Influence of freezing rate on microstructure and electrochemical properties of Mg-2%Ga alloys

Equilibrium freezing curve of Mg-2%Ga (mass fraction) alloy was calculated by CALPHALD method. Microstructures of the melted Mg-2%Ga alloys solidified by iron and copper moulds, respectively, were investigated using OM and SEM. Electrochemical properties of the Mg-2%Ga alloys with different freezing rates were measured by galvanostatic, potentiodynamic and electrochemical impedance spectroscopy tests. The results show that solidification by copper mould leads to intergranular Mg5Ga2 compounds with small size and large number density. Less adsorbent of Mg+ and oxide corrosion products occur on the surface of the Mg-2%Ga alloys solidified by copper mould, producing lower corrosion current density of 1.8×10−5 mA/cm2. In the galvanostatic tests with 100 mA/cm2 current density, more negative stable potential of −1.604 V exists in the Mg-2%Ga alloys solidified by iron mould due to the lower freezing rate, which leads to smaller inductive and capacity time constants as well as shorter activity time and better electrochemical activity.