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EIS study of a self-repairing microarc oxidation coating
Corrosion Science 53 (2011) 618–623
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
EIS study of a self-repairing microarc oxidation coating Lei Wen a, Y.M. Wang a,⇑, Yan Liu b, Y. Zhou a, L.X. Guo a, J.H. Ouyang a, D.C. Jia a a b
Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China The Key Laboratory for Engineering Bionics, Ministry of Education, Jilin University, Changchun 130025, China
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Article history: Received 28 August 2010 Accepted 14 October 2010 Available online 23 October 2010 Keywords: A. Aluminium B. EIS C. Oxide coatings C. Passive films
a b s t r a c t The self-repairing microarc oxidation (MAO) coating consisting of a bottom nanocrystalline layer covered by a top conversion ceramic coating was fabricated on 2024 Al alloy by a duplex process with surface mechanical attrition (SMAT) prior to microarc MAO treatment. A 20 lm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated by SMAT, and on which covered by a top MAO coating of 5 lm. The self-repairing property caused by the formation of a dense passive film at the damaged regions contacting the bottom nanocrystalline layer enhances the corrosion reisitance of the SMAT-MAO coating. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Aluminium alloys are widely used in aerospace and automotive industries due to their excellent properties of high strength-toweight ratio and good formability. Unfortunately, aluminum alloys are susceptible to corrosion, which greatly restricts their further application, especially in some adverse service environments. Microarc oxidation (MAO) can be used to provide a good corrosion resistant surface on aluminum alloys, and the anti-corrosion property of the MAO coating has been studied in many published literatures [1–11]. However, MAO can have a detrimental effect on the fatigue behaviour of aluminum alloy [12–14]. The combination of mechanical cold-work and microarc oxidation has been demonstrated to significantly improve the fatigue resistance compared with MAO process alone. Asquith et al. demonstrates that the loss in fatigue life can be recovered to a certain extent by the application of a suitable surface cold-work process prior to treatment [15]. In their study, shot-peening was employed as such a coldwork process and was demonstrated to recover the fatigue life of the MAO treated aluminum by approximately 85%. Surface mechanical attrition treatment (SMAT) [16], enabling to produce a more severe cold-work compared with the shot-peening, can induce grain refinement into nanometer scale in the surface layer of bulk metal sample. For example, a nanocrystalline layer in the surface of Al alloy has been successfully produced by Tao et al. [17]. In this paper, SMAT was chosen as the pre-treatment process of 2024 Al alloy, followed by the MAO treatment. In this way the spe-
⇑ Corresponding author. Tel.: +86 451 86402040x8403; fax: +86 451 86414291. E-mail address: [email protected] (Y.M. Wang). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.10.010
cially modified layers structure with a refined grain bottom layer covered by a top MAO coating was fabricated and would be beneficial to recover the fatigue life of the coated aluminum alloy. In our previous study (submitted elsewhere), it has been proved that SMAT-MAO coated aluminum alloy has a better fatigue life compared with single MAO coated one. However, it should be noted that such a specially modified structure will inevitably produce certain effects on corrosion behaviour of 2024 Al alloy. Unfortunately, how the separated layer structure (especially the refined grains bottom layer) affects the corrosion is not clear. Thus, this work focuses on the relationship between the specially modified layers microstructure and the electrochemical behaviour of the combined SMAT-MAO coated aluminum alloy, finally further understanding the influence of the bottom nanocrystalline layer on the electrochemical behaviour of coated alloy. 2. Experimental 2.1. Materials The material used in the experiment is 2024 Al alloy with nominal composition shown in Table 1. The specimens, 80 mm 80 mm 3 mm in dimension, were ground with 400#, 800# and 1200# abrasive papers, ultrasonically washed with acetone and distilled water, and dried for SMAT and MAO treatment. 2.2. Nanocrystalline modified layer preparation by SMAT SMAT was performed in vacuum using a SNC-1 type machine. A container of glass fibre reinforced plastic (GFRP) was placed in the steel chamber that was vibrated by a generator. ZrO2 balls with
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Fe
Si
Zn
Ti
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Al
4.36
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diameter of 6 mm, instead of the conventional steel balls used in SMAT, were placed at the bottom of GFRP container. This kind of set can effectively prevent the iron pollution products to incorporate into the sample surface. The vibration frequency of the chamber is 50 Hz.
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2.3. Preparation of modified layer by MAO The top ceramic conversion coatings were fabricated by MAO process on the surfaces of SMAT modified and original Al samples, respectively. The electrolyte of alkaline silicate was prepared from the solution of Na2SiO3 (6.0 g/L), NaOH (1.2 g/L), (NaPO3)6 (35.0 g/L) and Na2WO4 (6.0 g/L) in distilled water. A 65 kW microarc oxidation device provides the voltage waveforms, and the main pulse parameters, such as pulse duration, voltage amplitude and duty cycle during both positive and negative biasing can be adjusted independently. In the experiment, the electrical parameters were fixed as follows: voltage 600 V, frequency 600 Hz, duty cycle 10.0%, and the treatment time 10 min, thus the current varied with the duration of anodizing time. 2.4. Characterization of modified layer The microstructure of the alloy surface treated by SMAT, MAO, and the combination of the two methods, was investigated by
(311)
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Table 1 Chemical composition of 2024 Al alloy (wt.%).
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2Theta, degree Fig. 1. XRD patterns of 2024 Al alloy before and after SMAT: (a) base alloy and (b) SMAT.
X-ray diffraction (XRD) with a Philips X’pert diffractometer using Cu Ka in the range of 30–100°, the measurements were performed with a continuous scanning mode at a rate of 4°/min with an incident angle of 4°. The surface and cross-section morphologies of the modified surface were observed by a Hitachi S-4700 scanning electron microscopy (SEM). A Philips CM-12 transmission electron microscopy (TEM) was used to examine the microstructure of the nanocrystalline layer and determine the grain size in different depth along the section of the SMAT modified layer. To prepare TEM specimens, 0.2 mm thick slices were cut from the different depth of the surface modified layer. After mechanical grinding to
Fig. 2. TEM image of 2024 Al alloy before and after SMAT process: (a) base alloy; (b) surface, after SMAT; and (c) 20 lm from surface, after SMAT.
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40–50 lm, 3 mm-diameter discs were punched from the thin foils, which were then ion-milled with a small incident angle till perforation from single side only. 2.5. Electrochemical tests Electrochemical impedance spectroscopy (EIS) measurement was carried out using an IviumStat electrochemical workstation at room temperature, and a 3.5 wt.% NaCl solution was used as the corrosive medium. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate was used as the counter electrode. EIS was performed between 0.01 Hz and 100 kHz frequency. Amplitude of the sinusoidal voltage signal was 10 mV. By studying the characteristic change in the impedance spectra, failure mechanism of the coating can be revealed.
3. Results and discussion 3.1. Microstructure of nanocrystalline modified surface The XRD patterns of the 2024 Al alloy before and after SMAT are shown in Fig. 1. It can be found that, compared with the untreated 2024 Al alloy, there is an evident broadening of the Bragg-diffraction peaks and a slight shift in the position of diffractions after SMAT. The Bragg-diffraction peak broadening in the surface layer may be attributed to grain refinement and the shift of the diffraction peaks is caused by the development of microstrain. During SMAT, a nanocrystalline surface layer forms due to large grain boundary misorientations, dislocation blocks and microbands [17]. Quantitative XRD measurements indicate that the average grain
size of the treated surface layer was 52.8 nm after SMAT for 15 min. The bright-field TEM image of non-treated 2024 Al alloy, as shown in Fig. 2a, shows that the grain size of Al alloy substrate is around several micrometers. Fig. 2b and c show the bright-field TEM image of SMAT 2024 Al alloy. As shown in Fig. 2b, a nanocrystalline structure characterized by the equiaxed shape grains of about 20–180 nm can be found at about 5 lm depth bellow the surface of 2024 Al alloy after SMAT for 15 min. The grain size at 20 lm depth bellow the sample surface increases to several hundred nanometers, as shown in Fig. 2c. Therefore, it can be deduced that, after SMAT, a nanocrystalline layer of about 20 lm thickness was obtained at the surface of 2024 Al alloy sample. 3.2. Coating analysis Under the same experimental parameters (voltage 600 V, frequency 600 Hz, duty cycle 10.0%, and the treatment time 10 min), MAO coating and SMAT-MAO coating were fabricated on the original Al sample and the SMAT modified sample surface, respectively. The surface and cross-section morphologies of a simple MAO coated and a SMAT-MAO coated 2024 Al alloy are illustrated in Fig. 3, respectively. Observations on the surface of the coated 2024 Al alloy indicated that there are less micro pores at the surface of MAO coated alloy than that of SMAT-MAO coated one, as shown in Fig. 3a and b. This phenomenon can be due to the refined grains of the alloy surface after SMAT pre-treatment. The alloy after SMAT has a nanocrystalline surface with much more defect sites, at which it is easier to induce discharge during the MAO process. Therefore, more discharge channels (remained as micro pores at the surface) were formed at the surface of the
Fig. 3. Surface and cross-section morphologies of coated 2024 Al alloy surface morphology of (a) MAO coating; (b) SMAT-MAO coating; cross-section morphology of (c) MAO coating and (d) SMAT-MAO coating.
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Fig. 4. XRD patterns of coating: (a) MAO coating; (b) SMAT-MAO coating.
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3.3. Electrochemical study Electrochemical impedance spectroscopy was employed to investigate the corrosion characteristics of MAO and SMAT-MAO coated alloys during the long-term immersion tests (0.5–360 h) in 3.5 wt.% NaCl solution, and the resultant EIS plots of MAO and SMAT-MAO coatings are presented in Figs. 5 and 7. It can be found that, in the initial stage of immersion (0.5 h), the MAO coated alloy showed much better corrosion resistance compared with the SMAT-MAO coated one. This is attributed to the less micro pores at the surface of the MAO coating. As shown in Fig. 5a, it can be seen that the impedance plots illustrate clearly the deterioration process of the MAO coating in the long-term immersion. The impedance response indicates that the MAO coating was gradually penetrated by the corrosive solution during immersion. In the early stage of immersion (0.5–96 h), two time constants can be found in Fig. 5b. The one appearing at high frequency represents the dielectric property of the porous layer of MAO coating, while the other one at medium frequency corresponds to the barrier layer [18]. After immersion for 120 h, another time constant, corresponding to corrosion reaction under the MAO coating, appeared at low frequency, indicating that the MAO coating was penetrated by the electrolyte [19], and the galvanic corrosion cells had established under the MAO coating. As shown in Fig. 6a, an equivalent circuit (EC) was employed to fit the experimental data, and the physical model for the MAO
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SMAT-MAO coating. Fig. 3c and d show the cross-section morphologies of MAO and SMAT-MAO coated 2024 Al alloy, respectively. It can be found that the thickness of both coatings is similar, about 5 lm, under the same processing condition of MAO treatment. As mentioned above, a nanocrystalline layer of about 20 lm was formed at the surface of the alloy after SMAT. After the subsequent MAO treatment, the nanocrystalline layer was partially oxidized to form as the top ceramic coating, while the unoxidized nanocrystalline layer was remained as the bottom loading layer. Therefore, the finally formed SMAT-MAO coating consists of two parts: a top conversion ceramic coating and a bottom nanocrystalline layer adjacent to the coating. The XRD results revealed the presence of c-Al2O3 phase in both coatings, as shown in Fig. 4. No obvious difference can be distinguished from the XRD pattern between MAO and SMAT-MAO coated Al alloy. Since both of the coatings are not thick (only about 5 lm), the diffraction peak of Al alloy substrate is reflected obviously in the XRD patterns.
