Effect of phytic acid on the microstructure and corrosion resistance of Ni coating

Article abstract of DOI:10.1016/j.electacta.2010.05.054

In this work, the pure Ni coatings were synthesized on Q235 steel by using reverse pulsed electrodeposition technique in sulphate-based baths with 0, 0.1, 0.2 and 0.3 g/L phytic acid additive. The effect of phytic acid on the microstructure and micro-morp

Full text of DOI:10.1016/j.electacta.2010.05.054

Electrochimica Acta 55 (2010) 5990–5995
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effect of phytic acid on the microstructure and corrosion
resistance of Ni coating
Guozhe Menga,b,∗,1, Feilong Suna, Yawei Shaoab, Tao Zhanga,b
,
Fuhui Wanga,b, Chaofang Dongc, Xiaogang Lib,c
a Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University,
Ministry of Education, Harbin 150001, China
b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
c Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, the pure Ni coatings were synthesized on Q235 steel by using reverse pulsed electrode-
position technique in sulphate-based baths with 0, 0.1, 0.2 and 0.3 g/L phytic acid additive. The effect
of phytic acid on the microstructure and micro-morphology of the sample was observed by transmis-
sion electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. And the effect of
phytic acid on the corrosion resistance of the sample was studied by potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS) techniques. The results demonstrated that the addition
of phytic acid was in favor of the growth of nano-scale twins (NT) in the interior of grains, which was
due to the lowered stacking fault energies of Ni during the electrodeposition, and the typical morphol-
ogy of pyramidal islands on the surface. The results also demonstrated that the effect of phytic acid was
not monotonous with increasing concentration: the passive current density ip was minimum and the
charge transfer resistance Rt was maximum for the sample obtained from the bath with 0.2 g/L phytic
acid, indicating that the sample obtained from the bath with 0.2 g/L phytic acid showed the best corrosion
resistance.
Received 10 March 2010
Received in revised form 13 May 2010
Accepted 17 May 2010
Available online 24 May 2010
Keywords:
Nano-scale twins
Phytic acid
Reverse pulsed electrodeposition
TEM
Corrosion
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
certain materials [5,9]. Lu et al. synthesized a Cu coating with nano-
scale growth twins using a pulsed electrodeposition technique [10].
The concept of ‘grain boundary design and control’ has been
introduced by Watanabe in 1984 [1]. This concept was developed
as grain boundary engineering (GBE) by Palumbo [2]. And much
attention has been paid to the application of GBE since then [3–6]. It
was found that the property enhancement of these materials can be
obtained by the application of GBE due to an increase in the propor-
tion of coincidence site lattice (CSL) boundaries, which sometimes
owing to twinning.
They found that the mechanical and electrical properties of nano-
scale growth twins were remarkably enhanced compared with the
conventional crystalline and nanocrystalline counterparts as well.
In our previous work [11,12], we synthesized a NT nickel coating
by using pulsed electrodeposition technique and investigated the
effects of NT structure on their corrosion resistance in the solu-
tion with and without chloride ions. It was demonstrated that NT
structure noticeably enhanced the pitting corrosion resistance of
nickel.
It was known that the enhanced corrosion resistance was closely
related to the twin density in the metals or alloys [2,4,5]. High
twin densities could be easily obtained in metals and alloys with
low stacking fault energies (SFEs), such as stainless steels [4,13]
and Cu–Zn alloys [14], under proper conditions. But the twins
may be difficult to form in some materials with high SFEs (such
as Al and Ni) except under extreme conditions [15–17]. It has
been demonstrated that the pulsed electrodeposition technique
can be employed to synthesize metal coatings with NT structure
[10–12,18]. On the other hand, organic additive contents in the
electrodeposition bath also have effective role on the microstruc-
Grain size and special grain boundaries are two of the principal
control factors in the concept of GBE [7,8]. It has been demonstrated
that refining grain and CSL boundaries can improve properties of
∗
Corresponding author at: Corrosion and Protection Laboratory, Key Labora-
tory of Superlight Materials and Surface Technology, College of Materials Science &
Chemical Engineering, Harbin Engineering University, Ministry of Education, Nan-
tong ST 145, Harbin, Heilongjiang 150001, China. Tel.: +86 451 8251 9190;
fax: +86 451 8251 9190.
E-mail address: mengguozhe@hrbeu.edu.cn (G. Meng).
ISE member.
1
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.05.054

