Effect of electroless bath composition on the mechanical, chemical, and electrochemical properties of new NiP–C3N4 nanocomposite coatings

Surface & Coatings Technology 362 (2019) 239–251

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Effect of electroless bath composition on the mechanical, chemical, and electrochemical properties of new NiP–C3N4 nanocomposite coatings

T

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Eman M. Fayyada,1, Aboubakr M. Abdullaha, , Adel M.A. Mohamedb, George Jarjourac, Zoheir Farhatc, Mohammad K. Hassana a

Center for Advanced Materials, Qatar University, Doha P.O. Box 2713, Qatar Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Box 43721, Suez, Egypt c Department of Process Engineering and Applied Science, Materials Engineering Program, Dalhousie University, Halifax, NS B3J 2X4, Canada b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electroless Ni-P C3N4 Nanocomposite coatings Surfactant Corrosion EIS

C3N4 nanosheets, which have high hardness and excellent thermal and chemical stability, were used in the electroless nickel-phosphorous (NiP) bath to deposit the NiP–C3N4 nanocomposite coatings (NCCs) on API X100 carbon steel (CS). The effect of plating time, pH, surfactant type and C3N4 concentration on the mechanical, chemical, and electrochemical properties of the electroless NiP–C3N4 NCC were thoroughly stated. The morphology and structure of the NiP–C3N4 NCC were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The results indicated that the alkaline electroless bath produced a NiP–C3N4 NCC with a microhardness 250 HV200 higher than that produced from the acidic one. A small addition of polyvinyl pyrrolidone (PVP) as a surfactant was sufficient for increasing the homogeneity of the nanocomposite coating and, consequently, its microhardness. The corrosion performance of the NiP–C3N4 NCC prepared at the optimum conditions was studied in a 3.5 wt% NaCl solution through electrochemical impedance spectroscopy (EIS) and Tafel analysis techniques. An electroless bath of 0.5 g L−1 C3N4 offered a nanocomposite coating with the highest microhardness and superior corrosion resistance. It has an efficiency of approximately 96% that decreased gradually, losing about only 2% after one week and 20% after one month of immersion time in the aforementioned chloride solution.

1. Introduction The rising demand to mitigate corrosion and improve the wear resistance of the metals and alloys used for engineering applications cannot be ignored or overemphasized. Coatings, generally, are broadly used in the oil and gas industries. Electroless plated coatings, specifically, play a central role in the coating of the internal parts of pipes, valves, nuts, risers, pumps, and special tools for their unique capability to uniformly plate objects with intricate geometries [1–4]. Electroless plating is an autocatalytic chemical reduction process for ions in an aqueous solution. Electroless plated NiP (ENP) coating is a model for electroless plated alloys that are used in many industries. Although ENP coatings have excellent features such as good wear, corrosion resistance, and high hardness [5–8], the improvement of their properties via insertion of hard nanoparticles (e.g., SiC, TiC, W, TiN, etc.) and/or self-healing agents advances and revolutionizes this coating application [9–14]. The incorporation and distribution of second phase

nanoparticles throughout the NiP matrix are influenced by several factors, such as the concentration of the particles in the plating bath, speed of the agitation, plating time, pH and temperature of the bath, concentration of the nickel ions and the reducing agent, and the presence of a surfactant [15–19]. Surfactant is often added to increase the wettability of the ENP coating [20]. It is reported that cationic, anionic, and non-ionic surfactants affect the quality of the NiP composite coating [21,22]. The incorporation of hard carbon nitride (C3N4), characterized by high hardness and excellent thermal and chemical stability, in the NiP matrix has only been investigated in our previous work [23]. We used the electroless plating technique to prepare NiP and new NiP–C3N4 nanocomposite coatings and studied their morphology, roughness, microhardness, and corrosion properties with and without heat treatment [23]. The objective of the present work is to fully study this new electroless plated NiP–C3N4 nanocomposite coating. The study includes, firstly, investigating the effect of the various electroless bath

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Corresponding author. E-mail address: [email protected] (A.M. Abdullah). 1 Permanent Address: Physical Chemistry Department, National Research Centre, P.O. Box 12622, Dokki, Giza, Egypt. https://doi.org/10.1016/j.surfcoat.2019.01.087 Received 30 November 2018; Received in revised form 8 January 2019; Accepted 24 January 2019 Available online 26 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

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Secondly, the electroless NiP–C3N4 NCC was prepared with different C3N4 concentrations (0.25, 0.5, 1, and 2 g L−1) at the obtained optimum conditions, which produce the best NiP–C3N4 NCC regarding morphology and the mechanical properties, of plating time, pH, and surfactant type that were examined in the first part of the study. Carbon nitride (C3N4) was synthesized, as reported by our previous work [25]. The synthetization method of C3N4 can be shown in SI1.

parameters, such as the effect of plating time, pH, and surfactant type on the morphology, composition (P wt%), thickness, deposition rate, and microhardness of the NiP–C3N4 nanocomposite coatings. Secondly, the effects of C3N4 concentration on the morphology, composition, thickness, deposition rate, microhardness, and corrosion behavior of the nanocomposite coating prepared under the optimized conditions (plating time, pH, and surfactant type) were detailed. This study is significantly useful as it helps in optimizing the properties of the new NiP–C3N4 nanocomposite towards better mechanical and corrosion resistance properties.

2.3. Characterization of the coatings The evaluation of the surface morphology and elemental analysis of the synthesized coatings was done using a scanning electron microscope (SEM, Nova NanoSEM 450, ThermoFischer Scientific, Eindhoven, Netherland) and energy-dispersive spectroscopy (EDX, Bruker detector 127 eV, Bruker, Leiderdorp, Netherlands), respectively. The thickness of the ENP coating was also measured using SEM by cross-sectioning the coated specimens. Vickers microhardness tester (FM-ARS9000, FutureTech Corp., Tokyo, Japan) was utilized to measure the microhardness at a load of 200 g for 10 s. To get accurate results, the hardness measurements were repeated five times on each sample, and the average value was calculated. 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) atomic force microscope equipped with a silicon probe (Al reflex coated Veeco model–OLTESPA, Olympus, Tokyo, Japan; Spring constant: 2 Nm−1, resonant frequency: 70 kHz) was utilized for all roughness experiments. Measurements were conducted at ambient conditions using the Standard Topography AC in the air (tapping mode in air). C3N4 nanosheets were characterized using a Japanese JEOL 2100 F high-resolution transmission electron microscope (HR-TEM) operating at 200 kV and coupled with an energy dispersive x-ray unit. The sample preparation was conducted by mixing a small quantity of sample with ethanol and sonicating it for 15 min. The sample was then loaded onto the TEM grid. X-ray diffractometry (XRD, Miniflex2 Desktop, Cu K, Rigaku, Tokyo, Japan) was used to analyze the effect of the different concentrations of C3N4 on the structure and the phases of the NiP–C3N4 NCCs. For the zeta potential measurements, a suitable amount of C3N4 was dispersed with the surfactant in water at room temperature and their suspension was continuously stirred for 3 h. The zeta potential measurements were conducted using a Malvern Zetasizer NanoZS system (Royston, United Kingdom) at 632.8 nm He–Ne laser irradiation.

2. Experimental procedure 2.1. Materials and preparation of substrates API X100 carbon steel substrates, with a size of 20 × 30 × 10 mm3, were purchased from Tianjin Tiangang Guanye Co., Ltd. (Tianjin, China). The chemical composition of this steel (in wt%) is C (0.07), Si (0.30), Mn (1.90), Ni (0.33), Mo (0.17), Cu (0.20), Nb (0.05), and Ti (0.02), and Fe. The steel substrates were polished with 220, 320, 600, 1000, 1200, and 2000-grit emery papers, followed by polishing with the micro-polish suspension to obtain a mirror finish surface. Alkaline cleaning was done in a solution of (50 g L−1 NaOH, 30 g L−1 Na2CO3, and 30 g L−1 NaPO4) for 5 min at 70–80 °C. Finally, the substrates were etched in 15 wt% H2SO4 solution for 20 s. The substrates were then rinsed with plenty of deionized water after each of the pretreatment steps. The cleaned substrates were immersed immediately after cleaning in the electroless bath solution. All solutions were prepared from analytical grade reagents obtained from Sigma-Aldrich (Munich, Germany) using deionized water. 2.2. Bath of electroless deposition Firstly, the electroless NiP–C3N4 nanocomposite coating (NCC) was prepared at a fixed concentration of C3N4 (0.5 g L−1) with a different time, pH, and surfactant type. The main electroless NNC's bath composition and all operational parameters used are shown in Table 1. To evaluate the effect of the plating time on the NCC characteristics, different times, namely 1, 2, 3, and 5 h, were used where the other parameters were kept at “values shown in Table 1”. Similarly, to check the effect of the pH (4.5, 6.5, and 8) and type of surfactant, all parameters were as shown in Table 1, except the parameter under study. 0.05 g L−1 of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CETB), or polyvinylpyrrolidone (PVP) was used to study the effect of surfactant type as anionic, cationic, and non-ionic or polymeric surfactant, respectively. The plating solution was ultra-sonicated for 1 h to disperse the C3N4 uniformly in the electroless bath to avoid agglomeration. Then, a magnetic stirrer was used under 300 rpm during the electroless plating process. At the end of the process, the specimens were rinsed with deionized water and dried by being air blown. 10 wt% sulfuric acid and sodium hydroxide were used for the pH adjustment. Sodium citrate was used instead of sodium acetate as a complexing agent in the alkaline bath to reduce the probability of the formation of the nickel hydroxide that retards the adjustment of the alkaline pH value [24].

2.4. Corrosion study of the coatings 2.4.1. Electrochemical impedance spectroscopy measurements The corrosion resistance of both electroless NiP and NiP–C3N4 NCCs prepared at 3 h plating time, pH of 8, with PVP surfactant and different C3N4 concentrations was investigated by the electrochemical impedance spectroscopy (EIS) technique at 25 °C using a GAMRY 3000 potentiostat/galvanostat/ZRA (Warminster, PA, USA). A three-electrode cell was used, where the coated specimen, Ag/AgCl electrode, and graphite rod acted as the working, reference, and counter electrodes, respectively. A 10 mV AC excitation was used with the frequency range between 1 × 10−2 to 1 × 105 Hz was used. The recording EIS data were always started after the open circuit potential was stabilized. An area of 2 cm2 of the working electrode surface was exposed to a solution of 3.5 wt% sodium chloride. An immersion test was also used to measure the corrosion behavior of the NCC with the best concentration of C3N4 which maintained the highest corrosion resistance after 3, 24, 168, 336, and 720 h in 3.5 wt% NaCl solution.

Table 1 Composition and operational parameters of the electroless bath. Constituent

Value (g L−1)

Parameter

Value

NiSO4 NaH2PO2 Sodium acetate NH4Cl Thiourea C3N4

15 30 30 15 0.002 0.5

Temperature Stirring Plating time pH Surfactant –

85 ± 1 °C 300 rpm 2h 4.5 ± 0.1 None –

2.4.2. Potentiodynamic polarization (Tafel test) Potentiodynamic polarization (PP) tests were performed after immersing the specimens in a 3.5% (w/w) NaCl solution and holding them at the open-circuit potential for 20 min. Anodic and cathodic polarization curves were obtained using a scan rate of 0.167 mV s−1 within a 240

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Fig. 1. TEM micrograph of C3N4 powder.

potential window of −250 to 250 mV. The potential scanning always started from the cathodic potentials towards the anodic one.

the nodules gradually decreases, the coarse-grained structure disappears, and the distance between adjacent nodules becomes smaller, as seen in Fig. 2b and c. After 5 h of plating, C3N4 particles appeared as uniform protrusions distributed in large numbers over a whole NiP deposit surface. The protrusion structure makes the composite coating rougher than those composite coatings obtained at a plating time of < 5 h. Fig. 3a–d show the AFM images of the NiP–C3N4 NCCs after 1, 2, 3, and 5 h of plating time, respectively, in a non-tapping mode. It is observed that increasing the content of the C3N4 in the nanocomposite coating leads to little increase in its surface roughness during the first 3 h of plating time. After 5 h of plating time, the surface roughness of the nanocomposite coating increases about 12 nm compared to that one obtained after 3 h of plating time. Lowering the number of spherical nodules means decreasing the deposition of the nickel in the coating. Indeed, the EDX measurements shown in Table 2, which summarize the weight percentages of C, N, Ni, and P in the coating as a result of the change in the plating time, display that the nickel content in the coating is decreased about 10 wt% at a plating time of 5 h compared to that at 1 h. Moreover, the content of C and N increases as the deposition time increases, indicating that the physical deposition of the C3N4 in the NiP coating may occur before the co-deposition of Ni and P ions, as reported in similar work [27]. This may help in hindering the deposition of the Ni ions. Furthermore, the thickness of the NCC increases as the plating time increases despite the decrease in the deposition rate as the deposition time increases. The latter is attributed to the depletion of the ions in the plating bath as the deposition time increases. This is illustrated in Table 2. The deposition rate (R) (mg h−1 cm−2) of the ENP alloy could be calculated using the gravimetric method [28] and expressed in terms of the weight gain during the deposition process using the following equation,

