Introduction

Despite numerous surface protection approaches1,2, the creation of epoxy coatings has recently gained a lot of popularity3. Regrettably, by gaining access to the coating's defective areas, corrosive substances like oxygen and aggressive ions could destroy the metal surface4.

Nano-materials have gained significant attention in the development of anti-corrosion epoxy coatings due to their unique properties and potential benefits5,6. Nano-materials, such as nanoparticles or nanoclays, can be incorporated into epoxy coatings to create a highly effective barrier against corrosive agents7,8. The small size and high surface area of nano-materials enable them to form a dense and uniform protective layer, hindering the penetration of moisture, oxygen, and corrosive chemicals to the underlying substrate9,10.

Certain nano-materials possess self-healing capabilities, allowing them to repair any damage or defects in the coating. For example, nanocapsules or nanotubes filled with corrosion inhibitors can release their contents when the coating is damaged, effectively preventing further corrosion and extending the coating's lifespan11,12. Nanomaterials can improve the adhesion between the epoxy coating and the substrate. By modifying the surface properties of the nanomaterials or introducing functional groups, they can promote stronger interfacial bonding, reducing the risk of delamination or detachment of the coating13,14. Nanomaterials can act as active corrosion inhibitors within the epoxy matrix. For instance, nanoparticles like zinc oxide (ZnO), cerium oxide (CeO2), or graphene oxide (GO) can release corrosion-inhibiting ions or form a protective layer on the substrate, effectively slowing down the corrosion process15,16,17. Some nano-materials possess excellent UV stability, which is crucial for outdoor applications. By incorporating UV-absorbing or UV-reflecting nanoparticles, the epoxy coating can resist UV degradation, preventing discoloration, chalking, or loss of mechanical properties18. Nano-materials can enhance the mechanical properties of epoxy coatings, such as hardness, toughness, and abrasion resistance. Reinforcing nanofillers like carbon nanotubes (CNTs) or nanofibers can provide additional strength and durability to the coating, making it more resistant to physical damage19.

Nano-materials offer promising opportunities for developing advanced anti-corrosion epoxy coatings with improved performance, longevity, and environmental sustainability. Ongoing research and technological advancements in this field continue to expand the possibilities for utilizing nano-materials in corrosion protection applications.

Popular findings in the publications showed that nano-fillers could be used to fill the characteristic defects in the epoxy layer. For instance, based on an investigation by Othman et al.20, introducing an innovative method of enhancing water barrier and corrosion resistance capabilities by dispersing graphene oxide sheets in the epoxy coating by using the stable surface property of zinc oxide. Because the existence of nano-fillers may obstruct the electrolytes’/ions’ routes for transfer in the EP structure, the coating protective effectiveness can be greatly enhanced21,22,23. Majiidi et al.24 created tetragonal GO-ZnO-chitosan coatings to prevent mild steel substances from corroding.

One essential group of transition metals is cobalt sulfides, which are utilized in catalysts, lithium-ion batteries, supercapacitors, magnetic materials, and alkaline rechargeable batteries. The application of cobalt sulfide nanoparticles as coating materials remains challenge. As a result, takes into account for the first time the impacts of nano-CoS2 on the adhesion power and corrosion resistant of epoxy coatings.

Experimental details

Chemicals and materials

CoCl2.7H2O, sodium sulfide, ethyl alcohol were supplied from Alpha chemical company. Bisphenol A Epoxy Resins (PE) (Specifications: EEW = 182–192 (gr/eq), concentration (% w/w) = 100%) and polyamine epoxy hardener (content: > 90 wt%) (They were purchased from Egy-coating Company). The 3.5 wt% sodium chloride solution has been produced by diluting AR type 99.8% NaCl using a solution containing distilled water. The substrate was carbon steel pieces (sizes = 4.3 cm × 2.5 cm × 0.02 cm) having the corresponding composition (wt%): 0.08C; 0.5Mn; 0.006P; 0.03Cu; 0.015Cr; 0.012Ni; 0.06Si and balanced Fe. Before applying coatings, the carbon steel samples were polished with SiC paper (degrees = 400–1200) and cleaned with acetone and distilled water, respectively.

