| Due to the light weight, high specific strength and good thermal conductance,dimensional stability and magnetic shielding resistance and convenient recycling, the magnesium alloys are used more and more in the automotive, aerospace field, military products, digital products and so on in recent years. Especially for the automotive industry, with the aggravation of fuel shortages and environment deterioration, energy saving which requires lightweight automotive components including automotive wheel is an important direction for automotive research. The application of magnesium alloys in automobile is very helpful to energy saving of automobiles.However, the poor corrosion resistance of magnesium alloys limits their widely use in automobile wheel hub. Although many researches prove that magnesium alloy surface coating can enhance the corrosion resistance of magnesium alloys, there are little research about the effects of surface cracks on the corrosion resistance of magnesium alloy coatings in the domestic and abroad. And the generation of cracks is nearly unavoidable during its use.In the present work, two kinds of coatings by dutaiyin and micro-arc oxidation on the surface of cast magnesium alloy(AZ81+2%Ce) were prepared. The investigation on the effects of surface cracks on the corrosion resistance of two coatings were carried out by means of cross test, electrochemical test, saline soak, SEM and EDS analysis,hydrogen evolution test, etc. This investigation can provide basic data for the application of magnesium alloy automotive wheels. The main results are as follows:(1) The corrosion rate of Dutaiyin and MAO coating on magnesium alloy in different solution are less than the bare substrate, it shows that the two coatings can improve the corrosion resistance of the alloys. And Dutaiyin coating has better corrosion resistance than the MAO coating.(2) After the samples were shocked by using 0.98 J, 1.96 J, 2.94 J and 3.92 J impact energy, the corrosion resistance of the damaged Dutaiyin coating is worse than the intact one, the weakening degree is along with the increase of impact energy. Especially when the impact energy is greater than 3.92 J, the corrosion resistance decreases significantly due to the coating is damaged and the matrix form variables is very large, but its corrosion rate is still smaller than the bare substrate. When the total area of cracks is controlled, the corrosion speed of samples increases with the increase of the length ofsingle scratch whether the scratches reaches to matrix. The surface of Dutaiyin coating is very dense, the corrosion resistance almost all depends on the nature of the cracks when cracks formed on Dutaiyin coating.(3) After the samples were shocked by 0.98 J, 1.96 J, 2.94 J and 3.92 J impact energy,the corrosion resistance of the damaged MAO coating is worse than the intact one.Especially when the impact energy is greater than 2.94 J, the corrosion resistance decreases significantly due to the dense layer is damaged and the matrix form variables is very large, even the corrosion rate of the impact crack local is approaching the bare substrate. When the total length and area of crack is controlled, the corrosion speed of the scratches which are reach to matrix increases with the increase of the length of single scratch. Contrarily, the corrosion speed of the coating of the minor scratches which are not reach to the matrix are not change with the change of the length of single scratch. The surface of MAO coating has micropores, the corrosion resistance not all depends on the nature of the cracks but also depends on the performance of the coating itself when cracks formed on MAO coating. |
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