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1 Corrosion Science 5 (28) Contents lists available at ScienceDirect Corrosion Science journal homepage: Anodizing of magnesium alloy AZ31 in alkaline solutions with silicate under continuous sparking Liyuan Chai a, *, Xia Yu a, Zhihui Yang a, Yunyan Wang a, Masazumi Okido b a School of Metallurgical Science and Engineering, Central South University, 4183 Changsha, PR China b School of Engineering, Nagoya University, Nagoya , Japan article info abstract Article history: Received 6 August 27 Accepted 3 August 28 Available online 3 August 28 Keywords: A. Magnesium A. Alloy C. Anodic films Anodization is a useful technique for forming protective films on magnesium alloys and improves its corrosion resistance. Based on the alkaline electrolyte solution with primary oxysalt developed previously, the optimum secondary oxysalt was selected by comparing the anti-corrosion property of anodic film. The structure, component and surface morphology of anodic film and cross-section were analyzed using energy dispersion spectrometer (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion process was detected by electrochemical impedance spectroscopy (EIS). The results showed that secondary oxysalt addition resulted in different anodizing processes, sparking or nonsparking. Sodium silicate was the most favorable additive of electrolyte, in which anodic film with the strongest corrosion resistance was obtained. The effects of process parameters, such as silicate concentration, applied current density and temperature, were also investigated. High temperature did not improve anti-property of anodic film, while applying high current density resulted in more porous surface of film. Ó 28 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium is the 8th most abundant element on the earth [1]. It has some favorable properties that make it an excellent choice for a number of applications, such as a high ratio of strength to weight with a density that is only 2/3 of aluminum and 1/4 of iron, high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics and good machinability [2]. These properties make it valuable in a number of applications including automobile and computer parts, aerospace components, mobile phones, sporting goods, handheld tools and household equipment, especially in the fields where weight reduction is critical or particular technical requirements are needed. However, magnesium is a metal with an extremely negative equilibrium potential and it has an undesirable property of poor corrosion resistance. One of the most effective ways to prevent corrosion are to coat the base material. Magnesium with anodic coatings and its alloys are used to improve their corrosion resistance or to enhance the adhesion for organic coatings [3 4]. Although anodization is a useful technique for forming protective films on magnesium alloys and has been the target of extensive research, most previous studies have employed an electrolyte consisting mainly of a solution containing chromic acid or fluoride * Corresponding author. Tel.: ; fax: address: lychai@mail.csu.edu.cn (L. Chai). [5 6]. However, from the viewpoint of low environmental impact and the health hazards involved in handling chromate/fluoridebased baths, the development of an anodization process for magnesium alloys that use a chromium-free bath is highly anticipated. Recently, many researches focused on the selection of electrolyte, such as alkaline solution with additives just as phosphate, silicate, borate, and organic substances [7 1]. In literatures, the addition of oxysalt in electrolyte improved the corrosion resistance of magnesium alloys [8 9,11]. Therefore, the objectives of this study were to (1) select the optimum secondary oxysalt of the anodizing of magnesium alloy AZ31 under continuous sparking through investigating the structures, components and surface morphologies of anodic films by EDS, XRD and SEM; and (2) study the effects of some process parameters such as oxysalt concentration, applied current density and temperature on the anodization of magnesium alloy AZ31. The outcomes can provide information on the technological improvement of the anodization of magnesium alloy, AZ Experimental The specimens are AZ31 magnesium alloy plates, which contain 3 mass% Al and 1 mass% Zn. The specimens were left 1 cm 2 of the surface exposed. After polishing to.5 lm alumina powder, the electrodes were carefully degreased by water and acetone. An electrochemical system was constructed with Pt coil as a counter electrode and Ag/AgCl saturated with KCl as a reference electrode X/$ - see front matter Ó 28 Elsevier Ltd. All rights reserved. doi:1.116/j.corsci

