Corrosion Science 52 (2010) 1721–1727

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Corrosion Science

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Influence of fluoride ion on the electrochemical behaviour of b-Ti alloyfor dental implant application

Satendra Kumar a,*, T.S.N. Sankara Narayanan a,*, S. Saravana Kumar b

a National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, Indiab Sri R.K.M.V. College of Arts and Science, Coimbatore 641 020, India

a r t i c l e i n f o

Article history:Received 18 September 2009Accepted 15 January 2010Available online 20 January 2010

Keywords:A. TitaniumB. EISB. PolarizationB. X-ray diffractionC. Passive films

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.01.008

* Corresponding authors. Tel.: +91 44 2254 2077; fE-mail addresses: sat_nml@rediffmail.com (S. Ku

(T.S.N. Sankara Narayanan).

a b s t r a c t

The corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl containing 0.01, 0.03, 0.06 and 0.5 M NaF isevaluated and its protective ability is compared with that of CP-Ti and Ti–6Al–4V alloy, to ascertain theirsuitability for dental implant application. The steady state current density of CP-Ti and, Ti–15Mo and Ti–6Al–4V alloys in 0.15 M NaCl containing 0.03 M NaF at 200 mV vs. SCE is found to be 1, 2 and 6 lA/cm2,respectively, which indicate that all of them could offer a better corrosion resistance in the potentialrange that could exists in the oral environment.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Titanium and titanium alloys are widely used for many biomed-ical applications due to their low density, excellent biocompatibil-ity, corrosion resistance and mechanical properties [1–3]. Amongthe various types of Ti alloys, Ti–6Al–4V has been the choice inmany instances. However, studies have expressed concerns onthe use of Ti–6Al–4V alloy in spite of its better mechanical proper-ties and corrosion resistance. Leaching of V and Al from this alloycould cause long-term health problems such as, peripheral neurop-athy, osteomalacia and Alzheimer diseases [4–6]. The large modu-lus mismatch between the Ti–6Al–4V alloy (�110 GPa) and thebone (�10–40 GPa) could cause insufficient loading of the boneadjacent to the implant, leading to an eventual failure of the im-plant [7]. Considering the severity of leaching of the alloying ele-ments present in implant materials, there has been an increasinginterest among the biomaterial scientists to evaluate the corrosionresistance of the metallic biomaterials. Hence, in this regard manyuntreated and surface modified biomaterials were investigated fortheir passivity, corrosion and pitting resistance behaviour withvarying electrolyte concentration, pH, temperature, etc. [8–22].

Titanium and its alloys are also used as dental implants. In re-cent years, there has been a rapid increase in the utilization of fluo-rinated prophylactic gels and rinses in the odontological field.Many commercially available fluorinated gels contain very high

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concentrations of fluoride ions, up to 10,000 ppm (1 wt.%), with apH, ranging between 7.2 and 3.2 [23]. The negative influence offluoride on the corrosion of titanium and its alloys has been re-ported earlier [23–25]. Ti–0.5Pt, Ti–0.5Pd and Ti–30Cu–10Ag alloysoffer a better corrosion resistance than CP-Ti and, Ti–6Al–4V andTi–6Al–7Nb alloys in presence of fluoride since the presence ofPt, Pd, Cu and Ag in these alloys have the ability to promote pass-ivation [26,27].

In recent years, b-type titanium alloys, particularly the Ti–Moalloys have received considerable attention as a biomaterial sincethey offer significant benefits in terms of corrosion resistanceand mechanical biocompatibility [15,16,28–36]. Based on themicrostructural evolution and strengthening mechanisms of Ti–15Mo alloy, Nag et al. [29] have recommended it as one of thepromising biocompatible Ti alloy. Oliveira et al. [31] and Oliveiraand Guastaldi [32,33] have demonstrated the ability of Ti–Mo al-loys having 4–20 wt.% Mo to exhibit spontaneous passivation inRinger’s solution and to prevent pitting at potentials up to 8V(vs. SCE). Oliveira and Guastaldi [32,33] have recommended Ti–Mo alloys as a suitable biomaterial due to their ability to exhibitspontaneous passivation, better electrochemical stability of thepassive films formed on them and their excellent biocompatibility.Kumar and Sankara Narayanan [15] have reported that the corro-sion and pitting corrosion resistance of Ti–15Mo alloy are compa-rable with Ti–6Al–4V alloy but better than that of CP-Ti. Accordingto Alves et al. [35], though both Ti–10Mo and Ti–6Al–4V alloys ex-hibit similar electrochemical characteristics in 0.15 M NaCl con-taining 0.03 M NaF, the passive current density is relatively lowerfor Ti–10Mo alloy. According to Alves-Rezende et al. [36], both

1722 S. Kumar et al. / Corrosion Science 52 (2010) 1721–1727

CP-Ti and Ti–10Mo alloy rapidly attain a steady state potential in0.05 wt.% NaF whereas in commercial mouthwashes a similarpotential is reached very slowly. The corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl containing varying concentrations ofNaF was reported in our earlier paper [16]. The present paper aimsto evaluate the corrosion behaviour of Ti–15Mo alloy and tocompare its corrosion protective ability with that of CP-Ti andTi–6Al–4V alloy, to ascertain their suitability for dental implantapplications.

2. Materials and methods

CP-Ti (Grade 2) and Ti–6Al–4V alloy were purchased from M/sTi Anode Fabricators, Chennai, India whereas the Ti–15Mo alloywas received as a gift sample from National Institute for MaterialsResearch (NIMS), Japan. These alloys were used in as-received con-dition without any further treatment. Samples used for microstruc-tural characterization, structural characteristics and corrosionstudies were cut from a sheet of 2 mm thickness. The chemicalcomposition of CP-Ti (Grade 2) and, Ti–6Al–4V and Ti–15Mo alloysis given in Table 1. The microstructure of these Ti samples was as-sessed using a Leica DMLM optical microscope with image ana-lyzer software. The details of sample preparation and etchingwere described elsewhere [15,16,28]. The structural characteristicof the Ti alloys was evaluated by X-ray diffraction (XRD) measure-ment, using Cu-Ka radiation. The corrosion behaviour of all thethree Ti samples was evaluated using 0.15 M NaCl solution as thebase electrolyte to which varying concentrations of NaF (0.01,0.03, 0.06 and 0.5 M) was added to study the influence of fluoride.The base electrolyte and concentration of NaF were chosen basedon the earlier reports [27,34,37]. The fluoride concentration (interms of NaF) in some commercial toothpastes and prophylacticgels could vary from 0.1–2.0 wt.% [24]. Hence, in the present study,four different concentrations of NaF, namely, 0.01, 0.03, 0.06 and0.5 M, were chosen to assess the influence of fluoride on the corro-sion behaviour of all the three Ti samples.

Table 1Chemical composition (in wt.%) of the Ti alloys used in this study.

Element Composition (wt.%)

CP-Ti (Grade 2) Ti–6Al–4V alloy Ti–15Mo alloy

N 0.01 0.02 0.01C 0.03 0.03 0.02H 0.010 0.011 0.011Fe 0.20 0.22 0.012O 0.18 0.16 0.10Al – 6.12 –V – 3.93 –Mo – – 15.04Ti Balance Balance Balance

Fig. 1. Microstructure of: (a) CP-Ti; (b) Ti–

Potentiodynamic polarization and chronoamperometric/cur-rent–time transient (CTT) studies were carried out using a poten-tiostat/galvanostat of ACM instruments (model: Gill AC) to assessthe corrosion performance of Ti alloys. The details of the test meth-od were already described in our earlier papers [15–17]. The Ti orTi alloy sample forms the working electrode while a saturated cal-omel electrode (SCE) and a graphite rod were used as the referenceand auxiliary electrodes, respectively. These electrodes wereplaced within a flat cell in such a way that only 1 cm2 area of thesample was exposed to the electrolyte solution. Potentiodynamicpolarization studies were carried out at a scan rate of 100 mV/min. from �250 mV to +3000 mV vs. SCE with respect to open cir-cuit potential (OCP) to study the passivation behaviour. The scanrate chosen should be slow enough to reflect only the interfacialcorrosion process at every potential of the polarization scan anddoes not reflect charging of the surface capacitance in addition tothe corrosion process. In addition, one should maintain an appro-priate balance between the slow scan rates and quickly get the re-quired information. In addition, the scan rates should be keptsimilar to ensure consistent comparison between different alloysin a given environment and vice versa. A scan rate of 100 mV/min. has been used in our earlier studies [15–17,38].

Current–time transient (CTT) studies were performed at threedifferent potentials namely, +500 mV, +1250 mV and +2000 mVvs. SCE for 60 min. Since, the intraoral oxidation potential rangesbetween �58 mV and +212 mV vs. SCE, CTT studies were also con-ducted in 0.15 M NaCl containing 0.03 M NaF at an impressed po-tential of +200 mV vs. SCE for 30 min and the performance of CP-Tiand, Ti–6Al–4V and Ti–15Mo alloys were compared based on thesteady state current density. The surface morphology of the Ti sam-ples after subjecting them to CTT studies at +200 mV vs. SCE for30 min was assessed using scanning electron microscope. Allexperiments were conducted at 27 ± 1 �C, open to air and understatic conditions. Though many researchers have studied the corro-sion behaviour of Ti and its alloys at human body temperature(37 ± 1 �C), some of them have studied the corrosion and tribocor-rosion behaviour of Ti and its alloys at room temperature (24–30 �C) [21,39,40]. In a recent review on corrosion of alloys usedin dentistry, Upadhyaya et al. [41] have compiled many reportsin which the corrosion behaviour of alloys was evaluated at tem-peratures other than the human body temperature. The potentio-dynamic polarization and CTT studies were repeated at leastthree times to ensure reproducibility of the test results.

3. Results and discussion

3.1. Microstructure of CP-Ti and, Ti–6Al–4V and Ti–15Mo alloys

The microstructure of CP-Ti (Fig. 1a) exhibits a typical rapidlycooled metastable feather-like microstructure and consists of

6Al–4V alloy; and (c) Ti–15Mo alloy.

S. Kumar et al. / Corrosion Science 52 (2010) 1721–1727 1723

equiaxed a-grain with some twin bands. The microstructure of Ti–6Al–4V alloy reveals a + b structure (Fig. 1b) that consists of elon-gated a-grains (light) and intergranular b-grains (mottled or out-lined). In case of Ti–15Mo alloy, equiaxed b-grains are observedas the dominant phase (Fig. 1c) and the b-grains are found to behomogeneous with an even distribution. The XRD pattern of CP-Ti (Fig. 2a) is comprised entirely of hexagonal a-phase, the Ti–6Al–4V alloy (Fig. 2b) exhibits the presence of both a- and b-phases, whereas the Ti–15Mo alloy (Fig. 2c) exhibits the presenceof only the b-phase.

The microstructural features and structural characteristics of Tiand its alloys is largely a function of the phase content as well asthe alloying elements present in them. The a- and a + b-phasestructure of CP-Ti and Ti–6Al–4V alloy, respectively, has been wellestablished. Retention of the b-phase in Ti alloys with higher Moconcentrations has been reported earlier by Davis et al. [42]. Hoet al. [28] have reported that Ti–Mo alloys having 9 wt.% Mo havea significant amount of equiaxed b-phase whereas in alloys con-taining P10 wt.% Mo, the b-phase becomes the only dominantphase. Bania [43] has also reported that a minimum of 10 wt.%Mo is needed to stabilize the b-phase at room temperature.

Ho et al. [28] and Oliveira et al. [31] have suggested that thecrystal structure of the Ti–Mo alloys is sensitive to the Mo content.Based on the XRD measurements, they have confirmed a significantretention of the b-phase for the Ti alloy containing 10 wt.% Mo,while in alloys having higher Mo concentrations (15 and 20 wt.%)only the b-phase is retained [28,31]. The results of the presentstudy further confirms the retention of only the b-phase in theTi–15Mo alloy.

3.2. Potentiodynamic polarization studies of CP-Ti and, Ti–6Al–4V andTi–15Mo alloys

Potentiodynamic polarization curves of CP-Ti and, Ti–6Al–4Vand Ti–15Mo alloys in 0.15 M NaCl is shown in Fig. 3a and the cor-responding curves obtained with the addition of 0.06 and 0.5 MNaF in 0.15 M NaCl are shown in Fig. 3b and c, respectively. Theanodic branch of the polarization curves of all three Ti samplesexhibits an active–passive transition. With the addition of NaF inthe electrolyte medium (Fig. 3b and c), the active region of thepolarization curves of all the three Ti samples is extended to highercurrent region. In spite of the increase in active region, all three Ti

25 30 35

020406080

100

C.P Ti

Diffractio

0

406080

100

Ti-6Al-4V

Rel

ativ

e in

tens

ity

020406080

100

Ti-15Mo

α (1

0 0

)α

(10

0)

Fig. 2. X-ray diffraction pattern of: (a) CP-Ti; (b

samples exhibit the formation of a passive film in presence of fluo-ride ions when the potential is increased further in the anodicdirection. There is a strong dependence of the passive current den-sity of the Ti samples on the concentration of NaF in the electrolytesolution; increase in concentration of NaF increases the passivecurrent density. The average passive current density measured at+1200 mV vs. SCE is shown in Fig. 4.

The active region of the polarization curves of CP-Ti and, Ti–6Al–4V and Ti–15Mo alloys is due to the electro-chemical reactionof the sample in the electrolyte medium and formation of a porousor defective oxide layer whereas the passive region is associatedwith the formation of one or more protective oxide films. The shiftin the polarization curve towards higher current region followingthe addition of NaF in 0.15 M NaCl suggests the negative influenceof fluoride on the corrosion resistance of these Ti samples, which isalso confirmed by many researchers [23,24]. Reclaru and Meyer[44] have reported that the dissolution of Ti in the presence of fluo-ride is due to the formation of Ti–F complex, [TiF6]2� that form sol-uble salts with alkaline metals. Huang [24] has identified theformation of Na2TiF6 on the surface of Ti–6Al–4V alloy immersedin acidic artificial saliva (pH: 5) containing 0.1 wt.% of NaF, byXPS. Hence, it is evident that the addition of NaF to 0.15 M NaClwould lead to the incorporation of fluoride ions in the passiveoxide layer of Ti and its alloys, resulting in the formation of a por-ous layer. Depending on the concentration of NaF and the pH, tita-nium fluoride, titanium oxyfluoride or sodium titanium fluoridecould form on the surface of the Ti alloys [23,24,44]. Hence, it isapparent that the increase in active region of all the three Ti sam-ples of the present study in presence of fluoride ions is due to theformation of a porous or defective oxide layer.

In spite of the increase in active region, CP-Ti and, Ti–6Al–4Vand Ti–15Mo alloys exhibit the formation of a passive film in pres-ence of fluoride ions. The formation of passive oxide film on Ti andits alloys in fluoride containing electrolytes has been reported to bea function of the pH of the medium and, concentration of NaF anddissolved oxygen [23,24]. HF is considered responsible for destroy-ing the passive oxide film on Ti and its alloys and the concentrationof HF in turn depends on the pH of the medium. Once the passiveoxide layer on Ti is destroyed by fluoride attack, the rate of regen-eration of passive oxide layer is a linear function of the dissolvedoxygen concentration [23]. Hence, the effect of pH and, concentra-tion of NaF and dissolved oxygen, should be considered together. In

40 45 50

n angle (2θ)

β (1

10)

β (1

10)

α (1

0 1

)

α (0

002)

α (1

0 1

) (a)

(b)

(c)

) Ti–6Al–4V alloy; and (c) Ti–15Mo alloy.

Fig. 3. Potentiodynamic polarization curves of CP-Ti and, Ti–6Al–4V and Ti–15Moalloys in: (a) 0.15 M NaCl; (b) 0.15 M NaCl + 0.06 M NaF; and (c) 0.15 MNaCl + 0.5 M NaF.

Fig. 4. Average passive current density of CP-Ti and Ti–6Al–4V and Ti–15Mo alloysmeasured at 1200 mV vs. SCE as a function of the concentration of NaF in baseelectrolyte 0.15 M NaCl.

1724 S. Kumar et al. / Corrosion Science 52 (2010) 1721–1727

the present study, the electrolyte medium is less acidic (pH: 6.0),the concentration of NaF is in the range of 0.01–0.5 M and it isexposed to open air. The dissolved oxygen concentration of theelectrolyte solution is estimated to be around 6–8 ppm (0.0006–0.0008 wt.%). According to Nakagawa et al. [23] in acidic condi-tions, 30 ppm (0.003 wt.%) of HF could lead to destruction of thepassive film on Ti surface. Since the pH of the electrolyte used inthe present study is 6.0, the concentration of HF will be much low-er than 30 ppm (0.003 wt.%). Kwon et al. [45] have estimated theconcentration of HF of 0.05, 0.1 and 0.2 wt.% NaF solutions at pH

6 is only of the order of 12–13 ppm (0.0012–0.0013 wt.%) and con-firmed that at these HF concentrations Ti alloys could sustain theirpassive oxide layer. The availability of sufficient concentration ofdissolved oxygen would also enable regeneration of the passiveoxide layer on all the three Ti samples, even if it could have de-stroyed by the fluoride attack.

The dependence of passive current density of Ti and its alloys onthe concentration of NaF in the electrolyte medium has also beenreported earlier. An increase in passive current density with in-crease in concentration of fluoride ions in the medium has alsobeen observed for Ti–6Al–4V by Huang [24]. Among the three Tisamples studied, the average passive current density is low forCP-Ti compared to Ti–15Mo and Ti–6Al–4V alloys. This is due todissolution of the alloying elements (Mo in Ti–15Mo alloy and, Aland V in Ti–6Al–4V alloy), induced by the fluoride ions. There isnot much variation in the average passive current density of Ti-15Mo and Ti–6Al–4V alloys in 0.15 M NaCl containing 0.01 and0.03 M NaF. In 0.15 M NaCl containing 0.06 M NaF, the passive cur-rent density of Ti–6Al–4V alloy is relatively lower than Ti–15Mo al-loy. However, in 0.15 M NaCl containing 0.5 M NaF, the averagepassive current density of Ti–6Al–4V alloy is much lower thanCP-Ti and Ti–15Mo alloy. This can be attributed to the precipitationof aluminium and vanadium oxides along with the oxides of tita-nium on the surface of Ti–6Al–4V alloy. In contrast, in 0.15 M NaClcontaining 0.5 M NaF, the passive current density of Ti–15Mo alloyis much higher than CP-Ti and Ti–6Al–4V alloy.

3.3. Chronoamperometric studies of CP-Ti and, Ti–6Al–4V and Ti–15Mo alloys

The average steady state current density of CP-Ti and, Ti–6Al–4V and Ti–15Mo alloys in 0.15 M NaCl containing varying concen-trations of NaF (0, 0.01, 0.03, 0.06 and 0.5 M NaF) at differentapplied potentials are compiled in Table 2. The increase in averagesteady state current density with increase in concentration of NaFand increase in applied potential is due to the dissolution of theprotective oxide film as well as the substrate. In 0.15 M NaCl aswell as in 0.15 M NaCl containing 0.01, 0.03 and 0.06 M NaF, theaverage steady state current density is low for CP-Ti, followed byTi–15Mo alloy and Ti–6Al–4V alloy. However, in 0.15 M NaCl con-taining 0.5 M NaF, Ti–6Al–4V alloy exhibits the lowest steady state

Table 2Average steady state current density of the Ti alloys in 0.15 M NaCl containing varying concentrations of NaF at different impressed anodic potentials.

Type of Ti alloy Electrolyte medium Average steady state current densitya (mA/cm2) at different impressed anodic potentials

500 mV vs. SCE 1250 mV vs. SCE 2000 mV vs. SCE

CP-Ti 0.15 M NaCl 0.0005 ± 0.0002 0.0011 ± 0.0002 0.0020 ± 0.00020.15 M NaCl + 0.01 M NaF 0.0032 ± 0.0004 0.0073 ± 0.0003 0.0112 ± 0.00050.15 M NaCl + 0.03 M NaF 0.0262 ± 0.0010 0.0860 ± 0.0009 0.1440 ± 0.00100.15 M NaCl + 0.06 M NaF 0.0421 ± 0.0009 0.2159 ± 0.0011 0.4581 ± 0.00100.15 M NaCl + 0.5 M NaF 1.226 ± 0.0012 1.8856 ± 0.0014 3.5345 ± 0.0021

Ti–15Mo 0.15 M NaCl 0.0026 ± 0.0002 0.0120 ± 0.0002 0.0200 ± 0.00020.15M NaCl + 0.01 M NaF 0.0182 ± 0.0003 0.0274 ± 0.0004 0.1207 ± 0.00040.15 M NaCl + 0.03 M NaF 0.0858 ± 0.0009 0.1024 ± 0.0010 0.1840 ± 0.00080.15 M NaCl + 0.06 M NaF 0.1682 ± 0.0011 0.5296 ± 0.0012 0.7322 ± 0.00120.15 M NaCl + 0.5 M NaF 1.3048 ± 0.0013 4.6519 ± 0.0018 6.7558 ± 0.0022

Ti–6Al–4V 0.15 M NaCl 0.0063 ± 0.0002 0.0274 ± 0.0002 0.0313 ± 0.00020.15 M NaCl + 0.01 M NaF 0.0407 ± 0.0004 0.0827 ± 0.0004 0.1462 ± 0.00050.15 M NaCl + 0.03 M NaF 0.1503 ± 0.0009 0.2186 ± 0.0011 0.3427 ± 0.00100.15 M NaCl + 0.06 M NaF 0.2286 ± 0.0012 0.6280 ± 0.0012 0.7435 ± 0.00140.15 M NaCl + 0.5 M NaF 0.0176 ± 0.0010 0.0277 ± 0.0010 0.6727 ± 0.0012

a Average of five determinations.

S. Kumar et al. / Corrosion Science 52 (2010) 1721–1727 1725

current density, followed by CP-Ti and Ti–15Mo alloy. Al-Mayoufet al. [27] have also reported that the corrosion current densityof CP-Ti is lower than Ti–6Al–4V alloy in presence of 0.01 and0.05 M NaF whereas they have observed a reversal in trend whenthe concentration of NaF is increased to 0.5 M. Alves et al. [35] havereported that the passive current density of Ti–10Mo alloy is lowerthan Ti–6Al–4V alloy in 0.15 M NaCl containing 0.03 M NaF. Theresults of the current–time transient of the present study furtherconfirm the observations made earlier by other researchers[27,35].

The CTT studies performed in 0.15 M NaCl containing 0.03 MNaF at +200 mV vs. SCE for 30 min (Fig. 5) provides a comparisonof the performance of CP-Ti and, Ti–6Al–4V and Ti–15Mo alloys,in the intraoral oxidation potential (ranges between �58 mV and+212 mV vs. SCE). The steady state current density of CP-Ti and,Ti–15Mo and Ti–6Al–4V alloys in the intraoral oxidation potentialof +200 mV vs. SCE is found to be 1, 2 and 6 lA/cm2, respectively.The range of current density values indicate that all of them couldoffer better corrosion resistance in the potential range that couldexists in the oral environment. Among them, the corrosion protec-tive ability of CP-Ti and Ti–15Mo alloy are highly comparablewhereas the protective ability of Ti–6Al–4V alloy is relatively less.The surface morphology of the samples after subjecting them to

Fig. 5. Current–time transient curves of CP-Ti, Ti–15Mo and Ti–6Al–4V alloys in0.15 M NaCl containing 0.03 M NaF at 200 mV vs. SCE.

CTT studies in 0.15 M NaCl containing 0.03 M NaF at +200 mV vs.SCE for 30 min is shown in Fig. 6 and clearly indicate that the sur-face has became completely heterogeneous, resulting in a roughersurface following the corrosive attack of fluoride ions in the med-ium. In addition, following the corrosive attack in the grain bound-aries, the grains have become clearly visible in CP-Ti. In Ti–6Al–4Vand Ti–15Mo alloys, the dissolution of the alloying elements andtheir re-precipitation causes a different morphological featurecompared to that of CP-Ti.

4. Conclusions

The corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl solu-tion containing varying concentrations of NaF (0.01, 0.03, 0.06 and0.5 M) was evaluated and its corrosion performance was comparedwith that of CP-Ti and Ti–6Al–4V alloy to ascertain their suitabilityfor dental implant application. The study leads to the followingconclusions:

1. Ti–15Mo alloy exhibits the formation of a passive film in pres-ence of fluoride ions up to 0.5 M in the electrolyte solution, sim-ilar to CP-Ti and Ti–6Al–4V alloy.

2. There is a strong dependence of the passive current density onthe concentration of NaF in the electrolyte solution; increase inconcentration of NaF increases the passive current density.

3. The average steady state current density, measured from CTTstudies, increases with increase in concentration of NaF andincrease in impressed potential following the dissolution ofthe protective oxide film as well as the substrate.

4. In 0.15 M NaCl as well as in 0.15 M NaCl containing 0.01, 0.03and 0.06 M NaF, the steady state current density is low forCP-Ti compared to that of Ti–6Al–4V and Ti–15Mo alloyswhereas in 0.15 M NaCl containing 0.5 M NaF, Ti–6Al–4V alloyexhibits the lowest steady state current density.

5. The steady state current density of CP-Ti and, Ti–15Mo and Ti–6Al–4V alloys in 0.15 M NaCl containing 0.03 M NaF at 200 mVvs. SCE (intraoral oxidation potential) is found to be 1, 2 and6 lA/cm2, respectively. These values suggest that all of themcould offer better corrosion resistance in the potential rangethat could exists in the oral environment.

6. Based on the ability to form a stable passive film in presence offluoride ions and the low steady state current density in theintraoral oxidation potential, Ti–15Mo alloy can be a potentialcandidate for dental implant application.

Fig. 6. Scanning electron micrographs of: (a) CP-Ti; (b) Ti–6Al–4V alloy; and (c) Ti–15Mo alloy after subjecting them to current–time transient studies in 0.15 M NaClcontaining 0.03 M NaF at 200 mV for 30 min.

1726 S. Kumar et al. / Corrosion Science 52 (2010) 1721–1727

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.

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