Achieving superior anti-corrosion performance with spherical organic additives and synergistic barrier passivation mechanisms

Abstract

Developing effective additive materials for high-performance anti-corrosion coatings remains a significant challenge. While organic materials are increasingly recognized for their inherent properties, versatility, and ease of processing, their use as filler additives for corrosion protection is still in the early stages. In this study, we present a novel spherical organic material, termed NTAB, which exhibits enhanced electrochemical activity, superior molecular stability, and optimized electronic properties. NTAB is synthesized through a simple condensation reaction. The resulting NTAB organic material serves as an active filler in epoxy resin (EP), forming an anti-corrosion NTAB/EP coating. This coating demonstrates outstanding anti-corrosion performance, with an exceptionally low corrosion rate of 9.9 × 10⁻⁶ mm per annum and a corrosion inhibition efficiency of 99.70%. Notably, the NTAB/EP coating achieves an impressive |Z|_{0.01 Hz} value of 8.94 × 10⁹ Ω cm² after an extended immersion period of 154 days in a 3.5 wt% NaCl solution, significantly outperforming previously reported anti-corrosion coatings. Real-world evaluations in seawater environments further confirm the coating's durability, with no visible corrosion even near scratches. This remarkable longevity is attributed to the synergistic effects of the NTAB organic material, combining efficient physical barrier properties with chemical passivation, offering valuable insights for the development of anti-corrosion coatings in harsh environments.

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Jun Yang

Jun Yang received his PhD degree from Nanjing Tech University (P.R. China) in 2018. He worked as an exchange student at Nanyang Technological University from 2014 to 2018. He is now working as a lecturer in Jiangsu University of Science and Technology. He focuses on functional materials for energy and environment related applications, such as sodium batteries, aqueous batteries, capacitive deionization and organic anticorrosive coating repair. He has published more than 120 papers with over 5000 total citations.

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1. Introduction

Metals are widely used across various industries and everyday applications due to their exceptional mechanical strength, durability, and favorable chemical properties.^1–3 They are fundamental to modern technological advancements, playing a crucial role in the construction of large infrastructures, transportation vehicles, complex machinery, and consumer electronics. However, the persistent issue of metal corrosion presents a significant challenge in both engineering and industrial contexts, leading to considerable economic and safety concerns. Corrosion, a natural yet troublesome electrochemical process, gradually degrades metal surfaces through chemical or electrochemical reactions with their environment, posing serious risks to human safety, structural integrity, and environmental health.^4–6 The harmful effects of corrosion compromise the durability and functionality of metal components, resulting in costly maintenance, frequent replacements, and operational downtime. These challenges underscore the urgent need for effective corrosion management strategies to reduce economic losses, ensure safety, and promote sustainability.^7–9 Tackling corrosion not only extends the lifespan of metal structures but also contributes to resource conservation and environmental protection, making it a crucial area of research and innovation in materials science and engineering.^10,11

To effectively mitigate corrosion, it is crucial to implement robust and innovative protective measures. One of the most widely used and practical methods to prevent metal corrosion is the application of protective coatings,^12,13 which serve as physical barriers against corrosive agents such as moisture, oxygen, and salts. However, conventional coatings often face inherent limitations, including limited durability, susceptibility to mechanical damage, and poor adhesion, which can undermine their long-term effectiveness in preventing corrosion. These challenges highlight the need for advanced coating formulations with improved performance and resilience. A promising strategy involves integrating specialized additives into coating matrices, significantly enhancing barrier properties and strengthening defenses against corrosion.¹⁴ These additives not only reduce the infiltration of corrosive agents but also offer dual functionality by chemically reacting with corrosive species and impeding electrochemical reactions at the metal–coating interface. With these factors in mind, the primary challenge in developing next-generation protective coatings lies in identifying, designing, and utilizing innovative filler additives that provide high-efficiency corrosion protection while maintaining compatibility with the coating matrix.¹⁵

