Investigation On Film Formation Mechanism And Performances Of Complex Zinc Phosphate Coatings On Magnesium Alloy

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.Chromate conversion coating was usually performed on magnesium alloy substrates for preventing from corrosion. In consideration of the environment friendly technology, zinc phosphate coating should be one promising method for magnesium alloys anticorrosion and pretreatment process before paint, tends to replace chromate treatment where the environment polluted hexad chromium was involved.Zinc phosphate coatings have been successfully used to underneath the paint of steel and aluminum for many years because they can enhance the adhesion of the paint and substrates. Phosphatization of magnesium alloy is rather difficult in comparison with the traditional phosphatization of steel and aluminum because of high electrochemically activity of magnesium. In the acid phosphating bath resolving rate of magnesium must be restrained in order to obtain dense and high quality phosphate coating. Although the growing interests in magnesium alloys, there are only limited studies about phosphate treatments on the surface of magnesium alloy.Han et al. obtained a phosphate film of Mn3(PO4)2 on AZ31D magnesium alloy in a bath containing phosphate and manganese in 2003. Phosphate films were gained from phosphate-permanganate bath, which have micro cracks,similar microstructure to chromate conversion. A zinc phosphate (Zn3(PO4)2?4H2O) coating was revealed on AM 60 magnesium alloy in 2004 by Kouisni et al.The main works include:(1) Dense and crack-free complex zinc phosphate coatings have been prepared Dense and crack-free complex zinc phosphate coatings have been prepared in phosphating bath containing mainly H3PO4, ZnO and NaF. The main compositions of the coating are Zn3(PO4)2?4H2O, Zn, AlPO4 and MgZn2(PO4)2 etc. SEM has shown that very fine zinc particles surrounded phosphate crystals and filled in the interstice of the insoluble phosphate. The adhesion of magnesium alloy and the zinc phosphate coating was better than that of chromate conversion coating, due to the uneven structure of the phosphate coating.(2) The influence of sodium metanitrobenzene sulphonate on the zinc phosphate coating of magnesium alloySodium metanitrobenzene sulphonate, SMBS(the abridge will be used in the follows)was used to the accelerating reagent for replacing nitrite and its influence on the characteristics of the zinc phosphate coatings on AZ91D magnesium alloy was investigated. The results of this study showed that addition of sodium metanitrobenzene sulphonate greatly shortened the formation time of phosphate film from 55 minutes to about 4～5 minutes. The phosphating process can be accelerated because sodium metanitrobenzene sulphonate absorbed on the anode areas of the magnesium alloy to restrain the resolving of the substrate magnesium and to facilitate the nucleation of zinc phosphate. As a result, denser and fine phosphate crystals formed on the magnesium alloy substrate.(3) The influence of molybdate sodium on the zinc phosphate coating of magnesium alloyUniform and denser gray multiplex molybdate modified zinc phosphate coatings were obtained on the magnesium alloy AZ91D surfaces by using a phosphting bath containing molybdtate and corrosion inhibitor. Results indicated that the phosphate coatings consist mainly of Zn3(PO4)2·4H2O, and Zn. When molybdate was added in the phosphate bath, The concentration of metallic Zn increased and phosphate coatings became denser and non-crack. The phosphate coatings obtained from the bath containing 1.5g/L sodium molybdate exhibit more dense microstructure and have high concentration of metallic zinc.(4) The influence of the pH of the phosphating bath on the zinc phosphate coating of magnesium alloySEM and XRD indicate that fully compact and dense phosphate coatings were formed in the phosphate baths with pH in the range of 2.15～2.5. When the pH of the phosphating bath is lower than 2.15, the edges of slabs of crystal became round and the substrate was not fully covered by phosphate crystals. That is because the formed phosphate crystals tend to resolve in the strong acidic bath。When the pH of the phosphating bath is more than 2.50, phosphate crystals is difficult to form and the resolve of magnesium is too slow, because electrochemical deposition reaction can not start. As the pH of the bath decreases, zinc phosphate crystals of slab-like tends to growth along (020) orientation and crystals zinc concentration in the coating increases.(5)The influence of Metal inons of the phosphating bath on the zinc phosphate coating of magnesium alloyMagnesium alloy in the phosphating solution add salt (usually nitrate), such as Mo, Mn, Ca, Co and other potential than the metal salts , in favor of nuclei formation and grain refinement ,in favor of accelerating the phosphating process. Such as magnesium alloy phosphating , whether or not to participate in their film, to a large extent changed the phosphating film magnesium alloy composite microstructure. In this paper, the main aim is to test for the development of a magnesium alloy surface treated with preservatives prior to zinc phosphating film technology, the technology has a short processing time, low temperature, simple of the process , and good corrosion resistance for the magnesium alloy surface treated with preservatives to provide a new idea. Therefore this paper will define the surface of magnesium alloy AZ91D optimum phosphating film parameters, including the selection of additives and processing the best solution concentration of each component, pH value, pH regulator, control the film-forming time and temperature parameters, and a series of corrosion test methods on the conversion coating of corrosion resistance was measured and evaluated ;using SEM, XPS and other technology on the film morphology and Phase composition analysis, the final percentage of the conversion coating corrosion mechanism and the mechanism of membrane were studied for the magnesium alloy surface membrane into the process to provide theoretical guidance, the promotion of magnesium alloy into the surface membrane of the industrialization of this technology.