AM30鎂合金之錳酸鹽化成處理

Abstract

以往用於保護鋁合金之錳酸鹽化成處理目前被視為能夠提升鎂合金抗蝕性之重要製程。本研究於第一部分主要探討錳酸鹽濃度對於AM30鎂合金之錳酸鹽化成皮膜特性的影響，以及於第二部分研究浸泡於0.1 M酸性錳酸鹽化成液之皮膜成長機構和能夠提升溶液穩定性之鉬酸鈉添加劑對於後續生成皮膜性質所造成的影響。研究結果顯示，溶液中添加越多的錳酸鹽濃度降低整體皮膜的成長速率，並且於初期形成富含錳的緻密氧化層，因此利用此簡便且低成本之新製程可獲得附著性及抗蝕性極佳的薄膜。另一方面，添加鉬酸鈉於化成液中雖然可以完全地解決溶液之不穩定性，然而卻因為吸附或儲存於皮膜缺陷中之聚鉬酸鹽，而導致錳酸鹽化成皮膜的抗蝕性大幅下降。而以最佳製程條件下，錳酸鹽化成皮膜於24小時鹽霧試驗後的腐蝕面積降為0.07 %，顯示此皮膜具有優異的抗蝕效果。本論文最後則是針對最佳參數的錳酸鹽化成皮膜提出可能的成長機構。

Permanganate conversion coatings, which have been used as protective coatings for aluminum alloys, are now being considered the prominent process for protecting the magnesium alloys from corrosion. This study investigated the effect of permanganate concentration on the properties of permanganate conversion coating on AM30 magnesium alloys in the first part. Moreover, the evolution of conversion coating immersed in 0.1 M acidic permanganate solution and the characterization of the coating with the addition of sodium molybdate, which stabilized the solution markedly, were studied in the second part. Results show that increasing the permanganate concentration reduced the growth rate of the coating via the initial formation of compact layer comprising manganese oxides. As a consequence, a thin layer with superior adhesion and corrosion resistance was obtained using the innovative process with simplicity and low cost. Adding Na2MoO4 in the conversion bath stabilized the solution completely, however, deteriorating the corrosion resistance dramatically on account of acidic isopolymolybdate ions stored or absorbed on the coating. In the optimal processing conditions, the corrosion area dropped down to approximate 0.07 % after 24 hrs salt spray tests, indicating the excellent corrosion resistance of the permanganate conversion coating. The formation mechanism of permanganate conversion coating was discussed in detail, which emphasis on the evolution of the coating with optimal parameter.