Superamphiphobic and Electroactive Nanocomposite toward Self

Research Article www.acsami.org

Superamphiphobic and Electroactive Nanocomposite toward SelfCleaning, Antiwear, and Anticorrosion Coatings Ruixia Yuan,†,‡ Shiqi Wu,† Peng Yu,†,§ Baohui Wang,† Liwen Mu,‡ Xiguang Zhang,† Yixing Zhu,† Bing Wang,† Huaiyuan Wang,*,† and Jiahua Zhu*,†,‡

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Provincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, China ‡ Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States § Oil Refinery of Daqing Petrochemical Company, Daqing 163711, China ABSTRACT: Multifunctional coatings are in urgent demand in emerging fields. In this work, nanocomposite coatings with extraordinary self-cleaning, antiwear, and anticorrosion properties were prepared on aluminum substrate by a facile spraying technique. Core−shell structured polyaniline/functionalized carbon nanotubes (PANI/fCNTs) composite and nanosized silica were synergistically integrated into ethylene tetrafluoroethylene (ETFE) matrix to construct lotus-leaf-like structures, and 1H,1H,2H,2H- perfluorooctyltriethoxysilane (POTS) was used to decrease the surface energy. The composite coating with 6 wt % PANI/fCNTs possesses superamphiphobic property, with contact angles of 167°, 163°, and 159° toward water, glycerol, and ethylene glycol, respectively. This coating demonstrates stable nonwetting performance over a wide temperature range ( 150°) toward various oil and water liquids are arousing increasing interest owing to their wide range of industrial applications, such as antifouling, self-cleaning, anti-icing, and anticorrosion coatings.3−6 Especially, the superior physical barrier effect of superamphiphobic coatings could provide unexceptionable benefits to prevent the adhesion of water/oil onto the coating surface and finally restrict the ion permeation and corrosion process.7−9 Different technologies have been developed to prepare superamphiphobic surfaces including but not limit to spraying,10 chemical vapor deposition,11 and electrospinning.12 Nevertheless, successful application of existing coatings is extremely scarce © 2016 American Chemical Society

Received: April 3, 2016 Accepted: May 2, 2016 Published: May 2, 2016 12481

DOI: 10.1021/acsami.6b03961 ACS Appl. Mater. Interfaces 2016, 8, 12481−12493

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation procedures for PANI/fCNTs composite (a) and superamphiphobic ETFE-PANI/fCNTs coating (b).

increase the effectiveness of electron delocalization.27,28 Integration of CNTs and PANI has been reported to improve the conductivity and thermal stability of PANI materials and enhance the corrosion protection ability.27−29 However, the porous structure of bare PANI/CNTs composite poses huge concern when it is directly used as coating material, since pores facilitate the permeation of corrosive ions to coating/metal interface. To extend the functions and improve the performances, PANI or PANI/CNTs have been blended with other materials to form composites, such as epoxy resin and polyurethane.30,31 Nevertheless, rare work has been reported on incorporating conductive PANI/CNTs composite into superamphiphobic coating, which is expected to maximize the functionality of PANI/CNTs as well as the integration of superamphiphobicity toward anticorrosion coatings. In this study, superamphiphobic/electroactive nanocomposite coating with excellent mechanical durability and corrosion resistance was prepared on aluminum substrate via a facile spraying technique. Nano/micro hybrid structure was built on the aluminum surface by acid etching and hydrothermal reaction to enhance the coating adhesion ability. To overcome the insolubility of inert CNTs, functionalized CNTs (fCNTs) were attained by concentrated acid oxidation process. Conductive PANI/fCNTs composites with core−shell structures were synthesized via in situ polymerization method. The as-prepared PANI/fCNTs and nanosized silica particles were embedded into the ETFE coating as nanofiller. 1H,1H,2H,2Hperfluorooctyltriethoxysilane (POTS) with extremely low surface energy (19 mN/m) was added into the above coating to obtain superamphiphobic surface. The nonwettability, wear resistance, self-cleaning, bending ability, and thermal stability of the prepared coating were investigated. The anticorrosion behaviors and involved mechanisms were analyzed based on electrochemical corrosion measurement in saline solution. It is believed that this research will pave a new way to design the multifunctional coating for anticorrosion applications especially under mechanically erosive/abrasive environment.

