Pulse reverse plating of Ni-Co alloys: Deposition kinetics of Watts, sulfamate and chloride electrolytes

Article abstract of DOI:10.1016/j.electacta.2006.07.012

Fundamental electrochemical properties of electrolyte systems are the prerequisite for the development of a successful pulse deposition process. Three different electrolyte systems for the galvanic deposition of nickel cobalt alloys (chloride, Watts and sulfamate type) were investigated in order to reveal underlying deposition mechanisms and rate determining factors. The electrochemical experiments were supported by X-ray fluorescence analyses of the alloy composition in dependence on the current density and the type of bath. A special focus was set on the investigation of the passive (oxide) layer formation by the anodic pulses. The choice of electrolyte system strongly influences the reaction mechanism and thus the alloy deposition. Also the cobalt content within the deposited layer varied strongly in dependence on the electrolyte system used. While sulfamate and Watts baths show an ability for passive layer formation, the chloride bath exhibits a lower proneness to passivation, accompanied by pit formation.

Full text of DOI:10.1016/j.electacta.2006.07.012

Electrochimica Acta 52 (2006) 1145–1151
Pulse reverse plating of Ni–Co alloys: Deposition kinetics of
Watts, sulfamate and chloride electrolytes
Wolfgang E.G. Hansala,c,∗,1, Barbara Turyb, Martina Halmdiensta,c,
Magda Lakatos Varsanyib,1, Wolfgang Kautekc,1
´
a Happy Plating GmbH, Leobersdorfer Strasse 42, A-2560 Berndorf, Austria
b Bay Zoltan Institute for Materials Science and Technology, Fehervari ut 130, H-1116 Budapest, Hungary
c Department of Physical Chemistry, University of Vienna, Wa¨hringer Strasse 42, A-1090 Vienna, Austria
Received 14 March 2006; received in revised form 17 June 2006; accepted 10 July 2006
Available online 23 August 2006
Abstract
Fundamental electrochemical properties of electrolyte systems are the prerequisite for the development of a successful pulse deposition process.
Three different electrolyte systems for the galvanic deposition of nickel cobalt alloys (chloride, Watts and sulfamate type) were investigated in order
to reveal underlying deposition mechanisms and rate determining factors. The electrochemical experiments were supported by X-ray fluorescence
analyses of the alloy composition in dependence on the current density and the type of bath. A special focus was set on the investigation of the
passive (oxide) layer formation by the anodic pulses.
The choice of electrolyte system strongly influences the reaction mechanism and thus the alloy deposition. Also the cobalt content within the
deposited layer varied strongly in dependence on the electrolyte system used. While sulfamate and Watts baths show an ability for passive layer
formation, the chloride bath exhibits a lower proneness to passivation, accompanied by pit formation
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Nickel–cobalt; Passivation; Polarisation; Pulse reverse deposition; XRF
1. Introduction
Nevertheless, the highest hardness of that alloy is obtained at
a composition of 30 at.% Co and 70 at.% Ni [8]. At the same
composition the pulse or pulse reverse plated alloy shows higher
hardness than that obtained from direct current deposition [9].
Typical plating baths that are used for the electrodeposition of
such alloys are sulphate, sulfamate, Watts and chloride type
baths [10].
Several available references deal with the anomalous depo-
sition of Ni–Co [11–13,22]. In the case of Ni–Co alloy depo-
sition, the adsorption ability of Co(OH)+ is higher than that of
Ni(OH)+. Therefore, the Ni–Co alloy gets rich in Co resulting
in an anomalous deposition. One possible explanation for this
phenomenon is the formation of hydrogen during the electroly-
sis, as it enhances the formation of metal hydroxyl in the coating
[14]. The tendency for OH− inclusion in the coating is the high-
est in chloride solutions and is lower in sulphate and sulfamate
solutions.
