本研究利用純鋁及純鎂以直接熔煉的方式，來製備單相Mg17Al12(β相)材料，同時探討AZ91D鎂合金中各主要組成相之電化學性質。本實驗熔煉所採用的原料為純鎂(wt% = 56%)及純鋁(wt% = 44%)，混合加熱至720˚C並持溫25分鐘後爐冷，所得之試片以掃瞄式電子顯微鏡及X光繞射分析儀進行顯微組織觀察及結構分析。將Mg17Al12 (β相)與純鎂(α相)、純鋁於測試溶液中個別量測其開路電位及動電位極化曲線。電化學測試溶液為1M NaCl (pH = 2、6、8、10、12.2)溶液及0.5M / 0.75M / 1M NaF (pH = 12.2)溶液。最後再將所得之α相與β相以零電阻安培計測量耦合之伽凡尼電流。
實驗結果顯示，將純鎂及純鋁依適當質量比例混合後，在氬氣環境下可以製備取出單相的β相。在1M NaCl 溶液系統中，Mg17Al12 (β相)之開路電位高於α相。而在NaF (pH = 12.2)溶液系統中，β相開路電位及腐蝕電位並無明顯改變，但α相則會因為氟離子濃度的增加，開路電位會急速上升。並於0.75M / 1M NaF (pH = 12.2)溶液中，α相之開路電位會逐漸高於Mg17Al12 (β相)。極化曲線結果顯示α及β兩相在此溶液中皆有立即鈍化的現象，且具有寬廣之鈍化區。
伽凡尼耦合電流量測得知，在1M NaCl溶液系統中，由其電流方向可判斷在此溶液系統中，β相為陰極，α相為陽極。而於0.75M / 1M NaF (pH = 12.2)溶液中，電流方向會由負轉正。以電流方向轉變判斷此時α相及β相的極性有反轉之現象。β相為陽極，α相則為陰極。將極性反轉後之試片，以X光光電子能量散佈儀分析其鈍化膜的表面成分。可知造成極性反轉的原因為純鎂(α相)與溶液中氟離子反應，形成不易溶解之氟化鎂，使其由活潑相轉變為貴重相。

The electrochemical behavior of each constituent phase in AZ91D alloy was investigated. The Mg17Al12 (β) phase was prepared by melting Mg of 56 wt% and Al of 44 wt% in a crucible at 720˚C for 25 minutes. The mixed melt was cooled from 720˚C in a furnace and then removed from the crucible for subsequent analyses. The crystal structure and microstructure of the solidified melt were analyzed by SEM and XRD. The electrochemical tests of Mg17Al12 (β) and pure Mg were performed in 1M NaCl (pH = 2、6、8、10、12.2) and 0.5M / 0.75M / 1M NaF (pH = 12.2) solutions. The open circuit potential and polarization curves of Mg17Al12 (β), pure magnesium (α), and pure aluminum were measured in the above different solutions. Galvanic current density between Mg17Al12 (β) and pure magnesium (α) was also measured by a zero resistance ammeter.
In 1 M NaCl solution, the open circuit potential of β phase was higher than that of α phase. In NaF solution system, the open circuit potential and corrosion potential of β phase was independent of the concentration of NaF. However, the open circuit potential of α phase increased with increasing NaF concentration. In both 0.75M and 1 M NaF (pH = 12.2) solution, the open circuit potentials of α phase were higher than those of β phase. The polarization curves showed the both phases could be passivated rapidly and each had a wide passive region.
Galvanic current measurement in 1M NaCl solution showed that β phase was cathodic with respect to α phase. In either 0.75M or 1M NaF solution (pH = 12.2), β phase was initially cathodic with respect to α phase. However, a reversion of polarity was observed as the test was pro-longed. The results of XPS analyses revealed the relatve amount of MgF2 formed on α and β phases in NaF solution varied with immersion, which was probably responsible for the change of polarity, and that caused the reverse polarization.

[1] 葉哲政, “鎂合金在汽車產業之應用”, 金屬工業研究發展中心, 高雄市, 2003.

[2] G. L. Maker and J. Kruger, “Corrosion of magnesium”, International Materials Reviews, vol.38, no.3, p.138, 1993.

[3] 路全勝, “壓鑄業動向與前景分析-鎂、鋁、鋅合金”, 經濟部技術處,台北市, 1999.

