In Situ and Laboratory Investigation of the Anti-Corrosion and Anti-Fouling Efficacy of an Innovative Biocide-Free Coating for Naval Steels

Abstract

This study presents an in situ and laboratory evaluation of an innovative biocide-free nanocomposite coating designed to provide dual anti-corrosion and anti-fouling protection for EH36 naval steel in marine environments. The coating, comprising polyaniline nanorods, titanium dioxide nanoparticles, and Fe₃O₄-functionalized multiwalled carbon nanotubes embedded in a robust resin matrix, was systematically assessed through electrochemical, microscopic, and field-based methods. Laboratory immersion tests and extended exposures at two Mediterranean sea sites (Thessaloniki and Heraklion) revealed substantial improvements in corrosion resistance and significant suppression of marine biofouling over periods of up to 24 months. Electrochemical measurements demonstrated that coated specimens maintained a corrosion inhibition efficiency exceeding 93% throughout the study, exhibiting markedly lower corrosion current densities and higher charge transfer resistances than uncoated controls. Impedance spectroscopy and equivalent circuit modeling confirmed sustained barrier properties, while digital imaging and qualitative biological assessments showed reduced colonization by both micro- and macrofouling organisms. Comparative analysis with conventional biocidal and alternative eco-friendly coatings underscored the superior durability, environmental compatibility, and anti-fouling efficacy of the developed system. The results highlight the coating’s promise as a sustainable, high-performance solution for long-term protection of naval steels against the combined challenges of corrosion and biofouling in harsh marine settings.

1. Introduction

Naval vessels, offshore structures, and marine infrastructure rely extensively on steel due to its favorable mechanical properties and cost-effectiveness [1]. Prolonged immersion in seawater, however, subjects steel surfaces to two major threats: corrosion induced by aggressive saline environments and biofouling through the settlement and growth of marine organisms [2,3,4,5,6,7,8,9,10]. These dual forms of degradation increase maintenance costs, reduce vessel efficiency, and have pronounced environmental and economic consequences for global shipping and naval operations [11,12,13,14,15,16].

Corrosion of steel in marine environments is facilitated by the presence of dissolved oxygen, chloride ions, and microorganisms acting synergistically to accelerate localized material loss [17,18,19,20,21]. Simultaneously, marine biofouling—a process involving the adherence and proliferation of microorganisms, algae, and invertebrates on submerged surfaces—leads to significant drag on vessels, reduced fuel efficiency, and increased greenhouse gas emissions [22,23]. Traditional approaches to mitigating these hazards have relied on protective coatings, often embedding toxic biocides to deter fouling organisms or employing metallic layers to block corrosive agents [24,25,26,27,28,29,30,31,32,33,34,35,36]. However, the deployment of biocide-based antifouling paints has raised substantial environmental concerns [37,38,39,40,41,42]. The leaching of toxic chemicals, such as organotin compounds and copper-based agents, has been implicated in the disruption of aquatic ecosystems, prompting regulatory measures and a global push toward more sustainable solutions [43,44,45,46,47].

In response to environmental restrictions and escalating ecological awareness, research has shown growing emphasis on the development of biocide-free antifouling coatings that are both effective and environmentally benign. These innovative systems typically exploit physical, chemical, or nanostructured barriers to inhibit organism attachment and minimize corrosion processes [48,49,50,51,52,53,54,55].

Recent studies have demonstrated that biocide-free coatings, based on advanced polymer matrices or hybrid nanocomposites, can provide dual protection for naval steels. For instance, polymer-based antifouling materials—including fluoropolymers and hydrogels—introduce smooth, low-friction surfaces that reduce organism adhesion and enhance hydrophobicity, indirectly decreasing rates of both biofouling and corrosion [56]. Nanocomposite strategies, integrating conductive polymers such as polyaniline (PAni) with nanoparticles like TiO₂ and Fe₃O₄, have yielded promising results in laboratory and artificial seawater immersion tests [57,58]. These coatings exhibit barrier properties, mechanical robustness, and sustained corrosion mitigation, as well as significant longevity under prolonged marine exposure.

Moreover, the inclusion of functionalized nanostructures—such as graphene oxide, carbon dots, and silane-modified nanoparticles—has proven effective in increasing the impermeability of epoxy and acrylate coatings, further impeding the ingress of water and ions that facilitate corrosion and fouling [59,60,61,62,63,64]. In situ and extended field trials involving such coatings have reported not only marked reductions in corrosion rates (sometimes approaching 100% relative to uncoated controls) but also a marked diminution in macro- and microfouling settlement over durations exceeding six months [65].

The transition to biocide-free coatings aligns with the International Maritime Organization’s environmental directives and growing maritime regulations limiting the use and discharge of hazardous chemicals [66,67]. From an economic perspective, innovative antifouling systems have demonstrated potential to deliver tangible savings throughout the operational life of marine vessels. Life cycle assessments (LCA) and cost analyses have documented notable reductions in fuel consumption (up to ~9.6%), corresponding CO₂ emissions, and overall environmental impact—while also decreasing total maintenance expenditures by nearly 9% in comparison to conventional biocidal coatings [22].

Despite substantial advancements documented over the past decade, the effectiveness and durability of biocide-free coatings—particularly under real-world, in situ marine conditions—remain active fields of investigation. In situ studies are crucial to bridge the gap between controlled laboratory evaluations and the complex realities of full-scale deployment, where factors such as fluctuating temperatures, salinity, flow dynamics, and diverse fouling communities prevail [68].

Recent advances in biocide-free marine nanocoatings have centered on the integration of conductive polymers, metal oxides, and carbon nanomaterials to address growing environmental concerns and performance demands in marine antifouling and anticorrosion technologies. This approach offers a promising replacement to traditional toxic biocidal paints, leveraging nanotechnology-enabled synergy for both chemical passivation and robust antifouling efficacy [69].

Conductive polymers, notably polyaniline (PAni) and polypyrrole, have emerged as active agents against marine biofouling due to their unique redox and electrical properties. Laboratory and field studies have demonstrated that such polymers reduce marine organism attachment and provide electrochemical inhibition of corrosion without the environmental toxicity associated with copper- or organotin-based biocides [56]. For example, Baldissera et al. validated that PAni-based coatings retained antifouling efficacy for over twelve months during in situ testing, outperforming conventional commercial paints [70]. Although early work with conductive polymer additives delayed initial fouling, only recent nanocomposite architectures have yielded long-term activity [71].

The incorporation of metal oxides—such as titanium dioxide (TiO₂), zinc oxide (ZnO), and magnetite (Fe₃O₄)—in nanocomposite coatings enhances barrier properties, mechanical resilience, and even photocatalytic self-cleaning. Chitosan-ZnO nanocomposite coatings, for example, have shown superior inhibition of diatom and bacterial colonization compared to chitosan alone [72]. ZnO nanorods embedded in silicone matrices increased fouling resistance, lowered surface free energy, and maintained hydrophobicity during extended field trials in natural seawater, with higher nanorod concentrations promoting greater efficacy. These metal oxide nanostructures act as physical and chemical barriers while supporting long-term durability in harsh saline environments [71].

Recent research highlights the role of carbon nanomaterials, such as multiwalled carbon nanotubes (MWCNTs) and graphene derivatives, in improving nanocomposite dispersion, barrier integrity, and multi-modal protection [57,73]. Selim et al. reported on biocide-free silicone/graphene-silicon carbide coatings achieving hierarchical surface structures that deter fouling without toxic leachates [73]. MWCNTs functionalized with magnetite have been shown to synergize with conductive polymers and metal oxides, providing dense nanobarriers and sites for radical-driven organism inactivation [57].

This body of literature substantiates the technological leap embodied in the present work, setting new benchmarks for the multifunctionality, durability, and environmental compatibility of biocide-free marine coatings. The present work represents a significant advance in the field, uniquely combining polyaniline nanorods, titanium dioxide nanoparticles, and Fe₃O₄-functionalized MWCNTs in a robust resin matrix for EH36 naval steel protection. This composition leverages:

• Conductive polymer-mediated redox inhibition and passivation;
• Photocatalytic and barrier effects from TiO₂;
• Magnetite-modified nanotubes for mechanical robustness and synergistic anti-biofouling activity.

The novelty of combining polyaniline (PAni) nanorods, TiO₂ nanoparticles, and Fe₃O₄-functionalized multi-walled carbon nanotubes (MWCNTs) in marine coating technologies emerges from the synergistic integration of their distinct properties to address multifaceted challenges such as biofouling, corrosion, and mechanical durability [57]. Polyaniline nanorods provide intrinsic electronic conductivity, creating an electrostatic environment that disrupts the initial adhesion of negatively charged biofoulants, thereby imparting antifouling properties [58]. The TiO₂ nanoparticles contribute semiconducting photocatalytic activity, catalyzing the generation of reactive oxygen species like H₂O₂ under light exposure, which degrades organic matter and inhibits biofilm formation through eco-friendly mechanisms [74]. Meanwhile, the Fe₃O₄-functionalized MWCNTs enhance mechanical strength, barrier properties, and impart magnetic functionalities, which extend the coating’s resilience and reduce water permeability. The orientation of TiO₂-decorated PAni nanorods and horizontally aligned Fe₃O₄-MWCNTs further optimizes surface interactions to sustain antifouling efficacy and corrosion resistance over extended immersion in seawater [57].

This research is motivated by the pressing need for sustainable, high-performance surface protection strategies for naval steels. By conducting comprehensive laboratory analyses alongside field-based, in situ assessments, this study aims to evaluate the anti-corrosion and anti-fouling efficacy of a novel biocide-free coating. The goal is to elucidate the mechanisms, long-term performance, and potential practical applications of such innovations—thereby contributing to the fundamental understanding and technological progress of environmentally responsible marine protection systems.

2. Materials and Methods

2.1. Material

EH36 grade naval steel panels were used as the substrate. For laboratory assessments, coupons with dimensions 4 cm × 4 cm × 2 mm were prepared. For field exposure studies, larger panels of 20 cm × 20 cm × 2 mm were utilized.

The chemical composition (elemental %) of the EH36 steel, sourced from the supplier datasheet is listed in

Table 1. The chemical composition of EH36 grade naval steel.

