Robust superhydrophobic surface based on multiple hybrid coatings

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Applications of Polymer, Composite, and Coating Materials

Robust superhydrophobic surface based on multiple hybrid coatings for application in corrosion protection Yaya Zhou, Yibing Ma, Youyi Sun, Zhiyuan Xiong, Chunhong Qi, Yinghe Zhang, and Yaqing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19663 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Applied Materials & Interfaces

Robust Superhydrophobic Surface based on Multiple Hybrid Coatings for Application in Corrosion Protection Yaya Zhou1, Yibing Ma1, Youyi Sun1,*, Zhiyuan Xiong2, Chunhong Qi1, Yinghe Zhang3, Yaqing Liu1 1.Shanxi Province Key Laboratory of Functional Nanocomposites, North University of China, Taiyuan 030051, P.R.China. 2. Department of Chemical and Bio-molecular Engineering, The University of Melbourne, Victoria 3010, Australia. 3. Nanotechnology Department, Helmholtz Association, Hamburg 21502, Germany.

Abstract: A new class superhydrophobic surface based on multiple hybrid coatings is proposed and prepared to improve mechanical and reproduction stability. It does not only show large water contact angle (ca. 174.5o), but also a slight decrease (ca.6.4%) of water contact angle is observed after 100 mechanical abrasion cycles. Furthermore, the water contact angle is a slight change (relative standard deviation, 0.14%) for the three superhydrophobic surfaces prepared with the same procedure. The application of superhydrophobic multiple hybrid coatings in corrosion protection is further investigated by the Tafel polarization curves and electrochemical impedance spectroscopy. The superhydrophobic multiple hybrid coatings showed lower corrosion current (1.4×10-11A/cm2), lower corrosion rate (ca. 1.6×10-7 mm/year) and larger polarization resistance (7.9×104 MΩ·cm2) in 3.5wt% NaCl aqueous solution comparing to other superhydrophobic coatings reported in previous works. The work does not only confirm the formation of robust superhydrophobic surface for real application in corrosion protection, but also provide a new model of superhydrophobic surface based on multiple hybrid coatings with high mechanical, chemical and reproduction stability for various applications.

Keywords: Superhydrophobic surfaces, multiple hybrid coatings, low surface energy, corrosion protection, stability.

Responding author: Fax: 86-351-3559669 E-mail address: [email protected] (YY Sun)

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1. INTRODUCTION Superhydrophobic surfaces as a promising coating for corrosion protection have received lots of attentions due to effectively shield corrosive species, such as water and ions1. As well-known, the hydrophobicity of material surface is mainly attributed to its chemical structure and surface roughness. Generally, decreasing surface energy of materials is firstly used to improve hydrophobicity1. However, it was difficult to exceed 120o for water contact angle of smooth hydrophobic surface2. So, highly porous micro-structures or hierarchical structure were often needed and prepared on surface of metal substrate3,4. However, when these surfaces were damaged by external forces, the superhydrophobicity was easily reduced, resulting in a decrease of anti-corrosion performance. Furthermore, it is still a high challenge to control surface roughness or micro-structure by the current preparing methods of anti-corrosive superhydrophobic surfaces, such as chemical reaction method5, etching method6, hydrothermal method7, anodization method8, electrodeposition method9,10, sol-gel method11, nanocomposite coating method12, templating method13 and so on. So, most of present superhydrophobic surface were still difficult to be large-scale preparation. These problems restricted the real applications of superhydrophobic surface in corrosion protection industry. To this end, a facile production of superhydrophobic surface with good mechanical and reproduction stability are the key roles for the practical applications. Here, a new robust superhydrophobic surface based on multiple hybrid coatings was developed and prepared by a new synthesis process. The new synthesis process could be large-scale reproduction of superhydrophobic surfaces due to that the superhydrophobicity of coating was slightly affected by surface roughness or micro-structure. At the same reason, it also showed resilience and maintained its superhydrophobic and anti-corrosion properties under mechanical abrasion and chemical solution immersion. Furthermore, the superhydrophobic multiple hybrid coatings showed better anti-corrosion performance comparing to previous superhydrophobic anti-corrosion coatings. These results are very important to further design and develop superhydrophobic surface for real applications in industry. 2. EXPERIMENT SECTION 2.1. Materials FeCl2·4H2O, FeCl3·6H2O and NaOH were purchased from Shanghai Chemical Reagent Co., Ltd. ammonia solution (NH3·H2O), ethyl silicate (TEOS), urea, cetyl trimethyl ammonium bromide (CTAB), 1-amyl alcohol, cyclohexane, toluene, ethanol (C2H5OH) and hydrochloric acid (HCl) were purchased from Tianjin guangfu technology development Co., Ltd. N-octyl triethoxysiloxane was purchased from Aladdin. Epoxy and curing agent were purchased from Shanghai Yue Ke Composite Co., Ltd. Q235 was purchased from the local chemical market. These chemicals without further puriﬁcation were used in present work. 2.2. Preparation of Fe3O4@SiO2 nanocomposites modified with N-octyl triethoxysiloxane


