Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 896-903
Published Online September 2012 (
Electrodeposition of Cyclic Multilayer Zn-Co Films Using
Square Current Pulses and Investigations on
Their Corrosion Behaviors
Ramesh S. Bhat1, A. Chitaranjan Hegde2*
1Nitte Mahalinga Adyanthaya Memorial Institution of Technology (NMAMIT), Nitte, India
2National Institute of Technology Karnataka, Surathkal, India
Email: *
Received May 13, 2012; revised June 21, 2012; accepted July 12, 2012
The cyclic multilayer alloy (CMA) coatings of Zn-Co have been developed galvanostatically on mild steel (MS), using
single bath technique. Depositions were carried out using square current pulses. Corrosion behaviors of the coatings
were evaluated by potentiodynamic polarization and electrochemical impedance method, supported by dielectric spec-
troscopy. The cyclic cathode current densities (CCCD’s) and number of layers were optimized for highest corrosion
stability of the coatings. The CMA coating developed at 3.0/5.0 A/dm2, having 300 layers, represented as
(Zn-Co)3.0/5.0/300 was found to exhibit ~40 times better corrosion resistance compared to monolayer coating, developed
from same bath for same time. Substantial improvement in the corrosion resistance of CMA coatings is attributed to
layered coating, having alternatively different phase structures, evidenced by XRD study. The formation of multilayer
and corrosion mechanism was analyzed using Scanning Electron Microscopy.
Keywords: CMA Zn-Co Coatings; Corrosion Resistance; Dielectric Spectroscopy; X-Ray Diffraction (XRD); Scanning
Electron Microscopy (SEM) Study
1. Introduction
The composition modulated multilayer alloy, or simply,
cyclic multilayer alloy (CMA) coatings, are basically
consisting of successive layers of alloys having two/or
more compositions [1]. They may be developed from a
single bath containing two/or metals ions, by proper
simulation of cathode current densities, using sophistica-
ted power sources [2-4]. CMA coatings consist of a large
number of thin alternate alloy layers, and each layer plays
its own distinctive role in achieving preferred performa-
nces. The development of zinc-based CMA coatings for
the protection of steel substrates has been investigated
recently [3-5], and were found to exhibit the enhanced
corrosion performance. Kirilova et al. reported the ano-
dic behavior of composition modulated Zn-Co multilayer,
electrodeposited from single and dual baths [6]. The
coatings were developed under different conditions of
current density, and their anodic behaviors were studied.
CMA coatings obtained from a single bath was found to
dissolve at potentials between the dissolution potentials
of pure Co and pure Zn coatings. Zinc and Zn-Ni alloy
CMA coatings were electrodeposited on to a steel subs-
trate by the successive deposition of zinc and Zn-Ni alloy
sub layers from dual baths [7]. The coated samples were
evaluated in terms of the surface appearance, surface and
cross-sectional morphologies, as well as corrosion resist-
ance. The layered structure and the existence of micro
cracks caused by the internal stress in the thick Zn-Ni
alloy sublayers were observed. CMA coatings were
found to be more corrosion-resistant than the monolithic
coatings of zinc or Zn-Ni alloy of same thickness. The
possible reasons for the better protective performance of
Zn-Ni/Zn CMA coatings were given on the basis of the
analysis of micrographic features of zinc and Zn-Ni alloy.
A probable corrosion mechanism of zinc and Zn-Ni alloy
CMA coatings was also proposed. The zinc sub layers
beneath the Zn-Ni alloy top layer was found to dissolve
through the pores and micro-cracks existing in the Zn-Ni
alloy deposits existing during corrosion.
Varieties of zinc and Zn-Co alloy CMA coatings were
electrodeposited onto steel substrates using dual bath
technique [8]. The experimental results showed that the
zinc and Zn-Co alloy CMA coatings were more corro-
sion-resistant than corresponding monolithic coatings of
same thickness. The development of zinc and zinc-alloy
based coatings was reviewed by Wilcox [9,10]. The
*Corresponding author.
