Study on the electrochemical behavior of Mg and Al ions in LiCl-KCl melt and preparation of Mg-Al alloy

Sholong Li ,Yusi Che ,Chenyo Li ,Yongchun Shu ,Jilin He ,Bin Yng,c ,Jinxun Song,*

a Henan Province Industrial Technology Research Institute of Resources and Materials,Zhengzhou University,450001 Zhengzhou,China

b School of Material Science and Engineering,Zhengzhou University,Zhengzhou 450001,China

c National Engineering Laboratory of Vacuum Metallurgy,Kunming University of Science and Technology,650093 Kunming,Yunnan,China

Abstract The electrochemical behavior of Mg2+ and Al3+ in LiCl-KCl (mass 4:1) melt at 973K was studied on a Mo electrode systematically by cyclic voltammetry,square wave voltammetry and chronopotentiometry.The results showed that the reductions of Mg2+ and Al3+ were reversible processes controlled by the rate of the mass transfer.When Mg2+ and Al3+ coexisted in LiCl-KCl melt,they had no significan effect on the reduction potential of each other.The equilibrium potentials of Mg2+/Mg and Al3+/Al were obtained by open circuit potential method.Their apparent standard potentials were also calculated in this system and the values were -2.52V vs Cl2/Cl-,-1.66V vs Cl2/Cl-,respectively.Correspondingly,the apparent Gibbs free energies of Mg2+/Mg and Al3+/Al were -485.71kJ/mol,-480.78kJ/mol.Finally,potentiostatic electrolysis was performed on a Mo electrode in LiCl-KCl-MgCl2-AlCl3 (the mass ratio of MgCl2 to AlCl3 was 10:1) melt at different potentials.The components of the deposits were characterized by scanning electron microscope and energy dispersive spectroscopy.The study revealed that the content of Al in the deposit decreased as the overpotential increased and Al tended to segregate at the grain boundaries.

Keywords: Electrochemical behavior;Thermodynamic properties;Mg-Al alloy;Electrolysis.

1.Introduction

Mg and its alloys have a wide application in field of automobile,aerospace and bio-medicine due to the high specific strength,excellent bio-compatibility,and reduced cost,so they have been widely concerned in recent years [1-7].Aluminum is often used as an alloying element to improve the strength of magnesium alloys and the Mg-Al series are one type of the most popular commercial magnesium alloys with good mechanical property,castability and workability [8-11].The traditional process for preparation of Mg-Al alloys includes electrochemical metallurgy process of every metallic element and subsequent mixing smelting,which complicates the route.In addition,high energy cost and serious metal oxidation are also the drawbacks of conventional method [12-14].

A better method to prepare alloys is direct electrolysis deposition,which greatly reduces the oxidation of the alloys and decreases the energy cost [15,16].The electrochemical behavior of magnesium ions in chloride molten salts have been studied by many researchers,which are basically onestep reaction controlled by diffusion [13,17-20].Similarly,aluminum ions are reduced to metallic aluminum in chloride molten salt by one step and the reaction is generally a reversible or quasi-reversible process controlled by diffusion [16,21-23].So far,the preparation of magnesium alloys by molten salt electrodeposition has been widely investigated and various binary,ternary and quaternary magnesium alloys have been prepared.The researches on preparation of common magnesium alloys by molten salt electrolysis in recent years are summarized in Table 1.

Table 1 Preparation of magnesium alloy by molten salt electrolysis.

Table 2 The EDS analyses of the magnesium alloy obtained by potentiostatic electrolysis on Mo electrode in LiCl-KCl-MgCl2-AlCl3 melt.

Fig.1.Schematic diagram of electrodeposition device.

As the most widely used magnesium alloys,the study on the electrochemical behavior,thermodynamic properties of Mg,Al and the electrochemical deposition of Mg-Al alloys is not systematic.The present work is to investigate the electrochemical behavior,thermodynamic properties of magnesium ions,aluminum ions in molten salts systematically by a series of electrochemical methods.The compositions of Mg-Al alloys were regulated by controlling the electrolytic potential in LiCl-KCl molten salt.

2.Material and methods

The experimental facility was shown in Fig.1.The whole experiment was carried out under argon gas using a threeelectrode system,the working electrode was an inert molybdenum wire (1mm in diameter,99.94% purity);the counter electrode was a graphite rod (6mm in diameter,99.99% purity);the quasi-reference electrode was a platinum wire(1mm diameter,99.99% purity).All electrode materials were purchased from Macklin Biochemical Technology Co.,Ltd.Before experiment,electrodes need to be polished with 2000 mesh SiC sandpaper,then rinsed with absolute alcohol and dried.The working electrode was inserted into the molten salt to a depth of 4.0mm.A positive scan on a graphite electrode was performed prior to each test to determine the precipitation potential of the Cl2.Then potentials in this work were all calibrated to Cl2/Cl-.

Fig.2.Curve of the relationship between density and temperature of molten electrolyte salts and metallic Mg.

Chlorine will be produced and escapes from the liquid surface during the electrolysis process.If the density of magnesium alloys produced by electrolysis is less than that of molten salt,then the magnesium alloys will floa to the surface of the molten salt,which will react with chlorine,resulting in pollution of the products.Therefore,in order to reduce the contact between the deposits and chlorine,the density of molten salt must be less than that of magnesium alloy,so that magnesium alloy can sink to the bottom of the molten salt.All reagents used in this experiment,which were purchased from Aladdin Biochemical Technology Co.,Ltd,are of analytical grade.The molten salt used in this experiment was LiCl-KCl with mass ratio of 4:1,which was preheated at 473K for 6h under vacuum to remove residual moisture.The temperature of furnace was measured by a platinum-rhodium thermocouple with deviation of ±1.5K.The relationship between the density and temperature of magnesium and LiCl-KCl was shown in Fig.2 [27,28].Pure metallic magnesium is lighter than Mg-Al alloys,so LiCl-KCl molten salt can ensure that Mg-Al alloys can sink to the bottom of the molten salt regardless of the ratio of magnesium and aluminium.MgCl2and AlCl3were used as the magnesium ion source and aluminum ion source in this experiment.The experimental temperature was 973K,which was higher than the melting points of magnesium,aluminum and Mg-Al alloys,so the electrolyticproducts would deposit to the bottom of molten salt in liquid form.

The Mg-Al alloys were prepared in the LiCl-KCl-MgCl2-AlCl3melt on a molybdenum wire (1mm in diameter) by the potentiostatic electrolysis at a potential previously define by the cyclic voltammetry as shown in Table 2.It should be mentioned that most of the liquid deposits can hardly adhere to the molybdenum electrode.Thus,a magnesia crucible was used to collect the liquid deposits dropping from the electrode as shown in Fig.1.The counter electrode was a graphite rod (6mm in diameter).The deposited alloys were dried and carefully stored in a vacuum drying oven before subjected to further analysis.The surface of the obtained alloy particles should be polished with 2000 mesh SiC sandpaper in order to test its composition and chemical element distribution.

The electrochemical tests and electrodeposition were carried out with an AUTOLAB (PGSTAT 302N).Cyclic voltammetry,square wave voltammetry and chronopotentiometry were employed to investigate the electrochemical behavior of magnesium and aluminum ions.Scanning Electrons Microscope (SEM,FEI Quanta 250 FEG) and energy dispersive spectroscopy (EDS) were used to investigate the morphology and components of the alloys deposited under different parameters.

3.Results and discussion

3.1.Electrochemical behaviors of Mg2+ and Al3+

Fig.3 showed typical cyclic voltammograms obtained on a molybdenum electrode at 973K before and after the addition of 1.0wt.% MgCl2and 1.0wt.% AlCl3in the molten LiCl-KCl.The precipitation signal R0of the alkali metal Li was observed at around -3.30V in the blank LiCl-KCl melt,and there was no reaction occurred from -1.00V to -3.20V as shown in Fig.3(a).After the addition of MgCl2,the current density peaks of R1at around -2.80V can be clearly seen from Fig.3(b),corresponding to the reductions of Mg2+.After AlCl3was added,the current density peaks of R2was found at around -2.00V as shown in Fig.3(c),corresponding to the reductions of Al3+.The deposition potential of magnesium ions was significantl more negative than the deposition potential of aluminum ions.If magnesium ions and aluminum ions were simultaneously deposited to form an alloy,a greater driving force was required to cause them to be collectively discharged.Firstly,the electrochemical behavior of Mg2+and Al3+in LiCl-KCl molten salt was studied in detail.

Fig.3.Cyclic voltammograms at 973K on a Mo electrode at the scan rate of 100mV/s:(a) LiCl-KCl (mass 4:1);(b) LiCl-KCl-MgCl2 (1.0wt.%);(c)LiCl-KCl-MgCl2 (1.0wt.%)-AlCl3 (1.0wt.%).

Cyclic voltammograms recorded of 1.0wt.% MgCl2on a Mo electrode in LiCl-KCl melt at 973K with varied scan rates were presented in Fig.4.A pair of cathodic/anodic peaks which corresponded to the deposition and dissolution of magnesium was observed.The results of the current density peaks recorded clearly showed that cathodic peak current density increased with the increasing scan rate.A linear relationship between current density peaks and square root of the scan rates as shown in the inset (a) of Fig.4,which indicated that the electrode process of Mg2+was controlled by the rate of the mass transfer.The results of the voltammetric peak recorded at different scan rates showed that peak potential shifted little with the increasing scan rate as shown in the inset (b) of Fig.4,suggesting that,the deposition was a reversible process under this condition.When the electrochemical reaction was reversible and controlled by diffusion,Eq.(1) [29] could be applied to calculate the diffusion coefficien of Mg2+.

Fig.4.Cyclic voltammograms of LiCl-KCl-MgCl2 (1.0wt.%) at 973K on a Mo electrode.Scan rate:100-500mV/s.Inset(a):relationship between square root of scan rate and peak current density.Inset (b) relationship between the logarithm of scan rate and peak potential.

Fig.5.Square wave voltammograms of LiCl-KCl-MgCl2 (1.0wt.%) on a Mo electrode at 973K with varied frequency from 5Hz to 25Hz.Inset:relationship between square root of frequency and peak current density.

WhereIPis the peak current;νis scan rate;n is the number of exchange electrons;Ris the gas constant;Tis the absolute temperature;Fis the Faraday constant;Ais the surface area of the working electrode;Cis the bulk concentration of the reducible ions andDis the diffusion coefficient The diffusion coefficien of Mg2+can be calculated to be 6.81×10-5cm2/s.

Fig.5 was a set of square wave voltammograms at different frequencies in the LiCl-KCl-MgCl2(1.0wt.%) system (background current density has been subtracted).Square wave voltammetry is an accurate method which can be used to calculate the number of exchange electrons in a reversible electrochemical process with the following Eq.(2) [30].

Fig.6.Chronopotentiograms of LiCl-KCl-MgCl2 (1.0wt.%) on a Mo electrode at 973K with varied current density from 0.18 A/cm2 to 0.35 A/cm2.Inset:linear relationship between j and τ-1/2.

Where W1/2is the half-wave width of the reduction peak.The number of electrons transferred was determined to be 1.99,which confirm that Mg2+is reduced to metallic Mg by transferring two electrons in one step.As shown in the inset of Fig.5,the current density peak has a good linear relationship with the square root of the frequency.This result was consistent with the CV test results,further indicating that the reduction of Mg2+on the molybdenum electrode was a reversible process,which was also consistent with the results of previous literatures [31].

Chronopotentiometry was carried out to study the electrolysis process of Mg2+and the results were shown in Fig.6.When the current density was less than 0.22 A/cm2,there was only one potential platform.As the current density increased to more than 0.25 A/cm2,two potential platforms appeared.The potential platform near -2.75V was the discharge platform of Mg2+,and the potential platform near -3.30V was the discharge platform of alkali metal.The electrode process was converted from a simple discharge of Mg2+to a simultaneous discharge of Mg2+and Li+[15,16].

The transition time (τ) was obtained as shown in Fig.6.Several currents were applied,and the plot of current densities (j)versus τ-1/2gave a straight line as shown in the inset of Fig.6.Combined with above results of cyclic voltammetry,the electrochemical reduction of Mg2+can be considered as a diffusion controlled process,and the diffusion coefficien of Mg2+in the system can be calculated using the Sand Eq.(3) [32].The diffusion coefficien of Mg2+was 6.69×10-5cm2/s,which was very close to the value(6.81×10-5cm2/s) obtained by cyclic voltammetry.

Fig.7.Cyclic voltammograms of LiCl-KCl-AlCl3 (1.0wt.%) at 973K on a Mo electrode,scan rate:100-500mV/s.Inset(a):relationship between square root of scan rate and peak current density.Inset (b) relationship between the logarithm of scan rate and peak potential.

The electrochemical behavior of Al3+in LiCl-KCl molten salt was also studied by cyclic voltammetry.Fig.7 was a group of cyclic voltammograms of LiCl-KCl-AlCl3(1.0wt.%) with varied scan rates.The current density peak increased with the increasing of the scan rate.Moreover,the peak current density was proportional to the square root of the scan rate as shown in the inset (a) of Fig.7,which indicated that the electrode process of Al3+was controlled by the rate of the mass transfer.The results of the voltammetric peaks recorded at different scan rates showed that peak potential shifted little with the increasing scan rate as shown in the inset (b) of Fig.7,suggesting that,the deposition was a reversible process under these conditions,which was consistent with the results of previous literature [23].According to the Eq.(1),the diffusion coefficien of Al3+can be calculated to be 4.28×10-5cm2/s.

Square wave voltammetry was carried out at different frequencies in the LiCl-KCl-AlCl3(1.0wt.%) system,and the square wave voltammograms were shown in Fig.8 (background current density has been subtracted).The number of electrons transferred was determined to be 2.87 according to the Eq.(2),which confirme that Al3+was reduced to metallic Al by transferring three electrons in one step.As shown in the inset of Fig.8,the peak current density had a good linear relationship with the square root of the frequency.This result was consistent with the CV test results,further indicating that the reduction of Al3+on the molybdenum electrode was a reversible process.

In this paper,when Mg2+and Al3+coexisted in LiCl-KCl molten salt,their electrochemical behaviors were also studied.Fig.9 showed typical cyclic voltammograms of LiCl-KCl system in which Al3+and Mg2+coexisted.R1corresponded to the reduction of Mg2+,and R2corresponded to the reduction of Al3+.The peak potential of each peak did not change with the increasing of the scan rate,and the peak current density was proportional to the square root of the scan rate.R1and R2were still reversible processes and controlled by diffusion.As shown in Fig.9,the electrode processes of Al3+and Mg2+did not affect each other.

Fig.8.Square wave voltammograms of LiCl-KCl-AlCl3 (1.0wt.%) on a Mo electrode at 973K with varied frequency from 5Hz to 30Hz.Inset:relationship between square root of frequency and peak current density.

Fig.9.Cyclic voltammograms of LiCl-KCl-MgCl2 (1.0wt.%)-AlCl3(1.0wt.%) at 973K on a Mo electrode,scan rate:100-500mV/s.Inset (a):relationship between square root of scan rate and peak current density.Inset(b):relationship between the logarithm of scan rate and peak potential.

Fig.10 was a typical square wave voltammograms of the LiCl-KCl-MgCl2-AlCl3system.The result was consistent with the CV test results,further indicating that the reduction of Mg2+and Al3+on the molybdenum electrode were reversible processes.

Fig.10.Square wave voltammograms of LiCl-KCl-MgCl2 (1.0wt.%)-AlCl3(1.0wt.%) on a Mo electrode at 973K with varied frequency from 5Hz to 35Hz.Inset:relationship between square root of frequency and peak current density.

Fig.11.Chronopotentiograms of LiCl-KCl-MgCl2 (1.0wt.%)-AlCl3(1.0wt.%) on a Mo electrode at 973K with varied current density from 0.07 A/cm2 to 0.36 A/cm2.

Fig.11 showed the chronopotentiograms of LiCl-KCl-MgCl2(1.0wt.%)-AlCl3(1.0wt.%)system at different current densities.The current density gradually increased from 0.07 A/cm2to 0.36 A/cm2,and the electrode reaction gradually changed from the precipitation of Al to the co-precipitation of Mg,Al and alkali metals.The precipitation platform of Al can be seen clearly when the current density was 0.07 A/cm2.With the increasing of current density,the concentration of Al3+around the electrode surface gradually decreased due to the increasing of the electrode reaction rate.Meanwhile,the transition time of Al gradually shortened,and the precipitation platform of Mg can be seen clearly when the current density increased to 0.18 A/cm2.When the current density increased to 0.29 A/cm2,the discharge of Mg2+and Al3+cannot meet the current density,and alkali metals began to precipitate.

3.2.Apparent standard potentials of Mg2+/Mg and Al3+/Al

The reduction reactions of magnesium ions and aluminum ions in molten LiCl-KCl are reversible based on the above analysis.Reversibility is crucial in treating processes thermodynamically,since thermodynamics can strictly cover only systems which is in equilibrium.As a thermodynamic parameter,apparent standard potential has more universal values on mirroring the ionic strength of Mg2+in melt which basically dominates the electrochemical reactivity under the experimental conditions.The apparent standard potentials of Mg2+can be define as Eq.(4) [33].

Correspondingly,its equilibrium potential can be expressed by the Nernst equation as Eq.(6) [34].

and the relative Gibbs free energy for formation of MgCl2can be depicted as Eq.(8).

