Electropolishing of Ti-6Al-4V ELI Alloy: Surface Roughness and Wettability Characteristics

Abstract

This study evaluates electropolishing as a surface preparation method for the Ti-6Al-4V ELI alloy for implant applications, aiming to reduce surface roughness and improve wettability behavior. Alloy samples were subjected to electropolishing in a solution of acetic acid, perchloric acid, and glycerol, under different voltages (13–18 V) and times (5, 10, and 15 min). The surfaces were characterized by scanning electron microscopy, profilometry, and contact angle measurements. Higher voltages and longer anodizing times favored roughness reduction. The average Ra, calculated from samples analyzed in triplicate, was 70.207 nm, with a standard deviation of 19.260 nm, obtained under the condition of 18 V for 15 minutes. The largest contact angles, 112.62° and 110.74°, were observed at 14 V for 10 and 15 minutes, respectively, suggesting that factors beyond surface topography influence wettability. It was concluded that electropolishing is an effective method for preparing the surface of Ti-6Al-4V ELI, with potential for implant applications, and that the parameters should be adjusted according to the desired performance (surface finish, hydrophobicity, or hydrophilicity).

Keywords:
Ti-6Al-4V ELI; Electropolishing; Roughness; Surface wettability

1. Introduction

Ti-based implants are widely used in biomedical applications due to their excellent mechanical strength and biocompatibility. However, the surface bioinertia of these materials hinders effective integration with adjacent bone tissues. This limitation compromises osseointegration, defined as the direct structural and functional connection between the implant surface and the surrounding tissues, which may negatively affect the long-term durability and clinical success of implants. To overcome this challenge, implant surface modification becomes essential, since protein adsorption, followed by cell adhesion, proliferation, and calcification, is strongly influenced by surface properties, particularly topography and wettability¹,¹,³.

Although widely employed, the Ti-6Al-4V ELI alloy faces challenges such as aseptic loosening, infection, and insufficient osseointegration, which compromise its long-term performance. Surface roughness and composition influence bacterial adhesion and biofilm formation, which are critical factors for implant success⁴,⁵.

The control of surface roughness (Ra) and surface wettability through the contact angle (CA) is essential for biomedical applications. Wettability, determined from the contact angle, classifies the surface as hydrophilic (10° < Ө < 90°), hydrophobic (90° < Ө < 150°), superhydrophilic (Ө < 10°), or superhydrophobic (Ө > 150°). Hydrophilic surfaces minimize bacterial adhesion⁵,⁶.

Electropolishing is as an effective technique to reduce surface roughness, providing a smooth and bright finish through the controlled anodic dissolution of the metal in an acidic electrolyte bath, without altering the crystallographic contour⁷. This process improves surface quality, contributing to enhanced resistance to bacterial adhesion and improved osseointegration.

The electropolishing process involves immersing the sample in an electrolyte bath under temperature control, where it serves as the anode connected to the positive polarity of a power supply, while the negative polarity is connected to the cathode. The current flows from the anode, where the surface metal is dissolved into the electrolyte, toward the cathode. On the cathode surface, a reduction reaction occurs, producing hydrogen. The electrolyte typically consists of a concentrated acidic medium, often based on phosphoric acid, sulfuric acid, or a perchloric acid-acetic acid solution⁸.

Electropolishing is widely applied in the metal finishing industry, biomedical and semiconductor applications, pharmaceutical facilities, and niobium superconducting cavities. Electropolished titanium alloys are stress-free and free of inclusions, most of which exhibit a microscopically smooth or even mirror-like surface, ensuring enhanced corrosion resistance, surfaces free of microorganisms, and potential improvements in mechanical properties. Compared to conventional polishing, electropolishing is more effective, as it might not impair fatigue performance while reducing surface roughness⁷,⁹. This is particularly important for load-bearing implants, as it has been widely studied that surface discontinuities can act as stress concentrators and shorten fatigue life under cyclic loading¹⁰,¹¹.

The stages of the electropolishing process (reaction, descaling, smoothing, and leveling) as applied to the Ti-6Al-4V alloy are presented in Figure 1.

Figure 1
Electropolishing mechanism of the Ti-6Al-4V alloy, adapted from ⁹.

