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Contents lists available at ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

study of calcium carbonate/multiwalled-carbon nanotubes/chitosanomposite coatings on Ti–6Al–4V alloy for orthopedic implants

asha A. Ahmeda,b,∗, Amany M. Fekryc, R.A. Farghali a,c

Chemistry Department, Faculty of Science, Taif University, Saudi ArabiaForensic Chemistry Laboratories, Medico Legal Department, Ministry of Justice, Cairo, EgyptChemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt

r t i c l e i n f o

rticle history:eceived 25 June 2013eceived in revised form 12 August 2013ccepted 13 August 2013

a b s t r a c t

In an attempt to increase the stability, bioactivity and corrosion resistance of Ti–6Al–4V alloy, chi-tosan (CS) biocomposite coatings reinforced with multiwalled-carbon nanotubes (MWCNTs), and calciumcarbonate (CaCO3) for surface modification were utilized by electroless deposition. Scanning electronmicroscope (SEM), Fourier transform infrared spectroscopy (FTIR) reveals the formation of a compact

vailable online xxx

eywords:arbon nanotubehitosanalcium carbonate

and highly crosslinked coatings. Electrochemical techniques were used to investigate the coats stabilityand resistivity for orthopedic implants in simulated body fluid (SBF). The results show that Est value ismore positive in the following order: CaCO3/MWCNTs/CS > CS/MWCNTs > CS > MWCNTs. The calculatedicorr was 0.02 nA cm−2 for CaCO3/MWCNTs/CS which suggested a high corrosion resistance.

© 2013 Elsevier B.V. All rights reserved.

lectrochemical impedance

. Introduction

Metallic materials such as stainless steels, cobalt–chromiumlloys, pure titanium, Ti–6Al–4V alloys, and nickel–titanium shapeemory alloys (TiNi SMAs) are traditionally used as biocompatibleaterials to replace the structural components of the human body

1–3]. The Ti–6Al–4V and Ti–6Al–7Nb alloys are among the mostommonly used implant materials, particularly for dental, orthope-ic and osteosynthesis applications [4–6]. Approximately 20–30%f commercially available medical devices are made of Ti–6Al–4Vlloy. Good biocompatibility of that alloy has been attributed to theormation of passive, thin and adherent oxide film on its surface,hich protects the metallic substrate from the aggressive environ-ent [7,8]. On the other hand, this oxide film may not be sufficient

o eliminate the release of titanium, aluminum and vanadium fromhe alloy inside the body and these ions mix in the main bodytream. The release of even small amounts of these ions may causeocal irritation of the tissues surrounding the implant. Furthermore,elease of these particles onto tissue induces a significantly higherelease of proinflammatory and osteolytic mediators, which are

Please cite this article in press as: R.A. Ahmed, et al., A study of calcium caon Ti–6Al–4V alloy for orthopedic implants, Appl. Surf. Sci. (2013), http://d

esponsible for the aseptic loosening of the prosthesis [9]. More-ver, titanium, aluminum and vanadium ions can inhibit the apatiteormation in vivo and consequently, the mineralization process

∗ Corresponding author at: Chemistry Department, Faculty of Science, Taif Uni-ersity, Saudi Arabia. Tel.: +966 0562805809.

E-mail address: rashaauf@yahoo.com (R.A. Ahmed).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.08.056

at the bone–implant interface [10–15]. Sometimes, the infectionscan increase the corrosion due to the high temperature, metabolicproducts and pH modification [16,17].

Surface modification methods are thus essential to improve thecorrosion resistance and biocompatibility of titanium, impartingthe desired surface properties for implant applications. Most stud-ies on surface modification of titanium have focused mainly onthermal oxidation [18], hydrothermal synthesis [19], ion implan-tation [20], and colloidal processing [21]. Taking into accountthat titanium and its alloys are bioinert materials [22,23], andtheir osseointegration is a long process, various bioactive coatingswere proposed using hexamethyldisilazane [24], calcium phos-phate [25], bioactive glass [26], and various proteins [27].

As an important active material, chitosan has attractedconsiderable attention owing to its excellent biocompatibility,biodegradability, nontoxicity and adsorption properties [28–30]. Ithas been widely used in biomedical area, such as vehicle for drug,protein, gene delivery and scaffolds for tissue engineering [31–36].Chemical modification of CS to create new biofunctional materi-als is of primary interest because the created products would notchange the fundamental skeleton of CS and would keep the origi-nal physicochemical and biochemical properties of the introducedgroup.