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The Fe3O4@SiO2 nanocomposites modified with N-octyl triethoxysiloxane (Fe3O4@OTS-SiO2) was synthesized by the solution reaction method as shown in Scheme 1. Firstly, 3.6g FeCl2·4H2O and 6.1g FeCl3·6H2O were dissolved in 90.0mL deionized water. Then, the 92.5mL NaOH aqueous solution (1.33M) was dropped into above mixing solution under stirring at 50.0℃. The above mixing solution continued to react for 90.0min. The Fe3O4 nanoparticles were collected by a magnet and then washed by deionized water. After purification, the Fe3O4 nanoparticles were dispersed in mixing solution (ethanol:water:ammonia solution=20:5:1) to obtain Fe3O4 suspension solution (8.0mg/mL). Secondly, 2.0mL TEOS was added to 125.0mL Fe3O4 suspension solution (8.0mg/mL) under stirring and the reaction was continued for 6.0h at room temperature. The Fe3O4@SiO2 core-shell nanoparticles were also collected by a magnet and then washed by deionized water. After purification, the Fe3O4@SiO2 core-shell nanoparticles were dispersed in water to obtain Fe3O4@SiO2 suspension solution (16.7mg/mL). Furthermore, 1.2g urea, 2.0g CTAB, 3.0mL 1-amyl alcohol, 60.0mL cyclohexane and 5.0g TEOS was added to Fe3O4@SiO2 suspension solution. The above mixing solution was moved to Teflon-lined stainless steel autoclave and continued to react for 5.0h at 120.0℃. The Fe3O4@SiO2 modified with CTAB was also separated with a magnet and were washed for several times with deionized water to remove excess materials. And then pure Fe3O4@SiO2 modified with CTAB was dispersed in toluene solution to form stable Fe3O4@SiO2 modified with CTAB suspension solution (0.2g/mL). Thirdly, 20.0mL N-octyl trimethylsilyl was added to 50.0mL Fe3O4@SiO2 modified with CTAB suspension solution (0.2g/mL). The above mixing solution was moved to Teflon-lined stainless steel autoclave and continued to react for 20.0h at 120.0℃. The Fe3O4@OTS-SiO2 nanocomposites were also collected by a magnet and then washed by deionized water. Finally, the Fe3O4@OTS-SiO2 nanocomposites were dispersed in dimethybenzene to form paint. In a comparison, the Fe3O4@SiO2 nanocomposites without modification of N-octyl triethoxysiloxane were also prepared by the similar process. Scheme 1. 2.3. Preparation of superhydrophobic multiple hybrid coatings The superhydrophobic multiple hybrid coatings can be easily prepared by the spraying method as shown in Scheme 2. The bisphenol A type epoxy resin (EP), amine curing agent and defoaming agent (5:1:0.5) was mixed to form epoxy resin paint. The epoxy resin paint was sprayed on surface of Q235 iron sheet and was pre-cured for 2.0h at 60.0℃. Then, the Fe3O4@OTS-SiO2 dispersed solution was sprayed on surface of epoxy resin coating. Finally, the above composite coatings were further cured for 5.0h at 60℃ to form multiple hybrid coatings. In a comparison, the pure epoxy coating and Fe3O4@SiO2/EP coating was also prepared by the similar process. Scheme 2.



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2.4. Instruments and characterization The phase structure of samples was characterized by the X-ray diffraction with Cu Kα radiation diffraction (λ=0.154nm, 35.0kV and 40.0mA) in the scan range of 5.0-80.0° and scanning speed of 4.0° min-1. Chemical structure of samples was characterized by Fourier transform infrared (FT-IR) spectrometer (Thermo Nicolet 360) in the range of 4000-400.0cm-1. The magnetic properties of samples were evaluated by the vibrating magnetometer (VSM, Versolab, Quantum Design, USA) under an magnetic ﬁeld range of -11.0kOe to 11.0kOe. The surface structure of samples was characterized by Field emission scanning electron microscopy (FE-SEM, Su-8010 and FEI). The water contact angle (WCA) of samples was characterized by the equipment (DSA100, Germany). 2.5. Stability of the superhydrophobic surface The chemical stability was evaluated by measuring the contact angle value and electrochemical parameters of the superhydrophobic coating immersed in NaCl aqueous solution (3.5wt%), HCl aqueous solution (pH=0, 2.0mol/L), NaOH aqueous solution (pH=14, 2.0mol/L) and flowing water. The mechanical stability was evaluated by measuring the contact angle value and electrochemical parameters of the superhydrophobic coating under mechanical abrasion. The sample was moved on surface of sandpaper for 1000.0cm under a normal load of 100.0g. 2.6. Anti-corrosion performance The anti-corrosion performance was characterized by the potentiodynamic polarization and electrochemical impedance spectroscopy on an electrochemical workstation (CHI660C, Chenhua, Shanghai). In a three-electrode system, the superhydrophobic surface coated carbon steel, platinum plate and Hg/HgO electrode (SCE) were used as the working electrode, the counter and reference electrode, respectively. The corrosion current density (Icorr) and potential (Ecorr) were determined by using electrochemical analyzer software. The polarization resistance (Rp) was determined by the equation 16： Rp 

ba  bc 2.303Icorr  ba  bc 

(1)

Where the ba and bc are the slope of anodic and cathodic Tafel plots, respectively. The corrosion rate (Vcorr) was calculated according to the equation 214:

Vcorr 

AI corr  87600(mm / year ) n F

(2)

Where the A is formula weight (55.85g/mol), the ρ is density of iron (7.85g/cm3), the n is chemical valence of iron (2) and F is the Faraday constant (96485C/mol). 3. RESULTS AND DISCUSSION


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3.1. Design and preparation of superhydrophobic surface based on multiple hybrid coatings An new class multiple hybrid coatings consisted of superhydrophobic tocoat and adhesive ground coat was proposed and prepared by the spraying method for improving mechanical and reproduction stability of superhydrophobic surface as shown in Scheme 2. The tocoat was based on nanoaprticles with low surface energy, providing the superhydrophobicity of multiple hybrid coatings. The ground coat was based on polymer resin, which was acted as adhesive agents for adhesion nanoparticles on surface of substrate. Among the synthesis process, the one of most important steps was the synthesis of paints based on nanoparticles with low surface energy. Although, there was some works reporting the synthesis of nanoparticles with low surface energy15,16, however, the nanoparticles were poor stability in solution and the surface modification agents were high cost. So, the uniform superhydrophobic tocoat was difficult to be obtained and the reproduction stability was also poor. Here, a modified synthesis process was developed to improve reproduction stability and reduce the cost. Firstly, the Fe3O4 nanoparticles were prepared by the co-precipitation solution method to obtain stable Fe3O4 dispersion solution. Furthermore, it was also more facile for SiO2 coated on surface of Fe3O4. Secondly, the N-octyl triethoxysiloxane with low cost was acted as surface modification agents to reduce surface energy of nanoparticles and improve the stability of nanoparticles in dimethybenzene. Furthermore, it was chemically grafted on surface of SiO2 for improving the surface stability of nanoparticles. In addition, the dimethybenzene was acted as solution of paints, which could effectively improve the uniform properties of superhydrophobic tocoat and adhesive strength between nanoparticles and epoxy resin. As shown in inset of Fig.1B, the Fe3O4@OTS-SiO2 dispersion dimethybenzene solution could remain high stability for several days and the Fe3O4@OTS-SiO2 nanocomposites were easily separated from the dispersion solution by an magnet. As well-known, if the Fe3O4 nanoparticles and SiO2 were mechanical mixture, the Fe3O4 nanoparticles would be separated from the suspension while the SiO2 would remain in dispersion solution under an magnet. These results confirmed the formation of Fe3O4@OTS-SiO2 nanocomposites. The Fe3O4@OTS-SiO2 nanocomposites were further characterized by the XRD spectra, IR spectrum and VSM as shown in Figure 1A, 1B and 1C, respectively. Some strong diffraction peaks at 18.2° 、 29.9° 、 35.4° 、 37.1° 、 43.0° 、 53.3° 、 56.9° 、 62.6° and 74.1° were clearly observed, corresponding to the (111) , (220), (311), (400), (422), (511), (440), and (533) planes of Fe3O4 (JCPDS card, 79-0418)17 as. shown in Fig.1A. In addition, an weak peak at 21.5° was assigned to the amorphous SiO215. As seen in Figure 1B, the peak at 579.0cm-1 was assigned to the Fe-O bond of Fe3O417. In addition, the strongly absorbing peaks at 476.0cm-1, 1096.0cm-1 and 1632.0cm-1 were assigned to the vibration of Si-O-H bending, Si-O-Si stretching and H-O-H bending vibrations of N-octyl trimethylsilyl, respectively18. The absorbing bands around 2920.5cm-1 and 2849.0cm-1 were assigned to the CH2 of N-octyl trimethylsilyl. The absorbing peak at 1470.0cm-1 was assigned to the C-H of CTAB19. These results confirmed that the



