Decorated fibrous silica epoxy coating exhibiting anti-corrosion properties

Progress in Organic Coatings 127 (2019) 110–116

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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Decorated fibrous silica epoxy coating exhibiting anti-corrosion properties ⁎

T

Aziz Fihri , Dana Abdullatif, Hawra'a Bin Saad, Remi Mahfouz, Hameed Al-Baidary, ⁎ Mohamed Bouhrara Oil and Gas Network Integrity Division, Research & Development Center, Saudi Aramco, Dhahran, 31311, Saudi Arabia

A R T I C LE I N FO

A B S T R A C T

This article is dedicated to the memory of El Wacham Hada, admirable grandmother and an irreplaceable person.

In this paper we report a simple approach of preparation of fibrous superhydrophobic silica employing a low cost and scalable methodology. The prepared superhydrophobic fibrous silica was characterized using thermal gravimetric analysis, transmission electron microscopy, Fourier transform infrared spectrophotometry (FTIR) and 13C solid-state NMR spectroscopy. The findings revealed that the hydrophobic alkyl chains were introduced into silica particles via modification and the fibrous silica particles were modified from a hydrophilic to hydrophobic nature. TEM analysis revealed that the unique morphology of the fibrous silica remained intact after its modification with alkoxysilane reagents. The particles were spatially stuck on the surface of the epoxy resin coating with the purpose to create a rough surface with random micro/nano structures. The prepared superhydrophobic surface exhibited robust water-repellent surface, excellent durability and corrosion resistance. It not only introduces a cost-effective process for superhydrophobic modification of epoxy coating, but also provides a promising strategy to protect metals by simultaneously combining the protective functions of both superhydrophobic surface and organic protective coating, which could be employed for large-scale industrial fabrication of superhydrophobic epoxy coating onto steel materials.

Keywords: Fibroussilica Epoxy coatings Carbon steel Superhydrophobicity Corrosion resistance

1. Introduction Nature has developed materials with fascinating properties, in particular, lotus leaves which have an outstanding hydrophobic properties characterized by contact angles greater than 150° and a sliding angle less than 10° [1]. The origins of this phenomenon, called the lotus effect, have been studied early by the botanists Barthlott and Neinhuis [2]. They have established that this particular property is due to the presence of a double scale of roughness, micro- and nanometric, associated with hydrophobic surface chemistry. Both of these characteristics are, in fact, essential for the design of artificial superhydrophobic surfaces. Over the last twenty years, considerable attention has been paid to the development of superhydrophobic artificial surfaces [3]. This interest stems from the fact that superhydrophobic surfaces are generally multifunctional [4,5]. Indeed, they show self-cleaning and antifouling properties, and provide corrosion protection and reduction in hydrodynamic drag. The potential applications of these surfaces are considered in a wide variety of sectors such as automotive, aerospace, building, marine, photovoltaic panels, textiles, etc. Various methods have been adopted to design and produce such surfaces such as plasma deposition [6], sol-gel method [7], layer-by-layer assembly [8], laser etching [9], chemical vapor deposition [10], electrochemical

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deposition and others [11–13]. All these modern methodologies are successfully used for tailoring surface topography and improving the hydrophobicity by coating a designed rough surface with a thin low surface free energy monolayer. However, the majority of these processes associated with many challenges as they involve special accouterments, costly and toxic reagents, complicated and time-consuming experimental procedures which make them unpractical for large scaleup production. Nonetheless, one of the most effective ways to inhibit the corrosion of metallic materials is using organic coatings. The diffusion of water through the anticorrosive organic coating is the major contributor to losing adhesion between the substrate and the organic coating leading to its rapid degradation [14]. Therefore, many researches have been devoted to designing superhydrophobic polymeric coatings owing to the fact that the superhydrophobic surface and barrier properties tend to cooperatively improve the corrosion resistance of the polymeric coatings [15]. Recently, considerable effort has been dedicated to designing multifunctional superhydrophobic surfaces based on oxide materials such as zinc oxide [16], titanium oxide [17] copper oxide [18] fluorographene [19] and Fe3O4-filled carbon nanofibers [20]. Different polymers are used as matrices to design superhydrophobic coatings such as epoxy [19,21], polystyrene [22], polyurethane [23] and polydimethylsiloxane [24]. However, minor

Corresponding authors. E-mail addresses: Aziz.fi[email protected] (A. Fihri), [email protected] (M. Bouhrara).

https://doi.org/10.1016/j.porgcoat.2018.09.025 Received 28 August 2018; Received in revised form 17 September 2018; Accepted 24 September 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.

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perfectly. As a result, many studies of micro or nanostructured substrates that make the surface superhydrophobic are related to the Cassie-Baxter law [30,31]. Marmur proposes two different approaches for measuring the apparent contact angles [32]. The first experimental approach is to vibrate a sessile drop to overcome the energy barriers between the different metastable states and to reach the minimum of global energy. Wolansky and Marmur demonstrated that the drop is axisymmetric when this minimum of global energy is reached [33]. The second approach is to measure the advancing and receding contact angles and assimilate the apparent contact angle to its average. Marmur also suggested the measurement of angles on large drops of the order of magnitude of milliliters with an axisymmetric shape so that the Wenzel and Cassie-Baxter models are valid. Marmur suggested using the sessile drop technique for measuring the low contact angles and captive bubble method to measure high contact angles to minimize the dependence of the practical advancing and receding contact angles on the drop volume.

research has been devoted to the design of superhydrophobic surfaces based on inorganic fibers and only few publications have been reported. For instance, Shiratori et al. designed a superhydrophobic nanofibrous zinc oxide film surface with a contact angle of 165° using an electrospinning technique [25]. The authors attributed the superhydrophobicity of zinc oxide to the cooperative effects of the surface roughness generated by the fibrous structure and the hydrophobic fluoroalkylsilane modification. Ding et al. combined electrospinning, calcinations and surface modification techniques to develop nanoparticle decorated fibrous silica membranes showing high superhydrophobicity and highly flexible properties with a contact angle of 155° [26]. This was attributed to the increased surface roughness due to the nanometre-scale papillae on the fiber surfaces. However, the brittleness of inorganic surface fibrous particles considerably limits their practical applications. To date, as far as we know, a fundamental understanding of how to enhance their surface superhydrophobicity whilst retaining their flexibility remains a very important and challenging aspect. In the present work, we designed a superhydrophobic epoxy coating modified with fibrous silica to provide the epoxy surface with both low surface free energy and micro/nanostructure built by fibrous silica. The as-prepared coating exhibited long term durability and excellent corrosion resistance properties in 3.5 wt% NaCl solution.

3. Experimental 3.1. Materials Tetraethoxysilane (98%), cetyltrimethylammonium bromide (99%), hexadecyltrimethoxysilane (85%), urea (98%), trimethoxy(octyl)silane (97.5%), toluene (99.7%), ethanol (99.8%), cyclohexane (99.5%) and 1-pentanol (99%) reagents were analytical grades and were supplied by the Sigma-Aldrich chemical company and used directly as received without further purification. The isophoronediamine (> 99.7%) was purchased from BASF chemical company. The epoxy monomer used in this work was supplied by West System Inc and commercially known as 105 Epoxy Resin. This consists of a mixture of bisphenol-A type epoxy resin (> 50%) and bisphenol-F type epoxy resin (< 20%) with approximately 20% benzyl alcohol solvent.

