| Magnesium is one of the most abundant elements in the earth, which alloys have gained considerable interest as structural material for computer, mobile and aerospace application due to high strength to weight ratio, good electromagnetic shielding and damping characteristics. However, magnesium alloys also possess some disadvantageous properties including poor corrosion, wear resistance and high chemical reactivity, while the former has been the bottleneck for their widespread use in many applications. Some surface modification techniques have been developed for protection of Mg alloys. These techniques include electrochemical plating, conversion coatings, anodizing, laser surface alloying, organic coatings and so on. Recently, micro-arc oxidation (MAO) has been applied in surface treatment of Mg alloys more and more. Micro-arc oxidation, as a new surface modification technology developed from traditional researches. MAO treatment has been used extensively to modify the surface of magnesium alloys with higher corrosion resistance, higher hardness and improved wear-resistance properties, as well as enhanced paint adhesion.In our previous work, we have already obtained high corrosion resistance anodic coating with silicate and aluminate in alkaline borate electrolyte. The effects of phosphate concentration with or without aluminate and silicate on the morphology, composition and corrosion resistance of anodic coating formed with AC power source are investigated in this thesis. The properties of anodizing films were ascertained using potentiodynamic polarization method in conjunction with the scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. X-ray diffraction patterns show that the composition of the film is mainly consisted of MgO, MgSiO3 and MgAl2O4. The anodizing films are integrated firmly, due to sintering which is reinforced by phosphate. The corrosion current density of experiment test is about 1×10-8A/cm2, which indicates the phosphates can promote the corrosion resistance comparing with our previous work. In order to improve the corrosion resistance of anodizing film more, we manufactured the pulse power source to be used in the anodizing of AZ91D magnesium alloys. The parameters of equipment include wave form, frequency, duty cycle and voltage. We found that the higher frequency and the smaller duty cycle can reinforce the intensity of anodizing of AZ91D magnesium alloys.The corrosion processes of anodized films on AZ91D magnesium alloys in neutral 0.1mol/L NaCl solutions were investigated using electrochemical impedance spectroscopy (EIS) method for long immersion time.In order to investigate the effect of additive phosphate, EIS plots of the four different films were searched. Both Nyquist and Bode plots contain a low-frequency inductive component, which has been interpreted as a manifestation of pitting or has been attributed to the formation and precipitation of a salt film. Different corrosion process of these MAO films may due to their different morphologies structure and chemical composition. The Rt moves up and down when the electrolyte contains aluminates, which maybe owe to the self-sealing process of anodic film in solution contain Cl- but from pitting corrosion. The EIS results indicate that Si-P-film and Al-Si-P-film represent comparatively good corrosion resistant than Al-P-film and P-film in a long immersion test, which presents a litter different comparing with the polarization results.In order to control the process of anodizing process when using pulse source steadily, the parameters (frequency: 2000Hz, duty cycle: 0.5) were chosen. During the corrosion process of AZ91D magnesium alloy, the radii of capacitive arc diminish gradually with the elongation of corrosion time, but the low frequency range impedance corresponds to the inner layer properties don't decrease evidently. It can be seen that this film have a uniform and compact inner barrier layer comparatively.
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