Natural sustainable coatings for marine applications: advances, challenges, and future perspectives

3.1 Biocide-based coatings

Biocide-based coatings have long been the cornerstone of antifouling strategies in marine environments due to their effectiveness in preventing the settlement and growth of biofouling organisms such as algae, barnacles, and mussels. These coatings function by releasing biocidal agents that inhibit or kill fouling organisms upon contact, thereby maintaining the structural integrity and hydrodynamic efficiency of ships and marine structures. Historically, tributyltin (TBT) was one of the most effective antifouling agents, extensively used in marine coatings for its potent biocidal properties [32]. TBT’s efficiency stemmed from its ability to disrupt the growth and reproduction of a wide range of marine organisms, significantly reducing maintenance costs and improving vessel performance. However, the environmental consequences of TBT became evident over time. Its high toxicity, persistence in marine ecosystems, and tendency to bioaccumulate in the food chain led to severe ecological damage, including deformities in marine life such as oyster shell abnormalities and endocrine disruption in aquatic species. Due to these detrimental effects, the International Maritime Organization (IMO) implemented a global ban on TBT-based coatings in 2008 [33].

Following the TBT ban, the marine industry shifted towards metal-based biocides, primarily using copper and zinc compounds, as alternatives in antifouling coatings. These metals exhibit broad-spectrum antimicrobial properties, effectively controlling biofouling by disrupting the cellular functions of microorganisms. For instance, copper ions interfere with the enzymatic processes and protein structures of fouling organisms, while zinc oxide exhibits strong antibacterial activity, especially when incorporated into polymer matrices like PDMS to enhance fouling resistance [34]. Despite their effectiveness, metal-based biocides present significant environmental concerns [34], 35]. The continuous leaching of copper and zinc into marine environments can lead to metal accumulation in sediments, posing toxicity risks to non-target marine organisms and altering aquatic ecosystems [36], 37]. Moreover, the persistence of these metals in the environment raises issues of ecotoxicity, bioaccumulation, and the potential for antimicrobial resistance among marine microbial communities. As regulatory bodies increasingly impose stricter limits on metal-based biocide usage, the need for environmentally friendly, sustainable antifouling solutions becomes more urgent.

3.2 Fouling-release coatings

Fouling-release (FR) coatings have emerged as a promising alternative to traditional biocide-based antifouling systems, offering a less toxic approach to managing marine biofouling. Among these, polydimethylsiloxane (PDMS)-based coatings are the most widely used due to their unique hydrophobic and viscoelastic properties, which create a low-energy surface that discourages the firm attachment of marine organisms [38]. The hydrophobic nature of PDMS reduces the surface wettability, thereby minimizing the strength of adhesive bonds formed by fouling organisms such as algae, barnacles, and mussels. Additionally, the viscoelastic characteristics of PDMS enable the surface to deform under mechanical stress, which helps dislodge attached biofouling organisms through natural water flow or cleaning processes [30], 31], 39]. This fouling-release mechanism does not prevent initial organism settlement but makes it easier for fouling to be removed with minimal effort, reducing the need for frequent manual cleaning and lowering maintenance costs for marine vessels and infrastructure.

Despite these advantages, PDMS coatings face several limitations that hinder their widespread adoption in more demanding marine environments. One of the primary challenges is their low mechanical strength, which makes PDMS surfaces prone to abrasion, cracking, and damage under harsh conditions such as high-velocity water flow, mechanical impacts, or prolonged UV exposure [30], 31], 40], 41]. This mechanical weakness reduces the coating’s durability and necessitates frequent repairs or reapplication, increasing operational costs. Moreover, PDMS coatings exhibit limited resistance to diatom adhesion, which is a significant concern as diatoms often act as pioneer species in biofouling communities, creating a foundation for more complex biofilms and macrofouling organisms [42], 43]. To address these issues, researchers have explored the incorporation of reinforcing agents such as nano-zinc oxide particles (ZnO NPs) into PDMS matrices. ZnO NPs enhance the mechanical properties of PDMS coatings by improving their structural integrity, abrasion resistance, and antifouling performance. Additionally, ZnO exhibits antibacterial properties that can inhibit the initial microbial colonization, thereby reducing the risk of biofilm formation, which often facilitates the settlement of larger fouling organisms [38]. Despite these improvements, the long-term environmental impact of incorporating nanoparticles into marine coatings remains an area of active research, highlighting the need for continuous development of more sustainable fouling-release technologies.

3.3 Epoxy-based anticorrosion coatings

Epoxy-based anticorrosion coatings are extensively used in maritime applications, particularly for protecting steel structures such as ship hulls, offshore platforms, and underwater pipelines [44], [45], [46]. Their popularity stems from their exceptional adhesion properties, strong chemical resistance, and superior mechanical durability, which enable them to withstand harsh marine environments characterized by high salinity, moisture, and mechanical stress [47], [48], [49]. Epoxy coatings create a dense, impermeable barrier that prevents the penetration of water, oxygen, and corrosive ions like chloride, thereby mitigating the electrochemical reactions responsible for steel corrosion [46]. Additionally, their ability to bond effectively with metallic substrates ensures long-term protection against corrosion and physical degradation, reducing maintenance costs and extending the service life of maritime structures.

