Electrochemical studies of zinc–nickel codeposition in sulphate bath

Electrochemical studies of zinc–nickel

codeposition in sulphate bath

Mortaga M. Abou-Krisha

Faculty of Science, Chemistry Department, South Valley University, Qena, Egypt

Received 23 June 2004; received in revised form 30 January 2005; accepted 30 January 2005

Available online 12 March 2005

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2005) 1035–1048

Abstract

The electrodeposition of Zn–Ni alloys from a sulphate bath was studied under different conditions. The bath had the

composition 0.40 M sodium sulphate, 0.01 M sulphuric acid, 0.16 M boric acid, 0.20 M zinc sulphate and 0.20 M nickel

sulphate. It is found that the plating bath temperature has a great effect on the cyclic voltammograms, galvanostatic

measurements during electrodeposition, and consequently linear polarization resistance for corrosion study and the alloy

composition. Under the examined conditions, the electrodeposition of the alloys was of anomalous type. X-ray diffraction

measurements revealed that the alloys consisted of the d-phase (Ni3Zn22) or a mixture of the two phases d and g (Ni5Zn21). The

comparison between Ni deposition and Zn–Ni codeposition revealed that the remarkable inhibition of Ni deposition takes place

due to the presence of Zn2+ in the plating bath. The Ni deposition starts at �0.85 V in the bath of Ni deposition only, but the

deposition starts at more negative potentials in the codeposition bath although the concentration of Ni2+ is the same in the both

baths.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Zn–Ni alloy; Electroplating; Anomalous codeposition; Plating bath temperature; Electrochemical studies; Sulphate bath

1. Introduction

In recent years, great interest has been shown in the

possibilities offered by the electrodeposition of alloys,

mainly in the automotive industry. Usually the

mechanical and chemical properties of metals are

improved by alloying. In particular, it is known that

the mechanical properties of zinc electrodeposits can

be improved by alloying zinc with nickel [1,2]. Using

E-mail address: [email protected].

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.01.161

Zn–Ni alloy deposits on iron also increases their

corrosion resistance [3–8].

The electrodeposition of Zn–Ni alloys is classified

by Brenner [9] as an anomalous codeposition, where

zinc, which is the less noble metal, is preferentially

deposited. Although this phenomenon [10] has been

known since 1907, the codeposition mechanisms of

zinc and nickel are not well understood [11,12]. Many

studies have attempted to understand the characteristics

of the deposition process [13–16]. There are some

propositions to explain the anomalous codeposition of

Zn–Ni alloys. The first attributes the anomalous

.

M.M. Abou-Krisha / Applied Surface Science 252 (2005) 1035–10481036

codeposition to a local pH increase, which would

induce zinc hydroxide precipitation and would inhibit

the nickel deposition [17–19]. It was, however, later that

anomalous codeposition occurred even at low current

densities [20], where hydrogen formation is unable to

cause large alkalinization effects. Another proposition

is based on the underpotential deposition (UPD) of zinc

on nickel-rich zinc alloys or on nickel nuclei [21,22].

Two other papers on NiFe electrodeposition propose

different mechanisms. The mechanism of Lieder and

Biallozor [23], assumes that Ni2+ discharges first to

form a thin layer which chemisorbs water to form

adsorbed Ni(OH)+, competition between the Ni2+ and

Fe2+ to occupy active sites leads to the preferential

deposition of Fe. Matlosz [24] uses a two-step reaction

mechanism involving adsorbed monovalent intermedi-

ate ions for both electrodeposition of iron and nickel, as

single metals, and combines the two to develop a model

for codeposition. Anomalous effects are assumed to be

caused by preferential surface coverage due to

differences in Tafel rate constants for electrodeposition.

Keith Sasaki and Jan Talbot [25] proposed model

extends the one-dimensional diffusion modeling of

Grande and Talbot [26], a supportive or interpretive,

rather than a predictive, model of electrodeposition. A

main contribution of this model is the inclusion of

hydrogen adsorption and its effects on electrodeposi-

tion. Zech et al. [27] concluded that codeposition of

iron group metals leads to a reduction of the reaction

rate of the more noble component and an increase of

the reaction rate of the less noble component

compared to single metal deposition.