Cobalt sulfide nanoparticles synthesis

Cobalt disulfide nanoparticles were prepared by dissolving 2.379 g (0.01 mol) of CoCl2·7H2O in de-ionized water. In other flask, 0.02 mol of sodium sulfide was prepared. Then, sulfide solution was added slowly to the cobalt solution during stirring and the reaction completed at 80 °C. The previous mixture was turned gradually to black. The final CoS2 precipitate was washed several times using ethyl alcohol. The obtained CoS2 nanoparticles was dried at 50 °C then characterized.

Composite coating (EP/nano-CoS2) preparation

The nano-CoS2 particles were added to the dimethylformamide (DMF) and the mixture was sonicated for 60 min to improve the CoS2 particles dispersion in the EP. The nano-CoS2 particles (1.0% by total weight of EP and hardener) were introduced to the EP immediately, with the stirring mechanism running around 1800 rpm for 120 min. Polyamine hardener with balanced ratios has been added to the mixture. The viscosity was adjusted using butyl acetate.

Utilizing the dipping process, EP/nano-CoS2 nanocomposite coatings have been put on clean carbon steel substrates. The coated carbon steel specimens were left to cure over 1.0 h around 393 K. Hand carried micrometre (B.C. Ames Company.) was employed to determine the dry layer thickness of the coating. It had a value of 30 ± 5 μm.

Characterizations tools

The crystalline information of nano-CoS2 was obtained using X-ray powder diffractometer (XRD) (PanalyticalXPERT-PRO MPD—Netherlands).

A spectrum of Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer, USA) was employed for the identification of nano-CoS2.

The Zeta Potential distribution of CoS2 was investigated using Malvern Zetasizer ZS-HT, UK.

Electrochemical and mechanical tests

Gamry Reference 3000TM using open circuit potential (OCP) with voltage intensity 10 mV within the frequency region 1 Hz–100 kHz was used for the EIS investigations. For EIS data fitting, the Z-View-programme software was employed.

The electrochemical equivalent circuit (EEC) can be done by fitting the impedance data obtained from experiments to the circuit model. Z-View software can import ASCII text files generated by electrochemical stations and perform efficient EIS fits.

The fitted parameters will provide insight into the electrical behavior of the anti-corrosion coating system. It is important to validate the selected circuit model by comparing the modeled impedance response with experimental data not used during parameter estimation. This validation step helps ensure that the chosen circuit model accurately represents the electrical behavior of the anti-corrosion coating system and can be relied upon for further analysis.

The EIS test includes a Pt wire (counter electrode), a coated steel surface (working electrode), a saturated calomel electrode (SCE) (reference electrode), and a 3.5 wt percent sodium chloride (corrosive solution)25,26. The ASTM D4541 procedure was utilized to assess the adhesive strength of coatings employing a pull-off adhesion device (GM01-6.3 kN). A perpendicular pulling force is applied to a coating and substrate in pull-off adhesion examination. Both the coating and the metal surface must be cleaned before beginning the test. After that, the glue is ready and put on the metal surface, which is then attached to the covered surface. After then, the actuator of the machine is positioned over the coated surface, and pressure is exerted until the adhesion breaks.

As in prior studies, EP/(1.0%)nano-CoS2 coating preparation, Pull-off adhesion test and EIS analyses were conducted out27,28,29.

Results and discussion

Characterization

Figure 1 displays the XRD pattern of the synthesized cobalt sulfide nanoparticles. This pattern indicates that all diffraction lines relate to CoS2 in the cubic phase30. The peaks located at 26.16°, 31.70°, 35.92°, 39.24°, 45.42°, and 53.67° are corresponding to indices (111), (200), (210), (211), (220), and (222), respectively. The nano-scale of the formed cobalt sulfide nanoparticles confirmed by the broadening of the peaks (see Fig. 1). This is indicating that the synthesis of pure CoS2 nanostructures was successful.

