Research ArticleIn Vitro Corrosion Studies of Surface Modified NiTi Alloy forBiomedical Applications

Manju Chembath1 J N Balaraju1 and M Sujata2

1 Surface Engineering Division CSIR National Aerospace Laboratories Post Bag No 1779 Bangalore Karnataka 560017 India2Materials Science Division CSIR National Aerospace Laboratories Post Bag No 1779 Bangalore Karnataka 560017 India

Correspondence should be addressed to J N Balaraju jnbalarajurediffmailcom

Received 30 June 2014 Revised 1 October 2014 Accepted 21 October 2014 Published 20 November 2014

Academic Editor William A Brantley

Copyright copy 2014 Manju Chembath et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Electropolishing was conducted on NiTi alloy of composition 491 Ti-509 Ni at under potentiostatic regime at ambienttemperature using perchloric acid based electrolyte for 30 sec followed by passivation treatment in an inorganic electrolyteThe corrosion resistance and biocompatibility of the electropolished and passivated alloys were evaluated and compared withmechanically polished alloy Various characterization techniques like scanning electron microscopy atomic force microscopy andX-ray photoelectron spectroscopy were employed to analyze the properties of surface modified and mechanically polished alloysWater contact angle measurements made on the passivated alloy after electropolishing showed a contact angle of 356∘ whichwas about 58 lower compared to mechanically polished sample implying more hydrophilicity The electrochemical impedancestudies showed that for the passivated alloy threefold increase in the barrier layer resistance was obtained when compared toelectropolished alloy due to the formation of compact titanium oxide The oxide layer thickness of the passivated samples wasalmost 18 times higher than electropolished samples After 14 days immersion in Hanksrsquo solution the amount of nickel releasedwas 315 ppb which was nearly half of that obtained for mechanically polished NiTi alloy confirming better stability of the passivelayer

1 Introduction

Binary NiTi alloys containing 50-51 at Ni are widely usedfor biomedical applications owing to their unique shapememory as well as superelastic properties and are preferredover conventional implant materials like Co-Cr-Mo alloysand stainless steel for specific applications [1] These alloysare reported to exhibit surface passivity due to the presenceof native titanium oxide layer which prevents the alloy fromcorrosion under the influence of body fluids and hence theypossess superior biocompatibility compared to stainless steel[2 3] In spite of its remarkable properties nickel elution isstill a major issue of concern in using NiTi alloys as highnickel content creates serious health hazards when implantedinto human body Human physiological environment iscomplex and the biocompatibility of the material needs tobe established prior to use as an implant device Nickelelution on immersion of NiTi in simulated body fluids

(SBF) has been studied by various researchers [4ndash6] Sincenickel elution produces severe allergic issues many of theresearch works on NiTi alloys were focused on improving thebiocompatibility and corrosion resistance by suitable surfacemodification techniques [7ndash9] The corrosion behavior ofthe alloy has been analyzed in various simulated body fluidssuch as Tyrodersquos Hanksrsquo phosphate buffered saline (PBS)and 09 wt NaCl solutions [10ndash17] Results of these studiesshowed that the electrochemical behavior and the nickelrelease rate vary from one type of solution to the otheralthough the chloride content in these physiological solutionsremained almost the same Therefore the presence of otherions also plays a significant role in the electrochemicalbehavior of NiTiThe purity and ratio of individual elementsmethod of meltingcasting [18] and shape memory heattreatment also play a vital role in determining the biocom-patibilityThe formation of uniform compact and defect freeoxide on the surface was found to be beneficial in the place

Hindawi Publishing CorporationAdvances in BiomaterialsVolume 2014 Article ID 697491 13 pageshttpdxdoiorg1011552014697491

2 Advances in Biomaterials

of a thicker oxide layer [19] Since the corrosion resistanceof an alloy depends strongly on the surface structure and thecomposition selective tuning of the surface of the alloy usingchemical or electrochemical methods is expected to improveits corrosion resistance

Among the various surface modification techniquesdeveloped so far for NiTi alloys electropolishing process wasproved to be beneficial as it removes the outermost surfacelayer of the alloy formed during metal working operationsand is effective in reducing the surface roughness frommicro- to nanometer level enabling the surface to functionas a biomaterial [20 21] Electropolishing of NiTi alloysin various solutions has been developed and was found toimprove the bioapplicability [22 23] These processes can beoperated potentiostatically or galvanostatically at or belowroom temperature

In the present study the effect of electropolishing andpas-sivation on the corrosion resistance of biomedical grade NiTialloy has been carried out Low temperature electropolishingtreatments are generally preferred for obtaining good surfacefinish The present work is focused on electropolishing ofNiTi alloy in perchloric acid based solutions at ambient con-ditions for 30 sec which we consider as the lowest durationcompared to the literature reports [12 24] Electropolishingprocess was followed by passivation treatment in potassiumperiodate to form a passive film The oxidizing power ofpotassium periodate is utilized for various applications [25]Hence passivation using periodate solution was expected tomodify the surface properties of NiTi alloy also Mechanismof electropolishing and passivation and their in vitro corro-sion resistance and biocompatibility have been analyzed andcompared to those of mechanically polished NiTi alloy

2 Experimental

Vacuum arc melted and hot-rolled NiTi strips (509 at Ni)of 10mm width and 1mm thickness were employed in thisstudy The microstructure of NiTi alloy strip after etching inKrollrsquos reagent (HF HNO

3 andH

2O in the ratio 5 3 92) was

examined under secondary electron mode using scanningelectron microscope (SEM Carl Zeiss Supra 40 VP) Thecomposition of the phases present in the alloy was analyzedusing a CAMECA Electron Probe Micro Analyzer (EPMA)

The specimens of dimension 10mmtimes 10mmtimes 1mmwereabraded progressively using SiC grinding papers from 100to 600 grit and then ultrasonically cleaned in acetone anddistilled water separately for 15min After being air dried thesamples were electropolished at a constant voltage for 30 s atroom temperature in an electrolytic cell using stainless steel ascathodeThe electrolyte consisted of perchloric acid butanoland absolute alcohol in the ratio 1 3 1 The voltages used forelectropolishing (EP) were 15 20 and 25V After electropol-ishing the samples were ultrasonically cleaned in acetoneand distilled water consecutively for 15min and air driedFurther passivation treatment using potassiumperiodatewasgiven to selected specimensThe sample identification shownin Table 1 will be followed in this paper unless otherwisestated

Table 1 Details of sample identification and surface treatmentconditions

Sample identification Surface treatment

Bare NiTi Mechanically polishedNiTi alloy

EP20VMechanical polishingfollowed byelectropolishing at 20V

EP20VPIE

Mechanical polishingand electropolishing at20V followed bypassivation treatmentusing saturatedpotassium periodatesolution at 95∘C for 1 h

Themean roughness factor 119877119886 of mechanically and elec-

tropolished alloyswasmeasured over a length of 31mmusinga profilometer (Surtronic 3+ Taylor Hobson make) Thesurface topography and roughness analysis for as-polishedelectropolished and passivated samples were characterizedusing atomic force microscopy (AFM model SSI CSEMmake) under noncontactmodeTheprofile viewof the samplewas examined using nano profilometer For all the samplesimages were recorded over an area of 15 times 15 120583m2 Watercontact angle of the samples was measured by sessile dropmethod using a Contact Angle Analyzer (model Phoenix300 Plus from Ms Surface Electro Optics South Korea)Deionized water with a droplet volume of 8 120583L was drippedon the sample surface and the contact angle between the dropand the substrate was measured The surface morphologyof mechanically polished electropolished and passivatedsamples was examined using scanning electron microscope(Carl Zeiss Supra 40 VP) Surface analysis was performed byan X-ray photoelectron spectrometer (SPECS) equippedwitha hemispherical analyzer The X-ray source used was Al K120572radiation with a pass energy of 20 eV A vacuum generatorargon source having a pressure of 10minus9 Torr was used forsputtering The spectra for each sample were generated after2 minutes of argon sputtering and the binding energies werecomputed with reference to C1s peak located at 2845 eV Forall the analyses three samples were tested

The electrochemical methods employed to evaluate thebehavior of mechanically polished and surface-treated NiTialloys include potentiodynamic polarization and electro-chemical impedance spectroscopy (EIS) using CHI604Delectrochemical workstation The corrosion cell was firstcleaned with deionized water rinsed with phosphate bufferedsaline (PBS) solution and filled with approximately 260mLof PBS The cell with its contents was brought up to 37∘Cby placing it in a controlled temperature water bath ThePBS solution was purged with ultrahigh-purity nitrogen for30min prior to immersion of the specimen After immersionnitrogen purging continued for an additional 30min beforestarting the polarization tests A saturated calomel electrodewas used as the reference electrode and it was inserted intoa Luggin Capillary The surface area of the specimen in

Advances in Biomaterials 3

20120583m

(a)

10120583m

(b)

Figure 1 Secondary electron microstructures of the cross-section of NiTi strip (a) unetched and (b) etched in Krollrsquos reagent

contact with PBS was carefully calculated to increase theaccuracy of the corrosion parameters Polarization scanswere performed at a rate of 0167mVsminus1 Corrosion current(119864corr) and corrosion current density (119894corr) were estimatedby Tafel extrapolation to the cathodic and anodic part ofthe polarization curves respectively Corrosion rate wascalculated as per ASTM G102 minus 89 which is given as

Corrosion rate 119862 =(119870 times 119894corr times 119864119882)

120588

(1)

119870 = 119886 constant given by 327 times 10minus3mmsdotg120583Asdotcmsdotyear 119894corr= corrosion current density in 120583Acm2 119864119882 = equivalentweight of NiTi alloy in grams and 120588 = density of NiTi ingcm3