[4] G. Song, “Investigation on Corrosion of Magnesium and its Alloys”, Corrosion Science in the 21th Century. UMIST, 2003.

[5] G. Song and A. Atrens, “Corrosion Mechanisms of Magnesium Alloys”, Advanced Engineering Materials, vol.1, no.1, p.11,1999.

[6] R. Zeng, J. Zhang, W. Huang, W. Dietzel, K. U. Kainer, C. Blawert, W. Ke, “Review of studies on corrosion of magnesium alloys”, Trans.Nonferrous Met.Soc.China, vol.16, p.S763, 2006.

[7] 楊智超, “鎂合金材料特性及新製程發展”, 工業材料, p.72, 1999.

[8] 唐乃光, “鎂合金壓鑄技術手冊”, 經濟部技術處, 台北市, 2003.

[9] J. H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, “A TEM Investigation of Naturally Formed Oxide Films on Pure Magnesium”, Corrosion Science, vol.39, no.8, p.1397, 1997.

[10] H. B. Yao, Y. Li, A. T. S. Wee. “An XPS investigation of the oxidation / corrosion of melt-spun Mg”, Applied Surface Science , vol.158, p.112, 2000.

[11] M. Pourbaix, Atlas of Electrochemical Equilibrium in Aqueous Solution, NACE, Houston, TX,USA,1974.

[12] R. Ambat, N. N. Aung and W. Zhou, “Studies on the influence of chloride ion and pH on the corrosion and electrochemical behavior of AZ91D magnesium alloy”, Journal of Applied Electrochemistry, vol.30, p.865, 2000.

[13] E. Guali, W. Dietzel, and K. U. Kainer, “Testing of General and Localized Corrosion of Magnesium Alloys : A Critical Review”, Journal of Materials Engineering and Performance, vol.13, no.5, p.517, 2004.

[14] 陳欣蘋, 蔡文達, “壓鑄AZ91D鎂合金在3.5 wt% NaCl 水溶液中之腐蝕行為”, 中華民國防蝕工程學會年會, 台南, 2002.

[15] G.Song, A. Attens, D. Stjohn, J. Nairn, Y. Li, “The electrochemical corrosion of pure magnesium in 1N NaCl“, Corrosion Science, vol.39, no.5, p.855, 1997.

[16] G. Wada and S. Maekawa, “The Inhibitive Behavior of the Fluorid Ion and the Destruction of Its Protective Film in the Corrosion of Magnesium in Neutral Solutions”, Bull. Chem.Soc. Jpn, vol.37, p.998, 1964.

[17] A. K. Vijh, “Correlation between Atomic Number and Electrochemical Behavior : Anodic dissolution of Metals in Hydrogen Fluoride”, Electrochimica Acta, vol.16, p.441, 1971.

[18] P. M. Bradford, B. Case, G Dearnaley, J. F. Turner. “Ion Beam Analysis of Corrosion Films on A High Magnesium Alloy “, Corrosion Science, vol.16, p.747, 1976.

[19] E. Gulbrandsen, Johan Tafto, A. Olsen, “The Passive Behavior of Mg in Alkaline Fluoride Solutions. Electrochemical and Electron Microscopical Investigations”, Corrosion Science, vol.34, no.9, p.1423, 1993.

[20] S. Ono, K. Asami and N. Masuko, “Mechanism of Chemical Conversion Coating Films Growth on Magnesium and Magnesium Alloys”, Materials Transactions, vol.42, no.7, p.1225, 2001.

[21] S. Verdier, N. Van der Laak, S. Delalande, J. Metson, F. Dalard “The surface reactivity of a magnesium –aluminum alloy in acidic fluoride solutions studied by electrochemical techniques and XPS”, Applied Surface Science, vol.235, p.513, 2004.

[22] K. Y. Chiu, M. H. Wong, F. T. Cheng, H. C. Man, “Characterization and corrosion studies of fluoride conversion coating on degradable Mg implants”, surface & coating Technology, vol.202, p.590, 2007.

[23] J.E. Gray-Munro, B. Luan, L. Huntington, “The influence of surface microchemistry in protective film formation on multi-phase magnesium alloys”, Applied Surface Science, vol.254, p.2871, 2008.

[24] S. Mathieu, C. Rapin, J Hazan, P Steinmetz, “Corrosion behavior of high pressure die-cast and semi-solid cast AZ91D alloys”, Corrosion Science, vol.44, p.2737, 2002.

