2011 karahan

Study of effect of boric acid on Zn–Co alloy
electrodeposition from acid baths and on
composition, morphology and structure of
deposit
I. H. Karahan* and H. A. Çetinkara
The effect of boric acid as additive in electrodeposition of Zn–Co alloy coatings from an acidic
sulphate electrolyte containing NH4Cl, Na2SO4 and citric acid Na3C6H5O7, at ambient
temperature, without agitation has been investigated. The deposition process was investigated
by cyclic voltammetry. It was found that the current density first decreased and then increased,
due to adsorption of a boric acid complex and/or changes in the morphology, but the initial
deposition potential was not affected. The addition of boric acid to the bath led to the formation of
improved Zn–Co deposits, composed of coalesced globular fine grains smaller than y2 mm in
diameter. Also, all of the Zn–Co deposits studied consisted of g phases. The Co content in the
Zn–Co deposits produced in the presence of boric acid increased from 2?5 to 4?3 wt-%. It is
suggested that Zn–Co deposits produced in the bath containing 60 g L21 boric acid probably
offer sacrificial protection to the steel substrate.
Keywords: Electrodeposition, Zn–Co alloy, Corrosion, Boric acid, X-ray diffraction
Introduction
The application of zinc (Zn) alloy electrodeposited
coatings in various industrial sectors has been greatly
expanded as a consequence of their excellent corrosion
resistance, paintability and good formability. Iron (Fe),
cobalt (Co) and nickel (Ni) have been incorporated in
Zn plating baths to obtain coatings with higher
corrosion resistance.1–4 Typically, the initially formed
corrosion products are insoluble and form a stable
protective layer. Zn–Ni and Zn–Co deposits have shown
no whisker growth during humidity testing. This
characteristic is desirable in electric and electronic
applications.5 They can also be considered as substitutes
for toxic and high cost cadmium coatings.6–9
If Zn alloys have a sufficiently high amount of Zn,
they can still maintain a sufficiently negative potential to
steel and, yet, offer better corrosion protection than Zn
alone. During Zn codeposition with metals of the Fe
group, the less noble metal, Zn, is electrodeposited
preferentially; this phenomenon has been described as
anomalous codeposition by Brenner.10
The electrodeposition of Zn–Co alloys with controlled morphology and composition has been studied
extensively. The electrodeposition of Zn and Zn alloys
is usually conducted in both acidic and alkaline
Department of Physics, Faculty of Art and Science, University of Mustafa
Kemal, Antakya, Turkey
*Corresponding author, email [email protected]
ß 2011 Institute of Metal Finishing
Published by Maney on behalf of the Institute
Received 21 July 2010; accepted 12 October 2010
DOI 10.1179/174591911X12968393517774
solutions.11–14 Toxic and corrosive traditional cyanide
containing alkaline baths are sometimes used for
electroplating of Zn and Zn alloys.13,15
Kalantary16 investigated the corrosion performance
of four types of commercially used Zn alloy coatings,
namely Zn–Co, Zn–Fe, Zn–Ni and Zn–Mn. He concluded that Zn–Co alloy coatings containing 1%Co
demonstrate optimum corrosion resistance, similar to
those of Zn–Ni alloy electrodeposits. Ortiz-Aparicio
et al.17 studied the influence of Co on Zn electrodeposition from alkaline glycinate solutions. However, the
optimum content of Co in the coating and the protection
mechanism of such a coating are still controversial.
