Monitoring and Evaluation of Corrosion at the Interface of Zirconium Alloy Biomaterials Under Simulated Oxidative Biological Environment

Abstract

The present work investigates the electrochemical behavior of the Zr2.5Nb alloy in a biomedical context, emphasizing the influence of electrochemical oxidation treatment on its stability in simulated physiological environments. The alloy samples were oxidized in 1 M H₂SO₄ under controlled voltages (200–275 V) and times (1 min), identifying 200 V–1 min as the optimal condition for obtaining a uniform porous oxide layer with an average pore diameter of ~90 nm. The corrosion resistance was evaluated using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) in Ringer’s solution and Ringer’s solution containing 40 g/L H₂O₂ to simulate physiological and inflammatory conditions. Electrochemical tests revealed that electrochemically oxidized samples exhibited a polarization resistance up to 14.78 MΩ·cm², about 26 times higher than that of the untreated alloy (0.56 MΩ·cm²). After 77 h of immersion, the oxidized alloy maintained a high resistance (17.54 MΩ·cm²), confirming long-term stability. Scanning Electron Microscopy (SEM–EDX) and X-Ray Diffraction (XRD) analyses highlighted significant increases in oxygen content and the transformation from the monoclinic baddeleyite to the cubic arkelite phase of ZrO₂, contributing to enhanced corrosion resistance. These findings demonstrate that controlled electrochemical oxidation significantly improves the durability of Zr2.5Nb alloy in oxidative environments, supporting its potential for long-term biomedical implant applications.

1. Introduction

In the context of biomedical applications, a biomaterial is considered biocompatible when it fulfills the functional and physiological requirements specific to a particular use.

Biocompatibility is therefore context-dependent: a material may be well tolerated and function appropriately in one biological environment or medical scenario but may elicit adverse responses or fail to perform in a different context. For example, a metal alloy may be biocompatible as an orthopedic implant but could be unsuitable for use in cardiovascular devices due to different tissue interactions [1].

A major technological challenge in implant development is the identification and optimization of materials that meet stringent standards of biocompatibility and long-term performance under in vivo conditions. In this direction, recent research is focused on zirconium alloys, which, due to their variable crystal structure and chemical composition, offer promising prospects [2,3,4,5,6].

A notable example is the Zr2.5Nb alloy studied in this work, which, according to the review by Mehjabeen et al. [7], presents multiple biomedical advantages: high biocompatibility, superior corrosion resistance, absence of allergic reactions, increased hardness (~12 GPa), superior aesthetics in dental applications, radiopacity, and good osseointegration. However, it also presents some disadvantages, such as difficulties in intraoral finishing and relatively high costs.

Corrosion of implanted metallic materials is a key issue in assessing the durability and safety of biomaterials. Dissolved salts in body fluids—especially chloride ions (~103 mmol/L in blood plasma)—constitute an aggressive chemical environment, contributing to the acceleration of degradation processes of metallic implants [8,9,10,11]. In addition, electrochemical processes occurring at the metallic surface, particularly partial oxygen reduction reactions (ORRs), can lead to the formation of reactive oxygen species (ROS) such as H₂O₂ and HO₂⁻, which may induce oxidative stress on adjacent tissues [12,13].

Thus, corrosion stability becomes a critical requirement for biomaterials, along with mechanical properties and biocompatibility.

The Zr2.5Nb alloy has been shown to have a modulus of elasticity close to the human cortical bone (30 GPa), adequate mechanical strength, and remarkable fracture resistance. It also exhibits lower magnetic susceptibility and superior chemical stability over time compared to titanium alloys [14,15,16]. To improve performance in biological environments, metallic materials can be subjected to surface modification treatments that target the functional and biochemical properties of the implant. There are three broad categories of methods: (a) mechanical methods, (b) chemical methods, and (c) physical methods [17,18,19]. Among these, anodic oxidation is a well-established electrochemical process for the formation of oxide layers with insulating properties on metallic surfaces, to enhance their subsequent resistance to corrosion and wear in aggressive human intracorporeal environments [20,21,22,23]. It is a cost-effective, simple, and efficient method for the formation of a protective oxide layer, which considerably enhances the corrosion and wear resistance of implants in aggressive intracorporeal environments [20,21,22,23]. Its advantages include low cost, ease of application and precise control of the thickness and morphology of the oxide layer.

However, in the literature, there are still a limited number of studies that directly investigate the electrochemical oxidation of Zr2.5Nb alloy, despite its demonstrated potential for biomedical applications [14,15,16,23,24,25,26]. This gap highlights the need for further experimental research on its electrochemical behavior in simulated physiological environments, to establish optimal strategies for surface protection and functionalization.

The novelty of this study consists of the systematic investigation of the electrochemical oxidation process applied to the Zr2.5Nb alloy, a relatively recent material proposed for biomedical applications, under simulated environmental conditions, both physiological (Ringer’s solution) and inflammatory (Ringer + H₂O₂), an aspect rarely addressed in specialized literature. This study provides detailed comparative data on the corrosion behavior of oxidized and non-oxidized surfaces, correlated with morphological, compositional, and structural characterizations, thus contributing to increasing the durability of zirconium-based metallic implants.

2. Results and Discussion

2.1. Morphological and Compositional Analysis of the Surfaces of the Zr2.5Nb Alloy and the Obtained Oxide Films, Using Scanning Electron Microscopy (SEM–EDX)

SEM–EDX Analysis of Untreated and Electrochemically Modified Zr2.5Nb Alloy

Figure 1a shows the SEM–EDX analysis of the untreated Zr2.5Nb alloy. Figure 1(ai) displays the EDX spectrum of the elements detected at a certain ionization energy field (keV), and Figure 1(aii) the quantitative analysis of the detected elements while Figure 1(aiii) shows the micrograph of the studied surface at 10,000× magnification.

