Self-Healable Superomniphobic Surfaces for Corrosion Protection

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Self-healable superomniphobic surfaces for corrosion protection Mohammadamin Ezazi, Bishwash Shrestha, Nathan Klein, Duck Hyun Lee, Sungbaek Seo, and Gibum Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08855 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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ACS Applied Materials & Interfaces

Self-healable superomniphobic corrosion protection

surfaces

for

Mohammadamin Ezazi †, Bishwash Shrestha †, Nathan Klein †, Duck Hyun Lee ‡, Sungbaek Seo §, Gibum Kwon †* Department of Mechanical Engineering, University of Kansas, Lawrence, Kansas 66045, USA. †

‡

Green Materials and Process Group, Korea Institute of Industrial Technology, Ulsan, 44413,

Republic of Korea. § Department of Biomaterials Science, Pusan National University, Miryang, 50463, Republic of Korea. *Email: [email protected] Keywords: superomniphobic surface, corrosion protection, chemical durability, self-healing, epoxidized soybean oil Abstract Corrosion protective surfaces are of the utmost relevance to ensure long term stability and reliability of metals and alloys by limiting their interactions with corrosive species, such as water and ions. However, their practical applications are often limited either by the inability to repel low surface tension liquids such as oils and alcohols or by the poor mechanical durability. Here, a superomniphobic surface is reported that can display very high contact angles for both high and low surface tension liquids as well as the concentrated acids and bases. Such an extreme repellency allowed for approximately 20% of the corrosion rate compared to the conventional

superhydrophobic

corrosion

protective

coatings.

Furthermore,

the

superomniphobic surface can autonomously repair the mechanical damage at an elevated temperature (60 C) within a short period of time (60 s), and the surface can restore its intrinsic 1 ACS Paragon Plus Environment

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corrosion protection performance. Such superomniphobic surfaces thus offer a wide range of potential applications including pipelines with sustainable corrosion protection and rust inhibitors for steel in reinforced concrete. Introduction The corrosion of metals and alloys has been a grand challenge due to its detrimental impacts on the environment,1 energy and fuel infrastructure,2 as well as health,3 and safety.4 Of particular interest is the corrosion in aggressive liquids such as seawater, acids and bases that are typically found in a wide range of applications including offshore constructions,5 storage tanks6 and petrochemical plants.7 To date, a large volume of research has focused on improving the corrosion resistance of metals by introducing a protective coating (surface) with a liquid barrier property.8-11 Corrosion protective surfaces (CPSs) can isolate the underlying metallic substrate from corrosive attack by preventing the permeation of the corrosive liquids (e.g., seawater).12 In recent years, superhydrophobic surfaces displaying apparent contact angle (*) for water greater than 150 with a low contact angle hysteresis * (i.e., the difference between the advancing and receding contact angles) have demonstrated effective corrosion prevention in aqueous corrosive liquids.13-16 This can be attributed to a synergetic effect of a rough surface structure and a low solid surface energy (sv) enabling the surface to trap plastron between the contacting liquid and the solid structure, which can significantly reduce the contact area between them.17-19 In addition, the low surface energy layer can further prevent the diffusion of the corrosive liquid into the bulk coating.20 However, their wettability with low surface tension corrosive liquids is often not specified.13-15, 20 Typically, low surface tension liquids, such as various oils and alcohols, tend to readily wet and spread on most solid surfaces. It can be inferred that surfaces that repel low surface tension liquids can easily repel high surface tension liquids. Given that most corrosive liquids

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in real practice possess low surface tension,21 developing corrosion protective surfaces that can repel both high and low surface tension corrosive liquids is essential. Superomniphobic surfaces display apparent contact angles * that are greater than 150 and exhibit low contact angle hysteresis with essentially all contacting liquids.22, 23 In previous work, we

24, 25

and others

26, 27

explained how re-entrant surface texture in conjunction with

surface chemistry and roughness can be used to design superomniphobic surfaces. However, developing mechanically durable superomniphobic surfaces has thus far been challenging. This is primarily because their micro- and nanoscale surface texture is vulnerable to mechanical damage, which results in defects that may cause a loss of extreme repellency. Recently, few reports