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168 h 240 h 264 h 288 h 312 h 336 h 360 h 3
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coated alloy immersed after 120 h was also presented. In the proposed EC, Qp // Rp and Qb // Rb represent the response of porous and barrier layer of the MAO coating, respectively, and the pair Qdl // Rct is attributed to the charge transfer reactions taking place under the MAO coating. Using the EC proposed above, the experimental data can be well fitted, and the fitted result of the MAO coating immersed for 240 h was also illustrated in Fig. 6b. The impedance behaviour was obviously different for the SMAT-MAO coated alloy, as shown in Fig. 7a. It can be found that, the SMAT-MAO coated alloy showed a complex trend during the immersion in 3.5 wt.% NaCl solution. Only two time constants can be found during the whole immersion period, as shown in Fig. 7b. The one appearing at high frequency corresponds to the porous layer of SMAT-MAO coating, while the one at medium frequency corresponds to the barrier layer. It can be seen that, the position of peak at medium frequency remains unchanging with the immersion time from 0.5 to 72 h, indicating a stable property of the barrier layer of the SMAT-MAO coating. In addition, the decrease of peak height with immersion time indicates that the response of barrier layer becomes less capacitive. This phenomenon can be attributed to the penetration of electrolyte into the coating. After immersion for 96 h, as the immersion time increasing, the peak at medium frequency began to shift to the low frequency gradually, suggesting that the dielectric property of the barrier layer may be influenced due to the immersion. During the whole period of immersion, there was no third time constant, corresponding to corrosion reaction, appearing at the low frequency, indicating that the SMAT-MAO coating shows a more stable corrosion resistance compared with the simple MAO coating during the whole test.
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The improved corrosion resistance is possibly attributed to the repairing mechanism by the formation of dense passive film in the damaged region contacting the bottom nanocrystalline layer. When the top ceramic layer was attacked and broken through at the weak site, the active ions from the corrosion media will move through the collapsed layer and contact the interface adjacent to the bottom nanocrystalline layer. It is believed that the nanocrystalline layer has a high density of nucleation sites for passive films, which led to a high fraction of passive layers and low corrosion rate [20]. Furthermore, the smaller grain size of the material decrease the amount of chloride ions absorbed on the surface and promoted the formation of compact passive film, which significantly improved the corrosion resistance [21]. Once the top ceramic coating of SMAT-MAO coating was penetrated by NaCl solution, a dense passive film could form instantly at that damaged position near to the exposed surface of nanocrystalline layer, which in fact plays a role in repairing the damaged barrier layer of the top ceramic coating, resulting in the improvement of corrosion resistance. In this situation, the dense passive film formed on the bottom nanocrystalline layer of SMAT-MAO coating can be seen as a part of barrier layer of top ceramic coating. Based on the discussion above, a physical model and an equivalent circuit for SMAT-MAO coated alloy immersed in 3.5 wt.% NaCl solution are illustrated in Fig. 8a. Compared with that of the simple MAO coated alloy, EC for the SMAT-MAO coated one has no pair Qdl // Rct corresponding to the corrosion reactions. The experimental data can be well fitted by using the EC. The fitted result of the SMAT-MAO coating immersed for 240 h was presented in Fig. 8b. The impedance modulus at low frequency (for example, 0.1 Hz) can be considered as an indication of the corrosion resistance of
the tested sample [9,22,23]. In this way, the corrosion resistance values of the tested samples were determined from the magnitude of the impedance data at 0.1 Hz as a function of immersion time, and the values are presented in Fig. 9. It can be found that the resistance of MAO coated sample is very high (4026.6 kX cm2) at initial 0.5 h of immersion in 3.5 wt.% NaCl solution. With immersion time increasing, the modulus of MAO coated sample decreased from 4026.6 kX cm2 at 0.5 h, to 127.5 kX cm2 at 360 h, gradually, indicating the deterioration of MAO coating in NaCl solution. While, the resistance of the SMAT-MAO coated alloy shows relatively complex trend: decreases gradually in the initial 72 h of immersion, then increases with immersion time from 96 to 336 h, and when immersed for 360 h falls into the lowest value. In the early stage of immersion (0.5–72 h) in 3.5 wt.% NaCl solution, the SMAT-MAO coating undergoes degradation due to the seepage of electrolyte through the pores, resulting in the reduction of impedance, corresponding to the decrease of phase angle in EIS plot. However, with passage of time (after 96 h), the electrolyte penetrated the top ceramic coating, and reached the bottom nanocrystalline layer. Owing to the high oxidizing activity of the nanocrystalline in the bottom nanocrystalline layer of the SMAT-MAO coating, the more compact passive film can be easily generated at the damaged interface area of substrate/ceramic layer compared with that of the simple MAO coating. Thus the corrosion process can result in the formation of a stable passive film on the substrate [8], corresponding to the shift of time constant to the low frequency. With increasing immersion time from 72 to 336 h, more sites of the SMAT-MAO coating were penetrated by the solution gradually, followed by the effective self-repairing
