Electrochemical polymerization of polythiophene (PTh) was investigated on nickel coated mild steel (MS) electrode, in LiClO4 containing acetonitrile medium (ACN-LiClO4). Nickel layer (1 mum thick) was deposited galvanostatically, from a proper bath solution. Then, the synthesis of PTh film was achieved in 0.1 M thiophene containing ACN-LiClO4, by using cyclic voltammetry technique. The corrosion performances of nickel coated samples with and without polymer top coats were investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. The nickel coating behaved like a physical barrier and provided some protection to MS against corrosion. But its barrier property diminished significantly with time and failed to protect MS. It was shown that PTh top coat improved the barrier efficiency remarkably, and excellent protection efficiency was obtained against MS corrosion, for considerable exposure time in such aggressive environment. (C) 2004 Elsevier B.V. All rights reserved.