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Enhanced corrosion properties of pure Mg and AZ31 Mg alloy recycled by solid-state process
Materials Science and Engineering A 435–436 (2006) 275–281
Enhanced corrosion properties of pure Mg and AZ31 Mg alloy recycled by solid-state process Yasumasa Chino a,1 , Tetsuji Hoshika a,1 , Mamoru Mabuchi b,∗ a
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan b Department of Energy Science & Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 14 April 2006; received in revised form 26 June 2006; accepted 7 July 2006
Abstract The corrosion behaviors of pure Mg and AZ31 Mg alloy recycled by a solid-state process were investigated by salt (5 wt.% NaCl solution) immersion tests, and were compared with those of an ingot reference and an extrusion reference subjected to the same deformation history. The recycled specimen possessed superior corrosion resistance compared with the reference ones, contrary to anticipated apprehension. The enhancement of corrosion resistance for the recycled specimens was attributed to the presence of dense oxide contaminants which were distributed parallel to the extrusion direction. The addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. This suggests that the capacity of the oxides as corrosion barriers depends their elemental content. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Recycling; Extrusion; Corrosion; Oxide contaminant
1. Introduction Mg alloys exhibit high specific strength and stiffness, and their products have been applied for structural uses such as automobile parts and electric appliance cases [1,2]. To promote the contribution of Mg alloys to reducing the environmental load, it is necessary to develop useful recycling processes. To date, some recycling processes such as remelting [3,4] have been proposed and applied. However, the recycled Mg alloys often exhibit poorer service properties than the virgin ones because of contamination. Recently, recycling by a solid-state process, that is, solidstate recycling, has been proposed as a new method for Mg alloy scraps [5–10]. In solid-state recycling, Mg scraps such as machined chips are directly recycled by hot extrusion without remelting. Because microstructural control such as grain refinement and dispersion of contaminants can be achieved due
∗
Corresponding author. Tel.: +81 75 753 5404; fax: +81 75 753 5428. E-mail addresses: [email protected] (T. Hoshika), [email protected] (M. Mabuchi). 1 Tel.: +81 52 736 7461; fax: +81 52 736 7406. 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.019
to severe deformation during recycling, the recycled materials show excellent mechanical properties of high strength [5], ductility [5], and superplasticity [9]. Recently, it has been reported that the AZ31 Mg alloy recycled by a solid-state process with severe deformation exhibited higher strength, due to particledispersion strengthening by oxide contaminants, than a reference specimen which was subjected to the same deformation history [10]. Mg possesses the lowest standard potential among all engineering metals, and Mg and its alloys generally show poor corrosion resistance. Recently, it has been reported [11–14] that rapid solidification or powder metallurgy processing improves not only the mechanical properties, but also the corrosion resistance of Mg alloys due to microstructural control such as grain refinement. For solid-state recycling as well, because microstructural control of grain refinement is possible, solid-state-recycled Mg is expected to show superior corrosion resistance to a remeltingrecycled Mg. However, if the presence of contaminants has a detrimental influence on corrosion resistance, promotion of recycling will diminish. This report shows that recycled pure Mg and its alloy possess superior corrosion resistance compared with virgin specimens due to the presence of oxide contaminants from the scraps, con-
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trary to the anticipated apprehension. This finding will contribute to the realization of up-grade recycling. 2. Experimental procedure Machined chips of AZ31 Mg alloy (Mg–3 mass%Al– 1 mass%Zn–0.5 mass%Mn) and pure Mg were prepared. The machined chips of AZ31 Mg alloy are shown in Fig. 1. The average dimensions of the AZ31 Mg alloy and pure Mg chips were 12 mm × 2 mm × 0.1 mm and 12 mm × 2 mm × 0.3 mm, respectively. The machined chips were filled into a cylindrical container with a diameter of 40 mm and then extruded at 673 K at an extrusion ratio of 45:1 in air. Part of the extruded bar was annealed at 673 K for 30 min for grain growth. For corrosion testing, cylindrical specimens with a diameter of 6 mm and a length of 40 mm were used. For comparison, reference specimens were fabricated from a virgin ingot and from an extrusion from the virgin ingot, which was subjected to hot deformation under the same conditions as the recycled specimen. The specimens were abraded with grit 1000 SiC paper and degreased in ethanol. Salt immersion tests [15] were then performed for 3 days in 5 wt.% NaCl solution saturated with Mg(OH)2 , whose pH was 10.0, exposed to the ambient laboratory temperature. After testing, the corrosion products were stripped in boiling chromic acid solution (110 g CrO3 /L water), and the corrosion rate was determined from measurements of weight loss. Microstructures of the specimens were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Oxygen concentra-
Fig. 1. Machined chips of AZ31 Mg alloy.
tion in the specimens was measured by glow discharge mass spectrometry (GDMS), and concentrations of the other elements were detected by inductively coupled plasma atomic emission spectrometry (ICP-AES). Oxygen and aluminum distributions were analyzed by electron probe microanalyzer (EPMA) and energy-dispersive X-ray spectroscopy (EDS). 3. Results and discussion Microstructures of the AZ31 Mg alloy specimens are shown in Fig. 2, where (a) is the recycled specimen, (b) is the ingot reference specimen, and (c) is the extruded reference specimen.
