corrosion behaviour of ti–15mo alloy for dental implant applications
TRANSCRIPT
Corrosion behaviour of Ti–15Mo alloy for dental implantapplications
Satendra Kumar *, T.S.N. Sankara Narayanan *
National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, India
j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 5 0 0 – 5 0 7
a r t i c l e i n f o
Article history:
Received 7 November 2007
Received in revised form
7 March 2008
Accepted 25 March 2008
Keywords:
Corrosion
Titanium alloys
Biocompatibility
X-ray diffraction
Electrochemical characterization
Dental implant
a b s t r a c t
The corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl solution containing varying
concentrations of fluoride ions (190, 570, 1140 and 9500 ppm) is evaluated using potentio-
dynamic polarization, electrochemical impedance spectroscopy (EIS) and chronoampero-
metric/current–time transient (CTT) studies to ascertain its suitability for dental implant
applications. The study reveals that there is a strong dependence of the corrosion resistance
of Ti–15Mo alloy on the concentration of fluoride ions in the electrolyte medium. Increase in
fluoride ion concentration from 0 to 9500 ppm shifts the corrosion potential (Ecorr) from�275
to �457 mV vs. SCE, increases the corrosion current density (icorr) from 0.31 to 2.30 mA/cm2,
the passive current density (ipass) from 0.07 to 7.32 mA/cm2 and the double-layer capacitance
(Cdl) from 9.63 � 10�5 to 1.79 � 10�4 F and reduces the charge transfer resistance (Rct) from
6.58 � 104 to 6.64 � 103 V cm2. In spite of the active dissolution, the Ti–15Mo alloy exhibit
passivity at anodic potentials at all concentrations of the fluoride ions studied. In dental
implants since the exposure of the alloy will be limited only to its ‘neck’, the amount of Mo
ions released from Ti–15Mo alloy is not likely to have an adverse and hence, in terms of
biocompatibility this alloy seems to be acceptable for dental implant applications. The
results of the study suggest that Ti–15Mo alloy can be a suitable alternative for dental
implant applications.
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1. Introduction
Titanium and titanium alloys are widely used for many
biomedical applications due to their low density, excellent
biocompatibility, corrosion resistance and mechanical proper-
ties.1–4 Among the various types of Ti alloys, Ti–6Al–4V has
been the choice in many instances due to its ideal mechanical
properties and corrosion resistance for implant applications.
However, if the leaching of V and Al exceeds a threshold level,
then it may cause peripheral neuropathy, osteomalacia and
Alzheimer diseases.5–7 The development of Ti–6Al–7Nb8 and
Ti–5Al–2.5Fe,9 where Nb and Fe were substituted for V in Ti–
6Al–4V alloy, has not received much success as they still
contains Al.10,11
* Corresponding authors. Tel.: +91 44 2254 2077; fax: +91 44 2254 1027E-mail addresses: [email protected] (S. Kumar), tsnsn@redi
0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reservedoi:10.1016/j.jdent.2008.03.007
The titanium alloys developed in the early stage are mainly
a + b type ones. Recently, mechanical biocompatibility of
biomaterials is also regarded as an important criterion in the
selection of biomaterial. Hence, the research and development
on b-type titanium alloys, which are considered advantageous
in terms of mechanical biocompatibility, are increasing.12,13
Since the b-phase in Ti alloys exhibits a significantly lower
modulus than the a-phase, the development of low modulus
b-Ti alloys which retain a single b-phase microstructure on
rapid cooling from high temperatures assumes significance.
Several b-phase Ti alloys, having Nb, Ta, Zr and Mo as alloying
elements (b-stabilizer elements) such as, Ti–12Mo–6Zr–2Fe
and Ti–13Mo–7Zr–3Fe (‘TMZF’),14,15 Ti–15Mo–5Zr–3Al,16 Ti–
15Mo–3Nb–3O (‘TIMETAL 21Srx’),16 Ti–14Nb–13Zr,17 Ti–35Nb–
.ffmail.com (T.S.N. Sankara Narayanan).
d.
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7Zr–5Ta and Ti–34Nb–9Zr–8Ta (‘TNZT’),14,15 Ti–29Nb–4.6Zr–
13Ta,17 Ti–15Mo14, etc., were developed. Ho et al.18 and Oliveira
et al.19 have studied the structure and properties of a series of
binary Ti–Mo alloy with Mo content ranging up to 20 wt.%.
Based on the microstructural evolution and strengthening
mechanisms of Ti–15Mo alloy, Nag et al.15 have recommended
it as one of the promising biocompatible Ti alloy.
In the solution annealed condition, Ti–15Mo alloy has a
modulus of elasticity that is about two-third of the modulus of
Ti–6Al–4V alloy, along with considerably improved ductility
and fatigue properties.20,21 Ti–15Mo alloy has superior notch
sensitivity resistance when compared with conventional a + b
titanium alloys, Ti–6Al–4V ELI and Ti–6Al–7Nb.22 This is an
important material property to be considered for devices
subject to scratching and for components with holes, threads,
sharp edges, and in contact with other components in the
design of an implant. Considering the improved mechanical
properties and better corrosion resistance of the b-phase Ti–
15Mo alloy, it is worthwhile to evaluate its corrosion resistance
in a variety of simulated body fluids and is being explored by
our research group.
Titanium alloys have also been used in dental implants.
Since the oral environment could involve fluoride medium,
the degree of corrosion resistance offered by the Ti alloys in
fluoride containing medium becomes an important criterion
in the selection of a metallic biomaterial to be used in the oral
medium. Many researchers have confirmed the negative
influence of fluoride ions on the corrosion of titanium.23–31
During the last four decades, the use of gels and solutions
containing high concentrations of fluorides has indeed
become more frequent, reaching a noticeable impact on the
dental caries prevention.32 There has been a rapid increase in
the utilization of fluoridated prophylactic gels and rinses in
the odontological field. Many commercially available fluori-
nated gels contain very high concentrations of fluoride ions,
up to 10,000 ppm, with a pH, ranging between 7.2 and 3.2.28,33
Such a high concentration of fluoride ions would have a
deleterious influence on the corrosion resistance of Ti alloys.
Hence, it is essential to understand the electrochemical
behaviour of Ti alloys in fluoride medium.
The biocompatibility of Ti and its alloys in dental
application is decided based on the osseointegration response
and cell adhesion behaviour. Wang and Li34 have studied the
biocompatibility of Ti alloys for dental restoration. They have
found that Ti and its alloys were not mutagenic but no
significant difference in the cell attachment was observed.
Surface modification of Ti alloys, namely TiN coating, plasma
sprayed hydroxyapatite coatings, pulsed laser deposited of TiC
coating, etc., were explored to improve the biocompatibility of
Ti alloys for dental implant applications.35–37 It has been
established that the chemical properties of the oxide layer on
the Ti alloys play an important role in deciding its biocompat-
ibility with the surrounding tissues. If the medium is acidic
and contains appreciable amount of fluoride ions, then it
would lead to the formation of hydrofluoric acid (HF). When
the concentration of HF exceeds 30 ppm, the passive film on
the Ti alloy will get destructed and its mechanical properties
will be drastically affected.27 Alloying of certain elements
along with Ti is found to offer a better corrosion resistance in
fluoride containing medium.38,39 Alloying of 0.5 wt.% of Pt or
Pd with Ti promotes the formation of a passive film on the
titanium surface in presence of fluoride and offer better
corrosion resistance than that of CP–Ti, Ti–6Al–4V and Ti–6Al–
7Nb alloys.32 It is important to ascertain whether the b-phase
Ti–Mo alloys having 10–20 wt.% of Mo could offer a better
corrosion resistance in presence of fluoride. Alves et al.23 have
evaluated the corrosion resistance of Ti–10Mo alloy in 0.15 M
NaCl containing 570 ppm of fluoride ions and compared with
that of Ti–6Al–4V alloy. According to them, both Ti–10Mo and
Ti–6Al–4V alloys exhibit similar electrochemical character-
istics and the passive current density of Ti–10Mo alloy is
relatively lower.23 Much remains to be explored on the
corrosion behaviour of Ti–Mo alloys in fluoride containing
electrolytes. In this perspective, the present work aims to
study the corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl
containing varying concentrations of fluoride ions (190, 570,
1140 and 9500 ppm) and to predict its suitability for dental
implant applications.
2. Materials and methods
Ti–15Mo alloy (chemical composition, in wt.%—N: 0.01; C: 0.02;
H: 0.011; Fe: 0.012; O: 0.10; Mo: 15.04; Ti: balance) was used in
the present study. The microstructure of the Ti–15Mo alloy
was examined using a Leica DMLM optical microscope with
image analyzer software. For metallographic studies, the Ti–
15Mo alloy was mechanically polished using various grades of
SiC paper (60, 100, 220, 320, 400, 600, 1/0, 2/0, 3/0 and 4/0,
respectively) followed by polishing using a 0.3 mm diamond
paste and subsequently etched for 20–30 s approximately
using a mixture of 15 vol.% HNO3, 5 vol.% HF and balance
water. This etching solution has been used earlier by Ho et al.18
for evaluating the microstructure of Ti–Mo alloys containing
up to 20 wt% Mo. The structural characteristic of the Ti–15Mo
alloy was evaluated by X-ray diffraction (XRD) measurement,
using Cu Ka radiation. The microhardness was determined
using a Leica VMHTMOT microhardness tester at a load of
200 g applied for 15 s.
The corrosion behaviour of Ti–15Mo alloy was evaluated
using 0.15 M NaCl solution as the base electrolyte to which
varying concentrations of fluoride ions (190, 570, 1140 and
9500 ppm), was added (as NaF) to study the influence of
fluoride. Nakagawa et al.28 have suggested that many
commercially available fluorinated gels may contain up to
10,000 ppm of fluoride ions, with a pH ranging from 7.2 to 3.2.
Alves et al.23 have evaluated the corrosion resistance of Ti–
10Mo and Ti–6Al–4V alloys in 0.15 M NaCl containing 570 ppm
of fluoride ion. Based on these reports it was decided to use
0.15 M NaCl as the base electrolyte and to represent a wide
range, the concentration of fluoride ions were chosen as 190,
570, 1140 and 9500 ppm.
Potentiodynamic polarization, electrochemical impedance
spectroscopy (EIS) and chronoamperometric/current–time
transient (CTT) studies were carried out using a potentio-
stat/galvanostat/frequency response analyzer of ACM instru-
ments (model: Gill AC) to assess the corrosion performance of
the alloy. For corrosion studies, the Ti–15Mo alloy was
mechanically polished using various grades of SiC paper,
rinsed with deionized water, pickled using a mixture of
Fig. 1 – Microstructure of the Ti–15Mo alloy.
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35 vol.%, HNO3, 5 vol.% HF and balance water at 40 8C,
thoroughly rinsed in deionized water and dried using a
stream of compressed air. The cleaned Ti–15Mo alloy forms
the working electrode while a saturated calomel electrode
(SCE) and a graphite rod were used as the reference and
auxiliary electrodes, respectively. The electrodes were placed
within a flat cell in such a way that only 1 cm2 area of the
working electrode was exposed to the electrolyte solution.
Potentiodynamic polarization measurements were carried out
in the potential range from �250 mV in the cathodic direction
to +250 mV in the anodic direction from open-circuit potential
(OCP) vs. SCE at a scan rate of 100 mV/min and the corrosion
potential (Ecorr) and corrosion current density (icorr) were
determined from the polarization curves using Tafel extra-
polation method. The potentiodynamic polarization experi-
ments were also carried out in the potential range of �250 to
+3000 mV with respect to OCP vs. SCE at a scan rate of 100 mV/
min, to study the passivation behaviour and the average
passive current density (ipass) was calculated from the
polarization curves. EIS study was conducted by applying a
sinusoidal signal with an amplitude of 32 mV and the
electrode response was analyzed in the frequency range
between 10,000 and 0.01 Hz in 0.15 M NaCl containing varying
concentrations of fluoride ions (190, 570, 1140 and 9500 ppm),
at their respective OCP. The charge transfer resistance (Rct)
and double-layer capacitance (Cdl) values were determined
from the Nyquist plot by fitting the data using Boukamp
software. The CTT studies of Ti–15Mo alloy in 0.15 M NaCl
containing varying concentrations of fluoride ions (190, 570,
1140 and 9500 ppm), were performed at three different
potentials, namely, +500, +1250 and +2000 mV vs. SCE for
1 h. The potentiodynamic polarization, EIS and CTT studies
were repeated at least three times so as to ensure reprodu-
cibility of the test results.
Fig. 2 – Potentiodynamic polarization curve of Ti–15Mo
alloy in 0.15 M NaCl with varying concentrations of
fluoride ions (190, 570, 1140 and 9500 ppm): scan rate—
100 mV/min (potential in mV vs. SCE).
3. Results
3.1. Microstructure, structure and microhardness of theTi–15Mo alloy
The microstructure of the Ti–15Mo alloy (Fig. 1) reveals
equiaxed b-grains as the dominant phase, which is homo-
geneous and evenly distributed. The X-ray diffraction pattern
of the alloy indicates that only the b-phase is retained in the
structure (figure not shown). The microhardness of Ti–15Mo
alloy is found to be 238 � 5 HV0.2.
3.2. Potentiodynamic polarization studies of the Ti–15Moalloy
Potentiodynamic polarization studies of the Ti–15Mo alloy
were conducted in the potential range of�250 to +250 mV with
respect to OCP vs. SCE at a scan rate of 100 mV/min to observe
the effect of fluoride ions on the corrosion behaviour (Fig. 2).
Though the shape of the curves is quite similar, the active
region of the curves is extended to higher current region in
presence of fluoride ions. The corrosion potential (Ecorr) and
corrosion current density (icorr), calculated using Tafel extra-
polation method, are compiled in Table 1. There is a cathodic
shift in Ecorr from �275 to �457 mV vs. SCE and a remarkable
increase in icorr from 0.31 to 2.30 mA/cm2 with increase in
fluoride ion concentration from 0 to 9500 ppm.
To observe the effect of fluoride ion on the passivity of
Ti–15Mo alloy, the potentiodynamic polarization studies
were performed in the potential range of �250 to +3000 mV
with respect to OCP vs. SCE at a scan rate of 100 mV/min.
The anodic branch of the polarization curve exhibits an
active–passive transition in all the cases (Fig. 2). The average
passive current density (ipass) of the Ti–15Mo alloy is
measured from the polarization curves and compiled in
Table 1.
Table 1 – Corrosion potential (Ecorr), corrosion current density (icorr) and average passive current density (ipass) of Ti–15Moalloy in 0.15 M NaCl containing varying concentrations of fluoride ions
Electrolyte medium Corrosion potential,Ecorr (mV vs. SCE)
Corrosion currentdensity, icorr (mA/cm2)
Average passive currentdensity, ipass (mA/cm2)
0.15 M NaCl �275 0.31 0.07
0.15 M NaCl + 190 ppm of F� �282 0.65 0.11
0.15 M NaCl + 570 ppm of F� �376 1.22 0.42
0.15 M NaCl + 1140 ppm of F� �409 1.77 0.93
0.15 M NaCl + 9500 ppm of F� �457 2.30 7.32
Fig. 3 – Bode impedance and phase angle plots of Ti–15Mo
alloy in 0.15 M NaCl with varying concentrations of
fluoride ions (190, 570, 1140 and 9500 ppm): (*) 0 ppm; ( )
190 ppm; (*) 570 ppm; (&) 1140 ppm; (4) 9500 ppm of FS.
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3.3. Electrochemical impedance spectroscopy studies of theTi–15Mo alloy
Fig. 3 shows the Bode impedance and Bode phase angle plots of
the Ti–15Mo alloy in 0.15 M NaCl containing varying concen-
tration of fluoride ions (190, 570, 1140 and 9500 ppm), at their
respective OCP vs. SCE. The Bode impedance plot of Ti–15Mo
alloy clearly reveals the dependence of the impedance values
on the fluoride ion concentration in the base solution (0.15 M
NaCl), which is also reflected in the change in diameter of the
semicircle in the Nyquist plots (figure not shown). The Rct and
Cdl, determined from the Nyquist plots after fitting the data
using Boukamp software, are presented in Table 2. The Rct is
decreased from 6.58 � 104 to 6.64 � 103 V cm2 whereas the Cdl
Table 2 – Charge transfer resistance (Rct) and double-layer capacitance (Cdl) of the Ti–15Mo alloy in 0.15 M NaClcontaining varying concentrations of fluoride ions
Electrolyte medium Rct (V cm2) Cdl (F)
0.15 M NaCl 6.58 � 104 9.63 � 10�5
0.15 M NaCl + 190 ppm of F� 4.76 � 104 8.40 � 10�5
0.15 M NaCl + 570 ppm of F� 2.16 � 104 1.31 � 10�4
0.15 M NaCl + 1140 ppm of F� 1.19 � 104 1.57 � 10�4
0.15 M NaCl + 9500 ppm of F� 6.64 � 103 1.79 � 10�4
is increased from 9.63 � 10�5 to 1.79 � 10�4 F with increase in
fluoride ion concentration from 0 to 9500 ppm.
The Bode phase angle plots (Fig. 3) indicate that the phase
angle shift is nearly�308 at low frequency in 0.15 M NaCl and it
reduces to �58 with increase in fluoride ion concentration
from 0 to 9500 ppm in the base solution. However, at
intermediate frequencies, the phase angle maximum is
around �658 in 0.15 M NaCl and remained constant over a
wide range of frequency. The phase angle shift increases from
approximately �658 to �708 with increase in fluoride ion
concentration and the frequency range in which the phase
angle remains constant is also reduced. When the fluoride ion
concentration in the base solution is higher (9500 ppm), the
phase angle shift starts to reduce immediately after reaching
the peak value of �708. The flat portion of the phase-angle
spectrum reduces with increase in fluoride ion concentration
in 0.15 M NaCl solution.
3.4. Chronoamperometric/current–time transient studiesof the Ti–15Mo alloy
CTT studies of Ti–15Mo alloy in 0.15 M NaCl containing
varying concentrations of fluoride ions (190, 570, 1140 and
9500 ppm) were performed at three different potentials,
namely, +500, +1250 and +2000 mV vs. SCE for 1 h. The CTT
curve of Ti–15Mo alloy in 0.15 M NaCl containing 0, 190, 570
and 1140 ppm of fluoride ions, at +1250 mV vs. SCE is
presented as a representative curve in Fig. 4. In the absence
Fig. 4 – Current–time transient curves of Ti–15Mo in 0.15 M
NaCl with varying concentrations of fluoride ions (190, 570
and 1140 ppm) at +1250 mV vs. SCE.
Table 3 – Average steady state current density of the Ti–15Mo alloy in 0.15 M NaCl containing varying concentrations offluoride ions at different impressed potentials
Electrolyte medium Average steady state current density at different impressed potentials(mA/cm2)
+500 mV +1250 mV +2000 mV
0.15 M NaCl 0.003 0.012 0.020
0.15 M NaCl + 190 ppm of F� 0.018 0.027 0.121
0.15 M NaCl + 570 ppm of F� 0.086 0.102 0.184
0.15 M NaCl + 1140 ppm of F� 0.168 0.530 0.732
0.15 M NaCl + 9500 ppm of F� 1.305 4.652 6.756
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of fluoride ions as well as in presence of 190 ppm of fluoride
ions, the CTT curves exhibit a rapid decrease in current
density followed by a slow decay to attain a steady state. In
presence of 1140 ppm of fluoride ions, the CTT curves exhibit
a decrease in current density during the initial period,
reaching a minimum followed by an increase in current
density, reaching a peak current, which subsequently decays
to attain a steady state. In presence of 9500 ppm of fluoride
ions, the CTT curves exhibit a sharp increase in current
density during the initial periods which then decays to attain
steady state, at all applied potentials (figure not shown). The
average steady state current density calculated at all the three
potentials is complied in Table 3.
4. Discussion and conclusions
4.1. Microstructure, structure and microhardness of theTi–15Mo alloy
The retention of the b-phase in Ti alloys with higher Mo
content has been reported earlier by Davis et al.40 Ho et al.18
have reported that Ti–Mo alloys having 9 wt.% Mo have a
significant amount of equiaxed b-phase whereas in alloys
containing equal to or greater than 10 wt.% Mo, the b-phase
becomes the only dominant phase. Bania41 has also
reported that a minimum of 10 wt.% Mo is needed to fully
stabilize b-phase at room temperature. Ho et al.18 and
Oliveira et al.19 have suggested that the crystal structure of
the Ti–Mo alloys is sensitive to the Mo content. Based on the
XRD measurements, they have confirmed that a significant
retention of the b-phase for the Ti–Mo alloy containing
10 wt.% Mo, while in alloys having higher Mo concentrations
(15 and 20 wt.%) only the b-phase is retained. Ho et al.18
have reported that the hardness of Ti–Mo alloys (containing
6–20 wt.% Mo), in general, is higher than that of CP–Ti.
They have found that the hardness of Ti–7.5Mo alloy is
lower by 10.5% than that of Ti–6Al–4V alloy whereas the Ti–
20Mo alloy had a higher hardness than Ti–6Al–4V alloy.
The results of the present study reveal that the microhard-
ness of the Ti–15Mo alloy is lesser than the observation
made by Ho et al.18 As several factors could influence the
microhardness of Ti–Mo alloys, which include solid solution
strengthening, precipitation hardening, strain aging, grain
size and crystal structure/phase (a, a + b, b),18 the inter-
pretation of the measured microhardness values becomes
quite complex.
4.2. Potentiodynamic polarization studies of the Ti–15Moalloy
The shift in the active region of the polarization curves
towards higher current region (Fig. 2) suggesting the negative
influence of fluoride ions on the corrosion resistance of Ti–
15Mo alloy. The cathodic shift in Ecorr from �275 to �457 mV
vs. SCE and a remarkable increase in icorr from 0.31 to 2.30 mA/
cm2 with increase in fluoride ion concentration from 0 to
9500 ppm confirm this phenomenon. The negative influence
of fluoride ions on the corrosion resistance of Ti alloys has also
been confirmed by many researchers.23–31 Thomson-Neal
et al.42 have reported that 3 ppm NaF is sufficient to tarnish
the surface of Ti while autoclaving the Ti implants. Oshida
et al.33 have reported that commercially available fluoride
treatment agent could cause discoloration of Ti–6Al–4V
brackets. According to Schutz and Thomas,43 20 ppm of NaF
may destroy the protective oxide layer on Ti while Nakagawa
et al.27 have reported such an occurrence at 30 ppm of NaF.
Hence, the increase in the active region of Ti–15Mo alloy in
presence of fluoride ions is due to the formation of a porous or
defective oxide layer that reduces its corrosion protectiveness,
which is reflected in the Ecorr and icorr values.
The active–passive transition observed in the anodic
branch of the polarization curves, both in the absence and
in presence of 190, 570, 1140 and 9500 ppm of fluoride ions,
suggests that the presence of fluoride ions in 0.15 M NaCl did
not hinder the formation of passive oxide film on the surface
of the Ti–15Mo alloy. The occurrence of active–passivation
transition of Ti alloys in presence of fluoride ions has also been
reported earlier.30,44 Huang et al.30 have observed the active–
passive transition of Ti–6Al–4V alloy in acidic artificial saliva
(pH: 5) except when the NaF concentration is 0.5% whereas
Schmidt et al.44 have observed a similar phenomenon in
lactated Ringer’s serum (pH: 6.5) in presence of fluoride. The
occurrence of active–passive transition depends mainly on the
concentration of HF in the medium. According to Nakagawa
et al.,27 in acidic conditions, 30 ppm of HF could lead to the
destruction of the passive film on Ti surface. Since, the pH of
the electrolyte used in the present study is 6.0, the concentra-
tion of HF will be much lower than 30 ppm. Kwon et al.31 have
estimated the concentration of HF in solutions of 0.05, 0.1, and
0.2% NaF, i.e., 950, 1900 and 3800 ppm of fluoride ions, at pH 6
is of the order of only 12–13 ppm and confirmed that at these
HF concentrations the passive oxide layer on Ti alloys could be
sustained. Hence, the occurrence of active–passive transition
in 0.15 M NaCl as well as in presence of 190, 570, 1140 and
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9500 ppm of fluoride ions is due to the lower concentrations of
HF and confirms the ability of Ti–15Mo alloy to impart
passivity. The passive films are believed to be composed of
one or more protective oxide films.
The ipass is found to increase from 0.07 to 7.32 mA/cm2 with
increase in fluoride ion concentration from 0 to 9500 ppm. This
observation suggests that the oxide film formed on the surface
is having less insulating property, thus allowing the passage of
higher current for oxidation of species in the electrolyte.
Huang et al.30 have also reported about the increase in passive
current density of Ti–6Al–4V with increase in NaF concentra-
tion in the electrolyte. The increase in ipass of Ti–15Mo alloy
may be due to the dissolution of the alloying element, namely
Mo, induced by fluoride ions present in the medium.
4.3. Electrochemical impedance spectroscopy studies of theTi–15Mo alloy
The decrease in Rct from 6.58 � 104 to 6.64 � 103 V cm2 and
increase in Cdl from 9.63 � 10�5 to 1.79 � 10�4 F suggests the
negative influence of fluoride ions on the corrosion resistance
of Ti–15Mo alloy and support the observations made by
potentiodynamic polarization studies. The phase angle max-
imum and the phase shifts observed in the Bode phase angle
plots is typical for a passive surface indicating good corrosion
resistance with a near capacitive behaviour and indicate the
difficulty in charge transfer process. The two peaks observed
in phase angle plots indicate the involvement of two
relaxation time constants. The two distinct capacitive beha-
viours can be attributed due to the bi-layered oxide surface
consisting of a porous outer layer and a barrier inner layer.45,46
The change in the shape of the Bode plots as a function of
fluoride ion concentration is mainly caused by diffusion
phenomena due to dissolution reaction. Therefore, the ease of
charge transfer is more with the increase of fluoride ion
concentration in the base solution. Besides, at higher fluoride
ion concentration (9500 ppm) with 0.15 M NaCl, the phase
angle shift values are higher over the frequency range of 1000–
100 Hz. This is believed to be due to the formation of passive
oxide film consisting of oxides of titanium and molybdenum
immediately after the fast interaction of the alloy with fluoride
ion whereas the interaction is delayed in the absence or
presence of lower concentrations of fluoride ions. The passive
film formed in presence of 9500 ppm of fluoride ions may not
be stable for a longer time and starts dissolving due to the
negative effect of higher concentration of fluoride ion. This
could be one of the reasons for the decrease in phase angle just
after reaching the peak value. The changes in phase angle
confirm the negative influence of fluoride ion and support the
observation made in potentiodynamic polarization studies.
4.4. Chronoamperometric/current–time transient studiesof the Ti–15Mo alloy
The rapid decrease in current density followed by a slow decay
to attain a steady state observed in the CTT curves in the
absence as well as in presence of 190 ppm of fluoride ions can
be explained based on the decrease in active area due to the
growth of a passive film. The decrease in current density
during the initial period, reaching a minimum followed by an
increase in current density, reaching a peak current, which
subsequently decays to attain a steady state observed in
presence of 1140 ppm of fluoride ions can be due to the
instability of passive oxide film that leads to the exposure of
active surface area and the consequent repassivation or re-
precipitation of the oxides on the surface. The maximum of
the CTT curves may be associated with the active/passive
transition peak, which is commonly attributed to electro-
oxidation process or new phase formation. Alves et al.23 have
also observed a similar trend in the CTT curves of Ti–10Mo
alloy in 0.15 M NaCl containing 570 ppm of fluoride ions. The
electro-oxidation process may be mainly related to titanium or
titanium species along with the respective alloying element
species present in the initially formed film.
The extent of change in current as a function of time and
the average steady state current density has a strong
dependence on the concentration of fluoride ions in the
electrolyte medium and the applied potentials. The increase in
the average steady state current density with increase in
fluoride ion concentration and increase in applied potentials is
due to the dissolution of the protective oxide film as well as the
substrate (Table 3). It is important to note that in spite of the
increase in extent of dissolution with increase in applied
potential, the Ti–15Mo alloy exhibit passivity at all the three
potentials, namely, +500, +1250 and +2000 mV vs. SCE. Alves
et al.23 have reported that the passive current density of Ti–
10Mo alloy is lower than Ti–6Al–4V alloy in 0.15 M NaCl
containing 570 ppm of fluoride ions. Al-Mayouf et al.39 have
also reported that corrosion current density of Ti–6Al–4V alloy
is lower than CP–Ti and Ti–30Cu–10Ag alloy in presence of
9500 ppm of fluoride ions. The results of the current–time
transient of the present study further confirm the observa-
tions made earlier by other researchers.23,39
The CTT studies reveal greater extent of dissolution of the
Ti–15Mo alloy in presence of fluoride ions. Hence, the toxicity
of Mo should be considered as a criterion in the choice of Ti–
15Mo alloy to be used for dental implant application. Some
positive as well as negative influence of Mo has been reported
in the published literature.47–49 Mo is considered to be
instrumental in regulating the pH balance in the body. It
has been shown to act as a cofactor for a limited number of
enzymes in humans.47 However, excessive intake of Mo, in
rare cases, has been found to cause joint pain and swelling, leg
cramps and, a burning sensation upon urination. It has been
reported that 10–100 ppm of Mo ion could alter cell metabolic
functions.48 The recommended dietary allowance (RDA) for
adult men and women is 45 mg/day. The tolerable upper intake
level is 2 mg/day, a level based on impaired reproduction and
growth in animals.49 Fortunately, the exposure of dental
implants to fluoride ion containing gels, etc., would be limited
only to the ‘neck’ of the implant and for very short periods of
time. Hence, the amount of Mo ions released from Ti–15Mo
alloy is not likely to have an adverse effect and in terms of
biocompatibility Ti–15Mo alloy seem to be acceptable for
dental implant applications.
The corrosion behaviour of Ti–15Mo alloy in 0.15 M NaCl
solution containing varying concentrations of fluoride ions (190,
570, 1140 and 9500 ppm), was evaluated using potentiodynamic
polarization, electrochemical impedance spectroscopy and
chronoamperometric studies to ascertain the protective ability
j o u r n a l o f d e n t i s t r y 3 6 ( 2 0 0 8 ) 5 0 0 – 5 0 7506
of the alloy in presence of fluoride ion. The microstructure,
structural characteristics and hardness were also evaluated.
The study leads to the following conclusions:
� T
he Ti–15Mo alloy exhibits the presence of only theequiaxed b-phase, which is homogeneous and evenly
distributed and its microhardness is 238 � 5 HV0.2.
� T
he active–passive transition is observed in presence of allconcentrations of fluoride ions. However, the active region is
extended to higher current region in presence of fluoride
ions. In spite of the active dissolution in presence of fluoride
ions, the Ti–15Mo alloy exhibit passivity at anodic poten-
tials.
� T
here is a strong dependence of the Ecorr, icorr, ipass, Rct, Cdland the average steady state current density values of the
Ti–15Mo alloy on the concentration of fluoride ions in the
electrolyte medium. Increase in fluoride ion concentration
increases the icorr, ipass, average steady state current density
and Cdl values, causes a cathodic shift in Ecorr and a decrease
in Rct values, suggesting the negative influence of fluoride
ion and a decrease in corrosion protective ability of Ti–15Mo
alloy. The average steady state current density also exhibits
a linear dependence on the applied potential, suggesting the
dissolution of the protective oxide film as well as the
substrate.
� A
s the exposure of dental implants to fluoride ion containinggels, etc., would be limited only to the ‘neck’ of the implant
and for very short periods of time, the amount of Mo ions
released from Ti–15Mo alloy is not likely to have an adverse
effect. Hence, in terms of biocompatibility Ti–15Mo alloy
seem to be acceptable for dental implant applications.
� B
ased on the results of the study, Ti–15Mo alloy can be asuitable alternative for dental implant applications.
Acknowledgement
The authors express their sincere thanks to Prof. S.P.
Mehrotra, Director, National Metallurgical Laboratory, Jam-
shedpur, for his keen interest and permission to publish this
paper.
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