Copyright © 2012 SciRes. JMMCE
R. S. BHAT, A. C. HEGDE 897
principle of productions of CMA coatings, concentrating
on their applications as protective coatings for metal
surfaces was examined. Zn-Ni, Zn-Fe, Zn-Co and Zn-Mn
alloy coatings was reported, to deposit in multilayer for-
mat. Electrodeposition methods, bath chemistries, coat-
ing morphologies, and their performances against corro-
sion were also reviewed. Most of the work reported
above, explains the development of CMA Zn-M (M = Ni,
Co and Fe) coating using double bath technique (DBT),
in which successive layers of alternating composition
was obtained from two separate electrolytes having ei-
ther pure Zn2+ and M2+ ions, or Zn2+ and (Zn2+ + M2+)
ions. The deposition conditions were optimized and re-
sults were discussed. The coating behaviors were eva-
luated either by their dissolution potentials or by Ecorr
values, without quantifying their corrosion rates. In this
direction, Thangaraj et al. [11,12] reported optimization
of an acid chloride bath for production of CMA Zn-Co
coating, showing the highest corrosion stability.
In this work, it is attempted to develop a stable bath
having thiamine hydrochloride (THC) as additives, and
to optimize the deposition conditions for peak perform-
ance of the coatings against corrosion. The corrosion ra-
tes were calculated from Tafel extrapolation method,
supported by electrochemical impedance method. The
role of THC, in modulation of composition was tried to
identify by cyclic voltammetry (CV) study. The optimi-
zation of deposition conditions, in terms of cathode cur-
rent densities and number of layers for development of
CMA Zn-Co coating and its characterization, concen-
trating on their application as protective coatings on MS
are reported.
2. Experimental
To begin with, the bath constituents and operating para-
meters, for deposition of monolithic (monolayer) coatings
of Zn-Co were optimized using direct current (DC),
following the standard Hull cell method [13]. THC is a
water-soluble B-complex vitamin, freely soluble in water
and is reportedly compatible with many metal ions. Com-
patibility is dependent upon factors such as pH, concen-
tration, temperature and diluents used. CA is a colour-
less crystalline organic compound, one of the carboxylic
acids. The optimal cathode current density, pH and con-
centration of each constituent were arrived, based on the
brightness (visual observation), homogeneity and adhe-
sion (tape test) of coatings. The bath composition arrived
is: 50 g/L ZnSO4·7H2O, 15 g/L CoSO4·7H2O, 60 g/L
CH3-COONa, 4 g/L C6H8O7·H2O (CA), and 0.5 g/L
C12H17N4OSCl·HCl (THC) and optimal pH and temper-
ature are respectively, 4˚C and 30˚C. The electrolyte was
prepared using LR-grade chemicals and distilled water.
A PVC cell of 250 cm3 capacity was used for electrop-
lating, with cathode anode space of ~5 cm. The anode
was pure Zn with the same exposed area. The pre-cleaned
mild steel (MS) panels with 7.5 cm2 active surface area
were used as cathode. All depositions (monolayer and
multilayered) were carried out using DC Power Analyzer
(AGILENT, N6705A), and subsequent electrochemical
characterizations were made using potentiostat/galva-
nostat (VersaSTAT3, Princeton Research). All coatings
were carried out for 10 min for comparison purpose.
After deposition, the electrode was rinsed with distilled
water, and used for further investigation.
Cyclic Voltammetry (CV) study was performed in a
conventional three-electrode cell to better understand the
kinetics of electrodeposition, and to identify the role of
additives, namely THC and CA. Pure platinum foil with
a surface area of 1 cm2 was used as working electrode.