Fig. 2. Microstructures of AZ31 Mg alloy specimens: (a) recycled specimen, (b) ingot reference specimen, and (c) extruded reference specimen.
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Fig. 3. Oxide mapping by EPMA of recycled AZ31 Mg alloy.
The observed plane is perpendicular to the extrusion direction. The grain size was 7.8 m for the recycled specimen, 85.5 m for the ingot reference specimen, and 13.3 m for the extruded reference specimen. Enhancement of grain refinement for the recycled specimen is likely to be attributed to both dynamic recrystallization during hot deformation [16,17] and suppression of grain growth by oxide contaminants [10]. The observed grain size of pure Mg was 67.9 m for the recycled specimen, 2.42 mm for the ingot reference specimen and 101 m for the reference specimen. Oxide mapping by EPMA of the recycled AZ31 Mg alloy is shown in Fig. 3, where the observed plane is parallel to the extrusion direction. It is found that oxides were distributed parallel to the extrusion direction. The distance between the oxide lines was 10–20 m. Such oxides distributed parallel to the extrusion direction were not observed in the reference specimens. The oxides were probably mixed in from the surfaces of machined chips and were rearranged parallel to the extrusion direction during hot extrusion [10]. A transmission electron micrograph of the recycled AZ31 Mg alloy is shown in Fig. 4. Nordlien et al. [18] reported that
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the thickness of surface films formed naturally on pure Mg was about 20–50 nm. However, the oxide layers observed in Fig. 4 were much thicker than these from their observation. This may be attributed to the exposure of machined chips to hightemperature air during hot extrusion. Side surfaces of the AZ31 Mg alloy after 3 days of immersion are shown in Fig. 5, where (a) is the recycled specimen, (b) is the ingot reference specimen, and (c) is the extruded reference specimen. It should be noted that there was less damage at surfaces attacked by salt immersion for the recycled specimen compared with the reference specimens. Clearly, the recycled specimen exhibited superior corrosion resistance to the reference specimens. The corrosion rates, which are determined from weight loss after the 3 day immersion tests, for AZ31 Mg alloy and pure Mg are listed in Table 1. The corrosion rate of the recycled specimen was lower than those of the reference specimens for both pure Mg and AZ31 Mg alloy, indicating that enhancement of corrosion resistance occurs by solid-state recycling, independent of alloy composition. On the whole, the corrosion rates for AZ31 Mg alloys were lower than those for pure Mg. Additionally, the ratio of the corrosion rate of a recycled specimen to that of the extruded reference specimen for AZ31 Mg alloy (=52%) was lower than that for pure Mg (=64%), in which the corrosion ratio of AZ31 recycled specimen was the average value of the non-annealed specimen and the annealed specimen. Thus, addition of Al element accelerated the enhancement of corrosion resistance by solid-state recycling. Corrosion behavior depends on metallurgical factors such as the alloy content, other phase components, precipitates, segregation of alloying elements, impurities and grain size. Grain boundary phases are invariably cathodic compared with the grain interior, and no corrosion penetrates inwardly along the grain boundaries for Mg [19]. Song et al. [20] noted that the surface composition changes during corrosion and that networks of corrosion barriers tend to be formed as the grain size decreases. Therefore, one of possible reasons for the high corrosion resistance of the recycled specimen is grain refinement [21]. However, enhancement of corrosion resistance by grain refinement was not significant, as shown in Table 1. For example, although the AZ31 recycled specimen after annealing at 673 K had the larger grain size than the AZ31 extrusion reference specimen, the corrosion rate of the former was lower than that of Table 1 The corrosion rate and grain size for pure Mg and AZ31 Mg alloy Material AZ31 recycled specimen AZ31 recycled specimen (annealed at 673 K) AZ31 ingot reference specimen AZ31 extrusion reference specimen
Fig. 4. Transmission electron micrograph of recycled AZ31 Mg alloy.
Pure Mg recycled specimen Pure Mg ingot reference specimen Pure Mg extrusion reference specimen
Corrosion rate (mm/year)
Grain size (m)
4.1 3.5 8.4 7.3
7.8 16.1 85.5 13.3
35.7 150 67.9
67.9 2420 101
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Fig. 5. Side surfaces of AZ31 Mg alloy specimens after immersion for 3 days: (a) recycled specimen, (b) ingot reference specimen, and (c) extruded reference specimen.
