Studies of phase composition of Zn-Ni alloy obtained in acetate-chloride electrolyte by using XRD and potentiodynamic stripping

Article abstract of DOI:10.1016/j.electacta.2004.07.044

The phase composition of Zn-Ni alloy electrodeposited in acetate-chloride electrolyte has been studied as a function of ZnCl₂ concentration and cathodic current density (i_c) by the potentiodynamic stripping and XRD methods. It appeared to be dependent only on the Zn/Ni ratio in the alloy, irrespective of whether the result was attained by varying the cathodic current density or by changing the [Zn²⁺]/[Ni²⁺] ratio in electrolyte. A Zn-Ni alloy dissolving in the potential range of i_a, peak D can be obtained by the method of cyclic voltammetry. This phase is a compact black coating. It has been determined by potentiodynamic stripping that pure Ni oxidizes only in the range of positive potentials, while Zn-Ni alloy containing some quantity of Zn oxidizes in the range of negative potentials. It was determined that the preciser data of potentiodynamic stripping were obtained in electrolyte containing Cl^- ions. Zn-Ni alloy can be chromated only in the case when the η-phase makes up a sufficiently large portion of Zn-Ni alloy.

Full text of DOI:10.1016/j.electacta.2004.07.044

Electrochimica Acta 50 (2005) 1189–1196
Studies of phase composition of Zn–Ni alloy obtained in acetate-chloride
electrolyte by using XRD and potentiodynamic stripping
∗
ˇ
ˇ
˙
ˇ¯
˙
ˇ ˙
A. Petrauskas , L. Grinceviciene, A. Cesuniene, R. Juskenas
Institute of Chemistry, Electrochemistry of Metals, A. Gosˇtauto 9, LT-01108 Vilnius, Lithuania
Received 24 March 2004; received in revised form 26 May 2004; accepted 31 July 2004
Available online 11 September 2004
Abstract
The phase composition of Zn–Ni alloy electrodeposited in acetate-chloride electrolyte has been studied as a function of ZnCl2 concentration
and cathodic current density (ic) by the potentiodynamic stripping and XRD methods. It appeared to be dependent only on the Zn/Ni ratio
in the alloy, irrespective of whether the result was attained by varying the cathodic current density or by changing the [Zn2+]/[Ni2+] ratio in
electrolyte. A Zn–Ni alloy dissolving in the potential range of ia peak D can be obtained by the method of cyclic voltammetry. This phase is
a compact black coating. It has been determined by potentiodynamic stripping that pure Ni oxidizes only in the range of positive potentials,
while Zn–Ni alloy containing some quantity of Zn oxidizes in the range of negative potentials. It was determined that the preciser data of
potentiodynamic stripping were obtained in electrolyte containing Cl− ions. Zn–Ni alloy can be chromated only in the case when the ␩-phase
makes up a sufficiently large portion of Zn–Ni alloy.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Zn–Ni coatings; Acetate-chloride electrolyte; Stripping
1. Introduction
45 to 15 wt.% with increase in the potential value and the
␥-phase is dominant in the alloy.
The investigation of the mechanism of Zn–Ni codepo-
sition has been carried on since 1963 when Brenner [1]
thoroughly described anomalous codeposition of Zn and Ni.
Extensive studies of Zn–Ni deposition in electrolytes con-
taining ammonia ions have shown that normal or anomalous
codeposition takes place depending on the value of potential
[2,3] or cathodic current [4]. The authors [3] highlighted
three ranges of metal deposition. In the first range of small
potentials values (from −0.700 to −0.800 V) the coating
contains ≥95 wt.% Ni. 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
Investigation of this phenomenon is important not only
from the theoretical but also from practical point of view. It
has been known [4–7] that the electrodeposited Zn–Ni alloys
containing 10–15 wt.% Ni exhibit a much higher corrosion
resistance than the coatings of pure Zn, though they are also
distinguished for their anodic protection with respect to Fe.
It is believed that Zn–Ni alloys are of optimum composition
when they contain 13 wt.% Ni [7]. At present, black Zn–Ni
alloys are being applied more and more frequently for deco-
rative finishing of articles.
The mechanism of Zn–Ni codeposition has been in-
vestigated in various sulphate [4,2], chloride electrolytes
[8,9,3,10], with and without ammonium ions while acetate-
chloride electrolytes are much less investigated [7,11]. It has
been determined that anticorrosive and physical–mechanical
properties of Zn–Ni alloy coatings depend on their phase
composition. That is why the studies of phase composi-
tion of Zn–Ni alloys by the XRD and potentiodynamic
∗
Corresponding author. Tel.: +370 5 2612483; fax: +370 5 2617018.
E-mail address: acenin@ktl.mii.lt (A. Petrauskas).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.07.044

1190
A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
stripping methods have attracted the particular attention
[4–6,8,9,3,12–15]. In electrolytic Zn–Ni alloy coatings ␩-,
␣- and ␥-phases can be detected, while ␤- and ␦-phases
of Zn–Ni alloy can be obtained by the pirometallurgical
method [4] but they have not been detected in elec-
trodeposited coatings. However, some authors claim, that
␦-phase in electrodeposited Zn–Ni alloys was also detected
[16,13,10]. It has been found by the potentiodynamic
stripping method that there are four typical ia peaks A–D
of anodic dissolution on the potentiodynamic curves (PDC).
Peak A is attributed to the anodic dissolution of Zn from the
most electrochemical active the ␩-phase (1% Ni solution
in zinc). Some authors [8,14,15] observed ia peak A as a
double, therefore, they claimed that during stripping in this
peak the compact ␥-phase, which oxidizes at ia peak C can
be formed from a non-compact ␥ꢀ-phase. Peak B is attributed
to Zn dissolution from ␣-phase. The majority of authors
consider ia peak C as Zn dissolution from the ␥-phase, while
ia peak D is unambiguously attributed to anodic dissolution
of porous Ni which left after Zn oxidation from ␩-, ␣- and
␥-phases.
In our previous works it has been determined that under
the relevant deposition conditions black decorative coatings
of Zn–Ni alloy in acetate-chloride electrolyte without special
additives can be deposited. The thickness of these coatings
is proportional to the deposition time. Acetate-chloride elec-
trolyte was chosen as an object for our studies owing to its
considerable promise for industry.
The aim of this work was to investigate the phase com-
position of Zn–Ni alloys electrodeposited in acetate-chloride
electrolyte as a function of ic and the [Zn2+]/[Ni2+] ratio by
the potentiodynamic stripping and XRD methods.
Fig. 1. Potentiodynamic stripping (scan rate = 5 mV s−2) response in elec-
trolyte I of Zn–Ni alloys obtained in electrolyte I with various ZnCl2 concen-
trations (mol dm−3): (1) 0.0; (2) 0.0037; (3) 0.0047; (4) 0.0055; (5) 0.0073.
Time of electrodeposition t = 360 s; ic = 0.010 A cm−2. t = 20 ◦C; pH 5. Zn
wt.% in alloy: (1) 0.0; (2) 3.5; (3) 10.2; (4) 15.9; (5) 28.9.
Deposition of Zn–Ni alloy coatings was performed at pH
5. The rate of potential scan was v = 5 mV s−1. All the data
of potentiodynamic studies are presented in respect to the
saturated silver chloride electrode.
2. Experimental
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 content of Zn and Ni in the alloy was determined by
means of electron probe microanalysis with a microanalyser
JXA-50A (JEOL).
An atomic force microscope (AFM) SPM Explorer (Ther-
moMicroscope) was used to determine the average surface
roughness of Zn–Ni alloy.
Potentiodynamic studies were performed with a poten-
tiostat PI 50-1 in a thermostatic electrochemical cell ISE-2
on a Pt electrode with an area of 1 cm2. The nickel foil was
used as an anode. The anode was separated from the cath-
ode by a diaphragm. To prepare solutions the following salts
were used: Ni(CH3COO)2·4H2O; ZnCl2; KCl; CH3COOH;
H3BO3; MgSO4·7H2O. All the used reagents were at least
of pro analysis grade.
The Zn–Ni alloys were deposited in electrolyte I (Table 1)
with various concentrations of ZnCl2 for experiments under
galvanostatic or cyclic voltammetry conditions.
Potentiodynamic stripping of Zn–Ni alloy was performed
in electrolyte I or electrolyte II (Table 1).
3. Results and discussion
3.1. Stripping in chloride electrolyte
Table 1
Composition of the electrolytes
Fig. 1 shows potentiodynamic stripping response in elec-
trolyte I of electrodeposited Ni and Zn–Ni alloys. The PDC of
coating obtained in electrolyte I without ZnCl2 exhibits a typ-
ical ia peak H which is attributed to dissolution of pure Ni in
Concentration of electrolyte I, c
(mol dm−3
Ni(CH3COO)2·4H2O: 0.56; KCl: 0.91;
)
H3BO3: 0.5
Concentration of electrolyte II, c MgSO4·7H2O: 0.5; H3BO3: 0.5
(mol dm−3
)

A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
1191
Fig. 2. Potentiodynamic stripping (scan rate = 5 mV s−2) response (in elec-
trolyte II) of Zn–Ni alloys obtained in electrolyte I with different concen-
trations of ZnCl2 (mol dm−3): (1) 0.0; (2) 0.0055; (3) 0.0073. t = 360 s; ic =
0.010 A cm−2. t = 20 ◦C; pH 5. Zn wt.% in alloy: (1) 0.0; (2) 15.9; (3) 28.9.
the range of positive potentials (Fig. 1a, curve 1). The Zn–Ni
alloy obtained in electrolyte I containing 0.0037 mol dm−3
ZnCl2 has3.5 wt.%Zn, PDCshowsahighia peakHattributed
to dissolution of pure Ni (Fig. 1a, curve 2) and very low ia
peak also appears in the range of negative potentials (from
−0.4 to −0.2 V). After the concentration of ZnCl2 in the elec-
trolyte was increased up to 0.0047 mol dm−3, 10.1 wt.% Zn
was found in the alloy, but ia peak of Ni dissolution dom-
inates in PDC while in the range of negative potentials ia
peak somewhat increases (Fig. 1a, curve 3). When the elec-
trolyte contains 0.0055 mol dm−3 ZnCl2 the quantity of Zn
in the alloy increases up to 15.9 wt.%, and PDC shows two
ia peaks (Fig. 1b, curve 4). An ia peak of Zn dissolution
from its solid solution of uncertain concentration in nickel
(␣-phase) manifests itself in the range of negative potentials
(from −0.4 to −0.2 V), and ia peak H of pure Ni dissolu-
tion emerges in the range of positive potentials (from 0.4
to 0.8 V).
After concentration of ZnCl2 in electrolyte was increased
upto0.0073 mol dm−3, thequantityofZninthealloyreached
28.9 wt.% and PDC did not exhibit ia peak of Ni dissolu-
tion, but there was just ia peak of Zn–Ni alloy dissolution ia
peaks B + C and D (Fig. 1b, curve 5). This suggests that
Zn–Ni alloy electrodeposited under these conditions does
not contain phase of pure Ni therefore anodic dissolution
of all the phases can occur in the range of negative potentials
E < 0.0 V.
Fig. 3. Potentiodynamic stripping (scan rate = 5 mV s−2) response of the
Zn–Ni alloy obtained in electrolyte I with 0.22 mol dm−3 of ZnCl2: (a) elec-
trolyte I; (b) electrolyte II. For 1 and 1ꢀ ic = 0.010 A cm−2; t = 360 s (Zn
wt.% = 88.1); for 2 and 2ꢀ ic = 0.020 A cm−2; t = 180 s (Zn wt.% = 84.5). t
= 20 ◦C; pH 5.
When potentiodynamic stripping of Zn–Ni alloy (Zn =
15.9 wt.%) obtained in electrolyte I with 0.0055 mol dm−3
ZnCl2 was performed in sulphate electrolyte, the PDC
showed a rather broad ia peak with a vague top at 0.0 V in
3.2. Stripping in sulphate electrolyte
The data presented above were obtained in electrolyte I
containing 0.91 mol dm−3 KCl, while Zn–Ni alloy was dis-
solved under conditions when dissolution of Ni was stimu-
lated by Cl− ions. In Fig. 2 the curves of potentiodynamic
stripping response in electrolyte II (sulphate) are presented.
Under these conditions while potential was swept from the
potential of pure Ni exhibited in studied electrolyte up to E ≤
1.2 V, pure Ni was not dissolved and PDC did not exhibit any
ia peaks up to evolution of O2 at E ≥ 1.3 V (Fig. 2, curve 1).
Fig. 4. Cyclic voltammograms (v = 5 mV s−1) of Zn–Ni alloys obtained in
electrolyte I with 0.073 mol dm−3 ZnCl2 on Pt electrode. Curve 1: the first
cycle from Pt potential 0.3 to −1.25 V and back to 0.0 V. Curve 2: the second
cycle from 0.0 to −1.25 V and back to −0.4 V. Curves 3–6: the third to sixth
cycles from −0.4 to −1.25 V and back to −0.4 V. Curve 7: the seventh cycle
from −0.4 to −1.25 V and back to 0 V. Curve 8: 26th cycle from −1.25 to
1.3 V (after 25 cycles from −1.25 to −0.4 V and back to −1.25 V). t = 20 ◦C;
pH 5.

1192
A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
Fig. 5. AFM picture of Zn–Ni alloy obtained on Pt electrode (area 1 cm2) under the same conditions as in Fig. 4, Curve 8 only after 190 cycles; 5000×. The
¯
mean value of the average roughness of the Pt surface, Ra = 4.78 nm.
the potential range from −0.4 to 0.4 V (Fig. 2, curve 2). In
PDC recorded for Zn–Ni alloy (Zn = 28.9 wt.%) obtained in
electrolyte I with 0.0073 mol dm−3 ZnCl2 just ia peak man-
ifested itself in the range of the negative potentials (Fig. 2,
curve 3).
To study processes occurring in the potential range (from
−0.35 to 0.0 V) of ia peak D an experiment in order to ac-
cumulate a large quantity of Zn–Ni alloy that could further
dissolve only in ia peak D was performed. For this purpose
the alloy was deposited by cyclic polarization of Pt electrode,
i.e. by sweeping E from −1.25 to −0.4 V. Under these con-
ditions with decrease in E up to −1.25 V, all the Zn–Ni alloy
phases can be deposited, when E is increased up to −0.4 V
(the beginning of ia peak D) Zn is removed from ␩-, ␣- and
␥-phases leaving on the electrode only a newly formed phase,
dissolving at E >−0.4 V (at ia peak D). As is seen in Fig. 4,
curve 1, ia peak D obtained after the first cycle is small size,
while after 7 or 25 cycles (curves 7 and 8 respectively) it is
many times larger than the former. The Zn–Ni alloy coating
obtained by this method is black, compact and fine-grained
(Fig. 5).
Fig. 3 presents potential stripping response in chloride and
sulphate electrolytes of Zn–Ni alloys obtained in electrolyte
with 0.22 mol dm−3 ZnCl2. The alloys contained depending
on ic—88.1 and 84.5 wt.% Zn. Potentiodynamic dissolution
of the coatings containing 88.1 wt.% Zn, in both the chloride
and sulphate electrolytes resulted in PDCs which had nearly
analogous ia peaks A, B + C and D (Fig. 3, curves 1 and 1ꢀ).
PDCs of the coatings containing a lower quantity (84.5 wt.%)
of Zn recorded in the chloride electrolyte showed ia peaks A,
B + C and D (Fig. 3a, curve 2), while in sulphate electrolyte
ia peaks B + C and D merged into one (Fig. 3b, curve 2ꢀ).
Therefore, in our opinion, potentiodynamic stripping should
be performed only in electrolytes with Cl− ions, because in
a sulphate electrolyte the data were distorted by passivation
of Ni.
3.3. Formation of Zn–Ni alloy which dissolves during
potentiodynamic stripping in the potential range of ia
peak D
The data presented in Fig. 1 show, that pure Ni can dis-
solve in the range of positive potentials, but according to
references ia peak D is also attributed to Ni dissolution from
the porous Ni matrix. However, some authors claim that dur-
ing stripping the Ni/Zn ratio in alloy varies as Zn is removed
from the most active ␩- and ␥-phases, therefore, phase tran-
sition can take place and ␣- [14] or ␤-[15] phases can be
formed.
Fig. 6. XRD pattern of Zn–Ni phase formed during cycling for 6 h.

A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
1193
Table 2
Zn quantity (wt.%) in Zn–Ni alloy as a function of ZnCl2 concentration in electrolyte I and ic of electrodeposition: t = 20 ◦C; pH 5
ic (A cm−2
)
ZnCl2 (mol dm−3
)
0.029
0.058
0.11
0.22
0.020
0.010
0.005
a
18.8 (1)a
35.9 (4)
19.2 (2)
43.6 (5)
58.0 (7)
33.2 (3)
56.0 (6)
77.5 (9)
60.7 (8)
–
–
88.7 (10)
The numbers of XRD patterns (Fig. 7) and potentiodynamic stripping PDC (Fig. 9; curves 1–3 and 7–10) given in brackets help to arrange them in order
of increasing Zn quantity (wt.%) in Zn–Ni alloy.
Analysis of the coating obtained by this method after
190 cycles have shown the presence of 42.4 wt.% Zn and
57.6 wt.% Ni in the alloy. This composition is close to that
of the ␤-phase (NiZn). The 15.2 wt.% excess of Ni could
support the statement that at the first ia peaks only Zn disso-
lution from different phases of Zn–Ni alloy takes place and
Ni accumulates on the electrode. It could be expected that
during anodic dissolution of Zn–Ni alloy accumulated on the
electrode under the cycling double peak of ia attributed to Zn
and Ni will manifest itself on PDC. Because of this during
experiment the number of cycles was varied from 1 to 80,
however, in all cases during anodic dissolution only a single
ia peak was observed, which might be attributed to the anodic
dissolution of the ␤-phase formed during cycling. Moreover,
this Zn–Ni alloy phase accumulated during cycling and com-
prising 42 wt.% Zn actively dissolves in diluted (10%) HCl.
This was the reason why XRD studies were performed.
Fig. 6 shows a fragment of the pattern indicating that max-
imum the experimental peak is located between positions of
the XRD peaks corresponding to ␤-NiZn and pure Ni phases.
It could mean that Zn–Ni deposit obtained by cycling in po-
tential range from −1.25 to −0.4 V presents ␤-NiZn phase
with increased Ni content (there is 40% of Zn only according
to elemental analysis). On the other hand it could mean that
a solid solution of Zn in nickel was formed during cycling.
About 10% of zinc must be in the other phase—amorphous
pure zinc or ZnO—in the latter case.
Fig. 7 shows dependence of phase composition on Zn
quantity in Zn–Ni deposit. At low zinc quantities (∼19 wt.%)
the deposit consists of nickel and nickel rich ␣-phase,
which is inhomogeneous with respect to the zinc concen-
tration. Fig. 7a presents a fragment of the XRD pattern
no. 1 in Fig. 7. Deconvolution of the peak at 2Θ of ∼76◦
shows that it presents an overlap of two peaks (dashed
curves): maximum of the first peak correspond to d value
of 0.1264 nm and can be attributed to ␣-Ni phase and the
second one with d value of 0.1247 nm belongs to pure Ni
phase.
In different crystallites of ␣-phase the Zn quantity can
vary in the range from 0 to 30 wt.% (Zn dissolution limits
in nickel). As zinc quantity reaches ∼33 wt.% Ni–Zn elec-
trodeposit is composed of three phases: Ni, ␣-Ni–Zn and ␥-
Ni5Zn21. A negligible increase in Zn quantity (from 33.2 to
35.9 wt.%) significantly increases the content of ␥-Ni5Zn21
phase. However, this may be caused by the different current
densities used. When zinc quantity reaches 56 wt.% a pure
nickel phase becomes undetectable by XRD, though a small
quantity of Ni–Zn ␣-phase remains. As Zn amounts 60 wt.%
Ni–Zn ␣-phase becomes undetectable, as well. The quantity
of Ni reaches 33.5 wt.% and it is much higher than that re-
quired for the formation of ␥-Ni5Zn21 phase (i.e. ∼18 wt.%).
Other authors [17] state that Zn–Ni ␩-phase (solid solution
of up to 6 wt.% of Ni in zinc) was formed, however, we
did not detect such a phase. On the other hand, the same
authors state that a Zn solid solution in ␥-Ni5Zn21 phase
was formed and it caused an increase in lattice parameter
(from 0.892 to 0.894 nm). We observed the inverse effect
i.e. lattice parameter a of ␥-Ni5Zn21 phase decreased when
nickel content was higher than 15–18 wt.% (Fig. 8). The as-
sumption that nickel atoms in the lattice of ␥-Ni5Zn21 sub-
stitute some Zn atoms could explain why any Ni or ␣-Ni–Zn
phases are not observed despite the Ni excess in the de-
posit.
The data obtained suggest (Figs. 4 and 6) that Zn–Ni alloy
can be obtained by the cyclic voltammetry method, the latter
dissolves in the potential range of ia peak D.
3.4. Dependence of phase composition of Zn–Ni alloy
on Zn content in the alloy
When the relevant deposition conditions of alloy are cho-
sen, it is possible at various ic and different concentrations
of ZnCl2 to deposit Zn–Ni alloys containing the same Zn/Ni
ratio in it. For example the Zn–Ni alloys deposited in elec-
trolyte I containing 0.44 mol dm−3 ZnCl2 at ic = 0.40 A cm−2
and those deposited in electrolyte I containing 0.15 mol dm−3
ZnCl2 at ic = 0.0025 A cm−2 appeared to contain nearly the
same quantity of Zn, i.e. 85.7 and 85.0 wt.%, respectively.
Potentiodynamic stripping of these coatings resulted in anal-
ogous PDCs. That is why the specimens of Zn–Ni alloys
obtained at different ic and at various ZnCl2 concentrations
were prepared (Table 2).
A small quantity of ZnO was detected in some samples
electrodeposited under a current density of 0.010 A cm−2
(Fig. 7b, sample no. 8). Fig. 7b shows XRD pattern no. 8
in Fig. 7 giving evidence that ZnO is present in the electrode-
positofZn–Nialloycontaining60.7%ofZn. Intensitiesofthe
diffracted X-rays are given in logarithmic scale to make ZnO
peaks visible more definitely. It should be noted that ZnO was
found in the deposits containing the ␥-Ni5Zn21 phase. Other
authors [12], who studied Zn–Ni electrodeposition from am-
monium chloride solution, assert that ZnO incorporates into

1194
A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
Fig. 7. XRD patterns of Ni–Zn deposits with various Zn content. The deposition conditions for each sample are presented in Table 1; Cu K␣ radiation: (a) a
fragment of the XRD pattern no. 1 and (b) XRD pattern no. 8 for Zn–Ni electrodeposited alloy containing 60.7% of Zn.
deposit when the ␣-Ni–Zn phase is formed. The formation of
ZnO was caused by increase in pH in the vicinity of the cath-
ode surface due to intensive hydrogen evolution. According
to these authors hydrogen overvoltage is much higher on the
␥-Ni5Zn21 phase than that on the ␣-Ni–Zn phase. However,
it has been determined by other authors that it is ␥-Ni5Zn21
phase that is distinguished for a high catalytic activity in hy-
drogen evolution [18]. When Zn quantity in the Zn–Ni alloy
amounts 88.7% XRD peaks corresponding to pure Zn phase
appear on the pattern (Fig. 7, pattern no. 10). These peaks can
be attributed to Zn–Ni ␩-phase containing ≤1% of nickel as
well.
The stripping method is more advisable than XRD as it
makes it possible to quantitatively estimate the phase compo-
sition of Zn–Ni alloy. The only problem being that ␣-phase of
Zn–Ni alloy (solid zinc solution in nickel) is not of constant
composition, as Zn quantity in it can vary ≤30 wt.% and the
oxidation of this phase can occur in a wide potential range.
ia peak B of Zn dissolution from the ␣-phase often overlaps
ia peak C of Zn dissolution from the ␥-phase Because of this
reason the stripping data should be supported by the data of
XRD studies.
