Mechanism of Synergistic Corrosion and Radiation Protection of Hexamethylenetetramine and Benzotriazole for Bionic Superhydrophobic Coating on Q235 Steel

Abstract

Bionic superhydrophobic coatings were prepared on Q235 steel substrates by combining hexamethylenetetramine (HMTA) and benzotriazole (BTA) with methyltrimethoxysilane (MTMS), nano-silica, zinc oxide, and polydimethylsiloxane (PDMS). Three-dimensional morphology analysis revealed micro- and nanostructures in the coating. The coating’s corrosion resistance was demonstrated through electrochemical impedance spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS) analysis confirmed zinc oxide embedding within the micro- and nano-rough structures. The optimized bionic coating achieved a contact angle (CA) of 161.2° and a sliding angle (SA) of 2.0°. The bionic coatings demonstrated low adhesion, dynamic hydrophobicity, and self-cleaning properties when exposed to various liquids and contaminants. The corrosion inhibition mechanism of BTA and HMTA in superhydrophobic coatings involves a synergistic combination of chemisorption, complexation, and physical barrier effects. This MTMS-SiO₂-ZnO-PDMS-HMTA-BTA coating demonstrated the highest protection efficiency among the tested formulations. The optimized coating achieved a protection efficiency of 92.12%. Additionally, the bionic coating demonstrated effective UV resistance, maintaining a contact angle of 153.7° after 120 h of UV exposure.

1. Introduction

Bionic superhydrophobic coatings, recognized for their extreme water repellency, have attracted considerable attention in recent years for their potential applications in self-cleaning surfaces, anti-fouling materials, and corrosion-resistant coatings [1]. Such surfaces are inspired by natural phenomena like the lotus leaf effect, where water droplets roll off easily, removing dirt and other contaminants due to the hierarchical micro- and nanoscale structures combined with low-energy surface materials [2,3]. Developing these coatings, particularly for industrial substrates like steel, presents challenges such as ensuring long-term durability, environmental wear resistance, and sufficient corrosion protection [4,5].

Q235 steel is extensively used across various industries for its mechanical strength and cost-effectiveness [6,7]. However, its susceptibility to corrosion restricts its applicability in harsh environments, such as seawater or acidic conditions [8,9,10]. Consequently, there is an increasing demand for advanced coatings that provide superhydrophobicity and enhanced corrosion resistance, thereby extending the lifespan of steel substrates [11,12,13]. A promising strategy to address this issue involves incorporating corrosion inhibitors and hydrophobic agents into the coating formulation to improve performance under corrosive conditions [14,15,16].

Recent studies demonstrate that combining nanomaterials, such as nano-silica and zinc oxide, with organosilanes like methyltrimethoxysilane (MTMS) and polymers like polydimethylsiloxane (PDMS) effectively generates hierarchical micro- and nanostructures on steel surfaces [17,18,19]. These materials lower surface energy, allowing water droplets to exhibit high contact and low sliding angles—essential characteristics of superhydrophobic surfaces. The corrosion resistance of copper (Cu) can be enhanced using a combination of hyamine 1622 and benzotriazole inhibitors [20]. Benzotriazoles function as UV stabilizers and absorbers [21,22]. Researchers investigated the corrosion inhibition of mild steel in a 1M hydrochloric acid solution using HMTA, finding an inhibition rate of 86% at a dosage of 0.8 g/L [23]. Hexamethylenetetramine (HMTA) served as an effective additive for the microarc oxidation of magnesium alloys, achieving a maximum corrosion inhibition rate of 94.57% at a concentration of 360 g/L in a 3.5% NaCl solution [24]. Additionally, the addition of hexamethylenetetramine during pickling can effectively slow steel corrosion [25,26].

This study incorporated hexamethylenetetramine (HMTA) and benzotriazole (BTA) in the preparation of solid superhydrophobic coatings to investigate their combined corrosion inhibition mechanisms. This research focuses on developing a superhydrophobic coating for Q235 steel substrates by combining MTMS, SiO₂, ZnO, PDMS, HMTA, and BTA. The primary aim of this study is to investigate the synergistic effects of these materials in creating a durable, corrosion-resistant superhydrophobic surface. Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion resistance of the superhydrophobic coatings. The findings indicate potential applications in industries where protecting steel substrates from corrosion is essential, such as the marine, automotive, and construction sectors.

