Electrochemical study on the inhibitory effect of the underpotential deposition of zinc on Zn-Co alloy electrodeposition

Article abstract of DOI:10.1016/j.electacta.2005.08.002

Zn-Co alloy electrodeposition from chloride baths containing different Zn²⁺/Co²⁺ ratios was investigated by cyclic voltammetry and anodic linear sweep voltammetry using a Pt electrode. The peaks were attributed by means of EDX analysis, SEM and TEM observations performed on some alloys potentiostatically deposited. In the range of potential where zinc deposits underpotential, cyclic voltammetry showed a complex cathodic peak with one maximum and two shoulders, correlated with the deposition of different cobalt rich alloys. Up to four anodic peaks, two correlated with zinc oxidation from η and γ phases and two correlated with oxidation of solid solutions of zinc in cobalt, were observed. ALSV and TEM indicated that the remarkable increase in Zn content of the alloy, which occurs with a strong inhibition of the process at potentials more negative than that of the cathodic peak and more positive than the bulk deposition potential of zinc, is due to the deposition of γ phase. No inhibition of the alloy deposition process was observed with very low concentrations of zinc (<0.015 M) in the bath containing 0.19 M Co²⁺.

Full text of DOI:10.1016/j.electacta.2005.08.002

Electrochimica Acta 51 (2006) 2691–2697
Electrochemical study on the inhibitory effect of the underpotential
deposition of zinc on Zn–Co alloy electrodeposition
G. Roventi∗, T. Bellezze, R. Fratesi1
Department of Materials and Environmental Engineering and Physics, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy
Received 18 April 2005; received in revised form 4 August 2005; accepted 7 August 2005
Available online 5 October 2005
Abstract
Zn–Co alloy electrodeposition from chloride baths containing different Zn2+/Co2+ ratios was investigated by cyclic voltammetry and anodic
linear sweep voltammetry using a Pt electrode. The peaks were attributed by means of EDX analysis, SEM and TEM observations performed
on some alloys potentiostatically deposited. In the range of potential where zinc deposits underpotential, cyclic voltammetry showed a complex
cathodic peak with one maximum and two shoulders, correlated with the deposition of different cobalt rich alloys. Up to four anodic peaks,
two correlated with zinc oxidation from ␩ and ␥ phases and two correlated with oxidation of solid solutions of zinc in cobalt, were observed.
ALSV and TEM indicated that the remarkable increase in Zn content of the alloy, which occurs with a strong inhibition of the process at
potentials more negative than that of the cathodic peak and more positive than the bulk deposition potential of zinc, is due to the deposition of
␥ phase. No inhibition of the alloy deposition process was observed with very low concentrations of zinc (<0.015 M) in the bath containing
0.19 M Co2+.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Zn–Co alloy; Zn underpotential deposition; Cyclic voltammetry; Stripping
1. Introduction
which depend on the experimental conditions [2–4]. Some
authors [2] attributed this change to the transition from nor-
mal to anomalous co-deposition. Recently, both for Zn–Co
and Zn–Ni alloy deposition, it was found that the abrupt
change does not always correspond to the normal/anomalous
transition, because this change was observed even at condi-
tions where the transition does not occur [4–6].
Previously [4,6], by means of potentiostatic tests, the
obtainment of Zn–Co and Zn–Ni alloys, rich in cobalt or
nickel, respectively, at low polarization (from −0.700 to
−1.000 V versus Ag/AgCl) was found to be due to zinc
depositing at potentials more positive than its deposition
potential (underpotential deposition), driven by reduction of
the more noble metal, in agreement with the mechanism pro-
posed by Fabri Miranda et al. for Zn–Ni alloys [7]. The
underpotential deposition of Zn was observed also on a cobalt
electrode from Co-free solution [8].
