Electrodeposited Ni-B alloy coatings: Structure, corrosion resistance and mechanical properties

Article abstract of DOI:10.1016/j.electacta.2009.11.069

Ni-B alloy coatings with different boron content ranging from 4 to approximately 28 at.% were prepared by electrodeposition in a nickel-plating bath containing sodium decahydroclovodecaborate as a boron source. The influence of the boron concentration in the coatings on their structure, morphology, electrochemical and corrosion behavior, physico-mechanical and electrical properties was investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and other methods. It was found that the electrodeposited Ni-B coatings with relatively low boron content (≤8 at.%) are nanocrystalline and comprise a solid solution of boron in f.c.c. Ni lattice having a mixed substituted-interstitial type. Further increase in the boron content (up to 10-15 at.%) leads to the appearance of heterogeneous amorphous-nanocrystalline structure, and the coatings with a high boron content (20 at.% and above) are X-ray amorphous. Polarization measurements in neutral NaCl solutions showed that the Ni-B coatings with relatively low boron content demonstrate a potential region of low anodic currents associated with the passive film formation at the alloy surface. The anodic current in this potential region increases significantly with increasing the boron content above 10 at.%, suggesting the non-protective nature of the anodic film formed on the amorphous Ni-B alloys. Immersion tests monitored by EIS measurements revealed a significantly better corrosion performance of the Ni-B coatings with low boron content (4 at.%) in comparison with that of the amorphous coatings. The microhardness and wear resistance of the Ni-B coatings essentially increases with increasing the boron content. Maximum microhardness and wear resistance were found for the coatings containing 8 at.% B.

Full text of DOI:10.1016/j.electacta.2009.11.069

Electrochimica Acta 55 (2010) 2223–2231
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposited Ni–B alloy coatings: Structure, corrosion resistance and
mechanical properties
Yu.N. Bekish, S.K. Poznyak∗, L.S. Tsybulskaya, T.V. Gaevskaya
Research Institute for Physical Chemical Problems, Belarusian State University, Leningradskaya Str. 14, 220050 Minsk, Belarus
a r t i c l e i n f o
a b s t r a c t
Article history:
Ni–B alloy coatings with different boron content ranging from 4 to approximately 28 at.% were prepared
by electrodeposition in a nickel-plating bath containing sodium decahydroclovodecaborate as a boron
source. The influence of the boron concentration in the coatings on their structure, morphology, electro-
chemical and corrosion behavior, physico-mechanical and electrical properties was investigated using
X-ray diffractometry (XRD), scanning electron microscopy (SEM), potentiodynamic polarization, electro-
chemical impedance spectroscopy (EIS) and other methods. It was found that the electrodeposited Ni–B
coatings with relatively low boron content (≤8 at.%) are nanocrystalline and comprise a solid solution of
boron in f.c.c. Ni lattice having a mixed substituted-interstitial type. Further increase in the boron con-
tent (up to 10–15 at.%) leads to the appearance of heterogeneous amorphous-nanocrystalline structure,
and the coatings with a high boron content (20 at.% and above) are X-ray amorphous. Polarization mea-
surements in neutral NaCl solutions showed that the Ni–B coatings with relatively low boron content
demonstrate a potential region of low anodic currents associated with the passive film formation at the
alloy surface. The anodic current in this potential region increases significantly with increasing the boron
content above 10 at.%, suggesting the non-protective nature of the anodic film formed on the amorphous
Ni–B alloys. Immersion tests monitored by EIS measurements revealed a significantly better corrosion
performance of the Ni–B coatings with low boron content (4 at.%) in comparison with that of the amor-
phous coatings. The microhardness and wear resistance of the Ni–B coatings essentially increases with
increasing the boron content. Maximum microhardness and wear resistance were found for the coatings
containing 8 at.% B.