Most results reported on the electrodeposition of Zn–Co
alloy coatings showed that the maximum amount of Co
in their deposits was about 6–7 wt-%.18,19 The deposits
of Zn–Co alloys with Co content of more than 6 or 7
wt-% have not been widely reported.20,21
Kautek et al. explained corrosion activity suppression
by the elimination of the negatively charged Zn cation
vacancies by Co.22 Ramanauskas reported topography,
texture, lattice imperfections and superficial corrosion
product layer composition of the Zn–Co alloys and found
that structural properties of the alloy are of substantial
importance to superiority of corrosion resistance.23
Lichušina et al. deposited Zn–Co alloys with y15%Co
in an alkaline bath under equilibrium conditions exhibiting good appearance and a high corrosion resistance,
comparable with that of chromated conventional Zn–Co
coatings with low Co concentrations.24 Fei and Wilcox
obtained Zn–Co alloy deposits with a wide Co content
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Study of effect of boric acid on Zn–Co electrodeposition
range of 10–90%.25 The use of additives in electrodeposition baths is vital due to their influence on the
growth and structure of the deposits obtained.26–29
Generally, additives are added to a bath at very low
concentrations; their presence in the electrolyte promotes the formation of smooth and shiny coatings. The
specific activity of an additive is generally understood
in terms of its absorption onto the cathode surface
during electrodeposition. Additive molecules adsorbed
on the cathode surface can affect the activation
energy30 and the rate of charge transfer in the elecftrochemical reaction, and may also influence the
mechanism of electrocrystallisation.31,32 In the authors’
previous study investigating the relation of corrosion
properties with gelatin as additive in Zn–Fe alloys,33
the results indicated strong evidence of an increase in
corrosion potential with addition of gelatin to the
electrolyte. Mouanga et al.28 studied the influence of
coumarin on Zn–Co alloy obtained from a sulphate
bath. They found that well structured Zn–Co alloys
with a finer grain size were obtained in the presence of
coumarin and this additive affected the reduction of
Zn, but it had no effect on the reduction of Co. They
also noticed that the presence of coumarin in the
electrolyte resulted in structural refinement of alloy
deposits and increase in cathodic current efficiency.
Barbosa and Carlos34 used sorbitol for electrodeposition of Zn–Fe as a complexing agent in an alkaline
bath. They observed that it is useful for minimising the
potential difference of Fe and Zn. Wu et al. introduced
boric acid into an ammonia–citric acid plating bath of
Ni–W alloys. They found that boric acid acted as a
surfactant to impede the proton reduction.35
Heydari Gharahcheshmeh and Heydarzadeh Soh
studied Zn–Co deposition from an alkaline bath in the
presence of glycine. They showed that Co deposited at a
potential near to that of Zn together with successful
codeposition of Co and Zn. They also showed that
reduction–oxidation reactions of Zn–Co alloy deposits
were quasi-reversible and resulted in deviation of
electrodeposited alloys from the equilibrium phase
diagrams. The corrosion resistance of the deposits was
also highly influenced by the composition and morphology of the coatings.36
In this paper, electrodeposition of Zn–Co alloys from
a cyanide free alkaline bath in the presence of boric acid
as a complexing agent was studied using the potentiostatic deposition method. Cyclic voltammetry was also
used to study the codeposition of Zn and Co. Scanning
electron microscopy, energy dispersive spectroscopy and
X-ray diffraction (XRD) analyses of Zn–Co films were
applied to determine the morphology, composition and
structure of the coatings respectively. Finally, the
corrosion resistance of electrodeposited Zn–Co alloy
coatings has been investigated as a function of boric acid
content in the electrolyte.
Experimental
Electrodeposition of Zn–Co alloys
Zn12xCox alloys were prepared by electrodeposition under
potentiostatic conditions on aluminium and AISI 4140
steel disc substrates from a sulphate plating bath at
room temperature (Table 1). The chemical composition of
the AISI 4140 steel is 0?36C–0?80Mn–0?005Si–0?914Cr–
100
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0?30Ni–0?85Mo–0?075V–0?07S–0?143Cu–0?034P (wt-%).
The electrolytes were prepared using (18 MV cm) twice
distilled water. The pH value of the bath was adjusted to
y5 with hydrochloric acid and NaOH. The dimensions of
the deposits were y1?561?5 cm. Before the deposition,
the substrates are prepared in the standard industrial way:
chemical then electrolytic degreasing in sodium hydroxide
solution (40 g L21; 64?5uC) for 2 min followed by water
wash, mechanically grinding with silicon carbide papers
from 3 to 0?5 mm and velvet, chemical pickling and
activation in an acid medium (hydrochloric acid, 30% v/v)
for 10 s, rinsed with the twice distilled water and then dried
in air. Thus, the wettability and therefore the reactivity of
the cleaned substrate surface were at an optimum. After
these preparation steps, the substrates were ready to be
electrodeposited. The counter electrode was platinum. The
reference electrode used in all experiments was a saturated
calomel electrode.
The quantitative composition analysis of the electrodeposits was determined using a JEOL 6400 scanning
electron microscope with an energy dispersive spectrometer working at 15–30 kV. The compositions of the
films were determined using energy dispersive spectroscopy. The preferred orientations of the deposits were
determined by XRD analysis, using a Siemens D500 Xray diffractometer with Cu Ka radiation. The 2h range of
0–90u was recorded at a rate of 0?02u 2h/0?5 s. The
crystal phases were identified by comparing the 2h
values and intensities and the thickness of the deposits
was determined by an X-ray fluorescence spectrometer
(Canberra Ultra Low Energy Germanium Detector
GUL0035).
Electrochemical measurements
The electrochemical behaviours of the electrodeposited
Zn–Co alloys were analysed in 3 wt-% NaCl aqueous
solution at room temperature in a Pyrex glass cell. As
above, the counter electrode was platinum and the
reference electrode used in the electrochemical experiments was a saturated calomel electrode. The corrosion
behaviours of the samples were investigated by potentiodynamic polarisation technique. Polarisation measurements were performed with an electrochemical
analyser/workstation (model 1100; CH Instruments,
Inc., Austin, TX, USA) with a three electrode configuration. The exposed areas of the specimens were
y1 cm2. The specimens were embedded in a cold setting
resin and immersed into the solution until a steady open
circuit potential was reached. After equilibration
(45 min later), polarisation was started at a rate of
1 mV s21.
Table 1 Solution
compositions
electrodeposition
for
Zn–Co
Solution compositions
Levels
ZnSO4/M
CoSO4/M
C6H5Na3O7.2H2O/g L21
H3BO3/g L21
NH4Cl/g L21
Na2SO4/g L21
Solution pH
Temperature/uC
Deposition time/min
Voltage/V
0.5
0.1
25
0, 20, 40, 60 and 80
45
0.5
5
Room temperature
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Study of effect of boric acid on Zn–Co electrodeposition
2 Effect of H3BO3 on Co content of Zn–Co alloy electrodeposits in electrolyte at room temperature
1 Cyclic voltammograms for different H3BO3 values
Cyclic voltammetry was carried out using a proprietary system (model 1100; CH Instruments, Inc.), a
conventional three electrode cell with a steel plate as the
working electrode, platinum as the counter electrode
and saturated calomel as reference electrode at a sweep
rate of 10 mV s21. The solution was deaerated by
bubbling argon gas for at least 45 min before each
measurement. The temperature of the cell was kept at
room temperature; the pH values of solutions were
adjusted using a digital pH meter (¡0?1 accuracy).
Results and discussion
Figure 1 shows voltammetric curves for Zn–Co deposition from plating baths with various boric acid
concentrations. In solutions, during the forward scan
towards the negative direction, the cathodic current
increased sharply when the deposition began. As can be
seen, the deposition current density (CD) first decreased
and then increased as boric acid was added to the bath.
Comparing CD at the peak potential around 21?5 V, in
the absence and presence of 40 g L21 boric acid, it can
be seen that in the latter case, CD was reduced by
y48%. It has been reported that boric acid, polyalcohols and mannitol react in the molar ratio 1 : 2
respectively, to form a boric–polyalcohol complex,37–39
so that the presence of boric acid in the solution
hindered the diffusion of Zn2z and Co2z ions. The
voltammograms suggest that initial codeposition of Zn
and Co occurs on limiting current at around 21?11 V
and does not change in any of the baths studied,
implying that no complex was formed between Zn or Co
ions and the boric anion.
All the voltammograms show a gradual increase in the
anodic current which results in the formation of ‘humps’
on the anodic peaks. These ‘humps’ are considered as
the results of the dissolution of Co and Zn, which form
on the surface of the deposit during the cathodic scan.
Only at the cyclic voltammogram obtained from the
electrolyte containing 60 g L21 boric acid, was Co
dissolution not observed. Addition of boric acid in the
electrolyte caused an increase in both deposition and
dissolution CD.
Figure 2 shows the effect of H3BO3 concentration on
the Co content of Zn–Co alloy deposits. The additive
presence in the electrolyte is an important factor
affecting film composition. The Co concentration in
deposits depends on the electrodeposition CD, bath pH
and addition of boric acid. The additive changes the
formation mechanism of the film and thereby the film
composition.
At the boric acid concentration of 20 g L21 the Co
concentration of the film decreased suddenly from 3?1 to
2?5%. Then Co content increased gradually from 2?5%
to the maximum 4?3% with increasing H3BO3 concentration from 20 to 60 g L21 and decreased to 3?6%
thereafter. It should be pointed out that higher
concentration H3BO3 (.60 g L21) tended to raise the
solution viscosity and then to impede the mass
transportation of the metallic ions.35
Figure 3 shows XRD patterns of the electrodeposited
Zn–Co alloys deposited at different boric acid concentrations, varying from 0 to 80 g L21. The XRD analysis
showed that the films are polycrystalline in structure.
The XRD patterns of the deposits, whether from the
solution with boric acid or not, exhibit a preferred
orientation along the (101), (102), (103), (110) and (112)
planes.
In Fig. 3, with increasing value of the boric acid in the
electrolyte from 20 to 80 g L21, the intensity of (101)
preferred orientation was decreased, which led to the
decrease in corrosion resistance of the alloys. It can be
seen from the figure that increasing the boric acid
content decreased the (101) peaks. As the boric acid
value decreases the signals belonging to the g phase
become less intense.
Figure 4a–c shows the surface morphologies of Zn–
Co alloy deposits obtained from the plating bath
containing different concentrations of boric acid.
3 X-ray diffraction pattern obtained from Zn–Co coating
from electrolyte containing 0, 20, 40, 60 and 80 g L21
boric acid
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Study of effect of boric acid on Zn–Co electrodeposition
a 40 g L21; b 60 g L21; c 80 g L21
4 Scanning electron microscopy images of Zn–Co films electrodeposited at different boric acid contents in electrolyte
According to Fig. 4a and b, with the increase in boric
acid content in the bath from 40 to 60 g L21, the grain
size becomes more crystalline. However, further increase
in the boric acid content from 60 to 80 g L21 has a
reverse effect and causes finer grains (Fig. 4c). This can
stem from the increase in CD as a result of boric acid
increasing, due to the rise in the overpotential free
energy for formation of new nuclei in Zn–Co alloy
deposits that leads to higher nucleation rate, and hence
formation of coatings with smaller grain size.14
Figure 5 shows the results corresponding to the
influence of addition of 20, 40, 60 and 80 g L21 boric
acid on the corrosion resistance of electrodeposited Zn–
Co alloy from a sulphate bath in a 3 wt-% NaCl aqueous
solution. Two stages of passivation are observed in Zn–
Co alloy coatings. This can be an indication of the
formation of protective film in two stages. When Zn–Co
alloy is corroded, Zn begins to dissolve preferentially
according to equations (1)–(4)26,40
corroded coatings and create areas in which CD is
independent of potential.
The presence of Co in the coating enhances dissolution of Zn in NaCl solution. Consequently, the increase
in Zn dissolution causes its reaction with chloride ions in
NaCl solution and formation of zinc hydroxy chloride
according to equation (5)26,42
5Zn2z z8OH{ z2Cl{ zH2 O?Zn5 OH8 Cl2 :H2 O (5)
Zinc hydroxy chloride has a very low product of
solubility, and ensures higher protective ability for Zn–
Co alloy coatings.41,42
It is noticed that the alloy coating from a bath
containing 60 g L21 boric acid has the highest corrosion resistance among all coatings. While Zn–Co
alloy coating from a plating bath containing 80 g L21
boric acid has the lowest corrosion resistance compared with other alloy coatings. An increase in boric
acid content in the electrolyte up to 60 g L21 led to the
formation of the best Zn–Co deposits, composed of
coalesced globular fine grains smaller than y2 mm in
diameter and Zn–Co alloys presenting a lower degree
of corrosion.
ZnðsÞ?Zn2z ðaqÞz2e{
(1)
O2 ðgÞz2H2 OðlÞz4e{ ?4OH{ ðaqÞ
(2)
Zn2z ðaqÞz2OH{ ðaqÞ?ZnðOHÞ2 ðsÞ
(3)
Conclusions
ZnðOHÞ2 ðsÞ?ZnOðsÞzH2 O
(4)
Compact and bright Zn–Co binary alloys were electrodeposited satisfactorily on steel sheet from baths
containing cobalt sulphate, zinc sulphate, sodium
sulphate, sodium citrate and boric acid (pH55). The
effects of boric acid concentration on cathodic polarisation, composition and structure of the deposited alloys
were investigated.
The composition of the deposits is strongly dependent
upon the added boric acid content. The Co content in
the film increases with increasing boric acid content in
the bath up to a limiting level (60 g L21), but decreases
with the further increase in boric acid. These alloy
deposits can be used for sacrificial protection of steels
from corrosion. The deposited alloys formed from the
currently studied bath consisted of a single solid solution
phase with a hexagonal structure irrespective of the
composition of the alloys.
Thus, the protective films have complex compositions
including Zn(OH)2 and ZnO that cover the surfaces of the
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102
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