3. Results and discussion 3.1. Optimization of the plating time, pH, and surfactant type Based on the electroless bath parameters, the characteristics of the prepared NiP–C3N4 NCCs will be changed. In this part of the study, the effects of plating time, pH, and surfactant type of the electroless bath on the morphology, composition, thickness, deposition rate, and hardness of the NiP–C3N4 NCCs were studied at a fixed concentration of C3N4 (0.5 g L−1). C3N4 resembles sheets on a nano scale, as observed in TEM measurements shown in Fig. 1. In addition, SEM micrograph can be shown in SI2. 3.1.1. Effect of plating time It is reported that the incorporation and distribution of the hard particles throughout the NiP surface are influenced by the plating time [26]. Fig. 2a–d are SEM micrographs for the electroless plated NiP–C3N4 NCCs obtained at 1, 2, 3, and 5 h plating time, respectively, at a pH of 4.5 in the absence of any surfactant. At 1 h, the C3N4 are codeposited but poorly distributed throughout the NiP surface coating, with agglomeration in some places to a certain degree, even under the conditions of ultrasonic and magnetic stirring. These agglomerations appear as dark chunky zones in the SEM micrographs, as indicated by the arrows shown in Fig. 2a. EDX measurements of these dark zones are documented in Fig. 2e. It proves the presence of C and N at a higher content in these coating zones. For more clarification, Fig. SI3a and b show SEM micrographs at a higher magnification for the NiP–C3N4 NCC, prepared after 1 h of plating time, and its EDX mapping for C and N content. As the plating time passes, the C3N4 becomes homogeneously distributed throughout the NiP matrix. In addition, the number and size of

R = (Wf − Wi )/ AT t

(1)

where Wi and Wf (mg) are the weights of the substrate before and after 241

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a

b

C3 N 4

c

d

Norm. C (wt.%) 20.26 14.66 6.30 58.78

C N P Ni

Atom. C (at.%) 42.83 26.58 5.17 25.43

Error 3.88 3.16 0.29 1.68

e Fig. 2. SEM micrographs for the NiP–C3N4 NCCs obtained after (a) 1, (b) 2, (c) 3, and (d) 5 h of plating time, at a pH of 4.5, without surfactant, and with 0.5 g L−1 C3N4. (e) EDX measurements for the chunky zones that appeared in the NiP–C3N4 NCC shown in Fig. 1a.

plating, respectively, AT is the surface area of the substrate (cm2), and t is the plating duration (h). It is reported that microhardness is dependent on the content of the phosphorous in the coating. As it decreases, the microhardness

increases [28,29]. An increase of approximately 40 HV200 in the microhardness of the composite coating is observed with an increase in the plating time from 1 to 3 h, as shown in Table 2. After 5 h of deposition, the microhardness is greatly diminished about 100 HV200, 242

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a

b

20.0 nm

18.2 nm c

d

24.5 nm

36.2 nm

Fig. 3. AFM images (3D) of the surface of the NiP–C3N4 NCCs obtained after (a) 1, (b) 2, (c) 3, and (d) 5 h of plating time, at a pH of 4.5, without surfactant, and with 0.5 g L−1 C3N4.

has a heavily compact nodule-like structure with a uniform distribution of fine C3N4 inside the boundaries and spaces of the NiP coating, as previously reported [31], since no agglomerated dark areas of C3N4 appeared. As depicted in Fig. 4c and d, which are the magnified SEM micrograph of the rectangular area shown in Fig. 4b and its corresponding EDX mapping, respectively, there is a homogeneous distribution of the different elements present in the NiP–C3N4 NCC. For more clarification, Fig. SI4 shows the EDX mapping for each element separately. The EDX analysis summarized in Table 3 shows the quantities of the N, C, Ni, and P in the deposit as a function of the pH after 2 h of plating time. It is evident that the quantity of the Ni in the deposit increases with the increasing pH of the electroless solution bath and, consequently, this increased the deposition rate. This result is in good agreement with the results in the current literature [32,33]. Also, the percentage of the P in the deposit is 9.50 wt% at a pH of 4.5, which decreases to 6.88 in the alkaline solution. This is attributed to the retardation of the reduction reaction of P as the concentration of OH− ions in the bath increases [33]. NaH2PO2 is used in the bath as a reductant. The redox potential of hypophosphite ions (H2PO2−) and nickel ions are −0.5 and −0.25 V, respectively [33]. The deposition reactions for the Ni and P are presented as follows.

although the P wt% is decreased. This may be attributed to the nonuniform distribution of the C3N4 particles and the rough structure of the composite coating, which is described above in Figs. 2d and 3d, respectively. In addition, the increase in the amount of the C3N4 in the coating, which takes place after 5 h of deposition time, leads to an agglomeration which helps in the voids formation. This may be the reason that the microhardness decreases after 5 h of plating time. Therefore, it can be claimed that the microhardness is not controlled by the deposition time which is in accordance with various works in the literature [28,30]. It could be concluded that the optimum deposition time for the NiP–C3N4 NCC is at either 2 or 3 h, at which the nanocomposite has the most homogeneous structure and the highest microhardness. 3.1.2. Effect of pH variation The kinetics of the deposition process is greatly influenced by the pH of the electroless bath. Fig. 4a and b show the surface morphology of the NiP–C3N4 NCCs at a pH of 6.5 and 8, respectively, prepared after 2 h in the absence of any surfactant. It is clear that the surface of the composite coating prepared in a solution with a pH of 6.5 has more nodules with a lesser number of apparent dark spots of C3N4, which are distributed over all the surface, compared to that prepared in the solution of pH 4.5. This is shown in Figs. 4a and 2b, respectively. Fig. 4b shows that the surface of the composite coating prepared at a pH of 8

H2 PO2− + H2 O → H+ + HPO32 − + 2Hads

(2)

Table 2 EDX analysis, deposition rate, thickness, and the microhardness of the NiP–C3N4 NCCs obtained after different plating times, at a pH of 4.5, without surfactant, and with 0.5 g L−1 C3N4. Plating time (h)

N (wt%)

C (wt%)

Ni (wt%)

P (wt%)

Rate of deposition (mg h−1 cm−2)

Thickness (μm)

Hardness (HV200)

1 2 3 5

3.71 4.20 4.96 8.70

5.39 7.00 7.68 11.21

81.00 79.30 78.03 71.09

9.90 9.50 9.33 9.00

11.50 9.95 9.5 8.5

24 35 40 45

450 487 490 380

243

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a

b

c

d

Fig. 4. SEM micrographs of the NiP–C3N4 NCCs obtained at a pH of (a) 6.5 and (b) 8 for 2 h of plating time, without surfactant, and with 0.5 g L−1 C3N4. (c) The magnified SEM micrograph and (d) the corresponding EDX mapping of the rectangular area present in (b).

In the alkaline electroless bath, the NiP–C3N4 NCC that contains the lower percentage of C3N4 is the thickest one with the highest microhardness and the most compact structure compared to those obtained at neutral (pH 6.5) and acidic (pH 4.5) solutions (Table 3). This indicates that, to some extent, the lower content of C3N4 in the coating is, the more uniform the distribution of C3N4 in the NiP matrix and the higher the microhardness of the coating will be.

Table 3 EDX analysis, deposition rate, thickness, and the microhardness of the NiP–C3N4 NCCs obtained at different pH values, for 2 h plating time, without surfactant, and with 0.5 g L−1 C3N4. pH

N (wt%)

C (wt%)

Ni (wt%)

P (wt%)

Rate of deposition (mg h−1 cm−2)

Thickness (μm)

Hardness (HV200)

4.5 6.5 8.0

4.20 2.51 2.40

7.00 6.55 3.29

79.30 81.36 87.43

9.50 8.58 6.88

9.50 12.0 14.5

35 42 48

487 500 690

Ni2 + + 2Hads → Ni + 2H+

(3)

H2 PO2− + Hads → P + OH− + H2 O

(4)

3.1.3. Effect of surfactant Fig. 5a–c show the morphology of the NiP–C3N4 NCC deposited from the electroless bath that has anionic, cationic, or polymeric surfactants, respectively, after 2 h of plating time and at a pH of 4.5. It can be observed that the surfaces of the NiP–C3N4 NCCs that are deposited from the electroless bath with a surfactant (Fig. 5) are smoother and more homogeneous than those deposited from one without a surfactant (Fig. 2b). This is because the presence of the surfactant prevents, to some extent, the agglomeration of the added particles, leading to its uniform distribution and occupation for the spaces of the NiP matrix [34]. Additionally, the cross-sections of the NiP–C3N4 NCCs that are prepared in the presence and absence of surfactant show a clear change in the homogeneity and the structure of the composite coating on the substrate. A lack of defects at the interface between the substrate and

From Eq. (3), it can be concluded that the higher the concentration of H+ in the electroless bath is, the less the deposition of the Ni will be. At low pH values, the percentage of Ni content in the coating is low. As the pH increases, Reaction (3) is shifted to the right direction, causing a decrease in the concentration of H+, i.e., an increase in the Ni content in the coating. On the contrary, as the pH of the electroless bath decreases, the concentration of the OH− ions decrease and, consequently, the percentage of the P in the coating increases, which is shown in Table 3. 244

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a

b

c

d

e

f

g

Fig. 5. SEM micrographs of the NiP–C3N4 NCCs obtained from the electroless solution using (a) anionic (SDS), (b) cationic (CETB), and (c) polymeric (PVP) surfactants for 2 h of plating time at a pH of 4.5. The cross-section of the NiP–C3N4 NCC (d) with surfactant (PVP) and (e) without surfactant, and (f, g) the corresponding EDX mapping, respectively.

significant role in the shaping of the coating due to various interfacial behaviors [35,36]. Noticeably, the use of an anionic surfactant leads to the formation of many small distinct nodules that decrease the surface porosity of the composite coating (Fig. 5a). Using a cationic surfactant showed little terrains and pores on the composite coating surface (Fig. 5b). The smoothest and more compact surface morphology with a uniform distribution of C3N4 in the NiP matrix appeared when a polymeric surfactant was used, as shown in Fig. 5c and the corresponding EDX mapping (Fig. 5f). Furthermore, the addition of a surfactant in the electroless bath

the composite coating result in good adhesion and a compact structure with the use of the surfactant. This is evident in Fig. 5d, which shows the cross-section of the composite coating prepared in the presence of PVP. The EDX mapping of the NCC in Fig. 5d (Fig. 5f) illustrates the homogeneous distribution of the C3N4 in the NiP matrix. In contrast, some voids and some C3N4 agglomerated areas can be noticed when the composite coating is prepared without using the surfactant, as shown in Fig. 5e and the corresponding EDX mapping (Fig. 5g). It is notable that the surface morphology and the porosity of the coating are highly affected by the type of the surfactant, which plays a

245

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Table 4 EDX analysis, deposition rate, thickness, and the microhardness of the NiP–C3N4 NCCs obtained using different types of surfactants for 2 h of plating time at a pH of 4.5. Surfactant type

N (wt%)

C (wt%)

Ni (wt%)

P (wt%)

Rate of deposition (mg h−1 cm−2)

Thickness (μm)

Hardness (HV200)

– SDS

4.20 3.04

7.00 5.11

79.30 83.10

9.50 8.75

9.50 14.0

35 45

487 550

6.22

4.10

80.09

9.59

12.5

42

510

4.70

4.27

84.07

7.03

16.5

50

600

(Anionic) CETB (Cationic) PVP (Polymeric)

1.0, and 2.0 g L−1) of NiP–C3N4 NCC—prepared at optimum conditions (3 h of plating time, a pH of 8, and PVP as surfactant) acquired in the first part of this study—on its morphology, composition, thickness, deposition rate, and microhardness. An electroless NiP coating is prepared, under the same conditions, to use as a control specimen in the electrochemical studies.

increases the deposition rate of the composite coating to about 51%, on average. This is attributed to reducing the interfacial tensions between the hydrogen bubbles and the deposited substrate and/or the bubbles and the plating solution. Reducing surface tension leads to the quick removal of the adsorbed hydrogen gas bubbles. Since hydrogen gas is one of the products of the electroless deposition reduction reactions, removing it shifts the deposition reaction to the direction of the reduction of Ni2+ ions [36]. Accordingly, the presence of a surfactant enhances the deposition rate, as seen in Table 4. The lowest deposition rate of the composite coating occurs when the cationic surfactant (CETB) is added. This refers to the probability of the formation of ion pairs of the CET cations and the hypophosphite anions at the interface between the substrate and the plating solution, which can minimize the deposition process [36]. The deposition rate when an ionic surfactant is used is lower than that when a polymeric one is used. This may be attributed to its electrostatic interactions that inhibit the reduction of the Ni2+ and, hence, minimize the Ni content in the coating [37]. According to the EDX results in Table 4, the addition of a cationic surfactant increases the incorporated amount of C3N4 in the coating compared to the anionic and polymeric ones. This can be explained based on the zeta potential measurements, which showed that C3N4 in the absence of any surfactant has a zeta potential of −35 mV, which abruptly changes to +14 mV upon the addition of the cationic surfactant (CETB), as shown in Fig. SI5a and b. The large positive shift of the zeta potential enhances the possibility of the electrostatic adsorption of C3N4 on the cathodic sites [38–40], whereas the addition of the anionic surfactant shifts the zeta potential of the C3N4 to a more negative value (−40 mV) (Fig. SI5c). This leads to a decrease in the incorporation of C3N4 in the deposited coating, which is confirmed by the nitrogen content in the EDX results shown in Table 4. The presence of the polymeric surfactant with the C3N4 in the electroless bath relatively increased the zeta potential to −21 mV, as shown in Fig. SI5d. This is more positive than that of C3N4 in the surfactant-free electroless bath. This explains the increase of the incorporation of C3N4 to the coating in the presence of PVP compared to the surfactant-free case. Fig. SI6a and b and the corresponding EDX mapping (Fig. SI6A and B) of the crosssections of the NiP–C3N4 NCC obtained using the anionic and cationic surfactants, respectively, clarify the incorporation and distribution of C3N4 in each one. Although the incorporation of C3N4 in the composite coating when polymeric surfactant is used is lower than that when cationic surfactant is used and higher than that when an anionic surfactant is used, the former is found to have a higher microhardness. This may be attributed to the lower P content and the presence of C3N4 in an organized distribution in the NiP matrix forming a compact structure on the surface. As can be seen in Table 4, the higher incorporation of the C3N4 in the composite coating is not preferred as it results in a lower microhardness.

3.2.1. Morphologies, structure, and microhardness Fig. 6 depicts the surface morphologies of the NiP–C3N4 NNCs with and without different concentrations of C3N4. Table 5 reveals the average coating compositions with and without different concentrations of C3N4. The variation of C3N4 contents has various influences on the quality of the surface morphology of the composite coating. It is noted that the addition of 1 g L−1 or less of C3N4 in the electroless bath does not change the cauliflower-like nodule structure that distinguishes the amorphous NiP coating, which is proven in several references [41,42]. Nevertheless, these concentrations influence the size of the nodules and their uniformity. As the concentration of the C3N4 particles increases up to 0.5 g L−1, such nodules become smaller, smoother, and more uniform, as shown in Fig. 5b–c. Additionally, for more clarification, Fig. SI7a shows the trapping of C3N4 in the spaces of one grain of the NiP deposit. The corresponding EDX analysis is also shown beside Fig. SI7b. It can be claimed that the presence of C3N4 in the nodular boundaries affects the nodule growth accordingly. Despite the decrease in the size of the nodules with the increase of the concentration of C3N4 up to 1 g L−1, the incorporation percentage of the C3N4 increases, as shown in Table 5. An additional increase in the concentration of C3N4 to 2 g L−1 results in a higher incorporation percentage of C3N4 in the coating. Extreme incorporation of C3N4 leads to the formation of a fibrous-like structure that leads to the loss of uniformity, as shown in Fig. 6e. In this case, the surfactant cannot prevent the agglomeration of C3N4. Table 5 shows that increasing the C3N4 concentration in the electroless bath results in a reasonable decrease in the deposition rate, which decreases sharply at a C3N4 concentration of 2 g L−1. This can be attributed to the possible physical adsorption of some C3N4 sheets on the catalytic surface, resulting in the minimization of the available active sites for the deposition process. This leads to a decrease in the overall deposition rate [10]. Accordingly, the thickness of the composite coating decreases, as shown in Table 5. Additionally, it can be seen in Table 5 that the higher incorporation of C3N4 is associated with reducing the P wt%. As previously mentioned, a lower P wt% leads to a higher microhardness. However, there is no regular trend for the microhardness when the C3N4 concentration is increased, since the highest microhardness is observed at a C3N4 concentration of 0.5 and not 2 g L−1. Fig. 7a and b show the XRD patterns of C3N4 and electroless NiP and NiP–C3N4 nanocomposite coatings with different C3N4 concentrations (0.25, 0.5, 1, and 2 g L−1), respectively. The as-deposited NiP has a single broad peak at the 2θ position of 45°, as shown in Fig. 7b, which is correlated to the face-centered cubic (FCC) Ni (111) plane. Furthermore, XRD patterns for all different samples of the nanocomposite

3.2. Effect of C3N4 concentrations at optimum conditions In this part, the effect of different C3N4 concentrations (0.25, 0.50, 246

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a)

b)

c)

d)

e)

Fig. 6. SEM micrographs of the NiP–C3N4 NCCs prepared with (a) 0, (b) 0.25, (c) 0.50, (d) 1.0, and (e) 2.0 g L−1 C3N4 for 3 h of plating time, at a pH of 8, and in the presence of PVP as a surfactant.