According to EDX analysis, the main constituent elements of the untreated Zr2.5Nb alloy in decreasing order of mass percentage values are the following: Zr—92.81%; Nb—5.11%; Hf—0.82%. Another element such as oxygen (O₂) is identified because the Zr2.5Nb alloy, as well as many other metallic materials, is prone to the formation of thin oxide films after immediate exposure to the ambient environment. In our case, the incorporation percentage of ZrO₂ on the surface of the untreated Zr2.5Nb alloy is approximately 7.93%.

This result is obtained after converting the mass percentage of oxygen from the general analysis in relation to the molecular mass of zirconium dioxide (123.218 g/mol). In Figure 1(aiii), the surface morphology of the untreated Zr2.5Nb alloy is represented. Thus, the specific rough relief can be observed through unidirectional striations, a configuration resulting from the final processing by the manufacturer.

Figure 1b–d shows the SEM–EDX analysis for Zr2.5Nb alloy samples electrochemically oxidized in 1 mol/L sulfuric acid (H₂SO₄) for 1 min at different applied voltages (200 V, 250 V, 275 V). In this electrolyte, the zirconium samples underwent changes; thus, for Zr2.5Nb oxidized at 200 V for 1 min, the mass percentage of ZrO₂ formation is 54.11%, but in the case of samples electrochemically oxidized at potentials of 250 V and 275 V, a more substantial difference is observed with values of 62.14% and 62.87%, respectively.

Morphologically, on the Zr2.5Nb samples electrochemically oxidized in sulfuric acid, surfaces with different porosities and sizes are highlighted, and in the case of the sample oxidized in 1 M H₂SO₄ for 1 min at an applied potential of 275 V, conglomerated pores with larger sizes are observed that unite and form non-uniform crevices (trenches) with random distribution over the entire surface, as shown in Figure 1d.

In Figure 2a–c, SEM micrographs of the untreated and electrochemically oxidized Zr2.5Nb alloy are presented for morphological comparison at a higher resolution, a magnitude of 50,000×.

The micrographs in Figure 2a–c represent the surfaces of the Zr2.5Nb alloy after the electrochemical oxidation step in the 1 mol/L sulfuric acid solution, where a non-uniform dissolution of the native zirconium oxide film formed is observed, a phenomenon also encountered in other studies [27], with the presence of a high number of defects and reduced porosity, respectively, decreasing as the applied voltage increases.

From the analysis of Figure 2, a progressive increase in the diameter of the nanopores is observed with the increase in the applied voltage during the electrochemical oxidation process, maintaining the same treatment time (1 min) and the same electrolyte (H₂SO₄).

Thus, at a voltage of 200 V, the average value of the pore diameter is 91.58 nm, at a voltage of 250 V, the average value of the pore diameter increases to 124.30 nm while at a voltage of 275 V, the average value of the pore diameter reaches a diameter of 134.50 nm. This evolution indicates that the applied voltage plays an essential role in controlling the size of the nanopores, and increasing its value favors the formation of larger pores.

In addition to the EDX spectral analysis, it is also possible to use the analysis of the distribution maps of the identified elements; thus, in the case of the Zr2.5Nb alloy under study, the elements (O, Zr, Nb, Hf) are analyzed at a magnification of 10,000× of the SEM images.

In Figure 3, the distribution maps of the constituent elements on the surface of the untreated Zr2.5Nb alloy are displayed.

From the analysis of Figure 3, it can be seen that the untreated sample contains less oxygen as compared with the sample oxidized presented in Figure 4b. The spatial distributions of zirconium (Figure 3c), niobium (Figure 3d), and hafnium (Figure 3e) correspond to the expected composition of the alloy. Figure 4 shows the Zr2.5Nb alloy sample anodically oxidized in 1 mol/L sulfuric acid at a voltage of 200 V for a period of 1 min.

From the analysis of Figure 4, it is observed that the presence of oxygen is higher in the anodically oxidized sample, and the other elements are uniformly distributed without obvious changes compared to the untreated sample.

2.2. Structural Characterization by X-Ray Diffraction (XRD) of Untreated and Electrochemically Modified Zr2.5Nb Alloy

XRD analysis is based on the ability of the crystalline structures on the surface of a material to diffract X-rays in a characteristic way, which subsequently allows the precise determination of the morphology of the crystalline phases, based on the data obtained. With the help of XRD, the lattice parameters of the chemical element, the space group, the chemical composition, the macro stresses or the qualitative phase analysis can be investigated. Based on the intensity of the recorded peak, information about the crystallographic structure (atomic positions) can be obtained, as well as quantitative phase and texture analyses

Finally, the peak shape provides information about the structural phase boundaries of the sample (microstrain variations and crystallite size) [28]. In the field of materials science and engineering, XRD analysis has been developed to become a state-of-the-art technique, especially for qualitative and quantitative phase analysis, but also in investigations of crystallographic textures and residual stress measurements [29]. Figure 5a shows the XRD spectrum for the untreated Zr2.5Nb alloy and Figure 5b shows the XRD spectrum for the electrochemically oxidized and modified Zr2.5Nb alloy.

Thus, for the index noted as (1) in Figure 5a, zirconium dioxide (ZrO₂) baddeleyite, found in the crystallographic database of the mentioned application with Crystallography Open Database (COD) 96-901-6715, which belongs to the monoclinic crystallization system and the space group P 1 21/c 1, the following crystallographic planes are recognized: (011), (11-1), (111), (121), (211), (22-1), (22-2), and (113) at the 2θ angle levels (28.00°), (32.35°), (36.70°), (51.75°), (54.30°), (60.19°), (69.00°), and (72.59°).

