硫酸根與氯離子之協同作用對AZ31B鎂合金腐蝕行為與錳酸鹽化成處理的影響

Synergistic Action of Sulfate and Chloride on the Corrosion Behavior and Permanganate Conversion Coating of AZ31B Magnesium Alloy

陳奐彣(Huan-Wen Chen)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202001924

摘要

鎂合金具有低密度、高比強度、高比剛性、與良好散熱性，其輕金屬的特性在輕量化交通載具與航太產業結構材料上扮演重要的角色。然而鎂合金的工業應用受限於鎂本身的高化學活性，與不具保護能力的腐蝕產物，同時，隨著高度工業化，燃燒石化能源產生的空氣汙染，如PM2.5，也造成腐蝕環境的複雜化。因此為提升鎂合金抗蝕性，需了解鎂合金在不同離子環境下的腐蝕行為，以發展有效的防蝕處理，故本研究旨以即時影像、微結構分析、成分分析、與電化學分析，了解常見陰離子硫酸根與氯離子對於鎂合金AZ31B腐蝕行為之影響，並將其應用於錳酸鹽化成處理。
即時影像與析氫試驗結果顯示，添加硫酸根於氯離子的環境具有抑制腐蝕的效果，然而在短時間(< 25分鐘)的浸泡處理中，單純浸泡氯離子生成之腐蝕產物較單純浸泡硫酸根更具保護能力。TEM橫截面分析顯示，浸泡於氯離子溶液中生成之腐蝕產物膜厚為80 ± 30 nm的不均勻膜層；浸泡於有硫酸根的溶液中生成之腐蝕產物則連續且均勻，表示在氯離子溶液中，鎂合金主要受到局部腐蝕，再加入硫酸根後，硫酸根與氯離子競爭吸附的作用下抑制了局部腐蝕的發生，因而沉積連續且均勻之腐蝕產物層。在電化學交流阻抗(EIS)分析中，說明混合陰離子系統在硫酸根作用下形成之連續膜層，具有最大總阻抗值，表示硫酸根作為腐蝕抑制劑的效果為整平腐蝕產物與鎂合金底材之界面，沉積均勻腐蝕產物提升抗蝕能力。
本研究第二部分為將硫酸根腐蝕抑制的效果應用於錳酸鹽化成處理中，由SEM與TEM微結構結果得知，添加硫酸根確實形成均勻且連續之化成皮膜，均勻的皮膜厚度也有效減少脫水過程中裂紋的產生。然而以XPS對化成皮膜成分分析，含有硫酸根的化成系統，傾向生成以三價錳為主的三氧化二錳，相對於二氧化錳為主的化成皮膜三氧化二錳膜層疏鬆且為非晶結構，以電化學交流阻抗與動電位極化曲線對化成皮膜進行抗蝕性分析，雖然脫水裂紋減少，然而皮膜結構造成皮膜總阻抗隨著硫酸根比例增加而下降。

關鍵字

鎂合金 ； 腐蝕抑制劑 ； 錳酸鹽化成 ； 電化學交流阻抗 ； 表面分析

並列摘要

Magnesium alloy has low density, high specific strength and stiffness ,as well as good heat dissipation enabling the weight reduction of automotive and aerospace vehicles for more energy consumption efficient. However, magnesium has high chemical activity, and porous corrosion products which hamper to some extent the practical applications of magnesium alloys. Air pollution (like PM2.5) resulting from the extensive applications of fossil energy has become a major issue, which complicates the corrosive environment. Therefore, to understand the corrosion behavior of magnesium alloys in different ionic environments and to develop effective corrosion protection treatments are essential for improving the corrosion resistance of magnesium alloys. This study aims to understand the effect of sulfate and chloride ions on the corrosion behavior of magnesium alloy AZ31B and this understanding is applied to the permanganate conversion coating treatment.
The results of real-time image analysis and hydrogen evolution test show that the presence of sulfate in chloride-contanting corrosion environment has the effect of corrosion inhibition. However, in a short-term immersion treatment (< 25 minutes), the corrosion product formed in the chloride environment is more protective than that formed in the sulfate solution. Cross-sectional TEM was employed to characterize the sample after 25 min of immersion (when the dark corrosion product was produced.) The results show that when the magnesium alloy AZ31B was treated in the chloride solution, the thickness of the corrosion product layer is about 80 ± 30 nm; the corrosion products generated by immersion in a sulfate- containing solution are continuous and uniform. This indicates that the in the chloride solution, the magnesium alloy is mainly subject to localized corrosion. After the addition of sulfate, the competitive adsorption of chloride by sulfate inhibits the occurrence of localized corrosion, which, in turn, results in a uniform corrosion product layer. The EIS results shows that the continuous film formed by the addition of sulfate has the maximum impedance value, indicating that the mechanism of sulfate as a corrosion inhibitor is to level the interface between the corrosion product and the magnesium alloy substrate and facilitate the deposition of uniform corrosion products.
The second part of this study is to apply the sulfate (corrosion inhibitor) to the permanganate conversion coating treatment. The SEM top-view images and TEM cross-sectional images show that the addition of sulfate does form a uniform and continuous conversion coating, which is effective in reducing the formation of cracks during dehydration. However, the XPS results show that the addition of sulfate tends to form a loose and amorphous conversion coating containing trivalent manganese oxide. Hence, the total impedance of the conversion coating decreases with increasing sulfate content in the solution.

並列關鍵字

AZ31B magnesium alloys ； Corrosion inhibitor ； Surface analysis ； Electrochemical impedance spectroscopy ； Permanganate conversion coating

參考文獻

[1]. I.J. Polmear Light Alloys (3rd edition), Edward Arnold, London (1995) Chapter 5

[2]. Polmear, I.J., Recent Developments in Light Alloys. Materials Transactions, JIM, 1996. 37(1): p. 12-31.

[3]. Esmaily, M., et al., Fundamentals and advances in magnesium alloy corrosion. Progress in Materials Science, 2017. 89: p. 92-193.

[4]. Zhou, X., et al., Improvement of corrosion resistance of AZ91D magnesium alloy by holmium addition. Corrosion Science, 2006. 48(12): p. 4223-4233.

[5]. Schulz, R.B., V.C. Plantz, and D.R. Brush, Shielding theory and practice. IEEE Transactions on Electromagnetic Compatibility, 1988. 30(3): p. 187-201.