Cathodic inhibition and anomalous electrodeposition of Zn-Co alloys

Article abstract of DOI:10.1016/j.electacta.2007.02.077

The electrodeposition of Zn-Co alloys from chloride electrolytes was studied on steel substrate. Electrodeposition of Zn-Co alloys is usually divided into two potential regions, i.e. normal (positive to Zn deposition potential E_Zn⁰ = - 1.05 V versus SCE) and anomalous (negative to E_Zn⁰). In order to elucidate the deposition mechanism a complementary approach was used based on the combination of various electrochemical techniques. The morphology of the deposits and elemental composition analysis were determined by using SEM/EDX. It was found that the presence of Zn²⁺ in electrolyte inhibited Co²⁺ and H⁺ reductions in normal region. A critical potential was also noticed in the so-called normal deposition range above which a Co-enriched phase of Co-Zn alloy was favored and below that a severe mitigation to deposition occurred that was considered due to underpotential deposition (UPD) of Zn on the substrate and on active Co sites at either nucleation or growth stage. Beyond E_Zn⁰ the deposition is considered anomalous due to the fact that Zn deposits preferentially compared to the more noble Co. This anomalism was explained by the faster deposition kinetics of Zn as compared to Co on steel and could be overcome by either increasing the Co²⁺/Zn²⁺ ratios in the electrolyte or by carrying out the deposition at higher temperature.

Full text of DOI:10.1016/j.electacta.2007.02.077

Electrochimica Acta 52 (2007) 5444–5452
Cathodic inhibition and anomalous electrodeposition of Zn–Co alloys
a,∗
b
a
a,c
a,b
Z.F. Lodhi , J.M.C. Mol , W.J. Hamer , H.A. Terryn , J.H.W. De Wit
a
Netherlands Institute for Metals Research (NIMR), Mekelweg 2, 2600 GA Delft, The Netherlands
b
Delft University of Technology, Department of Materials Science & Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands
c
Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
Received 28 September 2006; received in revised form 16 February 2007; accepted 27 February 2007
Available online 4 March 2007
Abstract
The electrodeposition of Zn–Co alloys from chloride electrolytes was studied on steel substrate. Electrodeposition of Zn–Co alloys is usually
divided into two potential regions, i.e. normal (positive to Zn deposition potential E0 = −1.05 V versus SCE) and anomalous (negative to E ). In
0
Zn
Zn
order to elucidate the deposition mechanism a complementary approach was used based on the combination of various electrochemical techniques.
The morphology of the deposits and elemental composition analysis were determined by using SEM/EDX. It was found that the presence of Zn
2+
2
+
+
in electrolyte inhibited Co and H reductions in normal region. A critical potential was also noticed in the so-called normal deposition range
above which a Co-enriched phase of Co–Zn alloy was favored and below that a severe mitigation to deposition occurred that was considered due
to underpotential deposition (UPD) of Zn on the substrate and on active Co sites at either nucleation or growth stage. Beyond E0 the deposition
Zn
is considered anomalous due to the fact that Zn deposits preferentially compared to the more noble Co. This anomalism was explained by the
2
+
2+
faster deposition kinetics of Zn as compared to Co on steel and could be overcome by either increasing the Co /Zn ratios in the electrolyte or
by carrying out the deposition at higher temperature.
©
2007 Elsevier Ltd. All rights reserved.
Keywords: Anomalous electrodeposition; Zn–Co alloys; Underpotential deposition; Cathodic potentiodynamic polarization; Critical potential
1
. Introduction
[3,4] and Higashi et al. [5,6] proposed that the discharge of
2+
Co ions is hindered in the same manner by the formation
of Zn(OH)2 on cathode surface. Based on this theory, depo-
sition conditions that can cause an increase in surface pH would
enhance the anomalous codeposition. Tsuru et al. [9] have sup-
ported HSM theory by having a normal electrodeposition from
a methanol bath and showed that anomalous behavior occurs
when water is added to the electrolyte. Recently, the hydroxide
oscillation concept was proposed [10], according to which the
thickness of the hydroxide layer changes periodically and the
hydrogen reduction and cobalt deposition takes place when the
hydroxide layer is depleted. The pH increase due to the rapid
Zinc alloys with group eight metals (Ni, Co and Fe) have
attracted a lot of interest because these alloys exhibit signif-
icantly higher corrosion resistance than pure zinc and have a
potential to replace the banned cadmium coating. Electrodepo-
sition of these alloys is classified as anomalous by Brenner [1]
because the less noble zinc deposits preferentially under most
plating conditions. This anomalism has yet to be explained. Sev-
eral theories have been forwarded by various researchers [2–8],
but the most widespread one, and subject of controversy, is
the so-called ‘hydroxide suppression mechanism’ (HSM) [3–9].
This model, initially proposed by Dahm and Croll [2] for the
Ni–Fe alloys, suggests that the discharge of more noble ions
+
H reduction leads to the reformation of the layer and code-
position becomes anomalous. Higashi et al. [5] measured local
pH in the vicinity of the electrode using an antimony micro-
electrode during the Zn–Co electrodeposition process. With the
initial electrolyte pH 5.0, they observed a rise in pH that might
cause precipitation of Zn(OH)2 while inhibiting Co deposition.
But other researchers [11–14] (with the initial pH less than
4.0) could not find a significant pH increase that could result
in Zn(OH)2.
2+
(
i.e. Ni ) is hindered by the formation of Fe(OH)2 in respective
electrolytes, due to local pH rise, on cathode surface and there-
fore inhibits the codeposition of Ni. Decroly and co-workers
∗
Corresponding author. Tel.: +31 15 2783723; fax: +31 15 2786730.
E-mail address: z.f.lodhi@tudelft.nl (Z.F. Lodhi).
0
013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.02.077

Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
Table 1
5445
In contrast, several researchers have disagreed and objected
Compositions of the various Zn, Co and ZnCo alloys electrolytes used for
deposition
to the HSM theory. Gomes and Valles [15] observed that an
increase in the solution pH promote the normal codeposition
of Zn–Co alloys while Chassaing and Miranda [16,17] found
high Ni deposition at higher pH values during Zn–Ni electrode-
position in chloride and sulfate medium and suggest that the
deposition of Ni is activated with an increase in solution pH
of the Zn–Ni system, which is in contradiction with HSM. On
the basis of impedance measurements during alloy deposition,
Electrolyte
ZnCl2
CoCl2·6H2O
Molar ratio
Zn /Co
%Co2+
2
+
2+
(
g/L)
(g/L)
1
2
3
4
5
6
7
0
238
0
1.0
1.1
0
100
27.6
36.4
40
150
150
150
150
150
138
100
150
180
202
238
1.1/0.42
1.1/0.63
1.1/0.75
1.1/0.85
1.0/1.0
+
they put forward that a mixed intermediate of [ZnNi]ad plays
44
50
an important role in the preferential zinc deposition.
Some researchers [18–26] have investigated the deposition
kinetics of Zn and iron group metals at various molar concentra-
tions of electrolytes and reported the slower deposition kinetics
of iron group metals being responsible for the anomalism.
Several others [26–31] suggested that the anomalous code-
position process is associated with the underpotential deposition
lytical grade chemicals. The deposition is done on high strength
steel (AISI 4340) substrates. The chemical composition of AISI
4340 steel was 0.41% C, 0.73% Mn, 0.8% Cr, 1.74% Ni, 0.25%
Mo and 0.25% Si (wt%). The surface area of the substrates was
2
1
cm . Before deposition the samples were thoroughly cleaned
(
UPD) of the less noble metal on more noble metal substrate.
in alkaline solution (containing NaOH, Sodium glucconate,
Na3PO4, Na2CO3 and alkaline surfactant) followed by anodic
electrolytic cleaning in alkaline solution (containing NaOH,
Most of these researchers have tried to explain the anomalism
with UPD in a region more negative to the zinc deposition poten-
tial. The term UPD refers to the phenomenon of deposition of
less noble metal (i.e. zinc) on foreign substrates (i.e. steel) in the
potential region more positive to the equilibrium potential of the
less noble metal. The UPD of Zn has previously been reported
2
sodium glucconate and alkaline surfactant) for 3 min at 3 A/dm .
After a thorough rinse with distilled water the samples were
etched in 8% HCl for 3 min to neutralize the remnants of alka-
line solution and to activate the surface. Finally, the substrates
were rinsed again with water and alcohol and dried with air.
[
27–35]onseveralsubstrates, e.g. Fe, Co, Pt, Ag, Au, Cuandalso
steels. The UPD is usually considered limited to a monolayer but
several layers of zinc deposits have also been reported [34–36].
