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One-step hydrothermal process to fabricate superhydrophobic surface on magnesium alloy with enhanced corrosion resistance and self-cleaning performance
Accepted Manuscript Title: One-step Hydrothermal Process to Fabricate Superhydrophobic Surface on Magnesium Alloy with Enhanced Corrosion Resistance and Self-cleaning Performance Authors: Libang Feng, Yali Zhu, Jing Wang, Xueting Shi PII: DOI: Reference:
S0169-4332(17)31722-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.066 APSUSC 36266
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-3-2017 5-5-2017 5-6-2017
Please cite this article as: Libang Feng, Yali Zhu, Jing Wang, Xueting Shi, One-step Hydrothermal Process to Fabricate Superhydrophobic Surface on Magnesium Alloy with Enhanced Corrosion Resistance and Self-cleaning Performance, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step
Hydrothermal
Process
to
Fabricate
Superhydrophobic Surface on Magnesium Alloy with Enhanced
Corrosion
Resistance
and
Self-cleaning
Performance Libang Fenga *, Yali Zhua,b, Jing Wang a, Xueting Shia
a
School of Mechatronic Engineering, Lanzhou Jiaotong University, Lanzhou 730070 b
Department
of
Mechanical
and
Electrical
Engineering, Xi’an Railway Vocational &
Technical Institute, Xi’an 710026
Corresponding
author, E-mail: [email protected]
Graphical Abstract
A facile, environment-friendly, and cost-effective one-step hydrothermal route is proposed to fabricate the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic surface can greatly improve the corrosion resistance and self-cleaning performance of magnesium alloy due to its high water repellence.
1
Research Highlights
A facile one-step method for fabricating superhydrophobic surface on Mg alloy is developed.
The superhydrophobic surface has enhanced anti-corrosion performance.
The superhydrophobic surface exhibits excellent self-cleaning performance.
The mechanisms of corrosion resistance and self-cleaning performance are deduced.
Abstract: Superhydrophobic surfaces can exhibit anti-corrosion, anti-fogging, and self-cleaning performance due to their high water repellence. It is significant for industrial fabricating of superhydrophobic surface with a simple and environment-friendly method. Herein, a facile, environment-friendly, and cost-effective one-step hydrothermal route is proposed to fabricate the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic magnesium alloy surface presents the rough and hierarchical micro/nano- structure grafted with long hydrophobic alkyl chains via covalent bonds. Both electrochemical corrosion test and long term immersion in 3.5 wt.% of NaCl solution demonstrate that the superhydrophobic surface greatly improves the corrosion resistance of magnesium alloy. Meanwhile, the superhydrophobic magnesium alloy exhibits excellent self-cleaning performance. It is supposed that this facile method and remarkable properties of resultant superhydrophobic magnesium alloys have a promising future in expanding the application of magnesium alloys. Key words: Magnesium alloy; Superhydrophobicity; One-step; Hydrothermal process; Anti-corrosion; Self-cleaning
1. Introduction The corrosion of metals can cause a tremendous economic loss each year [1-3], and it has attracted much attention of developing new methods and technology to slow down the corrosion rate of metals [4-7]. Among various metals, magnesium and its alloy have stimulated considerable interest in aerospace, automobile, machinery, computer, electronic industry, and so on, thanks to their excellent characteristics as low density, high specific strength, high ductility, good thermal/electrical conductivity, electromagnetic compatibility, good castability, and abundant availability [8,9]. However, due to the low standard potential and high chemical reactivity, magnesium is one of very active metals and it is susceptible to erode in an aqueous environment, 2
moist atmosphere, or other corrosive media. And which has limited its large scale application in engineering fields. In order to improve the corrosion resistance and extend the application area of magnesium and its alloy, many strategies have been attempted, such as introducing rare earth elements [10], increasing the purity of alloy [11], using rapid solidification processing [12], introducing corrosive metal atoms [13], wet chemical etching [14], and so on. Recently, many studies have shown that it is an effective method of constructing superhydrophobic surfaces on various metals to enhance their corrosion resistance since the superhydrophobic surfaces can hinder the close contact of a surface with the corrosive species [15-19]. So it may be a preferred method to improve the corrosion resistance by endowing magnesium and its alloy with the superhydrophobicity. Various methods have been reported for preparing the superhydrophobic surface on magnesium and its alloy. For example, chemical conversion film, electrodeposition, anodization, chemical vapor deposition, laser treating, micro-arc oxidation, wet chemical etching, electrospinning, and so on [20-26]. However, in most cases, the method is subject to certain limitations, such as special equipment, complex operations, caustic reagents (for instance, HNO3, H2SO4, HF, H2O2, etc.), or expensive materials (namely, fluoride, etc.), and severe conditions. Consequently, these procedures/methods may cause a lot of problems as no universality, high cost, environment pollution, and so on [23-26]. Thus, it is quite needed of developing simple, inexpensive, versatile, and environment-friendly process for the fabrication of the superhydrophobic surfaces on magnesium alloys so as to promote the advantages for industrial large-scale application. Hydrothermal reaction is an efficient way for materials preparation and modification, and it has been used to prepare the superhydrophobic surfaces as well [27,28]. A few papers have been reported for the preparation of superhydrophobic surfaces on magnesium alloys. For example, Wang and co-workers [29] prepared the superhydrophobic magnesium alloy by the hydrothermal process and then modified the surfaces with fluoroalkylsilane, and results showed that the superhydrophobic surface had outstanding corrosion protection effect on magnesium alloy. Li et al. [30] also proposed a hydrothermal method to fabricate superhydrophobic surface on AZ91D magnesium alloy combining stearic acid modification. The as-prepared samples exhibited a considerable enhancement in corrosion resistance and superior anti-bacteria adhesion effect. However, two steps are involved in both routes for fabricating the superhydrophobic surfaces: the 3
first step is to construct a rough-structured surface via hydrothermal process and then to modify surfaces with low-surface-energy substances. Moreover, the caustic NaOH or expensive fluoroalkylsilane is used in the fabrication procedures. To overcome the inherent disadvantage and simplify the procedure, herein, a facile, environment-friendly, and cost-effective method is developed for preparing superhydrophobic magnesium alloys via only one-step hydrothermal process. Furthermore, the as-prepared superhydrophobic surface greatly enhances the corrosion resistance of magnesium alloy. Meanwhile, the superhydrophobic magnesium alloys show excellent self-cleaning performance. The presented method is simple, low-cost, and environment-friendly, which is significant for industrial fabricating of superhydrophobic surfaces with excellent anti-corrosion and self-cleaning performance. Therefore, it is supposed to have a promising future in expanding the application of magnesium alloys.
