Influence of Al Content on the Atmospheric Corrosion Behaviour of Magnesium-Aluminum Alloys

J. Mater. Sci. Technol., Vol.25 No.2, 2009 225 Influence of Al Content on the Atmospheric Corrosion Behaviour of Magnesium-Aluminum Alloys Ruiling Jia 1,3), Chuanwei Yan 2) and Fuhui Wang 1,2) 1) College of Materials Science & Chemistry Engineering, Key Laboratory of Superlight Materials and Surface Technology Ministry of Education, Harbin Engineering University, Harbin 150001, China 2) State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 3) College of Materials Science & Engineering, Inner Mongolia University of Technology, Hohhot 010051, China [Manuscript received September 28, 2007, in revised form December 26, 2007] The influence of Al content on the Mg-Al alloys corrosion performance during sodium chloride induced atmospheric corrosion has been studied. It was found that the corrosion rate of three Mg-Al alloys was accelerated with increasing Al content. The poor corrosion resistance was attributed to the galvanic coupling between the β phase and eutectic phase or α phase and the formation of porous corrosion products. KEY WRDS: Magnesium alloys; Al content; Atmospheric corrosion 1. Introduction In the last decades, the corrosion behavior of magnesium alloys has been investigated widely in aqueous solutions [1 9] ; therefore a better understanding of the corrosion behavior of magnesium alloys in solutions has been obtained. However, the corrosion behavior of Mg-Al alloys in atmosphere differs greatly from that in solutions, which has attracted increasing attention of some researchers [10 15]. At present, it has been mainly reported about the influence of factors such as humidity, immersion time, and deposition of sulfur dioxide, carbon dioxide and chloride ions on the atmospheric corrosion process of magnesium alloys, but the influence of microstructures on the atmospheric corrosion behavior of magnesium alloys hasn t been reported. However, the phase composition of magnesium alloys is one of the important factors for the atmospheric corrosion behavior, thus this paper took account of the influence of β phase in a dual phase (α+β) magnesium alloys on the corrosion behavior in atmosphere. It has been reported by Song et al. [16] that the β phase in magnesium alloys may reduce the reactive surface area, leading to the less possibility of corrosion, from which the conclusion could be drawn the conclusion that the higher volume fraction of β phase in magnesium alloys was benefit for the better corrosion resistance in solutions. However, the β phase is more noble than the α phase [17], and the galvanic corrosion cannot be ignored in some cases. Therefore, The aim of this work was to gain a better understanding of the role of β phase in the corrosion process induced by sodium chloride particles deposited on the surfaces of magnesium alloys in moist atmospheric environment. 2. Experimental Three magnesium-aluminum alloys were prepared by using commercially pure magnesium (99.99%) and pure aluminum (99.97%). The compositions were Corresponding author. Ph.D.; E-mail address: jrl014014@163.com (R.L. Jia). 3.4 mass%, 9.3 mass%, 20.7 mass% aluminum respectively. To protect the melt from oxidation and burning, the melting process was conducted under a flowing protective gas mixture of 30 ml/min SF 6 and 5 l/min N 2. The Mg-Al melts were held at 730 C and homogenized for about half an hour, and then cast into a metal mold. A specimen with a dimension of 10 mm 10 mm 3 mm was used for all the experiments. The specimens were sequentially polished by silicon carbide (SiC) paper and finally down to 0.5 µm diamond paste. The polished samples were ultrasonically cleaned for 20 min in an acetone bath, and then rinsed with ethanol, dried and stored in a desiccator. Samples were placed in a vacuum desiccator where placed some glyceryl solution, so that the humidity was kept at (93±2)%. The temperature was maintained at (25±1) C by means of a thermostatically controlled water tank. The atmosphere