Similarly, after the XRD spectral analysis of the zirconium alloy, the crystalline phase for zirconium (Zr) was identified with the index (2). According to the Crystallography Open Database (COD) 96-900-8524, this phase belongs to the hexagonal crystallization system, space group P63/mmc, with crystallographic planes (002), (101), (012), (110), (013), and (004) corresponding to the 2θ angles (40.68°), (42.64°), (56.32°), (67.21°), (75.35°), and (88.20°). Another type of zirconium identified and mentioned by index (3) is the orthorhombic crystalline form, with space group P b a m, being identified with Crystallography Open Database (COD) 96-231-0735 and the crystallographic plane (421) corresponding to the angle 2θ (38.32°).

In the case of the crystalline phase identified as niobium (Nb), index (4) in Figure 5a, only one crystallographic plane (200) was quantified at the 2θ angle (50.60°), according to Crystallography Open Database (COD) 96-153-4904 with the cubic crystallization system and space group F m −3 m. For the last identified phase—hafnium (Hf), denoted by (5) in Figure 5a—according to the database with COD 96-153-9054, the hexagonal crystallization system, and space group P63/mmc, only one crystallographic plane (112) is observed at the 2θ angle (81.65°).

It is worth noting that the crystallographic planes (002), (101), (012), (110), (013), and (004) for (Zr) zirconium, (111), (11-1), and (211) for zirconium dioxide (ZrO₂), and (200) with the β phase for niobium (Nb) are also mentioned in other experimental studies with the Zr2.5Nb alloy [30,31,32,33,34,35]. After analyzing Figure 5a, it can be observed that the untreated study sample contains a major Zr phase in a hexagonal structural system, with broader diffraction peaks compared to the position of the oxide diffraction lines at smaller 2θ angles, which indicates the possibility of intercalation of Nb and Hf atoms in the zirconium crystal lattice. This form of crystallographic organization is encountered in a study comparing the Zr2.5Nb alloy with ZrTa and ZrNbTa samples [34,35].

For the Zr2.5Nb alloy anodically oxidized at 200 V–1 min in sulfuric acid (Figure 5b), the difference from the untreated alloy is observed by the additional appearance of crystallographic planes (002), (21-1), and (222)) at the 2θ angles (38.88°), (49.19°), and (78.05°), and the absence of planes (11-1), (121), (211), (22-1), and (22-2) for the baddeleyite-type zirconium dioxide (ZrO₂) phase. Concomitantly with this modification, another excess crystalline phase (3) is identified, such as zirconium dioxide (ZrO₂) of the arkelitic type for only a single crystallographic plane (111) at the 2θ angle (35.20°), registered in the database under (COD) 96-500-0039, with the cubic crystallization system in a space group of F m -3 m.

In order to highlight the difference in electrochemical oxidation processes, the sequential comparison of XRD spectra was used according to the most representative crystallographic planes in terms of the intensities of the evolutionary peaks.

According to Figure 5a–d, the untreated Zr2.5Nb alloy samples and the one electrochemically oxidized at a potential of 200 V–1 min are compared at the level of the crystallographic plane (111) of baddeleyite-type zirconium dioxide (ZrO₂) and according to the sequence of the plane (111) of arkelite-type (ZrO₂).

Another tool used in XRD structural characterization is the objective differentiation of the average size of ZrO₂ crystallites, depending on the electrolyte used for anodic oxidation. The evaluation of the average sizes was performed using the Debye–Sherrer Equation (1) [36]:

$$D={\displaystyle \frac{k\lambda }{FWHM\ast \cos \theta }}$$

(1)

where D is the average crystallite size (Å); k is the form factor (0.9); λ is the wavelength of the X-ray radiation (1.790300 Å) Co Kα; θ is the Bragg angle (or diffraction angle); FWHM is the full width at half maximum of the characteristic peak of the peak (in Radians).

Applying this formula and considering that the height corresponds to the average crystallite dimensions calculated according to the (111) plane of ZrO₂ baddeleyite Figure 5c, the grain size has a value of 49.30 nm for the untreated sample. The size of the oxide grains increases to 50.21 nm in the case of the Zr2.5Nb sample oxidized in sulfuric acid.

In the case of zirconium dioxide (ZrO₂) arkelite at the (111) crystallization phase according to Figure 5d, the untreated sample does not identify the mentioned plane, and the average crystallite size increases to 40.09 nm for the sample anodically oxidized in H₂SO₄.

2.3. Corrosion Resistance Assessment of Untreated and Electrochemically Modified Zr2.5Nb Alloy in Both Physiological and Pathological Artificial Solutions

2.3.1. Evolution of the Open Circuit Potential (OCP) for Untreated and Electrochemically Modified Zr2.5Nb Alloy After Immersion in Physiological and Pathological Environments

The electrochemical behavior of the zirconium alloy (Zr2.5Nb) is evaluated by measuring the evolution of the open circuit potential (OCP) over time, under two different surface treatment conditions and subsequent immersion in the physiological Ringer’s solution. The OCP method is essential in evaluating the stability and reactivity of metallic surfaces in various corrosive environments. It allows for real-time monitoring of the open circuit potential of a material without applying an external current, thus providing valuable information about the processes of formation and dissolution of the oxide layer on its surface. Figure 6 shows the evolution of the open circuit potential for the Zr2.5Nb sample, both untreated and electrochemically oxidized in sulfuric acid, after immersion in the artificial Ringer’s solution, in two time steps: (a) t₁—5 h and (b) t₂—77 h.

