1 Introduction

In the past two decades, microstructural superhydrophobic and anticorrosion properties for the surfaces of metals and alloys have attracted extensive attention in industry and science. Owing to their excellent corrosion resistance, high mechanical strength, strong heat resistance, and good biocompatibility, titanium and titanium alloys are widely used in marine [1], aerospace [2], and medical equipment [3]. Several techniques have been explored to achieve superhydrophobicity and anticorrosive in microstructures on metals and alloy surfaces, such as direct laser writing [4,5,6,7], laser interference lithography and ion beam etching [8], layer-by-layer self-assembly [9, 10], high-speed wire electrical discharge machining [10, 11], electrical discharge machining and chemical etching [12], anodic/chemical etching [13,14,15,16,17,18], and electrodeposition [19,20,21,22,23]. Anodic etching is more prevalently adopted than these methods due to its unique advantages for fabricating superhydrophobic surfaces on metals and alloys. Such advantages include large-area, simplicity, and efficiency. The correlation between microstructures and electrochemical parameters is the most important scientific problem encountered in optimizing anodic etching techniques [13, 16,17,18].

The preparation of superhydrophobic surfaces by anodic etching involves two steps: the first step is to cover a rough surface with micro/nanostructures. The second is to decrease surface energy through chemical modification. For example, durable superhydrophobic Zn/ZnO/TiO2 surfaces with dendritic structures were prepared on Ti–6Al–4V substrate through anodic etching, electrodeposition, and annealing; the water contact angle (WCA) reached ~ 160° and rolling angle was less than 1° [24]. A superhydrophobic, durable, and abrasion-resistant surface was synthesized on Ti–6Al–4V substrate by anodic etching and modification [14]. The surface exhibited a contact angle of 158.8° ± 1.9° and a tilt angle of 5.3° ± 1.1°. In addition, chemical reagents, such as (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane, hexadecyltriethoxysilane [25], ethanolic stearic acid [26], and compound chemicals (MnCl2·4H2O:C16H32O2 ≈ 1:2.6 wt%) [27], were used to reduce surface energy and improve surface wettability. Furthermore, the pitting corrosion of titanium easily forms on oxide films in bromine ion solution [28]. Nevertheless, chemical reagents are prone to causing environmental pollution. Therefore, exploring a novel method to prepare superhydrophobic (WCA ≥ 150°) surfaces on titanium and titanium alloys without surface modification is necessary.

In this work, a superhydrophobic and anticorrosive surface was fabricated on Ti–6Al–4V through anodic etching in a mixed solution of NaBr and KCl. Anodic etching was conducted by changing the current densities and times. A scanning electron microscope (SEM) was used to observe the surface morphology after anodic etching. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) were used to investigate the composition and distribution of elements on the surface. A drop shape analyzer was employed to measure the water contact angle on the surface after anodic etching. In addition, anticorrosive properties were investigated in 3.5 wt% NaCl aqueous solution [11, 15] by using an electrochemical workstation. The results showed that the one-step anodic etching is an effective way to realize the superhydrophobic and anticorrosive surface for Ti–6Al–4V.

2 Experimental Details

2.1 Fabrication of Superhydrophobic Ti–6Al–4V Alloy Surfaces

Ti–6Al–4V substrate was treated by using a polishing machine. Prior to anodic etching, the Ti–6Al–4V substrate was ultrasonically cleaned in acetone (99%), ethanol (99%), and deionized water for 5 min, repeating three times. The Ti–6Al–4V substrate was sealed with paraffin, and an exposed surface with dimensions of 1 cm × 1 cm was treated through anodic etching.

Anodic etching was conducted by using a typical three-electrode model, i.e., the Ti–6Al–4V substrate, platinum electrode, and saturated calomel electrode (SCE) were used as working electrode (WE), counter electrode (CE), and reference electrode (RE), respectively. The distance between the WE and CE was 10 mm. In addition, the mixed electrolyte solution contained 15 wt% potassium chloride (KCl) and 15 wt% sodium bromide (NaBr). The experiments were carried out at a constant current density (i = 0.20, 0.25, 0.30, and 0.35 A cm−2). The reaction was performed for different durations (t = 300, 360, 420, 480, and 540 s). Subsequently, the samples were ultrasonically cleaned in acetone, ethanol, and deionized water for 5 min three times. Finally, the samples were dried in an oven at 60 °C for 2 h.

