Ideas Inspired by Nature to Combat Marine Biofouling and Corrosion

Given the increasingly worrying situation regarding available energy, all countries worldwide have agreed to actively develop marine resources. As a result of the expansion of marine resources, biofouling and corrosion issues encountered by marine engineering materials have attracted an increasing amount of attention.

Biofouling is a significant issue which hinders the development of marine technology. Solid surfaces serve as attachment points in marine ecosystems. Owing to the adhesion of fouling organisms, materials submerged in seawater develop a complex biofilm, and the surface of the material experiences numerous physical and chemical changes [1]. This damages the material and leads to biofouling [2]. There are four stages within biofouling. The first stage is the associated accumulation of organic macromolecules, such as proteins, polysaccharides, and glycoproteins, which make a surface more wettable. The attachment of organic molecules during the early phase of biofouling provides nutrition and attachment points for the life activities of bacteria and other microorganisms; this is the second stage in biofouling. After approximately 1–2 h, the bacteria and other microorganisms begin to adhere to the surface of the substance. The attachment of protozoa (microscopic unicellular eukaryotes), invertebrates, and algal larvae is the third stage in biofouling. The attachment of huge creatures, such as tunicates, coelenterates, mosses, and barnacles, constitutes the fourth and last stage in biofouling [3]. Marine biofouling can cause harm in various ways. For example, the accumulation of organisms on the surfaces of ships increases the roughness of ship surfaces and drag resistance, thereby resulting in increased sailing time and the wastage of fuel resources. Barnacles and other organisms that firmly adhere to the surfaces of ships may potentially cause corrosion, which shortens the service life of a ship and results in significant financial losses for the maritime transportation industry [4].

The complex electrochemical, chemical, and marine biological corrosion of metal components in seawater constantly occurs in harsh and complicated marine environments [5,6]. In marine environments, metallic materials are primarily electrochemically corroded [7]. The primary cause of electrochemical corrosion is when the electrode potential on the surface of a material is different because of the existence of various constituent elements in a metallic material. Therefore, the microbattery reaction occurs in the corrosive medium, thereby resulting in corrosion and damage to the metallic material [8]. Metal corrosion affects the strength, plasticity, and mechanical qualities of a material [9]. Additionally, it can harm materials’ structure, thus making it useless over time. According to statistics, the annual economic damage caused by marine corrosion amounts to approximately 1 % of the global gross domestic product (GDP). Therefore, a solution to maritime-corrosion-related issues must be found.

Humanity has been fighting against biofouling for more than 2000 years. To prevent biofouling, hulls were initially covered with metallic materials, such as lead, copper, and zinc. However, it was discovered that metallic materials react electrochemically with seawater, thereby accelerating the corrosion of the hull and shortening the antifouling period. In the last century, researchers discovered that tributyltin antifouling agents have a broad-spectrum bactericidal effect; hence, it was grafted onto acrylate to develop a tributyltin self-polishing antifouling coating, which has an excellent antifouling effect in practical applications. However, due to its solubility in seawater, it interferes with the endocrine system of marine mollusks and leads to sexual abnormalities, which have a significant impact on marine safety. Therefore, since 2008, countries have restricted the use of tributyltin antifouling coatings. The pressing demand for novel antifouling coatings is increasing the difficulties faced by coating researchers. In response to this issue, researchers have begun using cuprous oxide as an antifouling agent. However, it has considerably less bactericidal power than tributyltin, which means it needs to be used extensively. In addition, copper is a heavy metal element which damages the marine environment in certain ways. Therefore, the use of cuprous oxide antifouling chemicals is gradually becoming less widespread [10]. Marine anticorrosion coatings are types of functional coatings that can extend the service of marine engineering facilities and machinery. Traditional anticorrosion coatings predominantly include epoxy resin, polyurethane, fluorocarbon resin, and silicone coatings. Although traditional corrosion-resistant coatings have many excellent properties, they can be further improved.

• Epoxy resin coatings

Due to the high crosslinking density of epoxy resin, microcracks and holes will form in a coating made from this material. In addition, epoxy coatings have a certain level of porosity, which allows corrosive medium molecules to penetrate them, thereby reducing the anticorrosion performance of the coating [11].

• Polyurethane coatings

The anticorrosion capability of polyurethane coatings is compromised by polyurethane’s weak resistance to acids, alkalis, salts, and other media. Additionally, it slightly hydrolyzes in seawater [12].

• Fluorocarbon resin coatings

Fluorocarbon-resin-based coatings offer excellent chemical and UV resistance; however, their low adhesion and wettability affect their industrial applications [13].

• Silicone coatings

Although silicone coatings offer good chemical stability and resistance to acids and alkalis, their exorbitantly high cost prevents them from being widely used in industry. Additionally, due to their weak solvent resistance and adhesion, the effectiveness and stability of their anticorrosion property is decreased [14].

