Facile formation of superhydrophobic silica-based surface on aluminum substrate with tetraethylorthosilicate and vinyltriethoxysilane as co-precursor and its corrosion resistant performance in corrosive NaCl aqueous solution

Highlights

• Superhydrophobic coating with contact angle of 154.9° was created by a sol–gel method on aluminum substrate.

• Hierarchical micro-nanostructure terminated with vinyl groups was obtained.

• The surface remained superhydrophobic after immersion in NaCl solution for 120 min.

• The surface displayed excellent corrosion resistant performance in NaCl solution.

Abstract

A facile sol–gel method has been developed to create a superhydrophobic surface on aluminum substrate with tetraethylorthosilicate (TEOS) and vinyltriethoxysilane (VTES) as co-precursor at room temperature. Firstly, nanometer sized silica particles were self assembled on the substrate through the hydrolysis of TEOS. Then, the silica particles were modified with vinylsiloxane through the hydrolysis and condensation polymerization of VTES. The emphasis was focused on investigating the influence of the molar ratio between NH₃·H₂O and ethanol on the surface morphology and wetting property. The surface morphology was observed by scanning electron microscopy (SEM), the chemical composition and bonding state of the surface were explored by energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectra (XPS), and the wetting property of the surface was investigated by water contact angle measurement (WCA). The modified silica-based surface possessed the greatest static contact angle of 154.9°, exhibiting excellent superhydrophobic property. A hierarchical microstructure with spherical microparticles of around 2 μm decorated with nanoparticles of around 450 nm was observed on the film surface. The surface was covered by hydrophobic vinyl groups via the decoration of silica microparticles with vinyl-terminated siloxane nanoparticles as evidenced by EDS, FTIR and XPS. The corrosion resistant performance and durability of the superhydrophobic silica-based surface formed on aluminum substrate in corrosive NaCl solution were estimated by electrochemical impedance spectroscopy (EIS) measurements. The appropriate equivalent circuit model was put forward to fit and analyze the EIS data. The electrochemical result revealed that the corrosion resistant performance of aluminum was improved greatly by the superhydrophobic treatment.

Introduction

Superhydrophobic surfaces (water contact angle greater than 150°) have attracted much attention due to their wide range of potential applications in both academic research and industrial technology thanks to their outstanding properties of self-cleaning, anti-sticking, anti-icing and anticorrosion [1], [2], [3], [4], [5]. An electron microscopy study was performed by biologists Neinhuis and Barthlott [6] on the lotus leaf (the typical natural example of superhydrophobic phenomenon), which revealed that the combination of the pronounced surface roughness at micro-nanoscale with epicuticular wax-like materials on the surface should be responsible for the superhydrophobicity. On the basis of thorough understanding of biological superhydrophobic examples, conventionally, two strategies were developed to artificially construct superhydrophobic surfaces [4], [7]: one is to create a rough surface and modify it with low surface energy material; the other is to roughen the surface of hydrophobic materials (WCA > 90°). It can be clearly seen that the construction of rough surface is the sticking point for artificially fabricating superhydrophobic surface. Up to now, superhydrophobic surfaces have been prepared on a variety of materials such as polymer [8], [9], glass [10], [11], metal oxide [12], [13] and metal [14], [15], [16], [17], and the surface roughness was realized through various methods such as lithographic technique [18], [19], templating [20], assembly of particles [21], etching [22], [23], and crystal growth [24].

Recently, the implementation of superhydrophobic treatment on engineering materials such as steel, copper, zinc and magnesium shed a light on the puzzle of being susceptible to be corroded in the humid and corrosive environment [25], [26], [27], [28], [29], [30]. Ishizaki et al. [25], [26] created superhydrophobic surface on magnesium alloy through nanostructured cerium oxide coating and fluoroalkylsilane molecule modification, the electrochemical impedance spectra measurement revealed that the anticorrosion resistance of magnesium alloy was significantly improved by the superhydrophobic treatment. Wang et al. [27] proposed a potentiostatic electrolysis method to prepare zinc tetradecanoate film with superhydrophobicity on the surface of zinc, the contribution of the superhydrophobic film and air trapped in the film to the enhanced anticorrosion property was analyzed through the electrochemical measurements and appropriate equivalent circuit models. Liu et al. [28] reported an n-tetradecanoic acid etching method to fabricate superhydrophobic film on copper surface and the electrochemical impedance spectroscopy revealed that the corrosion rate of superhydrophobic copper in sea water decreased dramatically compared to bare copper due to the superhydrophobic property. The superhydrophobic surface has been proved to be effective for improving the corrosion resistant performance of engineering materials due to the water repelling and air capturing property.

Aluminum and aluminum alloys occupied an important position in modern industry because they demonstrated superior physical and mechanical properties such as low-specific weight, high-specific strength and high electrical capacity, which made them versatile for the aerospace, building, transportation, energy and other industries [31]. The aluminum resisted well against corrosion through a compact and chemically stable oxide film that spontaneously formed on the surface. However, when encountered with some harsh environment, such as Cl⁻ containing solution, the aluminum was prone to be corroded by the localized attack that resulted from the migration of aggressive ions through the film. Therefore, it's crucial to improve the anticorrosion property of the aluminum especially in corrosive environment for the convenience of wider range of applications. Some studies have already been reported on the fabrication and corrosion resistant property of the superhydrophobic aluminum. Liu et al. [32] prepared nano-TiO₂ coatings onto the anodized aluminum surface by a vacuum dip-coating method. The electrochemical measurements revealed that the nano-TiO₂ coatings significantly decreased the corrosion current densities and increased the values of polarization resistance simultaneously, indicating the nano-TiO₂ coatings exhibit excellent anticorrosion properties in seawater. Shi et al. [33] prepared superhydrophobic coating on aluminum substrate through acid etching followed by modification with nano-silica and fluorosilane, the electrochemical property over the time of exposure to 0.3 wt.% and 3 wt.% NaCl solution was investigated. It can be found that superhydrophobic treatment would be a promising solution for improving the anticorrosion performance of aluminum in aggressive environment by way of effectively suppressing the contact of water with the metal base due to its water-repelling property. However, more relevant experimental data still remained to be reported to understand the anti-corrosion behavior and figure out the corrosion mechanism of the superhydrophobic surface.

In addition, most of the reports created superhydrophobic surface on aluminum substrate through a two-step method including construction of rough surface and modification with low surface energy material, leading to tedious procedure and expensive modifier. It would be desirable for the scale production and extensive application of superhydrophobic surface if the building of rough microstructure and modification could be carried through in one step. Sol-gel technique of alkylsilane is a promising strategy to one-step fabricate superhydrophobic surface on engineering materials because it can create rough surface and introduce low surface energy alkyl groups to the surface at the same time through the hydrolysis and condensation of alkylsilane [34], [35], [36]. Yang et al. [35] and Xue et al. [36] reported the successful preparation of the superhydrophobic silica film by a simple sol-gel method using methyltriethoxysilane (MTES) and Vinyltriethoxylsilane (VTES) as precursor, respectively. However, they didn't evaluate the corrosion resistance and durability of the silica-based superhydrophobic film in corrosive medium.

In this paper, we report a facile sol-gel method with TEOS and VTES as co-precursor for the fabrication of a superhydrophobic surface on aluminum substrate at room temperature. The whole procedure was carried out by a one-step route without further modification and no special equipment was required. In addition, the corrosion resistant performance and durability of the superhydrophobic aluminum in corrosive NaCl aqueous solution was investigated using electrochemical and contact angle measurements.