polybenzoxazine based superhydrophobic coating with enhanced corrosion resistance and high thermal stability

Accepted Manuscript Title: Facile fabrication of epoxy/polybenzoxazine based superhydrophobic coating with enhanced corrosion resistance and high thermal stability Authors: Chang Lou, Rui Zhang, Xin Lu, Changlu Zhou, Zhong Xin PII: DOI: Reference:

S0927-7757(18)31590-5 https://doi.org/10.1016/j.colsurfa.2018.10.066 COLSUA 22952

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

13-8-2018 24-10-2018 24-10-2018

Please cite this article as: Lou C, Zhang R, Lu X, Zhou C, Xin Z, Facile fabrication of epoxy/polybenzoxazine based superhydrophobic coating with enhanced corrosion resistance and high thermal stability, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.10.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile fabrication of epoxy/polybenzoxazine based superhydrophobic coating with enhanced corrosion resistance and high thermal stability Chang Lou, Rui Zhang, Xin Lu*, Changlu Zhou, Zhong Xin

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Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, Department of Product

Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai

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200237, China.

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*Corresponding author. E-mail address: [email protected] (X. Lu)

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Graphical abstract

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Abstract A durable epoxy/polybenzoxazine based superhydrophobic coating with enhanced corrosion resistance and high thermal stability was prepared on mild steel by spray coating and thermal curing method. The curing reaction between epoxy and benzoxazine were investigated

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by Fourier transform infrared spectroscopy (FTIR). The epoxy/polybenzoxazine based superhydrophobic coating exhibited high thermal stability, desirable mechanical durability and

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good self-cleaning property with a static water contact angle of 165.2 ± 2.2 °. The results of electrochemical impedance spectroscopy (EIS) showed that the as-prepared superhydrophobic coating performs high corrosion resistance after immersion in 3.5 wt% NaCl aqueous solution

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for 30 days. It is believed that such a superhydrophobic coating has potential application in

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practical conditions that need waterproof, dustproof, heatproof and corrosion resistance.

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Keywords: Polybenzoxazine; Superhydrophobic; Corrosion; Durability; Mild Steel

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1. Introduction

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Superhydrophobic materials have attracted wide interests due to their varieties of applications in self-cleaning[1-2], antifouling[3-4], water-oil separation[5] ， corrosion

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resistance[6-9] and so forth in the past few decades. Generally speaking, the surface roughness and low surface energy material are two important roles in determining surface wettability[10].

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Commonly, rough structure can be prepared by various methods such as etching[11], electrodeposition[12], template imprinting[13], nanoparticles deposition[14] and so on. Meanwhile,

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it’s necessary to use substance with low surface energy such as expensive fluoride for fabricating superhydrophobic surfaces[15-17]. So nowadays developing cost-efficient nonfluorine superhydrophobic material is significant in practical applications. As a new type of low surface energy material, polybenzoxazine is promising for a stable superhydrophobic surface with low absorption of water[18], near zero shrinkage[19], good dielectric properties[20] and thermal stability[21]. Zhang[10] used pendent aliphatic chain3

substituted

benzoxazine and TiO2 nanoparticles to prepare superhydrophobic surface with

self-cleaning and reversible adhesion properties. Wang[22] prepared superhydrophobic surfaces with excellent abrasion resistance based on bisphenol A-aniline based benzoxazine and mesoporous SiO2. Recently, polybenzoxazine has attracted researchers’ attentions in metal protection field.

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Zhou[23] prepared silane-functional polybenzoxazine coating on steel, used as corrosion

protective coating. Poorteman[24] prepared para-phenylenediamine benzoxazine for barrier

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coating applications on 1050 aluminium alloys. Caldona[25] fabricated superhydrophobic coating with rubber modified benzoxazine and silica nanoparticles as protective coating from

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metallic corrosion. Moreover, favourable crosslinking density of the coating guarantees their

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ideal barrier ability which plays a crucial role in corrosion protection[26]. It is known that a

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stable chemical bond can be formed between benzoxazine and epoxy molecules because the

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epoxide group can react with the phenolic hydroxyl groups of polybenzoxazine[27], which could improve the crosslinking density of the polybenzoxazine and epoxy network[28]. we

prepared

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durable

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Herein,

and

low-cost

epoxy/polybenzoxazine

based

superhydrophobic coating on mild steel with enhanced corrosion resistance and high thermal

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stability by spraying and thermal curing method. The composite coatings exhibited a stable

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superhydrophobicity against from heat treatment and mechanical abrasion, a good self-cleaning property and a persistent high corrosion resistance. The corrosion protection mechanism of the superhydrophobic coating was also proposed.

