Controllable Construction and Corrosion Resistance Mechanism of Durable Superhydrophobic Micro-Nano Structure on Aluminum Alloy Surface

Abstract

Aluminum alloy corrosion resistance could be improved by micro-nanostructures on superhydrophobic surfaces, but inadequate mechanical stability remains a bottleneck concern in the sector. Herein, femtosecond laser processing and spray modification techniques are employed to fabricate “armor-style” micro-nanostructures on aluminum alloy surfaces. The construction of durable superhydrophobic surfaces was controllably constructed using this strategy. Applying a spray of hydrophobic nano silica onto the surface of aluminum alloys is an effective method for creating a low surface energy coating, while the femtosecond laser-processed “armor-style” micro-nano structure offers additional adhesion sites for the hydrophobic nano-silica. The findings indicated that the treated surface’s contact angle (CA) reached 152.5° while the slide angle (SA) was only 2.3°, exhibiting favorable superhydrophobic performance. Being worn 100 times with 400# sandpaper, the superhydrophobic surface retained a contact angle above 150°. Electrochemical tests demonstrated significant reductions in the self-corrosion current of superhydrophobic surfaces. Meanwhile, the impedance increased significantly, showing good thermal, mechanical, and chemical stability, enabling better sustainable use of aluminum alloys. These results will serve as a theoretical foundation for the surface protection of aluminum alloys.

1. Introduction

Aluminum alloy is known for its exceptional lightweight properties, high strength-to-weight ratio, and low overall density. Currently, this material is utilized in a wide variety of applications including marine engineering, ship transportation, and rail transit systems, among others [1,2,3]. However, prolonged exposure to corrosive environments could significantly affect the lifespan of components by accelerating corrosion behaviors [4]. Therefore, improving the corrosion resistance of aluminum materials has become a research focus.

One of the most widely used approaches to enhance the corrosion resistance of materials is to apply anti-corrosion coatings on the surface of substrates through techniques such as electroplating or spraying [5,6,7]. The anti-corrosion layer functions by obstructing the movement of corrosion ions toward the substrate, thus effectively preventing corrosion. However, anti-corrosion coatings are prone to defects such as cracking, peeling, blistering, and rusting [8,9]. Inspired by lotus leaves, there has been a growing interest in utilizing superhydrophobic surfaces to enhance corrosion resistance [10,11,12].

Over the last few decades, a plethora of techniques have been employed to prepare superhydrophobic surfaces [13,14,15,16,17,18]. Yu et al. [19] successfully fabricated a superhydrophobic surface by spray-coating aluminum with silica nanoparticles. Meanwhile, the prepared superhydrophobic material has exhibited outstanding self-cleaning performance, durability, and stability under harsh environmental conditions. Guo et al. [20] successfully constructed a superhydrophobic surface by employing a combination of electrolytic polishing, etching using a 0.7 g/L MgCl₂ aqueous solution, and modification via perfluorooctyltriethoxysilane. By utilizing anodization and low-temperature plasma treatment on an aluminum substrate, Wang et al. [21] successfully achieved a superhydrophobic surface. They further modified the surface with trichlorooctadecylsilane, resulting in a CA of 152.1° for the modified aluminum substrate surface. After performing plasma treatment on the anodized aluminum film, the surface exhibited micro-nanostructures that enabled a CA of 157.8°. Using chemical etching, Chen et al. [22] successfully fabricated a superhydrophobic surface on an aluminum substrate and conducted subsequent characterization through SEM, CA measurement, and optical methods. The obtained superhydrophobic surface had a CA of 154.8° and an SA of about 5°, with irregular micro-nano structures resulting from the etching process. The superhydrophobicity on the aluminum substrate exhibited a favorable self-cleaning performance.