2. Material and Methods 2.1 Materials Magnesium alloy (AZ91) plate was purchased from Qingdao Dexingsheng metal materials Co., Ltd. The chemical composition of AZ91 was 8.50-9.50% Al, 0.45-0.90% Zn, 0.17-0.40% Mn, 0.05% Si, 0.025% Cu, 0.001% Ni, 0.004% Fe, and Mg balance. Stearic acid (STA) was supplied by Shanghai Zhongqin Chemical Reagent Co., Ltd. Ethanol was purchased from Sinopharm Group Chemical Reagent Co., Ltd. 2.2 Fabrication of the superhydrophobic magnesium alloys The fabricating process of the superhydrophobic magnesium alloy surface is shown in Scheme.1. After polished with abrasive paper and cleaned ultrasonically with acetone/deionized water, AZ91 plate was introduced into a Teflon-lined stainless steel autoclave filled with deionized water, ethanol, and STA. The volume ratio of ethanol to deionized water was kept at 1:1.4 while STA concentration was 50 mmol/L. Then the autoclave was sealed and maintained at 80 oC for 10 hours. After the hydrothermal reaction finished, AZ91 plate was taken out and cleaned with ethanol for three times. Finally, the as-prepared sample was dried in air, and thus the superhydrophobic magnesium alloy was obtained. 2.3 Corrosion resistant performance evaluation The corrosion resistant performance of as-fabricated superhydrophobic magnesium alloys was 4
evaluated by both electrochemical corrosion test and long term immersion in 3.5 wt.% of NaCl solution. Electrochemical corrosion test was performed on a computer-controlled electrochemistry workstation (CHI660D, CH Instruments Inc., China) by potentiodynamic polarization in a three-electrode system: working electrode, a platinum stick counter electrode, and a saturated calomel reference electrode. Dynamic measurement of polarization plots in a Tafel model was acquired at a scan rate of 1 mV/s at room temperature when samples were exposed to corrosive solution for a short period of 10 min. Long-term durability of superhydrophobic magnesium alloys under continuous contact with saline water was carried out by immersing samples in 3.5 wt.% of NaCl solution at 25.0 ± 0.5 °C from 0 to 60 hours. After a definite time of immersion, the sample was taken out, dried under a nitrogen stream, and retained in a vacuum oven at 30 °C for 2 hours. Then, the wettability of the samples was measured. 2.4 Self-cleaning performance test The self-cleaning performance of as-fabricated superhydrophobic magnesium alloys was evaluated by measuring the ability of the rolling water droplets taking away the simulating contaminant particles, while the cigarette ash, carbon powder, and chalk dust were used as the simulating contaminant particles. The self-cleaning test was carried out by deliberately spreading contaminants to form thick layers on the magnesium alloy surfaces. Then a water droplet with a volume of 10 μL was dripped gently onto the sample surfaces under a slope angle of 6° above horizontal. 2.5 Characterization The wettability was evaluated by both static water contact angle (CA) and sliding angle (SA). In our research, the CA and SA for deionized water were measured with a 10 μL of droplet using a horizontal microscope with a protractor eyepiece (DSA 100, Kruss, Germany) at ambient temperature. The droplet was placed at five different spots for each sample surface and the average value was regarded as the contact angle. The error of the contact angle is ±1o. The sliding angle was measured by slowly tilting the level stage controlled with computer, and the tilted angle was adopted when the droplet began to roll in the downhill direction on the sample surfaces. Scanning electron microscopy (FE-SEM, JSM-6701F, Japan) was used to observe the surface 5
morphology. X-ray diffractometer (XRD, XRD-7000LX, Shimadzu, Japan) was used to characterize the phase structure. Fourier transform infrared spectra (FT-IR, VER-TEX 70, Germany) and energy dispersive X-ray spectroscopy (EDS, Inca X-Max, UK) were used to analyze the surface chemical composition.
3. Results and discussion 3.1 Surface morphology, structure, and wettability The superhydrophobic magnesium alloys are fabricated by one-step hydrothermal method. SEM images of the untreated magnesium alloy substrate and superhydrophobic surface are shown in Fig.1. The SEM image of untreated magnesium alloy surface (see as Fig.1a) displays a relatively flat and smooth surface while only abrasive grooves are observed. The untreated substrate surface takes a static water contact angle of ca. 38.3°, indicating the hydrophilic nature of the alloy surface. Fig.1b-d show surface morphologies of magnesium alloy after hydrothermal treatment. It can be noted that great changes happen at the magnesium alloy surface after hydrothermal process. The low-magnification SEM image in Fig.1b shows the surface is covered by a large number of bar-like and plate-like structure completely. It is clearly observed from the high magnification SEM images (Fig.1c and Fig.1d) that the as-prepared surface consists of many nano-plates. The plate length is between 1 μm and 5 μm while the plate thickness is about 50–100 nm. Moreover, it is worth noting from Fig.1d that the plate is composed of several layers. Meanwhile, a great deal of gaps can be easily found among plates. Consequently, a large number of cavities are resulted in the magnesium alloy substrate surface, and the wideness of cavities ranges from 100 nm to 2 μm. Therefore, a quite rough and hierarchical structure with both nanoand micro-scales presents at the magnesium alloy surface after hydrothermal treatment. In order to identify the surface species, phase structure is examined with XRD while the investigated XRD patterns of magnesium alloys both before and after hydrothermal treatment are shown in Fig.1e. It is apparent that the untreated magnesium alloy surface is mainly composed of Mg (JCPDF No. 35-0821). By contrast, XRD pattern has changed much after hydrothermal treatment, and some new peaks present in XRD pattern. The new peaks are attributed to MgO (JCPDF No. 30-0794) and Mg(OH)2 (JCPDF No. 44-1482), indicating that the magnesium alloy surface is mainly composed of MgO and Mg(OH)2 after hydrothermal treatment. It is supposed 6
that such species are generated by following reactions: H 2 O (l ) H Mg ( s ) 2 H
Mg
Mg
( aq .) OH
( aq .) Mg
(1)
( aq .)
2
( aq .) H 2 ( g )
2
( aq .) 2 OH
2
( aq .) H 2 O MgO ( s ) 2 H
(2)
( aq .) Mg ( OH ) 2 ( s )
(3)
(4)
Just based on the formation of MgO and Mg(OH)2 at magnesium alloy surface upon hydrothermal treatment, a quite rough and hierarchical surface with both nano- and micro-scaled structure is obtained. Meanwhile, the wettability of AZ91 surface changes greatly after the hydrothermal treatment. It can be observed from Fig.1f that the water droplet with a volume of 10 uL takes on a quasi-spherical shape. The static water contact angle increases to 153.8° compared to 38.3° of the substrate before hydrothermal treatment, while the sliding angle is about 4°. These results show that the wettability of the surface changes from hydrophilic to superhydrophobic after hydrothermal treatment. 3.2 Chemical composition analysis It is well known that the surface wettability is not only related to surface microstructure, but also is relevant to surface chemical composition. In our research, STA is introduced in the hydrothermal treatment. So the surface chemical structure may have much difference before and after hydrothermal treatment. The chemical composition of the as-prepared superhydrophobic surface is analyzed by FT-IR and EDS techniques, and the investigated spectra are shown in Fig.2. In the FT-IR spectrum of STA (see as Fig.2a), the absorption peaks present at 2920 cm-1 and 2850 cm-1 correspond to the symmetric and asymmetric stretching vibration of C-H groups. The absorption peak locates at 1700 cm-1 is ascribed to -COOH groups. Fig.2b shows FT-IR spectrum of the as-prepared superhydrophobic magnesium alloy. It can be found that the stretching vibration absorption peaks of C–H groups still exist. However, the –COOH absorption peak disappears. Instead, a new absorption peak arises at 1570 cm-1, and which stems from the stretching vibration of –COO- groups [31]. These results indicate that the chemical structure at magnesium alloy surfaces has changed markedly after hydrothermal treatment. Result from XRD indicates that Mg atom at AZ91 surfaces after hydrothermal treatment mainly presents in MgO and Mg(OH)2. On 7
the other hand, the chemical reaction between –COOH groups in STA molecules and –OH groups in MgO and Mg(OH)2 will take place in hydrothermal process. The reaction equation can be described as follows: Mg ( OH ) 2 ( s ) 2 CH
3
( CH
2
) 16 COOH ( l ) Mg ( CH
3
( CH
2
) 16 COO ) 2 ( s ) 2 H 2 O ( l )
(5)
Therefore, results from FT-IR confirm that the long aliphatic chains have been grafted onto the magnesium alloy surfaces with covalent bonds successfully upon hydrothermal treatment. However, Mg(CH3(CH2)16COO)2 presents in amorphous state and which cannot be detected with XRD technique. In order to further confirm the successful graft of STA chains at the magnesium alloy surface upon hydrothermal treatment, EDS technique is used. Fig.2c and Fig.2d show EDS spectra at the magnesium alloy surfaces before and after hydrothermal treatment, while the elemental weight ratio is listed in Table 1. It is obvious that Mg, C, O, and Al elements present at both the untreated and as-fabricated superhydrophobic AZ91 surfaces. As compared to those at the untreated AZ91 surface, the amount of Mg atom decreases distinctly while contents of C and O atoms rise evidently at the superhydrophobic magnesium alloy surface. The decrease of Mg atom and the increase of elemental C and O result from the generation of MgO and Mg(OH)2 as well as the graft of STA chains. The formation of MgO and Mg(OH)2 at magnesium alloy surface in hydrothermal process leads to a quite rough and hierarchical surface with both nano- and micro-scaled structure, while the graft of long hydrophobic aliphatic chains at the rough magnesium alloy surfaces results in the typical chemical composition. Consequently, the low surface energy of AZ91 can be resulted. In our research, the surface energy at as-prepared surfaces is about 19.3 mN·m−1, which is calculated by using three liquids method (i.e., water, diiodomethane, and ethylene glycol method). This low surface energy as well as specific hierarchical structure at the surfaces ensures the magnesium alloy with superhydrophobicity. It is well known that the rough and hierarchical morphology with both micro- and nano-scaled structure together with the hydrophobic surface composition play an important role on wettability of solid surface [32]. A great deal of air can be trapped in the cavities that generated in the hierarchical structure, and which can result in a larger water contact angle and a smaller sliding angle according to Cassie–Baxter equation [33]: 8
