On the relationship between corrosion inhibiting effect and molecular structure of 2,5-bis(n-pyridyl)-1,3,4-thiadiazole derivatives in acidic media: Ac impedance and DFT studies F. Bentiss a,⇑ , B. Mernari a , M. Traisnel b , H. Vezin c , M. Lagrenée d,⇑ a Laboratoire de Chimie de Coordination et d’Analytique (LCCA), Faculté des Sciences, Université Chouaib Doukkali, B.P. 20, M-24000 El Jadida, Morocco b Unité Matériaux et Transformations (UMET), Ingénierie des Systèmes Polymères, CNRS UMR 8207, ENSCL, B.P. 90108, F-59652 Villeneuve d’Ascq Cedex, France c Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), UMR-CNRS 8516, Université des Sciences et Technologies de Lille, Bâtiment C5, F-59655 Villeneuve d’Ascq Cedex, France d Unité de Catalyse et de Chimie du Solide (UCCS), UMR-CNRS 8181, ENSCL, B.P. 90108, F-59652 Villeneuve d’Ascq Cedex, France article info Article history: Received 29 June 2010 Accepted 29 September 2010 Available online 8 October 2010 Keywords: A. Mild steel A. Acidic solutions B. EIS C. Acid inhibition abstract The inhibition properties of 2,5-bis(n-pyridyl)-1,3,4-thiadiazoles (n-PTH) on corrosion of mild steel in dif- ferent acidic media (1 M HCl, 0.5 M H 2 SO 4 and 1 M HClO 4 ) were analyzed by electrochemical impedance spectroscopy (EIS). The n-PTH derivatives exhibit good inhibition properties in different acidic solutions and the calculated values of DG  ads revealed that the adsorption mechanism of n-PTH on steel surface is mainly due to chemisorption. While in 1 M HClO 4 , both 2-PTH and 4-PTH isomers stimulate the corrosion process especially at low concentrations. Quantum chemical calculations using the density functional theory (DFT) were performed on n-PTH derivatives to determine the relationship between molecular structure and their inhibition efficiencies. The results of the quantum chemical calculations and experi- mental inhibition efficiency were subjected to correlation analysis and indicate that the inhibition effects of n-PTH may be explained in terms of electronic properties. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Many heterocyclic compounds containing heteroatoms like N, O, S, have been reported to be effective inhibitors for the corrosion of steel in acid media by several authors [1–11]. The inhibition property of these compounds is attributed to their molecular struc- ture. The structure and the lone electron pairs in the heteroatoms are important features that determine the adsorption of these mol- ecules on the metallic surface. Generally, organic inhibitor mole- cules might physically or chemically adsorb on a corroding metal. In any case, adsorption is general over the metal surface and the resulting adsorption layer function as a barrier, isolating the metal from the corrosion [12]. The inhibition efficiency has found to be closely related to inhibitor adsorption abilities and molecular properties for different kinds of organic compounds [13–15]. The efficiency of an inhibitor does not only depend on its structure, but also on the characteristics of the environment in which it acts; the nature of the metal and other experimental conditions. Under certain conditions, the electronic structure of the organic inhibitors has a key influence on the corrosion inhibi- tion efficiency to the metal. The choice of effective inhibitors has been mostly done by using the empirical knowledge based on their macroscopically physico- chemical properties, mechanism of action and electron donating ability [16,17]. The most significant criteria involved in the selec- tion of an inhibitor are its hydrophobicity, molecular structure, electron density at their donor atoms, solubility and dispersibility. Recently, quantitative structure activity relationship (QSAR) has been a subject of intense interest in many disciplines of chemistry. The development of semi-empirical quantum chemical calcula- tions emphasizes the scientific approaches involved in the selec- tion of inhibitors by correlating the experimental data with quantum–chemical properties. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), charges on reactive centre, dipole moment (l) and conformations of molecules have been used to achieve the appropriate correla- tions [18–23]. In continuation of our earlier work on thiadiazole derivatives [1,24–33], we have investigated the corrosion behaviour of mild steel in different acidic media (1 M HCl, 0.5 M H 2 SO 4 and 1 M HClO 4 ) in the presence of 2,5-bis(n-pyridyl)-1,3,4-thiadiazoles (n- PTH) using electrochemical impedance spectroscopy (EIS). Also, the ab initio density functional theory (DFT) calculations were per- formed to correlate the structure of these 1,3,4-thiadiazole deriva- tives (both neutral and cationic forms) with their effectiveness in corrosion inhibition of steel in different acidic media. For this 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.09.063 ⇑ Corresponding authors. Tel.: +33 320 337 746; fax: +33 320 436 814 (F. Bentiss). E-mail addresses: fbentiss@enscl.fr (F. Bentiss), michel.lagrenee@ensc-lille.fr (M. Lagrenée). Corrosion Science 53 (2011) 487–495 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci