The Preparation and Properties of a Ni-SiO₂ Superamphiphobic Coating Obtained by Electrodeposition

Abstract

Superamphiphobic coatings have shown great potential in many fields such as with their anti-corrosion, high-temperature resistance, self-cleaning, and drag reduction properties. However, due to the poor stability of their coatings, it is difficult to apply them on a large scale. In this paper, two kinds of SiO₂ particles and nickel were co-deposited on the surface of steel to construct a micro/nano dual-scale structure by composite electrodeposition. The surface of the coating was then fluorinated with the low-surface-energy material 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (AC-FAS) to prepare a Ni-SiO₂ superamphiphobic coating. The coating has a water contact angle of 159° and an oil contact angle of 151°. The effect of nanoparticle concentration on the wettability and surface morphology of the coating was systematically studied. Comparative experiments revealed that the optimal micro/nanoparticle concentrations were 8 g/L of 20 nm SiO₂ and 2 g/L of 1 μm SiO₂. This preparation method greatly improves the corrosion resistance, wear resistance, chemical stability, and high-temperature resistance of the coating.

1. Introduction

Carbon steel is widely used in the oil and gas industry because of its excellent mechanical properties, abundant raw materials and straightforward smelting process. However, the surface of carbon steel is highly susceptible to wetting by droplets, leading to corrosion that significantly reduces the service life of storage and transportation equipment, such as pipelines. This corrosion results in substantial economic losses, resource wastage, and safety hazards for personnel [1]. Therefore, implementing effective anti-corrosion measures on carbon steel surfaces is crucial. Inspired by the self-cleaning properties of lotus leaves [2], researchers have demonstrated that superamphiphobic coatings can effectively prevent a metal surface from contacting corrosive media, thereby reducing the corrosion rate and greatly enhancing the metal’s corrosion resistance [3]. The application of superamphiphobic coatings on metal surfaces has thus become a key method for corrosion protection [4,5]. Additionally, superamphiphobic surfaces offer various functionalities, including drag reduction [6], anti-fogging [7], self-cleaning [8,9], self-healing [10,11,12], antibacterial properties [13], flame retardance [14,15,16], heat resistance [17,18], anti-electromagnetic interference [19], and directional liquid transport [20,21,22].

Superamphiphobic coatings can be obtained by composite electrodeposition [23], physical deposition [24], chemical vapor deposition [25], plasma etching [26], sol-gel processes [27], and template methods [28]. Wang et al. [29] used a combination of EDM and electrodeposition to prepare superamphiphobic surfaces on Al substrates with a water contact angle of 169.8 ± 3.0° and an oil contact angle of 154.5 ± 8.3°, but the coatings lost their superamphiphobic quality after 75 cm of abrasion with just 1000-grit sandpaper. Shen [23] et al. used a composite electrodeposition process to form Ni/nano-WO₃/CNT metal matrix composite (MMC) superamphiphobic coatings by introducing tungsten trioxide (WO₃) nanoparticles and carbon nanotubes (CNTs) into the deposited nickel metal, and the coatings lost their superamphiphobic after abrasion on a 600-grit sandpaper. Therefore, the preparation of stable superamphiphobic coatings by a simple and low-cost method is still a great challenge.

To date, superamphiphobic coatings have not seen widespread industrial application. Beyond the constraint of high production costs, a more critical issue is the generally poor stability of most superamphiphobic coatings. This instability arises from two primary factors. First, the micro/nanostructure of the superamphiphobic surface is relatively fragile; any external physical damage to this structure can directly impair the coating’s superamphiphobic properties. Second, exposure to corrosive substances can alter the chemical properties and surface structure of the coating, resulting in diminished corrosion resistance and durability. Several studies have shown that the co-deposition of nanomaterials into coatings by composite electrodeposition technique can not only refine the grain of the coating, but also effectively enhance the mechanical properties of the coating [30,31]. For example, the friction coefficient of the coating with the addition of La₂O₃ was reduced by 46.57% compared with that of the pure Ni-W coating [32]. With the addition of α-ZrP, the hardness of the Ni-B coating was as high as 1000.92 HV [33]. With the development of silica nanoparticles, researchers found that the microstructure, hardness, wettability and corrosion resistance of nickel-based composite coatings doped with silica nanoparticles were improved [34,35]. However, they only used single-particle-size silica particles and did not study the effect of different particle sizes of silica particles on the coating.

