Large-scale preparation of superhydrophobic cerium dioxide nanocomposite coating with UV resistance, mechanical robustness, and anti-corrosion properties

Surface & Coatings Technology 384 (2020) 125312

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Large-scale preparation of superhydrophobic cerium dioxide nanocomposite coating with UV resistance, mechanical robustness, and anti-corrosion properties

T

Kai Ana,b, Cai Longa,b, Yi Suia,b,e, Yongquan Qinga,b, Guangzhen Zhaoc, Zhenqiang Ana,b, ⁎ Linshan Wangd, Changsheng Liua,b, a

School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China Energy Resources and Power Engineering College, Northeast Electric Power University, Jilin 132012, China d College of Science, Northeastern University, Shenyang 110819, China e National Key Laboratory for Research and Comprehensive Utilization of Rare Earth Resources in Baiyun Obo, Baotou 014030, China b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Superhydrophobicity CeO2 UV-resistance Durability Self-cleaning Anti-corrosion

Superhydrophobic coatings have been developed widely, because of their self-cleaning, anti-fouling, and antiicing properties. However, there are critical challenges in terms of poor durability in harsh environments. Herein, we report an effective and simple strategy for large-scale fabrication of durable superhydrophobic composite coating, based on polydimethylsiloxane filled with fluoroalkyl silane functionalized cerium dioxide. The obtained composite coating showed satisfactory water repellency with water contact angle of 161 ± 2° and sliding angle of 4 ± 1°, mainly attributed to the construction of multilevel micro/nano scale structure. Such coatings possessed outstanding chemical stability, self-cleaning property, and adhesion strength. Notably, the coating was UV resistant and mechanically robust, which could also retain good superhydrophobicity after 240 h of UV irradiation, 30 abrasion cycles, and scratching, and could be applied to a variety of substrate surfaces. In addition, according to electrochemical test results, the superhydrophobic composite coating showed excellent corrosion barrier and active corrosion inhibition effect, due to synergy between trapped air films and cerium dioxide. These multiple key properties integrated into our superhydrophobic composite coating are expected to provide a wide range of applications in various extreme outdoor environments.

1. Introduction

amino silane film and modified it with CeO2 nanoparticles to improve the barrier and corrosion inhibition properties of composite films for carbon steel. Zhao et al. [20] added CeO2 nanoparticles into a multilayered coating using layer-by-layer self-assembly process on Mg alloys. The coating provided both corrosion resistance and self-healing ability. However, only few studies have focused on finding applications of cerium oxide for other properties of coatings, such as superhydrophobicity and UV resistance. Therefore, there is a vast opportunity and challenge to further explore the properties and applications of cerium oxide based coatings. Superhydrophobic surfaces, with large water contact angles (> 150°) and low sliding angles (< 10°), are important for their potentially different applications requiring anti-corrosion, self-cleaning, anti-fouling, oil–water separation, anti-icing, and drag reduction properties [21–27]. However, artificial superhydrophobic surfaces are

Cerium dioxide nanoparticles (CeO2) have unique chemical and electronic properties, due to which they find applications in various fields, such as catalysis [1–3], UV absorption [4,5], chemical-mechanical polishing [6], biomaterials [7,8], batteries [9,10], electronic devices [11,12], and coatings, among others [13–16]. Recently, the application of CeO2 nanoparticles in anti-corrosion coatings has attracted a lot of attention. The anti-corrosive properties and mechanisms of ceria and other cerium oxides/hydroxides have been studied in different coating matrixes and for different materials. Montemor et al. [17] improved the anti-corrosive performance of modified silane films by filling them with CeO2 nanoparticles. Fedel et al. [18] studied the mechanism and electrochemical behavior of cerium oxide nanoparticles as corrosion inhibitors for mild steel. Chen et al. [19] prepared a bis-

⁎

Corresponding author at: School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China. E-mail addresses: [email protected], [email protected] (C. Liu).

https://doi.org/10.1016/j.surfcoat.2019.125312 Received 11 October 2019; Received in revised form 21 December 2019; Accepted 25 December 2019 Available online 27 December 2019 0257-8972/ © 2019 Published by Elsevier B.V.

