Research Progress of Self-Healing Coatings on Ships Against Biological Pollution: A Review

Abstract

Marine biofouling is a well-established and significant challenge for the maritime industry. Self-healing coatings applied to ships have demonstrated superior surface properties, including enhanced corrosion resistance and the ability to mitigate biological contamination. Consequently, numerous studies have been conducted to assess different self-repairing coatings, which incorporate mechanisms such as microcapsules, dynamic covalent bonds, and ion exchange. This review begins with an introduction to the process of biofouling formation. It then provides a comprehensive outline of the self-healing coatings that have been developed to improve wear resistance, summarizing the advancements in this area. Finally, building upon these three coating systems, this paper offers a summary of the fabrication and protection technologies for self-healing coatings, including the preparation of micro/nano containers, corrosion warning mechanisms, and intelligent responsive protection. Furthermore, the review explores the future prospects of self-healing coatings, offering valuable insights for researchers in the field. The potential limitations of their application scenarios are also addressed.

1. Introduction

With ongoing economic development, marine transportation has become increasingly vital. About 71% of the Earth’s surface is submerged by water bodies, primarily oceans; maritime transportation is essential to the global transportation sector. In particular, the exploitation of marine resources and the growth of the marine economy heavily depend on marine engineering equipment, such as vessels, submarines, submersibles, marine exploration tools, and offshore platforms. However, ship hulls face considerable challenges due to biofouling and wear. The oceanic ecosystem is a complex habitat that sustains a diverse range of life forms, including seaweed, shellfish, microbes, and microorganisms. This study addresses the development of biological growth on submerged surfaces in marine environments and the advancements in technologies designed to prevent such growth [1]. Meanwhile, biofouling on ships increases frictional resistance during voyages, leading to higher fuel consumption. Hence, the effects of biofouling and friction-induced corrosion together result in economic losses and environmental degradation [2,3].

The underwater structure of the vessel is particularly vulnerable to colonization by aquatic life forms, promoting the formation of a biofilm. This biofilm functions as an adhesive layer, Encouraging the adherence of marine species to the vessel’s exterior [4], which in turn raises hydrodynamic resistance and lowers the efficiency of the vessel’s power system. In contrast, the complex structure of the hull below the waterline makes biofilm removal more challenging and costly. Additionally, when the ship’s surface is affected by biofouling or sustains mechanical damage, its overall safety performance may be compromised. Consequently, addressing biofouling on ships is essential from both economic and safety perspectives.

Researchers have been exploring effective protective strategies to ensure the safety, reliability, and longevity of ships. One such strategy that has proven to be widely used and highly effective is coating. Traditional coatings, including metal and inorganic types, are often combined to create coating systems [5,6,7]. However, over time, especially in marine environments, these coatings gradually lose their effectiveness. In contrast, organic coatings have become widely adopted in ship protection due to their significant improvements in corrosion and wear resistance. Specifically, organic coatings are highly regarded for their exceptional corrosion resistance [8,9]. Traditional organic coatings typically address only one need, such as corrosion protection or antifouling.

Organic coatings possessing self-healing characteristics have been formulated in recent years. These advanced coatings deliver corrosion protection and antifouling effects, while also possessing the capability to self-repair and shield the underlying substrate from corrosion. Moreover, self-healing coatings applied to ships exhibit exceptional mechanical properties, making them an increasingly prominent area of research [10]. Initially, inspired by biological systems, autonomous restoration coatings represent a category of smart coatings capable of independently repairing damage as it occurs. Secondly, these self-healing coatings can reduce the need for extensive repairs following coating failure, ultimately lowering the overall maintenance costs of ships. Finally, by inhibiting biofouling and promptly repairing any damage to the coating, self-healing coatings help maintain the smoothness of the hull surface. This, in turn, reduces hydrodynamic resistance, improves fuel efficiency, and lowers carbon emissions [11,12].

This review highlights the novel self-healing coating system that combines autonomous repair with enhanced antifouling properties, offering superior performance compared to traditional coatings. By addressing both biofouling and corrosion in a single layer, it enhances the longevity and durability of ships. This innovation reduces the environmental impact of marine transportation by minimizing maintenance requirements and fuel consumption, supporting both economic and environmental sustainability. The research provides a promising approach to achieving more sustainable ship maintenance in the future.

The organization of the current study is outlined below: Section 2 offers an overview of biological pollution in marine environments. Section 3 reviews the current research on self-healing coatings for ships. Then, Section 4 discusses corrosion strategies for preventing biofouling on ships. Finally, Section 5 presents the prospects and outlook for future developments. Figure 1 represents the structure of the review.

2. Marine Biofouling

2.1. Forms of Biological Pollutions

Marine biological pollution results from the buildup of both organic and inorganic materials in oceanic ecosystems. With the development of microbial biofilms, this process gradually evolves into a complex community consisting of larger aquatic organisms, including plants and animals [13]. Most fouling organisms tend to settle on hard surfaces, particularly the surfaces of ships. Figure 2 presents marine biofouling on ship hull surfaces. To date, more than 4000 species of organisms responsible for fouling have been documented globally [14]. Marine fouling organisms can be broadly categorized based on their size and the nature of their attachment to ship surfaces: microfouling organisms (micrometer scale) and macrofouling organisms (centimeter scale) [15,16].

Microfouling organisms mainly include microorganisms like bacteria, diatoms, fungi, and microalgae. Furthermore, macrofouling species are divided into two categories: flexible macrofouling species and rigid macrofouling species. Soft macrofouling organisms consist of large algae, soft corals, sea anemones, and sponges that lack rigid support structures. Hard macrofouling organisms consist of animals with rigid support structures, such as tube worms and barnacles [17].

Table 1. Typical fouling organisms and their by-products in the marine environment.

Category	Year *	Denomination	Metabolic Substance
Microfouling Organisms	2020	Bacteria [18]	Organic acids like lactic acid and acetic acid.
	2021	diatoms [19]	Polysaccharides, pigments (including pigments like xanthophylls, chlorophylls, and others), and UV-absorbing compounds similar to mycosporine amino acids.
	2022	fungi [20]	Carbohydrate, lipid, protein.
	2024	microalgae [21]	Plant sterols, aromatic substances (including phenolic acids, brominated phenols, root tannins, flavonoid derivatives, and related compounds), lipids, and nitrogen-containing compounds.
Soft Macrofouling Organisms	2020	Large algae [22]	Phenolic compounds, encompassing bromophenols, phthalic acid tannins, and flavonoids; terpenoids, comprising steroids and carotenoids; thio-terpenoids; nitrogen-containing compounds, including proteins, nitrogenous bases, and chlorophyll analogs; as well as saccharides and complex carbohydrates.
	2024	soft corals [23]	Hemiterpenes, diterpenes, steroids and fatty acids.
	2024	sea anemones [24]	Triterpenoid saponins, organic acids and coumarins.
	2024	sponges [25]	Steroidal compounds, terpenoids, cyclic peptides, alkaloids and pyrimidines.
Hard macrofouling organisms	2022	tube worms [26]	Biological compounds such as carbohydrates, amino acids, vitamins and cofactors, among others.
	2024	barnacles [27]	Chitin, calcite, chitinase and calcite hydrolase.

* The “year” column indicates the year of publication for the research findings.

presents typical fouling organisms and their by-products in the marine environment.

2.2. The Process of Biological Pollution

The adhesion of aquatic fouling species is a complex process that involves several interrelated phases. Typically, the development of marine fouling occurs in three essential stages: the formation of the conditioning layer, the proliferation of the biofilm, and the establishment of the fouling community [28]. Figure 3 presents the biological pollutants generated at different time intervals and their corresponding sizes.

Once a vessel’s exterior is exposed to seawater, substances such as polysaccharides, proteins, and glycoproteins rapidly interact with the surface through van der Waals forces, hydrogen bonds, and electrostatic attractions, leading to the formation of a conditioning layer [29]. This phenomenon is reversible and provides sustenance for the subsequent attachment of microorganisms.

Once the conditioning layer has formed, microorganisms like bacteria and diatoms attach to it within a day. The attached bacteria and algae produce extracellular matrix materials made up of polysaccharides, proteins, and glycoproteins, which facilitate the adherence of microorganisms to the surface. This results in a biofilm formed by water, organic components, microorganisms, and secreted by-products [30].

The formation of the biofilm, along with the adherence of algal cells and certain microorganisms, creates a nutrient-rich substrate, leading to the surface being covered in a slimy layer within a short period. Over the subsequent weeks, larger fouling species, such as crustaceans, bivalves, and marine plants, attach to the surface and proliferate, forming a complex biological community. Throughout a span of several months, large-scale fouling develops, leading to severe biofouling phenomena and once this process occurs, it is typically irreversible [13].

2.3. Impact of Biological Pollution

Research has demonstrated that biofouling on the ship’s surface raises frictional resistance. A light coating applied across the entire hull can lead to an increase in total resistance by 7%–9%, while a heavy coating can cause an increase of 15%–18%. In contrast, biofouling on the hull surface can lead to a total resistance increase of 20% to 30% [31]. There is a direct correlation between hull roughness and biofouling. According to calculations, for high-speed vessels, a 10–20 μm increase in hull roughness can cause a 5% increase in frictional resistance [32,33,34].

Biofouling not only increases the frictional resistance encountered by a ship during its operation but also affects the vessel’s weight distribution and stability. During operation, the ship is subject to both tangential and vertical forces. The tangential forces are primarily responsible for the frictional resistance encountered by the hull as it moves through the water, while the vertical forces influence the ship’s weight distribution and stability. The accumulation of fouling organisms alters the weight distribution on the hull, potentially impacting the vessel’s trim and stability in the water. Furthermore, as fouling species, the buildup of barnacles, algae, and other microorganisms on the vessel’s exterior results in an increase in surface irregularities, causing higher frictional resistance in the water. This increased resistance directly affects the ship’s propulsion efficiency, requiring more power to maintain its speed. As the vessel moves more slowly, the impact of this additional resistance becomes more pronounced, resulting in a 3%–4% increase in fuel consumption [35]. Additionally, biofouling can lead to a reduction in ship speed by up to 40%, as the increased resistance reduces its overall hydrodynamic performance [36,37].

To minimize these consequences, it is imperative to develop coatings and technologies with self-repairing characteristics. These coatings are successful in reducing or minimizing the adhesion of fouling species, boosting fuel efficiency, lowering maintenance demands, and enhancing the vessel’s overall efficiency and durability [38].

3. Current Research on Self-Healing Coatings for Ships

Self-repairing coatings are modeled after biological systems, which can independently or with minimal assistance repair physical damage and regain their original functionality. In the last twenty years, these coatings have gained significant attention because of their potential and exceptional performance. Based on the origin of the repair components, two main categories exist for self-repairing methods: “externally-triggered” and “internally-triggered” [39,40,41,42].

“External-triggered” self-healing includes microcapsules and ion-exchange systems. Microcapsules achieve self-healing by encapsulating healing agents, which are released from damaged areas or cracks to complete the repair process, thereby maintaining the protective performance of the coating. This type of self-healing is generally irreversible [43]. In contrast to microcapsules, ion-exchange coatings, as an “external-triggered” self-healing method, are reversible. When external conditions change, the ions originally adsorbed on the coating surface can be replaced by other ions, thus realizing a “reversible” process [44].

“Internal-triggered” self-healing primarily relies on dynamic covalent bonds between polymer chains. Dynamic covalent bonds represent reversible chemical linkages capable of breaking and reorganizing when physical or chemical reactions take place at the damaged site, triggered by external factors such as temperature, pH, light, or intermolecular interactions. Compared to “external-triggered” self-healing, “internal-triggered” self-healing is more environmentally friendly, as it does not require additional additives [45].

Table 2. Benefits and drawbacks of ship self-healing coatings.

Antifouling Types	Year *	Compound	Main Characteristics and Requirements	Advantages	Disadvantages
	2020	Microencapsulated polyurea with ethylenediamine as the central component [46]	High Self-healing Efficiency; Thermal Stability; High Core Content; Excellent Corrosion Resistance.	Enhanced Mechanical Properties; Improved Self-Healing Capability; Thermal Stability.	Poor Dispersion; Poor Degradability; High Cost.
	2021	ZnO-encapsulated MPDA microspheres responsive to UV, NIR, and acid/base stimuli [47]	Multiple Responsiveness; Superhydrophobicity Antibacterial and Antibiofouling Properties; Environmental Stability; Multifunctionality.	Effective Bacterial Inhibition; UV Resistance and Corrosion Resistance; Low Raw Material Cost.	UV Degradation; Slow Self-Healing Rate; Poor Compatibility.
Microcapsules	2022	Tung oil-PGMA@PANI microcapsules [48]	Self-healing; Corrosion Inhibition; Mechanical Stability; Uniform Dispersion; Long-term Durability.	Improved Self-Healing Capability; Enhanced Corrosion Resistance; Controlled Release.	Limited Mechanical Strength; Potential for Incomplete Healing; Environmental Sensitivity.
	2023	Salicylic acid@polyurea-formaldehyde [49]	Long-term Antibacterial Activity; Anti-corrosion; Controlled Release; and Environmental Durability.	Excellent long-term Antibacterial Properties; Controlled Release of Salicylic Acid; Enhanced Stability of Salicylic Acid.	Limited Mechanical Strength; Potential for Incomplete Release; Cost of Production.
	2024	Magnetically responsive microcapsules with self-healing properties [50]	Magnetically Responsive Self-healing; Corrosion-resistant; Antifouling; Well-dispersed Microcapsules for Enhanced Durability.	Controlled Release; Enhanced Durability and Protection; Eco-Friendly.	Complex Synthesis Process; High Production Costs; Limited Mechanical Strength.
	2020	Zinc alginate coatings applied through electrophoretic deposition [51]	Marine Antifouling; Zinc/calcium Ion Integration; Electrophoretic Deposition; Biocompatibility; Corrosion Resistance and Long-term Durability.	Improved Corrosion Resistance; Environmentally Friendly; Uniform Coating Thickness.	Limited Mechanical Strength; Thermal Instability; Limited Durability.
	2021	Polymeric salt-based zinc ions [52]	Self-polishing; Zinc-polyurethane Copolymer; Controlled Degradation; Marine Antifouling Efficiency; Biocide release; Long-term Durability.	Extended durability; Regulated marine fouling-resistant coatings; Enhanced stability.	High Production Costs; Limited Mechanical Strength; Potential Toxicity at High Concentrations.
Ion Exchange	2022	Natural rosinbased zinc (RZn-x) resins [53]	Hydrolysis-Controlled Self-polishing; Antibacterial; Anti-algal; Environmentally Friendly; Long-term Durability.	Eco-Friendly and Renewable; Good Corrosion Resistance; Low Cost.	Limited Thermal Stability; Poor UV Resistance; Limited Mechanical Strength.
	2022	IL-functionalized polystyrene nanospheres [54]	Ionic Liquid Functionalization; Antibacterial and Antifouling Properties; Wear Resistance; Self-Polishing Nanocomposite Coatings.	Enhanced Surface Properties; Increased Stability and Solubility; Enhanced Catalytic Activity.	Complex Synthesis Process; Potential Toxicity of Ionic Liquids; Limited Mechanical Strength.
	2024	Poly-ionic liquid-derived antimicrobial polyacrylate-3 coating [55]	Self-Renewing Antifouling; Ionic Exchange; Antibacterial and Anti-Algal Properties; Self-Cleaning; Optical Device Compatibility.	Effective Antimicrobial Properties; Durability and Long-lasting Protection; Flexibility and Adhesion.	High Production Cost; Environmental Sensitivity; Limited Mechanical Strength.
	2020	Dynamic disulfide bonds [56]	Self-Healing; Reversible Bonds; Shape-Memory Crack Closure; Enhanced Mechanical Properties; Phase Separation.	Thermally Responsive; Flexibility in Design; Enhanced Mechanical Properties.	Sensitivity to Reducing Agents; Limited Stability Under Harsh Conditions; Slower Reformation Rate.
	2021	Polydimethylsiloxane network fibers and reversible boronic ester linkages [57]	Ultra-Thin Self-Healing; Durable Hydrophobicity; Scratch Resistance; Superhydrophobicity; Scalable Fabrication; Environmentally Friendly.	Reversible Crosslinking; Improved Mechanical Properties; Durability.	Sensitivity to Environmental Factors; Lower Long-Term Mechanical Strength; Complexity in Synthesis.
Dynamic Covalent Bonds	2021	Polyacrylamide multiarmed polyethylene glycol [58]	Spray-Paintable Hydrogel; Antifouling; Self-Polishing; High Adhesion; Mechanical Stability; Seawater Degradation.	Improved Stability; Functional Flexibility; Controlled Release and Gel Formation.	Complex Synthesis; Environmental Persistence; Cost.
	2022	PDMS-based polymer [59]	Smart Self-Healing; Dual Antifouling and Anti-Corrosion; Silicone-Based; High Toughness; Environmentally Friendly; Strong Adhesion.	Flexibility and Elastomeric Properties; Thermal Stability; Optical Transparency.	Low Mechanical Strength; Low Adhesion; Cost.
	2022	Polydimethylsiloxane network [60]	Switchable Zwitterionic Ester; Antibacterial and Antifouling; Sustainable Capsaicin Release; Long-Term Performance; Environmental Adaptability.	Improved Mechanical Properties; Increased Durability; Enhanced Thermal Stability.	Brittleness at Low Crosslinking Densities; Difficult Processing; Difficult to Bond to Other Materials.
	2024	Polyurethane incorporating dimethylglyoxime [61]	Macrophage-Inspired Antifouling; Dynamic Surface; Free Radical Release; Self-Regenerating; Antibacterial and Antialgal.	Enhanced Metal Ion Binding; Improved Mechanical Properties; Enhanced Durability and Resistance.	Stability Issues; Processing Complexity; Cost.

