Preparation and Corrosion Resistance of Superhydrophobic Composite Coatings on Shot-Peened AA 7075-T6 Aluminum Alloy

Abstract

In order to further improve the corrosion resistance of 7075-T6 aluminum alloy after shot peening, corrosion-resistant superhydrophobic coatings (EP-HDTMS@SiO₂) containing epoxy resin (EP), cetyltrimethoxysilane (HDTMS), and nano-silica (SiO₂) were prepared by a simple spraying method on the surface of shot-peened AA 7075-T6 aluminum alloy. The effects of different EP/SiO₂ mass ratios on the micro-morphology, surface wettability, and corrosion resistance of the superhydrophobic composite coatings were analyzed. Due to the combination of microstructure and the modification of low surface energy organics, the contact angle of EP-HDTMS@SiO₂ coatings reached the superhydrophobic level (152.6°). The electrochemical tests showed that the corrosion current densities (I_corr) of the EP-HDTMS@SiO₂ composite coatings were both significantly lower than those of the EP-HDTMS coatings and matrix aluminum alloys. The addition of SiO₂ nanoparticles could improve the hydrophobicity and corrosion resistance of epoxy-based composite coatings. Due to the increase in surface roughness and epoxy resin, the shot-peened AA 7075-T6 alloy coating had high adhesion after the peel test. The prepared coatings also showed excellent corrosion resistance in the neutral salt spray test. This study provides a simple method for preparing stable superhydrophobic coatings on metal surfaces, which is expected to expand the application of 7075 aluminum alloy in harsh environments.

1. Introduction

7075 aluminum alloy is a Al-Zn-Mg-Cu ultra-high strength aluminum alloy, which is widely used in the aircraft industry [1]. 7075 aluminum alloy also has good corrosion resistance, but has a certain tendency for stress corrosion. In particular, Cl⁻ often leads to local pitting corrosion, intergranular corrosion (IGC), or stress corrosion cracking (SCC), which significantly affects the safety performance and service life of the components in marine environments [2,3,4]. Corrosion pits usually initially form at surface defects and often act as stress concentration locations, where cracks are more likely to form under external loads or tensile residual stresses. Theoretically, the stress corrosion behavior of aluminum alloy depends not only on the composition of the grain boundaries, the grain structure, and precipitated phase, but also on surface conditions such as residual stress and surface roughness [5,6,7]. Therefore, AA 7075-T6 aluminum alloy is expected to improve corrosion resistance through appropriate subsequent surface treatment.

Previous research has investigated the influence of various parameters of shot peening on stress corrosion behavior. Bao et al. [8] showed that the compressive residual stress and surface roughness of aluminum alloy AA 7075-T6 were influenced by the diameter of shots and the intensity of shot peening. Compared with the single shot-peening process, the dual shot-peening process had a higher compressive residual stress value and a lower surface roughness. In terms of the number and depth of cracks, it showed relatively better resistance to stress corrosion; however, all shot-peened samples showed surface pitting corrosion. The increased surface roughness often accelerated the process of localized pitting corrosion [9,10,11]. Sajid et al. [12] found that after improving the surface roughness of ASTM A36 steel, the local corrosion pitting tendency was alleviated. It was shown that the increased surface roughness could improve the wettability of the steel and thereby promote the corrosion process. In contrast, samples with lower surface roughness are significantly less susceptible to pitting corrosion, indicating that surface roughness is one of the important factors affecting the degree of surface corrosion pitting. Walter et al. [13] investigated the effect of surface roughness on the passivation and pitting behavior of AZ91 magnesium alloy in a chlorine-containing environment. As the surface roughness of magnesium alloy increased, the pitting tendency of the alloy also increased.

Due to their special surface wettability, superhydrophobic coatings have a lot of potential for applications in different fields, such as anti-corrosion [14], self-cleaning [15], anti-pollution [16], anti-icing [17], and drag reduction [18]. Inspired by the natural material template of lotus, researchers constructed an artificial superhydrophobic coating with a contact angle (>150°) and a low sliding angle (<10°) through a combination of a low-surface energy materials and a micro-nano bilayer coarse structure. In recent years, the application of superhydrophobic coatings in the corrosion protection of metal alloys has attracted great attention. The superhydrophobic coating can make the corrosion liquid to form a ball on the surface and slide off easily, thereby reducing the contact area and contact time between the corrosive medium and the protected surface [19]. In acidic water and seawater, the superhydrophobic film layer has a good barrier effect against chloride ions and acid–alkaline corrosive media, which can effectively inhibit the corrosion process and is expected to change the surface wettability and significantly improve the corrosion resistance of metal alloy [20].

Many methods for producing superhydrophobic coatings have been proposed so far. Compared with other methods, the spraying method is simple, low cost, and highly compatible with various substrates. The spraying method is considered as an important process for the large-scale production of superhydrophobic coatings on aluminum alloys [21]. However, the solvents and additives with low surface energy used in the preparation process are more expensive and usually contain fluorine or silane compounds, which cause varying degrees of environmental impact. As a low surface energy additive, hexadecyltrimethoxysilane is inexpensive, environmentally friendly, and is widely used as a superhydrophobic modification agent [22,23,24]. There are many nanoadditives available to modify the wettability of coatings, such as graphene and carbona [25,26], but they are expensive to prepare and have problems with dispersion and stability in some cases. SiO₂ nanoparticles have excellent chemical stability, good dispersion in organic solvents and polymer matrices, are more likely to form homogeneous micro-nanostructures in coatings, and are relatively inexpensive and environmentally friendly, making SiO₂ nanoparticles a very attractive option. Shells are composed of nanoparticles and polymers [27], and HDTMS@SiO₂ coatings have a particle/polymer design similar to the microstructure of simulated shells, and thus they exhibit excellent corrosion resistance and superhydrophobicity like shells.

In this research, a single-step spraying technique was used to prepare the superhydrophobic composite coating EP-HDTMS@SiO₂, aiming to protect the aluminum alloy 7075-T6 from pitting after shot-peening treatment. The coating suspension contained a fluorine-free modifier, epoxy resin, polyamide, and silica nanoparticles. The wettability, surface morphology, and surface chemical groups of the superhydrophobic coating were characterized, and its corrosion resistance and self-cleaning ability were investigated. Finally, the protection mechanism of the composite coating on the shot-peened aluminum alloy was further analyzed.

2. Materials and Methods

2.1. Materials

Aluminum alloy AA 7075-T6 was used; the specific composition and content are shown in Table 1. SiO₂ with a diameter of 30 nm, hexadecyltrimethoxysilane (HDTMS), epoxy resin E-44 (C₂₁H₂₄O₄), and polyamide 651 ((C₆H₁₁NO)_n) were purchased from Shanghai Yi En Chemical Technology Co., Ltd., Shanghai, China. Ammonia, sodium chloride (NaCl), and anhydrous ethanol were supplied by Sinopharm Group Chemical Reagent Co., Ltd., Ningbo, China. All reagents were used as received and no further purification was required.

2.2. Pre-Treatment of AA 7075-T6 Samples

AA 7075-T6 aluminum alloy samples were cut into blocks of 15 mm × 15 mm × 3 mm and polished with silicon carbide (SiC) abrasive paper (320, 800, and 2000 mesh numbers in sequence). Ultrasonic washing with acetone and ethanol for 15 min was applied to remove oil and stains. The surface of AA 7075-T6 aluminum alloy was treated with an air blasting machine from Kunshan Kasi Machinery Co., Ltd. in Suzhou, Jiangsu, China. The 0.18 mm ceramic balls were used, and the distance between the nozzle and the sample was 100 mm. The air pressure was 0.40 MPa, and the surface coverage of the shot-peening process was 100%. The prepared shot-peening sample was cleaned with ultrasonic waves in anhydrous ethanol for 10 min and then dried in cold air.

2.3. Preparation of EP-HDTMS@SiO₂ Superhydrophobic Coatings

SiO₂ nanoparticles with a weight of 0.6 g were ultrasonically dispersed in 25 mL of anhydrous ethanol for 30 min, and then HDTMS (0.8 g) was added into the solution. To promote the hydrolysis of HDTMS, the solution pH was adjusted to 7.6 by adding 1 mL of NH₃-H₂O at a concentration of 4 mol/L. The reaction lasted 12 h under constant magnetic stirring at room temperature, and an HDTMS-modified hydrophobic SiO₂ particle suspension was obtained. At the same time, epoxy resin (0.9 g) was added and mixed for 6 h with magnetic stirring. Before spraying treatment, the curing agent (0.45 g) was added to the mixing solution and mixed by ultrasonication at room temperature for 20 min to form an emulsion (i.e., paint). The emulsion was introduced into the barrel of the spray gun and, with a nozzle diameter of 1 mm and a pressure of 0.2 MPa, the suspended solution was sprayed about 10 cm away from the aluminum alloy plate. Numerous experiments and studies have shown that 60 °C and 12 h are the optimal curing conditions, so the coating samples were placed in a 60 °C oven and held for 12 h.

The formation mechanism of the superhydrophobic EP-HDTMS@SiO₂ coating can be explained as follows: under the catalysis of an alkaline solution, a long chain of HDTMS silane was hydrolyzed to a cetylsilanol. Each cetylsilanol contained three -OH groups and the silanol molecules reacted with each other to form polysiloxanes. On the other hand, due to the hydrophilicity of SiO₂, SiO₂ particles adsorbed a large amount of (-OH), which can be bonded with hydrogen on the surface of the solution. Finally, the long-chain silane condensed with SiO₂ particles to form Si-O-Si three-dimensional network structure nanoparticles [23]. Under the joint action of hydrophobic groups and hydroxyl groups, small gaps emerged between silica particles. These small gaps were filled with air and formed a micro-rough structure on the surface, achieving superhydrophobicity. The processing is shown in Figure 1.

2.4. Characterization

The surface morphology of the superhydrophobic EP-HDTMS@SiO₂ coating before and after the corrosion test was characterized using a field emission scanning electron microscope (Thermo Fisher Scientific, Nova Nano-450, Waltham, MA, USA) and an energy dispersive spectrometer (EDS, FEI, Nova Nano-450, USA). The water contact angle (WCA) of the prepared sample was measured using a 1 μL water drop with an interfacial tension meter (JC2000C1). A Fourier transform infrared spectrometer (FT-IR, Perkin Elmer Waltham, MA, USA) was used to analyze the chemical structure by characterizing the power scraped from the surface. The chemical composition of the composite coating was determined by X-Ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha, Waltham, MA, USA). The corrosion products of the aluminum alloy after the salt spray test were determined using an X-Ray diffractometer (XRD, D8 Advance Bruker AXS, Karlsruhe, Germany).

The electrochemical behavior of prepared samples was examined in a 3.5 wt% NaCl solution using an electrochemical workstation (CHI600e). The edge of the sprayed sample was sealed with wax oil to ensure that the exposed area was 1 cm². The operational setup consists of a standard measurement configuration with three electrodes, a counter electrode made of platinum, a reference electrode in the form of a saturated calomel electrode, and a working electrode that represents the sample itself. Polarization measurements were performed at a constant voltage sweep rate of 0.5 mV/s, allowing the calculation of the corrosion current density (I_corr) through the cathode-table slope. In addition, electrochemical impedance spectroscopy (EIS) was performed in a frequency range of 10⁻² to 10⁵ Hz using a polarization curve potential design of −1.6 to 0.2 V and an AC amplitude of 20 mV. To ensure the stability of the coating data, the sample was placed in an NaCl solution for 10 min before each electrochemical test. Zview software (2.0.0.10)was used to analyze and adjust the impedance spectrum of the sample. A KE-60B salt spray test machine (Foshan Huahe Vibration Testing Machine Manufacturing Co., Ltd., Foshan, Guangdong, China) was used for the salt spray test.

3. Results and Discussion

3.1. Surface Morphology of EP-HDTMS@SiO₂ Coatings

As shown in Figure 2(a₁,a₂), without the addition of SiO₂, the surface of the aluminum alloy was covered with an organic coating mainly composed of epoxy resin that was evenly distributed and flat, and had a good overall appearance. However, upon careful observation, it can be seen that there were some small pores in the coating, which were probably due to the pores formed by the evaporation process of anhydrous ethanol. The addition of nano-silicon dioxide made the surface contour of the coating rough, and valleys and micro peaks (several hundred nano-meters high) can be clearly seen with the SEM. When the mass ratio of EP to SiO₂ reaches 1.5, as shown in Figure 2(b₁), the accumulated nanoparticles of SiO₂ modified by HDTMS form many irregular structures, providing a rough structure and enough air pockets on the surface to achieve hydrophobicity. However, after superhydrophobic modification, some nanoparticles agglomerated to a certain extent and formed micro-sized agglomerates. These clusters were mainly composed of modified HDTMS@SiO₂ coated with epoxy resin (shown in Figure 2(b₂)). The contact angle measured here was below 120° because the epoxy content significantly exceeded the silica content. This suggested that obtaining a superhydrophobic rough surface structure is essential for low surfaces. Comparing Figure 2(c₂,d₂,e₂), it can be found that the surface roughness of the composite coating was higher compared to Figure 2(b₂), which allowed air transport and resulted in a larger contact angle on the surface of the composite coating. When a water droplet was introduced into the EP-HDTMS@SiO₂ coating, only a small part of it came into contact with the protruding micro/nano-structures, thereby reducing the contact area between the droplet and the nano-composite coating. As a result, the EP-HDTMS@SiO₂ nano-composite coating exhibited excellent superhydrophobicity with a contact angle of up to 150°. However, looking at Figure 2(c₁), one can see that there were some subtle cracks in the coating at this point. These cracks divided the entire surface coating into fragments due to the further increase in nanoparticles, making their distribution difficult. At the same time, the lack of epoxy resin in the composite coating can also cause cracks in the coating surface. These cracks in the film can be caused by volume shrinkage due to the evaporation of ethanol in the film. Therefore, excessive addition of silica and relative reduction in epoxy resin did not have a positive effect on the integrity of the coating.

