Effect of additives on electrodeposition of nanocrystalline zinc from acidic sulfate solutions

Article abstract of DOI:10.1149/1.2772093

The influence of polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB), benzalacetone (BA), and thiourea (TU) on pulse electrodeposition of zinc from acidic sulfate baths was investigated by X-ray diffraction, scanning electron microscopy, and potentiodynamic polarization techniques. The results show that a mixture of PEG and CTAB can fully dissolve BA in the concentrated zinc sulfate solutions. This ternary additive can increase the overpotential of zinc electrodeposition markedly, and then gives rise to forming a bright nanocrystalline zinc coating on steel substrate, with an average grain size of 52 nm. The combination of these four compounds is more powerful for improving the cathodic polarization and inhibit hydrogen evolution during zinc electrodeposition in comparison with ternary additive systems. A bright and compact zinc coating with a grain size of 43 nm and the (110)(100)(201) preferred orientations are produced due to the coexistence of all four compounds. These compounds act in a synergetic way, especially between BA and TU, leading to the grain refinement of zinc deposits.

Full text of DOI:10.1149/1.2772093

Journal of The Electrochemical Society, 154 ͑11͒ D567-D571 ͑2007͒
D567
0013-4651/2007/154͑11͒/D567/5/$20.00 © The Electrochemical Society
Effect of Additives on Electrodeposition of Nanocrystalline Zinc
from Acidic Sulfate Solutions
Moucheng Li,a,z Suzhen Luo,b Yuhai Qian,b Wenqi Zhang,a Lili Jiang,a and
Jianian Shena
aInstitute of Materials, Shanghai University, Shanghai 200072, China
bBaoshan Iron & Steel Company, Limited, Technology Center, Shanghai 201900, China
The influence of polyethylene glycol ͑PEG͒, cetyltrimethylammonium bromide ͑CTAB͒, benzalacetone ͑BA͒, and thiourea ͑TU͒
on pulse electrodeposition of zinc from acidic sulfate baths was investigated by X-ray diffraction, scanning electron microscopy,
and potentiodynamic polarization techniques. The results show that a mixture of PEG and CTAB can fully dissolve BA in the
concentrated zinc sulfate solutions. This ternary additive can increase the overpotential of zinc electrodeposition markedly, and
then gives rise to forming a bright nanocrystalline zinc coating on steel substrate, with an average grain size of 52 nm. The
combination of these four compounds is more powerful for improving the cathodic polarization and inhibit hydrogen evolution
during zinc electrodeposition in comparison with ternary additive systems. A bright and compact zinc coating with a grain size of
43 nm and the ͑110͒͑100͒͑201͒ preferred orientations are produced due to the coexistence of all four compounds. These com-
pounds act in a synergetic way, especially between BA and TU, leading to the grain refinement of zinc deposits.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2772093͔ All rights reserved.
Manuscript submitted January 15, 2007; revised manuscript received June 21, 2007. Available electronically August 30, 2007.
In recent years, nanosized metal electrodeposits have attracted
great attention due to the merits of nanostructured materials. In the
literature, many pure metals and alloys were electrodeposited with
grain size less than 100 nm, such as Ni,1 Cu,2 Pd,3 Zn–Ni,4
Ni–Fe–Cr,5 and so on.
with a grain size of 60 nm. However, benzalacetone with emulsifier
OP ͑polyethyleneglycol p-isooctylphenyl ether͒ as solubilizer dis-
solved incompletely in the concentrated zinc sulfate solutions, giv-
ing rise to forming turbid baths. The aim of this work is to have an
insight into the role of four commercial organic compounds in zinc
electrodeposition and then produce zinc coatings with smaller grain
size from the sulfate-based baths.
It is well known that electrocrystallization occurs by growth of
existing crystals or formation of new nuclei.6 The latter is favored at
high overpotential and low surface diffusion rates of adsorbed me-
tallic ions on the growing surface and then benefits formation of
nanocrystals.7-9 Practically, organic additives and pulse electrodepo-
sition are utilized most frequently to produce homogeneous and ul-
trafine deposits. Organic additives, added commonly as leveler and
brightener to electroplating baths, can increase the overpotential and
nucleation rate and inhibit the surface diffusion of adions. These
conditions also can be fulfilled by using pulse electroplating, be-
cause a higher peak current density is possible in comparison with
direct current plating, i.e., much higher than the limiting current
density attained during direct current plating in the same
electrolyte.9-11 Thus, organic additives and pulse control are power-
ful means to achieve grain refinement down to the nanosized dimen-
sion of metallic deposits.
Experimental
Electrolytic cell and sulfate-plating bath.— Zinc electrodeposi-
tion was carried out in a two-electrode cell containing 200 mL
sulfate-plating solution. A platinum foil served as the anode. Cold-
rolled low-carbon steel specimens were used as the cathode with an
exposed surface area of about 0.1 cm2 ͑i.e., electrodes with a di-
mension of 3.4 ϫ 3 mm͒. Prior to each plating experiment, the
specimen surface was ground with 800 grit waterproof abrasive pa-
per and then pickled in 10 wt % H2SO4 at room temperature for 30
s. The anode and cathode were fixed with a space of about 5 cm and
immersed vertically into the plating solution about 3 cm lower than
the solution surface. The plating solutions were made of
350 g L−1 ZnSO4·7H2O ͑i.e., about 1.22 mol L−1͒ with or without
organic additives, maintained at 23 2°C, and agitated slowly by a
magnetic stirrer. The pH value was adjusted to about 1 by dilute
H2SO4. Polyethylene glycol ͑PEG, C2nH4n+2On+1͒ with a mean mo-
lecular weight of 600 g mol−1, CTAB ͑C19H42BrN͒, benzalacetone
͑BA, C10H10O͒, and thiourea ͑TU, CH4N2S͒ were used separately or
in combination as additives. The studied solutions were listed as
follows:
Zinc electrodeposition on steel surfaces is an important industrial
process for protection of steels from corrosion. Nanocrystalline
zinc coatings had been produced in different electrolytes in order to
get better properties. In acetate-based bath without additives, several
nanocrystalline zinc deposits were obtained by cyclic voltammetry
and square wave pulsating potential methods, among which
the smallest grain size was about 68 nm.12 In chloride-based bath
with a mixture of thiourea and polyacrylamide, a zinc deposit with
an average grain size of 50 nm was produced by pulse current
control.13,14 In sulfate-based bath in the presence of surfactants
͓cetyltrimethylammonium bromide ͑CTAB͒, sodium dodecyl sulfate
͑SDS͒, and Triton X-100͔ and N2 bubbling, zinc coatings with dif-
ferent crystal shape and size ͑in the range 40–20 nm͒ were prepared
on stainless steel substrates by pulse current electrolysis.15 Youssef
et al. found that nanocrystalline zinc coatings had higher hardness
and corrosion resistance in alkaline solutions in comparison with
conventional polycrystalline ones.13,16
1. S0: ZnSO4·7H2O, the base solution
2. SP: S0 + PEG
3. SC: S0 + CTAB
4. SPC: S0 + PEG + CTAB
5. SCB: S0 + CTAB + BA
6. SPCB: S0 + PEG + CTAB + BA
7. SCT: S0 + CTAB + TU
8. SPT: S0 + PEG + TU
9. SPCBT: S0 + PEG + CTAB + BA + TU
In a previous work,17 a simple acidic sulfate-based bath without
nonzinc supporting electrolyte was studied at high current densities
on the basis of continuous zinc electroplating for steel plates. A
synergetic effect between the two additives ͑benzalacetone and thio-
urea͒ induced the formation of particle-like nanocrystalline zinc
In all cases, the concentrations of PEG, CTAB, BA, and TU were
2, 0.5, 0.5, and 1 g L−1, respectively. All chemicals were analytical
grade. Distilled water was used to make these solutions.
Electrodeposition procedure.— According to a previous study,17
zinc electrodeposits were prepared by applying a galvanostatic
square wave for 10 min with a pulse peak current density of
2 A cm−2. The current-on time and current-off time were set at 4
z E-mail: mouchengli@yahoo.com.cn
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D568
Journal of The Electrochemical Society, 154 ͑11͒ D567-D571 ͑2007͒
and 8 ms, respectively. A PAR ͑Princeton Application Research,
AMETEK, Inc.͒ system, which comprised an M273A potentiostat/
galvanostat and the PowerSuite software, was used to generate the
current waveform and supply the current.
Cathodic polarization curves were determined potentiodynami-
cally with a potential scan rate of 2 mV s−1 in the aforesaid elec-
trolytic cell using a saturated calomel electrode ͑SCE͒ as the refer-
ence electrode. Each curve was corrected for ohmic potential drop
deduced from the high-frequency electrode impedance. Potentiody-
namic scans and impedance measurements were conducted using the
above PAR system.
