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Electrodeposition of aluminum on magnesium alloy in aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid and its corrosion behavior
Electrochemistry Communications 9 (2007) 1602–1606 www.elsevier.com/locate/elecom
Electrodeposition of aluminum on magnesium alloy in aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid and its corrosion behavior Jeng-Kuei Chang a
a,*
, Su-Yau Chen a, Wen-Ta Tsai a, Ming-Jay Deng b, I-Wen Sun
b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Chemistry, National Cheng Kung University, Tainan, Taiwan Received 7 February 2007; received in revised form 1 March 2007; accepted 5 March 2007 Available online 12 March 2007
Abstract A dense and adhesive Al layer was successfully electrodeposited on a Mg alloy in aluminum chloride–1-ethyl-3-methylimidazolium chloride ionic liquid. The corrosion resistance of the uncoated and Al-coated samples was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization measurements in 3.5 wt% NaCl solution. It was confirmed that the protective Al layer significantly reduces the corrosion rate of the Mg alloy. However, the deposition potential was a crucial factor that governed the structure and therefore the protection capability of the Al layer. 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Corrosion; Electrodeposition; Al-coating; Ionic liquid
1. Introduction The natural abundance of environment-friendly magnesium (Mg) compounds and a combination of advantageous mechanical properties and considerably low density (1.7 g/cm2) have made Mg and its alloys the most promising materials for various applications in recent years. However, these materials exhibit poor corrosion resistance, thereby limiting their usage in hostile environments [1–3]. Conversion coatings are usually done to prevent the Mg substrate from rapid corrosion. However, previous reports [4,5] concluded that these coatings did not provide satisfactory protection. On the other hand, anodization treatment is another popular process that improves the corrosion resistance of Mg alloys [6–10]. Nevertheless, the ceramic oxide coating produced is generally nonconductive and rather rough. The fabrication of a metallic layer with good *
Corresponding author. E-mail address: [email protected] (J.-K. Chang).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.03.009
protective capabilities on the Mg alloy was seldom reported in the literature [11–13]. A conductive coating is of great interest because the low resistivity of the coated Mg alloy can be maintained and its electromagnetic shielding property will not be lost. Electrodeposition of a protective aluminum (Al) layer on the Mg alloy, which has never been explored to the best of our knowledge, is attempted in this study. Moreover, the improvement in corrosion resistance will be investigated for the first time. Al was chosen for the metallic protection layer on the Mg alloy due to four main factors. (1) Being a lightweight metal, Al does not increase the overall density significantly. Moreover, the corrosion resistance of Al and its alloys is satisfactory. (2) An Al-coated Mg alloy can generate a continuous Mg–Al intermetallic compound near the surface after proper heat treatment. It has been confirmed that this compound possesses good corrosion resistance [14–18]. (3) Since the anodization and electrolysis coloration techniques for Al are already well established, they should be directly applicable to a Mg alloy
J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
4Al2 Cl 7 þ 3e ! Al þ 7AlCl4
ð1Þ
Accordingly, this type of ionic liquid is employed in this study. Electroplating of Al on mild steel using the ionic liquid was also previously reported by Liu et al. [23]. 2. Experimental In the experiment, an acidic ionic liquid whose molar ratio of AlCl3–EMIC was 1.5:1 was prepared by the slow addition of a certain weight of AlCl3 (anhydrous powder, 99.99%, Aldrich) into EMIC (99%, Aldrich) at room temperature according to a previous paper [24]. The mixture was continuously stirred by a magnetic bar for one day to ensure uniformity. All chemicals were handled under a purified nitrogen atmosphere in a glove box (Vacuum Atmosphere Co.), in which the moisture and oxygen content were maintained below 1 ppm. A diecast AZ91D Mg alloy with 9.02 wt% Al and 0.49 wt% Zn was used in this study. Before electrodeposition, each sample was ground with SiC papers to a grit of 1000 in the glove box. The typical exposed area was 0.1 cm2 with the other portion sealed with a Teflon film. An Al layer was deposited on the Mg alloy by electrodeposition in the AlCl3–EMIC ionic liquid at 25 C. A three-electrode electrochemical system controlled by an AUTOLAB potentiostat was adopted for this experiment. The Mg alloy was assembled as the working electrode. In addition, Al wires were used for both the counter and reference electrodes. Cathodic deposition was performed under a constant potential mode. Potentials of 0.2 V and 0.4 V (vs. the Al wire) were applied to yield a total passed charge of 50 C/cm2. After deposition, the samples were thoroughly cleaned using distilled water and then dried in air. The morphology and chemical composition of the samples were examined with a scanning electron microscope (SEM, Philip XL-40FEG) and its auxiliary X-ray energy dispersive spectroscope (EDS). X-ray diffraction (XRD) analysis was also performed with a Rigaku diffractometer to explore the crystal structure. Deaerated 3.5 wt% NaCl testing solution was employed for corrosion resistance measurements. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. In
the potentiodynamic polarization test, the potential was scanned from 1.8 V to 0.5 V (vs. SCE) with a sweep rate of 1 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential. The detected frequency was in the range of 65,535–0.1 Hz with a voltage amplitude of 10 mV. 3. Results and discussion The experimental results indicate that the open circuit potential (OCP) of the AZ91D Mg alloy was approximately 0 V (vs. Al) and that it did not vary significantly with time in the AlCl3–EMIC ionic liquid at 25 C. Electrodeposition of Al was then performed by applying cathodic potential on the Mg substrate. Fig. 1 shows the variations in the current density versus time during the deposition processes, which were conducted at 0.2 V (curve a) and 0.4 V (curve b). As shown in the figure, the cathodic current for both the curves decreased rapidly in the beginning due to double layer charging. Afterward, the current density of the 0.2 V electrode remained stable at approximately 20 mA/cm2 throughout the deposition process, while the deposition current density of the 0.4 V electrode increased gradually from 20 to 50 mA/cm2. This current increase could be attributed to the progress of surface roughness during the electrodeposition process. For a total cathodic passed charge of 50 C/cm2, the time required to deposit Al at 0.2 V was 2550 s, revealing an overall deposition rate that is half that at 0.4 V. Both the Al-deposited samples were silver-gray and the surface coverage was quite satisfactory. The surface morphologies of the uncoated and Al-coated Mg alloys were further examined by SEM. Fig. 2a shows a micrograph
0
Current density (mA/cm2 )
with its surface entirely covered by an Al film. (4) Good recycling of materials can be maintained because Al is a primary alloy element for the AZ series Mg alloys. The extraction and refining of Al on inert electrodes in room temperature aluminum chloride–1-ethyl-3-methylimidazolium chloride (AlCl3–EMIC) ionic liquid have been reported [19]. This ionic liquid exhibits adjustable Lewis acidity depending on the molar ratio of AlCl3–EMIC [20–22]. Electrodeposition of Al can only be performed under the Lewis acidic condition when the molar ratio of AlCl3–EMIC is greater than one. The Al2 Cl 7 precursor, which is the dominant species in the ionic liquid, can be electrochemically reduced to the metallic form according to the following reaction [20–22]:
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Deposition time (second) Fig. 1. Variations of current density versus time during the deposition processes, which are performed at 0.2 V (curve a) and 0.4 V (curve b).
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J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
Fig. 2. SEM top-view micrographs of the (a) die-cast AZ91D Mg alloy substrate; (b) 0.2 V Al-deposited sample; (c) 0.4 V Al-deposited sample; (d) EDS analysis of the deposited laye and (e) SEM cross-section micrograph of the Al-coated Mg alloy.
