Corrosion, often metal corrosion, is the material destruction that leads to failure in function. It has been a major problem plaguing mankind dating back to ancient times. The first written description of corrosion appeared in the works of Plato (427–347 B.C.) [1
] and the first patent of a protective paint appeared in 1625 [2
]. Since a large number of metallic materials are developed and utilized in various fields such as marine oil and gas exploitation and transportation industries, the accompanying problem of corrosion becomes greater, which brings enormous economic loss and poses great threats to personal safety and the natural environment. According to the National Association of Corrosion Engineers International study 2016, the direct global corrosion cost was estimated to be $
2.5 trillion, which is equivalent to roughly 3.4% of the global Gross Domestic Product [3
]. This study also reported that implementing corrosion control/prevention practices could result in savings of 15%–35% of the cost of damage. Among a variety of anti-corrosion techniques including the passive coatings and active methods [4
], organic anti-corrosion coatings, which isolate and protect the substrate metal kinetically, are the most cost-effective and environmentally-friendly approach for the ever-pressing corrosion issue [5
The fundamental principles of corrosion reveal that material corrosion is thermodynamically spontaneous and kinetically mediated by corrosive substances such as H2
, and Cl−
. These substances invade and transport through the coating to the coating/substrate interface, which leads to an accelerated corrosion reaction [6
] that consists of chemical corrosion and electrochemical corrosion. Moreover, the electrochemical corrosion processes at a high rate and it is a complex process that includes anodic and cathodic reactions. The anodic reaction transfers metal atoms into metal cations. This process can be accelerated by Cl−
, especially the pitting corrosion process [7
]. On the other hand, the cathodic reaction generates OH−
followed by the aggregation of Na+
, and this process could accelerate the accumulation of corrosion products, which leads to peeling-off of the coating [9
]. These reactions, supplied by the penetrated corrosive substances, not only consume the metal substrate but also lead to the failure of the protective coatings. Therefore, the key to high-performance anti-corrosion coatings requires effective blocking of corrosive substances at the external surface and control of ion transportation within the coating [11
Among the rapid research progress in protective coatings, learning from nature to design novel anti-corrosion materials is one of the best ways for creating these materials. Over the course of evolution, nature develops ingenious strategies that can be implemented to address the issues of surface blocking and ion control in anti-corrosion coatings. One representative dealing with the blocking is the superhydrophobic coating bio-inspired from self-cleaning lotus leaves, which protects metals from corrosion and has been studied extensively. The mechanism involves an isolative air layer formed between the external corrosive solution and the substrate, usually fulfilled by the hierarchical structure and the chemical constituents of the coating. Once the corrosive medium penetrates into the coating as time goes by, controlling the transportation of corrosive ions becomes significant to delay and, thus, prevent a corrosive reaction. Research efforts in controlling the transmission of corrosive agents can be passive, e.g., by adding fillers, such as zinc particles [14
]. The mechanism involves the blocking and prolonging of the path of corrosive agents, and the preferred corrosion of zinc rather than steel due to a higher electrochemical activity. The other is an active approach that includes combining with conductive/reactive components to identify and interact with the ions selectively. A variety of ion-selective organic coatings doped with different ion exchange resins, first studied by Wang et al. [15
], show effective control of the moving direction of ions such as Cl−
. The ion-selective coating that interacts with one type of ions is called single polar coating. An anionic coating (cation-selective film) blocks the invasion of anions (e.g., Cl−
) and allows the passage of cations (e.g., Na+
), while a cationic coating blocks the transmission of cations. A bipolar coating composed of cationic and anionic layers, which can restrict both types of ions, shows promising application in metal protection. Besides, conductive polymers, such as polyaniline (PANI) and modified PANI [18
], have also been used for ion selective anti-corrosion coatings [8
]. However, the ion selective coatings can only adjust the transport of ions and cannot deal with the water infiltration, which deteriorate the protective function of the coatings greatly. Therefore, preventing the invasion of H2
O into the coating layer is another important consideration of developing advanced anti-corrosion coatings.