Characterization of zinc coatings.— The surface morphology
and grain size of zinc electrodeposits were observed using scanning
electron microscopy ͑SEM͒ ͑JSM 6700F͒. X-ray diffraction ͑XRD͒
analysis was conducted by using a Rigaku diffractometer ͑D/MAX
2550 V͒ with Cu K␣ irradiation ͑␭ = 0.15405 nm͒. The scanning
rate was 6° per min for 2␪ ranging from 30 to 90°, or 0.5° per min
from 41 to 46° for the line-broadened peak of zinc. The crystal size
was also estimated by the Scherrer’s equation D = K␭/B cos ␪,
where D is average crystal size, ␭ is the wavelength of the X-ray
irradiation, K is usually taken as 0.89, B is the full width half maxi-
mum ͑fwhm͒ of the diffraction peak corrected for the instrumental
line broadening using silicon as a standard, and ␪ is the diffraction
angle.18 Before XRD determination, zinc deposits with black sur-
faces were ground on filter paper to remove the black matter.
The texture of coating can be determined in terms of a preferred
orientation factor, p͑hkl͒, which is defined as19
Figure 1. ͑Color online͒ XRD patterns for zinc deposits obtained from so-
lutions SC, SP, and SPC ͑૽: ZnO; #: steel substrate͒.
Bonou et al.33 also observed a similar blocking effect from them. In
the same way, the adsorption of PEG might be enhanced by Br− ions
from CTAB, as these two compounds coexisted, which could be the
I͑hkl͒/Io͑hkl͒
p͑hkl͒ =
main reason for the formation of a smaller grain size in solution SPC
.
1
Nevertheless, the black appearance of the three layers indicated that
the solutions were in need of other brighteners for producing nano-
crystalline zinc with metallic luster.
͓I͑hkl͒/Io͑hkl͔͒
͚
n
where I͑hkl͒, Io͑hkl͒, and n are the measured intensity of ͑hkl͒
reflections, the theoretical intensity of ͑hkl͒ reflections, and the
number of reflections used in the analysis, respectively. In this treat-
ment, p͑hkl͒ is the relative fraction of crystals that have ͕hkl͖ plane
normals perpendicular to the surface of the electrodeposit. A value
of p͑hkl͒ larger than one indicates that the ͑hkl͒ reflection is more
highly oriented than the ͑hkl͒ reflection of a random powder sample.
BA in combination with CTAB and PEG as additives.— In
recent years, BA has been increasingly used as an additive in plating
baths to promote the formation of shiny zinc coatings.25-29 In the
present case, a mixture of CTAB and PEG gave rise to the full
dissolution of BA in the base solution, forming a transparent solu-
tion SPCB. CTAB also could dissolve BA fully on its own, while
PEG could not.
Zinc coatings electrodeposited from solution SPCB were bright
but were black from solution SCB. The reason might be that PEG, as
one type of nonionic surfactant, increased the solubility and bright-
ening power of BA.25 SEM observation ͑Fig. 2͒ found that the
bright coating was composed of particle-like nanocrystalline zinc.
Figure 3 gives the XRD patterns for them. For solution SCB, the
coating had a ͑110͒ preferred orientation, while there were
͑100͒͑110͒ orientations for solution SPCB. ZnO content in the bright
zinc coating was very low because the corresponding peaks were
extremely weak.
Results in Table I for solutions SPC and SPCB displayed that add-
ing BA to solution SPC offered a bright appearance to the zinc coat-
ing but almost no decrease in grain size. It seems that the mixture of
PEG and CTAB served as a grain refiner, while BA served mainly as
a brightener in the electrodepositing process of zinc. Actually, the
three additives should interact closely with each other, instead of
independently. In the literature,13,22,24,25 a synergetic effect was of-
ten noticed when several additives were added to the electroplating
Results and Discussion
Using additives in plating baths is a fundamental factor to affect
the growth and structure of the zinc electrodeposits. In order to
make the best of the synergetic effect between BA and TU during
zinc electrodeposition,17 PEG and CTAB were used to improve the
dissolution of BA in concentrated zinc sulfate solutions. Obviously,
all these compounds are common additives for zinc
electroplating.20-30 The following results attempted gradually to
show their effect and coeffect on the preparation of nanocrystalline
zinc coatings on steel specimens. By the way, a small amount of
ethanol or methanol was generally used to dissolve BA in aqueous
solutions,27,28 but the pre-experiments found that these two com-
pounds did not dissolve BA fully in the base solution S0, mainly due
to the high concentration of zinc sulfate.
