镁合金微弧氧化技术下有机环保型电解液的阳极氧化膜性能

Properties of Anodic Coatings Obtained in an Organic, Environmental Electrolyte by Micro Arc Oxidation on Magnesium Alloy Rongfa Zhang1,2, a, Shufang Zhang1,2,b, Jianchao Gong2,c, Wenlong Liu 2,d, Hejing Zhou2,e, Pengfei Guo2,f and Xiaoxiao Wu2,g 1Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, 330013, China 2School of Materials Science and Engineering, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, 330013, China arfzhang-10@163.com, b24725007@qq.com, c157434786@qq.com, d1010972710@qq.com, e441686258@qq.com, f379447364@qq.com, g351459068@qq.com Keywords: Magnesium Alloy; MAO; Electrolyte; Phytic Acid; Property. Abstract. In a solution containing 10g/L NaOH and 12g/L phytic acid, anodic coatings were obtained by micro arc oxidation (MAO) on AZ91HP magnesium alloy. The morphology, structure and composition of anodic coatings were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX). The corrosion resistance of magnesium alloy before and after MAO treatment was evaluated by immersion test and potentiodynamic polarization testing in 3.5wt. % NaCl solution. The coatings were evenly formed on the substrate and mainly composed of MgO. EDX analyses showed that phytic acid took part in the coating formation. Compared with the substrate, the corrosion resistance of magnesium alloy after MAO treatment was improved considerably. Introduction MAO, developed under the traditional anodization, can efficiently improve the corrosion and wear resistance of magnesium alloy and the coating properties are mainly determined by the used electrolytes. Due to the health and environmental pressure, some environmentally friendly processes have been developed in alkaline solutions containing inorganic oxysalts especially silicate [1]. Compared with inorganic electrolytes, organic substances were seldom used in MAO [2-4]. Although organic solvents were used to obtain anodic coatings with high corrosion resistance, addition of inorganic substances into the solution was necessary to form an anodic film [5]. Phytic acid (C6H18O24P6) is first known as the storage form of phosphorus in seeds [6] and is nontoxic, biocompatible and green to the environment [7]. As an organic macromolecule compound, phytic acid consists of 24 oxygen atoms, 12 hydroxyl groups and 6 phosphate carboxyl groups. The peculiar structure of phytic acid makes it be always negatively charged over a wide pH range from pH>2.0 [7], have good conductivity and powerful chelating capability with di- and trivalent cations such as Ca 2+ , Mg 2+ , Zn 2+ , Cu 2+ , Fe 3+ . Phytic acid and its salts have been used on biotechnology [7], prevention of metal corrosion [8] and conversion coatings [9-11]. In this paper, anodic coatings were successfully obtained in an alkaline solution only containing phytic acid. The coating properties such as surface morphology, structure, composition and corrosion resistance were investigated by SEM, XRD, EDX and potentiodynamic polarization testing. Experimental set-up An ingot of AZ91HP magnesium alloy was used and the chemical composition is as follows (in wt. %): Al 8.93, Zn 0.47, Mn 0.22, Si 0.03, Cu 0.002, Ni 0.001, Fe 0.001, and Mg balance. Prior to MAO treatment, samples were polished successively on SiC paper up to 1000 grit finish, degreased by acetone, washed with distilled water and dried in a cool air stream. The used solution was 12g/L phytic acid (purity >70.0%) with 4g/L NaOH, which was added to adjust the pH. The equipment for Advanced Materials Research Vols. 189-193 (2011) pp 1001-1004 Online available since 2011/Feb/21 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.189-193.1001 