Dogel and Freyland [34] has reported the deposition of three
consecutive layers on the substrate and Fujuwara and Enomoto
2.2. Electrolyte composition
The composition of blank solution was KCl 186 g/L (2.5 M),
[
36] has shown thick Zn-rich CuZn deposits in the underpo-
tential region. Nicol and Philip [27] and Ohtsuka and Komori
29] suggested that the UPD of the less noble metal gives rise
H3BO3 = 30 g/L (0.5 M) and NH4Cl = 20 g/L (0.4 M). All elec-
trodepositions, given in Table 1, were carried out at a constant
[
◦
bulk pH of 3.6 at 35 C and the electrolytes were continuously
to preferential or anomalous deposition, but this model has not
found much favor since anomalism occurs in the overpotential
deposition region. This approach was found unable to explain
how a thick (10–25 ␮m) Zn–Ni or Zn–Co alloys deposits (with
higher zinc content) can be formed by the UPD mechanism at
potentials more negative than the zinc equilibrium potential (i.e.
in the overpotential region) [26].
In the presented work the electrodeposition mechanism of
Zn, Co and Zn–Co alloys is studied by using cathodic poten-
tiodynamic polarization, potentiostatic deposition and anodic
stripping methods. The purpose is to elucidate various stages of
anomalism and inhibition at various potentials and to explain
which processes are responsible for it. Various existing theories
are analyzed in this work to study which theory covers the results
under the given operating conditions. Moreover, the effect of
varying the composition and temperature of the electrolyte in
overcoming anomalism is also studied.
stirred during deposition.
2.3. Potentiodynamic cathodic polarization measurement
Cathodic potentiodynamic polarization studies were carried
out in a three-electrode cell. The cathode (high strength steel
substrate) was immersed in aerated solutions (containing Zn,
Co, ZnCo and blank solutions) as mentioned in the composition
section above. Saturated Calomel Electrode (SCE) was used as
an external reference electrode and a Luggin capillary was used
to keep the reference electrode as close as possible to overcome
the ohmic resistance. A platinum mesh anode was used to com-
plete the cell. A Solarton 1286 computer controlled potentiostat
was used to apply the potential ranging from −0.4 V to −1.5 V
at a scan rate of 1 mV/s and the resulting current density was
recorded.
2
. Experimental
2.4. Potentiostatic electrodeposition
2
.1. Material preparation
Potentiostatic depositions of Zn, Co and Zn–Co alloy were
carried out at −0.78 V, −0.90 V and −1.10 V onto steel sub-
strates for 3–5 min. The current was monitored and recorded
during deposition. The potentiostatic cathodic measurements
were also conducted in a three-electrode cell, provided with steel
All coatings were electrodeposited from electrolytes pre-
pared from chloride salts of respective metals. All the
electrolytes were prepared from demineralised water and ana-

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Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
2
disc of an area 1 cm , as cathode. A platinum mesh was used
as anode and a saturated calomel electrode as a reference elec-
trode. The potential application to the working electrode and
the current measurement was done via Solarton 1286 computer
controlled potentiostat.
2
.5. Anodic stripping technique
Anodic stripping analysis is employed effectively for the in
situ characterization of the electrodeposition process and prod-
ucts of the potentiostatically obtained electrodeposits on steel
substrate. For anodic stripping the potentiostatic deposition was
carried out for a few seconds (time was adjusted with applied
potential) in order to obtain a thin deposit. The purpose of these
measurements was to perform a qualitative analysis and not
a quantitative one. The analysis was performed right after the
potentiostatic depositions, in situ, without removing the sample
from the respective electrolytes. The set up is the same as men-
tioned above for potentiostatic deposition. The stripping was set
to start from the deposition potential and was done at a scan rate
of 10 mV/s.
Fig. 2. Cathodic potentiodynamic electrodeposition on steel substrate. The
curves show changes in the deposition behavior with the variation in concentra-
tion of Co in the electrolyte: (curve 1) 100% Co, (curve 2) 100% Zn, (curve
2
+
3
) 27.6% Co, (curve 4) 36.4% Co, (curve 5) 40% Co and (curve 6) 44% Co. The
◦
polarization is carried out at 35 C, pH 3.5 at a scan rate of 1 mV/s.
