Electrochemical impedance spectroscopy and corrosion behaviour of Al2O3-Ni nano composite coatings

Article abstract of DOI:10.1016/j.electacta.2008.01.020

In this paper, the results on the electrochemical impedance spectroscopy and corrosion properties of electrodeposited nanostructured Al₂O₃-Ni composite coatings are presented. The nanocomposite coatings were obtained by codeposition of alumina nanoparticles (13 nm) with nickel during plating process. The coating thickness was 50 μm on steel support and an average of nano Al₂O₃ particles inside of coatings at 15 vol.% was present. The structure of the coatings was investigated by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure. The electrochemical behavior of the coatings in the corrosive solutions was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solutions 0.5 M sodium chloride and 0.5 M potassium sulphate were used in a three electrode open cell. The corrosion potential is shifted to more negative values for nanostructured coatings in 0.5 M sodium chloride. The polarization resistance in 0.5 M sodium chloride decreases in 24 h, but after that increases slowly. In 0.5 M potassium sulphate solution the polarization resistance decreases after 2 h and after 30 h of immersion the polarization resistance is higher than that of the beginning value. The corrosion rate calculated by polarization potentiodynamic curves obtained after 30 min from immersion in solution is smaller for nanostructured coatings in 0.5 M potassium sulphate (4.74 μm/year) and a little bit bigger in 0.5 M sodium chloride (5.03 μm/year).

Full text of DOI:10.1016/j.electacta.2008.01.020

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 4557–4563
Electrochemical impedance spectroscopy and corrosion behaviour of
Al O –Ni nano composite coatings
2
3
a,∗
a,1
b,1
a
Alina–Crina Ciubotariu , Lidia Benea , Magda Lakatos–Varsanyi , Viorel Dragan
a
Dunarea de Jos, University of Galati, Metallurgy and Materials Science Faculty, Competences Center Interfaces-Tribocorrosion-Electrochemical Systems,
CC-ITES, 47 Domneasca Street, 80008 Galati, Romania
b
Bay Zoltan Foundation, Institute for Materials Science and Technology, Budapest H–1116, Hungary
Received 28 September 2007; received in revised form 8 January 2008; accepted 9 January 2008
Available online 19 January 2008
Abstract
In this paper, the results on the electrochemical impedance spectroscopy and corrosion properties of electrodeposited nanostructured Al
2
3
O –Ni
composite coatings are presented. The nanocomposite coatings were obtained by codeposition of alumina nanoparticles (13 nm) with nickel during
plating process. The coating thickness was 50 ␮m on steel support and an average of nano Al particles inside of coatings at 15 vol.% was
present. The structure of the coatings was investigated by scanning electron microscopy (SEM). It has been found that the codeposition of Al
2
O
3
2
O
3
particles with nickel disturbs the nickel coating’s regular surface structure. The electrochemical behavior of the coatings in the corrosive solutions
was investigated by polarization potentiodynamic and electrochemical impedance spectroscopy methods. As electrochemical test solutions 0.5 M
sodium chloride and 0.5 M potassium sulphate were used in a three electrode open cell. The corrosion potential is shifted to more negative
values for nanostructured coatings in 0.5 M sodium chloride. The polarization resistance in 0.5 M sodium chloride decreases in 24 h, but after that
increases slowly. In 0.5 M potassium sulphate solution the polarization resistance decreases after 2 h and after 30 h of immersion the polarization
resistance is higher than that of the beginning value. The corrosion rate calculated by polarization potentiodynamic curves obtained after 30 min
from immersion in solution is smaller for nanostructured coatings in 0.5 M potassium sulphate (4.74 ␮m/year) and a little bit bigger in 0.5 M
sodium chloride (5.03 ␮m/year).
©
2008 Published by Elsevier Ltd.
Keywords: Al2O3–Ni nanostructured composite coatings; Electrochemical impedance spectroscopy; Potentiodynamic polarization; Sodium chloride; Potassium
sulphate
1
. Introduction
particles in the composite coatings. Furthermore, metal matrix
nanocomposite coatings exhibit unique magnetic, mechani-
cal and optical properties and are promising materials for
micro-devices. Nano-composite deposition may be useful in
fundamental studies of their nanometric nature.
