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Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method
Journal Pre-proof Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method Jie Zang, Sirong Yu, Guang Zhu, Xue Zhou PII:
S0257-8972(19)31069-2
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125078
Reference:
SCT 125078
To appear in:
Surface & Coatings Technology
Received Date: 4 September 2019 Revised Date:
16 October 2019
Accepted Date: 17 October 2019
Please cite this article as: J. Zang, S. Yu, G. Zhu, X. Zhou, Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125078. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method Jie Zang, Sirong Yu*, Guang Zhu, Xue Zhou College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China Corresponding author: Prof. Sirong Yu College of Material Science and Engineering China University of Petroleum Qingdao 266580 P.R. China Tel.: + 86 532-86983500-8815 Fax.: +86 532-86983670 E-mail address: [email protected] Abstract: The superhydrophobic surface was successfully prepared on aluminum alloy 6061 by anodization and lauric acid modification. The reaction was carried out for 1 h in a 0.3 M H3PO4 electrolyte at a voltage of 60 V to construct a honeycomb pore-needle microstructure with a water contact angle (WCA) up to around 160° and a sliding angle (SA) less than 2°. The chemical composition of superhydrophobic surface was characterized by energy disperse spectroscopy (EDS) and Fourier transform infrared spectrum (FTIR). And the result indicated that lauric acid was successfully combined on the surface of the oxide layer. Besides, the superhydrophobic surface exhibited good anti-icing property, and the reason for its delayed icing time was given from the Cassie equation and the classic nucleation theory. Moreover, the superhydrophobic surface exhibited low adhesion, excellent self-cleaning function, good
thermal stability, mechanical stability and corrosion resistance. Keywords: Aluminum alloy, anodic oxidation, superhydrophobicity, anti-icing, self-cleaning, thermal stability. 1. Introduction Aluminum resources are abundant in nature. Aluminum alloy is low density, high strength, high plasticity, good electrical and thermal conductivity, and is widely used in aviation, aerospace, automobile, machinery manufacturing, shipbuilding and chemical industries [1, 2]. However, when exposed to harsh environments, it is susceptible to contamination and corrosion. Especially in cold environment, water droplets will freeze on the surface. The occurrence of these conditions has greatly affected the performance and service life of aluminum alloy and even can cause huge economic losses and disaster accidents [3, 4]. Therefore, it is of great practical significance to study how to improve the protective performance of aluminum alloy. Since the 1970s, Barthlott and Neinhuis [5, 6] discovered that lotus leaves have a layer of fluff and some tiny waxes on the surface. This special surface structure enables lotus to clean itself, which is called "lotus effect". Thus, it has aroused widespread concern among scientists. Scientists have also discovered more and more natural superhydrophobic surfaces in nature, such as pitcher plants [7], water striders [8], cicada wing, gecko feet [9] etc. With the continuous development of superhydrophobic theory, it is found that the application of superhydrophobic surface technology to metals and their alloys will improve their performance, and it has broad application in self-cleaning [3, 5, 10], corrosion resistance [2, 11, 12] and anti-icing [13-15]. Two key points of preparing superhydrophobic surfaces are micro-nanoscale structures
and low surface energy [16]. Until now, there are many methods to prepare superhydrophobic surface, such as hydrothermal reaction [17], sol-gel processing [18], template method [19], electrospinning method [20], electrodeposition method [21] etc. Due to the disadvantages of these methods, such as high manufacturing cost, small processing scale, complex process or unstable prepared film layer, the application of the above methods is limited. By contrast, anodic oxidation is a simpler, cheaper and more effective surface treatment technology [22]. In this paper, the honeycomb pore-needle structure was constructed on the surface of aluminum alloy 6061 by anodizing. Then the sample was modified with lauric acid. Thus, the superhydrophobic surface was obtained through the synergy of alumina microstructure and low surface energy. The influences of experimental parameters (anodic oxidation voltage, anodic oxidation time and electrolyte concentration) on surface microstructure and wettability of samples were analyzed. In addition, anti-icing performance, low adhesion, self-cleaning, thermal stability, mechanical stability and corrosion resistance of superhydrophobic surface were studied. 2. Experimental 2.1. Materials Aluminum alloy 6061 was obtained from Shenzhen Ou Difu Material Co., Ltd., and cut into the size of 40 × 20 × 3 mm. Phosphoric acid (H3PO4, ≥ 85%) was purchased from West Long Chemical Co., Ltd. Anhydrous ethanol and sodium hydroxide (NaOH) were purchased from Shanghai Titan chemical Co., Ltd. Acetone and lauric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the chemical reagents were used as received without further purification. 2.2. Fabrication of superhydrophobic surface
Aluminum oxide layer was prepared on the surface of aluminum alloy 6061 by anodic oxidation. First, the samples were polished with sandpaper from 240# to 1500#, and ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 5 min respectively to remove contaminants. Then, the sample was etched in 0.5 M NaOH solution for 2 min to remove the oxide layer and degrease and then rinsed with deionized water. Subsequently, a sample (anode) and a graphite plate (cathode, 40 × 20 × 3 mm) were immersed in solution containing H3PO4, with a certain voltage and oxidation time. After anodizing, the obtained sample was immediately rinsed with deionized water, blow-dried and soaked in a 0.2 M ethanol solution of lauric acid for 1.5 h, subsequently dried at 100 °C for 30 min. 2.3. Characterizations and tests Water contact angle (WCA) was measured with a water droplet (3 µL) by a contact-angle measuring instrument (SL200B, USA, KINO) in air at room temperature. The average value of WCA was obtained with at least five measurements of each sample. The sliding angle (SA) was the tilted angle of the measured surface once the water droplet was rolling on the sample surface. The surface morphology was observed by a field emission scanning electron microscope (SEM, JEOL JSM-7200F) equipped with energy disperse spectroscopy (EDS, Oxford AZtec). Fourier transform infrared spectrum (FTIR) was recorded on a Fourier Transform Infrared Spectrometer in the range from 4000 to 500 cm-1. In order to verify the anti-icing performance of the prepared superhydrophobic surface, the freezing delay time test was carried out on the substrate and superhydrophobic sample. The substrate and the superhydrophobic sample were placed in a low temperature chamber at -20 °C, in order to lower the temperature of the samples to ambient temperature. Then, 10 µL
