- 學位論文
酸洗前處理對AZ91D鎂合金錫酸鹽化成皮膜微結構與性質的影響
Effect of Acid Pickling on the Microstructure and Properties of Stannate Conversion Coating on AZ91D Magnesium Alloy
摘要
本文探討不同酸洗前處理對AZ91D鎂合金錫酸鹽化成皮膜微結構的影響，並釐清酸洗處理後，錫酸鹽皮膜顆粒在AZ91D底材不同相上方成核機制，最後再以鹽霧試驗量測不同前處理後錫酸鹽化成皮膜之抗蝕性質。 實驗結果顯示以11wt% HF進行酸洗前處理後，SEM觀察下可看到底材表面局部區域有溶蝕的現象發生，而以0.5wt% HF進行酸洗前處理後，底材受到的溶蝕程度則較不嚴重，以0.5wt% HNO3進行酸洗處理後，α相受到侵蝕速率大於ß相，底材呈現凹凸不平整之表面狀態。在橫截面TEM 觀察下，以11wt% HF、0.5wt % HF與0.5wt% HNO3酸洗前處理後試樣，表面皆覆蓋了一層鈍化膜，比較鈍化膜微結構後發現，11wt% HF酸洗後所形成的鈍化膜較0.5wt% HNO3酸洗後所形成鈍化膜緻密，0.5wt% HF鈍化膜的緻密程度則介於11wt%HF與0.5wt% HNO3之間。在後續的化成處理中，以11wt% HF或0.5wt%HNO3酸洗前處理後的試樣，在30秒化成處理內皮膜顆粒先於ß相及其相鄰的位置上成核。將化成時間延長至10分鐘後，試樣表面則佈滿錫酸鹽化成皮膜。橫截面SEM觀察發現，經11wt% HF酸洗後化成皮膜顆粒的堆疊較為密集且顆粒與顆粒間縫隙較小，以0.5wt% HF或0.5wt% HNO3酸洗後化成皮膜顆粒的堆疊則會在局部區域，顆粒與顆粒間會出現略為粗大的縫隙。鹽霧試驗果則顯示經11wt%HF酸洗處理後，皮膜顆粒堆疊較為緊密，因此抗蝕性較為良好，當以0.5wt%HF、0.5wt%HNO3酸洗處理後，皮膜顆粒堆疊不夠緊密，進而產生過大的縫隙時，抗蝕性則會下降。
並列摘要
The purpose of this study is to explore the effect of various pickling pretreatments on the nucleation and microstructure of stannate conversion coating on AZ91D magnesium alloy, with emphasis on the effect of theα and β phases of AZ91D.The salt spray test was used to investigate the corrosion resistance of stannate-coated samples with different pickling pretreatments. Local dissolution was observed on the AZ91D substrate after pickling in 11 wt% HF solution. This local dissolution was less severe in 0.5 wt% HF solution. Theα phase was more severely attacked than theβ phase in 0.5wt% HNO3 solution, resulting in a relatively rough surface. Cross-sectional TEM showed the presence of a passive layer on the surface of the AZ91D substrate after pickling preatment. Among the various passive layers, the one formed in 11wt% HF solution was the most compact, whereas the counterpart formed in 0.5 wt% HNO3 solution was the most porous. After 30 s of conversion coating treatment, stannate conversion coating nucleated preferentially on the ß phase or the area near the ß phase for the substrate after pickling in 11 wt% HF solution or 0.5 wt% HNO3 solution. When the conversion treatment was prolonged to 10 min, the sample was nearly completely covered with the conversion coating. The compact density of stannate conversion coating on the 11 wt% HF-pretreated sample was higher than those on the 0.5 wt% HF-pretreated or 0.5 wt% HNO3-pretreated samples. For example, the interstices in between magnesium stannate particles were more frequently observed on the samples with 0.5 wt% HF or 0.5 wt% HNO3 pretreatments. As a result, the stannate-coated sample with 11 wt% HF pickling pretreatment had better corrosion resistance than the counterparts with 0.5 wt% HF or 0.5 wt% HNO3 pretreatments.