2 L. Chai et al. / Corrosion Science 5 (28) The experiments were performed in a bath containing 4 g l 1 of NaOH and 3 g l 1 of sodium borate with agitation. The anodic polarization curve was measured by a potentiostat with 6 mv s 1 of scan rate, and anodizing process was carried out at constant current density. The anti-corrosion polarization curves of the anodized specimens were measured from the open circuit potential with the scan speed of 1 mv s 1 in.99 g l 1 of NaCl g l 1 of Na 2 SO 4 solution at 25 C. The anodized specimens were analyzed with XRD. The morphologies of surface and cross-section were tested by SEM, and atomic ratios were obtained from EDS. The corrosion process was performed with an EIS Analyser (Salarton SI 126). After 3 min retard, impedance spectra were recorded over a frequency range of.1 1 khz using a single sinusoidal excitation signal. Current density, I / ma/cm Na 2 SiO 3 Na 2 MoO 4 Na 3 PO 4 NaAlO 2 none 3. Results and discussion 3.1. Selection of the secondary oxysalt as basic composition of electrolyte solution During the electrochemical process, unexpected effects could be produced for cooperative function between different ions in the solution. Based on the previous investigation of the authors [11], the secondary oxysalts were added into the alkaline electrolyte solution to investigate their effects on anodizing process and property of anodic film formed on magnesium alloy, AZ31. The primary oxysalt in this study was 3 g l 1 of sodium borate. Sodium silicate, sodium phosphate, sodium aluminate and sodium molybdate were chosen as the secondary oxysalts. Their concentration was 3 g l 1. The experiments were conducted at 1 ma cm 2 of constant applied current density at room temperature for 3 min. The voltage transients for AZ31 magnesium alloy in solutions with different oxysalts are shown in Fig. 1. At the same applied Potential, E / V (SHE) NaSiO 3 Na 3 PO 4 NaAlO 2 Na 2 Mo 2 O Time, t / s Fig. 1. E t curves of anodized AZ31 obtained in solution containing secondary oxysalts (primary oxysalts: 3 g l 1 of sodium borate, 1 ma cm 2, 3 min, room temperature) Fig. 2. Effects of secondary oxysalts on anti-corrosion properties of anodic films (1 ma cm 2, 3 min, room temperature). current density, the additions of the secondary oxysalt led to the great variation of anodizing process. During anodization conducted in solutions containing sodium silicate or sodium aluminate, the voltage increased rapidly with time at the beginning of the process and thereafter remained constant after 1 min. Extensive voltage oscillation and sparking occurred when the potential reached the breakdown point. However, in alkaline solution spiked with sodium phosphate and sodium molybdate, the potentials were kept constantly and rather low, about 3 V. During the whole procedure, no sparking occurred and the formation of anodic film was stable. Anti-corrosion properties of anodic films formed in different bath solutions were compared in this study (Fig. 2). Compared with basic alkaline electrolyte solution, the addition of sodium silicate resulted in the positive transfer of corrosion potential in the curve of potential against current density. But those phenomena were not found in an electrolyte containing sodium phosphate, sodium aluminate and sodium molybdate. The result indicates that the corrosion resistance of anodic film was greatly improved by the addition of sodium silicate. The surface morphology of anodic film was also observed in this study. The film formed in an electrolyte containing sodium phosphate or sodium molybdate appeared to be corroded, and there were some corrosive productions on surface (Fig. 3). From the SEM image of the anodic film surface formed in a solution containing sodium aluminate, holes of different sizes caused by sinter appeared on the surface of film and were connected with cracks, which may contribute to the inferior corrosion resistance. While the anodic film formed in solution containing sodium silicate was rather smooth with little cracks (Fig. 3). From the above mentioned results, sodium silicate could be chosen as the optimum Fig. 3. Effects of secondary oxysalts on morphologies of anodic films.