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Fig. 1. (a) Schematic illustration for fabrication of superhydrophobic FAS-CeO2/PDMS coating, (b) FT-IR spectra of CeO2, FAS-CeO2, and FAS-CeO2/PDMS coating.

properties. This provided a scope for expanding the applications of superhydrophobic coatings.

easily destroyed, due to their poor durability/chemical stability and low adhesion property, which gradually reduce the superhydrophobicity of coating. Moreover, when the coating is exposed to outdoor environment (ultraviolet light), changes in the chemical composition and structure occur at the surface, which affects its superhydrophobicity. Recently, many researchers have worked hard to overcome these limitations of superhydrophobic coatings. Li et al. [28] fabricated mechanically stable superhydrophobic surfaces with selfcleaning ability, which provided protection against damages due to mechanical abrasion and stretching. Liu et al. [29] reported a durable superhydrophobic PVDF composite coating, which possessed both selfhealing ability and UV resistance. Qing et al. [25] constructed a superhydrophobic coating using a sandpaper template. The surface exhibited exceptional mechanical robustness, pressure stability, and repellency to hot water. However, most of the research focused on improving only one property, but not on synergistically improving the comprehensive performance of the coating. The superhydrophobic coating possessing water-repellent property could significantly improve the barrier property for metal substrates by preventing the penetration of water and other aggressive species. So, till date, many researchers have focused on development of superhydrophobic surfaces, which also have anti-corrosion properties [30–32]. However, all of the relative works about anti-corrosion mechanism of superhydrophobic coatings are always limited in physical barrier theory. With changes in external temperature or pressure, the corrosive medium penetrates into the interior of the coating and destroys the metal substrate. Herein, a superhydrophobic composite coating of polydimethylsiloxane (PDMS) with modified cerium dioxide was fabricated, which had combined properties of durability, UV resistance, and corrosion resistance. First, cerium dioxide nanoparticles were prepared and modified with trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl) silane (FAS) to obtain low surface energy nanoparticles. Then, PDMS with outstanding wear-resistance and strength was used as a binder to enhance the adhesion between the nanoparticles and between coatings and substrates [28,31]. The as-fabricated stable surface displayed good superhydrophobicity and stability even in harsh environments (drops of acid or base, abrasion from sandpaper, ultraviolet irradiation, and damage due to cross-cutting). It could also be applied on a variety of substrates. Additionally, the coating provided an effective corrosion protection for the metal substrate. Compared to the existing works, our newly fabricated superhydrophobic surface was endowed with superhydrophobicity, UV-resistance, durability, and corrosion-resistance

2. Experimental 2.1. Materials Q235 carbon steel (20 mm × 40 mm × 1 mm) was used as substrate. Cerium nitrate hexahydrate (Ce(NO3)3·6H2O), ammonium bicarbonate (NH4HCO3), hydrochloric acid (HCl), and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Co. Ltd. Ethanol and n-hexane were supplied by Tianjin Fuyu Fine Chemical Co. Ltd. Trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane (FAS) was obtained from Shanghai Aladdin-Reagent Co. Ltd. Polydimethylsiloxane (PDMS) (Sylgard 184 silicone elastomer) with curing agent was purchased from Dow Corning Company. All reagents were of analytical grade and were used as received. 2.2. Preparation and modification of cerium dioxide (CeO2) nanoparticles CeO2 nanoparticles were synthesized by reverse setting method using Ce(NO3)3·6H2O (0.5 mol/L) and NH4HCO3 (1.5 mol/L) at 60 °C in the presence of hexadecyltrimethylammonium bromide catalyst. After washing and freeze-drying to obtain dispersed powder with low rate of impurity and aggregates, the precursor was placed in a muffle furnace (OTF-1200X, HEFEI KE JING Materials Technology Co. Ltd.) and heated to 500 °C for 3 h to complete the reaction. Modified CeO2 (FAS-CeO2) was prepared by hydrothermal reaction. Initially, CeO2 nanoparticles (0.5 g) were added into 20 mL ethanol and then the pH of the mixture was adjusted to 5 with hydrochloric acid solution (0.1 mol/L). Next, trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane (FAS) (0.2 g) was added to the solution and ultrasonicated for 20 min. The mixture was stirred for 2 h at 50 °C and then washed several times with ethanol. FAS-CeO2 was finally obtained after drying at 120 °C for 1 h. The procedures for preparation and modification of CeO2 are schematically shown in Fig. 1(a). 2.3. Fabrication of superhydrophobic composite (FAS-CeO2/PDMS) coating Fig. 1(a) illustrates the process for preparation of FAS-CeO2/PDMS composite coating. First, PDMS (Sylgard 184 silicone elastomer) and FAS-CeO2 (1,4 ratio by weight) were ultrasonically dispersed in n2

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2.8. Electrochemical corrosion test

hexane (15 g) for 10 min. Subsequently, this solution was magnetically stirred for 15 min and then mixed with Sylgard 184 curing agent. The mixture was then stirred ultrasonically for 10 min. The prepared solution was directly applied to the substrates by spraying. Then, the FASCeO2/PDMS samples were allowed to stand at room temperature for 30 min, before curing at 120 °C for 1 h.

