Enhanced Corrosion Resistance of Superhydrophobic Layered

Subscriber access provided by UNIV OF DURHAM

Applications of Polymer, Composite, and Coating Materials

Enhanced Corrosion Resistance of Superhydrophobic Layered Double Hydroxide (LDH) Films with Long-Term Stability on Al Substrate Yanhui Cao, Dajiang Zheng, Xueliang Li, Jinyan Lin, Cheng Wang, Shigang Dong, and Changjian Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02280 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Enhanced Corrosion Resistance of Superhydrophobic Layered Double Hydroxide (LDH) Films with Long-Term Stability on Al Substrate Yanhui Cao†, Dajiang Zheng‡, Xueliang Li†, Jinyan Lin‡, Cheng Wang†, Shigang Dong§, Changjian Lin†* †

State Key Laboratory of Physical Chemistry of Solid Surfaces, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, P.R. China. ‡ College of Materials, Xiamen University, Xiamen, Fujian 361005, P.R. China. § College of Energy, Xiamen University, Xiamen, Fujian 361005, P.R. China. Email-address: [email protected] ABSTRACT: A superhydrophobic ZnAl-LDH-La film was prepared by a hydrothermal method and further modification by laurate anions in this work. Comprehensive characterizations of this film were performed in terms of morphology, composition, structure, roughness and wettability by scanning electronic microscope (SEM), energy dispersive X-ray spectroscopy (EDS), the X-ray diffraction (XRD) pattern and a 3D laser scanning confocal microscope, etc. The long-term corrosion protection effect of this superhydrophobic film was investigated deeply by monitoring the changes of the electrochemical impedance spectroscopy (EIS) in a long time up to a month in 3.5 wt.% NaCl solution. In the meantime, the changes of contact angle (CA) were also recorded with evolution of the immersion time. The result indicated that the stable superhydrophobic ZnAl-LDH-La film was able to provide efficient protection for the underlying Al substrate in a long time. In addition, the capability of the superhydrophobic surface against harsh conditions including chemical damages and physical damages was emphatically investigated. It was found that the superhydrophobic surface was chemically stable towards acid (pH ≥ 3), alkali and heating, and it also exhibited high UV radiation resistance. This superhydrophobic coating maintained superhydrophobic for 7 days of radiation in an UV chamber equipped with 40 W ultraviolet lamp (λ = 254 nm), indicating superior ability of adapting to out-door environment. This comprehensive investigation of the superhydrophobic ZnAl-LDH-La film is considerably helpful to researchers and engineers to get deep insight into its potential for practical applications in the field of corrosion and protection. KEYWORDS: LDH films; superhydrophobicity; stability; corrosion protection; mechanism; practical application 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61

1. INTRODUCTION Layered double hydroxides (LDHs) with a general formula of [M1-x2+Mx3+(OH)2]x+(Ax/nn-)·mH2O are known as hydrotalcite-like compounds,1 where M2+ and M3+ are the divalent and trivalent metals. The schematic structure of LDHs is shown in Fig.1. The hydroxide layer consists of a large number of octahedral units, where metal cations stay in the centres and hydroxyl groups occupy the eight edges of the unit. As some M2+ cations are replaced by M3+ cations, the hydroxide layers are positively charged. As a result, extra anions are needed to get into the interlayer space of LDHs to compensate the extra positive charges on the hydroxide layers. Besides, the gallery can also accommodate neutral solvent molecules such as water molecules. LDHs have anion-exchange capability since the anions in the interlayer space can be replaced by other anions in the environment. Generally, the anion exchange reaction is governed by a dynamic equilibrium, suggesting that the anions in the gallery cannot be completely removed. It is worthy to note that the propensity for some anions available in solution to exchange with “guest” anions in the LDH structure depends on the charge of the anion involved. When the anions have higher charges, they usually have stronger interaction with the hydroxide layer and therefore it would be harder for them to be replaced by other anions.2 That is the reason why nitrates and chlorides can be easily displaceable from the LDH structure, whereas sulfate and carbonate anions are more difficult to remove by exchange. Therefore, nitrate intercalated LDHs are often used as precursors for preparation of LDHs loaded with more complex anions.

62 63 64 65 66 67 68 69 70

Figure 1 Schematic representation of the LDHs structure LDHs have attracted much attention of the researchers in the field of catalysis, waste water treatment, antifouling and UV blocking.3-8 The development of corrosion protective coatings for metal substrate is of great significance for a wide range of industrial applications and resources saving. In recent years, LDHs have been widely studied in the field of designing protective coatings for metallic materials. Generally, LDHs are applied in corrosion protection field in two forms including powders and 2


Page 2 of 27

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114

films. Many researchers added the inhibitor intercalated LDH powders into organic coatings or concrete with the intention of improving the corrosion resistance of the metals in these systems.9-10 In this case, the aims are two-fold: trapping aggressive chlorides and releasing inhibitor ions based on the anion-exchange capacity. Compared with LDH powders, the in-situ grown LDH films on Al and Mg substrates and their alloys demonstrate better corrosion protection effect due to their strong adhesion with the substrate. In addition to the roles of absorbing chlorides and releasing inhibitor anions, the film itself also acts as a strong physical barrier, impeding the attack of water molecules and aggressive anions. Researchers have intercalated a large variety of inhibitor ions into the LDH films such as vanadates, molybdates, 2-mercaptobenzothiazolates (MBT) and 8-hydroxyquinoline (8-HQ) based on the large interlayer space of LDHs.11-13 The results indicate that the inhibitor loaded LDHs present better corrosion protection effect for Al substrate in comparison with those without them. In addition to intercalating inhibitor anions into LDHs, researchers also modified them with low-surface-energy substance such as fluoroalkylsilane, aliphatic carboxylate or corresponding acid to obtain superhydrophobic surface.14-19 When the superhydrophobic surface was immersed in solution, an “air valley” film was formed on the surface acting as a strong physical barrier against water molecules and aggressive ions. Therefore, the corrosion resistance was dramatically improved in this way. Wang et al. prepared MgAl-LDH on Al substrate and then immersed the synthesized films in sodium oleate solution, sodium laurate solution and sodium stearate solution, respectively. They proposed that the negatively charged aliphatic carboxylate anions adsorbed onto the LDH surface through the electronic interaction. However, they didn’t give enough evidence to verify this point of view.17 Zhou et al. fabricated ZnAl-LDH films on AZ91D magnesium alloys and modified the films by the long chain fatty acid (stearic acid) to lower the surface energy. As a result, the corrosion current density was reduced more than two orders of magnitude after the superhydrophobic treatment, indicating that the corrosion resistance was dramatically improved. However, the superhydrophobicity decreased obviously dramatically under ultraviolet (UV) irradiation.18 Zhang et al. fabricated superhydrophobic LiAl-LDH on Al-Li alloy via in situ growth method and further chemical modification with 1H,1H,2H,2H-Perfluorodecyltrimethoxysilane (PFDTMS) and proposed the possible formation mechanism of the superhydrophobic surface.19 On one hand, the nest-like hierarchical structure constitutes the rough surface. On the other hand, the LDH surface is rich in hydroxyl groups and they can react with the hydroxyl groups of hydrolyzed PFDTMS by a condensation reaction. As a result, the PEDTMS was bonded chemically to the LDH surface. Zhang et al. and Lei et al. prepared superhydrophobic ZnAl-LDH on Al substrate and ZnAlCu-LDH on Cu foil with modification of sodium laurate. The XRD results verified the successful intercalation of laurates into the LDH gallery.15-16 The electrochemical test results demonstrated that the superhydrophobic coatings were able to provide effective corrosion protection for the substrate due to the chemical bonding of the intercalated low-surface-energy species. 3



