Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications

Highlights

• A hierarchical superhydrophobic coating (SHPC) was obtained by a cost effective method.

• Synergistic effect of micro-nanostructure and low surface energy is the key to SHPC.

• The SHPC retains a high contact angle above 150° after sandblasting for 60 s.

• The SHPC shows excellent corrosion resistance with protection efficiency of 99.75%.

• The SHPC has low ice adhesion strength around 0.065 MPa after 10 icing/de-icing cycles.

Abstract

Aluminum (Al) has been widely used in numerous applications, but it is prone to contamination or damage under harsh working environments. In this paper, a superhydrophobic coating (SHPC) on the Al surface was fabricated via a simple and cost effective method using anodization in sulfuric acid electrolyte followed by surface modification with inexpensive myristic acid. The as-prepared SHPC with hierarchical micro-nanostructure exhibited good superhydrophobicity with a static water contact angle (CA) of 155.2 ± 0.5° and a sliding angle (SA) of 3.5 ± 1.3°. The SHPC possessed both good mechanical and chemical stabilities: it retained a CA as high as 151.1 ± 0.1° after mechanical sandblasting for 60 s and was stable after dripping test using both acidic and alkaline solutions. Besides, after exposure to UV/water condensation cycles for 7 days, the coating remained superhydrophobic, indicating excellent weathering resistance. The prepared SHPC also demonstrated excellent self-cleaning and anti-icing performance. Ice adhesion strength as low as 0.065 ± 0.022 MPa was obtained for the optimized coating. Electrochemical measurement showed that there is a two-order of magnitude of reduction of the corrosion current density (J_corr) and the protection efficiency (P%) of the as-prepared SHPC has reached up to 99.75%.

Introduction

Aluminum (Al) and its alloys are important engineering materials owing to its abundance in nature, good ductility, low specific weight and excellent electrical conductivity. They have been widely used in many areas, especially in sports, aerospace, transportation and civilian industries. Al is known to develop a thin natural oxide layer in dry and non-salty environments, which could prevent itself from further corrosion. However, it is highly prone to corrosion in humid and salty environments [1], which will cause damage or malfunction of facilities and loss of esthetic values. Therefore, it is very important to form a surface layer on Al to protect it from a wide spectrum of mechanical and chemical attacks. One of the approaches is through transforming the hydrophilic nature of Al surface to be superhydrophobic.

Superhydrophobic surfaces with static water contact angles (CAs) higher than 150° and sliding angles (SAs) lower than 10° have attracted a great deal of interest in both scientific research and practical applications because of their unique properties, including water repellency [2], self-cleaning [3], [4], oil–water separation [5], anti-icing [6] and anti-corrosion [7]. The key to constructing superhydrophobic surface is to create a rough hierarchical micro-nanostructure on a low energy surface. In regards the broad applications of superhydrophobic surfaces in outdoor equipment, researchers have developed a great number of methods, including chemical etching [8], sol–gel [9], template [10], hydrothermal synthesis [11], electrospinning [12] and electrochemical processes [13].

Although there are numerous techniques to construct superhydrophobic surfaces, few products have been available for practical applications mainly due to their weak mechanical and poor chemical stabilities. It is important to note that most artificial superhydrophobic surfaces are easily damaged by even gentle physical rubbing, or finger scratching and so on. Besides, some superhydrophobic layers have weak adhesion with substrates, making it easy for them to be peeled off [14]. With respect to chemical stability, many superhydrophobic surfaces may lose their superhydrophobicity rapidly after exposure to harsh conditions, such as wet environment, strong acidic or alkali solutions. UV irradiation can accelerate their aging which leads to performance degradation and shortening of lifetime. To date, a variety of mechanically robust, chemically stable and UV resistant superhydrophobic surfaces have been reported. Wang et al. fabricated superhydrophobic sponges and fabrics with strong mechanical robustness by in situ growth of transition-metal oxides and metallic nanocrystals [15]. Li et al. obtained superhydrophobic cotton fabrics with good abrasion and laundering stability through the repeated graft-on-graft strategy [16]. Wu et al. created mechanically robust superhydrophobic coatings on glass substrates and glass fiber reinforced epoxy composites using sol–gel method [17]. Lai et al. produced a transparent superhydrophobic TiO₂-based coating with good chemical stability on indium tin oxide (ITO) glass [18]. Pan et al. prepared good UV blocking superhydrophobic cotton fabric using sol–gel and self-assembly method [19]. Xiu et al. obtained inorganic superhydrophobic silica coating with improved UV stability via sol–gel processing [20]. However, these researches were mainly carried out on glass, fabric and sponge substrates. To the best of our knowledge, very few publications have been available on the investigation of mechanically and chemically stable [21], UV resistant superhydrophobic surfaces on Al surfaces.

Currently, a number of approaches have been reported to fabricate superhydrophobic surfaces on Al and its alloys with outstanding corrosion resistance. Zhang et al. prepared a hierarchical superhydrophobic film which provides an effective corrosion-resistant coating for the underlying Al on PAO/Al substrates [22]. Barkhudarov et al. created superhydrophobic films as corrosion inhibitors on Al surfaces from a precursor solution containing mixed alkoxides 3,3,3-trifluoropropyl-trimethoxysilane and tetramethyl orthosilicate via a variation of the aerogel thin film process [23]. Yin et al. produced a superhydrophobic coating on Al alloy for corrosion protection by chemical etching followed by surface modification [24]. However, the researchers mainly focused on the improvement of corrosion potential (E_corr) and decrease of corrosion current density (J_corr), they did not pay much attention to the protection efficiency (P%) of the SHPCs for Al substrates.

Ice is prone to accumulation on Al surfaces in tough freezing weather, which can cause serious accidents and economic losses [25]. It might be easy to correlate icephobicity with superhydrophobicity because it seems that the water repellence would be a common requirement for it. However, not all the superhydrophobic surfaces can display good anti-icing property on Al surfaces [26], [27]. Besides, the durability of anti-icing property is an important consideration for any practical application. Kulinich et al. reported that superhydrophobic Al surfaces whose surface asperities were gradually damaged and even lost the superhydrophobicity during icing/de-icing cycles, showing that the anti-icing performance of the samples was significantly deteriorated [28], [29]. So it is necessary to create SHPCs on Al surfaces which are endowed with excellent enduring anti-icing property.

For practical applications, the outdoor surfaces are usually polluted by contaminants and dusts. So far, although there have been lots of reports on producing self-cleaning SHPCs, little detailed research is available to quantify their self-cleaning efficiency. This can be done by the color contrast which is directly related to the amount of artificial dirt on the surface [30].

Summarizing the above analysis, it is very important to construct a stable and corrosion-resistant SHPC with excellent anti-icing property and self-cleaning effect on the Al surface for its wide applications. However, systematic work about all the properties together on the superhydrophobic Al surfaces has been rarely reported. Besides, most reported methods are still subjected to certain limited conditions involving low efficiency, complicated procedure and high cost of production [31], [32]. In this work, a facile and low-cost method is used to construct SHPCs on Al surfaces. The fabrication process contains two steps: construction of rough hierarchical micro-nanostructure with nanotubes and chemical modification with inexpensive myristic acid. The as-prepared SHPC has a CA as high as 155.2 ± 0.5° and a SA as low as 3.5 ± 1.3°. The mechanical and chemical stabilities of the SHPC have been evaluated by micro-sandblasting test and dripping test using solutions with different pH values. The SHPC also shows excellent weathering resistance and highly improved corrosion resistance after exposure to the UV/water condensation cycles for 7 days and immersion in 3.5 wt.% NaCl solution. Furthermore, the low ice adhesion strength and dirt accumulation results demonstrate good anti-icing and self-cleaning performance of the as-prepared SHPC.