[25] R. Ambat, N. N. Aung, W. Zhou, “Evaluation of microstructure effects on corrosion behavior of AZ91D magnesium alloy”, Corrosion Science, vol.42, p.1433, 2000.

[26] 陳欣蘋, “顯微組織對壓鑄AZ91D鎂合金之腐蝕行為影響研究”, 國立成功大學材料科學及工程學系碩士論文，2002.

[27] G. Ballerini, U. Bardi, R. Bignucolo, G. Ceraolo, “About some corrosion mechanism of AZ91D magnesium alloy”, Corrosion Science, vol.47, p.2173, 2005.

[28] O. Lunder, J. E. Lein, T. K. Aune, K. Nisancioglu, “The Role of Mg17Al12 Phase in the Corrosion of Mg Alloy AZ91”, Corrosion, vol.45, no.9, p741, 1989.

[29] G. Song, A. Atrens, X. Wu, B. Zhang, “Corrosion Behavior of AZ21, AZ501, AZ91 in Sodium Chloride”, Corrosion Science, vol.40, no.10, p.1769, 1998.

[30] G. Song, A. Atrens, M. Dargusch, “Influence of microstructure on the corrosion of diecast AZ91D”, Corrosion Science, vol.41, p.249, 1999.

[31] S. Mathieu, C. Rapin, J. Steinmetz, P. Stenmetz, “A corrosion study of the main constituent phases of AZ91 magnesium alloys”, Corrosion Science, vol.45, p.2741, 2003.

[32] Z. Shi, G. Song, A. Atrens, “Influence of the β phase on the corrosion performance of anodized coatings on magnesium-aluminum alloys”, Corrosion Science, vol.47, p.2760, 2005.

[33] F. Andreatta, I. Apachitei, A. A. Kodentsov, J. Dzwonczyk, J. Duszczyk, “Volta potential of second phase particles in extruded AZ80 magnesium alloy”, Electrochimical Acta, vol.51, p.3551, 2006.

[34] M. Jonsson, D. Thierry, N. LeBozec, “The influence of microstructure on the corrosion behavior of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin porbe”, Corrosion Science, vol.48, p.1193, 2006.

[35] M. Anik, E. Korpe, E. Sen, “Effect of alloy microstructure on electroless NiP deposition behavior on Alloy AZ91”, Surface & Coating Technology, vol.201, p4702, 2007.

[36] A. Pardo, M. C. Merino, A. E. Coy, R. Arrabal, F. Viejo, E. Matykina, “Corrosion behavior of magnesium/aluminum alloys in 3.5 wt% NaCl”, Corrosion Science, vol.50, p.823, 2008.

[37] A. Pardo, M.C. Merino, A.E. Coy, F. Viejo, R. Arrabal, S. Feliú Jr. “Influence of microstructure and composition on the corrosion behavior of Mg/Al alloys in Chloride media”, Electrochimica Acta, vol.53, p.7890, 2008.

[38] 蕭鴻昱, “陽極化處理製程參數對壓鑄AZ91D鎂合金陽極化處理及陽極膜性質之影響研究”, 國立成功大學材料科學及工程學係博士論文, 2005.

[39] Y. Wang, G. Liu, Z. Fan, “Microstructure evolution of rheo-diecast AZ91D alloy during heat treatment”, Acta Materialia, vol.54, p.689, 2006.

[40] M. X. Zhang, P. M. Kelly, “Crystallography of Mg17Al12 precipitates in AZ91D alloy”, Scripta Materialia, vol.48, p.647, 2003.

[41] G. Song, A. L. Bowles, D. H. StJohn, “Corrosion resistance of aged die cast magnesium alloy AZ91D”, Materials Science and Engineering, vol.A366, p.74, 2004.

[42] N. N. Aung and W. Zhou, “Effect of heat treatment on corrosion and electrochemical behavior of AZ91D magnesium alloy”, Journal of Applied Electrochemistry, vol.32, p.1397, 2002.

[43] H. Okamoto, J. Phase Equilibria, vol.19, p.598, 1998.

[44] A. Jagminas, I. Vrublevsky, J. Kuzmarskytė, V. Jasulaitienė, “Composition, structure and electrical properties of aluminum barrier layers growth in fluoride-containing oxalic acid solutions”, Acta Materialia, vol.56, p.1390, 2008.

[45] Robertc.weast, “Handbook of chemistry and physics”, CRC, 1979.