The development of advanced filler additives for protective coatings has increasingly shifted toward organic compounds due to their natural abundance, structural versatility, and processability.^16,17 Unlike conventional inorganic additives such as chromates and phosphates, which raise environmental and toxicity concerns, organic fillers offer a sustainable alternative with tunable molecular architectures for tailored functionality.^18,19 Their superior performance stems from distinctive structural features, particularly electron-rich conjugated configurations with heteroatoms (S, N, and O), which enable strong chemisorption onto metal surfaces to form robust protective barriers.^20,21 Notably, organics like polythiophene (PTh),^22,23 polyaniline (PANi),^24,25 covalent organic frameworks (COFs),^26,27 and imine/anthraquinone compounds^28,29 have emerged as promising candidates due to their remarkable corrosion inhibition via redox activity and interfacial passivation mechanisms. Their heteroatom-rich backbones further enhance performance by acting as molecular anchors that block reactive sites and suppress electrochemical degradation.³⁰ However, key challenges remain, including limited long-term stability and corrosion inhibition under harsh conditions, along with insufficient dispersion within resin matrices. Ongoing research is vital for augmenting electrochemical activity and refining the electronic structure of organics, as well as for seamlessly incorporating them into coatings to fully amplify their capacity to improve the performance and durability of metals in a variety of harsh environments.³¹

In this work, we introduce a novel spherical organic compound, designated as NTAB, synthesized through a condensation reaction, which is characterized by its enhanced electrochemical activity, remarkable molecular stability, and optimized electronic properties. The NTAB organic material has been incorporated as an additive within an epoxy resin (EP) matrix to engineer a novel anti-corrosion NTAB/EP coating. Comprehensive electrochemical analyses demonstrate that the NTAB/EP coating possesses exceptional anticorrosive properties, including a remarkably low corrosion rate of 9.9 × 10⁻⁶ mm per annum, a high corrosion inhibition efficiency of 99.70%, and a substantial impedance modulus (|Z|_{0.01 Hz}) of 8.94 × 10⁹ Ω cm² even after 154 days of immersion in a 3.5 wt% NaCl solution, significantly exceeding the performance of previously reported coatings. To assess its practical applicability, the anti-corrosion coating has undergone additional testing in natural seawater environments, where it exhibits exceptional anticorrosive ability, showing no visible corrosion even near intentionally inflicted scratches. Further validation using the scanning vibrating electrode technique (SVET) and corrosion product analysis confirms the coating's long-term efficacy and exceptional resistance, attributed to its synergistic physical barrier and chemical passivation mechanisms, which collectively establish a durable and multi-layered defense against corrosion. The findings highlight the potential of NTAB/EP coating as a groundbreaking solution for advanced corrosion protection in demanding environments.

2. Results and discussion

The novel NTAB organic compound was synthesized via a condensation reaction using 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTD) and 3,3′-diaminobenzidine (DAB) as precursors, as illustrated in Fig. 1a. Scanning electron microscopy (SEM) images of the synthesized NTAB organic material reveal a well-defined spherical morphology (Fig. 1d), in contrast to the irregular forms of the NTD and DAB precursors (Fig. 1b and c), indicating a successful transformation into a more uniform and organized spherical structure during synthesis. Dynamic light scattering (DLS) analysis was performed to statistically validate the particle size distribution, while the formation mechanism of the spherical structure is discussed in detail in Fig. S1.† The X-ray diffraction (XRD) pattern of the organic material exhibits a prominent diffraction peak around 25.8° (Fig. 1e), suggesting strong π–π stacking interactions that enhance the molecular stability of the NTAB organic (Fig. 1g). Moreover, partial density of states (PDOS) analysis of the NTAB molecule (Fig. 1f) reveals overlapping hybridized orbitals near the Fermi level, highlighting its enhanced electrochemical activity. Upon condensation of the NTD and DAB precursors to form the NTAB molecule, there is a significant increase in conjugation, with isosurface rings extending across the entire molecular framework, as shown in the π-electron localization function (ELF-π) illustrated in Fig. 1h, signifying the establishment of a robust molecular backbone.

Fig. 1 (a) A schematic representation illustrating the synthetic pathway for microsphere-like NTAB derived from the NTD and DAB precursors. SEM images of (b) NTD, (c) DAB and (d) NTAB. (e) XRD pattern and (g) the corresponding structural diagram of the NTAB organic. (f) PDOS pattern of the NTAB molecule. (h) ELF-π plots of NTD, DAB and NTAB molecules.

