Possibility of raising the deposition rate of nickel coatings from a chloride electrolyte

Article abstract of DOI:10.1134/S1070427209020177

Conditions under which nickel coatings can be deposited from a chloride electrolyte at a high rate were studied. The mechanism of this process was suggested and experimentally confirmed. The mass-transfer rate of nickel-containing particles was determined

Full text of DOI:10.1134/S1070427209020177

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 2, pp. 255−261. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.I. Balakai, N.Yu. Kurnakova, A.V. Arzumanova, K.V. Balakai, I.V. Balakai, 2009, published in Zhurnal Prikladnoi Khimii,
2
009, Vol. 82, No. 2, pp. 262−267.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Possibility of Raising the Deposition Rate of Nickel Coatings
from a Chloride Electrolyte
V. I. Balakai, N. Yu. Kurnakova, A. V. Arzumanova, K. V. Balakai, and I. V. Balakai
South-Russian State Technical University, Novocherkassk, Rostov-on-Don oblast, Russia
Received February 28, 2008
Abstract—Conditions under which nickel coatings can be deposited from a chloride electrolyte at a high rate
were studied. The mechanism of this process was suggested and experimentally confirmed. The mass-transfer
rate of nickel-containing particles was determined by chronopotentiometry, with temperature raised from 20 to
5
0°C. The dependence of pH on the current density was determined. Physicochemical parameters of the coatings
s
(microhardness, internal stresses, porosity, luster, and adhesion) were measured.
DOI: 10.1134/S1070427209020177
As known [1−3] that it is possible to accelerate
high-performance electrolytes, it is necessary to lower
the pH in the solution bulk or choose more effective
buffering additives lest the pH value of the near-cathode
electrodeposi-tion of metal coatings from electrolytes in
which the discharge process involves, together with ions
of electrodeposited metals, also finely dispersed particles
of their compounds, capable of discharging to the metal
in the same range of potentials as the ions themselves. In
the course of nickel plating, a kinetically stable system of
finely dispersed particles may appear in the near-cathode
space and can intensify the electrodeposition process.
In the optimal case, this system can be likened to a
mobile system of pores, in which arise, in the course of
electrolysis, electrosurface phenomena that cause stirring
of the difficultly stirrable part of the diffusion layer.
This system should not be necessarily thermally stable.
It is sufficient that it should be kinetically stable during
electrolysis and possess certain optimal parameters that
enable the appearance of nonequilibrium electrosurface
phenomena. The finely dispersed particles generated in
the course of electrolysis may be composed of hydroxide
and basic salt species.
layer, pH , be shifted too strongly toward the onset pH
s
of hydrate formation for nickel, pH , thereby leading to
h
rapid coagulation and formation of coarsely dispersed
particles and their coagulation and, consequently, to their
hindered discharge.
Nickel chloride was chosen as the main component
for developing a high-performance electrolyte. Chloride
electrolytes offer wide opportunities for acceleration
of the nickel plating process, because the electrolyte
should not contain large amounts of multiply charged
ions, including sulfate ions, which exhibit coagulating
capacity with respect to sols. In addition, according to
[4], hydroxide species precipitated upon alkalization
of the solution are positively charged if the starting salt
is a chloride. If, however, the starting salt is a metal
sulfate, negatively charged hydroxide species are formed,
which is undesirable as regards the intensification of the
nickel-plating process. The stability of finely dispersed
hydroxide and basic salt particles in the near-cathode
layer in the presence of nickel chloride and their positive
charge can intensify the electrolysis process.
To be discharged at the cathode, the finely dispersed
particles should be positively charged. The charge can
be imparted to them via adsorption of nickel cations,
because it is known that inorganic ions contained in
the particle core are primarily adsorbed on dispersed
particles. As the cathode current density increases, the
alkalization of the near-cathode space should become
more pronounced because hydrogen evolution commonly
occurs together with nickel deposition at the cathode. In
A promising way to make progress in electroplating
is to devise and use high-performance electrolytes con-
taining finely dispersed compounds of a metal being
electrodeposited, which are reduced at the cathode [1, 2].
