- Email: [email protected]
Corrosion protection application of slippery liquid-infused porous surface based on aluminum foil
Applied Surface Science 423 (2017) 365–374
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Corrosion protection application of slippery liquid-infused porous surface based on aluminum foil Yanjing Tuo a , Haifeng Zhang a,∗ , Weiping Chen a , Xiaowei Liu a,b a b
MEMS Center, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Urban Water Resource & Environment(Harbin Institute of Technology), Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 April 2017 Received in revised form 8 June 2017 Accepted 15 June 2017 Available online 19 June 2017 Keywords: Corrosion Superhydrophobic SLIPS
a b s t r a c t Corrosion is a major problem for metal in marine systems. In this research, we fabricated a slippery liquidinfused porous surface (SLIPS) on the aluminum foil to protect the underneath metal. The as-fabricated samples were characterized with EDS, XRD, SEM, and contact angle meter. And the anti-corrosion property was evaluated by electrochemical measurements. The corrosion current density of SLIPS is ca.2 orders of magnitude lower than that of the untreated aluminum and superhydrophobic surface. And the impedance spectra of the SLIPS shows a large impedance semicircle with a diameter of several hundred k. cm2 . The SLIPS exhibits an outstanding corrosion protection capability, thus it has a broad application prospect in marine. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Metal corrosion is an eternal enemy in the industrial fields, especially in marine applications. In recent years, it has been found that superhydrophobic film has an anti-corrosion capability to protect the underneath metal [1–4]. When the superhydrophobic surface is immersed in seawater, air layer trapped in the gap of the superhydrophobic film can block the contact of seawater and metal substrate [5–7]. It has been proved that air layer trapped in superhydrophobic film is the essential contributor to the enhancement of barrier performance [8]. Due to capillary effect, at the interface between liquid and air, the capillary pressure prevents seawater from penetrating the superhydrophobic film [9]. As the immersion time increases, unfortunately, the trapped air layer will be slowly dissolved by seawater. The increase of contact area between seawater and metal substrate will cause the collapse of corrosion resistance. Owing to the temporary durability, the application of superhydrophobic surface has been hindered greatly in water phase [10–12]. This forces researchers to explore alternatives instead of the superhydrophobic surface. Nepenthes pitcher, which survives in tropical rainforests, uses micro-structure to lock in an intermediary liquid to obtain repellent surface [13]. It offers a compelling idea to acquire a stable anticorrosion surface. Slippery liquid-infused porous surfaces (SLIPS)
∗ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.apsusc.2017.06.170 0169-4332/© 2017 Elsevier B.V. All rights reserved.
inspired by Nepenthes can catch a thin film of lubricant immiscible with water phase by porous structures [14,15]. Therefore, it can work as an effective self-protective layer to prevent the invasion of water and corrosive ions [16–18]. Besides, the lubricant film is insoluble in seawater, allowing it to be stored in a rough surface for a longer time than air layer. Thus, replacing the air layer with lubricant film is a promising option to acquire a stable anticorrosion surface. In process of constructing SLIPS, three criteria should be met [19,20]. First, a rough hydrophobic surface is needed to lock in lubricant film, which can form a continuous, homogenous and smooth liquid-solid interface. Second, lubricant and external medium should be immiscible. Third, the affinity between lubricant and rough structure should be well matched. Fouling and corrosion are the two recognized problems in shortening metal life in marine environments. In previous study, SLIPS shows an excellent anti-fouling performance [21–24]. If SLIPS has admirable corrosion resistance, it will be widely applied in marine. Yang et al. have proven that the SLIPS fabricated on the low alloy steel can afford an effective self-standing layer to protect the steel substrate [20]. Slippery anodic aluminum oxide surfaces fabricated by Song et al. have excellent anticorrosion property [25]. In this study, we use a facile method to fabricate LDHs (LDHs are a class of synthetic anionic clays that consist of positively charged layers containing alternatively distributed divalent and trivalent cations in the sheets and charge balancing anions between the layers [26–28]) superhydrophobic surface on aluminum foil, then impregnate lubricant on it to form stable SLIPS. The lubricant can match well with the superhydrophobic surface, so the lubricant
366
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
film can’t be replaced by external seawater automatically. It is reasonable to infer that slippery LDHs surface can protect aluminum substrate from corrosion in seawater for a long time, but it should be proven. Electrochemical experiments were carried out in simulated seawater (3.5% sodium chloride aqueous solution) to evaluate the corrosion resistance of SLIPS and superhydrophobic surface. Results showed that the anti-corrosion property of SLIPS was better than the superhydrophobic surface in simulated seawater. Therefore, the SLIPS is a promising candidate to protect metal in marine applications. Moreover, the stabilities of SLIPS and superhydrophobic surface were tested under acidic and alkaline environments. We can conclude that the SLIPS doesn’t always have better corrosion resistance than the superhydrophobic surface. While the lubricant working as self-standing layer can protect aluminum substrate from corrosion in alkaline environment, SLIPS will be severely corroded in acidic condition. For the superhydrophobic surface, it’s just the opposite, which has a better anti-corrosion property in acidic corrosion. We believe this study will be meaningful in the research of corrosion resistance in different conditions.
form hydrogen bonds on the surface. Then the aluminum foil was hydrothermal synthesized in a mixed aqueous solution of Zn(NO3 )2 and C6 H12 N4 at 95 ◦ C for 3 h. In the mixed solution, the concentration of Zn(NO3 )2 and C6 H12 N4 was 0.05 M.
2.2.3. Surface modification and lubricant impregnation The sample after hydrothermal synthesis was superhydrophilic, so it must be modified by a functionalization reagent to reduce the surface energy. In this study, the sample was immersed in 0.5% FAS ethanol solution for 1 h at room temperature, then heated at 120 ◦ C for 1 h to obtain a superhydrophobic surface. To form a SLIPS, Krytox100 (20 L) was applied onto the superhydrophobic surface with a pipette. The surface was tilted at a small angle for several hours to remove the excess lubricant from the sample surface. With the lubricant coating, the SLIPS could repeal various liquids such as water, ethanol, rap oil, etc.
2.3. Characterization 2. Experimental 2.1. Materials and reagents The reagents used in this study including copper dichloride (CuCl2 . 6H2 O), hydrochloric acid (HCl), zinc nitrate (Zn(NO3 )2 . 6H2 O), hexamethylenetetramine (C6 H12 N4 ), sodium chloride(NaCl), sodium hydroxide (NaOH), fluoroalkylsilane (FAS), lubricant (Krytox100), acetone and ethanol. In the process of the experiment, these regents were used without further purification. The base material was industrial grade aluminum foil which was 1 mm thickness and 99% purity. Deionized water was used in all experiments. 2.2. Fabrication of SLIPS 2.2.1. Chemical etching The aluminum foil (10mm × 10mm × 1 mm) was polished by #1200 sandpaper and cleaned by acetone, ethanol and deionized water in sequence. Then the aluminum sample was chemically etched in a mixed solution for 4 min at room temperature. In the mixed solution, the concentration of HCl was 1 M and concentration of CuCl2 was 0.2 M. 2.2.2. Hydrothermal synthesis After chemical etching, the aluminum sample was thoroughly cleaned to remove the copper and dried at 350 ◦ C for 10 min to
2.3.1. Surface characterization Contact angle (CA) and sliding angle (SA) was measured by a contact angle meter system (JC2000D2A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at room temperature. The morphologies of aluminum foil after electrochemical etching and hydrothermal synthesis were observed with a field-emission scanning electron microscope (FE-SEM, TESCAN VEGA). Chemical composition of the as-prepared sample was characterized by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD).
2.3.2. Electrochemical experiments Electrochemical tests including polarization curves (Tafel) and electrochemical impedance spectra (EIS) were carried out on a computer-controlled electrochemical system (CHI660D, CH Instruments Inc.) in 3.5% NaCl aqueous solution at room temperature. The tests were carried out in a three-electrode system: a saturated calomel electrode (Ag/AgCl, 3 M KCl) was used as reference electrode, a graphite electrode was used as counter electrode and the as-prepared sample was used as working electrode. The area of the working electrode was 1 cm2 . Tafel curves were obtained with a sweeping range ±0.3 V versus the open circuit potential and with a scanning rate of 1 mV/s. EIS tests were performed at frequencies ranging from 105 Hz to 1 Hz at open circuit potential with an amplitude of perturbation voltage of 250mV [29].