2. Theory The wettability of a perfectly smooth, chemically homogeneous, insoluble and inert solid surface expressed by contact angle θ of a water droplet was firstly proposed by Young two centuries ago as illustrated in the Eq. (1):

COSθ =

γSV −γSL γLV

(1)

Where γSV, γSL and γLV are solid-vapor, solid-liquid and liquid-vapor interfacial surface tensions, respectively [27]. The contact angle depends only on the physicochemical nature of the three phases and is considered a property of the material-fluid couple independent of gravity. The majority of solid surfaces have defects and imperfections leading to surface roughness and in this case Young theory could not be applied. Wenzel was the first to identify the difference between the contact angle defined by Young θY and the contact angle measured on rough surfaces [28]. It connects the two angles by introducing a roughness factor r, defined as the ratio of the actual surface to the apparent surface. The apparent surface corresponds to the surface created by the projection of the real surface on a plane. The following Eq. (2) was thus proposed:

cos θW = r cos θY

3.2. Synthesis of fibrous silica particles Fibrous silica particles were successfully prepared by hydrothermal process instead of microwave assisted technique as was reported elsewhere [34,35]. A solution composed of tetraethoxysilane (20 g), cyclohexane (240 mL) and 1-pentanol (12 mL) was prepared and stirred at room temperature for 30 min. Simultaneously, a second solution composed of cetyltrimethylammonium bromide (8 g), urea (4.8 g), and deionized water (240 mL) was also prepared and stirred at room temperature for 30 min. The tetraethoxysilane solution was then added into the cetyltrimethylammonium bromide solution and left to stir at ambient temperature for 1 h. The resulting solution was then transferred into a sealed stainless steel autoclave and heated in an oven at 125 °C for 6 h. The autoclave was then gradually cooled to ambient temperature and the silica particles were collected by repeated centrifugation in deionized water and ethanol. After being dried overnight at room temperature, the as-synthesized silica was calcined under continuous air flow at 550 °C for 4 h yielding a pure white powder.

(2)

Where θW corresponds to the apparent contact angle in Wenzel’s theory. Since the value of r is greater than or equal to 1, the Eq. (2) predicts that the roughness enhances both the hydrophobicity and hydrophilicity nature of a surface. That is to say, if the intrinsic contact angle of a smooth surface is inferior to 90°, an increment in r decreases θw; but if the contact angle is superior to 90°, an increment in r increases θw. However, in the case of hydrophobic surfaces, the remaining air bubbles may be trapped in the cavities and in this case, the Wenzel model is no longer valid to define the apparent contact angle. Later on, Cassie and Baxter propose a new description of the apparent contact angle for chemically heterogeneous but smooth solid surfaces [29]. The apparent contact angle of Cassie-Baxter can be expressed based on a surface composed of two different homogeneous areas having a surface fractions f1 and f2 and contact angles θ1 and θ2 as shown in the Eq. (3):

cos θCB = f1 cos θ1 + f2 cos θ2

3.3. Preparation of superhydrophobic epoxy coating Prior to silanization, 10 g of the silica particles were dried for 1 h at 100 °C then placed immediately into a round-bottom flask and then 35 mL of hexadecyltrimethoxysilane and 200 mL of toluene were added and the resulting mixture was refluxed for 48 h. The suspension was cooled down to ambient temperature, before isolating the solid product from the solution by repeated centrifugation using toluene and ethanol. The collected silica powder was dried overnight at 100 °C and ground to a fine powder using a mortar; thus, superhydrophobic fibrous silica particles were successfully prepared so that it could be systematically characterized. To prepare the superhydrophobic epoxy coating, 2 g of bisphenol based epoxy resin monomer and 1 g of isophoronediamine

(3)

The authors manage to demonstrate that any surface with trapped air, such as duck feathers or the lotus leaves, acts as a composite surface having a chemical heterogeneity and therefore the Eq. (3) works 111

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5. Results and discussion

were vigorously mixed, the resulting epoxy pre-mixture was poured and spread over onto a cleaned carbon steel substrate using spin coater. Then, the modified fibrous silica powder was spatially dispersed onto the epoxy surface coating. Afterwards, the epoxy coating system was cured at 40 °C for 48 h, the excess of silica particles that were not anchored to epoxy surface was eliminated by air blowing followed by flowing ethanol and finally the prepared surface was placed in an oven and dried at 50 °C for 2 h. The thickness of the prepared coating was basically around 160 μm.

