本研究的目的在於探討不同釔添加量之二元鎂釔合金，其微結構差異對於腐蝕性質的影響。首先利用ICP-MS、XRD、SEM等對鎂釔合金樣品進行微結構的觀察。結果顯示Mg-1wt%Y樣品所含的鐵雜質元素遠大於其容許極限，且以雜質顆粒的形式隨機分布在樣品表面；Mg-7wt%Y樣品則具有含鋯的雜質顆粒，而釔則主要以連續網狀的Mg24Y5二次相分布於晶界上。
接著透過電化學量測搭配臨場光學顯微鏡監測樣品腐蝕電位和表面腐蝕形貌隨浸泡時間的演變。在Mg-1wt%Y的樣品上可觀察到嚴重的局部腐蝕，且擴展十分迅速；相比之下，Mg-7wt%Y的樣品上局部腐蝕擴展則較為緩慢，且其腐蝕速率隨著時間的進行並沒有太大的改變。
最後通過SEM/FIB系統進一步觀察腐蝕微結構之差別。結果顯示Mg-1wt%Y樣品的局部腐蝕受鐵雜質所主導且在擴展過程無阻礙，可輕易擴展至整個樣品表面；而Mg-7wt%Y樣品則由鋯顆粒的伽凡尼效應主導局部腐蝕的進行，但局部腐蝕的擴展會受到Mg24Y5二次相的阻擋，因此導致擴展過程較為緩慢。鎂釔合金隨著釔添加量的提高，會在晶界上形成連續網狀的二次相結構，其對於阻擋局部腐蝕的擴展及提昇合金抗蝕性有正面的影響。

The purpose of this study is to investigate the influence of alloy microstructure on the corrosion properties of binary magnesium-yttrium alloys with different yttrium additions. Firstly, ICP-MS, XRD, and SEM were used to study the microstructure of magnesium-yttrium alloy samples. The results show that the Mg-1wt%Y sample contains iron impurity far greater than the tolerance limit, and they are randomly distributed along the surface as impurity particles; while the Mg-7wt%Y sample contains impurity particles with zirconium, and the yttrium is mainly present in the continuous network Mg24Y5 second phase distributed on the grain boundaries.
Then, the evolution of corrosion potential and surface corrosion morphology of the samples with immersion time was monitored through electrochemical measurements and in situ optical microscopy. Severe localized corrosion was observed on the Mg-1wt%Y sample with rapid expansion. In contrast, the expansion of localized corrosion on the Mg-7wt%Y sample is slower, and the corrosion rate does not change much with immersion time.
Finally, the difference in corrosion microstructure was studied by SEM/FIB system. The results show that the localized corrosion on the Mg-1wt%Y sample is dominated by iron impurities and can easily expand to the entire sample surface without much hinderance. On the other hand, the localized corrosion on the Mg-7wt%Y sample is dominated by the galvanic effect of the zirconium particles, but its expansion was blocked by the Mg24Y5 second phase, which leads to a slower expansion rate. With increasing amount of yttrium addition, continuous network-like second phase can form on the grain boundaries in magnesium-yttrium alloy, which has a positive effect on blocking the localized corrosion expansion and improving the corrosion resistance.

[1] B.L.Mordike, Creep-resistant magnesium alloys, 324 (2002) 103–112.
[2] P.W.Chu, E.A.Marquis, Linking the microstructure of a heat-treated WE43 Mg alloy with its corrosion behavior, Corros. Sci. 101 (2015) 94–104.
[3] T.Cain, L.G.Bland, N.Birbilis, J.R.Scully, A Compilation of Corrosion Potentials for Magnesium Alloys, 70 (2014) 1043–1051.
[4] M.Esmaily, J.E.Svensson, S.Fajardo, N.Birbilis, G.S.Frankel, S.Virtanen, R.Arrabal, S.Thomas, L.G.Johansson, Fundamentals and advances in magnesium alloy corrosion, Prog. Mater. Sci. 89 (2017) 92–193.
[5] X.Liu, D.Shan, Y.Song, E. houHan, Influence of yttrium element on the corrosion behaviors of Mg–Y binary magnesium alloy, J. Magnes. Alloy. 5 (2017) 26–34.
