Co-deposition of Al-Zn on AZ91D magnesium alloy in AlCl3-1-ethyl-3-methylimidazolium chloride ionic liquid

Article abstract of DOI:10.1016/j.electacta.2009.11.050

The co-deposition of Al and Zn on AZ91D magnesium alloy from a Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl₃-EMIC, with a molar ratio of 60:40) ionic liquid containing 1 wt% ZnCl₂ at room temperature was studied. The effect of potential on the deposition rate, the microstructure and the chemical composition of the deposit was explored. The experimental results show that the simultaneous deposition of Al and Zn on Mg alloy can be achieved under properly controlled potential conditions. The deposition rate increased while the amount of Zn existing in the coating decreased with increasing negative deposition potential. In the ionic liquid studied, a uniform chemical composition of the coating was obtained when the deposition was performed at -0.2 V (vs. Al).

Full text of DOI:10.1016/j.electacta.2009.11.050

Electrochimica Acta 55 (2010) 2158–2162
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Co-deposition of Al–Zn on AZ91D magnesium alloy in
AlCl3–1-ethyl-3-methylimidazolium chloride ionic liquid
Szu-Jung Pana, Wen-Ta Tsaia,∗, Jeng-Kuei Changa, I-Wen Sunb
a Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
b Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The co-deposition of Al and Zn on AZ91D magnesium alloy from a Lewis acidic aluminum chloride–1-
ethyl-3-methylimidazolium chloride (AlCl3–EMIC, with a molar ratio of 60:40) ionic liquid containing
1 wt% ZnCl2 at room temperature was studied. The effect of potential on the deposition rate, the
microstructure and the chemical composition of the deposit was explored. The experimental results
show that the simultaneous deposition of Al and Zn on Mg alloy can be achieved under properly con-
trolled potential conditions. The deposition rate increased while the amount of Zn existing in the coating
decreased with increasing negative deposition potential. In the ionic liquid studied, a uniform chemical
composition of the coating was obtained when the deposition was performed at −0.2 V (vs. Al).
© 2009 Elsevier Ltd. All rights reserved.
Received 14 August 2009
Received in revised form
13 November 2009
Accepted 14 November 2009
Available online 22 November 2009
Keywords:
Ionic liquid
Magnesium alloy
Aluminum
Zinc
Co-electrodeposition
1. Introduction
between the substrate and the coating is another important char-
acteristic to be considered. To enhance the adhesion strength
Magnesium and its alloys are active and have poor corrosion
resistance even in mildly corrosive environments [1,2]. To expand
their application, surface treatments, such as anodization [3–6], are
frequently applied to increase the corrosion resistance of compo-
nents made of magnesium alloys. Although, electroplating using
an aqueous solution bath is commonly used for the surface finish-
ing of steels, it may not be feasible for magnesium alloys because
of their active nature in aqueous environments. The use of ionic
liquids as a substitute for aqueous electrolytes is thus of interest
and worthy of study. Electrodeposition of Al on copper, platinum
and tungsten, involving chloroaluminate ionic liquids, has been
reported by several investigators [7–10]. The co-deposition of Al
with other metallic components has also been explored [11–13].
A previous study proved that acidic aluminum chloride–1-ethyl-
3-methylimidazolium chloride ionic liquids (AlCl3–EMIC) can be
used as an electrolyte for the electrodeposition of Al on a mag-
nesium alloy surface [14]. Successful Al electrodeposition can be
performed in ionic liquids with proper AlCl3 to EMIC molar ratios
[15]. Improved corrosion resistance has been demonstrated in
polarization and electrochemical impedance spectroscopy (EIS)
studies [16]. Beside the corrosion resistance, the adhesion strength
between the metallic coating and the Mg alloy substrate, the forma-
tion a metallurgical bounding by employing heat treatment may be
applied. The chemical composition compatibility of the interdiffu-
sion zone after heat treatment may give rise to improved adhesion
strength. Since Al and Zn are the common major alloying elements
of the Mg alloys, co-deposition of Al and Zn may be beneficial for the
formation of an interdiffusion zone with a chemical composition
complying with the substrate. Thus, the feasibility of the simul-
taneous deposition of Al and Zn is the main focus of this study.