of deionized water droplet was added to the two samples. Record the time it took for the droplet to freeze. In order to evaluate the adhesion of the superhydrophobic surface, it was achieved by moving superhydrophobic sample approaching, contacting, and leaving 3 µL of water droplet, recording the process that water droplet approached, contacted, deformed, and separated from the surface. In addition, we designed an experiment to test the self-cleaning properties of untreated substrate and superhydrophobic sample. The solid pollution was simulated by covering the surface with a layer of fly-ash cenosphere. The untreated and superhydrophobic samples were tilted by 2°, and then a drop of about 0.1 mL of water droplet was dropped on the surface. Record the behavior of water droplets and their effects on impurities. Moreover, superhydrophobic samples were placed at different temperatures (30 °C, 60 °C, 90 °C, 120 °C, 150 °C, 180 °C, and 200 °C) for 1 h respectively to assess the thermal stability. After being cooled to room temperature, the WCAs were measured. Given the significant status of mechanical durability, we evaluated the anti-wear performance of the superhydrophobic surface. The specimen was loaded by a weight of 100 g and rubbed by 2000# SiC sandpaper at a constant speed under manual traction. The corrosion resistance of the samples was tested by potentiodynamic polarization testing in 3.5 wt.% NaCl solution operated at room temperature using a CorrTest CS310 electrochemical workstation equipped with a three-electrode cell system. The sample with the size of 1 cm2 was used as the working electrode, and platinum electrode was used as the counter electrode, and a saturated calomel electrode acted as the reference electrode. Polarization curves were acquired at a scanning rate of 1 mV/s from -1.1 to -0.4 V. 3. Results and discussion
3.1. Optimization of experimental parameters In order to obtain the optimal superhydrophobic surface, three experimental parameters were investigated: anodic oxidation voltage, anodic oxidation time and electrolyte concentration. Fig. 1 summarizes the results of these studies. Fig. 1a shows the influence of anodic oxidation voltage on the WCAs. The samples were anodized at the voltage of 25 - 70 V for 1 h in 0.3 M H3PO4 electrolytic solution, followed by surface modification with 0.2 M ethanol solution of lauric acid for 1.5 h, and subsequently dried at 100 °C for 30 min. As shown in Fig. 1a, when the anodic oxidation voltage was in the range of 25 - 65 V, the WCAs were higher than 150°. Especially, when the voltage was in the range of 55 - 65 V, the WCA can reach 155° or more. The WCA was peaked at 60 V with a value of 160.55°, and its SA was under 2°. Therefore, we used anodic oxidation voltage of 60 V in subsequent experiments. Fig. 1b shows the effect of anodic oxidation time on the WCAs. The samples were anodized for 0 - 80 minutes in 0.3 M H3PO4 electrolytic solution at 60 V, followed by the same surface modification and heat treatment. We found that the WCAs increased with the increase of anodic oxidation time, peaking at 1 h, and then WCAs decreased as time increased. Therefore, the optimum anodic oxidation time for fabricating superhydrophobic surfaces was 1 h. The effect of H3PO4 concentration on WCAs is shown in Fig. 1c. The samples were anodized at 60 V for 1 h in a H3PO4 electrolyte concentration range of 0 - 0.4 M, followed by surface modification and heat treatment similarly. It can be seen that the WCAs increased with the increase of electrolyte concentration, peaking at 0.3 M. 3.2. Surface morphology analysis
Different surface morphologies were obtained by changing the experimental parameters. Fig. 2 shows the superhydrophobic surface morphology under different experimental voltage (50 V, 55 V, 60 V, and 65 V). It was observed that the anodized film had a honeycomb-like structure. Moreover, as the voltage increased, the pore size increased, and the pore walls were dissolved and broken. As a result, a needle-like structure appeared. SEM images of different anodic oxidation time are shown in Fig. 3. At 20 min (Fig. 3a), the morphology of the surface was like a network structure with the WCA of 139.33°. When the experiment was carried out for 40 min, it was obvious that the pore depth and pore size increased as shown in Fig. 3b, and its WCA was 147.76°. Continue to increase the oxidation time, the pore wall was dissolved and broken, and the pore wall collapsed to form a pore-needle composite structure (Fig. 3c, d). Because such a structure could trap more air, their WCAs were above 150°, 160.55° and 157.37°, respectively. 3.3. Chemical composition of the superhydrophobic surface Fig. 4 shows the surface chemical composition of the aluminum alloy sample under different conditions. The chemical composition of the substrate was shown in Fig. 4a, showing that the surface was consisted of the elements of Al, Mg, Si, and C. Strictly speaking, large amount of Al element attributes to the aluminum alloy substrate. Moreover, Mg and Si are the main added alloying elements of the six-series aluminum alloy [23]. In addition, the existence of C element may be caused by impurities in the experimental environment. Compared to the substrate (Fig. 4a), the elements of O and P appeared on the superhydrophobic surface (Fig. 4b). The existence of O element was due to the formation of alumina in the sample after anodic oxidation. P element came from the electrolyte and might be adsorbed in the pores of the oxidation film. It was difficult to distinguish whether C was