* The “year” column indicates the year of publication for the research findings.

outlines the benefits and drawbacks of ship self-repairing coatings.

3.1. Microencapsulation Type

Microcapsules are crucial in the development of self-repairing coatings for vessels, providing methods to enhance the coatings’ durability and resistance. The refinement of microcapsule synthesis includes several critical elements that directly affect the structure and properties of the microcapsules, enabling the development of coatings with significant self-repair capabilities [62,63]. Microcapsules, embedded within the coating and containing healing agents, rupture upon coating damage, emitting the healing agents, which react with nearby materials to restore the cracks. The healing agents encapsulated in microcapsules can generally be classified into two categories: restoration agents and anti-corrosion compounds. Coatings containing microcapsules intended for autonomous restoration can self-repair damage and shield the surface from environmental factors, resulting in a prolonged coating lifespan [64,65,66].

To achieve higher encapsulation efficiency and better self-healing performance of microcapsules, modification is essential. In the study conducted by Ma et al. [46] in 2020, polyurea microcapsules were synthesized using ethylenediamine (EDA) as the primary component. The study assessed both the anti-corrosion performance of the composite layer and thoroughly evaluated its mechanical characteristics. Microencapsulated systems have been widely utilized in the creation of intelligent anti-corrosion coatings and various other self-repairing materials [40], their fragility remains a challenge compared to matrix materials or highly crosslinked polymers. Therefore, a robust microcapsule shell is typically required to meet stringent demands, such as high survival rates during manufacturing, solvent resistance, load-bearing capacity, and the capability to release healing agents effectively when damaged. These elements play a vital role in improving the efficiency of autonomous restoration. The results indicated that the synthesized microcapsules exhibited an excellent spherical form, with their mean diameter ranging from 0.54 to 0.70 μm. Relative to untreated central components, the microencapsulated systems exhibited enhanced thermal stability, with the central material composition reaching up to 56.00 wt%. The epoxy layer containing 5.0 wt% EDA-polyurea microencapsulated systems achieved a corrosion inhibition efficiency of 90.00%, significantly improving the layer’s resistance to external corrosion. Figure 4 describes the autonomous restoration process of the coating.

In the domain of maritime fouling control, mesoporous polydopamine microspheres have demonstrated exceptional performance as a smart responsive material. These MPDA microspheres, which exhibit good stimulus-responsive properties, can be applied in intelligent self-healing coatings [67,68,69,70]. In 2021, Ni et al. [47] drew inspiration from the self-healing properties of superhydrophobic surfaces observed in nature, including those on lotus petals and clover, and developed an innovative water-based smart self-repairing coating using ZnO-encapsulated MPDA microspheres responsive to UV, NIR, and acid/base stimuli. In the smart self-repairing coating, hydrophobic-modified SiO₂ and ZnO nanoparticles serve as protective layers for the responsive microspheres and functional additives, enhancing the coating’s antimicrobial effectiveness. The coating’s resistance to harsh conditions was verified by subjecting it to UV-enhanced weathering and immersing it in various solutions, including strong acids, strong bases, saline solutions, and artificial seawater.

Polyaniline (PANI), an eco-friendly and highly efficient corrosion suppressor, facilitates the formation of protective layers on metallic substrates, hindering the penetration of damaging agents, and is widely used in corrosion-resistant coatings [71,72,73,74]. Recently, investigators have effectively created dual-purpose microcapsule systems possessing both repair and corrosion-resistant capabilities through the encapsulation of inhibitor agents within microcapsules. Among these, PANI has been utilized for the preparation of functional microcapsules due to its unique performance advantages [75,76]. However, the corrosion-resistant mechanisms and self-repair actions of PANI microcapsules within protective layers have yet to be fully clarified in current research [77,78,79,80]. To resolve this problem, Wu et al. [48] successfully synthesized a novel high-strength PANI microcapsule in 2022 and incorporated it into a waterborne epoxy matrix to improve its corrosion resistance. The findings indicated that these microcapsules possess strong mechanical properties and exceptional corrosion resistance. More importantly, the incorporation of PANI microcapsules into the protective layer not only imparted considerable repair functions to the coating but also greatly enhanced its overall corrosion resistance. This research contributes to the development of innovative intelligent anti-corrosion coatings.

However, the aforementioned coatings exhibit limited efficacy in combating microorganisms such as bacteria and diatoms. To tackle this problem, Ma et al. [49] successfully fabricated microcapsules with polyurea-formaldehyde (PUF) as the outer layer and salicylic acid (SA) as the inner content in 2023. The developed SA@PUF microcapsules demonstrate exceptional persistent antibacterial characteristics, primarily attributed to the controlled release characteristics of SA antifoulant by PUF microcapsules, coupled with their dual functionality of inducing reactive oxygen species generation and bacterial inactivation. This research provides significant theoretical foundation and engineering support for the creation of novel coatings with prolonged antibacterial activity and superior anticorrosion performance. In a subsequent advancement, Liu et al. [50] developed a repairable microcapsule with magnetic responsiveness (MS-TO@CA), utilizing tung oil as the core material and calcium alginate as the encapsulating layer, which contains conductive polyaniline antifouling particles and Fe₃O₄ magnetic particles. This coating offers high stability, economic viability, and a low level of acidity but also effectively reduces the adhesion of fouling organisms. Furthermore, it facilitates the electrolysis of seawater to produce sodium hypochlorite, significantly inhibiting the attachment of fouling organisms. These findings lay the groundwork for protecting marine materials.

3.2. Ion-Exchange Type

The microcapsule-driven repair coating system is classified as a non-reversible repair system, and their restoration mechanism dictates that such coatings can only perform a single restoration. In contrast, reversible self-healing systems possess the ability to repair multiple times, demonstrating superior practical value. Among various reversible self-healing systems, ion-exchange coatings play a crucial role due to their unique self-healing properties [81,82,83,84]. The strong affinity between ion pairs not only promotes the reorganization of polymer chain structures but also enables infinite modulation of molecular structures [85]. Based on this characteristic, ion-exchange coatings exhibit broad application prospects within the scope of self-healing coatings.