3 3276 L. Chai et al. / Corrosion Science 5 (28) Fig. 4. The SEM image of cross-section of anodic film formed on magnesium alloy AZ31 (electrolyte: primary oxysalts + 9 g l 1 Na 2 SiO 3 +1gl 1 phenol, 1 ma cm 2, 3 min, room temperature). addition of the secondary oxysalt. The cross-section morphology of anodized sample was shown in Fig. 4. From this figure, it can be seen that the anodic film was combined with base metal tightly and the thickness of the anodic film was 2 lm. The corrosion process of anodized sample without sealing was also detected and analyzed by EIS (Fig. 5). The equivalent circuit of anodized AZ31 magnesium alloy was present in our previous study [12]. Based on the equivalent circuit and EIS patterns, the nonlinear fit curve of resistances was obtained and shown in Fig. 6, in which R s, R c, R po and R ct were resistances for solution, film, pores and corrosive reaction, respectively. Since the film without sealing was porous, the corrosive ions penetrated into the film and then reached the interface between anodic film and base R, k Ω /cm 2 7 R s 5 R c /R po R ct Immersion time,t/h Fig. 6. Fitting curve of corrosive resistances for anodized specimen. metal. As shown in Fig. 6, the corrosive resistances decreased rapidly within 1 h, and then remained relatively stable. The results implied that the corrosion process rapidly initiated within a short period, and thereafter this process slowed down because of anodic film formation Effect of sodium silicate concentration on anti-corrosion properties of anodic films Fig. 5. The EIS patterns of anodized specimen (electrolyte: primary oxysalts + 9 g l 1 Na 2 SiO 3 +1gl 1 phenol, 5 ma cm 2, 1 min, room temperature). Comparison of corrosion resistances of anodic films formed in solutions containing different concentrations of sodium silicate is shown in Fig. 7. The corrosion potential was positively transferred with increasing concentration when Na 2 SiO 3 concentration was lower than 9 g l 1. The largest positive transfer of corrosion potential was observed at 9 g l 1 of Na 2 SiO 3. Thereafter, corrosion potential was negatively transferred. The results indicated that the addition of 9 g l 1 could get the best improvement of anti-corrosion properties for anodic films. However, when Na 2 SiO 3 concentration was over 9 g l 1, the corrosion resistance decreased. The X-ray diffraction patterns of anodic films formed in solutions containing different concentrations of sodium silicate are shown in Fig. 8. When the concentration of sodium silicate in anodizing solution was 1 g l 1, a large amount of MgO could be found in the anodic films. However, with the increasing of sodium silicate concentration, the peak of MgO gradually diminished, and

4 L. Chai et al. / Corrosion Science 5 (28) Current density, I / ma/cm g/L 3g/L 6g/L 9g/L 12g/L Fig. 7. Effects of sodium silicate on anti-corrosion properties of anodic films (room temperature, 3 min, 1 ma cm 2 ). Intensity, I MgO Mg 12g/L 9g/L 3g/L 1g/L θ Fig. 8. Effects of Na 2 SiO 3 concentrations on compositions of anodic films. Table 1 Elemental components of anodic films formed in electrolyte with different Na 2 SiO 3 concentrations Concentration (g/l) O (wt%) Mg (wt%) Si (wt%) O (atomic Mg (atomic O/Mg applied current density, sparking moved more rapidly. Moreover, the little white sparks changed into large, yellow bright one with current density increasing. The anti-corrosion properties of anodic films formed at different applied current densities are presented in Fig. 9. The results revealed that with the increasing of applied current density, better corrosion protection of anodic film could be obtained. The voltage transients observed during anodizing processes conducted at different applied current densities are shown in Fig. 1. It was found that the voltage increased almost linearly at the initial stage of anodizing, then reached up a plateau and remained constant. The increasing of voltage with time was caused by the increment of coverage percentage to substrate and thickness of anodic film. Together with the voltage decreasing caused by the change of anodic film structure and property under sparking, the voltage was kept almost constant. To ensure the well-balanced growth of anodic film, there should be simultaneity of destruction of old film and appearance of new film. The destruction processes of anodic film included the breakdown, physical fusion and chemical dissolution. The extensive oscillation of voltage revealed that the growth of anodic film may be the competitive process of the three steps including destruction of old film, reparation of destroyed film and formation of new film. The latter two should be dominant [13]. Current density, I / ma/cm mA/cm 2 1mA/cm 2 3mA/cm 2 2mA/cm 2 4mA/cm 2 5mA/cm Potential, E / V (SHE).5 Fig. 9. Effects of applied current density on anti-corrosion properties of anodic films (primaryoxysalts + 9 g l 1 Na 2 SiO 3, room temperature, 3 min). 1 then finally disappeared. As seen in Table 1, the results of EDS revealed an increased concentration of silicon species but a constant atomic ratio of Mg to O for the films formed in electrolytes with more sodium silicate addition. From substance phase analysis in XRD patterns (Fig. 8), it was indicated that the species with element silicon or oxygen were amorphous. The possible explanation was that the molten productions of anodizing, which was caused by high temperature produced by sparking, were cooled down suddenly when touching the solution. And then the cooling was too prompt for the atoms of magnesium, silicon and oxygen to be arranged regularly according to the lattice structure Effect of applied current density on anti-corrosion properties of anodic films The different applied current densities led to the obvious variation of the experimental phenomena in this study. With increasing mA/cm 2 3mA/cm 2 1mA/cm 2 5mA/cm 2 4mA/cm 2 5mA/cm Time, t / s Fig. 1. E t curves of anodizing processes at different applied current densities.