Received 27 June 2009
Received in revised form
18 November 2009
Accepted 21 November 2009
Available online 26 November 2009
Keywords:
Ni–B coating
Electrodeposition
Corrosion resistance
Microhardness
Wear resistance
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
and different amine boranes (dimethylamine borane [17,18], pyri-
dine borane [19]). As a rule, electroless deposition is performed
In the last two decades, considerable attention has been given to
metal–metalloid alloys with nanocrystalline and amorphous struc-
ture which demonstrate a number of new properties in comparison
with those of pure metals or metal alloys. Among these materials,
Ni–B alloy coatings are promising due to their high hardness and
superior mechanical wear resistance [1–8]. After heat treatment,
the wear resistance of nickel–boron can equal or even exceed that
of hard chromium coatings [9–11].
Although a variety of techniques, such as thermal boriding of the
surface, melt quenching, sputtering in vacuum, are available for the
preparation of the Ni–B coatings, electro- and electroless plating
processes have received widespread acceptance due to their less
complex processing sequence and cost-effectiveness. Electroless
Ni–B coatings can be obtained from baths containing strong reduc-
ing agents such as sodium or potassium borohydride [1–3,8,10–16]
at high temperature (70–90 ◦C) and high pH values (13–14). Ni–B
alloy coatings can be also deposited electrochemically from the
baths containing sodium borohydride [20], dimethylamine borane
−
[4,21,22], trimethylamine borane [5], carborane ion (C2B9H12
)
[23] and sodium decahydroclovodecaborate [6]. It should be noted
that the electrochemical method offers several advantages in
comparison with the electroless one, such as high rate and low tem-
perature of deposition, stability of electrolyte, uniform distribution
of boron in the deposit, ease control of the plating process. How-
ever, the mechanism of deposition and properties of electroplated
Ni–B coatings have been less studied in comparison with those of
electroless coatings.
Along with enhanced microhardness, wear resistance and elec-
trical characteristics, practical application of Ni–B coatings requires
their high corrosion resistance depending on a number of fac-
tors such as chemical composition of the resultant coatings, their
morphology and structure. It should be noted that the researches
concerning the corrosion stability of electroless and electrode-
posited Ni–B coatings are scanty [13,16,24,25]. Sankara Narayanan
∗
Corresponding author. Tel.: +375 17 2264696; fax: +375 17 2264696.
E-mail address: poznyak@ua.pt (S.K. Poznyak).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.11.069

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and Seshadri [13] and Dadvand et al. [25] have pointed out that
the corrosion resistance of amorphous electroless Ni–B coatings
is less than that of electroless Ni–P ones. Anik et al. revealed that
the corrosion resistance of electroless Ni–B films increased with
increasing the boron content in the range from 4.5 to 8 wt.% [16].
In addition, the electroless Ni–B coatings were found to suffer a
decrease in corrosion resistance with heat treatment [13,16,25]. To
our knowledge, the effect of boron content and coating structure
on the corrosion resistance of electroplated Ni–B films has not yet
studied.
In our earlier work [6], we proposed the bath formulation for
electrochemical deposition of Ni–B coatings using sodium decahy-
droclovodecaborate as a source of boron and studied the influence
of the electrolyte composition and plating conditions on the com-
position, structure, microhardness and contact resistance of the
deposited alloys. Unlike other boron-containing additives, the use
of sodium decahydroclovodecaborate allows to deposit uncracked
Ni–B alloy coatings with a boron concentration varied over a wide
range from 1 to 30 at.% B.
performed using a Gamry potentiostat with a PCI4 Controller in
a frequency range from 5 × 104 to 3 × 10−3 Hz with a step of 7
points per decade. All the EIS spectra were recorded at open cir-
cuit potential with applied 10 mV sinusoidal perturbation. Before
the spectra recording, the system was allowed to attain a stable
open circuit potential. The impedance plots were fitted using dif-
ferent equivalent circuits by means of the Elchem Analyst software
from Gamry.
Tribotechnical tests of the coatings were performed using an
automated tribometer equipped by a special set-up for measure-
ment of a coefficient of friction. Tribotechnical characteristics were
measured under non-lubricated conditions at a specific load of
1 MPa. The film hardness (film thickness – 20 ␮m) was determined
using a Viskers microhardness tester with a load of 1 N for 10 s of
exposure, and the hardness values were averaged out of 10 deter-
minations. Transient (contact) resistance of the films was assessed
by the method based on measuring the resistance of a point contact
of a reference electrode with the film at a load of 40 g.
The main objective of the present work was to elucidate how the
corrosion stability and wear resistance of the electrodeposited Ni–B
alloys depend on their structure and composition. Potentiodynamic
polarization technique and electrochemical impedance spec-
troscopy were applied to investigate the corrosion properties of the
alloys. Furthermore, new data on the morphology and structure
of the Ni–B coatings deposited in decahydroclovodecaborate-
containing electrolyte were obtained.
3. Results and discussion
The introduction of sodium decahydroclovodecaborate in a
nickel-plating electrolyte results in codeposition of boron with
nickel. The boron content in the coating increases from 4 to approx-
imately 28 at.% with the increase in the concentration of sodium
decahydroclovodecaborate from 0.1 to 20 g L−1 (Fig. 1). The depo-
sition rate is 24 3 ␮m h−1 at jc = 0.02 A cm−2
.
The current efficiency of the Ni–B plating increases from 95
to 100% and above with increasing the concentration of sodium
decahydroclovodecaborate in electrolyte which is indicative of
the presence of a concurrent reaction of Ni(II) reduction by
sodium decahydroclovodecaborate at the cathode surface. When
the Na2B10H10 concentration exceeds 20 g L−1, this undesirable
chemical process begins to proceed with an appreciable rate in the
solution bulk giving fine-dispersed Ni particles in electrolyte.
2. Experimental
The Ni and Ni–B coatings were electrodeposited on a copper sub-
strate with an area of 3 cm2 from electrolyte containing 240 g L−1
NiSO4·7H2O, 36 g L−1 NiCl2·6H2O, 31 g L−1 H3BO3, 57 g L−1 Na2SO4
and 0.1–20 g L−1 sodium decahydroclovodecaborate (Na2B10H10).
The current density, temperature and pH of the electrolyte were
maintained at 0.02 A cm−2, 30 ◦C, 4.5, respectively. Hot rolled nickel
plate (99.7%) was used as an anode. Anode was placed parallel with
cathode in the bath, and the anode-to-cathode area ratio was 2:1.
Ni–B coatings with different boron content (hereafter referred to
as Ni–BX, where X is the boron concentration in atomic percents)
were produced by adjusting the Na2B10H10 concentration in the
bath. The film thickness was estimated by gravimetric analysis and
averages 20 ␮m. The boron content in the Ni–B coatings was deter-
mined by potentiometric titration in the presence of mannitol [26].
The structure of the coatings was characterized by HZG–4M diffrac-
tometer (Germany) with a CoK␣ radiation. The lattice parameter a
was determined using diffraction peaks (1 1 1), (2 0 0), (2 2 0) and
(3 1 1). The average grain size was estimated from the (1 1 1) peak
broadening using the Scherrer equation [27]. The surface morphol-
ogy of the deposits before and after corrosion tests was examined
by scanning electron microscopy (LEO 1420 microscope).
3.1. Structure of the coatings
Fig. 2 shows the results of the XRD study of Ni and Ni–B coat-
ings. All coatings have face-centered cubic lattice of nickel. The
main growth directions are [1 0 0] and [1 1 1]. The average grain
size estimated for Ni coatings is 35–40 nm. Incorporation of 4 at.%
B leads to the redistribution of the peak intensities correspond-
ing to the (1 1 1) and (2 0 0) planes and to the decrease in average
grain size up to 10–15 nm. The peak (1 1 1) becomes to be the most
The polarization measurements were performed using a Gamry
potentiostat and a conventional three-electrode two-compartment
Pyrex glass cell. A Pt foil and an Ag/AgCl/KCl (sat.) electrode
(+0.199 V vs. the SHE) with a Luggin probe were used as the counter
and reference electrodes, respectively. The working electrode with
an exposed area of 1 cm2 was used in these measurements. The
sweep rate was 0.5 mV s−1. All measurements were performed in
non-deaerated 3.5% aqueous NaCl solution.
