| Due to their low density and high strength-to-weight ratio, the demand for applications of magnesium alloys in industry is continuously increasing, however, its application has certainly restricted for the poor corrosion resistance caused by high intrinsic dissolution tendency, and low intensity and plastic formability. It was reported that the presence of impurity elements(Fe, Ni, Cu, Co), with high standard electrode potential acting as local cathodes caused local galvanic acceleration of corrosion. Therefore, it is very important to improve the purity of magnesium alloy, reduce the content of impurity elements and improve the corrosion resistance of magnesium alloys.Based on the existing purification technologies of magnesium alloys, this paper analyzed the effect of holding temperature on Fe solubility through the thermodynamic phase diagram calculation that the Fe solubility decreases with decreasing holding temperature. The commercial purity magnesium, commercial purity AZ and AM magnesium alloys were used in the present work. The impurity element Fe precipitated from Mg alloy melt naturally and settled to the bottom in the form of element or intermediate compounds by holding the melt at low temperature. And different purity magnesium alloy samples were obtained. In order to know the microstructure and properties of purified alloys, OM, XRD, SEM and EDS were employed to characterize the microstructure and impurity particles containing iron in Mg alloys. The corrosion rate of Mg alloys was studied by means of weight loss, hydrogen evolution and electrochemical polarization curves. The results of this research are as follows:(1) The AZ/AM Magnesium alloys have purified significantly by low temperature treatment, the appropriate holding temperature is between 630? and 650?, the Fe content was dropped to 10 ppm in AZ61 alloy, and the Fe content in AZ91 was below 35 ppm, and Fe content in AM60 and AM50 were less than 15 and 18 ppm respectively, after the low temperature melt treatment. The lowest Fe content in Mg melt is 24 ppm with temperature gradient treatment. The effect of melt holding time is obvious on the Fe content in Mg alloys, after 45 min holding, the Fe content will not continue to decrease.(2) The exist of impurity element Fe is in the form of Fe-Al-Mn particle, and the size of particles were 1-10?m which observed at the bottom of the ingot. And the impurity particles in Mg alloys were inclined to integrate spherical particles and formed belong second phases.(3) The velocity of impurity particles containing Fe, Al, and Mn is and the settlement distance of impurity particles is, the movement of impurity particles is mainly affected by the impurity particle size, and the temperature does not affect the movement significantly. The Mg melt will be purified effectively when holding at low temperature as long as possible.(4) The effect of Fe content on the microstructure is not obvious. The corrosion rate of magnesium alloys decreased significantly with decreasing of Fe content after melt low temperature treatment. Also, The corrosion rate of magnesium alloy is decreased with decreasing Fe/Mn ratio. The magnesium alloy galvanic corrosion rate is greatly reduced as the micro galvanic effect between impurities and matrix were weakened. Due to the decrease of the number of the second Phase, the corrosion rate of the solid solution treatment state is the minimum, the corrosion rate of the different heat treatment states are cast > aging state> solid solution. |
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