Figure 1
figure 1

XRD spectra of CoS2 nanoparticles.

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Debye-Scherer’s equation (D = (0.94λ/β cos ɵ) was used to calculate the mean particle size (D)31 of the synthesized CoS2.

Depending on the most sharp, and intense peak (i.e. 211) to substitute in Scherrer equation by using the value of β (the full width at half-maximum value (FWHM) in radians of XRD diffraction lines, the mean particle size of CoS2 nanoparticles is 19.38 nm.

FT-IR measurements were used as another technique to confirm the chemical structure of the synthesized CoS2 nanoparticles. Concerning Fig. 2, absorbance peaks appear at 3550.72 cm−1 and 1623.63 cm−1 are assigned to stretching vibration mode of the hydroxide groups that absorbed on the sulfide surface30. The characteristic bands related to sulfides are present at 1095.66 cm−1 and 671.36 cm−1 due to asymmetric and stretching modes, respectively32. Additionally, a small peak at 604.64 cm−1 is assigned to the stretching vibrations of the cobalt33.

Figure 2
figure 2

FT-IR spectra of CoS2 nanoparticles.

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The surface electric charges of the particles are reflected by the zeta potential. Zeta potential is a measure of how strongly charged particles repel one another through electrostatic forces34. The resistance of the particles to aggregation in a dispersion system is shown by a high degree of zeta potential, either positive or negative, and this suggests an apparent stability of the system35. The value of zeta potential of CoS2 nanoparticles was − 9.78 mV indicating good stability (see Fig. 3).

Figure 3
figure 3

Zeta potential distribution of CoS2.

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Compare the XRD results of the neat EP and EP/nano-CoS2 to understand the impact of nanoparticle incorporation on the material’s crystallographic properties. The XRD diagrams of neat EP and EP/nano-CoS2 are shown in Fig. 4. Broad, featureless peaks are usually visible when looking at the XRD pattern of neat EP. This is because the epoxy resin that is usually employed in coatings is amorphous. According to Fig. 4, the diffraction peak of the EP/nano-CoS2 vary negligible when compared to those of neat EP, suggesting that the CoS2 particles have minimal influence on the structure of EP resin.

Figure 4
figure 4

XRD diagrams of neat EP and EP/nano-CoS2.

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EP/nano-CoS2 coating corrosion protection characteristics

The corrosion performance of nano-composite coatings that included CoS2 nanoparticles was evaluated using EIS estimation. Upon 2 days of soaking in 3.5% NaCl liquid, the Nyquist (Fig. 5a), phase charts (Fig. 5b) and Bode-Impedance curves (Fig. 5c) of neat EP-coated steel and an EP/nano-CoS2 coating nano-composite are shown.

Figure 5
figure 5

Impedance spectra for carbon steel substrates protected with neat EP and EP/nano-CoS2 coating dipped in 3.5% NaCl liquid at 303 K include (a) Nyquist, (b) Bode-phase angle plots, (c) Bode-Impedance, (d) EEC of neat EP and EP/nano-CoS2 coating.

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As shown in Fig. 5a, the neat EP-coated carbon steel provides a twice constant inside the Nyquist plots. The behavior is caused by the permeability of the epoxy resin coating36. The first time constant semicircle was attributed to the epoxy layer's impedance (@high frequency)36. The peaks within the intermediate frequency range (101–103 Hz) indicate the pore blockage of corrosive substances in defective areas and the formation process of new coatings37. The electrochemical equivalent circuit (EEC) used to assess the apparent impedance variables for neat PE includes the electrolyte-resistance (Rs), coating-resistance (Rc), charge-transfer-resistance (Rct), coating-capacitance (Cc), and double-layer-capacitance (Cdl) (see Fig. 5d). Because of the heterogeneity of coating and double layer formed at the underlying surface, CPEC and CPEdl have to be used instead of the Cc and Cdl parameters, respectively.

