Effect of Amorphous Boron on the Microstructure and Corrosion Properties of Ni-W Coatings

Abstract

In this study, a nickel-tungsten/amorphous boron composite coating (Ni-W/B) was successfully deposited on carbon steel using the electrodeposition method. Electrodeposition was performed by dispersing varying quantities of amorphous boron (0, 0.5, 1, and 3 g/L) particles in a Ni-W electrolytic bath. The microstructure and morphology of composite coatings were characterized by scanning electron microscope (SEM). The electrochemical properties of the Ni-W alloy and Ni-W/B composite coatings were studied using electrochemical impedance spectroscopy (EIS), polarization curves, and immersion testing. It was found that the addition of amorphous boron particles to Ni-W coatings can significantly smooth the modified composite coatings and improve the corrosion resistance, probably by changing the corrosion from pitting to uniform corrosion with increasing boron content. The boron concentration of 0.5 g/L in the bath provided the highest corrosion resistance among all the modified coatings.

1. Introduction

Nanocomposite coatings are a popular method used to improve properties of materials because of their distinctive mechanical, tribological, and electrochemical properties compared to metal or alloy coatings. The electrodeposition method is commercially used to fabricate the coatings because it is a simple, controllable, and low-cost technique for developing the surface coating with various materials via plating modification [1]. Furthermore, the type and quantity of incorporated materials strongly affect the microstructure and properties of the composite coatings [1,2,3,4,5,6,7].

Nickel-tungsten (Ni-W) alloy coatings have been developed as a good alternative to hard chromium coatings to reduce health and environmental problems [8,9]. The coatings are used in various applications, such as automotive, aerospace, petroleum, and electronics industries, because Ni-W alloy exhibits high hardness and wear and corrosion resistance [9,10,11,12]. In previous works, it was reported that the particles incorporated in Ni-W coatings, such as Al₂O₃ [13], TiO₂ [14], diamond [15], SiC [16], TiN [17], CeO₂ [18], Y₂O₃ [19], and TaC [20], can increase the corrosion and wear resistance, hardness, and adhesion strength. The properties of the composite coatings highly depend on the character of the incorporated particles and their content in the deposits. According to the properties of incorporated particles [21,22] and the Ni-W coating application, amorphous boron particles are good choice to add to Ni-W coatings to improve their properties.

In our previous work, we reported that the original Ni-W alloy coatings and composite Ni-W coatings modified by adding amorphous boron particles (Ni-W/B) can be successfully deposited on carbon steel using the electrodeposition method [4,23,24]. The hardness and wear resistance of the coatings are significantly improved via the incorporation of an appropriate content of amorphous boron particles because these particles uniformly distribute and provide a smooth surface [4,23]. Furthermore, the incorporated particles in Ni-W coatings can enhance the corrosion resistance [1,2,3,17,18,19]. Therefore, the addition of amorphous boron also enhances the corrosion resistance of Ni-W coatings. In previous works, the addition of boron to the Ni-W alloy coatings enhanced the corrosion resistance [25,26,27]. However, few studies have examined the influence of various boron additions on the corrosion resistance in Ni-W/B composite coatings. Herein, Ni-W/B nanocomposite coatings were prepared with various contents of amorphous boron particles on carbon steel substrate, which is widely used in piping and automotive parts [28]. In addition, the boron content in Ni-W coatings can be easily controlled by adding amorphous boron particles. The corrosion resistance of the coatings was investigated as the function of boron content in 3.5 wt.% NaCl solution.

