Electrodeposition of magnesium from CaCl2-NaCl-KCl-MgCl 2 melts

Article abstract of DOI:10.1149/1.1758814

Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic (tungsten and molybdenum) and glassy carbon electrodes. Pure CaCl₂ melt and different alkali chloride solvents including binary CaCl₂-NaCl, equimolar composition, and ternary CaCl ₂-NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl₂ concentrations, were used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic magnesium (923 K) so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process in melts containing CaCl₂ due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Threedimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of the diffusion coefficient of Mg(II) ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was a stable process in the ternary mixture containing MgCl₂, which was related to the presence of 10 mol % KG. Moreover, in this melt a remarkable pseudopassivation process was found at high oyerpotentials.

Full text of DOI:10.1149/1.1758814

C508
Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
0013-4651/2004/151͑7͒/C508/6/$7.00 © The Electrochemical Society, Inc.
Electrodeposition of Magnesium from CaCl2-NaCl-KCl-MgCl2
Melts
a,z
b
A. M. Martınez, B. Børresen,a G. M. Haarberg,a, Y. Castrillejo, and
´
*
*
R. Tunold,a,
aDepartment of Materials Technology, Norwegian University of Science and Technology, 7491 Trondheim,
Norway
b
´
´
Departamento de Quımica Analıtica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid,
Spain
Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic ͑tungsten and molybdenum͒ and
glassy carbon electrodes. Pure CaCl2 melt and different alkali chloride solvents including binary CaCl2-NaCl, equimolar compo-
sition, and ternary CaCl2-NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl2 concentrations, were
used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic
magnesium ͑923 K͒ so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process
in melts containing CaCl2 due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Three-
dimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of
the diffusion coefficient of Mg͑II͒ ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was
a stable process in the ternary mixture containing MgCl2 , which was related to the presence of 10 mol % KCl. Moreover, in this
melt a remarkable pseudopassivation process was found at high overpotentials.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1758814͔ All rights reserved.
Manuscript submitted November 14, 2003; revised manuscript received December 18, 2003. Available electronically June 10,
2004.
Electrolysis of molten magnesium chloride and thermal reduc-
available, thus reducing the scientific dissemination. In addition,
complete and systematic works where different experimental param-
eters are varied ͑melt composition, working temperature, cathode,
etc.͒ are missing, especially those resembling industrial conditions.
A wide variety of cathode materials has been used, but due to
industrial applications, iron and stainless steel are the substrates
used by most of the authors,3-10 although tungsten, platinum, and
glassy carbon have also been used.10-14 Kannan et al.4 indicated that
the current efficiency in the electrolytic cells for magnesium de-
pends much upon the wettability of the cathode surface by the mol-
ten metal and the electrolyte, but due to several experimental restric-
tions in measuring wetting angles, only ex situ visual observations
of nine different steel cathodes after electrolysis were carried out.
All authors conclude that magnesium deposition from molten
chlorides is a simple diffusion-controlled process involving a two-
electron transfer. However, Kisza et al.15 found the magnesium re-
duction reaction on liquid magnesium electrodes and in pure molten
magnesium chloride at temperatures ranging from 750 to 810°C, to
follow a two-step electrode mechanism, the high-frequency process
being pure charge transfer with the low-frequency process showing
mixed charge-transfer-diffusion character.
tion of magnesium oxide are the two principal production processes
in use today. Although silicothermic reduction has become more
important in recent years, currently most of the production on a
worldwide scale is still by electrolysis.1
Different electrolytic methods can be distinguished, depending
on the cell feed ͑raw material and preparation of magnesium chlo-
ride͒, design of the cells, and utilization of the by-product, chlorine.
