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A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces
Applied Surface Science 266 (2013) 445–450
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces Jinlong Song a,∗ , Yao Lu a , Shuai Huang a , Xin Liu a , Libo Wu a,b , Wenji Xu a,∗ a b
School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China School of Mechanical Engineering, Dalian Ocean University, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 7 April 2012 Received in revised form 5 December 2012 Accepted 11 December 2012 Available online 20 December 2012 Keywords: Superhydrophobic surface Mg alloy Immersion
a b s t r a c t A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces is present here. Micro/nanometer-scale rough structures composed of micrometer-scale island-like rough structures and nanometer-scale sheets are generated on the Mg alloy surfaces after immersion in the aqueous CuSO4 solution. After ultrasonic cleaning, the micro/nanometer-scale rough structures are disappeared, whereas the lump-like rough structures appear on the Mg alloy surfaces. After modification with stearic acid, the as-prepared micro/nanometer-scale rough structures and the micrometer-scale lump-like rough structures all show superhydrophobicity. The contact angles of the water droplet on the aforementioned two structures are respectively 151.3◦ and 161.8◦ . The rolling angles are respectively 3◦ and 13◦ . The results indicate that the cooperation of suitable rough structures and stearic acid modification is responsible for the obtained superhydrophobicity on the Mg alloy surfaces. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces are the surfaces with the water contact angle larger than 150◦ and can be divided into two classes according to the size of the rolling angle. One is the low adhesive superhydrophobic surfaces with the rolling angle less than 10◦ . The other is the high adhesive superhydrophobic surfaces without any rolling angle, which allow water droplets to pin the surfaces even when the surfaces are turned upside down. The common low adhesive surfaces in nature are lotus leaves, rice leaves, and water strider’s legs [1,2]. The common high adhesive surfaces in nature are peanut leaves, rose petals, and the feet of some animals, such as geckos, flies, bees, and locusts [3]. Since Morra et al. [4] fabricated the superhydrophobic surfaces on PTFE substrates; the fabrication and application of superhydrophobic surfaces have attracted great attention in the field of biology, physics, chemistry, materials science, and mechanics. Superhydrophobic surfaces are of great prospect applied in the fields of military, communication, energy, environment, and biomedicine for their characteristics of high loading capacity, drag reduction, anti-icing, corrosion resistance, and self-cleaning. The use of superhydrophobic surfaces on the micro water-transport equipment can greatly improve the loading capacity, which can be used to prepare the micro aquatic robots for military purposes [5]. 70–80% of the ship resistance is attributed to the frictional
∗ Corresponding authors. Tel.: +86 411 84708422; fax: +86 411 84708422. E-mail addresses: [email protected] (J. Song), [email protected] (W. Xu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.063
resistance between ship and surrounding water flow. The use of superhydrophobic surfaces on ship can effectively reduce the frictional resistance and increase the sailing velocity. The use of superhydrophobic surfaces on the microflow devices can achieve the low resistance and no loss liquid transportation [6,7]. The use of superhydrophobic surfaces on the aircraft wings and propellers can inhibit the frost to prevent safety accidents, such as increasing flight resistance and aircraft crash, caused by ice and frost. The use of superhydrophobic surfaces on the high voltage transmission lines can effectively delay the formation of ice on the cables, insulators, poles and towers, enhance the AC flashover voltages under icing condition, and prevent safety accidents, such as short circuit and pour tower. The use of superhydrophobic surfaces on the satellite antenna receivers used in areas where it snows often can prevent signal interruption caused by snow. The use of superhydrophobic surfaces on the refrigeration equipment can inhibit the water bridge and ice coating on the evaporator, reduce the ventilation resistance and noise, and avoid the decrease of the heat exchange efficiency [8,9]. The marine engineering materials, such as, the sea working platforms, ships, and shells of the torpedoes, are easily corroded. The use of superhydrophobic surfaces on the aforementioned engineering materials can effectively isolate the contact between the substrates and sea water and increase the corrosion resistance [10]. The use of superhydrophobic surfaces on the glasses of buildings and automobiles can confer the self-cleaning property to the glasses. The garment and textile products with a superhydrophobicity have a waterproof breathable effect [11]. The use of superhydrophobic and superoleophobic surfaces on the petroleum pipeline can stop the petroleum from clinging to the pipe inner surfaces, reduce the loss of energy in the transportation, and
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Fig. 1. SEM images of (a)–(c) polished Mg alloy, (d)–(f) sample A, and (g)–(i) sample B surfaces.
prevent the pipes from being clogged [12]. Marine oil spillage causes serious ecological damage. The conventional methods used to solve oil spillage have limitations, such as low absorption capacity, poor recyclability, and so on. However, the use of superhydrophobic and superoleophilic surfaces on the separation of oil and water has numerous advantages, such as high oil absorption capacity and completely no absorbing water [13–15]. The use of superhydrophobic surfaces on the artificial vascular inner surfaces can reduce the platelet adhesion and improve the blood compatibility [16]. It is because that superhydrophobic surfaces have so many characteristics and applications and have an important influence on national defense and economy, many countries have made much research on the theory, fabrication method, and applications of superhydrophobic surfaces. Mg alloys are widely used because of their low specific gravity, high stiffness/weight ratios, good castability, good vibration, and shock adsorption. Because of the aforementioned superior characteristics, the fabrication of the superhydrophobic Mg alloy surfaces has attracted considerable interest among researchers. To date, many approaches, such as strong acids etching [17–19], micro-arc oxidation [20], chemical deposition [21–24], electrochemical etching [25], and electrochemical deposition [26], have been developed to fabricate the superhydropbobic Mg alloy surfaces. However, most of the mentioned approaches involve severe conditions, such as dangerous chemicals, high processing cost, low processing efficiency, complex device, and so on. To solve the aforementioned disadvantages, a simple, highly effective, and low-cost immersion approach was developed to fabricate the superhydrophobic Mg alloy surfaces.