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L. Wen et al. / Corrosion Science 53 (2011) 618–623
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A nanocrystalline surface layer was successfully synthesized on 2024 Al alloy by means of SMAT. The average grain size of surface layer was 52.8 nm when treated for 15 min. The SMAT-MAO coating, consisting of a top ceramic coating and a bottom nanocrystalline layer, was obtained by applying MAO process on the SMAT modified layer. During the whole immersion test, the corrosion resistance of the MAO coated alloy decreased gradually, while that of SMAT-MAO coated alloy decreased in the initial state, then increased. The difference of corrosion behaviour can be attributed to the different structure of the substrate adjacent to ceramic coating. When the solution penetrated the top ceramic coating of the SMAT-MAO coating to reach the bottom nanocrystalline layer, a dense passive film formed at the damaged region contacting the bottom nanocrystalline layer. Due to the formation of the dense passive film, SMAT coated alloy showed a more stable and improved corrosion resistance compared with that of simple MAO coated one. Acknowledgements
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References
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The partial supports from the NSFC grant nos. 50701014, 60776803 and 50905071, the program for New Century Excellent Talents in University of China (NCET-08-0166) and the opening project of Key Laboratory for Engineering Bionics (Jilin University, Ministry of Education) are gratefully acknowledged. The authors also thank Prof. T. Zhang for the assistance of corrosion test and analysis.
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Immersion time (h) Fig. 9. Corrosion resistance as a function of immersion time of MAO and SMATMAO coated 2024 Al alloy in 3.5 wt.% NaCl solution.
induced by the formation of dense passive film in nanocrystalline layer, resulting in that the impedance slightly increased with immersion time. However, after immersion for 360 h, the passive film was penetrated completely by the electrolyte, resulting in the sudden decrease of impedance. Compared with single MAO coating, SMAT-MAO coating shows a better and stable corrosion resistance. While anodizing in chromium baths is the traditional protecting method of 2024 Al alloy used in aeronautical applications, the corrosion resistance of substrate alloy has been enhanced by chromium anodizing coating [24]. Whether the SMAT-MAO coating could be an alternative to chromium anodizing or not needs to be further studied.
[1] R.C. Barik, J.A. Wharton, R.J.K. Wood, K.R. Stokes, R.L. Jones, Surf. Coat. Technol. 199 (2005) 158–167. [2] C.-E. Barchiche, E. Rocca, C. Juers, J. Hazan, J. Steinmetz, Electrochim. Acta 53 (2007) 417–425. [3] C.-E. Barchiche, E. Rocca, J. Hzan, Surf. Coat. Technol. 202 (2008) 4145–4152. [4] H. Duan, K. Du, C. Yan, F. Wang, Electrochim. Acta 51 (2006) 2898–2908. [5] H. Duan, C. Yan, F. Wang, Electrochim. Acta 52 (2007) 3785–3793. [6] J. Jiang, P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 51 (2009) 2483– 2492. [7] D.Y. Hwang, Y.M. Kim, D.Y. Park, B. Yoo, D.H. Shin, Electrochim. Acta 54 (2009) 5479–5485. [8] J. Jiang, P. Bala Srinivasan, C. Blawert, M. Stormer, W. Dietzel, Electrochim. Acta 54 (2009) 3842–3850. [9] J. Jiang, P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 52 (2010) 540– 547. [10] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corros. Sci. 50 (2008) 1744–1752. [11] P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 50 (2008) 2415–2418. [12] B. Rajasekaran, S. Ganesh Sundara Raman, S.V. Joshi, G. Sundararajan, Int. J. Fatigue 30 (2008) 1259–1266. [13] B. Rajasekaran, S. Ganesh Sundara Raman, L. Rama Krishna, S.V. Joshi, G. Sundararajan, Surf. Coat. Technol. 202 (2008) 1462–1469. [14] B. Lonyuk, I. Apachitei, J. Duszczyk, Surf. Coat. Technol. 201 (2007) 8688–8694. [15] D.T. Asquith, A.L. Yerokhin, J.R. Yates, A. Matthews, Thin Solid Films 515 (2006) 1187–1191. [16] K. Lu, J. Lu, J. Mater. Sci. Technol. 15 (1999) 193–197. [17] N.R. Tao, Z.B. Wang, W.P. Tong, M.L. Sui, J. Lu, K. Lu, Acta Mater. 50 (2002) 4603–4616. [18] V. Moutarlier, M.P. Gigandet, B. Normand, J. Pagetti, Corros. Sci. 47 (2005) 937– 951. [19] C. Liu, Q. Bi, A. Leyland, A. Matthews, Corros. Sci. 45 (2003) 1257–1273. [20] V. Afshari, C. Dehghanian, Corros. Sci. 51 (2009) 1844–1849. [21] L. Liu, Y. Li, F.H. Wang, Electrochim. Acta 53 (2008) 2453–2462. [22] S.J. Xia, R. Yue, R.G. Rateick Jr., V.I. Birss, J. Electrochem. Soc. 151 (3) (2004) B179–B187. [23] H. Yasuda, Q.S. Yu, M. Chen, Prog. Org. Coat. 41 (2001) 273–279. [24] Y.L. Huang, H. Shih, H.C. Huang, J. Daugherty, S. Wu, S. Ramanathan, C. Chang, F. Mansfeld, Corros. Sci. 50 (2008) 3569–3575.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
EIS study of a self-repairing microarc oxidation coating Lei Wen a, Y.M. Wang a,⇑, Yan Liu b, Y. Zhou a, L.X. Guo a, J.H. Ouyang a, D.C. Jia a a b
Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China The Key Laboratory for Engineering Bionics, Ministry of Education, Jilin University, Changchun 130025, China
a r t i c l e
i n f o
Article history: Received 28 August 2010 Accepted 14 October 2010 Available online 23 October 2010 Keywords: A. Aluminium B. EIS C. Oxide coatings C. Passive films
