Wet chemical reaction This method covers a broad range of different chemical reactions by which the microstructures required for superhydrophobic surfaces are directly synthesized.
Yin et al. By immersing in cerium nitrate hexahydrate solution, Ishizaki generated nanostructured cerium oxide films on the surface of magnesium alloy. From the polarization curves, the corrosion current density of superhydrophobic magnesium alloy surface was much smaller than that of untreated surface. These results suggest that after superhydrophobization, the anticorrosive performance of magnesium alloy was greatly improved.
In a recent study, superhydrophobic films were also built up on the surface of magnesium alloy by a simple immersion process in a solution containing ferric chloride, deionized water, tetradecanoic acid, and ethanol Fig. The main advantage of wet chemical reactions is that the fabrication of superhydrophobic surfaces requires only simple solution immersion. This also permits a certain level of flexibility since the method can be applied to substrates of different shapes and sizes.
The microstructures can be also obtained by selectively chemical etching of dislocations or impurities on metal surfaces. Liu et al.
Polarization curves confirmed the superior anticorrosive performance of the superhydrophobic surface was attributed to air inclusion in the microstructures under water. Compared to wet chemical reactions, the superhydrophobic microstructures are directly etched out from the bulk materials and are thus expected to be more stable against mechanical forces.
Similar to the two methods above, the hydrothermal method is another important top-down approach for fabrication of superhydrophobic anticorrosive surfaces. Hydrothermal methods are considered more environmentally friendly since the chemical used to create surface microstructures is only H 2 O or dilute H 2 O 2. For example, Ou et al. After treatment with perfluorooctyl trichlorosilane PFOTS , superhydrophobic surfaces were also obtained. As shown in Fig. Compared with etching method, the superhydrophobic structure prepared through hydrothermal method exhibited even better corrosion resistance higher corrosion potential and lower corrosion current density , which was co-contributed by the air cushion as resulted from the surface hydrophobicity and the barrier effect from the hydroxide layer grown on the metal substrates.
Superhydrophobic Surfaces | Max Planck Institute for Polymer Research
By means of anodization, micro or nanoscaled surface roughness can also be effectively generated to prepare superhydrophobic anticorrosive surfaces. Electrochemical impedance spectroscopy EIS results showed that a high corrosion resistance can be retained after immersion in 3. Xiao et al. The superhydrophobicity was found stable at pH 1— It was largely maintained in pure water and 1. One of the major advantages of anodization techniques is its high efficiency in producing surface microstructures.
For example, using a traditional Watts bath and a platinum anode, a pine-cone-like hierarchical micronanostructured nickel coating was electrochemically deposited onto copper cathode. The electrodeposition process can be further simplified so that roughness creation and surface hydrophobization can be finished in a single step.
Inorganic—organic sol—gel coatings can also be engineered with superhydrophobicity for anticorrosion purposes. Hu and coworkers developed an interesting electrodeposition assisted sol—gel approach Fig. The corrosion resistance of the obtained superhydrophobic coating was confirmed by much higher impedance modulus in EIS results.
Because of the high strength of the Si—O bond, inorganic—organic sol—gel coatings can demonstrate better thermal and UV stability than organic coatings. Superhydrophobic anticorrosive surfaces can also be obtained with a bottom-up approach by means of nanocomposite coatings. Many of the superhydrophobic nanocomposite coatings are sprayable and, therefore, share a unique advantage in the adaptability for large-scale fabrication. ZnO nanoparticles, modified by hydrophobic stearic acid, were mixed with fluorinated polysiloxane FPHS and sprayed on steel substrate Fig.
When a strong bonding is established between nanoparticles and resins, the mechanical durability of the coating can be significantly improved. In addition, nanoparticles, such as clays 98 or graphene, 99 have been incorporated to these coatings to decrease oxygen permeability.
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The templating method can be applied on diverse types of polymers and is capable of making very fine surface microstructures down to only a few nanometers. In addition, the templating method might not be able to produce excessively complex surface microstructures as damages may occur on both the samples and the templates during the peeling off procedure. Although superhydrophobic surfaces have received rapidly increasing popularity for their high corrosion resistance, existing studies mostly focused on their fabrications but lacked in-depth discussion over the electrochemical behaviors and anticorrosion mechanisms.
The microstructures of a superhydrophobic surface are capable of trapping air films, which greatly reduce the contact area between droplet and surface. Thus, it is generally believed that superhydrophobic surfaces can inhibit atmospheric corrosion by hindering the formation of an electrolyte film. This is expected to be useful for corrosion protection under rainy atmospheric environments. As a result, the protective air films cannot be formed. On a tilted surface, the salt particles tend to slip off due to the highly lubricating effect of trapped air Fig.
Moreover, the salt particles deposited from the actual marine atmosphere can be very small and easily reach into the microstructures of superhydrophobic surfaces. Therefore, deliquescence of salt is more likely to induce a Wenzel contact which pins the droplet to the surface. For a superhydrophobic surface immersed in water, the air film captured in the microstructures forms an additional nonconductive barrier, which to some extent isolates the surface from surrounding water and improves the overall anticorrosive performance Fig.
The existence of the air films has been confirmed visibly , and by confocal Raman spectroscopy. The superior inhibitive effect of superhydrophobic surface can be also more directly observed from the suppressed corrosion current density from polarization curves, by which the corrosion rate can be estimated.
To achieve superhydrophobicity, highly porous microstructures with large surface areas are often needed.
Superhydrophobic surfaces: a review on fundamentals, applications, and challenges
However, this makes the microstructure itself hardly a durable barrier against a corrosive environment. Yu et al. However, if immersed, the porous superhydrophobic layer was more vulnerable to water ingress which led to lower anticorrosive performance than that of a hydrophobic layer.
In Fig. Furthermore, despite the undeniable importance of surface roughness in hydrophobicity, further research is required to fully clarify the relationship between surface roughness of the microstructures and the resulting anticorrosive properties. This is partly due to the fact that the precise control of surface roughness is difficult through most of the current methods for preparing superhydrophobic anticorrosive surfaces. Improving the stability of superhydrophobic surfaces Essentially, all superhydrophobic surfaces are to a certain extent capable of withstanding chemical attacks in aqueous acid, alkaline, or salt solutions.
Nevertheless, because of their delicate porous structures required for superhydrophobicity, most of these surfaces do not have high mechanical durability. Integrity of the coatings may be also damaged, which causes defects, such as cracks, and results in decreased protectiveness against corrosion. To this end, improving the mechanical robustness has become one of the key issues in the development of superhydrophobic surfaces with long-lasting corrosion resistance.
Xiu et al. The nanostructures at the bottom were largely preserved, which was critical for retaining the surface superhydrophobicity. Interfacial strength among nanostructures, binding materials, and substrate is another critical factor affecting the mechanical durability of superhydrophobic structures.
By UV irradiation, Zhao et al. Another very important but essentially more straightforward strategy is to create superhydrophobic structures directly from bulk materials. A templating process sequentially using nanoporous anodic aluminum oxide and microporous silicon was applied to fabricate a hierarchical superhydrophobic surface out of hydrophilic metallic glass.
Self-healing materials are an emerging class of smart materials that when damaged are capable of autonomous repair or can be repaired under external stimuli, such as heat, light, and solvents. The self-healing of hydrophobicity can be realized by replenishing materials of low surface energy at the surface. For example, through chemical vapor deposition, a large amount of FAS molecules were deposited on the surface and into the bulk of porous polyelectrolyte multilayers to prepare the superhydrophobic surface. Similarly, mesoporous silica microparticles were used to store the molecules of octadecylamine, endowing the superhydrophobic surface with a self-healing function.
Compared with replenishing surface hydrophobic materials, the healing of superhydrophobicity by restoring the rough microstructures is even more difficult. However, liquids, such as water and acid, were capable of restoring such structure by swelling of coating, thereby restoring the superhydrophobic performance Fig.
More interestingly, the recovery of superhydrophobicity was more efficient by more acidic solution which would protonate the amine groups in poly ethyleneimine for faster swelling. Although not reported yet, the use of shape memory polymers SMP would represent another promising solution to endow self-healing function to superhydrophobic anticorrosive surfaces. SMPs are stimuli-sensitive materials which are able to recover to their original shapes from temporarily deformed shapes by heat, light, or other environmental triggers.
In corrosion researches, a major class of self-healing coatings are prepared with corrosion inhibitors as fillers. The damages to these surfaces can trigger the leaching of embedded corrosion inhibitors which form strongly adherent films at the defects via complexation with corrosion products such as metallic ions. A list of excellent articles can be referred to for the reviews of inhibitor-containing self-healing anticorrosive coatings. Even if the superhydrophobic topcoat is damaged, prolonged release of corrosion inhibitors from the underlying primer can still confer active corrosion protection at the defected region.
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