Electrochemical formation of Mg–Li alloys at solid magnesium electrode from LiCl–KCl melts

doi:10.1016/j.electacta.2007.11.043

Electrochimica Acta

Volume 53, Issue 8, 10 March 2008, Pages 3323-3328

https://doi.org/10.1016/j.electacta.2007.11.043 Get rights and content

Abstract

This work presents a study on electrochemical formation of Mg–Li alloys on solid magnesium electrode in a molten LiCl–KCl (50:50, wt.%) system at 753 K. Cyclic voltammetry and open circuit chronopotentiometry were employed to investigate the electrode reaction. For an Mg electrode, the electroreduction of Li(I) takes place at more positive potential values than at the inert W electrode indicating the formation of Mg–Li alloys. X-ray diffraction and scanning electron microscopy (SEM) analysis of the deposits indicated that α, α + β and β phases Mg–Li alloys with the thickness of 182, 365 and 2140 μm were obtained by potentiostatic electrolysis at −2.26, −2.30 and −2.39 V (vs. Ag/AgCl), respectively. The results showed that formation of α, α + β and β phases Mg–Li alloys could be controlled by applied potential. Lithium contents of Mg–Li alloys can be decreased via electrolysis at low temperature followed by thermal treatment at higher temperature. Mg–Li alloys with excellent mechanical properties can be produced by this novel method.

Introduction

Mg–Li alloys, known as the lightest metallic materials, have low density (1.3–1.6 g cm⁻³). It is 1.5–2.0 times less than that of aluminum alloys and similar to that of constructional plastics. Mg–Li alloys also show high specific stiffness, high electrical and thermal conductivities. These alloys have attracted great attention due to their merits, especially in the fields of aerospace, aircraft, and weapon [1], [2], [3].

Pure Mg has a hexagonal closed-packed (hcp, α) structure. The density of Mg–Li alloy decreases with an increase of lithium content, and the addition of Li increases ductility. The Mg–Li phase diagram [4] shows that Li is soluble in hcp structure up to 5 wt.%. As Li content exceeds 11.5 wt.%, Mg–Li alloys become a body-centered cubic (bcc, β) structure. Mechanical properties of the hcp α phase are worse compared with the bcc alloys, which are very good machinability and weldablility. Disadvantages of Mg–Li alloys with bcc structure are high chemical activity and poor corrosion resistivity. Some compromise would be an alloy with 8 wt.% of Li (a mixture of phases α + β) that might exhibit improved mechanical properties as well as good corrosion resistance [5].

At present, there are two methods including metal casting and thermoreduction in the industrial production of Mg–Li alloys. Both methods have many disadvantages, such as unhomogeneous alloy composition, complicated production process, serious metals burning and high-energy consumption. Consequently, preparation of Mg–Li alloys by molten salt electrochemical process is proposed. Castrillejo et al. [6], [7], [8], [9], [10] studied electrochemical behavior of Dy, Er, Gd, Pr and Ho on Al electrode in the eutectic LiCl–KCl, and produced corresponding Al-based alloys. Nohira and co-workers [11], [12], [13] investigated electrochemical formation and phase control of Sm–Ni, Dy–Ni and Pr–Ni alloys on Ni electrode. Smolinski et al. [14], [15] studied the preparation process of Mg–Li by electrolysis in molten salts in the temperature range of 773–903 K. Our group has first prepared different contents Mg–Li alloys via a two-electrode cell in a molten salt electrolyte (50 wt.% LiCl–50 wt.% KCl) at 693–783 K. The optimal temperature was 753 K, which corresponds to the highest current efficiency [16]. Thus, the experiment temperature was set at 753 K in this paper. Previous studies supplied some experiment parameters for production of Mg–Li alloys by molten salt electrochemical method. However, lithium content of all samples exceeded 25% (wt.%). It is difficult to apply them directly in industry. In order to accurately obtain different phases of Mg–Li alloys due to the close relationship between mechanical properties and phase structure, it is necessary to investigate the electrochemical behavior of Mg–Li alloys on Mg electrode in LiCl–KCl melts.

In this paper, electrochemical formation of Mg–Li alloys at solid magnesium electrode was investigated in LiCl–KCl (50:50, wt.%) melts at 753 K, three kinds of phases Mg–Li alloys were attained by potentiostatic electrolysis. A novel method was developed to decrease the lithium content of Mg–Li alloys for better mechanical properties.

Section snippets

Preparation and purification of the melts

The LiCl–KCl mixture (LiCl:KCl = 50:50 (wt.%), analytical grade) was contained in an alumina crucible placed in a quartz cell inside a electric furnace. The temperature of melts was measured with a nickel–chromium thermocouple sheathed by an alumina tube. The mixture was dried under vacuum for more than 72 h at 473 K to remove excess water. Following this dehydration procedure, metal ion impurities were further removed by pre-electrolysis for 4 h at −2.0 V (vs. Ag/AgCl). All experiments were

Cyclic voltammetry

Fig. 1 shows typical cyclic voltammograms of the LiCl–KCl solution at a W electrode (curve 1) and an Mg electrode (curves 2 and 3). In curve 1, a sharp increase in cathodic current from approximately −2.32 V (vs. Ag/AgCl) is observed. The cathodic signal A can be ascribed to the deposition of Li, since no alloys or intermetallic compounds exist for W-Li binary system at 753 K. In the reverse scan, an anodic peak A’ corresponding to the dissolution of Li is observed. The shape of the curves

Conclusions

Electrochemical formation of Mg–Li alloys on solid magnesium electrode was investigated from the LiCl–KCl (50:50, wt.%) melts at 753 K. Formation of three kinds of phases Mg–Li alloys depends on the applied potential. α, α + β and β phases Mg–Li alloys with the thickness of 182, 365 and 2140 μm were obtained by potentiostatic electrolysis at −2.26, −2.30 and −2.39 V (vs. Ag/AgCl), respectively. The results indicate that the growth rate of Mg–Li alloys at 753 K is extremely fast. Mg–Li alloys with

Acknowledgments

The work was financially supported by 863 project of Ministry of Science and Technology of China (2006AA03Z510), the Scientific Technology Project of Heilongjiang Province (GC06A212), and the Scientific Technology Bureau of Harbin 2006PFXXG006 The authors are grateful to professors Liyi He, Deyu Qiu and Jun Wang for their technical assistance and useful suggestions.

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Electrochemical formation of Mg–Li alloys at solid magnesium electrode from LiCl–KCl melts

Abstract

Introduction

Section snippets

Preparation and purification of the melts

Cyclic voltammetry

Conclusions

Acknowledgments

J. Alloys Compd.

Mater. Sci. Eng. A

Compos. Sci. Technol.

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta