AM30鎂合金硝酸鈰化成處理

Cerium Conversion Coatings on AM30 Magnesium Alloys

本研究於AM30鎂合金進行硝酸鈰化成處理。先前實驗發現硝酸鈰化成皮膜組成為由內而外的多孔層、緻密層以及髮狀層，且皮膜主要為氫氧化物組成；當加入過氧化氫後，溶液中的鈰離子氧化成四價鈰離子，化成皮膜在微結構上由內而外為多孔層與緻密層，在化成期間產生的氫氣泡脫離表面時生成起泡缺陷（blister）使得皮膜附著性不佳，若減緩其成膜速率可減少破壞的產生。本研究藉由改變濃度及溫度調控化成反應速率，以得到最佳之皮膜性質。利用掃描式電子顯微鏡（SEM）觀察其顯微結構；並以極化曲線、交流阻抗（EIS）以及鹽霧試驗來評估其皮膜抗蝕能力；再由百格試驗測試皮膜附著性。

In this study, cerium conversion coatings were conducted on AM30 magnesium plates. The previous works have shown a three-layer conversion coating formed in a Ce (III) solution, including a porous layer overlaying on the magnesium substrate, and an intermediate compact layer, followed by a fibrous layer as the major overlay. The coating is mainly composed of Mg hydroxides. Adding H2O2 to Ce (III) solution results in oxidation of Ce (III) to Ce (IV). The layered structure is altered to an inner porous layer and an outer compact layer. However, hydrogen evolution, a biproduct during conversion treatment, results in blisters, which deteriorate the coating adhesion. Reducing the conversion reaction rate effectively eliminates the blister problems. This study thus aims at obtaining an optimal Ce conversion coating by varying the solution concentration and temperature. Several techniques were adapted to investigate the microstructure and properties of the coatings, including the defect inspection by SEM; the corrosion resistance by polarization test, electrochemical impedance spectroscopy (EIS), and salt spray test; the coating adhesion by tape test (ASTM D3359).