3.2. Chemical Composition of EP-HDTMS@SiO₂ Coatings

As shown in the figure, only the elements C, O, and Si were detected in the spectra without the addition of nano-SiO₂ with atomic percentages of 84.84%, 13.95%, and 1.21%, respectively. However, it is clear from the EP-HDTMS@SiO₂ in Figure 3b that the proportions of the three main elements C, O, and Si are 59.08%, 29.95%, and 10.69%, respectively. The initially small amount of silicon element contained is mainly due to the small amount of Si element contained in hexadecyltrimethoxysilane. The significant increase in the content of O and Si elements in Figure 3b is due to the addition of SiO₂. From the distribution of nanoparticle elements in Figure 3c, it can be seen that the C, O, and Si indicate relatively uniform distribution of the coating by spraying.

The FT-IR test results are shown in Figure 4. The characteristic peaks of SiO₂ include a stretching vibration peak of 3440 cm⁻¹ for -OH, a bending vibration peak of 1627 cm⁻¹ for H-OH, an asymmetric stretching vibration peak of 1100 for Si-O-Si, a stretching vibration peak of Si-O-Si bonds at 802 cm⁻¹, and a bending vibration peak of Si-O-Si bonds at 470 cm⁻¹. At 1464 cm⁻¹, a new band formed due to the stretching vibration of the C-O group, indicating a covalent bond between the Si-OH group and the hexadecyl group of HDTMS [28]. More importantly, two new bands were discovered at 2922 and 2852 cm⁻¹, attributed to the stretching vibrations in the -CH₃ and -CH₂ bands of HDTMS, respectively. Especially, according to the FT-IR spectrum of SiO₂, the absorption peak at 1700 cm⁻¹ was carbonyl (C=O stretching), indicating that the double bonds in the carbonyl group were unstable [29]. For superhydrophobic EP-HDTMS@SiO₂ coatings, the characteristic absorption peak intensity of the Si-O bonds gradually increased, indicating that the nano-SiO₂ particles were successfully grafted onto the surface.

The chemical composition of the superhydrophobic EP-HDTMS@SiO₂ coating was further determined and characterized using XPS in Figure 5. The C1s energy level spectrum of the superhydrophobic EP-HDTMS@SiO₂ coating can be curve-fitted into three peak components, and the BEs of the C-Si, C-C, and C-O double bonds were 283.0 eV, 284.5 eV, and 286.4 eV, respectively [30]. The Si-2p spectra showed peak components at BEs of 103, 103.5, and 102.7 eV, corresponding to Si-O, Si-O-Si, and Si-O-C, respectively. Furthermore, the superhydrophobic coating EP-HDTMS@SiO₂ exhibited both Si-O-Si and Si-O-C peak components, indicating the presence of low surface energy silanes on the composite coatings [31]. The above results showed that HDTMS had successfully completed the modification through the covalent interaction between the silanol groups of the silane agent and SiO₂ [10], which were consistent with the previous FT-IR characterization.

3.3. Wettability of EP-HDTMS@SiO₂ Coatings

To investigate the influence of the mass ratio of EP and SiO₂ on the EP-HDTMS@SiO₂ coatings, samples with different mass ratios were prepared. The mass ratios of unadded SiO₂ and EP/SiO₂ were adjusted to 3:1, 3:2, 1:1, and 3:4 for a total of five groups, with the other components remaining unchanged. The results of the contact angle test are shown in Figure 6. The aluminum alloy surface showed good wettability with a contact angle of only 38.5° due to its low surface energy and high flatness. Pretreatment by shot peening increased the surface roughness and specific surface area, thereby increasing the surface energy. After shot-peening treatment, the contact angle increased to 68° and still showed hydrophilicity, indicating that the shot-peening method can change the wettability of the surface. Without the addition of nanoparticles, the contact angle of the epoxy resin coating was 90°, indicating that the epoxy resin itself had some hydrophilicity. By adding SiO₂ nanoparticles, the contact angle on the surface of the epoxy resin composite coatings could be significantly improved. As the amount of SiO₂ particles increased, the contact angle on the surface of the epoxy resin composite coating gradually increased. At a mass ratio of 3:2, the surface hydrophobicity of the nano-composite coating reached up to 150°, achieving superhydrophobicity. When the mass ratio of EP/SiO₂ nanoparticles was 1:1, the surface hydrophobicity of the nano-composite coating was the best with a WAC of 152°. This was because as the mass of EP-SiO₂ nanoparticles increased, not only did the surface roughness of the nano-composite coating improve, but also the content of HDTMS grafted on SiO₂ nanoparticles increased, which further reduced the surface energy. Consequently, the EP-HDTMS@SiO₂ nano-composite coating exhibited superhydrophobicity.

As the mass ratio of EP to SiO₂ continued to increase, the water contact angle (WAC) of the nano-composite EP-HDTMS@SiO₂ coating did not increase further and remained at about 152.6°. This phenomenon may be due to the fact that when the mass of SiO₂ nanoparticles increased to a certain level, the excessive number of nanoparticles prevented them from effectively adhering to the EP-HDTMS@SiO₂ coating. A further increase in nanoparticle mass may complicate the dispersion of nanoparticles, which may be due to increased interactions between nanoparticles, making it difficult for them to disperse evenly throughout the coating. This experiment could meet the requirements of superhydrophobicity under certain addition ratios.

3.4. Wettability of EP-HDTMS@SiO₂ Coatings with Different Solutions

To study the influence of pH on the contact angle of droplets of a superhydrophobic surface solution, acidic solutions with pH 1 ~ 6 were prepared with HCl solution, solutions with pH 8~14 were prepared with NaOH solution, and the liquid with pH 7 was ultrapure water. Through experiments, we found that the EP-HDTMS@SiO₂ coating showed good superhydrophobic properties in solutions with different pH values. In particular, when the pH value of the solution was in the range of 1 to 13, the contact angle of the droplets on the coating surface was larger than 140°, indicating that the coating has excellent resistance to both acidic and alkaline solutions, as shown in Figure 7a. Furthermore, common household liquids such as tea, coffee, milk, Coca-Cola, and salt water all showed extremely low adhesion to the surface of the prepared superhydrophobic coating, as shown in Figure 7b. These droplets could exist in a spherical shape on the surface of the coating and show the unique properties of an ultra-wet-phobic surface. Compared to other liquids, milk resulted in a smaller contact angle on the coated surface, indicating that milk had better diffusion and wettability on the coated surface. This phenomenon may be related to the molecular structure and composition of milk, such as sugar molecules, fats, etc.

3.5. Electrochemical Behavior of EP-HDTMS@SiO₂ Coatings

The electrochemical behavior of different mass ratios of EP to SiO₂ was investigated using electrochemical impedance spectroscopy. The Nyquist plot and Bode plot of the measured EIS data are shown in Figure 8. For the Nyquist diagram, the larger the impedance arc radius, the higher the corrosion resistance. At an EP to SiO₂ mass ratio of 1.5, the EP-HDTMS@SiO₂ coating exhibited the highest antipolarization performance. It can be seen from Figure 8a that the Nyquist curve was semicircular when the mass ratio of EP to SiO₂ was a maximum, and the diameter of the semicircular curve can usually be regarded as the resistance value of the test materials. The Nyquist curve of other coatings with different proportions consists of a semicircle and a tail, which is the “semicircle + tail” curve. According to the fitting circuit analysis, the corrosion resistance of the coating was mainly related to the internal resistance R₁ of the coating. The resistance values R₁ are listed in Table 2. The R₁ of all coatings with added SiO₂ nanoparticles was higher than that of EP-HDTMS coatings, indicating that the addition of SiO₂ can improve the corrosion resistance of the coatings. As the content of SiO₂ nanoparticles increased, the internal resistance R₁ first increased and then decreased, reaching its maximum value of 52 W Ω at 3:1. The rapid drop in internal resistance R₁ was related to the cracking caused by the insufficient amount of epoxy resin. The NaCl solution appears to quickly diffuse into the epoxy resin layer and reach the substrate. Figure 8b shows the phase angle curves of composite coatings prepared with different ratios. At high frequencies, the phase angle of the EP-HDTMS@SiO₂ coating approaches 90°, indicating that the coating acted as a perfect capacitor. At a mass ratio of EP to SiO₂ of 1.5, the phase angle of the coating remained at a high level over a wide intermediate frequency range. In the Bode diagram, the impedance modulus |Z| at low frequencies can typically represent the corrosion resistance of superhydrophobic coatings. From the graph, it can be seen that the EP-HDTMS@SiO₂ coating achieved the highest resistance when the mass ratio of EP to SiO₂ was 1.5. For the quantitative analysis, the Z_view equivalent circuit was used to accurately fit the EIS, as shown in Figure 8d. Among them, the R_s, R₁, and R₂ scans represented the solution resistance, coating resistance, and charge transfer resistance, respectively, while the CPE₁ and CPE₂ types correspond to the coating capacitance and double layer capacitance, respectively.

Figure 9 shows the polarization curves of all samples, and the corresponding corrosion potential (E_corr) and corrosion current density (I_corr) results are shown in Table 3. The corrosion resistance of superhydrophobic EP-HDTMS@SiO₂ coatings was evaluated not only via electrochemical impedance spectroscopy (EIS), but also using dynamic potential polarization (Tafel). In this study, measurements were performed at room temperature in a 3.5% NaCl solution. Figure 8 shows the polarization curves of a superhydrophobic surface with different mass ratios of EP and SiO₂. Tafel linear extrapolation was used to obtain electrochemical parameters of the samples, mainly including corrosion potential (E_corr) and corrosion current density (I_corr). The specific values are listed in Table 3. For the Tafel test, an increase in corrosion potential and a decrease in corrosion current represented an improvement in the corrosion resistance of the material. It was not difficult to see that the aluminum alloy after spraying organic coatings, regardless of the mass ratio of EP to SiO₂, had a more positive E_corr and a smaller I_corr compared to pure Al. Without the addition of nano-silica, the E_corr of the EP-HDTMS coating was −0.972 V and the I_corr was 1.321 × 10⁻⁷ A/cm⁻². When the mass ratio of EP to SiO₂ is 3:1, the minimum I_corr obtained is 2.295 × 10⁻⁸ A/cm⁻². Due to the presence of fine cracks on the surface of the coating, the I_corr at a mass ratio of EP to SiO₂ of 3:2 was 2.562 × 10⁻⁷ A/cm⁻². A surface containing an appropriate amount of nano-silica can effectively improve the corrosion resistance of the coating. Excess silica increased agglomeration and cracking of the coating, resulting in a reduction in the coating’s protection of the substrate’s corrosion resistance.

3.6. Self-Cleaning Performance of EP-HDTMS@SiO₂ Coatings

Figure 10 shows the self-cleaning effect of the hydrophobic coating on the surface of the aluminum alloy. When water droplets are applied to an untreated aluminum alloy surface, they diffuse and adhere to the surface, indicating the hydrophilicity of the surface. However, for aluminum alloys coated with the superhydrophobic EP-HDTMS@SiO₂ coating, the water droplets remained spherical and were cut off from the surface with the CuSO₄ powder, revealing its superhydrophobic properties. Under similar conditions, when a drop of water touches a surface made of superhydrophobic aluminum alloy that contains dirt particles such as toner, soot, and chalk dust, the water drop immediately beads up, taking the attached dirt particles with it. This process left almost no traces of contaminants on the surface, indicating that the adhesion between the contaminant particles and the superhydrophobic surface was weak. It was concluded that the superhydrophobic aluminum alloy coated with EP-HDTMS@SiO₂ had excellent self-cleaning properties. The mechanism for this self-cleaning performance lay in the interaction between the superhydrophobic surface and the rolling water droplets. Since the surface was superhydrophobic, water droplets did not spread upon contact with the surface but remained spherical. At the same time, due to the effect of surface tension, the water droplets reduced their surface area so that they wrapped tightly around the dirt particles and carried them away from the surface. This process not only removed the surface contamination but also ensured that the surface was clean and effectively prevented further contamination.

3.7. Adhesion of EP-HDTMS@SiO₂ Coatings

In practical applications, it is crucial to maintain the superhydrophobicity of the surface of superhydrophobic wood under external loads. To investigate mechanical durability, mechanical tests were performed including tape peeling and knife scraping. Tape test: The adhesive strength of the coating was evaluated according to ISO 2409–2007. As shown in the images of Figure 11(a₁,a₂), the tape peeling test was carried out by quickly peeling off the surface of superhydrophobic wood by repeatedly applying a high-viscosity tape with fingers. After mixing with epoxy resin, the polyamide hardener chemically reacts to form a high molecular weight polymer. Next came the cross-linking reaction. The polymer materials produced through polymerization reactions ultimately undergo curing reactions. After the cross-linking reaction is complete, the polyamide hardener continues to participate in the curing reaction, making the epoxy resin harder and more durable. A network structure was formed through radical addition reactions. These cross-linked networks can improve the mechanical properties and chemical stability of epoxy resin (as shown in Figure 11(c₂)). Shot-peening pretreatment improved the surface roughness and effectively improved the overall adhesion strength of the composite coating (see Figure 11(c₁)). At the same time, the addition of nanoparticles does not reduce the adhesive strength of the coating nor compensates for the difference in mechanical properties between the substrate and the coating due to its excellent thixotropy, thereby significantly improving the adhesive strength between the nano-composite coating and the substrate. In the knife cutting test, the bond strength was evaluated using the cross-cutting test method (ISO 2409). As shown in Figure 11(b₂), according to ASTM standards, there was no peeling phenomenon at the edge of the grid in the coating, and small pieces came off at the intersection of the cuts. The actual damage in the grid area did not exceed 5%, indicating that the adhesive force reached the 4B level and the coating was well bonded to the substrate.

