion protection on aluminum substrates as compared with a chromate treatment. Mechanisms of adhesion and corrosion protection of these silane films on aluminum substrates are proposed.