Effect of Anodizing Potential on the Surface Morphology and Corrosion Property of AZ31 Magnesium Alloy

Transcription

1 Materials Transactions, Vol. 51, No. 6 (21) pp. 119 to 1113 #21 The Japan Institute of Metals Effect of Anodizing Potential on the Surface Morphology and Corrosion Property of AZ31 Magnesium Alloy S. A. Salman 1, R. Mori 1, R. Ichino 2 and M. Okido 1 1 Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya , Japan 2 Ecotopia Science Institute, Nagoya University, Nagoya , Japan Anodizing is a functional method for coating magnesium alloys and improves its corrosion resistance. The anodizing process was performed on AZ31 magnesium alloy in 1 M NaOH alkaline solution at various applied potentials. The surface morphology and phase structure of the anodic film were analyzed using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion property of the anodic film was characterized using potentiodynamic polarization measurement, electrochemical impedance spectroscopy (EIS) and salt spray test. The anodic film is composed of two phases, magnesium hydroxides and magnesium oxides. At low anodizing potential, the main constituent of the anodic film is magnesium hydroxide. By increasing the anodizing potential, magnesium oxide grows to be the main constituent of the film. The results show that the anticorrosion property was enhanced by anodizing and the film formed at 1 V has the best corrosion resistance. [doi:1.232/matertrans.m2938] (Received November 17, 29; Accepted March 23, 21; Published May 19, 21) Keywords: AZ31 magnesium alloy, anodizing, alkaline solution, anticorrosion 1. Introduction Table 1 Chemical composition (mass%) of AZ31 Mg alloy. 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 density that is only 2/3 of aluminum alloy and 1/4 of steel, high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping and good machinability. 2) These properties make it an attractive target in a number of applications, especially in the fields where weight reduction is critical or particular technical requirements are needed. There are several applications of magnesium including automobile, computer parts, aerospace components, mobile phones, sporting goods, handheld tools and household equipment. Unfortunately, magnesium has also some inadequate characteristics that have delayed its wide scale use in many applications. Poor corrosion resistance is one of the major problems that prevent the widespread of magnesium alloy in many applications due to high chemical and electrochemical activity compared with other structural metals such as steel and Al alloys. There are two primary reasons for the poor corrosion resistance of magnesium alloys. The first reason is the internal galvanic corrosion by second phases or impurities, the second reason is the hydroxide film on magnesium is much less stable than passive films that form on metals such as Al alloys and stainless steels. Surface treatment was performed in order to achieve a good corrosion resistance such as electrochemical plating, chemical conversion, anodizing, thermal spraying, chemical and physical vapor deposition. Chromate bath was traditionally applied in spite of being not friendly to the environment. It is toxic to human and difficult to be recyclable. 3 5) In our previous work, we investigated the anodizing of AZ31 magnesium alloy in alkaline solution at 3V. 6,7) Anodizing in alkaline solution with co-precipitation of cerium oxide and aluminum oxide was also investigated in order to improve the anticorrosion property of magnesium alloy. 8,9) The anticorrosion and surface properties of AZ31 Al Zn Mn Si Cu Ni Fe Mg Others Imp <:1.1 <:3 Bal..3 magnesium alloy were significantly improved with cerium additives. In the present work, we performed the anodizing process at a various range of anodizing potential, 3 1 V. The anti-corrosion behaviors were evaluated using the anodic polarization curves and the electrochemical impedance spectroscopy (EIS). Furthermore, the surface morphologies and phase structure were detected using SEM and XRD. 2. Experimental Procedure 2.1 Specimens Commercially available AZ31 Mg alloys were used as the substrate. The chemical composition of the alloy is listed in Table 1. The surface of the alloy was polished up to # 2 emery paper followed by.5 mm alumina powders. The specimens were carefully cleaned with water, rinsed with acetone and dried under air. All of the experiment specimens were mounted using polytetrafluoroethylene (PTFE) resin tape, leaving 1 cm 2 surface area. 2.2 Anodizing Anodizing was performed at room temperature for 2 6 s in the aqueous electrolyte of 1 M NaOH using power supply (6 A, 5 V) to provide various ranges of constant potential to the anodizing cell. The anodizing potential used in this study was changed over the range from 3 to 11 V. AZ31 magnesium alloy is connected to a positive terminal of a power supply and platinum counter electrode is connected to a negative terminal. A digital multimeter and Ag/AgCl sat. KCl reference electrode was used to measure the actual potential near the surface of the specimen and keep at

