| Magnesium alloys have a number of desirable features, including low density, high strength-to-weight ratio, and good machinability and damping characteristics, good electrical and thermal conductivity. Because of its outstanding mechanics and physical performance, it has already attracted much attention on them, and its use in a variety of applications, particularly in aerospace, mechanical and electronics, automobile, is now growing rapidly. However, poor corrosion resistance has often prevented it from further applications. So how to improve its corrosion-resistance is an urgent topic of the day.Surface treatment on magnesium alloys were studied in details by means of scanning electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction(XRD) and electrochemistry method, which are including composite electrolyte, rare earths action, color microarc oxidation, electroless depositing Ni-P alloy on MAO ceramic film and chemical conversion, and so on.On the basic of researching the primary components of micro-arc oxidation electrolyte, the influences of metallic oxysalt, polyacid salt and surfactant on the anti-corrosion property and micro structure of the micro-arc oxidation ceramic film were studyed. The orthogonal design method was applied to optimize the MAO electrolyte formulas which contained composite electrolyte component. Then two good micro-arc oxidation electrolyte formulas were obtained. X-ray diffraction test indicated that the ceramic film was primary composed of MgO, MgAl2O4, Mg2SiO4, etc. Because of accession of composite additives, the micro-arc’s striking and termination voltage were decreased obviously, so the structure and corrosion resistance of oxidation film were improved greatly. The dynamic potential polarization study showed that ceramic oxidation film’s corrosion current density had been reduced about four orders of magnitude and the corrosion potential increased about0.34-0.37V on the dynamic potential polarization curve, and it’s AC impedance had been increased about13-15kΩ/cm2.The ceramic had higher growth speed in forepart and metaphase, namely, the growth speed of film descended gradually as the treatment time increasing. Proposed changes in micro-arc oxidation film performance in three stages, oxidation time10-20min, the oxide film thickness of the growing season; oxidation time20-30min, the oxide film from the corrosion potential and impedance values increased rapidly, enhanced corrosion resistance of the oxide film period; oxidation time30-40min, hardness of the oxide film growth. Oxidation time is greater than40min, dense oxide film significantly reduced corrosion resistance weakened.Effect of rare earth on micro-arc oxidation is studied by means of two methods, which are rare earth added into electrolyte and rare earth treatment as pre-treatment of micro-arc oxidation. The results show that when rare earths are added into electrolyte, magnesium alloys are obviously eroded and mico-arc oxidation is failed. But fine morphology, thickness and corrosion resistance of the oxidation coating are obtained with rare earth treatment as pre-treatment of micro-arc oxidation. The arc starting was easier and descended about10v. Comparison of different samples of rare earth salt solution for dealing with micro-arc oxidation treatment can be drawn, Neodymium oxide film for the appearance, thickness, corrosion resistance of the best. Rare earth neodymium salt solution obtained after a short treatment of magnesium alloy micro-arc oxidation film thickness distribution,"particles" small uniform; reduce arcing time, arc voltage and current density, reduce power consumption. Rare earth elements only change the micro-arc oxidation film surface morphology, had no effect on its surface composition.The effects of different electrolyte systems and different stain on the color of ceramic coating were investigated, and the basis compositions of the micro-arc oxidation electrolyte were established. The yellow and green ceramic coatings were obtained on magnesium alloy. The orthogonal design method was implicated to optimize the MAO electrolytes systems, then the better technical composition and operation conditions of the color coatings were gained. Phase structure of yellow micro-arc oxidation film of magnesium alloy is Mg, MgAl2O4, Al2O3, Mn2O3, MnO2and Mn8O5. Green micro-arc oxidation film phase structure of Mg, MgO, MgAl2O4and V3O5are found. Color micro-arc oxidation film growth mechanism is found. Micro-arc oxidation film depends not only on the apparent color of the oxide film in the content of colorants, more importantly, the surface morphology of the oxide film, which is tiny micro-arc oxidation channel. Coloring agent is mainly dispersed in small micro-arc channel around, but thick micro-arc channel decreases the role of colorants. Electrochemical test results show that corrosion current of yellow micro-arc oxidation film is changed from316μA·cm-1to0.105μA-cm-2compared with magnesium alloys, corrosion resistance of which is superior to the green ceramic membrane. Electroless plating Ni on micro-arc oxidation film and their properties are studied. Using simple silicate as electrolyte system, electroless plating coating on micro-arc oxidation film are obtained after low concentrations of palladium ion sensitization and activation process, properties of which is good adhesion functions, corrosion resistance and high hardness. This has greatly improved the properties of magnesium alloys. Alkaline and acidic electroless nickel plating on the micro-arc oxidation film is optimized.Micro-arc oxidation cycle test chemical bath results showed that bath time and the plating rate stability PdCl2down quickly with the increase the number of periods. Local defects increased and coating have many pores after the four cycles due to the accumulation of harmful ions and impurity precipitation bath. |
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