Electrochemical measurements were performed on an electrochemical workstation (CHI660E) with three-electrode arrangement using 3.5 wt.% NaCl solution at room temperature. The test was conducted until open circuit potential (OCP) attained a steady state. Electrochemical impedance spectroscopic (EIS) measurements were conducted at an amplitude of 10 mV as the sinusoidal perturbation, in the frequency range of 100 kHz to 10 MHz. ZSimpWin software was used to simulate the electrical parameters.

2.4. Characterizations The surface morphologies and chemical compositions of samples were studied by scanning electron microscopy (SEM, JSM-7001F and Zeiss Ultra Plus) at 15.0 kV. Atomic force microscopy (AFM, Bruker Nano-DIMENSION ICON) was employed to analyze the surface topography and roughness of coatings with a scanning scale of 5.0 μm × 5.0 μm. The elemental analysis and information regarding the chemical bonding of the coating was obtained by X-ray photoelectron spectroscopy (XPS, K-α) and Fourier transform infrared spectroscopy (FT-IR, Bruker–VERTEX 70).

3. Results and discussion 3.1. Surface chemical composition FT-IR spectra of CeO2, modified CeO2 (FAS-CeO2), and FAS-CeO2/ PDMS coating are presented in Fig. 1(b). The spectrum of CeO2 exhibited two characteristic peaks at 549 cm−1 and 721 cm−1, which were assigned to the CeeO stretching vibrations. These peaks were also seen in all the spectra in Fig. 1(b) [34,35]. The absorption peaks at 3425 cm−1 and 1616 cm−1 were due to stretching and bending vibrations, respectively, of –OH. FAS-CeO2 showed characteristic peaks of SieOeCe at 1060 cm−1. Moreover, new peaks appeared at 1145 cm−1 and 1205 cm−1 (stretching vibrations of CeF bond in –CF2–) and at 1246 cm−1 (stretching vibrations of CeF bond in –CF3) [36]. This was evidence that the CeO2 particles were successfully prepared and modified with FAS. In the spectrum of FAS-CeO2/PDMS, peaks for SieCH3 stretching vibrations of PDMS were observed at 2955 cm−1 and 1260 cm−1 [37,38]. Further, the absorption peaks at 1080 cm−1 and 802 cm−1 could be attributed to the stretching vibrations of SieOeSi and CeSi bonds, respectively [39,40]. Obviously, these peaks suggested the presence of PDMS moieties in FAS-CeO2/PDMS coating. Meanwhile, the characteristic peak of FAS-CeO2 also appeared in the spectrum of FAS-CeO2/PDMS. Results of FT-IR analysis indicated that CeO2, FASCeO2, and FAS-CeO2/PDMS coating were successfully prepared. XPS was further employed to study the chemical composition and bonding state of FAS-CeO2/PDMS coating. Strong peaks of cerium, fluorine, carbon, oxygen, and silicon could be observed in the survey spectrum of the coating (Fig. 2a). The high-resolution XPS spectra of Ce 3d, F 1s, and Si 2p are shown in Fig. 2 (b–d). For Ce 3d spectrum, peaks between 892.0 and 920.0 eV corresponded to Ce 3d5/2, whereas, peaks between 875.0 and 892.0 eV were assigned to Ce 3d3/2 [41–43]. The F 1s spectrum showed peaks at 686.9 eV and 687.8 eV attributed to –CF2–and –CF3– groups, respectively, both of which corresponded to fluorine in the FAS molecule [25]. The Si 2p spectrum in Fig. 2 (d) could be fitted into two peaks, corresponding to binding energies of 99.8 eV and 100.6 eV, associated with silicate in PDMS and SieO in FAS molecule, respectively [44]. The results reconfirmed the structure of FAS-CeO2/PDMS composite coating and were consistent with the results of FT-IR spectroscopy.

2.5. Wettability and self-cleaning ability Contact angles (CA) and sliding angles (SA) of the samples were measured using a contact angle measurement device (Dataphysics, OCA-20, Germany) with 5 μL water droplets. The average values of water contact angles were recorded by testing the same sample at five different locations. High-speed camera was used to record the bouncing process of droplets on the coating surface. The self-cleaning test was performed as follows: the sample was placed at a certain angle (10°) of inclination and then the graphite powder was dispersed on the surface of the superhydrophobic coating. Water droplets were then slowly dropped on the sample surface and optical photographs were recorded. 2.6. Chemical and UV stability The chemical durability of FAS-CeO2/PDMS coating was evaluated from CA and SA values of water droplets at different pH values and for different sodium chloride concentrations on the coating surface. The anti-UV ability of the FAS-CeO2/PDMS coating was tested from the wettability of coated surface at different time intervals under UV irradiation (λ = 365 nm). The CA and SA values were determined every 24 h. The average values were recorded by testing the same sample at five different locations. 2.7. Mechanical durability and adhesion The abrasion durability of the superhydrophobic coating was evaluated by the following method: the superhydrophobic sample surface (20 mm × 40 mm) was placed in a face-down position onto 1000 grit sandpaper and a 100 g load was placed on it (pressure = 1.3 kPa). It was then dragged 10 cm forward in one direction and then rotated at an angle of 90° to move another 10 cm, which constituted one cycle. The contact angles (CA) and sliding angles (SA) of the samples were measured after each cycle. The mechanical robustness of the coatings was evaluated using free-falling water droplet test [33]. Water droplets were allowed to fall freely from a height of 40 cm onto the coating surface, which was inclined at 45°. The rate was 180 droplets/min and it hit the surface at a speed of 2.8 m/s, which was equivalent to a working pressure of 3.9 kPa. The adhesion ability of FAS-CeO2/PDMS coating was investigated according to standard GB/T 9286 method. First, the coating was cut into 2 mm × 2 mm square and the metal surface was exposed using a utility knife. The adhesion test tape (3 M Scotch600) was pressed against the surface of the cross-cut area and then pulled out along the vertical direction.

3.2. Surface morphology and wettability The surface morphologies of CeO2 and FAS-CeO2 are shown in Fig. 3(a1, a2). The SEM image showed that the CeO2 particles were rodshaped with sizes between 400 and 600 nm. On further modification with FAS, the particle morphology did not change significantly and there was no agglomeration. Nevertheless, it could be seen from Fig. 3 (b1, b2) that CeO2 changed from hydrophilic to hydrophobic after modification with FAS, which indicated that FAS-CeO2 particles had lower surface energy. Fig. 3 (c) shows the morphology of the FAS-CeO2/ PDMS coating, wherein the FAS-CeO2 nanoparticles appeared partially agglomerated together. They along with PDMS covered the substrate to form a structure with micro-nano scale roughness. Meanwhile, from Fig. 3 (c) the contact angle was found to be 161 ± 2° and the sliding angle was 4 ± 1°. Besides, when the FAS-CeO2/PDMS coating was 3

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Fig. 2. XPS spectral analysis of FAS-CeO2/PDMS coating surface: (a) survey spectrum, (b) Ce 3d, (c) F 1s, and (d) Si 2p spectra.

the crater. Furthermore, FT-IR analysis indicated that UV irradiation caused slight decomposition of the organic functional groups. However, the characteristic absorption peaks of main functional groups could be seen in the spectrum of the coating after anti-UV performance testing. These results demonstrated the excellent UV resistance of FAS-CeO2/ PDMS coating. In addition, the self-cleaning process is shown in Fig. S2. The graphite powder particles on the coating could be easily washed away by water droplets, suggesting that the FAS-CeO2/PDMS coating had excellent self-cleaning ability. Besides, Fig. S3 demonstrates the potential application of FAS-CeO2/PDMS surface in anti-icing and the fabric treated by FAS-CeO2/PDMS in oil-water separation.

applied to substrates of different materials, it showed good superhydrophobicity. (For details, see Fig. S1.) AFM 2D and 3D images and section profiles were used to further analyze the surface topography of the FAS-CeO2/PDMS coating, as shown in Fig. 3 (d, e). The micro-nanoscale roughness could be seen clearly. The surface showed many peaks and valleys and the height of micro-structure ranged from −388 nm to 377 nm. The air pockets formed in the grooves by the micro-nano structure reduced the contact area of the water droplet on the surface, imparting anti-wetting properties to the surface. Hence, in case of FAS-CeO2/PDMS coating, the superhydrophobic properties were the result of both, low interfacial energy of FAS-CeO2 particles and micro-nano scale roughness. As shown in Fig. 3 (f), a water droplet was impacted on the FAS-CeO2/PDMS coating with a velocity of 2 ms−1. It bounded completely from the coating upon impact, without wetting. This demonstrated the superhydrophobic properties of FAS-CeO2/ PDMS coating.

3.4. Chemical and mechanical stability The influence of solution pH on the wettability of FAS-CeO2/PDMS coating was investigated, as shown in Fig. 5(a). It was clear that the contact angles of all water droplets were more than 150°, whereas the sliding angle remained below 10° at all pH values. Meanwhile, from Fig. S4, it was evident that the coating still retained its superhydrophobicity in presence of 5 mol/L sodium chloride concentration. All these results demonstrated the good chemical stability of FAS-CeO2/PDMS coating. Highly acidic/alkaline solutions and aqueous salt solutions had no significant effect on the surface wettability of the coating. The resistance of the coating against the water droplet impact was shown in Fig. 5 (b). The FAS-CeO2/PDMS coating can withstand impact of 194,400 droplets and still retains superhydrophobicity. The variation of surface morphology after exposure to 518,000 water droplets was investigated (shown in Fig. S5). The mechanical stability of superhydrophobic coating was evaluated by the sandpaper abrasion test. Fig. 5 (c) and (d) are the

3.3. Anti-UV and self-cleaning properties The UV resistance of FAS-CeO2/PDMS coating was evaluated from the influence of UV irradiation on the wettability as shown in Fig. 4(a). After exposure to UV irradiation for 240 h, the sliding angle increased from 3 ± 1° to 8 ± 2° and the contact angle was 151 ± 1°. Obviously, the FAS-CeO2/PDMS surface still retained its superhydrophobicity and also had good anti-UV properties. To further study the mechanism of UV resistance of FAS-CeO2/PDMS coating, changes in surface functional groups and morphology after UV irradiation were studied (Fig. 4b). It was evident that the FAS-CeO2/PDMS surface coating formed many craters, which appeared like the lunar surface. Meanwhile, the original rough structure of the coating still remained in 4

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Fig. 3. SEM surface images of (a1) CeO2, (a2) FAS-CeO2, optical images of water droplets on (b1) CeO2 and (b2) FAS-CeO2, (c) surface morphology of the FAS-CeO2/ PDMS coating, (d) AFM surface topography and 3D images of FAS-CeO2/PDMS coating, (e) Roughness profile extracted from the AFM image shown in (d), X: lateral distance and Z: height, (f) Time-lapse photographs of water droplets bouncing on the FAS-CeO2/PDMS coating.

micro-nano scale structure of the surface was destroyed after sandpaper abrasion, but a new micro-scale rough structure was formed. After combination with the action of FAS-CeO2, the coating still showed excellent superhydrophobicity. Since PDMS had good fixed adhesiveness with FAS-CeO2 nanoparticles [28,31], the coating could withstand 30 abrasion cycles, which demonstrated good mechanical stability. Fig. 6 (a) shows the surface and wettability of the coating after cross-cutting. The sample surface retained its superhydrophobicity with

schematic representations of one abrasion cycle and changes in water contact/sliding angles for different sandpaper abrasion cycles. The water contact/sliding angle of coating surface changed within a certain range (CA > 150°, SA < 6°) during the abrasion test. Obviously, the FAS-CeO2/PDMS coating still had good wear resistance even after 30 cycles. Furthermore, in order to study the mechanical resistance of the coating, the micromorphology of the surface after 30 abrasion cycles was studied as shown in Fig. S6. It was evident that the original

Fig. 4. (a) Influence of UV irradiation on the wettability of FAS-CeO2/PDMS coating (b) FT-IR spectra and SEM images of FAS-CeO2/PDMS coating, before and after exposure to UV radiations for 240 h. 5

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Fig. 5. Influence of (a) solution pH on the wettability of superhydrophobic FAS-CeO2/PDMS coating, (b) the variation of the CA and SA as a function of the number of water droplets in the free-falling water test, (c) schematic representation of one abrasion cycle and (d) the water contact/sliding angles as a function of abrasion cycles with sandpaper.

PDMS coating was obviously higher in the high frequency region. It is generally considered that the higher the impedance module, the larger is the peak of phase angle, that is, better is the anticorrosion performance [21,30,46]. Furthermore, the equivalent circuit models for EIS results are shown in Fig. 7(c–d) and the fitted electrochemical data is presented in Table 1. In the equivalent circuit, Rs, Rair, and Cair represented the solution resistance, air layer resistance, and inductance, respectively. The terms CPEc and CPEdl denoted the capacitive elements at the interface of coating/solution and substrate/solution. The term Rct implied the charge-transfer resistance and Rc was the coating resistance. After 8 h immersion, it was clear that the FAS-CeO2/PDMS coating exhibited a higher value of Rc, reaching 2.75 × 10^5 Ω·cm2, as compared to that of 1.35 × 10^4 Ω·cm2 for the PDMS coating. Besides, the CPEc of FAS-CeO2/PDMS coating was 6.89 × 10^-10, which was 5 orders of magnitude lower than that of the PDMS coating. As the immersion was prolonged to 24 h, the electrochemical parameters of FASCeO2/PDMS coating were still better than those of the PDMS coating. The results indicated that the FAS-CeO2/PDMS superhydrophobic

high WCA of 159 ± 2°. After cross-cut adhesion test, about 15% of the sample surface area was damaged, the adhesion ability of which could be classified as level 2 (Fig. 6d) [22,45]. Moreover, the FAS-CeO2/ PDMS coating still retained its high water-repellency (Fig. 6 b, c). This could be attributed to the compatibility and synergistic effects between FAS-CeO2 nanoparticles and PDMS, which resulted in significant reduction of cracking of the coating due to stress, which enhanced the adhesiveness. 3.5. Corrosion resistance The corrosion resistances of PDMS coating and FAS-CeO2/PDMS coatings were evaluated by electrochemical impedance spectroscopy. The Bode plots for samples after exposure to 3.5 wt.% NaCl solution for 8 h and 24 h are shown in Fig. 7(a) and (b). According to the Bode plots, for different soaking times, the FAS-CeO2/PDMS coating showed impedance modules at low frequency (|Z|0.01Hz) much greater than that of PDMS coating. In addition, the peak in phase angle of the FAS-CeO2/

Fig. 6. (a) Optical images and wettability of FAS-CeO2/PDMS coating after cross-cutting and (b, c) after adhesion test, (d) adhesion test results. 6

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Fig. 7. The Bode plots of PDMS coating and FAS-CeO2/PDMS superhydrophobic coating after 8 h (a) and 24 h (b) of exposure to 3.5 wt.% NaCl solution. Equivalent electrical circuits for EIS results of (c) PDMS coating and (d) FAS-CeO2/PDMS superhydrophobic coating.

coating could effectively prevent the corrosion of substrate. For understanding the protection offered by the coating in a better manner, the anti-corrosion mechanism of the FAS-CeO2/PDMS superhydrophobic coating is illustrated schematically in Fig. 8. At the beginning of immersion, the air cushion formed between the coating and the corrosive medium due to the hydrophobicity of the FAS-CeO2, served as the first line of defense of the coating. The air cushion effectively prevented the infiltration of water, chloride ions, and oxygen action and therefore acted as a physical barrier [31,32,47]. The second line of defense of the coating became functional when the corrosive medium broke through the air barrier and penetrated into the interiors of the coating. Owing to the unstable fluorite structure of CeO2, Ce4+ had a tendency to transform to Ce3+. This was followed by release of oxygen molecules with subsequent formation of oxygen vacancies [18,48–50]. Thus, CeO2 was converted to Ce2O3 in the corrosive medium (Eq. (1)). Then, Ce3+ was released into the corrosive medium to form cerium oxide, resulting from a larger ionic size than Ce4+, which would be oxidized to Ce4+ (Eq. (2)), which led to the formation of CeO2 in FAS-CeO2/PDMS coating (Eq. (3)) [18,20,51].

2CeO2 → Ce2 O3 +

1 O2 (Ce 4 + → Ce3 +) 2

4Ce3 + + O2 + 2H2 O → 4Ce 4 + + 4OH−

(2)

Ce 4 + + 4OH− → CeO2 ↓ + 2H2 O

(3)

As the immersion process continued, the corrosive medium penetrated through the coating to reach the Q235 surface, which was the third line of defense function. Ce3+ released from the FAS-CeO2, diffused into the metal surface and formed a barrier layer consisting of cerium oxides and/or hydroxides at the active cathodic sites of substrate surface, which inhibited the corrosion process. The corresponding reactions are as follows [17,51–53]: At anode

Fe → Fe2 + + 2e− At cathode

2H2 O + O2 + 4e− → 4OH− Ce3 + + 3OH− → Ce (OH )3

2Ce(OH)3 + 2OH− → 2CeO2 ↓ + 4H2 O + 2e−

(1)

Table. 1 Fitting of electrochemical data from modeling of Bode plots after 8 h and 24 h of immersion in 3.5 wt.% NaCl solution. Sample

Time (h)

Rct (ohm·cm2)

Rc (ohm· cm2)

CPEdl Y0 (S cm

PDMS coating FAS-CeO2/PDMS coating

8 24 8 24

1.77 1.38 1.85 2.12

× × × ×

10^4 10^4 10^4 10^4

3.23 9.74 4.19 1.24

× × × ×

−2

n

S )

10^-10 10^-11 10^-6 10^-9

n

CPEc Y0 (S cm

0.88 0.99 0.81 0.97

1.35 1.77 2.75 1.30

7

× × × ×

10^4 10^3 10^5 10^4

1.62 5.30 6.89 2.97

× × × ×

−2

n

S )

10^-5 10^-4 10^-10 10^-6

Rair (ohm·cm2)

Cair (F·cm−2)

1.34 × 10^4 5.98 × 10^3

1.75 × 10^-4 3.28 × 10^-10

n 0.57 0.42 0.88 0.43

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Fig. 8. Schematic representation of anti-corrosion mechanism of FAS-CeO2/PDMS superhydrophobic coating.

4. Conclusions

manuscript is approved by all authors for publication. I would like to declare on behalf of my authors that the work described was original research that not been published previously, and under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

In summary, a stable superhydrophobic composite coating was fabricated by mixing together modified micro/nanoparticles of cerium dioxide and PDMS that could resist sandpaper abrasion, blade scratching, UV irradiations, and corrosive medium. Compared to previous works [13,17,18,28–30], cerium dioxide nanoparticles introduced into the superhydrophobic coating in our present work, not only acted as low surface energy particles to construct a rough structure, but also provided barrier protection and active corrosion inhibition property to the coating. The FAS-CeO2/PDMS superhydrophobic coating was able to retain its superhydrophobicity even after 30 abrasion cycles and 240 h of UV irradiation. Besides, the obtained superhydrophobic composite coating showed excellent corrosion resistance, which was due to the air cushion (which acted as a physical barrier) and active corrosion inhibition effect of cerium dioxide. This coating could be applied to a variety of material surfaces to achieve superhydrophobic self-cleaning property, which showed water CA of 161 ± 2° and SA of 4 ± 1°. Importantly, the superhydrophobic coating had combined advantages of superhydrophobicity, UV-resistance, durability, and corrosion resistance. These properties provided advanced superhydrophobic coatings, generating better prospects for coating applications.

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Author contribution section 1. Guarantor of integrity of entire study: Kai An, Cai Long, Yi Sui, Yongquan Qing, Guangzhen Zhao, Zhenqiang An, Linshan Wang, Changsheng Liu. 2. Study concepts: Kai An, Cai Long, Yi Sui, Yongquan Qing, Linshan Wang, Changsheng Liu. 3. Study design: Kai An, Cai Long, Yi Sui, Yongquan Qing, Changsheng Liu. 4. Literature research: Kai An, Cai Long. 5. Experimental studies: Kai An, Cai Long, Yi Sui, Zhenqiang An. 6. Data acquisition: Kai An. 7. Data analysis/interpretation: Kai An, Guangzhen Zhao. 8. Manuscript preparation: Kai An, Cai Long, Yi Sui. 9. Manuscript defnition of intellectual content: Kai An, Yongquan Qing. 10. Manuscript editing: Kai An. 11. Manuscript revision/review: Kai An, Cai Long, Yongquan Qing. 12. Manuscript final version approval: Kai An, Cai Long, Yi Sui,Yongquan Qing, Guangzhen Zhao, Zhenqiang An, Linshan Wang, Changsheng Liu. Declaration of competing interest No conflict of interest exits in the submission of this manuscript, and 8

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