Despite these advantages, conventional epoxy coatings often rely on synthetic corrosion inhibitors (chromates, phosphates, and other chemical additives) to enhance their protective performance. While effective in preventing corrosion, these synthetic inhibitors pose significant environmental risks, particularly in marine ecosystems where they can leach into the water, accumulate in sediments, and exert toxic effects on aquatic organisms [50]. The release of hazardous substances from degraded coatings can contribute to marine pollution, bioaccumulation, and toxicity to non-target species, raising concerns about the sustainability of traditional epoxy-based systems.

To address these environmental challenges, recent research has focused on the development of eco-friendly alternatives, notably the incorporation of tannin-based natural inhibitors into epoxy coatings. Tannins, which are natural polyphenolic compounds derived from plant sources, exhibit strong chelating properties, enabling them to form stable metal-tannin complexes that enhance the corrosion resistance of steel surfaces [47]. When combined with metal ions such as zinc (Zn²⁺) or cerium (Ce³⁺), tannins create an additional protective layer that inhibits corrosion through passivation and barrier effects. Studies have demonstrated that tannin-metal complexes not only improve the anticorrosive performance of epoxy coatings but also reduce their ecotoxicity, making them more environmentally sustainable. Moreover, tannin-based inhibitors enhance the compatibility of the coating with the epoxy matrix, ensuring uniform dispersion and stable long-term performance. This innovative approach represents a promising step toward greener, more sustainable anticorrosion technologies for the maritime industry, aligning with global efforts to minimize the environmental footprint of industrial coatings.

3.4 Performance and environmental challenges of conventional coatings

Conventional marine coatings, while effective in the short term, face significant performance and environmental challenges that limit their long-term sustainability and efficacy. As illustrated in Figure 2, these challenges encompass issues related to durability, toxicity, antimicrobial resistance, and the increasing pressure from regulatory frameworks.

Figure 2:

Overview challenges of conventional coatings.

4.1 Bio-based polymers and resins

Bio-based polymers and resins are emerging as sustainable alternatives to traditional synthetic coatings in marine environments, offering eco-friendly solutions with excellent antifouling and anticorrosion properties (Table 1). These materials, derived from renewable biological sources, not only reduce environmental impact but also provide innovative functionalities tailored for maritime applications. One notable advancement is the development of transparent, flexible, and responsive amphiphilic coatings designed for full-cycle antifouling protection [57]. By incorporating fluorine-containing groups and natural extracts like citronellol, these coatings achieve an initial hydrophobic and active antifouling interface. Their unique ability to switch from hydrophobic to amphiphilic states in response to seawater enhances long-term antifouling performance. This dynamic switching mechanism, facilitated by silane-induced zwitterion generation, effectively inhibits bacterial and diatom attachment by over 90 % while maintaining flexibility and optical transparency, making it suitable for sensors, underwater probes, and marine photovoltaic devices.

Another promising bio-based antifoulant involves the use of apple cider vinegar combined with epoxy resin, which demonstrated significant efficacy in controlling biofouling Al Solami and Satheesh [59]. Laboratory and field studies showed that this natural product reduces biofilm formation and barnacle larval settlement with minimal toxicity to marine organisms. Panels coated with apple cider vinegar-based formulations exhibited a substantial reduction in fouling biomass, ascidians, and mussels, highlighting its potential as an environmentally friendly antifouling agent for marine structures.

The incorporation of dual-biomimetic polymer coatings inspired by natural shell structures and superhydrophobic surfaces has also shown promising results [57]. These coatings consist of alternating layers of hard epoxy resin enhanced with benzotriazole-functionalized silica for corrosion resistance and soft fluorocarbon resin reinforced with graphene oxide for mechanical durability. The addition of surface textures improves hydrophobicity and reduces wear, providing excellent corrosion protection and self-healing properties in harsh marine conditions.

In the realm of photocatalytic antifouling technologies, researchers have developed coatings based on polydimethylsiloxane (PDMS) infused with a ternary double Z-scheme photocatalyst composed of graphite-phase carbon nitride, titanium dioxide nanotubes, and long afterglow powders [60]. This innovative approach leverages visible light photocatalysis and fluorescence emission to inhibit the adhesion of fouling organisms, achieving remarkably low attachment rates for bacteria and diatoms under simulated natural conditions.

Furthermore, bio-based polymers have been enhanced with natural antimicrobial agents like borneol and silver nanoparticles (AgNPs) through in situ polymerization strategies [48]. Coatings developed with bisphenol A epoxy resin and glycidyl methacrylate-borneol polymers exhibited superior mechanical flexibility and antibacterial activity, with inhibition rates exceeding 95 % against common marine bacteria. These coatings also demonstrated excellent antifouling performance against biofilms and diatoms, offering a sustainable alternative to conventional toxic antifouling paints.

