Corrosion and wear resistant electrodeposited composite coatings

Article abstract of DOI:10.1016/j.electacta.2004.11.067

Composite electroplating is a method allowing to co-deposit fine particles of metallic or non metallic compounds into the plated layers in order to improve the surface properties. The aim of the present work was to compare the performance of pure nickel and Ni-SiC nano-structured composite coatings as far as corrosion, wear and abrasion resistance were concerned. The characteristics of the coatings were assessed by scanning electron microscopy, micro hardness test, Taber Abrader test, electrochemical impedance spectroscopy and wear corrosion measurements. Additionally accelerated salt spray tests were performed. The results obtained in this study indicate that the co-deposition of nickel and SiC nano-particles leads to uniform deposits possessing better abrasion, wear and corrosion properties.

Full text of DOI:10.1016/j.electacta.2004.11.067

Electrochimica Acta 50 (2005) 4551–4556
Corrosion and wear resistant electrodeposited composite coatings
Maria Lekkaa, Niki Kouloumbia,∗, Mauro Gajob, Pier Luigi Bonorac
a
National Technical University of Athens, 9 Heroon Politechniou Street, 15780 Athens, Greece
b
Elsy Rersearch snc, Conegliano, TV, Italy
c
University of Trento, Italy
Received 27 January 2004; received in revised form 18 October 2004; accepted 18 November 2004
Available online 24 June 2005
Abstract
Compositeelectroplatingisamethodallowingtoco-depositfineparticlesofmetallicornonmetalliccompoundsintotheplatedlayersinorder
to improve the surface properties. The aim of the present work was to compare the performance of pure nickel and Ni–SiC nano-structured
composite coatings as far as corrosion, wear and abrasion resistance were concerned. The characteristics of the coatings were assessed
by scanning electron microscopy, micro hardness test, Taber Abrader test, electrochemical impedance spectroscopy and wear corrosion
measurements. Additionally accelerated salt spray tests were performed. The results obtained in this study indicate that the co-deposition of
nickel and SiC nano-particles leads to uniform deposits possessing better abrasion, wear and corrosion properties.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Nickel; Silicon carbide; Nano-particles; Composite; Coating; Abrasion; Corrosion
1. Introduction
mechanical parts under friction, due to their high wear resis-
tance and the low cost of ceramic powder. The usual dimen-
sions of SiC powder in such applications are in the range
of micrometers. The shapes used vary from round to acicu-
lar while a mixture of them can also be used. The objective
of the present experimental work is the extension of these
researches when nano-crystals of silicon carbide (mean diam-
eter 20 nm) are codeposited.
The need for coatings with improved resistance to highly
aggressive environments is high as a result of a growing
demand for extended safe service life of industrial objects.
In our previous research work the corrosion and wear corro-
sion resistance of nitrides coatings [1], (TiCr)N and Ti/TiN
PVD coatings [2], hard chromium coatings[3], HVOF cermet
coatings[4] as well as of composite electrodeposited coatings
of nickel and nano-crystals of silicon carbide was studied
[5,6].
2. Experimental
The production of nano-composite films can be achieved
through electrochemical deposition of the matrix material
from a solution containing suspended particles such as:
oxides, carbides, nitrides, metal powder and s.a. This tech-
nique has been applied in the present work using nanometer
size SiC particles for the production of the composite coat-
ings.
Two types of samples were prepared. Samples coated with
purenickelcoatingandsamplescoatedwithNi + SiCcoating.
The electroplating bath used for the electrochemical depo-
sition of pure nickel coatings had the following composi-
tion: 240 g/l nickel sulphate (NiSO4), 45 g/l nickel chloride
(NiCl2), 30 g/l boric acid (H3BO3) and 10 g/l sodium dode-
cylsulphate (CH3(CH2)11OSO3Na). The pH of the solution
was about 4.5.
Ni–SiC composites have been studied extensively [7–10]
and commercialized successfully for the protection of
The nanostructured SiC-nickel coatings were electrode-
posited from a suspension of SiC nanoparticles (20 nm)
in the above mentioned electrolyte. Suspensions were pre-
∗
Corresponding author. Tel.: +30 210 7723101; fax: +30 210 8035906.
E-mail address: koni@chemeng.ntua.gr (N. Kouloumbi).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.11.067

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M. Lekka et al. / Electrochimica Acta 50 (2005) 4551–4556
pared by adding 20 g/l of SiC nanoparticles produced by
ENEA (Italy) and they were stirred for 24 h before the
deposition.
The substrate was standardized Q-panel of steel and the
anode pure nickel. The bath temperature was maintained
between 40 and 50 ◦C. The current density used for the elec-
trodeposition in all cases was equal to 1.5 A/dm2.
Study of the surface morphology, microhardness mea-
surements, abrasion resistance measurements, visual obser-
vations after exposure to salt spray cabinet, electrochemical
impendence spectroscopy (EIS) measurements and wear cor-
rosionmeasurementswerecarriedoutforthecharacterization
of the composite coatings of this work.
The morphology of the deposits was examined by scan-
ning electron microscopy (SEM).
Microhardness measurements were carried out using a
Vickers microhardness tester, applying 0.1 N load for 10 s
time.
The abrasion resistance of the coatings was determined
using a Taber Abrader model 5131. A coated sample is placed
ontoadiskwhichisrotatedatasteadyspeed, undertheweight
that is inflicted by two wheels of hard material which are also
rotated in opposite directions (Fig. 1). The wheels used were
CS10 type and an additional weight of 1000 g has been added
oneachwheel. Theabrasiontestwasconductedinthreesteps:
from 0 to 1000 cycles every 200 cycles, from 1000 to 10,000
every 500 and from 10,000 to 30,000 every 1000 cycles. The
abrasion resistance expressed by the loss of thickness in the
abraded area was calculated through the weight loss of the
coated specimens.
Fig. 1. Schematic presentation of Taber Abrader.
ally observed and the rust percentage of their surface was
calculated by digital image processing.
The E.I.S. measurements were carried out with a Solartron
FRA 1255, three electrode system over a frequency range
from 10 mHz to 100 kHz using an ac potential of 20 mV. A
3.5% NaCl solution was used as electrolyte. The reference
electrode was an Ag/AgCl electrode and the counter elec-
trode a platinum electrode. Measurements started immedi-
ately after the addition of the electrolyte and were performed
every 1 or 1.5 h till the appearance of corrosion attack on the
surface of the sample.
Wear corrosion tests were performed using an apparatus
built in the laboratory of Industrial Corrosion Control of the
Department of Materials Engineering of Trento University
[6] (Fig. 2). Samples coated by pure nickel coating or by
Ni + SiC coating embedded in resin were the working elec-
trode. An alumina cylinder played the role of an antagonist
The protective properties of the coatings were first exam-
ined by exposing 8 samples of each type of coated specimens
in a salt spray cabinet according to ASTM B117 for a total
time interval of 35 days. Every 5 days the samples were visu-
Fig. 2. Schematic presentation of wear corrosion system.

M. Lekka et al. / Electrochimica Acta 50 (2005) 4551–4556
4553
which was rotated on the coated surface with a rotation speed
of 200 rpm. An external load was applied on the working elec-
trode varying between 1 and 3 kg. 0.5 M Na2SO4 solution
was used as electrolyte. A platinum counter electrode and
an Ag/AgCl reference electrode were also used. The wear
parameter was expressed by the value of the friction factor
and corrosion potential, Ecorr, time dependence were simul-
taneously reported by computer software.
3. Results and discussion
3.1. Morphology
Fig. 4. Thickness loss vs. number of cycles during abrasion resistance mea-
surements.
A micrography of a pure nickel coating is shown in Fig. 3a
while one of nickel with nano-silicon carbides coating is
shown in Fig. 3b. Both coatings were electrodeposited at a
current density of 1.5 A/dm2 and had a thickness at about
20 ␮m. As it can be observed from Fig. 3 the composite coat-
ing is more compact than the pure one and consists of smaller
and rounder nickel grains. As a consequence better mechan-
ical and anticorrosive properties of the composite coating
should be expected. TEM analyses of samples are under way,
aimed to show up the SiC nanoparticles, not detectable by
SEM.
are presented in Fig. 4. The plot can be divided into three
parts. In the first part (0–1000 cycles) the two curves related
to the behaviour of the two types of coatings are similar to
each other. At the end of 1000 cycles the loss of pure nickel
coating is about 1.23 ␮m while that of the composite coat-
ing is about 1 ␮m. In the second part (1000–10,000 cycles)
the two curves present great difference between each other
and at the end of 10,000 cycles the loss of thickness of the
pure nickel coating was equal to 4.3 ␮m while that of the
composite coating is equal to 2.6 ␮m. Finally at the third part
(10,000–30,000 cycles) the declinations of the two curves
increase and after 21,000 cycles they remain stable with that
of the pure nickel coating presenting clearly higher value.
At the end of 30,000 cycles the loss of thickness of pure
nickel coating is about 14 ␮m while that of the composite
one is about 9 ␮m. The whole of these results prove that the
abrasion resistance of the composite coatings is about 35.7%
higher than that of the pure nickel one.
3.2. Mechanical properties
3.2.1. Microhardness measurements
Vickers microhardness measurements were performed on
pure nickel and on Ni + SiC coatings of 30 ␮m thickness. The
mean value of Vickers microhardness of pure nickel coat-
ings has been found of about 270 11 kg/mm2 while that
of composite coatings is about 356 10 kg/mm2. This result
shows that the codeposition of SiC nanoparticles ameliorates
the mechanical properties of the film by 30% microhardness
increase.
3.3. Anticorrosive properties
3.3.1. Exposure to salt spray cabinet
The time dependence of the degree of rust formed on the
surface of coated specimens, during their exposure in a salt
spray cabinet, is presented in Fig. 5. The rust percentage is
assessed by digital image processing. From the plot of Fig. 5 it
3.2.2. Abrasion resistance test
The loss of thickness in the abraded area of the coated
specimens was calculated from their weight loss. The results
Fig. 3. (a) SEM surface morphology of pure Ni coating, (b) SEM surface morphology of Ni + SiC coating.