The application of femtosecond laser processing technology for the preparation of superhydrophobic surfaces has garnered significant interest, especially for enhancing the corrosion resistance of substrates. Femtosecond laser technology as a convenient processing method was broadly employed in anti-corrosion and self-cleaning applications and has been identified as a promising technique for creating superhydrophobic surfaces [23,24]. Zhang et al. [25] used femtosecond laser processing to create a superhydrophobic surface with synergistic anti-wear and corrosion properties on FeMnSiCrNiNb shape memory alloy coatings. The study showed that due to the surface micro-nanostructure, the resulting superhydrophobic surface exhibited favorable superhydrophobicity, as well as good thermal and chemical stability. By utilizing nanosecond ultraviolet laser perpendicular-cross scanning, Xin et al. [26] successfully generated a micro-protrusion array structure on the surface of 5083 aluminum alloy. Their research findings demonstrate that the micro-nanostructure of the resulting superhydrophobic surface played a significant role in improving its corrosion resistance. Using femtosecond laser ablation, Song et al. [27] created a superhydrophobic aluminum surface with controllable adhesion and examined the impacts of critical laser processing parameters on the wetting properties of the laser-ablated surface. Their research outcomes indicated that the adhesion between water and the superhydrophobic surface was significantly influenced by adjusting the laser processing parameters, enabling effective modulation of adhesion from ultra-low to high levels. In addition, Trdan et al. [28] found that polarization resistance increased and anodic dissolution decreased after femtosecond laser shock peening treatment. At the same time, it could also maintain long-term stability in chloride ion solution.

In this experiment, a highly corrosion-resistant superhydrophobic surface was fabricated on aluminum through a combination of femtosecond laser processing and nano-silica spraying techniques. The synergistic effect of the micro-nanostructures generated via femtosecond laser processing and the uniform distribution of nano-silica effectively hindered the penetration of corrosive ions into the substrate, thus achieving a remarkable anti-corrosion effect. Importantly, this approach significantly enhanced the corrosion resistance without affecting other properties.

2. Experiment

2.1. Materials and Methods

Aluminum alloy (7075 aluminum alloy, Guiqian Aluminum Co., Ltd., Shanghai, China, Other elements are Si0.4, Fe0.5, Cu1.2–2.0, Mn0.3, Mg2.1–2.9, Cr0.18–0.28, Zn5.1–6.1, Ti0.2.) was used as the substrate for the experiment. Before processing, the surface was smoothed with 2000# sandpaper. The hydrophobic nano-silica was purchased from Maclean Chemical Co., Ltd., Collingwood, ON, Canada. The hydrophobic nano-silica was sprayed four times during the experiment.

Preparation of micro-nanostructures using femtosecond lasers (Advanced Optowave Corporation Co., Ltd., Ronkonkoma, NY, USA, AOFemto 20–200) operating under the following processing conditions: laser spot size of 2.529 mm, wavelength of 1035.43 nm, laser power of 40.30 W, pulse width of 362 fs, and frequency of 800 kHz [25].

2.2. Characterization Methods

The prepared sample was sectioned into a cubic shape with dimensions of 10 mm × 10 mm × 10 mm. Following the completion of sample preparation, the coating’s structure and phase were thoroughly analyzed utilizing X-ray diffraction (XRD, DX-2800B, Dandong Haoyuan Instrument Co., Ltd., Liaoning, China, 10°~90°, v = 0.06°/s) and scanning electron microscope (SEM, EM-30AX, COXEM) techniques. The element type and content were analyzed using EDS (Energy dispersive X-ray Analysis, Weipu).

The surface morphology was analyzed utilizing a 3D optical profiler (NewView™9000, NUOSD), while electrochemical performance was evaluated through a three-electrode testing system (CHI700E; CH Instrument Company, Ltd., Bee Cave, TX, USA; The Pt electrode area was 1 cm²) measuring polarization curves and impedance tests in 3.5 wt% NaCl solutions, a strong acid (pH = 1), and strong base (pH = 14) at room temperature. To determine the CA and SA, an optical CA measurement system (JC2000D1, China) was employed to measure a 10 μL water droplet’s CA and SA, with three measurements of each different position taken. Wear test using SiC sandpaper (400#), sandpaper weight of 100 g.

2.3. Superhydrophobic Surface Preparation

Before the spraying operation, the sample (a cube with a side length of 10 mm) treated with a femtosecond laser was immersed in alcohol and subjected to ultrasonic treatment for 20 min. Then, it was rinsed with deionized water followed by being dried at room temperature. A homogeneous solution was obtained by dispersing hydrophobic SiO₂ nanoparticles (15~20 nm, 1.2 g) in 50 mL of ethanol and stirring it at 600 rpm for 2 h, followed by a 30-min ultrasonic treatment. Finally, the suspension was applied onto the aluminum surface using a spray gun, followed by natural drying under room temperature, achieving a superhydrophobic effect [29,30,31]. The solution was sprayed with a pressure of 0.8 MPa, and the nozzle distance from the sample surface was 10 mm. The preparation process of superhydrophobic coating is indicated in Figure 1.

2.4. Chemical Stability and Thermal Stability Test

The evaluation of thermal stability for superhydrophobic surfaces was carried out by maintaining the sample at various temperatures (−70, −40, −10, …, 170, 200 °C) for two hours followed by measuring the CA on the surface to ascertain the retention of superhydrophobicity. Solutions with different pH values were prepared using NaOH and HCl at room temperature. By measuring CA in solutions with different pH values (pH = 1~14), the chemical stability of superhydrophobic surfaces can be determined.

2.5. Water Impact Resistance Test and Tape Adhesion Test

In this project, superhydrophobic surfaces were evaluated using high-speed impact water flow to determine their water impact resistance. To determine the average jet velocity v, we used the following formula for calculation:

$$v=\frac{4V}{\pi d^2}\Delta t$$

(1)

In this system, d is the diameter of the nozzle, and V is the volume of the jet of water in Δt time. The nozzle diameter used in the high-speed jetting system measured 2 mm. Within a 0.1 s interval, 5 mL of water could be expelled, resulting in a velocity v of 15.9 m/s.

Adhesion tests were carried out using a 10 mm wide tape. During each experiment, the central position of a piece of tape was adhered to the superhydrophobic surface, the two ends of the tape were then pulled down with force using fingers so that the tape was tightly attached to the superhydrophobic surface, which was then quickly pulled up. The process is illustrated in the diagram below (Figure 2) and was repeated 50 times.

3. Result and Discussion

3.1. Phase and Microstructure

The XRD patterns for the substrate and two “armor-style” structures are shown in Figure 3. A11 and A12 correspond to the XRD spectra of the surfaces of samples with block and triangular structures, while A13 represents the XRD spectrum of the substrate surface. By comparing these three lines, it can be seen that the surface phases of the samples before and after femtosecond laser treatment remain consistent, mainly composed of α phase and η phase.

As shown in Figure 4a, the surface of the block and triangular structure samples primarily consists of oxygen and silicon elements, resulting from the surface-coated nano silica dioxide. Upon further magnification, aluminum elements appear in the EDS spectrum, originating from the deep-layered aluminum alloy substrate of the sample, as depicted in Figure 4e,f.

Figure 5a,b show the 3D contour scanner images of the triangular and block structures. The depressions in the images were caused by laser ablation. The figure clearly shows that the surface treated through a femtosecond laser presents a “groove-shape” structure, leading to a significant rise in the surface roughness of the coating. Nonetheless, the femtosecond treatment elevates the surface energy of the coating, and the surface becomes more hydrophilic [32], resulting in a CA of 88°, illustrated in Figure 5c,d.

3.2. Surface Wettability

To achieve a superhydrophobic state on the surface, we used a spray-coating method with hydrophobic silica nanoparticles. To investigate the relationship between superhydrophobicity and the number of spray-coating cycles, different numbers of silica spray coatings were applied to the sample surface; the experimental results are shown in Figure 6a. The results showed that when the number of spray coatings was applied 1–3 times, the maximum CA was 146°, which did not reach the superhydrophobic effect. However, when the number of spray coatings reached four times or more, the CA stabilized at 150° or above, indicating better hydrophobicity. As shown in Figure 6c, no white particles were found on the pure superhydrophobic surface, while white particles were observed on the surface of the sample sprayed with nanoscale silica, as indicated by the arrows in Figure 6d,e.

