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Fabrication of ZIF-8@SiO2 Micro/Nano Hierarchical Superhydrophobic Surface on AZ31 Magnesium Alloy with Impressive Corrosion Resistance and Abrasion Resistance Cuiqing Wu, Qi Liu, Rongrong Chen, Jingyuan Liu, Hongsen Zhang, Rumin Li, Kazunobu Takahashi, Peili Liu, and Jun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16848 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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ACS Applied Materials & Interfaces

Fabrication of ZIF-8@SiO2 Micro/Nano Hierarchical Superhydrophobic Surface on AZ31 Magnesium Alloy with Impressive Corrosion Resistance and Abrasion Resistance Cuiqing Wu a, Qi Liu a, Rongrong Chen

a ,b*

, Jingyuan Liu a, Hongsen

Zhang a, Rumin Li a, Kazunobu Takahashi b, Peili Liu b, Jun Wang a, b* a

Key Laboratory of Superlight Material and Surface Technology, Ministry of

Education b

Institute of Advanced Marine Material, Harbin Engineering University, Harbin

150001, People’s Republic of China,

* Corresponding author: Email: [email protected], [email protected]

Keywords: AZ31 magnesium alloy, ZIF-8@SiO2, superhydrophobic, anti-attrition, corrosion resistance ABSTRACT ： Superhydrophobic coatings are highly promising for protecting material surfaces and for wide applications. In this study, superhydrophobic composites, comprising a rhombic-dodecahedral zeolitic imidazolate framework (ZIF-8@SiO2), have been manufactured onto AZ31 magnesium alloy via chemical etching and dip-coating methods to enhance stability and corrosion resistance. Herein, we report

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a simple strategy to modify hydrophobic hexadecyltrimethoxysilan (HDTMS) on ZIF-8@SiO2 to significantly improve the property of repelling water. We show that various liquids can be stable on its surface and maintain a contact angle higher than 150°. The morphologies and chemical composition were characterized by means of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FI-IR). addition, the anti-corrosion and anti-attrition properties of the film were assessed by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization and HT, respectively. Such a coating shows promising potential as a material for large scale fabrication.

1. INTRODUCTION Liquid

repellent

surfaces,

also

known

as

lotus 1-3

superhydrophobic surfaces, have been widely reported.

effect

and

It is generally

regarded that a superhydrophobic surface has a water contact angle (CA) higher than 150°and

a sliding angle (SA) less than 10°.

4-5

At the

time, the superhydrophobic surfaces have attracted a lot of attention because of their practical application in anti-corrosion, drag

reduction,

12

self-cleaning,

13-16

etc.

6-8

anti-icing,

However,

9-11

most

surfaces tend to lose their property of non-wettability due to which seriously affects their applicability. There are two methods for fabrication of superhydrophobic surfaces: one is to modify in situ the surface of various substrates, including glass,

17-18


copper,

19

aluminum

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alloy,

20

and magnesium alloy,

21-22

etc., The second is to modify a rough

substrate with superhydrophobic powder or particles,

23-25

which is easily

realized. Fabrication of a superhydrophobic surface depends on the surface chemical compositions and morphology. Recently, a number of studies have reported on superhydrophobic coatings. Alexander et al 26. prepared aluminum oxide nanoparticles modified with hydrocarbon chains. The modified nanoparticles can be coated onto various substrates. Yoon et al. reported about a transparent coating which was successfully formed onto substrate via electrospraying. Our group

28

have formed transition metal

ion functionalized hydrocarbon chains via a hydrothermal method. Unfortunately, few works have studied the abrasive resistance of the superhydrophobic coating. The weak anti-friction effect of the superhydrophobic coating is attributed to low adhesion strength between coating and substrate. This problem impeded the practical application of the superhydrophobic surface. The waterproof properties of the superhydrophobic surface, however, do not meet the needs of society. the development of science and technology, the multifunctionality of the superhydrophobic surface has become an important requirement. Nowadays, a wide variety of inorganic particles are used to prepare a functional surface, such as nano silica, Among

them,

Metal–organic

29

carbon black

frameworks


30

and MOFs.

