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Incorporation of multi-walled carbon nanotubes into the oxide layer on a 7075 Al alloy coated by plasma electrolytic oxidation: Coating structure and corrosion properties
Current Applied Physics 11 (2011) S55eS59
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Incorporation of multi-walled carbon nanotubes into the oxide layer on a 7075 Al alloy coated by plasma electrolytic oxidation: Coating structure and corrosion properties Kang Min Lee a, Young Gun Ko b, *, Dong Hyuk Shin a, ** a b
Department of Metallurgy and Materials Engineering, Hanyang University, Ansan 426-791, Republic of Korea School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 22 April 2011 Accepted 22 April 2011 Available online 14 July 2011
The influence of incorporating multi-walled carbon nanotubes (MWCNTs) on the coating structure and the corrosion response of a 7075 Al alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, a series of PEO coatings were prepared on 7075 alloy specimens in a silicate electrolyte with MWCNTs, and the results were compared to those without MWCNTs. Microstructure observations revealed that when MWCNTs were added to the electrolyte, the pore size and fraction of the oxide layer coated in the electrolyte incorporating MWCNTs were respectively smaller. Since the number of MWCNTs having negative potential within the present electrolyte moved into the oxide layer, which acts as an anode, some micro pores were filled effectively with MWCNTs, resulting in a denser coating layer. From the results of the electrochemical tests, the corrosion resistance of the present sample obtained by PEO coating in the electrolyte with MWCNTs was significantly improved. The electrochemical mechanism underlying the high corrosion resistance of the present sample was discussed based on the equivalent circuit model. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolytic oxidation Carbon nanotubes Coating Microstructure Corrosion
1. Introduction With rapid growth in various fields of electronics and in the automobile industry over the past two decades, Al and its alloys have seen wide use due to their excellent properties, such as high specific strength, good formability, and good electro-thermal conductivity [1e3]. In spite of these inherent attractive properties, the mechanical characteristics, including the hardness and surface strength of Al and its alloys, need to be enhanced [4e6]. To this end, approaches including chemical conversion coating, anodizing, and plasma electrolytic oxidation (PEO) have been studied [7e12]. Amongst these coating methods, the PEO technique has recently been receiving considerable interest, since it can provide strong adhesion even between metals and ceramics through the application of extremely high activation energy arising from an electrochemical cell [13]. For instance, sturdy passivation films with a micro length scale can be readily formed on Al alloy * Corresponding author. Tel.: þ82 53 810 2537. ** Corresponding author. Tel.: þ82 31 400 5224. E-mail addresses: [email protected] (Y. G. Ko), [email protected] (D. H. Shin). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.07.009
samples. After undergoing coating, however, they are accompanied by micro pores due to plasma attack. This in turn deteriorates, the corrosion properties of Al alloys. Such oxide layers depend upon coating variables, such as the electrolyte, substrate, and electrochemical parameters [14e16]. It was reported in earlier works that the use of secondary substances in the electrolyte could bring about the formation of an oxide layer that is suitable for preventing corrosion failure from severe atmospheres. Matykina et al. [17] reported the positive influence of zirconia incorporation on the corrosion resistance of PEO-coated pure Al. Jin et al. [18] found that the use of Fe micrograins as a secondary substance in the electrolyte led to enhanced surface hardness and wear resistance of Al alloys, since the oxide layer became denser with a lower population of micro pores. In line with this, multi-walled carbon nanotubes (MWCNTs), which offer excellent mechanical and electrical properties, have been added to the electrolyte prior to PEO coating [19e21]. The effect of the incorporation of MWCNTs in the electrolyte on the coating structure and the corrosion behavior of the oxide layer on a 7075 Al alloy fabricated by the PEO process were investigated in the present study. In addition, the electrochemical mechanism underlying high corrosion resistance is elucidated in relation to the equivalent circuit model.
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K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
Table 1 Chemical compositions of the electrolytes used for the present PEO coatings.
Bath A Bath B
Na2SiO3 (mol/L)
KOH (mol/L)
KF (mol/L)
MWCNT (mL/L)
0.16 0.16
0.27 0.27
0.09 0.09
e 150
Fig. 2. Coating time vs. voltage curves of the PEO coating of 7075 Al alloy samples using two electrolytes.
Fig. 1. Initial MTCNTs used in this study.
2. Experimental procedure The chemical composition of the 7075 Al alloy supplied from Alcoa Inc was Al-5.8Zn-2.6Mg-1.6Cu (in wt.%). Prior to PEO coating, the initial samples were cut into a rod shape with diameter and thickness of 20 and 10 mm, respectively. They were polished with #1000 sand paper, rinsed with distilled water, and ultrasonically
cleaned in ethanol. The PEO machine has an electrolyte cell consisting of a stainless steel cathode and a 7075 Al alloy anode. The distance between the cathode and anode was 50 mm in the electrolyte. The chemical compositions of the two electrolytes are listed in Table 1. MWCNTs, which consist of multiple rolled layers of graphite, with an average diameter of w30 nm were used (Fig. 1). PEO coatings were carried out using two electrolytes at a current density of 150 mA/cm2 for 600 s. The temperature of the electrolyte was stabilized at 293 K during PEO coating. The surface morphologies of each sample were observed using a field-emission scanning electron microscope (FE-SEM) with an acceleration voltage of 15 kV. Both potentio dynamic polarization and electrochemical
Fig. 3. SEM images showing surface structures of the oxide layers in the PEO-coated 7075 Al alloy samples under (a), (b) Bath A, and (c), (d) Bath B conditions.
K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
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impedance tests were performed to evaluate the corrosion response of the oxide layers using a Reference 600 potentiostat. 3. Results and discussion Fig. 2 presents the voltage vs. time curves of the present samples during PEO coating using two electrolytes with and without MWCNTs. At the initial stage of PEO coating, the responding voltage steeply increased due to the formation of an oxide layer that acts as a ceramic insulator. It was also apparent that the voltage values of Bath B were lower than those of Bath A. The breakdown voltages where an electron avalanche by high voltage at the interface between the electrolyte and metal substrate occurred, causing micro discharges and intense sparking at the anode surface, were observed to be w275 and w225 V for the Bath A and B conditions, respectively. The MWCNTs used in Bath B were reported to possess electro-chemically high capacity in nature owing to the symmetry and unique electronic structure of graphene [22]. Thus, they could be exploited for storing a dielectric micro-charge during PEO coating, resulting in the low breakdown voltage of Bath B. When the present coating was completed at 600 s, on the other hand, the final voltages of both samples were roughly equal, approaching w375 V. This indicated that the increasing rate of Bath B was faster than that of Bath A with an increased coating time. This is attributed to an appreciable change in the coating structure associated with the incorporation of MWCNTs into the oxide layer. In order to investigate the effects of the incorporation of MWCNTs on the formation of the oxide layer, the structures and morphologies of the oxide layers were characterized, as shown in Fig. 3. Fig. 3(a) and (b) display the typical structure of the oxide layer generated by PEO coating accompanied by visible plasma sparks, which led to well-developed micro pores on the surface. Similar to volcanic activities, molten oxide nodules close to micro pores were detected on the oxide layer due to rapid cooling as soon as the newly formed oxide contacted the relatively cool electrolyte. In the case of the oxide layer produced by PEO coating in the electrolyte with MWCNTs shown in Fig. 3(b), the number and the area fraction of the micro pores were observed to decrease. Interestingly, a number of MWCNTs that retained their initial shapes, which were interconnected with inner shells, were successfully incorporated inside the micro pores of the sample. Based on SEM observations with high magnification (Fig. 3(d)), we concluded that the 7.03 at% MWCNTs in the oxide layer played an important role in blocking the micro pores, giving rise to a denser oxide layer in Bath B than in Bath A. This allowed the oxide layer to work as an effective barrier to suppress corrosion. As a structurally stable substance, MWCNTs would not be dissolved and then ionized under the present conditions using a silicate electrolyte. This was supported by the absence of any significant change in the dimensions of the MWCNTs before and after PEO coating. An electrochemical reaction between the electrolyte and the MWCNTs was not responsible for their incorporation into the oxide layer during PEO coating. Instead, it is suggested that the mechanism by which MWCNTs are incorporated during the PEO process is electrophoresis, that is, the spontaneous motion of dispersed particles when an electric field is imparted to an aqueous electrolyte [23]. If the MWCNTs are incorporated in the region of the metal substrate that functions as an anode, the surface charge of the MWCNTs dispersed in the electrolyte should be negative. Fig. 4(a) presents the variation of the zeta potentials of the MWCNTs in distilled water as a function of pH and the present electrolyte. As the pH values of the solution increased, the zeta potential of the MWCNTs gradually decreased, falling to 60 mV in the present electrolyte. Hence, the MWCNTs having a negative charge readily moved toward the anode substrate. Moreover,
Fig. 4. (a) The zeta potential of MWCNTs with different pH values and the present electrolyte and (b) schematic illustration showing the addition of MWCNTs to the oxide layer during PEO coating.
