Q235 carbon steel substrates (size: 40 mm × 40 mm × 1 mm) were purchased from Shandong Shengxin Technology Co., Ltd. Hydrophilic aluminum oxide nanoparticles (Al₂O₃ NPs, 99.9%, 30 nm) were purchased from Shanghai Macklin Biochemical Co., Ltd. Polydimethylsiloxane (PDMS, Sylgard 184) and silicone elastomer curing agent were obtained from Dow Corning Corporation. Other reagents including n-hexane (C₆H₁₄, 97.0%), ethanol absolute (C₂H₆O, 99.7%), sodium chloride (NaCl, 99.5%), and methylene blue trihydrate (C₁₆H₁₈CIN₃S·3H₂O, 99.5%) were supplied by Sinopharm Chemical Reagent Co., Ltd, and 3 M VHB tape was supplied by 3 M China Limited. Silicon carbide abrasive paper (2000 grit) was provided by Suisun Co., Ltd. All chemical reagents were used as received without further purification.

A total of 1 g PDMS, 0.1 g curing agent, and different amounts of Al₂O₃ NPs were ultrasonically and magnetically dissolved in 10 ml n-hexane to obtain a milk-like suspension. Different mass ratios of Al₂O₃ NPs/PDMS (1:4, 1:2, 3:4, 1:1, 5:4, and 3:2) were used to achieve Al₂O₃@PDMS composite coatings. Prior to the spray coating process, Q235 carbon steel substrates were sanded and cleaned with ethanol absolute solution. A spraying gun with a nozzle diameter of 1 mm and a spraying pressure of 0.3-MPa was employed for spray coating treatment. The spray-coated Al₂O₃ NPs/PDMS composite coatings were cured at 100°C for 1 h after spraying. Various substrates including glass, aluminum alloy, 3D foam material, polyurethane plastic, wood, filter paper, and concrete block were used to prepare superhydrophobic Al₂O₃@PDMS coating according to the same procedure of Q235 carbon steel. The schematic illustration of the preparation process of the superhydrophobic Al₂O₃@PDMS composite coating is shown in Figure 1.

The micro-/nano-morphologies of different samples were observed by using a field-emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM450) equipped with energy-dispersive X-ray spectroscopy (EDS, Oxford X-MaxN50). X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was employed to analyze the chemical compositions of the fabricated superhydrophobic Al₂O₃@PDMS composite coating. The static water contact angles and sliding angles of Al₂O₃@PDMS composite coatings with different Al₂O₃/PDMS mass ratios were measured by a contact angle meter (Dataphysics OCA25) with 4 μL deionized water droplets.

The electrochemical tests of different samples were carried out using an electrochemical workstation (CorrTest CS2350H) in a typical three-electron cell. Platinum sheet, saturated calomel electrode, and testing samples were used as the counter electrode, the reference electrode, and the working electrode, respectively. The electrochemical impendence spectroscopy (EIS) and the potentiodynamic polarization of bare Q235 carbon steel and superhydrophobic Al₂O₃@PDMS composite coating were measured in 3.5 wt.% NaCl aqueous solution under open circuit potential with an amplitude of 10 mV. The testing frequency range of EIS is from 100 kHz to 10 mHz. The obtained EIS data were fitted with equivalent electrical circuit (EEC) using Zsimwin software. Potentiodynamic polarization curves were measured with respect to OCP in both the anodic and cathodic directions with a scanning speed of 0.167 mV/s. The corrosion current density (I _corr) and corrosion potential (E _corr) were calculated by extrapolating the linear portion of the curves with CS Studio software.

The mechanical robustness of the fabricated superhydrophobic Al₂O₃@PDMS composite coating was assessed using the tape-peeling test and sandpaper abrasion tests. The prepared superhydrophobic coating was pressed with 3 M tape loaded with 100 g for 30 s to ensure a uniform contact between the 3 M tape and coating for the tape-peeling test. After that, the 3 M tape was completely peeled away from the surface. This press-peeling process was defined as one testing cycle. The abrasion test was carried out by orienting the as-fabricated superhydrophobic coating toward the 2,000-grit sandpaper and placing a weight of 100 g on top of it. The superhydrophobic coating was then subjected to unidirectional drift with a speed of 1 cm/s. One abrasion cycle was defined as the reciprocating pulling of the sample in the horizontal direction over a distance of 20 cm. After various tape-peeling and sandpaper abrasion cycles, the water contact angles and sliding angels of the samples were measured using a contact angle meter.