28-30

have demonstrated mechanically durable superomniphobic surfaces by utilizing

monolithic fluorinated aerogels,28 or polymer-nanoparticle composites.29, 30 Even the most durable superomniphobic surface will eventually become damaged upon an extreme or repeated mechanical stress, which may cause irreversible wetting. Therefore superomniphobic surfaces that can repair the mechanical damage and restore their intrinsic physico-chemical properties would be highly desirable. Herein, we report a superomniphobic surface composed of the cross-linked blend (EFC) of epoxidized soybean oil (ESO), a perfluorinated epoxy (F-epoxy) and citric acid (CA) along with silica nanoparticles (SiO2). It can display very high contact angles for liquids including water, oils, alcohols, as well as concentrated acid and base. The copper substrate spray-coated with the superomniphobic blend exhibits corrosion rate of approximately 20% of the conventional superhydrophobic corrosion protection coatings when immersed in NaCl solution (3.5 wt%). Furthermore, our superomniphobic surface can autonomously repair the mechanical damage at an elevated temperature (60 C) within a short period of time (60 s), and restore its intrinsic chemical resistance and corrosion protection performance. Such superomniphobic surfaces thus offer a wide range of potential applications including pipelines with sustainable corrosion protection, marine heat exchanger, and rust inhibitors for steel in reinforced concrete. 3 ACS Paragon Plus Environment


Results and discussion In this work, we employed a single-step synthesis by reacting epoxidized soybean oil (ESO), 3-Perfluorooctyl-1,2-epoxypropane (F-epoxy), and citric acid (CA) with the weight ratio of 89:4:5 (see Experimental Section). Citric acid can protonate both ESO and F-epoxy and initiate epoxide ring-opening polymerization.31 This reaction results in the cross-linked ESO-F-epoxycitric acid (EFC) copolymer network. The synthesis method is shown in Scheme 1.

Scheme 1. Schematic illustrating a single step synthesis of the cross-linked ESO-F-epoxy-citric acid (EFC) copolymer network.

Superomniphobic surface was created using a facile spray method. Briefly, a copper (Cu) substrate was sprayed with a solution of ESO, F-epoxy, citric acid, and silica (SiO2) nanoparticles. We chose copper (Cu) substrate in this work because it is one of the most corrosion-vulnerable metallic materials compared to others such as stainless steel and titanium, which needs an effective corrosion protection.32 SiO2 was used to introduce the surface roughness with re-entrant texture (i.e., overhang or convex topography) (see Figure 1a and 1b). The re-entrant texture and the low solid surface energy (sv) can result in a robust Cassie-Baxter state with air trapped between the solid structure and the contacting liquid.33 The prepared surface showed very high contact angles for liquids including a concentrated acid (5M, hydrochloric acid, HCl) and a concentrated base (5M, sodium hydroxide, NaOH) (Figure 1c). 4 ACS Paragon Plus Environment

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Figure 1d shows the advancing and receding apparent contact angles as well as roll-off angles () of liquids with a wide range of surface tension values on the prepared surface. We can find that the surface displays *adv > 150 and  < 10 toward liquids with lv  25.3 mN m-1 (n-dodecane). Our experimentally measured roll-off angles (that is, the minimum angle by which the substrate must be tilted for the droplet to roll off from the surface)24 match reasonably well with the predictions based on the work by Furmidge 34 (Supporting Information, SI section 1). The key parameter influencing the surface chemistry is the composition of F-epoxy in the blend of ESO, F-epoxy, and citric acid (EFC). To systematically investigate the effect of Fepoxy composition on the superomniphobicity, we fabricated EFC blends with different compositions of F-epoxy and measured the contact angles. Our results indicate that EFC blend with 5 wt% F-epoxy exhibited the lowest solid surface energy (sv = 11.3 mN m-1, SI section 2). Figure 1e shows time sequence images of droplet (radius  1 mm) of n-hexadecane (hexadecane = 27.5 mN m-1) dropped under gravity from a height of  7 mm. A video illustrating the bouncing of n-hexadecane droplet on the superomniphobic EFC surface is included as SI (Movie S1).

