生物可降解植入物具有能在人體中降解而不需進行二次手術將其取出的特性，可用於取代現行的生醫植入物。鎂合金因具有良好的生物相容性與生物可降解性，是非常適合做為生物可降解植入物的材料。除此之外，鎂合金擁有與人體骨骼接近的機械性質，能夠有效抑制應力遮蔽效應的發生。不過目前鎂合金作為生物可降解植入物仍有初期腐蝕速率過快的問題，將導致植入物周遭局部酸鹼值上升與大量氫氣泡堆積及植入物機械性質的過早劣化。因此，我們需要針對鎂合金的腐蝕微結構與降解行為進行探討，以了解生物可降解鎂合金初期腐蝕速率過快的原因。
本研究使用ZX21鎂合金(Mg-2 wt%Zn-1 wt%Ca)浸泡於37°C的漢克平衡鹽溶液中，來探討生物可降解鎂合金的腐蝕微結構與降解行為。臨場光學顯微鏡觀察指出，ZX21鎂合金在浸泡一小時內就有局部腐蝕的發生。ZX21鎂合金的抗腐蝕性與腐蝕速率則透過電化學阻抗頻譜(EIS)分析搭配等效電路擬合、動電位極化曲線與重量改變量測等進行測量。所有量測結果均指出隨著浸泡時間增長，ZX21鎂合金的腐蝕速率也有加快的趨勢。此現象與浸泡期間局部腐蝕的持續擴展有所關聯。ZX21鎂合金中的二次相則透過雙束掃描式電子顯微鏡/聚焦離子束(SEM/FIB)系統搭配能量散布X光光譜(EDS)進行橫截面分析。經由EDS分析結果與X光繞射(XRD)圖譜發現ZX21鎂合金中內含有Ca2Mg6Zn3與Mg2Ca兩種二次相。在兩種二次相並列出現的情形下，兩者間的電位差距導致了局部的伽凡尼腐蝕效應，使電位較低的Mg2Ca優先溶解，進而促進周遭鎂底材的腐蝕及局部腐蝕的起始。此外，本研究亦在局部腐蝕的擴展前緣發現其擴展會受到Ca2Mg6Zn3二次相的阻擋。以上結果顯示ZX21鎂合金中的二次相在合金的降解行為中具有複雜的效應。
本研究也針對ZX21鎂合金在HBSS中浸泡24小時後的表面腐蝕膜層與局部腐蝕區域的腐蝕產物進行橫截面穿透式電子顯微鏡(TEM)、掃描穿透式電子顯微鏡(STEM)與EDS分析。選區電子繞射(SAED)與EDS分析指出表面腐蝕膜層主要由氫氧化鎂(Mg(OH)2)與二水合磷酸氫鈣(CaHPO4·2H2O, DCPD)所組成。而局部腐蝕區域的腐蝕產物則主要由氫氧化鎂組成。局部腐蝕區域內的氯訊號則說明氯離子促進了局部腐蝕的擴展。
最後，本研究也針對ZX21鎂合金的生物相容性進行細胞貼附實驗。實驗結果證實人骨肉瘤細胞(MG63)能成功貼附於ZX21鎂合金表面，說明ZX21鎂合金具有良好的生物相容性。

Biodegradable implants, which can be decomposed within the human body and eliminate the need for second removal surgeries, are alternatives to current biomedical implants. Magnesium (Mg) alloys are promising biodegradable implant materials owing to their excellent biocompatibility and biodegradability. Moreover, Mg alloys have mechanical properties close to human bones, minimizing the risk of stress shielding effect. However, one of the most critical challenges of using Mg alloys as biodegradable implants is their rapid initial corrosion rates, leading to an increase in local pH, hydrogen gas accumulation around the implants, and early deterioration of their mechanical properties. To deal with this problem, it is crucial to investigate the corrosion microstructure and degradation behavior of biodegradable Mg alloys.
Corrosion microstructure and degradation behavior of a biodegradable ZX21 (Mg-2 wt%Zn-1 wt%Ca) Mg alloy in Hanks’ balanced salt solution (HBSS) at 37°C are investigated in this thesis. Localized corrosion on ZX21 alloy was observed in the early stage of immersion (less than 1 hour) by in situ optical microscopic (OM) observations. Corrosion resistances and corrosion rates of the ZX21 alloy were measured by electrochemical impedance spectroscopy (EIS) with equivalent circuit fitting, potentiodynamic polarization curves, and weight loss measurements. The results show an increasing corrosion rate of ZX21 alloy with prolonged immersion, which is attributed to the continuous expansion of localized corrosion. A dual-beam scanning electron microscope/focused ion beam (SEM/FIB) system equipped with an energy dispersive X-ray spectroscopy (EDS) detector was employed to perform cross-sectional analysis of the ZX21 alloy second phases. EDS analysis and X-ray diffraction (XRD) identified two second phases in the ZX21 alloy: Ca2Mg6Zn3 and Mg2Ca. When these two second phases coexist side-by-side, their potential difference leads to microgalvanic corrosion and the preferential dissolution of the lower-potential Mg2Ca, subsequently promoting the dissolution of nearby Mg substrate and initiating localized corrosion. Nevertheless, when analyzing the localized corrosion propagation fronts, the Ca2Mg6Zn3 second phase was found to interact with or hinder the propagation of localized corrosion. These results indicate that the second phases in the ZX21 Mg alloy play complex roles in the degradation behavior.
Additionally, this study performed cross-sectional transmission electron microscope (TEM)/scanning transmission electron microscope (STEM)/EDS analysis of the general surface corrosion film and the corrosion products in the localized corrosion regions of ZX21 alloy after immersed in HBSS for 24 hours. Selected area electron diffraction (SAED) and EDS analysis show that the general surface corrosion film is mainly composed of magnesium hydroxide (Mg(OH)2) and dicalcium phosphate dihydrate (DCPD, CaHPO4·2H2O). Meanwhile, the corrosion products in the localized corrosion regions are primarily composed of Mg(OH)2. The presence of chlorine (Cl) signals in the localized corrosion regions indicates that the chloride (Cl-) ions promote the propagation of localized corrosion.
Lastly, this study also conducted cellular response experiments to assess the biocompatibility of ZX21 alloy. The results demonstrate successful adhesion of MG63 cells to the surface of ZX21 alloy, indicating good biocompatibility.

Abstract i
摘要 iii
致謝 v
Table of Contents vii
List of Figures ix
List of Tables xii
Chapter 1 Introduction 1
Chapter 2 Literature Review 3
2.1 Biodegradable Mg Implants 3
2.2 Alloying Elements in Biodegradable Mg Alloys 7
2.3 Selection of Simulated Physiological Environments 10
2.4 Mg-Zn-Ca Alloys and Their Corrosion Behaviors 16
Chapter 3 Experiments 22
3.1 Experiment Flow 22
3.2 Material and Sample Preparation 22
3.3 Alloy Substrate Analysis 23
3.4 Electrochemical and Corrosion Rate Measurements 24
3.5 Corrosion Microstructure Characterizations 28
3.6 Cellular Response Experiments 28
Chapter 4 Results 30
4.1 Alloy Substrate Analysis 30
4.2 Electrochemical and Corrosion Rate Measurements 32
4.2.1 Open Circuit Potential (OCP) Measurements and in situ OM Observations 32
4.2.2 Electrochemical Impedance Spectroscopy (EIS) Measurements 35
4.2.3 Potentiodynamic Polarization Curves and Corrosion Rates 38
4.2.4 Weight Loss Measurements 42
4.3 Corrosion Microstructure Characterizations 44
4.3.1 General Surface Corrosion Film 44
4.3.2 Localized Corrosion and the Effect of the Second Phases in ZX21 Alloy 50
4.4 Cellular Response Experiments 66
Chapter 5 Discussion 68
5.1 Corrosion Rates of ZX21 Alloy 68
5.2 Localized Corrosion and the Effect of the Second Phases in ZX21 Alloy 70
5.3 Comparison of Corrosion Film Microstructure 72
Chapter 6 Conclusions 76
Chapter 7 Future Prospects 78
References 80

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