In aquaculture applications, antifouling coatings combining biogenic antimicrobial agents (extracted from Houttuynia and Scutellarin) with low-surface-energy silicone resins have proven effective in reducing fouling adhesion on marine nets [61]. These coatings maintained high hydrophobicity, significantly inhibited bacterial growth, and reduced fouling adhesion in field tests, contributing to the sustainable development of marine aquaculture.

Moreover, superhydrophobic epoxy coatings with interfacial strengthening cells (IS-Cells) have been designed to enhance corrosion and fouling resistance [46]. These coatings exhibit outstanding barrier properties, maintaining corrosion-free surfaces even after prolonged exposure to saltwater environments. Their anti-fouling capabilities are attributed to the electrostatic repulsion between hydrophobic chains and fouling proteins, ensuring durability in harsh marine conditions.

Another innovative approach involves the electrosynthesis of cerium-containing silicon acrylate coatings, which offer excellent adhesion, hydrophobicity, and corrosion resistance [64]. These coatings demonstrated strong bonding to steel substrates, with corrosion protection efficiency reaching 93.9 % and maintaining structural integrity after extended exposure to marine environments.

Lastly, eco-friendly self-polishing antifouling coatings based on eugenol methacrylate provide a sustainable solution for biofouling control [63]. The hydrolysis of phenolic ester groups in the resin facilitates the release of natural, non-toxic eugenol into seawater, achieving effective antifouling without the ecological risks associated with traditional metal-based agents. The coatings displayed strong antibacterial and anti-algal properties, significantly reducing mussel attachment in marine environments.

In summary, bio-based polymers and resins represent a versatile and sustainable class of materials for marine antifouling and anticorrosion applications. Their ability to integrate natural extracts, dynamic functionalities, and eco-friendly mechanisms underscores their potential to replace traditional toxic coatings, contributing to greener maritime operations and environmental conservation.

4.2 Plant-derived oils and waxes

Plant-derived oils and waxes have emerged as eco-friendly alternatives to synthetic chemicals in marine antifouling and anticorrosion coatings due to their natural bioactivity, sustainability, and biodegradability. These materials offer significant potential to reduce environmental harm while maintaining effective protection against biofouling organisms and corrosion in harsh marine environments.

One promising example is the use of geranium essential oil blended with silicone to develop long-term antifouling coatings [65]. This formulation demonstrated the ability to inhibit the formation of biofilms, particularly against Psychrobacter adeliensis, and effectively reduced macrofouling adhesion for up to 11 months. Structural analyses, including 13C NMR and FTIR spectroscopy, confirmed the stability of the essential oil within the coating matrix without compromising thermal stability. Although some increase in coating porosity was observed after prolonged marine immersion, the antifouling performance remained consistent, highlighting the potential of essential oils as natural antifoulants with minimal environmental impact.

Similarly, polymerized tung oil (PTO) has been incorporated into urushiol-based benzoxazine copper polymer (UBCP) coatings, creating a composite with excellent antifouling properties [66]. The long alkane chains of PTO significantly enhanced the coating’s flexibility and reduced its surface modulus, weakening the adhesion of fouling organisms and making them easier to remove. In addition, a small concentration of copper ions released from the composite provided a synergistic antifouling effect. The coating demonstrated strong antifouling activity against bacteria such as Escherichia coli, Staphylococcus aureus, and V ibrio  alginolyticus, as well as microalgae like N. closterium and P. tricornutum, offering an effective, environmentally friendly solution for marine applications.

Natural surfactants derived from residual soybean oil were utilized to create a non-biocidal antifouling coating in a study by da Silva et al. [67]. The surfactants, obtained through fermentation and chemical modification processes, were incorporated into a natural resin-based matrix and applied to metal panels for field testing in the Port of Recife, Brazil. After 25 days of immersion, the coated panels showed a 30 % reduction in biofouling compared to untreated controls. The coatings containing laurate and hydroxylated acid exhibited the best antifouling activity, demonstrating the potential of natural surfactants to provide eco-friendly biofouling protection without the harmful effects associated with traditional biocidal coatings.

In addition to antifouling, plant-derived oils have shown promise as anticorrosion agents. Vázquez-Vélez et al. [68] evaluated fatty amides and anionic surfactants synthesized from inedible coconut and palm oils for their ability to inhibit corrosion on carbon steel surfaces. These compounds, formulated with natural wax and synthetic oils, were tested under accelerated corrosion conditions in a fog chamber simulating various environments, including marine and industrial-marine atmospheres. The fatty amide inhibitor from palm oil demonstrated outstanding corrosion protection, achieving 98.4 % efficiency in rural environments for 336 h and 98.8 % efficiency in polluted rural environments for 168 h. Additionally, under Prohesion cyclic conditions, the protective layer maintained up to 244 h of corrosion resistance with 99.8 % efficiency. Structural analyses using XRD and SEM confirmed the formation of protective films, primarily composed of Fe₃O₄, which effectively shielded the metal surface from corrosion.

Collectively, these studies underscore the versatility and effectiveness of plant-derived oils and waxes as sustainable materials for marine coatings. Their natural bioactivity, combined with strong antifouling and anticorrosion performance, makes them attractive candidates for the development of eco-friendly marine protection systems that align with global sustainability goals.

