| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === Magnesium is the lightest structural metal and its alloys are attractive to the automotive, aerospace and electronic industries for their high ratio of strength to weight. Unfortunately, magnesium alloys exhibit a very poor corrosion resistance and wear resistance. These weaknesses seriously influence the development and application of magnesium alloys. Micro-arc oxidation, which can generate oxide coatings on alloys’ surface, is a high voltage electrochemical surface treatment for magnesium alloys. This technique can remarkably promote the corrosion resistance and wear resistance of magnesium alloys.
In this study, LZ91 and AZ31 magnesium alloys are choosed to be anodized in silicate and phosphate based electrolytes. The effect of various process parameters, including electrolytic composition, current density, duty ratio, frequency, and working time, on MAO coatings was evaluated. Salt Fog Test, Potentiodynamic Polarization Test, and Electrochemical Impedence Spectroscopy (EIS) were conducted for corrosion analysis; Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD) were used to study the morphology, microstructure and composition of coatings.
Test results show that MAO treated magnesium samples with proper process parameter exhibit excellent corrosion resistance. The effect of cathodic current density and duty ratio is smaller than the one of frequency and working time. In all experiments, samples produced by 1000 Hz and 8 minutes were always rated better corrosion resistance. After 96 hours salt spray test, LZ-SAc 10% 1000 8 and AZ-SAc 10% 1000 8 had only 1 and 2 pits with total corrosion area 0.009% and 0.061%, respectively. Besides, by studying the microstructure of corroded smaples, flaking effect was observed. MAO treated samples may suffer from faster corrosion rate for this effect.
On the other hand, a color changing phenomenon was observed on MAO treated LZ91 samples. The test results of XRD, SEM and XPS indicate that the degree of crystallinity, morphology and surface composition of coatings remain no change; however, decaying corrosion resistance is showed by EIS data on 15 days idling sample. Therefore, the conversion of color and corrosion resistance may be contributed to the change of composition or chemical state in MAO coatings.
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