2. Experimental

2.1. Materials and Preparation

The Q235 steel substrate size was 20 mm × 20 mm × 1 mm. The Q235 steel was purchased from Taizhou Jurun Metal Co., Ltd. (Taizhou, China). Hydrofluoric acid (HF, 0.4 mol/L, 25 °C) was purchased from Nantong Mingxin Chemical Co., Ltd. (Nantong, China). Methyltrimethoxysilane (MTMS, purity ≥ 98.0%, 25 °C) was purchased from Runhui Biotechnology Co., Ltd. (Weihai, China). The nano-silica with a diameter of 500 ± 5 nm was purchased from Shanghai Kaiyin Chemical Co., Ltd. (Shanghai, China). Zinc oxide (ZnO, particle size 100 μm, analytical purity) was purchased from Tianjin Hengxing Fine Chemical Co., Ltd. (Tianjin, China). The polydimethylsiloxane (PDMS) was purchased from Dow Corning (Shanghai, China). Hexamethylenetetramine (HMTA) was purchased from Tianjin Hengxing Fine Chemical Co., Ltd. (Tianjin, China). Benzotriazole (BTA) was purchased from Tianjin Hengxing Fine Chemical Co., Ltd. (Tianjin, China). The water used in all experiments and tests was obtained from the UPC-III water purification system.

The Q235 steel surface was sanded with 280# sandpaper and then cleaned with alcohol-soaked wool. The substrate was subsequently activated with hydrofluoric acid for 1 min, rinsed with deionized water, and dried. The original formulation was optimized to improve the adhesion of the superhydrophobic coating to the substrate [27]. A PDMS-HMTA-BTA transition layer was prepared to act as a transition layer prior to sol application [28]. The transition layer formulation consisted of 0.25 g PDMS, 0.025 g curing agent, 0.025 g HMTA, and 0.0125 g BTA. These substances are mixed and stirred well and then applied dropwise to the Q235 steel substrate. The transition layer was achieved by heating the sample in a chamber furnace at 80 °C for 5 min. The optimized superhydrophobic coating formulation consisted of 5 mL MTMS, 1 g nano-silica, 0.1 g zinc oxide, 0.25 g PDMS, 0.025 g curing agent, 0.025 g HMTA, and 0.0125 g BTA. These substances were added, mixed, and magnetically stirred in a specific order to form a homogeneous sol. The sol was then uniformly drop-coated onto the transition layer. The sol-coated Q235 steel substrate was dried at room temperature (13–22 °C) for 24 h. The superhydrophobic coatings were achieved by heating the sample in a chamber furnace at 200 °C for 30 min. The basic formulation consisted of 5 mL MTMS, 1 g nano-silica, and 0.1 g zinc oxide. Other superhydrophobic coatings were formulated by adding the appropriate substances to this base. The formulation of different superhydrophobic coatings is shown in

Table 1. Formulation of different superhydrophobic coatings.

Sample	PDMS (g)	HMTA (g)	BTA (g)
MTMS-SiO2-ZnO	-	-	-
MTMS-SiO2-ZnO-PDMS	0.25	-	-
MTMS-SiO2-ZnO-HMTA	-	0.025	-
MTMS-SiO2-ZnO-BTA	-	-	0.0125
MTMS-SiO2-ZnO-PDMS-HMTA-BTA	0.25	0.025	0.0125

. The MTMS-SiO₂-ZnO coating is named C1. The MTMS-SiO₂-ZnO-PDMS coating is named C2. The MTMS-SiO₂-ZnO-HMTA coating is named C3. The MTMS-SiO₂-ZnO-BTA coating is named C4. The MTMS-SiO₂-ZnO-PDMS-HMTA-BTA coating is named C5.

The siloxane chain (-Si-O-Si-) on the PDMS molecule may react with the hydroxyl group (-OH) on the surface of SiO₂ and ZnO to form new Si-O-Si bonds and Si-O-Zn, forming a crosslinked siloxane network structure at 200 °C. The methyl group of the PDMS remains in the molecular structure of the PDMS, functioning as a hydrophobic group. MTMS undergoes a hydrolytic condensation reaction at 200 °C. The hydrolysis products contain much -CH₃, and the surfaces of nano-SiO₂ and micron ZnO contain much -OH. Highly crosslinked network polymers are formed by the above hydrolysis condensation reaction at 200 °C. -CH₃ was successfully grafted onto the surfaces of nano-SiO₂ and micron ZnO, which made the coatings exhibit superhydrophobicity. The surface hydrophobic modification process is shown in Figure 1. Hydrophobic methyl groups generated by the hydrolysis of MTMS replaced the hydrophilic -OH on the surfaces of nano-SiO₂ and micron ZnO.

