Influence of Si, Ca and Ag addition on corrosion behaviour of new wrought Mg-Zn alloys

The development of new wrought magnesium alloys for automotive industry has increased in recent years owing to their high potential as structural materials for low density and high strength/ weight ratio demands. However, the poor mechanical properties and low corrosion resistance of the magnesium alloys have led to searching a new kind of magnesium alloys for better strength, ductility and high corrosion resistance. The main objective of the present research is to investigate the mechanical properties and the corrosion behaviour of new magnesium alloys, Mg-Zn-Ag (ZQ) and Mg-Zn-Si-Ca(ZS) alloys. The ZQ6X and ZS6X-YCa alloys were prepared by using hot extrusion method. Hardness AC and DC polarisation tests were carried out on the extruded rods, which contain different amounts of silver or silicon and calcium. The potential difference in air between different phases and the matrix was examined using scanning Kelvin probe force microscopy. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. The results showed that the silver addition improved the mechanical properties but decreased the corrosion resistance. The addition of silicon and calcium also affected both mechanical properties and corrosion behaviour. These results can be explained by the effects of alloying elements on microstructure of Mg-Zn alloys such as grain size and precipitates caused by the change in precipitation and recrystallisation behaviour.