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M. Lekka et al. / Electrochimica Acta 50 (2005) 4551–4556
Fig. 5. Rust percentage vs. days of exposure in salt spray cabinet for samples
coated with pure nickel and Ni + SiC coating.
Fig. 6. Nyquist diagrams for both types of coatings vs. exposure time to
3.5% NaCl solution.
is evident that the composite coating presents a clearly better
protective behaviour compared to that of pure nickel coating.
For times up to 20 days the coverage of the coating area by
corrosion products of the samples with composite coatings
is only 1%, while in the same time that of the samples with
pure nickel coating reaches 6.5%. At the end of the expo-
sure (35 days) this rust coverage is 4.8% for the composite
coatings and 11.3% for the pure nickel coatings respectively.
Thus from the beginning of the exposure composite coatings
exhibit protective characteristics by 84.6% higher than the
pure Ni coating, behaviour which is maintained up to the end
of the exposure when this factor presents a value of 57.5%
presenting small fluctuations followed by a more or less
steady state condition up to about 925 h when the sample
starts to be corroded. As it is known nickel is covered by
a passive film of oxides [1] that protects it against corro-
sion. So it could be suggested that during the first 100 h of
exposure this film is transformed into a more stable phase.
Thus the increase of the coating resistance (Fig. 7a) and the
corresponding decrease of the coating capacitance (Fig. 7b)
could be attributed in the formation of that phase. On the
contrary the resistance of the pure nickel coating during the
same exposure interval presents great variations until it is
almost stabilized at a value which is quite lower than that
of the composite coating. The sharp decrease of the nickel
coating’s resistance after 714 h of exposure to the corrosive
environment indicates that the corrosion has started whereas
3.3.2. EIS measurements
The barrier characteristics of the coatings were evaluated
through the fitting of the experimental results to equivalent
circuits. A simple circuit comprising the electrolyte resis-
tance, the coating resistance and the coating capacitance was
generally used (R(QR) circuit). After 980 h of exposure for
the composite coating and 800 h for the pure coating a more
complex circuit was used in which a double layer capacitance
and a charge transfer resistance has been added at the metal
substrate-coating interface (R(Q(R(QR))) circuit).
From the Nyquist plot of both type of coatings (Fig. 6)
it is clear that just after the addition of the electrolyte the
two coatings present a very similar barrier response. After
one hundred hours of exposure the imaginary part of the
impedance of the composite coating is increased while that
of pure nickel coating remains stable. The difference in the
behaviour of the two types of coatings is maintained even
after 600 h of exposure with the composite coating present-
ing always a higher value of the Zimag part of the impedance.
Finally after 800 h of exposure the specimens coated with
pure nickel coating start to be corroded as shown trend of the
Nyquist plot.
The barrier parameters of the coatings, determined by the
fitting to the equivalent circuit, are presented in Fig. 7 versus
time. After about 100 h the composite coating presents higher
coating resistance values as well as a more even behavior.
During this exposure period its Rf value increases steadily
Fig. 7. (a) Coating resistance time dependence, (b) coating capacitance time
dependence.