G. Meng et al. / Electrochimica Acta 55 (2010) 5990–5995
5991
Table 1
Table 2
The electrolyte composition for reverse pulsed electrodeposition of
pure Ni coatings.
The electrodeposition parameters for reverse pulsed elec-
trodeposition of pure Ni coatings.
Ni2SO4·6H2O
NiCl2
H3BO3
C4H6O2
CH3(CH2)11OSO3Na
C6H18O24P6
240 g/L
30 g/L
30 g/L
0.2 g/L
0.1 g/L
0, 0.1, 0.2, 0.3 g/L
ia,p
tp,on
tp,off
tp
ia,r
tr,on
tr,off
tr
1.5 A/dm2
800 ␮s
3200 ␮s
100 ms
0.3 A/dm2
100 ␮s
1000 ␮s
10 ms
Total plating time
Temperature
pH
30 min
40 ◦C
4.2
ture and the resultant properties of metal coatings. The organic
additive, such as 2-butyne-1, 4-diol and phenolic derivative, could
modify the structure and surface topography of the deposits to a
large extent and produces smoother deposits [19,20]. Phytic acid
is a kind of polydentate binder molecule and has polyfunctional
groups [21,22], which can be used as an additive in the electrode-
position bath. It may absorb on the preferential locations of the
surface as an inhibitor [23] and then may facilitate the formation
of twin structure. Therefore, it is essential to investigate the effect
of phytic acid on the microstructure and corrosion resistance of Ni
coatings.
In this work, the reverse pulsed electrodeposition technique was
used to synthesize pure Ni coatings in sulphate-based electrolyte
with various concentrations of phytic acid. The effects of phytic acid
on the micro-morphologies, microstructures and then the corro-
sion properties of the pure Ni coatings were studied by SEM, TEM
and electrochemical methods.
ia,p and ia,r refer to the positive and reverse pulse average
current densities, respectively.
2.2. Analysis of microstructure
The microstructures of the electrodeposited samples were
observed by a TEM, which is a Jeol 2010 and operated at 200 kV.
The surface and cross-section micro-morphologies of the samples
were examined by SEM (JSM-6300).
2.3. Electrochemical experiments
All electrochemical studies were carried out in a three-electrode
cell. The counter and reference electrodes were Pt and Ag/AgCl (sat-
urated KCl), respectively. The working electrode was the Ni coating.
The potentiodynamic polarization measurements were performed
using a model 273 potentiostat/galvanostat (EG&G) with a poten-
tial scan rate of 0.333 mV/s. The EIS measurements were carried
out on an electrochemical workstation (IVIMSTATE). The scanning
frequency ranged from 100 kHz to 10 mHz and the perturbing AC
amplitude was 5 mV. The reproducibility of the presented data was
generally checked by duplicate or triplicate measurements and typ-
ical results were reported.
2. Experimental
2.1. Preparation of samples
The nickel coating was synthesized by using reverse pulsed elec-
trodeposition technique from a solution of NiSO4, the composition
of which was given in Table 1. The reverse pulsed electrodeposi-
tion was carried out galvanostatically by square wave pulses, which
were schematically shown in Fig. 1. The detail electrodeposition
parameters were shown in Table 2. The substrate for electrode-
position was Q235 steel with a chemical composition (wt%): C
0.14–0.22, Mn 0.30–0.65, Si ≤ 0.30, S ≤ 0.050, P ≤ 0.045 and Fe bal-
ance. The Q235 steel sheet was cut into 20 mm × 20 mm × 2 mm
and abraded with 1000 grit SiC paper and finally degreased with
acetone before electrodeposition. A highly purified (99.99%) nickel
sheet was used as the soluble anode.
All experiments were performed at 30 2 ◦C in 6.2 g/L
H3BO3 + 28.6 g/L Na2B4O7 buffer solutions with NaCl 5.85 g/L
(pH = 8.7). All the reagents used in this work were analytical pure.
Distilled water was used to prepare all aqueous solutions.
3. Results and discussion
3.1. Effect of phytic acid on the microstructure
TEM observations of the typical microstructure in as-deposited
Ni samples are shown in Fig. 2. Fig. 2a shows the bright-field TEM
image of Ni coating obtained in electrolyte without phytic acid.
It can be seen that it consists of irregular shaped grains, most of
which are roughly equiaxed in three dimensions and the grain
size is approximately 500 nm. The corresponding electron diffrac-
tion pattern is shown in Fig. 2b, which indicates that the coating
has the typical face-centered cubic (fcc) crystal structure (crystal
plane index are marked in the figure. A and B denotes interpla-
√
nar spacing, where A/B = 2). Fig. 2c shows the bright-field TEM
image of Ni coating obtained in electrolyte with 0.2 g/L phytic acid
and Fig. 2d shows the corresponding electron diffraction pattern.
It indicates that the coating consists of irregular equiaxed shaped
grains in three dimensions with a grain size of about 500 nm, which
is similar to the sample obtained in electrolyte without phytic
acid. However, the noticeable difference is that high densities of
growth twins are present in the interior of each grain, which is
of the {1 1 1}/[1 1 2] type (Fig. 2d). The diffraction spots of twins
and matrix form mirror symmetry along the crystal plane of (1 1 1).
Measurements of the lamella thickness show a wide distribution
ranging from about 50 to 220 nm (Fig. 3). The high density growth
Fig. 1. Schematic diagram of the wave shape of the reverse pulsed electrodeposition.