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composition of Fe3O4, SiO2, N-octyl trimethylsilyl and CTAB all presented the nanocomposites. The VSM of pure Fe3O4 nanoparticles and Fe3O4@OTS-SiO2 nanocomposites was also compared as shown in Figure 1C. They both showed super-paramagnetic character. The saturation magnetization (Ms) of pure Fe3O4 nanoparticles and Fe3O4@OTS-SiO2 nanocomposites was about 56.8emu/g and 16.7emu/g, respectively. The much lower Ms of the Fe3O4@OTS-SiO2 nanocomposites comparing to pure Fe3O4 nanoparticles could be attributed to the presence of nonmagnetic SiO2, N-octyl trimethylsilyl and CTAB in the nanocomposites. Figure 1. The micro-structure of Fe3O4@OTS-SiO2 nanocomposites was further determined by the SEM images as shown in Figure 2A-B. As shown in Figure 2A, Fe3O4 nanoparticles showed uniform size of ca.35.0nm and uniformly spherical shape. After coating with SiO2, the size of spherical particles obviously increased to 60.0nm~95.0nm. The change of size further confirmed that the SiO2 was coated on surface of Fe3O4 nanoparticles, in which the thickness of SiO2 shell was about more than 20.0nm. The core-shell structure of Fe3O4@OTS-SiO2 nanocomposites was firstly confirmed by the XPS spectrum as shown in Figure 2C. The wide-scan spectrum clearly showed the signals of O, C, and Si element, which was assigned to SiO2 and OTS, respectively. It was interesting that the signal of Fe element was not observed. As well-known, the element of surface within ca. 15.0nm can be detected by the XPS method as shown in Figure 2D20. So, the absence of Fe element further confirmed core-shell structure of Fe3O4@OTS-SiO2 nanocomposites. The core-shell structure was further characterized by the TEM image as shown in Figure 2E. The homogenous core-shell structure of the Fe3O4@OTS-SiO2 nanocomposites was clearly observed. The Fe3O4@OTS-SiO2 nanocomposites were well formed spheres with an approximate diameter of 80-100.0nm. The size of core and shell was about 22.0nm and 40.0nm, respectively, which were almost consistent with the results of SEM images. Furthermore, it clearly showed that the Fe3O4@OTS-SiO2 nanocomposites were consisted of dendrimeric fibers, resulting from self-assembly of SiO2 nanoparticles. However, the organic molecules of OTS were difficult to be observed due to form thin film. These results further indicated the formation of Fe3O4@OTS-SiO2 nanocomposites with core-shell structure. Figure 2. The surface micro-structure of hydrophobic surface based on nanocomposite coating and multiple hybrid coatings at the same chemical composition were further characterized and compared by the SEM images and non-contact white light profilometer as shown in Fig.3. Generally, the superhydrophobic surface was mainly based on nanocomposite coatings12,21-23. The nanoparticles and organic resin was firstly mixed to form paints, which was further coated on surface of substrate. In the model of nanocomposite coatings (in Figure 3A ), surface roughness was required to obtain superhydrophobicity, resulting from nanoparticles dispersed in organic resins23. In addition, the superhydrophobic surface based on


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nanocomposite coatings can be large-scale preparation by the spraying method. However, the nanocomposite coating based on Fe3O4@OTS-SiO2 and epoxy resin showed relatively low roughness (ca.0.13μm) as shown in Figure 3B and Figure 3E. It was well-known that surface roughness of nanocomposite coatings strongly depended on the chemical composition of paints and coating process11-23. So, the chemical composition and coating process should be further optimized to obtain enough large surface roughness. In addition, the nanocomposite coatings showed also relatively high surface energy (ca. 43.0mN/m). It was due to that the Fe3O4@OTS-SiO2 nanocomposites were easily coated by the epoxy resin with relatively high surface energy. Therefore, the present nanocomposite coating based on Fe3O4@OTS-SiO2 showed low contact angle (ca. 68.6o) and hydrophilicity as shown in inset of Fig.3B. These results indicated that the superhydrophobic surface was still difficultly obtained by means of nanocomposite coatings22. The superhydrophobic surface based on multiple hybrid coatings was further designed and proposed as shown in Figure 3A, too. In the model, the nanoparticles were directly coated and adhesion on surface of epoxy resin. As shown in inset of Figure 3C and Figure 3F, although the multiple hybrid coatings showed also relatively low surface roughness (ca.0.24μm), yet, it showed excellent superhydrophobicity (CA=174.5o)(in sMovie 1). The multiple hybrid coatings based on Fe3O4@SiO2 nanocomposites showed similar surface roughness (SA=0.21μm) as shown in Figure 3G, and lower hydrophobicity (CA=133.7o) comparing to Fe3O4@OTS-SiO2 nanocomposites. According to these results, the superhydrophobicity of present multiple hybrid coatings was mainly due to low surface energy (ca.7.2mN/m), and was few contributed by the surface roughness. From the perspective, superhydrophobicity of multiple hybrid coatings was few affected by the coating process, the superhydrophobic surface based on multiple hybrid coatings was more facile preparation comparing to nanocomposite coating. Figure 3. 3.2. Stability of superhydrophobic surface based on multiple hybrid coatings (1) Mechanical stability Generally, most of superhydrophobic surfaces were poor mechanical durability, which restricted their real applications in industry1,6. Here, the mechanical stability was also characterized by the mechanical abrasion test. As shown in Figure 4A and inset of Figure 4C, the multiple hybrid coatings still showed good superhydrophobicity (CA=161.8o) after 100th abrasion cycle. Figure 4B shows the water contact angles as function of abrasion cycles. The water contact angles slightly decreased from 173.3o to 161.8o after 80 abrasion cycles and then was almost no change with further increasing in abrasion cycles. By comparing the mechanical abrasion result of the superhydrophobic multiple hybrid coatings to that of others superhydrophobic surfaces as shown in Table 1. It clearly showed that present superhydrophobic multiple hybrid coatings had the best mechanical stablity. The result was attributed to that the surface