Fig. 1. EDS of the aluminum samples. (a) untreated aluminum. (b) hydrothermal synthesis for 3 h.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
367
3. Results and discussion 3.1. Composition and morphology of LDH film The energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to confirm the composition and crystalline characteristic of the hydrothermal synthesized film. As shown in Fig. 1, comparing the EDS spectrums of untreated aluminum with that of hydrothermal synthesized aluminum, we can conclude that the hydrothermal synthesized film is composed of Zn, Al and O elements. Further study on the crystalline characteristic is shown in Fig. 2, which exhibits that the hydrothermal synthesized film is the mixture of ZnO and Zn-Al LDH. There are two diffraction peaks at 11◦ and 23◦ characterizing Zn-Al LDH, which are in accordance with (003) and (006) respectively[30]. Two weak diffraction peaks are in the range of 30–40◦ characterizing the existence of ZnO[31]. Consequently, after hydrothermal synthesis, the aluminum surface is coated with ZnO and Zn-Al LDH film. The SEM images of the as-prepared sample show the morphologies of aluminum foil after chemical etching and hydrothermal synthesis. As shown in Fig. 3a, aluminum surface is covered with step-like micro-structure after chemical etching. Due to the connection of the micro-structure, some pits with the diameter about 5 m are formed on the surface. After hydrothermal synthesis, the morphology of the sample appears a significant change. Fig. 3b, 3c and 3d show the SEM images of aluminum which is hydrothermal synthesized for 1 h, 2 h and 3 h respectively. The thin film is
Fig. 2. XRD pattern of the aluminum that is hydrothermal synthesized in the mixed aqueous solution of Zn(NO3 )2 and C6 H12 N4 for 3 h.
consisted of staggered sheets, which form the flower-like structure. When the reaction time is 1 h, these staggered sheets cluster together and form small protrusions on aluminum surface. When it increases to 3 h, the small protrusions connect together forming an even surface. Besides, with the increase of reaction time, the staggered sheets become dense, leading to the reduction of the porosity on the surface.
Fig. 3. SEM images of aluminum samples after chemical reactions. (a) chemical etching for 4 min. (b) hydrothermal synthesis for 1 h. (c) hydrothermal synthesis for 2 h. (c) hydrothermal synthesis for 3 h.
368
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 4. Contact angles and sliding angles of hydrophobic surfaces and SLIPSs. (a), (b) and (c) are the CAs of hydrophobic surfaces hydrothermal synthesized for 1 h, 2 h and 3 h. (d), (e) and (f) are the SAs of hydrophobic surfaces hydrothermal synthesized for 1 h, 2 h and 3 h. (g), (h) and (i) are the SAs of SLIPSs hydrothermal synthesized for 1 h, 2 h and 3 h.
3.2. Wettability of hydrophobic surface and SLIPS After chemically modified by FAS, the as-prepared sample exhibits hydrophobic property. The surface wettability is changed from hydrophobic to superhydrophobic with the increase of hydrothermal synthesis time. As shown in Fig. 4, when the hydrothermal synthesis time is 1 h, the surface has a high adhesion to water droplets. When it is 3 h, the surface shows an excellent superhydrophobic property, on which the CA is 156◦ and SA is 3◦ . Then the Krytox100 is impregnated on the hydrophobic surfaces to form SLIPS. As shown in Fig. 4g ∼ i, the SAs are 17◦ , 4◦ and 3◦
to water droplets when the hydrothermal synthesis time is 1 h, 2 h and 3 h. This phenomenon suggests that the affinity between lubricant and superhydrophobic surface matches well. In the following experiments, all the superhydrophobic surfaces and SLIPSs are hydrothermal synthesized for 3 h.
3.3. Corrosion protection of superhydrophobic surface and SLIPS For the untreated aluminum, a thin layer of oxide film is spontaneously formed on its surface in air. However, the oxide film fails to prevent the occurrence of corrosion when the aluminum foil is
Fig. 5. Tafel curves of untreated aluminum foil (Al), superhydrophobic surface (Su) and SLIPS (SL). (a) immersion in simulated seawater for 2 h. (b) immersion in simulated seawater for 15d.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
369
Fig. 6. Nyquist plots of Al, Su and SL. (a) immersion in simulated seawater for 1d. (b) immersion in simulated seawater for 15d.
immersed in seawater for a long time. So, it is necessary to prepare an effective protection coating on aluminum foil. The air layer trapped in the gap of superhydrophobic surface insulates external liquid from the aluminum substrate, so that the corrosion effect on the underneath metal can be relieved. The lubricant, as one kind of liquid coating, is an alternative to prevent corrosion. In this study, polarization curves obtained by electrochemical experiments are used to evaluate the protection capability of superhydrophobic sur-
face and SLIPS to the aluminum substrate. As shown in Fig. 5a, the Tafel curves are tested after the as-prepared samples were immersed in 3.5% NaCl aqueous solution for 2 h. The corrosion current density of untreated aluminum is 6.17 × 10−8 A/cm2 . After the formation of superhydrophobic LDH layer on the aluminum surface, the corrosion current density decreases to 2.19 × 10−8 A/cm2 , indicating that the superhydrophobic layer reduces the corrosion rate and protects the underneath aluminum from corrosion attack.
Fig. 7. Bode plots of Al, Su and SL. (a) Bode-phase angle versus frequency plots of the samples immersed in simulated seawater for 1d. (b) Bode-|Z|versus frequency plots of the samples immersed in simulated seawater for 1d. (c) Bode-phase angle versus frequency plots of the samples immersed in simulated seawater for 15d. (d) Bode-|Z|versus frequency plots of the samples immersed in simulated seawater for 15d.
370
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 8. The equivalent circuits and schematic illustration of corrosion protection. (a), (c) superhydrophobic surface in 3.5% NaCl solution. (b), (d) SLIPS in 3.5% NaCl solution.
Table 1 The electrochemical parameters for Al, Su and SL. Samples
Rs
Cf 2
Al Su SL
Rf −2
(cm )
(F cm
41.7 193.1 20.7
0.504 0.021 0.154
)
Cdl 2
Rt −2
(cm )
(F cm
20459 1413 55.56
3.166 0.076 0.197
In fact, the SLIPS further enhances the corrosion protection. By using lubricant as the liquid coating, the corrosion current density decreases to 7.82 × 10−10 A/cm2 , which is ca.2 orders of magnitude lower than that of the untreated aluminum and superhydrophobic surface. Corrosion potential of the untreated aluminum, superhydrophobic surface and SLIPS is −0.726 V, −0.57 V and −0.657 V respectively. The corrosion potential moving to right demonstrates that both trapped air and lubricant are good physical barriers to isolate aluminum from 3.5% NaCl aqueous solution in a short time. After immersion for 15d, the corrosion rate of all the samples increases and the corrosion potential shifts to the left. As shown in Fig. 5b, the corrosion current density of untreated aluminum is 6.52 × 10−7 A/cm2 which is bigger than 6.17 × 10−8 A/cm2 , indicat-
)
W-R
W-T
W-P
– 17241 –
– 0.04 –
– 0.589 –
2
(cm ) 12839 36215 186260
ing that the corrosion have been occurred on the aluminum surface during the immersion in simulated seawater. The corrosion current density of superhydrophobic surface and SLIPS is 6.87 × 10−8 A/cm2 and 1.56 × 10−9 A/cm2 , which is ca.1 and ca.2 orders of magnitude lower than that of the untreated aluminum. The corrosion potential of untreated aluminum, superhydrophobic surface and SLIPS is −0.743 V, −0.723 V and −0.657 V respectively. For the superhydrophobic surface, the corrosion potential has an approximate 150 mV leap after 15d of immersing in simulated seawater. However, for the SLIPS, the corrosion potential was hardly changed. From the comparison of corrosion current density and potential, the lubricant film is more effective than the air layer to protect
Fig. 9. Optical photographs of superhydrophobic surface. (a) Immersed in simulated seawater for 1d. (b) Immersed in simulated seawater for 15d.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
371
Fig. 10. SEM images of the as-prepared samples which are immersed in 3.5% NaCl aqueous solution for 15d. (a) untreated aluminum. (b) superhydrophobic surface. (c) SLIPS.
the underneath aluminum from corrosion. The results indicate that SLIPS has excellent corrosion resistance in simulated seawater. To further explore the corrosion inhibition capability of the superhydrophobic surface and SLIPS, the electrochemical impedance spectra (EIS) is measured by an electrochemical method. As shown in Fig. 6a, the Nyquist plots of the as-prepared samples are tested after immersion in simulated seawater for 1d. The impedance spectra of the SLIPS shows a large impedance semicircle whose diameter is around several hundreds of k. cm2 . The impedance semicircle diameter of the superhydrophobic surface and the untreated aluminum is dozens of k. cm2 . These large impedance semicircles indicate that three samples all have good anti-corrosion properties after immersion for 1d, especially the
SLIPS. With the increase of immersion time, the semicircle diameter of the impedance spectra reduces gradually, revealing the progressively deterioration of corrosion resistance. The Fig. 6b shows the EIS plots of the three samples which are immersed in simulated seawater for 15d. The impedance semicircle diameter of the untreated aluminum drops to several hundreds of . cm2 , indicating the corrosion resistance is collapsed. However, the SLIPS still shows a large impedance semicircle with a diameter of dozens of k. cm2 , which is ca.2 and ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface respectively. Therefore, the SLIPS presents an excellent corrosion resistance, when it suffers the attack in the simulated seawater for a long time. The superhydrophobic surface also displays an anti-corrosion property to the
Fig. 11. Contact angles of superhydrophobic surface and sliding angles of SLIPS. (a), (b) CAs of superhydrophobic surface immersed in 0.1 M HCl solution and NaOH solution respectively. (c), (d) SAs of SLIPS immersed in 0.1 M HCl solution and NaOH solution respectively.
372
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 12. SEM images of superhydrophobic surface and SLIPS when they are immersed in acidic or alkaline solution for 10 min. (a), (b) superhydrophobic surface immersed in 0.1 M HCl solution and 0.1 M NaOH solution respectively. (c), (d) SLIPS immersed in 0.1 M HCl solution and 0.1 M NaOH solution respectively.
underneath aluminum, but the effect is just passable, especially when the immersion time is relatively long. The Bode plots of the as-prepared samples are shown in Fig. 7. When the samples are immersed in simulated seawater for 1d, the |Z| value of the SLIPS is ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface in the low frequency region. As the immersion time increases to 15d, the |Z| value of the SLIPS is ca.2 and ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface in the all frequency region. The results reflect that lubricant film endows SLIPS an excellent anti-corrosion property, which is consistent well with the result from Nyquist plots and Tafel curves. In Fig. 7a and b, untreated aluminum presents two time constants within the frequency range of the test, corresponding to the formation of the corrosion layer at 103 −102 HZ and corroding interface at 10−1HZ [9]. The equivalent circuit is shown in Fig. 8b, in which Rs is the solution resistance, Rf is the corrosion layer resistance, Rt is the charge-transfer resistance, Cdl and Cf are the constant phase elements modeling capacitance of double-layer and corrosion layer[32,33]. Fig. 8c shows the schematic illustration of superhydrophobic surface in 3.5% NaCl solution. The aluminum is covered with complex metal and air, so the EIS result is analyzed with the equivalent circuit shown in Fig. 8a. In this circuit, Rf is the film resistance, Cf is the constant phase element modeling capacitance of the composite film. SLIPS presents an excellent corrosion protection property because of the existence of lubricant film. According to the EIS result of SLIPS, the equivalent circuit is analyzed as shown in Fig. 8b. Table 1 is the fitted electrochemical
parameters of untreated aluminum, superhydrophobic surface and SLIPS immersed in 3.5% NaCl solution for 1d. Owing to the existence of protective coating (air layer or lubricant film), both the superhydrophobic surface and SLIPS present an anti-corrosion property. For the superhydrophobic surface, the trapped air works as isolated layer to prevent the penetration of the corrosive ions. However, with the increase of immersion time, the corrosive ions will seep into the air layer gradually. There is an interesting phenomenon to confirm the replacement of air by water. As shown in Fig. 9a, when the superhydrophobic surface is immersed in 3.5% NaCl solution for 1d, the surface is exceptionally bright. According to the theory of total reflection in physics, when light transfers from water to the interface of air with an incident angle higher than critical angle, the light is completely reflected [9,32–34]. Therefore, the bright surface indicates that air still be trapped in the gaps of superhydrophobic surface. As immersion time increases to 15d, the superhydrophobic surface is not bright, on which a large area of air is replaced. Thus, parts of LDH film are contact with corrosive solution directly, which leads to corrosion of underneath aluminum. For the SLIPS, the lubricant is full of the gaps on the rough surface. Since the lubricant is insoluble in the aqueous solution and the affinity between lubricant and hydrophobic surface is much higher than that between air and hydrophobic surface, the lubricant film can be stored in the gaps of the hydrophobic surface stably. Moreover, as the lubricant is liquid phase, the SLIPS presents a self-healing property. When the defect appears on the surface of SLIPS, the lubricant will spontaneously flow to repair it. Thus, the lubricant film can prohibit the occurrence of corro-
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
sion effectively. Fig. 10 shows the SEM images of the three samples that are immersed in 3.5% NaCl aqueous solution for 15d. As shown in Fig. 10a, there are some millimeter level pores on the surface, indicating the occurrence of pitting corrosion on the untreated aluminum. For the superhydrophobic sample, the entire LDH film is damaged, especially at the point of prominence shown in Fig. 10b. The image of SLIPS is shown in Fig. 10c, which is almost intact. This results further prove that the lubricant film can work as a robust barrier to improve the corrosion resistance enormously. Besides the corrosion resistance study of the superhydrophobic surface and SLIPS in simulated seawater, we also explore their anticorrosion properties in acidic and alkaline environments. Fig. 11a and b shows the CAs of the superhydrophobic surface when it is immersed in 0.1 M HCl and NaOH for different time, respectively. With the increase of immersion time, the CAs gradually decrease. But in the alkaline condition, there is a slight fluctuation in the decrease trend, which indicates that there may be new structures formed on the aluminum surface. As shown in Fig. 12a and b, the SEM images are the morphologies of superhydrophobic surface when it is immersed in 0.1 M HCl and NaOH corrosive solution for 10 min. The surface still retains the original structure and only appears a slight corrosion after immersing in acidic solution. However, immersion in alkaline solution, the surface is completely corroded and new microstructure is formed. The hydrophobic structure is more badly destroyed in the alkaline solution because OH− is more easily adsorbed to the fluoroalkylsilane molecules than H+ . The result shows that the superhydrophobic surface has a better anti-corrosion property in acidic environment than in alkaline environment. The SLIPS is also immersed in 0.1 M HCl and NaOH solution to study its anti-corrosion property. Contrast Fig. 11c and d, as the time prolongs, the SA of SLIPS immersed in HCl solution has a more rapid increase than that in NaOH solution. The water droplets even adhere to the surface completely when the SLIPS is immersed in 0.1 M HCl for 10 min. But it still keeps a low SA of water droplet after immersion in 0.1 M NaOH for 10 min. Results indicate that the Krytox100 lubricant is replaced by the hydrochloric acid gradually as the immersion time increases. As shown in Fig. 12c, the surface is completely corroded by the HCl solution. However, the sodium hydroxide can’t replace the lubricant film from the gaps of superhydrophobic surface. As shown in Fig. 12d, there is scarcely any corrosion on the surface of SLIPS, proving that the surface is protected intact by the lubricant film. The result exhibits that the lubricant film can build a barrier in an alkaline environment for a short time, but it is failure in an acidic environment. 4. Conclusion In this research, the superhydrophobic surface and SLIPS were fabricated on aluminum foil by a simple method. The CA of superhydrophobic surface can reach 156◦ and the SA of SLIPS is as low as 3◦ . Since the lubricant is immiscible with water phase, SLIPS exhibits better protection performance than superhydrophobic surface, especially when they are immersed in simulated seawater. In acidic and alkaline environments, however, the superhydrophobic surface and SLIPS present different anti-corrosion properties. The superhydrophobic surface has a better corrosion protection capability than the SLIPS in acidic environment. The results are just the opposite in alkaline solution. Therefore, this research can provide an effective strategy for corrosion protection of metal. Acknowledgements The work is supported by National Science Foundation of China (No. 61474034), National Basic Research Program of China
373
(No. 2012CB934100), Natural Science Foundation of Heilongjiang Province of China (No: F201418), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016TS 06).