The aim of this work was to investigate the

mechanism of Zn–Ni alloy deposition in sulphate

electrolytes. The results of the experimental approach,

based essentially on the analysis of the cyclic

voltammograms, galvanostatic measurements during

the electroplating, and linear polarization for corro-

sion study and X-ray diffractograms behaviour. Also

the effect of plating bath temperature on the alloy

composition and the morphology of the deposit were

studied.

2. Experimental

The electrodeposition of zinc, nickel and zinc

nickel alloys from sulphate bath has been conducted

under the examined conditions of bath temperature. A

detailed study has been made on the effect of the bath

composition and temperature on the cathode poten-

tials and cathode efficiency of zinc, nickel and zinc–

nickel alloy. Also the compositions of the alloys were

determined. The effect of these variables on the

morphology and the corrosion resistance of the

deposits were also investigated.

The electrolytic cell used for the present work was

of 100 cm3 capacity and contained three separate

compartments, two of them were used for fitting both

the working and the counter-electrodes. The third

compartment was used for fitting the reference

electrode, which is saturated Ag/AgCl electrode.

The platinum counter-electrode of large area (6 cm2)

was separated from the main bulk electrolyte by

means of C4 sintered glass disc to separate the anode

and the cathode compartments. Also, the reference

electrode was separated from the electrolyte via

sintered glass disc C4. The end of this electrode was

elongated into the Haber-Luggen capillary, which

placed at distance of 2pr (r is the radius of the Luggen

capillary) away from the working electrode surface.

Before each run the glass cell only was cleaned with

chromic/sulphuric acids mixture, but C4 sintered glass

with hot sulphuric acid, washed with first and second

distilled water. Then the cell filled with the 100 cm3 of

the electroplating solution at temperature 30.0 8C and

placed along the experiment in air thermostat to ensure

adjustment of temperature at 30.0 8C.

The electrolytes used for electrodeposition of Zn,

Ni and Zn–Ni alloys were freshly prepared using

Analar grade chemicals used without further purifica-

tion and dissolved in appropriate amount of doubly

distilled water. The pH of the standard bath used was

2.5. All experiments were duplicated and the

reproducibility for this type of measurements was

found to be satisfactory.

For electrodeposition of these metals and their

alloys, pure steel rod (99.98+%) of cross-sectional

area (0.196 cm2), in Teflon mount, in contact with

solution was used. Before each experiment the

electrode was mechanically polished with successive

grades of emery paper, degreased with ethyl alcohol,

rinsed with doubly distilled water and dried.

For electrochemical methods (cyclic voltammetric

behaviour, galvanostatic measurements and linear

polarization resistance technique) EG&G Potentio-

M.M. Abou-Krisha / Applied Surface Science 252 (2005) 1035–1048 1037

Table 1

Values of electrochemical corrosion measurements of the deposit on steel from a bath containing 0.20 M ZnSO4 (b and c), 0.20 M NiSO4 (a and

c), 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 10 mA cm�2 for 10 min at 30.0 8C

Deposit on steel (b) Zn only (c) Zn–Ni alloy (a) Ni only

ba (V decade�1) 1.878 13.99 23.48

bc (V decade�1) 4.265 0.9187 0.417

icorr. (A cm�2 � 10�3) 12.51 11.60 0.4388

Rp (kV) 0.1167 0.1233 1.295

Corr. rate (milli-inches year�1) 8987 8823 186.2

Ecorr. (corrosion potential/mV) �1000 �947 �383

stat/Galvanostat Model 273A controlled by a PC using

352 corrosion software was used.

