Investigation of Sn-Zn electrodeposition from acidic bath on EQCM

Article abstract of DOI:10.1016/j.jallcom.2010.10.161

Tin-zinc (Sn-Zn) alloy with low tin content was deposited on gold electrode and steel substrate with use of chronoamperometric technique from an acidic bath. In order to evaluate coating efficiency of Sn-Zn alloy in 0.5 M NaCl solution, open circuit potential-time curve (E_OCP-t), polarization curves, mass change of the electrode (Δm-t) using quartz crystal microbalance (QCM) were compared to those of pure Sn and Zn coatings. Anodic stripping measurements were carried out simultaneously with the mass loss of the deposit. Scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) analysis were performed to characterize the surface morphology. Anodic stripping experiment and EDS analysis indicated that Sn, Zn, and SnO₂ formed on the electrode surface when Sn-Zn was coated from acidic bath. Furthermore, local mapping demonstrated homogeneous distribution of Sn and Zn atoms throughout the surface.

Full text of DOI:10.1016/j.jallcom.2010.10.161

Journal of Alloys and Compounds 509 (2011) 1534–1537
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Investigation of Sn–Zn electrodeposition from acidic bath on EQCM
Mürsel Aricia, Hasan Nazira,∗, M. Levent Aksub
a Ankara University, Faculty of Science, Department of Chemistry Tandogan, 06100 Ankara, Turkey
b Gazi University, Faculty of Education, Department of Chemistry Education, Teknikokullar, 06500 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2010
Received in revised form 20 October 2010
Accepted 27 October 2010
Available online 4 November 2010
Tin–zinc (Sn–Zn) alloy with low tin content was deposited on gold electrode and steel substrate with use
of chronoamperometric technique from an acidic bath. In order to evaluate coating efficiency of Sn–Zn
alloy in 0.5 M NaCl solution, open circuit potential–time curve (EOCP–t), polarization curves, mass change
of the electrode (ꢀm–t) using quartz crystal microbalance (QCM) were compared to those of pure Sn
and Zn coatings. Anodic stripping measurements were carried out simultaneously with the mass loss
of the deposit. Scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) analysis
were performed to characterize the surface morphology. Anodic stripping experiment and EDS analysis
indicated that Sn, Zn, and SnO2 formed on the electrode surface when Sn–Zn was coated from acidic bath.
Furthermore, local mapping demonstrated homogeneous distribution of Sn and Zn atoms throughout the
surface.
Keywords:
Metals and alloys
Coating materials
Oxidation
SEM
© 2010 Elsevier B.V. All rights reserved.
Corrosion
1. Introduction
perometric technique from the acidic bath due to the fact that it
both provides good protection for ferrum based metals and is cost
Tin–zinc (Sn–Zn) alloys are used as sacrificial plated coatings
for the protection of ferrum based metals. These coatings provide,
high corrosion protection, frictional or anti-frictional properties [1],
ductility and solderability to base metal in question [2–5]. Sn–Zn
coatings are used in various industrial applications such as on the
chassis of electrical and electronic equipment owing to low elec-
trical contact resistance of Sn–Zn alloy [3,5,6]. Sn–Zn coatings are
also used as an alternative to cadmium and nickel coatings because
of their toxic and allergenic effects [2,5,7,8]. However, the first bath
used for Sn–Zn alloys was the cyanide bath. New tin–zinc bath com-
positions were investigated due to the toxicity of cyanide baths and
the difficultly to control the parameters [3,4,6]. Hence, there are dif-
ferent bath compositions alternative to the cyanide baths reported
in industry, emphasizing the Sn–Zn ratio being the primary fac-
tor which effects the coating performance. Some studies reported
that Sn–Zn coating with 70–80% Sn and 30–20% Zn in the alloy in
neutral bath had the best mechanical and protective effect [3,6,7].
Sn–Zn coatings with high tin content provide permanent protection
against rusting and uniform corrosion of metals [1]. At the same
time, some researchers studied Sn–Zn coatings with high zinc con-
tent in slightly acidic bath [1,9]. In this study, Sn–Zn alloy with low
Sn content was electroplated onto gold electrode using chronoam-
effective. Coating performance of Sn–Zn alloy was examined with
EQCM technique in 0.5 M NaCl solution. In addition, scanning elec-
tron microscopy (SEM) and energy dispersive X-ray spectroscopy
(EDS) were utilized to characterize surface morphology and analyze
coating composition before and after Sn–Zn deposit was exposed
to corrosion in 0.5 M NaCl.
2. Experimental
The EQCM measurements were carried out in a Model 400A time-resolved elec-
trochemical quartz crystal microbalance (CH Instrument, USA) linked to a computer
equipped with electrochemistry software (CHI400A). Electrochemical experiments
were performed in a conventional three-electrode cell with a capacity of 4 ml. The
working electrode used in QCM and anodic stripping experiments was gold AT-cut
quartz crystal (CHI125A). Surface area of a gold crystal was 0.204 cm2 and its nom-
inal resonant frequency was 7.995 MHz. ST-42 carbon steel with a surface area of
0.204 cm2 was also used as a working electrode in other electrochemical studies.
The reference and counter electrodes were a Ag/AgCl/KCl(sat) electrode (CHI111)
and a platinum rod (CHI115) respectively. Sn–Zn alloy coating was deposited from
the mixture of SnSO4·7H2O (5 g/L), ZnCl2·6H2O (75 g/L), NH4Cl (180 g/L) and pH was
adjusted to 1.04 using conc. H2SO4. Sn and Zn coatings were carried out following
bath compositions: Sn bath: SnSO4·7H2O (80 g/L) and 15 ml conc. H2SO4; Zn bath:
ZnCl2·6H2O (75 g/L) and NH4Cl (180 g/L). All reagents were analytical grade from
Merck. Only one side of the gold crystal was coated with Sn, Zn and Sn–Zn alloy
monitored current–mass (i–ꢁm) change at a potential range of −0.6 V to −1.2 V
by the use of chronoamperometric technique. The mass and theoretical thickness
changes of Sn–Zn, Sn and Zn coatings on the electrode were calculated by Sauerbrey
2
√
2f ꢂꢁm
o
equation as follows [10]:ꢁf = −
A
ꢃꢄ
∗
Corresponding author. Tel.: +90 312 2126720x1026–1035;
fax: +90 312 2232395.
where ꢀm is the mass change of the quartz crystal surface (g), ꢀf is measured
frequency (f1 − f0) (Hz) shift caused by ꢀm, fo is the resonant frequency of the fun-
damental mode of the crystal (Hz), A is the geometric area of the electrode (cm2), ꢄq
is shear modulus of quartz (2.947 × 1011 g cm−1s−2) and ꢃq is the density of quartz
E-mail addresses: arici mursel086@hotmail.com (M. Arici),
nazir@science.ankara.edu.tr (H. Nazir), maksu@gazi.edu.tr (M.L. Aksu).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.10.161

M. Arici et al. / Journal of Alloys and Compounds 509 (2011) 1534–1537
1535
Fig. 1. SEM, EDS and local mapping analysis images for Sn–Zn alloy deposited by using chronoamperometric technique on steel substrate at −1.2 V final potential.
(2.648 g cm−3). The theoretical thickness (Tf, cm) of the deposits was calculated from
ꢀm/ꢃ (density g/cm3).
with the change of the mass of the electrode was monitored for
approximately one minute. The amount and the thickness of the
coverage on the surface calculated by Sauerbrey equation were
62 ␮g and about 100 nm, respectively. The steel substrate was
coated with Sn–Zn alloy in the same manner. The SEM image (mag-
nification of 2000×) and EDX spectra of Sn–Zn coated steel are
shown Fig. 1. According to the EDS analysis the deposit was found
to contain 22.73 wt% Sn, 66.12 wt% Zn and 11.15 wt% O. EDS results
indicate that Sn are coated as Sn and SnO2 on the electrode surface
from acidic bath due to the oxidation. Since Sn2+ ions are oxidized
to Sn4+ ions in acidic medium, tin is deposited on the surface as
SnO2 as well as Sn [11]. The formation mechanism of SnO2 can be
given as follows:
In order to determine corrosion resistance of Sn–Zn coating, the change in depo-
sition was recorded by comparing with pure Sn and Zn coatings and steel for 5 h in
0.5 M NaCl at room temperature by the use of open circuit potential (OCP). The OCP
curves monitored for five hours. The polarization curves were taken after 5 h OCP
measurements. The polarization curves were scanned at 1 mV/s adjusting potentials
200 mV more cathodic and anodic of the OCP value. Mass changes of Sn, Sn–Zn and
Zn electrodes were monitored for 5 h at OCP in 0.5 M NaCl. Anodic stripping analysis
was carried out at a scan rate of 10 mV/s in 0.5 M NaCl solution immediately after
deposition. Morphology and composition of the coverage were examined with SEM
(JEOL JSM-7000F Field Emission SEM apparatus) in the presence and the absence of
0.5 M NaCl.
3. Results and discussion
SO42− + 2H2O + 2e− → SO2 + 4OH−
Sn2+ + 4OH− → SnO2 + H2O
3.1. Electrodeposition process and surface morphology and
compositional analysis of Sn–Zn deposit
Sn–Zn alloy was coated on one side of the gold electrode surface
using chronoamperometric technique. The change in the current
Fig. 2. OCP–t curves of Sn, Sn–Zn, Zn deposits and steel substrate in 0.5 M NaCl
Fig. 3. Polarization curves of Sn, Sn–Zn, Zn coatings and steel in 0.5 M NaCl after the
solution.
electrode was kept at the OCP for 5 h.

