Fabrication of inherent anti-corrosion superhydrophobic surfaces on

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Fabrication of inherent anti-corrosion superhydrophobic surfaces on metals Tengfei Xiang, Yang Han, Zongqi Guo, Rong Wang, Shunli Zheng, Sheng Li, Cheng Li, and Xianming Dai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00639 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Sustainable Chemistry & Engineering

Title: Fabrication of inherent anti-corrosion superhydrophobic surfaces on metals Author names and affiliations: Tengfei Xianga,b, Yang Hanb, Zongqi Guob, Rong Wangb, Shunli Zhengc, Sheng Lid, Cheng Lia*, Xianming Daib* a

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, 29 Jiangjun Avenue, Jiangning District, Nanjing, Jiangsu, 211106, P. R. China b

Department of Mechanical Engineering, University of Texas at Dallas, 800 W.

Campbell Rd., Richardson, TX 75080, USA c

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, 639798, Singapore d

Department of Physics, University of Texas at Dallas, 800 W. Campbell Rd.,

Richardson, TX 75080, USA Corresponding Author: Professor Cheng Li: Tel: +86-25-52112902 Email: [email protected] Professor Xianming Dai: Tel: +1-972-883-5449 Email: [email protected] Abstract: The corrosion of metal materials is detrimental to petroleum pipelines, marine ships, heat exchangers, et al. To date, existing technologies have focused on improving

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corrosion resistances, such as anodization and painting. Superhydrophobic surfaces, which can retain air gaps between the corrosive liquids and solid structures, have shown effective corrosion prevention, but the inherent material property is not corrosion resistant. Herein, we reported a novel superhydrophobic surface with excellent corrosion resistance by using electroplating method. The superhydrophobic surface consists of Zn-Ni and Zn-Co alloys, which are inherently corrosion resistant. We compared such a newly developed surface with superhydrophobic NiO/ZnO and ZnO coatings, respectively. The water contact angle of the novel superhydrophobic coating was larger than 150° after immersion into NaCl solution for 48 hours, while the other two coatings lost the superhydrophobicity. Furthermore, electrochemical measurement results demonstrated that our newly developed superhydrophobic coating exhibited the highest corrosion potential (-0.777 V) and lowest corrosion density (1.905×10-6 A cm-2). We envision that the inherently anti-corrosion superhydrophobic coating can effectively prevent the corrosion of industrial metals.

Keywords:

superhydrophobic

surface,

wettability,

prevention.


electroplating,

corrosion

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Introduction The degradation of metal and its alloys caused by chemical or electrochemical corrosion leads to detrimental impacts on the world’s economic and ecological system. Corrosion costs more than 4 trillion dollars a year globally1. For this aspect, a protective coating with liquid barrier property is commonly used for corrosion prevention. In the past decades, chromate coating was considered as one of the most effective coating to protect metals. However, the hexavalent chromium has been restricted worldwide due to its toxic and carcinogenic nature2. Therefore, scientists developed some other coatings as alternatives of the chromate coating. Mondal et al. designed a thin ceramic-graphene nanolaminate corrosion protection coating which consists of aluminum and titanium oxides, and this coating was deposited by atomic layer deposition (ALD) onto a thin reduced graphene oxide (rGO) nanoplatelet3. Vakili et al. fabricated an epoxy/polyamide coating based on cerium-based conversion film to improve the corrosion performance of coating4. Tran et al. presented a coating combined an electrodeposited capsular nanocomposite coating with an organic coating containing pH-responsive capsules for autonomous corrosion protection5. However, these methods are cost consuming, process complicating, and sophisticated instruments needed. In recent years, superhydrophobic surfaces (SHS), with the water contact angle (WCA) higher than 150° and contact angle hysteresis lower than 10° has developed rapidly, and its special wettability can broaden the applications of engineering metal surfaces6-11. By combining rough surface structure and low surface energy layer, SHS can trap a large amount of air pockets within the surface textures, which greatly reduced the contact area between corrosive liquids and solid textures12-14. In addition, the low surface energy layer further isolates the underneath metal from the external corrosive liquids15,16. Therefore, researchers fabricated SHS on metals to mitigate corrosion. Zhang et al. showed that the intercalation of laurate anions within Zn-Al layered double hydroxide film on Al substrate can obtain a SHS, which provided an excellent corrosion resistance for the underlying Al15. Weng et al. prepared a novel nanostructured electroactive polymer surface which mimicked from “Xanthosoma ACS Paragon Plus Environment


