Journal
of Chemical
Technology
Metallurgy,
48,2013,
3, 2013
Journal
of Chemical
Technology
and and
Metallurgy,
48, 3,
308-315
STATIONARY AND PULSE ELECTRODEPOSITION OF CoNi AND CoNiCu COATINGS
K. Ignatova1, Y. Marcheva2
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
2
Technical University – Sofia, Sofia 1000, Bulgaria
1
Received 23 November 2012
Accepted 15 May 2013
ABSTRACT
The kinetics of stationary and pulse potentiostatic electrodeposition of Co, Ni and Cu as well as the possibility for
their co-deposition in alloys CoNi and CoNiCu from slightly acidic citrate electrolyte was investigated. The morphologies
and the elemental composition of coatings were studied using SEM and EDSA analysis respectively. It was established that
at pulse deposition of CoNi the coatings were with smooth surface, crystalline structure with rounded crystals and average
size about 200-300 nm at frequency 500 Hz. In the case of triple alloy CoNiCu deposition, with the increase of frequency
up to 1 000 Hz a finer nanosized structure was formed, with the Ni content up to 27 %. The Co content in the triple alloy
was not influenced substantially by the pulse frequencies and was about 71-76 %, while copper content decreased from
8 % to less than 5 %. The X-Ray analysis indicated that copper, cobalt, and nickel crystallize in cubic lattice (fcc) in all
studied alloy coatings. Besides the cubic phases of the three metals, the presence of cobalt-containing phase with hexagonal
crystal lattice (h.p.c.) was ascertained.
Keywords: alloy electrodeposition, pulse potential modes, voltammetry, morphology, elemental and phase composition of coathings.
INTRODUCTION
In recent years an increasing interest exists towards
electrodeposition of Co and CoNi-alloyed [1-7] and
multi-layer coatings [6-8]. The particular interest towards nano-sized Co alloys is due to their increasing
application in magnetosensor technologies and magnetoelectronics where miniaturization of items is the
underlying purpose [9-14]. Due to their high hardness,
wear resistance, endurance and corrosion resistance,
the cobalt alloys are widely used in medicine, nuclearpower systems, chemical- and oil industry [12, 15, 16].
The hardness of nano-sized coatings increases with the
reciprocal of the square root of crystals grain size according to the Hall-Petch dependency [17].
The СоNi coatings are deposited mainly from sulphate [18] and citrate [1,6,7,19] electrolytes. However,
the literature does not provide detailed data about the
kinetics and the deposition conditions of these alloys.
The citrate electrolyte is used in recent years because of
its ability to serve as a buffer, to form complexes, and to
308
add coating luster, thus avoiding the need of introduction
of special organic additives [7]. The difficulties in using
this electrolyte come from its stability. It was found [21],
that the stability of citrate electrolyte for deposition of
CuNi coating can be controlled by modifying the pH.
It decreases upon reaching рН levels bellow 4, which
corresponds to complexes CuHCit, Cu2Cit2-2 and NiНCit.
The stable electrolyte corresponds to рН = 5-6.
The phase content of CoNi changes depending on
the deposition conditions [2, 3, 25]. The alloys, deposited at more positive potentials contain ε-phase with
hexagonal close-packed (hcp) lattice and α-phase with
cubic lattice (fcс), the proportion between phases remains constant [25]. The CoNi alloys, deposited at more
negative potentials contain pure cobalt and mixture of
α- and ε-phases. Anomalous galvanostatic deposition in
glucinate bath of CoNi [21] and potentiostatic deposition
of CoNi and CoNiCu in citrate bath [25] were performed.
It was established formation of CoNi solid solution with
hexagonal close packed lattice [21] and solid solution of
NiCoCu with face centered cubic lattice [25].
