Bio-inspired slippery zinc phosphate coating for sustainable corrosion

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Bio-inspired slippery zinc phosphate coating for sustainable corrosion protection Tengfei Xiang, Shunli Zheng, Min Zhang, Hisham Rabia Sadig, and Cheng Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02345 • Publication Date (Web): 01 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Sustainable Chemistry & Engineering

Title: Bio-inspired slippery zinc phosphate coating for sustainable corrosion protection Author names and affiliations: Tengfei Xianga, Shunli Zhengb, Min Zhangc, Hisham Rabia Sadiga, Cheng Lia*

a

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

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

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

Nanyang Avenue, Singapore, 639798, Singapore c

School of Mathematics and Computational Science, Anqing Normal University, 1318

Jixian north road, Yixiu District, Anqing, Anhui, 246000, China

Corresponding Author: Professor Cheng Li Address: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Jiangjun Avenue, Jiangning District, Nanjing, Jiangsu, 210016, P R China Tel: +86-25-52112902 Fax: +86-25-52112626 Email: [email protected]

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Abstract In this work, a slippery zinc phosphate coating composed with homogeneous pores was successfully fabricated by a facile method which was inspired by nepenthes pitcher plants. The as-prepared coating exhibited superhydrophobicity with a large water contact angle (WCA) of 156.6° after modification with fluoroalkylsilane (FAS) ethanol solution. A slippery surface with highly water-repellency was achieved by completely filling the Krytox 100 lubricant into the pores. Generally, based on the stable and low-volatile lubricant, a slippery surface can provide a better corrosion resistance than that of superhydrophobic surface for metals. Herein, this novel slippery coating also displayed an efficient and sustainable anti-corrosion performance. The corrosion resistance was enhanced by 7 orders of magnitude compared with that of bare mild steel substrate. More importantly, the corrosion inhibition efficiency of this coating is still higher than 99.99 % even after immersion in NaCl solution for 6 weeks, demonstrating an excellent long-term stability for corrosion protection. The coating has great potential to be used as anti-bacterial, anti-icing and water harvesting films. Keywords: Slippery surface; Phosphate coating; Superhydrophobic; Corrosion resistance


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Introduction Corrosion occurrence is a big issue in the world, which undermines the global economy and environment. Many industries including automobiles, marine and pipeline are suffering from corrosion severely1-4. Especially for many future materials which cannot be replaced because of the cost consideration and a long service time requirement. Thus, the knowledge and technology for sustainable corrosion protection is in great demand. Generally, a coating is used for isolating the bulk materials from corrosive attack5-12. However, some coatings based on organics are not environmentally friendly. Besides, it would accelerate the corrosion rate when the coating is broken13, 14. Thus, a green coating with long-term stability is urgent to be developed. In recent years, large numbers of novel materials have been developed which were bio-inspired from lotus leaves, rose petals and nepenthes pitcher plants15-19. As one of the brilliant plants, nepenthes pitcher plants use its special structure to lock-in a medium liquid to protect surface20. The well matched solid and liquid surface energies combining with a rough structure, generate a stable state in which the liquid fills the structure and the liquid forms a perfect overlying film21. Bio-inspired from nepenthes pitcher plants, Aizenberg and her co-workers first fabricated a slippery liquid infused porous surface (SLIPS), which shows superior liquid repellency with self-healing property19. More significantly, they made a definition of the requirements to form a defect-free slippery surface. First of all, the infused liquid must wick into and adhere within the substrate stably. This requirement can be satisfied by using a micro-nano structure combining with chemical affinity for the liquid. For the liquid with a long-term storage, a semi-closed porous structure is a better choice rather than an opened porous structure. Secondly, the chemical affinity between the substrate and the infused liquid should be higher than that between the external liquid and the substrate (a lower total energy is the preferred wetting state). To satisfy this requirement, the total interfacial energies of the substrate that are entirely wetted by the immiscible test liquid A (EA = RγS-A + γA), or a lubricating liquid B with El (El = RγS-B + γB-A + γA) or without E2 (E2 = RγS-B ACS Paragon Plus Environment


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+ γB) a fully wetted immiscible test liquid standing on top of it should meet the following relationship19, 22: △E1 = E1 - EA < 0

(1)

△E2 = E2 - EA < 0

(2)

where R is the roughness of the substrate, γS-A, γS-B, γB-A, γA and γB represent the surface energies of the solid-liquid A interface, solid-liquid B interface, liquid A-liquid B interface, liquid A-vapor interface, and liquid B-vapor interface, respectively. And the third requirement is that the infused liquid and the tests liquids must be immiscible. The last two requirements can be gratified by using low surface tension perfluorinated liquids with low-volatilility as the infusion oil which is immiscible with most of liquids and therefore can form a slippery interface on the substrate (△E1 < 0, △E2 < 0). Most of the porous structures fabricated on metals were based on layered double hydroxide (LDH), anodic aluminum oxide (AAO) and nanoparticles23-29. However, the metals which can be used to fabricate LDH and AAO directly are limited. Besides, the incorporation of nanoparticles into the coatings should be demonstrated weak adhesion between substrate and particles. Thus, a coating can be used to fabricate on metals by chemical conversion or other means with high adhesion is in urgent need. Zinc phosphate (ZP) coating, a traditional green corrosion-resistant conversion coating30-36, which owns a special semi-closed porous structure and a large adhesion with substrate, is an excellent candidate coating to offer a large space to store the infused liquid. Compared with other porous coatings, phosphate coating can be fabricated on metals with high adhesion quickly, and the pores can be controlled easily by changing the parameters of the phosphate reaction. Followed by surface functionalization to match with the chemical nature of the infused lubricant, a stable slippery surface can be formed. To date, even though numerous scientists have studied the corrosion resistance of ZP coating, few researchers investigated the anti-corrosion performance of a slippery ZP coating and its long-term stability. In this work, we first fabricated a ZP coating based on Zn-Ni (ZN) coating and then infiltrated non-volatile and immiscible lubricant to generate slippery surface. Then, we focus on study the ACS Paragon Plus Environment

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corrosion protection performance of the slippery surface. Experiment Materials and chemicals Mild steel (composition in wt. %: C ≤ 0.1, Mn ≤ 0.5, P ≤ 0.035, S ≤ 0.035, with the rest being of Fe) was cut into 5 cm × 2 cm × 0.1 cm as substrates, the composition of ZN plating bath was given in our previous works6, 37. Briefly, it contains 35 g·L-1 boric acid (H3BO3), 75

g·L-1 zinc chloride (ZnCl2), 110 g·L-1 nickel chloride

hexahydrate (NiCl2·6H2O), 35 g·L-1 ammonium chloride (NH4Cl), 200 g·L-1 potassium chloride (KCl), 20 g·L-1 potassium citrate monohydrate (K3C6H5O7·H2O) and 0.1 g·L-1 sodium dodecylbenzene sulfonate (C18H29NaO3S). The pH of bath plating was kept at 5.0 ~ 5.5, which was adjusted by hydrochloric acid (HCl) and sodium hydroxide (NaOH). Zinc dihydrogen phosphate [Zn(H2PO4)2·2H2O], phosphoric acid (H3PO4), nickel nitrate hexahydrate [Ni(NO3)2·6H2O], manganese nitrate dihydrate [Mn(NO3)2·2H2O], sodium chloride (NaCl), ethanol and the plating chemicals were all analytical pure grade and purchased from Nanjing Chemical Reagent Co., Ltd., China. Fluoroalkylsilane (FAS) was obtained from Sigma-Aldrich. Lubricant (Krytox 100) was offered by Dupont. All the chemicals were used without any further treatment. Self-made deionized water was used in the experiment process. Fabrication of ZP coating Before electroplating, the substrates were degreased, acid pickled and activation firstly. The details also can be seen in our previous works6, 37. After that, the ZN coating was deposited under the current density of 4 A·dm-2 at 35 °C for 20 min by a direct current power supply, the plating bath was stirred at a speed of 200 r·min-1 during the plating process. Subsequently, the ZN coating was immersed into the 100 mL phosphate solution which contains 40 g·L-1 Zn(H2PO4)2·2H2O, 20 g·L-1 H3PO4, 4 g·L-1 Ni(NO3)2·6H2O and 11 g·L-1 Mn(NO3)2·2H2O for 3, 5 and 10 min respectively. The pH was kept at 1 ~ 2. The temperature of the solution was kept by a water bath at 40 °C. Finally, the sample was cleaned by deionized water and dried in air, and the ZP coating was obtained. Sample modification and lubricant infusion ACS Paragon Plus Environment


For a perfect wettability by the lubricant, ZP coating was first modified by FAS solution (10 mM FAS of ethanol solution) for 30 min and then dried at 70 °C for 2 h in an oven. Afterward, 20 µL lubricant Krytox 100 was homogeneously infused into the phosphate coating by a spin-coater with a speed of 2000 r·min-1. After infusion, these samples were placed vertically at ambient environment for 12 h to drain off the excess liquid. For a better readability, the total abbreviation was summarized here. ZN and ZP coating represent Zn-Ni and Zinc phosphate coating (based on Zn-Ni coating), respectively. MZP stands for the modified zinc phosphate coating, SZP on behalf of the slippery zinc phosphate coating while after the lubricant infused into the modified ZP coating. Characterization The morphologies and elemental compositions of samples were observed by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) equipped with an energy dispersive spectroscopy (EDS). The structure of ZN and ZP coatings were detected by an X-ray diffraction (XRD, D8 Advance, Bruker, Germany) using filtered Cu Kα as a radiation source (λ = 0.15406 nm) at a scanning rate of 4°/min from 10° to 90° of 2θ. The water contact angle (WCA) and contact angle hysteresis (CAH) was measured by a contact angle meter (SL200B, Solon Tech., China) with a 6 µL deionized water droplet. Five different positions on each sample were tested to obtain the average WCA value. The corrosion resistance of samples was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves in 3.5 wt. % NaCl solution (pH = 5.9) using an electrochemical workstation (CHI 750C, Shanghai Chenhua Instrument Corporation, China) at room temperature. Before testing, the samples were immersed into NaCl solution for 30 min to obtain stable open circuit potentials. The measurements were performed with a standard three-electrode system, a saturated calomel electrode (SCE) and a platinum (Pt) electrode were acted as a reference electrode and a counter electrode, respectively. The tested sample with an exposed area of 1 cm2 was used as the work electrode. The corrosion potential (Ecorr) and corrosion current density (Icorr) can be acquired from the potentiodynamic ACS Paragon Plus Environment

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polarization curves (scanning rate of 1 mV·s-1) using extrapolation method. The EIS was carried out at the open circuit potential from 10-2 Hz to 105 Hz with a sinusoidal perturbation signal of 5 mV amplitude. Results and discussion Surface morphologies of samples To investigate the effect of phosphating time on the morphologies of ZP coating, the coating formed with different reaction time were detected. As shown in Fig. 1 (a), the ZN coating exhibited a flat and dense structure composed of “sand-like” substances with an average diameter about 500 nm. After immersion into the phosphate solution for 3 min, the ZP coating showed a “honeycomb” hierarchical structure which can be seen in Fig. 1 (b), and the “cells” were consisted with homogeneous nano-pores which like flower petals with a diameter of 250 nm. While prolonging the reaction time to 5 min, the size of the nano-pores showed an expanded trend and the diameter is about 300 nm, the depth of these pores looks deeper. However, when the reaction time reached 10 min, the pores transferred to collapse. This phenomenon may be related to the ZP coating formation process. During the formation of ZP coating, the crystals would be extruded with each other, and the pores may be broken in this process, especially for the hydrogen escaping31, 38. Another reason is that the ZP coating would be dissolved in the phosphate solution after the substrate was covered by the coating completely for a long time33, 34.



Figure 1. The surface morphologies with different magnification of (a) ZN coating, (b), (c) and (d) ZP coating generated with 3 min, 5 min and 10 min, respectively. Surface composition of samples XRD technique was used to detect the surface composition of ZN and ZP coatings. As depicted in Fig. 2 (a), the peaks in the position of 42.8°, 62.3°, 78.7° and 89.4° are corresponding to the (411), (600), (721) and (811) planes of γ-Ni5Zn21 (JCPDS No. 06-0653), several Fe peaks can be seen in ZN coating demonstrating that the substrate can be detected. After reacted in the phosphate solution with different time, some new peaks appeared. It can be seen that after reacting for 3 min, three weak peaks were generated in the position of 9.7°, 19.4° and 31.3° which corresponding to the (020), (220) (241) planes of Zn3(PO4)2·4H2O (JCPDS No. 37-0465). While prolong the reaction time, these peaks intensity become sharper, demonstrated a better phosphate coating. Besides, there is a peak in 46.8° also matches well with the position of (371) plane of Zn3(PO4)2·4H2O. Moreover, a low


