Electrodeposition of Zn-Co alloys with pulse containing reverse current

Article abstract of DOI:10.1016/j.electacta.2004.11.014

The electrodeposition of Zn-Co alloy deposits was studied by means of pulse-plating technique. The surface morphologies of Zn-Co alloy deposits were examined using scanning electron microscopy (SEM), and an attendant energy dispersive X-ray analyser (EDA) was used to analyse the composition of Zn-Co alloy deposits. Results obtained showed that the average current density and reverse current density amongst all the variables investigated had very strong effects on the cobalt content in the Zn-Co alloy deposits. It is possible to electrodeposit Zn-Co alloy coatings with a very wide cobalt content range of 10-90 wt.% by modulating pulse parameters. Grain size, surface appearance and internal stress in the deposits were also improved significantly by introducing reverse current.

Full text of DOI:10.1016/j.electacta.2004.11.014

Electrochimica Acta 50 (2005) 2693–2698
Electrodeposition of Zn–Co alloys with pulse containing
reverse current
Jing-Yin Feia,∗, G.D. Wilcoxb
a
Department of Applied Chemistry, Northwestern Polytechnical University, Xi’an 710072, China
b
Institute of Polymer Technology and Materials Engineering, Loughborough University, Loughborough LE11 3TU, UK
Received 19 August 2004; received in revised form 20 October 2004; accepted 6 November 2004
Available online 7 January 2005
Abstract
The electrodeposition of Zn–Co alloy deposits was studied by means of pulse-plating technique. The surface morphologies of Zn–Co alloy
deposits were examined using scanning electron microscopy (SEM), and an attendant energy dispersive X-ray analyser (EDA) was used
to analyse the composition of Zn–Co alloy deposits. Results obtained showed that the average current density and reverse current density
amongst all the variables investigated had very strong effects on the cobalt content in the Zn–Co alloy deposits. It is possible to electrodeposit
Zn–Co alloy coatings with a very wide cobalt content range of 10–90 wt.% by modulating pulse parameters. Grain size, surface appearance
and internal stress in the deposits were also improved significantly by introducing reverse current.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Pulse plating; Zn–Co alloys
1. Introduction
or 7 wt.% have not been widely reported [14,15]. This may
be due to the perceived limitations arising from anomalous
co-deposition according to the definition of anomalous co-
deposition by Brenner [16], since the preferential deposition
of zinc was observed under a wide range of conditions, while
the content of noble element cobalt in the deposits was much
less than that in the bath [17–19]. Bahrololoom et al. [15] re-
ported that the deposition mechanism changed from anoma-
lous to equilibrium co-deposition and consequently high-
cobalt-containing Zn–Co alloys were obtained if the Co3+
ions produced from anodic reaction were prevented from
transferring to the catholyte portion of the bath. Although
an attempt has been made by Bahrololoom et al. [15] using
a two-compartment glass cell, which was separated by a sin-
tered ceramics membrane to achieve Zn–Co alloy coatings
with high percentages of cobalt content (up to 70 wt.%) in
Zn–Co alloy deposits, the potential problem with this method
is the large electrical resistance of the ceramic membrane,
which will convert large amount of power into heat and limit
the plating current.
Electrodepositions of zinc-based alloys have been inves-
tigated extensively for decades [1–4]. Of the several zinc al-
loysavailable, electrodepositedZn–Coalloycoatingwithlow
percentage of alloying element cobalt (<1 wt.%) is the most
commercially viable option as it is cost effective and exhibits
better corrosion resistance (three- or four-fold) over the exist-
ing zinc coating system [5]. Instead of being used as promis-
ing anti-corrosion coatings, electrodeposited Zn–Co alloys
with controlled morphology and composition have also been
the subject of many studies as a consequence of their ob-
served catalytic activity, very similar to Zn–Ni alloys [6–11].
These two completely different functions of the Zn–Co al-
loys require very different composition and morphologies.
However, most results reported on the electrodeposition of
Zn–Co alloy coatings showed that the maximum amount of
cobalt in their deposits was about 6–7 wt.% [12,13]. The de-
posits of Zn–Co alloys with cobalt content of more than 6
∗
The application of a pulse current (PC) instead of a direct
current (DC) may be a possible approach to achieve Zn–Co
Corresponding author. Tel.: +86 29 83053800; fax: +86 29 88491000.
E-mail address: jyfei@nwpu.edu.cn (J.-Y. Fei).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.11.014