of the bare diecast AZ91D Mg alloy, demonstrating its dual-phase microstructure. As displayed in this SEM photo, the Mg alloy consisted of two distinguishable portions: a gray area that represents a Mg solid solution phase (a) and a white area that represents an intermetallic phase (b, Mg17Al12). Fig. 2b shows the electrode electrodeposited at 0.2 V. A faceted microstructure with the crystal size in the range of 510 lm was clearly observed by SEM. Moreover, the deposited layer appeared quite dense and uniformly covered the entire Mg alloy substrate, regardless of the areas of the a or b phases. Surface observation result of the 0.4 V deposited electrode is shown in Fig. 2c; this figure shows that the deposition potential certainly caused a significant change in the morphology of the deposit. Regular nodules were observed in the SEM photo. It was noted that several small cracks could be found within the deposit
whose structure did not appear to be as compact as that of the 0.2 V deposited electrode. The chemical compositions of both the deposits were inspected with an EDS and the results revealed an identical spectrum, as shown in Fig. 2d. The analytical data revealed that the deposits were pure Al (no other element could be recognized). Furthermore, no residual salts were detected throughout the sample surface, thereby indicating that the incorporation of the ionic liquid can be neglected. Fig. 2e shows a typical crosssectional SEM micrograph of the Al-coated Mg alloy. The uniform and continuous Al layer adhered properly on the substrate and exhibited excellent deposition qualities. As observed in this figure, the thickness of the deposited layer was approximately 20 lm, which is close to the theoretical value according to Faraday’s electrolysis law. This result suggested that the deposition current efficiency was quite
J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
Al
-1.2
Potential (V vs. SCE)
high and the thickness of the deposit could be easily controlled by adjusting the total charge applied. Crystal structure analyses were also carried out using XRD, and the acquired diffraction patterns are shown in Fig. 3. Curve a in the figure was recorded from the bare Mg alloy, again revealing its dual-phase nature as described above. Curves b and c represent the 0.2 V and 0.4 V deposited samples, respectively. The diffraction peaks attributed to pure Al with a face-centered cubic structure (FCC) were clearly detected from both the samples. The signals from the Mg alloy substrates were highly suppressed since their surfaces were covered by the deposited Al layers. However, a close comparison of the diffraction patterns revealed that the substrate intensity of the 0.4 V deposited sample was higher than that of the other sample. This was attributed to the less compact characteristic of the 0.4 V deposited Al layer (with cracks), as previously observed from SEM. Since the continuous Al layer was successfully deposited on the Mg alloy substrate, how the corrosion resistance was improved due to the metallic film was then extensively examined by OCP, EIS, and potentiodynamic polarization measurements. Fig. 4 shows the OCPs of the uncoated and Al-coated Mg alloys in 3.5 wt% NaCl aqueous solution as a function of time. The bare Mg alloy (curve a) yielded a very negative potential of below 1.6 V (vs. SCE) indicating its extremely active nature. As shown in this figure, the OCPs clearly shifted toward the noble direction due to the protective Al layer. The OCPs of the 0.2 V (curve b) and 0.4 V (curve c) Alcoated samples were observed at 1.42 V and 1.52 V,
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b
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c a -1.6
-1.8 0
200
400
600
800
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1200
Time (second) Fig. 4. OCPs of the various samples as a function of time in 3.5 wt% NaCl solution. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively.
respectively. The EIS data of the samples were also collected at the OCPs to further evaluate the corrosion resistance. Fig. 5 demonstrates the Nyquist plot of the various samples. The single semicircle of each spectrum characterized a one time constant equivalent circuit. Nonlinear least square (NLLS) fitting revealed that the polarization resistances (Rp) of the bare Mg alloy, 0.2 V Al-coated,
-6E+3
Al
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c
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Al Al
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Arbitrary intensity
-4E+3 Al
b
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Al
Al
-2E+3 α
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Diffraction angel (2 theta) Fig. 3. X-ray diffraction patterns of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Aldeposited samples, respectively.
1E+3
2E+3
3E+3
4E+3
5E+3
6E+3
2 Z' (Ω -cm )
Fig. 5. Nyquist plot of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively. The measurement was performed in 3.5 wt% NaCl solution.
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J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
as compared to that of the bare Mg alloy. This result was consistent with that obtained from the EIS measurement. In addition, occurrence of the passivation breakdown was probably due to pitting corrosion, which is quite common for aluminum in a solution containing chlorides [25,26].
-0.6
Potential (V vs. SCE)
-0.8
b
c
a
-1.0
4. Conclusions
-1.2
-1.4
-1.6
-1.8 1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
2
Current density (A/cm ) Fig. 6. Potentiodynamic polarization curves of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively. The measurement was performed in 3.5 wt% NaCl solution.