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2. Experimental 2.1 Materials

Phenol, dodecylamine, formaldehyde aqueous solution (37 wt%) and 4, 4’diaminodiphenyl methane (MDA) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Chloroform, acetone, xylene and n-butyl alcohol were provided from Shanghai Lingfeng 4

Chemical Corp. All chemicals were analytical grade and used as received without any further purification. Epoxy resins (type E-51, 0.53 eq/100 g) were obtained from Wuxi Changgan Chemical Engineering Co. Ltd. Silica nanoparticle (approximately 20 nm diameter) was purchased from Nanjing XFNANO Materials Tech Co.,Ltd. The Mild Steel (MS, composition in wt%: C: 0.12 %, Mn: 0.5 %, S: 0.004 %, P: 0.035 %, Si ≤ 0.3 %, Fe: balance) was purchased

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from Biuged Laboratory Instruments (Guangzhou) Co., Ltd. 2.2 Synthesis of benzoxazine monomer

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A pendent aliphatic benzoxazine, namely 4-dodecyl-3, 4-bihydro-2H-1, 3 benzoxazine was synthesized through the Mannich reaction according to literature[29]. 18.82 g phenol (0.2

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mol) and 49.62 g aqueous formaldehyde solution (37 wt%) were dissolved in 200 mL

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chloroform and poured into a three-neck flask at a temperature of 65 ℃. Then 37.07 g

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dodecylamine (0.2 mol) dissolved in 100 mL chloroform was added into the flask. The reaction

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was conducted for 3 h under continuous stirring. The crude products were washed with sodium hydroxide solution (1 M) for three times and then with water for four times. After being placed

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in anhydrous sodium sulfate overnight, the products were filtered and separated out by drying solvent and then the white and viscous product can be obtained and cryopreserved. The

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synthesis procedure is illustrated in Scheme 1. 1H NMR (400MHz, CDCl3): 3.99 ppm (Ar-CH2-

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N), 4.86 ppm (O–CH2–N), 6.76~7.11 ppm (Ar).

Scheme 1. Synthesis of benzoxazine monomer (P-da)

2.3 Preparation of the epoxy primer Epoxy coatings have been widely used for metal protection as primers. The bare MS (about 15 cm × 7 cm × 0.1 cm) was used as substrate, which was abraded with 400 grit sandpaper and then cleaned with acetone in an ultrasonic bath for 1 h to remove oil on its surface before coating. 5

Then, the epoxy and curing agent MDA (7:3 in mass ratio) were mixed with the solvent consisting of xylene and n-butanol (7:3, V/V). The solution was ultrasonically mixed for 1 h and then was sprayed onto the treated MS. Finally, the samples were cured in oven at 80 ℃ for 2 h and then 150 ℃ for 2 h, and the primers were prepared. The thickness of epoxy primer is 25±2 μm.

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2.4 Preparation of superhydrophobic topcoat

To improve coating adhesion of topcoat, epoxy/polybenzoxazine/SiO2 coatings were

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prepared upon the epoxy primer. P-da and epoxy (4:6 in mass ratio) were dissolved in mixed solution of xylene and n-butanol (7:3, V/V). Subsequently, the designated amounts of SiO2

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nanoparticles (20 wt% of monomers) were dispersed into the mixed solution. The mixture was

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held in an ultrasonic bath for 2 h to form a homogeneous colloidal dispersion and then the

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mixture was sprayed onto primer. Finally, the samples were cured in oven at 100 ℃ for 1h and

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then 200 ℃ for 2 h. In addition, the epoxy/polybenzoxazine coating without nanoparticles was also prepared considered as a reference sample. Samples with or without nanoparticles were

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denoted as PEBS or PEB, and the thickness of them are 41±3 μm and 43±2 μm, respectively. 2.5 Measurement and characterization

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2.5.1 Chemical structure characterization

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FTIR measurements of P-da, epoxy monomers and polymers were performed on a Nicolet iS10 FTIR spectrometer. A solution of monomer in acetone was directly dropped onto a KBr plate and a monomer film was formed by drying the solvent. The polymer films were prepared

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by further curing the as-prepared monomer film, of which the curing temperature and time were the same as mentioned above. Moreover, 1H NMR spectroscopy was performed using a German Bruker AVANCE III at a proton frequency of 400 MHz to characterize the chemical structure of the P-da monomer.