In this paper, silica micro/nanoparticles of varying sizes and nickel were co-deposited onto the surface of carbon steel using composite electrodeposition. The advantages of the high hardness, high strength, high abrasion resistance, corrosion resistance, oxidation resistance, and chemical stability of silicon dioxide itself and nickel metal together are utilized to construct a micro/nano dual-scale structure. The resulting Ni-SiO₂ superamphiphobic coating significantly reduces the corrosion rate of the steel. Additionally, the robust micro/nanostructure on the coating’s surface ensures that its superamphiphobic properties are retained even after wear, thereby extending the coating’s lifespan.

2. Materials and Methods

2.1. Solvent Configuration and Preparation of Superamphiphobic Coating

The plating solution used in the composite electrodeposition process is the Watt plating solution, with its composition detailed in

Table 1. Composition and content of bath.

Reagent Name	Chemical Formula	Concentration
Nickel sulfate	NiSO4·6H2O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)	240 g/L
Nickel chloride	NiCl2·6H2O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)	40 g/L
Boric acid	H3BO3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)	35 g/L
Sodium lauryl sulfate	C12H25SO4Na (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)	0.15 g/L
20 nm SiO2	SiO2 (Lavasi (Beijing) New Material Technology Co., Ltd., Beijing, China)	0, 2, 4, 6, 8 g/L
1 μm SiO2	SiO2 (Lavasi (Beijing) New Material Technology Co., Ltd., Beijing, China)	0, 2, 4, 6, 8 g/L

. The main salt of the plating solution is NiSO₄, which supplies Ni²⁺ ions for the electrodeposition process. Additionally, NiCl₂ is included to prevent passivation of the anode during electrodeposition. H₃BO₃ was added to maintain the pH value stability of the plating solution. Sodium dodecyl sulfate (SDS, Macklin, Shanghai, China) was used as an anionic surfactant to prevent the agglomeration of silica particles in the solution. The combinations of different SiO₂ micro/nanoparticles are shown in

Table 2. Composition of different SiO₂ micro/nanoparticles.

Test	20 nm SiO2 Concentration (g/L)	1 μm SiO2 Concentration (g/L)
1	8	0
2	6	2
3	4	4
4	2	6
5	0	8

. By varying the concentration of SiO₂ micro/nanoparticles of different sizes, various coatings were produced.

ASTM A106B steel with a size of 30 mm × 20 mm × 2 mm was chosen as the substrate. Wires were welded to the back of the test piece, which was then encapsulated with epoxy resin. The surface to be plated was sequentially polished using 180 # to 2000 # sandpaper.

The test piece was subsequently placed in an alkali washing solution at 65 °C for 25 min. The alkali washing solution comprised 30 g/L sodium hydroxide, 40 g/L sodium carbonate, 25 g/L sodium silicate, and 20 g/L sodium phosphate. Finally, the sample surface was acid-washed and activated with a 10% dilute hydrochloric acid solution, then dried for later use.

The composite electrodeposition experiment was conducted using a two-electrode system, as illustrated in Figure 1. A nickel plate (60 mm × 40 mm × 5 mm) served as the anode, while ASTM A106B steel (30 mm × 20 mm × 2 mm) functioned as the cathode, with a spacing of 4 cm between them.

The experimental parameters for electrodeposition are detailed in

Table 3. Electrodeposition process parameters.

Experimental Parameters	Numerical Setting
Temperature (°C)	50
pH	4.6
Mixing speed (r/min)	150
Electrodeposited current density (A/dm2)	10
Electrodeposition time (min)	20

. The concentration of SiO₂ particles of different sizes was varied to produce distinct coatings. To ensure the homogeneous dispersion of the particles, the plating solution was subjected to ultrasonic treatment for 25 min prior to electrodeposition. After electrodeposition, anhydrous ethanol as a solvent, AC-FAS (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) as a solute, and a trace of hydrochloric acid were used to obtain the fluorosilane solution. The coated test piece was placed in an 80 °C fluorosilane solution for heat treatment for 3 h. The surface energy of the coating is greatly reduced after AC-FAS modification, which provides the coating with super-double hydrophobicity. Finally, the test piece was placed in a drying oven at a constant temperature of 115 °C for 1 h to enhance the bonding between the surface modifier and the coating.