cos f cos 0 f 1
(6)
Where f represents the area fraction of the water/solid interface, and θ and θ0 denote the water contact angle on the rough surface and smooth surface modified by materials with low surface energy, respectively. In our research, the water contact angles at the rough surface and smooth surface grafted long aliphatic chains are 153.8° and 83.7° in order. Thus, it is calculated that f herein is 9.26%. It indicates that the contact area fraction of water/air interface can reach 90.74%. Thereupon, it can be concluded that the contact area between the water and air accounts for the overwhelming majority of contact area. The large contact area between the water and air is quite effective in preventing the penetration of water from intruding into the rough and hierarchical surface. Consequently, the as-prepared magnesium alloy surfaces win the superhydrophobicity. 3.3 Corrosion resistance of the superhydrophobic magnesium alloys In order to evaluate the corrosion protection of the generated superhydrophobic surface on magnesium alloys, electrochemical corrosion test is performed. The investigated potentiodynamic polarization plots of both untreated and as-prepared superhydrophobic magnesium alloys are shown in Fig.3, while which are obtained after samples being exposed to 3.5 wt.% of NaCl solution for 10 min. Meanwhile, the important parameters, such as the corrosion potential (Ecorr) and corrosion current density (Icorr) derived from the potentiodynamic polarization curves are listed in Table 2. The potentiodynamic polarization is widely used to evaluate the instantaneous corrosion rate of a tested specimen, while parameters like Ecorr and Icorr can reflect the electrochemical corrosion behavior directly. Generally, it is believed that a higher Ecorr represents a better corrosion resistance and a lower Icorr value corresponds to a lower corrosion rate [34]. It can be found from Fig.3 and Table 2 that Ecorr of the untreated magnesium alloy in 3.5 wt.% NaCl solution is -1.50 V. By contrast, it increases positively to -1.34 V for the as-prepared superhydrophobic magnesium alloy. Meanwhile, Icorr decreases from 1.48×10-4 A·cm-2 to 3.32×10-7 A·cm-2 after hydrothermal treatment. As compared to the untreated substrate, Icorr of the superhydrophobic magnesium alloy decreases by nearly 3 orders of magnitude. The decrease of Icorr and the increase of Ecorr suggest that the as-prepared superhydrophobic magnesium alloy exhibits an excellent corrosion resistance in NaCl corrosive media as compared to the untreated one. This shows that the corrosion resistance of magnesium alloy has been improved greatly by 9
our one-step hydrothermal treatment. The corrosion behavior from the electrochemical corrosion test above is based on a short term immersion process, herein, 10 min. To further assess the long-term durability and stability of the as-fabricated surface under the attack of corrosive species, such as water, oxygen, and chloride ions, the as-prepared superhydrophobic magnesium alloys are immersed into 3.5 wt.% of NaCl solution for a much longer period, i.e., several hours to tens hours. The surface wettability evolution, including static contact angle and the sliding angle, at superhydrophobic magnesium alloy surface was measured, as shown in Fig.4. It is obvious that the static contact angle at the as-prepared superhydrophobic surfaces decreases gradually from more than 150° to about 20° when the immersion time in the NaCl solution extends to 60 hours. On the contrary, the sliding angle increases from 4° to ca. 90° upon exposed in the corrosive solution. The wettability variation indicates that materials with low surface energy grafted on the rough magnesium alloy surfaces are deteriorated gradually with the immersion time extending. Additionally, it can also be observed that the static contact angle decreases slowly before 30 hours of immersion while it decreases more rapidly after this time. By contrast, the sliding angle increases much slowly before 15 hours of exposure in corrosive media. It enhances quite rapidly up to 90o from 15 hours to 35 hours by contraries. Whereafter, it remains unchanged on the whole. The reason that leads to this result might be hydrophobic molecules are packed closely and the corrosion reaction is suppressed at the earlier stage. However, part hydrophobic films will finally be deteriorated under the persistent attack of corrosive species. Consequently, some defects, such as nano- or micro- holes will be engendered, and which will facilitate some corrosion conversely and then bring about faster deterioration of the hydrophobic film. In order to further understand the corrosion protection of the superhydrophobic surface on magnesium alloys, morphologies of both untreated and superhydrophobic magnesium alloys after immersed in 3.5 wt.% of NaCl solution for 20 hours are observed with SEM and the images are shown in Fig.4 as well. Fig.4b shows the untreated magnesium alloy surface exposed in NaCl solution for 20 hours. It is obvious that a great deal of granular and protuberant corrosion products arise at the substrate surface, indicating that the severe corrosion reaction has taken place and the substrate has been eroded seriously. By contrast, Fig.4c shows that only a small quantity of corrosion products present at the superhydrophobic magnesium alloy surface after immersed in 10
NaCl solution for 20 hours, and which manifests that slight corrosion happens for the superhydrophobic magnesium alloy. So the corrosion resistance of magnesium alloys has been improved greatly by endowing magnesium alloys with superhydrophobicity. Thereupon, results above show that the superhydrophobic magnesium alloys can endure much longer period in corrosive media as compared to the untreated ones, and the generated superhydrophobic surface on magnesium alloys can protect magnesium alloys from severe corrosion effectively. Consequently, it can be concluded that the superhydrophobic surface shows long-term corrosion protection for magnesium alloy substrate. It is well known that once the magnesium alloy without treatment is immersed in the corrosive medium (such as NaCl solution), water and corrosive species, such as O2 and Cl-, etc., can easily contact with the substrate directly and react with it [35]. Consequently, corrosion products as Mg(OH)2, MgO, and Mg(OH)Cl, and so on, will be engendered. Hence, the magnesium alloy surface can be eroded severely and massive corrosion products present at the surface, just as what has been displayed schematically in Scheme 2. On the contrary, a great deal of air is entrapped in cavities of rough and hierarchical superhydrophobic magnesium alloy surface, and which can restrain water and corrosive species approach and contact with the substrate effectively when the superhydrophobic magnesium alloy is immersed in the corrosive medium. Consequently, the magnesium alloy substrate is protected from corrosion by the superhydrophobic surface. So the corrosive rate of the superhydrophobic magnesium alloy can be reduced markedly as compare to that of the untreated substrate. However, with the immersion time in the corrosive medium extending, slight corrosion can also happen for the superhydrophobic magnesium alloys. The corrosion protection of the superhydrophobic surface as well as the slight corrosion behavior of the superhydrophobic magnesium alloys can be inferred as follows, also schematically illustrated in Scheme 2: the hydrophobic alkyl molecules are exposed to corrosive solution directly at the superhydrophobic film/liquid interface. In the earlier stage, the hydrophobic molecules pack closely and the corrosive species can be prevented from approaching the surface of magnesium alloys. Consequently, the corrosive reaction can be suppressed. However, under the persistent attack of water and corrosive species, part chemical interfacial bonding might tend to decompose, for instance, hydrolyze. Thus some hydrophobic molecules will dissociate from the substrate. 11
Consequently, defects in the superhydrophobic shielding film, such as nano- or micro- holes, will be engendered. Under the situation above, water molecules and corrosive species will intrude into the superhydrophobic film/vapor interface. Then they may penetrate the superhydrophobic film to reach the magnesium alloy substrate. As a result, the corrosion reaction will be triggered and which will further lead to the dissociation of more hydrophobic chains. So the corrosion behavior will become serious gradually with the immersion time in NaCl solution exceeds 30 hours. Therefore, it can be concluded that the superhydrophobic surface has outstanding protection effect on magnesium alloy substrate from corrosion, especially at the earlier stage in corrosive media. However, the protection effect will reduce gradually under the persistent attack of corrosive species. Once the superhydrophobic film has been broken through by the corrosive species and cracks are engendered in the superhydrophobic surface, the protection effect will reduce and even lose. 3.4 Self-cleaning performance of the superhydrophobic magnesium alloys The self-cleaning performance is an important engineering application index for superhydrophobic materials. In our research, the self-cleaning performance is assessed by observing whether the simulated contaminant particles (herein, carbon powder, chalk dust, and cigarette ash) spread on tilted magnesium alloy surfaces can be carried away by the rolling water droplets easily. The test images are shown in Fig.5. Firstly, cigarette ash is spread and coated onto the untreated magnesium alloy surface, and then the sample is tilted for a inclination angle of 6° above horizontal. As the surface is hydrophilic, the water droplet spreads out immediately and wettens the surface when it is dripped onto the tilted surface gently. So no appreciable removal of cigarette ash is noticed, indicating that the cigarette ash cannot be taken away from the untreated surface, as shown in top left picture in Fig.5. Similarly, carbon powder and chalk dust dispersed on untreated magnesium alloy surfaces cannot be taken away by the water droplets as well for the alike reason. These results indicate that the untreated magnesium alloy surface has not self-cleaning performance. By contrast, once water droplets are applied onto superhydrophobic magnesium alloy surfaces covered with simulated contaminant particles (such as cigarette ash, carbon powder, and chalk dust) under the similar conditions, water droplets roll down surfaces immediately since they cannot wetten the surfaces, as shown in right pictures in Fig.5. Meanwhile, contaminant particles 12
are transferred to the outside surfaces of rolling water droplets and taken off from the superhydrophobic surfaces. Moreover, no contaminant particles are visible on the rolling path of water droplets while the path at superhydrophobic surface remains dry and clean. These manifest that the adhesive force between contaminant particles and the superhydrophobic magnesium alloy surfaces is quite weak. Thus contaminant particles can be taken away from superhydrophobic surfaces by the rolling water droplets without difficulty. Thereupon, it shows that the superhydrophobic magnesium alloys have excellent self-cleaning performance. More importantly, the self-cleaning effect can be repeated for 20 times and above, so the as-prepared superhydrophobic magnesium alloys can maintain their clean surfaces without any pollution for a long time. The self-cleaning mechanism of superhydrophobic magnesium alloys is due to the distinctive surface structure and composition. A large number of air is trapped inside the superhydrophobic surfaces, and which serves as a “air cushion”. Consequently, the superhydrophobic magnesium alloy surface cannot be fouled easily. Even if the surface has been fouled by contaminants, the contaminants can be cleaned by the water droplets (such as rain droplets) or breeze without difficulty as the adhesion between contaminants and superhydrophobic surfaces is quite weak. Thereupon, the samples regain cleanliness in a short time. Therefore, the superhydrophobic magnesium alloys can keep a clean surface over a long period of time, and it is a significant feature for practical applications.
4. Conclusions In summary, a one-step hydrothermal process is developed to prepare the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic magnesium alloy surface presents rough and hierarchical structure while long hydrophobic alkyl chains are grafted onto the surface with covalent bonds. Consequently, the superhydrophobic surface with a water contact angle of 153.8° and a low sliding angle of ca. 4° is obtained. The as-prepared superhydrophobic surface can provide protect effect for magnesium alloys. As a result, the magnesium alloys show excellent corrosion resistance and self-cleaning performance due to the superhydrophobic surfaces. We believe that this method provides a novel, simple, and cost-effective process to improve the corrosion resistance and self-fouling performance of magnesium alloys. Moreover, it is profitable for the practical application of magnesium alloy as an engineering material. 13
Acknowledgements This research is supported by National Natural Science Foundation of China (Grant No. 21161012).
14
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superhydrophobic surface with controlled adhesion by electrodeposition, Chem. Eng. J., 248 (2014) 440-447. [11] K.A. Khalil, E.M. Sherif, A.A. Almajid, Corrosion passivation in simulated body fluid of magnesium/hydroxyapatite nanocomposites sintered by high frequency induction heating, Int. J. Electrochem. Sci., 6 (2011) 6184-6199. [12] J.S. Liao, M. Hotta, Y. Mori, Improved corrosion resistance of a high-strength Mg–Al–Mn–Ca magnesium alloy made by rapid solidification powder metallurgy, Mater. Sci. Eng. A, 544 (2012) 10-20. [13] J.H. Li, Q. Liu, Y.L. Wang, R.R. Chen, K. Takahashi, R.M. Li, L.H. Liu, J. Wang, Formation of a corrosion-resistant and anti-icing superhydrophobic surface on magnesium alloy via a single-step method, J. Electrochem. Soc., 163 (2016) C213-C220. [14] 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, Appl. Phys. Lett., 92 (2008) 183103. [15] W.J. Stępniowski, T. Durejko, M. Michalska-Domańska, M. Łazińska, J. Aniszewska, Characterization of nanoporous anodic aluminum oxide formed on laser pre-treated aluminum, Mater. Charact., 122 (2016) 130-136. [16] S. Sarbada, Y.C. Shin, Superhydrophobic contoured surfaces created on metal and polymer using a femtosecond laser, Appl. Surf. Sci., 405 (2017) 465-475. [17] J.L. Zhang, C.D. Gu, Y.Y. Tong, W. Yan, J.P. Tu, A smart superhydrophobic coating on AZ31B magnesium alloy with self-healing effect, Adv. Mater. Interfaces, 3 (2016) 1500694. [18] S.H. Wang, X.W. Guo, Y.J. Xie, L.H. Liu, H.Y. Yang, R.Y. Zhu, J. Gong, L.M. Peng, W.J. Ding, Preparation of superhydrophobic silica film on Mg-Nd-Zn-Zr magnesium alloy with enhanced corrosion resistance by combining micro-arc oxidation and sol-gel method, Surf. Coat. Technol., 213 (2012) 192-201. [19] G.S. Wu, W. Dai, L.X. Song, A.Y. Wang, Surface microstructurization of a sputtered magnesium thin film via a solution-immersion route, Mater. Lett., 64 (2010) 475-478. [20] Q. Liu, D.X. Chen, Z.X. Kang, One-step electrodeposition process to fabricate corrosion resistant superhydrophobic surface on magnesium alloy, ACS Appl. Mater. Interfaces, 7 (2015) 1859-1867. 16
[21] A.D. Forero-López, I.L. Lehr, S.B. Saidman, Anodisation of AZ91D magnesium alloy in molybdate solution for corrosion protection, J. Alloys Compd., 702 (2017) 338-345. [22] S. Kiahosseini, A. Afshar, M.M. Larijani, M. Yousefpour, Structural and corrosion characterization of hydroxyapatite/zirconium nitride-coated AZ91 magnesium alloy by ion beam sputtering, Appl. Surf. Sci., 401(2017) 172-180. [23] T. Darmanin. F. Guittard, Recent advances in the potential applications of bioinspired superhydrophobic materials, J. Mater. Chem. A, 2 (2014) 16319-16359. [24] T. Ishizaki, J. Hieda, N. Saito, N. Saito, O. Takai, Corrosion resistance and chemical stability of superhydrophobic film deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition, Electrochim. Acta, 55 (2010) 7094-7101. [25] L.B. Feng, Y.L. Zhu, W.B. Fan, Y.P. Wang, X.H. Qiang, Y.H. Liu, Fabrication and corrosion resistance of superhydrophobic magnesium alloy, Appl. Phys. A, 120 (2015) 561-570. [26] L. Zhao, Q. Liu, R. Gao, J. Wang, W.L. Yang, L.H. Liu, One-step method for the fabrication of superhydrophobic surface on magnesium alloy and its corrosion protection, antifouling performance, Corros. Sci., 80 (2014) 177-183. [27] Y. Zhu, Q. Zhao, Y.H. Zhang, G. Wu, Hydrothermal synthesis of protective coating on magnesium alloy using de-ionized water, Surf. Coat. Technol., 206 (2012) 2961-2966. [28] X.X. Guo, S.L. Xu, L.L. Zhao, W. Lu, F.Z. Zhang, D.G. Evans, X. Duan, One-Step hydrothermal crystallization of a layered double hydroxide/alumina bilayer film on aluminum and its corrosion resistance properties, Langmuir, 25 (2009) 9894-9897. [29] J.F. Ou, W.H. Hu, M.S. Xue, F.J. Wang, W. Li, Superhydrophobic surfaces on light alloy substrates fabricated by a versatile process and their corrosion protection, ACS Appl. Mater. Interfaces, 5 (2013) 3101-3107. [30] Z.W. Wang, Y.L. Su, Q. Li, Y. Liu, Z.X. She, F.N. Chen, L.Q. Li, X.X. Zhang, P. Zhang, Researching a highly anti-corrosion superhydrophobic film fabricated on AZ91D magnesium alloy and its anti-bacteria adhesion effect, Mater. Charact., 99 (2015) 200-209. [31] B. Zhang, Y. Li, B. Hou, One-step electrodeposition fabrication of a superhydrophobic surface on an aluminum substrate with enhanced self-cleaning and anticorrosion properties. RSC Adv., 5 (2015) 100000–100010. [32] T. Schutzius, S. Jung, T. Maitra, G. Graeber, M. Köhme, D. Poulikakos, Spontaneous droplet 17
trampolining on rigid superhydrophobic surfaces. Nature, 527 (2015) 82-85. [33] A.B.D. Cassie, S. Baxter, Surface roughness and contact angle, Trans. Faraday Soc. 40 (1944) 546-551. [34] I. Vilaró, J. Yagüe, S. Borrós, Superhydrophobic copper surfaces with anticorrosion properties fabricated by solventless CVD methods. ACS Appl. Mater. Interfaces, 9 (2017) 1057-1065. [35] T.M. Mukhametkaliyev, M.A. Surmeneva, A. Vladescu, C.M. Cotrut, M. Braic, M. Dinu, M.D. Vranceanu, I. Pana, M. Mueller, R.A. Surmenev, A biodegradable AZ91 magnesium alloy coated with a thin nanostructured hydroxyapatite for improving the corrosion resistance, Mater. Sci. Eng. C, 75 (2017) 95-103.
