Reverse pulse electrodeposition of Zn-Ni alloys from a chloride bath

Article abstract of DOI:10.1016/j.jallcom.2009.07.163

The properties of electrodeposited zinc-nickel alloys including corrosion resistance are largely controlled by their chemical compositions and structures. The present work systematically investigates the influence of pulse parameters for the reverse pulse

Full text of DOI:10.1016/j.jallcom.2009.07.163

Journal of Alloys and Compounds 487 (2009) 479–482
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Reverse pulse electrodeposition of Zn–Ni alloys from a chloride bath
Yuttanant Boonyongmaneerata,b,∗, Sawalee Saenapitaka, Kanokwan Saengkiettiyuta
a Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330, Thailand
b Center of Innovative Nanotechnology, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The properties of electrodeposited zinc–nickel alloys including corrosion resistance are largely controlled
by their chemical compositions and structures. The present work systematically investigates the influence
of pulse parameters for the reverse pulse electrodeposition of zinc–nickel alloys in a chloride bath on
the composition and structure of the deposits. Different degrees of zinc and nickel dissolutions occur
upon varying the anodic current density between 0 and 0.1 A/cm2, yielding ␥-phase coatings with nickel
content ranging from 11 to 17 wt.%. Deposits with much higher nickel content are obtained by prolonging
the dezincification period with increased pulse reverse duration. Furthermore, anodic pulse can also
introduce grain refinement and pore formation to the structure. Effective tailoring of the zinc–nickel
alloys’ characteristics has been demonstrated using the reverse pulse plating technique.
© 2009 Elsevier B.V. All rights reserved.
Received 5 June 2009
Accepted 29 July 2009
Available online 5 August 2009
Keywords:
Zinc–nickel
Deposition
Anodic pulse
Dealloying
Coating materials
Microstructure
1. Introduction
uniform morphology, as compared to the direct current cases [5,6].
Furthermore, the increase of pulse peak-current density results in
Electrodeposited zinc (Zn) has been widely used in a variety
of applications, including coatings for automotive and electronic
parts. This is owed to its decent cathodic and barrier protective
natures. By incorporating an element from the 8th-group, such as
Ni, Fe and Co into the system, the Zn alloy coatings thus formed
can exhibit much enhanced anti-corrosion properties. For exam-
ple, Zn–Ni alloys with a pure ␥-phase and Ni varied between 8 and
14 wt.% show five times better corrosion protection than unalloyed
zinc deposits [1]. By modifying the compositions of the metal salts
and bath additives, electrodeposited Zn–Ni alloys of various com-
positions can be fabricated under a conventional direct current (DC)
mode [2,3].
A number of studies have recently investigated the viability of
pulse-current electrodeposition for better-controlling of the struc-
ture and properties of Zn–Ni deposits [4–8]. Pulse plating, which
refers to the electrodeposition process where current is imposed
in a periodic manner with a rectangular waveform, allows re-
nucleation of discharged ions and desorption of impurities during
current-off periods. Therefore, the process usually yields homoge-
neous coatings with fine grains, uniform morphology and limited
amount of defects. In particular, it has been observed that the pulse
plating of Zn–Ni alloys gives deposits of smaller grain size and more
grain refinement and enrichment of Ni content in Zn–Ni deposits
[5,8].
Alternatively, Zn–Ni alloys may be electrodeposited under a
reverse pulse (RP) mode, in which the cathodic pulse waveform
is coupled with periodic anodic pulse. During anodic pulsing peri-
ods, the deposited species may be dissoluted into a plating bath
at different rates, and re-nucleation of the deposit may also be
well-promoted. Furthermore, the limiting current is relatively high
under the RP mode. Reverse pulsing could, therefore, serve as a
potential strategy to tailor both the chemical composition and
structure of Zn–Ni alloys synthesized from single bath chemistry.
While a large number of publications have concentrated on direct
current and pulse current plating, a study of reverse pulsing in
general is very limited.
Reverse pulsing has been shown recently to permit a precise
control of solute content during the electrodeposition of Ni–W
alloys, which in turn influences grain size and hence hardness of
the coatings [9,10]. As for Zn–Ni alloys, limited studies on reverse
pulse plating, solely conducted on the alkaline [11] and sulfate elec-
trolyte [12] systems, have suggested that increment of Ni content in
the deposits is induced upon application of pulse anodic current. In
the present study, we systematically investigate the reverse pulse
plating of Zn–Ni alloys from a chloride bath, which is known to
exhibit relatively high conductivity and is commonly used indus-
trially [5,13]. The influence of the plating parameters, particularly
anodic peak-current density and pulse duration, on the deposits’
composition is examined in relation to microstructure and corre-
sponding phases.
∗
Corresponding author at: Metallurgy and Materials Science Research Institute,
Chulalongkorn University, Soi Chula 12, Phyathai Rd., Pathumwan, Bangkok 10330,
Thailand. Tel.: +66 2 218 4243; fax: +66 2 611 7586.
E-mail address: yuttanant.b@chula.ac.th (Y. Boonyongmaneerat).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.07.163