The electrodeposition of Zn–Co alloys is of interest
because these alloys have a significantly higher corrosion
resistance than pure zinc. The electrodeposition of zinc alloys
with group eight metals (Ni, Co and Fe) is classified as
anomalous co-deposition, according to the Brenner defini-
tion [1], because the less noble metal deposits preferably on
the cathode with respect to the more noble one. However,
in certain experimental conditions [2–4], the co-deposition
of zinc alloys with group eight metals is not anomalous
and deposits containing a percentage of the more noble
metal higher than that of the bath can be obtained. On per-
forming galvanostatic electrodepositions of zinc alloys, an
abrupt change in potential, alloy composition and current
efficiency can be observed at certain current density values,
Previous work on Zn–Co electrodeposition [4] showed
that in the normal co-deposition range the polarization curve
exhibits a maximum, followed by a strong decrease in current
∗
Corresponding author. Tel.: +39 071 2204272; fax: +39 071 2204729.
E-mail address: g.roventi@univpm.it (G. Roventi).
ISE member.
1
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.08.002

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G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
density at potentials more negative than about −0.840 V.
Analysis of the partial current density curves indicated that,
under the experimental conditions used, the maximum can-
not be attributed to hydrogen reduction only. This inhibition
is probably the cause of the abrupt change in potential, alloy
composition and current efficiency observed during galvano-
static electrodepositions when the current density applied
drives the cathodic potential in this range. At potentials more
negative than about −1.000 V, depending on the experimental
conditions, the equilibrium potential of zinc is reached and
very zinc-rich alloys are obtained, due to the large differences
between the exchange current densities of zinc and nickel or
cobalt [9–11].
Both for Zn–Co and Zn–Ni alloys, the deposition of alloys
of different composition, morphology and structure depend-
ing on the cathodic potential was observed [4,6]. In particular,
the maximum in the polarization curve, obtained by plotting
as a function of the potential the stationary values of j mea-
sured during potentiostatic electrodepositions, coincideswith
a maximum in the internal stress of the deposits [4,6]. Previ-
ous results on Zn–Co alloy [4] showed that X-ray analysis is
not always able to identify the deposited phases because this
alloy is poorly crystallized in a wide range of composition,
with a large amount of crystallites too small for resolution by
this technique.
The aim of this work was to study the Zn–Co alloy
electrodeposition from chloride baths without complexing
agents by cyclic voltammetry (CV) and anodic linear sweep
voltammetry (ALSV), to identify the deposited phases at the
various potentials and to investigate the cause of the inhi-
bition observed at low polarizations. Scanning electronic
microscopy (SEM) and transmission electronic microscopy
(TEM) observations together with energy dispersive X-ray
(EDX) analysis were carried out on some deposits obtained
potentiostatically. The attention was particularly focused on
the range of potential where zinc deposits underpotential.
platinum substrate was used because the corrosion potential
of iron in the deposition bath (−0.600 V versus Ag/AgCl)
is lower than that of cobalt and cobalt-rich alloy dissolu-
tion potential. Stripping analysis was performed immediately
after potentiostatic depositions at different potentials without
removing the electrode from the solution and selecting, in
each case, an initial potential for which deposition did not
occur. The deposition and stripping charges were calculated
by integrating the current/time curves. All the potentiody-
namic polarizations were carried out using a scanning rate
of 5 mV s−1, chosen after preliminary tests. A conventional
three-electrode cell was used. The working electrode was
mounted in a flat specimen holder with an exposed area of
0.71 cm2; the counter-electrode was a platinum spiral and the
reference electrode was a Ag/AgCl electrode, mounted inside
a Luggin capillary, whose tip was placed next to the working
electrode surface. Before each experiment, solutions were
deaerated with N2 inside the cell. Electrochemical measure-
ments were performed using an EG & G Princeton Applied
Research potentiostat/galvanostat Mod. 273 controlled by a
personal computer. In order to determine the percentage com-
position of the electrodeposited alloys, some deposits were
stripped in a minimum volume of 1:3 HCl solution and anal-
ysed for cobalt and zinc by means of an inductively coupled
plasma spectrometer (Perkin-Elmer Optima 3200 XL).
The deposits obtained potentiostatically were character-
ized by means of a Philips XL 20 scanning electron micro-
scope and an energy-dispersive X-ray spectrometer (Edax
PV9800). The phases of the deposit obtained potentiostati-
cally at −0.96 V were identified by means of a Philips CM
200 transmission electron microscope.
