M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

ava i l ab l e a t www.sc i enced i r ec t . com

www.e l sev i e r . com/ loca te /matcha r

Thermal oxidation of CP Ti — An electrochemical andstructural characterization

Satendra Kumara, T.S.N. Sankara Narayanana,⁎,S. Ganesh Sundara Ramanb, S.K. Seshadrib

aNational Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai-600 113, IndiabIndian Institute of Technology Madras, Chennai-600 036, India

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +91 44 2254 207E-mail addresses: sat_nml@rediffmail.com

1044-5803/$ – see front matter © 2010 Elsevidoi:10.1016/j.matchar.2010.03.002

A B S T R A C T

Article history:Received 10 June 2009Received in revised form3 March 2010Accepted 4 March 2010

In the present study commercially pure titanium (CP Ti) samples were oxidized thermally atthree different temperatures (500, 650 and 800 °C) for 24 h and evaluation of theirmorphological and structural characteristics, microhardness and corrosion resistance inRinger's solution was done. The corrosion protective ability of thermally oxidized materialsshows a strong dependence on the nature and thickness of the surface oxide layer. Based onthe corrosion protective ability, the untreated and thermally oxidized samples can beranked as follows: CP Ti (800 °C)>CP Ti (650 °C)>CP Ti (500 °C)>untreated CP Ti.

© 2010 Elsevier Inc. All rights reserved.

Keywords:Thermal oxidationCP TitaniumX-ray diffractionElectrochemical characterizationBiomedical application

1. Introduction

Titanium and titanium alloys are widely used as bio-implantmaterials, particularly for orthopaedic and osteosynthesisapplicationsdue to their lowdensity, excellent biocompatibility,corrosion resistance andmechanical properties [1–4]. One of theimportant reasons for choosing thesematerials is their ability toshowpassivity. The passive layer, typically of 4–6 nmthickness,comprises of either amorphous or poorly crystallized non-stoichiometric TiO2. This layer is highly stable, providesprotection against the harmful effects of aggressive environ-ments and is responsible for the higher corrosion resistance.However, it has been reported that under in vivo conditions thestability of the passive layer could be altered [5]. Analyses ofretrieved implants have shown accumulation of ions on tissues

7; fax: +91 44 2254 1027.(S. Kumar), tsnsn@redif

er Inc. All rights reserved

adjacent to the implant [5]. It may be due to the inferiormechanical properties of thenative formsofTiO2 films and theymight have got disrupted at very low shear stresses, even byrubbing against soft tissues [6]. The passive layer is subse-quently formed upon reaction with the local environment.However, the wear debris and the metal ions released duringfracture of the passive layer can cause adverse tissue reactions.Fretting and sliding wear conditions can also lead to fracture ofthe passive layer [7–10] and under extreme conditions, they cancause loosening and eventual failure of the implant. The abovelimitations preclude the use of Ti and its alloys for articulatingsurfaces.

Numerous surface modification methods such as chemicaltreatment (acid and alkali treatment) [11,12], electrochemicaltreatment (anodic oxidation) [13], sol–gel [14], chemical vapour

fmail.com (T.S.N.S. Narayanan).

.

590 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

deposition [15], physical vapour deposition [16], plasma spraydeposition [17], ion implantation [18], thermal oxidation [19],etc. havebeen explored to impart thedesired surfaceproperties,especially the corrosion and tribo-corrosion resistance, on thecurrently used implant materials. One of the simplest methodsto generate a barrier layer on titanium is to treat it thermally in afurnace inair atmosphere,whichproducesa surfaceoxide layer.Thermal oxidation treatments aimed to obtain in situ ceramiccoatings, mainly based on rutile, can offer thick, highlycrystalline oxide films, which are accompanied with thedissolution of oxygen beneath them. This method has alreadybeen investigated formanybiomaterialswitha focus to improvethe hardness and wear resistance [20–23]. However, only a fewreports are available on the corrosion behaviour of thermallyoxidized commercially pure (CP) Ti. A comparative study on thecorrosion resistance of thermally oxidized CP Ti at differenttemperatures is rather limited.

The thermally oxidized surface of titanium shows betterproperties than the others, since it produces a thick, highlycrystalline rutile oxide film [24]. However, the oxide layerformed by thermal oxidation, at high temperatures (>800 °C)and for prolonged time duration, results in a thick TiO2 layerthat leads to spallation of oxide layer [25]. The large volumeratio of rutile to Ti (1.73) [26], large lattice mismatch and thelarge difference in coefficient of thermal expansion betweenthe rutile and titanium are considered responsible for thespallation of the oxide layer from the substrate [27]. Thus, anextensive study is needed to find the proper oxidationtemperature and time that can produce an adherent andthick layer of surface oxide, preferably with rutile structuresince rutile phase is more inert and resistant to attack in hotreducing acids compared to the anatase phase [24].

To ascertain the suitability of an implant material, severalproperties must be evaluated. Amongst these, corrosionresistance is of special interest, because the metal ionsreleased from the implant to the surrounding tissues maygive rise to biocompatibility problems. The present study aims(i) to study thermal oxidation behaviour of CP Ti at differenttemperatures, namely, 500, 650 and 800 °C for 24 h; and (ii) toevaluate the corrosion resistance of thermally oxidized CP Tiin Ringer's solution (simulated body fluid environment); andto identify the suitable conditions for the thermal oxidation ofCP Ti.

2. Experimental Details

CP Ti grade-2 (chemical composition in wt.%: N: 0.01; C: 0.03;H: 0.01; Fe: 0.20; O: 0.18 and Ti: Balance) was used to study thethermal oxidation behaviour. Before subjecting CP Ti samplesfor oxidation process, they were mechanically polished usingvarious grades of SiC paper (60, 100, 220, 320, 400, 600 and 1/0,respectively) at a very slow speed, thoroughly rinsed indeionized water and dried using a stream of compressed air.Proper care is taken during polishing so that the extent ofplastic deformation will be very less to cause any significantchange in the oxidation kinetics/mechanism. After polishing,the samples were rinsed in deionized water and dried using astream of compressed air. Thermal oxidation was carried outin atmospheric air in a rectangular furnace (LENTON make) at

500, 650 and 800 °C for 24 h. The samples were kept inside thefurnace. Subsequently they were heated to the requiredtemperature at a heating rate of 5 °C/min. Once the temper-ature of the furnace was reached to the required temperatureset using digital controller the time for oxidation wasrecorded. After oxidation treatment to the set temperatureand time, furnace was automatically cut off from the powersupply, coolingwas done in a regular fashion, and the sampleswere allowed to cool in the furnace itself. The maximumtemperature employed for oxidation was restricted to 800 °Cfor 24 h because thermal oxidation of CP Ti done at tempera-tures higher than 800 °C and for longer time leads todebonding of the oxide layer [25,26]. Samples thermallyoxidized at 1100 °C for 24 h followed by furnace cooling exhibitspallation of oxide layers.

The phase constituents of untreated and thermally oxidizedsamples and the nature of the oxide film formed on the surfaceat different temperatures were determined using X-ray diffrac-tion measurements(D8 DISCOVER, Bruker AXS) with Cu-Kα

radiation in the 2θ range of 20 to 90° at a scan speed of 1 s/stepwith an increment of 0.1°. The thickness of the oxide layer aswell as its surface morphology was assessed by scanningelectron microscopy. Atomic force microscopy was used toassess the surface morphology of the representative sampleoxidized at 800 °C for 24 h and the sectional analysis was alsodone to measure the roughness of the oxides. Atomic forcemicroscopy was used to understand the growth of the oxidelayer at nanometer scale. The microhardness of the untreatedand thermally oxidized samples was measured at the surfaceusing a Leica VMHTMOT microhardness tester using Vicker'sdiamond pyramid indenter at a load of 200 gf applied for 15s.Seven indentations were made on each sample and the valueswere averaged out. Roughness measurements were performedon the surface of the samples before and after the treatmentsusing a Mitutoyo SJ-301 stylus surface profilometer.

The corrosion behaviour of untreated and thermallyoxidized samples was evaluated by potentiodynamic polari-zation and electrochemical impedance spectroscopic studiesusing a potentiostat/galvanostat/frequency response analyzerof ACM instruments (model: Gill AC). Ringer's solution, havinga chemical composition (in g/l) of 9 NaCl, 0.24 CaCl2, 0.43 KCland 0.2 NaHCO3 (pH: 7.8), that chemically simulates thephysiological medium of the human body, which is alsoreferred as simulated body fluid, was used as the electrolytesolution. The temperature was maintained to 27±1 °C duringthe experiments. Before performing the corrosion studies, theuntreated CP Ti sample was mechanically polished usingvarious grades of SiC paper (60, 100, 220, 320, 400, 600, 1/0, 2/0,3/0 and 4/0, respectively), followed by 0.3 µm alumina paste toa mirror finish, rinsed with deionized water, pickled using amixture of 35 vol.% HNO3, 5 vol.% HF and balance water at40 °C, as described by Schutz and Thomas [28], thoroughlyrinsed in deionized water and dried using a stream ofcompressed air. Whereas, the thermally oxidized sampleswere rinsed in deionized water and dried using a stream ofcompressed air. The cleaned samples, either untreated orthermally oxidized, formed the working electrode while asaturated calomel electrode and a graphite rod served as thereference and auxiliary electrodes, respectively. These elec-trodes were placed in a flat cell in such a way that only 1 cm2

Fig. 1 – Surface morphology of the oxidized CP Ti at (a) 500 (b) 650 and (c) 800 °C for 24 h in air atmosphere.

591M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

area of the working electrode was exposed to the electrolytesolution.

Potentiodynamic polarization tests were carried out at ascan rate of 100 mV/min from −250 to +3000 mV vs. saturatedcalomel electrode (SCE) with respect to the open circuitpotential. Before polarization tests the samples were kept for60 min for stabilization. A very high anodic potential(+3000 mV vs. SCE) was chosen to study the passivationbehaviour of untreated and thermally oxidized samples inRinger's solution. The corrosion potential and corrosioncurrent density were determined from the polarization curvesusing Tafel extrapolationmethod. The passive current densitywas determined from the anodic region of the polarizationcurve. Electrochemical impedance spectroscopy study wasconducted by applying sinusoidal signal amplitude of 32 mVand the electrode response was analyzed in the frequencyrange between 10,000 Hz and 0.01 Hz in Ringer's solution attheir respective open circuit potential. The charge transferresistance and double layer capacitance values were deter-mined from the Nyquist plot by fitting the data using

Boukamp software. The potentiodynamic polarization andelectrochemical impedance spectroscopy studies were repeat-ed at least 3 times to ensure reproducibility of the test results.

3. Results and Discussion

3.1. Surface Morphology and Thickness of the Oxide FilmFormed on CP Ti

The surfacemorphology of thermally oxidized CP Ti at 500, 650and 800 °C for 24 h clearly reveals the presence of oxide scalesthroughout the surface without spallation, irrespective of theoxidation temperature (Fig. 1 (a–c)). A closer examination ofthe surface morphology of oxidized samples at differenttemperatures gives an idea about the nature of the oxidefilm. Surface morphology clearly revealed a thin oxide scale isformed over the substrate at 500 °C, which is relatively smoothwhen compared to samples oxidized at 650 and 800 °C (Fig. 1).For samples oxidized at 650 °C, small grains of TiO2 are

Fig. 3 – AFM surface morphology and the section analysis ofthe sample oxidized at 800 °C for 24 h.

592 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

observed due to the agglomeration of oxide which makes thescale thicker whereas for samples oxidized at 800 °C, thesurface is completely covered with the oxide islands (con-firmed by the X-ray diffraction (XRD)) and the grains areoriented perpendicular to the substrate. The surface morphol-ogy of the oxide film formed on the samples oxidized at 500,650 and 800 °C suggests that nucleation of oxide takes placethroughout the surface of the samples when it immediatelycomes in contact with oxygen. The growth mode involves theformation of a thin oxide scale followed by its agglomerationand growth to completely cover the surface. Similar observa-tions were also made earlier by Garcia-Alonso et al. [29] intheir study on oxidation of Ti6Al4V alloy at 700 °C for 1h. Theyhave reported that the surface is fully covered with oxideislands and the morphology, size and distribution areconsistent with the α-phase of the substrate. According toSiva Rama Krishna et al. [23], the oxide layer formed afteroxidation of CP Ti at 850 °C for 5 h followed by either furnace orair cooling is comprised of square-based columnar grains withangular tips. The cross-sectional view of CP Ti oxidized at650 °C for 24 h reveals the presence of a thick and homogenousoxide layer of approximately 30 µm thick over the substrate(Fig. 2). The atomic forcemicroscopy (AFM) surface topographyclearly reveals the growth of the oxide layer is homogenousand the direction of growth is perpendicular to the substratethat supports the observation made by scanning electronmicroscopy (SEM) surface morphology. The sectional analysisof the sample oxidized at 800 °C for 24 h is given in Fig. 3. Theroot mean square roughness value of the sample measuredfrom section analysis using AFM topography was found to145.69 nm. Roughness measurements were performed on thesamples before and after the thermally oxidized sample at 500,650 and 800 °C temperature for 24 h time duration, usingsurface profile meter. The mean surface roughness (Ra) isdetermined and compiled in Table 1. While the untreatedsamples have a Ra of 0.15 µm, after the treatment and with the

Fig. 2 – Scanning electron micrograph showing the crosssection of CP Ti sample thermally oxidized at 650 °C for 24 hin air atmosphere.

increase in temperature the roughness value increase up to0.45 µm is observed. The rougher surface of the treatedsamples can be ascribed to the growth mechanism of theoxide layer: with the used treatment conditions, as temper-ature increases this layer becomes porous and develops astratified structure, increasing the surface roughness [30,31].

3.2. Structural Characteristics of the Untreated andThermally Oxidized CP Ti

The XRD patterns of untreated and thermally oxidized CP Tisamples are shown in Fig. 4. Untreated CP Ti is entirelycomprised of hexagonal α-phase (denoted as ‘Ti’ in Fig. 4).Since the depth of penetration of Cu-Kα radiation is in therange of 10 to 20 µm, the presence of α-Ti peaks in the XRDpattern of thermally oxidized CP Ti samples with thinneroxide layer is quite evident. The XRD patterns of thermallyoxidized samples exhibit the presence of rutile and oxygendiffused Ti (Ti(O)) as the predominant phase along with asmall amount of α-Ti. The samples oxidized at 500 °C showspredominantly the α-Ti, and Ti(O) peaks. The samplesoxidized at 650 °C exhibit the presence of α-Ti, Ti(O) and rutilepeaks (denoted as ‘R’ in Fig. 4) with rutile being thepredominant one. The samples oxidized at 800 °C exhibit

Table 1 – Surfacemicrohardness and corrosion parameters of CP Ti in untreated and thermally oxidized conditions (500, 650and 800 °C for 24 h).

Condition Surface Roughness,Ra (µm)

Surface microhardness(HV0.2)

icorr (µA/cm2) Ecorr (mV) ipass (µA/cm2) Rct (Ω cm2) Cdl (F)

Untreated 0.15 178±6 11.7×10−1 −474 25.12 3.1×104 9.9×10−5

Oxidized at 500 °C 0.35 200±7 2.2×10−2 −275 0.67 5.2×106 3.8×10−6

Oxidized at 650 °C 0.39 443±20 2.9×10−4 −396 0.05 3.8×107 2.7×10−8

Oxidized atz 800 °C 0.45 1186±42 3.9×10−4 −291 0.05 2.3×108 2.5×10−9

593M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

only the rutile peaks. Similar observations were also made byother researchers [20,23,29]. Siva Rama Krishna et al. [23] havereported the formation of Ti(O) at temperatures less than700 °C and rutile at and above 800 °C as the dominant phasefollowing oxidation of titanium. Guleryuz and Cimenoglu[19,32] have reported the presence of anatase phase forsamples oxidized at 600 °C for 24 and 48 h whereas rutilewas the only dominated phase when the samples wereoxidized at 650 °C for 48 h. However, no anatase phase isfound in samples oxidized at all the three temperature studiedin the present work.

The presence of only the rutile phase suggests the formationof a thick oxide film on samples oxidized at 800 °C; the presenceofα-Ti andTi(O) peaksat 500 °C indicates the formationof a thinoxide filmwhereas thepresenceofα-Ti, Ti(O) andrutile at 650 °Cpoint out that the thicknessof the oxide layerwould lie betweenthoseoxidizedat500 °Cand800 °C.ThepresenceofbothTi(O) aswell as rutilepeakssuggests that the extent of rutile formation isrelatively less at 650 °C, which would otherwise lead to theabsence of the Ti(O) peaks [33]. Though an oxygen diffused zoneis likely to format and above 800 °C, the rutile layer formed is sothick that the X-rays could not penetrate up to the Ti(O) region.

3.3. Microhardness of Untreated and Thermally OxidizedCP Ti

The microhardness of untreated and thermally oxidizedsamples measured on the surface of the oxide layer is given

Fig. 4 – XRD pattern of untreated and thermally treated CP Tisamples at different temperatures such as 500, 650 and800 °C for 24 h time duration.

in Table 1. The microhardness of untreated CP Ti is 178±6HV0.2. Thermally oxidized samples exhibit a significantincrease in hardness due to the formation of hard oxide layersand the strains evolved during the dissolution of oxygenbeneath the oxide layer of the substrate [34,35]. Almost a sixfold increase in hardness is observed for samples oxidized at800 °C compared to that of untreated. The scattering inmicrohardness appears to increase with increase in oxidationtemperature. This may be due to the microstructural inho-mogenities, i.e., the formation of oxide islands with theirgrains oriented perpendicular to the surface. It has beenreported elsewhere that thermal oxidation would lead toincrease in surface roughness of the oxide film due to thedifferential oxidation rate of individual grains of the polycrys-talline alloy [36,37]. The increase in surface roughness couldalso be a contributing factor for the scattering of the hardnessvalues.

3.4. Potentiodynamic Polarization Studies of Untreatedand Thermally Oxidized CP Ti

The potentiodynamic polarization curves of untreated andthermally oxidized CP Ti in Ringer's solution in the potentialrange of −250 mV to +3000 mV vs. saturated calomelelectrode (SCE) with respect to open circuit potential (OCP)are shown in Fig. 5. The higher anodic potential (+3000 mVvs. SCE) was chosen to observe the effect of oxidationconditions and consequently the oxide layer thickness on

Fig. 5 – Potentiodynamic polarization curves of untreated andthermally treated CP Ti samples at different temperaturessuch as 500, 650 and 800 °C for 24 h time duration.

Fig. 6 – (a) Nyquist plots of untreated and thermally treated CPTi samples at different temperatures such as 500, 650 and800 °C for 24 h time duration; [Inset: High frequency regionsof the Nyquist plots in expanded scale]. (b) Bode Impedanceplots of untreated and thermally treated CP Ti samples atdifferent temperatures such as 500, 650 and 800 °C for 24 htime duration. (c) Bode theta-angle plots of untreated andthermally treated CP Ti samples at different temperaturessuch as 500, 650 and 800 °C for 24 h time duration.

594 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

the passivity and breakdown of the oxide layer, if any. Thecorrosion potential (Ecorr), corrosion current density (icorr) andpassive current density (ipass) of the untreated and thermallyoxidized samples are given in Table 1. The thermally oxidizedsamples exhibit a shift in Ecorr towards the noble direction(from −474 to −291 mV vs. SCE) and a significant decrease inicorr (from 1.17 to 3.9×10−4 µA/cm2) compared to that ofuntreated. The extent of decrease in icorr increases withincrease in oxidation temperature from 500 to 800 °C (Fig. 5and Table 1).

The anodic branch of the polarization curve exhibits anactive–passive transition in all the cases. However, the activeregion of the polarization curves of the thermally oxidizedsamples is extended towards lower current region, suggestingthe ability of the thermally formed oxide layer to offer animprovement in corrosion resistance. The decrease in ipass(from 0.67 to 0.05 µA/cm2) with increase in oxidation temper-ature (increase in oxide layer thickness) suggests that theoxide film formed on the surface of samples possess a betterinsulating property and hinders the passage of higher currentfor further chemical reaction/oxidation of species in theelectrolyte (Fig. 5 and Table 1). The ipass of sample oxidizedat 800 °C is about 500 times lower when compared to that ofuntreated.

The decrease in icorr and ipass of the thermally oxidizedcompared to the untreated samples suggest an increase inthickness of the oxide layer and an increase in their corrosionprotective ability with increase in oxidation temperature.Several researchers have also confirmed the fact thatthermally oxidized CP Ti and Ti6Al4V alloy could offer abetter corrosion resistance in a variety of environments.Garcia-Alonso et al. [29] have reported that the ipass ofTi6Al4V alloy, thermally oxidized at 500 and 700 °C for 1 h,in Ringer's solution, is very low and quite similar to the ipassobserved for the thermally oxidized CP Ti samples of thepresent study. Guleryuz and Cimenoglu [19] have reportedthat Ti6Al4V alloy thermally oxidized at 600 °C offersexcellent corrosion resistance when compared to untreatedTi6Al4V alloy when they are subjected to acceleratedcorrosion test in 5 M HCl solution. According to them, noloss in weight is observed even after 36 h due to theformation of a thick and stable oxide film on the surface ofthe alloy. Bloyce et al. [24] have compared the corrosionresistance of untreated, plasma nitrided (PN), thermallyoxidized (TO), palladium-treated thermally oxidized (PTO)CP Ti in 3.5% NaCl solution at room temperature bypotentiodynamic polarization studies. Both TO and PTO CPTi samples exhibit a shift in Ecorr towards the noble directionand a decrease in icorr compared to that of PN and untreatedCP Ti. They have also evaluated the corrosion resistance ofthese samples by the accelerated corrosion test that involvesimmersion in boiling 20% HCl. Both TO and PTO CP Tisamples increase the lifetime by a factor of about 13 and 27,respectively compared to PN CP Ti. The results of the presentstudy further confirm the fact that thermally oxidized CP Tisamples could offer excellent corrosion resistance comparedto untreated CP Ti. Based on the corrosion protective ability,the untreated and thermally oxidized CP Ti samples of thepresent study can be ranked as follows: CP Ti (800 °C)>CP Ti(650 °C)>CP Ti (500 °C)>untreated CP Ti.

Fig. 7 – Morphology of the untreated (a) and thermallyoxidized (650 °C for 24 h) CP Ti (b) after subjecting them forpolarization studies in Ringer's solution.

595M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

3.5. Electrochemical Impedance Spectroscopic (EIS) Studiesof Untreated and Thermally Oxidized CP Ti

The Nyquist plots of untreated and thermally oxidized CP Tisamples in Ringer's solution, at their respective OCP's, areshown in Fig. 6(a). The Nyquist plots of untreated and samplesubjected to oxidation at 500 °C exhibit only a single semicircle.The sample oxidized at 650 °C exhibits a semicircle in the highfrequency region, followedbyaWarburg diffusion tail in the lowfrequency region. The sample oxidized at 800 °C exhibits asemicircle in thehigh frequency region followed by a loop in thelow frequency region. The diameter of the semicircle is higherfor thermally oxidized samples compared to untreated andamong the oxidized samples the diameter of the semicircleincreases with increase in oxidation temperature (Inset of Fig. 6(a)). The observed variation in the curves suggests a strongdependenceof thenatureand thicknessof theoxide layeron theoxidation temperature.

The charge transfer resistance (Rct) and double layer capac-itance (Cdl) values, determined after fitting the data usingBoukamp software, are given in Table 1. The increase in Rct anddecrease in Cdl values with increase in oxidation temperaturesuggest an increase in the corrosion protective ability ofthermally oxidized samples and support the observationsmade by potentiodynamic polarization studies. Based on thecorrosion protective ability, the untreated and thermally oxi-dized CP Ti samples of the present study can be ranked asfollows: CP Ti (800 °C)>CP Ti (650 °C)>CP Ti (500 °C)>untreatedCP Ti. For samples oxidized at 800 °C, the increase in Rct value isalmost 10,000 times compared to untreated, which confirms theability of thermally oxidized samples to offer an excellentcorrosion resistance.

In order to get a better insight about the mechanism, Bodeimpedance (Fig. 6(b)) and Bode phase angle (Fig. 6(c)) plotswere made. The Bode impedance plot reveals that theimpedance value increases with increase in oxidation tem-perature from 500 to 800 °C (Fig. 6(b)) and further confirms theobservations made from the Nyquist plot. The Bode phaseangle plots (Fig. 6(c)) exhibit much variation, which providesan understanding of the corrosion mechanism. Untreatedsample exhibit a phase angle maximum of −65° at anintermediate frequency of around 8 Hz, which suddenlydeclines to −20° at 0.01 Hz. The sample oxidized at 500 °Cexhibits a relatively higher phase angle maximum of about−78°, which remains constant over a wide frequency range of10 Hz to 0.5 Hz, before it declines to −50°. The occurrence of aconstant phase angle maximum over a wide range offrequency is typical of passive surfaces, which indicates anear capacitive behaviour. Such an occurrence also describesthe dielectric properties of the oxide film; the difficultyencountered in charge transfer process and, ascertains agood corrosion protective ability of the oxide film. The phaseangle plots of samples thermally oxidized at 650 and 800 °Cstarts from −90° at 10,000 Hz and continuously decrease to−22° and −10°, respectively at 0.01 Hz. The continuousdecrease of the phase shift in the entire frequency range(10,000 to 0.01 Hz) can be explained using a transmissiveboundary model, which considers the fact that only thebottom of the hole is conducting whereas its wall is isolating[38].

The occurrence of a single semicircle for untreated andthermally oxidized samples at 500 °C indicates the involve-ment of a single time constant. The occurrence of a singlephase angle maximum in the phase angle plot furtherconfirms the same. Garcia-Alonso et al. [29] have also reporteda single time constant for untreated Ti6Al4V alloy as well asthe alloy thermally oxidized at 500 °C for 1 h. The occurrenceof a Warburg diffusion tail in the low frequency region of theNyquist plot of sample oxidized at 650 °C indicates that thecorrosion process is diffusion controlled; the loop observed atthe low frequency region of the sample oxidized at 800 °Cindicates diffusion of electrolyte through the pores in theoxide film. The low frequency dispersions, observed in thephase angle plots of these samples, confirm the involvementof a diffusion controlled process and diffusion through pores.The occurrence of two inflections in the phase angle plotindicates the involvement of two time constants. The

596 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

involvement of two time constants has also been reportedearlier by Garcia-Alonso et al. [29] for Ti6Al4V alloy oxidized at700 °C for 1 h.

The surface morphology of untreated and thermallyoxidized (650 °C for 24 h) samples after subjecting them topolarization tests in Ringer's solution is given in Fig. 7. It isevident that etching of grain boundary occurred for untreatedCP–Ti whereas the thick oxide layer present on thermallyoxidized CP–Ti prevents the corrosion attack, which signifiedits improved corrosion protective ability.

4. Conclusions

The corrosion resistance of untreated and thermally oxidized(at 500, 650 and 800 °C for 24 h) CP Ti in Ringer's solution wasevaluated by potentiodynamic polarization and EIS studies toascertain their corrosion protective ability for implant appli-cation. The surface morphology, microhardness and structur-al characteristics were also studied. The study leads to thefollowing conclusions:

◉ The surface morphology of thermally oxidized samplereveals the presence of oxide scales throughout the surfacewithout spallation, irrespective of the oxidation tempera-ture. The growth mode involves the formation of a thinoxide scale followed by its agglomeration and growth tocompletely cover the surface.

◉ The surface morphology changes from a thin adherentsurface layer at 500 °C to the formation of small grainstructure at 650 °C. The surface is completely covered withoxide islands with the grains oriented perpendicular to thesubstrate at 800 °C.

◉ CP Ti sample oxidized at 800 °C exhibits the formation of athick oxide film that contains only the rutilephase; bothTi(O)and α-Ti are found to be present on samples oxidized at500 °C and α-Ti , Ti(O) and rutile are formed at 650 °C.

◉ Thermally oxidized samples exhibit a significant increase inhardness. Almost a six fold increase in hardness is observedfor samplesoxidizedat 800 °C compared to that ofuntreated.

◉ >The values of icorr, ipass, Rct and Cdl of thermally oxidizedsamples show a strong dependence on the nature andthickness of the surface oxide layer. The ipass of the sampleoxidized at 800 °C (0.05 µA/cm2) is about 500 times lowercompared to that of untreated (25.12 µA/cm2), which is alsoreflected in the significant increase in the Rct value of thesesamples (from 3.1×104 to 2.3×108 Ωcm2), suggesting theexcellent corrosion resistance of the thermally oxidizedsamples.

◉ Based on the corrosion protective ability, the untreated andthermally oxidized samples can be ranked as follows: CP Ti(800 °C)>CP Ti (650 °C)>CP Ti (500 °C)>untreated CP Ti.

◉ From corrosion point of view, thermal oxidation of CP Ti at800 °C for 24 h is a promising surface treatment and it can bea suitable alternative to the untreated CP Ti as a bio-implant.

◉ Further studies on mechanical properties, tribological andtribocorrosion behaviour and bio-compatibility are neededto justify the choice of thermally oxidized CP Ti as asuitable implant material. Our future papers will addressthese aspects.

Acknowledgement

The authors express their sincere thanks to Prof. S.P. Mehrotra,Director, National Metallurgical Laboratory, Jamshedpur, for hiskeen interest and permission to publish this paper.

R E F E R E N C E S

[1] Kovacs P, Davidson JA. Chemical and electrochemical aspectsof the biocompatibility of titanium and its alloys. In: BrownSA, Lemons JE, editors. Medical Applications of Titanium andits Alloys: The Materials and Biological IssuesASTM STP 1272;1996. p. 163–78.

[2] Hunt JA, Stoichet M. Biomaterials: surface interactions. CurrOpin Solid State Mater Sci 2001;5:161–2.

[3] Williams DF. Titanium and titanium alloys. In: Williams DF,editor. Biocompatibility of clinical implant materials, Vol. II.CRC Press; 1981. p. 9–44.

[4] LongMJ, RackHJ. Titaniumalloys in total joint replacement— amaterials science perspective. Biomats 1998;19:1621–39.

[5] Mu Y, Kobayashi T, Sumita M, Yamamoto A, Hanawa T. Metalion release from titaniumwith active oxygen species generatedby ratmacrophages in vitro. J BiomedMater Res 2000;49:238–43.

[6] Lilley PA, Walker PS, Blunn GW. Wear of titanium by softtissue. Transactions of the 4th Word Biomaterials Congress,Berlin; April 24–28 1992. p. 227.

[7] Thull R, SchaldachM. Corrosion of Highly StressedOrthopaedicJoint Replacements. Berlin: Springer; 1976. p. 242–56.

[8] Brown SA, Hughes PJ, Merrit K. In vitro studies of frettingcorrosion of orthopaedic materials. J Orthop Res 1988;6:572–9.

[9] Hoeppner DW, Chandrasekaran V. Fretting in orthopaedicimplants: a review. Wear 1994;173:189–97.

[10] Rabbe LM, Rieu J, Lopez A, Combrade P. Fretting deteriorationof orthopaedic implant materials. Search for solutions. ClinMater 1994;15:221–6.

[11] Sittig C, Textor M, Spencer ND, Wieland M, Vallotton PH.Surface characterization of implant materials CP–Ti,Ti–6Al–7Nb and Ti–6Al–4 V with different pretreatments.J Mater Sci Mater Med 1999;10:35–46.

[12] Wen HB, Wolke JG, Wijn JR, Liu Q, Cui FZ, de Groot K. Fastprecipitation of calcium phosphate layers on titanium inducedby simple chemical treatments. Biomats 1997;18:1471–8.

[13] Nie X, Leyland A, Matthews A. Abrasive wear/corrosionproperties and TEM analysis of Al2O3 coatings fabricatedusing plasma electrolysis. Surf Coat Technol 2002;149:245–51.

[14] Brendel T, Engel A, Russel C. Hydroxyapatite coatings by apolymeric route. J Mater Sci Mater Med 1992;3:175–9.

[15] Andreazza P, De Barros MI, Andreazza-Vignolle C, Rats D,Vandenbulcke L. In-depth structural X-ray investigation ofPECVD grown diamond films on titanium alloys. Thin SolidFilms 1998;319:62–6.

[16] Uchida M, Nihira N, Mitsuo A, Toyoda K, Kubota K, Aizawa T.Friction and wear properties of CrAlN and CrVN filmsdeposited by cathodic arc ion plating method. Surf CoatTechnol 2004;177–178:627–30.

[17] Kweh SWK, Khor KA, Cheang P. An in vitro investigation ofplasma sprayed hydroxyapatite (HA) coatings produced withflame-spheroidized feedstock. Biomats 2002;23:775–85.

[18] HanawaT,KamiuraY,YamamotoS,KohgoT,AmemiyaA,UkaiH, et al. Early bone formation around calcium–ion-implantedtitanium inserted into rat tibia. J Biomed Mater Res 1997;36:131–6.

[19] Guleryuz H, Cimenoglu H. Effect of thermal oxidation oncorrosion and corrosion–wear behaviour of a Ti–6Al–4V alloy.Biomats 2004;25:3325–33.

597M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 5 8 9 – 5 9 7

[20] Lopez MF, Jimenez JA, Gutierrez A. Corrosion study ofsurface-modified vanadium-free titanium alloys. ElectrochimActa 2003;48:1395–401.

[21] Velten D, Biehl V, Aubertin F, Valeske B, Possart W, Breme J.Preparation of TiO(2) layers on cp–Ti and Ti6Al4V by thermaland anodic oxidation and by sol–gel coating techniques andtheir characterization. J Biomed Mater Res 2002;59:18–28.

[22] Escudero ML, Gonzalez-Carrasco JL, Garcıa-Alonso C, RamırezE. Electrochemical impedance spectroscopy of preoxidizedMA 956 superalloy during in vitro experiments. Biomats1995;16:735–40.

[23] Siva Rama Krishna D, Brama YL, Sun Y. Thick rutile layer ontitanium for tribological applications. Tribol Int 2007;40:329–34.

[24] Bloyce A, Qi-Y z, Dong H, Bell T. Surface modification oftitanium alloys for combined improvements in corrosion andwear resistance. Surf Coat Technol 1998;107:125–32.

[25] Dong H, Bloyce A, Morton PH, Bell T. Titanium'95 Science andTechnology; 1995. p. 1999–2006.

[26] Burns GP. Titanium dioxide dielectric films formed by rapidthermal oxidation. J Appl Phys 1989;65(5):2095–7.

[27] Boettcher C. Deep case hardening of titanium alloys withoxygen. Surf Eng 2000;16:148–52.

[28] Schutz RW, Thomas DE. Corrosion of Titanium and TitaniumAlloys, ASM Handbook, Corrosion, Vol.13, ASM International,p. 671.

[29] Garcia-Alonso MC, Saldana L, Valles G, Gonzalez-Carrasco JL,Gonzalez-Cabrero J, Martinez ME, et al. In vitro corrosion

behaviour and osteoblast response of thermally oxidisedTi6Al4V alloy. Biomats 2003;24:19–26.

[30] Dong H, Bloyce A, Morton PH, Bell T. Surface engineering toimprove tribological performance of Ti–6Al–4V. Surf Eng1997;13:402–6.

[31] Bertrand G, Jarraya K, Chaix JM. Morphology of oxide scalesformed on titanium. Oxid Met 1983;21:1–19.

[32] Guleryuz H, Cimenoglu H. Surfacemodification of a Ti–6Al–4Valloy by thermal oxidation. Surf Coat Technol 2005;192:164–70.

[33] Ting CC, Chen SY, Liu DM. Preferential growth of thin rutileTiO2 films upon thermal oxidation of sputtered Ti films. ThinSolid Films 2002;402:290–5.

[34] Weissman S, Shrier A. In: Jaffee RI, Promisel NE, editors. TheScience, Technology and Application of Titanium. London:Pergamon Press; 1968. p. 441.

[35] Borgioli F, Galvanetto E, Galliano FP, Bacci T. Air treatment ofpure titanium by furnace and glow-discharge processes. SurfCoat Technol 2001;141:103–7.

[36] Dearnley PA, Dahm KL, Çimenoglu H. The corrosion–wearbehaviour of thermally oxidised CP–Ti and Ti–6Al–4V. Wear2004;256:469–79.

[37] Kofstad P. High Temperature Corrosion. Essex: ElsevierApplied Science; 1988.

[38] Mueller Y, Tognini R, Mayer J, Virtanen S. Anodized titaniumand stainless steel in contact with CFRP: an electrochemicalapproach considering galvanic corrosion. J Biomed Mater Res2007;82A:936–46.