Figure 6b illustrates the contact and detachment process of water droplets on the superhydrophobic surface. It can be observed from the figure that because of the favorable superhydrophobicity of the surface, the droplet cannot adhere to the surface and exhibits extremely weak adhesion. The adhesive work could be quantitatively calculated using the Young-Dupré equation [30]:

$$W_{LS}={\gamma }_L\left(1+\cos \theta \right)$$

(2)

where W_LS stands for the adhesive work when the droplet is fully separated from the superhydrophobic surface, while L and S stand for liquid and solid phases, respectively. The recorded data indicate a surface tension value of 72 mN/m for water droplets at a temperature of 25 °C, alongside a CA of 151° with the superhydrophobic surface, leading to a derived adhesive coefficient of 0.00903 J/m² for the superhydrophobic surface, indicating that the micro-nanostructure enables the superhydrophobic surface to exhibit extremely weak adhesion force.

Figure 7a illustrates the presence of two wetting states for the superhydrophobic surface: the Wenzel state [33] (the droplet becomes ensnared within crevices and loses the ability to roll) and the Cassie state [34] (the droplets only remain at the top and could smooth rolling). As shown in Figure 7b,c, on the prepared “armor-style” superhydrophobic coating surface, water droplets stained with methylene blue form a spherical shape due to the repellency of the superhydrophobic coating.

Following this, submerging the samples into deionized water, NaOH solution with a pH = 14, and HCl solution with a pH = 1 for 5 s each produced a silver-white air layer on the coating’s surface, as indicated by Figure 7d–f. Figure 8a indicates the immersion and detachment processes of block structure samples in deionized water dyed with methylene blue. It can be observed from the figures that after leaving the solution, the surface of the samples remained dry, indicating that the wetting state of the sample surface was identified as the Cassie state, which was also observed in strong acid and strong alkaline solutions. In the case of unprocessed samples, upon immersion in solution, no air layer was formed and upon removal from the solution, the surface cannot maintain dryness. In addition, to further explore the sample surface’s self-cleaning efficacy, a small amount of sodium chloride was placed obliquely over the sample, and a solution of methylene blue was dropped on the surface of the sample, as indicated in Figure 8b. As the droplet rolled off, the sodium chloride was quickly removed from the sample surface, demonstrating favorable self-cleaning performance. The lotus effect is the name given to this phenomenon [35], and its principle model is indicated by Figure 8c. The existence of this effect could effectively protect the surface of the workpiece in harsh working environments. However, for unprocessed aluminum, the dropped droplets quickly adhered to the sample surface and did not exhibit self-cleaning properties.

3.3. Corrosion Resistance

As shown in Figure 9a, to evaluate the corrosion resistance performance of superhydrophobic aluminum coated with nano silica spray treated with a femtosecond laser, the polarization curves of three types of samples were tested in NaCl solution. The higher the Ecorr value, the less susceptible the sample was to corrosion, while a lower Icorr value suggested a slower corrosion rate for the sample. It can be observed from the figure that the self-corrosion potential of pure aluminum reached −1.25 V, but the self-corrosion voltage of the prepared “armor-style” superhydrophobic structure increased by 0.6 V, indicating that the superhydrophobic coating exhibits a low level of activity, leading to a marked enhancement in its anti-corrosion properties. Moreover, the self-corrosion current also decreased, and the surface corrosion rate also decreased to some extent, as shown in

Table 1. Corrosion parameters extracted from polarization curves.

Sample	Rp (kΩ·cm−2)	βa (Vdec−1)	βc (Vdec−1)	B (mV)	Icorr (μA·cm−2)	Ecorr (V)
Pure aluminum	5.04	0.28	0.10	32.65	6.45	−1.23
Block structure	8.13	0.10	0.34	33.36	4.11	−0.60
Triangular structure	8.43	0.19	0.25	46.48	5.57	−0.67

.

The Rp value can be obtained from the Stern–Geary equation [36]:

$$R\mathrm{p}=\frac{B}{Icorr}$$

(3)

In this context, B denotes the Stern–Geary constant, which could be derived from the anode and cathode Tafel constants βa and βc:

$$B=\frac{\beta \mathrm{a}\times \beta \mathrm{c}}{2.303\times \left(\beta a+\beta c\right)}$$

(4)

βa and βc in the formula could be calculated using data from polarization curves. As per the data presented in Table 1, the polarization resistance (Rp) of both block and triangular structures is higher than that of pure aluminum samples. This indicates that the femtosecond laser-treated samples exhibit improved corrosion resistance capabilities.

Nyquist radius [37] reflects corrosion resistance, with a larger radius indicating stronger corrosion resistance. The Nyquist radius of the block and triangular structured samples in Figure 9b is significantly larger than that of pure aluminum, indicating improved corrosion resistance after processing. The equivalent circuit used to calculate the EIS results of the samples is indicated in Figure 10, and the fitted data are indicated in

Table 2. EIS parameters obtained from analog circuits.

Sample	Rs (Ω·cm2)	Cdl (F cm−2)	CPE (Ω−1cm−2sn)		Rct (kΩ·cm2)
			Q	n
Pure aluminum	121.40	5.37 × 10−7	2.74 × 10−6	0.83	10.22
Block structure	8.72	3.74 × 10−5	4.18 × 10−5	0.83	13.56
Triangular structure	72.61	5.23 × 10−5	5.03 × 10−5	0.66	21.44

, where Rs, Rct, and CPE are associated with solution resistance, interfacial charge transfer resistance, and constant phase element, respectively.

The capacitance (C) of the sample is calculated using the resistance R and CPE (Q). The formula is:

$$C=\frac{{\left(QR\right)}^{\frac{1}{\mathrm{n}}}}{R}$$

(5)

As per the EIS data, the treated samples manifest lower Rs values relative to pure aluminum; concurrently, both block and triangular structural specimens demonstrate higher Rct values compared with unprocessed aluminum, aligning with the results obtained from the Nyquist plot.

As is well known, “gas valleys” [38,39] are formed in the micro-nano peak structure under the Cassie state. The presence of “gas valleys” could obstruct the contact of corrosive ions with the substrate surface, such as Cl⁻, thereby significantly improving the corrosion resistance [40], as shown in Figure 11.

3.4. Stability

The comprehensive performance of superhydrophobic surfaces is significantly influenced by the chemical and thermal stability of the surface. Figure 12a illustrates the variation of CAs of two samples within a temperature range of −70 °C to 200 °C. The results indicate that the superhydrophobic surface, which has undergone ultralow and ultrahigh temperature treatments, maintains a CA of over 150°, exhibiting favorable superhydrophobic performance. To gain deeper insights into the chemical stability of the superhydrophobic surface, a deep analysis of the CAs exhibited by specimens in solutions with varying pH values (1~14) was conducted, as indicated by Figure 12b. Remarkably, the findings suggest that the chemical stability of both samples is in harmony, as they exhibit a sustained CA of over 150° even in the presence of harsh acidic environments. The superhydrophobic surface’s remarkable capability to resist corrosion experiences a noteworthy boost under an acidic environment. This observation remains in perfect agreement with the outcomes drawn from the polarization curve. However, in alkaline solutions with pH = 11~14, the CAs of both samples decreased to below 150°, losing their superhydrophobic effect. This indicates that in strongly alkaline environments, the alkaline solution reacts with the silica particles on the sample surface. Therefore, the superhydrophobic performance of the sample surface is reduced. The above tests on the superhydrophobic surface of the sample confirm its favorable thermal stability and chemical stability.

As indicated by Figure 12c, the CA variation of the two samples and the substrate after different water flow impact times are displayed. After 15 high-speed water jet impacts, the surface CA of the sample remained above 150°, indicating that the sample surface still had favorable superhydrophobic properties. After 50 high-speed water jet impacts, the surface CA of the sample was about 130°. However, the CAs exhibited by the substrate surface experienced a rapid decrease upon exposure to increasing degrees of water flow impact; after undergoing as few as 50 cycles, the surface CA of the substrate was only 95°. The above experiments prove that the micro-nanostructure on the surface of the sample has a good protective effect. Because the micro-nanostructure provides more contact points for the hydrophobic material (silica) on the surface, it could protect it from being washed away to some extent, thereby reducing the wear effects of high-speed water jet impact. Furthermore, the presence of “gas valleys” within the micro-nanostructures under the Cassie state results in a heightened pressure differential between air and liquid phases, thereby further enhancing the protective attributes of the surface. Conversely, in the absence of such protective micro-nanostructures, an untreated substrate under the impact of high-speed water flow, the superhydrophobic properties of the surface will decrease rapidly.

In addition, tape adhesion experiments were performed on the surfaces of pure aluminum, block, and triangular structured samples. The experimental process is shown in Figure 13, which demonstrates the effect of tape adhesion on the sample surface. The tape reduces the superhydrophobicity of the sample surface by adhering to and removing the nano-silica particles on the surface, and the superhydrophobic stability of the samples was evaluated by measuring the CA of the sample surface. The experimental results in Figure 12d show that after 50 tape adhesion experiments, the surface CA of the triangular structure sample remained at 150°, while the surface CA of the block structure sample remained at 150° after 30 tape adhesion experiments. In contrast, the pure aluminum sample lost its superhydrophobicity after only three tape adhesion experiments. These results indicate that the micro-nano structures on the sample surface play a protective role for the nano-silica particles, allowing the superhydrophobic properties to be maintained and demonstrating good stability on the sample surface.

3.5. Durability

As shown in Figure 14a, a reciprocating wear test was conducted on the superhydrophobic surface to further verify its favorable wear resistance. A weight of 100 g was placed on the 400# SiC sandpaper, and the reciprocating wear experiment was carried out on the superhydrophobic surface. The moving speed of the sandpaper was 5 cm/s [31]. As the number of experiments increased, the CA of the superhydrophobic surface gradually decreased, while the sliding angle gradually increased, as indicated by Figure 14b. It is possible to divide the wear of the superhydrophobic surface into several stages when testing its wear resistance. In the initial wear stage, the micro-nano structure of the superhydrophobic surface was slightly compressed but still maintained favorable superhydrophobic properties (CA > 150°, SA < 10°), indicating strong anti-wear stability. As the number of wear tests increased, the groove structure wear became more severe, but the nano-silica still adhered to the surface of the superhydrophobic layer, and the CA remained slightly below 150°, retaining good superhydrophobicity. At the end of the wear tests, the micro-nano groove structure was completely worn away, and the nano-silica was only left in the grooves, with a stable CA of 124°. As the sample surface lost its micro-nano structure, the surface wetting state had changed from Cassie state to Wenzel state.

4. Conclusions

In summary, femtosecond laser processing and nano-silica spray coating technologies were used to create a high-performance superhydrophobic layer on an aluminum substrate surface. The phase of the sample surface remained essentially unaltered after femtosecond laser processing; it appeared as α and η phase. The superhydrophobic surface’s mountain-like micro-nano structure furnished additional adhesion points for nanoparticulate silica; this transformed the surface from hydrophilic to superhydrophobic The hydrophobic effect of the sample surface attained superhydrophobicity after four spray coats, with a Cassie state surface wetting condition, in the surface wetting performance test. Furthermore, the superhydrophobic surface had good self-cleaning properties, and surface spots were swiftly transported away by droplets. The thermal and chemical stability tests were carried out over a wide temperature and pH range, and the sample surface’s superhydrophobicity was successfully sustained, showing acceptable thermal and chemical stability. The wear resistance test on a 400# SiC sandpaper validated the superhydrophobic surface’s superior stability.