The surface wettability of spray-coated Al₂O₃@PDMS composite coatings was investigated to understand and validate the optimal parameters. Figure 2 depicts the variation of water contact angles and sliding angles of Al₂O₃@PDMS composite coatings with different Al₂O₃/PDMS mass ratios including 1:4, 1:2, 3:4, 1:1, 5:4, and 3:2. It can be seen that the water contact angles of the coatings gradually increased as the Al₂O₃/PDMS mass ratio increased. The contact angle of the Al₂O₃@PDMS composite coating was 119.6 ± 4.8°, 123.2 ± 2.2°, 127.9 ± 1.5°, and 146.9 ± 3.5°, corresponding to the Al₂O₃/PDMS mass ratios of 1:4, 1:2, 3:4, and 1:1, respectively. When the Al₂O₃/PDMS mass ratio is increased to 5:4, the water contact angle and sliding angle of the surface are 157.0 ± 3.5° and 6.7 ± 0.5°, respectively, displaying a typical superhydrophobic property. The water contact angle and the sliding angle are 156.5 ± 2.6° and 4.5 ± 0.5° when the Al₂O₃/PDMS mass ratio is 3:2. Surface superhydrophobicity was achieved through the combination of Al₂O₃ NP–induced roughness and low–surface energy PDMS molecules. The optimal Al₂O₃/PDMS mass ratio for developing superhydrophobic Al₂O₃@PDMS composite coating is 5:4, which was used to fabricate superhydrophobic samples for the characterizations and performance evaluations listed as follows.

The dynamic contacting process of a water droplet on the as-prepared superhydrophobic Al₂O₃@PDMS composite coating is depicted in Figure 3. As the water droplet moves down from the microsyringe, it gradually closes and contacts with the as-prepared superhydrophobic Al2O3@PDMS composite coating (Figures 3A,B). As it is pressed further, the water droplet is pushed up and moved up along the neck of the microsyringe (Figure 3C). The water droplet detached and separated from the superhydrophobic Al₂O₃@PDMS coating as the microsyringe moved up, eventually hanging on the needlepoint of the microsyringe (Figures 3D–F). Throughout the dynamic process, the superhydrophobic surface remains non-wetting with no water traces remaining. As a result, the adhesion force between the superhydrophobic surface and the water droplet is extremely low, which contributes to the easy rolling behavior of liquids on the as-fabricated superhydrophobic Al₂O₃@PDMS coating.

FE-SEM, EDS, and XPS techniques were used to observe the micro-/nano-structures and chemical compositions of the samples. Figures 4A,B present the SEM images of the bare Q235 carbon steel and superhydrophobic Al₂O₃@PDMS coating. The surface morphology of bare Q235 carbon steel is relatively smooth, with few sandpaper polishing marks, while for the superhydrophobic Al₂O₃@PDMS coating (shown in Figure 4B), some micro-sized clusters and protrusions with nano-sized particles (Figure 4B inset) were packed on the coating surface, resulting in a remarkably improved surface roughness. Theses micro- and nano-hierarchical structures can trap air to form an extremely thin layer of air cushion. The air cushion can significantly reduce the contacting area between the superhydrophobic Al₂O₃@PDMS coating and water droplets, which is crucial for the non-wetting Cassie–Baxter air–liquid–solid contact. As a result, the water droplets barely penetrate the as-fabricated superhydrophobic Al₂O₃@PDMS coating, demonstrating stable water-repellent superhydrophobicity.

Besides, the two-dimensional EDS mapping result of the fabricated superhydrophobic Al₂O₃@PDMS coating is presented in Figure 4C. According to the EDS mappings, the C, O, Al, and Si elements of the spray-coated superhydrophobic Al₂O₃@PDMS coating showed a uniform distribution. Figure 4D shows the full XPS spectrum of the as-prepared superhydrophobic Al₂O₃@PDMS coating. In this elemental analysis, five peak signals of Al2p, Si2p, Si2s, C1s, and O1s can be clearly observed in the XPS survey at the binding energies of 74.8, 102.4, 153.8, 284.8, and 532.7 eV, respectively. The results of the elemental testing indicate that the Al₂O₃ NPs@PDMS composite coating with superhydrophobicity was successfully produced.

The corrosion resistance behaviors of bare Q235 carbon steel and the as-prepared superhydrophobic Al₂O₃@PDMS coating in 3.5 wt.% NaCl aqueous solution were evaluated from the electrochemical points of view. Electrochemical impedance spectroscopy (EIS) is one of the most common techniques applied for the evaluation and analysis of the protective properties of coatings for metallic materials (Jeyaram et al., 2020; Mei et al., 2020). Figure 5 presents the EIS plots and the equivalent circuit models of bare Q235 carbon steel and the superhydrophobic Al₂O₃@PDMS coating. Figure 5A and its inset image display the Nyquist plots of the Q235 carbon steel and superhydrophobic Al₂O₃@PDMS coating. The diameter of the capacitance arcs of the superhydrophobic Al₂O₃@PDMS coating is significantly larger than that of the bare Q235 carbon steel, demonstrating a superior anticorrosion properties. Figure 5B shows the Bode plots of log |Z| vs. log frequency of the Q235 carbon steel and superhydrophobic Al₂O₃@PDMS coating. The impedance modulus data at low frequency (|Z|_0.01 Hz) are commonly used as an intuitive index of the anticorrosion barrier performance of protective coatings (Mishra et al., 2020; Wiering et al., 2021). According to the Bode plots of log |Z| vs. log frequency, the |Z|_0.01 Hz of the superhydrophobic Al₂O₃@PDMS coating is 1.53 × 10⁷ Ω·cm², which is four orders of magnitude higher than bare Q235 carbon steel (1.31 × 10³ Ω·cm²), indicating that the superhydrophobic Al₂O₃@PDMS coating can lead to excellent corrosion protection properties.

To perform a quantitative comparison, the EIS data were fitted using equivalent circuit models. Figures 5C,D show the equivalent circuit model of the bare Q235 carbon steel and the superhydrophobic Al₂O₃@PDMS coating. The R _s(Q _dl R _ct) equivalent circuit model was used for the EIS fitting of bare Q235 carbon steel. R_s, Q_dl, and R_ct refer to solution resistance, constant phase element of the electric double layer, and charge transfer resistance at Q235 carbon steel/electrolyte interface, respectively. As a comparison, the R _s{Q _f[R _f(Q _dl R _ct)]} equivalent circuit model was employed for the as-prepared superhydrophobic Al₂O₃@PDMS coating, in which Q _f and R _f represent the constant phase element and film resistance of superhydrophobic Al₂O₃@PDMS coating, respectively. The corresponding fitting parameters are provided in Table 1. The lower Q_dl and higher R_ct values of the superhydrophobic Al₂O₃@PDMS coating suggest a conspicuously reduced corrosion rate.

Generally, a lower corrosion current density (I _corr) or a higher corrosion potential (E _corr) in a typical potentiodynamic polarization curve corresponds to a lower corrosion rate and higher corrosion resistance (Mishra and Balasubramaniam, 2004; Liu et al., 2016). Figure 6 shows the potentiodynamic polarization curves of bare Q235 carbon steel and the as-prepared superhydrophobic Al₂O₃@PDMS coating in 3.5 wt.% NaCl aqueous solution. The corrosion potential (E _corr) and corrosion current density (I _corr) of the as-prepared superhydrophobic Al₂O₃@PDMS coating are −0.52 V and 3.74 × 10^–10 A/cm², respectively, indicating a remarkable difference from that of the bare Q235 carbon steel (E _corr = −0.80 V, I _corr = 4.66 × 10^–6 A/cm²). The E _corr of the as-fabricated superhydrophobic Al₂O₃@PDMS coating shifted toward the positive direction by 0.27 V and the I _corr decreased by more than four orders of magnitude. The significant positive shift of the E _corr could be attributed to the improved protective performance of the as-fabricated superhydrophobic Al₂O₃@PDMS coating. The corresponding corrosion inhibition efficiency (η) could be calculated using the equation as follows (Liu et al., 2020; Ma et al., 2020): η = I c o r r b a r e − I c o r r c o a t i n g I c o r r b a r e × 100 % . (1)

According to Eq. 1, the η value of the as-fabricated superhydrophobic Al₂O₃@PDMS coating was calculated to be 99.992%, supporting the conclusion that the spray-coated superhydrophobic Al₂O₃@PDMS coating provides an effective barrier for corrosion suppression.

Atmospheric corrosion of metallic materials is a spontaneous degradation process resulting from interactions with its surrounding environment (Pei et al., 2020). More frequently, the atmospheric corrosion processes are initiated by surface wetting due to the moisture condensation and hygroscopic properties of salts and pollutants deposited on the surface (Koushik et al., 2021). Thus, understanding the interaction and relationship between NaCl salt particles and the atmospheric environment with high relative humidity remains a major challenge. To address this issue, the hygroscopic and deliquesce behaviors of single and double NaCl salt particles were investigated, as shown in Figure 7.

Figures 7A–D depict the deliquescence process of a single NaCl particle on a horizontally placed superhydrophobic Al₂O₃@PDMS coating in an atmospheric condition with 80 ± 2% relative humidity. Water vapor condensed on the surface of the solid NaCl particle during the deliquesce process, forming a saline solution. The solid NaCl particle was completely dissolved after deliquesce for 95 min, presenting a spherical saline liquid droplet. Owing to the liquid-repellent superhydrophobicity, the saline solution could not spread and wet the coating, resulting in a good protective performance against NaCl salt deliquesce–induced atmospheric corrosion attack. In addition, the deliquesce behavior of double NaCl salt particles on the prepared superhydrophobic Al₂O₃@PDMS coating was first carried out and studied, as depicted in Figures 7E–H. At the beginning of the deliquescence process, the evolution of the double NaCl particles was similar to that of the single NaCl salt deliquesce. The solution film formed over the double NaCl particles, resulting in larger spherical droplets as deliquescence time passed. With the deliquescence time prolonged to 195 min 6 s (Figure 7H), it is worth noting that the two saline solution droplets self-coalesced instantaneously and eventually became a single saline droplet. The self-coalescence–induced salt deliquescence behavior of the as-fabricated superhydrophobic Al₂O₃@PDMS coating demonstrates that the Cassie–Baxter interfacial phase contacts of the superhydrophobic surface can serve as an efficient barrier to suppress the formation of thin saline liquid electrolyte film and protect the underlying substrate from being corroded under the atmospheric service environments with high relative humidity.

The adaptability of superhydrophobic coatings is critical for real-world applications. The preparation procedure should be adaptable to various substrates. To demonstrate the substrate-independent property of the as-fabricated superhydrophobic Al₂O₃@PDMS coating, eight typical substrates were used to study the large-scale adaptability performance, including Q235 carbon steel, glass, aluminum alloy, 3D foam material, polyurethane plastic, wood, filter paper, and concrete block. Figures 8A–H show the optical images of the spray-coated superhydrophobic Al₂O₃@PDMS coating on different substrates. The dyed blue water droplets maintained a spherical shape on each spray-coated substrate, illustrating a typical Cassie–Baxter interfacial phase contact. The water contact angles and sliding angles of the obtained superhydrophobic coating on different substrates are presented in Table 2. The contact angles were all greater than 150° with sliding angles less than 8°, indicating excellent superhydrophobicity. It is noteworthy that the facile and substrate-independent spray coating method and the fluorine-free superhydrophobic Al₂O₃@PDMS coating will provide a suitable large-scale technique that can be easily applied to various substrates and has potential applications for water repellency, self-cleaning, corrosion resistance, and so on.

Mechanical robustness plays a key role for functional applications. Tape peeling and sandpaper abrasion tests are commonly conducted to evaluate the mechanical robustness and durability of superhydrophobic materials. Figures 9A,B present the variations of water contact angles and sliding angles of the superhydrophobic Al₂O₃@PDMS coating after different cycles in the tape peeling and sandpaper abrasion tests. After 32 tape peeling cycles, the as-prepared superhydrophobic Al₂O₃@PDMS coating remained its superhydrophobic property with a contact angle of 152.4 ± 2.2° and a sliding angle of 9.8 ± 0.6°. The sandpaper abrasion test was performed to confirm the mechanical durability. As shown in Figure 9B, the contact angles of the superhydrophobic Al₂O₃@PDMS coating decrease by only 5° after 20 abrasion cycles (i.e., 400 cm length) with a contact angle of 153.6 ± 1.8°. The results of the tape peeling and sandpaper abrasion tests show that the fabricated superhydrophobic Al₂O₃@PDMS coating possesses excellent mechanical robustness and durability. The cross-linked PDMS can firmly bind the Al₂O₃ NPs and contribute to the improvement of the mechanical strength of the coating.