M. Lekka et al. / Electrochimica Acta 50 (2005) 4551–4556
4555
Fig. 8. Friction factor evolution vs. exposure for different applied loads.
the corrosion initiation of the sample with composite coating
is observed only after 925 h.
3.3.3. Wear corrosion measurements
The time dependence of the friction factor for both types
of coatings and for the three applied loads is shown in Fig. 8.
In all cases the friction factor of both coatings increases with
the load increase. The friction factor values of the composite
coating remain more or less stable for each applied load with
an exception in the case of 3 kg load where they present a
tendency to be increased after the first hour of abrasion. The
pure nickel coating’s friction factor presents the highest val-
ues immediately after the application of the additional load
and then after it presents a decay to lower values followed by
a steady state condition.
This behavior can be explained taking into consideration
the differences in the morphology of the two types of coat-
ings. The pure coating is characterized by larger and angular
grains, due to which the friction factor of this coating remains
high at the beginning of each loading till the smoothing of
the surface grains and then it decreases. In the contrary the
composite coating is more compact and thus the friction fac-
tor values are quite stable. The increase of the friction factor
in the case of 3 kg load could be attributed to a higher com-
posite coating’s abrasion resistance with the increase of the
load offered by the nanoparticles of silicon carbon.
During this test the corrosion potential was also recorded
(Fig. 9). Throughout this test the open circuit potential of the
sample with the composite coating presented values close to
the potential of passive nickel. A small decrease of potential
values were observed only with the increase of the applied
load proving that the wear process almost does not influ-
ence the protective characteristics of the coating. On the
contrary the potential of the sample with pure nickel coat-
ing remains close to the potential of bare steel in the same
corrosive environment. Only in the two first measurements
just after the start up of the test, the potential is close to
that of passive nickel. This result suggests that in the case
of pure nickel coating the wear process destroys the nickel
oxides passive film so that the underlaying microporous pure
Fig. 9. Corrosion potential evolution vs. exposure time for different applied
loads.
nickel layer allows the electrolyte to enter and reach the
substrate.
In the case of the composite coating the nanoparticles of
silicon carbon prevent the total damage of the passive film.
The passive film that covers pure nickel coating is composed
of two molecular layers of NiO and a molecular layer of
Ni(OH)2 [11]. The nano-particles of SiC being much bigger
than these layers, but not so big to give rise to the so called
chunk effect, are able to protect this film in the way shown in
Fig. 10. For the confirmation of this suggestion the abraded
area of pure nickel coated specimens was repassivated with
nitric acid and the corrosion potential was measured with a
micro-electrode. A value equal to −0.12 V has been found
which is close to that of passive nickel. Thereafter the wear
corrosion test was repeated in the same area and just after the
start of wear process the potential presented a decay reaching
once again the value of −0.48 V which characterizes the bare
specimen.
Fig. 10. Schematic presentation of the influence of the presence of SiC
nanoparticles to the coatings’ abrasion resistance.

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M. Lekka et al. / Electrochimica Acta 50 (2005) 4551–4556
4. Conclusions
ing the steel substrate against corrosion even under abrasive
conditions.
The codeposition of nickel with nanoparticles of sili-
con carbide changes the morphology of the coating giving
smaller, moreroundnickelgrainsand, asaresult, thecompos-
ite coating seems more compact. Consequently the composite
coating presents better mechanical and anticorrosive proper-
ties.
Amelioration of the mechanical properties of the compos-
ite coating results in a 30% increase of the coating microhard-
ness and a 35.7% higher abrasion resistance.
Acknowledgements
The authors wish to thank Luca Benedetti and Lorenzo
Ghirardelli, Dipartimento di Ingegneria dei Materialli, Uni-
`
versita di Trento, for technical assistance and Carlo Bersan-
`
nini, Dipartimento di Ingegneria dei Materialli, Universita di
Trento, for SEM analysis.
All measurements results prove the higher anticorrosive
characteristics of the composite coatings. After 35 days of
salt spray exposure the rust percentage on specimens with
composite coatings is about 4.8%, while that on specimens
with pure nickel coating is equal to 11.3%. EIS measurements
also proved that the composite coating presents higher coat-
ingresistancevalues, whichremainstableforlongerexposure
period before corrosion of steel substrate stats, while those
of pure nickel coatings start before.
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