稀土釹與旋鍛製程對AZ71鎂合金腐蝕特性之影響

鎂合金因輕量化、高比強度、好的熱傳導性質而廣泛應用於3C產品中；但也因鎂合金的化學活性高，因此提升鎂合金之抗蝕能力為當務之急。本研究於AZ71鎂合金中添加0.5、1.0、1.5 wt.%稀土釹，探討各成分在鑄態、均質化熱處理、旋鍛後退火250°C、350°C等溫度，四種製程之抗腐蝕特性。由實驗結果顯示，AZ71鎂合金在添加1.0 wt.%稀土釹時具有最佳抗蝕能力；而由交流阻抗試驗觀察，旋鍛退火350°C後，整體之電荷轉移電阻值(Ra)較大，抗蝕能力最好。此外極化試驗的結果也顯示，腐蝕電流密度(Icorr)大小由鑄態的10-6 A/cm2下降至旋鍛退火後的10-7 A/cm2，其中以350°C旋鍛退火後的腐蝕電流密度最低，整體抗蝕性最佳。由浸泡試驗觀察得知，腐蝕最初皆由含有錳、矽等雜質元素之析出物附近開始產生伽凡尼腐蝕；添加稀土釹提升鎂合金抗蝕能力的主要機制為，因固溶在基地相中的Nd有減輕伽凡尼腐蝕電位差的效果，且在添加1.0wt.% Nd的AZ71中，Al-Nd析出相均勻散布，因此該合金在四種製程下皆有良好的抗蝕能力；但當過量添加1.5 wt.%稀土釹時，因大量團聚的析出相(Al-Nd)，使局部伽凡尼腐蝕增加，導致抗蝕效果皆不佳，顯示過量的添加稀土釹，不利於鎂合金之抗蝕性質。

關鍵字

AZ71鎂合金；旋轉鍛造；抗腐蝕；交流阻抗；極化

並列摘要

Due to the advantages of high specific strength and reasonable thermal conductivity, magnesium alloys are extensively utilized in 3C product applications. However, the deficient corrosion resistance of magnesium alloys limit their use. In this study, 0.5, 1.0 and 1.5 wt.% of Nd were doped into AZ71 magnesium alloys. The corrosion resistance of these alloys in as-cast, homogenized, rotary swaged, and 250°C and 350°C annealed conditions were studied. According to electrochemical impedance tests, 1.0 wt.% Nd added AZ71 alloy demonstrated the highest total charge transfer resistance after swaging and 350°C annealing, giving the optimum corrosion resistance. The polarization results then showed that the corrosion current densities (Icorr) dropped from 10-6 A/cm2 to 10-7 A/cm2 after annealing. Therefore, the specimens swaged and annealed at 350°C also had the lowest Icorr and the best total corrosion resistance. The 1.0 wt.% Nd doped alloy demonstrated excellent corrosion resistance ability in the four processes studied. It was observed that the impurities of Mn- and Si-containing precipitation initiated local galvanic corrosion at the early stage of corrosion. The Nd-containing matrix phase and homogeneous precipitation of Al-Nd could both reduce the Galvanic corrosion. When 1.5 wt.% Nd was doped to AZ71, the local galvanic corrosion dramatically increases due to the agglomeration of Al-Nd precipitates. Over-doping of Nd to magnesium alloys becomes harmful to their corrosion resistance properties.

並列關鍵字

AZ71 magnesium alloys ； rotary swaging ； corrosion resistance ； Electrochemical Impedance Spectroscopy ； Polarization

參考文獻

[97] 陳宇辰，「稀土釹與旋鍛製程對AZ71鎂合金之影響」，國立台北科技大學碩士論文，2013，第60-90頁。

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稀土釹與旋鍛製程對AZ71鎂合金腐蝕特性之影響

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