Sagittifolium-leaves” revealed enhanced corrosion protection for cold-rolled steel17. However, the layered double hydroxides only can fabricated on several kinds of metals, and the polymer surface could not offer a long-term stability. Thus, a coating can fabricated on any metal substrates with long-term stability is urgent to be developed. Qing et al. fabricated a rough structure by Zn electroplating, which is inherently protect steel by sacrificing itself firstly based on its lower potential. Besides, it further coated with fluorinated TiO2 nano particles for a better anti-corrosion performance18. Su et al. deposited Ni coating on Cu substrate on account of Ni can inherently protect substrate with a higher potential to make corrosion happens more difficult, and after assembling with low surface energy component, it can enhance the corrosion resistance with superhydrophobicity19. Li et al. prepared Co coating which owns similar properties with Ni on Mg alloy by electroplating, showing good chemical, mechanical stability and outstanding corrosion protection20. Even though all the above metals can provide anti-corrosion property, a Zn-Ni and Zn-Co coating can supply better anti-corrosion performance than unitary Zn, Ni and Co coating ascribed to the dense and flat structure as well as the synergistic effect of Zn, Ni and Co elements21-23. Nevertheless, a flat Zn-Ni or Zn-Co coating is hard to evolve to SHS to improve its anti-corrosion performance. Herein, a novel nano-porous Zn-Ni-Co SHS was developed on mild steels (MS) through electroplating together with surface modification of myristic acid. Two different kinds of coatings (porous NiO/ZnO and nanowire ZnO) which were both containing Zn element were also prepared to make comparisons with the as-prepared Zn-Ni-Co SHS in anti-corrosion and the stability property. Additionally, the self-cleaning property of the as-prepared SHS was examined. Experimental section Materials and Chemicals Mild steel (composition in wt. %: C ≤ 0.1, Mn ≤ 0.5, P ≤ 0.035, S ≤ 0.035, with the rest being Fe) was cut into 50 mm × 20 mm × 1 mm as substrates. Because of the different potential of Zn and Ni, using a Zn, Ni or Zn-Ni alloy anode would lead to a change of ions concentration in plating bath. Thus, a titanium mesh coated with ACS Paragon Plus Environment

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ruthenium oxide was used as inert anode for Zn-Ni electroplating to avoid this drawback. The composition of the plating bath is given in our previous work24. All the chemicals

including

zinc

nitrate

tetrahydrate

[Zn(NO3)2·6H2O],

hexamethylenetetramine (C6H12N4), cobalt sulfate heptahydrate (CoSO4·7H2O), sodium sulfate (Na2SO4), sodium chloride (NaCl), myristic acid [CH3(CH2)12COOH, 95 %] and ethanol (C2H5OH) were purchased from Sigma-Aldrich and used without any further treatment. Sample preparation Preparation of Zn-Ni-Co coating Before Zn-Ni electroplating, MS was immersed into the mixed solution of 35 g·L-1 Na2CO3, 25 g·L-1 NaOH and 35 g·L-1 Na3PO4 at 75 °C for degreasing. Afterward, MS was immersed into 125 ml·L-1 HCl solution for 5 min to remove the possible metal oxide and the protective Zn coating. Subsequently, it was activated in 30 ml·L-1 HCl solution for dozens of seconds and dried in air. After that, the pretreated MS was electroplated in the Zn-Ni plating bath under 4 A·dm-2 for 20 min. At last, the Zn-Ni-Co coating was obtained by immersing the Zn-Ni coating in 0.1 M CoSO4·7H2O for 10 min at 25 °C, and then dried in air. For a succinct description, Zn-Ni and Zn-Ni-Co coating were referred as ZN and ZNC coating, respectively. Preparation of porous NiO/ZnO coating The fabrication procedure was followed by the previous report25. Briefly, the resulting ZN coating was immersed into 0.01 M NiCl2·6H2O at 60 °C for 60 min, then rinsed by deionized water and dried in air. Finally, the porous NiO/ZnO coating was annealed at 160 °C for 150 min in oven. These samples were called NZ here. Preparation of ZnO nanowire coating For the fabrication of ZnO nanowire, a ZnO seed layer needs to be coated on ZN coating firstly as growth nucleus. Here a magnetron sputter system (AJA 1500-F) was used to deposit nano ZnO seed on ZN coating at 100 W for 15 min with argon (Ar) flows. Then equimolar (50 mM) solutions of Zn(NO3)2·6H2O and C6H12N4 were used to grow ZnO nanowire in an autoclave at 90 °C for 240 min26,27. Finally, the sample were taken out from the autoclave, sonicated for several seconds and dried in air. ACS Paragon Plus Environment