Element	Chemical Composition Elemental %
Iron (Fe)	balanced
Carbon (C)	0.170%
Silicon (Si)	0.100%
Manganese (Mn)	0.900%
Phosphorus (P)	0.035%
Sulfur (S)	0.035%
Aluminum (Al)	0.015%
Titanium (Ti)	0.020%
Copper (Cu)	0.350%
Chromium (Cr)	0.200%
Nickel (Ni)	0.400%
Molybdenum (Mo)	0.080%
Niobium (Nb)	0.020%
Vanadium (V)	0.050%

.

2.2. Synthesis of Antifouling Coating

2.2.1. Preparation of Polyaniline (PAni) Nanorods

Following the dissolution of aniline monomer in 1 M HCl, the solution was chilled in an ice bath. To initiate the oxidative polymerization process, ammonium peroxydisulfate (APS) was introduced dropwise while being agitated vigorously. The process was maintained at a temperature between 0 and 5 °C for six hours to facilitate the complete synthesis of polyaniline nanorods. Centrifugation was employed to collect the product, which was subsequently purified many times with methanol and deionized water before being dried at 50 °C.

2.2.2. Functionalization of PAni Nanorods with TiO₂

TiO₂ nanoparticles were dispersed in deionized water using ultrasonication. To achieve a uniform coating, polyaniline nanorods were included into the TiO₂ suspension and subjected to ultrasonic sonication [57]. The PAni/TiO₂ hybrid was subjected to vacuum washing and subsequent drying.

2.2.3. Synthesis of MWCNTs/Fe₃O₄ Nanocomposites

Following four hours of refluxing in 7 M HNO₃ to oxidize the virgin MWCNTs, they were filtered and subsequently neutralized with deionized water for purification. An aqueous solution containing FeCl₃·6H₂O and FeCl₂·4H₂O in a molar ratio of 2:1 was prepared under a nitrogen environment for the deposition of magnetite. Upon introducing the oxidized MWCNTs into the iron salt solution, NH₃ was incrementally added until the pH reached a minimum of 10, leading to the deposition of Fe₃O₄ nanoparticles on the MWCNTs via co-precipitation. The composite was vacuum-dried following cleaning with ethanol and water.

2.2.4. Final Coating Preparation and Application

The PAni/TiO₂ and MWCNTs/Fe₃O₄ nanomaterials were combined with water-soluble resin in a mass ratio of 1:1:2 (PAni/TiO₂: MWCNTs/Fe₃O₄: resin). The chemistry of the resin matrix plays a critical role in determining the dispersion of nanoparticles and the adhesion properties of the resulting nanocomposite coating. The well-formulated resin matrix facilitates uniform distribution of functionalized nanoparticles—such as polyaniline nanorods, titanium dioxide, and Fe₃O₄-modified carbon nanotubes—by providing compatible chemical interactions and sufficient steric stabilization to prevent agglomeration. Optimal matrix chemistry enhances nanoparticle surface affinity, promoting effective wetting and interfacial bonding that are essential for homogeneity and mechanical integrity. Moreover, the resin’s chemical structure influences the coating’s adhesion to the substrate by dictating its polarity, crosslinking density, and curing behavior, thereby affecting interfacial strength and resistance to delamination under marine exposure conditions. Consequently, tailoring resin chemistry to balance hydrophilicity, toughness, and chemical compatibility with incorporated nanomaterials is paramount to achieving durable coatings with superior barrier properties, sustained anti-corrosive activity, and strong substrate adherence in harsh aquatic environments.

Mechanical agitation and ultrasonication facilitated homogeneous dispersion. Additives necessary for stability, adhesion, and other purposes were integrated. Steel panels were degreased with acetone, polished with sandpaper, and degreased again. The formulated nanocomposite antifouling paint was immediately applied on one side of the panels and their edges with an airless sprayer (SATA MINIJET 4400B HVLP, SATA GmbH & Co. KG, Kornwestheim, Germany) and an airless compressor. Thereafter, they were cured at room temperature for a minimum of 24 h before characterization or exposure testing. Prior to the final application of the coatings to the panels, several tests were made in order to adjust the pressure value (~2 bar) to the lowest setting with a good spray pattern. Thus, a controlled thickness and a proper coverage of the paint were achieved.

This protocol produces a multifunctional, biocide-free antifouling coating integrating conducting polymer, photocatalytic nanostructures, and magnetite-modified nanotubes within a robust resin matrix.

The X-ray diffraction patterns (XRD) of different nanocomposites were using a Bruker D8 Focus X-ray diffractometer with CuKα radiation (Bruker AXS SE, Karlsruhe, Germany). The morphology analysis of nanocomposites and antifouling coatings were characterized by a JEOL 2100 HR microscope (JEOL, Tokyo, Japan), transmission electron microscope (TEM, JEOL, Tokyo, Japan) using bright field images mode and 80 kV accelerating voltage.

2.3. Laboratory Immersion Testing in Artificial Sea Water (ASW)

Electrochemical characterization was undertaken to assess the corrosion resistance and electrochemical behavior of the developed biocide-free nanocomposite antifouling coating applied on EH36 naval steel substrates.

All measurements were conducted employing a conventional three-electrode electrochemical cell assembly, comprising the coated or uncoated EH36 steel specimen as the working electrode (exposed area approximately 1 cm²), a platinum mesh serving as the counter electrode, and a saturated calomel electrode (SCE) or silver/silver chloride (Ag/AgCl) electrode as the reference electrode. Laboratory evaluations utilized artificial seawater prepared in accordance with ASTM D1141 standards [75]. Tests were performed at a controlled temperature of approximately 27 °C for different immersion times (0.5, 1, 6, 12, and 24 months). Before the electrochemical evaluation, open circuit potentials (OCP) were monitored to determine the thermodynamic stability and corrosion tendency of the coated and uncoated surfaces.

Potentiodynamic polarization curves were recorded by scanning the electrode potential from −250 mV to +250 mV relative to OCP at a sweep rate of 1 mV/s. By utilizing CorrView v.36a software in Rp mode, it was possible to fit the polarization curves and analyze the corrosion behavior. Rp mode allows for the determination of parameters like corrosion potential (E_corr), corrosion current density (I_corr), and mechanistic insight into anodic and cathodic reactions governing corrosion processes.

EIS measurements were performed over a frequency range of 100 kHz to 10 mHz with a 10 mV sinusoidal perturbation applied around the OCP. Analysis of Nyquist spectrum permitted extraction of parameters related to coating resistance, charge transfer resistance at the metal/coating interface, and interfacial capacitance. Equivalent electrical circuit models were fitted to the data to quantify the coating’s barrier performance and corrosion protection efficacy.

Corrosion rates Equation (1) were quantitatively determined from electrochemical data and expressed in terms of millimeters per year (mm/year), enabling comparative evaluation between coated and bare steel specimens.

$$CR={\displaystyle \frac{i_{corr}\times K\times EW}{n\times \rho }},$$

(1)

where, I_corr represents the corrosion current density (μA/cm²) of the evaluated sample, K is a conversion constant (3.27 × 10⁻³ for mm/year when I_corr in μA/cm², EW in g/eq, ρ in g/cm³), EW equivalent weight of metal (g/eq), calculated based on alloy composition, n is the number of electrons exchanged (for Fe: n = 2), and ρ represents the density of the metal.

For iron-based steels, with ρ = 7.85 g/cm³ and n = 2, Equation (1) is rewritten as follows:

$$\mathrm{C}\mathrm{R}(\mathrm{m}\mathrm{m}/\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r})={\displaystyle \frac{0.00327\times {\mathrm{{\rm I}}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}\times 27.92}{2\times 7.85}},$$

(2)

where, 27.92 g/eq is the equivalent weight for Fe.

The corrosion protection efficiency (n%) of the coating was calculated by using the Equations (3) and (4):

$$\eta \%=\left(1-{\displaystyle \frac{{{\rm I}}_{corr}\left(coated\right)}{{{\rm I}}_{corr}\left(uncoated\right)}}\right)\times 100\%,$$

(3)

where, I_corr (coated) represents the corrosion current of the sample with antifouling paint, and I_corr (uncoated) denotes the corrosion current of the uncoated sample.

$$\mathsf{\eta }\mathrm{\%}=\left(1-{\displaystyle \frac{{\mathrm{R}}_{\mathrm{c}\mathrm{t}}\left(\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\right)}{{\mathrm{R}}_{\mathrm{c}\mathrm{t}}\left(\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\right)}}\right)\times 100\mathrm{\%}$$

(4)

where, R_ct (coated) represents the charge transfer resistance of the sample with antifouling paint, and R_ct (uncoated) denotes the charge transfer resistance of the uncoated sample.

2.4. In Situ (Field) Seawater Immersion

Sets of coated and uncoated steel panels were deployed in coastal waters at multiple Greek locations (Thessaloniki and Heraklion) for identical periods as laboratory tests. Samples were retrieved at predefined intervals (1, 6, 12, and 24 months), rinsed in deionized water and ethanol, and dried at room temperature. The extent and nature of biological fouling were assessed by macroscopic examination and image analysis.

Thessaloniki, located on the northwest coast of the Aegean Sea, experiences a semi-enclosed marine environment heavily influenced by urban and industrial activities. This area is subject to moderate to high anthropogenic pressures, including the discharge of wastewater, heavy metals, and elevated nutrient levels, which contribute to eutrophication and perturbations in biogeochemical cycles. Seasonal seawater temperatures range from approximately 12 °C in winter to 28 °C in summer, with salinity values around 37 PSU affected by freshwater inputs from the Thermaic Gulf. Prolonged stratification periods and episodic bottom-water hypoxia, driven by organic matter decomposition, have been documented, factors that can exacerbate corrosion and modify fouling assemblages on submerged surfaces [76].

Heraklion, situated on the northern coast of Crete in the eastern Mediterranean, presents a comparatively oligotrophic marine environment characterized by limited anthropogenic impact. Hydrologically, the site features high water clarity, more stable and slightly elevated salinity around 38 PSU, and temperatures varying from approximately 15 °C in winter to 27 °C in summer. The well-mixed water column and reduced nutrient inputs result in lower incidence of hypoxic events and a distinct biological community structure, often leading to reduced fouling intensity and different corrosion dynamics than those observed in more industrialized northern Aegean areas (e.g., Thessaloniki).