The equilibrium potential of Mg2+/Mg in this system was obtained by open circuit potential method as shown in Fig.12(a),and the equilibrium potential was about -2.74V.Therefore,apparent standard potential and apparent Gibbs free energy can be obtained as -2.52V,-485.71kJ/mol,respectively.Similarly,the equilibrium potential of Al3+/Al was obtained by the open circuit potential method(Fig.12(b)),and apparent standard potential and apparent Gibbs free energy can be found to be -1.66V,-480.78kJ/mol,respectively.The standard Gibbs free energies at 973K for the reactions of magnesium ions and aluminum ions can be obtained from HSC Chemistry,and then the standard reduction potentials can be calculated according to Eq.(8).The standard reduction potential of Mg ion is -2.53V,which is only 0.01V different from the apparent standard potential.Meanwhile,the different between standard reduction potential (-1.77V) and apparent standard potential of Al ion is 0.11V,which is comparatively small.The apparent standard potentials measured under this system is relatively close to the standard reduction potentials.

Fig.12.Open circuit voltammograms.(a) LiCl-KCl-MgCl2 (1.0wt.%) at 973K after electrolysis at -3.80V vs Cl2/Cl- for 3s;(b) LiCl-KCl-AlCl3 (1.0wt.%)at 973K after electrolysis at -3.80V vs Cl2/Cl- for 3s.Working electrode:Mo;counter electrode:graphite.

Fig.13.SEM and EDS analysis (the area enclosed by orange line in the SEM image) of the electrolytic deposits at vary potentials from -2.20V to -3.00V vs Cl2/Cl-.(a) The electrolytic potential is -2.20V;(b) The electrolytic potential is -2.90V;(c) The electrolytic potential is -3.00V.

3.3.Deposits of potentiostatic electrolysis

Fig.14.SEM and EDS analysis of the electrolytic deposits at -3.00V vs Cl2/Cl-.

Potentiostatic electrolysis was carried out in the LiCl-KCl-MgCl2-AlCl3(mass 200 g-50 g-25 g-2.5g) melt on a molybdenum wire at 973K,and the deposits collected in the magnesia crucible.Fig.13 showed the morphology and composition of the electrolyzed products after polishing at different potentials through SEM and EDS.The deposit at -2.20VvsCl2/Cl-manifests the formation of metallic Al as shown in Fig.13(a).Since the deposition potential of Mg was far below -2.20V,the product was almost all Al with only a trace amount of metallic Mg.When the potential was negatively shifted to the deposition potential of magnesium ions,Mg and Al were simultaneously precipitated.When the potential was -2.90VvsCl2/Cl-,the electrolytic product was Mg-Al alloy (Fig.13(b)).As the potential was negatively shifted to-3.00VvsCl2/Cl-,the content of Al in the product droped from 3.23wt.% to 0.55wt.% (Fig.13(c)).These experimental data and test results were summarized in Table 2.Since the electrochemical processes of Mg ions and Al ions were controlled by diffusion,it was easier for Al ions to reach the limit current as the over-potential increases because of the lower concentration of Al ions in the melt.Although the deposition rates of both increased with the increasing over-potential,it was obvious that the deposition rates of Mg ions increased faster than those of Al ions.This can explain why the ratio of Mg/Al in the product increases with the increasing overpotential.

When the sample was etched with a solution of 4.0vol.%HNO3in alcohol with 30s,some grain boundaries can be observed as shown in Fig.14.The content of Al at the grain boundary was significantl higher than that in the unit cell,which indicated Al was more likely to segregate at the grain boundary during the nucleation or solidification

4.Conclusions

The paper systematically investigated the electrochemical reduction mechanism of magnesium ions and aluminum ions on a Mo electrode in the molten LiCl-KCl,and revealed the influenc of applied potentials on the Al content in Mg-Al alloy.Conclusionsare as follows.

(1) The reductions of Mg2+and Al3+are reversible processes controlled by the rate of the mass transfer.When Mg2+and Al3+coexist in LiCl-KCl melt,they have no significan effect on the reduction potential of each other.

(2) The equilibrium potentials of Mg2+/Mg and Al3+/Al were obtained by open circuit potential method.Their apparent standard potentials were also calculated in this system and the values are-2.52VvsCl2/Cl-,-1.66VvsCl2/Cl-,respectively.Correspondingly,the Gibbs free energy of Mg2+and Al3+are -485.71kJ/mol,-480.78kJ/mol.

(3) Potentiostatic electrolysis was performed on a Mo electrode in LiCl-KCl-MgCl2-AlCl3(the mass ratio of MgCl2and AlCl3was 10:1) melt at different potentials.The results showed that the content of Al in the deposit decreased as the over-potential increased and Al tended to segregate at the grain boundaries.

Acknowledgement

The authors thank the National Natural Science Foundation of China (Grant No.51804277).