Lower the surface energy and endowing the superhydrophobicity All the samples were immersed into 0.01 M myristic acid ethanol solution for 120 min at ambient temperature and then dried in air. The schematic of the preparation process was shown in Fig. 1.

Figure 1. Schematic image of the preparation processes of ZNC coating, NZ coating and ZnO coating, respectively. All the coatings were based on ZN coating. Characterization Field emission scanning electron microscope (FESEM, ZEISS SUPRA40) was utilized to characterize the surfaces morphologies. Phase composition and crystal structures were investigated by X-ray diffraction (XRD, Rigaku Smartlab) using filtered Cu Kα as a radiation source at a scanning rate of 5°/min from 10° to 90° of 2θ. The WCA and sliding angle (SA) of samples were detected via a contact angle meter system (Model 500, Rame-hart) based on a sessile drop measuring method with a water droplet volume of 4 µL at room temperature. The data were obtained by taking the average of measurements on five different points for each sample. All the electrochemical measurements were carried out in 3.5 wt. % NaCl solution (pH = 5.9) at room temperature by an electrochemical workstation (CHI 750C). Measurements were conducted in three-electrode cell with a saturated calomel ACS Paragon Plus Environment

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electrode (SCE) as the reference electrode, a platinum (Pt) electrode as the counter electrode, and the sample with an exposed area of 1 cm × 1 cm as the work electrode. Before electrochemical measurements, all the samples were immersed in the 3.5 wt. % NaCl solution for 60 min to obtain a stable open circuit potential (OCP). The electrochemical impedance spectroscopy (EIS) was performed at OCP in the range from 10−2 Hz to 105 Hz with a sinusoidal signal perturbation of 5 mV. The corrosion potential (Ecorr) and corrosion current density (Icorr) was obtained by the Tafel extrapolation method from the polarization curves at a scan rate of 2 mV·s−1. Results and discussion Surfaces characterization SEM images of different coatings were shown in Fig. 2. All the coatings are modified by myristic acid. The myristic acid only adds a subnanometer thick on the original surface that the change of surface morphologies can be neglected28,29. The morphology of ZN showed very dense and angular grained in Fig. 2. Because of the special structure, the WCA of ZN coating after modification is as large as 110 ± 3.2°. However, both the ZNC and NZ coatings exhibited porous structure. The pores of ZNC coating were irregular and showed different depth, resulting in a rougher structure and large specific surface area compared with that of ZN coating. There is no doubt that the porous structure of ZNC coating was generated by the chemical replacement reaction. In addition, in Fig. 2 (c), the NZ coating showed a “flower-like” structure was produced by the dehydration process. Besides, the ZnO coating grew in autoclave showed an ordered nanowire structure with a diameter of 50 ~ 100 nm in Fig. 2 (d). The gap between the nanowires is about 50 nm. The WCA of ZNC coating was 155.1°, which is less than those of NZ coating (158.1°) and ZnO coating (157.7°). This could be related to the surface roughness because the chemical modification of organic matters is same.



