Preparation And Performance Study Of Silane Films On Metal Surface

Metal surface pre-treatment technology before painting can significantly improve the adhesion of the coating and the metallic matrix, improve the corrosion resistance of the entire coating and provide protection between processes to avoid rust. It has an important impact on the overall quality of the coating. But the commonly used passivation technology and phosphate processing technology are both high energy consumption and will cause environmental pollution. With the development of automotive and other manufacturing industries, coating technology has entered a new era of technology innovation and optimization. Research and application of environmentally friendly, energy-saving coating pre-treatment technology is imminent. In this paper, the fluorine-free titanium salt conversion film and silane conversion film techniques were studied systematically. With combining the advantages of both technologies, the composite film of titanium salt conversion film and silane conversion film which is green, corrosion resistance excellent was obtained.Macroscopic morphologies, tafel curves analysis and electrochemical impedance spectroscopy analysis of titanium salt conversion film with different complexing agents were studied to comparing the effects of different complexing agents on quality and corrosion resistance of the film and to investigating the corrosion mechanism. The results showed that oxalic acid and sodium citrate can replace the currently used fiuorinated complexing agent to form a stable, environmentally friendly fluorine-free titanium salt treatment system. The optimal concentrations of oxalic acid and sodium citrate are respectively 1.5g/L and 0.8-1 g/L. The corrosion mechanisms of titanium salt conversion films using oxalic acid and sodium citrate as complexing agents and using of fluorine-containing complexing agent were different. They respectively have the cathode suppression effect and anode suppression effect on the corrosion reaction, and the film using oxalic acid and sodium citrate has better corrosion resistance. By studying EIS results of the titanium salt conversion film in salt solution with the mass fraction of 3.5% for different immersion time can find that, the corrosion resistance is mainly provided by the interface layer between film and matrix, the strong complexation of phytic acid and oxalic acid with iron ions make the interface layer grow into reticular macromolecules more dense at the initial stage of corrosion, which can inhibit the cathodic mass diffusion effectively and lead to increasing of the resistance value of interface layer. The self repair function can slow down the corrosion rate of the matrix.The optimal hydrolysis time of BTESPT solution and γ-AEAPS solution are respectively 6d and 4h which obtained by measuring the conductivity of the silane solutions and corrosion resistance of the silane films. Contact angle measurements, salt spray test and electrochemical impedance spectroscopy test were used to investigate the performances of BTESPT film doping tannic acid inhibitor and γ-AEAPS film doping phytic acid inhibitor. The BTESPT film doping tannic acid have a certain improvement in drophobicity and corrosion resistance, the tannic acid is negative catalytic inhibitor. γ-AEAPS film doping phytic acid inhibitor also have a certain improvement in drophobicity and corrosion resistance, the phytic acid is covering inhibitor. By contrasting EIS results of mixed silane films of BTESPT and γ-AEAPS in different proportions can find that corrosion resistance of mixed silane film which one BTESPT and γ-AEAPS at ratio 4/1 is the best. Infrared spectroscopic analysis showed that silane molecules were cross-linked with Si-O-Si bond that can effectively prevent the intrusion of corrosive substances. The result of electrochemical impedance spectroscopy of mixed silane film for different immersion time in 3.5% sodium chloride solution showed that, due to the hydrophilic difference of the two silanes, a gradient film composed of strong polar matrix, hydrophilic γ-AEAPS, strong hydrophobic BTESPT and the interface formed by interaction of matrix and silane was formed. It improved the adhesion of film and matrix, hydrophobicity and density of the, exhibited excellent corrosion resistance.With compositing of titanium salt conversion film and mixed silane film under the optimal conditions, the resistance value of the conversion film can reach to 5×105Ω·cm2. IR spectra showed that the titanium salt conversion film and silane film were linked by Ti-O-Si bond, through it the organic film and the inorganic film growed intergrated. The titanium salt conversion film has more crosslinkable point, there was a greater degree of crosslinking of silane film on surface of titanium salt conversion film than on surface of steel matrix, which can enhance the film density and binding force between the film layers. Polarization curve analysis showed that, titanium salt conversion film corrosion protection was primarily depend on the inhibition of the cathode reaction, the silane film was mainly depend on the inhibition of the anode reaction, and the composite conversion film had strong inhibitory effects on both cathode and anode. The mechanism study through electrochemical impedance spectroscopy showed the composite film gived full play to the advantages of silane conversion film and titanium salt conversion film. The outer hydrophobic strong silane conversion film formed a physical barrier to the matrix and the inorganic layer, prevented the invasion of corrosive medium, after penetration of corrosion medium through the silane film, titanium salt conversion film gived a second protection with different mechanism.