| Magnesium alloy exhibits many ascendant properties, such as high intensity, high stiffness, very good damping behavior and damping capacity, and it can de made into manufacturing accessories in stead of steel, aluminum alloy and plastic. So in the future, magnesium alloy will be widely used in automobile manufacturing, electronic industries, aviation and spaceflight, and other domains. About 90% of the component of magnesium alloy is Mg, and the chemical stability of pure Mg is rather poor, also its electrode potential is low (E0= ?2.37V), approximate 2V lower than Fe, 0. 7V lower than Al, so it is usually used as the immolate anode in cathodic protection or as the additive in the organic films to protect other metal. In most of the mediums, Mg and its alloys are not stable, and they have no corrosion-resisting property, and all those mentioned above restrict the application of magnesium to a certain extent. In order to improve the corrosion-resisting property of magnesium alloy, on the one hand, with a view to alloy designing and boosting the foundry condition, we should reduce the content of the impurities, and append other metal element, to make magnesium alloy with high purity and corrosion-resisting property in high temperature; on the other hand, considering the surface modification, we can create some protective films on the surface of the magnesium alloy, to improve its corrosion-resisting property. And the exterior films can made in these ways, electrochemistry method, chemistry method, heat treatment method, vacuum method and other physics methods. And some new methods of surface treatment also can be used, such as composite transformation film, laser surface treatment, PVD, chromium nitride coating, electro deposition of aluminum alloy on the surface of magnesium alloy and so on. The purpose of forming the magnesium alloy transforming film is to enhance the corrosion-resisting property, and the film also can be regarded as the rendering of certain coatings. The way of forming such a coating can be divided into two types by the kind of the film-forming additive, one is phosphate series, the other is chromate series. At present, chromatizing method is a relative mature method of transforming film treatment, which the chemical process takes place in the liquid of which the main component are chromic anhydride and bichromate.Because of chromadizing based on the Cr 6 + is venomousness, goes against the environment, the application of chromadizing is gradually restricted both interio and overseas. For the moment, there is not a non-chrome passivation technics which can completely take place of the chromadizing, so it has become urgent affairs to find a green passivation technics which can take place of the chromadizing. After studying several conversion coating technecs for magnesium alloy which are used now, we choose the tannin passivation technics to enhance the anticorrosion property of magnesium alloy. Now, the tannin passivation technics is mainly used in those aspects as follows:1. the most extensive application of this technics is the dope for rust-transformation, and it is also well investigated. 2. the application in zincification passivation, which could greatly enhance the combination of the paintings and the corrosion-resisting property. 3. the application in aluminum. This kind of dope has no pollution, and it also can effectively enhance the adhesive power and corrosion-resisting property. So this kind of film is called the third generation transforming film. 4. and in copper. To get the tannic conversion coating, generally have three steps, first dissolving the metal in the solution; secondly form the coating; thirdly equilibrium of dissolving and forming. When the tannin is used as the rust-transformation agent, chemical reaction will take place between the tannin and the metal, the main component of production, to form some stable coordination which has reticulation structure, and the reaction can make the production effectively translate into protective films, and hard hangs together with the metal matrix.By all those mentioned above, we can find that till now, the tannin conve- rsion coating is mostly used for surface treatment of metallic materials such as Fe, Zn, Al and Cu, and there is no use of tannin conversion coating on magnesium alloy, so we chose the research program about the use of tannin transform ting film treatment technics on magnesium alloy, with the purpose to extend the application of magnesium alloy. Coming by a large numbers of experiment, finally we find a new technecis about tannin transformating film for magnesium alloy surface. And after the farther experiment, we find that if we add a kind of corrosion inhibitor(ammonium metavanadate) into the basic electrolyte, the corrosion-resisting property of the tannin transformting film will boost prominently. It is be studied that how those factors affect the erosion resistance of the tannic conversion coating, such as temperature, surface area of the sample and pH. It is also be detailedly studied the affect of the time and conditioning agent of pH. The anticorrosion of conversion coating forming with 10min is better than 5min. Although the self-corrosion potential of the coating adjusted by HCl and is minus than HNO3, after the dropping experiment and the corrosion-accelerating experiment, we found that the tannic conversion coating adjusted by HCl has better corrosion resistance. This is because adjusted by the HCl, the film appears obvious passivation space interval.We observe the micrographs of the tannic conversion coating with SEM(Scanning Electron Microscope). The result shows that if we use HCL to adjust the coating-forming, the tannic conversion coating which costs 5min and 10min to form is very compact, and the grain of the micrographs is quite symmetrical. However the film, that its coating-forming time is 10min, has a lot of cracks on its surface, which can decreases the corrosion current and weaken the corrosion destroy; and there are some white substances around the grain, which is Mg3 (PO4 )2 studied by XPS and FT-IR. Those substances can make the coating more compact and anticorrosion.We also analyze the component and structure of the tannic conversion coating with FT-IR and XPS. It shows that the conversion coating is composed of organism, Mg3(PO4 )2 and some crystal water. In the study of FT-IR, we do not find the ubiquinone group which can prove that the tannin is not redoxed.It shows the mechanism of forming the tannic conversion coating. When the Mg matrix is put into the electrolyte, it is eroded and some small hole s are created, the atoms of Al and Zn gradually expose, then the chemical reaction takes place among these atoms and the hydroxyl group and carbolxyl of the tannin, and the complexation reaction takes place between Zn and hydroxyl group of tannin, to form huge organic molecule which covers the surface of the matrix, which can prevent the contact the magnesium with the ion in the solution. And the conversion coating is contact with the base of the magnesium alloy, so the adjacencies powerful. The soluble Mg2+ can react with PO4 3- which could get the indiscerptible substance Mg3(PO4 )2.The anticorrosion of the tannic conversion coating is influenced by three mechanism. 1. the conversion coating act as the obstruction which can prevent the contact magnesium with corrosion ion. 2. the organic conversion coating has two kinds of group which is hydrophile and hydrophobe. The hydrophile group is located inside and the hydrophobe group is located outside. So the organic conversion coating also act as barrier to prevent the transition of electron. 3. the conversion coating has passivation which could minish the corrosion current and improve its anticorrosion.
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