derived from environmental impurities or lauric acid. Therefore, the FTIR spectra were combined to determine whether the surface was combined with lauric acid. FTIR spectra of superhydrophobic surface (a) and lauric acid (b) were shown in Fig. 5. For lauric acid, it could be seen from Fig. 5b that the peaks appeared at 2918 and 2850 cm-1 which corresponded to the asymmetric and the symmetric stretching vibrations of C-H in the -CH2 groups [24], respectively. Moreover, the same absorption peaks appeared in the similar position (2922 and 2852 cm-1) of the superhydrophobic surface. The peak at 1703 cm-1 (b) was attributed to the -COO- groups. However, on the superhydrophobic surface (a), the peak of -COO- shifted, and the new absorption peaks appeared at 1573 and 1470 cm-1. The asymmetric and symmetric stretching vibration peaks of -COO- at 1605 and 1474 cm-1 were also found in other experiment [24]. This indicated that lauric acid was not only retained on the surface by physical adsorption, but was partially chemically bound [25]. Therefore, long chain branches of hydrocarbon base were attached to the surface, endowing the surface of aluminum alloy with superhydrophobicity. 3.4. Anti-icing property Excellent anti-icing performance can be evaluated from two aspects, one is to delay the freezing time, and the other is to reduce the binding force of ice particles with samples [26]. Under the overcooled condition, the freezing delay time test could explain the anti-icing property well [27-29]. At the beginning, the water droplet appeared spherical on the superhydrophobic sample in Fig. 6a. The water droplet on the substrate was spread out, so there was a large contact area between the solid state and water droplet. Ice formation of water droplet was observed at regular intervals. After repeated experiments, it was determined that the water droplet on the
substrate was iced at about 111 s. However, the icing time on the superhydrophobic surface could all exceed 15 min (900 s), as shown in Fig. 6. The experimental results showed that the superhydrophobic surface could greatly prolong the icing time compared to the untreated substrate. Meanwhile, the ice particle crystallized on the superhydrophobic surface was more likely to peel off than the ice particles on the substrate. The extension of the icing time and the easy peeling of the ice particles made the superhydrophobic surface have the anti-icing function. According to the Cassie equation [30], ܿߠݏ = ݂ଵ ܿߠ ݏଵ + ݂ଶ ܿߠ ݏଶ = ݂ଵ ܿߠ ݏଵ + ݂ଵ − 1
(a)
where θ1 was the WCA of the substrate surface after modification (90.37°), f1 and f2 were the fraction of the solid area and the air area to the total area, respectively (f2 = 1 - f1). And θc was the WCA of the superhydrophobic surface (160.55°). Therefore, f1 = 0.058 and f2 = 0.942, there was a large amount of air trapped in the microstructures. Thus, the superhydrophobic surface had a relatively smaller contact area, and the trapped air in the microstructures had a good barrier effect. The above two points allowed the same volume of water droplets to have a lower heat conductivity on the superhydrophobic surface than the surface of the substrate. Such experimental data results can be explained by thermodynamics. Because of the roughness of the hydrophobic surface, water droplets crystallize into heterogeneous nuclei on the surface. The formation of heterogeneous nuclei also requires overcoming the energy barrier [29]. According to the classic nucleation theory [31, 32], we can calculate the free energy barrier of nucleation by the following formulas (b, c). ∆( = ܩ
ଶగோ య ଷ
݂(ߠ) =
∆݃ + 2ߨܴ ଶ ݎ௦ )݂(ߠ) (ଶା௦ ఏ)(ଵି௦ ఏ)మ ସ
(b) (c)
where R is the radius of a water droplet. γsl is the solid-liquid interfacial tension. ∆g is the
Gibb's energy density difference between the water droplet and ice, and θ is the WCA. The above formulas show that the free energy barrier of the nucleation (∆G) and f(θ) are in direct proportion. And f(θ) increases with the increase of θ. Obviously, the superhydrophobic surface shows bigger ∆G, suggesting that it exhibits better anti-icing performance. 3.5. Low adhesion and self-cleaning properties As can be seen from the Fig. 7, the water droplet was deformed as the height of the sample increased, but a small WCA and contact area were still maintained. As the height of the sample decreased, water droplet was separated from the sample, and there was no droplet residue on the surface of the sample. The result displayed that the superhydrophobic surface had a very feeble adhesive force to the water droplet, which exhibits lower adhesion behavior and showed an excellent superhydrophobicity. Self-cleaning effect is an important property of superhydrophobic surfaces for various applications. Therefore, contamination test was carried out on the surface of superhydrophobic sample and untreated substrate. About 0.1 mL of water droplets were dropped on the surface of the impurity-filled substrate and the superhydrophobic sample, respectively. As shown in Fig. 8a, the water droplets were spread on the substrate; on the superhydrophobic surface (Fig. 8b), the water droplets rolled off and carried away the fly ash cenospheres, leaving a clean track on the surface. Therefore, the result distinctly demonstrates that the superhydrophobic surface exhibits excellent self-cleaning property. This phenomenon is consistent with the “Lotus Effect” and provides a good application prospect for outside coating. 3.6. Thermal stability analysis In order to verify whether the superhydrophobic surface prepared in this paper can
maintain excellent superhydrophobic properties under high temperature environment, the thermal stability of the sample was tested. The samples were placed in vacuum drying boxes at 30, 60, 90, 120, 150, 180 and 200 °C for 1 h respectively, and then the WCAs were measured. As shown in the Fig. 9a, when the sample was kept at 30 - 120 °C, the WCAs of the superhydrophobic surface did not change substantially. The WCAs decreased rapidly when the temperature was higher than 120 °C. At 180 °C, the WCAs were less than 150° so that the sample lost superhydrophobicity. It could be learnt from the result that the surface gradually lost superhydrophobicity with increasing temperature. The high temperature might cause volatilization or decomposition of the organic modification on the superhydrophobic surface. Moreover, when the samples that had lost their superhydrophobicity were re-modified, they restored superhydrophobicity. The morphology of sample after 200 °C heat treatment was shown in the illustration of Fig. 9a, and it showed that the structure had not changed. Fig. 9b showed the Thermogravimetry (TG) and the Derivative Thermogravimetry (DTG) curves of lauric acid. The curves in Fig. 9b indicated that lauric acid decomposed and the weight lost with the increase of temperature. It could be learnt from Fig. 9b that the weight lost about 2.245% and the derivative of mass loss was 0.072 mg/°C at 120 °C. But when the temperature was 180 °C, the weight lost 15.882% and the derivative of mass loss reached up to 0.322 mg/°C. Therefore, the higher the temperature, the faster lauric acid decomposition, which eventually led to a reduction in superhydrophobicity. 3.7. Mechanical stability The evolution of WCAs was recorded and presented in Fig. 10a. It can be clearly seen that the WCAs of superhydrophobic surface decreased with the increase of the wear distance.
