| The possibility of using silane coupling agents as replacements for chromate treatments was investigated for aluminum substrates. In order to understand the influence of deposition parameters on silane film formation, pure Al substrates were used to study the interaction between silane coupling agents and aluminum surfaces. The silane films formed on pure A1 substrates from aqueous solutions were characterized by ellipsometry, contact angle measurements, reflection absorption infrared spectroscopy, x-ray photoelectron spectroscopy, and Time-of Flight Secondary Ion Mass Spectrometry. The deposition parameters studied include solution concentration, solution dipping time and pH value of the applied solution. It appears that silane film formation involved a true equilibrium of hydrolysis and condensation in aqueous solution. It has been found that the silane film thickness obtained depends primarily on the solution concentration and is independent of the solution dipping time. The molecular orientation of the applied silane film is determined by the pH value of the applied silane solution and the isoelectic point of the metal substrates. The deposition window in terms of pH value for A1 substrates is between 4 and 7. The total surface energy of silane-coated A1 substrate decreases with film aging time, the decreased rate, however, is determined by the nature of silane coupling agents.; Based on the results obtained above, a pretreatment, which involved two-step bis-(triethoxysilyl) ethane and gamma-aminopropyltriethoxysilane, was developed for Al substrates with commercial polyester and polyurethane paints. The results of salt spray testing, cyclic corrosion testing, and electrochemical impedance spectroscopy showed that this treatment provided the same level of corrosion performance as the treatment of phosphating plus a final chromate rinse. The likely reasons for excellent performance are discussed in terms of the physical and chemical characteristics of the treatment layers and their stability in the under-film corrosion environment.; Salt spray test results showed that two-step BTSE/gamma-APS could provide the same or better corrosion protection than that offered by a standard chromate treatment. Polyester or polyurethane powder-painted Al panels, pretreated with this treatment, also displayed excellent corrosion performances in the salt spray test and filiform corrosion test. Thus, the advantage of this treatment is that is combines a temporary rust prevention and a permanent prepaint treatment into one simple approach.; Electrochemical impedance spectroscopy was used to study the various pre-paint treatments under the polyester paint. It provides more detailed mechanistic information about corrosion processes than salt spray test and filiform corrosion test. However, combination of corrosion tests that involve different corrosion mechanisms may still be necessary to have a better understanding of corrosion behavior of a new treatment. |
