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Article

Pulse Electrodeposited Super-Hydrophobic Ni-Co/WS2 Nanocomposite Coatings with Enhanced Corrosion-Resistance

by
Meijiao Wang
1,†,
Zixiao Xue
2,3,†,
Shaojiu Yan
1,4,
Jin He
3,
Qiuyue Shao
5,
Wen Ge
5,* and
Baodi Lu
5
1
Key Laboratory of Geological Survey and Evaluation of Ministry of Education, China University of Geosciences, Wuhan 430078, China
2
Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
3
Hangzhou Institute for Advanced Study, UCAS, Hangzhou 310024, China
4
Research Center of Graphene Applications, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
5
Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430078, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2022, 12(12), 1897; https://doi.org/10.3390/coatings12121897
Submission received: 30 October 2022 / Revised: 27 November 2022 / Accepted: 2 December 2022 / Published: 5 December 2022
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Abstract

:
The hydrophobicity and corrosion resistance of composite coatings can be effectively improved by changing the electrodeposition method and adding inorganic nanoparticles. In this work, the incorporation of WS2 nanoparticles significantly increased the surface roughness of Ni-Co coatings. The best hydrophobicity and corrosion resistance of the Ni-Co/WS2 nanocomposite coatings (water contact angle of 144.7°) were obtained in the direct current electrodeposition mode when the current density was 3 A/dm2 and the electrodeposition time was 50 min. Compared with direct current electrodeposition, the pulsed current electrodeposition method was more conducive to improving the electrodeposition performance of the nanocomposite coatings. Under the conditions of a current density of 3 A/dm2, pulse duty cycle of 70%, and pulse frequency of 1000 Hz, the nanocomposite coatings reached a superhydrophobic state (water contact angle of 153.8°). The nanocomposite coatings had a slower corrosion rate and larger impedance modulus in this state, and thus the corrosion resistance was superior. The wetting state of the Ni-Co/WS2 nanocomposite coating surface was closer to the Cassie–Baxter model. The protective air layer formed by the layered rough microstructures significantly reduced the actual contact area between the liquid and the substrate, achieving excellent hydrophobic and corrosion resistance properties.

1. Introduction

Fabricating superhydrophobic coatings is an effective way to improve the corrosion resistance of metal substrates, as the superhydrophobic coatings can effectively reduce the contact area between the metal substrate and the corrosive liquid [1,2]. One of the most well-known metallic coatings is nickel (Ni) coating, which can protect the surface of engineering components against environmental attacks such as corrosion and wear [3,4]. The addition of alloying elements such as Cu, Co, W, Ti, and Mo to Ni coatings, to form Ni matrix composite coatings, can further enhance their performance [5,6,7,8,9,10]. Among these, Ni-Co coating is attracting attention as one of the most promising alternatives for the replacement of hard-chromium coatings [11].
It is now well-established that creating micro- and nano-scale rough structures can make a metal surface superhydrophobic [12]. Many superhydrophobic surfaces (both artificial and natural) typically exhibit micro- or nanosized roughness, as well as hierarchical structures, which somehow can influence the surface’s water repellence [13]. In recent years, the number of works on enhancing composite coatings with second phase particles, to increase these properties to a better level, has been increasing rapidly [14,15,16,17,18,19,20]. With the rapid development of nanotechnology, various nanoparticles such as Fe2O3, Al2O3, TiO2, SiO2, ZrO2, WC, WS2, and MoS2 have been introduced into coatings [21,22,23,24,25]. Nanoparticle-embedded composite coatings impart improved properties or new functionalities to metallic coatings such as nickel and cobalt [26,27,28]. For example, introducing WS2 and WC into Ni coatings can create fine microstructures on the surface, leading to an increased surface roughness. The rough surface makes the Ni/WC-WS2 composite coatings more hydrophobic [29], conforming to the principle of the Wenzel approach [30].
Various methods, such as the sol-gel method [31,32], plasma electrolytic oxidation method [33], sputtering [34], physical vapor deposition [35], chemical vapor deposition [36], and electrochemical deposition [37,38], are currently used to prepare metal-based composite coatings. Among these methods, electrochemical deposition is considered a powerful tool for fabricating superhydrophobic composite coatings on a metal substrate, due to its simple equipment requirements, environmental friendliness, low cost, and high efficiency. Recently, superhydrophobic WS2 composite surfaces, including Ni/WS2 [39], Ni-P/WS2 [40], and Ni-W-P/WS2 [41], have been obtained through traditional direct current (DC) electrodeposition. DC preparation is simple and does not require expensive equipment or specialized reaction environments. However, the tunable parameters in DC are highly limited. This limits the diversity of microstructure morphologies that can be created. Additionally, the continuous discharging in DC often causes ion depletion at the electrode, which affects the coating quality.
In contrast, pulse current (PC) electrodeposition is a facile technique for fabricating metallic coatings [42,43]. Its periodical discharge reduces the occurrence of ion depletion. PC electrodeposition allows precise control of the surface micro-structure morphology through parameter tuning. These unique parameters provide the opportunity to perturb the adsorption–desorption phenomenon at the cathode/electrolyte interface, which has a considerable impact on the nucleation rate and the growth rate of crystals during electro-crystallization, and ultimately decides the size and shape of crystals [44]. The controllable surface microstructures facilitate the composite coatings reaching a superhydrophobic state, which can protect metal materials from corrosion in marine or other industrial environments. Therefore, PC is a superior alternative to DC electrodeposition for fabricating superhydrophobic composite coatings with enhanced corrosion-resistance. Nemes et al. [45] studied the influence of the current regime on the electrodeposition of Zn–CeO2 composite coatings using both direct and pulse current electrodeposition techniques and revealed that a controlled pulsed-current regime was preferable to a direct-current approach for the electro-codeposition process, offering more compact deposits, with better anti-corrosion properties. They noticed the corrosion current density values corresponding to samples prepared in the PC regime were approximately 2–4 times smaller than those with direct current. Ganji et al. [46] applied a nickel-iron-titanium carbide (Ni-Fe-TiC) nanocomposite to St 14 low-carbon steel and obtained an optimum coating via pulse electrodeposition, which increased the corrosion potential from −0.675 V to −0.332 V and decreased the corrosion current density from 157.200 μA/cm2 to 0.790 μA/cm2. Xia et al. [47] fabricated Ni-TiN/SiC nano-coatings using pulse current electrodeposition, and Ni-TiN/SiC deposited at 4 A/dm2 had the smallest corrosion current density, equal to 8.12 × 10−6 A/cm2, demonstrating the best corrosion resistance.
While PC electrodeposition has been used to prepare various metal matrix composite coatings, the study of this unique deposition method for WS2 nanoparticle-doped Ni-Co composite coatings has not yet been reported. As such, systematic investigations on the effect of electrodeposition conditions on the development of the microstructure, hydrophobic properties, and corrosion resistance of composite coatings have not been conducted. In addition, WS2 is often used as an additive to fluid and solid lubricants, due to its special layer structure [48,49]. Zhang et al. [50] showed that a small amount of WS2 additives can remarkably reduce friction and wear. Wu et al. [51] found that reasonable embedding of WS2 can produce dense microstructures when it is composited with different matrices. However, studies on the promising combination of WS2 nanoparticles and metallic coatings have been very limited. Thus, there is still a lot of room for the development of Ni-Co-based composite coatings.
In this work, we present the fabrication of Ni-Co/WS2 nanocomposite coatings via DC and PC electrodeposition. The variations in morphological structure, chemical composition, hydrophobic properties, and corrosion resistance behavior, which depend on electrodeposition parameters and the WS2 content of the coating, were systematically investigated and compared. The results guided parameter optimizations, enabling the fabrication of a superhydrophobic nanocomposite coating with excellent corrosion resistance. In contrast, the nanocomposite coatings fabricated by DC electrodeposition could not reach a superhydrophobic state. Finally, how the superhydrophobic properties were imparted to the nanocomposite coating by PC electrodeposition is discussed.