2
.6. Microscopic and composition analysis
as mentioned in previous section. The polarization curves from
the blank electrolyte (with and without N2 purging) are plotted
to identify the contribution of hydrogen and oxygen reduction
reactions from the blank electrolyte during actual metallic depo-
sition. It is shown that the current density increases with the
increase in cathodic potential mainly due to the reduction reac-
A JEOL 6400 scanning electron microscope (SEM) was used
to examine the surface morphology of the coatings after elec-
trodeposition. The elemental composition of the electrodeposits
was determined with energy dispersive X-rays (EDX) working
at 15–20 kV in association with SEM.
+
tion of H ions that result in H2 evolution. However, the compari-
son of both curves shows some contribution of oxygen reduction
reaction in the initial part of the curves. The hydrogen evolution
increases with the increase in cathodic potentials, also observed
by the formation of tiny bubbles at the cathode surface area.
Fig. 2 shows the cathodic potentiodynamic polarization
curves for zinc, cobalt and ZnCo alloys (electrolytes 1–6) on
steel substrate from their respective plating solutions, as given
in Table 1, under the operating conditions mentioned above. In
case of pure zinc, at potentials below the rest potential (−0.59 V)
till−1.04 Vthecurrentdensitydoesnotincreasethesamewayas
it did for blank electrolyte. As the potential reaches −1.05 V, the
currentdensityincreasesabruptlyandcorrespondstotheonsetof
zinc deposition. The current density continues to increase until it
reaches −1.25 V and beyond which a limiting stage appears. The
limiting current density is found to increase with the increase in
temperature and stirring rate of the electrolyte (not shown here).
In case of cobalt, the electrodeposition starts at about −0.60 V
in the given electrolyte. The current density increases markedly
below −0.60 V and continues to do so until it reaches −0.82 V.
At this potential the current density does not increase further
and the deposition reaches its limiting stage.
3
. Results
3
.1. Cathodic potentiodynamic polarization curves
Fig. 1 shows the cathodic potentiodynamic polarization
2+
2+
curves for blank electrolyte (in which no Zn or Co ions are
present) at 25 C and rest of the operating conditions are same
◦
In case of cathodic potentiodynamic polarization for the
Zn–Co alloys (curves 3–6), the curves follow pure cobalt curve
in region “A” beyond the rest potential. Nevertheless, the current
density in region A is suppressed and the curves deviate from the
cobalt curve with the increase of Zn2 ions in electrolytes and
Fig. 1. Cathodic potentiodynamic polarization curves on steel substrate. The
two curves elucidate the contribution of hydrogen and oxygen reduction (with
and without N2 purging) during the polarization. The polarization is carried out
+
2
+
with increasing cathodic potential. The presence of Zn ions
in electrolyte appears to be responsible for the suppression of
◦
at 25 C, pH 3.5 and at a scan rate of 1 mV/s.

Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
5447
current density. With the further increase in potential, as shown
in curves 3–6, a critical potential (Cp) is observed. Below Cp,
the current density decreases abruptly (marked as region B) and
the deposition in this region suffers from severe inhibition, as
shown by the drop in cathodic current density. With the increase
2+
2+
in Zn /Co ratio in electrolyte the current density correspond-
ing to Cp decreases and the Zn–Co alloy deposition curves also
shift further away from the Co deposition curve in both regions
A and B. The current density continues to decrease till the poten-
tial reaches −1.04 V to −1.05 V for bath 3–6, respectively, at
which the current density increases distinctly (marked as region
C). This sudden rise in current density indicates that whatever
was the cause of mitigation is removed and the current density
increases till −1.25 V followed by a region of limiting current
density.
3
.2. Electrodeposition and analysis in three regions
Fig. 4. In situ anodic stripping at a scan rate 10 mV/s of ZnCo alloy electrode-
posited from electrolytes 1, 3, 4 and 5 at −0.78 V after deposition for 20 s at
◦
3
5 C.
In Fig. 2, during potentiodynamic polarization in Zn–Co
electrolyte, three regions are identified. In this section, the poten-
tiostatic deposition is performed in all the three regions and
the obtained deposits are analyzed by anodic stripping method.
One of the reasons of carrying out the potentiostatic deposition
in all three regions is to rule out the influence of any initially
deposited Co layer on the substrate formed during the poten-
tiodynamic cathodic polarization, as shown in Fig. 2. While for
anodic stripping, the coatings in the mentioned three regions
were obtained for 15–20 s of potentiostatic deposition in order
to obtain a thin layer for a good and fast qualitative analysis
with distinctive peaks. The morphology and composition of the
deposits are analyzed by using SEM/EDX analysis.
sity increases rapidly at the onset of the test and then decreases
steadily over a longer period of time. In case of curves 4 and 5 the
deposition seems to be time dependent and increases rapidly in
the beginning but at a decreased rate over a longer period of time.
2
+
The current density increases with the increase in Co concen-
trations in the electrolyte but in general the current density values
2
areverylowinthispotentialregionA(i.e. 0.0003–0.001 A/cm ).
The in situ anodic stripping results, shown in Fig. 4, are
performed on deposits obtained after 20 s of potentiostatic depo-
sition. The purpose of anodic stripping is to find out the role of
zinc in causing inhibition of the deposition process. The deposits
are stripped anodically right after the deposition under the same
conditions in order to verify the findings shown in Fig. 3 and to
see what phases are obtained in the deposits at certain potentials.
The anodic peak related to pure cobalt is observed at −0.17 V,
a potential value similar to that found in previous research [37].
The stripping peaks A4, corresponding to highly Co-enriched
phase, have shifted towards more negative potential than pure Co
3
.2.1. Potentiostatic deposition and anodic stripping curves
in region A
Fig. 3 shows the potentiostatic deposition curves at −0.78 V,
in the region marked as “A” in Fig. 2, for electrolytes 3–5.
◦
The potentiostatic deposition is carried out for 5 min at 35 C
under the given conditions. In case of curve 3 the current den-
2
+
2+
peak and shift further away with the increase of Zn /Co ratio
in electrolytes. It is shown that for the same duration of deposi-
tion the peaks vary in heights and increase with the increase of
2+
2+
Co /Zn ratio in the electrolytes.
3
.2.2. Morphology and composition of the deposits in
region A
The composition analysis of deposits is done by EDX and
the results are shown in Fig. 5A. The wt% cobalt in the deposit
2+
increases with the increase of Co concentration in the elec-
trolyte. The composition of the deposits indicates a relatively
higher wt% of Co (91–97% Co) than wt% of Co2+ in the elec-
trolyte. It also verifies the presence of Zn (3–9%) in the deposit.
The SEM micrograph of Zn–Co alloy deposit obtained poten-
tiostatically at −0.78 V from bath 5 is shown in Fig. 5B. The
EDX composition analysis reveals that the deposit contains 6%
of Zn and the rest is cobalt. The composition can be related to
␣-phase of Zn–Co alloy that has a maximum Zn solubility of
Fig. 3. Potentiostatic electrodeposition from electrolytes 3–5 electrodeposited
◦
at −0.78 V (region A). The deposition was carried out at 35 C, at a pH of 3.5
30%, as reported in literature [22].
and stirring rate of 350 rpm.

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Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
Fig. 6. Potentiostatic electrodeposition from electrolytes 3–5 electrodeposited
◦
at −0.94 V (region B). The deposition was carried out at 35 C.
3.2.4. Morphology and composition of deposits in region B
Fig. 8 shows the SEM micrograph of the deposit obtained at
−
0.94 V, where the cathodic deposition is severely inhibited, as
shown in Figs. 2 and 6. The deposit, after 3 h of deposition, is
very thin and EDX composition analysis reveals approximately
3–5% Zn in deposit and the rest is cobalt.
3.2.5. Electrodeposition in region C
In this section, the deposition behavior in region C is con-
sidered, where the current density increases markedly, as shown
in Fig. 2. In order to explain the deposition behavior in region
C, the composition of Co in Zn–Co alloy deposits (analyzed
by EDX) versus Co2 concentration in electrolyte is plotted in
Fig. 9. The deposits are obtained potentiostatically at −1.1 V and
+
◦
at 35 C from the electrolytes containing various molar concen-
Fig. 5. (A) Variation of Co content in the Zn–Co alloy deposit as a function of
◦
2+
variation in concentration of Co in electrolyte deposited at −0.78 V at 35 C,
tration of Co . It is found that the amount of cobalt in deposit
increases with the increase of Co2 concentration in the elec-
trolyte but the compositions of alloys remain always lower than
the composition reference line (CRL), defined as:
+
determined by EDX. The amount of Zn in electrolyte is 1.1 M. (B) Micrograph
◦
of deposit obtained from electrolyte 5 at 35 C at −0.78 V. Deposit contains
∼6% Zn.