The dehydration methods ͑producing anhydrous magnesium chlo-
ride͒ are all energy intensive and expensive but give many benefits,
mainly related to minimal consumption of the graphite electrodes,
which permits a fixed cell configuration and recovery of concen-
trated chlorine as a by-product.2
Good wettability of the cathode substrate by the melt and the
magnesium deposit is one of the most important factors in electro-
lytic production of magnesium. Experimental results have shown
that the magnitude of the magnesium losses during the electrolysis is
principally determined by the conditions of the deposition of mag-
nesium on the cathode, the coalescence of the magnesium droplets
in the electrolyte, and by the design of the electrolyzer.2 These pa-
rameters depend on the physicochemical properties of the electro-
lytes, particularly on their interfacial properties. For example, if the
interfacial tension at the electrolyte/cathode is relatively higher than
at the cathode/liquid magnesium, liquid magnesium is deposited on
the cathode as a continuous layer and readily coalesces into large
droplets. Under such conditions the losses of metallic magnesium
during the electrolysis are minimal and maximum current yield is
attained. Major amounts of dispersed magnesium in the electrolyte,
which is difficult to harvest, are then absent.
Some authors also performed nucleation studies during magne-
sium electrodeposition on foreign substrates. It was found that the
magnesium electrodeposition follows an instantaneous nucleation
process with three-dimensional growth of the deposit.5,8,16
Many authors studied the effect of impurities, mainly metal17 and
oxide ions6,12,18,19 in the electrodeposition process. In hydrous melts
a passivation effect in cathodic polarization was found, which was
explained by formation of insoluble MgO at the electrode surface.18
It is also pointed out that the MgO adsorbed on magnesium particles
interferes with the cohesion of Mg droplets.19
Cathodes of steel are used in industrial electrolytic production of
magnesium due to their stability and low cost, but they have poor
wetting properties and in addition, the magnesium produced is con-
taminated by iron due to corrosion. Thus, alternative cathode mate-
rials should be proposed.
Extensive electrochemical studies of magnesium in molten chlo-
rides have been carried out through the years. The effect of the
cathode material and impurities in the electrodeposition process
were the main subjects of those investigations. Most of these articles
are written in Chinese and Russian and only English abstracts are
Børresen et al. studied the charge-transfer and diffusion kinetics
of the magnesium electrodeposition process from halide melts,
mainly pure magnesium chloride melt.20 Authors also studied the
influence of oxide ions as well as metal dissolution in the elec-
trodeposition process. Spectroscopic studies of the so-called metal
fog were also carried out.21
Because alkali and alkaline-earth fluorides are used as effective
agents to prevent passivation of the cathode surface to a consider-
able extent, many authors also studied the influence of fluoride ions
in the magnesium discharge.7,20,22,23 Observations showed that fluo-
rides cause active breakdown of the passivating films, which gradu-
ally peels away from the cathode.22 Rao23 concluded that the ben-
* Electrochemical Society Active Member.
z E-mail: martinez@material.ntnu.no
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Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
C509
Figure 1. Voltammograms obtained in the pure CaCl2 melt with different
MgCl2 contents. T ϭ 1123 K. Tungsten substrate. ϭ 0.2 V sϪ1
Figure 2. Chronoamperograms obtained in the pure CaCl2 melt containing
5 mol % MgCl2 . T ϭ 1123 K. Tungsten substrate. Applied potentials vs.
Mg reference electrode.
.
v
eficial effect of sodium fluoride addition is attributed to the fluxing
action of fluoride on the surface oxide of the electrolytically pro-
duced magnesium droplets. It is also reported that the addition of
large amounts of fluoride into magnesium electrolytes may lead to
undesirable discharge of alkali and/or alkaline-earth metals at the
cathode if the limiting-current region is reached.22
The present study is part of a research project with the goal of
understanding the factors that affect the formation of a magnesium
deposit with the appropriate characteristics. Different cathode mate-
rials were tested: glassy carbon rods and metallic wires of tungsten
and molybdenum, although the reported results are mainly focused
on the use of tungsten electrodes in calcium chloride-based melts.
tammograms obtained on a tungsten electrode when various
amounts of MgCl2 were added to the pure CaCl2 melt. The wave
B/B corresponds to electrodeposition of liquid Mg which is reoxi-
Ј
dized in the anodic scan. In this case, and contrary to what has been
observed in other molten chloride media20 ͑discussed later in this
paper͒, no evidence of underpotential deposition ͑UPD͒ of Mg was
observed on any of the employed substrates. C/C corresponds to
Ј
formation and subsequent reoxidation of calcium metal.