Elemental distribution mapping images confirm a uniform distribution of carbon, nitrogen, and oxygen atoms throughout the entire NTAB organic framework, as clearly shown in Fig. 2a. The solid-state ¹³C NMR spectrum provides detailed insights into the chemical structure of the NTAB organic material. As shown in Fig. 2b, three distinct peaks at chemical shifts of 130.5, 125.7, and 115.9 ppm correspond to carbon atoms in the C=C/C–C configurations within the naphthalene and biphenyl rings. Additionally, two peaks at 158.5 ppm are attributed to carbon atoms in the C=O bond, while a peak at 143.2 ppm signifies carbon atoms in the C=N bond. The Fourier transform infrared (FT-IR) spectrum of NTAB organic is presented in Fig. S2.† Characteristic absorption peaks are observed at 1586 and 1630 cm⁻¹, corresponding to the stretching vibrations of C=N and C=O bonds of the NTAB organic, respectively. Besides, the characteristic absorption peaks at 1384 and 1350 cm⁻¹ are assigned to the stretching vibrations C–N and C–C bonds of the NTAB organic. To further investigate the molecular structure and bonding composition, X-ray photoelectron spectroscopy (XPS) was employed. The high-resolution C 1s spectrum (Fig. 2d) reveals five distinct subpeaks, assignable to C=C (285.2 eV), C=N (285.6 eV), C–N (286.2 eV), C=O (288.8 eV), and π–π interactions (291.7 eV). The presence of stable π–π interactions is further supported by reduced density gradient (RDG) simulation (Fig. 2c), where a green blade is observed in the region of −0.02 to 0.00 a.u. of the (±ρ) symbol.^32,33 Additional confirmation of the C–N and C=N bonds within the NTAB organic material is provided by the high-resolution N 1s spectrum (Fig. 2e), which displays two subpeaks at approximately 400.2 and 398.2 eV. Similarly, the high-resolution O 1s spectrum (Fig. 2f) exhibits a prominent peak at 531.1 eV, corresponding to the C=O bond. These comprehensive analyses confirm the well-defined chemical structure and bonding interactions within the NTAB organic material.

Fig. 2 (a) Elemental distribution mapping images, (b) solid-state ¹³C NMR spectrum, and (c) plots of RDG vs. sign (λ₂)ρ of the NTAB organic. High-resolution (d) C 1s, (e) N 1s and (f) O 1s XPS spectra of the NTAB organic.

The thermal stability and electronic properties of the NTAB organic material were further investigated. Thermogravimetric analysis (TGA) under an inert atmosphere, presented in Fig. 3a, reveals that the NTAB organic material, with a robust molecular backbone, retains approximately 83% of its weight even at 600 °C. The observed mass loss profile reveals a multi-stage decomposition process: the initial weight reduction (50–250 °C) can be attributed to the volatilization of residual solvents and degradation of thermally labile terminal groups, while subsequent decomposition (250–400 °C) likely involves cleavage of unreacted amino groups, accompanied by the release of small molecules such as H₂O and NH₃. The high-temperature stability (>400 °C) reflects the intrinsic robustness of the core molecular architecture of the NTAB organic material, where thermal decomposition requires breaking of strong covalent bonds in the aromatic backbone. In contrast, the NTD and DAB precursors decompose violently around 400 °C, with almost total mass loss. Beyond their robust structural integrity, the NTAB organic material exhibits exceptional electronic properties. A smaller optical energy gap (E_g) is known to correlate with enhanced electrochemical properties. As shown in Fig. 3b, the E_g value of the NTAB organic material is remarkably narrow at only 1.61 eV, significantly lower than the E_g values of the two precursors (2.08 and 2.93 eV), highlighting the enhanced electrochemical activity of the NTAB organic material. Fig. 3c compares the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for NTD, DAB, and NTAB organic (see Table S1† for additional details). The synthesized NTAB organic exhibits the narrowest HOMO–LUMO energy gap at approximately ∼2.56 eV (Fig. 3d). This reduced energy gap enhances molecular polarization, promoting electron delocalization across the conjugated NTAB organic material framework. The exceptional backbone strength and improved electrical properties of NTAB organic material can be attributed to the extended π-conjugation system, which demonstrates continuous electron delocalization across the entire molecular framework. The enhanced electrochemical activity is supported by the narrow band gap and reduced HOMO–LUMO energy gap, facilitating efficient electron transfer and molecular polarization. These characteristics arise from the orbital hybridization near the Fermi level, where overlapping hybridized orbitals contribute to the superior redox activity and charge transport capabilities of the NTAB organic material. Therefore, the condensation reaction between NTD and DAB precursors effectively creates the highly conjugated configuration of the NTAB organic material, leading to improved electron delocalization and structural stability compared to the individual precursors.

