简介概要

Characterization of ceramic coating on ZK60 magnesium alloy prepared in a dual electrolyte system by micro-arc oxidation

来源期刊：Rare Metals2013年第5期

论文作者：Ze-Xin Wang Jing Chen Sheng Lu

文章页码：459 - 464

摘要：Micro-arc oxidation（MAO）process was carried out in an optimized dual electrolyte system to fabricate a compact,smooth,and corrosion resistant coating on ZK60 Mg alloy.The microstructural characteristics of coating were investigated by scanning electron microscopy（SEM）coupled with an energy dispersive spectrometer（EDS）and X-ray diffraction（XRD）.Test of mass loss was conducted at a 3.5%NaCl solution to assess the resistance to corrosion.The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment.The results reveal that MgAl2O4and MgO are the main phases of ceramic coating obtained in the dual electrolyte system.The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 gám-2áh-1,which demonstrates excellent corrosion resistance.This is mainly due to the compact,uniform coating with high bonding strength.

稀有金属 (英文版) 2013,32(05),459-464

Characterization of ceramic coating on ZK60 magnesium alloy prepared in a dual electrolyte system by micro-arc oxidation

Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Science and Technology

Zhenjiang Naisi New Materials Co., Ltd.

摘要：

Micro-arc oxidation (MAO) process was carried out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy.The microstructural characteristics of coating were investigated by scanning electron microscopy (SEM) coupled with an energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) .Test of mass loss was conducted at a 3.5%NaCl solution to assess the resistance to corrosion.The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment.The results reveal that MgAl2O4and MgO are the main phases of ceramic coating obtained in the dual electrolyte system.The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 gám-2áh-1, which demonstrates excellent corrosion resistance.This is mainly due to the compact, uniform coating with high bonding strength.

收稿日期：16 April 2013

基金：supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Key Laboratory of Advanced Welding Technology of Jiangsu Province, China (No. JSAWT-11);

Characterization of ceramic coating on ZK60 magnesium alloy prepared in a dual electrolyte system by micro-arc oxidation

Ze-Xin Wang Wei-Gang Lv Jing Chen Sheng Lu

Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Science and Technology

Zhenjiang Naisi New Materials Co., Ltd.

Abstract：

Micro-arc oxidation (MAO) process was carried out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coating were investigated by scanning electron microscopy (SEM) coupled with an energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) . Test of mass loss was conducted at a 3.5 % NaCl solution to assess the resistance to corrosion. The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment. The results reveal that MgAl2 O4 and MgO are the main phases of ceramic coating obtained in the dual electrolyte system. The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 gám-2áh-1, which demonstrates excellent corrosion resistance. This is mainly due to the compact, uniform coating with high bonding strength.

Keyword：

ZK60 magnesium alloys; Micro-arc oxidation; Characterization; Dual electrolyte system;

Author： Ze-Xin Wang e-mail: wzxlwg@126.com;

Received： 16 April 2013

1 Introduction

Magnesium alloys are considered as the most promis ing and the lightest metallic structural material in th twenty-first century, having been used for automobile industry, aerospace components, and communication and computer parts due to their excellent physical and mechanical properties[1–3].However, magnesium alloys are highly susceptible to corrosion, especially, in acidic environments and in salt-water conditions, which seriously limit their widespread use in many applications[4, 5].This makes protective surface treatment an essential and practicable part of the manufacturing process for Mg alloys.Micro-arc oxidation (MAO) is an advanced surface treatment technique employed to form relatively thick, ceramic coatings on magnesium, aluminum, titanium, and other valve metals[6–9].

It is well known that properties of coatings fabricated by MAO mainly depend on the electrolyte and electric parameters.Composition of electrolyte acts as an important factor for its effects on the color, thickness, phase composition, and electrochemical corrosion behavior of MAO coating, while electric parameters also play a very important role[10].Only with reasonable electric parameters can MAO process carry on well and guarantee good coatings fabricated on substrates.

It is reported that most of MAO process are carried out in an electrolyte system with only one of silicate, aluminate, or phosphate as the main film forming agent.And the coating is easy to form in silicate system, while the corrosion resistance of the coating fabricated in aluminate system is better relatively compared with phosphate system.However, the film forming ability and corrosion resistance of coating prepared in any single electrolyte system still remain problems.Therefore, some researches paid attention to dual electrolyte systems for a solution to these problems.The present authors conducted MAO process in a dual electrolyte system consisting of aluminate and phosphate and easily made coating with improved corrosion resistance[11].Up to now, properties and preparation process of coating in dual electrolyte system are lack of systematic research.

In this study, MAO process was carried out in an optimized dual electrolyte system[11]coupled with optimized processing parameters to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy.The corrosion resistance, microstructure, bonding strength, and other characteristics of the coating were investigated.

2 Experimental

The microstructural characteristics of coating, coating thickness, and phase constituent were investigated by scanning electron microscope (SEM, JSM-6480) and X-ray diffraction (XRD, Shimadzu XRD-6000) .Tests of potentiodynamic polarization and weight loss were conducted at a 3.5%Na Cl solution to assess the resistance to corrosion of the coating.

The macro-morphology and surface roughness of the coating were investigated by digital camera and Olympus confocal laser scanning microscopy (OLS4000) .The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment.Other properties of coating were tested by Vickers hardness, Perkin Elmer diamond thermo gravimetric (TG) /DTA, and friction and wear experiments.

3 Results and discussion

3.1 Analysis of coating phases

3.2 Corrosion resistance of MAO coating

3.2.1 Corrosion rate

Corrosion rates were tested by means of immersion test in3.5%Na Cl solution.The corrosion rate of the optimized sample is only 0.0061 g.m^-2.h^-1, which demonstrates excellent corrosion resistance with only 0.68%of0.9032 g.m^-2.h^-1of ZK60 alloy substrate.

3.2.2 Electrochemical impedance spectroscopy (EIS)

The electrochemical corrosion behaviors of MAO coating in3.5%Na Cl solution were studied by means of EIS, and EIS of ZK60 alloy and optimized MAO coating is presented in Fig.2.It can be seen that impedance spectroscopy of ZK60alloy is composed of a capacitive loop at high frequency range and an inductive loop at low frequency range, while the capacitive loop of optimized MAO coating is bigger than that of ZK60 alloy substrate.And the impedance value of the optimized sample is increased by four orders compared with bare ZK60 alloy.It indicates that the corrosion resistance of optimized MAO coating is greatly improved, which is consistent with the result of corrosion rates tested by means of immersion test.

3.2.3 Potential dynamic polarization curves

Potential dynamic polarization curves were tested by a potentiostat in 3.5%Na Cl solution.The polarization curves of optimized coating and ZK60 substrate are shown in Fig.3.It is found that the shapes of anodic part are dissimilar, which might be attributed to the passivation of the protective coating.

Fig.1 XRD patterns of MAO coating

Fig.2 Impedance diagrams in 3.5%Na Cl solution for magnesium alloys a without or b with micro-spark ceramic coating

Fig.3 Polar curves of MAO coating and magnesium alloy substrate

Table 1 Electrochemical parameters from curves of Zeta potential curves 下载原图

Table 1 Electrochemical parameters from curves of Zeta potential curves

Corrosion potential (E_corr) and corrosion current density (i_corr) of bare ZK60 alloy and optimized sample are listed in Table 1.It is obvious that the i_corrvalue of optimized sample is 16.87 × 10^-9which is decreased approximately by three orders.This indicates a better corrosion resistance of coating might prevent corrosive mediums from penetrating into the substrate, thus reducing the corrosion rate effectively.

Figure 4 demonstrates the surface and cross-section morphologies of the optimized coating.It is observed that the coating is uniform and smooth with very tiny voids or pores and fewer micro-cracks, which may account for the better corrosion resistance of the coating.