3. Results and discussion
Potentiostatic electrodepositions in the potential range
from −0.760 to −1.100 V were performed on iron and plat-
inum cathodes from the bath containing 0.19 M Co2+ and
0.40 M Zn2+ (Co2+ percentage in the bath (Cob): 30 wt.%),
in order to study the effect of the change in substrate. The
results indicated that the change in substrate does not signif-
icantly affect the polarization curves and the composition of
the deposits (Fig. 1). On the contrary, some tests performed
on glassy carbon electrode showed a strong decrease in the
deposition current density, which starts to increase only when
the electrode surface was totally covered by the alloy. These
results agree with those of other authors [8,12] that found a
strong inhibition of the process on glassy carbon cathode.
Fig. 2 shows the cyclic voltammogram obtained from the
same bath used for the potentiostatic tests. The scan was
started from the rest potential of Pt (−0.100 mV) and reversed
at −1.100 V. The curve shows that the alloy deposition starts
at about −0.750 V, according to the potentiostatic tests results
(Fig. 1); the cathodic current peak, observed in the polar-
ization curve obtained by means of the potentiostatic tests
(Fig. 1), is a complex peak with one maximum (C2) and
2. Experimental details
Zn–Co alloy potentiostatic electrodepositions were car-
ried out at 55 ◦C for 200 s, using a bath of the
following composition: 54.57 g dm−3 (0.40 M) ZnCl2;
45.30 g dm−3 (0.19 M) CoCl2·6H2O; 26 g dm−3 (0.42 M)
H3BO3; 220 g dm−3 (2.95 M) KCl (pH 4.2). This bath was
chosen in order to correlate of the present results with those
obtained previously [4]. Cyclic voltammetry was carried out
from similar baths, changing the concentrations of Zn2+ (0,
0.0015, 0.015, 0.1 and 0.4 M) or Co2+ (0, 0.002, 0.02, 0.1 and
0.19 M), while maintaining constant Co2+ (0.19 M) or Zn2+
(0.4 M) concentrations, respectively. All the solutions were
prepared with doubly distilled water and analytical grade
reagents.
Potentiostatic electrodepositions were carried out on mild
steel and platinum disks. Cyclic voltammetry and anodic lin-
ear sweep voltammetry were performed on a platinum disk; a

G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
2693
Fig. 1. Polarization curves and effect of the deposition potential on zinc
percentage (Znd) in the Zn–Co alloy electrodeposition on iron (ꢀ) and on
platinum (ꢁ) substrate. 0.40 M Zn2+; 0.19 M Co2+. CRL: composition ref-
erence line.
two shoulders (C1 and C3). In the reverse scan, C2 slightly
increases and the deposition continues until about −0.670 V,
at potential values where the deposition does not occur in the
direct scan; this fact indicates that the presence of the alloy
on the cathode surface enhances the process. Three peaks
(A1–A3) appear in the anodic scan.
To attribute the peaks observed by CV, a wide range of
Zn2+ and Co2+ concentrations in the bath were used. Fig. 3
shows cyclic voltammograms obtained by adding different
concentrations of Co2+ to the bath containing 0.40 M Zn2+.
Pure zinc deposition (Fig. 3a) starts at about −1.070 V; in
fact, on performing CV with less cathodic reverse poten-
tials, no anodic peaks were observed. The pure zinc oxidation
peak (A4) was found at about −0.980 V, in agreement with
the results obtained in chloride media by other authors [13].