Fig. 10a showed that superhydrophobic surface still maintained superhydrophobicity when the wear distance was less than 300 cm, indicating an excellent mechanical property. The good mechanical durability of the fabricated superhydrophobic 6061 surface could be attributed to the good hardness of the anodic oxide film. Moreover, the morphology of superhydrophobic surface after abrasion test was observed by SEM. The morphology of superhydrophobic surface after 300 cm abrasion test was shown in Fig 10b. The original honeycomb pore-needle structure (Fig. 2c) was destroyed so that the hierarchical structure was no longer apparent. Such a structure cannot trap as much air as before, resulting in a lower WCA. The result further demonstrates that microstructure is one of the conditions for constructing superhydrophobicity. 3.8. Corrosion resistance and mechanism The corrosion behaviors of the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface were investigated via polarization curves in 3.5 wt.% NaCl solution. The corrosion tendency of the sample can be judged by measuring the corrosion potential. And higher corrosion potential indicates better corrosion resistance [33, 34]. The corrosion potential increased from -0.735 V for the untreated aluminum alloy (Fig. 11b) to -0.709 V for the superhydrophobic aluminum alloy (Fig. 11a). Moreover, the corrosion current density of the untreated substrate, 2.235×10−6 A/cm2, was much higher than that of the superhydrophobic sample, 9.333×10−9 A/cm2. It is worth noting that the corrosion current density decreased significantly after the construction of superhydrophobic surface on aluminum alloy. To make easier to understand, we show a simple anti-corrosion diagram in Fig. 12. When a fabricated superhydrophobic 6061 substrate was immersed in corrosive environment, the
water-repellent feature of superhydrophobic surface contributes to the Cassie contact between solid surface and corrosive media. Air cushion trapped in the honeycomb pore-needle structure isolates the contact between the substrate and the corrosion irons, such as Cl− etc. Such an air cushion can inhibit the electron transfer between the aluminum alloy substrate and the corrosive media, which effectively protects the underlying aluminum alloy substrate. Therefore, it means that the superhydrophobic surface is very effective in protecting the aluminum alloy from corrosion. Besides, the stability of the air cushion trapped in the surface morphology plays a key role in determining its corrosion resistance. 4. Conclusion In conclusion, a superhydrophobic surface with honeycomb pore-needle structure was successfully prepared by two-step method of adopting an environmentally friendly anodizing and low energy modification. The superhydrophobic aluminum alloy 6061 exhibits extremely low surface adhesion force and excellent self-cleaning capacity for contaminants. It is worth mentioning that the area fraction of the gas-liquid interface of the superhydrophobic surface calculated by the Cassie equation is as high as 94.2%. Therefore, the superhydrophobic surface exhibits low thermal conductivity and high nucleation free energy barrier, which makes the icing time prolonged. As a result, it shows good anti-icing performance. Meanwhile, the surface of the superhydrophobic aluminum alloy exhibits good thermal stability, mechanical stability and corrosion resistance, which provides a guarantee for its application in harsh environments. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province of China (No. ZR2019MEM020).
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Figure captions: Fig. 1. Effects of (a) anodic oxidation voltage, (b) anodic oxidation time, and (c) electrolyte concentration on WCAs. Fig. 2. Surface morphologies of superhydrophobic surface under different anodizing voltage: (a) 50 V, (b) 55 V, (c) 60 V, and (d) 65 V. Fig. 3. Surface morphologies of superhydrophobic surface under different anodizing time: (a) 20 min, (b) 40 min, (c) 60 min, and (d) 80 min. Fig. 4. EDS spectra of (a) 6061 aluminum alloy substrate, (b) superhydrophobic sample. Fig. 5. FTIR spectra of (a) superhydrophobic sample and (b) lauric acid. Fig. 6. The freezing process of water droplets on substrate and superhydrophobic sample surface under different time: (a) 0 s, (b) 111 s, and (c) 900 s. Fig. 7. Images of a water droplet (3 µL) suspending on the needle, contacting, pressing and departing from the superhydrophobic surface. The arrows represent the moving direction of the sample. Fig. 8. Self-cleaning process on the surface of substrate (a) and superhydrophobic surface (b). Fig. 9. (a) The change of WCAs on the superhydrophobic surface with different heating temperature. The illustration was the morphology of sample after 200 °C heat treatment. (b) Thermogravimetry (TG) and Derivative Thermogravimetry (DTG) curves of lauric acid. Fig. 10. (a) The evolution of WCAs with wear distance. (b) The morphology of superhydrophobic surface after 300 cm abrasion test. Fig. 11. Potentiodynamic polarization curves of (a) the superhydrophobic aluminum alloy and (b) the untreated aluminum alloy measured in 3.5 wt.% NaCl aqueous solution. Fig. 12. Schematic illustration of corrosion and protection.
Highlights A facile and effective anodic oxidation method is used to prepare the superhydrophobic surface. The superhydrophobic surface shows good anti-icing due to delay in icing time. The superhydrophobic surface exhibits low adhesion, good self-cleaning performance, excellent thermal stability, mechanical stability and corrosion resistance.
