Electrochemical Characteristics of a Biomedical Ti70Zr20Nb7.5Ta2.5 Refractory High Entropy Alloy in an Artificial Saliva Solution

High entropy alloys are a new type of multi-component material with improved mechanical properties that could be useful in medical implants. The corrosion behavior of a biomedical Ti70Zr20Nb7.5Ta2.5 alloy was examined and matched with that of commercial Ti and the traditional biomaterial Ti6Al4V in artificial saliva. Moreover, the impact of different pH and concentrations of fluoride ions on the corrosion behavior of Ti70Zr20Nb7.5Ta2.5 was also investigated. The Ecorr decreases in the following order: Ti70Zr20Nb7.5Ta2.5 > Ti6Al4V > Ti. The steady-state potential of the OCP indicates that the corrosion resistance decreases in the same order: Ti70Zr20Nb7.5Ta2.5 > Ti6Al4V > Ti. The Ti70Zr20Nb7.5Ta2.5 immersed in saliva at low pH (pH 2.0) and a high fluoride ion concentration (2000 ppm) suffers from cracking and exhibits the lowest resistance to corrosion compared to the sample immersed in the saliva without and with low concentrations of fluoride ions (0–1000 ppm) and at high pH values (5.0 and 7.0). These data reveal that if the fluoride ion concentrations are enhanced or the pH falls, the alloy corrosion resistance reduces. The EIS data show that the passive layer is made up of a duplex outer and inner oxide layer and that the alloy's resistance to corrosion in fluoride-containing solutions has been significantly reduced. Additionally, the data demonstrate that a Ti70Zr20Nb7.5Ta2.5 alloy's corrosion resistance rises with increasing immersion time with and without fluoride ions. According to the X-ray photoelectron spectroscopy investigation, the protective passive oxides include TiO2, ZrO2, Nb2O5, and Ta2O5. The alloy Ti70Zr20Nb7.5Ta2.5 can be considered as a promising material suitable for usage as a biomaterial among all the materials tested in this work.


Introduction
The introduction of the high entropy alloy (HEA) as a cutting-edge concept was developed to produce the materials for medical implants using particular HEA designs [1]. High entropy alloys (HEAs) are a new type of multi-component material that has unique and excellent properties, with each component having the same chance of filling lattice points [2,3]. Although high entropy alloys have a complex composition, they are usually made up of stable and flexible two-phase or single-phase structures [4]. According to a recent study, the bulk of high entropy alloys utilized in medical implants are made up of non-toxic and hypoallergenic refractory elements [5]. Ti and hypoallergenic and non-toxic elements from groups IV and V are commonly used as major components in biomedical HEAs, with copper and cobalt elements added to the matrix. For example, three TiTaHfbased HEAs were recently discovered and displayed great biocompatibility in immersion testing, referring that these novel HEAs have the potential to be utilized as long-term implant materials [6]. The addition of Nb and Zr components has made a major contribution to material corrosion performance improvement.
Commercial titanium and titanium alloys are the preferred materials in dentistry for dental implant systems and the frames of implant-supported prostheses [7,8] as a result of their advantages over other similar materials, including, low densities [3], lack of toxicity [4], great biocompatibility [5] and chemical inertia [2]. However, the corrosion process is an important phenomenon in defining the biocompatibility of dental alloys. The production of an adhering Ti oxide layer on Ti or Ti alloy surfaces determines the resistance to corrosion [9,10]. Moreover, in some acidic conditions in the oral cavity, titanium oxide films can be corroded, releasing ions that may be harmful to the tissues of the mouth [11]. Although the traditional Ti 6 Al 4 V was among the earliest Ti biomaterials utilized in devices and implantable components, several fears have been raised about its use since small amounts of V or Al appear to be poisonous and have an unfavorable influence on organs and living tissues [12,13]. The element of Al can build up in the thyroid, spleen, liver, brain, kidney, and other organs and tissues [14]. The V element can also lead to bone weakening, nerve problems, and anemia. On the other hand, the elastic modulus of the Ti 6 Al 4 V alloy (about 120 GPa) is still higher compared to real bone (10∼35 GPa) [15]. This could be hindering osseointegration and actual implant compatibility [16,17]. Therefore, the development of novel titanium alloys containing non-toxic elements (Mo, Zr, Fe, Ta, Nb, etc.) is becoming essential [18]. The oral cavity's environment is highly corrosive and varies from person to another, which over time can damage restorative materials, particularly those created from metals or alloys. Niinomi and co-workers produced a β-type Ti alloy [19], consisting of non-toxic and non-allergic elements like Ta, Zr and Nb, to obtain excellent mechanical performance and a considerably lower Young's modulus. Previous research has shown that by performing heat treatment or thermo-mechanical treatments on β-type Ti alloys can have a variety of mechanical properties by undergoing heat treatment or thermo-mechanical treatments [19,20]. In bodily fluids and air, β-type Ti alloy has been shown to have high resistance to corrosion [21][22][23][24][25]. This points out that this alloy could be utilized in orthodontic appliances.
On the other hand, fluoride ions found in toothpastes, saliva, and any commercial mouthwashes used daily to prevent tooth decay exacerbated corrosion issues. According to several researchers, fluoride ions damage the protective oxide coatings generated on the metallic dental alloy surface [26][27][28][29][30][31][32]. Furthermore, since the fluoride ions permeate the oxide film, acidic fluoride saliva (pH ≤ 3.5) promotes the onset of pitting corrosion [26,[33][34][35]. As a result, some basic research is still required to recognize the corrosion behavior of a Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy in the environment of the oral cavity. Dental treatments containing fluoride are usually utilized to prevent the formation of cavities and plaque. Fluoride concentrations in fluorinated products range from 1000 to 1500 parts per million in toothpaste to 10,000-20,000 parts per million in gels and 200 ppm in buccal rinses, with pH levels changing from neutral to acidic. The existence of a low pH and fluorides results in the creation of HF 2 − and HF species [36], which can dissolve the protective passive layers on titanium alloys and create Ti fluoride or oxyfluoride complexes [37].
The goal of this study was to test the impact of different concentrations of F − ions (representative of the oral cavity) and the change in pH on the corrosion behavior of a biomedical Ti 70 Zr 20 Nb 7.5 Ta 2.5 HEA in artificial saliva solution (ASS). The corrosion behavior of this alloy was examined and matched with that of commercial titanium and the traditional biomaterial Ti 6 Al 4 V alloy. The electrochemical methods, like electrochemical impedance spectroscopy, potentiodynamic polarization, and open-circuit potential techniques, complemented with SEM, were carried out to achieve this goal.

Experimental
The study was conducted to contrast the electrochemical corrosion behavior of the HEA Ti 70 Zr 20 Nb 7.5 Ta 2.5 with that of the traditional biomedical Ti 6 Al 4 V alloy and commercial Ti (their chemical compositions are given in Table 1) in an artificial saliva solution (ASS). The present investigated Ti 70 Zr 20 Nb 7.5 Ta 2.5 HEA is characterized by its excellent mechanical properties such as relatively high hardness (330 Hv), low elastic modulus (46 GPa), and high compressive strength (2600 PMa), which promotes it as a good implant material. The samples were made by epoxy cold resin mounting of alloys, with regions for exposure to the electrolyte of 1.0 cm 2 for the Ti, Ti 6 Al 4 V, and Ti 70 Zr 20 Nb 7.5 Ta 2.5 , respectively. Three-electrode cell were used for the electrochemical studies. The samples of the three materials were provided by Japan Coating Center Co. Ltd., Kanagawa, Japan, which were used as the working electrodes. A Pt electrode was utilized as an auxiliary electrode. A silver/silver chloride electrode was utilized as a reference electrode for all potential measurements. The electrode is mechanically polished using emery paper with various grades, i.e., 500, 800, and finally 1200, before being rinsed with acetone and distilled water before each experiment. Finally, the electrode is activated in 1.0 M HF for 1.0 min, then washed thoroughly with double-distilled water. The measurements were taken at 37 °C using Fusayama ASS (chemical composition: 0.9 g l −1 KCl, 0.4 g l −1 NaCl, 0.69 g l −1 NaH 2 PO 4 , 1 g l −1 urea, and 0.795 g l −1 CaCl 2 ·2H 2 O) [38]. Saliva with 500, 1000, and 2000 ppm fluoride ion concentrations was made with the addition of NaF. The Fusayam a saliva pH was changed to 2.0 and 7.0 by utilizing dilute hydrochloric acid and sodium hydroxide solutions, respectively. The electrochemical experiments were done utilizing a 1000 Gamry Instrument Potentiostat/Galvanostat/ZRA. The potentiodynamic polarization curves were implemented in the potential range from − 1.2 to + 3.0 V with a 5 mVs −1 sweep rate. The evolution of the open circuit potential (OCP) was measured for 1.4 h in the ASS solution at 37 °C. The electrochemical impedance spectroscopy (EIS) diagrams were plotted after 1800s of immersion in ASS in a frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV peak-to-peak. The experiments were performed after 30 min of immersion. The electrochemical tests were done in duplicate. The samples surface morphology was examined using a SEM (scanning electron microscopy) model; the JEOL JEM-1200EX II Electron Microscope. XPS (X-ray photoelectron spectroscopy) was collected on K-ALPHA (Thermo Fisher Scientific, USA) with monochromatic X-ray Al Kα radiation-10 to 1350 eV spot size of 400 µm at pressure 10-9 mbar with a full-spectrum pass energy of 200 eV and a narrow-spectrum of 50 eV. Figure 1 displays the PPC's of Ti, Ti 6 Al 4 V, and Ti 70 Zr 20 Nb 7.5 Ta 2.5 samples in an ASS under similar conditions at 37 °C. The potential was recorded by scanning the potential beginning from − 1.2 V, where H 2 evolution is dominating, up to an anodic potential of + 3.0 V with a 5 mVs −1 sweep rate. All samples exhibit active-passive behavior. The long range of passivity up to + 3.0 V indicates the presence of a protective oxide film on the surface in each case. The corrosion behavior was estimated utilizing the following parameters: cathodic and anodic Tafel slopes (β a , β c ), polarization resistance (R p ), corrosion current (i corr ), corrosion potential (E corr ), passive current density (i pass ), and corrosion rate (CR  Table 2). This means that the Ti 70 Zr 20 Nb 7.5 Ta 2.5 has a more positive E corr value and consequently possesses the strongest corrosion resistance in the ASS at 37 °C. This behavior can be attributed to its microstructure, which has a β-type phase and favors the production of a spontaneous passivation film, dislodging a corrosion potential in favor of more noble values. It has been shown that materials tend to corrode when β a < β c , while for passivation when β a > β c [39]. On the other hand, Ti 70 Zr 20 Nb 7.5 Ta 2.5 possesses the lowest i pass while Ti possesses the highest i pass . The material corrosion resistance is better if the values of i corr and/or CR as well as the i pass are low and R p is high [39]. Taking all these parameters into account, it is clear that the Ti 70 Zr 20 Nb 7.5 Ta 2.5 sample immersed in the ASS at 37 °C possesses the highest corrosion resistance and the lowest i corr and i pass values compared to the other two samples of Ti 6 Al 4 V alloy and individual Ti. From all the above data, the addition of Nb, Zr, and Ta elements significantly improves the corrosion resistance of Ti 70 Zr 20 Nb 7.5 Ta 2.5 . Due to a high electronegativity and low polarizability of the fluoride ion, it 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 [40]. 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 The following electrochemical parameters were calculated and are listed in Table 3: i pass , i corr , E corr , E p , |E corr − E p | shows inclination to form passive film (minimum values indicate easy and good passivation). It was observed that the sample of Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy immersed in saliva at a low pH (pH 2.0) and high fluoride ion concentration Table 2 Electrochemical parameters obtained for a Ti, Ti6Al4V, and Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy in artificial saliva solution at 37 °C     Table 4. The decrease in i corr , i pass , and CR with increasing the time of immersion without and with fluoride ions suggests that Ti 70 Zr 20 Nb 7.5 Ta 2.5 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.     Under these circumstances, a drop in the values of open circuit potential could be due to thevariation 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 TiO 2 oxide), is decreased as H + ion concentration is increased, thus reducing their resistance to corrosion [43]. Under the same circumstances, commercially pure Ti exhibited similar behavior [44].  Fig. 7a, all curves are found to be depressed semicircles, which is due to the frequency dispersion [45]. This result indicates that the charge transfer controls the corrosion of the three samples. In Bode plots (Fig. 7b)  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 [46][47][48], while a circuit with two-time constants was utilized to represent the existence of a duplex outer and inner oxide layer [49][50][51][52].

Electrochemical Impedance Spectroscopy (EIS)
Pan [51] utilized modified models to account for the plugging of pores by products of corrosion, while Ibris [53] 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 Ti 70 Zr 20 Nb 7.5 Ta 2.5 ) was achieved utilizing the EC presented in Fig. 8. The components of this EC are: R s is the resistance of the electrolyte, R 1 and R 2 are the resistances of the outer and inner oxide film, respectively. CPE 1 and CPE 2 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 [54]: 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 (R s , R 1 , CPE 1 , R 2 and CPE 2 ) of the suggested EC are listed in Table 5 (1) for all analyzed samples. The summation of the outer oxide film resistances and the inner oxide film (R 1 + R 2 ) is the polarization resistance (R p ). The passive layer, according to the suggested model, is made up of two films: The resistance R 2 values of the compact inner film are much higher than those of the porous outer film, R1 (Table 5), pointing out that the inner compact oxide film is more resistant to species transport and charge transfer. The CPE 1 of the porous outer film capacitances is greater than the CPE 2 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 [18]. The exponent values n 1 and n 2 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. [55].
EIS studies of Ti 70 Zr 20 Nb 7.5 Ta 2.5 HEA were performed at open circuit potentials with 30 min of immersion in an ASS at various fluoride ion concentrations and pH. Nyquist plots (Fig. 9a, b) show capacitive semicircles that reduce in size as the fluoride ion concentration rises and the pH falls.  In Bode plots (Fig. 10a), as the fluoride ion concentration promotes from 0 to 2000 ppm, the Z mod sharply diminishes from 41.64 to 0.38 kΩ cm 2 and the maximum phase angles reduce from around − 75.87° to − 35.80° when they go slightly to a higher frequency region. In Bode plots (Fig. 10b), when the pH promotes from 2.0 to 7.0, the (Z mod ) increases greatly from 0.11 to 21.88 kΩ cm 2 and the maximum phase angles increase sharply from about − 9.55° to − 74.31°. 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. 10a, b), confirming the dual nature of the passive oxide film on Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy in various ASS. The values of R s , R 1 , CPE 1 , R 2 and CPE 2 were obtained by fitting the Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy experimental impedance results in an ASS with changing pH and concentrations of fluoride ions (Fig. 8) are listed in Table 6. When Ti 70 Zr 20 Nb 7.5 Ta 2.5 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 (R 2 ) is greater than the porous outer film resistance (R 1 ). 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 Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy does not exhibit corrosion resistance. When the pH decreases and fluoride ion concentration increases, the capacitances of the compact inner (CPE 2 ) and porous outer (CPE 1 ) films increase, indicating that the two oxide films thickness decreases due to dissolution. CPE 2 is higher than that of CPE 1 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 HF 2 − species but not the presence of fluoride ions was responsible for titanium dissolution [36] and other metals [56] in an acidic fluoride environment.
(2)   (iii). For 1000 total F and pH 5, F − and HF 2 − have the same concentrations and HF is dominant. The Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy was severely attacked under these circumstances. The surface is tarnished after 48 h of exposure, and it has a rough, porous appearance with deep grooves that detect the metallographic microstructure of the Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy (Fig. 13c).
Therefore, the production of HF 2 − and HF is accountable for the low corrosion resistance of a Ti 70 Zr 20 Nb 7.5 Ta 2.5 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).
EIS studies of Ti 70 Zr 20 Nb 7.5 Ta 2.5 HEA were performed at OCP after various times of immersion in an ASS in the absence and presence of F − ions (Fig. 11a, b). Without and   (Fig. 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 [57]. 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, O 2 or OH penetrating the fluoride reacted with the matrix's bottom to generate a passive film [58]. As a result, the corrosion rate Fitting parameters reveal that R 1 , and R 2 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 [57]. Also, they reveal larger R 2 values than R 1 ones with increasing immersion time ( Table 7), referring that the compact inner film has greater resistance to species transport and charge transfer through it. Due to slightly greater corrosion processes [59], the resistance values of the two layers, R 1 and R 2 are lower with fluoride ions than without them. The CPE 1 and CPE 2 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 CPE 1 is superior to the CPE 2 , pointing out that the resistance to corrosion is attributed to the inner oxide film. The CPE 1 and CPE 2 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 [54].

SEM and XPS Measurements
To be able to elucidate the Ti 70 Zr 20 Nb 7.5 Ta 2.5 surface morphology after immersion, SEM was utilized to characterize the alloy surface. The surface morphology of Ti 70 Zr 20 Nb 7.5 Ta 2.5 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 Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy at pH 7.0 and immersion time of 168 h 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 h (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 [60,61]. Some studies [60][61][62] 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 Ti 70 Zr 20 Nb 7.5 Ta 2.5 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 (sample 2) (Fig. 14c), the local passive film on the Ti 70 Zr 20 Nb 7.5 Ta 2.5 surface includes Ti 4+ , Nb 5+ , Zr 4+ , Ta 5+ , Ca 2+ , 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 8). In this table, the binding energies are consistent with Vasilescu et al. [59] and the handbook of X-ray photoelectron spectroscopy [63], and display doublet peaks for the existence of protective oxides TiO 2 , Nb 2 O 5 , ZrO 2 , and Ta 2 O 5 on the Ti 70 Zr 20 Nb 7.5 Ta 2.5 surface. The identical O 1 s peaks at 531.08, 532.5, and 533.3 eV, indicate the existence of three oxygen species (sample 1), O 2− , OH − , and absorbed H 2 O, respectively (Fig. 15a, Table 8). Figure 15b (sample 2) shows the identical O 1 s peaks at 531.08 and 532.5, indicate the existence of two oxygen species, O 2− and OH − respectively. These data indicate the existence of oxygen in the surface film as hydroxide and oxide. Figure 16a and b (sample 2) shows 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 (Ti 4+ , 459.01 eV 2p 3/2 and 464.7 eV 2p 1/2 ) and trivalence (Ti 3+ , 458.3 eV 2p 3/2 and 463.4 eV 2p 1/2 ) oxide states. In Fig. 16d (sample 2), the Ti element in the passive films also exists in  and are both found in the Ti 4+ at those energies. According to prior research, the fluorides (TiF 4 ), oxyfluorides (TiOF 2 ), or hexafluorides (TiF 6 2− ) are most likely the sources of the titanium-fluorine bond [64][65][66][67]. As found in the later work [68], since the soluble TiF 6 2− 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 Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy under the above circumstances. ( Table 7 includes a list of the binding energies described above). Figure 16e-g represents the peaks for Nb3d (like Nb 5+ , Nb 2 O 5 ), Zr 3d (like Zr 4+ , ZrO 2 ), and Ta 4F (like Ta 5+ , Ta 2 O 5 ) in all samples [63,[69][70][71]].

Conclusion
The corrosion behavior of a biomedical Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy was examined and matched with that of commercial titanium and the traditional biomaterial Ti 6 Al 4 V alloy in an artificial saliva solution. Moreover, the impact of different concentrations of F − ions and pH on the corrosion behavior of Ti 70 Zr 20 Nb 7.5 Ta 2.5 was also investigated. The corrosion potential decreases in the following order: Ti 70 Zr 20 Nb 7.5 Ta 2.5 > Ti 6 Al 4 V > Ti. The steady-state potential of the OCP, which agrees with the result of the PPC curves, indicates that the corrosion resistance decreases in the same order: Ti 70 Zr 20 Nb 7.5 Ta 2.5 > Ti 6 Al 4 V > Ti. It was observed that the Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy immersed in saliva at a high concentration of fluoride ions (2000 ppm) and low pH (pH 2.0) suffers from cracking and exhibits the lowest corrosion resistance compared to the sample immersed in the saliva solution without and with low concentrations of fluoride ions (0-1000 ppm) and at high pH values (5.0 and 7.0). These data reveal that if the fluoride ion concentrations enhance or the pH falls, corrosion resistance of the alloy reduces. The EIS data shows that the passive layer is made up of a duplex outer and inner oxide layer, and that the alloy's resistance to corrosion in fluoride-containing solutions has been significantly reduced. Additionally, the data demonstrates that a Ti 70 Zr 20 Nb 7.5 Ta 2.5 alloy's corrosion resistance rises with increasing immersion time in both the presence and absence of F − ions. The protective passive oxides, according to XPS (X-ray photoelectron spectroscopy) investigation, include TiO 2 , ZrO 2 , Nb 2 O 5 , and TaO. Based on the aforementioned conclusions, the alloy Ti 70 Zr 20 Nb 7.5 Ta 2.5 can be regarded as the best suitable for usage as a biomaterial among all the materials tested in this work.

Author Contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by IHE, MAMI, and NFEB. The first draft of the manuscript was written by all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Availability of Data and Materials
All data generated or analyzed during this study are included in this published article.