2. Materials and Methods

The Ni-W/B composite coatings were prepared on commercial-grade carbon steel (AISI 1040) substrates in a Ni-W electrolyte bath by adding amorphous boron particles, as described in our previous work [4]. Typically, 18 g/L nickel sulphate hexahydrate (NiSO₄·6H₂O), 53 g/L sodium tungstate dihydrate (Na₂WO₄·2H₂O), 168 g/L trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 31 g/L ammonium chloride (NH₄Cl), 18 g/L sodium bromide (NaBr), and various concentrations of amorphous boron particles (0, 0.5, 1, and 3 g/L) were dissolved in DI water. The steel specimens with dimensions of 20 mm × 30 mm × 1 mm were used for all the electrodeposition experiments. Before electrodeposition, the steel substrates were mechanically cleaned to remove impurities. Then, the substrates were immersed in the NaOH and HCl solution, followed by rinsing with DI water. The pre-cleaned substrates and Pt mesh were applied as the cathode and anode for electrodeposition, respectively. During the electrodeposition, a constant current density of 0.1 A/cm² was applied for 1 h at the bath temperature of 75 °C. All the electrodeposition experiments were carried out in a 250 mL beaker, with a magnetic bar stirrer continuously rotating at the speed of 150 rpm.

A scanning electron microscope, SEM (Hitachi SU3500, Tokyo, Japan), was used to study the surface microstructure and morphology of both Ni-W and Ni-W/B composite coatings. A three-electrode potentiostat (µAutolab/FRA2, Metrohm Autolab B.V, Utrecht, Netherland) using a Ag/AgCl reference electrode, a platinum plate as a counter electrode, and a sample as a working electrode was used to investigate the electrochemical properties of the coatings in 3.5 wt.% NaCl solution (pH = 8) via electrochemical impedance spectroscopy (EIS, Metrohm Autolab B.V, Utrecht, Netherland) measurement and potentiodynamic polarization testing. All electroplated coatings were cleaned with ultrasonic cleanser, degreased, and immersed in the NaCl solution for 1 h. Then, the open circuit potential (OCP) value was measured for 30 min. EIS testing was performed in the frequency range between 10 mHz and 10 kHz at an amplitude of 10 mV. The potentiodynamic polarization tests were performed at a scan rate of 0.016 mV/s with the potential range within 250 mV (vs. SCE) of the OCP value. The electrochemical testing was conducted using NOVA software (version 2.0).

In addition, the corrosion rate of pure Ni-W (boron concentration of 0 g/L) and modified Ni-W coatings with different boron contents (boron concentrations of 0.5, 1, and 3 g/L) was observed by immersion testing. The testing was performed in 3.5 wt.% NaCl solution at ambient temperature. The weight of all samples before and after the immersion testing was measured by an electronic balance with an accuracy of 0.01 mg. All samples after testing were ultrasonically cleaned with ethanol and dried before weighing. The weight loss was calculated from the difference in the samples’ weight before and after testing.

3. Results

According to our previous work [4], the Ni-W/B composite coatings can be successfully prepared. Figure 1 illustrates SEM images of the Ni-W and Ni-W/B coatings. It can be seen that the surface of Ni-W/B composite coatings is smoother than that of the Ni-W alloy coating (see Figure 1), which is in good agreement with the previous work [4]. Moreover, the thickness of the coatings is in the range of 47–49 μm in samples having addition of 1 and 3 g/L of boron [4]. A previous study reported that the boron content in Ni-W coatings does not significantly influence the film thickness. The surface roughness was measured by a surface profilometer. The Ni-W coating has an average surface roughness (Ra) of 1.58 ± 0.1 µm, while the Ni-W/B coatings have Ra values of 1.38 ± 0.05, 1.26 ± 0.07, and 0.93 ± 0.04 µm, for boron concentrations of 0.5, 1, and 3 g/L in the plating solution, respectively. The incorporation of boron provided a smooth surface of the coatings because the incorporated particles act as nucleation sites and enable grain refinement, resulting in smaller grains [19,29,30], as shown in previous works [4,23,31]. In addition, boron particles retard the crystalline growth of the Ni-W matrix coating, leading to a smoother and more uniform surface [4,29,30].

Open-circuit potential (OCP) as a function of time at various boron concentrations in the Ni-W plating bath is shown in Figure 2. The OCP indicates that the potential is significantly changed in the NaCl solution. The OCP obtained from the Ni-W coating is lower than that of the Ni-W/B coating samples, and further decreases slowly with the increase in time. In contrast, the OCP of all Ni-W/B coatings first decreases with increasing time. After 1800 s (30 min) of immersion, the OCP value becomes stable. The OCP results show that the Ni-W/B samples exhibit better chemical stability than the Ni-W sample. Similar results were observed in the Ni-W/MWCNT composite coating system [3].