2.2. Characterization

The wettability of the coating was evaluated by a commercial CA instrument (Attension Theta by Biolin Scientific, Danderyd, Sweden). The test droplets were 4 μL distilled water droplets. The test sample was fixed on the sample table, a drop was placed on the sample table, the sample table was automatically tilted, the drop rolled down, and then the value of the SA was recorded. Three random locations were selected at room temperature on the coating surface. The fitting method was Laplace fitting. A digital camera (Canon EOS 90D, Tokyo, Japan) was used to take photographs. The microscope images were taken using a Keyence VHX-2000 Digital Microscope (Keyence VHX-2000, Osaka, Japan). High-speed visualization of a water droplet bouncing on the superhydrophobic coating was achieved using a high-speed camera (Qian Yan Lang 5F10, Shijiazhuang, China) with a frame rate of 1000 fps at 25 °C. An electrochemical workstation (Ke Si Te CS2350H, Wuhan, China) was used to test electrochemical impedance spectra. An X-ray photoelectron spectrometer was used (Shimadzu AXIS ULTRA DLD, Kyoto, Japan): voltage 15 kV, current 10 mA, total spectral throughput 160 eV, monochromatic aluminum target, power 150 W (15 kV, 10 mA). The full-spectrum energy was 160 eV, and the analytical area was 300 μm × 700 μm. The wavelength of the UV lamp was 365 nm, power 70 W (CCS LNSP-200UV3-365-FNNR, Kyoto, Japan). The surface of the coated samples was characterized using a 3D confocal white light interferometer (Sneox, SENSOFAR, Barcelona, Spain). The C5 superhydrophobic coating and ZnO surfaces were characterized by Raman spectroscopy (RAMAN, Bruker, Ettlingen, Germany). The spectrum range was 400~2200 cm⁻¹. The excitation wavelength was 785 nm. The power was 20 mW. The spectrum linewidth was less than 0.2 nm.

3. Results and Discussion

3.1. Wettability and Morphology

The wettability and three-dimensional morphology of the superhydrophobic coating surfaces are shown in Figure 2. The C1 superhydrophobic coating exhibits a contact angle (CA) of approximately 153.9° and a sliding angle (SA) of about 3.0°. Coatings with different composite formulations have a slight increase in their surface contact angle. In particular, the C5 superhydrophobic coating has a surface contact angle of 161.2° and a sliding angle of 2°. The contact angle increased by 7.3°, and the sliding angle decreased by 1° compared to the superhydrophobic coating with the C1 base formulation. The non-homogeneous rough structure of the coated surface, combined with the low surface energy provided by MTMS and PDMS, creates a superhydrophobic surface. The substantial gaps between particles effectively trap air. This trapped air minimizes the actual contact area between the solid surface and water droplets, significantly enhancing the hydrophobicity of the coating surface [29]. In the testing and characterization below, the superhydrophobic coatings are referred to as C5 coatings if not otherwise specified.

3.2. Low Adhesion

Figure 3 presents the low adhesion test results on the superhydrophobic coating surface. Figure 3a demonstrates the process of a water droplet contacting, deforming, and then detaching from the superhydrophobic surface. Even under significant deformation due to applied pressure, the water droplet easily detaches from the superhydrophobic surface. This result confirms that the superhydrophobic coating prevents liquid infiltration and maintains low adhesion. Low adhesion is essential for enabling the self-cleaning capabilities of the superhydrophobic coating [30]. Figure 3b presents an image of a water jet impacting the superhydrophobic surface. The thin air cushion on the superhydrophobic surface causes the water jet to contact the air layer first, subsequently ejecting with minimal energy loss [31]. Figure 3c displays an image of a water jet impacting the hydrophilic Q235 steel surface. Due to the hydrophilic nature of Q235 steel, the water jet spreads across the surface.

3.3. Droplet Impact Characteristics

Dynamic hydrophobicity is a crucial characteristic of superhydrophobic materials, reflecting their ability to repel impacting droplets. The primary evaluation metrics include droplet contact time on the material surface and the dynamic hydrophobicity during contact. Among these, the Weber number ($We$) and Reynolds number ($Re$) are two key dimensionless parameters, defined as follows [32]:

$$\mathrm{W}\mathrm{e}={\displaystyle \frac{\mathrm{\rho }{\mathrm{D}}_0{\mathrm{V}}^2}{\mathrm{\sigma }}}$$

(1)

$$\mathrm{R}\mathrm{e}={\displaystyle \frac{\mathrm{\rho }{\mathrm{D}}_0\mathrm{V}}{\mathrm{\mu }}}$$

(2)

here, ρ is the density of the liquid (1 g$·$cm⁻³), σ is the solid–liquid interfacial tension (72 mN$·$m⁻¹), $V$ is the impact velocity of the water drop, $D_0$ is the diameter of the initial liquid drop, and $\mu$ is the viscosity of the liquid (1.0 × 10⁻³ N$·$S$·$m⁻²). In this experiment, a droplet with a radius of 1 mm hits the surface at the speed of 1 m$·$s⁻¹, and $We$ and $Re$ are calculated to be about 27.8 and 2234.9, respectively.