115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158

It can be summarized that the published work in the field of superhydrophobic LDH films remains an insufficient study on their long-term stability, which is one of the most crucial issues for practical applications. In addition, the properties closely related to practical applications such as chemical stability and mechanical durability were rarely investigated. A detailed summary was made as follows. Firstly, the study on the corrosion resistance effect of such sample was only restricted to a short time of several hours. As a result, the long-term corrosion resistance of superhydrophobic surface remained a huge challenge. It is well known that the result of long-term corrosion resistance is of valuable significance for the practical applications in industry. Along with monitoring the long-term corrosion protection effect of superhydrophobic LDH samples, the recording of the contact angle changes of the immersed surface in NaCl solutions would be helpful to analyse the corrosion protection mechanism. Secondly, a series of important properties closely related to the practical applications of such superhydrophobic LDH surface were not reported in the literature. To be specific, the chemical stability when contacting solutions with various pH values and when it is subjected to heating is worthy of being studied. In addition, mechanical durability against physical damages is also needed to be deeply investigated. Another important property is the resistance against UV light. It’s worthy to note that it is significantly important for a superhydrophobic coating to possess UV-shielding property for practical outdoor use. In the reported literature, a great number of superhydrophobic coatings become superhydrophilic after UV exposure as a result of the chemical changes caused by UV radiation.20-22 The transformation from superhydrophobic state to superhydrophilic state is able to make the metal substrate more susceptible to the attack of water molecules and chlorides, therefore, corrosion can be initiated quickly. If the LDH films can maintain its superhyophobicity for a longer time under UV irradiation, the corrosion resistance can be enhanced greatly due to the air film on its surface as a result of the superhydrophobicity. Therefore, testing the UV-durability of the superhydrophobic LDH films is of great significance for the application of corrosion protection in outdoor environment. In our work, we carried out a comprehensive study of superhydrophobic ZnAl-LDH-La films for corrosion protection in outdoor environment including chemical stability, mechanical durability and long-term corrosion resistance, which have been rarely reported until now. To be specific, we emphasize this work as follows. Firstly, the long-term corrosion protection effect of ZnAl-LDH-La films was extensively investigated. Meanwhile, the changes of contact angle of the superhydrophobic surface were recorded with the evolution of immersion time. The mechanism of corrosion protection was analyzed reasonably based on the combined result of the changes of corrosion resistance and contact angle in a long time up to a month in 3.5 wt.% NaCl solution. This result provided instructive significance for researchers and engineers working in corrosion protection. Secondly, various kinds of damage may be experienced by the superhydrophobic coatings when they are applied in practical environments. Therefore, it is necessary to study the capability of the superhydrophobic surface against harsh conditions including chemical and physical damages. The chemical stability to acid, alkali and a wide variety of common liquid 4


Page 4 of 27

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202

was investigated. Besides, the resistance against higher temperature and UV light was also studied. The result is meaningful to the outdoor application, especially in summer or hot areas where the coatings suffer from serious UV or thermal aging. In addition, the mechanical durability was demonstrated by ultrasonic treatment and tape peeling test since the prepared film would be inevitably subjected to physical scratches or other types of damage in practical application. Apparently, this comprehensive investigation of the capability against various environmental damages of the superhydrophobic ZnAl-LDH-La film is helpful for further exploration in practical applications. 2. EXPERIMENTAL SECTION 2.1 Materials Al (≥ 99.9 wt.%) plates were provided by Huahuibo Platinum Nonferrous Metal Co. Ltd, Shenzhen, China, and they were cut into 2.3 cm × 2 cm × 0.3 cm. Zinc nitrate, ammonium nitrate, sodium hydroxide and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. 25 wt.% NH3·H2O and sodium chloride were supplied by Guangdong Guanghua Sci-Tech Co. Ltd., sodium laurate (La) was ordered from Aladdin Industrial Corporation. All chemicals were used as received without further treatment. All the solutions were prepared with deionized water. 2.2 Pretreatment of Al Plates In a typical procedure, the Al plates were sequentially abraded with emery sand paper of grade 400, 800 and 1200, cleaned in ethanol ultrasonically for 10 min and then dried in air naturally. In order to remove the oxide film on the surface, the abraded Al plates were then etched in 0.1 M NaOH for 120 s at room temperature. 2.3 Synthesis of LDH Films The surface treated Al substrates were vertically immersed in a mixed solution of 0.05 M ZnNO3 and 0.3 M NH4NO3. The pH was controlled to 6.5 by 1.0 wt.% NH3· H2O. The hydrothermal reaction was performed under hydrothermal treatment (85 °C, 12 h). After that, the samples were washed by ethanol and dried in air naturally. The obtained samples were denoted as ZnAl-LDH. 2.4 Chemical Modification In the chemical modification process, sodium laurate was used to lower the surface energy according to the previous literature.15-16 ZnAl-LDH was immersed in 0.05 M aqueous sodium laurate solution for 4 h at 50 °C in the oven. Then the samples were taken out of the solution, rinsed by ethanol and finally dried in air. The final product ZnAl-LDH-La was obtained. Besides, Al plates were immersed in sodium laurate solution for 4 h at 50 °C and the final product was labelled as Al-La for comparison. 2.5 Surface Characterizations The morphology of the surface of different samples was obtained by scanning electronic microscope (SEM, Hitachi FE-SEM 4800). And the element composition was also analysed by energy dispersive X-ray spectroscopy (EDS). A 3D laser scanning confocal microscope system (VK-X200, Keyence, Japan) was used for surface roughness measurements. The (X-ray diffraction) XRD patterns were used to 5



203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246

characterize the crystal structure of the samples. The corresponding conditions of the wide-angle XRD spectra were listed as follows: 40 kV, 30 mA, Cu Kα radiation (λ = 1.5406 Å), and the samples were tested from 5-80° at a scanning rate of 10 °/min. And the small-angle XRD pattern was performed according to the following conditions: 35 kV, 15 mA, Cu Kα radiation (λ = 1.5406 Å), and the samples were tested from 2-10° at a scanning rate of 1 °/min. X’Pert HighScore Version 1.0 was used for data processing. Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR, Nicolet IS10 Thermo Scientific, USA) spectra were obtained in the range of 4000-525 cm-1 at a resolution of 4 cm-1 using 16 scans, and diamond was used as the crystal. The chemical composition of the surface film of ZnAl-LDH-La was characterized by XPS spectrometry (Thermo Fisher 250Xi) with X-ray source of Al Kα (hν = 1486.6 eV). The incident beam diameter used was 500 µm. The analyser was operated in constant pass energy of 20.0 eV. The binding energies were referenced to the C 1s signal at 284.8 eV. And Avantage Version 5.974 software was used for all data processing. The static water contact angle (CA) was measured by a water drop volume of 4 µL with a dosing rate of 1 µL/s using a contact-angle meter (DSA100, Dataphysics, Germany) at ambient temperature. The CA values reported in the form of mean values were calculated from measurements made on five different locations of the same surface. 2.6 Chemical Stability Tests The chemical stability of the fabricated superhydrophobic surface towards droplets at different pH values was studied by dropping aqueous solution with pH values ranging from 1 to 6 and 13. In addition, droplets of tea, coffee, soybean milk were also dropped onto the as-prepared surface to check its stability when contacting different kinds of solutions, and digital images were recorded. 2.7 UV-durability Test A UV chamber equipped with a 40 W ultraviolet (λ = 254 nm) high-pressure mercury lamp (Philips, made in Poland) was used for irradiation. The samples were placed in the UV chamber for up to 7 d and the static CA of water was measured every day. 2.8 Mechanical Stability Tests The mechanical stability of the superhydrophobicity was estimated under ultrasonication (100 W, 99% amplitude) for 10 min. The CA was recorded before and after ultrasonication, respectively. Tape-peeling test was also performed to assess the adhesion to the substrate of the as-prepared superhydrophobic films according to ATSM D3359-97. In a typical procedure, the lattice patterns were obtained by two perpendicular scratches from a special knife (QFH-HG600, Guangdong). The scotch adhesive tape was pressed onto the as-cut surface by fingers for 120 s to ensure good contact, and then it was removed by peeling it off. The digital images and CA before and after peeling were collected by Digital Microscope (Leica DVM6, Germany) and the contact-angle meter, respectively. 2.9 Study on Self-Cleaning Property The self-cleaning property of the as-prepared surfaces was tested by deposition of graphite powder on the as-prepared surface and subsequently cleaned by the 6