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M. Arici et al. / Journal of Alloys and Compounds 509 (2011) 1534–1537
Fig. 5. Stripping voltammogram at ꢅ = 10 mV s−1 of deposit simultaneously mass
changing of deposit in 0.5 M NaCl solution.
Fig. 4. Mass change of Sn, Sn–Zn, Zn coatings versus time at the electrode surface
in 0.5 M NaCl at OCP.
at the end of 5 h. These results indicate that Sn–Zn bath prepared
from acidic bath provides good protection for ferrum based met-
als. Moreover, polarization curves of Sn, Sn–Zn, Zn coatings and
steel in 0.5 M NaCl are shown in Fig. 3 after OCP was recorded
for 5 h. As shown in Fig. 3, break down potential of Sn–Zn coat-
ing is more positive than that of Zn coating and the pure steel and
more negative than that of Sn coating. These results showed that
Sn (22.73 wt%)–Zn alloy coating obtained with acidic bath is more
resistance than pure Zn coating.
Local mapping analysis of alloy component and analysis of its
EDX spectra also showed the formation of Sn, Zn and SnO2 on the
surface.
3.2. OCP and polarizing curves measurement
The open-circuit potential against time (EOCP–t) curves of the
pure steel and Sn, Sn–Zn, Zn coatings on steel substrate measured
for 5 h in 0.5 M NaCl solution are shown in Fig. 2. As seen from Fig. 2,
OCP value of Sn–Zn deposit is more positive than those of Zn deposit
and the pure steel and more negative than that of pure tin deposit
3.3. Mass change
The mass losses of Sn, Sn–Zn, Zn deposits with time in 0.5 M
NaCl were followed up to 5 h using EQCM at OCP. The slopes of
Fig. 6. SEM images and EDX spectra for Sn–Zn deposit after 5 h immersion in 0.5 M NaCl solution.

M. Arici et al. / Journal of Alloys and Compounds 509 (2011) 1534–1537
1537
curves are proportional to corrosion rates of the coatings [3,12]. As
seen from Fig. 4, the total mass losses of Sn, Sn–Zn and Zn deposit
are −9.3 ␮g, −16.3 ␮g and −23.05 ␮g, respectively at the end of 5 h.
These results show that Sn–Zn deposit is more resistance than pure
Zn deposit. Moreover, there was a rapid mass loss observed for the
Sn–Zn coverage for the first 6000s because of the dissolution of Zn
in the deposit. The mass loss was observed to decrease after 9000s
due to decrease in the amount of Zn in the deposit.
sizes SnO2 nano particles in deposit and Zn and Sn undergoes cor-
rosion in 0.5 M NaCl at the end of 5 h. EDS analysis result after a
corrosion time reveals that oxide and chloride layers are formed
on the surface. According to EDS analysis, zinc oxide and zinc chlo-
ride can be found as corrosion products of zinc in 0.5 M NaCl on the
surface [13,14].
4. Conclusions
3.4. Potentiodynamic anodic stripping
Sn–Zn alloy was electroplated with chronoamperometric tech-
nique from acidic bath. The investigation of the corrosion resistance
of Sn–Zn coating compared to Sn and Zn coating by the use OCP
and polarization curves showed that Sn–Zn coatings coated from
acidic bath provide good protection to the steel matrix. Moreover,
according to QCM result, Sn–Zn alloy coating with low tin content
is more resistance than pure Zn coating. According to potentiody-
namic anodic stripping and EDS results, the coverage was found
to contain Sn, Zn and SnO2 in the deposit and the amount Zn in
the deposit was higher than the other components. SEM pictures
showed the presence of SnO2 nanoparticles in the deposit. Low Sn
content Sn–Zn coating by the use of an acidic bath gave highly
satisfactory results regarding to the protection of steel from the
corrosion process.
Potentiodynamic anodic stripping was carried out simultane-
ously with the mass change of coated surface to elucidate the
coverage of Sn–Zn layer upon the surface. Anodic stripping experi-
ment was carried out using the gold working electrode. Fig. 5 shows
simultaneous anodic stripping curve and mass change for Sn–Zn
deposit. The deposited film was stripped in the reverse direction (in
anodic direction) in 0.5 M NaCl after Sn–Zn alloy was coated under
the same conditions (temperature, pH and mass change of deposit)
in order to show the presence of SnO2 in the deposit. The peaks
heights of the anodic stripping curve can also be used to estimate
proportion of tin and zinc in deposit [5]. As shown by Fig. 5, anodic
stripping starts with the dissolution of Zn (P1) and is followed by the
dissolution of Sn (P2). Anodic dissolution peaks of metallic Zn and
Sn are observed at −0.725 V and −0.394 V. The stripping peak P3,
corresponding to Sn+4 ion (−0.073 V), shows the presence of SnO2
in deposit. Moreover, as seen from mass change in Fig. 5, Zn ratio
is higher than the other constituents in the deposit. Mass change
curve indicates that the mass loss of Zn in deposit is −32 ␮g up to
−0.675 V it was Zn which is mainly consumed in the deposit. Mass
loss of Sn is −8.2 ␮g up to −0.38 V in the deposit. These results show
that mass change and anodic stripping experiment verify that the
coverage obtained from the acidic bath was rich in Zn and Sn was
coated on the surface as Sn and SnO2.
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