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Study on Preparation of Mg-Sr Alloy by Molten Salt Electrolysis

Author: FanShuDi
Tutor: LuGuiMin；LiXiaoSong
School: East China University of Science and Technology
Course: Materials Engineering
Keywords: molten salt electrolysis Mg-Sr alloy codeposition current efficiency MgCl2-SrCl2-KCl molten salt
CLC: TF82
Type: Master's thesis
Year: 2012
Downloads: 87
Quote: 0
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Abstract


Magnesium alloy as green engineering materials have gained more and more attentions owe to their features of low density, high strength to weigh, rigidity to weigh ratio, good damping capacity, good electromagnetic shielding properties and convenient recycling. Magnesium alloys have wide applications in the aircraft, communication industries and electronics. But Magnesium alloys have poor mechanics capability when the temperature is over 120℃, especially the capability of creep-resistant, corrosion-resistant and hot crack-resistant. Adding alkaline earth metal element Strontium to Magnesium alloys can significantly improve the heat-resistant of Magnesium alloys. The mixing and fusing method is commonly used to prepare Mg-Sr alloy. The disadvantages of this method are long process and high energy consumptions. In the process of melting Mg-Sr alloy, the alloying elements are burning serious and macro segregation. In view of the disadvantages of the mixing and fusing method, Mg-Sr alloy are prepared by electrochemical codeposition in metal chloride’s molten salts in this paper.In this thesis, the Mg-Sr alloy is firstly prepared through electo-codeposition method in a molten MgCl2-SrCl2-KCl system at 700℃. The electrochemical behavior of Mg2+ and electro-codeposition mechanism of Mg-Sr alloy were both studied by electrochemical measurement technology such as cyclic voltammetry, chronoamperometry and chronoptentiometry in a molten MgCl2-SrCl2-KCl system at W electrode at 700℃.The results show that in a molten MgCl2-SrCl2-KCl system, Mg2+ cathodic reduction reaction is quasi reversible two-electron charge transfer process, the speed control step is diffusion step which is caused by concentration polarization. The Mg2+ cathodic reduction process reversible has decreased with an increase in potential scan rate, current intensity and MgCl2 concentration in the molten salts. The Mg2+ diffusion coefficient numerical value is in the order of 10-5 cm2/s magnitude. The Mg2+ diffusion activation energy is 55.7kJ/mol.The experiment results show that Strontium is in the formation of under potential deposition on the pre-deposition Magnesium to lead to the formation of a Mg-Sr alloy. The actual precipitation potential of Strontium reduces nearly 0.5V because of the depolarization effects of Strontium activity reduced by formed Mg-Sr alloy. The codeposition potential condition of Magnesium, Strontium to form Mg-Sr alloy is:When electrode potential is more negative than -1.5V, the Magnesium will precipitate; when electrode potential is more negative than -2.0V, the Magnesium and Strontium will both deposit. The speed control step of codeposition process of Magnesium and Strontium is not diffusion control step. The codeposition current condition of Magnesium, Strontium to form Mg-Sr alloy by chronoptentiometry is:the codeposition of Mg and Sr occurs at cathode current densities higher than 0.71 A/cm2 in (2wt%)MgCl2-SrCl2-KCl molten salt system; the codeposition of Mg and Sr occurs at cathode current densities higher than 1.57A/cm2 in (5wt%)MgCl2-SrCl2-KCl molten salt system; the codeposition of Mg and Sr occurs at cathode current densities higher than 2.83 A/cm2 in (10wt%)MgCl2-SrCl2-KCl molten salt system.The electrolytic process parameters such as electrolytic temperature, current intensity, electrolytic time and MgCl2 concentration are optimized by researching on the effect of electrolytic efficiency and composition of the alloy. Codeposition of Mg-Sr alloy were feasible at a temperature of 700℃at cathodic current density of 8.0A/cm2 and Mg2+/Sr2+of near 1/4.4 in the molten salt system. The Mg-Sr alloys which co deposit in the molten salt need to take out every two hours. The current efficiency of the electrolytic process will reach nearly 85% which has 40 points ahead of the current efficiency of electrolytic preparation of only metal Strontium.

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