Effect of oxalate anions on zinc electrodeposition from an acidic sulphate bath

Article abstract of DOI:10.1149/1.3575640

Zinc coatings were electrodeposited on steel substrate. The effect of oxalate anions as chelating agents on nucleation and growth mechanism, surface morphology, predominant texture and corrosion behaviour of zinc electrodeposits was investigated. Evaluation of nucleation mode of zinc in the presence and absence of oxalate anions showed that the instantaneous nucleation is predominant in both cases. From the AFM images, the nuclei densities for zinc deposition were 3.5 10⁹ and 2 10⁹ cm^-2 in the absence and presence of oxalate anions, respectively. These are one order of magnitude higher than those obtained by analysis. Electrochemical impedance spectroscopy revealed that in the bath containing oxalate anions, the growth of deposit is semi-infinite diffusion controlled. While in the absence of oxalate anions, the charge transfer control was governed. In the absence of oxalate anions, the texture components of zinc coating were mostly basal and low angle pyramidal planes, whereas in the presence of oxalate anions, the high angle pyramidal and prism planes were dominant. Furthermore, oxalate anions changed the zinc surface morphology from packets of hexagonal platelets to a granular nature. In the presence of oxalate anions, the zinc coating produced had a better corrosion resistance than that deposited from bath without oxalate anions.

Full text of DOI:10.1149/1.3575640

Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
0
D377
013-4651/2011/158(6)/D377/7/$28.00 VC The Electrochemical Society
Effect of Oxalate Anions on Zinc Electrodeposition from an
Acidic Sulphate Bath
z
S. Khorsand, K. Raeissi, and M. A. Golozar
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
Zinc coatings were electrodeposited on steel substrate. The effect of oxalate anions as chelating agents on nucleation and growth
mechanism, surface morphology, predominant texture and corrosion behaviour of zinc electrodeposits was investigated. Evalua-
tion of nucleation mode of zinc in the presence and absence of oxalate anions showed that the instantaneous nucleation is predomi-
9
9
ꢁ2
nant in both cases. From the AFM images, the nuclei densities for zinc deposition were 3.5 ꢀ 10 and 2 ꢀ 10 cm in the absence
and presence of oxalate anions, respectively. These are one order of magnitude higher than those obtained by analysis. Electro-
chemical impedance spectroscopy revealed that in the bath containing oxalate anions, the growth of deposit is semi-infinite diffu-
sion controlled. While in the absence of oxalate anions, the charge transfer control was governed. In the absence of oxalate anions,
the texture components of zinc coating were mostly basal and low angle pyramidal planes, whereas in the presence of oxalate
anions, the high angle pyramidal and prism planes were dominant. Furthermore, oxalate anions changed the zinc surface morphol-
ogy from packets of hexagonal platelets to a granular nature. In the presence of oxalate anions, the zinc coating produced had a
better corrosion resistance than that deposited from bath without oxalate anions.
VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3575640] All rights reserved.
Manuscript submitted December 8, 2010; revised manuscript received March 17, 2011. Published April 13, 2011.
Zinc coatings are widely used in order to protect steels against
corrosion because zinc is anodic to steel and sacrificially protects
the base metal at low cost. It is well known that the corrosion re-
in its molecular structure contributes in chelating. So far, the role of
oxalate anions in zinc electrodeposition has not been fully studied.
Thus, in the present work, attempts were made to investigate the
effect of oxalate anion, as an additive in sulphate bath, on nucleation
and growth mechanism, surface morphology, predominant texture
and corrosion behaviour of zinc electrodeposits.
1
sistance, formability and paintability of the coatings are associated
2
–5
with the surface morphology and predominant texture. The zinc
coatings can be applied using various techniques including electro-
deposition. The properties of electrodeposits including the surface
6
morphology and texture are strongly affected by various factors. It
Experimental
has been shown that the temperature and pH of the bath, deposition
current density, substrate surface preparation and texture intensity
of substrate influence the surface morphology and texture of zinc
Zinc coatings were electrodeposited galvanostatically by apply-
ing direct current. Using a digital coulometer model BHP 2050, dep-
5
–9
ꢂ
osition was performed at 25 C in a cell with graphite electrode as
deposits. In addition, it is well known that the additives have also
significant effect on the properties of electrodeposited coatings.
