bInstitute of Applied Physics, Jiangxi Academy of Sciences, Shangfang Road 108, Nanchang, Jiangxi Province 330029, PR China
Mild steel is a most widely-used metal materials in engineering and industrial applications due to its low price and acceptable properties . However, mild steel could not be suitable served in the harsh environment because of its highly susceptible to corrosion and mediocre mechanical strengths . Failures (such as corrosion or wear) often occur on the surfaces of mild steel devices . Therefore, preparation of an protective coating is one of the best known methods for broadening the application fields of mild steel [4, 5]. In recent years, electrodeposition technology has been widely used because it is really a convenient, practical and inexpensive method for engineering application . Numbers of metal or alloy (such as Ni , Zn , Cr , Ni-W [10, 11], Ni-Co , Zn-Ni  et al.) have been electrodeposited as the protective coating on the surface of mild steel. Among these coatings, Ni-W alloy coating has drawn lots of interests as a candidate to replace hard chromium, because of its low toxicity for aquatic species .
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In general, the purpose of researches on electroplated Ni-W alloy coating is how to enhance their hardness and corrosion resistance. Particularly, incorporation a second ceramic particles into the growing metal or alloy matrix during the electroplating process is a effective method. The composite coatings always exhibited enhanced mechanical and corrosion properties [14-17]. For this reason, a large amount of researches have been drawn on Ni-W nanocomposite coatings (Ni-W-Al2O3, Ni-W-SiO2 ,Ni-W-TiO2, Ni-W-diamond  and Ni-W-SiC, et. al). The ceramic particles used as the second-phase in the composite coatings, more or less, would promote the corrosion resistance, hardness or wear-resistance [23-25]. Although nano-Cr2O3 particles have been certified as a favorable and considerable incorporated ceramic particles in Ni or Co coating [26, 27], there is no report about nano Cr2O3 particles employed in electrodeposition of Ni-W nanocomposite coatings so far. In this paper, in order to improve the performance of Ni-W alloy coating which is know as a environment friendly protective coating with excellent for mild steel, Ni-W-Cr2O3 nanocomposite coating was electrodepositied in the sulfate-citric bath containing various of Cr2O3 nanoparticles for improving both its hardness and corrosion resistance.
Mild steel (1-1 cm2, Q235, Baosteel Co., Ltd. in Shanghai, China) was used as cathode and a platinum plate (1-1 cm2, Xianren instrument Co., Ltd. in Shanghai, China) was employed as the anode. The mild steel was mechanically polished by 800, 1200 and 2000 grit emery-paper and then ultrasonically cleaned in acetone for 600 s. The cleaned mild steel was activated in 10% (w/v) HCl solution for 30 s and then washed with distilled water. The base consist of electrolyte solution is as follows: 26.3 g/L NiSO4·6H2O, 98.95 g/L Na2WO4·2H2O, 147.05 g/L Na3C6H5O7·2H2O, 26.75 g/L NH4Cl, 0.3 g/L NaBr. Before electrodeposition, nano-Cr2O3 particles was added into the electrodeposition bath and then dispersed by ultrasonic concussion (3600 s) to break up agglomerates. The electroplating current density and time were 4 A/dm2 and 1800 s.
2.2. Coatings characterization
The surface morphology was studied using a scanning electron microscope (SEM, JEOL JSM-6700F), supplied with an EDS spectrometer (Oxford Instruments, UK) for determining the chemical compositions of the coatings. The phase compositions of electrodeposited coatings were characterised by X-ray diffraction (XRD, D/max-2200) with Cu Kα radiation, operating at 40 kV and 40 mA, scanning from 20° to 100° with the step of 0.02°.
The surface microhardness ofNi-W-Cr2O3 nanocomposite coatings were measured using a microhardness tester (VH-3) at an applied load of 9.8 N for 15 s, each sample was tesetd five times for averaging. The corrosion behavior of the obtained coating was evaluated in 3.5 wt.% NaCl solution by using an electrochemical workstation (CHI660E). All experiments were conducted in a conventional three-electrode cell (consisting of the electro-deposied coating as a working electrode, Pt sheet as a counter electrode and SCE as a reference electrode). The potentiodynamic polarization test (Tafel) of electro-deposied coating was tested from -800 mV to -400 mV with a scan rate of 1 mV, while mild steel was tested from -900 mV to -600 mV. Electrochemical impedance spectroscopy (EIS) was conducted at Ecorr, with voltage perturbation amplitude of 10 mV in the frequency range from 105 Hz to 10-2Hz. All electrochemical tests are carried out at room temperature (25 oC).
The characterization of nano-Cr2O3particles was carried out by using TEM and XRD analysis, the results are displayed in Fig. 1. It is showed that the particles are free of secondary phases except Cr2O3, which is consists of polyhedral structure with the mean diameter of about 40 nm. Inevitably, there are some degree of agglomeration between the nano-particles.
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The composition of electroplated W alloy coatings can be analysed by EDS as the previous studies . The W content and Cr2O3 in the electroplated coatings as a function of Cr2O3 addition in the electroplating bath are displayed in Fig. 2. The Cr2O3 content is corresponding to detected Cr element ratio in Ni-W-Cr2O3 nanocomposite coatings. As shown in Fig. 2, with the increase of Cr2O3 concentration in electroplating bath, the Cr2O3 particles incorporated in the coating increase rapidly when the Cr2O3 concentration is low (≤5 g/L). While it increases gradually when the Cr2O3 concentration is in range of 10-20 g/L. A deviation from the Langmuir adsorption behavior in the high Cr2O3 concentration solution is observed, which is caused by some particles would sedimentate by gravity in hydrodynamic conditions of without agitated. In addition, the results reveal that W content corresponding decreases with the increase of Cr2O3 addition in electroplating bath. This is because that the sufficiently high overpotentials is in favour of deposition of W atom. Once the Cr2O3 nano-particles adsorbed on cathode surface, it could form as nucleation sites and accordingly reduce the overpotentials. As a result, the deposition of W atom is inhibited, while Ni itself can also be deposited from its complex with citrate.
Fig. 3 shows the XRD patterns of the coatings electrodeposited in the bath with and without Cr2O3 nano-particles. In the bath without Cr2O3 nano-particles (shown in Fig.3a), the pattern of obtained coating consists of a broad peak from 41° to 47°, indicating the amorphous nature of the Ni-W alloy coating. The amorphous structure should be electrodeposited under the pretense of the deposition rate is high compared to the exchange rate, which implies that all metal atoms are immediately discharged once they get to cathode surface. Thus, high content of W in the alloy must be observed, which is confirmed by the EDS result (45.8 wt.%, shown in Fig 2). What more, the amorphous characteristic also can be demonstrated by the SEM micrograph of Ni-W alloy coating (Fig.4a). As the results reported in the literatures by O. Younes  and T. Yamasaki , the electrodeposited Ni-W alloy coatings presented as an amorphous state when tungsten composition ranged from 20 to 40 at.%. While the structure of deposited would transform once the Cr2O3 nano-particles existed in the bath, Ni-W-Cr2O3 nanocomposite coatings exhibit crystalline fcc structure of Ni-W alloy and Cr2O3 phases. The reason for this phenomenon is that the reduced overpotentials caused by the adsorbed Cr2O3 nano-particles on cathode surface would lead to deposition of crystalline phase, which is thermodynamically more stable than the amorphous phase . Simultaneously, an unidentified peak at 2θ≈41.4– is presented in the patterns of the Ni-W-Cr2O3 composite coatings. Similar peak have been observed by I. Mizushima et. al  and R. Jušk-nas et. al . The former proposed that it is the codeposition of nanocrystalline Ni(-W) and Ni-W-C phases . While R. Jušk-nas et. al claimed this peak corresponded to NiWO4. However, so far this anomalous peak remains unidentified. As the increasing of Cr2O3 nano-particles addition in solution, the intensity for diffraction peak of Ni-W (111) increases and unidentified line profile decreases, indicating that grain sizes of the Ni-W crystallites increase and the unidentified phase in the composite coatings gradual reduce.
Fig. 4 shows the surface morphology of the coatings electrodeposited in baths containing different amount of Cr2O3 nano-particles. In all cases, the coatings are compact, uniform and crack-free, which can provide a barrier to protect substrate material. In comparison of Ni-W coating which shows a typical amorphous characteristic which is absence of grain boundaries, Ni-W-Cr2O3 composite coatings is consisted of irregular crystal structures, uniform distributed ultrafine Cr2O3 particles and some arresting big nodules, which is caused by Cr2O3 agglomerates codeposited with Ni-W as metal electrocrystallized. With the increase of Cr2O3addition in the solution, the Cr2O3 particles corresponding increase and the nodules trend to be unobvious. The reason may be that Cr2O3agglomerates become much more serious in the high concentration solution and then precipitate by settlement. Thus, the possibility for agglomerates absorbed on the vertically cathode surface and formation of nodules reduce during the electrodeposition process. Generally, homogeneous distribution of incorporated ceramic particles in composite coating would be benefit to enhance its properties .