However, the mechanism by which the electrodeposition
2
55

256
BALAKAI et al.
from these electrolytes can be intensified remains
unclear.
should not exceed 1 min [5]. The pHs value was measured
with a microscopic glass electrode.
To accumulate new facts useful for elucidating an
implementable mechanism, fundamental aspects of
nickel electrodeposition from a chloride nickel-plating
electrolyte were studied.
The phase composition of the nickel coatings was
studied with a DRON-1.5 X-ray diffractometer (CuKα
radiation, nickel filter). The microhardness of the coatings
was measured with a PMT-3 instrument at indenter loads
of 5–50 g. The internal stresses (Is) were determined
by the flexible-cathode method; adhesion to the base,
by bending to an angle of 90° till fracture; porosity,
by application of a filter paper; and luster, by FB-1A
glossmeter.
EXPERIMENTAL
The study was carried out in a chloride nickel-
plating electrolyte of composition (M): nickel chloride
−3
Figure 1 shows how the product jτ1/2 depends on the
current density for the chloride nickel-plating electrolyte
at different temperatures. It can be seen that the product
hexahydrate 1.0, boric acid 0.4, 1,4-buyinediol 5 × 10 ;
pH 1.0 at different temperatures. The electrolyte was
prepared with distilled water from reagents of analytically
pure grade. Polarization dependences were obtained with
a PI-50-1-1 potentiostat by the potentiodynamic method at
1
/2
jτ is independent of the current density both at room
temperature and under heating.
−1
a potential sweep rate of 1 mV s . Chronopotentiograms
and potential-decay curves were measured with a PI-
Figure 2 shows the temperature dependences of the
product jτ1/2 in a chloride electrolyte and, for comparison,
5
0-1-1 potentiostat with a PR-8 programming unit,
in a sulfate electrolyte, both containing the same amounts
of nickel cations. For diffusion-controlled processes, the
mass-transfer rate is proportional to the squared product
synchronized with an S-8-12 storage oscilloscope. All
measurements were made in a YaSE-2 electrochemical
cell thermostated with a UTU-4 ultrathermostat. As
electrodes under study served nickel plates with working
areas of 3×3 (chronopotentiometric measurements) and
1/2
jτ . For the chloride nickel-plating electrolyte, the mass-
transfer rate increases by a factor of 11.1 on raising the
temperature from 20 to 50°C, and by a factor of 14.8, in
the range 50–60°C. The total increase in the mass-transfer
rate on raising the temperature from 20 to 60°C is by more
than a factor of 163.3.
1
0×10 mm (polarization measurements and potential-
decay curves). The inoperative sides of the electrodes
were insulated with an epoxide compound. The potential
of the electrodes under study was measured relative to
a saturated silver chloride reference electrode and was
recalculated to the hydrogen scale of potentials. The
chronopotentiograms were obtained at current densities
of 20–150 A dm−2 at different electrolyte temperatures.
To exclude the influence of the natural convection, the
current density was chosen so that the transient time
The sulfate nickel-plating electrolyte had the following
composition (M): nickel sulfate heptahydrate 1.0, sodium
−3
chloride 0.3, boric acid 0.4, 1,4-butynediol 5 × 10 ; pH
1.0. In this electrolyte, the mass-transfer rate increases
by a factor of 6.5 on raising the temperature from 20
to 50°C, and by a factor of 10.7, in the range 50–60°C.
The total increase in the mass-transfer rate on raising the
6
0
j, A dm−
2
t, °C
Fig. 1. Product jτ1/2 vs. the current density j at various tem-
peratures of the chloride electrolyte. t (°C): (1) 20, (2) 40, (3)
Fig. 2. Product jτ1/2 vs. the temperature t of (1) chloride and
(2) sulfate electrolytes.
5
0, and (4) 60.
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POSSIBILITY OF RAISING THE DEPOSITION RATE OF NICKEL COATINGS
257
temperature from 20 to 60°C is by approximately a factor
of 69.7. Sodium chloride was additionally introduced into
the sulfate nickel-plating electrolyte for making the ionic
strength of the solution the same as that of the chloride
electrolyte and to improve the dissolution of nickel
anodes. The increase in the mass-transfer rate on raising
the temperature from 20 to 60°C in the chloride electrolyte
is 2.3 times that in the sulfate electrolyte. Introduction of
The nondiffusion nature of the limiting cathode
current densities is also indicated by the high temperature
−1
coefficients, whose values are as follows (% deg ): in
the temperature range 20–60°C: 2.0 at pH 1.0, 2.1 at pH
3.0, 2.2 at pH 5.0; and in the temperature range 50–60°C:
4.8 at pH 1.0, 5.0 at pH 3.0, and 5.6 at pH 5.0 (Fig. 3).
Commonly, the temperature coefficient W=100Δj / j Δt
l
l
−
1
for the limiting diffusion current is 1.6–1.9 % deg
−
1
1
,4-butynediol into the chloride electrolyte in an amount
(for hydrogen ions, only 1.19 % deg ) [8]. Sure, the
temperature coefficients can be judged from results of
potentiometric measurements on solid electrodes only
approximately. However, their values are anomalously
high in the case in question.
of 5 × 10−3 M does not change the mass-transfer rate
both at room temperature and upon heating, because the
1
/2
dependence of the product jτ on the current density is
the same in both cases.
The experimental data obtained can make it possible to
determine fundamental aspects of nickel electrodeposition
under the conditions under consideration. For example,
calculation of the diffusion coefficient of nickel ions at
a temperature of 20°C by the Sand equation gives a value
The fact that the cathode current density is independent
of the square root of the rotation rate of the disk electrode
(in the range from 30 to 140 rps) and the limiting
working cathode current density is independent of the
potential sweep rate (at 30 to 80 mV s ) also points to
the nondiffusion nature of the limitations in the working
range of potentials.
−1
−9
2 −1
of 0.58 × 10 m s , which is in approximate agreement
with reference data (for ultimately diluted solutions, D =
−9
2 −1
0
.72 × 10 m s ) [7].
Comparing the working intervals of the cathode
−
9
current densities and the dependence of the pH value
on the cathode current density, one can notice that high-
quality deposits start to be formed at current densities at
Calculation for 60°C gives a value 77.58 × 10
s
2
−1
m s , which is unlikely because the true diffusion
coefficient has a more gradual temperature dependence
in this temperature range. Consequently, discharging
nickel compounds are not delivered to the cathode
surface via diffusion, with the product jτ1/2 increasing
more steeply at higher temperatures, especially above
which pH reaches pH . The latter value for a chloride
s
h
nickel-plating electrolyte of the same composition, but
without an additive, is approximately 4.1 at a temperature
of 20°C and 3.6 at 55°C [9].
5
0°C. Presumably, as temperature increases, hydrolysis
As can be seen in Fig. 4, pH increases with the
s
becomes more pronounced and a more finely dispersed
solid phase composed of basic nickel compounds
is formed. A considerable increase in the amount of
finely dispersed particles in the chloride nickel-plating
electrolyte in the temperature range 20–60°C was also
noted in [1].
cathode current density and tends to a certain limiting
value at both 20 and 60°C. In electrodeposition at pH 1.0
from the electrolyte under development, pH tends to 6.1
s
at 20°C, which coincides with the data of [10], and to 5.0
at 60°C. It follows from the dependences obtained that,
−2
even at j = 1 A dm , pH reaches a value at which basic
s
t, °C
jc, A dm−
2
Fig. 3. Limiting working cathode current density j vs. the tem-
l
perature t of the chloride electrolyte. pH: (1) 1.0, (2) 3.0, and
Fig. 4. pH vs. the cathode current density j at chloride elec-
s
c
(3) 5.0.
trolyte temperatures of (1) 20 and (2) 60°C.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 2 2009

258
BALAKAI et al.
nickel compounds are formed, which favors formation of
colloid particles of nickel compounds in the near-cathode
layer, and just at j >1 A dm high-quality deposits are
colloids and fine suspensions, formed near the cathode
surface, is a typical capillary system, i.e., a capillary-
porous formation with a solid, but not rigid skeleton
pierced by a system of pores filled by the electrolyte
solution. These pores have an arbitrary shape and
structure, which vary in the course of time because of the
continuous involvement of the pore-forming substance
−2
obtained at both 20 and 60°C.