This peak has been attributed to the dissolution of zinc from
␩ phase (hexagonal structure). By adding up to 0.002 M Co2+
to the bath (curve not shown in the figure), only a slight shift
towards more positive potentials of the zinc dissolution peak
was found, due to the presence in the alloy of a small amount
Fig. 3. Cyclic voltammograms obtained with various Co2+ concentrations
in the bath containing 0.4 M Zn2+. (a) 0 M Co2+, (b) 0.02 M Co2+ and (c)
0.10 M Co2+; 5 mV s−1
.
of Co, which increases its oxidation potential. No anodic
peaks at more noble potentials appear, indicating that a solid
solution of cobalt in zinc ␩ phase is deposited [14]. The curve
obtained from the bath with 0.02 M Co2+ (Fig. 3b) shows in
addition to A4 an anodic peak at −0.450 mV; on magnify-
ing the curve (inset in Fig. 3b) a small reduction wave around
−0.800 Vinthedirectscanandareductionpeakinthereverse
scan can be observed. The charge related to the anodic peak
at −0.450 mV is almost equal to the sum of that related to the
reduction wave and that of the reduction peak; this suggests
that the second anodic peak is mainly related to the oxida-
tion of the alloy obtained at potentials more anodic than that
of the bulk deposition of zinc. On cutting off the reverse
scan at −0.800 V, EDX analysis performed on the cathode
revealed that the deposit contains Zn 40%. On increasing
Co2+ content in the bath to 0.10 M (Fig. 3c), the cathodic
curve becomes similar to that obtained from the bath with
0.19 M Co2+ (Fig. 2). The anodic curve does not show the
peak of the Zn dissolution from ␩ phase (the current density
Fig. 2. Cyclic voltammogram. 0.40 M Zn2+; 0.19 M Co2+; 5 mV s−1
.

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G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
of the reverse scan is almost the same as the direct one in
the range of potential where ␩ phase dissolution occurs), but
shows peak A1, shifted by ∼80 mV towards more cathodic
potentials with respect to that obtained from the bath with
0.19 M Co2+ (Fig. 2). An anodic peak at about −0.600 V
has also been found by other authors [11,14] and has been
attributed to the oxidation of zinc from ␥ phase (Co5Zn21, bcc
structure, equilibrium zinc percentage 82%). The presence of
A1 and the absence of A4 indicate that the phase prevalently
deposited at the more cathodic potentials changes from ␩ to
␥. The shift of the peak potential with respect to that obtained
from the bath with 0.19 M Co2+ is due to the higher zinc per-
centage in the deposit. On cutting off the reverse scan after A1
peak (−0.580 V), EDX analysis performed on the remaining
deposit indicates that it contains about 9% Zn; so A2 and A3
are related both to the dissolution of Co remaining after Zn
oxidation from ␥ phase and to the oxidation of cobalt rich
Zn–Co alloy deposited in correspondence of the reduction
peak.
Fig. 4 shows the effect of different concentrations of Zn2+
in the bath containing 0.19 M Co2+. The curve related to the
pure cobalt bath (Fig. 4a) shows that the cathodic current den-
sity reaches a limit value because the process is under mass
transfer control [15,16]. The corresponding anodic peak was
observed at −0.140 V, a potential value similar to that found
by other authors for the dissolution of electrodeposited pure
cobalt [12,15]. Previous results [4] showed that pure cobalt
deposited in these experimental conditions is poorly crystal-
lized and X-ray diffrattograms showed only low peaks related
to the presence of ␣ phase (hexagonal structure). A structure
prevalently hexagonal-close-packed (from 96 to 100%) has
also been found by other authors for pure cobalt deposited
from chloride media [17,18]. It is noteworthy that this peak
is very large: the presence of a large shoulder may imply that
the cobalt deposits with different energy states. Two oxida-
tion waves for pure cobalt deposited from low melting point
molten salts have been found by other authors [8]. The curve
obtained by adding 0.0015 M Zn2+ shows the appearance of
C1 in the cathodic scan, which leads to a slight increase in
the whole cathodic curve (Fig. 4b). The anodic curve does
not change appreciably: only a slight increase in A3 can be
observed as a consequence of the higher cathodic current.