Electrochemical impedance spectroscopy (EIS) measurements
were carried out at room temperature during immersion tests (3.5%
NaCl solution) in a three-electrode cell consisting of a saturated
calomel reference electrode, a platinum foil as the counter elec-
trode and the working electrode with an exposed area of 3 cm2.
The cell was placed in a Faraday cage to avoid any interference
with external electromagnetic fields. The EIS measurements were
Fig. 1. Dependence of the boron content in Ni–B coatings on sodium decahydro-
clovodecaborate concentration in the electroplating bath.

Yu.N. Bekish et al. / Electrochimica Acta 55 (2010) 2223–2231
2225
intensive and the relative intensity of the peaks (2 2 0) and (3 1 1)
increases slightly. With increasing the boron content up to 8 at.%,
the intensity of peaks (2 0 0), (2 2 0) and (3 1 1) reduces. These peaks
disappear at 10 at.% B and an amorphous phase arises (Fig. 2). The
average grain size decreases up to 5–7 nm. The coatings with the B
content more than 20 at.% are X-ray amorphous (Fig. 2).
arately at freshly prepared electrodes at a low scanning rate of
0.5 mV s−1. As can be seen from Fig. 4 increasing the B content does
not influence noticeably the rate of the oxygen electroreduction
occurring in the potential range from −0.3 to −0.8 V, and at more
negative potentials only a slight increase in the overpotential for
hydrogen evolution is observed.
When incorporating 4 at.% B in the coating, the f.c.c. lattice
parameter a decreases from 3.524 to 3.508 Å. This decrease in the
lattice spacing can be related to the replacement of nickel atoms
having bigger atomic radius (rNi = 1.24 Å) by boron atoms with
smaller atomic radius (rNi = 0.97 Å). The f.c.c. lattice parameter for
the Ni–B alloy, calculated under assumption that the Ni atoms are
replaced by B atoms, is significantly less (acalc = 3.493 Å for alloy
with 4 at.% B) than the experimentally observed one. Since it is
known that boron atoms due to their small size can occupy octahe-
dral interstitial sites in the f.c.c. Ni lattice, it is possible to assume
that nanocrystalline Ni–B coatings are a solid solution of boron in
the f.c.c. Ni lattice having a mixed substituted-interstitial type. The
formation of the substitutional solid solution can be indicative of
the codeposition of boron in the elementary form. When increas-
ing the boron content up to 8 at.% B, the lattice parameter is not
appreciably changed. The estimation of the lattice parameter is dif-
ficult for the Ni–B coatings with the boron content more than 8 at.%,
because the peak (1 1 1) is strongly broadened and the other peaks
disappear.
Unlike the cathodic reactions, the boron doping exerts a stronger
effect on the anodic processes of film oxidation. The electrode-
posited boron-free Ni coatings and Ni–B coatings with a relatively
low B content (≤10 at.%) are characterized by the presence of a
region of low anodic currents in the potential range from corro-
sion potential to about 0 V (Fig. 4). After stopping the potential
sweep in this region, the current drops rapidly up to very low values
(less than 1 ␮A cm−2), indicating that the passive oxide film forma-
tion occurs in this potential region (Fig. 5, insert). At more positive
potentials, the breakdown of the passive film takes place due to
the interaction of the aggressive chloride ions with the passivated
surface [28], and the anodic current begins to grow exponentially.