(MOFs),

as

31

novel


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organic-inorganic particles have received more attention in potential applications but seldom have appeared in superhydrophobic reports due the water-sensitivity of most MOFs. Given the polyhedral structure of MOFs, the modification of MOFs foreshadows a significant role. MOFs, also called porous crystalline coordination polymers

32

include

Zeolitic Imidazolate Frameworks (ZIFs), 33 and consist of metal ions and various organic ligands yields，which exhibit an orderly arrangement of pores and great high surface area. 34 MOFs and nanoparticles also play an important role for combining the functionalities of each component. In particular, the combination of ZIFs with other components, which can enhance chemical and biological properties, have recently received much attention. 35 Zeolite imidazole frameworks (ZIFs), which belong to porous crystalline coordination polymers (MOFs), exhibit good chemical and flexibility, and have wide application in catalysis and separation. 36-37 In all of the ZIFs, zeolitic imidazolate framework-8 (ZIF-8) has been widely applied in many applications due to its rhombic dodecahedral structure and porosity. Fortunately, we selected SiO2 in situ formed on ZIF-8 with a special core/shell structure because of its cheap and facile fabrication.

38

In addition, it does not change the primary morphology,

the shell of SiO2 can be easily modified. Studies about superhydrophobic silica and compounds have been reported, but few studies have reported


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the successful synthesis of ZIF-8@SiO2 , with regard to abrasive resistance. In this work, we describe our recent progress in the synthesis of the superhydrophobic surface, which is nanometer ZIF-8 covered with SiO2 (ZIF-8@SiO2). This was synthesized for tackling the poor durability and weak

abrasion

performance

of

the

superhydrophobic

surface.

Hydrophobic modification was achieved in alkoxysilane, which involved 5 vol.% hexadecyltrimethoxysilane (HDTMS) dissolved in 75/25 (v/v) ethanol/water solvent and the use of acetic acid to adjust pH=3.5. Finally, the etched Mg alloy, which presented micrometer morphology, was dipped into the hydrophobic ZIF-8@SiO2 n-hexane solution. The as-prepared superhydrophobic surface with hierarchical structure on magnesium alloy not only performed well as a water repellant but also exhibited

advantageous

properties,

including

excellent

corrosion

resistance, mechanical and chemical stability, which satisfies the needs of society and offered promising application potential. 2. EXPERIMENTAL SECTION 2.1. Materials. Magnesium alloy AZ31 (composition:2.98 wt% Al, 0.88 wt% Zn, 0.38 wt% Mn, 0.0135 wt% Si, 0.0027 wt% Fe, 0.002 wt% 0.001 wt% Cu, Mg) was cut into 30mm×30mm×2mm sheets and used as the substrate. Zinc nitrate tetrahydrate( Zn(NO3)2.6H2O, 98%) , 2-methylimidazole (H-MeIM, 99%), tetraethoxysilane (TEOS, 95%) and



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hexadecyltrimethoxysilane (HDTMS, 98%)were purchased from Aladdin, cetyl trimethyl ammonium bromide (CTAB), methanol, hydrochloric acid, acetic acid, deionized water, ammonium hydroxide (25%), absolute ethanol were used as analytical grade and purchased from Tianjin Benchmark Chemical Regent, China. 2.2. Synthesis of ZIF-8@SiO2 core-shell particles. At room temperature, ZIF-8 nanocrystals were synthesized via a co-precipitation procedure, which is depicted in a previous report. 39 The specific process is as follows: 732 mg of Zn(NO3).6H2O was dissolved in 50 mL methanol, and 162.2 mg of 2-methylimidazole was dissolved in 50 mL methanol under continuous stirring for 10 min, respectively. Then the latter was poured into the former solution and stirred for 5 min. The solution mixture was kept for 24 h without stirring. Subsequently, the turbid liquid was centrifuged and washed with methanol three times. After that, the final product ZIF-8 was dried in an oven at 60 ℃ overnight. ZIF-8 was coated with SiO2 based on a sol-gel process that included hydrolysis and condensation of tetraethylorthosilicate (TEOS) in ethanol, forming a core-shell structure of ZIF-8@SiO2. 80 mg of ZIF-8 particles 100 mg CTAB were dispersed in ethanol (80 mL) and ammonium hydroxide (0.2 mL) used to adjust pH=11.The mixture was ultrasonicated for 2 min, then 0.125 mL of TEOS was added to the above solution under vigorous stirring for 18 h. Finally, the product was centrifuged and rinsed


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with a molar ratio (1:1) of deionized water and ethanol three times, and dried in an oven at 70 ℃. The core-shell structure of ZIF-8@SiO2 was formed. 2.3. The modification of ZIF-8@SiO2 particles. The ZIF-8@SiO2 particles were modified by HTMS to improve their hydrophobicity. The silane solution included a 5 v% silane coupling agent that dissolved in a mixed solution which contained 75/25 (v/v) ethanol/ water and acetic acid to adjust pH=4.5. The mixed solution was stirred for 24 h and 2 mg of ZIF-8@SiO2 particles were added to the above solution under stirring for 12 h. Finally, the as-prepared particles were centrifuged and washed with deionized water and ethanol, and dried at 70 ℃for 12 h. 2.4. Pretreatment of the AZ31Mg alloy. First, magnesium alloy substrate was abraded with 600#, 1000# and 2000# silicon carbide paper to remove the oxide/hydroxide layer. Secondly, The Mg alloy was ultrasonically degreased in absolute ethanol for 10 min to remove impurities. Finally, it was washed with deionized water and dried with a hairdryer. 2.5. Fabrication of superhydrophobic surface. A mass of 4 mg of ZIF-8@SiO2 particles coated with HTMS were dispersed in 10 mL of n-hexane by ultrasonication for 10 min. The treated AZ31 alloy was dipped into the solution at 80 ℃

for 2 h in an oven. The

superhydrophobic coating was successfully formed on AZ31 magnesium alloy. ACS Paragon Plus Environment


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2.6. Characterization. The phase composition and structures of the samples were investigated by X-ray diffraction analysis (XRD, Rigaku TTR-III, Cu Kα ,λ=0.15406 nm). Images from a scanning electron microscope (SEM) characterized the surface morphologies, carried out using

a

JEOL

energy-dispersive

JSM-6480A X-ray

microscope

spectrum

(EDS).

equipped Transmission

with

an

electron

microscope (TEM) was used to observe the morphology of the specimen. Fourier transform infrared spectroscopy (FTIR) was carried out for ZIF-8, ZIF-8@SiO2 and ZIF-8@SiO2 coated with HTMS. Static contact angles were recorded on a FTA200 drop shape analysis system at room temperature. The property of anti-attrition was measured by HT Scratch Testing Machine (HT-1000) and the corrosion resistances of properties of sample and bare magnesium alloy substrate were measured by an electrochemical workstation (Zennium, IM6, Germany) using 3.5 wt% NaCl aqueous solution in a three-electrode system, where the sample with exposed area of 1 cm2 as the working electrode (WE), saturated calomel electrode (SCE) as reference electrode (RE), a platinum mesh as counter electrode (CE). Potentiodynamic polarization tests were performed at a scan rate of 1 mV/s. The potential range was studied from -400 mV of the open circuit potential (OCP) value in the cathodic regime, to 1600 mV in theRESULTS anodic regime. 3. AND DISCUSSION 3.1. Characterization of ZIF-8@SiO2 particles. Fig. 1 describes the


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XRD of ZIF-8 and ZIF-8@SiO2 particles and modified ZIF-8@SiO2 particles. As shown in Figure 1a, the image exhibits a number of sharp peaks between 5°and 90°, indicating their high crystallinity and all of the sharp diffraction peaks could be assigned to (011), (002), (112), (022), (013) and (222) peaks. Based on these results, it is shown that a rhombic dodecahedral structure ZIF-8 is formed. From Figure 1b, silica is an amorphous solid, therefore, we can detect a new broad diffraction peak at around 23°and the residual peaks can be indexed to diffraction peaks of ZIF-8. Notably, after the ZIF-8@SiO2 decoration process (Figure 1c), the diffraction peak tends to migrate to a low angle (around 22°), which suggests that HDTMS has formed a new crystalline structure. From the literature, it may be due to the self-polymerization of silane coupling 40

To further confirm ZIF-8 and SiO2 particles were prepared, we used FTIR analysis. Figure 2 shows the FTIR spectra of the ZIF-8 and ZIF-8@SiO2 and modified particles. For the ZIF-8 particles, there are characteristic peaks at 2926 cm-1, 1600 cm-1, 1145 cm-1 and 995 cm-1, assigned to vibrations of C-H, C=N and C-N in the imidazole ring, respectively.

41

These peaks are also found from ZIF-8@SiO2 particles.

Meanwhile, two new peaks at about 1090 cm-1 and 471 cm-1 appear that attributed to stretching vibrations of Si-O-Si and bending vibration of the Si-O. For the modified particles, we observe two peaks at 2918 cm-1 and



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2850 cm-1,

42

which belong to the -CH2- asymmetric and symmetric

vibrations of the HDTMS. In addition, the peaks at 1115 cm-1 and 1030 cm-1 are assigned to the asymmetric and symmetric vibrations of Si-O-Si HDTMS. These results indicate that silica covers ZIF-8 and HDTMS the surface of ZIF-8@SiO2, which is in agreement with that previously reported. 43

Figure 1. XRD patterns of the ZIF-8 particles (a), ZIF-8@SiO2 particles (b), modified ZIF-8@SiO2 particles (c).


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Figure 2. FTIR of the ZIF-8 particles (a), ZIF-8@SiO2 particles (b), modified ZIF-8@SiO2 particles (c). 3.2. Morphology of superhydrophobic surface and wettability. Surface measurements and water contact angle (CA) of the bare AZ31 surface, etched AZ31 surface, ZIF-8 and ZIF-8@SiO2 particles at magnifications are presented in Figure 3. As shown in Figure 3a, there is no obvious morphology on the bare AZ31 alloy, and the CA is about 35° （inset in Figure 3a）. However, after the etching process, the surface is covered with lots of irregularly shaped micro holes (Figure 3b). Because Mg atoms on the substrate may be dissolved in the acid solution, the surface becomes more rougher, but the surface energy is still high, so the CA is about 0°(inset in Figure 3b). On the basis of the ZIF-8 particles (Figure 3c), the SEM image shows a hexagonal shape with an average of 500nm, a higher resolution image is shown in Figure 3d. With SiO2 ACS Paragon Plus Environment


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coated onto ZIF-8, it is clearly seen that the image still retains its rhombic dodecahedral structure (Figure 3e). However, compared with the particles, there is a significant appearance that the edges and corners of dodecahedron structure are not obvious (Figure 3f). It further certified SiO2 is successfully coated on ZIF-8. These results are consistent with XRD diagrams. Figure 3g shows that when the particles of ZIF-8@SiO2 are modified with HDTMS, the morphology remains unchanged and the CA was approximately 130° (inset in Figure 3g). As shown in Figure when hydrophobic particles of ZIF-8@SiO2 are deposited on the etched Mg

alloy,

roughness

and

low

surface

energy

result

in

with a CA value of 153° (insert in Figure 3h). Another important observation is that the superhydrophobic surface can also repel daily liquids (Figure 3i). In order to further confirm the preparation of ZIF-8@SiO2 crystals, as-obtained ZIF-8 particles and modified with SiO2 were carefully characterized by TEM, respectively. Figure 4a shows well the rhombic dodecahedral morphology of ZIF-8 seeds with size about 500nm, which corresponds to the size of SEM image. In contrast, when incorporated with SiO2, the edge of ZIF-8 with rough structure is clearly observed (Figure 4b). It indicates that nanoscaled structure SiO2 is successfully formed. These results are in good agreement with XRD and FT-IR. The superhydrophobicity of the sample can be attributed to the


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hierarchical structure, when a water droplet or other daily-used liquids are put on the surface, a three-phase of solid/water/air develops, which is consistent with the Cassie model.

44

The aerated rough structure filled

air can protect the substrate from wetting by liquid droplet. Thus, the drops are in contact with the air phase without contact with the solid surface. The high CA can be interpreted according to the Cassie-Baxter equation 45: Cosθ =f cosθ1+ f-1

(1)

Where θ and θ1 are the contact angles of a liquid droplet on the treated surface and untreated surface, respectively. f is the fraction of liquid contact with solid. From the contact angle measurements, θ=153°, θ1= 35° and f=0.06 can be calculated from Eq. (1). The low value of f indicates that about 6% of the contact area of the liquid droplet contact with the superhydrophobic surface and about 94 % contact with air. These results indicate that a micro/nano rough structure contributes to the superhydrophobility of the film.



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Figure 3. SEM images of samples: (a) Bare Mg alloy, (b) Etched Mg alloy, (c, d) ZIF-8 particles with different magnification; (e, f) ZIF-8@SiO2 particles with different magnification; (g) ZIF-8@SiO2 particles modified with HDTMS; (h) coating; (i) daily-used liquids on the superhydrophobic surface, and the inset in (a), (b), (g) and (h) show the CA of water droplet on the samples.


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Figure 4. TEM images of ZIF-8 (a), and ZIF-8@SiO2 (b) particles. 3.3. Stability of superhydrophobic surface. In view of the application of the superhydrophobic surface in the harsh conditions, the chemical and long-term stability, as well as mechanical durability were measured. 3.3.1. Chemical and long-term stability. The chemical stability of the superhydrophobic surface was investigated by liquids at different pH values (pH=2-12). From Figure 5a, there is no obvious difference water droplets and different pH values droplets and the contact angles varying from 150°to 153°. According to these results, the existence of the air layer that trapped in the microstructure reduces the contact area between the droplet and the solid surface. At the same time, the contact angle (CA) is greater than 150°, indicating a good chemical stability. In addition, an immersion test was carried out to estimate the long-term stability of the superhydrophobic coating. Figure 5b shows the between the CA of superhydrophobic film and the immersion time in



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ethanol solution. The CA was almost constant after immersion in ethanol solution for 3 days. It can be seen from Figure 5b, accompanied by the extension of the immersion time, the contact angle slightly changed. the superhydrophobic magnesium alloy was immersed in ethanol solution, the ethanol solution will wet the surface due to the surface tension of the ethanol below the solid surface tension. However, the ethanol molecules stayed on the surface will evaporate rapidly and not destroy the surface structure that increases the physical barrier to the water droplet to some extent so the initial contact angle did not change. The above results that the coated sample can resistant to ethanol, which is important for the practical application of the superhydrophobic film.

Figure 5. The relationship between the CA and the different pH values of liquids on the as-prepared specimen (a), and variation of CA after immersion into ethanol solution with different times (b). 3.3.2. Mechanical stability. The friction and wear test was conducted explain the mechanical stability of the superhydrophobic surface. Figure 6a shows the friction coefficients of bare alloy, etched alloy and ACS Paragon Plus Environment

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superhydrophobic alloy. It is clearly observed that the friction coefficients of bare alloy and etched alloy have fluctuated considerably, while the superhydrophobic alloy has only fluctuated slightly and the friction coefficients are lower than the bare and etched samples. Figure 6b shows the relationship between friction coefficient and abrasion loss. The bare sample and etched samples have friction coefficients of 0.488 and 0.52 abrasion losses of 1 × 10^-3 mm3/Nm and 1.4 × 10^-3 mm3/Nm, However, for the as-prepared sample, the friction coefficients is 0.288 abrasion loss is 5×10^-4 mm3/Nm, lower than those of bare and etched samples. These results demonstrates that the inherent wear resistance of silicon dioxide and the micropores interact strongly with the nano polyhedron, resulting in the superhydrophobic layer protect the AZ31 from wearing, and therefore increasing wearability.. The images shown in Figure 7 together with SEM tracks caused by friction and wear tests in superhydrophobic layers further show that the superhydrophobic coating has the function of reducing wear. The SEM track (Figure 7a) depicts the superhydrophobic film after testing, which shows clearly that from EDS analysis the film is not removed and there many cracks and granules in the SEM image. In addition, compared to Figure 7b, Zn, Si, C, O, N have not disappeared and Mg appeared after testing (Figure 7c). The quality is much less, indicating that the layer has not been completely worn. So the film exhibits a good response for



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environments situation, where there is exposure for a long time.

Figure 6. Friction coefficient and abrasion loss of samples: (a) Friction coefficient, (b) Abrasion loss.

Figure 7. SEM image for superhydropobic Mg alloy after friction and test for 20 min (a), EDS spectra before and after friction and wear test (b, ACS Paragon Plus Environment

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c). 3.4. Ice-repellent properties of the superhydrophobic coating. Considering industrial applications, the anti-icing property has attracted great interest recently. In this work, we speculate that the film possesses anti-icing performance. 46 Two plates of bare Mg alloy (on the left) and superhydrophobic Mg alloy (on the right) were horizontally placed on a glass dish, in an ambient of temperature of -15℃. Then, to assist in observing the freezing process, two drops of water doped with methyl orange, were carefully set on the bare alloy and superhydrophobic coating, respectively. The color of the water would changes from deep orange to light orange.

47

It was

observed that the water on the two surfaces does not change for about 10s (Figure 8a). After 300 s, the left sample freezes at the edge while there in no change with the right sample (Figure 8b). At 500 s, the surface of bare Mg alloy appears misty, meanwhile, the water droplet completely freezes on the bare substrate, at the same time, the as-prepared sample has not begun to change (Figure 8c). As shown in Figure 8d, the water droplet on the right sample has not completely frozen after about 900 s. From Figure 8e, the coated and uncoated alloys were both frozen at 1200 s. In addition, it was easy to remove the ice from the superhydrophobic surface, and after deicing, the surface is still clean. But for the untreated Mg alloy, the ice is difficult to remove from the surface (Figure 8f).



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In order to further prove the potential of ice-repellent of the superhydrophobic surface, we put the two samples at the edge of the glass dish. Two drops of liquids were carefully dropped on the untreated AZ31 Mg alloy and superhydrophobic AZ31 Mg alloy, respectively. At 10 s, the liquid stays on the bare alloy, nevertheless, the spherical liquid rolls on the glass dish (Figure 8g). About 1200 s later, we observed the water has frozen on both alloy samples surfaces (Figure 8h). These results indicate that the superhydrophobic surface can suppress ice formation and is more resistant to freezing than the bare sample. These results are consistent with the Cassie model, as mentioned above.

Figure 8. A comparison of surface icing property between the untreated substrate (left) and superhydrophobic surface substrate (right). The frozen time of the two samples are shown in (a) 10 s, (b) 300 s, (c) 500 s, (d) 900 s, (e) 1200 s, and (f) the surface after de-icing, (g) 10 s, (h) 1200 s. 3.5. Corrosion resistance of the superhydrophobic surface. Magnesium alloy is easy to corrode, especially in a corrosive medium. ACS Paragon Plus Environment

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Here, the anticorrosion property was conducted by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in a three electrode system in 3.5 wt% NaCl aqueous solution. Figure 9 shows that