Moreover, the OCP value shows that the Ni-W coatings modified by the addition of various boron concentrations (Ni-W/B) have higher potential than that of the original (see Figure 2). This indicates that more positive OCP values and increasing galvanic corrosion resistance were achieved by adding boron particles due to retarding the penetration of the NaCl solution on the Ni-W/B coatings. Furthermore, the Ni-W/B coatings prepared by adding 0.5 g/L of boron in the plating bath increased slightly after 500 s immersion in 3.5 wt.% NaCl solution, which demonstrates that this sample has the most chemical stability compared to other samples. This is because a passive film of W-rich oxide may form on the Ni-W matrix coating [10] as a very thin coating layer and affect the corrosion resistance together with the boron particles.

The Nyquist plot for the Ni-W and Ni-W/B coatings with various boron concentrations is shown in Figure 3. The impedance spectra shape of all coatings is a single semicircle. The size of the semicircle demonstrates the corrosion resistance of the samples. The incorporation of amorphous boron particles does not change the shapes of the EIS, but it can increase the radius of the Nyquist plot. The real impedance (Z′) of the Ni-W/B coatings is larger than that of the Ni-W coatings, indicating that boron addition can enhance the corrosion resistance of Ni-W coatings. The Ni-W/B coating with a boron concentration of 0.5 g/L has the largest semicircle radius (see Figure 3). From the obtained results, boron particles influence the coating structure modification because the deposited boron particles in Ni-W coatings are uniformly distributed and fill crevices, gaps, and micro-holes of the coatings [26,32,33,34]. This leads to a reduction in surface defects.

The Bode modulus can indicate (1) impedance (Z) in the high-frequency region as the solution resistance (R_s) and (2) Z in the low-frequency region as the polarization resistance (R_p) or corrosion resistance of the samples. Figure 4 depicts the Bode modulus of the Ni-W and Ni-W/B coatings. The results demonstrate that the Ni-W coating has the lowest corrosion resistance, while the highest resistance was obtained from the Ni-W/B coating with the addition of a boron concentration of 0.5 g/L (see R_p at a frequency of 0.01 Hz). The result from the Bode modulus is in good agreement with the Nyquist plot.

Figure 5 shows the Tafel curves of both the Ni-W alloy and Ni-W/B composite coating with various boron concentrations obtained by potentiodynamic polarization testing in the 3.5 wt.% NaCl solution. Extrapolating the linear segment of the cathodic and anodic curves from the Tafel curve to their intersection was used to calculate the corrosion potential (E_corr) and corrosion current density (I_corr) values, as shown in

Table 1. OCP value, corrosion potential, and corrosion current density on Ni-W/B and Ni-W/B.

Boron (g/L)	OCP vs. SCE(V)	Ecorr vs. SCE (V)	Icorr (µA/cm2)
0	−0.562	−0.579	25.594
0.5	−0.483	−0.489	5.869
1	−0.515	−0.529	3.070
3	−0.502	−0.524	5.365

.

According to electrochemical theory, the current density is inversely proportional to the charge transfer resistance (R_ct). The obtained data in Table 1 show that Ni-W/B nanocomposite coatings with varying boron concentrations have higher E_corr and lower I_corr than those of Ni-W coatings. Thus, the incorporation of boron particles can increase R_ct and decrease I_corr values [35]. The coating prepared with a boron concentration of 0.5 g/L appears to have the highest corrosion potential, with the value of −0.489 V. Since the optimum boron content affects the uniform boron particle distribution in Ni-W coatings, this leads to maximum corrosion resistance because the particles increase the inactive areas between the NaCl solution and composite coating [36,37]. Second, an increase in boron content provides a smaller grain size leading to a higher grain boundary, and then increases surface energy because boron is a grain-refining element. This accelerates passive film formation, leading to better corrosion resistance [36], which was found in the sample with boron content of 0.5 g/L.

The immersion test was further carried out to evaluate the corrosion behavior for Ni-W and Ni-W/B coatings. The surface color of the Ni-W/B coatings changed. Meanwhile, the Ni-W coating showed a more severe visible change, and its surface become rough and lost brightness. The surface morphology, as shown in Figure 6, can be observed by SEM. The SEM images show that the surface of the Ni-W coating has fewer corroded holes than that of the Ni-W/B coatings. In contrast, the weight loss of the Ni-W coating is higher than that of the Ni-W/B coatings, leading to less corrosion resistance (see

Table 2. Weight loss of the Ni-W and Ni-W/B coatings after immersion testing in 3.5 wt.% NaCl solution for 200 h.

Boron (g/L)	Initial Weight (g)	After Immersion (g)	Weight Loss (g)
0	4.9038	4.8925	0.0113
0.5	4.8663	4.8652	0.0011
1	4.8962	4.8943	0.0019
3	4.6225	4.6188	0.0037

). Therefore, the corroded holes in the Ni-W coating appear to be deeper, which means Ni-W is more susceptible to pitting corrosion. In addition, the corroded surface gradually changed from localized corrosion to uniform corrosion when boron concentration was increased in the plating bath (Figure 6a–d). This trend is beneficial due to uniform corrosion’s predictable failure, compared to the often unpredictable rapid rupture failure of localized corrosion. In Figure 6c,d, no cracking is observed on the surfaces. On the other hand, in Figure 6a,b, cracks are clearly shown along grain boundaries. These cracks reinforce the corrosion process by being channels for mass transfer, thus worsening the corroded surface. Table 2 shows the weight loss data of Ni-W and Ni-W/B coatings obtained from the immersion test for 200 h in the 3.5 wt.% NaCl solution. It can be seen that that the presence of boron particles in the Ni-W coating significantly decreases the corrosion rate of the coating, and adding a boron concentration of 0.5 g/L provided the best corrosion resistance compared to the others. Moreover, the coating surface appears to have fewer visible holes as the boron concentration increases, as the condition changed from pitting corrosion to uniform corrosion (see Figure 6). The corrosion resistance of the Ni-W/B coating with a boron concentration of 0.5 g/L is about 10 times better than that of Ni-W. This finding is in good agreement with the result obtained from the electrochemical testing.

Figure 7 shows the relationship between boron concentration, tungsten content in the coating, and corrosion current. Increasing boron concentration from 0 to 3 g/L leads to decreasing the tungsten content in the coating. It also sharply reduces the corrosion current as boron concentration decreases from 0 to 0.5 g/L, with the lowest corrosion current shown at a boron concentration of 1 g/L. In general cases, the grain boundary is highly susceptible to pitting corrosion. Figure 6a clearly illustrates this event, as many large holes can be seen along the grain boundary. Hence, in this experiment, boron particles were added to limit the susceptibility of the grain boundary to corrosion. Furthermore, the coatings obtained with the incorporation of boron become much denser and smoother than that of Ni-W. The densely packed coating can significantly inhibit the corrosion and provide better corrosion protection to the substrate than that of Ni-W. The addition of boron particles to the Ni-W alloy coating can prevent corrosion in two main ways. Firstly, the boron particles fill crevices and micro-holes in the Ni-W matrix, creating a physical barrier against pitting corrosion. This leads to denser and smoother coatings [38]. Secondly, the homogeneous distribution of boron particles raises the corrosion potential and reduces the corrosion current density. This results in uniform corrosion and inhibition of localized corrosion [39].

4. Conclusions

A Ni-W/B composite coating was successfully electroplated on low carbon steel. Addition of amorphous boron particles to the Ni-W plating bath can increase the corrosion resistance of Ni-W-based coatings. Adding a boron concentration of 0.5 g/L provided the highest corrosion resistance since boron particles: were uniformly distributed in the Ni-W matrix coating; refined grains and reduced porosities, leading to a smooth surface and reducing pitting corrosion; and tended to change the corrosion condition to uniform corrosion, which is preferable.