[6] G.Ben-Hamu, D.Eliezer, K.S.Shin, S.Cohen, The relation between microstructure and corrosion behavior of Mg-Y-RE-Zr alloys, J. Alloys Compd. 431 (2007) 269–276.
[7] X.Zhang, Y.Li, C.Wang, Corrosion and electrochemical behavior of Mg − Y alloys in 3.5 % NaCl solution, Trans. Nonferrous Met. Soc. China. 23 (2013) 1226–1236.
[8] M.Liu, P.Schmutz, P.J.Uggowitzer, G.Song, A.Atrens, The influence of yttrium (Y) on the corrosion of Mg-Y binary alloys, Corros. Sci. 52 (2010) 3687–3701.
[9] H.D.Zhao, G.W.Qin, Y.P.Ren, W.L.Pei, D.Chen, Y.Guo, The maximum solubility of y in α-Mg and composition ranges of Mg 24Y5 - X and Mg2Y1 - X intermetallic phases in Mg-Y binary system, J. Alloys Compd. 509 (2011) 627–631.
[10] A.D.Sudholz, K.Gusieva, X.B.Chen, B.C.Muddle, M.A.Gibson, N.Birbilis, Electrochemical behaviour and corrosion of Mg-Y alloys, Corros. Sci. 53 (2011) 2277–2282.
[11] J.H.Zhang, H.F.Liu, W.Sun, H.Y.Lu, D.X.Tang, J.Meng, Influence of Structure and Ionic Radius on Solubility Limit in the Mg–Re Systems, Mater. Sci. Forum. 561–565 (2007) 143–146.
[12] X.Zhang, K.Zhang, X.Deng, L.Hongwei, L.Yongjun, M.Minglong, L.Ning, Y.Wang, Corrosion behavior of Mg–Y alloy in NaCl aqueous solution, Prog. Nat. Sci. Mater. Int. 22 (2012) 169–174.
[13] H.B.Yao, Y.Li, A.T.S.Wee, Passivity behavior of melt-spun Mg-Y Alloys, Electrochim. Acta. 48 (2003) 4197–4204.
[14] M.Wang, H.Zhou, L.Wang, Effect of Yttrium and Cerium Addition on Microstructure and Mechanical Properties of AM50 Magnesium Alloy, J. Rare Earths. 25 (2007) 233–237.
[15] Q.A.Li, Q.Zhang, C.Q.Li, Y.G.Wang, Microstructure and mechanical properties of Mg-Y alloys, Adv. Mater. Res. 239–242 (2011) 352–355.
[16] 邱垂泓/Davy C.Chiu, 鎂合金成形技術, 台灣輕金屬協會. (2013).
[17] I.J.Polmear, Magnesium alloys and applications, 10 (1994).
[18] B.L.Mordike, T.Ebert, Magnesium Properties - applications - potential, Mater. Sci. Eng. A. 302 (2001) 37–45.
[19] G.L.Song, A.Atrens, Corrosion mechanisms of magnesium alloys, Adv. Eng. Mater. 1 (1999) 11–33.
[20] A.Prasad, P.J.Uggowitzer, Z.Shi, A.Atrens, Production of high purity magnesium alloys by melt purification with Zr, Adv. Eng. Mater. 14 (2012) 477–490.
[21] C.Sanchez, G.Nussbaum, P.Azavant, H.Octor, Elevated temperature behaviour of rapidly solidified magnesium alloys containing rare earths, Mater. Sci. Eng. A. 221 (1996) 48–57.
[22] G.L.Song, Corrosion of magnesium alloys, Corros. Magnes. Alloy. (2011) 1–640.
[23] M.Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, (n.d.).
[24] S.Fajardo, G.S.Frankel, Effect of impurities on the enhanced catalytic activity for hydrogen evolution in high purity magnesium, Electrochim. Acta. 165 (2015) 255–267.
[25] G.Williams, H.N.Mcmurray, Localized Corrosion of Magnesium in Chloride-Containing Technique, (2008) 340–349.
[26] E.Ghali, W.Dietzel, K.Kainer, General and Localized Corrosion of Magnesium Alloys : A Critical Review, 13 (2004) 7–23.
[27] L.Yang, X.Zhou, M.Curioni, S.Pawar, H.Liu, Z.Fan, G.Scamans, G.Thompson, Corrosion Behavior of Pure Magnesium with Low Iron Content in 3.5 wt% NaCl Solution, J. Electrochem. Soc. 162 (2015) C362–C368.
[28] G.L.Song, A.Atrens, Understanding Magnesium Corrosion A Framework for Improved Alloy Performance, (2003) 837–858.
[29] H.Okamoto, Mg-Y (magnesium-yttrium), J. Phase Equilibria Diffus. 31 (2010) 199.
[30] Y.M.Kim, C.D.Yim, H.S.Kim, B.S.You, Key factor influencing the ignition resistance of magnesium alloys at elevated temperatures, Scr. Mater. 65 (2011) 958–961.
[31] Fridman, Tutorial a beginner ’ s guide to ICP-MS, Spectroscopy. 16 (2001) 38–55.
[32] C.Growth, N.P.Company, A.Energy, Letter to the editors etchant for revealing dislocations in magnesium oxide single crystals Toshio Harada, 44 (1978) 635–637.
[33] S.Feliu, Electrochemical impedance spectroscopy for the measurement of the corrosion rate of magnesium alloys: Brief review and challenges, Metals (Basel). 10 (2020) 1–23.
[34] B.Myers, TEM Sample Preparation with the FIB / SEM TEM Sample Prep with FIB Bulk-Out U-Cut, (2009).
[35] D.Qiu, M.X.Zhang, P.M.Kelly, Crystallography of heterogeneous nucleation of Mg grains on Al2Y nucleation particles in an Mg-10 wt.% Y alloy, Scr. Mater. 61 (2009) 312–315.
[36] D.Qiu, M.X.Zhang, J.A.Taylor, P.M.Kelly, A new approach to designing a grain refiner for Mg casting alloys and its use in Mg-Y-based alloys, Acta Mater. 57 (2009) 3052–3059.
[37] P.W.Chu, E.LeMire, E.A.Marquis, Microstructure of localized corrosion front on Mg alloys and the relationship with hydrogen evolution, Corros. Sci. 128 (2017) 253–264.
[38] R.K.S.Raman, The role of microstructure in localized corrosion of magnesium alloys, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 35 A (2004) 2525–2531.
[39] R.Arrabal, A.Pardo, M.C.Merino, M.Mohedano, P.Casajús, K.Paucar, G.Garcés, Effect of Nd on the corrosion behaviour of AM50 and AZ91D magnesium alloys in 3.5 wt .% NaCl solution, Corros. Sci. 55 (2012) 301–312.
[40] M.Sun, A.Yerokhin, M.Y.Bychkova, D.VShtansky, E.A.Levashov, A.Matthews, Self-healing Plasma Electrolytic Oxidation Coatings Doped with Benzotriazole Loaded Halloysite Nanotubes on AM50 Magnesium Alloy, Eval. Program Plann. (2016).
[41] P.L.Bonora, M.Andrei, A.Eliezer, E.M.Gutman, Corrosion behaviour of stressed magnesium alloys, 44 (2002) 729–749.
[42] R.Arrabal, A.Pardo, M.C.Merino, M.Mohedano, P.Casajús, K.Paucar, G.Garcés, Effect of Nd on the corrosion behaviour of AM50 and AZ91D magnesium alloys in 3.5 wt .% NaCl solution, Corros. Sci. 55 (2012) 301–312.
[43] M.Liu, S.Zanna, H.Ardelean, I.Frateur, P.Schmutz, G.Song, A.Atrens, P.Marcus, A first quantitative XPS study of the surface films formed, by exposure to water, on Mg and on the Mg-Al intermetallics: Al3Mg2 and Mg17Al12, Corros. Sci. 51 (2009) 1115–1127.
[44] G.Baril, N.Pébère, Corrosion of pure magnesium in aerated and deaerated sodium sulphate solutions, Corros. Sci. 43 (2001) 471–484.
[45] A.E.Coy, F.Viejo, P.Skeldon, G.E.Thompson, Susceptibility of rare-earth-magnesium alloys to micro-galvanic corrosion, Corros. Sci. 52 (2010) 3896–3906.