The boron doping leads to the shift of the corrosion potential to
the negative direction. The anodic polarization curves are changed
most significantly in going from nanocrystalline to amorphous Ni–B
coatings when increasing the B content from 10 to 20 at.% and
higher. An appreciable growth of the anodic current is observed
for the amorphous coatings in the potential range from −0.3 to
0 V, and a current step appears at potentials from −0.16 to −0.18 V
(Fig. 4). Fig. 5 shows the current vs. time dependence recorded at a
fixed potential of −0.16 V chosen in the region of the current step.
After stopping the potential sweep the current initially continues
to grow with time and then begins to drop gradually until reach-
ing quasi-stationary values (70–80 ␮A cm−2) which are essentially
higher than those observed for the nanocrystalline Ni–B coatings.
SEM inspection of the surface morphology after 7 h of holding the
Ni–B20 electrode at a potential of −0.16 V showed that a distinc-
tive etching of the surface with the formation of fine (hundreds
of nanometers) and larger (about 1 ␮m) pits takes place, i.e. under
these conditions the anodic pitting dissolution of the coating occurs
(Fig. 6).
3.2. Morphology of the coatings
Fig. 3 shows the surface morphology of the boron-free Ni coat-
ings and Ni–B alloy ones with different boron content. The rough
and dull surface of the Ni coating (Fig. 3) consisting of pyramidal
crystallites turns into more smooth and bright one as boron atoms
are incorporated into the film and their content increases. At 4 at.%
B, the surface becomes nodular and spherical particles differ widely
in size (Fig. 3). The nodular structure with “orange peel” shape
becomes more clearly defined and the size of spherical particles
becomes more uniform in going to the X-ray amorphous coatings.
Thus, the results of the polarization measurements testify that
a high content of boron in the Ni–B coatings can lead to apprecia-
ble deterioration of the corrosion resistance of these materials in
neutral chloride solutions.
3.3. Polarization measurements
Fig. 4 presents typical polarization curves for the Ni and Ni–B
coatings with different B content in 3.5% NaCl solution. Anodic and
cathodic branches of the polarization curves were recorded sep-
3.4. Immersion corrosion tests and EIS measurements
Prolonged immersion tests monitored by impedance measure-
ments were performed to gain a better understanding of the
corrosion behavior of Ni–B coatings. Impedance spectra of the Ni
and Ni–B samples were recorded at different times after immer-
sion of the samples in non-deaerated 3.5% NaCl solution. Typical
impedance spectra after 24 and 168 h of immersion are shown in
Fig. 7. The Nyquist impedance diagrams have the form of slightly
depressed semicircle, and in most cases, only one time constant can
be observed on the Bode plots in the frequency range studied. In
the case of amorphous Ni–B20 and Ni–B26 coatings, Warburg diffu-
sion behavior was observed in the low frequency region, indicating
that the corrosion mechanism is controlled not only by a charge
transfer step but also by a diffusion process. A simple equivalent
circuit composed of the solution resistance, Rsol, in series with a
parallel connection of the capacitance of double layer, Qdl, and the
charge transfer resistance, Rct, was used for interpretation of the
impedance spectra revealing only one time constant for nanocrys-
talline Ni and Ni–B coatings (Fig. 8a). After 168 h of immersion,
an additional time constant appears on the impedance spectrum
of the Ni–B10 coatings in low frequency region (0.1–1 Hz) (Fig. 9).
This time constant can be associated with the corrosion processes
in pits appeared, as will be shown below, on the sample surface
Fig. 2. X-ray diffraction patterns of nickel and Ni–B coatings.

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during the immersion test. In this case, we used a more compli-
cated equivalent circuit for fitting of impedance spectra with two
additional elements, Qcor and Rcor, for describing the corrosion pro-
cess in the pits (Fig. 8b). For amorphous Ni–B coatings, a Warburg
element describing the diffusion process was added to the equiv-
alent circuit (Fig. 8c). In the equivalent circuits used, the double
layer capacitance Cdl and the capacitance Ccor are represented by a
constant phase element (Q), which accounts for the deviation from
Fig. 3. SEM micrographs of the surface of as-deposited Ni and Ni–B coatings and the coatings after immersion tests in 3.5% NaCl solution for 3 weeks.

