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

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. Ti 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 of 37◦C.


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][4][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.Ti 3+ 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 of 37 • C.

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 −250 mV 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 −250 mV versus OCP.

Results and Discussion
It can be seen from Figure 1 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.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.Figure 2 shows the pH-potential Pourbaix diagram for titaniumwater 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 TiO 2 in the Pourbaix diagram indicating the fact that under steady-state conditions, the alloys form a stable oxide layer of TiO 2 .
Figure 3 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.
The corrosion rate was calculated using the expression [14] Corrosion rate mm year where E.W. = equivalent weight, I corr = current density in μA/cm 2 , and ρ = density in g/cm 3 .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 −4 mm/year [16].The difference between the passivation potential and E corr 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.

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 of TiO 2 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.

Figure 1 :
Figure 1: Open circuit potential values of alloys as a function of time at 37 • C.

Table 1 :
Open circuit potential values of alloys with change in time at 37 • C.

Table 2 :
Corrosion parameters for the forward and reverse scans for the alloys at 37 • C after seven days.

Table 3 :
Passivation parameters for the alloys at 37 • C after seven days.Material E pass (mV) I pass (μA/cm 2 ) E corr (mV)