The potentiodynamic cathodic polarization mea-

surements (the cathodic part of the cyclic voltammo-

grams) are conducted during the electrodeposition of

the parent metals and their alloys from the selected

baths on steel cathodes. The influence of the examined

conditions on the E–i profile was studied. The scan

rate in all experiments was 5.0 mV s�1. The Zn–Ni

alloys were stripped (the anodic part of the cyclic

voltammograms) under potentiodynamic conditions

in the same electrolyte from which they were

deposited. Previously, it was verified that no replace-

ment reaction between Ni2+ and Zn2+ took place [28].

The surface morphology of the deposit was

evaluated by a scanning electron microscope (JSM-

5500 LV, SEM, JEOL, Japan). X-ray diffractometry

(XRD) model D5000 Siemens diffractometer was

Table 2

Values of Ni and Zn amount in the deposit, total mass of the deposit, % N

thickness and electrochemical corrosion measurements of the deposit on s

H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 10 mA cm�2 for 10 min at di

Bath temperature 25.0 8C

Ni amount in the deposit (10�6 g) 42

Zn amount in the deposit (10�6 g) 324

Total mass of the deposit (10�6 g) 366

Ni content (%) 11.5

Zn content (%) 88.5

Ni Current efficiency, eNi (%) 11.8

Zn Current efficiency, eZn (%) 81.3

Zn–Ni deposit current efficiency, etotal (%) 93.1

Thickness of the deposit (mm) 2.54

ba (V decade�1) 2.165

bc (V decade�1) 3.711

icorr. (A cm�2 � 10�3) 16.46

Rp (kV) 0.1156

Corr. rate (milli-inches year�1) 9321

Ecorr. (corrosion potential/mV) �961

used to identify the phases of Zn–Ni alloys deposited.

The instrument is equipped with a copper anode

generating Ni-filtered Cu Ka radiation (l = 1.5418 A,

40 kV, 30 mA). An on-line data acquisition and

handling system facilitated an automatic JCPDS

library search and match (Diffrac software, Siemens)

for phase identification purposes.

The galvanostatic measurements of steel were

conducted by keeping the current at a constant level

10.0 mA cm�2 for 10 min, in the examined solution,

and the potential–time relation was plotted. In order to

determine the percentage composition of deposit, the

deposited was dissolved in 20 cm3 of 3 M HNO3, then

diluted with doubly distilled water up to 100 cm3. A

suitable diluted solution was then analyzed to

ascertain the Zn and Ni contents in the deposited

alloy using atomic absorption spectroscopy (Varian

SpectrAA 55). The Zn and Ni contents in the deposit

i and Zn content, current efficiencies % (Ni, Zn and Zn–Ni deposit),

teel from a bath containing 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M

fferent bath temperature

30.0 8C 35.0 8C 40.0 8C 50.0 8C

43 46 50 61

319 314 309 287

362 360 359 348

11.9 12.8 13.9 17.5

88.1 87.2 86.1 82.5

12.0 12.9 14.0 17.1

80.1 78.8 77.6 72.0

92.1 91.7 91.6 89.1

2.51 2.49 2.48 2.38

13.99 10.13 15.19 17.56

0.9187 0.6779 0.7759 0.3146

15.60 10.39 8.980 2.985

0.1233 0.1354 0.1461 0.1877

8823 9321 1778 1655

�947 �980 �935 �882


were confirmed by EDS (energy-dispersive X-ray

spectrometer) system with link Isis1 software and

model 6587 X-ray detector (Oxford, UK).

Using the resultant analysis, the film thickness and

the cathode current efficiency of the deposit from the

selected baths on steel were calculated. Also, the

thickness measured with scanning electron micro-

scope (cross-section) to confirm the calculated results.

The thickness of the deposited alloy layer is

estimated from the mass of deposit, the densities of Zn

(dZn = 7.14 g cm�3) and Ni (dNi = 8.90 g cm�3) and

the surface area (0.196 cm2). Using the following

equation

h ¼ mt=ðda � saÞThe thickness can be calculated as the height of the film

(h).mt is the total mass of the deposit, sa the surface area

and da the alloy density which equal (dZn(mZn/

mt) + dNi(mNi/mt)), where mZn the Zn amount in the

deposit, mNi the Ni amount in the deposit.

