Electrochemical techniques were used to investigate the effect of concentration of three new thiadiazole derivatives (I-III) on the corrosion behavior of C-steel in 0.5 M NaCl solution through the analysis of electrochemical measurements including open circuit potential (OCP), Tafel polarization and electrochemical impedance spectroscopy (EIS). Polarization curves showed that the compounds studied act as anodic type inhibitors, where the inhibition efficiency increases with increase in inhibitor concentration and decreases with rise in temperature. An adherent layer of inhibitor molecules on the surface is proposed to account for their inhibitive action in which the organic molecules adsorb on the active anodic sites following Langmuir isotherm. The thermodynamic parameters of adsorption and corrosion processes were determined and discussed. The results also indicated that pitting potential at higher anodic polarization of C-steel in 0.5 M NaCl solution becomes more positive the higher the concentration of the additive, suggesting that these inhibitors acts as retarding catalyst for pitting corrosion. EIS data confirm well the electrochemical dc results and the results are all in good agreement with the calculated quantum chemical HOMO and LUMO energies of the tested molecules, as well as with surface examination via scanning electron microscope. (C) 2010 Elsevier B.V. All rights reserved.