以pH值探討AZ31鎂合金磷酸鹽/錳酸鹽化成皮膜成長特性

Abstract

隨著鎂合金的應用日益廣泛，提升其抗蝕性成為相當重要的課題。長期以來，鎂合金之鈍化處理以鉻酸鹽處理為主，但由於鉻酸鹽對環境及人體具有毒性，已逐漸被世界各國所禁用。在許多取代鉻酸鹽的處理系統當中，本實驗針對極具潛力之磷酸鹽/錳酸鹽系統，以微結構分析技術與電化學量測方法，探討化成液pH値對化成皮膜性質影響之研究。 實驗結果發現，在pH值4.0的化成液中，皮膜呈現兩層結構，分別為與底材相接觸的多孔層，與批覆於多孔層上之柱狀層。當化成液pH値提升至5.7後，皮膜除了多孔層與柱狀層之外，還有結晶性的磷酸鹽析出物批覆其上。相較於pH值4.0下生成之皮膜，pH值5.7下所得的皮膜較薄且顏色較淺，同時具有良好的光澤度與附著性，且在極化曲線量測中表現出較低的腐蝕電流。 此外，本實驗利用厚度量測與SEM橫截面影像觀察，針對磷酸鹽/錳酸鹽化成皮膜進行動力學參數之討論。結果顯示，在pH4.0化成液中處理時，構成皮膜主體之柱狀層在完全潤濕的情形下對於底材之保護性相當有限，而抗蝕性主要來自於多孔層。提升pH值後，磷酸鹽結晶的析出可以抑制柱狀層的成長，進而降低裂紋的生成，並獲得較厚的多孔層，提升皮膜抗蝕性。

Magnesium alloys have become a group of the most widely used materials for their outstanding mechanical properties. However, the poor corrosion resistance is a critical weakness of most magnesium alloys. Among the various surface modification treatments, chromate conversion coating has received most attention in the past decades; however, the use of hexavalent chromate is prohibited due to environmental concerns. The phosphate/permanganate system, which is more environmental friendly and has been shown to have comparable corrosion resistance as compared to the chromate counterpart, is now the developing system with most expectation. In former researches, the pH value has been mentioned to be the key factor of the phosphate/permanganate conversion coating. Nonetheless, the pH effects on both growth behavior and coating properties are less well studied. Therefore, this study, with the aids of microstructural analyses and electrochemical measurements, aims at detailing the pH effects on the phosphate/permanganate conversion coating on AZ31 magnesium alloys. The results show that the coating formed under pH 4.0 is composed of two layers: the inner porous layer contacting the substrate, and the outer cellular layer, which shows litter protection during the conversion coating treatment. On the contrary, the coating formed under pH 5.7 is covered by crystalline struvite (MgNH4PO4•6H2O) precipitates, which seems to inhibit the formation of the cellular layer. Compared to the coating formed under pH 4.0, the thinner coating with lighter color formed under pH 5.7 shows enhanced adhesion property and better performance in corrosion test.