To measure the corrosion resistance of the

deposited, the linear polarization resistance technique

was used. In this technique, the steel with a Zn–Ni

coating, galvanostatically, on it was washed and

Fig. 1. E–i curves for steel in 0.20 M ZnSO4 (b and c), 0.20 M NiSO4 (a a

5 mV s�1 at 30.0 8C.

transferred into the electrolytic cell containing

100.0 cm3 of 0.025 M HCl in order to dissolve the

coating anodically. To begin the measurements, the

sample was introduced into the cell immediately after

electroplating, rinsed with double distilled water and

was allowed to reach equilibrium. The anodic

dissolution of deposits in a voltammetric mode was

conducted at a potential scan rate of 2 mV s�1. The

values of electrochemical corrosion measurements of

the coatings, Ecorr. is the corrosion potential, Icorr. the

corrosion current, Rp the polarization resistance, bc the

cathodic and ba the anodic Tafel constants and

corrosion rate, were obtained and represented in

Tables 1 and 2.

3. Results and discussion

3.1. Comparison between deposited Zn, Ni

and Zn–Ni codeposited

Potentiodynamic cathodic polarization measure-

ments of steel in bath solution at 30.0 8C in the

nd c), 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 and scan rate


Fig. 2. i–E curves (cyclic voltammograms) for steel in (a) 0.16 M H3BO3, (b) 0.40 M Na2SO4, (c) 0.01 M H2SO4 and (d) 0.01 M

H2SO4 + 0.40 M Na2SO4 + 0.16 M H3BO3, and scan rate 5 mV s�1 at 30.0 8C.

presence of Zn2+ or Ni2+ alone and Zn2+ + Ni2+

together were represented in Fig. 1. From this figure, it

is observed that the deposition of Zn starts at about

�1.14 V which takes the same shape and close to the

potential of Zn–Ni codeposition (about �1.12 V). On

the other hand, from the same figure it is obvious that

the Ni deposition starts at about �0.85 V and the

growth of deposited layer increases rapidly when the

potential shifts to more negative values. Also, the

polarization curve of the alloy deposition lies between

the polarization curves of each deposition Zn and Ni.

This position suggested that the codeposition enable

Zn to deposit at more positive potential (i.e. shifts the

deposition potential of Zn to less negative values) and

increase that of Ni to more negative ones due to the

presence of Ni2+ which facilitates Zn deposition [27].

The cathodic peak started at about �0.5 V may be due

to the hydrogen evolution, which increase as the pH of

the bath decrease. This peak is similar to that appears

in the absence of both Zn2+ and Ni2+, so this peak

probably appears mainly due to the presence of H2SO4

(Fig. 2). In the presence of Ni2+, this cathodic peak

was decreased, due to the surface effect occurs with

the addition of Zn2+ and/or Ni2+. This is ascribed to

that the addition of Zn2+ and/or Ni2+ (especially Ni2+)

decreases the adsorption of H+ and consequently

hydrogen evolution. This may be attributed to start the

competitive adsorption between Zn2+ and/or Ni2+ (or

its monovalent intermediate) and H+ [23–27].

In Fig. 3, cyclic voltammograms behaviour of steel

at 30.0 8C in the bath solutions were represented. This

figure has only one anodic peak at about �0.97 V

which corresponds to the anodic dissolution of Zn

deposited alone in the absence of Ni. Whereas two

anodic peaks were observed for Zn–Ni alloy deposit

corresponding to the formation of two phases (Fig. 4),

d-phase ((Ni3Zn22) the first dissolution anodic peak)

and g-phase ((Ni5Zn21) the second dissolution anodic

peak). The third peak corresponding to the dissolution

of Ni. The behaviour of the layer growth of the Zn

deposition in the cathodic direction is seems to be

similar to that of the Zn–Ni codeposition. Taking into

consideration that the potential of the anodic dissolu-

tion of the d-phase is more positive than that of the Zn

deposition, the dissolution of the d-phase in the

positive direction also relatively close to that of Zn


Fig. 3. i–E curves (cyclic voltammograms) for steel in 0.20 M ZnSO4 (b and c), 0.20 M NiSO4 (a and c), 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M

H3BO3 and scan rate 5 mV s�1 at 30.0 8C.

