A Lubricant-Sandwiched Coating with Long-Term Stable Anticorrosion

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A Lubricant-Sandwiched Coating with LongTerm Stable Anticorrosion Performance Mizuki Tenjimbayashi, Sachiko Nishioka, Yuta Kobayashi, Koki Kawase, Jiatu Li, Jyunichiro Abe, and Seimei Shiratori Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03913 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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A Lubricant-Sandwiched Coating with Long-Term Stable Anticorrosion Performance. Mizuki Tenjimbayashi†‡, Sachiko Nishioka†‡, Yuta Kobayashi†, Koki Kawase, Jiatu Li†, Jyunichiro Abe†, and Seimei Shiratori†* †

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Yokohama, 223-8522, Japan. ‡

Equally contributed author

*

[email protected]

KEYWORDS; slippery surface; anti-corrosion; hydrophobicity; phase separation; self-secretion

ABSTRACT

Lubricant-infused surface(s) (LIS) bio-inspired by the Nepenthes pitcher plant are receiving enormous attention owing to their excellent hydrophobicity as well as their self-healing ability. Thus, they have been applied as anticorrosion coatings. However, the loss of lubricant mediated by vapor or other liquids deteriorates their functions. Herein, we introduce a lubricant-inserted (sandwiched) microporous triple-layered surface (LIMITS) that prevents the sudden loss of lubricant. The sandwiched lubricant gradually self-secretes toward the surface, resulting in

ACS Paragon Plus Environment

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long-term stability even under water. The LIMITS prevented the corrosion of the Fe plate for at least 45 days, which is much superior to a conventional LIS coating. This work opens an avenue for the application of slippery coating materials that are stable under water and will also promote the development of anticorrosion coating in various industries.

INTRODUCTION

Metal materials are of both fundamental and industrial significance because of their potential applications such as in construction,1 vehicles,2 and industrial plants.3 The corrosion of metal materials significantly undermines the global economy and the environment, at a scale of several percent of the gross domestic product of industrialized countries.4 For example, although Fe is one of the most common and essential engineering materials in a wide range of industries, it is very actively ionized and has poor corrosion resistance, which degrades its performance.5 To prevent the corrosion of Fe, the construction of corrosive media–metal interfaces has been intensively researched, and various strategies for preparing materials with anticorrosion properties have been reported such as plating,6–8 preparing sacrificial electrodes,9–12 and covering with castor oil.13

Recently, antiwater coatings developed by mimicking materials from nature have received much attention as passive ways to isolate the bulk materials from corrosive attack.14–22 Because corrosion is initiated on the interface between aqueous media and the materials, hydrophobic surfaces are effective for anticorrosion.22 Although the structure of a lotus-leaf offers a


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remarkable template for designing superhydrophobic surfaces by introducing a microscopic air layer on a controlled rough structure to protect a coating from water invasion, it is difficult to prevent the invasion from nanosized and microsized liquids such as vapor or fog.23–26

Inspired by the Nepenthes pitcher plant, Wong et al. reported a state-of-the-art water-repellent material that forms a water-immiscible lubricant layer immobilized on the surface by a microporous solid layer.27 Because of the introduction of a seamless hydrophobic lubricant layer, a lubricant-infused surface (LIS) can effectively protect the bulk material from attack by water and is stable under pressure. Therefore, it is anticipated to be used for various applications including anticorrosion.28–31

The challenge to design a LIS with long-term stability remains difficult owing to the loss of the lubricant layer by evaporation or attack by water.32–34 The lubricant layer can easily be removed under aqueous conditions, even though the LIS is thermodynamically stable. Therefore, methods to mitigate the degradation of the lubricant or the loss of the LIS are of current interest. Urata et al. developed thermoresponsive organogels that form a lubricant layer only under desirable temperature conditions to prevent the sudden loss of the lubricant.35 Aizenberg’s group developed a self-replenishing LIS and a self-secreting LIS.36 To the best of our knowledge, besides these reported methods, which have succeeded in improving the stability of LIS in air, there have been no reports of LIS with long-term stability and resistance to the loss of lubricant. Specifically, the preparation of an LIS that is stable under water has not been researched, even though there is a strong requirement for it in practice.

Here, we report a strategy for forming a lubricant layer on metal materials that is stable under water by covering the bulk with a lubricant sandwiched in a microporous triple-layered surface


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(LIMITS), in which the lubricant layer is inserted between two of the microporous solids (i.e., an LIS consists of a lubricant-solid-substrate layer structure whereas a LIMITS consists of a solid-lubricant-solid-substrate layer structure) to obtain an anticorrosion coating with long-term stability as long as lubricant layer exists. We have previously designed a superoleophilic self-standing porous surface (SPS) through a non-solvent induced phase separation technique.37 This technology was used to prepare LIMITS by simply covering the LIS with SPS, which works not only to protect the materials from water attack but also to prevent lubricant loss. Because of the introduction of the top SPS layer, the secretion of the lubricant to the surface is restricted. When the lubricant on the surface is lost, more lubricant is slightly secreted. Note that the LIMITS is conceptually different with previously researched self-secreting LIS36, 38 as well as a conventional LIS27 as in Figure 1.

In this article, we studied the wetting mechanism and morphology of LIMITS. We also investigated the underwater stability of LIMITS on an Fe plate through detailed comparison with the anticorrosion performance of LIS and SPS through wettability and electrochemical analysis. We believe that our design of a lubricant-inserted multilayer with controlled self-secretion and long-term stability provides new insights for the development of practical liquid-infused materials especially for industrial application.

EXPERIMENTAL SECTION


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Materials.

Poly(vinylidene

fluoride-co-hexafluoropropylene)

(PVDF-HFP,

(-CH2CF2-)m[-CF2CF(CF3)-]n, Mw∼400,000, Mn∼130,000, m:n = 10:1 (molar ratio)) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Dibutyl phthalate (DBP, 99.5%), acetone (99.5%), ethanol (EtOH, 99.5%), potassium hydrate (KOH), and hydrochloric acid (HCl) were purchased from Kanto Chemical (Tokyo, Japan). Perfluoropolyether (PFPE; Krytox 103) was purchased from DuPont (Wilmington, USA). Deionized water (resistance 18.2 MΩ cm) was obtained using Aquarius GS-500.CPW (Advantec Toyo Kaisha, Ltd., Japan). Iron (Fe) plates were kindly gifted by Honda Industry Co., Ltd. (Ibaraki, Japan). Commercial castor oil coating solution (Panehakuri®) was obtained from KONDOTEC Co., Ltd. (Osaka, Japan).

Preparation of the Fe plate. The Fe plates were ultrasonicated in 5 wt.% KOH aq. for 30 minutes, and rinsed thoroughly. The plates were then ultrasonicated in 10 wt.% HCl aq. for 30 minutes, and rinsed thoroughly. The obtained plates were dried under a flow of N2 (See Figure S1 for photo images of the Fe plates before and after the cleaning process).

Design of the self-standing porous surface (SPS). The fabrication protocol of the SPS has been described in our previous work.37 Briefly, PVDF-HFP (2.0 g) and DBP (4.0 g) were dissolved in acetone (24.0 g) by stirring and then heated at 50 °C in a glass container for 1.0 hour. Subsequently, the mixture was uniformly shaped into a membrane with a thickness of 4.24 µm by dipping the glass substrate into the cocktail at a speed of 10 mm/s and drying for 5 min. The dried membrane was washed with ethanol to make a porous surface owing to the extraction of DBP.


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Design of the lubricant infused surface (LIS). LIS was fabricated by impregnating 5 µL/cm2 of PFPE as a lubricant on a SPS coating. Two types of LIS with different SPS membrane thicknesses were prepared. The LIS* was prepared by lubricating an SPS with a thickness of 4.24 µm. The LIS** was prepared by lubricating an SPS with a thickness of 9.03 µm, which was prepared by changing the dipping speed of the SPS solution to 13.3 mm/s (see Figure S2 for the effect of dipping speed on the thickness and morphology of the SPSs). Design of the lubricant-inserted microporous triple-layered surface (LIMITS). LIMITS was fabricated by covering LIS* with SPS (thickness: 4.24 µm). Characterization. The morphology of the coatings was analyzed by low-vacuum field emission scanning electron microscopy (SEM; Inspect S50, FEI, Hillsboro, USA) and a 3D laser scanning microscope (VK-9710, Keyence, Osaka, Japan). Contact angles were measured by analyzing the digital camera images with Image J software (Wayne Rasband). The chemical composition of the SPS was measured by X-ray photoelectron spectroscopy (XPS, JPS-9010TR; JEOL Ltd., Akishima, Japan). Anticorrosion tests. The anticorrosion properties of the materials were analyzed by immersing the samples into NaCl 3.5 wt.% aq. solution. These samples were analyzed by surface images and electrochemical measurements as a function of time τ (day). The electrochemical measurements were carried out using an ECstat-301 (EC FRONTIER Co., Ltd, Kyoto, Japan) electrochemical workstation in a standard three electrode system. An Ag/AgCl electrode in saturated KCl aqueous solution served as the reference electrode and a Pt plate served as the counter electrode. A NaCl 3.5 wt.% aq. solution that was open to room air and not deoxygenated was used as the electrolyte. The open-circuit voltage (OCV) of each sample was measured after


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stabilization for 10 min in the electrolyte. Then, the polarization measurements were performed at an OCV range of ±300 mV at a scanning rate of 1 mV s-1. Uncoated Fe plate after cleaning process is compared as a control group.