From Figure 6a, it is observed that the first 300 min of exposure of the samples provided clear insights into the behavior of the untreated and oxidized alloy. The untreated Zr2.5Nb sample (curve 1) shows an increase in the OCP value in the first 300 min of monitoring without reaching the equilibrium state. This sample starts from a potential of −394 mV vs. Ag/AgCl at immersion and reaches a potential of −313 mV vs. Ag/AgCl at the end of the measurement. The potential difference between the value of the potential at immersion and the value at the end of the 300 min in this case is ΔE = 81 mV vs. Ag/AgCl.

The potential shift towards more positive values indicates a rapid formation of the natively formed protective oxide layer on the surface of the material, but as quickly as it forms, it dissolves [37,38,39]. This behavior is also observed in the literature and by other authors [40]. Maciej Sowa and his team [41,42] argue that the presence of a thin native oxide film on the surface of the Zr2.5Nb alloy, not subjected to electrochemical treatment, determines an increase in the open circuit potential due to the reduced permeability of the oxide layer for the corrosive environment, while the slight decrease in the potential observed in the samples treated by electrochemical oxidation can be attributed to the superficial cracks in the porous structure of the formed oxide layer. N. Veaceslav and collaborators [43] also observed a shift of the open circuit potential towards more positive values in the case of untreated Zr2.5Nb alloy

The sample treated with sulfuric acid (curve 2) showed a distinct evolution, with an OCP that is stabilized after approximately 30 min around the value of −372 mV vs. Ag/AgCl. The stability of the open circuit potential suggests the formation of a very stable oxide layer, capable of preventing dissolution and conferring immunity to the corrosive environment [37,38,39]. This constancy indicates that, in this case, the electrochemical oxidation treatment in H₂SO₄ solution is more effective in providing protection against corrosion compared to the untreated sample.

The OCP method has demonstrated its utility in evaluating the electrochemical behavior of untreated and electrochemically oxidized zirconium alloys in corrosive environments. By continuously monitoring the open circuit potential, it was highlighted how different surface treatments influence the stability of the oxide layer. In particular, a decrease in OCP indicates the dissolution of the protective oxide layer, while an increase suggests its reformation. A constant OCP signifies the achievement of an equilibrium or immunity state, where the oxide layer remains intact and effective in protecting the material. These results provide valuable insights for optimizing the surface treatments of Zr2.5Nb alloys, in order to improve their corrosion resistance in medical applications.

From Figure 6b, after 3 days of immersion, the samples showed a similar behavior to that shown in Figure 6a. The untreated sample and the one electrochemically oxidized with H₂SO₄ maintained their OCPs relatively constant during the tests. This behavior suggests that the formed oxide layer neither dissolves nor forms.

Figure 6c,d shows the evolution of the open circuit potential (OCP) for the Zr2.5Nb sample, both untreated and electrochemically oxidized in sulfuric acid, after immersion in artificial Ringer’s solution with 40 g/L H₂O₂ in two time steps: (a) t₁—5 h and (b) t₂—77 h.

Comparing Figure 6a–d, it can be seen that samples immersed in Ringer’s solution with the addition of 40 g/L H₂O₂ showed more positive potential values compared to those immersed in Ringer’s solution without hydrogen peroxide.

The addition of hydrogen peroxide to Ringer’s solution plays an important role, since H₂O₂ is a strong oxidizing agent, which accelerates redox processes on the surface of metallic materials. In this context, H₂O₂ can induce the formation of a passivating layer on the alloy surface, providing additional protection against corrosion. In addition, solutions containing H₂O₂ are often used to simulate more aggressive conditions, relevant for biomedical applications, such as inflamed body environments or those exposed to oxidative stress. Thus, the study of the interaction of alloys with H₂O₂ is essential to evaluate their durability and safety in medical implants or other similar applications [37,38,39,44,45,46,47]. This behavior has also been observed in the literature and by other authors on different materials [37,39,44,45].

2.3.2. Electrochemical Impedance Spectroscopy (EIS) of Untreated and Electrochemically Modified Zr2.5Nb Alloy After Immersion in Physiological and Pathological Environment

Electrochemical impedance spectroscopy is an electrochemical technique based on the application of an alternating current to characterize the processes occurring at the interface of the material (electrode) and the corrosive medium (electrolyte). This method provides a wide range of information about the kinetics of the processes occurring at the electrode–electrolyte interface and is used in various fields, such as the study of corrosion processes, the characterization of semiconductors, the analysis of batteries, and the study of the kinetics of electrochemical deposition [37]. In studies of reactivity and corrosion of metallic surfaces, electrochemical impedance spectroscopy (EIS) plots provide complete information about the kinetics of complex processes or reactions occurring at the electrode–electrolyte interface (e.g., the corrosive environment in which the material under study is immersed) [37].

The EIS plots are presented both in the complex (Nyquist) plane, where the real part is represented on the abscissa and the imaginary part is represented on the ordinate, and in the (Bode) plane with double graphical illustration, where the logarithmic frequency part is represented on the abscissa and the logarithmic impedance modulus or phase angle is represented on the ordinate. The complex representation is often used in the literature because it allows for easy identification of the equivalent circuit elements that are used to fit the recorded experimental data and to determine the polarization resistance or specific resistance [37].

Thus, the Nyquist and Bode plots for untreated and electrochemically oxidized Zr2.5Nb samples were fitted using the equivalent electrical circuits shown in Figure 7.

Where R_e is the electrolyte resistance, R_p is the polarization resistance, CPE_p represents the constant phase element of the surfaces of the biomaterials under study, R_ox is the resistance of the electrochemically developed oxide, and CPE_ox is the constant phase of the electrochemically developed oxide [48,49,50].

Figure 8 shows the EIS evolution in Nyquist representation for both untreated and electrochemically oxidized Zr2.5Nb samples after immersion in artificial Ringer’s solution. The measurement was performed in two time steps: (a) t₁—5 h and (b) t₂—77 h.