Fig. 3 (a) TGA curves obtained under an inert atmosphere, (b) UV-vis absorption spectra, (c) HOMO–LUMO distributions and (d) the corresponding LUMO–HOMO energy levels for the NTD, DAB and NTAB molecules.

Fig. 4a illustrates the fabrication of the NTAB/EP coating, achieved by uniformly applying an epoxy coating (EP) containing spherical NTAB organic material onto a Q235 steel substrate via spin coating. To ensure the homogeneity of the NTAB dispersion in the EP matrix, we systematically evaluated the dispersion stability through experimental measurements, with the corresponding results presented in Fig. S3 and S4.† For comparison, bare Q235 steel, pure EP coating, and NTAB/EP coating were subsequently immersed in a 3.5 wt% NaCl solution at ambient temperature. Fig. 4b delineates the open circuit potential (OCP) curves for bare Q235 steel, pure EP, and NTAB/EP coatings. Obviously, the NTAB/EP coating stands out with an OCP value of −0.06 V, much higher than the −0.15 V observed for the pure EP coating and the −0.45 V for the bare Q235. The reduced corrosion potential exhibited by the NTAB/EP coating signifies its enhanced capacity to withstand degradation when subjected to corrosive agents. To further substantiate these findings, the OCP values were monitored over a five-week immersion period, as illustrated in Fig. 4c. The results reveal a concerning decline in the OCP values of both bare Q235 and pure EP coatings during the immersion period, indicating increased corrosion activity and vulnerability to environmental factors. In stark contrast, the NTAB/EP coating demonstrates remarkable stability, maintaining positive OCP values throughout the five-week immersion period. This stability underscores the coating's long-lasting protective capabilities against corrosion.^34–36

Fig. 4 (a) A fabrication diagram of the NTAB/EP coating onto Q235 steel through spin coating. (b) OCP curves for Q235, pure EP and NTAB/EP coatings. (c) The corresponding OCP evolutions over different immersion durations.

Fig. 5a presents the polarization curves analyzed using Tafel extrapolation to determine the corrosion potential (E_corr) and corrosion current density (I_corr). The E_corr provides insight into the thermodynamic stability of the coating; a more positive E_corr value indicates a more stable coating, which is less prone to initiating corrosion. The NTAB/EP coating manifests the highest E_corr value of −0.33 V, as observed in Fig. 5b. This positive shift in E_corr signifies that the NTAB/EP coating offers enhanced protection against corrosion compared to both bare Q235 steel and pure EP coating. The corrosion current density is another essential parameter that reflects the kinetics of the corrosion process. A diminished I_corr value correlates with a reduced rate of metal dissolution, indicating stronger corrosion resistance.^37,38 Notably, the NTAB/EP coating demonstrates a minimal I_corr value of 8.5 × 10⁻¹⁰ A cm⁻², which is significantly lower than that of both bare Q235 steel and pure EP coating. Furthermore, the corrosion rate (V_corr), which quantifies the speed of corrosion, was calculated for each coating (Table S2†). The NTAB/EP coating showcases an exceptionally low V_corr value of 9.9 × 10⁻⁶ mm per annum, further emphasizing its superior performance in protecting against corrosion. Through quantitative analysis, the NTAB/EP coating (80 μm thickness) demonstrates a high corrosion inhibition efficiency of 99.70%. Even at reduced thicknesses of 30 and 50 μm, the NTAB/EP coatings maintain remarkably high corrosion inhibition efficiencies of 99.10% and 99.32% (Fig. S5, S6 and Table S3†), respectively. These performance metrics significantly outperform those of other reported anti-corrosion coatings, such as Phen/HDMIC (73.80%, 80 μm), PPZ/PMMA (81.38%, 80 μm), BTA/MoS₂/HAp/ZIF8 (60.28%, 80 μm), La + CNT (82.29%, 50 μm), and ECO (96.80%, 30 μm) coatings, as depicted in Fig. 5c.^39–43 Additional details on the NTAB/EP coating with varying amounts of NTAB organic material are provided in Fig. S7 and Table S4.†

Fig. 5 (a) Polarization curves and (b) the calculated E_corr, I_corr and V_corr values for Q235, pure EP and NTAB/EP coatings. (c) A comparative diagram illustrating the corrosion inhibition efficiency of the NTAB/EP coatings with varying thicknesses (30, 50, and 80 μm) alongside other reported anti-corrosion coatings within similar thickness ranges.

Nyquist and Bode plots, derived from electrochemical impedance spectroscopy (EIS) data using an equivalent circuit model (Fig. S8†), are powerful tools for evaluating the corrosion resistance of coatings.^44–46Fig. 6a–f present the Nyquist and Bode plots, along with the corresponding phase angle curves, for pure EP coatings (Fig. 6a–c) and NTAB/EP coatings (Fig. 6d–f) after 1 day and 56 days of immersion in a 3.5 wt% NaCl solution. A higher coating resistance (R_c), reflected by a larger semicircle on the Nyquist plot, indicates better protection.^47,48 A higher impedance modulus at 0.01 Hz (|Z|_{0.01 Hz}) in the Bode plot suggests the coating's effectiveness in preventing corrosion reactions.^49,50 The phase angle at 10 Hz (θ_{10 Hz}) reveals the coating's capacitive behavior.⁵¹ A phase angle closer to 90° implies effective insulation, whereas a lower angle indicates potential degradation. Fig. 6g displays that the NTAB/EP coating achieves outstanding values of R_c (1.74 × 10¹¹ Ω cm²), |Z|_{0.01 Hz} (1.44 × 10¹¹ Ω cm²), and θ_{10 Hz} (89.92°), far surpassing those of the pure EP coating. Notably, after 56 days of immersion, the NTAB/EP coating maintains high values of R_c (4.52 × 10¹⁰ Ω cm²), |Z|_{0.01 Hz} (3.56 × 10¹⁰ Ω cm²), and θ_{10 Hz} (88.98°), confirming its excellent long-term barrier and anticorrosive properties. The breakpoint frequency (f_b), at a phase angle of −45° helps evaluate coating delamination.⁵² A higher f_b value typically indicates more extensive delamination and increased corrosion. Initially, the pure EP coating shows a significantly higher f_b value (17.30 Hz) compared to the NTAB/EP coating (0.014 Hz), as seen in Fig. 6h. Moreover, the f_b value of the pure EP coating increases over time, indicating electrolyte penetration and delamination expansion. In contrast, the f_b value of the NTAB/EP coating remains low (0.017 Hz after 56 days), suggesting superior barrier performance and resistance to delamination.

Fig. 6 Nyquist and Bode plots, accompanied by the corresponding phase angle curves of (a–c) pure EP and (d–f) NTAB/EP coatings after 1 day and 56 days of immersion in 3.5 wt% NaCl solution. (g) A comparative analysis of the evolution of R_c, |Z|_{0.01 Hz} and θ_{10 Hz} values, along with (h) f_b values for pure EP and NTAB/EP coatings after 1 day and 56 days of immersion.

