J. Mater. Sci. Technol., 2010, 26(11), 1016-1020. Effects of Current Density on the Microstructure and the Corrosion Resistance of Alumina Coatings Embedded with SiC Nano-particles Produced by Micro-arc Oxidation Yue Yang 1) and Yaohui Liu 2) 1) Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China 2) Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130025, China [Manuscript received July 23, 2010, in revised form September 13, 2010] To improve the surface corrosion resistance of the alumina films fabricated by micro-arc oxidation (MAO), Al 2 O 3 coatings at different current densities (5, 7 and 10 A/dm 2 ) were produced on aluminum alloys by adding SiC nano-particles into electrolyte during MAO process. The morphology and phase composition of the coatings were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Furthermore, the corrosion performance of the coatings was evaluated via a three-electrode system in a 3.5 wt pct NaCl solution. From the obtained morphology of alumina coatings, it was believed that the Al 2 O 3 coatings embedded with SiC nano-particles were formed. The electrochemical impedance spectroscopy (EIS) plots and potentiodynamic polarization plots of the Al 2 O 3 coatings with and without SiC nano-particles at different current densities reveal that the Al 2 O 3 coatings with SiC nano-particles formed at 10 A/dm 2 showed the better corrosion resistance than the other coatings produced at 5 and 7 A/dm 2. KEY WORDS: Micro-arc oxidation; SiC nano-particles; Corrosion resistance 1. Introduction Recently, micro-arc oxidation (MAO), derived from conventional anodic oxidation, has attracted much attention because MAO is a room-temperature electrochemical process suitable for the formation of uniform coatings on the substrate [1,2]. According to the previous reports [3,4], the corrosion resistance and wear resistance of the metals and their alloys after MAO process can be significantly improved. In the MAO process including surface micro-arc discharge, diffusion and electrophoresis, various kinds of conventional ceramic coatings containing adventitious elements have been synthesized on metal substrates such as Al, Ti, Mg and their alloys [5,6]. However, as- Corresponding author. Tel.: +86 431 85716421; Fax: +86 431 85716426; E-mail: yangyue@mail.ccut.edu.cn (Y. Yang). deposited alumina coatings have porous microstructure, which has strong effects on the corrosion resistance of MAO Al 2 O 3 coatings [7,8]. In this investigation, an attempt has been made to improve the corrosion resistance of porous ceramic coating by adding nano-particles into the electrolyte in MAO process. It has been indicated that some nano-powders such as SiO 2 can be embedded in the porous ceramic coatings on Ti in the form of nano-particles [9,10]. In the present work, in order to enhance the corrosion resistance of MAO Al 2 O 3 coatings, SiC nano-particles were embedded in Al 2 O 3 coatings with different current densities. The surface morphology and corrosion resistance of the Al 2 O 3 coatings with SiC nano-particles were characterized, and the enhancement mechanism brought about by SiC nano-particles embedding was also discussed.
Y. Yang et al.: J. Mater. Sci. Technol., 2010, 26(11), 1016 1020 1017 Table 1 Composition of aluminium alloy 6060 (wt pct) Cu Si Fe Mn Mg Zn Cr Ti Al 0.10 0.3 0.6 0.1 0.3 0.10 0.35 0.6 0.15 0.05 0.10 Bal. 2. Experimental Aluminum alloy 6060 pieces with dimension of 10 mm 20 mm 3 mm were used as substrates for alumina coating deposition (chemical composition was shown in Table 1). The size of SiC nano-particles was about 400 nm. Prior to the MAO treatment, the specimens were polished with SiC abrasive paper (up to 1000 grit) and degreased in acetone and distilled water. MAO units used in this work mainly consist of electrolyte bath with stirring and cooling systems and a pulsed DC current mode power supply with approximate 1000 V of the maximum voltage amplitude. Samples and a stainless steel bar were used as the anode and cathode, respectively. The MAO process was performed in alkaline electrolyte containing Na 2 SiO 3 15 g/l, NaOH 3 g/l and SiC 2 g/l. The temperature of the