| Based on the semi-solid forming process technology, the most popular AZ91D magnesium alloy was chosen as the research objective according to the characteristics of semi-solid thixoforming. Through the immersing weight lose tests, collecting hydrogen experiments, chemical analyses and electrochemical methods, the corrosion behavior of the thixoformed AZ91D magnesium alloy has been systematically researched in different concentration sodium chloride, different pH value aqueous solution and Lanzhou atmosphere. The research has focused on the differences of corrosion behavior between thixoformed AZ91D magnesium alloy and Conventional Permanent Mold Cast (PMC) AZ91D magnesium alloy. The results are instructive in the practical use of thixoformed magnesium alloy. The main conclusions of the research are as follow:1. Through weight lose tests and collecting hydrogen experiments, the corrosion resistant of the thixoformed AZ91D magnesium alloy in sodium chloride aqueous solution is higher than that of PMC alloy. The corrosion rate decreases with the decrease of concentration of sodium chloride. In the same concentration of sodium chloride, the initial corrosion rate of the thixoformed AZ91D magnesium alloy is higher than that of PMC magnesium alloy, but the corrosion rate of the thixoformed AZ91D magnesium alloy decreases sharply with the time increased, and the following corrosion rate of the thixoformed AZ91D magnesium alloy is lower than that of PMC alloy.2. The results of the EPMA of corrosion surface and chemical analysis of corrosion products have shown: the corrosion phase of thixoformed AZ91D magnesium alloy in sodium chloride aqueous solution is mainly primaryαand eutecticα. The corrosion degree of the eutecticαis higher than the primaryα. The main corroded element is magnesium, and aluminum is lightly corroded, and zinc is rarely corroded.3. The corrosion of thixoformed AZ91D magnesium alloy is electrochemical corrosion. The corrosion behavior is concerned with the ratio of the area of the cathode and anode of the galvanic couple. In the different part of the specimen, the different solidification condition leads to different quantity of the phases, and the different area ratio of the cathode and anode of the galvanic couple. In sodium chloride aqueous solution, the corrosion resistant of the specimen surface is higher than that of the inner part in the initial time, but the corrosion difference of the inner and outer part decreased in the following corrosion.4. The corrosion rate of thixoformed AZ91D magnesium alloy decreases after solid solution treatment, and the corrosion rate of magnesium alloy after aging treatment is lower than that of solid solution treatment, which is caused by the change of microstructure.5. The weight loses tests results in different pH aqueous solution has shown: the corrosion rate decreases with the increase of pH value. In the same pH value aqueous solution, the corrosion rate of thixoformed AZ91D magnesium alloy is higher than that of PMC alloy. The difference of initial corrosion rate of the two alloys is obvious, but with the corrosion continues, the corrosion rate difference decreases.6. The results of the chemical analysis to the corrosion products in different pH aqueous solution have shown: The main corroded element of the thixoformed AZ91D magnesium alloy is magnesium, and aluminum is lightly corroded, and zinc is rarely corroded.7. In the atmosphere environment, the outdoor specimen corrosion mainly happens at the primary a, and the indoor specimen corrosion mainly happens in the eutecticα.8. The four-stage corrosion model has been suggested according to the corrosion behavior of thixoformed AZ91D magnesium alloy. The first stage is the primary films of magnesia become Mg (OH)2 and thickness of Mg (OH)2 increase. The second stage is the corrosion reaction happens, and corroded products' layer forms. The third stage is the corrosion products' layer exists steadily with the progress of corrosion. The fourth stage is the corrosion products' layer destroys, and corrosion happens again in the layer destroyed area. |
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