Fig. 4. XRD patterns of electrodeposited Zn–Ni on steel, obtained at potential �1.13 V holds for 15 min, from a bath containing 0.20 M ZnSO4,

0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 30.0 8C.


Fig. 5. E–t curves for steel in 0.20 M ZnSO4 (b and c), 0.20 M NiSO4 (a and c), 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 10 mA cm�2

for 10 min at 30.0 8C.

Fig. 6. log i–E curves for steel, plated from a bath containing 0.20 M ZnSO4 (b and c), 0.20 M NiSO4 (a and c), 0.01 M H2SO4, 0.40 M Na2SO4,

0.16 M H3BO3 at 10 mA cm�2 for 10 min at 30.0 8C, in 0.025 M HCl at 30.0 8C.


Fig. 7. E–i curves for steel in 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 and scan rate 5 mV s�1 at different

bath temperatures.

deposit alone. This can be understood from the fact

that the Zn–Ni deposit is mainly contained Zn. The

Zn–Ni codeposition started at about �1.12 Vand there

is no Ni cathodic peak at �0.85 V as mentioned

before. The reason for this behavior is ascribed to the

presence of Zn2+, which inhibits Ni2+ electrodeposi-

tion. Similar results were reported by Ohtsuka et al.

[29] during study of the initial layer formation of the

preferential Zn deposition during Zn–Ni electroplat-

ing using another bath nearly like the present bath.

In Zn–Ni codeposition bath, the deposited layer

does not grow at the potential �1.12 V (the cathodic

current decays with potential as seen in Fig. 3) which

corresponds to high overvoltage for the Ni deposition.

Also, in Zn electrodeposition, the cathodic current

peak decays with potential after the initial spike means

that the nucleus growth of the Zn deposition is

probably not induced at the potential of about

�1.14 V. In the Ni deposition bath in the absence

of Zn2+, however, the nucleus growth of Ni deposition

(the cathodic peak increases gradually with potential

as seen in Fig. 3) is induced at the smaller overvoltage

than the above potential. This means that the adsorbed

Zn2+ (or its monovalent intermediate) inhibits the

nucleus growth of the Ni deposition in the Zn–Ni

codeposition bath.

It is interesting to mention that the height of the

anodic peak (the peak current) of the deposited Zn

dissolution was higher than that in the case of the

codeposited Zn–Ni alloy dissolution. This means that

the amount of Zn in the alloy less than that in a single

metal deposit and gives an indication to the formation

of Ni in the alloy deposited.

Fig. 5 shows the potential–time dependence for the

deposition of Zn, Ni and Zn–Ni alloy on steel at

10 mA cm�2 for 10 min. It is clear that the deposition

of Ni needs low overpotential to create the initial

nucleus [30] and the deposit grow at low potentials.

The deposition of Zn takes place with higher

nucleation overpotential and grows at high potential.

The Zn–Ni codeposited at moderate overpotential; this

is due to the deposition of Ni is strongly inhibited by

the presence of Zn2+, while the deposition of Zn is

induced by the presence of Ni2+.

Linear polarization tests (Fig. 6) were done using a

steel-coated galvanostatically by pure Zn and Ni and


Zn–Ni alloy. From the measured corrosion potential

(Ecorr.) listed in Table 2, it is found that the anodic peak

of the alloy lies at more positive potential than that of

plated Zn alone, this indicate that the plated Zn–Ni

have high resistance relative to plated Zn only. It is

appear also that the corrosion rate and current were

decreased with the presence of Ni in the deposit, but

the polarization resistance increased. Thus, the

improvement achieved in the corrosion resistance of

the alloy deposits can explained by the presence of Ni.