potentiodynamic

polarization

curves

of

the

as-prepared

superhydrophobic sample after immersion in 3.5 wt% NaCl aqueous solution for 3 h, 6 h, 9 h, 24 h, 72 h, 144 h and of the bare and etched samples after immersion for 3h. The results of the test are summed up in Table 1. On the basis of Figure 9 and Table 1, it is shown that the corrosion current density (icorr) decreases after the magnesium alloy being coated. Due to the corrosion potential (Ecorr) mainly explains the thermodynamic performance of the materials. Hence the anticorrosion performance be assessed by Ecorr. Another evaluation standard is icorr, which for the untreated alloy and etched alloy are 1.57×10-5 A/cm2 and 1.52×10-5 A/cm2 after immersion in 3.5 wt% NaCl aqueous solution for 3h, respectively, whereas that of the coated substrate is 2.17×10-7 A/cm2 (Figure 9a). The icorr of the superhydrophobic film formed on AZ31 Mg alloy decreases two orders of magnitude compared to the untreated and etched substrates. After immersion for 6 h and 9 h, the icorr of the coated samples were approximately 2.69×10-7 A/cm2 and 4.49×10-7 A/cm2, respectively. In addition, a significant change in the icorr after immersion the 3.5 wt% NaCl aqueous solution for 24h, 72h, and 144h (Figure 9b),



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icorr of the coated samples increased by about one order and two orders magnitude compared to that sample that immersion in NaCl solution for 3h.Yet, it still lower than that that of bare Mg alloy. These results indicate that coated surface has a high resistance to corrosion for a long time. Figure 9, the value of the icorr of the superhydrophobic surface is much lower than that of bare magnesium alloy after immersion in the 3.5 wt% NaCl aqueous solution for different times. From literature reported,

48-49

lower corrosion current densities signify lower corrosion rate, that is, anticorrosion property. So we can draw a conclusion that the composite surface formed on the substrate improves the corrosion resistance of which is ascribed to the effective inhibition of infiltration of corrosive-acting ions. For explaining the microstructure of the superhydrophobic substrates after immersion in 3.5 wt% NaCl aqueous solution, SEM images were measured. Figure 10 displays the SEM images of the superhydrophobic after immersion in NaCl solution for 3 h (a), 6 h (b), 9 h (c), 24 h (d), 72 (e) and 144 h (f), as well as the bare and the etched samples after immersion for 3 h (g-h), respectively. Compared with the morphology before test (Figure 3a, b, h), there is little change for coated alloys after immersion for 3 h (Figure 10a). After 6 h, a few pits appear on the surface but the surface remains uniform (Figure 10b). When the immersion time


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was increased to 9 h, a few pits appear on the coating (Figure 10c). Accompanied by an increase in immersion time, the pits gradually larger with the emergence of corrosion products (Figure 10d, e). It can be clearly seen that after immersed for 144 h (Figure 10f), pitting corrosion phenomenon appeares on the coated surface, which is likely to result in decrease of impedance value. To some extent, these pores can provide an area of accumulation of corrosion products, which protect the substrate from invasion of a corrosive solution, but the protection time will not last long. As shown in Figure 10g and Figure 10h, fort both the bare Mg alloy and etched Mg alloy surfaces, a large number of corroded products and spiral-shaped pits appear after immersion for 3 h, respectively. The SEM images further prove that a superhydrophobic surface can effectively improve the corrosion resistance of the Mg alloy substrate. In order to further characterize corrosion resistance, we used EIS to analyze the anticorrosion of the sample. Figure 11 depicts the Nyquist of the superhydrophobic surface, untreated specimen and etched after immersion in 3.5 wt% NaCl aqueous solution for 3 h. The bare and etched specimens shown two capacitive semicircles in high and medium frequency ranges in the enlarged Nyquist plot (insert in Figure 11). It contains a capacitive loop at high frequency and an inductive loop at medium frequency. The emergence of the inductive loop is ascribed to intermediate products, derived from the dissolved of Mg, deposited on the



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specimen surface. In comparison, the as-prepared superhydrophobic film has a single capacitive semicircle at high frequency, with diameter of loop much larger than the bare and etched Mg alloys, indicating that the Mg alloy possesses good anticorrosion property. According to previous works,

50

the larger of the diameter of the capacitive loop in the Nyquist

plots reply the greater corrosion resistance of the working electrode. 12 shows that Nyquist plot of immersion in 3.5 wt% NaCl solution with different times at room temperature. From Figure 12a, with the increase immersion time, the diameter of capacitive loop decreases, which demonstrates

that

the

corrosion

resistance

of

the

as-prepared

superhydrophobic film is degraded. However, it is still more than that of bare Mg substrate. The above mentioned results are consistent with the results of the potentiodynamic polarization curves and the description of the images (Figure 10). Figure 12b is an enlarged Nyquist plot that immersion in 3.5 wt% NaCl solution for 24 h, 72 h, 144 h, 480 h and 600 It can be seen that the radius of curvature for the treated Mg alloy clearly decreases, indicating that the air layer on the solid surface gradually disappears and the electrolyte penetrates into the product layer. It is worth considering that immersion in 3.5 wt% NaCl solution for 3 h, the |Z| for superhydrophobic specimen approximates 105 Ω·cm2, three orders of magnitude higher than those of untreated and etched specimens after 3 h immersion, which are about 102 Ω·cm2 (Figure 13a), demonstrating that


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coated AZ31 alloy is not corroded and the corrosion resistance of the substrate is enhanced. For the untreated and etched Mg alloys, two time constants exist around low and high frequency regions, respectively. Compared to the abovementioned samples, the new time constant of the superhydrophobic sample appeares at a low frequency region of about 1Hz (Figure 13b), which is attributed to the charge transfer process at the interface between Mg alloy and the superhydrophobic surface. Another time constant appears at high frequency region about 104 Hz, which is ascribed to the resistance of the superhydrophobic surface. These results demonstrate that the superhydrophobic coating protects the substrate from corroding.

Figure 9. Potentiodynamic polarization curves for the bare and modified surface after immersion into 3.5 wt% NaCl aqueous solution for different times.



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Figure 10. Microstructure of the superhydrophobic samples, untreated Mg alloy and etched Mg alloy with immersion in 3.5 wt% NaCl solution for different times.

Figure 11. Nyquist plots for the bare and modified surfaces in 3.5 wt% NaCl aqueous solution for 3 h.


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Figure 12. EIS measurement for as-prepared superhydrophobic surface after immersion in 3.5 wt% NaCl solution for different times: (a) Nyquist plots, and (b) magnified Nyquist plots. Table 1 Electrochemical Parameters of Bare Mg Alloy, Etched Mg Alloy and Superhydrophobic Surface Mg Alloy in 3.5 wt% NaCl Aqueous Solution. Sample

Time (h)

Ecorr (V/(SCE))

icorr (µA.cm-2)

Untreated AZ31

3

-1.174

15.7

Etched AZ31

3

-1.172

15.2

Coated AZ31

3

-1.196

0.217

6

-1.154

0.269

9

-1.189

0.449

24

-1.159

1.78

72

-1.156

3.13

144

-1.156

10.8



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Figure 13. EIS diagrams of the bare, etched, superhydrophobic surfaces after immersion in 3.5 wt% NaCl aqueous solution for 3 h. (a) Bode plots of |Z| versus frequency, (b) Bode plots of phase angle versus frequency. 4. CONCLUSIONS The core/shell structure of ZIF-8@SiO2 superhydrophobic film was successfully formed on etched AZ31 Mg alloy substrate by a simple dip-immersion method. The as-prepared rhombic dodecahedral zeolitic imidazolate framework surface with micro/nano hierarchical structure had a contact angle of 153°. Ascribed to the micro/nano structure coating induced by chemical etching and dip-immersion, the surface showed excellent chemical stability and long-term durability with long term exposure to ethanol as well as mechanical durability by friction and wear test. The surface also developed a significantly improved anticorrosion property compared to untreated Mg alloy. We believe that the method is adaptable large scale fabrication of a superhydrophobic coating on magnesium alloy. In addition, we anticipate a promising potential to


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transfer this technology to other metal substrates for important applications. AUTHOR INFORMATION Corresponding Authors *Email: [email protected], *Email: [email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC 51301050), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang Province（B2015021）, International Science & Technology Cooperation Program of China (2015DFA50050) and the Magor Project of Science and Technology of Heilongjiang Province (GA14A101).

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