| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === With excellent specific strengh and low density, magnesium alloys are extensively used in light products, including electrical appliances, bicycles and automobiles. However, since magnesium is chemically reactive (standard reduction potential -2.37 V vs. SHE), surface conversion coating treatments are therefore indispensable for improving the corrosion resistance of magnesium alloys. Hexavalent chromium conversion treatment was widely used in conventional conversion coating treatment, but has been limited by RoHs due to its high toxicity. Thus, the development of alternatives to hexavalent chromium conversion coating treatment is now an urgent necessity.
In this study, AZ91D and AZ31B magnesium alloys were used as experimental materials. Meanwhile, permanganate coatings, which are widely used for surface treatment in industries, are adapted to have fundamental discussions. The surface morphology of the coating was investigated by scanning electron microscopy (SEM). The microstructure and thickness of the coating were characterized by cross-sectional transmission electron microscopy (TEM). The compositions of the coating were investigated by energy dispersive spectrometry (EDS) and x-ray photoelectron spectrometry (XPS). Moreover, the corrosion resistance of the coating was measured by polarization test, electrochemical impedance spectroscopy (EIS) and salt spray test (SST). The adhesion of the coating was measured by the tape adhesion test according to ASTM D3359-97 standard. Finally, the resistivity of the coating was measured by Loresta Meter.
The Experiments are divided into two parts. In the first section of the dissertation, an optimal preparation condition of permanganate conversion treatment is studied. The excellent properties of the permanganate conversion coating is obtained by adding phosphate (KH2PO4) and manganese nitrate (Mn(NO3)2) in the permanganate conversion solution. The Guyard reaction provides a new route to form a thin, nearly crack-free MnO2-rich conversion coating on magnesium alloys. The conversion coating exhibited a two-layered structure: a compact layer major overlay and a porous layer directly contacting with the substrates. Dense passive films were generated on the surface of the magnesium alloy with the interaction of MnO4-and Mn2+, which the thickness of about 200~400 nm and consist of the Mg, O, and Mn composition. Experiment results indicated that the coating formed on magnesium alloys may provide enhanced corrosion protection.
The second section of the dissertation investigated the properties of the optimal permanganate conversion coating proposed in the first section. Thus, with increasing immersion time (48 h), the coating’s lifetime would be definitely responded. Based on the EIS measurement, structure and composition analysis of the conversion coated AZ31 was proposed for understanding the corrosion process of passive film. With the form of the impedance spectrum could be determined the mechanism of the corrosion process. An acid chromate bath, Dow 1, was also used for comparison. The results of the electrochemical measurements and the salt spray tests demonstrate that the corrosion resistance of the AZ31 alloy has been markedly improved by the permanganate conversion treatment. However, the optimal permanganate conversion coating still has corrosion resistance inferior to the chromate (Dow 1) conversion coating. This is likely due to the unique self-healing capability of the chromate conversion coating.
The formation mechanism of permanganate conversion coating was discussed in detail, which emphasis on the evolution of the coating. Moreover, we discussed how the Mg17Al12 (β) phase affects the microstructure of the coating and the aging time of conversion solution.
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