| As the lightest structural materials, Mg-Li alloys possess low densities of1.35-1.65g/cm3. Besides the superlight property, Mg-Li alloys have some other advantages, such ashigh specific strength, high specific stiffness, good property of electromagnetic shielding,good heat-conductivity and electricity-conductivity, etc. Therefore, Mg-Li alloy is deemed asone of prospective advanced materials having wide applications in the fields of aerospace,weapon equipment, electric equipment, traffic equipment, and sports equipment, etc. However,Mg and Li, which are the two main elements in Mg-Li alloys, are both very active. Thismakes Mg-Li base alloys very active with poor corrosion resistance, which becomes the keyproblem restricting the wide application of Mg-Li alloys.In this dissertation, the corrosion characteristics of three common Mg-Li base alloys withdifferent phase compositon are studied. Then two surface treatment methods, molten saltsubstitution and diffusion and micro-arc oxidation, are used to prepare coatings on Mg-Libase alloys. Besides the parameters optimizations for these two methods, the thermodynamicsand dynamics of the aluminum diffusion during the process of molten salt substitution anddiffusion are studied. The effects of the additives, sodium aluminate and alumina sol. on theprocess of micro-arc oxidation are also studied. The research content and the results are asfollow:The microstructure, corrosion resistance and high temperature oxidization behavior ofthree common Mg-Li base alloys, LAZ532, LAZ832and LAZ1432, are researched. OM andXRD analysis show that, LAZ532is composed of (Mg) and AlLi. AlLi phase mainly existsat the grain boundary of (Mg) in the form of reticulation, and a small amount of it exists atthe inner of (Mg) grains. In LAZ832, there are (Mg),(Li) and AlLi. AlLi exists in the(Li) in the form of particle. In LAZ1432, there are (Li) and AlLi. AlLi phase exists in thelarge scale (Li) in the form of block. Hydrogen evolution, weight loss, electrochemicalpotentiodynamic polarization, et al. are used to evalute the corrosion characteristics of thethree Mg-Li base alloys. Results show that the three Mg-Li base alloys all possess poorcorrosion resistance. With the increase of Li content, the corrosion resistance becomes poorerand poorer. The sequence of the corrosion resistant capability of the three alloys is: LAZ532>LAZ832>LAZ1432. The corrosive electric current density difference among thethree alloys is about one order of magnitude. The high temperature oxidations of the threealloys were carried at25-225℃for0-24hours. Results show that, as for the oxidationresistance, LAZ532is best, LAZ832is the second, and LAZ1432is the poorest. Comparingthe phases in the three alloys,(Mg) phase possesses the best oxidation resistance, AlLi phaseis the second, and β(Li) phase is the poorest. After hight temperature oxidation, the corrosionresistances of the three alloys are all better than those of the alloys before oxidaiton. However,with the increase of corrosion time, the oxide film will be destroyed, making the corrosionrate increase. Among the three alloys, the corrosion rate of LAZ1432after high temperatureoxidation is obviously higher than those of LAZ532and LAZ832, because of many cavitiesexisting in the oxide film of LAZ1432.Molten salt substitution and diffusion treatment was used to prepare metallic coat onLAZ532. The opitimized parameters are listed as follow: molten salt composition(AlCl3:NaCl=1:1, mol. ratio), temperature (300~400℃), treatment time (2-8h). Under theseexperimental parameters, a good Al-riched alloy layer with a thickness of about10μm can beprepared on the surface of LAZ532. The bonding between the alloy layer and LAZ532ismetallurgical bonding. The alloy layer is mainly composed of Mg17Al12, and small amount ofsolid solution with the elements of Mg, Li and Al. Temperature and treatment time are twoimportant parameters affecting the formation of diffusion layer.350℃+8his goog parameters.After treatment, the corrosion resistance of the alloy is improved obviously. The corrosionpotential increases by0.2V, and the corrosion current deceases by one order of magnitude.The corrosion resistance improvement can be attributed to the continuous Mg17Al12, whichcan hinder the matrix alloy from further corrosion. The formation of alloy layer includes threeprocesses, substitution, diffusion and phase transformation.To botain a ceramic coat on Mg-Li alloy, micro-arc oxidization treatment was carried inthe silicate electrolyte. The effects of the additives of sodium aluminate and alumina sol onthe film were also researched. The optimization parameters are listed as follow:Na2SiO3·9H2O4g, NaOH5g, Na2B4O7·10H2O3g, distilled water1L, positive duty rate20%,electric current0.3A. After the micro-arc oxidization treatment with these parameters, thecorrosion current of the alloy decreases by two orders of magnitude compared with the alloywithout surface treatment. When the sodium aluminate is added in silicate electrolyte, the electric conductivity of the electrolyte will decrease, and the amount of micro-pores on theoxidation film will decrease obviously. With the increase of sodium aluminate content, thethickness and electric resistivity of film both increase, although the corrosion resistance of theoxidation film does not improve obviously. When the sodium aluminate content is2g/L, thecorrosion resistance of the corresponding oxidation film is the best. When alumina sol isadded into electrolyte, the amount and size of pores on the micro-arc oxidation film bothdecrease obviously. With the increase of alumina sol content, electric resistivity of the filmincreases, and the corrosion resistance of the film is also improved. When the alumina solcontent is7ml/600ml, the corrosion resistance of the corresponding film is the best. |
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