Electrochemical anodization of cast titanium alloys in oxalic acid for biomedical applications

Abstract

Titanium and its alloys have numerous biomedical applications thanks to the composition and morphology of their oxide film. In this study, the colorful oxide films were formed by anodizing cast Ti-6Al-4V and Ti-6Al-7Nb alloys in a 10% oxalic acid solution for 30 s at different voltages (20–80 V) of a direct current power supply. Atomic force microscopy was used as an accurate tool to measure the surface roughness of thin films on the nanometer scale. Scanning electron microscopy and X-ray diffraction were performed to analyze surface morphology and phase structure. According to the results, the produced titanium oxide layer showed high surface roughness, which increased with increasing anodizing voltage. The impact of anodizing voltages on the color and roughness of anodized layers was surveyed. The corrosion resistance of the anodized samples was studied in simulated body fluid at pH 7.4 and a temperature of 37 °C utilizing electrochemical impedance spectroscopy and the potentiodynamic polarization method. The anodized samples for both alloys at 40 V were at the optimal voltage, leading to a TiO₂ layer formation with the best compromise between oxide thickness and corrosion resistance. Also, findings showed that TiO₂ films produced on Ti-6Al-7Nb alloys had superior surface roughness properties compared to those of Ti-6Al-4V alloys, making them more appropriate for orthopedic applications. From the obtained data and the fruitful discussion, it was found that the utilized procedure is simple, low-cost, and repeatable. Therefore, anodization in 10% oxalic acid proved a viable alternative for the surface finishing of titanium alloys for biomedical applications.

Graphical abstract

Keywords

DC anodization / Ti-6Al-4V alloy / Ti-6Al-7Nb alloy / oxide film / corrosion

1 Introduction

Titanium and its alloys are utilized extensively in various industries due to their unique traits, such as their low modulus of elasticity, elevated strength-to-weight ratio, great biocompatibility, and corrosion resistance [1]. Titanium alloys are popular as implant materials for orthopedic and dental applications due to their favorable mechanical properties and biocompatibility [2,3]. Surface properties like chemical composition and topography influence this biocompatibility. The naturally formed TiO₂ layer in the initial stages of the osseointegration process is not optimal for bone adhesion and growth [4,5]. Furthermore, post-surgical problems include implant-associated infection and mechanical loosening, compromising long-term performance. Ti-6Al-4V is the most commonly utilized α + β Ti alloy and is frequently employed for surgical repair and replacement implants, such as intramedullary nails, hip joints, dental implants, and bone plates [6,7]. However, the release of V and Al ions, which have toxic effects and result in various health problems over time, such as systemic dermatitis, peripheral neuropathy, and Alzheimerʼs, makes long-term implantation of Ti-6Al-4V raise safety concerns [8,9]. Then, researchers created V-free Ti alloys such as Ti-6Al-7Nb, which had mechanical and metallurgical characteristics almost similar to Ti-6Al-4V [10]. Despite the effectiveness of V-free Ti alloys, the Al content of the materials might contribute to various osteal and neural issues. As a result of the aforementioned defects, surface modification can be used to overcome these problems while keeping the required bulk properties [11].

Anodization is an electrochemical surface modification process applied on metallic surfaces to alter oxide layer thickness, composition, and morphology [12,13]. It is a low-cost, one-step method that involves applying a constant voltage or current between the working and counter electrodes of an electrochemical cell. This causes metal atoms, particularly those of titanium and its alloys, to oxidize to Ti⁴⁺ ions that bond with oxide anions from the electrolyte, thus gradually yielding an oxide layer on the metal surface [14]. Based on the properties of the oxide that forms, titanium and its alloys can be anodized in three ways: the first way is the traditional anodization, producing compact, thin, and colored films; the second way is the anodization in fluoride-containing electrolytes, producing nanotubular oxides, and the third way is the high voltage anodization, known as plasma electrolytic oxidation or micro-arc oxidation [15,16].