Much attention has been focused on the development of
ceramic/metalnanocompositesbecausesuchmaterialsofferout-
standing mechanical and multifunctional properties [1].
Composite coatings are produced by codeposition of inert
particles into a metal matrix from an electrolytic or electroless
bath being considered as metal matrix composite (MMC) coat-
ings. This technique is receiving increased interest because of
its ability to produce films with excellent mechanical proper-
ties such as wear resistance, corrosion resistance and lubrication
A nano Al2O3–Ni composite coating is used primarily to
increase the wear resistance of metal surfaces in micro-devices.
Although the micro Al2O3–Ni composite coating grades have
progressed deeply, certain problems persist in its preparation.
The volume content of alumina particles in Al2O3–Ni com-
posite coatings cannot be controlled quantitatively and the
particles are frequently agglomerated together in the composite
coatings. The alumina weight content in the composite coatings
can be promoted from 3.5% to 14.6% using pulse-reverse elec-
trodeposition. The mechanical properties are also improved.
Although the process has been developed for a long time, much
recent research effort has been spent on electrodeposition of
[
2–6]. These properties depend on the morphology of the inert
∗
Corresponding author. Fax: +40 236460754.
E-mail address: Alina.Ciubotariu@ugal.ro (A. Ciubotariu).
ISE active member.
1
0
013-4686/$ – see front matter © 2008 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2008.01.020

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A. Ciubotariu et al. / Electrochimica Acta 53 (2008) 4557–4563
nanostructured coatings. The well-dispersed nanosized inert
particles in a metal matrix do not only enhance the mechanical
property, but also open up potential applications of the materials
in microdevices. Coatings of Al2O3–Ni are highly abrasion
and heat resistant and have quite good anticorrosion proper-
ties. They find application as coatings of engine cylinders,
high-pressure valves and dies and in the production of musical
instruments, drill fittings, car accessories and small aircraft
and electrotechnical parts [7]. Among the processes to produce
nanostructured composites, the electrodeposition technique has
further demonstrated that a smoother surface, a better bonding
between particles and metal and higher microhardness can be
achieved. Different types of nanocomposites such as ZrO2–Ni,
SiC–Ni, PZT–Ni, Al2O3–Ni, diamond–Ni and Al2O3–Cu have
been successfully produced by direct, pulse current, pulse
reverse current electrodeposition [8,9].
in the literature are often inconsistent. The authors Szczygiel
and Kolodziej reported in paper [17] the corrosion resistance of
Al2O3–Ni composite coatings in 0.5 M sodium sulphate solu-
tion.
This paper focuses on electrochemical corrosion behaviour of
nanostructured Al2O3–Ni composite coatings obtained by elec-
trodeposition process. The corrosion behaviour was investigated
by electrochemical impedance spectroscopy and polarization
potentiodynamic methods in 0.5 M sodium chloride and 0.5 M
potassium sulphate solutions.
2. Experimental
Nickel (Ni) and composite (Al2O3–Ni) coatings were elec-
trochemically deposited from a Watts bath with the following
composition: 1 M NiSO4·7H2O, 0.1 M NiCl2·6H2O, 0.2 M
−
3
The codeposition of Al2O3 was also studied by Kuo et al.
H3BO3, 1.2 × 10 M sodium dodecyl sulphate (SDS). The pH
of the solution was about 4. Suspension was preparated by
adding Al2O3 nanoparticles (13 nm) to the solution to give a
concentration of 20 g/L in the nickel plating electrolyte. Elec-
[
10,11] using pulse current and ultrasonic energy treatment. The
corrosion properties of composite coatings obtained were not
discussed.
◦
Nanocomposite Al2O3–Ni films electrodeposited from a sus-
pension of Al2O3 nanoparticles was studied in aqueous nickel
sulphamate solution. The volume fraction of particles incor-
porated increased with electrode rotation rate and decreased
with deposition current density. The saturation magnetiza-
tion showed a weak dependence on particle concentration
trodeposition took place in the bath at a temperature of 40 C
2
and a current density of 2 Ad m for a time sufficient to obtain
a 50 ␮m thick coating. The suspension bath was stirred by
a mechanical stirrer at a constant rotational speed of about
300 rpm.