In addition, a small oxidation wave D , related to an inflection of
Ј
the cathodic wave D, is observed. This is most likely related to the
formation of a Ca-Mg alloy ͑dissolution of liquid calcium in the
already deposited liquid magnesium͒. Dissolution of calcium metal
into the melt can also be observed in Fig. 1 by the shape of oxida-
Experimental
The experiments were carried out inside a glove box ͑VAC
model HE-43-2͒ in an inert argon atmosphere. The chloride mix-
tures were melted in a glassy carbon crucible ͑Carbone Lorraine͒
placed in a quartz cell inside a furnace. A Eurotherm model 2404
programmable device controlled the temperature of the furnace
͑Ϯ2°C͒, which was measured with a thermocouple ͑Pt/Pt-Rh 10%͒
protected by an alumina tube inserted into the melt.
The working electrodes consisted of glassy carbon rods ͑3 mm
diam, Tokai Carbon Co., grade GC-20S͒ or tungsten and molybde-
num wires ͑1 mm diam, Alfa, 99.995%͒. Another glassy carbon rod,
the glassy carbon crucible, or a magnesium electrode was used as
counter electrode. The reference electrode consisted of a magnesium
electrode ͑solid magnesium rod or magnesium pool, depending on
the working temperature, Riedel-DeHaen, 99.8%͒. In a few experi-
ments, a Ag͉AgCl electrode was used. The preparation of such an
electrode has been described previously ͑e.g., Ref. 24͒. Unless oth-
erwise stated, all potentials are referred to the Mg͉Mg͑II͒ system.
The lower end of the glassy carbon and tungsten electrodes was
polished thoroughly by using SiC paper and 1 ␮m diamond paste.
They were then cleaned in ethanol using an ultrasonic bath followed
by heating under high vacuum.
tion wave C , which is typical for oxidation of dissolved species
Ј
rather than a deposit. This is due to the high solubility of calcium in
CaCl2 melts, reported as 2.7 mol % at 1093 K.25
Deposition processes on foreign substrates may often be studied
by potentiostatic current transients. The current tends to increase
with time if the deposited phase is conducting. The time dependency
of the current increase depends on the morphology or shape of the
growth of the deposited nuclei in a microscale. Theoretical current-
time behavior for various situations is reported in the literature.26,27
During the growth process the current increases until the diffusion
fields of the independent growth centers are overlapping. For longer
times, an increasing current indicates a growth of the deposit on a
macroscale.
Current transients recorded on tungsten substrates in this melt
͑see Fig. 2͒ show characteristic features due to nucleation and
growth phenomena of the magnesium deposit at high overpotentials
and MgCl2 concentrations around 5 mol % and higher. The increase
in current in the chronoamperometric curves is due to nucleation and
growth of the magnesium droplets, which after some time tend to
coalesce, giving a steady value of the current.
Purification of NaCl and KCl consisted of heating the salt to
600°C ͑approximately͒ under vacuum. Dehydrated CaCl2 and
MgCl2 were prepared from commercial CaCl2•2H2O and
MgCl2•6H2O, respectively, by drying them carefully under HCl at-
mosphere. The dried MgCl2 was subsequently distilled under
vacuum. The dried CaCl2 was used without further purification.
The pure salts were stored and handled inside a glove box ͑M-
BRAUN Labmaster 130͒ with inert argon atmosphere. The moisture
and oxygen levels inside the box were kept below 1 ppm.
The rising part of the curves was analyzed according to the cri-
teria from the literature,26,27 indicating that the nucleation process is
three-dimensional with instantaneous growth of the nuclei ͑see in-
serted graph in Fig. 2͒.
