Study of Microstructure and Corrosion Behavior of Cast Zn-Al-Mg Alloys

Zn-based alloys have found increasing interest as orthopedic biodegradable implantable materials, hence it was the aim of this work to investigate the microstructure and corrosion behavior of Zn-0.5Al-xMg cast alloys with different Mg additions in simulated body fluid (SBF). The cast samples were prepared using a simple stir casting method and the molten alloys were poured in a cast iron mold. The SEM results showed that adding Mg greatly influenced the microstructure of the Zn-based alloys where the degree of fineness of the microstructure increased with a rise in the Mg content. Moreover, polarization measurements revealed that the Zn-0.5Al-0.6Mg alloy attained the lowest degradation rate of 0.33 mm/year as compared to the other investigated alloys which complies the requirements of ideal corrosion rates for biodegradable bone implants. This corrosion rate helps the implantable metal alloy to last in the body until healing of the bone tissue proceeds. The fine structure and uniform distribution of Aluminum oxide and MgZn2 intermetallic phases along the grain boundaries were most likely the main factors in the superior corrosion stability of the Zn-0.5Al-0.6Mg alloy in SBF. However, higher concentrations of Mg (1 wt%) lowered the corrosion resistance of the Zn-Al-Mg alloy which was attributed to the accelerated galvanic corrosion between Zn and Mg-2 Zn-11 phases and the inhomogeneous distribution of corrosion products on the alloy surface due to the increased grain size and the coarse structure of the Zn alloy.