| Summary: | 鎂合金在目前行動及手持式電子產品中，是一種受歡迎的外殼材料，由於夲身電化學活性較大，表面抗腐蝕的能力格外重要. 抗腐蝕有眾多方法，電化學電鍍，陽極處理, 化學置換鍍層等等，其一是以化學置換鍍層法，在表面形成碳酸鈣層，隔絕外界環境可能對鎂合金造成的腐蝕。 實驗樣品以AZ91D鎂鋁合金當試片， 浸泡在碳酸鈣溶液，在表面形成 LDH (Layered Double Hydroxide) 以及碳酸鈣 CaCO3層，藉由層狀披覆保護底材對抗腐蝕，使用不同的檢測儀器SEM/ EDS 對表面形態結構及元素分佈做觀察分析，AZ91D初期經過碳酸水溶液的腐蝕會形成表面龜裂的情形，表面元素已轉換成含有大量的碳(C)及氧(O)，使用 FT-IR對形成的物質種類做比對確認，再以 GAXRD低掠角X光繞射分析儀針對表面物質的結晶程度做觀察，峰值從原先前驅層LDH的出現但不明顯的狀態經過鹼性碳酸水溶液浸泡後轉變成峰值明顯的Mg ,Al-hydrotalcite(Mg6Al2(OH)16CO3.4H2O) LDH，顯示前驅層形成結晶的趨勢明確，後段CaCO3階段則顯現出結晶型態大多屬於Calcite，這和浸泡在過飽和的碳酸鈣溶液有密切關係。由電化學極化實驗TAFEL曲線尋找腐蝕電位及電流評估抗腐蝕的能力，經 LDH及碳酸鈣成長的試片，其腐蝕電位上升，腐蝕電流有下降的趨勢，顯示對抗腐蝕能力有明顯提升。 試片在某些區域發現到覆蓋不甚均勻的情形，從表面SEM觀察可看到少量的孔洞，這種結構很可能對抗腐蝕層的功能造成威脅與傷害，針對此使用 FIB做定點切割，再由FE-SEM觀察及比較孔洞和碳酸鈣覆蓋區域的縱切面，藉以瞭解結構上的差異並推論形成的原因，可作為製程上改善的依據。 從觀察及量測的過程發現，試片表面碳酸鈣成長較差的區域，其底下的前驅層則比旁邊成長較佳區域為薄，甚至有區域發現沒有碳酸鈣，其底下也沒出現應有的前驅層，這說明前驅層對碳酸鈣的成長具有很大的影響，亦即試片浸泡初期腐蝕是必然的，但是維持它的腐蝕深度及腐蝕的均勻性卻是必要的。 There are a lot of methods to enhance the anti-corrosion capability of Magnesium alloy. Electrochemical plating, anodizing process, chemical conversion coating etc, are those of them. One method is to use chemical conversion coating to form a CaCO3 layer onto the Mg alloy to isolate it from the surroundings. This could prevent it from the possibilities of corrosion to the base material. This experiment used AZ91D Mg-Al alloy , the widely used in electronic industry, and dip into CaCO3 solutions to form LDH and CaCO3 layer for corrosion resistance. First is to dip samples into carbonic acid to form precursor layer. SEM/EDS show that the surface exists many crack lines and already change to high amount of oxygen and carbon. FTIR and GAXRD detect the LDH grain structure after dipping into alkaline carbonic acid solution. And finally dipping into CaCO3 solutions, SEM show it is mostly calcite grain type and polarization curves indicate the corrosion resistance capability is enhanced. We may observe some pores on the surface by SEM inspection. This kind of structure should decrease the capability of corrosion resistance. Using Focus Ion Beam for digging a hole across the poor and well coated areas, compare the under structure by FE-SEM inspection. Under 4~5kx magnification, a precursor layer with cracks are found between CaCO3 and base substrate. After observation, the poor CaCO3 coating area has thinner precursor layer; furthermore there is an area without CaCO3, the precursor layer could not be found. This indicates the precursor layer plays an important role in the formation of CaCO3 coatings. The preliminary corrosion should be necessary and how to keep corrosion a certain depth and uniformity is essential in the process of CaCO3 coating formation. 中文摘要……………………………………………………………………………II Abstract…………………………………………………………………………III 目次………………………………………………………………………………V 圖表目次…………………………………………………………………………VI 第一章 緒論………………………………………………………………………1 1.1 前言…………………………………………………………………………1 1.2 研究方向與內容……………………………………………………………1 第二章 實驗設計……………………………………………………………………5 2.1 實驗試片前處理準備………………………………………………………5 2.2 實驗步驟及方法……………………………………………………………5 2.3 表面成長狀態觀察…………………………………………………………6 2.4 傅立葉轉換紅外線 FT-IR 光譜分析……………………………………6 2.5 結晶結構 GAXRD分析……………………………………………………7 2.6電化學極化實驗……………………………………………………………7 2.7 FIB 對試片做定點切割及 FE-SEM 觀察量測……………………………8 第三章 實驗結果…………………………………………………………………11 3.1 SEM 及 EDS 對試片表面的觀察…………………………………………11 3.2 傅立葉紅外線分析 FT-IR………………………………………………11 3.3 GAXRD 結晶結構分析……………………………………………………12 3.4 電化學極化實驗及腐蝕電流電位………………………………………12 3.5 FIB 及 FE-SEM 結構觀察………………………………………………13 第四章 結論及討論………………………………………………………………35 參考文獻……………………………………………………………………………37
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