The high surface tension of this type of molten chlorides, which
often implies a high interfacial tension between the deposit, Mg, and
the electrode, W, makes possible the formation of a deposit formed
by magnesium ‘‘globules’’ rather than homogeneous/continuous lay-
ers.
The CaCl2-NaCl-MgCl2 system at 823 and 1000 K.—
Voltammograms on W substrates obtained in the CaCl2-NaCl ͑45:55
mol %͒ mixture containing dilute MgCl2 solutions at a temperature
Results
The CaCl2-MgCl2 system at 1123 K.—Figure 1 shows some vol-
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C510
Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
Figure 4. Voltammogram obtained in molten CaCl2-NaCl ͑45:55 mol %͒
melt at 823 K containing 0.6 mol % MgCl2 . Glassy carbon substrate.
v
ϭ 0.2 V sϪ1
.
ing 0.6 mol % MgCl2 on a glassy carbon electrode ͑working tem-
perature 823 K͒. Figure 4 shows a typical voltammogram obtained
during electrodeposition of solid magnesium.
Apart from the electrochemical signals corresponding to magne-
sium electrodeposition/oxidation (B/B ), one observes a high back-
Ј
ground current ͑F͒. Taking into account the voltammogram obtained
in the pure melt ͑without MgCl2), one may assume that it is mainly
due to sodium deposition which is enhanced by Na intercalation into
the carbon structure. The current wave C is due both to Na-C inter-
calation compounds and Ca-Mg alloy formation, although the first
process is probably predominant.
In any case, the shape of the B/B wave is characteristic of the
Ј
formation of a new phase. This has been confirmed by examination
of the voltammograms, because the ratio of the anodic to the ca-
thodic current was higher than unity, and the ratio of the total anodic
to the total cathodic charge was close to unity and independent of
scan rate.
It was possible to calculate the diffusion coefficient of Mg͑II͒
ions by cyclic voltammetry ͑CV͒ and convolution of voltammo-
grams by applying Eq. 1 and 2, respectively
Figure 3. Voltammograms obtained in molten CaCl2-NaCl ͑45:55 mol %͒
melt at ͑a͒ 823 and ͑b͒ 1000 K containing 0.6 mol % MgCl2 at different
cathodic limits. Tungsten substrate, ϭ 0.2 V sϪ1
.
v
below the melting point of magnesium ͑see Fig. 3a͒ showed similar
features to those obtained under the same conditions in an equimolar
CaCl2-NaCl melt, which were previously reported.28
3/2
I ϭ 0.61 nF͒ RT͒Ϫ1/2ACoD
͓1͔
͓2͔
͑
͑
1/2v1/2
p
1/2
*
m
ϭ nFACoD
The electrochemical reduction of Mg͑II͒ ion also takes place in a
single step (B/B ). The current peaks D/D and E/E probably cor-
Ј
Ј
Ј
where Ip is the peak current of the voltammograms, Co the bulk
concentration of the electroactive species, D the diffusion coeffi-
respond to the formation of solid Ca-Mg alloys29,30 due to dissolu-
tion of calcium in the solid magnesium already deposited on the
*
cient, A the electrode surface area, and m the maximum value of
tungsten electrode. D/D and E/E could also be due to Na-Mg alloy
Ј
Ј
the convoluted current of the voltammograms.
formation, although the solubility of liquid sodium in solid magne-
The results obtained on the different substrates are gathered in
Table I. Results obtained on glassy carbon electrodes are considered
to be inconsistent due to the high background current from Na in-
tercalation in the carbon lattice, although the difference is rather
small.
sium is small.29,31 The cathodic limit of the melt (C/C system͒
Ј
corresponds to liquid sodium and/or solid calcium deposition and
subsequent reoxidation. Voltammograms obtained at molybdenum
substrates showed similar features.