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 120.203.52.15-24/02/12,03:00:47) MAO was a MAOI-50C power supply and treatment was performed under a constant current control mode. The electrical parameters were fixed as follows: current density 40mA/cm 2 , frequency 2000Hz, duty cycle 20% and treatment time 3min. The phase composition of anodic coatings was analyzed by using a D8ADVANCE X-ray diffractometer with Cu Kα radiation. Surface and cross-section morphologies of anodic coatings were observed by a scanning electron microscope (SEM QUANTA 200). Potentiodynamic polarization testing was conducted in 3.5wt. % NaCl solution using a CHI760C electrochemical workstation to evaluate the corrosion resistance. The quiet time was 120s and scan was conducted with a constant rate of 0.001V/s from initial potential of –1.7V towards more noble direction until the film breakdown occurred. Results and discussion Appearance of anodized samples. The anodized sample obtained in the solution of 10g/L NaOH is shown in Fig.1a. It was clear that no continual coatings were formed on the sample surface after MAO treatment. However, after addition of 12g/L phytic acid, the coatings were developed (Fig.1b), which indicated that phytic acid could promote the coating formation. (a) (b) Fig. 1 Pictures of the anodized samples obtained in solutions of 10g/L NaOH (a) and 10g/L NaOH with 12g/L phytic acid (b) Surface and cross-section morphologies of anodic coatings. Surface and cross-section morphologies of anodic coatings are shown in Fig 2. Anodic coatings were typically porous and the largest pore size was 1µm. The distance between the two adjacent pores was between 0.6µm to 2µm (Fig.2a). From Fig. 2b, the coating was evenly formed in different regions of the substrate and was about 6µm in thickness. (a) (b) Fig. 2 Morphologies of anodic coatings: (a) surface and (b) cross-section EDX analysis showed that anodic coatings contained C, O, Mg, Al and P. C and P came from phytic acid in the solution, which indicated that phytic acid took part in the coating formation and Epoxy resin Anodic coating Substrate 1002 Manufacturing Process Technology entered into anodic coatings. The coating compositions were as follows (in at. %): C 14.98%, O 33.35%, Mg 41.84%, Al 2.81%, P 7.02%. Phase and chemical compositions of anodic coatings. Fig.3 shows the X-ray diffraction patterns of the substrate and anodic coatings obtained by MAO treatment. Fig.3. XRD patterns of magnesium alloy before (a) and after (b) MAO treatment According to the XRD patterns, the substrate consisted of the solid solution phase of Mg (α phase) and the intermetallic compound phase Mg17Al12 (β phase). Anodic coatings were composed of MgO, Mg and Mg17Al12. The intensity of peaks corresponding to the substrate was very strong, which might be that the coatings were porous and the X-ray could penetrate into the coatings to the substrate. As an organic substance, phytic acid or its compounds did not appear in the XRD pattern. Corrosion resistance of AZ91HP before and after MAO. After immersed in 3.5wt. % NaCl solution for 24h, many, large corrosion pits were observed on the substrate (Fig. 4a). However, only one corrosion pit was developed on the edge of the sample treated in the solution containing phytic acid (Fig. 4b). These indicated that MAO could improve the corrosion resistance of magnesium alloy significantly, which was verified by potentiodynamic polarization testing in 3.5wt. % NaCl solution shown in Fig.5. (a) (b) Fig.4 The appearance of the substrate (a) and the anodized sample (b) immersed in 3.5wt.