Electrochemical Behavior of Three CP Titanium Dental Implants in Artificial Saliva

The electrochemical behavior of three titanium dental implants purchased on the Brazilian market was evaluated in artificial saliva using open-circuit potential measurements, electrochemical impedance spectroscopy and potentiodynamic polarization. The three implants are passive in this medium but their corrosion resistance and the stability of their passive oxide films are quite different. Although the three implants were made of commercially pure grade titanium, their corrosion current densities and passive current densities were shown to differ from one to two orders of magnitude.


Introduction
Titanium has been widely used for dental implants and restorations due to its good osseointegration, excellent corrosion resistance and biocompatibility in biological fluids and high resistance/weight ratio 1 .The influence of contaminants and surface treatments of titanium implants (alumina-blasting, acid etching, anodization, hydroxyapatite coating, for example) on osseous integration has been extensively studied.Many works have been also published on the corrosion behavior of commercially pure titanium in artificial saliva, Ringer, Hanks solutions and others [2][3][4][5][6] .All the authors agree that the titanium corrosion resistance in these media is due to the formation of an adherent and highly protective oxide film on its surface which is mainly formed of TiO 2 [7] .Nevertheless, this oxide can lose its chemical stability under certain conditions.In fluoride medium for example, titanium can be severely attacked depending on the pH due to the formation of stable titanium fluoride and oxy-fluoride compounds 8 .In spite of the abundant bibliography on titanium corrosion properties, no study was found about the corrosion resistance of commercial titanium implants.Although titanium implants are considered as highly corrosion resistant materials for applications in body fluids, the determination of their corrosion rate is of great importance.Indeed, some studies indicated that titanium release in the organism, even at low levels, can lead from local discoloration of the tissues to serious inflammatory reactions [9][10] .
The aim of the present work was to study the corrosion behavior of three titanium dental implants purchased on the Brazilian market in artificial saliva, using open-circuit potential measurements, electrochemical impedance spectroscopy and potentiodynamic polarization.

Experimental
The dental implants (namely A, B and C) investigated in the present work were obtained on the Brazilian market from three different suppliers and are made of grade 2, 3 and 4 CP-titanium, respectively.Their chemical composition according to ASTM F67-95 standard 11 is reported in Table 1.
Samples were cut from the implants for microstructure analysis, mounted in resin, grounded until 2400 grit, polished using an aqueous The open-circuit potentials tend to stabilize with time.Their values after 3h exposure, -0.235, -0.046 and +0.075 V/SCE for A, B and C implants, respectively (or 0.007, 0.196 and 0.317 V/SHE), show that C implant presents the noblest character, following by B implant and finally A implant.Nevertheless, all these potentials are found in the stability region of TiO 2 oxide in the Ti-H 2 O Pourbaix diagram for solutions of pH 5.5 [13] .A comparison of the implants corrosion potentials in artificial saliva with published data is a difficult task as the OCP of CP-Ti is highly dependent on heat-and mechanical treatments, surface preparation of the samples and exposure time.For example Schiff 14 and Al-Mayouf 15 reported OCPs of +0.025 V/SCE and -0.331 V/SCE for CP-Ti in Fusayama artificial saliva, respectively.

Results and Discussion
Figure 3 shows the polarization curves of the three Ti implants in artificial saliva.The anodic polarization curves have the same shape and are typical of passive behavior.Nevertheless, great differences in the values of anodic current density are evident and this suggests that the passive films formed on the three implants do not have the same stability and that these implants present difference in corrosion resistance.presents an excellent corrosion resistance when its corrosion rate is in the 25-100 µm/year range 16 .Nevertheless, metallic implants generally do not fail due to corrosion damage but due to tissue irritation or infection caused by the corrosion products.Qualitative estimates suggest that implants must corrode at rates of 0.25 µm/year or less to avoid the possibility of tissue reaction 16 .Only C implant corrosion rate is close to this limiting value.EIS experiments were carried out at OCP in artificial saliva.The Bode diagrams presented in Figures 4 a and b show that the metal/ solution interface for the three Ti implants has a predominantly capacitive behavior, which is characteristic of passive materials.Nevertheless the capacitive behavior is limited to a narrower frequency range for A and B implants, 10 1 -10 2 and 10 -1 -10 2 Hz, respectively.The C Ti implant presents a capacitive behavior in a larger frequency range,10 -1 -10 3 Hz.This is related to the higher stability of the passive film formed on C implant, which has already been observed from its polarization curve (Figure 3) that shows the lowest values of the anodic current density.The impedance values measured at low frequencies (Figure 4a) indicate that the increasing order of corrosion resistance of the Ti implants in artificial saliva is: A < B < C.
The difference in chemical composition of the three Ti implants (grade 2, 3 and 4 for A, B and C, respectively) cannot explain the superior resistance of C implant and the lowest resistance of A im- The corrosion current density obtained by Tafel extrapolation method and the passive current density measured at 1 V/SCE are reported in Table 2.The results show that the increasing order of corrosion resistance of the Ti implants is: A < B < C.
There is difference of two orders of magnitude between corrosion current densities of A and C implants.Schiff 14 and Al-Mayouf 15 also reported very different corrosion current density for CP-Ti in Fusayama artificial saliva, 10 -6 and 3.10 -8 A cm -2 , respectively.
Considering the corrosion current densities reported in Table 2 and using the Faraday's law for a four-electron transfer, the calculated corrosion rates of A, B and C implants are 43.0,5.0 and 0.5 µm/year, respectively.These values depict the excellent corrosion resistance of the three implants, since it is usually admitted that a metal or alloy    plant.Indeed, the general corrosion resistance of commercially pure Ti grades is quite similar and our results showed differences of one to two orders of magnitude for corrosion current densities of the implants.ASTM grade 2 is only more resistant than ASTM grades 3 and 4 to stress corrosion cracking in chloride media.The difference in Ti grain size (Figure 1) cannot also explain the difference in corrosion resistance of the three Ti implants.Indeed, for most metals and alloys a decrease in corrosion resistance is usually observed when the grain size diminishes [17][18] .From the grain size standpoint, the increasing order of corrosion resistance should be C < B < A. An improvement in corrosion resistance of CP-Ti grade 2 in simulated body fluid was already observed but for ultra fine grained microstructure (sub-micron grain size) 19 .Then, other reasons such as heat-and mechanical treatments during manufacture of the implants may be raised to explain the significant difference in the corrosion resistance of the three implants.

Conclusions
The corrosion behavior of three commercially pure Ti dental implants purchased on the Brazilian market from three different suppliers was evaluated in artificial saliva using electrochemical techniques.
The three implants present a passive behavior in this medium and high corrosion resistance.Nevertheless their corrosion resistance and the stability of their passive oxide films are quite different.
The corrosion current densities of the three implants were shown to differ from one to two orders of magnitude.
Nor the chemical composition neither the Ti grain size of the implants could explain this difference in corrosion resistance.

Figure 2
Figure 2 presents the variation of the open-circuit potential (OCP) of the three Ti implants as a function of exposure time in artificial saliva.All OCPs shift in the noble direction with time which depicts the growth of a passive oxide film on the three implants in this medium.The potential of both A and B implants presents some oscillations, which can be related to some instability of their oxide films.These oscillations are not observed for C implant.The open-circuit potentials tend to stabilize with time.Their values after 3h exposure, -0.235, -0.046 and +0.075 V/SCE for A, B and C implants, respectively (or 0.007, 0.196 and 0.317 V/SHE), show that C implant presents the noblest character, following by B implant and finally A implant.Nevertheless, all these potentials are found in the stability region of TiO 2 oxide in the Ti-H 2 O Pourbaix

Figure 2 .
Figure 2. Variation of open-circuit potential for the three Ti implants in artificial saliva.

Figure 3 .Table 2 .
Figure 3. Polarization curves of the three Ti implants in artificial saliva.

Figure 4 .
Figure 4. Bode diagrams of the three Ti implants obtained at OCP in artificial saliva: (a) impedance modulus vs frequency and (b) phase angle vs frequency.

Table 1 .
Chemical composition of the A, B and C CP-Ti dental implants.