Fabrication of durable anticorrosion superhydrophobic surfaces on aluminum substrates via a facile one-step electrodeposition approach

Abstract

With unique water-repellent and self-cleaning properties, engineering metallic materials with superhydrophobicity endows them with greatly enhanced corrosion resistance. Herein, a facile and controllable one-step electrodeposition approach was employed to fabricate a superhydrophobic surface (SHPS) on an aluminum (Al) substrate as a barrier against corrosion media. The wettability, morphology, and chemical composition of the consequent SHPS were characterized by contact angle (CA), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS), respectively. The anti-contamination and anticorrosion behaviors of the resultant SHPS were investigated by self-cleaning test, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The electrochemical results indicate that the resultant SHPS possessed greatly enhanced corrosion resistance, and were able to reduce the corrosion of a bare Al surface, with an inhibition efficiency of 99.96%. Furthermore, the as-fabricated SHPS maintained excellent durability and stability after air exposure, deionized water immersion, and 3.5 wt% NaCl solution immersion. We believe that SHPS fabricated by a versatile one-step electrodeposition approach would make it possible to develop engineering materials with durable self-cleaning and anticorrosion properties for use in rugged environments.

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1. Introduction

As a fundamental property of solid surfaces,^1,2 wettability has a considerable effect on the applications of engineering metallic materials. Inspired by the self-cleaning lotus leaf^3–7 and water-repellent legs of the water strider,⁸ superhydrophobic surfaces (SHPS), with a water contact angle (CA) greater than 150° and a sliding angle (SA) less than 10°, are attracting growing interest for their fabrication and potential applications.^9–14

Aluminum (Al) is the most abundant metallic element on earth. Al and Al alloys are essential structural engineering materials and are widely used in aerospace, automobiles, mechanical apparatus, packaging, construction, and household items due to their characteristics of low weight, high specific strength, excellent thermal/electrical conductivity, and good castability. However, their inherent susceptibility to corrosion media influences their performance in corrosive environments, leading to malfunctions, economic loss, and even disasters. Therefore, it is necessary to enhance their corrosion resistance in harsh corrosive media, which will greatly extend their lifetime and industrial applications.

Recently, the achievement of a superhydrophobic state of metallic surfaces^15–19 has become a promising emerging technology that addresses many of the classical shortcomings of traditional corrosion protection approaches, such as cathodic protection,^20,21 organic/inorganic coatings,^22,23 corrosion inhibitors,^24–27 etc. The superhydrophobicity of Al or Al alloy substrates can create an interface that retains air within the surface structure, minimizing the contact area with the liquid and resulting in highly nonwetting and anticorrosion properties. In recent years, some literature studies^28–30 reported the preparation of SHPS as an efficient anticorrosion barrier for Al or its alloys. For example, Ou et al.³¹ fabricated two types of SHPS using hydrothermal and chemical etching methods on Al alloy substrates and investigated their anticorrosion behaviors, suggesting which were the better choices for corrosion protection. Liu et al.³² deposited graphene film on an Al alloy substrate and achieved excellent corrosion resistance and mechanical abrasion resistance. Lv et al.³³ prepared SHPS on an Al substrate via surface roughening by NaClO and surface passivation by hexadecyltrimethoxysilane, and investigated the short-term and long-term corrosion resistance for immersing the sample in sodium chloride (NaCl) solutions. Vengatesh and Kulandainathan³⁴ reported a facile method for the fabrication of self-lubricating superhydrophobic anodic aluminum oxide (AAO) surfaces with improved corrosion resistance, which could have a high impact in broadening the potential applications of Al or its alloys. Lu et al.³⁵ demonstrated the advantages of employing SHPS as a barrier against atmospheric corrosion induced by salt deliquescence in a marine environment. Boinovich et al.³⁶ prepared a SHPS on an Al alloy substrate via an efficient nanosecond laser treatment followed by surface modification, and evaluated the resistance enhancement to pitting corrosion in NaCl solutions. More recently, the fabrication of durable SHPS on Al substrate by etching and ZnO nanoparticle deposition were reported by Rezayi et al.,³⁷ with the surfaces showing a remarkable decrease in corrosion current density and long-term stability improvement versus immersion in 3.5 wt% NaCl solution.

