Correlating the microstructure of the die-chill skin and the corrosion properties for a hot-chamber die-cast AZ91D magnesium alloy

The corrosion of a hot-chamber die-cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure, to elucidate the role of die-chill skin in corrosion. The die-chill skin was composed of a thin layer of chill zone and a thick layer of an interdendritic Al-rich alpha-Mg/Al12Mg17 beta-phase particle/alpha-Mg grain composite microstructures. The chill zone (4 +/- 1 mu m in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles. Beneath the chill zone, Al12Mg17 beta particles were irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich alpha phase (also known as eutectic alpha) was in the interdendritic network, which occupied a higher volume fraction than the beta phase in the die-skin layer. Corrosion characteristics were studied via constant-immersion and electrochemical tests. Although previous studies have ascribed the fine microstructure to good corrosion resistance for the AZ91D alloy, the present study showed severe corrosion of the sample with a die skin in chloride solution. Moreover, the sample without the die skin on the surface corroded more slowly. The inferior corrosion performance of the die skin was considered to be related to the high volume fraction of the interdendritic network of Al-rich alpha phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich alpha phase does not increase the corrosion resistance of the AZ91D alloy.