ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2015, Vol. 51, No. 5, pp. 873–884. © Pleiades Publishing, Ltd., 2015. 873 1 1. INTRODUCTION The synthesis of new organic molecules offers var- ious molecular structures containing heteroatoms and substituents for corrosion protection in acid pickling of metals. The most synthesized compounds are the nitrogen heterocyclic compounds, which are known to be excellent complex or chelate forming substances with metals [1–7]. The choice of the inhibitor is based on two considerations: first it could be synthesized conveniently from relatively cheap raw materials, sec- ondly, it contains the electron cloud on the aromatic ring or, the electronegative atoms such as nitrogen and oxygen in the relatively long chain compounds [8, 9]. Pyrazolic compounds are good inhibitors of iron corrosion in sulphuric and hydrochloric acid [10–15]. Their inhibiting effect is closely related to the molecu- lar structure containing two nitrogen atoms in the pyrazolic ring. Generally, the heterogeneous organic compounds having higher basicity and electron density on the het- ero atoms such as N, O, and S, have tendency to resist 1 The article is published in the original. corrosion. Nitrogen and oxygen are the active centers for the process of adsorption on the metal surface. These compounds can adsorb on the metal surface, block the active sites on the surface and thereby reduce the corrosion attack. The efficiency of these com- pounds as corrosion inhibitors can be attributed to the number of mobile electron pair present, the π orbital character of free electrons and the electron density around nitrogen and oxygen atoms [16–18]. In the present study, the inhibitive effect of length of the chain between pyrazolic synthesized com- pounds, 1,1-bis(3-ethoxycarbonyl-5-methylpyrazolyl)- methane(P1) and 1,4-bis(3-ethoxycarbonyl-5-meth- ylpyrazolyl)-butane(P2), were investigated on mild steel in 1 M HCl solution. The results were analyzed with the aim of determining the inhibition efficiency of these compounds at different temperatures and Adsorption Properties and Inhibition of Mild Steel Corrosion in 1 M HCl Solution by Some Bipyrazolic Derivatives: Experimental and Theoretical Investigations 1 H. Elmsellem a , T. Harit b , A. Aouniti a , F. Malek b , A. Riahi c , A. Chetouani a, d , and B. Hammouti a a Laboratoire de Chimie Appliquée et environnement (LCAE-URAC18), Faculté des Sciences, Université Mohammed Premier, Oujda, Morocco b Faculte des Sciences-Universite Mohamed Premier, Laboratoire de Chimie Organique, Macromoleculaire et Produits Naturels-URAC 25, Bd Mohamed VI, BP: 717, 60000 Oujda, Morocco c Université de Reims Champagne-Ardenne, Institut de Chimie Moléculaire de Reims (ICMR)-Groupe Méthodologie en Synthése Organique, CNRS UMR 6229, Bat. Europol’Agro-Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France d Laboratoire de chimie physique, Centre Régionale des Métiers de l’Education et de Formation “CRMEF”, Région de l’Orientale, 60000 Oujda, Morocco e-mail: ahmedchetouani70@hotmail.com Received October 17, 2014 Abstract—The corrosion inhibition of mild steel in 1 M HCl by 1,1-bis(3-ethoxycarbonyl-5-methylpyra- zolyl)-methane (P1) and 1,4-bis(3-ethoxycarbonyl-5-methylpyrazolyl)-butane (P2) has been studied at 308 K using electrochemical and weight loss measurements. Polarization curves reveal that the used compounds are mixed type inhibitors. Results show that inhibition efficiency increases when the inhibitor concentration increases. The comparative study of inhibitive performance of the two bipyrazolic derivatives revealed that P2 is more effective than P1. The adsorption of P2 on steel surface obeyed Langmuir’s adsorption isotherm. The kinetic and thermodynamic parameters for mild steel corrosion and inhibitor adsorption, respectively, were determined and discussed. Obvious correlation was found between corrosion inhibition efficiency and quan- tum chemical parameters, using the Density Functional Theory method (DFT). The obtained theoretical results have been compared with the experimental results. DOI: 10.1134/S207020511505007X Table 1. Chemical composition of steel specimens Elements Fe C Si P Mn S Al Mass % 99.21 0.21 0.38 0.08 0.05 0.05 0.01 PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION