添加氟硼酸鹽對鋁合金三價鉻化成皮膜成長行為影響研究

The Behavior of Fluoroborate during the Formation of Trivalent Chromate Conversion Coatings on Aluminum Alloys

本研究主要觀察三價鉻溶液組成中加入氟硼酸鹽對於成膜過程的影響，並嘗試討論氟硼酸鹽於三價鉻成膜過程中所扮演的效用。實驗中分別使用開環電位紀錄法（OCP）記錄其成膜時間與電位變化的情形，使用陽極極化曲線分析法評估氟硼酸鹽對鋁基材的影響，皮膜的微結構使用SEM進行分析與觀察，皮膜的組成則使用XPS進行分析。
由SEM的觀察發現當化成液中含有氟硼酸鹽時可以減少皮膜上的裂紋產生，由XPS的縱深分析與極化曲線測試結果可證實氟硼酸鹽的添加，可以促進鋁基材的溶解增加皮膜的厚度，其皮膜的組成份呈現外層為富有鋯化合物內層為富有鋁化合物的成份梯度變化結構層，並且經由各元素的能譜圖分析後確定皮膜主要的成份組成為Cr2O3、Cr(OH)3、Al2O3、Al(OH)3、ZrO2、Zr(OH)4及AlF3。

The aim of this study was to characterize the role of tetrafluoroborate ions during the formation of trivalent chromate conversion coatings on aluminum alloys. At first, the open circuit potential (OCP) of the aluminum alloy in a trivalent-chromate containing solution was continuously monitored as the immersion time increased. The impact of tetrafluoroborate ions on the coatings was evaluated in a 3.5 wt.% NaCl solution at room temperature via anodic polarization analysis. The structure and valence state of the coatings were examined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively.
The results indicated that the coatings were smooth and dense after being prepared in trivalent chromate baths that contain tetrafluoroborate ions. The XPS depth analysis and polarization analysis results showed the addition of tetrafluoroborate ions could promote the dissolution of the aluminum substrate and increase the thickness of the coatings. According to the XPS analysis, the trivalent chromium conversion coatings were composed of Cr2O3, Cr(OH)3, Al2O3, Al(OH)3, ZrO2, Zr(OH)4, and AlF3, and the coatings exhibited a double-layer structure, of which the outer layer was zirconium-rich and the inner layer was aluminum-rich.