High-speed visualization of a water droplet bouncing on the superhydrophobic coating was recorded at 1000 fps and 25 °C. The droplet’s falling, spreading, and bouncing behaviors were captured. As shown in Figure 4, water droplets on the Q235 steel surface adhere after 13.6 ms of motion. Figure 4c demonstrates that after impacting the Q235 steel surface, the droplet reaches its maximum spread diameter in 3.8 ms. In contrast, Figure 5 illustrates that water droplets bounce off the treated surface at 12.4 ms. Figure 5c indicates that on the superhydrophobic surface, the droplet spreads quickly, reaching its maximum diameter in 2.6 ms, and fully bounces off after retracting for 7.8 ms (Figure 5e). This observation illustrates the superior water-repellent properties of the superhydrophobic coating. This effect arises from the significant air trapped within the micro-nano structure of the superhydrophobic coating, which reduces the contact area between droplets and the surface. Consequently, the reduced viscous forces between the droplet and coating result in superior dynamic hydrophobicity [33]. Analysis suggests that the thin air cushion on the superhydrophobic surface minimizes energy dissipation during droplet spreading and retraction, allowing the droplet to retain sufficient energy for rapid retraction and subsequent bouncing after reaching its maximum spread diameter [31].

3.4. Self-Cleaning

Figure 6 illustrates the self-cleaning performance of the superhydrophobic coating with various liquids. The coating’s self-cleaning properties were evaluated by immersing it in water, green tea, milk, orange juice, cooking wine, and coffee. After vertical removal from each liquid, the coated Q235 steel surface remained clean and dry. The air cushion between the coating’s hierarchical rough surface and the liquid effectively prevents contamination [31]. The coating’s excellent self-cleaning and anti-fouling properties enhance its potential applications. Throughout immersion and removal, the superhydrophobic coating surface remained unwetted by all tested liquids.

Sand and chalk powder were selected as solid contaminants for the self-cleaning test. Figure 7 presents microscope images of the chalk powder and sand employed in the self-cleaning test. Based on image scaling, the average particle sizes of the simulated contaminants were 10 μm and 150 μm, respectively.

The self-cleaning properties of superhydrophobic surfaces protect against contamination, presenting substantial potential for diverse applications. Self-cleaning performance was evaluated using chalk powder (10 μm) and sand (150 μm) as contaminants. The test involved 1.5 min of water cleaning using 10 mL of water with minimal kinetic energy in the droplets. Most particles were removed from the rough micro- and nano-layered structure, although approximately 15%–25% remained [27]. Under the same test conditions, about 15%–25% of particles remained on the surface of the C1 coating. About 15%–20% of particles remained on the surface of the C5 coating. The self-cleaning performance of the latter is slightly better than that of the former. The self-cleaning process of chalk dust on the C5 superhydrophobic surface is shown in Figure 8. The self-cleaning process of sand on the C5 superhydrophobic surface is shown in Figure 9. The water droplet carries contaminants in its path as it rolls down. The C5 coating produced in this study exhibited a contact angle (CA) of 161.2° and a sliding angle (SA) of 2.0°. Compared with previous studies, the contact angle increased by 7.3°, while the sliding angle decreased by 1.0°. Two factors may contribute to this improvement. First, air trapped within the microstructure reduces the droplet’s contact area with the superhydrophobic surface. Additionally, the high capillary force of water droplets and the low adhesion of contaminants in this particle size range to the superhydrophobic surface enhance the self-cleaning performance.

As shown in Figure 10, the Si 2p peak in the C1 coating appears between 100 and 110 eV. This peak indicates the chemical states of silicon atoms, including Si-O-Si and Si-C bonds. Si-O-Si bonds are typically from siloxane networks formed by MTMS during the condensation reaction. Si-C bonds can be attributed to methyl groups (-CH₃) attached to silicon from MTMS, contributing to the hydrophobicity of the surface. The C 1s peak typically appears between 280 and 290 eV. This peak may indicate carbon–carbon and carbon–hydrogen bonds, potentially from residual MTMS methyl groups, surface contamination, or carbon-containing functional groups. The O 1s peak is usually observed between 528 and 532 eV. This peak reveals the chemical environment of oxygen atoms, indicating Si-O, Si-O-C, or possibly C=O bonds. The O 1s peak reveals contributions from various oxygen species: Si-O bonds from SiO₂ or MTMS-derived siloxane structures, Si-O-C bonds from crosslinked MTMS networks, and potential C=O species if there are traces of organic contaminants. The Zn 3p peak appears between 102 and 105 eV. This peak indicates the chemical state of zinc atoms, specifically Zn-O bonds. Additional signals include Zn LM1 through LM7 peaks and O KL1 through KL3 peaks. Zn 2p 1/2 and Zn 2p 3/2 spin-orbit splitting peaks are observed, along with characteristic Zn 3s, Zn 2s, and Zn 3p peaks. The Zn 2p 1/2 and Zn 2p 3/2 peaks confirm the presence of ZnO. Zn LM1 through Zn LM7 peaks indicate zinc’s electronic transitions. These multiple XPS signals result from Zn’s numerous inner electrons. O KL1, O KL2, and O KL3 peaks result from oxygen’s Auger transitions, supporting the interpretation of various oxygen environments (Si-O, Zn-O).