3.8. Salt Spray Test for EP-HDTMS@SiO₂ Coatings

The substrate was placed in a closed cabinet at 35 °C (±1) in a salt spray environment created by a 5 wt% NaCl solution to evaluate the corrosion resistance of bare aluminum alloys and coated samples. To complete the test, the corrosion resistance of the bare alloy and EP-HDTMS@SiO₂ coating was tested for 120 h in a neutral salt spray test chamber. The neutral salt spray test showed that the bare aluminum alloy has significantly low corrosion resistance. After 24 h of exposure, a noticeable accumulation of white corrosion oxides can be observed on the surface. Over time, the surface of the alloy became covered with large, loose corrosion products in irregular clumps or foam-like shapes. These corrosion products gradually increased in thickness and expanded outward, ultimately resulting in a significant corrosion area. The XRD analysis showed that the composition of the corrosion product was basically the same, mainly consisting of aluminum alloy and the corrosion product Al₂O₃. The composition may include AlO(OH), Al(OH)₃, and other intermediate phases. As the exposure time increased, the intensity of the Al peak gradually decreased, while the intensity of the Al₂O₃ peak progressively increased. NaCl showed remarkable hygroscopic properties, with the dissolved Cl ions in a thin liquid film increasing the conductivity of the solution and thus accelerating the corrosion process. At the same time, Cl^- was an active ion with strong adsorption and erosivity. Cl^- and O₂ were competitively adsorbed or first adsorbed through a thin liquid film at the active site on the surface of aluminum alloy, then Cl⁻ further reacted with Al(OH)₃ to produce water-soluble AlCl₃ (as shown in Figure 12a) [29]. Accordingly, the film layer of the superhydrophobic EP-HDTMS@SiO₂ coating fell off and corrosion products appeared on the substrate surface after 96 h of soaking, indicating that the solution penetrated into the interface of the alloy substrate. At this point, the visible aluminum alloy is still covered by the clear coating, indicating that the coating still provides protection. The experiment was carried out in accordance with the AETM B117-2016 standard and, at that time, the coating had a protection level of 5, which means that there were breaks on the edges. Figure 12d shows the weight change per day for the superhydrophobic EP-HDTMS@SiO₂ coating and the salt spray corrosion of the substrate. After 24 h of corrosion, 7075 aluminum alloy corrodes heavily and produces more new material due to its low corrosion resistance. The superhydrophobic EP-HDTMS@SiO₂ coating provides excellent protection and the corrosion products slowly increase. Based on all these results, EP-HDTMS@SiO₂ coating significantly improved the corrosion resistance of 7075 aluminum alloy in immersion tests or cyclic wet and dry immersion tests. When the coating is immersed in water, the formation of an air film on the coating surface prevents water droplets from penetrating the microstructure. This air film acts as an additional barrier and effectively isolates the coating from water, soluble ions, and oxygen.

In the salt spray chamber, the salt mist generated consists of ultrafine electrolyte particles that are significantly smaller than the average distance between adjacent micropeak valleys on the coating surface. This creates a high susceptibility for the electrolyte particles to be deposited at the bottom of the cavities between the peaks and valleys, which in turn form the electrolyte layer. The similarity of the size of these electrolyte particles with the microstructure of the coating makes the electrolyte layer between the peaks and valleys more vulnerable to water, which accelerates the aggregation process of the electrolyte particles; this may lead to a reduction in the corrosion resistance of the coating. Furthermore, the corrosion starts from the coating to the substrate and leads to peeling of the edges of the substrate coating.

3.9. Mechanism of Ant-Corrosion Resistance for EP-HDTMS@SiO₂ Coatings

The mechanism of EP-HDTMS@SiO₂ coating to improve the ant-corrosion resistance of aluminum alloy against pitting is shown in Figure 13. In the microscopic space, a unique rough surface structure with low surface energy was jointly constructed by mixing epoxy resin, silica, and hexadecyltrimethoxysilane. The EP-HDTMS@SiO₂ coating conformed well to the stable Cassie–Baxter wetting model, and the air present in the rough structure created a stable pore as a physical barrier to prevent corrosion. The nano-scale holes present on this surface can trap air, block corrosive ions such as Cl^-, and prevent direct contact between the substrate and corrosive media. Due to the capillary action in the pores and grooves of the coating, the resulting high capillary negative pressure and antigravity are transmitted more easily; the Laplace pressure, as defined in Formula (1), can push seawater out of the pores, where $ϴ$ is the contact angle, $\gamma$ is the surface tension, g is the gravitational acceleration, R is the capillary radius, and $\rho$ is the liquid density.

(1)$\mathrm{H}={\displaystyle \frac{2\gamma cosϴ}{\rho gR}}$

In addition, during the preparation of the superhydrophobic EP-HDTMS@SiO₂ coating, a large amount of C-H occupies the surface interaction sites, which neatly arranges outward to result in hydrophobicity. Even in long-term corrosive environments, rough surface microstructures can be destroyed and hydrophobic agents can be desorbed, leading to the loss of the first two protection strategies. The organic EP-HDTMS@SiO₂ coating, which mainly consists of epoxy resin, can still effectively suppress the diffusion of corrosive media for a period of time [32].