References [1] J. Liang, Y. Hu, Y. Wu, H. Chen, Facile formation of superhydrophobic silica-based surface on aluminum substrate with tetraethylorthosilicate and vinyltriethoxysilane as co-precursor and its corrosion resistant performance in corrosive NaCl aqueous solution, Surf. Coat. Tech. 240 (2014) 145–153. [2] L. Feng, Y. Che, Y. Liu, X. Qiang, Y. Wang, Fabrication of superhydrophobic aluminium alloy surface with excellent corrosion resistance by a facile and environment-friendly method, Appl. Surf. Sci. 283 (2013) 367–374. [3] C. Liu, F. Su, J. Liang, P. Huang, Facile fabrication of superhydrophobic cerium coating with micro-nano flower-like structure and excellent corrosion resistance, Surf. Coat. Tech. 258 (2014) 580–586. [4] L. Feng, H. Zhang, Z. Wang, et al., Superhydrophobic aluminum alloy surface: Fabrication, structure, and corrosion resistance, Colloid Surf. A. 441 (3) (2014) 319–325. [5] P. Wang, D. Zhang, R. Qiu, B. Hou, Super-hydrophobic film prepared on zinc as corrosion barrier, Corros. Sci. 53 (2011) 2080–2086. [6] W. Peng, R. Qiu, D. Zhang, Z. Lin, B. Hou, Fabricated super-hydrophobic film with potentiostatic electrolysis method on copper for corrosion protection, Electrochim. Acta. 56 (2010) 517–522. [7] F. Zhang, S. Chen, L. Dong, Y. Lei, L. Tao, Y. Yin, Preparation of superhydrophobic films on titanium as effective corrosion barriers, Appl. Surf. Sci. 257 (2011) 2587–2591. [8] P. Wang, D. Zhang, R. Qiu, J. Wu, Y. Wan, Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere, Corros. Sci. 69 (2013) 23–30. [9] P. Wang, D. Zhang, R. Qiu, Liquid/solid contact mode of super-hydrophobic film in aqueous solution and its effect on corrosion resistance, Corros. Sci. 54 (2012) 77–84. [10] R. Qiu, D. Zhang, P. Wang, Superhydrophobic-carbon fibre growth on a zinc surface for corrosion inhibition, Corros. Sci. 66 (2013) 350–359. [11] C. Lee, C.J. Kim, Wetting and active dewetting processes of hierarchically constructed superhydrophobic surfaces fully immersed in water, J. Microelectromech. S. 21 (2012) 712–720. [12] A.S. Mohamed, H.V. Tafreshi, M. Gadelhak, Superhydrophobic surfaces: from the lotus leaf to the submarine, Cr. Mecanique. 340 (2012) 18–34. [13] P. Wang P, D. Zhang, Z. Lu, Slippery liquid-infused porous surface bio-inspired by pitcher plant for marine anti-biofouling application, Colloid Surf. B. 136 (2015) 240–247. [14] H.F. Bohn, W. Federle, Insect aquaplaning: nepenthes pitcher plants capture prey with the peristome a fully wettable water-lubricated anisotropic surface, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 14138–14143. [15] T.S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, J. Aizenberg, Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity, Nature 477 (2011) 443–447. [16] T. Song, Q. Liu, M. Zhang, R. Chen, K. Takahashi, X. Jing, L. Liu, J. Wang, Multiple sheet-layered super slippery surfaces based on anodic aluminium oxide and its anticorrosion property, RSC Adv. 5 (2015) 70080–70085. [17] R. Qiu, Q. Zhang, P. Wang, L. Jiang, W. Guo, Fabrication of slippery liquid-infused porous surface based on carbon fiber with enhanced corrosion inhibition property, Colloid Surf. A. 453 (2014) 132–141. [18] P. Wang, Z. Lu, D. Zhang, Slippery liquid-infused porous surfaces fabricated on aluminum as a barrier to corrosion induced by sulfate reducing bacteria, Corros. Sci. 93 (2015) 159–166. [19] P. Kim, M.J. Kreder, J. Alvarenga, J. Aizenberg, Hierarchical or not? Effect of the length scale and hierarchy of the surface roughness on omniphobicity of lubricant-infused substrates, Nano. Lett 13 (2013) 1793–1799. [20] S. Yang, R. Qiu, H. Song, P. Wang, Z. Shi, Y. Wang, Slippery liquid-infused porous surface based on perfluorinated lubricant/iron tetradecanoate: preparation and corrosion protection application, Appl. Surf. Sci. 328 (2015) 491–500. [21] A.K. Epstein, T.S. Wong, R.A. Belisle, Liquid-infused structured surfaces with exceptional anti-biofouling performance, Proc. Natl. Acad. Sci. U. S. A. 109 (2012) 13182. [22] X. Yao, S.S. Dunn, P. Kim P, Fluorogel elastomers with tunable transparency, elasticity, shape-memory, and antifouling properties, Angewandte Chemie 53 (2014) 4418. [23] P. Wang, D. Zhang, Z. Lu, S. Sun, Fabrication of slippery lubricant-infused porous surface for inhibition of microbially influenced corrosion, Acs Appl. Mater. Inter. 8 (2015) 1120–1127. [24] L. Xiao, J. Li, S. Mieszkin, F. Di, A.S. Clare, M.E. Callow, Slippery liquid-infused porous surfaces showing marine antibiofouling properties, Acs. Appl. Mater. Inter. 5 (2015) 10074–10080. [25] T. Song, Q. Liu, J. Liu, W. Yang, R. Chen, X. Jing, Fabrication of super slippery sheet-layered and porous anodic aluminium oxide surfaces and its anticorrosion property, Appl. Surf. Sci 355 (2015) 495–501. [26] X. Guo, F. Zhang, S. Xu, D.G. Evans, X. Duan, Preparation of layered double hydroxide films with different orientations on the opposite sides of a glass
374
[27]
[28]
[29]
[30]
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374 substrate by in situ hydrothermal crystallization, Chem. Commun. 44 (2009) 6836–6838. Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, W.S. Seo, Surface precipitation of highly porous hydrotalcite-like film on Al from a zinc aqueous solution, Langmuir 22 (2006) 3521–3527. Y. Kuang, L. Zhao, S. Zhang, F. Zhang, M. Dong, M. Xu, Morphologies, preparations and applications of layered double hydroxide micro-/Nanostructures, Materials 3 (2010) 5220–5235. R. Qiu, Q. Zhang, P. Wang, L. Jiang, J. Hou, W. Guo, Fabrication of slippery liquid-infused porous surface based on carbon fiber with enhanced corrosion inhibition property, Colloid Surface A. 453 (2014) 132–141. Y. Guo, Q. Wang, T. Wang, Facile fabrication of superhydrophobic surface with micro/nanoscale binary structures on aluminum substrate, Appl. Surf. Sci. 257 (2011) 5831–5836.
[31] M. Zhou, X. Pang, L. Wei, K. Gao, Insitu grown superhydrophobic Zn–Al layered double hydroxides films on magnesium alloy to improve corrosion properties, Appl. Surf. Sci. 337 (2015) 172–177. [32] P. Wang, D. Zhang, R. Qiu, Y. Wan, J. Wu, Green approach to fabrication of a super-hydrophobic film on copper and the consequent corrosion resistance, Corros. Sci. 80 (2014) 366–373. [33] P. Wang, D. Zhang, R. Qiu, J. Wu, Super-hydrophobic metal-complex film fabricated electrochemically on copper as a barrier to corrosive medium, Corros. Sci. 83 (2014) 317–326. [34] P. Wang, D. Zhang, Z. Lu, Advantage of super-hydrophobic surface as a barrier against atmospheric corrosion induced by salt deliquescence, Corros. Sci. 90 (2015) 23–32.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Corrosion protection application of slippery liquid-infused porous surface based on aluminum foil Yanjing Tuo a , Haifeng Zhang a,∗ , Weiping Chen a , Xiaowei Liu a,b a b
MEMS Center, Harbin Institute of Technology, Harbin 150001, China State Key Laboratory of Urban Water Resource & Environment(Harbin Institute of Technology), Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 April 2017 Received in revised form 8 June 2017 Accepted 15 June 2017 Available online 19 June 2017 Keywords: Corrosion Superhydrophobic SLIPS
a b s t r a c t Corrosion is a major problem for metal in marine systems. In this research, we fabricated a slippery liquidinfused porous surface (SLIPS) on the aluminum foil to protect the underneath metal. The as-fabricated samples were characterized with EDS, XRD, SEM, and contact angle meter. And the anti-corrosion property was evaluated by electrochemical measurements. The corrosion current density of SLIPS is ca.2 orders of magnitude lower than that of the untreated aluminum and superhydrophobic surface. And the impedance spectra of the SLIPS shows a large impedance semicircle with a diameter of several hundred k. cm2 . The SLIPS exhibits an outstanding corrosion protection capability, thus it has a broad application prospect in marine. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Metal corrosion is an eternal enemy in the industrial fields, especially in marine applications. In recent years, it has been found that superhydrophobic film has an anti-corrosion capability to protect the underneath metal [1–4]. When the superhydrophobic surface is immersed in seawater, air layer trapped in the gap of the superhydrophobic film can block the contact of seawater and metal substrate [5–7]. It has been proved that air layer trapped in superhydrophobic film is the essential contributor to the enhancement of barrier performance [8]. Due to capillary effect, at the interface between liquid and air, the capillary pressure prevents seawater from penetrating the superhydrophobic film [9]. As the immersion time increases, unfortunately, the trapped air layer will be slowly dissolved by seawater. The increase of contact area between seawater and metal substrate will cause the collapse of corrosion resistance. Owing to the temporary durability, the application of superhydrophobic surface has been hindered greatly in water phase [10–12]. This forces researchers to explore alternatives instead of the superhydrophobic surface. Nepenthes pitcher, which survives in tropical rainforests, uses micro-structure to lock in an intermediary liquid to obtain repellent surface [13]. It offers a compelling idea to acquire a stable anticorrosion surface. Slippery liquid-infused porous surfaces (SLIPS)
∗ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.apsusc.2017.06.170 0169-4332/© 2017 Elsevier B.V. All rights reserved.
inspired by Nepenthes can catch a thin film of lubricant immiscible with water phase by porous structures [14,15]. Therefore, it can work as an effective self-protective layer to prevent the invasion of water and corrosive ions [16–18]. Besides, the lubricant film is insoluble in seawater, allowing it to be stored in a rough surface for a longer time than air layer. Thus, replacing the air layer with lubricant film is a promising option to acquire a stable anticorrosion surface. In process of constructing SLIPS, three criteria should be met [19,20]. First, a rough hydrophobic surface is needed to lock in lubricant film, which can form a continuous, homogenous and smooth liquid-solid interface. Second, lubricant and external medium should be immiscible. Third, the affinity between lubricant and rough structure should be well matched. Fouling and corrosion are the two recognized problems in shortening metal life in marine environments. In previous study, SLIPS shows an excellent anti-fouling performance [21–24]. If SLIPS has admirable corrosion resistance, it will be widely applied in marine. Yang et al. have proven that the SLIPS fabricated on the low alloy steel can afford an effective self-standing layer to protect the steel substrate [20]. Slippery anodic aluminum oxide surfaces fabricated by Song et al. have excellent anticorrosion property [25]. In this study, we use a facile method to fabricate LDHs (LDHs are a class of synthetic anionic clays that consist of positively charged layers containing alternatively distributed divalent and trivalent cations in the sheets and charge balancing anions between the layers [26–28]) superhydrophobic surface on aluminum foil, then impregnate lubricant on it to form stable SLIPS. The lubricant can match well with the superhydrophobic surface, so the lubricant
366
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
film can’t be replaced by external seawater automatically. It is reasonable to infer that slippery LDHs surface can protect aluminum substrate from corrosion in seawater for a long time, but it should be proven. Electrochemical experiments were carried out in simulated seawater (3.5% sodium chloride aqueous solution) to evaluate the corrosion resistance of SLIPS and superhydrophobic surface. Results showed that the anti-corrosion property of SLIPS was better than the superhydrophobic surface in simulated seawater. Therefore, the SLIPS is a promising candidate to protect metal in marine applications. Moreover, the stabilities of SLIPS and superhydrophobic surface were tested under acidic and alkaline environments. We can conclude that the SLIPS doesn’t always have better corrosion resistance than the superhydrophobic surface. While the lubricant working as self-standing layer can protect aluminum substrate from corrosion in alkaline environment, SLIPS will be severely corroded in acidic condition. For the superhydrophobic surface, it’s just the opposite, which has a better anti-corrosion property in acidic corrosion. We believe this study will be meaningful in the research of corrosion resistance in different conditions.
form hydrogen bonds on the surface. Then the aluminum foil was hydrothermal synthesized in a mixed aqueous solution of Zn(NO3 )2 and C6 H12 N4 at 95 ◦ C for 3 h. In the mixed solution, the concentration of Zn(NO3 )2 and C6 H12 N4 was 0.05 M.
2.2.3. Surface modification and lubricant impregnation The sample after hydrothermal synthesis was superhydrophilic, so it must be modified by a functionalization reagent to reduce the surface energy. In this study, the sample was immersed in 0.5% FAS ethanol solution for 1 h at room temperature, then heated at 120 ◦ C for 1 h to obtain a superhydrophobic surface. To form a SLIPS, Krytox100 (20 L) was applied onto the superhydrophobic surface with a pipette. The surface was tilted at a small angle for several hours to remove the excess lubricant from the sample surface. With the lubricant coating, the SLIPS could repeal various liquids such as water, ethanol, rap oil, etc.
2.3. Characterization 2. Experimental 2.1. Materials and reagents The reagents used in this study including copper dichloride (CuCl2 . 6H2 O), hydrochloric acid (HCl), zinc nitrate (Zn(NO3 )2 . 6H2 O), hexamethylenetetramine (C6 H12 N4 ), sodium chloride(NaCl), sodium hydroxide (NaOH), fluoroalkylsilane (FAS), lubricant (Krytox100), acetone and ethanol. In the process of the experiment, these regents were used without further purification. The base material was industrial grade aluminum foil which was 1 mm thickness and 99% purity. Deionized water was used in all experiments. 2.2. Fabrication of SLIPS 2.2.1. Chemical etching The aluminum foil (10mm × 10mm × 1 mm) was polished by #1200 sandpaper and cleaned by acetone, ethanol and deionized water in sequence. Then the aluminum sample was chemically etched in a mixed solution for 4 min at room temperature. In the mixed solution, the concentration of HCl was 1 M and concentration of CuCl2 was 0.2 M. 2.2.2. Hydrothermal synthesis After chemical etching, the aluminum sample was thoroughly cleaned to remove the copper and dried at 350 ◦ C for 10 min to
2.3.1. Surface characterization Contact angle (CA) and sliding angle (SA) was measured by a contact angle meter system (JC2000D2A, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at room temperature. The morphologies of aluminum foil after electrochemical etching and hydrothermal synthesis were observed with a field-emission scanning electron microscope (FE-SEM, TESCAN VEGA). Chemical composition of the as-prepared sample was characterized by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD).
2.3.2. Electrochemical experiments Electrochemical tests including polarization curves (Tafel) and electrochemical impedance spectra (EIS) were carried out on a computer-controlled electrochemical system (CHI660D, CH Instruments Inc.) in 3.5% NaCl aqueous solution at room temperature. The tests were carried out in a three-electrode system: a saturated calomel electrode (Ag/AgCl, 3 M KCl) was used as reference electrode, a graphite electrode was used as counter electrode and the as-prepared sample was used as working electrode. The area of the working electrode was 1 cm2 . Tafel curves were obtained with a sweeping range ±0.3 V versus the open circuit potential and with a scanning rate of 1 mV/s. EIS tests were performed at frequencies ranging from 105 Hz to 1 Hz at open circuit potential with an amplitude of perturbation voltage of 250mV [29].
Fig. 1. EDS of the aluminum samples. (a) untreated aluminum. (b) hydrothermal synthesis for 3 h.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
367
3. Results and discussion 3.1. Composition and morphology of LDH film The energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to confirm the composition and crystalline characteristic of the hydrothermal synthesized film. As shown in Fig. 1, comparing the EDS spectrums of untreated aluminum with that of hydrothermal synthesized aluminum, we can conclude that the hydrothermal synthesized film is composed of Zn, Al and O elements. Further study on the crystalline characteristic is shown in Fig. 2, which exhibits that the hydrothermal synthesized film is the mixture of ZnO and Zn-Al LDH. There are two diffraction peaks at 11◦ and 23◦ characterizing Zn-Al LDH, which are in accordance with (003) and (006) respectively[30]. Two weak diffraction peaks are in the range of 30–40◦ characterizing the existence of ZnO[31]. Consequently, after hydrothermal synthesis, the aluminum surface is coated with ZnO and Zn-Al LDH film. The SEM images of the as-prepared sample show the morphologies of aluminum foil after chemical etching and hydrothermal synthesis. As shown in Fig. 3a, aluminum surface is covered with step-like micro-structure after chemical etching. Due to the connection of the micro-structure, some pits with the diameter about 5 m are formed on the surface. After hydrothermal synthesis, the morphology of the sample appears a significant change. Fig. 3b, 3c and 3d show the SEM images of aluminum which is hydrothermal synthesized for 1 h, 2 h and 3 h respectively. The thin film is
Fig. 2. XRD pattern of the aluminum that is hydrothermal synthesized in the mixed aqueous solution of Zn(NO3 )2 and C6 H12 N4 for 3 h.
consisted of staggered sheets, which form the flower-like structure. When the reaction time is 1 h, these staggered sheets cluster together and form small protrusions on aluminum surface. When it increases to 3 h, the small protrusions connect together forming an even surface. Besides, with the increase of reaction time, the staggered sheets become dense, leading to the reduction of the porosity on the surface.
Fig. 3. SEM images of aluminum samples after chemical reactions. (a) chemical etching for 4 min. (b) hydrothermal synthesis for 1 h. (c) hydrothermal synthesis for 2 h. (c) hydrothermal synthesis for 3 h.
368
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 4. Contact angles and sliding angles of hydrophobic surfaces and SLIPSs. (a), (b) and (c) are the CAs of hydrophobic surfaces hydrothermal synthesized for 1 h, 2 h and 3 h. (d), (e) and (f) are the SAs of hydrophobic surfaces hydrothermal synthesized for 1 h, 2 h and 3 h. (g), (h) and (i) are the SAs of SLIPSs hydrothermal synthesized for 1 h, 2 h and 3 h.
3.2. Wettability of hydrophobic surface and SLIPS After chemically modified by FAS, the as-prepared sample exhibits hydrophobic property. The surface wettability is changed from hydrophobic to superhydrophobic with the increase of hydrothermal synthesis time. As shown in Fig. 4, when the hydrothermal synthesis time is 1 h, the surface has a high adhesion to water droplets. When it is 3 h, the surface shows an excellent superhydrophobic property, on which the CA is 156◦ and SA is 3◦ . Then the Krytox100 is impregnated on the hydrophobic surfaces to form SLIPS. As shown in Fig. 4g ∼ i, the SAs are 17◦ , 4◦ and 3◦
to water droplets when the hydrothermal synthesis time is 1 h, 2 h and 3 h. This phenomenon suggests that the affinity between lubricant and superhydrophobic surface matches well. In the following experiments, all the superhydrophobic surfaces and SLIPSs are hydrothermal synthesized for 3 h.
3.3. Corrosion protection of superhydrophobic surface and SLIPS For the untreated aluminum, a thin layer of oxide film is spontaneously formed on its surface in air. However, the oxide film fails to prevent the occurrence of corrosion when the aluminum foil is
Fig. 5. Tafel curves of untreated aluminum foil (Al), superhydrophobic surface (Su) and SLIPS (SL). (a) immersion in simulated seawater for 2 h. (b) immersion in simulated seawater for 15d.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
369
Fig. 6. Nyquist plots of Al, Su and SL. (a) immersion in simulated seawater for 1d. (b) immersion in simulated seawater for 15d.
immersed in seawater for a long time. So, it is necessary to prepare an effective protection coating on aluminum foil. The air layer trapped in the gap of superhydrophobic surface insulates external liquid from the aluminum substrate, so that the corrosion effect on the underneath metal can be relieved. The lubricant, as one kind of liquid coating, is an alternative to prevent corrosion. In this study, polarization curves obtained by electrochemical experiments are used to evaluate the protection capability of superhydrophobic sur-
face and SLIPS to the aluminum substrate. As shown in Fig. 5a, the Tafel curves are tested after the as-prepared samples were immersed in 3.5% NaCl aqueous solution for 2 h. The corrosion current density of untreated aluminum is 6.17 × 10−8 A/cm2 . After the formation of superhydrophobic LDH layer on the aluminum surface, the corrosion current density decreases to 2.19 × 10−8 A/cm2 , indicating that the superhydrophobic layer reduces the corrosion rate and protects the underneath aluminum from corrosion attack.
Fig. 7. Bode plots of Al, Su and SL. (a) Bode-phase angle versus frequency plots of the samples immersed in simulated seawater for 1d. (b) Bode-|Z|versus frequency plots of the samples immersed in simulated seawater for 1d. (c) Bode-phase angle versus frequency plots of the samples immersed in simulated seawater for 15d. (d) Bode-|Z|versus frequency plots of the samples immersed in simulated seawater for 15d.
370
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 8. The equivalent circuits and schematic illustration of corrosion protection. (a), (c) superhydrophobic surface in 3.5% NaCl solution. (b), (d) SLIPS in 3.5% NaCl solution.
Table 1 The electrochemical parameters for Al, Su and SL. Samples
Rs
Cf 2
Al Su SL
Rf −2
(cm )
(F cm
41.7 193.1 20.7
0.504 0.021 0.154
)
Cdl 2
Rt −2
(cm )
(F cm
20459 1413 55.56
3.166 0.076 0.197
In fact, the SLIPS further enhances the corrosion protection. By using lubricant as the liquid coating, the corrosion current density decreases to 7.82 × 10−10 A/cm2 , which is ca.2 orders of magnitude lower than that of the untreated aluminum and superhydrophobic surface. Corrosion potential of the untreated aluminum, superhydrophobic surface and SLIPS is −0.726 V, −0.57 V and −0.657 V respectively. The corrosion potential moving to right demonstrates that both trapped air and lubricant are good physical barriers to isolate aluminum from 3.5% NaCl aqueous solution in a short time. After immersion for 15d, the corrosion rate of all the samples increases and the corrosion potential shifts to the left. As shown in Fig. 5b, the corrosion current density of untreated aluminum is 6.52 × 10−7 A/cm2 which is bigger than 6.17 × 10−8 A/cm2 , indicat-
)
W-R
W-T
W-P
– 17241 –
– 0.04 –
– 0.589 –
2
(cm ) 12839 36215 186260
ing that the corrosion have been occurred on the aluminum surface during the immersion in simulated seawater. The corrosion current density of superhydrophobic surface and SLIPS is 6.87 × 10−8 A/cm2 and 1.56 × 10−9 A/cm2 , which is ca.1 and ca.2 orders of magnitude lower than that of the untreated aluminum. The corrosion potential of untreated aluminum, superhydrophobic surface and SLIPS is −0.743 V, −0.723 V and −0.657 V respectively. For the superhydrophobic surface, the corrosion potential has an approximate 150 mV leap after 15d of immersing in simulated seawater. However, for the SLIPS, the corrosion potential was hardly changed. From the comparison of corrosion current density and potential, the lubricant film is more effective than the air layer to protect
Fig. 9. Optical photographs of superhydrophobic surface. (a) Immersed in simulated seawater for 1d. (b) Immersed in simulated seawater for 15d.
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
371
Fig. 10. SEM images of the as-prepared samples which are immersed in 3.5% NaCl aqueous solution for 15d. (a) untreated aluminum. (b) superhydrophobic surface. (c) SLIPS.
the underneath aluminum from corrosion. The results indicate that SLIPS has excellent corrosion resistance in simulated seawater. To further explore the corrosion inhibition capability of the superhydrophobic surface and SLIPS, the electrochemical impedance spectra (EIS) is measured by an electrochemical method. As shown in Fig. 6a, the Nyquist plots of the as-prepared samples are tested after immersion in simulated seawater for 1d. The impedance spectra of the SLIPS shows a large impedance semicircle whose diameter is around several hundreds of k. cm2 . The impedance semicircle diameter of the superhydrophobic surface and the untreated aluminum is dozens of k. cm2 . These large impedance semicircles indicate that three samples all have good anti-corrosion properties after immersion for 1d, especially the
SLIPS. With the increase of immersion time, the semicircle diameter of the impedance spectra reduces gradually, revealing the progressively deterioration of corrosion resistance. The Fig. 6b shows the EIS plots of the three samples which are immersed in simulated seawater for 15d. The impedance semicircle diameter of the untreated aluminum drops to several hundreds of . cm2 , indicating the corrosion resistance is collapsed. However, the SLIPS still shows a large impedance semicircle with a diameter of dozens of k. cm2 , which is ca.2 and ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface respectively. Therefore, the SLIPS presents an excellent corrosion resistance, when it suffers the attack in the simulated seawater for a long time. The superhydrophobic surface also displays an anti-corrosion property to the
Fig. 11. Contact angles of superhydrophobic surface and sliding angles of SLIPS. (a), (b) CAs of superhydrophobic surface immersed in 0.1 M HCl solution and NaOH solution respectively. (c), (d) SAs of SLIPS immersed in 0.1 M HCl solution and NaOH solution respectively.
372
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
Fig. 12. SEM images of superhydrophobic surface and SLIPS when they are immersed in acidic or alkaline solution for 10 min. (a), (b) superhydrophobic surface immersed in 0.1 M HCl solution and 0.1 M NaOH solution respectively. (c), (d) SLIPS immersed in 0.1 M HCl solution and 0.1 M NaOH solution respectively.
underneath aluminum, but the effect is just passable, especially when the immersion time is relatively long. The Bode plots of the as-prepared samples are shown in Fig. 7. When the samples are immersed in simulated seawater for 1d, the |Z| value of the SLIPS is ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface in the low frequency region. As the immersion time increases to 15d, the |Z| value of the SLIPS is ca.2 and ca.1 orders of magnitude larger than the untreated aluminum and superhydrophobic surface in the all frequency region. The results reflect that lubricant film endows SLIPS an excellent anti-corrosion property, which is consistent well with the result from Nyquist plots and Tafel curves. In Fig. 7a and b, untreated aluminum presents two time constants within the frequency range of the test, corresponding to the formation of the corrosion layer at 103 −102 HZ and corroding interface at 10−1HZ [9]. The equivalent circuit is shown in Fig. 8b, in which Rs is the solution resistance, Rf is the corrosion layer resistance, Rt is the charge-transfer resistance, Cdl and Cf are the constant phase elements modeling capacitance of double-layer and corrosion layer[32,33]. Fig. 8c shows the schematic illustration of superhydrophobic surface in 3.5% NaCl solution. The aluminum is covered with complex metal and air, so the EIS result is analyzed with the equivalent circuit shown in Fig. 8a. In this circuit, Rf is the film resistance, Cf is the constant phase element modeling capacitance of the composite film. SLIPS presents an excellent corrosion protection property because of the existence of lubricant film. According to the EIS result of SLIPS, the equivalent circuit is analyzed as shown in Fig. 8b. Table 1 is the fitted electrochemical
parameters of untreated aluminum, superhydrophobic surface and SLIPS immersed in 3.5% NaCl solution for 1d. Owing to the existence of protective coating (air layer or lubricant film), both the superhydrophobic surface and SLIPS present an anti-corrosion property. For the superhydrophobic surface, the trapped air works as isolated layer to prevent the penetration of the corrosive ions. However, with the increase of immersion time, the corrosive ions will seep into the air layer gradually. There is an interesting phenomenon to confirm the replacement of air by water. As shown in Fig. 9a, when the superhydrophobic surface is immersed in 3.5% NaCl solution for 1d, the surface is exceptionally bright. According to the theory of total reflection in physics, when light transfers from water to the interface of air with an incident angle higher than critical angle, the light is completely reflected [9,32–34]. Therefore, the bright surface indicates that air still be trapped in the gaps of superhydrophobic surface. As immersion time increases to 15d, the superhydrophobic surface is not bright, on which a large area of air is replaced. Thus, parts of LDH film are contact with corrosive solution directly, which leads to corrosion of underneath aluminum. For the SLIPS, the lubricant is full of the gaps on the rough surface. Since the lubricant is insoluble in the aqueous solution and the affinity between lubricant and hydrophobic surface is much higher than that between air and hydrophobic surface, the lubricant film can be stored in the gaps of the hydrophobic surface stably. Moreover, as the lubricant is liquid phase, the SLIPS presents a self-healing property. When the defect appears on the surface of SLIPS, the lubricant will spontaneously flow to repair it. Thus, the lubricant film can prohibit the occurrence of corro-
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374
sion effectively. Fig. 10 shows the SEM images of the three samples that are immersed in 3.5% NaCl aqueous solution for 15d. As shown in Fig. 10a, there are some millimeter level pores on the surface, indicating the occurrence of pitting corrosion on the untreated aluminum. For the superhydrophobic sample, the entire LDH film is damaged, especially at the point of prominence shown in Fig. 10b. The image of SLIPS is shown in Fig. 10c, which is almost intact. This results further prove that the lubricant film can work as a robust barrier to improve the corrosion resistance enormously. Besides the corrosion resistance study of the superhydrophobic surface and SLIPS in simulated seawater, we also explore their anticorrosion properties in acidic and alkaline environments. Fig. 11a and b shows the CAs of the superhydrophobic surface when it is immersed in 0.1 M HCl and NaOH for different time, respectively. With the increase of immersion time, the CAs gradually decrease. But in the alkaline condition, there is a slight fluctuation in the decrease trend, which indicates that there may be new structures formed on the aluminum surface. As shown in Fig. 12a and b, the SEM images are the morphologies of superhydrophobic surface when it is immersed in 0.1 M HCl and NaOH corrosive solution for 10 min. The surface still retains the original structure and only appears a slight corrosion after immersing in acidic solution. However, immersion in alkaline solution, the surface is completely corroded and new microstructure is formed. The hydrophobic structure is more badly destroyed in the alkaline solution because OH− is more easily adsorbed to the fluoroalkylsilane molecules than H+ . The result shows that the superhydrophobic surface has a better anti-corrosion property in acidic environment than in alkaline environment. The SLIPS is also immersed in 0.1 M HCl and NaOH solution to study its anti-corrosion property. Contrast Fig. 11c and d, as the time prolongs, the SA of SLIPS immersed in HCl solution has a more rapid increase than that in NaOH solution. The water droplets even adhere to the surface completely when the SLIPS is immersed in 0.1 M HCl for 10 min. But it still keeps a low SA of water droplet after immersion in 0.1 M NaOH for 10 min. Results indicate that the Krytox100 lubricant is replaced by the hydrochloric acid gradually as the immersion time increases. As shown in Fig. 12c, the surface is completely corroded by the HCl solution. However, the sodium hydroxide can’t replace the lubricant film from the gaps of superhydrophobic surface. As shown in Fig. 12d, there is scarcely any corrosion on the surface of SLIPS, proving that the surface is protected intact by the lubricant film. The result exhibits that the lubricant film can build a barrier in an alkaline environment for a short time, but it is failure in an acidic environment. 4. Conclusion In this research, the superhydrophobic surface and SLIPS were fabricated on aluminum foil by a simple method. The CA of superhydrophobic surface can reach 156◦ and the SA of SLIPS is as low as 3◦ . Since the lubricant is immiscible with water phase, SLIPS exhibits better protection performance than superhydrophobic surface, especially when they are immersed in simulated seawater. In acidic and alkaline environments, however, the superhydrophobic surface and SLIPS present different anti-corrosion properties. The superhydrophobic surface has a better corrosion protection capability than the SLIPS in acidic environment. The results are just the opposite in alkaline solution. Therefore, this research can provide an effective strategy for corrosion protection of metal. Acknowledgements The work is supported by National Science Foundation of China (No. 61474034), National Basic Research Program of China
373
(No. 2012CB934100), Natural Science Foundation of Heilongjiang Province of China (No: F201418), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2016TS 06).
References [1] J. Liang, Y. Hu, Y. Wu, H. Chen, Facile formation of superhydrophobic silica-based surface on aluminum substrate with tetraethylorthosilicate and vinyltriethoxysilane as co-precursor and its corrosion resistant performance in corrosive NaCl aqueous solution, Surf. Coat. Tech. 240 (2014) 145–153. [2] L. Feng, Y. Che, Y. Liu, X. Qiang, Y. Wang, Fabrication of superhydrophobic aluminium alloy surface with excellent corrosion resistance by a facile and environment-friendly method, Appl. Surf. Sci. 283 (2013) 367–374. [3] C. Liu, F. Su, J. Liang, P. Huang, Facile fabrication of superhydrophobic cerium coating with micro-nano flower-like structure and excellent corrosion resistance, Surf. Coat. Tech. 258 (2014) 580–586. [4] L. Feng, H. Zhang, Z. Wang, et al., Superhydrophobic aluminum alloy surface: Fabrication, structure, and corrosion resistance, Colloid Surf. A. 441 (3) (2014) 319–325. [5] P. Wang, D. Zhang, R. Qiu, B. Hou, Super-hydrophobic film prepared on zinc as corrosion barrier, Corros. Sci. 53 (2011) 2080–2086. [6] W. Peng, R. Qiu, D. Zhang, Z. Lin, B. Hou, Fabricated super-hydrophobic film with potentiostatic electrolysis method on copper for corrosion protection, Electrochim. Acta. 56 (2010) 517–522. [7] F. Zhang, S. Chen, L. Dong, Y. Lei, L. Tao, Y. Yin, Preparation of superhydrophobic films on titanium as effective corrosion barriers, Appl. Surf. Sci. 257 (2011) 2587–2591. [8] P. Wang, D. Zhang, R. Qiu, J. Wu, Y. Wan, Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere, Corros. Sci. 69 (2013) 23–30. [9] P. Wang, D. Zhang, R. Qiu, Liquid/solid contact mode of super-hydrophobic film in aqueous solution and its effect on corrosion resistance, Corros. Sci. 54 (2012) 77–84. [10] R. Qiu, D. Zhang, P. Wang, Superhydrophobic-carbon fibre growth on a zinc surface for corrosion inhibition, Corros. Sci. 66 (2013) 350–359. [11] C. Lee, C.J. Kim, Wetting and active dewetting processes of hierarchically constructed superhydrophobic surfaces fully immersed in water, J. Microelectromech. S. 21 (2012) 712–720. [12] A.S. Mohamed, H.V. Tafreshi, M. Gadelhak, Superhydrophobic surfaces: from the lotus leaf to the submarine, Cr. Mecanique. 340 (2012) 18–34. [13] P. Wang P, D. Zhang, Z. Lu, Slippery liquid-infused porous surface bio-inspired by pitcher plant for marine anti-biofouling application, Colloid Surf. B. 136 (2015) 240–247. [14] H.F. Bohn, W. Federle, Insect aquaplaning: nepenthes pitcher plants capture prey with the peristome a fully wettable water-lubricated anisotropic surface, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 14138–14143. [15] T.S. Wong, S.H. Kang, S.K.Y. Tang, E.J. Smythe, B.D. Hatton, A. Grinthal, J. Aizenberg, Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity, Nature 477 (2011) 443–447. [16] T. Song, Q. Liu, M. Zhang, R. Chen, K. Takahashi, X. Jing, L. Liu, J. Wang, Multiple sheet-layered super slippery surfaces based on anodic aluminium oxide and its anticorrosion property, RSC Adv. 5 (2015) 70080–70085. [17] R. Qiu, Q. Zhang, P. Wang, L. Jiang, W. Guo, Fabrication of slippery liquid-infused porous surface based on carbon fiber with enhanced corrosion inhibition property, Colloid Surf. A. 453 (2014) 132–141. [18] P. Wang, Z. Lu, D. Zhang, Slippery liquid-infused porous surfaces fabricated on aluminum as a barrier to corrosion induced by sulfate reducing bacteria, Corros. Sci. 93 (2015) 159–166. [19] P. Kim, M.J. Kreder, J. Alvarenga, J. Aizenberg, Hierarchical or not? Effect of the length scale and hierarchy of the surface roughness on omniphobicity of lubricant-infused substrates, Nano. Lett 13 (2013) 1793–1799. [20] S. Yang, R. Qiu, H. Song, P. Wang, Z. Shi, Y. Wang, Slippery liquid-infused porous surface based on perfluorinated lubricant/iron tetradecanoate: preparation and corrosion protection application, Appl. Surf. Sci. 328 (2015) 491–500. [21] A.K. Epstein, T.S. Wong, R.A. Belisle, Liquid-infused structured surfaces with exceptional anti-biofouling performance, Proc. Natl. Acad. Sci. U. S. A. 109 (2012) 13182. [22] X. Yao, S.S. Dunn, P. Kim P, Fluorogel elastomers with tunable transparency, elasticity, shape-memory, and antifouling properties, Angewandte Chemie 53 (2014) 4418. [23] P. Wang, D. Zhang, Z. Lu, S. Sun, Fabrication of slippery lubricant-infused porous surface for inhibition of microbially influenced corrosion, Acs Appl. Mater. Inter. 8 (2015) 1120–1127. [24] L. Xiao, J. Li, S. Mieszkin, F. Di, A.S. Clare, M.E. Callow, Slippery liquid-infused porous surfaces showing marine antibiofouling properties, Acs. Appl. Mater. Inter. 5 (2015) 10074–10080. [25] T. Song, Q. Liu, J. Liu, W. Yang, R. Chen, X. Jing, Fabrication of super slippery sheet-layered and porous anodic aluminium oxide surfaces and its anticorrosion property, Appl. Surf. Sci 355 (2015) 495–501. [26] X. Guo, F. Zhang, S. Xu, D.G. Evans, X. Duan, Preparation of layered double hydroxide films with different orientations on the opposite sides of a glass
374
[27]
[28]
[29]
[30]
Y. Tuo et al. / Applied Surface Science 423 (2017) 365–374 substrate by in situ hydrothermal crystallization, Chem. Commun. 44 (2009) 6836–6838. Y.F. Gao, M. Nagai, Y. Masuda, F. Sato, W.S. Seo, Surface precipitation of highly porous hydrotalcite-like film on Al from a zinc aqueous solution, Langmuir 22 (2006) 3521–3527. Y. Kuang, L. Zhao, S. Zhang, F. Zhang, M. Dong, M. Xu, Morphologies, preparations and applications of layered double hydroxide micro-/Nanostructures, Materials 3 (2010) 5220–5235. R. Qiu, Q. Zhang, P. Wang, L. Jiang, J. Hou, W. Guo, Fabrication of slippery liquid-infused porous surface based on carbon fiber with enhanced corrosion inhibition property, Colloid Surface A. 453 (2014) 132–141. Y. Guo, Q. Wang, T. Wang, Facile fabrication of superhydrophobic surface with micro/nanoscale binary structures on aluminum substrate, Appl. Surf. Sci. 257 (2011) 5831–5836.
[31] M. Zhou, X. Pang, L. Wei, K. Gao, Insitu grown superhydrophobic Zn–Al layered double hydroxides films on magnesium alloy to improve corrosion properties, Appl. Surf. Sci. 337 (2015) 172–177. [32] P. Wang, D. Zhang, R. Qiu, Y. Wan, J. Wu, Green approach to fabrication of a super-hydrophobic film on copper and the consequent corrosion resistance, Corros. Sci. 80 (2014) 366–373. [33] P. Wang, D. Zhang, R. Qiu, J. Wu, Super-hydrophobic metal-complex film fabricated electrochemically on copper as a barrier to corrosive medium, Corros. Sci. 83 (2014) 317–326. [34] P. Wang, D. Zhang, Z. Lu, Advantage of super-hydrophobic surface as a barrier against atmospheric corrosion induced by salt deliquescence, Corros. Sci. 90 (2015) 23–32.