4.3 Natural fibers and nanomaterials

Natural fibers and nanomaterials are increasingly being explored for their potential in enhancing the performance of marine coatings, offering both antifouling and anticorrosion benefits as shown in Table 2. These materials, derived from renewable biological sources and advanced nanotechnologies, improve mechanical strength, environmental resistance, and biocompatibility while reducing the ecological impact associated with conventional coatings.

One innovative approach involves the incorporation of acylated chitin fibers coated with polydopamine (PDA) into bamboo-plastic composites (BPCs) [58]. This self-assembled, crosslinked structure significantly improved the mechanical properties of the composites, increasing tensile strength by 73.64 % and impact strength by 63.57 %. The enhanced performance is attributed to improved interfacial compatibility, allowing for better stress distribution and strain dissipation. Additionally, the modified BPCs exhibited superior thermal stability, crystallization behavior, and moderate hydrophobicity, making them suitable for marine applications where durability is critical.

Another promising development is the use of zwitterionic cellulose nanocrystals (CNCs) combined with deacetylated chitin nanofibers (DECHNs) and SiO₂ nanoparticles to create eco-friendly marine antifouling coatings [69]. The zwitterionic properties, derived from the combination of positively and negatively charged components, contributed to a 93 % antibacterial rate and 88 % antialgae activity. When tested in actual marine environments, these coatings maintained 93 % biofouling resistance over 60 days, showcasing their potential as sustainable alternatives to toxic antifouling agents.

Inspired by marine invertebrates, researchers have also developed antioxidant biomaterials based on cysteine-rich thrombospondin-1 type I repeat-like (TSRL) proteins [70]. These proteins self-assemble into water-resistant coatings or redox-responsive hydrogels, exhibiting superior antioxidant activity compared to traditional agents like glutathione and ascorbic acid. TSRL-based coatings effectively protect against oxidative stress and UV-induced damage, suggesting potential applications in both marine coatings and biomedical devices. The incorporation of superparamagnetic nanoparticles coated with agar derived from red marine algae (Gelidium robustum) has shown promise for both biomedical and marine applications [71]. Magnetite (Fe₃O₄) and cobalt ferrite (CoFe₂O₄) nanoparticles displayed high crystallinity and biocompatibility, with strong magnetic responses suitable for antifouling applications where electromagnetic fields can be used to deter biofouling organisms.

However, not all nanomaterials are without environmental risks. The ecotoxicity of copper oxide nanoparticles (CuO NPs) has been evaluated, revealing concentration-dependent toxicity to marine organisms such as rotifers, shrimps, and copepods [72]. Although CuO NPs offer antifouling properties, their leaching potential and impact on marine biodiversity raise concerns, emphasizing the need for safer nanomaterial designs in marine coatings. Carbon nanotube (CNT)-modified polydimethylsiloxane (PDMS) nanocomposites have demonstrated improved antifouling (AF) properties in natural seawater [73]. The incorporation of low CNT concentrations (0.1 % wt) enhanced surface wettability, altered nanostructures, and significantly reduced the colonization of pioneer microbial communities, contributing to more effective fouling resistance.

In addition, capsaicin-loaded nanocomposites have been introduced as natural antifoulants with oriented nanotopography to suppress biofouling [74]. By chemically bonding capsaicin to CoFe₂O₄/gelatin magnetic nanoparticles and incorporating them into PDMS-based coatings, researchers achieved long-lasting antifouling performance without the rapid leaching issues typically associated with natural product antifoulants. The combination of nanotopographical structures and controlled capsaicin release significantly enhanced antifouling efficacy under marine conditions. Collectively, these studies demonstrate the versatility and effectiveness of natural fibers and nanomaterials in marine coatings. Their ability to improve mechanical strength, provide antifouling protection, and reduce environmental toxicity highlights their potential as sustainable solutions for maritime industries seeking to balance performance with ecological safety.

4.4 Marine-derived biopolymers (e.g., Chitosan, Alginate)

Marine-derived biopolymers, particularly chitosan and alginate, have gained significant attention in the development of eco-friendly coatings due to their biodegradability, biocompatibility, and antimicrobial properties as shown in Table 3. These biopolymers, derived from marine organisms such as crustacean shells and brown algae, offer sustainable alternatives to traditional synthetic materials, providing effective antifouling, anticorrosion, and antimicrobial performance in marine environments.

In a study by Soleimani et al. [39], a novel hydrophobic/hydrophilic hybrid (HHH) coating was developed by combining polydimethylsiloxane (PDMS) with chitosan and sodium polyacrylate. The coatings demonstrated both antibacterial activity and foul-release properties, reducing pseudo-barnacle adhesion strength to as low as 0.04 MPa and achieving 9.8 % fouling coverage after 30 days of seawater immersion. The optimized coating exhibited a high water contact angle (116.05°), indicating strong hydrophobicity, which contributes to antifouling effectiveness, making it a promising solution for aquaculture applications.