Fig. 8. SEM photographs of electrodeposited Zn–Ni on steel from a bath

Na2SO4, 0.16 M H3BO3 at 10 mA cm�2 for 10 min at different bath temper

3.2. Effect of plating bath temperature

The influence of the bath temperature on the

potentiodynamic cathodic polarization curves was

studied as shown in Fig. 7. The E–i curves were

carried out for the electrodeposition of Zn–Ni alloy

at different plating bath temperatures. As shown

from these curves, in general, the potentials are

shifted to more positive direction with the bath

temperature from 25.0 to 50.0 8C. The temperature

containing 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M H2SO4, 0.40 M

atures: (a) at 25.0 8C, (b) at 35.0 8C, (c) at 40.0 8C and (d) at 50.0 8C.


Fig. 8. (Continued ).

rise is followed by increase in Ni content and

decrease in Zn content of the alloy as shown in

Table 2. In order to evaluate the effect of plating bath

temperature on the morphological characteristic of

Zn–Ni anomalous deposit alloy films, SEM was used

as shown in Fig. 8. From this figure, the micrographs

taken for film deposited at 25.0 8C (Fig. 8a), one can

see that the coatings are not uniform and contain a

large number of voids. Upon further increase in

plating bath temperature to 35.0 8C (Fig. 8b) no

major change in morphology is observed, but, the

compactness is increased and the voids are

decreased. Curiously, the most uniform and compact

deposit, which consists of many large grain size

films, is obtained at 40.0 8C (Fig. 8c). When the

plating bath temperature is raised to 50.0 8C (Fig. 8d)

there is a transition to a fine-grained structure and a

full surface coverage was obtained. It has already

been shown that the potentiodynamic cathodic

polarization curves depend on the electrolysis

temperature. Increasing the temperature from 25 to

50 8C activate nickel deposition, thus producing a

higher nickel content in alloys at 50.0 8C. Such

behavior is primarily the result of intrinsically slow

nickel kinetics. Also, this may be ascribed to the

redissolution of Zn deposited in the acidic medium

and to increase the diffusion process at elevated

temperatures [31]. Since the rate of dissolution of Zn


Fig. 9. XRD patterns of electrodeposited Zn–Ni on steel, obtained at potential �1.13 V holds for 15 min, from a bath containing 0.20 M ZnSO4,

0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 50.0 8C.

Fig. 10. i–E curves (cyclic voltammograms) for steel in 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 and scan

rate 5 mV s�1 at different bath temperatures.


Fig. 11. E–t curves for steel in 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at 10 mA cm�2 for 10 min at

different bath temperatures.

Fig. 12. log i–E curves for steel, plated from a bath containing 0.20 M ZnSO4, 0.20 M NiSO4, 0.01 M H2SO4, 0.40 M Na2SO4, 0.16 M H3BO3 at

10 mA cm�2 for 10 min at different bath temperatures, in 0.025 M HCl at 30.0 8C.


was much higher than that of Ni, it seemed that the

Ni content in the alloy layer increased at those

temperatures.

It is interesting to mention that the cathodic peak,

which probably due to the presence of H2SO4 and

started at about �0.5 V, increases with the temperature

rise. Also, appearances of new cathodic peak

approximately started at �0.87 V at the elevated

temperature. The position of this peak is Ni deposition

and may be attributed to the induced deposition of Ni

at this temperature. By analysis, the sharply increase

of Ni content at this temperature was detected [31].

