B. KUCHARSKA et al.: THE EFFECT OF CURRENT TYPES ON THE MICROSTRUCTURE AND ...
403–411
THE EFFECT OF CURRENT TYPES ON THE MICROSTRUCTURE
AND CORROSION PROPERTIES OF Ni/NANOAl2O3 COMPOSITE
COATINGS
VPLIV VRSTE TOKA NA MIKROSTRUKTURO IN LASTNOSTI
KOROZIJE PREVLEK NA Ni/NANOAl2O3 KOMPOZITIH
Beata Kucharska, Agnieszka Krawczynska, Krzysztof Ro¿niatowski,
Joanna Zdunek, Karol Poplawski, Jerzy Robert Sobiecki
Warsaw University of Technology, Woloska 141, 01-407 Warszawa, Poland
b.kucharska@inmat.pw.edu.pl
Prejem rokopisa – received: 2015-12-16; sprejem za objavo – accepted for publication: 2016-09-02
doi:10.17222/mit.2015.347
Nickel matrix composite coatings with a ceramic disperse phase have been widely investigated due to their enhanced properties,
such as higher hardness and wear resistance in comparison to pure nickel. This paper describes the research on nickel and
nickel-alumina coatings. The coatings were obtained from a Watts bath with the presence of nickel grain-growth inhibitors by
DC (direct current), PC (pulse current) and PRC (pulsed reverse current) plating. The study includes the composite coatings of
microcrystalline and nanocrystalline Ni matrix and nanometric Al2O3 particles. In order to ensure uniform co-embedding of the
disperse phase particles with a nickel matrix and producing a stable suspension, mechanical agitation was also used. It was
proved in our previous investigations that mechanical agitation is the best way of embedding nano-alumina particles in a nickel
matrix. The effect of the electroplating techniques on the microstructure (SEM – scanning electron microscopy, STEM –
scanning transmission electron microscopy, XRD – X-ray diffraction, surface profile) of Ni and Ni/Al2O3 composite coatings
was investigated. In order to evaluate the corrosion resistance of the produced coatings, the corrosion studies were carried out by
electrochemical impedance spectroscopy and the potentiodynamic method in a 0.5-M NaCl solution. Bode diagrams obtained
by impedance spectroscopy method were established. The equivalent electric circuit and its parameters were determined to
interpret impedance spectra. The corrosion current and corrosion potential were determined. Investigations of the corrosion
damage to the produced surface layers were performed by scanning microscope techniques. The completed studies have shown
that the type of current significantly affects the structure of the nickel and composite coatings, as well as their corrosion
properties.
Keywords: nanocomposites, Ni/Al2O3 coatings, corrosion resistance, pulse electrodeposition
Matrike nikljevih kompozitnih oblog, razpr{enih s kerami~no fazo, so pogosto raziskovali zaradi njihovih izbolj{anih lastnosti,
kot na primer zaradi ve~je trdote in odpornosti proti obrabi, v primerjavi s ~istim nikljem. ^lanek opisuje raziskave na nikljevih
in nikelj-aluminijevih premazih. Prevleke so bili narejene iz Watts kopeli s prisotnostjo inhibitorjev rasti zrn niklja s povr{insko
obdelavo z enosmernim (DC), impulznim (PC) in impulzno-povratnim (PRC) tokom. [tudija vklju~uje mikrokristalne in
nanokristalne prevleke kompozitov Ni matrike in nanometrskih Al2O3 delcev. Da bi zagotovili enotno delovanje vgrajevanja
disperzne faze delcev z matriko Ni in pripravo stabilne suspenzije, je bilo uporabljeno tudi mehansko spodbujanje z me{anjem.
