Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications

    Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications Shunli Zheng, Cheng Li, Qitao Fu, Wei Hu, Tengfei Xiang, Qi Wang, Mengping Du, Xingchen Liu, Zhong Chen PII: DOI: Reference:

S0264-1275(15)31016-9 doi: 10.1016/j.matdes.2015.12.155 JMADE 1179

To appear in: Received date: Revised date: Accepted date:

14 May 2015 25 December 2015 28 December 2015

Please cite this article as: Shunli Zheng, Cheng Li, Qitao Fu, Wei Hu, Tengfei Xiang, Qi Wang, Mengping Du, Xingchen Liu, Zhong Chen, Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications, (2015), doi: 10.1016/j.matdes.2015.12.155

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ACCEPTED MANUSCRIPT Title: Development of Stable Superhydrophobic Coatings on Aluminum Surface for Corrosion-resistant, Self-cleaning, and Anti-icing Applications Author names and affiliations: , Cheng Li

a,*

, Qitao Fu b, Wei Hu a, Tengfei Xiang a, Qi Wang a,

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a,b

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Shunli Zheng

Mengping Du a, Xingchen Liu a, Zhong Chen b,*

College of Materials Science and Technology, Nanjing University of Aeronautics and

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a

Astronautics, Nanjing, Jiangsu 210016, PR China

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, 639798, Singapore

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Corresponding Authors:

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b

Professor Cheng Li:

Address: College of Materials Science and Technology, Nanjing University of

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Aeronautics and Astronautics, 29 Jiangjun Avenue, Jiangning District,

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Tel: +86-25-52112902

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Nanjing, Jiangsu 210016, PR China

Fax: +86-25-52112626

Email: [email protected]

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Professor Zhong Chen:

Address: School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

Tel: +65-67904256 Fax: +65-67909081 Email: [email protected]

ACCEPTED MANUSCRIPT Abstract Aluminum (Al) has been widely used in numerous applications, but it is prone to

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contamination or damage under harsh working environments. In this paper, a

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superhydrophobic coating (SHPC) on the Al surface was fabricated via a simple and cost effective method using anodization in sulfuric acid electrolyte followed by

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surface modification with inexpensive myristic acid. The as-prepared SHPC with hierarchical micro-nanostructure exhibited good superhydrophobicity with a static

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water contact angle (CA) of 155.2 ± 0.5° and a sliding angle (SA) of 3.5 ± 1.3°. The SHPC possessed both good mechanical and chemical stabilities: it retained a CA as

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high as 151.1 ± 0.1° after mechanical sandblasting for 60 s and was stable after dripping test using both acidic and alkaline solutions. Besides, after exposure to UV / water condensation cycles for 7 days, the coating remained superhydrophobic,

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indicating excellent weathering resistance. The prepared SHPC also demonstrated

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excellent self-cleaning and anti-icing performance. Ice adhesion strength as low as

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0.065 ± 0.022 MPa was obtained for the optimized coating. Electrochemical measurement showed that there is a two-order of magnitude of reduction of the corrosion current density (Jcorr) and the protection efficiency ( P% ) of the as-prepared

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SHPC has reached up to 99.75%.

Keywords: Superhydrophobic surface; Aluminum; Stability; Corrosion resistance; Self-cleaning; Anti-icing

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Introduction

Aluminum (Al) and its alloys are important engineering materials owing to its abundance in nature, good ductility, low specific weight and excellent electrical conductivity. They have been widely used in many areas, especially in sports, aerospace, transportation and civilian industries. Al is known to develop a thin natural oxide layer in dry and non-salty environments, which could prevent itself from further

ACCEPTED MANUSCRIPT corrosion. However, it is highly prone to corrosion in humid and salty environments [1], which will cause damage or malfunction of facilities and loss of aesthetic values.

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Therefore, it is very important to form a surface layer on Al to protect it from a wide

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spectrum of mechanical and chemical attacks. One of the approaches is through transforming the hydrophilic nature of Al surface to be superhydrophobic.

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Superhydrophobic surfaces with static water contact angles (CAs) higher than 150° and sliding angles (SAs) lower than 10° have attracted a great deal of interest in both

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scientific research and practical applications because of their unique properties, including water repellency [2], self-cleaning [3, 4], oil-water separation [5], anti-icing

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[6] and anti-corrosion [7]. The key to constructing superhydrophobic surface is to create a rough hierarchical micro-nanostructure on a low energy surface. In regards the broad applications of superhydrophobic surfaces in outdoor equipment,

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researchers have developed a great number of methods, including chemical etching

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[8], sol-gel [9], template [10], hydrothermal synthesis [11], electrospinning [12] and

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electrochemical processes [13].

Although there are numerous techniques to construct superhydrophobic surfaces, few products have been available for practical applications mainly due to their weak

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mechanical and poor chemical stabilities. It is important to note that most artificial superhydrophobic surfaces are easily damaged by even gentle physical rubbing, or finger scratching and so on. Besides, some superhydrophobic layers have weak adhesion with substrates, making it easy for them to be peeled off [14]. With respect to

chemical

stability,

many

superhydrophobic

surfaces

may

lose

their

superhydrophobicity rapidly after exposure to harsh conditions, such as wet environment, strong acidic or alkali solutions, etc. UV irradiation can accelerate their aging which leads to performance degradation and shortening of lifetime. To date, a variety of mechanically robust, chemically stable and UV resistant superhydrophobic surfaces have been reported. Wang et al. fabricated superhydrophobic sponges and fabrics with strong mechanical robustness by in situ growth of transition-metal oxides

ACCEPTED MANUSCRIPT and metallic nanocrystals [15]. Li et al. obtained superhydrophobic cotton fabrics with good abrasion and laundering stability through the repeated graft-on-graft strategy

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[16]. Wu et al. created mechanically robust superhydrophobic coatings on glass

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substrates and glass fiber reinforced epoxy composites using sol-gel method [17]. Lai et al. produced a transparent superhydrophobic TiO2-based coating with good

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chemical stability on indium tin oxide (ITO) glass [18]. Pan et al. prepared good UV blocking superhydrophobic cotton fabric using sol-gel and self-assembly method [19].

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Xiu et al. obtained inorganic superhydrophobic silica coating with improved UV stability via sol-gel processing [20]. However, these researches were mainly carried

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out on glass, fabric and sponge substrates. To the best of our knowledge, very few publications have been available on the investigation of mechanically and chemically stable [21], UV resistant superhydrophobic surfaces on Al surfaces.

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Currently, a number of approaches have been reported to fabricate superhydrophobic

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surfaces on Al and its alloys with outstanding corrosion resistance. Zhang et al.

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prepared a hierarchical superhydrophobic film which provides an effective corrosion-resistant coating for the underlying Al on PAO/Al substrates [22]. Barkhudarov et al. created superhydrophobic films as corrosion inhibitors on Al from

a

precursor

solution

containing

mixed

alkoxides

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surfaces

3,3,3-trifluoropropyl-trimethoxysilane and tetramethyl orthosilicate via a variation of the aerogel thin film process [23]. Yin et al. produced a superhydrophobic coating on Al alloy for corrosion protection by chemical etching followed by surface modification [24]. However, the researchers mainly focused on the improvement of corrosion potential (Ecorr) and decrease of corrosion current density (Jcorr), they did not pay much attention to the protection efficiency ( P% ) of the SHPCs for Al substrates. Ice is prone to accumulation on Al surfaces in tough freezing weather, which can cause serious accidents and economic losses [25]. It might be easy to correlate icephobicity with superhydrophobicity because it seems that the water repellence would be a common requirement for it. However, not all the superhydrophobic

ACCEPTED MANUSCRIPT surfaces can display good anti-icing property on Al surfaces [26, 27]. Besides, the durability of anti-icing property is an important consideration for any practical

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application. Kulinich et al. reported superhydrophobic Al surfaces whose surface

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asperities were gradually damaged and even lost the superhydrephobicity during icing/de-icing cycles, showing that the anti-icing performance of the samples was

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significantly deteriorated [28, 29]. So it is necessary to create SHPCs on Al surfaces which are endowed with excellent enduring anti-icing property.

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For practical applications, the outdoor surfaces are usually polluted by contaminants and dusts. So far, although there have been lots of reports on producing self-cleaning

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SHPCs, little detailed research is available to quantify their self-cleaning efficiency. This can be done by the color contrast which is directly related to the amount of artificial dirt on the surface [30].

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Summarizing the above analysis, it is very important to construct a stable and

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corrosion-resistant SHPC with excellent anti-icing property and self-cleaning effect

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on the Al surface for its wide applications. However, systematic work about all the properties together on the superhydrophobic Al surfaces has been rarely reported. Besides, most reported methods are still subjected to certain limited conditions

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involving low efficiency, complicated procedure and high cost of production [31, 32]. In this work, a facile and low-cost method is used to construct SHPCs on Al surfaces. The fabrication process contains two steps: construction of rough hierarchical micro-nanostructure with nanotubes and chemical modification with inexpensive myristic acid. The as-prepared SHPC has a CA as high as 155.2 ± 0.5° and a SA as low as 3.5 ± 1.3°. The mechanical and chemical stabilities of the SHPC have been evaluated by micro-sandblasting test and dripping test using solutions with different pH values. The SHPC also shows excellent weathering resistance and highly improved corrosion resistance after exposure to the UV / water condensation cycles for 7 days and immersion in 3.5 wt.% NaCl solution. Furthermore, the low ice

ACCEPTED MANUSCRIPT adhesion strength and dirt accumulation results demonstrate good anti-icing and

Experimental

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self-cleaning performance of the as-prepared SHPC.

2.1 Materials

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The Al plates (purity: 99.9%, thickness: 0.05 mm) were purchased from Art Friend & Buona Vista Pte Ltd, Singapore. Myristic acid [CH3(CH2)12COOH, purity: 95%],

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sodium hydroxide, nitric acid, sulfuric acid, hydrochloric acid and sodium chloride were supplied by Sigma-Aldrich, USA. Anhydrous ethanol was provided by EMD

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Millipore Corporation, Germany. In this experiment, all reagents were analytical grade

2.2 Sample preparation

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and deionized (DI) water was used to prepare the aqueous solutions.

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First, the Al plates were mechanically polished using 1000 grid emery papers to

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remove the native oxide on the surfaces. Next, they were ultrasonically degreased in sequence using 5 wt.% NaOH and 5 wt.% HNO3 solutions for 2 min, respectively, and DI water for 5 min before drying in air. A cleaned Al plate of 2.5 cm × 2.5 cm

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size as the anode and two lead (Pb) plates as the double cathodes were separated by a distance of 2.5 cm in a 15 wt.% H2SO4 electrolyte solution. The anodic oxidation was carried out with vigorously magnetic stirring under different voltages from 10 V to 22 V for 1 h. During the process, the temperature was kept at 25 °C using a water bath. After the anodization, the samples were thoroughly rinsed and ultrasonically cleaned with DI water and then dried. And then, the samples were modified with molten myristic acid at 70 °C for 30 min followed by immersing into anhydrous ethanol for 2 min and ultrasonically treated in DI water for 5 min to remove the excessive acid. Finally, the sample was heated at 80 °C for 1 h in the oven. The samples with myristic acid modification were denoted as MA-x and the anodized Al samples without surface

ACCEPTED MANUSCRIPT modification used as reference were denoted as AAO-x, where x represents the value

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of anodization voltage. The schematic fabrication process is shown in Fig. 1.

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Fig. 1. Schematic for fabrication of the samples.

2.3 Sample characterization

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The CA and SA were measured with a contact angle measurement device (OCA20, DataPhysics Corporation, Germany). All samples were measured five times on different positions with 4 μL water droplets at ambient temperature. The surface

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morphology and chemical composition were characterized by a field emission

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scanning electron microscope (FESEM, JEOL JSM-6340F, Japan). The surface

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roughness was measured by an atomic force microscope (AFM, Asylum Research Cypher S, Oxford Instruments Company, USA) with a scan size of 10 μm ×10 μm. The mechanical stability of the obtained samples was evaluated using a

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home-designed micro-sandblaster tester. The chemical stability was analysed by dripping water with different pH values on the surface for 5 s. The pH was adjusted by different amount of HCl or NaOH. The weathering resistance was tested by a UV / Condensation weathering instrument (ATLAS material testing technology LLC, USA) according to ASTM G154-12a for 7 days. The procedure contains UV exposure for 4 h and water spray and condensation for 4 h sequently. During the UV process, eight UV lamps (295-400 nm) with power intensity of 0.77 W/m2 each were used. The electrochemical corrosion measurement was performed in 3.5 wt.% NaCl solution (pH = 6.0) at ambient temperature using an electrochemical workstation (CHI 750C, Shanghai Chenhua Instrument Corporation). The sample with an exposed

ACCEPTED MANUSCRIPT area of 1 cm2 was used as the working electrode (WE). The saturated calomel electrode (SCE) and a platinum (Pt) sheet were uesd as the reference electrode (RE)

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and the counter electrode (CE) respectively. The potentiodynamic polarization curves

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were used to evaluate the corrosion resistance with a constant scan rate of 0.01 V/s. In

before the electrochemical measurement began.

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order to achieve a steady state, the sample was kept in the test environment for 1 h

The anti-icing behavior was measured by a home-designed adhesion tester placed

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inside an icing chamber, as shown in Fig. 2. To carry out this measurement, an teflon mold with inner diameter of 18 mm was filled with DI water, covered by the Al

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samples and then kept in the icing chamber at –10 °C for at least 24 h to form an ice column on the samples. During an adhesion test, when the piston contacts the ice column, the push force, driven by compressed air, will be increased at a constant rate

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until the ice column is separated from the sample surfaces. The corresponding force

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the following equation:

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was recorded by the force gauge. The ice adhesion strength is calculated according to

 F/A

(1)

where F was the force and A was the contact area between ice column and sample

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surface.

Fig. 2. Schematic of the home-designed adhesion tester.

The self-cleaning test was conducted by immersing the samples into 15 g/L artificial dirty solution after thorough mechanical mixing. The solution consisted of nano-clay, silica particles, carbon black, oil and salts. The lightness reading (L) of pristine coatings and immersed coatings (which were dried overnight and heated at 40 °C for

ACCEPTED MANUSCRIPT 30 min) was measured by a spectrophotometer (Elektro Physik, Germany). Each sample was measured five times at different locations. The lightness value represents

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the brightness reflected from the surfaces, and is related to the amount of dirt on the

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surface. The change in the lightness readings was calculated to evaluate the dirt

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accumulation on the samples.

Results and discussion

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3.1 CA and SA of the coatings under different anodization voltages The relationship between anodization voltage and CA as well as SA is shown in Fig. 3

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and Table 1. It can be clearly seen that the CA of the coatings increased from 114.1 ± 2.7° to 155.2 ± 0.5° with increasing anodization voltage from 0 V to 22 V. It is noticed that the CA improved drastically when the anodization voltage reached 20 V.

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However, when the anodization voltage increased to 22 V, both the CA and SA of the

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coatings slightly worsened from 155.2 ± 0.5° and 3.5 ± 1.3° to 152.8 ± 0.3° and 7.0 ±

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1.3°.

It is well known that trapped air in the grooves of superhydrophobic surfaces can lead to the decrease of the contact area between liquid and solid. Both the Wenzel and

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Cassie-Baxter models can result in a high CA, but only the Cassie-Baxter model (with an air gap) can lead to a very low SA [33]. Therefore, the fractional contact area of both MA-20 and MA-22 coatings was calculated using the Cassie-Baxter equation [34]: cos   f1 cos 1  f 2 cos 2

(2)

where   is apparent contact angle; 1 and  2 are the intrinsic CAs on components 1 and 2; f1 and f 2 are the corresponding surface area fractions of each component ( f1  f 2  1 ). Here, f 2 is the air fraction on the superhydrophobic surface, which means  2 = 180°. Thus equation (2) can be expressed as:

ACCEPTED MANUSCRIPT cos  f1 (cos 1  1)  1

(3)

Assuming that the CA on the smooth Al surface modified by myristic acid is 114.1 ±

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2.7°, it is estimated that f1 for MA-20 and MA-22 superhydrophobic coatings is

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0.1559 and 0.1869, respectively. That means 84.41% of water droplets are in contact

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with air for MA-20 coating and only 81.31% for MA-22 coating. These results demonstrate that the more trapped air in the coating, the higher CA on the surface. In

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such a case, the optimal anodization voltage to obtain the SHPC is 20 V.

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Fig. 3. CAs on different coatings.

Table 1. The CA and SA of MA-20 and MA-22 coatings. CA (°)

SA (°)

MA-20

155.2 ± 0.5

3.5 ± 1.3

MA-22

152.8 ± 0.3

7.0 ± 1.3

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Samples

3.2 Surface morphology and chemical composition The surface morphology is shown in Fig. 4. From Figs. 4(a) and (b), it can be seen that the MA-0 coating is very smooth. As shown in Figs. 4(c) and (d), the dense nanotubes were generated on the anodized Al surface with uniform nanopores of average diameter about 40 nm under the voltage of 20 V. It is obvious that the rough surface was achieved on the MA-20 coating which was completely covered by micro-scale rough structures with size range from 5 μm to 20 μm, as shown in Fig.

ACCEPTED MANUSCRIPT 4(e). And from the high magnification image of Fig. 4(f), it can be seen that the nanopores on these rough structures, indicating that the MA-20 SHPC has a

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hierarchical micro-nanostructure. Comparing Figs. 4(c) and (f), the surface

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morphologies of undecorated and decorated anodized Al surface shows no obvious difference, which suggests that only a very small amount of myristic acid has been

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assembled on the SHPC.

Fig. 4. FESEM images of the coatings: (a-b) MA-0 at different magnifications; (c-d) top view and cross section of AAO-20; (e-f) MA-20 at different magnifications.

Fig. 5 shows AFM scan images of the coatings. Fig 5 (a) shows the MA-0 coating has a root-mean-square roughness (Rq) of 69.9 nm, which is too flat to construct a superhydrophobic surface. After anodization at the voltage of 20 V, the Al surface with Rq of 788.3 nm, as shown in Fig. 5(b), was covered by nanotubes, which is in

ACCEPTED MANUSCRIPT agreement with the observation from Figs. 4(c) and (d). Fig. 5 (c) shows that the Rq of the superhydrophobic MA-20 coating decreased to 219.4 nm, which suggests that the

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rougher than MA-0 coating, the Al plate before anodization.

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myristic acid might have partially filled or covered the nanopores. But it is still much

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Fig. 5 AFM images of the coatings: (a) MA-0; (b) AAO-20; (c) MA-20.

Fig. 6 shows the EDS spectra of different coatings. As can be seen in Figs. 6 (a) and (b), the MA-0 coating is composed of C, O and Al while the AAO-20 surface consists of O, Al and S which is from sulfate ions (SO42-) incorporate in the AAO films during anodization in sulfuric acid [35, 36]. In contrast, both C and S appeared on the MA-20 coating, which demonstrates that myristic acid was anchored onto the anodized Al surface. With reference to other research [37], the myristic acid does not attach to the surface physically but reacts with released Al3+ to form Al carboxylate. The chemical reaction is as follows: Al3+ + CH3(CH2)12COOH → Al[CH3(CH2)12COO]3 + 3H+

(4)

ACCEPTED MANUSCRIPT According to the above analyses, it can be concluded that the synergy of hierarchical structure and chemical composition is the key to realize the surperhydrophobic

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surface.

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Fig. 6. EDS spectra of the coatings: (a) MA-0; (b) AAO-20; (c) MA-20.

3.3 Stability

Considering the practical application of the SHPCs in the outdoor environment, the

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mechanical and chemical stabilities as well as weathering resistance were

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investigated.

3.3.1 Mechanical Stability The micro-sandblasting test was carried out to evaluate the mechanical stability of the

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SHPCs. As shown in Fig. 7, the abrasive gun is placed 15 cm vertically above the sample surfaces with a constant exposed area (diameter = 17.6 mm). 63 μm SiO2 particles were blasted to the sample surface under 30 kPa pressure for 60 s. After the test, the coating was ultrasonically cleaned in DI water for 2 min and dried at 60 °C for 1 h in the oven.

Fig. 7. Schematic of the micro-sandblasting test.

ACCEPTED MANUSCRIPT After the sandblasting, MA-20 coating still retained superhydrophobicity with a CA of 151.1 ± 0.1° while MA-0 coating became hydrophilic with a CA of 73.2 ± 3.0°, as

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shown in Fig. 8. The insets in Fig. 8 display that the MA-0 coating surface was

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destroyed completely while no obvious change was observed on the MA-20 coating

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surface, which confirms the as-prepared SHPC has robust mechanical stability.

Fig. 8. CAs of MA-0 and MA-20 coatings before and after micro-sandblasting test for

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60 s. The insets are the images of corresponding coatings before and after the test.

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3.3.2 Chemical stability

The relationship between the CA of superhydrophobic MA-20 coating and the immersion time in DI water is depicted in Fig. 9. As shown in Fig. 9, the CA was

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slightly decreased to 152.4 ± 0.2° from 155.2 ± 0.5° after immersion in DI water for 19 days. Thus the immersion time in DI water has no significant effect on the surface states.

Fig. 9. Influence of immersion time in DI water on the CA of MA-20 coating for 19 days.

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The as-prepared MA-20 coating can stand not only DI water but also both acidic and

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alkaline solutions. Fig. 10 displays the change of CA on MA-20 coating with the

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water droplets of different pH values. It can be seen that there is no obvious fluctuation on CA and the coating maintained superhydrophobic over a wide range of

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pH values from 1 to 12. These results reveal that the MA-20 coating has good

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chemical stability in both acidic and alkaline conditions.

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Fig. 10. Influence of water droplets with different pH values on the CA of MA-20 coating for 5 s.

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3.3.3 Weathering resistance UV stability of the SHPCs is also important especially for outdoor surfaces. As shown in Fig. 11, the MA-20 coating still remained superhydrophobic with a CA above 150° while the CA of MA-0 coating reduced drastically from 114.1 ± 2.7° to 82.7 ± 3.4° after weathering resistance test for 7 days. The insets in Fig. 11 clearly display that there is a large amount of water on the MA-0 coating as compared to that on the MA-20 coating. It suggests that the MA-0 coating which turned hydrophilic attracts water to attach to its surface. On the other hand, due to the superhydrophobic nature of the MA-20 coating, the water droplets can easily slide off the surface because of the Cassie-Baxter mode of contact as discussed before.

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Fig. 11. CAs of MA-0 and MA-20 coatings before and after weathering resistance test

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for 7 days. The insets are the images of corresponding coatings after the test.

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The surface morphology for both MA-0 and MA-20 coatings after the weathering resistance test is shown in Fig. 12. Comparing Figs. 4(a) and 12(a), it is clear that there are some pits and holes on the MA-0 surface, indicating that the native

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protective layer on the MA-0 surface is not resistant to continuous UV irradiation and

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water spray. On the contrary, we found that the microstructure remains unchanged on

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the MA-20 coated surface by comparing Fig. 4(e) with Fig. 12(b). This indicates that the SHPC has a substantial improvement on the weathering resistance for Al

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substrates.

Fig. 12. FESEM images of the coatings after weathering resistance test for 7 days: (a) MA-0; (b) MA-20.

ACCEPTED MANUSCRIPT 3.4 Corrosion resistance The potentiodynamic polarization curve is a useful method to evaluate the

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instantaneous corrosion rate of the substrates. Higher corrosion potentiel (Ecorr) and

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lower corrosion current density (Jcorr), as well as higher polarization resistance (Rp), correspond to better corrosion resistance. Additionally, the protection efficiency ( P% )

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can be calculated by the following equation [38]:

P%  100[ Rp1 (uncoated )  Rp1 (coated )] / Rp1 (uncoated )

(5)

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Fig. 13 depicts the potentiodynamic polarization curves for bare Al surface, MA-0 and MA-20 coatings in 3.5 wt.% NaCl solution. The Ecorr, Jcorr and Rp were obtained

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by Tafel extrapolation, as listed in Table 2.

As seen in Fig. 13 and Table 2, it can be found that Ecorr and Jcorr changed remarkably

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when the Al surface became superhydrophobic. The Ecorr of the superhydrophobic

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MA-20 coating was 59 mV more positive than the bare Al substrate and 39 mV higher than the hydrophobic MA-0 coating, and the corresponding Jcorr (1.527×10-9

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A∙cm-2) was reduced by 2 orders of magnitude as compared to that of bare Al substrate. Meanwhile, the as-prepared superhydrophobic MA-20 coating showed higher Rp, which was 408 times that of the bare Al and 147 times that of the MA-0

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coating. Comparing the P% of both MA-0 and MA-20 coatings, the MA-20 coating showed a P% of 99.75% for Al substrate, which was much higher than that of MA-0 coating (63.93%). Additionally, we have made a comparison with other reported anticorrosion superhydrophobic Al and its alloy surfaces. Yin et al. [39] and He et al. [40] produced superhydrophobic films on Al as corrosion protection, and it was found that the Jcorr and P% were about 10-7 A∙cm-2 and 96%. Saleema et al. obtained a superhydrophobic Al alloy surface, but both the Jcorr and Rp of the hydrophilic surface and the superhydrophobic surface do not show significant difference, which demonstrates that there is not substantial improvement in the corrosion resistance [41]. Feng et al. prepared a superhydrophobic surface on Al alloy. The Jcorr of the superhydrophobic surface (5.01×10-5 A∙cm-2) is reduced by 1 order of

ACCEPTED MANUSCRIPT magnitude compared with the clean bare Al alloy (7.26×10-4 A∙cm-2) [42]. Our results show that the as-prepared superhydrophobic MA-20 coating has a much better

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corrosion resistance than the previously reported superhydrophobic Al and its alloy

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surfaces.

Fig.13. Potentiodynamic polarization curves of different samples: Al, MA-0 and

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MA-20.

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Table 2. Electrochemical parameters of potentiodynamic polarization curves for the

Samples

MA-0

MA-20

Jcorr (A∙cm-2)

Rp (Ω∙cm2)

P (%)

-0.629

3.060×10-7

8.451×104

-

-0.609

2.214×10-7

2.343×105

63.93

-0.570

1.527×10-9

3.448×107

99.75

Ecorr (V vs. SCE)

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Al

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samples.

3.5 Anti-icing property Ice could easily accumulate on Al surfaces at low temperatures. In this study, the ice adhesion strength was investigated to evaluate the anti-icing performance. As shown in Fig. 14, the superhydrophobic MA-20 coating exhibited the lowest ice adhesion strength (0.065 ± 0.022 MPa) among the samples at -10 °C and is still superhydrophobic after 10 icing/de-icing cycles. In comparison, Kulinich et al. reported a superhydrophobic Al surface with ice adhesion strength from about 0.055 MPa to 0.110 MPa and CA less than 150° after the samples were repeated 10

ACCEPTED MANUSCRIPT icing/de-icing cycles [29]. The result shows that the MA-20 coating has a low ice adhesion strength and a higher CA. The hydrophilic Al and superhydrophilic AAO-20

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surfaces have high ice adhesion strength of 1.024 ± 0.283 MPa and 1.137 ± 0.110

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MPa, respectively, which indicates that these two hydrophilic surfaces can be partly or completely wetted to form strong bonding with ice [43]. However, for the

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superhydrophobic MA-20 coating, the trapped air between the ice and coating surface makes great contribution to the reported low adhesion strength. According to the

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classical nucleation theory [44, 45], the heterogeneous nucleation is easier on concave or flat surfaces than on convex surfaces. Due to the trapped air, the water droplets can

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only stay on the top of convex parts and thus it is difficult to form nucleation. In addition, the trapped air can also reduce the actual contact area between the ice and coating surface, which is in favour of minimizing the mechanical anchoring effect.

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There exists a correlation between the CA and the ice adhesion for the surfaces: the

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higher the CA, the lower the ice adhesion [46].

Fig. 14. Ice adhesion strength of the samples: Al; AAO-20 and MA-20.

It was observed that a large fraction of ice stuck on the Al and AAO-20 surfaces while very small amount of residual ice was left on the superhydrophobic MA-20 coating after the icing/de-icing test, as shown in Fig. 15. This further confirms that the as-prepared SHPC has excellent anti-icing property.

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Fig. 15. Images of the samples after icing/de-icing test for 10 cycles: (a) Al; (b) AAO-20; (c) MA-20.

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3.6 Self-cleaning effect

Fig. 16 shows the self-cleaning performance of the samples. After immersion in the

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dirty solution and drying, there was a lot of dirt accumulated on the Al and AAO-20 surfaces, as seen in Figs. 16(d) and (e). In contrast, little dirt can be seen on the

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MA-20 coating, as shown in Fig. 16(f). After water spray, the MA-20 coating was as

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clean as before, as shown in Fig. 16(i). However, both Al and AAO-20 surfaces were still covered by a large amount of dirt, as shown in Figs. 16(g) and (h). The

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observation indicates that it is difficult for the dirt to attach to the superhydrophobic surface and the dirt can be easily taken away by water spraying. The reason is the

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joint action of high capillary forces induced by water droplets and weak adhesion of the powder to the superhydrophobic surface [47].

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Fig. 16. Self-cleaning process of the samples. Pristine samples before test: (a) Al (b)

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AAO-20 (c) MA-20; Immersed samples after immersion in the dirty solution for 1

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min: (d) Al (e) AAO-20 (f) MA-20; Clean samples after water spray and drying: (g) Al (h) AAO-20 (i) MA-20.

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The dirt accumulation on the SHPCs was further quantified with the reflection brightness. The lightness values of the pristine samples and immersed samples after drying were recorded as L1 and L2. The difference between L1 and L2, which is defined as ΔL, represents the extent of dirt accumulated on the surface after immersion. The percentage of dirt accumulation on the samples can be calculated as ΔL / L1 × 100. The details are shown in Table 3. It can be clearly seen from Table 3 that the MA-20 coating has the lowest percentage of dirt accumulation compared to Al and AAO-20 surfaces, which is in great agreement with the observation in Fig. 16. Consequently, the as-prepared SHPC has an excellent contamination resistance and self-cleaning efficacy, which are important for practical applications.

ACCEPTED MANUSCRIPT Table 3. Lightness values of the samples before and after immersion in the artificial dirty solution. L2

(ΔL = L1 - L2)

Al

75.61

62.86

12.75

AAO-20

64.93

57.54

MA-20

64.34

64.21

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Dirt accumulation (%) ΔL / L1 × 100

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L1

16.86

7.39

11.38

0.13

0.20

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Samples

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ΔL

Conclusions

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In this study, a self-cleaning SHPC with a CA of 155.2 ± 0.5° and a SA of 3.5 ± 1.3° has been successfully constructed on Al surface using a facile and low-cost method by

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the combination of electrochemical anodization and chemical modification. The

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superhydrophobicity on Al surfaces is derived from the synergistic effect of rough hierarchical micro-nanostructure and the low energy surface after the treatment. The

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as-prepared self-cleaning SHPC has not only good mechanical and chemical stabilities, excellent weathering resistance, but also highly improved corrosion

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resistance and good anti-icing performance. This preparation process offers an effective strategy for fabrication of SHPCs on Al surfaces and shows promising applications including outdoor sports equipment, transportation facilities and other industrial facilities.

Acknowledgments The authors would like to thank the PhD Abroad Short-term Visiting Project of Nanjing University of Aeronautics and Astronautics and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions as well as the Project of NUAA-UT Group Joint Laboratory of Advanced Electronic Materials. We also thank Dr. Danping Wang and Dr. Xin Zhao for the FESEM morphology characterization.

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References

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[1] Ghahremaninezhad A, Dolati A. A study on electrochemical growth behavior of the Co–Ni alloy nanowires in anodic aluminum oxide template. J Alloys Compd. 2009;480:275-8. [2] Singh AV, Rahman A, Sudhir Kumar NVG, Aditi AS, Galluzzi M, Bovio S, et al. Bio-inspired approaches to design smart fabrics. Mater Des. 2012;36:829-39. [3] Wang Z, Li Q, She Z, Chen F, Li L. Low-cost and large-scale fabrication method for an environmentally-friendly superhydrophobic coating on magnesium alloy. J Mater Chem. 2012;22:4097-105. [4] Kumar D, Wu X, Fu Q, Ho JWC, Kanhere PD, Li L, et al. Hydrophobic sol–gel coatings based on polydimethylsiloxane for self-cleaning applications. Mater Des. 2015;86:855-62. [5] Huang JY, Li SH, Ge MZ, Wang LN, Xing TL, Chen GQ, et al. Robust superhydrophobic TiO2@fabrics for UV shielding, self-cleaning and oil-water separation. J Mater Chem A. 2015;3:2825-32. [6] Fu Q, Wu X, Kumar D, Ho JWC, Kanhere PD, Srikanth N, et al. Development of sol–gel icephobic coatings: effect of surface roughness and surface energy. ACS Appl Mater Interfaces. 2014;6:20685-92. [7] Zheng S, Li C, Fu Q, Li M, Hu W, Wang Q, et al. Fabrication of self-cleaning superhydrophobic surface on aluminum alloys with excellent corrosion resistance. Surf Coat Technol. 2015;276:341-8. [8] Qian B, Shen Z. Fabrication of superhydrophobic surfaces by dislocation-selective chemical etching on aluminum, copper, and zinc substrates. Langmuir. 2005;21:9007-9. [9] Zhao Q, Wu LYL, Huang H, Liu Y. Ambient-curable superhydrophobic fabric coating prepared by water-based non-fluorinated formulation. Mater Des. 2016;92:541-5. [10] Kannarpady GK, Khedir KR, Ishihara H, Woo J, Oshin OD, Trigwell S, et al. Controlled growth of self-organized hexagonal arrays of metallic nanorods using template-assisted glancing angle deposition for superhydrophobic applications. ACS Appl Mater Interfaces. 2011;3:2332-40. [11] Shen YZ, Tao HJ, Chen SL, Zhu LM, Wang T, Tao J. Icephobic/anti-icing potential of superhydrophobic Ti6Al4V surfaces with hierarchical textures. RSC Adv. 2015;5:1666-72. [12] Ganesh VA, Nair AS, Raut HK, Yuan Tan TT, He C, Ramakrishna S, et al. Superhydrophobic fluorinated POSS-PVDF-HFP nanocomposite coating on glass by electrospinning. J Mater Chem. 2012;22:18479-85. [13] Lai Y, Pan F, Xu C, Fuchs H, Chi L. In situ surface-modification-induced superhydrophobic patterns with reversible wettability and adhesion. Adv Mater. 2013;25:1682-6.

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

[14] Zhang Z, Zhu X, Yang J, Xu X, Men X, Zhou X. Facile fabrication of superoleophobic surfaces with enhanced corrosion resistance and easy repairability. Appl Phys A: Mater Sci Process. 2012;108:601-6. [15] Wang B, Li J, Wang G, Liang W, Zhang Y, Shi L, et al. Methodology for robust superhydrophobic fabrics and sponges from in situ growth of transition metal/metal oxide nanocrystals with thiol modification and their applications in oil/water separation. ACS Appl Mater Interfaces. 2013;5:1827-39. [16] Li SH, Huang JY, Ge MZ, Li SW, Xing TL, Chen GQ, et al. Controlled grafting superhydrophobic cellulose surface with environmentally-friendly short fluoroalkyl chains by ATRP. Mater Des. 2015;85:815-22. [17] Wu X, Fu Q, Kumar D, Ho JWC, Kanhere P, Zhou H, et al. Mechanically robust superhydrophobic and superoleophobic coatings derived by sol–gel method. Mater Des. 2016;89:1302-9. [18] Lai Y, Tang Y, Gong J, Gong D, Chi L, Lin C, et al. Transparent superhydrophobic/superhydrophilic TiO2-based coatings for self-cleaning and anti-fogging. J Mater Chem. 2012;22:7420-6. [19] Pan C, Shen L, Shang S, Xing Y. Preparation of superhydrophobic and UV blocking cotton fabric via sol–gel method and self-assembly. Appl Surf Sci. 2012;259:110-7. [20] Xiu Y, Hess DW, Wong CP. UV and thermally stable superhydrophobic coatings from sol–gel processing. J Colloid Interface Sci. 2008;326:465-70. [21] Peng S, Tian D, Yang X, Deng W. Highly efficient and large-scale fabrication of superhydrophobic alumina surface with strong stability based on self-congregated alumina nanowires. ACS Appl Mater Interfaces. 2014;6:4831-41. [22] Zhang F, Zhao L, Chen H, Xu S, Evans DG, Duan X. Corrosion resistance of superhydrophobic layered double hydroxide films on aluminum. Angew Chem Int Ed. 2008;47:2466-9. [23] Barkhudarov PM, Shah PB, Watkins EB, Doshi DA, Brinker CJ, Majewski J. Corrosion inhibition using superhydrophobic films. Corros Sci. 2008;50:897-902. [24] Yin B, Fang L, Hu J, Tang AQ, He J, Mao JH. A facile method for fabrication of superhydrophobic coating on aluminum alloy. Surf Interface Anal. 2012;44:439-44. [25] Tarquini S, Antonini C, Amirfazli A, Marengo M, Palacios J. Investigation of ice shedding properties of superhydrophobic coatings on helicopter blades. Cold Reg Sci Technol. 2014;100:50-8. [26] Zou M, Beckford S, Wei R, Ellis C, Hatton G, Miller MA. Effects of surface roughness and energy on ice adhesion strength. Appl Surf Sci. 2011;257:3786-92. [27] Javan-Mashmool M, Volat C, Farzaneh M. A new method for measuring ice adhesion strength at an ice–substrate interface. Hydrol Process. 2006;20:645-55. [28] Kulinich SA, Farhadi S, Nose K, Du XW. Superhydrophobic surfaces: are they really ice-repellent? Langmuir. 2011;27:25-9. [29] Farhadi S, Farzaneh M, Kulinich SA. Anti-icing performance of superhydrophobic surfaces. Appl Surf Sci. 2011;257:6264-9.

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

[30] Kumar D, Wu X, Fu Q, Ho JWC, Kanhere PD, Li L, et al. Development of durable self-cleaning coatings using organic-inorganic hybrid sol–gel method. Appl Surf Sci. 2015;344:205-12. [31] Liu Y, Liu J, Li S, Han Z, Yu S, Ren L. Fabrication of biomimetic super-hydrophobic surface on aluminum alloy. J Mater Sci. 2014;49:1624-9. [32] Liao R, Zuo Z, Guo C, Yuan Y, Zhuang A. Fabrication of superhydrophobic surface on aluminum by continuous chemical etching and its anti-icing property. Appl Surf Sci. 2014;317:701-9. [33] Liu W, Luo Y, Sun L, Wu R, Jiang H, Liu Y. Fabrication of the superhydrophobic surface on aluminum alloy by anodizing and polymeric coating. Appl Surf Sci. 2013;264:872-8. [34] Cassie ABD, Baxter S. Wettability of porous surfaces. Trans Faraday Soc. 1944;40:546-51. [35] Li S, Wang J, Li Y, Zhang X, Wang G, Wang C. Photoluminescent properties of anodic aluminum oxide films formed in a mixture of malonic and sulfuric acid. Superlattice Microst. 2014;75:294-302. [36] Wang JY, Li C, Yin CY, Wang YH, Zheng SL. Ultrasmall nanopores obtained by electric field enhanced one-step anodisation of aluminium alloy. Surf Coat Technol. 2014;258:615-23. [37] Tao Y. Structural comparison of self-assembled monolayers of n-alkanoic acids on the surfaces of silver, copper, and aluminum. J Am Chem Soc. 1993;115:4350-8. [38] Peng C-W, Chang K-C, Weng C-J, Lai M-C, Hsu C-H, Hsu S-C, et al. UV-curable nanocasting technique to prepare bio-mimetic super-hydrophobic non-fluorinated polymeric surfaces for advanced anticorrosive coatings. Polym Chem. 2013;4:926-32. [39] Yin Y, Liu T, Chen S, Liu T, Cheng S. Structure stability and corrosion inhibition of super-hydrophobic film on aluminum in seawater. Appl Surf Sci. 2008;255:2978-84. [40] He T, Wang Y, Zhang Y, lv Q, Xu T, Liu T. Super-hydrophobic surface treatment as corrosion protection for aluminum in seawater. Corros Sci. 2009;51:1757-61. [41] Saleema N, Sarkar D, Gallant D, Paynter R, Chen X. Chemical nature of superhydrophobic aluminum alloy surfaces produced via a one-step process using fluoroalkyl-silane in a base medium. ACS Appl Mater Interfaces. 2011;3:4775-81. [42] Feng L, Zhang H, Wang Z, Liu Y. Superhydrophobic aluminum alloy surface: fabrication, structure, and corrosion resistance. Colloids Surf A. 2014;441:319-25. [43] Hassan MF, Lee HP, Lim SP. The variation of ice adhesion strength with substrate surface roughness. Meas Sci Technol. 2010;21:075701. [44] Jamieson MJ, Nicholson CE, Cooper SJ. First study on the effects of interfacial curvature and additive interfacial density on heterogeneous nucleation. Ice crystallization in oil-in-water emulsions and nanoemulsions with added 1-heptacosanol. Cryst Growth Des. 2005;5:451-9.

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

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T

[45] Fletcher NH. Size effect in heterogeneous nucleation. J Chem Phys. 1958;29:572-6. [46] Dotan A, Dodiuk H, Laforte C, Kenig S. The relationship between water wetting and ice adhesion. J Adhes Sci Technol. 2009;23:1907-15. [47] Su F, Yao K. Facile fabrication of superhydrophobic surface with excellent mechanical abrasion and corrosion resistance on copper substrate by a novel method. ACS Appl Mater Interfaces. 2014;6:8762-70.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights •

A hierarchical superhydrophobic coating (SHPC) was obtained by a cost

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Synergistic effect of micro-nanostructure and low surface energy is the key to

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effective method.

SHPC.

The SHPC retains a high contact angle above 150° after sandblasting for 60 s.

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The SHPC shows excellent corrosion resistance with protection efficiency of

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99.75%.

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icing/de-icing cycles.

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The SHPC has low ice adhesion strength around 0.065 MPa after 10

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•