Advantages of CoS2 nano-particles on the corrosion resistance and adhesiveness of epoxy coatings

Researchers face significant challenges because of the inadequate corrosion resistance and weak adherence of epoxy (EP) coatings. We deal with these issues here by means of a novel nano-composite coating (EP/nano-CoS2). In order to create a composite coating (EP/nano-CoS2), CoS2 nanoparticles (nano-CoS2) were prepared and incorporated to an epoxy (EP) resin. The synthesized CoS2 was characterized using XRD and FT-IR spectroscopic techniques. The mean particle size was determined using Scherer equation and found to be 19.38 nm. The zeta potential was also determined (− 9.78 mV). Electrochemical impedance spectroscopies (EIS) as well as pull-off assessments were used to quantify the EP/nano-CoS2 coating’s anti-corrosion capabilities and adhesive power. The findings demonstrate that the EIS variables of the EP/nano-CoS2 composite coating are markedly improved when compared to raw EP coating. The corrosion resistance or even adhesion of EP protective layer can be markedly increased by using the synthesized nanoparticles as nano-fillers.

www.nature.com/scientificreports/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 enhanced [21][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-CoS 2 on the adhesion power and corrosion resistant of epoxy coatings.

Cobalt sulfide nanoparticles synthesis
Cobalt disulfide nanoparticles were prepared by dissolving 2.379 g (0.01 mol) of CoCl 2 •7H 2 O 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 CoS 2 precipitate was washed several times using ethyl alcohol.The obtained CoS 2 nanoparticles was dried at 50 °C then characterized.

Composite coating (EP/nano-CoS 2 ) preparation
The nano-CoS 2 particles were added to the dimethylformamide (DMF) and the mixture was sonicated for 60 min to improve the CoS 2 particles dispersion in the EP.The nano-CoS 2 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-CoS 2 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.
A spectrum of Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer, USA) was employed for the identification of nano-CoS 2 .
The Zeta Potential distribution of CoS 2 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 (GM01-6.3kN).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.
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 CoS 2 nanoparticles is 19.38 nm.
FT-IR measurements were used as another technique to confirm the chemical structure of the synthesized CoS 2 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 surface 30 .The characteristic bands related to sulfides are present at 1095.66 cm −1 and 671.36 cm −1 due to asymmetric and stretching modes, respectively 32 .Additionally, a small peak at 604.64 cm −1 is assigned to the stretching vibrations of the cobalt 33 .
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 forces 34 .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 system 35 .The value of zeta potential of CoS 2 nanoparticles was − 9.78 mV indicating good stability (see Fig. 3).
Compare the XRD results of the neat EP and EP/nano-CoS 2 to understand the impact of nanoparticle incorporation on the material's crystallographic properties.The XRD diagrams of neat EP and EP/nano-CoS 2 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

EP/nano-CoS 2 coating corrosion protection characteristics
The corrosion performance of nano-composite coatings that included CoS 2 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-CoS 2 coating nano-composite are shown.
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 coating 36 .The first time constant semicircle was attributed to the epoxy layer's impedance (@high frequency) 36 .The peaks within the intermediate frequency range (10 1 -10 3 Hz) indicate the pore blockage of corrosive substances in defective areas and the formation process of new coatings 37 .The electrochemical equivalent circuit (EEC) used to assess the apparent impedance variables for neat PE includes the electrolyte-resistance (R s ), coating-resistance (R c ), charge-transfer-resistance (R ct ), coating-capacitance (C c ), and double-layer-capacitance (C dl ) (see Fig. 5d).Because of the heterogeneity of coating and double layer formed at the underlying surface, CPE C and CPE dl have to be used instead of the C c and C dl parameters, respectively.
The R c and C c values for neat EP are 10.13 Mohm cm 2 and 1.2 × 10 -8 F cm −2 , respectively.For the same neat EP, the R ct and C dl values are 3.2 Mohm cm 2 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 corrosion 38 .Coating debonding is brought on by the creation of corrosion products from the anodic process and the release of hydrogen gas at the cathodic process 39,40 .In the EP/nano-CoS 2 coating -coated steel situation (see Fig. 5a-c), quite a semicircle is observed in addition to an increment in R c to 51.6 Mohm cm 2 and a reduction in C c level to 3.2 × 10 -10 F cm −2 .The comparable EEC for steel with the EP/nano-CoS 2 coating is shown in Fig. 5d.An EP/nano-CoS 2 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-CoS 2 improves epoxy's stiffness and anti-corrosion properties.Noticeably, by combining nano-CoS 2 into the epoxy composite, corrosion of covered carbon steel seemed to be reduced significantly.
Nano-CoS 2 coatings act as a physical shield, preventing corrosive charged particles from dispersing through the coating and preventing corrosion 41 .Furthermore, the incorporation of Nano-CoS 2 helps improve the mechanical features of the epoxy coating 42 .
Pull-off tests were used to assess the overall impact of nano-CoS 2 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-CoS 2 are shown in Fig. 6a and b respectively.
The average AS of the neat EP coating (without nano-CoS 2 ) became 4.20 MPa, whereas the EP/nano-CoS 2 coating provided considerably higher values of 12.33 MPa.As a consequence, incorporating CoS 2 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-CoS 2 43 .The considerable increase in resistance of the neat EP modified by nano CoS 2 having a significant specific surface area can be attributed to the following reasons.Actually, nano-CoS 2 typically fills in voids and pinholes in epoxy coatings, which lowers the cured epoxy coating's total free volume and raises its cross-linking density 44 .Within the epoxy matrix, nano-CoS 2 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 CoS 2 nanoparticles may reduce EP dis-aggregation during curing, resulting in a more uniform coating 44 .

Conclusion
This work is the first to examine the effect of nano-CoS 2 on the adhesion strength and corrosion resistance of epoxy coatings.The preparation of CoS 2 nanoparticles (19.38 nm) and their incorporation into an epoxy (EP) resin were done in order to make a composite coating (EP/nano-CoS 2 ).In the case of the steel with the EP/nano-CoS 2 coating, a distorted semicircle is seen, along with an increase in R c to 51.6 Mohm cm 2 and a decrease in C c level to 3.2 10 -10 F cm −2 .While the EP/nano-CoS 2 coating produced noticeably higher AS values of 12.33 MPa, the average AS of the clean EP coating (without nano-CoS 2 ) 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.
Figure1displays the XRD pattern of the synthesized cobalt sulfide nanoparticles.This pattern indicates that all diffraction lines relate to CoS 2 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 CoS 2 nanostructures was successful.Debye-Scherer's equation (D = (0.94λ/β cos ɵ) was used to calculate the mean particle size (D)31 of the synthesized CoS 2 .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 CoS 2 nanoparticles is 19.38 nm.FT-IR measurements were used as another technique to confirm the chemical structure of the synthesized CoS 2 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 .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 CoS 2 nanoparticles was − 9.78 mV indicating good stability (see Fig.3).Compare the XRD results of the neat EP and EP/nano-CoS 2 to understand the impact of nanoparticle incorporation on the material's crystallographic properties.The XRD diagrams of neat EP and EP/nano-CoS 2 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

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-CoS 2 (b).