2.2 Surface Characterization

The surface morphologies were observed using a scanning electron microscope (SEM, FEI Quant FEG 250). The chemical composition and valency of the surface were investigated using EDS (INCA Energy, Oxford 135 Ins) and XPS (Escalab 250Xi, Thermo Fisher Scientific). Furthermore, the Krüss drop shape analyzer (DSA 100, Germany Cruise) was used to measure the WCAs at room temperature (~ 24 °C). The droplet volume was 2 μL, and WCAs were recorded at different positions for each sample.

The microdisplacement platform (L-310, PI, Germany) and the balance (TP-214, Denver Instrument, USA) were adopted to measure the adhesion of these hydrophobic surfaces as shown in Fig. S1. Before the experiment, the sample was placed on balance, adjusting the sight to zero. A ring with a diameter of 1.5 mm at the terminal of the metal wire (Diameter of 1 mm) was connected to the micro-displacement platform. A microinjector was used to inject deionized water 5 μL into the metal ring. The microdisplacement platform was descended to control the height of the droplet. Then, the ascent rate of the micro-displacement platform was 0.1 mm s−1.

2.3 Electrochemical Characterization

The anticorrosive properties of the superhydrophobic surfaces in 3.5 wt% NaCl solution were also researched by an electrochemical workstation (Potentiostat A4000, Princeton Applied Research, USA) [11, 15]. The samples were paraffin encapsulated and only left a 10 mm × 10 mm surface. The corrosion behaviors were analyzed from the polarization curves.

3 Results and Discussion

3.1 Wettability

Figure 1 shows the SEM image of the morphology and WCAs of the Ti–6Al–4V before and after anodic etching. The surface of surface of the polished Ti–6Al–4V substrate was smooth, as shown in Fig. 1a, and the WCA reached about 92° in inset in Fig. 1a, exhibiting a weak hydrophobicity. For another, a lot of microstructures and nanopores were distributed on the surface after anodic etching in the mixed solution of 15 wt%, potassium chloride (KCl) and 15 wt% sodium bromide (NaBr) at current density 0.25 A cm−2 for 420 s, as shown in Fig. 1b, and the WCA reached 160.9° as shown in the inset, exhibiting the superhydrophobicity. These microstructures and nanopores contributed to the superhydrophobic properties [4, 5, 8, 10, 13].

Fig. 1
figure 1

SEM image of surface morphology and optical image of contact angle (inset) of Ti–6Al–4V alloy: a polished and b after anodic etching

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Figure 2 shows the WCAs on the surface of Ti–6Al–4V after anodic etching at different etching times (300, 360, 420, 480, and 540 s) and current densities (0.20, 0.25, 0.30, and 0.35 A cm−2). Figure 2a shows the optical images of the WCAs on the surfaces. We noticed that all samples were hydrophobic, and a few arrived at the superhydrophobicity. The minimum WCA was 116.0° larger than the WCA 96.5° on commercial Ti–6Al–4V substrate [14] and the WCA 92° after the polishing process in this work, and the maximum WCA arrived at 160.9°. Moreover, the WCAs were close to or over 150° after anodic etching at 0.25 A cm−2, indicating that superhydrophobicity was easily realized at this condition. Figure 2b depicts the variation of the WCAs with etching time and current density. From curves 1–4 in Fig. 2b, the etching time had a negligible effect on the WCAs at the current densities 0.20–0.30 A cm−2. Nevertheless, the superhydrophobicity was also shown after anodic etching for 540 s at the current density 0.30 A cm−2 (see curve 3). In addition, curve 4 of the WCAs showed a nonlinear increase at the current densities 0.35 A cm−2. These phenomena were due to the weak controllability in the electrochemical system, resulting in some errors in the WCA. However, the optimal current density may be approximately 0.25 A cm 2 to fabricate the superhydrophobic surface for Ti–6Al–4V substrate by one-step anodic etching.