From Figure 8a, it is observed that after 5 h of immersion, the untreated zirconium alloy (curve 1) has a polarization resistance value approximately 26 times lower compared to the electrochemically oxidized sample (curve 2). The untreated alloy, Zr2.5Nb, has a polarization resistance (R_p) of 0.56 MΩ·cm², a relatively low value, indicating a reduced protection against corrosion. This suggests that the untreated alloy does not form an effective protective oxide film and is more prone to electrochemical attack.

The Zr2.5Nb sample electrochemically oxidized in sulfuric acid (H₂SO₄) solution (Figure 8a, curve (2)) had the highest polarization resistance value, 14.78 MΩ·cm². This treatment produced an extremely efficient protective oxide film, offering superior corrosion protection compared to the untreated alloy. This increase in polarization resistance indicates better passivation and more efficient formation of a protective layer on the alloy surface, thus reducing the corrosion rate.

This phenomenon can be explained by the fact that sulfuric acid favors the formation of a denser and more adherent oxide layer, capable of limiting the transport of ions and electrons compared to the native oxide layer formed on the surface of the untreated alloy [47].

After 77 h of immersion (Figure 8b), significant variations in polarization resistance (R_p) are observed, which reflect changes in the corrosion behavior of the material, depending on the treatment applied.

From Figure 8b, it is observed that the polarization resistance for the untreated sample (R_p = 0.134 MΩ·cm²) is significantly lower than for the electrochemically modified sample, indicating a low protection against corrosion. This suggests that the untreated alloy does not form an efficient protective oxide film, which makes it more vulnerable to electrochemical attack in the tested environment (Ringer’s solution). In the case of the Zr2.5Nb sample oxidized in sulfuric acid (H₂SO₄) solution, the polarization resistance is the highest, reaching a value of 17.54 MΩ·cm². This denotes the formation of an extremely efficient oxide film, with much better protection against corrosion compared to the untreated alloy. Also, comparing the two immersion times studied, it can be stated that with increasing immersion time, the value of polarization resistance increases for both studied samples. However, the electrochemical treatment applied to the Zr2.5Nb alloy proved to be more effective, demonstrating significantly better protection than the unoxidized samples.

Figure 9 shows the electrochemical behavior of the Zr2.5Nb alloy in Ringer’s solution, highlighting the significant differences in corrosion resistance depending on the treatment applied. The untreated alloy (curve 1) presents the lowest impedance over the entire frequency range, indicating inadequate passivation and low corrosion protection. This behavior suggests that the protective layer formed under natural conditions is insufficient to limit the corrosive electrochemical processes.

The highest impedance value is recorded for the sample electrochemically oxidized in sulfuric acid solution (curve 2), indicating optimal passivation and maximum corrosion protection. The oxide layer formed in this environment appears to be denser and more adherent, which significantly limits charge transfer and corrosive processes. These results emphasize the critical influence of the oxidizing environment on the corrosion resistance properties of the Zr2.5Nb alloy, demonstrating that oxidation in sulfuric acid solution generates superior electrochemical protection.

The electrochemical analysis of the Zr2.5Nb alloy in Ringer’s solution is also performed using Bode diagrams, which show the variation of the phase angle as a function of frequency (Figure 10), for two distinct experimental conditions: the untreated alloy and the electrochemically oxidized alloy in sulfuric acid solution (H₂SO₄). These measurements allowed for the evaluation of the electrochemical behavior and corrosion protection of the material depending on the treatment applied. In the case of the untreated alloy (curve 1), the phase angle reaches a maximum value of approximately −70°. This value suggests relatively low protection of the alloy against corrosion, which indicates a less stable passivating film and less effective in preventing electrochemical degradation processes. The results obtained for the untreated sample reflect the active electrochemical behavior of the material, without significant protection offered by a passivating layer.

In the case of the alloy electrochemically oxidized in sulfuric acid solution (curve 2), the phase angle reaches a maximum value of approximately −85°, the highest value observed among the analyzed samples. This suggests the formation of an extremely stable and efficient oxide film, which offers the best protection against corrosion among the tested treatments. The result indicates that the electrochemical treatment in sulfuric acid generates a superior quality oxide layer, capable of significantly reducing the rate of corrosion processes. The analysis of the Bode diagram highlights an obvious trend in terms of improving the electrochemical protection of the Zr2.5Nb alloy depending on the treatments applied. Thus, the measured phase angle values suggest that the electrochemical oxidation treatments lead to an increase in the stability of the oxide film and, implicitly, to a more efficient protection against corrosion.

In contrast, the untreated alloy presented the lowest phase angle value, reflecting inadequate corrosion protection. These results highlight the importance of electrochemical treatment conditions in optimizing the corrosion protection performance of zirconium alloys for use as biomaterials and other industrial applications.

Table 1. The determined values of the representative equivalent electrical circuit for each type of surface analyzed, after experimental fitting of Zr2.5Nb samples, both untreated and electrochemically oxidized in 1 M H₂SO₄, after immersion in Ringer’s solution.

Equivalent Electrical Circuit Elements/Measuring Unit	Untreated Zr2.5Nb EIS1/EIS3	Oxidized Zr2.5Nb Eox 200 V–1 min 1 M H2SO4 EIS1/EIS3
Rs [Ω cm2]	46.92 / 48.59	66.53 / 59.37
CPE-P [F/cm2]	6.999 × 10−6 / 5.887 × 10−6	6.422 × 10−8 / 2.296 × 10−9
α	0.920 / 0.924	0.977 / 0.971
Rp [MΩ cm2]	0.5581 / 0.1342	16.82 × 10−6 / 3.375 × 10−6
CPE-Tox [F/cm2]	-	1.447 × 10−9 / 6.411 × 10−8
α	-	0.831 / 0.976
Rp [MΩ cm2]	-	14.7810 / 17.5420

presents the determined values of the representative equivalent electrical circuit for each type of surface analyzed, after experimental fitting of both untreated and electrochemically oxidized Zr2.5Nb samples in 1 M H2SO4 after immersion in Ringer’s solution.

From Table 1, it can be observed that the evolution of the α parameter over time (EIS₁/EIS₃) highlights the changes in uniformity and dielectric behavior of the passive film formed on the Zr2.5Nb alloy surface. For the untreated sample, the α values remain relatively low and nearly constant, indicating a heterogeneous surface characterized by a non-uniform distribution of defects and increased roughness. This behavior is typical of a thin and discontinuous natural oxide film, whose stability changes only slightly during immersion [38].

In the case of the sample electrochemically oxidized in 1 M H₂SO₄, the α values are higher compared to the untreated surface and remain close between EIS₁ and EIS₃, suggesting a behavior closer to that of an ideal capacitor. This indicates the formation of a dense, homogeneous, and time-stable passive layer with a uniform distribution of dielectric properties. The increase in α after oxidation can be associated with a reduction in surface roughness and an improvement in film continuity, leading to enhanced electrochemical protection [38].

The results obtained from the equivalent electrical circuit analysis demonstrate that the electrochemical treatments applied to the Zr2.5Nb alloy significantly influence its electrochemical behavior. The sulfuric acid treatment led to a significant increase in corrosion resistance and the formation of a more stable oxide film, while the untreated sample showed a more active electrochemical behavior, with a lower corrosion protection, compared to the electrochemically treated samples.

Figure 11 presents the comparative diagrams of the electrochemical impedance spectroscopy (EIS) in Nyquist representation for both untreated and electrochemically oxidized Zr2.5Nb samples after immersion in artificial Ringer’s solution with 40 g/L H₂O₂, to simulate infection environment. The measurements are also performed for the two times studied.

Comparing Figure 8 and Figure 11, it can be seen that samples immersed in Ringer’s solution with the addition of 40 g/L H₂O₂ showed lower R_p values compared to those immersed in Ringer’s solution without hydrogen peroxide, for both times studied.

Figure 12 and Figure 13 present the EIS diagrams in Bode format with the representation of the impedance modulus as a function of the logarithm of frequency and with the representation of the phase angle as a function of the logarithm of frequency for both untreated and electrochemically oxidized Zr2.5Nb samples after immersion in Ringer’s solution with 40 g/L H₂O₂. The measurements are performed for the two studied times.

Analyzing the diagrams in Figure 12, it can be seen that the impedance of the Zr2.5Nb alloy immersed in Ringer’s solution with the addition of 40 g/L H₂O₂ is lower compared to the impedance obtained for the electrochemically oxidized sample.

From Figure 13, it is observed that the highest value of the phase angle, approximately −80°, is recorded for the alloy electrochemically oxidized in sulfuric acid solution (curve 2), which reflects an improved capacitive behavior and an optimal passivation compared to the other studied surfaces. The values of the equivalent circuit elements used for the simulation of the experimental data are presented in

Table 2. The determined values of the representative equivalent electrical circuit for each type of surface analyzed, after experimental fitting of Zr2.5Nb samples, both untreated and electrochemically oxidized in 1 M H₂SO₄, after immersion in the solution composed of Ringer with 40 g/L H₂O₂.

Equivalent Electrical Circuit Elements/Measuring Unit	Untreated Zr2.5Nb EIS1/EIS3	Oxidized Zr2.5Nb Eox 200 V–1 min 1 M H2SO4 EIS1/EIS3
Rs [Ω cm2]	62.22 / 60.9	87.79 / 67.70
CPE-P [F/cm2]	7.466 × 10−6 / 6.451 × 10−6	6.189 × 10−8 / 7.984 × 10−8
α	0.952 / 0.952	0.998 / 0.998
Rp [MΩ cm2]	0.5257 / 0.1397	0.14625 / 0.02361
CPE-Tox [F/cm2]	-	3.366 × 10−8 / 6.943 × 10−8
α	-	0.735 / 0.450
Rp [MΩ cm2]	-	4.0304 / 6.7985

.

From Table 2, it can be observed that the evolution of the α parameter (EIS₁/EIS₃) for both the untreated and anodized Zr2.5Nb samples immersed in Ringer’s solution containing 40 g/L H₂O₂ reflects changes in the dielectric uniformity and stability of the passive films. For the untreated alloy, α values remain relatively low and almost constant (0.95), indicating a heterogeneous and defect-rich surface with a non-uniform oxide layer that shows limited protective capability under oxidative conditions.

In contrast, the anodized Zr2.5Nb sample initially exhibits higher α values (0.99), consistent with the formation of a compact and homogeneous anodic oxide layer. However, a slight decrease in α after prolonged immersion (EIS₃) suggests that exposure to the strongly oxidative environment containing H₂O₂ induces partial modification or degradation of the oxide film. This decrease indicates a minor increase in surface heterogeneity, likely due to localized dissolution or structural rearrangement within the anodic oxide film.

Overall, although the anodized surface maintains a higher α compared to the untreated alloy, the observed reduction over time highlights the sensitivity of the passive film to aggressive oxidative conditions, suggesting that hydrogen peroxide can gradually affect its dielectric homogeneity and long-term stability.

The electrochemically oxidized sample exhibits higher R_p values compared to the untreated sample, indicating greater stability in the oxidative environment.

The passivation layer capacity decreases significantly for the samples oxidized in H₂SO₄, suggesting the formation of a denser and less porous protective layer compared to the untreated sample.

The electrochemically oxidized samples have significantly higher polarization resistance values than the untreated sample, suggesting improved corrosion protection.

These values suggest that electrochemical oxidation treatments in sulfuric acid solutions contribute to the formation of superior protective layers on the surface of the Zr2.5Nb alloy, thus optimizing its corrosion resistance in solutions similar to the physiological environment.

To highlight the evolution of the polarization resistance (R_p) obtained from EIS measurements for the Zr2.5Nb alloy, Figure 14 and Figure 15 present bar graphs, in which the R_p values are illustrated for the Zr2.5Nb alloys both in the untreated state and after anodic oxidation in sulfuric acid solution, immersed in Ringer’s solution and Ringer’s solution with 40 g/L H₂O₂ at both evaluated measurement times.

From Figure 14, we can see that the polarization resistance increases with the application of anodic oxidation treatment compared to the untreated sample. It is also observed that increasing the immersion time of the samples leads to an increase in R_p values.

From Figure 15, we can see that the physio-pathological environment composed of Ringer’s solution with 40 g/L H₂O₂ significantly influences all the surfaces studied.

Comparing Figure 14 and Figure 15, the most sensitive surfaces to Ringer’s solution with 40 g/L H₂O₂ proved to be in the case of electrochemically oxidized samples, but nevertheless they retained superior anti-corrosion properties to untreated samples.

In conclusion, the formation of a protective layer on the surface of a bio-alloy can reduce the corrosion rate of the implanted material in human pathophysiological environments frequently encountered post-operatively.

3. Materials and Methods

3.1. Zr2.5Nb Alloy

For this research, the Zr2.5Nb alloy is used, which is supplied by Evek GmbH, Mülheim an der Ruhr, Germany, in the form of a plate with initial dimensions of 500 mm × 500 mm × 1.5 mm. For the experiments, small plates cut to dimensions of 25 mm × 25 mm × 1.5 mm were used. The chemical composition of the Zr2.5Nb alloy is presented in

Table 3. Chemical composition of Zr2.5Nb alloy [wt.%].

Be	Hf	Ni	Cr	Ti	Al	O	Pb	Nb	Zr
0.003	0.01	0.02	0.02	0.007	0.008	0.06–0.1	0.005	2.4–2.7	Rest

, and the mechanical properties of the material are detailed in

Table 4. Mechanical properties at 20 °C of the Zr2.5Nb alloy.

Alloy	Modulus of Elasticity (Young) [GPa]	Breaking Strength [MPa]	Elongation δ [%]	Hardness HB [kgf/mm2]
Zr2.5Nb	95	569	28	64–67

.

After cutting, a chemical degreasing step is applied with a 50 g/L NaOH solution, followed by a chemical pickling step in a dilute hydrochloric acid solution (HCl 1:1), both of which have the role of removing organic impurities and superficial oxidations. These treatments allow for the complete elimination of residual particles and traces of grease or contaminants from the surface of the samples. Next, the samples are rinsed with distilled water to eliminate traces of degreasers.

3.2. Electrochemical Oxidation Process

Electrochemical oxidation is performed using a TDK Lambda Gen300-8 (TDK-Lambda Corporation, Tokyo, Japan) electrochemical device in a classic electrochemical cell composed of two electrodes. Thus, the anode is the sample to be oxidized, and the cathode is a platinum–rhodium mesh, being inert in the oxidation solution. The cell also includes a cooling system to maintain the temperature of the acidic solution (200 mL of 1 M H₂SO₄) constant throughout the process. This configuration allows for precise control of the electrochemical oxidation process, favoring the formation of uniform nanostructures on the surface of the investigated alloy. Thus, once the circuit is closed, redox processes are initiated in the electrochemical anodization cell, using electrolytes such as sulfuric acid. The oxidation of the superficial layer, which functions as the anode, determines the release of metal cations, while reduction takes place at the cathode. Subsequently, the metal cations generated at the anode react with oxygen in the water, resulting in the formation of a uniform oxide layer on the metal surface. During the electrochemical oxidation process, an exposed surface area of 6.25 cm² of the sample was used as the anode, while the remaining surface was insulated to ensure a uniform oxide growth over the active area.

For the subsequent corrosion measurements, the exposed area was reduced to 5 cm² by additionally insulating part of the sample edges. This was done to minimize the influence of edge effects and surface irregularities, as the borders of the specimens tend to be rougher and may interfere with the accuracy and reproducibility of the electrochemical measurements.

Several experimental parameters were varied in the electrochemical oxidation process, including electrolytes, different values of the applied voltage (200 V, 250 V and 275 V) and different treatment times. As a result of the experiments, it was found that the most effective parameters were obtained in sulfuric acid solution, at a voltage of 200 V and an oxidation time of 1 min, conditions in which the most uniform oxide layers with the size of the pores formed in the nanometric range were formed. All electrochemical oxidation tests were performed at room temperature, 22 ± 1 °C, and were repeated 5 times to verify the reproducibility of the data.

3.3. Description of the Corrosion Test Method

The corrosion resistance of the samples is studied on a PGZ301 electrochemical apparatus with integrated VoltaMaster4 version 7.10 software, in an electrochemical cell composed of three electrodes (the working electrode is the surface to be studied, the working counter electrode (a Pt-Rh network) and a reference electrode (Ag/AgCl). The volume of electrolyte used was 150 mL simplified Ringer’s solution, and simplified Ringer’s solution with the addition of 40 g/L H₂O₂, which was used to simulate a highly oxidative environment associated with inflammation processes in the human body. The chemical composition and physico-chemical parameters of simplified Ringer’s solution are shown in

Table 5. Chemical composition and physico-chemical parameters of simplified Ringer’s solution and simplified Ringer’s solution with inflammatory compound.

Nr. Crt.	Chemical Compound	Solution Name Simplified Ringer [g/L]	Simplified Ringer + 40 g/L H2O2
1	NaCl	8.402	8.402
2	KCl	0.302	0.302
3	CaCl2	0.298	0.298
4	H2O (distilled water)	Rest	Rest
5	pH	6.67	5.85
6	Conductivity [mS/cm]	14.4	12.3
7	Salinity [ppt]	8.4	7.1

. The characterization of the solutions in terms of pH, electrical conductivity and salinity is performed using a sensiIONTM + MM374 electrochemical multimeter (Hach, Loveland, CO, USA).

The electrochemical reagents used for the preparation of simplified Ringer’s solution were purchased from Sigma-Aldrich (Darmstadt, Germany). The 30% hydrogen peroxide used for the preparation of the electrolyte with properties similar to the inflammatory medium was also purchased from the same company.

In corrosion tests, the use of simplified Ringer’s solution and hydrogen peroxide (H₂O₂) is motivated by the need to simulate the real conditions of the human internal environment in which metallic implants are to be placed. Simplified Ringer’s solution, with an ionic composition close to that of blood plasma, is frequently used as a reference medium for evaluating the stability of materials under physiological conditions. It provides a buffered environment, with a neutral pH thus reflecting the normal state of the body, in the absence of inflammatory processes or other postoperative complications.

On the other hand, hydrogen peroxide (H₂O₂) is used to reproduce the inflammatory environment characteristic of the initial post-implantation stage. This is naturally generated by the body’s immune system, especially by leukocytes (neutrophils, macrophages), through the so-called oxidative burst, in which reactive oxygen species (ROS), including H₂O₂, are released. Thus, the addition of hydrogen peroxide to simplified Ringer’s solution allows for the investigation of the electrochemical behavior of the material in a pronounced oxidative environment, similar to that associated with inflammation and cellular oxidative stress.

Therefore, the choice of the two media, simplified Ringer’s solution and the solution with the addition of H₂O₂, offers a complex and relevant experimental approach, allowing for the evaluation of the corrosion resistance of the surfaces obtained both under normal physiological conditions and in the context of the challenges imposed by the pathological inflammatory environment.

The electrochemical experimental protocol applied for the study of electrochemical corrosion consisted of the application of two electrochemical methods: open circuit potential evolution (OCP) and electrochemical impedance spectroscopy (EIS) at a free potential, in alternating current with amplitude AC = 10 mV, at a variable frequency from 100 kHz to 10 mHz, where data recording is performed every 20 s. For fitting the EIS experimental data, Z_View 3.4 software is used, the chi-square value obtained from fitting the experimental data being less than 10⁻³.

The experiments were carried out at a controlled temperature of 37 ± 1.5 °C, corresponding to the physiological temperature of the human body, using a thermostated system to maintain constant solution temperature during the tests.

3.4. Morphological and Compositional Characterization (SEM–EDX)

The surface morphological and compositional analysis of the Zr2.5Nb alloy and the obtained oxide films were performed using the FEI QUANTA 200 SEM (FEI Company, Hillsboro, OH, USA) scanning electron microscope (SEM) and the energy dispersive X-ray analyzer (EDX) coupled to the EDAX Genesis data acquisition 5.10 software.

3.5. Structural Analysis with X-Ray Diffractometer

X-ray diffraction (XRD) measurements were performed on a Dron-3 equipment using Co Kα (λ = 1.790300 Å). The X-ray diffractometer was operated at a voltage of 30 kV and a current of 20 mA, with a step of 0.05°/s, an exposure of 3 s and a total time/sample of 2 h and 13 min, in a range of 2θ = 15–90°. The obtained diffraction patterns were analyzed using the Match! 3.16 software, and the reference database library used was the Crystallography Open Database (COD).

4. Conclusions

The results of this study demonstrate the superiority of the electrochemical oxidation treatment compared to the untreated Zr2.5Nb alloy in forming oxide layers with enhanced anti-corrosion properties. The treatment performed in 1 M H₂SO₄ at 200 V for 1 min proved to be the most effective condition for obtaining a homogeneous oxide film with nanometric porosity, which significantly improves the protective behavior of the alloy in simulated physiological and inflammatory environments.

SEM–EDX analysis revealed that electrochemical oxidation under these conditions promotes the formation of uniformly distributed nanopores and leads to a noticeable increase in oxygen content, confirming effective surface oxidation compared to the untreated sample. XRD analysis indicated that electrochemical oxidation induces a structural transformation from the monoclinic baddeleyite (ZrO₂) phase, characteristic of the untreated alloy, to the more stable tetragonal arkelite (ZrO₂) phase. This transformation contributes to the improved corrosion resistance of the treated material through the formation of a denser and more stable oxide layer.

Electrochemical investigations confirmed that anodically oxidized samples exhibit more positive open circuit potential values and significantly higher polarization resistance compared to untreated ones. These findings demonstrate that the electrochemical oxidation process produces a compact and adherent oxide layer that effectively limits electrochemical reactions at the metal–electrolyte interface.

Even though the addition of hydrogen peroxide (H₂O₂) simulating an inflammatory environment negatively affected the overall corrosion behavior, the oxidized samples maintained superior stability and higher resistance compared to untreated ones. This observation suggests that the oxide film generated by the electrochemical oxidation process offers long term protection, even under oxidative stress conditions.

In conclusion, anodic oxidation in sulfuric acid at 200 V for 1 min represents an optimal surface treatment for biomedical applications, promoting the development of durable, corrosion-resistant oxide layers that can extend implant longevity under physiological and oxidative conditions relevant to the in vivo environment.