Dokazano je bilo, da je mehanska spodbuda najbolj{i na~in utrjevanja delcev aluminijevega oksida v Ni matriki. Raziskan je bil
u~inek tehnik povr{inske obdelave na mikrostrukturo (SEM – elektronska mikroskopija, STEM – transmisijska elektronska
mikroskopija, XRD – rentgenska difrakcija, povr{inski profil) niklja in Ni/Al2O3 kompozita. Da bi ocenili odpornost proti koro-
ziji pri proizvedenih premazih, so bile izvedene {tudije korozije, ki smo jih izvedli z elektrokemijsko impedan~no
spektroskopijo in potenciodinamskim postopkom v 0,5 M NaCl raztopini. Vzpostavljeni Bodovi diagrami so bili dobljeni z
metodo impedan~ne spektroskopije. Enakomerni elektri~ni krog in njegovi parametri so bili dolo~eni za razlago impedan~ne
spektroskopije. Dolo~ena sta bila tok korozije in korozijski potencial. Preiskave korozijskih po{kodb proizvedenih povr{inskih
slojev so bile izvedene s tehnikami mikroskopskega skeniranja. Kon~ne preiskave so pokazale, da tip toka pomembno vpliva na
strukturi nikljevih in kompozitnih prevlek, tako kot na korozijske lastnosti.
Klju~ne besede: nanokompoziti, Ni/Al2O3 premazi, odpornost proti koroziji, elektrodepozicija
1 INTRODUCTION
The introduction of new materials for transport and
industry is mainly aimed at improving the performance
characteristics of machine parts and equipment, often
working in aggressive environments. Nanocomposite
engineering metal coatings are an inspiring material
group introduced in the past few years. There are various
techniques of producing metallic nanocomposite mate-
rials. Electrodeposition is a simple, inexpensive and
versatile one, being widely used in industry.1–4 The
structure and properties of composite surface coatings
can be designed through a careful selection of the matrix
and dispersed phase. Among the wide variety of coatings
used for tribological applications, nickel composite
coatings with alumina particles appear to have a very
promising potential. The size and the amount of the
inserted reinforced alumina phase determines the pro-
perties of Ni/Al2O3 composite coatings.4,6 Until now,
most of the works were carried out using Al2O3 micro-
particles.2,4,7–10 However, the recent emergence of
nanotechnologies has led to scientific and technological
interest in the electrodeposition of Ni/Al2O3 nanocom-
Materiali in tehnologije / Materials and technology 51 (2017) 3, 403–411 403
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:621.793.7:544.653.23 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)403(2017)
posite coatings with Al2O3 particles smaller than 100 nm,
mainly devoted to an increase in the abrasion resistance
of material surfaces.3,5–6,11–12
These performance characteristics are affected pri-
marily by the process parameters, including the electro-
lyte composition (e.g., the use of organic growth
inhibitors), as well as the type (DC, PC, PRC), amplitude
and frequency of the used current. Proper optimization of
electrodeposition process of Ni/nmAl2O3 coatings is faced
with the fundamental problem of the agglomeration of
particles in the electrolyte, and therefore an insufficient
and uneven distribution of nanoparticles in the nickel
matrix.5,6 In the study the effect of the addition of a
growth inhibitor and nano-alumina powder disperse
phase, as well as the type of current (DC, PC, PRC) on
the corrosion properties of nickel and composite
Ni/Al2O3 coatings were examined.
Hard and wear-resistant Ni/Al2O3 coatings should
also have good anti-corrosion properties in the environ-
ment in which they are operated.2 The researchers’ re-
ports include mainly corrosion rate determination.3,4,11–15
This allows a good comparison of materials in terms of
corrosion resistance, but does not give information about
the causes of the corrosion changes and the mechanism
of corrosion. Electrochemical impedance spectroscopy
can provide significant information regarding the
corrosion mechanisms and susceptibility of materials to
corrosion in the exposed environment. In the present
work the nickel and composite coatings were examined
both by the potentiodynamic method (setting the corro-
sion current, corrosion potential, polarization resistance)
as well as electrochemical impedance spectroscopy.