Fig. 2
figure 2

a WCAs after anodic etching obtained at constant current densities 0.20–0.35 A cm−2 for 300 − 540 s. b Change of the WCAs with increasing the current densities and etching times

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3.2 Analysis of EDS and XPS

EDS and XPS were conducted on the local region of the microstructure, as shown in Figs. 3 and 4, respectively, to reveal the distribution of elements after anodic etching. Figure 3a shows that the main elements on the surface are composed of Ti, Al, V, and O. Figures 3b, c, and f show that the distribution of the Al and O elements were in agreement with the topography of the surface, indicating that aluminum oxide might exist mainly on the surface. In addition, Figs. 3d and e show that Ti and V elements are distributed on the surface of the whole region. Nevertheless, the intensity of Ti was higher than that of V in Fig. 3a. This finding is in accordance with the distribution of the elements for the Ti–6Al–4V substrate [16,17,18]. Furthermore, the valence states of O and Ti on the superhydrophobic surface are presented in Fig. 4. The XPS spectrum of O 1 s is exhibited in Fig. 4a. In addition to surface oxygen defects, H–O and Ti–O bonds were present on the surface layer. Nevertheless, the intensity of H–O bonds was weaker than that of Ti–O. The existence of the H–O bonds might contribute to the adsorption of water molecules. Figure 4b shows that Ti4+ predominated, verifying the presence of TiO2 [29,30,31]. Meanwhile, Fig. 4b also reveals the presence of a minor amount of Ti3+ species on the surface, which could be ascribed to Ti2O3 that resulted from the incomplete oxidation of Ti element. These results indicated that the surface was mainly covered by the composite with TiO2 [32, 33].

Fig. 3
figure 3

EDS mapping of the superhydrophobic surface. a Surface elemental analysis, b SEM image of a selective area, element distribution of c aluminum, d titanium, e vanadium, and f oxygen

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Fig. 4
figure 4

XPS spectra for O 1s and Ti 2p of the Ti–6Al–4V surface after anodic etching

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3.3 SEM Surface Morphology

Figure 5 showed the SEM images with micro/nanostructures and nanopores on the surfaces etched at 0.25 A cm−2 for 300 and 480 s. Figures 5a and b show the presence of some irregular microscale structures on the surfaces. Moreover, numerous irregular and nanopore structures were found on the surfaces. Furthermore, the irregular microscale structures reduced (Fig. 5b), and nanostructures and nanopores increased after anodic etching for 420 s. These nanostructures and pores might provide the surface with resistance to water droplet infiltration [34,35,36], resulting in the change of the WCA, as shown in Fig. 2a. The roughness of the whole surface might have an effect on wettability [32, 37, 38].

Fig. 5
figure 5

SEM images of microstructure surfaces etched at 0.25 A cm−2 for a 300 s and b 480 s

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3.4 Surface Adhesions

To investigate the influence on the adhesiveness of the surface by the wettability, adhesive force was also tested by using a balance and microdisplacement platform (Fig. S1). The change in the adhesive forces is also illustrated in Fig. 6. Curves 1–4 in Fig. 6 illustrate that the adhesive force initially decreased and then slightly fluctuated with etching time. This change was inversely related to the change in WCAs (Fig. 4). In addition, the minimum adhesive force reached approximately 130.9 ± 6.1 mN (Fig. 6, curve 2). This change corresponded to the maximum WCA of 160.9° (Fig. 4a), which was obtained through anodic etching at 0.25 A cm−2 for 420 s. The low adhesive forces between ~ 130 and ~ 145 mN concentrated in the marked oval area in Fig. 6. Furthermore, adhesive forces were less than 225 mN, indicating that one-step anodic etching can reduce surface adhesiveness.

Fig. 6
figure 6

Change in the adhesion forces of hydrophobic and superhydrophobic surfaces at different current densities and etching times

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3.5 Anticorrosive Property

The anticorrosive behaviors of hydrophobic and superhydrophobic surfaces were also examined in this work. Figure 7 presents the Tafel curves of Ti–6Al–4V etched in 3.5 wt% NaCl electrolyte. The passive current densities in the Tafel curve are important for evaluating the anticorrosive properties of metals, especially passive metals [29, 30, 34]. The transition between activation and passivation was not obvious for almost all samples (Fig. 7). These potentials could maintain a low current density over a wide range, as shown in Figs. 7a–c, indicating that spontaneous passivation occurred with low current density [32]. However, passivation disappeared when the sample was etched at the high current density of 0.35 A cm−2, as shown in Fig. 7d. Meanwhile, the current density of the passivation reached ~ 1.23 × 10−8 to 2.88 × 10−8 A cm−2 and ~ 1.45 × 10−8 to 5.01 × 10−8 A cm−2 (Fig. 7b) when anodic etching was performed at 0.25 A cm−2 for 360 and 420 s. The passivated current densities were about one to two orders of magnitude higher than those in Figs. 7a and c, indicating that good anticorrosive property was obtained after one-step anodic etching [16,17,18].

Fig. 7
figure 7

Polarization curves of the hydrophobic and superhydrophobic surfaces for different reaction times (300, 360, 420, 480, and 540 s) at the current densities a 0.20 A cm−2, b 0.25 A cm−2, c 0.30 A cm−2, and d 0.35 A cm−2

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Corrosion potential (Ecorr) and corrosion current density (icorr) are key parameters for evaluating the corrosion properties of metal and alloy [39]. They are shown in Table 1 and were obtained on the basis of the Tafel curves shown in Fig. 7. In fact, the extrapolation of these Tafel curves of load transfer controlled corrosion involved these icorr and Ecorr. The curves of Ecorr and corrosion current densities (icorr (\(\text{lg}\left|{i}\right|\))) were formed with etching times, as shown in Fig. 8 on the basis of the data in Table 1. All Ecorr (obtained at 0.35 A cm−2) were larger than the Ecorr of the treated sample (− 0.3929 V, also see Fig. S2) and others in Fig. 8a from curves 1–4. The maximum Ecorr reached ~ 0.5109 V in curve 4 (the sample obtained at 0.35 A cm−2 for 420 s) and had increased by 230% compared with the Ecorr (− 0.3929 V) of the untreated sample. The high corrosion potential is beneficial for enhancing the anticorrosive property [40]. We could also see that almost icorr (obtained at 0.25, 0.30, and 0.35 A cm−2) were less than the icorr (\(\text{lg}\left|{i}\right|\)= − 6.0949 A cm−2; i =  ~ 8.04 × 10−7 A cm−2, see Fig. S2) untreated sample (also in Table 1) in Fig. 8b from curves 1–4. The data of the Ecorr and icorr show that one-step anodic etching is an effective strategy for increasing the anticorrosiveness of Ti–6Al–4V.

Table 1 Corrosion potential (Ecorr) and corrosion current (icorr) of all samples
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Fig. 8
figure 8

Corrosion potential (Ecorr) and current densities (\(i\) corr, Log \(\left|{i}\right|\)) in 3.5 wt% NaCl aqueous solution: a curves of Ecorr (V) and b \(i\) corr (A cm−2)

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4 Summary

An anticorrosive superhydrophobic surface was directly prepared on Ti–6Al–4V through one-step anodic etching. The WCAs showed that superhydrophobicity could be easily obtained through one-step anodic etching at a current density from ~ 0.25 to ~ 0.30 A cm−2. Furthermore, the maximum WCA could reach 160.9° after anodic etching for ~ 420 s. Furthermore, EDS and XPS characterizations revealed that the metal oxide film on the surface was the main component of TiO2, indicating that spontaneous passivation occurred. Moreover, corrosion potentials and corrosion current densities showed that the anticorrosive property could be simultaneously realized through one-step anodic etching. Although the passivated potential disappeared at a current density of ~ 0.35 A cm−2, the corrosion potential increased. This effect could also enhance the anticorrosive property. In summary, one-step anodic etching is an effective way to confer superhydrophobicity and anticorrosiveness to Ti–6Al–4V.