Pollutant loads at Thessaloniki include elevated concentrations of trace metals such as copper, zinc, and lead, predominantly from industrial effluents and urban runoff. These pollutants influence microbial colonization and biofilm diversity, thereby accelerating corrosive processes on steel substrates. In contrast, Heraklion’s coastal waters generally exhibit reduced contamination levels, with trace metals primarily originating from natural sources and minimal maritime traffic, which affects the corrosion and fouling patterns distinctively at this site. Thessaloniki’s enclosed gulf with limited water exchange favors accumulation of corrosive agents and fouling organisms. Heraklion’s open coastal waters promote pollutant dispersal and reduce localized corrosive effects, underscoring the necessity of site-specific evaluation of antifouling systems. These environmental nuances are crucial for interpreting protective coating performance and tailoring sustainable marine infrastructure protection strategies.

The detailed chemical compositions of seawater from Thessaloniki and Heraklion along with artificial seawater (ASW) standards, are summarized in

Table 2. The chemical compositions of seawater from Thessaloniki and Heraklion along with artificial seawater (ASW) standards.

Parameter	Thessaloniki Seawater	Heraklion Seawater	Artificial Seawater (ASTM D1141)
Salinity (‰)	37.0–39.0	38.5–39.5	35.0
Temperature (°C)	12–28	15–27	Controlled to ~27
pH	7.9–8.3	8.0–8.4	8.1
Major ions (mg/L)
Sodium (Na+)	10,500–11,000	11,000–11,200	10,800
Chloride (Cl−)	19,000–20,000	19,500–20,500	19,350
Sulfate (SO42−)	~2500–2700	~2600–2800	2700
Magnesium (Mg2+)	1300–1400	1350–1450	1290
Calcium (Ca2+)	400–450	420–460	410
Potassium (K+)	380–400	390–410	380
Trace metals (µg/L)	Elevated Cu, Zn, Pb due to pollution	Lower trace metal levels	Minimal to none
Nutrients	Higher nutrient (N, P) concentrations	Lower nutrient concentrations	Not typically present

.

Thessaloniki coastal seawater is influenced by riverine inputs and anthropogenic activity, leading to slightly higher variability in salinity and elevated trace metal and nutrient concentrations. Heraklion seawater exhibits more stable, oligotrophic conditions with slightly higher salinity and lower pollution levels. Artificial seawater per ASTM D1141 [75] replicates the average major ion concentrations of seawater but omits pollutants and exhibits very controlled chemical parameters for laboratory reproducibility. These detailed seawater characteristics impact corrosion and biofouling behavior of naval steels and coatings differently in the two field locations tested in the study and explain variations noted in biofouling and corrosion outcomes.

3. Results

3.1. Characterization of Antifouling Coating

Τhe X-ray diffraction (XRD) pattern of TiO₂@PAni (titanium dioxide-functionalized polyaniline nanotubes) typically shows features representing both components—titania (TiO₂) and polyaniline (PAni)—which confirm successful composite formation (Figure 1).

The sharp, intense peaks evident in the lowermost curve in Figure 1 correspond to crystalline TiO₂. The main characteristic peaks for anatase phase TiO₂ typically appear at: 25.3° (101), 37.8° (004), 48.0° (200), 53.9° (105), 55.1° (211), 62.7° (204). The pattern matches standard anatase TiO₂ according to JCPDS card no. 21-1272, confirming its presence as the primary crystalline phase.

The middle curve in Figure 1 corresponds to the PAni diffraction and shows broad, less intense peaks at approximately 2θ ≈ 20° and 25°, which are signatures of the partially crystalline/semi-amorphous structure of polyaniline. These broad features indicate periodicity along the polymer chain and interplanar distances typical for PAni. The broad nature of both the 20° and 25° peaks in Figure 1 suggests that polyaniline synthesized in this work exhibits partial crystallinity rather than highly ordered crystal structure.

The crystallinity index of polyaniline (PAni) in Figure 1 can be quantitatively determined by analyzing the relative intensity and breadth of the X-ray diffraction (XRD) peaks associated with its crystalline and amorphous domains. For PAni, the crystallinity index is commonly calculated as the ratio of the intensity of the main crystalline peak (around 2θ ≈ 25°) to the intensity of the amorphous background (around 2θ ≈ 20°). The crystallinity index was 1.8, indicating moderate semicrystalline character.

The top trace (PAni-TiO₂) exhibits a combination of sharp TiO₂ peaks at expected positions (overlapping with pure TiO₂), indicating retention of crystal structure within the composite, and broad PAni features seen as humps underlying the sharper TiO₂ reflections. No new peaks appear, suggesting that the composite is a physical hybrid, without the formation of new crystalline phases. The TiO₂ peaks in the composite may appear slightly broader or less intense due to smaller crystallite sizes, interaction with the polymer matrix, or reduced crystallinity.

The XRD analysis unequivocally demonstrates that the Fe₃O₄@MWCNTs nanocomposite contains both the crystalline magnetite phase and the graphitic carbon structure of the nanotubes (Figure 2). The main Fe₃O₄ peaks at ~30.1°, 35.5°, 43.2°, 53.5°, 57.2°, 62.8°, and 74.3° confirm the magnetite spinel structure, while the broad peak at 26° is retained from the underlying MWCNTs. This pattern confirms successful decoration of MWCNTs with Fe₃O₄ nanoparticles, with high purity and no formation of additional phases.

Transmission electron microscopy (TEM) bright field imaging provides direct visualization of the nanostructure in TiO₂@PAni (titanium dioxide–decorated polyaniline) nanocomposites (Figure 3a). TEM bright field images confirm the morphological integration of TiO₂ and PAni at the nanoscale, with clear visualization of PAni nanorods and continuous TiO₂ nanoparticles decoration, supporting the formation of the intended functional nanocomposite for advanced antifouling application.

The crystallite size of nanoparticles visible in Figure 1 and Figure 2 can be estimated using the Scherrer equation, which relates the width of the most intense X-ray diffraction (XRD) peaks to the mean size of coherently scattering domains (crystallites). The estimated sizes are in the ranges of 32.8 nm for TiO₂ and 5.8 nm for Fe₃O₄ (closely matching those observed in TEM micrographs).

Figure 3a demonstrate the intimate contact between PAni and TiO₂, with PAni clearly delimiting the boundaries of individual TiO₂ particles, indicating a successful surface-decorated structure. TEM image confirms that TiO₂ is evenly distributed and polyaniline nanorod is well-covered by TiO₂ nanoparticle. The PAni manifests as a lower contrast and lighter gray nanorods. TiO₂ appears as high contrast spheroidal nanoparticles on the surface of the PAni nanorods. TiO₂ nanoparticles show clear shapes, ranging from 20 to 50 nm in diameter. The good dispersion with minimal agglomeration of TiO₂ nanoparticles in the polymer nanotubes is sought for optimized properties.

TEM bright field image of Fe₃O₄@MWCNTs (Figure 3b) show clear dark spots (Fe₃O₄) attached to the lighter, tube-shaped background (MWCNTs). More specifically, MWCNTs appear as extended, medium-gray contrast tubular structures with hollow cores, often ~10 nm in diameter. Magnetite (Fe₃O₄) nanoparticles are seen as darker, nearly spherical dots with higher electron density. They are generally 5 nm in diameter and attach either uniformly or in clusters the outer walls of the nanotubes. TEM image displays a homogeneous distribution of Fe₃O₄ nanoparticles with minimal aggregation across the MWCNT surfaces.

3.2. Laboratory Immersion Tests

Figure 4 presents the temporal evolution of the open circuit potential (OCP) for EH36 steel samples, both uncoated and coated with the biocide-free antifouling coating after varying exposure durations. The OCP measurements shown in Figure 4 were recorded at two-hour intervals, ensuring continuous monitoring of the electrochemical behavior throughout the entire immersion period. This high-frequency sampling approach allowed for detailed tracking of open circuit potential changes without relying on discrete data points alone. As a result, the OCP curves are presented as continuous, accurately reflecting the real-time evolution of corrosion protection for both coated and uncoated samples.

The uncoated steel exhibited the most negative potentials, indicative of active corrosion. In contrast, coated samples, immersed for 1, 6, 12, and 24 months, demonstrated increasingly noble (less negative) OCP values, signifying enhanced corrosion protection over time. This progressive shift highlights the long-term electrochemical stability imparted by the innovative coating.

Figure 5 depicts the potentiodynamic polarization curves of EH36 naval steel specimens, comparing the uncoated control with samples protected by the biocide-free antifouling coating over exposure periods of 1, 6, 12, and 24 months. The uncoated steel (black curve) exhibits the highest corrosion current densities and the most negative corrosion potentials (E_corr), indicative of pronounced electrochemical activity and accelerated corrosion in the marine environment. These findings are reinforced by the steep anodic and cathodic branches in the polarization profile, reflecting minimal barrier protection and high susceptibility to both metal dissolution and oxygen reduction reactions.

In contrast, all coated samples display significant shifts toward lower current densities and less negative corrosion potentials. Especially after longer exposure times (12 and 24 months), the curves become markedly flatter, denoting substantial suppression of electrochemical processes at the steel/coating interface. The corrosion potential moves in a more noble direction, while the passivation behavior is enhanced, particularly for the coating aged for 24 months (the current does not increase quickly with increasing applied potential—indicating that the surface is highly resistant to further corrosion). These features collectively indicate a robust and persistent protection afforded by the innovative nanocomposite coating.

Furthermore, quantitative analysis reveals a progressive decrease in corrosion current density (I_corr) with increasing immersion time, suggesting that the coating maintains or even improves its barrier and electrochemical inhibitory properties over time. The observed trends clearly demonstrate the long-term effectiveness of the biocide-free antifouling approach, as the coated steel remains stably passivated and exhibits minimal corrosion risk throughout the extended marine exposure period. These results underscore the coating’s potential for sustainable, durable protection in challenging naval applications.

Table 3. Electrochemical parameters obtained for uncoated and coated steel with the antifouling biocide-free coating immersed in ASW solution for various exposure periods.