2
Steel plate with an area of 2 cm was used as the cathode; a
[
12].
Electrochemical responses contain valuable information
pure Ni plate was used as the anode. Prior to electroplating, the
substrates were mechanically polished to 0.08–0.12 ␮m surface
roughness and then a sequence of cleanings was performed to
remove contamination on the substrate surface. The steel sub-
strate was activated in a mixed acidic bath at room temperature
before electroplating.
both on interfacial structures and dynamics of interfacial
processes. The established practice of the electrochemical
characterisation implies sequential probing of the electro-
chemical system with various techniques, highly specialised
in particular aspects of ac and dc responses. Each of the
components of the electrochemical response has a notable
potential for interfacial studies: a double electric layer is
the thinnest probe, which helps to detect even the slightest
changes on the interface by the variation of the double layer
capacitance. The losses of opportunities originate from the
above-mentioned disintegration of the electrochemical probing
between different techniques and the difficulties of components
separation in complex responses, which hinder the develop-
ment of practically feasible integrated probing procedures for
the comprehensive characterisation of electrochemical systems
Before electrochemical corrosion investigations, the surfaces
of the Ni coatings and Al2O3–Ni composite coatings were exam-
ined with scanning electron microscope type AMRAY 1830.
For electrochemical corrosion measurements were used a
VOLTAMASTER 4 complete SOFTWARE, a three-electrode
open cell with nanostructured Al2O3–Ni composite coatings
as working electrode (WE), a platinum gauze as counter
electrode (CE) and Ag/AgCl as reference electrode (RE)
(ERE = +200 mV/ENH). As test solutions 0.5 M sodium chloride
and 0.5 M potassium sulphate were used at room temperature
◦
(20 ± 1 C).
[
13–15].
Polarization methods such as potentiodynamic polarization,
EIS measurements used initial frequency (I.F.) 100 kHz, final
frequency (F.F.) 100 mHz, AC sine wave amplitude of 10 mV;
frequency per decade: 10 Hz, delay before integration 1 s. The
impedance spectra were recorded on different samples after dif-
ferent times from immersion (30 min, 2 h, 6 h, 24 h and 30 h).
All the recorded impedance spectra were analyzed as Nyquist
diagrams.
potentio-staircase and cyclic voltammetry are often used for
laboratory corrosion testing. These techniques can provide sig-
nificant useful information regarding the corrosion mechanisms,
corrosion rate and susceptibility of specific materials to corro-
sion in designated environments. Polarization methods involve
changing the potential of the working electrode and monitoring
the current which is produced as a function of time or potential
For PD measurements were used initial potential (I.P.)
−800 mV (Ag/AgCl), final potential (F.P.) +100 mV (Ag/AgCl)
and a scan rate of 1 mV/s. The polarization potentiodynamic
curves were recorded after 30 min of immersion. The corro-
sion current density (icorr) for the particular specimens was
determined by extrapolating the anode and cathode Tafel
curves.
[
16].
The fact that the second-phase particles affect the nickel
composite coating’s surface morphology and protective prop-
erties in different ways and to different degrees are the reason
why the corrosion test results for coatings Al2O3–Ni reported

A. Ciubotariu et al. / Electrochimica Acta 53 (2008) 4557–4563
4559
3
. Results and discussions
Figs. 1(AandB)and2(AandB)comparethesurfacestructure
of pure nickel coating and nanostructured Al2O3–Ni composite
coatings under a scanning microscope. The surface of a nickel
is made up of regular pyramidal crystals with a uniform grain
size. The Al2O3 particles codeposit with nickel radically change
the structure of the metal: disorder the regular crystal structure
and the structure of the nickel matrix becomes finely crys-
talline. Al2O3 nanoparticles show a distinct tendency to form
conglomerates uniformly distributed over the whole surface of
the coating. Some of them protrude above the surface of the
nickel matrix while other, despite being deeply embedded in it,
are not covered by a layer of the metal because of their dielectric
properties.