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energy (ca.11.6mN/m) of superhydrophobic surface was slight increased after mechanical abrasion comparing to superhydrophobic surface without abrasion (ca.7.2mN/m). Furthermore, it showed similar micro-structure for present superhydrophobic surface before and after abrasion as shown in Figure 4C and Figure 3C, respectively, which was also responsible for the slight change of superhydrophobicity. These results indicated the formation of superhydrophobic surface based on multiple hybrid coatings with good mechanical stability . As well-known, the real sea water was flow, producing a fluid force. It attacked the surface of material substrate, which also damaged superhydrophobic surface24. So, the mechanical stability of superhydrophobic surface should be also evaluated by the flowing water test. The flowing water test was carried out on the glass beaker, in which the sample was immersion in flowing NaCl aqueous solutions containing sands under stirring (in inset of Figure 4D). Figure 4D shows the water contact angle of present superhydrophobic surface as function of immersion time. The water contact angle was almost no change over time, in which a contact angle of 173.0o ±2.0 was still maintained after 60.0 min. These results confirmed that superhydrophobic surface based on multiple hybrid coatings also showed good mechanical stability in flowing water. Although, there was some works reporting the good mechanical stability of superhydrophobic surface16,25, yet, it showed relatively lower superhydrophobicity. Furthermore, the superhydrophobic surface was generally based on the fluorinated polymer, which was difficult to synthesize and high cost. These problems also restricted its large-scale industrial applications. Contrarily, the present superhydrophobic surface was not only good mechanical stability, but also good superhydrophobicity and low cost. It is believed that present superhydrophobic surface based on multiple hybrid coatings can be used in various applications due to be good mechanical stability. Figure 4. Table 1. (2) Chemical stability The chemical stability of superhydrophobic surface based on multiple hybrid coatings was also important for their industrial application due to that the anti-corrosion coating was often used in chemical environment15,

21.

Therefore, the chemical stability of present superhydrophobic surface was also

determined by immersion in static NaCl aqueous solutions (3.5wt%), strong acid solution (pH=0) and strong alkaline solution (pH=14.0). As shown in Figure 5A-C, the water contact angle of superhydrophobic surface was almost no change over time, in which a contact angle of 174.0±0.5o was still maintained after 10 days. In addition, a spherical water droplet was clearly observed on superhydrophobic surface after immersion in NaCl aqueous solutions for 10 days as shown in inset of Figure 5. From a wettability perspective, present superhydrophobic surface possessed good chemical stability in NaCl aqueous solutions, acidic and alkaline environments. By comparing the chemical stability of the superhydrophobic multiple


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hybrid coatings to that of previous superhydrophobic surfaces as shown in Table 2. Present superhydrophobic multiple hybrid coatings also showed better chemical stability comparing to previous superhydrophobic surfaces. These results were also attributed to that the surface energy of present superhydrophobic surface showed slight change after immersion in NaCl aqueous solutions (ca.7.6mN/m), acidic (ca.8.0mN/m) and alkaline (ca.7.8mN/m) environments comparing to superhydrophobic surface without treatment. Figure 5. Table 2. 3.3. Anti-corrosion performance of superhydrophobic surface based on multiple hybrid coatings Figure 6A shows the optical photograph of water droplets and water contact angel on the bare metal and

surface-coated

metal.

The

multiple

hybrid

coatings

based

on

Fe3O4@SiO2/EP

and

Fe3O4@OTS-SiO2/EP both showed hydrophobicity1. In a comparison, the bare metal and pure epoxy coating showed hydrophilicty1. The Tafel polarisation curves of bare metal (Q235) and surface-coated metal with various hydrophobicity were further determined and compared as shown in Figure 6B. The values of Icorr, Ecorr, Rp and Vcorr were calculated and concluded in Table 3. The Ecorr was shifted in the positive direction with increasing in hydrophobicity as shown in Figure 6B. Especially, the Ecorr shifted positively for about 0.72V when the anodized Fe was covered with superhydrophobic surface as shown in Table 3. This suggested that the superhydrophobic surface based on multiple hybrid coatings mainly retarded the dissolution of Fe between the interface of the Fe surface and seawater26. In a comparison, the Icorr, Rp and Vcorr of surface-coated metal were far lower than that of bare metal. At the same time, the Icorr, Rp and Vcorr of surface-coated metal obviously decreased with increasing in hydrophobicity of coating as shown in Table 3. The Icorr, Rp and Vcorr of super-hydrophobic surface coated metal were all reduced by about seven orders of magnitude comparing to bare metal (in Table 3). As well-known, the anti-corrosive coating with lower Icorr or Vcorr showed a better anti-corrosive performance1,

21.

Obviously, according to the test results, the superhydrophobic surfaces showed more

effective protection of metal immersed in seawater comparing to other anti-corrosive coatings with low hydrophobicity. The result was attributed to the formed air films between superhydrophobic surface and water as shown in Figure 6C. Contrarily, it was difficult to form air films for low hydrophobic surface or hydrophilic surface. The air films formed an additional nonconductive barrier and isolated layer, improving the overall anti-corrosive performance1, 27. In addition, a additional ground epoxy resin coating was also important for good anti-corrosive performance14. These results indicated that present superhydrophobic surface based on multiple hybrid coatings could be used in corrosion protection industry. Figure 6. Table 3.



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The good anti-corrosion performance of superhydrophobic surface based on multiple hybrid coatings was also confirmed by the EIS. Figure 7A shows the Nyquist plots of all samples in presence of 3.5wt% NaCl aqueous solution. The Nyquist plot was obviously consisted of a high-frequency semicircle and a low-frequency straight line for the bare metal (in Figure 7B), suggesting that a diffusion reaction was the main process28. In a comparison, there was a Warburg characteristic properties in the low frequency for surface-coated metal, which was due to the restriction of diffusion reaction between corrosion products and metal. This result indicated the good barrier property of present superhydrophobic surface29. As well-known, the high-frequency semicircle represented the charge transfer resistance (Rct) at the metal/aqueous solution interface28. A larger semicircle showed a higher Rct value, indicating a longer path of the ion diffusion from aqueous solution to metal substrate. As shown in Figure 7A and 7B, the Rct value of bare metal was far smaller than that of surface-coated metal. Especially, the Rct value (170.2MΩ·cm2) of superhydrophobic surface coated metal was improved by about six orders of magnitude comparing to bare metal. In addition, the Rct value of surface-coated metal was shown to increase with increasing in hydrophobic performance of coating. Generally, it is a larger Rct value, indicating a lower corrosion rate30. Figure 7C shows impedance module plots of all samples immersed in NaCl aqueous solution. It clearly also showed that modulus of the surface-coated metal increased with increasing in hydrophobic performance. The modulus of superhydrophobic surface coated metal was improved to be six orders comparing to bare metal. The phase angle plots of all samples were also compared as shown in Figure 7D. The phase angle value of present superhydrophobic surface-coated metal was also obviously larger comparing to other samples at high frequencies, indicating good barrier behavior of superhydrophobic surface31. These results further confirmed the formation of superhydrophobic surface with good anti-corrosion performance. Figure 7. The long-term stability of superhydrophobic surface immersed in NaCl aqueous solution was a crucial parameter for the real application in corrosion protection of metal. Figure 8 shows Tafel polarization curves of surface-coated metal before and after immersion in NaCl aqueous solution for 10 days. The Ecorr of low hydrophobic or hydrophilic surface (eg. pure EP and Fe3O4@SiO2/EP) coated metal was obviously shifted to negative direction after 10 days as shown in Figure 8A and B, respectively. Furthermore, the Icorr and Rp also increased and decreased by about one order of magnitude after 10 days, respectively. In a comparison, the Tafel polarization curves of superhydrophobic surface coated metal were slight change before and after immersion in NaCl aqueous solution. The Icorr and Rp also slight increased, which was both in the order of 10-11 before and after immersion in NaCl aqueous solution as shown in Table 3. These results indicated the formation of superhydrophobic surface with the long-term stability in seawater, and


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the long-term stability strongly depended on hydrophobicity of coatings. So, when designing the long-term anti-corrosion coating, the hydrophobicity should be also considered. Figure 8. The long-term stability of present superhydrophobic surface in corrosion protect of metal was further confirmed by EIS measurement as shown in Figure 9. The high-frequency semicircle was obviously reduced for pure EP and Fe3O4@SiO2/EP coated metal after 10 days as shown in Figure 9A and B, respectively. The Rct value of surface-coated metal was reduced by about one order of magnitude as shown in Table 3. In a comparison, the Fe3O4@OTS-SiO2/EP coated metal showed similar Nyquist plots before and after immersion in NaCl aqueous solution as shown in Figure 9C. Furthermore, the Rct value was almost no change as shown in Table 3. It was well-known that the good corrosion protection of superhydrophobic surface was mainly due to form air film1, 27. So, the stability of air films was key role for the long-term corrosion protection of superhydrophobic surface. However, most of air film were only maintained for several hours and few days32,33. The long-term stability of present superhydrophobic surface in corrosion protect was mainly attributed to underwater stability of air film between superhydrophobic surface and surrounding water, restricting the diffusion of ion and oxygen in water to metal substrates. Even, the air film still remained no change after immersion in NaCl aqueous solution for 10 days as shown in Figure 9D. The good underwater stability was firstly attributed to that the superhydrophobicity of present superhydrophobic surface mainly resulted from low surface energy. In addition, the low roughness of present coating was also key role for the good underwater stability of air film. Generally, the superhydrophobic surface was based on micro and nanoscale hierarchical structure with high roughness. Compared to a low roughness, the air firm was easily damaged and water could easily penetrate into the hierarchical structure with high roughness1, 34. All results further confirmed that present superhydrophobic surface also enhanced long-term stability of anti-corrosion coatings. Figure 9. 3.4. Reproduction stability of superhydrophobic surface based on multiple hybrid coatings The reproduction stability of superhydrophobic surface was very important for real application in corrosion protection. However, reproduction stability of superhydrophobic surface was generally poor due to that the surface roughness or micro-structure was difficult to be precisely controlled. Here, the reproduction stability of the superhydrophobic surface based on multiple hybrid coatings towards superhydrophobicity and corrosion protect was also evaluated as shown in Figure 10. It clearly showed that the water contact angle was a slight change for the reproduction 3 samples as shown in Figure 10A. The relative standard deviation (RSD) value of the water contact angle was calculated to be about 0.14%. The Tafel polarisation curves and Nyquist plots of the reproduction 3 samples were also characterized and compared as shown in Figure 10B and Figure 10C, respectively. All samples showed the similar Tafel



Page 12 of 33

polarisation curves and Nyquist plots. Furthermore, the Ecorr, Icorr and Rct were also slight change as shown in Table 4. The RSD value of the Ecorr and Rct was about 1.8% and 2.4%, respectively. These results indicated that the superhydrophobic surface based on multiple hybrid coatings also exhibited good reproduction stability. Figure 10. Table 4. The anti-corrosion performance of the present and previous superhydrophobic surfaces was concluded as shown in Table 5. It could be observed that the present superhydrophobic surface showed larger water contact angle, lower Icorr and larger Rp comparing to previous works5-13, 21-23, 35-48. Here, the higher superhydrophobicity and better anti-corrosion property was attributed to multiple hybrid structure as shown in Scheme 3. Firstly, the thin topcoat based on nanoparticles provided superhydrophobicity, resulting from low surface energy and island nano-structure. The air film between superhydrophobic surface and surrounding water formed an additional nonconductive barrier, which effectively retard the diffusion process of ion and oxygen in water. Secondly, the epoxy resin was acted as protection coating, which also effectively retard the diffusion of ionic and oxygen in water into metal substrate. Even if the superhydrophobic topcoat is damaged, the epoxy resin can still provide corrosion protection at the damaged region. These can effectively restrict ionic diffusion between anodic and cathodic areas, suppressing the formation of corrosion products. Furthermore, present superhydrophobic surface also showed better good mechanical and chemical stability comparing to previous superhydrophobic surface as shown in Table 1 and 2. It was also attributed to multiple hybrid structure of superhydrophobic surface. Firstly, the superhydrophobicity of multiple hybrid coatings was mainly based on low surface energy, and slightly affected by the surface roughness. As well-known, the surface energy mainly depended on the chemical structure, which was good mechanical and chemical stability. So, even, the surface roughness of superhydrophobic surface was reduced, the superhydrophobicity was also slight reduced. Secondly, the nanoparticles were adhesive on surface of metal by the epoxy resin, enhancing the mechanical and chemical stability of superhydrophobic surface as shown Scheme 3. Thirdly, the organic molecule with low surface energy was chemically grafted to surface of nanoparticles as shown in Scheme 1. It obviously showed better mechanical and chemical stability comparing to nanoaprticles modified organic molecule by physical modification. Scheme 3. Table 5. 4. CONCLUSIONS In summary, a new class superhydrophobic surface based on multiple hybrid coatings was proposed and prepared by the spraying method for application in corrosion protect. The superhydrophobic surface


Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


showed a robust resistance to mechanical abrasion and chemical immersion. Furthermore, it also exhibited

good reproduction stability and excellent an-corrosion performance. Therefore, the superhydrophobic surface based on multiple hybrid coatings has the potentiality of large-scale production for industrial use.. ACKNOWLEDGMENTS The authors are grateful for the support of the National Natural Science Foundation of China under grants (51773184 and U1810114), and the Shanxi Provincial Natural Science Foundation of China (201701D121046 and 201803D421081). SUPPORTTING INFORMATION AVAILABLE: Additional video of water droplets on the superhydrophobic surface based on multiple hybrid coatings. REFERENCES (1)Zhang, D.; Wang, L.; Qian, H.; Li, X. Superhydrophobic Surfaces for Corrosion Protection: A Review of Recent Progresses and Future Directions. J. Coatings Technol. Res. 2016, 13, 11–29. (2)Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust Omniphobic Surfaces. Proc. Natl. Acad. Sci. 2008, 105, 18200–18205. (3)Yuan, S.; Pehkonen, S. O.; Liang, B.; Ting, Y. P.; Neoh, K. G.; Kang, E. T. Superhydrophobic Fluoropolymer-Modified Copper Surface via Surface Graft Polymerization for Corrosion Protection. Corros. Sci. 2011, 53, 2738–2747. (4)Wu, C.; Liu, Q.; Liu, J.; Chen, R.; Takahashi, K.; Liu, L.; Li, R.; Liu, P.; Wang, J. Hierarchical Flower like Double-Layer Superhydrophobic Films Fabricated on AZ31 for Corrosion Protection and Self-Cleaning. New J. Chem. 2017, 41, 12767–12776. (5)Niu, S.; Fang, Y.; Qiu, R.; Qiu, Z.; Xiao, Y.; Wang, P.; Chen, M. Superhydrophobic Film Based on Cu-Dodecanethiol Complex: Preparation and Corrosion Inhibition for Cu. Colloids Surfaces A Physicochem. Eng. Asp. 2018, 550, 65–73. (6)Ma, Q.; Tong, Z.; Wang, W.; Dong, G. Fabricating Robust and Repairable Superhydrophobic Surface on Carbon Steel by Nanosecond Laser Texturing for Corrosion Protection. Appl. Surf. Sci. 2018, 455, 748–757. (7) Peng, F.; Wang, D.; Ma, X.; Zhu, H.; Qiao, Y.; Liu, X. “ Petal Effect ” -Inspired Superhydrophobic and Highly Adhesive Coating on Magnesium with Enhanced Corrosion Resistance and Biocompatibility. Sci. China. Mater. 2018, 61, 629–642. (8) Yin, B.; Fang, L.; Tang, A. Q.; Huang, Q. L.; Hu, J.; Mao, J. H.; Bai, G.; Bai, H. Novel Strategy in Increasing Stability and Corrosion Resistance for Super-Hydrophobic Coating on Aluminum Alloy Surfaces. Appl. Surf. Sci. 2011, 258, 580–585. (9)Xiang, T.; Zheng, S.; Zhang, M.; Sadig, H. R.; Li, C. Bioinspired Slippery Zinc Phosphate Coating for Sustainable Corrosion Protection. ACS Sustain. Chem. Eng. 2018, 6, 10960–10968.



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Table 1. Mechanical stability of superhydrophobic surfaces with various structures. Structure/Chemical composition

Load

Abrasion distance (cm)

Stability (Contact angle)

Reference

Fe3O4@OTS-SiO2/EP multiple hybrid coatings

100g

1000

173.3° to 161.8°

Present work

Porous carbon steel/ perfluorodecyltriethoxysilane Cu-Zn/hierarchical structured Porous MWCNTs-Co/a-C:H film Porous Au film

100g

200

161o to 156o

6

100g 100g

10 200

>150o 158° to 150°

34 40

300g

200

169° to 106°

43

Table 2. Chemical stability of superhydrophobic surfaces with various structures. Structure/Chemical composition Fe3O4@OTS-SiO2/EP multiple hybrid coatings Double-layer/octadecyltric hlorosilane (OTS) PorousMg(OH)2/Mg-Al/so diumoleate Porous SiO2/fluoroalkylsilane SiO2/PFOTES ASO/PTW-SiO2/PPS nanocompositescoating Porous Au film

pH

Immersion time 10 days

Stability (Contact angle) 174±0.5°

Reference

1.0h

160°~156.5°

4

1~13(0.1mol/L, NaOH -----or HCl solutions) 1~13 1.0h

62°~150°

7

144o~157o

11

1~14(HCl solutions) 1~14

and

NaOH ------

>155o

12

12.0h

>155o

21

1~14(HCl solutions)

and

NaOH ------

>150o

43

0~14 (2.0mol/L, HCl and NaOH solutions) 2~12


Present work

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Analysis results of Tafel polarisation curves and electrochemical impedance spectroscopy of superhydrophobic surfaces with various structures. Sample Bare Metal Pure EP

0d 10d

Fe3O4@Si

0d

O2/EP

10d

Fe3O4@OT

0d

S-SiO2/EP

10d

Contact

Ecorr

Icorr

Rp

Rct

Vcorr

angle

(V)

(A/cm2)

(MΩ cm2)

(MΩ cm2)

(mm/year)