10
anode and a saturated calomel electrode (SCE) as the reference. The
reference electrode was set up close to the cathode surface via a
Luggin capillary, filled up with bath solution. The current density
The presence of additives in electroplating bath is extremely impor-
tant, basically due to their influence on the nucleation and growth of
deposits. The adsorbed additives can cover some parts of cathode
surface and influence the mechanism of zinc electrodeposition,
ꢁ
2
was 100 mA cm and the plating time was set to 110 s. A plain
low carbon steel sheet with 2 mm thickness was used as substrate,
after punching to discs shape with 1 cm surface area. Specimens
2
6
which would decrease the nucleation rate.
11
ꢂ
were sealed in a stainless steel foil sack and annealed at 880 C for 3
h. After annealing, specimens were soaked in 1000 mL hydrochloric
Some researchers have investigated the influence of surfactants
on the properties of zinc deposits. Their results have shown that in
the presence of surfactants, uniform surface morphology and smaller
acid solution (37%) containing 20 g antimony trioxide (Sb O ) and
2
3
1
2
5
2
0 g stannous chloride (SnCl ) as inhibitors (according to ASTM
grains are obtained. Alvarez et al. have also investigated the very
initial stages of zinc electrodeposition process on highly oriented
pyrolytic graphite (HOPG) from a sulphate bath in the presence of
gelatine. Their results indicate that the nucleation mechanism
changes from instantaneous to progressive due to the formation of
gelatine film on HOPG surface. Other organic additives such as thi-
standard G1) to remove the iron oxide film formed during annealing,
and then neutralized in 5% NaOH solution for 10 min. Specimens
were mechanically polished to a mirror finish using a sequence of
abrasive SiC papers (80–4000 grit) and then polished with 0.3 ꢀm
alumina. Specimens were then washed with water and ethanol
respectively, and finally dried quickly. Prior to electrodeposition,
specimens were cleaned ultrasonically in ethanol for 20 min, acti-
vated in 10 wt% sulphuric acid at room temperature for 20 s,
washed in distilled water and then immediately placed in the elec-
troplating bath.
1
0
6
ourea and benzalaceton and polyethylene glycol, thiamine hydro-
chloride and 1-butyl-3-methylimidazolium hydrogen sulphate-
1
[BMIM] HSO (Ref. 13) are also the most common additives used
4
to improve the properties of zinc electrodeposits.
Despite of the fact that the complexing agents can play an impor-
tant role in electrolytic baths, few works exist concerning their
ꢁ
1
The electrodeposition bath contained (620 g L ) ZnSO
75 g L ) Na SO as supporting electrolyte and (63 g L ) oxalic
4
ꢃ7H
2
O,
ꢁ
1
ꢁ1
1
4
(
acid (C
effects. Torent-Burg’es et al. have studied the effect of tartarate
anion, as a chelating agent, on the electrodeposition of zinc from a
sulphate solution on vitreous carbon electrodes. Their results
showed that in the presence of tartarate anions, the electrodeposition
process starts at a more positive potential than in the absence of tar-
tarate. In addition, tartarate anion allows deposits to be obtained
with a more uniform composition and morphology during the elec-
2
4
2
H
2
O
4
ꢃ2H
2
O) as chelating agent. All solutions were prepared
by dissolving the reagents in double distilled water. The pH of the
bath was adjusted to 2.0 by adding sodium hydroxide or sulphuric
acid.
The electrochemical cell was connected to an EG&G (model
63A) computer–controlled potentiostat/galvanostat. An EG&G ac
2
15
responser (model 1025) was coupled with the mentioned potentio-
stat/galvanostat to read the ac impedance data. The counter electrode
was a platinum wire and the reference electrode was a saturated calo-
mel electrode (SCE). All the potential values were referred to SCE.
Cyclic voltametery experiments were carried out from open circuit
trodeposition process.
2
ꢁ
) is a reducing agent as well as a chelating
agent for metal cations. The oxygen atoms associated with hydrogen
Oxalate anion (C
2
O
4
z
ꢁ1
E-mail: k_raeissi@cc.iut.ac.ir
potentials to ꢁ 1.4V with a scan rate of 40 mV s . The corrosion
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D378
Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
protection of zinc coatings was studied by exposing the samples in a
ꢂ
ous nucleation, all the nuclei form in the beginning of process. So,
the number of nuclei would be constant during the deposition pro-
salt spray chamber (ASTM B-117: NaCl 5%, 35 C).
