- 學位論文
LZ91鎂合金之過錳酸鹽皮膜微結構與耐蝕性質
Microstructure and corrosion resistance of permanganate conversion coating on LZ91 magnesium alloy
摘要
LZ91雙相鎂合金相較於傳統AZ系列鎂合金，其密度更低且質量更輕，再加上其具有較佳的機械加工性以及延展性，使其在結構件上的應用逐漸被重視。但是由於LZ91雙相微結構所誘發的伽凡尼效應導致其耐蝕性更差而限縮使用範圍。因此本研究以錳酸鹽化成處理於底材表面披覆一層鈍化層冀以提升材料整體抗蝕能力，同時調控溶液溫度以尋求擁有最佳抗蝕能力之參數。 為了解化成溶液溫度以及底材雙相效應對於過錳酸鹽化成皮膜披覆於LZ91鎂合金的性質與成膜機制以及其抗蝕能力之影響，本研究之化成溶液為0.1M過錳酸鉀並以濃硫酸調至pH=1.5，再將LZ91底材分別浸泡於不同溫度(15℃、25℃、40℃、60℃)溶液中進行10 s化成反應。皮膜之表面形貌及橫截面微結構是藉由掃描式電子顯微鏡與穿透式電子顯微鏡觀測而得。皮膜之組成成分則是以化學分析電子光譜儀以及歐傑電子能譜儀測得。皮膜抗蝕能力之評估是以動電位極化曲線、交流頻譜阻抗分析及鹽霧試驗機檢測而得。實驗結果顯示經不同溫度化成反應所製得之皮膜均由MgO, MnOx, Mg(OH)2所組成，其顯示化成反應中鎂鋰金屬之氧化/溶出即伴隨著過錳酸根之還原而形成錳氧化物沉積於皮膜中。皮膜厚度則隨著化成溫度升高而有逐漸減薄之趨勢。此外，15℃條件下化成之皮膜佈滿脫水裂紋，影響其抗蝕能力之表現，隨著化成溫度逐漸升高，脫水裂紋也明顯被抑制之現象。鹽霧試驗結果顯示，四組參數下所製得之皮膜均能通過12小時鹽霧試驗。最後，極化曲線以及交流頻譜阻抗則顯示，於溫度40℃條件下化成之皮膜擁有最佳之抗蝕能力。此外，經由底材浸泡於不同溫度化成溶液中析氫測試之結果顯示，提高化成溶液溫度有助於成膜反應加快，且皮膜鈍化表現較佳。然而，經由橫截面觀察結果顯示，本研究採用之酸性化成系統由於其反應性極高，在雙相底材上快速驅動界面pH值上升並促使成膜反應發生，因此可以抑制底材雙相效應對於皮膜不均的影響，使皮膜短時間內能較有效地於底材表面全面披覆。
並列摘要
LZ series magnesium alloys have lower density and better formability than AZ magnesium alloys that have found many applications. The LZ magnesium alloys are thus potential for structural applications. However, the LZ alloy has poor corrosion resistance resulting from galvanic corrosion prevailing in the and phase. This study, therefore, aimed at developing a permanganate conversion solution for LZ91 alloys. Moreover, the effect of conversion bath temperature was detailed so as to prepare the permanganate conversion coating effectively enhancing the corrosion resistance of LZ91 alloys. Permanganate conversion coatings were formed on LZ91 alloys in the permanganate solution with pH value of 1.5 at different temperatures, 15℃、25℃、40℃、60℃, respectively. The surface and cross-section morphology of the permanganate coating were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XPS and Auger spectroscopy were employed to measure the composition of the permanganate coating. The corrosion resistance of the permanganate-coated LZ91 alloys was measured by potentiodynamic polarization, EIS, and salt spray tests. Results show that the permanganate coating was composed of MgO, MnO2, and Mg(OH)2 , indicating that permanganate ions was reduced to manganese oxides when magnesium and lithium were oxidized and dissolved. The thickness of the permanganate coating decreased with increasing bath temperature. the permanganate coating formed at 15℃ was relatively thick and suffered severe cracking after dehydration. Increasing permanganate bath temperature retarded the growth of the permanganate coating and coating thus formed had better corrosion resistance. Although the permanganate-coated LZ91 sustained after 12h of the salt spray test regardless of the permanganate bath temperature, the EIS and polarization measurements showed that the coatings formed at 40℃ afforded better protection over the LZ91 alloy. Finally, hydrogen evolution experiment showed that elevating permanganate bath temperature promoted the reaction and optimized the passivation of the coating. Nevertheless, the effect of dual phase induced galvanic corrosion on the conversion mechanism was not notable in such a strongly acidic solution. A full coverage of permanganate coating on the LZ91 alloy can be obtained in a short term of conversion treatment.
參考文獻
延伸閱讀
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