# Electrochemical Performance of Ti-Based Commercial Biomaterials

Authors: E. Porcayo-Palafox; S. I. Carrera-Chavez; S. R. Casolco; J. Porcayo-Calderon; V. M. Salinas-Bravo
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

In order to determine the electrochemical behavior against the corrosion of different commercial biomaterials, in this study the results of the evaluation of different titanium implants are reported. The commercial implants evaluated were purchased randomly with different suppliers. The different biomaterials were subjected to studies of potentiodynamic polarization curves, open circuit potential measurements, linear polarization resistance measurements, and electrochemical impedance spectroscopy measurements in a 0.9% NaCl solution. The results showed that the chemical composition of the biomaterials corresponds to commercially pure Ti or to the alloy Ti6Al4V. In addition, although all the biomaterials showed a high resistance to corrosion, notable differences were observed in their performance. These differences were associated with the thermomechanical processes during the manufacture of the biomaterial, which affected its microstructure.


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## Body

## 1. Introduction

The demand for highly specialized materials for the manufacture of different components used as implants, such as simple bone screws, pins, and rods until the manufacture of total hip replacement, is increasing every day. An objective of the replacement of a bone structure damaged by a metallic implant is that the implant assumes the same function as the replaced part and that its useful life is as long as possible. However, biomaterials are susceptible to failure because both their mechanical and chemical properties are different from those of the bone structure. The most common failures of commercial biomaterials have been attributed mainly to corrosion, wear, lack of biocompatibility, low fatigue strength, low fracture toughness, and differences in the modulus of elasticity among others. Therefore, it is a common need for painful and expensive revision surgeries to prevent premature failure of the implant. This has motivated the development of biomaterials with the longest useful life and the best possible biocompatibility [1].Most commercial biomaterials are manufactured mainly from stainless steel, Co-Cr alloys, Ti, and Ti alloys [2]. Each material has its advantages and disadvantages; for example, stainless steel has moderate mechanical properties and good corrosion resistance, Co-Cr alloys have greater strength, hardness, and biocompatibility, and Ti and its alloys show excellent mechanical properties and corrosion resistance in addition to excellent biocompatibility [1, 3].However, despite the excellent properties of the biomaterials used to manufacture implants, their premature failure can occur due to factors associated with material properties, manufacturing defects, incorrect position within the human body, etc. [2, 4–8]. Regarding the properties of the materials, these properties determine the phenomena that occur in the implant-biological environment interface [9–13].Currently, the most used biomaterials for the manufacture of implants are Ti and its alloys; however, in general terms, the different suppliers simply market them as Ti biomaterials, without specifying if they are made of nonalloyed titanium or any of their alloys. This situation can be critical if one takes into account that, in addition, the chemical composition for a particular type of alloy can also vary [14, 15]. The chemical composition variation of an alloy can be a determinant in the life of the implant because the surface properties of the implant can be compromised.Therefore, in this study, the performance against corrosion of different components used as implants was determined. The different biomaterials were randomly acquired with suppliers from different countries. The biomaterials were evaluated by electrochemical techniques, and their performance was correlated in terms of their chemical composition and their microstructural characteristics.


## 2. Experimental Procedure

### 2.1. Materials

The materials to be evaluated consisted of various Ti-based commercial biomaterials, in addition to a commercially pure Ti bar (Ti CP), and according to the manufacturer (Goodfellow), its purity is 99.6% with an annealing heat treatment.The elemental chemical analysis performed by X-ray energy dispersive spectroscopy (EDS) (Bruker, model XFlash 6 ∣ 30) indicated that the composition of the different biomaterials corresponded to nonalloyed Ti and to the alloy Ti6Al4V. The chemical compositions of the different biomaterials were within the limits set by the ASTM F67 and ASTM F1472 standards. Nonalloyed Ti biomaterials were purchased from suppliers in Switzerland and Mexico and those from the Ti6Al4V alloy with suppliers from the USA, Germany, China, Brazil, and India. For simplicity, the different biomaterials will be referred in terms of their country of origin, except for the Ti CP.For the corrosion tests, samples of the different biomaterials were cut. Each sample was welded with a copper conductor wire to one of its faces using the spot-welding technique, and in this condition, they were encapsulated in an acrylic resin to be used as working electrodes in the different corrosion tests using electrochemical techniques. The encapsulation of the samples allows to assure a controlled reaction area where the corrosion phenomenon is carried out.


### 2.2. Surface Finishing

The surface condition of the materials to be evaluated is of great importance since it strongly influences the results of the electrochemical tests. Therefore, in this study, the surface finish used was that suggested by various ASTM standards (F746, F1801, and G61), namely, roughing the surface with an abrasive paper to 600 grit. Later, the samples were degreased with acetone, washed with ethyl alcohol and distilled water, and finally dried with hot air.


### 2.3. Corrosive Medium

Since chloride ions are the most critical component that determines the performance against corrosion of the biomaterials, in this study a solution of 0.9% NaCl (% weight) at 37°C was used as a corrosive medium. The pH of the solution was measured and adjusted to 7.0 ± 0.2 with dilute ammonium hydroxide. Different ASTM standards (F746 and F1801) recommend the use of this solution because it simulates more closely the real body fluids, and the use of more complex solutions, as those suggested by the ASTM standard F2129, may favor inhibitory effects that counteract the activity of the chloride ion.


### 2.4. Electrochemical Evaluation

Electrochemical evaluation was carried out in a three electrode electrochemical cell. As a working electrode, the encapsulated biomaterials were used, a saturated calomel electrode as a reference electrode and graphite bar as a counter electrode. The different electrochemical measurements used were potentiodynamic polarization curves (PPC), open circuit potential measurements (OCP), linear polarization resistance measurements (LPR), and electrochemical impedance spectroscopy measurements (EIS). Additionally, cyclic polarization curves (CPC) were made on those biomaterials that showed a pitting potential according to the PPC measurements.Potentiodynamic polarization curves were performed by sweeping the potential from −400 to 2500 mV with respect to its corrosion potential at 10 mV/min scanning rate. Before carrying out the test, the open circuit potential of the working electrode was measured for one hour. CPC were made according to the ASTM standard F2129. OCP measurements were carried out by recording the potential of the working electrode with respect to the reference electrode at intervals of one hour for 100 hours. In LPR measurements, the working electrode was polarized ±10 mV with respect to its open circuit potential at 10 mV/min scanning rate, and the polarization resistance value was determined from the slope of theE-i ratio obtained. The measurements were made at one-hour intervals for 100 hours. The electrochemical impedance spectra were obtained by applying an amplitude perturbation of 10 mV with respect to the open circuit potential of the working electrode in a frequency range from 100 kHz to 0.01 Hz. These measurements were performed for 100 hours at time intervals of 0, 3, 6, 9, 12, 18, 24, 50, 75, and 100 hours. The measurements of EIS, LPR, and OCP were made in a potentiostat-galvanostat Gamry Interface 1000, and the PPC were made in a potentiostat-galvanostat GillAC.


## 2.1. Materials

The materials to be evaluated consisted of various Ti-based commercial biomaterials, in addition to a commercially pure Ti bar (Ti CP), and according to the manufacturer (Goodfellow), its purity is 99.6% with an annealing heat treatment.The elemental chemical analysis performed by X-ray energy dispersive spectroscopy (EDS) (Bruker, model XFlash 6 ∣ 30) indicated that the composition of the different biomaterials corresponded to nonalloyed Ti and to the alloy Ti6Al4V. The chemical compositions of the different biomaterials were within the limits set by the ASTM F67 and ASTM F1472 standards. Nonalloyed Ti biomaterials were purchased from suppliers in Switzerland and Mexico and those from the Ti6Al4V alloy with suppliers from the USA, Germany, China, Brazil, and India. For simplicity, the different biomaterials will be referred in terms of their country of origin, except for the Ti CP.For the corrosion tests, samples of the different biomaterials were cut. Each sample was welded with a copper conductor wire to one of its faces using the spot-welding technique, and in this condition, they were encapsulated in an acrylic resin to be used as working electrodes in the different corrosion tests using electrochemical techniques. The encapsulation of the samples allows to assure a controlled reaction area where the corrosion phenomenon is carried out.


## 2.2. Surface Finishing

The surface condition of the materials to be evaluated is of great importance since it strongly influences the results of the electrochemical tests. Therefore, in this study, the surface finish used was that suggested by various ASTM standards (F746, F1801, and G61), namely, roughing the surface with an abrasive paper to 600 grit. Later, the samples were degreased with acetone, washed with ethyl alcohol and distilled water, and finally dried with hot air.


## 2.3. Corrosive Medium

Since chloride ions are the most critical component that determines the performance against corrosion of the biomaterials, in this study a solution of 0.9% NaCl (% weight) at 37°C was used as a corrosive medium. The pH of the solution was measured and adjusted to 7.0 ± 0.2 with dilute ammonium hydroxide. Different ASTM standards (F746 and F1801) recommend the use of this solution because it simulates more closely the real body fluids, and the use of more complex solutions, as those suggested by the ASTM standard F2129, may favor inhibitory effects that counteract the activity of the chloride ion.


## 2.4. Electrochemical Evaluation

Electrochemical evaluation was carried out in a three electrode electrochemical cell. As a working electrode, the encapsulated biomaterials were used, a saturated calomel electrode as a reference electrode and graphite bar as a counter electrode. The different electrochemical measurements used were potentiodynamic polarization curves (PPC), open circuit potential measurements (OCP), linear polarization resistance measurements (LPR), and electrochemical impedance spectroscopy measurements (EIS). Additionally, cyclic polarization curves (CPC) were made on those biomaterials that showed a pitting potential according to the PPC measurements.Potentiodynamic polarization curves were performed by sweeping the potential from −400 to 2500 mV with respect to its corrosion potential at 10 mV/min scanning rate. Before carrying out the test, the open circuit potential of the working electrode was measured for one hour. CPC were made according to the ASTM standard F2129. OCP measurements were carried out by recording the potential of the working electrode with respect to the reference electrode at intervals of one hour for 100 hours. In LPR measurements, the working electrode was polarized ±10 mV with respect to its open circuit potential at 10 mV/min scanning rate, and the polarization resistance value was determined from the slope of theE-i ratio obtained. The measurements were made at one-hour intervals for 100 hours. The electrochemical impedance spectra were obtained by applying an amplitude perturbation of 10 mV with respect to the open circuit potential of the working electrode in a frequency range from 100 kHz to 0.01 Hz. These measurements were performed for 100 hours at time intervals of 0, 3, 6, 9, 12, 18, 24, 50, 75, and 100 hours. The measurements of EIS, LPR, and OCP were made in a potentiostat-galvanostat Gamry Interface 1000, and the PPC were made in a potentiostat-galvanostat GillAC.


## 3. Results and Discussion

### 3.1. Potentiodynamic Polarization Curves

Figure1 shows the potentiodynamic polarization curves of Ti biomaterials evaluated in 0.9% NaCl solution at 37°C. From Figure 1, it is observed that the Ti CP shows an active-pseudopassive behavior in the evaluated potential range. At potentials 400 mV above its corrosion potential, it tends to form a pseudopassive zone up to about 1800 mV, and at higher potentials, it tends to form a passive zone. Ti Mexico biomaterial shows a similar behavior to that of Ti CP. It shows a more noble 80 mV corrosion potential with respect to CP Ti and instabilities in its pseudopassive zone, possibly due to the breakdown and regeneration of its protective oxide (TiO2) [11–13]. On the contrary, Ti Switzerland showed a different behavior. Its corrosion potential is slightly more active (40 mV) than the corrosion potential of Ti CP; approximately 300 mV above its corrosion potential, it shows a passive zone in a range of approximately 1000 mV, and at higher potential shows the same behavior of the other Ti biomaterials. The range of stability of the passive or pseudopassive zone can be dependent on the scanning rate and the composition of the physiological solution; for example, it has been reported that the passive zone of the Ti is around 2000 mV or greater than 3000 mV with respect to its corrosion potential [16]. The difference with the values reported in this study is due to the different scanning rate used (5 y 4 mV/min) and the presence of the other compounds in the electrolyte. In this evaluation, the electrolyte contained only chloride ions, and the scanning rate was 10 mV/min. It is also mentioned that the dispersion of data observed in different sources may also be due to the reaction area of the working electrode; for example, in very small samples, it is possible to observe the breakdown of the passive zone to lower potentials due to the possible presence of crevice corrosion in the alloy-resin interface [17]. In this study and due to the shape of the biomaterials, the working electrode area of Ti CP and Ti Suiza was similar and that of Ti México was 50% lower. Nevertheless, the Ti Switzerland showed the development of a passive zone and the Ti CP and Ti México only a pseudopassive zone.Figure 1
Potentiodynamic polarization curves for Ti biomaterials in 0.9% NaCl solution at 37°C (0.1667 mVs−1).Table1 shows the electrochemical parameters values obtained from the polarization curves of Ti biomaterials. From Table 1, it can be seen that the corrosion rates (Icorr) of the three Ti biomaterials are small and their values very similar. Ti Switzerland showed the lowest anodic slope which indicates a higher initial corrosion rate, and this could have favored a rapid formation of its protective oxide and therefore the development of a visible passive zone.Table 1
Electrochemical parameters from the polarization curves of the different biomaterials evaluated.
Biomaterials
Chemical composition
Ecorr (mV)
Ba (mv/dec)
Bc (mV/dec)
Icorr (mA/cm2)
Ti CP
Ti
−316.0
343
231
3.6∗ 10−4
Mexico
Ti
−230.5
388
286
6.6∗ 10−5
Switzerland
Ti
−354.2
234
204
1.1∗ 10−4
USA
Ti6Al4V
−447.4
261
181
9.3∗ 10−5
Germany
Ti6Al4V
−472.5
233
171
8.3∗ 10−5
China
Ti6Al4V
−409.9
222
153
9.0∗ 10−5
India
Ti6Al4V
−144.0
355
345
4.1∗ 10−4
Brazil
Ti6Al4V
−246.4
185
256
8.8∗ 10−5Figure2 shows the potentiodynamic polarization curves of the Ti6Al4V biomaterials evaluated in 0.9% NaCl solution at 37°C. From the polarization curves, the great similarity in behavior of the Ti6Al4V alloys of the USA, Germany, and China is evident. All have the same behavior with the formation of a passive zone that starts between 300 and 400 mV with respect to its corrosion potential and ends between 1400 and 1500 mV with respect to its corrosion potential and, at higher potentials, show the formation of a pseudopassive zone. However, the biomaterials of India and Brazil behaved completely different. Although both showed the nobler potential values, they were not able to develop a passive zone. In general, both alloys showed the formation of a wide pseudopassive zone from 100 mV to approximately 2000 mV with respect to their corrosion potential. In the case of Ti6Al4V Brazil, its pseudopassive zone showed much instability, and Ti6Al4V India showed a pitting potential with an increase in its current density up to 3 orders of magnitude.Figure 2
Potentiodynamic polarization curves for Ti6Al4V biomaterials in 0.9% NaCl solution at 37°C (0.1667 mVs−1).Table1 shows the values of the electrochemical parameters obtained from the polarization curves of the Ti6Al4V alloys. From Table 1, it can be seen that Ti6Al4V biomaterials from USA, Germany, and China show similar corrosion potentials and that the biomaterials from India and Brazil showed the nobler corrosion potentials. Similarly, the anodic slopes of the Ti6Al4V biomaterials from the USA, Germany, and China showed similar values (similar to the Ti Switzerland); however, the Ti6Al4V India showed the highest anodic slope (similar to that of Ti CP and Ti Mexico), and the Ti6Al4V Brazil showed the lowest anodic slope of all the biomaterials evaluated. Regarding their corrosion rates, it is observed that all Ti6Al4V biomaterials show low corrosion rates and their values are within the same order of magnitude.Previous analysis shows that the evaluated biomaterials have similar corrosion rates; this indicates that their passive layer is of the same nature. However, at potentials higher than their corrosion potential, two different behaviors were observed; namely, some biomaterials were able to show a passive zone and others only a pseudopassive zone. The results seem to show a correlation between the value of the anodic slope and the capacity of the biomaterial to develop a passive zone. The biomaterials that showed the formation of a pseudopassive zone are those that present the extreme values of the anodic slope. On the one hand, a big value of the anodic slope can be associated with a rapid growth of the protective oxide and therefore a greater probability of defects in its structure, and on the other hand, a low anodic slope can be associated with a slow growth of the protective oxide and therefore a greater probability of contact between the aggressive species and the bare metal surface. These differences may be associated with the manufacturing processes of the biomaterials and/or their shaping processes.Potentiodynamic polarization curves are a destructive test that only shows the initial behavior of the material when it is subject to large disturbances from its equilibrium condition, and therefore, it is necessary to perform evaluations at a longer immersion time in order to determine the evolution of its behavior in function of time.Figure3 shows the cyclic polarization curve of Ti6Al4V India. This additional measurement was carried out because this biomaterial was the only one that showed a pitting potential in the evaluated potential range. According to the inverse sweep, it is observed that this biomaterial does not present a protection potential, indicating its susceptibility to crevice corrosion.Figure 3
Cyclic polarization curve for Ti6Al4V India in 0.9% NaCl solution at 37°C (0.1667 mVs−1).


### 3.2. Open Circuit Potential Measurements

A simple way to study the formation of protective films on the surface of the biomaterial is to monitor the evolution of its OCP value as a function of time. In a simple way, it can be established that an increase in the OCP value indicates the formation of a passive film, and a decrease is indicative of its breakage, dissolution, or nonformation, and a value without significant variation indicates a stable surface [18]. These measurements are important because the passive layer plays an important role in the biocompatibility of the implant-tissue relationship, and the biocompatibility of the implant is determined by the stability of its oxide film and its ability to incorporate ions from the solution.Figure4 shows the variation of the open circuit potential as a function of time for the biomaterials evaluated in a 0.9% NaCl solution at 37°C for 100 hours. From Figure 4, it can be seen that regardless of the chemical composition of the biomaterials, their OCP values show an ascending behavior towards nobler potentials without reaching the steady state (except Ti6Al4V India). This indicates that the biomaterials tend to the continuous formation of their passive layer. In general, slight disturbances are observed which indicate the rupture and repassivation of the protective oxide. The observed tendency is attributed to the fact that all tend to form on their surface a passive film formed by a single oxide (TiO2), unlike most of the alloys which form passive layers composed of multiple oxides, where this condition makes them more susceptible to failure due to the differences that exist in the corrosion resistance of each oxide [19]. It has been reported that, in the range of observed OCP variations (−450 to 150 mV) and in a wide range of pH values, the thermodynamically stable species is TiO2 [20]. Additionally, it is indicated that in fact the passive layer is formed by two layers, a dense layer adhered to the alloy and on it a porous layer [13,17,19,21–24]. It is suggested that while the dense layer acts as a barrier and confers its corrosion resistance to the biomaterial, the porous layer provides its ability for osseointegration. The porous oxide layers can be hydrated, and the electrolyte ions can be incorporated into the pores and then precipitated causing either corrosion or self-sealing. This behavior of excellent stability of Ti and its alloys makes them attractive as biomaterials due to their biocompatibility and corrosion resistance. Its excellent behavior has been attributed to its thin layer (2 to 10 nm) of protective oxide strongly adhered to its surface. In addition, it has been reported that Ti can spontaneously regenerate its surface layer in milliseconds even in poorly oxygenated media [19,25–28].Figure 4
OCP measurements of the biomaterials for 100 hours in a 0.9% NaCl solution at 37°C.On the contrary, it is interesting to note that, in the evaluated time interval of the Ti biomaterials, the Ti CP was the one that showed the lowest increase in its OCP value (232 mV) with respect to that shown by the Ti Switzerland (400 mV) and Ti Mexico (510 mV). Regarding the Ti6Al4V biomaterials, those from Brazil and India showed the lowest increase (266 mV and 286 mV, respectively) with respect to the other Ti6Al4V biomaterials whose increase was from 400 to 500 mV. These differences may indicate a slow growth rate of the passive layer (lower ΔE) under conditions of free corrosion potential.


### 3.3. Linear Polarization Resistance Measurements

Figure5 shows the variation as a function of time of the linear polarization resistance of the biomaterials evaluated in a 0.9% NaCl solution at 37°C for 100 hours.Figure 5
LPR measurements of the biomaterials for 100 hours in a 0.9% NaCl solution at 37°C.From Figure5, it is observed that although all the biomaterials showed polarization resistance values greater than 10,000 ohms-cm2, their behavior in general is located in three zones: those with polarization resistance values in the order of 1 ∗ 106 ohms-cm2 (Ti Switzerland, Ti6Al4V USA, and Ti6Al4V Germany), those with polarization resistance values of the order of 1–4 ∗ 105 ohms-cm2 (Ti CP, Ti Mexico, Ti6Al4V Brazil, and Ti6Al4V China), and those with polarization resistance values less than 1 ∗ 105 ohms-cm2 (Ti6Al4V India). These values of polarization resistance indicate a high resistance to corrosion in environments rich in chlorides due to the ability of biomaterials to form a passive layer with high resistance to corrosion.Except for Ti6Al4V India, all other biomaterials showed a tendency to increase their polarization resistance values as a function of time. It is possible that the differences observed are due to the thermomechanical process to which each biomaterial has been exposed. The thermomechanical processes can affect their microstructural characteristics.Figure6 shows the microstructure of nonalloyed Ti biomaterials. In general, it is observed that all biomaterials show an equiaxial structure formed by the α-phase (hcp). In particular, the Ti Switzerland showed a microstructure with a larger grain size (10–20 microns), and the other biomaterials showed a microstructure with the smaller grain size (<10 microns). In all cases, the presence of α′ acicular martensite is also evident, and the Ti CP shows a high content of it. According to the manufacturer’s data, the Ti CP was subjected to an annealing process; their thermomechanical history of the other biomaterials (Ti and Ti6Al4V) is unknown. It is reported that the presence of α-acicular martensite is attributed mainly to high cooling rates [29, 30]. Although the main microstructure corresponds to that expected for pure Ti, namely, a compact hexagonal allotropic crystalline phase (α-phase) stable up to 882°C, it is observed that the corrosion resistance is influenced by the grain size of the α-phase and the content of secondary phases. This indicates that the corrosion resistance of the biomaterial is greater to larger grain size and lower content of secondary phases.Figure 6
Microstructural characteristics of nonalloyed Ti biomaterials: (a) Switzerland; (b) Mexico; (c) Ti CP.
(a)
(b)
(c)Figure7 shows the microstructure of Ti6Al4V biomaterials. In the case of Ti alloys, its microstructure is composed of two phases (α + β), and its quantity and distribution is a function of the thermomechanical processes to which they have been exposed. At room temperature, the two-phase microstructure improves the mechanical properties of the alloy, and the thermomechanical processes must guarantee the presence of the two phases, as well as their distribution, homogeneity, and grain size [30, 31]. Depending on the thermomechanical processes, the alloy can present three types of basic microstructures, namely, laminar, equiaxial, or bimodal (duplex) [2, 32]. From Figure 7, it is observed that, in general, all the biomaterials show a two-phase microstructure formed by the aluminum-rich α-phase (dark gray) and the vanadium-rich β-phase (light gray).Figure 7
Microstructural characteristics of the Ti6Al4V biomaterials: (a) USA; (b) Germany; (c) China; (d) Brazil; (e) India.
(a)
(b)
(c)
(d)
(e)In particular, it is observed that the Ti6Al4V Germany shows a bimodal microstructure with presence of the secondaryα, β and α-phases. The presence of the secondary α-phase (interspersed in the β-phase) is associated with slow cooling rates of the alloy [2, 30, 32]; in addition, the presence of the β-phase in the form of thin sheets is common in treated alloys thermally at temperatures around 800°C [31]. Ti6Al4V USA shows a microstructure of equiaxed grains (α-phase) with the presence of fine crystals of the β-phase distributed mainly in grain boundaries. Both materials showed the largest grain size of the Ti6Al4V biomaterials (>5 microns). On the contrary, Ti6Al4V from China, Brazil, and India show an equiaxial microstructure (α-phase) with a large amount of β-phase crystals in grain boundaries (1 micron average size) and a large amount of β-phase (nanometric size) inside the α-phase. Apparently, there was no adequate control during the thermomechanical treatment of these alloys. Again, it is observed that the best corrosion resistance was shown by those Ti6Al4V biomaterials with the larger grain size of the primary phase and an ordered arrangement of the secondary phase.


### 3.4. Electrochemical Impedance Spectroscopy Measurements

Figure8 shows the impedance spectra of nonalloyed Ti biomaterials after 100 hours of immersion in 0.9% NaCl solution at 37°C. From the Nyquist diagram, it is possible to observe that the different Ti biomaterials exhibited a similar capacitive response, that is, the formation of a capacitive semicircle with different diameters. Ti CP showed the smaller diameter (lower charge transfer resistance) and the Ti Switzerland the larger diameter (greater charge transfer resistance). It is not possible to observe more details of the corrosion process because the spectrum only presents approximately 20% of the experimental data, and these correspond to that information of the low frequency region. However, the analysis of the Bode diagram in its impedance module format allows to see that the biomaterials show similar characteristics, namely, the development of the low frequency plateau and the increase of the impedance module with the frequency, but it is not possible to observe the development of the low frequency plateau. This indicates that the corrosion resistance of biomaterials is greater than the last value of |Z| registered. However, it is possible to establish that Ti CP is the biomaterial with the lowest corrosion resistance, and Ti Switzerland has the highest corrosion resistance, as it was observed in the Nyquist diagram. The results are consistent with the polarization resistance measurements (Figure 5). On the contrary, the analysis of the Bode diagram in its phase angle format allows observing that, in the high frequency region, the phase angle of all biomaterials tends to zero at frequencies greater than 10 k Hertz. From the intermediate frequency region to the low frequency region, it is observed that the Ti Switzerland shows an apparent plateau with a phase angle greater than 80°; similarly, the Ti Mexico also shows an apparent plateau but with a higher phase angle at 75°. This may be due to the overlap of at least two time constants. Only the CP Ti showed the clear formation of two-phase angle maxima (time constants), where the first one has a maximum of about 75° and the second one of 65°. In all cases, it is observed that the phase angle tends to zero at frequencies lower than 0.01 Hertz. The magnitude of the phase angle also indicates that the Ti Switzerland showed the highest corrosion resistance (greater phase angle) and the Ti CP the lowest resistance to corrosion (lower phase angle).Figure 8
Electrochemical impedance spectra of nonalloyed Ti biomaterials after 100 hours of immersion in a 0.9% NaCl solution at 37°C.
(a)
(b)
(c)Figure9 shows the morphological aspects of Ti biomaterials after 100 hours of immersion in the 0.9% NaCl solution at 37°C. The different surface aspects of biomaterials show a surface free of corrosion products and superficial attack. This shows the high corrosion resistance of the biomaterials in their free corrosion condition.Figure 9
Morphological aspects of Ti biomaterials after the corrosion test in 0.9% NaCl solution at 37°C for 100 hours: (a) Ti Switzerland; (b) Ti Mexico; (c) Ti CP.
(a)
(b)
(c)Figure10 shows the impedance spectra of the Ti6Al4V biomaterials after 100 hours of immersion in 0.9% NaCl solution at 37°C. The great similarity of the impedance spectra with those obtained from Ti biomaterials is clear. This is because the electrochemical response of the biomaterials of both Ti and Ti6Al4V is dependent on their ability to develop the same protective oxide on their surface. Nevertheless, it is possible to observe some differences. Similarly, from the Nyquist diagram, it is observed that the different Ti6Al4V biomaterials exhibit a very similar capacitive response; that is, the formation of a capacitive semicircle with different diameters is observed. The biomaterials from India and Brazil showed the smallest diameter (lowest charge transfer resistance) and the Ti6Al4V Germany the largest diameter (highest charge transfer resistance). The biomaterials USA and China apparently show the same behavior. As indicated, it is not possible to observe more details of the corrosion process because the Nyquist diagrams only show approximately 20% of the experimental data, and these correspond to that information of the low frequency region. However, the analysis of the Bode diagram in its impedance module format allows to see that the different Ti6Al4V alloys show similar characteristics, namely, the development of the low frequency plateau and the increase of the impedance module with the frequency, but not it is possible to observe the evident development of the low frequency plateau. This indicates that the corrosion resistance of Ti6Al4V biomaterials is greater than the last |Z| value registered. However, it is possible to observe that Ti6Al4V India is the biomaterial with the lower corrosion resistance and the Ti6Al4V Germany with the highest corrosion resistance. On the contrary, the analysis of the Bode diagram in its phase angle format (°) shows that, in the high frequency region, the phase angle of all Ti6Al4V alloys tends to zero at frequencies greater than 10 k Hertz, and in the intermediate frequency region, all show their maximum phase angle (approximately 80° for Germany, USA, and China and 70° for Brazil and India). At lower frequencies (low frequency region), the phase angle tends to decrease and the presence of another time constant is evident as was observed in the case of the Ti biomaterials. As indicated, this is due to the overlap of at least 2 time constants. Only Ti6Al4V Germany showed a constant zone of the maximum phase angle. Based on the maximum values of phase angle, it is evident that both Ti6Al4V Germany and Ti6Al4V USA show the highest corrosion resistance because they develop on their surface a protective oxide with better capacitive properties than the other Ti6Al4V biomaterials. The Ti6Al4V India shows the development of a protective oxide with the lowest capacitive properties and therefore with lower corrosion resistance.Figure 10
Electrochemical impedance spectra of the Ti6Al4V biomaterials after 100 hours of immersion in a 0.9% NaCl solution at 37°C.
(a)
(b)
(c)Figure11 shows the morphological aspects of the Ti6Al4V biomaterials after 100 hours of immersion in the 0.9% NaCl solution at 37°C. Once again, all the biomaterials show a surface free of corrosion products, with the marks of the initial surface finishing. These surface aspects show that Ti6Al4V alloys have a high corrosion resistance under free corrosion conditions.Figure 11
Morphological aspects of Ti6Al4V biomaterials after the corrosion test in 0.9% NaCl solution at 37°C for 100 hours: (a) Ti6Al4V USA; (b) Ti6Al4V Germany; (c) Ti6Al4V China; (d) Ti6Al4V Brazil; (e) Ti6Al4V India.
(a)
(b)
(c)
(d)
(e)The analysis of the impedance spectra of the biomaterials of both nonalloyed Ti and Ti6Al4V alloy shows that although each group has a similar chemical composition, the evolution of its spectra is completely different. These differences may be due to the different thermomechanical history of each biomaterial, and their microstructural characteristics are an indication of this (Figures6 and 7). According to the microstructures and evolution of the impedance spectra, it is observed that the biomaterials with the best capacitive properties of their surface are those that show the largest grain size and minor presence of secondary phases in the case of Ti nonalloys and the larger grain size and better arrangement of the secondary phase in the case of Ti6Al4V alloys.


## 3.1. Potentiodynamic Polarization Curves

Figure1 shows the potentiodynamic polarization curves of Ti biomaterials evaluated in 0.9% NaCl solution at 37°C. From Figure 1, it is observed that the Ti CP shows an active-pseudopassive behavior in the evaluated potential range. At potentials 400 mV above its corrosion potential, it tends to form a pseudopassive zone up to about 1800 mV, and at higher potentials, it tends to form a passive zone. Ti Mexico biomaterial shows a similar behavior to that of Ti CP. It shows a more noble 80 mV corrosion potential with respect to CP Ti and instabilities in its pseudopassive zone, possibly due to the breakdown and regeneration of its protective oxide (TiO2) [11–13]. On the contrary, Ti Switzerland showed a different behavior. Its corrosion potential is slightly more active (40 mV) than the corrosion potential of Ti CP; approximately 300 mV above its corrosion potential, it shows a passive zone in a range of approximately 1000 mV, and at higher potential shows the same behavior of the other Ti biomaterials. The range of stability of the passive or pseudopassive zone can be dependent on the scanning rate and the composition of the physiological solution; for example, it has been reported that the passive zone of the Ti is around 2000 mV or greater than 3000 mV with respect to its corrosion potential [16]. The difference with the values reported in this study is due to the different scanning rate used (5 y 4 mV/min) and the presence of the other compounds in the electrolyte. In this evaluation, the electrolyte contained only chloride ions, and the scanning rate was 10 mV/min. It is also mentioned that the dispersion of data observed in different sources may also be due to the reaction area of the working electrode; for example, in very small samples, it is possible to observe the breakdown of the passive zone to lower potentials due to the possible presence of crevice corrosion in the alloy-resin interface [17]. In this study and due to the shape of the biomaterials, the working electrode area of Ti CP and Ti Suiza was similar and that of Ti México was 50% lower. Nevertheless, the Ti Switzerland showed the development of a passive zone and the Ti CP and Ti México only a pseudopassive zone.Figure 1
Potentiodynamic polarization curves for Ti biomaterials in 0.9% NaCl solution at 37°C (0.1667 mVs−1).Table1 shows the electrochemical parameters values obtained from the polarization curves of Ti biomaterials. From Table 1, it can be seen that the corrosion rates (Icorr) of the three Ti biomaterials are small and their values very similar. Ti Switzerland showed the lowest anodic slope which indicates a higher initial corrosion rate, and this could have favored a rapid formation of its protective oxide and therefore the development of a visible passive zone.Table 1
Electrochemical parameters from the polarization curves of the different biomaterials evaluated.
Biomaterials
Chemical composition
Ecorr (mV)
Ba (mv/dec)
Bc (mV/dec)
Icorr (mA/cm2)
Ti CP
Ti
−316.0
343
231
3.6∗ 10−4
Mexico
Ti
−230.5
388
286
6.6∗ 10−5
Switzerland
Ti
−354.2
234
204
1.1∗ 10−4
USA
Ti6Al4V
−447.4
261
181
9.3∗ 10−5
Germany
Ti6Al4V
−472.5
233
171
8.3∗ 10−5
China
Ti6Al4V
−409.9
222
153
9.0∗ 10−5
India
Ti6Al4V
−144.0
355
345
4.1∗ 10−4
Brazil
Ti6Al4V
−246.4
185
256
8.8∗ 10−5Figure2 shows the potentiodynamic polarization curves of the Ti6Al4V biomaterials evaluated in 0.9% NaCl solution at 37°C. From the polarization curves, the great similarity in behavior of the Ti6Al4V alloys of the USA, Germany, and China is evident. All have the same behavior with the formation of a passive zone that starts between 300 and 400 mV with respect to its corrosion potential and ends between 1400 and 1500 mV with respect to its corrosion potential and, at higher potentials, show the formation of a pseudopassive zone. However, the biomaterials of India and Brazil behaved completely different. Although both showed the nobler potential values, they were not able to develop a passive zone. In general, both alloys showed the formation of a wide pseudopassive zone from 100 mV to approximately 2000 mV with respect to their corrosion potential. In the case of Ti6Al4V Brazil, its pseudopassive zone showed much instability, and Ti6Al4V India showed a pitting potential with an increase in its current density up to 3 orders of magnitude.Figure 2
Potentiodynamic polarization curves for Ti6Al4V biomaterials in 0.9% NaCl solution at 37°C (0.1667 mVs−1).Table1 shows the values of the electrochemical parameters obtained from the polarization curves of the Ti6Al4V alloys. From Table 1, it can be seen that Ti6Al4V biomaterials from USA, Germany, and China show similar corrosion potentials and that the biomaterials from India and Brazil showed the nobler corrosion potentials. Similarly, the anodic slopes of the Ti6Al4V biomaterials from the USA, Germany, and China showed similar values (similar to the Ti Switzerland); however, the Ti6Al4V India showed the highest anodic slope (similar to that of Ti CP and Ti Mexico), and the Ti6Al4V Brazil showed the lowest anodic slope of all the biomaterials evaluated. Regarding their corrosion rates, it is observed that all Ti6Al4V biomaterials show low corrosion rates and their values are within the same order of magnitude.Previous analysis shows that the evaluated biomaterials have similar corrosion rates; this indicates that their passive layer is of the same nature. However, at potentials higher than their corrosion potential, two different behaviors were observed; namely, some biomaterials were able to show a passive zone and others only a pseudopassive zone. The results seem to show a correlation between the value of the anodic slope and the capacity of the biomaterial to develop a passive zone. The biomaterials that showed the formation of a pseudopassive zone are those that present the extreme values of the anodic slope. On the one hand, a big value of the anodic slope can be associated with a rapid growth of the protective oxide and therefore a greater probability of defects in its structure, and on the other hand, a low anodic slope can be associated with a slow growth of the protective oxide and therefore a greater probability of contact between the aggressive species and the bare metal surface. These differences may be associated with the manufacturing processes of the biomaterials and/or their shaping processes.Potentiodynamic polarization curves are a destructive test that only shows the initial behavior of the material when it is subject to large disturbances from its equilibrium condition, and therefore, it is necessary to perform evaluations at a longer immersion time in order to determine the evolution of its behavior in function of time.Figure3 shows the cyclic polarization curve of Ti6Al4V India. This additional measurement was carried out because this biomaterial was the only one that showed a pitting potential in the evaluated potential range. According to the inverse sweep, it is observed that this biomaterial does not present a protection potential, indicating its susceptibility to crevice corrosion.Figure 3
Cyclic polarization curve for Ti6Al4V India in 0.9% NaCl solution at 37°C (0.1667 mVs−1).


## 3.2. Open Circuit Potential Measurements

A simple way to study the formation of protective films on the surface of the biomaterial is to monitor the evolution of its OCP value as a function of time. In a simple way, it can be established that an increase in the OCP value indicates the formation of a passive film, and a decrease is indicative of its breakage, dissolution, or nonformation, and a value without significant variation indicates a stable surface [18]. These measurements are important because the passive layer plays an important role in the biocompatibility of the implant-tissue relationship, and the biocompatibility of the implant is determined by the stability of its oxide film and its ability to incorporate ions from the solution.Figure4 shows the variation of the open circuit potential as a function of time for the biomaterials evaluated in a 0.9% NaCl solution at 37°C for 100 hours. From Figure 4, it can be seen that regardless of the chemical composition of the biomaterials, their OCP values show an ascending behavior towards nobler potentials without reaching the steady state (except Ti6Al4V India). This indicates that the biomaterials tend to the continuous formation of their passive layer. In general, slight disturbances are observed which indicate the rupture and repassivation of the protective oxide. The observed tendency is attributed to the fact that all tend to form on their surface a passive film formed by a single oxide (TiO2), unlike most of the alloys which form passive layers composed of multiple oxides, where this condition makes them more susceptible to failure due to the differences that exist in the corrosion resistance of each oxide [19]. It has been reported that, in the range of observed OCP variations (−450 to 150 mV) and in a wide range of pH values, the thermodynamically stable species is TiO2 [20]. Additionally, it is indicated that in fact the passive layer is formed by two layers, a dense layer adhered to the alloy and on it a porous layer [13,17,19,21–24]. It is suggested that while the dense layer acts as a barrier and confers its corrosion resistance to the biomaterial, the porous layer provides its ability for osseointegration. The porous oxide layers can be hydrated, and the electrolyte ions can be incorporated into the pores and then precipitated causing either corrosion or self-sealing. This behavior of excellent stability of Ti and its alloys makes them attractive as biomaterials due to their biocompatibility and corrosion resistance. Its excellent behavior has been attributed to its thin layer (2 to 10 nm) of protective oxide strongly adhered to its surface. In addition, it has been reported that Ti can spontaneously regenerate its surface layer in milliseconds even in poorly oxygenated media [19,25–28].Figure 4
OCP measurements of the biomaterials for 100 hours in a 0.9% NaCl solution at 37°C.On the contrary, it is interesting to note that, in the evaluated time interval of the Ti biomaterials, the Ti CP was the one that showed the lowest increase in its OCP value (232 mV) with respect to that shown by the Ti Switzerland (400 mV) and Ti Mexico (510 mV). Regarding the Ti6Al4V biomaterials, those from Brazil and India showed the lowest increase (266 mV and 286 mV, respectively) with respect to the other Ti6Al4V biomaterials whose increase was from 400 to 500 mV. These differences may indicate a slow growth rate of the passive layer (lower ΔE) under conditions of free corrosion potential.


## 3.3. Linear Polarization Resistance Measurements

Figure5 shows the variation as a function of time of the linear polarization resistance of the biomaterials evaluated in a 0.9% NaCl solution at 37°C for 100 hours.Figure 5
LPR measurements of the biomaterials for 100 hours in a 0.9% NaCl solution at 37°C.From Figure5, it is observed that although all the biomaterials showed polarization resistance values greater than 10,000 ohms-cm2, their behavior in general is located in three zones: those with polarization resistance values in the order of 1 ∗ 106 ohms-cm2 (Ti Switzerland, Ti6Al4V USA, and Ti6Al4V Germany), those with polarization resistance values of the order of 1–4 ∗ 105 ohms-cm2 (Ti CP, Ti Mexico, Ti6Al4V Brazil, and Ti6Al4V China), and those with polarization resistance values less than 1 ∗ 105 ohms-cm2 (Ti6Al4V India). These values of polarization resistance indicate a high resistance to corrosion in environments rich in chlorides due to the ability of biomaterials to form a passive layer with high resistance to corrosion.Except for Ti6Al4V India, all other biomaterials showed a tendency to increase their polarization resistance values as a function of time. It is possible that the differences observed are due to the thermomechanical process to which each biomaterial has been exposed. The thermomechanical processes can affect their microstructural characteristics.Figure6 shows the microstructure of nonalloyed Ti biomaterials. In general, it is observed that all biomaterials show an equiaxial structure formed by the α-phase (hcp). In particular, the Ti Switzerland showed a microstructure with a larger grain size (10–20 microns), and the other biomaterials showed a microstructure with the smaller grain size (<10 microns). In all cases, the presence of α′ acicular martensite is also evident, and the Ti CP shows a high content of it. According to the manufacturer’s data, the Ti CP was subjected to an annealing process; their thermomechanical history of the other biomaterials (Ti and Ti6Al4V) is unknown. It is reported that the presence of α-acicular martensite is attributed mainly to high cooling rates [29, 30]. Although the main microstructure corresponds to that expected for pure Ti, namely, a compact hexagonal allotropic crystalline phase (α-phase) stable up to 882°C, it is observed that the corrosion resistance is influenced by the grain size of the α-phase and the content of secondary phases. This indicates that the corrosion resistance of the biomaterial is greater to larger grain size and lower content of secondary phases.Figure 6
Microstructural characteristics of nonalloyed Ti biomaterials: (a) Switzerland; (b) Mexico; (c) Ti CP.
(a)
(b)
(c)Figure7 shows the microstructure of Ti6Al4V biomaterials. In the case of Ti alloys, its microstructure is composed of two phases (α + β), and its quantity and distribution is a function of the thermomechanical processes to which they have been exposed. At room temperature, the two-phase microstructure improves the mechanical properties of the alloy, and the thermomechanical processes must guarantee the presence of the two phases, as well as their distribution, homogeneity, and grain size [30, 31]. Depending on the thermomechanical processes, the alloy can present three types of basic microstructures, namely, laminar, equiaxial, or bimodal (duplex) [2, 32]. From Figure 7, it is observed that, in general, all the biomaterials show a two-phase microstructure formed by the aluminum-rich α-phase (dark gray) and the vanadium-rich β-phase (light gray).Figure 7
Microstructural characteristics of the Ti6Al4V biomaterials: (a) USA; (b) Germany; (c) China; (d) Brazil; (e) India.
(a)
(b)
(c)
(d)
(e)In particular, it is observed that the Ti6Al4V Germany shows a bimodal microstructure with presence of the secondaryα, β and α-phases. The presence of the secondary α-phase (interspersed in the β-phase) is associated with slow cooling rates of the alloy [2, 30, 32]; in addition, the presence of the β-phase in the form of thin sheets is common in treated alloys thermally at temperatures around 800°C [31]. Ti6Al4V USA shows a microstructure of equiaxed grains (α-phase) with the presence of fine crystals of the β-phase distributed mainly in grain boundaries. Both materials showed the largest grain size of the Ti6Al4V biomaterials (>5 microns). On the contrary, Ti6Al4V from China, Brazil, and India show an equiaxial microstructure (α-phase) with a large amount of β-phase crystals in grain boundaries (1 micron average size) and a large amount of β-phase (nanometric size) inside the α-phase. Apparently, there was no adequate control during the thermomechanical treatment of these alloys. Again, it is observed that the best corrosion resistance was shown by those Ti6Al4V biomaterials with the larger grain size of the primary phase and an ordered arrangement of the secondary phase.


## 3.4. Electrochemical Impedance Spectroscopy Measurements

Figure8 shows the impedance spectra of nonalloyed Ti biomaterials after 100 hours of immersion in 0.9% NaCl solution at 37°C. From the Nyquist diagram, it is possible to observe that the different Ti biomaterials exhibited a similar capacitive response, that is, the formation of a capacitive semicircle with different diameters. Ti CP showed the smaller diameter (lower charge transfer resistance) and the Ti Switzerland the larger diameter (greater charge transfer resistance). It is not possible to observe more details of the corrosion process because the spectrum only presents approximately 20% of the experimental data, and these correspond to that information of the low frequency region. However, the analysis of the Bode diagram in its impedance module format allows to see that the biomaterials show similar characteristics, namely, the development of the low frequency plateau and the increase of the impedance module with the frequency, but it is not possible to observe the development of the low frequency plateau. This indicates that the corrosion resistance of biomaterials is greater than the last value of |Z| registered. However, it is possible to establish that Ti CP is the biomaterial with the lowest corrosion resistance, and Ti Switzerland has the highest corrosion resistance, as it was observed in the Nyquist diagram. The results are consistent with the polarization resistance measurements (Figure 5). On the contrary, the analysis of the Bode diagram in its phase angle format allows observing that, in the high frequency region, the phase angle of all biomaterials tends to zero at frequencies greater than 10 k Hertz. From the intermediate frequency region to the low frequency region, it is observed that the Ti Switzerland shows an apparent plateau with a phase angle greater than 80°; similarly, the Ti Mexico also shows an apparent plateau but with a higher phase angle at 75°. This may be due to the overlap of at least two time constants. Only the CP Ti showed the clear formation of two-phase angle maxima (time constants), where the first one has a maximum of about 75° and the second one of 65°. In all cases, it is observed that the phase angle tends to zero at frequencies lower than 0.01 Hertz. The magnitude of the phase angle also indicates that the Ti Switzerland showed the highest corrosion resistance (greater phase angle) and the Ti CP the lowest resistance to corrosion (lower phase angle).Figure 8
Electrochemical impedance spectra of nonalloyed Ti biomaterials after 100 hours of immersion in a 0.9% NaCl solution at 37°C.
(a)
(b)
(c)Figure9 shows the morphological aspects of Ti biomaterials after 100 hours of immersion in the 0.9% NaCl solution at 37°C. The different surface aspects of biomaterials show a surface free of corrosion products and superficial attack. This shows the high corrosion resistance of the biomaterials in their free corrosion condition.Figure 9
Morphological aspects of Ti biomaterials after the corrosion test in 0.9% NaCl solution at 37°C for 100 hours: (a) Ti Switzerland; (b) Ti Mexico; (c) Ti CP.
(a)
(b)
(c)Figure10 shows the impedance spectra of the Ti6Al4V biomaterials after 100 hours of immersion in 0.9% NaCl solution at 37°C. The great similarity of the impedance spectra with those obtained from Ti biomaterials is clear. This is because the electrochemical response of the biomaterials of both Ti and Ti6Al4V is dependent on their ability to develop the same protective oxide on their surface. Nevertheless, it is possible to observe some differences. Similarly, from the Nyquist diagram, it is observed that the different Ti6Al4V biomaterials exhibit a very similar capacitive response; that is, the formation of a capacitive semicircle with different diameters is observed. The biomaterials from India and Brazil showed the smallest diameter (lowest charge transfer resistance) and the Ti6Al4V Germany the largest diameter (highest charge transfer resistance). The biomaterials USA and China apparently show the same behavior. As indicated, it is not possible to observe more details of the corrosion process because the Nyquist diagrams only show approximately 20% of the experimental data, and these correspond to that information of the low frequency region. However, the analysis of the Bode diagram in its impedance module format allows to see that the different Ti6Al4V alloys show similar characteristics, namely, the development of the low frequency plateau and the increase of the impedance module with the frequency, but not it is possible to observe the evident development of the low frequency plateau. This indicates that the corrosion resistance of Ti6Al4V biomaterials is greater than the last |Z| value registered. However, it is possible to observe that Ti6Al4V India is the biomaterial with the lower corrosion resistance and the Ti6Al4V Germany with the highest corrosion resistance. On the contrary, the analysis of the Bode diagram in its phase angle format (°) shows that, in the high frequency region, the phase angle of all Ti6Al4V alloys tends to zero at frequencies greater than 10 k Hertz, and in the intermediate frequency region, all show their maximum phase angle (approximately 80° for Germany, USA, and China and 70° for Brazil and India). At lower frequencies (low frequency region), the phase angle tends to decrease and the presence of another time constant is evident as was observed in the case of the Ti biomaterials. As indicated, this is due to the overlap of at least 2 time constants. Only Ti6Al4V Germany showed a constant zone of the maximum phase angle. Based on the maximum values of phase angle, it is evident that both Ti6Al4V Germany and Ti6Al4V USA show the highest corrosion resistance because they develop on their surface a protective oxide with better capacitive properties than the other Ti6Al4V biomaterials. The Ti6Al4V India shows the development of a protective oxide with the lowest capacitive properties and therefore with lower corrosion resistance.Figure 10
Electrochemical impedance spectra of the Ti6Al4V biomaterials after 100 hours of immersion in a 0.9% NaCl solution at 37°C.
(a)
(b)
(c)Figure11 shows the morphological aspects of the Ti6Al4V biomaterials after 100 hours of immersion in the 0.9% NaCl solution at 37°C. Once again, all the biomaterials show a surface free of corrosion products, with the marks of the initial surface finishing. These surface aspects show that Ti6Al4V alloys have a high corrosion resistance under free corrosion conditions.Figure 11
Morphological aspects of Ti6Al4V biomaterials after the corrosion test in 0.9% NaCl solution at 37°C for 100 hours: (a) Ti6Al4V USA; (b) Ti6Al4V Germany; (c) Ti6Al4V China; (d) Ti6Al4V Brazil; (e) Ti6Al4V India.
(a)
(b)
(c)
(d)
(e)The analysis of the impedance spectra of the biomaterials of both nonalloyed Ti and Ti6Al4V alloy shows that although each group has a similar chemical composition, the evolution of its spectra is completely different. These differences may be due to the different thermomechanical history of each biomaterial, and their microstructural characteristics are an indication of this (Figures6 and 7). According to the microstructures and evolution of the impedance spectra, it is observed that the biomaterials with the best capacitive properties of their surface are those that show the largest grain size and minor presence of secondary phases in the case of Ti nonalloys and the larger grain size and better arrangement of the secondary phase in the case of Ti6Al4V alloys.


## 4. Conclusions

The evaluated biomaterials purchased as Ti implants, and their chemical composition indicated that they correspond to nonalloyed Ti and Ti6Al4V alloy. However, the supplier does not specify this difference or the thermomechanical history of the biomaterial. Its electrochemical evaluation showed that all have a high corrosion resistance in a 0.9% NaCl solution. However, there are notable differences in their performance regardless of their chemical composition. The potentiodynamic polarization curves showed that the different biomaterials have the capacity to form a passive or pseudopassive zone if they are disturbed from their equilibrium state. Open circuit potential measurements showed that all biomaterials tend to develop a passive layer on their surface upon contact with the corrosive medium. Apparently, its growth rate determines its protective capacity. In general, linear polarization resistance measurements showed a constant increase in the corrosion resistance of the biomaterials due to the development of its protective oxide. Electrochemical impedance measurements showed large values of charge transfer resistance for all biomaterials, and the evolution of their spectra indicates that their corrosion resistance is due to the development of a bilayer protective oxide. The differences in the observed behaviors seem to be associated with the different microstructures of the biomaterials. In general, biomaterials with a high corrosion resistance are those with a large grain size and either with the minor presence of secondary phases or with an ordered arrangement of the secondary phase.



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*Source: 4352360-2019-01-17.xml*