1
7
The surface morphology of zinc electrodeposits was studied
using a scanning electron microscope (SEM) (model Philips Xl30).
A standard Nanoscope III atomic force microscope (AFM) was
employed in contact mode to observe the surface topography of the
zinc electrodeposits at the initial stages. Samples were prepared gal-
cess and all nuclei have similar age and size. In progressive
nucleation, the number of nuclei increases during the process.
Thus, the age of nuclei will be different. Figures 2a and 2b show
7
a typical family of potentiostatic current-time transients for zinc
deposition in the presence and absence of oxalate anions, respec-
tively. The transients have the characteristic shape of diffusion
ꢁ2
vanostatically at 100 mA cm for 3 s. The coating texture¸ was
examined by X-ray diffractometer (XRD) (model Philips XCpert
MPD). The diffractometer was operated using Cu-Ka at an acceler-
ating voltage of 40 kV and a current of 40 mA.
1
8
controlled three-dimensional nucleation. It is known that in the
first stages of nucleation, the nuclei grow independently and the
growth is controlled by hemispherical diffusion. As the time
passes, the diffusional fields around nuclei overlap each other and
the nucleation process terminates with a large proportion of the
Results and discussion
7,17
surface remained un-nucleated.
controlled by extension of linear diffusion to the area of electrode
Finally, the growth of nuclei is
Voltammetric study.— Figure 1 shows cyclic voltametery plots
of zinc electrodeposit in two sulphate baths, with and without oxa-
late anions. As it is noted, there is a wide peak for each cathodic
scan at potentials within ꢁ700 and ꢁ1020 mV. This peak disap-
pears just before the redox potential of zinc deposition (around
7
,17
surface.
As proposed by Scharifker, the nucleation mode can be deter-
mined by comparing the non-dimensional plots of (i/im) versus
2
m
(t/t ) according to Eq. 1a for instantaneous nucleation and Eq. 1b
17,19,20
for progressive nucleation
ꢁ
1020 mV). The existence of such a wide peak indicates an irre-
versible process, which represents hydrogen reduction on the steel
ꢀ
ꢁ
2
16
16
surface. According to Casanova et al., this peak arises due to the
hydrogen evolution and formation of an adsorbed zinc monolayer
just before the zinc crystallization. Zinc reduction occurs at poten-
tials lower than its redox potential which can be seen as a tail on
each plot. The large peak in the anodic range corresponds to dissolu-
tion of zinc. The redox potential is exactly the potential at which the
current density is brought to zero, (i.e., ꢁ1020 mV), and is almost
the same in both baths. This means that the complexion of some
zinc ions by the oxalate anions had no changes on redox potential of
zinc, but, it affects electrodeposition overpotential. The readings
indicate that the zinc electrodeposition overvoltage is higher in the
presence of oxalate ions than that in the absence of oxalate ions.
This can be easily seen by comparing the tail of cathodic scans
below ꢁ1020 mV. The shift of the zinc reduction potential to a
more negative value can be related to the complexion of zinc ions
i
1:9542
ðt=tmÞ
2
¼
f1 ꢁ exp½ꢁ1:2564ðt=tmÞꢄg
[1a]
[1b]
im
ꢀ
ꢁ
2
i
1:2254
ðt=tmÞ
2
2
¼
f1 ꢁ exp½ꢁ2:3367ðt=tmÞ ꢄg
im
The curves are obtained from potentiostatic current-time transient
plots, where the time (t ) can be determined from the maximum
m
17
current density (im).
The calculated dimensionless (i/i
stantaneous and progressive plots have been shown in Figs. 3a and
b. In these figures, the thick and thin lines correspond to extreme
2
m
m
) versus (t/t ) curves for in-
3
12
by the oxalate anions. In addition, the intensity of anodic peak,
which corresponds to the zinc dissolution, decreases for the coating
deposited in the presence of oxalate anions. Therefore, it can be con-
cluded that the oxalate anions would decrease the mass of zinc elec-
trodeposited. This is in coincidence with above discussion.
Nucleation of zinc.— Nucleation mode.— In order to determine
1
7
the nucleation mode, Scharifker’s model was applied. Two
forms of nucleation which can be observed during the electrode-
position process are instantaneous and progressive. In instantane-
Figure 1. Cyclic voltametery plots in sulphate bath with oxalic acid (curve
Figure 2. Family of potentiostatic current-time transient plots obtained dur-
ing the electrodeposition of Zinc onto steel (a) with oxalate anions (b) with-
out oxalate anions.
ꢁ1
a) and without oxalic acid (curve b), scan rate ¼ 40 mV s , vertex
pot ¼ꢁ1.4 V.
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Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
D379
Figure 4. AFM micrograph of mechanically polished surface, (a) 3D view,
b) top view and (c) section analysis diagram.
(
obtained in oxalate-free bath is about 12.07 nm (Fig. 5c), while
the Ra of those obtained from oxalate-containing bath (Fig. 6c) is
about 14.20 nm.
2
ꢂ
Figure 3. Dimensionless curves (i/i ) vs (t/t ) at 25 C and pH 2. Key: (~)
m m
ꢁ
1.25 V, (^) ꢁ 1.3 V, (h) ꢁ 1.35 V, (*) ꢁ 1.4 V. (thick line) instantaneous
Compared with bath with oxalate anions (Fig. 6a), in the
bath with no oxalate anions the zinc deposit obtained at the
early stages of electrodeposition creates more isolated islands
nucleation mode, and (thin line) progressive nucleation mode; (a) with oxa-
late anions (b) without oxalate anions.
(Fig. 5a). This means that in the latter case more nucleation
occurs. Nuclei measurement showed that the average nuclei
instantaneous and progressive nucleation, respectively. According
to Scharifker, at the beginning of each transient, the current den-
sity increases due to an increase in the electro-active area. In this
7
,17
stage, the nuclei grow in size and/or number.
During this stage,
expansion of diffusion zones around each nucleus occurs. The
decrease in current density after the maximum current density
would be due to overlapping the diffusional fields around these
2
1
nuclei. In addition, hydrogen evolution reaction can disperse
zinc ions from the cathode surface and cause decay after the maxi-
2
2
mum. Some of the discrepancies that are observed could possi-
bly be due to non-uniform coverage of the substrate surface by
diffusional zones, which could be due to non-uniform distribution
2
3
of active sites for nucleation on the steel substrate. Non-uniform
distribution of active sites for nucleation on steel substrate is gen-
erally expected because of heterogeneity and defects, which usu-
ally exist on a commercial steel surface. It has been claimed that
the experimental results obtained on a carbon substrate show bet-
2
4
ter correspondence. As can be seen, in both cases (with and
without oxalate additions), the experimental data lie near the theo-
rical instantaneous curves. Thus, the instantaneous nucleation
would be predominant.
AFM observation.— Figure 4 shows the AFM image of mechani-
cally polished steel surface used as the substrate. The AFM image
of steel substrate was scanned over a 1 ꢀ 1 ꢀm of surface area.
The average roughness value (Ra) of the surface was about 10 nm.
AFM images of the initial stages of zinc electrodepositon from
both baths at ꢁ1350 mV after 3 s are shown in Figs. 5 and 6.
Figure 5a shows the isolated islands developed on all areas of the
steel substrate. The average roughness value (Ra) of the islands
Figure 5. AFM micrograph of zinc electrodeposited galvanostatically at 100
mA cm for 3 s in the absence of oxalate ions, (a) 3D view, (b) top view
and (c) section analysis diagram.
ꢁ
2
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D380
Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
ꢄ
ꢄ
ꢂ
ꢃꢅꢅ
1
ꢁ expðꢁP3tÞ
P3
i3DꢁdcðtÞ ¼ P4tꢁ
1=2
ꢀ
1 ꢁ exp ꢁP2 t ꢁ
[
4]
1
2
FD =2C
p
where, P4 ¼
total) is given by:
and the total current due to overall process
1
=2
(
i
ꢆ
ꢇ
ꢅ
ꢁ1=2
itotalðtÞ ¼ P þ P4t
1
ꢄ
ꢄ
ꢂ
ꢃꢅꢅ
1
ꢁ expðꢁP3tÞ
P3
ꢀ
1 ꢁ exp ꢁP2 t ꢁ
[5]
[6]
where
ꢀ
ꢁ
1=2
2
CM
ꢅ
P ¼ P1
1
pq
Fitting the experimental current-time transients by non-linear rela-
*
1 2 3 4
tionship of Eq. 5, P , P , P , and P parameters are obtained. From
these parameters, electrodeposition processing parameters such as
N, A and kPR could be obtained. Table I shows the above mentioned
*
1
parameters obtained at ꢁ1350 mV. From P
, the kPR is calculated in
ꢁ
8
ꢁ2 ꢁ1
the order of 10 mol cm
value of k in the same order for nickel and cobalt electrodeposi-
s . Other authors have also found the
PR
18,25
tion.
