Al-Si hypoeutectic alloys produced by casting are mostly used in the automotive industry, especially for engine blocks. They have the advantage of low weight associated with low coefficient of thermal expansion and excellent mechanical properties. The corrosion resistance of these alloys in coastal area, particularly in seawater environment is not well known. In this investigation, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation have been used to evaluate the corrosion resistance of Cu free and 0.5wt% Cu content Al-6Si-0.5Mg alloy in simulated seawater environment. The 0.5wt% Cu addition to the Al-6Si-0.5Mg alloy showed that Cu decreased susceptibility to electrochemical corrosion compared to the Cu free Al-6Si-0.5Mg alloy. The magnitude of open circuit potential (OCP), corrosion potential (Ecorr) and pitting corrosion potential (Epit) of Al-6Si-0.5Mg alloy in simulated seawater were shifted to the more noble direction due to 0.5wt% Cu addition and thermal modification.