Titanium anodizing is performed in acid and salt solutions. Usually, oxalic acid (C₂H₂O₄), a strong acid, can be used as an anodizing electrolyte in concentrations of 3% up to 10% [17], producing an interference-colored oxide film on titanium [18,19]. The surface color of titanium varies with the oxide thickness, which depends on the applied voltage and electrolyte type. Oxalic acid plays a specific role in the anodizing process of medical implants; apart from forming a barrier layer in the biological environment, it also provides a desirable porous surface that supports implant fixation and bone growth into open pore spaces, thereby improving the bone-implant interface and avoiding implantation failure [20,21]. A TiO₂ layer typically has oxygen vacancies in the lattice of the TiO₂ crystal, and these serve as important sites for the adsorption and stimulation of numerous surface processes. Particularly, H₂O dissociation causes the production of two OH groups on the surface [22]. These OH groups enhance the uptake of Ca²⁺ ions and encourage the hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) creation, which are essential elements in the coating development. Hydroxyapatite, is a ceramic biomaterial, enhances bone integration with the implant. However, the process of anodizing titanium alloys with oxalic acid still needs to be fully understood and requires additional research.

This study compared thin oxide films grown on Ti-6Al-4V and Ti-6Al-7Nb alloys by anodization in a 10% oxalic acid solution for 30 s at 0–80 V of a direct current (DC) power supply. This research aims to provide a suitable surface and enhance the titanium alloyʼs lifetime (corrosion rate) as a biomedical implant inside the human body. The tested alloysʼ surface topography and morphological characteristics were characterized using atomic force microscopy (AFM). The corrosion behavior of alloys was investigated in simulated body fluid (SBF).

2 Experimental

2.1 Fabrication of titanium alloy specimens

The investigation specimens used in the current study were prepared by remelting pieces of Ti-6Al-4V and Ti-6Al-7Nb alloys (provided by Baoji Xuhe Titanium Metal Co., Ltd.) in a vacuum arc furnace (model LHD 1250). The process starts by inserting pieces of each alloy in the melting chamber inside a copper-cooled mold, as shown in Fig.1. This was followed by evacuating the melting chamber to avoid titanium reactivity and ensure a clean melting process. When the required vacuum level was reached, a high voltage was applied between the tungsten electrode and the copper mold. This resulted in an arc initiation, which melted the charge inside the mold. To guarantee homogeneity, electromagnetic stirring was used, and the samples were remelted several times until sound rod samples were obtained. The figure shows the melting process and the mold wherein the charge was inserted. The obtained rods were then finished using a turning machine and cut into 80 mm diameter and 10 mm high discs. The chemical composition of the samples was analyzed by an inductively coupled plasma atomic-emission spectroscopy method, as shown below in Tab.1.

Fig.1 The copper mold used for preparing the rod-shaped samples and the molten charge inside the vacuum chamber of the arc furnace.

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Tab.1 Chemical composition of Ti-6Al-4V and Ti-6Al-7Nb alloys

Element	Al	Nb	V	Ta	Fe	C	O	N	Others	Ti
Ti-6Al-4V	6.1	0.01	4.00	0	0.10	0.02	0.03	0.01	< 0.4	Bal.
Ti-6Al-7Nb	6.2	6.80	0.01	< 0.05	0.03	< 0.01	0.14	< 0.01	< 0.4	Bal.

2.2 Preparation of the samples

The Ti alloy discs were fixed in an epoxy resin block, exposing an area of 0.5 cm². When immersed in an electrolyte solution, a teflon-coated stainless-steel rod held each disc in place. The discs were mechanically polished with SiC sheets up to 2400 grit and then cleaned ultrasonically in deionized water for 5 min. The Ti alloy discs were etched in a mixture of 80 mL·L⁻¹ HNO₃, 60 mL·L⁻¹ HF, and 150 mL·L⁻¹ H₂O₂ for 2 min at room temperature. After ultrasonic cleaning, the samples were in deionized water for 5 min and air-dried.

2.3 Anodization of the samples

A DC power supply was utilized to power the anodization process potentiostatically in a 10% oxalic acid solution for 30 s at room temperature in the voltage range of 20–80 V. The designed electrochemical cell has two electrodes: a cathode made of the platinum basket and an anode made of a sample of Ti alloy. The cathode and anode were separated by about 4 cm.

2.4 Surface characterization

Macrographs taken by a stereoscope were used to measure the average grain. Optical micrographs were not ideal for calculations since the grain size was large enough to be seen by the naked eye. The grain size was measured using the linear intercept method [23], where random straight lines were drawn on the micrograph. The length of each line (considering the magnification) was then divided by the number of intersections. Finally, the average of all the lines was considered and repeated for several micrographs.

The surface roughness and morphology of the tested samples were inspected using an AFM (Anton Paar-Tosca™ 200, USA). After anodization, images were recorded at randomly selected locations. A scanning size of 10 µm × 10 µm with a resolution of 400 × 400 was acquired using an arrow NCR tapping cantilever. To evaluate the surface roughness, the AFM data were processed according to ISO 25178 using Tosca analysis software, a specialized program. Scanning electron microscope (SEM, JEOL-JSM-5410, Japan) was used to determine the oxide film thickness on anodized Ti alloy by preparing a cross-section of selected samples. X-ray diffraction (XRD) thin film, model “BRUKER-D8 DISCOVER” operated at 40 kV and 40 mA, with a CuKα radiation; (λ = 0.154 nm) was used for identifying the surface constituting elements/compounds after anodization.

2.5 Electrochemical test

Electrochemical measurements were performed at 37 °C utilizing a potentiostat/galvanostat (Auto Laboratory PGSTAT 302N, Netherlands) in SBF solution containing (g·L⁻¹(NaCl 8, CaCl₂ 0.14, KCl 0.4, NaHCO₃ 0.35, C₆H₁₂O₆ 1, NaH₂PO₄ 0.1, MgCl₂·6H₂O 0.1, Na₂HPO₄·2H₂O 0.06, and MgSO₄·7H₂O 0.06 at pH 7.4 [24,25]. A three-electrode cell with 100 mL SBF was used, with Ag/AgCl as the reference electrode, platinum as the counter electrode, and titanium or anodized titanium alloys as the working electrodes. The potentiodynamic studies started once the open-circuit potentials had stabilized for 30 min. A perturbation amplitude of 5 mV was used for electrochemical impedance spectroscopy (EIS) in the frequency range of 10 MHz–100 kHz. Potentiodynamic polarization curves between ‒0.8 and 1.0 V were measured at a scan rate of 1 mV·s⁻¹.

3 Results and discussion

3.1 Microstructure of the prepared samples

Fig.2 displays the microstructure of the as-cast samples. As known for the cast microstructure of the two-phase titanium alloys, it comprises Widmanstädten α with prior β grains. It is remarkable here that the grain size of the Ti-6Al-7Nb specimen, shown in Fig.2(b), is finer than that of Ti-6Al-4V presented in Fig.2(a). According to the grain size measurements, the average grain size for Ti-6Al-4V was calculated to be ca. 4.6 mm, while for Ti-6Al-7Nb, it was ca. 3.2 mm. An example of the macrographs used for calculations is shown in Fig.2. The differences in the original microstructure and the beta phase stabilizing elements (V and Nb) between the two alloys are expected to influence their response to the anodization. It was previously reported that these two alloys responded differently upon surface modification using oxidation [26,27].

Fig.2 Stereo macrographs of (a) Ti-6Al-4V and (b) Ti-6Al-7Nb as-cast alloys.