2. Experimental 20 mm × 30 mm × 2 mm AZ61 Mg alloy plates composed of 6.29 wt% Al, 0.98 wt% Zn, 0.37 wt% Mn, and Mg balance (Dongguan Jiahao Die Materials Co., China) were polished mechanically using 1500# metallographic abrasive paper to remove the surface oxide layer. The plates were then cleaned ultrasonically in sequence with alcohol and deionized water for 1 min. After drying, the cleaned Mg alloy plates were immersed in the 0.2 mol/L aqueous CuSO4 solution at 80 ± 5 ◦ C for 10 min, and then washed thoroughly with deionized water. The immersed Mg alloy plates were marked as sample A. We found that, after immersion, the sample A was covered with a layer of green film. Some sample A were cleaned ultrasonically with deionized water until the green film was removed completely, and the gray substrates appeared. The gray substrates were marked as sample B. Finally, the as-obtained sample A and B were immersed in a 0.05 mol/L ethanol solution of stearic acid for 3 h and subsequently dried at a 120 ◦ C for 10 min. The surface morphology of the samples was characterized using a scanning electron microscope (SEM, JSM-6360LV, Japan). The chemical composition of the samples was characterized using an Xray diffractometer system (XRD-6000, Japan), an energy-dispersive X-ray spectroscope (EDS, INCA Energy, Oxford Ins.), and a Fouriertransform infrared spectrophotometer (FTIR, JACSCO, Japan). The X-ray source was a Cu K␣ radiation ( = 0.15418 nm), which was operated at 40 kV and 40 mA within the range of 5◦ and 100◦ at a scanning rate of 2 = 0.026◦ /min. The water contact angle and rolling angle were measured according to the sessile-drop method using an optical contact angle meter (Krüss, DSA100, Germany)
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at ambient temperature. Water droplets (5 L) were carefully dropped onto the surfaces of the plates, and the average value of five measurements obtained at the different positions in the samples was adopted as the final contact angle. The water rolling angle was defined as the angle at which the water drop began to roll off the gradually inclined surface. 3. Results and discussion 3.1. Surface morphology To study the changes of the microstructure on the Mg alloy surfaces, the morphology of the sample surfaces at the different treatment approach has been observed carefully with SEM. Fig. 1 shows the SEM images of the polished Mg alloy surfaces, sample A, and sample B at different magnifications. It can be seen from Fig. 1(a)–(c) that the polished Mg alloy surfaces appear smooth except for several scratches generated by polishing. After immersion in the aqueous CuSO4 solution for 10 min, a layer of rough green film was obtained on the Mg alloy surfaces (sample A). Fig. 1(d) shows the SEM images of the sample A surfaces at low magnification. Some island-like structures can be found distributed unevenly on the sample A surfaces in a random pattern with diameters ranging from 15 m to 50 m. The island-like structures are separated from each other, forming voids. As can be seen from the magnified image of the sample A (Fig. 1(e) and (f)), many nanosheets with an edge length of about 500 nm and a thickness of about 70 nm are present on the surfaces of the island-like structures and voids. Thus, after immersion, hierarchical micro/nanometerscale rough structures composed of micrometer-scale island-like structures and nanometer-scale sheets are formed on the Mg alloy surfaces. After ultrasonic cleaning, the green film completely drops out, leaving gray rough structures. Fig. 1(g)–(i) shows the SEM images of the sample B surfaces. Lump-like structures 5–20 m in diameter divided by cracks are uniformly distributed on the sample B surfaces. The lump-like structure surfaces are rather smooth without any relatively finer rough structures. 3.2. Chemical composition The analyses of surface chemical composition were carried out using XRD and EDS. Figs. 2 and 3 show the XRD patterns and EDS spectra of the polished Mg alloy, sample A, and sample B surfaces. 12 diffraction peaks detected on the polished Mg alloy surfaces are attributed to Mg (JCPDS Card No. 04-0770). After immersion in the aqueous CuSO4 solution for 10 min, the characteristic peaks of alkali copper sulfate [Cu4 SO4 (OH)6 and Cu3 (SO4 )2 (OH)2 ·4H2 O] are detected besides other diffraction peaks attributed to Mg (JCPDS Card No.13-0398, 37-0525), indicating that new composition is generated in the reaction between Mg alloys and aqueous CuSO4
Fig. 2. XRD patterns of (a) polished Mg alloy, (b) sample A, and (c) sample B surfaces.
solution. It also can be seen in Fig. 3(a) and (b) that, compared with the polished Mg alloy surfaces, elements S, O, and Cu are present on the sample A surfaces, further confirming that the hierarchical micro/nanometer-scale rough structures shown in Fig. 1(d)–(f) are alkali copper sulfate. Fig. 2(c) shows that the sample B surfaces exhibit a similar XRD pattern with the polished Mg alloy surfaces besides two very weak diffraction peaks attributed to alkali copper sulfate. In addition, the content of elements S, O, and Cu in Fig. 3(c) are much less than that in Fig. 3(b). The results of XRD and EDS indicate that the content of alkali copper sulfate is very little, and the main composition of the lump-like structures shown in Fig. 1(g)–(i) is Mg. 3.3. Reaction mechanism There is a reversible hydrolysis equilibrium reaction between Cu2+ ions and H2 O molecules in the aqueous CuSO4 solution, which produce H+ ions and make the solution acidic (Eq. (1)). Mg is extremely easy to react selectively with H+ ions to produce Mg2+ ions and H2 because of a high chemical activation of Mg and a strong ability of H+ ions to acquire the electron pair (Eq. (2)). With the continuous consumption of H+ in the solution, the hydrolysis equilibrium reaction moves right, leading to the generation of Cu(OH)2 . The generated Cu(OH)2 reacts with residual CuSO4 in the solution to form sparingly soluble green alkali copper sulfate, which attach onto the sample A surfaces (Eqs. (3) and (4)). The phase diagram of the AZ61 Mg alloys shows a typical Mg–Al binary phase diagram. The alloys were composed of two main phases, namely: the Mg-rich ␣ phase and the Al-rich (M17 Al12 )  phase [27]. The electrode potential of the ␣ phase is lower than that of the  phase [28]. Compared with the  phase, the ␣ phase of
Fig. 3. EDS spectra of (a) polished Mg alloy, (b) sample A, and (c) sample B surfaces.
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Fig. 4. FTIR spectra of (a) stearic acid, (b) sample A without stearic acid modification, and (c) stearic acid modified sample A.
Mg alloys is easily corroded by H+ , resulting in the higher corrosion velocity of the ␣ phase. Thus, the lump-like structures obtained on the sample B surfaces after ultrasonic cleaning is due to the uneven corrosion of the ␣ phase and  phase. Cu2+ + 2H2 O Cu(OH)2 + 2H+ +
Mg + 2H = Mg
2+
(1)
+ H2
(2)
Cu(OH)2 + CuSO4 = Cu4 (OH)6 SO4
(3)
Cu(OH)2 + 2CuSO4 + 4H2 O = Cu3 (SO4 )2 (OH)2 ·4H2 O
(4)
3.4. Stearic acid modification The low-cost stearic acid was employed to reduce the surface energy of the Mg alloy surfaces. Fig. 4(a)–(c) shows the FTIR spectra of stearic acid, the sample A without stearic acid modification, and the stearic acid modified sample A. Compared with the sample A without stearic acid modification (Fig. 4(b)), many absorption bands are detected on the stearic acid modified sample A, indicating that stearic acid modification affects the chemical composition of the sample A surfaces. It is also found the free COO
Fig. 5. FTIR spectra of (a) stearic acid, (b) sample B without stearic acid modification, and (c) stearic acid modified sample B.
band from stearic acid at 1702 cm−1 (Fig. 4(a)) is no longer present in the spectrum of the strearic acid modified sample A. A new band appears at 1585 cm−1 (Fig. 4(c)) corresponding to coordinated COO groups. Additionally, from the spectra of Fig. 4(a) and (c), there is no obvious change of the bands standing for CH3 (2956 cm−1 ) and CH2 (2916 and 2850 cm−1 ) groups. The FTIR measurements demonstrate that stearic acid has been successfully grafted onto the sample A surfaces after immersion in the ethanol solution of stearic acid. It is well known that stearic acid is widely used to fabricate superhydrophobic surfaces because of its lower surface energy. The CH3 and CH2 groups could significally lower the surface energy and enhance the hydrophobicity of the sample A surfaces. Similarly, the two absorption peaks at 1562 cm−1 and 1469 cm−1 , which are ascribed to the asymmetric and symmetric stretching vibration of coordinated COO groups, are detected on the stearic acid modified sample B (Fig. 5), indicating that stearic acid has been successfully grafted onto the sample B surfaces after immersion in the ethanol solution of stearic acid. Fig. 6 shows the SEM images of the stearic acid modified sample A and sample B surfaces. It can be seen that the surface structures have little difference with Fig. 1, indicating the effect of the stearic acid modification on the surface morphology is small.
Fig. 6. SEM images of stearic acid modified (a)–(c) sample A and (d)–(f) sample B surfaces.
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Fig. 7. Images of water droplets on (a) polished Mg alloy surfaces, (b) sample A surfaces without stearic acid modification, (c) sample B surfaces without stearic acid modification, (d) stearic acid modified sample A surfaces, and (e) stearic acid modified sample B surfaces. (f) Digital image of three water droplets on stearic acid modified sample A surfaces.
3.5. Wettability Fig. 7 shows the images of the water droplets on the different Mg alloy surfaces. The water contact angle of the polished Mg alloy surfaces is approximately 30.9◦ , indicating hydrophilicity (Fig. 7(a)). After obtaining the hierarchical micro/nanometer-scale rough structures, the sample A surfaces without stearic acid modification exhibit superhydrophilicity. The water droplets completely spread on the sample A surfaces at a water contact angle of approximately 0◦ (Fig. 7(b)). After obtaining the lump-like structures via ultrasonic cleaning, the hydrophilicity is obtained again. The water contact angle of the sample B surfaces without stearic acid modification is 35.7◦ (Fig. 7(c)). When the sample A and sample B are modified by stearic acid with a low surface energy, the wettability changes from superhydrophilicity or hydrophilicity to superhydrophobicity. The water contact and rolling angles of a water droplet on the stearic acid modified sample A and sample B surfaces are respectively 151.3◦ , 3◦ , and 161.8◦ , 13◦ (Fig. 7(d)–(f)), indicating that the cooperation of the rough structures and stearic acid modification is responsible for the superhydrophobicity of the as prepared Mg alloy surfaces. Jiang et al. [1] have reported that nanostructures on the surfaces are effective for obtaining the large water contact angles. However, the obtained superhydrophobicity on the stearic acid modified sample B surfaces only containing
the micrometer-scale lump-like structures indicates that suitable micrometer-scale rough structures also can induce a high water contact angle. However, the rolling angle of the surfaces only containing the micrometer-scale rough structures is much larger than that of the surfaces containing the micro/nanometer-scale rough structures. The stability of the resultant superhydrophobic surfaces in air was evaluated. The experiment results show that the superhydrophobicity can be maintained at least for 8 months of storage in air. The water contact and rolling angles of the stearic acid modified sample A and sample B surfaces after exposure in air for 8 months are respectively 150.5◦ , 2◦ , and 161.5◦ , 12.5◦ , indicating the resultant superhydrophobic surfaces have good long-term stability in air. 4. Conclusion In summary, we have developed a simple, highly effective, and low-cost immersion approach to fabricate the superhydrophobic Mg alloy surfaces. The measurements of SEM, XRD, and EDS indicate that the hierarchical micro/nanometer-scale rough structures composed of the micrometer-scale island-like structures and nanosheets, which are obtained on the Mg alloy surfaces after immersion in the aqueous CuSO4 solution for 10 min, are alkali copper sulfate. The lump-like rough structures obtained
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after ultrasonic cleaning are Mg. The FTIR measurements indicate that stearic acid has been successfully grafted onto the rough structures after immersion in the ethanol solution of stearic acid. The water contact and rolling angles of a water droplet on the stearic acid modified hierarchical micro/nanometer-scale rough structures and lump-like rough structures are respectively 151.3◦ , 3◦ , and 161.8◦ , 13◦ , showing superhydrophobicity. The study also finds that the superhydrophobicity can be obtained on the surfaces only containing the micrometer-scale rough structures. However, the rolling angle of the surfaces only containing the micrometer-scale rough structures is much larger than that of the surfaces containing the micro/nanometer-scale rough structures. The approach can be easily scaled up to create the large-area uniform superhydrophobic surfaces and can be applied in commercial manufacturing. Acknowledgments This work was financially supported by National Natural Science Foundation of China (NSFC, Grant No. 90923022) and Scholarship Award for Excellent Doctoral Student Granted by Ministry of Education.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
