Electrochemical behaviour of cast Al-Zn alloys in natural chloride solution were investigated by potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). Results from open circuit potential (OCP) measurement against immersion time showed a stable free corrosion process was achieved after 15 kiloseconds and alloys with more Sn contents had shifted OCP value towards more negative direction. Potentiodynamic polarisation curves showed that the corrosion potential (Ecorr) of aluminium alloys with Sn addition were much active compared to alloy without Sn. The anodic curves were characterized by no sign of passive current due active dissolution on the surface of the Al-Zn-Sn alloy. SEM images show that the presence of Sn in Al-Zn alloys produces better and uniform dissolution morphology. EIS results confirm that the presence of Sn is beneficial in improving anodic dissolution of Al-Zn alloys by reducing resistance to polarization (Rp). The presence of 0.1%wt. Sn in Al-Zn alloy has been found to be useful in activating electrochemical reaction at alloy-solution interface based on inductive loop in EIS diagram.