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peak of Zn3(PO4)2·4H2O and Mn2Zn(PO4)2 also can be detected. However, when the reaction time was increase to 10 min, another two peaks appeared in 35.9° and 74.3° which consist with (NiHPO4)·3H2O (JCPDS No. 39-0706) and Ni7P3 (JCPDS No. 03-1101). That may be related to that once the zinc phosphate coating was dissolved, the surface is progressively enriched in Ni by a dissolution-re-deposition process with a consequent potential shift towards more anodic potentials33-35, and as a result, some compounds which containing Ni formed, not zinc phosphate coating. This was in keeping with the morphological changes. Thus, in this work, we only focus on researching the ZP coating with the reaction time of 5 min (a better phosphate coating). To confirm the composition of ZP coating, EDS was applied to monitor the elements of coating. Mn element can be seen in Fig. 2 (b) with a low content, which is fitted well with the XRD consequence. As a whole, the ZP coating was mainly composed of Zn3(PO4)2·4H2O.

Figure 2. (a) The XRD patterns of ZN coating reacted in the phosphate solution with different time, (b) the EDS spectra of ZN coating after reacted in phosphate solution for 5 min. Surface wettability The chemical affinity can be evaluated by the surface wettability. Fig. 3 exhibited the images of 6 µL water droplets on the as-prepared coatings. It can be seen that the WCA of ZN coating is about 85.2°, which may be related to the surface roughness and passivation. While after phosphating treatment, water droplet spread on the surface quickly and permeated through the surface, and the WCA sharply ACS Paragon Plus Environment


decreased to 0°. The porous structure of ZP coating must be responsible for the superhydrophilicity. After decorating with FAS, the water droplet can stay on the surface with a minimized contact area with WCA about 156.6°, as shown in Fig. 3(c). The change of the microstructure can be ignored because the FAS was only as a subnanometer layer39, 40. The superhydrophobicity of MZP coating was result from the “air cushion” due to the trapped air among the porous structure40-42. Moreover, the WCA decreased to 122.8° after infusion of the Krytox 100, as shown in Fig. 3(d). The Krytox 100 exhibits a low surface tension about 16-20 mN m−1 while water owns about 72.5 mN m−1 at 26 °C25. Thus, combining with the rough structure, it meets the second requirement very well. The water immiscible lubricant would completely occupy the pores of MZP coating because of the well chemical affinity, leaving no space for air. Therefore, the contact interface transferred from the solid-air-water state to solid-lubricant-water state. Moreover, water can slide off from the coating with a low CAH about 8° revealing superior water repellency. The sliding process can be seen in Fig. 4 (e), which were obtained from video shots and showed a slow sliding velocity on this coating.

Figure 3. The contact angle images of 6 µL water droplet on (a) ZN coating, (b) ZP coating, (c) MZP coating and (d) SZP coating. (e) Images of the slippery process with 6 µL water droplet on SZP coating. Corrosion resistance Corrosion protection performance of samples As an effective and powerful measurement, the EIS technique was used to ACS Paragon Plus Environment

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evaluate the anti-corrosion performance of samples. EIS experiments were all conducted in 3.5 wt. % NaCl aqueous solution. The Bode-module and Bode-phase plots can be seen in Figs. 4 (a) and (b). As a general rule, a larger |Z| in low frequency indicates a better corrosion resistance40. Herein, it can be observed that the |Z| of SZP coating is the largest, and there is no doubt that the value of bare mild steel is the lowest. Besides, the |Z| of ZP coating is larger than that of ZN coating which means that the phosphate coating can provide a better corrosion protection for mild steel in the NaCl solution. Moreover, the MZP coating can offer further corrosion protection compared with ZP coating, with a larger value of |Z| in low frequency. From the Bode-phase plot, the SZP coating exhibited an extremely wide and high phase angle in a very broad frequency domain, suggesting that a highly stable and compact coating was provided by the SZP coating43, 44. As for the phase plots of these coatings, the large phase angle peak could indicate the interaction of two time constants44. According to previous works40-42, the charges transferred in coatings system should pass the solution-coating and coating-substrate interface during the process of electrochemical test. Therefore, an equivalent circuit as shown in Fig. 4 (c) was used to fit the EIS results of the coating. Where Rs on behalf of the solution resistance, Rc represent the coating resistance, Rct corresponds to the charge transfer resistance of the interface of coating and substrate while the Cdl means the capacitance of double electrode layer. The CPEc was used to replace the coating capacitance due to the distribution of relaxation times resulting from different degrees of heterogeneities at the electrode surface45. The CPE can be mathematically expressed as below45, 46: ZCPE = [Q0 (jω)n]-1

