Interfacial Regulation of Hard-Anodized Aluminum Alloy for Fabrication of a Wear-Resistant Superhydrophobic F-SiO₂@PU Composite Coating with Enhanced Long-Term Corrosion Protection

1. Introduction

Aluminum alloys are among the most extensively utilized lightweight structural materials owing to their high specific strength, excellent thermal and electrical conductivity, superior machinability [1,2,3]. These attributes have promoted their widespread application in aerospace engineering, marine equipment, transportation systems, and electronic packaging [4,5]. Nevertheless, the naturally formed oxide film on aluminum alloys is relatively thin and vulnerable to mechanical damage, chloride ion penetration, and chemical attack [6,7]. Under aggressive service conditions, such as high humidity, saline atmospheres, acidic or alkaline media, and cyclic environmental fluctuations, localized degradation processes including pitting corrosion, crevice corrosion, and intergranular corrosion can be readily initiated [6,7,8]. Once the passive film is disrupted, corrosion propagates rapidly through electrochemical reactions, resulting in material loss, deterioration of mechanical integrity, and eventual structural failure [9]. These corrosion-induced damages not only shorten the service life of aluminum alloy components but also compromise the operational safety and reliability of engineering systems [10]. Therefore, effective surface protection strategies with long-term corrosion resistance and environmental adaptability are urgently needed for the practical application of aluminum alloys.

Inspired by the micro-nano structures and low surface energy of natural biological interfaces such as lotus leaves [11], dandelion seeds [12], water striders [13], and springtail [14], the superhydrophobic surface engineering provides a new design concept for the protection of aluminum alloy. By constructing micro-nano rough structures and combining with low surface energy modification, a stable gas film can be formed on the surface, effectively blocking the contact between the substrate and corrosive media, thereby achieving self-cleaning and anti-corrosion functions [15,16,17].

To address these issues, research at home and abroad mainly focuses on enhancing the mechanical properties of the coating and optimizing the organic-inorganic composite interface [18,19,20]. One approach enhances coating hardness and scratch resistance by incorporating inorganic hard particles such as SiO₂ and Al₂O₃ [18,21]. However, this strategy often increases coating brittleness and compromises deformation coordination, making the coating prone to microcracks and localized failure under bending or impact loads [22].Another approach involves directly introducing inorganic particles into the polymer matrix, enhancing their dispersion and interface interaction through surface modification [23]. However, during the modification process, the consumption of surface active groups weakens the chemical bonding between the particles and the polymer, making the interface combination more dependent on weak physical forces. Under friction and shear stress, the particles are prone to detachment and form interface defects, accelerating the penetration of water and ions [19,23]. From an essential perspective, the core bottleneck of the existing system lies in the lack of a multi-scale, continuous, and stable interface chemical coupling mechanism between the organic matrix, inorganic fillers, and metal substrate [24].

Among various polymeric matrices, PU has attracted extensive attention in the field of protective coatings owing to its excellent flexibility, strong adhesion, outstanding mechanical toughness, and good environmental stability [25,26]. The presence of abundant urethane linkages and reactive functional groups enables PU to form strong interfacial interactions with both inorganic fillers and metallic substrates. Moreover, its elastomeric network can effectively dissipate external stress through elastic deformation, thereby alleviating stress concentration and suppressing crack initiation and propagation during mechanical loading [27]. These characteristics make PU a promising candidate for constructing durable superhydrophobic coatings with enhanced wear resistance and corrosion protection [28]. Nevertheless, the intrinsic surface energy of PU remains relatively high, making it difficult to achieve stable superhydrophobicity without the incorporation of micro/nano-scale rough structures and low-surface-energy components [29]. Furthermore, the interfacial compatibility between modified inorganic particles and the PU matrix is often insufficient, and the bonding strength at the coating/substrate interface remains limited [30]. As a result, particle detachment, interfacial debonding, and structural degradation may still occur under long-term mechanical wear and corrosive environments, leading to a gradual deterioration of protective performance.

To address the challenges of insufficient interfacial adhesion and the difficulty in simultaneously achieving high wear resistance and long-term corrosion protection in conventional superhydrophobic coatings, a F-SiO₂@PU composite superhydrophobic coating was developed on 6061 aluminum alloy through an anodization-assisted interfacial engineering strategy. An anodic oxide layer with a porous architecture and abundant hydroxyl groups was first constructed to regulate surface reactivity and enhance interfacial bonding. Subsequently, FDTS-modified SiO₂ nanoparticles were incorporated into the PU matrix to establish a stable micro/nano hierarchical structure with low surface energy. The hydroxyl-rich anodized surface further reacted with isocyanate groups in the PU matrix, forming a strongly coupled substrate–polymer–particle interface. The resulting coating exhibited excellent mechanical durability, maintaining a water contact angle of 150 ± 2° after 10,000 cm abrasion and 200 tape-peeling cycles. Moreover, superior corrosion resistance was achieved in 3.5 wt.% NaCl solution, with a low-frequency impedance modulus of log|Z|_{0.1 Hz} = 8.0 and a corrosion current density as low as 5.36 × 10^-11A cm^-2. The enhanced performance is attributed to the synergistic effect of interfacial chemical anchoring, low-surface-energy shielding, and stress dissipation provided by the elastic PU network. This work provides an effective interfacial regulation strategy for the development of mechanically robust and long-lasting superhydrophobic coatings on aluminum alloys.

2. Experimental

2.1. Materials

6061 aluminum alloy plates (20 mm × 20 mm × 2 mm) were employed as substrates. The diatomite particles were purchased from Sainke Experimental Equipment (Jinan) Co., Ltd. Hydroxide aluminum and sodium hydroxide were both purchased from Fu Chen (Tianjin) Chemical Reagent Co., Ltd. 1H, 1H, 2H, 2H-perfluorooctyl trichlorosilane (FDTS), Capstone FS-3100 were all purchased from Aladdin (Shanghai) Reagent Co., Ltd. Tetraethyl orthosilicate (TEOS) was purchased from Tianjin Da Mao Chemical Reagent Factory. Anhydrous ethanol, deionized water, ammonia water (25 wt%) were purchased from Tianjin Fu Yu Fine Chemical Co., Ltd. Nano silica sol (30 wt%) were purchased from Guangzhou Fu Er Chemical Technology Co., Ltd. Waterborne polyurethane resin (50 wt%) was purchased from Jining Huakai Resin Co., Ltd. Hydrochloric acid was purchased from Shenyi Reagent Factory in Shenyang. Sodium chloride was purchased from Tianjin Yong Da Chemical Reagent Co., Ltd.

2.2. The Preparation of the Hydroxylated Substrate

Aluminum alloy specimens were mechanically polished and successively subjected to degreasing, alkaline etching, acid neutralization, and desmutting prior to anodization. Hard anodization was performed in a 130 g L^-1H₂SO₄ electrolyte at 0 °C for 50 min, resulting in the formation of anodic aluminum oxide (AAO) films. The obtained AAO samples were subsequently treated in boiling deionized water to promote hydration of the oxide layer and generate a hydroxyl-rich surface. The thickness of the anodic oxide films, determined using a coating thickness tester, ranged from 60 to 80 μm.

2.3. Preparation of Modified Nanoparticles

2.3.1. Preparation of F-Al(OH)₃@Siltite Nanoparticles

To prepare fluorinated Al(OH)₃@diatomite particles, 0.3 g of Al(OH)₃ and 1.0 g of diatomite were dispersed in a premixed solution containing 20 mL of absolute ethanol, 2 mL of deionized water, and 1 mL of ammonia. The suspension was stirred at 50 °C for 1 h to ensure uniform dispersion. Subsequently, 0.5 mL of 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FDTS) was dissolved in 20 g of n-hexane and ultrasonicated for 10 min. The obtained FDTS solution was added dropwise to the suspension and allowed to react at 50 °C for 24 h. During this process, FDTS molecules were grafted onto the surface hydroxyl groups of Al(OH)₃ and diatomite via silanization, thereby reducing the surface energy of the particles. Finally, the modified particles were dried, ground, and sieved through a 200-mesh screen to obtain fluorinated Al(OH)₃@diatomite composite particles (F-Al(OH)₃@diatomite).

2.3.2. Preparation of F-SiO₂ Solution

To prepare fluorinated SiO₂ nanoparticles, 0.5 mL of tetraethyl orthosilicate (TEOS) and 1.0 g of silica sol were dispersed in a premixed solution containing 20 mL of absolute ethanol, 2 mL of deionized water, and 1 mL of ammonia. The mixture was stirred at 50 °C for 1 h to obtain a uniform silica precursor suspension. Subsequently, 0.1–0.5 mL of 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FDTS) was dissolved in 20 g of n-hexane and ultrasonically treated for 20 min. The resulting FDTS solution was added dropwise to the precursor suspension and maintained at 50 °C for 24 h. Through the silanization reaction between FDTS and surface hydroxyl groups on the silica particles, fluorinated SiO₂ nanoparticles with hierarchical roughness and low surface energy were obtained. The resulting suspension was designated as F-SiO₂.

2.4. Preparation of F-SiO₂@PU Coating

The F-SiO₂@PU coating was prepared via a two-step spray deposition process. First, 5 g of polyurethane was mixed with 5 mL of deionized water, and 0.03 g of FS-3100 was added to improve wettability and reduce the suspension’s surface tension. Subsequently, 1.3 g of F-Al(OH)₃@diatomite particles was introduced and uniformly dispersed under magnetic stirring. The resulting suspension was sprayed onto the hydroxylated aluminum substrate, which was maintained at 100 °C, to form a particle-reinforced polyurethane interlayer. Before complete curing of the polyurethane matrix, 5 mL of F-SiO₂ nanodispersion was deposited onto the coating surface by spray coating. The spraying parameters were set to an air pressure of 0.4 MPa, a spray distance of 15 cm, and three spraying cycles of 5 s each, separated by 20 s intervals. This sequential deposition strategy enabled the formation of a robust hierarchical micro/nanostructured surface with low surface energy. The final coating, denoted as F-SiO₂@PU, exhibited an average thickness of 120.5 ± 1.2 μm.

2.5. Characterization (Instruments and Characterization)

The wettability of the coatings was evaluated by measuring the water contact angle (CA) and sliding angle (SA) using a JC2000D1 contact angle analyzer. Surface morphology and roughness were characterized by FE-SEM (S-4800, Hitachi, Japan) and white-light interferometry, respectively. The chemical composition, surface functional groups, and thermal stability of the coatings were analyzed by XPS, FTIR, and thermogravimetric analysis (TGA). Corrosion resistance was assessed through immersion tests in 0.1 M HCl, 1 M NaOH, and 3.5 wt.% NaCl solutions, together with potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements performed on a VersaSTAT-3F electrochemical workstation (AMETEK, USA). The mechanical durability of the coatings was evaluated using a sandpaper abrasion test with 2000-grit SiC sandpaper under a 200 g load, and the corresponding changes in CA and SA were recorded after abrasion.

3. Results and Discussion

3.1. Morphology and Chemical Composition of Fluorinated SiO₂ Nanoparticles

Formation mechanism of the F-SiO₂@PU superhydrophobic coating on anodized aluminum alloy, as illustrated in Figure 1, a porous anodic oxide film is first generated on the aluminum alloy surface through anodization, providing a rough micro/nanostructure and abundant anchoring sites. Subsequent hydroxylation treatment introduces a high density of surface hydroxyl groups, which significantly enhances the interfacial affinity and adhesion between the anodic oxide layer and the PU coating. The sprayed PU layer penetrates and fills the anodic pores, forming a mechanically interlocked and compact barrier structure. Finally, fluorinated SiO₂ nanoparticles are deposited onto the PU surface, constructing a hierarchical micro/nanostructure with low surface energy. The synergistic effect of interfacial bonding, physical interlocking, and trapped air within the rough surface effectively suppresses electrolyte penetration, thereby imparting excellent superhydrophobicity, mechanical durability, and long-term corrosion resistance to the coating.

The surface morphology of the SiO₂ nanoparticles after fluorination was as shown in Figure 2a. Following FDTS treatment, the particles maintained their spherical morphology while developing a rougher nanoscale texture due to the formation of a fluorinated silane layer. The chemical compositions of the particles were further analyzed using FTIR and XPS. The FTIR spectra of F-SiO₂ exhibited characteristic absorption bands corresponding to fluorinated alkyl groups, confirming the successful introduction of fluorosilane molecules onto the silica surface. The FTIR spectra of unmodified SiO₂ and F-SiO₂ are shown in Figure 2b. The absorption peaks at 1090 cm^-1 and 800 cm^-1 for unmodified SiO₂ correspond to the asymmetric and symmetric stretching vibrations of Si-O-Si bonds [31], respectively-characteristic peaks of a silica framework that demonstrate the presence of a complete siloxane network in the prepared particles. After FDTS modification, the samples retained the characteristic Si-O–Si absorption peaks near 1090 cm^-1 and 800 cm^-1 while exhibiting new peaks at 2963 cm^-1 and 2845 cm^-1, corresponding to the stretching vibrations of the -CH₂ groups in the organic segments; additionally, peaks at 1200 cm^-1and 1150 cm^-1represent the stretching vibrations of C-F bonds [32].

XPS analysis further verified the presence of fluorine-containing species through the appearance of distinct F 1s signals. The full XPS spectrum of F-SiO₂ (Figure 2c) clearly exhibits a series of characteristic electronic transition signals, including the F 1s peak at 689.2 eV, the C 1s peak at 284.8 eV, the O 1s peak at 533.8 eV, as well as the Si 2s and Si 2p peaks at 153.6 eV and 103.6 eV, respectively. The presence of the highly intense F-related characteristic signal in these spectral lines provides preliminary and compelling qualitative evidence that the FDTS molecule has successfully been introduced and coated onto the surface layer of the nano-silica particles. The high-resolution C 1s energy spectrum is shown in Figure 2d; sub-peaks at 284.8 eV, 286.3 eV, and 288.5 eV correspond to the C-C (or C-H), C-O, and C=O bonds, respectively. Peaks at 291.6 eV and 293.5 eV in the higher-energy region are attributed to the -CF₂ groups and terminal -CF₃ groups within the fluorinated organic chain [33]. Due to fluorine’s strong electronegativity, the electron cloud density around the connected carbon atoms decreases significantly, resulting in a substantial shift of their core electron binding energies toward higher values. The high-resolution F1s spectrum is shown in Figure 2e, where the characteristic peaks at -CF_2- (689.6 eV) and -CF₃- (688.9 eV) indicate that the fluorinated segments in the FDTS molecule were preserved during modification; the main peak at O1s (Figure 2f) primarily corresponds to Si-O-Si (533.8 eV), demonstrating that the modified sample retains a stable silica framework. The increased carbon and fluorine contents observed after modification provided direct evidence for the covalent grafting of FDTS onto the SiO₂ surface.

Figure 3 illustrates the surface morphology of aluminum alloy substrates after anodizing, hydroxylation, and F-SiO₂@PU coating treatment. Figure 3a shows the aluminum alloy substrate with mechanical scratches on its surface, exhibiting a contact angle of 95°. The anodized surface (Figure 3b) exhibits a rough, porous structure composed of irregular oxide flakes and particles. After hydroxylation (Figure 3c), numerous fine hydration products form on the surface, partially filling the pores and resulting in a denser, more uniform morphology. This change is attributed to the hydration of Al₂O₃ into boehmite, thereby enhancing the density of the oxide layer. Upon deposition of the F-SiO₂@PU coating (Figure 3d), the uniform distribution of SiO₂ nanoparticles within the polyurethane matrix creates a hierarchical micro/nanostructure. Increased surface roughness facilitates air trapping at the solid-liquid interface, promoting the formation of a stable Cassie-Baxter state; this leads to a contact angle of 161.5° and improved superhydrophobicity and corrosion resistance of the coating.

Figure 3e, f, g, and h respectively demonstrate white-light interferometric morphology images and surface roughness measurements of aluminum alloy substrates, anodized samples, hydroxylated samples, and the F-SiO₂@PU composite superhydrophobic coatings. These images reveal significant variations in three-dimensional surface characteristics among samples under different conditions, with surface roughness increasing progressively. The aluminum alloy substrate exhibits a relatively smooth surface with minimal ripples; distinct parallel striations are observed along specific directions, accompanied by slight peak-valley variations, without pronounced protrusions or depressions, with an average surface roughness (Ra) of only 0.339 μm. After anodization (Figure 3f), the original directional texture diminishes, giving way to a more uniform planar relief structure with slightly increased roughness, indicating that anodization establishes an initial framework of roughness on the substrate surface. Hydroxylation treatment further modifies the surface morphology: while hydration products (boehmite) fill deep pores, they create densely distributed particulate protrusions, resulting in a dense interlocking peak-valley pattern with Ra rising to 0.445 μm—demonstrating that pore sealing enhances surface roughness. The incorporation of F-SiO₂@PU introduces additional complexity: instead of simple continuous ripples, the surface displays densely arranged micro-protrusions and interconnected peak-valley structures, forming a more intricate three-dimensional profile with significantly elevated surface roughness (Ra = 2.135 μm).

Thermogravimetric analysis was conducted to evaluate the thermal stability of the modified nanoparticles. As shown in Figure 4, pristine SiO₂ exhibited only minor weight loss associated with the removal of physically adsorbed water and surface hydroxyl groups. In contrast, F-SiO₂ displayed a distinct two-step decomposition behavior. The first weight-loss stage observed between 150 and 250 °C was mainly attributed to the evaporation of residual solvents and low-molecular-weight siloxane species [34]. A second pronounced decomposition stage occurred in the range of 450–600 °C, corresponding to the thermal degradation of grafted perfluoroalkyl chains [35]. The significantly lower residual mass fraction of F-SiO₂ compared with pristine SiO₂ indicated a high density of fluorinated molecules grafted onto the particle surface. Moreover, negligible weight loss below 400 °C indicated excellent thermal stability, which is advantageous for subsequent coating fabrication and practical service applications.

Collectively, the morphology, chemical composition, and thermal analysis results confirm that FDTS molecules were successfully grafted onto the SiO₂ nanoparticles. The resulting fluorinated particles possess both low surface energy and enhanced nanoscale roughness, providing the fundamental building blocks required for constructing durable superhydrophobic coatings.

3.2. Wettability and Self-Cleaning Performance of F-SiO₂@PU Composite Coating

The wettability and self-cleaning behaviors of the F-SiO₂@PU composite coating were systematically investigated to evaluate its surface functionality. Representative self-cleaning tests using sand, soil, and carbon black contaminants are shown in Figure 5. Regardless of contaminant type, water droplets readily rolled across the coating surface and effectively removed adhered particles, leaving the surface clean. This phenomenon demonstrates excellent self-cleaning capability and confirms the successful establishment of a lotus-leaf-inspired superhydrophobic interface.

The outstanding self-cleaning performance originates from the hierarchical micro/nanostructure combined with the fluorinated low-surface-energy chemistry. When contaminants are deposited on the coating surface, the actual contact area between the particles and the substrate remains extremely small due to trapped air pockets. As water droplets roll across the surface, capillary forces at the liquid-particle interface exceed the weak adhesion forces between contaminants and the coating, thereby enabling efficient contaminant removal.

The dynamic wetting behavior was further characterized through sliding-angle measurements. As illustrated in Figure 5d, the coating exhibited an ultralow sliding angle of approximately 1°, indicating negligible contact-angle hysteresis and excellent droplet mobility. Such behavior is characteristic of a stable Cassie-Baxter wetting state, in which water droplets rest predominantly on an air-solid composite interface rather than directly contacting the solid substrate.

To further assess liquid-solid adhesion, a droplet adhesion test was performed by compressing and subsequently withdrawing a 5 μL water droplet from the coating surface (Figure 5f). Throughout the loading-unloading process, the droplet maintained a nearly spherical morphology and detached instantly upon withdrawal of the external force. No liquid bridge formation, droplet fragmentation, or residual liquid traces were observed. These observations indicate extremely weak interfacial adhesion and confirm the presence of a stable trapped-air layer at the coating surface.

The superior wetting behavior can be attributed to the synergistic effect of the fluorinated SiO₂ nanoparticles and the PU-supported hierarchical architecture. The fluorinated alkyl chains substantially reduce the surface free energy, whereas the multiscale rough structure amplifies hydrophobicity by stabilizing the Cassie-Baxter state. Simultaneously, the PU matrix enhances particle fixation and structural integrity, thereby preserving the micro/nanostructure required for sustained superhydrophobic performance.

Therefore, the F-SiO₂@PU composite coating exhibits excellent water repellency, ultralow liquid adhesion, and efficient self-cleaning capability, which are highly desirable characteristics for long-term anti-corrosion and anti-fouling applications.

3.3. Mechanical Durability and Environmental Stability

Mechanical durability is one of the most critical requirements for the practical application of superhydrophobic coatings. To evaluate the wear resistance of the F-SiO₂@PU composite coating, abrasion tests were conducted using 2000-grit SiC sandpaper under a constant load of 200 g（Figure 6a）. The evolution of water contact angle (CA) and sliding angle (SA) as a function of abrasion distance is presented in Figure 6b.

For both coatings, the CA gradually decreased. At the same time, the SA increased with increasing abrasion distance, indicating progressive damage to the surface micro/nanostructures and partial removal of the low-surface-energy fluorinated layer. Such deterioration inevitably weakens the hierarchical structure’s air-trapping capability and increases the actual liquid-solid contact area. However, the F-SiO₂@PU composite coating exhibited significantly superior abrasion resistance compared with the F-SiO₂ coating. Even after an abrasion distance of 10,000 cm, the F-SiO₂@PU coating maintained a CA of approximately 150 ± 2° and an SA of only 8 ± 1.7°. In contrast, the F-SiO₂ coating experienced a substantial loss of superhydrophobicity, with the CA decreasing to approximately 135 ± 2° and the SA exceeding 10°.

The enhanced abrasion resistance can be attributed to the synergistic contribution of the PU matrix and the anodized aluminum interface. The flexible PU network effectively encapsulates and immobilizes the fluorinated nanoparticles, thereby reducing particle detachment during frictional loading. Simultaneously, the elastic nature of the polymer matrix facilitates stress redistribution and energy dissipation, minimizing local stress concentration and suppressing structural collapse. Moreover, the hydroxyl-rich anodized layer promotes strong interfacial interactions with the PU matrix, significantly improving coating adhesion and overall structural stability. Consequently, the hierarchical roughness required for superhydrophobicity can be preserved even after prolonged mechanical wear.

The adhesion stability of the coating was further evaluated using repeated tape-peeling tests. As shown in Figure 6c, both coatings experienced a gradual decline in wettability performance with increasing peeling cycles. Nevertheless, the F-SiO₂@PU coating consistently outperformed the F-SiO₂ coating throughout the entire testing period. After 200 tape-peeling cycles, the composite coating still exhibited a CA of 151 ± 2° and an SA of 6 ± 1.5°, retaining its superhydrophobic character. In contrast, the CA of the F-SiO₂ coating decreased to approximately 129 ± 2°, while the SA increased to 29 ± 1.4°, indicating a transition from a stable Cassie-Baxter state toward a partially wetted state.

The superior anti-peeling performance further confirms the effectiveness of interfacial regulation in improving coating durability. The strong adhesion between the PU matrix and anodized substrate prevents large-scale delamination. At the same time, the polymer-assisted fixation of fluorinated nanoparticles suppresses the removal of the functional surface layer during repeated peeling.

Long-term storage stability is another important indicator for evaluating the practical applicability of superhydrophobic coatings. Therefore, the wettability evolution of the F-SiO₂@PU coating was monitored over 90 days under ambient laboratory conditions. As shown in Figure 6d, the CA decreased only slightly from 161.5 ± 2° to 156 ± 2°, while the SA remained below 4°. Such minimal variation indicates that neither significant structural collapse nor fluorosilane desorption occurred during prolonged storage. The retained superhydrophobicity demonstrates excellent environmental durability and long-term stability.

Thermal stability was investigated by exposing the coating to temperatures ranging from 50 to 200 °C for 2 h. As shown in Figure 6e, the coating maintained a CA above 155° and an SA below 3° throughout the entire temperature range. The negligible change in wettability suggests that the fluorinated SiO₂ nanostructure and PU matrix remained structurally intact under thermal exposure. This behavior is consistent with the excellent thermal stability demonstrated by the TGA results and indicates that the coating is suitable for applications involving elevated operating temperatures.

The chemical stability of the coating was further evaluated over a wide pH range from 1 to 14. As shown in Figure 6f, the CA remained within 157–161.5°, while the SA exhibited only minor fluctuations. Even under strongly acidic or alkaline conditions, the coating preserved its superhydrophobic characteristics, indicating excellent resistance to chemical degradation. The robust micro/nanostructure and chemically stable fluorinated surface layer effectively maintained the Cassie-Baxter wetting state, thereby preventing complete wetting of the coating surface.

Overall, the F-SiO₂@PU composite coating exhibits outstanding resistance to abrasion, peeling, thermal exposure, long-term storage, and chemical attack. These remarkable durability characteristics originate from the synergistic interaction among the hard-anodized interface, the flexible PU binding network, and the fluorinated hierarchical rough structure. The combination of strong interfacial adhesion, efficient stress dissipation, and stable surface roughness enables the coating to retain its superhydrophobic functionality under various harsh service conditions, thereby providing a solid foundation for long-term corrosion protection.

3.4. Corrosion Protection Performance

The corrosion-protection performance of bare aluminum alloy (Al), PU coating, F-SiO₂ coating, and F-SiO₂@PU composite coating was systematically evaluated in a 3.5 wt% NaCl solution using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The corresponding impedance spectra, Nyquist plots, Bode plots, and polarization curves are presented in Figure 7.

The impedance modulus is widely recognized as an important indicator for evaluating the barrier properties of protective coatings. As shown in Figure 7a, the F-SiO₂@PU composite coating exhibited the highest impedance modulus over the entire frequency range, indicating superior resistance to electrolyte penetration. In particular, the low-frequency impedance modulus (log|Z|₀.₁Hz) reached 8.0, which was significantly higher than those of the F-SiO₂ coating (7.7), PU coating (6.9), and bare aluminum alloy (3.6). Since low-frequency impedance is closely associated with a coating’s ability to suppress charge transfer and ionic transport, the remarkably high impedance value indicates the excellent barrier performance of the composite coating.

Further evidence is provided by the Nyquist plots shown in Figure 7b. The F-SiO₂@PU coating displayed the largest capacitive semicircle radius among all samples, indicating a substantially increased charge-transfer resistance and a lower corrosion reaction rate at the coating/substrate interface. In contrast, the bare aluminum alloy exhibited only a small capacitive loop, suggesting rapid electrochemical corrosion and poor resistance to aggressive chloride ions. The enlarged semicircle observed for the composite coating indicates effective suppression of electrochemical processes at the metal surface.