All the experiments were repeated thrice to check therepeatability

The effect of surfacemodification on themetal ion releasewas assessed by measuring the amount of nickel releasedafter immersion in Hanksrsquo solution The samples were keptin 5mL of Hanksrsquo solution and incubated in a thermostaticchamber at 37∘C for 2 weeks The solutions were periodicallywithdrawn and replaced with fresh solution Nickel elutedout was quantitatively measured using atomic absorptionspectrophotometer (AAS GBC Scientific Equipment Ltd)

The composition of PBS and Hanksrsquo solution is given inTable 2

3 Results and Discussion

31 Substrate Characteristics Secondary electron micro-graphs of polished and etched NiTi samples showed thatthe structure consisted of uniformly distributed parti-clesprecipitates of size 1-2 120583m in a matrix of polycrystallineNiTi grains (Figure 1(a)) EPMA analysis conducted on thesecond phase particles showed that they were Ti-enrichedwith an approximate composition corresponding to Ti

2Ni

The average size of the NiTi grains was measured to be 30 120583m(Figure 1(b))

Electropolishing process was optimized at 20V for 30 secBased on visual observations it was found that below andabove this voltage electropolishing was not efficient At

Table 2 Composition of simulated body fluids used in the presentstudy

Chemicals PBS (gL) Hanksrsquo (gL)NaCl 8 8CaCl2 014KCl 02 04MgCl2sdot6H2O 01MgSO4sdot7H2O 01NaHCO3 035Na2HPO4 115Na2HPO4sdot2H2O 012KH2PO4 02 006Glucose 1

an applied voltage of 15 V electropolished surface appeareddull whereas at 25V the surface appeared bright but wasfound to be rough with large number of undulations andwavy pattern Secondary electron micrographs depicting themorphology of mechanically polished electropolished andpassivated NiTi samples were given in Figure 2 Scratchesand grooves due to mechanical polishing were seen clearlyin bare NiTi After electropolishing at 20V the surfaceappeared smoother and the polishing marks were completelyeliminated Surface of the passivated sample appeared dullcompared to electropolished sample possibly due to theformation of oxide layer

32 Topography and Wetting Behavior Studies revealed thatsurface roughness has a strong influence on the corrosionresistance of NiTi implant materials [26 27] The roughnessvalues obtained in the present study for the untreatedelectropolished and passivated sampleswere given inTable 3

The mechanically polished (bare) sample exhibited anaverage roughness of 768 nm 119877

119886value obtained for the

electropolished and passivated sample using profilometerwas 662 and 1512 nm respectively The average roughnessvalues obtained from AFM for mechanically polished NiTiEP20V and EP20VPIE were 17 nm 25 nm and 308 nmrespectivelyThe increase in roughness value after passivationcould be due to the formation of titaniumoxide at the surface

4 Advances in Biomaterials

5120583m

(a)

5120583m

(b)

5120583m

(c)

Figure 2 Surface morphology of (a) bare NiTi (b) EP20V and (c) EP20VPIE

Table 3 Average surface roughness values of various NiTi samplesfrom surface profilometer and AFMmeasurements

Sample Surface roughness 119877119886

(nm)Profilometry AFM

Bare NiTi 768 plusmn 68 17 plusmn 01EP20V 662 plusmn 52 25 plusmn 12EP20VPIE 1512 plusmn 131 308 plusmn 24

The topographical images of untreated and surface mod-ified NiTi alloy obtained using AFM are shown in Figures3(a)ndash3(c)

Nonuniform ldquostreakyrdquo pattern was clearly visible foruntreated sample which was due to mechanical polishingAfter electropolishing at 20V there was a noticeable changein the topography of the surface wherein it appeared asuniform distribution of nanospikes over the entire areaThe topography of the passivated sample exhibited severalbright nodules which have resulted due to modification ofthe surface during interaction with the electrolyte Thesesamples displayed more than twelve-fold increase in rough-ness compared to electropolished samplesThese results werequantitatively in agreement with profilometry measurement(Figures 4(a)ndash4(c))

There was a mismatch in the 119877119886values obtained using

AFM and profilometer the values being higher in the lattercase However the trend in changes in the surface roughnessdue to surface modification was found similar irrespectiveof the technique used for measurement It may be notedthat there is a marginal change in the 119877

119886values due to

electropolishing On the other hand passivationwas found toresult in substantial increase in 119877

119886values While the increase

was found to be nearly 25 times in the case of profilometryAFM measurements indicated almost 12-fold increase inthe surface roughness The difference in the mismatch of119877119886values obtained in the two techniques used could be

attributed to scan lengtharea used for measurements Inthe present study the scan length for profilometric mea-surements and AFM was 31mm and 15 120583m respectivelyLonger scan lengths appeared to have resulted in a higherroughness value Similar observations were mentioned byEliaz and Nissan in the case of stainless steel [28] In additionto the initial surface conditioning the 119877

119886value was found

to be dependent on the magnitude of the surface areameasured Hence a diverse range of 119877

119886values was reported

in the literature for electropolished NiTi alloys [10 12 19]The roughness value obtained from AFM was found to be232 nm after electropolishing in perchloric acid solutionwhen measurements were conducted over an area of 50 times50 120583m2 [12] On the other hand Cisse et al reported slightlylower roughness value (17 nm) when the scan area waslimited to 2 times 2120583m2 [19]

Graphical representation of contact angle observed fordifferent samples studiedwas given in Figure 5 Contact angleof the mechanically polished (bare NiTi) sample was 856∘Electropolishing at 20V showed a contact angle similar tothat ofmechanically polished samples A significant change inthe wetting behavior of the alloy was noticed after passivationprocess wherein the contact angle observed was 356∘ whichwas about 58 lower compared to bare NiTi indicatingmorehydrophilicity

One of the necessary criteria for a biocompatible materialis that the surface should have good wetting properties [2930] The marked lowering of contact angle exhibited by thepassivated alloy relative to other surfaces may be attributed

Advances in Biomaterials 5

y 15 x15

(120583m)

(120583m)

34

(nm

)

0

(a)

y 15 x15

23

0

(120583m)

(120583m)

(nm

)

(b)

y 15 x15

020

000

(120583m)

(120583m

)

(120583m)

(c)

Figure 3 AFM images of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

to the transition of metal topography namely formation ofoxide associated with pores and enhanced roughness [31ndash33] In the current investigation AFM image (Figure 3(c))and the profile view (Figure 4(c)) support the formation ofmicrosized nodules over these surfaces compared to elec-tropolished samples Based on the above it can be concludedthat the wettability of NiTi can be improved by potassiumperiodate passivation treatment

33 Surface Analysis Figure 6 depicts the XPS survey spec-trum of mechanically polished sample which was almostidentical to that of surface modified samples The presenceof Ni Ti O and some amount of carbon due to physicaladsorption of carbon containing molecules from the atmo-sphere was identified

Surface chemical concentration of each elementwas givenin Table 4 The nickel content was around 11 at the surfacefor bare NiTi which reduced to 08 after passivation TheTiNi ratio for bare NiTi after sputtering was 18 Electropol-ishing process resulted in increasing the ratio to 38 However

Table 4 Atomic concentration of Ni and Ti for untreated andsurface treated NiTi alloys from XPS data

Sample Ti (at) Ni (at) O (at) TiNi ratioBare NiTi 191 106 703 18EP20V 269 71 660 38EP20VPIE 241 08 751 301

after periodate treatment the TiNi ratio was found to be301 Among all the samples the surface concentration ofnickel was the least for the passivated sample Hence it canbe conjectured that electropolishing followed by passivationassists in reducing the content of nickel over the surface dueto preferential oxidation of titanium to titanium dioxide

Figures 7(a)ndash7(c) showed detailed Ti 2p spectra for allthe three samples The main peak at 4587 eV could beassigned to Ti 2p

32in +4 oxidation state The doublet

separation was 61 eV For bare and electropolished samplea low intense peak at 4546 eV was due to the presence

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

2 Advances in Biomaterials

of a thicker oxide layer [19] Since the corrosion resistanceof an alloy depends strongly on the surface structure and thecomposition selective tuning of the surface of the alloy usingchemical or electrochemical methods is expected to improveits corrosion resistance

Among the various surface modification techniquesdeveloped so far for NiTi alloys electropolishing process wasproved to be beneficial as it removes the outermost surfacelayer of the alloy formed during metal working operationsand is effective in reducing the surface roughness frommicro- to nanometer level enabling the surface to functionas a biomaterial [20 21] Electropolishing of NiTi alloysin various solutions has been developed and was found toimprove the bioapplicability [22 23] These processes can beoperated potentiostatically or galvanostatically at or belowroom temperature

In the present study the effect of electropolishing andpas-sivation on the corrosion resistance of biomedical grade NiTialloy has been carried out Low temperature electropolishingtreatments are generally preferred for obtaining good surfacefinish The present work is focused on electropolishing ofNiTi alloy in perchloric acid based solutions at ambient con-ditions for 30 sec which we consider as the lowest durationcompared to the literature reports [12 24] Electropolishingprocess was followed by passivation treatment in potassiumperiodate to form a passive film The oxidizing power ofpotassium periodate is utilized for various applications [25]Hence passivation using periodate solution was expected tomodify the surface properties of NiTi alloy also Mechanismof electropolishing and passivation and their in vitro corro-sion resistance and biocompatibility have been analyzed andcompared to those of mechanically polished NiTi alloy

2 Experimental

Vacuum arc melted and hot-rolled NiTi strips (509 at Ni)of 10mm width and 1mm thickness were employed in thisstudy The microstructure of NiTi alloy strip after etching inKrollrsquos reagent (HF HNO

3 andH

2O in the ratio 5 3 92) was

examined under secondary electron mode using scanningelectron microscope (SEM Carl Zeiss Supra 40 VP) Thecomposition of the phases present in the alloy was analyzedusing a CAMECA Electron Probe Micro Analyzer (EPMA)

The specimens of dimension 10mmtimes 10mmtimes 1mmwereabraded progressively using SiC grinding papers from 100to 600 grit and then ultrasonically cleaned in acetone anddistilled water separately for 15min After being air dried thesamples were electropolished at a constant voltage for 30 s atroom temperature in an electrolytic cell using stainless steel ascathodeThe electrolyte consisted of perchloric acid butanoland absolute alcohol in the ratio 1 3 1 The voltages used forelectropolishing (EP) were 15 20 and 25V After electropol-ishing the samples were ultrasonically cleaned in acetoneand distilled water consecutively for 15min and air driedFurther passivation treatment using potassiumperiodatewasgiven to selected specimensThe sample identification shownin Table 1 will be followed in this paper unless otherwisestated

Table 1 Details of sample identification and surface treatmentconditions

Sample identification Surface treatment

Bare NiTi Mechanically polishedNiTi alloy

EP20VMechanical polishingfollowed byelectropolishing at 20V

EP20VPIE

Mechanical polishingand electropolishing at20V followed bypassivation treatmentusing saturatedpotassium periodatesolution at 95∘C for 1 h

Themean roughness factor 119877119886 of mechanically and elec-

tropolished alloyswasmeasured over a length of 31mmusinga profilometer (Surtronic 3+ Taylor Hobson make) Thesurface topography and roughness analysis for as-polishedelectropolished and passivated samples were characterizedusing atomic force microscopy (AFM model SSI CSEMmake) under noncontactmodeTheprofile viewof the samplewas examined using nano profilometer For all the samplesimages were recorded over an area of 15 times 15 120583m2 Watercontact angle of the samples was measured by sessile dropmethod using a Contact Angle Analyzer (model Phoenix300 Plus from Ms Surface Electro Optics South Korea)Deionized water with a droplet volume of 8 120583L was drippedon the sample surface and the contact angle between the dropand the substrate was measured The surface morphologyof mechanically polished electropolished and passivatedsamples was examined using scanning electron microscope(Carl Zeiss Supra 40 VP) Surface analysis was performed byan X-ray photoelectron spectrometer (SPECS) equippedwitha hemispherical analyzer The X-ray source used was Al K120572radiation with a pass energy of 20 eV A vacuum generatorargon source having a pressure of 10minus9 Torr was used forsputtering The spectra for each sample were generated after2 minutes of argon sputtering and the binding energies werecomputed with reference to C1s peak located at 2845 eV Forall the analyses three samples were tested

The electrochemical methods employed to evaluate thebehavior of mechanically polished and surface-treated NiTialloys include potentiodynamic polarization and electro-chemical impedance spectroscopy (EIS) using CHI604Delectrochemical workstation The corrosion cell was firstcleaned with deionized water rinsed with phosphate bufferedsaline (PBS) solution and filled with approximately 260mLof PBS The cell with its contents was brought up to 37∘Cby placing it in a controlled temperature water bath ThePBS solution was purged with ultrahigh-purity nitrogen for30min prior to immersion of the specimen After immersionnitrogen purging continued for an additional 30min beforestarting the polarization tests A saturated calomel electrodewas used as the reference electrode and it was inserted intoa Luggin Capillary The surface area of the specimen in

Advances in Biomaterials 3

20120583m

(a)

10120583m

(b)

Figure 1 Secondary electron microstructures of the cross-section of NiTi strip (a) unetched and (b) etched in Krollrsquos reagent

contact with PBS was carefully calculated to increase theaccuracy of the corrosion parameters Polarization scanswere performed at a rate of 0167mVsminus1 Corrosion current(119864corr) and corrosion current density (119894corr) were estimatedby Tafel extrapolation to the cathodic and anodic part ofthe polarization curves respectively Corrosion rate wascalculated as per ASTM G102 minus 89 which is given as

Corrosion rate 119862 =(119870 times 119894corr times 119864119882)

120588

(1)

119870 = 119886 constant given by 327 times 10minus3mmsdotg120583Asdotcmsdotyear 119894corr= corrosion current density in 120583Acm2 119864119882 = equivalentweight of NiTi alloy in grams and 120588 = density of NiTi ingcm3

All the experiments were repeated thrice to check therepeatability

The effect of surfacemodification on themetal ion releasewas assessed by measuring the amount of nickel releasedafter immersion in Hanksrsquo solution The samples were keptin 5mL of Hanksrsquo solution and incubated in a thermostaticchamber at 37∘C for 2 weeks The solutions were periodicallywithdrawn and replaced with fresh solution Nickel elutedout was quantitatively measured using atomic absorptionspectrophotometer (AAS GBC Scientific Equipment Ltd)

The composition of PBS and Hanksrsquo solution is given inTable 2

3 Results and Discussion

31 Substrate Characteristics Secondary electron micro-graphs of polished and etched NiTi samples showed thatthe structure consisted of uniformly distributed parti-clesprecipitates of size 1-2 120583m in a matrix of polycrystallineNiTi grains (Figure 1(a)) EPMA analysis conducted on thesecond phase particles showed that they were Ti-enrichedwith an approximate composition corresponding to Ti

2Ni

The average size of the NiTi grains was measured to be 30 120583m(Figure 1(b))

Electropolishing process was optimized at 20V for 30 secBased on visual observations it was found that below andabove this voltage electropolishing was not efficient At

Table 2 Composition of simulated body fluids used in the presentstudy

Chemicals PBS (gL) Hanksrsquo (gL)NaCl 8 8CaCl2 014KCl 02 04MgCl2sdot6H2O 01MgSO4sdot7H2O 01NaHCO3 035Na2HPO4 115Na2HPO4sdot2H2O 012KH2PO4 02 006Glucose 1

an applied voltage of 15 V electropolished surface appeareddull whereas at 25V the surface appeared bright but wasfound to be rough with large number of undulations andwavy pattern Secondary electron micrographs depicting themorphology of mechanically polished electropolished andpassivated NiTi samples were given in Figure 2 Scratchesand grooves due to mechanical polishing were seen clearlyin bare NiTi After electropolishing at 20V the surfaceappeared smoother and the polishing marks were completelyeliminated Surface of the passivated sample appeared dullcompared to electropolished sample possibly due to theformation of oxide layer

32 Topography and Wetting Behavior Studies revealed thatsurface roughness has a strong influence on the corrosionresistance of NiTi implant materials [26 27] The roughnessvalues obtained in the present study for the untreatedelectropolished and passivated sampleswere given inTable 3

The mechanically polished (bare) sample exhibited anaverage roughness of 768 nm 119877

119886value obtained for the

electropolished and passivated sample using profilometerwas 662 and 1512 nm respectively The average roughnessvalues obtained from AFM for mechanically polished NiTiEP20V and EP20VPIE were 17 nm 25 nm and 308 nmrespectivelyThe increase in roughness value after passivationcould be due to the formation of titaniumoxide at the surface

4 Advances in Biomaterials

5120583m

(a)

5120583m

(b)

5120583m

(c)

Figure 2 Surface morphology of (a) bare NiTi (b) EP20V and (c) EP20VPIE

Table 3 Average surface roughness values of various NiTi samplesfrom surface profilometer and AFMmeasurements

Sample Surface roughness 119877119886

(nm)Profilometry AFM

Bare NiTi 768 plusmn 68 17 plusmn 01EP20V 662 plusmn 52 25 plusmn 12EP20VPIE 1512 plusmn 131 308 plusmn 24

The topographical images of untreated and surface mod-ified NiTi alloy obtained using AFM are shown in Figures3(a)ndash3(c)

Nonuniform ldquostreakyrdquo pattern was clearly visible foruntreated sample which was due to mechanical polishingAfter electropolishing at 20V there was a noticeable changein the topography of the surface wherein it appeared asuniform distribution of nanospikes over the entire areaThe topography of the passivated sample exhibited severalbright nodules which have resulted due to modification ofthe surface during interaction with the electrolyte Thesesamples displayed more than twelve-fold increase in rough-ness compared to electropolished samplesThese results werequantitatively in agreement with profilometry measurement(Figures 4(a)ndash4(c))

There was a mismatch in the 119877119886values obtained using

AFM and profilometer the values being higher in the lattercase However the trend in changes in the surface roughnessdue to surface modification was found similar irrespectiveof the technique used for measurement It may be notedthat there is a marginal change in the 119877

119886values due to

electropolishing On the other hand passivationwas found toresult in substantial increase in 119877

119886values While the increase

was found to be nearly 25 times in the case of profilometryAFM measurements indicated almost 12-fold increase inthe surface roughness The difference in the mismatch of119877119886values obtained in the two techniques used could be

attributed to scan lengtharea used for measurements Inthe present study the scan length for profilometric mea-surements and AFM was 31mm and 15 120583m respectivelyLonger scan lengths appeared to have resulted in a higherroughness value Similar observations were mentioned byEliaz and Nissan in the case of stainless steel [28] In additionto the initial surface conditioning the 119877

119886value was found

to be dependent on the magnitude of the surface areameasured Hence a diverse range of 119877

119886values was reported

in the literature for electropolished NiTi alloys [10 12 19]The roughness value obtained from AFM was found to be232 nm after electropolishing in perchloric acid solutionwhen measurements were conducted over an area of 50 times50 120583m2 [12] On the other hand Cisse et al reported slightlylower roughness value (17 nm) when the scan area waslimited to 2 times 2120583m2 [19]

Graphical representation of contact angle observed fordifferent samples studiedwas given in Figure 5 Contact angleof the mechanically polished (bare NiTi) sample was 856∘Electropolishing at 20V showed a contact angle similar tothat ofmechanically polished samples A significant change inthe wetting behavior of the alloy was noticed after passivationprocess wherein the contact angle observed was 356∘ whichwas about 58 lower compared to bare NiTi indicatingmorehydrophilicity

One of the necessary criteria for a biocompatible materialis that the surface should have good wetting properties [2930] The marked lowering of contact angle exhibited by thepassivated alloy relative to other surfaces may be attributed

Advances in Biomaterials 5

y 15 x15

(120583m)

(120583m)

34

(nm

)

0

(a)

y 15 x15

23

0

(120583m)

(120583m)

(nm

)

(b)

y 15 x15

020

000

(120583m)

(120583m

)

(120583m)

(c)

Figure 3 AFM images of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

to the transition of metal topography namely formation ofoxide associated with pores and enhanced roughness [31ndash33] In the current investigation AFM image (Figure 3(c))and the profile view (Figure 4(c)) support the formation ofmicrosized nodules over these surfaces compared to elec-tropolished samples Based on the above it can be concludedthat the wettability of NiTi can be improved by potassiumperiodate passivation treatment

33 Surface Analysis Figure 6 depicts the XPS survey spec-trum of mechanically polished sample which was almostidentical to that of surface modified samples The presenceof Ni Ti O and some amount of carbon due to physicaladsorption of carbon containing molecules from the atmo-sphere was identified

Surface chemical concentration of each elementwas givenin Table 4 The nickel content was around 11 at the surfacefor bare NiTi which reduced to 08 after passivation TheTiNi ratio for bare NiTi after sputtering was 18 Electropol-ishing process resulted in increasing the ratio to 38 However

Table 4 Atomic concentration of Ni and Ti for untreated andsurface treated NiTi alloys from XPS data

Sample Ti (at) Ni (at) O (at) TiNi ratioBare NiTi 191 106 703 18EP20V 269 71 660 38EP20VPIE 241 08 751 301

after periodate treatment the TiNi ratio was found to be301 Among all the samples the surface concentration ofnickel was the least for the passivated sample Hence it canbe conjectured that electropolishing followed by passivationassists in reducing the content of nickel over the surface dueto preferential oxidation of titanium to titanium dioxide

Figures 7(a)ndash7(c) showed detailed Ti 2p spectra for allthe three samples The main peak at 4587 eV could beassigned to Ti 2p

32in +4 oxidation state The doublet

separation was 61 eV For bare and electropolished samplea low intense peak at 4546 eV was due to the presence

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Journal ofNanomaterials

Advances in Biomaterials 3

20120583m

(a)

10120583m

(b)

Figure 1 Secondary electron microstructures of the cross-section of NiTi strip (a) unetched and (b) etched in Krollrsquos reagent

contact with PBS was carefully calculated to increase theaccuracy of the corrosion parameters Polarization scanswere performed at a rate of 0167mVsminus1 Corrosion current(119864corr) and corrosion current density (119894corr) were estimatedby Tafel extrapolation to the cathodic and anodic part ofthe polarization curves respectively Corrosion rate wascalculated as per ASTM G102 minus 89 which is given as

Corrosion rate 119862 =(119870 times 119894corr times 119864119882)

120588

(1)

119870 = 119886 constant given by 327 times 10minus3mmsdotg120583Asdotcmsdotyear 119894corr= corrosion current density in 120583Acm2 119864119882 = equivalentweight of NiTi alloy in grams and 120588 = density of NiTi ingcm3

All the experiments were repeated thrice to check therepeatability

The effect of surfacemodification on themetal ion releasewas assessed by measuring the amount of nickel releasedafter immersion in Hanksrsquo solution The samples were keptin 5mL of Hanksrsquo solution and incubated in a thermostaticchamber at 37∘C for 2 weeks The solutions were periodicallywithdrawn and replaced with fresh solution Nickel elutedout was quantitatively measured using atomic absorptionspectrophotometer (AAS GBC Scientific Equipment Ltd)

The composition of PBS and Hanksrsquo solution is given inTable 2

3 Results and Discussion

31 Substrate Characteristics Secondary electron micro-graphs of polished and etched NiTi samples showed thatthe structure consisted of uniformly distributed parti-clesprecipitates of size 1-2 120583m in a matrix of polycrystallineNiTi grains (Figure 1(a)) EPMA analysis conducted on thesecond phase particles showed that they were Ti-enrichedwith an approximate composition corresponding to Ti

2Ni

The average size of the NiTi grains was measured to be 30 120583m(Figure 1(b))

Electropolishing process was optimized at 20V for 30 secBased on visual observations it was found that below andabove this voltage electropolishing was not efficient At

Table 2 Composition of simulated body fluids used in the presentstudy

Chemicals PBS (gL) Hanksrsquo (gL)NaCl 8 8CaCl2 014KCl 02 04MgCl2sdot6H2O 01MgSO4sdot7H2O 01NaHCO3 035Na2HPO4 115Na2HPO4sdot2H2O 012KH2PO4 02 006Glucose 1

an applied voltage of 15 V electropolished surface appeareddull whereas at 25V the surface appeared bright but wasfound to be rough with large number of undulations andwavy pattern Secondary electron micrographs depicting themorphology of mechanically polished electropolished andpassivated NiTi samples were given in Figure 2 Scratchesand grooves due to mechanical polishing were seen clearlyin bare NiTi After electropolishing at 20V the surfaceappeared smoother and the polishing marks were completelyeliminated Surface of the passivated sample appeared dullcompared to electropolished sample possibly due to theformation of oxide layer

32 Topography and Wetting Behavior Studies revealed thatsurface roughness has a strong influence on the corrosionresistance of NiTi implant materials [26 27] The roughnessvalues obtained in the present study for the untreatedelectropolished and passivated sampleswere given inTable 3

The mechanically polished (bare) sample exhibited anaverage roughness of 768 nm 119877

119886value obtained for the

electropolished and passivated sample using profilometerwas 662 and 1512 nm respectively The average roughnessvalues obtained from AFM for mechanically polished NiTiEP20V and EP20VPIE were 17 nm 25 nm and 308 nmrespectivelyThe increase in roughness value after passivationcould be due to the formation of titaniumoxide at the surface

4 Advances in Biomaterials

5120583m

(a)

5120583m

(b)

5120583m

(c)

Figure 2 Surface morphology of (a) bare NiTi (b) EP20V and (c) EP20VPIE

Table 3 Average surface roughness values of various NiTi samplesfrom surface profilometer and AFMmeasurements

Sample Surface roughness 119877119886

(nm)Profilometry AFM

Bare NiTi 768 plusmn 68 17 plusmn 01EP20V 662 plusmn 52 25 plusmn 12EP20VPIE 1512 plusmn 131 308 plusmn 24

The topographical images of untreated and surface mod-ified NiTi alloy obtained using AFM are shown in Figures3(a)ndash3(c)

Nonuniform ldquostreakyrdquo pattern was clearly visible foruntreated sample which was due to mechanical polishingAfter electropolishing at 20V there was a noticeable changein the topography of the surface wherein it appeared asuniform distribution of nanospikes over the entire areaThe topography of the passivated sample exhibited severalbright nodules which have resulted due to modification ofthe surface during interaction with the electrolyte Thesesamples displayed more than twelve-fold increase in rough-ness compared to electropolished samplesThese results werequantitatively in agreement with profilometry measurement(Figures 4(a)ndash4(c))

There was a mismatch in the 119877119886values obtained using

AFM and profilometer the values being higher in the lattercase However the trend in changes in the surface roughnessdue to surface modification was found similar irrespectiveof the technique used for measurement It may be notedthat there is a marginal change in the 119877

119886values due to

electropolishing On the other hand passivationwas found toresult in substantial increase in 119877

119886values While the increase

was found to be nearly 25 times in the case of profilometryAFM measurements indicated almost 12-fold increase inthe surface roughness The difference in the mismatch of119877119886values obtained in the two techniques used could be

attributed to scan lengtharea used for measurements Inthe present study the scan length for profilometric mea-surements and AFM was 31mm and 15 120583m respectivelyLonger scan lengths appeared to have resulted in a higherroughness value Similar observations were mentioned byEliaz and Nissan in the case of stainless steel [28] In additionto the initial surface conditioning the 119877

119886value was found

to be dependent on the magnitude of the surface areameasured Hence a diverse range of 119877

119886values was reported

in the literature for electropolished NiTi alloys [10 12 19]The roughness value obtained from AFM was found to be232 nm after electropolishing in perchloric acid solutionwhen measurements were conducted over an area of 50 times50 120583m2 [12] On the other hand Cisse et al reported slightlylower roughness value (17 nm) when the scan area waslimited to 2 times 2120583m2 [19]

Graphical representation of contact angle observed fordifferent samples studiedwas given in Figure 5 Contact angleof the mechanically polished (bare NiTi) sample was 856∘Electropolishing at 20V showed a contact angle similar tothat ofmechanically polished samples A significant change inthe wetting behavior of the alloy was noticed after passivationprocess wherein the contact angle observed was 356∘ whichwas about 58 lower compared to bare NiTi indicatingmorehydrophilicity

One of the necessary criteria for a biocompatible materialis that the surface should have good wetting properties [2930] The marked lowering of contact angle exhibited by thepassivated alloy relative to other surfaces may be attributed

Advances in Biomaterials 5

y 15 x15

(120583m)

(120583m)

34

(nm

)

0

(a)

y 15 x15

23

0

(120583m)

(120583m)

(nm

)

(b)

y 15 x15

020

000

(120583m)

(120583m

)

(120583m)

(c)

Figure 3 AFM images of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

to the transition of metal topography namely formation ofoxide associated with pores and enhanced roughness [31ndash33] In the current investigation AFM image (Figure 3(c))and the profile view (Figure 4(c)) support the formation ofmicrosized nodules over these surfaces compared to elec-tropolished samples Based on the above it can be concludedthat the wettability of NiTi can be improved by potassiumperiodate passivation treatment

33 Surface Analysis Figure 6 depicts the XPS survey spec-trum of mechanically polished sample which was almostidentical to that of surface modified samples The presenceof Ni Ti O and some amount of carbon due to physicaladsorption of carbon containing molecules from the atmo-sphere was identified

Surface chemical concentration of each elementwas givenin Table 4 The nickel content was around 11 at the surfacefor bare NiTi which reduced to 08 after passivation TheTiNi ratio for bare NiTi after sputtering was 18 Electropol-ishing process resulted in increasing the ratio to 38 However

Table 4 Atomic concentration of Ni and Ti for untreated andsurface treated NiTi alloys from XPS data

Sample Ti (at) Ni (at) O (at) TiNi ratioBare NiTi 191 106 703 18EP20V 269 71 660 38EP20VPIE 241 08 751 301

after periodate treatment the TiNi ratio was found to be301 Among all the samples the surface concentration ofnickel was the least for the passivated sample Hence it canbe conjectured that electropolishing followed by passivationassists in reducing the content of nickel over the surface dueto preferential oxidation of titanium to titanium dioxide

Figures 7(a)ndash7(c) showed detailed Ti 2p spectra for allthe three samples The main peak at 4587 eV could beassigned to Ti 2p

32in +4 oxidation state The doublet

separation was 61 eV For bare and electropolished samplea low intense peak at 4546 eV was due to the presence

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

4 Advances in Biomaterials

5120583m

(a)

5120583m

(b)

5120583m

(c)

Figure 2 Surface morphology of (a) bare NiTi (b) EP20V and (c) EP20VPIE

Table 3 Average surface roughness values of various NiTi samplesfrom surface profilometer and AFMmeasurements

Sample Surface roughness 119877119886

(nm)Profilometry AFM

Bare NiTi 768 plusmn 68 17 plusmn 01EP20V 662 plusmn 52 25 plusmn 12EP20VPIE 1512 plusmn 131 308 plusmn 24

The topographical images of untreated and surface mod-ified NiTi alloy obtained using AFM are shown in Figures3(a)ndash3(c)

Nonuniform ldquostreakyrdquo pattern was clearly visible foruntreated sample which was due to mechanical polishingAfter electropolishing at 20V there was a noticeable changein the topography of the surface wherein it appeared asuniform distribution of nanospikes over the entire areaThe topography of the passivated sample exhibited severalbright nodules which have resulted due to modification ofthe surface during interaction with the electrolyte Thesesamples displayed more than twelve-fold increase in rough-ness compared to electropolished samplesThese results werequantitatively in agreement with profilometry measurement(Figures 4(a)ndash4(c))

There was a mismatch in the 119877119886values obtained using

AFM and profilometer the values being higher in the lattercase However the trend in changes in the surface roughnessdue to surface modification was found similar irrespectiveof the technique used for measurement It may be notedthat there is a marginal change in the 119877

119886values due to

electropolishing On the other hand passivationwas found toresult in substantial increase in 119877

119886values While the increase

was found to be nearly 25 times in the case of profilometryAFM measurements indicated almost 12-fold increase inthe surface roughness The difference in the mismatch of119877119886values obtained in the two techniques used could be

attributed to scan lengtharea used for measurements Inthe present study the scan length for profilometric mea-surements and AFM was 31mm and 15 120583m respectivelyLonger scan lengths appeared to have resulted in a higherroughness value Similar observations were mentioned byEliaz and Nissan in the case of stainless steel [28] In additionto the initial surface conditioning the 119877

119886value was found

to be dependent on the magnitude of the surface areameasured Hence a diverse range of 119877

119886values was reported

in the literature for electropolished NiTi alloys [10 12 19]The roughness value obtained from AFM was found to be232 nm after electropolishing in perchloric acid solutionwhen measurements were conducted over an area of 50 times50 120583m2 [12] On the other hand Cisse et al reported slightlylower roughness value (17 nm) when the scan area waslimited to 2 times 2120583m2 [19]

Graphical representation of contact angle observed fordifferent samples studiedwas given in Figure 5 Contact angleof the mechanically polished (bare NiTi) sample was 856∘Electropolishing at 20V showed a contact angle similar tothat ofmechanically polished samples A significant change inthe wetting behavior of the alloy was noticed after passivationprocess wherein the contact angle observed was 356∘ whichwas about 58 lower compared to bare NiTi indicatingmorehydrophilicity

One of the necessary criteria for a biocompatible materialis that the surface should have good wetting properties [2930] The marked lowering of contact angle exhibited by thepassivated alloy relative to other surfaces may be attributed

Advances in Biomaterials 5

y 15 x15

(120583m)

(120583m)

34

(nm

)

0

(a)

y 15 x15

23

0

(120583m)

(120583m)

(nm

)

(b)

y 15 x15

020

000

(120583m)

(120583m

)

(120583m)

(c)

Figure 3 AFM images of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

to the transition of metal topography namely formation ofoxide associated with pores and enhanced roughness [31ndash33] In the current investigation AFM image (Figure 3(c))and the profile view (Figure 4(c)) support the formation ofmicrosized nodules over these surfaces compared to elec-tropolished samples Based on the above it can be concludedthat the wettability of NiTi can be improved by potassiumperiodate passivation treatment

33 Surface Analysis Figure 6 depicts the XPS survey spec-trum of mechanically polished sample which was almostidentical to that of surface modified samples The presenceof Ni Ti O and some amount of carbon due to physicaladsorption of carbon containing molecules from the atmo-sphere was identified

Surface chemical concentration of each elementwas givenin Table 4 The nickel content was around 11 at the surfacefor bare NiTi which reduced to 08 after passivation TheTiNi ratio for bare NiTi after sputtering was 18 Electropol-ishing process resulted in increasing the ratio to 38 However

Table 4 Atomic concentration of Ni and Ti for untreated andsurface treated NiTi alloys from XPS data

Sample Ti (at) Ni (at) O (at) TiNi ratioBare NiTi 191 106 703 18EP20V 269 71 660 38EP20VPIE 241 08 751 301

after periodate treatment the TiNi ratio was found to be301 Among all the samples the surface concentration ofnickel was the least for the passivated sample Hence it canbe conjectured that electropolishing followed by passivationassists in reducing the content of nickel over the surface dueto preferential oxidation of titanium to titanium dioxide

Figures 7(a)ndash7(c) showed detailed Ti 2p spectra for allthe three samples The main peak at 4587 eV could beassigned to Ti 2p

32in +4 oxidation state The doublet

separation was 61 eV For bare and electropolished samplea low intense peak at 4546 eV was due to the presence

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Advances in Biomaterials 5

y 15 x15

(120583m)

(120583m)

34

(nm

)

0

(a)

y 15 x15

23

0

(120583m)

(120583m)

(nm

)

(b)

y 15 x15

020

000

(120583m)

(120583m

)

(120583m)

(c)

Figure 3 AFM images of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

to the transition of metal topography namely formation ofoxide associated with pores and enhanced roughness [31ndash33] In the current investigation AFM image (Figure 3(c))and the profile view (Figure 4(c)) support the formation ofmicrosized nodules over these surfaces compared to elec-tropolished samples Based on the above it can be concludedthat the wettability of NiTi can be improved by potassiumperiodate passivation treatment

33 Surface Analysis Figure 6 depicts the XPS survey spec-trum of mechanically polished sample which was almostidentical to that of surface modified samples The presenceof Ni Ti O and some amount of carbon due to physicaladsorption of carbon containing molecules from the atmo-sphere was identified

Surface chemical concentration of each elementwas givenin Table 4 The nickel content was around 11 at the surfacefor bare NiTi which reduced to 08 after passivation TheTiNi ratio for bare NiTi after sputtering was 18 Electropol-ishing process resulted in increasing the ratio to 38 However

Table 4 Atomic concentration of Ni and Ti for untreated andsurface treated NiTi alloys from XPS data

Sample Ti (at) Ni (at) O (at) TiNi ratioBare NiTi 191 106 703 18EP20V 269 71 660 38EP20VPIE 241 08 751 301

after periodate treatment the TiNi ratio was found to be301 Among all the samples the surface concentration ofnickel was the least for the passivated sample Hence it canbe conjectured that electropolishing followed by passivationassists in reducing the content of nickel over the surface dueto preferential oxidation of titanium to titanium dioxide

Figures 7(a)ndash7(c) showed detailed Ti 2p spectra for allthe three samples The main peak at 4587 eV could beassigned to Ti 2p

32in +4 oxidation state The doublet

separation was 61 eV For bare and electropolished samplea low intense peak at 4546 eV was due to the presence

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

6 Advances in Biomaterials

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(a)

10

0

minus10

y(n

m)

00 05 10 15

x (120583m)

Texture

(b)

020

015

010

005

00000 05 10 15

x (120583m)

Profile 1

y(120583

m)

(c)

Figure 4 Profile view of various surface treated NiTi alloys (a) bare NiTi (b) EP20V and (c) EP20VPIE

0

20

40

60

80

100

EP20VPIEEP20VBare NiTi

Con

tact

angl

e (de

g)

Figure 5 Variation of the contact angles observed in bare andsurface modified NiTi samples

of unoxidized titanium present in the NiTi alloy For thissample existence of titanium in metallic state would result inreduced thickness of the passive layer as quoted by Vojtechet al [34] After passivation the corresponding peak wasabsent due to the complete oxidation of titanium underthe experimental conditionsThe thickness and compactnesswere expected to be more due to passivation treatment

The high resolution photoelectron spectra of nickel inthe 2p region for mechanically polished electropolishedand passivated samples were given in Figures 8(a)ndash8(c)The spectra of bare NiTi and electropolished sample lookedalmost similar The prominent peak at 8523 eV could beattributed to the binding energy of Ni 2p

32in the elemental

formThe spin orbit separation of 2p32

and 2p12

was 171 eVA satellite peak at 8593 eV which is the characteristics of

0 200 400 600 800 10000

5000

10000

15000

20000

25000

NiL

MN

NiL

MN

Inte

nsity

(cps

)

Binding energy (eV)

C1s

Ti 2

pO

1sTi

2sN

iLM

N

Ni 2

p

OKL

L

O2s

Figure 6 XPS survey spectrum for mechanically polished NiTi

Ni 2p32

was also seen in the figure For passivated samplethe peaks were noisy due to low concentration of nickel atthe surface Consequently nickel might have diffused inwardsduring the passivation process [35] The absence of peakfor nickel in +2 oxidation states and the selective oxidationof titanium are in accordance with the Gibbs free energiesfor the formation of NiO and TiO

2 which are minus2117 and

minus8888 kJmolminus1 respectively [36]

34 Electrochemical Behavior Potentiodynamic polarizationcurves obtained for mechanically polished and surface mod-ified NiTi specimens in PBS solution were displayed inFigure 9 and the parameters were given in Table 5

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Advances in Biomaterials 7

Table 5 Potentiodynamic polarization results of untreated and treated alloys

Sample 119864corr (mV) 119868corr (nAcm2) 119864

119887

(mV) 119868119887

(120583Acm2) Corrosion rate (mmyear)Bare NiTi minus470 plusmn 28 770 plusmn 36 490 plusmn 35 19 plusmn 07 698 times 10minus3

EP20V minus292 plusmn 14 6 plusmn 03 1142 plusmn 78 42 plusmn 01 535 times 10minus5

EP20VPIE minus215plusmn 15 5 plusmn 03 1010 plusmn 70 036 plusmn 008 426 times 10minus5

450 455 460 465 470

0

1000

2000

3000

4000

5000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(a)

450 455 460 465 470

0

2000

4000

6000

8000

10000

Ti

Inte

nsity

(cps

)

Binding energy (eV)

Ti 2p32

Ti 2p12

(b)

450 455 460 465 470

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

(cps

)

Binding energy (eV)

minus1000

Ti 2p32

Ti 2p12

(c)

Figure 7 High resolution XPS spectra for Ti 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

Corrosion potential 119864corr is a measure of stability ofthe surface towards corrosion when immersed in corrosivemedia From Figure 9 it was evident that bare NiTi alloyexhibited an activepassive transient behavior during anodicpolarization Similar transient peak was also observed by Huet al for bare NiTi alloy in 09 NaCl solution while nosuch behavior could be noticed in case of treated samples[17] Bare NiTi alloy revealed a corrosion potential of minus047Vversus SCE The breakdown of the passive film and theinception of pitting attack occurred at lower anodic potentialof 049V indicated by sharp increase in current density for

small change in potentials The corrosion potential of all thetreated samples was found to be nobler than untreated onesAn excellent biocompatible material should exhibit higherbreakdown potential and minimum passive current densityover a wide range of potentials which ensures good passivityat the surface [37]The corrosion current density observed forbare NiTi was 77 times 10minus7 Acmminus2 For all the treated samplesthere was almost two-magnitude decrease in corrosion cur-rent density which was in the order of 10minus9 Acmminus2 Sun andWang reported that after the surface treatment on NiTi alloythe corrosion current density was in the order of 10minus6 Acmminus2

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

8 Advances in Biomaterials

840 850 860 870 880 890

0

500

1000

1500

2000

2500

3000

3500

4000

Binding energy (eV)

Inte

nsity

(cps

)

minus500

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(a)

840 850 860 870 880 890

0

1000

2000

3000

4000

Inte

nsity

(cps

)

Binding energy (eV)

Ni 2p32

Ni 2p12

Ni 2p32sat Ni 2p12sat

(b)

840 850 860 870 880 8907700

7800

7900

8000

8100

8200

8300

8400

8500

8600

Inte

nsity

(cps

)

Binding energy (eV)

Ni

(c)

Figure 8 High resolution XPS spectra for Ni 2p region (a) bare NiTi (b) EP20V and (c) EP20VPIE

when analyzed in PBS solution [38] The diversity in thephysical chemical and electrochemical properties of NiTialloy could be correlated to the differences in processingparameters and the composition of the alloy The breakdownpotential 119864

119887 of EP20V and EP20VPIE was 1140mV and

1008mV respectively The passive current density 119894119887 for

mechanically polished sample was 19 times 10minus5 Acmminus2 Therewas one-order magnitude decrease in passive current densityfor EP20V For EP20VPIE sample there was two-ordermagnitude decrease in passive current density the value wasaround 36 times 10minus7 Acmminus2 signifyingmore passive behavior Inthe biomedical application point of view implanted materialshould retain its passivity to prevent the failure of thedevice Several parameters influence the corrosion behaviorof implanted material such as localized pH temperaturetribological effect and ionic concentrations Hence it canbe expected that a stable and more passive surface can givebetter corrosion resistance for these applications Therefore

from Figure 9 it was evident that passivation process afterelectropolishing resulted in lowering the corrosion currentdensity and exhibited more noble corrosion potential Thisindicates the effective improvement in the corrosion resis-tance performance due to the passivation treatment Forelectropolished sample the anodic curve displayed activebehavior until around 05 V and the formed passive layer wasstable until 11 V However for passivated sample the anodiccurve exhibited passivity from 0V which extended until 1 VTherefore the range of passive behavior was the highest forpassivated samples

In PBS solution mechanically polished NiTi alloy exhib-ited highest rate of corrosion (698 times 10minus3mmyear) Inthe case of electropolished and passivated samples the rateof corrosion was in the order of 10minus5mmyear which wasalmost two orders lower than bare NiTi This indicated thatuntreated alloy was more susceptible for corrosion than thesamples subjected to electropolishing The material loss due

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Advances in Biomaterials 9

0

00 02 04 06 08 10 12 14 16

(b) EP20V

(c)

(c) EP20VPIE

(b)

Potential (mV versus SCE)

(a)

(a) MP NiTi

Curr

ent d

ensit

y (lo

g i A

cm

2 )

minus11

minus10

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

minus08 minus06 minus04 minus02

Figure 9 Potentiodynamic polarization curves of untreated andtreated alloys

Pits

50120583m

Figure 10 Optical micrograph of bare NiTi alloy after potentiody-namic polarization test

to corrosion can be further reduced by passivation treatmentin periodate solution

The optical image of bare NiTi alloy after potentiody-namic polarization test is shown in Figure 10 Pits couldbe seen on the sample surface as a result of the attack ofchloride ions present in the medium Bare NiTi sampleswere more susceptible to pitting corrosion than the surfacetreated NiTi alloy Pitting usually initiates whenever thereis a defect in the native oxide layer During polarizationonce a pit is formed the parent material gets exposed tothe electrolyte solution The inner anodic site is prone tocorrosion resulting in faster dissolution of the material Thepolarization curves of the surface treated samples did notexhibit pitting type of corrosionwhichmeans that the surfacewas nearly defect-free after electropolishing and passivationThese results suggest that electropolishing and passivationimprove the corrosion resistance of NiTi alloy as indicatedby more positive corrosion potential and lower corrosioncurrent density

Electrochemical impedance spectroscopy (EIS) studiesare a useful technique which give quantitative informationon the mechanism of corrosion of metals when immersed

in an electrolytic solution Figures 11(a)ndash11(c) show the Bodeplots obtained from as-polished and surface treated NiTialloy in PBS solution These experimental results are fitted toappropriate equivalent circuits as shown in Figure 12

In the circuit 119877119904represents the electrolyte resistance

between the working electrode and reference electrode 119877119901

is the double electrochemical layer resistance associated withthe charge transfer resistance at the electrolyte-porous layerinterface and 119862

119901is its capacitance 119877

119887is resistance of the

barrier layer and 119862119887is barrier layer capacitance In order

to account for nonideal frequency response it is commonlyaccepted to employ constant phase element (CPE) denotedby 119876 which has a noninteger power dependence on thefrequency instead of pure capacitance The impedance of aCPE is defined as

119885CPE = 119884minus1

(119895120596)minus119899

(2)

where 119884 is the proportional factor 119895 is radic minus 1 120596 is thefrequency and minus1 lt 119899 lt 1 has the meaning of a phase shift If119899 = 1119876 is pure capacitance and if 119899 = 0119876 is pure resistance

The equivalent circuit used for fitting the experimentaldata of the present study has been found to be similarto earlier proposed circuit for Ti and its alloys [39ndash41]The fitting quality was evaluated by chi-square value whichwas found to be in the order of 10minus3-10minus4 and the relativeerror values were below 10 The fitting parameters used tosimulate EIS data for NiTi alloy of different surface finishesare given in Table 6

In the present study a bell shaped Bode plot wasobtained for bare NiTi (Figure 11(a)) Hang et al reportedsimilar behavior for NiTi substrate in PBS solution [42]The electropolished and passivated alloys exhibited two-timeconstant behavior consisting of outer porous layer whoseresistance is 119877

119901and an inner barrier layer whose resistance is

119877119887 These surfaces showed a typical behavior of a corrosion

resistant surface exhibiting a near capacitive response asillustrated by a phase angle close to minus90∘ over a wide rangeof frequencies suggesting that a very stable passive film wasformed after surface treatment of NiTi alloy with a doublelayer structure The polarization resistance value of bareNiTi sample was given by 38 times 104Ωcm2 The resistanceof the porous layer for both electropolished and passivatedsamples was respectively 67 and 42Ωcm2 The barrierlayer resistances of these samples were found to be in theorder of 106Ωcm2 Therefore the outer porous layer was notefficient in preventing the attack of corrosive ions but theinner barrier layer could withstand their attack There was100 times increase in polarization resistance (119877

119901+ 119877119887) values

for all the surface treated alloys in comparison with untreatedalloys indicating that electropolishing and passivation cancontrol the charge transfer at substrateelectrolyte interfaceand hence improved the corrosion resistance Further the119877119887value for the passivated sample was almost three times

higher than electropolished samples In general the term ldquo119899rdquosignifies the surface roughness of the working electrode Thedeviation of ldquo119899rdquo fromunity indicates an uneven surface finishThe ldquo119899rdquo valuewas found to be the least (Table 6) for passivatedsamples due to the chemical reaction at the solutionsample

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

10 Advances in Biomaterials

Table 6 Fitted values for simulative EIS spectra of untreated and treated alloys

Sample Circuit 119876119901

(Ss119899cmminus2) 119899 119877119901

(Ωcm2) 119876119887

(Ss119899cmminus2) 119899 119877119887

(Ωcm2)Bare NiTi 119877(119876119877) 36 times 10minus5 094 38 times 10+4

EP20V 119877(119876(119877(119876119877))) 42 times 10minus7 094 6718 79 times 10minus6 096 12 times 10+6

EP20VPIE 119877(119876(119877(119876119877))) 54 times 10minus6 098 42 43 times 10minus7 088 30 times 10+6

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

100 0

101

102

103

104

105

|Z|

(Ohm

)

minus20

minus40

minus60

minus80

120579(d

eg)

(a)

Frequency (Hz)

100

101

102

103

104

105

106

107

10minus2 10minus1 100 101 102 103 104 1050

minus20

minus40

minus60

minus80

120579(d

eg)

|Z|

(Ohm

)

(b)

100

101

102

103

104

105

106

107

|Z|

(Ohm

)

Frequency (Hz)10minus2 10minus1 100 101 102 103 104 105

0

minus20

minus40

minus60

minus80

120579(d

eg)

(c)

Figure 11 Bode plots of NiTi alloy with various surface treatments (a) bare NiTi (b) EP20V and (c) EP20VPIE

Qb

Rs

Rb

(a)

Qb

Rs

Rb

Qp

Rp

(b)

Figure 12 Equivalent circuit for the interpretation of experimental Bode diagrams of (a) bare NiTi and (b) electropolished and passivatedalloys

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Advances in Biomaterials 11

interface which modified the surface finish The 119877119886value

obtained fromAFM studies also confirmed a rougher surfaceafter post-treatment

The barrier layer capacitance 119876119887 for bare NiTi EP20V

and EP20VPIE was 36 79 and 043 120583Ssncmminus2 respectivelywhich means that the thickness of the barrier layer formedaftermechanical polishing was very thin compared to surfacetreated alloys The capacitance 119862 and the thickness 119889 arerelated by the equation 119862 = 120576120576

0119860119889 where 120576 is the dielectric

constant of the barrier 1205760the vacuum permittivity 119860 the

area and 119889 the thickness of the oxide layer Therefore thehigher the capacitance the lower the thickness of the oxideformed The oxide layer thickness increased almost 85 timesafter electropolishing and passivation when compared tomechanically polished samples The barrier layer resistance119877119887 also gave the same conclusion in which the barrier layer

resistance of bare NiTi was almost 100 times lower thanthat of surface treated alloys The thickness of the oxidelayer of passivated samples was almost 18 times higher thanelectropolished samples This indicated better protectioncapacity of the oxide formed after post-treatment

35 Nickel Release Surface modification by electropolishingprocess aids in removing the defective passive layer and dueto the formation of uniform surface a more homogenouspassive layer will be formed which can effectively preventthe release of nickel The results of cumulative nickel releaserate measured as a function of immersion duration in Hanksrsquosolution for a period of 14 days were given in Figure 13

It was evident from Figure 13 that electropolishing pro-cess significantly reduced the nickel elution compared tobare NiTi Passivated NiTi alloy showed lowest nickel ionrelease The trend in release rate was altered due to surfacemodification even though it could not completely prevent thenickel elution For bareNiTi alloy the amount of nickel elutedwas 640 ppb after 2 weeks of immersion Passivated surfacewhich has the lowest water contact angle showed minimalamount of nickel release among all the samples This maybe due to the increase in the thickness andor compactnessof the passive titania layer which reduced the harmful nickelelution

Native titaniumoxide would be formed spontaneously onthe surface of fresh cut NiTi alloy due to surface oxidationeven at ambient conditions After mechanical polishing thesurface possesses several scratch defects The native oxideformed onNiTi alloymay not be uniform due to the presenceof such defects Electropolishing process aids in forming ahomogenous surface and so the oxide layer formed wouldbe almost defect free During the process of electropolishingbecause of the applied potential a polishing film will form atthe anode surfacewhich controls the anodic dissolution of thesubstrate Peaks which receive higher current densities willbe selectively etched compared to the valleys resulting in asmooth surface finish Along with anodic dissolution oxygenevolution will also occur at the anode Various reactions

4 6 8 10 12 14100

200

300

400

500

600

700

Days

Bare NiTiEP20VEP20VPIE

Nic

kel r

eleas

e (pp

bcm

2)

Figure 13 Nickel ion release measured as a function of immersionduration in Hanksrsquo solution

occurring at the electrodes during electropolishing can bewritten as

2H2O (aq)

997888rarr O2(g) + 4H+ (aq) + 4eminus (Anodic oxidation)

2H2O (aq) + 2eminus

997888rarr H2(g) + 2OHminus (aq) (Cathodic reduction)

(3)

Nickel existing in nonoxidized state is more liable to dissolu-tion and oxidation [34] Initial reaction at the anode will befield assisted dissolution which may result in the migrationof titanium and nickel ions to the polishing filmelectrolyteinterface and it chemically dissolves in perchloric acid

TiO2+ + 4ClO4

minus

997888rarr Ti (ClO4)4

+

1

2

O2+ 2eminus

Ni2+ + 2ClO4

minus

997888rarr Ni (ClO4)2

(4)

Due to the applied field the outward diffusion of nickel fromthe substrate surface will be more and hence the electrolyticsolution will be enriched with nickel and NiTi alloy surfacewith titanium even though the bulk composition remainsunchanged

Electropolished and passivated samples after polarizationtest did not showanypits at the surface which is a characteris-tic feature of an inclusionprecipitate free surface Passivationusing saturated potassium periodate at 95∘C resulted in theoxidation of NiTi alloy The improved surface oxidation ofNiTi alloy was also evident in the AFM images in which thenanosize peaks formed after electropolishing were convertedtomicrosize peaks after passivation Capacitive behavior overa wide range of frequencies supported the compactness ofthe oxide layer formed although some amount of unoxidized

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

12 Advances in Biomaterials

titanium existed at the surface of the electropolished samplePassivating the electropolished samples in periodate solutionassisted in the complete oxidation of titanium to titania andhence the compactness of the oxide was further enhancedThis was also supported by in vitro nickel release analysiswhich showed that due to passivation nickel ion release wassignificantly reduced when compared to untreatedNiTi alloy

The present study established that electropolishing andpostpassivation treatment result in a remarkable increasein the corrosion resistance and biocompatibility of NiTialloy The study was mainly focused on establishing theelectrochemical behavior of the surface modified alloy onexposure to simulated body fluids for short term period Butfrom a biomaterial application point of view the materialneeds to be evaluated further for its electrochemical behaviorand nickel release rate on exposure to longer time durationAnother important aspect of the use of these materialsfor implant applications requires establishing osseointegra-tion An understanding of osseointegration behavior can beachieved by studying the growth characteristics of hydrox-yapatite on the NiTi alloy surface on exposure to simulatedbody fluids and these studies show a great promise for futureresearch

4 Conclusions

In the present study electropolishing of equiatomicNiTi alloywas carried out using perchloric acid based solution Goodelectropolished surface was obtained within a short durationof 30 sec Passivation at 95∘C using potassium periodatesolution improved the hydrophilicity of the alloy due tothe formation of microsized nodules distributed over thesurface The passive film formed after surface treatment wasmore compact and uniform and no pits could be noticed asobserved for mechanically polished samples The TiNi ratiosubstantially increased after passivation The nickel contentat the surface of the passivated sample was the least andhence the amount of nickel eluted out was also minimumThe barrier layer resistance increased thrice when comparedto electropolished samples due to the increased stability ofthe oxide layer formed after passivation Electropolishing inperchloric acid based electrolyte and passivation in potas-sium periodate solution would be beneficial for enhancingthe biomedical properties of NiTi shape memory alloys

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors thank the Director NAL Council of Scientificand Industrial Research (CSIR) New Delhi for giving per-mission to publish this work The authors also thank presentand past Heads SED for the support The authors wish tothank Dr S K Bhaumik and his group for providing theNiTi alloy used in this study Help received fromMr Praveen

Kumar V for AFM Mrs Premlatha for surface roughnessmeasurement Mr Srinivas G for XRD Mr Siju for SEMMr Bharathidasan T for contact angle measurements DrParthasarathi Bera for XPS and Mrs Anila Kumari for AASis greatly appreciated One of the authors (Manju Chembath)acknowledges the financial assistance in the form of SeniorResearch Fellowship from CSIR-National Aerospace Labora-tories India

References

[1] M Es-Souni M Es-Souni and H Fischer-Brandies ldquoAssessingthe biocompatibility of NiTi shape memory alloys used formedical applicationsrdquo Analytical and Bioanalytical Chemistryvol 381 no 3 pp 557ndash567 2005

[2] C Trepanier R Venugopalan and A R Pelton Shape MemoryImplants edited by L Yahia NDC 2000

[3] P Rocher L El Medawar J-C Hornez M Traisnel J Bremeand H F Hildebrand ldquoBiocorrosion and biocompatibility ofNiTi alloysrdquo European Cells and Materials vol 9 no 1 pp 23ndash24 2005

[4] B Yuan M Lai Y Gao C Y Chung and M Zhu ldquoThe effectof pore characteristics on Ni suppression of porous NiTi shapememory alloysmodified by surface treatmentrdquoThin Solid Filmsvol 519 no 15 pp 5297ndash5301 2011

[5] Z D Cui H C Man and X J Yang ldquoThe corrosion and nickelrelease behavior of laser surface-melted NiTi shape memoryalloy in Hanksrsquo solutionrdquo Surface and Coatings Technology vol192 no 2-3 pp 347ndash353 2005

[6] S A Bernard V K Balla N M Davies S Bose and ABandyopadhyay ldquoBone cell-materials interactions and Ni ionrelease of anodized equiatomic NiTi alloyrdquo Acta Biomaterialiavol 7 no 4 pp 1902ndash1912 2011

[7] L A Bravo A G de Cabanes J M Manero E Ruperez and FJ Gil ldquoNiTi superelastic orthodontic archwires with polyamidecoatingrdquo Journal ofMaterials ScienceMaterials inMedicine vol25 no 2 pp 555ndash560 2014

[8] F Sun K N Sask J L Brash and I Zhitomirsky ldquoSurfacemodifications of Nitinol for biomedical applicationsrdquo Colloidsand Surfaces B Biointerfaces vol 67 no 1 pp 132ndash139 2008

[9] Y Cheng and Y F Zheng ldquoThe corrosion behavior andhemocompatibility of TiNi alloys coated with DLC by plasmabased ion implantationrdquo Surface and Coatings Technology vol200 no 14-15 pp 4543ndash4548 2006

[10] W Simka M Kaczmarek A Baron-Wiechec G Nawrat JMarciniak and J Zak ldquoElectropolishing and passivation ofNiTishape memory alloyrdquo Electrochimica Acta vol 55 no 7 pp2437ndash2441 2010

[11] M Kaczmarek W Simka A Baron J Szewczenko and JMarciniak ldquoElectrochemical behavior of Ni-Ti alloy after sur-face modificationrdquo Journal of Achievements in Materials andManufacturing Engineering vol 18 pp 111ndash114 2006

[12] W Wu X Liu H Han D Yang and S Lu ldquoElectropolishingof NiTi for improving biocompatibilityrdquo Journal of MaterialsScience and Technology vol 24 no 6 pp 926ndash930 2008

[13] G Bolat DMareci S Iacoban N Cimpoesu andCMunteanuldquoThe estimation of corrosion behavior of NiTi and NiTiNballoys using dynamic electrochemical impedance spectroscopyrdquoJournal of Spectroscopy vol 2013 Article ID 714920 7 pages2013

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Advances in Biomaterials 13

[14] B G Pound ldquoSusceptibility of nitinol to localized corrosionrdquoJournal of Biomedical Materials Research Part A vol 77 no 1pp 185ndash191 2006

[15] W Haider and N Munroe ldquoAssessment of corrosion resistanceand metal ion leaching of nitinol alloysrdquo Journal of MaterialsEngineering and Performance vol 20 no 4-5 pp 812ndash815 2011

[16] R A Silva I P Silva and B Rondot ldquoEffect of surfacetreatments on anodic oxide film growth and electrochemicalproperties of tantalum used for biomedical applicationsrdquo Jour-nal of Biomaterials Applications vol 21 pp 93ndash103 2006

[17] T Hu Y C Xin S L Wu et al ldquoCorrosion behavior onorthopedic NiTi alloy with nanocrystallineamorphous sur-facerdquoMaterials Chemistry and Physics vol 126 no 1-2 pp 102ndash107 2011

[18] W Haider N Munroe C Pulletikurthi P K S Gill andS Amruthaluri ldquoA comparative biocompatibility analysis ofternary nitinol alloysrdquo Journal of Materials Engineering andPerformance vol 18 no 5-6 pp 760ndash764 2009

[19] O Cisse O SavadogoMWu and LH Yahia ldquoEffect of surfacetreatment of NiTi alloy on its corrosion behavior in Hanksrsquosolutionrdquo Journal of Biomedical Materials Research vol 61 no3 pp 339ndash345 2002

[20] T Hryniewicz ldquoConcept of microsmoothing in electropolish-ing processrdquo Surface amp Coatings Technology vol 64 no 2 pp75ndash80 1994

[21] L Neelakantan M Valtiner G Eggeler and A W Hasse ldquoSur-face chemistry and topographical changes of an electropolishedNiTi shapememory alloyrdquo Physica Status Solidi (A) Applicationsand Materials Science vol 207 no 4 pp 807ndash811 2010

[22] C L Chu R M Wang T Hu et al ldquoSurface structureand biomedical properties of chemically polished and elec-tropolished NiTi shape memory alloysrdquo Materials Science andEngineering C vol 28 no 8 pp 1430ndash1434 2008

[23] D Batalu and H Guoqiu ldquoImprovement of the corrosionresistance of equiatomic NiTi shape memory alloy for medicalimplants by the electropolishing methodrdquo UPB Scientific Bul-letin B vol 71 p 832 2009

[24] K Fushimi M Stratmann and A W Hassel ldquoElectropolishingof NiTi shape memory alloys in methanolic H

2

SO4

rdquo Elec-trochimica Acta vol 52 no 3 pp 1290ndash1295 2006

[25] F Feigl and V Anger Spot Tests in Inorganic Analysis ElsevierScience BV Amsterdam The Netherlands 6th edition 2012

[26] J-X Liu D-Z Yang F Shi and Y-J Cai ldquoSol-gel depositedTiO2

film on NiTi surgical alloy for biocompatibility improve-mentrdquoThin Solid Films vol 429 no 1-2 pp 225ndash230 2003

[27] J H Yu L CWu J T Hsu Y Y Chang H H Huang andH LHuang ldquoSurface roughness and topography of four commonlyused types of orthodontic archwirerdquo Journal of Medical andBiological Engineering vol 31 no 5 pp 367ndash370 2011

[28] N Eliaz and O Nissan ldquoInnovative processes for electropol-ishing of medical devices made of stainless steelsrdquo Journal ofBiomedical Materials Research A vol 83 no 2 pp 546ndash5572007

[29] F L Nie Y F Zheng Y Cheng S C Wei and R ZValiev ldquoIn vitro corrosion and cytotoxicity on microcrystallinenanocrystalline and amorphous NiTi alloy fabricated by highpressure torsionrdquoMaterials Letters vol 64 no 8 pp 983ndash9862010

[30] T Hu C-L Chu L-H Yin et al ldquoIn vitro biocompatibilityof titanium-nickel alloy with titanium oxide film by H

2

O2

oxidationrdquo Transactions of Nonferrous Metals Society of Chinavol 17 no 3 pp 553ndash557 2007

[31] X Zhu J Chen L Scheideler R Reichl and J Geis-GerstorferldquoEffects of topography and composition of titanium surfaceoxides on osteoblast responsesrdquo Biomaterials vol 25 no 18 pp4087ndash4103 2004

[32] C C Annarelli J Fornazero R Cohen J Bert and J-LBesse ldquoColloidal protein solutions as a new standard sensorfor adhesive wettability measurementsrdquo Journal of Colloid andInterface Science vol 213 no 2 pp 386ndash394 1999

[33] Z Huan L E Fratila-Apachitei I Apachitei and J DuszczykldquoPorous NiTi surfaces for biomedical applicationsrdquo AppliedSurface Science vol 258 no 13 pp 5244ndash5249 2012

[34] D Vojtech J Fojt L Joska and P Novak ldquoSurface treatmentof NiTi shape memory alloy and its influence on corrosionbehaviorrdquo Surface and Coatings Technology vol 204 no 23 pp3895ndash3901 2010

[35] D Vojtech M Voderova J Fojt P Novak and T KubasekldquoSurface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloyrdquo Applied Surface Science vol257 no 5 pp 1573ndash1582 2010

[36] D R Lide CRC Handbook of Chemistry and Physics Taylor ampFrancis Group Boca Raton Fla USA 89th edition 2008

[37] X-J Yan and D-Z Yang ldquoCorrosion resistance of a laser spot-welded joint of Ni-Ti wire in simulated human body fluidsrdquoJournal of Biomedical Materials Research vol 77 no 1 pp 97ndash102 2006

[38] T Sun and M Wang ldquoA comparative study on titania layersformed onTi Ti-6Al-4V andNiTi shapememory alloy througha low temperature oxidation processrdquo Surface and CoatingsTechnology vol 205 no 1 pp 92ndash101 2010

[39] H Maleki-Ghaleh V Khalili J Khalil-Allafi and M JavidildquoHydroxyapatite coating on NiTi shape memory alloy byelectrophoretic deposition processrdquo Surface and Coatings Tech-nology vol 208 pp 57ndash63 2012

[40] I Milosev T Kosec and H-H Strehblow ldquoXPS and EIS studyof the passive film formed on orthopaedic Ti-6Al-7Nb alloy inHankrsquos physiological solutionrdquo Electrochimica Acta vol 53 no9 pp 3547ndash3558 2008

[41] M Attarchi M Mazloumi I Behckam and S K SadrnezhaadldquoEIS study of porous NiTi biomedical alloy in simulated bodyfluidrdquoMaterials and Corrosion vol 60 no 11 pp 871ndash875 2009

[42] R Hang S Ma and P K Chu ldquoCorrosion behavior of DLC-coated NiTi alloy in the presence of serum proteinsrdquo Diamondand Related Materials vol 19 no 10 pp 1230ndash1234 2010

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CeramicsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CompositesJournal of

NanoparticlesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

International Journal of

Biomaterials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MetallurgyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BioMed Research International

MaterialsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Nano

materials

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofNanomaterials