Abstract
To increase the utilization of die-cast Mg alloys with various shapes in a variety of environments, the corrosion behaviors of commercial die-cast Mg alloys with different thicknesses were investigated in neutral and alkali solutions at ambient temperature. A decrease in the thickness of a specimen leads to an increase in cooling and solidification rates, which, in turn, decreases the size of the eutectic β phases and the interphase distance, thus improving the hardness of the specimen. Specimens with relatively large β phases were more corroded under neutral conditions due to severe galvanic corrosion at the interface between α-Mg and the β phases, whereas they were protected by passivation films formed on the substrate in the alkaline solution. However, in the case of the alloy with thin thickness and high solidification rate, the fine β phases improved corrosion resistance by forming a net structure that acted as a barrier to corrosion propagation of the α matrix. These results suggest that the size and distribution of the eutectic phases should be appropriately controlled, depending on the environment.
Highlights
The low corrosion resistance of Mg and its alloys limits its widespread use in many applications, in those involving atmospheres that contribute to pitting corrosion [5,6]
The cooling rate of die-cast specimens, which generally varies with the thickness of the specimen, affects the size and distribution of the β phase
This study investigated the effects of cooling rates and the resulting microstructures of die-cast AZ91 specimens on mechanical properties and corrosion behaviors in both neutral and alkaline solutions
Summary
The energy efficiency of designs can be enhanced by using high-performance lightweight structural materials. Magnesium (Mg), which is of interest due to its low specific gravity (1.74 g/cm3 ), holds great potential for use in automotive and aerospace applications [1,2]. Die-casting techniques are frequently used in the manufacture of Mg components because they can produce high-quality parts at a high production rate and reasonable cost [3]. Commercial AZ91 (Mg-9 wt.% Al-1 wt.% Zn) alloy is the most widely used Mg-Al die-cast alloy, due to its high fatigue strength and excellent castability for even the most complex and thin-walled parts [4]. The low corrosion resistance of Mg and its alloys limits its widespread use in many applications, in those involving atmospheres that contribute to pitting corrosion [5,6]. The corrosion reaction of Mg and its alloys are similar in both neutral and alkaline media, and the overall reaction can be expressed as
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