Effect of cooling rate on corrosion resistance and behavior of micro-alloyed cast AZ91-Ca-Y alloy

Micro-alloying is an effective approach for improving the corrosion resistance of cast AZ91. However, the effect of micro-alloyed elements on corrosion resistance can be varied depending on the solidification rate influencing the diffusion and precipitation behavior of micro-alloying elements. This study investigated the effects of the cooling rate on the microstructure and corrosion behavior of micro-Ca and -Y alloyed cast AZ91 alloy (i.e., AZXW9100). To achieve various cooling rates, the alloys were prepared using three methods: steel mold casting (SMC), copper step mold casting (CSMC), and high-pressure die casting (HPDC). The corrosion behavior was analyzed through weight loss measurements, electrochemical impedance spectroscopy, and corrosion morphology observations. The results showed that the key microstructural factors influencing corrosion resistance differed between short- and long-term corrosion. As the cooling rate increased, the short-term corrosion rate was lowered from 0.91 mm/y (SMC) to 0.38 mm/y (HPDC), which was attributed to the decrease in the total area fractions of the eutectic α and β phases acting as galvanic corrosion sources. The long-term corrosion rate was reduced from 17.20 mm/y (SMC) to 0.71 mm/y (HPDC), which was revealed to be due to the enhanced connectivity of the β phase acting as corrosion barriers. Meanwhile, the increase in the cooling rate led to a modification of the Zn molar ratio in the β phase, reducing the Volta potential of the β phase from 101.8 mV to 66.9 mV. This reduction in the Volta potential of the main galvanic source also contributed to improved corrosion resistance. The HPDC AZXW9100 alloy produced in this study exhibited the lowest corrosion rate compared to other alloys. These findings suggest that controlling the cooling rate is a promising strategy for enhancing the corrosion resistance of AZXW9100 alloys.