Figure 2. FESEM images of hydrophobic (a) ZN coating, (b) ZNC coating, (c) NZ coating and (d) ZnO coating. The insert figures at top right corner are the large magnification images while the left corner figures are the corresponding WCAs. To identify the composition of coatings, XRD technique was utilized to characterize the crystal structure. As shown in Fig. 3, the basic coating exhibited two strong peaks in 2θ of 42.7° and 78.7° which corresponding to the peaks of (411) and (721) planes of γ-Ni5Zn21 (JCPDS No. 06-0653). It demonstrated that the ZN coating has been successfully deposited on MS. As for the magenta line in Fig. 3, the peak in 34.4° (002) and 36.2° (101) as well as 76.9° (202) could be indexed as the hexagonal wurtzite structure of ZnO which was consistent with the standard card (JCPDS No. 36-1451). And the peak in 39.1° represented a new phase of ZnO (JCPDS No. 21-1486) which must be produced by the magnetron sputter system, as for the wurtzite structure which is the thermodynamically most feasible form of the hydrothermal anisotropic growth30, 31. The dark cyan line displayed lots of weak peaks lower than 30° which is owing to Ni(OH)2 and Zn(OH)2 produced by immersing the ZN coating into NiCl2·6H2O solution, and NiO and ZnO generated after annealing for 2h. The top red line stands for peak of ZN coating reacted by CoSO4·7H2O. A new peak appeared in 35.1° which matches well with the characteristic peak of CoZn13 (JCPDS No. 29-0523). Moreover, the peak in 42.8° changed to 42.6° indicating that the Ni5Zn21 transferred to NiZn3 (JCPDS No.47-1019) after replaced with Co2+. The decrease of Zn element in Zn-Ni alloy and the generation of CoZn13 must be responsible for the formation of porous ZNC coating, which was agree well with the ACS Paragon Plus Environment

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SEM morphologies.

Figure 3. XRD patterns of different coatings. XRD patterns of samples after immersion Most of the superhydrophobic porous surfaces cannot withstand liquid pressure for a long time and the surface would be damaged. Therefore, the XRD technique was utilized to analyze the surface components of the immersed coatings (Immersed in 3.5 wt. % NaCl solution for 48 h). As shown in Fig. 4, a strong peak of Zn(OH)2 (JCPDS No. 20-1435) in low degree domain can be detected from the NZ, ZnO and ZN coatings according to XRD patterns, which means the coatings were corroded severely. Moreover, Fe(OH)3 (JCPDS No. 38-0032) can be also detected in these coatings, revealing that the substrate was corroded. However, only a weak peak of Zn(OH)2 can be found in low degree region from the ZNC coating. Therefore, it can be concluded that the ZNC coating showed a better stability in NaCl solution even though a slight Fe(OH)3 (JCPDS No. 22-0346) peak in 62.7° and a Zn(OH)2 (JCPDS No. 20-1437) peak in 66.0° can be seen.



Figure 4. XRD patterns of samples after immersion in 3.5 wt. % NaCl solution for 48 h at room temperature. Surfaces wettability To confirm the effect of myristic acid on the wettability, the CAs of samples before and after modification were both tested to make a comparison. In Fig. 5, the CA of ZN coating is about 35° and raise to 110° after modification with myristic acid. The CA of ZNC coating was also changed from 24.5° to 155.1° after modification, the ZNC coating transferred from hydrophilicity to superhydrophobicity due to the combination of rough structure and low surface energy14. The decrease of CAs from 35° of ZN to 24.5° of ZNC coating can be explained using Wenzel model, namely, the hydrophilicity of intrinsic hydrophilic surfaces is enhanced by surface roughness increasing32,33. Furthermore, based on the sharply increased CAs of above two modified samples, it can be concluded that the myristic acid can lower the surface tension. For the nanowire ZnO, the CA is dramatically increased from 0° (superhydrophilic) to 157.7° after modification. Besides, all the SAs of the superhydrophobic ZNC, NZ and ZnO coatings were lower than 10° and not showed here. In addition, the WCAs of the superhydrophobic samples after immersed into 3.5 wt. % NaCl for 48 h were also detected, which is helpful to understand the anti-pressure and anti-corrosion property. As can be seen in Fig. 5, the CAs of ACS Paragon Plus Environment

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samples were all decreased after immersion into 3.5 wt. % NaCl solution for 48 h at room temperature. The WCA of ZNC was only decreased by 4.4° and still showed superhydrophobicity.

