Mechanically robust superhydrophobic porous anodized AA5083 for marine corrosion protection

Mechanically robust superhydrophobic porous anodized AA5083 for marine corrosion protection

Corrosion Science xxx (xxxx) xxxx Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci Mech...

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Corrosion Science xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Mechanically robust superhydrophobic porous anodized AA5083 for marine corrosion protection ⁎

Binbin Zhanga,b,d, , Weichen Xua,b,d, Qingjun Zhua,b,d, Yuanyuan Sunc,d, Yantao Lia,b,d,



a CAS Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, No.7 Nanhai Road, Qingdao 266071, PR China b Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology (Qingdao), No.1 Wenhai Road, Qingdao 266237, PR China c CAS Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, No.7 Nanhai Road, Qingdao 266071, PR China d Center for Ocean Mega-Science, Chinese Academy of Sciences, No.7 Nanhai Road, Qingdao 266071, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Aluminum alloy Anodisation Superhydrophobic Corrosion resistance Robust

Given the widely usage and corrosion tendency of AA5083, it is crucial and highly desired to transform intrinsically hydrophilic AA5083 to be water-repellent superhydrophobicity. In this paper, we constructed porous anodized AA5083 and achieved superhydrophobicity by fluorosilane self-assembly. The surface topography, roughness and chemical compositions were investigated by SEM, AFM and XPS. The corrosion-resisting behavior was revealed by EIS, demonstrating prominent improvement of corrosion resistance. In addition, the superhydrophobic AA5083 exhibits ultralow adhesion force, excellent self-cleaning and mechanical stability. We greatly anticipate the construction of superhydrophobic surfaces with mechanical robustness on AA5083 will effectively broaden its promising applications.

1. Introduction

separation [21,22], anti-icing/frosting [23,24], water collection [25,26], microdroplet manipulation [27,28], Surface-enhanced Raman scattering [29,30] etc., especially in marine corrosion resistant [31–34] and bio-fouling suppression [35–37]. Various artificial superhydrophobic surfaces have been successfully achieved to endow metallic substrates with enhanced corrosion resistance. For instance, Niu et al. [38] achieved superhydrophobicity on copper by the coordination with dodecanethiol based on an electrooxidation-deposition route, achieving a greatly decrease (nearly 4 orders of the magnitude) of corrosion current density. Wu et al. [39] produced a repairable superhydrophobic Co5Zn21 surface by immersing and annealing process on a zinc substrate, revealing a high corrosion inhibition efficiency of 98.51%. Ye et al. [40] developed a superhydrophobic oligoaniline-containing silica pre-process coating for corrosion protection of Q235 carbon steel, decreasing the corrosion current density by more than 3 orders of the magnitude. As for aluminum alloys or high-purity aluminum materials, some instructive research work involving the fabrication and corrosion protection of superhydrophobic surfaces have been reported, including 6061 aluminum alloy [41,42], 2024 aluminum alloy [43,44], 5A05 aluminum alloy [45], 7075 aluminum alloy [46], Al-Mg alloy [47], high-purity aluminum [48,49] etc.

With the advantages of moderate strength, good welding ability and machinability, 5083 aluminum alloy (AA5083) is considered as pivotal materials for maritime applications including shipbuilding, offshore platforms, and port-wharf construction, etc. Chlorides, as aggressive ions, are encountered in marine environments, causing a detrimental effect on the intrinsic corrosion resistance and resulting in intergranular/pitting corrosion of aluminum alloys [1,2]. Thus, it is highly desirable and necessary to impart excellent corrosion resistant performance to aluminum alloys. Over the years, extensive literatures have been published concerning about the treatment of aluminum alloys for corrosion resistance improvement [3–8]. Among these corrosion resistant approaches [9–14], a surface treatment for achieving superhydrophobicity of metallic substrates has recently attracted the attention of scientists and engineers, addressing many shortcomings of traditional corrosion protective strategies. Owing to the remarkable water-proofing ability and comprehensive superiority, the exploitation of natural-inspired super anti-wetting surfaces [15,16] find promising applications in many fields involving self-cleaning [17,18], drag reduction [19,20], oil-water

⁎ Corresponding authors at: CAS Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, No.7 Nanhai Road, Qingdao 266071, PR China. E-mail addresses: [email protected] (B. Zhang), [email protected] (Y. Li).

