3.3 Electrochemical Impedance Spectroscopy (EIS)
Figure 3 displays the Nyquist plots of Ti, Ti6Al4V, and Ti70Zr20Nb7.5Ta2.5 electrodes after 30 min of immersion in an ASS at 37°C. From Fig. 3a, all curves are found to be depressed semicircles, which is due to the frequency dispersion [41]. This result indicates that the charge transfer controls the corrosion of the three samples. In Bode plots (Fig. 3b) the value of modulus of impedance (Zmod) in the case of Ti70Zr20Nb7.5Ta2.5 (41.64 kΩ cm2) is larger than the values of Zmod in the cases of Ti6Al4V (30.28 kΩ cm2), and Ti (11.35 kΩ cm2), pointing out the improvement in corrosion resistance of the protective oxide layer of Ti70Zr20Nb7.5Ta2.5 alloy in saliva solution. Moreover, the largest value of phase angle in the case of Ti70Zr20Nb7.5Ta2.5 alloy (-75.87◦) compared to Ti6Al4V alloy (-74.8◦) and Ti (-65.45◦) suggests the existence of a more protective passive film of Ti70Zr20Nb7.5Ta2.5 alloy in saliva solution. In the literature, various equivalent circuit (EC) models have been utilized to describe the titanium alloy/aqueous solution interface. An EC with a one-time constant was utilized to depict the existence of an inner oxide layer [42–44], whilst a circuit with two-time constants was utilized to represent the existence of a duplex outer and inner oxide layer [45–48].
Pan [47] utilized modified models to account for the plugging of pores by products of corrosion, while Ibris [49] utilized modified models to account for the space charge layer contribution. After analyzing the impedance spectra taken at the OCP with several electrical circuit models, a successful fit of the entire data set for all samples (Ti, Ti6Al4V, and Ti70Zr20Nb7.5Ta2.5) was achieved utilizing the EC presented in Fig. 4. The components of this EC are: Rs is the resistance of the electrolyte, R1 and R2 are the resistances of the outer and inner oxide film respectively. CPE1 and CPE2 are the capacitances of the outer and inner oxide films, respectively. The CPE (constant phase element) was used in place of pure capacitors in the fitting technique to achieve good compatibility between the experimental and simulated results.
The CPE impedance was calculated by [50]:
where ω is the angular frequency, n is correlated to irregular current distribution due to surface inhomogeneity or roughness, and C is the capacitance. The basic parameters (Rs, R1, CPE1, R2 and CPE2) of the suggested EC are listed in Table 2 for all analyzed samples. The summation of the outer oxide film resistances and the inner oxide film (R1 + R2) is the polarization resistance (Rp). The passive layer, according to the suggested model, is made up of two films: The resistance R2 values of the compact inner film are much higher than those of the porous outer film, R1 (Table 2), pointing out that the inner compact oxide film is more resistant to species transport and charge transfer. The CPE1 of the porous outer film capacitances is greater than the CPE2 of the compact inner layer capacitances, indicating that the compact inner film is accountable for the corrosion resistance. These data show that the compact inner film is mainly responsible for the protection proceed via the passive film [19]. The exponent values n1 and n2 are lower than 1, which is correlated to defects due to roughness, heterogeneity of the surface, porous film creation, adsorption film on the surface, etc. [51].
Table 2
Fitting parameters of EIS obtained for a Ti, Ti6Al4V and Ti70Zr20Nb7.5Ta2.5 alloy in artificial saliva solution at 37 ᵒC.
Material
|
Rs
(Ω cm2)
|
R1
(kΩ cm2)
|
CPE1
(F cm2 Hz1 − n1)
|
n1
|
R2
(kΩ cm2)
|
CPE2
(F cm2 Hz1 − n2)
|
n2
|
Ti
|
83.90
|
0.066
|
381.9x10− 6
|
0.824
|
11.54
|
129.0x10− 6
|
0.966
|
Ti6Al4V
|
80.94
|
0.082
|
239.0x10− 6
|
0.898
|
33.67
|
88.10x10− 6
|
0.892
|
Ti70Zr20Nb7.5Ta2.5
|
91.09
|
0.095
|
145.6x10− 6
|
0.895
|
86.16
|
72.30x10− 6
|
0.890
|
3.4 The impact of different F− ion concentrations and pH
Due to its high electronegativity and low polarizability, the fluoride ion has a strong affinity. Fluoride ions in the solution greatly attracted other ions, and molecules in comparison to other halide ions which simultaneously accelerated the dissolution of other matters [52]. This is thought to be due to the F− ions ability to aggressively dissolve matter. The sole aggressive ions for the compact inner oxide film of Ti and Ti alloys are fluoride ions. As a result, their presence may potentially cause pitting and crevice corrosion processes to cause localized corrosive degradation. In reality, the corrosion in F solutions is dependent on the pH and the generation of hydrogen fluoride from the dissociation of sodium fluoride in high concentrations or from the bonding association of hydrogen and fluoride ions in low pH solutions.
Figure 5 (a,b) depicts the PPCs of Ti70Zr20Nb7.5Ta2.5 HEA in an ASS at various fluoride ion concentrations and pH with a 5 mVs− 1 sweep rate at 37°C. The results support the hypothesis that the passive or active state is determined by fluoride ion concentrations (expressed as parts per million) and pH. Indeed, the anodic polarization curves form an exemplary passive layer for pH 5/0 fluoride ions, pH 5/500 fluoride ions, pH 5/1000 fluoride ions, and pH 7/1000 fluoride ions circumstances, whereas the existence of an active/passive transition peak is an indication of spontaneous active behavior for pH 5/2000 fluoride ions and pH 2/1000 fluoride ions circumstances.
The following electrochemical parameters were calculated and are listed in Table 3: ipass, icorr, Ecorr, Ep, |Ecorr - Ep| shows inclination to form passive film (minimum values indicate easy and good passivation). It was observed that the sample of Ti70Zr20Nb7.5Ta2.5 alloy immersed in saliva at a low pH (pH 2.0) and high fluoride ion concentration (2000 ppm) and low pH (pH 2.0) exhibits the lowest corrosion resistance and the highest icorr and ipass values compared to the sample immersed in the ASS without and with low concentrations of fluoride ions (0-1000 ppm) and at high pH values (5.0 and 7.0). This means that the creation of a protective (passive) film is unaffected by the presence of low F− ion concentration in a neutral solution (1000 fluoride ions and pH 7.0), but the protective oxide coating has deteriorated in acidic and high fluoride-ion concentration solutions [53, 54].
Table 3
Electrochemical parameters obtained for a Ti70Zr20Nb7.5Ta2.5 alloy in artificial saliva with different concentrations of F− ions and different pH at 37 ᵒC.
|
pH 5.0
|
1000 F−
|
|
0 F−
|
500 F−
|
1000 F−
|
2000 F−
|
pH 7.0
|
pH 5.0
|
pH 2.0
|
icorr /A cm− 2
|
6.50x10− 6
|
1.52x10− 5
|
1.12x10− 4
|
2.60x10− 4
|
2.47x10− 5
|
1.12x10− 4
|
1.17x10− 3
|
- Ecorr /V
|
0.429
|
0.595
|
0.693
|
0.926
|
0.441
|
0.693
|
0.829
|
βa (V dec− 1)
|
0.195
|
0.119
|
0.333
|
0.464
|
0.258
|
0.333
|
0.698
|
βc (V dec− 1)
|
0.117
|
0.175
|
0.750
|
0.951
|
0.321
|
0.750
|
0.344
|
-Epass / V
|
0.289
|
0.364
|
0.439
|
0.600
|
0.229
|
0.439
|
0.369
|
ipass mA cm− 2
|
0.068
|
0.143
|
0.239
|
0.767
|
0.118
|
0.239
|
0.393
|
CR / mpy
|
3.371
|
7.905
|
57.99
|
134.8
|
12.80
|
57.99
|
606.9
|
EIS studies were performed at open circuit potentials with 3 hours of immersion in an ASS with fluoride ions. Nyquist plots (Fig. 6a, b) show capacitive semicircles that reduce in size as the fluoride ion concentration rises and the pH falls. pH 5.0/0 fluoride ions and pH 7.0/1000 fluoride ions have much greater impedances than those of pH 5.0/500 fluoride ions, pH 5.0/1000 fluoride ions, pH 5.0/2000 fluoride ions and pH 2.0/1000 fluoride ions. These data reveal that if the fluoride ion concentrations enhance or the pH falls, corrosion resistance of the alloy reduces. Under circumstances equivalent to fluoride ion concentrations and pH, Nyquist plots were obtained of the identical form and order for the impedance values for Ti-23Ta alloy [50].
In Bode plots (Fig. 7a), as the fluoride ion concentration promotes from 0 to 2000 ppm, the Zmod sharply diminishes from 41.64 to 0.38 kΩ cm2 and the maximum phase angles reduce from around − 75.87o to -35.80o when they go slightly to a higher frequency region. In Bode plots (Fig. 7b), when the pH promotes from 2.0 to 7.0, the (Zmod) increases greatly from 0.11 to 21.88 kΩ cm2 and the maximum phase angles increase sharply from about − 9.55o to -74.31o. These results indicate that a reduction in pH and an excess in the concentration of fluoride ions facilitate the dissolution process of the oxide film. This model successfully matched the experimental results in both the active and passive states (Fig. 7a,b), confirming the dual nature of the passive oxide film on Ti70Zr20Nb7.5Ta2.5 alloy in various ASS. The values of Rs, R1, CPE1, R2 and CPE2 were obtained by fitting the Ti70Zr20Nb7.5Ta2.5 alloy experimental impedance results in an ASS with changing pH and concentrations of fluoride ions (Fig. 4) are listed in Table 4. When Ti70Zr20Nb7.5Ta2.5 alloy exhibits excellent corrosion resistance (high values of impedance), such as at pH 5.0 /0 fluoride ions and pH 7.0/1000 fluoride ions, the compact inner film resistance (R2) is greater than the porous outer film resistance (R1). This reveals that the compact inner film is more resistant to species transport and charge transfer. The relatively low resistance of the compact film measured at high concentrations of fluoride ions (pH 5.0/2000) and at low pH (pH 2.0/1000) is evidence that the Ti70Zr20Nb7.5Ta2.5 alloy does not exhibit corrosion resistance. when the pH decreases and fluoride ion concentration increases, the capacitances of the compact inner (CPE2) and porous outer (CPE1) films increase, indicating that the two oxide films thickness decreases due to dissolution. CPE2 is higher than that of CPE1 in the studied conditions of the pH and concentration of fluoride ions, indicating that the compact inner film is thinner than the porous outer film. It was discovered that the existence of HF and HF2− species but not the presence of fluoride ions was responsible for titanium dissolution [55] and other metals [56] in an acidic fluoride environment.
Table 4
Fitting parameters of EIS obtained for a Ti70Zr20Nb7.5Ta2.5 alloy in artificial saliva with a different concentration of NaF and different pH at 37 ᵒC.