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J.-Y. Fei, G.D. Wilcox / Electrochimica Acta 50 (2005) 2693–2698
alloys with a very wide range of alloy compositions and prop-
erties by simply varying the applied pulse parameters. A re-
view of the theory and practice of pulse plating is available
[20]. In the electroprocess using pulsed current (PC), three
parameters, the peak current density, the on-time and the off-
time, at least, can be varied independently. If a pulse with
reverse current is used, even more parameters could be used
to modulate the composition and morphology of Zn–Co al-
loy deposits. Furthermore, it is possible for zinc component
in Zn–Co alloys to be removed preferentially when convert-
ing current from cathodic to anodic, which may result in the
deposition of high-cobalt-containing Zn–Co alloys. In the
present work, the PC electrodeposition of Zn–Co alloy from
a sulphate bath has been studied. The bath was free of ad-
ditives such as levelers or brighteners. The range of average
current density (Iav) of pulse plating was chosen in the same
range as the current density normally used to produce the DC
deposits. The composition and morphology of the deposited
alloys were analysed as a function of the PC parameters.
Fig. 1. A typical pulse waveform for the electrodeposition of Zn–Co alloy
deposits.
Zn–Co deposit is nT (the value of n depending on the de-
posit thickness). The frequency (f) used for pulse plating was
in the range of 1–500 Hz. It should be noted that the pulse
waveform will be distorted by the charging or discharging of
the electrical double layer at the electrode–solution interface
if the frequency used is much higher than 500 Hz, the pulsed
current is virtually ‘a direct current’, and the potential benefit
of pulse plating is lost. The range of average current densities
(Iav) investigated in this work varied from 5 to 320 mA/cm2,
2. Experimental
Thesubstrateforelectroplatingwasmildsteelpanels(with
a plated area of 4 cm × 4 cm). Pretreatment before plating
consisted of an initial soak in an alkaline cleaner (40 g l−1
sodium carbonate + 5 g l−1 sodium hydroxide) at 60 ◦C for
5 min followed by rinsing in tap water, then degreasing by us-
ingcathodic cleaning ina solution containing 25 g l−1 sodium
carbonate, 25 g l−1 sodium hydroxide and 50 g l−1 tri-sodium
phosphate at ambient temperature. After rinsing in tap wa-
ter, etching in (50% v/v) S.G. 1.18 HCl, followed by further
rinsing in tap water, the panels were electroplated in accor-
dance with the pre-determined conditions. The trials on the
effects of pulse plating parameters on the cobalt content in
the Zn–Co alloy deposits were carried out by varying one
parameter and keeping the others constant. Electrodeposited
Zn–Co alloy coatings were characterised by their surface ap-
pearance, composition and surface morphologies. The sur-
face morphologies were evaluated using scanning electron
microscopy (SEM), and an attendant energy dispersive X-
ray analyser (EDA) was used to analyse the composition of
Zn–Co alloy deposits.
and the value for reverse anodic peak current densities (Ip−
)
was chosen to be a fraction of the positive peak current den-
sity (Ip+), namely Ip− = xIp+ (x = 0, 0.2, 0.4, 0.6 or 0.8). For a
given average current density (Iav) and the value of fraction
Although some literatures are available on the electrode-
position of Zn–Co alloys, very little attention has been given
to the pulse plating of this alloy [21]. In the present work
a computer-aided pulse plater unit (CAPP, Axel Akermenn
A/S) was used for the electrodeposition of Zn–Co alloy de-
posits. The basic waveform used for the investigation of
the effects of pulse parameters is shown in Fig. 1. Symbols
marked in the schematic diagram, Ip+, Ip−, Iav, T, tc, and ta,
stand for positive peak current density (Ip+), reverse peak cur-
rent density (Ip−), average current density (Iav), cycle time
(T), cathodic time (tc) and anodic time (ta), respectively. The
cathodic time (tc) and anodic time (ta) were kept the same
in all cases during pulse plating. The plating time for every
Fig. 2. Effect of cobalt ion content in the bath on cobalt content in Zn–Co
alloy deposits.