https://doi.org/10.1016/j.corsci.2019.06.031 Received 18 January 2019; Received in revised form 17 June 2019; Accepted 30 June 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Binbin Zhang, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.06.031

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In our previous research works [50,51], we have fabricated superhydrophobic surfaces on aluminum substrates through electrochemical approaches and achieved multifunctional properties involving selfcleaning, corrosion resistant and bio-fouling suppression etc. However, up to now, transforming the intrinsic hydrophilicity of AA5083 surface to be superhydrophobic has rarely been reported [52,53]. In view of the widely usage and corrosion tendency of AA5083 materials, it is crucial and highly desired to achieve superhydrophobic AA5083 surface and evaluate the surface wettability, self-cleaning and corrosion protective behaviors in detail. Furthermore, the working conditions of superhydrophobic surfaces, especially mechanical damage, severely destroy water-repellent superhydrophobicity. Therefore, it is meritorious and noteworthy to endow surface superhydrophobicity with enhanced mechanical stability. Thus, herein, facile and pragmatic oxalic acid-based anodisation and fluorosilane self-assembly treatment were utilized to fabricate superhydrophobic AA5083 surface with anodized porous architecture. The surface morphology, roughness, chemical compositions, wettability, self-cleaning ability, corrosion resistant behavior and mechanical stability were investigated successively in detail. It is demonstrated that the fabrication and utilization of robust superhydrophobic anodized AA5083 surface with excellent multi-functions could be an effective strategy for marine corrosion protection and widely applicable for many fields.

Scientific Escalab 250Xi) was utilized to record the surface chemical compositions of the prepared sample. The water contact angles and sliding angles of the samples were measured with 5 μL deionized water droplets by Dataphysics OCA 25 (Germany) at a constant room temperature of 298 K.

2. Experimental section

2.5. Abrasion test

2.1. Materials and reagents

The specimen was loaded by a weight of 100 g and rubbed by 1000 grade SiC sandpaper at a constant speed under manual traction. The single trip of abrasion distance is 10 cm. The round trip of 20 cm is defined as a cycle.

2.4. Electrochemical properties The electrochemical performances of the prepared samples were investigated by AMETEK Parstat 4000+ electrochemical workstation (USA). All the electrochemical measurements were carried out at a constant room temperature of 298 K in a three-electrode cell with 3.5 wt. % NaCl aqueous solution as electrolyte. A Pt mesh and saturated silver/silver chloride was served as counter electrode and reference electrode, respectively. A sinusoidal perturbation signal with peak-topeak amplitude of 40 mV (with respect to open circuit potential, OCP) was employed for the measurement of electrochemical impedance spectroscopy (EIS). The impedance spectra were acquired under the frequency range from 100 kHz to 10 mHz. Before the electrochemical measurement, the specimen was immersed in 3.5 wt. % NaCl aqueous solution for more than one hour to acquire a steady system state. The variations of the open circuit potential (OCP) of pristine AA5083 and as-fabricated superhydrophobic AA5083 during immersion in 3.5 wt.% NaCl aqueous solution for 3600 s was presented in Figure S1.

5083 aluminum alloy (AA5083) was purchased from Dongguan Wanxing Metal Co.,Ltd and tailored to 25 mm × 20 mm size through cutting machine. The main composition of the AA5083 was 4.0–4.9 % Mg, 0.4% Si, 0.1–0.4% Fe, 0.4–1.0 % Mn, 0.25% Zn, 0.1% Cu, 0.15% Ti, 0.05–0.25% Cr and the balance was Al. Oxalic acid (98%) was purchased from Sigma-Aldrich. Ethanol (99.7%), graphite powder (99.85%) and sodium chloride (99.5%) were supplied by Sinopharm Chemical Reagent Co.,Ltd. Silica micropowder (99.8%) and manganese monoxide (99%) were bought from Aladdin Industrial Corporation. 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES, C16F17H19O3Si, 96%) was acquired from Shanghai Macklin Biochemical Co.,Ltd. All the reagents were utilized as received without further purification.