Page 6 of 27

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290

directed movement of water droplets using an injection syringe. The digital pictures were recorded in the self-cleaning process. 2.10 Electrochemical Experiments The electrochemical corrosion tests were carried out in a three-electrode cell, where the platinum sheet was used as the counter electrode, the saturated calomel electrode (SCE, 0.242 V vs. SHE) was used as the reference electrode and the prepared sample with LDH film with an exposure area of 1 cm2 was used as the working electrode. The cell was placed in a Faraday cage to avoid external electromagnetic interference in the environment. The samples were immersed in 3.5 wt.% NaCl solution in 1 h to reach a steady state before they were subjected to electrochemical impedance spectra (EIS) test and the polarization test successively. The EIS test was carried out at open circuit potential (OCP) and the selected frequency range was from 100 kHz to 10 mHz with an amplitude of 10 mV. The EIS data were fitted using different equivalent circuits with Zview 3.1. The polarization curves were recorded from - 0.2 V vs. OCP (SCE) to 0.2 V vs. OCP (SCE) at 1 mV/s. All the potentials in this paper are referred to SCE. All the electrochemical tests were repeated at least twice to guarantee the reproducibility. 3. RESULTS AND DISCUSSION 3.1 Characterizations Figure 2 depicts the SEM images and EDS detections of the different samples. It can be seen clearly from Figure 2a that there were many scratches and pits on the abraded Al substrate. After immersion in sodium laurate solution, the samples presented smoother morphology (Figure 2b). The possible reasons may be that the Al surface dissolved in the sodium laurate solution and therefore less scratches and pits can be observed on the surface. When the LDH films grew on the Al substrate, the morphology changed completely. According to Figure 2c, the curved plate-like LDH microcrystals were perpendicular to the substrate and covered the entire substrate surface. This nest-like microstructure is in good accordance with the reported literature.12 After treatment with sodium laurate, the morphology of the ZnAl-LDHs film transformed from packed sheet to flower-like structure. As can be seen from Figure 2d, the platelets of LDH crystallites distributed randomly on the surface. It was reported that the intercalation of laurate into the LDH gallery induced considerable stress in the film, which can be relieved by the formation of the disorder of arrangement on the surface.15-16 The EDS results showed that the abraded Al surface was mainly composed of Al and O, and a small percentage of C was caused by contamination in the air. After the abraded Al was treated with the sodium laurate solution, the content of Al decreased by nearly 30% and the content of C and O increased obviously compared with that of the abraded Al. This result supported the above explanation for the morphology changes. When the ZnAl-LDH film grew on the Al substrate, Zn element appeared with an atomic percentage of 13.51% due to the formation of ZnAl-LDH and also O element increased dramatically. After the ZnAl-LDH film was modified by sodium laurate solution, C increased markedly to an atomic percentage of 75.93%, indicating that laurates were successfully adsorbed or 7



291 292 293 294 295 296 297

298 299 300 301 302 303 304 305 306 307 308 309

bonded with the ZnAl-LDHs. The atomic ratio of C/O was 6:1 in the laurate anions, while the C/O ratio obtained in the EDS spectra of ZnAl-LDH-La was 4.3:1, which was less than 6:1 since oxygen also constituted the LDHs platelets. Figure 2d and e show the side-view of the ZnAl-LDH and ZnAl-LDH-La films, respectively. The thickness of the ZnAl-LDH film was approximately 2 µm. After the intercalation of laurate anions in the gallery, the thickness of the films increased a bit with a approximate value of 2.5 µm.

Figure 2 The top-view SEM images and EDS results of (a) blank Al substrate, (b) Al-La, (c) ZnAl-LDH and (d) ZnAl-LDH-La; The side-view SEM images of (e) ZnAl-LDH and (f) ZnAl-LDH-La The wide-angle and small-angle XRD patterns of the samples are shown in Figure 3a,b and c, respectively. As can be seen from Figure 3a, the peaks at around 39°, 45°, 65° and 78° of all the samples were attributed to the reflection peaks from Al substrate. As the intensity of the peaks from Al substrate was extremely high, the peaks assigned to LDH cannot be seen clearly. Therefore, the magnification of Figure 3a from 5° to 38° is shown in Figure 3b. According to Figure 3b, the peaks at around 26°of all the samples belonged to the reflection peaks of the Al2O3. As can be seen 8


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325

326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346

from the curve a and b in Figure 3a and b, it demonstrated that the peaks of Al substrate and Al-La were at the same position in the XRD patterns, indicating no or only small amount of new species was formed on the surface of Al-La. In curve c of Figure 3b, the peaks observed at 9.9°, 19.9° and 35.1° corresponded to the (003), (006) and (012) plane of ZnAl-LDH, indicating typical layered structure of LDH structures. The basal spacing was 0.893 nm, which can be estimated from the position of (003) plane using Bragg’s law. This gallery height was in good accordance with the reported literature.23 After treatment with sodium laurate solution, the obtained ZnAl-LDH-La showed peaks at 2.6°, 5.2°, 7.8°, 10.4° and 13.0° (Fig.3b curve d and Fig.3c), which can be indexed as the (003), (006), (009), (0012) and (0015) reflections of a LDH phase intercalated with laurates.15-16 The d-spacing of 3.40 nm corresponding to (003) plane was obtained based on the Bragg’s law. This value was in good accordance with the value expected for a bilayer of laurate anions arranged in a tilted orientation within the interlayer.15-16, 24 This result suggests that the laurate anions have been intercalated into the gallery of ZnAl-LDH-La successfully.

Figure 3 (a) and (b)The wide-angle XRD patterns of different samples (curve a: Al substrate, curve b: Al-La, curve c: ZnAl-LDH, curve d: ZnAl-LDH-La, please note that Figure 3b is the magnification of Figure 3a from 5° to 38°), (c) the small angle XRD pattern of ZnAl-LDH-La Figure S1 (Supporting Information) presents the ATR-FTIR spectra of ZnAl-LDH and ZnAl-LDH-La on Al surface, respectively. As can be seen from Figure S1a, a broad peak in the range of 3000 to 3600 cm-1 can be attributed to H-O-H stretching vibration of water molecules in the gallery of ZnAl-LDH. And the peak at 1637 cm-1 corresponded to the bending vibration of water molecules. In addition, the band observed at 1345 cm-1 can be assigned to the stretching vibration of the interlayer NO3- in ZnAl-LDH.25 The peak occurred at 607 cm-1 belonged to Al-O vibration. In the spectra of ZnAl-LDH-La (Figure S1b), the stretching vibration bands at 2918 and 2849 cm-1 can be ascribed to the alkyl C-H groups.17 Additionally, the peaks observed at 1595 and 1409 cm-1 were due to the asymmetric and symmetric vibration of COO- groups.26 This result verified the existence of laurate anions in the interlayer of LDHs structures. Besides, it is worthy to note that the absorption band of NO3- was hardly observed, suggesting that most of the nitrate anions in the gallery were replaced by the laurate anions. Based on the above analysis of the ATR-FTIR results, it can be concluded that laurate anions were successfully intercalated into the 9



