Scholarly article on topic 'Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step Spraying Method'

Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step Spraying Method Academic research paper on "Chemical sciences"

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{Superhydrophobicity / Polymer / "Corrosion resistance" / Adhesion}

Abstract of research paper on Chemical sciences, author of scientific article — Wael I. El Dessouky, Rafik Abbas, Wagih A. Sadik, Abdel Ghaffar M. El Demerdash, Ahmed El Hefnawy

Abstract Superhydrophobic metal substrates have been fabricated by a simple spraying method. The processes of decreasing surface free energy and increasing surface roughness have been accomplished in one step via the addition of functionalized silica (silica nano particles with octyltriethoxysilane) to adhesive polymer. The method is simple, cost-effective and can be applied on the large industrial scale. Scanning electron microscopy (SEM) was used for surface morphology analysis, showing the roughness produced by surface treatment. The wettability of the micro-nano silica film varied from hydrophilicity (water contact angle 88°) to superhydrophobicity (water contact angle 156.9°), while sliding contact angles dramatically decreased (<5°) by adding Functionalized silica and/or adhesive polymer. Roughness increased with silica increment which improves the wettability. The coatings were electrochemically characterized by electrochemical impedance spectroscopy (EIS) and Tafel polarization curves; it was found that both systems had good performance against corrosion in 3.5% sodium chloride solution. Furthermore, the stability of the coated layer on copper substrate was investigated.

Academic research paper on topic "Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step Spraying Method"

Accepted Manuscript

Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step Spraying Method

Arabian Journal of Chemistry

Wael I. El Dessouky, Rafik Abbas, Wagih A. Sadik, Abdel Ghaffar M. El Demerdash, Ahmed El Hefnawy

PII: DOI:

Reference:

S1878-5352(15)00339-1 http://dx.doi.org/10.1016/j.arabjc.2015.12.011 ARABJC 1820

To appear in:

Arabian Journal of Chemistry

Received Date: 19 August 2015

Accepted Date: 18 December 2015

Please cite this article as: W.I. El Dessouky, R. Abbas, W.A. Sadik, A.G.M. El Demerdash, A.E. Hefnawy, Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step Spraying Method, Arabian Journal of Chemistry (2015), doi: http://dx.doi.org/10.1016/j.arabjc.2015.12.011

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Improved Adhesion of Superhydrophobic layer on metal Surfaces via One Step

Spraying Method

Wael I. El Dessouky1, Rafik Abbas2, Wagih A. Sadik3, Abdel Ghaffar M. El Demerdash4, Ahmed El Hefnawy 5

Institute of Graduate Studies and Research, Alexandria University, Egypt.

Abstract

Superhydrophobic metal substrates have been fabricated by a simple spraying method. The processes of decreasing surface free energy and increasing surface roughness have been accomplished in one step via the adding of functionalized silica (silica nano particles with octyltriethoxysilane) to adhesive polymer. The method is simple, cost-effective and can be applied on the large industrial scale. Scanning electron microscopy (SEM) was used for surface morphology analysis, showing the roughness produced by surface treatment. The wettability of the micro-nano silica film varied from hydrophilicity (water contact angle 88o) to superhydrophobicity (water contact angle 156.9o), while sliding contact angles dramatically decreased (< 5o) by adding Functionalized silica and/ or adhesive polymer. Roughness increased with silica increment which improves the wettability. The coatings were electrochemically characterized by electrochemical impedance spectroscopy (EIS) and Tafel polarization curves; it was found that both systems had good performance against corrosion in 3.5 % sodium chloride solution. Furthermore, the stability of the coated layer on copper substrate was investigated.