3. Results and discussion 3.1. Surface morphology As it is known, appropriate surface architecture plays a crucial role for superhydrophobic construction. Fig. 1 shows the SEM images of the anodized AA5083 specimens under different anodisation voltage condition. Fig. 1a shows the surface morphology of pristine AA5083 surface. It could be clearly observed that the pristine AA5083 substrate is relatively flat and only some grooves owing to the grinding pre-treatment of AA5083 substrate could be seen. The SEM images of anodized AA5083 surfaces under different external voltage condition were illustrated in Fig. 1b-f. After the anodisation reaction under 25 V and 30 V for 10 min (Fig. 1b and c), the AA5083 surface hardly shows any noticeable changes except for a formation of few nanopores. As the anodisation voltage increased to 35 V (Fig. 1d), some visible changes begin to take place on the AA5083 surface. More nanopores could be found, resulting in a relatively rough surface. With the continuously increasing of anodisation voltage to 40 V (Fig. 1e), a typical hierarchical structure formed on the AA5083 surface with obvious cavities and protrusions, presenting a porous anodized surface architecture. The micro/nanoscale rough structure of AA5083 surface benefits the formation of air cushion among structural gaps, which is favorable for the final waterrepellent Cassie contact. When the anodisation voltage climbed to 45 V (Fig. 1f), multi-scale porous structures with larger size could be seen. However, owing to the warming of anodisation system and the boiling of oxalic acid aqueous solution, the anodisation electrolyte evaporates rapidly. A terrible loss of electrolyte impacts the reaction system, which reaction time cannot last for 10 min. Therefore, we deduce that 40 V is more suitable for the construction of dual scale anodized porous rough architecture. Surface roughness plays a key role in determining wettability. In general, root mean square roughness (Rq) and average roughness (Ra) are widely used for surface roughness description. The AFM 3D images

2.2. Sample fabrication AA5083 specimens were grinded with 200, 400, 800, 1200, 2000 grade SiC paper successively. Before anodisation, the specimen was ultrasonically cleaned using absolute ethanol and deionized water for 15 min, respectively. Then, the specimen was promptly transferred into anodisation cell with a conventional two electrodes configuration (Anode: AA5083 specimen, Cathode: Pt sheet). The anodisation reaction was conducted in 0.3 M oxalic acid aqueous solution for 10 min with vigorous stirring under different voltage. After the reaction, the anodized AA5083 specimen was rinsed for two times with deionized water and absolute ethanol to eliminate the residual anodisation electrolyte. The anodized AA5083 specimen was then immersed into 1 vol. % PFDTES/ absolute ethanol solution for 20 min at a constant room temperature of 298 K, then followed by 100 °C heat treatment for 10 min. 2.3. Characterization techniques The topography of the fabricated sample surface was studied using Japan Hitachi S4300 N field-emission scanning electron microscope (FE-SEM). The surface roughness and topographic fluctuation were characterized by atomic force microscopy (AFM, Germany Bruker Multimode 8). X-ray photoelectron spectroscopy (XPS, Thermo 2

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Fig. 1. SEM images of (a) Pristine AA5083 and anodized AA5083 surface for 10 min under different anodisation voltage: (b) 25 V, (c) 30 V, (d) 35 V, (e) 40 V and (f) 45 V.

Fig. 2. AFM 3D images and topographic fluctuation of pristine AA5083 surface (a–b) and superhydrophobic AA5083 surface (c–d).

and topographic fluctuation of pristine AA5083 and superhydrophobic AA5083 surface were revealed in Fig. 2. As shown in Fig. 2a, the surface of pristine AA5083 specimen is relatively smooth with the values of Rq, Ra and Rmax being 43.3 nm, 35.2 nm and 332 nm respectively. From the topographic fluctuation of pristine AA5083 surface, as presented in Fig. 2b, it can be found that the topographic relief height of pristine AA5083 sample is approximately 175 nm. As a comparison, after the anodisation of AA5083 under constant voltage of 40 V, a superhydrophobic roughened porous structure can be clearly observed in Fig. 2c with a significantly increase of the values of Rq, Ra and Rmax being 244 nm, 190 nm and 1811 nm respectively. From the topographic fluctuation of superhydrophobic AA5083 surface, as depicted in Fig. 2d, it can be confirmed that the remarkably enhanced topographic relief height is about 1.35 μm. These results manifest that the anodized porous structure provide a significant improvement of surface roughness, which is beneficial for the formation of air cushion and the achievement of water-repellent superhydrophobicity.

