Corrosion inhibition of mild steel in hydrochloric acid by some pyridine derivatives: An experimental and quantum chemical study K.R. Ansari a , M.A. Quraishi a, *, Ambrish Singh b a Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India b Department of Chemistry, Lovely Faculty of Technology and Sciences, Lovely Professional University, Phagwara, India Introduction Mild steel is widely used in fabrication of reaction vessels, storage tanks and petroleum refineries. Corrosion of mild steel is a fundamental academic and industrial concern that has received a considerable amount of attention [1]. Among numerous corrosion prevention measurements, the use of inhibitors is one of the most efficient alternatives to protect metals against corrosion, especially in acidic media due to its advantages of economy, high efficiency, and wide applicability [2–4]. The pyridine nucleus is one of the most important N-hetero- cycles found in natural products, pharmaceuticals and functional materials [5]. These N-heterocyclic compounds not only play an important role in biological activity but also act as a potential corrosion inhibitor [6–8]. Survey of literature reveals that pyridine compounds are effective corrosion inhibitors [9–12], which incited us to use them as a corrosion inhibitor for mild steel. The inhibition efficiency of previously reported pyridine derivatives at different acid concentration and temperature ranges (59–96%) at (1176–5400 mg L 1 ) inhibitor concentration [9,10,13–15]. But in our case best inhibition efficiency of 97.6% at only 400 mg L 1 observed in 1 M HCl. In the present study three pyridine derivatives namely 2-amino-6-methoxy-4-(4-methoxylphenyl) pyridine-3,5-dicar- bonitrile (PC-1), 2-amino-6-methoxy-4-(4-methylphenyl) pyri- dine-3,5-dicarbonitrile (PC-2) and 2-amino-6-methoxy-4- phenylpyridine-3,5-dicarbonitrile (PC-3) were investigated as corrosion inhibitor for mild steel in 1 M HCl by using gravimetric measurements, electrochemical impedance spectroscopy and polarization measurements. The surfaces of the mild steel specimens were examined by Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Quan- tum chemical study. To date, no literature is available reporting the use of these heterocyclic compounds as corrosion inhibitors. Experimental Materials and chemicals The inhibitors were synthesized according to the reported literature [16]. The molecular structure, IUPAC name and abbreviation of the inhibitors are given in Table 1. These inhibitors were purified by recrystallization from methanol. Corrosion tests were performed on the mild steel strips with the following composition (wt%): C = 0.076, Mn = 0.192, P = 0.012, Journal of Industrial and Engineering Chemistry 25 (2015) 89–98 A R T I C L E I N F O Article history: Received 25 February 2014 Received in revised form 27 September 2014 Accepted 18 October 2014 Available online 27 October 2014 Keywords: Mild steel Gravimetric measurements SEM–EDX A B S T R A C T The adsorption and corrosion inhibition effect of three pyridine derivatives namely 2-amino-6- methoxy-4-(4-methoxylphenyl) pyridine-3,5-dicarbonitrile (PC-1), 2-amino-6-methoxy-4-(4-methyl- phenyl) pyridine-3,5-dicarbonitrile (PC-2) and 2-amino-6-methoxy-4-phenylpyridine-3,5-dicarboni- trile (PC-3) on mild steel in 1 M HCl was investigated using gravimetric, electrochemical impedance spectroscopy, potentiodynamic polarization techniques and quantum chemical study. PCs are mixed- type inhibitor mainly cathodic. Activation energy (E a ), equilibrium constant (K ads ), free energy of adsorption (DG8 ads ) were calculated and discussed. The studied inhibitors (PCs) obeyed Langmuir’s adsorption isotherm. Protective film of PCs on mild steel surface was investigated by using SEM and EDX. Quantum chemical parameters were carried out in the framework of the density functional theory (DFT). ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +91 9307025126; fax: +91 542 2368428. E-mail addresses: maquraishi@rediffmail.com, maquraishi.apc@itbhu.ac.in (M.A. Quraishi). Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry jou r n al h o mep ag e: w ww .elsevier .co m /loc ate/jiec http://dx.doi.org/10.1016/j.jiec.2014.10.017 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.