AZ31鎂合金硝酸鈰化成皮膜結構與性質研究

Microstructure and Properties of Cerium Nitrate Conversion Coating on AZ31 Magnesium alloy

10.6342/NTU.2008.02576

李偉任

硝酸鈰化成處理 ； 橫截面穿透式電子顯微鏡觀察 ； 極化曲線 ； 交流阻抗 ； 百格試驗 ； Cerium nitrate conversion coating treatment ； Cross-sectional TEM ； Polarization curve ； Electrochemical impedance spectroscopy ； Adhesion test

國立臺灣大學材料科學與工程學系學位論文

2008年

博士

林招松

繁體中文

本研究於AZ31B鎂合金表面進行硝酸鈰化成處理，探討過氧化氫、前處理、黏度調整和重複浸泡等製程的影響。使用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)及穿透式電子顯微鏡(TEM)觀察巨觀和微觀顯微結構，並以半定量成份分析(EDX)和擇區繞射分析皮膜成份，搭配化學分析電子光譜儀(ESCA)、低掠角X光繞射儀(GA-XRD)鑑定皮膜組成。以極化曲線、交流阻抗(EIS)分析皮膜抗蝕能力，另以百格試驗評估化成皮膜附著性。 結果顯示硝酸鈰化成皮膜由外至內為髮狀層、緻密層和多孔層，皮膜組成以氫氧化物為主，當化成皮膜脫水時會造成裂紋和缺陷，導致皮膜容易脫落。在化成液中添加過氧化氫可將Ce3+氧化成Ce4+，且溶液pH下降溶氧度上升，化成反應加速，並使皮膜色澤由灰色變成金黃色，皮膜結構由外至內為緻密層和多孔層。但因反應速度增快導致氫氣泡生成加速，在化成期間氣泡脫離表面造成皮膜損傷和剝離。因此在化成液中添加甘油調整劑，可減少化成時氫氣泡生成對皮膜的破壞，以及降低脫水裂紋寬度和連結度。重複浸泡處理可得到最佳化的皮膜，以化成時開路電位為監控標準分隔化成時間，以達到控制化成皮膜厚度和缺陷成長。針對重要製程參數分析並討論化成皮膜成長機制和缺陷成因，在AZ31B鎂合金硝酸鈰化成處理本研究可解決附著性、抗蝕性不佳等缺點。

Cerium conversion coating on AZ31B magnesium has been studied, with emphasis on the pretreatment, the addition of hydrogen peroxide, the viscosity of the solution, and the repeated immersion process. The surface morphology of the coating was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructure and thickness of the coating were characterized by cross-sectional transmission electron microscopy (TEM). The composition and phase of the coating were investigated by energy dispersive spectrometry (EDX), x-ray photoelectron spectrometry (XPS), and glancing angle x-ray diffraction (GA-XRD). Moreover, the corrosion resistance of the coating was measured by polarization test and electrochemical impedance spectroscopy (EIS). Finally, the adhesion of the coating was measured by the tape adhesion test according to ASTM D3359-97 standard. The conversion coating formed in Ce(NO3)3 solution exhibited a three-layered structure: a fibrous major overlay; a compact layer as the immediate layer; and a porous layer directly contacting with the subtracts. The coating was mainly composed of hydroxide, and suffered server cracking and peel off after dehydration. Adding hydrogen peroxide to the Ce(NO3)3 solution changed its color from colorless to orange as Ce3+ was oxidized to Ce4+. Meanwhile, the solution pH decreased to 3 while the dissolved oxygen concentration increased. The addition of hydrogen peroxide accelerated the reaction of coating formation, and the resultant golden coating consisted of a compact outer layer and a porous inner layer. On the other hand, intensive hydrogen bubble evolution during immersion caused blisters and partial peel-off of the coating. To retard the reaction of coating formation, glycerol was added to the Ce(NO3)3/H2O2 solution. The present of glycerol in the solution suppressed the evolution of hydrogen bubbles, and the resultant coating underwent less extents of cracking. A repeated immersion treatment was further employed to improve the adhesion and corrosion resistance of the coating. The improvement was achieved by controlling the thickness and defects of the coating grown during each step of immersion of which the immersion time was chosen based on the open circuit potential of the plate during conversion coating treatment. Finally, the growth and defect formation mechanism of the cerium conversion coating was discussed in detail.

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