and 0.4 V Al-coated samples were 500, 5000, and 2000 X cm2, respectively. As compared to that of the bare Mg alloy, the Rp value of the 0.2 V Al-coated sample is ten times greater; this clearly indicates that the Al coating led to an improvement in corrosion resistance. The lower protective performance of the 0.4 V deposited layer was attributed to its loose and cracked structure, as mentioned previously. Fig. 6 shows the polarization curves of the three samples. It was noted that the corrosion potentials (Ecorr) were slightly different from the OCPs shown in Fig. 4. Since surface conditions of the electrodes could be modified during the polarization test because the applied potential was swept from 1.8 V, the potential drifts were reasonable. Such phenomenon is common in many electrochemical systems. A highly active dissolution behavior with regard to the bare Mg alloy was observed in curve a. Above the Ecorr, the anodic current increased sharply and soon reached the limiting current density (approximately 1 · 101 A/cm2). However, the electrodeposited Al layers can effectively prevent the Mg alloy substrates from rapid corrosion, as demonstrated in curves b and c in this figure. Both the polarization curves were characterized by a wide passive region extending from 1.4 V to 0.7 V followed by a passivation breakdown. The formation of an alumina protective layer on the surface of the deposited Al contributed to the passivity phenomenon. The passive current densities of the 0.2 V Al-coated and 0.4 V Al-coated samples were approximately 1 · 105 and 5 · 104 A/cm2, respectively, thereby revealing that the dissolution rates were highly suppressed
From the above results, it can be concluded that a dense, continuous, and adhesive Al layer can be electrodeposited on a Mg alloy in AlCl3–EMIC ionic liquid at 25 C. The considerable enhancement in the corrosion resistance of the Mg alloy due to this protective film was clearly confirmed. However, the deposition potential should be carefully considered, since a high deposition rate would bring out loose structure and small cracks within the Al layer and consequently cause a reduction in its protection capability. In addition, both heat treatment and anodization of the Al-coated Mg alloy are attempted to further improve either the corrosion resistance or the surface finishing (including electrolysis coloring). This study is still underway and will be published elsewhere in the near future. References [1] G.L. Makar, J. Kruger, J. Electrochem. Soc. 137 (1990) 414. [2] G. Song, A. Atrens, D. St. John, X. Wu, J. Nairn, Corr. Sci. 39 (1997) 1981. [3] R. Ambat, N.N. Aung, W. Zhou, Corr. Sci. 42 (2000) 1433. [4] M. Forsyth, P.C. Howlett, S.K. Tan, D.R. MacFarlane, N. Birbills, Electrochem. Solid-State Lett. 9 (2006) B52. [5] G. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11. [6] Y. Zhang, C. Yan, F. Wang, H. Lou, C. Cao, Surf. Coat. Technol. 161 (2002) 36. [7] Z. Shi, G. Song, A. Atrens, Corr. Sci. 47 (2005) 2760. [8] H.Y. Hsiao, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [9] H.Y. Hsiao, H.C. Tsung, W.T. Tsai, Surf. Coat. Technol. 199 (2005) 127. [10] H.Y. Hsiao, P. Chung, W.T. Tsai, Corr. Sci. 49 (2007) 781. [11] H. Huo, Y. Li, F. Wang, Corr. Sci. 46 (2004) 1467. [12] R. Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [13] Z. Liu, W. Gao, Surf. Coat. Technol. 200 (2006) 5087. [14] O. Lunder, J.E. Lein, T.K. Aune, K. Nisancioglu, Corrosion 45 (1989) 741. [15] G. Song, A. Atrens, X. Wu, B. Zhang, Corr. Sci. 40 (1998) 1769. [16] G. Song, A. Atrens, M. Dargusch, Corr. Sci. 41 (1999) 249. [17] G. Song, Adv. Eng. Mater. 7 (2005) 563. [18] H.Y. Hsiao, W.T. Tsai, J. Mater. Res. 20 (2005) 2763. [19] T. Jiang, M.J.C. Brym, G. Dube´, A. Lasia, G.M. Brisard, Surf. Coat. Technol. 201 (2006) 1. [20] P.K. Lai, M.S. Kazacos, J. Electroanal. Chem. 248 (1988) 431. [21] R.T. Carlin, R.A. Osteryoung, J. Electrochem. Soc. 136 (1989) 1409. [22] T.J. Melton, J. Joyce, J.T. Maloy, J.A. Boon, J.S. Wikes, J. Electrochem. Soc. 137 (1990) 3865. [23] Q.X. Liu, S. Zein El Abedin, F. Endres, Surf. Coat. Technol. 201 (2006) 1352. [24] R.L. Perry, K.M. Jones, W.D. Scott, Q. Liao, C.L. Hussey, J. Chem. Eng. Data 40 (1995) 615. [25] Z. Szklarska-Smialowska, Corr. Sci. 41 (1999) 1743. [26] S.T. Pride, J.R. Scully, J.L. Hudson, J. Electrochem. Soc. 141 (1994) 3028.