8
The value of N, in order of 10 cm , is obtained from fit-
ꢁ2
7
ꢁ2
ting i-t transients. The N values are 31 ꢀ 10 cm in the absence
Figure 6. AFM micrograph of zinc electrodeposited galvanostatically at 100
7
ꢁ2
and 22 ꢀ 10 cm in the presence of oxalate anions, which are at
ꢁ2
mA cm for 3 s in the presence of oxalate ions, (a) 3D view, (b) top view
and (c) section analysis diagram.
least one order of magnitude less than those obtained by AFM (i.e.,
3
9
ꢁ2
9
ꢁ2
.5 ꢀ 10 cm and 2 ꢀ 10 cm in the absence and presence of ox-
alate anions). The disagreement between values of N obtained from
the chronoamperometric analysis and those obtained by observa-
tions has also been pointed out in literature. Using SEM, Torent-
14
14
9
ꢁ2
9
ꢁ2
9
Burg’es et al. have found nuclei density in order of 10 cm , 2
orders of magnitude higher than those obtained by chronoampero-
metric analysis.
density (N) for zinc deposition is 3.5 ꢀ 10 cm and 2 ꢀ 10
ꢁ
2
cm
in the absence and presence of oxalate anions, respec-
tively. These values are estimated by direct counting the nuclei
i.e., obvious isolated islands) observed on AFM top view
It has been reported that the values of A for zinc electrodeposi-
(
ꢁ
1 14
tion are within 1–100 s
transient curves (Table I) are in agreement with above results.
.
The A values obtained by fitting i-t
images in Figs. 5b and 6b. The nuclei density values for zinc
deposition reported by some authors using AFM or SEM obser-
vations are in the range of 10 –10 cm .
9
10
ꢁ2 14
The analysis of current transients obtained requires that the pro-
ton reduction (PR) reaction is taken into account in addition to the
nucleation process. The current density (iPR) associated with the
Nucleation rate constant.— As seen in Fig. 2, the current transients
obtained in the bath containing oxalate anions is qualitatively simi-
lar to that obtained in the bath with no oxalate anions. The main dif-
ference observed is the lower current density obtained in the bath
with oxalate anions, which means that a lower current density is
associated with nucleation and growth of zinc deposit in this case.
This is in a good agreement with the previous results obtained by
cyclic voltametery and AFM observations. Consequently, a lower
nucleation rate is expected in the presence of oxalate anions.
1
8
proton reduction reaction is given by
iPRðtÞ ¼ P1SðtÞ
[2]
with P ¼ zPRF kPR, where zPRF is the molar charge transferred dur-
1
ing the proton reduction process, kPR is the rate constant of the pro-
ton reduction reaction and
Considering the nucleation rate constant (A) and nuclei density
(N) at ꢁ1350 mV, the term A ꢀ N can be calculated from Eq. 7,
where ZF is the moles of charge transfers during electrodeposition,
C is the bulk concentration of metal ion, M is the molar mass of the
ꢀ
ꢁ
ꢄ
ꢄ
ꢂ
ꢃꢅꢅ
1
=2
2
CM
1 ꢁ expðꢁP3tÞ
P3
SðtÞ ¼
ꢀ 1 ꢁ exp ꢁP2 t ꢁ
[3]
pq
15
deposit, and q is the density of the deposited material
where C is the bulk concentration of metal ion, M is the molar mass
of the deposit, q is the density of the deposited material, P ¼ Np D
2
2 2
q
Z F
1=2 3=2
1
/2
(
the nucleation rate constant, respectively.
8pC/q) and P
3
¼ A, with N and A being the nuclei density and
A ꢀ N ¼ 0:2898ð
Þ
C
[7]
2
3
1
8
8pM
i t
m m
The current associated with the zinc reduction process is
given by
ꢁ1
ꢁ3
1
8
For zinc coatings, M is 65.37 g mol and q is 7.14 g cm . By sub-
stituting these values in Eq. 7 and considering the values of N
Table I. Best-fit parameters and kinetic data resulting from analysis of the experimental current-time transient in Fig. 2 at 21350 mV according
to the model described by Eq. (5).