Before each experiment, the electrode was activated by
immersion in 1:1 HNO3. The CV experiments were
conducted in a quiescent solution, without purging. Initi-
ally, three scan rates were evaluated viz. 10, 20 and 50
mV·s–1, in order to identify which scanning rate gives a
better readability of data. The choice fell on the 10
mV·s–1 scan rate and therefore the remaining tests were
carried out at this scan rate only. The scan began from 0
V in the positive direction, up to +0.5 V. Then, the scan
was reversed to the negative direction, down to –1.5 V,
and finally reversed back to +0.5 V. The phase structure
of the electrodeposits under different c.d. was analyzed
using X-ray Diffractometer (Bruker AXS) using CuKα-
radiation, (λ = 1.5405А, 30 kV). Scanning Electron Mic-
roscopy (SEM), Model JSM-6380 LA from JEOL, Japan,
was used to examine the multilayer formation and its
deterioration after corrosion. Generally, the direct current
resulted in the coatings having constant composition,
called monolithic coatings. But, periodic change in the
current density allowed the growth of layers on substrate
with periodic change in the chemical compositions [9].
i.e. Pulses of low current density results in layers of low
Co content, and pulses of high current density results in
layers of high Co content. The instrument was set to cy-
cle between two different cathode current densities,
called cyclic cathode current densities (CCCD’s) in re-
petitive way. While the thickness of the each layer was
controlled by the duration of each pulse, the composition
of each layer is decided by the current density applied.
The total number of layers was fixed appropriately by
adjusting the time for each cycle. Thus, CMA coatings of
different configurations were produced. Such multilayer
coatings are hereafter represented as (Zn-Co)1/2/n, where
(Zn-Co) represents alloy of Zn and Co, and 1 and 2 repr-
esent the cathode current density that is made to cycle
between, and “n” represents the total number of layers
Copyright © 2012 SciRes. JMMCE
formed during total deposition time (10 min).
The corrosion behavior of coatings was measured by
electrochemical DC and AC techniques, in a three-elec-
trode configuration cell. All electrochemical potentials
referred in this work are indicated relative to Ag/AgCl/
sat electrode. The 5% NaCl solution (30˚C) was used
as corrosion medium, throughout this study. Potentiody-
namic polarization behavior was studied in a potential
ramp of ±250 mV from open circuit potential (OCP) at
scan rate of 1 mV·s1. The corrosion rates of the coatings
were evaluated by the equation,
CR=0.0033 iμAcm
density of the alloygcm
where CR is the corrosion rate, in mm·y1 and icorr is the
corrosion current density. Electrochemical Impedance
spectroscopy (EIS) study was carried out in the fre-
quency limit from 100 kHz to 20 mHz. Before corrosion
study, the open circuit potential (OCP) for each test
specimen was fixed. This is accomplished by immersing
it in corrosion medium for 300 sec to reach equilibrium
potential. The chemical composition of the coatings was
determined by stripping the known amount of electrode-
posit into 2 N HCl, and wt% Co was estimated colori-
metrically [14]. While thicknesses of the coatings were
estimated by Faraday’s law, it was verified by measure-
ments, using a Digital Thickness Meter (Coatmeasure,
Model M & C).
3. Results and Discussions
3.1. Cyclic Voltammetric Study
CV is the universal electrochemical technique, used ei-
ther to understand the electrode kinetics or to elucidate
reaction mechanisms, including the quantitative analysis.
The technique consists of varying the electrode potential
in a linear fashion between two limits [15]. It was obse-
rved that THC in combination with CA has significant
effect on the homogeneity and brightness of the coatings.
Hence, the process and product of Zn-Co deposition was
studied by CV method, on adding CA and THC, indiv-
idually and in combination, into the electrolytic bath. In
absence of CA, the electrochemical oxidation curve
showed multiple peaks, as shown in Figure 1(a). This
corresponds to the dissolution of the metals in the alloy
via. different intermediate phases [16]. After adding CA,
a small change in the shape of the voltammogram was
found (Figure 1(b)). It indicates that CA has not involv-
ed in complexation of metal ions. Further, when THC
was added, the shape of voltammogram changed drasti-
cally, with one major peak at –0.80 V and one minor
peak at –0.30 V, corresponding to dissolution of alloys at
two different phases (Figure 1(c)). Lastly, when CA and
THC were added in combination, the intensity of the first
peak has increased with slight distortion to the right
(Figure 1(d)). Hence, it may be inferred that THC
improves the deposit character, by forming complex with
metal ions, and CA is acting as brightener, by getting
reinforced into crystal lattice by the action of THC [17].