Fig. 1 shows the FTIR spectra of pure and functionalized silica samples. For pure silica, the broad peaks located at 800 cm−1 can be assigned to the SieOH stretching vibration and the peak at 1100 cm−1 can be attributed to the SieOeSi stretching vibration. In case of hydrophobic silica particles, the absorption bands located at 2920 and 2853 cm−1 can be attributed to the asymmetric and symmetric stretching of the CH2 group [36]. These findings indicate clearly that the silica particles had been successfully modified by the organosilane group. The thermal behavior of the functionalized silica sample was investigated by thermogravimetric analysis and the corresponding thermograms are displayed in Fig. 2. In fact, the figure shows two steps of weight loss. The first weight loss occurred between 25 and 70 °C and may be assigned to the evaporation of physisorbed water. The second weight loss occurred between 200 and 650 °C is due to the loss of the covalently bonded octylsilane group. It was reported that the weight loss of silane layers does not occur at a particular temperature since the organic layer degrades gradually from the silica surface up to 650 °C [37].The number of formed siloxane bonds and the amount of alkoxysilane anchored to the silica surface depend on the reaction time, reaction temperature and the degree of the accessibility of silanol groups. The FTIR and TGA findings confirm the modification of the fibrous silica surface. The solid-state CPMAS 13C NMR spectrum of the functionalized silica fibrous silica is shown Fig. 3. The signals at 59.5 ppm and 50.1 ppm can be attributed to the methyl group of residual silane (CH3eOeSi) or surface-bound methoxy groups [38]. This demonstrates that methyl groups have different chemical environments. The signal at 13 ppm can be attributed to the terminal methyl group of the hexadecyltrimethoxysilane monomer. The peak located at 10.1 ppm can be attributed to methylene group of (CH2Si). Having different chemical environments eCH2- aliphatic linear carbons from alkylsilanes monomers appear at 32.2–17.9 ppm [39,40]. It has been reported that the alkylsilanes can be covalently anchored to the silica surface to generate robust bonds between the hydrolyzed alkylsilanes and the hydroxyl groups of the silica [41].The reaction mechanism of alkylsilanes and silica surfaces is still unclear and not fully explored but is believed to take place by addition and condensation reactions pathways. It has also been reported that formation of more than one siloxane bond is favored with the presence of a high density of silanol groups on the silica surface [42]. The TEM analysis demonstrates that the produced silica has a fibrous structure with ordered and uniform dendritic fibers spreading out

4. Characterization and measurement Thermogravimetric analysis-Derivative Thermogravimetry (TGADTG) was conducted under air in a TA Instruments SDT Q600 at a heating rate of 10 °C/min between 25 and 800 °C. Infrared spectra were recorded on a Nicolet FTIR is50 FT-IR with Smart iTR diamond ATR cell (Thermo Scientific). The measurements of contact angles were performed using an optical tensiometer (Attension Theta, Biolin Scientific, Germany) with a 4 μL of water-drop and the drops were fitted with a Young-Laplace formula to determine the static contact angle in the Theta software. An environmental scanning electron microscope (SEM) ESEMQ400 was used to observe the morphology of the prepared microstructure superhydrophobic surface. A small portion of the powdered sample was spatially attached on the surface of the epoxy resin coating placed on aluminum stub holder. Then the mounted sample was coated with a thin layer of gold to prevent charging during the analysis. Transmission electron microscopy (TEM) images were obtained using a TitanG2 80–300 instrument equipped with an image-corrector from CEOS. Solid state 13C{1H} cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectra were conducted on a Varian 500 MHz NMR spectrometer. The potentiodynamic polarization measurements were conducted at room temperature in a 3.5 wt% NaCl aqueous solution using Princeton Applied Research AMETEK PARSTAT 4000 potentiostat with a three-electrode system. A platinum wire was used as the counter electrode, a saturated calomel electrode was employed as reference electrode and the sample with an exposed area of 1 cm2 served as the working electrode. The measurements were performed in the frequency range between 10 MHz and 100 kHz. The polarization curves were obtained at a sweep rate of 0.17 mVs−1 from −0.25 to 0.08 V. The measurements were scanned from cathodic to anodic direction with a scan rate of 0.5 mVs-1.