It is of great scientific significance and industrial value to fabricate a functional SHPS on Al or its alloys. As to the fabrication, some strategies have been reported to fabricate an SHPS on Al or its alloys, mainly including spin-coating,³² boiling water treatment,³⁸ chemical etching,^39–45 sol–gel processing,^46,47 hydrothermal method,³¹ anodization,^48–52 laser treating,⁵³ high speed wire electrical discharge machining (HS-WEDM) technology,⁵⁴ chemical vapor deposition,^55,56 and an in situ growth process.^57,58 However, many of these methods consist of two general steps, namely, creating rough hierarchical topographies and lowing the surface energy, which have obvious shortcomings for practical applications, such as them involving a multistep procedure, the need for special apparatus and expensive reagents, and they are time-consuming. Therefore, if the two steps involved in the traditional preparation of a SHPS could be accomplished in just a single step, then the complexity of fabricating a SHPS would be eliminated. Recently, researchers have developed several solution-immersion one-step methods to construct an SHPS on Al or Al alloy substrates. For instance, Saleema^59,60 reported a simple one-step process to render a SHPS on an Al alloy substrate by immersing the substrate in an aqueous solution containing sodium hydroxide (NaOH) and fluoroalkyl-silane (FAS-17) molecules. Zhang et al.⁶¹ employed a low-cost one-step method to fabricate a SHPS on an Al alloy, simply by immersing the substrates in a solution containing hydrochloric and fatty acid molecules. Ou et al.⁶² reported a simple strategy to prepare a SHPS on 5083 Al alloy by immersing the substrates in a solution containing 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (PFOTS), ethanol, and H₂O/H₂O₂. Such few one-step examples are impressive for their easy operation; however, these methods are still subject to certain limitations, such as being highly toxic and corrosive, difficult to popularize, etc. To the best knowledge of the authors, studies about simple and universally feasible one-step techniques suitable for a wide range of materials are relatively scarce. In particular, few literature studies have reported fabricating a SHPS on an Al substrate via a one-step electrodeposition method and used it as a barrier against marine corrosion.

In this paper, a facile one-step electrodeposition approach was used to fabricate a SHPS on an Al substrate, providing it with a lotus-like self-cleaning effect and greatly enhanced corrosion resistance. The surface wettability, morphology, and chemical composition were investigated, respectively. The anti-contamination performance of the as-prepared SHPS was evaluated by the water sliding test and self-cleaning test. The anticorrosion behaviors of the surface were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) curves. The durability and stability of the resultant SHPS was evaluated through air exposure, deionized water immersion and a 3.5 wt% NaCl immersion test. This work offers a facile route to precisely control the surface roughness and wettability. In addition, this work provides metallic materials a potential solution for corrosion protection under harsh corrosive environments.

2. Experimental details

2.1. Materials and reagents

High-purity (99.999%) aluminum foils (25 mm × 20 mm) with 0.5 mm thickness were purchased from Beijing Goodwill Metal Technology Co., Ltd. Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O), acetone, and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. Hexadecanoic acid (HA) was purchased from BBI Life Sciences. Manganese monoxide (MnO, 99%) was purchased from Aladdin. All the reagents were of analytical grade and used without further purification. Deionized water with a resistivity of 18.0 MΩ cm was used in all the experiments.