The literature on electropolishing and surface treatment of Ti-6Al-4V presents relevant advances but still reveals important gaps, particularly regarding the understanding of how electropolishing parameters affect functional surface properties; the elucidation of mechanisms linking the resulting topography to reduced bacterial adhesion; the influence of additive manufacturing parameters on the electrochemical response and surface behavior after treatment; the impact of environmentally sustainable electropolishing on surface integrity and structural reliability; the lack of consolidated parameters for electrolyte-free processes; and the need for deeper investigation into the stability of the oxide layer formed and its biological interactions In this context, investigations into the electropolishing of Ti-6Al-4V ELI are essential for enhancing surface quality, improving biological performance, and establishing more robust guidelines for biomedical applications, thereby positioning electropolishing as an innovative and promising approach for the development of high-performance metallic implants⁸,¹²,¹³.

In light of the above, the present study aims to validate electropolishing as a surface preparation method for the Ti-6Al-4V ELI alloy in implant applications, with the objective of reducing surface roughness and obtaining surfaces that minimize bacterial adhesion. The experimental procedure employed electrolytes specifically formulated for the alloy under investigation and was conducted within the polishing plateau. Among the characterization techniques used, optical profilometry and contact-angle measurements were considered essential, as they classify the surface and identify the condition most conducive to osseointegration and reduced bacterial adhesion—requirements that are critical for biomedical applications.

2. Materials and Methods

The Ti-6Al-4V ELI alloy, recommended for surgical implant applications according to ASTM F136¹⁴, was adopted for this study. The samples were sectioned using a Struers Minitom cutting machine at 200 rpm, resulting in disks with a diameter of 14 mm and a thickness of 3 mm. Subsequently, the samples were cleaned in an Instruterm ultrasonic bath (LU-200-220) using acetone and deionized water for 5 minutes each, followed by drying with ethyl alcohol and cold air.

Following the ASTM E3 standard¹⁵, the samples were hot-mounted using an Arotec PRE-30Mi mounting press and polished on an Arotec Aropol VV-PU polishing machine at a rotational speed of 300 rpm, employing silicon carbide abrasive papers with grit sizes #420, #600, and #1200. Manual grinding was carried out under light and consistent pressure, with the grinding direction rotated by 90° for each subsequent abrasive paper until the scratches from the previous grit were completely removed. After grinding, the samples were cleaned again and then subjected to electropolishing.

Electropolishing was performed using a Buehler Electromet – Polishing & Etching system with an electrolyte solution composed of acetic acid (96%), perchloric acid (26.4%), and glycerol (6%). The process was carried out under constant voltage between 13 and 18 V for durations of 5, 10, and 15 minutes, using a stainless-steel cathode. The current density (A/cm²) and temperature parameters (°C) were displayed and controlled by the equipment. The current density ranged from 0.1 to 0.3 A/cm², and the temperature from 22 to 24 °C. After electropolishing, the samples were cleaned ultrasonically for 5 minutes in acetone and deionized water.

Surface roughness and topography were evaluated using an optical profilometer (WYKO NT1100) with Vision software. The center of each sample was used as the reference point for Ra measurements. Scanning Electron Microscopy (SEM), in a FEI Inspect S50 operating in secondary electron mode, was used to analyze the surface topography. In addition, contact angle measurements were performed to evaluate the wettability of the surfaces using a SEO Phoenix drop shape analysis system under ambient conditions of 23°C. For the procedure, deionized water with a volume of 3 μL and a needle with an internal diameter of 0.30 mm, supplied with a syringe, were used.

3. Results and Discussion

Figure 2 shows the micrograph of the Ti-6Al-4V ELI alloy surface after the manual grinding step. The scratches caused by the sandpaper are clearly visible. The surface exhibits well-defined and oriented parallel grooves, which indicate a relatively uniform material removal and a still elevated roughness level, characteristic of the initial stages of metallographic preparation. Small debris and adhered particles are observed between these grooves, a condition frequently reported prior to the fine polishing and ultrasonic cleaning steps. This morphological pattern is consistent with previous studies describing the abrasive wear mechanisms and the formation of oriented scratch marks in titanium alloys subjected to mechanical grinding¹⁶,¹⁷.

Figure 2
SEM micrograph of the Ti-6Al-4V ELI alloy after manual grinding.

Before starting electropolishing, the current density (mA/mm²) as a function of voltage (V) was obtained, as shown in Figure 3, with the aim of identifying the characteristic plateau region of electrolytic polishing. A solution composed of acetic acid, perchloric acid, and glycerol was used, as recommended by the Standard Guide for Electrolytic Polishing of Metallographic Specimens¹⁶ and widely adopted by several authors to determine the electropolishing curve. Electropolishing times were set at 5, 10, and 15 minutes for all samples, with the applied voltage varying within the range defined by the curve (13–18 V).

Figure 3
Current density curve as a function of applied voltage for the Ti-6Al-4V ELI alloy.

The curve shows three distinct regions: the chemical attack region, observed around 12 V; the electropolishing plateau, between 13 and 18 V; and a transition region toward localized corrosion (pitting) beyond the plateau. The passivation region is not clearly defined in the curve, which may be attributed to the low current density at the beginning of the plateau (approximately 0.2 A/cm²). In the passivation zone, a passive oxide layer forms on the anodic surface, reducing current density as the voltage increases. The experimental curve obtained in this study aligns with the classical behavior described in the literature⁷, accurately reflecting the typical electrochemical steps of the Ti-6Al-4V ELI electropolishing process.

The micrographs of the surfaces illustrate the electropolishing process: reaction, descaling, and surface leveling, as presented in Figure 4. With time kept constant, for any of the three durations analyzed, an increase in voltage tends to produce smoother finishes, particularly at voltages above 17 V. An exception occurs at 10 minutes, where the surface treated at 18 V exhibits poorer surface finish. Conversely, when the voltage is fixed and the electropolishing time is increased, the surface morphology remains almost unchanged for voltages between 13 and 16 V, with more significant morphological changes occurring only at higher voltages.

Figure 4
SEM micrographs of the samples subjected to different electropolishing conditions. Each horizontal row represents a processing time, while each vertical column corresponds to the applied voltage. The values indicated in the upper left corner of each micrograph refer to the average roughness (Ra) measured for each experimental condition.

Microscopy was employed in the study published by Zhang et al.⁹ to analyze the surface morphology of the Ti-6Al-4V ELI alloy after electropolishing. It was possible to identify the stages of the electropolishing process, including reaction and flaking, reaction sites, and surface smoothing. These findings corroborate the data presented in Figure 4⁹.

The greater reduction in roughness observed in the samples electropolished at higher voltages (17–18 V) is directly related to the electrochemical mechanisms’ characteristic of the electropolishing regime. At higher potentials, the anodic surface of the Ti-6Al-4V ELI alloy attains the appropriate conditions for the formation of a viscous layer rich in dissolved species, whose behavior is governed by diffusion-limited mass transport. In this regime, dissolution occurs preferentially at surface asperities due to the higher local electrochemical field intensity, promoting micro-geometric leveling and a pronounced reduction in roughness. A reduction in and rapid repassivation of the TiO₂ film also occurs, enabling continuous and more homogeneous surface dissolution. The combined phenomena of selective dissolution, diffusional control, and mitigated repassivation explain why voltages in the 17–18 V range yield smoother and more uniformly leveled surfaces, as confirmed by the SEM micrographs and the reduced roughness values obtained in the present study⁹,¹⁷-¹⁹.

Figure 5 presents representative optical profilometry maps of the control surface and two electropolished conditions, highlighting the clear contrast in topographic responses. The sample electropolished at 18 V for 15 minutes exhibited the lowest Ra value among all conditions, revealing a uniform surface with attenuated topography characterized by a narrow height distribution dominated by low-amplitude features. This visual homogeneity is consistent with the transition to a stable electropolishing regime, in which the selective dissolution of asperities and diffusion-limited mass transport promote effective micro-leveling. In contrast, the surface treated at 15 V for 5 minutes despite undergoing electropolishing shows a broader and more heterogeneous topographic profile, with pronounced peaks and valleys that exceed even those observed in the manually ground control sample. This behavior aligns with the incomplete establishment of the viscous anodic layer, resulting in non-uniform dissolution and, consequently, the highest roughness values recorded (250 nm, compared to 160 nm for the control). Taken together, these results demonstrate that only at sufficiently high voltages does the Ti-6Al-4V ELI alloy enter the regime required for consistent surface leveling, leading to substantial roughness reduction and enhanced morphological uniformity¹⁷,¹⁹,²⁰.

Figure 5
Optical profilometry results for three samples: on the left, a control sample subjected only to manual grinding; in the center, a sample electropolished at 15 V for 5 minutes; and on the right, a sample electropolished at 18 V for 15 minutes.

The graphs in Figure 6 show the temperature and current density data obtained during electropolishing at different voltages for 5 minutes, as well as the corresponding average roughness and contact angle values.

Figure 6
Data obtained during the electropolishing process carried out for 5 minutes as a function of applied voltage: on the left, temperature and current density; on the right, average roughness (Ra) and contact angle.

Over the last 5 minutes, a consistent trend of decreasing roughness with increasing applied voltage was observed, a behavior directly associated with the increase in experimental current density. The lowest average Ra (70.207 nm) was obtained at 18 V, where the current density reached its maximum, promoting more efficient anodic dissolution and, consequently, greater surface leveling. In contrast, the highest roughness (231.477 nm) was recorded at 13 V, with a current density (~0.2 A/cm²), resulting in insufficient and irregular dissolution. The temperature remained essentially constant across all applied voltages, indicating a negligible influence on the electropolishing process. The contact angle of the surfaces between 14 and 18 V was 10° < θ < 90°, classified as hydrophilic, except for 13 V, where the observed value was 107.9°, considered hydrophobic.

Yan et al.¹⁸ showed, for the Ti-6Al-4V alloy, that contact angles with polar liquids, such as water, exhibited a clear correlation with roughness: rougher surfaces had higher advancing angles and greater hysteresis, indicating lower wettability, likely due to air entrapment in the irregularities (Cassie-Baxter model). Conversely, for nonpolar liquids such as diiodomethane, contact angles varied little with roughness, reflecting the insensitivity of London dispersion interactions to surface topography. These observations highlight that surface wettability is governed by a combination of physical factors (surface roughness) and chemical factors (surface composition)²¹.

Leite et al.¹⁹ found only contact angles above 90° for commercially pure Ti subjected to a double acid etch, classifying the surfaces as hydrophobic. The authors emphasized a dependence between roughness parameters and contact angle but found no linear relationship. In the present study, considering the 5-minute electropolishing time, no clear and direct correlation between surface roughness and wettability (as represented by the contact angle) was observed²².

The high electropolishing efficiency observed even at short processing times, particularly under elevated voltages, can be attributed to the rapid establishment of the diffusional regime characteristic of anodic dissolution, enabled by the substantially increased current density. In this regime, a viscous electrolyte layer enriched with dissolved metallic species is rapidly formed at the metal electrolyte interface, effectively moderating the repassivation kinetics of the TiO₂ passive film and promoting the preferential dissolution of surface asperities. This combination of suppressed repassivation and diffusion-controlled mass transport accelerates microtopographical leveling, allowing significant reductions in surface roughness to be achieved within only a few minutes of processing. Conversely, at lower voltages where the current density is insufficient to establish or sustain the viscous layer, the process becomes dominated by rapid formation of a passive oxide film, resulting in non-uniform dissolution and more irregular surfaces. These findings demonstrate that, for the Ti-6Al-4V ELI alloy, electropolishing performance is governed primarily by the magnitude of the applied current density rather than by the duration of treatment, reinforcing the central role of electrochemical kinetics in achieving effective surface smoothing⁹,¹⁷.

For the electropolishing performed for 10 minutes, the corresponding graphs are shown in Figure 7. The lowest roughness was obtained at 17 V, reaching an average of 116.787 nm, a value lower than all those obtained at 5 minutes. In contrast, the voltage of 18 V, during the same period, produced the highest Ra value in the group (182.573 nm), despite the higher current density (0.3 A/cm²), suggesting surface degradation associated with excessive energy input. The largest contact angle in this set was recorded at 14 V, reaching 112.62°, and classified as hydrophobic. Hydrophilic surfaces were observed for all other contact angle values. The results indicate that the lowest roughness values were obtained at 17 V due to the stable establishment of the diffusional regime of anodic dissolution, in which the formation of a viscous layer at the metal–electrolyte interface suppresses TiO₂ repassivation and promotes microtopographical leveling¹⁷,²⁰. At 18 V, however, the excess energy induced electrochemical instability and surface degradation, even under higher current density, a behavior consistent with previous reports of localized attack in titanium alloys. These results further confirm that electropolishing efficiency is governed primarily by current density rather than processing time alone. Moreover, the higher contact angles observed at moderate voltages suggest that wettability is more strongly associated with the characteristics of the TiO₂ passive film than with surface roughness²³.

Figure 7
Data obtained during the electropolishing process performed for 10 minutes as a function of applied voltage: on the left, temperature and current density; on the right, average roughness (Ra) and contact angle.

In the 15-minute electropolishing group, as shown in Figure 8, the best overall surface finish was obtained, with a minimum roughness of 70.207 nm, also at 18 V, with a current density of 0.3 A/cm² and a contact angle of 76.4°, classifying the surface as hydrophilic. The results obtained are consistent with the findings of Zhang et al.⁹. When the current density reaches sufficiently high levels, the system remains stably within the diffusional regime of anodic dissolution, enabling efficient and continuous surface leveling. Current density is therefore more decisive than processing time alone. The highest contact angle for this duration was 110.74°, again at 14 V, with a relatively low Ra (76.927 nm). The consistency of this behavior across the three evaluated durations suggests that moderate voltages favor hydrophobicity, even without producing the lowest roughness values.

Figure 8
Data obtained during the electropolishing process performed for 15 minutes as a function of applied voltage: on the left, temperature and current density; on the right, average roughness (Ra) and contact angle.

Overall, the data indicate a trend of decreasing roughness with increasing voltage, particularly when combined with higher current densities, and this trend is more pronounced at longer durations. The 18 V condition, especially with 15 minutes of electropolishing, produced the smoothest surface. Conversely, the highest contact angles were obtained at intermediate voltages (13–14 V), suggesting that, under certain regimes, rougher topography can contribute to increased hydrophobicity, consistent with the Cassie-Baxter models²⁴.

According to Dubiel et al.²², surfaces cannot be classified solely as hydrophilic or hydrophobic to characterize their interaction with the biological environment, nor to predict implant osseointegration. Although contact angle is widely used to assess wettability of solid surfaces in the presence of a gaseous phase, this condition does not accurately represent environments encountered in cell cultures or within the human body. Furthermore, a balance between wettability and surface finish must be considered, as high roughness is known to adversely affect mechanical fatigue behavior. Silva et al.²³ identified an Rz roughness of 2 μm as a threshold for reduced fatigue strength in Ti-6Al-4V²⁵,²⁶.

Mechanical, chemical, and electrolytic polishing of Ti-6Al-4V alloy were investigated with the aim of reducing dental implant infection caused by bacterial accumulation on the surface. The most favorable outcomes were achieved with electropolishing, which resulted in a surface roughness of 10 nm, a contact angle of 92°, a surface oxygen content of 54 at.%, and demonstrated bacterial inhibition. Surface roughness affects the extent of bacterial adhesion⁸.

A study by Chatpaiboonwat et al.²⁴ on the electropolishing of Ti-6Al-4V ELI alloys fabricated by laser powder bed fusion (L-PBF) for dental applications showed that surface roughness was significantly reduced at an applied potential of 15 V with a current density of 0.3 A/cm², decreasing from 6.28 ± 0.65 µm to 2.12 ± 0.33 µm, along with an improvement in corrosion resistance²⁷.

Acquesta et al.²⁵ investigated the effect of electropolishing treatment using an environmentally friendly electrolyte solution and an applied potential of 25 V—on the fatigue properties of Ti-6Al-4V specimens produced by electron beam melting (EBM). After 15 minutes of electropolishing, surface roughness was reduced by 50%, and after 60 minutes, the reduction reached approximately 64%, accompanied by a mass loss of 4.2%. Fatigue analysis demonstrated that the mean fatigue life of the electropolished samples was 250% higher than that of the as-built samples under the same stress conditions (Sₘ_ax = 378 MPa; R = 0.1). Environmentally friendly electropolishing therefore appears to be a promising surface-enhancement technique, particularly for additively manufactured components.

Dry electropolishing was investigated by Bezerra et al.²⁶ on the Ti-6Al-4V ELI alloy for biomedical applications. A polishing plateau between 13 and 18 V for 30 minutes was adopted. The ANOVA statistical test confirmed a significant reduction in Ra from 193 ± 18 nm to 143 ± 24 nm at 17 V. After electropolishing, hydrophilic surfaces were obtained, with contact angles of 77 ± 6° at 16 V and 59 ± 4° for the control. Dry electropolishing is essential for biomedical applications, as it promotes osseointegration and reduces bacterial adhesion¹⁴.

Swain et al.²⁷ showed that optimizing cutting parameters in the dry turning of Ti-6Al-4V alloy cutting speed, feed rate, and depth of cut significantly improves machinability by reducing surface roughness, tool wear, and vibration. The combined application of the Taguchi method and Principal Component Analysis enabled the identification of efficient and sustainable conditions, reinforcing the feasibility of coolant-free machining processes. Although focused on machining, the study presents a conceptual connection to electropolishing, since surfaces machined under optimized parameters become more homogeneous and better suited for subsequent electrochemical treatment. Electropolishing, in turn, removes microdefects and enhances final surface integrity. Thus, the integration of optimized machining and electropolishing constitutes a complementary and effective strategy for producing Ti-6Al-4V ELI components with superior surface quality²⁷.

The results of the present study indicate that, for applications requiring the best possible surface finish quality, the optimal condition is electropolishing for 15 minutes at 18 V, yielding a contact angle of 76.4° and a hydrophilic surface. For applications prioritizing surface hydrophobicity, voltages between 13 and 14 V with durations of 10–15 minutes are recommended, even if this results in slightly higher surface roughness. It should be noted that contact angle behavior is multifactorial and not solely dependent on roughness, being also influenced by factors such as surface composition and surface free energy.

4. Limitations

The electropolishing of Ti-6Al-4V ELI alloy presents several limitations. The efficiency of the electropolishing process requires high and carefully controlled voltages because the passive oxide layer (TiO₂) hinders homogeneous anodic dissolution. Strict control of voltage or current density parameters is necessary to prevent pitting and localized corrosion. Variations in electrolyte temperature or agitation rate can alter the viscosity of the viscous layer and the dissolution rate, leading to poor reproducibility. Excessive polishing may modify the microstructure and the geometry of the component. When traditional acid-based electrolytes are used, there are significant chemical risks, requiring appropriate biosafety procedures for handling.

5. Conclusions

Electropolishing of the Ti-6Al-4V ELI alloy in a solution of perchloric acid, acetic acid, and glycerol yielded significant results. The best surface finish was achieved with electropolishing at 18 V for 15 minutes, reaching an average roughness (Ra) of 70.207 nm, which yielded a contact angle of 76.4°, characteristic of a hydrophilic surface. It was observed that higher voltages, combined with longer durations, promoted more effective surface smoothing. Conversely, the highest hydrophobicity was recorded at intermediate voltages, between 13 and 14 V, regardless of roughness reduction. Moreover, no direct correlation was observed between surface roughness and contact angle under any of the conditions analyzed.

Optical profilometry was fundamental for quantifying the reduction in surface roughness during the electropolishing of Ti-6Al-4V ELI, while SEM analysis revealed the morphological evolution of the surface throughout the process. Together, these techniques confirmed the effectiveness of electropolishing in producing a smoother and more homogeneous surface.

Future research should focus on the use of environmentally friendly electrolytes, the advancement of dry electropolishing, the development of hybrid polishing methods, and the implementation of advanced process-control strategies to enhance efficiency and reproducibility. Furthermore, optimizing the treatment of complex surfaces and internal structures typical of additively manufactured components remains a significant challenge.

6. Acknowledgments

The authors gratefully acknowledge financial support from the National Council for Scientific and Technological Development (CNPq), the Coordination for the Improvement of Higher Education Personnel (CAPES), the São Paulo Research Foundation (FAPESP), and the Financing Agency for Studies and Projects (FINEP). Special thanks are extended to Polyana Alves Radi Gonçalves for her assistance with profilometry analyses.

• Data Availability
  All the data supporting the findings of this study are provided within the article itself.

7. References

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Edited by

• Associate Editor:
  Ana Sofia de Oliveira.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

All the data supporting the findings of this study are provided within the article itself.

Publication Dates

• Publication in this collection
  16 Feb 2026
• Date of issue
  2026

History

• Received
  29 Aug 2025
• Reviewed
  27 Nov 2025
• Accepted
  11 Jan 2026