In the face of increasingly severe marine biofouling and corrosion problems, traditional antifouling and anticorrosion coatings have several limitations and cannot be widely used in the marine industry. Therefore, environmentally friendly and effective antifouling and anticorrosion coatings must be developed immediately. During long-term evolution, natural organisms have developed their own antifouling and anticorrosion strategies to ease the pressure of existence (Figure 1). For instance, numerous medium- and large-sized marine organisms, such as sharks and dolphins, can withstand the parasitism and adhesion of marine algae and other microbes for extended periods of time. Therefore, the application of natural antifouling and anticorrosion methods as coating systems has gradually become a new trend within sustainable development.

The current mainstream bionic antifouling technologies can be predominantly categorized into six categories.

• Bionic microstructure antifouling coatings

Surfaces with microstructures, such as lotus leaves, shark skin, and gecko feet, are common in nature. Extensive research has confirmed that coatings with biomimetic microstructures have strong antifouling capabilities [15]. The theory of surface-structure-induced wettability describes the antifouling mechanism present within these structures. According to this theory, surface microstructures induce changes to the surface’s wettability. A hydrophobic surface can transform into a superhydrophobic surface when the surface roughness increases. However, as the surface roughness increases, hydrophilic surfaces with a water contact angle of less than 90° may become superhydrophilic. This is comparable to the antifouling mechanisms of fouling-releasing and amphiphilic polymeric materials. Biomimetic microstructured antifouling coatings are environmentally friendly. However, the preparation process for biomimetic microstructured antifouling coatings is complex, and they have a limited construction area, high cost, and low long-term stability in seawater. Therefore, the development of low-cost and durable biomimetic microstructured antifouling coatings needs to be the focus of future research.

• Bionic ultra-slippery surfaces

In 2011, Aizenberg was inspired by Nepenthes to inject a lubricating fluid into a porous matrix to replace the trapped air in the pores of a superhydrophobic surface. In this process, lubricating oil is injected into a porous surface to form an oil film that repels almost any liquid or solid material. Therefore, the surface structure can effectively prevent fouling adhesion [16]. However, inevitably, the lubricating oil in a super-slippery surface is gradually lost due to the shearing of seawater. Therefore, future studies should gradually develop a super-slippery surface with a long life and shear resistance.

• Natural antifouling agents

The surface of algae in the ocean secretes active substances that inhibit the attachment of fouling organisms. These naturally active substances can be extracted, purified, and used as antifouling agents. In addition, capsaicin and piperine extracted from terrestrial plants (such as capsicum and pepper) can inhibit the attachment of marine organisms [17]. Unfortunately, industrialization is hindered by the comparatively high extraction costs associated with natural antifouling reagents. Therefore, the preparation of reagents similar to natural antifouling reagents via chemical synthesis has become the most common method. Moreover, note that to exert the antifouling performance of natural antifouling agents, a suitable carrier must be used to ensure the stable and long-term release of natural antifouling agents in seawater.

• Enzymes

Given that enzymes can degrade proteins, numerous marine organisms secrete enzymes to prevent biofouling from adhesion. Based on this concept, researchers have added enzymes to antifouling coatings or grafted them into polymers to achieve antifouling properties. However, the experimental results of research into marine antifouling materials based on enzymes have rarely been reported, and their actual antifouling effects in the ocean still need to be confirmed [18].

• Bionic dynamic surfaces

In the ocean, several corals have many soft tentacles on their surfaces, which can swing under the action of the current. By swinging their tentacles, coral forms a dynamic surface that prevents fouling organisms from adhering to it. Inspired by this approach, our research group prepared cylindrical imitations of coral tentacles in a previous study. In our study, it was found that artificial coral tentacles under high-frequency oscillation could inhibit the attachment of fouling organisms [19]. Unavoidably, antifouling materials based on dynamic surfaces are expensive to prepare and have not been tested in actual sea conditions. Therefore, it is crucial to verify whether they have stable, long-lasting, and broad-spectrum antifouling capabilities.

• Fouling-resistant materials

The skin mucus of fish and amphibians is characterized by softness and hydrophilicity. Mucus is induced to form a hydration layer on the surface via hydrogen bonding and electrostatic action. This hydration layer forms a physical barrier to fouling organisms, thereby inhibiting fouling adhesion [20]. Researchers have been inspired to prepare fouling-resistant materials with similar hydration layers. The most commonly investigated materials that resist fouling are polyethylene glycol, zwitterionic polymers, and hydrogels. After being submerged in seawater, the hydrophilic molecules or ions of these materials interact with water molecules to form a hydration layer on the surface, which inhibits fouling adhesion. However, this type of coating suffers from poor mechanical properties and adhesion to the substrate, which hinders its industrialization.

The current common bionic corrosion protection technologies can be predominantly categorized into four categories.

• Bionic superhydrophobic surfaces

Lotus leaves achieve superhydrophobicity by utilizing air that is trapped in their micro/nanohierarchical structures. Therefore, erosion from external water is blocked and removed. Inspired by superhydrophobic surfaces, air that is trapped by the micro/nanostructures can be used to inhibit the erosion of the substrate via corrosive medium [21]. Unfortunately, artificially prepared superhydrophobic coatings are complicated and expensive to create, and the trapped air is easily squeezed by seawater and gradually disappears. This eventually causes the coating to lose its anticorrosion capability.

• Bionic ultra-slippery surfaces

The bionic ultra-slippery surface of the bionic pitcher plant has not only antifouling, but also anticorrosion properties. This is due to the excellent hydrophobicity of the lubricant on the bionic ultra-slippery surface, which prevents seawater from penetrating the substrate surface. The limitations of this surface are described in the antifouling section.

• Super adhesion coating

Marine mussels exhibit impressive interfacial adhesion due to the mussel adhesion protein and the DOPA functional group that is located in the protein. It provides mussels with a special and unique anti-moisture adhesion capacity and allows them to firmly adhere to solid surfaces even in the presence of severe currents [22]. As is well known, the coating exhibits strong adhesion to ensure the structural integrity and reliability of the anticorrosion function. Inspired by mussels, numerous scholars have prepared polymer coatings that imitate mussel proteins to enhance interfacial adhesion and thus enhance anticorrosion performance.

• Transport control coating

In nature, some organisms can control the flow of liquids and ions through their own tissue structures. Nanochannels in Escherichia coli bacteria regulate ion transport by stabilizing target ions in the inner center of the membrane. Furthermore, some organisms achieve ion transport via fluid rectification. A typical example is epidermal bladder cells in quinoa stems, which sequester excess sodium from metabolically active cells to alleviate osmotic stress. Inspired by the bioselective control of liquid and ion transport, their application in anticorrosion coatings can effectively block the transport of corrosive media. Typically, fillers (e.g., graphene, carbon nanotubes, and zinc) are added to coatings as physical and chemical barriers to resist the transport of corrosive media. Another way to prolong the transport time of corrosive media is to prepare smart coatings of micro- and nanocapsules. These capsules release healing agents in response to external stimuli; these agents can repair coating defects and prolong the transmission channel of corrosive media [23]. Although transport control coatings have effective anticorrosion properties, their preparation procedure is complex and does not support widespread industrial development. Another crucial problem is the uneven distribution of fillers in the coating.

Given the increasing prevalence of biofouling and marine corrosion, solutions must be quickly determined. Therefore, this article describes nature-inspired antifouling and anticorrosion coatings. It explains the ability of natural organisms to withstand biofouling and corrosive media. In our opinion, this will serve as an inspiration for the development of future bionic antifouling and anticorrosion coatings. However, current bionic antifouling and anticorrosion coatings still have great limitations, and therefore, further studies should focus on the following four aspects:

• Although some naturally occurring organisms have strong antifouling properties, we are unable to fully recreate these properties; therefore, the coatings that are prepared have poor broad-spectrum performance. In future, multiple antifouling strategies could be integrated into the same coating to address the limitations of a single biomimetic antifouling strategy. In addition, current bionic antifouling coatings have the complex preparation processes, high costs, and poor mechanical properties. Thus, future development must evolve in the direction of the preparation of coatings with high strength and simple preparation methods.
• At present, anticorrosion coatings based on bionic structures are difficult to process. Additionally, it is necessary to investigate whether they have long-term anticorrosion performance in actual sea environments. Therefore, future anticorrosion coatings based on biomimetic structures should be developed with a focus on simplicity and long-term effectiveness.
• In addition to biofouling and marine corrosion in the ocean, the erosion of sediment and the cavitation of rotating parts, such as propellers, are urgent issues. Therefore, the development of coatings that are merely antifouling or anticorrosion coatings has limited applications. The development of multifunctional and integrated marine biomimetic coatings is expected to become a trend in the future.
• Smart coatings represent a trend in future developments [24,25,26]. Therefore, antifouling and anticorrosion coatings prepared at present should be developed with a focus on smart properties. In particular, smart features, such as self-healing and shape memory properties, can be added to the coatings. When coatings are stimulated by the outside world, feedback could be provided immediately to guarantee the functional integrity of the coatings, which would lengthen their service lives.

Figure 1. Bionic antifouling and anticorrosion strategies.

Figure 1. Bionic antifouling and anticorrosion strategies.