(3)

Where Q0 is a proportionality factor, j is an imaginary unit while ω is an angular frequency (ω = 2πf), n is an adjustable parameter with the value always between -1 for a prefect inductor and 1 for a perfect capacitor. With regard to the bare mild steel, there is only a thin passive oxide layer as the barrier layer, so the corrosive ions can penetrate into substrate easily. Thus, the equivalent circuit in Fig. 4 (d) can be used to fitting the EIS result of bare mild steel. The barrier layer resistance (approximate to the Rct) of each coating were obtained from the fitting results and showed in Fig 4 (e). ACS Paragon Plus Environment


The Rbarrier of bare mild steel is 613.9 Ω cm2 while the value for ZN coating is 5047 Ω cm2 which means the ZN coating can further protect mild steel from corrosion. In the meantime, the ZP and MZP coatings also show better corrosion resistance than ZN coating. Most importantly, the SZP coating displayed an unbelievable corrosion resistance and the Rct is as large as 1.091 × 1010 Ω cm2, which is 5 orders of magnitude higher than that of MZP coating. The above results fit well with those of Bode-module and Bode-phase plots. As a consequence, the SZP coating demonstrated a fantastic anti-corrosion performance.

Figure 4. Electrochemical impedance spectroscopy (EIS) measured in 3.5 wt. % NaCl solution. (a) Impedance (|Z|) and (b) phase of Bode plots for bare mild steel, ZN, ZP, MZP and SZP coatings. (c) and (d) the equivalent circuits for the four coatings and mild steel substrate, respectively. (e) Resistance of the barrier layer for samples obtained from modeling results. Potentiodynamic polarization (Tafel) method was further adopted to quantitative estimate the anti-corrosion performance of samples. The corrosion potential (Ecorr) and corrosion current density (Icorr) were calculated from the plots by extrapolation method. As can be seen in Fig. 5 (a), the plots of bare mild steel, ZN, ZP and MZP coatings were all exhibited a lower cathodic slope than anodic slope, which means


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that the Ecorr changed slowly with the increasing potential, it implies that the coatings and the bare mild steel were protected from the hydrogen evolution reaction [46]. Besides, the Ecorr of ZN, ZP and MZP is lower than that of bare mild steel, indicating that these coatings protected substrate from corrosion by cathodic protection and would sacrifice themselves firstly47. The Ecorr of ZP and MZP coatings is higher than that of ZN coating must be related to the phosphate film and the trapped air among the porous structure, respectively38, 48. While after infusing lubricant into the modified phosphate coating, the Ecorr transferred towards to positive direction, revealed a lower corrosion probability29. In addition, the Icorr of samples were calculated and displayed in Fig. 5 (b). The bare mild steel showed the largest Icorr (9.209 × 10-5 A cm-2), demonstrating the fastest corrosion rate49. It can be also found that the Icorr of ZN and other coatings were lower than that of bare substrate. Excitingly, the Icorr of SZP is dramatically decreased to 1.927 × 10-11 A cm-2, revealing an extremely low corrosion rate. The corrosion inhibition efficiency was calculated by the following equation50: Ei = (Icorrb - Icorrc)/ Icorrb × 100 %

(4)

where Icorrb and Icorrc are the corrosion current density of bare substrate and coated samples, respectively. It can be calculated that the Ei of ZN, ZP, MZP and SZP coatings is about 85.31 %, 92.61 %, 99.47 % and 99.99 %, respectively, as shown in Fig. 5 (b). These results were coincident with the above resistance values, indicating that the corrosive ions are strongly restrained by the protective property of coatings.

Figure 5. (a) The potentiodynamic polarization plots of samples, (b) the corrosion current density calculated from the plots.



Corrosion inhibitive mechanism As can be seen in Fig. 6 (a), the thickness of the MZP coating is about 1.5 µm, and the morphology still kept porous. When immersing the coating into water, a white layer appeared, demonstrated the existing of the air layer. In Fig. 6 (b), the morphology of this coating cannot be seen clearly because it was filled with lubricant, it protected the coating very well even immersing this coating into water. Thus, based on the FESEM images, the WCH, XRD, EDS results as well as the corrosion resistance results, the corrosion inhibitive mechanism for the coatings can be proposed. The ZP coating was almost composed of Zn3(PO4)2·4H2O which has very low conductivity, the corrosive ions can hardly transferred freely51, 52. Moreover, the micro galvanic corrosion can only occur among the phosphate pores separately53. Furthermore, the transport of oxygen to substrate is hindered by the protective phosphate coating between the substrate and the electrolyte due to the homogeneous porous structure54. Thus, the corrosion resistance of ZP coating is higher than that of ZN coating. With regard to the MZP coating with superhydrophobicity [as shown in Fig. 6 (a)], a large amount of air trapped inside the unique structure as the “air cushion” can minimize the contact area between NaCl aqueous solution and the coating, thus preventing the NaCl solution penetrating into the surface10, 11. Secondly, the “capillarity” acts as a critical role in the protection process50, the corrosive ions of Cl- can be repelled by the Laplace pressure. Thirdly, the FAS and air layers own high potential and large impedance, which can result in a well anti-corrosion performance40. For the SZP coating, the lubricant would fill the pores and the air must be pushed out, as shown in Fig. 6 (b). The lubricant can serve as a barrier layer to prevent the corrosive ions permeating into substrate23-27. The lubricant Krytox 100 is very stable in aqueous solution and immiscible with water, therefore, it can provide a superior corrosion protection for the mild steel substrate.


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Figure 6. (a) and (b) are the cross section images of MZP and SZP coatings, the images on top right corner are those with high-resolution while the bottom left corner images are the coatings immersed in water. (c) and (d) are the schematic illustrations of MZP and SZP coatings tested in 3.5 wt. % NaCl solution. Sustainable corrosion protection For most of superhydrophobic coatings, they can’t offer sustainable protection for substrate due to its brittle special structure. When the structure was broken, it can become a pass route for corrosive ions and accelerate the corrosion rate. In addition, the “air cushion” will be depleted with a long time and could not provide as a protection layer55, 56. Thus, it’s a challenge for the superhydrophobic coating to keep its superhydrophobicity when immersion in aqueous solution for a long time. As an alternative coating, lubricant-infused slippery coating exhibits a better stability in aqueous solution19. As shown in Fig. 7 (a), the SZP coating exhibited a large contact angle of 118.4 ± 4.1° even after immersion in NaCl solution for 6 weeks, which is only 4.4° lower compared with that before test, demonstrating an outstanding stability. To illustrate the sustainable protection performance of SZP coating, electrochemical measurement was also applied. As can be seen in Fig. 7 (b), the value of |Z| in low frequency decreased with increasing of the immersion time. Meanwhile, the Bode-phase plots were also presented a declined trend. The phase angle sharply



decreased in the middle frequency domain after immersion for 4 weeks for the SZP coating, which demonstrated that the anti-corrosion performance was decreased with a long immersion time. That likely due to the lubricant being partly removed from the substrate because of the positive spreading coefficient of lubricant in aqueous solution57, and part of the lubricant may be dissolved in the water limitedly given its relatively low viscosity, making the lubricant layer thinner58-60. Thus, the corrosive ions can penetrate into the coating/metal interface easier from the tip or the edge of the phosphate coating and result in the decrease of phase angle61. Also, the barrier resistance was also fitted by the same equivalent circuit in Fig. 4 (c). Even after immersion for 6 weeks, the Rbarrier can still keep as large as 5.339 × 108 Ω cm2, much higher than that of MZP coating of about 3 orders of magnitude. The corresponding results can be seen in Fig. 7 (d). As a result, the above results indicated that SZP coating has a remarkable sustainable corrosion protection performance for the bare mild steel substrate.

