Study On Preparation And Corrosion Resistance Of Composite Coatings Using Combined Microarc Oxidation And SiO₂ Sol On Magnesium Alloy

In this paper, SiO2 sol was prepared by hydrolyzing of organic alkoxide, and the dipping-costing method was employed to prepare the SiO2 inorganic layer onto ceramic coating treated by micro-arc oxidation(MAO)of AZ31B magnesium alloy. The microstructure and composition, binding mechanism and corrosion resistance of composite MAO/SiO2 coatings were investigated. The Fourier transformed infrared spectrometry(FT-IR) analysis was used to identify the composition of SiO2 sol. The thickness of ceramic coating was measured by eddy current thickness gauge. The roughness of ceramic coating and composite coatings were tested by surface roughness measure gauge. The surface and cross-section morphologies of ceramic coating and composite coatings were observed by scanning electron microscope(SEM). The changing of elemental composition and chemical binding state on surface of ceramic coating and composite coatings were analyzed by energy dispersive spectrometry(EDS) and X-ray photoelectron spectrometer(XPS). The corrosion resistance comparison between the MAO treated samples and composite treated samples were carried out by total immersion test, salt spray test and electrochemical test. The effect of thickness and surface morphologies of ceramic coating on structure and corrosion resistance of composite coatings were discussed.Some conclusions can be obtained. The transparent and homogeneous SiO2 sol was prepared using tetraethoxysilane(TEOS) and H2O as precursors, ethyl alcohol as solvent and hydrochloric acid as catalyst, and it can be proved that SiO2 colloidal particle existed in the solution. According to the micro-structure of composite coatings, it could be seen that SiO2 colloidal particle immersed into and filled up the porous and other surplus SiO2 sol formed effective sealing layer on surface of ceramic coating. The molar ratio of TEOS and H2O affected the quality of SiO2 layer on ceramic coating markedly. The binding type of composite coatings is mainly physical binding presented by acting force among molecules and mechanical gnathodynam. Compared with ceramic coating, the corrosion current densities of the samples with composite MAO/SiO2 coatings decreased by two orders. And results of total immersion tests show that the corrosion rate of samples reduced from 10.32g/m2(MAO treatment) to 3.12g/m2 (MAO coating sealed by SiO2 sol). The SiO2 layer provides corrosion protection by sealing the pores on the ceramic coating and acting as a barrier. The composite MAO/SiO2 coatings can suppress the corrosion process by preventing the corrosive ions from transferring or diffusing to the magnesium alloy substrate. The improving corrosion resistance of composite MAO/SiO2 coatings also relies on the inhibition of MgSiO3. The thickness of ceramic coating, the filling effect of SiO2 colloidal particle on the porous and structural integrity and chemical stability of SiO2 layer exert a joint influence on the corrosion resistance of composite MAO/SiO2 coatings.