Studies The Preparation Of Oxide, Polymer Coating Based Silica On AZ91D Magnesium Alloy And Its Corrosion Resistace

Magnesium alloys,which are considered as green engineering materials of 21 century,have unique characteristics of low density,high strength-to-weight ratio,high rigidity and elastic modulus,good recyclability,excellent castability and cutting properties.Unfortunately,magnesium and its alloys are highly susceptible to corrosion,particularly in salt-spray conditions.This has limited its use in the automotive and aerospace industries,where exposure to harsh service conditions is unavoidable.As relation to the development span of the magnesium and its alloy,the corrosion resistance of the magnesium alloy has become the focus of the worldwide.Thus it is of great significance to investigate the surface modification technology of magnesium alloy.Based on the summary of corrosion mechanism and the currently surface modification technologies of magnesium alloy,several protection technology on AZ91D magnesium alloy and its relation mechanism were investigated using electrochemical impedance spectroscopy, potentiodynamie polarization test,fourier transform infrared spectrum as well as scanning electron microscope in this paper.The major research efforts of the present study are as follows:1.The SiO₂ film was studied for AZ91D magnesium alloy.Silicon dioxide sols which can be directly applied on the surface of AZ91D magnesium alloy to improve its corrosion resistance were prepared by hydrolyzing organic alkoxide,the dipping-costing method was employed to prepare the SiO₂ coating.In this study,tetraethyl orthosilicate(TEOS) and triethoxyvinylsilane(VTEO) were employed as the precursors to prepare SiO₂(TV) sol,as a comparison,the other SiO₂(T) sol prepared using only tetraethyl orthosilicate(TEOS) as precursor was also investigated.Fourier transform infrared(FT-IR) spectrum and scanning electron microscope(SEM) were performed to analyze the structure and the surface morphology of the SiO₂(TV) sol and SiO₂(T) sol.The corrosion resistance of the AZ91D magnesium alloy coated by silicon dioxide coatings was examined using electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization tests in 3.5 wt.%NaCl aqueous solution.The results indicate that the corrosion resistance of the substrate is greatly improved by the silicon dioxide coatings obtained from SiO₂(TV) sol. 2.Polymer coating based silicon was explored.To investigate the effect of functional groups in the silicon polymer coating on the corrosion resistance of AZ91D magnesium alloy,various polymer coatings were prepared from the precursor's intercombination among tetraethyl orthosilicate(TEOS),triethoxyvinylsilane(VTEO),3-glycidoxypropyltrimethoxysilane(GPTMS) and 3-aminopropyltriethoxysilane(APTS) via sol-gel method.The surface morphology,the hydrophobic property and the thickness of these coatings were characterized by scanning electron microscope(SEM),contact angle goniometer and JB-5C system,respectively.The result shows that the coating thickness was about 2μm.The Fourier transform infrared(FT-IR) spectra were used to analyze the structure differences among the various gels.It demonstrated that electron with drawing group made the v_as(Si-O-Si) shift to shorter wave number while electron donating group made the peak shift to higher wave number.Electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization tests were employed to confirm the corrosion resistance ability of these sol-gel films.The results showed that the smaller steric hindrance of the functional groups on the silica precursor made the hydrolysis and condensation reaction easily and the relevant film had a better corrosion resistance.3.Considering the environmental friendly and cost-effective,molybdate conversion coatings have been successfully prepared on magnesium alloy to improve its corrosion resistance.For molybdate conversion treatment,various conditions including the pH of the molybdate baths, immersion time and bath temperature were investigated using electrochemical measurements.The corrosion resistance of the AZ91D magnesium alloy was improved to some extent by the conversion coating with the optimal conversion parameters(7.3 g/L(NH₄)₆Mo₇O₂₄·6H₂O solution with pH 5 for 30 min at 30℃).In order to get a further improvement of corrosion protection for AZ91D magnesium alloy,three layers of silicon sol-gel coatings were successfully deposited on the molybdate conversion coating pre-applied to AZ91D alloy.4.Take use of the passivation of magnesium and its alloy in contact with the HF solution,the composite technology-passivation layer and sol-gel film-was used to improve the corrosion resistance of magnesium alloy.The electrochemical impedance spectroscopy "(EIS) and potentiodynamic polarization tests in 3.5 wt.%NaCl aqueous solution were used to test the corrosion resistance of passivation layer and silica sol(F/Si),as well as passivation layer and lanthanum sol(F/La) composite film.The results showed that the F/La film had limited protection effect because of its non- uniformity.However,the F/Si film could offer better corrosion resistance to magnesium alloy.