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G. Meng et al. / Electrochimica Acta 55 (2010) 5990–5995
Fig. 2. TEM observations of the typical microstructure in as-deposited Ni samples. A bright-field TEM image (a) and the electron diffraction pattern (b) of Ni coating obtained
from the bath without phytic acid; a bright-field TEM image (c) and the electron diffraction pattern (d) of Ni coating obtained from the bath with 0.2 g/L phytic.
twins separate submicron-sized grains into nanometer-thick twin
lamellar structures.
adsorption of various species (especially hydrogen species) on the
crystal faces of the growing nickel crystals [24–27].
From what have discussed above, it can be concluded that the
organic additive of phytic acid in the bath changes the microstruc-
ture of pure Ni coating. Phytic acid is in favor of the growth of twins
in Ni sample, which has been reported to be difficult to form twins
due to its high SFEs [15–17]. Therefore, the SFEs of Ni must be low-
ered during the deposition when phytic acid was added into the
bath as an additive. This may be the result of modification of the
surface state by the adsorption of phytic acid, in terms of that the
direction of preferential growth was determined by a competitive
3.2. Effect of phytic acid on the micro-morphology
SEM observations of the surface micro-morphologies of Ni coat-
ings obtained in electrolyte with 0, 0.1, 0.2 and 0.3 g/L phytic acid
are shown in Fig. 4. It can be seen that pyramidal islands appear on
the sample obtained from the solution with phytic acid. When the
content of phytic acid was 0.2 g/L, the finest islands were observed
(Fig. 4c).
Fig. 5 shows SEM observations of the cross-section morpholo-
gies of Ni coatings obtained in electrolyte with 0, 0.1, 0.2 and 0.3 g/L
phytic acid. It can be found that the thickness of the samples varied
with the concentration of phytic acid in the bath (Fig. 5a–d). The
thickness of the sample obtained in the bath with 0.3 g/L phytic acid
is maximal (about 17.5–26.3 ␮m), while the thickness of the sam-
ple obtained in the bath with 0.1 g/L phytic acid is minimal (about
4.4–8.8 ␮m). Among these samples, the sample obtained in the bath
with 0.2 g/L phytic acid is relatively uniform.
3.3. Effect of phytic acid on corrosion property
The pitting potential (Eb) and passive current density (ip) are
important parameters to evaluate the corrosion resistance of pas-
sive metal. Potentiodynamic polarization curves of the samples
obtained in the baths with 0, 0.1, 0.2 and 0.3 g/L phytic acid are
shown in Fig. 6a. It can be observed that all the samples show
self-passive behavior in borate buffer solution with 5.85 g/L NaCl.
However, their corrosion potential, corrosion current density, pit-
Fig. 3. The statistical distributions for thickness of the twin lamellae.

G. Meng et al. / Electrochimica Acta 55 (2010) 5990–5995
5993
Fig. 4. SEM observations of the surface micro-morphologies of Ni coatings obtained from the bath with 0 g/L (a), 0.1 g/L (b), 0.2 g/L (c) and 0.3 g/L (d) phytic acid.
Fig. 5. SEM observations of the cross-section micro-morphologies of Ni coatings obtained from the bath with 0 g/L (a), 0.1 g/L (b), 0.2 g/L (c) and 0.3 g/L (d) phytic acid.

