Synthesis, Characterization, and Application of Novel Ni-P-Carbon Nitride Nanocomposites

Abstract

Dispersion of 2D carbon nitride (C₃N₄) nanosheets into a nickel phosphorous (NiP) matrix was successfully achieved by ultrasonication during the electroless plating of NiP from an acidic bath. The morphology and thickness, elemental analysis, phases, roughness, and wettability for as-plated and heat-treated nanocomposite were determined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy, and contact angle measurements, respectively. C₃N₄ showed a homogeneous distribution morphology in the nanocomposite that changed from amorphous in case of the NiP to a mixed crystalline-amorphous structure in the NiP-C₃N₄ nanocomposite. The microhardness and corrosion resistance of the as-plated nanocomposite and the heat-treated nanocomposite coating were significantly enhanced compared to the Ni-P. The nanocomposite showed a superior corrosion protection efficiency of ~95%, as observed from the electrochemical impedance spectroscopy (EIS) measurements. On the other hand, the microhardness of the nanocomposite was significantly increased from 780 to reach 1175 HV₂₀₀ for NiP and NiP-C₃N₄, respectively.

1. Introduction

Enhancing the corrosion protection for oil and gas pipelines continues to motivate intensive research efforts to find new coatings or modify the existing ones. Electroless deposited NiP coatings, obtained by an autocatalytic process, are characterized by a combination of many unique properties such as good wear and corrosion protection efficiency, uniformity of coating thickness, and a higher hardness [1]. These properties opened the field for NiP to be used in different industries [2]. However, to improve these properties, the incorporation of nanoparticles in the NiP matrix has been tried extensively recently [3,4,5,6,7,8]. The performance of the co-deposition of hard second-phase nanoparticles such as Al₂O₃, TiN, B₄C, ZrO₂, SiC, TiO₂, CNT, graphite, and diamond [9,10,11,12,13,14,15,16,17] with the Ni-P matrix was investigated, and has shown an enhancement in terms of microhardness, as well as corrosion and wear resistances. Nevertheless, the Ni-P matrix with the insertion of the carbon nitride (C₃N₄) nano-sheets has not been reported. The prediction of the possible existence of the C₃N₄ compound is credited to Cohen and Liu [18,19]. Due to its high hardness and excellent thermal and chemical stability, C₃N₄ has attracted significant interest. C₃N₄ properties are essentially the same as those of diamond [20], i.e., the mechanical and tribological characteristics, as well as the corrosion resistance of its composite coatings, are expected to be excellent [21]. The goal of the present study is to prepare a new NiP-C₃N₄ nanocomposite with better corrosion resistance and mechanical properties through the incorporation of 2D C₃N₄ nanosheets during the electroless deposition of the NiP alloy. In addition, a comparative study between the new nanocomposite and the original C₃N₄-free alloy will be carried out to show the superior performance of the electroless deposited NiP-C₃N₄ nanocomposite compared to the NiP alloys.

2. Experimental

2.1. Materials, Solutions, and Preparation

Electroless NiP and NiP-C₃N₄ nanocomposite coatings were deposited on an API X120 C-steel that was starting to be used recently in the oil and gas industry. The chemical composition (in wt %) of the API X120 steel that purchased from Tianjin Tiangang Guanye Co., Ltd., (Tianjin, China) is shown in

Table 1. The chemical composition (in wt %) of the substrate.

C	Si	Mn	Ni	Cr	Mo	Cu	V	Fe
0.129	0.101	0.541	0.017	0.039	0.0013	0.015	0.25	balance

.

Prior to the electroless deposition, the specimens were grinded with different grits of emery paper up to 2000, then polished with micro-polish alumina suspension (1 and 3 µm) to obtain a mirror finishing surface. After that degreasing the specimens in ultrasonicated acetone bath for 15 min was done, followed by alkaline cleaning for 5 min at 80 °C then electro-alkaline cleaning for 2 min at 70 °C with I = 2 A·cm⁻² and acid etching in 15 wt % H₂SO₄ solution for 20 s. The used alkaline cleaning solution consists of 50 g·L⁻¹ NaOH, 30 g·L⁻¹ Na₂CO₃ and 30 g·L⁻¹ Na₃PO₄ while that used in electro-alkaline cleaning consists of 15 g·L⁻¹ NaOH, 25 g·L⁻¹ Na₂CO₃ and 25 g·L⁻¹ Na₃PO₄.

After each of the pretreatment steps, the specimens were washed with deionized water. Pretreated substrate coupons of 20 × 30 × 10 mm³ were used in the electroless deposition process. All used solutions are analytical-grade reagents from Sigma-Aldrich (St. Louis, MO, USA).

The electroless bath for the NiP coating contained 15 g·L⁻¹ NiSO₄·6H₂O, 30 g·L⁻¹ NaH₂PO₂·H₂O, 20 g·L⁻¹ lactic acid, 20 g·L⁻¹ citric acid, and 0.002 g·L⁻¹ thiourea, while the electroless NiP-C₃N₄ nanocomposite coating bath contained 15 g·L⁻¹ NiSO₄·6H₂O, 30 g·L⁻¹ NaH₂PO₂·H₂O, 15 g·L⁻¹ ammonium chloride, 30 g·L⁻¹ sodium citrate, and 0.002 g·L⁻¹ thiourea. NaOH was used to adjust the pH of the NiP and NiP-C₃N₄ plating baths to 4.5 and 8, respectively. The two baths were maintained at 85 °C. To obtain well-suspended C₃N₄ nanosheets, which are synthesized and characterized in our previous work [22], in the electroless bath, 0.5 g of C₃N₄ was added to a 100 mL of the plating solution that includes also 0.02 g·L⁻¹ of sodium dodecyl sulfate as the surfactant. Then, the solution was mixed with ultrasonic probe for 2 h. Finally, the mixture was poured into the original plating bath stirred at 300 rpm using a magnetic stirrer. The coupons were immersed in the bath and the plating process lasted for 2 h. Then, the coupons were removed from the bath, rinsed with deionized water, and dried with blowing air. After plating, three samples of each coat underwent heat treatment (HT) at 400 °C for 1 h under vacuum to study the change in the properties of the NiP and NiP-C₃N₄ nanocomposite after the HT.

2.2. Characterization

The surface morphology and the elemental analysis of the nanocomposite were performed using a scanning electron microscope (SEM, Nova NanoSEM 450, Thermo Fisher Scientific, Eindhoven, The Netherlands) coupled with an energy-dispersive X-ray spectroscopy (EDX, Bruker detector 127 eV, Bruker, Leiderdorp, The Netherlands). The analysis of the different phases of the nanocomposite coating was performed using X-ray diffractometry (XRD, Miniflex2 Desktop, Cu Kα, Rigaku, Tokyo, Japan). A Vickers microhardness tester (FM-ARS9000, Future-Tech Corp., Tokyo, Japan) was used to measure the microhardness at a load of 200 g for 10 s. The measurements were repeated five times on each sample and the mean of the results was considered. Atomic Force Microscopy (AFM) was used to inspect the heterogeneities (surface roughness) of the coated specimens. An MFP3D Asylum research (Asylum Research, Santa Barbara, CA, USA) AFM equipped with a silicon probe (Al reflex coated Veeco model–OLTESPA, Olympus, Tokyo, Japan; Spring constant: 2 N·m⁻¹, resonant frequency: 70 kHz) was utilized for all roughness experiments. Measurements were conducted at ambient conditions using the Standard Topography AC in air (tapping mode in air). Contact angle measurements were conducted using a DataPhysics OCA35, DataPhysics Instruments GmbH, Filderstadt, Germany). Four microliters of deionized water were used as the probing liquid. To achieve accurate results, at least five contact angles were measured, and their average is reported.

2.3. Corrosion Study

The electrochemical impedance spectroscopy (EIS) was performed using a three-electrode cell at 25 °C in a 3.5% (w/w) NaCl solution utilizing a Gamry electrochemical workstation (Gamry Instruments, Warminster, PA, USA). The corrosion resistance of the electroless-plated NiP and the NiP-C₃N₄ nanocomposite was examined. An area of 2 cm² of the coated specimen was the working electrode, while an Ag/AgCl electrode and a graphite rod were used as the reference and the counter electrodes, respectively. A 10 mV AC amplitude was used, and the frequency varied from 0.01 Hz to 100 kHz. All EIS data were recorded after a steady-state, open-circuit potential was achieved. A 3.5 wt % sodium chloride solution was utilized to expose a 2 cm² area of the surface. Following the immersion of the specimens in a 3.5% (w/w) NaCl solution for 3 h and keeping them at an open-circuit potential for 20 min, potentiodynamic polarization tests (Tafel analysis) were performed. A scan rate of 0.167 mV·s⁻¹ and a potential range of ±250 mV vs. the open circuit potential was used to acquire the anodic and cathodic polarization curves.

3. Results and Discussion

3.1. Surface Morphology of the Ni-P and Ni-P-C₃N₄ Coatings

The SEM surface morphology of the as-plated NiP and NiP-C₃N₄ nanocomposite layers are represented in Figure 1a,b, respectively. The surfaces of both coatings exhibit a typical “cauliflower-like” morphology that shows many grains, and each contains many fine granules. Although the cauliflower morphology is a common feature for the electroless-plated NiP alloys, it can be observed that there are spaces between the nodules that facilitate the diffusion of the corrosive media to the substrate, as shown in Figure 1a. Figure 1b reveals a fine microstructure of the NiP-C₃N₄ nanocomposite coating compared to the NiP coatings that are shown in Figure 1a. This may be attributed to the uniform distribution of the C₃N₄ nanosheets in the NiP composite coating surface that results in a more fine compact structure with higher surface roughness, as shown later in AFM measurements. Figure 1c,d showed the EDX spectra of the as-plated NiP and NiP-C₃N₄ coatings that illustrate the presence of nitrogen in the nanocomposite, which proved the successful co-deposition of the C₃N₄ nanosheets into the NiP matrix. As seen in Figure 1e, for more illustration, the EDX mapping proves that the C₃N₄ nanosheets are uniformly and homogeneously distributed in the NiP matrix.

Furthermore, examination of Figure 1c,d shows that the NiP and NiP-C₃N₄ coatings contain approximately 10.48 and 8.76 wt % phosphorus, respectively. According to [23,24], the microstructure of ENP (Electroless NiP) coatings has been reported to be either amorphous or crystalline, or both, depending on the phosphorous content involved. For instance, crystalline, mixed amorphous-crystalline, and amorphous structures have been reported for low (1–5 wt %), medium (6–9 wt %), and high (10–13 wt %) phosphorous ENP coatings, respectively. Therefore, the microstructure of the NiP coating is amorphous (see XRD results in Section 3.3), whereas that of the NiP-C₃N₄ coating tends to be a mixed crystalline-amorphous, as depicted by XRD results in Section 3.3.

After HT at 400 °C for 1 h, the granular morphology of the as-plated NiP coating is gradually diminished and becomes smoother, as shown in Figure 1f. Comparison of the as-plated NiP-C₃N₄ composite coating to the heat-treated composite coating reveals that there is a difference in the nodular morphology, and the particles become larger and more agglomerated, as shown in Figure 1g. The change in the morphology with HT may be attributed to the different diffusion coefficients for the different phases in the composite coatings.

The cross-section morphology of as-plated NiP and NiP-C₃N₄ coatings was checked using SEM, as shown in Figure 2. It was revealed that the thickness of the NiP coating is approximately 52 µm, whereas the NiP-C₃N₄ coating has a thickness of 35 µm. This indicates that the existence of the reinforcing phase (C₃N₄) in the coating decreased the thickness considerably. Decreasing the thickness reveals the low deposition rate of the NiP-C₃N₄ coating compared to that for the NiP one. This may be attributed to the possibility of the physical adsorption of some C₃N₄ particles on the catalytic surface that result in the minimization of the available active sites for the deposition process that decreases the overall deposition rate [25]. In addition, there are no defects or cracks observed at the substrate-coating interface. This demonstrates the good adhesion of the coatings. The thickness of both coatings does not change after the HT.