to the desiccator was that of laboratory air, which was purified of C 2 and dried. A 0.22 mg/cm 2 sodium chloride was deposited onto each specimen surface before the specimens were exposed in the vacuum desiccator. The exposure duration was between 10 days and 40 days. The microstructures and corrosion morphologies of the alloy substrate were investigated by scanning electron microscopy (SEM) (XL-30FEG), and the analysis of corrosion products was performed by X- ray diffraction (XRD). Elemental distribution in the cross section of corrosion products was measured by electron probe microanalysis (EPMA, 1610, Japan). 3. Results and Discussion 3.1 Microstructure According to the phase diagram of Mg-Al binary system, the solid solubility of Al in Mg is about 2% at room temperature. Therefore, all the prepared alloys contained a solid solution of aluminum in magnesium called α phase and Mg 17 Al 12 intermetallics called β phase. Figure 1 shows the microstructures of the Mg-3Al alloy, which contains a few β phase along the grain

226 J. Mater. Sci. Technol., Vol.25 No.2, 2009 Fig. 1 Microstructure of Mg-3Al alloy boundaries. Figure 2 represents SEM morphologies of Mg-9Al alloy and Mg-21Al alloy. It could be seen that there are not only α phase and β phase but also the eutectic phase existing in both alloys. With the increasing of Al content, the volume fraction of β phase and eutectic phase increased obviously. The eutectic phase in Mg-9Al alloy is located at grain boundaries and the β phase represents the white netted structure, while the dark grey zones is identified as the combination of α phase and eutectic phase. In the microstructure of Mg-21Al, the white zones are mainly composed of β phase and a little of α phase. Figure 2(c) is the partly amplified image of Mg-21Al, from which it could be seen clearly that eutectic phase existed in parallel forms. Table 1 Regression values of A and B using in Eq. (1) Alloy A B Mg-3Al 0.97528 0.23269 Mg-9Al 1.15619 0.31366 Mg-21Al 3.26837 0.44307 3.2 Corrosion kinetics Identical characteristics are revealed in the corrosion kinetics curves of the three Mg-Al alloys (Fig. 3). The corrosion data of three Mg-Al alloys approximately follow the equation C = At B (1) where C is the mass gain (mg); t is the exposure time (d); A and B are constants. The value of A shows the mass gain in the unit time. B reflects the variety of mass gain with time. The regression values of A and B using in Eq. (1) are listed in Table 1. From the corrosion kinetics results, it is found that the corrosion rates increase with the increasing of Al content in the alloys, which is related to the microstructures of these three Mg-Al alloys. The corroded surface morphologies of the Mg-Al alloy substrate are observed by SEM (Fig. 4(a c)) after 40 d exposure. As can be seen, Mg-3Al is characterized in part by the feature of pitting corrosion, and there are deep cavities in the substrates of Mg-9Al and Mg-21Al alloy. The corrosion zones spread along the α phase while the β phase remains in the surface, which indicated the occurrence of microgalvanic corrosion. The results showed that the microgalvanic effects between α phase and β phase should be taken into consideration even if the corrosion occurs in the atmospheric environment. Based on the principles of galvanic corrosion, the cathode-to-anodic area ratio is one of the predominant factors determining the rate of galvanic corrosion. Galvanic corrosion is accelerated when the cathode-to-anode area ratio is greater than 0.5 [18]. For Mg-3Al alloy, it is unlikely to cause significant galvanic effects because of such a small cathode (β phase) in it. Though the cathode (β phase)-to-anode (matrix) area ratio does not exceed 0.5 in Mg-9Al alloy, β phase could form a cell with eutectic phase. The geometrical conditions of galvanic corrosion in Mg-21Al alloy is more favorable because the cathode (β phase)-to-anode (α phase or eutectic phase) area ratio approaches to 1.0, thus giving rise to the heavy galvanic corrosion. 3.3 Cross-section morphologies The cross-section morphologies of the alloys exposured for 40 days are shown in Fig. 5(a c). It can be seen that the Mg-3Al alloy seemed to be suffered from relatively light pit corrosion, formed a gray-black corrosion film with a depth of about 30 µm (Fig. 5(a)). For Mg-9Al and Mg-21Al alloys, there are deep pits in the substrates and the rather thicker corrosion products, which illustrates they have been suffered from more severe corrosion (Fig. 5(b, c)). It is found that with the increasing of Al content in Mg-Al alloys, the corrosion products have been loosed, facilitating the formation of more macro defects. The possible mechanism may be that due to the serious corrosion of the substrates in Mg-9Al and Mg-21Al alloys, when the formed corrosion products grow outward continuously, the corrosion products originally formed on the etch-pits may be forced to be piled up or fell out Fig. 2 SEM morphologies of: (a) Mg-9Al alloy, (b) Mg-21Al alloy, (c) the partly amplified image of Mg-21Al

J. Mater. Sci. Technol., Vol.25 No.2, 2009 227 Mass gain / mg cm -2 9 8 7 6 5 4 3 2 1 Mg-3Al Mg-9Al Mg-21Al 0 0 10 20 30 40 Exposure / d Fig.3 Mass changes of magnesium alloys vs exposure time at 93% RH and 25 C toward the surroundings. Consequently, it is difficult to form compact corrosion products and the formation of defects is inevitable. Figure 6 shows the results of X-ray diffraction, which indicates that brucite (Mg(H) 2 ) is the main corrosion product and,, (H) 3 C1 0 also exist in the corrosion products of three Mg-Al alloys. As can be seen, there are evidence for the existence of MgAl 2 4 spinel and Mg 17 Al 12 in Mg-9Al and Mg-21Al alloys. According to XRD results, the formation process of corrosion products may be described as follows. The existence of NaCl accelerates the corrosion of Mg-Al alloys. As NaCl is easy to absorb water, it could induce the pitting corrosion of the alloy because of the local high concentration of Cl. Moreover, NaCl has the characteristic of secondary spreading. In other words, it tends to spread peripherad after absorbing water, which could form a thin liquid membrane on the surface of the alloy, thereby a thin film of NaCl electrolyte with an approximate concentration of 1 mol/l is formed gradually [19], resulting in the preferred electrochemical corrosion at the deposition sites and in the adjacency of NaCl particles. The following reactions may occur in this case. Mg 2e + (anodic reaction) (2) 2 + + 4e 4H (cathodic reaction) (3) + + 2H Mg(H) 2 (4) 2+ + +3H +C1 C1(H) 3 0 (5) + + 2C1 or + + 2C1 + (6) In the Mg-9Al and Mg-21Al alloys, the following reactions also occur: Al 3e A1 3+ (anodic reaction) (7) Under the atmospheric condition, oxygen could reach the surface of substrates more easily and in a fast rate, which could be supplemented continuously. Thus, there are enough opportunities for the oxygen to react with the substrate to form MgAl 2 4. The Mg 17 Al 12 phase detected in corrosion products may be caused by the entering of the fallout second phase from substrates into the corrosion product when the substrates is corroded. Fig. 4 Corroded morphologies of substrate: (a) Mg-3Al, (b) Mg-9Al, (c) Mg-21Al alloy after 40 days exposure (corrosion products removed) Fig. 5 Cross section morphologies: (a) 3Al-Mg alloy, (b) 9Al-Mg alloy, (c) 21Al-Mg alloy after exposure for 40 days

228 J. Mater. Sci. Technol., Vol.25 No.2, 2009 (a) Mg-3Al Mg(H) 2 Cl(H) 3 (b) Mg-9Al Mg(H) 2 Cl(H) 3 MgAl 2 4 Mg 17 Al 12 (c) Mg-21Al Mg(H) 2 Cl(H) 3 MgAl 2 4 Mg 17 Al 12 20 30 40 50 60 70 80 20 30 40 50 60 70 80 20 30 40 50 60 70 80 Fig. 6 XRD patterns of corrosion products of magnesium alloys with different Al content after exposure for 40 days Fig. 7 EPMA of: (a) Mg-3Al alloy, (b) Mg-9Al alloy, (c) Mg-21Al alloy The distribution characteristic of corrosion products has been investigated by EPMA surface scanning, as shown in Fig. 7. It can be seen that the distribution of Mg and element in the surface layer of corrosion products in Mg-3Al