Another innovative application of chitosan was presented by Muthamma et al. [75], who modified chitosan with pyrene-1-carboxaldehyde and fluorene-2-carboxaldehyde to create a smart fluorescent material (FCSSB). This modified chitosan acted as both a binder and fluorophore in eco-friendly formulations for anti-counterfeit applications. The resulting coatings displayed blue fluorescence under UV light, good rub resistance, and color stability after UV exposure, demonstrating the versatility of chitosan beyond marine coatings.

In the field of anticorrosion, Al Kiey et al. [76] developed chitosan–gum Arabic nanocomposites (CGACs) as sustainable anticorrosive coatings for mild steel in saline environments. The coatings achieved a remarkable protection efficiency of 96.6 % at a concentration of 200 ppm, effectively blocking corrosive chloride ions. The water contact angle increased from 50.7° (uncoated) to 101.2° (coated), indicating improved surface hydrophobicity, while SEM and AFM analyses confirmed the uniform coverage and stability of the protective layer.

The interaction between cerium oxide nanoparticles (CeO₂ NPs) and biopolymers like chitosan and alginate was investigated by Nigro et al. [77]. The coatings influenced the surface charge, stability, and toxicity of the nanoparticles when exposed to marine mussels (Mytilus galloprovincialis). Coating with chitosan enhanced antioxidant enzyme activities, while alginate coatings triggered oxidative stress. Despite similar bioaccumulation levels, the distinct toxicological outcomes underscore the role of biopolymer coatings in modulating nanoparticle behavior in aquatic environments.

In antifouling applications, Lima et al. [78] demonstrated the antibiofilm performance of chitosan-based surfaces in marine conditions. Coatings made from poly (lactic acid) (PLA) and chitosan reduced biofilm thickness by 36 % and decreased the number of culturable cells by up to 68 % compared to controls. The antimicrobial activity was attributed to chitosan’s ability to disrupt bacterial cell membranes, emphasizing its potential as an eco-friendly antifouling agent.

Muhring-Salamone et al. [79] advanced the development of low-fouling marine coatings by creating nitric oxide-releasing polysaccharide-based hybrid materials. These coatings, composed of heparin, alginate, and siloxanes, demonstrated strong antifouling performance against marine bacteria (Cobetia marina) and diatoms (Navicula perminuta), with nitric oxide acting as a natural signaling molecule to inhibit biofouling. Collectively, these studies highlight the versatile applications of marine-derived biopolymers like chitosan and alginate in developing sustainable, multifunctional coatings. Their natural antimicrobial properties, environmental compatibility, and ability to be chemically modified for enhanced performance make them promising candidates for marine antifouling and, anticorrosion.

4.5 Other eco-friendly additives and fillers

The advancement of eco-friendly additives and fillers in marine coatings is pivotal for enhancing antifouling and anticorrosion performance while mitigating environmental harm. Traditional coatings often rely on toxic chemicals such as heavy metals, which accumulate in marine ecosystems and pose long-term ecological risks. In contrast, recent research focuses on incorporating bioinspired proteins, natural fillers, functional nanomaterials, and environmentally benign composites. These materials work through diverse mechanisms, including surface modification, molecular interactions, and barrier effects, to provide sustainable and effective protection in harsh marine environments.

One notable development is the use of barnacle cement protein (rMrCP20) as a bioinspired corrosion inhibitor, introduced by Bui et al. [80]. Derived from the adhesive cement of Megabalanus rosa, rMrCP20 exhibits strong corrosion resistance due to its unique biophysical properties. The protein forms a robust, adsorptive layer on steel surfaces through hydrophilic and hydrophobic interactions, creating a physical barrier that limits the penetration of corrosive agents such as chloride ions and water molecules. Additionally, rMrCP20 interacts with iron ions released from the steel substrate, stabilizing them through coordination bonds and forming a passive layer that increases the coating’s impedance. This dual mechanism – surface adsorption and iron ion chelation – delays corrosion by reducing the electrochemical reactivity of the metal surface. The Langmuir adsorption isotherm explains the protein’s monolayer adsorption behavior, while electrochemical impedance spectroscopy (EIS) confirms enhanced charge transfer resistance.

In another study, Yu et al. [49] developed a secondary doped polyaniline-modified basalt flake composite epoxy coating. By incorporating fluorinated basalt/polyaniline (FBP), the coating achieved superior hydrophobicity and anticorrosion performance. The secondary doping of polyaniline with perfluorododecanoic acid introduces fluorine groups that significantly reduce surface energy, resulting in a water contact angle of 147.6°, indicative of strong hydrophobicity. This hydrophobic layer prevents moisture infiltration, a key factor in corrosion initiation. The basalt flakes create a “tortuous path” within the epoxy matrix, increasing the diffusion distance for corrosive agents and thereby enhancing the barrier effect. Additionally, polyaniline contributes to corrosion protection through its redox-active properties, facilitating the formation of passive oxide films on the metal surface. The Cassie-Baxter model explains the superhydrophobic behavior, while percolation theory describes the disruption of diffusion pathways by the basalt fillers.

