釩酸根對 AZ31 和 AZ91 鎂合金硝酸鈰化成皮膜結構與性質之影響

Influence of Vanadate Additive on the Structure and Properties of Cerium Conversion Coatings on AZ31 and AZ91 Magnesium Alloys

羅文昕(Wen-Hsin Lo)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.01382

摘要

鎂合金應用日益廣泛，但因過去大量使用之六價鉻化成溶液，對人體及環境會造成危害，近年來大量研究投入發展鎂合金化成系統之替代溶液。其中以稀土鹽類中之鈰鹽具有抑制金屬腐蝕效果，為一主要發展研究之方向。然而鈰鹽系統化成反應時間較長，添加過氧化氫可有效提升反應速率，卻又因加速反應使得氫氣泡大量生成，造成具表面缺陷之皮膜結構，缺陷面積高達 58 %。為抑制缺陷形成獲得良好性質之鈰鹽化成皮膜，藉添加釩酸鹽至鈰鹽化成溶液中，以獲得一緻密性、附著性及抗蝕性皆佳之皮膜。本研究針對，直接添加釩酸鹽及先行調整釩酸鹽 pH 值後之鎂合金鈰鹽化成皮膜，進行微結構、電化學分析與鹽霧試驗抗蝕性評估。
結果顯示直接添加釩酸鹽之鈰鹽化成溶液，雖溶液易發生 CeO2 沉澱，但此混合化成溶液適當之化成反應速率，可產生較薄但緻密、附著性佳，且表面缺陷少之雙層結構化成皮膜。由電化學及鹽霧分析上，抗蝕性有大幅提升。此外，增加添入釩酸鹽之濃度可獲得更佳之抗蝕性皮膜。為提升此混合化成溶液穩定性，先行調整釩酸鹽溶液 pH 值後再添加成混合化成溶液，可於化成短時間內獲得較高抗蝕性之化成薄膜，同時，抑制缺陷形成，皮膜缺陷面積僅 36 %，單位面積缺陷也具縮小之趨勢。透過比較添加不同濃度以及調整 pH 值所得之鈰-釩酸鹽混合化成溶液，本研究討論不同系統之化成皮膜成長機制以及缺陷成因，藉結果提出最佳之鎂合金鈰-釩混合化成皮膜之可能改良方案。

關鍵字

AZ31 鎂合金 ； AZ91 鎂合金 ； 鈰鹽化成處理 ； 釩酸鹽化成處理 ； 抗蝕性

並列摘要

Magnesium-based alloys have attracted many attentions these years because of their outstanding properties. However, the inadequate corrosion resistance limits the application of magnesium and its alloys. Many studies have established the feasibility of the conversion coating of rare earth metal salts on Mg alloys. Especially, the characteristic of inhibition of metal corrosion by cerium salt extends the research interest in the field of conversion coating. However, a long period of conversion time is necesssary to from a cerium conversion coating. Although adding H2O2 accelerates the conversion rate, the excess hydrogen evolution leads to the blisters on the coating, which has a blister area of 58 %. The present study adds metavanadate into cerium-based conversion solution in order to obtain a conversion coating with dense structure for improved adhesion and corrosion resistance. The analyses of microstructure observations, EIS and salt spray test are conducted to investigate the conversion coating on AZ31 and AZ91.
The results indicate the conversion solution containing metavanadate produces a thin bilayered coating without blisters on Mg alloys. The corrosion resistance is improved significantly. Moreover, increasing the concentration of metavanadate promotes the influence of vanadate on the properties of the conversion coating. Furthermore, this work adjusts the pH value of metavanadate solution prior to the addition to the conversion solution, for improving the stability of conversion solution. Adding the solution of metavanadate with pH 2.8-2.9, the conversion coating exhibits good corrosion resistance within short conversion times. Meanwhile, the formation of defect is inhibited. The surface defects area can be reduced to 36 %, and the size of each blister becomes much smaller. The present study compares the different cerium-based conversion coatings with adding metavanadate. Finally, the mechanisms of conversion and defect formation are discussed in details.

並列關鍵字

AZ31 magnesium alloys ； AZ91 magnesium alloys ； cerium conversion coating ； vanadate conversion coating ； corrosion resistance

參考文獻

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