RESULTS AND DISCUSSION

Surface design and morphological analysis The surface design of the LIMITS is based on the concept of protecting the loss of the lubricant that results from contact with other liquids or evaporation by covering the lubricant with a porous outermost solid layer. Thus, a self-standing porous PVDF-HFP layer was first prepared using a phase separation technique according to our previous study.37 The LIMITS was prepared by the insertion of a PFPE lubricant between two porous PVDF-HFP layers (Figure 2). Owing to the porous structure and surface fluorine group of SPS (see XPS spectra in Figure 2 and Figure S3), PFPE wets SPS with superoleophilic state (CA0, in which γ is the interfacial tension between the lubricant (o), water (w), or air (a).39 The wetting ridge39 created by the lubricant around the water droplet on the LIMITS indicated that the


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LIMITS self-replenishes the surface with lubricant. The water contact angle (WCA: θwa) on each coating is shown in Figure 4B. Although all the surfaces are hydrophobic (>90º), the WCAs on SPS and LIMITS are higher than that on LIS because the rough SPS layer is exposed on the SPS and LIMITS surfaces (see Figure 3B), which enhances their hydrophobicity according to Wenzel’s rule.40 The WCAs of SPS and LIMITS were almost same, even though SPS was rougher than LIMITS, which indicated that the WCA on a homogeneous lubricated oil surface was larger than on a smooth homogeneous PVDF-HFP surface. Although both the LIS* and LIS** were smooth and the surfaces were completely covered with PFPE lubricant, the WCA on LIS* was slightly smaller than that on LIS**. That may be because the water droplets sink to contact with the SPS underlayer and the degree of contact is affected by the thickness of the SPS underlayer.41 Because LIS** was more hydrophobic than LIS*, we compared the stability of the immobilized lubricants on LIS** with LIMITS. The LIS** was designed to adjust the SPS layer so that it had the same thickness as the sum of the two SPS underlayers used for LIMITS. We compared the lubricant stability of LIS** and LIMITS by monitoring the mass changes of the lubricant oil under air at 50 ºC, as shown in Figure 4C.33 Although the mass of lubricant in the LIS** decreased by 75.3% and 80.1% after 24 h and 48 h, respectively, the mass of lubricant in the LIMITS was 75.2% after 24h and 47.5% after 48 h thanks to the top SPS layer regulating the secretion of the lubricant. The water sliding angle over time was monitored to investigate the long-term stability of the surface lubricant in air (Figure 4D). When the surfaces were filled with lubricating oil, water droplets slid off the surfaces at low tilting angle owing to the slipperiness of the surfaces. Although LIS** and LIMITS were more stable than LIS* in terms of water repellency, their hydrophobicity slightly deteriorated after 40 days, which indicated the surface lubrication was lost owing to evaporation. However, the water sliding angle after removal of the


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top layer was the same value as the one before the test. This indicated that the inserted lubricant was not lost and the sandwiched lubricating oil was reserved even after 40 days.

Investigation of lubricant wettability under water and saltwater We investigated the lubricant wettability under water and under saltwater, as shown in Figure 5, which is critical for discussing underwater stability. Although the lubricant is superoleophilic (lubricant contact angle 0

(3)

in which the subscript s indicates SPS. Here, as in Figure 5D, the lubricant contact angle under water (θow) and lubricant contact angle in air (θoa) are expressed as:

cosθ

=

γ − γ γ

(4)


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cosθ

γ − γ γ

=

(5)

Hence, the relationship between θow and θoa is expressed as:

γ − γ

cosθ

= γ − γ cosθ

(6)

From equation (3) and (6), the inequality is given by:

γ − γ

cosθ

= γ − γ cosθ

γ ∴! " cosθ γ

< γ − γ cosθ

< cosθ

(7)

(8)

However, from the contact angle measurement, the relationship between θow and θoa is obtained as

cosθ

≫ cosθ

(9)

Thus, from equation (8) and (9), the inequality:

γ

≫γ

(10)

is obtained. Therefore, the low oil-air interfacial tension was critical for the decrease in the oleophilicity. Moreover, the oil contact angle under saltwater was slightly higher than that under water, indicating that conventional LISs were not appropriate for practical application such as anticorrosion against seawater. Especially, the lubricant layer becomes instable as the increase of flow rate.43 Thus, under high Reynolds number condition such as seas or rivers, the stable lubricant layer formation is quite important.


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Anticorrosion performances We evaluated the corrosion resistance of the coatings on a Fe plate (Figure 6). The coatings were immersed in salt-water and the change in color was monitored as a function of immersion time, τ (Figure 6A). Visually, it was observed that the anticorrosion performance was superior in the order of LIMITS>LIS>control (uncoated). In particular, LIMITS on a Fe plate remained pristine for at least 45 days under seawater (better than commercial castor oil coatings, see Figure S8). Compared with previous studies using hydrophobic lubricant layer and a LIS for anticorrosion, anti-corrosion performance of LIMITS (for 45 days) is quite long-term stabile and high resistant to the loss of lubricant22, 29, 31. Regardless of the thickness of the underlayer, the corrosion of LIS started at τ=3 and the corrosion area was spread across the whole Fe plate at τ=45. This indicated that the superoleophilic underlayer did not adhere well with the lubricating oil underwater. The surface corrosion properties were also evaluated using an electrochemical workstation. As shown in Figure 6B, linear polarization measurements were performed and the corrosion current densities (icorr) were calculated according to the intersection point of the two slopes (βc for cathodic tafel slope and βa for anodic tafel slope) in the tafel plots according to the following equation:44

$%&'' =

() (% *. , -. () + (%

(11)

in which Rp represents the corrosion resistance, which is determined by the current (∆i) at ± 25 mV (∆E). The values of βc, βa, Rp, and icorr are summarized in Table S1 and Figure S9.

-. =

∆$ ∆1

(12)


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The inhibition efficiency was calculated using the following equation:45

$∗%&'' − $%&'' 2 = 344

∗ $%&''

(13)

where icorr*and icorr are uninhibited (control) and inhibited corrosion current densities, respectively. The OCP of the control, LIS*, LIS** changed from -467 mV to -276 mV, -181 mV to -249 mV, -191 mV to -216 mV, respectively (Figure 6B). These OCP shift were attributed to the change of surface composition. The OCP of the control increased owing to the formation of a passivating oxide layer, whereas the decrease in the OCP of LIS*and LIS** was attributed to the water attack to the lubricant layer. In contrast, the OCP of LIMITS did not show a significant change (from -193 mV to -194 mV (Figure 6B)), which indicated that there was almost no change in the surface chemistry of the LIMITS during the 7 days corrosion test. However, the OCP value itself does not provide any information about the corrosion rate.46 More importantly, LIMITS maintained a high Rp and low icorr value, as shown in Figure S9 (∆Rp and ∆icorr were only 10.34% and 13.36%, respectively; Table S1), which suggested that LIMITS could effectively suppress the corrosion of the surface by retaining the lubricant and preserving its performance. Figure S9 also shows the Rp of LIS*and LIS** were drastically decreased (∆Rp were 93.50% and 97.60, respectively; Table S1) owing to the loss of the lubricant. As a result, the icorr of LIS*and LIS** increased (∆icorr were 93.21% and 96.21 %, respectively; Table S1) and corrosion was promoted, although the Rp and icorr values were superior to that of LIMITS. The LIS** exhibited slightly better anticorrosion properties than LIS* because of the higher thickness of its SPS layer.


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The inhibition efficiency calculated by equation (13) shows the effects of inhibition. As seven days passed, the inhibition efficiency of LIMITS was 72.03%, which is 95.06% of the initial value, while that of LIS* and LIS** were 19.18% and 59.49%, which are only 20.30% and 60.46% of initial value (Table S1, and Figure S10). Since the surface of LIMITS is composite of lubricant and SPS, the initial inhibition efficiency is lower than fully lubricated surface. However, the inhibition efficiency of LIMITS is quite stable owing to the self-secretion of lubricant onto the surface. According to these results, the LIMITS showed stable anticorrosion properties for a long time by preventing the evaporation of the lubricant.47

CONCLUSIONS

An advanced anticorrosion coating material, LIMITS, in which the lubricant is inserted between oleophilic porous layers, was designed and prepared using a phase separation technique. The LIMITS had long-term lubricant stability and was better at preventing the corrosion of an Fe plate than a conventional LIS, as indicated by photographic images and electrochemical measurements. Furthermore, the underwater oil contact angle measurements and the theoretical analysis indicated that stable lubrication underwater is difficult to achieve by using conventional lubricating strategy but easy with LIMITS. Owing to the facile and scalable fabrication of LIMITS, the coating technique can be used for the development of practical surface coatings on not only Fe plates but other metal plates to provide stable corrosion-resistance properties under water. We believe the strategy to improve life-time of LIS is not limited in anticorrosion


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application but also other potential use, such as anti-icing coating48 and anti-biofouling medical tube49. For further improvement of lubricant life-time, optimization of solid texture combined with sandwich structure may be required.50


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FIGURES

Figure 1. Schematic illustrations showing the concept of (A) Conventional liquid infused surface (LIS),27 (B) Self-secretion type LIS,36 and (C) lubricant-inserted (sandwiched) microporous triple-layered surface (LIMITS, this work). The difference between (A) and (B-C) is whether they self-secrete or not. Previous research of self-secretion type slippery surface replenishes lubricant layer by embedded lubricating particle moves on the surface (B). 36, 38 LIMITS supplies lubricant on a surface with sandwiched lubricant layer.


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Figure 2. Schematic illustrations showing the design procedure for the preparation of anticorrosion coatings on an Fe plate. A self-standing porous PVDF-HFP surface (SPS) was fabricated according to our previous study.37 Inserted spectrum is x-ray photon microscopy analysis of SPS. A lubricant infused surface (LIS) was fabricated by impregnating the lubricant in the SPS. A lubricant-inserted microporous triple layered surface (LIMITS) was obtained by covering the LIS by self-standing SPS to decrease the surface lubricant area. Sample information in this work is also summarized. Liquid thickness is estimated by dividing liquid volume with coating area.


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Figure 3. Surface morphology analysis of SPS, LIS*, and LIMITS by (A) scanning electron microscopy and (B) laser scanning microscopy. (See Figure S4 for surface morphology analysis of LIS**)


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Figure 4. Wettability analysis of the coatings. (A) Photographic images of a 10 µL water droplet on SPS, LIS, and LIMITS. (B) Water contact angles on the coatings. (C) The loss of lubricant on a LIS** and LIMITS as a function of time in air at 50 ºC. (D) Comparison of water sliding angle change on LIS*, LIS**, and LIMITS as a function of time at room temperature. The green circles indicate the sliding angle after peeling off a top layer of LIMITS after 40 days.


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Figure 5. Study of the underwater lubricant stability. (A, B) Photographic images of a lubricant droplet (10 µL) on SPS under (A) water containing 3.5 wt. % NaCl (salt water) and (B) pure water. (C) Lubricant contact angles under salt water and water (θow). (D) Schematic illustrations of lubricating oil droplet making contact with SPS under water showing the interfacial tension (γ) between the solid (s), lubricant (o), or water (w).


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Figure 6. Corrosion resistance of the coatings. (A) Time-lapse photographic images of an Fe plate surface coated with a control (uncoated), LIS*, LIS**, and LIMITS under salt water conditions as a function of immersion time τ. (B) Tafel polarization curves of Fe plates coated with a control (black triangle for τ=0 and black circle for τ=7), LIS* (red triangle for τ=0 and red circle for τ=7), LIS** (blue triangle for τ=0 and blue circle for τ=7), and LIMITS (yellow triangle for τ=0 and yellow circle for τ=7).


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano. ####### AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses †Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Yokohama, 223-8522, Japan. Author Contributions M.T. designed the experiment and write a paper. M. T. and S. N. conducted the experiment and analyzed the data. M.T., S. N., Y. K. discussed the data. K. K., J. L., and J. A. provided scientific advice. S. S. supervised the project, provided scientific advice and commented on the manuscript. ‡M.T. and S. N. contributed equally. Funding Sources This work was supported by JSPS KAKENHI (grant number JP 16J06070). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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We are grateful to our laboratory member and Prof. Walter Navarrini in Politecnico di Milano whose meticulous comments were of enormous help. M. T. thanks predoctoral fellowship (PD) from Japan Society of Promotion of Science (JSPS). ABBREVIATIONS LIS, Lubricant infused surface; SPS, Self-standable porous surface; LIMITS, Lubricant inserted microporous triple-layered surface.

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A Lubricant-Sandwiched Coating with Long-Term Stable Anticorrosion

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