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Preparation and Corrosion Resistance of the Composite Coatings Treated by Micro Arc Oxidation and Sol-gel on Magnesium Alloy AZ91D

Author: ShangWei
Tutor: ChenBaiZhen；ShiXiChang
School: Central South University
Course: Electrochemical Engineering
Keywords: Magnesium alloy Micro-arc oxidation Composite film Corrosion resistance Colored composite film
CLC: TG174.4
Type: PhD thesis
Year: 2011
Downloads: 521
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Abstract

Magnesium alloy is the lightest metal structural materials, has broad application prospects in the automotive, aerospace and electronics industry, but poor corrosion resistance limits the application of magnesium alloys. Micro-arc oxidation technology is recognized in recent years, the most promising one of the magnesium alloy surface treatment methods. It is evolved on the basis of ordinary anodized surface treatment technology, in Al, Mg, Ti and other valve metals and their alloys surface in situ formation of ceramic coating, which greatly improved metal corrosion, wear performance. The porous structure of the micro-arc oxidation and its anti-corrosion potential problems. Magnesium alloy micro-arc oxidation electrolyte solution into two categories, one for Cr (VI)-containing compound as the main component electrolyte solution; and the other as the main component of phosphate and / or fluoride electrolyte solution. These electrolyte solution contain extremely harmful substances to the environment, therefore, the development of environment-friendly magnesium alloy micro-arc oxidation electrolyte system, in-depth to explore the micro-arc oxidation process, the structure and properties of the film and its growth process is very necessary. To further improve the corrosion resistance of magnesium alloy, to improve the structure of the micro-arc oxidation film, the surface of magnesium alloy composite film was prepared and systematically study the binding mechanism between the composition of the composite film, the relationship between structure and properties, as well as film , on a wide range of applications of the magnesium alloy also has important significance. Firstly the single factor and orthogonal experiments, the system of micro-arc oxidation electrolyte main film-forming agent, auxiliary agent, additives and other ingredients and their concentration on the performance of micro-arc oxidation process and the film, the development of a green-magnesium alloy micro-arc oxidation electrolyte system, and analyzes the role of each component in the magnesium alloy micro-arc oxidation process. The developed green magnesium alloy micro-arc oxidation electrolyte system: sodium aluminate 30g/dm3, the sodium hydroxide 20g/dm3, montmorillonite 3g/dm3, gum arabic 2g/dm3. Taguchi experimental design to study the impact of process parameters on the surface morphology and corrosion resistance of micro-arc oxidation film to optimize the micro-arc oxidation process parameters. By scanning electron microscopy, spectroscopy, X-diffraction, polarization curves and AC impedance test methods, analysis of micro-arc oxidation film morphology, composition, structure and corrosion resistance. The results show that the best micro-arc oxidation process conditions: the termination voltage 180V, oxidation time 30 min, pulse frequency 50Hz, 30% duty cycle. Prepared under the optimized process micro-arc oxidation film porous structure, with higher levels of A1, O elements, film mainly by MgO.MgAl2O4 Al12Mg17, corrosion resistance than AZ91D magnesium alloy matrix has been improved to some extent. Magnesium alloy micro-arc oxidation on the basis of micro-arc oxidation and sol-gel technology is a combination of magnesium alloy micro-arc oxidation film surface prepared by the sol-gel layer with a certain thickness to form a composite film. By scanning electron microscopy, spectroscopy, X-diffraction, differential thermal - TG, IR, polarization curve, AC impedance and cyclic anodic polarization curve analytical test methods to optimize the sol preparation process and sol-gel layer deposition process, the system composite membrane morphology, composition, structure and characteristics and corrosion resistance. The results showed that the best sol preparation process: SI: Zr molar ratio of 2:1, bath time 30min, water bath temperature of 50 ° C, pH 3. Top sol-gel film deposition process: the gelling process, the drying temperature controlled at 80 ° C right-drying time of 1 h, a curing temperature of 150 ° C, curing time of 0.5h, dipping 3 times. Composite membrane prepared in the optimization process is a hybrid structure of microcrystalline and glassy surface and fewer defects and better adhesion and dense, with strong corrosion resistance. The growth of micro-arc oxidation film, analyzed at different growth stages of the oxide film morphology, composition and corrosion resistance. For the first time the Gaussian03 analysis software to calculate the bond length in the system before and after the adsorption of the sol-gel layer, state of charge and the binding energy of the bound state between the composite film, from the molecular or atomic level. The results showed that in the morphology of the different stages of growth of the oxide film structure and its corrosion resistance has a corresponding relationship. Sol-gel films are more stable in the micro-arc oxidation film surface adsorption, the binding force between the layer of the composite film, providing a theoretical basis for the Preparation of AZ91D magnesium alloy high-performance anti-corrosion coating. The polarization curves and electrochemical impedance depth study of the the corrosion expansion process of micro-arc oxidation and composite membranes in different corrosive media. And targeted different equivalent circuit model, carried out a detailed analysis of the corrosion behavior of micro-arc oxidation and composite membranes in different corrosive media and corrosion period. The results show that, in 3.5% sodium chloride solution, 3.5% sodium sulfate solution and simulated seawater solution, micro-arc oxidation and composite membranes showed some similarities Electrochemical Corrosion Behavior of some different characteristics. Periods in different corrosive media and the type of corrosion, corrosion control process is changed. Finally, the magnesium alloy surface was successfully prepared colored composite film, and scanning electron microscopy, spectroscopy, infrared spectroscopy, polarization curves and cyclic anodic polarization curve analytical test methods to explore the morphology of the colored composite membrane structure, resistance to corrosion and corrosion mechanism. The results showed that the surface of the composite membrane prepared colored smooth, good coloring effect. Add CuSO4 · 5H2O the colored composite membrane prepared corrosion resistance.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology
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