2 111 S. A. Salman, R. Mori, R. Ichino and M. Okido Table 2 constant value throughout the anodizing process. After the treatment, the specimens were carefully rinsed using distilled water and dried under air before analysis. 2.3 Morphology and structure of anodic film The morphology and microstructure of the anodic films were observed with a Hitachi S-8 scanning electron microscope (SEM), an optical microscopy and digital camera. The crystallographic was identified with X-ray diffraction (XRD). The thickness of the anodic films was measured using SEM. 2.4 Corrosion measurements Polarization technique The potentiodynamic polarization tests were carried out using a Solartron 1285 Potentiostate from Solartron Analytical, Farnborough, United Kingdom. The measurements were controlled by Scribner Associates Corrware electrochemical experiment software. The anodic and cathodic polarization curves were measured in 17 mm NaCl and.1 M Na 2 SO 4 solution at 298 K with a scanning rate of 1 mv/s AC impedance spectroscopy Electrochemical impedance spectroscopy of the immersed specimens was measured in 3.5 mass% NaCl solutions in order to accelerate the corrosion rate of the anodic films. The impedance test was performed at 298 K using a Solarton 1287 electrochemical interface and a Solarton 126 frequency response analyzer with a frequency range from 1, to.1 Hz. in amplitude of 1 mv peak-to-peak AC voltage. The measurements were controlled by Scribner Associates Z plot electrochemical experiment software Salt spray test The salt spray and humidity tests of the anodic films were carried out in 5% NaCl solution using salt spray test machine, the test conditions are shown in Table Results and Discussion Salt spray test conditions. Temp. ( C) humidity (%) Time (h) Salt spray (5 mass% NaCl) dry wet The current transients for AZ31 magnesium alloy in 1 M NaOH solution is shown in Fig. 1. Anodizing was performed at different potentials 3 1 V, the current density differs with the anodizing potential changes. The anodic film formed at anodizing potential has a dark gray color. The current density does not flow immediately when the potential was applied. With progress of time the current density increased due to the dissolution reaction of magnesium as represented by reaction (1). Then, magnesium ions reacted with OH ions, forming Mg(OH) 2 as described by reaction (2). 1) The current density reached its maximum value (.2 A cm 2 ) at about 9 s and then remains constant until the end of anodizing process because the stationary of dissolution and film formation. Current density, i / A cm V 3V 1V 1-3 1V Anodizing time, t/s Fig. 1 Current change during constant potential anodizing of AZ31 magnesium alloy in 1 M NaOH solution at 298 K. Mg! Mg 2þ þ 2e ð1þ Mg 2þ þ 2OH! Mg(OH) 2 ð2þ Magnesium oxide can also form according to reaction (3). Several reports demonstrated that these structures can be converted into each other by either hydration or dehydration procedures. 11,12) Mg(OH) 2! MgO þ H 2 O ð3þ On the contrary, with increasing of the anodizing potential 1 1 V, the current flowed immediately and it decreased with increasing the time. Anodizing at 1, 2 V has the lowest current density values. Anodizing potential of 7 1 V nearly has the same structure of the current transients. Subsequently, we only show the curve of 1 V. At the first 2 s, anodizing process is accompanied with intense sparking. Consequently, the current density sharply decreased and became approximately constant at about 3 s. The decreasing in the current density value at anodizing potential 1 1 V referred to the formation of more protected anodic film in comparison to low potential anodizing as will be discussed in a later part of this paper. Figure 2 shows the surface images of the anodic films, anodizing at shows a very rough surface as shown in Fig. 2(a). Anodizing at 1 V shows a porous structure, the diameter of the pore was about.84 mm as shown in Fig. 2(b). Figure 2(c), (d) shows that the surface becomes smoother with increasing of the anodizing potential from 2 to 7 V. Intense sparking was observed at anodizing potential 8 V and 1 V. The initial structure of the film formed by sparking is porous microstructure with circular or elliptic pores as shown in Fig. 2(e), (f). The average pore size at 1 V was.21 mm. Film thickness has been measured as a function of anodizing potential as shown in Fig. 3, the thickness of the anodic film reached its maximum value (2.6 mm) at anodizing potential with gray color appearance. With increasing the anodizing potential 1 2 V, the thickness of the film decreased to be.18 and.15 mm respectively, and it increased again with increasing the anodizing potential 2 1 V. The anodic film at 1 V shows a white color compact film with thickness about 1.2 mm. When the anodizing potential increased more than 1 V, an intense spark was occurring continuously with randomly increasing and decreasing of

3 Effect of Anodizing Potential on the Surface Morphology and Corrosion Property of AZ31 Magnesium Alloy 1111 (a) (b) (c) (d) (e) (f) 3 µ m Fig. 2 Surface morphologies of AZ31 magnesium alloy after anodizing at (a), (b) 1 V, (c) 2 V, (d) 7 V, (e) 8 V, and (f) 1 V. 3 Mg(OH) 2 MgO Mg Film thickness / µ m 2 1 (a) (b) 5 1 Anodizing potential, E / V vs. Ag/AgCl sat. KCl (c) Fig. 3 Film thickness as a function of anodizing potential. (d) current density. Furthermore, no film was formed. Figure 4 shows the XRD patterns of AZ31 magnesium alloy after anodizing in 1 M NaOH solution at different anodizing potential. Figure 4(a), (b), (c) shows that the anodic films formed at anodizing potential 3, 1, 2 V were mainly composed of magnesium. Magnesium hydroxide and magnesium oxide peaks were also detected on the surface indicating that the anodic film is most probably formed from the magnesium hydroxides and oxides. Magnesium hydroxide peaks were not observed at anodizing potential 7, 1 V as shown in Fig. 4(d), (e). In contrast, magnesium oxide peaks was increased at 7 V, and it has the highest value at 1 V. These results demonstrate that at high potential anodizing, the main constituent of the anodic film is magnesium oxide. Furthermore, the magnesium hydroxide peaks were wider than magnesium oxide peaks indicating (e) θ (degree, CuKα ) Fig. 4 The XRD pattern of AZ31 magnesium alloy after anodizing at (a), (b) 1 V, (c) 2 V, (d) 7 V, and (e) 1 V. a good crystallinity of Magnesium oxide compared with magnesium hydroxide. In order to evaluate and compare the corrosion properties of anodic films, anodic and cathodic polarization curves were measured in 17 mm NaCl and.1 M Na 2 SO 4 solution.

4 1112 S. A. Salman, R. Mori, R. Ichino and M. Okido 1 Non-treated specimen 1 V.5 Current density, i / ma cm V E Non-treated specimen E / V Fig Anodizing potential, E / V vs. Ag/AgCl sat. KCl Variation of E as a function of anodizing potential Potential, E / V vs. Ag/AgCl sat. KCl 1 4 Fig. 5 Anodic and cathodic polarization curves for AZ31 magnesium alloys before and after anodizing at 3 and 1 V in 17 mm NaCl and.1 M Na 2 SO 4 solution. Generally, the anodic polarization represents the dissolution of magnesium, the cathodic polarization curves are assumed to represent the cathodic hydrogen evolution through water reduction. The anodic polarization curves reveal clearly that the pitting potential was moved toward more positive value with anodizing. Therefore, corrosion resistance has improved with the anodizing treatment. The higher values of pitting potential ( 1:34 and 1:39 V) were observed with anodizing at and 1 V respectively as shown in Fig. 5. The current density increased sharply after the potential exceeded the pitting potential and finally the pits appeared on the surface. On the other hand, the cathodic polarization curves were measured in order to get more information about the corrosion behaviors of the anodic film. The cathodic overpotential of anodic films has negative potentials in comparison to non-treated specimens, which indicates the increase of hydrogen evolution overpotential by the anodic passive films which mainly formed from magnesium oxide/hydroxide. When we compare anodic films formed at and 1 V, the specimen of 1 V has the best corrosion resistance during the cathodic polarization. However, the specimen of has the best corrosion resistance during the