| The Dacromet coating is an most efficient anticorrosion method to the surface of metallic materials so far. The coretechnology of such coating is that a Dacromet coating is formed on the surface of metallic materials by using chromate binder. However, because the chromium ions especially hexavalent chromium are unhealthful, the use of chromate binder has been forbidden in many countries from the perspective of environmental protection. Therefor, in order to meet the requirement of environmental protection and completely abandon the use of toxic hexavalent chromium ions, it is very necessary to develop a technology that the non-chrome Dacromet coating is prepared through using nontoxic binder as a substitute for toxic chromate and keep the excellent corrosion resistance and heat resistance as well as low contamination during the preparation. Based on the above backgroud, the investigation was carried.Water-solubility glycidoxypropyltrimethoxysilane (γ-GPS) and alcohol-soluble vinyltriethoxysilane (VTES) were selected as adhesive silane coupling agent in this paper. The composition of the silane treating solution and the treatment processes were obtained by two rounds of orthogonal test using copper sulfate dropping test.The surface morphology, composition and structure of silane film on the surface of metallic matrix were studied by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and X-ray energy dispersive spectroscopy (EDS). The experimental results showed that silane film mainly consisted of the elements including Si, C,0 etc; not a simple physical adsorption but a chemical bonding to form Si-O-Me bond happened at the interface between silane and metal substrate, and the network structure of-Si-O-Si- was formed by condensation between silanols, that is, a transparent silane film was formed on metal surface.Corrosion resistances of silane films on the surface of Q235 steel, hot dip galvanized steel sheet and pure aluminum substrates were studied by means of polarization curves, Electrochemical Impedance Spectroscopy (EIS), SEM and EDS. The results showed that the silane films on the metal substrate surface can reduce the diffusion velocity of corrosive agents including O2, H2O, Cl- and so on into the metallic matrix and improving the corrosion resistance. If doping 1×10-3mol/L cerium nitrate into VTES solution, the insoluble compounds such as cerium oxide or hydroxide deposited or filled the network structure of Si-O-Si, Si-O-Me, which covered active area of metallic matrix and effectively inhibit the corrosion of metallic matrix.Through choosing water-soluble γ-GPS instead of chromic anhydride as binder and selecting coating solution of Dacromet as the reference of other constituents, the elementary composition of Chromium-free Dacromet firstly was selected and determined by an experiment of single factor, then the proportioning of every main constituents was determined by orthogonal tests, and finally the formula of coating solution and the preparation process of chromium-free Dacromet were determined by investigating the combination property of the coating including the appearance, adhesion and corrosion resistance and so on. Optimization formula of the coating was as follows:35%(mass percent, the same below) of metal powder (ratio of Zn to Al is 6:1),12% of wetting agent and dispersant,12% of binder γ-GPS,0.6% of corrosion inhibitor,0.1% of thickening agent,0.5% of defoamer and other surface additives, the remaining of deionized water; parameters of preparation process:drying for 10-15 min at 80℃, Curing for about 30 min at about 290℃.The method for modifying the flake aluminium by cerium-salt-doped silane coating was discussed, modified chromium-free Dacromet coating was prepared through substituting ordinary aluminum powder with the flake aluminium modified by cerium-salt-doped silane. With the technologies such as SEM, XPS, FTIR, EDS and so on, it was determined that the surface of flake aluminium was successfully coated by cerium-salt-doped VTES films. Anticorrosion efficiency of coated flake aluminium was increased to 72%.The surface morphology, composition and corrosion resistance of chromium-free Dacromet coating and modified chromium-free Dacromet coating were studied by SEM, EDS, Open circuit potential, polarization curve, EIS. The results showed that both of two kinds of coatings indicated the structure which is that flaked powders of zinc and aluminum piled up layer upon layer; the surfaces of the zinc and aluminum powders were coated by silane film, not including element Cr, which showed that the coating mainly consisted of zinc and aluminum powders as well as silane binder. The electrochemical corrosion process of two kinds of coatings in 3.5% NaCl solution included three different stages. At the initial stage of immersion, the metallic powders in the coating started to activate due to the attack of the electrolyte, the open circuit potential of the system is negative and shift to positive with coating corrosion. Corrosion current density decreased due to the corrosion speed of the coated aluminium powders slow down in modified coating. At the middle stage of immersion, the coating mainly exhibited the cathodic protection of metallic powders in the coating to the substrate through victimization of anode. Corrosion rate of aluminum and zinc powder in modified coating reduced, the accumulation rate of corrosion products slowed down, the positive movement of corrosion potential had slower speed, the corrosion potential of system last a longer time in -0.9～-0.75 V(vs.SCE) potential interval. The cathodic protection of modified coating to the substrate through victimization of anode last longer period than that of chromium-free Dacromet coating. At the ultimate stage of immersion, the coating has no cathodic protection to the substrate through victimization of anode, but the barrier protection of the coating predominated. Positive movement of corrosion potential of system had very high speed, the process last for shorter time. The neutral salt spray test showed that the time of modified coating for resisting salt spray corrosion is longer for 100 hours than that of chromium-free Dacromet. |
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