The effect of the concentration and type of buffering
additives on the limiting current densities in the chloride
nickel-plating electrolyte was studied in [11–13]. It was
shown that the limiting current densities increase with
the concentration of buffering additives. For example,
the limiting current density grows as follows: as the
(nickel hydroxide in the given case) in the electrochemical
processes. Various electrosurface phenomena arise in such
a layer under the action of the electric field of the cathode
(e.g., electrophoresis, electroosmosis, dipolophoresis,
diffusiophoresis), and just these phenomena are the reason
why the deposition of metals and alloys is accelerated.
−1
concentration of an acid is raised from 10 to 60 g l , by
a factor of 1.6 for aminoacetic acid, 1.9 for sulfosalicylic
acid, and 2.2 for sulfamic acid; by a factor of 1.7 as the
concentration of boric acid is raised from 20 to 40 g
For electrolytes in which nickel ions are discharged
simultaneously with the reduction of finely dispersed
nickel compounds, the following behavior is observed.
While the discharge potential is not reached and condi-
tions for discharge of nickel-containing finely dispersed
particles at the cathode are not created, these compounds
can be incorporated into the coating without being
discharged. When, however, the potential shifts to a more
electronegative region, their more complete discharge
begins. Electrons from the cathode and(or) evolving
hydrogen can serve as reducing agents. Such a mechanism
involving finely dispersed compounds of the metal being
electrodeposited can strongly affect the properties of the
coatings obtained.
−1
l , by a factor of 1.5 for ammonium chloride, etc. An
increase in the concentration of buffering additives leads
to a decrease in pH [14].
s
It is known [15, 16] that, in electrodeposition of
nickel, the near-cathode layer is alkalinized and there
appear conditions for formation of finely dispersed
nickel compounds, whose presence is confirmed by
electron microscopy. Nickel hydroxide is present in the
coatings, and the sensitive method of radioactive isotopes
quantitatively demonstrated that galvanic nickel deposits
contain hydroxide compounds [17, 18]. The involvement
of finely dispersed nickel compounds in the cathode
process is also indicated by the fact that the limiting
current density in the potentiodynamic characteristics of
the cathode process becomes two times lower in the case
of electrolysis with a cathode coated with an agar-agar gel,
which is a filter for finely dispersed nickel compounds.
Important parameters of nickel coatings are luster,
microhardness, internal stresses, adhesion, corrosion
resistance, uniformity of distribution over the surface
of intricately shaped articles, and porosity. In view of
the specific mechanism of reduction of finely dispersed
nickel compounds at the cathode, compared with nickel
discharge from ions, it would be expected that unreduced
molecules of nickel compounds contained in a dispersed
particles will be incorporated into a nickel coating,
especially at low current densities. This can change the
coating properties.
It is believed that the increase in the limiting diffusion
current in electrolyte solutions with temperature is
a common feature. In chloride electrolytes, the limiting
working current density is an order of magnitude higher
in this case, which points to a specific mechanism of
reduction of nickel ions in these solutions [1, 2].
The fact that the output capacity of nickel plating is
at a maximum at pH 1.0 and temperature of 60°C can
be understood as follows. In this case, a reducible finely
dispersed system of nickel compounds appears in the near-
cathode layer. This system has the optimal parameters for
appearance of equilibrium and nonequilibrium electro-
surface phenomena in the difficultly stirrable part of the
diffusion layer, which include the optimal dispersity,
composition, and homogeneity of the system, its certain
stability, interparticle distances in the system, etc.
