Electrochemical polymerization of polyindole (PI) was investigated on nickel-coated mild steel (MS/Ni) electrode in LiClO4 containing acetonitrile medium (ACN-LiClO4). One-micrometer-thick nickel coating was electrodeposited on MS galvanostatically in a proper bath solution. Then, the synthesis of PI film was achieved in 0.2 M indole containing ACN-LiClO4 by using cyclic voltammetry technique. It was proved that the synthesized PI film was very smooth and homogeneous. The corrosion performances of MS/Ni- and PI-top-coated MS/Ni samples were investigated properly in 3.5% NaCl solution. For this aim, electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential-time (E-ocp-t) diagrams were used. The porous nickel coating exhibited a physical barrier property on the surface and provided some protection to MS. But its permeability increased significantly with time and failed to protect MS. It was shown that PI-top-coated Ni coating exhibited much better protection efficiency to MS for considerable periods in such aggressive medium. The polymer film exhibited effective barrier behavior and lowered the permeability against the attack of corrosive environment. (c) 2004 Elsevier B.V. All rights reserved.