2 Fi 163

1Research Assistant, Materials Science Department, Institute of Graduate Studies and Research, Alexandria University, 163, Horreya Av., El-Shatby 21526, P.O. Box: 832, Alexandria, Egypt. email: Wael200374@yahoo.com. Phone: +201223829252 (corresponding author)

2 Full Professor, Materials Science Department, Institute of Graduate Studies and Research, Alexandria University, 63, Horreya Av., El-Shatby 21526, P.O. Box: 832, Alexandria, Egypt. email:

rafk.abbas@,yahoo.com. Phone: +201001519157

3 Associate Professor, Materials Science Department, Institute of Graduate Studies and Research, Alexandria University, 163, Horreya Av., El-Shatby 21526, P.O. Box: 832, Alexandria, Egypt. email:

wagih sadik@yahoo.com. Phone: +201009407971

4 Associate Professor, Materials Science Department, Institute of Graduate Studies and Research, Alexandria University, 163, Horreya Av., El-Shatby 21526, P.O. Box: 832, Alexandria, Egypt. email: eldemerd@yahoo.co.uk. Phone: +201228048544

5 Associate Professor, Materials Science Department, Institute of Graduate Studies and Research, Alexandria University, 163, Horreya Av., El-Shatby 21526, P.O. Box: 832, Alexandria, Egypt. email: hefn5@yahoo.com. Phone:+201224726336

Keywords: Superhydrophobicity; Metal surface; Contact angle; Silane; Polymers; Corrosion resistance; Adhesion. 1. Introduction

Recently the superhydrophobic surfaces with a water contact angle higher than 150o have received much interest, due to their convenience in many fields, Such as self-cleaning, snow inhibition and contamination inhibition [1-4]. These widespread applications have motivated great efforts to improve fabrication techniques of superhydrophobic surface. In recent years, it has been found that, if a surface with a rough or micro textured structure has a low interfacial free energy, the CA can reach almost 180°, and the surface will remain dry as a water droplet easily slides across it. The typical example is the self-cleaning of nature's lotus leaf, on which the CA is about (161) ° and the sliding angle (SA) only 2°. So far, many methods have been used for fabricating the superhydrophobic surface [5-8]. All of these different methods for fabricating the superhydrophobic surfaces are almost based on the two main methods, which are to create the rough structure on the hydrophobic surface and to modify a rough surface by materials with low surface free energy. Therefore, the roughness is crucial for the preparation of the superhydrophobic surface [9]. Many methods for obtaining the biomimic structure, array, irregular pores or particles with micro/nano-size have been applied to fabricate the rough surface [10-13].

r partiel

or w hydr

It is well known that the wettability of solid surface eould be characterized by contact angel (CA)

of water droplets. If the CA of a surface is lower than 90o, the surface is described as a

ydrophilic surface. In contrast, if that of a surface is higher than 90o, the surface is described as

hydrophobic surface for smooth surfaces; CA can be represented by Young's equation. For

rough surfaces, the wetting state can be described by the Wenzel [14, 15] and cassie-baxter

models [16]. The Wenzel model assumes that the liquid is in intimate contact everywhere with

the rough surface, and completely fill any surface structures. The roughness factor in the Wenzel

equation can enhance the natural state of material, and gives hydrophilic surfaces a lower contact angle and hydrophobic surfaces a larger contact angle. The cassie-Baxter model introduces another new wetting state, in which the liquid allows very low friction during droplet movement, which introduces a low sliding angle. Both the Cassie- Baxter and Wenzel states can cause high static contact angles, but only the cassie-Baxter state can lead to very low sliding angle.

Spraying is thought to be one effective technique to create artificial superhydrophobic surfaces due to its simplicity, low-cost and low-temperature method. Hui et al. fabricated superhydrophobic copper meshes by spraying an emulsion of n-octadecanethiol and silver nitrate in ethanol onto the copper mesh with nitrogen gas by a spray gun. The alkanethiols contain long-chain alkyl groups that have low surface free energy. The advantage of this method is that in case of mechanical damage of the surface, it can simply be repaired by partial spraying [17]. Other organic materials were used via spraying process such as silica and Functionalized silica. Functionalized silica SiO2 particles not only generated a firm dual size rough surface but also facilitated its further hydrophobization Most of the widely used organosilanes have one organic substituent and three hydrolysable substituents. In the vast majority of surface treatment applications, the alkoxy groups of trialkoxysilanes are hydrolyzed to form silanol-containing species. Reaction of these silanes involves four steps. Initially, hydrolysis of the three labile groups occurs. Condensation to oligomers follows. The oligomers then hydrogen bond with OH groups of the substrate. Finally, during drying or curing, a covalent linkage is formed with the substrate with concomitant loss of water. Although described sequentially, these reactions can occur simultaneously after initial hydrolysis step [18]. At the interface, there is usually only one bond between silicon of the organosilanes to the substrate surface. The two remaining silanol groups are present either in condensed or free form. The R group remains available for covalent

reaction or physical interaction with other phases. Many researches improved superhydrophobic layers but very few researchers improve durability (adhesion) of the superhydrophobic layer.

Many researches aim to fabricate superhydrophobic coating with little attention in improving the layer stability and suffer from low durability. In this paper, we reported a polymer functionalized silica film with superhydrophobic properties fabricated by one-step spray casting functionalized silica-polymer (i.e., epoxy, polystyrene, polyurethane, or ethylene vinyl acetate) suspensions with excellent adhesion property. Such a film shows not only highly stable superhydrophobic property with water CA of about 155o and a slide angle (SA) of less than 5o but also has an excellent corrosion resistance in 3.5 wt. % NaCl solution.

2. Experimental

2.1. Materials

Copper alloy, Aluminum alloy and Iron alloy substrates with 3 mm thickness were supplied by The Egyptian Company for Copper Works, Alexandria, Egypt. Silicon dioxide nanopowders (99.5%- 15 nm) were purchased from MKNANO, Canada. Octyltriethoxysilane (OCTES) was purchased from Zhejiang Feidian Chemical Company Ltd., China. Tetrahydrofuran (THF) was purchased from Central Drug House Ltd. Ethylene vinyl acetate (EVA) was purchased from Arkoma. Polystyrene (PS) E251D was purchased from Styrenex Company, Egypt. Epoxy Chemapoxy 150 and Polyurethane (PU) were purchased from Chemicals for Modern Building International Company, Egypt. Ethanol 95 % was purchased from Elnasr pharmaceutical Chemical Company, Alexandria, Egypt. All reagents were used as received without further purification.

2.2. Preparation of hierarchical superhydrophobic coatings

All substrate specimens (copper, aluminum and steel) were prepared with dimensions of 60 mm

x 25 mm x 0.3 mm. The substrates cleaned by ethanol followed by acetone and dried in air.

Functionalized silica nanoparticles (FS) are Silicon dioxide nanopowders treated with silane coupling gents. Three grams of OCTES, with long alkyl chain, were added dropwise to the hydrolysis ratio of ethanol to distilled water (ethanol ETOH: distilled water DW) 95: 5 after vigorously stirred for approximately 15 min at room temperature. Then, the solution was agitated for 3 hours [19, 20]. Different concentrations of silica nanoparticles (i.e., 1, 1.5, 2 and 2.5 wt. %) were prepared by adding 1, 1.5, 2 and 2.5 gm of silica to 100 ml ethanol and DW. Silicon dioxide nanopowders with previous percentage were added to the solution and agitated for extra 3 hours. After agitation the solution was kept in closed flask for 2 days at room conditions. Different polymers (i.e., Epoxy, PU, PS or EVA) with specific amounts were added different

samples of the functionalized silica (FS) solutions. The spraying distance was 25 cm then the

e for 2 4 hours. Th

t the spray rate t the spray rate

substrate was held in room temperature for 24 hours. The coating solution was sprayed with nitrogen gas by a spray gun nozzle at the spray rate of 5 cm3 per minute during the spray deposition.

„ Samplc

VCA) ai

Water contact angle (WCA) and sliding angle are considered the main values in superhydrophobicity classification. CA measurements were performed using Gaosuo USB digital microscope. All the angles were determined by averaging values measured at five points on different locations on each sample surface. Surface morphology of samples was examined using scanning electron microscope (SEM, JSM-6390A). Samples were coated with a thin layer of gold using a sputtering machine prior to examination with SEM. Surface roughness mathematical value (Ra) was examined using Mituoyo SJ-201. Substrate surface was tested in ten different locations were the average value was supplied as surface roughness. Adhesion improvement between superhydrophobic layer and substrate studied. The adhesion strength is influences of the durability of the superhydrophobic layer. Adhesion strength of different prepared

superhydrophobic substrates was evaluated using the tape test (ASTM D-3395). Thickness of the superhydrophobic layer was also studied using (elecometer-465).

2.4. Electrochemical Measurements

The corrosion behavior of the blank copper alloy and different superhydrophobic surfaces were investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in NaCl solution (3.5 wt. %) using electrochemical system (GAMRY PCI4G750 instruments) at room temperature a typical-three electrode was used in electrochemical testing with reference electrode (saturated calomel electrode), a platinum wire as the counter electrode (auxiliary electrode) and the sample with an exposed area of 1.0 cm2 as the working electrode as demonstrated. EIS were performed in the frequency range between 10 MHz and 100 kHz. Polarization curves (Tafel plots) were acquired at a scanning rate of 1 mV/s from -0.25 to 0.25 V. The corrosion current density (icorr) for the specimens was determined by extrapolating the anodic and cathodic Tafel slopes. All EIS spectra were analyzed in Nyquist representation.

3. Results and discussion

3.1. Surface morphology

5. All EIS

Fig. 1 shows SEM images under different Functionalized silica (FS) content sprayed on copper alloy substrates. Fig. 1 (a) shows the surface morphology of the film prepared with 1 wt. % FS content in solutions. It's clear that the irregular forms of silica nano particles on substrate surface morphology also clarify the coating layer rough surface. A high-magnification SEM image the opper surface (Fig. 1b) shows that the particles morphology includes hierarchical micro/nano-structures. Fig.1(c) demonstrates the coating layer with higher functionalized nano silica of 2.5 wt. % in the sprayed solutions it shows that the particles become more uniform, smaller, more intensive and connected with each other as the silica percentage increased. And from the higher

magnification SEM (Fig. 1b), we can find that these small particles completely cover the surface.

morp copp

It is clear that many irregular clusters randomly distribute on the FS/polymer coating surfaces. These clusters provide the micrometer-scale roughness, while the inclusion of nanoparticles provides the nanoscale roughness on top of the micrometer sized clusters. Such particle organization results in a hierarchical microscale-to-nanoscale roughness, which is known to

Fig. 1. SEM images coated copper surface with FS at different silica concentration (a) 1 wt. %; (b) the corresponding high magnification; (c) 2.5 wt. % and (d) the corresponding high magnification.

Fig. 2 shows SEM images of copper sprayed with coating layer of Functionalized silica with

epoxy as adhesive polymer. Fig. 2 (a) shows the surface morphology of the film prepared with 1

wt. % FS and 1 wt. % epoxy that increases the bond between FS particles causing silica particles to coagulate with each other. A high-magnification SEM image of the copper surface shows the polymer and the FS particles in position (1) and position (2), respectively (Fig. 2 (b)). Increasing polymer 2 wt. % and silica particles 2.5 wt. % shows highly assembled and durable pattern Fig. 2 (c and d). Adhesives polymer form bridge-like creates strong bond increasing durability and bonding between silica particles. Fig. 2 (e and f) shows same bridge like forms form bonding silica particles with adhesive polymer.

e forms f

Fig. 2. SEM images coated copper surface by FS with epoxy at different concentration (a) 1 wt.

% silica with 1 % epoxy (b) the corresponding high magnification; (c) 2.5 wt. % silica with 2 %

epoxy; (d) the corresponding high magnification; (e) and (f) high magnification with different positions.

3.2. The wetting behavior of the prepared surface

The surface wettability of the obtained superhydrophobic surface was studied by CA measurements. Fig. 3 displays the profiles of CA of the obtained superhydrophobic surface as function of silica content on FS/polymer. It can be seen from Fig. 3, without any polymer addition at 1 wt. % FS the CA is 155o and increased to 162o at 2.5 wt. %. This result demonstrates that silica achieve high roughness. As is reported, the superhydrophobic property can be achieved by an especial surface chemistry or surface geometrical microstructure [21]. This wettability can be well explained by Wenzel's equation [22]:

cos9 = rcos9o Eq. (1)

Where 9 represents the apparent contact angle of water on the actual surface, 9o is the equilibrium contact angle on a smooth flat surface and r is defined as the surface roughness ratio. According to Eq. (1), with increasing the roughness, the wettability of hydrophobic surfaces decreases. Introducing of epoxy as adhesive polymer with 1 wt. % decreases CA from 155o to 146.2o at 1 wt. % FS and decreases from 162o to156.9o with 2.5 wt. % FS. Still, low epoxy content shows weak stability coating. Although, increasing the FS improves the CA it reduces to 149.2o for 2.5 wt. % FS and 3 wt. % adhesive polymer due to high polymer content. So, optimization should be done to attain superhydrophobicity and good stability which achieved at wt. % FS and 2 wt. % epoxy. Similar trend found with different polymers on various metal substrates illustrate in Fig. 4.

optim 2 wt

Fig. 3. The contact angle as a function of silica percentage (wt. %) with different polymers percentage (wt. %).

Fig. 4. The contact angles at optimum silica percentage (2.5 wt. %) with optimum polymer percentage (2 wt. %) using different polymers types at various metals.

3.3. Surface roughness

Surface roughness (Ra) is very important criteria in superhydrophobicity evaluation. Where, the

hydrophobicity phenomena depend on surface tension and surface roughness. It is obvious that

the surface roughness increases the hydrophobicity which directly proportional to the roughness of the coated layer. Fig. 5 depicts the variation of surface roughness of coated copper surfaces with FS. The surface roughness 240 nm on as-received bare copper surface was found to increase to 600 nm, and 1600 nm at low content (1 wt. %) and high content (2.5 wt. %) of FS, respectively. This phenomenon is due to the unique effect of the hierarchical structure of sprayed rough silica on copper surface which revealed by the SEM images. By introducing the adhesive polymer, the roughness reduces as result of polymer layer bonding with FS where the polymer layer bond between FS particles and fills the vacant gabs between silica particles. The Ra reduces from 600 nm to 400 nm by adding 3 wt. % polymer with 1wt% FS, and from 1600 nm to 1076 nm by adding 3 wt. % polymer with 2.5 wt. % FS. The same results were obtained by using other polymers (PS, PU and EVA) on different metal substrate. The optimum silica and polymer content are 2 wt. %. The optimum performance of polymers on the surface roughness of different metals is summarized in Fig. 6.

Fig. 5. Variation of surface roughness with silica percentage (wt. %) at different polymer percentage.

Fig. 6. Variation of surface roughness at optimum silica percentage (2.5 wt. %) with optimum polymer percentage (2 wt. %) using different polymers types at various metals.

3.4. Layer Thickness

Coating thickness was studied using elecometer-465 apparatus, the average of five different measurements used to determine the change in coating layer thickness with different parameters. According to the data obtained, the layer thicknesses described in Fig. 7 clearly shows insignificant difference in thickness between the coatings applied to 1 wt. % and 2 wt. % as 3.91 |im and 4.1 |im, respectively. By adding adhesive polymer increase layer thickness to average of 8 |im for 3 wt. % epoxy with 1 wt. % FS and at high FS/epoxy 3wt% and 2.5wt% average thickness was 12 |im. It can thus be seen that in fact the concentration of polymer applied was the only factor having a significant effect on final coating thickness, regardless of the type of polymer. Obviously in SEM images the polymer layer formed a thick bottom layer between FS and metal substrates. The different polymers were applied in the same way, using the same proceeding; their chemical composition had comparable specific densities and resin viscosities.

Fig. 7. Variation of coating layer thickness with percentage.

rcentage at different polymer

3.5. Corrosion Resistance Performance 3.5.1. Electrochemical impedance spectroscopy (EIS)

Fig. 8 depicts the Nyquist plots and Bode plots recorded for the bare substrate and resulting superhydrophobic surface in neutral 3.5 wt. % NaCl solutions. Fig. 8a shows the EIS results in the form of Nyquist plots, in which the imaginary impedance (Zim) is plotted against the real

impedance (Zre). The results show quite different capacitive loops that can be attributed to the charge transfer resistance of the corrosion process (Rct). Compared with the untreated copper alloy substrate, the capacitive loops of the prepared superhydrophobic surface have a widespread trend attributed to a protective surface film of Si coating. It is well known that the diameter of the capacitive loop related to Rct in the Nyquist plots represents the impedance of the samples. The value of Rct for the superhydrophobic surface (5.05 kQ) is much larger than that of the

untreated substrate surface (0.091 kQ), which indicates the superhydrophobic film has largely

improved the corrosion properties. The same previous data is also displayed using Bode plot show the relation between the frequency (log f) with the equivalent impedance (log z) shown in Fig.8 b. For better understanding the mechanisms of the corrosive processes, which occur at the surface of the samples studied, we have applied the fitting of the experimental impedance spectra using the appropriate equivalent electric circuits. The equivalent circuits are shown in Fig. 9 and the fitted parameters are summarized in Table 1. The equivalent circuit representing the electrochemical behavior of the untreated surface shows only one time constant (Fig. 9a). In this circuit, Rct the charge transfer resistance, Cdl the double layer capacitance, Rs the solution resistance. In the case of the silica modified superhydrophobic surface, the equivalent circuit model should have two time constants in the corresponding impedance spectra. The Rct||Cdl elements in Fig. 9b showed the impedance with the interface reaction between the silica modified superhydrophobic film and the copper substrate. Cc would normally be assigned to the capacitance of the intact coating (a function of factors such as film thickness and defect structure). Its value is much smaller than a typical double layer capacitance. The resistance Rpo (pore resistance) could be attributed to the resistance of ion conducting paths that develop in the coating (governed mainly by pore dimensions). Additionally, the inhibition efficiency (IE) of the coated modified silica was used to evaluate the corrosion protection performance of superhydrophobic surface, which can be calculated by the following equation [23]:

Rrf — Rrtn IE = —-— x 100

Where Rct, Rcto is charge transfer resistance of copper alloy covered with and without superhydrophobic coatings, respectively. Inhibition efficiency of superhydrophobic coatings increases up to 99 % and showing excellent inhibition effect with increasing silica content closely related to their wettability and larger contact angle.

Table (1). Electrochemical model impedance parameters derived from EIS.

Specimen

Cu substrate 1% Silica 1.5% Silica 2% Silica

Rpo (kQcm)

Cc (MJ)

Ru (Ü) IE %

124 0.656

2.91 0.276

0.981 0.686

3.84 0.429

0.186 0.210 0.375

0.113 0.721 0.317 0.102 0.27 0.624

2.5 0.198 2.18 9.25

Fig. 8. (a) the Nyquist plots obtained from the blank copper alloy substrate and the superhydrophobic surfaces after treating, using different silica wt. % in neutral 3.5 wt. % NaCl solution (b) Bode diagrams of measured EIS.

Fig. 9. Equivalent circuits of the studied

(a) uncoated and (b) superhydrophobic

specimens.

arization eous cor

3.5.2. Potentiodynamic polarizatio To investigate the instantaneous corrosion rate, the potentiodynamic polarization curves (Tafel) is employed. Fig. 11 shows the potentiodynamic polarization curves of untreated copper alloy and prepared superhydrophobic surface in 3.5wt%NaCl solution using the Tafel extrapolation method. The results of potentiodynamic polarization test are summarized in Table 2. The result clearly shows that the corrosion potential (Ecorr) positively increases from -0.76 V of the eated copper alloy to -0.665 V of the sample of prepared superhydrophobic surface. This suggests that the film of silica/silane mainly retards the dissolution of copper between the interface of the copper surface and seawater According to the potentiodynamic polarization curves (Tafel) linear extrapolation method, the corrosion current density (icorr) and the corrosion rate (mpy) of the different sample can be got from Tafel curve (Table 2). The corrosion current

untre sugg

density (icorr) of the prepared superhydrophobic surface (2 wt. % Si) is 0.124 pA/cm , while untreated copper alloy is 15.8 pA/cm2 and the corrosion rate of the sample after silica coating (0.238 mpy) high significant decreases compared with the untreated copper (5.83 mpy). From results obtained by potentiodynamic polarization and electrochemical impedance spectroscopy, it can be concluded that copper alloy substrate is easily penetrated by the CP in the seawater. So it cannot improve the corrosion resistance obviously. However, once the surface was chemical modified by silica/silane, the specimen can show excellent anticorrosion properties because of the superhydrophobicity phenomena.

Table (2). The results of potentiodynamic polarization test

st of the c

of superhydrophobic coatings 3.5wt% NaCl solution.

opper alloy and after spraying

Specimen Cu substrate

1 % SiO2 1.5 % SiO2

2 % SiO2

icorr (pA cm 15.8 1.94

_0.124

Ecorr (V) Corrosion rate (mpy)

-0.75 5.83

-0.760 0.592

-0.713 0.445

-0.665 0.238

Fig. 10. Potentiodynamic polarization curves of the copper alloy and different silica wt. % in 3.5 wt. % NaCl solution.

3.6. Durability

It is well-known that the adhesion is a highly important parameter for the practical application of coatings. In this study, the adhesion of sprayed superhydrophobic surface was performed by using Tape Test (ASTM D 3359: Standard Test Methods for Measuring Adhesion by Tape Test). Fig. 11 shows the optical images of the superhydrophobic surface after the tape test. Examination results summarized in table (2) where it was illuminated from test results that samples without adhesive polymer shows good contact angles and stable layer on substrate but as soon tape introduced, all the superhydrophobic layer was removed from substrate and contact angles dropped from 155o to 118.4o with FS 1 wt. %, and from 163.5o to141.35o with samples contains 2.5 wt. % FS, these samples shows according the ASTM standard evaluation of (0B) as shown in Fig. 11 (a). Introducing adhesive polymers shows very noticed improvement in durability, where samples with 1 wt. % adhesive polymer demonstrations drop in contact angles by 5o for low FS percentages to about 2o FS with 2.5 wt. % in same time the adhesion test showed very good improvement from (0B) to (2B) and (3B), as shown in Fig. 11 (b). Samples with polymer percentages 2 wt. % showed average of 1o or 2o drop in contact angles after adhesion test while results of adhesion improved to from (4B) to (5B) depend on polymer concentration and substrates. The 3 wt. % polymer improved test results and classified as (5B) as shown in Fig. 11 (d). No delamination or detachment of the film at the edges and within the square lattice was observed. Undoubtedly, this indicates excellent adhesion property with less than 1o drop in angles [24].

bserved. contact a

Table (3) the results of adhesion test of different FS percentages with different adhesive polymer (epoxy) wt. %

FS % CA (o) before test CA (o) after test Adhesion evaluation

Without adhesive polymer (0%)

1% 155 118.44 0B

1.5% 158.3 123.65 0B

2% 160 134.1 0B

2.5% 163.5 141.35 0B

Polymer (1 %)

1% 146.2 141.12

1.5% 153.94 149.33 2B-3B

2% 154.81 152.07 * 2B-3B

2.5% 156.98 15487 2B-3B

Polymer (2 %)

1% 128.65 127.22 4B

1.5% 140 137.14 4B

2% 152.17 150.92 4B-5B

r 2.5% 154.39 153.75 4B-5B

Polymer (3 %)

1% 119.82 119.0 5B

1.5% 138.34 138.0 5B

2% 154.33 154.1 5B

2.5% 155.05 154.61 5B

Fig. 11. The optical images of the superhydrophobic surface after the tape test (a) sample without adhesive polymer; (b) sample with 1 wt. % adhesive polymer ,(c) sample with 2 wt. % adhesive polymer and (d) samples with 3 wt. % adhesive polymer.

4. Conclusions

A simple and cost effective one step spraying process was applied to prepare highly stable superhydrophobic metal surface based on silane and polymer. This method is time-saving and does not need special equipment or severe conditions. The efficient spray coating method was adopted effectively to get uniform superhydrophobic coatings and it can be used for almost in any kind of substrates like glass, plastics, metals, fabrics and so on. This Treatment showed CA of 155° and SA less than 5°. The FS micro-nano particles form hierarchical micro-nano structure enabled increased air to be trapped within the coating layer increases the surface water repellency and roughness but with poor adhesion. Unlike addition of polymers to the FS, the coating shows excellent stability and can be fabricated on different metal surfaces to prevent metal from corrosion. Therefore, these FS/polymer composite coatings exhibit the strong potential for industrial applications.

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