Immersion Time (Months)	Ecorr (V)	Ιcorr (μA cm−2)	CR (mm/Year)	Mass Loss (Grams)	η (%)
uncoated	−0.697	2.428	1.41 × 10−2	9.20 × 10−4 (±0.32)	-
1	−0.360	0.017	9.89 × 10−5	6.47 × 10−5 (±0.42)	99.30%
6	−0.481	0.073	4.25 × 10−4	1.67 × 10−4 (±0.31)	96.99%
12	−0.512	0.077	4.48 × 10−4	3.52 × 10−4 (±0.30)	96.83%
24	−0.637	0.141	8.20 × 10−4	7.07 × 10−4 (±0.31)	94.19%

presents the electrochemical parameters and corrosion performance indicators for EH36 steel samples, both uncoated and coated with the innovative biocide-free antifouling coating, over various immersion intervals. The uncoated steel exhibited a highly negative corrosion potential (E_corr: −0.697 V) and the highest corrosion current density (I_corr: 2.428 μA cm⁻²), resulting in a significant corrosion rate (CR: 1.41 × 10⁻² mm/year). In sharp contrast, the coated samples showed dramatic improvements: after just 1 month of immersion, E_corr shifted to −0.360 V and I_corr was reduced to 0.017 μA cm⁻², corresponding to a remarkably low CR of 9.89 × 10⁻⁵ mm/year and a protection efficiency (η) of 99.30%.

Upon extending the immersion period to 6, 12, and 24 months, the corrosion parameters for coated samples indicate a gradual and controlled decline in protective performance, with η values of 96.99%, 96.83%, and 94.19%, respectively. Despite this slight decrease, CR values for coated steel remain substantially lower than the uncoated reference throughout the entire duration, never exceeding 8,20 × 10⁻⁴ mm/year after 24 months. The E_corr values of all coated samples stay within the less negative range, reflecting improved thermodynamic stability even after prolonged exposure to the simulated marine environment.

Statistical analysis (one-way ANOVA) confirms that the observed differences in I_corr and CR between coated and uncoated groups are highly significant (p < 0.001), emphasizing the distinct and reliable barrier performance imparted by the biocide-free coating. The small and consistent I_corr values for coated samples across replicates suggest high reproducibility and stable long-term protection. The gradual increases in I_corr and CR over time were statistically insignificant between 6 and 24 months (p > 0.05), supporting the coating’s ability to withstand long-term mechanical and chemical stresses without significant degradation.

The uncoated steel exhibited a notably high corrosion rate of 1.41 × 10⁻² mm/year, corresponding to a substantial mass loss of 9.20 × 10⁻⁴ g. This highlights the pronounced material degradation occurring in aggressive marine environments without protective measures. Upon application of the biocide-free nanocomposite coating, a marked reduction in corrosion rates was observed, with the CR decreasing to 9.89 × 10⁻⁵, 4.25 × 10⁻⁴, 4.48 × 10⁻⁴, and 8.20 × 10⁻⁴ mm/year after 1, 6, 12, and 24 months of exposure, respectively. Correspondingly, the mass loss exhibited a consistent decline relative to uncoated samples, with values of 6.47 × 10⁻⁵, 1.67 × 10⁻⁴, 3.52 × 10⁻⁴, and 7.07 × 10⁻⁴ g over the same timeframes. These results demonstrate a clear positive correlation between corrosion rate and mass loss, where reductions in CR due to the protective coating directly translate into diminished material loss. Although an incremental increase in both CR and mass loss is evident with prolonged immersion, the coated steel consistently maintained significantly lower values than the unprotected counterpart, underscoring the coating’s sustained efficacy in mitigating corrosion-induced degradation over two years. This sustained protection reflects the enhanced barrier characteristics and electrochemical stability conferred by the nanocomposite coating under harsh marine conditions.

In comparison, the uncoated steel exhibited rapid and severe corrosion under identical conditions. Even the oldest immersed coated samples (24 months) maintained corrosion rates more than an order of magnitude lower, and continued to provide over 94% protection relative to the uncoated control. Collectively, these findings underscore the durable and robust corrosion inhibition afforded by the biocide-free antifouling coating across all exposure periods. The data support its promising potential as a long-term, environmentally friendly solution for the protection of naval steels in harsh marine environments.

Figure 6 presents a comparative analysis of corrosion rate (CR) and mass loss for both uncoated and nanocomposite-coated EH36 naval steel panels after various immersion durations in ASW, revealing substantial differences in performance between the two systems. The uncoated specimens displayed a consistently high corrosion rate, initially measured at 1.41 × 10⁻² mm/year and associated with a mass loss of 9.20 × 10⁻⁴ g after 24 months, highlighting the pronounced susceptibility of bare steel to rapid material degradation under aggressive marine conditions. In contrast, application of the biocide-free nanocomposite coating resulted in a drastic and immediate decrease in CR, reaching as low as 9.89 × 10⁻⁵ mm/year at the 1-month mark and gradually increasing to only 8.20 × 10⁻⁴ mm/year after 24 months—remaining over an order of magnitude lower than the uncoated control at every time point. Correspondingly, the coated steel exhibited a notably reduced mass loss trajectory: mass loss values ranged from 6.47 × 10⁻⁵ g at 1 month to just 7.07 × 10⁻⁴ g at 24 months, consistently outpacing the uncoated material in terms of material preservation.

The comparative temporal trends depicted in Figure 6 emphasize the long-term durability of the nanocomposite coating as opposed to the rapid deterioration experienced by the uncoated steel. While the coated specimens showed a slight, incremental increase in both CR and mass loss with extended immersion—likely attributable to minor water uptake and gradual resin dissolution—the overall rates remained minimal, and the efficacy of the coating as a corrosion barrier persisted throughout the 24-month experimental period. Statistical analysis further substantiates this finding, with the differences in CR and mass loss between coated and uncoated steels being highly significant (p < 0.001 p < 0.001 p < 0.001), whereas incremental increases within the coated set from 6 to 24 months did not reach statistical significance (p > 0.05 p > 0.05 p > 0.05), indicating stable, reproducible protection over time. Collectively, these results confirm that the innovative, biocide-free nanocomposite coating confers robust and sustained protection against corrosion and material loss when compared to unregulated marine exposure, establishing its suitability as an environmentally responsible and high-performance solution for naval steel preservation.

Figure 7a presents the electrochemical impedance spectroscopy (EIS) results for EH36 steel specimens, both uncoated and coated with the biocide-free antifouling system, depicted as Nyquist plots after immersion periods of 1, 6, 12, and 24 months. The main graph demonstrates the evolution in impedance behavior over time for the coated samples, while the inset contrasts these results with the markedly lower impedance of the uncoated steel. All coated samples exhibit highly capacitive, depressed semicircular arcs characteristic of effective barrier properties, with the magnitude of real (Zreal) and imaginary (−Zimaginary) components greatly exceeding those of the bare control.

Notably, the impedance arc radius for the 1-month coated sample is the largest and continues to exhibit high values up to around 150,000 Ω cm², indicating superior corrosion resistance and low charge transfer at the metal/coating interface in the early stage. As immersion time increases to 6, 12, and 24 months, the arc dimensions slightly decrease, reflecting a gradual reduction in barrier performance. However, even after 24 months, the coated samples still maintain substantially larger impedance radii (approaching 50,000 Ω cm²), signifying robust and persistent protection compared to the uncoated substrate, whose arc radius remains below 6000 Ω cm² (as shown in the inset).

These trends are further supported by the non-linear shape of the Nyquist plots, which is typical for organic coatings in corrosive environments and likely arises from a combination of coating capacitance and double-layer behavior at the steel interface. The sustained high impedance of the coated samples across all time points indicates their ability to mitigate electrolyte penetration, suppress electrochemical reactions, and maintain low corrosion current densities as confirmed by polarization analysis.

Figure 7b presents the Bode phase diagrams for uncoated and coated EH36 naval steel samples, illustrating the evolution of phase angle as a function of frequency after immersion in artificial seawater (ASW) for varying durations. The uncoated steel displays a lower phase angle across the entire frequency spectrum, indicative of its limited barrier properties and high susceptibility to ionic penetration and corrosion processes in the ASW environment. In contrast, the coated specimens demonstrate significantly elevated and sustained phase angles at intermediate frequencies (10–1000 Hz), as well as broader frequency ranges displaying capacitive behavior, which underscores the robust barrier efficacy and improved dielectric response imparted by the biocide-free nanocomposite coating. The gradual decrease in phase angle with increasing immersion time for coated samples suggests minor water uptake or localized relaxation phenomena, yet values consistently remain well above those observed for unprotected steel, reflecting the coating’s durable protection and capacitive performance over the entire exposure period.

Statistical evaluation reinforces the significance of these differences, with the impedance modulus of coated samples exhibiting values at least an order of magnitude higher than those of uncoated steel (p < 0.001), across replicate experiments. Collectively, the EIS data demonstrate that the biocide-free coating confers exceptional long-term electrochemical stability to EH36 steel in marine environments. Even after 24 months of exposure, the coating retains effective barrier properties and resists significant degradation, underscoring its potential as a durable, environmentally responsible solution for corrosion and fouling control in naval steel applications.

The fitting accuracy of the electrochemical impedance spectroscopy (EIS) data presented in Figure 7 was assessed through least-squares minimization of the difference between experimental and simulated impedance spectra, as quantified by the χ² statistic. Across all immersion durations and sample conditions, the calculated χ² values remained consistently below 10⁻³, indicating an excellent agreement between the experimental data and the equivalent circuit models utilized for both coated and uncoated EH36 steel. Such low χ² values confirm the reliability of the parameter extraction and validate the suitability of the proposed electrical circuit representations in describing the impedance behavior of the examined antifouling systems.

Polyaniline nanorods are known for their intrinsic electrical conductivity and redox activity, which facilitate the formation of a passivating oxide layer on the steel substrate, thereby inhibiting anodic dissolution and enhancing charge transfer resistance. This redox mediation can stabilize corrosion potentials and reduce corrosion current densities by suppressing localized corrosion sites. Meanwhile, the semiconducting and photocatalytic properties of TiO₂ nanoparticles enable the generation of reactive oxygen species under ambient light exposure, which actively degrade organic matter deposited on the coating surface. This photocatalytic activity not only assists in biofilm disruption but also contributes to the self-cleaning ability of the coating, reducing fouling accumulation and its associated corrosive underlayers. Such photocatalytic degradation mechanisms, combined with increased coating barrier properties, explain the sustained impedance and lowered corrosion rates observed in electrochemical impedance spectroscopy (EIS) and polarization data.

