The Passive Technology And Technical Parameters Research Of Silane Treatment For Hot Dip Galvanized Plate

Hot dip galvanized steel plate has been widely used all over the world because of better anti-corrosion properties, but it is easy to erode to form white and loose products which can affect the appearance and corrosion resistance in moist environment. The treatment of passivation is an effective method to improve the performance of corrosion resistance and coating. Generally, as a very effective passivated agent, the chromate was used widely in the world for a long time. But the treatment of passivation can bring serious environmental pollution. Now, there isn't any nontoxic passivated agent, which has self-own intellectual property rights, can replace the chromate passivation. In this article, the organic/inorganic composite passive system, which consists of nontoxic and non-contaminative silane(BTESPT) as the main ingredient and the inorganic additive as the accessorial ingredient, was prepared.The non-contaminative BTESPT passive system was researched in this thesis. The composition and technical parameters of the BTESPT passive solution was systematically studied by single factor experiments using the accelerated test methods of Lead acetate dropping test, neutral salt spray test and so all. The 2-round orthogonal test method were used to determinate the best composition and technical parameters of the BTESPT passive solution. The basic compositions of the BTESPT passive solution were as following:BTESPT,5vol.%; deionised water,14 vol.%; acetic acid,0.5 vol.%; ethanol,81.5vol.%. The technical parameters of the passivation were as following:passive solution temperature,35℃; passive solution hydrolysis time,72h; treatment time,5s; curing temperature,120℃; curing time, 30min. By adding 0.001 mol/L cerium nitrate and 0.02g/L Silica Sol, the non-contaminative BTESPT passive system was successfully improved, which made it better than contaminative passive system.The Electron Microscope, Atomic Force Microscope and Metallurgical Microscope were used to character the micromorphology on the surface of silane passive films. It was found that there were some microcracks on the surface of the BTESPT passive films. When adding additives such as cerium nitrate or silica sol to modify the BTESPT passive solution, the microcracks were further covered and suppressed, the passive films become more denser so that it reduced the eroding agents such as O2, H2O and Cl- to enter the surface of the zinc. The electrochemical corrosion of the zinc was controlled to improve the corrosion resistance.The characterization of the passive films was performed by Fourier transform infrared spectroscopy(FTIR) and X-ray photoelectron spectroscopy(XPS).The test results showed that passive films consist mainly of such organic functional groups as SiOH, Si-O-Si, Si-O-Zn, CH2,-SiO-. The XPS analysis gave information of the chemical composition of the passive films. The XPS testing results showed that consist mainly of such elements as C,O, Si, S, Zn. Sputtering depth and narrow scanning results showed that the outer layers were SiOH,Si-O-Si,Si-O-C. The inner layers were Si-O-Si, Si-O-Zn, etc. The interface layers might consists of ZnO,ZnS,ZnSiO3. Moreover, it was inferred the thickness of passive films is about 200nm.The corrosion performance of the passive films was evaluated by accelerated test methods of Lead acetate dropping test, neutral salt spray test. The silane film could give efficient protect to the HDG, but it was not as better as the contaminative film as the film did not have the self-healing ability. The film improved by 0.001 mol/L cerium nitrate or 0.02g/L Silica Sol acted better.The corrosion performance of the passive films was evaluated by Tafel polarization cure and electrochemical impedance spectroscopy (EIS). The results were shown that the self-corrosion potential obviously shifted to positive direction, corrosion polarization was controlled by anode process, self-corrosion tendency is significantly reduced. The EIS spectrums of the passive films were composed of two capacitive loops in first quadrant. The beeline that slope is one does not appear in the low frequency district. The corrosive system was controlled by the electrochemical corrosion control system.The corrosion resistance of BTESPT passivating treatment is better than that of chromate treatment, and its technical parameters are simple, the stability of passive solution is good. Therefore, it is the most promising alternative technologies of chromate treatment.