K. Ignatova, Y. Marcheva
It was established [7] that alloying the CoCu system
with low amounts of Ni, can improve the properties of
the thin films. The low nickel percentages in CoCu coatings can favor the segregations of small ferromagnetic
particles and increase of the magnetoresistance; decrease
the stress in the copper/ferromagnetic interface and
improvement in the corrosion resistance of the deposits [7]. The possibility of preparing CoNiCu granular
magnetoresistive coatings is proposed as alternative to
Cu-Co-Ni/Cu multilayers preparation [26].
The possibility of simultaneous CoNiCu was tested in
sulphate-citrate bath [24] with copper incorporated in CoNi
deposits in amounts 5 to 60 mass %. It would be interesting also to study the possibility for codeposition of ternary
CoCuNi alloys from citrate solutions in pulse conditions.
The subject of the present article is to study the
ability for codeposition of Cu, Ni, and Co in CoNi and
CoNiCu coatings from citrate electrolyte using stationary and pulse potentiostatic electrodeposition, as well
as the morphology, elemental and phase composition
of these alloys.
potential. For the purpose a pulse generator was used
connected to the input of an especially designed potentiostat connected to the three-electrode cell. The
average values of ΔĒ (calculated as difference between
the potential at current and the equilibrium potential)
and the average current Iav were measured respectively
by means of digital voltmeter with high input voltage,
and milliammeter. The amplitude values of polarization ΔΕp were measured using an oscilloscope. The
theoretical relation between the average (ΔĒ) and
the amplitude (ΔΕp) values of polarization in potentiostatic mode with rectangular shape of the pulses is:
EXPERIMENTAL
(=
f
The experimental studies were carried out in
three-electrode cell with 150 dm3 total volume at room
temperature (20oC±1oC) with disk-shaped cathode from
pure electrolytic copper (surface area 1cm2) and Pt plate
anode. The anode surface was more than 30 times larger
than that of the cathode. Saturated calomel electrode
(SCE) was used as reference electrode.
The study was carried out in slightly acidic citrate bath with content: CuSO4 from 4 to 12 g dm-3;
CoSO4.7H2O from 60 to 90 g dm-3; NiSO4 from 40 to
60 g dm-3; Na citrate 50 g dm-3.
The рН=5.3-5.5 of electrolyte was measured using
рН-meter and adjusted with NaOH and Н2SO4. The kinetics of deposition was studied by means of the method
of linear and cyclic voltammetry (CV) with potentioscan
type Wenking (Germany), enabling varying the sweep
rate from 0 to 150 mVs-1. The electrodeposition of cobalt
coatings was carried out both in stationary potentiostatic
and pulse mode with rectangular potentiostatic pulses,
with varying the applied potential (the polarizations in
pulse mode, resp.) and the pulse frequencies.
The pulse deposition of coatings was carried out
through potentiostatic pulses of rectangular shape of
τ
p
∆ E = θ .∆E p , where θ = τ + τ ;
p
z
τp – time of pulses, and
τz – time of pauses between pulses.
The relations
∆ E − I av and ∆E p − I av
were determined for all pulse frequencies
1
, Hz , T= τ p + τ z ) ,
T
θ = 0,5 .
The morphology and the elemental composition
were investigated using SEM and Energy Dispersive
Spectral Analysis (EDSA) respectively through equipment of Oxford Instruments, JSM-6390- Jeol.
The phases presented in the deposited coatings
were identified using X-Ray analysis. For the purpose,
a vertical automatic powder diffractometer Philips PW
1050 was used with secondary graphite monochromator
operating with Cu Kα radiation and scintillation counter.
The diffraction curves were recorded in angular interval
from 10 to 100 degrees 2θ with step 0.04 degrees 2θ and
exposure 1 sec.
RESULTS AND DISCUSSION
Deposition of Cu, Co and Ni
In order to get information on the nature of polarization in deposition of Cu, Co and Ni, the polarization
dependencies in the corresponding electrolytes were
obtained with varying the concentration of the basic
salts in the solution (Fig. 1, a-c) on copper electrode at
potential sweep rate 30 mV s-1.