2.2. Characterization

The water contact angle (WCA) and sliding angle (SA) of the coating surface were measured using an automatic contact angle measuring instrument (SDC-350, SINDIN, Guangzhou, China) to characterize the wettability of the coating. The surface morphology of the coating was analyzed by scanning electron microscopy (JSM-7200F, JEOL, Tokyo, Japan). The 3D morphology of the Ni-SiO₂ superamphiphobic coating surface was obtained through 3D confocal morphology testing (Axio Imager A2m, Zeiss, Oberkochen, German). The elemental composition and content of the coating were determined using EDS (JSM-7200F, JEOL, Tokyo, Japan).

2.3. Coating Chemical Stability Test

The Ni-SiO₂ superamphiphobic coating was immersed in 3.5 wt.% NaCl solution, and the water and oil contact angles of the sample surface were measured at specific intervals to evaluate the amphiphobic stability of the coating in a corrosive environment.

2.4. Coating Corrosion Resistance Test

In this experiment, a PARSTAT 2273 (AMETEK, Newark, DE, USA) electrochemical workstation was used for electrochemical impedance spectroscopy (EIS) and polarization curve tests. A standard three-electrode system was constructed, with the bare steel substrate, unmodified pure nickel coating, and Ni-SiO₂ superamphiphobic coating serving as the working electrodes. A saturated potassium chloride electrode (SCE) was used as the reference electrode, and a platinum electrode as the auxiliary electrode. Electrochemical tests were conducted in a 3.5 wt.% NaCl aqueous solution as the corrosive medium.

The polarization curve test range was set to ±250 mV relative to the open circuit potential, with a scanning rate of 0.3 mV·s⁻¹. The Tafel curve was extrapolated and fitted using Power Suite software (version number 2.5.8.0, AMETEK, Newark, DE, USA). For the electrochemical impedance spectroscopy test, the voltage amplitude was 10 mV, and the frequency ranged from 10 mHz to 100 kHz. The electrochemical behavior of the Ni-SiO₂ superamphiphobic coating was compared and analyzed by continuously monitoring the AC impedance spectrum of the coating in the 3.5 wt.% NaCl solution. The corrosion test cycles were conducted at intervals of 1 h, 24 h, 2 days, 4 days, 8 days, and 16 days. ZView software (version number 3.1, AMETEK, Newark, DE, USA) was used to construct the equivalent circuit model and fit the impedance spectrum data.

2.5. Wear Resistance Test of the Coating

Firstly, the hardness of steel, the pure nickel coating, and the Ni-SiO₂ superamphiphobic coating were tested. The hardness of the sample was tested using an FM-800 (D) micro W Vickers hardness tester (FTFuture-Tech, Tokyo, Japan). The pressure loading time was 15 s, and the load was 500 g. Five different points on the surface of each sample were tested for hardness, and the average value was recorded as the final hardness value.

Subsequently, a sandpaper friction experiment was conducted. As shown in Figure 2, a 100 g weight was placed on the coating to apply a constant pressure. The coating surface was placed face down on 400-mesh SiC sandpaper and dragged at a constant speed of 1.4 cm/s. The wear resistance of the coating was characterized by measuring the relationship between the water–oil contact angle and the friction distance on the coating surface.

2.6. High-Temperature Resistance Test of the Coating

The high-temperature resistance of Ni-SiO₂ superhydrophobic coatings was tested by conducting high-temperature experiments. The coatings were heated in incubators at 100, 150, 200, 250, and 300 °C for 1 h each. After the coatings cooled to room temperature, the surface was rinsed with deionized water followed by anhydrous ethanol. The contact angle of the coating surface was then measured to evaluate its high-temperature resistance.