According to the data of [19, 20], the layer of stabilized
As the cathode current density increases from 5 to
1
0 A dm−2 at the electrolyte temperature of 60°C, the
microhardness of the coatings first increases and then
starts to decreases and does so up to a cathode current
−
2
density of 10 A dm (Fig. 5). The same run of the
dependence is observed at 20°C.
For the conventional electrolytes, in which discharge
of a metal occurs from its ions, such a dependence
is unusual. However, for the electrolyte whose high
performance is accounted for by a specific mechanism
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POSSIBILITY OF RAISING THE DEPOSITION RATE OF NICKEL COATINGS
259
−
2
jc, A dm
jc, A dm−2
Fig. 5. Microhardness H of nickel coatings vs. the cathode
Fig. 7. Porosity N of nickel coatings vs. the cathode current
current density j . Chloride electrolyte; the same for Figs. 6–8.
density j . Deposition temperature (°C): (1, 2) 20 and (3, 4) 60.
c
c
Deposition temperature (°C): (1) 20 and (2) 60; the same for
Coating thickness (μm): (1, 3) 0.5 and (2, 4) 1.0.
Fig. 6.
L
jc, A dm−
2
jc, A dm−2
Fig. 6. Internal stresses Is of nickel coatings vs. the cathode
Fig. 8. Luster L of nickel coatings vs. the cathode current den-
current density j . Coating thickness 12 μm.
sity j . Deposition temperature (°C): (1) 20, (2) 40.
c
c
of reduction of finely dispersed compounds of the
electrodeposited metal, together with its ions, such
a dependence of the coating microhardness on the current
density indirectly confirms the specificity of the cathode
process. If the reduction potential of finely dispersed
particles is not reached, or if it is already reached, but the
reduction is incomplete because of the polydispersity of
the particles, then the amount of inclusions in the coating
increases. This changes the structure-sensitive properties.
If the potential of complete reduction of finely dispersed
nickel compounds is reached and these compounds are
monodisperse, the amount of inclusions of unreduced
particles in a coating becomes smaller and, therefore, the
microhardness decreases.
in electrolytes containing finely dispersed compounds of
a metal being electrodeposited. A similar dependence is
observed for the porosity (Fig. 7).
The dependence of the luster on the cathode current
density is shown in Fig. 8.
The maximum luster of the nickel coatings is
observed at approximately the same current densities
as the maximum microhardness and Is. This points to
pronounced changes in the electrolyte layer adjacent to
the cathode in these electrolysis modes.
According to the two-factor theory of luster formation
[
21], two conditions should be satisfied for obtaining
lustrous coatings: (1) finely dispersed particles of the type
MA] (where M is a metal ion, and A, anion) that can be
[
The dependence of the Is of nickel coatings on the
current density and electrolyte temperature is shown in
Fig. 6. The measurements were made at the instant when
the electrolysis was terminated. Tensile stresses were
observed. The type of the dependence of such a structure-
sensitive parameter as Is confirms what was stated above
about changes in the microhardness of coatings formed
reduced by hydrogen or electrons from the cathode should
be formed in the near-cathode layer and (2) forces should
be created that align the finely dispersed particles being
reduced with the shape of the liquid 1–liquid 2 phase
boundary. Here, liquid 1 is the electrolyte in its bulk,
and liquid 2, that near the cathode, where the viscosity is
increased because of the presence of dispersed particles.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 2 2009

260
BALAKAI et al.
It is the surface tension forces at this boundary that create
a liquid-like surface of lustrous coatings.
(5) As the cathode current density increases, the
microhardness, internal stresses, porosity, and luster pass
through a maximum.
As the working current density increases, the pH
value in the near-cathode layers increases because of
the simultaneous, with nickel deposition, hydrogen
evolution. At a pH exceeding a certain value (depending
on temperature and composition and concentration of
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(
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1.4 as temperature increases from 50 to 60°C.
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2
(
−1
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