To further assess the long-term anticorrosive performance of the NTAB/EP coating, Nyquist and Bode plots were recorded over an extended period of 7 to 154 days, as shown in Fig. 7a, b and S9.† Throughout this period, only a single capacitance arc is observed in the Nyquist plots, indicating a consistent electrochemical response and a uniform corrosion process,^53,54 while the NTAB/EP coating maintains its structural integrity. Fig. 7c depicts the evolution of the |Z|_{0.01 Hz} values for the NTAB/EP coating over the 154 day immersion period. The analysis reveals that the |Z|_{0.01 Hz} values nearly remain consistent throughout the entire immersion duration. Notably, on the 154th day, the |Z|_{0.01 Hz} value holds steady at an impressive 8.94 × 10⁹ Ω cm², significantly outperforming other reported coatings (Fig. 7d).^55–59 This exceptional performance underscores the NTAB/EP coating's remarkable durability and long-term corrosion resistance, making it an ideal solution for prolonged protective applications.

Fig. 7 Electrochemical impedance characterization of NTAB/EP coating: (a) Nyquist plots during early-stage immersion (7–56 days) and (b) long-term immersion (98–154 days) in 3.5 wt% NaCl solution. (c) Temporal evolution of low-frequency impedance modulus (|Z|_{0.01 Hz}) for NTAB/EP coating at selected immersion intervals. (d) Comparative analysis of |Z|_{0.01 Hz} values between the NTAB/EP coating and previously reported advanced anti-corrosion coatings.

Fig. 8a illustrates the contact angles for both pure EP and NTAB/EP coatings, highlighting a significant difference in their surface properties. A larger contact angle indicates a more hydrophobic surface, which effectively prevents the spread of corrosive agents and acts as a barrier to penetration.^60,61 The pure EP coating displays a moderate degree of hydrophobicity, characterized by a relatively low contact angle of 75.36°. In contrast, the NTAB/EP coating reveals a remarkable increase in contact angle to 122.53°. This enhanced hydrophobicity is likely attributable to the NTAB microspheres modifying the surface energy of the coating, rendering it energetically unfavorable for corrosive substances to spread, thereby minimizing contact and diminishing the potential for corrosive interactions with the underlying metal. Moreover, the adhesion strength of the EP coating is recorded at 1.67 MPa, whereas the adhesion strength of the NTAB/EP coating exhibits a significant rise to 2.12 MPa, as depicted in Fig. 8b. This enhanced adhesion reflects a stronger interfacial bond between the coating and the substrate, crucial for the coating's efficacy and longevity.⁶² After 240 h, the NTAB/EP coating retains remarkable adhesion integrity, exhibiting only a marginal reduction in bond strength from 2.22 MPa to 1.93 MPa (Fig. S10†). These findings from the contact angle and adhesion tests demonstrate that the NTAB/EP coating shows exceptional dispersibility and compatibility, enhancing its anti-permeability properties. More information about the water absorption characteristics of the NTAB/EP coating is provided in Fig. S11.†

Fig. 8 (a) Contact angles and (b) adhesion strengths of pure EP and NTAB/EP coatings.

To evaluate the potential application of the coating in real-world environments, we tested the anti-corrosion effectiveness of the NTAB/EP coating using seawater from the Bohai Sea to simulate marine exposure conditions. As shown in Fig. 9a, digital images of both pure EP and NTAB/EP-coated steels were taken after 1 day and 30 days of immersion. After just 1 day, both coatings appear intact, suggesting initial effectiveness in resisting corrosion. However, after 30 days, the pure EP-coated steel shows extensive corrosion, with significant rust and degradation, indicating its failure to protect the steel in the harsh marine environment. In contrast, the NTAB/EP-coated steel remains free from visible corrosion, even around intentionally scratched areas where the coating was compromised.^63,64 The corresponding XRD spectra after immersion are presented in Fig. 9b. The pure EP-coated Q235 steel shows distinct diffraction peaks of corrosion intermediates (β-FeOOH and α-Fe₂(OH)₃Cl), signaling severe corrosion. In contrast, these peaks are absent in the NTAB/EP-coated steel, which instead shows peaks at 27.52°, 31.94°, and 56.68°, corresponding to Fe₃O₄ and Fe₂O₃, indicating the formation of a passivation film that enhances the passivation capability of the NTAB/EP coating.^65,66 Additional SEM and AFM images after immersion are provided in Fig. S12 and S13,† further confirming the superior anticorrosive performance of the NTAB/EP coating under real marine conditions. Detailed results of the salt spray test are shown in Fig. S14.†

Fig. 9 (a) Photographs of pure EP and NTAB/EP coated steels immersed in Bohai Sea water and (b) the corresponding XRD patterns recorded before and after a 30 day immersion period. (c) Current density distribution maps for pure EP and NTAB/EP coatings acquired through the SVET technique following immersion in Bohai Sea water. (d) A comparative analysis of current densities for pure EP and NTAB/EP coatings over 24 h.