The toxicity of copper and silver ions has been widely confirmed, while the toxicity of nanoparticles remains a subject of academic debate. In this context, the development of coatings with excellent biocompatibility has become a significant research focus [86,87]. In 2020, Nassif et al. [51] successfully developed a biomass-based alginate hydrogel system. Studies have shown that zinc/calcium alginate composites not only effectively inhibit the adhesion of microalgae to material surfaces but also exhibit low biological toxicity, making them highly valuable for environmental applications. As an anionic polysaccharide, this research provides a systematic analysis of alginate, exploring for the first time its role in zinc alginate electrophoretic coatings and their impact on marine microorganisms, including bacteria and microalgae. The study indicated that divalent cations can be effectively embedded into the coating system via ion exchange with surrounding solution cations. Additionally, this approach is regarded as a promising antifouling technology due to its affordability, straightforward preparation process, and eco-friendliness. However, for the effective application of this technology, the corrosion resistance and longevity of the coatings continue to be significant challenges that need further resolution.

Copolymers with excellent mechanical strength and effective self-polishing functions are highly valuable in the field of marine fouling-resistant coatings. Degradable polyurethane materials have recently become a focal point in the marine antifouling area, thanks to their unique attributes. However, unlike traditional self-polishing acrylate materials, conventional degradable polyurethanes typically exhibit main-chain degradation characteristics and lack surface degradation capabilities [88,89,90]. This limitation has driven researchers to focus on developing self-polishing polyurethane materials with surface degradation abilities to meet practical application requirements. Drawing on the use of acrylate-based polymers in aquatic biofouling-resistant coatings and the research foundation of self-polishing properties in waterborne polyurethanes [91,92,93], Dai et al. [52] successfully synthesized a series of innovative zinc-based polyurethane copolymers in 2021. By incorporating zinc cations as polymeric salts, this material facilitates ionic interchange with sodium ions in saline environments. Through this distinctive ionic interchange process, the zinc-based polyurethane copolymers gradually change into a soluble form and undergo regulated leaching under the influence of seawater currents. Notably, marine field test results demonstrate that antifouling coatings based on this material exhibit outstanding antifouling performance, with effectiveness lasting over 12 months. These research findings fully demonstrate that zinc-containing polyurethane copolymers, as candidate materials for long-lasting and controllable marine antifouling coatings, possess significant advantages and application potential.

Multifunctional resins, along with multifunctional copolymer biofouling-resistant coatings, are crucial in the advancement of high-efficiency antifouling layers. As a naturally derived substance, rosin has been employed in biofouling-resistant coatings for a long time, with early preservative and antifouling formulations primarily using rosin as a key ingredient. However, due to its structural characteristics, pure rosin resin is susceptible to seawater penetration, leading to excessive release of antifoulants and a shortened antifouling period. This has led to a growing research focus on developing novel multifunctional resin materials by introducing particular functional groups into rosin [94,95]. Within this context, in 2021, Zhou et al. [53] successfully prepared rosin-based zinc (RZn-x) resin using Zn(OH)₂ and antibacterial natural rosin through a relatively simple and environmentally friendly synthesis method. This material, based on non-toxic ingredients, ensures its eco-friendly attributes and antimicrobial efficacy. Studies show that the Zn²⁺ and Cu²⁺ ions in zinc/copper-based resins can participate in ionic interactions with Na⁺ in seawater. This feature not only provides the resin with remarkable self-polishing capabilities but also grants it some antimicrobial effects. By integrating the nontoxic, antimicrobial, solubility, and self-polishing advantages of natural rosin resin, and incorporating metal ions into the natural rosin structure, a rosin-infused resin exhibiting adjustable polishing rates was synthesized. This innovation offers new possibilities for developing long-lasting, environmentally friendly antifouling coatings, marking a significant advancement within the area of marine antifouling coatings.

Furthermore, ionic liquids (ILs), particularly those containing imidazolium and ammonium groups, can function similarly to cationic antimicrobial agents, effectively inhibiting the growth of bacteria and fungi [96]. By carefully adjusting the composition of anions and cations, significant alterations may be made to the chemical and biological characteristics of these materials, thereby providing potential uses in biofouling resistance. In 2022, Wang et al. [54] successfully synthesized IL-functionalized polystyrene microcapsules through a method combining ring-opening reactions and surface-initiated atom transfer polymerization. Experimental findings showed that the addition of imidazolium moieties and ILs provided the functionalized microcapsules with outstanding antimicrobial properties. Additionally, the microcapsules effectively inhibited the adhesion of Porphyra and Dunaliella, showcasing their promising antifouling performance.

Research has shown that boron-based ionic liquids possess higher thermal stability compared to imidazolium and other common ionic liquids, and demonstrate superior stability in alkaline and nucleophilic environments (without acidic protons), while also being more cost-effective [97,98]. For increasing the thermal stability of boron-based ionic liquids, reduce their viscosity, improve gas transport properties, and strengthen their stability under alkaline and reducing conditions, Li et al. [55] successfully synthesized a bactericidal polyacrylate-3 coating (AFC-3) based on polyionic liquids in 2024. The synthesis was carried out via free radical photopolymerization, with isooctyl acrylate (2EHA), butyl acrylate, and the ionic liquid monomer acting as the starting materials. This coating not only exhibits antimicrobial properties via a contact inhibition mechanism but also enables self-renewal and dynamic switching of hydrophilic surfaces through ion exchange between PF6⁻ in IL-PF6 and Cl⁻ in artificial seawater. Test outcomes show that the AFC-3 layer attains antimicrobial efficiencies of 100% for Escherichia coli and 99.96% for Staphylococcus aureus. Furthermore, the persistent ion exchange between PF6⁻ and Cl⁻ contributes to an enhanced resistance of the coating to protein adsorption. Consequently, the AFC-3 layer demonstrates excellent biofouling resistance and holds significant potential as a biofouling-resistant coating for marine settings, making it suitable for a broader range of maritime applications. Biofouling is eliminated through the exchange of PF6⁻ and Cl⁻ ions.

3.3. Dynamic Covalent Bond Type

While materials such as microcapsules and ions are frequently employed in the development of self-repair coatings, these approaches may compromise coating performance and present potential hazards to the marine ecosystem. Therefore, in the last few years, there has been an increasing focus on self-repairing coatings that utilize dynamic covalent bonds (DCBs) [99,100,101]. The reversible and reorganizable nature of dynamic covalent bonds has drawn considerable interest. By integrating dynamic covalent bonds, polymer materials exhibit remarkable characteristics, such as improved flexibility, self-repair capability, responsiveness, adjustable mechanical properties, and reusability [102,103,104,105,106].

Automatic healing is difficult to achieve, as it typically requires manual intervention to reconnect the damaged parts of the original sample before healing can begin. Furthermore, it is exceedingly difficult to manually close cracks in the polymer coating before healing progresses. Therefore, the autonomous crack closure capability should be considered when employing internal self-healing mechanisms in self-healing coatings [107]. In 2020, Chen et al. [56] designed and prepared a polyurea–urethane/epoxy resin self-healing blend coating, incorporating dynamic covalent bonds into the domains, matrix, and interfaces, thereby improving the coating’s performance. Additionally, shape memory enables this coating to close cracks and facilitate healing. The study investigated the structure, morphology, and properties of the substances and layers, and assessed the self-repair abilities of the layer. The findings indicated that the polyurea–urethane/epoxy resin blend could be an effective candidate material for self-repairing polymer coatings.

Dynamic covalent polymer networks in vitreous materials, featuring associative or network-maintaining bonds, exhibit not only re-processability but also self-healing abilities [108,109,110]. Although vitreous chemicals have been applied to achieve waterproofing or self-healing properties, previous studies have not successfully developed ultrathin coatings (with thicknesses less than 100 nm) that simultaneously exhibit strong self-healing and hydrophobicity [111,112,113,114,115]. In 2021, Ma et al. [57] explored the development and fabrication of a glass-like ultrathin layer that integrates polydimethylsiloxane (PDMS) network chains with dynamic boronate ester crosslinking (dyn-PDMS) to utilize the natural hydrophobicity of silicones. The dynamic covalent bonds confer self-repairing and damage-resistant characteristics. Applied to rough aluminum, the dyn-PDMS coating exhibits superhydrophobic properties. Compared to fluorine-based chemicals, which raise environmental and health concerns, this PDMS vitreous coating provides a more sustainable option [116,117]. This research introduces an innovative method for producing ultrathin, long-lasting, self-repairing layers.

Moreover, hydrophilic coatings generally exhibit weak adhesion to various substrates, making their application more challenging. Despite their significant potential, the inadequate substrate adhesion and low longevity of these coatings pose substantial challenges for their practical implementation. Inspired by surface covalent crosslinking methods, which are utilized to enhance the attachment of hydrogels to diverse surfaces [118,119], Yang et al. [58] proposed a novel, paintable hydrogel coating in 2021 specifically designed for marine antifouling. This coating employs a robust hydrogel composed of polyacrylamide multi-arm polyethylene glycol (PAM-PEG) and incorporates epoxy groups to ensure strong adhesion to diverse surfaces while creating covalent bonds within the hydrogel layer. The research revealed that the coating exhibited outstanding biofouling resistance, effectively repelling a range of biomolecules, carbohydrates, microorganisms, and lipid. Furthermore, in marine environments, this hydrogel coating experiences gradual degradation, allowing the release of fouling molecules and organisms. Due to its excellent antifouling properties, ease of application, with excellent adhesion, this innovative paintable antifouling hydrogel coating shows significant potential for broad application in marine antifouling and related industries.

Despite their use, the materials cited above show soft properties and poor mechanical strength. Research indicates that more robust reversible covalent bonding typically results in enhanced structural integrity yet reduced repair capability, whereas less intense bonding interactions can boost autonomous restoration and elongation performance, though often compromising overall durability [112]. In 2022, Sun et al. [59] developed a novel polydimethylsiloxane (PDMS)-based material with exceptional structural integrity and repair abilities, along with enhanced resistance to biofouling and corrosion. The improvement of both structural characteristics and repair functions is promoted by the cooperative action of various reversible covalent linkages, integrated into the PDMS polymer chain to autonomously form a dynamic supramolecular polymer framework. Robust hydrogen interactions play a key role in providing toughness and elasticity, while flexible disulfide bonds assist in dissipating strain energy and boosting self-healing performance through effective reversible bond breaking and rearrangement.

Due to the intricate nature of the marine environment, hydrophobic PDMS coatings still demonstrate inadequate resistance to other marine contaminants, particularly mucus made up of proteins, bacteria, and extracellular polymers. The presence of mucus can contribute to biofilm formation, promoting the adhesion of larger organisms. Amphoteric ion polymers have been established as effective materials for enhancing PDMS’s anti-biofouling properties [120,121,122,123]. In 2022, Zhang et al. [60] pioneered the creation of a multifunctional antimicrobial biofouling layer for marine uses by linking convertible amphoteric ion esters and capsaicin polymers with a polydimethylsiloxane (PDMS) network. By utilizing its positive charge, the amphoteric ion ester targets and eliminates bacteria, while enzymatic breakdown into its amphoteric ion component leads to the release of dead bacterial cells. Meanwhile, the chemically bound capsaicin molecules can also be gradually released through enzymatic degradation, deterring marine organisms. Experiments conducted in the Yellow Sea confirmed that the layer exhibited excellent biofouling resistance, lasting for no less than 261 days. This study presents a promising approach for the development of effective and eco-friendly marine antimicrobial biofouling layers.

Although these biofouling-resistant coatings have been studied to inhibit the adhesion of proteins, microorganisms, and biofilm development, their mechanical properties and static biofouling resistance are still limited. A low concentration of antifouling agents may not effectively prevent fouling organism attachment, whereas excessive application could harm the aquatic ecosystem [124]. Consequently, it is essential to design an effective biofouling-resistant coating with an active antifouling mechanism. In 2024, Wang et al. [61], inspired by the way macrophages protect against pathogens through the generation of reactive oxygen species, carefully modified dynamic oxime-polyurethane covalent bonds to successfully fabricate a marine biofouling-resistant layer composed of polyurethane and dimethylglyoxime. The alkyl radicals (R) generated after the cleavage of the oxime-polyurethane bond effectively inhibit the settling of marine biofouling organisms. The inherent dynamic surface features of the coating also impede biofouling adhesion, contributing to sustained antifouling activity. The potential mechanisms underlying the biofouling resistance of the layers are explained in light of the previously discussed results.

4. Construction and Protection of Self-Healing Coatings for Marine Applications

4.1. Preparation of Micro/Nano Containers

The synthesis of micro/nano containers is a critical direction for advancing self-healing coatings in marine applications, as these containers greatly improve the longevity of self-repairing coatings under corrosive conditions (

Table 3. Advantages and disadvantages of antifouling strategies for ship self-repairing coatings.

Antifouling Strategies	Year *	Technology	Main Characteristics and Requirements	Advantages	Disadvantages
	2020	Pickering Emulsion Polymerization [128]	Solid Particle Stabilization; High Emulsion Stability; Surface Modification; Multifunctional Coatings; Self-Healing and Antimicrobial Properties.	Environmentally friendly; Stable; Multifunctional.	High cost; Complex process; Limited monomer selection.
	2021	Covalent Organic Framework (COF) [129]	2D Nanostructure; High Porosity; Chemical Stability; Low Interlayer Interactions; Functional Group Availability; Improved Dispersion.	High specific surface area and porosity; Strong structural designability; Excellent stability.	Stringent synthesis conditions; Poor processing performance; Relatively high cost.
Microencapsulation Type	2022	In situ polymerization [130]	Core–Shell Structure; Controlled Polymerization; Stable Emulsification; High Encapsulation Efficiency; Thermal Stability; Versatility.	Strong interfacial bonding; Simple process; Wide application.	Limited monomer selection; Difficult reaction control; Relatively high cost.
	2023	Covalent Organic Framework [131]	2D Structure; High Dispersion; Physical Barrier; Epoxy Resin Reactivity; Corrosion Resistance; Compatibility with Nanomaterials.	High specific surface area and porosity; Strong structural designability; Excellent stability.	Stringent synthesis conditions; Poor processing performance; Relatively high cost.
	2024	Interfacial polymerization [132]	Core–Shell Structure; Rapid Polymerization; High Core Content; Controllable Shell Thickness; Good Dispersion; Thermal Stability.	Fast reaction rate; Easy to control; Wide application.	Limited monomer selection; High requirements for interfacial stability; Difficulties in large-scale production.
	2020	Damage probes and corrosion inhibitors [133]	Damage Detection; Corrosion Inhibition; Controlled Release; Dual Functionality; Efficient Loading; Long-Term Protection.	High sensitivity; Accurate positioning; Various types.	Relatively high cost; Complex process; Environmental impact.
	2021	Fluorescent probe [134]	Fluorescent On–Off Mechanism; Corrosion Sensitivity; Fluorescence Enhancement; Premature Exposure Protection; Targeted Release; High Selectivity.	High sensitivity; Accurate positioning; Various types.	Relatively high cost; Complex process; Environmental impact.
Corrosion Early Warning Mechanism	2022	pH-sensitive molecules [135]	Corrosion Inhibition; pH-Responsive Release; Localized Protection; Targeted Release; Continuous Effectiveness.	High sensitivity; Environmental friendliness; Wide application.	Relatively high cost; Complex process; Environmental impact.
	2024	Fluorescent agents and cations [136]	Fluorescent Visualization; Early Damage Detection; UV-Triggered Polymerization; Self-Healing Activation; Good Compatibility.	High sensitivity; Environmental friendliness; Wide application.	Background interference; Toxicity; Relatively high cost.
	2024	Fluorescent probe [137]	Fluorescence On–Off Behavior; pH and Metal Ion Sensitivity; Early Corrosion Detection; High Selectivity; Fluorescence Enhancement; Corrosion-Specific Response.	High sensitivity; Accurate positioning; Various types.	Relatively high cost; Complex process; Environmental impact.
	2020	pH variation [138]	pH-Responsive Release; Controlled CAP Release; Enhanced Antibacterial in Alkaline pH; Long-Term Stability; Dynamic Control of Antifouling.	Environmental responsiveness; Durability; Environmental friendliness.	High pH control requirements; Limited repair effectiveness; Complexity.
	2021	Near-infrared (NIR) response [70]	NIR-Responsive; Photothermal Effect; Repeatable Self-Healing; Thermal Response; Durability and Reversibility.	Precise control; No chemical stimulation required; Efficient local repair.	Relatively high cost; Strong dependence on NIR light source; Limited penetration ability.
Smart Responsive Protection	2022	pH variation [139]	pH-Induced Release; Controlled Antibacterial Properties; Long-Term Stability; Responsive to pH Variation.	Environmental responsiveness; Durability; Environmental friendliness.	High pH control requirements; Limited repair effectiveness; Complexity.
	2022	Near-infrared (NIR) response [140]	NIR-Induced Self-Healing; Photothermal Effect; Recovery of Superhydrophobicity; Repeatable Performance; Localized Healing.	Precise control; No chemical stimulation required; Efficient local repair.	Relatively high cost; Strong dependence on NIR light source; Limited penetration ability.
	2023	Ultraviolet (UV) light [141]	UV-Triggered Self-Healing; Photocatalytic Effect; Controlled Polymerization; Efficiency and Control; Repeatability.	Fast Response; Selective Activation; Reversibility.	Dependency on UV Light; Limited Depth of Penetration; Material Compatibility.
	2024	Disulfide bonds and hydrogen bonds [142]	Synergistic Self-Healing; Thermal Reversibility; Chain Mobility; Mechanical Reinforcement; Enhanced Adhesion.	Reversibility; Environmental resistance; Wide application.	Slower repair speed; Temperature sensitivity; Relatively high cost.

