One-step method for the fabrication of superhydrophobic surface on magnesium alloy and its corrosion protection, antifouling performance
Introduction
The phenomenon of superhydrophobicity which is known as “lotus effect” exists widely on the plant surfaces and the bodies of some animals and insects [1]. The design of artificial superhydrophobic surfaces with water contact angles of more than 150° is nature inspiredly. Over the past few decades, many efforts have been made to replicate the micro/nanostructures with their special water-repellent and self-cleaning properties [2]. Practical applications include the production of anti-biofouling paints [3], reducing fluid resistances [4], corrosion resistance [5], self-cleaning surfaces [6], stain resistant textiles [7], and anti-icing coating [8].
After decades of development, there are various methods to create the superhydrophobic surface such as chemical vapor deposition [9], [10], chemical etching [11], [12], sol–gel techniques [13], [14], [15], hydrothermal synthesis [16], physical vapor deposition [17], layer-by-layer self-assembly [18], and electrospinning [19]. Generally, the way to fabricate an artificial superhydrophobic surface involves two steps: the first step is to form a surface with a micro/nanostructured roughness and then to modify that surface using low-surface-energy substances [20], [21], [22]. For example, Ishizaki and co-workers fabricated a superhydrophobic surface on a magnesium alloy coated with nanostructured cerium oxide film and used fluoroalkylsilane (FAS) to enhance the surface hydrophobicity [23]. Wang et al. prepared a conformal silica shell on the surface of a ZnO nanorod by a bioinspired layer-by-layer deposition method and the ZnO/SiO2 nanorod array was modified with an octadecyltrimethoxysilane self-assembled monolayer [24]. However, the fabrication of such micro/nanostructures usually requires special conditions, expensive materials and complicated procedures, limiting its widespread use. Furthermore, most existing methods involve the use of biological poison materials, such as FAS and rf-sputtered Teflon, to achieve a low energy surface.
Therefore, if both steps can be made in just one step, the process of preparing superhydrophobic surface would be simplified [21]. Organic acid solution immersion is one method to achieve surface roughness and surface energy reduction [21]. As long chain aliphatic acid having hydrophobic hydrocarbon side effects, so it reacts with substrates and also play a role in the surface modification. Because of this, tetradecanoic acid is to be taken as a model system. A surface treated with tetradecanoic acid is environment-friendly [25] and fulfils this purpose; the tetradecanoic acid is safe to use with its natural existence of long chain fatty acids.
Magnesium alloy is one of the lightest engineering materials. Due to its superior properties, such as low density, high weight ratios and good heat dissipation, it is expected to be an excellent material for reducing vehicle weight, increasing equipment strength and reducing fuel consumption [26], [27], [28]. However, the extremely drawback is low corrosion resistance and chemical stability in corrosive medium. A superhydrophobic coating would be a promising technology for improving corrosion protection because it would inhibit the contact of a surface with environmental humidity. Jiang et al. fabricated a superhydrophobic surface on a Mg–Li alloy, followed by immersion and annealing processes using FAS [29].
In this paper, we report a simple, one-step and environment-friendly process for the formation of a tetradecanoic acid iron (Fe(CH3(CH2)12COO)3) superhydrophobic surface on AZ31 magnesium alloy. The coating with micron rough structure and high contact angle produces AZ31 magnesium alloy with desirable corrosion protection and antifouling properties.
Section snippets
Materials
All reagents were of analytical grade and used as received without further purification. Magnesium alloy AZ31, 1.5 mm think, was used as the substrate (composition: 2.98 wt% Al, 0.88 wt% Zn, 0.38 wt% Mn, 0.0135 wt% Si, 0.0027 wt% Fe, 0.002 wt% Ni, 0.001 wt% Cu, and the remaining is Mg).
Preparation of the magnesium alloy
In a typical process, preparation for the substrates was the following: magnesium alloy AZ31 substrates, 30 mm × 20 mm × 1.5 mm, were abraded with silicon carbide papers (from 600 # to 2000 #), and then ultrasonically degreased
Morphology and wettability of the superhydrophobic surface
The morphology of the as-prepared superhydrophobic surface was studied from SEM images. The micrograph in Fig. 1a shows a large number of tufted microstructures evenly covering the substrate surface. And it can be seen there is a few microcracks on the surface (the yellow circle as shown in figure). The irregular arrangement of these micro clusters is composed of numerous nanosheets, 300–400 nm thick, length of 1–2 μm, with neighboring lamellae randomly overlapping and joining each other (Fig. 1
Conclusions
A simple, one-step and environment-friendly method to fabricate a superhydrophobic surface on magnesium alloy by a simple immersion process with a solution containing ferric chloride, deionized water, tetradecanoic acid and ethanol. According to the results of XPS and FT-IR analysis, the superhydrophobic micron rough structure has a high contact angle 165° ± 2° and is composed of ferric myristate. The superhydrophobic film shows an excellent performance of corrosion resistance when immersed in a
Acknowledgments
This work was supported by Special Innovation Talents of Harbin Science and Technology (2011RFQXG016), Fundamental Research Funds of the Central University (HEUCFZ), Key Program of the Natural Science Foundation of Heilongjiang Province (ZD201219), Program of International S&T Cooperation special project (2013DFA50480), Special Innovation Talents of Harbin Science and Technology (2012RFXXG104).
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