Figure 1. a-b) Scanning electron microscopy (SEM) images of the surface of a glass slide sprayed with EFC with SiO2 blend showing re-entrant texture with low and high magnification. c) Photograph of four droplets including water, n-dodecane, hydrochloric acid (HCl, 5M), and sodium hydroxide (NaOH, 5M) on superomniphobic EFC surface. d) Advancing and receding apparent contact angles, as well as roll-off angles for liquids with various surface tension on superomniphobic EFC surface. e) Series of images illustrating a droplet of n-hexadecane bouncing on our superomniphobic EFC surface. 5 ACS Paragon Plus Environment


Our superomniphobic EFC surface possesses exceptional chemical resistance as it can cause essentially all liquids to roll off. We tested the chemical durability by immersing a copper substrate coated with superomniphobic EFC blend in a concentrated base (5 M, NaOH) and measuring the apparent contact angles as a function of immersion time. We measured the contact angles for NaCl solution (3.5 wt%) possessing a low surface tension (lv = 28.5 mN m1)

(see Experimental Section). Figure 2a shows that the advancing apparent contact angles

decreased 2 while the receding contact angles decreased 3. For comparison, we tested the conventional superhydrophobic and hydrophobic corrosion protective coatings using a neat ESO and commercially available paint (Pond Shield) (see Experimental Section and SI section 3). The solid surface energy was computed as 25.0 mN m-1 and 28.6 mN m-1, respectively, for a neat ESO and commercial paint. The advancing apparent contact angles decreased 29 and 31 on a neat ESO and a commercial paint coated surface, respectively, while the receding contact angles became zero after 24 hours of immersion (Figure 2a). This can be attributed to the alteration of surface chemistry caused by hydrolysis of the ester groups (SI section 4), which may result in the permeation of corrosive species such as water and ions (e.g., chloride Cl-). Consequently, the underlying copper substrates underwent the corrosion reaction and produce cupric hydroxide (Cu(OH)2) or cuprous oxide (Cu2O) on their surface (see the insets in Figure 2a). The chemical resistance to concentrated acid (5 M, hydrochloric acid, HCl) was tested. Figure 2b shows the measured * for a low surface tension NaCl solution (3.5 wt%) on superomniphobic EFC, neat ESO and commercial paint coated Cu substrates as a function of immersion time. As expected, the advancing apparent contact angles decreased 27 and 28 on a neat ESO and a commercial paint coated surface, respectively, while the receding apparent contact angles became zero after 24 hours (Figure 2b), which is caused by the alternation of surface chemistry (SI section 4). In contrast, our superomniphobic EFC surface displayed 6 ACS Paragon Plus Environment

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negligible change of only 1° and 2° for advancing and receding apparent contact angles, respectively. Such an exceptional chemical resistance allows our superomniphobic EFC surface to cause a liquid droplet to roll off after being immersed in a concentrated acid for 48 hours.

Figure 2. a-b) Time-dependent advancing and receding apparent contact angles for a NaCl solution (3.5 wt%) with a low surface tension (lv = 28.5 mN m-1) on the copper substrate coated with superomniphobic EFC blend, neat ESO and commercial paint (Pond Shield) submerged in a) concentrated base (NaOH, 5M) and b) in concentrated acid (HCl, 5M). The insets in a) show SEM images of the underlying copper substrates after 48 hours immersion that were initially coated with (i) superomniphobic EFC, (ii) neat ESO, and (iii) commercial paint. To demonstrate the corrosion resistance, we used the potentiodynamic polarization method.35 Tafel plots were measured after the stable open-circuit potential was obtained (see Experimental Section). The corrosion potential (Ecorr) and corrosion current density (Icorr) were determined by the extrapolation method.36 Figure 3a shows the measured Tafel plots of the Cu substrates coated with superomniphobic EFC blend, superhydrophobic neat ESO, and commercial paint in NaCl solution (lv = 76.3 mN m-1, 3.5 wt.%). The Ecorr and Icorr of superomniphobic EFC surface are -81.8 mV and 15.4 nA cm-2, respectively. In comparison, those values of neat ESO and commercial paint were -142.0 mV and 23.8 nA cm-2, and -201.0 mV and 89.9 nA cm-2, respectively. On the basis of electrochemical kinetics of corrosion,36 a more positive Ecorr value corresponds to a lower corrosion probability, while the Icorr is a 7 ACS Paragon Plus Environment