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Figure Captions
Fig.1 SEM images of magnesium alloy surfaces before (a) and after (b,c,d) hydrothermal treatment; XRD patterns (e) and wettability of a 10 µL of water droplet on magnesium alloy surface (f) after hydrothermal treatment. The insert in panel f shows a water droplet sliding image with a sliding angle of 4°.
Fig.2 Chemical composition analysis with FT-IR spectra (a,b) and EDS spectra (c,d): (a) stearic acid, (b,d) superhydrophobic magnesium alloy, and (c) untreated substrate.
19
Fig.3 Potentiodynamic polarization plots of untreated and superhydrophobic magnesium alloys in 3.5 wt.% of NaCl solution.
Fig.4 (a) Surface wettability variation as a function of immersion time for superhydrophobic magnesium alloy, and SEM images of untreated (b) and superhydrophobic (c) magnesium alloy surfaces after immersion in NaCl solution for 20 hours.
20
Fig.5 Self-cleaning effect of contaminant particles coated on untreated and superhydrophobic magnesium alloy surfaces.
21
Scheme 1 Schematic illustration for the fabrication of superhydrophobic surface on magnesium alloy with one-step hydrothermal process.
Scheme 2 Schematic diagram of interface model for the mechanism of corrosion resistance.
22
Table 1 The weight content (wt.%) data at the untreated and superhydrophobic magnesium alloy surfaces. Sample Untreated Superhydrophobic
Mg
C
O
Al
71.07
20.31
6.63
1.99
9.67
65.96
23.70
0.67
Table 2 Corrosion potential (Ecorr) and current density (Icorr) of untreated and superhydrophobic magnesium alloys in 3.5 wt.% of NaCl solution. Sample
Ecorr (V)
Icorr (A·cm-2 )
Untreated
-1.50
1.48×10-4
Superhydrophobic
-1.34
3.32×10-7
23
S0169-4332(17)31722-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.066 APSUSC 36266
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-3-2017 5-5-2017 5-6-2017
Please cite this article as: Libang Feng, Yali Zhu, Jing Wang, Xueting Shi, One-step Hydrothermal Process to Fabricate Superhydrophobic Surface on Magnesium Alloy with Enhanced Corrosion Resistance and Self-cleaning Performance, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step
Hydrothermal
Process
to
Fabricate
Superhydrophobic Surface on Magnesium Alloy with Enhanced
Corrosion
Resistance
and
Self-cleaning
Performance Libang Fenga *, Yali Zhua,b, Jing Wang a, Xueting Shia
a
School of Mechatronic Engineering, Lanzhou Jiaotong University, Lanzhou 730070 b
Department
of
Mechanical
and
Electrical
Engineering, Xi’an Railway Vocational &
Technical Institute, Xi’an 710026
Corresponding
author, E-mail: [email protected]
Graphical Abstract
A facile, environment-friendly, and cost-effective one-step hydrothermal route is proposed to fabricate the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic surface can greatly improve the corrosion resistance and self-cleaning performance of magnesium alloy due to its high water repellence.
1
Research Highlights
A facile one-step method for fabricating superhydrophobic surface on Mg alloy is developed.
The superhydrophobic surface has enhanced anti-corrosion performance.
The superhydrophobic surface exhibits excellent self-cleaning performance.
The mechanisms of corrosion resistance and self-cleaning performance are deduced.
Abstract: Superhydrophobic surfaces can exhibit anti-corrosion, anti-fogging, and self-cleaning performance due to their high water repellence. It is significant for industrial fabricating of superhydrophobic surface with a simple and environment-friendly method. Herein, a facile, environment-friendly, and cost-effective one-step hydrothermal route is proposed to fabricate the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic magnesium alloy surface presents the rough and hierarchical micro/nano- structure grafted with long hydrophobic alkyl chains via covalent bonds. Both electrochemical corrosion test and long term immersion in 3.5 wt.% of NaCl solution demonstrate that the superhydrophobic surface greatly improves the corrosion resistance of magnesium alloy. Meanwhile, the superhydrophobic magnesium alloy exhibits excellent self-cleaning performance. It is supposed that this facile method and remarkable properties of resultant superhydrophobic magnesium alloys have a promising future in expanding the application of magnesium alloys. Key words: Magnesium alloy; Superhydrophobicity; One-step; Hydrothermal process; Anti-corrosion; Self-cleaning
1. Introduction The corrosion of metals can cause a tremendous economic loss each year [1-3], and it has attracted much attention of developing new methods and technology to slow down the corrosion rate of metals [4-7]. Among various metals, magnesium and its alloy have stimulated considerable interest in aerospace, automobile, machinery, computer, electronic industry, and so on, thanks to their excellent characteristics as low density, high specific strength, high ductility, good thermal/electrical conductivity, electromagnetic compatibility, good castability, and abundant availability [8,9]. However, due to the low standard potential and high chemical reactivity, magnesium is one of very active metals and it is susceptible to erode in an aqueous environment, 2
moist atmosphere, or other corrosive media. And which has limited its large scale application in engineering fields. In order to improve the corrosion resistance and extend the application area of magnesium and its alloy, many strategies have been attempted, such as introducing rare earth elements [10], increasing the purity of alloy [11], using rapid solidification processing [12], introducing corrosive metal atoms [13], wet chemical etching [14], and so on. Recently, many studies have shown that it is an effective method of constructing superhydrophobic surfaces on various metals to enhance their corrosion resistance since the superhydrophobic surfaces can hinder the close contact of a surface with the corrosive species [15-19]. So it may be a preferred method to improve the corrosion resistance by endowing magnesium and its alloy with the superhydrophobicity. Various methods have been reported for preparing the superhydrophobic surface on magnesium and its alloy. For example, chemical conversion film, electrodeposition, anodization, chemical vapor deposition, laser treating, micro-arc oxidation, wet chemical etching, electrospinning, and so on [20-26]. However, in most cases, the method is subject to certain limitations, such as special equipment, complex operations, caustic reagents (for instance, HNO3, H2SO4, HF, H2O2, etc.), or expensive materials (namely, fluoride, etc.), and severe conditions. Consequently, these procedures/methods may cause a lot of problems as no universality, high cost, environment pollution, and so on [23-26]. Thus, it is quite needed of developing simple, inexpensive, versatile, and environment-friendly process for the fabrication of the superhydrophobic surfaces on magnesium alloys so as to promote the advantages for industrial large-scale application. Hydrothermal reaction is an efficient way for materials preparation and modification, and it has been used to prepare the superhydrophobic surfaces as well [27,28]. A few papers have been reported for the preparation of superhydrophobic surfaces on magnesium alloys. For example, Wang and co-workers [29] prepared the superhydrophobic magnesium alloy by the hydrothermal process and then modified the surfaces with fluoroalkylsilane, and results showed that the superhydrophobic surface had outstanding corrosion protection effect on magnesium alloy. Li et al. [30] also proposed a hydrothermal method to fabricate superhydrophobic surface on AZ91D magnesium alloy combining stearic acid modification. The as-prepared samples exhibited a considerable enhancement in corrosion resistance and superior anti-bacteria adhesion effect. However, two steps are involved in both routes for fabricating the superhydrophobic surfaces: the 3
first step is to construct a rough-structured surface via hydrothermal process and then to modify surfaces with low-surface-energy substances. Moreover, the caustic NaOH or expensive fluoroalkylsilane is used in the fabrication procedures. To overcome the inherent disadvantage and simplify the procedure, herein, a facile, environment-friendly, and cost-effective method is developed for preparing superhydrophobic magnesium alloys via only one-step hydrothermal process. Furthermore, the as-prepared superhydrophobic surface greatly enhances the corrosion resistance of magnesium alloy. Meanwhile, the superhydrophobic magnesium alloys show excellent self-cleaning performance. The presented method is simple, low-cost, and environment-friendly, which is significant for industrial fabricating of superhydrophobic surfaces with excellent anti-corrosion and self-cleaning performance. Therefore, it is supposed to have a promising future in expanding the application of magnesium alloys.
2. Material and Methods 2.1 Materials Magnesium alloy (AZ91) plate was purchased from Qingdao Dexingsheng metal materials Co., Ltd. The chemical composition of AZ91 was 8.50-9.50% Al, 0.45-0.90% Zn, 0.17-0.40% Mn, 0.05% Si, 0.025% Cu, 0.001% Ni, 0.004% Fe, and Mg balance. Stearic acid (STA) was supplied by Shanghai Zhongqin Chemical Reagent Co., Ltd. Ethanol was purchased from Sinopharm Group Chemical Reagent Co., Ltd. 2.2 Fabrication of the superhydrophobic magnesium alloys The fabricating process of the superhydrophobic magnesium alloy surface is shown in Scheme.1. After polished with abrasive paper and cleaned ultrasonically with acetone/deionized water, AZ91 plate was introduced into a Teflon-lined stainless steel autoclave filled with deionized water, ethanol, and STA. The volume ratio of ethanol to deionized water was kept at 1:1.4 while STA concentration was 50 mmol/L. Then the autoclave was sealed and maintained at 80 oC for 10 hours. After the hydrothermal reaction finished, AZ91 plate was taken out and cleaned with ethanol for three times. Finally, the as-prepared sample was dried in air, and thus the superhydrophobic magnesium alloy was obtained. 2.3 Corrosion resistant performance evaluation The corrosion resistant performance of as-fabricated superhydrophobic magnesium alloys was 4
evaluated by both electrochemical corrosion test and long term immersion in 3.5 wt.% of NaCl solution. Electrochemical corrosion test was performed on a computer-controlled electrochemistry workstation (CHI660D, CH Instruments Inc., China) by potentiodynamic polarization in a three-electrode system: working electrode, a platinum stick counter electrode, and a saturated calomel reference electrode. Dynamic measurement of polarization plots in a Tafel model was acquired at a scan rate of 1 mV/s at room temperature when samples were exposed to corrosive solution for a short period of 10 min. Long-term durability of superhydrophobic magnesium alloys under continuous contact with saline water was carried out by immersing samples in 3.5 wt.% of NaCl solution at 25.0 ± 0.5 °C from 0 to 60 hours. After a definite time of immersion, the sample was taken out, dried under a nitrogen stream, and retained in a vacuum oven at 30 °C for 2 hours. Then, the wettability of the samples was measured. 2.4 Self-cleaning performance test The self-cleaning performance of as-fabricated superhydrophobic magnesium alloys was evaluated by measuring the ability of the rolling water droplets taking away the simulating contaminant particles, while the cigarette ash, carbon powder, and chalk dust were used as the simulating contaminant particles. The self-cleaning test was carried out by deliberately spreading contaminants to form thick layers on the magnesium alloy surfaces. Then a water droplet with a volume of 10 μL was dripped gently onto the sample surfaces under a slope angle of 6° above horizontal. 2.5 Characterization The wettability was evaluated by both static water contact angle (CA) and sliding angle (SA). In our research, the CA and SA for deionized water were measured with a 10 μL of droplet using a horizontal microscope with a protractor eyepiece (DSA 100, Kruss, Germany) at ambient temperature. The droplet was placed at five different spots for each sample surface and the average value was regarded as the contact angle. The error of the contact angle is ±1o. The sliding angle was measured by slowly tilting the level stage controlled with computer, and the tilted angle was adopted when the droplet began to roll in the downhill direction on the sample surfaces. Scanning electron microscopy (FE-SEM, JSM-6701F, Japan) was used to observe the surface 5
morphology. X-ray diffractometer (XRD, XRD-7000LX, Shimadzu, Japan) was used to characterize the phase structure. Fourier transform infrared spectra (FT-IR, VER-TEX 70, Germany) and energy dispersive X-ray spectroscopy (EDS, Inca X-Max, UK) were used to analyze the surface chemical composition.
3. Results and discussion 3.1 Surface morphology, structure, and wettability The superhydrophobic magnesium alloys are fabricated by one-step hydrothermal method. SEM images of the untreated magnesium alloy substrate and superhydrophobic surface are shown in Fig.1. The SEM image of untreated magnesium alloy surface (see as Fig.1a) displays a relatively flat and smooth surface while only abrasive grooves are observed. The untreated substrate surface takes a static water contact angle of ca. 38.3°, indicating the hydrophilic nature of the alloy surface. Fig.1b-d show surface morphologies of magnesium alloy after hydrothermal treatment. It can be noted that great changes happen at the magnesium alloy surface after hydrothermal process. The low-magnification SEM image in Fig.1b shows the surface is covered by a large number of bar-like and plate-like structure completely. It is clearly observed from the high magnification SEM images (Fig.1c and Fig.1d) that the as-prepared surface consists of many nano-plates. The plate length is between 1 μm and 5 μm while the plate thickness is about 50–100 nm. Moreover, it is worth noting from Fig.1d that the plate is composed of several layers. Meanwhile, a great deal of gaps can be easily found among plates. Consequently, a large number of cavities are resulted in the magnesium alloy substrate surface, and the wideness of cavities ranges from 100 nm to 2 μm. Therefore, a quite rough and hierarchical structure with both nanoand micro-scales presents at the magnesium alloy surface after hydrothermal treatment. In order to identify the surface species, phase structure is examined with XRD while the investigated XRD patterns of magnesium alloys both before and after hydrothermal treatment are shown in Fig.1e. It is apparent that the untreated magnesium alloy surface is mainly composed of Mg (JCPDF No. 35-0821). By contrast, XRD pattern has changed much after hydrothermal treatment, and some new peaks present in XRD pattern. The new peaks are attributed to MgO (JCPDF No. 30-0794) and Mg(OH)2 (JCPDF No. 44-1482), indicating that the magnesium alloy surface is mainly composed of MgO and Mg(OH)2 after hydrothermal treatment. It is supposed 6
that such species are generated by following reactions: H 2 O (l ) H Mg ( s ) 2 H
Mg
Mg
( aq .) OH
( aq .) Mg
(1)
( aq .)
