Influence of Co2+ and Cu2+ on the phase composition of Zn-Ni alloy

Article abstract of DOI:10.1016/j.electacta.2006.01.064

Influence of Co²⁺ and Cu²⁺ on codeposition of Zn-Ni in acetate-chloride electrolyte have been investigated by the potentiodynamic stripping and XRD methods. In our earlier work it has been determined that the quantity of the η-phase

Full text of DOI:10.1016/j.electacta.2006.01.064

Electrochimica Acta 51 (2006) 6135–6139
Influence of Co2+ and Cu2+ on the phase composition of Zn–Ni alloy
∗
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ˇ
˙
ˇ¯
˙
ˇ ˙
A. Petrauskas , L. Grinceviciene, A. Cesuniene, R. Juskenas
Institute of Chemistry, Gosˇtauto 9, LT-01108 Vilnius, Lithuania
Received 27 July 2005; received in revised form 18 January 2006; accepted 26 January 2006
Available online 9 May 2006
Abstract
Influence of Co2+ and Cu2+ on codeposition of Zn–Ni in acetate–chloride electrolyte have been investigated by the potentiodynamic stripping
and XRD methods. In our earlier work it has been determined that the quantity of the ␩-phase in the alloy increased with decrease in the quantity
of Ni when codeposition of Zn–Ni took place in acetate–chloride electrolyte. Meanwhile, both the quantity of Ni and that of the ␩-phase in the
alloys decreased, when Co or Cu were incorporated in the alloy. Data of XRD have shown that the quantity of ␥-phase in the alloy increased with
increase in the quantities of Co2+ or Cu2+ in acetate–chloride electrolyte. When concentration of [Co2+] (0.15 mol dm−3) was approximately 19
times as high as that of [Cu2+] (0.008 mol dm−3), the influence of Co2+ and that of Cu2+ on the phase composition of alloys were nearly similar.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Zn–Ni alloy; Electrodeposition; Co2+; Cu2+ potentiodynamic stripping; XRD
1. Introduction
of ␥- and ␣-phases. Commercially attractive deposits formed
with a nickel content of say, 12–15% nickel are therefore purely
␥-phase [1].
Electrodeposits of Ni are cathodic in respect to Fe, but they
have the allergic impact on human. Deposit of Zn is sacrifi-
cial anode, therefore provides cathodic protection to Fe, but this
coating is fast corrosive. To solve this problem the various pas-
sivations of Zn coatings are performed. Mostly, the chromating
process, which has hazardous impact on environment, is used.
Zn–Ni alloy coatings, containing up to 12 at.% Ni [1] provide
cathodic protection to Fe, and they possess more higher corro-
sion resistance than that of Zn deposits. Consequently, Zn–Ni
alloy coatings are more promising than Zn coatings, because
Zn–Ni alloys can be used without passivation. Most widely
used Zn–Co alloys containing from 0.5 wt.% Co [2] possess
inadequate corrosion resistance, so they are passivated. Many
references deal with the investigation of deposition of binary
Zn–Ni [3–6] and Zn–Co [1,2,7–9,10–12] alloy, their phase com-
position and corrosion resistance. The quantity of Ni in the alloy
causesthephasecompositionofZn–Nialloy[4], whilethecorro-
sion resistance of Zn–Ni alloy depends on the phase composition
of Zn–Ni alloy [3–6]. Alloys of 10–17% nickel are single ␥-
phase coatings. Coatings of less than 10% are mixture of ␥- and
␩-phases, whist nickel contents greater than 17% are mixture
The phase composition of Zn–Ni alloy depends on deposition
conditions: the current density (ic) or deposition potential (Ec),
temperature and the ratio of [Zn2+]/[Ni2+] in electrolyte. With
variation in the values of deposition potential of Zn–Ni alloy not
only phase composition of alloy undergoes changes, but changes
of mechanism of Zn–Ni codeposition also take place [13]. The
authors [13] highlighted three ranges of metal deposition. In the
first range of low potential values (from −0.7 to −0.8 V) the
coating contains ≥95 wt.% Ni and the normal Zn–Ni codeposi-
tion occurs. In the second range of intermediate potential values
the alloy contains approximately 75 wt.% Ni and the cathodic
current efficiency of the alloy drops to its lowest value. In the
third range of high potential values (from −1.04 to −1.12 V)
the quantity of Ni decreases from 45 to 15 wt.% with increase
in the potential value, the anomalous Zn–Ni codeposition takes
place and the ␥-phase is dominant in the alloy. The analogous
phenomenon was observed in acetate–chloride electrolyte by
varying the ratio [Zn2+]/[Ni2+] [14]. When the Zn–Ni alloy
was deposited in electrolyte containing the low [Zn2+]/[Ni2+]
ratios, normal codeposition of Zn–Ni occurred. With increase
in the [Zn2+]/[Ni2+] ratio in electrolyte the current efficiency
(CE) of Zn–Ni alloy passed through minimum, whereas normal
codeposition of Zn–Ni became an anomalous one. When ic was
increased, the quantity of Ni in the Zn–Ni alloy increased, while
∗
Corresponding author. Tel.: +370 5 264 92 12; fax: +307 5 264 97 74.
E-mail address: acenin@ktl.mii.lt (A. Petrauskas).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.01.064

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A. Petrauskas et al. / Electrochimica Acta 51 (2006) 6135–6139
that of the most active phase ␩ in alloy decreased, which affected
the increase in the quantity of ␣- and ␥-phases in Zn–Ni alloy.
The deposition temperature has also considerable influence
on the phase composition of Zn–Ni alloy [14]. A significant
increase in the quantity of Ni in Zn–Ni alloy was observed
when deposition temperature was increased from 20 to 50 ◦C,
the quantity of Ni increased in the alloy and the phase composi-
tion of Zn–Ni alloy was changed. In this alloy the ␩-phase was
absent or quantity of it was very low.
and potentiodynamic stripping processes were performed in a
non-agitated electrolyte. All the potentials of potentiodynamic
studies were referred to the saturated silver chloride electrode.
An X-ray diffraction (XRD) investigation of Zn–Ni elec-
trodeposits was carried out using an X-ray diffractometer D8
(Bruker AXS). Cu K␣ radiation and a continuous scan mode
with a scan rate of 1◦ min−1 were used.
The quantity of Zn, Ni, Co and Cu in the coatings was deter-
mined by the means of electron probe microanalysis with a
microanalyser JXA-50A (JEOL).
Potentiodynamic stripping and XRD methods are used to
investigate phase composition of Zn–Ni alloys.
It has been founded by the potentiodynamic stripping method
that there were four ia peaks A–D of anodic dissolution on
potentiodynamic curves (PDC) [15–17]. Peaks A, B and C are
attributed to dissolution of Zn from ␩-, ␣- and ␥-phases of Zn–Ni
alloy, respectively [15]. Peak D corresponds to dissolution of
Zn–Ni alloy containing 40 at.% Zn and 60 at.% Ni [18]. Areas
of ia peaks A–D correspond to the electric charge used during
dissolution of separate phase of Zn–Ni alloy and it allows mak-
ing quantitative analysis of phase composition of Zn–Ni alloys.
Most of references deal with the deposition of Zn–Ni and
Zn–Co. Only a few works were devoted to ternary alloys of
Zn–Ni–Co[19,20]. CuasZnhasgreatinfluenceonNideposition
[21].
3. Results and discussion
3.1. Influence of Co2+ on codeposition of Zn–Ni
3.1.1. Stripping results
The potentiodynamic stripping response (potential scan
rate ν = 0.005 V s−1) of electrolytic Co coatings deposited in
acetate–chloride electrolyte under galvanostatic conditions at
20 ◦C shows that Co oxidizes in the range of potentials from
−0.30 to 0.30 V. The range of Co oxidation potentials is not
affected by the deposition time in the range from 180 to 360 s
(Fig. 1, curve 1, 2).
The potentiodynamic stripping response (ν = 0.005 V s−1) of
alloys deposited in electrolyte containing 0.1 mol dm−3 [Zn2+]
and 0.5 mol dm−3 [Co2+] ([Zn2+]/[Co2+] = 1:5) shows that the
oxidation process of alloy is markedly shifted towards the nega-
tive potentials (Fig. 1, curve 3). The oxidation process began at
E = −0.92 V and finished at E < 0.0 V, but major part of Zn–Co
alloy was oxidized in the potential range from −0.6 to −0.3 V.