anodic polarization. Figure 6 shows the E values of all anodic films (3 1 V). E is defined as the difference between the potential value at.5 ma cm 2 (anodic side) and the potential value :5 ma cm 2 (cathodic side). E of anodic films formed at 3 8 V has nearly the same value.25 V and the lowest E values were observed at 1 and 2 anodizing potential around.22 V. The highest E values.33,.39, and.4 were observed at 9, 3 and 1 V anodizing potential respectively. Electrochemical impedance spectroscopy measurement (EIS) and salt spray test were performed to ensure the best corrosion resistance anodic film. Corrosion rate determination is associated with the charge transfer resistance (Rct) by using EIS technique. The charge transfer resistance is equal to the diameter of the semicircle in the complex plane graph (nyquist diagram). Figure 7 shows the time dependence of the charge transfer resistance, which R ct / Ω cm Immersion time, t / s 1 V 3 Fig. 7 Change in Rct of AZ31 magnesium alloy in 3.5 mass% NaCl solution before and after anodizing at 3 and 1 V. is corresponding to corrosion rate. The charge transfer resistance increases by increasing the anodizing potential. The specimen anodized at 1 V has much higher Rct value than anodic film. Therefore, the anodic film formed at 1 V has a more corrosion resistance than the anodic film formed at. It is also worth mentioning that the corrosion pits were observed with the naked eye on the surface of specimens after progress for 6 s. However, it was observed after 18 s at anodizing potential 1 V. Surface morphologies of the anodic films formed at 3 and 1 V before and after the salt spray test were shown in Fig. 8. Before the salt spray test, the surface of the anodic film formed at was dark gray because the formation of thick magnesium hydroxide film. After the salt spray test, the surface at the was rough with many pits were visible. The diameter of the observed pits reached 1 mm. On the other hand, the surface of the anodic film formed at 1 V a white color. It may be because the formation of a compact magnesium oxide film. After the salt spray test, the anodic film formed at 1 V has a uniform surface as before the salt spray test. Very few pits were observed on the surface, the maximum diameter of the pits was about 2 mm. Salt spray test results show good agreement with EIS and cathodic polarization results. We can conclude that the film formed at 1 V has excellent corrosion resistance compared with those formed at lower potential.

5 Effect of Anodizing Potential on the Surface Morphology and Corrosion Property of AZ31 Magnesium Alloy 1113 Before salt spray test After salt spray test 1 V 3 mm 3 mm 4 µ m Fig. 8 Surface morphologies of anodic film formed on AZ31 magnesium alloy before and after salt spray test. 4. Conclusion The film formed at has a dark gray color with rough surface, it mainly consisted of magnesium hydroxide, and it was thicker than all those films formed at higher potential. On the other hand, the anodic film formed at 1 V has a smooth surface with a white color. It may be because the formation of a compact magnesium oxide film. The film formed at 1 V has the lowest hydrogen overpotential, and it has the best corrosion resistance in this work. Acknowledgement The authors thank Dr. Kuroda for his kind advice and guidance in carrying out this work. The financial assistance from The Ministry of Education, Culture, Sports, Science and Technology of Japan (Monbukagakusho) is gratefully acknowledged. REFERENCES 1) G. L. Song and A. Atrens: Adv. Eng. Mater. 1 (1999) ) M. Yekehtaz, K. Baba, R. Hatada, S. Flege, F. Sittner and W. Ensinger: Nucl. Inst. Methods Phys. Res., Section B 267 (29) ) S-J. Kim, Y. Zhou, R. Ichino, M. Okido and S. Tanikawa: Metal. Mater. Int. 9 (23) ) S-J. Kim and M. Okido: Bull. Korean Chem. Soc. 24 (23) ) H. Umehara, M. Takaya and Y. Kojima: J. Jpn. Inst. Light Met. 5 (2) ) S. A. Salman, R. Ichino and M. Okido: Proc. 8th Int. Conf. on Advanced Surface Engineering, Tokyo, (26) p ) S. A. Salman, R. Ichino and M. Okido: Surf. Eng. 24 (28) ) S. A. Salman, R. Ichino and M. Okido: Trans. Nonferr. Metal. Soc. China 19 (29) ) S. A. Salman, R. Ichino and M. Okido: Mater. Trans. 49 (28) ) Y. Mizutani, S. Kim, R. Ichino and M. Okido: Surf. Coat. Technol (23) ) G. L. Song and A. Atrens: Proc. 6th Int. Conf. Magnesium Alloys and Their Applications, (K U Kainer, Weinheim, Germany, 24) pp ) M. Laska, J. Valtyni and P. Feller: Cryst. Res. Technol. 28 (1993)