Fe₃O₄-functionalized MWCNTs impart mechanical reinforcement and a tortuous path within the coating matrix, substantially enhancing its physical barrier properties against ion and water ingress, as evidenced by increased coating resistance and decreased capacitance values. Their magnetic properties may additionally influence fouling settlement by affecting microbial adhesion processes, while their high aspect ratio and surface functionalization improve dispersion and interfacial bonding within the resin matrix, promoting coating integrity under marine exposure. Moreover, the Fe₃O₄@MWCNTs may act as radical generation sites or electron sinks, further amplifying photocatalytic or redox-mediated antifouling effects in synergy with TiO₂ and PAni. Unraveling these complex interplays between electrochemical activity, photocatalysis, mechanical barrier elevation, and magnetic influences will provide crucial mechanistic insight supporting the interpretation of the coating’s durable corrosion protection and antifouling properties observed over extended immersion periods. This knowledge could inform optimization of nanofiller ratios and distribution strategies to enhance synergistic effects in next-generation marine coatings.

Figure 8 illustrates the corresponding electrical circuits utilized for simulating electrochemical impedance measurements of uncoated (Figure 8a) and coated naval steel surfaces (Figure 8b). The electrical circuit (Figure 8a) illustrates the uncoated steel sample, comprising a solution resistance (Rₛ) in series with the parallel arrangement of charge transfer resistance (R_ct) and double-layer capacitance (CPE_dl). Electrical circuit (Figure 8b) by introducing a more complex architecture of a single-layer antifouling coating system that encompasses both barrier and interfacial processes, as well as the integration of supplementary resistive and capacitive elements to address diffusion phenomena.

The equivalent circuit comprises the following elements: The ASW solution resistance (Rₛ) denotes the resistance of the ASW solution between the reference and working electrodes, whereas the coating constant phase element (CPE_c) models the non-ideal dielectric response of the organic coating, indicating water uptake or dielectric constant changes. Coating resistance (R_c) indicates the barrier effectiveness of the coating to ionic movement, with elevated values signifying superior integrity. Charge transfer resistance (R_ct reflects the resistance to corrosion processes at the metal/coating interface, serving as a direct measure of corrosion protection. The double layer constant phase element (CPE_dl) pertains to the electrochemical double-layer at the steel/coating interface, while the diffusion constant phase element (CPE_diff) and diffusion resistance (R_diff) are associated with the diffusion process across the metal/coating interface.

Table 4. Equivalent circuit parameters obtained for uncoated and coated steel.

Immersion Time (Months)	Rs (Ω)	CPEdl (F × cm−2)	ndl	Rct	CPEc (F × cm−2)	nc	Rc (Ω × cm2)	CPEdiff (F × cm−2)	ndiff	Rdiff (Ω × cm2)	η (%)
uncoated	50	1.70 × 10−3	0.80	1143
1	162	6.40 × 10−7	0.45	6.88 × 104	1.16 × 10−8	0.75	6140	8.80 × 10−6	0.45	3.80 × 105	98.34
6	200	2.40 × 10−7	0.85	4.58 × 104	9.70 × 10−7	0.43	5030	4.60 × 10−5	0.40	1.10 × 105	97.50
12	191	6.40 × 10−7	0.61	3.80 × 104	4.85 × 10−7	0.70	3710	2.00 × 10−6	0.80	5.70 × 104	96.99
24	157	6.61 × 10−6	0.80	1.78 × 104	6.70 × 10−9	0.80	3410	4.70 × 10−6	0.60	1.60 × 105	93.58

summarizes the fitting results derived from Nyquist impedance spectra for EH36 steel, uncoated and coated with the biocide-free antifouling system, at various immersion periods. The uncoated steel demonstrates relatively low values of charge transfer resistance (R_ct) and pore resistance (R_diff), reflecting the facile access of electrolyte and aggressive ions to the steel surface, which accelerates corrosion processes. High values of the constant phase elements (CPE_dl and CPE_diff) for the uncoated sample are indicative of an unimpeded double-layer formation at the electrode interface, typical of active corrosion in marine environments.

In stark contrast, the coated specimens show dramatically increased coating resistance (R_c) and charge transfer resistance (R_ct) immediately upon application of the antifouling layer. This enhanced impedance response signifies that the coating acts as an effective barrier, impeding both ionic transport and electrochemical reactions at the metal surface. Capacitance values (CPE_c) for the coated steel are considerably lower, highlighting restricted dielectric relaxation and minimal electrolyte penetration, while n_c values indicate a near-ideal capacitive behavior in the early immersion stages.

Over prolonged exposure (6 to 24 months), there is a gradual reduction of R_ct and R_diff in coated samples, paired with a slight rise in CPE_dl and alterations in n_c. These changes suggest minor water absorption or localized ion ingress, leading to subtle modifications in the coating’s dielectric properties. Nonetheless, the overall decline remains modest, and the impedance values for coated specimens consistently surpass those of uncoated steel by orders of magnitude.

Throughout all immersion intervals, the coated system maintains high levels of electrochemical protection. Elevated resistances and suppressed capacitance values across time points underscore the coating’s persistent barrier function and ability to slow down the corrosion kinetics of EH36 steel in saline environments. The results from equivalent circuit modeling therefore corroborate the strong and durable anti-corrosive performance of the biocide-free antifouling coating, aligning well with the trends observed from polarization and direct mass loss measurements.

The values in the last column of Table 3, representing the corrosion protection efficiency (η%), demonstrate consistently high performance for the biocide-free antifouling coating over the examined immersion periods. The η% was calculated by using the Equation (4). Immediately following application, the coating achieves an efficiency of 98.34%, reflecting its exceptional ability to inhibit corrosion processes from the outset. Although there is a gradual decline in efficiency with extended immersion (dropping to 97.49% at 6 months, 96.99% at 12 months, and 93.58% at 24 months), the protection remains robust throughout. This slight reduction over time likely results from minor water uptake or localized electrolyte penetration into the coating matrix as exposure progresses. Nonetheless, η% values persistently remain above 93% for up to two years, underscoring the long-term stability and effectiveness of the coating in providing enduring electrochemical protection to EH36 steel in harsh marine environments. The consistently high efficiency further confirms the coating’s suitability for durable, environmentally friendly corrosion control.

The SEM cross-sectional images presented in Figure 9 provide a detailed visualization of the morphological characteristics of the biocide-free antifouling coating applied to EH36 naval steel after varied immersion durations in artificial seawater. Across the examined samples, the interface between the steel substrate and the coating layer is generally well-defined at initial stages. The overall homogeneity and continuity of the protective film appear to diminish with extended exposure times. Notably, after prolonged immersion, the coating displays marked signs of thinning, surface roughening, and localized surface detachment, suggesting progressive resin loss and compromised integrity under aggressive corrosive marine conditions.

An estimation of the coating thickness, derived from the SEM scale bars and direct image measurements, places the initial antifouling layer at approximately (234 ± 12) μm upon application. This value is corroborated by the width enclosed by the yellow dashed lines visible in each cross-sectional micrograph, which collectively demarcate the upper and lower boundaries of the antifouling film. However, after immersion intervals of 6, 12, and 24 months, the thickness of the coating declines significantly, reflecting not only physical abrasion and chemical attack but also substantial solubilization and leaching of the water-soluble resin matrix into the surrounding ASW.

The reduced layer thickness is primarily attributed to the dissolution of the resin component over time in the aquatic environment. The depletion of matrix material facilitates the development of a uniform protective barrier, ultimately leading to uninterrupted coverage that prevents the steel substrate from direct exposure to seawater, hence decelerating both corrosion and biofouling processes. Thus, the observed development of a cohesive protective coating during subsequent immersion phases underscores a significant benefit in the longevity of water-soluble resin-based antifouling systems. Additional material optimization is necessary to achieve sustained barrier performance under severe marine environments.

3.3. Field Immersion Tests

The presented images in Figure 10 offers a systematic visual documentation of EH36 steel samples, both uncoated and coated with an innovative biocide-free antifouling system, following immersion in natural seawater environments at Thessaloniki and Heraklion for periods ranging from zero to twenty-four months. Three columns are used to organize the visual data: the first displays uncoated controls, while the second and third show samples protected by the antifouling coating and exposed at the two distinct geographic locations. At each time point, representative digital photographs, each including a 10 cm scale bar, allow for side-by-side qualitative assessment of macroscopic corrosion and fouling phenomena as a function of both immersion duration and protective strategy.

At the outset (0 months), all samples—uncoated and coated alike—display a uniform, intact appearance, free of visible corrosion products or biological deposits. The uncoated steel is characterized by a clean metallic sheen, with no oxidation or fouling evident. Similarly, the coated samples from both Thessaloniki and Heraklion present a homogeneous reddish surface attributed to the biocide-free paint layer, with no signs of degradation, microfouling, or discoloration. This initial state establishes a clear baseline for subsequent comparisons, confirming the uniform application and integrity of the protective coating prior to exposure.

After one month of immersion (early exposure), pronounced differentiation emerges between the uncoated and coated samples. The uncoated steel rapidly develops a heterogeneous layer of rust-colored corrosion products and incipient biofilm, with the formation of distinct brown to orange patches indicative of active, localized oxidation and initial microbial colonization. By contrast, both coated samples, regardless of deployment site, retain a largely unaltered surface morphology, with only minimal discoloration and virtually no macroscopic fouling or corrosion observed. These images visually corroborate the early-stage efficacy of the antifouling barrier in arresting marine corrosion and inhibiting biofouling.

Upon prolonging the immersion to 6 and 12 months (intermediate exposure), the uncoated steel undergoes significant visual degradation. The surface is almost entirely obscured by thick layers of corrosion scale interspersed with marine biofouling—likely a mix of algae, microorganisms, and possibly larger sessile invertebrates—resulting in a rough, darkened appearance that signifies advanced material deterioration. In sharp contrast, the coated samples for both Thessaloniki and Heraklion continue to display substantial surface preservation. While some areas exhibit isolated fouling spots and minor staining, the underlying red coating remains readily apparent, and the degree of corrosion or bioaccumulation is conspicuously lower than that of the unprotected controls.