4. Conclusions

In this study, a superhydrophobic EP-HDTMS@SiO₂ coating was successfully prepared on the surface of shot-peened 7075-T6 alloy. The coating exhibits excellent hydrophobic properties for various liquids, with a maximum static contact angle of 152.6°. In addition, the EP-HDTMS@SiO₂ coating not only has exceptional self-cleaning properties, but also shows good mechanical stability. Electrochemical test results show that the E_corr of the superhydrophobic composite coating increased by 0.639 V, while the I_corr decreased by four orders of magnitude compared to pure aluminum alloy. In the salt spray corrosion test, the EP-HDTMS@SiO₂ coating maintained its protective performance after 120 h. This research offers a simple method to prepare superhydrophobic coatings with high practical application.

Author Contributions

Writing—original draft, R.D., Z.L. and Q.Y.; writing—review and editing, K.Z. and V.J. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the research reported in this paper.

Footnotes

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Figures and Tables

Figure 1. Processing to prepare superhydrophobic EP-HDTMS@SiO2 coatings on shot-peened 7075-T6 aluminum alloy. (1. Mechanism of Superhydrophobic EP-HDTMS@SiO2 Coating Formation. 2. Processing of Superhydrophobic EP-HDTMS@SiO2 Coating.).

Figure 2. Low and high magnification SEM images of EP/SiO2 composite coatings with different ratios, (a1,a2) no SiO2 was added; (b1,b2) 3:1; (c1,c2) 3:2; (d1,d2) 1:1; (e1,e2) 3:4.

Figure 3. (a,b) EDS without nanosized SiO2 and EP/SiO2 mass ratio of 3:2; (c) mapping with an EP/SiO2 ratio of 3:2.

Figure 4. EP-HDTMS@SiO2 FT-IR spectroscopy of superhydrophobic coatings.

Figure 5. EP-HDTMS@SiO2 XPS of superhydrophobic coatings: (a) full spectrum; (b) C 1s; (c) O 1s; (d) Si 2p.

Figure 6. WAC data of aluminum alloy, pretreated aluminum alloy, and coated surface with different contents of SiO2 nanoparticles.

Figure 7. Wettability of EP-HDTMS@SiO2 superhydrophobic coating with 3:2 mass ratio of EP to SiO2 on different liquids: (a) different pH; (b) different solutions.

Figure 8. (a) Bode impedance plots with different EP/SiO2 ratios; (b) phase diagram; (c) freq Z diagram; (d) circuit fitting diagram.

Figure 9. Tafel curves for the coatings and substrate.

Figure 10. (a1,b1,c1) EP-HDTMS@SiO2 self-cleaning comparison between superhydrophobic coating with 3:2 mass ratio of EP to SiO2 and substrate (a2,b2,c2), blue powder is CuSO4. (a3,b3,c3) Self-cleaning theory diagram.

Figure 11. (a1,a2) Tape peeling test. (b1,b2) Knife scraping test. (c1) Shot peening SEM image. (c2) Schematic of coating–substrate bonding.

Figure 12. (a) Macroscopic morphologies of EP-HDTMS@SiO2 superhydrophobic coating and substrate in different days under salt spray test. (b,c) EP-HDTMS@SiO2 XRD before and after salt spray test between superhydrophobic coating and substrate. (d) Weight change of EP-HDTMS@SiO2 superhydrophobic coating and substrate.

Figure 13. (a) Schematic diagram of aluminum alloy pitting corrosion. (b) Schematic diagram of non-added nano-silica coating. (c) EP-HDTMS@SiO2 coating corrosion resistance schematic diagram.

Table 1

Composition and mass fractions of AA 7075-T6 aluminum alloy.

Element	Cu	Mn	Mg	Zn	Cr	Ti	Si	Fe
Content	1.2–2.0	0.30	2.1–2.9	5.1–6.1	0.18–0.28	0.20	0.40	0.50

Table 2

EIS fitting results of coatings with different compositions.

EP:SiO2	Without SiO2	3:1	3:2	1:1	3:4
RS/Ω	13.34	7.004	8.492	12.89	13.38
R1/Ω	41,238	333,640	520,971	3917	5498
R2/Ω	125,140	583,180	690,701	9919	—

Table 3

Tafel corrosion current densities and corrosion potentials for the coatings and substrate.

EP:SiO2	Without SiO2	3:1	3:2	1:1	3:4	Pure Al
Ecorr vs. SCE (V)	−0.972	−0.790	−0.828	−1.197	−1.174	−1.429
Icorr (A cm−2)	1.321 × 10−7	2.295 × 10−8	2.562 × 10−7	8.606 × 10−6	4.097 × 10−6	2.398 × 10−4

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