简介概要

Synthesis and properties of electrodeposited Ni-CeO₂ nano-composite coatings

来源期刊：Rare Metals2018年第2期

论文作者：Hui Jin Yi-Yong Wang Yu-Ting Wang Hong-Bing Yang

文章页码：148 - 153

摘要：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.

稀有金属(英文版) 2018,37(02),148-153

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

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 [ 1, 2, 3] .Many kinds of insoluble nanoparticles can be codeposited with metal by electrodeposition method to form nano-composite coatings with good properties of high hardness,excellent corrosion resistance,good wear resistance,etc. [ 4, 5, 6] .

Recently,rare earth oxide nanoparticles,such as Y₂O₃ [ 7, 8] ,CeO₂ [ 9, 10] and La₂O₃ [ 11] ,have been also used in the process of composite electrodeposition owing to their special physical and chemical properties.CeO₂ is a kind of powerful rare earth oxides with many excellent properties [ 12, 13] .It can make composite coatings have good corrosion resistance,tribological properties,hardness,oxidation resistance,etc. [ 14, 15] .Therefore,all kinds of researchers have devoted themselves to studying the composite coating with nano-CeO₂ to improve the properties of the composite coating.For instance,Zhang et al. [ 16] reported that codeposition of Ni and CeO₂ with different CeO₂ contents and the oxidation resistance at 800℃was improved.Compared with the pure Ni coating,NiCeO₂ coatings had a significant reduction in the isothermal oxidation rate.The reason was that the codeposited CeO₂nanoparticles blocked the outdiffusion of Ni along the grain boundaries.Xiong et al. [ 17] investigated the microhardness and wear resistance of Ni-CeO₂ composite coatings.It was elucidated that the incorporation of CeO₂to the Ni matrix decreased the grain size of coatings.Owing to the dispersion strengthening effect,the mechanical properties had the optimum value with the CeO₂ concentration 10 g·L^-1.Kasturibai and Kalaignan [ 18] prepared CeO₂-reinforced nickel nano-composite coatings in the nickel acetate bath.Experimental results showed that the microhardness value of the nickel nanocomposite coating was higher than that of pure nickel.The Ni-CeO₂ coatings have higher corrosion resistant nature in3.5%NaCl.However,few researches have been done on the microstructure of Ni-CeO₂ coatings and the corrosion resistance with the polarization curves and the alternating current (AC)-impedance measurement.

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 (I_corr) and corrosion potential (E_corr) 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.

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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 (E_corr),corrosion current (I_corr),polarization resistance (R_p),Tafel slopes b_a and b_c 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).AndI_corr 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 [ 19] .

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

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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 [ 20] .The equivalent electrical circuit of R_s (the electrolyte bulk resistance)-R_ct (the charge transfer resistance)/CPE (the constant phase angle element) was used to acquire the parameters in Table 3.In Table 3,n is the strength of the dispersion effect,which can be interpreted as the degree of surface roughness.Because of the distribution of relaxation time,which results from inhomogeneities at the micro-or nano-level such as the surface roughness phenomenon,the use of CPE is usually required.With the addition of CeO₂ nanoparticles in the coatings,R_ct values increase and CPE values decrease for Ni-CeO₂ nano-composite coatings.It is indicated that Ni-CeO₂ nano-composite coatings have more excellent corrosion resistance than pure nickel coatings.Therefore,it seems that the incorporation of CeO₂ nanoparticles plays a very important role in improving the corrosion resistance of the coatings.This may be explained by the following reasons.(1) Nanometer CeO₂ particles serve as inert physical barriers in the coating.They can hinder the initiation and propagation of corrosion defect.(2) Nanometer CeO₂particles dispersed in the Ni coating can form a lot of microcorrosion cells.They promote the anodic polarization.So they can inhibit the pitting corrosion and localized corrosion caused by crystal defects or micropores on the surface,and promote the uniform corrosion in the coating.

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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 I_corr 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.