When the working temperature was increased above the melting
point of metallic magnesium ͑see Fig. 3b͒, the voltammetric curves
show a more pronounced UPD of calcium on the liquid magnesium
already deposited at the tungsten substrate, probably due to forma-
tion of a Ca-Mg liquid solution.29 The solubility of Ca and/or Na
metal in the electrolyte increases with temperature, as seen from the
The diffusion coefficient values obtained directly from CV are
lower than those obtained by convolution of the voltammetric
curves. This could be attributed to the simultaneous Ca/Na deposi-
tion which would influence to a larger extent when the time scale of
the measurements is increased.
Chronoamperometric curves obtained in the dilute MgCl2 system
͑0.6 mol %͒ showed Cottrell behavior for all the substrates. How-
ever, at higher magnesium chloride contents ͑5-10 mol %͒, the chro-
noamperometric curves evidenced nucleation and growth of a liquid
or solid Mg deposit on metallic electrodes ͑W and Mo͒ at high
overpotentials ͑see Fig. 5a and b͒, which cause an increase of the
electroactive area. It is also expected that the interfacial properties
are changing due to Ca-Mg alloy formation.
shape of the reoxidation peak C .
Ј
One should emphasize the absence of magnesium UPD on both
metallic substrates. When using carbon substrates a significant de-
polarization of the reduction of sodium ions is usually observed due
to sodium penetration into the carbon lattice to form a very stable
sodium-carbon bond. Despite this, it was possible to study magne-
sium deposition in the molten CaCl2-NaCl ͑45:55 mol %͒ contain-
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Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
C511
Table I. Diffusion coefficient of Mg„II… species obtained by dif-
ferent electrochemical techniques in the molten CaCl2-NaCl
„45:55 mol %… melt at different temperatures.
Dគ 105 ͑cm2 sϪ1
͒
Technique
T ͑K͒
Substrate
Voltammetry
823
W
Mo
GC
0.7
0.5
0.6
1000
823
W
Mo
1.5
1.7
Convolution
W
Mo
GC
1.3
1.2
1.5
The rising part of the curves was analyzed according to the cri-
teria from the literature,26,27 indicating that the nucleation process
also here is three-dimensional with instantaneous growth of the nu-
clei ͑see inserted graph in Fig. 5a and b͒.
The CaCl2-NaCl-KCl-MgCl2 system at 1000 K.—Figure 6 shows
a typical voltammogram obtained in CaCl2-NaCl-KCl ͑35:55:10
mol %͒-5 mol % MgCl2 at 1000 K when using a tungsten substrate.
Molybdenum electrodes showed a similar behavior.
The current peaks A/A were associated to UPD of magnesium
Ј
on tungsten. The charge of these current peaks corresponds to the
formation of 1-2 monolayers. The reason why UPD appears in this
melt and not in the other calcium-based melts previously studied is
attributed to the differences in surface tensions of the melts, which
affects the interfacial properties.2
Figure 6. ͑a͒ Voltammogram obtained in molten CaCl2-NaCl-KCl ͑35:55:10
mol %͒ melt at 1000 K containing 5 mol % MgCl . Tungsten substrate.
v
2
ϭ 0.2 V sϪ1. ͑b͒ Detail of the voltammetric curve shown in ͑a͒.
Figure 7 shows a CV obtained under the same conditions as in
Fig. 6, but in this case the reversal potential was much more ca-
thodic. Apart from the current peaks A/A and B/B ͑the first one
Ј
Ј
exists but is hidden by the large scale of the figure͒, a curious effect
was observed. The current suddenly decreases at a cathodic potential
corresponding to the point labeled 1 up to point 2, where the current
increases again ͑cathodic limit of the melt͒. In the reverse scan the
Figure 7. Voltammograms obtained in pure molten CaCl2-NaCl-KCl
͑35:55:10 mol %͒ melt at 1000 K and containing 5 mol % MgCl2 . Tungsten
Figure 5. Chronoamperograms obtained in molten CaCl2-NaCl ͑45:55 mol
%͒ melt containing 5 mol % MgCl2 . T ϭ (a) 823 and ͑b͒ 1000 K. Tungsten
substrate. Applied potentials vs. Mg reference electrode.