2.2. Surface polishing

The aluminum foils were degreased in acetone and ethanol by ultrasonication for 10 min and rinsed in deionized water and ethanol before use, then dried using a blower. The cleaned and dried specimen was electropolished to reduce the surface oxidation film in a mixture of perchloric acid and ethanol (HClO₄/C₂H₅OH = 1 : 4 in volumetric ratio) under an applied potential of 20 V for 3 min at 4 °C with strong and uniform agitation of the electrolyte using a magnetic stirrer.

2.3. Fabrication of SHPS via one-step electrodeposition

Cerium(III) nitrate hexahydrate (0.05 M) and hexadecanoic acid (HA, 0.2 M) were immersed in ethanol under stirring until a uniform electrolyte solution was obtained. A Pt plate and polished Al foil were taken as the anode and cathode in an electrolyte cell, and a direct current (DC) voltage was applied to the electrodes. The distance of the two electrodes was 2 cm. The electrodeposition process was carried out under an applied potential of 30 V for 1 h at 40 °C. After electrodeposition, the working electrodes were rinsed thoroughly several times with ethanol and dried under atmospheric conditions.

2.4. Characterizations

SEM images were measured with a field-emission scanning electron microscope with an energy dispersive spectroscopy (EDS) instrument under a vacuum environment at 15 kV (Hitachi S-3400N, Japan). The roughness of the surfaces was measured by an atomic force microscope (AFM, Multimode 8, Bruker) in the tapping mode. The AFM scan area was 10 μm × 10 μm. The chemical compositions of the samples were studied by FTIR (Thermo Scientific NICOLET iS10) and XPS spectra (Thermo Scientific ESCALAB 250Xi). The measurements were performed using monochromated Al Kα irradiation and the chamber pressure was about 3 × 10⁻⁸ Torr under the test conditions. The binding energy of adventitious carbon (C1s: 284.8 eV) was used as a basic reference. FTIR analyses were performed in the range of 400–4000 cm⁻¹, with a resolution of 4 cm⁻¹. The contact angles (CAs) of water droplets (∼4 μL) were measured with a contact angle meter (JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at ambient temperature. The average and standard deviation values of the CAs were obtained via more than five measurements conducted at different locations on each specimen. The SAs were measured by slowly tilting the sample stage until the drops started moving using ∼4 μL of water droplets.

The electrochemical measurements were carried out in 3.5 wt% NaCl corrosion aqueous solutions on Gamry Reference 3000 (USA) in a three-electrode system: using a platinum plate, saturated calomel electrode (SCE), and the samples (bare Al surface and SHPS) as the counter, reference, and working electrodes, respectively. The surface area of the test samples open to the corrosion solution was 1 cm². EIS measurements were carried out at an open circuit potential with an AC amplitude of 15 mV over a frequency range of 10 mHz to 100 kHz. Polarization curves were subsequently performed from −250 mV to +250 mV versus an open circuit potential (OCP) at a scanning rate of 1 mV s⁻¹. Prior to the measurement, each sample was immersed in 3.5 wt% NaCl solution for 1 h to approach a steady state and establish the open circuit potential. Impedance data were analyzed using ZSimpWin software. Each electrochemical test was carried out at least three times to ensure reproducible results. All the electrochemical measurements were conducted at room temperature (298 K).

3. Results and discussion

3.1. Schematic illustration and electrodeposition mechanism

Fig. 1 shows a schematic illustration of the fabrication process of the SHPS on the Al substrate through a one-step electrodeposition approach. The electrodeposition process was carried out on the electropolished Al substrate in an ethanol solution containing cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) and hexadecanoic acid (HA). The electrodeposition mechanism can be explained as follows:⁶³ when a DC voltage was applied to the electrodes, HA molecules in the electrolyte solution reacted with the Ce³⁺ ions near the cathode to form cerium hexadecanoate and hydrogen ions (H⁺). Meanwhile, the concentration of the free H⁺ ions in the electrolyte solution increased quickly and some of them gained electrons to generate H₂ gas, thereby stirring the electrolyte. The reaction preferentially proceeded in the vicinity of the cathode of the growing particles, where the concentration of H⁺ was decreased, and the growth was mainly controlled by the reaction. The electrodeposition mechanism can be generated by the following reaction equations:

Ce³⁺ + 3CH₃(CH₂)₁₂COOH → Ce(CH₃(CH₂)₁₂COO)₃ + 3H⁺ (1)

3H⁺ + 3e⁻ → 3/2H₂ (2)

Fig. 1 Schematic for the preparation process of the SHPS on an Al substrate based on a one-step electrodeposition method.