This result indicates that a low amount of zinc is incorporated
in the cobalt lattice without perceptive changes in the electro-
chemical characteristics of the alloy. With 0.015 M Zn2+ in
the bath, a slight decrease in the deposition current at poten-
tials more cathodic than −0.950 V (Fig. 4c) was observed,
indicating an inhibition of the cathodic process; the anodic
curve shows the appearance of A2 at −0.250 V and a decrease
of A3. Peak A2 potential corresponds to that of the shoulder in
thecurveofpurecobaltoxidation(Fig. 4a), soitcanberelated
to the dissolution of a solid solution of zinc in cobalt at a
higher energy than that oxidized in peak A3. The alloys poten-
tiostatically electrodeposited in the potential zone of peak C1
(−0.780 V) from pure Co bath and from the baths containing
0.0015 and 0.015 M Zn2+ were black and dull, almost similar
Fig. 4. Cyclic voltammograms obtained with various Zn2+ concentrations
in the bath containing 0.19 M Cob. (a) 0 M Zn2+, (b) 0.0015 M Zn2+, (c)
0.015 M Zn2+ and (d) 0.10 M Zn2+; 5 mV s−1
.
to the pure cobalt deposit. SEM images of these alloys show
that the increase in zinc content leads to a strong decrease
in grain size (Fig. 5a and b, related to the deposits obtained
from a zinc-free bath and from the bath containing 0.015 M
Zn2+, respectively). Their Zn percentage (≤2.6%) is in the
range of the equilibrium solubility of Zn in electrodeposited
cobalt ␣ phase (<3% [19]). Alloys obtained potentiostatically
from the bath containing 0.015 M Zn2+ at −0.945 V and at
more negative potentials, where a slight inhibition of the co-
deposition was found, were grey bright and strongly cracked.
The deposit obtained at −0.945 V (in the potential zone of
peak C2) contains 9.5% Zn; on depositing at potentials more
cathodic than −0.945 V, the alloy zinc content (9.6%) does
not increase appreciably, but the deposit morphology has a
fluffy aspect (Fig. 5c). On performing cyclic voltammetry
with reverse potential −0.800 V, only A3 was observed in the
anodic scan. Therefore, peak A2 can be attributed to the dis-
solution of the alloy deposited in the potential zone of peak

G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
2695
Fig. 5. SEM images of some Zn–Co alloys obtained potentiostatically with various Zn2+ concentrations in the bath containing 0.19 M Cob: (a) 0 M Zn2+
−0.780 V; (b) 0.015 M Zn2+, −0.780 V, 2.6% Znd; (c) 0.015 M Zn2+, −1.070 V, 9.6% Znd; (d) 0.10 M Zn2+, −0.920 V, 19.5% Znd.
,
C2. With 0.10 M Zn2+ in the bath (Fig. 4d), the direct cathodic
scan shows an abrupt decrease in current density at potentials
more negative than −0.900 V; the curve becomes similar to
that observed with 0.4 M Zn2+ and C3 appears, which remark-
ably increases in the reverse scan. Both A2 and A3 decrease
and shift towards more negative potentials, according to the
higher zinc content; the absence of A1 indicates that no appre-
ciable amount of ␥ phase is deposited. The zinc percentage
of the deposit obtained potentiostatically at −0.920 V (in the
potential zone of peak C3) from the bath containing 0.10 M
Zn2+ reaches 19.5%; the deposit shows a nodular morphol-
ogy with very low grain size (Fig. 5d). Similar structures are
attributed to a renucleation process, due to the incorporation
of impurities to the grain boundaries, which stops the grain
growth and leads to a globular deposits [20]. Probably, in
this case zinc acts as an impurity. Other authors attributed the
nodular structures in Zn–Co alloys to the coexistence of ␥
and ␣ phases [19]. However, in this case peak A1, correlated
to the dissolution of ␥ phase, was not observed in the anodic
curve (Fig. 4d). On increasing Zn2+ content in the bath to
0.40 M, the alloy obtained potentiostatically in the potential
zone of peak C3 (−0.880 V) contains 20.5% Zn (Fig. 1); it is
noteworthy that this small increase in zinc content causes a
further strong inhibition of the process, in fact the deposition
current density decreases from 14.9 to 7.0 mA cm−2 (Figs.
4d and 2, respectively).
