Facile and fast fabrication method for mechanically robust superhydrophobic surface on aluminum foil

Highlights

• The superhydrophobic surface is fabricated by a simple method.

• The wettability of aluminum surface can be regulated by the surface morphology.

• The alumina surface remains superhydrophobic in a wide pH range of 1–12.

• The anti-corrosion properties of as-prepared surfaces are obviously improved.

Abstract

This work reports a simple method to fabricate superhydrophobic surfaces on aluminum foil via high-field electrochemical anodization in oxalic acid aqueous solution. Different morphologies of these superhydrophobic surfaces can be obtained by adjusting anodization time and current. These surfaces reveal remarkable superhydrophobicity after the modification of fluorinated silane solution. The maximum contact angle (CA), 163° ± 2°, can be easily obtained by this simple method. And the sliding angle (SA) is less than 2°. The superhydrophobic surface shows a long-term stability over a wide pH range. The corrosion current density of the superhydrophobic aluminum is reduced by about one order of magnitude compared to the untreated aluminum. This fabricated superhydrophobic surface has a promising application in anti-corrosion fields.

Introduction

In nature, many plants and animals demonstrate excellent water repellency because of their specially structured superhydrophobic surfaces, such as lotus leaves and water strider’s legs [1], [2], [3]. By convention, superhydrophobic surfaces are defined as surfaces with which the CA of a liquid water droplet is bigger than 150° [4], [5]. In the past decades, superhydrophobic surfaces have been manufactured mainly by two methodologies. One is to create a rough structure on a hydrophobic surface (CA > 90°) and the other is to modify a rough surface by low surface energy materials [6], [7], [8], [9]. Superhydrophobic surfaces have the distinctive capability of trapping air pockets in gaps at the solid–liquid interface so that the contact area between surfaces and water droplets can be greatly reduced. This superhydrophobic behavior has a wide variety of practical applications in drag reduction and corrosion resistance of micro-electro-mechanical systems (MEMS) [10], [11], [12].

Aluminum and its alloys are widely used in MEMS fields as basic materials for numerous micro-mechanical components due to their substantial advantages such as low cost, high plasticity, good conductivity, and easy fabrication [13]. However, they are actually reactive materials and prone to wearing, which could hinder their practical applications. To overcome this bottleneck, researchers have attempted to fabricate superhydrophobic surfaces with excellent stability, low friction, and good corrosion resistance on aluminum substrate [14], [15], [16]. Feng et al. fabricated superhydrophobic surfaces on aluminum alloys using the method of boiling water treatment and stearic acid modification [17]. Song et al. presented a facile chemical deposition process to fabricate superhydrophobic Cu surfaces on aluminum substrates [18]. Besides the superhydrophobic surfaces mentioned above, alumina films have also been widely used in industry due to their superior properties which can provide resistance to harsh environments such as high temperatures, large wearing, and highly corrosive environments. Recently, alumina surfaces with nanostructures including nanotips, nanowire arrays, and nanowire pyramids have been prepared on aluminum to improve superhydrophobic properties. Liu et al. reported a facile way to form nanowire forests on alumina surface via electrochemical etching and then postmodified the surface with hydrophobic materials in order to achieve super oil repellency [19]. Wang et al. prepared a superhydrophobic surface with nanopore structures on aluminum via chemical acid etching and electrochemical etching [20]. Many techniques and strategies such as phase separation [21], electrochemical deposition [22], plasma treatment [23], sol–gel processing [24], electrospinning [25], and solution immersion [26] had also been introduced to enhance the hydrophobicity of alumina surfaces [27], [28], [29], [30]. However, these techniques involve multi-step procedures, harsh condition, or specialized reagent and equipment. In this paper, we propose a fabricating method of diversely nanostructured alumina films via one-step high-field anodization. According to our present studies, alumina micro/nanostructures can be easily obtained by a simple, purely electrochemical method with high efficiency.