considering the high current density of 150 mA/cm2, we believe that this electrophoretic action was facilitated by the plasma state, and consequently electrochemically negative-charged MWCNTs were uniformly incorporated during the processing time of 600 s, as illustrated in Fig. 4(b). The corrosion properties of the bare and PEO-coated 7075 Al alloy samples were evaluated by a potentio dynamic polarization test in a 3.5 wt% NaCl solution. Polarization curves of the samples are shown in Fig. 5. The corrosion potential (Ecorr) and corrosion current density (icorr) were determined by using the Tafel extrapolation of potentio dynamic polarization curves, and the polarization resistance values (Rp) were calculated based on the SterneGeary equation given below, where ba and bc are the anodic and cathodic Tafel slopes [24].
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K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59 Table 3 Fitting results of the PEO-coated 7075 Al alloy samples by electrochemical plots based on the equivalent circuit model. The impedances of the two constant phase elements are defined by a dimensionless n parameter. Subscripts e, o, and i denote the electrolyte and the outer and inner layers, respectively.
Bath A Bath B
Re (U cm2)
Ro (U cm2)
CPEo-n
Ri (U cm2)
CPEi-n
30.3 30.2
3.95 105 6.38 106
0.869 0.887
1.35 107 2.17 108
0.751 0.512
resistance of Bath B provided better corrosion resistance against the corrosive solution than that of Bath A. Using the relationship proposed by Liu et al. [25], the pore level of the oxide layer was quantitatively analyzed using the electrochemical parameters obtained from the polarization curves viz.,
P ¼
Fig. 5. Potentio dynamic polarization curves of the bare sample as a control experiment and the PEO-coated 7075 Al alloy samples under Bath A and B conditions. Table 2 Results of potentio dynamic polarization tests of the PEO-coated 7075 Al alloy samples in 3.5 wt.% NaCl solution.
bare 7075 Al alloy Bath A Bath B
Rp ¼
Ecorr (V)
icorr (A/cm2)
bc (V)
ba (V)
Rp (U cm2)
0.604 0.477 0.411
2.31 107 4.60 109 8.33 1010
0.242 0.154 0.131
0.194 0.112 0.116
2.03 105 6.12 106 3.21 107
ba bc 2:303icorr ðba þ bc Þ
(1)
These polarization analysis results are tabulated in Table 2. The PEO-coated sample in Bath B had a corrosion potential of 0.411 V (vs. Ag/AgCl), which was higher than that in Bath A. In contrast, the corrosion current density of the PEO-coated sample in Bath A showed 10-fold higher corrosion potential than that in Bath B. Considering that the low corrosion current density and/or the high corrosion potential led to a good corrosion resistance [12]. Through the addition of MWCNTs into the oxide layer, the polarization
Rpb Rp
10
DE corr
bab
(2)
where P is a dimensionless value indicating the porosity of the oxide layer, Rpb and bab are the polarization resistance and the anodic Tafel slope of the bare sample, Rp is the polarization resistance of the PEO-coated sample, and DEcorr is the difference in the corrosion potential between the uncoated and coated samples. The oxide layer porosity (P) of the Bath A and B samples was measured to be 2.45 103 and 1.36 104, respectively. This suggests that the incorporation of MWCNTs into the oxide layer was beneficial for enhancing the corrosion performance due to the relative change in the amount of micro pores. The penetration of chlorine ions through the micro pores in the oxide layer is expected to be difficult in the case of the oxide layer containing MWCNTs, since the number and fraction of micro pores in the oxide layer decreased, which was in accordance with the above observation. The corrosion characteristics of the PEO-coated samples with and without MWCNTs were investigated in a quantitative manner by electrochemical impedance spectroscopy in the same solution as used above [26]. From Fig. 6, the sample coated in Bath B exhibited better electrochemical performance than that in Bath A. The equivalent circuit (EC) model was employed to simulate the experimental data [27,28]. The electrolyte resistance (Re) of a 3.5 wt % NaCl solution was connected in series to that of the oxide layer, and the resistances of two oxide layers (Ro and Ri) were parallel to their constant phase element (CPE) related to the structural in homogeneity of the oxide layer. With the Nyquist plots based on the EC model, best fitting was conducted between the experimental data and iterated results. The modeled values of electrochemical parameters are listed in Table 3. It is worth mentioning that the polarization resistances of the inner and outer oxide layers of the sample in Bath B were substantially higher than those in Bath A. As a result, the corrosion protection property of the sample in Bath B was superior to that in Bath A, which is attributed to the incorporation of a sufficient amount of MWCNTs to suppress the pore level of the oxide layer.
4. Summary
Fig. 6. Electrochemical impedance data of the PEO-coated 7075 Al alloy samples and the fitting results of their electrochemical plot based on the equivalent circuit model.
The influence of the addition of MWCNTs to the electrolyte on the coating structure and corrosion properties of the oxide layer on a 7075 Al alloy coated by the PEO process was studied. MWCNTs having a negative surface charge in the present electrolyte were successfully incorporated through electrophoresis. The MWCNTs found near micro pores played an important role in reducing the size and fraction of the micro pores. The corrosion resistance, as determined by the corrosion current density, corrosion potential,
K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
and impedance values, of the oxide layer containing MWCNTs was thereby improved. Acknowledgment This work was supported by the National Research Foundation of Korea (No 2010-0000812). References [1] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Corros. Sci. 49 (2007) 672. [2] X. Nie, A. Leyland, H.W. Song, A.L. Yerokhin, S.J. Dowey, A. Matthews, Surf. Coat. Technol. 116e119 (1999) 1055. [3] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085. [4] G.H. Lv, H. Chen, W.C. Gu, W.R. Feng, L. Li, E.W. Niu, X.H. Zhang, S.Z. Yang, Curr. Appl. Phys. 9 (2009) 324. [5] R.C. Barik, J.A. Wharton, R.J.K. Wood, K.R. Stokes, R.L. Jones, Surf. Coat. Technol. 199 (2005) 158. [6] G. Sabatini, L. Ceschini, C. Martini, J.A. Williams, I.M. Hutchings, Mater. Design 31 (2010) 816. [7] H.G. Moon, H.W. Jang, J.-S. Kim, H.-H. Park, S.-J. Yoon, Electron. Mater. Lett. 6 (2010) 135. [8] Y.M. Kim, D.Y. Hwang, C.W. Lee, B. Yoo, D.H. Shin, Kor. J. Met. Mater. 48 (2010) 49.
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[9] L. Malkinski, J.H. Lim, W.S. Chae, H.O. Lee, E.-M. Kim, J.-S. Jung, Electron. Mater. Lett. 5 (2009) 87. [10] J. Li, H. Cai, X. Xue, B. Jiang, Mater. Lett. 64 (2010) 2102. [11] H.G. Moon, H.W. Jang, J.-S. Kim, H.-H. Park, S.-J. Yoon, Electron. Mater. Lett. 6 (2010) 31. [12] Y.G. Ko, S. Namgung, D.H. Shin, Surf. Coat. Technol. 205 (2010) 2525e2531. [13] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [14] R.O. Hussein, D.O. Northwood, X. Nie, J. Vac, Sci. Technol. A 28 (2010) 766. [15] L. Wen, Y. Wang, Y. Zhou, J.H. Ouyang, L. Guo, D. Jia, Corros. Sci. 52 (2010) 2687. [16] N. Godja, N. Kiss, Ch. Löcker, A. Schindel, A. Gavrilovic, J. Wosik, R. Mann, J. Wendrinsky, A. Merstallinger, G.E. Nauer, Tribol. Int. 43 (2010) 1253. [17] E. Matykina, R. Arrabal, F. Monfort, P. Skeldon, G.E. Thompson, Appl. Surf. Sci. 255 (2008) 2830. [18] F. Jin, P.K. Chu, H. Tong, J. Zhao, Appl. Surf. Sci. 253 (2006) 863. [19] S. Iijima, Nature 354 (1991) 56. [20] J. Hong, J. Lee, C.K. Hong, S.E. Shim, Curr. Appl. Phys. 10 (2010) 359. [21] M.J. Kim, J.B. Yoo, Electron. Mater. Lett. 4 (2008) 57. [22] C. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997) 1480. [23] A.S. Dukhin, P.J. Goetz, Ultrasound for Characterizing Colloids. Elsevier, 2002. [24] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. [25] C. Liu, Q. Bi, A. Leyland, A. Matthews, Corros. Sci. 45 (2003) 1243. [26] H.M. Cho, Y.J. Park, J.W. Yeon, H.C. Shin, Electron. Mater. Lett. 5 (2009) 169. [27] Y.G. Ko, K.M. Lee, K.R. Shin, D.H. Shin, Kor. J. Met. Mater. 48 (2010) 724. [28] L. Wen, Y.M. Wang, Y. Liu, L.X. Guo, J.H. Ouyang, D.C. Jia, Corros. Sci. 53 (2011) 618.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Incorporation of multi-walled carbon nanotubes into the oxide layer on a 7075 Al alloy coated by plasma electrolytic oxidation: Coating structure and corrosion properties Kang Min Lee a, Young Gun Ko b, *, Dong Hyuk Shin a, ** a b
Department of Metallurgy and Materials Engineering, Hanyang University, Ansan 426-791, Republic of Korea School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 22 April 2011 Accepted 22 April 2011 Available online 14 July 2011
The influence of incorporating multi-walled carbon nanotubes (MWCNTs) on the coating structure and the corrosion response of a 7075 Al alloy coated by plasma electrolytic oxidation (PEO) was investigated. For this purpose, a series of PEO coatings were prepared on 7075 alloy specimens in a silicate electrolyte with MWCNTs, and the results were compared to those without MWCNTs. Microstructure observations revealed that when MWCNTs were added to the electrolyte, the pore size and fraction of the oxide layer coated in the electrolyte incorporating MWCNTs were respectively smaller. Since the number of MWCNTs having negative potential within the present electrolyte moved into the oxide layer, which acts as an anode, some micro pores were filled effectively with MWCNTs, resulting in a denser coating layer. From the results of the electrochemical tests, the corrosion resistance of the present sample obtained by PEO coating in the electrolyte with MWCNTs was significantly improved. The electrochemical mechanism underlying the high corrosion resistance of the present sample was discussed based on the equivalent circuit model. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolytic oxidation Carbon nanotubes Coating Microstructure Corrosion