5 3278 L. Chai et al. / Corrosion Science 5 (28) The E t curves in Fig. 1. indicated that the voltage would become higher with the increasing of applied current density. In the anodizing process, the formation of anodic film on the electrode could lead to the increasing of voltage at constant current or decreasing of current density at constant potential in the electrolyte solution with certain ingredient. According to Zhang et al. [6], there was a close correlation between voltage and film thickness for the same base metal. They pointed that the thicker the film was, the higher the voltage was. Therefore, the effects of current density on anodic film anti-corrosion property could contribute to the increased thickness resulted from the increased current density, which led to the improvement of anodic film anti-corrosion property. The structures of anodic film were not changed greatly by different current densities, which were indicated by X-ray diffraction patterns (data not shown). Elemental analysis of EDS showed that the increasing of applied current density had little effect on silicon content in the film, while the atomic ratio of O to Mg increased with the increasing applied current density (Table 2). SEM images of AZ31 alloy surface after anodizing at different applied current densities are shown in Fig. 11. The morphology of the film was nonuniform under different applied current densities. When current density was less than 2 ma cm 2, a rough surface was observed in anodizing film. When current density reached up to 2 ma cm 2, the film surface was smooth with less protuberance. With further increasing of current density, more rough areas with more pores could be observed on the film surface. The possible reason contributed to the rapid movement of sparking, leading Table 2 Elemental components of anodic films prepared at different applied current densities Current density (ma/cm 2 ) O (wt%) Mg (wt%) Si (wt%) O (atomic Mg (atomic O/Mg to more pores, and smaller particles resulted from repeating destruction. From the analysis of several properties of anodic film mentioned above, the results revealed that applied current density had two aspects of effect on the film. On the one hand, the thickness of anodic film, especial thickness of the compact layer was increased for voltage augment, which improved anti-corrosion property. On the other hand, the number of increased pores impaired corrosion resistance of film. For the point view of corrosion protection of compact layer, the former one played more important role on the film. Therefore, the anti-corrosion property of anodic film was improved by the increment of current density Effect of temperature on anti-corrosion properties of anodic films The corrosive polarization curves of anodic films formed at different temperatures are shown in Fig. 12. The higher the solution temperature was, the weaker the corrosion protection was. This could be explained in several ways. Firstly, the growth of anodic film was depressed by high temperature. Generally, the reaction Current density, I / macm o C 6 o C 25 o C 8 o C Fig. 12. Effects of temperature on anti-corrosion properties of anodic films (primary oxysalts + 9 g l 1 Na 2 SiO 3,5mAcm 2, 3 min). Fig. 11. Effects of applied current density on surface morphologies of anodic films.