PEG and/or CTAB as additives.— PEG and CTAB used sepa-
rately or together resulted in black zinc electrodeposits. Zinc depos-
its were characterized by XRD, as shown in Fig. 1. The coatings
showed the line broadening of diffraction peaks and the
͑100͒͑110͒͑112͒ preferred orientations. In addition, the response of
steel substrate and ZnO was detected. Table I gives the calculated
grain size of zinc deposits. It can be seen that nanocrystalline zinc
coatings were obtained from the three solutions SP, SC, and SPC, and
the combination of PEG and CTAB in solution, SPC, led to a marked
decrease of the grain size, about 20 nm, in comparison with the
separate cases of SP and SC. As for Cu deposition, Kelly et al.31,32
found that Cl− ions induced the adsorption of a nearly complete
monolayer of PEG molecules in the form of collapsed spheres.
Table I. The grain size and surface luster of zinc coatings depos-
ited from different baths.
Solution SC
SP
SPC
SCB
SPCB
SCT SPT
78 90
SPCBT
Grain
77
72
53
69
52
43
size ͑nm͒
Luster
Black Black Black Black Bright Gray Yellow Bright
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Journal of The Electrochemical Society, 154 ͑11͒ D567-D571 ͑2007͒
D569
Figure 2. SEM morphology of zinc electrodeposited from solution SPCB
.
Figure 4. SEM morphology of zinc electrodeposited from solution SPCBT
.
baths. The additives could form a strong adsorbed layer, even dif-
ferent kinds of associates, and then exerted a greater influence on
zinc electrodeposition than the separate ones.
the absence of it. This meant that TU did not interact with PEG or
CTAB to reduce the grain size of zinc deposits, but the opposite was
true. The result for solution SPCBT indicated that adding TU to solu-
tion SPCB decreased the grain size from 52 to 43 nm. It can be
deduced that a synergetic effect existed between TU and BA and
played an important role in the depositing process of nanocrystalline
zinc coatings. The interaction mechanism of TU and BA in the
deposition process of zinc had been scarcely investigated. According
to Cu deposition,34 TU might adsorb on the electrode and/or even
form complexes with metallic ions during electrodepositing, leading
to a promotion of the nucleation. The action of BA was principally
on the electrode surface, blocking the discharge of metallic ions by
creating a barrier in the vicinity of the electrode surface.26 Mockute
and Bernotiene25 supposed that the formation of different associates
by BA with other additives ͑e.g., benzoic acid, and even the reduced
compounds of BA͒ was the reason for the emerged synergistic effect
during zinc electrodeposition, because the associates occupied a
larger area on the electrode surface than separate additives. There-
TU-combined mixtures as additives.— The influence of TU-
combined mixtures was studied, and a bright zinc coating was elec-
trodeposited from solution SPCBT. The SEM image in Fig. 4 clearly
shows that this coating consisted of nanocrystalline zinc. Zinc layers
obtained from solutions SPT and SCT did not have bright surface
qualities. Figure 5 presents the XRD patterns of these zinc coatings.
In the case of solution SPCBT, zinc deposits displayed the
͑110͒͑100͒͑201͒ preferred orientations, the line broadening of dif-
fraction peaks, and the existence of ZnO. As for solutions SPT and
SCT, ZnO almost was not found and the coatings had the ͑110͒͑100͒
orientations and the ͑100͒͑110͒͑112͒ preferred orientations, respec-
tively.
A comparison of solutions SP, SC, SPT, and SCT from Table I
indicated that the grain size was larger in the presence of TU than in
Figure 3. ͑Color online͒ XRD patterns for zinc deposits obtained from so-
lutions SCB and SPCB ͑૽: ZnO; #: steel substrate͒.
Figure 5. ͑Color online͒ XRD patterns for zinc deposits obtained from so-
lutions SCT, SPT, and SPCBT ͑૽: ZnO; #: steel substrate͒.
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D570
Journal of The Electrochemical Society, 154 ͑11͒ D567-D571 ͑2007͒
model for zinc deposition in acidic electrolytes.35,37 This might be
caused by the blocking effect of additives adsorbed on the electrode
surface, which led to the increase of local pH values with hydrogen
evolution. Gomes and daSalva Pereira obtained similar results.15
The quasi plateaus appeared in the polarization curves before the
rapid deposition of zinc, especially in solution SPCBT. In these po-
tential domains, the current response could be attributed to the pre-
dominant hydrogen evolution on the adsorbed species.37 Further-
more, compared with solution SPCB, the current density was much
lower in solution SPCBT, which implied that the hydrogen evolution
became slower during zinc electrodeposition in the presence of TU,
as observed by Song et al.21 This change in the lower potential
regions was mainly due to the dominant effect between TU and BA
in solution SPCBT and the partial desorption of PEG in solution
SPCB.