Fig. 9a, curve 1 shows a representative example of the case
when all peaks overlap during potentiodynamic stripping. In
this case only on the basis of the data of XRD studies one can
assert that this PDC reflects Zn dissolution from the ␣-phase
of uncertain composition. However, it still remains unclear
if it is Zn dissolution from the ␣-phase or it is the disso-
lution of the ␣-phase itself. This ia peak is shifted 55 mV
in the direction of negative potentials in relation to ia peak
D. If assume that ia peak D is attributed to Ni matrix dis-
solution, then in the Fig. 9a, curve 1 Ni dissolution was not
observed.
The data of XRD studies have shown that the ␥-phase
appears in the alloy only when Zn content reached 33.2 wt.%
(Fig. 7, curve 3), meanwhile there are two anodic peaks B +
C and D on the PDC of alloy, containing 19.2 wt.% Zn.
ia peak B + C is attributed to the Zn dissolution from
the ␣-and ␥-phases, however, XRD patterns show no of
the presence ␥-phase in Zn–Ni alloy. It suggests that the

A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
1195
Fig. 8. Dependence of ␥-Ni5Zn21 phase lattice parameter a on Zn quantity
(wt.%) in Ni–Zn deposit.
potentiodynamic stripping method better represents phase
composition of Zn–Ni alloy.
With further increase in Zn quantity in the alloy from
58.0 to 77.5 wt.% (Fig. 9b, curves 7 and 9), the ia peak
B + C in PDC increases from 0.037 to 0.057 A cm−2
.
This can be explained by increase in the quantity of the
␥-phase in Zn–Ni alloy. The PDC of Zn–Ni alloy, de-
posited from electrolyte I with 0.022 mol dm−3 ZnCl2 at
ic = 0.005 A cm−2 (88.7 wt.% Zn) (Fig. 9c, curve 10) is
quite different from PDCs represented in Fig. 9a and b. In
this case, the PDC exhibits ia peak A which is attributed to
the dissolution of Zn from the ␩-phase (Fig. 9c, curve 10).
This also was confirmed by XRD pattern (Fig. 7, pattern
no. 10).
Acidic chromating solution containing 0.31 mol dm−3
Cr6+ and 1.7 ml dm−3 H2SO4 (d = 1.84 g/cm3) is used for
passivation of Zn coatings in the industry. During passivation
a layer of Zn coating partially dissolved, and passivating film
of Cr(OH)3 and Zn(OH)2 covered the surface. It has been
noticed that Zn–Ni alloys with Ni >20 wt.% and with pre-
vailing the ␥-phase did not react with passivation solutions
and, therefore, a passivating rainbow film was not formed
on their surface. The sample of Zn–Ni alloy marked as no.
10 contained 88.7 wt.% Zn and large quantity of ␩-phase
(1 wt.% Ni solution in Zn) (Table 2 and Fig. 9c, curve 10, ia
peak A).
After exposure of such alloy to chromating solution for
some time, ia peak A is not observed during potentiodynamic
stripping. The reason is that a chromating solution removed
Zn from the ␩-phase, because Ni cannot be dissolved under
these conditions. When potentiodynamic stripping is per-
formed after chemical dissolution of the coating a distinct in-
creaseinia peakB+Cisobserved. Itsuggeststhatunderthese
conditions a transition of the non-compact ␥-phase occurs
and the quantity of the ␥-phase increases. An analogous phe-
nomenon was also observed by the authors of [15]. If assume,
that during cycling the Zn–Ni alloy, which can dissolve in the
Fig. 9. Curves (1–3 and 7–10) of potentiodynamic stripping (in electrolyte
I) response of Zn–Ni alloys deposited in electrolyte I at various ZnCl2 con-
centrations and various ic. The conditions of deposition indicated in Table 2.