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G. Meng et al. / Electrochimica Acta 55 (2010) 5990–5995
Fig. 7. The Nyquist plots (a) and Bode plots (b) of EIS of pure Ni coatings obtained
from the bath with 0, 0.1, 0.2 and 0.3 g/L phytic acid. The solid lines are the fitting
results using the equivalent electrical circuit in Fig. 8a.
Fig. 6. Potentiodynamic polarization curves (a) and passive current densities (b) of
pure Ni coatings obtained from the bath with 0, 0.1, 0.2 and 0.3 g/L phytic acid.
ting potential, passive current density and passive potential zone
are clearly different. For the sample synthesized from the bath
without phytic acid, the value of Eb is about 200 mV. While for
the samples synthesized from the baths with phytic acid, the val-
ues of Eb vary from 310 to 350 mV. The increase of Eb indicates
the enhancement of pitting corrosion resistance of the samples.
The lower the value of ip is, the better the corrosion resistance of
passive metal. Fig. 6b shows the passive current densities ip of the
samples as a function of the different concentration of phytic acid.
It can be observed that the ip decreases first and then increases
with the increase of phytic acid concentration. And it reaches to
the minimum value when the concentration of phytic acid is 0.2 g/L.
Another noticeable difference is that all the values of ip of the sam-
ples obtained in the baths with the addition of phytic acid are
smaller than that of the samples obtained in the baths without the
addition of phytic acid. This indicates that the corrosion resistance
of the NT samples are enhanced compared with the conventional
coatings, which is in accordance with our previous work [11–12];
the sample obtained in the bath with 0.2 g/L phytic acid shows the
best corrosion resistance.
Fig. 7 shows the EIS of the samples obtained from the various
baths in borate buffer solution with 5.85 g/L NaCl, which are pre-
sented as Nyquist plots and Bode plots. The equivalent electrical
circuit (Fig. 8a) is used to fit the impedance data, where Rs is the
solution resistance, CPEf is the constant phase element (CPE) of the
passive film capacitance, Rpore designates the electrolytic resistance
in the defects of the passive film, CPEdl is the CPE of the double layer
capacitance and Rt designates the charge transfer resistance of Ni,
respectively [28–31]. This model assumes that the passive film does
not totally cover the metal surface and cannot be considered as a
perfect layer. In fact, neither real surfaces of solids in the active
range nor passive film on the metals or alloys can be considered to
be ideally perfect [32]. In Fig. 7, the solid lines are the fitting results
using the equivalent electrical circuit in Fig. 8a.
It can be found that the value of impedance of the samples
increases first and then decreases as the concentration of phytic
acid increases. And it reaches to the highest point when the con-
centration of phytic acid is 0.2 g/L (Fig. 7b). The charge transfer
resistance Rt of the samples can be obtained through the fitting
with the equivalent electrical circuit, which are plotted as a func-
tion of the addition of phytic acid in Fig. 8b. It can be observed
that the value of Rt increases first and then decreases with the
increase of the addition of phytic acid in the baths. And a maxi-
mum value of Rt is observed when the concentration of phytic acid
is 0.2 g/L. This indicates that the sample obtained from the bath
contaning 0.2 g/L phytic acid shows the best corrosion resistance,
which is in line with the results obtained from potentiodynamic
polarization measurements. It may be closely related to the high
density of NT (Fig. 2c), the fine and homogeneous morpholo-
gies (Figs. 4c and 5c) of the sample. Our previous work [33] has
showed that the passive film formed on NT sample was more inte-
grated and compact; the composition of the film was modified and
more homogeneous across the cross-section; and the adsorption
capability of chloride ions on the film was markedly attenuated.
These changes decreased the susceptibility of NT sample to pit-
ting corrosion and were responsible for its enhanced corrosion
resistance.

G. Meng et al. / Electrochimica Acta 55 (2010) 5990–5995
5995
The samples obtained from the bath with 0.2 g/L phytic acid shows
the best corrosion resistance.
Acknowledgements
The authors acknowledge the financial support from the
National Nature Science Found of China under the contract No.
50971050, the financial support from the Ministry of Science and
Technology of China (No. 2005DKA10400) and the Fundamental
Research Funds for the Central Universities (no. HEUCF101005).
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The pure Ni coatings were synthesized by using reverse pulsed
electrodeposition technique in sulphate-based baths with 0, 0.1,
0.2 and 0.3 g/L phytic acid additive. The addition of phytic acid
markedly affected on the microstructure, micro-morphology and
corrosion property of pure Ni coatings. The addition of phytic acid
was in favor of the growth of NT in the interior of grains, which was
due to the lowered SFEs of Ni during the electrodeposition. The
addition of phytic acid also affects the corrosion resistance of pure
Ni coatings. The effect of phytic acid on the corrosion resistance
of the sample was not monotonous with increasing concentration.