In the C5 coating, the XPS spectra still show the characteristic peaks of O KL1, O 1s, C 1s, Si 2p, and Si 2s, but the Zn peaks have disappeared. Si 2p and Si 2s peaks remain, confirming the presence of siloxane and Si-C bonds derived from MTMS and PDMS. The C 1s peak becomes more prominent, indicating increased organic content due to PDMS, HMTA, and BTA. PDMS contributes methyl (-CH₃) groups. HMTA and BTA introduce additional carbon-containing functional groups. The O 1s peak persists, indicating contributions from Si-O, Si-O-C, and possible organic oxygen species. The absence of Zn 2p, Zn LM, and Zn 3p peaks indicates that zinc signals are no longer detectable. The possible explanations are as follows. A reaction among MTMS, PDMS, HMTA, and BTA has likely created an encapsulating layer that shields ZnO. XPS analysis has a limited probing depth (typically 5–10 nm). If the encapsulating layer exceeds this depth, ZnO would be undetectable. This encapsulation might result from the complex chemical interactions as follows. Condensation of MTMS and PDMS formed a dense siloxane network. HMTA and BTA interacted with ZnO or formed a protective organic–inorganic composite layer.

Raman spectroscopy of the superhydrophobic coating and ZnO are shown in Figure 11. In the blue ZnO Raman spectrum, an important characteristic peak of the ZnO lattice vibration appears clearly at 437 cm⁻¹. In the red superhydrophobic coating Raman spectrum, the characteristic peak at 437 cm⁻¹ of ZnO disappears. It is analyzed that the five chemicals, MTMS, SiO₂, PDMS, HMTA, and BTA, form a uniform covering layer that encapsulates the ZnO particles inside. The interaction between ZnO and the external laser signal is weakened, which leads to the fact that its Raman characteristic peaks cannot be detected effectively. This is consistent with the phenomenon of Raman signal attenuation, whereby Raman light cannot penetrate into the ZnO particles and excite their characteristic vibrations when the particles are covered by a thick layer or buried in a coating.

3.5. Electrochemical Impedance Spectroscopy (EIS)

Figure 12 presents the kinetic potential curves of superhydrophobic surfaces on Q235 steel substrates with various formulations in a 3.5 wt.% NaCl solution. The working electrode was a sample with a 1.0 cm² test surface area. A saturated calomel electrode was the reference electrode, and a platinum plate was the auxiliary electrode. Sample polarization curves were measured within a scanning potential range of −0.3 V to +0.3 V at a rate of 1 mV/s. Electrochemical impedance spectra were recorded using a sinusoidal perturbation potential of 5 mV and a frequency range of 10⁵ to 10⁻² Hz.

In a typical polarization curve, a lower corrosion current density (J_corr) or higher corrosion potential (E_corr) indicates improved corrosion resistance [28]. Figure 12 displays the Tafel curves of bare Q235 steel and Q235 steel coated with superhydrophobic coatings of various formulations. The Tafel extrapolation method was applied to obtain the corrosion potential (E_corr) and corrosion current density (J_corr), with fitting results shown in

Table 2. Tafel curve fitting results of different samples.

Sample	${\mathit{E}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}}/\mathbf{V}$	${\mathit{J}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}}/(\mathbf{A}·{\mathbf{c}\mathbf{m}}^{-2})$	$\mathit{\eta }/\mathbf{\%}$
Bare Q235 steel	−0.62	2.83 × 10−5	-
MTMS-SiO2-ZnO	−0.57	1.68 × 10−5	40.64
MTMS-SiO2-ZnO-BTA	−0.56	4.32 × 10−6	84.73
MTMS-SiO2-ZnO-PDMS	−0.53	2.33 × 10−6	91.77
MTMS-SiO2-ZnO-HMTA	−0.51	2.91 × 10−6	89.72
MTMS-SiO2-ZnO-PDMS-HMTA-BTA	−0.50	2.23 × 10−6	92.12

. The corrosion potential of bare Q235 steel was −0.62 V, while the C5 superhydrophobic coating achieved a corrosion potential of −0.50 V. Superhydrophobic coatings including C2, C3, C4, and C5 demonstrated corrosion current densities an order of magnitude lower than bare Q235 steel. The corrosion inhibition efficiency, η, is calculated as follows:

$$\mathrm{\eta }=\left[\left({\mathrm{J}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^0-{\mathrm{J}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}\right)/{\mathrm{J}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^0\right]\times 100\mathrm{\%}$$

(3)

The corrosion potential of C5 is shifted in the positive direction (from −0.57 V to −0.50 V) compared to that of C1, indicating a better inhibition of the corrosion reaction and a decrease in the tendency to corrode. The higher corrosion current density of C1 indicates that the corrosion current density of C5 is significantly reduced to 2.23 × 10⁻⁶ A·cm⁻², indicating that it effectively hinders the generation of a corrosion current and has a significant inhibition effect. The corrosion inhibition efficiency of C1 is only 40.64%, which indicates that the inhibition effect of the base structure on corrosion is limited. The corrosion inhibition efficiency of C5 is 92.12%, which is a significant increase of more than 50 percentage points, indicating that the synergistic effect of HMTA and BTA significantly improves the corrosion inhibition performance.

The notable shifts in corrosion potential are linked to the rough, superhydrophobic micro- and nanostructures created by various materials. The enhanced corrosion resistance of various superhydrophobic coatings is closely tied to their unique surface structures. Air trapped within the nanostructured gaps significantly expands the air–liquid interface, effectively preventing the corrosive medium from reaching the substrate [28].

The effectiveness of compounded corrosion inhibitors depends on factors like inhibitor type and formulation ratio. The synergistic coefficient (S) is used to quantify the degree of synergism among compounded inhibitors. In this study, the synergy coefficient (S) for hexamethylenetetramine (HMTA, H) and benzotriazole (BTA, B) is calculated as follows [34]:

$$\mathrm{S}={\displaystyle \frac{1-{\mathrm{\eta }}_{\mathrm{H}}-{\mathrm{\eta }}_{\mathrm{B}}+{\mathrm{\eta }}_{\mathrm{H}\mathrm{B}}}{1-{\mathrm{\eta }}_{\mathrm{H}\mathrm{B}}}}$$

(4)

here, ${\eta }_H$ and ${\eta }_B$ represent the corrosion inhibition rates of HMTA and BTA individually, while ${\eta }_{HB}$ represents the inhibition rate when HMTA is compounded with BTA. If the synergistic coefficient (S) is ≤1, it suggests that the compounded corrosion inhibitor has an insignificant synergistic effect, or there may even be an antagonistic effect. When the synergistic coefficient (S) is >1, it indicates a significant synergistic effect of the compounded corrosion inhibitor; higher S values suggest more potent synergy. The synergistic coefficient of the superhydrophobic coating, formulated with 5 mL MTMS, 1 g SiO₂, 0.1 g ZnO, 0.25 g PDMS, 0.025 g HMTA, and 0.0125 g BTA, is calculated to be 2.24, indicating a notable synergistic effect between HMTA and BTA.

The synergistic corrosion inhibition mechanism of HMTA combined with BTA functions through multiple processes. First, BTA chemisorbs onto the Q235 steel surface through its π-bonds and lone electron pairs, forming an adsorptive film that effectively protects the Q235 steel surface from corrosive Cl⁻ ions, preventing oxidation. HMTA similarly adsorbs onto the Q235 steel surface through its nitrogen atoms, establishing a secondary protective layer that interacts with the BTA film, strengthening the coating’s barrier against Cl⁻ ions. Second, complexation and metal passivation take place. BTA molecules, rich in π-bonds and lone electron pairs, complex with Fe²⁺ ions to form stable coordination compounds. This complex inhibits electrochemical reactions on the metal surface, reducing metal dissolution and forming a passivation layer on Q235 steel, further preventing corrosion. While HMTA is less potent than BTA, its nitrogen atoms weakly complex with the Q235 steel surface, further enhancing passivation. BTA and HMTA each possess independent corrosion inhibition properties, enhancing overall inhibition efficiency through synergistic effects. BTA primarily provides chemical adsorption and complexation, forming a protective layer, while HMTA offers additional physical adsorption and Cl⁻ inhibition, enhancing the coating system’s resistance to Cl⁻ ions. Consequently, the C5 superhydrophobic coating, leveraging HMTA and BTA complexation, achieves a corrosion inhibition efficiency of 92.12%, the highest among the tested formulations. This coating’s inhibition efficiency is 51.48% greater than that of the C1 base formulation. Overall, the complex corrosion inhibition mechanism of BTA and HMTA in superhydrophobic coatings is accomplished through synergistic chemisorption, complexation, and physical barrier effects.

