AZ31鎂合金之硝酸鈰化成處理
Date Issued
2004
Date
2004
Author(s)
Fang, Ssu-Kai
DOI
zh-TW
Abstract
Cerium conversion coatings were made on AZ31 magnesium plates. In addition to the basic cerium nitrate solution, the effects of solution temperature, pH and the content of hydrogen peroxide on the formation and microstructure of the coatings were also investigated.
Experimental results indicate that upon immersing in the solution, AZ31 plates dissolved to form magnesium ions and trace of aluminum ions. Meanwhile, the pH at the interface rose because of hydrogen liberation. This increase in interfacial pH caused the precipitation of magnesium and aluminum hydroxides. The coating formed in the cerium nitrate solution was characterized by TEM to consist of three layers with a porous layer overlaying the magnesium substrate, an intermediate compact layer followed by a fibrous layer as the major overlay. SEM observations showed that numerous cracks formed on the coating due to desorption of water molecules. Furthermore, the openings of the cracks increased with increasing immersion time, and more badly, might lead to the peel-off of the coating.
Reducing the solution pH by adding nitric acid inhibited the growth of the coating. Adding hydrogen peroxide to the clear cerium nitrate solution resulted in an orange solution, indicating that Ce3+ was oxidized to Ce4+. The coating formed in the solution with the addition of hydrogen peroxide comprised primarily the cerium oxide as the major overlay and a porous magnesium hydroxide layer intimately contacted with the substrate. Partial detachment of the cerium oxides from the substrate was observed and was ascribed to liberation of hydrogen bubbles. Finally, the growth rate of the coating increased with increasing solution temperature by means of enhanced mass transportation.
Subjects
稀土金屬
化成處理
鎂合金
硝酸鈰
Rare earth metal
conversion coatings
magnesium alloys
Type
thesis