The phases in the Zn–Ni alloys influenced by bath

temperature and were identified by X-ray diffraction,

which are represented in Fig. 9. The phases of the

deposits depend on the nickel content in the

electrodeposited alloy. Under the examined condi-

tions, the deposits obtained consisted of a mixture of

two phases (d and g) corresponding to the first two

anodic peaks which appear in cyclic voltammograms

(Fig. 10). At the elevated bath temperature, the g-

phase content increased and the d-phase decreased in

the deposit content (i.e. increase the corrosion

resistance). Also Fig. 4 shows that the deposit at

30.0 8C contains d-phase mainly, while at 50 8C(Fig. 9) contains mainly g-phase. These results agree

with others reported earlier data [32,33]. In the cyclic

voltammograms the two anodic peaks appear approxi-

mately at �0.97 V, shifted to more positive potentials

and its height decreases with increasing the tempera-

ture, and �0.67 V, which its height increases with

increasing the temperature. Indicating possibly that

the zinc dissolving from d and g phases gives rise to

the first peak and the second peak. The third peak

related the Ni dissolution, which increases with the

temperature rise. The cyclic voltammograms are

therefore a convenient spectrum-like diagram depict-

ing the various phases that are exposed during

corrosion of the deposit [32].

The galvanostatic measurements in Fig. 11 shows

that there is some potential trembling observed,

probably because bubbles of hydrogen blocked part of

the electrode surface [34]. Also inspection of the data

reveals that increasing the temperature of the plating

bath decreases the cathodic potential of the alloy

deposition. It is clear that the codeposition at 50 8C,

low overpotential is needed to create the initial

nucleus, but at 25 8C more overpotential is needed.

This is may be due to that the Ni2+, which needs low

overpotential to create the initial nucleus [30],

increases with temperature rise.

When the temperature maintained between 25.0

and 50.0 8C at current density 10 mA cm�2, deposits

with 11–18% nickel content were readily obtained and

current efficiency of the alloy (Table 2) decreased due

to the decrease of Zn content which represent the main

alloy component. Also, decreasing in the thickness of

the deposited layer follows these changes. This may be

due to the decrease of Zn content on the alloy which

posses the lower density (dZn = 7.14 g cm�3 and

dNi = 8.90 g cm�3).

Fig. 12 represented the anodic linear polarization

curves at different bath plating temperature. It is clear

from this figure and the measured electrochemical

corrosion values recorded in Table 2 that the corrosion

potential decreases with the temperature rise. Also, it

is clear that the polarization resistance of the deposit

increased with increasing Ni content of the alloy, but

the corrosion rate and current were decreased. Thus

the improvement achieved in the corrosion resistance

of deposits can be explained by the increases of Ni

content.

Thus, based on the results in Table 2, it is possible

to select an appropriate operating conditions to obtain

an optimum nickel concentration in the range of 11–

13% that provides best corrosion resistance for this

types of alloy on steel substrate [35,36].

4. Conclusions

The present study revealed that the Zn deposition

was observed to start at about �1.14 V, which close to

Zn–Ni codeposition (about �1.12 V). The Ni deposi-

tion started at about �0.85 V and the deposited layer

rapidly growth with increase of the negative potential.

Also, the polarization curve of the alloy deposition lies

between the polarization curves of the separate

deposition of Zn and Ni. This position suggested that

the codeposition enable Zn to deposit at more positive

potential and increasing that of Ni due to that the Ni2+

facilitates the Zn deposition.

It is clear that the deposition of Ni needs low

overpotential to create the initial nucleus and the

deposit grow at low potentials. The depositions of Zn

take place with higher nucleation overpotential and


grow at high potential. The Zn–Ni codeposited at

moderate overpotential; this due to the deposition of

Ni is strongly inhibited by the presence of Zn2+, while

the deposition of Zn is induced by the presence of

Ni2+. Also, the plated Zn–Ni had the best resistance of

the coating tested than plated Zn only. Thus, the

improvement achieved in the corrosion resistance of

alloy deposits can explained by the presence of Ni.

Inspection of the data reveals that increasing the

temperature of the plating bath decreases the cathodic

potential of the alloy deposition. It is clear also that the

codeposition at 50 8C low overpotential is needed to

create the initial nucleus but at 25 8C higher

overpotential is needed. This is may be due to that

the Ni2+, which needs low overpotential to create the

initial nucleus, increases with temperature rise.

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Electrochemical studies of zinc–nickel codeposition in sulphate bath

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