a b s t r a c t The self-repairing microarc oxidation (MAO) coating consisting of a bottom nanocrystalline layer covered by a top conversion ceramic coating was fabricated on 2024 Al alloy by a duplex process with surface mechanical attrition (SMAT) prior to microarc MAO treatment. A 20 lm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated by SMAT, and on which covered by a top MAO coating of 5 lm. The self-repairing property caused by the formation of a dense passive film at the damaged regions contacting the bottom nanocrystalline layer enhances the corrosion reisitance of the SMAT-MAO coating. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Aluminium alloys are widely used in aerospace and automotive industries due to their excellent properties of high strength-toweight ratio and good formability. Unfortunately, aluminum alloys are susceptible to corrosion, which greatly restricts their further application, especially in some adverse service environments. Microarc oxidation (MAO) can be used to provide a good corrosion resistant surface on aluminum alloys, and the anti-corrosion property of the MAO coating has been studied in many published literatures [1–11]. However, MAO can have a detrimental effect on the fatigue behaviour of aluminum alloy [12–14]. The combination of mechanical cold-work and microarc oxidation has been demonstrated to significantly improve the fatigue resistance compared with MAO process alone. Asquith et al. demonstrates that the loss in fatigue life can be recovered to a certain extent by the application of a suitable surface cold-work process prior to treatment [15]. In their study, shot-peening was employed as such a coldwork process and was demonstrated to recover the fatigue life of the MAO treated aluminum by approximately 85%. Surface mechanical attrition treatment (SMAT) [16], enabling to produce a more severe cold-work compared with the shot-peening, can induce grain refinement into nanometer scale in the surface layer of bulk metal sample. For example, a nanocrystalline layer in the surface of Al alloy has been successfully produced by Tao et al. [17]. In this paper, SMAT was chosen as the pre-treatment process of 2024 Al alloy, followed by the MAO treatment. In this way the spe-
⇑ Corresponding author. Tel.: +86 451 86402040x8403; fax: +86 451 86414291. E-mail address: [email protected] (Y.M. Wang). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.10.010
cially modified layers structure with a refined grain bottom layer covered by a top MAO coating was fabricated and would be beneficial to recover the fatigue life of the coated aluminum alloy. In our previous study (submitted elsewhere), it has been proved that SMAT-MAO coated aluminum alloy has a better fatigue life compared with single MAO coated one. However, it should be noted that such a specially modified structure will inevitably produce certain effects on corrosion behaviour of 2024 Al alloy. Unfortunately, how the separated layer structure (especially the refined grains bottom layer) affects the corrosion is not clear. Thus, this work focuses on the relationship between the specially modified layers microstructure and the electrochemical behaviour of the combined SMAT-MAO coated aluminum alloy, finally further understanding the influence of the bottom nanocrystalline layer on the electrochemical behaviour of coated alloy. 2. Experimental 2.1. Materials The material used in the experiment is 2024 Al alloy with nominal composition shown in Table 1. The specimens, 80 mm 80 mm 3 mm in dimension, were ground with 400#, 800# and 1200# abrasive papers, ultrasonically washed with acetone and distilled water, and dried for SMAT and MAO treatment. 2.2. Nanocrystalline modified layer preparation by SMAT SMAT was performed in vacuum using a SNC-1 type machine. A container of glass fibre reinforced plastic (GFRP) was placed in the steel chamber that was vibrated by a generator. ZrO2 balls with
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diameter of 6 mm, instead of the conventional steel balls used in SMAT, were placed at the bottom of GFRP container. This kind of set can effectively prevent the iron pollution products to incorporate into the sample surface. The vibration frequency of the chamber is 50 Hz.
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2.3. Preparation of modified layer by MAO The top ceramic conversion coatings were fabricated by MAO process on the surfaces of SMAT modified and original Al samples, respectively. The electrolyte of alkaline silicate was prepared from the solution of Na2SiO3 (6.0 g/L), NaOH (1.2 g/L), (NaPO3)6 (35.0 g/L) and Na2WO4 (6.0 g/L) in distilled water. A 65 kW microarc oxidation device provides the voltage waveforms, and the main pulse parameters, such as pulse duration, voltage amplitude and duty cycle during both positive and negative biasing can be adjusted independently. In the experiment, the electrical parameters were fixed as follows: voltage 600 V, frequency 600 Hz, duty cycle 10.0%, and the treatment time 10 min, thus the current varied with the duration of anodizing time. 2.4. Characterization of modified layer The microstructure of the alloy surface treated by SMAT, MAO, and the combination of the two methods, was investigated by
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Table 1 Chemical composition of 2024 Al alloy (wt.%).
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2Theta, degree Fig. 1. XRD patterns of 2024 Al alloy before and after SMAT: (a) base alloy and (b) SMAT.