22 It was reported that a vigorous hydrogen evolution was able
to achieve porous copper and tin electrodeposits.39,40 Thus, as a
result of the different hydrogen evolution rate, zinc deposits pre-
pared from solutions SPCBT ͑Fig. 4͒ were evidently more compact
and homogeneous than from solution SPCB ͑Fig. 2͒.
Conclusion
Figure 6. ͑Color online͒ Polarization curves for steel electrodes in solutions
Organic compounds PEG, CTAB, BA, and TU, used separately
or in combination as additives, have great effect on the morphology
and structure of zinc electrodeposits obtained in concentrated acidic
sulfate baths by a high pulse current control.
S0, SPCB, and SPCBT
.
fore, the presence of a synergistic effect between TU and BA was
possibly due to the simultaneous formation of complexes by zinc
ions and TU and associates by TU and BA, and even involving
PEG, CTAB, and the decomposed matters of these additives.
A mixture of PEG and CTAB, compared with the separate ones,
leads to the formation of a nanocrystalline zinc coating with much
smaller grain size and burnt deposits. This mixture is able to en-
hance the solubility of BA in the concentrated zinc sulfate baths,
forming transparent baths. A mixed additive of these three com-
pounds shows a strong inhibiting effect, which promotes a shift of
zinc deposition potentials to much more negative values in compari-
son with additive-free case. As a result, bright nanocrystalline zinc
with an average grain size of 52 nm and the ͑100͒͑110͒ preferred
orientations is produced in the presence of this ternary additive.
Compared with the above ternary additive, all four compounds
used together in the plating bath can shift zinc deposition potentials
to more negative values and more powerfully inhibit the hydrogen
evolution during zinc electrodeposition. The synergetic effect of
these compounds, especially for BA and TU, is of great influence on
the zinc deposition process to achieve the grain refinement. As for
this four-additive bath, the electrodeposited zinc is bright and com-
pact, with a particle-shaped morphology, an average grain size of 43
nm, and the ͑110͒͑100͒͑201͒ preferred orientations. In addition, a
small amount of ZnO is identified in the deposits.
Polarization curves.— Bright nanocrystalline zinc coatings were
obtained from solutions SPCB and SPCBT, while only nanolaminated
zinc was produced from the base solution S0.17 Figure 6 gives the
polarization curves for steel substrates in these three solutions to
show the influence of the two mixed additives on zinc electrodepo-
sition. In addition, the polarization curves for the other six solutions
were not discussed here because the electrodeposited zinc layers had
poor appearance, especially burnt deposits.
In the case of base solution S0, a cathodic peak appeared in the
potential range −0.7 to −1.1 VSCE, which was related to the hydro-
gen evolution and formation of adsorbates ͑e.g., ZnHad, ZnOad and
ZnOHad͒.35,36 Zinc started to deposit at about −1.1 VSCE. The rapid
deposition at more negative potentials led to formation of a quasi-
vertical curve, similar to the results in Ref. 15, 37, and 38. The data
with current density larger than 50 mA cm−2 were deleted in view
of the marked ohmic drop. In solution SPCB, the analogous hydrogen
evolution peak was markedly suppressed, which could be ascribed
to the inhibitive action of organic additives, especially PEG. It was
reported that PEG could form a strongly adsorbed layer with a well-
ordered structure on the cathode surface and prevented protons from
accessing the electrode, leading to the suppression of hydrogen
evolution.22,24 However, in comparing solution SPCBT with SPCB, the
evolution peak was enhanced in the presence of TU. This was pos-
sibly due to the conflicting effect of PEG and TU in this potential
region. As mentioned above, the addition of TU to solution SP ͑i.e.,
solution SPT͒ resulted in the increase of grain size from 72 to 90 nm,
which suggested these two additives competed with each other dur-
ing the adsorption process and then gave rise to a reduction in the
overall adsorption, as observed between PEG and benzoic acid in
chloride solutions.22
Acknowledgments
Financial support provided by Natural Science Foundation of
China ͑NSFC͒ in combination with Shanghai BaoSteel Group Cor-
poration ͑grant no. 50471105͒ is greatly appreciated.
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