(FEP)/carbon nanofibers (CNFs) composite coating, which revealed outstanding antiwear property with negligible structure damage after 10 000 times abrasion and well-retained water contact angle (WCA) of 141 ± 1.2°.16 ETFE possesses the advantages of low surface energy, excellent mechanical strength, high thermal stability, self-lubrication, and chemical resistance to acids, bases, and solvents.17,18 It has been widely used in aerospace, automotive, petrochemical, medical, microelectronics, and electrical industries.19 Mindivan fabricated tungsten carbide thermal spray coating by using ETFE particles as solid lubricant which led to a noticeable increase in wear resistance along with a decrease in friction coefficient.18 The inherent nonsticky ETFE is expected to improve the superamphiphobic durability. However, to the best of our knowledge, ETFE as film-forming material for superamphiphobic surface has not been examined so far. In addition, it is well-recognized that both appropriate surface roughness and low surface energy are essential for constructing superamphiphobic surface. Nanoparticles/microparticles or fibers, for instance, SiO2, Al2O3, CNFs, carbon nanotubes (CNTs), and graphene, are mostly used as filler in the coating to create a hierarchically structured rough surface.20 Unfortunately, once a scratch or hole forms in the coating during the long-term use process, pitting corrosion could be induced and accelerated due to small anode/large cathode galvanic corrosion especially in environments that contain corrosive ions such as chloride ion.1 Different from above fillers, the electrically conductive polyaniline (PANI) was found to provide anodic protection in pinholes due to its electroactive redox property.2,21,22 Electroactive coating can act as a mediate between anodic and cathodic reactions taking place at metal/ solution interface and create a passive layer to reduce the metal dissolution.23−25 CNTs exhibit excellent Young’s modulus, superior mechanical strength, and high electrical and thermal conductivity, which are broadly applied as reinforcement in high performance and multifunctional composites.26 It has been proposed that the interaction between the quinoid ring of PANI and the CNTs network could facilitate charge-transfer process and 12482


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Figure 2. SEM and TEM images of fCNTs (a, b) and PANI/fCNTs composite (c, d). the above mixture on the pretreated aluminum plate with an air spray gun under a pressure of 6 bar. The composite coating with nano/ micro hierarchical surface structures was finally obtained after curing at 300 °C for 1.5 h. 2.4. Characterization. The morphology of the resultant PANI/ fCNTs and composite coatings was observed using a Zeiss SIGMA field emission scanning electron microscope (SEM) and FEI scanning transmission electron microscope (TEM) with an accelerating voltage of 120 kV. The chemical composition was inspected by Tensor27 Fourier transform infrared spectroscopy (FT-IR), X-ray energy dispersive spectrometry (EDS) using an Oxford Instruments X-Max 80 mm 2 silicon drift detector, as well as X-ray photoelectron spectroscopy (XPS) (PHI VersaProbe II) with Al Kα line excitation source. The UV−vis spectrum of PANI/fCNTs composite in ethanol was measured on a Hitachi 4100 UV−vis spectrophotometer. The wettabilities of the composite coatings were evaluated by measuring the CAs and sliding angles (SAs) with approximately 5 μL liquid droplets using contact angle meter (JGW-360A, Chengdeshi Shipeng Detection Equipment Co. Ltd.). Average values were taken from at least five measurements at different positions of the coating surface. The variation of CAs and SAs toward water and ethylene glycol was investigated after immersing into acidic solution (pH = 1) and 3.5 wt % NaCl aqueous solutions. The adhesive strength of the composite coating was investigated using cross-cut method based on GB/T 9286 standard testing method. The wear-resistant ability of the coating was evaluated using Taber wear and abrasion testers (JST3393). The electrochemical corrosion measurement was conducted in 3.5 wt % NaCl solution by electrochemical workstation (METEK VersaSTAR 4), with Pt wire and saturated calomel electrode (SCE) as counter electrode and reference electrode, respectively. The corrosion potential (Ecorr) of the aluminum substrates was obtained from the open circuit potential at the equilibrium state of the system. Tafel plots were obtained by scanning the potential from −250 to 250 mV above Ecorr at a scan rate of 20 mV/min. Corrosion current (Icorr) was determined through superimposing a straight line along the linear portion of the cathodic or anodic curve and extrapolating it through Ecorr. Electrochemical impedance spectroscopic (EIS) measurement was carried out in the frequency range of 105−10−2 Hz, with a sinusoidal signal perturbation of 10 mV.

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial ETFE powders were purchased from DuPont (USA). Aniline, ammonium persulfate (APS), cobalt sulfate (CoSO4), sulfuric acid (H2SO4, 98%), nitric acid (HNO3), ethylene glycol, glycerol, ethanol, ethyl acetate, and silica particles with average diameter of about 40 nm were provided by Aladdin Reagent Co. Ltd. (China). POTS and multiwalled CNTs were supplied by SigmaAldrich Co. (USA) and Beijing Boyu New Material Technology Co. Ltd. (China), respectively. 2.2. Preparation of PANI/fCNTs Composite. To obtain carboxylate functionalized CNTs (fCNTs), CNTs were magnetically stirred with a mixture of H2SO4 and HNO3 (3:1 v/v) at 80 °C for 12 h. The suspension was then neutralizated to pH 7.0, collected on a 0.22 μm Millipore membrane filter, and dried under vacuum at 60 °C to get the resulting products fCNTs. In a typical synthesis of PANI/ fCNTs composite, the as-prepared fCNTs (0.04 g) and aniline (0.4 g) were first added to 200 mL of HCl solution (0.5 mol/L) and ultrasonically stirred for 12 h at room temperature. A fresh solution with APS (0.49 g) and CoSO4 (0.05 g) in 50 mL of HCl solution (0.5 mol/L) was then rapidly transferred to the above solution. The polymerization reaction was carried out under magnetic stirring at room temperature for 24 h. The dark green precipitate was filtered with 0.22 μm Millipore membrane, thoroughly washed with deionized water and ethanol several times to remove aniline monomers and oligomers, and finally dried at 50 °C under vacuum for 24 h (Figure 1a). Bare PANI was also prepared as control following the above procedure without adding fCNTs. 2.3. Preparation of Coating. The pristine aluminum plate (1100 grade, 80 mm × 80 mm × 1 mm) was first polished with 600 mesh sandpapers in one direction, ultrasonically washed in absolute alcohol, and dried under nitrogen flow. Afterward, the polished aluminum was etched in 3.0 M hydrochloric acid for 30 min and washed thoroughly with deionized water and absolute alcohol. After that, the etched aluminum was boiled in water for 30 min. The typical fabrication process of ETFE-PANI/fCNTs composite coatings is illustrated in Figure 1b. Specifically, 1.0 g of ETFE powder was dispersed in 30 mL of ethyl acetate under ultrasonic stirring for 30 min. Then, 0.05 g of SiO2, 0.2 g of POTS, and a certain amount of PANI/fCNTs were added and ultrasonically stirred for 2 h to form a well-dispersed solution. The eventual coating was obtained by spraying 12483


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Figure 3. FT-IR spectra (a), UV−vis spectrum (b), XPS spectrum (c), and N 1s peak deconvolution analysis (d) of PANI/fCNTs composite.

than pure PANI, which is attributed to the fact that the sp2 carbons in fCNTs molecular could compete with the chloride ions and thus perturb the H-bonding environment and increase the N−H stretching.26 Furthermore, the characteristic band appearing at 1141 cm−1 as the “electron-like absorption” peak (−NquinoidN−) is considered to be a measure of delocalization electrons and thus a characteristic peak of PANI conductivity.27 The intensity of this peak for PANI/fCNTs is significantly increased and shifted to lower frequency at 1132 cm−1, indicating its relatively higher protonated state.26 This suggests that the strong PANI/fCNTs interactions may result in fCNTs functioning as a chemical dopant for PANI and increasing the effective degree of electron delocalization in PANI chains. Due to the large aspect ratio and surface area of fCNTs, they may serve as “conducting bridges” connecting PANI conducting domains and increasing the effective percolation with longer conjugation lengths, while other dopants do not change much in terms of charge mobility.33 Highly doped state of the dark green PANI/fCNTs product could also be proved by the strong peak centered at about 750 nm with a delocalized polaron tail extending into the nearinfrared region clearly observed in the UV−vis spectrum (Figure 3b).34,35 XPS results demonstrate that the PANI/ fCNTs nanocomposite is mainly composed of C, N, O, and Cl elements (Figure 3c). The N 1s spectrum is also identical to the doped PANI, and the doping level calculated by N+/N ratio is about 0.21 (Figure 3d), which is in good agreement with the literature.34 3.2. Coating Wettability. The PANI/fCNTs nanocomposite was added into ETFE with different loadings varying from 0 to 6 wt % to optimize the coating wettability. Further increase of PANI/fCNTs above 6 wt % leads to megascopic cracks on the coating surface. High water repellency with WCA from 164° to 167° and SAs below 1° were achieved after adding PANI/fCNTs (Figure 4a).

3. RESULTS AND DISCUSSION 3.1. Microstructure Investigation of PANI/fCNTs Composite. The as-prepared fCNTs show tangled hollow tubes with a rather smooth surface (Figure 2a,b). The successful carboxylation of CNTs surface by acid treatment is verified by the appearance of peaks at 1341 and 1033 cm−1 in FT-IR spectrum associated with the stretching vibrations of carboxyl groups (Figure 3a). The hydrophilic −COOH groups on the fCNTs surface could solvate in the water due to their polarity, and the aniline will adsorb on the fCNTs surface. Because of the site-selective interaction between the quinoid ring of PANI and fCNTs, the insoluble polyaniline oligomers and polymers chains would be constrained to grow around the CNTs for minimization of the overall interfacial energy of fCNTs.27,32 As a result, the tubelike PANI/fCNTs composite network with coaxial core−shell morphology is formed as evidenced by SEM image (Figure 2c). The diameter of each carbon nanotube is about 35−55 nm. A PANI layer with thickness of about 10 nm was deposited on the fCNTs surface (Figure 2d). The successful synthesis of PANI/fCNTs nanocomposite could also be evidenced by the characteristic peaks in FTIR spectrum of PANI/fCNTs composite at 1658 cm−1 (CN stretching of quinoid ring), 1300 cm−1 (C−N stretching of benzenoid structure), and 800−830 cm−1 (the out-of-plane bending mode of C−H bond in the aromatic ring), which are identical to those of pure PANI (Figure 3a).28,33 The FT-IR spectrum of PANI/fCNTs also exhibits quinoid and benzenoid ring vibrations at about 1593 and 1498 cm−1, revealing its emeraldine salt state.27 Moreover, a notable increase in peak intensity ratio of quinoid versus benzenoid ring is observed for the PANI/fCNTs compared to bare PANI. This result suggests that interactions between PANI and fCNTs via π-stacking could promote and stabilize the quinoid ring structure of PANI. In addition, the signal in the N−H stretching region near 3426 cm−1 is stronger in the composite 12484


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Figure 4. Effect of PANI/fCNTs content on the wettability of ETFE composite coating toward water (a), glycerol (b), and ethylene glycol (c). Effect of solution pH on the superhydrophobicity of the ETFE composite coating with 6 wt % PANI/fCNTs (d).

Figure 5. Antifouling tests of uncoated aluminum plate (a1, a2) and ETFE-PANI/fCNTs-6 composite coating (b1, b2) by immersion into sludge. Nonwetting tests with sludge (c1, c2), mixture of H2O and ethylene glycol (EG) (d), and concentrated H2SO4 (e).

As displayed in Figure 5, the superamphiphobic ETFEPANI/fCNTs-6 composite coating could remain clean even after 100 cycles of immersion, while the uncoated aluminum plate was seriously contaminated even after first immersion in the sludge. It can be also clearly observed that the droplets of sludge, mixture of water and ethyl glycerol (9:1, v/v), and highly corrosive concentrated H2SO4 (98%) tend to easily slide down from the coating surface without any trace left. High WCAs of 159° and 160° were maintained after the flow of water/ethyl glycerol and concentrated H2SO4. Such outstanding antifouling and self-cleaning abilities offer promising

Furthermore, the glycerol CA was improved from 157° to 163° and the ethylene glycol CA from 152° to 159° as PANI/fCNTs increased from 0 to 6 wt % (Figure 4b,c). The SAs of both glycerol and ethylene glycol droplets reduced gradually with increasing PANI/fCNTs. The coating with optimized superamphiphobicity (with 6 wt % PANI/fCNTs and named as ETFE-PANI/fCNTs-6) is thus further investigated in the following studies. It is worth mentioning that this coating also demonstrated stable water-repellent ability with high CAs (161−167°) and low SAs (0.7−1°) toward corrosive solutions from pH 1 to 14 (Figure 4d). 12485


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Figure 6. SEM images of polished aluminum substrate (a), acid-etched aluminum (b), aluminum handled with boiling water (c), ETFE-PANI/ fCNTs composite coating (d, e, f). Inset: Optical image of water (W), glycerol (G), and ethylene glycol (EG) droplets.

Figure 7. EDS spectra of the treated aluminum substrates (a) and pure ETFE coating and ETFE-PANI/fCNTs-6 composite coating (b).

appearance of silicon and nitrogen peaks in the EDS results (Figure 7b). The coating is estimated to be about 260 μm, and the etched aluminum surface can be clearly observed (Figure 6f). The fabricated hierarchical structure is similar to the natural lotus leaf where the air pockets can be used to develop an air film and thus reduce the contact area between the coating surface and liquid droplets.6 It should be pointed out that the pure ETFE coating with flat surface possesses lower water contact angle of 89.5°, even though its fluorine content (44.9%) is relatively higher than that of ETFE-PANI/fCNTs-6 composite coating (36.1%) (Figure 7b). This suggests that both well designed surface roughness and the extremely low surface energy contribute to the resultant superamphiphobicity and excellent self-cleaning ability. In the FT-IR spectrum of ETFE-PANI/fCNTs-6 composite coating, the peaks at 1450 (−CH2 deformation vibration), 1350 (−CH3 deformation vibration), and 775 cm−1 (−CH2 in plane vibration) are mainly from ETFE molecules (Figure 8). The strong adsorption bands at 1202 and 1149 cm−1 are associated with the stretching vibration of −CF2, which originate from ETFE and POTS molecules. The peaks located at 1080 and

capabilities of the coatings in long-term industrial applications in rigorous environment. 3.3. Coating Morphology and Composition Analysis. To understand better such a superamphiphobic coating, the morphology and chemical composition of the coating were explored. First, a rough aluminum surface could be obtained after polishing with sandpaper (Figure 6a), while microscaled honeycomb-like structures were formed by hydrochloric acid etching (Figure 6b). Interestingly, nano/micro roughness on the aluminum substrate was finally achieved after boiling in water (Figure 6c). In addition, the WCA of the aluminum plate turned from 32° to 0° after the etching process. This should be related to the formation of hydrophilic aluminum oxide following reaction I as evidenced by the increasing oxygen content in EDS spectrum (Figure 7a).36 Al + H 2O → Al 2O3 ·x H 2O + H 2↑ ⏐

(I)

The ETFE-PANI/fCNTs-6 composite coating exhibited microsized papillae with nanoparticles and interconnected nanofibers on the surface (Figure 6d,e). These nanostructures are composed of SiO2 and PANI/fCNTs as confirmed by the 12486


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strong adhesive strength of the composite coating is mainly attributed to the interlocking effect between the nanostructured/microstructured aluminum surface and cross-linked ETFE macromolecule network structure. Furthermore, the ETFE-PANI/fCNTs-6 composite coating after the cross-cut test still had high water repellency. This implies that the strong interface adhesion between the coating and aluminum substrate could prevent quick diffusion of corrosive solutions. 3.4.2. Wear Resistance. To estimate the wear resistance of the ETFE-PANI/fCNTs-6 composite coating, the abrasion resistance test was performed and compared with the commercial fluorocarbon coating. As displayed in Figure 10a, the friction wheels are covered by 800-mesh sandpaper and the coated aluminum plate was fixed under the friction wheels with 100 kPa (Figure 10a). The commercial fluorocarbon coating exhibited serious damage after 3400 cycles, and the CAs to water and ethylene glycol decreased from 91° and 72° to 71° and 51°, respectively (Figure 10b). However, the surface of ETFE-PANI/fCNTs-6 composite coating showed slight damage even after 45 000 times abrasion. In addition, the CAs of the rubbed coating to water and ethylene glycol can still remain 149° and 140°, respectively (Figure 10c). The excellent wear-resistance ability of the composite coating should be mainly attributed to the fabricated hierarchical structure which could capture the wear debris. Furthermore, the large amount of air trapped in the multiscale topographies is able to relieve the friction damage.15,16 Self-lubrication property of ETFE macromolecules and embedded CNTs are also beneficial for the coating to withstand long-time abrasion without a significant reduction in super-repellency. Such robust surface is essential to maintain the surface barrier effect in practical applications. 3.4.3. Bending Resistance. Bending test reveals the overall mechanical properties of coating including tensile strength, Weibull modulus, adhesion, and the intrinsic thin film stress. It is clearly observed that the pure ETFE coating exhibited obvious microcracks, although no macroscopic fracture was observed after bending 30 times (Figure 11a). Moreover, the ETFE composite coating without PANI/fCNTs (named as

Figure 8. FTIR spectra of pure SiO2, POTS, ETFE, and ETFE-PANI/ fCNTs-6 composite coating.

981 cm−1 correspond to the stretching vibration of carbon chain in ETFE and POTS molecules. The appearance of a peak at around 3600 cm−1 is related to Si−O−H stretching vibration of SiO2.37 It seems that the characteristic adsorption bands of PANI/fCNTs are overlapped with the peaks of ETFE. 3.4. Mechanical Behaviors of the Prepared Coating. 3.4.1. Adhesion Strength. The adhesive strength of the superamphiphobic coating was investigated to evaluate its physical barrier function (Figure 9). The coating was scribbled into 2 mm × 2 mm girding by using a razor to expose the substrate, and then the testing tape was pressed and pulled to remove it from the scored surface within 5 min. The ETFEPANI/fCNTs-6 composite coating showed excellent adhesion onto the pretreated aluminum surface with negligible peeling at the intersection of the scratches after removing the tape, which can be classified as grade 1 adhesion. The coating sprayed on the untreated aluminum substrate can be peeled off easily. The

Figure 9. ETFE-PANI/fCNTs-6 composite coating before (a) and after (b) the cross cut tape test and the afterward water-dropped test (c, d, e). 12487


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Figure 10. Schematic illustration of the abrasion resistance test (a), influence of friction process on the wettabilities of commercial fluorocarbon coating (b) and the ETFE-PANI/fCNTs-6 (c). Inset: Optical images of water and ethylene glycol droplets on the coating before and after abrasion.

Figure 11. SEM and optical images of pure ETFE coating (a1, a2, a3), ETFE-PANI/fCNTs-0 (b1, b2, b3), and ETFE-PANI/fCNTs-6 composite coating (c1, c2, c3) before and after bending tests.

ETFE-PANI/fCNTs-0) was detached after bending test (Figure 11b). However, cracks and stripping were not observed in the ETFE-PANI/fCNTs-6 composite coating after the bending test (Figure 11c), demonstrating its excellent bending resistance. Such improvement could be ascribed to the superior

interfacial interaction between ETFE macromolecular chains and large aspect-ratio PANI/fCNTs. 3.4.4. Heat Resistance. The thermal stability of the ETFEPANI/fCNTs-6 coating was examined by thermogravimetric (TG) analysis. As shown in Figure 12a, the coating kept stable 12488


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Figure 12. TG thermogram (a) and wettability (b) of the ETFE composite coating with 6 wt % PANI/fCNTs.

Figure 13. CAs toward water (a) and ethylene glycol (b) and optical images (c1,c2 and c3) of ETFE composite coatings after immersing in 1 mol/L HCl for different times.

below 460 °C and the weight loss was less than 5%. The coating surface wettability after annealing treatment for 2 h under different temperatures is shown in Figure 12b. The coating surface could maintain superamphiphobicity under 400 °C, while it loses its antiwetting characteristics at above 500 °C. Accordingly, the composite coating reveals stable antiwettability ability over a wide temperature range possibly due to the high bonding energy of CF2 bonds in ETFE macromolecules. 3.5. Corrosion Resistance of the Nanocomposite Coating. 3.5.1. Corrosion Resistance. After immersing in 3.5 wt % NaCl solution for 90 days, the contact angles toward water and ethylene glycol on the ETFE-PANI/fCNTs-6 coating retained 160 ± 0.9° and 151 ± 0.7°, respectively. Furthermore, the ETFE-PANI/fCNTs-6 coating in 1.0 mol/L HCl solution displayed superior and durable superamphiphobicity compared to ETFE-PANI/fCNTs-0 coating (Figure 13a,b). Apparent silver mirror effect can be observed due to the high water repellent ability of the surface (Figure 13c). It can be explained by the excellent chemical resistance of ETFE to acids, bases, and organic solvents. Besides, the ETFE-PANI/fCNTs-6

coating possesses well designed microstructure/nanostructure and low surface energy and thus effectively prevents infiltration of liquid. These results demonstrate that ETFE composite coating with conductive PANI/fCNTs could offer an enhanced corrosion protection for aluminum substrate in harsh corrosive environment. 3.5.2. Potentiodynamic Polarization Analysis. The corrosion protection efficiency of the as-prepared coatings was also investigated in 3.5 wt % NaCl aqueous solution. The typical potentiodynamic polarization measurements versus time (Figure 14) and the electrochemical parameters obtained from the polarization curves (Table 1) provide direct evidence of the coating stability during exposure to corrosive environment. The much higher current density of untreated aluminum could be related to the initiation of pitting corrosion, as confirmed in the following EIS results. Compared with the Ecorr of uncoated Al substrate (−797 mV), the ETFE composite coatings exhibited a drastic shift to anodic region after 1 d immersion, thus revealing improved corrosion resistance. Especially, positive Ecorr (129 mV) and extremely low corrosion 12489