2. Experimental

2.1. Materials and Specimen Preparation

The substrate used in this study was 40 mm × 50 mm × 1 mm copper plate. For substrate pretreatment, one side of the copper sheet was sealed with tape and the other side was polished with sandpaper (from 800 to 2000 grade in turn). Then, to clean the substrate: 1. Rinse using distilled water; 2. Sonicate in anhydrous ethanol for 10 min, to remove oil stains; 3. Rinse using distilled water. Subsequently, the cleaned substrate was activated in 10% w/w hydrochloric acid for 10 s and then rinsed with distilled water. Then, the substrate was ready for electrodeposition.
The electroplating bath was based on a Watts Bath, and the composition of the plating solution is described in Table 1. The names of the compositions are nickel sulfate (NiSO4), chloride nickel (NiCl2), cobalt sulfate (CoSO4), boric acid (H3BO3), cetyltrimethylammonium bromide (CTAB), and tungsten sulfide (WS2). The electrolyte was sonicated for 20 min in a water bath before electrodeposition. WS2 nanoparticles were supplied by Macklin Biochemical Technology Co., Ltd. (Shanghai, China). The rest of the above experimental materials were supplied by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).

2.2. The Electrodeposition Process

The electrodeposition system of Ni-Co/WS2 composite coating included a power supply, anode, cathode, and plating solution (Figure S1). The power supply used in this experiment was a SMD number controlled dual pulse electroplating power supply (model SMD-30, Da Shun Plating Equipment Co., Ltd., Handan, China), and the current type was direct current (Figure 1a) or rectangular wave pulse current (Figure 1b). The pretreated copper plate was used as the cathode, and a soluble nickel plate with a size of 100 mm × 100 mm × 5 mm was used as the anode. The composition of plating solution was mentioned above. Table 2 shows the deposition parameters. The temperature of electroplating solution was controlled at 40 °C using a constant temperature water bath, and the stirring speed was controlled at 300 r/min with a multifunctional electric device (Guo Hua Electric Co., Ltd., Changzhou, China). In the direct current electrodeposition experiments, the current density and deposition time were varied. In the pulsed electrodeposition experiments, the deposition time was fixed for all samples, and the pulse duty cycle and pulse frequency could be regulated in addition to the current density variation. After the electrodeposition process, 0.02 mol/L stearic acid/ethanol solution was used as the modification solution to modify the surface of coating, and then the plated sheet was put into the modification solution and subjected to ultrasound for 8 min. Finally, it was removed and blown dry with cold air.

2.3. Characterization

The surface morphologies of the coatings were observed using a scanning electron microscope (SEM, model SU8010, Hitachi Co., Ltd., Tokyo, Japan) in SE mode, using an applied voltage of 15.0 kV and a working distance of 10.1 mm. Meanwhile, the corresponding surface elements were detected using a matching energy dispersive spectrometer (EDS, model Genesis 60, EDAX Inc., Mahwah, NJ, USA). The 3D profile and the corresponding surface roughness of the samples were measured using an ultra-depth-of-field 3D microscope (model VHX-1000C, Keyence Co., Ltd., Osaka, Japan). The measurement of water contact angle (WCA) was performed using an optical contact angle meter (model JC2000C1, Zhong Chen Digital Equipment Co., Ltd., Shanghai, China), and the WCA value was the average of five measurements made with 5 µL distilled water droplets. The electrochemical performance of the composite coatings was tested with an INTERFACE 1000E electrochemical workstation (Gamry Instrument Inc., Warminster, PA, USA) in 3.5 wt.% NaCl solution. A three-electrode system was used for the corrosion tests, at room temperature, in which an experimental specimen was used as the working electrode, with a saturated calomel electrode (SCE) as a reference, and a platinum electrode as an auxiliary. The Tafel polarization curves were measured at a constant scan speed of 1 mV/s, starting from 300 mV in the negative direction of the open-circuit potential and ending at 300 mV in the positive direction of the open-circuit potential. Electrochemical impedance spectroscopy (EIS) tests were carried out in a frequency range of 0.1–105 Hz with a test area of 1 cm2 for the working electrode.