8
ꢁ7
8
*
1
D ( ꢀ 10 )
N ( ꢀ 10
)
A
(s
K
(mol cm
pr (ꢀ 10 )
P
(mA cm
4
P
(s
3
P
(s
2
P
(mA cm )
Bath
2
cm s
ꢁ1
ꢁ2
ꢁ1
ꢁ2 ꢁ1
ꢁ2 1/2
ꢁ1
ꢁ1
ꢁ2
(
)
(cm
)
)
s
)
s
)
)
)
0
0
.42
.30
31
22
12
5.8
7.2
11.7
15.3
12.9
12
5.84
0.335
0.18
0.78
1.26
Sulphate bath without oxalic acid
Sulphate bath with oxalic acid
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Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
D381
double layer construction and charge transferring and an inductive
developed at low frequencies. The induction is associated with an
adsorption process of electrochemical active specie such as zinc
7,26
hydroxides.
From Fig. 7b, it can be seen that the capacitive loop at high fre-
quencies is very similar to what is observed in Fig. 7a. However, in
the low frequency region, a different behaviour is observed. As is
evident from Fig. 7b, a low-frequency straight line with an angle
ꢂ
near 45 against the real axis is appeared, indicating Warburg
behaviour. Generally, the Warburg behaviour arises due to the pres-
ence of diffusion barriers in electrochemical systems. The chelating
agents may produce a surface film layer during the electrodeposi-
þ2
tion, which provides a diffusion barrier against Zn ion movements
across the metal/solution interface. The Warburg impedance (Z
observed whenever the reaction is under partial or complete mass
w
) is
27
transport control by diffusion. The term is reserved for the special
case of semi-infinite linear diffusion. In the condition of semi-infi-
nite linear diffusion, Z for a system at equilibrium can be
w
27
expressed by
0:5
Zw ¼ rxxꢁ ꢁ jrxxꢁ0:5
[8]
x
where r is the Warburg constant, x is the ac angular frequency,
Zw can be considered as a series combination of Warburg capaci-
tance (C
lowing equations
w
) and a psudoresistance (R
w
), which are given by the fol-
27
Cw ¼ rꢁx
1
x
ꢁ0:5
[9]
Rw ¼ rxxꢁ
0:5
[10]
The appearance of Warburg impedance means that the growth pro-
cess of deposit is intensively under semi-infinite diffusion control.
Therefore, it seems that the presence of oxalate anions, via bouncing
the diffusion of Zn (II) ions across the metal/solution interface, leads
to a retardation of the zinc electrodeposition kinetics. Consequently,
zinc growth can proceed through a semi-infinite length of diffusion
pathway.
Figure 7. (Color online) Nyquist plots drawn at ꢁ 1350 mV in the bath (a)
without oxalate anions, (b) with oxalate anions
The electrical equivalent circuits (ECs) are determined by apply-
ing the Zview software and are inserted in Figs. 7a and 7b. In the
estimated from AFM observations, the term A can be calculated
ECs, Rct is the charge transfer resistance, R
inductive loop, L is the inductor referred to the adsorption of elec-
trochemical active specie and R is the resistance of the inductor. R
is the uncompensated solution resistance, W is the Warburg resist-
ance, R is polarization resistance and CPE is the constant phase ele-
1 1
and L are elements of
ꢁ
1
ꢁ1
which is about 0.04 s in the presence and 0.1 s in the absence of
oxalate anions, respectively. These values are about 2 orders of mag-
nitude lower than those obtained by i-t transient analysis (Table I).
Anyway, the value of A in the absence of oxalate anions is twice of
that in the presence of oxalate anions. This means that the number
of nuclei created on the steel substrate decreases to about half mag-
nitude in the electrodeposition bath containing oxalate anions. This
denotes on the inhibitive effect of oxalate anions on nucleation of
zinc.
1
1
S
1
p
ment. When the Nyquist plot contains a “depressed” semicircle with
the centre under the real axis, the parallel network charge transfer
resistance-double layer capacity (Rct–Cdl) is usually a poor approxi-
mation. The depression is related to roughness and in homogeneities
28
of the solid surface and is a characteristic for solid electrodes. For
improving the fitness, CPE is used.The impedance of CPE is
expressed by the following equation
Growth of zinc deposits.— Electrochemical impedance spectros-
copy.— In order to study the effect of oxalate anions on zinc
electrodeposition mechanism, EIS measurements were performed in
the frequency range of 100 kHz–10 mHz, at ꢁ1350 mV. Figure 7
shows the Nyquist plots of experimental and simulated impedance
spectra for zinc deposition from baths without (Fig. 7a) and with
n ꢁ1
ZQ ¼ ½Y0ðjxÞ ꢄ
[11]
ꢁ
2
where Z
Q
ꢁ
is the CPE impedance (X cm ), Y
0
is the CPE admit-
1
n
ꢁ2
tance (X s cm ). The parameter n is a constant, which shows
the degree of deviation from ideal capacitive behaviour.