The Bode phase-angle plots shown in Figure 7c provide additional insight into the protective behavior of the coatings. The F-SiO₂@PU coating maintained phase angles between approximately 55° and 80° over a broad frequency range, with a maximum approaching 80°. Such behavior is characteristic of highly capacitive coatings with excellent barrier properties. By comparison, the PU and F-SiO₂ coatings exhibited lower phase angles, particularly in the medium- and low-frequency regions, indicating a reduced ability to impede electrolyte diffusion. The bare aluminum alloy maintained phase angles below 10° throughout the frequency range, reflecting its poor corrosion resistance.

Potentiodynamic polarization measurements were conducted to assess the corrosion kinetics of the different samples quantitatively. As shown in Figure 7d, bare aluminum alloy exhibited a corrosion potential (Ecorr) of −0.76 V and a corrosion current density (Icorr) of 1.87 × 10^-6 A cm^-2, indicating a strong tendency toward corrosion. The application of the PU coating shifted the corrosion potential positively to −0.66 V. It reduced the corrosion current density to 2.36 × 10^-9 A cm^-2, demonstrating the protective effect of the dense polymer barrier. Similarly, the F-SiO₂ coating exhibited an Ecorr value of −0.47 V and an Icorr value of 6.35 × 10^-9 A cm^-2, suggesting that the fluorinated rough surface effectively retarded electrolyte penetration.

Among all samples, the F-SiO₂@PU composite coating exhibited the most positive corrosion potential (−0.44 V) and the lowest corrosion current density (5.36 × 10^-11 A cm^-2). Compared with bare aluminum alloy, the corrosion current density decreased by approximately five orders of magnitude, confirming the exceptional corrosion protection capability of the composite coating. Such a dramatic reduction in corrosion activity demonstrates that the coating effectively isolates the metallic substrate from the corrosive environment.

3.5. Long-Term Corrosion Resistance in Acidic, Alkaline, and Saline Media

To evaluate the long-term corrosion protection capability of the F-SiO₂@PU composite coating under harsh service conditions, immersion tests were conducted in 0.1 M HCl, 1 M NaOH, and 3.5 wt.% NaCl solutions for 7 and 14 days. The evolution of electrochemical impedance spectra and polarization behavior was systematically analyzed to reveal the degradation mechanisms of the coating in different corrosive environments.

3.5.1. Corrosion Behavior in Acidic Environment

The electrochemical performance of the F-SiO₂@PU coating after immersion in 0.1 M HCl solution is shown in Figure 8. As the immersion time increased, a gradual decrease in impedance modulus and a slight increase in corrosion current density were observed, indicating progressive penetration of corrosive species into the coating system. Nevertheless, even after 14 days of exposure, the coating maintained a low-frequency impedance modulus above 10⁶ Ω·cm² and a corrosion current density significantly lower than that of bare aluminum alloy.

The degradation process in acidic media is primarily associated with the high concentration of H⁺ ions. Due to their small ionic radius and strong mobility, H⁺ ions can gradually diffuse through microscopic defects and pores within the coating. Once reaching the coating/substrate interface, localized dissolution of the oxide layer and aluminum substrate may occur. In addition, prolonged exposure to acid may partially hydrolyze interfacial chemical bonds, reducing coating compactness.

Despite these challenges, the corrosion resistance of the composite coating remained remarkably high throughout the immersion period. The dense PU matrix effectively slowed proton transport, while the fluorinated superhydrophobic surface minimized electrolyte retention on the coating surface. Furthermore, the trapped air layer within the hierarchical roughness reduced the effective contact area between the acidic solution and the coating, thereby delaying electrolyte penetration and suppressing interfacial corrosion reactions.

3.5.2. Corrosion Behavior in Alkaline Environment

Figure 9 presents the electrochemical behavior of the coating after immersion in 1 M NaOH solution. Compared with acidic conditions, a more pronounced decrease in the impedance modulus was observed as immersion time increased, indicating that alkaline media posed a greater challenge to the coating system.

The degradation mechanism under alkaline conditions differs substantially from that in acidic environments. Hydroxide ions (OH^-) can attack siloxane bonds within the fluorinated silica network, gradually weakening the integrity of the micro/nanostructure. Simultaneously, prolonged exposure to strong alkali may induce partial hydrolysis of urethane linkages within the PU matrix, resulting in localized softening of the polymer barrier. Such processes increase the probability of electrolyte transport through the coating.

Despite the aggressive alkaline environment, the F-SiO₂@PU coating still maintained relatively high impedance and low corrosion current densities after 14 days of immersion. This behavior demonstrates that the synergistic protection mechanism remained largely effective. The fluorinated surface continued to provide substantial liquid repellency, while the PU layer retained sufficient structural integrity to restrict electrolyte diffusion. Moreover, the anodized layer beneath the coating acted as an additional protective barrier, further delaying substrate degradation.

3.5.3. Corrosion Behavior in Saline Environment

The corrosion resistance of the coating in 3.5 wt.% NaCl solution is shown in Figure 10. Compared with acidic and alkaline environments, the coating exhibited the highest electrochemical stability in saline solution. After 7 and 14 days of immersion, only minor changes in impedance modulus and corrosion current density were detected.

The primary corrosive species in saline environments are chloride ions (Cl^-), which are well known for their ability to penetrate passive films and initiate localized corrosion on aluminum alloys. For conventional coatings, the gradual diffusion of chloride ions often leads to interfacial delamination and pitting corrosion. In contrast, the F-SiO₂@PU coating effectively inhibited chloride transport through multiple protective mechanisms.

The superhydrophobic top layer substantially reduced the residence time of electrolyte droplets on the surface. Meanwhile, the trapped air layer within the hierarchical micro/nanostructure acted as an additional diffusion barrier, significantly increasing the transport pathway for chloride ions. Even when limited amounts of electrolyte penetrated the outer layer, the dense PU matrix effectively restricted further migration toward the substrate. Consequently, the corrosion activity at the metal/coating interface remained extremely low throughout the immersion period.

3.6. Wear-Resistant Superhydrophobic Corrosion Protection Mechanism

The superior performance of the F-SiO₂@PU coating can be attributed to the synergistic integration of a hard-anodized interfacial layer, a flexible PU binding network, and a fluorinated hierarchical rough structure. The anodized layer strengthens coating adhesion through interfacial anchoring, the PU matrix enhances mechanical robustness through stress dissipation and particle fixation, and the fluorinated SiO₂ layer maintains a stable Cassie-Baxter state by combining hierarchical roughness with low surface energy. Together with the trapped-air barrier effect, these features establish a multi-level protection mechanism that effectively resists mechanical damage and inhibits the transport of corrosive species, resulting in durable superhydrophobicity and outstanding long-term corrosion protection.

Figure 11. (a) Mechanism diagram of the superhydrophobic and corrosion-resistant properties of the F-SiO₂@PU coating; (b) Mechanism diagram of the wear-resistant properties of the F-SiO₂@PU coating.

Figure 11. (a) Mechanism diagram of the superhydrophobic and corrosion-resistant properties of the F-SiO₂@PU coating; (b) Mechanism diagram of the wear-resistant properties of the F-SiO₂@PU coating.

4. Conclusions

A wear-resistant superhydrophobic F-SiO₂@PU composite coating was successfully fabricated on hard-anodized aluminum alloy via a facile spray-coating process. Benefiting from the synergistic effects of interfacial regulation and hierarchical micro/nanostructure construction, the coating exhibited excellent superhydrophobicity, mechanical durability, and corrosion resistance. At an FDTS dosage of 0.3 mL, uniformly distributed fluorinated SiO₂ particles generated a well-developed rough structure, resulting in a water contact angle of 161.5 ± 2° and a sliding angle of 1 ± 1.5°. The incorporation of the PU layer significantly enhanced interfacial adhesion and structural integrity, enabling the coating to retain a contact angle of approximately 150° after 10,000 cm abrasion and 200 tape-peeling cycles. Electrochemical measurements demonstrated outstanding corrosion protection performance, with a low-frequency impedance modulus of log|Z|₀.₁Hz = 8.0 and an ultralow corrosion current density of 5.36 × 10^-11 A cm^-2 in 3.5 wt.% NaCl solution, representing a reduction of approximately five orders of magnitude compared with bare aluminum alloy. Furthermore, the coating maintained high corrosion resistance in acidic, alkaline, and saline environments. The enhanced performance can be attributed to the strong interfacial bonding provided by the anodized layer, the dense barrier effect of the PU matrix, and the stable air-trapping capability of the fluorinated SiO₂ hierarchical structure, which collectively inhibit the ingress of corrosive species and improve resistance to mechanical damage. These findings demonstrate an effective strategy for developing durable superhydrophobic coatings that simultaneously enhance wear resistance and provide long-term corrosion protection for aluminum alloy applications.