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Fig. 1. (a–e) Surface morphology of Zn–Ni alloys deposited with forward current density (If) of 0.08 A/cm2 and reverse current density (Ir) of (a) 0 A/cm2, (b) 0.04 A/cm2, (c)
0.06 A/cm2, (d) 0.08 A/cm2, and (e) 0.1 A/cm2 (tf = 20 ms; tr = 3 ms). The 5 ␮m scale bar is for all parts of the figure.
2. Experimental
current densities and reverse pulse durations, respectively. The cor-
responding chemical composition and phase compositions of the
specimens are presented in Figs. 3a and b and 4a and b.
A set of electrodeposited Zn–Ni alloys was prepared in a plating bath with
aqueous bath chemistry: 100 g/l ZnCl2, 200 g/l NiCl·6H2O, 40 g/l H3BO3, with pH
of 3.5, and the temperature maintained at 30 ◦C. Platinum mesh and copper
(1.0 cm × 2.4 cm), polished to a mirror-like finish and activated by 10% H2SO4, were
used as an anode and a cathode, respectively. A magnetic bar stirrer was con-
tinuously rotated at 200 rpm upon deposition. Periodic pulse current, consisting
of forward (cathodic) peak-current density (If) of 0.08 A/cm2 and reverse (anodic)
peak-current density (Ir) of 0, 0.04, 0.06, 0.08, or 0.1 A/cm2, was applied to the elec-
trodes using a pulse rectifier (Dynatronix, DUPR 10-3-6). The forward and reverse
pulse durations (tf and tr) were respectively 20 and 3 ms. Additionally, the elec-
trodeposition was carried out with Ir = 0.04 A/cm2 and tr = 3, 5, 10, and 20 ms. This
gives forward duty cycles of 87, 80, 67, and 50%. The deposition time was 30 min
in all cases. X-ray diffractrometer (XRD), energy-dispersive spectroscopy (EDS), and
scanning electron microscopy (SEM) were employed to analyze the composition and
surface morphology of the specimens.
The deposit, processed without anodic pulse current
(Ir = 0 A/cm2), is characterized by a typical nodular morphol-
ogy and solely consists of ␥-phase with a preferential (3 3 0)
crystallographic orientation (Fig. 1a). As pulse reverse current
was employed at 0.04 A/cm2, the resulting deposit exhibits a
similar form of surface morphology, but with finer grains and a
more compact structure (Fig. 1b). The Ni content of the deposit
is increased from 11 wt.% in the non-anodic case to 14 wt.% in
this condition, as shown in Fig. 3a. Incrementing Ir to 0.06 A/cm2
leads to further enhancement of Ni content to about 17%. Fur-
thermore, such moderately high pulse current density induces the
formation of ␥-phase with (6 0 0) and (2 2 2) orientations in the
deposit, and this results in pyramidal crystallites, as evidenced
in Fig. 1c. For Ir beyond 0.06 A/cm2, however, a reduction of Ni
content occurs, and the deposits resume a nodular morphology
(Fig. 1d and e). Additionally, the specimens appear to have higher
porosity, especially for high Ir of 0.1 A/cm2. A strong (3 3 0) texture
3. Results and discussion
All Zn–Ni deposits obtained from the study are intact and exhibit
uniform structure. Figs. 1a–e and 2a–c show surface morphology
of the specimens, which were fabricated using different reverse
Fig. 2. (a–c) Surface morphology of Zn–Ni alloys deposited with forward pulse durations (tf) of 20 ms and reverse pulse durations (tr) of (a) 5 ms, (b) 10 ms, and (c) 20 ms
(If = 0.08 A/cm2; Ir = 0.04 A/cm2). The 5 ␮m scale bar is for all parts of the figure.

Y. Boonyongmaneerat et al. / Journal of Alloys and Compounds 487 (2009) 479–482
481
while the alloyed Ni may form a strong passive film preventing
it from rapid dissolution [12]. For Ir exceeding 0.08 A/cm2, how-
ever, Ni content is reduced. Here, the electrode may be polarized
to the point at which zinc starts to form a protective film on its
surface, while the passivation of Ni starts to breakdown. When the
reverse pulse duration is increased with the reverse current density
maintained at 0.04 A/cm2, the deposits apparently fall in the zinc
preferential-dissolution regime and undergo extended period of
anodic dissolution. In effect, the relative Ni content in the deposits
increases significantly with pulse duration (Fig. 3b).
The resulting chemical compositions in the deposits as con-
trolled by the applications of cathodic and anodic pulses directly
influence the formation of phase structure and microstructure of
the materials (Fig. 3a and b). The specimens with Ni content varied
between ∼10 and 20 wt.% are characterized by a single ␥-phase.
In general, the ␥-phase with (3 3 0) orientation dominates and the
deposits exhibit nodular structure. For the deposits with Ni con-
tent above ∼15%, the ␥-phase with (6 0 0) orientation becomes
dominant, leading to the formation of pyramidal crystallites [14].
The phase of the deposits with Ni content exceeding ∼40% is pure
␣, which is a solid solution of Zn in Ni. The anodic current with
Fig. 3. (a and b) Nickel content in the electrodeposited Zn–Ni alloys as a function of
(a) reverse current density and (b) reverse pulse duration.
also remains in these high reverse-current deposits, but with
lower intensity and broader XRD peaks, suggesting grain size
refinement.
On the other hand, the pulse duration study reveals a monoton-
ically increasing trend of Ni content in the deposits with reverse
pulse duration (Fig. 3b). In particular, as tr is increased from 3
to 5 ms (Ir = 0.04 A/cm2), the Ni content in the deposit reaches
20 wt.%, coupled with marked increase of relative intensity of
the ␥ (6 0 0) XRD peak and a formation of pyramidal microstruc-
ture (Fig. 2a). Increasing tr beyond 5 ms, however, the dominant
phase starts to alter from ␥ to Ni-rich ␣-phase. The deposit with
tr = 10 ms is characterized by porous structure, consisting of inter-
connected, round ␣-phase crystallites (Fig. 2b) with 42% Ni. With
the longest reverse pulse duration of 20 ms, the deposit finally
composes entirely of Ni with sub-micron pores dispersed in the
structure (Fig. 2c).
The study has therefore demonstrated that application of pulse
reverse current considerably influences the relative contents of Ni
and Zn in the Zn–Ni alloy coatings. Without anodic pulsing, the
coating conforms to anomalous-typed deposit in which the con-
tent of the more noble metal (i.e., Ni) is relatively low. With the
introduction of anodic pulse, the Ni content gradually increases
(Fig. 3a). This is largely attributed to preferential dissolution of
Zn in the deposits. In particular, zinc dissolves from the ␥-phase,
Fig. 4. (a and b) X-ray diffraction patterns of Zn–Ni alloys deposited with different
(a) reverse current densities and (b) reverse pulse durations. Note that the copper
peaks belong to the copper substrates.

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relatively short pulse duration (tr = 3 ms) also provides grain refine-
ment, as exemplified by the specimen with Ir = 0.04 A/cm2 (Fig. 1b).
The promotion of such fine-grained structure may be attributed
to the increase of the content of Ni atoms that segregate to grain
boundary and thermodymically stabilize small-sized grains [15].
Furthermore, owing to metal dissolutions upon anodic pulsing,
porosity can be formed in the deposits, as evidenced in those
processed with high reverse current density (i.e., Ir > 0.08 A/cm2)
or moderate current density but with long pulse duration (e.g.,
Ir = 0.04 A/cm2, tr > 5 ms).
Acknowledgements
This research was supported by the Iron and Steel Institute of
Thailand (ISIT) (contract 022/2552). YB also acknowledges the Cen-
ter of Innovative Nanotechnology, Chulalongkorn University for
support (award CIN2551-1.2).
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The application of reverse pulse thus influences the relative
content of the alloying elements, the phase structure and the
microstructure of electrodeposited Zn–Ni coatings. By varying
the anodic current density between 0 and 0.1 A/cm2, different
degrees of metal dissolution occur, yielding ␥-phase coatings
with Ni content ranging from 11 to 17 wt.%. Deposits with much
higher Ni contents are obtained by prolonging the dezincifica-
tion period with increased pulse reverse duration. The secondary
effects of anodic pulse include grain refinement and pore forma-
tion. Reverse pulse electrodeposition technique therefore provides
a simple but efficient route for tailoring the characteristics of
Zn–Ni alloys over a broad range of compositions and struc-
tures.
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