29
(
Fig. 7b) oxalate anions, respectively. As seen, the correlation
between experimental and simulated data is recognized to be accept-
able. According to Fig. 7a, the impedance plot shows two loops
including a capacitive at high frequencies which arises due to the
In Table II, the calculated values of circuit elements are summar-
ised. The results show that the oxalate containing bath possesses a
larger value of R . The R may be proportional to the surface
p
p
Table II. Parameters extracted from Nyquist plots in Fig. 7.
2
ꢁ2 ꢁ1 ꢁ0.5
2
2
ꢁ2
2
Rp (X cm )
W
1
(cm
X
s
)
R
1
(X cm )
L
1
(H)
R
ct (X cm )
CPE (ꢀF cm
)
R
s
(X cm )
Bath
3
5
.0
.2
—
7.3
17.55
—
0.0011
—
3.85
3.0
28
15
7.3
8.6
Sulphate bath without oxalic acid
Sulphate bath with oxalic acid
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D382
Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
Figure 8. Morphology of zinc electrodeposited on mechanically polished
ꢁ
2
steel substrate at 100 mA cm after 110 s, (a) in the absence of oxalate
anions, (b) in the presence of oxalate anions.
5
diffusion of zinc adions on substrate surface. Thus, a larger value
of R denotes on retarded growth of deposit.
p
SEM observation.— Figure 8a shows the SEM images of zinc coat-
ing deposited from the bath with no oxalate anions. The zinc depos-
Figure 10. RTCs of zinc coating electrodeposited in the bath (a) with oxalic
anions and (b) without oxalate anions.
ꢁ
2
its were galvanostatically grown at 100 mA cm on steel substrate
for 110 s. It can be seen that the zinc coatings consist of thin plate-
lets of hexagonal crystals, packed on the steel substrate. The packets
of hexagonal platelets with various orientations are stacked on each
other, all over the substrate surface. As seen, the coatings exhibit a
uniform and compact appearance. This result is in a good agreement
tern and IR(hkl) refers to the intensity of hkl plane in standard zinc
powder which is compared with the standard compiled by the Joint
Committee on Powder Diffraction and Standards (JCPDS), with
5
32
with the result obtained previously.
In the presence of oxalate anions, a zinc coating with dull
included card #04-0831.
In the absence of oxalate anions, in addition to basal (00.2)
plane, low angle pyramidal {10.3} and {10.4} planes can be consid-
ered as the texture components. It is important to notice that if the
relative texture coefficient (RTC) value is more than 1, the corre-
3
0
appearance was obtained. Some researchers believe that the crys-
tallographic orientation is related to the surface crystal appearance.
The dull surface appearance is due to the pyramidal (10.2 or 10.1)
3
1
33
or prismatic (10.0) grain orientations. As seen from Fig. 8b, the
presence of oxalate anions in the bath changes the surface morphol-
ogy of the zinc coating markedly. Surface morphology of zinc coat-
ing is changed from packets of hexagonal to granular nature, most
probably due to the diffusion controlled condition governed by the
presence of oxalate anions.
sponding texture is preferred. By adding oxalate anions, high
angle pyramidal {10.1}, prism {10.0} and {11.0} planes are
appeared as the texture components (Fig. 10). Such differences in
texture can be explained in terms of the diffusion controlled condi-
tion built up by the presence of oxalate anions in the electrodeposi-
tion bath, which has been discussed before. The granular surface
morphology obtained from the bath containing oxalate anions
X-ray analysis.— Figure 9 shows XRD spectra of zinc coatings
obtained in both baths (i.e. with and without oxalate anions). The
relative texture coefficients (RTC) of deposits were determined
using Eq. 12 (Ref. 31) and are shown in Fig. 10
(
Fig. 8b) is developed due to the presence of high angle pyramidal
and prism texture components. From the above discussion, it can be
concluded that the oxalate anions evidently play an important role
on development of high angle pyramidal and prism texture compo-
nents in the zinc electrodeposit coating.