substrate. ϭ 0.2 V sϪ1
.
v
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C512
Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
In addition, it is well known that the intrinsic interfacial proper-
ties of the molten chlorides can change because of the presence of
moisture in the electrolyte2 being a cause of inferior wetting of the
cathode by the melt. It is known from industrial practice that if the
electrolysis cell is not sufficiently dry, which is difficult to achieve
due to the hygroscopic nature of the chlorides used, the moisture
present during the electrolysis prevents metallic magnesium from
wetting the cathode properly, irrespective of the composition of the
electrolyte.2
All these points make it difficult to extract definitive conclusions
supported by literature data.
Interfacial properties of CaCl2ϪxMgCl2 melts may be responsible
for forming three-dimensional deposits ͑magnesium spheres͒ rather
than layers. It is expected that the magnesium spheres will coalesce
to form a uniform deposit if there is a good wetting of the cathode
by metallic magnesium. The results obtained in this type of melt
have basically shown no existence of monolayers of magnesium but
nucleation and growth phenomena of the magnesium deposits on the
metallic substrates. Nucleation and growth phenomena are more im-
portant when increasing the content of MgCl2 in the melts due to an
increase in the growth rate of the nuclei.33 There is a significant
solubility of calcium metal in the magnesium deposit, giving a
Mg-Ca liquid solution. This leads to an important UPD of calcium,
which is more pronounced at lower contents of MgCl2 in the melt
Figure 8. Chronoamperograms obtained in molten CaCl2-NaCl-KCl
͑35:55:10 mol %͒ melt containing 5 mol % MgCl2 . T ϭ 1000 K. Tungsten
substrate. Applied potentials vs. Mg reference electrode.
cathodic current decreases again when reaching point 2 and in-
creases again at the same point as in the forward scan ͑point 1͒.
Chronoamperometric curves showed that this behavior could be
due to a passivation process. From Fig. 8 one observes that the
curves have the expected features down to Ϫ0.300 V. At applied
cathodic potentials ranging from Ϫ0.350 to Ϫ0.600 V, the curves
have the typical habits of a pseudo-passivation process.27 These po-
tential values correspond to the potential region from point 1 to 2 in
the voltammogram from Fig. 7.
This behavior could be explained by the formation of a discon-
tinuous passivation layer the nature of which could have two expla-
nations: ͑i͒ local formation of a solid compound ͑e.g., KCl•CaCl2 ,
melting point 749°C͒ at some points of the electrode surface, and ͑ii͒
formation of a discontinuous passivating oxide layer ͑MgO͒ on top
of the already deposited metallic magnesium.
due to an increase in the calcium chloride activity.31
In the case of the CaCl2-NaCl-xMgCl2 melts the results are
similar, showing that nucleation and growth are important phenom-
ena in the formation of both solid and liquid magnesium deposits,
although continuous layers are formed easier when depositing mag-
nesium in the liquid state ͑steady currents in the chronoamperomet-
ric curves are reached earlier͒. Due to solubility of calcium metal in
magnesium, UPD of calcium also takes place in this kind of melt. In
the case of a dilute solution of MgCl2 in the molten mixture, current
waves corresponding to the formation of solid Ca-Mg alloys or for-
mation of a liquid Ca-Mg solution at temperature below or above
the melting point of magnesium, respectively, are present. It is less
likely that the signals are due to Na-Mg alloy formation due to low
solubility of sodium in magnesium.
The results were somewhat different when using
CaCl2-NaCl-KCl ͑35:55:10 mol %͒ melt containing 5 mol % MgCl2
at 1000 K as electrolyte. KCl has a positive effect on the wetting
angle,2 thus enabling the formation of a stable magnesium mono-
layer on tungsten and molybdenum substrates. There is another im-
portant feature in this type of melt and that is the passivation process
that takes place at high cathodic overpotentials ͑around Ϫ0.350 V͒
and which has been observed specially in the chronoamperometric
curves. The nature of the passivating layer is uncertain, but one can
think of two possible explanations:
1. The local concentration of the different molten chlorides at
some points in the electrode interface is such that the formation of
the stable compound KCaCl3 takes place. This compound is solid at
the working temperature ͑melting point 749°C͒ and its precipitation
could cause a pseudopassivation of the electroactive surface.