In the electrolytes both for copper and nickel depo-
309
Journal of Chemical Technology and Metallurgy, 48, 3, 2013
sition (Fig.1а and Fig.1с), plateaus of the current are
formed that increase with the increase of concentration
of salts in the solution. In the case of cobalt deposition of
(Fig.1,b), instead of plateau - a pronounced cathode peak
of the current is reached that increases with the increase
of concentration of cobalt and it is shifted towards more
negative values of potential. The increase of current with
the concentration in the three electrolytes suggests that
a part of the polarization is related to diffusion limitations. Evidence of this are the dependencies (Fig.2, a-c)
indicating the impact of the potential sweep rate on polarization characteristics in electrolytes for Cu, Co and
Ni. In all electrolytes the increase of the potential sweep
rate results in increase of the current corresponding to
40
-3
1
2
3
50
40
4 g dm CuSO4
8 g dm-3CuSO4
12 g dm-3CuSO4
1
30
30
ic,mA cm-2
2
i,mA cm-2
v, mV s-1: 1- 30
(Cu)
2- 50
3- 80
35
3
20
25
20
3
15
2
10
10
0
1
5
0
0,0
0,5
1,0
1,5
2,0
0,0
0,5
1,0
-E(SCE),V
60
i,mA cm-2
40
-3
25 g dm CoSO47H2
60 g dm-3 CoSO4H2O
90 g dm-3 CoSO4H2O
3 2
30
1
3
20
1
3
40
2
30
1
20
2
10
10
0
0
0,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
24
v, mV s-1: 1- 30
(Ni)
2- 50
3- 80
22
20
18
40
-3
ic, mA cm-2
30
25
2
14
1
12
10
8
20
6
15
3
10
4
2
2
1
0
5
0
3
16
1 - 40 g dm NiSO4
2 - 50 g dm3 NiSO4
3 - 60 g dm-3 NiSO4
35
i, mA cm-2
0,2
-E(SCE),V
-E(SCE),V
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
-E(SCE),V
0,0
0,5
1,0
1,5
2,0
-E(SCE),V
Fig. 1. Effect of concentration of components in citrate
electrolyte for deposition of Cu (a), Co (b), and Ni (c); 50
g dm-3 Na citratе, v= 30 mVs-1.
310
2,0
v, mV s-1: 1- 30
(Co)
2- 50
3- 80
50
ic,mA cm-2
1
2
3
50
1,5
-E(SCE),V
Fig. 2. Voltammerograms in citrate electrolyte at different
potential sweep rates, v: (1) 30 mVs-1; (2) 50 mV s-1; (3) 80
mVs-1 in electrolytes for deposition of Cu, Co, and Ni with
content: (а) 12 g dm-3 CuSO4; (b) 60 g dm-3 CoSO4.7H2O; (с)
60 g dm-3 NiSO4; (а-с) 50 g dm-3 Na citratе.
K. Ignatova, Y. Marcheva
i, mA cm-2
30
25
0,4 m Ni +
50 g dm-3 Na citrat
20
v=60 mV s-1
1*
15
10
2*
0
-5
0,0
0,5
1,0
1,5
2,0
-E,V(SCE)
Fig. 3 Cyclic voltammograms (v= 30 mV s-1) in electrolyte
for Ni with content: 60 g dm-3 NiSO4; 50 g dm-3 Na citrate:
1, 1* - first, resp. second repeat in straight direction; 2* second and subsequent repeats in reverse direction.
70
1- Ni
2- Co
3- NiCo
60
50
3
40
2
30
20
1
10
0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
-E(SCE),V
70
1- Cu
2- Ni
3- Co
4- CuNiCo
60
50
ic, mA cm-2
Codeposition of metals in CoNi and CoNiCu alloys
The possibility for codeposition of Co, Cu, and Ni
in alloy coatings can be evaluated through comparison
of the polarization dependencies for their separately
deposition to the dependency at the simultaneous presence of the corresponding metals in the solution, as it is
made in Fig. 4 (a) for CoNi, and in Fig. 4b for CoNiCu.
The curve taken at simultaneous presence of Ni
and Co (Fig. 4a) is almost a sum of deposition curves
of copper and cobalt. The data show that the deposition
potentials of Co and Ni on Cu substrate are very close
and almost coincide in the range from -1.2 V to -1.4V
(SCE), just in the way as the values of their standard
potentials are close to each other: Е0(Co2+/Co) = -0,277
V; Е0(Ni2+/Ni) = -0,250 V. Most likely the closeness of
the deposition potentials is due to the closeness of the
corresponding stability constants of the citrate complexes of both metals.
1
5
ic, mA cm-2
the cathode maximum either in the plateau in deposition
of Cu (Fig. 2a) and Ni (Fig. 2c), or in the cathode peak
in deposition of Со (Fig. 2b).
In electrolyte for Cu deposition at lower potential
sweep rates (Fig. 2,a), two pronounced plateaus in the
curves of current occur, corresponding to the two subsequent stages of electron transfer. Between them a chemical stage exists also, which is due to the reaction of copper
disproportion. The described mechanism was proved by
the authors in amine-nitrate electrolyte [22] and is also
proved by other authors in other electrolytes [23].
There is no pronounced plateau of limiting current
in the electrolyte for nickel deposition (Fig. 2c) at all
potential sweep rates. After 2-3 repeats of cyclic voltammograms (Fig. 3), when the electrode is coated with
nickel, the plateau gradually disappear and the impact
of the potential sweep rate is slight. This indicates that
the larger share in the polarization of nickel deposition
has an activation nature. The absence of anode branch in
the cyclic curves (Fig. 3) is also in favor of that claim.
From the kinetics results obtained we can conclude
that the electroactive forms in the citrate electrolyte are
complex compounds of copper, cobalt and nickel ions
with specific mechanism of deposition, with predominance of the diffusion polarization for Cu and Co and
activation one for Ni. In the investigated electrolyte
composition the deposition of cobalt initiates at –1.0 V,
of nickel initiates at –1.1 V and of copper at –1.25 V.
4
40
30
3
20
2
10
0
1
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
-E(SCE),V
Fig. 4. Comparison between the curves of individual deposition
and co-deposition of alloys (v = 30 mVs-1): (a) CoNi: Ni (1), Co
(2), and CoNi (3); and for: (b) CoNiCu: Cu (1), Ni (2), Co (3),
and CoNiCu (4) in electrolytes with content: 4 g dm-3 CuSO4;
90 g dm-3 CoSO4 7H2O; 60 g dm-3 NiSO4; 50 g dm-3 Na citratе.
311
Journal of Chemical Technology and Metallurgy, 48, 3, 2013
Fig. 4,b shows the curves of deposition of each of
the metals (Cu, Ni, Co) and the curve of their codeposition from electrolyte, containing the three components.
The deposition potentials of metals converge in the
range -1.3 V to -1.42 (SCE), which could be the range
of potentials for codeposition of CоNiCu alloy coatings
in stationary mode.
Deposition of CoNi and CoNiCu alloys in potentiostatic pulse mode
The effect of the pulse frequency (varied in the range
from 100 Hz to 10 000Hz, θ=0,5) on the dependencies
∆ E − I av (Fig. 5, curves 1-6) and ∆E p − I av (Fig. 5, curves
1* - 6*) was studied. The data for CoNi deposition are
similar to those for CoNiCu and this is the reason the
dependencies for the first alloy only to be presented
(Fig. 5).