I
=I
TðhklÞ RðhklÞ
RTC ¼
½12ꢄ
n
Salt-spray results.— The sustainability of coated specimens to chlo-
ride corrosion attack can be assessed by salt-spray tests. In these
tests, the more corrosion resistance of coatings, the longer is the
time for corrosion products to be appeared on the coating surface. In
the case of steel substrates, appearance of red rust on the coating
surface is the sign of substrate corrosion. The results obtained (time
for red rust observations) for the coatings produced in baths with
and without oxalate anions are shown in Table III. The results show
that the time of red rusting is longer for zinc coating obtained in the
presence of oxalate anions (i.e., 64 h) than the zinc coating produced
in oxalate-free bath (i.e., 40 h). This indicates that the zinc coating
produced in the presence of oxalate anions has a more corrosion
protective property.
P
ð
I
=IRðhklÞÞ ꢀ 1=n
TðhklÞ
1
where n is the number of planes selected from X-ray diffraction pat-
tern, IT(hkl) is the peak intensity of hkl plane in X-ray diffraction pat-
Table III. Salt spray tests results.
64 h
40 h
Spray time
—
Emerging of
ed rust spots
Rustless
Zinc coating in the
absence of oxalate anions
Zinc coating in the
Figure 9. X-ray diffraction patterns of zinc coatings deposited on mechani-
ꢁ2
Emerging of red
rust spots
cally polished steel substrate at 100 mA cm after 110 s (a) with oxalic
anions and (b) without oxalate anions.
presence of oxalate anions
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Journal of The Electrochemical Society, 158 (6) D377-D383 (2011)
D383
¸
6. J. C. Ballesteros, P. DCıaz-Arista, Y. Meas, R. Ortega, and G. Trejo, Electrochim.
Acta, 52, 3686 (2007).
Conclusions
7
.
K. Raeissi, A. Saatchi, and M. A. Golozar, J. Appl. Electrochem., 33, 635 (2003).
8. K. Raeissi, A. Saatchi, and M. A. Golozar, Trans. Inst. Met. Finish., 81, 186
2003).
1
. The nucleation mode of zinc coatings on steel substrate is in-
(
stantaneous and no change is observed by the presence of oxalate
anions in bath. However, a less number of nuclei are formed on steel
substrate in the presence of oxalate anions.
9
.
K. Raeissi, M. R. Bateni, A. Saatchi, M. A. Golozar, and J. A. Szpunar, Surf. Coat.
Technol., 201, 3116 (2006).
0. M. C. Li, L. L. Jiang, W. Q. Zhang, Y. H. Qian, S. Z. Luo, and J. N. Shen, J. Solid
1
2
. In the presence of oxalate anions, different impedance behav-
State Electrochem., 11, 549 (2007).
1
1
1
1
1. A. Gomes and M. I. da Silva Pereira, Electrochim. Acta, 51, 1342 (2006).
2. A. E. Alvarez and D. R. Salinas, J. Electroanal. Chem., 566, 393 (2004).
3. Q. Zhang and Y. Hua, J. Appl. Electrochem., 39, 261 (2009).
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alate anions, the growth of deposit is controlled by semi-infinite
diffusion, which differs from charge transfer control condition
observed in the absence of oxalate anions.
15. E. Guaus and J. Torrent-Burgues, J. Electroanal. Chem., 575, 301 (2005).
1
1
1
6. T. Casanova, F. Soto, M. Eyraud, and J. Crousier, Corros. Sci., 39, 529 (1997).
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trochim. Acta, 50, 4736 (2005).
3
. The presence of oxalate anions in the bath can change the sur-
face morphology of the zinc coating from packets of hexagonal pla-
telets to granular morphology. In oxalate-containing bath, higher
percentages of high angle pyramidal and prism planes develop at the
expense of basal plane.
19. G. A. Gunawardena, G. Hills, I. Montenegro, and B. Scharifker, J. Electroanal.
Chem., 138, 225 (1982).
2
2
2
0. J. Mostany, J. Mozota, and B. Scharifker, J. Electroanal. Chem., 177, 25 (1984).
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4
. The zinc coatings produced in the presence of oxalate anions
showed better substrate corrosion protectivity, as compared with
zinc coatings obtained in the bath without oxalate anions.
23. L. Heerman and A. Tarallo, J. Electroanal. Chem., 470, 70 (1999).
2
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