Electrochemical impedance spectroscopy (EIS) is a widely used and effective method for assessing the corrosion resistance of metallic materials [35]. The EIS test provides critical kinetic information, including charge transfer, double-layer capacitance, and the impedance of the coating or film. Figure 13 presents the Nyquist plots for various specimens in a 3.5 wt.% NaCl solution. The electrochemical impedance spectra’s low, medium, and high-frequency regions correspond to different properties of the sample–solution interface. The corrosion resistance of the coating is indicated by the low-frequency region in the Bode amplitude plot (Figure 14), the mid- and high-frequency regions in the Bode phase plot (Figure 15), and the capacitive arc diameter in the Nyquist plot. A larger capacitive arc diameter and a higher low-frequency impedance indicate more excellent corrosion resistance [36].

In the low-frequency region of the Bode amplitude–frequency plot, the modal value at 0.01 Hz for bare Q235 steel is 0.109 kΩ∙cm², while for the C5 superhydrophobic coating, it is 6.498 kΩ∙cm²—an order of magnitude higher than that of bare Q235 steel.

The equivalent circuit model, shown in Figure 16, was used to analyze further the corrosion behavior of each specimen in a 3.5 wt.% NaCl solution. The fitting results are presented in

Table 3. Equivalent circuit fitting data of EIS spectra for different specimens.

Sample	Cc/(F·cm−2)	Rc/(Ω·cm2)	Cdl/(F·cm−2)	Rct/(Ω·cm2)
Q235 Steel	-	-	1.637 × 10−1	191.6
Superhydrophobic Coating	2.85 × 10−5	365.7	3.622 × 10−5	7346

. For bare Q235 steel, R_s is the solution resistance, which indicates the resistance of the electrolyte solution (3.5 wt.% NaCl). R_ct is the charge transfer resistance, which describes the process of charge transfer between the metal surface and the corrosion solution, and directly reflects the ease of the corrosion reaction. C_dl is the bilayer capacitance, which indicates the charge distribution region formed on the metal surface. There is no protective coating on the surface of the bare Q235 steel. The metal is in direct contact with the electrolyte, resulting in a lower charge transfer resistance R_ct and indicating a more active corrosion reaction. For the C5 coating, R_s is the solution resistance, which is the same as that of bare Q235 steel. R_c is the coating resistance, which indicates the ability of the superhydrophobic coating to block ions or currents, with higher values indicating better densification and protection. C_c is the coating capacitance, which describes the coating’s ability to store electric current, reflecting the R_ct is the charge transfer resistance, which indicates the ease of corrosion reaction occurring in the contact area between the metal and the electrolyte under the coating. C_dl is the double-layer capacitance, which indicates the charge distribution area formed on the metal surface under the coating.

The presence of the coating effectively hinders the corrosion reaction by increasing the values of the coating resistance R_c and the charge transfer resistance R_ct. The increase in R_c indicates that the superhydrophobic coating forms a dense micro-nanostructure, which prevents the penetration of the electrolyte. The micro-nanostructure of the superhydrophobic coating introduced a thin layer of air, which effectively blocked the contact of the NaCl solution with the metal substrate. The air layer reduces the effective area for interfacial charge transfer, further increasing the R_ct. The air layer also causes the coating to exhibit a lower capacitance value of C_c, indicating that the coating has low permeability and good dielectric properties.

Analysis indicates that the micro-nano structure of the superhydrophobic surface can capture and retain a thin air layer, isolating most of the liquid from the coating’s surface. In the NaCl solution, water molecules and dissolved ions, including Cl⁻, cannot directly contact the solid coating surface. Cl⁻ must first cross the liquid–air interface to reach the pore structure; however, this interface presents a high energy barrier, effectively excluding Cl⁻ and making it difficult for these ions to enter the tiny pores, thus inhibiting Cl⁻ penetration. Cl⁻ ions, with a small ionic radius (0.181 nm), would typically diffuse easily into tiny pores. The superhydrophobic surface’s micro- and nanoscale pores, combined with the air layer, increase the diffusion potential energy of Cl⁻. Capillary action and surface tension further work to prevent the entry of Cl⁻ from the NaCl solution.

3.6. Ultraviolet Radiation Resistance

Figure 17 presents the UV–visible absorption spectra of the C5 superhydrophobic coatings, which display distinct absorption peaks in the UV–visible range of 200–370 nm.

Three samples were selected for testing, with three random test points chosen on each sample. The superhydrophobic coatings were irradiated under a UV lamp, and surface contact and sliding angles were measured after specific intervals. Figure 18 presents these values. As UV exposure time increased, the contact angle decreased while the sliding angle increased. After 120 h of irradiation, the contact angle decreased to 153.7°, and the sliding angle rose to 7.3°. Despite these changes, the coating retained strong superhydrophobicity after 120 h of UV exposure, demonstrating effective UV resistance.

Benzotriazole light stabilizers in superhydrophobic coatings exhibit strong photochemical properties, absorbing UV light in the 280–400 nm range. Upon absorption, they transition from the ground state to an excited state, releasing or dissipating the energy as heat [37]. Analysis indicates that benzotriazole undergoes electron excitation and proton transfer under UV irradiation, forming charged excited-state intermediates. This process involves electron redistribution within the benzotriazole molecular structure, with conjugated π–electron transitions and proton transfer occurring after UV photon absorption.

Zinc oxide in superhydrophobic coatings is a wide-bandgap semiconductor with a bandgap of approximately 3.37 eV. It absorbs high-energy UV photons via electron excitation, allowing it to absorb and scatter UV light (280–400 nm), thus preventing UV-induced damage to superhydrophobic surface materials. When zinc oxide absorbs UV light, photon energy excites electrons from the valence band to the conduction band, generating electrons (e⁻) in the conduction band and holes (h⁺) in the valence band. This electron–hole pair generation is the primary mechanism for UV absorption by zinc oxide. This photogenerated carrier composite process converts the energy of the UV light into thermal energy, reducing the impact of the UV light on the coating and substrate. Below is a simplified reaction equation representing this UV photon absorption by zinc oxide:

$$\mathrm{Z}\mathrm{n}\mathrm{O}+\mathrm{h}\mathrm{\nu }\to {\mathrm{e}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d}}^{-}+{\mathrm{h}}_{\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d}}^{+}$$

(5)

When ZnO is irradiated with UV light, the photocatalytic reaction produces reactive oxygen species. These reactive oxygen species are highly oxidizing and attack the PDMS chains, leading to their decomposition. The ZnO particles separate free electrons and holes. The holes on the ZnO particles are strongly oxidizing and can react with the chemical bonds on the surface of the PDMS to generate free radicals. These free radicals may attack the silicon–oxygen bond (Si-O) or methyl side groups (-CH₃) of PDMS, leading to its chemical degradation. Hydroxyl radicals or superoxide radicals attack the Si-O-Si backbone of PDMS, leading to the breakage of the backbone and the formation of silicon–oxygen groups and free radicals. Methyl groups(-CH₃) on PDMS are readily oxidized to methanol or formaldehyde. The reduction of hydrophobic methyl groups also results in a slight decrease in the superhydrophobicity of the coating. ZnO not only produces electrons and holes under UV irradiation, but may also be accompanied by thermal effects. Photogenerated electrons and holes may undergo complexation, i.e., the recombination of electrons and holes, inside or on the surface of ZnO particles, releasing energy. This compounding is usually non-radiative, and the energy is dissipated in the form of heat. The heat generated by the compounding process may propagate to the surroundings through lattice vibrations.

Additionally, the literature indicates that nano-silica absorbs UV light at 230 nm [38]. The size and morphology of SiO₂ particles determine their light-scattering properties. Nano-silica has a high surface energy and nanoscale structure, which can scatter UV light several times when uniformly distributed in the coating, effectively prolonging the UV light propagation path and reducing the energy penetration of UV radiation. Regarding the C1 coating in reference [27], the surface contact angle of the superhydrophobic coating was still as high as 153° after 120 h of UV illumination, and the sliding angle ranged from 3.0 to 5.3°. The contact angle of the C5 superhydrophobic coating decreased to 153.7°, and the sliding angle rose to 7.3°. Thus, the C5 superhydrophobic coating incorporates different kinds of UV absorbers, enhancing its UV radiation protection capabilities.

4. Conclusions

The superhydrophobic coatings developed in this study demonstrated superior wettability, minimal adhesion, dynamic hydrophobicity, and effective self-cleaning properties. The optimized MTMS-SiO₂-ZnO-PDMS-HMTA-BTA formulation successfully formed a micro-nanostructure capable of trapping air, thus reducing the water contact area with the coating surface. This structure produced high contact angles and low sliding angles, indicating superior water repellency and reduced adhesion. The coatings exhibited remarkable self-cleaning abilities, including in challenging liquids such as coffee and milk, and effectively repelled both small and large solid contaminants. Electrochemical tests demonstrated that the superhydrophobic coatings greatly improved corrosion resistance in a 3.5 wt.% NaCl solution. The corrosion potential shifted positively, and the corrosion current density showed a significant decrease compared to untreated Q235 steel. The MTMS-SiO₂-ZnO-PDMS-HMTA-BTA formulation achieved maximal corrosion inhibition efficiency, due to the synergistic effects of the PDMS transition layer and dual inhibitors, HMTA and BTA. In addition, the superhydrophobic coating offers substantial UV protection. Overall, the study demonstrated that the prepared superhydrophobic coatings are highly effective in anti-wetting and anti-corrosion applications, extending their potential for real-world use in harsh environments.