PDC (11; 12) obtained after immersion the sample of Zn–Ni alloy marked
as no. 10 into chromating solution containing 0.31 mol dm−3 Cr6+ and
1.7 ml dm−3 H2SO4 (d = 1.84 g/cm3) for 180 and 600 s, respectively; v =
5 mV s−1. t = 20 ◦C; pH 5.
potential range of ia peak D, accumulates on the electrode
(Fig. 4, curves 7 and 8), the processes occurring in Fig. 9 at ia
peaks D should be assigned to the dissolution of this alloy, but
not to the dissolution of pure Ni. Obviously, this assumption
requires further investigations; therefore, thorough studies of
the processes taking place during potentiodynamic stripping
in ia peaks C and D were scheduled to start in the nearest
future.

1196
A. Petrauskas et al. / Electrochimica Acta 50 (2005) 1189–1196
4. Conclusions
[3] F. Elkhatabi, M. Benballa, M. Sarret, C. M
44 (10) (1999) 1645.
u¨ller, Electrochem. Acta
[4] D.E. Hall, Plating Surf. Finish 70 (11) (1983) 59.
[5] J. Garcia, J. Barcelo, M. Sarret, C. Mu¨ller, J. Pregonas, J. Appl.
Electrochem. 24 (1994) 1249.
[6] M. Gavrila, J.P. Millet, H. Mazille, D. Marchandise, J.M. Cuntz,
Surf. Coat. Technol. 123 (2000) 164.
[7] E. Beltowska-Lehman, P. Ozga, Z. Swaitek, C. Lupi, Surf. Coat.
Technol. 151–152 (2002) 444.
[8] S.S. Swathijaran, J. Electrochem. Soc. 133 (4) (1986) 671.
[9] F. Elkhatabi, M. Sarret, C. Mu¨ller, J. Electroanal. Chem. 404 (1)
(1996) 45.
[10] Z. Wu, L. Fedrizzi, P.I. Bonora, Surf. Coat. Technol. 85 (3) (1996)
170.
[11] S.S. Abd El Rehim, E.E. Foaud, S.M. Abd El Wahab, H.H. Hassan,
Eletrochem. Acta 41 (9) (1996) 1413.
[12] C. Mu¨ller, M. Sarret, M. Benballa, Electrochem. Acta 46 (18) (2001)
2811.
[13] J. Stevanovic, S. Gojkovic, A. Despic, M. Obradovic, V. Nakic,
Electrochem. Acta 43 (7) (1998) 705.
1. It was determined that the preciser data of potentiody-
namic stripping were obtained in electrolyte containing
Cl− ions.
2. During potentiodynamic stripping dissolution of pure Ni
occurs only in the range of positive potentials.
3. It has been assumed that by using the cyclic voltammetry
method (from −1.25 to −0.4 V) a considerable quantity
of Zn–Ni alloy, which dissolves in the range of potentials
of ia peak D, can be accumulate on the electrode.
4. Phase composition of Zn–Ni alloy depends only on the
ratio Zn/Ni in it.
5. Zn–Ni alloy can be chromated only in the case when the
␩-phase makes up a sufficiently large portion of Zn–Ni
alloy.
[14] F. Elkhatabi, G. Barcelo, M. Sarret, C. Muller, J. Electronal. Chem.
419 (1) (1996) 71.
[15] Y.-P. Lin, R. Selman, J. Electrochem. Soc. 140 (5) (1993) 1299.
[16] L. Felloni, R. Fratesi, E. Quadrini, G. Roventi, J. Appl. Electrochem.
17 (3) (1987) 574.
References
[1] A. Brenner, Electrodeposition of Alloys, vol. 1, Academic Press,
New York, 1963, p. 77.
[2] F.J. Fabri Miranda, O.E. Barcia, S.L. Diaz, O.R. Mattos, R. Wiart,
Eletrochem. Acta 41 (7–8) (1996) 1041.
[17] C. M
85.
u¨ller, M. Sarret, M. Benballa, J. Eletroanal. Chem. 519 (2002)
[18] C.-C. Hu, C.-H. Tsay, A. Bai, Electrochem. Acta 48 (7) (2003) 907.