electrolyte was controlled at 25 30 C and a constant oxidation time 45 min was maintained. The frequency was 500 Hz and the current density was fixed at 5, 7 and 10 A/dm 2 by controlling the voltage amplitude in the deposition process, respectively. The phase composition of the coatings was identified by X-ray diffraction (XRD, Model D/Max 2500PC Rigaku, Japan) operated with CuKα. The X-ray generator settings were 50 kv and 50 ma, and the scans were acquired from 20 to 80 deg. (in 2θ). The polished cross-section of the coatings were studied by scanning electron microscopy (SEM, JSM-5500) and the surface morphologies of the samples were observed by field emission scanning electron microscope (FE-SEM, JSM6700F JEOL, Japan). The surface roughness of the coatings was measured with a Confocal laser scanning microscope (Olympus OLS3000, Japan). The corrosion behaviors of the coatings without and with SiC nano-particles prepared at different current densities were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test through Autolab PGSTAT 302 electrochemical analyzer in 3.5 wt pct NaCl solution at room temperature. The electrochemical measurement was conducted by using a conventional three electrodes electrochemical cell with a saturated calomel electrode (SCE) as the reference, a Pt foil as the counter electrode, and the samples with the area of 1 cm 2 as the working electrode. For EIS study, AC impedance measurements were made with the amplitude of 10 mv about the open-circuit potential vs frequency from 10 mhz to 1 MHz. The polarization scan rate was controlled at 0.2 mv/s. The electrochemical parameters of corrosion potential, corrosion current density and polarization resistance were analyzed. Fig. 1 XRD results of alumina coatings with SiC nanoparticles formed at different current densities: (a) 5 A/dm 2, (b) 7 A/dm 2, (c) 10 A/dm 2 3. Results and Discussion 3.1 Effect of current density on the composition and morphology of the composite MAO coating Figure 1 shows the phase compositions of the alumina coatings embedded with SiC nano-particles at different current densities. It can be found that the Al 2 O 3 -SiC composite coatings are mainly composed of α-al 2 O 3 and γ-al 2 O 3. However, compared with the coating formed at current density 10 A/dm 2, SiC peaks at 2θ=26.667 and 60.102 deg. cannot be found in the XRD pattern when the applied current density were 5 and 7 A/dm 2. XRD patterns were in good accordance with FE-SEM observation results (as shown in Fig. 2), which also confirmed the precipitation of SiC nano-particles. As well known, continuous and long time discharge at 10 A/dm 2 results in high temperature in the discharge channels, which is propitious to the deposition of SiC nano-particles. So peaks at 26.667 deg. can be detected in the coating produced at 10 A/dm 2. Figure 2 shows the surface morphology of the Al 2 O 3 coatings with SiC nano-particles formed at different current densities. From the obtained morphology of the composite alumina coatings, it was believed that the Al 2 O 3 coatings embedded with SiC nano-particles were formed. When the current density reached 10 A/dm 2, more dispersed SiC nano-particles were found on the surface, just as shown in Fig. 2(c). To investigate the oxide coating interface, the crosssection morphologies of the fabricated oxide coatings with and without SiC nano-particles on the aluminum alloy were characterized. From Fig. 3, it can be found that the coatings bound firmly with the aluminum
1018 Y. Yang et al.: J. Mater. Sci. Technol., 2010, 26(11), 1016 1020 Fig. 2 FESEM of MAO coatings with SiC nano-particles formed at different current density: (a) 5 A/dm 2, (b) 7 A/dm 2, (c) 10 A/dm 2 Fig. 3 Cross-section images of MAO coatings formed at different current density (A matrix, B dense layer, C porous layer): (a) 5 A/dm 2, (b) 7 A/dm 2, (c) 10 A/dm 2, (d) 10 A/dm 2 (without SiC nano-particles incorpration) alloy substrate and no cracks in the cross-section of the oxide coating were found. However, some pores were detected in the coatings embedded with SiC nano-particles produced at current density 5 and 7 A/dm 2, respectively. On the other