- 學位論文
AZ31B 和 WE43 鎂合金在氯化鈉水溶液中的腐蝕行為對比研究
Comparative studies of corrosion of AZ31B and WE43 magnesium alloys in sodium chloride solution
摘要
鎂合金因其具低密度與高比強度特性，使其成為機械構件輕量化的優先選擇，同時鎂合金具有生物相容性，為可生物降解植入物的潛在應用材料。然而鎂的高化學活性，使其在使用過程中易受到腐蝕。因此，深入了解鎂合金的腐蝕機理有助於我們找到解決此問題的有效途徑。 在本研究中以AZ31B及WE43兩種商用鎂合金作為研究材料，探討其浸泡於3.5wt% NaCl 水溶液中的 24 小時之腐蝕行為。根據析氫試驗、失重測量、橫截面觀察、電化學量測結果顯示，WE43浸泡在3.5 wt% NaCl水溶液中24小時，表現出較佳之抗蝕性，同時在即時攝影觀察中發現，WE43於浸泡24小時內並未表現出腐蝕產物breakdown之現象，為了瞭解WE43具較佳抗蝕性之原因，對浸泡24小時後的AZ31B與WE43腐蝕產物進行TEM分析，二者腐蝕產物主要皆由MgO與Mg(OH)2所組成，透過XPS與TEM line-scan分析得到WE43腐蝕產物內層有Y2O3 和 Y(OH)3 的富集，而AZ31B在TEM line-scan分析中並未顯示出在氧化膜層中具有Al的富集。 WE43腐蝕產物層內層中的富釔層，可歸因於底材中富釔析出物的存在，Mg和Y擁有相同電位，但Y2O3相較於MgO更能穩定存在於中性環境中，因此會優先形成。WE43相較於AZ31B具有較佳抗蝕性的原因有三：WE43為均勻腐蝕，且其主要二次相Mg24Y5並不會造成嚴重的伽凡尼腐蝕，因此具有連續且均勻的腐蝕產物層；內層Y2O3 和 Y(OH)3因擁有合適之PB ratio，因此提升了腐蝕產物層緻密度，同時Y2O3 因擁有較低之IEP，在中性及鹼性環境中不利於氯離子吸附，進而降低後續反應發生之機率。
並列摘要
Magnesium alloy attracts attention in many engineering metals due to its low density (Mg ~1.7g.cm-3 ) and high specific strength and stiffness. As a consequence, magnesium alloys have found many applications where the weight of the structure is of great importance, such as 3C, bicycle, and automotive transportation industries. Moreover, magnesium is biocompatible and has potential applications for biodegradable implants. However, magnesium is chemically active and most magnesium alloys tend to suffer corrosion during services. Therefore, gaining insight into the corrosion mechanism of magnesium alloys helps us to find out an effective way to solve this problem. In this study, two commercial magnesium alloys, AZ31B and WE43 were used as materials to investigate the corrosion behaviors immersed in 3.5wt% NaCl solution for 24 hours. According to the results of hydrogen evolution measurement, weight loss measurement, cross-sectional observation, and electrochemical measurement, WE43 showed a better corrosion resistance in 3.5 wt% NaCl solution for 24 hours. The breakdown of the corrosion film formed on WE43 was not observed in 24 hours. TEM analysis was performed on the corrosion products of AZ31B and WE43 after immersion for 24 hours to understand the main reason for WE43 has better corrosion resistance. The corrosion products of AZ31B and WE43 were mainly composed of MgO and Mg(OH)2. The XPS and TEM line-scan analysis indicated that the inner layer of the WE43 corrosion film was enriched with Y2O3 and Y(OH)3, while the Al-rich layer did not detect in AZ31B corrosion film. The yttrium-rich layer in the inner layer of the WE43 corrosion film can be attributed to the presence of yttrium-rich precipitates in the matrix. Mg and Y had the same standard electrode potential, but Y2O3 formed preferentially due to its higher stability than MgO in a neutral environment. WE43 exhibited a better corrosion resistance attributed to the following mechanism: (1) WE43 showed general corrosion and the main secondary phase Mg24Y5 did not cause serious Galvanic corrosion, resulting in a continuous and uniform corrosion film. (2) The Y-rich layer with a moderate PB ratio in the inner layer of the corrosion film increased the compactness. (3) Adsorption of Cl− was less favored on the Y2O3 surface due to its low IEP, which limited the possibility for further corrosion reaction.