3.2. Structures of the Ni-P and Ni-P-C₃N₄ Coatings

XRD patterns of both as-plated and heat-treated Ni-P and Ni-P-C₃N₄ layers are represented in Figure 3a,b, respectively. It is observed that the diffraction pattern of both NiP and NiP-C₃N₄ composite coatings before the HT has only a single broad peak at 44.5°, which is related to a face-centered cubic (FCC) Ni (111) plane, as shown in Figure 3a. The peaks representing the C₃N₄ particles that appear in the inset of Figure 3a are not detected in the diffraction pattern of the composite coating shown in Figure 3a. This may be attributed to the low quantity of C₃N₄ and high density of Ni diffraction peaks. According to the EDX results, the NiP-C₃N₄ composite coating microstructure is a mixture of amorphous and crystalline phases. Based on the full width at half maximum (FWHM), it is found that the FWHM of the NiP and NiP-C₃N₄ composite coatings is 7.63 and 6.34, respectively. Therefore, it is concluded that the presence of C₃N₄ in the coating promotes the formation of crystalline phase; see a similar case in Ref. [26].

After HT, the as-plated NiP coating crystallized mainly as Ni₃P particles on the surface. As the amorphous structure is metastable, its peak decreased after HT and crystalline Ni; also, Ni₃P phases are formed. It is clear that the diffraction pattern of the C₃N₄ nanocomposite coating coincides with that of the NiP coating, i.e., the presence of C₃N₄ nanosheets in the coating has not affected the phase angle (peaks positions) of the coating, and new peaks have not appeared. Moreover, it can be observed that the intensity of the resulted peaks decreases dramatically for the NiP-C₃N₄ nanocomposite coating compared to C₃N₄-free one. This can explained by the decrease in the amount of deposited Ni and P as a result of presence of C₃N₄ in the coating. This is illustrated in the EDX charts shown above in Figure 1h,i, in which the Ni and P contents decreased by 3.2 wt % and 22.9 wt %, respectively. In addition, as illustrated in Figure 1h, increasing the P(wt %) in the heat-treated NiP coating results in smaller Ni diffraction peaks.

3.3. Contact Angle Measurements

The surface roughness of coatings, in addition to its directly related properties, e.g., hydrophilicity and hydrophobicity, are important properties to be studied. The higher the surface roughness is, the more hydrophobic and corrosion-resistant the coating will be. Figure 4 shows the water contact angles (WCAs) of the substrate (API X120 steel), the NiP, and the NiP-C₃N₄ nanocomposite coatings before and after the HT at 400 °C for 1 h.

The WCA for steel is 86°, i.e., less than 90°, indicating its hydrophilic nature. Both as-plated NiP and NiP-C₃N₄ coatings are hydrophobic, as their contact angles are found to be 105° and 109°, respectively. After HT, a decrease in the water contact angle is observed for both NiP and the nanocomposite coatings (Figure 4d,e). Because of the recrystallization of the nickel and a phase transition that led to the change in the surface chemistry and roughness of the NiP coatings, the as-plated coatings became hydrophilic after HT [27].

3.4. Surface Roughness of Coatings

The Atomic force microscopy (AFM) was used to measure the surface roughness of the NiP and NiP-C₃N₄ coatings before and after the heat treatment, as shown in Figure 5. The surface roughness of the NiP-C₃N₄ composite coating in the presence of C₃N₄ nanosheets in the NiP coating increased. The surface roughness of the as-plated NiP coating is 22 nm, whereas the NiP-C₃N₄ shows a surface roughness of approximately 43 nm. This proves the increased hydrophobicity of the NiP-C₃N₄ coating relative to that of the NiP coating. It is obvious that the surface roughness of the NiP coating and the C₃N₄ composite coating after the heat treatment decreased by about half of their initial values. This was attributed to the recrystallization of the coatings and the formation of the Ni₃P and Ni crystals that are more stable and show ordered structures. Therefore, the roughness of the NiP-C₃N₄ composite coating is always higher than that of the NiP coating, even after reduction of the roughness upon the heat treatment.

3.5. Microhardness Measurements

The microhardness of the NiP and NiP nanocomposite coatings before and after the heat treatment were performed in addition to that of the C-steel substrate for comparison, as illustrated in Figure 6. The measured microhardness of the substrate is approximately 166 HV₂₀₀. The electroless deposition of the NiP and NiP-C₃N₄ leads to an increase in the microhardness to 406 and 645 HV₂₀₀, respectively. The increase in the microhardness upon the dispersion of the C₃N₄ nanosheets is attributed to the dispersion hardening effect caused via the incorporation of the nanoparticles into the composite coatings. As reported in [28,29], the uniform distribution of the nanoparticles in the matrix could restrain the growth of the alloy grains and the plastic deformation of the coating, leading to the stabilization of the dislocation and thus increasing the microhardness.

Upon heat treatment of NiP and NiP-C₃N₄ nanocomposites, the microhardness was increased significantly to 780 and 1175 HV₂₀₀, respectively. At that temperature (400 °C), the P atoms are forced to adapt to the crystal structure of the Ni. This adaptation resulted in the formation of a coherent relationship that leads to a distortion of the local stress field. When the Ni/P ratio gathers in a sufficient quantity, the intermetallic compound Ni₃P precipitates and keeps a coherent relationship with the Ni. The hardness of the Ni₃P is gets higher when the temperature is increased, since it is formed mainly by metallic and ionic bonds, resulting in a coherent precipitation strengthening effect and an improvement of the microhardness [30]. Regarding the NiP-C₃N₄ composite, the presence of Ni₃P as the hard phase in addition to the C₃N₄ provides an extra factor for increasing its microhardness after the heat treatment. Although the intensity of the Ni₃P peaks in the NiP-C₃N₄ XRD chart is lower than the corresponding ones in the XRD chart of the C₃N₄—free coating, the hardness is higher in the case of NiP-C₃N₄ coating, indicating that the C₃N₄ compensated for the decrease in the Ni₃P content, which is an advantage for the new coating. In addition, it worth mentioning that the thickness of the coating in case of the NiP-C₃N₄ coating is 30% less than that in case of the NiP one.

3.6. Corrosion Measurements

3.6.1. Electrochemical Impedance Spectroscopy (EIS)

Figure 7a,b show the Bode and the phase angle plots of the EIS spectra that are measured at open circuit potential for the substrate with the as-plated and heat-treated NiP, as well as NiP-C₃N₄ nanocomposite coatings immersed in a 3.5 wt % NaCl solution at the room temperature. The larger the value of the |Z| at low frequencies is, the better the corrosion protection properties of the coating will be [31]. Inspection of Figure 7a shows that the values of |Z| at 0.01 Hz for the as-plated NiP and NiP-C₃N₄ nanocomposite coatings, as well as for the heat-treated coatings, are much higher than that of the substrate, confirming the corrosion protection properties of both coatings before and after the heat treatment. The high corrosion resistance is attributed to the presence of phosphorus [32]. Generally, for a Ni-based coating, when the nickel starts to dissolve in the corrosive media, the phosphorus starts to react with water to form a film of adsorbed hypophosphite anions, preventing further hydration of the nickel. Consequently, the corrosion resistance of the coating is increased [33]. Moreover, the as-plated NiP-C₃N₄ composite coating offers higher corrosion protection ability compared to that of the as-plated NiP coating despite the smaller phosphorous content in the former, as seen from the EDX results. This finding indicates the strong protective ability of the C₃N₄ nanosheets that enhance the polarization resistance of the NiP nanocomposite coating in 3.5 wt % NaCl solution to reach a maximum value of 9225 Ω·cm², as shown in

Table 2. Electrochemical parameters obtained by fitting the measured data shown in Figure 7 using the equivalent circuits shown in Figure 8 of the substrate, NiP, and NiP-C₃N₄ composite coatings with and without heat treatment.

Type of Coating	Rs (Ω·cm2)	Rpo (Ω·cm2)	CPEcoat (µF·cm−2·s−n)	Rp (Ω·cm2)	CPEdl (µF·cm−2·s−n)	W (S·s1/2)	n	IE (%)
Substrate	22.4	–	–	445	526.5	–	–	–
NiP	17.6	167.50	28.4	2336	294	–	0.7	81
NiP-C3N4	21.9	–	–	9225	39	1.021 × 10−3	0.9	95
NiP(HT)	15.6	16.9	35	1597	324.6	–	0.65	72
NiP-C3N4(HT)	23.4	–	–	2990	170.00	3.879 × 10−3	0.8	85

. This can be attributed, as mentioned above, to the uniform distribution of C₃N₄ nanosheets throughout the coating, which support forming a more compact structure, blocking the defects in the NiP coating, inhibiting the diffusion of the chloride ions to the substrate, and enhancing the corrosion resistance as shown in a similar situation in [10].

In addition, in Figure 7a, the Bode plots of the as-plated NiP and NiP-C₃N₄ nanocomposite coatings display different shapes in the recorded frequency regions, illustrating that different fundamental processes occur on their surfaces. This behavior is more conspicuous in the plots of the phase angle versus frequency (Figure 7b). The Bode and phase angle plots for the NiP coating show two broad peaks at the analyzed frequency range, which confirms two-time constants behavior. The first relaxation process is related to the coating layer that can be checked at the higher and intermediate frequencies, whereas the second relaxation process is observed at the lower frequencies and represents the electrochemical behavior at the interface of the substrate and the coating [34]. The Bode and phase angle plots for the NiP-C₃N₄ nanocomposite coating show one time-constant behavior, as shown in Figure 7b.

The equivalent circuits that are used in analyzing the EIS measured spectra for both the as-plated NiP and NiP-C₃N₄ coatings with and without heat treatment are depicted in Figure 8a,b, respectively. The circuit in Figure 8a includes the solution resistance (R_s), the high frequency time constant (R₁·CPE₁), and the low frequency time constant (R₂·CPE₂). The high frequency time constant (R₁·CPE₁) corresponds to the areas covered with the coating and can be represented by the coating admittance (CPE_coat) and the pore resistance (R_po). The low frequency time constant is assigned to the polarization resistance (R_p) and the admittance associated with the double layer capacitance (CPE_dl). The equivalent circuit in Figure 8b consists of the solution resistance, the double layer capacity, the polarization resistance, and the Warburg diffusion element (W). The electrochemical parameters derived from fitting the measured data using the equivalent circuits are listed in Table 2. As is clearly shown in this table, the increased polarization resistance is related to the presence of the C₃N₄ nanosheets. Moreover, the NiP-C₃N₄ composite coating has the lowest double layer capacitance (39 µF·cm⁻²·s⁻ⁿ) and the higher value of n (0.9) compared to those of the NiP coating. Taken together, these characteristics lead to the superior protection efficiency of the composite coating reaching as high as 95%. The protection efficiency of the NiP coating is approximately 70.9%, which is less than that of the composite due to its porosity, which allows the aggressive chloride ions to diffuse into the substrate.

After heat treatment, the polarization resistances of both heat-treated NiP and NiP-C₃N₄ are decreased compared to the corresponding polarization resistances for the as-plated coatings, but are still much greater than that of the substrate. The protective ability of the as-plated NiP and NiP-C₃N₄ composite coatings decreases after the heat treatment by approximately 11% and 9.5%, respectively, as shown in Table 2. However, the protection efficiency of the heat-treated NiP-C₃N₄ composite coating is still higher by 13% and 4% than that of the heat-treated and as-plated NiP coatings, respectively. The decrease in the corrosion protection observed for both heat-treated coatings is due to the formation of nickel phosphide (Ni₃P) that reduces the phosphorus content of the remaining material and transforms the coating from amorphous to crystalline. Previous work has shown that the amorphous alloys have better corrosion resistance than their corresponding crystalline due to the formation of glassy films that passivate their surfaces [33].

The n values for both NiP and NiP-C₃N₄ coatings before and after the heat treatment lie between 0.6 and 0.9 (Table 2). This indicates that the system is far from the ideal capacitive behavior. The deviation from the ideal capacitive behavior is related to the inhomogeneity of the coating surface attributed to the roughness and surface porosity of the coating. According to the obtained values of the CPE_dl presented in Table 2, the NiP-C₃N₄ composite coating before and after the heat treatment has the most homogeneous surface with a lower porosity compared to that of the NiP coating. Consequently, a dense NiP-C₃N₄ composite coating is formed on the substrate that is slightly affected by high temperatures.

3.6.2. Tafel Analysis

The potentiodynamic polarization curves for the substrate and electroless NiP, as well as NiP-C₃N₄ coatings before and after the heat treatment in 3.5 wt % NaCl solution at room temperature, are shown in Figure 9. The electrochemical parameters (corrosion potential (E_corr), corrosion current density (i_corr), cathodic and anodic Tafel slopes, and corrosion inhibition efficiency IE) are presented in

Table 3. Electrochemical parameters of different coatings before and after heat treatment derived from polarization curves shown in Figure 9.

Type of Coating	Ecorr (mV)	icorr (µA·cm−2)	ba (V/decade)	bc (V/decade)	Corrosion Rate (mpy)	IE (%)
Substrate	−607	21.4	0.1	0.17	3.55	–
NiP	−542	6.9	0.09	0.15	2.91	71.4
NiP-C3N4	−309	1.8	0.15	0.12	1.4	91.5
NiP(HT)	−546	8.4	0.04	0.11	6.5	60.4
NiP-C3N4(HT)	−444	3.8	0.11	0.12	2.26	82

. E_corr of the NiP coating (−542 mV) is shifted significantly in the anodic direction compared to the C-steel, and that of the NiP-C₃N₄ is even more anodically shifted to −309 mV. i_corr of the substrate, NiP, and NiP-C₃N₄ composite coatings are 21.4, 6.9, and 1.8 µA·cm⁻², respectively. The decrease in the i_corr of the NiP-C₃N₄ composite coating compared to that of the NiP coating reveals a better corrosion resistance for the new composite coating. After heat treatment, E_corr of the NiP and the NiP-C₃N₄ composite coatings are shifted in the cathodic direction. In addition, the i_corr values of the NiP and NiP-C₃N₄ coatings are slightly increased compared to those before the heat treatment. This is due to the formation of crystalline Ni₃P. Consequently, an increase in i_corr results in a decrease in the IE to 60% and 82% for the heat-treated NiP and NiP-C₃N₄ coatings, respectively. It is worth mentioning that the IE of the heat-treated NiP-C₃N₄ composite coating is still higher than those of the NiP coating before and after the heat treatment. This is attributed to the existence of C₃N₄, which enhances the corrosion resistance by forming a more compact composite structure via uniform dispersion into the NiP matrix (confirmed by EDX mapping) and blocking the defects in the NiP coating [29].

4. Conclusions

The electroless deposition of the NiP-C₃N₄ nanocomposite coating is successfully achieved by sonicating the C₃N₄ nanosheets in the electroless NiP bath under the same conditions used for NiP electroless deposition. The morphology, structure, roughness, wettability, hardness, and corrosion resistance of the novel electroless NiP-C₃N₄ nanocomposite coating in comparison with the conventional NiP coating before and after heat treatment show superior properties of the new nanocomposite compared to the NiP alloy. The structure of the as-deposited NiP coating is amorphous, whereas that of the as-deposited NiP-C₃N₄ composite coating is crystalline-amorphous. The microstructure of the NiP coating is affected by the existence of the C₃N₄ nanosheets, which are uniformly spread within the NiP matrix. The existence of the C₃N₄ nanosheets and the heat treatment significantly enhances the microhardness of the NiP coating. The orderly presence of the C₃N₄ nanosheets in the coating led to the increase in the protection efficiency of the as-plated composite coating in a 3.5 wt % NaCl solution to 95% based on EIS results. After heat treatment, the formation of the crystalline Ni₃P phase slightly decreased the corrosion resistance of both the NiP and NiP-C₃N₄ nanocomposite coatings. However, the NiP-C₃N₄ nanocomposite still shows a corrosion protection efficiency that is higher than that of the NiP even before heat treatment.