3
.2.3. Potentiostatic deposition and anodic stripping in
region B
In this section, the deposition behavior in region B is consid-
ered, where the current density decreases distinctly, as shown in
Fig. 2. In order to observe the deposition behavior in region B
the potentiostatic deposition of Zn–Co alloys is carried out from
◦
electrolyte 3–5 at −0.94 V at 35 C, as shown in Fig. 6. It is
shown that the current density increases sharply during the first
few seconds followed by a reduction and then it decreases grad-
ually at a lower rate over the rest of deposition time. It is also
noticeable that the current density increases with the increase
of the cobalt concentration in the electrolyte but in general the
deposition rate is rather low.
Fig. 7 shows the anodic stripping of the deposits obtained
potentiostatically during 20 s. It is noteworthy that only one peak
is visible, which increases in height with the increase in Co2+
concentration in the electrolyte. In addition, it is also shown that
the alloys peaks shift towards pure cobalt (more positive) with
the decrease in Zn2 ratio in electrolytes.
Fig. 7. In situ anodic stripping at a scan rate 10 mV/s of ZnCo alloy electrode-
posited from electrolytes 3–5 at −0.94 V after deposition for 15 s at 35 C.
+
◦

Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
5449
Fig. 10. Potentiostatic electrodeposition from electrolytes 3–5 electrodeposited
◦
at −1.1 V (region C). The deposition was carried out at 35 C.
◦
Fig. 8. Micrograph of deposit obtained from electrolyte 5 at 35 C at −0.94 V.
Deposit contains ∼93% Co and the rest is zinc.
terms of current density. However in contrast to the findings
of Fig. 3, it is evident that an increase of Co2 concentration
in electrolyte results in a reduction of cathodic current den-
sity. On the other hand an increase of Zn /Co ratio in the
electrolyte results in an increase in the deposition rate in this
region.
+
c(Co2
+
)
CRL = [c(Co2+
(1)
+ Zn2+)]
2+
2+
2
+
2+
where c(Co ) and c(Zn ) are the concentrations of cobalt and
zinc in electrolytes.
It is also noteworthy that a significant increase in wt% Co
in deposit occurs beyond 0.8 M Co in the electrolyte. In case of
electrolyte 7, the wt% Co in the deposit becomes very high and
the composition becomes closer to the CRL.
The anodic stripping curves shown in Fig. 11 indicates a
shift in all peaks towards more positive potential and also Co
enriched peaks as a result of increase in Co /Zn ratio in the
electrolytes. It is also shown that the anodic stripping starts
with the dissolution of Zn-enriched phase (as shown by peak
2+
2+
3
.2.6. Potentiostatic deposition and anodic stripping curves
A ) and is followed by Zn–Co phases relatively richer in Co (as
shown by peak A ). After dissolution of phase corresponding
to peak A , the highly Co-enriched peaks A3 and A4 are
1
2
2
in region C
Fig. 10 shows the potentiostatic deposition curves at −1.1 V,
in the region marked as “C” in Fig. 2, for electrolytes 3–5.
The current density increases rapidly during the first few sec-
onds and then increases gradually over the rest of deposition
period of time. As shown in Fig. 10, the curves in general show
a higher deposition rate as compared to regions A and B in
visible. It is worth noticing that the peak A is visible for curve
3
corresponding to electrolyte 3 while it appears as a left shoulder
of peak A for electrolytes 4 and 5. However, the peak A is
4
4
only noticeable for electrolyte 4 and 5 and is not found for
electrolyte 3. The peaks heights of the anodic stripping curves,
Fig. 9. Variation of Co content in the Zn–Co alloy deposit as a function of
variation in concentration of Co in electrolyte deposited at −1.1 V at constant
◦
conditionsof35 C. TheamountofZninelectrolyteis1.1 Mandthecomposition
Fig. 11. In situ anodic stripping at a scan rate 10 mV/s of ZnCo alloy electrode-
posited from electrolytes 3–5 at −1.1 V at 35 C.
◦
was determined by EDX.

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Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
Fig. 14. Cathodic potentiodynamic polarization curves showing the effect of
◦
temperature on the electrodeposition behavior. The curves are obtained from
bath 7 at pH 3.5 and at constant stirring of 350 rpm.
Fig. 12. Micrograph of deposit obtained from electrolyte 5 at 35 C at −1.1 V.
Deposit contains 18% Co.
3
.3. Effects of temperature on the deposition behavior
shown in Fig. 11, verify the lessening of deposition with the
increase of Co2 concentration in the electrolyte, as shown in
Fig. 10.