2 EXPERIMENTAL PART
Nickel matrix composite coatings with a dispersed
nanometric alumina phase (Aldrich Chemistry) were
produced by electrochemical reduction on a copper
substrate (75×15×1) mm in a Watts bath modified (or
not) with benzoic sulfimide (saccharin) (Table 1). For
comparison the nickel coatings were also produced with
the same process parameters. The electrodeposition
process consists of several steps, including preliminary
and regular surface preparation (grinding, degreasing,
digestion and finally electroplating). The processes were
carried out at 45 °C at pH 4.2±0.1 using direct, pulsed
and pulse reversed current. The diagram and parameters
are shown in Figure 1. All the coatings have a thickness
of approximately 30 μm, with the exception of the
coatings to be examined by STEM technique (150–200
μm), for which the preparation has differed from the
conventional procedure (additional application of
electrospark cutting, grinding to a thickness of 4 μm and
electrochemical polishing). The thickness was estimated
using SEM techniques and the study was conducted on
metallographic cross-sections. In order to provide the
transport of alumina particles into cathode proximate
areas, the bath was mechanically stirred at 400 min–1.
Table 1: Bath composition
Tabela 1: Sestava kopeli
Component Concentration (mol L–1)
NiSO4· 6 H2O 1,14
NiCl2· 6 H2O 0,17
H3BO3 0,57
Saccharin 0,014
Al2O3 0,098
A structure analysis of the alumina powder, the
nickel and the nickel-alumina composite coatings was
carried out using a Hitachi SU-70 (SEM) and Hitachi
HD-2700 (STEM), respectively, scanning and scanning-
transmission electron microscopes (both equipped with
EDS – energy-dispersive X-ray spectroscopy device).
The phase composition, crystalline size, and residual
stresses of the deposits were studied by the XRD
technique using Cu-K	 (Philips PW-1830). The surface
roughness was measured using a Wyko NT9300 light
interference optical microscope.
Examinations and analyses of the corrosive proper-
ties of nickel and nickel-alumina coatings were made
with the use of potentiostat ATLAS Sollich, AtlasCorr
and AtlasLab computer programs. A three-electrode
measurement system included a measurement vessel
with a saturated calomel electrode (as a reference elec-
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404 Materiali in tehnologije / Materials and technology 51 (2017) 3, 403–411
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Figure 1: Scheme of impulse current and process parameters: a) DC, b) PC, c) PRC
Slika 1: Shema impulza toka in procesni parametri: a) DC, b) PC, c) PRC
trode), platinum mesh (as a counter electrode) and
examined coatings (as a working electrode). The corro-
sion resistance was tested in the 0.5-M NaCl solution at
20 °C. The samples, before corrosive properties exami-
nation, were stabilized for 60 min in the target system as
an open circuit. Research by electrochemical impedance
spectroscopy was carried out in the corrosion potential
surroundings in the frequency range 100 kHz to10 MHz
of the10 mV amplitude.
The corrosion study was performed using the poten-
tiodynamic method. For determining the polarization
curves of the tested materials the measurements started
from the potentials having values lower by 200 mV up to
the values higher by 200 mV from the predetermined
open-circuit potential with the potential scan rate equal
to 0.2 mV/s and then up to 500 mV with a scan rate of
0.8 mV/s. The obtained curves were analyzed by the
Tafel straight-lines method by keeping the conditions
inherent in this method. The surface morphology of the
examined materials after potentiodynamic tests was
analyzed by scanning electron microscopy.
3 RESULTS AND DISCUSSION
Al2O3 powder was used as the dispersed phase in the
Ni/Al2O3 composite coating formation process. The
Al2O3 particles are presented in Figures 2a and 2b in
bright-field (BF) and Z-contrast modes, respectively. We
can estimate from these images that their size is in the
nanometric regime and it varies from 5 nm to 25 nm.
Furthermore, the particles have the tendency to agglome-
rate, creating larger Figures 2a and 2b and smaller
agglomerates, consisting of a few particles (Figure 2c).
Images taken at higher magnifications (Figure 2d)
showing crystallographic plates demonstrate that at least
some of these nanoparticles are crystalline.