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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DOI:
https://doi.org/10.1016/j.surfcoat.2019.125078
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SCT 125078
To appear in:
Surface & Coatings Technology
Received Date: 4 September 2019 Revised Date:
16 October 2019
Accepted Date: 17 October 2019
Please cite this article as: J. Zang, S. Yu, G. Zhu, X. Zhou, Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125078. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Fabrication of superhydrophobic surface on aluminum alloy 6061 by a facile and effective anodic oxidation method Jie Zang, Sirong Yu*, Guang Zhu, Xue Zhou College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China Corresponding author: Prof. Sirong Yu College of Material Science and Engineering China University of Petroleum Qingdao 266580 P.R. China Tel.: + 86 532-86983500-8815 Fax.: +86 532-86983670 E-mail address: [email protected] Abstract: The superhydrophobic surface was successfully prepared on aluminum alloy 6061 by anodization and lauric acid modification. The reaction was carried out for 1 h in a 0.3 M H3PO4 electrolyte at a voltage of 60 V to construct a honeycomb pore-needle microstructure with a water contact angle (WCA) up to around 160° and a sliding angle (SA) less than 2°. The chemical composition of superhydrophobic surface was characterized by energy disperse spectroscopy (EDS) and Fourier transform infrared spectrum (FTIR). And the result indicated that lauric acid was successfully combined on the surface of the oxide layer. Besides, the superhydrophobic surface exhibited good anti-icing property, and the reason for its delayed icing time was given from the Cassie equation and the classic nucleation theory. Moreover, the superhydrophobic surface exhibited low adhesion, excellent self-cleaning function, good
thermal stability, mechanical stability and corrosion resistance. Keywords: Aluminum alloy, anodic oxidation, superhydrophobicity, anti-icing, self-cleaning, thermal stability. 1. Introduction Aluminum resources are abundant in nature. Aluminum alloy is low density, high strength, high plasticity, good electrical and thermal conductivity, and is widely used in aviation, aerospace, automobile, machinery manufacturing, shipbuilding and chemical industries [1, 2]. However, when exposed to harsh environments, it is susceptible to contamination and corrosion. Especially in cold environment, water droplets will freeze on the surface. The occurrence of these conditions has greatly affected the performance and service life of aluminum alloy and even can cause huge economic losses and disaster accidents [3, 4]. Therefore, it is of great practical significance to study how to improve the protective performance of aluminum alloy. Since the 1970s, Barthlott and Neinhuis [5, 6] discovered that lotus leaves have a layer of fluff and some tiny waxes on the surface. This special surface structure enables lotus to clean itself, which is called "lotus effect". Thus, it has aroused widespread concern among scientists. Scientists have also discovered more and more natural superhydrophobic surfaces in nature, such as pitcher plants [7], water striders [8], cicada wing, gecko feet [9] etc. With the continuous development of superhydrophobic theory, it is found that the application of superhydrophobic surface technology to metals and their alloys will improve their performance, and it has broad application in self-cleaning [3, 5, 10], corrosion resistance [2, 11, 12] and anti-icing [13-15]. Two key points of preparing superhydrophobic surfaces are micro-nanoscale structures
and low surface energy [16]. Until now, there are many methods to prepare superhydrophobic surface, such as hydrothermal reaction [17], sol-gel processing [18], template method [19], electrospinning method [20], electrodeposition method [21] etc. Due to the disadvantages of these methods, such as high manufacturing cost, small processing scale, complex process or unstable prepared film layer, the application of the above methods is limited. By contrast, anodic oxidation is a simpler, cheaper and more effective surface treatment technology [22]. In this paper, the honeycomb pore-needle structure was constructed on the surface of aluminum alloy 6061 by anodizing. Then the sample was modified with lauric acid. Thus, the superhydrophobic surface was obtained through the synergy of alumina microstructure and low surface energy. The influences of experimental parameters (anodic oxidation voltage, anodic oxidation time and electrolyte concentration) on surface microstructure and wettability of samples were analyzed. In addition, anti-icing performance, low adhesion, self-cleaning, thermal stability, mechanical stability and corrosion resistance of superhydrophobic surface were studied. 2. Experimental 2.1. Materials Aluminum alloy 6061 was obtained from Shenzhen Ou Difu Material Co., Ltd., and cut into the size of 40 × 20 × 3 mm. Phosphoric acid (H3PO4, ≥ 85%) was purchased from West Long Chemical Co., Ltd. Anhydrous ethanol and sodium hydroxide (NaOH) were purchased from Shanghai Titan chemical Co., Ltd. Acetone and lauric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the chemical reagents were used as received without further purification. 2.2. Fabrication of superhydrophobic surface
Aluminum oxide layer was prepared on the surface of aluminum alloy 6061 by anodic oxidation. First, the samples were polished with sandpaper from 240# to 1500#, and ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 5 min respectively to remove contaminants. Then, the sample was etched in 0.5 M NaOH solution for 2 min to remove the oxide layer and degrease and then rinsed with deionized water. Subsequently, a sample (anode) and a graphite plate (cathode, 40 × 20 × 3 mm) were immersed in solution containing H3PO4, with a certain voltage and oxidation time. After anodizing, the obtained sample was immediately rinsed with deionized water, blow-dried and soaked in a 0.2 M ethanol solution of lauric acid for 1.5 h, subsequently dried at 100 °C for 30 min. 2.3. Characterizations and tests Water contact angle (WCA) was measured with a water droplet (3 µL) by a contact-angle measuring instrument (SL200B, USA, KINO) in air at room temperature. The average value of WCA was obtained with at least five measurements of each sample. The sliding angle (SA) was the tilted angle of the measured surface once the water droplet was rolling on the sample surface. The surface morphology was observed by a field emission scanning electron microscope (SEM, JEOL JSM-7200F) equipped with energy disperse spectroscopy (EDS, Oxford AZtec). Fourier transform infrared spectrum (FTIR) was recorded on a Fourier Transform Infrared Spectrometer in the range from 4000 to 500 cm-1. In order to verify the anti-icing performance of the prepared superhydrophobic surface, the freezing delay time test was carried out on the substrate and superhydrophobic sample. The substrate and the superhydrophobic sample were placed in a low temperature chamber at -20 °C, in order to lower the temperature of the samples to ambient temperature. Then, 10 µL
of deionized water droplet was added to the two samples. Record the time it took for the droplet to freeze. In order to evaluate the adhesion of the superhydrophobic surface, it was achieved by moving superhydrophobic sample approaching, contacting, and leaving 3 µL of water droplet, recording the process that water droplet approached, contacted, deformed, and separated from the surface. In addition, we designed an experiment to test the self-cleaning properties of untreated substrate and superhydrophobic sample. The solid pollution was simulated by covering the surface with a layer of fly-ash cenosphere. The untreated and superhydrophobic samples were tilted by 2°, and then a drop of about 0.1 mL of water droplet was dropped on the surface. Record the behavior of water droplets and their effects on impurities. Moreover, superhydrophobic samples were placed at different temperatures (30 °C, 60 °C, 90 °C, 120 °C, 150 °C, 180 °C, and 200 °C) for 1 h respectively to assess the thermal stability. After being cooled to room temperature, the WCAs were measured. Given the significant status of mechanical durability, we evaluated the anti-wear performance of the superhydrophobic surface. The specimen was loaded by a weight of 100 g and rubbed by 2000# SiC sandpaper at a constant speed under manual traction. The corrosion resistance of the samples was tested by potentiodynamic polarization testing in 3.5 wt.% NaCl solution operated at room temperature using a CorrTest CS310 electrochemical workstation equipped with a three-electrode cell system. The sample with the size of 1 cm2 was used as the working electrode, and platinum electrode was used as the counter electrode, and a saturated calomel electrode acted as the reference electrode. Polarization curves were acquired at a scanning rate of 1 mV/s from -1.1 to -0.4 V. 3. Results and discussion
3.1. Optimization of experimental parameters In order to obtain the optimal superhydrophobic surface, three experimental parameters were investigated: anodic oxidation voltage, anodic oxidation time and electrolyte concentration. Fig. 1 summarizes the results of these studies. Fig. 1a shows the influence of anodic oxidation voltage on the WCAs. The samples were anodized at the voltage of 25 - 70 V for 1 h in 0.3 M H3PO4 electrolytic solution, followed by surface modification with 0.2 M ethanol solution of lauric acid for 1.5 h, and subsequently dried at 100 °C for 30 min. As shown in Fig. 1a, when the anodic oxidation voltage was in the range of 25 - 65 V, the WCAs were higher than 150°. Especially, when the voltage was in the range of 55 - 65 V, the WCA can reach 155° or more. The WCA was peaked at 60 V with a value of 160.55°, and its SA was under 2°. Therefore, we used anodic oxidation voltage of 60 V in subsequent experiments. Fig. 1b shows the effect of anodic oxidation time on the WCAs. The samples were anodized for 0 - 80 minutes in 0.3 M H3PO4 electrolytic solution at 60 V, followed by the same surface modification and heat treatment. We found that the WCAs increased with the increase of anodic oxidation time, peaking at 1 h, and then WCAs decreased as time increased. Therefore, the optimum anodic oxidation time for fabricating superhydrophobic surfaces was 1 h. The effect of H3PO4 concentration on WCAs is shown in Fig. 1c. The samples were anodized at 60 V for 1 h in a H3PO4 electrolyte concentration range of 0 - 0.4 M, followed by surface modification and heat treatment similarly. It can be seen that the WCAs increased with the increase of electrolyte concentration, peaking at 0.3 M. 3.2. Surface morphology analysis
Different surface morphologies were obtained by changing the experimental parameters. Fig. 2 shows the superhydrophobic surface morphology under different experimental voltage (50 V, 55 V, 60 V, and 65 V). It was observed that the anodized film had a honeycomb-like structure. Moreover, as the voltage increased, the pore size increased, and the pore walls were dissolved and broken. As a result, a needle-like structure appeared. SEM images of different anodic oxidation time are shown in Fig. 3. At 20 min (Fig. 3a), the morphology of the surface was like a network structure with the WCA of 139.33°. When the experiment was carried out for 40 min, it was obvious that the pore depth and pore size increased as shown in Fig. 3b, and its WCA was 147.76°. Continue to increase the oxidation time, the pore wall was dissolved and broken, and the pore wall collapsed to form a pore-needle composite structure (Fig. 3c, d). Because such a structure could trap more air, their WCAs were above 150°, 160.55° and 157.37°, respectively. 3.3. Chemical composition of the superhydrophobic surface Fig. 4 shows the surface chemical composition of the aluminum alloy sample under different conditions. The chemical composition of the substrate was shown in Fig. 4a, showing that the surface was consisted of the elements of Al, Mg, Si, and C. Strictly speaking, large amount of Al element attributes to the aluminum alloy substrate. Moreover, Mg and Si are the main added alloying elements of the six-series aluminum alloy [23]. In addition, the existence of C element may be caused by impurities in the experimental environment. Compared to the substrate (Fig. 4a), the elements of O and P appeared on the superhydrophobic surface (Fig. 4b). The existence of O element was due to the formation of alumina in the sample after anodic oxidation. P element came from the electrolyte and might be adsorbed in the pores of the oxidation film. It was difficult to distinguish whether C was
derived from environmental impurities or lauric acid. Therefore, the FTIR spectra were combined to determine whether the surface was combined with lauric acid. FTIR spectra of superhydrophobic surface (a) and lauric acid (b) were shown in Fig. 5. For lauric acid, it could be seen from Fig. 5b that the peaks appeared at 2918 and 2850 cm-1 which corresponded to the asymmetric and the symmetric stretching vibrations of C-H in the -CH2 groups [24], respectively. Moreover, the same absorption peaks appeared in the similar position (2922 and 2852 cm-1) of the superhydrophobic surface. The peak at 1703 cm-1 (b) was attributed to the -COO- groups. However, on the superhydrophobic surface (a), the peak of -COO- shifted, and the new absorption peaks appeared at 1573 and 1470 cm-1. The asymmetric and symmetric stretching vibration peaks of -COO- at 1605 and 1474 cm-1 were also found in other experiment [24]. This indicated that lauric acid was not only retained on the surface by physical adsorption, but was partially chemically bound [25]. Therefore, long chain branches of hydrocarbon base were attached to the surface, endowing the surface of aluminum alloy with superhydrophobicity. 3.4. Anti-icing property Excellent anti-icing performance can be evaluated from two aspects, one is to delay the freezing time, and the other is to reduce the binding force of ice particles with samples [26]. Under the overcooled condition, the freezing delay time test could explain the anti-icing property well [27-29]. At the beginning, the water droplet appeared spherical on the superhydrophobic sample in Fig. 6a. The water droplet on the substrate was spread out, so there was a large contact area between the solid state and water droplet. Ice formation of water droplet was observed at regular intervals. After repeated experiments, it was determined that the water droplet on the
substrate was iced at about 111 s. However, the icing time on the superhydrophobic surface could all exceed 15 min (900 s), as shown in Fig. 6. The experimental results showed that the superhydrophobic surface could greatly prolong the icing time compared to the untreated substrate. Meanwhile, the ice particle crystallized on the superhydrophobic surface was more likely to peel off than the ice particles on the substrate. The extension of the icing time and the easy peeling of the ice particles made the superhydrophobic surface have the anti-icing function. According to the Cassie equation [30], ܿߠݏ = ݂ଵ ܿߠ ݏଵ + ݂ଶ ܿߠ ݏଶ = ݂ଵ ܿߠ ݏଵ + ݂ଵ − 1
(a)
where θ1 was the WCA of the substrate surface after modification (90.37°), f1 and f2 were the fraction of the solid area and the air area to the total area, respectively (f2 = 1 - f1). And θc was the WCA of the superhydrophobic surface (160.55°). Therefore, f1 = 0.058 and f2 = 0.942, there was a large amount of air trapped in the microstructures. Thus, the superhydrophobic surface had a relatively smaller contact area, and the trapped air in the microstructures had a good barrier effect. The above two points allowed the same volume of water droplets to have a lower heat conductivity on the superhydrophobic surface than the surface of the substrate. Such experimental data results can be explained by thermodynamics. Because of the roughness of the hydrophobic surface, water droplets crystallize into heterogeneous nuclei on the surface. The formation of heterogeneous nuclei also requires overcoming the energy barrier [29]. According to the classic nucleation theory [31, 32], we can calculate the free energy barrier of nucleation by the following formulas (b, c). ∆( = ܩ
ଶగோ య ଷ
݂(ߠ) =
∆݃ + 2ߨܴ ଶ ݎ௦ )݂(ߠ) (ଶା௦ ఏ)(ଵି௦ ఏ)మ ସ
(b) (c)
where R is the radius of a water droplet. γsl is the solid-liquid interfacial tension. ∆g is the
Gibb's energy density difference between the water droplet and ice, and θ is the WCA. The above formulas show that the free energy barrier of the nucleation (∆G) and f(θ) are in direct proportion. And f(θ) increases with the increase of θ. Obviously, the superhydrophobic surface shows bigger ∆G, suggesting that it exhibits better anti-icing performance. 3.5. Low adhesion and self-cleaning properties As can be seen from the Fig. 7, the water droplet was deformed as the height of the sample increased, but a small WCA and contact area were still maintained. As the height of the sample decreased, water droplet was separated from the sample, and there was no droplet residue on the surface of the sample. The result displayed that the superhydrophobic surface had a very feeble adhesive force to the water droplet, which exhibits lower adhesion behavior and showed an excellent superhydrophobicity. Self-cleaning effect is an important property of superhydrophobic surfaces for various applications. Therefore, contamination test was carried out on the surface of superhydrophobic sample and untreated substrate. About 0.1 mL of water droplets were dropped on the surface of the impurity-filled substrate and the superhydrophobic sample, respectively. As shown in Fig. 8a, the water droplets were spread on the substrate; on the superhydrophobic surface (Fig. 8b), the water droplets rolled off and carried away the fly ash cenospheres, leaving a clean track on the surface. Therefore, the result distinctly demonstrates that the superhydrophobic surface exhibits excellent self-cleaning property. This phenomenon is consistent with the “Lotus Effect” and provides a good application prospect for outside coating. 3.6. Thermal stability analysis In order to verify whether the superhydrophobic surface prepared in this paper can
maintain excellent superhydrophobic properties under high temperature environment, the thermal stability of the sample was tested. The samples were placed in vacuum drying boxes at 30, 60, 90, 120, 150, 180 and 200 °C for 1 h respectively, and then the WCAs were measured. As shown in the Fig. 9a, when the sample was kept at 30 - 120 °C, the WCAs of the superhydrophobic surface did not change substantially. The WCAs decreased rapidly when the temperature was higher than 120 °C. At 180 °C, the WCAs were less than 150° so that the sample lost superhydrophobicity. It could be learnt from the result that the surface gradually lost superhydrophobicity with increasing temperature. The high temperature might cause volatilization or decomposition of the organic modification on the superhydrophobic surface. Moreover, when the samples that had lost their superhydrophobicity were re-modified, they restored superhydrophobicity. The morphology of sample after 200 °C heat treatment was shown in the illustration of Fig. 9a, and it showed that the structure had not changed. Fig. 9b showed the Thermogravimetry (TG) and the Derivative Thermogravimetry (DTG) curves of lauric acid. The curves in Fig. 9b indicated that lauric acid decomposed and the weight lost with the increase of temperature. It could be learnt from Fig. 9b that the weight lost about 2.245% and the derivative of mass loss was 0.072 mg/°C at 120 °C. But when the temperature was 180 °C, the weight lost 15.882% and the derivative of mass loss reached up to 0.322 mg/°C. Therefore, the higher the temperature, the faster lauric acid decomposition, which eventually led to a reduction in superhydrophobicity. 3.7. Mechanical stability The evolution of WCAs was recorded and presented in Fig. 10a. It can be clearly seen that the WCAs of superhydrophobic surface decreased with the increase of the wear distance.
Fig. 10a showed that superhydrophobic surface still maintained superhydrophobicity when the wear distance was less than 300 cm, indicating an excellent mechanical property. The good mechanical durability of the fabricated superhydrophobic 6061 surface could be attributed to the good hardness of the anodic oxide film. Moreover, the morphology of superhydrophobic surface after abrasion test was observed by SEM. The morphology of superhydrophobic surface after 300 cm abrasion test was shown in Fig 10b. The original honeycomb pore-needle structure (Fig. 2c) was destroyed so that the hierarchical structure was no longer apparent. Such a structure cannot trap as much air as before, resulting in a lower WCA. The result further demonstrates that microstructure is one of the conditions for constructing superhydrophobicity. 3.8. Corrosion resistance and mechanism The corrosion behaviors of the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface were investigated via polarization curves in 3.5 wt.% NaCl solution. The corrosion tendency of the sample can be judged by measuring the corrosion potential. And higher corrosion potential indicates better corrosion resistance [33, 34]. The corrosion potential increased from -0.735 V for the untreated aluminum alloy (Fig. 11b) to -0.709 V for the superhydrophobic aluminum alloy (Fig. 11a). Moreover, the corrosion current density of the untreated substrate, 2.235×10−6 A/cm2, was much higher than that of the superhydrophobic sample, 9.333×10−9 A/cm2. It is worth noting that the corrosion current density decreased significantly after the construction of superhydrophobic surface on aluminum alloy. To make easier to understand, we show a simple anti-corrosion diagram in Fig. 12. When a fabricated superhydrophobic 6061 substrate was immersed in corrosive environment, the
water-repellent feature of superhydrophobic surface contributes to the Cassie contact between solid surface and corrosive media. Air cushion trapped in the honeycomb pore-needle structure isolates the contact between the substrate and the corrosion irons, such as Cl− etc. Such an air cushion can inhibit the electron transfer between the aluminum alloy substrate and the corrosive media, which effectively protects the underlying aluminum alloy substrate. Therefore, it means that the superhydrophobic surface is very effective in protecting the aluminum alloy from corrosion. Besides, the stability of the air cushion trapped in the surface morphology plays a key role in determining its corrosion resistance. 4. Conclusion In conclusion, a superhydrophobic surface with honeycomb pore-needle structure was successfully prepared by two-step method of adopting an environmentally friendly anodizing and low energy modification. The superhydrophobic aluminum alloy 6061 exhibits extremely low surface adhesion force and excellent self-cleaning capacity for contaminants. It is worth mentioning that the area fraction of the gas-liquid interface of the superhydrophobic surface calculated by the Cassie equation is as high as 94.2%. Therefore, the superhydrophobic surface exhibits low thermal conductivity and high nucleation free energy barrier, which makes the icing time prolonged. As a result, it shows good anti-icing performance. Meanwhile, the surface of the superhydrophobic aluminum alloy exhibits good thermal stability, mechanical stability and corrosion resistance, which provides a guarantee for its application in harsh environments. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province of China (No. ZR2019MEM020).
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Figure captions: Fig. 1. Effects of (a) anodic oxidation voltage, (b) anodic oxidation time, and (c) electrolyte concentration on WCAs. Fig. 2. Surface morphologies of superhydrophobic surface under different anodizing voltage: (a) 50 V, (b) 55 V, (c) 60 V, and (d) 65 V. Fig. 3. Surface morphologies of superhydrophobic surface under different anodizing time: (a) 20 min, (b) 40 min, (c) 60 min, and (d) 80 min. Fig. 4. EDS spectra of (a) 6061 aluminum alloy substrate, (b) superhydrophobic sample. Fig. 5. FTIR spectra of (a) superhydrophobic sample and (b) lauric acid. Fig. 6. The freezing process of water droplets on substrate and superhydrophobic sample surface under different time: (a) 0 s, (b) 111 s, and (c) 900 s. Fig. 7. Images of a water droplet (3 µL) suspending on the needle, contacting, pressing and departing from the superhydrophobic surface. The arrows represent the moving direction of the sample. Fig. 8. Self-cleaning process on the surface of substrate (a) and superhydrophobic surface (b). Fig. 9. (a) The change of WCAs on the superhydrophobic surface with different heating temperature. The illustration was the morphology of sample after 200 °C heat treatment. (b) Thermogravimetry (TG) and Derivative Thermogravimetry (DTG) curves of lauric acid. Fig. 10. (a) The evolution of WCAs with wear distance. (b) The morphology of superhydrophobic surface after 300 cm abrasion test. Fig. 11. Potentiodynamic polarization curves of (a) the superhydrophobic aluminum alloy and (b) the untreated aluminum alloy measured in 3.5 wt.% NaCl aqueous solution. Fig. 12. Schematic illustration of corrosion and protection.
Highlights A facile and effective anodic oxidation method is used to prepare the superhydrophobic surface. The superhydrophobic surface shows good anti-icing due to delay in icing time. The superhydrophobic surface exhibits low adhesion, good self-cleaning performance, excellent thermal stability, mechanical stability and corrosion resistance.
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: