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Applied Surface Science 364 (2016) 264–272

Contents lists available at ScienceDirect

Applied Surface Science

 j ournal homepage : www.elsevier .com/ locate /apsusc

Microstructural, phase evolution and corrosion properties of siliconcarbide reinforced pulse electrodeposited nickel–tungsten compositecoatings

Swarnima Singha, M. Sribalajia, Nitin P. Wasekarb, Srikant Joshib, G. Sundararajanb,Raghuvir Singhc, Anup Kumar Keshria,∗

a Materials Science and Engineering, Indian Institute of TechnologyPatna,Navin Government Polytechnic Campus, PatliputraColony, Patna, Bihar 800013,

Indiab International Advanced Research Centre for PowderMetallurgy & NewMaterials(ARCI) Hyderabad, Balapur P.O., Hyderabad, AndhraPradesh 500005,

Indiac

CSIR-NationalMetallurgical Laboratory, Jamshedpur, Jharkhand 831007, India

a r t i c l e i n f o

 Article history:

Received 2 October 2015

Received in revised form

13 December 2015

Accepted 21 December 2015

Available online 23 December 2015

Keywords:

Pulse electrodeposition

Nickel–tungsten alloy

Silicon carbide

Surface morphologyPhase evolution

Corrosion

a b s t r a c t

Silicon carbide (SiC) reinforced nickel–tungsten (Ni–W) coatings were successfully fabricated on steel

substrate by pulse electrodeposition method (PED) and the amount of SiC was varied as 0 g/l, 2g/l, and

5 g/l in Ni–W coating. Effect of subsequent addition of SiC on microstructures, phases and on corrosion

property of the coating was investigated. Field emission scanning electron microscopy (FE-SEM) image

of the surface morphology of  the coating showed the transformation from the dome like structure to

turtle shell like structure. X-ray diffraction (XRD) of Ni–W–5g/l SiC showed the disappearance of (220)

plane of  Ni(W), peak splitting in major peak of  Ni(W) and formation of  distinct peak of  W(Ni) solid

solution. Absence of  (220) plane, peak splitting and presence of W(Ni) solid solution was explained by

the high resolution transmission electron microscopy (HR-TEM) images. Tafel polarization plot was used

to study the corrosion property of the coatings in 0.5 M NaCl solution. Ni–W–5g/lSiC coating was showed

higher corrosion resistance (i.e. ∼21% increase in corrosion potential, E corr) compared to Ni–W coating.Two simultaneous phenomena have been identified for the enhanced corrosion resistance of Ni–W–5g/l

SiC coating. (a) Presence of crystallographic texture (b) formation of continuous double barrier layer of 

NiWO4 and SiO2.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Nickel–tungsten (Ni–W) coatings have attracted significant

attentions in various industrial components like valves, pipes,

pumps, heat exchangers in automobiles, aerospace, energy and

petroleumindustries, largely due to theirexcellentcorrosionresis-

tant property [1–3]. In all these applications, Ni–W coatings are

more prone to harsh corrosive environments. Even, a minimal

corrosion in these coatings can lead to development of cracks or

pores resulting the breakdown of the components [4]. In order

to further improve the corrosion resistance of Ni–W coatings, a

number of ternary Ni–W–X composite coatings have been devel-

oped (where X= TiO2, SiC, BN, SiO2, MWCNT) [5–10]. Most of the

Ni–Wbasedcompositecoatings arefabricatedbyelectrodeposition

∗ Corresponding author.

E-mail address: anup@iitp.ac.in (A.K.Keshri).

methodbecauseof itsdensemicrostructure,higherdeposition rate,

reproducibility, easy operation, no post-deposition treatment and

lower cost [8,11]. Electrodeposition technique could be performed

with direct and pulse current, where the pulse electrodeposition

(PED) method has superior benefits on direct current electrode-

position. PED technique could provide the deposits with desired

compositions,optimized structure andporositybymodifying pulse

parameters, reduced bath additives, uniform/fine surface finish

with higher deposition rate [6,12,13]. In addition, PED technique

absorbs lesshydrogen, which leadstodensecoatingandultimately

results in increased corrosion resistant of the coating [6,12,13].

Several researchershaveutilizedPEDtechniqueandreinforcedvar-

ious reinforcement in Ni–W based compositecoating andreported

significant improvement in corrosion resistance of the coatings

[5,6,8,10,14].

Goldasteh et al. reinforced TiO2 in pulseelectrodeposited Ni–W

coating and evaluated the corrosion resistance in 0.5M NaCl solu-

tion [5]. Substantial improvement of ∼62% in corrosion resistance

http://dx.doi.org/10.1016/j.apsusc.2015.12.179

0169-4332/© 2015 Elsevier B.V. All rightsreserved.

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S. Singh et al./ Applied Surface Science 364 (2016) 264–272 265

was observed in Ni–W–TiO2  coating compared to Ni–W coating,

whichwasattributedtotwosimultaneousphenomena.Firstly,TiO2

particles acted as an inert physical barrier for initiation and devel-

opmentof corrosion in Ni–W matrix. Secondly, uniform dispersion

of TiO2 resulted the formation of microcells throughout thematrix

and hence, inhibits localized corrosion [5]. Similarly, Kumar et al.

reported relative improvement of ∼79% in corrosion resistance of 

pulse electrodeposited Ni–W–TiO2  coating in 3.5 wt.% NaCl solu-

tion [6]. Enhancement in corrosion resistance was attributed to

the uniform dispersion of TiO2   that prevents the dissolution of 

Ni–W by absorbing corrosive anodic sites [6]. Sassi et al. found

similar reason for the 99% improvement in corrosion resistance

of pulse electrodeposited Ni–W–SiO2   coating compare to Ni–W

coating [10].

In another study, Sangeetha et al. reported the 42% improve-

ment in corrosion resistance on reinforcement of boron nitride

(BN) in pulse electrodeposited Ni–W coating in 3.5wt.% NaCl solu-

tion [8]. Further, increased charge transfer resistance of ∼40%

was observed for Ni–W–BN than Ni–W after electrochemical

impedance spectroscopy (EIS) experiment. Increase in corrosion

resistanceandcharge transfer resistance was attributed to the uni-

form distribution of BN and formation of passive film over Ni–W

matrix.

Recently, Li et al. fabricated Ni–W–(0–5g/l) MWCNT (multi-walled carbon nanotube) composite coating using PED technique

and showed relative improvement of ∼78% in corrosion resistance

[14]. Further, charge transfer resistance of Ni–W coating increased

by∼22%on reinforcement of MWCNT. Significant enhancement in

corrosion resistance of Ni–W–MWCNT coating was attributed to

two parallel phenomena. Firstly, MWCNT couldfill thegaps, cracks

and micro holes of Ni–W coating and could act as physical barrier

to thegrowth of corrosiondefect. Secondly,uniform distributionof 

MWCNT could lead to uniform corrosionand inhibits the localized

corrosion [14].

In all of the above mentioned studies, effect of several rein-

forcement on corrosion property of pulse electrodeposited Ni–W

coating has been seen. Recently, silicon carbide (SiC) has attracted

considerable attentions as a potential reinforcing agent due to itsexcellentpropertiessuchas resistanceto thermal shock, oxidation,

andcorrosionathightemperatureandpressure[15], highhardness,

highthermal stability, and highchemical resistance [16]. Yao et al.

reinforced 10g/l SiC in Ni–W coating and fabricated the Ni–W–SiC

compositecoatingusingdirect currentelectrodeposition.Enhance-

mentof ∼60% incorrosion resistancewasfoundonadding 10g/l SiC

in Ni–W coating, which acts as an physical barrier to initiation and

development of corrosion [7]. In addition, SiCplayed an important

role in grain refining as well as in restricting the growth of nickel

grain [17,18].

However, there is scarcity of the work reporting the fabrica-

tion of Ni–W–SiC composite coating using “ pulse electrodeposition

method”. Further, effect of SiC on microstructure, phases and

corrosion resistance is completely unknown for the pulse elec-trodeposited Ni–W–SiC composite coating.

Motivated by this scenario, the focus of the present communi-

cation deals with the fabrication of the Ni–W composite coatings

with varying amount of SiC content using pulse electrodeposition

method. Microstructural and surface morphology of the coatings

are characterized by field emission scanning electron microscopy

(FE-SEM) and energy dispersiveX-ray spectroscopy (EDAX). Phase

evolution of the coatings with different amount of SiC content

are investigated by X-ray diffraction (XRD) and high resolution

transmission electron microscopy (HR-TEM). Corrosion behaviour

of the Ni–W–SiC composite coating in 0.5M NaCl solution is

evaluated by performing Tafel polarization. Post-polarization bar-

rier layer is identified using X-ray photoelectron spectroscopy

(XPS).

 Table 1

PulseelectrodepositionparametersfordepositingNi–W–SiCcoatingon plaincarbon

steel.

Current density (A/dm2) 10

Temperature (◦C) 75±1

Stirring speed (rpm) 300

Time (h) 1

Duty cycle 87%

pH 8.75

2. Experimental procedure

 2.1. Pulse electrodeposited Ni–W–SiC coating 

Pulse electrodeposited Ni–W coating with varying amount of 

SiC was obtained from the International Advanced Research Cen-

tre for Powder Metallurgy & New Materials (ARCI) laboratory,

Hyderabad, India. Two different compositions i.e. Ni–W–2g/l SiC

and Ni–W–5g/l SiC coating were deposited on plain carbon steel

(60mm×60mm×5mm). Pulse electrodeposited Ni–W coating

was used as a control sample. During the pulse electrodeposition

process, solution was magnetically stirred at the speed of around

∼300rpm in order to keep the SiC particles suspended in solu-

tion. Low carbon steel was chosen as cathode and Nickel as anode,distance between them was maintained at 3 cm. Pulse electrode-

positionparameters fordepositing thecoatinghave been shown in

Table 1.

 2.2. Microstructural and phase characterizations

Cross-sectional andsurface morphology of the pulse electrode-

posited coatings were examined using a field emission scanning

electronmicroscopeofmakeS-4800,HITACHI,JapanandX-Max50,

Oxford Instruments,UK respectively.EnergyDispersiveX-raySpec-

troscopy(EDAX)attachedwith FE-SEM (S-4800)wasusedforX-ray

mappingof Ni–W–2g/lSiCandNi–W–5g/l SiCcoatings. Inaddition,

surface morphology of Ni–W, Ni–W–2g/l SiC and Ni–W–5g/l SiC

coatingsafterpolarizationin0.5M NaClsolutionwere examinedbyusing optical microscope (BX51, OLYMPUS, Japan). Phase analysis

was performed on Ni–W, Ni–W–2g/l SiC and Ni–W–5g/l SiC com-

posite coatings using X-ray diffraction (TTRAXIII, Rigaku, Japan)of 

CuK   radiation of 1.54 A wavelength. The scanning range and the

scanningrate were maintained as 20–80◦ and 2◦/min respectively.

Further, high resolution transmission electron microscope (HR-

TEM) (JEM 2100, JEOL, Japan) operating at an accelerating voltage

of 300kV was used to validate the phases in Ni–W–SiC composite

coatings.

 2.3. Corrosion test 

Theelectrochemical investigationof thecoatingwasperformed

in0.5M NaClsolutionby usingPotentiostat/Galvanostat (Reference600, Gamry, UK). The electrochemical measurements were done

using conventional three electrode system at room temperature.

Thepulseelectrodeposited coatingsactsas workingelectrodewith

the exposure area of 1cm2, whereas saturated calomel electrode

(SCE) and graphite electrode was used as reference and counter

electrodes respectively. All the three coatings were cleaned using

acetone and then cleaned by distilled water before conducting

electrochemical measurements. The open circuit potential (OCP)

measurement was performed on all the coatings in 0.5M NaCl

solution for1 hour toattaina stabilized potential. Theanodicpolar-

ization test was carried outby scanningthesamples from cathodic

region −0.25V to anodic region 2V at scan rate of 100mV/min,

with respect to OCP condition. Echem AnalystTM software (Gamry

instruments,USA) is beingused forthe estimation of corrosionrate

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266 S. Singh et al./ Applied Surface Science 364 (2016) 264–272

of all three coatings, which uses the standard Tafel method (ASTM

standardG102-89) for calibrating it. This standard methoduses an

Eq. (1), derived from the Faraday’s law, as shown below.

Corrosion rate=K 1icorr EW

d  (1)

where, icorr   is the corrosion current density in A/cm2 which was

calculated by drawing a tangential line on the anodic as well as

on the cathodic branches of tafel plot. EW is the equivalent weight

of working electrode in grams/equivalent, d denotes thedensity in

g/cm3 andK 1 is theunitforcorrosionratewhich is1.288×105 mpy.

 2.4. X-ray photoelectron spectroscopy (XPS) analysis of polarized

Ni–W–SiC coating 

X-ray photoelectron spectroscopy (XPS) (SPECS, Germany)

using Al monochromator (binding energy: 1486.6eV) and Mg-K(binding energy: 1253.6eV) source, operating at a vacuum more

than 1×10−9 Torr was used to investigate the probable forma-

tion of barrier layer in polarized Ni–W–SiC coating. The survey

spectra was carried out from 0 to 1000eV to know the elements

present in polarized Ni–W–SiC composite coating with pass ener-

gies of50eV. Theslowscan were carriedoutfor Ni2p (900–830eV),

W4f  (45–24eV), Si2p (115–85 eV), C1s (300–270eV) with pass

energy of 25eV. The deconvolution of XPS spectra was done using

Computer Aided Surface Analysis (CASA) XPS software and Ni2p,

W4f, Si2p peak were calibrated by C1s peak at binding energy

of 284.6eV.

3. Results and discussion

 3.1. Microstructural characterization

Fig. 1(a–c) represents the cross-sectional FE-SEM images of 

Ni–W,Ni–W–2g/lSiCandNi–W–5g/l SiCcoatingsrespectively. The

cross-sectional images revealed that coating was dense and crack

free. Coating thickness of Ni–W was found to be ∼23m which

didnot change on addition of 2g/l SiC particles. However, addition

of 5g/l SiC particles in the Ni–W coating significantly reduced the

coating thickness from∼23m to∼5m. Thisdrastic reductionof 

∼78% in coating thickness could be attributed to the incorporation

of relatively higheramount of SiCparticles that mightacts asphys-

ical barrier and hinder the rate of the grain growth. Borkar et al.

found the similar trend (i.e. 42% reduction) in coating thicknesson

addition of Al2O3   (40 g/l) in electrodeposited Ni. They attributed

the reduction in coating thickness to grain growth hindrance by

Al2O3 particle [19].

Fig. 2(a–c) illustrates the FE-SEM images of the surface mor-

phology of Ni–W, Ni–W–2g/l SiC and Ni–W–5g/l SiC coatings

respectively. The surface morphology of all three coatings shows

the distinct features. As-deposited N–W coating exhibits the

equiaxed dome like structure which changed to coarse turtle shell

like structure on adding 2g/l SiC and further changes to finer tur-

tle shell like structure upon subsequent addition of 5g/l SiC. Few

researchers havereported themicrostructural transformation, due

to reinforcement of ternary element in Ni based matrix [5,17,20].

However, the reason for this kind of transformation has not been

reported.

This change in surface morphology could be attributed to the

uniformdistributionofSiCovertheNi–Wmatrixwhich accelerates

the grain refinementprocess. Fig. 3(a and b) shows the X-ray map-ping of the surface of Ni–W–2g/l SiC and Ni–W–5g/l SiC showing

theuniformdistributionofSi particlesover Ni–Wmatrix.The effect

of theseuniformly distributedSiCparticleson the grain refinement

process could be understood using the schematic.

Fig. 4 is the schematic showing the step-by-step grain refine-

ment process on adding SiC. Fig. 4(a) explains the formation of 

equiaxial dome like structure in Ni–W coating due to the homo-

geneous nucleation of Ni–W grains. On adding 2g/l SiC particles

to the solution, equiaxial dome like structure transformed into

coarse turtle shell like structure which might be the resultant of 

twoopposingphenomena.Firstly,SiC actsas nucleation siteswhich

favours the nucleation of more Ni–W grain. However, the growth

of these nucleated Ni–W grain could be restricted by the SiC itself,

Fig. 1. represents thecross-sectional FE-SEMimages of pulse electrodeposited (a) Ni–W, (b)Ni–W–2g/l SiCand (c) Ni–W–5g/l SiC coatings.

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268 S. Singh et al./ Applied Surface Science 364 (2016) 264–272

Fig. 4. The grain refinement mechanisms in (a) Ni–W, (b) Ni–W–2g/l SiC and (c)

Ni–W–5g/l SiC coatings.

