ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2011, Vol. 47, No. 4, pp. 517–527. © Pleiades Publishing, Ltd., 2011. 517 1 INTRODUCTION Iron and its alloys are widely used in industrial pro- cesses because of their good mechanical and economic characteristics. Acidic solutions also have huge applica- tions for industrial processes [1, 2]. Corrosion is an undesirable process and there have been many methods applied to prevent it. The most common way of these methods is to develop an adsorption layer on the metal surface by using organic inhibitors. They are commonly used in acid media [3–7]. Organic inhibitors that have heteroatoms such as nitrogen, sulphur and oxygen are in effective inhibitor classifications for acidic media [8– 12]. The efficiency of these inhibitors is due to their electronic and physicochemical characteristics. This depends on the functional groups of inhibitor molecules and their aromatic π electrons and steric effects [13, 14]. There are two types of interactions that are physical and chemical adsorptions occur between inhibitor and metal surface. Active sites of metal surface and the charge of hydrophilic groups of inhibitors interact as physical adsorption, while partially covalent bonds exist by electrostatic interactions or chemical adsorption where there is a charge transfer from inhibitor to metal surface. Charge and nature of metals, charge dispersion of molecules and the type of interactions between metal surface and organic molecules affect the adsorption [15–18]. The aromatic amines have great interests in recent years as corrosion inhibitors. They are also used 1 The article is published in the original. in industrial applications. The target molecule of this study “benzidine” is generally used for textile, cosmet- ics, azo dye production and synthesis of some chemicals [19]. The aim of this study is to investigate on the inhi- bition effect of benzidine on the corrosion of mild steel in hydrochloric acid medium in both short and long immersion times. EXPERIMENTAL The experiments were performed on a MS surface in the following chemical composition 0.097 C, 0.00321 Pb, 0.488 Cu, 0.117 Cr, 0.032 P, 0.099 Si, 0.012 V, 0.004 Nb, 0.054 Mo, 0.07 S, 0.018 Sn, 0.01 W, 0.0042 Co, 0.137 Ni, 0.459 Mn and 98.42 Fe (wt %). The spec- imens were embedded in polyester, which had a surface area of 0.785 cm 2 that was in contact with the corrosive media and Cu-wire was used for the electrical connec- tion. The surface preparation of the specimens was car- ried out using abrasive paper (150, 600 and 1200 grade), degreased with acetone and washed in doubly distilled water and dried at ambient temperature. Benzidine and hydrochloric acid (37%) were purchased from Merck Chemical Company. The chemical formula of benzi- dine was given in Fig. 1. The experiments were carried out in the aerated 1.0 M HCl solutions without and with 1.0–75.0 mM benzidine and before each analysis; fresh solutions were prepared and used. Electrochemical experiments were carried out by using CHI 660B model electrochemical Inhibition Efficiency of Benzidine for Mild Steel in Acidic Media 1 Demet Özkir and Emel Bayol Department of Chemistry, Faculty of Science and Art, Nigde University, 51200, Nigde, Turkey e-mail: emelbayol@nigde.edu.tr Received July 15, 2009 Abstract—In this study, the inhibition effect of different concentrations of benzidine possessing amine groups in its structure on the corrosion behavior of mild steel (MS) in 1.0 M HCl solution at 293 K temper- ature was practiced in both short and long immersion times by measuring electrochemical impedance spec- troscopy (EIS), hydrogen evolution (V H2 – t) and change of open circuit potential (E ocp – t). For short-term tests, potentiodynamic polarization and linear polarization resistance (R lp ) were also studied. Polarization data indicate that this compound act as mixed-type inhibitor for mild steel in 1.0 M HCl. With regard to the results cited, it was identified that the increase in the inhibitor efficiencies with concentration was emanated from the adsorption of benzidine molecules on the electrode surface. Pertinent to this adsorption, it followed Langmuir isotherm. Adsorption equilibrium constant and adsorption free energy were determined. The effect of temperature on the corrosion behaviour in the absence of any inhibitor and in the presence of 75 mM ben- zidine was studied in the range of 293–323 K. Activation energies were calculated from the obtained corrosion rates at different temperatures. Furthermore, in order to identify the adsorption mechanism of the inhibitor on the metal surface, the zero-charge potential (PZC) was determined by EIS measurement tech- nique. Surface morphology of mild steel electrodes were emerged by means of the scanning electron micro- scope. DOI: 10.1134/S2070205111040150 E a * ( ) PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION