# Electrochemical Behavior of Titanium and Its Alloys as  Dental Implants in Normal Saline

Authors: Rahul Bhola; Shaily M. Bhola; Brajendra Mishra; David L. Olson
Journal: Research Letters in Physical Chemistry
 (2009)

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## Abstract

The electrochemical behavior of pure titanium and titanium alloys in a simulated body fluid (normal saline solution) has been tested, and the results have been reported. The significance of the results for dental use has been discussed. The tests also serve as a screening test for the best alloy system for more comprehensive long-term investigations.


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## Body

## 1. Introduction

Commercially pure titanium and titanium alloys are known for their use
in dental practice owing to their good corrosion resistance, biocompatibility,
and biofunctionality in the human body [1].Titanium is a thermodynamically reactive metal as suggested by its
relatively negative reversible potential in the electrochemical series [2].
It gets readily oxidized during exposure to air and electrolytes to form oxides,
hydrated complexes, and aqueous cationic species. The oxides and hydrated complexes
act as barrier layers between the titanium surface and the surrounding
environment and suppress the subsequent oxidation of titanium across the
metal/barrier layer/solution interface. Even if the barrier layer gets
disrupted, it can get reformed very easily, leading to spontaneous
repassivation. A review of literature reveals that extensive work has been done
on the use of titanium alloys as prostheses when the alloy comes in contact
with saliva [3–5]. Whereas
when titanium alloys are used as implants, the alloy remains in contact with
saliva only during the transition period of healing of the bony socket and
gingiva (gum) but when it is capped with the crown, it is in contact with only
bone cells for the rest of its life. The aim of this study is to keep in focus
the use of titanium alloy as an implant inside the bone tissue and hence normal
saline solution was used to simulate the extracellular interstitial environment.
The decrease in pH can occur inside the bone tissue owing to surgical trauma, infection [6, 7] as well as the healing process of the bony socket. Ti3+ ions, if produced, can hydrolyze to further acidify the area [8].
Acidification can also be observed if crevice corrosion occurs in the clefts between
the implant and the crown [5]. According to Black [9],
the potential of a metallic biomaterial can range from −1 to 1.2 V versus SCE
in the human body. It is, thus, significant to study the electrochemical
behavior of titanium in this potential range.Electrochemical corrosion behavior of commercially pure titanium (grades
Ti1 & Ti2) and its alloys, Ti64, TiOs, Ti15Mo, and TMZF in normal saline
solution was analyzed at the body temperature of37°C.


## 2. Experimental

### 2.1. Materials Preparation

Titanium alloy grades Ti1, Ti2, Ti64, TiOs, Ti15Mo, and TMZF of
compositions Ti1 (0.1%C, 0.2%Fe, 0.015%H, 0.03%N, 0.18%O, and 99.47% Ti), Ti2
(0.1%C, 0.3%Fe, 0.015%H, 0.03%N, 0.25%O, and 99.30%Ti), Ti64 (0.1%C, 0.2%Fe,
0.015%H, 0.03%N, 0.2%O, 6%Al, 4%V, and 89.45%Ti), TiOs (0.05%C, 0.05%Fe,
0.015%H, 0.02%N, 0.15%O, 35.5% Nb, 5.7%Ta, 7.3%Zr, and 51.21%Ti), Ti15Mo
(0.05%C, 0.1%Fe, 0.015%H, 0.01%N, 0.15%O, 15%Mo, and 84.67%Ti), and TMZF
(0.02%C, 2%Fe, 0.02%H, 0.01%N, 0.18%O, 6%Zr, 12%Mo, and 79.77%Ti) were used for
the present investigation. Available cuboidal and cylindrical rods were cut to
expose cross-section areas of 0.855cm2 for Ti1, 1.0cm2 for Ti2, 1.5525cm2 for Ti64, 1.3270cm2 for TiOs, 0.4869cm2 for
Ti15Mo, and 2.7606cm2 for TMZF as working electrodes. Normal saline
solution (5.26gL−1 sodium chloride, 2.22gL−1 anhy. sodium
acetate, 5.02gL−1 sodium gluconate, 0.37gL−1 potassium chloride,
and 0.3gL−1 magnesium chloride hexahydrate) having pH 6.6 was used
to carry out the electrochemical testing of alloys.


### 2.2. Measurements

A Princeton Applied Research Potentiostat 273A was used for the
electrochemical measurements. A three-electrode cell assembly consisting of
titanium alloy as the working electrode, platinum wire as the counter electrode,
and a saturated calomel electrode as the reference electrode was used. 450 mL
of normal saline solution was used for the immersion
and electrochemical testing of the alloys.Open Circuit Potential (OCP)
Open circuit potential values of the alloys were
measured just after immersion, followed by a duration of 2, 6, and 24 hours up to
seven days.Tafel
After the end
of the seven days, Tafel curves of various alloys were obtained by polarizing
each electrode from−250mV versus OCP to 2 V with an ASTM scan rate of 1 mV/s [10].Cyclic Polarization
Cyclic
polarization measurements were extended after Tafel measurements by choosing a
vertex potential of 2 V, at which the scan was reversed, and the final
potential was−250mV versus OCP.


## 2.1. Materials Preparation

Titanium alloy grades Ti1, Ti2, Ti64, TiOs, Ti15Mo, and TMZF of
compositions Ti1 (0.1%C, 0.2%Fe, 0.015%H, 0.03%N, 0.18%O, and 99.47% Ti), Ti2
(0.1%C, 0.3%Fe, 0.015%H, 0.03%N, 0.25%O, and 99.30%Ti), Ti64 (0.1%C, 0.2%Fe,
0.015%H, 0.03%N, 0.2%O, 6%Al, 4%V, and 89.45%Ti), TiOs (0.05%C, 0.05%Fe,
0.015%H, 0.02%N, 0.15%O, 35.5% Nb, 5.7%Ta, 7.3%Zr, and 51.21%Ti), Ti15Mo
(0.05%C, 0.1%Fe, 0.015%H, 0.01%N, 0.15%O, 15%Mo, and 84.67%Ti), and TMZF
(0.02%C, 2%Fe, 0.02%H, 0.01%N, 0.18%O, 6%Zr, 12%Mo, and 79.77%Ti) were used for
the present investigation. Available cuboidal and cylindrical rods were cut to
expose cross-section areas of 0.855cm2 for Ti1, 1.0cm2 for Ti2, 1.5525cm2 for Ti64, 1.3270cm2 for TiOs, 0.4869cm2 for
Ti15Mo, and 2.7606cm2 for TMZF as working electrodes. Normal saline
solution (5.26gL−1 sodium chloride, 2.22gL−1 anhy. sodium
acetate, 5.02gL−1 sodium gluconate, 0.37gL−1 potassium chloride,
and 0.3gL−1 magnesium chloride hexahydrate) having pH 6.6 was used
to carry out the electrochemical testing of alloys.


## 2.2. Measurements

A Princeton Applied Research Potentiostat 273A was used for the
electrochemical measurements. A three-electrode cell assembly consisting of
titanium alloy as the working electrode, platinum wire as the counter electrode,
and a saturated calomel electrode as the reference electrode was used. 450 mL
of normal saline solution was used for the immersion
and electrochemical testing of the alloys.Open Circuit Potential (OCP)
Open circuit potential values of the alloys were
measured just after immersion, followed by a duration of 2, 6, and 24 hours up to
seven days.Tafel
After the end
of the seven days, Tafel curves of various alloys were obtained by polarizing
each electrode from−250mV versus OCP to 2 V with an ASTM scan rate of 1 mV/s [10].Cyclic Polarization
Cyclic
polarization measurements were extended after Tafel measurements by choosing a
vertex potential of 2 V, at which the scan was reversed, and the final
potential was−250mV versus OCP.


## 3. Results and Discussion

It can be seen from Figure1 and Table 1 that the OCP values for all
alloys have shown an overall increase for the time duration of seven days,
despite showing a small decrease in a few intervals.Table 1
Open circuit
potential values of alloys with change in time at37°C.
MaterialOpen circuit potential (mV)On immersion2 hrs.6 hrs.1 day2 days3 days4 days5 days6 days7 daysTi1−107−144−156−152−148−131−121−125−117−120Ti2−168−143−140−134−131−113−111−111−110−105Ti64−147−135−118−118−89−65−56−46−44−41TiOs−202−248−252−223−151−175−164−149−142−140Ti15Mo−148−179−181−179−142−68−82−57−47−27TMZF−184−218−227−235−208−175−160−133−120−109Figure 1
Open circuit
potential values of alloys as a function of time at37°C.The OCP shift in the noble direction for the alloys suggests the
formation of a passive film that acts as a barrier for metal dissolution and
reduces the corrosion rate. The shift can also suggest that the composition of
corrosive medium might be changing in the implant cavity but since large volume
of solution is used in testing, that can be neglected. The potential increase
shows that the alloys become thermodynamically more stable with time. According
to Blackwood et al. [11], the shift in OCP to positive values lowers
the corrosion rate by reducing the driving force of the cathodic reaction and
increasing the thickness of the passive oxide film. The initial decrease
observed in the OCP values for Ti1, TiOs, and TMZF suggests the initial
dissolution of the air-formed oxide film till the formation of a new oxide in
solution [12].The pH values of all alloys in normal saline solution of pH 6.6 were less
than 8 till seven days of immersion. Figure2 shows the pH-potential Pourbaix
diagram for titanium-water system at 37°C [13], with
potential also shown with respect to SCE on another y-axis. The corresponding
region of pH (6.6–8) and potential to which the present system is studied is similar to that of TiO2 in the Pourbaix diagram indicating the fact that under
steady-state conditions, the alloys form a stable oxide layer of TiO2.Figure 2
Pourbaix diagram
for titanium at37°C.Figure3 shows the potentiodynamic curves for the alloys at 37°C. The
corresponding corrosion parameters are given in Table 2, and passivation
parameters are given
in Table 3.Table 2
Corrosion
parameters for the forward and reverse scans for the alloys at37°C after seven days.
MaterialForward scanReverse scanEcorr(mV)Icorr(nA/cm2)Corr.rate*1013(mm/y)Ecorr(mV)Icorr(nA/cm2)Ti1−298.20.9343.43369.62.27Ti2−691.65.883381.8Ti64−2200.7082.65427.30.597TiOs−193.30.2220.64322.10.00531Ti15Mo−139.94.6815.67133.73.19TMZF−256.71.264.19225.90.658Table 3
Passivation
parameters for the alloys at37°C   after seven days.
MaterialEpass(mV)Ipass(μA/cm2)Ecorr(mV)Epass−Ecorr(mV)Ti11460.013−298.2442.2Ti23590.006−69428Ti64−1040.00173−220116TiOs−720.00143−193.3121.3Ti15Mo56910.02−139.9708.9TMZF6442.37−256.7900.7Figure 3
Potentiodynamic
curves for the alloys at37°C.The corrosion rate was calculated using the expression [14] (1)Corrosionrate(mmyear)=0.00327×E.W.×Icorrρ,   where E.W.  =  equivalent weight, Icorr  =  current density in μA/cm2, and ρ  =  density in g/cm3.The corrosion rates observed are in the order Ti15Mo> Ti2 > TMZF > Ti1 > Ti64 > TiOs.
The corrosion rates observed are very low and belong to the outstanding
corrosion resistance category of less than 0.02 mm/year as classified by Fontana [15]. Studies have further revealed that to prevent tissue damage
and irritation, the corrosion rate of a metallic implant should be less than
2.5×10−4mm/year [16]. The difference between the
passivation potential and Ecorr can provide an indication of the
ease of passivation of the alloys. The lower the difference is, the easier the passivation will be. The difference follows the
order TMZF > Ti15Mo > Ti1 > Ti2 > TiOs > Ti64.Cyclic polarization method is a highly useful method for determining the
susceptibility of a metal or alloy to pitting [17]. From Figures
4(a)–4(f), it can be
seen that at the vertex potential of 2 V when the scan reverses its direction,
the reverse scan starts left of the forward scan curve, that is, towards the
low current density region. This type of the cyclic polarization curve is known
to resist localized corrosion [18]. It is also observed that the
reverse scan curves meet the forward scan curve along the passive range. The
reverse scan curves show lower current densities as can be seen in Table 2 except for Ti1 and Ti2. The potentials for the reverse scan curves are more
positive than those for the forward scan. These results show that a stable oxide film
is formed during the forward scan.Cyclic
polarization curves for alloys (a)–(f) at37°C.
(a)
Ti1(b)
Ti2(c)
Ti64(d)
TiOs(e)
Ti15Mo(f)
TMZF


## 4. Conclusions

(1)
In normal saline solution, all alloys exhibit a high corrosion
resistance, and the corrosion rates observed fall in the acceptable range for
biocompatibility of metallic implants.(2)
All alloys show considerable ennoblement and form a stable oxide
film ofTiO2 as indicated by their corresponding pH and potential
position in the Pourbaix diagram.(3)
From the cyclic polarization curves obtained, it can be concluded
that the alloys resist localized corrosion.



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*Source: 574359-2009-03-26.xml*