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3.2 Anodization process

The anodization of Ti-6Al-4V and Ti-6Al-7Nb was done in 10% oxalic acid for 30 s at different voltages from 20 to 80 V of a DC power supply. The oxide films were successfully formed, and the different anodizing voltages provided different colors, as shown in Tab.2. The polished Ti alloy without anodization displays a brilliant metallic shine. After anodization, it is generally acknowledged that the colors seen are caused by the light interference phenomenon of reflected light between the oxide film surfaces [28]. As light rays fell on an anodized surface, owing to the different pores on the oxide layer, light reflection took place at different angles; thus, diverse, scattered images were created for each oxide layer with various surface colors [29].

Tab.2 Color of oxide films produced on the Ti-6Al-4V and Ti-6Al-7Nb alloys in the presence of 10% oxalic acid for 30 s at different DC voltages (0–80 V)

Voltage	Ti-6Al-4V			Ti-6Al-7Nb
	Color	Image		Color	Image
0 V	Silvery			Silvery
20 V	Blue			Crimson
40 V	Light green			Green
60 V	Dark yellow			Dark yellow
80 V	Dark blue with purple spots			Dark blue with purple spots

3.3 Surface roughness

AFM photographs were taken to examine the surface morphology and roughness of the oxide film produced on Ti-6Al-4V and Ti-6Al-7Nb alloy surfaces at different voltages. These characteristics are crucial because they affect the materialsʼ biocompatibility and corrosion behavior. Also, surface features impact the functional activity of cells interacting with biomaterials. The three-dimensional and cross-sectional profiles of Ti-6Al-4V and Ti-6Al-7Nb oxidized surfaces are displayed in Fig.3 and Fig.4, respectively. These images reveal that each sample has a different morphology, and the surface at 0 V (before anodization) is almost smooth and homogeneous, with an average surface roughness S_a (94 and 127 nm) and a root mean square S_q (112 and 153 nm) for Ti-6Al-4V and Ti-6Al-7Nb, respectively. The formation of the oxide layer during anodization was uneven, and the presence of defects increased roughness. Moreover, as the anodizing voltage increases, the roughness of the oxide films produced on both alloys continues to increase until they reach a maximum value at 80 V. Others have reported comparable results [30–32]. Tab.3 presents S_a and S_q values obtained from AFM. Rough surfaces increase the accessible surface area for cell adhesion, which may undergo osteoblast differentiation to form a bone matrix. Successful osseointegration improves clinical results [33,34]. According to the results, the surface roughness of anodized Ti-6Al-7Nb is greater than that of anodized Ti-6Al-4V, making them more suitable for orthopedic applications since rough surfaces facilitate implant osseointegration.

Fig.3 Three-dimensional images and cross-sectional profiles of anodic oxide films on Ti-6Al-4V alloy in 10% oxalic acid for 30 s at anodization voltages of (a) 0, (b) 20, (c) 40, (d) 60, and (e) 80 V.

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Tab.3 Sa and Sq for anodized Ti-6Al-4V and Ti-6Al-7Nb alloy surface at different voltages (0–80 V)

Applied DC voltage	Ti-6Al-4V			Ti-6Al-7Nb
	Sa/nm	Sq/nm		Sa/nm	Sq/nm
0 V	94	112		127	153
20 V	108	128		131	155
40 V	154	181		196	234
60 V	188	208		228	264
80 V	324	397		388	481

Fig.4 Three-dimensional images and cross-sectional profiles of anodic oxide films on Ti-6Al-7Nb alloy in 10% oxalic acid for 30 s at anodization voltages of (a) 0, (b) 20, (c) 40, (d) 60, and (e) 80 V.

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Typical line profiles from section analysis were generated for anodized Ti-6Al-4V and Ti-6Al-7Nb surfaces and presented in Fig.3 and Fig.4. The heights of the peaks in line profiles for anodizing Ti-6Al-4V and Ti-6Al-7Nb increased with an increase in anodizing voltage. The pronounced elevations in the size of the peaks indicated that the TiO₂ filmsʼ thickness increased with anodization voltages. The average thickness of the oxide layer depends on the applied voltage and increases with increasing voltages from 0 to 80 V [35,36]. The results of the line profile confirm much rougher properties of anodized Ti-6Al-7Nb, which is consistent with the AFM topography images.

SEM analyzed the average thickness of the oxide layer (TiO₂). Fig.5 and Fig.6 present SEM images of cross sections for Ti-6Al-4V and Ti-6Al-7Nb alloy subjected to anodization in 10% oxalic acid for 30 s at anodization voltages of 20 and 80 V. The anodizing process is expected to form a continuous oxide layer with thickness increases with the applied voltage. A general view of the oxide layer on Ti-6Al-4V alloy in 10% oxalic acid for 30 s at anodization voltages of 20 and 80 V is shown in Fig.5(a) and 5(c). It was apparent that there was a significant difference between the thickness of the oxide layer developed at different voltages. Fig.5(b) presented the thickness of TiO₂ at anodization 20 V ranging between ca. 5–6.9 μm, and that formed at 80 V was formed between ca. 14.3–14.8 μm as represented in Fig.5(d).

Fig.5 SEM image of cross-sections of TiO2 layer on Ti-6Al-4V alloy in 10% oxalic acid for 30 s at anodization voltages of (a, b) 20 and (c, d) 80 V.

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Fig.6 SEM image of cross-sections of TiO2 layer on Ti-6Al-7Nb alloy in 10% oxalic acid for 30 s at anodization voltages of (a, b) 20 and (c, d) 80 V.

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Fig.6(a) and 6(c) shows a general outlook of the oxide layer on Ti-6Al-7Nb alloy anodized at voltages 20 and 80 V. Fig.6(b) revealed that the thickness of the oxide layer developed on the surface of Ti-6Al-7Nb alloy anodized at 20 V ranged between ca. 7.4–9.8 μm. Fig.6(d) showed that the thickness of the oxide layer at anodized voltage 80 V was found in the range of ca. 19–20.8 μm, confirming that increasing voltage greatly enhanced the formation of TiO₂. These results confirm the outcomes of this study, the Ti-6Al-7Nb alloy is the most adequate alloy with excellent osteointegration properties.

Generally, anodization is a conventional electrochemical method to generate a protecting dense barrier-type oxide coating on Ti alloys. During anodization, the oxidized metal species Ti⁴⁺ generated at the metal/oxide interface (Eq. (1)) moves outward. In contrast oxygen ions O²⁻ formed by the deprotonation of H₂O (Eq. (2)) at the oxide/electrolyte interface moves inward. Thus, according to Eq. (3), compact titanium oxide is formed at both interfaces through this field-assisted ion migration. During anodization, the oxide film thickens if the electric field is powerful enough to provoke ionic conduction across the oxide.

(1)$\mathrm{T}\mathrm{i}\to {\mathrm{T}\mathrm{i}}^{4+}+{4\mathrm{e}}^{-}$

(2)${\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}^{2-}+{2\mathrm{H}}^{+}$

(3)${\mathrm{T}\mathrm{i}}^{4+}+{2\mathrm{O}}^{2-}\to {\mathrm{T}\mathrm{i}\mathrm{O}}_2$

where the overall reaction is

(4)$\mathrm{T}\mathrm{i}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{T}\mathrm{i}{\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$

The total reaction forms a stable oxide film, TiO₂, and its growth is enhanced by increasing the applied voltage (0–80V). This film provides an efficient corrosion and leakage barrier against toxic ions, increasing biocompatibility [5,37,38].

Figures S1 and S2 (cf. Electronic Supplementary Material) depict two-dimensional images of bare and anodized Ti-6Al-4V and Ti-6Al-7Nb alloys. Bare alloys, Ti-6Al-4V and Ti-6Al-7Nb in Figs. S1(a) and S2(a), exhibit homogeneous and uniform surfaces. Figures S1(b–e) and S2(b–e) show that compact TiO₂ growth is proportional to an applied voltage and affects roughness. High voltages promote the inward transfer of O^2‒ into the metal film interface and the diffusion of Ti⁴⁺ ions from the titanium alloys to the electrolyte interface at the anode, which thickens the oxide film.

The chemical phase composition of the TiO₂ layer was confirmed by XRD analysis. Fig.7 shows the XRD patterns of the Ti-6Al-4V and Ti-6Al-7Nb alloy after anodization at 40 V, respectively (optimal condition). All the samples show that the oxide layer mainly consists of the anatase TiO₂ phase with peaks appearing at main diffraction peaks observed at 2θ = 38.2°, 53.5°, and 62.2° for both alloys. These results were in good agreement with JCPDS Card No. 21-1272. Moreover, the 40.3° and 70.7° peaks are attributed to titanium substrate with standard JCPDS Card No. 44-1294.

Fig.7 XRD patterns on the (a) Ti-6Al-4V and (b) Ti-6Al-7Nb alloys anodized at 40 V.

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3.4 Electrochemical analysis

Electrochemical studies in SBF solution at 37 °C were carried out to establish whether the produced oxides give corrosion protection to the substrate. Fig.8 present the potentiodynamic polarization curves of Ti-6Al-4V and Ti-6Al-7Nb alloys anodized in 10% oxalic acid for 30 s. According to the polarization diagrams, anodization improves surface resistance for both alloys via shifting the corrosion potential in a noble (positive) direction and decreasing corrosion current density compared to non-anodized titanium samples. The results suggest that anodization can protect titanium alloys and increase their corrosion resistance in SBF. The anodic film was a barrier to block the solution from penetrating the metal surface [39]. The optimal anodization voltage for both alloys is 40 V with stable TiO₂ film. However, Ti-6Al-7Nb showed lower corrosion current densities than Ti-6Al-4V, demonstrating a more stable oxide film for the Ti-6Al-7Nb alloy.

Fig.8 Potentiodynamic polarization curves of anodized (a) Ti-6Al-4V and (b) Ti-6Al-7Nb in 37 °C SBF solution.

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The improved oxide film development and better corrosion performance of the alloy Ti-6Al-7Nb could be ascribed to the Nb presence. A previous study reported the positive effects of adding Nb to Ti-based alloys on surface film stabilization [40]. Moreover, Nb cations lower the anion vacancy concentration on the titanium oxide film, improving surface film passivation. Lower titanium oxidation states produce anions [41,42]. Therefore, the existence of Nb in the passive film of Ti-6Al-7Nb may be the reason for its low corrosion current density compared to Ti-6Al-4V [43].

Using Tafel slopes, the Stern-Geary equation shows that the polarization resistance is related to the corrosion current [44,45]:

(5)$R_{\mathrm{p}}=\frac{{\beta }_{\mathrm{a}}{\beta }_{\mathrm{c}}}{2.3\times i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}\left({\beta }_{\mathrm{a}}{+\beta }_{\mathrm{c}}\right)},$

where β_a and β_c are the anodic and cathodic Tafel slopes.The corrosion current density (i_corr) is proportional to the corrosion rate (R_i), as shown in Eq. (6):

(6)$R_{\mathrm{i}}=K{i}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}(eq.wt)/d,$

where R_i is given in mm·yr⁻¹, i_corr in μA·cm⁻², K = 3.27 × 10^‒3 mm·g·μA⁻¹·cm⁻¹·yr⁻¹, eq.wt is the equivalent weight for Ti under oxidizing condition, and d is the alloy density, 4.5 g·cm⁻³. Equation (7) was used to calculate the corrosion protection efficiency (PE, %) of anodized titanium alloys immersed in SBF solutions at 37 °C [44–46]:

(7)$\mathrm{P}\mathrm{E}=1-\frac{i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}}{i_{{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^{\circ }}}\times 100\mathrm{\%},$

where i_corr° is the corrosion current densities before anodization, and i_corr is the corrosion current densities of the titanium alloy after anodization. The corrosion characteristics presented in the polarization curves, including corrosion potential (E_corr), corrosion current density (I_corr), polarization resistance (R_p), corrosion rate (R_i), and PE are recorded in Tab.4.

Tab.4 Electrochemical parameters and corrosion rates obtained by polarization tests

	Specimens	Ecorr/V	Icorr/(μA·cm−2)	Rp/kΩ	Ri/(mm·yr−1)	PE/%
Ti-6Al-7Nb	0 V	‒0.098	1.9 × 10‒3	8.6	6.9 × 10‒4	–
	20 V	‒0.059	1.5 × 10‒4	65.0	5.4 × 10‒4	92.1
	40 V	0.190	6.3 × 10‒6	94.0	2.2 × 10‒5	99.6
	60 V	0.120	3.1 × 10‒4	29.0	7.2 × 10‒4	83.7
	80 V	0.021	5.1 × 10‒4	22.0	9.3 × 10‒4	73.7
Ti-6Al-4V	0 V	‒0.294	2.5 × 10‒3	6.4	8.7 × 10‒4	–
	20 V	‒0.220	3.1 × 10‒4	56.0	7.2 × 10‒4	87.6
	40 V	0.046	1.5 × 10‒5	89.3	1.1 × 10‒4	99.4
	60 V	0.035	5.2 × 10‒4	27.7	1.9 × 10‒4	79.2
	80 V	‒0.024	7.9 × 10‒4	21.2	6.7 × 10‒4	68.4

Utilizing the EIS technique, the Nyquist plots of anodized Ti-6Al-4V and Ti-6Al-7Nb alloys anodized in 10% oxalic acid for 30 s were examined in a 37 °C SBF solution. The results are shown in Fig.9(a) and Fig.9(b) for Ti-6Al-4V and Ti-6Al-7Nb, respectively. The impedance investigation complements the previously obtained polarization data. The Ti-6Al-7Nb showed the best corrosion resistance compared to the Ti-6Al-4V. The electrodes with protective oxide films produced by anodization present superior impedance values compared to the non-anodized; this behavior corresponds to the enhanced corrosion resistance of the uniform, compact passive films. The most stable oxide films were obtained at an applied voltage of 40 V; these results agree with the AFM, where an increase in the applied voltage over 40 V increases the roughness, which facilitates cell adhesion and osseointegration of the implant but may increase the metal ion release due to the penetration of the electrolyte, decreasing the corrosion resistance.

Fig.9 Nyquist plot curves of anodized (a) Ti-6Al-4V and (b) Ti-6Al-7Nb in 37 °C SBF solution.

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The resistance of the passive film of Ti-6Al-4V alloy was less than that of Ti-6Al-7Nb alloy. In the case of Ti-6Al-4V alloy, the vanadium oxide formed on the surface dissolute because of the Cl ions present in the SBF solution. Vanadium dissolution causes the generation and diffusion of vacancies in the oxide layer of the Ti-6Al-4V alloy [41].

The data were fitted with an equivalent circuit to enable an accurate analysis of the impedance diagrams as represented in Fig.10. In the case of the non-anodized Ti-6Al-4V and Ti-6Al-7Nb alloys, the equivalent circuit is the simple Randelʼs circuit (Fig.10(a)). This circuit involves a constant phase element (CPE) in series with the solution resistance R_s, and parallel to the polarization resistance (charge transfer resistance) of the alloy surface R_p. For the anodized electrode, the equivalent circuit fitted was two-time constants suggesting the formation of two layers (Fig.10(b)), with solution resistance represented as R_s, double layer capacitance of the outer passive layer of the oxide film represented as CPE₁, and CPE₂ associated with the inner passive layer of the oxide. The charge transfer resistances of the outer layer of the metal/electrolyte interface are represented as R₁ and those of the inner layer as R₂. The passive oxide films on Ti-based alloys are made of two superimposed oxide layers. The first outer passive oxide layer on the top surface is porous, and the second inner passive oxide layer displays a dense barrier-like structure.

Fig.10 Equivalent circuits employed for modeling of experimental EIS data (a) with one time constant and (b) with two-time constants.

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The action of the capacitor is compensated by the CPE due to the surface heterogeneity of the surface and is defined by its impedance value:

(8)$Z_{\mathrm{C}\mathrm{P}\mathrm{E}}={\left[C{\left(j\omega \right)}^{\alpha }\right]}^{-1},$

where α is a surface heterogeneity exponent, 0 ≤ α ≤ 1, j is an imaginary number (j = (‒1)^1/2), ω = 2πf is the angular frequency in rad·s⁻¹, and f is the frequency in Hz = s^‒1 [47]. The outcomes of the EIS analysis are represented in Tab.5.

Tab.5 Parameters of electrochemical impedance

	Specimen	Rs/Ω	R1/kΩ	CPE1/μF	α1	R2/kΩ	CPE2/μF	α2	PE/%
Ti-6Al-7Nb	0 V	76	6.20	139.0	0.85	–	–	–	–
	20 V	42	0.34	18.6	0.82	72	403	0.76	91.4
	40 V	45	0.38	15.6	0.83	142	220	0.75	95.5
	60 V	48	0.39	4.9	0.81	63	391	0.78	90.2
	80 V	52	0.43	3.3	0.84	51	430	0.77	88.0
Ti-6Al-4V	0 V	66	4.20	198.0	0.89	–	–	–	–
	20 V	39	0.22	13.6	0.85	40	332	0.75	89.6
	40 V	35	0.23	8.9	0.83	84	90	0.78	95.0
	60 V	46	0.24	3.6	0.84	24	342	0.77	82.7
	80 V	72	0.27	2.7	0.86	18	385	0.76	77.0

Equation (9) calculates the PE of anodized Ti-6Al-7Nb and Ti-6Al-4V alloys in SBF solution:

(9)$\mathrm{P}\mathrm{E}=1-\frac{R_{\mathrm{p}}^{\circ }}{R_{\mathrm{p}}}\times 100\mathrm{\%},$

where $R_{\mathrm{p}}^{\circ }$ is the resistances for the titanium alloys before anodization and R_p is the resistances for the titanium alloys after anodization.

Finally, it is worth mentioning that roughness is an important factor controlling implant osseointegration, as there is a relation between surface roughness and cell behavior. A rougher surface increases bone-to-implant contact and maximizes the response of bone cells during tissue healing around dental implants, allowing better osteogenic cell adhesion [48]. Accordingly, in our study, roughening titanium surface through anodization technique provided the required rough oxide surface, as revealed by the AFM results. In a meta-analysis conducted in 2016, it was revealed that anodized surfaces delivered a low implant failure probability [49], additionally; another study proved that anodized titanium surfaces significantly increased blood clot retention and thus favored osseointegration better than non-anodized titanium surfaces [50].

Therefore, according to the work mentioned and within the limitation of the study, anodizing titanium alloys in 10% oxalic acid represented a quick, simple, and inexpensive strategy to improve bone-implant interface and osseointegration. Moreover, the best corrosion performance was acquired by anodizing Ti-6Al-7Nb alloy, which can be suggested for biomedical applications. However, further investigations are required to inspect the bioactivity and osseointegration potential of the proposed anodized alloys on experimental animals in vivo.

4 Conclusions

Based on the findings of this study, it was concluded that anodization of Ti-6Al-7Nb and Ti-6Al-4V alloys in 10% oxalic acid solution for 30 s at 20–80 V could successfully produce protective, corrosion resistant, TiO₂ oxide films with different colors, surface morphologies, and a roughness according to the anodization voltage utilized and that the optimal anodization voltage for both alloys is 40 V, with more stable oxide film and superior corrosion resistance for Ti-6Al-7Nb alloy, suggesting its use in the biomedical field.

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Competing Interests

The authors declare that they have no competing interests.

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