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measure of the corrosion rate. Therefore, it can be inferred that our superomniphobic EFC surface possesses more effective corrosion protection compared to the conventional superhydrophobic (or hydrophobic) coatings. The result can be attributed to the plastron formed between the solid structure and the contacting NaCl solution which served as an additional nonconductive barrier, enhancing the overall corrosion protection performance.14. The difference in the corrosion protection performance between the superomniphobic surface and the conventional superhydrophobic (or hydrophobic) surfaces was further demonstrated in a low surface tension NaCl solution (lv = 28.5 mN m-1) (Figure 3b). The superomniphobic EFC surface exhibited slightly shifted Ecorr (-99.5 mV) to a negative direction and an increased Icorr (71.8 nA cm-2). In contrast, we found that the values of Ecorr and Icorr of neat ESO and commercial paint significantly changed to -131.0 mV and 229.0 nA cm-2, and 214.0 mV and 364.0 nA cm-2, respectively. The results suggested that conventional superhydrophobic (or hydrophobic) coatings are susceptible to the corrosion in a low surface tension corrosive liquid. Utilizing the obtained Icorr values, we estimated the corrosion rate (r, m year-1) according to Equation 1 36: r=

kIcorrMn

(1)

n

Here, Mn, , and n are the molar mass, density, and the valence of metal, respectively. k is a corrosion rate constant. Figure 3c shows the calculated corrosion rates of Cu substrates coated with the superomniphobic EFC blend, neat ESO, and commercial paint in NaCl solutions. Superomniphobic EFC surface showed corrosion rate of 0.35 m year-1 and 1.6 m year-1 in a high and low surface tension NaCl solution, respectively. Commercial paint exhibited 2.1 m year-1 and 8.5 m year-1 whereas neat ESO showed 0.6 m year-1 and 5.3 m year-1 in a high and low surface tension NaCl solution, respectively. Although the results indicate that our superomniphobic surface can delay the corrosion of the underlying Cu substrate, please note


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that it will be eventually corroded due to its non-zero Icorr value. Further, the Icorr values increases when our superomniphobic surface is fully wetted (i.e., Wenzel state, see SI section 5).

Figure 3. a-b) Measured current density of copper substrates coated with superomniphobic EFC, neat ESO and commercial paint in NaCl solution (3.5 wt%) with a) a high surface tension (lv = 76.3 mN m-1) and b) low surface tension (lv = 28.5 mN m-1). c) The estimated corrosion rate of copper substrates coated with superomniphobic EFC, neat ESO and commercial paint in NaCl solutions. In addition, 100% corrosion protection is challenging because corrosion-related ions or molecular species can diffuse through the coating layer over time.37 We evaluated the corrosion protection performance of the superomniphobic EFC coating (thickness = 702 m) in a low surface tension NaCl solution for a prolonged time (144 hours). In order to quantify the diffusion of NaCl solution through the coatings, we first experimentally measured the diffused mass (Mt) of NaCl solution (see Experimental Section). Mt is scaled with Mo which is the maximum possible diffused mass of NaCl solution through the coating. Mo was experimentally determined (see Experimental Section). The values of Mt/Mo were fitted using a onedimensional Fickian diffusion model 38 given by: (2)

where D and l are the diffusion coefficient and the thickness of coating. We can estimate the D by calculating the slope of the plot when Mt/Mo < 0.5 (see Experimental Section). The value of D of our superomniphobic EFC coating is 6.9  10-5 cm2 day-1 whereas those of neat ESO and 9 ACS Paragon Plus Environment