J.-Y. Fei, G.D. Wilcox / Electrochimica Acta 50 (2005) 2693–2698
2695
Table 1
Bath used for Zn–Co alloy pulse plating
ZnSO4·7H2O
CoSO4·7H2O
Na2SO4·10H2O
(NH4)2SO4
H3BO3
60 g l−1 (0.2 M)
140 g l−1 (0.5 M)
80 g l−1
50 g l−1
20 g l−1
pH
T (◦C)
2
55–60
(x), the correlations between Ip+, Ip−, Iav and x are as follows:
2Iav
Ip+
1 − x
Ip− = xIp+
=
(1)
(2)
Among all the variables (Ip+, Ip−, Iav, T, tc, ta and f) mentioned
above, only three of them were independent. Iav, x and f were
chosen as the independent parameters for the present study.
Given the values of Iav, x, f (or T) and the thickness of deposit,
a variety of pulse current patterns were created by inputting
Ip+, Ip−, T and number of cycles (n), into a user-friendly
program system.
Generally, it is easier to obtain a pulse power supply with a
frequency of 50 Hz due to the lower cost of making a big out-
put pulse power plater of this frequency. So, the investigation
on the effect of waveform parameters on the cobalt content
in the Zn–Co alloy deposits was carried out at the frequency
of 50 Hz. Afterwards, the effect of frequency on the cobalt
content in Zn–Co alloy deposit was undertaken. Fig. 1 illus-
trates a typical pulse current waveform pattern based on the
basic conditions of Iav = 30 mA/cm2, x = 0.4, and T = 20 ms.
Fig. 3. Effect of current density on cobalt content in Zn–Co alloy deposits.
tion of Zn–Co alloy using pulse plating technique, a set
of sulphate-based baths was examined to select a suitable
bath constituents for further investigation. By referring to
the results obtained by Bahrololoom et al. [15], all the baths
tested contain the same total main salt concentration of
0.7 M (namely ZnSO4·7H2O + CoSO4·7H2O = 0.7 M) with
the variation range of cobalt sulphate concentration from 0.1
to 0.6 M, namely, the values of [Co2+/(Zn2+ + Co2+)] in per-
centage were changed from bath to bath, whilst the concen-
trations of other chemicals, such as 80 g l−1 Na2SO4·10H2O,
50 g l−1 (NH4)2SO4, 20 g l−1 H3BO3 werekeptconstant. The
electrodeposition of Zn–Co alloy was carried out by employ-
ing a direct current density of 50 mA/cm2, pH of 2 and tem-
perature range of 55–60 ◦C. The results are shown in Fig. 2.
As can be seen in this figure, the cobalt content in the
deposits is low and increases very gradually, not being influ-
enced significantly by variations of the cobalt concentration
in the bath if the cobalt ion content in the bath is <55 wt.%.
It is evident that the electrodeposition of Zn–Co alloy is
an anomalous co-deposition according to Brenner’s defini-
tion [16]. However, cobalt content in the deposits increases
rapidly, reaching the maximum of 17 wt.%, if cobalt ion
3. Results and discussions
Actually, Zn–Co alloy coatings can be obtained using
baths consisting of various compositions and operating con-
ditions. In this work, the bath used for the investigation
of Zn–Co alloy pulse electrodeposition was mainly based
on the formulation reported by Bahrololoom et al. [15]
because it is possible to get Zn–Co alloy coatings with
a wide range of cobalt content. Before the electrodeposi-
Fig. 4. Surface morphologies of Zn–Co alloy deposits with a very wide range of cobalt content obtained by modulating pulse parameters.

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Fig. 8. Effect of frequency (Hz) on cobalt content in Zn–Co alloy (x = 0.2,
av = 30 mA/cm2).
Fig. 5. Effect of fraction (x) on cobalt content in Zn–Co alloy deposit.
I
content in the bath exceeds 60 wt.%. Despite that the cobalt
content in the deposits increases gradually with increasing
cobalt ion concentration in the bath, using a less concentrated
bath has the advantage of being more economical for practical
applications. Furthermore, deposits with uniform and bright
surface appearance could be obtained at the widest current
density range amongst the baths examined by using the bath
containing Co2+ = 0.5 M and Zn2+ = 0.2 M. So, the bath for-
mulation used for further investigations on the effect of pulse
plating variables was illustrated in Table 1.
Being different from the two-compartment cell technique
used by Bahrololoom et al. [15], this investigation on pulse
plating was carried out in a single cell using a platinised tita-
nium mesh as an anode. Fig. 3 shows the variation of cobalt
content in Zn–Co alloy deposits with average current density
(Iav). It is evident that all the curves illustrate a similar profile.
The cobalt content in the deposits is more than 90 wt.% if the
average current density (Iav) is less than 10 mA/cm2. This de-
creases rapidly to 10–20 wt.% at the average current density
of 20 mA/cm2, then increases gradually with further increase
Fig. 6. Relationship between zinc content in Zn–Co alloy deposits and the
reverse fraction (x).
Fig. 7. Surface morphologies of Zn–Co alloy deposits with and without reverse current.