2. Formation of a discontinuous MgO passivating layer on top of
the previous deposited drops of liquid magnesium. The formation of
MgO can be explained by the dramatic decrease of magnesium ox-
ide solubility at the electrode surface, where magnesium chloride
concentration drops to zero.34
Discussion
Interfacial properties of the molten media are important to con-
sider regarding interpretation of the results obtained in the elec-
trodeposition studies of magnesium from calcium chloride melts.
Other phenomena that could affect the electrodeposition process of
magnesium could be metal solubility and formation of metal fog or
interactions with the substrate, which are also related to interfacial
properties. However, it is difficult to obtain information about all
those properties and then extract the proper conclusions.
Surface tension values of different molten chlorides at the gas
interface are found in the literature.2 According to some authors the
surface tension at the gas interface of molten magnesium, calcium,
potassium, and sodium chlorides and their mixtures corresponds to
the surface tension of these mixtures at a solid interface. Then, if the
surface tension of the chlorides in the studied molten systems is
known at the gas interface, an approximate idea of their surface
tensions at a solid cathode interface may be obtained. Accordingly,
the variation of the surface tension of the electrolyte with its com-
position and temperature must be known. However, it is expected
that different values will be obtained depending on the solid cathode
material. Most of the data found in the literature correspond to iron,
graphite, and MgSiO3 substrates.32 Moreover, it is unsatisfactory to
consider the wetting angle of the cathode surface by the electrolyte
as the only criterion of wettability of the cathode by the liquid mag-
nesium, although experimental results support this theory in the case
of iron substrates.2
Conclusion
The formation of monolayers of magnesium on the solid sub-
strates, tungsten and molybdenum, was found to become stable by
increasing the content of KCl and decreasing CaCl2 in the molten
chloride. Homogeneous layers of deposits that cover the whole cath-
ode surface were then obtained.
The currently studied calcium chloride-based melts without any
potassium chloride give rise to three-dimensional magnesium depos-
its that grow macroscopically on the substrates. This also affects the
According to data obtained in the literature32 there is no clear
correlation between surface tension and interfacial tension of several
molten chlorides. It is then difficult to predict interfacial properties
by simply considering the surface tension values usually listed in the
literature.
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Journal of The Electrochemical Society, 151 ͑7͒ C508-C513 ͑2004͒
C513
3. M. L. Rudnitskii, V. B. Freidlin, V. N. Devyatkin, A. E. Stolina, and V. L. Rud-
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calculation of the diffusion coefficient of electroactive species by
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alternatively, to the precipitation of solid KCl•CaCl2 at some points
on the electrode surface.
From the obtained results one could give some recommendations
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studied for electrolytic production of magnesium. When it comes to
substrates, glassy carbon does not seem to be a good choice due to
formation of intermetallic compounds with the cations present in
any of the studied molten chlorides. Tungsten and molybdenum
could very well be used because both of them have a similar behav-
ior and possess good wetting properties.
Moreover, the obtained results showed that in the studied cal-
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due to the existing passivation phenomena. CaCl2-NaCl-MgCl2 and
CaCl2-MgCl2 mixtures possess similar characteristics. However,
when dealing with industrial processes there are some other aspects
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Acknowledgments
The Research Council of Norway and Norsk Hydro ASA are
acknowledged for financial support. A.M.M. also thanks the ‘‘Sec-
retarıa de Educacion, Universidades, Investigacion y Desarrollo’’
͑Spain͒ for a postdoctoral grant. Oddmund Wallevik and Dr. Chris-
tian Rosenkilde, Norsk Hydro ASA, are acknowledged for fruitful
discussions.
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Canada.
Norwegian University of Science and Technology assisted in meeting the
publication costs of this article.
˚
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