Electrodeposition of aluminum on magnesium alloy in aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid and its corrosion behavior Jeng-Kuei Chang a
a,*
, Su-Yau Chen a, Wen-Ta Tsai a, Ming-Jay Deng b, I-Wen Sun
b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Chemistry, National Cheng Kung University, Tainan, Taiwan Received 7 February 2007; received in revised form 1 March 2007; accepted 5 March 2007 Available online 12 March 2007
Abstract A dense and adhesive Al layer was successfully electrodeposited on a Mg alloy in aluminum chloride–1-ethyl-3-methylimidazolium chloride ionic liquid. The corrosion resistance of the uncoated and Al-coated samples was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization measurements in 3.5 wt% NaCl solution. It was confirmed that the protective Al layer significantly reduces the corrosion rate of the Mg alloy. However, the deposition potential was a crucial factor that governed the structure and therefore the protection capability of the Al layer. 2007 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Corrosion; Electrodeposition; Al-coating; Ionic liquid
1. Introduction The natural abundance of environment-friendly magnesium (Mg) compounds and a combination of advantageous mechanical properties and considerably low density (1.7 g/cm2) have made Mg and its alloys the most promising materials for various applications in recent years. However, these materials exhibit poor corrosion resistance, thereby limiting their usage in hostile environments [1–3]. Conversion coatings are usually done to prevent the Mg substrate from rapid corrosion. However, previous reports [4,5] concluded that these coatings did not provide satisfactory protection. On the other hand, anodization treatment is another popular process that improves the corrosion resistance of Mg alloys [6–10]. Nevertheless, the ceramic oxide coating produced is generally nonconductive and rather rough. The fabrication of a metallic layer with good *
Corresponding author. E-mail address: [email protected] (J.-K. Chang).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.03.009
protective capabilities on the Mg alloy was seldom reported in the literature [11–13]. A conductive coating is of great interest because the low resistivity of the coated Mg alloy can be maintained and its electromagnetic shielding property will not be lost. Electrodeposition of a protective aluminum (Al) layer on the Mg alloy, which has never been explored to the best of our knowledge, is attempted in this study. Moreover, the improvement in corrosion resistance will be investigated for the first time. Al was chosen for the metallic protection layer on the Mg alloy due to four main factors. (1) Being a lightweight metal, Al does not increase the overall density significantly. Moreover, the corrosion resistance of Al and its alloys is satisfactory. (2) An Al-coated Mg alloy can generate a continuous Mg–Al intermetallic compound near the surface after proper heat treatment. It has been confirmed that this compound possesses good corrosion resistance [14–18]. (3) Since the anodization and electrolysis coloration techniques for Al are already well established, they should be directly applicable to a Mg alloy
J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
4Al2 Cl 7 þ 3e ! Al þ 7AlCl4
ð1Þ
Accordingly, this type of ionic liquid is employed in this study. Electroplating of Al on mild steel using the ionic liquid was also previously reported by Liu et al. [23]. 2. Experimental In the experiment, an acidic ionic liquid whose molar ratio of AlCl3–EMIC was 1.5:1 was prepared by the slow addition of a certain weight of AlCl3 (anhydrous powder, 99.99%, Aldrich) into EMIC (99%, Aldrich) at room temperature according to a previous paper [24]. The mixture was continuously stirred by a magnetic bar for one day to ensure uniformity. All chemicals were handled under a purified nitrogen atmosphere in a glove box (Vacuum Atmosphere Co.), in which the moisture and oxygen content were maintained below 1 ppm. A diecast AZ91D Mg alloy with 9.02 wt% Al and 0.49 wt% Zn was used in this study. Before electrodeposition, each sample was ground with SiC papers to a grit of 1000 in the glove box. The typical exposed area was 0.1 cm2 with the other portion sealed with a Teflon film. An Al layer was deposited on the Mg alloy by electrodeposition in the AlCl3–EMIC ionic liquid at 25 C. A three-electrode electrochemical system controlled by an AUTOLAB potentiostat was adopted for this experiment. The Mg alloy was assembled as the working electrode. In addition, Al wires were used for both the counter and reference electrodes. Cathodic deposition was performed under a constant potential mode. Potentials of 0.2 V and 0.4 V (vs. the Al wire) were applied to yield a total passed charge of 50 C/cm2. After deposition, the samples were thoroughly cleaned using distilled water and then dried in air. The morphology and chemical composition of the samples were examined with a scanning electron microscope (SEM, Philip XL-40FEG) and its auxiliary X-ray energy dispersive spectroscope (EDS). X-ray diffraction (XRD) analysis was also performed with a Rigaku diffractometer to explore the crystal structure. Deaerated 3.5 wt% NaCl testing solution was employed for corrosion resistance measurements. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. In
the potentiodynamic polarization test, the potential was scanned from 1.8 V to 0.5 V (vs. SCE) with a sweep rate of 1 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential. The detected frequency was in the range of 65,535–0.1 Hz with a voltage amplitude of 10 mV. 3. Results and discussion The experimental results indicate that the open circuit potential (OCP) of the AZ91D Mg alloy was approximately 0 V (vs. Al) and that it did not vary significantly with time in the AlCl3–EMIC ionic liquid at 25 C. Electrodeposition of Al was then performed by applying cathodic potential on the Mg substrate. Fig. 1 shows the variations in the current density versus time during the deposition processes, which were conducted at 0.2 V (curve a) and 0.4 V (curve b). As shown in the figure, the cathodic current for both the curves decreased rapidly in the beginning due to double layer charging. Afterward, the current density of the 0.2 V electrode remained stable at approximately 20 mA/cm2 throughout the deposition process, while the deposition current density of the 0.4 V electrode increased gradually from 20 to 50 mA/cm2. This current increase could be attributed to the progress of surface roughness during the electrodeposition process. For a total cathodic passed charge of 50 C/cm2, the time required to deposit Al at 0.2 V was 2550 s, revealing an overall deposition rate that is half that at 0.4 V. Both the Al-deposited samples were silver-gray and the surface coverage was quite satisfactory. The surface morphologies of the uncoated and Al-coated Mg alloys were further examined by SEM. Fig. 2a shows a micrograph
0
Current density (mA/cm2 )
with its surface entirely covered by an Al film. (4) Good recycling of materials can be maintained because Al is a primary alloy element for the AZ series Mg alloys. The extraction and refining of Al on inert electrodes in room temperature aluminum chloride–1-ethyl-3-methylimidazolium chloride (AlCl3–EMIC) ionic liquid have been reported [19]. This ionic liquid exhibits adjustable Lewis acidity depending on the molar ratio of AlCl3–EMIC [20–22]. Electrodeposition of Al can only be performed under the Lewis acidic condition when the molar ratio of AlCl3–EMIC is greater than one. The Al2 Cl 7 precursor, which is the dominant species in the ionic liquid, can be electrochemically reduced to the metallic form according to the following reaction [20–22]:
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a
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b
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500
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1500
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Deposition time (second) Fig. 1. Variations of current density versus time during the deposition processes, which are performed at 0.2 V (curve a) and 0.4 V (curve b).
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J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
Fig. 2. SEM top-view micrographs of the (a) die-cast AZ91D Mg alloy substrate; (b) 0.2 V Al-deposited sample; (c) 0.4 V Al-deposited sample; (d) EDS analysis of the deposited laye and (e) SEM cross-section micrograph of the Al-coated Mg alloy.
of the bare diecast AZ91D Mg alloy, demonstrating its dual-phase microstructure. As displayed in this SEM photo, the Mg alloy consisted of two distinguishable portions: a gray area that represents a Mg solid solution phase (a) and a white area that represents an intermetallic phase (b, Mg17Al12). Fig. 2b shows the electrode electrodeposited at 0.2 V. A faceted microstructure with the crystal size in the range of 510 lm was clearly observed by SEM. Moreover, the deposited layer appeared quite dense and uniformly covered the entire Mg alloy substrate, regardless of the areas of the a or b phases. Surface observation result of the 0.4 V deposited electrode is shown in Fig. 2c; this figure shows that the deposition potential certainly caused a significant change in the morphology of the deposit. Regular nodules were observed in the SEM photo. It was noted that several small cracks could be found within the deposit
whose structure did not appear to be as compact as that of the 0.2 V deposited electrode. The chemical compositions of both the deposits were inspected with an EDS and the results revealed an identical spectrum, as shown in Fig. 2d. The analytical data revealed that the deposits were pure Al (no other element could be recognized). Furthermore, no residual salts were detected throughout the sample surface, thereby indicating that the incorporation of the ionic liquid can be neglected. Fig. 2e shows a typical crosssectional SEM micrograph of the Al-coated Mg alloy. The uniform and continuous Al layer adhered properly on the substrate and exhibited excellent deposition qualities. As observed in this figure, the thickness of the deposited layer was approximately 20 lm, which is close to the theoretical value according to Faraday’s electrolysis law. This result suggested that the deposition current efficiency was quite
J.-K. Chang et al. / Electrochemistry Communications 9 (2007) 1602–1606
Al
-1.2
Potential (V vs. SCE)
high and the thickness of the deposit could be easily controlled by adjusting the total charge applied. Crystal structure analyses were also carried out using XRD, and the acquired diffraction patterns are shown in Fig. 3. Curve a in the figure was recorded from the bare Mg alloy, again revealing its dual-phase nature as described above. Curves b and c represent the 0.2 V and 0.4 V deposited samples, respectively. The diffraction peaks attributed to pure Al with a face-centered cubic structure (FCC) were clearly detected from both the samples. The signals from the Mg alloy substrates were highly suppressed since their surfaces were covered by the deposited Al layers. However, a close comparison of the diffraction patterns revealed that the substrate intensity of the 0.4 V deposited sample was higher than that of the other sample. This was attributed to the less compact characteristic of the 0.4 V deposited Al layer (with cracks), as previously observed from SEM. Since the continuous Al layer was successfully deposited on the Mg alloy substrate, how the corrosion resistance was improved due to the metallic film was then extensively examined by OCP, EIS, and potentiodynamic polarization measurements. Fig. 4 shows the OCPs of the uncoated and Al-coated Mg alloys in 3.5 wt% NaCl aqueous solution as a function of time. The bare Mg alloy (curve a) yielded a very negative potential of below 1.6 V (vs. SCE) indicating its extremely active nature. As shown in this figure, the OCPs clearly shifted toward the noble direction due to the protective Al layer. The OCPs of the 0.2 V (curve b) and 0.4 V (curve c) Alcoated samples were observed at 1.42 V and 1.52 V,
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c a -1.6
-1.8 0
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600
800
1000
1200
Time (second) Fig. 4. OCPs of the various samples as a function of time in 3.5 wt% NaCl solution. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively.
respectively. The EIS data of the samples were also collected at the OCPs to further evaluate the corrosion resistance. Fig. 5 demonstrates the Nyquist plot of the various samples. The single semicircle of each spectrum characterized a one time constant equivalent circuit. Nonlinear least square (NLLS) fitting revealed that the polarization resistances (Rp) of the bare Mg alloy, 0.2 V Al-coated,
-6E+3
Al
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c
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Al Al
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a
αα
0E+0 0E+0
20
b
-1E+3
a
30
40
50
60
70
80
Diffraction angel (2 theta) Fig. 3. X-ray diffraction patterns of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Aldeposited samples, respectively.
1E+3
2E+3
3E+3
4E+3
5E+3
6E+3
2 Z' (Ω -cm )
Fig. 5. Nyquist plot of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively. The measurement was performed in 3.5 wt% NaCl solution.
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as compared to that of the bare Mg alloy. This result was consistent with that obtained from the EIS measurement. In addition, occurrence of the passivation breakdown was probably due to pitting corrosion, which is quite common for aluminum in a solution containing chlorides [25,26].
-0.6
Potential (V vs. SCE)
-0.8
b
c
a
-1.0
4. Conclusions
-1.2
-1.4
-1.6
-1.8 1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
2
Current density (A/cm ) Fig. 6. Potentiodynamic polarization curves of the various samples. Curve a, b and c present the bare Mg alloy, 0.2 V Al-deposited samples, and 0.4 V Al-deposited samples, respectively. The measurement was performed in 3.5 wt% NaCl solution.