The corrosion investigation of each sample began with mon-
itoring the open circuit potential after immersion into the testing
solutions till reaching a relatively stable stationary value (see
Fig. 3).
The Nyquist plot representation of impedance spectra per-
formed in 0.5 M sodium chloride and 0.5 M potassium sulphate
solutions at different times of immersion: after 30 min, 2 h, 6 h,
2
4 h and 30 h are shown in Figs. 4 and 5.
Fig. 2. (A and B) SEM surface morphology of pure nickel electroplating and
nanostructured Al2O3–Ni composite coatings (20 ␮m).
Fig. 3. Variation of the potential of nanostructured Al2O3–Ni composite coat-
ings function of time for 0.5 M sodium chloride and 0.5 M potassium sulphate
solutions.
Fig. 1. (A and B) SEM surface morphology of pure nickel electroplating and
nanostructured Al2O3–Ni composite coatings (5 ␮m).

4
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A. Ciubotariu et al. / Electrochimica Acta 53 (2008) 4557–4563
Fig. 4. Nyquist diagrams of impedance spectrum for nanostructured Al2O3–Ni
composite coatings in 0.5 M sodium chloride.
Fig. 5. Nyquist diagrams of impedance spectrum for nanostructured Al2O3–Ni
composite coatings in 0.5 M potassium sulphate.
An equivalent electrical circuit was proposed to account for
the experimental impedance spectra [18]. In most cases, this
circuit, represented in Fig. 7(A) allows to obtain an excel-
lent agreement between experimental and simulated impedance
plots. The experimental data of our work was simulated with
this equivalent circuit.
neous surfaces and has given rise to extensive studies. CPE are
not pure capacitors but components depending on frequency.
In Fig. 7 is represented one experimental diagram together
with the simulation curve described by the equivalent cir-
cuit from Fig. 6. It could be observed that the experimental
impedancedatafitverywellwiththeequivalentcircuitproposed.
The corresponding calculated polarization resistance and
capacitance values from impedance diagrams for nanostructured
Al2O3–Ni composite coatings in 0.5 M sodium chloride and
0.5 M potassium sulphate solutions using the equivalent circuit
from Fig. 7 are presented in Table 1.
The impedance could be described by the following equation:
Rp
Z = Re +
(with 0 < β ≤ 1)
(1)
β
1
+ (jωτ)
The value of beta (β) accounts for the fact that the centers
of the capacitive arcs of circles are under the axis of real part,
Fig. 6(B).
The value of β determines the amplitude of the depression
and the importance of the discrepancy between the Randles’
It was observed that the polarization resistance of nanocom-
posite coatings in 0.5 M sodium chloride decreases in 24 h,
but after that increases slowly. After 30 min of immer-
2
circuit model with a pure capacitor C , and the circuit with the
sion the polarization resistance is 26.30 kOhms cm , after
dl
2
CPE: the borderline case of the Randles’ circuit is found if β = 1.
CPE are a constant phase elements, accounting for the fact
that the centres of the capacitive arcs of the circle are under the
axis of real part. This feature of capacitive arcs is encountered in
all electrochemical impedance studies performed on inhomoge-
24 h is 12.00 kOhms cm and after 30 h from immersion is
2
12.44 kOhms cm .
The polarization resistance of nanocomposite coatings in
0.5 M potassium sulphate solution decreases during 2 h (after
2
2
30 min is 26.93 kOhms cm and 25.25 kOhms cm after 2 h),
Fig. 6. (A) Equivalent circuit to calculate the polarization resistance from impedance data with a CPE: Re – electrolyte resistance between the reference electrode
and the working electrode; CPE – the double layer capacity in parallel with the polarization resistance Rp; (B) Circuit with a parallel combination of a resistance R
and a CPE, and the corresponding Nyquist impedance plot.

A. Ciubotariu et al. / Electrochimica Acta 53 (2008) 4557–4563
4561
Fig. 8. Comparative polarization potentiodynamic curves for nanostructured
Al2O3–Ni composite coatings in 0.5 M sodium chloride and 0.5 M potassium
sulphate solutions obtained after 30 min from immersion time (log scale).