All coatings deposited from the basic Watts bath (Ni,
Ni/Al2O3) have microcrystalline nickel structures (Fig-
ures 3a to 3f). In the case of the use of the pulse reverse
current the smallest grains are visible in the nickel and
composite coatings. In turn the nickel and composite
coatings produced in the electrolyte with grain growth
inhibitor addition have a nanocrystalline structure
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Figure 3: Morphology of microcrystalline coatings: a) nickel (DC), b) nickel (PC), c) nickel (PRC), d) Ni/Al2O3 (DC), e) Ni/Al2O3 (PC),
f) Ni/Al2O3 (PRC)
Slika 3: Morfologija mikrokristalini~nih prevlek: a) nikelj (enosmerni tok), b) nikelj (pulzni tok), c) nikelj (impulznopovratni tok), d) Ni/Al2O3
(enosmerni tok), e) Ni/Al2O3 (pulzni tok), f) Ni/Al2O3 (impulznopovratni tok)
Figure 2: Microstructure of Al2O3 powder in: a) bright-field mode, b)
Z-contrast mode, c) secondary-electron mode, d) bright-field mode
Slika 2: Mikrostruktura Al2O3 prahu: a) svetlo polje, b) Z-kontrast,
c) sekundarni elektronski na~in, d) svetlo polje
(Figures 4a to 4f). The quality of the composite coatings
with a nickel matrix and alumina dispersed phase
depends on the formation of an appropriate structure
with embedded alumina nanoparticles. The Al2O3 parti-
cles co-deposited in the nickel matrix cause a significant
change in the morphology of its surface, as well the
microcrystalline Ni matrix, as the nanocrystalline Ni
matrix (Figures 3a to 3c). The all Niμm/Al2O3 have large
amounts of embedded disperse phase on its surface,
which occurs often as agglomerates. In turn in the case
of nanocrystalline composite coatings alumina is better
fixed and distributed in the matrix. The nanocomposite
coatings deposited with PRC were characterized by the
largest surface roughness.
Due to this, one of the coatings was designated to
TEM studies. The microstructure of the Ninm/Al2O3 coat-
ing obtained in the modified bath with PC was examined
by STEM technique (Figure 5). The Al2O3 particles
embedded in the matrix can easily be recognised as they
stick out of the surface in Figure 5a) taken in SE-mode.
Their existence was confirmed by X-ray EDS analysis
(Figure 5d). In BF-mode (Figure 5b) as well as in
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406 Materiali in tehnologije / Materials and technology 51 (2017) 3, 403–411
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Figure 5: Microstructure and EDS result of Ni/Al2O3 coating obtained in the modified bath with PC: a) SE-mode, b) BF-mode, c) Z-contrast
mode, d) EDS results
Slika 5: Mikrostruktura in EDS-analiza Ni/Al2O3 prevleke, pridobljene v spremenjeni kopeli z impulzivnim tokom (PC): a) SE-na~in,
b) BF-na~in, c) Z-kontrastni na~in, d) rezultati EDS-analize
Figure 4: Morphology of nanocrystalline coatings: a) nickel (DC), b) nickel (PC), c) nickel (PRC), d) Ni/Al2O3 (DC), e) Ni/Al2O3 (PC), f)
Ni/Al2O3 (PRC)
Slika 4: Morfologija nanokristalini~nih prevlek: a) nikelj (enosmerni tok), b) nikelj (pulzni tok), c) nikelj (impulznopovratni tok), d) Ni/Al2O3
(enosmerni tok), e) Ni/Al2O3 (pulzni tok), f) Ni/Al2O3 (impulznopovratni tok)
Z-contrast mode (Figure 5c) it is difficult to distinguish
them from the matrix. In the matrix grains with an
average 30 nm in diameter are present. In most of the
grains dislocations can be found. Some of them are
divided into sub-grains. Some contain annealing nano-
twins.
X-ray analysis was performed to examine the effect
of organic additive and the Al2O3 powder in the Watts
bath on the structure of the nickel and composite coat-
ings produced. The diffraction patterns of the coatings
deposited with the DC current are shown in Figure 6.