Nickel-based alloys are used in a wide variety of applications
for aerospace, energy generation and corrosion protection, espe-
cially in an environment where materials have to withstand high
temperatures and oxidizing conditions [1,2]. Among these mate-
rials, Ni–Co alloys have been gaining popularity because they
exhibit other functional properties than corrosion resistance; for
example magnetic materials and electro catalysis in hydrometal-
lurgy [3–7]. Due to their extraordinary hardness combined with a
high corrosion resistance, Ni–Co alloys are under consideration
as a potential system for hard chromium replacement.
Ni–Co alloys can be easily produced by electrodeposition
from aqueous media in a broad variety of alloy composition.
∗
Corresponding author. Tel.: +43 2672 819 75 951;
In some experiments a rotating cylinder Hull cell was used
to investigate the Ni, Co and Ni–Co deposition kinetics in a
sulphate solution [15]. When both Ni and Co are kinetically
fax: +43 2672 819 75 915.
E-mail address: wh@happyplating.at (W.E.G. Hansal).
ISE-Member.
1
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.07.012

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W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
Table 1
controlled, then the less noble Co metal inhibits the deposition
of the more noble Ni. When Co reaches its limiting current this
inhibition effect disappears.
Composition of the different bath, used for the deposition
Bath type
Chemicals
Composition (M)
Additives are usually used for improving the properties both
of the bath (e.g. throwing power, current efficiency) and of the
properties of the deposited layer (e.g. hardness, appearance)
[16–19]. The benefits of the addition of organic additives to the
plating electrolytes are multiple. Nevertheless the possibilities
by just altering the bath chemistry are limited.
A modern electrochemical method that improves the qual-
ity of electrochemically deposited metal layers is the pulse
reverse technique [10], in which anodic pulses are applied after
a sequence of cathodic current pulses interrupted by phases of
zero current, the so-called off-times. During these anodic pulses
partial dissolution of the previously deposited layer takes place
theoretically. However, the deposited alloy can be passivated
depending on the bath type and the electrochemical properties
of the deposited layer [3,20]. Accordingly, deposited alloy coat-
ings do not act as homogenous layers anymore, but as multilayer
structures consisting of metal alloy and metal oxide films with
varying physical and chemical properties.
Sulfamate bath
Ni-sulfamate
Ni-bromide
1.65
0.18
0.06
0.49
0.01
CoCl2·6H2O
H3BO3
C10H5(NaSO3)3·H2O
NiCl2·6H2O
CoCl2·6H2O
NH4Cl
H3BO3
C10H5(NaSO3)3·H2O
NiSO4·6H2O
NiCl2·6H2O
CoCl2·6H2O
H3BO3
Chloride bath
Watts type bath
Co bath
0.6
0.06
0.7
0.1
0.01
0.170
1.250
0.06
0.650
0.01
C10H5(NaSO3)3·H2O
CoCl2·6H2O
0.06
0.650
0.01
H3BO3
C10H5(NaSO3)3·H2O
Beside the increased uniformity and homogeneity of the
deposits using pulse reverse plating, pulse deposition is also
reported to decrease the level of internal stress within the deposit
[8]. Especiallyfortechnicalapplicationsthatrequirehigherlayer
thicknesses or even need to build up structures (electroforming)
the level of internal stress can be a relevant or critical factor.
The aim of this present work is to compare the deposition
kinetic properties of nickel cobalt alloys and its components in
three commonly used plating baths, i.e. sulfamate, Watts and
chloride. The electrochemical determination of these critical
values were performed by applying rotating disc voltamme-
try in order to get information about the film formation at
lower and higher overpotentials. The appearance of passiva-
tion during anodic polarisation controls the anodic dissolution
during any (anodic) reverse pulse. Therefore, the anodic polar-
isation was investigated by cyclic voltammetry and transient
measurements.
2.2. Rotating disc voltammetry
Linear sweep voltammetry measurements were carried out
in order to examine the electrodeposition kinetics of Ni, Co
and Ni–Co by means of a potentiostat (Jaissle PGU-20V-2A)
connected with a rotating disc electrode. The current/potential
plots were recorded at a scan rate of 2 mV s−1 at different
rotation speeds (100–3000 rpm) on steel rotating disc elec-
trodes with a surface area of 1 cm2. The potential was swept
500 mV relative to the open circuit potential in the cathodic
direction.
2.3. Electrodeposition
For the deposition experiments, steel discs, with an area of
1 cm2 were used as substrates. Before each experiment, the sub-
strate surfaces were polished with emery paper and washed with
isopropyl alcohol. All deposits were produced in a thickness of
10 0.5 ␮m (the thickness of the deposits was measured using
X-ray fluorescence) by pulse current plating using a computer
controlled pulse reverse power supply system (PE86 Plating
2. Experimental
2.1. Materials
Electronic)applyingpeakcurrentdensitiesofupto0.05 A cm−2
Both the on-time (ton) and the off-time (toff) were kept constant
at a value of 5 ms. A Pt–Ti basket, filled with depolarized nickel
pellets acted as a counter electrode.
.
The electrodeposition of the Ni–Co alloys was carried out
from three different types of baths using analytical reagent grade
chemicals (Table 1). The temperature of these baths was kept
constant at 40 ◦C for all experiments. Since the anomalous depo-
sition of Ni–Co is well known, the quantity of Co was kept low
in order to deposit cobalt under mass transfer control. The pH-
values of the three electrolyte systems ranged between 3 and 4
(sulfamate type 3.9; chloride 3.8; Watts type 3.6; all measured
at 22 ◦C). For the examination of the deposition kinetics of the
single alloy elements, pure Ni electrolytes prepared from each of
the plating baths, and a pure Co solution prepared from chloride
salts were used. A mercury sulphate electrode (0.5 M sulphuric
acid) was used as reference. All experiments were repeated three
times to verify the reproducibility.
2.4. Alloy composition
The alloy composition of the deposited nickel–cobalt
layers was derived by X-ray fluorescence measurements
¨
(Rontgenanalytik Maxxi 5 PIN). A semiconductor detector
instead of a classical Geiger counter was used for sufficient res-
olution of the nickel and cobalt peaks. The dependency of the
alloy composition on the pulse current density was determined
for all three plating electrolytes.

W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
1147
2.5. Cyclic voltammetry and chrono-amperometry
lations between the rotating speed and the limiting current. Up
to the potential value of −1000 mV the limiting current value
of Co is comparatively high even though the Co concentration
was at least ten times less than the concentration of Ni in each of
the plating baths. This clearly shows the anomalous behaviour.
As the intersection of the plot for Co is zero, it shows diffusion
control.
Intersections of Ni plots are above zero value. However, the
differences are not significant and it is possible that both kinetic
and mass transport control the electrode reactions. On the other
hand, it was stated [15] that nickel is only kinetically controlled
during the deposition, whereas cobalt is mass-controlled during
theNi–Coalloydeposition. Accordingtotheresultsofthiswork,
the probability for a kinetic control is higher in the chloride bath
and decreases in Watts bath and sulfamate bath, respectively.
Hydrogen evolution, as a side reaction, starts from the poten-
tial value of −1050 mV in Watts bath (Fig. 1A). Its reduction
potential shifts to more negative potentials in the case of chlo-
ride and sulfamate bath. The highest reduction overpotential of
hydrogen was found in the case of Co.
The electrochemical polarisation and passivity of the plated
surfaces in the respective plating electrolytes was investigated in
a three-electrode potentiostatic cell arrangement (Jaissle PGU-
20V-2A). A Pt–Ti basket, containing nickel pellets, was used as
a counter electrode. In all of these experiments, the potential was
swept at a rate of 10 mV s−1. Potential step measurements were
carried out in the native plating solutions in order to examine
the kinetics of the passive layer formation relevant for reverse
pulse deposition. The potential was fixed at −700 mV for 120 s
at the beginning of each experiment, followed by potential steps
to potential values in the passive region that had been selected
from previous studies on cyclic voltammetry measurements.
3. Results and discussion
3.1. Rotating disc voltammetry
For many applications, a certain composition of the alloy is
required for reaching the relevant physical, chemical or mechan-
ical phenomena. In the case of electrochemical deposition, the
composition of the alloy clearly depends on the deposition kinet-
ics of the alloy elements. If the Tafel slopes of electrode reactions
ofthetwo componentsareparallel, the(pulse) deposition param-
eters have no influence on the alloy composition [3]. However, if
their slopes are not parallel, i.e. the electrode kinetics differ, the
(pulse) deposition parameters have a strong effect on the alloy
composition. Therefore, polarisation curves are of special inter-
est reflecting the nature of the deposition process and providing
information on the deposition mechanism.
The relation between the current and the potential is essential,
whenthekineticsoftheelectrochemicalreactionisstudied. Gen-
erally, two types of deposition mechanisms exist. If the element
follows a kinetically controlled deposition then the Tafel equa-
tion describes the electrode process. If the deposition process is
under mass transport control then the current is determined by
the Levich equation.
Polarisation graphs recorded on pure Ni, Co and Ni–Co alloy
surfaces in different plating baths are illustrated in Fig. 1. In
each bath, up to the potential value of −1050 mV, Co has sig-
nificantly higher limiting current and more positive deposition
potentials than that of Ni and Ni–Co. At lower potential values,
Ni–Co cathodic polarisation plots are overlapped by that of Ni,
which leads to the assumption that the kinetics of the Ni–Co
electrodeposition is probably determined by the Ni deposition
kinetic. This phenomenon, however, changes, at higher potential
values in the chloride bath, when the cathodic Ni–Co polarisa-
tion curve is overlapped by the Co deposition. This might be due
to the stronger hydrogen evolution which increases the local pH.
That might result in different deposition kinetics [14].
The comparison of the cathodic polarisation graphs depend-
ing on the plating bath type is shown in Fig. 2. Similarly to the
polarisation plots for the pure Ni deposition in Fig. 1B(a), the
limiting current is the smallest for the sulfamate bath and the
highest for the chloride bath. The values of the reduction poten-
tial of hydrogen are the lowest for the Watts bath and increases
for the chloride and the sulfamate bath.
Since the deposition potentials of nickel and cobalt are very
similar the alloy deposition takes place from uncomplexed ions
[21]. Taking the pH-values of the three baths into consideration
the shift of the hydrogen reduction potential clearly follows the
pH values. The Watts electrolyte with the lowest pH-value and
thus the highest H+ concentration shows the lowest hydrogen
reduction potential. The bath with the highest pH-value and thus
the lowest H+ concentration exhibits the hydrogen reduction at
the highest potential of all three investigated electrolytes.
From the measured potential shift in the different baths, the
formation of the most stress free coating will be most likely
be obtained from the sulfamate bath since the lowest hydrogen
evolution during the deposition can be expected. The exact deter-
mination of the level of internal stress within the deposits will
require a closer investigation.
The limiting current depends on the thickness of the diffusion
layer. Thus, it is necessary to apply well-controlled convection
conditions by using a rotating disc electrode (RDE). In this case,
the ratio between the square-root of the rotation speed and the
limiting current is linear:
Il = 0.62nFD2/3C0∞ν1/6ω1/2
(1)
whereν isthekinematicsviscosity, Il thelimitingcurrentdensity,
D the diffusion coefficient, C0∞ the bulk concentration and ω is
the angular velocity. The limiting current density is measured
and its inverse value is plotted against the square root of the
rotation rate, producing a Levich plot. Overall, based on the
Levich equation, it can be stated that if the intersection of an
I–ω1/s plot is zero, then the electrode reaction is mass transport
controlled. In any other cases, the electrode reaction is under
mixed or kinetic control.