Liu et al. [81] explored low surface energy (LSE) composite marine antifouling coatings enhanced with antibacterial agents. LSE coatings reduce the adhesion strength of fouling organisms by minimizing van der Waals forces at the surface, making it easier for hydrodynamic forces to remove biofouling. The addition of nanoscale metal particles, such as silver and copper, further enhances antifouling performance by generating oxidative stress that disrupts bacterial cell membranes, preventing biofilm formation. This dual-action mechanism – physical deterrence through low surface energy and chemical inhibition via antibacterial agents – offers comprehensive protection against marine biofouling. The Baier curve illustrates the relationship between surface energy and fouling resistance, while DLVO theory (Derjaguin–Landau–Verwey–Overbeek) explains the balance of attractive and repulsive forces governing microbial adhesion.

A novel approach to environmentally friendly material design is presented by Zaheer et al. [82]; who developed asphaltene-coated bitumen microcapsules for safe midstream transportation. These microcapsules, approximately 1.25 mm in diameter, are formed by leveraging the self-aggregation properties of asphaltenes to create cross-linked shells around lighter bitumen fractions. The asphaltene shells provide mechanical strength and environmental resistance without the need for additional chemical additives. This encapsulation mechanism creates a robust barrier that prevents oxidation and reduces the risk of volatile organic compound (VOC) emissions during transportation. The core–shell model describes the mechanical stability of these microcapsules, while fracture mechanics principles explain their ability to withstand high stresses (up to 576 kN/m²), making them suitable for road and marine transport.

El-Sheshtawy et al. [83] synthesized an eco-friendly polyurethane acrylate (PUA)/natural filler-based composite using the rhizome water extract of Costus speciosus. This composite demonstrates strong antifouling activity by creating a slippery, low-adhesion surface that prevents the settlement of fouling organisms. The natural filler’s bioactive compounds, such as saponins and phenolic acids, exhibit bactericidal properties, reducing the growth of biofilm-forming bacteria like E. coli and Pseudomonas aeruginosa. The antimicrobial mechanism is based on membrane disruption theory, where bioactive compounds increase cell membrane permeability, leading to cell death. The Wenzel model explains how surface roughness affects wettability, contributing to the coating’s antifouling effectiveness.

Collectively, these studies highlight the diverse mechanisms through which eco-friendly additives and fillers enhance marine coatings. By leveraging biological inspiration, nanotechnology, and advanced surface engineering, these materials offer multifunctional protection against corrosion and biofouling. They not only improve the durability and performance of marine coatings but also align with global efforts to reduce the environmental footprint of maritime industries. The future of marine coatings lies in the continued integration of sustainable materials that combine physical barriers, chemical modifications, and biological interactions for holistic environmental protection.

5.1 Durability and long-term performance

One of the primary challenges facing natural sustainable coatings is their durability and long-term performance under harsh marine conditions. Marine environments expose coatings to extreme stressors such as saltwater corrosion, UV radiation, temperature fluctuations, mechanical abrasion, and biofouling pressures. Unlike conventional coatings that rely on synthetic polymers and heavy-metal biocides, natural coatings often lack the same level of chemical stability and mechanical robustness.

For instance, while bio-based polymers like chitosan and alginate offer excellent antifouling properties, they are prone to hydrolytic degradation and limited resistance to prolonged UV exposure. The rMrCP20 protein-based corrosion inhibitor demonstrated impressive short-term protection due to its strong adsorption and Fe-ion chelation capabilities. However, proteins can undergo denaturation or biodegradation over time, reducing their long-term efficacy.

Similarly, polyaniline-modified basalt flake composite coatings showed excellent hydrophobicity and corrosion resistance in controlled environments. However, their performance may decline due to mechanical fatigue and environmental stress cracking during extended exposure to fluctuating marine conditions. The loss of hydrophobic properties after repeated mechanical stress cycles indicates the need for improved structural reinforcement and cross-linking strategies to enhance durability.

Moreover, low surface energy (LSE) antifouling coatings, while effective against initial biofouling, suffer from poor static antifouling performance. In low-flow or stagnant water environments, the lack of hydrodynamic shear forces reduces their self-cleaning effectiveness, allowing biofilms to accumulate. This highlights the need for hybrid systems that combine passive barrier mechanisms with active antifouling functionalities for long-term performance.

5.2 Scaling up for industrial applications

Scaling up the production of natural sustainable coatings from laboratory research to industrial-scale applications presents significant technical and economic challenges. Many natural materials used in eco-friendly coatings, such as marine-derived biopolymers, plant extracts, and bioinspired proteins, face issues related to raw material availability, processing complexity, and cost-effectiveness.

For example, the production of recombinant proteins like rMrCP20 requires biotechnological fermentation systems, which involve high costs for culture media, purification processes, and quality control. The scalability of protein-based inhibitors is limited by the low yields and batch-to-batch variability inherent in biological production systems.

Similarly, while polyaniline-modified basalt composites show great potential for enhancing coating performance, the secondary doping process and the use of fluorinated materials can be energy-intensive and cost-prohibitive at large scales. The complex synthesis steps required for functionalizing basalt flakes with polyaniline limit their feasibility for mass production.