3. Results and Discussion

3.1. Effect of SiO₂ Particle Concentration of Different Particle Sizes in the Plating Solution on the Coating

The water contact angle and oil contact angle of the composite coating at various concentrations of micro/nano silica particles were measured, and the results are shown in Figure 3 and Figure 4. At a concentration of 8 g/L of 20 nm SiO₂ particles in the plating solution, without 1 μm SiO₂ micron particles, the static contact angles of water and oil are 136° and 129°, respectively. The dynamic rolling angles for water and oil are 19° and 22°.

When the concentration of 20 nm SiO₂ nanoparticles was adjusted to 6 g/L, and 1 μm SiO₂ micron particles were added at 2 g/L, the static contact angles of water and oil increased to 159° and 151°, respectively, and the rolling angles approached 0°, demonstrating excellent superamphiphobic characteristics.

Conversely, reducing the concentration of 20 nm SiO₂ nanoparticles to 2 g/L while increasing the concentration of 1 μm SiO₂ micron particles to 6 g/L resulted in decreased amphiphobic performance. Further, at 8 g/L of 1 μm SiO₂ micron particles without 20 nm SiO₂ nanoparticles, the static contact angles of water and oil decreased to 141° and 135°, respectively. The dynamic rolling angles increased to 15° and 17°, indicating a loss of superamphiphobicity.

These findings highlight the critical role of particle size and concentration in achieving and maintaining superamphiphobic properties in the coating.

Figure 5 illustrates the microstructure of coatings formed by different concentrations of silica particles. In Figure 5a, when the concentration of 20 nm SiO₂ nanoparticles is 8 g/L without 1 μm SiO₂ micron particles, the surface morphology shows a large number of uniformly distributed nano-sized SiO₂ particles. However, the absence of micron-sized particles results in an overall smooth coating surface that fails to form a micro/nano dual-scale structure, leading to only general amphiphobic performance.

In Figure 5b, at a nanoparticle concentration of 6 g/L and a micron particle concentration of 2 g/L, the surface morphology reveals deep ravines and peaks formed between the micron particles. Figure 6 presents a partially enlarged view of Figure 5b, highlighting how silica particles of both sizes serve as adsorption sites for the crystallization and growth of nickel metal. On the surface of these particles, nickel ions form smaller-scale spike-like structures, and numerous densely arranged nanoparticles populate the ravines between the micron protrusions, creating a micro/nano dual-scale structure that enhances the coating’s amphiphobic ability. At this point, the WCA and OCA are at their maximum. In Figure 5c, with a further reduction in the concentration of 20 nm SiO₂ nanoparticles, the surface morphology shows a decrease in nano-scale protrusions, leading to slightly reduced surface roughness and decreased amphiphobic properties.

In Figure 5d, at a micron particle concentration of 6 g/L, the surface morphology indicates fewer nano-level protrusions and more micron-level particles, resulting in a smoother surface and a decline in amphiphobic performance. Finally, Figure 5e depicts the surface morphology when the concentration of micron particles is 8 g/L without any nanoparticles. The coating surface features large clusters and irregularly agglomerated nano-ions. The number of protrusions formed by nano-silica is significantly reduced, causing a slight reduction in superhydrophobicity, with the static contact angles of water and oil dropping to 141° and 135°, respectively.

The superamphiphobicity of the coatings is obtained only when the concentration of 20 nm sized SiO₂ in the plating solution is 6 g/L and the concentration of 1 µm sized SiO₂ is 2 g/L. These observations highlight the importance of balancing the concentration of nano- and micron-sized particles to achieve optimal micro/nano dual-scale structures, which are crucial for superior amphiphobic performance in the coatings. SiO₂ particles are the key factor in achieving the superhydrophobic/superoleophobic properties.

The superamphiphobic coatings obtained at the optimal micro/nanoparticle concentrations (6 g/L for 20 nm SiO₂ and 2 g/L for 1 μm SiO₂) will be used in subsequent experiments. To determine the elemental composition and content of the Ni-SiO₂ superamphiphobic coating, an EDS test was conducted. The results, shown in Figure 7, reveal that the coating comprises five elements: Ni, F, Si, C, and O. Nickel is the most abundant element at 41.03%. Silicon accounts for 6.81%, and oxygen is 9.45%, indicating the successful deposition of silica particles on the coating surface. The atomic percentages of fluorine and carbon are 23.19% and 19.52%, respectively, demonstrating that AC-FAS effectively modified the composite coating surface. Figure 8 presents the distribution map of each element on the Ni-SiO₂ coating surface. The uniform distribution of various elements suggests that both the metal ions in the plating solution and the deposition process of SiO₂ on the electrode surface during electrodeposition are relatively uniform. This uniformity enhances the coating’s performance and quality. Figure 9 shows the 3D morphology of the Ni- SiO₂ superamphiphobic coating surface. The coating surface features protrusions of varying scales, representing the micro/nano dual-scale structure formed by embedded silica particles and nickel. The average surface roughness (Ra) of the coating is 2.23 μm. Figure 10 displays the cross-sectional scanning electron microscope image of the Ni-SiO₂ superamphiphobic coating. The thickness of the coating is 14.2 μm, with no noticeable pinholes, cracks, or other defects, indicating strong adhesion between the coating and the substrate.

3.2. Chemical Stability of the Coating

The change in the contact angle of the Ni-SiO₂ superamphiphobic coating in 3.5 wt.% NaCl solution over time is shown in Figure 11. After 1 h of immersion, the water and oil contact angles were 156° and 146°, respectively. When the immersion time was extended to 4 h, the water and oil contact angles decreased to 147° and 138°, respectively. The reduction in amphiphobicity can be attributed to the initial Cassie mode [36] contact between the coating and the corrosive solution. As immersion time increases, the solution gradually contacts the coating surface completely, transitioning to the Wenzel mode [37]. This shift, along with the adsorption of corrosive ions on the coating surface, disrupts the micro/nanostructure and increases surface energy, reducing the coating’s amphiphobicity [38]. Additionally, the corrosion damaged the micro/nanostructures, further altering the contact mode and decreasing hydrophobicity. After 72 h of immersion, the water and oil contact angles of the coating further decreased to 124° and 114°, respectively. Despite this reduction, the coating maintained hydrophobic and oleophobic properties, demonstrating strong chemical stability in the 3.5 wt.% NaCl solution.

3.3. Corrosion Resistance of the Coating

3.3.1. Polarization Curve Analysis

Figure 12 presents the potentiodynamic polarization curves of steel, the pure nickel coating, and the Ni-SiO₂ superamphiphobic coating immersed in 3.5 wt.% NaCl solution for 30 min. The corrosion potential (E_corr) and corrosion current density (i_corr) of each sample were summarized in

Table 4. Electrochemical parameters after fitting the polarization curves of different samples.

Sample Name	Ecorr (mVSCE)	icorr (A·cm−2)	βa (mV·dec−1)	βc (mV·dec−1)	η
Steel	−513.29	7.62 × 10−5	83.6	1011.3	--
Pure nickel coating	−466.31	3.06 × 10−5	206.9	132.6	59.8%
Ni-SiO2 superamphiphobic coating	−360.97	8.32 × 10−7	277.4	103.3	98.9%

. Then, the corrosion efficiency of the pure nickel coating and superamphiphobic coating were calculated according to Equation (1). In the formula, i_corr,sub and i_corr,coated represent the corrosion current density of steel and coating, respectively.

$$\eta ={\displaystyle \frac{i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r},\mathrm{s}\mathrm{u}\mathrm{b}}-i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r},\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}}}{i_{\mathrm{corr},\mathrm{s}\mathrm{u}\mathrm{b}}}}\times 100\%$$

(1)

The E_corr of the nickel-plated sample is significantly positive, and the i_corr is slightly reduced, indicating improved corrosion resistance. The inhibition efficiency η is only about 59.8%. In contrast, the Ni-SiO₂ superamphiphobic coating exhibits the highest E_corr and a corrosion current density that is two orders of magnitude lower than that of both the steel and pure nickel coating. The inhibition efficiency of the Ni-SiO₂ superamphiphobic coating is exceptionally high at 98.9%, demonstrating its outstanding anti-corrosion performance. This significant improvement underscores the effectiveness of the superamphiphobic coating in protecting against corrosion in harsh environments.

3.3.2. EIS Analysis

The Nyquist plots of different samples after soaking in a corrosion medium for 0.5 h are shown in Figure 13. Clearly, the capacitive arc radius of the sample with the superhydrophobic coating was significantly larger than that of the bare carbon steel substrate and the nickel coating, a symbol of the excellent anti-corrosion effect.

Figure 14 and Figure 15 show the EIS of steel samples in 3.5 wt.% NaCl solution. During the electrochemical corrosion process, the anode undergoes an oxidation reaction, and the cathode undergoes an oxygen absorption reaction. After 16 days of immersion, significant yellow and black corrosion products were observed adhering to the surface of the steel.

To better understand the corrosion mechanism of steel in 3.5 wt.% NaCl solution, an equivalent circuit is constructed, as shown in Figure 16. Here, R_s represents solution resistance, R_f is the resistance of the corrosion product film on the steel surface, R_ct denotes charge transfer resistance, and C_f and C_dl denote the capacitance of the corrosion product film and the double layer capacitance at the interface of steel and the solution, respectively. The fitting values of the AC impedance spectra of each circuit element are shown in

Table 5. The parameters of the equivalent circuit elements of steel with time.

Time	Rf (Ω·cm2)	Cf (F·cm−2)	Rct (Ω·cm2)	Cdl		χ2
				Y0 (Ω−1·cm−2·sn)	n
1 h	8.312	3.224 × 10−4	900.1	1.371 × 10−3	0.688	3.77 × 10−4
24 h	10.561	3.301 × 10−4	877.5	4.009 × 10−3	0.720	3.53 × 10−3
2 days	5.688	3.325 × 10−4	730.5	2.932 × 10−3	0.733	1.12 × 10−4
4 days	6.097	2.432 × 10−4	573.9	2.188 × 10−3	0.701	1.45 × 10−3
8 days	5.933	1.401 × 10−4	772.4	4.177 × 10−3	0.519	3.87 × 10−4
16 days	12.032	2.256 × 10−4	583.1	4.202 × 10−3	0.628	3.51 × 10−4

. The charge transfer resistance of steel shows a decreasing trend with immersion time.

Figure 17 and Figure 18 are the AC impedance spectra of the pure nickel coating with time. The impedance spectrum of pure nickel coating was fitted using the equivalent circuit shown in Figure 19b, and the fitted values of the circuit elements are listed in

Table 6. The parameters of the equivalent circuit elements of freshly prepared pure Ni coating with time.

Time	Rf (Ω·cm2)	Cf (F·cm−2)	Rct (Ω·cm2)	Cdl		χ2
				Y0 (Ω−1·cm−2·sn)	n
1 h	25.61	2.451 × 10−5	12,911	4.323 × 10−5	0.736	3.31 × 10−3
24 h	36.29	7.026 × 10−5	9017	8.145 × 10−5	0.697	5.15 × 10−3
2 days	44.51	7.309 × 10−5	8216	1.378 × 10−4	0.745	4.01 × 10−4
4 days	33.97	8.991 × 10−5	8133	2.891 × 10−4	0.733	3.23 × 10−4
8 days	29.06	8.635 × 10−5	7514	1.941 × 10−4	0.789	7.92 × 10−4
16 days	30.12	7.157 × 10−5	7056	1.033 × 10−4	0.751	8.73 × 10−4

. The overall trend shows a decrease in charge transfer resistance as the corrosive medium gradually alters the coating’s surface structure, indicating a gradual decline in corrosion resistance.

Figure 20 and Figure 21 show the Nyquist diagram and Bode diagram of the AC impedance spectrum of Ni-SiO₂ superamphiphobic coating in 3.5 wt.% NaCl solution for different times. The Nyquist plot initially exhibits a significant capacitive arc resistance, attributed to the presence of an air interlayer on the coating’s surface during the early stages of soaking. By the 2nd day of immersion, this air interlayer dissipates, as indicated by a decrease in the phase angle peak to a single, narrower peak corresponding to a single time constant. By the 16th day, no new phase angle peak was observed, suggesting that the corrosive medium had not penetrated the interface between the coating and the metal substrate [39].