X-ray diffraction (XRD) with a Philips X’pert diffractometer using Cu Ka in the range of 30–100°, the measurements were performed with a continuous scanning mode at a rate of 4°/min with an incident angle of 4°. The surface and cross-section morphologies of the modified surface were observed by a Hitachi S-4700 scanning electron microscopy (SEM). A Philips CM-12 transmission electron microscopy (TEM) was used to examine the microstructure of the nanocrystalline layer and determine the grain size in different depth along the section of the SMAT modified layer. To prepare TEM specimens, 0.2 mm thick slices were cut from the different depth of the surface modified layer. After mechanical grinding to
Fig. 2. TEM image of 2024 Al alloy before and after SMAT process: (a) base alloy; (b) surface, after SMAT; and (c) 20 lm from surface, after SMAT.
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40–50 lm, 3 mm-diameter discs were punched from the thin foils, which were then ion-milled with a small incident angle till perforation from single side only. 2.5. Electrochemical tests Electrochemical impedance spectroscopy (EIS) measurement was carried out using an IviumStat electrochemical workstation at room temperature, and a 3.5 wt.% NaCl solution was used as the corrosive medium. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate was used as the counter electrode. EIS was performed between 0.01 Hz and 100 kHz frequency. Amplitude of the sinusoidal voltage signal was 10 mV. By studying the characteristic change in the impedance spectra, failure mechanism of the coating can be revealed.
3. Results and discussion 3.1. Microstructure of nanocrystalline modified surface The XRD patterns of the 2024 Al alloy before and after SMAT are shown in Fig. 1. It can be found that, compared with the untreated 2024 Al alloy, there is an evident broadening of the Bragg-diffraction peaks and a slight shift in the position of diffractions after SMAT. The Bragg-diffraction peak broadening in the surface layer may be attributed to grain refinement and the shift of the diffraction peaks is caused by the development of microstrain. During SMAT, a nanocrystalline surface layer forms due to large grain boundary misorientations, dislocation blocks and microbands [17]. Quantitative XRD measurements indicate that the average grain
size of the treated surface layer was 52.8 nm after SMAT for 15 min. The bright-field TEM image of non-treated 2024 Al alloy, as shown in Fig. 2a, shows that the grain size of Al alloy substrate is around several micrometers. Fig. 2b and c show the bright-field TEM image of SMAT 2024 Al alloy. As shown in Fig. 2b, a nanocrystalline structure characterized by the equiaxed shape grains of about 20–180 nm can be found at about 5 lm depth bellow the surface of 2024 Al alloy after SMAT for 15 min. The grain size at 20 lm depth bellow the sample surface increases to several hundred nanometers, as shown in Fig. 2c. Therefore, it can be deduced that, after SMAT, a nanocrystalline layer of about 20 lm thickness was obtained at the surface of 2024 Al alloy sample. 3.2. Coating analysis Under the same experimental parameters (voltage 600 V, frequency 600 Hz, duty cycle 10.0%, and the treatment time 10 min), MAO coating and SMAT-MAO coating were fabricated on the original Al sample and the SMAT modified sample surface, respectively. The surface and cross-section morphologies of a simple MAO coated and a SMAT-MAO coated 2024 Al alloy are illustrated in Fig. 3, respectively. Observations on the surface of the coated 2024 Al alloy indicated that there are less micro pores at the surface of MAO coated alloy than that of SMAT-MAO coated one, as shown in Fig. 3a and b. This phenomenon can be due to the refined grains of the alloy surface after SMAT pre-treatment. The alloy after SMAT has a nanocrystalline surface with much more defect sites, at which it is easier to induce discharge during the MAO process. Therefore, more discharge channels (remained as micro pores at the surface) were formed at the surface of the
Fig. 3. Surface and cross-section morphologies of coated 2024 Al alloy surface morphology of (a) MAO coating; (b) SMAT-MAO coating; cross-section morphology of (c) MAO coating and (d) SMAT-MAO coating.
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3.3. Electrochemical study Electrochemical impedance spectroscopy was employed to investigate the corrosion characteristics of MAO and SMAT-MAO coated alloys during the long-term immersion tests (0.5–360 h) in 3.5 wt.% NaCl solution, and the resultant EIS plots of MAO and SMAT-MAO coatings are presented in Figs. 5 and 7. It can be found that, in the initial stage of immersion (0.5 h), the MAO coated alloy showed much better corrosion resistance compared with the SMAT-MAO coated one. This is attributed to the less micro pores at the surface of the MAO coating. As shown in Fig. 5a, it can be seen that the impedance plots illustrate clearly the deterioration process of the MAO coating in the long-term immersion. The impedance response indicates that the MAO coating was gradually penetrated by the corrosive solution during immersion. In the early stage of immersion (0.5–96 h), two time constants can be found in Fig. 5b. The one appearing at high frequency represents the dielectric property of the porous layer of MAO coating, while the other one at medium frequency corresponds to the barrier layer [18]. After immersion for 120 h, another time constant, corresponding to corrosion reaction under the MAO coating, appeared at low frequency, indicating that the MAO coating was penetrated by the electrolyte [19], and the galvanic corrosion cells had established under the MAO coating. As shown in Fig. 6a, an equivalent circuit (EC) was employed to fit the experimental data, and the physical model for the MAO
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SMAT-MAO coating. Fig. 3c and d show the cross-section morphologies of MAO and SMAT-MAO coated 2024 Al alloy, respectively. It can be found that the thickness of both coatings is similar, about 5 lm, under the same processing condition of MAO treatment. As mentioned above, a nanocrystalline layer of about 20 lm was formed at the surface of the alloy after SMAT. After the subsequent MAO treatment, the nanocrystalline layer was partially oxidized to form as the top ceramic coating, while the unoxidized nanocrystalline layer was remained as the bottom loading layer. Therefore, the finally formed SMAT-MAO coating consists of two parts: a top conversion ceramic coating and a bottom nanocrystalline layer adjacent to the coating. The XRD results revealed the presence of c-Al2O3 phase in both coatings, as shown in Fig. 4. No obvious difference can be distinguished from the XRD pattern between MAO and SMAT-MAO coated Al alloy. Since both of the coatings are not thick (only about 5 lm), the diffraction peak of Al alloy substrate is reflected obviously in the XRD patterns.