Yu.N. Bekish et al. / Electrochimica Acta 55 (2010) 2223–2231
2227
the ideal dielectric behavior of the non-ideal capacitor. The capac-
itance values for the capacitive elements in the equivalent circuits
were calculated using the following equation:
n−1
C = Q(ωmax
)
,
(1)
where Q is the constant phase element, ωmax the frequency at which
the imaginary impedance reaches a maximum for the correspond-
ing time constant [29].
The calculated equivalent circuit parameters at different times
of sample immersion in chloride solution are presented in
Figs. 10 and 11. It should be noted that the behavior of nanocrys-
talline boron-free Ni coatings and Ni–B ones with a relatively low
B content (≤10 at.%) is different from that of the amorphous Ni–B
coatings with a high B content. At the beginning of immersion, the
nanocryslalline coatings demonstrate the Rct values in the range
from 50 to 100 kOhm cm2 (Fig. 10a, inset). The charge transfer resis-
tance increases rapidly (especially in the first hour) with immersion
time, reaches maximum values and then begins to decrease. For
the Ni–B4 coatings, maximum values of Rct range from 7100 to
7500 kOhm cm2 as compared with those (5800 to 6000 kOhm cm2)
for boron-free Ni coatings (Fig. 10a). When increasing the B con-
tent up to 10 at.%, the Rct max decreases several times. The double
layer capacitance of Ni and Ni–B4 coatings remains in a rather nar-
row interval (21–26 ␮F cm−2) and is close to the values usually
attributed to the double layer capacitance on metals (Fig. 11a).
For the Ni–B10 coatings, the Cdl initially varies insignificantly
(21–23 ␮F cm−2). Then after 170 h of immersion, the capacitance
starts to grow reaching limiting values of 31–33 ␮F cm−2 after 300 h
of immersion (Fig. 11a).
Fig. 4. Polarization curves of Ni and Ni–B coatings in 3.5% aqueous NaCl solution.
Scanning rate was 0.5 mV s−1
.
The initial rapid rise in the charge transfer resistance for
nanocrystalline Ni and Ni–B coatings can be related to the for-
mation and growth of a thin passive film on the metal surface.
SEM studies showed that the surface morphology of the nanocrys-
talline Ni and Ni–B coatings is not appreciably changed after the
immersion test, indicating that the formed passive film is very
thin (Fig. 3a–c). Although these coatings have a dense micro- or
nanocrystalline structure, corrosion-active species from electrolyte
can penetrate gradually to the copper substrate along the grain
boundaries or finest pores or pinholes and initiate the corrosion
process, resulting in the formation of corrosion pits on the surface.
As a consequence, the charge transfer resistance of the electrode
system starts to drop after several days of immersion. Inspection
of the sample surface after the immersion tests actually revealed
several corrosion pits on the Ni and Ni–B10 coatings.
Fig. 5. The current vs. time curves recorded during anodic oxidation of the Ni–B20
and Ni–B4 (insert) coatings in 3.5% NaCl solution at a fixed potential of −0.16
and −0.06 V, correspondingly. Initially the potential was swept from the corro-
sion potential to −0.16 V for Ni–B20 and −0.06 V for Ni–B4 at a scanning rate of
0.5 mV s−1
.