At the conclusion of the 24-month exposure period (long-term exposure), severe corrosion progression is evident in the uncoated sample. The steel is almost completely masked by thick, dark corrosion layers and dense biological encrustations, presenting a visually degraded and heavily pitted surface. In contrast, the coated specimens, though showing evidence of moderate biofouling and occasional local coating wear—more pronounced in Heraklion than Thessaloniki—still maintain significant portions of their protective film. The overall surface integrity remains intact, with the majority of the red coating discernible amidst isolated fouling clusters. This long-term preservation underscores the durability and sustained barrier function of the biocide-free coating, even under continuous, aggressive marine exposure.

A comparative examination of the photographic record highlights the superior protection afforded by the biocide-free coating relative to bare steel at all time points and both locations. The consistent retention of coating color and minimization of corrosion and fouling, particularly in the first year, demonstrate the effectiveness of the protective strategy. Notably, samples from Heraklion display somewhat more pronounced surface changes and fouling in the second year than their Thessaloniki counterparts, possibly reflecting localized differences in seawater composition, biological activity, or hydrodynamic regime. Nonetheless, in both marine contexts, the antifouling coating markedly mitigates steel degradation and bioaccumulation, validating its potential as an environmentally friendly solution for long-term marine steel protection.

The presented

Table 5. Qualitative evaluation of colonization species based on the duration of exposure for coated and untreated naval steel specimens.

			Uncoated Sample					Coated Sample
Locations		Fouling Organisms	0	1 m	6 m	12 m	24 m	0	1 m	6 m	12 m	24 m
Thessaloniki	Micro-fouling	Biofilm	Χ	√	√	√	√	Χ	Χ	√	√	√
		Fungi and Protozoa	Χ	√	√	√	√	Χ	Χ	√	√	√
		Ultra spores	Χ	√	√	√	√	Χ	Χ	Χ	√	√
		Brown algae	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Multicellular algae	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Tunicate	Χ	√	√	√	√	Χ	Χ	Χ	Χ	√
	Macro-fouling	Macroalgae	Χ	√	√	√	√	Χ	Χ	Χ	Χ	√
		Bryozoans	Χ	√	√	√	√	Χ	Χ	Χ	Χ	Χ
		Solitary anemones	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Hydrates	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Tubeworms	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Sedentary polychaetes	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Ascidians	Χ	Χ	√	Χ	Χ	Χ	Χ	Χ	Χ	Χ
		Sponges	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Mussels	Χ	Χ	Χ	Χ	√	Χ	Χ	Χ	Χ	Χ
		Oysters	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Barnacles	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
			Uncoated Sample					Coated Sample
Locations		Fouling Organisms	0	1 m	6 m	12 m	24 m	0	1 m	6 m	12 m	24 m
Heraklion	Micro-fouling	Biofilm	Χ	√	√	√	√	Χ	Χ	√	√	√
		Fungi and Protozoa	Χ	√	√	√	√	Χ	Χ	√	√	√
		Ultra spores	Χ	√	√	√	√	Χ	Χ	Χ	√	√
		Brown algae	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	√
		Multicellular algae	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Tunicate	Χ	√	√	√	√	Χ	Χ	Χ	Χ	√
	Macro-fouling	Macroalgae	Χ	√	√	√	√	Χ	Χ	Χ	Χ	√
		Bryozoans	Χ	√	√	√	√	Χ	Χ	Χ	Χ	√
		Solitary anemones	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Hydrates	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Tubeworms	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Sedentary polychaetes	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Ascidians	Χ	Χ	√	Χ	Χ	Χ	Χ	Χ	Χ	Χ
		Sponges	Χ	Χ	√	√	√	Χ	Χ	Χ	Χ	Χ
		Mussels	Χ	Χ	Χ	Χ	√	Χ	Χ	Χ	Χ	Χ
		Oysters	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ
		Barnacles	Χ	Χ	Χ	√	√	Χ	Χ	Χ	Χ	Χ

offers a comprehensive overview of the time-dependent qualitative colonization by various fouling organisms on both uncoated and coated naval steel samples, exposed in the coastal environments of Thessaloniki and Heraklion. The evaluation differentiates between microfouling (including biofilm, fungi, protozoa, and unicellular algae) and macrofouling organisms (such as macroalgae, bryozoans, tubeworms, and bivalves) across several discrete immersion periods, ranging from initial exposure (0 months) to a maximum duration of 24 months. Table 4 provides a qualitative assessment of colonization species according to the immersion duration, allowing the assessment of the biocide-free coating’s antifouling efficacy against a broad spectrum of marine colonizers. Table 5 features a color scheme that visually represents the degree of fouling, from red (severe fouling) to green (low fouling).

At the onset of immersion (1 month), both sites—Thessaloniki and Heraklion—exhibited rapid establishment of biofilm on uncoated specimens, accompanied by early signs of fungal, protozoal, and algal colonization. These early microfouling communities act as pioneer species, facilitating progressive ecological succession. In marked contrast, the coated samples display a pronounced absence or significant delay in the settlement of these primary colonizers, demonstrating the immediate antifouling effect of the biocide-free coating, which persists through the early months of exposure. Only biofilm is occasionally observed on coated surfaces, with all other microfouling groups notably absent.

As immersion time extends to 6, 12, and 24 months, the uncoated steel surfaces become progressively dominated by diverse and complex macrofouling assemblages, including macroalgae, bryozoans, tubeworms, ascidians, sponges, mussels, oysters, and barnacles. These organisms not only increase surface roughness and mass but also exacerbate corrosive processes and maintenance challenges. In stark contrast, the coated specimens generally maintain effective resistance to the majority of macrofouling taxa throughout the evaluation period. While occasional settlement of certain groups (such as macroalgae or solitary anemones) is recorded after extended immersion, the incidence and diversity of colonizing species remain substantially lower compared to the uncoated controls, particularly for hard-fouling invertebrates.

Comparative analysis between Thessaloniki and Heraklion reveals subtle site-specific differences in the sequence and intensity of species colonization, likely attributable to local environmental conditions and biodiversity. However, the overall trend is unambiguous: the biocide-free coating consistently delays or suppresses both the onset and progression of multi-taxa fouling communities relative to bare steel. This pronounced reduction in both micro- and macrofouling across multiple months and locations highlights the coating’s robust and broad-spectrum antifouling capability. In summary, the qualitative data strongly support the conclusion that the innovative coating provides durable, environmentally friendly protection against a wide range of biofouling pressures encountered in real-world marine settings.

Seasonal variations play a critical role in influencing both the composition of fouling species and the corrosion rates observed during the 24-month field exposure of EH36 naval steel in different marine environments. In coastal Mediterranean waters such as those at Thessaloniki and Heraklion, fluctuations in seawater temperature, salinity, and nutrient levels occur throughout the year (Table 2), driving changes in biological activity and ecological dynamics of fouling communities. Warmer months typically promote accelerated growth and settlement of microfouling organisms, including biofilms, fungi, protozoa, and algae, which constitute the initial conditioning layers that facilitate subsequent colonization by macrofouling taxa. Conversely, colder seasons tend to reduce metabolic activity, slowing fouling development and biofilm maturation. These seasonal biological cycles inherently mediate the temporal patterns of fouling community structure and density observed on naval steel surfaces during prolonged immersion.

Alongside biotic influences, abiotic factors driven by seasonal changes substantially affect the corrosion behavior of steel substrates exposed in situ. Temperature variations modulate the kinetics of electrochemical reactions responsible for corrosion, with higher temperatures during spring and summer generally accelerating corrosion rates due to enhanced ion mobility and microbial metabolism. Variations in salinity and dissolved oxygen concentration, often coupled with seasonal stratification and mixing events, influence both uniform and localized corrosion mechanisms. For instance, periods of hypoxia induced by organic matter decomposition during warmer months may exacerbate anaerobic microbial influenced corrosion (MIC), whereas cooler, oxygen-rich conditions in winter may favor more uniform corrosive attack. In this study, such seasonal fluctuations contributed to the observed oscillations in corrosion current densities and mass loss on uncoated steel panels, reflecting the dynamic interplay between environmental parameters and material degradation over time.

The integration of long-term field data over the full seasonal cycle further underscored the protective efficacy of the biocide-free nanocomposite coating under these variable conditions. Despite the intrinsic seasonal variation in fouling pressures and corrosiveness of the seawater, coated specimens consistently exhibited delayed biofouling succession and substantially lower corrosion rates compared to uncoated controls across all immersion intervals. While seasonal peaks in fouling intensity were noted, particularly during warmer months, the coating’s multifunctional barrier and antifouling properties mitigated both micro- and macro-organism settlement effectively throughout the exposure duration (Figure 11). Seasonal differences between the more eutrophic, variable Thessaloniki site and the oligotrophic, stable Heraklion site also influenced the extent and species composition of fouling communities, highlighting the necessity of site-specific evaluation for comprehensive understanding of coating performance in real-world marine environments.

The biological assessment of micro- and macro-fouling on EH36 naval steel coated with the biocide-free nanocomposite system reveals a robust antifouling efficacy throughout a prolonged 24-month field deployment, as illustrated in Figure 10. In both tested Mediterranean sites—Thessaloniki and Heraklion—the coated specimens consistently exhibited a substantial reduction in the establishment and maturation of microfouling communities, including biofilms, fungi, protozoa, and unicellular algae, especially during periods of peak biological activity driven by seasonal environmental fluctuations. In contrast to the rapid and dense colonization observed on uncoated control panels, the nanocomposite-coated surfaces displayed delayed and significantly attenuated biofilm formation. This effect is attributed to the coating’s integrated physicochemical mechanisms, primarily its modified surface energy, redox activity, and photocatalytic degradation of organic matter by embedded TiO₂ nanoparticles, which collectively disrupt the initial microbial adhesion processes—a prerequisite for subsequent macrofouling succession.

In the context of macrofouling pressures, the coated panels demonstrated persistent resistance to the settlement and growth of complex assemblages of macroalgae, bryozoans, tubeworms, bivalves, and other hard-fouling invertebrates, as visualized in Figure 10. Even during seasonal peaks in fouling intensity, particularly in the warmer months, the biocide-free nanocomposite barrier minimized surface roughness and maintained the majority of its protective film, effectively suppressing both the extent and diversity of macrofouling taxa compared to the heavily encrusted and corroded uncoated controls. These results corroborate the coating’s multi-modal antifouling function, arising from the synergy of electrical conductivity (polyaniline), photocatalytic activity (TiO₂), and mechanical reinforcement (Fe₃O₄-functionalized MWCNTs), which not only inhibits biotic settlement but also stabilizes the steel’s passive electrochemical state against corrosion. Overall, the biological data affirm that the developed nanocomposite coating offers durable, broad-spectrum protection in real marine environments, and dramatically mitigates the ecological and operational impacts of marine biofouling on naval steels.