The ohmic drop of solution (between the Lugin
capillary and the working electrode), as well as the
polarization, used to charge the double electric layer
(DEL), are also included in the values of the average
and amplitude polarization. While the first becomes
negligible with increasing the polarization, the second
80
1-6 average polarization
1*-6* amplitude polariration
7--- stationary regime
1
70
2
3
5
6
60
average current, mA
4
1*
7
2*
50
3*
40
30
4*
20
5*
10
6*
0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
average (1-6) and amlitude (1*-6*) polarization,V
Fig. 5. Relation between the average cathode polarization
and average current ∆E p − I av (curves 1-4), and between the
amplitude polarization and average current ∆E p − I av (curves
1* - 4* ) at pulse frequencies: 100 Hz (1, 1*); 500 Hz (2,
2*); 1,000 Hz (3, 3*); 2,500 Hz (4, 4*); in electrolyte for
CoNi: 90 g dm-3 CоSO4 7H2O; 60 g dm-3 NiSO4; 50 g dm-3
Na citrate; curve 5 (dashed): polarization dependence in
stationary mode in the same composition.
312
is significant and is about one third of the total polarization, especially at frequencies above 1 000 Hz. At such
frequencies the time of pulses is of the order of the time
for loading DEL (τ p << 0,5ms) . Although the deposition
at frequencies higher than 1 000 Hz is ineffective due to
the mentioned reasons, CoNi coatings were deposited
at frequencies 5 000 Hz and even 10 000 Hz in order to
determine the effect of the pulse mode on the structure
of alloys in such non-standard conditions.
With the increase of frequency of pulses from 100
Hz to 10 000 Hz, the average polarization (Fig. 5, curves
1-6) is lower than that in stationary mode (Fig. 5, curve
7), which is explained with the possibility to decrease
the diffusion limitations due to relaxation of the diffusion
gradient during pauses (effect, more significant at lower
frequencies). In the same time the increase of frequency
of pulses results in almost double increase of the amplitude values of polarization (Fig. 5, curves 1*-6*) in
the frequency range investigated. Since the polarization
is a measure for saturation, it is reasonable to expect
achieving more fine crystal structure of coatings when
higher frequencies of pulses are applied.
Morphology and elemental content of the alloys
NiCo
Coatings of CoNi and CoNiCu were deposited in a
wide range of variation of pulse frequencies (from 500
Hz to 10 000 Hz for CoNi and from 500 Hz to 1 000
Hz for CoNiCu), θ=0,5. With the aim the stationary and
pulse plate conditions to be comparative, an average
polarization corresponded to an average current Iav (S =
1cm2), was applied (for CoNi Iav = 35 mA, for CoNiCu
Iav = 45 mA)
The SEM images of Ni-Co stationary deposited coatings are shown in Fig. 6а, b and pulse deposited coatings
(Fig. 6c-f). The elemental content (in wt.%) according
to the data from EDSA analysis is given in the capture
to the figure. The pulse deposited coatings have low Ni
content (up to 12 mass %) compared to the stationary
deposited coatings. At high frequencies presence of
oxides could be found in the coating.
The application of pulse mode especially the higher
pulse frequencies results in increased share of the more
oval crystals compared to the needle-shaped ones, their
size decreases (reaching 200-400 nm) and the surface
smoothes as a whole. The increase of pulse frequency
above 1 000 Hz results in deposition of alloys with high
K. Ignatova, Y. Marcheva
a)
b)
c)
d)
e)
f)
Fig. 6. SEM and EDSA of CoNi coatings deposited in stationary mode (a, b) and in pulse mode (c-f) at pulse frequencies:
500 Hz (c); 1000 Hz (d); 5000 Hz (e); 10000 Hz (f) in composition: 4 gd m-3 CuSO4; 90 g dm-3 CоSO4.7H2O; 60 g dm-3
NiSO4; 50 g dm-3 Na citrate. The applied deposition polarizations correspond to the same average current Iav =45 mA (S=1
cm2) in the polarization curves from Fig. 5. Coating composition: (a) 76 % Co-21 % Ni (3 % O); (b) 76 % Co-21 % Ni
(3 % O); (c) 88 % Co -12 % Ni ; (d) 96 % Co- 4 % Ni; (e) 89 % Co-5 %Ni (6 % O); (f) 90 % Co-5 % Ni (5 % O).
