| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 93 === This study details the effects of solution temperature and potassium permanganate (KMnO4) concentration on the properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy. The growth the coating was substantially suppressed as the solution temperature was increased from 40 to 60℃. Moreover, the coating formed at 40℃ exhibited a porous structure mainly composed of oxygen, magnesium, phosphorus and manganese species, and had poor adhesion to the substrate as characterized by SEM. Cross-sectional TEM further revealed that when immersed in the solution with 10 gl-1 KMnO4, the coating formed at 40℃ comprised an cellular overlay and a porous inner layer contacting with the substrate, while that formed at 60℃ exhibited a three-layered structure: a cellular overlay, an amorphous intermediate layer and a porous inner layer.
Increasing solution temperature improved the coating adhesion to the substrate. Adding more KMnO4 to the solution resulted in the coating having higher impedance resistance and hence better corrosion protective properties after a short period of salt spray test. The corrosion resistance of the coating, however, degraded markedly during prolonged salt spray test. This is due to the thinner coating formed in the solution containing larger amounts of KMnO4. Finally, the coating after salt spray test contained more manganese species than the as-plated coating. Manganese oxide, which was likely present in the coating, seemed to enhance the corrosion resistance of phosphate/permanganate coating.
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