Cyclic voltammograms show the deposition of several dif-
ferent phases during the potential sweep. The deposition of
cobalt rich alloys during the reverse scan can lead to phase
transitions of the zinc rich alloys deposited at the lower poten-
tials; this could explain why A1 does not appear in the curve
of Fig. 4d. To investigate if ␥ phase Co5Zn21 deposits in
the range of potential where the co-deposition is strongly
inhibited, stripping analyses after potentiostatic depositions
from the bath containing 0.4 M Zn2+ were also performed.
Only A3 peak was observed on the curves related to the
deposits obtained at −0.800 V (Fig. 6a, curve 1). The strip-
ping curves of the alloys deposited at −0.840 and −0.880 V
(in the potential zone of peaks C2 and C3, respectively) show
the presence of A2 and a progressive decrease of A3 (Fig. 6a,
curves 2 and 3, respectively). During the stripping tests of
the alloys obtained in the range from −0.880 to −1.000 V
it was very difficult to find an initial potential value where
the current was zero. In fact, even upon changing the initial
stripping potential of 1 mV, a low anodic or cathodic current
was always observed. In some cases, the current was initially
anodicandthenbecamerapidlycathodic. Thisfactcouldindi-
cate the occurrence of two opposite reactions (dissolution and

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G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
Fig. 6. Potentiodynamic stripping curves obtained after potentiostatic depo-
sitionatdifferentpotentialsfromthebathcontaining0.40 MZn2+ and0.19 M
Co2+: (a) curve 1: −0.800 V, 15.6% Znd; curve 2: −0.840 V, 20.5% Znd;
curve 3: −0.880 V, 19.2% Znd; (b) curve 4: −0.960 V, 39.5% Znd; curve 5:
−1.050 mV, 76.2% Znd; 5 mV s−1
.
deposition) at these potentials. The stripping curve related to
deposits obtained at a potential corresponding to the mini-
mum in deposition current density (around −0.960 V) shows
a small peak at −0.600 V (Fig. 6b, curve 4). The peak poten-
tial corresponds to the dissolution of zinc from ␥ phase, but
the charge related to the peak is less than 2% of the total
one, very low compared to the strong increase in zinc con-
tent with respect to the deposit obtained at −0.880 V (from
22 to 46%, Fig. 1). The comparison of the oxidation and the
deposition charge showed that the former (0.85 C) is remark-
ably lower than the latter (1.35 C); this difference is higher
than that expected on the basis of the current efficiency data
(ηalloy = 81%) and may indicate a reduction process during
the anodic stripping. The small peak at −0.600 V is proba-
bly the result of two simultaneous reactions: the dissolution
of the zinc-rich ␥ phase and the reduction of a cobalt rich
phase or hydrogen discharge. The curve related to the oxi-
dation of the alloy obtained at −1050 mV (Fig. 6b, curve 5)
clearly shows peak A1. It is noteworthy that when the peak
at −0.600 V is present, A2 disappears; this suggests that the
deposition of ␥ phase suppresses the incorporation of zinc in
the cobalt lattice.
Fig. 7. (a) TEM image (dark field) of the alloy obtained potentiostatically
at −0.960 V from the bath containing 0.40 M Zn2+ and 0.19 M Co2+. (b)
Corresponding electron diffraction pattern.
out on the basis of JPDF 22–521 and of the results of other
authors [8,14].
4. Conclusions
In the range of potential where underpotential zinc depo-
sition occurs, cyclic voltammograms performed in chloride
baths containing various Zn2+/Co2+ ratios showed a complex
cathodic peak with one maximum (C2) and two shoulders (C1
and C3). Four anodic peaks (A1–A4), which indicate the for-
mation of different phases during the potential sweep, were
observed:
TEM observations (Fig. 7a) on the deposit obtained poten-
tiostatically at −0.960 V show that the alloy is made of
crystallites, with dimensions ranging from 15 to about 30 nm,
uniformly distributed in an amorphous matrix. The electron
diffraction pattern of the crystalline phase corresponds to the
␥ phase (Co5Zn21) (Fig. 7b). The identification was carried

G. Roventi et al. / Electrochimica Acta 51 (2006) 2691–2697
2697
- At potentials more positive than C1 solid solutions of Zn
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with very low concentrations of zinc (<0.015 M) in the bath
containing 0.19 M cobalt.