| The density of magnesium is 1740kg/m3, which is the lightest metal. The weight of magnesium is 1/1.5 of aluminum and 1/4 of zinc when compared at the same volume. In recent years, the consumption of magnesium is enlarged owing to the thought of energy-saving and environmental protection, especially in transportation. In transportation, the lighter weight of vehicle is more and more required. If magnesium alloy can be make up some parts, we can lighten the weight of vehicle largely. Furthermore, magnesium also is the good material to make up hard-disk of computer and electrical equipment. Magnesium also is a kind of good desulphuring agent in iron and steel industry. Because of the increasing consumption in iron and steel industry, the amount of magnesium will increase steadily.But the electrode potential is so negative (Eo=-2.34v) and the ratio of PB is 0.79 that it can not produce protective oxidation film like aluminum. Therefore, the anti-corrosion of magnesium is so poor that it is apt to corrosion in most environment. In order to apply magnesium in industry, we must treat it by surface processes.To improve the performance of magnesium, the surface treatment means are: chemical oxidation process, electrochemical oxidation process (anodic oxidation process, coloring, chromate coating treatment. But methods as above all have various limitation.Electroless nickel process is an effctive surface treatment means, especially for difficult-to-dump metal such as magnesium, aluminum and zinc. According to the relevant documents, there is hardly successful example by electoless nickel plating on magnesium. Therefore, electroless nickel on magnesium is a new research direction and has large development room. If this process succeed, it must propel the development of other electroless treatment means.According to the experiment of electroless nickel on magnesium, we studied the process condition, especially the importance of pretreatment, acid wash process, zinc-dipping process. In our process, we use phosphor acid insteadof chromium trioxide and ferric chloride, and use layer of dipping zinc, cyanide copper and nickel as bottom. Then we got excellent coating.The study of acid wash procedure indicates: the various ingredients and conditions in documents took on large difference. Some of them cannot succeed at all, and the other methods need complicated operational requirement. Therefore, processes as above cannot apply to industry conveniently. Aiming to this problem, we decided a new acid wash ingredient independently, which we washed magnesium in 85% phosphor acid at room temperature. This process can not only lead to excellent coating, but preparation is convenient, which is suitable for industrial production conveniently.Optimal zinc-dipping process conditions were decided by SEM (Scanning Electron Microscope). SEM results show that, the performance of zinc coating gets better along with the increasing of time and concentration of dipping-zinc solution. When the time reached 15 minutes, the zinc coating can cover the surface of magnesium and faults inside the magnesium evenly. Enough concentration of zinc-dipping solution is required, else the dots on surface due to corrosion are fatal.Hardness and thickness of coating were decided by HX-1 microscopic hardness meter, and phase structure was decided by Japan Scientific 3015 X-ray diffraction meter. Results show that, there are nickel, copper and nickel-copper compound in coating, which are mainly Ni-P non-crystal phase.Adhesive stress was decided by bending experiment and file experiment, too. Results show that, coating has good adhesive stress, so it has excellent anti-corrosion performance.According to above experimental results, we reached a conclusion that, we can get excellent adhensive-stress coating throught this process, which has smooth surface, no bubble and make up the fault of magnesium in hardness, anti-corrosion and decoration. In a word, this process is practicable. |
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