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2.5.2 Surface properties A Data Physics OCA20 goniometer was used to measure static water contact angles (WCAs) of each sample at room temperature. A deionized water droplet of 5 μL was injected onto surface of the sample. WCAs of 9 points were measured, then the average value was calculated. The surface morphologies of the superhydrophobic surface were observed by an S-

of 15 kV. The sample was sputtered with gold before SEM observation.

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2.5.3 Self-cleaning property

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3400 scanning electron microscope (SEM, Hitachi, S-3400, Japan) at an acceleration voltage

The superhydrophobic PEBS coating was placed on a petri dish with a slant angle of about

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15 ° to investigate its self-cleaning property. Graphite powders scraped from a pencil were used

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as model contaminations and were artificially fallen down onto the surface of PEBS coating.

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The PEB coating was studied as a comparison. The water was introduced from a syringe onto

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the surface vertically. Make sure same volume of water be dripped and observe the cleanliness of the surface.

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2.5.4 Stability and durability properties

The mechanically durable performance of the PEBS coating was studied by using a

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sandpaper scratch test. In detail, the prepared coating was placed under a piece of sandpaper

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face to face. A 200 grams of weight was put upon the back of the sandpaper. The sandpaper was moved by force in one direction at a constant velocity as possible as we can to ensure there was no slide between the sandpaper and the weight. A ruler was used to measure the distance of

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scratch and the water contact angle of the samples with every movement of 20 cm was recorded in the process of abrasion test. The high temperature resistant ability of PEBS coating was investigated by heat treatment at temperatures ranged from 50 ℃ ~300 ℃ for 1 h. The water contact angle of samples was measured after heat treatment. 7

2.5.5 The corrosion resistant property Electrochemical impedance spectroscopy (EIS) measurements for the evaluation of corrosion resistance were conducted using AMETEK instruments VersaSTAT 3 workstation, with a stainless steel cylinder as the counter electrode, an Ag/AgCl (Saturated KCl) electrode as the reference electrode and the samples as working electrodes of which exposed area was

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about 14 cm2. All tests were performed in 3.5 wt% NaCl aqueous solution as corrosive medium

at ambient temperature. The working electrode was first immersed in corrosive medium for 30

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min before achieving a stable open-circuit potential (Eocp). EIS measurements were performed

in the frequency range from 105 to 10-2 Hz at the Eocp with an amplitude of 30 mV. Moreover,

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this study conducted long-term immersing test for both coatings. Samples were placed in

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constant temperature and humidity and tested at regular intervals. Zview software was used to

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fit EIS data and calculate electrochemical parameter of samples.

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3. Results and discussion

3.1 Curing reaction between benzoxazine and epoxy

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In order to investigate the chemical structure of P-da monomer, epoxy, PEB and PEBS,

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FTIR were undertaken for all these samples as shown in Fig. 1.

Fig. 1. FTIR spectra of P-da monomer, epoxy, PEB and PEBS 8

The absorption peak observed at 934 cm-1 is assigned to the oxazine ring in P-da monomer [30-31]. The 1034 cm-1 and 1376 cm-1 absorption peaks are primarily assigned to the C-O-C symmetric stretching vibration and -CH2 bending vibration. The characteristic peaks of epoxy appear at 813 cm-1 and 916 cm-1. Moreover, the new band peak at 1103 cm-1 is ascribed to the stretching vibration of Si-O-Si that results from the incorporation of silica nanoparticles. Upon

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heat treatment, as the curves of PEB and PEBS illustrate, the peak at 934 cm-1 disappears, suggesting that the oxazine ring has opened completely and the cross-linking network of

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polybenzoxazine has formed. In addition, the characteristic peaks of epoxy also disappear entirely in PEB and PEBS, which is due to the fact that the active phenolic hydroxyl group of

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polybenzoxazine can play a role of catalyst in curing reaction of epoxide group[27, 32]. So

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polymerization of epoxy and ring-opening polymerization of benzoxazine proceed concurrently,

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which is exhibited in Scheme 2.