a)
a*)
b)
b*)
Fig. 7. SEM and EDSA of CoNiCu coatings deposited in pulse mode with pulse frequencies: 500 Hz (a); 1 000 Hz (b)
in electrolyte: 4 g dm-3 CuSO4; 90 g dm-3 CоSO4.7H2O; 60 g dm-3 NiSO4; 50 g dm-3 Na-citrate. The applied deposition
polarizations correspond to the same average current Iav = 35 mA (S = 1 cm2).
313
Journal of Chemical Technology and Metallurgy, 48, 3, 2013
Fig. 8. Х-Ray diffraction curves of CoNiCu coatings (a)
and CoNi coatings (b-f) deposited in stationary (b) and
pulse mode at frequencies of pulses: 500 Hz (c); 1 000 Hz
(d); 5 000 Hz (e); 10 000 Hz (f) in electrolytes of content:
4 gdm-3 CuSO4; 90 g dm-3 CоSO4.7H2O; 60 g dm-3; 50 g
dm-3 Na citrate. The applied deposition polarizations correspond to average currents Iav = 35 mA (a) and Iav =45
mA (b-f); S=1 cm2.
cobalt content (about 90 mass % Со). The blurred SEM
images at higher frequencies can be explained with obtaining nanosized structure and smoothing the surface.
CuNiCo
Triple Cu-Ni-Co alloy coatings were pulse deposited, the data of the morphology and the elemental
content are presented in Fig. 7.
It was found that the CuNiCo alloy coating deposited
with pulse frequency 500 Hz have larger crystals and are
more inhomogeneous compared to CoNi coatings (Fig.
6с and Fig. 7a). The increase of pulse frequency up to 1
000 Hz results in obtaining rounded, more uniform, and
finer crystals of average diameter 400-500 nm (Fig.7,b).
Moreover, the percentage of nickel in the alloys increases
(up to 27 wt % Ni). The percentage of cobalt, however,
does not change significantly and remains about 71-76
wt %, while the copper content decreases from 8 wt %
to less than 5 wt % at the potentials applied.
Phase content of NiCo and CuNiCo coatings
All diffractograms are characterized with relatively
wide peaks due to their fine nanosized structure. This
makes difficult the interpretation of data since sometimes the identification of a certain phase, and mostly
314
the phases of cobalt, is made upon a single peak only.
The X-Ray analysis indicates that CoNi stationary
coatings are characterized with the highest and narrowest reflections (Fig. 8 b) corresponding to their larger
crystal structure in comparison to the pulse deposited
coatings (Fig. 8 с-g). This result well agrees with the
SEM observations of the same samples. In all alloy
coatings copper, cobalt, and nickel crystallize in cubic
lattice (f.c.c.). Only in the triple CoNiCu coating deposited at pulse frequency 1 000 Hz (Fig. 8 а), it was found
presence of cobalt both with face-centered cubic crystal
lattice (f.c.c.) and hexagonal crystal lattice (h.p.c.). The
clearly pronounced spectrum of copper is mainly due to
the fact that the layers are relatively thin (about 3 µm)
and are deposited on copper.
CONCLUSIONS
It was found that at potentiostatic pulse deposition
and higher frequency coatings with smooth surface and
finer crystals with rounded shape were produced. The
average grain size was about 200-300 nm for CoNi and
500 nm for CoNiCu. The optimal frequency of pulses
for obtaining fine-crystal nanosized alloys of maximum
homogeneity in both cases is 1 000 Hz. In CoNi coatings
the metals crystallize in cubic lattice (f.c.c.). In CoNiCu
coatings except phases of cubic copper and nickel, it
was found presence of cobalt both with face centered
cubic lattice (f.c.c.) and hexagonal crystal lattice (f.c.c.)
at pulse frequency 1 000 Hz.
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