After the one-step electrodeposition process, SHPS on the Al substrate was obtained. The characterizations of the resultant SHPS are investigated in the following sections, including surface morphology, wettability, chemical composition, self-cleaning, and anticorrosion properties.

3.2. Surface morphology and wettability

The surface topography of the as-prepared SHPS on the Al substrate was characterized by SEM and AFM. Fig. 2 shows the typical SEM images and AFM tapping mode topographical images of the resultant SHPS. After the electrodeposition process, the low-magnification image (Fig. 2a) indicates that the SHPS is not smooth and consists of a large quantity of well-dispersed micro-papillae structure. It further reveals that the papilla-like pattern has a diameter of about ∼2–3 μm in Fig. 2b, and that a typical single micro papilla includes a nanoscale structure. The higher-magnification image (Fig. 2c) demonstrates that these micro-papillae consist of some flower-like structures and are built with many staggered nanoplatelets as building blocks. AFM tapping mode topographical images were used to investigate the nanostructure of the single papilla. Fig. 2d and e exhibit enlarged observation images of the micro-papillae and flower-like microcluster, showing that the rough surface is composed of numerous irregular tiny nanoplatelets intercrossed with each other and seem perpendicular to the Al substrate. The typical thickness of the nanoplatelets is about ∼90 nm. Meanwhile, many messy gaps appeared between these nanoplatelets, resulting in a larger fraction of air trapped in the irregular void spaces and grooves.

Fig. 2 Typical SEM images of the resultant SHPS at different magnifications ((a) – 1000×, (b) – 2500×, (c) – 7500×) and AFM tapping mode topographical images of the papillae (d and e). The inset of (a) is the static contact angle of water droplets (∼4 μL) on the as-prepared surface.

As is known, roughness influences the behaviors of liquid droplets in both thermodynamic and hydrodynamic aspects, and plays an important role in determining the wettability of the surface wettability, especially to achieve surface superhydrophobicity. Enhanced surface roughness can increase the surface static contact angle and make the surface water repellent. Therefore, as a well-established tool, the AFM technique was used to observe the topography and roughness of the as-prepared SHPS.

Fig. 3 shows the corresponding AFM tapping mode 3D images of the micro-papillae structure and nanoplatelets on a single papilla, illustrating the surface roughness of the as-prepared SHPS. The scanning area of Fig. 3a is 10 × 10 μm², and the average roughness (R_a) and root-mean-square roughness (R_q) was found to be 278 nm and 348 nm, respectively. As for Fig. 3b, the scanning area is 2 × 2 μm², and the R_a and R_q values are 95.1 nm and 122 nm, individually. Compared with Fig. 2, the 3D images of Fig. 3 are more intuitive to show the corresponding morphology of the as-prepared SHPS. It can be concluded from the images that the surface has nano–micro-binary structures, which can capture a large amount of air. Therefore, this provides a beneficial geometric condition for final formation of the superhydrophobicity.

Fig. 3 AFM tapping mode 3D images of the micro-papillae structure and nanoplatelets on a single papilla. The 3D images clearly show the formation of hierarchical roughness. Scanning area: (a) 10 × 10 μm², (b) 2 × 2 μm².

According to the surface morphology analysis of Fig. 2 and 3, the topography of the as-fabricated SHPS is advantageous in leading to excellent superhydrophobic and low adhesive properties, with a static water contact angle as high as 167.4° and a sliding angle less than 2°. The corresponding photo of the water droplet on the SHPS is shown in the inset of Fig. 2a, exhibiting an excellent superhydrophobicity. The dual-scale (micro-papillae and nanoplatelets) roughness, together with the low surface energy of hexadecanoic acid, accounts for the superhydrophobicity.

The wettability of a surface is dependent on both the surface topography and chemical composition. Generally, the superhydrophobicity of the surface can be explained by the Cassie–Baxter model,⁶⁴ where the SHPS is regarded as a porous medium composed of air pockets. The apparent contact angle is formulated as follows:

cos θ_r = f₁ cos θ − f₂ (3)

where θ_r represent the water contact angle of the as-fabricated SHPS micro/nanostructured surface; θ is the contact angle of the smooth Al surface after hexadecanoic acid modification; and f₁ and f₂ are the area fraction of the water–solid and water–air interface, respectively.

According to this equation, the θ_r of an SHPS increases as the fraction of the water–air interface (f₂) increases. According to the calculation, the value of f₂ of the as-prepared SHPS is estimated to be 0.963, which means that the air occupies about 96.3% of the contact area between the water droplet and the SHPS. The resultant SHPS has a large fraction of trapped air between the grooves, which can reduce the contact area between water droplets and the solid surface and may be responsible for the consequent surface superhydrophobicity.

3.3. Chemical composition

FTIR, XPS, and EDS spectra were utilized to analyze the chemical composition of the as-prepared SHPS. Fig. 4a presents the FTIR spectra of pure hexadecanoic acid (HA) and the resultant SHPS. In the high-frequency region of the two curves, the absorption peaks at 2914 cm⁻¹ and 2847 cm⁻¹ are attributed to C–H symmetric and asymmetric stretching vibrations, respectively. In the low-frequency region, the peak for the carboxyl group (–COO) of HA at 1697 cm⁻¹ is no longer present at the SHPS, and two new absorption peaks appear at 1540 cm⁻¹ and 1445 cm⁻¹, corresponding to the appearance of carboxylate (cerium hexadecanoate).

Fig. 4 Chemical composition analysis of the bare Al surface and SHPS. (a) FTIR, (b) XPS, (c) EDS, (d) O1s region, and (e) C1s region.

Fig. 4b reveals the XPS survey spectrum of the as-prepared SHPS. According to the figure, the presence of Ce, C, and O on the SHPS can be observed. Fig. 4d and e show the O1s and C1s decomposition-fitted curves of the XPS spectra, respectively, which were investigated for further confirmation of the chemical composition of the as-prepared SHPS. As a result, the O1s spectrum of the as-prepared surface shows two main peaks: a peak at a binding energy of 531.6 eV, which is duo to the C=O bond, and a peak located at 529.9 eV, which can be assigned to the C–O– bond. The C1s spectrum consists of two peaks: a peak at 284.8 eV, which is attributed to –CH₂, and a peak located at 288.6 eV, which is attributed to the O=C–O– (carboxyl) group of HA. The XPS results indicate that HA was successfully deposited onto the specimen surface. In addition, Fig. 4c shows the EDS regional analysis of the as-prepared SHPS. It was found that the presence of C, O, and Ce on the resultant SHPS can provide evidence of the electrodeposition product. These results prove that HA is grafted on the original bare Al surface, and the outermost surface is mainly composed of low surface energy CH₃(CH₂)₁₄COO⁻, which contribute to the surface superhydrophobicity. On the basis of the above analysis and chemical valences of Ce³⁺ and CH₃(CH₂)₁₄COO⁻ in the electrolyte solution, we can deduce that cerium hexadecanoate, Ce[CH₃(CH₂)₁₄COO]₃, was formed on the original Al substrate.

