本研究探討以AlCl3-EMIC-ZnCl2離子液體為電解質，在活潑之AZ91D鎂合金基材表面共鍍鋁鋅合金的電化學行為，並對不同製程條件下製作之鋁鋅合金鍍層性質加以分析。在離子液體中共鍍鋁鋅合金時，ZnCl2的添加量、沉積電位、沉積總庫倫數等因素對電鍍行為以及鍍層性質的影響，是主要探討的重點。而後續熱處理對鍍層特性的影響，亦加以分析。各種不同製程條件下製備之鋁鋅合金鍍膜，分別利用掃描式電子顯微鏡（Scanning electron microscopy，SEM）觀察其表面形貌及橫截面組織，以能量散射光譜儀（Energy dispersive spectroscopy，EDS）對鍍層進行化學成分鑑定，以原子力顯微鏡（Atomic force microscopy，AFM）量測鍍層的表面粗糙度值（Ra）。鍍層之結晶結構則利用X光繞射分析儀（X-ray diffractometer，XRD）分析，其化學組態則利用X光光電子能譜儀（X-ray photoelectron spectroscopy，XPS）分析。針對鍍層之電化學性質則是將試片浸置於3.5 wt%氯化鈉水溶液中，利用恆電位儀進行開路電位（Open circuit potential，OCP）量測及動電位極化曲線（Potentiodynamic polarization curve）測試，藉以了解鍍層之耐蝕性質。
研究結果顯示在AlCl3-EMIC-ZnCl2離子液體中可有效的在AZ91D鎂合金上析鍍出鋁及鋁鋅鍍層，鋅的還原電位約為+0.19 VAl，鋁的還原電位為 -0.08 VAl。增加ZnCl2的添加量會使得離子液體的路易士酸度下降，使電鍍沉積速率減緩；但隨著ZnCl2添加量的增加，鍍層中鋅含量的比例也會提高。當沉積電位高於-0.2 VAl時，主要是以鋅的還原為主；而當沉積電位在-0.2 ~ -0.35 VAl，鍍層中鋁鋅含量是呈線性關係增減。但當電位低於-0.45 VAl時，此時鍍層以鋁為主且因電流密度過大，使表面鍍層的附著性不佳。控制沉積總量的結果顯示鋁鋅鍍層實際厚度比以法拉第定律計算之理論值大，也比純鋁鍍層厚，顯示鋁鋅鍍層內部不如純鋁鍍層緻密。XRD分析結果顯示，鋁鋅鍍層中的鋅以兩種型態存在，其一是固溶於鋁基地相中，其二是以純鋅的微晶相（第二相）存在。XPS分析結果顯示，當鋅以固溶狀態存在時，鋁原子之電子束縛能會降低。
經短暫後續熱處理後（不論是200 ℃或300 ℃），鍍層表面形貌無明顯變化。但施以200 ℃、12小時熱處理後，鍍層表面有越趨平整的現象發生，使鍍膜表面結晶顆粒的特徵變得不明顯。若施以300 ℃、12小時後續熱處理時，鍍層表面的形貌更為平坦，但表面有富鋅相結晶顆粒生成，XRD分析結果顯示在鍍膜與基材介面有β相（Mg17Al12）的生成。在硬度測試方面，不論是200 ℃或300 ℃熱處理，均可有效提升鍍層的硬度值，且隨著熱處理時間的增長，硬度值也持續上升，以300 ℃、12小時熱處理之條件，其硬度值可達308 Hv，遠比AZ91D鎂合金基材（84 Hv）高出許多。
在3.5 wt% NaCl水溶液中，以不同電鍍條件所製備之各種鍍膜具有不同的動電位極化曲線，其腐蝕電位、腐蝕電流密度、鈍化區範圍以及鈍化電流密度等，皆隨電鍍所採用的電荷量、電解液中ZnCl2濃度的變化、鍍後熱處理的溫度與時間等因素而有顯著的區別。其中純鋁鍍膜具有最寬廣的鈍化區與最低的鈍化電流密度，其耐蝕性質最佳。含鋅的鋁鋅合金初鍍膜，因為化學成分不均勻，其鈍化區縮短且鈍化電流密度升高，顯示與鋅共鍍時，會導致鋁鍍膜耐蝕性質的退化。經過200 oC/12h熱處理後，其極化曲線幾乎與純鋁鍍層一樣，耐蝕性獲得改善。

The co-elecrodeposition behavior and material characterization of AlZn coatings formed in AlCl3-EMIC-ZnCl2 ionic liquid on AZ91D magnesium alloy surface were investigated. The effects of ZnCl2 addition, deposition potential, and total applied charge on the co-deposition of Al and Zn in ionic liquid were focused. The role of heat treatment on affecting the properties of the AlZn coating was also explored. The characteristics of the co-deposited AlZn coatings prepared under different conditions were analyzed by employing scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc. The changes in surface morphology, cross section microstructure, chemical composition, crystal structure and phase transformation of the various deposits accompanied with the electrodeposition conditions were examined. The electrochemical properties of various AlZn coatings, including open circuit potential and potentiodynamic polarization curve, were measured to evaluate their corrosion performance in 3.5 wt% NaCl solution.
This study demonstrates that the processes for Al and AlZn co-deposition on AZ91D magnesium alloy from a Lewis acidic AlCl3-EMIC-ZnCl2 ionic liquid has been successfully established. In the AlCl3-EMIC-ZnCl2 ionic liquid electrolyte, the reduction potential of Zn was about +0.19 VAl, and -0.08 VAl for Al reduction. Increasing the amount of ZnCl2 in AlCl3-EMIC ionic liquid not only caused a decrease in the Lewis acidity, but also reduced the deposition rate. The experimental results showed that the amount of Zn in the deposit increased with increasing ZnCl2 concentration in the electrolyte. Under constant applied potential condition, over 90 mole% of Zn reduction was observed when the deposition potential was higher than -0.2 VAl. In the potential range between -0.2 to -0.35 VAl, co-deposition of Al and Zn occurred and the amount of AlZn coating increased with increasing potential. At potentials below -0.45 VAl, the deposits mainly consisted of rich Al. Under such condition, the high deposition rate gave rise to a loss in the adhesion between the coating and the substrate. Increasing the total deposition charge resulted in an increase in the coating thickness. However, the thickness of the AlZn coating measured was greater than the theoretical value calculated according to Faraday’s law, indicating the appearance of defect within the AlZn coating, which was less compact than pure Al coating. Based on the results of XRD, Zn existed either in solid solution state or as a segregate phase in the AlZn coatings. The substitution of Al by Zn in the AlZn coating also confirmed in the XPS analysis, revealing a reduced Al 2p electron binding energy due to Zn solid solution. Heat treatment at 200 oC for 12 hour caused a change of the surface morphology of the coating from well-defined crystalline appearance to that more flat feature. When the coated specimen was heat treated at 300 oC for 12 hour, the surface became even more flat accompanied with the formation of Zn-rich crystalline phase and Al-rich crystalline phase. A significant increase in hardness was observed with heat treatment at 200 oC or 300 oC. The hardness of the AlZn coating heat treated at 300 oC/12h was 308 Hv, which was much higher than that of the AZ91D magnesium alloy substrate (84 Hv).
The electrochemical properties of various coatings prepared under different conditions were evaluated in 3.5 wt% NaCl solution. The electrochemical parameters including corrosion potential (Ecorr), corrosion current density (icorr), passive range, and passivation current density (ipass), etc., were determined. The experimental results showed that pure Al coating had the broadest passive region and the lowest passivation current density indicating excellent corrosion resistance in the NaCl solution. For AlZn coatings, the passive range became narrow along with a higher passive current density, indicating a lower corrosion resistance as compared with pure Al coating. The inferior corrosion resistance of the as-deposited AlZn coating was attributed to the non-uniform chemical composition and a higher defect density within the coating. Aftering 200 oC/12h heat treatment, the polarization curves were almost the same as pure Al coating indicating corrosion resistance of the AlZn deposited film was improved.

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