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coated alloy immersed after 120 h was also presented. In the proposed EC, Qp // Rp and Qb // Rb represent the response of porous and barrier layer of the MAO coating, respectively, and the pair Qdl // Rct is attributed to the charge transfer reactions taking place under the MAO coating. Using the EC proposed above, the experimental data can be well fitted, and the fitted result of the MAO coating immersed for 240 h was also illustrated in Fig. 6b. The impedance behaviour was obviously different for the SMAT-MAO coated alloy, as shown in Fig. 7a. It can be found that, the SMAT-MAO coated alloy showed a complex trend during the immersion in 3.5 wt.% NaCl solution. Only two time constants can be found during the whole immersion period, as shown in Fig. 7b. The one appearing at high frequency corresponds to the porous layer of SMAT-MAO coating, while the one at medium frequency corresponds to the barrier layer. It can be seen that, the position of peak at medium frequency remains unchanging with the immersion time from 0.5 to 72 h, indicating a stable property of the barrier layer of the SMAT-MAO coating. In addition, the decrease of peak height with immersion time indicates that the response of barrier layer becomes less capacitive. This phenomenon can be attributed to the penetration of electrolyte into the coating. After immersion for 96 h, as the immersion time increasing, the peak at medium frequency began to shift to the low frequency gradually, suggesting that the dielectric property of the barrier layer may be influenced due to the immersion. During the whole period of immersion, there was no third time constant, corresponding to corrosion reaction, appearing at the low frequency, indicating that the SMAT-MAO coating shows a more stable corrosion resistance compared with the simple MAO coating during the whole test.
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The improved corrosion resistance is possibly attributed to the repairing mechanism by the formation of dense passive film in the damaged region contacting the bottom nanocrystalline layer. When the top ceramic layer was attacked and broken through at the weak site, the active ions from the corrosion media will move through the collapsed layer and contact the interface adjacent to the bottom nanocrystalline layer. It is believed that the nanocrystalline layer has a high density of nucleation sites for passive films, which led to a high fraction of passive layers and low corrosion rate [20]. Furthermore, the smaller grain size of the material decrease the amount of chloride ions absorbed on the surface and promoted the formation of compact passive film, which significantly improved the corrosion resistance [21]. Once the top ceramic coating of SMAT-MAO coating was penetrated by NaCl solution, a dense passive film could form instantly at that damaged position near to the exposed surface of nanocrystalline layer, which in fact plays a role in repairing the damaged barrier layer of the top ceramic coating, resulting in the improvement of corrosion resistance. In this situation, the dense passive film formed on the bottom nanocrystalline layer of SMAT-MAO coating can be seen as a part of barrier layer of top ceramic coating. Based on the discussion above, a physical model and an equivalent circuit for SMAT-MAO coated alloy immersed in 3.5 wt.% NaCl solution are illustrated in Fig. 8a. Compared with that of the simple MAO coated alloy, EC for the SMAT-MAO coated one has no pair Qdl // Rct corresponding to the corrosion reactions. The experimental data can be well fitted by using the EC. The fitted result of the SMAT-MAO coating immersed for 240 h was presented in Fig. 8b. The impedance modulus at low frequency (for example, 0.1 Hz) can be considered as an indication of the corrosion resistance of
the tested sample [9,22,23]. In this way, the corrosion resistance values of the tested samples were determined from the magnitude of the impedance data at 0.1 Hz as a function of immersion time, and the values are presented in Fig. 9. It can be found that the resistance of MAO coated sample is very high (4026.6 kX cm2) at initial 0.5 h of immersion in 3.5 wt.% NaCl solution. With immersion time increasing, the modulus of MAO coated sample decreased from 4026.6 kX cm2 at 0.5 h, to 127.5 kX cm2 at 360 h, gradually, indicating the deterioration of MAO coating in NaCl solution. While, the resistance of the SMAT-MAO coated alloy shows relatively complex trend: decreases gradually in the initial 72 h of immersion, then increases with immersion time from 96 to 336 h, and when immersed for 360 h falls into the lowest value. In the early stage of immersion (0.5–72 h) in 3.5 wt.% NaCl solution, the SMAT-MAO coating undergoes degradation due to the seepage of electrolyte through the pores, resulting in the reduction of impedance, corresponding to the decrease of phase angle in EIS plot. However, with passage of time (after 96 h), the electrolyte penetrated the top ceramic coating, and reached the bottom nanocrystalline layer. Owing to the high oxidizing activity of the nanocrystalline in the bottom nanocrystalline layer of the SMAT-MAO coating, the more compact passive film can be easily generated at the damaged interface area of substrate/ceramic layer compared with that of the simple MAO coating. Thus the corrosion process can result in the formation of a stable passive film on the substrate [8], corresponding to the shift of time constant to the low frequency. With increasing immersion time from 72 to 336 h, more sites of the SMAT-MAO coating were penetrated by the solution gradually, followed by the effective self-repairing
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A nanocrystalline surface layer was successfully synthesized on 2024 Al alloy by means of SMAT. The average grain size of surface layer was 52.8 nm when treated for 15 min. The SMAT-MAO coating, consisting of a top ceramic coating and a bottom nanocrystalline layer, was obtained by applying MAO process on the SMAT modified layer. During the whole immersion test, the corrosion resistance of the MAO coated alloy decreased gradually, while that of SMAT-MAO coated alloy decreased in the initial state, then increased. The difference of corrosion behaviour can be attributed to the different structure of the substrate adjacent to ceramic coating. When the solution penetrated the top ceramic coating of the SMAT-MAO coating to reach the bottom nanocrystalline layer, a dense passive film formed at the damaged region contacting the bottom nanocrystalline layer. Due to the formation of the dense passive film, SMAT coated alloy showed a more stable and improved corrosion resistance compared with that of simple MAO coated one. Acknowledgements
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References
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The partial supports from the NSFC grant nos. 50701014, 60776803 and 50905071, the program for New Century Excellent Talents in University of China (NCET-08-0166) and the opening project of Key Laboratory for Engineering Bionics (Jilin University, Ministry of Education) are gratefully acknowledged. The authors also thank Prof. T. Zhang for the assistance of corrosion test and analysis.
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induced by the formation of dense passive film in nanocrystalline layer, resulting in that the impedance slightly increased with immersion time. However, after immersion for 360 h, the passive film was penetrated completely by the electrolyte, resulting in the sudden decrease of impedance. Compared with single MAO coating, SMAT-MAO coating shows a better and stable corrosion resistance. While anodizing in chromium baths is the traditional protecting method of 2024 Al alloy used in aeronautical applications, the corrosion resistance of substrate alloy has been enhanced by chromium anodizing coating [24]. Whether the SMAT-MAO coating could be an alternative to chromium anodizing or not needs to be further studied.
[1] R.C. Barik, J.A. Wharton, R.J.K. Wood, K.R. Stokes, R.L. Jones, Surf. Coat. Technol. 199 (2005) 158–167. [2] C.-E. Barchiche, E. Rocca, C. Juers, J. Hazan, J. Steinmetz, Electrochim. Acta 53 (2007) 417–425. [3] C.-E. Barchiche, E. Rocca, J. Hzan, Surf. Coat. Technol. 202 (2008) 4145–4152. [4] H. Duan, K. Du, C. Yan, F. Wang, Electrochim. Acta 51 (2006) 2898–2908. [5] H. Duan, C. Yan, F. Wang, Electrochim. Acta 52 (2007) 3785–3793. [6] J. Jiang, P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 51 (2009) 2483– 2492. [7] D.Y. Hwang, Y.M. Kim, D.Y. Park, B. Yoo, D.H. Shin, Electrochim. Acta 54 (2009) 5479–5485. [8] J. Jiang, P. Bala Srinivasan, C. Blawert, M. Stormer, W. Dietzel, Electrochim. Acta 54 (2009) 3842–3850. [9] J. Jiang, P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 52 (2010) 540– 547. [10] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corros. Sci. 50 (2008) 1744–1752. [11] P. Bala Srinivasan, C. Blawert, W. Dietzel, Corros. Sci. 50 (2008) 2415–2418. [12] B. Rajasekaran, S. Ganesh Sundara Raman, S.V. Joshi, G. Sundararajan, Int. J. Fatigue 30 (2008) 1259–1266. [13] B. Rajasekaran, S. Ganesh Sundara Raman, L. Rama Krishna, S.V. Joshi, G. Sundararajan, Surf. Coat. Technol. 202 (2008) 1462–1469. [14] B. Lonyuk, I. Apachitei, J. Duszczyk, Surf. Coat. Technol. 201 (2007) 8688–8694. [15] D.T. Asquith, A.L. Yerokhin, J.R. Yates, A. Matthews, Thin Solid Films 515 (2006) 1187–1191. [16] K. Lu, J. Lu, J. Mater. Sci. Technol. 15 (1999) 193–197. [17] N.R. Tao, Z.B. Wang, W.P. Tong, M.L. Sui, J. Lu, K. Lu, Acta Mater. 50 (2002) 4603–4616. [18] V. Moutarlier, M.P. Gigandet, B. Normand, J. Pagetti, Corros. Sci. 47 (2005) 937– 951. [19] C. Liu, Q. Bi, A. Leyland, A. Matthews, Corros. Sci. 45 (2003) 1257–1273. [20] V. Afshari, C. Dehghanian, Corros. Sci. 51 (2009) 1844–1849. [21] L. Liu, Y. Li, F.H. Wang, Electrochim. Acta 53 (2008) 2453–2462. [22] S.J. Xia, R. Yue, R.G. Rateick Jr., V.I. Birss, J. Electrochem. Soc. 151 (3) (2004) B179–B187. [23] H. Yasuda, Q.S. Yu, M. Chen, Prog. Org. Coat. 41 (2001) 273–279. [24] Y.L. Huang, H. Shih, H.C. Huang, J. Daugherty, S. Wu, S. Ramanathan, C. Chang, F. Mansfeld, Corros. Sci. 50 (2008) 3569–3575.