3.2. X-ray Diffraction Study
The corrosion resistance of Zn-Co alloy depends practi-
cally on the wt% Co in the deposit, and consequently, its
phase structure depending on c.d. employed for its
deposition [18-23]. Hence, an effective modulation in
composition can be achieved by successive layering of
alloys having distinct phase difference, by proper selec-
tion of CCCD’s. This is accomplished by taking the XRD
patterns of Zn-Co alloy (monolayer) developed from the
same bath, at different current densities. Accordingly, the
XRD patterns of Zn-Co alloy at different c.d. are shown
in Figure 2. It may be observed that the intensity of the
peak corresponding Zn (110) increases progressively
with c.d. (from 2.0 - 5.0 A/dm2), and has reached maxi-
mum at 5.0 A/dm2. In addition to this, reflections corre-
sponding Zn (101) and Zn (110) have become more dist-
inct with increase of current density. Further, the peak cor-
responding to Zn (100) was observed as the current
density was increased from 2.0 A/dm2 to 5.0 A/dm2. Thus,
X-ray diffraction study clearly indicates that the variation
in cathodic current density brings significant change in
phase structure of coatings, allowing the better modu-
lation in composition, in conjunction with number layers.
Figure 1. Cyclic voltammograms for Zn-Co alloy bath, de-
monstrating the effects of THC. Working electrode: Pt,
pH = 4.0, T = 30˚C, v = 10 mV·s–1, (a) Without additive; (b)
With CA; (c) With THC; (d) With THC + CA.
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Copyright © 2012 SciRes. JMMCE
c.d. on wt% Co, thickness, corrosion rate (CR) and app-
earance of the coatings were reported in Table 1. The
corrosion data showed that the coating at 4.0 A/dm2 is
found to be bright, with least corrosion rate (7.08 × 102
mm·y1), compared to coatings at other current densities.
Hence, it has been taken as its optimal corrosion rate.
3.4. CMA Zn-Co Coating
3.4.1. Optimization of Cyclic Current Densities
The electrochemical deposition of CMA coatings was
accomplished by galvanodynamic cycling of the working
electrode between pre-set current ranges in an aqueous
solution containing Zn2+ and Co2+ ions. As mentioned
earlier, even a small change in the wt% of Zn-Co alloy
may result in significant properties change due to change
in the phase structure. Initially, multilayer alloy coatings
having 10 layers were developed at different sets of
CCCD’s to increase their corrosion resistance. Among
the various sets tried, the less corrosion rate was measured
in the coatings produced with difference of 3.0 A/dm2,
between CCCD’s (i.e. at 2.0/5.0 A/dm2 and 3.0/5.0
A/dm2). These coatings were found to be bright and
uniform. This combination of CCCD’s has been selected
for studying the effect of layering, as described in the
following section and in Table 2.
Figure 2. X-ray diffraction profiles of electrodeposits obta-
ined on MS from optimal bath at different current density
as mentioned on the plot.
3.3. Monolithic Zn-Co Coating
Wide spectrum of Zn-Co alloy formed on Hull cell panel
showed that current density (c.d.) plays an important role
in deciding the properties of the deposit. The effects of
Table 1. Effect of current density on the deposit characters of monolithic Zn-Co alloy.
c.d. (A/dm2) wt% Co Thickness
Vickers Hardness
Ecorr (V) vs.
icorr (µA·cm2)CR×102 (mm·y1) Appearance of the
1.0 0.39 8.2 152 1.295 6.57 9.64 Grayish white
2.0 0.46 10.6 185 1.302 5.768 8.46 Grayish
3.0 0.60 13.5 195 1.300 5.221 7.66 Bright
4.0 1.06 17.7 259 1.290 4.831 7.08 Bright
5.0 1.08 209 276 1.335 5.362 7.86 Porous
Table 2. Effect of overall number of layering on corrosion properties of Zn-Co CMA coatings obtained with 2.0 - 5.0 A/dm2
and 3.0 - 5.0 A/dm2 CCCD’s.
Coating configuration Number of
Deposition time for
each layer (sec)
Average thickness
of layer (nm)
Ecorr (V) vs.
Ag/AgCl/Clsat icorr/A·cm2 CR×102
10 60 2500
1.101 2.975 4.33
20 30 1250
1.086 1.804 2.63
60 10 417
1.151 1.019 1.48
120 5 208
1.112 0.789 1.15
300 2 83
1.116 0.234 0.34
600 1 41
1.110 5.697 8.36
10 60 2500
1.149 1.777 2.58
20 30 1250
1.197 1.286 1.87
60 10 417
1.195 0.512 0.75
120 5 208
1.196 0.276 0.40
300 2 83
1.198 0.108 0.16
600 1 41
1.194 3.51 5.15
3.4.2. Optimization of Overall Number of Layers
The properties of CMA electrodeposits, including their
corrosion resistance, may often to be improved by increas-
ing the total number of layers (usually, up to an optimal
number), as long as the adhesion between layers is not
deteriorated. Therefore, at the optimal combination of
current densities (3.0/5.0 A/dm2), CMA coatings with 10,
20, 60, 120, 300, and 600 layers were produced. As obs-
erved in Table 2, the corrosion rate decreased drastically
as the overall number of layers increased only up to 300
layers, and then increased. The lowest corrosion rate
(0.16 × 10-2 mm·y1) was observed for a multilayer
coatings, represented by (Zn-Co)3.0/5.0/300 configuration,
while for the same number of layers at CCCD’s of 2.0/5.0
A/dm2, the corrosion rate was 0.34 × 102 mm·y1 , which
is more when compared to the earlier. Hence, CMA
(Zn-Co)3.0/5.0/300 configuration, is taken as optimal. From
the total observed thickness (about 25 µm), the average
thickness of each layer in (Zn-Co)3.0/5.0/300 coating was
calculated. For this configuration, the average thickness of
each layer is found to be ~83.0 nm, as shown in Table 2.
The corrosion rate was found to increase at higher de-
gree of layering (i.e. >300 layers), shown in Table 2.
This may be attributed to the less relaxation time for re-
distribution of metal ions at the diffusion layer, during
plating [17]. The phenomenon may be explained as fol-
lows: During plating, metal ions from the bulk of the
electrolyte diffuse towards the cathode and to get dis-
charge as metal atom, and this process of diffusion is
mainly controlled by the cathode current density. As the
number of layers increased, the time for the deposition of
each layer, say, (Zn-Co)1 is small (as the total time for
deposition remains same, 10 min). At higher degree of
layering, there is no sufficient time for metal ions to re-
lax (against diffusion under given current density) and to
get deposit on cathode, with modulation in composition.
In other words, the cathode current density is cycling so
fast that ions cannot diffuse towards cathode with modu-
lation. As a result, at higher degree of layering modula-
tion in composition is not likely to take place, or CMA
deposit is tending towards monolayer with less corrosion
3.5. Corrosion Study
3.5.1. Tafel’s Polarization Study
Potentiodynamic polarization curves of CMA (Zn-Co)3.0/5.0
coating with different number of layers is shown in
Figure 3. Tafel extrapolation on such curves resulted in
determination of the corrosion potential, Ecorr corrosion
current density, icorr and corrosion rate, CR as listed in
Table 2. The progressive decrease of corrosion current
density (icorr) with number of layers indicated that imp-
roved corrosion resistance is due to layering of alloys,
having distinct properties. It may be observed that the
passivation current of Zn-Co coatings remained almost
same, regardless of the number of layers. This indicates
that the improved corrosion resistance is not due to the
corrosion product formed, but due to the delayed, or
blocked path of corrosion agent, due to layering, as will
be discussed later. Polarization curve shown in Figure 3
demonstrates that CMA coating with (Zn-Co)3.0/5.0/300
configuration is the most corrosion resistant.
3.5.2. Electrochemical Impedance Spectroscopy
Though potentiodynamic polarization technique is used
to study the kinetics of an electrode reaction, the result is
often corrupted by side effects such as the charging curr-
ents of the double layer observed on a time-scale of the
order of a millisecond, or by the ohmic drop associated to
the experimental setup [15]. The response of reversible
electrochemical systems studied in the presence of an oh-
mic drop unfortunately resembled the response of kin-
etically slow systems. The best way of differentiating the
kinetics of an electrode reaction from experimental side
effects is to use an excitation function covering a large
time domain. The most common of these techniques is
EIS where the electrode potential excitation function is a
sine wave of variable frequencies [24]. Accordingly, the
corrosion behavior of the coatings can be assessed, in
terms of electrical double layer (EDL) capacitor model,
with treating the substrate and medium as parallel plates,
and the coating in between them, as dielectrics of the
capacitor. When AC signal of small amplitude of ±10
mV is applied over a wide frequency limit are used, the
impedance data points obtained are the response of the
corrosion circuit, consisting of capacitance C, inductance,
Figure 3. Potentiodynamic polarization curves of CMA
(Zn-Co)3.0/5.0 coatings with different number of layers at
scan rate 1 mV·s1.
Copyright © 2012 SciRes. JMMCE
R. S. BHAT, A. C. HEGDE 901
L and resistance, R. The impedance data points obtained,
over the wide range of frequency is indicative of its
corrosion stability. Figure 4 shows the EIS Nyquist plots
of (Zn-Co)3.0/5.0 coatings, with different number of layers.
Impedance signals clearly indicate that the polarization
resistance, RP of the coatings increased progressively
with the number of layers (up to 300 layers), and then de-
creased (600 layers).
3.6. Dielectric Spectroscopy
The EIS data points can be used to calculate the relative
permittivity, εr of the coatings from film thickness, δ and
area, A and capacitance C, using the equation
where, ε0 is permittivity of the vacuum. Improved corro-
sion resistance of CMA coatings can be explained in
terms of the effect of time dependent electric field (i.e.
frequency response). Figure 5 shows the variation of
relative permittivity versus frequency, of the coatings
having different number of layers. It was observed that
the value of εr for all coatings is high at low frequency
which are diminished as the frequency is increased. At
low frequency side, the decrease of εr with increase of
number of layers indicates that the dielectric barrier of
coatings has increased with layering. This attributes to
the increased interfacial polarization effect, caused by the
heterogeneous media consisting of phases with different
dielectric permittivity [25]. There are many causes for
heterogeneity in materials, but concerning the CMA coa-
tings discussed in present work is related to interfaces
created by layering. Thus it may be summarized as the
peak corrosion resistance of CMA (Zn-Co)3.0/5.0/300 coat-
ing is due to the decreased permittivity of the coating.
