| Magnesium alloy has ideal mechanical compatibility, biocompatibility and degradability, which implies its widely developing prospects in clinical medicine. However, the medical application of magnesium alloy is restricted severely by its high corrosion rate. Metal surface silanization has been developing as a novel environmentally-friendly technology for corrosion protection in recent years, which has been successfully applied on the suface of stainless steel, aluminum alloy, magnesium alloy, etc. In this thesis, electrochemically-assisted deposition (short for electro-deposition) was used for silane treatment on AZ31B magnesium alloy. The optimum amounts during electro-deposition of KH-560, KH-570 and BTSE silane agents were studied by using electrochemical workstation, scanning electron microscope, energy spectrometer, etc and by means of simulated degradation experiment in vitro respectively, and effects of three kinds of silane films formed by the same electro-deposting process on corrosion resistance of magnesium alloy were compared. Based on optimum amounts of three silane agents, three kinds of silane films were modified by adding nano-SiO2 and Ce3+ respectively, and corrosion resistance of silane films formed on magnesium alloy before and after modification was compared. It is indicated that:KH-560 silane film, KH-570 silane film and BTSE silane film were prepared successfully on AZ31B magnesium alloy by electro-deposition respectively. The results of 48h simulated body fluid degradation experiment in vitro show that the corrosion rate of magnesium alloy can be reduced effectively by formation of all three silane films. And pitting corrosion occurs firstly on silane film rather than magnesium alloy. Among all three kinds of silane films, KH-570 silane film has maximum thickness, which is compact and uniform. As a result, KH-570 silane film has the best corrosion resistance. According to the results of potentiodynamic polarization curves measurement, the free corrosion potential of magnesium alloy was improved and its corrosion current density decreases to a great extent after electro-depositing three silane films. There are different optimum amounts of KH-560, KH-570 and BTSE during electro-deposition on magnesium alloy (6ml,8ml and 4ml respectively). The electrochemical impedance spectroscopies of magnesium alloys after three different silane electro-deposition treatments have similar features:In Nyquist plot, there is a capacitance arc in high frequency area and middle and low frequency area respectively, and the former has a bigger arc radius, while the inductance arc of bare magnesium alloy in lowest frequency area disappears. In impedance-frequency Bode plot, the value of impedance increases greatly after silanization, and corrosion resistance of AZ31B magnesium alloy is improved effectively; in phase angle-frequency Bode plot, all three silane agents have high phase angle values in low frequency area with optimum amounts, and the peaks of phase angle-frequency curves tend to low frequency range, while in middle and high frequency area, the phase angle values of magnesium alloys after different silanizations are improved steadily, which implies tight bonding of Si-O-Si and Mg-O-Si in silane films. By using ZSimpWin electrochemical analysis software, the equivalent circuit model (ECM) of magnesium alloy with silane film is founded. With optimum amount (8ml), KH-570 silane film has a stable Cc value (10-7 F·cm-2) and maximum thickness, which means that KH-570 silane film has the best corrosion resistance.By adding nano-SiO2 or Ce3+modifier, the improvements of KH-560, KH-570 and BTSE silane films on corrosion resistance of AZ31B magnesium alloy were developed further. Compared with nano-SiO2, Ce3+ has a more significant modification function. After electrochemically-assisted deposition, Ce3+ has a synergistic reaction with silane film and the ability of self-repairment is given to modified silane film, which can increase film thickness and decrease film porosity effectively. |
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