硝酸根與硫酸根對AZ31B鎂合金腐蝕行為與化成反應的影響

Effect of NO3- and SO42- Anions on the Corrosion Behavior and Conversion Coating Reaction of AZ31B Magnesium Alloy

張晉愷(Chin-Kai Chang)

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(2019年)

https://doi.org/10.6342/NTU201901880

摘要

鎂合金低密度、高比強度與良好的散熱性等特性在現在追求節能減碳的趨勢上成為一個重要的輕量化結構材料。然而鎂合金的高化學活性與多孔的腐蝕產物造成其抗蝕性不佳，大大限制了鎂合金的應用。此外，隨著現代高度工業化的影響下，空氣汙染物中所包含的許多陰離子都會影響到鎂合金的腐蝕。因此為了改善鎂合金的抗蝕性，了解鎂合金在不同陰離子下的腐蝕行為與發展有效的防蝕處理是重要的。
本研究利用腐蝕產物微結構分析與成分分析搭配電化學性質分析，探討AZ31B鎂合金在硝酸根與硫酸根水溶液中的腐蝕行為與腐蝕機制。並且在後續引入鋁離子，進行硝酸鋁與硫酸鋁的化成處理，試圖提升鎂合金的抗蝕能力與更加了解硝酸根與硫酸根對於鎂合金的作用。TEM橫截面微結構結果顯示，當AZ31鎂合金浸泡於硝酸根水溶液中，浸泡時間由1分鐘增加至25分鐘，其腐蝕產物膜層由70 nm成長到95 nm，總厚度只成長25 nm。而在硫酸根水溶液中，當浸泡時間由1分鐘增加至25分鐘，腐蝕產物膜層由厚度不均的90~250 nm成長到了450 nm。此結果表示，鎂合金的溶解與沉積反應在硝酸根水溶液中有所被抑制。同時，浸泡於陰離子溶液中的析氫試驗也證實此結果。AZ31浸泡於硫酸根溶液中的析氫量比在硝酸根溶液中多非常多，表示在硫酸根水溶液中，鎂合金的腐蝕反應確實沒有被抑制，反而持續地大量發生。此外，XPS的結果則顯示，抑制鎂底材的腐蝕反應發生的機制為:在硝酸根作用下，鎂合金的表面存在一層較連續且緻密的氧化鎂膜層以提供保護。EIS的結果則顯示在硝酸根溶液中的腐蝕產物，其總阻抗值最大，表示在硝酸根作用下的腐蝕產物確實是較具保護力的。化成處理的部分，由SEM與TEM微結構結果得知，硫酸鋁化成皮膜總厚度達830 nm的三層結構且皮膜呈現許多脫水裂紋。硝酸鋁化成皮膜則是厚度均勻約110 nm的單層結構。析氫試驗則顯示在硫酸鋁水溶液中產生大量的氫氣，表示鎂底材大量溶解與沉積反應導致劇烈的析氫反應。最後，電化學分析的結果顯示，硝酸鋁化成系統可以有效的提升AZ31的抗蝕能力。然而，對於硫酸鋁化成系統，因為大量的脫水裂紋，無法有效的提升AZ31的抗蝕能力。

關鍵字

AZ31B鎂合金 ； 陰離子作用 ； 表面分析 ； 電化學 ； 化成處理

並列摘要

In light of low density, high specific strength, and good heat dissipation properties, magnesium alloys are promising for lightweight structure applications in the pursuit of energy saving and carbon dioxide reduction. However, the high chemical activity and porous corrosion products of magnesium alloys cause poor corrosion resistance, which greatly limits the applications of magnesium alloys. In addition, due to the modern industrialization, many anions contained in air pollutants generally affect the corrosion behavior of magnesium alloys. Therefore, in order to improve the corrosion resistance of magnesium alloys, it is imperative to understand the corrosion behavior of magnesium alloys under different anions and to develop effective corrosion protection treatments.
In this study, the corrosion behavior and corrosion mechanism of AZ31B magnesium alloy in nitrate and sulfate aqueous solution were investigated using microstructure characterization, composition measurement and electrochemical analysis of the corrosion products. Moreover, aluminum ions were introduced to conduct the conversion coating treatment in aluminum nitrate and aluminum sulfate solutions, in an attempt to improve the corrosion resistance of magnesium alloys. The TEM cross-sectional images showed that as the AZ31 magnesium alloy was immersed in nitrate solution for 1 min to 25 min, the corrosion product layer grew from 70 nm to 90 nm. However, as the immersion in sulfate solution was increased from 1 min to 25 min, the corrosion product layer grew from 90-250 nm to 450 nm. This indicated that the dissolution and precipitation reaction of AZ31 magnesium alloy was inhibited in nitrate solution. The hydrogen evolution test in anion solutions further confirmed this inhibitory effect. The amount of hydrogen evolution of the AZ31 immersed in sulfate solution was much higher than that in nitrate solution, indicating that the corrosion reaction of magnesium alloy prevailed in sulfate solution and continued to a larger extent. Moreover, the XPS results showed that the mechanism for inhibiting the corrosion reaction of magnesium is that in nitrate solution, a relatively continuous and dense layer of magnesium oxide was present on the surface of the magnesium substrate, which provides protection against corrosion attack. The EIS results showed that the total impedance of the corrosion products formed in nitrate solution was larger than that formed in sulfate solution, indicating that the corrosion products formed in the presence of nitrate anions are indeed more protective. In the part of the conversion coating treatment, the SEM top-view images and TEM cross-sectional micrographs showed that the conversion coating layer formed in aluminum sulfate solution was a three-layer structure with total thickness of 830 nm and contained many dehydration cracks. In contrast, the conversion coating layer formed in aluminum nitrate solution was uniform in thickness of about 110 nm. The hydrogen evolution test showed that hydrogen bubbles were generated abundantly in aluminum sulfate solution, indicating that dissolution and precipitation reactions occur abundantly, resulting in a vigorous hydrogen evolution reaction. Finally, the results of electrochemical analysis showed that the aluminum nitrate conversion coating system can effectively improve the corrosion resistance of AZ31. Nevertheless, for the aluminum sulfate conversion coating system, the corrosion resistance of the AZ31 could not be effectively improved due to the presence of a large number of dehydration cracks.

並列關鍵字

AZ31B magnesium alloys ； effect of anions ； surface analysis ； electrochemical analysis ； conversion coating

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