4. Discussion

The results of this comprehensive study highlight the effectiveness of the novel biocide-free nanocomposite coating in providing long-term protection against both corrosion and marine biofouling on EH36 naval steel. The combination of laboratory and in situ immersion tests spanning up to 24 months demonstrated that the coating notably suppresses the electrochemical activity and colonization of marine organisms when compared with uncoated steel controls. This performance places the developed system at the forefront of current efforts to advance environmentally responsible surface protection for marine vessels.

The electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy, consistently indicated dramatic improvements in corrosion resistance for coated specimens. Corrosion current densities (I_corr) and corrosion rates (CR) of coated steel remained over an order of magnitude lower than those of uncoated samples throughout the experimental timeline, with corrosion protection efficiency (η) above 93% even after two years of continuous exposure. The observed shift in corrosion potential (E_corr) towards more noble values, coupled with flatter polarization curves and sustained high impedance modulus in Nyquist plots, demonstrate robust and persistent barrier function. These results strongly corroborate recent literature reporting similar behavior for nanocomposite and conductive polymer-based antifouling coatings, such as those incorporating polyaniline or titania phases, where long-term electrochemical stability and passivation have been attributed to improved interfacial properties and reduced ion permeability [6].

Field exposures in distinct marine environments—Thessaloniki and Heraklion—consistently substantiated laboratory findings, confirming real-world applicability. Visual inspection and digital image analysis of retrieved samples revealed that the biocide-free coating substantially reduced both the severity and diversity of fouling and corrosion products. Uncoated steel quickly succumbed to thick rust layers and intense macrofouling by organisms including barnacles, algae, and tubeworms, whereas the coated steel largely preserved its integrity and repelled colonization, especially in the first year. Although marginal increases in biofilm and some macrofouling were observed on coated samples after two years, the extent and impact were markedly less than on the bare substrate, supporting the coating’s durable performance.

Mechanistically, the superior efficacy of this antifouling system is credited to its multifunctional composition: the synergistic integration of conductive polyaniline nanorods, photocatalytic TiO₂ nanoparticles, and magnetite-decorated carbon nanotubes, all dispersed within a robust resin matrix. Each component serves a protective and functional role: PAni contributes via redox-mediated inhibition and electrical conductivity; TiO₂ introduces photocatalytic self-cleaning and microbial deterrence; Fe₃O₄@MWCNTs enhance barrier characteristics and provide potential sites for radical-driven inactivation of adhered organisms. This hybrid approach creates a dense, impermeable, and self-renewing nanocomposite film, in line with the most promising nanotechnology-enabled antifouling solutions emerging in recent research [13].

When assessing these findings in context with other recent advances in antifouling science, several trends and contrasts become apparent. Compared to traditional copper- or zinc-based biocidal paints, the present nanocomposite system eliminates the risk of toxic leachates and environmental persistence—key issues recently emphasized by regulators and marine scientists [10]. Conventional biocide-based coatings have been shown in multiple studies to provide only transient fouling protection and modest corrosion inhibition, their efficacy sharply declining as active substances are depleted and micropores develop in the matrix [22]. In contrast, the present biocide-free coating maintains high barrier resistances and low capacitance values over extended durations, attesting to minimal ion and water ingress.

The antifouling efficacy of the developed nanocomposite coating is principally attributed to its capacity to modify surface energy characteristics and exert electrochemical inhibition against biofouling organism settlement. By altering the surface energy, the coating creates an unfavorable environment for initial microbial adhesion and subsequent biofilm formation, thereby reducing the propensity for colonization by fouling organisms. This modification is achieved through the incorporation of functionalized nanomaterials such as polyaniline nanorods and titanium dioxide nanoparticles, which collectively alter surface wettability and reduce surface free energy, impeding the attachment of biological fouling species. Concurrently, the coating’s electrochemical properties contribute to antifouling by stabilizing the steel substrate’s passive state and inhibiting electrochemical reactions that facilitate microbial proliferation and corrosion-induced biofilm development. The conductive polyaniline domains provide redox-active sites that disrupt the electrochemical conditions conducive to fouling, while the photocatalytic activity of TiO₂ nanoparticles generates reactive oxygen species that degrade organic matter. Together, these mechanisms synergistically reduce both microbial adhesion and corrosion processes, establishing a durable, biocide-free barrier that effectively mitigates biofouling under prolonged marine exposure.

When compared with emerging eco-friendly systems, such as silicone-based fouling release coatings and hydrophilic/hydrophobic amphiphilic strategies, the current coating demonstrates advantages in both mechanical robustness and long-term stability. Silicone elastomers, though excellent at reducing initial organism adhesion, are known to exhibit mechanical degradation, eventual microplastic release, and susceptibility to damage in turbulent or abrasive conditions [19]. In the present study, the nanocomposite retained its functional performance even after two years, a level of resilience echoed by recent reports on polymer/nanoparticle hybrid coatings but rarely matched by soft, foul-release alternatives.

Biomimetic or naturally-derived coatings, including those utilizing marine peptides, polysaccharides, or plant-based extracts, offer nontoxicity and some anti-settlement effects but often fall short in durability and corrosion resistance, especially in the face of multi-species colonization or aggressive seawater chemistry. The dual functionality and persistence of the present nanocomposite system—resisting both corrosion and a broad spectrum of biofoulers—positions it ahead in terms of operational longevity and overall environmental benefit.

The qualitative and quantitative analyses performed in situ underscore the complex interplay between environmental conditions and coating performance. Notably, mild differences in performance between Thessaloniki and Heraklion could be linked to variations in local biofouling pressure, temperature, and seawater composition, emphasizing the importance of testing antifouling systems under diverse, real-world conditions. Nevertheless, in both environments, the developed coating consistently delayed or suppressed multi-taxa fouling communities that rapidly colonized bare steel, aligning closely with recent observations for field-tested nanostructured and non-biocidal coatings in Mediterranean settings [77].

A summary comparison with key antifouling coating types is presented in

Table 6. Comparison of major antifouling coating classes with respect to mechanism, protection, durability, and environmental performance.

Coating Type	Antifouling Mechanism	Corrosion Protection	Environmental Profile	Durability	Typical η (%) (2 Years)	Ref.
Biocide-Based (Cu, Zn, organotin)	Toxic leaching and biocidal action	Moderate	Toxic; leaching restrictions	Moderate–short-lived	70–85	[22]
Silicone Foul-Release	Low adhesion/friction	Low–moderate	Non-toxic, but microplastics risk	Moderate	80–88	[19]
Biomimetic/Bio-derived	Surface topography, natural extracts	Low–moderate	Fully green, biodegradable	Low–moderate	70–90	[10]
Polymeric Nanocomposite	Barrier, redox, photocatalytic, etc.	High	Non-toxic, no leaching, robust	High	93–99	[6], (this study)

.

Collectively, these findings reinforce the growing scientific consensus that advanced nanocomposite, biocide-free coatings offer a credible and sustainable alternative to conventional antifouling strategies. The present study not only validates the multi-year, real-world stability and protection offered by the developed system but also underscores the necessity for future work to address scaling, cost-effectiveness, and full life cycle environmental impacts. The continuous decline in protection efficiency observed after prolonged immersion, though minor, signals an area for further materials optimization—potentially through matrix reinforcement, further tuning of nanofiller content, or advanced self-healing chemistries, as suggested by recent polymer and surface science advances [78].

The proposed biocide-free nanocomposite antifouling paint exhibits significantly superior performance compared to commercial antifouling coatings, particularly those based on toxic biocidal agents such as copper, zinc, and organotin compounds (

Table 7. Comparison of commercial antifouling coatings with respect to composition, performance metrics, environmental profile, and durability.

Coating/Trade Name	Main Biocidal Composition	Key Performance Metrics	Environmental Profile	Typical Longevity
International Intersmooth 7465HS SPC	Copper oxide + ZnO + triazine herbicide	A total of 70–90% fouling protection efficiency (12–18 mo); increases fuel efficiency by ~6%	Moderate to high toxicity (Cu, Zn leaching, organics)	12–18 months
Jotun Sea Quantum Ultra	Copper oxide + zinc pyrithione	A total of 75–93% protection (1 yr, various routes); 7.5 g/m2/day Cu release	Cu leaching restricted in many ports	24 months
Hempel Hempasil X3 (Foul-release)	Siloxane (biocide-free, control)	A total of 60–75% protection (macro-fouling); needs silicone topcoat	Biocide-free, microplastics risk	18–24 months
Selektope™-containing paints (e.g., Chugoku SEA PREMIA)	Copper oxide + Selektope™ (Medetomidine)	A total of 70–90% (per barnacle settlement tests); up to 80% lower Cu leaching	Reduced copper load (but not zero)	12–20 months
International Intercept 8500LPP	Copper oxide + rosin, self-polishing copolymer	A total of 60–80% biocidal efficiency (in tropical trials); Tin-free	Cu leaching, organics; lower than organotin	12 months
Tributyltin (TBT)-based coatings (historic)	Tributyltin oxide, Cu2O	A total of >95% (up to 5 years, now banned); extremely effective	Extremely toxic; banned for global use	60+ months (banned)

). Unlike conventional systems, which maintain only moderate corrosion protection efficiencies that rapidly decline due to biocide depletion and matrix degradation, the innovative nanocomposite coating consistently achieves corrosion protection efficiencies exceeding 93% and demonstrates robust barrier properties over two years of continuous marine exposure. Electrochemical and field analyses reveal that the coated specimens experience dramatically reduced corrosion rates and suppressed biofouling colonization, thereby preserving steel integrity and minimizing maintenance requirements. Moreover, the environmental profile of this nanocomposite system is notably favorable, as it prevents the release of hazardous substances and maintains long-term mechanical durability, representing a sustainable and high-performance alternative to standard commercial antifouling technologies in naval applications.

