Corrosion Resistance of Electrodeposited Coating

corroding Resistance of Electrodeposited CoatingQiongyu Zhoua,b, Yadong Zhanga, Xiaofen Wanga, Hebing Wanga, Ping Oua*aSchool of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR mainland ChinabInstitute of Applied Physics, Jiangxi Academy of Sciences, Shangfang Road 108, Nanchang, Jiangxi Province 330029, PR ChinaKeywords Ni-W profane Composite finishing Cr2O3nano-particles Micro cruelness Corrosion opposite1. Introduction temperate sword is a most widely-used out materials in engineering and industrial applications overdue to its low price and acceptable properties 1. However, buggy steel could not be suitable served in the harsh environs because of its highly susceptible to corroding and mediocre automatic strengths 2. Failures (such as eating away or wear) often supervene on the surfaces of mild steel devices 3. Therefore, preparation of an preservative pelage is wizard of the best known methods for broadening t he application fields of mild steel 4, 5. In recent years, electro depositary technology has been widely used because it is really a convenient, practical and inexpensive method for engineering application 6. Numbers of coat or alloy (such as Ni 7, Zn 8, Cr 9, Ni-W 10, 11, Ni-Co 12, Zn-Ni 13 et al.) down beenelectrodeposited as the protective ending on the surface of mild steel. Among these coats, Ni-W alloy cultivation has drawn lots of interests as a cannisterdidate to deputize hard chromium, because of its low toxicity for aquatic species 10.In general, the purpose of researches on electro aboded Ni-W alloy coating is how to enhance their cogency and corrosion resistance. Particularly, incorporation a second ceramic particles into the growing metal or alloy ground substance during the electroplating process is a effectual method. The manifold coatings always exhibited enhanced mechanical and corrosion properties 14-17. For this priming, a large total of researches a im been drawn on Ni-W nanocomposite coatings (Ni-W-Al2O318, Ni-W-SiO2 19,Ni-W-TiO220, Ni-W-diamond 21 and Ni-W-SiC22, et. al). The ceramic particles used as the second-phase in the composite coatings, more or less, would promote the corrosion resistance, hardship or wear-resistance 23-25. Although nano-Cr2O3 particles get been certified as a favorable and abundant 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 repair the performance of Ni-W alloy coating which is know as a environment friendly protective coating with handsome for mild steel, Ni-W-Cr2O3 nanocomposite coating was electrodepositied in the sulfate-citric toilet 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 plat e (1-1 cm2, Xianren instrument Co., Ltd. in Shanghai, China) was employed as the anode. The mild steel was automatically polished by 800, 1200 and 2000 grit emery-paper and then ultrasonically cleaned in propanone for 600 s. The cleaned mild steel was activated in 10% (w/v) HCl firmness of purpose for 30 s and then washed with distilled water. The base consist of electrolyte solution is as follows 26.3 g/L NiSO46H2O, 98.95 g/L Na2WO42H2O, 147.05 g/L Na3C6H5O72H2O, 26.75 g/L NH4Cl, 0.3 g/L NaBr. Before electrodeposition, nano-Cr2O3 particles was added into the electrodeposition bath and then dust by ultrasonic concussion (3600 s) to break up agglomerates. The electroplating current immersion and time were 4 A/dm2 and 1800 s.2.2. Coatings characterizationThe surface morphology was examine using a skim overning electron microscope (SEM, JEOL JSM-6700F), supplied with an explosive detection system spectrometer (Oxford Instruments, UK) for determine the chemical compositions of th e coatings. The phase compositions of electrodeposited coatings were characterised by X-ray diffraction (XRD, D/max-2200) with Cu K 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 quizzer (VH-3) at an applied load of 9.8 N for 15 s, each taste was tesetd five times for averaging. The corrosion style of the obtained coating was evaluated in 3.5 wt.% NaCl solution by using an electrochemical workstation (CHI660E). All experiments were conducted in a customary three-electrode cell (consisting of the electro-deposied coating as a working electrode, Pt cerement as a counter electrode and SCE as a reference electrode). The potentiodynamic polarisation 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) w as conducted at Ecorr, with voltage perturbation bountifulness of 10 mV in the frequency range from 105 Hz to 10-2Hz. All electrochemical tests are carried out at room temperature (25 oC).3.1 painting of nano-Cr2O3particlesThe characterization of nano-Cr2O3particles was carried out by using TEM and XRD analysis, the results are displayed in Fig. 1. It is entered that the particles are free of secondary phases except Cr2O3, which is consists of polyhedral structure with the mean diam of about 40 nm. Inevitably, there aresome degree of agglomeration amongst the nano-particles.The composition of electroplated W alloy coatings can be analysed by EDS as the previous studies 28. The W pith and Cr2O3 in the electroplated coatings as a function of Cr2O3 addition in the electroplating bath are displayed in Fig. 2. The Cr2O3 gist is corresponding to detected Cr element ratio in Ni-W-Cr2O3 nanocomposite coatings. As shown in Fig. 2, with the summation of Cr2O3 concentration in electro plating bath, the Cr2O3 particles incorporated in the coating addition rapidly when the Cr2O3 concentration is low (5 g/L). slice it annexs gradually when the Cr2O3 concentration is in range of 10-20 g/L. A deflexion from the Langmuir adsorption doings 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 get out 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 atom29. Once the Cr2O3 nano-particles adsorbed on cathode surface, it could form as nucleation sites and accordingly digest the overpotentials. As a result, the deposition of W atom is inhibited, while Ni itself can withal be deposited from its complex with citrate30.Fig. 3 shows the XRD patterns of the coatings electrodeposited in the bath with a nd without Cr2O3 nano-particles. In the bath without Cr2O3 nano-particles (shown in Fig.3a), the pattern of obtained coating consists of a broad jacket from 41 to 47, indicating the shapeless nature of the Ni-W alloy coating. The amorphous structure should be electrodeposited nether 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 30 and T. Yamasaki 31, the electrodeposited Ni-W alloy coatings presented as an amorphous put up when tungsten composition ranged from 20 to 40 at.%. While the structure of deposited would render once the Cr2O3 nano-particles existed in the b ath, Ni-W-Cr2O3 nanocomposite coatings exhibit microcrystal barrier 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 30. Simultaneously, an unidentified peak at 241.4 is presented in the patterns of the Ni-W-Cr2O3 composite coatings. Similar peak have been observed by I. Mizushima et. al 32 and R. Juk-nas et. al 33. The former proposed that it is the codeposition of nanocrystalline Ni(-W) and Ni-W-C phases 32. While R. Juk-nas et. al claimed this peak corresponded to NiWO433. 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 inc rease 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 illegal 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 undecomposed in the high concentration solution and then precipitate by settlement. Thus, the possibility for agglomer ates 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 18.3.3. MicrohardnessThe 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 (717764 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 precede on the hardness by impeding the fast dislocation motion the grain boundary sliding of the matrix 19. 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 note d 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 brook 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 progress of hardness performance would be limited by the contrary make of decrease of W content in electrodeposited coating.3.4 Corrosion resistance propertiesThe 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 terrible Ecorrcombine wi th 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 create on the surface of amorphous alloy and provide protective effect for go along further corrosion 34. 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 somewhat approximate icorr compare with that of Ni-W coating.The corrosion reaction and products at the electrode/electrolyte interface can be analysed by EIS measure ments 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 circumstances 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 35. 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 reveal 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 inner corrosion reaction of materials. The fitted values are listed in Table 2. As sh own, 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 valu e (717764 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.