1. Introduction With rapid growth in various fields of electronics and in the automobile industry over the past two decades, Al and its alloys have seen wide use due to their excellent properties, such as high specific strength, good formability, and good electro-thermal conductivity [1e3]. In spite of these inherent attractive properties, the mechanical characteristics, including the hardness and surface strength of Al and its alloys, need to be enhanced [4e6]. To this end, approaches including chemical conversion coating, anodizing, and plasma electrolytic oxidation (PEO) have been studied [7e12]. Amongst these coating methods, the PEO technique has recently been receiving considerable interest, since it can provide strong adhesion even between metals and ceramics through the application of extremely high activation energy arising from an electrochemical cell [13]. For instance, sturdy passivation films with a micro length scale can be readily formed on Al alloy * Corresponding author. Tel.: þ82 53 810 2537. ** Corresponding author. Tel.: þ82 31 400 5224. E-mail addresses: [email protected] (Y. G. Ko), [email protected] (D. H. Shin). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.07.009
samples. After undergoing coating, however, they are accompanied by micro pores due to plasma attack. This in turn deteriorates, the corrosion properties of Al alloys. Such oxide layers depend upon coating variables, such as the electrolyte, substrate, and electrochemical parameters [14e16]. It was reported in earlier works that the use of secondary substances in the electrolyte could bring about the formation of an oxide layer that is suitable for preventing corrosion failure from severe atmospheres. Matykina et al. [17] reported the positive influence of zirconia incorporation on the corrosion resistance of PEO-coated pure Al. Jin et al. [18] found that the use of Fe micrograins as a secondary substance in the electrolyte led to enhanced surface hardness and wear resistance of Al alloys, since the oxide layer became denser with a lower population of micro pores. In line with this, multi-walled carbon nanotubes (MWCNTs), which offer excellent mechanical and electrical properties, have been added to the electrolyte prior to PEO coating [19e21]. The effect of the incorporation of MWCNTs in the electrolyte on the coating structure and the corrosion behavior of the oxide layer on a 7075 Al alloy fabricated by the PEO process were investigated in the present study. In addition, the electrochemical mechanism underlying high corrosion resistance is elucidated in relation to the equivalent circuit model.
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K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
Table 1 Chemical compositions of the electrolytes used for the present PEO coatings.
Bath A Bath B
Na2SiO3 (mol/L)
KOH (mol/L)
KF (mol/L)
MWCNT (mL/L)
0.16 0.16
0.27 0.27
0.09 0.09
e 150
Fig. 2. Coating time vs. voltage curves of the PEO coating of 7075 Al alloy samples using two electrolytes.
Fig. 1. Initial MTCNTs used in this study.
2. Experimental procedure The chemical composition of the 7075 Al alloy supplied from Alcoa Inc was Al-5.8Zn-2.6Mg-1.6Cu (in wt.%). Prior to PEO coating, the initial samples were cut into a rod shape with diameter and thickness of 20 and 10 mm, respectively. They were polished with #1000 sand paper, rinsed with distilled water, and ultrasonically
cleaned in ethanol. The PEO machine has an electrolyte cell consisting of a stainless steel cathode and a 7075 Al alloy anode. The distance between the cathode and anode was 50 mm in the electrolyte. The chemical compositions of the two electrolytes are listed in Table 1. MWCNTs, which consist of multiple rolled layers of graphite, with an average diameter of w30 nm were used (Fig. 1). PEO coatings were carried out using two electrolytes at a current density of 150 mA/cm2 for 600 s. The temperature of the electrolyte was stabilized at 293 K during PEO coating. The surface morphologies of each sample were observed using a field-emission scanning electron microscope (FE-SEM) with an acceleration voltage of 15 kV. Both potentio dynamic polarization and electrochemical
Fig. 3. SEM images showing surface structures of the oxide layers in the PEO-coated 7075 Al alloy samples under (a), (b) Bath A, and (c), (d) Bath B conditions.
