AZ91D鎂合金非鉻型化成皮膜結構與性質研究

Abstract

本研究第一部份將探討AZ91D鎂合金在壓鑄過程中壓鑄件表面產生的凝固激冷層(die-chill skin)對其後續錫酸鹽化成處理的皮膜成長特性及抗蝕性之影響。實驗中利用低掠角X-ray繞射及穿透式電子顯微鏡分別對具有die-chill skin及移除die-chill skin 的底材做分析與比較，並在二種底材試樣經過60秒及600秒之錫酸鹽化成處理後，利用掃描式電子顯微鏡觀察化成皮膜表面形貌，以極化曲線量測結果評估各種試片抗蝕能力的差異，並以低掠角X-ray繞射分析鑑定化成皮膜的組成。實驗發現，具有die-chill skin的底材試樣在低掠角X-ray繞射圖譜中可比對到氧化鎂(MgO)的訊號峰，此外在穿透式電子顯微鏡的觀察下發現，具有die-chill skin的底材表面孔洞處有明顯的氧化物的存在。在600秒錫酸鹽化成處理後，利用SEM觀察發現具有die-chill skin的底材上錫酸鹽皮膜分布較不均勻的現象。在極化曲線上，經過600秒之錫酸鹽化成處理後，具有die-chill skin的底材有較高的腐蝕電位及較低的陽極極化電流值。 第二部分實驗將針對AZ91D雙相鎂合金於不同化成溶液中，化成皮膜成長行為之差異做一詳細比較及探討，特別著重於微結構的分析。實驗結果顯示在鹼性錫酸鹽化成系統中，由於在化成初期β相及β相周圍的鎂離子較容易溶出，因此錫酸鹽皮膜在初期成核時會傾向先在β相及β相周圍形成，而隨著化成時間的增加，這些球狀皮膜的覆蓋率也會隨之上升。當AZ91D鎂合金浸泡在酸性硝酸鈰化成溶液中時，化成初期則是會在底材表面形成一層均勻的皮膜，同時由於α相處會持續有氧化及還原反應大量發生，因此化成過後會在α相處觀察到blister 的缺陷產生。而隨著化成時間的增加，硝酸鈰化成皮膜上會有越來越多的皮膜脫落的情況發生。 論文的最後一部分實驗將針對AZ91D鎂合金在硝酸鈰化成處理製程中的前處理液進行相關的研究，主要著重於不含氫氟酸及氟化物之前處理液的開發。實驗結果發現，相較於氫氟酸及鹽酸前處理的試樣，AZ91D鎂合金經由釩酸鹽前處理後，將可良好的控制後續硝酸鈰化成的反應速率，對於降低化成處理過程中局部起泡(blisters)缺陷與皮膜脫落的(peel-off coating)有明顯助益。此外，經由鹽霧試驗、極化曲線及交流阻抗分析的結果，說明了釩酸鹽前處理的試樣在經過化成處理後，所得到的較為連續的化成皮膜能提供給鎂底材更加的保護性。

In the first part of this paper, the effect of the different die-casting microstructures on the stannate conversion coating treatment has been investigated on AZ91D magnesium alloy, with emphasis on the die-chill skin and α/β (Mg17Al12) eutectic structure. Prior to the immersion in the stannate bath, AZ91D substrates were pickled in 11wt﹪HF solution at room temperature, respectively. After thorough cleaning in running water, the substrates were immersed in a stannate bath for 60s and 600s. SEM, TEM, EDS and XRD were employed to analyze the surface morphology and detailed microstructure of the coating on the AZ91D substrate. In addition, the electrochemical polarization measurement was performed to compare the corrosion resistance of the coating on the AZ91D with and without the die-chill skin. The results showed that the die-chill skin contain a lot of pores and those pores are covered with oxide. Besides, the stannate coatings on the substrate with die-chill skin after 10 minutes immersion show higher corrosion potential and smaller anodic current. In the second part, the formation characteristics of conversion coating on the AZ91D magnesium alloy were discussed. Comparing to the α phase, the hydroxides films formed on the β phase at the beginning of conversion process provide insufficient protection. Since the galvanic effect between β phase and eutectic α + βcould occur most easily, the nucleation of stannate conversion coating depending on sufficient Mg2+ took place predominantly in the vicinity of β phase. In the contrast, formation of ceirum conversion coating on AZ91D was largely relied on the increase of the pH value. Since α phase is more active than β phase in the acid solution, the severe Mg2+ dissolution causes the production of H2 bubbles, leading the formation of blisters on the α phase at the beginning of cerium conversion process. In the last part of this papar, a new approach for alternative of toxic hydrofluoric acid (HF) in the magnesium pretreatment process was investigated. The hydrochloric acid, hydrofluoric acid and vanadate solution were made on the AZ91D magnesium alloy before cerium/hydrogen peroxide conversion coating respectively. The microstructure of coatings was observed by transmission electron microscopy (TEM), and the electrochemical impedance spectroscopy (EIS)、Polarization curves and salt spray test (SST) were carried out to evaluate the corrosion resistance. The results show that, comparing with HF and HCl pretreated samples, the passive layer formed in vanadate-containing bath (pH=8) effectively restrained the reaction rate of Ce conversion coating and apparently reduced the number of blisters and partial peel-off of the coating. Both the EIS and SST results show that the better corrosion resistance coating can be obtained when the samples were pretreated in the vanadate-containing bath.