+
In this section, the effect of an increase in temperature on the
anomalism is investigated. Fig. 14 shows the potentiodynamic
polarization curves for electrolyte 7 and shows the effect of
temperature (26–62 C) on the deposition behavior. It is shown
◦
3
.2.7. Morphology and composition of deposits in region C
The deposit obtained from electrolyte 5 at −1.1 V and at
that the current density corresponding to Cp increases with the
increase in electrolyte temperature. It is also noteworthy that
the region of severe inhibition though does not completely dis-
appear but reduces with the increase in temperature, as shown
by the comparison of the curves. The deposition rate in general
increases and it appears that the cause of inhibition fades away
with the increase in temperature.
◦
5 C is shown by the micrograph in Fig. 12. The EDX analy-
3
sis of deposit shows that it contains approximately 18 wt% Co
which is far less then CRL. The EDX analysis reveals the pres-
ence of more than two areas with different compositions at the
surface, which is also confirmed by the emergence of several
peaks as shown by anodic stripping in Fig. 11. Fig. 13 shows
the micrograph of Zn–Co alloy deposited from electrolyte 7 at
−
1.1 V and some micro cracks are also visible at the surface.
The composition analysis shows more than 42% Co in alloys,
which is still less than CRL.
3.4. Local pH measurement
In order to verify the HSM theory and to ascertain the
formation of Zn(OH)2 at the cathode surface, the local pH mea-
surement experiment was carried out. A significant raise in local
pH (>5.0) can be related to the formation of Zn(OH)2 at the cath-
ode. The experiment was carried out as stated in literature [14]
◦
Fig. 13. Micrograph of deposit obtained from electrolyte 7 at 35 C at −1.1 V.
Fig. 15. The pH measurements at potentiostatic electrodeposition from elec-
trolyte 6 at −1.1 V. The deposition was carried out at stirring rate of 350 rpm.
The deposit contains 42% Co.

Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
5451
on a Pt mesh at room temperature. However, instead of using
the rotating disc electrode the solution was stirred at 350 rpm.
Fig. 15 shows the graph of pH change with time during poten-
tiostatic electrodeposition from baths 6 at −1.1 V. During 2 min
deposition time a deposit of approximately 2 ␮m was achieved.
It is shown that during the potentiostatic electrodeposition the
local pH value does not increase considerably from the initial
value of 3.7.
in the deposit at potentials positive to E0 = −1.05 V is only
Zn
possible through UPD mechanism. The potential at which UPD
takes place is defined by Kolb and Gerischer [39] by a relation-
ship, i.e. ꢀE = 0.5 ꢀφ, where ꢀE is the underpotential shift and
ꢀφ is the difference in work function values between the foreign
metallic substrate and the electrodeposited metal. The UPD is
expected when the work function value φ of the deposited metal
is smaller than that of the substrate metal. Therefore, consider-
ing φ, the UPD of Zn on steel and Co is not unexpected and UPD
interaction of Zn with steel and Co can occur at potentials more
positive than the Nernst zinc deposition potential [23,24,31].
It is considered that in both regions A and B the deposition
of Co takes place via progressive nucleation and three dimen-
sional growth mechanisms. In the presence of Zn the nucleation
rate becomes much lower due to UPD of Zn than in electrolyte
without any Zn [41]. The sub-monolayer Zn deposited on the
substrate inhibits the formation of clusters or nuclei of Co on the
substrate hereby limiting the space available for Co deposition.
Furthermore, a Co cluster that has the critical size for sponta-
neous growth resembles bulk Co and UPD of Zn on Co cluster
also occurs and inhibits it further from growth. This inhibition
of Co at the cluster growth stage due to the interaction with zinc
is also suggested by some authors [23]. In addition, the presence
4
. Discussion
The electrodeposition of Zn–Co alloys is a complex process
that involves inhibition and anomalism in different potential
regions. It is divided into three potential regions, i.e. a nor-
mal deposition region A (positive to Zn deposition potential
0
E
= −1.05 V versus SCE), an anomalous deposition region
Zn
0
Zn
C (negative to E ) and the region B located in between. A crit-
ical potential “Cp” is also reported here subsequent to region A
for the first time in the polarization curves (to our knowledge)
beyond which the deposition is sharply mitigated.
The deposition of Co starts around −0.6 V through nucle-
ation at a few active sites and increase steadily with potential
until −0.75 V mainly through growth mechanisms, also reported
by other researchers [3,37,38]. In case of Zn deposition curve
2+
of a higher amount of Zn ions in the cathodic diffusion layer
may also interfere with the replenishment of Co2+ ions in the
cathodic layer.