347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363

interlayer galleries of the LDH by anion-exchange process. Figure 4 shows XPS results of the ZnAl-LDH-La surface. It can be seen from the survey spectra in Figure 4a that oxygen, carbon, aluminium and zinc are the main composition of the film. The Al 2p spectra displayed only one peak at 74.2 eV, implying the existence of aluminium-containing hydroxides (Figure 4b). In Figure 4c, Zn 2p1/2 and Zn 2p3/2 were located at 1045.0 and 1022.1 eV, which corresponded to the reported data for Zn/Al hydrotalcite.12 The above results verified the existence of hydroxides of aluminium and zinc. Figure 4d is the spectra of C 1s spectra. The fitted peaks at 284.8, 285.1 and 288.4 eV can be attributed to -CH2, -CH3 and the -COOgroup of the laurate anions, respectively.27 The reason why alkyl and carboxylates were detected here is that some laurates adsorbed on the surface. Both the adsorbed and the intercalated laurate anions contributed to the superhydrophobicity of the film. The O 1s mainly consisted of three peaks (Figure 4e). The peak at 531.1 eV was ascribed to the presence of oxygen in the metallic hydroxide formed and the peak at 531.6 eV was due to the C=O bond (Figure 4e),12, 27 which supported the presence of the laurates on the surface. In addition, the peak located at 532.4 eV corresponded to the adsorbed water molecules on the film.12

364 365 366 367 368 369 370 371 372 373 374 375 376

Figure 4 (a) The XPS survey spectra of the ZnAl-LDH-La surface and XPS high-resolution spectra of the ZnAl-LDH-La surface: (b) Al 2p, (c) Zn 2p, (d) C 1s and (e) O 1s The 3D roughness profiles of different samples obtained by laser microscope are shown in Figure 5. The abraded Al substrate showed a surface roughness of 1.380 ± 0.006 µm. And the surface roughness was decreased to 0.836 ± 0.354 µm after treatment with sodium laurate solution. The surface roughness of the ZnAl-LDH and ZnAl-LDH-La increased remarkably to 3.162 ± 0.247 µm and 3.912 ± 0.335 µm, respectively. These results are in perfect agreement with the SEM images. As was stated in the analysis of the SEM images, less scratches and pits were observed on Al surface as it dissolved in the sodium laurate solution, therefore, the average roughness 10


Page 10 of 27

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


377 378 379

lowered slightly. Additionally, the modification of laurates rendered microcrystals randomly present on the surface, leading to the increased roughness of the surface in comparison with that of ZnAl-LDH.

380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

Figure 5 Laser microscopy images of four different samples (a) Al substrate, (b) Al-La, (c) ZnAl-LDH and (d) ZnAl-LDH-La The surface wettability of different samples was evaluated by water contact angle measurements and the corresponding CA results are shown in Figure 6. The CA of the abraded Al substrate was 36.6 ± 0.4°, indicating hydrophilic nature of the abraded Al. After immersion in sodium laurate solution, the obtained Al-La substrate became hydrophobic with a CA of 128.0 ± 0.5° due to the introduction of low-surface-energy substance. When ZnAl-LDH microcrystals grew on the Al substrate, CA decreased dramatically compared with the abraded Al, which can be attributed to the increased roughness of the surface due to the existence of ZnAl-LDH platelets. After the ZnAl-LDH was modified by laurate, the final product of ZnAl-LDH-La demonstrated superhydrophobicity with a CA of more than 150°. The roughness results in Figure 5 strongly support this wettability results. The corresponding theoretical foundation is presented as follows. According to the Wenzel’s equation (eq 1), the relationship between the surface roughness and Wenzel contact angle can be obtained.28 cos (1) Where is the Wenzel contact angle, represents the equilibrium contact angle on the ideal surface, and r is the ratio of the real surface area to the apparent surface area, the so-called surface roughness. Based on the above equation, it can be concluded that increasing the surface roughness will lead to a higher contact angle 11



402 403 404 405 406

when the equilibrium contact angle is more than 90°, and increasing the surface roughness will lead to a lower contact angle when is less than 90°. Comparison of the CA in Figure 6a and c, b and d was in perfect agreement with this conclusion. Besides, the average roughness and CA are intuitively shown in Figure S2 (Supporting Information) by bar graph.

407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426

Figure 6 Shapes of water droplets on the surface of different samples and corresponding water contact angles (a) Al substrate (b) Al-La (c) ZnAl-LDH (d) ZnAl-LDH-La 3.2 Chemical Stability Different types of damage may be experienced by the superhydrophobic coatings when they are applied in practical conditions. Therefore, it is of great significance to study the capability of the superhydrophobic surface against harsh conditions including chemical and physical damages. The thermal stability of the superhydrophobic surface was tested by exposing the samples in 100 °C atmosphere for 8 h (Figure 7a). The CA kept almost unchanged after heating, indicating excellent thermal stability of ZnAl-LDH-La. Besides, UV-durability of superhydrophobic materials is also crucial for their practical outdoor exposure as the decrease of superhydrophobicity caused by UV radiation is able to make the metal substrate more susceptible to the initiation of corrosion. In order to assess the UV-durability of the as-prepared superhydrophobic surfaces, the samples were exposed to UV light for 7 d and the wettability was recorded by videos every day (not shown here). Figure 7b shows the wettability before and after UV irradiation for 7 d. The CA remained nearly stable during 7 d of irradiation, indicating strong resistance to UV irradiation, which is superior to most of the superhydrophobic 12


Page 12 of 27

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


427 428 429 430 431 432 433 434 435

surfaces reported in the literatures.29-30 This may be due to the UV blocking properties of LDH layer.7 On one hand, some UV light can be absorbed by the LDH layer due to the existence of zinc in the hydroxide layer. Lin et al. reported that the extent of UV absorption increased with the increasing amount of zinc in the layers as a result of the decreasing band gap energy, which was confirmed by density functional theory calculations.8 On the other hand, UV radiation can be screened by the crystallite, which can be explained by the Mie scattering theory.8 It can be concluded that the prepared ZnAl-LDH-La has strong resistance against ultraviolet rays and it is quite suitable for outdoor application.

436 437 438 439

Figure 7 The CA changes of ZnAl-LDH-La in two conditions (a) heated at 100 °C for 8 h and (b) UV radiation for 7 d

440 441 442 443

Figure 8 (a, b and c) Digital photos of droplets of soybean milk, tea, coffee, acid (pH from 1 to 6) and alkali (pH = 13) on ZnAl-LDH-La surface; the SEM images of ZnAl-LDH-La (d) as-prepared, (e) after contact with alkali droplet (pH = 13) and (f) 13



444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487

after contact with acid (pH = 1); the EDS results (g, h and i) corresponding to the SEM images (d, e and f), respectively. Moreover, the chemical stability when contacting acid, alkali and a wide variety of common liquid was also investigated. It’s worthy to note that the underlying reasons were also explored carefully as follows. As is shown in Figure 8a, b and c, the soybean milk, tea, coffee, acid (pH from 3 to 6) and alkali (pH = 13) droplets with about 10 µL existed on ZnAl-LDH-La surface with a spherical shape, while acid droplets (pH = 1 and 2) spread with small CA on ZnAl-LDH-La surface. This result indicates that the as-prepared superhydrophobic surface is able to keep superhydrophobic when contacting with various droplets except for acid with a pH lower than 3. In order to analyse the underlying reasons deeply, the SEM images and the corresponding EDS results of ZnAl-LDH-La after contacting with alkali (pH = 13) and acid (pH = 1) were obtained (Figure 8e, f and h, i). The SEM and EDS results of the as prepared ZnAl-LDH-La are also shown for comparison (Figure 8d and g). As can be seen from Figure 8e and h, the surface morphology still presented randomly arranged microcrystals and the element composition was similar to that of the as prepared one. According to this result, it can be concluded that the superhydrophobic ZnAl-LDH-La surface presents amazing stability when contact with concentrated alkali. However, this stability disappeared when the surface contact with concentrated acid (pH < 3). In Figure 8f, the surface morphology changed completely in comparison with that of the as prepared one and the microcrystals cannot be observed. The ZnAl-LDH is actually mainly composed of hydroxide layers and intercalated anions, and the hydroxides may dissolve in the rigorous acid solution. The EDS result in Figure 8i supports this explanation powerfully. As can be seen from Figure 8i, C element decreased significantly and element of Al and O increased considerably, which was caused by the dissolution of the hydroxides in acid and Al substrate was exposed. In addition, the trace existence of Zn element also suggested the dissolution of hydroxides on the surface. 3.3 Mechanical Stability It is well-known that the mechanical stability is highly needed for practical application of materials since this film would be inevitably subjected to physical scratches or other types of physical damages in practical application. Therefore, the mechanical stability of superhydrophobic surfaces should be taken into consideration. Herein, the mechanical durability was demonstrated by ultrasonic treatment and tape peeling test. As can be seen from Figure 9, the mechanical stability was investigated by an ultrasonication process. The CA decreased from 152.7 ± 1.4° to 144.8 ± 6.4° after ultrasonication for 10 min, indicating slight recession of the superhydrophobicity. The SEM images of the sample before and after ultrasonication are shown in Figure 9b and e. It is clear that some slight aggregation appeared on the surface after ultrasonication process in comparison with the as-prepared one. This result suggests that the microstructure can be destroyed slightly by strong vibration. Besides, the EDS results are also present in Figure 9. The element composition remained almost 14


Page 14 of 27

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


488 489 490

491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510

unchanged, indicating the laurate anions were chemically bonded with the gallery of ZnAl-LDH. This result also implies that this film is able to remain superhydrophobic for a long time in above various environments.