It showed that pure Co was absent in the alloy, the deposit was
Zn–Co alloy and ␣- and ␥-phases were dominant. The quantities
of ␣- and ␥-phases in the alloy decrease and that of ␩-phase in
the alloy increases several time when the [Zn2+]/[Co2+] ratio in
electrolyte was increased up to 1:1 (Fig. 1, curve 4), because
a major part of alloy was oxidized in the potential range from
−0.9 to −0.6 V (Fig. 1, curve 4).
The aim of present work was to investigate the influence both
of Co2+ and Cu2+ on the phase composition of Zn–Ni alloy
deposited in acetate–chloride electrolyte under galvanostatic
conditions by potentiodynamic stripping and XRD methods.
2. Experimental
Potentiodynamic studies were performed with a potentiostat
PI 50–1 in a thermostatic electrochemical cell ISE-2 on a Pt
electrode with an area of 1 cm2. The foils of nickel, zinc, copper
and cobalt were used as anodes. The anodes were separated from
the cathode by a diaphragm. To prepare solutions the following
salts were used: Ni(CH3COO)2·4H2O; Zn(CH3COO)2·2H2O;
Cu(CH3COO)2·1H2O; Co(CH3COO)2·4H2O; KCl and H3BO3.
All the used reagents were atleast of pro analysis grade.
Electrodeposits of cobalt, zinc copper and Zn–Ni alloy were
obtained in electrolytes (Table 1) under galvanostatic conditions
at pH 5.
Ternary alloys of Zn–Ni–Co or Zn–Ni–Cu were deposited in
electrolyte IV with various concentrations of [Co2+] or [Cu2+],
respectively, under galvanostatic conditions at pH 5.
Potentiodynamic stripping of all electrodeposits, Zn–Ni alloy
and ternary alloys was performed in the same electrolytes which
were used to obtain the electrodeposits and alloys. Deposi-
tion and potentiodynamic stripping were performed at 20 ◦C.
The rate of potential scan (ν) was 0.005 V s−1. The deposition
The potentiodynamic stripping response of alloy deposited in
the electrolyte with the [Zn2+]/[Co2+] ratio = 5:1 (Fig. 2, curve 2)
is almost the same as that of Zn coating (Fig. 2, curve 1). It sug-
gests that Zn, which contains the ␩-phase of Zn–Co alloy as an
impurity, dominates in the alloy. The ia peaks of dissolution of ␣-
and ␥-phases of Zn–Co alloy in the potential range from −0.6 to
−0.3 V emerge in potentiodynamic stripping curves under strip-
ping of Zn–Co alloy deposited in electrolyte with a decreased
[Zn2+]/[Co2+] ratio = 2:1 (Fig. 2, curve 3).
The potentiodynamic stripping curves of Zn–Ni alloy
deposited in electrolyte with 0.25 mol dm−3 [Zn2+] and
0.5 mol dm−3 [Ni2+] ([Zn2+]/[Ni2+] = 1:2)indicatesthatallthree
Table 1
Compositions of the electrolytes
Concentration of electrolyte I, c (mol dm−3
)
Co(CH3COO)2·4H2O (0.5), KCl (0.91), H3BO3 (0.5)
Concentration of electrolyte II, c (mol dm−3
)
)
)
Zn(CH3COO)2·2H2O (0.5), KCl (0.91), H3BO3 (0.5)
Concentration of electrolyte III, c (mol dm−3
Concentration of electrolyte IV, c (mol dm−3
Cu(CH3COO)2·1H2O (0.5), KCl (0.91), H3BO3 (0.5)
Ni(CH3COO)2·4H2O (0.5), Zn(CH3COO)2·2H2O (0.25), KCl (0.91), H3BO3 (0.5)

A. Petrauskas et al. / Electrochimica Acta 51 (2006) 6135–6139
6137
Fig. 1. Potentiodynamic stripping response (ν = 0.005 V s−1) of various coat-
ings: curve 1: Co coating deposited in electrolyte I, τ = 180 s. Curve 2: the
same as curve I, τ = 360 s. Curves 3 and 4: Zn–Co alloy deposited in elec-
trolyte I with various concentrations of [Zn2+] (mol dm−3): 0.1 (3) and 0.5 (4).
ic = 0.010 A cm−2, τ = 360 s, t = 20 ◦C, pH 5.
Fig. 3. Potentiodynamic stripping response (ν = 0.005 V s−1) of various coat-
ings: curve 1: Zn–Ni alloy deposited in electrolyte IV. Curves 2 and 3:
Zn–Ni–Co alloy deposited in electrolyte IV with various concentrations of
[Co2+] (mol dm−3): 0.15 (2) and 0.25 (3). ic = 0.010 A cm−2, τ = 360 s, t = 20 ◦C,
pH 5.
changes the codeposition of Zn–Ni and those changes could
affect the corrosion resistance of alloy. The data presented in
Table 2 explain such influence of Co2+.
When Co incorporates in the alloy, it partly substitutes Ni,
therefore, the quantity of Ni in the alloy decreases, but the total
quantity of Ni and Co in the alloy increases. XRD studies con-
firmed such changes in the phase composition of alloy.
3.1.2. Data of the XRD studies
All coatings studied present ␥-Ni5Zn21 phase. The XRD pat-
tern for the sample deposited in IV electrolyte shows presence of
pure zinc or maybe ␩-phase, i.e. the solid solution of Ni in zinc
(Fig. 4, curve 1). XRD peaks of the ␥-phase are slightly shifted
towards lower diffraction angles with respect to those presented
in PDF Nr 6–653 and indicate a solid zinc solution in the ␥-
phase [13]. The lattice parameter of the ␥-phase increases with
an increase in the quantity of Co in the coating (Fig. 5). The
increase in the lattice parameter can be caused by the partial
replacement of nickel atoms with those of cobalt as the atomic
Fig. 2. Potentiodynamic stripping response (ν = 0.005 V s−1) of various coat-
ings: curve 1: Zn coating deposited in electrolyte II, τ = 360 s. Curves 2 and 3:
Zn–Co alloy deposited in electrolyte II with various concentrations of [Co2+
]
(mol dm−3): 0.1 (2) and 0.25 (3). ic = 0.010 A cm−2, τ = 360 s, t = 20 ◦C, pH 5.
˚
radii of the latter is greater (1.253 and 1.246 A for Co and Ni,
respectively).
phases ␩-, ␣-, and ␥-of Zn–Ni alloy are present in the alloy
(Fig. 3, curve 1. The data suggest that the ␩-phase (a solid 1% Ni
solution in Zn) which corresponds to ia peak A dominates in the
Zn–Ni alloy (Fig. 3, curve 1). When 0.15 mol dm−3 [Co2+] was
added to the above mentioned electrolyte, ia peak A decreased
while ia peaks B + C and D increased (Fig. 3, curve 2). When
concentration of [Co2+] was increased up to 0.25 mol dm−3 ia
peak A decreased even more markedly. It suggests that Co2+
3.2. Influence of Cu2+ on codeposition of Zn–Ni
3.2.1. Stripping results
Potentiodynamic stripping response (ν = 0.005 V s−1) of Cu
coating deposited in acetate–chloride electrolyte under galvano-
Table 2
Dependence of quantity of Zn, Ni, Co and Cu in the alloys on concentrations of [Co2+] or [Cu2+] in electrolyte IV
Electrolytes
Zn (at.%)
Ni (at.%)
Co (at.%)
Cu (at.%)
Electrolyte IV
89.3
86.2
85.1
83.2
78.5
10.7
8.3
6.8
10.3
8.6
–
–
–
–
6.5
12.9
Electrolyte IV + 0.15 mol dm−3 [Co2+
Electrolyte IV + 0.25 mol dm−3 [Co2+
]
]
5.5
8.1
–
Electrolyte IV + 0.008 mol dm−3 [Cu2+
Electrolyte IV + 0.024 mol dm−3 [Cu2+
]
]
–
The alloys were deposited under galvanostatic conditions ic = 0.010 A cm−2. Deposition time (τ) was 1200 s.

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A. Petrauskas et al. / Electrochimica Acta 51 (2006) 6135–6139
Fig. 4. XRD patterns of the alloys deposited in electrolyte IV with various
concentrationsof [Co2+] (mol dm−3): (1)0; (2)0.15;(3)0.25. ic = 0.010 A cm−2
τ = 1200 s, t = 20 ◦C, pH 5.