The superior properties and potential applications of carbon

rbonate/multiwalled-carbon nanotubes/chitosan composite coatingsx.doi.org/10.1016/j.apsusc.2013.08.056

nanotubes (CNTs) increasingly attract scientific and technologicalinterest. Their high length/diameter ratio, strength, elastic mod-ulus, flexibility, stiffness, large energy absorbing capacity, uniqueconductivity and chemical stability along with other excellent

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roperties [37–41] have led to the use of the CNTs as a novelber for a variety of composite materials. Likewise, MWCNTs are

relatively new carbon allotrope with high aspect ratio, out-tanding strength, unique atomic structure, thermal conductivitynd biocompatibility [42–44]. Making nanostructure is suitableor biomedical coatings [45–48]. Further, the biocompatibility ofNTs in orthopedic application is also established by studies show-

ng accelerated bone growth (in vitro) and osteoblast proliferation49,50], which fortifies its candidature as orthopedic biomaterial.owever, expanded use of CNTs in living systems suffered from theytotoxicity of CNTs. But the cytotoxic effect is mostly attributed tother factors, as presence of metallic catalyst particles, agglomer-tion and not due to CNT itself [50–52]. On the other hand, manyorks have reported functionalization of CNT with organic poly-ers through covalent, noncovalent or wrapping methods [53–59].ost of these efforts were devoted to improve the reactivity, solu-

ion process ability, and biological performance of CNT, which willxpand the orthopedic applications of CNTs.

Calcium carbonate, a natural mineral with great biocompat-bility, has been widely used in industry, technology, medicine,

icrocapsule fabrication, and many other bio-related fields [60].he combination of calcium with MWCNTs is being investigatedo exploit the extraordinary mechanical properties of MWCNTs toeinforce bioactive materials. The addition of CaCO3 to MWCNTs ishus expected to improve the composite coat by filling the carbon

icrotubes, forming dense bioactive film, preventing aluminumnd vanadium from leaching off to blood stream. This combina-ion improves corrosion resistance and induces an ideal interfacialonding between calcium and MWCNTs that could be ultimatelyesponsible for superior stability and resistivity. This compositeas achieved through monitoring the change in current–voltage

haracteristics of the samples upon immersion for 5 days in SBF. SBForresponds to a fluid environment mimicking the ion concentra-ion of human blood plasma and it is currently used to investigatehe surface bio-reactivity of biomaterials designed for orthopedicpplications.

The aim of this work is to improve the bioactivity, stabil-ty and corrosion resistance of Ti–6Al–4V alloy modified with CSoating, MWCNTs coating, MWCNTs/CS composite coatings andaCO3/MWCNTs/CS composite coatings and to compare those coatssing different electrochemical techniques and surface examina-ion. The expected improvement of the corrosion resistance ofi–6Al–4V alloy due to the modification would make it an excellentandidate for biomedical applications especially in simulated bodyuid (SBF) for orthopedic implants.

. Experimental

A Ti–6Al–4V alloy supplied from Johnson and Malthey (England)ith composition (wt%); 5.7 Al, 3.85 V, 0.18 Fe, 0.038 C, 0.106 O,

.035 N and the rest is titanium. The alloy was welded to an electri-al wire and fixed with Araldite epoxy resin in a glass tube leavingross-sectional area of 0.1 cm2. The surface of the test electrodeas mechanically polished by emery papers with 400 up to 1000

rit to ensure the same surface roughness. The electrode was thenegreased in acetone, rinsed with ethanol.

The simulated biological fluid (SBF), recommended by Fekry61], contains NaCl 8.74 g/L, NaHCO3 0.35 g/L, Na2HPO4 0.06 g/L andaH2PO4 0.06 g/L. The pH of the solution was 7.4. All reagents usedre Analar and SBF solution is prepared using triply distilled water.

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MWCNTs (Shenzhen Nanotech Port Co., Ltd.) with a diameter of0–80 nm were purified with 2.6 M HNO3 at 120 ◦C for 24 h. Then,he purified MWCNTs were treated with a 1:3 (v/v) mixture of HNO365%) and H2SO4 (98%) at room temperature for 8 h with continuous

Scheme 1. Schematic illustration of synthesis of chitosan doped with MWCNTs andCaCO3 particles.

ultrasonication. The product was centrifuged, washed with ethanoland dried under vacuum at 60 ◦C overnight.

CS from crab shells (85% deacetylated) was purchased fromSigma. CS gel was prepared as received by adding 0.50 g to 98.0 mLwater and gradually adding 2.0 mL of glacial acetic acid to the solu-tion to maintain the pH near 3, and stirred for 2 h [62,63]. After thatundissolved material was filtered. MWCNTs particles are preparedby sonication for 30 min of 0.01 g in 5 mL methanol and 5 mL DMF.

MWCNTs/CS coat mixture was prepared by mixing both solutionwith doubling the weight for both MWCNTs and CS then makingultrasonication for 30 min to achieve homogenous solution. 0.01 gof CaCO3 was added to MWCNTs/CS mixture and sonicated for30 min; Scheme 1 shows the preparation of CaCO3/MWCNTs/CScoat.

In order to form a layer of each coat on the surface of Ti–6Al–4Valloy, the electrode was dipped in the respective solution and rinsedusing ultra-pure water, and blot-dried, the electrode was thendipped in glutaraldehyde for 1 min and left to dry for 24 h. Theglutaraldehyde solution served as a crosslinking agent, by whichaldehyde groups can react with the primary amino group in (CS).

The electrochemical cell used was a typical three-electrode, onefitted with a large platinum sheet of size 15 mm × 20 mm × 2 mmas a counter electrode (CE), saturated calomel (SCE) as a refer-ence electrode (RE) and the alloy as the working electrode (WE).Cathodic and anodic polarization curves were scanned from −0.8 Vto 0.0 V for coated Ti–6Al–4V alloys with a scan rate of 1.0 mV s−1.The impedance diagrams were recorded at the free immersionpotential (OCP) by applying a 10 mV sinusoidal potential through afrequency domain from 100 kHz down to 100 mHz. The instrumentused is the electrochemical workstation IM6e Zahner-elektrik,GmbH (Kronach, Germany). The SEM micrographs were taken usinga JEOL JXA-840A electron probe microanalyzer. The electrochemi-cal were always carried inside an air thermostat which was kept at37 ◦C. To verify the fabrication of chitosan on the electrode and toelucidate the effect of addition of MWCNTs and CaCO3 on the prop-erties of the coatings, FTIR spectra of pure chitosan, MWCNTs/CSand CaCO3/MWCNTs/CS coats were obtained by means of a FTIRapparatus (Bruker).

3. Results and discussion

3.1. Characterization of the deposited layers

Fig. 1a–d shows the representative SEM images of MWCNTs, CS,MWCNTs/CS, and CaCO3/MWCNTs/CS coats over Ti–6Al–4V alloy,respectively. As shown in Fig. 1a, a typically tube like structurefor the MWCNTs. A smooth, uniform structure coat appeared inFig. 1b represented the CS compact layer. Different morphologieslike cloudy growth and wooly structure growing on Ti–6Al–4V alloy

rbonate/multiwalled-carbon nanotubes/chitosan composite coatingsx.doi.org/10.1016/j.apsusc.2013.08.056

surface are shown in Fig. 1c indicating the presence of MWCNTs/CScomposite. The deposit shown in Fig. 1d reveals that the morphol-ogy of calcium carbonate coat is a granular type.

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CNTs

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Fig. 1. SEM images scans for (a) MWCNTs, (b) CS, (c) MW

.2. FTIR analysis

The FTIR spectra of CS, MWCNTs/CS, and CaCO3/MWCNTs/CSoats are illustrated in Fig. 2, the figure shows the effect of intro-ucing MWCNTs and CaCO3 to the CS coating. In the pure CSpectrum, the band at 3312 cm−1 is attributed to O H stretchingnd overlaps with that of N H stretching of d-glucosamine. Bandst 2924 cm−1 and 2867 cm−1 are assigned to C H stretching andhe band at 1640 cm−1 is due to the C O (acetyl) bond. The band at568 cm−1 is related to N H stretching and the band at 1408 cm−1

Please cite this article in press as: R.A. Ahmed, et al., A study of calcium caon Ti–6Al–4V alloy for orthopedic implants, Appl. Surf. Sci. (2013), http://d

orresponds to the asymmetrical C H bending of the CH2 group.s can be observed in Fig. 2, FTIR spectra of the MWCNTs/CS,nd CaCO3/MWCNTs/CS coatings are shifted to lower amounts ofransmittance as a result of introduction of MWCNTs and CaCO3 to

ig. 2. FTIR spectra of coatings obtained for; (a) CS, (b) MWCNTs/CS and (c)aCO3/MWCNTs/CS.

/CS and (d) CaCO3/MWCNTs/CS coated TiT–6Al–4V alloy.

the CS gel. This result may be due to the formation of more bondingthat produced between NH3

+ groups of CS chains and MWCNTsand CaCO3 particles. These bonds are responsible for increasedabsorption of infrared radiation, thus lowering transmittance levelin the spectra. Moreover, these results indicate the formationof a composite with high crosslinking and consequently moreprotected surface.

3.3. Open circuit potential and impedance measurements

The open circuit potential (OCP) of coated Ti–6Al–4V alloy withMWCNTs, CS, both of them or CaCO3/MWCNTs/CS was studiedwith immersion time in SBF solution at 37 ◦C. Fig. 3 shows thevariation with time of steady state potential (Est) for the fourcoated electrodes in SBF at 37 ◦C. It was found that film resistivityincreases for all studied electrodes with increasing of immersiontime. The degree of ennobling in Est value may be due to the rela-tive stability of the spontaneous passive film formed on the alloyssurfaces in SBF. Est value was found to be more positive in the fol-lowing order: CaCO3/MWCNTs/CS > CS/MWCNTs > CS > MWCNTs.MWCNTs coated Ti–6Al–4V alloy is the lowest protected alloy withmost negative Est value and then CS coated Ti–6Al–4V alloy wasbetter where Est value shifts toward more positive value; however,by mixing both MWCNTs and CS as a coat, the protection for thetested Ti alloy becomes better compared with each coat alone. Thismay be attributed to the formation of complex nanotube net whichacts as a barrier and precludes the electrolyte through the coating.Furthermore, the uniform distribution of the MWCNTs may con-

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tribute to the corrosion resistance improvement through inhibitinglocalized corrosion and promoting the homogenous one. On addingCaCO3 salt to the coat, CaCO3/MWCNTs/CS coated Ti–6Al–4V alloywas found to be the best protected one as reflected from Est values.

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Fig. 4. (a) Bode and (b) Nyquist plots of MWCNTs coated Ti–6Al–4V alloy in SBF

ig. 3. Variation with time of steady state potential (Est) for the four coated elec-rodes in SBF at 37 ◦C.

fter water evaporation and dehydration, the CS matrix shrink-ge, leaving MWCNTs micropores in their vicinity. Furthermore,fter dehydration, due to the shrinkage in the inner walls of MWC-Ts, there would be a lot of empty space. CaCO3 act as fillers for

hese empty spaces, and forms a compact layer with excellent cor-osion resistance and low chemical reactivity at coating/electrolytenterface. Moreover, it can act as physical barrier to the corrosionrocess.

The results of the impedance measurements for the Ti–Al–4Vlloy in SBF solution at 37 ◦C were illustrated in previous work byekry et al. [61]. In this paper coated Ti–Al–4V alloy with MWCNTs,S, both of them or CaCO3/MWCNTs/CS was studied. Fig. 4a and bhows both Bode and Nyquist plots of MWCNTs coated Ti–6Al–4Vlloy, respectively, in SBF solution at 37 ◦C. As shown in Fig. 4a,hase diagram is so broad with phase angle maximum (�max) ∼ 70◦,max increases gradually with increasing immersion time till 120 h.his means that the film is stable for 5 days. As there is no sharpncrease with time but still slow increment for both phase angle

aximum and impedance values. This is also clear from Nyquistlot (Fig. 4b), the arc diameter increases with increasing immersionime, so that impedance value increases.