3.2. Chemical composition The achievement of lower surface energy is a major factor during the fabrication of superhydrophobic surfaces. To study the chemical compositions of the prepared superhydrophobic AA5083 surface, the XPS spectra of fabricated superhydrophobic AA5083 surface were obtained, as shown in Fig. 3. The XPS full spectrum shown in Fig. 3a proves the existence of C, O, F, Si and Al on the prepared superhydrophobic AA5083 surface. According to the high resolution spectrum of C 1s, as shown in Fig. 3b, four components including eCF3 group located at 291.6 eV, eCF2 group located at 290.1 eV, eCeOe bonds located at 285.6 eV and eCeCe bonds located at 284.2 eV can be respectively verified. Fig. 3c shows the high resolution spectrum of O 1s, which involves three characteristic peaks at 532.9 eV, 531.8 eV and 530.8 eV representing SieOeSi bond, eOeCe bond and eOeSie bond respectively. From the high resolution spectrum of F 1s shown in Fig. 3d, strong F 1 s peak can be confirmed at 688.7 eV resulting from F3

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Fig. 3. XPS spectra of the fabricated superhydrophobic AA5083 surface: (a) Full spectrum, (b) C 1s spectrum, (c) O 1s spectrum, (d) F 1s spectrum, (e) Si 2p spectrum and (f) Al 2p spectrum.

continuously increasing of anodisation voltage to 40 V, the contact angle of the prepared AA5083 surface reaches to the maximum 165 ± 2°, meanwhile, achieving the lowest sliding angle 1 ± 1°. These static contact angle and sliding angle of the fabricated AA5083 surface under 40 V demonstrates the successful achievement of waterrepellent superhydrophobicity of AA5083. The inset of Fig. 4a shows the water droplets picture on the superhydrophobic AA5083 surface, indicating typical spherical water droplets and excellent water-proofing ability. As the anodisation voltage further increased to 45 V, the decreasing of contact angle viz 156 ± 4° and increasing of sliding angle viz 2 ± 1° were observed. Thus, we could obtain the optimal anodisation voltage combining the analysis of surface morphology and wettability, which is 40 V in this experimental system. Fig. 4b and c present a comparison of contact angle photographs between pristine AA5083 and optimal anodized superhydrophobic AA5083 surface. An impressive increasing of contact angle from 81 ± 1° of pristine AA5083 to 165 ± 2° of superhydrophobic AA5083 was observed. These results make the porous anodized AA5083 surface ideal candidates for achieving water-repellent superhydrophobicity. The surface adhesion force characterization of pristine AA5083 and as-prepared superhydrophobic AA5083 surface was shown in Fig. 5a and b. We use the microsyringe with water droplet of Dataphysics OCA

C covalent bonds. As displayed in Fig. 3e, high resolution spectrum of Si 2p presents a peak at the binding energy of 102.4 eV, which can be assigned to Si-O bonds of PFDTES molecules. Fig. 3f shows high resolution of Al 2p spectrum, Al 2p peak at the binding energy of 74.8 eV which is attributed to the existence OeAleO bonds of Al2O3 after anodisation reaction. These information revealed from XPS spectra demonstrate the PFDTES molecules were successfully grafted on the anodized AA5083 surface. The outermost anodized AA5083 surface is comprised of eCF3 and eCF2 groups with extremely low surface energy, which contributing to the consequently water-repellent superhydrophobicity. 3.3. Wettability behavior Given the great influence of anodisation voltage on the surface morphology and roughness of AA5083 substrate, it conspicuously affects the surface wettability of anodized AA5083 after fluorosilane modification. Fig. 4a presents the influence of anodisation voltage on the contact angles and sliding angles of the prepared AA5083 surface. When the anodisation voltage is 25 V, the contact angle of AA5083 surface is 112 ± 3°. As the voltage increased to 30 V and 35 V, the contact angle rises slightly to 128 ± 1° and 132 ± 2°. With the

Fig. 4. The influence of anodisation voltage on the contact angles and sliding angles of the prepared anodized AA5083 surface (a) and the contact angle photographs of pristine AA5083 (b) and the optimal anodized superhydrophobic AA5083 surface (c). 4

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Fig. 5. The surface adhesion force characterization of pristine AA5083 (a) and as-prepared superhydrophobic AA5083 surface (b). The dynamic water dropping (c) and water flow impact (d) of superhydrophobic AA5083.