,
static conditions indicates that Cu oxidizes in ia peaks E and F
in the potentials from range from −0.16 to 0.45 V) (Fig. 6(a)).
When 0.008 mol dm−3 [Cu2+] was added to electrolyte
containing 0.25 mol dm−3 [Zn2+] and 0.5 mol dm−3 [Ni2+]
([Zn2+]/[Ni2+] = 1:2), the potentiodynamic stripping response
of the alloy deposited in this electrolyte showed a significant
decrease in the quantity of ␩-phase (ia peak A) in the alloy
(Fig. 6(b), curve 2). The similar decrease in the quantity of ␩-
phase in the alloy was also observed in the case of Co2+ (Fig. 3).
It should be noted that only one ia peak E which is correspon-
dent to dissolution of Cu emerged on PDC (Fig. 6(b), curve2).
It suggests that pat of Cu enters into the lattice of Zn–Ni alloy.
Potentiodynamicstrippingresponseofcoatingdepositedinelec-
trolyte with 0.024 mol dm−3 of [Cu2+] does not show the ia peak
A (Fig. 6(b), curve 3). It indicates that ␩-phase is absent in the
alloy or quantity of this phase is very low. The data presented
in Table 2 show that despite the low concentration of [Cu2+]
(0.008 mol dm−3) in electrolyte, thesufficientlylargequantityof
Cu (6.5 at.%) was detected in the alloy though the quantity of Ni
in the alloy slightly decreased. An analogous phenomenon was
reported in [21]. The data of [4,14] have shown that the quantity
Fig. 6. Potentiodynamic stripping response (ν = 0.005 V s−1) of various coat-
ings: (a) Cu coating deposited in electrolyte III. (b) Curve 1: Zn–Ni alloy
deposited in electrolyte IV. Curves 2 and 3: Zn–Ni–Cu alloy deposited in elec-
trolyte IV with various concentrations of [Cu2+] (mol dm−3): 0.008 (2) and 0.024
(3). ic = 0.010 A cm−2, τ = 360 s, t = 20 ◦C, pH 5.
of ␩-phase in Zn–Ni alloy increased with decrease in the quan-
tity of Ni in Zn–Ni alloy. The data presented in Table 2 show that
the quantity of Ni in Zn–Ni–Cu alloy decreases with increase
in quantity of Cu in the alloy, however, the quantity of ␩-phase
decreases several times (Fig. 6(b), peak A). It allows making an
assumption that Cu enters into the lattice of Zn–Ni alloy.
Fig. 7. XRD patterns of alloys deposited in electrolyte IV with various concen-
trations of [Cu2+] (mol dm−3): (1) 0.008; (2) 0. 24. ic = 0.010 A cm−2, τ = 1200 s,
t = 20 ◦C, pH 5.
Fig. 5. The lattice parameter of the ␥-phase as function of cobalt quantity in the
alloys deposited at ic = 0.010 A cm−2, τ = 1200 s, t = 20 ◦C, pH 5.

A. Petrauskas et al. / Electrochimica Acta 51 (2006) 6135–6139
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6139
3.2.2. Data of the XRD studies
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The increase in the quantity of Cu in the coating has caused a
very significant increment in the amount of crystallites of the ␥-
Ni5Zn21 phase, which had crystallographic plane (1 1 1) parallel
tothesurface(Fig. 7). Thiscrystallographicplanehasthehighest
packing density of atoms, which is responsible for the lowest
dissolution velocity of the plane. The lattice parameter of the
coating, which contained Cu, was slightly smaller than that of
the Zn–Ni coating. It allows making assumption that a part of
the Zn atoms was replaced by that of Cu.
4. Conclusions
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The data obtained suggest that the phase composition of
Zn–Ni alloy can be varied not only by the means mentioned in
the Introduction (ic or Ec, temperature, the ratio of [Zn2+]/[Ni2+]
in electrolyte) but introducing the additional ions into the elec-
trolyte, as well. The incorporation of Co or Cu changes the phase
composition of ternary alloy and increases the quantity of the ␣-
and ␥-phases in the alloy.
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