To account for the corrosion behavior of the coatings, an equiv-lent electrical circuit model, as is given in Fig. 4 inset, was utilizedo simulate the metal/solution interface and to analyze the Nyquistnd Bode plots. Fitting of the plots requires a three-time constantquivalent electrical circuit [37]. In this model, Rs refers to solu-ion resistance and R1, R2 and R3 refer to resistance of the threeormed layers, where R1 is the inner most layer. For the three resis-ive layers, there is a corresponding three parallel constant-phaselements (C1, C2 and C3), a constant-phase element (CPE) was usednstead of the ideal capacitance to account for inhomogenity ofhe surface [61]. The impedance of a phase element is defined asCPE = [C(jw)˛]−1, where −1 ≤ ≤ 1. The value of is associated withhe inhomogenity and surface defects. The resistance and capaci-ance values of the three layers are given in Table 1.

Fig. 5a and b shows both Bode and Nyquist plots of CS coatedi–6Al–4V alloy, respectively, with immersion time in SBF solu-ion at 37 ◦C. Fig. 5a shows that the phase diagram containswo maximum phases with phase angle maximum ∼65◦ and 70◦

hich is near to that for alloy coated with MWCNTs only. Thempedance which is inversely proportional with conductivity, itsalues increase with increasing time, however with higher values

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han those formed on MWCNTs coated Ti–6Al–4V alloy. This is alsolear from Nyquist plot (Fig. 5b). The model used to fit Fig. 5a and bas shown inset and fitting data is given in Table 1. The equivalent

ircuit is two time constant model including a Warburg impedance

with immersion time at 37 ◦C. Three time constant equivalent circuit model usedfor fitting (inset).

(W), which is in series for the outer porous layer resistance R2 [61],that can be linked to ion diffusion through the passive film and it isshown in Nyquist plot Fig. 5b. This Warburg impedance indicatesthat the corrosion mechanism is controlled not only by a charge-transfer process but also by a diffusion process. R2 is in parallelto double layer capacitance of outer porous layer C2 and R1 is theinner compact layer resistance [61] accompanied with double layercapacitance of inner compact layer C1. Also C1 and C2 are CPE’s andnot pure capacitances. This indicates that CS forms two layers onthe electrode surface. The inner layer is compacted and the outerone is porous. These results indicate that the protection providedby the passive layer is predominantly due to the inner compactlayer. Fig. 6a and b shows both Bode and Nyquist plots of MWC-NTs/CS coated Ti–6Al–4V alloy, respectively, with immersion timein SBF solution at 37 ◦C. Fig. 6 for MWCNTs/CS coat shows the samediagram like that of Fig. 5 for CS coat. This indicates that the pre-vailing coat is meanly CS, however, MWCNTs improves greatly theimpedance value for Ti–6Al–4V alloy. This may be due to MWC-NTs fills the pores in the porous outer layer of CS film. From theseresults, it is evident that the MWCNTs play an important role inincreasing the impedance of the composite coating by incorporatedin the porous layer of CS leading to increase in surface protection.The same model in Fig. 5 of two time constants is used for fittingFig. 6a and b.

After adding 0.01 g of CaCO3 to MWCNTs/CS coat, a highimpedance values were obtained for medium to low frequencies

rbonate/multiwalled-carbon nanotubes/chitosan composite coatingsx.doi.org/10.1016/j.apsusc.2013.08.056

with increasing time suggesting a high corrosion resistance in theelectrolyte used. Fig. 7a and b shows Bode and Nyquist plots forCaCO3/MWCNTs/CS coated Ti–6Al–4V alloy, respectively, in SBF at

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Table 1Impedance parameters of coated Ti–6Al–4V alloy in SBF with immersion time at 37 ◦C.

Coated Ti–6Al–4V Time (h) R1 (M� cm2) C1 (�−1 cm−2 S�) R2 (� cm2) C2 (�−1 cm−2 s�) R3 (K� cm2) C3 (�F cm−2) W (K� cm s−1/2) Rs

MWCNTs 5.00 0.5 4.5 3.2 8.8 3.3 39.0 – 12.415.0 0.9 3.6 3.7 7.5 3.7 36.7 – 12.330.0 1.1 2.9 4.1 6.9 3.9 34.6 – 12.660.0 1.5 2.3 4.3 5.7 4.2 30.3 – 18.2

120 1.9 1.7 4.7 4.4 4.5 28.1 – 18.6

CS 5.00 0.6 3.6 4330 8.1 – – 3.7 23.415.0 1.3 2.7 4800 7.3 – – 4.2 23.230.0 1.7 1.9 5100 6.2 – – 4.5 23.160.0 2.1 1.5 5500 5.0 – – 4.9 23.6

120 2.4 0.9 5800 4.2 – – 5.2 23.7

MWCNTs/CS 5.00 0.9 2.3 4500 7.5 – – 4.7 5.315.0 1.4 2.1 4900 7.0 – – 5.2 5.430.0 1.8 1.7 5300 5.9 – – 5.5 5.260.0 2.5 0.9 5900 4.7 – – 5.9 5.6

120 3.1 0.7 6300 3.0 – – 6.4 5.5

CaCO3/MWCNTs/CS 5.00 1.6 1.1 6400 6.1 – – – 19.415.0 2.5 1.0 7200 5.5 – – – 18.2

3aagot

FiW

30.0 3.1 0.6 8100

60.0 5.7 0.4 9300

120 7.6 0.3 9900

7 ◦C with immersion time. Phase diagrams showing phase anglespproaching 80◦ indicate a highly capacitive behavior typical for

compact passive film [61]. Adding CaCO3 changes phase dia-

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ram shape and leads to an increase in impedance values thanther used coats. The improvement of the corrosion resistance ofhe CaCO3/MWCNTs/CS composite coating can be related to the

ig. 5. (a) Bode and (b) Nyquist plots of CS coated Ti–6Al–4V alloy in SBF withmmersion time at 37 ◦C. Two time constant equivalent circuit model including

arburg impedance used for fitting (inset).

5.1 – – – 19.13.9 – – – 20.32.3 – – – 20.2

acceleration of the chemical passivation and thereby the thick-ening of the calcium carbonate-rich film. This can be seen fromthe schematic diagram, in which the electrically inert particles ofCaCO3 with low ionic conductivity properties incorporated inside

rbonate/multiwalled-carbon nanotubes/chitosan composite coatingsx.doi.org/10.1016/j.apsusc.2013.08.056

the MWCNTs and act as an inert filler to its pores, leading to a rapidformation of a thicker Ca-rich film which blocks the electron trans-fer, and in the same time this composite fills the porous outer layer

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Fig. 6. (a) Bode and (b) Nyquist plots of MWCNTs/CS coated Ti–6Al–4V alloy in SBFwith immersion time at 37 ◦C.

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Fig. 7. (a) Bode and (b) Nyquist plots of CaCO3/MWCNTs/CS coated Ti–6Al–4V alloyiu

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and CaCO3 have lower levels of icorr compared to the samples that

n SBF with immersion time at 37 ◦C. Two time constant equivalent circuit modelsed for fitting (inset).

f CS increasing the protection of electrode surface. The model usedo fit Fig. 7a and b is shown inset. This model indicates that the filmonsists of two layers as in Fekry et al. previous work [61].

Results of fitting data are given in Table 1 for all used coatsn Ti–Al–4V alloy. According to Table 1, the incorporation of bothWCNTs and CaCO3 into the composite coating increases the

harge transfer resistance and thereby the corrosion resistance ofhe coating. However, MWCNTs alone without Cs does not pro-ect the electrode surface well due to its conductivity. The resultsndicate that the protection provided by the passive layer is pre-ominantly due to the inner layer for all coats. So, the corrosionesistance of the Ti–4V–6Al alloy is ascribed to the inner most layeresistance R1. On comparing all these data with that previouslybtained by Fekry et al. [61] for uncoated one. It was found thathe impedance values are higher for coated electrodes. Therefore,ny of the used coats improves the tested alloy against corrosion.or all coats, the impedance values increase with immersion time

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n SBF solution till 5 days and the four coats are so stable withime. CaCO3/MWCNTs/CS coated Ti–6Al–4V alloy is the most pro-ected alloy due to the incorporation of MWCNTs and CaCO3 in the

able 2orrosion parameters of coated Ti–6Al–4V alloy in SBF after 5 h of immersion time at 37 ◦

Coated Ti–6Al–4V Ecorr (mV) icorr (nA cm

MWCNTs −396 0.18

CS −393 0.10

MWCNTs/CS −392 0.06

CaCO3/MWCNTs/CS −387 0.02

Fig. 8. Polarization scans for (a) MWCNTs, (b) CS, (c) MWCNTs/CS and (d)CaCO3/MWCNTs/CS after immersion for 5 h in SBF at 37 ◦C.

outer layer which may serve as separator layers. That prevents closetouch between the electrolyte and the metal surface. As mentionedbefore, these results may be attributed to the role of CaCO3 andMWCNTs in modifying the structure of CS by refilling its pores. Thenanoscale deposited MWCNTs reduce the surface and structuraldefects by filling the crevices, gaps and micron holes of the coat[37–40,64]. Furthermore, incorporation of CaCO3 into the coatinglayer leads to the formation of a denser and more homogenous coat.

3.4. Potentiodynamic polarization

Polarization behavior of tested coated electrodes was followedby scanning from −0.8 to 0.0 V vs. SCE (Fig. 8) after 5 h of immer-sion using potentiodynamic polarization measurements at a scanrate of 1.0 mV s−1 in SBF solution. Prior to the potential scan, theelectrode was left under open circuit conditions in the respec-tive solution for 5 h until a steady free corrosion potential (Est)value was recorded. The electrochemical parameters shown inTable 2 were obtained by analyzing the I/E data as describedelsewhere [61]. The corrosion potential (Ecorr) and current den-sity (icorr) were calculated by Tafel extrapolation method for thecathodic branches of the polarization curves. As an illustrationof the relative stability of the surface film on coated Ti–6Al–4Valloy in SBF solution, icorr values are found to decrease and Ecorr

values shifts positively in the same order as obtained previouslyfrom open circuit potential and impedance measurements as fol-lows: CaCO3/MWCNTs/CS > CS/MWCNTs > CS > MWCNTs. MWCNTscoated Ti–6Al–4V alloy. According to the electrochemical param-eters (icorr, Ecorr, ˇa and ˇc) given in Table 2, it is confirmed thatall coatings lead to protection of the substrate compared to thebare surface. But it can be seen that sample containing MWCNTs

rbonate/multiwalled-carbon nanotubes/chitosan composite coatingsx.doi.org/10.1016/j.apsusc.2013.08.056

C.

−2) ˇa (mV dec−1) ˇc (mV dec−1)

10.5 −18.812.3 −17.312.5 −14.511.1 −12.1

are coated with only MWCNTs/CS, MWCNTs and CS This meansthat by introducing MWCNTs and CaCO3 to the coating, corrosionresistance would be increased compared to coatings that contain

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nly MWCNTs/CS, MWCNTs and CS. Additionally, the incorpora-ion of the MWCNTs and CaCO3 decreases cathodic Tafel slopehile increasing the anodic one. The lower ˇc value correspond-

ng to the CaCO3/MWCNTs/CS composite coating clearly indicateshat the rate of cathodic half reaction (most probably hydrogenvolution) is higher for the CaCO3/MWCNTs containing layer. Thiss due to the low hydrogen overvoltage on MWCNTs [65,66]. Itlso causes the corrosion potential of the CaCO3/MWCNTs/CS com-osite coating to shift toward more noble direction compared tother coating curves. Moreover, the value of icorr is much lower foraCO3/MWCNTs/CS coated Ti–6Al–4V alloy which agree with EISesults.

. Conclusions

This work describes a novel method for improving the sta-ility and corrosion resistance of Ti–6Al–4V alloy. Overall, theseoats are simple to fabricate, gives high compacted and uniformoatings. SEM and FTIR techniques were applied for study of char-cterization of the coats. An electrochemical technique has beenollowed to study simultaneously both the stability and corro-ion resistance of the coats in SBF. The results reveal that TheaCO3/MWCNTs/CS film exhibited higher Est and stronger corro-ion resistance than CS/MWCNTs, CS, MWCNTs and pure Ti–6Al–4Vlloy, suggesting much better long-term stability in physiologicalnvironment. Finally, the promising feature of the nanocompos-te coat could serve as a versatile platform for the protection ofi–6Al–4V alloy used in orthopedic implants.

cknowledgements

The authors are grateful for the financial support of Chem-stry Department (University of Taif, Kingdom of Saudi Arabia) andaculty of Science Cairo University to carry out the above investi-ations.

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