Figure 4. Nyquist plots of CMA Zn-Co coating system with
(Zn-Co)3.0/5.0 configuration having different number of la-
3.7. Comparison between Monolayer and CMA
Zn-Co Coatings
On comparing the corrosion rate of (Zn-Co)3.0/5.0/300, giv-
en in Table 2, with that of monolayer Zn-Co alloy, at 4.0
A/dm2 (Table 1), it was found that the CMA coating is
~40 times higher corrosion resistant. Potentiodynamic
polarization behaviors of monolithic and CMA coatings
(under optimal conditions) is shown in Figure 6. A drastic
decrease of corrosion current, icorr was observed when the
coating pattern was changed from monolayer to multilayer
type. The high corrosion resistance of CMA coatings can
be envisaged as, the failures like pores, crevices occurring
in one layer is blocked or neutralized by the successively
deposited coating layers, and thus the corrosion agent’s
Figure 5. Relative permittivity of CMA (Zn-Co)3.0/5.0 coat-
ings with varying number of layers as function of fre-
Figure 6. Comparison of polarization behaviors of monolay-
er (Zn-Co)4.0 alloy and CMA (Zn-Co)3.0/5.0 coatings of same
Copyright © 2012 SciRes. JMMCE
path is delayed or blocked. The improved corrosion resis-
tance afforded by CMA coating can also explained in
terms of the formation of alternate layers of alloys with
low and high wt% of Co. In other words, the protection
efficacy of CMA Zn-Co coatings is due to barrier effect
of Zn-Co alloy layer with high wt% Co and the sacrifi-
cial effect of Zn-Co alloy layer with low wt% Co [8]. A
small compositional change in alternate layers, corre-
sponding to deposits at 3.0 and 5.0 A/dm2 has brought a
significant change in the phase structure of the alloys, as
evidenced by XRD study [26].
3.8. SEM Study
Surface morphology of monolithic alloy coatings, marked
as Figure 7(a), displayed a uniform and crack-free mor-
phology. Formation of alternate layers of alloys having
distinctive properties was confirmed by SEM. Cross sect-
ional view of (Zn-Co)3.0/5.0/10 is shown in Figure 7(b). The
poor contrast may be due to marginal difference in
chemical composition (0.60 wt% Co at 3.0 A/dm2 and
1.08 wt% Co at 5.0 A/dm2) of alloys in each layer. Ins-
pection of the microscopic appearance of surface, after
corrosion tests was used to demonstrate the formation of
successive layers of alloys, during deposition and to
understand the reason for improved corrosion resistance.
By subjecting the coating to dissolve by corrosion test, a
region displaying the layers, with distinction can be
obtained. This has been accomplished by the anodic
polarization up to +250 mV vs. OCP in 5% NaCl. Then
the corroded specimen was washed with distilled water
and examined, under SEM. Figure 7(c) exhibits CMA
(Zn-Co)3.0/5.0/10, after corrosion test. Hence, it may
inferred that the increased corrosion stability of CMA
coatings is due to successive layers of alloys having dif-
ferent degree of pores and cracks, which allowed the sel-
ective dissolution of metals in alternate layers with low
and high wt% of Co.
4. Conclusions
CMA coatings of Zn-Co have been developed on MS
by proper manipulation of cathodic current densities
and number of layers.
CV study demonstrated that THC improves the depo-
sit character by forming complex with metal ions, and
CA acts as brightener by getting reinforced into the
crystal lattice by the action of THC.
The XRD study revealed that the improved corrosion
resistance afforded by CMA coating is due to altern-
ate layers of alloys, having different phase structures.
The CMA (Zn-Co) coating, having 300 layers, depos-
ited at 3.0 and 5.0 A/dm2 found to show the least cor-
rosion rate (0.16 × 10–2 mm·y–1) compared to that of
monolithic alloy (7.08 × 10–2 mm·y1) of same thick-
Corrosion resistance of CMA coating increased with
number of layers only up to a certain optimal level
(300 layers) and then decreased.
Increase of corrosion rate at higher number of layers
is attributed to less relaxation time for redistribution
of metal ions (Zn2+ and Co2+) at the diffusion layer,
during deposition. In other words, at higher layering
the CMA coating tends to become monolithic.
SEM analysis confirms the formation of multilayer
during deposition, and evidenced the extended protection
by successively deposited alloy coating layers having
different degree of pores and crevices.
(c) Layers
Mild steel
Figure 7. SEM image of monolayer Zn-Co alloy deposit (a),
CMA (Zn-Co)3.0/5.0/10 coating (b); Cross sectional view after
corrosion test displaying the layer formation (c).
Copyright © 2012 SciRes. JMMCE
Copyright © 2012 SciRes. JMMCE
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