superhydrophobic AA5083 surface. For the self-cleaning process, the superhydrophobic AA5083 substrate with contaminants were subjected to water droplets using disposable syringe. The optical pictures before and after water cleaning were recorded with a camera for convenient comparison. Fig. 6 shows the self-cleaning behavior of the fabricated superhydrophobic AA5083 surface using different contaminants: Graphite powder (Fig. 6a), Silica powder (Fig. 6b), MnO powder (Fig. 6c) and sand (Fig. 6d). Water droplets were dropped slowing on the contaminated superhydrophobic AA5083 specimens. As revealed, water droplets roll off superhydrophobic AA5083 surface at a small tilt angles, collecting contaminant particles along the way without leaving any trace of water. With the dropping of water droplets, some blank tracks can be observed. As the continuously water dropping, four kinds of contaminants were all efficiently detaching from the superhydrophobic AA5083 surface. Eventually, the fabricated superhydrophobic AA5083 surfaces were cleaned totally and no contaminants were observed left behind on the surfaces. These data clearly demonstrates that the prepared superhydrophobic AA5083 surfaces have superior self-cleaning efficiency.

25 instrument gradually approaching and touching the solid specimen surface. Fig. 5a presents the approach, touch, absorb and moisten of water droplet on pristine AA5083 surface. The water droplet can be easily absorbed wetting the surface because of the intrinsic surface hydrophilicity, suggesting a high surface adhesion force of AA5083 substrate. As a comparison, the water droplet could completely depart from the prepared superhydrophobic AA5083 surface after tightly contacts, without leaving any water trace, as shown in Fig. 5b. So, it can be clearly seen that the surface adhesion force of the fabricated superhydrophobic AA5083 surface was greatly reduced resulting from the surface water-repellence property and typical Cassie contact between water droplet and solid surface. Besides, the dynamic water dropping process upon the prepared superhydrophobic AA5083 surface was recorded, as shown in Fig. 5c. With the water dropping, the water droplets transiently contact the superhydrophobic surface, then bouncing or sliding away easily from the specimen without leaving any water stain, suggesting an excellent water-proofing property. For many artificial superhydrophobic surfaces, the hierarchical micro/nano structure as well as surface wettability, usually disturbed/distracted under the impact of a water flow jet or water flow dropping impact [54]. To confirm the stability of trapped air cushion of fabricated superhydrophobic AA5083 surface, in our study, a 5 mL syringe was completely filled by water and a water flow jet was produced under normal force. The water flow jet was kept 2 cm higher above the superhydrophobic AA5083 surface at a certain tilt angle. During the water flow jet process, it can be clearly observed that the water flow was reflected out the surface just like the reflection of light. The water flow dropping impact test was carried out under a water faucet. As the water flow dropping of faucet, water hits the surface and immediately repelled off the superhydrophobic AA5083 surface easily, as shown in Fig. 5d. It is verified that the air pocket of the fabricated superhydrophobic AA5083 surface cannot be squeezed out by the impact of water flow jet and water flow dropping, illustrating a stable water repellent property.

3.5. Corrosion resistance and mechanism In order to study the corrosion behavior of the two types of specimens, electrochemical impedance spectroscopy (EIS) test of pristine AA5083 and as-fabricated superhydrophobic AA5083 surfaces were carried out, as shown in Fig. 7. In general, the information regarding with corrosion kinetic process could be revealed by EIS plots. Fig. 7a and b show the EIS results of the prepared superhydrophobic AA5083 and pristine AA5083 surface in 3.5 wt. % NaCl aqueous solution. Both the EIS of superhydrophobic and pristine AA5083 system present capacitive characteristics. As for superhydrophobic AA5083 surface, the Nyquist plot was composed of two overlapped capacitive loops, whose diameter of capacitive arc is much larger than pristine AA5083 substrate. In addition, according to the Bode plots showing phase angle vs. frequency (Fig. 7d), we can clearly observe two time constants corresponding to two electrode process corresponding to superhydrophobic film/electrolyte interface at the high frequency range and film/metal interface at the low frequency range respectively. For pristine AA5083 (Fig. 7b), the Nyquist plots depict that it features with single and smaller capacitive loop. Only one time constant could be observed in the Bode phase angle vs. frequency plots, as presented in Fig. 7d, which

3.4. Self-cleaning ability Lotus leaves usually stay contamination-free owing to their selfcleaning ability. To confirm the self-cleaning ability of the fabricated superhydrophobic AA5083 surface, various particles including graphite powder, silica powder, MnO powder and sand were utilized as contaminants. We randomly spread different kinds of contaminants on the 5

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Fig. 6. The self-cleaning behavior of the fabricated superhydrophobic AA5083 surface using different contaminants: (a) Graphite powder, (b) Silica powder, (c) MnO powder and (d) sand.

is five orders of magnitude higher than that of pristine AA5083 substrate. The significant promotion of impedance modulus value could be attributed to the water-repellent and anti-wetting surface of the

are derived from the electric double layer and charge transfer resistance. Fig. 7c shows the Bode plots of log |Z| vs. frequency. The impedance modulus value of the as-prepared superhydrophobic surface

Fig. 7. EIS plots and fitting results of as-prepared superhydrophobic AA5083 (a) and pristine AA5083 (b), (c) Bode plots of log |Z| vs. frequency and (d) phase angle vs. frequency for pristine AA5083 and superhydrophobic AA5083.

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fabricated superhydrophobic AA5083 surface. To quantify the corrosion resistance of different samples, electrochemical equivalent circuits were established to fit the EIS results, as shown in the insertion of Fig. 7a and 7b. For as-prepared superhydrophobic AA5083 surface, the capacitive semicircle was formed by two overlapping semicircles, indicating that the diameter of these two arcs are similar. Two electrochemical processes, including that at the superhydrophobic film/electrolyte interface at high-frequency and charge-transfer process occurring at the metal/film interface at lowfrequency, can be assigned to this system. As for pristine AA5083 surface, the Nyquist spectrum exhibits only one capacitive semicircle, which can be attributed to the fact that no film formed on the grinded AA5083 surface. Thus, the characterization of the determined curve was the charge-transfer process at the corrosive solution and AA5083 interface. One time constant was used to fit EIS results of aluminum alloys previously [10,55]. Rs, Rfilm and Rct represent solution, film and charge transfer resistance, respectively. Qf, and Qdl represent the superhydrophobic film capacitance and doublelayer capacitance. In this study, all the constant phase elements Q were employed to model the capacitance because of the heterogeneity of the electrode surface [56]. In addition, Table 1 lists the electrochemical parameters obtained from the fitting results of pristine AA5083 and as-fabricated superhydrophobic AA5083 surface in 3.5 wt. % NaCl aqueous solution. We can observe from Table 1 that the Rct0 value of pristine AA5083 and the Rct of prepared superhydrophobic AA5083 are 7.54 × 103 Ω·cm2 and 8.31 × 108 Ω·cm2 respectively, revealing a significant improvement of charge transfer resistance by five orders of magnitude. Besides, the Qdl value of pristine AA5083 surface and superhydrophobic AA5083 surface are respectively 6.76 × 10−5 Ω−1 cm-2 sn and 1.35 × 10−9 Ω1 cm−2 sn. The lower Qdl value and high Rct value of the prepared superhydrophobic AA5083 surface suggests that the charge transfer process of corrosive ions is difficult to occur. In addition, Rct values are frequently employed to calculate the inhibition efficiency (η) of the specimen using the formula: η = (Rct Rct0)/ Rct [51] in which Rct and Rct0 represent the charge transfer resistance of superhydrophobic AA5083 specimen and pristine AA5083 specimen. According to the Rct and Rct0 values shown in Table 1, we can calculate the inhibition efficiency in this case to be approximately 99.99%, indicating an impressive corrosion protection performance in marine condition. To make easier to understand, we show a corrosion and protection diagram in Fig. 8. For pristine AA5083 substrate, the corrosive media could wet the surface easily, promoting the diffusion and transfer of the corrosion ions, such as Cl− etc. Thus, localized corrosion will occur on the pristine AA5083 substrate because of the high electrochemical activity at some specific points. As a comparison, when a fabricated superhydrophobic AA5083 substrate was immersed in marine corrosive environment, the water-repellent superhydrophobicity of surface contributes to the Cassie contact between solid surface and corrosive

Fig. 8. The corrosion and protection mechanism diagram.

media. So, the corrosive ions and liquids are isolated from most of the superhydrophobic AA5083 surface, making it hard to penetrate into the micro-nano structure and the underlying substrate. The stability of air cushion trapped in the surface morphology plays a key role in determining the anti-corrosion ability.

3.6. Mechanical stability and long-term durability The two essential features required for superhydrophobic, viz. micro/nanostructure and low surface energy, contribute to the waterrepellent property because of the trapped air cushion. However, these characteristics are highly susceptible to mechanical abrasion, which could alter the surface morphology and chemistry. Given the significant status of mechanical durability, we evaluated the anti-wear performance of the as-prepared superhydrophobic AA5083 surface. The schematic illustration of abrasion test employed was shown in Fig. 9a, as described in the experimental Section 2.5. The evolution of contact angles were recoded and presented in Fig. 9b. It can be clearly seen that the contact angle of superhydrophobic AA5083 substrate maintained above 150° after 22 abrasion cycles, indicating an excellent mechanical property. A sharp reduce of contact angel was observed dropping to 146 ± 2° after 24 abrasion cycles, as shown in the inset of Fig. 9b. The excellent mechanical durability of the fabricated superhydrophobic AA5083 surface could be attributed to the high hardness of the anodized Al2O3 film. In order to investigate the long-term durability of the prepared superhydrophobic AA5083 surface, air exposure and 3.5 wt. % NaCl immersion tests were carried out. The evolution of contact angle and sliding angle after long-term exposure were recorded, as shown in Fig. 9c and d. After 60-day air exposure and 20-day 3.5 wt. % NaCl immersion, the contact angles of the as-fabricated superhydrophobic AA5083 specimen were still higher than 150°. The XPS spectra after the immersion test was shown in Figure S2, demonstrating no change in the composition of the surface elements, suggesting a superior long-term stability.

Table 1 The electrochemical parameters obtained from the fitting results of pristine AA5083 and superhydrophobic AA5083 surface in 3.5 wt. % NaCl aqueous solution. Parameters

Rs (Ω·cm2) Qf (Ω−1·cm-2·sn) n1 Rf (Ω·cm2) Qdl (Ω−1·cm-2·sn) n2 Rct (Ω·cm2) η (%)

Pristine AA5083

Superhydrophobic AA5083

Simulated data

Error (%)

Simulated data

Error (%)

23.61 / / / 6.76 × 10−5 0.81 7.54 × 103 /

± 0.39 / / / ± 0.71 ± 0.17 ± 0.64 /

273.54 7.35 × 10−10 0.89 1.34 × 107 1.35 × 10−9 0.86 8.31 × 108 99.99 %

± 4.62 ± 6.89 ± 0.76 ± 10.01 ± 7.35 ± 2.64 ± 11.46 /

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Fig. 9. (a) Schematic illustration of abrasion test employed to evaluate the mechanical stability of the fabricated superhydrophobic AA5083 surface. (b) The evolution of contact angle with abrasion cycles. (c–d) The evolution of contact angles and sliding angles after air exposure and 3.5 wt. % NaCl solution immersion.

4. Conclusion To sum up, we fabricated superhydrophobic porous anodized AA5083 surface with a contact angle of 165 ± 2° and sliding angle of 1 ± 1°. The superhydrophobic AA5083 exhibits extremely low surface adhesion force and excellent self-cleaning capacity for various contaminants. The Rct0 value of pristine AA5083 and the Rct of prepared superhydrophobic AA5083 are 7.54 × 103 Ω cm2 and 8.31 × 108 Ω·cm2 respectively, revealing a significant improvement of charge transfer resistance by five orders of magnitude. The abrasion test suggests that the contact angle of superhydrophobic AA5083 substrate maintained above 150° after 22 abrasion cycles, presenting a mechanically robust property. After 60-day air exposure and 20-day 3.5 wt. % NaCl immersion, the superhydrophobic specimen retains its superior water-repellent and anti-wetting performance. Thus, the superhydrophobicity transformation of AA5083 is expected to provide one kind of promising protective material and broaden its maritime applications.

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Data availability

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The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Acknowledgements This research work was financially supported by the National Natural Science Foundation of China (Grant No. 41806089) and the Strategic Priority Program of Chinese Academy of Sciences (Grant No. XDA13040401). In addition, I would like to express my gratitude to Prof. Dahai Xia (School of Materials Science and Engineering, Tianjin University) for his electrochemical advice in the revision process.

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.corsci.2019.06.031.

[15] [16]

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