the latter. Thus, the superior corrosion resistance for the recycled specimens cannot be explained only as the effect of grain refinement. The chemical compositions of the recycled specimen and the extrusion reference specimen for AZ31 Mg alloy are listed in Table 2. For reference, the data of JIS standard AZ31B [22] is also given in Table 2. It is known that metals with low hydrogen overvoltage constitute efficient cathodes and cause severe galvanic corrosion of Mg. For example, impurities of Fe, Ni, Cu and Co significantly accelerate corrosion even at concentrations less than 0.2% [23]. One of the candidate contaminants during solidstate recycling is Fe because of the invasion of Fe from containers during extrusion. The Fe concentrations of both specimens were lower than that of the JIS standard (=0.005%), although the Fe concentration of the recycled specimen was slightly higher than that of the reference specimen. Thus, the influence of Fe impurity was negligible. On the other hand, the oxygen concentration of the recycled specimen was two orders of magnitude higher than that of the reference specimen. Therefore, it is suggested that the superior corrosion resistance of the recycled specimens
is related to the presence of oxides distributed parallel to the extrusion direction. SEI and oxygen images from EPMA at the specimen surface after the salt immersion tests are shown in Fig. 6. In the SEI images, the white areas are the portions where ongoing corrosion is suppressed. It is of interest to note that the white areas did not always coincide with the areas of high oxygen concentration for the reference specimen. The oxide layer at the surface of Mg is often porous, because the Mg/MgO system is associated with a relatively low Pilling–Bedworth ratio [24,25]. Nordlien et al. [18] investigated the surface films formed naturally on pure Mg and reported that the surface films consisted of a thin (20–50 nm) oxide layer with an amorphous structure. This result indicates that a Mg oxide surface formed in air at room temperature does not always exhibit high corrosion resistance because of its thin and porous structure. The fact that the white areas do not coincide with the areas of high oxygen concentration for the reference specimen suggests that the local corrosion resistance capacity depends on substrate conditions such as grain boundaries and other phases for the reference specimen.
Table 2 Chemical compositions (mass%) of recycled specimen and extruded reference specimen for A231 Mg alloy Alloy
Al
Zn
Mn
Si
Cu
Ni
Fe
O
AZ31 (JISH4204) Recycled specimen Extruded reference specimen
2.4–3.6 2.89 2.89
0.5–1.5 0.88 0.87
≥0.15 0.37 0.39
≤0.10 0.0045 0.0075
≤0.05 0.0017 0.0026
≤0.005 0.0001 0.0004
≤0.005 0.0026 0.0009
– 878 ppm 7.1 ppm
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Fig. 6. SEI and oxygen images by EPMA at specimen surface after salt immersion tests.
For the recycled specimen, however, the white areas coincided with the area of high oxygen concentration, demonstrating that the presence of oxides enhances the corrosion resistance for the recycled specimen. Besides, interval of the white areas was about 10–20 m, which well agreed with interval of the oxide layers in the recycled specimen as shown in Fig. 3. Therefore, it is likely that the superior corrosion resistance for the recycled specimen is attributed to the oxide contaminants which come from the surfaces of the scraps.
18 wt.% at equilibrium [18]. However, the oxygen concentration for the recycled specimen was only 0.1%, as shown in Table 2. It should be pointed out that such a small oxide content played a vital role as a corrosion barrier. The oxides were distributed parallel to the extrusion direction, indicating that networks of tube-like oxide layers cover the Mg matrix, as illustrated in Fig. 7. Such networks of tube-like oxide layers effectively serve as a corrosion barrier even for small contents of oxides.
4. Discussion It is known that other phases strongly affect the corrosion resistance of Mg-based materials [26,27]. For Mg–Al alloys, the -phase serves as a galvanic cathode and accelerates the corrosion process of the ␣-phase for a low volume fraction of the -phase [26,28]. However, the oxides do not serve as a galvanic cathode even at a low volume fraction. As shown in Figs. 3 and 4, the thick oxide contaminants in the recycled specimen were distributed continuously. Therefore, the oxides in the recycled specimens are considered to act effectively as a corrosion barrier. On the other hand, it has been reported that when the volume fraction of the -phase is large, the -phase acts as an anodic barrier to inhibit the overall corrosion because networks of the -phase are formed [12,18,29,30]. The fraction of the -phase is
Fig. 7. Schematic illustration of networks of tube-like oxide layers covering Mg matrix.
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The previous studies [31,32] revealed that oxide concentration in the recycled specimen is closely related to shape of recycled machined chips. The oxygen concentration increased with the total surface area of the machined chips in the recycled specimen, indicating that the size of the machined chips is one of the important factors in the control of the oxide concentration level. It would be easy to image that large oxide concentration contributes to high corrosion resistance of the recycled specimen because interval of networks of tube-like oxide layers becomes narrower. On the other hand, the previous studies [9,31,32] also revealed that introduction of large amount of oxide layer deteriorates low elongation at elevated temperature. The deterioration in elongation is caused by excessive cavity nucleation around oxide layer. The tradeoff relationship between corrosion resistance and formability should be considered for practical use of the recycled Mg alloy. Another important result in the corrosion investigation for the recycled specimen is that the addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. EDS analyses of aluminum and oxygen around the oxide in the recycled AZ31 Mg alloy specimen are shown in Fig. 8(a and b), respectively. It can be seen that the oxide had a higher aluminum concentration than the Mg matrix. Nordlien et al. [33] showed that alumina components in a Mg–Al alloy forms a continuous skeletal structure in a oxide layer, such that the oxide properties become more predominantly determined by the properties of the alumina component, when aluminum contents in Mg alloys exceed about 4%. They also suggested that the alumina oxide layers are regarded to have much better passivating properties than Mg(OH)2 and MgO layers. On the other hand, the consumption of aluminum by oxide formation in the recycled specimen would reduce the total amount of a solute aluminum element, which should decrease corrosion resistance of the recycled specimen [19]. However, the amount of aluminum consumption should be negligible level because of small amount of oxygen (less than 0.1 mass%) in the
Fig. 8. EDS analyses of aluminum (a) and oxygen (b) around oxide in recycled AZ31 Mg alloy specimen.
recycled specimen. Therefore, alumina formation in the oxide layer is probably responsible for the enhancement of corrosion resistance by addition of aluminum. 5. Summary The corrosion behaviors of pure Mg and AZ31 Mg alloy recycled by a solid-state process were compared with those of an ingot reference and an extrusion reference subjected to the same processing history. Salt immersion tests showed that the recycled specimen possesses superior corrosion resistance to the references. The enhancement of corrosion resistance for the recycled specimens is likely attributed to networks of thick oxide layers parallel to the extrusion direction. Another important result for the recycled specimen is that addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. This suggests that the capacity of the oxides as corrosion barriers depends on their elemental content. The recycled Mg and its alloy fabricated by solid-state processing exhibited not only better mechanical properties [10] but also superior corrosion resistance compared with the virgin specimens. This finding may lead to the realization of up-grade recycling. References [1] T. Ebert, B.L. Mordike, Mater. Sci. Eng. A302 (2001) 37–45. [2] Y. Nishikawa, A. Takara, Mater. Sci. Forum. 426–432 (2003) 569–574. [3] M. Inoue, M. Iwai, S. Kamado, Y. Kojima, T. Itoh, M. Sudama, J. Jpn. Inst. Light Met. 49 (1999) 277–281. [4] J.F. King, A. Hopkins, S. Thistlethwaite, in: G.W. Lorimer (Ed.), Proceedings of Third International Magnesium Conference, The University Press Cambridge, Cambridge, 1997, pp. 51–61. [5] M. Mabuchi, K. Kubota, K. Higashi, Mater. Trans., JIM 36 (1995) 1249–1254. [6] Y. Chino, K. Kishihara, K. Shimojima, Y. Yamada, C.E. Wen, H. Iwasaki, M. Mabuchi, J. Jpn. Inst. Met. 65 (2001) 621–626. [7] H. Watanabe, K. Moriwaki, T. Mukai, K. Ishikawa, M. Kohzu, K. Higashi, J. Mater. Sci. 36 (2001) 5007–5011. [8] K. Kondoh, T. Luangvaranunt, T. Aizawa, J. Jpn. Inst. Light Met. 51 (2001) 516–520. [9] Y. Chino, K. Kishihara, K. Shimojima, H. Hosokawa, Y. Yamada, C.E. Wen, H. Iwasaki, M. Mabuchi, Mater. Trans. 43 (2002) 2437–2442. [10] Y. Chino, T. Hoshika, J.S. Lee, M. Mabuchi, J. Mater. Res. 21 (2006) 754–760. [11] G. Neite, K. Kubota, K. Higashi, F. Hehmann, in: R.W. Cahn, P. Haasen, E.J. Kramer, K.H. Matucha (Eds.), Materials Science and Technology, Structure and Properities of Nonferrous Alloys, vol. 8, VCH, Weinheim, 1996, pp. 113–212. [12] H. Alves, U. Koster, E. Aghion, D. Eliezer, Mater. Technol. 16 (2001) 110–126. [13] D. Daloz, P. Steinmetz, G. Michot, Corrosion 53 (1997) 944–954. [14] K.S.N. Govind, M.C. Mittal, K. Lal, R.K. Mahanti, C.S. Sivaramakrishnan, Mater. Sci. Eng. A 304–306 (2001) 520–523. [15] Japan Standard Association (Ed.), Method of Alkaline Salt Corrosion Testing for Magnesium and Magnesium Alloys, Japan Standard Association, Tokyo, 2003 (JIS H0541). [16] A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Mater. 49 (2001) 1199–1207. [17] M.T. P˙erez-Prado, J.A. del Valle, J.M. Contreras, O.A. Ruano, Scripta Mater. 50 (2004) 661–665. [18] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corros. Sci. 39 (1997) 1397–1414.
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[27] S. Mathieu, C. Rapin, J. Hazan, P. Steinmetz, Corros. Sci. 44 (2002) 2737–2756. [28] R.K.S. Raman, Metall. Mater. Trans. A 35 (2004) 2527–2533. [29] O. Lunder, J.E. Lein, T.K. Aune, K. Nisancioglu, Corrosion 45 (1989) 741–748. [30] G. Song, A. Atrens, M. Dargusch, Corros. Sci. 41 (1999) 249–273. [31] Y. Chino, M. Mabuchi, M. Kobata, K. Shimojima, H. Hosokawa, Y. Yamada, H. Iwasaki, Mater. Trans. 45 (2004) 361–364. [32] Y. Chino, M. Mabuchi, H. Iwasaki, A. Yamamoto, H. Tsubakino, Mater. Trans. 45 (2004) 2509–2515. [33] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 143 (1996) 2564–2572.
Enhanced corrosion properties of pure Mg and AZ31 Mg alloy recycled by solid-state process Yasumasa Chino a,1 , Tetsuji Hoshika a,1 , Mamoru Mabuchi b,∗ a
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan b Department of Energy Science & Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 14 April 2006; received in revised form 26 June 2006; accepted 7 July 2006
Abstract The corrosion behaviors of pure Mg and AZ31 Mg alloy recycled by a solid-state process were investigated by salt (5 wt.% NaCl solution) immersion tests, and were compared with those of an ingot reference and an extrusion reference subjected to the same deformation history. The recycled specimen possessed superior corrosion resistance compared with the reference ones, contrary to anticipated apprehension. The enhancement of corrosion resistance for the recycled specimens was attributed to the presence of dense oxide contaminants which were distributed parallel to the extrusion direction. The addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. This suggests that the capacity of the oxides as corrosion barriers depends their elemental content. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Recycling; Extrusion; Corrosion; Oxide contaminant
1. Introduction Mg alloys exhibit high specific strength and stiffness, and their products have been applied for structural uses such as automobile parts and electric appliance cases [1,2]. To promote the contribution of Mg alloys to reducing the environmental load, it is necessary to develop useful recycling processes. To date, some recycling processes such as remelting [3,4] have been proposed and applied. However, the recycled Mg alloys often exhibit poorer service properties than the virgin ones because of contamination. Recently, recycling by a solid-state process, that is, solidstate recycling, has been proposed as a new method for Mg alloy scraps [5–10]. In solid-state recycling, Mg scraps such as machined chips are directly recycled by hot extrusion without remelting. Because microstructural control such as grain refinement and dispersion of contaminants can be achieved due
∗
Corresponding author. Tel.: +81 75 753 5404; fax: +81 75 753 5428. E-mail addresses: [email protected] (T. Hoshika), [email protected] (M. Mabuchi). 1 Tel.: +81 52 736 7461; fax: +81 52 736 7406. 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.019
to severe deformation during recycling, the recycled materials show excellent mechanical properties of high strength [5], ductility [5], and superplasticity [9]. Recently, it has been reported that the AZ31 Mg alloy recycled by a solid-state process with severe deformation exhibited higher strength, due to particledispersion strengthening by oxide contaminants, than a reference specimen which was subjected to the same deformation history [10]. Mg possesses the lowest standard potential among all engineering metals, and Mg and its alloys generally show poor corrosion resistance. Recently, it has been reported [11–14] that rapid solidification or powder metallurgy processing improves not only the mechanical properties, but also the corrosion resistance of Mg alloys due to microstructural control such as grain refinement. For solid-state recycling as well, because microstructural control of grain refinement is possible, solid-state-recycled Mg is expected to show superior corrosion resistance to a remeltingrecycled Mg. However, if the presence of contaminants has a detrimental influence on corrosion resistance, promotion of recycling will diminish. This report shows that recycled pure Mg and its alloy possess superior corrosion resistance compared with virgin specimens due to the presence of oxide contaminants from the scraps, con-
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trary to the anticipated apprehension. This finding will contribute to the realization of up-grade recycling. 2. Experimental procedure Machined chips of AZ31 Mg alloy (Mg–3 mass%Al– 1 mass%Zn–0.5 mass%Mn) and pure Mg were prepared. The machined chips of AZ31 Mg alloy are shown in Fig. 1. The average dimensions of the AZ31 Mg alloy and pure Mg chips were 12 mm × 2 mm × 0.1 mm and 12 mm × 2 mm × 0.3 mm, respectively. The machined chips were filled into a cylindrical container with a diameter of 40 mm and then extruded at 673 K at an extrusion ratio of 45:1 in air. Part of the extruded bar was annealed at 673 K for 30 min for grain growth. For corrosion testing, cylindrical specimens with a diameter of 6 mm and a length of 40 mm were used. For comparison, reference specimens were fabricated from a virgin ingot and from an extrusion from the virgin ingot, which was subjected to hot deformation under the same conditions as the recycled specimen. The specimens were abraded with grit 1000 SiC paper and degreased in ethanol. Salt immersion tests [15] were then performed for 3 days in 5 wt.% NaCl solution saturated with Mg(OH)2 , whose pH was 10.0, exposed to the ambient laboratory temperature. After testing, the corrosion products were stripped in boiling chromic acid solution (110 g CrO3 /L water), and the corrosion rate was determined from measurements of weight loss. Microstructures of the specimens were observed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Oxygen concentra-
Fig. 1. Machined chips of AZ31 Mg alloy.
tion in the specimens was measured by glow discharge mass spectrometry (GDMS), and concentrations of the other elements were detected by inductively coupled plasma atomic emission spectrometry (ICP-AES). Oxygen and aluminum distributions were analyzed by electron probe microanalyzer (EPMA) and energy-dispersive X-ray spectroscopy (EDS). 3. Results and discussion Microstructures of the AZ31 Mg alloy specimens are shown in Fig. 2, where (a) is the recycled specimen, (b) is the ingot reference specimen, and (c) is the extruded reference specimen.
Fig. 2. Microstructures of AZ31 Mg alloy specimens: (a) recycled specimen, (b) ingot reference specimen, and (c) extruded reference specimen.
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Fig. 3. Oxide mapping by EPMA of recycled AZ31 Mg alloy.
The observed plane is perpendicular to the extrusion direction. The grain size was 7.8 m for the recycled specimen, 85.5 m for the ingot reference specimen, and 13.3 m for the extruded reference specimen. Enhancement of grain refinement for the recycled specimen is likely to be attributed to both dynamic recrystallization during hot deformation [16,17] and suppression of grain growth by oxide contaminants [10]. The observed grain size of pure Mg was 67.9 m for the recycled specimen, 2.42 mm for the ingot reference specimen and 101 m for the reference specimen. Oxide mapping by EPMA of the recycled AZ31 Mg alloy is shown in Fig. 3, where the observed plane is parallel to the extrusion direction. It is found that oxides were distributed parallel to the extrusion direction. The distance between the oxide lines was 10–20 m. Such oxides distributed parallel to the extrusion direction were not observed in the reference specimens. The oxides were probably mixed in from the surfaces of machined chips and were rearranged parallel to the extrusion direction during hot extrusion [10]. A transmission electron micrograph of the recycled AZ31 Mg alloy is shown in Fig. 4. Nordlien et al. [18] reported that
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the thickness of surface films formed naturally on pure Mg was about 20–50 nm. However, the oxide layers observed in Fig. 4 were much thicker than these from their observation. This may be attributed to the exposure of machined chips to hightemperature air during hot extrusion. Side surfaces of the AZ31 Mg alloy after 3 days of immersion are shown in Fig. 5, where (a) is the recycled specimen, (b) is the ingot reference specimen, and (c) is the extruded reference specimen. It should be noted that there was less damage at surfaces attacked by salt immersion for the recycled specimen compared with the reference specimens. Clearly, the recycled specimen exhibited superior corrosion resistance to the reference specimens. The corrosion rates, which are determined from weight loss after the 3 day immersion tests, for AZ31 Mg alloy and pure Mg are listed in Table 1. The corrosion rate of the recycled specimen was lower than those of the reference specimens for both pure Mg and AZ31 Mg alloy, indicating that enhancement of corrosion resistance occurs by solid-state recycling, independent of alloy composition. On the whole, the corrosion rates for AZ31 Mg alloys were lower than those for pure Mg. Additionally, the ratio of the corrosion rate of a recycled specimen to that of the extruded reference specimen for AZ31 Mg alloy (=52%) was lower than that for pure Mg (=64%), in which the corrosion ratio of AZ31 recycled specimen was the average value of the non-annealed specimen and the annealed specimen. Thus, addition of Al element accelerated the enhancement of corrosion resistance by solid-state recycling. Corrosion behavior depends on metallurgical factors such as the alloy content, other phase components, precipitates, segregation of alloying elements, impurities and grain size. Grain boundary phases are invariably cathodic compared with the grain interior, and no corrosion penetrates inwardly along the grain boundaries for Mg [19]. Song et al. [20] noted that the surface composition changes during corrosion and that networks of corrosion barriers tend to be formed as the grain size decreases. Therefore, one of possible reasons for the high corrosion resistance of the recycled specimen is grain refinement [21]. However, enhancement of corrosion resistance by grain refinement was not significant, as shown in Table 1. For example, although the AZ31 recycled specimen after annealing at 673 K had the larger grain size than the AZ31 extrusion reference specimen, the corrosion rate of the former was lower than that of Table 1 The corrosion rate and grain size for pure Mg and AZ31 Mg alloy Material AZ31 recycled specimen AZ31 recycled specimen (annealed at 673 K) AZ31 ingot reference specimen AZ31 extrusion reference specimen
Fig. 4. Transmission electron micrograph of recycled AZ31 Mg alloy.
Pure Mg recycled specimen Pure Mg ingot reference specimen Pure Mg extrusion reference specimen
Corrosion rate (mm/year)
Grain size (m)
4.1 3.5 8.4 7.3
7.8 16.1 85.5 13.3
35.7 150 67.9
67.9 2420 101
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Fig. 5. Side surfaces of AZ31 Mg alloy specimens after immersion for 3 days: (a) recycled specimen, (b) ingot reference specimen, and (c) extruded reference specimen.
the latter. Thus, the superior corrosion resistance for the recycled specimens cannot be explained only as the effect of grain refinement. The chemical compositions of the recycled specimen and the extrusion reference specimen for AZ31 Mg alloy are listed in Table 2. For reference, the data of JIS standard AZ31B [22] is also given in Table 2. It is known that metals with low hydrogen overvoltage constitute efficient cathodes and cause severe galvanic corrosion of Mg. For example, impurities of Fe, Ni, Cu and Co significantly accelerate corrosion even at concentrations less than 0.2% [23]. One of the candidate contaminants during solidstate recycling is Fe because of the invasion of Fe from containers during extrusion. The Fe concentrations of both specimens were lower than that of the JIS standard (=0.005%), although the Fe concentration of the recycled specimen was slightly higher than that of the reference specimen. Thus, the influence of Fe impurity was negligible. On the other hand, the oxygen concentration of the recycled specimen was two orders of magnitude higher than that of the reference specimen. Therefore, it is suggested that the superior corrosion resistance of the recycled specimens
is related to the presence of oxides distributed parallel to the extrusion direction. SEI and oxygen images from EPMA at the specimen surface after the salt immersion tests are shown in Fig. 6. In the SEI images, the white areas are the portions where ongoing corrosion is suppressed. It is of interest to note that the white areas did not always coincide with the areas of high oxygen concentration for the reference specimen. The oxide layer at the surface of Mg is often porous, because the Mg/MgO system is associated with a relatively low Pilling–Bedworth ratio [24,25]. Nordlien et al. [18] investigated the surface films formed naturally on pure Mg and reported that the surface films consisted of a thin (20–50 nm) oxide layer with an amorphous structure. This result indicates that a Mg oxide surface formed in air at room temperature does not always exhibit high corrosion resistance because of its thin and porous structure. The fact that the white areas do not coincide with the areas of high oxygen concentration for the reference specimen suggests that the local corrosion resistance capacity depends on substrate conditions such as grain boundaries and other phases for the reference specimen.
Table 2 Chemical compositions (mass%) of recycled specimen and extruded reference specimen for A231 Mg alloy Alloy
Al
Zn
Mn
Si
Cu
Ni
Fe
O
AZ31 (JISH4204) Recycled specimen Extruded reference specimen
2.4–3.6 2.89 2.89
0.5–1.5 0.88 0.87
≥0.15 0.37 0.39
≤0.10 0.0045 0.0075
≤0.05 0.0017 0.0026
≤0.005 0.0001 0.0004
≤0.005 0.0026 0.0009
– 878 ppm 7.1 ppm
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Fig. 6. SEI and oxygen images by EPMA at specimen surface after salt immersion tests.
For the recycled specimen, however, the white areas coincided with the area of high oxygen concentration, demonstrating that the presence of oxides enhances the corrosion resistance for the recycled specimen. Besides, interval of the white areas was about 10–20 m, which well agreed with interval of the oxide layers in the recycled specimen as shown in Fig. 3. Therefore, it is likely that the superior corrosion resistance for the recycled specimen is attributed to the oxide contaminants which come from the surfaces of the scraps.
18 wt.% at equilibrium [18]. However, the oxygen concentration for the recycled specimen was only 0.1%, as shown in Table 2. It should be pointed out that such a small oxide content played a vital role as a corrosion barrier. The oxides were distributed parallel to the extrusion direction, indicating that networks of tube-like oxide layers cover the Mg matrix, as illustrated in Fig. 7. Such networks of tube-like oxide layers effectively serve as a corrosion barrier even for small contents of oxides.
4. Discussion It is known that other phases strongly affect the corrosion resistance of Mg-based materials [26,27]. For Mg–Al alloys, the -phase serves as a galvanic cathode and accelerates the corrosion process of the ␣-phase for a low volume fraction of the -phase [26,28]. However, the oxides do not serve as a galvanic cathode even at a low volume fraction. As shown in Figs. 3 and 4, the thick oxide contaminants in the recycled specimen were distributed continuously. Therefore, the oxides in the recycled specimens are considered to act effectively as a corrosion barrier. On the other hand, it has been reported that when the volume fraction of the -phase is large, the -phase acts as an anodic barrier to inhibit the overall corrosion because networks of the -phase are formed [12,18,29,30]. The fraction of the -phase is
Fig. 7. Schematic illustration of networks of tube-like oxide layers covering Mg matrix.
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The previous studies [31,32] revealed that oxide concentration in the recycled specimen is closely related to shape of recycled machined chips. The oxygen concentration increased with the total surface area of the machined chips in the recycled specimen, indicating that the size of the machined chips is one of the important factors in the control of the oxide concentration level. It would be easy to image that large oxide concentration contributes to high corrosion resistance of the recycled specimen because interval of networks of tube-like oxide layers becomes narrower. On the other hand, the previous studies [9,31,32] also revealed that introduction of large amount of oxide layer deteriorates low elongation at elevated temperature. The deterioration in elongation is caused by excessive cavity nucleation around oxide layer. The tradeoff relationship between corrosion resistance and formability should be considered for practical use of the recycled Mg alloy. Another important result in the corrosion investigation for the recycled specimen is that the addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. EDS analyses of aluminum and oxygen around the oxide in the recycled AZ31 Mg alloy specimen are shown in Fig. 8(a and b), respectively. It can be seen that the oxide had a higher aluminum concentration than the Mg matrix. Nordlien et al. [33] showed that alumina components in a Mg–Al alloy forms a continuous skeletal structure in a oxide layer, such that the oxide properties become more predominantly determined by the properties of the alumina component, when aluminum contents in Mg alloys exceed about 4%. They also suggested that the alumina oxide layers are regarded to have much better passivating properties than Mg(OH)2 and MgO layers. On the other hand, the consumption of aluminum by oxide formation in the recycled specimen would reduce the total amount of a solute aluminum element, which should decrease corrosion resistance of the recycled specimen [19]. However, the amount of aluminum consumption should be negligible level because of small amount of oxygen (less than 0.1 mass%) in the
Fig. 8. EDS analyses of aluminum (a) and oxygen (b) around oxide in recycled AZ31 Mg alloy specimen.
recycled specimen. Therefore, alumina formation in the oxide layer is probably responsible for the enhancement of corrosion resistance by addition of aluminum. 5. Summary The corrosion behaviors of pure Mg and AZ31 Mg alloy recycled by a solid-state process were compared with those of an ingot reference and an extrusion reference subjected to the same processing history. Salt immersion tests showed that the recycled specimen possesses superior corrosion resistance to the references. The enhancement of corrosion resistance for the recycled specimens is likely attributed to networks of thick oxide layers parallel to the extrusion direction. Another important result for the recycled specimen is that addition of Al accelerated the enhancement of corrosion resistance by solid-state recycling. This suggests that the capacity of the oxides as corrosion barriers depends on their elemental content. The recycled Mg and its alloy fabricated by solid-state processing exhibited not only better mechanical properties [10] but also superior corrosion resistance compared with the virgin specimens. This finding may lead to the realization of up-grade recycling. References [1] T. Ebert, B.L. Mordike, Mater. Sci. Eng. A302 (2001) 37–45. [2] Y. Nishikawa, A. Takara, Mater. Sci. Forum. 426–432 (2003) 569–574. [3] M. Inoue, M. Iwai, S. Kamado, Y. Kojima, T. Itoh, M. Sudama, J. Jpn. Inst. Light Met. 49 (1999) 277–281. [4] J.F. King, A. Hopkins, S. Thistlethwaite, in: G.W. Lorimer (Ed.), Proceedings of Third International Magnesium Conference, The University Press Cambridge, Cambridge, 1997, pp. 51–61. [5] M. Mabuchi, K. Kubota, K. Higashi, Mater. Trans., JIM 36 (1995) 1249–1254. [6] Y. Chino, K. Kishihara, K. Shimojima, Y. Yamada, C.E. Wen, H. Iwasaki, M. Mabuchi, J. Jpn. Inst. Met. 65 (2001) 621–626. [7] H. Watanabe, K. Moriwaki, T. Mukai, K. Ishikawa, M. Kohzu, K. Higashi, J. Mater. Sci. 36 (2001) 5007–5011. [8] K. Kondoh, T. Luangvaranunt, T. Aizawa, J. Jpn. Inst. Light Met. 51 (2001) 516–520. [9] Y. Chino, K. Kishihara, K. Shimojima, H. Hosokawa, Y. Yamada, C.E. Wen, H. Iwasaki, M. Mabuchi, Mater. Trans. 43 (2002) 2437–2442. [10] Y. Chino, T. Hoshika, J.S. Lee, M. Mabuchi, J. Mater. Res. 21 (2006) 754–760. [11] G. Neite, K. Kubota, K. Higashi, F. Hehmann, in: R.W. Cahn, P. Haasen, E.J. Kramer, K.H. Matucha (Eds.), Materials Science and Technology, Structure and Properities of Nonferrous Alloys, vol. 8, VCH, Weinheim, 1996, pp. 113–212. [12] H. Alves, U. Koster, E. Aghion, D. Eliezer, Mater. Technol. 16 (2001) 110–126. [13] D. Daloz, P. Steinmetz, G. Michot, Corrosion 53 (1997) 944–954. [14] K.S.N. Govind, M.C. Mittal, K. Lal, R.K. Mahanti, C.S. Sivaramakrishnan, Mater. Sci. Eng. A 304–306 (2001) 520–523. [15] Japan Standard Association (Ed.), Method of Alkaline Salt Corrosion Testing for Magnesium and Magnesium Alloys, Japan Standard Association, Tokyo, 2003 (JIS H0541). [16] A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Mater. 49 (2001) 1199–1207. [17] M.T. P˙erez-Prado, J.A. del Valle, J.M. Contreras, O.A. Ruano, Scripta Mater. 50 (2004) 661–665. [18] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corros. Sci. 39 (1997) 1397–1414.
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