Scheme 2. Curing reaction of P-da and epoxy

3.2 Surface morphology and hydrophobicity of PEBS coating Fig. 2. shows the WCAs of different samples and the insets are the corresponding WCA images of each sample. Bare MS performs hydrophilicity with a WCA of 74.3 ± 1.0 °. The WCA of epoxy coating is ~87.5 ± 1.5 ° which shows hydrophilic property due to the large amounts of 9

hydroxyl in epoxy resin. When P-da is introduced into epoxy coatings, the WCA grows to 92.1 ± 1.2 °, showing slight hydrophobicity, which is attribute to the low surface energy of benzoxazine (P-da). After incorporating SiO2 nanoparticles into system, the WCA of PEBS

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coating increases to 165.2 ± 2.2 °, due to the improvement of surface roughness.

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Fig. 2. WCAs of bare MS, epoxy, PEB and PEBS coatings

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Fig. 3. SEM images of PBES surface at different magnifications: (a) 1000× and (b) 30000×

SEM images of PEBS surface at different magnifications are shown in Fig. 3. SEM images

of PEBS indicate that silica nanoparticles cover the surface of sample and form roughness with

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numerous micro-nano hierarchical structures, which constitutes the composite interface and endows an appropriate volume of air trapped in the micro- and nano-scale binary structure[33]. Fig. 3(b) is an enlarged view of Fig. 3(a), demonstrating that silica nanoparticles conglomerate together so that they can form rough structures with micro- and nano-scale, leading to the superhydrophobicity of the PEBS coating. 10

3.3 Self-cleaning property of PEBS coating Superhydrophobic materials with self-cleaning property are significant due to their potential or practical applications. For the PEBS coating, the graphite powders are removed thoroughly as water droplets dispensed from a syringe needle roll down, which is illustrated in Fig. 4 (a1, a2, a3). Therefore, the dirty surface becomes clean again. Meanwhile, a control

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experiment was taken on the PEB coating. The graphite powders and water are still remained and adhere to the surface after droplets washing as seen in Fig. 4 (b1, b2, b3). The self-cleaning

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property of the superhydrophobic PEBS coating makes it possible to be applied on protecting

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metal from dirty and corrosive medium.

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Fig. 4. Self-cleaning property test: (a) PEBS coating and (b) PEB coating

3.4 Stability and durability of PEBS coating. The stability and durability of superhydrophobic coatings are major concerns in practical

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complex environments, such as mechanical scratch and extremely high temperature. The asprepared PEBS superhydrophobic coating was used for stability and durability tests. Mechanical durability is significant for the superhydrophobic surface and a sandpaper abrasion test has been carried out to measure the durability of the PEBS coating. As the abrasion distance increases, the water contact angles perform a slight drop. After the sample is scratched for 200 11

cm, it still possesses superhydrophobicity as shown in Fig. 5, which indicate that the PEBS surface exhibited favourable mechanical durability. Fig. 6 shows the results of heat resistance of the PEBS coating. We can see that the PEBS coating can maintain high superhydrophobicity after heat treatment of 250 ℃ for 1h with a WCA over 162.8±4.0 °. Even after heat treatment of 300 ℃ for 1 h, the WCA of PEBS can still

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approach to 156.3±4.6 °, demonstrating that the PEBS coating performs desirable thermal

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stability.

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Fig. 5. The mechanically durable test of the PEBS coating by using 400 # sandpaper under 200 g

Fig. 6. Variations in WCAs of PEBS coating after heat treatment for 1 h at different temperatures

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3.5 Corrosion resistance of PEB and PEBS coatings All the samples were studied by EIS tests in 3.5 wt% NaCl solution at room temperature. Fig. 7 shows the Bode impedance plots (a) and phase angle plots (b) of bare MS, PEB and PEBS coatings. The impedance plots of the PEB and PEBS coatings are nearly straight lines with slopes of -1 in the entire frequency range which indicates the pure capacitance behaviour

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of both coatings. The impedance modulus (|Z| value) at the lowest frequency (|Z|0.01Hz) in Bode

impendence plots can semi-quantitatively reflect the barrier performance of the coatings[34-35]

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and further indicate the protective properties of coatings[36]. Compared with the |Z|0.01Hz value

of bare MS (3.24×103 Ω·cm2), |Z|0.01Hz value of PEB and PEBS samples are 1.69×1011 Ω·cm2

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and 1.16×1011 Ω·cm2, respectively, which are over seven orders of magnitude higher than that

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of the bare MS (Fig. 7a).

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The phase angle is another important parameter indicating the barrier property of the

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coatings against corrosive medium in wet environments[37]. Peaks appearing in different frequency ranges generally correspond to different time constants in the phase angle plots.