The microhardnesses of Ni-W and Ni-W-Cr2O3 composite coatings are showed in Fig. 5. Compared with Ni-W coating (687 HV0.1), Ni-W-Cr2O3 composite coatings exhibite a considerable increase in microhardness (717~764 HV0.1). And the harness increase with the increase of Cr2O3concentration in the bath. Similar trend is usual observed in previous publication [18, 19]. The nano-particles incorporated in alloy coatings would positively contribute on the hardness by impeding the fast dislocation movement the grain boundary sliding of the matrix . As a result, the hardness is direct relate to the incorporated Cr2O3particles in the coating, which increased with the Cr2O3 concentration in the bath ( as shown in Fig. 1). It is noted that the increase in hardness of the Ni-W-Cr2O3 composite coating are limited when the Cr2O3 concentration in the bath increase from 10 g/L to 20 g/L. As the research published previously, both W content and incorporated nano-particles would contribute to the hardness of W alloy coating [11, 20]. With the increase of Cr2O3 concentration in bath, the increase of Cr2O3 in electrodeposited coating would result in increased hardness, However, the promotion of hardness performance would be limited by the contrary effects of decrease of W content in electrodeposited coating.
3.4 Corrosion resistance properties
The corrosion resistance of electrodeposited coating was evaluated by polarization curves and EIS, the result displayed in Fig. 6 and Fig. 7, respectively. The corrosion parameters (Ecorr , icorr) extracted form polarization curves in Fig. 6 are listed in Table 1. It is revealed that both amorphous Ni-W coating and crystalline Ni-W-Cr2O3 nanocomposite coatings show noble Ecorrcombine with low icorr compared with mild steel substrate. This means, the compact electrodeposited coatings can provide an effective protection for mild steel substrate.
A passivation region (-0.55V~-0.45V) is observed in the anode area of polarization curves for Ni-W coating. Passive layer is often formed on the surface of amorphous alloy and provide protective effect for prevent further corrosion . Meanwhile, the Ni-W coating show a lowest icorr in all electroplated coatings, indicating a most excellent corrosion resistance. In addition, with the increase of Cr2O3 concentration in electroplating bath, the corrosion resistance of obtainedNi-W-Cr2O3 nanocomposite coating became better in view of a gradual increase of icorr. When the Cr2O3 concentration in the bath increased to 20 g/L, Ni-W-Cr2O3 nanocomposite coating show a fairly approximate icorr compare with that of Ni-W coating.
The corrosion reaction and products at the electrode/electrolyte interface can be analysed by EIS measurements in conjunction with impedance fitting. Fig. 7 show Nyquist plots of mild steel and electrodeposited coatings obtained in baths with different amount of Cr2O3 nano-particles. The plots for electrodeposited coating and mild steel substrate are consist of a continuous circle arcs, meaning that aggressive ions (Cl–) can not across the compact coating and only one primary interfacial reactions occured between the coating surface (or mild steel sample) and electrolyte.
To model this corrosion behavior, suitable equivalent circuits showed in Fig. 8 was employed . In this equivalent circuit, Rs is solution resistance, Cdl is double-layer capacitance formed in the substrate/electrolyte interface, CPE is a constant phase element for revealing the non-ideal dielectric properties of the coatings, and Rct is the charge transfer resistance of the coating (or substrate) interface, which relate to the intrinsic corrosion reaction of materials. The fitted values are listed in Table 2. As shown, Rct value of Ni-W-Cr2O3 nanocomposite coating electrodeposited in the bath containing 2 g/L Cr2O3is much smaller than that of Ni-W coating. This is because passive layer formed on Ni-W coating surface would prevent corrosion reaction, while no passive behavior have been observed for the crystalline Ni-W-Cr2O3 nanocomposite coating. What more, the Rct values of Ni-W-Cr2O3 nanocomposite coating increase with the increase of Cr2O3concentration in the bath, and the Rct value of Ni-W-Cr2O3 nanocomposite coating electrodeposited in the bath containing 2 g/L Cr2O3is quite close to the Rct value of the Ni-W coating, meaning that this Ni-W-Cr2O3 nanocomposite coating have an excellent corrosion resistance as amorphous Ni-W coating.
Compact Ni-W-Cr2O3 nanocomposite coatings were electrodeposited on mild steel from sulfate-citrate bath containing Cr2O3nano-particles. Compared with Ni-W coating (687 HV0.1), Ni-W-Cr2O3 composite coatings exhibite a considerable increase in microhardness value (717~764 HV0.1). In addition, incorporation of little Cr2O3nano-particles into amorphous Ni-W coating would transform its structure to crystalline, which resulted in no passive behavior occurred on the coating surface and decrease of corrosion resistance. However, the corrosion resistance of Ni-W-Cr2O3 coating could be improved with the increase of Cr2O3concentration in the bath. Finally, a excellent Ni-W-Cr2O3 nanocomposite coating with approximate corrosion resistance and much higher hardness compared with Ni-W coating can be obtained in the bath containing 20 g/L Cr2O3nano-particles. This Ni-W-Cr2O3 nanocomposite coating can be considered as an ideal protective coating to broaden the application of mild steel.
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