% NaCl for 24h According to Fig.5, the corrosion resistance of the samples before and after MAO was evidently different. Compared with the substrate sample, the sample after MAO has more positive corrosion potential and much lower corrosion current density. These indicated that the corrosion resistance of magnesium alloy had been improved by MAO treatment. 20 30 40 50 60 70 80 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 In te n s it y 2 θ/( ο ) Mg Mg 17 Al 12 MgO a b Advanced Materials Research Vols. 189-193 1003 Fig.5 Potentiodynamic polarization curves of magnesium alloy in 3.5 wt.% NaCl solution before and after MAO treatment In the process of MAO, anions in the solution such as phytic acid ions and OH - move to the anode under the electric field. When phytic acid ions arrive at the anode/electrolyte interface, they will combine with positively charged metal ions such as Mg 2+ , Al 3+ and Zn 2+ from the substrate metal to form chelate compounds. According to the used metal substrate, the obtained phytates are mainly as magnesium phytate. Because magnesium phytate are insoluble in water solution (approx. 4×10 -4 M) [8], the sample after MAO treatment in the solution containing phytic acid achieves excellent corrosion resistance. Conclusions Anodic coatings were obtained by MAO treatment on AZ91HP magnesium alloy in an alkaline solution containing phytic acid. The porous coatings were uniformly formed on the substrate and mainly composed of MgO. The coatings contained phosphate species, which indicated that phytic acid took part in the coating formation. After MAO treatment, the corrosion resistance of magnesium alloy was improved considerably. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51061007). References [1] L.Y. Chai, X.Yu, Z.H. Yang, Y.Y. Wang and M.Okido: Corros. Sci. Vol. 50 (2008), p. 3274 [2] H.F. Guo and M.Z. An: Thin Solid Films Vol. 500 (2006), p. 186 [3] D.Wu, X.D. Liu, K.Lu, Y.P. Zhang and H.Wang: Appl. Surf. Sci. Vol. 255 (2009), p. 7115 [4] G.X. Guo, M.Z. An, P.X. Yang, H.X. Li and C.N. Su: J. Alloys Compd. Vol. 482 (2009), p. 487 [5] A.Yabuki and M.Sakai: Corros. Sci. Vol. 51 (2009), p. 793 [6] K.Dost and O.Tokul: Anal. Chim. Acta Vol. 558 (2006), p. 22 [7] L.Z. Yang, H.Y. Liu and N.F. Hu: Electrochem. Commun. Vol. 9 (2007), p. 1057 [8] T.Notoya, V.O. Alego and D.P. Schweinsberg: Corrs. Sci. Vol. 37 (1995), p. 55 [9] J.R. Liu, Y.N. Guo and W.D. Huang: Surf. Coat. Technol. Vol. 201 (2006), p. 1536 [10] X.F. Cui, Y.Li, Q.F. Li, G.Jin, M.H. Ding and F.H. Wang: Mater. Chem. Phys. Vol. 111 (2008), p. 503 [11] F.S. Pan, X.Yang and D.F. Zhang: Appl. Surf. Sci. Vol. 255 (2009), p. 8363 -1.72-1.70-1.68-1.66-1.64-1.62-1.60-1.58-1.56-1.54-1.52-1.50-1.48-1.46-1.44-1.42-1.40 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 lo g (C u rr e n t /A /c m 2 ) Potential /V Uncoated Coated 1004 Manufacturing Process Technology Manufacturing Process Technology 10.4028/www.scientific.net/AMR.189-193 Properties of Anodic Coatings Obtained in an Organic, Environmental Electrolyte by Micro Arc Oxidation on Magnesium Alloy 10.4028/www.scientific.net/AMR.189-193.1001 DOI References [1] L.Y. Chai, X.Yu, Z.H. Yang, Y.Y. Wang and M.Okido: Corros. Sci. Vol. 50 (2008), p. 3274 doi:10.1016/j.corsci.2008.08.038 [2] H.F. Guo and M.Z. An: Thin Solid Films Vol. 500 (2006), p. 186 doi:10.1016/j.tsf.2005.11.045 [3] D.Wu, X.D. Liu, K.Lu, Y.P. Zhang and H.Wang: Appl. Surf. Sci. Vol. 255 (2009), p. 7115 doi:10.1016/j.apsusc.2009.04.157 [4] G.X. Guo, M.Z. An, P.X. Yang, H.X. Li and C.N. Su: J. Alloys Compd. Vol. 482 (2009), p. 487 doi:10.1016/j.jallcom.2009.04.053 [10] X.F. Cui, Y.Li, Q.F. Li, G.Jin, M.H. Ding and F.H. Wang: Mater. Chem. Phys. Vol. 111 (2008), p. 503 doi:10.1016/j.matchemphys.2008.05.009