To elucidate the electrochemical behavior of Ni-SiO₂, the corrosion process was divided into initial and late immersion stages, each described separately by the equivalent circuit shown in Figure 19.

Table 7. The parameters of the equivalent circuit elements of Ni-SiO₂ superamphiphobic coating with time.

Time	Rair Ω·cm2	Cair F·cm−2	Rf Ω·cm2	Cf F·cm−2	Rct Ω·cm2	Cdl		χ2
						Y0 (Ω−1·cm−2·sn)	n
1 h	3.270 × 105	3.021 × 10−5	29.19	2.406 × 10−6	6.826 × 105	2.537 × 10−5	0.703	4.91 × 10−3
1 day	1.532 × 105	4.488 × 10−5	28.17	4.153 × 10−6	2.121 × 105	1.505 × 10−5	0.739	3.22 × 10−4
2 days	1.231 × 105	4.617 × 10−5	33.51	5.961 × 10−6	1.307 × 105	3.022 × 10−5	0.621	6.81 × 10−4
4 days	--	--	38.12	1.158 × 10−5	12,103	4.165 × 10−5	0.608	4.51 × 10−4
8 days	--	--	36.38	1.235 × 10−5	9871	2.315 × 10−5	0.599	6.44 × 10−4
16 days	--	--	35.61	1.375 × 10−5	8962	8.188 × 10−5	0.608	8.09 × 10−4

presents the AC impedance spectrum fitting values for each circuit element. Notably, the initial air interlayer resistance peaked at approximately 10⁵ ohms·cm² after 1 h, suggesting significant corrosion inhibition during the initial period. However, as time increased, R_air decreased progressively, indicating air interlayer dissipation. Throughout the test period, the coating’s resistance remained significantly lower than the charge transfer resistance, highlighting the latter’s critical role in the corrosion process within the electric double layer.

This underscores the superior corrosion resistance of the Ni-SiO₂ superamphiphobic coating, attributed to its micro/nano-structured surface creating air-filled grooves that hinder wetting and penetration by corrosive liquids [40].

To quantitatively assess corrosion resistance over time, the polarization resistance (R_p) of steel, pure nickel coating, and Ni-SiO₂ superamphiphobic coating was calculated. Equation (2) calculates the polarization resistance of Ni-SiO₂ superamphiphobic coating, while Equation (3) calculates the polarization resistance of steel and pure nickel coating:

$$R_{\mathrm{p}}=R_{air}+R_{\mathrm{f}}+R_{ct}$$

(2)

$$R_p=R_{\mathrm{f}}+R_{ct}$$

(3)

Table 8. The polarization resistance of steel, pure Ni coating, and Ni-SiO₂ superamphiphobic coating (Ω·cm²).

Time	Steel	Pure Nickel Coating	Ni-SiO2 Superamphiphobic Coating
1 h	908.41	1.291 × 104	1.100 × 107
24 h	888.06	9.053 × 103	3.653 × 105
2 days	736.19	8.216 × 103	2.238 × 105
4 days	580.00	8.133 × 103	1.214 × 104
8 days	778.33	7.514 × 103	9.907 × 103
16 days	595.13	7.056 × 103	8.997 × 103

presents the polarization resistance values for each sample at different times. Notably, the polarization resistance of Ni-SiO₂ superamphiphobic coating remained high at 1.100 × 10⁷ Ω·cm², indicating effective corrosion protection by the air interlayer on the coating surface. Even after 16 days, the polarization resistance remained at 8.398 × 10³ Ω·cm², albeit showing a decreasing trend due to the formation of Ni(OH)₂ during the corrosion process. This compound benefits from the micro/nano-bumps on the coating surface, providing numerous active sites for its growth and the formation of a passivation layer that further inhibits corrosion [41].

In conclusion, the Ni-SiO₂ superamphiphobic coating exhibits exceptional corrosion resistance compared to steel and pure nickel coating. Its unique micro/nanostructure and air interlayer effectively prevent the penetration of corrosive media, providing long-term protection against corrosion.

3.4. The Wear Resistance of the Coating

Figure 22 shows the Vickers hardness values of steel, pure nickel coating, and Ni-SiO₂ superamphiphobic coating. The Ni-SiO₂ coating exhibits the highest microhardness, approximately 1.2 times that of pure nickel coating and 1.96 times that of steel. This significant increase in microhardness is attributed to the inherent hardness of silica particles. The co-deposition of these particles with nickel enhances the overall hardness of the coating.

Figure 23, Figure 24 and Figure 25 depict the water–oil contact angle changes with friction distance, wear resistance, and scanning electron microscope (SEM) images after wear testing. The water–oil contact angle of the Ni-SiO₂ superamphiphobic coating shows minimal change even after 400 cm of wear, maintaining its superamphiphobic properties. In contrast, the AC-FAS-modified pure nickel superhydrophobic coating loses its superhydrophobicity after 40 cm of wear, with the water static contact angle dropping sharply to 143°. SEM images reveal severe surface damage and wear marks on the pure nickel coating, whereas the Ni-SiO₂ superamphiphobic coating shows minimal wear due to its high hardness and strong adhesion to the substrate. Although the tops of the micron-scale structures are abraded, the nanoscale structures and surface modifiers remain intact, resulting in significantly less abrasion of the superamphiphobic coating. The micro/nano dual-scale structure of the Ni-SiO₂ coating remains preserved, maintaining its amphiphobic performance even after extended wear.

3.5. The High-Temperature Resistance of the Coating

Figure 26 demonstrates the static contact angles of water and oil on Ni-SiO₂ superamphiphobic coatings at different temperatures. Up to 250 °C, the contact angles remain stable at 153° for water and 147° for oil, indicating robust superamphiphobic behavior where droplets maintain spherical shapes on the coating surface. However, at 300 °C, the coating transitions abruptly from a superamphiphobic state to a hydrophilic and lipophilic state, with water and oil droplets spreading fully on the surface. Figure 27’s SEM image confirms that high-temperature treatment at 300 °C does not compromise the surface morphology or integrity of the coating.

Figure 28 presents the results of energy-dispersive X-ray spectroscopy (EDS). Comparison with Figure 8 reveals that the percentage of fluorine (F) atoms on the coating surface undergoes a significant decrease after the high-temperature treatment at 300 °C. This reduction indicates the loss of the low-surface-energy fluorocarbon material (AC-FAS) from the coating surface. This is due to the boiling point of AC-FAS of about 300 °C; when the temperature was heated to 300 °C, AC-FAS began to evaporate; after losing the modification of AC-FAS, the surface energy of the coating rose sharply, leading to the conversion from superamphiphobic to hydrophilic and lipophilic properties.

In conclusion, the Ni-SiO₂ superamphiphobic coating exhibits exceptional high-temperature stability, maintaining strong amphiphobic properties up to 250 °C. The coating’s high hardness and robust micro/nanostructure contribute to its superior wear resistance and durability under harsh environmental conditions.

4. Conclusions

A Ni-SiO₂ superamphiphobic coating was prepared using composite electrodeposition. The optimal process was determined by adjusting the concentration of SiO₂ particles in the plating solution to 6 g/L for 20 nm silica particles and 2 g/L for 1 μm silica particles. Electrodeposition was performed at a current density of 10 A/dm² for 20 min, followed by heating in a fluorosilane solution at 80 °C for 3 h and curing in a drying oven at 115 °C for 1 h. The resulting coating exhibited excellent superamphiphobic properties, with contact angles for water and oil of 159° and 151°, respectively, and a rolling angle close to 0°.

The coating also demonstrated good chemical stability and corrosion resistance. Its short-term corrosion resistance in 3.5 wt.% NaCl solution was remarkable, with a corrosion current density two orders of magnitude lower than that of steel and pure nickel coating, and a corrosion efficiency of 98.9%. After 16 days of immersion in 3.5 wt.% NaCl solution, the polarization resistance of the coating remained 15.12 times that of steel and 1.27 times that of pure nickel coating, indicating that the corrosive medium did not penetrate the interface between the coating and the substrate.

Additionally, the coating exhibited a hardness of up to 331.3 HV. The coating shows good wear resistance and high-temperature resistance under the wear resistance test and high-temperature test.