All the examined coatings are characterized by a
crystalline structure. The profile diffraction lines indicate
that the nickel coatings deposited in the base bath have
the largest dimensions of the grains (79 nm and 88 nm,
as estimated by the Scherrer equation). All the coatings
produced with the addition of saccharine have nanocrys-
talline nickel structures. This is revealed by both the
XRD patterns (Figure 6) and the images of the micro-
structure obtained by the SEM (Figures 3 and 4).
The diffraction line profiles (Figure 6) indicate that
the composite coatings are characterized by a smaller di-
mension of crystallites: 26 nm and 18 nm and by 23nm
and 15 nm, respectively, in comparison to micro- and
nanocrystalline nickel.
The surface roughness of the examined coatings is
shown in Figures 7 and 8. It can be seen from the 3D
view of the surface profiles that the surface roughness is
the highest in the case of the PRC coatings, followed by
the DC and finally the PC coatings. The coatings
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Figure 6: X-ray diffractogram of the nickel and Ni/Al2O3 composite
coatings produced with DC in a Watts bath and a Watts bath with
saccharine additives
Slika 6: XRD-analiza niklja in Al2O3 prevlek kompozitov, narejenih z
enosmernim tokom in z Watts-ovo kopeljo z dodatki saharina
Figure 7: Surface roughness of microcrystalline coatings: a) nickel (DC), b) nickel (PC), c) nickel (PRC), d) Ni/Al2O3 (DC), e) Ni/Al2O3 (PC),
f) Ni/Al2O3 (PRC)
Slika 7: Hrapavost povr{ine mikrokristalini~nih prevlek: a) nikelj (enosmerni tok), b) nikelj (pulzni tok), c) nikelj (impulznopovratni tok),
d) Ni/Al2O3 (enosmerni tok), e) Ni/Al2O3 (pulzni tok), f) Ni/Al2O3 (impulznopovratni tok)
deposited in the bath without organic additives are
characterized by greater roughness (Figure 7). In the
case of the DC and PRC microcrystalline coatings, the
addition of Al2O3 makes the surface more irregular,
which is visible both in 3D images and from the surface
parameter characteristics (Table 2).
Table 2: Surface-roughness parameters
Tabela 2: Parametri povr{inske hrapavosti
Ra (nm) Rq (nm) Rt (nm)
Niμm
DC 162 207 1650
PC 162 206 1520
PRC 332 423 2930
Niμm/Al2O3
DC 286 388 3840
PC 159 213 1870
PRC 516 672 5120
Ninm
DC 100 132 1340
PC 80 103 992
PRC 141 178 1160
Ninm/Al2O3
DC 112 144 1050
PC 55 70 525
PRC 179 229 1970
Table 3: Characteristic parameters obtained as a result of potentio-
dynamic tests
Tabela 3: Karakteristike parametrov, pridobljene kot rezultat
potenciodinami~nih testov
Material Current Ecorr(mV)
icorr
(ìA cm–2)
Niμm PRC -175 0.013
Niμm/Al2O3
DC -218 0.048
PC -182 1.25
PRC -189 0.062
Ninm PRC -165 0.22
Ninm/Al2O3
DC -147 0.056
PC -186 1.208
PRC -172 0.230
The results of the potentiodynamic method examina-
tions are presented at Figure 9. The characteristic values
are presented in Table 3. The potentiodynamic tests
revealed that the nickel and composite coatings (with the
exception of the composite coatings produced with PC)
deposited in the basic Watts bath have good and similar
corrosion properties, while the Ninm and Ninm/Al2O3
coatings obtained in the modified electrolyte (with a
nano-nickel matrix) are characterized by increased corro-
sion potential and current density (Table 3).
