Surface Engineering

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Zn–Ni compositionally modulated multilayered

alloy coatings for improved corrosion resistance

Ramesh S. Bhat , P. Nagaraj & Sharada Priyadarshini

To cite this article: Ramesh S. Bhat , P. Nagaraj & Sharada Priyadarshini (2020): Zn–Ni
compositionally modulated multilayered alloy coatings for improved corrosion resistance, Surface
Engineering, DOI: 10.1080/02670844.2020.1812479

To link to this article: https://doi.org/10.1080/02670844.2020.1812479

Published online: 31 Aug 2020.

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SURFACE ENGINEERING
https://doi.org/10.1080/02670844.2020.1812479

Zn–Ni compositionally modulated multilayered alloy coatings for improved

corrosion resistance
a b
Ramesh S. Bhat , P. Nagaraj and Sharada Priyadarshinic
a
Department of Chemistry, NMAM Institute of Technology, Karnataka, India; bDepartment of Chemistry, Yenepoya Institute of Technology,
Karnataka, India; cDepartment of Chemistry, Sahyadri College of Engineering and Management, Karnataka, India

ABSTRACT ARTICLE HISTORY

Zinc–Nickel monolayer (single layer) and compositionally modulated multilayer alloy coatings Received 22 May 2020
were developed on to mild steel from acidic sulphate bath; and their corrosion performance Revised 22 July 2020
were studied using potentiodynamic polarization and electrochemical impedance Accepted 13 August 2020
spectroscopy methods. The coating layers were formed galvanostatically by single bath
KEYWORDS
technique, using square current pulses and triangular current pulses. For the best efficiency CMMA coating; SEM; square
of the deposits against corrosion, the switched cathode current density and the number of pulse; triangular pulse;
layers were optimized. Under optimized conditions, Zn–Ni multilayer alloy coatings were corrosion resistance;
produced at the current density 3.0/5.0 A dm−2 having 300 layers were ∼65 (square) and ∼48 monolayer
(triangular) times more corrosion resistant respectively than the single layer alloy coating of
the same thickness. Scanning electron microscopy has been employed to investigate, the
formation of multilayer alloy coatings as well as the morphology of the surface before and
after corrosion studies.

Introduction
Another such category of materials comprises a class
Electrodeposition, also known as electroplating or of materials commonly known as Compositionally
simply plating, is an inexpensive technique for protec- Modulated Multilayer Materials (CMMM). It consists
tive and improving the functionality of parts used in a of alternate thin (a few nanometers in thickness) layers
wide range of industries, including home appliances, of metal or alloy. The multilayered deposits may pos-
jewellery, automobile, aircraft/aerospace and electronics, sess exceptional and some time, special functional
electrical and tools for machinery items [1]. Metal coat- properties that cannot be achieved in normal metallur-
ings have been widely used in surface protection or for gical alloys. The compositionally modulated multilayer
decorative applications. Zinc and its alloy coatings find alloy (CMMA) coatings will increase the corrosion
numerous applications as sacrificial metal coatings [2]. resistance [13–16]. Such properties include Giant mag-
For several years, thick Zn coatings have been used to neto-resistance (GMR), enhanced hardness, wear and
provide economic protection for metal parts. Slowly corrosion resistance, optical x-ray properties, mag-
zinc alloys replaced the conventional zinc coatings, neto-optical properties, perpendicular magnetism,
because of their improved efficiency at elevated tempera- superconducting properties, etc. The most important
ture [3]. Iron group metals like nickel, cobalt, etc. alloyed requirement of CMMA coatings for displaying these
with zinc provides better protection efficiency than zinc properties is the thickness of each layer in the coating.
coating [2–5]. Zinc–Nickel alloy coatings can be It should be as thin as possible with well-defined
obtained by an electroplating process in an acidic or demarcation without diffusion of interlay. A periodic
alkaline bath. The Zn–Ni alloy coating, where the wt- change in each layer’s composition/structure can be
%Ni content is 12–16, shows the highest corrosion accomplished by bringing about periodic changes in
resistance [6]. For a range of parameters, it has been the deposition conditions. That is, by adjusting the
observed that the process takes place anomalously, phase of mass transfer with respect to changes in cath-
since zinc being less noble metal is preferentially depos- ode current density, temperature, etc. [17]. Nano-
ited. This may be explained by ‘hydroxide suppression layered materials are produced either by dry or wet
mechanism’, where zinc hydroxy compounds inhibit processes. In the dry process, there are numerous
the nickel deposition [7–10]. Vasilache et al. described methods for manufacturing CMMA coatings; physical
the mechanism of electrochemical deposition of nickel vapour deposition (PVD), chemical vapour deposition
and zinc–nickel alloy [11,12]. (CVD) [18], and electrodeposition (wet process) [19].
The materials with ultra-fine microstructure are In the electrolytic method, the two techniques
promising as products of new generation materials. used in CMMA coatings are single-bath (SBT) and

CONTACT Ramesh S. Bhat, rameshbhat@nitte.edu.in Department of Chemistry, NMAM Institute of Technology, Nitte 574110, Karnataka, India
© 2020 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute
2 R. S. BHAT ET AL.

dual-bath (DBT). In SBT, a single solution containing Table 1. The bath compositions and working conditions for
all the constituent ions required for the multilayer electrodeposition of Zn–Ni alloy.
deposition is used. During deposition, plating poten- Bath constituents Amount (g L−1) Operating parameters

tial/current is changed in an alternate manner with ZnSO4·7H2O 120 pH: 4.0

NiSO4·7H2O 100 Temperature: 303 K
or without variation of mass transfer towards the sur- C6H8O7·H2O 4.0 Anode: Pure zinc
face of the cathode. This method helps to control the CH3COONa·3H2O 70 Current density: 3.0 A dm−2
C12H17N4OSCl·HCl 0.6
quantitative composition of the alloys. In DBT, the
substrate is either manually or automatically trans-
ferred between the different plating solutions. During air drying. The bath components and working par-
DBT, there is not only increased amount of consump- ameters were designed using direct current (DC) for
tion of chemicals and water, but also there is chance Zn–Ni single layer deposition, following the Hull cell
that the oxide layer may be formed on the substrate method [23]. Thiamine Hydrochloride (THC) is one
surface [19–22]. of the B-complex vitamins. It is soluble in water. It is
The present work, report the synthesis of Zn–Ni a complexing agent and is generally well-matched
CMMA coatings from acidic sulphate bath onto MS with several metal ions. Compatibility depends on vari-
using square, triangular current pulses. The thiamine ables that are used such as pH, concentration, tempera-
hydrochloride (THC) was used as additive. The cor- ture and diluents. Citric acid (CA) is a crystalline
rosion performances are compared with its single colourless organic compound, serves as a buffer to
layer coating. The effect of square and triangular cur- balance the pH of the electrolyte. The composition
rent pulses on coating characters and corrosion beha- and operating condition of the baths were listed in
viours are discussed. The variation of wt-%Ni in the Table 1 for achieving homogeneous bright deposit on
coatings, is the key reason for better corrosion resist- to MS substrate.
ance of Zn–Ni CMMA coatings. An electrolytic glass cell, of 250 ml capacity was
used for deposition. The single layer and multilayer
coatings were carried out using DC power analyzer
Experimental method (N6705C, Key sight Technologies) for 10 min, for com-
parison purpose. Potentiostat/Galvanostat (CH-Instru-
Materials preparation
ments) with standard tri-electrode system was used to
A flat commercial grade MS or commercial steel speci- carry out electrochemical studies. The calomel (SCE),
men of approximately 4 cm × 2.5 cm area of the sheet platinum and MS were used as reference, auxiliary
was taken as cathode material and a sheet of zinc of and working electrodes respectively. The corrosion
the same area was taken as anode. Emery paper of characteristics of the coatings were conducted, in
fine grade was used for surface cleaning. Then the sur- 3.5% Sodium chloride solution by PP and EIS method.
face was washed using the solution of sodium carbon- A potential boundary of ±200 mV from open circuit
ate. The surface is descaled by dipping in 5% HCl for potential (OCP) was used for polarization study, at a
one minute at room temperature (303 K) followed by scan rate of 1 mV s−1. The frequency range, from
rinsing with deionised water and finally cleaned with 100 KHz to 10 MHz was used for EIS study with per-
trichloroethylene. The MS substrate and sheet of zinc turbing signal of 10 mV. The thickness of the coating
was purchased from Mangalore, a High-tech sales cor- was evaluated by Faraday law and cross examined by
poration. The electrolytic solutions were formulated digital thickness tester. Scanning Electron Microscopy
using laboratory grade chemicals and deionised (SEM), Model JSM-6380 LA from JEOL, Japan, in
water. Electroplating was very much dependent on the magnification of 1000× was used to observe the sur-
pH and time. The pH increases, the formation of face morphology, the layers formed during coating and
metal hydroxide deposited on the substrate resulting its deterioration after corrosion tests. The hardness of
increase in thickness and also the time for deposition the coating was measured using a micro hardness
changes, thickness also changes. Other factors like meter. The compositions of the coatings were calcu-
temperature, concentrations of electrolyte, applied lated using standard the Colorimetric method [24].
potential/current are directly influence the mor- Zn–Ni CMMA coating with alternatively different
phology, electrochemical behaviour, and particularly compositions were defined as: (Zn–Ni)1/2/n (where1
the coating thickness. So totally pH and time largely and 2 signify the two different switched cathode cur-
depends on thickness of the coating. The pH of the rent densities, and ‘n’ is the number of layers formed
plating solution was adjusted to 4 by adding required during the total plating period, i.e. 10 min). Zn–Ni
quantity of sulphuric acid. The experiments were car- CMMA coatings were developed and characterized
ried out under atmospheric conditions (303 K) and at with different configurations. All conditions of depo-
a magnetic stirring speed of 200 rpm with the placing sition were kept constant, except current density. Alter-
time of 10 min. The plated samples were cleaned natively, predetermined cathode current densities were
with water for 20 s after coating, which is followed by applied by proper power source set-up. Single layer and
 SURFACE ENGINEERING 3

wt-%Ni in deposit as shown in Table 2. It may be

ascribed to zinc and Nickel (dZn = 7.14 g cm−3 and
dNi = 8.9 g cm−3) in the deposit by inherent high den-
sity. But at very high CD the coating was very bulky
and porous, with decreased hardness. Thick and por-
ous deposit at high CD is due to the formation of
metal hydroxide caused by quick hydrogen evolution
during plating [25].
The polarization curves of various coating CD’s
were determined at their OCP, the corrosion CD’s
(icorr) and corrosion potential (Ecorr) were estimated
by extrapolation from the intercepts on the Tafel
Figure 1. Schematic representation of different power patterns slopes, obtained with respect to the calomel electrode.
used for the deposition of Zn–Ni single layer and multilayer The corrosive behaviour of Zn–Ni alloy coating in cor-
alloy coatings, using DC power Analyzer (N6705C, Keysight, rosive media (3.5 wt.% NaCl) is closely correlated with
Technologies, USA): (A) constant current for monolayer Zn-Ni the potential for corrosion. The corrosion resistance of
coating, (B) square and (C) triangular current pulse. the coatings relates primarily to structural morphologies
and chemical compositions [26]. The main cathodic
multilayer composition-graded coatings were pro- reduction of pure Zn and Zn alloy has been shown to
duced using DC, square and triangular current pulses, be the reduction of oxygen, which is the rate control
as shown schematically in Figure 1. stage for corrosion [27]. However, the superior cor-
rosion resistance of the deposits corresponds to the
more negative corrosion potential (Ecorr) values or
Results and discussion lower corrosion current (icorr) [28]. Table 2 shows that
Direct current electrodeposition of Zn–Ni alloy the Ecorr and icorr values of Zn–Ni coatings from sul-
phate bath. However, as the coating CD was increased,
The alloy of Zinc–nickel was deposited on MS using the icorr decreased, and more positive Ecorr were recorded
direct current in a galvanostatic mode at different cur- indicates that the corrosion rate is controlled more by
rent densities. The Ni contents of the deposit and cor- cathodic reaction showing that Zn–Ni alloy coatings
rosion resistance were studied. As the current density behave as sacrificial coating. It was observed that the
increases, the weight % of nickel also increases as deposit at 3.0 A dm−2 with ∼2.9 wt.%Ni (From Table
reported in Table 2. The corrosion resistance of the 2) showed least corrosion rate (4.99 µA cm−2). Hence
deposit increases with the increase in Ni content up it is inferred that at 3.0 A dm−2, added THC has reduced
to a certain extent and then at higher CD’s, the cor- the availability of free Ni+2 ions in the solution by proper
rosion resistance decreases due to porous surface struc- complexation, and improved the homogeneity of coat-
ture of the deposit. The changes in composition, ings and reduced the corrosion rate. But at higher coat-
thickness, hardness and appearance of the coatings ing CD, the icorr increased may be due to porosity of the
with deposition CD’s are listed in Table 2. The thick- deposit. Moreover, Abou-Krisha [29] found that
ness of the deposits was found to be increases substan- increasing the plating current density improved Ni con-
tially with applied CD’s as shown in Table 2. The linear tent in the coating and shifted the Ecorr toward more
dependency of thickness of the deposit with CD may be positive potential, which led to higher corrosion resist-
explained by the fact that, at very high CD, adsorbed ance properties.
metal hydroxide film at cathode (due to local decrease
of pH by evolution of hydrogen) likely to get occluded
in the crystal lattice of deposits [25]. Bath temperature
Mechanism of electrodeposition
has also prominent role on the thickness, composition
of nobler metal content (Ni) and appearance of the The mechanism of Zn–Ni alloy deposition on to MS
deposit as exhibited by other Zn–Fe group metal alloys. substrate, the chemical reactions that occur on the
The hardness of the coating was found to increase with cathode (MS) follow two steps, as described in

Table 2. Variation of wt.%Ni, VHN, thickness, and corrosion parameters of Zn–Ni coatings obtained at different CD’s.
-Ecorr icorr
CD’s (A dm−2) Wt.%Ni VHN Thickness (µm) V vs. SCE. (µA cm−2) Appearance of deposit
1.0 1.3 131 6.2 1.316 13.30 White
2.0 2.1 146 11.8 1.286 5.74 Grayish white
3.0 2.9 160 19.2 1.342 4.99 Bright
4.0 3.5 180 24.8 1.224 6.15 Bright
5.0 6.5 198 28.3 1.286 7.55 Porous bright
4 R. S. BHAT ET AL.

[11,12]. Zinc ions and Nickel ions are deposited on the in the coatings with a difference of 3.0 and
MS substrate. In addition, as Zn+2 ions combine with 5.0 A dm−2 between SCCD’s. These coatings were
hydrogen ion to form ZnH+, it must take into account found to be smooth, uniform and have been selected
the secondary reactions, in the same way that Ni+2 ions for studying the effect of layering. Thus, the same
combine with hydrogen to form NiH+. These inter- was taken as optimal SCCD’s for the production of
mediate species, formed during the adsorption process, Zn–Ni CMMA coatings. The wt-% Ni in the coating
finally decompose to form metallic Zn and metallic Ni, at 3.0 and 5.0 A dm−2 were found to be 2.9 and 6.5,
respectively. respectively.
The electrochemical reactions that occur could be
described as follows
Optimization of total number of layers for better
Ni+2 + e−  Ni+
ads corrosion resistance
Ni+ −
ads + e  Ni
The corrosion resistance and other properties of
Ni + H+  NiH+
ads CMMA deposits can be enhanced by increasing the
NiH+ + −
ads + H + 2e  Ni + H2 number of layers up to an optimum number provided
Zn+2 + e−  Zn+
ads the hold between the layers is not affected [33]. The
Zn+ −
ads + e  Zn
optimal SCCD was identified before (3 and
Zn + H+  ZnH+ 5 A dm−2), The CMMA Zn–Ni coatings with 10, 20,
ads
60, 120, 300 and 600 layers were developed. From
ZnH+ + −
ads + H + 2e  Zn + H2 Table 3, it is evident that as the number of layers
Ni+2 and Zn+2 are dissolved, hydrolysed or not, as met- increases up to 300, the corrosion resistance increases
allic ions. NiH+ +
ads and ZnHads which may or may not
and then decreased in both square and triangular
contain hydroxyl group are adsorbed monovalently pulses. The lowest icorr values observed for square
in intermediate reactions. Ni and Zn are the metallic and triangular current pulses are 0.07 and
deposits of nickel and zinc, respectively [30–32]. 0.10 µA cm−2 respectively. They are represented as
Same mechanism takes place in a multilayer coating (Zn–Ni) 3/5/300/square and (Zn–Ni)3/5/300/triangular as
of different SCCD’s to form different layers with differ- listed in Table 3. The configuration of CMMA (Zn–
ent compositions of Zn–Ni alloys. Hence corrosion Ni) 3/5/300/square is found to be maximum for the coating
resistance of multilayer deposits increases as compared system in order to achieve the high degree of resistance
with monolayer’s alloy coatings. towards the corrosion.
As the degree of layering increases (such as 600
layers), the corrosion resistance decreases in all current
pulses. This is since as the number of layers increases,
CMMA coating
solutes find less time to get redistributed in the diffu-
Optimization of switched cathode current sion layer [33]. Upon increase in the number of layers,
densities (SCCD’s) the time available for each layer to deposit is limited
(because the overall deposition period remains the
A small change in the concentration of metal ions, can
same). As a result, variation in the composition is not
affect the major changes in the nature of Zn-M (M =
likely to occur at a high degree of layering. In other
Ni, Co and Fe) CMMA coatings. Prabhu Ganesan et
words, CMMA coating tends toward monolayer, show-
al. deposited Zn–Ni alloy by potentiostatic method
ing less resistance to corrosion. As a result of interlayer
from SBT [22], where the Ni content varied by apply-
diffusion, CMMA coating having 600 layers and Zn–Ni
ing a varying potential as a function of the thickness of
the coatings. With this opportunity, modulation in Zn–
Ni CMMA coatings was attempted using various cur- Table 3. Corrosion data of multiple layer of (Zn–Ni) 3/5/square
rent pulses like square and triangular. A specific con- and (Zn–Ni) 3/5/triangular coatings, developed from the optimal
bath for the same length of time.
trol of SCCD’s has allowed the development of Zn–
(CCCD’s) -Ecorr icorr
Ni multilayer’s with different composition and there- (A dm−2) No. of layers V vs.SCE (µA cm−2)
fore different properties. The most important require- (Zn–Ni)3.0/5.0/square 10 1.255 2.11
ment for CMMA materials to exhibit improved 20 1.224 2.08
60 1.193 1.07
property is a simple layer demarcation; without diffu- 120 1.116 0.19
sion of inter-layers. To attain this, SCCD’s should be 300 1.081 0.07
600 1.108 4.76
carefully chosen before going for a higher degree of (Zn–Ni)3.0/5.0/triangular 10 1.100 3.78
layering. To start with, the coating with only 10 layers 20 1.149 2.40
60 1.194 0.77
was developed at different sets of SCCD’s in both 120 1.218 0.23
square and triangular pulses. Among the various sets 300 1.212 0.10
tried, the highest corrosion resistance was measured 600 1.197 5.96
 SURFACE ENGINEERING 5

Table 4. Comparison of corrosion data of (Zn–Ni)3.0, multiple

layers of (Zn–Ni)3.0/5.0/300/square, and (Zn–Ni)3.0/5.0/300/triangular
coatings developed from the optimal bath for the same
length of time.
-Ecorr icorr
Coating configuration V vs.SCE (µA cm−2)
Monolayer (Zn–Ni)3.0 1.32 4.99
CMA (Zn–Ni)3.0/5.0/300/square 1.08 0.07
CMA (Zn–Ni)3.0/5.0/300/triangular 1.21 0.10

monolayer alloy coating at 3.0 A dm−2 both the have

same corrosion resistance. The CMMA (Zn–Ni)3.0/5.0/
300/square and (Zn–Ni)3.0/5.0/300/triangular, however, dis-
played a minimum icorr values (0.07 and Figure 3. Comparative account of CR’s of monolayer Zn–Ni
0.10 µA cm−2) relative to the optimum Zn–Ni single alloy, multiple layers of (Zn–Ni)3.0/5.0/300/square and (Zn–N)3.0/
layer coating (Table 2). Accordingly, (Zn–Ni)3.0/5.0/ 5.0/300/triangular developed from the optimal bath for the same

300/square was obtained as the best CMMA coating

length of time.
configuration for superior corrosion efficiency.
with multilayer coating, it can be pointed that the Ecorr
values have been shifted towards anodic side (less nega-
Corrosion study tive potential), indicating that the multilayer coating
have passivated or nobler behaviour of the coatings [34].
Tafel analysis
A relative account of resistance towards corrosion of
The corrosion data reported in Table 4 reveals that the Zn–Ni CMMA deposits using square and triangular
corrosion resistance of Zn–Ni CMMA coatings is current pulses in relation to that of monolayer Zn–Ni
more than that of monolayer Zn–Ni coating obtained alloy coating (all under optimal conditions) in 3.5%
from the same optimized bath. It is due to advanced NaCl solution is shown in Figure 3.
material properties achieved through nanostructured
multilayer coatings, having micro-sized grain structures.
When the total number of layers increases, the corrosion Electrochemical impedance study
rate decreases in both square and triangular current EIS, also known as AC impedance spectroscopy, is an
pulses. Hence, for the enhanced resistance towards cor- effective method for obtaining useful evidence of the
rosion, the number of interfaces separating two distinct improved corrosion resistance [35]. In Nyquist plots,
compositions becomes significant. The potentiodynamic it is normal to plot the data as imaginary impedance
polarization curves of coatings obtained using direct versus real impedance with provision to distinguish
current; square and triangular current pulses are the contribution of polarization resistance (Rp) from
shown in Figure 2. On comparing monolayer coating the solution resistance (Rs). The impedance response
of monolayer and CMMA Zn–Ni coatings, developed
using unlike types of power patterns such as square, tri-
angular, etc. is shown in Figure 4. It is clear that the

Figure 2. Comparison of Tafel curves for monolayer (Zn–Ni)3.0 Figure 4. Comparison of EIS curves for monolayer (Zn–Ni)3.0
and multiple layers of (Zn–Ni)3.0/5.0/300/square and CMMA (Zn– and multiple layers of (Zn–Ni)3.0/5.0/300/square and (Zn–Ni)3.0/
Ni)3.0/5.0/300/triangular coatings developed from the optimal 5.0/300/triangular coatings developed from the optimal bath for
bath for the same length of time. the same length of time.
6 R. S. BHAT ET AL.

increase in the radius of the semicircle demonstrated Surface structure study

that Zn–Ni CMMA coatings display the better resist-
The surface morphology of single layer coating was
ance towards corrosion than monolayer (Zn–Ni)3.0
found to be very smooth, uniform and bright, as
coating. There exists a large reactance of an electrical
shown in Figure 5(a). Formation of single layer coating
double layer capacitor at the interface of the base
with no modulation in composition was confirmed
metal and the medium. Hence there is a large decrease
from its cross sectional view shown in Figure 5(b).
in the corrosion rate of the CMMA coating compared
The coatings were subjected to corrosion study at
to single layer coating. In the case of CMMA, pores of
±200 mV vs. OCP in 3.5% NaCl solution, in order to
one layer will be blocked by the next layers preventing
understand the corrosion mechanism; the corroded
corrosion. Alternate layers of alloys with low and high
specimens were washed with distilled water and exam-
wt-% Ni also contribute towards the increased cor-
ined under SEM. The image of Zn–Ni alloy coating
rosion resistance of Zn–Ni CMMA coatings.
(3 A dm−2), displaying the surface covered with cor-
rosion product is shown in shown in Figure 5(c). A
part of the corrosion products must have been
detached from the coating surface during the cor-
rosion, and it appears as pits. This indicates that the
corrosion followed the selective dissolution of low wt-
% noble metal (Ni) in the deposit.
The surface morphology of (Zn–Ni)3/5/10/square and
(Zn–Ni)3/5/10/triangular coatings, as shown in Figure 6
(a,b) was found to be comparatively smoother, uni-
form, bright and free of cracks. A cross-sectional
view of CMMA coatings (Zn–Ni)3/5/20/square and (Zn–
Ni)3/5/20/triangular reveals the formation of layers with
distinctive properties in Figure 7(a,b) respectively. In

Figure 5. Surface images of single layer Zn–Ni deposits: sur- Figure 6. Surface morphology of multiple layer coating sys-
face image of (Zn–Ni)3 coating (a), cross-sectional inspection tems: (Zn–Ni)3/5/20/square (a) and (Zn–Ni)3/5/20/triangular (b),
(b), surface after corrosion analysis (c) under optimal condition. under optimal condition.
 SURFACE ENGINEERING 7

Figure 7. Cross-sectional examination of multiple layer coat-

ings: (Zn-Ni)3/5/20/square (a) and (Zn–Ni)3/5/20/triangular (b) under
optimal condition.
Figure 8. SEM image of multiple layer coating systems after
alternative layers, the good contrast in the cross-sec- corrosion test: (Zn–Ni)3/5/4/square (a) and (Zn–Ni)3/5/4/triangular
tional view of multiple layer coatings may be due to (b) under optimal condition.
the small disparity in chemical composition of alloys.
The surface images of corrosion undergone Zn–Ni layer barrier effect, with high wt-%Ni (6.9) and Zn–
CMMA coatings with 6 layers (for better distinction) Ni layer sacrificial influence, with less wt-% Ni (2.9).
represented as, (Zn–Ni)3/5/6/square and (Zn–Ni)3/5/6/ Thus, it displays how alternate layers of varying differ-
triangular are shown in Figure 8(a,b). The inspection of
ent compositions are getting degraded during
the microscopic appearance of the surface allows corrosion.
understanding of corrosion mechanism; with potential
explanations for increased corrosion resistance. The
Conclusion
probability of high corrosion protection is due to the
fact that failures such as pores, crevices structure that The Zn–Ni single layer and CMMA coatings have been
occur in the case of a single layer deposited in the depo- developed from the optimized bath using DC, square
sition phase can be neutralized by the successively and triangular current pulses and following con-
deposited layers of the coating and thus the path of clusions are drawn:
the corrosion agents is no longer or blocked [18].
This is why multilayer coating takes more time for . The Zn–Ni CMMA coatings were developed
the corrosive agent to penetrate into the substrate through SBT using THC as an additive and cor-
material through coating defects than in the case of rosion behaviours were studied.
monolayer coating. In other terms, it extends or blocks . As the number of layers increases up to 300, the
the direction of the corrosive agents. Zn–Ni alloy layer, corrosion resistance goes on increasing. Beyond
with less wt-% Ni beneath the high wt-% Ni top layer 300 layers, corrosion resistance decreases due to
dissolves through the pores and micro-cracks existing the availability of less relaxation time for the
in the CMMA coatings existing during corrosion redistribution of metal ions at the dispersal
[36,37]. Overall protective efficiency of Zn–Ni layer. (Above 300 layers, it tends to become a single
CMMA coatings can be explicated by the Zn–Ni layer).
8 R. S. BHAT ET AL.

. The significant progress in the resistance towards sulfate–acetate baths. Surf Coat Technol. 2002;151–
corrosion of Zn–Ni CMMA coatings are attributed 152:444–448.
to gradually changing composition in alternate [10] Byk TV, Gaevskaya TV, Tsybulskaya LS. Effect of elec-
trodeposition conditions on the composition, micro-
layers due to gradually changing current densities structure, and corrosion resistance of Zn-Ni alloy
during deposition. coatings. Surf Coat Technol. 2008;202:5817–5823.
. The coatings ∼65 (Square current pulse) and ∼ 48 [11] Vasilache T, Gutt S, Sandu I, et al. Electrochemical
(Triangular current pulse) times better resistance mechanism of nickel and zinc-nickel alloy electrodepo-
towards corrosion than corresponding single layer sition. Recent Pat Corros Sci. 2010;2:1–5.
[12] Bard AJ. Electrochemical methods. Fundamentals and
alloy coating.
applications. New York: John Wiley and Sons; 2001.
. The formation of multilayer coating is confirmed by [13] Barrel G, Maximovich S. Preparation of composition-
SEM images. The enhanced protection of the modulated films by alternate electrodeposition from
CMMA is due to the blockage of pores by the suc- different electrolytes. J Phys Colloques. 1990;51:C4-
cessive layers. 291–C4-297.
[14] Rahsepar M, Bahrololoom ME. Corrosion study of Ni/
Zn compositionally modulated multilayer coatings
using electrochemical impedance spectroscopy.
Acknowledgement Corros Sci. 2009;51:2537–2543.
[15] Fei J, Wilcox GD. Electrodeposition of zinc–nickel
I thankful to the Principal, NMAM Institute of Technology, compositionally modulated multilayer coatings and
Nitte for providing the instrumental facilities. their corrosion behaviours. Surf Coat Technol.
2006;200:3533–3539.
[16] Fei J, Liang G, Xin W, et al. Surface modification
Disclosure statement with zinc and Zn-Ni alloy compositionally
No potential conflict of interest was reported by the author(s). modulated multilayer coatings. J Iron Steel Res Int.
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