Fig. 7. Nyquist diagrams of impedance spectrum of experimental data (square
points) and fitting curve (solid line) for nanostructured Al2O3–Ni composite
coatings in 0.5 M sodium chloride solution after 24 h of immersion.
•
the quantity, (βa·βc)/(βa + βc), is referred to as the Tafel con-
stant.
then the polarization resistance increases. So that after 30 h
of immersion the polarization resistance (29.88 kOhms cm ) is
a little bit higher than that the value calculated from Nyquist
impedance diagrams after 30 min of immersion, but in the same
order of magnitude.
The corrosion potential (Ecorr), corrosion current density
icorr) and polarization resistance (Rp), which were obtained
from the potentiodynamic polarization curves are summarized
in Table 2.
2
(
The corrosion potential is shifted to more negative values for
nanostructured coatings in 0.5 M sodium chloride (−359.2 mV)
and to positive values for same coatings in 0.5 M potassium
sulphate (−158.8 mV).
The capacitance values are smaller in 0.5 M potassium sul-
2
phate solution (between 4.13 and 5.25 ␮F/cm ) than in 0.5 M
2
sodium chloride solution (between 8.09 and 15.32 ␮F/cm ).
The performed potentiodynamic diagrams for nanostructured
Al2O3–Ni composite coatings in 0.5 M sodium chloride and
The polarization resistance values calculated with Stern-
Geary formula from potentiodynamic diagrams were in good
agreement with those obtained from impedance measurements.
From potentiodynamic polarization curves the polarization
0
.5 M potassium sulphate solutions after 30 min of immersion
are presented in Fig. 8.
In corrosion, quantitative information on corrosion currents
and corrosion potentials can be extracted from the slope of the
curves, using the Stern-Geary equation, as follows [19]:
2
resistance is 25.03 kOhms cm in 0.5 M sodium chloride and
in 0.5 M potassium sulphate the polarization resistance is
2
3
2.17 kOhms cm .From impedance diagrams the polarization
2
ꢀ
ꢁ
resistance is between 26.30 kOhms cm (after 30 min of immer-
1
βa × βc
2
icorr =
(2)
sion) and 12.44 kOhms cm (after 30 h of immersion) in 0.5 M
2
.303Rp βa + βc
2
sodium chloride and between 26.93 kOhms cm (after 30 min of
2
immersion) and 39.88 kOhms cm (after 30 h of immersion) in
2
•
•
•
icorr is the corrosion current density in Amps/cm ;
Rp is the corrosion resistance in ohms cm ;
0.5 M potassium sulphate.
2
From experimental data it was observed that corrosion rate
2
βa is the anodic Tafel slope in Volts/decade or mV/decade of
current density;
(corrosion current) is 0.50 ␮A/cm for Al2O3–Ni nanocom-
posite coatings in 0.5 M potassium sulphate. In 0.5 M sodium
chloride solution was obtained a little smaller value by
•
βc is the cathodic Tafel slope in Volts/decade or mV/decade
of current density;
2
0.43 ␮A/cm for same nanocomposite coatings. VOLTAMAS-
Table 1
Polarization resistance and capacitance values of nanostructured Al2O3–Ni composite coatings in 0.5 M sodium chloride and 0.5 M potassium sulphate solutions
after different time of immersion calculated with equivalent circuit from Fig. 7
Solution
Data from impedance spectra
Time (h)
.5
0
2
6
24
30
2
Rp (kOhms cm )
C (␮F/cm )
26.30
8.83
19.39
15.32
19.07
11.11
12.00
11.77
12.44
8.09
0
.5 M sodium chloride
2
2
Rp (kOhms cm )
C (␮F/cm )
26.93
4.90
25.25
5.25
25.37
4.13
26.56
4.95
29.88
4.79
0
.5 M potassium sulphate
2

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A. Ciubotariu et al. / Electrochimica Acta 53 (2008) 4557–4563
Table 2
Polarization resistance values of nanostructured Al2O3–Ni composite coatings calculated from polarization potentiodynamic curves obtained after 30 min from
immersion in solutions
Solution
Ecorr (mV), Ag/AgCl
−359.2
icorr (␮A/cm2)
βa (mV/decade)
βc (mV/decade)
Rp from Tafel (kꢀ cm2)
Corrosion (␮m/year)
0
0
.5 M sodium chloride
.5 M potassium sulphate −158.8
0.43
0.50
77.1
126.5
61.8
128.5
25.03
32.17
5.03
4.74
TER 4 calculates and displays the corrosion rate Corr. in
mm/year: this rate is calculated from the icorr: corrosion current
density found, the D density, the M atomic mass and V valence
entered in the Tafel dialogue box. The calculation is performed
as follows:
culated value from corrosion current density is 4.74 ␮m/year)
and for 0.5 M sodium chloride the rate of corrosion is a little
bit higher (calculated value from corrosion current density is
5.03 ␮m/year).
