Superhydrophobic surfaces for corrosion protection: a review of recent progresses and future directions
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Due to their superior water-repelling effects, superhydrophobic surfaces have received increasing attention as a promising solution to corrosion of metallic materials. The present article introduces the fundamental theories behind superhydrophobicity followed by a comprehensive review of the recent progresses of this rapidly growing field over the past 5 years. A critical discussion over anticorrosion mechanisms of superhydrophobic surfaces is also provided. For many realistic applications, future efforts are pressingly demanded to prolong the corrosion resistance of these superhydrophobic surfaces. To this end, several important strategies and examples in designing stable, self-healable, or inhibitor-loaded superhydrophobic surfaces are discussed.
KeywordsSuperhydrophobic surface Corrosion protection Wetting Microstructures Self-healing
Inspired by nature (e.g., lotus leaves,1 cicada’s wing,2 mosquito compound eyes3 and rose petals4), superhydrophobic surfaces are generally defined as surfaces which have water contact angles (θ) higher than 150°. Depending on the contact angle hysteresis (CAH) values, superhydrophobicity can be further categorized into different states.5 While some superhydrophobic surfaces might exhibit high CAH and strong adhesion to water (also known as adhesive superhydrophobicity),4,6,7 the surfaces of interest in this paper refer to those having very low CAHs and thus high water repellency. These surfaces have received continued attention for their broad applications, such as self-cleaning, antifogging and frosting, and drag reduction. With the unique water-repelling feature of these surfaces, they are also capable of reducing deterioration of metal surfaces caused by corrosion in aqueous media. In recent years, improving the corrosion resistance of metallic materials by surface superhydrophobization has become one of the hottest research areas in corrosion and protection. Compared to existing literature which has frequently reviewed the theories, preparation, and applications of superhydrophobic surfaces,8, 9, 10, 11, 12, 13, 14, 15 this review mainly focuses on the most recent developments of superhydrophobic surfaces for anticorrosion purposes. The authors review the typical preparation techniques of superhydrophobic anticorrosive surfaces and their anticorrosive performance, and provide a critical discussion of some mechanistic aspects of corrosion protection based on superhydrophobicity. Finally, some important future perspectives are also highlighted to provide references for developing superhydrophobic surface with long-lasting corrosion resistance.
The theory of superhydrophobic surfaces
Wettability of a smooth surface
Wettability of a rough surface
Application of superhydrophobic surfaces for corrosion protection
Increasing surface hydrophobicity often results in decreased corrosion rate of metals by limiting their interactions with corrosive species, such as water and ions. For organic anticorrosive coatings, this would retard the diffusion process of water molecules and prolong the coating’s protectiveness against corrosion of underlying metal substrates.19 As previously explained, the hydrophobicity of a material surface mainly depends on its intrinsic chemical properties and surface microstructures. To improve surface hydrophobicity, incorporation with low surface energy materials is often first considered.20, 21, 22, 23, 24 For example, the surface energy of silicone resin may be as low as 22 mN/m.25 Fluorine-containing resin is of even lower surface energy (~10 mN/m26). However, water contact angle of smooth hydrophobic surface can hardly exceed 120°. By introducing surface roughness, the hydrophobicity of a surface can be further increased, with contact angle even higher than 150°, achieving the superhydrophobic state. To enhance the corrosion resistance of metallic materials, different methods have been used to create rough microstructures on their surfaces to endow superhydrophobicity. Among these methods, the most common ones include wet chemical reaction, etching, hydrothermal method, anodization, electrodeposition, sol–gel method, nanocomposite coating, and templating.
Wet chemical reaction
Once water has penetrated the air barrier after prolonged immersion, a Cassie contact may be changed to a Wenzel contact and the anticorrosive performance of superhydrophobic surface declines (Fig. 12b). For example, Wang et al. intentionally depleted the trapped air in superhydrophobic copper surfaces by a solvent replacing method from ethanol to water.34 Compared to a regular superhydrophobic surface, the deaerated superhydrophobic surface formed a Wenzel contact with the surrounding water. Without the air barrier, both polarization curves and EIS spectra confirmed a dramatically lower corrosion resistance. Ejenstam et al. prepared alkyl ketene dimer (AKD) wax coatings with a lotus-like surface (high contact angle, low CAH) and a rose-like surface (high contact angle, high CAH) and tracked the variations of their EIS spectra for up to 17 days.108 Compared to a lotus-like surface, water can easily penetrate into the microscaled structures on the rose-like surface. Therefore, despite a similarly high contact angle, the absence of air pockets resulted in a rapid decay of impedance modulus for the rose-like surface when immersed during the EIS studies.
Obviously, the effectiveness of superhydrophobic surfaces to protect immersed structures largely depends on the underwater stability of the air films which rapidly decays with increasing hydraulic pressure,115, 116, 117 flow,118 or salinity119 of the surrounding fluids. In fact, almost none of the surfaces developed so far have shown an acceptably long underwater superhydrophobicity for practical applications.117 Most of them failed within hours and rarely for days. Xu et al. have shown that the air film trapped under water in a specially designed trench can be retained for >1200 h if maintained at a shallow position and with minimal environmental fluctuation.117 However, actual corrosion environments hardly meet such requirements.
Future trends in development of superhydrophobic surfaces with long-lasting corrosion resistance
Improving the stability of superhydrophobic surfaces
Self-healing capability of superhydrophobic surfaces
Inhibitor-containing superhydrophobic coatings
Superhydrophobic surfaces represent a significant technological breakthrough in recent studies over corrosion protection of metals. By reducing water contacting area and time or forming additional air barrier films, superhydrophobic surfaces can minimize the interaction between metal substrates and aqueous corrosive species and produce superior anticorrosive performances. Of the reported fabrication methods, most are laboratory-scaled and not yet ready for production of large superhydrophobic surfaces that could be potentially interesting for many real applications. This can be attributed to the complexity of hierarchical micro/nanostructures as well as the costly low surface energy materials. For these superhydrophobic surfaces, another major concern is their capability of demonstrating long-lasting corrosion resistance. Thus, future works are still required with focuses on enhancing stability of superhydrophobic structures. Alternatively, self-healing functionalities and/or corrosion inhibitors may be introduced for the design of smart superhydrophobic surfaces that can repair their anticorrosive performance autonomously or with minimal external intervention.
This work is supported by National Natural Science Foundation of China (No. 51401018) and the National Basic Research Program of China (973 Program project, No. 2014CB643300).
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