The potential galvanic effects arising between Fe₃O₄ nanoparticles and EH36 naval steel represent a critical consideration in the design of nanocomposite marine coatings, given the electrochemical differences between the magnetite phase and the steel substrate. Fe₃O₄, possessing distinct electrochemical properties and electrical conductivity, could theoretically act as cathodic sites when in direct electrical contact with steel, thereby accelerating localized anodic dissolution of the steel through galvanic coupling. Such an effect would undermine the corrosion resistance of the substrate and compromise the coating’s protective function. However, the innovative nanocomposite coating design effectively mitigates this risk by embedding Fe₃O₄-functionalized multiwalled carbon nanotubes (MWCNTs) within an electrically insulating, robust resin matrix that spatially isolates the magnetite nanoparticles from direct contact with the steel surface. Additionally, the uniform dispersion of these nanomaterials, combined with the barrier properties imparted by the polymeric matrix and the conductive polyaniline-TiO₂ network, prevents the establishment of continuous galvanic pathways. This architectural strategy restricts electron flow and ion transport necessary for galvanic corrosion processes, thus preserving the substrate integrity while leveraging the mechanical reinforcement and antifouling benefits conferred by Fe₃O₄@MWCNTs. Consequently, the coating sustains its corrosion inhibition efficacy by balancing multifunctionality and electrochemical compatibility, addressing a key challenge in the integration of conductive and magnetic nanomaterials in marine protective systems.

The cost implications of deploying the novel biocide-free nanocomposite coating system for marine steel protection must be critically evaluated in comparison to conventional marine coatings, which predominantly rely on toxic biocides such as copper and zinc compounds. While the initial material costs of advanced nanomaterials—including polyaniline nanorods, titanium dioxide nanoparticles, and Fe₃O₄-functionalized multiwalled carbon nanotubes—are generally higher than those of traditional coating constituents, the extended service life and superior corrosion and biofouling resistance demonstrated in both laboratory and in situ conditions confer potential long-term economic benefits. Specifically, the nanocomposite’s capacity to maintain a corrosion protection efficiency exceeding 93% over 24 months translates into markedly reduced maintenance frequency, decreased dry-docking intervals, and lower fuel consumption due to minimized hull roughness. These operational savings can offset the higher upfront investment, particularly over the typical lifespan of naval and commercial vessels. Furthermore, elimination of toxic leachates mitigates environmental compliance costs and potential liabilities associated with restrictions on biocidal substances, thereby enhancing the cost-effectiveness of the nanocomposite system from a regulatory and sustainability perspective.

Scalability considerations for the nanocomposite coating involve both manufacturing and application challenges relative to standard marine coatings. The synthesis and integration of multifunctional nanomaterials require precise control over particle size, dispersion, and functionalization to ensure reproducible coating performance—a complexity not generally present with conventional formulations. However, the processes employed—including oxidative polymerization for polyaniline, co-precipitation for Fe₃O₄@MWCNTs, and ultrasonic dispersion techniques—are amenable to scale-up using established industrial chemical engineering methods. The coating application protocols, involving airless spraying of nanoparticle-resin mixtures without the need for primers, align with existing marine coating application infrastructure, facilitating adoption. Key hurdles remain in optimizing large-scale uniform dispersion, maintaining batch-to-batch consistency, and controlling production costs associated with high-purity nanomaterials. Ongoing advancements in nanomaterial production technologies and cost reductions in carbon nanotube manufacture bode well for future scalability. Overall, while the nanocomposite system presents higher complexity than conventional coatings, its demonstrated durability and multifunctionality hold promise for economically viable scale-up and widespread industrial implementation in marine protection applications. The cost implications of implementing the novel biocide-free nanocomposite coating system for marine steel protection, relative to standard marine coatings, reflect a trade-off between higher initial material expenses and substantial long-term savings. The advanced components, including polyaniline nanorods, titanium dioxide nanoparticles, and Fe₃O₄-functionalized multiwalled carbon nanotubes, inherently increase formulation costs compared to traditional biocidal paints based on copper or zinc compounds. However, the nanocomposite’s demonstrated durability, with corrosion protection efficiencies exceeding 93% across 24 months and significant antifouling effects, can reduce maintenance frequency, dry-docking intervals, and fuel consumption due to less biofouling-induced drag. These operational savings may offset higher upfront costs over the service life of naval and commercial vessels. Additionally, the environmentally benign profile of the biocide-free system mitigates regulatory compliance costs and potential environmental liabilities, enhancing its overall economic viability in a regulatory landscape increasingly averse to toxic antifouling agents.

Regarding scalability, the synthesis and production of the multifunctional nanocomposite coating pose challenges beyond those encountered with conventional marine paints, which typically involve simpler formulations. Precise control is necessary throughout the preparation of polyaniline nanorods, TiO₂ nanoparticles, and Fe₃O₄-functionalized carbon nanotubes to ensure uniform dispersion, functionalization, and batch consistency, which are crucial for coating performance. Nonetheless, the employed chemical routes—oxidative polymerization, co-precipitation, and ultrasonication—are adaptable to industrial scale-up processes within current chemical manufacturing frameworks. Application of the coating via standard brush or spin-coating further facilitates integration within established marine coating workflows. While production costs associated with high-purity nanomaterials and process complexity remain higher, ongoing advances in nanomaterial fabrication and economies of scale are expected to improve cost efficiency. Overall, despite increased complexity compared to standard coatings, the system’s robust multifunctionality and long-term benefits present a promising path toward industrial scalability and commercial deployment in sustainable marine protection.

While the environmental compatibility of the developed biocide-free nanocomposite coating is a key advantage emphasized in this study, it is important to acknowledge the absence of direct leachate analysis and ecotoxicity testing data as a limitation. Although the coating composition excludes traditional toxic biocides and incorporates non-leaching inorganic nanomaterials such as polyaniline, TiO₂, and Fe₃O₄-functionalized MWCNTs—recognized for their inert or benign environmental profiles—comprehensive evaluation of potential nanoparticle release, transformation, and ecological impacts under prolonged marine exposure remains unexplored. The lack of empirical leachate quantification and standardized ecotoxicological assessments means that possible adverse effects on non-target aquatic organisms and marine ecosystems cannot be fully ruled out at this stage. Future research integrating advanced analytical techniques to monitor nanoparticle dispersion and systematic bioassays to assess acute and chronic toxicities would be essential to robustly validate the environmental safety claims and support regulatory approval for large-scale deployment in marine environments. This gap underscores the necessity for holistic life-cycle impact assessments alongside performance evaluations to ensure sustainable adoption of nanotechnology-enabled antifouling coatings.

In conclusion, the developed biocide-free nanocomposite coating provides an effective, durable, and environmentally sustainable strategy for mitigating both corrosion and biofouling on marine steels. Its superior performance over conventional and alternative eco-friendly coatings, confirmed by rigorous laboratory and long-term field data, supports its candidacy for broader deployment in naval and commercial marine contexts.

5. Conclusions

This study systematically demonstrated the outstanding long-term anti-corrosion and anti-fouling performance of an innovative biocide-free nanocomposite coating applied to EH36 naval steel, evaluated through both laboratory trials and in situ marine exposures. The novel coating, comprising a synergistic combination of polyaniline nanorods, TiO₂ nanoparticles, and Fe₃O₄-decorated multiwalled carbon nanotubes in a robust resin matrix, afforded a substantial protective barrier that effectively inhibited both electrochemical corrosion processes and the settlement of marine biofouling organisms.

Across immersion periods extending to 24 months, electrochemical assessments revealed that coated samples consistently displayed markedly reduced corrosion current densities and corrosion rates, with protection efficiencies exceeding 93% even after prolonged exposure. Open circuit potential measurements and electrochemical impedance spectroscopy confirmed the sustained integrity of the barrier, as evidenced by more noble corrosion potentials, elevated charge transfer resistances, and depressed capacitance signatures compared to uncoated steel. Field deployments in both Thessaloniki and Heraklion substantiated these findings under real-world conditions, where visual inspection and qualitative biological surveys documented significantly lower levels of corrosion products and reduced micro- and macrofouling colonization on coated specimens.

The numerical and statistical analysis of the electrochemical data strongly substantiates the designation of “more noble corrosion potentials” for EH36 steel specimens coated with the biocide-free antifouling system. According to Table 3, the corrosion potential (E_corr) of uncoated steel measured after immersion in artificial seawater was −0.697 V, reflecting a highly active and susceptible surface toward anodic dissolution. In marked contrast, the coated samples exhibited significantly less negative corrosion potentials, with E_corr shifting to −0.360 V after 1 month and remaining consistently more noble over extended immersion periods (−0.481 V at 6 months, −0.512 V at 12 months, and −0.637 V at 24 months). Statistical analysis performed using one-way ANOVA revealed that these shifts in E_corr between coated and uncoated groups are highly significant (p < 0.001), confirming that the coating induces a robust and reproducible enhancement in the electrochemical stability of the steel surface. This sustained elevation toward less negative, more noble potentials demonstrates the coating’s efficacy in passivating the substrate and decreasing its thermodynamic tendency toward corrosion, thereby supporting the expression “more noble corrosion potentials” with rigorous quantitative and statistical evidence.

Comparison with conventional biocidal coatings and alternative eco-friendly strategies underscores the robustness and multi-functionality of the present nanocomposite system. Unlike traditional coatings reliant on toxic leaching or predominantly physical fouling release, the biocide-free coating here provided both chemical passivation and antifouling efficacy, without adverse environmental impact or significant material degradation over time. The minimal decrease in protective performance observed with increasing immersion duration suggests excellent durability and highlights the coating’s suitability for long-term marine deployment.

In summary, this work provides strong evidence that advanced, nanotechnology-enabled, biocide-free coatings can deliver reliable and persistent protection against the dual threats of corrosion and biofouling in harsh saline environments. These findings support the adoption of such coatings as a sustainable alternative in the marine and naval industries, aligning with current regulatory directives and environmental stewardship objectives. Further optimization focused on scaling, application methods, and cost-effectiveness will be essential for widespread industrial adoption, but the present results signify a critical advance toward greener and more durable marine protection technologies.

6. Patents

There is one patent resulting from the work reported in this manuscript: WO2024224120A1, Electrically anisotropic antifouling coatings, BFP Advanced Technologies, 2024.