Effective control of ion-containing fluids can also be observed in nature, e.g., the mangrove plants which survive and thrive in the marine intertidal environments featuring high humidity and high salinity through salt secretion [19
]. The harsh habitat is close to the anti-corrosion coatings that protect the substrate metals in marine surroundings [20
], and the strategies utilized by the mangrove through salt glands are vivid inspirations for developing ion-control coatings, formulated first in this work. In an aim to develop high-performance anti-corrosion coatings addressing the control of ion transport and external blocking, we explore the salt secretion of the mangrove, and, for the first time, fabricate mangrove-inspired anti-corrosion coatings employing ion-selective resins and hydrophobic surface construction. The structural and functional features of the salt glands on the mangrove (Ceriops tagal (perr.) C. B. Rob
) are presented, and the bio-inspired, bipolar hydrophobic coatings were fabricated to exclude external H2
O and corrosive ions (Cl−
). Our results show that the bio-inspired anti-corrosion coatings exhibit excellent properties in restraining the corrosive ion intrusion and transport within the coating, which leads to significantly improved anti-corrosion performance.
2. Materials and Methods
2.1. Observation of the Mangrove Leaves
The optical images of living mangrove were taken by a digital camera. Mature mangrove (Ceriops tagal (perr.) C. B. Rob) leaves and some branches of the mangrove were collected from Shenzhen Bay. The mangrove branches were cultured in nutrition solutions that were diluted by 100 mL tap water and 100 mL 3.5 wt % NaCl solution, respectively, for eight days.
For scanning electron microscopy, mangrove leaf samples were sputter-coated using a Leica EM ACE200 Automatic low vacuum coating apparatus (platinum, 30 s) (Leica, Wetzlar, Germany), and then observed by a ZEISS SUPRA55 Field emission sweep electron microscope (Carl Zeiss, Jena, Germany). The elemental mappings through energy-dispersive X-ray spectroscopy (EDS) of the samples were scanned by an Oxford X-Max 20 Electrically cooled X-ray spectrometer (Oxford, England).
2.2. Fabrication of the Mangrove-Inspired Coatings
Epoxy varnish (E-44 bisphenol A epoxy resin) was used as the film forming material. Single-polar coatings (two types) were obtained by doping different ion-selective resins, the 719 (202) strong base styrene anion exchange resin, and the 732 strong acid styrene cation exchange resin, respectively, into the epoxy varnish as paints. The ion exchange resins and epoxy varnish were diluted by xylene, and the doping concentrations were 0, 2.5 wt %, 5 wt %, 10 wt %, and 20 wt % for both groups. After the paints were evenly distributed in the epoxy varnish, TY-650 polyamide was mixed to cure the epoxy varnish. Then they were brush-coated on silica gel plate and also metal substrates (Q235) to obtain the single-polar coatings and the coated metal samples, respectively.
Q235 steel with the size of 10 mm × 10 mm × 5 mm was the metal substrate and the main elements are (wt %), C 0.127, Si 0.15, Mn 0.41, P 0.018, S 0.019, Fe balance. All metal substrates were polished with water on graded sandpapers (150#, 400#, 600#, 800#, and 1000#) step-by-step and were linked with copper wire by soldering. Metal samples were sealed by epoxy resin with a working surface of 10 mm × 10 mm exposed. Sealed samples were polished with water phase sandpapers (150#, 400#, 600#, 800#, and 1000#,) step-by-step, washed by anhydrous ethanol, dried, and kept in a dryer until utilization.
For the bipolar, hydrophobic coatings, a hydrophobic surface layer was fabricated using a template method. A superhydrophobic silicon plate was fabricated following the method in previous work [21
]. The silicon nanowires and grooves were fabricated based on a 425-mm thick silicon wafer. A standard Micro-Electro-Mechanical System process technology was employed to fabricate rough structures on a silicon surface, which consists of two essential structural features, silicon micropillars, and silicon grooves. A photolithography process was first used to selectively cover a photoresist on a silicon wafer, which was followed by reactive ion etching (RIE) to etch the wafer areas that are not protected by the photoresist, and deep RIE was used to further etch the silicon substrate. This process formed silicon micropillars. The deep RIE process included cyclic passivation and etching modes in which C4F8 and SF6 were used. In the etching cycle, the SF6 flow rate was 130 sc·cm and platen power was set at 12 W. In the passivation cycle, the C4F8 flow rate was 85 sc·cm. Lastly, the photoresist was removed and deep RIE was used to further etch the silicon substrate covered by photoresist, which formed a silicon groove. Then, the surface fabrication process was completed. The prepared silicon wafer was taken as an original template, and PDMS was applied to copy the structure on the coating surface. The thicknesses of all fabricated coatings were measured by a micrometer. The thicknesses of bare metal substrates were monitored at three different points, and the average of all the measurements was taken as the thickness of metals (TM). The total thickness of coatings and metal substrate (TT) were measured by the same processes. Then the thickness of the coating is the difference of the TT and the TM. For the first type of single polar coatings, the coating thicknesses of the fabricated coatings and control groups were kept the same (45 ± 5 μm). For the second type of bipolar coatings, the total coating thicknesses of the fabricated bipolar coatings and control groups were kept the same (90 ± 5 μm), since those were fabricated layer-by-layer.
2.3. Electrochemical Measurements
Electrochemical tests including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and polarization curves were performed on CHI760E. The electrochemical experiments were carried out in 3.5 wt % NaCl aqueous solution and a three-electrode configuration was applied, including the as-prepared samples (Q235 steel coated with the epoxy varnish, the bioinspired single-polar coatings, and the bioinspired bipolar, hydrophobic coatings), platinum plate, and saturated calomel electrode (SCE) as working, counter, and reference electrodes, respectively. If there is no other specific indication, all potentials reported in this paper are taking SCE as the reference. EIS tests were performed in a frequency range of 105 through 0.01 Hz at the open circuit potential with an amplitude of 10 mV. Potentiodynamic polarization curves were obtained by setting the sweeping range of ±300 mV versus the rest potential value, and a rate of 1 mV/s was employed for scanning. The measured results were further analyzed by using software Cview.
2.4. Wettability and Ion-Resistant Property
The surface wettability was measured using a contact angle meter (DSA-100, KRüSS Instruments, Hamburg, Germany) recording the contacting scenario of a water droplet to the surface. For the ion-selectivity analysis, the fabricated single-polar coatings were fixed in a custom-designed equipment, as shown in Figure 5a, between the 3.5 wt % NaCl solution (30 mL) on the left side and the ultrapure water (100 mL) on the right side. With increasing time at fixed intervals, 100 μL solution on the left side was taken out and the concentrations of Cl− and Na+ were measured by ion chromatography (ICS-900, DIONEX, Sunnyvale, CA, USA) and inductively coupled the plasma mass spectrometer (iCAP Q, Thermo Scientific, Bremen, Germany).
To control the corrosive substances, e.g., water, Na+, and Cl−, is the key for a high-performance anti-corrosion coating to inhibit the corrosion reaction. Strategies developed by nature could provide numerous ingenious designs for dealing with that issue. In this study, we investigated the mangrove salt glands, which are distributed on both surfaces of the leaves (Ceriops tagal (perr.) C. B. Rob), and the salt secretion of the salt glands. Inspired by the function of controlling transport of ions into and out of the plant, we designed single-polar and bipolar coatings that have different ion-selective abilities and, thus, control of transport of Na+ and Cl−. We further fabricated mangrove-inspired, bipolar hydrophobic coatings that have a top-down protective ability. Our electrochemical evaluations show that, among the manufactured mangrove-inspired protective coatings, the bipolar, hydrophobic coatings (H, Cationic/Anionic) possess significant outstanding and long-term anti-corrosion performance.