In the case of natural filler-based composites, the extraction of bioactive compounds from plants like C. speciosus may face seasonal supply fluctuations and variability in composition, affecting the consistency of the final product. Furthermore, the processing of natural fillers often requires additional steps such as purification, stabilization, and compatibilization with polymer matrices, increasing production costs.

Logistical challenges also arise when integrating natural coatings into existing industrial processes. Traditional marine coatings rely on well-established application techniques such as spray coating and dip coating, which may require modifications to accommodate the rheological properties of bio-based materials. This adds to the capital investment needed for equipment upgrades and workforce retraining.

5.3 Regulatory and certification barriers

Navigating the regulatory landscape and obtaining certifications for natural sustainable coatings is another significant barrier to their commercial adoption. Marine coatings must comply with stringent international standards set by organizations such as the International Maritime Organization (IMO), Environmental Protection Agency (EPA), and various regional maritime safety authorities.

Many eco-friendly coatings, especially those containing novel biopolymers, nanomaterials, or bioactive compounds, face regulatory uncertainties due to the lack of established safety guidelines for these materials. For instance, while chitosan and alginate are generally recognized as safe in biomedical and food applications, their ecotoxicological impacts in marine environments require comprehensive evaluation. The interaction of natural polymers with marine ecosystems, including potential effects on non-target organisms, must be thoroughly assessed through long-term environmental impact studies.

Moreover, coatings incorporating nanostructured fillers or functionalized materials (e.g., polyaniline-doped composites) are subject to additional scrutiny regarding nanoparticle release, bioaccumulation, and toxicity. The lack of standardized testing protocols for these materials slows down the approval process and creates regulatory bottlenecks.

Certification processes also impose hurdles. Marine coatings must undergo rigorous performance testing for parameters such as corrosion resistance, antifouling efficacy, mechanical durability, and environmental safety. These tests are often time-consuming and costly, requiring accelerated weathering tests, salt spray tests, and real-world field trials that simulate marine conditions over extended periods. Furthermore, regulatory discrepancies between regions complicate the global deployment of sustainable coatings. A coating approved in one jurisdiction may face additional testing or modifications to meet the specific requirements of another. This adds to the development timeline and increases costs for manufacturers seeking international market access.

5.4 Market availability and characterization challenges of natural sustainable coatings

Although natural sustainable coatings have received increasing research attention, their commercial market availability remains limited, particularly for marine applications requiring long-term antifouling and anticorrosion performance. Many of the bio-based materials studied – such as chitosan composites [39], 78], barnacle cement proteins [80], plant-derived essential oils [65], and polymerized tung oil/urushiol systems [66] – have demonstrated promising laboratory-scale performance. However, translating these materials into commercially viable marine coatings remains a major challenge.

At present, a few pilot-scale products incorporating rosin-modified antifouling matrices and chitosan-based coatings have been proposed, mostly for niche uses such as aquaculture nets and recreational vessels [33], 61]. Nevertheless, large-scale deployment for critical sectors like commercial shipping or offshore energy structures is still rare. The key barriers include inconsistent performance under dynamic marine environments, limited mechanical durability, and degradation under UV and salt exposure. For example, while chitosan-based and alginate-based coatings have shown antifouling activity against marine bacteria and diatoms [77], 78], concerns remain about long-term stability and mechanical adhesion to metallic substrates.

A critical issue contributing to slow market uptake is the lack of standardized material characterization protocols specifically tailored for natural marine coatings. Traditional standards, such as ASTM D3623 for antifouling and ISO 12944 for corrosion resistance, are often applied; however, they do not fully capture the behavior of natural materials. Natural coatings based on plant oils (e.g., citronellol-modified amphiphilic coatings) and marine polysaccharides (e.g., sodium alginate-tannic acid systems) may exhibit dynamic wettability, photocatalytic effects, or bioactive degradation, which conventional mechanical or salt spray tests do not fully address.

Moreover, variability in natural raw materials – such as essential oil composition, chitosan molecular weight, or tannin structure – introduces batch-to-batch inconsistencies that complicate reproducibility. Studies such as Montoya et al. [34] demonstrated that even small variations in biocide release rates (e.g., copper ion release supported by ZSM-5 zeolites) critically influence antifouling effectiveness and environmental safety, yet such variations are not systematically controlled in current bio-based formulations.

In terms of mechanical and corrosion characterization, coatings such as secondary doped polyaniline basalt composites [49] and IS-cell enhanced epoxy coatings [46] represent important progress, but often rely on advanced techniques like electrochemical impedance spectroscopy (EIS), SEM, and ATR-FTIR for analysis. These techniques are essential but still lack universally accepted benchmarks for comparing natural versus synthetic coatings on parameters such as biofilm resistance, crack propagation, or coating lifetime.

Finally, economic and logistical challenges also impede commercialization. Natural-based coatings such as those incorporating barnacle cement proteins or plant-extract antifoulants require sophisticated extraction and stabilization processes, raising production costs. Furthermore, issues such as short shelf-life, sensitivity to storage conditions, and limited compatibility with existing industrial coating systems further hinder market entry.