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commercial paint are 36.1  10-5 cm2 day-1 and D = 338.5  10-5 cm2 day-1, respectively. Diffusion of a high surface tension NaCl solution through the superomniphobic EFC, neat ESO and commercial paint coatings are discussed in SI section 6. Figure 4 shows the experimentally measured diffused mass (Mt) of NaCl solution into our EFC coating as a function of immersion time. The superomniphobic EFC coating was completely saturated (i.e., Mt/Mo  1) after 108 hours whereas neat ESO and commercial paint took 47 hours and 0.3 hours, respectively. Note that the thickness of neat ESO and commercial paint was 701 m and 700.1 m, respectively. This result implies that the conventional superhydrophobic

(or

hydrophobic)

coatings

may

fail

significantly

earlier

than

superomniphobic coating in a low surface tension corrosive liquid.

Figure 4. A plot of measured diffused mass of 3.5 wt% NaCl solution (Mt) with a low surface tension through the coatings as a function of time (hour1/2). Mt is scaled with Mo which is the maximum possible mass of the diffused NaCl solution. Inset: a zoomed-in image showing Mt/Mo during the first 4 hours.


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Practical application of the corrosion protective surfaces requires good mechanical durability. However, under external abrasion, the mechanical durability can be readily compromised (SI section 7). A surface that can autonomously repair the mechanical damage and restore the intrinsic properties would overcome these limitations. Our superomniphobic EFC surface can self-heal a scratch at an elevated temperature (T = 60 C) within a short period of time (60 s). Figure 5a shows time sequence images for self-healing of a scratch on the superomiphobic EFC surface. A scratch ( 45 m wide and  70 m deep) was made on the surface (Figure 5a(i)). When the temperature reaches at 60 C, the scratch becomes narrow (Figure 5a(ii)) and nearly invisible after 60 s (Figure 5a(iii)). The healing process can be attributed to the viscoelastic flow of the EFC blend at the glass transition temperature (Tg).39 The Tg of our EFC blend is approximately 56 C (SI section 8). A video illustrating the selfhealing of our superomniphobic EFC surface is included as supporting information (Movie S2). Figure 5b shows the relations of the percentage (%) healing efficiency of the superomniphobic EFC surface at different temperature (60 C, 80 C and 100 C). The healing efficiency (%) is defined as 100(wo-wt/wo), where wo and wt are the width of the scratch at t = 0 and t. The results indicate that the healing process can be facilitated at a higher temperature. Such temperature-dependent healing can be a result of the transesterification bond-exchange (TBE) reaction

40

which typically takes place in a cross-linked epoxy network at an elevated

temperature (see the inset in Figure 5b). TBE reaction can enhance the viscoelastic flow by releasing the internal stress in the cross-linked network.40 Our superomniphobic EFC surface’s ability to restore its corrosion protection performance was evaluated by measuring the polarization resistance (Rp) after self-healing and comparing it with the value of the as-prepared sample. The Rp was obtained from the slope (Rp = V i-1) of the Linear Polarization Resistance (LPR) plot.41 We measured the LPR plots for the surface with a scratch immersed in low surface tension NaCl solution (SI section 9). The 11 ACS Paragon Plus Environment


temperature was maintained at 60 C. Figure 5c shows the in situ Rp values of superomniphobic EFC surface during the self-healing. Initially, the Rp was 0.67 k implying that NaCl solution can directly contact the Cu substrate. Upon the onset of self-healing, the Rp gradually increased and eventually reached at the value of 15.3 k. Note that the Rp of as-prepared superomniphobic EFC surface was 15.9 k (SI section 9). In comparison, the Rp of neat ESO and commercial paint during self-healing are also provided in Figure 5c. Similar to our superomniphobic EFC surface, the Rp of neat ESO surface increased and reached at 5.8 k which is close to its pristine value of 5.9 k (SI section 9). This is because a neat ESO can repair the scratch at 60 C (SI section 10). In contrast, the Rp of the commercial paint remained almost constant (0.67 k) indicating that the scratch was not healed. Finally, our superomniphobic EFC surface demonstrated that it can restore the corrosion protection against concentrated acid after healing the mechanical damage. Figure 5d shows time-sequence images of the Cu substrate coated with superomniphobic EFC with a scratch ( 80 m wide and  70 m deep) submerged in concentrated nitric acid (5 M) at 60 C (Figure 5d(i)). The underlying Cu reacted with nitric acid and vigorously generated bubbles (Figure 5d(ii)). After the completion of healing, no bubbles were generated suggesting that the Cu substrate became protected (Figure 5d(iii)). A video illustrating our superomniphobic EFC surface’s self-healing in nitric acid is included as supporting information (Movie S3).


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Figure 5. a) Time-sequence SEM images of repairing the scratch on superomniphobic EFC surface at 60 C. The insets show the energy dispersive spectroscopy (EDS) image of fluorine before and after repairing the scratch (scale bar = 100 m). b) A plot of % healing efficiency of superomniphobic EFC surfaces at 60C, 80C, and 100C. The inset shows schematic illustrating molecular rearrangement induced by transesterification bond exchange reaction. c) Polarization resistance (Rp) of superomniphobic EFC, neat ESO, and commercial paint coated copper substrates during self-healing process. d) Series of images showing the self-healing of the scratch on superomniphobic EFC coated copper substrate submerged in nitric acid (5M) and the recovery of chemical resistance. Conclusions In summary, we have developed a superomniphobic surface exhibiting very high contact angles for virtually all liquids and exceptional chemical durability toward both concentrated acid and base. The superomniphobic surface displayed approximately 20% of the corrosion rate compared to the conventional superhydrophobic (or hydrophobic) corrosion protective coatings in NaCl solutions (3.5 wt%). We demonstrated that it can autonomously repair the mechanical damage at an elevated temperature and restore the intrinsic corrosion protection and chemical resistance. We anticipate that such superomniphobic surfaces will enable sustainable technologies in corrosion protection, rust inhibitors, and marine heat exchangers. 13 ACS Paragon Plus Environment


Experimental Section Corrosion protection surfaces fabrication: (i) superomniphobic EFC surface: A solution of epoxidized soybean oil (ESO), 3-Perfluorooctyl-1,2-epoxypropane (F-epoxy), citric acid (CA), and silica nanoparticles (SiO2, average diameter  250 nm) were prepared in ethanol. The overall concentration of the solute (ESO, F-epoxy, CA, and SiO2) was 48 wt%. The weight ratio of ESO, F-epoxy, CA, and SiO2 was 89:4:5:1. Please note that this composition results in the lowest solid surface energy of the EFC blend (see SI section 2). The solution was then sprayed onto a copper (Cu) substrate followed by polymerization at 120 C for 60 mins. (ii) neat superhydrophobic ESO surface: A solution of ESO, CA, and SiO2 were prepared in ethanol. The overall concentration of the solute (ESO, CA and SiO2) was 48 wt%. The weight ratio of ESO, CA, and SiO2 was 94:5:1. The solution was sprayed onto a Cu substrate followed by polymerization at 120 C for 60 mins. (iii) Commercial paint: Pond Shield was purchased from Pond Armor and prepared by following the manufacturer’s instruction. Briefly, the epoxy resin and the curing agent were mixed in 2:1 weight ratio followed by vigorous stirring at 700 rpm for 10 mins. The Cu substrate was spin-coated with the solution followed by curing for 24 hours at room temperature (22 C). All coatings were prepared on copper substrates with 2 cm  1 cm  0.05 cm. Potentiodynamic polarization method 35: A Cu electrode was treated with a desired coating. A platinum and Ag/AgCl were used as counter and reference electrodes, respectively. Electrolyte solutions were NaCl solution (3.5 wt%) with either high (lv = 76.3 mN m-1) or low (lv = 28.5 mN m-1) surface tension. The Tafel plots were obtained at a scan rate of 10 mV s-1. Self-healing test: A scratch was manually created using a scriber. Optical microscopy was used to confirm that the underlying Cu substrate was exposed. The temperature was maintained at 60 C during the self-healing test. We utilized Gamry Interface 1000 electrochemical workstation and Gamry Framework software. 14 ACS Paragon Plus Environment

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Contact angle measurement: All contact angles were measured by advancing or receding about 3 μL of liquid onto the surface (Ramé-Hart 190-U1 goniometer). Measurement of Mt and Mo: The coatings were immersed in 3.5 wt% NaCl solution for 144 hours. The mass of the entire coating was periodically measured. The diffused mass of NaCl solution to the coating (Mt) over time was determined by subtracting the mass of as-prepared coating. The maximum possible mass of 3.5 wt% NaCl solution diffused to the coating (Mo) was determined by submerging it in a solution for 144 hours at which no further mass change was observed. Diffusion coefficient (D) estimation42: The diffusion coefficient (D) of the coatings were estimated by calculating the slope of Mt/Mo curves (see Figure 4 in the main text). A linear line was created by connecting zero and the point of the curve at which Mt/Mo is 0.5. The slope of this linear line was denoted as the diffusion coefficient. Preparation of low surface tension 3.5 wt% NaCl solution: First we mixed ethanol and DI water with 1:1 weight ratio. Then we added NaCl to the solution to make the overall concentration of 3.5 wt%. Scanning electron microscopy (SEM): The morphology of the surfaces was imaged using field emission scanning electron microscope (FE-SEM, FEI Versa 3D Dual beam) at an accelerating voltage of 10 kV. To prevent charging, all the surfaces were sputtered coated with a thin layer of gold ( 10 nm). Energy dispersive x-ray spectroscopy (EDS): The elemental analysis of the superomiphobic EFC surface before and after self-healing was performed by EDS. The EDS area mapping was obtained by using energy of 10 eV. Coating thickness measurement: We utilized an optical profilometry (Veeco Wyko NT 1100) to determine the coating thickness. The coating was clearly cut using a scalpel. Then the stepped edge of the coating was scanned with the optical profiler at a scan rate of 500 nm per second.



Further, we performed cross-sectional SEM. The stepped edge height was imaged and directly measured by SEM software. The measurements were conducted at least 5 different places. Supporting information Estimation of roll off angles on superomniphobic EFC surface, Estimation of the solid surface energy of EFC blends; Wettability of superhydrophobic ESO and hydrophobic commercial paint; FTIR analysis of various surfaces after immersion in concentrated acid and concentrated base; The corrosion protection in the Wenzel state; Diffusion of high surface tension NaCl solution (3.5 wt%) through various surfaces; Mechanical durability tests using linear abraser; Characterizing glass transition temperature (Tg) of superomniphobic EFC surface; Linear polarization resistance of various surfaces; Self-healing of a neat ESO surface; Movie showing bouncing of n-hexadecane droplet on the superomniphobic EFC surface; Movie showing superomniphobic EFC surface can repair the surface scratch at elevated temperature; Movie showing recovery of chemical resistance of superomniphobic EFC surface after repairing. Author information Corresponding Author Email: [email protected] Author contributions G.K. designed the research. M.E., B.S., N.K., performed experiments. M.E., B.S., N.K., D.H.L., S.S., analysed the data. M.E., B.S., G.K., wrote the manuscript. Notes The authors declare no competing financial interest. Acknowledgements This research was supported by the University of Kansas and the Kansas Corn Commission under grants 2234508-099-MESUPPRV and 1000350. This research has been conducted with the support of the Korea Institute of Industrial Technology (KITECH US-18-0001). We thank Dr. Matt O’Reilly at the University of Kansas for use of facilities. 16 ACS Paragon Plus Environment

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Self-Healable Superomniphobic Surfaces for Corrosion Protection

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