This work carries out an analysis of the structure obtained in gravity chill cast Al-5.3 at. % Zn with additions of 5.3. 6.5, 7.5 and 11.5 at. % Mg. The as-cast microstructure consisted of alpha-Al dendrites, the intermetallic tau in alpha-Al solid solution and eutectic alpha+tau in interdendritic regions. The electrochemical behavior of alloys in both as-cast and heat treated ingots was investigated in 3% NaCl solution at room temperature. Results showed that the eutectic in interdendritic regions and the intermetallic in alpha-Al solid solution have an impact on the electrochemical efficiency of the alloys under study which are designed to be used as Al-sacrificial anodes. To correlate the effect of structure with electrochemical efficiency, it was analyzed the vertical section parallel to the Al-Mg side for a constant concentration of 5.3 at. % Zn together with growth temperature of phases obtained during solidification. Then, the range of solidification front velocities was predicted, where the alpha-Al and intermetallic tau phases grew simultaneously. These results, together with the predicted variation with growth velocity of solute concentration in the alpha-Al solid solution and the electrochemical efficiency values were used to select an alloy with an Al-5.34 at. % Zn-6.53 at. % Mg composition, which after heat treatment can be used as Al-sacrificial anode.