(
between −0.8 and −1.04 V), the current density remains lower
than that for blank electrolyte. It appears that addition of
Zn in electrolyte has suppressed the other reduction reac-
tions (mainly H reduction). This suppression of H reduction
prior to −1.05 V is related to the UPD of zinc on steel sub-
strate, as the UPD of Zn has previously been reported [27–35]
on several substrates including steels. It is known that Zn has
a lower exchange current density for H reduction (i.e. in the
order of 10
1
2
+
It is realized that it is difficult to determine the deposition
kinetics of Zn and Co at higher overpotential (in region C and
>−1.2 V) due to complexity of polarization curves in that region
and lack of information (i.e. not covered in this paper). However,
some researchers have calculated and compared the deposition
kinetics of both Zn and Co at equilibrium and various concen-
trations of electrolytes [18,20,23,24,40] and concluded that Zn
has faster deposition kinetics than Co. The anomalism found in
region C is scrutinized under both deposition kinetics and HSM
theories.
+
+
+
−
10.5
2
A/cm ) as compared to Fe (approximately
−5.5
2
0
A/cm ) [25]. Therefore the low cathodic current density
recorded in case of pure Zn deposition on steel substrate prior
to −1.04 V is due to UPD of Zn, which suppressed hydrogen
evolution reaction (HER).
The mechanisms related to the deposition of Zn–Co alloys,
on steel substrates, in the three regions A–C can be explained as
follows.
On the basis of former theory, as there exists a competition
2+
2+
between Zn and Co to occupy the active sites on the sub-
strate, Zn2 deposits preferentially due to its faster deposition
kinetics as compared to cobalt. This is supported by the poten-
tiostatic deposition, in which an increase of Co2 concentration
in electrolyte though resulted in a comparatively higher amount
of cobalt in deposits but reduced the cathodic current density for
deposition. The higher Co (wt%) in deposit with the increase of
+
+
The curves in region A, prior to Cp, indicate that the depo-
sition rate increases steadily with the increase in cathodic
potential. However, the curves are suppressed compared to the
pure Co curve and Cp shift towards more positive potentials
2+
2+
2+
due to increase of Zn /Co ratio in the electrolytes. During
deposition in region B, i.e. below Cp, a pronounced inhibition
in deposition and an abrupt decrease in current density occur.
The anodic stripping curves, for both region A and B veri-
fied the presence of Co-enriched Zn–Co alloy phase, usually
referred as ␣ phase according to some researchers [22], which
is located at potentials more negative to pure cobalt. It is illus-
trated that the peaks shift to further negative values and decrease
in heights with the increase of Zn2 ratio in the electrolytes. The
EDX analysis of deposits obtained in regions A and B confirms
the presence of small amount (approximately 5%) of Zn in the
deposit. The microstructure is in agreement with the findings of
some other researchers [3,35]. In principle, the addition of zinc
Co concentration in the electrolyte is attributed to an increase
in Co2 ions in cathodic layer while due to slower deposition
+
kinetics it lowers the overall deposition rate of the alloy. The
2+
2+
increase of Co /Zn ratios in electrolyte assists in overcom-
ing the so-called anomalism to a great extent. On the contrary,
2+
2+
the increase of Zn /Co ratio in electrolyte results in a higher
deposition rate and more Zn in deposit. This may be attributed
to an increase in Zn2 ions (that has faster deposition kinetics) in
the cathodic layer and as a result elevates the overall deposition
rate of alloy.
+
+
It is found that the increase in temperature assists in increas-
ing the deposition rate of Co and overcoming the anomalism
during deposition. It may be attributed to the increase in depo-

5
452
Z.F. Lodhi et al. / Electrochimica Acta 52 (2007) 5444–5452
sition kinetics of Co at elevated temperature that results in
increasing Co2 ions in the cathodic layer and faster replenish-
ment during deposition. It is noteworthy that with the increase in
temperature the onset of region C shifts to more positive poten-
tials with higher amount of cobalt in the deposit. However, it is
also noticeable that the regions A and B though reduced did not
disappear with the rise in temperature, where UPD is considered
to play role, as discussed earlier.
is approximately 3.7, the local pH at the cathode surface during
deposition does not rise high enough to form Zn(OH)2. There-
fore, the HSM theory does not seem to play a role in anomalous
codeposition of Zn–Co alloy.
+
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[
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(
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(
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