Synthesis and properties of electrodeposited Ni-CeO2 nano-composite coatings
School of Materials and Metallurgy, University of Science and Technology Liaoning
School of Material Science and Engineering, Shenyang University of Technology
Laser Advanced Manufacturing Technology Center, University of Science and Technology Liaoning
收稿日期：14 December 2015
基金：financially supported by the National Natural Science Foundation of China (No.61674141);the Natural Science Foundation of Liaoning Province Department of Science and Technology (No.201602401);the Natural Science Foundation of Liaoning Province Department of Education (No.L2015259);Anshan City Science and Technology Plan Project (No.20153413);the National Training Programs of Innovation and Entrepreneurship for Undergraduates (No.201610146027);
Synthesis and properties of electrodeposited Ni-CeO2 nano-composite coatings
Hui Jin Yi-Yong Wang Yu-Ting Wang Hong-Bing Yang
School of Materials and Metallurgy, University of Science and Technology Liaoning
School of Material Science and Engineering, Shenyang University of Technology
Laser Advanced Manufacturing Technology Center, University of Science and Technology Liaoning
Abstract：
Current machinery requires metallic materials to have better surface properties. Based on an orthogonal experimental design and analysis method, the CeO2-reinforced nickel nano-composite coatings were prepared by direct current electrodeposition in a nickel sulfate bath containing CeO2 nanoparticles. Statistical results indicate that current density is the most significant variable in the electrodeposition processing, while temperature is the least important factor. The microstructure of Ni and Ni-CeO2 nano-composite coatings was characterized by scanning electron microscopy(SEM) equipped with energy-dispersive spectroscopy(EDS), and X-ray diffraction(XRD).The microhardness of the Ni coating is enhanced by the incorporation of CeO2 nanoparticles. Potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) were used to characterize the corrosion behavior of Ni and Ni-CeO2 coatings. These studies show that NiCeO2 coating has better corrosion resistance compared to Ni coating.
Keyword：
Electrodeposition; Nano-composite coatings; CeO2 nanoparticles; Corrosion resistance;
Author： Yi-Yong Wang,e-mail: hui313@163.com;
Received： 14 December 2015
1 Introduction
At present,the industrial objects demand for the extended safe service life increasingly,so the composites need to have enhanced resistance in the aggressive environments.Composite electroplating method can codeposit the insoluble metallic or nonmetallic compounds in a metal or alloy matrix to produce the metal matrix composite coatings
Recently,rare earth oxide nanoparticles,such as Y2O3
In this paper,Ni-CeO2 nano-composite coatings were prepared onto copper sheet through the method of electrodeposition.The microstructure and corrosion resistance properties of the Ni-CeO2 nano-composite coatings were studied compared with those of pure Ni coating,and the related mechanism was discussed.
2 Experimental
2.1 Materials and instrument
The polished copper sheet with dimension of25 mm×25 mm×3 mm was used as the cathode.The polished nickel sheet (purity≥99.5%) with dimension of50 mm×25 mm×2 mm was used as the anode.Before electrodeposition,the copper sheet was degreased in ethyl alcohol and acetone,activated in 10 wt%HCl solution,and then washed with high-purity water.CeO2 nanoparticles(20 nm) were supplied by the Aladdin Industrial Corporation.All the nano-composite coatings were prepared by direct current power supply as power source.
2.2 Preparation and electrodeposition conditions
The electrodeposition of Ni-CeO2 and pure Ni coatings was carried out by nickel sulfate bath under magnetic stirring conditions.All the chemicals used for electrodeposition experiments were of analytically pure grade.High-purity water was used for the preparation for the plating bath.The composition of the plating bath was nickel sulfate of 300.0 g-L-1,nickel chloride of80.0 g·L-1,boric acid of 30.0 g·L-1,sodium dodecyl sulfate of 0.1 g-L-1.Before electrodeposition,CeO2nanoparticles were fully dispersed in the plating bath by magnetic stirring with the spin speed of 180 r·min-1 for1 h.The boric acid was used as buffer agent.The sodium dodecyl sulfate was used as surfactant,and the nickel chloride was used as anode activator.The electrodeposition time was 2 h.
2.3 Material characterization
The surface morphology and microstructural evolution of coatings were investigated by scanning electron microscopy (SEM,JSM 6480LV) equipped with energy-dispersive spectroscopy (EDS).The microstructure of coatings was carried out by X-ray diffractometer (XRD,X'Pert Powder).Vickers microhardness was measured under an applied load of 1.961 N for 15 s.The Vickers microhardness values had been measured for a set of five points across the length of the diagonal from which a mean value was calculated.Electrochemical impedance and Tafel polarization curves were measured by an electrochemical workstation (Autolab PGSTAT 302).The corrosion experiments were performed in 3.5 wt%NaCl solution at room temperature.The coating prepared with an exposed area of 1 cm2 was used as working electrode.An Ag/AgCl(sat.KC1) electrode was used as the reference electrode,and Pt foil (1 cm2) was used as the counter electrode.The linear polarization curves were carried out for the potential range from-0.5 to 0 V.The sweep rate was 1.0 mV·s-1.From the polarization curves,corrosion current (Icorr) and corrosion potential (Ecorr) values of the coatings were calculated by Tafel extrapolation method.Electrochemical impedance was carried out in 3.5 wt%NaCl solution in the same three electrode system.The impedance curves were recorded in the frequency range from 10,000.0 to 0.1 Hz.
3 Results and discussion
3.1 Orthogonal experimental analysis
With the orthogonal designing experiment method,the NiCeO2 nano-composite coatings were studied.Four factors on operating parameters of Ni-CeO2 nano-composite coatings,namely temperature,current density,concentration of CeO2 and pH,were chosen for the orthogonal experimental study.The influence of the four factors on the prepared composite coatings is determined by microhardness.The factors and their test levels employed are all listed in Table 1.
The orthogonal array L9 (34) was selected to arrange the experimental conditions,and the specimens were electrodeposited according to the experimental conditions in Table 1.Based on the preliminary optimization results of single-factor experimental design,three levels of each factor were selected:temperature (A):50,55 and 60℃,current density (B):2,3 and 4 A·dm-2,concentration of CeO2 (C):0.5,1.0 and 1.5 g-L-1,pH (D):3.8,4.2 and 5.0.The experimental results and the effect of different factors on the microhardness of coatings are shown in Table 1.
Table 1 Results of orthogonal test for Ni-CeO2 nano-composite coatings treatment
Based on R values (range value) in Table 1,the effect orders of the four factors are:B>C>D>A.The current density is found to be the most important determinant for microhardness.The concentration of CeO2 and pH is the second and third most important,respectively.Comparatively,temperature is not found to be important for microhardness.In Table 1,the factors that give the optimal microhardness value are A2B1C2D3;in other words,the optimum experimental conditions of electrodeposited NiCeO2 nano-composite coatings treatment are temperature of 55℃,current density of 2 A·dm-2,concentration of CeO2 of 1.0 g·L-1 and pH of 5.0.The experiment of the optimum conditions is just right the forth experiment in Table 1,which also has the highest microhardness value(HV 993.52),higher than that of pure nickel coatings (HV387.20).
3.2 Microstructure
SEM images of the pure nickel coatings and the NiCeO2 nano-composite coatings are shown in Fig.1.Both coatings are smooth,which have no pores and cracks.Pure nickel coatings show a regular pyramidal structure due to a typical nickel growth texture,as shown in Fig.1a.However,with the addition of CeO2nanoparticles,the micromorphology of the Ni-CeO2coatings is changed to spherical structure,as shown in Fig.1b.XRD pattern of Ni-CeO2 nano-composite coatings is shown in Fig.2.By the side of the Ni peaks,CeO2 peaks exist,proving the incorporation of CeO2nanoparticles.CeO2 peaks corresponding to (111),(220)and (331) are observed beside Ni peaks (111),(200),(220) and (311).
EDS spectrum of Ni-CeO2 nano-composite coating obtained under the optimum conditions is shown in Fig.3.It shows the existence of Ce peak,indicating the presence of CeO2 nanoparticles in the nickel matrix.It is confirmed that copper substrate is covered with Ni-CeO2 nano-composite coating,which is in good accordance with previous analysis.EDS spectrum of Ni-CeO2 nano-composite coatings gives the average contents of Ni,O and Ce as93.79 wt%,4.12 wt%and 2.09 wt%,respectively.
Fig.1 SEM images of surface topographies of a pure Ni and b Ni-CeO2 coatings
Fig.2 XRD pattern of Ni-CeO2 nano-composite coatings
Fig.3 EDS analysis spectrum of Ni-CeO2 nano-composite coatings
3.3 Tafel measurements and electrochemical impedance measurements
Tafel curves of pure nickle and Ni-CeO2 nano-composite coatings were measured in 3.5 wt%NaCl solution (in Fig.4).Corrosion potential (Ecorr),corrosion current (Icorr),polarization resistance (Rp),Tafel slopes ba and bc are obtained from Tafel curves (Table 2).The corrosion potential of Ni-CeO2 nano-composite coating (-0.221 V)is less negative than that of pure nickle coating(-0.225 V).AndIcorr value of Ni-CeO2 nano-composite coating (2.021μA·cm-2) is lower compared to that of pure nickle coating (6.505μA·cm-2).These show that the corrosion resistance of Ni-CeO2 nano-composite coating is more excellent than that of pure nickle coating.The addition of CeO2 nanoparticles can improve the corrosion properties of Ni coatings
Electrochemical impedance curves of pure nickel and Ni-CeO2 nano-composite coatings are shown in Fig.5,where Z'is the real part of impedance and Z"is the imaginary part of impedance.The Nyquist curves
Fig.4 Potentiodynamic polarization curves of pure Ni and Ni-CeO2nano-composite coatings
Table 2 Derived results from potentiodynamic polarization measurements
Fig.5 Nyquist diagrams of impedance spectrum of pure Ni and Ni-CeO2 nano-composite coatings (inset being equivalent electrical circuit)
measured are composed of one capacitive loop due to the charge transfer process.The more prominent frequency arcs are obtained for the copper matrix covered with NiCeO2 nano-composite coating
Table 3 Parameters resulted from impedance measurements of pure Ni and Ni-CeO2 nano-composite coatings
4 Conclusion
From the results of the experiments,it can be drawn that Ni-CeO2 nano-composite coatings can be prepared in nickel sulfate plating solution.The addition of CeO2nanoparticles to the nickel coating leads to the change of microstructure,surface roughness and chemical constitution.It is worth noting that the addition of CeO2nanoparticles to the nickel coating results in the noticeable increase in microhardness.The Vickers microhardness value of Ni-CeO2 nano-composite coatings (HV 993.52) is higher than that of pure nickel coatings (HV 387.20).The Icorr value of Ni-CeO2 nano-composite coatings is obviously less than that of pure nickel coating.So the addition of CeO2 nanoparticles to the coating results in decent improvement in the corrosion resistance which can be probably attributed to the inert physical barriers effect and the microcorrosion cells effect in the coatings.
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