The scanning vibrating electrode technique (SVET, Fig. S15†) is a powerful method for converting corrosion potential signals around artificial scratches (5 mm in length) into current density signals at various time intervals during immersion in Bohai Sea water. Fig. 9c shows the current density profiles for both pure EP and NTAB/EP coatings. Over time, the corrosion current density of the pure EP coating (0.42–0.45 μA cm⁻²) significantly increases after 24 hours of immersion. In contrast, the NTAB/EP coating exhibits much lower anodic current densities (0.27–0.30 μA cm⁻²) after the same period, indicating a markedly reduced tendency for corrosion, as shown in Fig. 9d. These results emphasize the superior corrosion resistance of the NTAB/EP coating, underscoring its potential for practical applications, especially in environments requiring long-term protection of metal surfaces.

The incorporation of NTAB organic matter into protective coatings offers a highly effective strategy for mitigating the penetration of corrosive media, achieved through two primary mechanisms (Fig. 10). First, the unique three-dimensional microsphere-like structure of the NTAB organic material acts as a robust physical barrier. These microspheres effectively fill the micropores in the epoxy coating, creating a pronounced “maze effect” to hinder the penetration of corrosive media and delay the onset of corrosion. Second, the intrinsic electrochemical activity of the NTAB organic material plays a critical role in corrosion resistance. The abundant active sites, particularly carbonyl and amino groups, as revealed by molecular electrostatic potential (MESP) analysis, enable NTAB organic material to participate in electrochemical reactions with electrons released during metal dissolution. As metallic ions like iron (Fe) dissolve, they release electrons that are captured by NTAB organic material, stabilizing it in a reduced state. This process facilitates the conversion of ferrous (Fe²⁺) and ferric (Fe³⁺) ions into passivated iron oxides, specifically Fe₃O₄ (magnetite) and Fe₂O₃ (hematite). Together, the physical barrier and chemical passivation create a multi-faceted defense system, significantly enhancing the NTAB/EP coating's ability to resist corrosion and prolonging its protective lifespan in harsh environments.

Fig. 10 A schematic illustration of the corrosion protection mechanism of microsphere-like NTAB organic matter serving as an anti-corrosion additive.

3. Conclusion

In conclusion, this research led to the development of a spherical NTAB organic material synthesized through a simple condensation reaction and characterized by enhanced redox activity, substantial molecular stability, and a tailored electronic structure. When incorporated into the EP matrix as active fillers, the NTAB organic material facilitates the creation of an NTAB/EP anti-corrosion coating with exceptional protective properties. This is evidenced by an extraordinarily low corrosion rate of 9.9 × 10⁻⁶ mm per annum and an impressive corrosion inhibition efficiency of 99.70%. The NTAB/EP coating also exhibits remarkable long-term performance, achieving an |Z|_{0.01 Hz} value of 8.94 × 10⁹ Ω cm² following extensive 154 day immersion in a 3.5 wt% NaCl solution, marking significant improvement over existing anti-corrosion coatings. Additional tests under real seawater conditions further reveal the coating's exceptional anti-corrosive ability, maintaining its uncorroded appearance even under physical abrasion. The outstanding durability of the NTAB/EP coating can be attributed to the synergistic effects of a robust physical barrier and effective chemical passivation, contributing significantly to the field of anti-corrosion technology and setting the stage for future generations of multifunctional protective coatings.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†]. More detailed data are available on request from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge funding support from the China Postdoctoral Science Foundation (2022M711686), the Science and Technology Planning Social Development Project of Zhenjiang City (SJC20240100056), the National Institute of Education, Singapore, under its Academic Research Fund (RG3/23), the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (RG88/23), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX25_2564).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01862e

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