Bode and the corresponding phase angle plots of steel coupons coated with NiP–C3N4 NCs with different concentrations of C3N4 (0, 0.25, 0.5, 1 and 2 g L−1) and immersed in 3.5 wt% NaCl solution at room temperature at open-circuit potential. The coating is highly protective when it has a high impedance |Z| at low frequencies, i.e., high polarization resistance (Rp) and low capacitances [43]. Evidently, the |Z| value at the low frequency of the composite coatings with and without different concentrations of C3N4 is higher than that of the substrate. This proves that the protective properties against corrosion for NiP and NiP–C3N4 coatings are due to the presence of phosphorus [44]. Also, the protection ability of the NiP–C3N4 NCCs deposited from solutions with concentrations up to 1 g L−1 of C3N4 are higher than that of NiP/C3N4-free coating. Moreover, Fig. 8 illustrates that changing the amount of C3N4 in the electroless bath affects the Bode and the phase angle plots. As the concentration of C3N4 in the electroless bath increases up to 0.5 g L−1, the coating resistance increases. Increasing C3N4 concentration to 1 g L−1 reduces the Rp by 40 and 70% compared to NCCs deposited from baths with 0.25 and 0.5 g L−1, respectively. However, it still has approximately 5% improvement in the protection efficiency compared to that of the NiP coating. A further increase in the

coatings have the same peak at the same 2θ position of 45°, without a distinct identification peak for C3N4. This identification peak appears at the 2θ position of 27°, as shown in Fig. 7a. This may be attributed to the small amount and fine size of the C3N4, as well as the high density of Ni diffraction peaks [6,7]. It is well known that the electroless NiP coatings with high phosphorus content (10–13 wt%) have an amorphous microstructure, whereas the ones with low (1–5 wt%) or medium (5–10 wt %) phosphorous content have a crystalline or semi-crystalline microstructure. Therefore, the as-plated NiP coatings have amorphous microstructures, whereas the NiP–C3N4 nanocomposite has a semi-crystalline or mixed amorphous-crystalline microstructure, in line with EDX results. Moreover, the full width at half-maximum (FWHM) value obtained from the XRD pattern of the NiP coatings is 0.2558, whereas those of the nanocomposite coatings with 0.25, 0.5, 1, and 2 g L−1 C3N4 concentrations are 0.2430, 0.2240, 0.2179, and 0.2047, respectively. This implies that the presence of C3N4 in the NiP matrix leads to refining the NiP nodules and boosts the crystalline phase formation. 3.2.2. Corrosion studies 3.2.2.1. Electrochemical impedance spectroscopy (EIS). Fig. 8 shows the

Table 5 Electroless NiP–C3N4 NCCs with and without C3N4, in different concentrations, obtained after 3 h of plating time, at a pH of 8, and in the presence of PVP as a surfactant. C3N4 content (g L−1)

N (wt%)

C (wt%)

Ni (wt%)

P (wt%)

Rate of deposition (mg h−1 cm−2)

Thickness (μm)

Hardness (HV200)

0.00 0.25 0.50 1.00 2.00

– 1.21 2.68 5.90 16.82

– 3.62 4.57 10.16 27.56

89.09 86.66 85.97 77.12 51.40

10.91 8.51 6.78 6.82 4.22

15.0 14.0 13.5 10.5 5.00

55.0 47.8 42.5 33.0 27.0

510 560 740 655 337

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8000

a

1600

C 3 N4

1400

0.25 g/L C 3N4 0.50 g/L C 3N4

6000

1000 800 600

1.00 g/L C 3N4

Intensity (cps)

1200

100

Intensity (cps)

0.00 g/L C 3N4

b

2.00 g/L C 3N4 4000

400

2000

200 0 10

20

30

40

50

60

0

70

20

30

40

50

60

70

80

2 theta (degree)

2 theta (degree)

Fig. 7. XRD pattern of (a) C3N4 and (b) NiP and NiP–C3N4 nanocomposite coatings with different concentrations of C3N4 (0.25, 0.50, 1, and 2 g L−1).

C3N4 concentration to 2 g L−1 results in a sharp decrease in the corrosion resistance of the NiP–C3N4 NCC. At this high concentration of C3N4 (2 g L−1), as previously discussed, the deposition rate decreases. This is because the presence of a high percentage of C3N4 in the coating blocks the active sites for the deposition of more NiP. At the same time, the presence of C3N4 in a large amount in the electroless bath may act as eroding particles that have a harsh effect on the deposited NiP layer as they form a coating with some voids or scratches. It is noted that Bode and phase angle plots for the NiP–C3N4 NCCs that are deposited from baths with lower concentrations of C3N4 (0.25–1 g L−1) are different in shape compared with those that are deposited from baths with 0 and 2 g L−1 of C3N4. The Bode and phase angle plots for the former show one-time constant behavior, as shown in Fig. 9b. The baths with no and 2 g L−1 of C3N4 exhibit broad peaks and reveal the presence of two-time constant behavior (two capacitive responses), as shown in Fig. 9a. The equivalent electric circuits that demonstrate the EIS spectra and simulate the electrochemical behavior of the coatings are depicted in Fig. 9. The circuit in Fig. 9a includes the solution resistance (Rs), the pore resistance (Rpo), the polarization resistance (Rp), and two constant phase elements: CPEcoat and CPEdl for the coating and the metal/solution interfaces, respectively. The CPEcoat and Rpo represent the highfrequency time constant, while the CPEdl and Rp are assigned for the low-frequency time constant. The circuit in Fig. 9b, which represents a one-time constant, comprises only Rs, Rp, CPEdl, and a Warburg diffusion element (W), which illustrates the presence of electrolyte diffusion. In addition, as the value of the phase angle increases, the protection of the coating increases.

The values of Rs, Rpo, Rp, CPEcoat, CPEdl, and W were derived from fitting the measured EIS data for the electroless NiP and NiP–C3N4 coatings using the equivalent circuits shown in Fig. 9. These values are compiled in Table 6. The Rp is enhanced with the NiP–C3N4 NCCs deposited from baths with lower concentrations of C3N4, reaching to a maximum value of 14,560 Ω cm2 at a C3N4 concentration of 0.5 g L−1. This shows about 96% protection efficiency. About 15% depression in the protection efficiency is noticed for the NiP–C3N4 NCC deposited from the bath with a concentration of 2 g L−1 C3N4 compared to that of the NiP coating. The n values of the composite coatings ranged between 0.55 and 0.9, as shown in Table 6, indicating the non-homogeneity of all surfaces' coatings, which does not exhibit ideal capacitive behavior. These differences in the n values are due to the variations in the degree of the surface coating homogeneity from one coating to another. It is noted that the most homogeneous composite coating (i.e., the one that has the highest n value) is one that is deposited from the electroless bath with 0.5 g L−1 C3N4. This coating, additionally, has the lowest value of CPEdl (93.5 μF cm−2). Therefore, it can be concluded that the NiP–C3N4 NCC deposited from the bath with a C3N4 concentration of 0.5 g L−1 is the most homogeneous with the least porosity. Moreover, the C3N4 concentration of 0.5 g L−1 is the optimum one for obtaining the most efficient protection of the NiP–C3N4 NCC in a 3.5 wt% NaCl solution. The Bode/phase plots for the NiP–C3N4 NCCs deposited from a bath with a C3N4 concentration of 0.5 g L−1 (which has the highest corrosion protection) at different times of immersion (i.e., 3, 24, 168, 336, and 720 h) in a 3.5 wt% NaCl solution are shown in Fig. 10. It can be concluded that the immersion time has a significantly different effect on

20 104

CS 0.00 g/l C 3N 4

CS 0.00 g/l C3N4

0

0.25 g/l C3N4 o

1.00 g/l C3N4

.cm2 |Z|,

phase angle,

103

0.25 g/l C 3N 4 0.50 g/l C 3N 4

0.50 g/l C3N4 2.00 g/l C3N4

102

1.00 g/l C 3N 4

-20

2.00 g/l C 3N 4

-40

-60

101 10-2

a

-80 10-1

100

101

102

103

104

105

10-2

Frequency, Hz

b 10-1

100

101

102

103

104

105

Frequency, Hz

Fig. 8. (a) The Bode and (b) corresponding phase angle plots of the substrate, and electroless NiP–C3N4 NCCs with and without different concentrations of C3N4 in a 3.5 wt% NaCl solution at room temperature. 248

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Fig. 9. Equivalent electric circuits of the electroless NiP–C3N4 NCCs with and without C3N4 in a 3.5 wt% NaCl solution. Table 6 Electrochemical parameters obtained by fitting the measured EIS data shown in (Fig. 8) using the equivalent circuit (Fig. 9) for the substrate, and electroless NiP–C3N4 NCCs with and without different concentrations of C3N4. Concentration of C3N4 (g L−1)

Rs (Ω cm2)

Rpo (Ω cm2)

CPEcoat (μF cm−2)

Rp (Ω cm2)

CPEdl (μF cm−2)

W (S s1/2)

n

IE (%)

Substrate 000 0.25 0.50 1.00 2.00

12.3 11.3 16.0 12.3 14.3 10.8

– 187.5 – – – 44.5

– 88.3 – – – 102

575 3300 7500 14,560 4350 1770

435 231 134.6 93.5 170 273

– – 2.160e-3 3.787e-3 2.087e-3 –

– 0.75 0.80 0.90 0.65 0.60

– 82.5 92.0 96.0 86.7 67.5

20 3h 24 h 168 h 336 h 720 h

0

-20

|Z|,

.cm2

103

increases, the polarization resistance of the NiP–C3N4 NCCs decreases and, consequently, the corrosion resistance decreases. Noticeably, the depression in the protection efficiency of the NiP–C3N4 NCCs after one week (168 h) of immersion in the brine solution is only about 4% compared to that after 3 h of immersion. This confirms that the composite coating has good efficiency. With prolonged immersion times, the Rp magnitudes of the NiP–C3N4 NCCs gradually decrease and, consequently, the corrosion resistance also decreases. This can be seen in Fig. 10 and Table 7. This is attributed to the tendency of the chloride ions (Cl−) to be preferentially adsorbed on certain sites of the coatings, such as lattice defects and higher Ni containing areas, as soon as the coatings are submerged in the NaCl solution. This leads to the corrosion of the coating through the formation of soluble NiCl2 [45]. This process continues as the immersion time increases, which results in a decrease in the corrosion resistance of the coating, as seen from the Rp values in Table 7. It is worth mentioning that the NiP–C3N4 NCC has a superior corrosion resistance that reaches 96% and lasts for 30 days, as it only loses 20% of its protection efficiency (Table 7).

-40 102

phase angle, o

104

-60

-80

101 10-2

10-1

100

101

102

103

104

105

Frequency, Hz

Fig. 10. The Bode and corresponding phase angle plots of the NiP–C3N4 NCCs that are deposited from a bath with a C3N4 concentration of 0.5 g L−1 after different times of immersion in a 3.5 wt% NaCl solution at room temperature.

3.2.2.2. Potentiodynamic polarization (PP). For further investigation of the corrosion behavior of the newly developed NiP–C3N4 NCC, the potentiodynamic polarization method is used. The polarization curves (Tafel curves) of the NiP and NiP–C3N4 coatings with different concentrations of C3N4 in a 3.5 wt% NaCl solution are shown in Fig. 11a. Table 8 lists the electrochemical parameters, e.g., corrosion potential (Ecorr), corrosion current (icorr), and anodic and cathodic Tafel slopes (ba and bc) that were derived from the Tafel curves of the coatings. It can be seen broadly that increasing the C3N4 concentration

the electrochemical behavior of the electroless plated NiP–C3N4 NCC. The Bode and phase angle plots for the NiP–C3N4 NCCs after 3, 24, and 168 h of immersion display a one-time constant behavior at the whole frequency domain. At immersion times higher than 168 h, the Bode and phase angle plots reveal a two-time constant behavior, as shown in Fig. 10. The corresponding equivalent circuits that are used to fit the measured EIS data are shown in Fig. 9a and b, and their electrochemical parameters values are shown in Table 7. As the time of immersion

Table 7 Electrochemical parameters obtained by fitting the measured EIS data (Fig. 10) using the equivalent circuit (Fig. 9) for the deposited NiP–C3N4 NCCs after different times of immersion in a 3.5 wt% NaCl solution at room temperature. Immersion time (h)

Rs (Ω cm2)

Rpo (Ω cm2)

CPEcoat (μF cm−2)

Rp (Ω cm2)

CPEdl (μF cm−2)

W (S s1/2)

n

IE (%)

3 24 168 336 720

14.6 13.2 11.2 11.8 10.3

– – – 178 134

– – – 24 36.5

14,560 12,200 9200 4000 2340

93.5 115 132 156 239

3.787e-3 1.021e-3 580.3e-4 – –

0.90 0.8 0.75 0.7 0.7

96.0 95.3 93.8 85.6 75.4

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-0.1 E, V vs. Ag/AgCl

-0.2

E (V vs. Ag/AgCl)

b

0.0

a

0.0

-0.4 -0.6

-0.2 -0.3

-0.8

CS 0.00 g/l C3N4 0.25 g/l C3N4

-1.0

0.50 g/l C3N4 1.00 g/l C3N4

-0.5

2.00 g/l C3N4

-0.6

-1.2 -9

-8

-7

-6

-5

-4

-3

-0.4

3h 24 h 168 h 336 h 720 h -10

-2

-9

-8

-7

-6

-5

-4

-3

-2

Log i, A.cm-2

Log i, A.cm-2

Fig. 11. Tafel polarization curves for (a) the substrate and the electroless NiP–C3N4 NCCs with and without different concentrations of C3N4 and (b) the NiP–C3N4 NCC deposited from the bath with 0.5 g L−1 of C3N4 after different times of immersion in a 3.5 wt% NaCl solution at room temperature. The scan rate is 0.167 m V s−1.

microhardness of the NiP–C3N4 NCC is studied. In addition, the effect of different C3N4 concentrations (0.25, 0.50, 1.0, and 2.0 g L−1) on the structure, mechanical, and electrochemical properties of the NiP–C3N4 NCC produced at the optimum parameters (i.e., 3 h of plating time, a pH of 8, and with the PVP surfactant) are investigated. The following conclusions can be drawn from the current study:

shifts the Ecorr towards more noble values, accompanied with a decrease in the corrosion current up to 0.5 g L−1 C3N4. A further increase in the concentration of C3N4 (1 and 2 g L−1) results in a further increase in the icorr and a negative shift in the Ecorr compared to those of the composite coatings with lower C3N4 concentrations (0.25 and 0.5 g L−1). Therefore, the NiP–C3N4 NCC with a C3N4 concentration higher than 1 g L−1 shows no significant benefit in corrosion protection compared to that of the NiP coating. Fig. 11b shows the Tafel polarization plots for the NiP–C3N4 NCC deposited from the electroless bath with 0.5 g L−1 of C3N4 after different times of immersion in a 3.5 wt% NaCl solution at room temperature. The obtained electrochemical parameters from the polarization curves using Tafel extrapolation are summarized in Table 9. It is obvious that increasing the immersion time shifts the Ecorr of the NiP–C3N4 NCC towards more negative potentials and increases the icorr, i.e., the corrosion rate of the NiP–C3N4 NCC increases and the protection efficiency decreases as the immersion time progresses. It should also be pointed out that the NiP–C3N4 NCC still has about 75% protection efficiency compared to the substrate after 30 days of immersion in NaCl. This proves the good quality of the nanocomposite coating, as the uniform distribution of C3N4 through the NiP matrix retards the diffusion of the corrosive solution to the substrate as previously illustrated. Comparing the results of the EIS and the Tafel polarization curves confirms that both techniques are effective in studying the corrosion behavior of these types of coating, as the difference in the obtained protection efficiencies is within the 5% range.

1- Further increase in the plating time until 5 h in an acidic electroless bath (pH of 4.5) yields NiP–C3N4 NCC with the least microhardness and a large amount of agglomerated C3N4. 2- The alkaline electroless bath (pH of 8) produced a NiP–C3N4 NCC with heavily compact nodulated structures, a uniform distribution of the C3N4, and a higher microhardness. 3- The addition of the surfactant during the deposition process increases the deposition rate about 51% on average, making a clear change in the surface morphology of the composite coating with a void-free cross-section. Although the cationic surfactant (CETB) increases the incorporation percentage of C3N4 in the coating, the polymeric one (PVP) leads to composite coating with the highest microhardness. 4- The NiP–C3N4 NCC has a cauliflower-like nodule structure similar to NiP coating. Addition of smaller concentrations of C3N4 (0.25, 0.5 and 1 g L−1) in the electroless bath results in a composite coating with more compact and higher microhardness than C3N4-free coating and that one containing 0.5 g L−1 C3N4 has the highest compactness and microhardness. Further increase in C3N4 concentration (2 g L−1) produces a NiP–C3N4 NCC with less uniformity, a fibrous-like structure and the lowest microhardness. 5- Based on EIS and potentiodynamic polarization results, the corrosion protection efficiency of the NiP coating significantly improves by the presence of small amounts of C3N4 (0.25, 0.5, and 1 g L−1). However, the addition of higher concentrations of C3N4 (2 g L−1) decreases its protection efficiency by about 15%. The NiP–C3N4 NCC

4. Conclusions The effect of different times of plating (1, 2, 3, and 5 h), pH (4.5, 6.5, and 8), and type of surfactant (anionic (SDS), cationic (CETB) and polymeric (PVP)) on the morphology, composition, deposition rate, and

Table 8 Electrochemical parameters of the substrate and the electroless deposited NiP–C3N4 NCCs with and without different concentrations of C3N4 derived from polarization curves shown in Fig. 11a. Concentration of C3N4 (g L−1)

Ecorr (mV)

icorr (μA cm−2)

ba (V decade−1)

bc (V decade−1)

Corrosion Rate (mpy)

IE (%)

Substrate 0.00 0.25 0.50 1.00 2.00

−607 −522 −336 −252 −365 −459

21.8 4.50 2.30 1.50 3.50 8.30

0.22 0.18 0.18 0.13 0.20 0.20

0.35 0.25 0.23 0.18 0.27 0.32

5.99 2.99 2.04 1.25 2.59 4.50

– 79.4 89.4 93.0 83.9 61.9

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Table 9 Electrochemical parameters of the electroless deposited NiP–C3N4 NCC after different times of immersion in a 3.5 wt% NaCl solution at room temperature, derived from the polarization curves shown in Fig. 11b. Immersion time (h)

Ecorr (mV)

icorr (μA cm−2)

ba (V decade−1)

bc (V decade−1)

Corrosion Rate (mpy)

IE (%)

3 24 168 336 720

252 265 291 312 363

1.5 2.1 3.0 4.2 5.3

0.13 0.21 0.40 0.20 0.18

0.18 0.14 0.27 0.16 0.17

1.25 1.85 2.24 3.69 5.84

93.0 90.4 86.0 80.7 75.7

has the highest protection efficiency (about 96%) with 0.5 g L−1 C3N4, which drops to 76% after 30 days of immersion in the 3.5 wt% NaCl solution.

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