* The “year” column indicates the year of publication for the research findings.

). The production of organic micro/nano containers is a complex process with multiple stages, including polymerization to create the containers, encapsulating active ingredients, and eliminating by-products and solvents. Currently, commonly employed methods include Pickering emulsion templating, in situ polymerization, and interfacial polymerization. As an innovative nanomaterial, two-dimensional covalent organic networks hold significant potential for future uses [125,126,127].

By offering precise control over the morphology and structure of polymer particles, Pickering emulsion polymerization offers distinct advantages in the formation of containers with controlled shapes and dimensions. This approach enables highly customizable encapsulation processes, and the resulting polymer particles generally display outstanding mechanical properties, including hardness, strength, and wear resistance [143,144]. Furthermore, Pickering emulsion technology allows for the combination of corrosion resistance and antibacterial properties within one layer. Such dual-functional self-healing coatings can provide corrosion protection while simultaneously inhibiting bacterial growth [145]. For instance, Qian et al. [128], in 2020, developed a water-based anticorrosion coating using Pickering nanocontainers. They employed flaxseed oil as the core material and surface-modified Ludox TMA particles as stabilizers to synthesize polysulfone (PSF)/SiO₂ hybrid capsules, achieving a self-healing anticorrosion effect. Through surface modification of the nanocontainers, the silica particles not only gained enhanced stability but also exhibited antibacterial activity. This significantly improved the coating’s antibacterial performance against Gram-positive bacteria, with an inhibition efficiency exceeding 90%. Such dual-purpose coatings offer valuable perspectives for the development of future self-repairing coatings utilizing micro-/nanocontainers. It is important to highlight that Pickering emulsion polymerization is mainly effective for the polymerization of hydrophobic or low-polarity monomers, which somewhat restricts its scalability and wider industrial applicability.

In situ polymerization has become a widely adopted method for the production of microcapsules. This approach involves adding reactants either inside [146] or outside the core material to initiate the polymerization reaction, which results in the creation of a polymer shell. Compared to other techniques, in situ polymerization is more effective at encapsulating complex substances. This benefit stems from the fact that polymerization takes place within the bulk phase, instead of at the interface of the core substance, resulting in a denser and more stable polymer shell structure [147,148,149]. In 2022, Li et al. [130] employed in situ polymerization to successfully synthesize styrene and benzoyl peroxide (BPO) microcapsules, using urea-formaldehyde and melamine-formaldehyde copolymer composites as the shell materials. They investigated the possible use of these microcapsules in polymer-based self-healing materials. By optimizing the synthesis procedure of BPO microcapsules, the authors identified the ideal conditions to achieve the best overall performance. Despite the significant success of in situ polymerization on a laboratory scale, scaling up to industrial production still faces several challenges, including equipment adaptability, precise process control, and cost efficiency.

Interfacial polymerization is a technique that induces polymerization reactions at the interface between two phases (e.g., water/oil or solid/liquid), resulting in the creation of a stable polymer structure. This approach facilitates the creation of a strong interfacial bond within the coating, which in turn strengthens adhesion to the substrate and boosts its durability and stability. Furthermore, interfacial polymerization typically occurs under mild conditions and can be easily implemented through simple solution immersion or coating processes. The preparation process is relatively straightforward and cost-effective, allowing for the rapid formation of functional coatings. During interfacial polymerization, two common polymerization mechanisms are anionic polymerization and polycondensation/addition polymerization. Among these, polycondensation/addition polymerization is commonly employed in the production of self-repairing coatings for marine uses [150,151,152]. In 2024, Zhang et al. [132] successfully synthesized hexamethylene diisocyanate (HMDI) microcapsules through interfacial polymerization and thoroughly analyzed their structure and characteristics. Subsequently, the synthesized microcapsules were integrated into an epoxy resin composite to create a self-repairing corrosion-resistant layer. The experimental findings showed that the layer demonstrated self-repair abilities when damaged and efficiently prevented deterioration of the metallic substrate in a sodium chloride solution. The research emphasizes the considerable potential of interfacial polymerization in the development and use of self-repairing coatings. In particular, this approach proves highly beneficial for marine coatings and other materials exposed to harsh environments over extended periods, offering improved corrosion resistance and prolonged service life.

Recently, covalent organic frameworks (COFs) have gained considerable interest in the area of corrosion-resistant coatings because of their distinctive self-repair properties. Studies have shown that incorporating COFs into layers can significantly enhance their resistance to corrosion and enable self-repair upon damage. Due to their adaptable, porous configurations, high surface areas, and adjustable pore sizes, COFs have become a potential strategy for creating self-repairing materials. They are also considered a novel method for offering dynamic corrosion resistance to metallic surfaces [153,154,155,156]. In a 2021 study, Liu et al. [129] successfully synthesized multilayer-stacked two-dimensional covalent organic framework (2D-COF) nanocontainers and incorporated them into self-healing epoxy coatings. Due to the relatively weak interlayer interactions of 2D-COFs, they demonstrate remarkable integration and dispersion in polymer-based corrosion-resistant layers. Specifically, the monomers used to synthesize TpPa-1 2D-COFs contain amine groups, which can interact with epoxy groups during curing, thereby enhancing the structural stability and corrosion protection of the layer. Furthermore, the 2D nanosheets of TpPa-1 possess a nanoporous structure with pore diameters of approximately 2 nm, making them ideal nanocontainers for loading corrosion inhibitors. In a 2023 study, Zhang et al. [131] utilized the sol–gel technique to synthesize mesoporous silica nanocontainers (MSN-CS) and further enhanced the distribution of 2D-COFs within epoxy layers. By attaching MSN-CS nanospheres containing corrosion inhibitors to the surface of TpPa, the research team effectively created MSN-CS/TpPa self-repairing nanocomposites. These materials exhibited exceptional corrosion resistance and self-repair abilities, providing a durable solution for the prolonged safeguarding of epoxy layers. Despite these advancements, the creation of epoxy layers with extended corrosion resistance continues to be a significant hurdle, especially in harsh marine conditions. Therefore, exploring the application potential of COF-based self-healing anti-corrosion coatings is of great significance for achieving long-lasting protection and reducing maintenance costs, holding both substantial research value and engineering importance.

4.2. Corrosion Early Warning Mechanism

Polymeric substances are susceptible to minor damage, which is often challenging to identify. If not promptly addressed, these damages can progressively compromise the material’s structural stability, eventually leading to material failure. To overcome this challenge, self-healing technologies have been introduced, offering not only automatic detection but also the capability to repair the damage. For ship protective layers, the emergence of visible deterioration often indicates that the damage has reached a critical stage. Hence, the capacity to identify initial deterioration beneath the layer and in damaged regions holds considerable practical importance [157,158,159]. Currently, widely used methods for early damage detection rely on specific indicators that exhibit distinct visual changes when damage occurs. Mechanical damage is typically linked to localized physical or chemical alterations, and monitoring these changes can aid in the effective detection of damage. A common approach for monitoring involves employing microcapsules or fibers that enclose visual probes [160,161], such as fluorescent dyes [162,163], pH-sensitive molecules [164,165,166] and aggregation-induced emission agents [167,168,169]. Upon physical impact, the microcontainers rupture and discharge the indicators, offering a visual cue to locate the damage site. Another method is the use of mechanochromic polymers, which can convert mechanical stimuli into visible optical changes [170,171,172,173].

Following mechanical damage, repair agents or corrosion inhibitors are released to fill the defects, restoring the coating’s protective function. Combining damage detection and self-repair capabilities within the same material can improve reliability [174,175,176,177]. In 2020, Liu [133] designed a coating with a corrosion warning mechanism. The electrochemical corrosion caused by damage can quickly trigger a warning within just five minutes through a prominent orange-red color. Beyond its warning function, the smart coating exhibits efficient repair abilities in the damaged regions, as evidenced by the vanishing of the electrochemical admittance signal. This straightforward yet effective method depends on a single active element to enable self-warning and repair functions, providing a promising approach to enhance the safety and longevity of other synthetic materials.

Direct incorporation of fluorescent probes may cause their early exposure to complex corrosive environments, rendering them vulnerable to interference from the coating or corrosive substances, potentially leading to fluorescence signal quenching and loss of corrosion detection capability. To tackle this challenge, the incorporation of layered double hydroxides as carriers effectively facilitates the encapsulation, gradual release, and regulated release of substances [178,179,180,181,182]. In 2021, Lv et al. [134] developed an iron ion- and proton-responsive fluorescence switch probe—rhodamine B hydrazone (RBA)—for corrosion detection. In epoxy coatings, the RBA/LDH composite can detect corrosion through fluorescence signals and accurately identify the locations of corrosion and coating defects. Additionally, the layered structure and excellent dispersion of the RBA/LDH improve the coating’s shielding effect, thereby enhancing its corrosion resistance.

Currently, the restricted capacity for encapsulation and the unregulated release of microcontainers present obstacles to the wider use of corrosion detection systems. Zeolite imidazole frameworks (ZIFs), a type of metal–organic network composed of imidazole ligands and metal ions, have attracted significant attention in the field of intelligent materials [183,184]. Benzimidazole, a commonly employed corrosion suppressant, also functions as the organic ligand in ZIFs. Remarkably, the interaction between the ZIF ligands and metal ions exhibits pH responsiveness, emphasizing its potential for creating self-repairing corrosion-resistant coatings [185,186]. In 2022, Liu [135] created nanocontainers by modifying hydroxyapatite plates with zeolite imidazole networks, which were used to enclose corrosion indicators. When localized corrosion occurs, Phen molecules are quickly released and interact with metal ions, generating a noticeable red color at the damaged area, thus allowing for real-time corrosion detection.

The combination of self-repair and self-alert functions can collectively enhance the overall effectiveness of coatings, significantly extending their service life and protecting the substrate. In a 2024 study, Gong et al. [136] prepared a coating with SiO₂ as the wall material using an in situ interface polymerization method. This coating, composed of epoxy resin, fluorescent agents, cations, and other components, was applied to a PVC fabric to create a self-healing and self-warning coating via a scraping method. The coating demonstrated outstanding repair, alerting, and structural characteristics. When the coating surface developed a cut and began to expand, it was able to release fluorescent active components and repair agents. Under UV light, the scratched area displayed yellow fluorescence, indicating the specific location of the coating damage.

Monitoring the early damage of protective coatings and metal structures before critical failures occur is crucial for preventing potential safety crises. Although previous studies have demonstrated the effectiveness of self-reporting coatings, achieving layered visual monitoring of coating damage and localized corrosion, along with quantitatively assessing the intensity of corrosion warning, remains a challenge. In a 2024 study, Cheng et al. [137] developed a novel anti-corrosion coating system by crosslinking a polyurethane network with fluorescent dye-loaded nanosensors. When the coating experiences localized damage, the sensor releases 2′, 7′-dichlorofluorescein (DCF) indicators, which emit green light due to the aggregation-induced fluorescence suppression effect of DCF molecules. As the damage progresses to the magnesium alloy substrate, the DCF probes undergo a ring-opening reaction and produce noticeable red light at the initial corrosion location. Additionally, with the help of a specially designed image processing program, the captured images are converted into detailed grayscale values, enabling the quantitative assessment of early-stage corrosion. This study provides a visual monitoring system for in-depth analysis of early corrosion stages and lays the foundation for the development of self-monitoring corrosion-resistant coatings.

4.3. Smart Responsive Protection

Developing high-performance self-repair coatings for corrosion resistance requires a thorough understanding of how these layers respond to the different stimuli caused by corrosion. At present, the self-healing mechanism generally relies on external stimuli, including heat, ultraviolet light, pH fluctuations, or corrosive ions [187,188,189,190].

During corrosion, the local pH of the environment experiences considerable changes. This characteristic is precisely identified by pH-responsive units within smart coatings, thereby triggering self-healing mechanisms [191,192]. In 2020, Hao et al. [138] developed a series of pH-triggered multilayer films based on Capsaicin. Corrosion leads to significant fluctuations in the local environmental pH. The study evaluated the pH-sensitive characteristics of the fabricated films by quantifying the released capsaicin concentration in solutions with varying pH levels and employing the plate colony method. Additionally, it compared the pH-sensitive behavior and antimicrobial longevity of the films in both acidic and basic solutions, providing valuable insights for the development of pH-responsive intelligent biofouling-resistant and antimicrobial layers designed for marine environments. Furthermore, in 2022, Hao et al. [139] designed a multilayer coating using Polydopamine/Tannic Acid (PDA/TA) to achieve pH-controlled release behavior of Allicin@Chitosan (ALL@CS), aiming for antibacterial and antifouling effects. This coating showed strong effectiveness against E. coli, S. aureus, and P. aeruginosa. Owing to the protonation of chitosan’s amine sites in acidic conditions and deprotonation in alkaline conditions, Allicin (ALL) was released as a biocide, exhibiting excellent antibacterial performance. Experimental results indicated that the coating possessed sustained pH-responsive antibacterial activity with an efficiency of approximately 87%, demonstrating its potential in controlled-release and antimicrobial properties. This offers new approaches for creating innovative antifouling coatings tailored for marine environments.

In the area of intelligent, responsive self-repairing protective layers, temperature-sensitive and light-responsive shape memory self-healing coatings have become the most commonly used approaches for corrosion resistance. Light-responsive coatings, in particular, have attracted considerable interest because of their distinct benefits, including the ability to target specific damaged areas, rapid reaction time, and potential for remote activation. These distinctive features not only prevent undesired side reactions during the repair process but also minimize thermal harm to the intact coating regions. The integration of near-infrared (NIR) light activation technology has further enhanced the performance of light-responsive self-healing coatings. By utilizing NIR light, these coatings enable precise damage localization and repair without direct physical contact. This non-invasive repair method not only enhances repair efficiency but also minimizes the effect on adjacent undamaged areas, thereby prolonging the overall lifespan of the coating [193,194,195].

In recent years, notable advancements have been made in the development of near-infrared (NIR) responsive intelligent self-repair coatings. In 2021, Ni et al. [70] successfully created a new self-repairing coating system with NIR-responsive properties., which is particularly innovative as its healing process does not require additional reactive materials. The researchers synthesized the coating using fluorine-free, non-toxic materials with excellent biocompatibility, successfully achieving superhydrophobic properties. Experimental results indicated that when the coating is affected by external environmental factors, exposure to near-infrared light alone can trigger the spontaneous restoration of both superhydrophobicity and anti-biofouling properties.

Building upon this foundation, Li et al. [140] made further advancements in 2022 by developing a multifunctional superhydrophobic coating system that integrates ambient temperature stability, mechanical robustness, self-repair capability, and responsiveness to near-infrared stimulation. The coating features a unique bilayer structure, comprising a self-repairing polyurethane (PU) interface strengthening layer and a hydrophobic layer composed of flower-shaped ZnO composite particles coated with polydopamine (PDA). Studies show that the PU matrix exhibits exceptional self-repair abilities, due to the combined effect of reversible hydrogen bonds and disulfide linkages. Due to the self-repair capabilities and interface strengthening of PU, along with the remarkable photothermal conversion properties of PDA, this hybrid coating demonstrates outstanding mechanical strength and undergoes quick restoration when exposed to near-infrared stimulation. Additionally, the intelligent layer exhibits remarkable self-cleaning properties and improved resistance to corrosion. This study provides valuable insights for developing high-performance, stable, and reversible superhydrophobic coatings, emphasizing their potential for engineering applications.

Polymers based on ionic and hydrogen interactions exhibit excellent structural integrity while maintaining high self-repair efficiency [196,197]. Furthermore, under ultraviolet (UV) irradiation, zinc oxide (ZnO) can generate photoelectrons, and the resulting hot carriers induce lattice vibrations, thereby increasing the material temperature [198,199]. As a photothermal material, ZnO quickly reacts to light stimuli and promotes the rapid healing of damaged regions [200]. Various doping methods have been proposed to enhance the photothermal performance of ZnO. For example, incorporating anions, cations, or rare earth metals into ZnO materials can significantly enhance charge segregation efficiency and mitigate recombination problems [201]. In a 2023 study, Feng [141] developed a photo-responsive system based on hydrogen and ionic interactions, using UV light to initiate the self-repair process. This system consists of a covalently crosslinked zinc–diacetyl–dioxime–polyurethane coordination network featuring three reversible bonds. The combination of hydrogen interactions and metal–ligand coordination bonds significantly enhances the self-repair ability. Through molecular dynamics simulations, the self-healing behavior of three-dimensional microcracks and its temperature dependence were investigated, providing molecular-level insights into the self-healing mechanism. Moreover, copper-doped ZnO provides metal–ligand interactions that speed up the self-repair process, while simultaneously improving the photothermal response and antimicrobial characteristics of polyurethane.

Polydimethylsiloxane (PDMS) is widely used in marine antifouling coatings, but its controlled release of antifouling agents and mechanical properties still require improvement. To address this, Liu et al. [142], in 2024, developed an innovative self-regulating marine biofouling-resistant layer by combining the biofouling agent release strategy inspired by carp and the effectiveness of low-surface-energy materials. The layer demonstrates significant eugenol-controlled release capability and high self-repair efficiency. In seawater, the coating regulates eugenol release by modifying its inherent hydrophilicity and surface antifouling agent concentration. Leveraging the combined effect of disulfide and hydrogen interactions, the layer attains a self-repair efficiency of up to 94.06%. Moreover, the polyurea-modified PDMS substrate imparts the layer with exceptional corrosion resistance. The study introduces an effective strategy for advancing eco-friendly and long-lasting biofouling-resistant coatings for marine applications.

5. Prospects and Outlook

Significant advancements have been made in the research and development of antifouling self-healing coatings for ships, yet substantial scientific and technical challenges persist in achieving their widespread adoption. The development of these coatings necessitates deep integration and collaborative innovation across multiple disciplines, including materials science, mechanical engineering, chemistry, and environmental engineering. This process requires not only systematic investigation into the mechanical properties, thermodynamic characteristics, and stability of materials but also a thorough understanding of the chemical reaction dynamics and the fundamental self-healing mechanisms of the coating components.

Experimental validation of the self-healing performance is a critical step in assessing its practical applicability, particularly in evaluating the repair capabilities of coatings subjected to impact, friction, or other mechanical stresses. Additionally, assessing the environmental compatibility of coatings is essential to ensure that all components meet pollution-free and environmentally friendly standards. This requirement is especially crucial in marine engineering, where stringent environmental regulations are enforced. Given that self-healing coatings may contain potentially toxic chemicals or specialized solvents, comprehensive environmental impact assessments must be conducted, covering biodegradability, chemical stability, and long-term durability in natural environments.

Since each stage of the development process requires the integration of specialized knowledge from multiple disciplines, achieving breakthroughs in the short term remains challenging. From material selection to environmental impact evaluation, the entire research process is complex and demands collaborative efforts from interdisciplinary teams. While the interdisciplinary nature of the research increases complexity and extends development timelines, it also establishes a solid scientific foundation for the development of high-performance, multifunctional self-healing coatings.

The manufacturing process of self-healing coatings is intricate and costly. To achieve self-healing functionality, intelligent materials typically need to undergo specific curing or crosslinking processes. These processes rely on advanced equipment, specialized materials, and precision control technologies, resulting in low production efficiency and significant cost increases. Specifically, the realization of self-healing functionality often depends on mechanisms such as microcapsules, dynamic covalent bonds, or ionic liquids, which operate through specific chemical reactions or physical processes. Consequently, the performance of materials must be precisely controlled during both the research and manufacturing processes. For instance, composite materials containing microcapsules can significantly enhance the self-healing ability of the coating, but their preparation and application require complex technological support. Moreover, the curing or crosslinking processes of coatings necessitate stringent condition controls to ensure the desired physical and chemical properties, further increasing the complexity of the processes and the demands on production equipment.

To achieve optimal self-healing effects, precise adjustments must be made to the materials’ mechanical properties, thermodynamic characteristics, and chemical stability. This high precision requirement not only extends the research and development cycle but also leads to frequent experimental adjustments, increasing technical uncertainty. Due to the complexities of the technologies and processes involved, the production efficiency of self-healing coatings remains low, and the unit costs are high. Furthermore, the use of advanced equipment and specialized materials further drives up manufacturing costs, presenting significant economic challenges for large-scale applications.

In practical applications, while self-healing coating technology demonstrates considerable potential, it also faces several limitations as follows:

• The amount of healing agent stored in microcapsules is limited, and in cases of severe coating damage, it may not be able to provide sufficient repair materials. Additionally, the randomness of microcapsule rupture makes it difficult to precisely control the release location and dosage of the healing agent, which may result in insufficient local repair. Particularly in the marine environment, high humidity and temperature fluctuations accelerate the degradation or failure of healing agents, weakening the coating’s long-term self-healing ability.
• Dynamic covalent bonds, although providing some self-healing capability, typically result in slower repair processes that are insufficient for larger cracks or deep damage. Moreover, the reversibility of dynamic covalent bonds is highly sensitive to environmental factors such as temperature and humidity, and excessive repair cycles may degrade material performance, shortening the effective lifespan of the coating. Simultaneously, the repair effect’s high sensitivity to environmental conditions (e.g., temperature, humidity, and pH) may cause instability in the repair effect under complex environmental conditions.
• Ionic exchange reactions are primarily used to repair surface damage, and their effectiveness for deep cracks or complex damage is often limited. Furthermore, ionic exchange reactions are relatively slow, and the repair process is time-consuming, which fails to meet the demands for rapid repairs of ship coatings. Additionally, the repair effect of ionic exchange coatings is also vulnerable to environmental factors such as seawater, humidity, and temperature, further limiting their practical application.

Despite these challenges, the demand for high-performance ship coatings is increasing with the continuous growth of the global shipping industry, particularly due to the expansion of international trade and the flourishing marine economy. Thus, the prospects for self-healing coating technology remain promising. As a key technology for enhancing ship performance, self-healing coatings are expected to play a significant role in future markets. To overcome current technological bottlenecks and promote large-scale application, future research must focus on material optimization, process improvement, and environmental adaptability, while also exploring more efficient and cost-effective production methods for the commercial promotion and industrial application of this technology.