and 0.4 V Al-coated samples were 500, 5000, and 2000 X cm2, respectively. As compared to that of the bare Mg alloy, the Rp value of the 0.2 V Al-coated sample is ten times greater; this clearly indicates that the Al coating led to an improvement in corrosion resistance. The lower protective performance of the 0.4 V deposited layer was attributed to its loose and cracked structure, as mentioned previously. Fig. 6 shows the polarization curves of the three samples. It was noted that the corrosion potentials (Ecorr) were slightly different from the OCPs shown in Fig. 4. Since surface conditions of the electrodes could be modified during the polarization test because the applied potential was swept from 1.8 V, the potential drifts were reasonable. Such phenomenon is common in many electrochemical systems. A highly active dissolution behavior with regard to the bare Mg alloy was observed in curve a. Above the Ecorr, the anodic current increased sharply and soon reached the limiting current density (approximately 1 · 101 A/cm2). However, the electrodeposited Al layers can effectively prevent the Mg alloy substrates from rapid corrosion, as demonstrated in curves b and c in this figure. Both the polarization curves were characterized by a wide passive region extending from 1.4 V to 0.7 V followed by a passivation breakdown. The formation of an alumina protective layer on the surface of the deposited Al contributed to the passivity phenomenon. The passive current densities of the 0.2 V Al-coated and 0.4 V Al-coated samples were approximately 1 · 105 and 5 · 104 A/cm2, respectively, thereby revealing that the dissolution rates were highly suppressed
From the above results, it can be concluded that a dense, continuous, and adhesive Al layer can be electrodeposited on a Mg alloy in AlCl3–EMIC ionic liquid at 25 C. The considerable enhancement in the corrosion resistance of the Mg alloy due to this protective film was clearly confirmed. However, the deposition potential should be carefully considered, since a high deposition rate would bring out loose structure and small cracks within the Al layer and consequently cause a reduction in its protection capability. In addition, both heat treatment and anodization of the Al-coated Mg alloy are attempted to further improve either the corrosion resistance or the surface finishing (including electrolysis coloring). This study is still underway and will be published elsewhere in the near future. References [1] G.L. Makar, J. Kruger, J. Electrochem. Soc. 137 (1990) 414. [2] G. Song, A. Atrens, D. St. John, X. Wu, J. Nairn, Corr. Sci. 39 (1997) 1981. [3] R. Ambat, N.N. Aung, W. Zhou, Corr. Sci. 42 (2000) 1433. [4] M. Forsyth, P.C. Howlett, S.K. Tan, D.R. MacFarlane, N. Birbills, Electrochem. Solid-State Lett. 9 (2006) B52. [5] G. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11. [6] Y. Zhang, C. Yan, F. Wang, H. Lou, C. Cao, Surf. Coat. Technol. 161 (2002) 36. [7] Z. Shi, G. Song, A. Atrens, Corr. Sci. 47 (2005) 2760. [8] H.Y. Hsiao, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [9] H.Y. Hsiao, H.C. Tsung, W.T. Tsai, Surf. Coat. Technol. 199 (2005) 127. [10] H.Y. Hsiao, P. Chung, W.T. Tsai, Corr. Sci. 49 (2007) 781. [11] H. Huo, Y. Li, F. Wang, Corr. Sci. 46 (2004) 1467. [12] R. Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [13] Z. Liu, W. Gao, Surf. Coat. Technol. 200 (2006) 5087. [14] O. Lunder, J.E. Lein, T.K. Aune, K. Nisancioglu, Corrosion 45 (1989) 741. [15] G. Song, A. Atrens, X. Wu, B. Zhang, Corr. Sci. 40 (1998) 1769. [16] G. Song, A. Atrens, M. Dargusch, Corr. Sci. 41 (1999) 249. [17] G. Song, Adv. Eng. Mater. 7 (2005) 563. [18] H.Y. Hsiao, W.T. Tsai, J. Mater. Res. 20 (2005) 2763. [19] T. Jiang, M.J.C. Brym, G. Dube´, A. Lasia, G.M. Brisard, Surf. Coat. Technol. 201 (2006) 1. [20] P.K. Lai, M.S. Kazacos, J. Electroanal. Chem. 248 (1988) 431. [21] R.T. Carlin, R.A. Osteryoung, J. Electrochem. Soc. 136 (1989) 1409. [22] T.J. Melton, J. Joyce, J.T. Maloy, J.A. Boon, J.S. Wikes, J. Electrochem. Soc. 137 (1990) 3865. [23] Q.X. Liu, S. Zein El Abedin, F. Endres, Surf. Coat. Technol. 201 (2006) 1352. [24] R.L. Perry, K.M. Jones, W.D. Scott, Q. Liao, C.L. Hussey, J. Chem. Eng. Data 40 (1995) 615. [25] Z. Szklarska-Smialowska, Corr. Sci. 41 (1999) 1743. [26] S.T. Pride, J.R. Scully, J.L. Hudson, J. Electrochem. Soc. 141 (1994) 3028.