ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2009, Vol. 45, No. 1, pp. 46–53. © Pleiades Publishing, Ltd., 2009. 46 1 INTRODUCTION Acidic solutions are used in many industrial areas. The most important application are acid pickling, industrial acid cleaning, acid descaling, and oil well acidising [1, 2]. Various methods are investigated to protect the metals from corrosion in acidic solutions. The studies of inhibitive effect of organic molecules are progressive. Organic molecules rich in heteroatoms as oxygen, sulfide and nitrogen provide the best protection from metal corrosion [3–13]. Various organic com- pounds used as inhibitors in industrial applications blockade the metal surface by adsorbing on the metal surface physically and chemically. Adsorbed inhibitors prevent the occurrence of cathodic or anodic reactions or both of them. The adsorption effect of inhibitive molecules depends on the structure of a molecule, corrosion envi- ronment, the charge and nature of metal surface, and the interaction type between metal surface and organic molecule [7–11]. In recent studies, the search on compounds such as azol, aminoacids, aminoesters, and pyridine, contain- ing nitrogen and sulfur, are in progress [7–15]. Oguzie et al. found methionine functions as an inhibitor of mild steel corrosion in 0.5 M H 2 SO 4 solution. A mixed- inhibiting mechanism is proposed for the protective effect increased with concentration [16]. On the other 1 The text was submitted by the authors in English. hand, quantum chemical studies have been successfully performed to link the corrosion inhibition efficiency with molecular structure levels for some kinds of organic compounds, e.g. imidazole [17], amides [18, 19], etc. The aim of the present study was to examine inhib- itive action of the afore mentioned inhibitors toward the corrosion of iron in HCl solution and to investigate structure and inhibitor relationship. EXPERIMENTAL Hydrocloric acid (HCl, Merch, 35–37%) methion- ine, and tyrosine (Sigma Aldrich) were used as received. Armco iron was used for the electrochemical measurements. Iron electrode was armored in polyes- ter, with a surface area of 0.785 cm 2 in contact with the corrosive media. The electrode was first polished suc- cessively with metallographic emery paper up to 600 grits. The electrode was then washed with distilled water, degreased with acetone, washed using distilled water again, and then inserted immediately into the glass cell containing 250 ml of the electrolyte solution. Polarization experiments were carried out in a con- ventional three-electrode electrochemical cell. Pt elec- trode was the counter one, Saturated Calomel Electrode (SCE) was as the reference one, and iron electrode was the working. Polarization curves were recorded by changing the electrode potential from –250 to +250 mV MOLECULAR AND SUPRAMOLECULAR STRUCTURES AT THE INTERFACES Inhibition Effects of Methionine and Tyrosine on Corrosion of Iron in HCl Solution: Electrochemical, FTIR, and Quantum-Chemical Study 1 S. Zor, F. Kandemirli, and M. Bingul Department of Chemistry, Kocaeli University, Kocaeli, 41380, Turkey Received October 26, 2007 Abstract—The inhibiting effect of methionine and tyrosine on the corrosion of iron is researched electrochem- ically in 0.1M HCl, Quantum chemical calculations were performed. The level of HF with the 6–311G(d,p) basis set for methionine and tyrosine. Corrosion current density has been determined by polarization measures and the inhibition effect was calculated. With an increase in the concentration of inhibitor, the effectiveness of inhibition increases. The highest inhibition is determined as 97.8% at 100 ppm methionine. The effect of tem- perature is determined by chronoammetric measures. Surface analysis is performed with FTIR spectroscopy. Methionine and tyrosine adsorb on the iron surface according to Langmiur isotherm. The highest occupied and the lowest unoccupied molecular orbital energy, as well as Mulliken and atomic charges with hydrogens summed into heavy atoms of C, N, O, S atoms and of methionine, tyrosine, and protonated forms have been examined. PACS numbers: 81.65.Kn, 68.43.-h DOI: 10.1134/S2070205109010079