3. Results and Discussion

3.1. WS2-Induced Hydrophobic Microstructures on Ni-Co/WS2 Nanocomposite Coating Surface via Direct Current Electrodeposition

The Ni-Co/WS2 nanocomposite coatings were created using DC electrodeposition. The presence of WS2 nanoparticles in the Ni-Co coatings was confirmed by EDS analysis. Figure S2 shows that the nanocomposite coatings were only composed of Ni, Co, W, and S elements. Among these, the atomic ratio of W and S was measured as 1:2, very close to the nominal WS2 doping ratio.
WS2 nanoparticles exhibited a multilayer film structure at nanometer scale (Figure S3), corresponding to the description of a 2D-layered compound for WS2 and other transition-metal chalcogenides. The multilayer structure of WS2 suggested its potential for modifying the surface morphology of Ni-Co coatings.
The surface morphology of the Ni-Co/WS2 nanocomposite coatings was analyzed by SEM (Figure 2). Rough structures were observed on the copper substrate after DC electrodeposition, which were manifested as many micron-sized clustered protrusions. The clusters were cross-linked to each other and there were many pores distributed among them. Observed at 4000 times magnification, the protrusions were actually formed by layer-by-layer stacking of micro-/nano-scale sheet crystals of varying sizes, showing a stacked sheet-like shape.
The distribution of Ni, Co, W, and S elements in the Ni-Co/WS2 nanocomposite coating was traced using EDS element mapping, to further confirm the correspondence between the distribution of WS2 nanoparticles and the surface microstructure. Figure 3 shows that the Ni and Co elements basically covered all areas of the coating layer, while the distribution of W and S elements tended to correspond to cauliflower-like protrusions. Due to the difference in resistivity between WS2 nanoparticles and metal ions (6.84 × 10−8 Ω·m for nickel, 6.64 × 10−8 Ω·m for cobalt and 0.24 Ω·m for WS2) [52], the resistivity of WS2 was much higher than that of Ni and Co. Thus, Ni2+ and Co2+, which were partly in solution and partly adsorbed on the surface of the WS2 nanoparticles, gained electrons first and deposited them on the cathode surface. Subsequently, WS2 nanoparticles were deposited, to form cauliflower-like microstructures.
By comparing the differences in the microscopic morphology of the nanocomposite coating surface when only the current density was varied, the effect of different current densities on the hydrophobicity of the composite coatings was examined. The hydrophobicity of Ni-Co/WS2 nanocomposite coating was evaluated using the static water contact angle (WCA). Larger WCAs corresponded to higher hydrophobicities. According to Tafel’s formula, the magnitude of current density can change the cathodic overpotential, which in turn changes the nucleation rate and the crystal growth rate during electrodeposition. To achieve a high hydrophobicity, the current density in DC electrodeposition was optimized for obtaining a large WCA. Figure 4a shows the effect of the current density. As the current density increased from 1 to 5 A/dm2, the WCA curve first rose and then falls (Figure 4a). At the current density of 3 A/dm2, the WCA reached the maximum of 144.7°. At this time, the contact area between the water droplet and coating surface was minimal (Figure S4), indicating high hydrophobicity.
The correlation between the WCA and surface morphology was then investigated using SEM (Figure 4b–f). The microstructures showed sparse distributions, with large cluster sizes at the current density of 1 A/dm2 (Figure 4b). When the current density was gradually increased from 1 A/dm2 to 3 A/dm2 (Figure 4b–d), the large and irregular clusters on the surface of the composite coating gradually became finer and more dense. At the current density of 3 A/dm2, the dense clusters with small sizes were uniformly distributed (Figure 4d). This pattern of surface microstructure corresponds to large WCAs. When the current density was increased to 4 A/dm2 and above (Figure 4e,f), the distribution of clusters became uneven, and some of the clusters squeezed each other, to aggregate. This process corresponded to a gradual decrease in WCA. The results suggest that fine and uniformly distributed microstructures may increase the surface roughness, and thus lead to a large WCA.
An increased WCA indicates that the contact between a metal substrate and corrosion liquid will be reduced, and the anti-corrosion performance of the coating will be improved. To test the anti-corrosion performance of the current Ni-Co/WS2 nanocomposite coating, the potential dynamic polarization curve was measured using 3.5 wt.% NaCl as the corrosion liquid (Figure 5a). The curve was fit using the Tafel extrapolation method, to obtain corrosion parameters (Table 3). AC impedance tests were also performed and the Nyquist plot for this is shown in Figure 5b. The larger the radius of the capacitive arc in the Nyquist plot, the larger the impedance modulus, indicating that the anti-corrosion performance of the nanocomposite coating was stronger [53]. The experimental variable for the corrosion resistance test was current density, allowing a parallel comparison with the WCA and microstructures.
The results showed that the nanocomposite coating exhibited the most positive corrosion potential, the lowest corrosion current density, and the slowest corrosion rate at the current density of 3 A/dm2. As seen in the Nyquist plot, the impedance modulus of the coating was also at its maximum at this time, and this coating had the strongest corrosion resistance. At current densities of 2 and 4 A/dm2, the corrosion rate of the coating increased and the impedance modulus decreased. When the current density was 1 and 5 A/dm2, the corrosion rate of the coating was further accelerated and the impedance modulus was significantly inferior. An equivalent electrical circuit was adopted to fit the Nyquist plots and is shown in Figure 6. In the EEC, the Rs was the electrolyte resistance. The CPE was the constant phase element, instead of a pure double layer capacitor. The Rp could characterize the charge transfer resistance, evaluating the anti-corrosion behavior of the coatings. The fit parameters are listed in Table 4. It can be seen that the largest Rp value was obtained at the current density of 3 A/dm2, indicating a better corrosion resistance than the coatings prepared at other current densities. Moreover, the Rp values showed a trend of increasing and then decreasing as the current density was increased from 1 A/dm2 to 5 A/dm2, which is consistent with the results obtained from the Tafel polarization curves.
The results for corrosion resistance positively correlated with the WCA. When the WCA reached its maximum, the corrosion resistance was best. In summary, the doping of WS2 can improve the corrosion resistance of the coating, and reduce the contact between metal substrate and corrosion liquid by inducing microstructures on the coating surface.
Further attempts were made to increase the WCA by optimizing the electrodeposition time. The current density was kept constant at 3 A/dm2, and the electrodeposition time was adjusted to 20 min, 30 min, 40 min, 50 min, 60 min, and 70 min in turn, observing the changes of WCA. It can be seen that the maximum WCA (144.7°) was achieved when the deposition time was 50 min (Figure S5). This angle was the optimum value when the current density was optimized. This suggests that manipulating parameters in DC electrodeposition is difficult, to further improve the WCA. A limitation of DC is that constant electrodeposition causes concentration polarization, leading to inhomogeneous dispersion of chemical compositions in the system. This optimum value of WCA (144.7°) was promising for high its hydrophobicity and corrosion resistance. However, the coating surface had not yet reached a superhydrophobic state (threshold: WCA > 150°). To further improve the WCA, we switched from DC to PC electrodeposition.

3.2. Improving the Surface Microstructures of Ni-Co/WS2 Nanocomposite Coating via Pulse Electrodeposition

A Ni-Co/WS2 nanocomposite coating was prepared by PC electrodeposition. The element composition was analyzed by EDS and compared with the coating prepared by DC. In both electrodepositions, the current density was set to 3 A/dm2. Table 5 shows that WS2 nanoparticles were successfully incorporated into the coating by PC electrodeposition. Furthermore, the WS2 content in the PC-prepared coating was higher than that of the DC-prepared coating. This suggests that PC electrodeposition is promising for creating surface microstructures, since these structures are induced by WS2.
The parameters in PC electrodeposition were further optimized for the hydrophobicity of the composite coating, using WCA as the readout; these were the pulse duty cycle, pulse frequency, and current density. Pulse duty cycle is the ratio of on-time to total cycle time in a pulse cycle, and the pulse duty cycle reflects the ratio of pulse current on and off time. Pulse frequency is the number of pulses generated per unit of time. These pulse parameters can control the waveform, frequency, on-off ratio, and intensity of the output electrical signal, allowing the electrodeposition process to vary over a wide range, thus providing strong support for improving coating performance.
The optimization was performed using the control variable method, i.e., fixing two parameters and adjusting the third. As a result, the optimal values were 70%, 1000 Hz, and 3 A/dm2 for the pulse duty cycle, pulse frequency, and current density, respectively (Figure 7a–c). When PC electrodeposition was performed using these optimized parameters, the composite coating surface could reach a superhydrophobic state, with a WCA of 153.8°. In a superhydrophobic state, the surface microstructures were uniformly dispersed, with fine-sized clusters (Figure 7d–f). This pattern of surface microstructure corresponded to a large WCA.
In parameter optimization, the WCA can generally be enhanced by increasing the parameter values. However, after the coating reaches the superhydrophobic state, further increasing the parameter value decreases the WCA (Figure 7a–c). The correlation between WCA and PC parameters was investigated using SEM images, to compare surface microstructures. For pulse duty cycle optimization, the coating surface appeared rough at high pulse duty cycles (Figure S6), corresponding to a large WCA. The pulse duty cycle of 90% led to a decrease in WCA, because the on-time of the current is too long at too high a pulse duty cycle, leading to excessive grain growth (Figure 7g). For pulse frequency optimization, at low frequency, the clusters of the microstructures appeared bulky, with large gaps between each other (Figure S7). At high frequencies, the clusters became thinner and distributed uniformly, corresponding to a large WCA. A 1250 Hz frequency caused the WCA to decline (Figure 7h), probably owing to the limited discharge time at this high frequency, which slowed down the process of deposition. For current density optimization, the increase of current density created fine-sized microstructures (Figure S8), probably through reducing the grain size in the electro-crystallization. Too a high current density can produce clusters with too small sizes. These tiny-sized clusters accumulate densely and cause the microstructures to be squeezed, destroying the even distribution pattern (Figure 7i).
Next, the pulse current (PC) was compared to DC using the WCA of nanocomposite coatings as the readout. The coatings prepared by PC showed a significantly larger WCA at all current densities tested (Figure 8). In contrast, the DC-prepared coatings showed a small WCA and large data deviations, especially at small current densities (Figure 8, black box). Similarly, for the PC- and DC-prepared coatings, the WCA reached a maximum at a current density of 3 A/dm2 and then decreased when the current density further increased. Notably, only the PC-prepared coatings could reach a superhydrophobic state (WCA > 150°). In conclusion, the Ni-Co/WS2 nanocomposite coatings prepared by PC had superior hydrophobic properties.
The corrosion resistance was then compared between the PC- and DC-prepared coatings at a fixed current density of 3 A/dm2. The corrosion parameters in Table 6 were obtained by fitting the potential dynamic polarization curves (Figure 9a), and it can be intuitively seen that the coatings prepared by pulsed electrodeposition had a better corrosion potential, lower corrosion current density, and slower corrosion rate. The results in the Nyquist plot (Figure 9b) also show that the capacitive arc radius of the PC-prepared coating was significantly larger than that of the DC-prepared coating, and the corresponding impedance modulus was also larger. The electrochemical parameters obtained by fitting the Nyquist plots with the equivalent electrical circuit are shown in Table 7. The results show that the Rp value of the PC-prepared coating was much larger than that of the DC-prepared coating. Therefore, the Ni-Co/WS2 nanocomposite coatings prepared by pulsed electrodeposition had superior corrosion resistance.
To understand the superior properties of the PC-prepared coatings, the surface microstructures of PC- and DC-prepared coatings were investigated using SEM. Under low magnification, both PC- and DC-prepared coatings showed a uniform and dense distribution on their surface (Figure 10a,b). Interestingly, the rough structure of the PC-prepared coating was more three-dimensional and the clusters showed obvious high and low undulations, while the surface morphology of the DC-prepared coating surface was relatively flat (Figure 10c,d). Under high magnification, the layered deposition effect of the PC-prepared coatings was more obvious, with more layers and more complex and finer grains at the top of the clusters (Figure 10e,f). Therefore, the top of the PC-prepared coating had less contact area with liquid, and the coating contained more tiny spaces for storing air. These characteristics may be the key factors allowing the PC-prepared coatings to reach a superhydrophobic state and superior corrosion resistance.

3.3. Mechanistic Understanding for the Hydrophobic Properties and Anti-Corrosion Behavior of the Ni-Co/WS2 Nanocomposite Coating

The details of the surface microstructures were further analyzed, to explore their connection with the hydrophobic properties. The Ni-Co/WS2 nanocomposite coatings were prepared by PC at different nano-WS2 concentrations. The variation of WS2 content was expected to change the distribution of the surface microstructures, facilitating observation and comparison by SEM. As a result, a WS2 content of 1 g/L was sufficient to induce rough surface microstructures that increased the WCA ~37° (Figure 11c). As the WS2 content increased, the surface microstructures became dense and fine in size, and the gaps among the clusters gradually contracted into small and uniformly distributed pores, with a corresponding increase in the WCA (Figure 11c–e). However, when the WS2 content equaled to 15 g/L, the originally uniformly dispersed clusters began to overlap and plate, the rough structures were found to be less three-dimensional under magnification, and the WCA decreased accordingly (Figure 11f). The differences between Figure 11e,f suggest that, in addition to the surface distribution, the roughness and space within the surface structures also contributed to hydrophobic properties.
An ultra-deep field 3D microscope was employed to quantify the surface roughness using the roughness average (Ra), a parameter associated with hydrophobic properties. This also allowed analyzing the depth of the surface microstructures on the Z-axis. At a WS2 content of 1 g/L, mountain-like structures were generated on the surface (Figure 12b). These structures greatly increased the Ra, compared with the coating surface containing no WS2 (Figure 12a). The Ra reached a maximum when the distribution of mountain-like structures were the densest (Figure 12d). However, at a WS2 content of 15 g/L, the mountain-like structures became mound-like (Figure 12e), from steep and high protrusions to wide and gentle bulks, and the gaps among the microstructures also increased significantly, causing a decrease in Ra. In conclusion, the 3D microscope supplemented the details of surface microstructures. When the nanocomposite coating reached a superhydrophobic states, the microstructures exhibited a dense distribution of mountain-like structures, corresponding to the highest Ra.
The above results suggest that the formation of micro-/nano-scale rough structures allowed the composite coatings to obtain excellent hydrophobic properties and corrosion resistance. The hydrophobicity of the coating surface was quantified using the water contact angle (WCA). The larger the WCA, the higher the hydrophobicity. The concept of WCA can be explained by the Cassie–Baxter model. This model describes the contact between a water droplet and a rough interface, and well fits the case of Ni-Co/WS2 nanocomposite coatings. It considers two contact states of the droplet, namely “liquid–solid” and “liquid–gas”. As shown in Figure 13, many rough microstructures were generated on the surface of the coating, and there were finer protrusions standing on top of the surface microstructures. The pores between these protrusions became filled with air, forming a thin air layer. This air layer constituted the “liquid–gas” interface between the droplet and the surface. According to the Cassie–Baxter equation [54], the “liquid–gas” contact is represented by f2 and the “liquid–solid” contact is represented by f1. The sum of f1 and f2 is 1. θ1 is the contact angle of the liquid on an ideal smooth surface. For a hydrophobic surface (−1 < cosθ1 < 0), the smaller the “liquid–solid” contact area and the greater the contact area of air in the pores, the larger the WCA (θ in the Cassie–Baxter equation) and the better the hydrophobic performance. The increase of f2 contributes more to a larger WCA, because the factor of f2 is −1, a consistently larger weight than that of f1. This means that the pores hidden in the surface rough structures are important for increasing the hydrophobic properties.
Cassie–Baxter Equation (1):
cos θ = f1 cosθ1 − f2, −1 < cosθ1 < 0
Comparing the microstructures created by DC and PC, the PC-created structures were more layered, richer, and contain more pores (Figure 10). These layered and sophisticated microstructures are key factors in the coatings to reach a superhydrophobic state. The question is how micro-/nano-scale structures with these geometric characteristics are preferentially created by PC instead of DC.
To answer this question, the formation of surface microstructures should be described in details. Figure 13 gives a schematic diagram. According to the modified Guglielmi model, the electrochemical co-deposition process of nanoparticles and metal coatings consists of several successive adsorption stages [55,56,57]. First, WS2 nanoparticles adsorb free Ni2+ and Co2+ on the surface, due to its high surface energy, forming larger charged particles. Then, the charged particles and metal ions in solution move toward the cathode under mechanical agitation and form a loose adsorption on the outer side of the compact electric double layer of the electrode. Subsequently, under cooperative interactions, both due to thermal fluctuations and due to the influence of an electric field in a double layer, the particles in a loose adsorbed state remove ionic components and solvate shells adsorbed on their surface. In addition, the particles enter the compact electric double layer and come into direct contact with the electrode surface, forming irreversible electrochemical strong adsorption. Finally, the nanoparticles are captured by the growing metal and form a nanocomposite deposition layer on the electrode surface. The multilayer structure of WS2 disturbs the orderly accumulation of metal atoms, allowing the deposition to develop in a three-dimensional direction, and eventually generates cauliflower-like microstructures.
The crystal growth of WS2 is suggested as explaining the differences between DC and PC in creating surface microstructures. In DC electrodeposition, nucleation mainly happens at the beginning and the nucleus grows continuously, resulting in low nucleation numbers and large crystals. In contrast, PC electrodeposition offers more opportunities for nucleation and limits the over-growth of the nucleus, resulting in high nucleation numbers and small crystals. As a result, PC-created microstructures consist of partially-grown crystals and contain more pores that are favorable for the formation of a “liquid–gas” interface.
Interestingly, the crystal growth of WS2 can be controlled by the parameters in the electrodeposition process. By adjusting the current density in DC electrodeposition, the desired surface microstructures can be created. The current density can control the size and number of WS2 nucleation by influencing the overpotential [58,59]. However, the optimization of current density does not allow the composite coating to reach a superhydrophobic state (Figure 4). The reason behind this is the concentration polarization that occurs at the cathode interface. Concentration polarization indicates an ion concentration gradient between the electrode surface and the solution, which is caused by the continuous discharges in DC electrodeposition. In contrast, the concentration polarization can be largely alleviated in PC electrodeposition, because the pulse interval allows the ion concentration at the cathode–solution interface to be replenished (green arrow in Figure 14). In addition, more tunable parameters are available in PC to fine-tune the crystal growth of WS2, which makes the fabrication of superhydrophobic coatings possible.

4. Conclusions

Ni-Co/WS2 nanocomposite coatings were obtained with the use of direct current and pulsed current electrodeposition, respectively. Among the two, enhanced corrosion-resistance superhydrophobic Ni-Co/WS2 nanocomposite coatings were successfully prepared using the pulsed electrodeposition method. The main conclusions could be drawn as follows:
The electrodeposition mode and the corresponding electrodeposition parameters can regulate the deposition, crystallization, and growth process of nanoparticles and metal atoms, forming many micro-/nano-scale protruding structures on the surface of the substrate, increasing the surface roughness of the coating. The air remaining in the pores of these micro/nanoscale protruding structures forms a thin air layer, which largely reduces the contact area between the liquid and the coating surface, allowing the coating to reach a hydrophobic or superhydrophobic state. The limited liquid–solid contact area and protective air layer can protect the substrate surface from infiltration by corrosive media, which results in a composite coating with a higher charge transfer resistance and lower corrosion rate, ultimately exhibiting excellent corrosion resistance.
By optimizing the electrodeposition parameters, more hydrophobic and corrosion-resistant coatings can be obtained. In DC electrodeposition mode, the best hydrophobicity of the prepared composite coatings (water contact angle of 144.7°) could be achieved when the current density was 3 A/dm2 and the electrodeposition time was 50 min. While, the best corrosion resistance of the coating was also achieved under these conditions. Compared with DC electrodeposition, pulsed electrodeposition makes it easier to obtain micro-/nano-scale coatings with a smaller grain size, by controlling the periodic conduction and disconnection of the plating circuit. At the same time, pulsed electrodeposition can alleviate concentration polarization, which significantly improves the electrodeposition of composite coatings. The SEM surface morphology and WCA test results showed that under the conditions of a current density of 3 A/dm2, pulse duty cycle of 70%, and pulse frequency of 1000 Hz, the surface of the composite coating showed a rough cluster structure with uniform distribution and suitable pore size, at which time the static contact angle of Ni-Co/WS2 nanocomposite coating reached 153.8°, showing superhydrophobic properties and the best corrosion resistance.
In addition, the incorporation of WS2 nanoparticles is also an important factor affecting the state of metal ion deposition and the surface morphology of composite coatings. WS2 nanoparticles exhibit a multilayer structure at nanoscale. Its incorporation can form a fine and dense cauliflower-like cluster structure on the coating surface, and many pores are distributed between the cluster structures, which can enhance the surface roughness of the coating. However, if the nano-WS2 content is too high, the clusters will be too densely accumulated and the surface roughness will also decrease. The optimal content of nano-WS2 doped into composite coatings was 10 g/L, at which time the coating showed a superhydrophobic state. Correspondingly, the roughness of the coating reached a maximum value. The electrochemical performance test results showed that the corrosion rate of the composite coatings in this state was the smallest and the impedance modulus was the highest, which gave the Ni-Co/WS2 nanocomposite coatings better corrosion resistance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings12121897/s1, Figure S1. Diagram of the electrodeposition experimental device; Figure S2. EDS spectrum of Ni-Co/WS2 nanocomposite coatings prepared by DC electrodeposition; Figure S3. The SEM image of WS2 nanoparticle; Figure S4. The WCA images of Ni-Co/WS2 composite coatings prepared by DC electrodeposition, with different current densities; Figure S5. WCA images of Ni-Co/WS2 composite coatings prepared by DC electrodeposition, with different electrodeposition times; Figure S6. SEM images of the Ni-Co/WS2 composite coatings prepared at different pulse duty cycles; Figure S7. SEM images of the Ni-Co/WS2 composite coatings prepared at different pulse frequencies; Figure S8. SEM images of the Ni-Co/WS2 composite coatings prepared at different current densities by PC electrodeposition.