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Peaks in high frequency range (104~105 Hz) reflect the barrier performance of coatings. Peaks appearing in intermediate frequency range (1~103 Hz) correspond to the presence of micro

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pores on coatings while that in low frequency range (10-2~1 Hz) can be attributed to a corrosion

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process occurred on the interface between the metal and the coating[38]. As shown in Fig. 7(b), the phase angle plot of the MS sample only has one peak in low frequency range, indicating the occurrence of corrosion on the MS surface. As to the other two samples, the phase angle plots

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of them both maintain at approximately -90 ° for the whole frequency range, which means they correlate well with the behaviour of pure capacitance and further demonstrates their excellent corrosion resistance ability at the initial stage of immersion.

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Fig. 7. (a) Bode impedance plots and (b) phase angle plots of MS, PEB and PEBS coatings

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Using equivalent circuit models to analyse EIS results can make it clear to understand how coatings protect metallic substrate. A simple model Rs (CdlRct) shown in Fig. 8(a) can be used

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to fit corrosion process of the MS due to its one time constant in the phase angle plot, where Rs

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is solution resistance, Rct is charge transfer resistance that appears in electrode process and is in

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an inverse ratio to corrosion rate, and Cdl is capacitance of the electrical double layer. When it

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comes to the other two samples, a model Rs (CPEcRc) in Fig. 8(b) can fit them very well because both of them exhibit a nearly pure capacitance behaviour. Rc and CPEc represent the resistance

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and constant phase element of coatings, respectively. Using CPE instead of pure capacitance makes it more accurate to fit the given non-ideal behaviour of coatings[39]. The electrochemical

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parameters of samples calculated from fitting results are shown in Table 1. The PEBS coating

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possesses higher coating resistance which is approximately 3 times enhancement compared with the PEB coating. The immersion test will further elucidate their performance variation in

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corrosion resistance.

Fig. 8. Equivalent circuit models of (a) MS, (b) PEB and PEBS coatings

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To further investigate their long-term corrosion protection, the PEB and PEBS coated samples were immersed in 3.5 wt% NaCl aqueous solution for 30 days. The impendence modulus is directly related to the resistance and capacitance of the coatings, which reflect the corrosion resistant performance of the coatings. Low resistance and high capacitance of the coatings will lead to a decreasing of |Z| value[40]. In addition, the capacitance of the coatings

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will increase as the corrosive medium permeates into the coatings gradually[41]. Regarding the PEB coating, as illustrated in Fig. 9 (a, b), the Bode impedance modulus and phase angle in low

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frequency range trends to fall with increasing immersion time. After 30 days of immersion, |Z|0.01Hz declines from 1.69×1011 Ω·cm2 to 5.10×1010 Ω·cm2, and Model (b) can fit EIS data at

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this stage. As to corrosion resistance performance of the PEBS coating, we can see overlapping

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straight lines shown in Fig. 9 (c, d) within the whole 30 days in the entire frequency range, with

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|Z|0.01Hz only decreased slightly from 1.16 × 1011 to 7.51 ×1010 Ω·cm2. The phase angle curves

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keep near to -90 ° in high and intermediate frequency range and start to decline in low frequency range as immersion time prolongs. The phase angle plot of the PEBS coating can be still higher

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than that of the PEB coating after immersing for 30 days. The results of Bode plots indicate that the PEBS coating exhibit pure capacitance behaviour in the whole 30 days of immersion, which

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verifies the rationality of using model B to fit its EIS data. The PEBS coating performs desirable

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protection to metallic substrate against water permeation because of its superhydrophobicity

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and barrier ability.

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Fig. 9. Bode impedance plots and phase angle plots of (a, b) PEB and (c, d) PEBS coatings,

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respectively immersed in 3.5 wt% NaCl aqueous solution for 30 days and solid lines are fitted results

Table 1 summarizes the results of the fitted parameters in 30 days of immersion. We can

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see that each model is well fitted and applicable to the fitted plots with low Chi-Squared (less

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than 6 ‰). Generally, Rc represents the porosity and the degree of degradation of coatings. It’s

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usually related to the amounts of micro-pores and channels vertical to substrates inside the

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coatings and corrosive medium pass through these pores and channels to reach the interface of coating/metal[42]. As shown in Table 1, Rc decreases as immersing time prolongs, which

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indicates the permeation of corrosive medium. The Rc drops from 3.12 ×1012 Ω·cm2 to 1.84×1011 Ω·cm2 for the PEBS coating after 30 days immersion in 3.5 wt% NaCl aqueous

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solution, while it drops from 1.04×1012 Ω·cm2 to 6.7×1010 for the PEB coating. Moreover, due

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to the high dielectric constant of water, the adsorption of water makes the dielectric constant of the coating increase, leading to an overall rising trend of CPEc.

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Table 1 Electrochemical parameters fitted from EIS results of the PEB and PEBS coatings in immersion tests PEB

PEBS

Days

Chi-Squ /‰

Rc /Ω·cm2

CPEc /F·cm-2

Chi-Squ /‰

Rc /GΩ·cm2

CPEc /nF·cm-2

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0.5

1.04×1012

8.9×10-11

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3.12×1012

1.15×10-10

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2.7

1.77×1011

9.5×10-11

1.3

2.97×1011

1.32×10-10

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2.8

1.27×1011

9.5×10-11

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2.62×1011

1.38×10-10

30

2.9

6.70×1010

9.5×10-11

2.9

1.84×1011

1.45×10-10

As to the PEBS coating, model (b) fits its EIS data well from initial stage to 30 days of immersion with Chi-Squared less than 3‰ as illustrated in Table 1. As the immersion test proceeds, Rc value decreases and remains above 1011 Ω·cm2 in the whole 30 days. This can

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provide a strong qualitative evidence of the extraordinary barrier performance. A decreasing Rc

and increasing CPEc as observed may be attributed to the fact that the corrosive medium

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gradually infiltrates the surface and starts to penetrate into the coating. The PEBS coating exhibits nearly pure capacitance behaviour because its CPE exponents (n) for all period are

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3.6 Corrosion protection mechanism of PEBS coating

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higher than 0.97, which further indicates an outstanding corrosion resistance[13].

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The incorporation of P-da makes the PEBS coating possess low surface energy. The

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introducing of the silica nanoparticles constructs roughness on the surface of the PEBS coating with micro- and nano-structure, constituting a superhydrophobic surface. The rough surface of

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superhydrophobic material possess cavity structures that can reserve air so that it can form a continuous layer of air film. As shown in Fig. 10, it can be observed that the surface of PEBS

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coating immersed into 3.5 wt% NaCl aqueous solution is exceptionally bright when viewed

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from the side. This phenomenon demonstrates that air can be trapped in rough surface of

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superhydrophobic coatings, according to the theory of total reflection[43].

Fig.10 Digital photograph of superhydrophobic PEBS coating immersed in 3.5 wt% NaCl aqueous solution 17

When the PEBS samples are exposed to aggressive corrosive medium, the air film can exist between corrosive medium and surface of sample, isolating metallic substrate from corrosive medium to protect metal as illustrated in Fig. 11. After long-term of immersion in corrosive medium, the air film in the interface of coating/metallic substrate dissolves in aqueous phase gradually. Then, corrosive medium takes place of the air film and contacts the surface of

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coating directly. In this stage, the barrier performance of the organic coating plays an important role in its protective effect. The curing process of epoxy and P-da makes an interrelated dense

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network because of the reaction between epoxy ring and phenolic hydroxyl group in

benzoxazine through oxzine-etherification reaction, which could ensure its barrier performance.

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Furthermore, agglomerated silica nanoparticles dispersed heterogeneously in the coating hinder

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corrosive medium from passing through the coating by making the paths more circuitous. As a

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result，the two factors reduce the possibility that corrosive medium permeates through the

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superhydrophobic coating and endow the coating with desirable corrosion resistance.

Fig. 11. Corrosion protection mechanism of superhydrophobic PEBS coated MS

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Conclusions

In summary, we reported a facile fabrication of epoxy/polybenzoxazine based

superhydrophobic coating (PEBS) on MS substrates by spraying and thermal curing. FTIR results confirm the formation of a dual crosslinking network between epoxy and polybenzoxazine. The PEBS coating with a static WCA 165.2 ° exhibited good self-cleaning 18

property. The as-prepared superhydrophobic coating is stable even after heat treatment at 300 ℃ for 1 h and it also possesses excellent mechanical durability. The EIS results showed that the superhydrophobic coating performs high corrosion resistance after immersion in 3.5 wt% NaCl aqueous solution for 30 days. As a result, we anticipate that the epoxy/polybenzoxazine based superhydrophobic coating has promising application in conditions that need waterproof,

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dustproof and corrosion resistance.

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Acknowledgements

This work was financially supported by National Natural Science Foundation of China (21776080，21776091) and the Fundamental Research Funds for the Central Universities

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(22A1817025).

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