Study On Optimum Preparation Process And Performance Of Silane Films On Aluminum

Silanization of aluminum is a new and environment-friendly surface treatmenttechnology, which has been developed recently in order to replace the pollutivechromate passivation and phosphating treatments effectively. However, it should benoticed that the traditional silanization solutions are alcohol-rich, which has causedsome restriction for its industrial applications. With regard to this deficiency, awater-based silanization solution is adopted for silanization of aluminum materials inthis paper, and the performance of silane films is optimized in this system.（1） Optimization of preparation process of silane films in water-based silanizationsolution. Water-based silanization solution was prepared by using a biphasic hydrolysissystem, and the effects of curing time and temperature on performance ofvinyltriethoxy silane （VTES） films were studied by signal factor experiments. Theresults show that the optimum curing parameters of VTES are cured at100oC for30min. Bis-[triethoxysilylpropyl] tetrasulfide （BTESPT） films on aluminum were prepared byusing pulse electrodeposition technique, the protective performance of silane filmsprepared under different potential and time ratio were assessed by electrochemicalimpedance spectroscopy （EIS） and potentiodynamic polarization. The resultsdemonstrate that the optimum pulse electrodeposition parameters of silane films are:positive deposition potential of-0.6VSCE, negative deposition potential of-1.6VSCEand deposition time ratio of1:1. The porosity of BTESPT film deposited under thiscondition is only0.04%, and its protection efficiency is92.45%. The surfacetopography and chemical composition of silane-coated aluminum samples werecharacterized by scanning electron microscopy and X-ray energy dispersivespectroscopy （SEM/EDS）. It can be concluded that aluminum samples are covered byBTESPT-based coating of188nm thick after dipping in water-based silanizationsolution, and silane films prepared by electrodeposition are found to be more uniform,compact and rougher.（2） Preparation and characterization of silane-cerium hybrid films. Preparationprocess of silane-cerium hybrid films in water-based silanization solution wasoptimized by orthogonal tests, and the optimum preparation parameters of hybrid filmsare obtained as follow: cerium salt concentration of0.01mol/L, H₂O₂concentration of15ml/L, dipping time of90min, and dipping temperature of35oC. The protectiveperformance of silane-cerium hybrid films were assessed by electrochemical methods, the durability of hybrid films were tested by the method of immersing salts water, andthe surface topography and chemical composition of hybrid films were characterizedby SEM/EDS. Results of EIS and polarization curves indicate that the density andhydrophobicity of silane-cerium hybrid films are better than silane films andcerium-based conversion coatings. Meanwhile, hybrid films can obviously restrictcathode and anode reaction on aluminum substrate, which reduce the corrosion currentdensity to about10^-7A/cm²and effectively improve the corrosion resistance ofaluminum. Durability tests in NaCl solution demonstrate that cerium-doped silanefilms have a certain "self-healing" capability and chromate alternative corrosiveresistance, which make the R_ctof hybrid films remain at about10⁵·cm²after7-dimmersion in salts water. SEM and EDS results show that doping of cerium salt canincrease the thickness of silane films to about500nm and effectively improve theuniformity and density of films.（3） Preparation and characterization of functional bis-silane films. Bis-silanefilms （BTESPT/VTES and BTESPT/γ-glycidoxypropyltrimethoxy silane （γ-GPS））were prepared by using two-step methods in water-based silanization solution, andacrylic coatings （AC） and two-component epoxy coatings （EC） were painted onbis-silane-coated aluminum respectively. The protective performance of bis-silanefilms was assessed by electrochemical methods, the durability of bis-silane/organiccoating-treated samples were tested by the method of immersing in salts water. Themaximum corrosion resistance of10⁶·cm²of BTESPT/VTES andBTESPT/γ-GPS-treated aluminum samples was observed in the EIS results, which islarger than that of BTESPT, VTES and γ-GPS films. Results of polarization curves alsoshow that Ecorrof aluminum samples shift to more positive values after bis-silanecoating, which means improvement of corrosion resistance of metal substrate.Durability tests in NaCl solution indicate that corrosion resistance ofBTESPT/VTES/AC-and BTESPT/γ-GPS/EC-treated samples are respectivelymaintained at9.50×10⁶·cm²and6.37×1010·cm²after90-d immersion in saltswater, which is significantly higher than that of silane/organic coating-treated Alsamples. The bonding strength of organic coatings and bis-silane-coated aluminumswere determined by the pull-off tests. The results show that the compatibility ofbis-silane films and organic coatings are ideal, and the matching performance ofBTESPT/γ-GPS/EC is better than BTESPT/VTSEAC. SEM and EDS results ofBTESPT/γ-GPS demonstrate that the bis-silane films composed of inner BTESPT filmof about200nm thick and the outer γ-GPS film of about180nm thick are uniform and compact. And deposition of bis-silane films can increase the overall thickness andsurface roughness of films which can lead to effective improvement of bondingstrength between organic coatings and aluminums.