Figure 9 The wettability, morphology and element composition of ZnAl-LDH-La (a, b, c) before and (d, e, f) after the ultrasonication process The mechanical stability was also evaluated by Scribe-Grid Test (ASTM D 3359-97). Figure S3 (Supporting Information) shows the optical images of the superhydrophobic surface before and after peeling off the tape. As can be seen clearly from Figure S3b, no delamination or detachment of the film at the edges and within the square lattice was observed. According to ASTM D 3359-97 standard, the adhesion of the film with the substrate is close to the 5A level, indicating good adhesion with the substrate. Additionally, the CA of the surface was changed from 152.7 ± 1.4° to 144.6 ± 3.0°. The slight decrease of CA may be due to slight destruction of the surface by the tape, however, its effect on superhydrophobicity was very slight. Apparently, the above comprehensive investigation of the capability against various damages of the superhydrophobic ZnAl-LDH-La film is considerably helpful to researchers and engineers to get deep insight into its potential for practical applications. 3.4 Self-cleaning Behaviour The self-cleaning behaviour was tested with black graphite powder as the 15



511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527

contaminant. The self-cleaning process was recorded by digital images in Figure S4 (Supporting Information). It is clear that the spherical water droplet began to roll quickly on the slightly tilted surface and removed the graphite powders along its rolling trace. The free energy of the surface modified by laurates was extremely low, therefore, the adhesion between powder and the surface was weak. As a result, the powder can be easily removed by water droplets. The self-cleaning process demonstrated that the dusted surface became completely clean after water droplets rolled down from the surface. This result suggested that the superhydrophobic surface was able to prevent substrates from pollution in practical applications. 3.5 Anti-corrosion Effect The corrosion behaviour of the prepared ZnAl-LDH-La was evaluated by the EIS and polarization measurements. Figure 10 and Figure 11 depict the Nyquist plots and Bode plots of different samples in 3.5 wt.% NaCl solution after immersion for 1 h, respectively. It can be seen clearly from Figure 11a that the low frequency impedance modulus increased considerably for ZnAl-LDH-La compared to other samples. It is worthy to note that the impedance modulus of ZnAl-LDH-La was nearly two orders of magnitude higher than that of the Al substrate at 0.01 Hz.

528 529 530 531

Figure 10 (a) Nyquist plots of different samples in 3.5 wt.% NaCl solution after immersion for 1 h, (b) the magnifying graph of (a)

532 533 16


Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577

Figure 11 (a) Impedance-frequency Bode plots, (b) Phase-frequency Bode plots of different samples in 3.5 wt.% NaCl solution after immersion for 1 h In order to interpret the obtained EIS results clearly, the equivalent circuits are shown in Figure 12. In the case of reference system (Al substrate) and Al-La, only one broad phase angle peak is observed at intermediate frequencies (≈ 100 Hz) in the phase-frequency Bode plots in Figure 11b, which can be attributed to the overlapping of two phase angle peaks. These two phase angle peaks correspond to the aluminium oxide on the metal surface and the corrosion process, respectively. The equivalent circuit in Figure 12a was used to represent the impedance response of this system. In this equivalent, Rs is the solution resistance, Rox and CPEox corresponds to the oxide layer. Rct presents the charge transfer resistance whose value is a measurement of electron transfer across the surface, and CPEdl is the constant phase element instead of the capacitances to demonstrate non-ideal capacitive behaviour of filmed samples. In the case of ZnAl-LDH system, the EIS data can be fitted by an equivalent circuit with three time constants (Figure 12b).31 The time constant at higher frequencies (> 103 Hz) is attributed to the LDH film response, represented by a CPELDHs in parallel with the pore resistance of the LDH film (RLDHs). The second time constant occurring at intermediate frequencies (≈ 100 - 101 Hz), is related to the aluminium oxide, which can be presented by a CPEox in parallel with oxide resistance Rox. And the third time constant (< 10-1 Hz) is associated with electrochemical activity of the corrosion process (CPEdl and Rct). For the sample of ZnAl-LDH-La, the electrochemical behaviour was more complex compared with that of ZnAl-LDH due to the formation of another film (the air valley film) on the superhydrophobic surface. Considering the insulation property of air, a constant phase element CPEair was introduced to characterize the trapped air.32 The corresponding equivalent circuit is shown in Figure 12c. The obtained parameters by fitting the EIS data based on the proposed equivalent circuit are listed in Table 1. As can be seen from Table 1, the value of Rox for the blank Al was extremely small compared to other samples as the aluminium oxide layer on its surface was not compact enough and it suffered serious attack of chloride anions in the solution. As for the sample of Al-La, the value of Rox was larger to some extent compared to that of Al substrate, which may result from the protection effect of laurates. In addition, the value of Rct for Al-La was also slightly larger than that of Al, suggesting the limited protection by modification of laurates for Al substrate. As for ZnAl-LDH-La, the nair was 1 and the Yair was extremely small, indicating that the air trapped in the film behaved as dielectric for a pure parallel plate capacitor, which can inhibit electron transfer between the electrolyte and substrate.32 Besides, the RLDHs of ZnAl-LDH-La was larger than that of ZnAl-LDH to some degree, indicating that the LDH layer of ZnAl-LDH-La presented stronger protective effect for the substrate. And the YLDHs value was remarkably smaller in comparison with ZnAl-LDH. Usually, the YLDHs depicts the rate of water absorption and the hydrolytic stability of the coatings in aggressive solutions.33-34 Based on this result, it can be concluded that the LDH layer of ZnAl-LDHs-La was protected effectively by the air film on the 17



578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602

superhydrophobic surface in 3.5 wt.% NaCl solution. The oxide layer between the Al substrate and the LDH layer is the last barrier between the aggressive medium and the substrate, therefore, the compactness and integrity of this layer are of great importance. When the samples do not undergo corrosion activity, the resistance of the oxide layer can be regarded as one powerful standard to evaluate the protection property of the coatings.33 It is worthy noting that the Rox of ZnAl-LDH-La was approximately one order of magnitude higher than that of ZnAl-LDH, which suggested that the oxide layer was much more compact and was protected greatly by the LDH layer and air film outside. Similarly, the Yox corresponds to the water adsorption and the hydrolytic degradation of the oxide layer. As can be seen from Table 1, the value of Yox of ZnAl-LDH was larger than that of ZnAl-LDH-La, implying the more compact and less porous structure of the oxide layer of ZnAl-LDH-La. And we cannot get the accurate fitting results of Rct since the corresponding time constant did not appear in the Phase-frequency Bode plots (corrosion did not occur at this time). This result demonstrates that the films protected the substrate well from the aggressive medium and the underlying substrate was far from corrosion activities. The ZnAl-LDH films worked as a strong physical barrier, which impeded the attack of water molecules and aggressive chlorides. Also, the ZnAl-LDH was able to absorb some chlorides into its gallery by anion-exchange reaction. As for the ZnAl-LDH-La, the modification of laurates enabled it to become superhydrophobic and an air film existed on its surface, which acted as another physical barrier and prevented the attack of water molecules and aggressive chlorides in addition to the ZnAl-LDH platelets, resulting in a better protection effect for the underlying substrate.

603 604 605 606 607 608

Figure 12 Equivalent circuits used to simulate EIS data of (a) Al and Al-La, (b) ZnAl-LDH and (c) ZnAl-LDH-La after immersion in 3.5 wt.% NaCl solution for 1 h

18


Page 18 of 27

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


609

610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628

Table 1 The fitting parameters from EIS spectra depicted in Figure 10 Parameters Al Al-La ZnAl-LDH ZnAl-LDH-La 2 Rs (Ω cm ) 15.58±0.51 14.98±0.20 16.35±0.35 89.95±4.65 Yair CPEair \ \ \ 1.32±1.26 (10-11Ω-1 cm-2 sn) nair \ \ \ 1.00 RLDHs (kΩ cm2) \ \ 99.49±23.91 121.43±2.38 YLDHs \ \ 14.43±0.60 1.24±0.89 -6 -1 CPELDHs (10 Ω cm-2 sn) nLDHs \ \ 0.91±0.01 0.4780±0.0151 2 Rox (kΩ cm ) 0.14±0.02 10.09±0.06 123.65±19.90 2510.9±1316.2 Yox 9.46±0.50 28.56±2.27 8.19±1.77 7.45±3.47 CPEox (10-6Ω-1 cm-2 sn) nox 0.76±0.01 0.80±0.02 0.95±0.10 0.72±0.17 2 Rct (kΩ cm ) 18.98±0.30 31.55±6.50 * * Ydl 0.33±0.10 2.72±0.33 * * CPEdl (10-5Ω-1 cm-2 sn) ndl 0.77±0.01 0.90±0.03 * * (*) Low frequency data was not well fitted because the parameter is out of the frequency range Figure 13 shows the comparison of the polarization curves of the different samples in 3.5 wt.% NaCl solution after immersion for 40 min. According to Figure 13, the corrosion potential of blank Al substrate was the most negative among all the samples with a high corrosion current density. When Al was treated by sodium laurate solution, the obtained Al-La presented a more positive corrosion potential, however, a similar corrosion current density. This means that modification of laurate cannot improve the corrosion resistance remarkably, which was in good agreement with the EIS result. The ZnAl-LDH samples presented an obviously lower corrosion current density in comparison with Al and Al-La. And the lowest corrosion current density was obtained for ZnAl-LDH-La, suggesting superior ability of corrosion protection. Additionally, the Ecorr shifted to positive direction by nearly 200 mV, due to the air trapped on the film, which can reduce susceptibility of the underlying Al substrate. The corrosion potential (Ecorr), corrosion current density (icorr) and the Tafel constants (βc and βa) were obtained by extrapolating the polarization data by General Purpose Electrochemical System (GPES) equipped with Corrosion Rate Procedure according to the following equation:35-36 .( )

.( )

629

exp

630 631 632 633

The related fitting data obtained by the fitting procedure are shown in Table 2. It can be clearly seen from Table 2 that the corrosion current density of ZnAl-LDH-La was nearly two orders of magnitude lower than that of Al, indicating excellent corrosion protection.

"#

$ − exp

"

$&

19


(2)


634 635 636 637 638

Figure 13 Polarization curves of different samples in 3.5 wt.% NaCl solution after immersion for 40 min Table 2 The electrochemical parameters estimated from polarization data in Figure 13 b

E

samples

639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656

Page 20 of 27

b a

corr

i c

-1

corr -1

-2

/mV(SCE)

/mV dec

/ mV dec

/ µA cm

Al

-937±19

644±246

-336±74

4.45±1.36

Al-La

-736±12

43±10

-290±69

5.13±0.28

ZnAl-LDHs

-832±11

756±187

-422±101

0.117±0.035

ZnAl-LDHs-La

-755±32

323±12

-359±16

0.0674±0.0225

In addition to the above presented EIS and polarization results of ZnAl-LDH-La in a short time (several hours), the long-term corrosion protection effect of ZnAl-LDH-La films was deeply investigated as well since it is of great significance for its industrial application in the corrosion and protection field. Figure 14 presents the Bode plots of various samples at different immersion times up to 28 d in 3.5 wt.% NaCl solution. Obviously, the superhydrophobic ZnAl-LDH-La showed the highest impedance modulus |Z| at low frequency during the whole immersion period of 28 d, suggesting the best corrosion resistance for the underlying substrate in comparison with the bare Al substrate and the sample with ZnAl-LDH films. As shown in Figure 14a, the Bode plots of the bare Al substrate were composed of the capacitive zone (> 10-1 Hz) and the resistance zone (< 10-1 Hz) in the initial immersion period (1 h and 1 d). The presence of the resistive zone indicated that the bare Al substrate could not resist the Cl- attack in the NaCl solution and corrosion activity occurred on its surface.37 The impedance at low frequency increased with the increased immersion time due to the precipitation of corrosion products, which acted as a physical barrier partially against the aggressive anions and water molecules. However, the impedance at 0.01 Hz was still much smaller than that of the substrate with ZnAl-LDH-La film. 20


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700

As for the ZnAl-LDH (Figure 14b), the impedance kept stable up to 21 d in 3.5 wt.% NaCl solution and it decreased significantly after immersion of 28 d. On one hand, the intercalated nitrates could take part in the anion-exchange reaction with chlorides in NaCl solution, resulting in a relatively smaller concentration of chlorides. To prove this, the XRD characterization of the ZnAl-LDH sample after immersion in 3.5 wt.% NaCl soluiton for 2 h was performed and the result was shown in Figure S5 (Supporting Information). The appearance of a new peak at about 11.4°indicated that some chlorides have been adsorbed into the gallery of LDH by anion-exchange between the nitrate and chloride anions.38 On the other hand, the LDH layer itself could act as a strong physical barrier against chlorides. The significant decrease of the impedance may be due to the breakdown of the LDH layer after immersion in the aggressive media for a long time. In Figure 14 c, the impedance at high frequencies in the initial immersion time (1 h and 1 d) was much higher than that in a longer immersion time. After immersion in NaCl solution for more than one day, this impedance value declined due to the gradual escaping of the entrapped air layer on the rough surface. The impedance at low frequency was still much higher than that of the bare substrate and the substrate with ZnAl-LDH films even though it decreased gradually with the increased immersion time. It is worthy to note that the laurate anions intercalated in the gallery contributed to the enhanced corrosion protection in two ways. On one hand, the modification of laurates endowed the film with superhydrophobicity and the trapped air impeded the ingress of water molecules and chlorides, and on the other hand, laurates interacted strongly with the interlayer and it prevented chlorides from attacking the LDH layer. Therefore, it would be hard for the chloride ions to get into the LDH gallery by anion exchange reaction. As is shown in Figure S6 (Supporting Information), the XRD result of ZnAl-LDH-La after immersion in 3.5 wt.% NaCl solution for 2 h was similar to that of the as-prepared ZnAl-LDH-La shown in Figure 3b (curve d), verifying that anion-exchange reaction did not take place and chlorides failed to intercalate into the LDH gallery. Along with the measurement of EIS, the water CA of the superhydrophobic ZnAl-LDH-La was also monitored all the time to estimate its durability. Figure 14d demonstrates that CA changes of the ZnAl-LDH-La in 3.5 wt.% NaCl solution. It can be seen that the ZnAl-LDH-La film was able to maintain its superhydrophobic property within the initial immersion time of 24 h. After that, it dropped gradually, however, it kept above 140° during the immersion of the first week, suggesting excellent stability of the film. Then it continued to decrease until more than 120° after immersion of 28 d. The changes of CA are in good agreement with the impedance changes shown in Figure 14c. The results in Figure 14 present the long-term stability of the fabricated superhydrophobic film, which is superior to the superhydrophobic surfaces reported in the literature.39-40 This excellent long-term stability of the superhydrophobic surface can be ascribed to the stable micro/nano structures on its surface and the laurates in the gallery with stable orientation, where the polar carboxylic group were intercalated into the interlayer by electrostatic interaction and the alkyl chain stretched outside. Base on the above results, it can be concluded that the highly efficient long-term protection effect of ZnAl-LDH-La for the underlying Al substrate was attributed to 21



701 702 703

704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726

the excellent stability of the superhydrophobic surface in 3.5 wt.% NaCl solution for a long time.