3.4. Low adhesion and self-cleaning test

The nonsticking behavior of the as-fabricated SHPS was also investigated. Fig. 5a shows the water droplet (∼4 μL) sliding process with respect to the as-prepared SHPS on a fixed angle of 2.0°. The water droplet slides on the surface quite quickly in a very short time (about 200 ms), with a fixed sliding angle of 2.0°, demonstrating an excellent low adhesive superhydrophobicity property.

Fig. 5 Water droplet sliding process (a) with a fixed angle of 2.0°, showing the low adhesive superhydrophobicity and demonstration of the self-cleaning ability of bare Al surface (b) and SHPS (c).

Functional surfaces simultaneously possessing a high static water contact angle and a low sliding angle have an important application in the field of anti-contamination. The self-cleaning ability facilitates reliable and durable applications of water-repellent substrates in complex environments.

Fig. 5b and c show the self-cleaning effect of the bare Al surface and the as-prepared SHPS placed at a fixed slope angle. First, the bare Al surface and SHPS were contaminated deliberately with manganese monoxide (MnO) powder as dust. To reduce the surface energy, the hydrophobic dust, herein MnO particles, tended to accumulate at the air/water interface. Subsequently, the water was dropped to the upper side of the surface by a plastic dropper. The rolling water droplet was able to pick up the hydrophobic dust without influence on its motion, resulting from the minimized contact area between the droplet and the SHPS (i.e., via a “roll-to-clean” mechanism). According to Fig. 5b and c, when water droplets roll off the SHPS, they entrain the dust and contaminants along with them. This is not the case for bare Al surfaces, where the dust remains. The SHPS can efficiently clear off the hydrophobic dust under several water droplets, whereas the bare Al surface did not have a competitive performance. The hydrophobic dust might adhere to the bare Al surface and cannot be completely removed even under water flushing. This experiment indicates that the as-prepared SHPS has an excellent lotus-like self-cleaning effect.

3.5. Anticorrosion performance and surface durability

The corrosion resistance of the as-prepared SHPS is a key factor in determining the possibilities of the SHPS in fundamental research and practical applications. The electrochemical behavior of the SHPS was investigated in a 3.5 wt% NaCl corrosion solution. The SHPS can form a successful barrier against moisture/water diffusion through hydrophobic particles underlying the Al surface, where corrosion can be initiated. Tafel plots were measured when a stable open circuit potential was obtained after the samples had been exposed to 3.5 wt% NaCl solution for a certain period. Fig. 6a exhibits the potentiodynamic polarization curves of the bare Al surface and SHPS after immersion in 3.5 wt% NaCl aqueous solution for 1 h. According to the Tafel extrapolation from the potentiodynamic polarization curves, important electrochemical parameters, such as the corrosion potential (E_corr) and the corrosion current density (I_corr), for the bare Al surface and SHPS can be obtained. In general, in a typical polarization curve, the lower I_corr value and positive E_corr value indicate a lower corrosion dynamic rate and lower corrosion thermodynamic tendency. It should be noted that the E_corr of the SHPS (−0.706 V) is more positive than that of the untreated bare Al surface (−1.098 V). In addition, I_corr of the SHPS (2.864 × 10⁻¹⁰ A cm⁻²) decreased by more than 2 orders of magnitude compared to that of the bare Al surface (9.647 × 10⁻⁸ A cm⁻²). Thus, we could conclude that the as-prepared SHPS is effective and has long-term potential for improving the corrosion resistance of the Al substrate.

Fig. 6 Potentiodynamic polarization (a), EIS plots and fittings (b–d), and equivalent circuit (e and f).

EIS is generally recognized as an effective and revealing method for the corrosion characterization of coated metals. In the present work, the EIS measurements were performed under an open circuit potential in 3.5 wt% NaCl corrosive solutions with a frequency range from 10 mHz to 100 kHz. Fig. 6b–d shows the EIS plots and their fitting results for both the bare Al surface and SHPS in 3.5 wt% NaCl solution. It is clear that the diameter of the capacitive arc of SHPS is much higher than that of the bare Al surface (Fig. 6b), and simultaneously, the value of |Z| of SHPS is improved by 3 orders of magnitude (Fig. 6c), indicating that the SHPS can effectively prevent the corrosion ions from the underlying aluminum. From the Bode-phase angle versus frequency plots of the SHPS (Fig. 6d), it can be seen that it presented two time constants within the tested frequency range, which correspond to the electrode process of the superhydrophobic layer at the higher frequency (10⁵ to 10² Hz) and the layer/metal interface at the lower frequency (10⁻² to 10² Hz); while for the bare Al surface, there is only one time constant, suggesting that there is only one electrochemical process happening on the metal surface. In this case, the EIS results for both the bare Al surface and SHPS can be analyzed with the equivalent circuits shown in Fig. 6d and e, in which R_s is the solution resistance, R_ct is the charge-transfer resistance, R_f is the resistance of the film (superhydrophobic layer), and Q_dl and Q_f are the constant phase elements modeling the capacitance of the double-layer and superhydrophobic layer, respectively. The impedance of the constant phase element is defined as:^65,66

where Y₀ represents the modulus, j is the imaginary number, ω is the angular frequency, and n is the phase.

The |Z| value for the SHPS (Fig. 6c) is higher than 10⁶ Ω cm², which means that the layer can play a good role in protection of the base metal, whereby corrosive ions, such as, water, oxygen ions, chloride ions, are prevented from participating in the electrochemical reaction. Thus, it is reasonable to analyze the EIS plots with a series-parallel equivalent circuit regarding the superhydrophobic layer as the capacitor (Fig. 6f), and the film resistance with an R–Q combination in parallel. Table 1 shows the electrochemical parameters obtained from simulation of the EIS results of the SHPS and bare Al surface in 3.5 wt% NaCl solution. Generally, the inhibition efficiency (η) is calculated with the following formula:^67,68

where R⁰_ct is the charge-transfer resistance of the bare Al surface and R_ct is the charge-transfer resistance of the SHPS.

Table 1 Electrochemical parameters obtained from simulation of the EIS results of the bare Al surface and SHPS in 3.5 wt% NaCl solution

Samples	Rs (Ω cm^2)	Qf		Rf (Ω cm^2)	Qdl		Rct (Ω cm^2)	η (%)
		Y (F cm^−2)	n		Y (F cm^−2)	n
Bare Al surface	4.38	—	—	—	3.66 × 10^−6	0.96	3.05 × 10^4	99.96
SHPS	2.17	3.64 × 10^−8	0.74	4.77 × 10^4	1.02 × 10^−7	0.82	7.11 × 10^7

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In Table 1, the Q_dl value and R_ct value of the bare aluminum are 3.66 × 10⁻⁶ F cm⁻² and 3.05 × 10⁴ Ω cm², respectively, while for SHPS, the two parameters are 1.02 × 10⁻⁷ F cm⁻² and 7.11 × 10⁷ Ω cm², individually. The higher Q_dl value and lower R_ct value of the bare Al surface indicates that the charge-transfer process happens more easily than that of SHPS. Furthermore, the inhibition efficiency of the superhydrophobic layer to corrosion of the substrate was calculated as 99.96%, indicating that the as-fabricated SHPS can effectively protect the underlying aluminum substrate from corrosion. It has been proven that air can be trapped in the hierarchical structure of the SHPS. The air cushion trapped in the SHPS can greatly enhance the anticorrosion performance of the substrate by inhibiting the penetration of corrosive medium into the surface. As a result, corrosive Cl⁻ cannot penetrate into the Al substrate, so that the SHPS can effectively separate the corrosive medium and protect the underlying aluminum substrate, indicating that the SHPS can inhibit corrosion of the substrate in a marine atmosphere or in natural seawater.