However,

the

NZ

coating

and

ZnO

lost

their

superhydrophobicity. The WCA of NZ coating decreased with 50.5° may be related to the less depth of pores as compared with that of ZNC coating. Thus, the trapped air can be easily pressed out under liquid pressure for a long time. With regard to the ZnO nanowire, the area of trapped air must be occupied by liquid in a certain extent and transferred from Cassie state to Wenzel state after immersion into liquid solution. The above results are consistent with the observation of XRD results that the ZNC coating can remain its crystal structure after immersion in NaCl solution.

Figure 5. The CAs of samples before and after modification as well as after immersion into 3.5 wt. % NaCl for 48 h, the WCA of ZnO nanowire is 0 here. Chemical stability and durability Coatings that can resist not only water but also other chemicals can be used in a wide range of applications. Here NaCl and Na2SO4 were used as corrosion mediums to estimate the chemical stability of coatings. It can be found that all the CAs of coatings still showed superhydrophobicity except the ZN coating, as shown in Fig. 6 (a). Another interesting finding was that the CAs of NaCl and Na2SO4 droplets on these coatings had a slight decrease. The CAs of NaCl droplets on these four kinds of coating (ZNC, NZ, ZnO and ZN) were declined for only 1°, 1.8°, 1.5° and 1° compared with that of WCAs, and the CAs of Na2SO4 on these coatings were lowered



for 0.8°, 1.3°, 1.1° and 0.5°, respectively. This phenomenon may be related to the specific adsorption of anions34,35. The Cl- has a strong specific adsorption at the interface of coating and solution while the SO42- owns a weak specific adsorption. Thus, the CA of NaCl droplets showed the minimum value. It was also confirmed that Cl- could corroded the coatings on the above section. Moreover, the electrical conductivity of these saline solutions may also contribute to this phenomenon. Even so, all the SHSs can repel these solutions effectively. To further investigate the chemical stability, solutions with different pH values were used here (adjusted by HCl and NH3·H2O). As depicted in Fig. 6 (b), both NZ and ZnO nanowire coatings showed slight decrease in CA in low pH and high pH regions while the ZNC showed a quite stable CA in a large pH region. The reason is that ZnO can react with acidic and basic solutions, resulting in reduced CAs on NZ and ZnO coatings. On the contrary, the ZNC coating was composed of Zn-Ni and Zn-Co alloys, which can be stable in harsh environment. To evaluate the long-term durability of samples, the CAs of the samples exposed in ambient environment were tested. From Fig. 6 (c), it can be observed that the CAs of samples were all decreased with the exposure time increasing. And after exposure for 4 weeks, ZnO coating lost its superhydrophobicity with a CA about 145.3°. That’s because the myristic acid decoration on surfaces was just bonded to the surface via a bidentate interaction of carboxylate with two oxygen atoms and would degraded gradually with the exposed time36. It is worth noting that ZNC showed the largest CA as compared with the other coatings. Furthermore, Fig. 6 (d) showed that the ZNC coating existed excellent self-cleaning property even after exposure for 4 weeks in ambient environment. Carbon black was spread on the surface and 20 µL water droplet dropped on the surface with a tilt angle about 6º, the carbon black were slide off with the droplet. Moreover, in Fig. 6 (d), the uncoated area was corroded with rust and the coated area was completed. This phenomenon further confirmed the superior stability and long-term durability of ZNC coating.


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Figure 6. (a) The CAs of SHSs in different chemicals (water, 3.5 wt. % NaCl, and 3.5 wt. % Na2SO4); (b) Droplets with different pH values on the SHSs; (c) WCA of SHSs with different rest times; (d) The self-cleaning test of ZNC after exposed in ambient environment for 4 weeks. Mechanical stability Furthermore, the coatings must be able to withstand surface wear while applied during its lifetime service. Mechanical abrasion test is one of an effective method to evaluate the mechanical stability of SHS. Here a 500 g weight was placed on a sandpaper of 1200 grit and samples were set under the sandpaper. Then pull the sandpaper in one direction for 5 cm one time at a constant speed of 1 cm/s. The schematic figure is shown in Fig. 7 (a). It can be seen in Fig. 7 (b), after abrasion for 20 times, the ZNC coating and NZ coating still showed superhydrophobic with WCA of 152.5° (decreased for 2.6°) and 152.4° (decreased for 5.6°) respectively. Nevertheless, the WCA of ZnO coating decreased to 148.5° and lost the superhydrophobicity. And after abrasion for 50 times, the WCA of ZNC and NZ as well as ZnO coating decreased to 147.6°, 145.4° and 135.6°, respectively. The above results demonstrated that the ZNC coating exhibited the best anti-abrasion



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performance and the porous ZNC is more stable than ZnO nanowire structure in keeping air. That’s because ZNC coating was composed of Zn-Ni and Zn-Co alloy which possess a strong mechanical stability in nature compared with metal oxide22,37-40. With respect to the strong stability of porous structure, it can be explained by two reasons. One is that the structure itself can support each other when withstanding abrasion. The other one must be relateded to the semi-closed system and exhibit a higher internal pressure while abrasion, thus it can keep air within the structure better compared with the open system of nanowire41,42.

Figure 7. (a) The schematic image of abrasion process for moving with 5 cm in a direction of one time. (b) The CAs of samples after withstanding the abrasion process for 0, 20 and 50 times. Corrosion resistance The

corrosion

resistance

of

samples

was

quantitative

measured

by

potentiodynamic polarization (Tafel) plots. The corrosion potential (Ecorr) and corrosion current density (Icorr) were measured by extrapolation method, the ordinate and the abscissa of the intersection of anodic slope (βa) and the cathodic slope (βc) represented the Icorr and Ecorr values, respectively43. As can be seen in Fig. 8 (a) and Table 1, the ZNC coating displayed the maximum anodic slope and minimum cathodic slope, which means the current density was changed slowly with the increased potential in cathodic branch44. Besides, the cathodic slope value showed lower than that of anodic slope. This implied that the samples were protected from the hydrogen evolution reaction45. In such curves, a more positive Ecorr corresponds to a lower corrosion probability, while the Icorr is a measurement of the corrosion rate46.


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The Ecorr of ZN coating is -0.952 V and the Icorr is 2.987×10-5 A·cm-2. The Ecorr of ZNC and NZ coating becomes more positive (-0.782 V and -0.777 V) and the corresponding Icorr reduced to 1.905×10-6 A·cm-2 and 3.425×10-6 A·cm-2, respectively, which means that the superhydrophobic ZNC and NZ coating reduce both the corrosion probability and the corrosion rate47. The Ecorr of ZnO coating was also increased as compared with that of ZN coating. However, its anti-corrosion performance is limited comparing with that of ZNC and NZ coatings. This may be related to its high activity which can easily generate Zn(OH)2 in NaCl solution even though it showed superhydrophobicity, and the analogous conclusions were obtained in XRD patterns. The mechanism of the excellent corrosion resistance of ZNC coating and NZ coating may be related to the following reasons. The first is that the trapped air among the porous structure can provide an effective protection to the coating from being attacked by chloride48. In addition, the air gap formed among the structure could repel the corrosion ions into coatings by Laplace pressure49. Furthermore, the Zn-Ni and Zn-Co alloy can protect substrate better than NiO and ZnO as well as Zn(OH)2 coatings based on the outstanding stability of metal alloys21,22,50. Thus, the superhydrophobic ZNC coating can provide the best corrosion prevention performance. As a powerful and complementary electrochemical technique, EIS was also used to evaluate the corrosion performance of coatings. Fig. 8 (b) showed the Nyquist plots of coatings, the insert figure is the amplified diagram in the high frequency region. The semicircle of Nyquist plots in high frequency is ascribed to the interfacial charge transfer reaction45. The results indicated that the superhydrophobic coatings provide more charge transfer resistance compared with ZN coating. And in mid frequency range, the ZnO coating must be corroded by corrosive ions and showed a corrosion resistance almost equally to that of the ZN coating. In low frequency range, the superhydrophobic ZNC coating still showed a large semicircle, revealing an excellent anti-corrosion performance. However, the NZ coating brought out an inductance loop in low frequency range. This may be related to the pitting corrosion. The corrosion products dissolved during the electrochemical reaction process and lead to substrate ACS Paragon Plus Environment


