| 其他摘要 | The application of chromate conversion coating and primer containing chromate has a long history. However, due to the carcinogenic effect of chromate, the above conversion coating and primer are being restricted in use or will be inhibited in the future. The research on the substitute of chromate has been focused in recent years. At present, the increasing effort is being put of the research and development on the conversion coating without chromate and low-content chromate primer, but little work was done in domestic universities or institutions. China is now carrying out the plan of Large Airplane, so the need of study on coating system on plane is urgent. It is undoubtedly of practical significance to research on the non-chromate silane-based conversion coating and nano-composite epoxy primer on aluminum alloy in this paper. The preparation and performance of the non-chromate silane-based Self-assembled Nano-phase Particles Coating was studied using different experimental methods. The effect of typical inorganic and organic corrosion inhibitors on the anti-corrosion performance of self-assembled coatings was studied. Moreover, the protective ability of silane-based coating on AZ91D magnesium alloy and sealing effect of silane-based coating on AZ91D phosphated conversion coating were discussed. The effect of nano-particles on corrosion resistance of epoxy primer was studied as well. First, the mechanism of film forming of Self-assembled Nano-phase Particles Coating was studied. Experimental results showed that nano-sized siloxane particles were formed through the hydrolysis and condensation of the precursors, GPTMS(3-glycidoxypropyltrimethoxysilane) and TMOS(tetramethoxysilane). The size of the particles decreased with the increase of distilled water. After diethylentriamine cured, the particles were crosslinked to form self-assembled coating. The coating was prone to form pinholes and defects if it was cured at room temperature, and the addition of anti-foam agent was required to reduce the bubbling, while the even coating can be prepared by high-temperature curing. FTIR of sol and cured film proved the formation of Si-O-Si unit and disappearing of epoxy group on GPTMS. Second, EIS (Electrochemical Impedance spectra) was used to investigate the effect of several corrosion inhibitors on the corrosion resistance of self-assembled coatings cured by different curing agent. The equivalent electrical circuits fitted with the corrosion process of the coatings were put forward and the change of the parameters of the equivalent electrical circuits was investigated. The fitted results showed that, two time constants appeared at the initial period of immersion, and three time constants that represent pitting corrosion of aluminum alloy appeared at the intermediated period of immersion of coatings without inhibitor and with MBT(mercaptobenzothiazole), indicating the addition of MBT did not improve the corrosion resistance of the coating. On the contrary, three time constants appeared at the end period of immersion or did not appear for the coatings with the addition of CeCl3 and MPD, also high Rox was obtained, indicating the excellent corrosion inhibition of the inhibitors. In contrast to CeCl3, addition of Ce(NO3)3 could play self-healing effect on aluminum alloy in the immersion process. Fitted results also showed that the coatings cured by diethylentriamine had the best corrosion resistance, the use of longer-chain amine weakened the corrosion resistance. Then, the protection of self-assembled coating on AZ91D magnesium alloy was explored. Polarization curves showed that the current density of crack-sealed coating by self-assembled coating was lower by two orders of magnitude than un-sealed phosphated conversion coating on AZ91D magnesium alloy. EIS results showed one time constant in the immersion of sealed coating. Also no crack was found on the sealed coating with the addition of MPD after immersed in Harrison solution for 96h. The above showed that the sealed coating had excellent corrosion resistance. At last, the effect of nano-particles on the corrosion resistance of epoxy primer on aircraft was investigated. The results of Electrochemical Impedance Spectrum (EIS) and salt spray test indicated that the 1%(wt.%) addition of nano-TiO2/SiO2 significantly improved the anticorrosion resistance of epoxy resin and primer. The enhancement of the anticorrosion resistance can be explained by the formation of layered ring structure of GPTMS treated nano-SiO2. The multi-hydroxyls on SiO2 promoted the formation of network structure combined with GPTMS. First, the coating is more compact due to the framework of layer nano-SiO2 particles. Second, due to the lengthened transport path, water and Cl- ion are more difficult to arrive at the coating/substrate interface. Accordingly, the bubble and delamination are difficult to occur to destroy the stable structure. |
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