K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
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impedance tests were performed to evaluate the corrosion response of the oxide layers using a Reference 600 potentiostat. 3. Results and discussion Fig. 2 presents the voltage vs. time curves of the present samples during PEO coating using two electrolytes with and without MWCNTs. At the initial stage of PEO coating, the responding voltage steeply increased due to the formation of an oxide layer that acts as a ceramic insulator. It was also apparent that the voltage values of Bath B were lower than those of Bath A. The breakdown voltages where an electron avalanche by high voltage at the interface between the electrolyte and metal substrate occurred, causing micro discharges and intense sparking at the anode surface, were observed to be w275 and w225 V for the Bath A and B conditions, respectively. The MWCNTs used in Bath B were reported to possess electro-chemically high capacity in nature owing to the symmetry and unique electronic structure of graphene [22]. Thus, they could be exploited for storing a dielectric micro-charge during PEO coating, resulting in the low breakdown voltage of Bath B. When the present coating was completed at 600 s, on the other hand, the final voltages of both samples were roughly equal, approaching w375 V. This indicated that the increasing rate of Bath B was faster than that of Bath A with an increased coating time. This is attributed to an appreciable change in the coating structure associated with the incorporation of MWCNTs into the oxide layer. In order to investigate the effects of the incorporation of MWCNTs on the formation of the oxide layer, the structures and morphologies of the oxide layers were characterized, as shown in Fig. 3. Fig. 3(a) and (b) display the typical structure of the oxide layer generated by PEO coating accompanied by visible plasma sparks, which led to well-developed micro pores on the surface. Similar to volcanic activities, molten oxide nodules close to micro pores were detected on the oxide layer due to rapid cooling as soon as the newly formed oxide contacted the relatively cool electrolyte. In the case of the oxide layer produced by PEO coating in the electrolyte with MWCNTs shown in Fig. 3(b), the number and the area fraction of the micro pores were observed to decrease. Interestingly, a number of MWCNTs that retained their initial shapes, which were interconnected with inner shells, were successfully incorporated inside the micro pores of the sample. Based on SEM observations with high magnification (Fig. 3(d)), we concluded that the 7.03 at% MWCNTs in the oxide layer played an important role in blocking the micro pores, giving rise to a denser oxide layer in Bath B than in Bath A. This allowed the oxide layer to work as an effective barrier to suppress corrosion. As a structurally stable substance, MWCNTs would not be dissolved and then ionized under the present conditions using a silicate electrolyte. This was supported by the absence of any significant change in the dimensions of the MWCNTs before and after PEO coating. An electrochemical reaction between the electrolyte and the MWCNTs was not responsible for their incorporation into the oxide layer during PEO coating. Instead, it is suggested that the mechanism by which MWCNTs are incorporated during the PEO process is electrophoresis, that is, the spontaneous motion of dispersed particles when an electric field is imparted to an aqueous electrolyte [23]. If the MWCNTs are incorporated in the region of the metal substrate that functions as an anode, the surface charge of the MWCNTs dispersed in the electrolyte should be negative. Fig. 4(a) presents the variation of the zeta potentials of the MWCNTs in distilled water as a function of pH and the present electrolyte. As the pH values of the solution increased, the zeta potential of the MWCNTs gradually decreased, falling to 60 mV in the present electrolyte. Hence, the MWCNTs having a negative charge readily moved toward the anode substrate. Moreover,
Fig. 4. (a) The zeta potential of MWCNTs with different pH values and the present electrolyte and (b) schematic illustration showing the addition of MWCNTs to the oxide layer during PEO coating.
considering the high current density of 150 mA/cm2, we believe that this electrophoretic action was facilitated by the plasma state, and consequently electrochemically negative-charged MWCNTs were uniformly incorporated during the processing time of 600 s, as illustrated in Fig. 4(b). The corrosion properties of the bare and PEO-coated 7075 Al alloy samples were evaluated by a potentio dynamic polarization test in a 3.5 wt% NaCl solution. Polarization curves of the samples are shown in Fig. 5. The corrosion potential (Ecorr) and corrosion current density (icorr) were determined by using the Tafel extrapolation of potentio dynamic polarization curves, and the polarization resistance values (Rp) were calculated based on the SterneGeary equation given below, where ba and bc are the anodic and cathodic Tafel slopes [24].
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K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59 Table 3 Fitting results of the PEO-coated 7075 Al alloy samples by electrochemical plots based on the equivalent circuit model. The impedances of the two constant phase elements are defined by a dimensionless n parameter. Subscripts e, o, and i denote the electrolyte and the outer and inner layers, respectively.
Bath A Bath B
Re (U cm2)
Ro (U cm2)
CPEo-n
Ri (U cm2)
CPEi-n
30.3 30.2
3.95 105 6.38 106
0.869 0.887
1.35 107 2.17 108
0.751 0.512
resistance of Bath B provided better corrosion resistance against the corrosive solution than that of Bath A. Using the relationship proposed by Liu et al. [25], the pore level of the oxide layer was quantitatively analyzed using the electrochemical parameters obtained from the polarization curves viz.,
P ¼
Fig. 5. Potentio dynamic polarization curves of the bare sample as a control experiment and the PEO-coated 7075 Al alloy samples under Bath A and B conditions. Table 2 Results of potentio dynamic polarization tests of the PEO-coated 7075 Al alloy samples in 3.5 wt.% NaCl solution.
bare 7075 Al alloy Bath A Bath B
Rp ¼
Ecorr (V)
icorr (A/cm2)
bc (V)
ba (V)
Rp (U cm2)
0.604 0.477 0.411
2.31 107 4.60 109 8.33 1010
0.242 0.154 0.131
0.194 0.112 0.116
2.03 105 6.12 106 3.21 107
ba bc 2:303icorr ðba þ bc Þ
(1)
These polarization analysis results are tabulated in Table 2. The PEO-coated sample in Bath B had a corrosion potential of 0.411 V (vs. Ag/AgCl), which was higher than that in Bath A. In contrast, the corrosion current density of the PEO-coated sample in Bath A showed 10-fold higher corrosion potential than that in Bath B. Considering that the low corrosion current density and/or the high corrosion potential led to a good corrosion resistance [12]. Through the addition of MWCNTs into the oxide layer, the polarization
Rpb Rp
10
DE corr
bab
(2)
where P is a dimensionless value indicating the porosity of the oxide layer, Rpb and bab are the polarization resistance and the anodic Tafel slope of the bare sample, Rp is the polarization resistance of the PEO-coated sample, and DEcorr is the difference in the corrosion potential between the uncoated and coated samples. The oxide layer porosity (P) of the Bath A and B samples was measured to be 2.45 103 and 1.36 104, respectively. This suggests that the incorporation of MWCNTs into the oxide layer was beneficial for enhancing the corrosion performance due to the relative change in the amount of micro pores. The penetration of chlorine ions through the micro pores in the oxide layer is expected to be difficult in the case of the oxide layer containing MWCNTs, since the number and fraction of micro pores in the oxide layer decreased, which was in accordance with the above observation. The corrosion characteristics of the PEO-coated samples with and without MWCNTs were investigated in a quantitative manner by electrochemical impedance spectroscopy in the same solution as used above [26]. From Fig. 6, the sample coated in Bath B exhibited better electrochemical performance than that in Bath A. The equivalent circuit (EC) model was employed to simulate the experimental data [27,28]. The electrolyte resistance (Re) of a 3.5 wt % NaCl solution was connected in series to that of the oxide layer, and the resistances of two oxide layers (Ro and Ri) were parallel to their constant phase element (CPE) related to the structural in homogeneity of the oxide layer. With the Nyquist plots based on the EC model, best fitting was conducted between the experimental data and iterated results. The modeled values of electrochemical parameters are listed in Table 3. It is worth mentioning that the polarization resistances of the inner and outer oxide layers of the sample in Bath B were substantially higher than those in Bath A. As a result, the corrosion protection property of the sample in Bath B was superior to that in Bath A, which is attributed to the incorporation of a sufficient amount of MWCNTs to suppress the pore level of the oxide layer.
4. Summary
Fig. 6. Electrochemical impedance data of the PEO-coated 7075 Al alloy samples and the fitting results of their electrochemical plot based on the equivalent circuit model.
The influence of the addition of MWCNTs to the electrolyte on the coating structure and corrosion properties of the oxide layer on a 7075 Al alloy coated by the PEO process was studied. MWCNTs having a negative surface charge in the present electrolyte were successfully incorporated through electrophoresis. The MWCNTs found near micro pores played an important role in reducing the size and fraction of the micro pores. The corrosion resistance, as determined by the corrosion current density, corrosion potential,
K.M. Lee et al. / Current Applied Physics 11 (2011) S55eS59
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