Other behavior is observed in the case of amorphous Ni–B
coatings with a high B content. On immersing into NaCl solu-
tion, these coatings demonstrate significantly lower Rct values
(15–25 kOhm cm2) in comparison with those for the nanocrys-
talline ones (Fig. 10b). Moreover, the charge transfer resistance
even decreases slightly during the first hour of immersion, indi-
cating that the active corrosion process occurs on the metal surface
(Fig. 10b, inset). Then the Rct begins to grow, reaches a maximum,
decreases and finally again grows after approximately 170 h of
immersion. It should be noted that the maximum values of Rct
for amorphous Ni–B coatings are significantly lower (fall in the
range 100–150 kOhm cm2) than those for nanocrystalline Ni and
Ni–B coatings. Some rise in the Rct after an hour may be asso-
ciated with the formation of a film of corrosion products on the
surface which is not so protective as in the case of nanocrys-
talline Ni and Ni–B coatings. Further decrease in the Rct seems to
be related to the appearance of minute local pits on the corroded
surface. Accumulation of the corrosion products impedes further
action of the electrolyte, leading to the Warburg diffusion behavior
and to gradual decrease in the corrosion rate. Actually Ni–B20 and
Ni–B26 samples after corrosion tests demonstrate the appearance
of colored film on the surface. SEM examination of the corroded
Fig. 6. SEM micrographs (at two different magnifications) of the surface of Ni–B20
electrode after keeping in 3.5% NaCl solution for 7 h at a fixed potential of −0.16 V.

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Ni–B20 and Ni–B26 coatings revealed the presence of a film of cor-
rosion products with a specific nanostructure (Fig. 3). The enhanced
Cdl values observed for these samples after prolonged immersion
are probably related to the high porosity of the corroded surface
(Fig. 11b).
Thus, the results of EIS studies are in good agreement with the
data of polarization measurements and testify that the corrosion
resistance of nanocrystalline Ni–B coatings is essentially higher
than that of amorphous Ni–B coatings with a high B content. This
enhanced corrosion resistance is due to the formation of a com-
pact protective oxide film on the metal surface. The Ni–B coatings
with a low boron content (4 at.%) demonstrate the maximum cor-
rosion resistance in chloride solutions which may be caused by a
sharp decrease in the grain size for these coatings in comparison
with boron-free Ni ones. This assumption is based on the results
of Wang et al. who revealed that the corrosion resistance of the
electrodeposited Ni coatings increased significantly as the grain
size decreased from microcrystalline to nanocrystalline [30]. They
explained this effect by the more rapid formation of a compact
passive film on nanocrystalline Ni owing to a higher density of
nucleation sites (grain boundaries and dislocations inside grains)
for the passive film.
Fig. 7. Bode and Nyquist plots of the Ni and Ni–B coatings after 24 and 168 h of immersion in 3.5% NaCl solution.

Yu.N. Bekish et al. / Electrochimica Acta 55 (2010) 2223–2231
2229
Fig. 10. Time evolution of the charge transfer resistance of (a) Ni and nanocrys-
talline Ni–B coatings and (b) amorphous Ni–B coatings during the immersion in
3.5% NaCl solution. Insets show time evolution of the charge transfer resistance at
the beginning of immersion.
3.5. Mechanical and electrical properties of the coatings
Fig. 8. Equivalent electrical circuit models used to analyze the EIS data for Ni and
Ni–B coatings.
Data on the influence of composition of Ni–B coatings on their
mechanical and electrical properties are presented in Fig. 12 and
Table 1. As can be seen from Fig. 12, the rapid transition to a linear
stage of steady-state wear is characteristic of the boron-free nickel
As the boron content increases up to 10 at.% in the Ni–B coatings,
an amorphous phase appears in them and their corrosion resistance
decreases. Probably in such heterogeneous coatings the corrosion
processes can be accelerated due to the formation of local minute
galvanic couples between crystalline and amorphous phases. In
addition, the increase in the boron content can lead to degrada-
tion of protective properties of the forming passive film due to the
increase in its defectiveness and loosening. This effect is most pro-
nounced in the case of amorphous Ni–B coatings. They have no
region of film passivity on the anodic polarization curves, and dur-
ing prolonged immersion tests, a relatively thick film of corrosion
products is formed on the coating surface, which does not fulfill
the function of a protective film to prevent the development of
corrosion processes.
Fig. 11. Time evolution of the double layer capacitance of (a) Ni and nanocrystalline
Ni–B coatings and (b) amorphous Ni–B coatings during the immersion in 3.5% NaCl
solution.
Fig. 9. Bode plot of the Ni–B10 coating after 168 h of immersion in 3.5% NaCl solu-
tion.

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the breaking strength of materials during frictional interaction or
cyclic loading [32], the formation of crystalline domains in amor-
phous coatings under friction can result in the deterioration of
wear-resisting properties of such materials. At the same time,
gain in stability of amorphous phases should be accompanied by
a rise in the wear resistance of the coatings. Thus, the materi-
als with a metastable amorphous-crystalline structure possessing
an enhanced tendency to the mechanical crystallization, such as
Ni–B10 coatings, should demonstrate low tribotechnical charac-
teristics.
The Ni–B coatings with nanocrystalline structure have a rather
low contact resistance (3–4.5 mOhm) only slightly higher than that
of boron-free Ni (Table 1). At the same time, the amorphous Ni–B
coatings demonstrate sufficiently high values of the contact resis-
tance (13–17 mOhm).
4. Conclusions
The following main conclusions can be drawn from this study:
(1) At the boron content ≤8 at.%, electrodeposited Ni–B coatings
have nanocrystalline structure and are comprised of a solid
solution of boron in f.c.c. Ni lattice having a mixed substituted-
interstitial type. The Ni–B coatings with a medium boron
content (approximately 10 at.%) demonstrate amorphous-
nanocrystalline structure, and the coatings with a high boron
content (20 at.% and above) are X-ray amorphous.
(2) The corrosion resistance of the nanocrystalline Ni–B coatings is
essentially higher than that of the amorphous ones. The Ni–B
coatings with a relatively low boron content (4 at.%) demon-
strate a maximal corrosion resistance which even exceeds the
corrosion resistance of boron-free Ni coatings.
(3) The increase of the boron content in the homogeneous
nanocrystalline Ni–B coatings leads to a significant increase in
their microhardness and wear resistance. For the coatings with
heterogeneous amorphous-nanocrystalline structure, these
mechanical characteristics are somewhat deteriorated. The fur-
ther amorphization of the Ni–B coatings at a high B content
again results in an improvement of the mechanical properties.
Fig. 12. The wear and friction coefficient vs. sliding distance for Ni and Ni–B coatings.
coating. The Ni coating wears out completely after a sliding distance
of 100 m. The wear rate for the Ni coating is 39.1 × 10−2 mg m−1
(Table 1), the friction coefficient ranges from 0.7 to 0.85 (Fig. 12) and
the microhardness is 2400 MPa (Table 1). Incorporation of boron
into the Ni coating leads to a significant increase in its microhard-
ness, wear resistance and friction coefficient (Fig. 12 and Table 1).
The lowest wear rate (0.2 × 10−2 mg m−1) and the highest micro-
hardness (8500 MPa) are observed for the nanocrystalline Ni–B8
coating. The appearance of amorphous phase in the Ni–B coatings
at a boron content of 10 at.% leads to the decrease in microhard-
ness and wear resistance. For the amorphous Ni–B coatings, the
microhardness and wear resistance again increase in comparison
with Ni–B10 coatings. Thus, the appreciable deterioration of the
mechanical properties is observed for the Ni–B coatings including
both nanocrystalline and amorphous phases. The following con-
sideration could explain this effect. It is known that along with
heating, the plastic deformation can initiate the crystallization of
amorphous materials. This effect was named as mechanical crys-
tallization [31]. The formation of crystalline domains in amorphous
coatings on friction can result in appearance of considerable tensile
stresses at the interface due to a substantial difference in spe-
cific atomic volumes of amorphous and crystalline phases. Since
it is known that interfacial deformations can significantly decrease
Thus, the electrodeposited nanocrystalline Ni–B coatings with
relatively low boron content demonstrate rather high corrosion
resistance, essentially enhanced microhardness and wear resis-
tance, rather low contact resistance and are promising for different
applications.
Acknowledgement
This work was partially supported by the Belarusian Republican
Foundation for Fundamental Research (Project no. X09 GKNT-006).
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