Figure 7. (a) WCA of SZP coating immersion in 3.5 wt.% NaCl solution for different time. (b) and (c) are the corresponding impedance (|Z|) and phase of Bode plots. (d) Resistance of the barrier layer obtained from modeling results. The Tafel plots of SZP coating after immersion in 3.5 wt. % NaCl solution for ACS Paragon Plus Environment

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different time were shown in Fig. 8 (a). It can be found that the Ecorr shifted to negative position after immersion in NaCl solution, indicating that the corrosion would occurs easily. However, with the increasing immersion time, the Ecorr of the coating had a slight rise, a thin lubricant-water layer would be generated between the coating and solution57, and thus the corrosion potential increased. The Icorr of samples were calculated and displayed in Fig. 8 (b). There is no doubt that the Icorr increased after immersion in NaCl solution. However, the value is still far less than that of MZP coating even after immersion for 6 weeks (1.373 × 10-10 A cm-2). The Ei of the immersed samples were all higher than 99.99%. It can be concluded that the SZP exhibited a glaring sustainable corrosion protection performance.

Figure 8. The potentiodynamic polarization plots of SZP coating immersed in 3.5 wt. % NaCl solution for different time and (b) the corrosion current density calculated from the plots. Conclusions We have developed a novel slippery zinc phosphate coating on mild steel substrate successfully by a simple and environmentally friendly method. The morphology of this coating was investigated and showed a homogeneous porous structure. The XRD and EDS results demonstrated that the coating was mainly composed of Zn3(PO4)2·4H2O. It showed superhydrophobicity with a WCA of 156.6° after modification with the low surface energy material of FAS. When infusing the lubricant into the surface, the WCA decreased to 122.8°. Even though the WCA decreased, the anti-corrosion property was increased sharply due to the excellent water-repellency of lubricant. The corrosion resistance of SZP coating was significantly enhanced by 5 orders of magnitude compared with that of MZP coating. ACS Paragon Plus Environment


After immersion this novel SZP coating into NaCl solution for 6 weeks, it still exhibited a superior anti-corrosion performance with corrosion inhibition efficiency higher than 99.99%. This novel coating will be of great significance to retard the long-standing corrosion problem and will enlarge the application of engineering metals. Acknowledgements: The authors would like to thank the project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions and the Funding of Jiangsu Innovation Program for Graduate Education (No. KYLX16_0340). 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) Fujita, S.; Mizuno, D. Corrosion and Corrosion Test Methods of Zinc Coated Steel Sheets on Automobiles, Corros. Sci. 2007, 49, 211-219. (3) Zhang, B. B.; Li, J. R.; Zhao, X.; Hu, X. H.; Yang, L. H.; Wang, N.; Li, Y. T.; Hou, B. R. Biomimetic One Step Fabrication of Manganese Stearate Superhydrophobic Surface as an Efficient Barrier Against Marine Corrosion and Chlorella Vulgaris-Induced Biofouling, Chem. Eng. J. 2016, 306, 441-451. (4) Wang, D. H.; Bierwagen, G. P. Sol-gel Coatings on Metals for Corrosion Protection, Prog. Org. Coat. 2009, 64, 327-338. (5) Böhm, S. Graphene Against Corrosion, Nat. Nanotechnol. 2014, 9, 741. (6) Xiang, T. F.; Han, Y.; Guo, Z. Q.; Wang, R.; Zheng, S. L.; Li, S.; Li, C.; Dai, X. M. Fabrication of Inherent Anticorrosion Superhydrophobic Surfaces on Metals, ACS Sustainable Chem. Eng. 2018, 6, 5598-5606. (7) Zhu, C.; Fu, Y. J.; Liu, C. A.; Liu, Y.; Hu, L. L.; Liu, J.; Bello, I.; Li, H.; Liu, N. Y.; Guo, S. J.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Kang, Z. H. Carbon Dots as Fillers Inducing Healing/Self-healing and Anticorrosion Properties in Polymers, Adv. Mater. 2017, 29, 1701399. (8) Leal, D. A.; Riegel-Vidotti, I. C.; Ferreira, M. G. S.; Marinoa, C. E. B. Smart Coating Based on Double Stimuli-responsive Microcapsules Containing Linseed ACS Paragon Plus Environment

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Synopsis: A novel slippery zinc phosphate coating was fabricated via a facile method for sustainable corrosion protection.


Bio-inspired slippery zinc phosphate coating for sustainable corrosion

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