|
PH 5.0
|
1000 F−
|
|
0 F−
|
500 F−
|
1000 F−
|
2000 F−
|
pH7.0
|
pH 5.0
|
pH 2.0
|
RS (Ω cm2)
|
29.84x10− 6
|
62.37x10− 6
|
18.82x10− 6
|
36.29x10− 6
|
82.61x10− 6
|
18.82x10− 6
|
89.20x10− 9
|
R1 (Ω cm2)
|
71.08
|
63.07
|
62.47
|
48.35
|
98.71
|
62.47
|
26.47
|
CPE1 (F cm2 Hz1 − n1)
|
49.56x10− 9
|
56.59x10− 9
|
62.90x10− 9
|
63.93x10− 6
|
54.52x10− 9
|
62.90x10− 9
|
98.56x10− 9
|
n1
|
0.844
|
0.839
|
0.852
|
0.845
|
0.910
|
0.852
|
0.814
|
R2 (Ω cm2)
|
43.23x103
|
5.50x103
|
2.00x103
|
410.4
|
19.33x103
|
2.007x103
|
32.82
|
CPE2 (F cm2 Hz1 − n1)
|
250.8x10− 6
|
539.4x10− 6
|
224.9x10− 6
|
1.275x10− 3
|
73.65x10− 6
|
224.9x10− 6
|
1.389x10− 3
|
n2
|
0.851
|
0.980
|
0.785
|
0.855
|
0.803
|
0.875
|
0.873
|
HF ⇋ H+ + F− (3)
HF2− ⇋ HF + F− (4)
K1= [H+][F−]/[HF] = 1.30×10− 3 mol L− 1 (5)
K2 = [HF][F−]/[HF2−] = 0.104 mol L− 1 (6)
[Total F] = [F−] + [HF] + 2[HF2−] (7)
The accounts utilizing our experimental conditions display that [55]:
(i) Only F− is present in the solution at 1000 total F− and pH 7.0. The polished surface of a Ti70Zr20Nb7.5Ta2.5 sample is shown in the next section (Fig. 13a) after being exposed to saliva with a pH of 7.0 and 1000 fluoride ions for 168 hours. The surface is mirror-like, compact, and smooth, resembling untested samples in appearance. There was no weight loss seen and no indication of a localized attack. A passive state will agree with this.
(ii) F− is dominated by concentrations of 1000 and 2000 F at pH 5.0. For 1000 total F, HF2− and HF have low concentrations relative to F−, but significant concentrations for 2000 total F.
(iii) For 1000 total F and pH 5, F and HF2− have the same concentrations and HF is dominant. The Ti70Zr20Nb7.5Ta2.5 alloy was severely attacked under these circumstances. The surface is tarnished after 48 hours of exposure, and it has a rough, porous appearance with deep grooves that detect the metallographic microstructure of the Ti70Zr20Nb7.5Ta2.5 alloy (Fig. 13c).
Therefore, the production of HF2− and HF is accountable for the low corrosion resistance of a Ti70Zr20Nb7.5Ta2.5 alloy under pH 5.0/2000 fluoride ions and pH 2.0/1000 fluoride ions conditions (proven by the low oxide layer resistance and fast rate of corrosion).
Figure 8 shows the open-circuit potential behavior as a time function of Ti70Zr20Nb7.5Ta2.5 samples in an ASS, recorded at various fluoride ion concentrations and pH. After exposure for a short time, the steady state value of EOCP was attained. When the pH is constant (pH 5.0), the final value of open circuit potential reduces with an increase in the concentration of fluoride ions, but when the fluoride ion concentration is constant (1000 ppm), the OCP value greatly increases as the pH rises. This shows that the pH and fluoride ions concentration affect the corrosion behavior of Ti70Zr20Nb7.5Ta2.5 alloy in ASS.
OCP values can be seen in two different ranges. In the conditions of pH 5.0/0 fluoride ions, pH 5.0/1000 fluoride ions, and pH 7.0/1000 fluoride ions, open circuit potentials showed noble values (-0.48 to − 0.0003 V), which are referred to as the passive state. However, at the active behavior conditions of pH 5.0/2000 fluoride ions and pH 2.0 /1000 fluoride ions, the open circuit potential values are significantly less noble, -0.93 and − 0.85 V, respectively.
Under these circumstances, a drop in the values of open circuit potential could be due to the variation in the oxide chemical composition, which causes an increase in the concentration of H+ ions in the electrical double layer that has arisen on the surface. It's well known that, in general, the stability of the oxides formed on the surface of alloys (particularly TiO2 oxide), is decreased as H + ion concentration is increased, thus reducing their resistance to corrosion [57]. Under the same circumstances, commercially pure Ti exhibited similar behavior [58].
Potentiodynamic polarization curves of Ti70Zr20Nb7.5Ta2.5 HEA recorded after different immersion times in ASS and ASS containing 1000 ppm fluoride ions with a sweep rate of 5 mVs− 1 at 37°C are shown in Figs. 9, 10. It was shown that the polarization curves without and with fluoride ions are shifted towards more positive potential values with increasing time of immersion, referring that the corrosion resistance is improved. The average values of the ipass and the base parameters of corrosion in an ASS with different immersion times are summarized in Table 5. The decrease in icorr, ipass, and CR with increasing the time of immersion without and with fluoride ions suggests that Ti70Zr20Nb7.5Ta2.5 alloy has high corrosion resistance after long-term electrolyte contact. It should be noted that these values without fluoride ions are lower than in the presence of them, indicating the corrosion resistance is much better without fluoride ions.
Table 5
Electrochemical parameters obtained for the Ti70Zr20Nb7.5Ta2.5 alloy in artificial saliva and artificial saliva containing 1000 ppm F− ions with various immersion time at 37 ᵒC.
F−/
ppm
|
Time/
hours
|
iCorr
A cm− 2
|
-ECorr
/ V
|
βa
(V dec− 1)
|
βc
(V dec− 1)
|
Epass
/ V
|
ipass
mA cm− 2
|
CR
mpy
|
|
200
|
5.10x10− 6
|
0.365
|
0.156
|
0.197
|
0.076
|
0.161
|
2.644
|
0
|
400
|
1.68x10− 7
|
0.385
|
0.090
|
0.063
|
0.125
|
0.010
|
0.870
|
|
600
|
1.26x10− 7
|
0.225
|
0.306
|
0.183
|
0.200
|
0.010
|
0.376
|
|
200
|
1.20x10− 5
|
0.302
|
0.991
|
0.300
|
0.175
|
0.174
|
6.211
|
1000
|
400
|
4.38x10− 6
|
0.163
|
0.674
|
0.352
|
-0.180
|
0.129
|
2.271
|
|
600
|
2.63x10− 6
|
0.194
|
0.558
|
0.187
|
-0.110
|
0.148
|
1.363
|
EIS studies were performed at OCP after various times of immersion in an ASS in the absence and presence of F− ions (Figs. 11a, b). Without and with fluoride ions, Nyquist plots show capacitive semicircles that increase in size as immersion time increases. These data display that a Ti70Zr20Nb7.5Ta2.5 alloy corrosion resistance enhances when the time of immersion increases without and with F− ions. Without fluoride ions, Bode plots show that the high values of the Zmod (57.543 − 0.066 kΩ cm2) and high phase angle values (-66.04 o -72.19 o) present as the immersion time increases. With fluoride ions, Zmod increases from 20.892 to 30.902 kΩ cm2 and phase angle increases from − 70.57 o to -76.42 o as the immersion time increases (Figs. 12a,b). As a result, the corrosion rate of the alloy in an ASS without and with F− ions reduces with longer times of immersion.
Without F− ions, with longer immersion times in ASS, the spontaneously generated oxide film on the alloy surface becomes more resistant [59]. With F− ions, during the corrosion process, free F− ions in an ASS was reduced, resulting in an increase in holes in the surface fluorides. Then, at the interface between the matrix and the fluorides, O2 or OH penetrating the fluoride reacted with the matrix's bottom to generate a passive film [60]. As a result, the corrosion rate of the alloy in an ASS without and with F− ions reduces with longer times of immersion.
Fitting parameters reveal that R1, and R2 increase with increasing the immersion time, referring that the protection of the generated oxide film on the alloy enhances with an increase in time of immersion in an ASS without and with fluoride ions [59]. Also, they reveal larger R2 values than R1 ones with increasing immersion time (Table 6), referring that the compact inner film has greater resistance to species transport and charge transfer through it. Due to slightly greater corrosion processes [61], the resistance values of the two layers, R1 and R2 are lower with fluoride ions than without them. The CPE1 and CPE2 enhance with decreasing the time of immersion, indicating that the thickness of two oxide films has decreased due to their dissolution at low immersion time. The CPE1 is superior to the CPE2, pointing out that the resistance to corrosion is attributed to the inner oxide film. The CPE1and CPE2 values are higher in the presence of F− ions and have a longer immersion time than in the absence of them. This result reveals that the thickness of two oxide layers has decreased due to their dissolution [50].
Table 6
Fitting parameters of EIS obtained for a Ti70Zr20Nb7.5Ta2.5 alloy in artificial saliva and artificial saliva containing 1000 ppm NaF with various immersion time at 37ᵒC.
F−/
ppm
|
Time/
hours
|
Rs
(Ω cm2)
|
R1
(kΩ cm2)
|
CPE1
(F cm2 Hz1 − n1)
|
n1
|
R2
(kΩ cm2)
|
CPE2
(F cm2 Hz1 − n2)
|
n2
|
|
200
|
34.39x10− 6
|
0.106
|
174.1x10− 6
|
0.846
|
61.62
|
52.30x10− 9
|
0.855
|
0
|
400
|
283.4x10− 6
|
0.128
|
163.9x10− 6
|
0.761
|
115.2
|
27.10x10− 9
|
0.856
|
|
600
|
1.36x10− 3
|
0.148
|
89.41x10− 6
|
0.840
|
123.0
|
22.91x10− 9
|
0.838
|
|
200
|
93.08x10− 6
|
0.044
|
320.4x10− 6
|
0.834
|
34.52
|
29.25x10− 9
|
0858
|
1000
|
400
|
234.2x10− 6
|
0.045
|
114.2x10− 6
|
0.807
|
39.58
|
22.73x10− 9
|
0.874
|
|
600
|
58.11x10− 9
|
0.051
|
67.51x10− 6
|
0.882
|
75.51
|
14.23x10− 9
|
0.885
|
To be able to elucidate the Ti70Zr20Nb7.5Ta2.5 surface morphology after immersion, SEM was utilized to characterize the alloy surface. The surface morphology of Ti70Zr20Nb7.5Ta2.5 alloy at various pH after various immersion times of 168 h, 120 h and 48 h and pH 7.0, 5.0, and 2.0 in ASS with 1000 ppm F− ions are depicted in Fig. 13a–c. The surface of the Ti70Zr20Nb7.5Ta2.5 alloy at pH 7.0 and immersion time of 168 hours is mirror-like, compact, and smooth, resembling an untested sample in appearance. There was no indication of a localized attack seen despite the existence of fluoride ions referring to a passive state. The surface shows rough and irritated scratches at pH 5.0 and an immersion time of 120 hours (Fig. 13b). Whereas at pH 2 / 48 h, the surface is characterized by localized corrosion and the formation of microcracks (Fig. 13c). Since the grain boundary and its surroundings in an HF or fluoride ion-containing solution have different elements, the intergranular precipitation is more likely to cause severe intergranular corrosion [62, 63]. Some studies [64–66] also showed that the HF solution's attack was slightly more severe near grain boundaries than it was inside of grains. So, the intergranular surface product was more susceptible to cracking, and the exposed area was subject to more severe attack. Finally, these results indicate that increasing pH and time of immersion improve the passive film and thus decrease the Ti70Zr20Nb7.5Ta2.5 alloy corrosion rate.
XPS was used to identify the passive film compositions generated on the Ti70Zr20Nb7.5Ta2.5 alloy surface after varied immersion times (200, 400, and 600 h) in an ASS without and with F− ions. Peaks corresponding to O, F, and Ti elements could be seen in the measured XPS spectra. As seen from the survey spectrum for two samples, after 600 h immersion in an ASS (sample 1) (Fig. 14a) and after 600 h immersion in an ASS containing 1000 fluoride ions (sample2) (Fig. 14c), the local passive film on the Ti70Zr20Nb7.5Ta2.5 surface includes Ti4+, Nb5+, Zr 4+, Ta5+, Ca2+, C and oxygen ions in sample 1 and the passive film on the surface of sample 2 contains the same ions as well as sodium and fluoride ions (Table 7). In this table, the binding energies are consistent with Vasilescu et al. [61] and the handbook of X-ray photoelectron spectroscopy [67], and display doublet peaks for the existence of protective oxides TiO2, Nb2O5, ZrO2, and Ta2O5 on the Ti70Zr20Nb7.5Ta2.5 surface. The identical O 1s peaks at 531.08, 532.5, and 533.3 eV, indicate the existence of three oxygen species (sample 1), O2−, OH−, and absorbed H2O, respectively (Fig. 15a, Table 7). Figure 15b (sample 2) shows the identical O 1s peaks at 531.08 and 532.5, indicate the existence of two oxygen species, O2− and OH− respectively. These data indicate the existence of oxygen in the surface film as hydroxide and oxide. Figures 16a and b (sample 2) show the spectra of Na1s and F1s respectively. The peak of F1s, is correlated to the bonding of fluorine-titanium (F-(Ti), 684.7 eV). The spectra of Ti2p seen in Fig. 16c (sample 2) allowed researchers to determine that the Ti element in passive films exists in the tetravalence (Ti4+, 459.01 eV 2p3/2 and 464.7 eV 2p1/2) and trivalence (Ti3+, 458.3 eV 2p3/2 and 463.4 eV 2p1/2) oxide states. In Fig. 16d (sample 2), the Ti element in the passive films also exists in the trivalence (Ti3+, 458 eV 2p3/2 and 462 eV 2p1/2) and the titanium-fluorine bonding state (Ti4+(F), 459.3 eV 2p3/2 and 465 eV 2p1/2) and tetravalence (Ti4+) the titanium-oxygen bonding state (Ti 4+(O), 459.0 eV 2p3/2 and 464.4 eV 2p1/2) and are both found in the Ti4+ at those energies. According to prior research, the fluorides (TiF4), oxyfluorides (TiOF2), or hexafluorides (TiF62− ) are most likely the sources of the titanium-fluorine bond [68–71]. As found in the later work [72], Since the soluble TiF62− is improbable to be responsible for the high intensity of the distinctive spectra of F1s, the metal-fluorine bonding observed in the passive film should be attributed to the fluorides and oxyfluorides. In conclusion, Ti fluorides, oxides, and hydroxides make up the majority of the passive films that form on the surface of the Ti70Zr20Nb7.5Ta2.5 alloy under the above circumstances. (Table 7 includes a list of the binding energies described above.) Figs. 16e, f and g represent the peaks for Nb3d (like Nb5+, Nb2O5), Zr 3d (like Zr4+, ZrO2), and Ta 4F (like Ta5+, Ta2O5) in all samples [67, 73–75].
Table 7
Binding energies of the main XPS peaks
Sample
|
O1s (eV)
|
Ti2p 3/2 (eV)
|
Nb3d 5/2 (eV)
|
Zr3d 5/2 (eV)
|
Ta4f5/2
|
Na1s (eV)
|
F1s (ev)
|
Blank
|
531.08 532.50
533.30
|
459.36
|
207.68
|
183.08
|
31.26
|
1071.93
|
685.45
|
Saliva 600 h
|
532.88 531.88 530.69
|
459.01
|
206.81
|
191.02
|
22.62
|
1071.99
|
684.91
|
Saliva + 1000 F 600 h
|
531.94 530.51
|
459.02
|
210.08
|
191.54
|
22.38
|
1072.43
|
686.04
|