熱處理對壓鑄AZ91D鎂合金陽極膜之耐蝕性質的影響

Effect of Baking Process on Corrosion Behavior of Anodized Die Cast AZ91D Mg Alloy

本研究探討經陽極化處理之AZ91D鎂合金，經不同溫度、時間之熱處理，對陽極膜耐蝕性質之影響。陽極化處理是於3 M KOH＋0.21 M Na3PO4＋0.6 M KF＋0.15 MAl(NO3)3電解液中進行，採用定電流（10 mA/cm2）及定電位（90 V）兩階段的方式處理。陽極化處理後再進行不同溫度及時間的熱處理，以嘗試改變其耐蝕性質。經不同熱處理程序之試片則於3.5 wt% NaCl水溶液利用電化學交流阻抗頻譜分析來評估其耐蝕性質。實驗結果顯示：經過陽極化處理，生成以MgO為主要結晶成分，並包含Mg(OH)2、MgAl2O4、Al2O3以及Al(OH)3的陽極膜。施以適當的熱處理，可以增加陽極膜中MgO、MgAl2O4、Al2O3的比例，而大幅增進其極化阻抗。在150℃的溫度下，經過60分鐘的熱處理，可以將極化阻抗增加至286,000Ω-cm2。

The effect of anodization, with or without post heat treatment, on corrosion resistance of die cast AZ91D magnesium alloy was investigated in this study. AZ91D Mg alloy was anodized in 3 M KOH ＋ 0.21 M Na3PO4 ＋0.6 M KF ＋ 0.15 M Al(NO3)3 electrolyte. Anodization was conducted in a two-step process; consisting of a constant current density (10 mA/cm2) stage followed by a constant voltage (90 V) stage. Post heat treatments at different temperatures with various periods of time were applied to some of the anodized Mg alloys. Corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) in is 3.5 wt% of NaCl aqueous solution. The experimental results showed that MgO was the main component in the anodic film which also contained some Mg(OH)2, MgAl2O4, Al2O3and Al(OH)3. After optimum baking process, the amounts of MgO and Al2O3 were increasing. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. Moreover, the maximum polarization resistance of 286,000 Ω-cm2 could be obtained after heating at 150 ℃ for 60 minutes.