word, it is clear that the average coating thickness and the ratio of the compact coating increased as the applied current density increased. The coatings embedded with SiC nano-particles fabricated at 5 and 7 A/dm 2 for 45 min were about 15 and 20 µm thick, respectively, while the coating formed at 10 A/dm 2 were about 26 µm thick. This indicates that the coatings grow faster with the current density 10 A/dm 2. This is because at higher current density, the rate of formation of MAO coating is more. Furthermore, the deposition of SiC nanoparticle has no obvious effect on the thickness of the composite coatings just as shown in Fig. 3(c) and (d). The surface morphologies of the coatings are shown in Fig. 4. In the composite oxide coatings, the crack-like structure was observed when the applied current density was 5 and 7 A/dm 2 (Fig. 4(a) and (b)). When the applied current density was 10 A/dm 2, the smaller pores with diameters about 5 µm were formed just as shown in Fig. 4(c). Simultaneously, more oxide product was produced from the discharge channels on the oxide coating without SiC
Y. Yang et al.: J. Mater. Sci. Technol., 2010, 26(11), 1016 1020 1019 Fig. 4 Surface morphologies of the coatings formed at different current density: (a) 5 A/dm 2, (b) 7 A/dm 2, (c) 10 A/dm 2, (d) 10 A/dm 2 (without SiC nanoparticles incorpration) deposition formed at current density 10 A/dm 2 just as shown in Fig. 4(d), which is connected to the larger and long-lived spark discharges during the MAO process. From the experiments, it can be observed that the spark was smaller and short-lived for the SiC nano-particles in the electrolyte at the same current density 10 A/dm 2. Hence, the nano-particles had a strong effect on the pores size formed from the discharge channels on the ceramic surface in the oxide coating. Furthermore, the alumina coating compared to the composite coatings has a high relative surface roughness, just as shown in Table 2. When the ox- Table 2 Thickness and surface roughness of MAO coatings with and without SiC nano-particles formed at different current density Current density/(a/dm 2 ) Ra/µm Thickness/µm 10 (without SiC nano-particle) 11.76 25.94 5 (with SiC nano-particle) 5.87 14.65 7 (with SiC nano-particle) 7.52 20.43 10 (with SiC nano-particle) 8.02 26.78 ide coating without SiC incorporation was formed at current density 10 A/dm 2, the surface roughness was 11.76 µm (Ra=11.76). While the surface roughness of the oxide coating embedded with SiC nano-particles formed at current density 10 A/dm 2 was 8.02 µm. The surface roughness of the composite oxide coating formed at current density 5 and 7 A/dm 2 was 5.87 and 7.52 µm, respectively. According to the above discussion, the incorporation of SiC nano-particles in the electrolyte could reduce the roughness by reducing the oxide product produced from the discharge channels on the oxide coating. 3.2 Effect of current density on the corrosion resistance of the composite MAO coating In the present work, EIS and potentiodynamic polarization were used to characterize the corrosion behavior of the Al 2 O 3 coatings with and without SiC nano-particles formed at different current densities in 3.5 wt pct NaCl solutions. The EIS plots and the potentiodynamic polarization plots are presented in Figs. 5 and 6, respectively. In Fig. 5, it appears to be only one loop for all the coatings which indicates the existence of only one time constant in the frequency by a simple equivalent circuit consisting of a resistance R p and a constant phase element Q in parallel [11]. The value of R p for ceramic coatings with SiC nanoparticles formed at current density 10 A/dm 2 is an increase of two orders of magnitude compared with that of the as-deposited ceramic coating. From Fig. 6, the corrosion resistance of the samples after SiC nanoparticles incorporation increased obviously. The corrosion potential (E corr ) of the coatings without SiC incorporation produced at current density 10 A/dm 2 is much higher than that of the composite coatings. As for the corrosion current density (i corr ),
1020 Y. Yang et al.: J. Mater. Sci. Technol., 2010, 26(11), 1016 1020 Fig. 5 EIS plots for MAO coatings with and without SiC nano-particles at different current density: (a) 10 A/dm 2 without SiC nano-particles, (b) 5 A/dm 2 with SiC nano-particles, (c) 7 A/dm 2 with SiC nano-particles, (d) 10 A/dm 2 with SiC nano-particles Fig. 6 Potentiodynamic polarization curves for MAO coatings with and without SiC nano-particles at different current density: (a) 10 A/dm 2 without SiC nano-particles, (b) 5 A/dm 2 with SiC nanoparticles, (c) 7 A/dm 2 with SiC nano-particles, (d) 10 A/dm 2 with SiC nano-particles the samples with Al 2 O 3 -SiC composite coatings exhibited lower corrosion current density than the asdeposited oxide coating. Furthermore, it should be noted that the corrosion current density of the coatings embedded with SiC nano-particles fabricated at 10 A/dm 2 is far lower than that of coatings produced at 5 and 7 A/dm 2. As described above, the corrosion resistance of the composite coating was greatly improved when they was produced at 10 A/dm 2. This may be attributed to the morphologies and roughness of the coatings discussed above. It is well known that the main corrosion form of alumina coating in NaCl solution is pitting corrosion. Therefore, the micropores of the coatings have detrimental effect on the corrosion performance. For the composite oxide coatings, SiC nano-particles could embed into the microarc discharge channels through the diffusion and electrophoresis by the introduction of SiC nano-particles in the electrolyte during MAO. Larger pores augment the real exposed area to the corrosive solution and they may concentrate more corrosion medium than little ones. Thus, the Al 2 O 3 coatings embedded with SiC nano-particle at current density 10 A/dm 2 showed the better corrosion resistance than the coatings formed at 5 and 7 A/dm 2. 4. Conclusions (1) Al 2 O 3 coatings embedded with SiC nanoparticles at different current densities were successfully fabricated on aluminum alloy by micro-arc oxidation. Incorporation of SiC nano-particles was accomplished by adding SiC nano-particles into the electrolyte in MAO process. (2) The Al 2 O 3 -SiC composite coatings are mainly composed of α-al 2 O 3 and γ-al 2 O 3. The thickness and growth rate of the coating increases with increasing applied current density. Simultaneously, more SiC nano-particles randomly dispersed on the surface were found when the current density reached 10 A/dm 2. Furthermore, the incorporation of SiC nano-particles in the electrolyte could reduce the roughness of the coatings. (3) The samples with Al 2 O 3 -SiC composite coatings exhibited lower corrosion current density than the as-deposited oxide coating. Furthermore, the corrosion current density of the coatings embedded with SiC nano-particles fabricated at 10 A/dm 2 is far lower than that of the coatings produced at 5 and 7 A/dm 2. Hence, the Al 2 O 3 coatings embedded with SiC nanoparticle formed at current density 10 A/dm 2 showed the best corrosion resistance than those coatings. Acknowledgement This work was financially supported by the Ministry of Education of the People s Republic of China (Contract No. 210051). REFERENCES [1 ] G.H. Lv, H. Chen, W.C. Gu, L. Li, E.W. Niu, X.H. Zhang and S.Z. Yang: J. Mater. Process. Technol., 2008, 208, 9. [2 ] A.L. Yerokhin., X. Nie, A. Leyland, A. Matthews and S.J. Dowey: Surf. Coat. Technol., 1999, 122, 73. [3 ] Y.M. Wang, B.L. Jiang, L.X. Guo and T.Q. Lei: Appl. Surf. Sci., 2006, 252, 2989. [4 ] H.H. Wu, J.B. Wang, B.Y. Long, B.H. Long, Z.S. Jin, N.D. Wang, F.R. Yu and D.M. Bi: Appl. Surf. Sci., 2005, 252, 1545. [5 ] Y. Zhang, Y.L. Chen, D.Z. Yu and T.F. Zhang: J. Chin. Soc. Corros. Protect., 2010, 30, 222. (in Chinese) [6 ] A.L. Yerokhin, X. Nie, A. Leyland and A. Matthews: Surf. Coat. Technol., 2000, 130, 195. [7 ] J. Liang, L. Hu and J. Hao: Appl. Surf. Sci., 2007, 253, 6939. [8 ] H. Guo, M. An, S. Xu and H. Huo: Mater. Lett., 2006, 60, 1538. [9 ] V. Raj and M.M. Ali: J. Mater. Process. Technol., 2009, 20, 5341. [10] T. Qiu, X.L. Wu, F.Y. Jin, A.P. Huang and P.K. Chu: Appl. Surf. Sci., 2007, 253, 3987. [11] D.R. Annett, C. Schurer, G. Irmer and E. Muller: Surf. Coat. Technol., 2004, 177-178, 830.