| Magnesium alloys have been widely used in aerospace, automotive and electronic manufacturing industry and other fields because of a series of excellent performances. It is considered to be the most potential for the development and application of "green engineering materials" in the 21st century. However, magnesium has a high chemical reactivity, it is susceptible to corrosion which can cause severe pitting in the metal resulting in a decreased mechanical stability and unattractive appearance. All these have greatly limited the application of magnesium alloy. The surface protection technology of magnesium alloy is to form a barrier between matrix and corrosion medium for blocking the formation of corrosion current, therefore it is an effective anti-corrosion method. Until the metallurgical control and the development of new alloys have been made decisive progress, the adoption of a variety of surface treatment technology to control corrosion of magnesium alloy is the most important question to investigate. As to the surface protection technology for magnesium alloys, there are some questions need to face, such as environmental pollution, cost, complexity of the pre-treatment steps, the poor adhesive ability, as well as the poor corrosion resistance, etc. In the present work, we attempted to improve two of the above mentioned problems, i.e., environmental pollution and the poor adhesive ability, by choosing environmentally friendly organic compounds with multi-functional group, as well as deposition in the preparation of the surface organic conversion coating with a reactive functional group in aqueous solution.In order to provide the necessary information for the preparation of organic conversion coating in aqueous solutions, the corrosion of magnesium alloys in aqueous solution was investigated. The results showed that when the pH value was less than 11.5, magnesium alloy is susceptible to oxidation by H + and corrosion; when the pH value was more than 11.5, magnesium alloy is stable. The polarization behavior caused by working potential is divided into three regions, cathode polarization region, passive region, anode polarization region. In the region of anode polarization, magnesium alloy can be eroded. Oxidation corrosion of magnesium alloy is affected by the pH value: with the solution pH value increases, passivation region become wider, and the the region of anode polarization shifted positively.The method of choosing organic corrosion inhibitor for magnesium alloy was established on the basis of previous reports concerning other surface protection technologies of magnesium alloy and other metal corrosion inhibition technologies. The way of "concerns on structure (choice of the multi-functional group organic compounds with oxygen, nitrogen, sulfur or phosphorus atoms)—chemical precipitation experiment (in acidic or alkaline conditions, the formation of insoluble materials)—electrochemical corrosion experiments to test the anti-corrosion property—surface morphology observition and the surface composition analyzation" is the selection process and three types of environment-friendly organic corrosion inhibitors for magnesium alloy, p-nitro-benzene-azo-resorcinol (PNBAR), phytic acid (PA) and acidum tannicum (AT), were successfully selected.Technology for preparing PNBAR conversion coating on magnesium alloy surface by electrochemical deposition method in alkaline solution was established. The pH value and working potential were taken into account in the process of preparing organic conversion coating. Magnesium alloy surface oxidation and the deposition of PNBAR are synchronous by controlling the working potential. The PNBAR conversion coating was prepared through polarizing 30 minutes with constant potential (3.2V) in the PNBAR solution (5mmol/L) of pH 13. PNBAR bonds to the surface of magnesium alloy through covalent bond Mg-ON, the corrosion potential was shifted positively about 110 mV by conversion coating when compared with control, and its adhesive ability reached to Grade one(in accordance with GB / T 9286).Four methods of preparation phytic acid conversion coating on magnesium alloy surface were proposed according to the characteristics of the molecular structure of phytic acid: 1) esterification by phytic acid with surface hydrated magnesium ion, 2) the reaction between surface oxide and phytic acid, 3) deposition of phytic acid with the surface metal in acidic solution or 4) deposition in alkaline phytic acid solution. The conversion coating could not be formed by dehydration and oxide reaction because of low activity of solid-phase reaction. In 0.5mg/ml phytic acid solution with pH 13, phytic acid conversion coating was formed by polarizing for 30 minutes at 3.2V potential. In solution with pH 13, phytic acid and magnesium ions form insoluble of [Mg5(H2L)].22H2O compound, generation of magnesium hydroxide become the competitive reaction to phytic acid deposition, so phytic acid deposited together with magnesium hydroxide. In 0.5mg/ml acidic phytic acid solution with pH 5, an orderly generation and compact phytic acid conversion coating formed after 20 minutes of immersion. The conversion coating has the thickness of 4μm~5μm, and has better corrosion resistance than the coating formed under alkaline condition, the corrosion potential was shifted positively about 185 mV, and the adhesive ability also reached to Grade one(in accordance with GB / T 9286).The influence of chemical composition and state of magnesium alloy on forming process and properties of phytic acid conversion coating in the acidic condition was studied. In the acidic condition, phytic acid can be deposited on magnesium alloy surface through forming insolubility compounds with multivalent elemental ions (except magnesium ion) of magnesium alloy and formed conversion coating. For the common magnesium alloy, it is notable that the elements of aluminum and zirconium play a vital role in the process of phytic acid conversion coating formation because of their concentration and performance in the alloys.
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