J.-Y. Fei, G.D. Wilcox / Electrochimica Acta 50 (2005) 2693–2698
2697
Fig. 9. Surface morphologies of zinc–cobalt alloy electrodeposited at different frequencies.
in average current density (Iav). The surface appearance of
the deposits obtained at low current density was uniform but
dull, whilst the deposits electrodeposited at average current
densities higher than 20 mA/cm2 were uniform and bright.
Micrographs A and B in Fig. 4 illustrate the surface mor-
phologies of Zn–Co alloy deposits with very different cobalt
contents obtained by varying average current density and re-
verse pulse component. Obviously, the Zn–Co alloy deposits
were uniform and micro-crack-free over this wide range of
cobalt contents.
The large variation of cobalt content in the deposits can
also be obtained by modulating the reverse fraction (x) be-
sides adjusting the average current density (Iav). Reverse cur-
rent, amongst the operating parameters investigated, has the
greatest effect on the cobalt content in Zn–Co alloy deposits.
Fig. 5 shows the variation of cobalt content with reverse frac-
tion (x) at different average current densities (Iav). Except for
at low average current density (<10 mA/cm2), as shown by
curves 1 and 2 in Fig. 5, reverse current density has a great
effect on cobalt content when the average current density is
more than 20 mA/cm2. A significant variation of cobalt con-
tent in the Zn–Co alloy deposits was obtained by changing
the reverse current (Ip−). For a given average current density
(Iav), it is evident that higher cobalt contents correspond to
higher reverse fraction values (x).
internal stress occurred in the Zn–Co alloys electrodeposited
without reverse anodic current may be associated with the
hydrogen incorporated in the deposits as reported by Hillier
and Robinson [22]. The hydrogen co-deposited during the
cathodic current flows could be removed by inverting current
from cathodic to anodic, which results in the deposition of
less stressed Zn–Co alloys [20]. The grain refinement may
be attributed to the high nucleation rate accompanying high
over-potentials resulting from the high peak current densities
(since the values of peak current density both Ip+, and Ip− are
proportional to the values of x) and the dissolution of large
grains during the reverse current cycle.
The effect of frequency (in Hz) on the cobalt content
and the microstructural characteristics of Zn–Co alloy de-
posits was carried out in the frequency range of 1–500 Hz.
Fig. 8 shows the variation of the cobalt content in Zn–Co
deposits with frequency. It can be seen that the cobalt con-
tent decreases very slowly with increasing frequency, and
remains relatively constant at about 12–16 wt.% in the fre-
quency range tested. However, the micrographs of Zn–Co
alloy deposits were affected significantly. As can be seen in
Fig. 9, micro-cracks occurred at frequencies lower than 5 Hz.
No micro-cracks could be found if the frequency was more
than 10 Hz, which means that the Zn–Co alloy deposits with
low internal stress could be obtained under these conditions.
Fig. 6 illustrates the variation of zinc content in Zn–Co
alloys with reverse fraction (x) electrodeposited at the aver-
age current density of 50 mA/cm2. Obviously, zinc content
in Zn–Co alloys decreases with increasing the fraction of an-
odic current. It is likely that the zinc component in the Zn–Co
alloy deposit obtained during the cathodic part of the cycle
dissolves selectively as the reverse current flows, resulting in
a decrease in the zinc content of the deposit [20].
Fig. 7 shows the surface morphologies of Zn–Co al-
loy deposits obtained at the same average current density
(Iav = 50 mA/cm2) with and without reverse current, respec-
tively. The deposits prepared without reverse current pre-
sented big grain size and micro-cracks (see micrograph A in
Fig. 7), whereas the deposits obtained using reverse current
showed a relatively smooth surface and posses refined grains
(see micrograph B in Fig. 7). This suggests that the internal
stress could perhaps be reduced to some extent and the grain
refinement occur if the reverse current was used. The high
4. Conclusions
The results obtained from the investigation on the effect of
bath constituents and pulse parameters on the cobalt content
in Zn–Co alloy deposits and surface morphologies show that
the cobalt content in Zn–Co alloy deposits and the surface mi-
crostructures depend mainly on the average current density
(Iav) and the value of the reverse pulse fraction (x). Zn–Co
alloy deposits with a wide cobalt content range of 10–90%
could be obtained by adjusting average current density (Iav)
and the value of fraction (x). Compared to the deposits ob-
tained without reverse current, fine grain and more compact
crystal structures could be electrodeposited by using pulse
with reverse current. Internal stress of Zn–Co alloy deposits
could also be reduced to some extent by introducing a reverse
current. Although the frequency of the pulse has a relatively

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J.-Y. Fei, G.D. Wilcox / Electrochimica Acta 50 (2005) 2693–2698
little effect on the cobalt content in Zn–Co alloy deposits, it
changes the microstructure of the deposits.
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Acknowledgements
Jing-Yin Fei thanks China Scholarship Council and North-
western Polytechnical University for their financial support
during his sabbatical visit to Loughborough University, UK.
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