The polarization resistance of nanocomposite coatings in
0
.5 M sodium chloride decreases during 24 h of immersion,
2
icorr(A/cm ) × M(g)
but after that increases slowly. Thus after 30 min of immer-
sion the polarization resistance is 26.30 kOhms cm , decreasing
after 24 h at 12.00 kOhms cm . The polarization resistance
of nanocomposite coatings in 0.5 M potassium sulphate solu-
tion decreases during 2 h (after 30 min having the value of
Corr =
× 3270
(3)
3
2
D(g/cm ) × V
2
with 3270 = 0.01 × [1 year (in seconds)/96497.8] and
9
6497.8 = 1 Faraday in Coulombs.
Comparing these values with the value reported for the polar-
ization resistance and corrosion rate of nickel pure coating [20]
2
2
2
6.93 kOhms cm and 25.25 kOhms cm after 2 h), after that the
polarization resistance increases. So that after 30 h of immer-
sion, the polarization resistance (29.88 kOhms cm ) is a little
2
2
at Rp = 8.20 ohms cm and icorr = 4.1 ␮A/cm obtained in 0.5 M
sodium sulphate (similar to potassium sulphate) the conclusion
is that Al2O3–Ni nanocomposite coatings have a better corro-
sion resistance and a smaller corrosion rate. Also it was reported
that the corrosion rate for the Ni coating and composite coating
Al2O3–Ni in a 0.5 M sodium sulphate solution decreases during
the first day of exposure. With time the corrosion current for
the Ni coating increases markedly, reaching 360 nA/cm . After
4 days the rate of corrosion of Al2O3–Ni composite coating is
three times lower than that of the Ni coating [17].
2
bit higher than the value calculated from Nyquist impedance
diagrams after 30 min of immersion, but in the same order of
magnitude.
The nanostructured Al2O3–Ni composite coatings have a bet-
ter corrosion resistance than pure nickel coating in agreement
with others publication data.
The better corrosion resistance behaviour of Al2O3–Ni com-
posite coatings could be due to the fine surface structure of
composite coating compared with pure nickel coating as well
as to the incorporation of Al2O3 nanoparticles into composite
coatings acting as insulators on composite surface.
2
1
This improvement of corrosion resistance could be due to
the fine surface structure of composite coating compared with
pure nickel coating as well as to the incorporation of Al2O3
nanoparticles into composite coatings.
Acknowledgments
4
. Conclusions
The authors gratefully acknowledge the European Project
COST Action D19 Chemistry: “Chemical Functionality Specific
to the Nanometer Scale”, Working Group number: D19/009/03,
The surface morphology of nanostructured Al2O3–Ni com-
posite layers is different compared with pure nickel coatings:
the regular crystal structure characteristic of electroplated nickel
coatings was disturbed. The Al2O3 particles embedded in the
nickelmatrixperturbthe nickelgrowth during electrodeposition.
Electrochemical impedance spectroscopy and polarization
potentiodynamic methods are powerful techniques to investigate
the corrosion protection of nanostructured Al2O3–Ni composite
coatings. The polarization resistance values obtained from both
methods are in good agreement.
“
Development of new nanostructured functional materials”,
STSM – Grant Ref No D19 – 00933 and CNCSIS (research
grant no. 1347/2005-2006) for financial support.
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