Though, the co-deposition of Al with Ti, Zr, Hf, V, Cr, Ni, Co, and Cu
in ionic liquids on inert substrates, such as glassy carbon, W, Ni,
Cu, and Pt has also been reported [11–13,17], Al–Zn co-deposition,
especially on magnesium alloys, has yet to be investigated. In this
investigation, the co-deposition of Al–Zn on an active magnesium
alloy is conducted in ionic liquid. The material characteristics of the
deposited film are examined and analyzed.
2. Experimental
An acidic ionic liquid with AlCl3 to EMIC molar ratio of 60:40
was prepared by the slow addition of AlCl3 (anhydrous powder,
99.99%, Aldrich) into EMIC (99%, Aldrich) at room temperature. The
mixture was continuously stirred by a magnetic paddle for 24 h to
ensure uniformity. For brevity, the AlCl3–EMIC mixture consisting
of a 60% molar fraction of AlCl3 is denoted as 60 m/o ionic liquid
∗
Corresponding author. Tel.: +886 6 2757575x62927; fax: +886 6 2754395.
E-mail address: wttsai@mail.ncku.edu.tw (W.-T. Tsai).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.11.050

S.-J. Pan et al. / Electrochimica Acta 55 (2010) 2158–2162
2159
in this paper. This electrolyte was used for plain Al deposition. For
the co-deposition of Al and Zn, 1 wt% anhydrous ZnCl2 (99.99%)
was added and dissolved directly in the AlCl3–EMIC ionic liquid to
prepare a plating bath with the desired composition. All chemicals
were handled in a glove box under a purified nitrogen atmosphere.
Within the glove box, both the moisture content and the oxygen
content were maintained below 1 ppm.
1.5 V. Curve (a) in Fig. 1 displays the cyclic voltammogram obtained
in 60 m/o AlCl3–EMIC without ZnCl2 addition, in the potential range
of −0.5 to 2.5 V. As reported previously [14–16], when the AlCl3
molar ratio exceeds 50%, the following reaction takes place:
−
AlCl4− + AlCl3 → Al2Cl7
(1)
Under cathodic polarization condition, the following reaction can
occur to form metallic Al:
A diecast AZ91D Mg alloy with 9.02 wt% Al and 0.49 wt% Zn was
used as the substrate in this study. Before electrodeposition, each
sample was ground with SiC papers to a grit of 2000 inside the
glove box. A three-electrode cell controlled by an EG&G Model 273A
potentiostat/galvanostat was employed for electroplating. The Mg
alloy, with a total exposed surface area of about 0.06 cm2, was
assembled as the working electrode while an Al wire placed in a
separate fritted glass tube containing the 60 m/o ionic liquid was
used as the reference electrode. The counter electrode for plain Al
deposition was an Al spiral, which was directly immersed in the
plating bath; for Al–Zn co-deposition, a Zn spiral was used instead.
Electrodeposition was performed at room temperature under con-
stant potential mode. The potentials applied were −0.1, −0.2, −0.3,
4Al2Cl7− + 3e− → Al + 7AlCl4
(2)
−
In curve (a), there is a noticeable increase in the cathodic current
density at potentials below approximately −0.15 V, corresponding
to the reduction of the Al2Cl7− precursor in the ionic liquid to form
Al deposit. The intercept of the curves for the forward and the back-
ward scans is at approximately −0.08 V, above which Al deposition
does not occur. The existence of a cathodic current loop, as shown
in curve (a), indicates that nucleation overpotential was required
for Al deposition on the glassy carbon surface. On the reverse scan,
the sharp anodic peak in curve (a) corresponds to the dissolution
of Al formed in the forward scan.