Author Contributions

Methodology, W.G., M.W., and S.Y.; validation, Q.S. and B.L.; formal analysis, Z.X., J.H., and B.L.; investigation, M.W. and Q.S.; resources, W.G.; writing—original draft preparation, M.W. and Q.S.; writing—review and editing, W.G., S.Y., Z.X., and J.H.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of direct current waveform (a) and pulse current waveform (b). The current density (3 A/dm2), pulse period (1 ms), and duty cycle ratio (70%) were the best values for the experimentally obtained parameters.
Figure 1. Schematic diagram of direct current waveform (a) and pulse current waveform (b). The current density (3 A/dm2), pulse period (1 ms), and duty cycle ratio (70%) were the best values for the experimentally obtained parameters.
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Figure 2. The SEM images of the Ni-Co/WS2 nanocomposite coatings at different magnifications: (a) 500 times; (b) 1000 times; (c) 1500 times; (d) 4000 times. The parameters for DC electrodeposition: current density, 3 A/dm2; deposition time, 50 min; WS2 concentration, 10 g/L.
Figure 2. The SEM images of the Ni-Co/WS2 nanocomposite coatings at different magnifications: (a) 500 times; (b) 1000 times; (c) 1500 times; (d) 4000 times. The parameters for DC electrodeposition: current density, 3 A/dm2; deposition time, 50 min; WS2 concentration, 10 g/L.
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Figure 3. (a,b) SEM image of the Ni-Co/WS2 nanocomposite coating and the corresponding EDS mappings of (c) Ni, (d) Co, (e) W, and (f) S.
Figure 3. (a,b) SEM image of the Ni-Co/WS2 nanocomposite coating and the corresponding EDS mappings of (c) Ni, (d) Co, (e) W, and (f) S.
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Figure 4. The WCA change trend (a) and SEM images (b–f) of the Ni-Co/WS2 nanocomposite coatings prepared under different current densities: (b) 1 A/dm2; (c) 2 A/dm2; (d) 3 A/dm2; (e) 4 A/dm2; (f) 5 A/dm2.
Figure 4. The WCA change trend (a) and SEM images (b–f) of the Ni-Co/WS2 nanocomposite coatings prepared under different current densities: (b) 1 A/dm2; (c) 2 A/dm2; (d) 3 A/dm2; (e) 4 A/dm2; (f) 5 A/dm2.
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Figure 5. Tafel polarization curves (a) and Nyquist plot (b) of the Ni-Co/WS2 nanocomposite coatings prepared under different current densities.
Figure 5. Tafel polarization curves (a) and Nyquist plot (b) of the Ni-Co/WS2 nanocomposite coatings prepared under different current densities.
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Figure 6. The equivalent electrical circuit for fitting the EIS of the coatings.
Figure 6. The equivalent electrical circuit for fitting the EIS of the coatings.
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Figure 7. The WCA change trends of the Ni-Co/WS2 nanocomposite coatings prepared under different duty cycles (a), frequencies (b), and current densities (c); and SEM images at a duty ratio of 70% (d) and 90% (g), frequency of 1000 Hz (e), and 1250 Hz (h), and current densities of 3 A/dm2 (f) and 5 A/dm2 (i).
Figure 7. The WCA change trends of the Ni-Co/WS2 nanocomposite coatings prepared under different duty cycles (a), frequencies (b), and current densities (c); and SEM images at a duty ratio of 70% (d) and 90% (g), frequency of 1000 Hz (e), and 1250 Hz (h), and current densities of 3 A/dm2 (f) and 5 A/dm2 (i).
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Figure 8. Boxplot comparing the WCA of Ni-Co/WS2 nanocomposite coatings prepared by different electrodeposition methods. * p < 0.05, ** p < 0.01, *** p < 0.001, two-sided student’s t-test.
Figure 8. Boxplot comparing the WCA of Ni-Co/WS2 nanocomposite coatings prepared by different electrodeposition methods. * p < 0.05, ** p < 0.01, *** p < 0.001, two-sided student’s t-test.
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Figure 9. Tafel polarization curves (a) and Nyquist plot (b) of the Ni-Co/WS2 nanocomposite coatings prepared with different electrodeposition methods.
Figure 9. Tafel polarization curves (a) and Nyquist plot (b) of the Ni-Co/WS2 nanocomposite coatings prepared with different electrodeposition methods.
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Figure 10. SEM images of the Ni-Co/WS2 nanocomposite coatings prepared by PC (a,c,e) and DC (b,d,f). The magnification was 500 times, 1000 times, and 1500 times for the first, second, and third rows, respectively.
Figure 10. SEM images of the Ni-Co/WS2 nanocomposite coatings prepared by PC (a,c,e) and DC (b,d,f). The magnification was 500 times, 1000 times, and 1500 times for the first, second, and third rows, respectively.
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Figure 11. The WCA change trend (a) and SEM images (b–f) of the Ni-Co/WS2 nanocomposite coatings prepared with different concentrations of WS2.
Figure 11. The WCA change trend (a) and SEM images (b–f) of the Ni-Co/WS2 nanocomposite coatings prepared with different concentrations of WS2.
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Figure 12. The 3D profile and surface roughness average (Ra) of the Ni-Co/WS2 nanocomposite coatings prepared with different concentrations of WS2.
Figure 12. The 3D profile and surface roughness average (Ra) of the Ni-Co/WS2 nanocomposite coatings prepared with different concentrations of WS2.
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Figure 13. Schematic diagram of the mechanism of micro-/nano-scale rough structures in enhancing the hydrophobic properties of composite coating surfaces.
Figure 13. Schematic diagram of the mechanism of micro-/nano-scale rough structures in enhancing the hydrophobic properties of composite coating surfaces.
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Figure 14. Schematic diagram of the mechanism of forming micro/nanoscale structures on substrates using DC and PC electrodeposition.
Figure 14. Schematic diagram of the mechanism of forming micro/nanoscale structures on substrates using DC and PC electrodeposition.
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Table
Table 1. The composition of the electroless plating solution.
Table 1. The composition of the electroless plating solution.
CompositionConcentration (g/L)Function
WS20, 1, 5, 10, 15nanoparticles
NiSO4·6H2O200main salt ions
CoSO4·7H2O25main salt ions
NiCl2·7H2O40increase conductivity
H3BO340buffer pH
CTAB0.1dispersant of nanoparticles, reduce agglomeration
Table
Table 2. Parameters of the electrodeposition process.
Table 2. Parameters of the electrodeposition process.
DC and PC ParameterValue
pH3 ± 0.1
Temperature (°C)40
stirring speed300 r/min
Current density (A/dm2)1, 2, 3, 4, 5
Electrodeposition time (min)20, 30, 40, 50, 60, 70
PC Specific ParameterValue
Duty ratio30%, 50%, 70%, 90%
Pulse frequency (Hz)50, 100, 200, 500, 1000, 1250
Table
Table 3. The corrosion parameters of the Ni-Co/WS2 nanocomposite coatings under different current densities.
Table 3. The corrosion parameters of the Ni-Co/WS2 nanocomposite coatings under different current densities.
SampleEcorr (V)icorr (A/cm2)Corrosion Rate (mm/a)
DC-j-1−0.2755.29 × 10−50.618
DC-j-2−0.2161.06 × 10−61.24 × 10−2
DC-j-3−0.2226.17 × 10−77.22 × 10−3
DC-j-4−0.2583.76 × 10−64.40 × 10−2
DC-j-5−0.3281.49 × 10−41.74
j, current density; Ecorr, corrosion potential; icorr, corrosion current density.
Table
Table 4. Equivalent electrical circuit parameters obtained by fitting the Nyquist plots of the coatings prepared under different current densities.
Table 4. Equivalent electrical circuit parameters obtained by fitting the Nyquist plots of the coatings prepared under different current densities.
SampleRs
(Ω·cm2)		CPE
Ω−1·sn·cm−2		Rp
(Ω·cm2)
DC-j-19.0371.30 × 10−31810
DC-j-28.7981.53 × 10−48253
DC-j-36.1777.04 × 10−516,929
DC-j-415.42.64 × 10−44939
DC-j-57.3472.16 × 10−31029
j, current density.
Table
Table 5. Elemental content of the Ni-Co/WS2 nanocomposite coatings prepared by PC and DC electrodeposition.
Table 5. Elemental content of the Ni-Co/WS2 nanocomposite coatings prepared by PC and DC electrodeposition.
Wt.%NiCoWS
Sample Number
PC-j-342.6515.1431.2710.94
DC-j-339.1427.1624.878.83
PC, pulse current; DC, direct current; j, current density.
Table
Table 6. The corrosion parameters of the Ni-Co/WS2 nanocomposite coatings with different electrodeposition methods.
Table 6. The corrosion parameters of the Ni-Co/WS2 nanocomposite coatings with different electrodeposition methods.
SampleEcorr (V)icorr (A/cm2)Corrosion Rate (mm/a)
DC-j-3−0.2226.17 × 10−77.22 × 10−3
PC-j-3−0.1951.32 × 10−71.54 × 10−3
DC, direct current; PC, pulse current; j, current density; Ecorr, corrosion potential; icorr, corrosion current density.
Table
Table 7. The equivalent electrical circuit parameters obtained by fitting the EIS of the coatings prepared with different electrodeposition methods.
Table 7. The equivalent electrical circuit parameters obtained by fitting the EIS of the coatings prepared with different electrodeposition methods.
SampleRs
(Ω·cm2)		CPE
Ω−1·sn·cm−2		Rp
(Ω·cm2)
DC-j-36.1777.04 × 10−516,929
PC-j-310.175.86 × 10−523,617
DC, direct current; PC, pulse current; j, current density.
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Wang, M.; Xue, Z.; Yan, S.; He, J.; Shao, Q.; Ge, W.; Lu, B. Pulse Electrodeposited Super-Hydrophobic Ni-Co/WS2 Nanocomposite Coatings with Enhanced Corrosion-Resistance. Coatings 2022, 12, 1897. https://doi.org/10.3390/coatings12121897

AMA Style

Wang M, Xue Z, Yan S, He J, Shao Q, Ge W, Lu B. Pulse Electrodeposited Super-Hydrophobic Ni-Co/WS2 Nanocomposite Coatings with Enhanced Corrosion-Resistance. Coatings. 2022; 12(12):1897. https://doi.org/10.3390/coatings12121897

Chicago/Turabian Style

Wang, Meijiao, Zixiao Xue, Shaojiu Yan, Jin He, Qiuyue Shao, Wen Ge, and Baodi Lu. 2022. "Pulse Electrodeposited Super-Hydrophobic Ni-Co/WS2 Nanocomposite Coatings with Enhanced Corrosion-Resistance" Coatings 12, no. 12: 1897. https://doi.org/10.3390/coatings12121897

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