The inhibition effect of three Schiff bases (SB-I SB-II and SB-III) and its synergistic effect with KI on the corrosion of carbon steel in 0.5 M H2SO4 solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Scanning electron microscopy (SEM) was used to characterize the steel surface. The inhibition efficiency increases with the concentration of the Schiff bases and increased further with the presence of KI. The thermodynamic parameters K-ads and Delta G degrees(ads) are calculated and discussed. The probable inhibitive mechanism is proposed from the viewpoint of adsorption theory. (C) 2013 Elsevier Ltd. All rights reserved.