In Fig. 1, curves (b) and (c) show the cyclic voltammograms for
various potential ranges with the addition of 1 wt% ZnCl2 to the
60 m/o ionic liquid. Curve (b) depicts the cyclic voltammogram
obtained at potentials ranging from 0 to 1.5 V. The cathodic scan
shows that an abrupt increase in current density occurred at about
+0.19 V, indicating the reduction of Zn. The current loop shown in
curve (b) suggests a nucleation rate controlled deposition process
for zinc reduction. The intercept of the reverse scan at zero cur-
rent density was found at +0.24 V, above which Zn could not be
reduced. The peak in the anodic scan is associated with the dissolu-
tion of Zn. The cyclic voltammogram for an enlarged potential scan
range of −0.5 to 2.5 V is shown as curve (c) in Fig. 1. Two cathodic
humps were observed showing the sequential reduction of Zn and
Al. The initial potential for Zn reduction, as indicated in curve (c)
was +0.19 V, identical to that revealed in curve (b). A decrease in
the current density in the first hump in curve (c) suggests that the
depletion of Zn2+ ions on the electrode surface occurred during the
cathodic scan. The potential at which the current density began
to rise again corresponds to Al reduction. As revealed in curve (a),
the reduction of Al started at potentials below −0.08 V; thus, the
co-deposition of Zn and Al can occur below this potential.
and −0.4 V, respectively, to yield a total passed charge of 50 C cm−2
.
After deposition, the samples were thoroughly cleaned with dis-
tilled ethanol and then dried in air. The surface morphologies and
chemical compositions of the deposits were examined with a scan-
ning electron microscope (SEM, Hitachi SU-1500) and its auxiliary
X-ray energy dispersive spectroscope (EDS).
3. Results and discussion
Fig. 1 shows the cyclic voltammograms of a glassy carbon elec-
trode obtained at room temperature in the 60 m/o ionic liquid
with/without 1 wt% ZnCl2. The voltammograms were obtained by
scanning the potential at a rate of 0.05 V s−1 from open circuit
potential (approximately 1.0 V) towards the negative direction to
a preset value, and then reversed in the anodic direction to 2.5 or
During reverse scanning, the rates for both Al and Zn reductions
decreased. When the potential reached approximately −0.08 V,
deposition of Al ceased but Zn2+ could still be reduced until +0.24 V.
Thus, in the potential range of −0.08 to +0.24 V, the reduction of
Zn2+ to form Zn and the dissolution of Al to form Al3+ simultane-
ously took place. The (a1) peak in curve (c) corresponds to the net
current density during scanning in this potential range. The second
anodic peak (a2) is associated with the dissolution of both Al and
Zn.
Electrodeposition of Al or Al–Zn on AZ91D Mg alloy was per-
formed under constant applied potential condition. The variation
of current density with time for Al deposition in 60 m/o ionic liquid
(without ZnCl2) at an applied potential of −0.2 V is shown as curve
(a) in Fig. 2. The current density increased rapidly at the beginning
and then reached a steady state value at about −22.5 mA cm−2
.
With the addition of 1 wt% ZnCl2 in the 60 m/o ionic liquid, the
variation of current density with time at an applied potential of
−0.2 V is shown as curve (b) in Fig. 2. As in curve (a), a rapid
increase of the current density was followed by a steady growth
of the Al–Zn deposit. However, the steady state current density in
curve (b) is about −18 mA cm−2, lower than that of curve (a). For
a total cathodic passed charge of 50 C cm−2, the times required to
deposit Al and Al–Zn at −0.2 V were 2250 and 2640 s, respectively.
The addition of ZnCl2 decreased the deposition rate. It has been
pointed out that the ZnCl2–EMIC ionic liquids having a ZnCl2 mole
Fig. 1. Cyclic voltammograms on a glassy carbon electrode at room temperature
and at a scan rate of 50 mV S−1: (a) 60 m/o AlCl3–EMIC (scan range: −0.5 to 2.5 V),
(b) 1 wt% ZnCl2 + 60 m/o AlCl3–EMIC (scan range: 0 to 1.5 V) and (c) 1 wt% ZnCl2 + 60
m/o AlCl3–EMIC (scan range: −0.5 to 2.5 V).

