Protective treatments of Al alloy with silanes

The protective characteristics of films formed on 7075 Al Alloy by treatment with several silanes have been examined. The following silanes were tested as filming agents: n-propyltrimethoxysilane, n-octyltrimethoxysilane, octadecyltrimethoxysilane, vinyltrimethoxysilane, 3-(trimethoxysilyl)propyl methacrylate, bis1,2-trimethoxysilylethane, bis 3,3’-trimethoxysilylpropylamine. The films obtained by immersion in aqueous solutions at 80°C and in hydroalcoholic solutions at room temperature were compared. The influence of the silane solutions pH, between 3.5 and 8.0, and the pre-treatment time were analysed. The inhibiting efficiency of the films was evaluated by polarization resistance measurements in 0.1 N NaCl solution at 25°C. Polarization curves and electrochemical impedance spectra were measured as a function of immersion time in the aggressive solution to evaluate the modification of the protective characteristics of the films. The results showed that the films prepared with hydroalcoholic solutions at room temperature generally had better protective characteristics than those prepared with aqueous solutions at 80°C. The pH of the silane solutions did not greatly influence the inhibiting efficiency of the films with the exception of vinylsilane and propylsilane derivatives. The protective characteristics of the films decreased as the immersion time in the aggressive solution increased, but in some cases an inhibiting effect of over 90% was observed after 10 days of immersion. Polarization curves showed that the films usually blocked the cathodic reaction of oxygen reduction, whereas the effect on the anodic dissolution was not remarkable