67.3o

-1.02

2.5×10-4

3.0×10-3

3.8×10-4

2.9

-0.38

3.1×10-9

346.9

12.9

3.6×10-5

-0.55

4.2×10-8

29.3

1.1

4.9×10-4

-0.31

2.1×10-10

4998.0

77.6

2.5×10-6

-0.55

3.5×10-9

326.3

1.1

4.9×10-4

-0.18

1.4×10-11

7.9×104

170.2

1.6×10-7

-0.19

9.0×10-11

1.2×104

161.6

1.1×10-6

62.6o 133.7o 174.5o

Table 4. Analysis results of three Fe3O4@OTS-SiO2/EP samples with same preparation process. Sample

Fe3O4@OTS-SiO2/EP RDS(%)

Contact angle

Ecorr

Icorr

Rct

Vcorr

(V)

(A/cm2)

(MΩ cm2)

(mm/year)

1st

174.5o

-0.18

1.4×10-11

170.2

1.6×10-7

2ed

173.8o

-0.19

2.4×10-11

160.5

2.8×10-7

3rd

174.2o

-0.18

1.4×10-11

168.6

1.6×10-7

0.14

1.8

-------

2.4

-------



Page 20 of 33

Table 5.The water contact angle and electrochemical parameters of superhydrophobic surfaces with various compositions and structures. Preparation

Chemical

composition/ Contact

methods

Structure

Spraying

Fe3O4@OTS-SiO2/EP

Icorr

Rp

angle (o)

(A/cm2)

(MΩ cm2)

174.5

1.4×10-11

7.6×104

multiple hybrid coatings

Reference Present work

108.0

5.3×10-8

1.2

22

163.0

2.2×10-8

------

23

160.0

3.3×10-9

9.9

12

161.0

3.3×10-11

1.6×103

21

1.2×10-7

0.18

6

167.0

4.1×10-7

5.6×103

41

153.6

×10−6

------

45

156.0

3.2×10-9

11.3

5

165.0

2.6×10-8

------

46

Pore PAni/CS/ZS

150.7

8.7×10-8

1.1

42

Porous Al2O3/Triethox-

167.7

5.1×10−7

------

8

161.5

2.3×10−9

------

47

Biomimetic polyaniline arry

160.0

5.0×10−8

1.7

13

Epoxy-acrylate leaf-like arry

153.0

2.3×10−6

9.4×10-3

48

Ni arry Porous

156.0

2.6×10-7

0.7

39

MWCNTs-Co/a-C:H film

158.1°

4.2×10-10

2.0

40

PDMS/TiO2 nanocomposites coating TiO2/PMMA nanocomposites coating SiO2/PFOTES nanocomposites coating ASO/PTW-SiO2/PPS nanocomposites coating laser etching

Porous

carbon

steel/ 161.1

perfluorodecyltriethoxysilane chemical etching

Porous CuO/Perfluorodecyltrichoxysilane

Wet

chemical Porous complex Cu(II)-

reaction

laurylamine Porous Cu-dodecanethiol complex Porous Fe(CH3(CH2)12COO)3

Anodization

yoctylsilane Porous Al2O3/perfluorooctadecanoic acid

Templating

Electrodeposition


Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Porous Ni-P/Cu/Ni

155.0

4.0×10-6

------

10

Porous Zinc

156.6

2.0×10-11

1.1×104

9

169.0

5.2×10-5

3.0×10-2

43

158.0

3.7× 10-8

4.2

11

PVDF/SA 155.0

8.1×10-9

8.7

35

phosphate/fluoroalkylsilane Chemical

Porous Au film

deposition Sol-gel

Porous SiO2/fluoroalkylsilane

Electrospinning

Porous nanofibrous

Porous Cu2O/stearic acid

157.7

8.0×10-9

2.5

36

Micro-arc

Porous

159.0

3.5×10−8

~1.0

37

oxidation

MAO-(PA@Ce)n-FAS Porous MAO/SA

155.5

5.4×10-8

10.0

44

Hydrothermal

Porous

151.2

9.8× 10-7

1.2

7

method

Mg(OH)2/Mg-Al/sodiumolea 162.0

1.4×10-9

3.7

38

Etchinghydrothermal

te Dealloying

WO3@TiO2 nanoflake arry

/liquid deposition



Page 22 of 33

Scheme 1. Schematic preparation process of Fe3O4@OTS-SiO2 nanocomposites.

TEOS Fe3O4

EtOH/H2O/NH3

SiO2

Fe3O4

CTAB Urea/1-amyl alcohol/ Cyclohexane/H2O

120 ℃ TEOS /5h CTAB

N-octyl trimethylsilyl 120℃/20h

N-octyl trimethylsilyl TEOS

Scheme 2. Schematic preparation process of superhydrophobic surface based on multiple hybrid coatings.


Page 23 of 33

Scheme 3. Schematic corrosion protection mechanism of superhydrophobic surface based on multiple hybrid structure. ClO2

NaCl solution

Superhydrophobic surface

Air film

Polymer adhesive coating

Metal

280

A

SiO2

40

B

-1 579.0cm 472.0cm

2849.0cm

-1

-1

Fe3O4

-1

-1 30 3444.0cm 2920.5cm

200

1632.0cm -1 1470.0cm -1

T(%)

Intensity(a.u)

240

160

20

120 1096.0cm

80

10

20

30

40

2Theta/ 60

o

50

60

70

10 4000

3000

2000

-1

1000 -1

Wavenumber(cm )

C

40

Ms(emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

20

b

0 -20 -40 -60 -10000

-5000

0

Applied (Oe)

5000

10000

Figure 1. (A) XRD and (B) FT-IR spectrum of Fe3O4@OTS-SiO2 nanocomposites, (C) VSM curves of (a) pure Fe3O4 nanoparticles and (b) Fe3O4@OTS-SiO2 nanocomposites. The inset of (B) is the optical photograph of Fe3O4@OTS-SiO2 dispersion solution with and without magnet.



A

Page 24 of 33

B

85.0nm 35.0nm

D

O1s

C

X-rays

OTS

Electron C1s

ca.20.0nm O(A)

Fe3O4 35.0nm

Si2p Si

2s

0

>20.0nm 300

600

900

SiO2

1200

Binding energy(eV)

E

Shell Core

Core

Figure 2. SEM image of (A) pure Fe3O4 nanoparticles and (B) Fe3O4@OTS-SiO2 nanocomposites, (C) XPS spectrum of Fe3O4@OTS-SiO2 nanocomposites, (D) Schematic illustration of the XPS test of Fe3O4@OTS-SiO2 nanocomposites, (E) TEM image of Fe3O4@OTS-SiO2 nanocomposites.


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A a

B

Water Metal Water

b

C

CA:68.6o

resin

nanoparticle

Metal

500nm

CA:174.5o

D

500nm

500nm

E

SA=0.13μm

G

SA=0.21μm

CA:137.7o

F


SA=0.24μm


Figure 3. (A) Schematic superhydrophobic surface based on (a) multiple hybrid and (b) nanocomposite coatings, SEM image of (B) nanocomposite coatings and (C) multiple hybrid coatings based on Fe3O4@OTS-SiO2, (D) multiple hybrid coatings based on Fe3O4@SiO2; Non-contact white light profilometer of (E) nanocomposite coatings and (F) multiple hybrid coatings based on Fe3O4@OTS-SiO2, (G) multiple hybrid coatings based on Fe3O4@SiO2. 180

B a

b

Moving

Contact angless(degree)

A

CA:167.5o

170 160 150 140 130

0

20

40

60

80

100

Cycle 180

C

D

500nm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

170 160 150

Sample

140 Water containing sands

130

0

15

30

45

60

Time(min)

Figure 4. (A) optical photographs of (a) sandpaper abrasion test, (b) the superhydrophobicity of multiple hybrid coatings after 100th abrasion cycle (load 50.0g), (B) water contact angle of multiple hybrid coatings as function of abrasion cycle, (C) SEM image of multiple hybrid coatings after 100th abrasion cycles, (D) water contact angle of multiple hybrid coatings as function of immersion time in flowing NaCl aqueous solutions containing sands (10.0mg/mL). The inset of (C) is optical photograph of water droplets on the hydrophobic surface. The inset of (D) is the optical photograph of flowing water test.


Page 27 of 33

180

A Contact angless(degree)


180 170 160 150 140 130

0

180


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

4

6

Immersion Time (d)

8

10

B

170 160 150 140 130

0

2

4

6

Immersion Time (d)

8

10

C

170 160 150 140 130

0

2

4

6

8

10

Immersion Time (d)

Figure 5. The contact angle of superhydrophobic surface as function of immersion time in (A) static NaCl aqueous solutions (3.5wt%), (B) acid solution (pH=0) and (C) alkaline solution (pH=14.0). The inset of (A-C) is optical photograph of water droplets on the hydrophobic surface.



A 67.3o

-2

a

b 174.5o

133.7o c

d

-2

a

B

-4

62.6o

LogI(A cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

-6

b

-8

d

-10

c

-12

C

b

a

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

ESCE(V)

c

d Air film

Figure 6. (A) optical photograph of water droplets on surface of bare metal (a) without coating and with coating of (b) pure epoxy, (c) Fe3O4@SiO2/EP, (d) Fe3O4@SiO2/OTS/EP, (B) potentiodynamic polarization curves and (C) optical photograph of bare metal (a) without coating and with coating of (b) pure epoxy, (c) Fe3O4@SiO2/EP, (d) Fe3O4@SiO2/OTS/EP immersed in 3.5wt% NaCl aqueous solution. The inset of (A) is the profile images of the water contact angle of corresponding coating.


Page 29 of 33

A

0.16

2

60

-Z''/KΩ cm

-Z''/MΩ cm

2

80

c

40

a 0

30

60

90

120

Z'/MΩ cm

0.08

a 0.00 0.0

180

0.1

2

0.2

Z'/KΩ cm

0.3

D

C

c b

a 2

d

c

80

4

b

60 40 20

a

0

0

-2

0.4

2

100

Phase angle/deg

2

150

d

6

0.12

d

b

0

8

B

0.04

20

Log(|Z|/MΩ cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

0

1

2

3

4

5

-2

Log(f/Hz)

0

2

4

6

Log(f/Hz)

Figure 7. (A) Nyquist plots of (a) bare metal and metal coated with various surfaces of (b) pure epoxy, (c) Fe3O4@SiO2/EP, (d) Fe3O4@OTS-SiO2/EP; (B) Enlarged Nyquist Plots of bare metal, (C) Bode plots of |Z| vs. frequency and (D) Bode plots of phase angle vs. Frequency for (a) bare metal and metal coated with various surfaces of (b) pure epoxy, (c) Fe3O4@SiO2/EP, (d) Fe3O4@OTS-SiO2/EP.



-6

-6

A

B

-7 -2

LogI(A cm )

-2

LogI(A cm )

-7 -8

0d 10d

-9

0d 10d

-12

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

ESCE(V)

-2

-9

-11

-11

-9

-8

-10

-10

LogI(A cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

0.2

0.0

-0.2

-0.4

-0.6

-0.8

ESCE(V)

C

-10 0d 10d

-11 -12 -13 0.2

0.0

-0.2

-0.4

-0.6

ESCE(V)

Figure 8. Tafel polarization curves of metal coated with various surfaces of (A) EP, (B) Fe3O4@SiO2/EP and (C) Fe3O4@OTS-SiO2/EP after immersion in 3.5wt% NaCl aqueous solution for 0 and 10 days.


Page 31 of 33

6.0

A

2

-Z''/MΩ cm

2

-Z''/MΩ cm

3.0

1.5

0.0

0d 10d

30

20

10

0

2

4

6

8

Z'/MΩ cm

2

10

12

14

0

0

20

40

Z'/MΩ cm 0d 10d

C

80

2

B

40

0d 10d

4.5

-Z''/MΩ cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D

0d

60

80

2

5d

60

40

10d

20

0

0

30

60

90

120

Z'/MΩ cm

150

180

2

Figure 9. Nyquist plots and Impedance module plots of metal coated with various surfaces of (A) EP, (B)Fe3O4@SiO2/EP and (C)Fe3O4@OTS-SiO2/EP after immersion in 3.5wt% NaCl aqueous solution for 0 and 10 days; (D) the optical photograph of air film on hydrophobic surface as function of immersion time.



-9

A

160

B

-10 -2

LogI(A cm )

Contact angle(deg)

200

120 80

1st 2ed 3rd

-11 -12 -13

40

-14

0

1

100

2

3

C

0.2

0.0

-0.2

-0.4

-0.6

ESCE(V)

1st 2ed 3rd

2

80

-Z''/MΩ cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

60 40 20 0

0

30

60

90

120

Z'/MΩ cm

150

180

2

Figure 10. (A) Water contact angle, (B) Tafel polarisation curves and (C) Nyquist plots of three superhydrophobic surfaces prepared by the same process. The inset of (A) is optical photograph of water droplets on hydrophobic surface and the profile images of the water contact angle.


Page 33 of 33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Robust superhydrophobic surface based on multiple hybrid coatings

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