2
( aq .) H 2 ( g )
2
( aq .) 2 OH
2
( aq .) H 2 O MgO ( s ) 2 H
(2)
( aq .) Mg ( OH ) 2 ( s )
(3)
(4)
Just based on the formation of MgO and Mg(OH)2 at magnesium alloy surface upon hydrothermal treatment, a quite rough and hierarchical surface with both nano- and micro-scaled structure is obtained. Meanwhile, the wettability of AZ91 surface changes greatly after the hydrothermal treatment. It can be observed from Fig.1f that the water droplet with a volume of 10 uL takes on a quasi-spherical shape. The static water contact angle increases to 153.8° compared to 38.3° of the substrate before hydrothermal treatment, while the sliding angle is about 4°. These results show that the wettability of the surface changes from hydrophilic to superhydrophobic after hydrothermal treatment. 3.2 Chemical composition analysis It is well known that the surface wettability is not only related to surface microstructure, but also is relevant to surface chemical composition. In our research, STA is introduced in the hydrothermal treatment. So the surface chemical structure may have much difference before and after hydrothermal treatment. The chemical composition of the as-prepared superhydrophobic surface is analyzed by FT-IR and EDS techniques, and the investigated spectra are shown in Fig.2. In the FT-IR spectrum of STA (see as Fig.2a), the absorption peaks present at 2920 cm-1 and 2850 cm-1 correspond to the symmetric and asymmetric stretching vibration of C-H groups. The absorption peak locates at 1700 cm-1 is ascribed to -COOH groups. Fig.2b shows FT-IR spectrum of the as-prepared superhydrophobic magnesium alloy. It can be found that the stretching vibration absorption peaks of C–H groups still exist. However, the –COOH absorption peak disappears. Instead, a new absorption peak arises at 1570 cm-1, and which stems from the stretching vibration of –COO- groups [31]. These results indicate that the chemical structure at magnesium alloy surfaces has changed markedly after hydrothermal treatment. Result from XRD indicates that Mg atom at AZ91 surfaces after hydrothermal treatment mainly presents in MgO and Mg(OH)2. On 7
the other hand, the chemical reaction between –COOH groups in STA molecules and –OH groups in MgO and Mg(OH)2 will take place in hydrothermal process. The reaction equation can be described as follows: Mg ( OH ) 2 ( s ) 2 CH
3
( CH
2
) 16 COOH ( l ) Mg ( CH
3
( CH
2
) 16 COO ) 2 ( s ) 2 H 2 O ( l )
(5)
Therefore, results from FT-IR confirm that the long aliphatic chains have been grafted onto the magnesium alloy surfaces with covalent bonds successfully upon hydrothermal treatment. However, Mg(CH3(CH2)16COO)2 presents in amorphous state and which cannot be detected with XRD technique. In order to further confirm the successful graft of STA chains at the magnesium alloy surface upon hydrothermal treatment, EDS technique is used. Fig.2c and Fig.2d show EDS spectra at the magnesium alloy surfaces before and after hydrothermal treatment, while the elemental weight ratio is listed in Table 1. It is obvious that Mg, C, O, and Al elements present at both the untreated and as-fabricated superhydrophobic AZ91 surfaces. As compared to those at the untreated AZ91 surface, the amount of Mg atom decreases distinctly while contents of C and O atoms rise evidently at the superhydrophobic magnesium alloy surface. The decrease of Mg atom and the increase of elemental C and O result from the generation of MgO and Mg(OH)2 as well as the graft of STA chains. The formation of MgO and Mg(OH)2 at magnesium alloy surface in hydrothermal process leads to a quite rough and hierarchical surface with both nano- and micro-scaled structure, while the graft of long hydrophobic aliphatic chains at the rough magnesium alloy surfaces results in the typical chemical composition. Consequently, the low surface energy of AZ91 can be resulted. In our research, the surface energy at as-prepared surfaces is about 19.3 mN·m−1, which is calculated by using three liquids method (i.e., water, diiodomethane, and ethylene glycol method). This low surface energy as well as specific hierarchical structure at the surfaces ensures the magnesium alloy with superhydrophobicity. It is well known that the rough and hierarchical morphology with both micro- and nano-scaled structure together with the hydrophobic surface composition play an important role on wettability of solid surface [32]. A great deal of air can be trapped in the cavities that generated in the hierarchical structure, and which can result in a larger water contact angle and a smaller sliding angle according to Cassie–Baxter equation [33]: 8
cos f cos 0 f 1
(6)
Where f represents the area fraction of the water/solid interface, and θ and θ0 denote the water contact angle on the rough surface and smooth surface modified by materials with low surface energy, respectively. In our research, the water contact angles at the rough surface and smooth surface grafted long aliphatic chains are 153.8° and 83.7° in order. Thus, it is calculated that f herein is 9.26%. It indicates that the contact area fraction of water/air interface can reach 90.74%. Thereupon, it can be concluded that the contact area between the water and air accounts for the overwhelming majority of contact area. The large contact area between the water and air is quite effective in preventing the penetration of water from intruding into the rough and hierarchical surface. Consequently, the as-prepared magnesium alloy surfaces win the superhydrophobicity. 3.3 Corrosion resistance of the superhydrophobic magnesium alloys In order to evaluate the corrosion protection of the generated superhydrophobic surface on magnesium alloys, electrochemical corrosion test is performed. The investigated potentiodynamic polarization plots of both untreated and as-prepared superhydrophobic magnesium alloys are shown in Fig.3, while which are obtained after samples being exposed to 3.5 wt.% of NaCl solution for 10 min. Meanwhile, the important parameters, such as the corrosion potential (Ecorr) and corrosion current density (Icorr) derived from the potentiodynamic polarization curves are listed in Table 2. The potentiodynamic polarization is widely used to evaluate the instantaneous corrosion rate of a tested specimen, while parameters like Ecorr and Icorr can reflect the electrochemical corrosion behavior directly. Generally, it is believed that a higher Ecorr represents a better corrosion resistance and a lower Icorr value corresponds to a lower corrosion rate [34]. It can be found from Fig.3 and Table 2 that Ecorr of the untreated magnesium alloy in 3.5 wt.% NaCl solution is -1.50 V. By contrast, it increases positively to -1.34 V for the as-prepared superhydrophobic magnesium alloy. Meanwhile, Icorr decreases from 1.48×10-4 A·cm-2 to 3.32×10-7 A·cm-2 after hydrothermal treatment. As compared to the untreated substrate, Icorr of the superhydrophobic magnesium alloy decreases by nearly 3 orders of magnitude. The decrease of Icorr and the increase of Ecorr suggest that the as-prepared superhydrophobic magnesium alloy exhibits an excellent corrosion resistance in NaCl corrosive media as compared to the untreated one. This shows that the corrosion resistance of magnesium alloy has been improved greatly by 9
our one-step hydrothermal treatment. The corrosion behavior from the electrochemical corrosion test above is based on a short term immersion process, herein, 10 min. To further assess the long-term durability and stability of the as-fabricated surface under the attack of corrosive species, such as water, oxygen, and chloride ions, the as-prepared superhydrophobic magnesium alloys are immersed into 3.5 wt.% of NaCl solution for a much longer period, i.e., several hours to tens hours. The surface wettability evolution, including static contact angle and the sliding angle, at superhydrophobic magnesium alloy surface was measured, as shown in Fig.4. It is obvious that the static contact angle at the as-prepared superhydrophobic surfaces decreases gradually from more than 150° to about 20° when the immersion time in the NaCl solution extends to 60 hours. On the contrary, the sliding angle increases from 4° to ca. 90° upon exposed in the corrosive solution. The wettability variation indicates that materials with low surface energy grafted on the rough magnesium alloy surfaces are deteriorated gradually with the immersion time extending. Additionally, it can also be observed that the static contact angle decreases slowly before 30 hours of immersion while it decreases more rapidly after this time. By contrast, the sliding angle increases much slowly before 15 hours of exposure in corrosive media. It enhances quite rapidly up to 90o from 15 hours to 35 hours by contraries. Whereafter, it remains unchanged on the whole. The reason that leads to this result might be hydrophobic molecules are packed closely and the corrosion reaction is suppressed at the earlier stage. However, part hydrophobic films will finally be deteriorated under the persistent attack of corrosive species. Consequently, some defects, such as nano- or micro- holes will be engendered, and which will facilitate some corrosion conversely and then bring about faster deterioration of the hydrophobic film. In order to further understand the corrosion protection of the superhydrophobic surface on magnesium alloys, morphologies of both untreated and superhydrophobic magnesium alloys after immersed in 3.5 wt.% of NaCl solution for 20 hours are observed with SEM and the images are shown in Fig.4 as well. Fig.4b shows the untreated magnesium alloy surface exposed in NaCl solution for 20 hours. It is obvious that a great deal of granular and protuberant corrosion products arise at the substrate surface, indicating that the severe corrosion reaction has taken place and the substrate has been eroded seriously. By contrast, Fig.4c shows that only a small quantity of corrosion products present at the superhydrophobic magnesium alloy surface after immersed in 10