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S.-J. Pan et al. / Electrochimica Acta 55 (2010) 2158–2162
Fig. 2. Effects of ionic liquid composition and applied potential on the variation of
current density with time during electrodeposition.
fraction less than 0.33 are basic [18,19]. In this investigation, the
amount of ZnCl2 added into the acidic 60 m/o AlCl3–EMIC ionic
liquid was small. Thus, it was very likely that 1 wt% ZnCl2 addition
would neutralize and decrease the acidity of the 60 m/o AlCl3–EMIC
ionic liquid. As a result, the concentration of Al2Cl7− decreases with
the decrease in acidity. It has been reported [15] that the deposi-
tion rate decreases as the acidity of the electrolyte decreases. The
relatively low reduction rate observed with the addition of 1 wt%
ZnCl2 was probably attributed to the increase in basicity of the ionic
liquid.
When the deposition potentials were controlled at −0.3 and
−0.4 V, respectively, the cathodic steady state current densities
increased, as shown in curves (c) and (d) in Fig. 2. For a total cathodic
passed charge of 50 C cm−2, the times required to deposit Al–Zn
at −0.3 and −0.4 V were 1920 and 1330 s, respectively. The results
show that the deposition rate increased with increasing magnitude
of cathodic potential. At a total cathodic passed charge of 50 C cm−2
,
the thickness of the deposit was approximately 15–18 ␮m. The
deposition rate depends not only on the applied potential, but also
on the applied current density or AlCl3 concentration, as reported
previously [14–16].
Fig. 3. (a) SEM micrograph, (b) EDS result, and (c) cross-section micrograph of the
deposit formed in 1 wt% ZnCl2 + 60 m/o AlCl3–EMIC ionic liquid at −0.1 V.
Surface morphologies of various Al–Zn coated AZ91D alloys
were examined by SEM. Under an applied potential of −0.1 V, a
very rough surface was observed, as shown in Fig. 3(a). Outward
extruded dendrites formed on the surface. These dendrites were
mainly composed of Zn, as identified by EDS and shown in Fig. 3(b).
The reduction of Zn2+ ions from the ionic liquid was thus confirmed.
As revealed in Fig. 1, both Zn and Al could be reduced at potentials
below −0.08 V. However, since Zn could be reduced at a more pos-
itive potential than that of Al, the rate of Zn reduction should be
faster than that of Al at the deposition potential of −0.1 V. The low
Al peak appeared in the EDS spectrum shown in Fig. 3(b) confirms
that a smaller amount of Al was deposited on the Mg alloy sur-
face. The cross-section SEM micrograph shown in Fig. 3(c) clearly
reveals the microstructure of the deposit formed at −0.1 V. The for-
mation of dendrites gave rise to a very rough surface. Some of the
dendrites were found encapsulated by Al–Zn deposits. EDS analy-
sis shows that the dendrites (white spots) were pure Zn while the
surrounding areas (grey region) consisted of Al and Zn.
morphology of the AZ91D alloy after electrodeposition in 1 wt%
ZnCl2 + 60 m/o AlCl3–EMIC ionic liquid at −0.2 V. The surface of the
deposited film was flat as compared to that formed at −0.1 V. The
corresponding cross-section SEM micrograph is shown in Fig. 4(b),
which shows that the deposit was uniform in thickness. EDS results
show that the distributions of Al, Zn, and Mg across the whole thick-
ness are displayed in Fig. 4(c). As shown in this figure, co-deposition
with uniform chemical composition was obtained when electrode-
position was performed in the above ionic liquid at −0.2 V.