Firstly, low intensity peak of Ni(W) at 50◦ and 75.08◦ disappeared

in Ni–W–5g/l SiC composite coating, which might be due to the

reductionof Ni content in the coating asa resultof relatively higher

amount of SiC addition.

Secondly, peak splitting between 43◦ to 45◦ in Ni–W–5g/l SiC

coating. Goldasteh et al. also found the similar dominant (200)

W(Ni) solid solution peak emerged from the (111) Ni peak result-

ing thepeaksplittingin pulseelectrodeposited Ni–W–TiO2 coating

[6]. However,mechanismofpeaksplittingis notmentionedin their

study.

Peak splitting might bedue to the strain in the latticesof Niand

W due to the addition of relatively higher amount of SiC. In order

toconfirm this,micro-strainhas been calculated forNi–W–2g/lSiC

and Ni–W–5g/l SiC coating with using Eq. (2).

ε =ˇ

4tan    (2)

where, ε isthemicro-strain,ˇ isbroadeningduetodeformationand

 is theBraggangle,whichhasbeentakenfromtheX-raydiffraction

pattern of both the coating. Micro-strain was found just double

i.e. 2.255 for Ni–W–5g/l SiC compared to Ni–W–2g/l SiC which is

1.205. Excessive amount of strain in the Ni–W–5g/l SiC might be

the reason for the peak splitting in Ni(W) phase.

Fig.5. X-ray diffraction patternsof pulseelectrodeposited(a) Ni–W,(b) Ni–W–2g/l

SiC and (c) Ni–W–5g/l SiC coatings.

Fig. 6. (a)The HR-TEMimages of pulse electrodeposited Ni–W–5g/l SiC composite

coating (b) magnified image of (a) showing the presence of SiC crystallites, Ni(W)

and W(Ni). IFFT (Inverse Fast Fourier Transformation) image confirmed the lattice

spacing of SiC crystallites, Ni(W) and W(Ni).

Thirdly, an additional peak observed for W(Ni) in Ni–W–5g/l

SiC composite coating. This could be due to the addition of higher

amount of SiC which leads to segregation of Ni rich i.e. Ni(W) and

W rich i.e. W(Ni)phase. Signatureof W(Ni)hasbeen seenin diffrac-

tionpatternofNi–W–5g/l SiC,which might have significant impact

on corrosion behaviour of coating. Further, the formation of W(Ni)

phaseonadditionofSiCcanbereconfirmed bytheHR-TEM images.

Fig. 6(a) displays the HR-TEM image of Ni–W–5g/l SiC coating,

whereblackpatches areobservedthroughoutthe coating. A higher

magnification imageon theregionshown in Fig. 6(a) clearly shows

the presence of lattice fringes in black patches. The Inverse Fast

Fourier Transformed (IFFT) imageof blackpatches showeda lattice

spacing of 0.25nm that corresponds to (102) plane of SiC (JCPDScard no.: 00-049-1428) crystallites. The IFFT images of the other

markedregionsindicatedinFig.6(b),confirmsthepresenceof(111)

plane of Ni(W) solid solution as well as (200) and (220) planes

of W(Ni) solid solution, which matches well with the diffraction

pattern ofNi–W–5 g/lSiC coating (Fig. 5c). This indicatesthesegre-

gationofNi(W) andW(Ni)phaseon incorporation of SiC. However,

the lattices of these W(Ni) and Ni(W) planes was found distorted,

which might be due to the lattice disturbance caused by the incor-

poration of relatively higher amount of hexagonally close packed

(HCP)SiC particlesin FCCNi(W) lattices.Around theSiCcrystallites,

existence of amorphous region was observed (Fig. 6b), whichis in

accordance with the broadness of spectra in diffraction pattern as

observed in Fig. 5(c).

 3.3. Corrosion behaviour 

Corrosion behaviour of all pulse electrodeposited coatings was

evaluated by the corrosion potential (E corr) and corrosion current

density (icorr) values afterpolarizationof eachcoating. Further, cor-

rosion behaviour was analyzed by estimating the corrosion rate of 

all three coatings.

The anodic polarization curves of Ni–W, Ni–W–2g/l SiC and

Ni–W–5g/l SiC coatings were obtained in 0.5M NaCl solution as

shown in Fig. 7(a–c) respectively. The E corr   and icorr   values were

obtained by extrapolating the anodic and cathodic branches using

Tafel method and tabulated in Table 2. It is evident from Table 2,

with increasing the SiC content in Ni–W composite coating, icorr

value decreased. Further, E corr   showed an increasing trend with

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S. Singh et al./ Applied Surface Science 364 (2016) 264–272 269

Fig. 7. The anodic polarization curvesof pulse electrodeposited Ni–W, Ni–W–2g/l SiC, Ni–W–5g/l SiC coatings.

increasing SiC content and this was found higher (−0.479 V) for

the Ni–W–5g/l SiC composite coating. Total increase of ∼21% in

E corr  was observed on reinforcement of 5g/l SiC in Ni–W coating.

Relatively higher E corr and lowest icorr indicates the enhanced cor-

rosionresistance of Ni–W–5g/l SiC coating compared to Ni–W and

Ni–W–2g/l SiC coatings.

Corrosion rate of all three coating was estimated and tabu-lated in Table 2. Ni–W coating shows the highest corrosion rate of 

5.639mpy,whichreducedto 5.427mpy forNi–W–2g/l SiCand fur-

ther reduced to 4.698mpy for Ni–W–5g/l SiC coating. This clearly

indicates the effect of reinforcement of 5g/l SiC in Ni–W coating

which lowered the corrosion rate by∼17%.

Enhanced corrosion resistance of Ni–W–5g/l SiC was majorly

been attributed to the two simultaneous phenomena (a) presence

of crystallographic texture i.e. (200) of W(Ni) and (b) formation of 

barrier layer after polarization. Several researchers have reported

that a grain with preferred orientation shows higher corrosion

resistance [22,23]. The pronounced orientation in Ni–W–5g/l SiC

coating was found to be (200) texture of W(Ni) as seen previ-

ously in X-ray diffraction pattern (Fig. 5c) and HRTEM (Fig. 6). This

was reconfirmed by carrying out the texture content calculationon (200) and (111) textures of Ni–W–5g/l SiC coating using the

following Eq. (3) [22].

C (200)  =1

1+ 1.25I (111)

I (200)

(3)

 Table 2

summarizes the icorr and E corr after polarization, corrosion rate and inhibition effi-

ciency of pulseelectrodeposited Ni–W–(0–5g/l) SiC coating.

Sample nomenclatures E corr (V) icorr (A/cm2) Corrosion rate (mpy)

Ni–W −0.582 12.6×10−6 5.639

Ni–W–2g/l SiC −0.574 12.2×10−6 5.427

Ni–W–5g/l SiC −0.479 10.9×10−6 4.698

where, I (111) and I (200) are the intensities of (111) and(200) diffrac-

tion peaks respectively. It is evident from this calculation that the

extentof preferredorientation of (200) texture present Ni–W–5g/l

SiC was 18% higher than the (111) texture. The higher preferred

orientation of (200)plane couldstrongly attribute to theenhanced

corrosion resistance of Ni–W–5g/l SiC coating. Wang et al. did

the laser surface treatment of 321 austenitic steel and found thehigher preferred orientation of (200) textures, which contributed

towards improved pitting corrosion resistance [22]. Venegas

et al. also reported the similar results in agreement of Wang

et al. [23].

In order to further investigate, the pitting corrosion resistance

was evaluated after polarization of all three coatings using an

optical microscope. Fig. 8(a–c) shows the pits formed in Ni–W,

Ni–W–2g/l SiC and Ni–W–5g/l SiC coatings after polarization in

0.5M NaCl solution. The average pit size of Ni–W coating was

observed to be∼326m (Fig. 8a), which reduced to∼265m (i.e.

∼18% reduction) on adding 2g/l SiC in Ni–W matrix (Fig. 8b). Fur-

ther, addition of 5 g/l SiC to Ni–W matrix has significantly reduced

the pit size to an average value of ∼50m (i.e. ∼84% reduction)

compared to Ni–W coating. The large reduction in the pit size of 5 g/l SiC coating might be due to the presenceof pronounced (200)

texture because of the SiC particles. Shi et al. also reported the

restrictions in the growth of corrosive pits due to the presence of 

SiC particles [20].