NaCl solution for 20 hours, and which manifests that slight corrosion happens for the superhydrophobic magnesium alloy. So the corrosion resistance of magnesium alloys has been improved greatly by endowing magnesium alloys with superhydrophobicity. Thereupon, results above show that the superhydrophobic magnesium alloys can endure much longer period in corrosive media as compared to the untreated ones, and the generated superhydrophobic surface on magnesium alloys can protect magnesium alloys from severe corrosion effectively. Consequently, it can be concluded that the superhydrophobic surface shows long-term corrosion protection for magnesium alloy substrate. It is well known that once the magnesium alloy without treatment is immersed in the corrosive medium (such as NaCl solution), water and corrosive species, such as O2 and Cl-, etc., can easily contact with the substrate directly and react with it [35]. Consequently, corrosion products as Mg(OH)2, MgO, and Mg(OH)Cl, and so on, will be engendered. Hence, the magnesium alloy surface can be eroded severely and massive corrosion products present at the surface, just as what has been displayed schematically in Scheme 2. On the contrary, a great deal of air is entrapped in cavities of rough and hierarchical superhydrophobic magnesium alloy surface, and which can restrain water and corrosive species approach and contact with the substrate effectively when the superhydrophobic magnesium alloy is immersed in the corrosive medium. Consequently, the magnesium alloy substrate is protected from corrosion by the superhydrophobic surface. So the corrosive rate of the superhydrophobic magnesium alloy can be reduced markedly as compare to that of the untreated substrate. However, with the immersion time in the corrosive medium extending, slight corrosion can also happen for the superhydrophobic magnesium alloys. The corrosion protection of the superhydrophobic surface as well as the slight corrosion behavior of the superhydrophobic magnesium alloys can be inferred as follows, also schematically illustrated in Scheme 2: the hydrophobic alkyl molecules are exposed to corrosive solution directly at the superhydrophobic film/liquid interface. In the earlier stage, the hydrophobic molecules pack closely and the corrosive species can be prevented from approaching the surface of magnesium alloys. Consequently, the corrosive reaction can be suppressed. However, under the persistent attack of water and corrosive species, part chemical interfacial bonding might tend to decompose, for instance, hydrolyze. Thus some hydrophobic molecules will dissociate from the substrate. 11
Consequently, defects in the superhydrophobic shielding film, such as nano- or micro- holes, will be engendered. Under the situation above, water molecules and corrosive species will intrude into the superhydrophobic film/vapor interface. Then they may penetrate the superhydrophobic film to reach the magnesium alloy substrate. As a result, the corrosion reaction will be triggered and which will further lead to the dissociation of more hydrophobic chains. So the corrosion behavior will become serious gradually with the immersion time in NaCl solution exceeds 30 hours. Therefore, it can be concluded that the superhydrophobic surface has outstanding protection effect on magnesium alloy substrate from corrosion, especially at the earlier stage in corrosive media. However, the protection effect will reduce gradually under the persistent attack of corrosive species. Once the superhydrophobic film has been broken through by the corrosive species and cracks are engendered in the superhydrophobic surface, the protection effect will reduce and even lose. 3.4 Self-cleaning performance of the superhydrophobic magnesium alloys The self-cleaning performance is an important engineering application index for superhydrophobic materials. In our research, the self-cleaning performance is assessed by observing whether the simulated contaminant particles (herein, carbon powder, chalk dust, and cigarette ash) spread on tilted magnesium alloy surfaces can be carried away by the rolling water droplets easily. The test images are shown in Fig.5. Firstly, cigarette ash is spread and coated onto the untreated magnesium alloy surface, and then the sample is tilted for a inclination angle of 6° above horizontal. As the surface is hydrophilic, the water droplet spreads out immediately and wettens the surface when it is dripped onto the tilted surface gently. So no appreciable removal of cigarette ash is noticed, indicating that the cigarette ash cannot be taken away from the untreated surface, as shown in top left picture in Fig.5. Similarly, carbon powder and chalk dust dispersed on untreated magnesium alloy surfaces cannot be taken away by the water droplets as well for the alike reason. These results indicate that the untreated magnesium alloy surface has not self-cleaning performance. By contrast, once water droplets are applied onto superhydrophobic magnesium alloy surfaces covered with simulated contaminant particles (such as cigarette ash, carbon powder, and chalk dust) under the similar conditions, water droplets roll down surfaces immediately since they cannot wetten the surfaces, as shown in right pictures in Fig.5. Meanwhile, contaminant particles 12
are transferred to the outside surfaces of rolling water droplets and taken off from the superhydrophobic surfaces. Moreover, no contaminant particles are visible on the rolling path of water droplets while the path at superhydrophobic surface remains dry and clean. These manifest that the adhesive force between contaminant particles and the superhydrophobic magnesium alloy surfaces is quite weak. Thus contaminant particles can be taken away from superhydrophobic surfaces by the rolling water droplets without difficulty. Thereupon, it shows that the superhydrophobic magnesium alloys have excellent self-cleaning performance. More importantly, the self-cleaning effect can be repeated for 20 times and above, so the as-prepared superhydrophobic magnesium alloys can maintain their clean surfaces without any pollution for a long time. The self-cleaning mechanism of superhydrophobic magnesium alloys is due to the distinctive surface structure and composition. A large number of air is trapped inside the superhydrophobic surfaces, and which serves as a “air cushion”. Consequently, the superhydrophobic magnesium alloy surface cannot be fouled easily. Even if the surface has been fouled by contaminants, the contaminants can be cleaned by the water droplets (such as rain droplets) or breeze without difficulty as the adhesion between contaminants and superhydrophobic surfaces is quite weak. Thereupon, the samples regain cleanliness in a short time. Therefore, the superhydrophobic magnesium alloys can keep a clean surface over a long period of time, and it is a significant feature for practical applications.
4. Conclusions In summary, a one-step hydrothermal process is developed to prepare the superhydrophobic surface on magnesium alloy. The as-prepared superhydrophobic magnesium alloy surface presents rough and hierarchical structure while long hydrophobic alkyl chains are grafted onto the surface with covalent bonds. Consequently, the superhydrophobic surface with a water contact angle of 153.8° and a low sliding angle of ca. 4° is obtained. The as-prepared superhydrophobic surface can provide protect effect for magnesium alloys. As a result, the magnesium alloys show excellent corrosion resistance and self-cleaning performance due to the superhydrophobic surfaces. We believe that this method provides a novel, simple, and cost-effective process to improve the corrosion resistance and self-fouling performance of magnesium alloys. Moreover, it is profitable for the practical application of magnesium alloy as an engineering material. 13
Acknowledgements This research is supported by National Natural Science Foundation of China (Grant No. 21161012).
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Figure Captions
Fig.1 SEM images of magnesium alloy surfaces before (a) and after (b,c,d) hydrothermal treatment; XRD patterns (e) and wettability of a 10 µL of water droplet on magnesium alloy surface (f) after hydrothermal treatment. The insert in panel f shows a water droplet sliding image with a sliding angle of 4°.
Fig.2 Chemical composition analysis with FT-IR spectra (a,b) and EDS spectra (c,d): (a) stearic acid, (b,d) superhydrophobic magnesium alloy, and (c) untreated substrate.
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Fig.3 Potentiodynamic polarization plots of untreated and superhydrophobic magnesium alloys in 3.5 wt.% of NaCl solution.
Fig.4 (a) Surface wettability variation as a function of immersion time for superhydrophobic magnesium alloy, and SEM images of untreated (b) and superhydrophobic (c) magnesium alloy surfaces after immersion in NaCl solution for 20 hours.
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Fig.5 Self-cleaning effect of contaminant particles coated on untreated and superhydrophobic magnesium alloy surfaces.
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Scheme 1 Schematic illustration for the fabrication of superhydrophobic surface on magnesium alloy with one-step hydrothermal process.
Scheme 2 Schematic diagram of interface model for the mechanism of corrosion resistance.
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Table 1 The weight content (wt.%) data at the untreated and superhydrophobic magnesium alloy surfaces. Sample Untreated Superhydrophobic
Mg
C
O
Al
71.07
20.31
6.63
1.99
9.67
65.96
23.70
0.67
Table 2 Corrosion potential (Ecorr) and current density (Icorr) of untreated and superhydrophobic magnesium alloys in 3.5 wt.% of NaCl solution. Sample
Ecorr (V)
Icorr (A·cm-2 )
Untreated
-1.50
1.48×10-4
Superhydrophobic
-1.34
3.32×10-7
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