Influence of Dendritic Fragmentation through Mg Addition on the Electrochemical Characteristics of Zn–0.5 wt% Al Alloy

Abstract

Microstructure refinement is known to have a significant effect on the electrochemical properties of the metallic alloys. The current research investigated the electrochemical properties of Zn–0.5 wt% Al alloy as influenced by the microstructure changes due to Mg addition. Magnesium was added as alloying element during the melting process in amounts that ranged between 0.1 and 0.9 wt%. Some specimens (0.2, 0.5, 0.7, 0.8, 0.9 wt% Mg) with obvious microstructure changes were selected to evaluate their electrochemical properties. The samples were immersed in 3.5% NaCl solution for 3, 8 and 30 days, and the corrosion rate was then calculated based on the weight loss after removing the corrosion products. Polarization tests were also performed, and both of Tafel curves and impedance circles were recorded. It was observed that Mg additions refined Zn dendrites and significantly reduced their size from 165 μm without Mg down to 20 μm with 0.7 wt% Mg where the dendritic morphology changed to the polygonal shape. However, the dendritic morphology was retained with further increase in Mg content. As a result of microstructure refinement and the observed uniform distribution of Al in the interdendritic regions with Mg addition, the corrosion resistance of (Zn–0.5Al) alloy increased and the corrosion rate was reduced from (~ 0.00099 mm/year) without Mg down to (~ 0.0005 mm/year) at 0.7 wt% Mg. Increasing the amount of Mg to 0.8 wt% raised the corrosion rate to (~ 0.0015 mm/year) and affected negatively the corrosion resistance of the alloy.