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Materials Chemistry and Physics 138 (2013) 565e572

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Thermal oxidation of titanium: Evaluation of corrosion resistanceas a function of cooling rate

M. Jamesh a, T.S.N. Sankara Narayanan b, Paul K. Chu a,*

aDepartment of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong KongbDepartment of Dental Biomaterials and Institute of Oral Bioscience, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea

h i g h l i g h t s

< Thermal oxidation (TO) of Ti was performed at different cooling conditions.< Faster cooling rate does not hamper the corrosion resistance of TO Ti at 650 �C (14 h).< Faster cooling rate hampers the corrosion resistance of TO Ti at 850 �C (6 h).

a r t i c l e i n f o

Article history:Received 21 July 2012Received in revised form27 November 2012Accepted 2 December 2012

Keywords:CorrosionBiomaterialsElectrochemical propertiesHeat treatment

* Corresponding author. Tel.: þ852 3442 7724; fax:E-mail addresses: jamesh2011@gmail.com (M. Jam

(P.K. Chu).

0254-0584/$ e see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2012.12.020

a b s t r a c t

Commercial pure titanium (CP-Ti) samples were subjected to thermal oxidation (TO) at 650 �C for 14 hand 850 �C for 6 h under different cooling conditions viz. furnace, air, and water cooling. XRD reveals theformation of the rutile phase and a-Ti on the CP-Ti TO at 650 �C and the rutile phase on the CP-Ti TO at850 �C. On the 650 �C CP-Ti, faster cooling leads to the formation of oxide scales on the surface withoutspallation whereas on the 850 �C sample, faster cooling conditions lead to the formation of oxide scaleswith spallation. Electrochemical studies reveal that the faster cooling rate has no deleterious effect on thecorrosion resistance of TO CP-Ti at 650 �C for 14 h whereas the faster cooling rate has deleterious effecton the corrosion resistance of TO CP-Ti at 850 �C for 6 h in the 0.9% NaCl solution.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Titanium and its alloys are suitable for biomedical applicationsdue to their well-established corrosion resistance and biocompat-ibility. Their excellent corrosion resistance arises from the surfacepassive oxide film which consists of mainly TiO2 and provideschemical inertness in many aqueous media as well as biocompat-ibility in a biological environment [1e4]. However, it has been re-ported that under in vivo conditions, the stability of the passivelayer can be altered [5] and analysis of retrieved implants hasshown accumulation of ions in tissues adjacent to the implant [5]. Itmay be due to the inferior mechanical properties of the native formof TiO2 which may be disrupted at very low shear stress, even byrubbing against soft tissues [6]. Wear debris and metal ionsreleased during fracture of the passive layer can cause adversarytissue reactions. Fretting and sliding wear conditions can also leadto fracture of the passive layer [7e10] and under extreme

þ852 3442 0542..esh), paul.chu@cityu.edu.hk

All rights reserved.

conditions, they can cause loosening and eventual failure of theimplant. These limitations precludewider use of Ti and its alloys forarticulating surfaces.

Thermal oxidation (TO) tends to improve the surface charac-teristics of titanium and its alloys. Oxidation, particularly ata temperature above 200 �C, promotes the development of a crys-talline oxide film. Increasing the temperature induces the forma-tion of a thicker oxide layer, which is accompanied by dissolution ofoxygen beneath it [11]. The formation of a mechanically stable andchemically resistant oxide layers bodes well for the corrosion andwear behavior of titanium and its alloys [12e15]. It has been re-ported that the properties of the oxide layer formed during thermaloxidation of Ti and its alloys are influenced by the experimentalconditions such as the treatment temperature, time, and mode ofcooling [15e22]. Increasing the temperature during TO of CP-Tiincreases the hardness and roughness [16]. A sixfold increase inhardness is observed from CP-Ti samples oxidized at 800 �C for 48 hcompared to the untreated CP-Ti. Ebrahimi et al. [18] have reportedthat a longer treatment time increases the surface hardness andhardened layer thickness on thermally oxidized Tie4Ale2V alloys.Siva Rama Krishna et al. [17] have reported that thermal oxidation

20 30 40 50 60 70 80 90

TiTiTi

Ti

Ti

Ti

TiTi

Ti

Diffraction Angle (2θ)

Rela

tiv

e In

ten

sit

y

R- Rutile

RRRRRR

RRR

R

Untreated CP Ti

WC 850 ˚C/6 h

WC 650 ˚C/14 h

FC 850 ˚C/6 h

AC 650 ˚C/14 h

FC 650 ˚C/14 h

AC 850 ˚C/6 h

Fig. 1. XRD spectrum of untreated and thermally oxidized CP-Ti at different coolingrate (FC e Furnace cooled, AC e Air cooled, WC e Water cooled).

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572566

of CP-Ti followed by air cooling (fast cooling) leads to poor adhesivestrength, inferior friction characteristics, and poor wear resistance.Leinenbach and Eifler [19] have reported that thermal oxidation ofCP-Ti at 570 �C for 3 h followed by air cooling (faster cooling rate)gives rise to a poor fatigue limit. Nevertheless, Garcia-Alonso et al.[15] have reported that thermal oxidation of Ti6Al4V alloy followedby air cooling (fast cooling) produces enhanced osteoblastic cellattachment. In addition, thermal oxidation does not undermine thein vitro corrosion behavior and biocompatibility of the materials.However, Zhang, et al. [23] have recently reported that thermaloxidation of Tie5Ag alloy at 700 �C in air accompanied by watercooling for 2 h (fast cooling) improves the corrosion resistance. Tothe best of our knowledge, the corrosion behavior of thermallyoxidized titanium prepared under different cooling conditions hasseldom been explored but a better understanding is of bothfundamental and technical importance. In this present study, weaim at evaluating the corrosion behavior of thermally oxidizedcommercially pure titanium (CP-Ti) cooled under different condi-tions (furnace, air, and water) in 0.9% NaCl solutions.

2. Experimental details

CP-Ti (Grade 2) samples with dimensions of 1 � 1 � 0.2 cm3

were used as the substrate materials. Before thermal oxidation, theCP-Ti samples were ground using SiC papers with different gritsizes followed by mechanical polishing with 1 mm alumina paste toa mirror finish. They were ultrasonically cleaned in distilled waterfor 5 min and dried by compressed air. Thermal oxidation of the CP-Ti samples was performed in a muffle furnace in air at 650 �C for14 h and 850 �C for 6 h at a constant heating rate of 5 �C/min. TheCP-Ti samples were placed inside the furnace, and the temperaturewas increased at a constant rate (the thermal treatment includesramping step). After reaching 650 �C or 850 �C, the CP-Ti sampleswere left at this temperature for 14 h or 6 h (not including theramp-up step). Afterwards, the samples were allowed to cool inthree different ways viz. furnace cooling, air cooling, and watercooling.

The phase constituents of the samples were determined byconventional and glancing-angle X-ray diffraction (XRD, PHILIPS X’

Pert MPO Pro) using Cu-Ka radiation. The microstructure wasexamined by optical microscopy (LOM) and scanning electronmicroscopy (SEM). The corrosion behavior of the untreated and TOCP-Ti was evaluated by electrochemical impedance spectroscopy(EIS) and potentiodynamic polarization conducted using thepotentiostat/galvanostat/frequency response analyzer on the Zah-ner Zennium electrochemical workstation at 37 �C. The details ofthe electrochemical experiments can be found in our earlier papers[24e26]. Before the corrosion studies, the untreated CP-Ti sampleswere polished with various grades of SiC paper followed bymechanical polishing with 1 mm alumina paste to a mirror finish,pickled using a mixture of 35 vol.%, HNO3, 5 vol.% HF, and balancewater for 60e70 s. This was done to remove the naturally formedoxide layer. The samples were ultrasonically cleaned in distilledwater for 5 min and dried by compressed air. The cleaned untreatedCP-Ti or TO CP-Ti samples formed the working electrode whereasa saturated calomel electrode (SCE) and graphite rod served as thereference and auxiliary electrodes, respectively. These electrodeswere placed in a cell in such a way that only 1 cm2 area of theworking electrode was exposed to the 0.9% NaCl solution. EISstudies of untreated and thermally oxidized samples were con-ducted at their respective open circuit potentials (OCPs). Theimpedance spectra were recorded using an excitation voltage of10 mV rms (root mean square) in the frequency range between10 kHz and 0.01 Hz. Based on the nature of the Nyquist plots, anequivalent electrical circuit was proposed to account for the

corrosion behavior. The electrochemical parameters were obtainedby fitting the data using the proposed model. The Bode impedanceand phase angle plots were also recorded to assess the corrosionperformance. Potentiodynamic polarization tests were carried outat a scanning rate of 1 mV s�1 from�250 toþ3000 mV vs. SCE withrespect to the open circuit potential (OCP). A high potential (uptoþ3000mV) was employed to study the passivation behavior. Thecorrosion potential (Ecorr) and corrosion current density (icorr) weredetermined from the polarization curves using the Tafel extrapo-lation method. To compare the passivation ability of the untreatedand TO CP-Ti, the passive current densities (ipass) were determinedat þ1000, þ2000 and þ3000 mV versus SCE. The EIS and poten-tiodynamic polarization studies were repeated at least three timesto ensure reproducibility.

3. Results and discussion

The XRD patterns of the untreated and TO CP-Ti samples aredepicted in Fig.1. The untreated CP-Ti comprisesmainly a-Ti (Fig.1).The low angle XRD patterns of the TO samples at 650 �C for 14 h atdifferent cooling rates suggest the presence of a-Ti and TiO2 withthe rutile structure. As the X-ray penetration is beyond the thick-ness of the oxide layer, the peaks from alpha titanium are alsodetected. The XRD patterns of the TO samples at 850 �C for 6 h atdifferent cooling rates suggest the presence of TiO2 with the rutilestructure. By increasing the temperature, the oxide layer becomesthicker. As the X-ray penetration is smaller than the thickness ofthe oxide layer, the peaks from alpha titanium are not detectedby XRD pattern. The observation is consistent with other reports[15e17,20,23,27].

The surface morphology of the thermally oxidized samples at650 �C for 14 h clearly reveals the presence of oxide scales on thesurface without spallation, irrespective of the cooling conditions(Fig. 2aec). After oxidation for 14 h, the oxide layer covers the entiresurface (Fig. 2aec). Obviously, the oxidation time must be longenough to ensure full coverage. Garcia-Alonso et al. [15] have re-ported that the surface of Tie6Ale4V alloy is fully covered bysurface oxide scales in just 1 h when the samples are oxidized at700 �C. The cross-sectional view of the CP-Ti sample oxidized at650 �C for 14 h accompanied by cooling at different rates reveals thepresence of aw1 mm thick oxide layer followed by aw4 mmoxygendiffusion zone beneath it. The thickness of the oxide layer remains

Fig. 2. Surface morphology of thermally oxidized CP-Ti (a) furnace cooled at 650 �C/14 h, (b) air cooled at 650 �C/14 h, (c) water cooled at 650 �C/14 h, (d) furnace cooled at 850 �C/6 h, (e) air cooled at 850 �C/6 h and (f) water cooled at 850 �C/6 h.

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572 567

unchanged under different cooing conditions (Fig. 3aec). Thesurface morphology of the thermally oxidized samples at 850 �C for6 h clearly reveals the presence of oxide scales (Fig. 2def). Spall-ation of the oxide scale is not observed from the furnace cooledsamples whereas spallation is observed from both the air andwatercooled samples. The cross sectional view of the CP-Ti sampleoxidized at 850 �C for 6 h and cooled at different rates also supportsthe above observation (Fig. 3def) also revealing the presence of

a w20 mm thick oxide layer and a w22 mm thick oxygen diffusionzone underneath. Cracking and spallation are more severe on thewater cooled samples than air cooled samples. A thick TiO2 layer isprone to spallation due to the large volume ratio of rutile to Ti (1.73)[28], large lattice mismatch, and difference in the coefficient ofthermal expansion [29e31]. However, spallation of the oxide layeris not observed from the furnace cooled samples (Fig. 3d) becauseof the small cooling rate. Spallation of the oxide layer on the air

Fig. 3. Cross sectional morphology of thermally oxidized CP-Ti (a) furnace cooled at 650 �C/14 h, (b) air cooled at 650 �C/14 h, (c) water cooled at 650 �C/14 h, (d) furnace cooled at850 �C/6 h, (e) air cooled at 850 �C/6 h and (f) water cooled at 850 �C/6 h.

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572568

cooled samples (Fig. 3e) and spallation with severe cracks of theoxide layer on the water cooled samples (Fig. 3f) are due to fastcooling which results in a large thermal stress arising from thedifference in the coefficients of thermal expansion between the Tiand rutile layer [17].

The Nyquist plots of the untreated and TO CP-Ti in 0.9% NaCl attheir respective OCP’s vs. SCE are shown in Fig. 4. The EIS data canbe fitted by the three different equivalent electrical circuit (EEC)models illustrated in Fig. 4. The EIS spectrum of the untreated CP-Tiis characterized by a single semicircle, suggesting the involvementof a single time constant. In these EEC (inset of Fig. 4a), RS repre-sents the solution resistance. A constant-phase element (Q) isincluded in the EEC instead of the capacitance to represent the shiftfrom an ideal capacitor. The EEC proposed for the untreated CP-Ticlosely simulates the formation of a compact layer with a polari-zation resistance (RP) and a capacitance (QP). Similar models havebeen used to explain the corrosion behavior of Ti and Ti6Al4V alloy

[15,23]. The EIS spectrum of TO CP-Ti at 650 �C for 14 h is charac-terized by two time constants, irrespective of the cooling condi-tions. The EEC proposed for TO CP-Ti (650 �C for 14 h at differentcooling condition) closely simulates the oxide layer formed on thesurface of the CP-Ti consisting of two layers. In this EEC (inset inFig. 4bed), RS is the solution resistance and ROL and RP represent theresistance of the oxide layer and passive film of Ti substraterespectively. QOL and QP designate the capacitance of the oxidelayer and passive film of the Ti substrate, respectively. Zhang et al.and Garcia et al. have also used a similar model to explain thecorrosion behavior of TO Ti5Ag and TO Ti6Al4V alloys in artificialsaliva and Ringer’s solution, respectively [15,23]. The EIS spectrumof the furnace cooled TO CP-Ti at 850 �C for 6 h is characterized bya single semicircle followed by a Warburg diffusion tail. The EECproposed for the TO CP-Ti closely simulates the oxide layer formedon the surface of the CP-Ti followed by Warburg diffusion. In thisEEC (inset of Fig. 4e), RS is the solution resistance, ROL and QOL

Fig. 4. Nyquist plots and their corresponding equivalent circuit of untreated and thermally oxidized CP-Ti at different cooling rate (FC e Furnace cooled, AC e Air cooled, WC e

Water cooled) in 0.9% NaCl at 37 �C.

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572 569

Table 1EIS parameters of untreated and thermally oxidized CP-Ti in 0.9% NaCl at 37 �C (FC e Furnace cooled, AC e Air cooled, WC e Water cooled).

CP-Ti samples ROL (Ohm cm2) QOL (mho sn/cm2) n1 RP (Ohm cm2) QP (mho sn/cm2) n2 W (mho s0.5/cm2) c2

Untreated 4.38 � 105 1.45 � 10�5 0.92 3.12 � 10�4

FC TO 650� C/14 h 2.33 � 105 1.33 � 10�7 0.94 4.65 � 107 1.42 � 10�7 0.65 6.33 � 10�4

AC TO 650� C/14 h 1.98 � 105 1.62 � 10�7 0.94 2.82 � 107 1.63 � 10�7 0.63 3.76 � 10�4

WC TO 650� C/14 h 1.71 � 105 1.74 � 10�7 0.94 2.29 � 107 3.06 � 10�7 0.61 7.85 � 10�4

FC TO 850� C/6 h 1.06 � 106 5.93 � 10�7 0.93 2.36 � 106 2.09 � 10�3

AC TO 850� C/6 h 1.98 � 106 1.033 � 10�5 0.89 2.28 � 10�4

WC TO 850� C/6 h 2.57 � 105 5.19 � 10�5 0.85 4.77 � 10�4

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572570

represent the resistance and capacitance of the oxide layer,respectively, andW denotesWarburg diffusion. The EIS spectrum ofthe air and water cooled TO CP-Ti at 850 �C for 6 h is characterizedby a single semicircle, suggesting the involvement of a single timeconstant which may be due to spallation of the oxide layer on thesurface of the titanium (Fig. 3e and f). In this EEC (inset of Fig. 4f andg), RS is the solution resistance and RP and QP denote the resistanceand capacitance of the oxygen diffused Ti substrate, respectively.The validity of these models is confirmed based on the better non-linear least square fits of the experimental data to within 5% error.The EIS parameters determined from the Nyquist plots after fittingthe data are listed in Table 1. The good agreement between the

Fig. 5. Bode impedance (a) and Bode phase angle (b) plots of untreated and thermallyoxidized CP-Ti at different cooling rate (FC e Furnace cooled, AC e Air cooled, WC e

Water cooled) in 0.9% NaCl at 37 �C.

experimental data and fitted data can be obtained with c2 of about10�3. The important factor from the corrosion protection perspec-tive is the charge transfer resistance which controls the rate of theelectrochemical processes at the metal/electrolyte interface. Thelarger resistance, RP, and smaller capacitance, QP, for TO CP-Ti at650 �C for 14 h under different cooling conditions suggest bettercorrosion resistance than the untreated CP-Ti. Moreover, there is noconsiderable difference between the resistance and capacitancewith respect to the cooling conditions, indicating that faster coolinghas no deleterious effects on the corrosion resistance of TO CP-Ti at650 �C for 14 h. Zhang et al. [23] have also observed improvementin the corrosion resistance from the thermally oxidized Tie5Agalloy after water cooling. The bigger resistance, ROL, and smallercapacitance, QOL, of the furnace cooled TO CP-Ti at 850 �C for 6 hsuggest better corrosion resistance than the untreated CP-Ti. Like-wise, the larger resistance, RP, and smaller capacitance, QP, of the aircooled TO CP-Ti at 850 �C for 6 h imply slightly better corrosionresistance than the untreated CP-Ti. However, the smaller resis-tance, RP, and larger capacitance, QP, of the water cooled TO CP-Ti at850 �C for 6 h indicate poorer corrosion resistance than theuntreated CP-Ti. Based on the SEM results (Fig. 3f), formation ofsurface cracks and spallation of the oxide layer are the probablereasons for the worse corrosion resistance.

The Bode plots (Fig. 5) exhibit variations which may providebetter understanding of the corrosion mechanism. The Bodeimpedance plot (Fig. 5a) indicates higher impedance at both higherand lower frequencies for the TO CP-Ti at 650 �C for 14 h (differentcooling conditions) compared to the untreated CP-Ti. The Bodephase angle plot (Fig. 5b) shows that the phase angle maximum

Fig. 6. Potentiodynamic polarization curves of untreated and thermally oxidized CP-Tiat different cooling rate (FC e Furnace cooled, AC e Air cooled, WC e Water cooled) in0.9% NaCl at 37 �C.

Table 2Corrosion potential (Ecorr), corrosion current density (icorr) and passive current density (ipass) of untreated and thermally oxidized (TO) CP-Ti at different cooling rate (FC e

Furnace cooled, AC e Air cooled, WC e Water cooled) in 0.9% NaCl solution calculated from potentiodynamic polarization studies.

CP Ti samples Ecorr (mV vs SCE) icorr (mA/cm2) ipass at (mA/cm2)1000 mV vs SCE

ipass at (mA/cm2)2000 mV vs SCE

ipass at (mA/cm2)3000 mV vs SCE

Untreated �451 1.1 2.8 4.4 7.0FC TO 650� C/14 h �119 1.8 � 10�3 4.5 � 10�3 5.5 � 10�3 1.1 � 10�2

AC TO 650� C/14 h �114 1.9 � 10�3 7.4 � 10�3 1.3 � 10�2 2.9 � 10�2

WC TO 650� C/14 h �170 2.3 � 10�3 1.2 � 10�2 1.9 � 10�2 3.5 � 10�2

FC TO 850� C/6 h �251 1.4 � 10�2 2.6 � 10�1 4.0 � 10�1 6.9 � 10�1

AC TO 850� C/6 h �176 1.2 � 10�1 4.1 � 10�1 5.4 1.2 � 102

WC TO 850� C/6 h �132 1.9 5.8 2.2 � 101 1.4 � 102

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572 571

over a wide range of frequency is typical of passive surfaces andindicates a near capacitive behavior for TO CP-Ti at 650 �C for 14 h(different cooling conditions) compared to the untreated CP-Ti. Inaddition, there is no considerable difference between the imped-ance and phase angle shift with respect to the cooling conditions.This further indicates that a larger cooling rate has no deleteriouseffects on the corrosion resistance of TO CP-Ti at 650 �C for 14 h. TheBode impedance plot (Fig. 5a) indicates higher impedance at bothhigh and low frequencies for furnace cooled TO CP-Ti at 850 �C for6 h compared to the untreated CP-Ti. This implies better corrosionresistance than the untreated CP-Ti. However, the Bode impedanceplot (Fig. 5a) indicates higher impedance at higher frequencies butlower impedance at lower frequencies and the Bode Phase angleplot (Fig. 5b) shows the higher phase angle at higher frequenciesbut lower phase angle at lower frequencies for the furnace cooledTO CP-Ti at 850 �C for 6 h compared to the TO CP-Ti at 650 �C for14 h (different cooling conditions). Hence, the oxide layer formedon the TO CP-Ti at 650 �C for 14 h (different cooling conditions) ismore compact and protective compared to the furnace cooled TOCP-Ti at 850 �C for 6 h which shows the diffusion phenomena. Thelarger Bode impedance and higher phase angle maxima observedfrom the air cooled TO CP-Ti at 850 �C for 6 h than the untreated CP-Ti indicate that the corrosion property is slightly improved.However, the Bode impedance plot (Fig. 5a) indicates lowerimpedance at both high and low frequencies for air cooled TO CP-Tiat 850 �C for 6 h compared to the furnace cooled TO CP-Ti at 850 �Cfor 6 h. Therefore, fast cooling at relatively high temperature hasa deleterious effect on the corrosion property of the CP-Ti. The Bodeimpedance plot (Fig. 5a) indicates smaller impedance at both highand low frequencies and the Bode Phase angle plot (Fig. 5b) showsthe lower phase angle maxima for water cooled TO CP-Ti at 850 �Cfor 6 h compared to the untreated CP-Ti. Thus, it has poorercorrosion resistance than the untreated CP-Ti.

The potentiodynamic polarization curves of the untreated andthermally oxidized CP-Ti in 0.9% NaCl are shown in Fig. 6. Thecorrosion potential (Ecorr), corrosion current density (icorr) andpassive current density (ipass) of the untreated and thermallyoxidized samples are given in Table 2. The anodic branch of thepolarization curve exhibits an activeepassive transition in all thecases. However, the active region of the polarization curves of theTO CP-Ti at 650 �C for 14 h (different cooling conditions) isextended toward the lower current region. There is a shift in Ecorrtoward the noble direction together with a significant decrease inicorr compared to that of untreated CP-Ti clearly suggesting bettercorrosion resistance. There is no considerable difference betweenthe corrosion current density (icorr) with respect to the coolingconditions. This indicates that faster cooling has no deleteriouseffects on the corrosion resistance of the TO CP-Ti at 650 �C for 14 h.However, the corrosion current density (icorr) and passive currentdensity (ipass) diminish slightly for faster cooling, indicating thata larger cooling rate affects slightly the corrosion resistance of TOCP-Ti at 650 �C for 14 h. The smaller corrosion current density (icorr)

observed from the furnace cooled TO CP-Ti at 850 �C for 6 h indi-cates better corrosion resistance than the untreated CP-Ti. Never-theless, its corrosion current density (icorr) is larger than that of theTO CP-Ti at 650 �C for 14 h (different cooling conditions). Therelatively thin oxide layer formed at 650 �C for 14 h is morecompact and protective against corrosion than the relatively thickoxide layer formed at 850 �C for 6 h. A smaller corrosion currentdensity (icorr) is observed from the air cooled TO CP-Ti at 850 �C for6 h than the untreated CP-Ti. However, its passive current density(ipass) is not stable and continuously increases showing higher ipass(1.2 � 102 mA cm�2) at 3000 mV vs SCE than untreated CP-Ti(7.0 mA cm�2). Therefore, the air cooled TO CP-Ti at 850 �C for 6 hoffers poorer corrosion resistance than the untreated CP-Ti. Thelarger corrosion current density (icorr) and passive current density(ipass) observed from the water cooled TO CP-Ti at 850 �C for 6 hindicates inferior corrosion resistance compared to the untreatedCP-Ti. Based on the SEM micrographs (Fig. 3e and f), surface cracksand spallation are the reasons for the worse corrosion resistance ofthe air and water cooled specimens.

4. Conclusion

The corrosion resistance of untreated and thermally oxidized(650 �C for 14 h and 850 �C for 6 h followed by different coolingconditions viz. furnace, air, and water cooling) CP-Ti in the 0.9%NaCl solution was evaluated by EIS and potentiodynamic polari-zation to assess the corrosion protective ability for biomedicalapplications. The following conclusions can be drawn.

� XRD measurements reveal the formation of the rutile phaseand a-Ti on the CP-Ti TO at 650 �C for 14 h and the rutile phaseon the CP-Ti TO at 850 �C for 6 h irrespective of the coolingconditions.

� The surface morphology and cross-sectional view of thethermally oxidized samples at 650 �C for 14 h disclose oxidescales on the surface without spallation and the thickness ofthe oxide layer remains unchanged, irrespective of the cool-ing conditions. The surface morphology and cross-sectionalview of the thermally oxidized samples at 850 �C for 6 hshow oxide scales. Spallation of the oxide scale is notobserved from the furnace cooled samples whereas spallationof oxide scale is observed from both the air and water cooledsamples. Cracking and spallation are more severe on thewater cooled samples than air cooled samples.

� The larger resistance, smaller capacitance, lower corrosioncurrent density, icorr, and smaller passive current density, ipass,of the TO CP-Ti at 650 �C for 14 h (different cooling condi-tions) suggest that the oxide layer formed is more compactand protective against corrosion thereby offering bettercorrosion resistance than the untreated CP-Ti. The EIS andpolarization results reveal that faster cooling decreases the

M. Jamesh et al. / Materials Chemistry and Physics 138 (2013) 565e572572

corrosion resistance slightly but has no deleterious effects onthe corrosion resistance of the TO CP-Ti at 650 �C for 14 h.

� The larger resistance, smaller capacitance, lower corrosioncurrent density, icorr, and smaller passive current density, ipass,of the furnace cooled TO CP-Ti at 850 �C for 6 h lead to bettercorrosion resistance than the untreated CP-Ti. Even though ithas a relatively thicker oxide layer, it offers poorer corrosionresistance than TO CP-Ti at 650 �C for 14 h under differentcooling conditions because of diffusion of electrolytesthrough the pores of the oxide layer.

� The EIS and polarization results further disclose that fastercooling imposes deleterious effects on the corrosion resis-tance of the TO CP-Ti at 850 �C for 6 h.

Acknowledgments

This studywas financially supported by the Hong Kong ResearchGrants Council (RGC) General Research Funds (GRF) No. CityU112510.

References

[1] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in Medicine,Springer, Heidelberg, 2001.

[2] R. Boyer, G. Welsch, E.W. Collings, Materials Properties Handbook: TitaniumAlloys, ASM International, Materials Park, Ohio, 1994.

[3] M.J. Donachie, Titanium: A Technical Guide, ASM International, Materials Park,Ohio, 1989.

[4] X. Liu, P.K. Chu, C. Ding, Mater. Sci. Eng. R 47 (2004) 49e121.[5] Y. Mu, T. Kobayashi, M. Sumita, A. Yamamoto, T. Hanawa, J. Biomed. Mater.

Res. 49 (2000) 238e243.

[6] P.A. Lilley, P.S. Walker, G.W. Blunn, presented at transactions of the fourthword biomaterials congress, Berlin, Germany, April 24e28, 1992, pp. 227.

[7] R. Thull, M. Schaldach, Advances in Artificial Hip and Knee Joint Technology,Springer-Verlag, Berlin, Heidelberg, New York, 1976.

[8] S.A. Brown, P.J. Hughes, K. Merrit, J. Orthop. Res. 6 (1988) 572e579.[9] D.W. Hoeppner, V. Chandrasekaran, Wear 173 (1994) 189.

[10] L.M. Rabbe, J. Rieu, A. Lopez, P. Combrade, Clin. Mater. 15 (1994) 221e226.[11] P. Kofstad, High Temperature Corrosion, Elsevier Applied Science Publishers

Ltd., Essex, England, 1988.[12] M. Long, H.J. Rack, Biomaterials 19 (1998) 1621e1639.[13] A. K. Mishra, J. A. Davidson, R. A. Poggie, P. Kovacs, T. J. FitzGerald, presented at

the material and biological issues, Phoenix, Arizona, November 15e16, 1994,pp. 96e112.

[14] M.F. Lopez, J.A. Jimenez, A. Gutierrez, Electrochim. Acta 48 (2003) 1395e1401.[15] M.C. Garcia-Alonso, L. Saldana, G. Valles, J.L. Gonzalez-Garrasco, J. Gonzalez-

Cabrero, M.E. Martinez, E. Gil-Garay, L. Munuera, Biomaterials 24 (2003)19e26.

[16] S. Kumar, T.S.N. Sankara Narayanan, S. Ganesh Sundara Raman, S.K. Seshadri,Mater. Charact. 61 (2010) 589e597.

[17] D. Siva Rama Krishna, Y.L. Brama, Y. Sun, Tribol. Int. 40 (2007) 329e334.[18] A.R. Ebrahimi, F. Zarei, R.A. Khosroshahi, Surf. Coat. Technol. 203 (2008)

199e203.[19] C. Leinenbach, D. Eifler, Acta Biomater. 5 (2009) 2810e2819.[20] A. Ashrafizadeh, F. Ashrafizadeh, J. Alloys Compd. 480 (2009) 849e852.[21] S. Kumar, T.S.N. Sankara Narayanan, S. Ganesh Sundara Raman, S.K. Seshadri,

Mater. Chem. Phys. 119 (2010) 337e346.[22] J. Hu, Y. Wang, D. Wang, Adv. Mat. Res. 146e147 (2011) 1536e1539.[23] B.B. Zhang, B.L. Wang, L. Li, Y.F. Zheng, Dent. Mater. 27 (2011) 214e220.[24] Y. Xin, T. Hu, P.K. Chu, Corros. Sci. 53 (2011) 1522e1528.[25] R.Q. Hang, S.L. Ma, V. Ji, P.K. Chu, Electrochim. Acta 55 (2010) 5551e5560.[26] M. Jamesh, S. Kumar, T.S.N. Sankara Narayanan, Corros. Sci. 53 (2011)

645e654.[27] M. Jamesh, S. Kumar, T.S.N. Sankara Narayanan, J. Mater. Eng. Perform. 21

(2012) 900e906.[28] G.P. Burns, J. Appl. Phys. 65 (1989) 2095e2097.[29] C. Boettcher, Deep Surf. Eng. 16 (2000) 148e152.[30] T. Fukuzuka, K. Shimogori, H. Satoh, F. Kamikubo, 80 (1980) 2783e2791.[31] H. Dong, X.Y. Li, Mater. Sci. Eng. A 80 (1980) 2783e2791.