exposure in solution, and then corrosive ions can penetrate into substrate though these pits51. As a whole, a larger radius of the semicircle indicates a better corrosion resistance of the coatings. In addition, the corresponding impedance modulus and phase angle plots were shown in Figs. 8 (c) and (d). It can be seen that the module value of NZ coating was decreased in low frequency region, which was coincided with the above corrosion hypothesis. And the value of ZNC (2.28 × 105 Ω cm2) is higher than that of ZN (2.45 × 103 Ω cm2), ZnO (2.61 × 103 Ω cm2) and NZ (6.65 × 104 Ω cm2) coatings in 10-2 Hz. Besides, ZN coating showed a higher corrosion resistance than that of ZnO coating at medium frequency domain may be the result of the corrosion products covered on the ZN coating. Furthermore, in Bode-Phase angle plots, the ZNC coating shows a high and wide phase angle in mid frequency region, indicating a better corrosion resistance52. Both Bode-Module and Bode-Phase angle plots agree well with the results obtained from the above polarization plots. In such a case, three different equivalent circuit models can be used to fit the EIS plots for a better understanding of corrosion process. The model in Fig. 8 (e) can fit well with the EIS plots of ZN coating. Where Rs is the electrolyte resistance, Rc is the resistance of coating, Cdl means the double layer capacitance between the coating and electrolyte, Rct represents the charge transfer resistance between the coating and substrate, CPEc denotes the constant phase element modeling the coating53. The EIS plots of ZnO and ZNC coating can be fitted using the model in Fig. 8 (f). Ra and Ca correspond to the resistance and capacitance of air which is trapped within the structures. Cc||Rc are the capacitance and resistance of coatings. Rct and Cdl present the same function of that in model (e). The different circuit element between Fig. 8 (g) and Fig. 8 (f) is the inductive impedance (L) which is shown in this model and can match well with the EIS plots of NZ coating. The obtained ZNC coating shows the porous structure and the existence of air/liquid interface can reduce the corrosion which is consistent with the observation from XRD patterns.


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Figure 8. The electrochemical measurements obtained in 3.5 wt. % NaCl solution. (a) Tafel plots of ZN coating, superhydrophobic ZNC, NZ, ZnO coatings, (b) Nyquist plots of ZN coating and the superhydrophobic coatings, the insert figure is the plots in high frequency range, (c) Bode impedance plots and (d) phase plots of ZN coating and superhydrophobic coatings. (e)-(g) the corresponding equivalent circuit models used for fitting the EIS plots. Table 1. Corrosion parameters calculated from Tafel plots. samples

Ecorr (V)

Icorr (A cm-2)

βa (mv dec-1)

-βc (mv dec-1)

ZNC

-0.777

1.905×10-6

99.67

10.27

NZ

-0.782

3.425×10-6

89.56

28.09

ZnO

-0.877

1.369×10-5

78.14

29.79

ZN

-0.952

2.987×10-5

51.86

46.67

The corrosion resistance of these coatings after exposure for 4 weeks in ambient



environment was also measured by Tafel plots and EIS plots. Combined the results of Fig. 9 (a) with Table 2, it can be concluded that the ZNC coating shows the best anti-corrosion performance even though both the Ecorr and Icorr were decreased as compared with that before exposure. The Ecorr of ZnO, NZ and ZNC coatings were all higher than that of ZN coating, which can be related to the presence of “air cushion”. EIS results showed that the ZNC coating exhibited the largest semicircle radius which means it owns the largest corrosion resistance. This is also supported by Tafel results. Besides, inductance arcs can be seen in the plots of ZNC and NZ coatings. Similar results have been discussed in the above part. From the Bode-Module plots, it can be seen clearly that the ZNC coatings process a high module and two reasonable equivalent circuit models were used to understand the mechanism of corrosion process. The model in Fig. 9 (e) can be employed to simulate the corrosion process of ZnO coating because it lost the superhydrophobicity after exposure for 4 weeks and the air existed among the structure can be ignored. The model in Fig. 9 (f) can be performed for NZ and ZNC coatings and the elements present the same function of that in Fig. 8 (g). As a whole, the ZNC coating exhibits the best long-term durability and anti-corrosion performance as compared with the porous NZ coating and ZnO nanowire coating.


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Figure 9. The coatings exposed in ambient environment for 4 weeks. (a) Tafel plots of ZN coating, superhydrophobic ZNC, NZ, ZnO coatings, (b) Nyquist plots of ZN coating and the superhydrophobic coatings (c) Bode impedance plots and (d) phase plots of ZN coating and the superhydrophobic coatings. (e)-(f) the corresponding equivalent circuit models used for fitting the EIS plots of coatings. Table 2. Corrosion parameters calculated from Tafel plots of coatings exposed in ambient environment for four weeks. samples

Ecorr (V)

Icorr (A cm-2)

βa (mv dec-1)

-βc (mv dec-1)

ZNC

-0.799

1.445×10-5

78.35

42.87

NZ

-0.802

2.571×10-5

75.94

28.77



Page 20 of 27

ZnO

-0.804

3.230×10-5

94.99

27.87

ZN

-1.036

8.186×10-4

68.32

66.24

Conclusions In this study, a novel porous ZNC coating has been developed via electroplating and chemical replacement reaction. Another NZ porous structure and ZnO nanowire coating are also fabricated for comparison. After modification with myristic acid, the nanoporous ZNC coating shows superhydrophobicity with a WCA of 155.1° ± 3.5o and SA of 2.2°. The XRD result shows that the ZNC coating was composed of Zn-Ni and Zn-Co alloys. This ZNC coating remains WCA higher than 150° even after immersion in NaCl solution for 48 h, exhibiting excellent corrosion resistant stability. Besides, it can resist abrasion with a 500 g weight on 1200 grit sandpaper for more than 20 times.

Moreover, the electrochemical measurements demonstrate a

prominent corrosion resistance with a corrosion potential of -0.777 V and a corrosion density of 1.905×10-6 A·cm-2. This novel ZNC coating can be easily fabricated on many other metal materials, which can enlarge the applications of metals and have potential to solve the long-standing corrosion damages of industrial metals.


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Acknowledgements: The authors acknowledge the startup funding from the University of Texas at Dallas. T. Xiang acknowledges the PhD Abroad Short-Term Visiting Project of Nanjing University of Aeronautics and Astronautics, the Funding of Jiangsu Innovation Program for Graduate Education (No. KYLX16_0340) as well as the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. We also thank Prof. Hongbing Lu for his guidance on the work. References: (1) Li, X. G.; Zhang, D. W.; Liu, Z. Y.; Li, Z. C.; Du, W.; Dong, C. F. Materials Science: Share Corrosion Data. Nature 2015, 527, 441-442. (2) Xia, L.; Akiyama, E.; Frankel, G, McCreery, R. Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings Equilibrium Aspects of Cr VI Concentration. J. Electrochem. Soc. 2000, 147, 2556-2562. (3) Mondal, J.; Marques, A.; Aarik, L.; Kozlova, J.; Simoes, A.; Sammelselg, V. Development of a Thin Ceramic-Graphene Nanolaminate Coating for Corrosion Protection of Stainless Steel. Corros. Sci. 2016, 105, 161-169. (4) Vakili, H.; Ramezanzadeh, B.; Amini, R. The Corrosion Performance and Adhesion Properties of The Epoxy Coating Applied on The Steel Substrates Treated by Cerium-Based Conversion Coatings. Corros. Sci. 2015, 94, 466-475. (5) Tran, T. H.; Vimalanandan, A.; Genchev, G.; Fickert, J.; Landfester, K.; Crespy, D.; Rohwerder, M. Regenerative Nano-Hybrid Coating Tailored for Autonomous Corrosion Protection. Adv. Mater. 2015, 27, 3825-3830. (6) Lai,

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TOC/Abstract Graphic

Synopsis: A novel superhydrophobic porous surface was successfully fabricated by environmentally friendly electroplating and chemical replacement method for anti-corrosion.


Fabrication of inherent anti-corrosion superhydrophobic surfaces on

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