| Magnesium alloys have excellent physical properties which are regarded as green metallic structure material. In21st century, the new material field pays more attention to it. Thanks to its lively electrochemical properties, the parts of magnesium alloys don’t have an advantage of overall performance in the corrosion environment. The surface modification technology can significantly improve the adaptability for external environment on magnesium alloys. Metallic coating technique is a relatively simple and excellent method of surface modification on magnesium alloys. The literatures in international and domestic are reported more about means and methods of research results on aluminum and other materials. But there are very small innovative technical studies on magnesium alloys. The key step of surface metallic coating technology on magnesium alloys is pretreatment procedure. Pickling and activation are two main technologies among numerous pretreatment processes on magnesium alloys. At the same time, the surface metallic coating technology also need improvement and innovation which can provide new technology for application of surface treatment on magnesium alloys. The main results were gained as following:1. It can be concluded that the newly developed Na2MO4pickling solution can replace traditional CrO3pickling solution. The ions CrO42-and NO3-in CrO3+HNO3pickling solution have very strong etching function to the substrate of magnesium alloys. The main outcome of CrOOH has an effect of temporary protection to the substrate. After pickling in the H3PO4and HNO3mixed solution, there is a piece of phosphating film with Al3PO4and Mg3(PO4)2formed, which can prevent the direct response between the pickling solution and substrate. After pickling in the Na2MO4and H3PO4mixed solution, there are molybdenum acid salts film formed. If it is adequately thick, it can also play an important role on protection of the substrate. Sodium molybdate acid liquid has a number of prosperous features such as environmental protection, non-toxic, safety features. The etching degree of the substrate in sodium molybdate pickling solution is comparative to in chromic acid pickling solution, and the security of sodium molybdate pickling solution is better than the phosphoric acid pickling solution.2. After the NH4HF2activation, the adaptive coating can also obtained on electroless nickel plating, so the NH4HF2activation can replace HF activation technology which has volatility. The K4P2O7activation can effectively remove oxide after pickling procedure. The soluble outcome of [Mg(P2O7)]2-can guarantee the substrate surface completely exposed. Because of the excellent conductive performance, the activation technology can be used in directly electroless nickel plating. NH4HF2activation and HF activation can form fluorine activation membrane, but the HF activation has thicker density than NH4HF2activation. It means that the reactive activation of NH4HF2membrane is higher than that of HF membrane. In control of activation time in NH4HF2solution, the coverage of MgF2film can be controlled.3. Electroless nickel plating on complex surface of magnesium alloys can get nickel coating with excellent adhesion. The Had will form adhere to the substrate surface and the Ni2+will be deoxidize to nickel during the process of H2PO2-hydrolysis. The main reaction mechanism is the formation of nickel crystal nucleus. The displacement response occurs only in the early stage, and the later reaction is mainly the reduction of nickel deposit which is guided from atomic hydrogen.4.[Ni2++mL-n]+4H2PO2-+H2O'Ni+P+3HPO32-+4H++3/2H2+mL-n is the total reactivate equation of electroless nickel plating on magnesium alloys. The deposition rate of nickel reduction, adhesion and corrosion resistance of the electroless nickel plating are mainly affected by the reactivate concentration of [Ni2+]、[H2PO2-]、[L-n], pH value, temperature and the stabilizer of thiourea.5. The process of zinc immersion was improved. The new solution contains F-, EDTA and small additive of NiSO4and can increase density of and uniformity of the zinc film. The MgF2and Mg(OH)2mixed membrane and zinc film on the surface of substrate can form quickly. They would adhere to the substrate surface and result in potential increasing, the dissolution activity reducing and conductivity unchanging. These features can improve the quality of plating nickel.6. Nickel plating is mature processes and they can be used in industrial production. The main reaction is the reduction of Ni2+Current density, pH value and temperature are the main factors of effecting adhesion and corrosion resistance of the nickel plating. F-can promote anodic dissolution between1.0mol·dm-3and1.5mol·dm-3in the nickel plating process. The activation of anodic dissolution is strengthened when the temperature increases. After electroless nickel plating or electroplating, the latter Cu/Ni/Cr combination coatings can further enhance corrosion resistance.7. Electrochemical nickel plating process has advantages of high plating rate and thick coating in electroplating. It also has merits of deep plating and symmetrical coating in eletroless nickel plating. It avoids effectively deficiency from the former two plating technologies. The ion Ni2+is reduced in role of NaH2PO2which can effectively improve the current efficiency and decrease the cathode polarization. It can also improve refinement of the electrochemical deposition nickel grain.8. Research innovations in this paper are as follows:(1) One of innovations in this dissertation is the investigation of sodium molybdate pickling solution, which has a number of prosperous features such as environmental protection, non-toxic, safety features. It can replace deleterious chromium anhydride pickling solution. It has the capability of finer symmetrical etching and there are molybdenum acid salts film formed on the surface to prevent extremely thick cavity from serious partial corrosion which can go against nickel plating. The pickling solution is adaptive both to electroless nickel platting and electroplating.(2) The soluble outcome of [Mg(P2O7)]2-in K4P2O7activation solution can guarantee the surface of magnesium alloys completely exposed. The new surface after activation is suitable for direct nickel electroplating. MgF2film will be formed after NH4HF2activation. The coverage of MgF2film can be controlled through in control of activation time. NH4HF2activation can replace dangerous HF activation to get coating with good adhesion.(3) Electrochemical nickel plating process is first suggested. The ion Ni2+is reduced in role of NaH2PO2which can effectively improve the current efficiency and decrease the cathode polarization. It can also improve refinement of the electrochemical deposition nickel grain. Experimental results showed that the nickel coating has the merits of uniformity in electroless nickel plating and high speed in electroplating nickel. |
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