The Rc and Cc values for neat EP are 10.13 Mohm cm2 and 1.2 × 10–8 F cm−2, respectively. For the same neat EP, the Rct and Cdl values are 3.2 Mohm cm2 and 4.6 × 10–8 F cm−2, respectively. This suggests that the neat epoxy layer appears to possess a low level of corrosion impedance.

The substrate in this case exhibits unusual signs of the very corrosive effects. On steel surfaces, chloride ions speed up general corrosion and produce pitting corrosion38. Coating debonding is brought on by the creation of corrosion products from the anodic process and the release of hydrogen gas at the cathodic process39,40.

In the EP/nano-CoS2 coating -coated steel situation (see Fig. 5a–c), quite a semicircle is observed in addition to an increment in Rc to 51.6 Mohm cm2 and a reduction in Cc level to 3.2 × 10–10 F cm−2. The comparable EEC for steel with the EP/nano-CoS2 coating is shown in Fig. 5d. An EP/nano-CoS2 coating showed a greater phase angle in comparison to a tidy EP coating, implying that it likely be more adaptable and flexible (see Fig. 5b). It can be inferred from this that adding nano-CoS2 improves epoxy’s stiffness and anti-corrosion properties. Noticeably, by combining nano-CoS2 into the epoxy composite, corrosion of covered carbon steel seemed to be reduced significantly.

Nano-CoS2 coatings act as a physical shield, preventing corrosive charged particles from dispersing through the coating and preventing corrosion41. Furthermore, the incorporation of Nano-CoS2 helps improve the mechanical features of the epoxy coating42.

Pull-off tests were used to assess the overall impact of nano-CoS2 on the adhesion strength (AS) of EP coating. The photographic images of pull-off wet adhesion test in 3.5 wt.% NaCl solution for neat EP and EP/nano-CoS2 are shown in Fig. 6a and b respectively.

Figure 6
figure 6

The photographic images of pull-off wet adhesion test in 3.5 wt.% NaCl solution for neat EP (a) and EP/nano-CoS2 (b).

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The average AS of the neat EP coating (without nano-CoS2) became 4.20 MPa, whereas the EP/nano-CoS2 coating provided considerably higher values of 12.33 MPa. As a consequence, incorporating CoS2 into the EP coating considerably enhanced the coating's adhesion force. This increase in AS could be likened to the restraint of indentation caused by the increased physical interactions between both the EP resin and nano-CoS243.

The considerable increase in resistance of the neat EP modified by nano CoS2 having a significant specific surface area can be attributed to the following reasons. Actually, nano-CoS2 typically fills in voids and pinholes in epoxy coatings, which lowers the cured epoxy coating’s total free volume and raises its cross-linking density44. Within the epoxy matrix, nano-CoS2 can serve as reinforcing fillers. Enhanced mechanical properties are a result of their high aspect ratio and significant surface area-to-volume ratio. Furthermore, the CoS2 nanoparticles may reduce EP dis-aggregation during curing, resulting in a more uniform coating44.

Conclusion

This work is the first to examine the effect of nano-CoS2 on the adhesion strength and corrosion resistance of epoxy coatings. The preparation of CoS2 nanoparticles (19.38 nm) and their incorporation into an epoxy (EP) resin were done in order to make a composite coating (EP/nano-CoS2). In the case of the steel with the EP/nano-CoS2 coating, a distorted semicircle is seen, along with an increase in Rc to 51.6 Mohm cm2 and a decrease in Cc level to 3.2 10–10 F cm−2. While the EP/nano-CoS2 coating produced noticeably higher AS values of 12.33 MPa, the average AS of the clean EP coating (without nano-CoS2) was 4.20 MPa. The final conclusion demonstrated that using the synthesized nanoparticles as nano-fillers can significantly improve the corrosion resistance or even adhesion of the EP protective layer.