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Figure 9: Polarization curves of Ni and Ni/Al2O3 coatings deposited
with different current parameters
Slika 9: Polarizacijske krivulje niklja in Ni/Al2O3 prevlek pri raz-
li~nih parametrih toka
Figure 8: Surface roughness of nanocrystalline coatings: a) nickel
(DC), b) nickel (PC), c) nickel (PRC), d) Ni/Al2O3 (DC), e) Ni/Al2O3
(PC), f) Ni/Al2O3 (PRC)
Slika 8: Hrapavost povr{ine nanokristalini~nih prevlek: a) nikelj
(enosmerni tok), b) nikelj (pulzni tok), c) nikelj (impulznopovratni
tok), d) Ni/Al2O3 (enosmerni tok), e) Ni/Al2O3 (pulzni tok),
f) Ni/Al2O3 (impulznopovratni tok)
Together with an increase of the co-deposited
alumina greater etching is visible (Figure 10) in com-
parison with the nickel coatings. Analyses of the elec-
trodeposited nickel and composite coatings after
exhibited in corrosion environments have demonstrated a
pitting corrosion on the surfaces of the electrodeposited
materials, except for the Niμm and Niμm/Al2O3 deposited
with PRC. The smallest corrosion degradation of the
surface occurs in the case of the microcrystalline coat-
ings (Figure 10).
The measured impedance spectra for the nickel and
composite coatings deposited at different current para-
meters in the 0.5-M NaCl are shown as Bode diagrams in
Figures 11 and 12. The fitting of the spectra obtained
during the measurements based on the two equivalent
electric circuits (Table 4) and enables an evaluation of
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Figure 10: Morphology of the Ni and Ni/Al2O3 coatings after corrosion examinations: a) Niμm, (PRC), b) Ninm (PRC), c) Niμm/Al2O3 (DC),
d) Niμm/Al2O3 (PC), e) Niμm/Al2O3 (PRC), f) Ninm/Al2O3 (DC), g) Ninm/Al2O3 (PC), h) Ninm/Al2O3 (PRC)
Slika 10: Morfologija nikljevih in Ni/Al2O3 prevlek po korozijskih pregledih: a) Niμm, (PRC), b) Ninm (PRC), c) Niμm/Al2O3 (DC),
d) Niμm/Al2O3 (PC), e) Niμm/Al2O3 (PRC), f) Ninm/Al2O3 (DC), g) Ninm/Al2O3 (PC), h) Ninm/Al2O3 (PRC)
Table 4: Electric parameters of corrosion systems and schemes of the equivalent electric circuit for corroded coatings
Tabela 4: Elektri~ni parametri korozijskih sistemov in shem ekvivalentnega elektri~nega tokokroga za korodirane prevleke
Material Current Electricalequivalent circuit
Electrochemical parameters
Rs
(cm2)
R1
(kcm2)
Y01
(ìFcm–2s(n–1)) n1
R2
(kcm2)
Y02
(Fcm–2s(n–1)) n2
Niμm PRC 14.0 409 23.0 0.95 — — —
Niμm/Al2O3
DC 14.2 790 26.2 0.91 — — —
PC 15.1 0.316 21.8 0.86 567 11.8 0.9
PRC 14.0 73.2 39.6 0.91 20.1 215.4 1.3
Ninm PRC 14.3 101 18.8 0.88 — — —
Ninm/Al2O3
DC 14.9 112 23.8 0.95 — — —
PC 14.3 0.000000049 13.2 0.94 75.6 6.5 0.4
PRC 13.5 126 17.5 0.92 — — —
the corrosion properties. The equivalent electric circuits
represent the process occurring in the corrosion system
and allow us to define the parameters of these processes.
In the electric models the element Rs reflects the
resistance of the corrosion environment. The resistance
of the electrolyte was comparable in all cases (Table 4).
In contrast the element R1 characterizes the rate of
corrosion process and it is connected with the resistance
of the charge transfer across the interface. The capacity
of these coatings reproduces the element Y01. Two addi-
tional elements in the second equivalent electric circuit:
capacity of the surface area (Y02) and the resistance of
the electrolyte in the area of the material (R2) are used in
the cases of the coatings with higher roughness.
The simple equivalent electric circuit (Table 4) was used
in the case of the micro- and nanocrystalline nickel
coatings deposited with PRC and the composite nickel
coatings (both micro- and nanocrystalline nickel matrix)
deposited with DC. It is related to the different structures
of this coatings. For the Niμm and Niμm/Al2O3 coatings
deposited with PRC an increase in the phase-angle maxi-
mum is observed at low frequencies in contrast to the
nickel and composite coatings with a nano-nickel matrix
deposited with PRC (Figure 11). The Niμm/Al2O3 coating
deposited with DC has the highest R1 value (790 k cm2,
Table 4) and the widest range of maximum phase angle
(Figure 12). In contrast the micro- and nanocrystalline
composite coatings deposited with PC have the lowest
R1, which indicates that there are surface-blocking pro-
cesses.
Based on the analysis of the impedance phase-angle
spectra, we found that microcrystalline coatings without
Al2O3 phase are characterized by the highest corrosion
resistance. Impedance phase-angle values have the maxi-
mum in a wide frequency range.
The incorporation of the ceramic phase in the
microcrystalline nickel matrix results in the presence of
boundaries between the embedded particles and the
matrix, which is a place of corrosive attack. In particu-
lalr, the Al2O3 phase in the case of coatings with
microcrystalline matrix is incorporated in the form of
several hundred nm agglomerates (Figure 3d to 3f).
From the analysis of the impedance phase-angle
spectra, we can also conclude that coatings with nano-
crystalline nickel matrix have a decreased corrosion
resistance. It is associated with a significantly larger
number of grain boundaries, which also take the place of
potential corrosive attack. Composite coatings with a
nanocrystalline nickel matrix are characterized by
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Figure 12: Bode spectra of the Ni/Al2O3 coatings deposited with DC,
PC and PRC
Slika 12: Bode spektri Ni in Ni/Al2O3 prevlek pridobljenih z DC, PC
in PRC
Figure 11: Bode spectra of the Ni and Ni/Al2O3 coatings deposited
with PRC
Slika 11: Bode spektri Ni in Ni/Al2O3 prevlek pridobljenih z PRC
enhanced corrosion resistance compared to nanocrys-
talline nickel coating (wider range of maximum values
of phase angle in the frequency range), which may either
be due to the embedding of non-agglomerated, chemi-
cally resistant Al2O3 (Figures 4d to 4f) and the presence
of the phase AlNi3 (Figure 6).
In the case of coatings produced by the PC current
method, a larger amount of built-in Al2O3 particles (Fig-
ures 3e and 4e) is observed, resulting in a higher amount
of interfacial boundary. These connections of the nickel
matrix and the nanometric alumina particles are the
target of corrosive attacks and therefore it results in the
worst possible corrosion resistance.
4 CONCLUSIONS
Composite coatings are characterized by a smaller
grain size compared to the pure nickel coatings obtained
under the same conditions. Nickel and composite coat-
ings deposited in a bath containing saccharine have a
nanometric dimension of the crystallites, as compared to
coatings produced in a basic Watts bath. In the case of
the Ninm/Al2O3 composite coating, the presence of the
AlNi3 phase is also observed.
The surface roughness is the highest in the case of
PRC coatings, followed by the DC and lastly the PC
coatings. The coatings deposited in the bath with organic
additives are characterized by a lower roughness.
Both the nickel and composite coatings deposited in
the base Watts bath (microcrystalline coatings) have
better corrosion properties in comparison with nanocrys-
talline coatings. The higher corrosion current and the
greater destruction process (especially pitting corrosion)
are visible in the case of nanocrystalline coatings.
The composite coatings deposited both in a base
Watts bath and modified Watts bath with PC and PRC
are characterized by different corrosion properties in
comparison with the nickel and composite coatings
deposited with DC. They are described by means of an
advanced equivalent electric circuit (five-partial circuit).
The best corrosion properties, which are determined,
e.g., by the phase-angle maximum and the corrosion
destruction of surface, exhibit the Niμm and Niμm/Al2O3
coatings deposited with PRC. The increase of the phase
angle is observed at low frequencies in contrast to nickel
and composite coatings with a nano-nickel matrix depo-
sited with PRC.
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