alloy is homogeneous (Fig. 7(a)-Mg, ), indicating the even distribution of corrosion products; while the distribution of Al element shows there is no the formation of Al-rich corrosion products (Fig. 7(a)-Al). However, there are a little of Al-rich phase distributed in the corrosion products of Mg-9Al alloy, and relatively more Al-rich phase in the corrosion products of Mg-21Al alloy, which are in the nonuniform distribution (Fig. 7(c)-Al). The results of EPMA are consistent with XRD, which indicates that the amount of Al-rich phase in the corrosion products increase as the Al content of Mg-Al alloys increasing. Therefore, it is difficult to form a compact corrosion product film when the Al-rich phase and other phase exhibit the interlaced distribution in the corrosion products. The protective effect of corrosion products is determined by their uniform and compact characteristics. If there are many macro defects such as cracks or pores in the corrosion products, they could accelerate the ionic diffusion and facilitate the process of moisture moving through the corrosion product onto the substrate. As mentioned earlier, as the Al content increasing, the corrosion products become loose and porous, and the composition and distribution of corrosion products are not homogenous, which decreasing

J. Mater. Sci. Technol., Vol.25 No.2, 2009 229 the protective effects of corrosion products, accelerating the corrosion of Mg-Al alloys. 4. Conclusions (1) The corrosion rate of Mg-Al alloy with different aluminum content induced by sodium chloride in moist atmospheric was strongly dependent on the microstructures. As the content of Al increasing, the volume fraction of β phase increased, leading to the relatively high corrosion rate of Mg-Al alloys. (2) The corrosion behavior of Mg-Al alloys is determined partially by galvanic coupling between the β phase and the α phase or eutectic phase. With the increasing of the volume fraction of β phase, the cathodic effects of β phase becomes more obvious and significant. (3) The Al content of Mg-Al alloys has an important influence on the barrier function of corrosion products. As the content of Al increasing, the distribution of corrosion products is not uniform and there are some macro defects, which is one of the main reasons for the increasing of corrosion rate in Mg-Al alloys. Acknowledgements The financial support by the National Natural Science Foundation of China (Grant No. 50571105) is acknowledged. The authors are also pleased to show their deep thanks to Dr. Tao Zhang for his discussion and suggestion. REFERENCES [1 ]. Lunder, T.K.R. Aune and K. Nisancioglu: Corrosion, 1987, 43(5), 291. [2 ] G.L. Song and A. Atrens: Adv. Eng. Mater., 2003, 5(12), 837. [3 ] T. Beldjoudi, C. Fiaud and L. Robbiola: Corrosion, 1993, 49(9), 738. [4 ]. Lunder, J.E. Lein, T.K.R. Aune and K. Nisancioglu: Corrosion, 1989, 45(9), 741. [5 ] P. Uzan, D. Eliezer and E. Aghion: Synthesis of Lightweight Metals., 1999, 3, 171. [6 ] G.L. Song, A. Atrens and M. Dargusch: Corros. Sci., 1999, 41, 249. [7 ] S. Mathieu, C. Rapin, J. Hazan: Corros. Sci., 2003, 45, 2741. [8 ] R. Ambat, N.N. Aung and W. Zhou: Corros. Sci., 2000, 42, 1433. [9 ] T. Zhang, Y. Li and F.H. Wang: Corros. Sci., 2006, 48, 1249. [10] R. Lindstrom, L.G. Johansson, G.E. Thompson, P. Skeldon and J.E. Svensson: Corros. Sci., 2004, 46, 1441. [11] R. Lindstrom, L.G. Johansson and J.E. Svensson: Mater. Corros., 2003, 54, 587. [12] N. LeBozec, M. Jonsson and D. Thierry: Corrosion, 2004, 60(4), 356. [13] M. Jonsson, D. Persson and D. Thierry: Corros. Sci., 2007, 49(3), 1540. [14] C. Lin and X.G. Li: Rare Metals, 2006, 25(2), 190. [15] Y. Wan and C.W. Yan: Chin. J. Nonferrous Met., 2006, 16(1), 176. (in Chinese) [16] G.L. Song, A. Atrens, X.L. Wu and B. Zhang: Corros. Sci., 1998, 40(10), 1769. [17] M. Jonsson, D. Thierry and N. LeBozec: Corros. Sci., 2006, 48(5), 1193. [18] R.K. Singh: Metall. Mater. Trans., 2004, 35A, 2525. [19] A.K. Neufeld, I.S. Cole, A.M. Bond and S.A. Furman: Corros. Sci., 2002, 44, 555.