Figure 14 Bode plots of (a) bare Al substrate, (b) ZnAl-LDH, (c) ZnAl-LDH-La with different immersion times in 3.5 wt.% NaCl solution and (d) the water contact angle changes of ZnAl-LDH-La with time in 3.5 wt.% NaCl solution Figure 15 presents the schematic illustration of the structure of ZnAl-LDH-La film for understanding its corrosion protection mechanism. As shown in Figure 15, the whole system is composed of five parts, including the corrosive medium, the air film, the LDH layer, the oxide layer and Al substrate. Chlorides act as aggressive anions in the system (3.5 wt.% NaCl solution) by intruding into different protective layers and then attacking the underlying substrate. Additionally, water molecules could diffuse through different protective layers and reach the substrate. The air film, the LDH layer and the oxide layer constitute the whole protective layers, all of which contribute to impeding the invading of water molecules and chlorides. The air valley film exists on the superhydrophobic surface of the LDH layer due to the rough structure caused by the perpendicular growth of the LDH platelets and also the introduction of the low-surface-energy species (laurate anions). This air film is almost insulative, which can prevent the electron transfer between the medium and the substrate. In the LDH layer, the laurate anions are intercalated into the gallery of LDH layer with their long carbon chains stretching outside, repelling the permeation of water molecules and chlorides. In addition, the LDH layer itself could act as another barrier preventing the attack of water molecules and chlorides. Besides, the intermediate oxide film acts as the last barrier between the Al substrate and the corrosive medium. This dense and the 22


Page 22 of 27

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


727 728 729 730

731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761

compact oxide film is of great importance for the protection of the underlying substrate. In summary, it can be concluded that the prepared ZnAl-LDH-La modified by sodium laurate are able to provide efficient protection for the Al metal.

Figure 15 Schematic illustration of the corrosion protection mechanism of the ZnAl-LDH-La film 4. CONCLUSIONS In this work, a laurate intercalated ZnAl-LDH-La was fabricated successfully. The XRD result verified the intercalation of laurate anions into the gallery of the LDH structure. It is revealed that the as-prepared superhydrophobic ZnAl-LDH-La has remarkable chemical stability in various harsh environment (eg. high-temperature, UV irradiation and acidic/alkali solutions), suggesting robustness of the as-prepared surface to chemical damages. The obtained ZnAl-LDH-La presents excellent long-term corrosion protection for the underlying Al substrate in 3.5 wt.% NaCl solution, demonstrating its great potential of practical application in corrosion protection field. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. ATR-FTIR spectra results of ZnAl-LDH before and after treatment by laurates; Bar graph of average roughness and average contact angles of Al, Al-La, ZnAl-LDH and ZnAl-LDH-La; Optical images of ZnAl-LDH-La before and after peeling off the tape; The process of the self-cleaning behaviour of ZnAl-LDH-La sample; The XRD results of ZnAl-LDH and ZnAl-LDH-La after immersion in 3.5 wt.% NaCl solution for 2 h (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Changjian Lin: 0000-0003-0032-8420 23



762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805

NOTES The authors declare no completing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21621091, 21773199) and International S&T Cooperation Program of China (2014DFG52350). REFERENCES (1) Vega, J. M.; Granizo, N.; de la Fuente, D.; Simancas, J.; Morcillo, M. Corrosion Inhibition of Aluminum by Coatings Formulated with Al–Zn–vanadate Hydrotalcite. Prog. Org. Coat. 2011, 70 (4), 213-219. (2) Salak, A. N.; Tedim, J.; Kuznetsova, A. I.; Zheludkevich, M. L.; Ferreira, M. G. S. Anion Exchange in Zn–Al Layered Double Hydroxides: In Situ X-ray Diffraction Study. Chem. Phys. Lett. 2010, 495 (1-3), 73-76. (3) Li, Y.; Zhang, L.; Xiang, X.; Yan, D.; Li, F. Engineering of ZnCo-Layered Double Hydroxide Nanowalls toward High-efficiency Electrochemical Water Oxidation. J. Mater. Chem. 2014, 2 (33), 13250-13258. (4) Tang, Y.; Fang, X.; Zhang, X.; Fernandes, G.; Yan, Y.; Yan, D.; Xiang, X.; He, J. Space-Confined Earth-Abundant Bifunctional Electrocatalyst for High-Efficiency Water Splitting. ACS Appl. Mater. Interfaces 2017, 9 (42), 36762-36771. (5) Yang, M.; Gu, L.; Yang, B.; Wang, L.; Sun, Z.; Zheng, J.; Zhang, J.; Hou, J.; Lin, C. Antifouling Composites with Self-adaptive Controlled Release Based on an Active Compound Intercalated into Layered Double Hydroxides. Appl. Surf. Sci. 2017, 426, 185-193. (6) Guo, X.; Zhang, F.; Peng, Q.; Xu, S.; Lei, X.; Evans, D. G.; Duan, X. Layered Double Hydroxide/Eggshell Membrane: An Inorganic Biocomposite Membrane as an Efficient Adsorbent for Cr(VI) Removal. Chem. Eng. J. 2011, 166 (1), 81-87. (7) Wang, G.; Xu, S.; Xia, C.; Yan, D.; Lin, Y.; Wei, M. Fabrication of Host–guest UV-blocking Materials by Intercalation of Fluorescent Anions into Layered Double Hydroxides. RSC Adv. 2015, 5 (30), 23708-23714. (8) Wang, G.; Rao, D.; Li, K.; Lin, Y. UV Blocking by Mg–Zn–Al Layered Double Hydroxides for the Protection of Asphalt Road Surfaces. Ind. Eng. Chem. Res. 2014, 53 (11), 4165-4172. (9) Cao, Y.; Dong, S.; Zheng, D.; Wang, J.; Zhang, X.; Du, R.; Song, G.; Lin, C. Multifunctional Inhibition Based on Layered Double Hydroxides to Comprehensively Control Corrosion of Carbon Steel in Concrete. Corros. Sci. 2017, 126, 166-179. (10) Tedim, J.; Poznyak, S. K.; Kuznetsova, A.; Raps, D.; Hack, T.; Zheludkevich, M. L.; Ferreira, M. G. Enhancement of Active Corrosion Protection via Combination of Inhibitor-Loaded Nanocontainers. ACS Appl. Mater. Interfaces 2010, 2(5), 1528-1535. (11) Tedim, J.; Zheludkevich, M. L.; Salak, A. N.; Lisenkov, A.; Ferreira, M. G. S. Nanostructured LDH-Container Layer with Active Protection Functionality. J. Mater. Chem. 2011, 21 (39), 15464-15470. (12) Zhang, Y.; Liu, J.; Li, Y.; Yu, M.; Li, S.; Xue, B. Fabrication of Inhibitor 24


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849

Anion-Intercalated Layered Double Hydroxide Host Films on Aluminum Alloy 2024 and Their Anticorrosion Properties. J. Coat. Technol. Res. 2015, 12 (2), 293-302. (13) Wang, L.; Zong, Q.; Sun, W.; Yang, Z.; Liu, G. Chemical Modification of Hydrotalcite Coating for Enhanced Corrosion Resistance. Corros. Sci. 2015, 93, 256-266. (14) Chen, H.; Zhang, F.; Fu, S.; Duan, X. In Situ Microstructure Control of Oriented Layered Double Hydroxide Monolayer Films with Curved Hexagonal Crystals as Superhydrophobic Materials. Adv. Mater. 2006, 18 (23), 3089-3093. (15) Zhang, F.; Zhao, L.; Chen, H.; Xu, S.; Evans, D. G.; Duan, X. Corrosion Resistance of Superhydrophobic Layered Double Hydroxide Films on Aluminum. Angew. Chem. Int. Ed. 2008, 47 (13), 2466-2469. (16) Lei, X.; Wang, L.; Zhao, X.; Chang, Z.; Jiang, M.; Yan, D.; Sun, X. Oriented CuZnAl Ternary Layered Double Hydroxide Films: In Situ Hydrothermal Growth and Anticorrosion Properties. Ind. Eng. Chem. Res. 2013, 52 (50), 17934-17940. (17) Wang, Y.; Zhang, D.; Lu, Z. Hydrophobic Mg–Al Layered Double Hydroxide Film on Aluminum: Fabrication and Microbiologically Influenced Corrosion Resistance Properties. Colloid. Surface. A 2015, 474, 44-51. (18) Zhou, M.; Pang, X.; Wei, L.; Gao, K. Insitu Grown Superhydrophobic Zn–Al Layered Double Hydroxides Films on Magnesium Alloy to Improve Corrosion Properties. Appl. Surf. Sci. 2015, 337, 172-177. (19) Zhang, Y.; Liu, J.; Li, Y.; Yu, M.; Li, S.; Xue, B. A Facile Approach to Superhydrophobic LiAl-layered Double Hydroxide Film on Al–Li Alloy Substrate. J. Coat. Technol. Res. 2015, 12 (3), 595-601. (20) Lai, Y.; Lin, L.; Pan, F.; Huang, J.; Song, R.; Huang, Y.; Lin, C.; Fuchs, H.; Chi, L. Bioinspired Patterning with Extreme Wettability Contrast on TiO2 Nanotube Array Surface: a Versatile Platform for Biomedical Applications. Small 2013, 9 (17), 2945-2953. (21) Zhang, B. Y.; Lu, S. X.; Xu, W. G.; Cheng, Y. Y. Controllable Wettability and Morphology of Electrodeposited Surfaces on Zinc Substrates. Appl. Surf. Sci. 2016, 360, 904-914. (22) Qing, Y. Q.; Yang, C. N.; Sun, Y. Z.; Zheng, Y. S.; Wang, X. D.; Shang, Y.; Wang, L. S.; Liu, C. S. Facile Fabrication of Superhydrophobic Surfaces with Corrosion Resistance by Nanocomposite Coating of TiO2 and Polydimethylsiloxane. Colloid Surf. A 2015, 484, 471-477. (23) Poznyak, S. K.; Tedim, J.; Rodrigues, L. M.; Salak, A. N.; Zheludkevich, M. L.; Dick, L. F. P.; Ferreira, M. G. S. Novel Inorganic Host Layered Double Hydroxides Intercalated with Guest Organic Inhibitors for Anticorrosion Applications. ACS Appl. Mater. Interfaces 2009, 1 (10), 2353-2362. (24) Gerds, N.; Katiyar, V.; Koch, C. B.; Risbo, J.; Plackett, D.; Hansen, H. C. B. Synthesis and Characterization of Laurate-Intercalated Mg–Al Layered Double Hydroxide Prepared by Coprecipitation. Appl. Clay. Sci. 2012, 65-66, 143-151. (25) Zhang, G.; Wu, L.; Tang, A.; Zhang, S.; Yuan, B.; Zheng, Z.; Pan, F. A Novel Approach to Fabricate Protective Layered Double Hydroxide Films on the Surface of Anodized Mg-Al Alloy. Adv. Mater. Interfaces 2017, 4 (12), 1700163. 25



850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893

(26) Hang, T. T. X.; Truc, T. A.; Duong, N. T.; Vu, P. G.; Hoang, T. Preparation and Characterization of Nanocontainers of Corrosion Inhibitor Based on Layered Double Hydroxides. Appl. Clay. Sci. 2012, 67-68, 18-25. (27) Zhu, J. Y.; Wan, H. R.; Hu, X. F. A Rapid One-Step Process for the Construction of Corrosion-Resistant Bionic Superhydrophobic Surfaces. Prog. Org. Coat. 2016, 100, 56-62. (28) N. Wenzel, R. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988-994. (29) Li, L. J.; Huang, T.; Lei, J. L.; He, J. X.; Qu, L. F.; Huang, P. L.; Zhou, W.; Li, N. B.; Pan, F. S. Robust Biomimetic-Structural Superhydrophobic Surface on Aluminum Alloy. ACS Appl. Mater. Interfaces 2015, 7 (3), 1449-1457. (30) Wang, N.; Xiong, D. S.; Deng, Y. L.; Shi, Y.; Wang, K. Mechanically Robust Superhydrophobic Steel Surface with Anti-Icing, UV-Durability, and Corrosion Resistance Properties. ACS Appl. Mater. Interfaces 2015, 7 (11), 6260-6272. (31) Tedim, J.; Zheludkevich, M. L.; Bastos, A. C.; Salak, A. N.; Lisenkov, A. D.; Ferreira, M. G. S. Influence of Preparation Conditions of Layered Double Hydroxide Conversion Films on Corrosion Protection. Electrochim. Acta 2014, 117, 164-171. (32) Wang, P.; Zhang, D.; Qiu, R.; Hou, B. Super-Hydrophobic Film Prepared on Zinc as Corrosion Barrier. Corros. Sci. 2011, 53 (6), 2080-2086. (33) Yu, M.; Liang, M.; Liu, J.; Li, S.; Xue, B.; Zhao, H. Effect of Chelating Agent Acetylacetone on Corrosion Protection Properties of Silane-Zirconium Sol–Gel Coatings. Appl. Surf. Sci. 2016, 363, 229-239. (34) Yasakau, K. A.; Tedim, J.; Zheludkevich, M. L.; Ferreira, M. G. S. Active Corrosion Protection by Nanoparticles and Conversion Films of Layered Double Hydroxides. Corrosion 2014, 70 (5), 436-445. (35) Wang, R.; Luo, S.; Liu, M.; Xue, Y. Electrochemical Corrosion Performance of Cr and Al Alloy Steels Using a J55 Carbon Steel as Base Alloy. Corros. Sci. 2014, 85, 270-279. (36) Zhou, X.; Yang, H.; Wang, F. Investigation on the Inhibition Behavior of a Pentaerythritol Glycoside for Carbon Steel in 3.5% NaCl Saturated Ca(OH)2 Solution. Corros. Sci. 2012, 54, 193-200. (37) Ramezanzadeh, B.; Niroumandrad, S.; Ahmadi, A.; Mahdavian, M.; Moghadam, M. H. M. Enhancement of Barrier and Corrosion Protection Performance of an Epoxy Coating through Wet Transfer of Amino Functionalized Graphene Oxide. Corros. Sci. 2016, 103, 283-304. (38) Tedim, J.; Kuznetsova, A.; Salak, A. N.; Montemor, F.; Snihirova, D.; Pilz, M.; Zheludkevich, M. L.; Ferreira, M. G. S. Zn–Al Layered Double Hydroxides as Chloride Nanotraps in Active Protective Coatings. Corros. Sci. 2012, 55, 1-4. (39) She, Z. X.; Li, Q.; Wang, Z. W.; Li, L. Q.; Chen, F. N.; Zhou, J. C. Novel Method for Controllable Fabrication of a Superhydrophobic CuO Surface on AZ91D Magnesium Alloy. ACS Appl. Mater. Interfaces 2012, 4 (8), 4348-4356. (40) Wu, L. K.; Zhang, X. F.; Hu, J. M. Corrosion Protection of Mild Steel by One-Step Electrodeposition of Superhydrophobic Silica Film. Corros. Sci. 2014, 85, 482-487. 26


Page 26 of 27

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


894 895

TOC graphic

896 897 898 899 900 901 902 903 904 905 906 907

27


Enhanced Corrosion Resistance of Superhydrophobic Layered

Recommend Documents