| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 94 === Abstract
Effects of electrolyte composition, current density/potential applied during anodizing and heat treatment on composition, structure and protection of anodic films formed on die cast AZ91D magnesium alloy by sparking were investigated in this study. The base anodizing electrolyte was 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF. Effects of additive on sparking behavior and composition of protective anodic film formed was through adding Al(NO)3 and Na2SiO3 in base electrolyte. The composition of anodic films formed on AZ91D magnesium alloy after heat treatment were analyzed and their electrochemical properties were investigated.
Microstructures of Mg alloy and anodic film were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The chemical properties and crystal structures of anodic film were determined by X-ray photoelectron spectroscopy and glance angle XRD. Corrosion resistance of anodized AZ91D Mg alloy was evaluated by electrochemical impendance spectroscopy (EIS), immersion test and salt spray test.
The results showed that Al(NO3)3 made sparks become more uniform and the uniformity of anodic film was increasing as the concentration of Al(NO3)3 was increasing. The anodic film was mainly composed of MgO. The addition of Al(NO3)3 into the base electrolyte results in the formation of Al2O3 and Al(OH)3 in the anodic film. The maximum amount of Al2O3 was found in the anodic film when the alloy was anodized in the electrolyte containing 0.15 M Al(NO3)3. The results of EIS analysis and morphological examination showed that the MgO anodic film modified with Al2O3 exhibited the superior corrosiom resistance for AZ91D Mg alloy. The polarization resistance of anodic film was about 13700 Ω-cm2. The anodic film was mainly composed of MgO. The additions of Na2SiO3 into the electrolyte enhanced the formations of silicate in the anodic film. Increased corrosion resistance of the anodic film was found when AZ91D Mg alloy was anodized in 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF + 0.15 M Al(NO3)3 with 0.1 M Na2SiO3 addition.
The cooling rate had a significant influence on the microstructure evolution of the AZ91D magnesium alloy after solution heat treatment at 440℃ for 20 h in N2 atmosphere. A single-phase microstructure was observed when the alloy was quenched in water after solution heat treatment. However, a duplex structure consisting of both α and high density β phases was found on the metal surface if the solution-annealed alloy was cooled in air. The polarization resistance for the alloy had highest volume ratio of β phase was relative higher than both as-cast and annealing/water quench Mg alloy. The differences in microstructure of the heat treated AZ91D magnesium alloy gave rise to a significant change in the property of the anodic film formed in 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF + 0.15 M Al(NO3)3 electrolyte. The highest polarization resistance of anodic film was found for that formed on annealed and air-cooled alloy. The presence of Al-rich β phase on the surface gave rise to the formation of a more protective and uniform anodic film which consisted of a great amount of Al2O3.
The degree of crystallinity was increasing after baking and the amount of oxides were increased by dehydration reaction. The polarization resistances of anodized Mg alloys were improved significantly by increasing amount of oxide in anodic film. An optimum value of polarization resistance of anodic film was obtained after baking at 150 ℃ for 2 hr followed by air cooling. The polarization resistance was about 280 kΩ-cm2. but the polarization resistance was decreasing when the baking temperature was obove 150 ℃.
The corrosion resistances of anodized AZ91D Mg alloys were also evaluated by immersion test and salt spray test. The relation between electrochemical impedance spectroscopy and immersion, salt spray test was evolution. The results show that the polarization resistance of as-cast AZ91D Mg alloy was very low. In immersion and salt spray test, localized corrosion appeared quickly and extended fast. Anodizztion treatment enhanced corrosion resistance of AZ91D Mg alloy greatly. During immersion test, localized corrosion appeared and extended slowly as the polarization resistance of anodic film was higher. During salt spray test, the microstructure of anodic film was changed with polarization resistance of anodic film.
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