In conclusion, while laboratory research on natural sustainable coatings has significantly advanced, the transition to commercial marine applications is constrained by performance uncertainties, standardization gaps, and economic hurdles. To bridge this gap, future efforts must focus on developing unified international testing standards specific to natural materials, improving raw material supply chain control, and demonstrating field-scale performance validation under real marine conditions. Only by addressing these integrated challenges can natural sustainable coatings move from promising academic studies to impactful industrial solutions.

7.1 Emerging materials and technologies

The development of new materials and advanced technologies is pivotal in addressing the limitations of conventional marine coatings. Recent research has explored bioinspired proteins, nanostructured materials, and hybrid composites that offer enhanced performance and environmental safety.

For instance, the use of barnacle cement protein (rMrCP20) as a bioinspired corrosion inhibitor [80] represents a significant advancement. This protein exhibits strong adhesion properties and forms a stable protective layer on metal surfaces, effectively delaying corrosion. The future trend involves genetic engineering and recombinant technology to produce similar proteins with tailored functionalities, improving both corrosion resistance and coating durability in harsh marine environments.

Another promising material is the secondary doped polyaniline-modified basalt flake composite, which demonstrates exceptional hydrophobicity and anticorrosion properties. The incorporation of fluorinated functional groups and nanostructured fillers enhances barrier properties and reduces water absorption. Moving forward, nanotechnology will play a critical role in developing coatings with self-healing capabilities, stimuli-responsive behavior, and adaptive surface properties that respond to changes in the marine environment.

Additionally, low surface energy (LSE) coatings combined with antimicrobial agents are expected to evolve further. The integration of nanoparticles and advanced polymers will enhance their antifouling efficiency while reducing environmental impact. The application of graphene-based materials, metal-organic frameworks (MOFs), and bio-nanocomposites is also gaining attention for their potential to improve mechanical strength, corrosion resistance, and fouling prevention.

7.2 Innovations in bio-based anti-fouling solutions

The future of antifouling technologies is shifting towards bio-based solutions that eliminate the need for toxic biocides traditionally used in marine coatings. Innovations in this area focus on natural compounds, marine-derived biopolymers, and bioinspired antifouling strategies that mimic natural defense mechanisms found in marine organisms.

For example, coatings utilizing chitosan and alginate have shown promise due to their antimicrobial properties and biodegradability. Studies like those of El-Sheshtawy et al. [83] highlight the potential of PUA/natural filler composites derived from C. speciosus, which reduce bacterial adhesion and biofilm formation. Future research will likely explore synergistic combinations of natural compounds, such as essential oils, tannins, and microalgal extracts, to enhance antifouling performance while maintaining environmental safety.

Another emerging trend is the development of bioinspired surface structures that deter fouling through physical mechanisms rather than chemical toxicity. Inspired by the micro- and nano-patterned surfaces of marine organisms like sharks and mollusks, these coatings create topographical barriers that prevent the attachment of fouling organisms. Photocatalytic coatings, such as those incorporating graphitic carbon nitride (g-C₃N₄) and titanium dioxide nanotubes (TNTs), are also being explored for their ability to degrade organic contaminants and inhibit biofouling under sunlight exposure [60]. Furthermore, the use of eco-friendly natural surfactants and bioactive compounds from plant sources, as demonstrated by da Silva et al. [67]; is gaining traction. These bio-based additives can be incorporated into polymer matrices to enhance antimicrobial activity and reduce the environmental impact of marine coatings.

7.3 Integration with smart and functional coating systems

The integration of smart technologies into marine coatings represents a transformative trend in the field. Smart coatings are designed to respond to environmental stimuli such as temperature, salinity, pH, or mechanical stress, enabling them to adapt to changing marine conditions for improved performance and longevity.

One example of this is the development of self-healing coatings, where microencapsulated healing agents are embedded within the coating matrix. When mechanical damage occurs, these capsules release the healing agent, autonomously repairing the coating and restoring its protective function. This approach significantly reduces maintenance costs and extends the service life of marine structures.

The incorporation of sensors and responsive materials into coatings is also an emerging trend. For instance, coatings embedded with fluorescent indicators can provide real-time feedback on the integrity of the coating or the presence of biofouling. Such systems can be used for early detection of corrosion or fouling buildup, allowing for timely maintenance interventions. Smart coatings with temperature-sensitive polymers can adjust their properties to optimize antifouling performance in varying climates.

Moreover, the concept of multifunctional coatings is gaining momentum. Future marine coatings are expected to combine multiple functionalities, such as anticorrosion, antifouling, self-cleaning, and even energy-harvesting capabilities. For example, coatings with photothermal properties can harness solar energy to create localized heating, deterring biofouling and reducing drag on ship hulls.

The use of Internet of Things (IoT) technology to monitor the performance of smart coatings is another exciting prospect. By integrating wireless sensors and data analytics, it will be possible to track coating degradation, fouling rates, and environmental conditions in real time, optimizing maintenance schedules and reducing operational costs.