These results from the polarization and EIS further demonstrate that the as-prepared SHPS provides excellent and long-term anticorrosion abilities, which can suppress the contact of the Al substrate with the corrosive solution. It is believed that the SHPS can easily trap a large amount of air among the hierarchical rough structures. When the SHPS is immersed in a corrosive solution, such an “air cushion” behaves as a dielectric for a pure parallel plate capacitor and inhibits the electron transfer between the electrolyte and the Al substrate, thereby significantly improving the corrosion resistance of the Al substrate.

As of today, the durability of the SHPS is a challenging area to work with, in terms of its industrial applications. In view of this, we attempted to evaluate the durability of the as-prepared SHPS in different aspects: (a) exposed to air for 8 months at ambient temperature, (b) immersed in deionized water for 7 days, (c) exposed in 3.5 wt% NaCl aqueous solution for a 72 h, and then checked their wettability. Fig. 7a–c shows the variation of the static water contact angle against time duration. It can be seen that the resulting SHPS can maintain good superhydrophobicity in air, deionized water, and 3.5 wt% NaCl aqueous solution for a certain time. The values of the water contact angle vary little, indicating that the as-prepared SHPS has a long-term stability and durability. It is worth noting that the wear resistance of the SHPS is of great significance for wide industrial applications in the future. The mechanical stability of the as-prepared SHPS was characterized by an abrasion test, as shown in Fig. 7d. The as-prepared SHPS was dragged to move on 1000 grit SiC sandpaper in one direction under a pressure of 1.3 kPa. Changes in the CAs of the SHPS were evaluated. The test showed that the resultant surface still maintain a CA above 150° after abrasion for 500 mm. However, after abrasion for 600 mm, the CA of the surface decreased to 142.6 ± 4.8°. Despite this, the as-fabricated SHPS showed a good mechanical durability to some extent. These data above clearly demonstrate that the SHPS fabricated via this one-step electrodeposition technique was extremely stable and had a high degree of durability and repeatability.

Fig. 7 Variation of the static water contact angle against time duration: (a) air exposure, (b) deionized water immersion, (c) 3.5 wt% NaCl immersion. (d) Schematic diagram of the abrasion test and CA of the SHPS as a function of abrasion length.

4. Conclusion

In summary, a durable self-cleaning SHPS on an Al substrate was synthesized as an efficient barrier against corrosive solutions through a facile one-step electrodeposition approach. After the one-step electrodeposition process, a micro/nano hierarchical papillae structure consisting of a large quantity of staggered nanoplatelets as building blocks was obtained, endowing the substrate with unique superhydrophobicity. The dual-scale surface roughness and the low surface energy of HA accounts for the final superhydrophobicity. The resultant SHPS showed a CA of 167.4° and a SA of less than 2.0°. It also exhibited an excellent lotus-like nonsticking self-cleaning ability. As for anticorrosion properties, the E_corr of the SHPS (−0.706 V) was more positive than that of the untreated bare Al surface (−1.098 V). In addition, I_corr of the SHPS (2.864 × 10⁻¹⁰ A cm⁻²) decreased by more than 2 orders of magnitude compared to that of the bare Al surface (9.647 × 10⁻⁸ A cm⁻²). The corrosion inhibition efficiency was as high as 99.96%. Air exposure, deionized water immersion, and 3.5 wt% NaCl immersion test results indicated that the resultant SHPS possessed good durability and stability. These results demonstrate the advantages of the as-prepared SHPS as an effective barrier against hostile corrosive media. We believe that the facile one-step electrodeposition technique presented here may provide a valid strategy in the design of a SHPS on metallic materials and expand the potential practical applications of a SHPS for anti-contamination, anticorrosion, or anti-biofouling applications in harsh marine environments.

Acknowledgements

This study was financially supported by National Natural Science Foundation of China (Grant No. 41276074) and Basic Research Project of Science & Technology Plan of Qingdao (Grant No. 13-1-4-122-jch).

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