At an applied potential of −0.4 V, a granular surface, similar to
that shown in Fig. 4(a), was observed. The cross-section SEM micro-
graph and the EDS line scan results are demonstrated in Fig. 5. As
shown in this figure, the film had a low surface roughness with a rel-
atively uniform thickness, as compared with that formed at −0.1 V.
However, EDS results show that the Al content was much higher
than that of Zn. In addition, Zn was not distributed uniformly across
the whole thickness of the deposit. A serration of chemical compo-
sition, especially for Zn, was observed. In the 1 wt% ZnCl2 + 60 m/o
At more negative potentials, a granular surface was observed.
Fig. 4(a) demonstrated the SEM micrograph showing the surface

S.-J. Pan et al. / Electrochimica Acta 55 (2010) 2158–2162
2161
Fig. 5. (a) SEM micrograph and (b) EDS line scans for the deposit formed in 1 wt%
ZnCl2 + 60 m/o AlCl3–EMIC ionic liquid at −0.4 V.
Fig. 4. (a) SEM micrograph, (b) cross-section image, and (c) EDS line scans for the
deposit formed in 1 wt% ZnCl2 + 60 m/o AlCl3–EMIC ionic liquid at −0.2 V.
AlCl3–EMIC ionic liquid, the concentration of the precursor from
−
which Zn could be reduced was much lower than that of Al2Cl7
.
Under a more negative applied potential, the amount of Zn reduced
from the electrolyte was thus less than that of Al. The reduction
of Zn2+ on the electrode surface resulted in its depletion. Once it
occurred, a decrease in Zn2+ reduction was observed, leading to
the composition serration as shown in Fig. 5. A non-homogenous
chemical composition in the deposit was also found when the elec-
trodeposition was performed at −0.3 V.
Fig. 6. Effect of deposition potential on the variation of Zn content (determined by
EDS) in the Al–Zn deposit formed in 1 wt% ZnCl2 + 60 m/o AlCl3–EMIC ionic liquid.
homogeneous deposit, proper adjustments of the ionic liquid com-
position and the reduction overpotential are required.
The above results demonstrate that the co-deposition of Al–Zn
in ionic liquid can be obtained. The amounts of Al and Zn reduced
from the electrolyte depend on the potential applied. The variation
of average Zn content in the deposit with potential, determined by
EDS, is shown in Fig. 6. As can be seen in this figure, the amount of
Zn reduced from the electrolyte decreased as the potential became
more negative. It should be noted, however, that the chemical com-
position was not uniform across the whole thickness, except for
that electrodeposited at −0.2 V. In order to obtained a chemically
4. Conclusions
Cyclic voltammograms show that the reductions of Zn and Al on
a glassy carbon electrode can occur in ZnCl2-containing AlCl3–EMIC
ionic liquid. The potential for Zn reduction was more positive than
that for Al. The co-deposition of Al–Zn on AZ91D Mg alloy was

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achieved under constant applied potential condition. In the acidic
60 m/o AlCl3–EMIC ionic liquid, a decrease in the deposition rate
at −0.2 V (vs. Al) was observed with the addition of 1 wt% ZnCl2.
An increase in deposition rate, however, was observed when the
deposition potential was decreased from −0.2 to −0.4 V. At −0.1 V,
the deposit formed on the AZ91D Mg alloy was primarily composed
of Zn dendrites with a rough surface. Under a total applied charge
of 50 C cm−2, a 15–18 ␮m-thick deposit with a relatively flat sur-
face was obtained when the potential was kept below −0.2 V. The
chemical composition of the deposit was uniform when deposited
at −0.2 V, but it became non-uniform at a more negative applied
potential. The non-uniform chemical composition of the Al–Zn co-
deposit was attributed to the fast reduction of Al at a more negative
potential and the relatively low ZnCl2 content in the ionic liquid.
The average Zn content in the deposit decreased with decreasing
deposition potential.
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Acknowledgement
The authors are grateful to the National Science Coun-
cil of the Republic of China for partially supporting the use
of instruments for surface characterization under grant NSC
96–2221–E–006–109.