References
[20]
[1] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q. Liu, L. Jiang, D.B. Zhu, Super-hydrophobic surfaces: from natural to artificial, Advanced Materials 14 (2002) 1857–1860. [2] X. Gao, L. Jiang, Water-repellent legs of water striders, Nature 432 (2004) 36. [3] Y.K. Lai, Z. Chen, C.J. Lin, A review on the recent progress in superhydrophobic surfaces with special adhesions, Science China Chemistry 41 (2011) 609–628. [4] M. Morra, E. Occhiello, F. Garbassi, Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene), Langmuir 5 (1989) 872–876. [5] Q.M. Pan, J. Liu, Q. Zhu, A water strider-like model with large and stable loading capacity fabricated from superhydrophobic copper foils, ACS Applied Materials and Interfaces 2 (2010) 2026–2030. [6] G. McHale, N.J. Shirtcliffe, C.R. Evans, M.I. Newton, Terminal velocity and drag reduction measurements on superhydrophobic spheres, Applied Physics Letters 94 (2009) 064104–64111. [7] M. Zhou, J. Li, C.X. Wu, X.K. Zhou, L. Cai, Fluid drag reduction on superhydrophobic surfaces coated with carbon nanotube forests (CNTs), Soft Matter 7 (2011) 4391–4396. [8] Y. Yang, J. Li, J.L. Hu, Y.S. Zhao, X.L. Jiang, C.X. Sun, Experimental study on icing properties of superhydrophobic coatings on insulators, High Voltage Engineering 36 (2010) 621–626. [9] C. Antonini, M. Innocenti, T. Horn, M. Marengo, A. Amirfazli, Understanding the effect of superhydrophobic coatings on energy reduction in anti-icing systems, Cold Regions Science and Technology 67 (2011) 58–67. [10] L.B. Boinovich, S.V. Gnedenkov, D.A. Alpysbaeva, V.S. Egorkin, A.M. Emelyanenko, S.L. Sinebryukhov, A.K. Zaretskaya, Corrosion resistance of composite
[21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
coatings on low-carbon steel containing hydrophobic and superhydrophobic layers in combination with oxide sublayers, Corrosion Science 55 (2012) 238–245. J. Zimmermann, F.A. Reifler, G. Fortunato, L. Gerhardt, S. Seeger, A simple, one-step approach to durable and robust superhydrophobic textiles, Advanced Functional Materials 18 (2008) 3662–3669. J.M. Xi, L. Feng, L. Jiang, A general approach for fabrication of superhydrophobic and superamphiphobic surfaces, Applied Physics Letters 92 (2008) 053102–153102. Q. Zhu, Q.M. Pan, F.T. Liu, Facile removal and collection of oils from water surfaces through superhydrophobic and superoleophilic sponges, Journal of Physical Chemistry C 115 (2011) 17464–17470. J. Wu, J. Chen, K. Qasim, J. Xia, W. Lei, B.P. Wang, A hierarchical mesh film with superhydrophobic and superoleophilic properties for oil and water separation, Journal of Chemical Technology and Biotechnology 87 (2012) 427–430. C.H. Su, Y.Q. Xu, W. Zhang, Y. Liu, J. Li, Porous ceramic membrane with superhydrophobic and superoleophilic surface for reclaiming oil from oily water, Applied Surface Science 258 (2012) 2233–2319. L. Chen, D. Han, L. Jiang, Study and fabrication of biomimetic vascular interface materials, Journal of Southeast University (Medical Science Edition) 30 (2011) 224–227. B. Yin, L. Fang, J. Hu, A.Q. Tang, W.H. Wei, J. He, Preparation and properties of super-hydrophobic coating on magnesium alloy, Applied Surface Science 257 (2010) 1666–1671. Y.H. Wang, W. Wang, L. Zhong, J. Wang, Q.L. Jiang, X.Y. Guo, Super-hydrophobic surface on pure magnesium substrate by wet chemical method, Applied Surface Science 256 (2010) 3837–3840. K.S. Liu, M.L. Zhang, J. Zhai, J. Wang, L. Jiang, Bioinspired construction of Mg–Li alloys surfaces with stable superhydrophobicity and improved corrosion resistance, Applied Physics Letters 92 (2008), 183103–183103-3. Z.X. Kang, X.M. Lai, F. Wang, Y. Long, Y.Y. Li, Preparation of super-hydrophobic duplex-treated film on surface of Mg–Mn–Ce magnesium alloy and its corrosion resistance, Chinese Journal of Nonferrous Metals 21 (2011) 283–289. T. Ishizaki, N. Saito, Rapid formation of a superhydrophobic surface on a magnesium alloy coated with a cerium oxide film by a simple immersion process at room temperature and its chemical stability, Langmuir 26 (2010) 9749–9755. T. Ishizaki, Y. Masuda, M. Sakamoto, Corrosion resistance and durability of superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution, Langmuir 27 (2011) 4780–4788. J. Wang, D.D. Li, Q. Liu, X. Yin, Y. Zhang, X.Y. Jing, M.L. Zhang, Fabrication of hydrophobic surface with hierarchical structure on Mg alloy and its corrosion resistance, Electrochimica Acta 55 (2010) 6897–6906. J. Wang, D.D. Li, R. Gao, Q. Liu, X.Y. Jing, Y.L. Wang, Y. He, M.L. Zhang, Z.H. Jiang, Construction of superhydrophobic hydromagnesite films on the Mg alloy, Materials Chemistry and Physics 12 (9) (2011) 154–160. W.J. Xu, J.L. Song, J. Sun, Y. Lu, Z.Y. Yu, Rapid fabrication of large-area, corrosionresistant superhydrophobic Mg alloy surfaces, ACS Applied Materials and Interfaces 3 (2011) 4404–4414. Z.W. Wang, Q. Li, Z.X. She, F.N. Chen, L.Q. Li, Low-cost and large-scale fabrication method for an environmentally-friendly superhydrophobic coating on magnesium alloy, Journal of Materials Chemistry 22 (2012) 4097–4105. G. Ballerini, U. Bardi, R. Bignucolo, G. Ceraolo, About some corrosion mechanisms of AZ91D magnesium alloy, Corrosion Science 47 (2005) 2173–2184. G. Galicia, N. Pébère, B. Tribollet, V. Vivier, Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy, Corrosion Science 51 (2009) 1789–1794.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces Jinlong Song a,∗ , Yao Lu a , Shuai Huang a , Xin Liu a , Libo Wu a,b , Wenji Xu a,∗ a b
School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China School of Mechanical Engineering, Dalian Ocean University, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 7 April 2012 Received in revised form 5 December 2012 Accepted 11 December 2012 Available online 20 December 2012 Keywords: Superhydrophobic surface Mg alloy Immersion
a b s t r a c t A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces is present here. Micro/nanometer-scale rough structures composed of micrometer-scale island-like rough structures and nanometer-scale sheets are generated on the Mg alloy surfaces after immersion in the aqueous CuSO4 solution. After ultrasonic cleaning, the micro/nanometer-scale rough structures are disappeared, whereas the lump-like rough structures appear on the Mg alloy surfaces. After modification with stearic acid, the as-prepared micro/nanometer-scale rough structures and the micrometer-scale lump-like rough structures all show superhydrophobicity. The contact angles of the water droplet on the aforementioned two structures are respectively 151.3◦ and 161.8◦ . The rolling angles are respectively 3◦ and 13◦ . The results indicate that the cooperation of suitable rough structures and stearic acid modification is responsible for the obtained superhydrophobicity on the Mg alloy surfaces. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobic surfaces are the surfaces with the water contact angle larger than 150◦ and can be divided into two classes according to the size of the rolling angle. One is the low adhesive superhydrophobic surfaces with the rolling angle less than 10◦ . The other is the high adhesive superhydrophobic surfaces without any rolling angle, which allow water droplets to pin the surfaces even when the surfaces are turned upside down. The common low adhesive surfaces in nature are lotus leaves, rice leaves, and water strider’s legs [1,2]. The common high adhesive surfaces in nature are peanut leaves, rose petals, and the feet of some animals, such as geckos, flies, bees, and locusts [3]. Since Morra et al. [4] fabricated the superhydrophobic surfaces on PTFE substrates; the fabrication and application of superhydrophobic surfaces have attracted great attention in the field of biology, physics, chemistry, materials science, and mechanics. Superhydrophobic surfaces are of great prospect applied in the fields of military, communication, energy, environment, and biomedicine for their characteristics of high loading capacity, drag reduction, anti-icing, corrosion resistance, and self-cleaning. The use of superhydrophobic surfaces on the micro water-transport equipment can greatly improve the loading capacity, which can be used to prepare the micro aquatic robots for military purposes [5]. 70–80% of the ship resistance is attributed to the frictional
∗ Corresponding authors. Tel.: +86 411 84708422; fax: +86 411 84708422. E-mail addresses: [email protected] (J. Song), [email protected] (W. Xu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.063
resistance between ship and surrounding water flow. The use of superhydrophobic surfaces on ship can effectively reduce the frictional resistance and increase the sailing velocity. The use of superhydrophobic surfaces on the microflow devices can achieve the low resistance and no loss liquid transportation [6,7]. The use of superhydrophobic surfaces on the aircraft wings and propellers can inhibit the frost to prevent safety accidents, such as increasing flight resistance and aircraft crash, caused by ice and frost. The use of superhydrophobic surfaces on the high voltage transmission lines can effectively delay the formation of ice on the cables, insulators, poles and towers, enhance the AC flashover voltages under icing condition, and prevent safety accidents, such as short circuit and pour tower. The use of superhydrophobic surfaces on the satellite antenna receivers used in areas where it snows often can prevent signal interruption caused by snow. The use of superhydrophobic surfaces on the refrigeration equipment can inhibit the water bridge and ice coating on the evaporator, reduce the ventilation resistance and noise, and avoid the decrease of the heat exchange efficiency [8,9]. The marine engineering materials, such as, the sea working platforms, ships, and shells of the torpedoes, are easily corroded. The use of superhydrophobic surfaces on the aforementioned engineering materials can effectively isolate the contact between the substrates and sea water and increase the corrosion resistance [10]. The use of superhydrophobic surfaces on the glasses of buildings and automobiles can confer the self-cleaning property to the glasses. The garment and textile products with a superhydrophobicity have a waterproof breathable effect [11]. The use of superhydrophobic and superoleophobic surfaces on the petroleum pipeline can stop the petroleum from clinging to the pipe inner surfaces, reduce the loss of energy in the transportation, and
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Fig. 1. SEM images of (a)–(c) polished Mg alloy, (d)–(f) sample A, and (g)–(i) sample B surfaces.
prevent the pipes from being clogged [12]. Marine oil spillage causes serious ecological damage. The conventional methods used to solve oil spillage have limitations, such as low absorption capacity, poor recyclability, and so on. However, the use of superhydrophobic and superoleophilic surfaces on the separation of oil and water has numerous advantages, such as high oil absorption capacity and completely no absorbing water [13–15]. The use of superhydrophobic surfaces on the artificial vascular inner surfaces can reduce the platelet adhesion and improve the blood compatibility [16]. It is because that superhydrophobic surfaces have so many characteristics and applications and have an important influence on national defense and economy, many countries have made much research on the theory, fabrication method, and applications of superhydrophobic surfaces. Mg alloys are widely used because of their low specific gravity, high stiffness/weight ratios, good castability, good vibration, and shock adsorption. Because of the aforementioned superior characteristics, the fabrication of the superhydrophobic Mg alloy surfaces has attracted considerable interest among researchers. To date, many approaches, such as strong acids etching [17–19], micro-arc oxidation [20], chemical deposition [21–24], electrochemical etching [25], and electrochemical deposition [26], have been developed to fabricate the superhydropbobic Mg alloy surfaces. However, most of the mentioned approaches involve severe conditions, such as dangerous chemicals, high processing cost, low processing efficiency, complex device, and so on. To solve the aforementioned disadvantages, a simple, highly effective, and low-cost immersion approach was developed to fabricate the superhydrophobic Mg alloy surfaces.
2. Experimental 20 mm × 30 mm × 2 mm AZ61 Mg alloy plates composed of 6.29 wt% Al, 0.98 wt% Zn, 0.37 wt% Mn, and Mg balance (Dongguan Jiahao Die Materials Co., China) were polished mechanically using 1500# metallographic abrasive paper to remove the surface oxide layer. The plates were then cleaned ultrasonically in sequence with alcohol and deionized water for 1 min. After drying, the cleaned Mg alloy plates were immersed in the 0.2 mol/L aqueous CuSO4 solution at 80 ± 5 ◦ C for 10 min, and then washed thoroughly with deionized water. The immersed Mg alloy plates were marked as sample A. We found that, after immersion, the sample A was covered with a layer of green film. Some sample A were cleaned ultrasonically with deionized water until the green film was removed completely, and the gray substrates appeared. The gray substrates were marked as sample B. Finally, the as-obtained sample A and B were immersed in a 0.05 mol/L ethanol solution of stearic acid for 3 h and subsequently dried at a 120 ◦ C for 10 min. The surface morphology of the samples was characterized using a scanning electron microscope (SEM, JSM-6360LV, Japan). The chemical composition of the samples was characterized using an Xray diffractometer system (XRD-6000, Japan), an energy-dispersive X-ray spectroscope (EDS, INCA Energy, Oxford Ins.), and a Fouriertransform infrared spectrophotometer (FTIR, JACSCO, Japan). The X-ray source was a Cu K␣ radiation ( = 0.15418 nm), which was operated at 40 kV and 40 mA within the range of 5◦ and 100◦ at a scanning rate of 2 = 0.026◦ /min. The water contact angle and rolling angle were measured according to the sessile-drop method using an optical contact angle meter (Krüss, DSA100, Germany)
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at ambient temperature. Water droplets (5 L) were carefully dropped onto the surfaces of the plates, and the average value of five measurements obtained at the different positions in the samples was adopted as the final contact angle. The water rolling angle was defined as the angle at which the water drop began to roll off the gradually inclined surface. 3. Results and discussion 3.1. Surface morphology To study the changes of the microstructure on the Mg alloy surfaces, the morphology of the sample surfaces at the different treatment approach has been observed carefully with SEM. Fig. 1 shows the SEM images of the polished Mg alloy surfaces, sample A, and sample B at different magnifications. It can be seen from Fig. 1(a)–(c) that the polished Mg alloy surfaces appear smooth except for several scratches generated by polishing. After immersion in the aqueous CuSO4 solution for 10 min, a layer of rough green film was obtained on the Mg alloy surfaces (sample A). Fig. 1(d) shows the SEM images of the sample A surfaces at low magnification. Some island-like structures can be found distributed unevenly on the sample A surfaces in a random pattern with diameters ranging from 15 m to 50 m. The island-like structures are separated from each other, forming voids. As can be seen from the magnified image of the sample A (Fig. 1(e) and (f)), many nanosheets with an edge length of about 500 nm and a thickness of about 70 nm are present on the surfaces of the island-like structures and voids. Thus, after immersion, hierarchical micro/nanometerscale rough structures composed of micrometer-scale island-like structures and nanometer-scale sheets are formed on the Mg alloy surfaces. After ultrasonic cleaning, the green film completely drops out, leaving gray rough structures. Fig. 1(g)–(i) shows the SEM images of the sample B surfaces. Lump-like structures 5–20 m in diameter divided by cracks are uniformly distributed on the sample B surfaces. The lump-like structure surfaces are rather smooth without any relatively finer rough structures. 3.2. Chemical composition The analyses of surface chemical composition were carried out using XRD and EDS. Figs. 2 and 3 show the XRD patterns and EDS spectra of the polished Mg alloy, sample A, and sample B surfaces. 12 diffraction peaks detected on the polished Mg alloy surfaces are attributed to Mg (JCPDS Card No. 04-0770). After immersion in the aqueous CuSO4 solution for 10 min, the characteristic peaks of alkali copper sulfate [Cu4 SO4 (OH)6 and Cu3 (SO4 )2 (OH)2 ·4H2 O] are detected besides other diffraction peaks attributed to Mg (JCPDS Card No.13-0398, 37-0525), indicating that new composition is generated in the reaction between Mg alloys and aqueous CuSO4
Fig. 2. XRD patterns of (a) polished Mg alloy, (b) sample A, and (c) sample B surfaces.
solution. It also can be seen in Fig. 3(a) and (b) that, compared with the polished Mg alloy surfaces, elements S, O, and Cu are present on the sample A surfaces, further confirming that the hierarchical micro/nanometer-scale rough structures shown in Fig. 1(d)–(f) are alkali copper sulfate. Fig. 2(c) shows that the sample B surfaces exhibit a similar XRD pattern with the polished Mg alloy surfaces besides two very weak diffraction peaks attributed to alkali copper sulfate. In addition, the content of elements S, O, and Cu in Fig. 3(c) are much less than that in Fig. 3(b). The results of XRD and EDS indicate that the content of alkali copper sulfate is very little, and the main composition of the lump-like structures shown in Fig. 1(g)–(i) is Mg. 3.3. Reaction mechanism There is a reversible hydrolysis equilibrium reaction between Cu2+ ions and H2 O molecules in the aqueous CuSO4 solution, which produce H+ ions and make the solution acidic (Eq. (1)). Mg is extremely easy to react selectively with H+ ions to produce Mg2+ ions and H2 because of a high chemical activation of Mg and a strong ability of H+ ions to acquire the electron pair (Eq. (2)). With the continuous consumption of H+ in the solution, the hydrolysis equilibrium reaction moves right, leading to the generation of Cu(OH)2 . The generated Cu(OH)2 reacts with residual CuSO4 in the solution to form sparingly soluble green alkali copper sulfate, which attach onto the sample A surfaces (Eqs. (3) and (4)). The phase diagram of the AZ61 Mg alloys shows a typical Mg–Al binary phase diagram. The alloys were composed of two main phases, namely: the Mg-rich ␣ phase and the Al-rich (M17 Al12 )  phase [27]. The electrode potential of the ␣ phase is lower than that of the  phase [28]. Compared with the  phase, the ␣ phase of
Fig. 3. EDS spectra of (a) polished Mg alloy, (b) sample A, and (c) sample B surfaces.
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Fig. 4. FTIR spectra of (a) stearic acid, (b) sample A without stearic acid modification, and (c) stearic acid modified sample A.
Mg alloys is easily corroded by H+ , resulting in the higher corrosion velocity of the ␣ phase. Thus, the lump-like structures obtained on the sample B surfaces after ultrasonic cleaning is due to the uneven corrosion of the ␣ phase and  phase. Cu2+ + 2H2 O Cu(OH)2 + 2H+ +
Mg + 2H = Mg
2+
(1)
+ H2
(2)
Cu(OH)2 + CuSO4 = Cu4 (OH)6 SO4
(3)
Cu(OH)2 + 2CuSO4 + 4H2 O = Cu3 (SO4 )2 (OH)2 ·4H2 O
(4)
3.4. Stearic acid modification The low-cost stearic acid was employed to reduce the surface energy of the Mg alloy surfaces. Fig. 4(a)–(c) shows the FTIR spectra of stearic acid, the sample A without stearic acid modification, and the stearic acid modified sample A. Compared with the sample A without stearic acid modification (Fig. 4(b)), many absorption bands are detected on the stearic acid modified sample A, indicating that stearic acid modification affects the chemical composition of the sample A surfaces. It is also found the free COO
Fig. 5. FTIR spectra of (a) stearic acid, (b) sample B without stearic acid modification, and (c) stearic acid modified sample B.
band from stearic acid at 1702 cm−1 (Fig. 4(a)) is no longer present in the spectrum of the strearic acid modified sample A. A new band appears at 1585 cm−1 (Fig. 4(c)) corresponding to coordinated COO groups. Additionally, from the spectra of Fig. 4(a) and (c), there is no obvious change of the bands standing for CH3 (2956 cm−1 ) and CH2 (2916 and 2850 cm−1 ) groups. The FTIR measurements demonstrate that stearic acid has been successfully grafted onto the sample A surfaces after immersion in the ethanol solution of stearic acid. It is well known that stearic acid is widely used to fabricate superhydrophobic surfaces because of its lower surface energy. The CH3 and CH2 groups could significally lower the surface energy and enhance the hydrophobicity of the sample A surfaces. Similarly, the two absorption peaks at 1562 cm−1 and 1469 cm−1 , which are ascribed to the asymmetric and symmetric stretching vibration of coordinated COO groups, are detected on the stearic acid modified sample B (Fig. 5), indicating that stearic acid has been successfully grafted onto the sample B surfaces after immersion in the ethanol solution of stearic acid. Fig. 6 shows the SEM images of the stearic acid modified sample A and sample B surfaces. It can be seen that the surface structures have little difference with Fig. 1, indicating the effect of the stearic acid modification on the surface morphology is small.
Fig. 6. SEM images of stearic acid modified (a)–(c) sample A and (d)–(f) sample B surfaces.
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Fig. 7. Images of water droplets on (a) polished Mg alloy surfaces, (b) sample A surfaces without stearic acid modification, (c) sample B surfaces without stearic acid modification, (d) stearic acid modified sample A surfaces, and (e) stearic acid modified sample B surfaces. (f) Digital image of three water droplets on stearic acid modified sample A surfaces.
3.5. Wettability Fig. 7 shows the images of the water droplets on the different Mg alloy surfaces. The water contact angle of the polished Mg alloy surfaces is approximately 30.9◦ , indicating hydrophilicity (Fig. 7(a)). After obtaining the hierarchical micro/nanometer-scale rough structures, the sample A surfaces without stearic acid modification exhibit superhydrophilicity. The water droplets completely spread on the sample A surfaces at a water contact angle of approximately 0◦ (Fig. 7(b)). After obtaining the lump-like structures via ultrasonic cleaning, the hydrophilicity is obtained again. The water contact angle of the sample B surfaces without stearic acid modification is 35.7◦ (Fig. 7(c)). When the sample A and sample B are modified by stearic acid with a low surface energy, the wettability changes from superhydrophilicity or hydrophilicity to superhydrophobicity. The water contact and rolling angles of a water droplet on the stearic acid modified sample A and sample B surfaces are respectively 151.3◦ , 3◦ , and 161.8◦ , 13◦ (Fig. 7(d)–(f)), indicating that the cooperation of the rough structures and stearic acid modification is responsible for the superhydrophobicity of the as prepared Mg alloy surfaces. Jiang et al. [1] have reported that nanostructures on the surfaces are effective for obtaining the large water contact angles. However, the obtained superhydrophobicity on the stearic acid modified sample B surfaces only containing
the micrometer-scale lump-like structures indicates that suitable micrometer-scale rough structures also can induce a high water contact angle. However, the rolling angle of the surfaces only containing the micrometer-scale rough structures is much larger than that of the surfaces containing the micro/nanometer-scale rough structures. The stability of the resultant superhydrophobic surfaces in air was evaluated. The experiment results show that the superhydrophobicity can be maintained at least for 8 months of storage in air. The water contact and rolling angles of the stearic acid modified sample A and sample B surfaces after exposure in air for 8 months are respectively 150.5◦ , 2◦ , and 161.5◦ , 12.5◦ , indicating the resultant superhydrophobic surfaces have good long-term stability in air. 4. Conclusion In summary, we have developed a simple, highly effective, and low-cost immersion approach to fabricate the superhydrophobic Mg alloy surfaces. The measurements of SEM, XRD, and EDS indicate that the hierarchical micro/nanometer-scale rough structures composed of the micrometer-scale island-like structures and nanosheets, which are obtained on the Mg alloy surfaces after immersion in the aqueous CuSO4 solution for 10 min, are alkali copper sulfate. The lump-like rough structures obtained
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after ultrasonic cleaning are Mg. The FTIR measurements indicate that stearic acid has been successfully grafted onto the rough structures after immersion in the ethanol solution of stearic acid. The water contact and rolling angles of a water droplet on the stearic acid modified hierarchical micro/nanometer-scale rough structures and lump-like rough structures are respectively 151.3◦ , 3◦ , and 161.8◦ , 13◦ , showing superhydrophobicity. The study also finds that the superhydrophobicity can be obtained on the surfaces only containing the micrometer-scale rough structures. However, the rolling angle of the surfaces only containing the micrometer-scale rough structures is much larger than that of the surfaces containing the micro/nanometer-scale rough structures. The approach can be easily scaled up to create the large-area uniform superhydrophobic surfaces and can be applied in commercial manufacturing. Acknowledgments This work was financially supported by National Natural Science Foundation of China (NSFC, Grant No. 90923022) and Scholarship Award for Excellent Doctoral Student Granted by Ministry of Education.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
References
[20]
[1] L. Feng, S.H. Li, Y.S. Li, H.J. Li, L.J. Zhang, J. Zhai, Y.L. Song, B.Q. Liu, L. Jiang, D.B. Zhu, Super-hydrophobic surfaces: from natural to artificial, Advanced Materials 14 (2002) 1857–1860. [2] X. Gao, L. Jiang, Water-repellent legs of water striders, Nature 432 (2004) 36. [3] Y.K. Lai, Z. Chen, C.J. Lin, A review on the recent progress in superhydrophobic surfaces with special adhesions, Science China Chemistry 41 (2011) 609–628. [4] M. Morra, E. Occhiello, F. Garbassi, Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene), Langmuir 5 (1989) 872–876. [5] Q.M. Pan, J. Liu, Q. Zhu, A water strider-like model with large and stable loading capacity fabricated from superhydrophobic copper foils, ACS Applied Materials and Interfaces 2 (2010) 2026–2030. [6] G. McHale, N.J. Shirtcliffe, C.R. Evans, M.I. Newton, Terminal velocity and drag reduction measurements on superhydrophobic spheres, Applied Physics Letters 94 (2009) 064104–64111. [7] M. Zhou, J. Li, C.X. Wu, X.K. Zhou, L. Cai, Fluid drag reduction on superhydrophobic surfaces coated with carbon nanotube forests (CNTs), Soft Matter 7 (2011) 4391–4396. [8] Y. Yang, J. Li, J.L. Hu, Y.S. Zhao, X.L. Jiang, C.X. Sun, Experimental study on icing properties of superhydrophobic coatings on insulators, High Voltage Engineering 36 (2010) 621–626. [9] C. Antonini, M. Innocenti, T. Horn, M. Marengo, A. Amirfazli, Understanding the effect of superhydrophobic coatings on energy reduction in anti-icing systems, Cold Regions Science and Technology 67 (2011) 58–67. [10] L.B. Boinovich, S.V. Gnedenkov, D.A. Alpysbaeva, V.S. Egorkin, A.M. Emelyanenko, S.L. Sinebryukhov, A.K. Zaretskaya, Corrosion resistance of composite
[21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
coatings on low-carbon steel containing hydrophobic and superhydrophobic layers in combination with oxide sublayers, Corrosion Science 55 (2012) 238–245. J. Zimmermann, F.A. Reifler, G. Fortunato, L. Gerhardt, S. Seeger, A simple, one-step approach to durable and robust superhydrophobic textiles, Advanced Functional Materials 18 (2008) 3662–3669. J.M. Xi, L. Feng, L. Jiang, A general approach for fabrication of superhydrophobic and superamphiphobic surfaces, Applied Physics Letters 92 (2008) 053102–153102. Q. Zhu, Q.M. Pan, F.T. Liu, Facile removal and collection of oils from water surfaces through superhydrophobic and superoleophilic sponges, Journal of Physical Chemistry C 115 (2011) 17464–17470. J. Wu, J. Chen, K. Qasim, J. Xia, W. Lei, B.P. Wang, A hierarchical mesh film with superhydrophobic and superoleophilic properties for oil and water separation, Journal of Chemical Technology and Biotechnology 87 (2012) 427–430. C.H. Su, Y.Q. Xu, W. Zhang, Y. Liu, J. Li, Porous ceramic membrane with superhydrophobic and superoleophilic surface for reclaiming oil from oily water, Applied Surface Science 258 (2012) 2233–2319. L. Chen, D. Han, L. Jiang, Study and fabrication of biomimetic vascular interface materials, Journal of Southeast University (Medical Science Edition) 30 (2011) 224–227. B. Yin, L. Fang, J. Hu, A.Q. Tang, W.H. Wei, J. He, Preparation and properties of super-hydrophobic coating on magnesium alloy, Applied Surface Science 257 (2010) 1666–1671. Y.H. Wang, W. Wang, L. Zhong, J. Wang, Q.L. Jiang, X.Y. Guo, Super-hydrophobic surface on pure magnesium substrate by wet chemical method, Applied Surface Science 256 (2010) 3837–3840. K.S. Liu, M.L. Zhang, J. Zhai, J. Wang, L. Jiang, Bioinspired construction of Mg–Li alloys surfaces with stable superhydrophobicity and improved corrosion resistance, Applied Physics Letters 92 (2008), 183103–183103-3. Z.X. Kang, X.M. Lai, F. Wang, Y. Long, Y.Y. Li, Preparation of super-hydrophobic duplex-treated film on surface of Mg–Mn–Ce magnesium alloy and its corrosion resistance, Chinese Journal of Nonferrous Metals 21 (2011) 283–289. T. Ishizaki, N. Saito, Rapid formation of a superhydrophobic surface on a magnesium alloy coated with a cerium oxide film by a simple immersion process at room temperature and its chemical stability, Langmuir 26 (2010) 9749–9755. T. Ishizaki, Y. Masuda, M. Sakamoto, Corrosion resistance and durability of superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution, Langmuir 27 (2011) 4780–4788. J. Wang, D.D. Li, Q. Liu, X. Yin, Y. Zhang, X.Y. Jing, M.L. Zhang, Fabrication of hydrophobic surface with hierarchical structure on Mg alloy and its corrosion resistance, Electrochimica Acta 55 (2010) 6897–6906. J. Wang, D.D. Li, R. Gao, Q. Liu, X.Y. Jing, Y.L. Wang, Y. He, M.L. Zhang, Z.H. Jiang, Construction of superhydrophobic hydromagnesite films on the Mg alloy, Materials Chemistry and Physics 12 (9) (2011) 154–160. W.J. Xu, J.L. Song, J. Sun, Y. Lu, Z.Y. Yu, Rapid fabrication of large-area, corrosionresistant superhydrophobic Mg alloy surfaces, ACS Applied Materials and Interfaces 3 (2011) 4404–4414. Z.W. Wang, Q. Li, Z.X. She, F.N. Chen, L.Q. Li, Low-cost and large-scale fabrication method for an environmentally-friendly superhydrophobic coating on magnesium alloy, Journal of Materials Chemistry 22 (2012) 4097–4105. G. Ballerini, U. Bardi, R. Bignucolo, G. Ceraolo, About some corrosion mechanisms of AZ91D magnesium alloy, Corrosion Science 47 (2005) 2173–2184. G. Galicia, N. Pébère, B. Tribollet, V. Vivier, Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy, Corrosion Science 51 (2009) 1789–1794.