3.3 Hardness of MAO coating

Hardness of MAO coating is presented in Table 2.Due to high hardness of ceramic and especially the compact ceramic coating of optimized MAO coating, the hardness value reaches to HV 726 which is seven times as hard as that of ZK60 alloy substrate (HV 110) .Meanwhile, hardness of the interface layer also increases in some extent which can be attributed to the coating side.

Fig.4 SEM images of MAO coating:a surface and b cross section

Table 2 Hardness of magnesium alloy substrate and optimized MAO coating 下载原图

Table 2 Hardness of magnesium alloy substrate and optimized MAO coating

Fig.5 Macro-morphology of MAO coating

Fig.6 3D surface images of MAO coatings

3.4 Appearance and roughness of MAO coating

The optimized coating obtained in this work is uniform with a comfortable touching feeling.The macro-morphology of the optimized coating with offwhite, smooth, and fine appearance was demonstrated in Fig.5.The surface of the coating is very dense seldom with micro-cracks and pores, which also confirms the results of surface roughness testing as shown in Fig.6.

The roughness of the coating was measured by Olympus laser scanning confocal microscopy (OLS) .The average roughness R_aof the coating is only 0.264 lm and the stable roughness curve of the coating indicates that the coating surface exhibits good homogeneity.

3.5 Bonding strength between substrate and MAO coating

The bonding strength between the coating and ZK60 substrate was evaluated by scratch experiment.The coating surface was cleared and dried before test, and then scratched with a tool into grids (the size of each grid is2 mm × 2 mm) .Subsequently, the grids were cleaned along diagonal with banister brush.Then adhesive tape was used to adhere to the coating surface tightly for less than 5 min and then torn out smoothly with about 60°angle within 0.5–1.0 s.

Figure 7 shows surface morphologies of the MAO coating before and after the scratching experiment.It can be found that there are almost no changes on the surface of the coating except edges of several grids being damaged.Areas of damaged coating is 2%–3%of the total surface and the bonding strength reaches to the first stand according to the national standard[GB/T-1998eqv ISO:1992].

3.6 TG analysis of MAO coating

Figure 8 shows the TG analysis curve of MAO coating.The weight loss is very low with a value of 3.07 (wt%) when the optimized sample was gradually heated to 357°C.This weight loss can be attributed to the volatilization of moisture and residual solvents along with the decomposition of fluoride and hydroxide[14, 15].It indicates that the MAO coating exhibits excellent thermal stability.

Fig.7 Surface morphologies of MAO coating a before and b after of drawing pane

Fig.8 TG of MAO coating

Fig.9 Friction coefficient of optimized coating under dry friction

3.7 Abrasion behavior

Abrasion experiments were carried out by means of Universal Tribometer UMT-2 with a steel ball as the friction counterpart, at constant liner rate of 50 mm.s^-1under different load.As shown in Fig.9, the friction coefficient is 0.3744, 0.3201, and 0.2892 under load of 5, 7, and 9 N, respectively.Obviously, MAO coating exhibits lower friction coefficient under a higher load.

Morphologies of worn surface of MAO coating under different loads are shown in Fig.10.After abrasion experiment, the grooves on the surface of MAO are small at load of 5 N.When the applied load increases, the width and depth of grooves also increase.From the energy dispersive spectrometer results of the worn coating surface, it is proved that the worn surface is Fe-rich.This is because during the friction the surface temperature increases and the friction pair, and steel ball tend to soften and are scratched by the hard MAO coating.Therefore, an adhesive wear occurs at the coating surface with steel debris left on the coating surface.

4 Conclusion

The optimized MAO coating prepared on ZK60 alloy in the optimized dual electrolyte was mainly composed of Mg Al₂O₄and Mg O.The ceramic coating is very hard and smooth with surface hardness of HV 726 and R_aof0.264 lm.Its corrosion resistance is improved a lot due to the smooth, compact coating.TG experiment shows that the coating is hardly decomposed under high temperature, which indicates excellent thermal stability.The bonding strength between the coating and substrate achieves the top one level of the China national corresponding standard.