| Magnesium and its alloys are very important light alloys. Magnesium alloys has obvious advantage in energy saving and machining efficiency improving, due to low density, high specific intensity, good metal cutting performance, well casting properties, and so on. With the in-depth study of magnesium alloys, they have been widely used in automobile industry, aerospace equipment, military and national defenses. Also, because of the well thermal conductivity, electrical conductivity and electromagnetic shielding, magnesium alloys are widely applied to 3c products(computers, communications, and consumer electronics). However, due to their high chemical and electrochemical activity, magnesium alloys are easily oxidized and corroded, which limited their large-scale use.In this paper, the author aims to modify AZ31 magnesium alloy specimens with organic/inorganic composite coating to improve their corrosion resistance.First, the author tries to modify the specimens with organic film in dopamine solution. The influence of concentration of dopamine, pH value and reaction time on the result of experiment is discussed. And a series of methods were used to characterize the performance of the coatings, such as field-emission scanning electron microscopy(FE-SEM),energy-dispersive X-ray spectrum(EDS) and X-ray photoelectron spectroscopy(XPS), tape test, etc. The results show that integrity organic film can be obtained in the condition: pH value is 10, the concentration of dopamine aqueous solution is 1 mg/mL, reaction time is 20 h to 25 h. The film with good adhesion is composed of poly-dopamine.Then, the author studies the preparation of corrosion resistant films on magnesium alloys by hydrothermal method. Scanning electron microscopy(SEM) was used to study the morphologies of the films. By adjusting the hydrothermal reaction conditions with the help of surfactant, the nanometer-scale filament-like structure, nanometer-scale spheroidal structure, and micrometer-scale island-like and nanometer-scale sheets hierarchical structures. X-ray diffraction(XRD) results show the main phase of the film is Mg(OH)2and the thicknesses of the films are controlled by temperature. Electrochemical tests prove that the treated specimens show much better corrosion resistant. Comparing these specimens with AZ31 magnesium alloys, the corrosion potential shifts positively for 250 mV, corrosion current density decreases about 2 orders. Tape test shows the adhesion of magnesium hydroxide film to substrate is well.Also, the author prepares poly-dopamine/magnesium hydroxide composite film combining aforementioned hydrothermal and chemical impregnation method. Electrochemical test result shows that this composite film much better than single poly-dopamine and single magnesium hydroxide film.Combining with poly-dopamine, the best corrosion resistant films can be gotten when the hydrothermal temperature is 160℃and the reaction time is 6 h. Comparing with AZ31 magnesium alloys, the best composite film make the corrosion potential shifts positively about 300 mV and corrosion current density decreases about 3 orders. |
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