Another reason responsible for higher corrosion resistance of 

Ni–W–5g/l SiC coating could be the formation of barrier layer

over the coating. Many researchers claimed the presence of oxide

based barrier layer as significant factor forenhancingthe corrosion

resistanceof electrodeposited coatings[1,10,24]. In order tounder-

stand the barrier layer formation, XPS analysis was performed

on the polarized coating having least amount of SiC (Ni–W–2g/l

SiC).

Theformationof NiWO4 andSiO2 layerwas experimentallycon-

firmed by performing XPS analyses on polarized Ni–W–2g/l SiC

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S. Singh et al./ Applied Surface Science 364 (2016) 264–272 271

Fig. 10. The corrosion mechanisms of pulse electrodeposited (a) Ni–W, (b) Ni–W

2g/l SiC, (c) Ni–W 5g/l SiC coating.

XPSspectra confirmed theformationofNiWO4 and SiO2 barrier

layer whichcanenhance thecorrosionresistance.Formationofbar-

rier layer on all these three coatings could be understood using a

schematic as shown in Fig. 10.Fig. 10(a) is the schematic showing the formation of discon-

tinuous NiWO4   layer in the Ni–W coating. Discontinuous layer of 

NiWO4 mightbe attributed topresenceof the coarsergrainin Ni–W

coating as discussed earlier. Few literatures have reported the for-

mation of uniform barrier layer to fine grained microstructures

and vice versa [28,29]. This discontinuous NiWO4   barrier layer

contributes towards enhanced corrosion resistance of the Ni–W

coating.

Fig. 10(b) is the schematicshowingthe formationof NiWO4 and

SiO2   layer in the Ni–W–2g/l SiC coating. Since, grain is relatively

finercomparetoNi–W, thefilmformation could starttransforming

fromdiscontinuous tothecontinuous layerresultingenhancedcor-

rosion resistance of Ni–W–2g/l SiC compared to Ni–W coating.

Fig. 10(c) is the schematicshowing theformation ofdoublebar-

rier layer i.e. NiWO4  and SiO2 layer in the Ni–W–5g/l SiC coating.

Since,grain ismuch finer compare toNi–W andNi–W–2g/l SiC, the

film formation could be almost continuous stopping the penetra-

tionofCl− ioninside thecoating,resulting thedrastic enhancement

in corrosion resistance of Ni–W–5g/l SiC compared to Ni–W and

Ni–W–2g/l SiC coatings.

Apart from the presence of crystallographic texture and forma-

tion of barrier layer,micro-cell also forms on addition of SiC. Since,

standard potential of SiC was more positive than Ni–W, SiC act as

a cathode and Ni–Wact as ananode. So, SiC particles might help in

inhibiting thelocalizedcorrosion.Also,additionof relativelyexcess

amount of SiC particle might help in lowering the metallic area

required for corrosion, leading to enhanced corrosion resistances

of the SiC reinforced Ni–W coating.

4. Conclusion

Pulse electrodeposited SiC reinforced Ni–W coating was fab-

ricated and effect of SiC on microstructure, phases and corrosion

property was investigated. The inclusion of SiC in Ni–W coating

transformed the surface morphology from dome shaped structure

to turtle shell like structure. Ni rich phase i.e. Ni(W) and W rich

phase i.e. W(Ni) was segregated by the reinforcement of 5g/l SiC.

Signature peak ofW(Ni) solid solutionwasseen in the XRDspectra.

Increase of ∼21% in E corr indicates the higher corrosion resistance

of Ni–W coating with 5g/l SiC. Enhanced corrosion resistance was

majorly been attributed to the presence of crystallographic tex-

tures and the formation of continuous double layer of NiWO4  and

SiO2 which acted as barrier to the initiation and development of 

corrosion.

 Acknowledgements

Authors at Indian Instituteof Technology Patna (IIT-P)acknowl-

edge the Department of Physics, IIT-P for providing XRD

characterization facility. Authors also thank Mr. Y.N. Singh Babu,

PhD scholar, CSIR-NML for carrying out XPS analysis and Mr.Biswajyoti Mukherjee, PhDscholar, IITPatna for their constructive

suggestions.

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