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site coating is attributed to the shielding effect of superamphiphobic surface and the higher interaction energies between the PANI and fCNTs.28 3.5.3. EIS Analysis. The evolution of electrochemical impedance spectra for coated Al substrates as a function of immersion time in 3.5 wt % NaCl solution was investigated to evaluate the corrosion protection activity of coatings and probe the anticorrosion mechanism (Figure 15). Larger semicircle diameter in Nyquist spectra (charge transfer resistance) corresponds to a lower corrosion rate.2 In the case of uncoated Al substrate after 0.5 h immersion, the Nyquist spectrum exhibits a semicircle-like shape (Figure 15a). Its Bode plot displays one time constant at lower frequencies, which is associated with the capacitance of electrochemical double layer on the solid/electrolyte interface (Figure 16a). This resulted from corrosion process occurring at the metal surface as evidenced in potentiodynamic polarization curve. The impedance spectra of the pure ETFE coating possess capacitive loops at high and medium frequencies and a tail at low frequencies (Figure 15b). The capacitive loops at medium frequencies might be related to the charge transfer of corrosion reaction and the double layer capacitance at the electrode surface, while the loop at low frequencies could be attributed to the charge transfer resistance.38 The significant decrease in both loops over immersion time indicates the decreased anticorrosion performance of the coating, which could be attributed to the proceeding of localized corrosion attack on the coating cracks and the subsequent breakdown of partial protective layer. The impedance magnitudes of the superamphiphobic coatings are much higher than the pure ETFE coating and bare Al substrates regardless of PANI/fCNTs addition. However, the EIS spectra in the case of superamphiphobic/ electroactive coating display features obviously different from the insulated superamphiphobic coating. In general, the composite coating with 6 wt % conductive PANI/fCNTs possesses higher total impedance than the coating without PANI/fCNTs due to its better water repellency property (Figure 16). Furthermore, the Nyquist plots for the ETFEPANI/fCNTs-0 coated Al exhibit one semicircle over the whole frequency range during the entire exposure period, implying a capacitive behavior and barrier type protection (Figure 15c). The diameter of the semicircle in the Nyquist plots reduced gradually with immersion time, whereas for ETFE-PANI/ fCNTs-6, the Nyquist plots show a near linear relationship between the imaginary and real parts of the impedance (Figure 15d), which can be attributed to the dispersion of conductive PANI/fCNTs in the coating.21 It should be noted that no obvious decrease could be observed in the impedance magnitude over the whole exposure time. Furthermore, the long-term anticorrosion performance of the superamphiphobic/electroactive ETFE-PANI/fCNTs-6 coating could also be reflected by the high phase angles (∼80°) over a

Figure 14. Potentiodynamic polarization curves for the uncoated aluminum substrate after 0.5 h (a), ETFE-PANI/fCNTs-0 coating after 1 d (c) and 90 d (b) immersion in 3.5 wt % NaCl solution, ETFE-PANI/fCNTs-6 coating after 1 d (e) and 90 d (d) immersion in 3.5 wt % NaCl solution.

current density (6.1 × 10−11 A/cm2) were obtained for the ETFE-PANI/fCNTs-6 coated Al substrate. However, after 90 d immersion, the corrosion potential decreased to 51 and −792 mV for the ETFE composite coating with and without PANI/ fCNT. Furthermore, the corrosion current density in ETFEPANI/fCNTs-6 is estimated to be about 3.5 × 10−10 A/cm2, which is about 3 and 5 orders of magnitude lower than the ETFE-PANI/fCNTs-0 coated aluminum and bare aluminum substrate, respectively. This reveals the long-term anticorrosion performance of superamphiphobic/electroactive multifunctional coating, attributed to the superior water-repellent ability and passive hydroxide film formed on the aluminum substrate.38 The corrosion rate (CR) and corrosion protection efficiency (PE, %) are obtained from the measured Icorr values according to eq 1 and eq 2.28,39 CR = 3270 ×

IcorrM Vd

(1)

⎛ Ic ⎞ ⎟ × 100 PE (%) = ⎜1 − corr 0 Icorr ⎠ ⎝

(2)

where 3270 = 0.01 × [1 year (in s)/96 497.8] and 96 497.8 = 1 F in Coulombs. M, V, and d represent the atomic mass, the valence, and the density of substrate, respectively. In addition, I0corr and Iccorr are the corrosion current of bare Al substrate and coated Al substrate, respectively. The ETFE-PANI/fCNTs-6 nanocomposite coating exhibits durable corrosion resistance property for the Al substrate under the free corrosion potential condition, with extremely lower corrosion rate (0.004 μm/year) and higher protection performance (99.997%) after 90 d immersion. The superior corrosion protection performance of ETFE-PANI/fCNTs-6 nanocompo-

Table 1. Potentiodynamic Polarization Parameter Values for Uncoated and Coated Al Substrates sample

immersion time (h)

Ecorr (mV)

uncoated ETFE-PANI/fCNTs-0

0.5 24 (1 d) 2160 (90 d) 24 (1 d) 2160 (90 d)

−797 −205 −792 129 51

ETFE-PANI/fCNTs-6

12490

Icorr (A/cm2) 1.3 1.0 1.2 6.1 3.5

× × × × ×

10−5 10−8 10−7 10−11 10−10

CR (μm/year)

PE (%)

152 0.12 1.40 0.001 0.004

99.923 99.077 99.999 99.997


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Figure 15. Nyquist plots of bare Al substrate (a), pure ETFE coating (b), ETFE-PANI/fCNTs-0 (c), and ETFE-PANI/CNTs-6 composite coatings (d) as a function of exposure time.

Figure 16. Bode plots of bare Al substrate (a, d), pure ETFE coating (b, e), and ETFE-PANI/fCNTs-6 composite coating (c, f) as a function of exposure time.

wide range in frequency during the whole immersion period (Figure 16f). Comparatively, two time constants at high and low frequencies could be differentiated in the impedance spectra for pure ETFE coated Al substrates within 60 d (Figure 16e). The time constant at high frequencies is related to the responses of the electrolyte/coating interface, and the time constant at lower frequencies is associated with the corrosion process happening at the electrolyte/substrate interface. Only one time constant at around 100 Hz could be found for the pure ETFE coated sample after 90 d immersion, possibly due to the corrosive chloride ions penetration through pinholes.21,31

The rough surface of ETFE-PANI/fCNTs-6 coating possesses micropores that are able to trap air at the solid− liquid interface and offer a stable air layer to prevent the penetration of water and chloride ions through the coating, thus enhancing the barrier property. In addition, the electroactive redox effect of PANI/fCNTs in composite coatings could induce the formation of a passive metal oxide layer on the aluminum substrate and provide a long-term active protection for aluminum substrate.38 It is suggested that the synergistic integration of superamphiphobicity of the coating surface and electroactivity of conductive PANI/fCNTs offers the best corrosion protection performance of the composite coating. 12491


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4. CONCLUSIONS Conductive PANI/fCNTs nanocomposite with core−shell structure was successfully prepared via in situ polymerization method. Superamphiphobic/electroactive ETFE composite coating on aluminum substrate was obtained by employing 6 wt % PANI/fCNTs and nanosized SiO2 as nanofiller. The superior antifouling to sludge, concentrated sulfuric acid, and mixture of water and ethyl glycerol can be ascribed to the hierarchical rough surface and the chemical resistant property of ETFE. The strong adhesion ability of the ETFE-PANI/ fCNTs coating is achieved by nano/micro roughness of the aluminum plate via acid etching process. The ETFE-PANI/ fCNTs-6 coating demonstrates a remarkable improvement in wear-/bending-resistance and antiwetting ability. The coating exhibits outstanding antiwettability up to 400 °C. The ETFEPANI/fCNTs-6 coated aluminum possessed extremely low corrosion rate (0.004 μm/year) and higher protection performance (99.997%) even after immersion in 3.5 wt % NaCl solution for 90 d. On the basis of the EIS analysis, the incorporation of PANI/fCNTs into the ETFE composite coating plays an essential role in long-term anticorrosion performance of the coating. It is expected that the robust multifunctional coatings have great potential in practical applications especially in harsh environments such as high temperature and corrosive environments.

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AUTHOR INFORMATION

Corresponding Authors

*H.W.: e-mail, [email protected]; phone, 86-459-6503083. *J.Z.: e-mail, [email protected]; phone, 1-330-972-6859. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the start-up fund of The University of Akron, National Science Foundation of China (Grants 51175066, 21507009), China Postdoctoral Science Foundation (Grant 2014M551215), Heilongjiang Educational Committee Foundation (Grant 12531077), Science Foundation of Northeast Petroleum University (Grant 2013NQ111). Acknowledgement is also made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (Grant 55570-DNI10).

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REFERENCES

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