6 L. Chai et al. / Corrosion Science 5 (28) Table 3 Elemental components of anodic films prepared at different temperatures Temperature ( o C) activation energy and conductivity of solution could be enhanced by increasing temperature, which would make the formation of new film and the destruction of old film. However, the influence of temperature on the rate of above procedure differed [6]. Some reports revealed that the destruction of old film would be faster than the formation of new film, leading to the decreased growth of anodic film [7]. Secondly, heat was released for electric breakdown and discharge of plasma during anodizing process with sparking. When the heat was exhausted continuously, compact film could be formed. Moreover, the accumulation of large amounts of water vapor under high temperature of solution resulted in the formation of large size holes, which would weaken anodic film. In this study, the effects of temperature on element amount were also studied. As shown in Table 3, the largest amount of silicon species in the film was obtained at 25 C. Thereafter, its amount decreased with minor variation. Conversely, the amount of oxygen species increased with increasing temperature, especially the atomic ratio of O:Mg was greatly enhanced. This suggested that the high temperature of solution contributed to the generation of compound containing oxygen but inhibited the formation of compound containing silicon. 4. Conclusion O (wt%) Mg (wt%) Si (wt%) O (atomic Mg (atomic O/Mg This study showed that the secondary oxysalts resulted in distinct influences on anodizing process of magnesium alloy AZ31. Sparking was observed under sodium silicate or sodium aluminate addition, but this symptom does not exist under sodium phosphate and sodium molybdate additions. Among the selected oxysalts, sodium silicate could contribute to obtaining of anodic film with the best anti-corrosion property, and the different concentrations of sodium silicate would cause the change of anodic film structure, and the optimum additive concentration in electrolyte solution was 9 g l 1. The corrosion resistance of anodic film was closely related to the applied current density. Under high applied current density, strong corrosion resistance and porous surface of anodic film were obtained. The solution temperature had a negative effect on the anti-corrosion property of anodic film with applied current density. Probably, the large quantity of heat caused by sparking could not be released effectively. References [1] E.F. Emley, Principle of Magnesium Technology, Pergamon Press Ltd., Headington Hill Hall, Oxford, [2] D. Eliezer, E. Aghiion, F.H. Froes, Magnesium science, technology and applications, Advanced Performance Materials 5 (3) (1998) [3] Y. Kojima, Platform science and technology for advanced magnesium alloys, Materials Science Forum 35 (3) (2) [4] H. Kuwahara, Y. Al-Abdullat, M. Ohta, S. Tsutsumi, K. Ikeuchi, N. Mazaki, T. Aizauta, Surface reaction of magnesium in Hank s solutions, Materials Science Forum 35 (3) (2) [5] S. Ono, N. Masuko, Anodic films grown on magnesium and magnesium alloys in fluoride solution, Materials Science Forum 419 (4) (23) [6] Y. Zhang, C. Yan, F. Wang, W. Li, Electrochemical behavior of anodized Mg alloy AZ91D in chloride containing aqueous solution, Corrosion Science 47 (25) [7] A.K. Sharma, R.U. Rnai, K. Giri, Studies on anodization of magnesium alloy for thermal control Applications, Metal Finishing 95 (3) (1997) [8] Q.Z. Cai, L.S. Wang, B.K. Wei, Q.X. Liu, Electrochemical performance of microarc oxidation films formed on AZ91D magnesium alloy in silicate and phosphate electrolytes, Surface & Coating Technology 2 (12 13) (26) [9] W.P. Li, L.Q. Zhu, Y.H. Li, B. Zhao, Growth characterization of anodic film on AZ91D magnesium alloy in an electrolyte of Na 2 SiO 3 and KF, Journal of University of Science and Technology Beijing 13 (5) (26) [1] C.S. Wu, Z. Zhang, F.H. Cao, L.J. Zhang, J.Q. Zhang, C.N. Cao, Study on the anodizing of AZ31 magnesium alloys in alkaline borate solutions, Applied Surface Science 253 (27) [11] L. Chai, X. Yu, M. Okida, Anodizing of magnesium alloy AZ31 in alkiline NaOH solution, in: TMS Fall Extration & Processing Meeting: Sohn International Symposium, August 27 31, 26, pp [12] X. Yu, Study on Anodizing Process and Fundamentals of Magnesium Alloy AZ31, Doctoral Dissertation of Central South University, 26 (in Chinese). [13] Y.J. Zhang, C.W. Yan, F.H. Wang, H.Y. Lou, C.N. Cao, Study on the environmentally friendly anodizing of AZ91D magnesium alloy, Surface & Coatings Technology 161 (1) (22)