Fig. 1. FT-IR spectra of fibrous silica before (a) and after hydrophobization (b). 112

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Fig. 2. TGA (a) and DTG (b) curves of fibrous silica functionalized with hexadecyltrimethoxysilane.

the air pockets are not formed and a longer immersion results in the formation of corrosion products and breakdown of the superhydrophobic surface. Fig. 6 displays the relationship between the water contact angle and the immersion time of the prepared superhydrophobic epoxy coating based on fibrous silica particles. The experimental results revealed a contact angle of 150° that was maintained stable after 60 days of immersion in the sodium chloride solution. These findings indicate a good durability of the as prepared superhydrophobic surface of the coating system. The literature includes reports of similar findings that indicate a maintained chemical stability to the salt aqueous solution for some materials such as fluorinated polysiloxane-zinc nanocomposite coatings and superhydrophobic epoxy coating modified by fluorographene [19,44]. It is noteworthy that the modification of the fibrous silica was also performed with trimethoxy(octyl)silane under identical conditions described above and its wetting properties were evaluated and then compared to those of the hydrophobic silica particles functionalized with hexadecyltrimethoxysilane. The main objective of this experimental trial was to investigate the effect of the chain length of

from the center of the particle in all directions as illustrated in the Fig. 4. This unique morphology remained intact after its modification with hexadecyltrimethoxysilane. The average diameter of the fibrous silica microspheres was ranged from 380 to 460 nm and the distance between the two fibers is approximately 15 nm. The silanization of the fibers does not result in significant change of the size of the obtained silica particles. It is worth noting that the SEM micrographs of the prepared superhydrophobic epoxy surface revealed that fibrous silica particles have different sizes and shapes randomly attached over the epoxy surface. This resulted in the formation of densely packed protrusions after the curing process was complete. Moreover, the particles became larger due to agglomeration as shown in Fig. 5. The chemical stability of superhydrophobic surfaces is an essential key factor determining their performance and their feasibility for industrial applications. The immersion of coating systems in 3.5 wt% NaCl aqueous solution is widely used as a standard test to evaluate the long-term chemical stability and the durability of superhydrophobic surfaces [43]. Indeed, if the superhydrophobic surface is not stable, the salty solution can easily penetrate through the microstructure of the coating system reaching the metal and initiating corrosion. In this case,

Fig. 3.

13

C {1H} CP/MAS spectrum of fibrous silica modified with hexadecyltrimethoxysilane. 113

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Fig. 4. TEM images of calcined fibrous silica particles (a) and calcined fibrous silica particle modified with hexadecyltrimethoxysilane (b).

employs two classical well established models of Wenzel and CassieBaxter [28,29]. According to these two models, the chemical composition and geometrical microstructure of the surface are the two main parameters to consider when determining the wettability of solid materials. Therefore, the superhydrophobicity of the epoxy coating fibrous silica could be mainly due to cooperative effects of the low surface energy of the hydrocarbon chains and the surface roughness generated by the fibrous structure of silica surface and as well as the formation of the SieOeSi network [26]. The anti-corrosion properties of the superhydrophobic surface are a crucial parameter to determine their performance for any practical industrial applications. The corrosion resistance of the prepared superhydrophobic fibrous silica epoxy coating was evaluated ina 3.5 wt% NaCl aqueous solution utilizing an electrochemical workstation. Fig. 7 illustrates the potentiodynamic polarization curves of a bare carbon steel substrate and prepared superhydrophobic epoxy surface. The electrochemical parameters of the corrosion were obtained using the Tafel extrapolation from the potentiodynamic polarization curves. The corrosion potential (Ecorr) of carbon steel coated with superhydrophobic epoxy coating is more higher compared to that of the bare carbon steel substrate. The increase in corrosion potential shows that the superhydrophobic silica epoxy coating enhanced the corrosion resistance of the carbon steel substrate. Moreover, the corrosion current density (Icorr) of the superhydrophobic fibrous silica epoxy on the carbon steel substrate decreased compared to that of the bare substrate, 84-fold decrease from the bare carbon steel. It is very important to emphasize that in a typical potentiodynamic polarization curve, a lower

Fig. 5. SEM of the prepared superhydrophobic epoxy surface.

alkylsilanes coupling agent on the hydrophobicity behavior of the prepared epoxy coating. Interestingly, the contact angle measured was found to be 149° indicating that the contact angle value does not increase as the alkyl chain length of grafted alkylsilanes increases. This value is higher compared to those obtained with hydrophobic silica particles produced by the Ströber method and modified by trimethoxy (octyl)silane, where the contact angle measured was found to be 135° [43]. The evaluation of the wettability behavior of solid surfaces

Fig. 6. Change in the contact angle as a function of the immersion time of the as prepared superhydrophobic surface in a 3.5 wt% NaCl aqueous solution. 114

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Fig. 7. Potentiodynamic polarization curves of the bare carbon steel and the modified silica epoxy coating in 3.5 wt.% NaCl solution.

possesses excellent durability in corrosive environments. The results of the polarization curves also demonstrated that the fibrous silica epoxy coating provides excellent anti-corrosive properties. This could be ascribed to the combined effects of the high roughness and the low surface energy of the grafted hydrophobic chains. The use of fibrous silica fillers in epoxy polymer matrices opens the door to designing new generation of polymeric materials with enhanced properties. These properties include mechanical performance, thermal stability and flame retardancy. When these materials are enhanced, they become promising for many applications such as the oil-water separation, novel easy-clean coatings and in electronic. Further studies in the application of these materials are underway in our laboratory.

Table 1 Corrosion potential (Ecorr), corrosion current density (Icorr) and corrosion rate of uncoated and coated substrates. Sample

Ecorr (V)

Icorr (A/cm2)

Rcorr (mm/year)

Uncoated substrate Coated substrate

−1.2 −1.07

7.2 × 10−6 6.05 × 10−8

8.4 × 10−2 7.05 × 10−4

corrosion current density typically indicates a lower corrosion rate and better anti-corrosion properties [45,46]. Fitting results of the polarization curves obtained by Tafel extrapolation are illustrated in Table 1. The Ecorr of the untreated substrate was −1.2V vs. SCE and shifted positively to −1.07 V vs. SCE once the substrate was coated with superhydrophobic fibrous silica epoxy. Additionally, the corrosion rate (Rcorr, mm/y) is employed to quantitatively evaluate the anti-corrosion efficiency of the prepared superhydrophobic surface and it can be calculated by the following Eq. (4) [19]:

R corr =

KMn Icorr npm

Acknowledgments We are very thankful to Abdullah Ghamdi and Waleed Al-Obaid for their day to day aid in laboratory life. References

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Where k is a constant that defines the units for the corrosion rate (3268.5 mol/A), Mn is the molecular weight of metal (g/mol), n is the number of charge-transfer, ρm denotes the metal density (g/cm3). It can be seen that the corrosion rate of the untreated substrate was 8.4 × 10−2 and significantly decreased after applying the epoxy silica coating to 7.05 × 10-4. It can be concluded, according to these findings that the prepared superhydrophobic surface provided excellent anticorrosion performance. Once the superhydrophobic surface is immersed in a sodium chloride solution, the air pockets act as a dielectric for a pure parallel plate capacitor and prevent the electron transfer between the electrolyte and the carbon steel substrate. Therefore, the superhydrophobic silica epoxy coating improved the corrosion performance of the substrate. 6. Conclusion In the present work, a superhydrophobic silica particles with unique morphology and dendritic fibrous textural properties were successfully prepared using a simple and cost-effective process. The fibrous silica particles with low-surface-energy were randomly attached on the surface of the epoxy resin coating to create roughness in hierarchical micro/nanostructure on a carbon steel substrate. Moreover, the resulted surface exhibited outstanding superhydrophobic properties and 115

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