The Synthesis And Properties Of Silane Based Coatings For Hot Dip Galvanized Layer

Zinc coating is a common material used in corrosion protection of steel. Due to great potential difference between the substrate and the zinc coating, zinc has high dissolution rate. In moist environment, the galvanized plate is easily corroded to form white products or become dark, which can affect the appearance of hot dip galvanized steel and decrease corrosion resistance. Chromate conversion coatings have been used widely to decrease the dissolution rate of zinc. However, the hexavalent chromium salts is a kind of extremely poisonous and carcinogenic material which has the mutafacient function. In order to make pretreatment technology satisfy environment-friendly demand, the chromate(VI)-free pretreatment for galvanized steel has increasingly developed. In this paper, the inorganic/organic compound treating system was prepared, in which nontoxic, nonpolluted bis-[triethoxysilylpropyl]tetrasulfide silane (BTESPT), gamma-aminopropyltriethoxysilane (y-APS) and inorganic additive is used as the main ingredient and the accessorial ingredient, respectively.The composition of the BTESPT treating solution was designed and sieved by single factor experiments using the accelerated test methods of lead acetate dropping test, neutral salt spray test and electrochemical corrosion test. The compositions of the treating solution were studied to determine the experimental parameters. The basic compositions of the BTESPT treating solution were as following: BTESPT 5vol.%, deionised water 14vol.%, acetic acid 0.5vol.%, ethanol 81vol%. The temperature for hydrolysis of the BTESPT treating solution is 35℃.On the basis of neutral salt spray test grading standards, the types and amounts of additives including rare earth nitrate, nano-oxides and corrosion inhibitors were studied using the contrastive experiment methods. The types and amounts of the selected additives of BTESPT treating solution were cerium nitrate 0.001mol/L, Silica Sol 0.005-0.02g/L, BTA 0.001g/L respectively.The parallel test method and orthogonal test method were used to determinate the dipping time, the hydrolysis time of the treating solution, the curing time and temperature of the BTESPT treating coating and the effects of experimental parameters on the properties of the treating coating. When the technical parameters change, BTESPT treating coating has a good corrosion resistance in the condition of the treating solution hydrolysis time 72h, dipping time 5s, curing temperature 120℃, curing time 30min.The corrosion performance of the BTESPT treating film was evaluated by Tafel polarization cure and electrochemical impedance spectroscopy(EIS). The results showed that the corrosion polarization was controlled by anode process, the self-corrosion potential obviously shifted toward positive direction, the self-corrosion tendency was significantly reduced. The EIS spectrums of the treating film were composed of two capacitive loops in first quadrant. The beeline with the slope is 1 didn't appear in the low frequency domain. The corrosive system was controlled by the electrochemical control.The characterization of the silane treating film was analysed by Fourier transform infrared spectroscopy(FTIR). The test results showed that silane treating film consisted mainly of organic functional groups including SiOH, Si-O-Si, Si-O-Zn, CH2,-SiO-. The chemical compositions of the silane treating film were observed by X-ray photoelectron spectroscopy (XPS). The XPS testing results showed that treating film mainly consisted of the elements including C,O, Si, S, Zn. Sputtering depth and narrow scanning results showed that the outer layers of the BTESPT treating film composed of SiOH, Si-O-Si, Si-O-C, the inner layers composed of Si-O-Si, Si-O-Zn, etc, and the interface layers consisted of ZnO, ZnS, ZnSiO3. The thickness of BTESPT treating film was about 200 nm.The morphology of the BTESPT treating film was observed with Scanning Electron Microscope and Metallurgical Microscope. It was found the treating film was uniform, dense, integrity, nano-structure, but there were some microcracks on the surface of the treating film. When adding additives including cerium nitrate, silica sol and benzotriazole(BTA) to modify the BTESPT treating solution, the microcracks of treating film were further covered and decreased using the insoluble compounds such as cerium oxide or hydroxide, inert silicate to fill the network structure of Si-O-Si, Si-O-Zn, the treating film of Silane became more denser, reducing corrosive agents including O2,H2O and Cl-to enter the surface of the zinc and improving the corrosion resistance.According to 1 grade neutral salt spray test standards, the compositions of the mixed silane treating solution of BTESPT/γ-APS were determined by contrastive Test. The mixed silane treating solution consisted of BTESPT 1-4vol.%,γ-APS 2-4vol.%, deionized water 28vol.%, the rest is ethanol, pH value of 4 to 6. Becauseγ-APS structure contained-NH2 hydrophilic groups, BTESPT was more fully hydrolyzed, and condensation of silanol was inhibited, which significantly improved the corrosion resistance of the mixed silane films.