Fig. 1 compares the polarisation curves of individual nickel
and cobalt depositions from different electrolytes and the corre-
All three baths clearly show a limiting current region. The
two-step reduction mechanism of the reactive species Ni2+ is

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W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
Fig. 1. (A) Polarisation graphs of (a) Co in chloride, Ni in different electrolytes. Ni, Co and Ni–Co in (b) Watts bath, (c) chloride bath and (d) sulfamate bath. (B)
Levich plots of (a) Ni, Co and Ni–Co in Watts bath, (b) Co in chloride, Ni in different electrolytes, (c) Ni, Co and Ni-Co in chloride bath and (d) Ni, Co and Ni-Co
in sulfamate bath at the rotation speed of 100 rpm.

W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
1149
Fig. 2. (A) Levich plots of Ni–Co alloys deposited from chloride, Watts and sulfamate bath. (B) Polarisation graphs of Ni–Co alloys deposited from chloride, Watts
and sulfamate bath at the rotation speed of 100 rpm.
clearly visible by a shoulder in the polarisation plot for the chlo-
ride bath. This shoulder is also visible for the sulfamate bath
but less distinct while it was not found for the Watts electrolyte.
This result refers to a very fast reduction of the Ni+ ions to Ni0
at the electrode surface in case of the Watts bath.
current density was measured. From this behaviour an activation
controlled nickel deposition can be concluded for the three types
of electrolytes investigated.
The ratio between the nickel and the cobalt ions in the original
solutions has to be considered for the comparison of the absolute
values of the cobalt deposited in the coatings. This ratio is 10%
of cobalt in the chloride bath, about 4.2% in the Watts bath and
about 3.3% in the sulfamate bath. Thus, comparing the three
electrolytes, the cobalt content in the deposits clearly follows the
cobalt content in the electrolyte used. For all three systems the
cobalt content in the deposit is much higher in the deposit than
in the electrolyte used, also following the anomalous deposition.
The enrichment of the cobalt content in the deposit will lead to
a depletion of cobalt ions in the electrolyte if the replenishment
is not performed accordingly. The determination of this effect
will be subject of further studies (Fig. 3).
3.2. Alloy composition in dependence on the
electrochemical parameters
According to the rotating disc electrode measurements
(Fig. 1A(c) and B(c)), in the chloride bath at higher potential
values, the alloy deposition is closer to Co that may result in a
higher Co content in the alloy [10] compared to the other elec-
trolytes. In the case of higher Co content, voltammetric peaks
show higher anodic currents that result in a higher dissolution
rate of the metal. Also a shift in the anodic peak position shows
a Co-like behaviour, leading to a higher dissolution of the com-
ponents rather than to the passivation of the alloy.
3.3. Cyclic voltammetry and chronoamperometry
These derivations were proven by conducting alloy composi-
tion measurements using X-ray fluorescence. Nickel cobalt alloy
layers were deposited from all three electrolytes using different
pulse current densities. As stated, based on the electrochemi-
cal measurements, the cobalt content varied in dependence on
the electrolyte used. For all current densities in the investigated
region the highest cobalt content was found for the chloride type
electrolyte system. Values of the cobalt content up to 34 at.% in
the layer at lower current densities decreased at higher current
densities down to about 23 at.% for the chloride electrolyte sys-
tem. The cobalt content in the layers deposited from the Watts
bath ranged from 26 at.% in the low current region to 15 at.%
in the high current region. The sulfamate electrolyte led to the
lowest cobalt content in the deposits. Just 15 at.% in the low
current region and about 8 at.% in the high current region were
measured. These results are in a remarkable accordance to the
results of the electrochemical measurements.
Under certain conditions a metal alloy coating builds up a thin
oxide overlayer that is usually energetically more stable than
the coating itself [1,2]. However, in the case of pulse reverse
Since the nickel–cobalt deposition is anomalous an increase
of the nickel content in the deposited layer with the increase of
the applied current density is expected. For all three electrolytes
systems a linearly decrease of the cobalt content with increasing
Fig. 3. Composition of the plated Ni–Co-alloys in dependence of the current
density for the three investigated Ni–Co-alloy electrolyte systems.

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W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
lution of the alloy started at the lowest (least anodic) potential.
The dissolution peak can be found at a slightly more positive
potential for the chloride bath while the dissolution of the alloy
is shifted clearly to the anodic region in case of the sulfamate
bath. These results give a dependency of the dissolution poten-
tial on the amounts of halogenides ions within the solution. The
bath with the highest chloride content, the Watts bath, does not
show a passivation effect and allows dissolution of the alloy at a
relatively low potential. This is also valid for the chloride elec-
trolyte. The sulfamate system, not containing chlorides and just
a small amount of bromides, leads to a passivation effect and
thus the most anodic dissolution potential.
Fig. 5 represents the logarithmic form of the current–time
transients recorded on Ni–Co samples in their parent solution.
Based on the results of the cyclic voltammograms, the potential
was swept from −700 to −430 mV in sulfamate and Watts bath.
In the chloride bath the potential was swept to −160 mV. The
highest dissolution tendency is observed in the chloride bath.
Passive layer formation starts almost immediately in the case of
the sulfamate system, but it needs more time to develop at the
Watts and chloride systems. A decrease in the current shows the
formation of a passive layer on the Ni–Co surfaces in each bath.
However, this passive layer is not stable, and the current starts
to increase again, representing the dissolution of the alloy.
Comparing the results of the cyclic voltammetry with the
potential step measurements in all three types of electrolytes
investigated, it was found that those alloys that were deposited
from the chloride containing electrolyte have lower proneness
to passivation [12] together with the possibility for pit forma-
tion. The tendency for pit formation decreases and the ability
for passive layer formation increases by decreasing the Cl−
Fig. 4. Plots of cyclic voltammetry recorded on Ni–Co layers in the solution
used for their deposition. Sweep rate is 10 mV s−1
.
plating, this phenomenon is undesired as oxide layers hinder the
dissolution during the anodic phase and transform the general
(electrochemical) behaviour of the deposited alloy towards that
of a multilayer. Therefore, it is important to clarify the conditions
of passivity in different plating electrolytes.
A comparison of the voltammograms for the Ni–Co alloys
deposited in the three different plating electrolytes is presented
in Fig. 4. The voltammograms exhibit a similar shape: there are
no separated Ni and Co peaks and the wide peaks indicate the
dissolution of Ni–Co as an alloy. However, there is a consid-
erable difference between the anodic peak positions: −520 mV
in the Watts bath, −460 mV in the sulfamate and −300 mV in
the chloride bath. According to these measurements the Watts
bath exhibited the lowest proneness for passivation. The disso-
Fig. 5. Potential step measurements on Ni–Co layer in the solution used for their deposition. (A and B) From −700 to −430 mV; (C) from −700 to −160 mV.

W.E.G. Hansal et al. / Electrochimica Acta 52 (2006) 1145–1151
1151
content as is the case in the bath type series—chloride–Watts–
sulfamate.
the sulfamate bath while both the Watts and the chloride bath
will show an effective anodic dissolution.
Acknowledgements
4. Conclusion
This work has been carried out by the support of R&T Inno-
vation Found as well as the Austrian Exchange Service (OAD)
Academic Cooperation and Mobility Unit, in the frame of the
HungarianandAustrianBilateralIntergovernmentalS&TCoop-
eration.
This work revealed the importance of the fundamental elec-
trochemical properties of electrolyte systems for any devel-
opment of a proper pulse deposition process. The choice of
electrolyte system will not only determine the deposition mech-
anism and thus the properties of the deposited alloy but also the
efficiency of any anodic current pulses during the anodic phase
of a reverse pulse deposition.
¨
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Comparing the electrolytes investigated in this work, the
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The kinetic control is not dominant for the nickel deposition in
case of the Watts and sulfamate baths.
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