2-Amino-5-nitro-4,6-diarylcyclohex-1-ene-1,3,3-tricarbonitriles as new and effective corrosion inhibitors for mild steel in 1 M HCl: Experimental and theoretical studies Chandrabhan Verma a , M.A. Quraishi a, ⁎, Ambrish Singh b a Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India b State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, South west Petroleum University, Chengdu, Sichuan 610500, China abstract article info Article history: Received 24 June 2015 Received in revised form 12 September 2015 Accepted 14 October 2015 Available online 11 November 2015 Keywords: 2-Amino-5-nitro-4,6-diarylcyclohex-1-ene- 1,3,3-tricarbonitriles Mild steel Acid corrosion Adsorption SEM/EDX/AFM Quantum chemical calculation The influence of three 2-amino-5-nitro-4,6-diarylcyclohex-2-ene-1,3,3-tricarbonitrile (ANDT) derivatives namely, 2-amino-5-nitro-4,6-diphenylcyclohex-1-ene-1,3,3-tricarbonitrile (ANDT-1), 2-amino-4,6-bis-(4- methoxy-phenyl)-5-nitro-cyclohex-2-ene-1,3,3-tricarbonitrile (ANDT-2) and 2-amino-4,6-bis-(3-hydroxy-4- methoxy-phenyl)-5-nitro-cyclohex-2-ene-1,3,3-tricarbonitrile (ANDT-3) on mild steel corrosion in 1 M HCl has been investigated using experimental and theoretical techniques. Among the all studied inhibitors the ANDT-3 showed maximum inhibition efficiency of 98.96% at 25 mg L -1 . Adsorption of the ANDTs on mild steel surface follows the Langmuir adsorption isotherm. Polarization studies show that ANDTs act as mixed type inhibitors. SEM, EDX and AFM techniques were used to confirm the existence of adherent protective film of inhibitors on the mild steel surface. A quantum chemical calculation based on the DFT-theory was used to fur- ther corroborate the mechanism of inhibition. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Mild steel is widely used in the chemical, petrochemical and petroleum industries because of its low cost, and high mechanical and thermal stability [1,2]. However, it undergoes rapid corrosion when it is exposed to acid solutions during acid pickling, industrial cleaning and descaling processes [3–7]. To reduce the attack of acid on metal surfaces, corrosion inhibitors are added in the acid solutions. Use of organic compounds containing heteroatoms (such as N, O, S, and P), aromatic rings and/or π-electrons are considered as one of the most practical and economic methods to prevent metallic corrosion [8]. Previously existing data show that organic compounds act by forming surface film at metal/electrolyte interfaces [9]. Generally, adsorption of these inhibitors is influenced by numerous factors including chemical structure, electronic distribution, and nature of metal and testing medium [10]. Literature survey reveals that heterocyclics containing nitrile groups (carbonitriles) act as efficient corrosion inhibitors for metals in different testing media [11–21]. In view of this, three carbonitrile compounds namely synthesized 2- amino-5-nitro-4,6-diarylcyclohex-2-ene-1,3,3-tricarbonitrile (ANDT) derivatives namely, 2-amino-5-nitro-4,6-diphenylcyclohex-1-ene-1,3,3- tricarbonitrile (ANDT-1), 2-amino-4,6-bis-(4-methoxy-phenyl)-5- nitro-cyclohex-2-ene-1,3,3-tricarbonitrile (ANDT-2) and 2-amino-4,6- bis-(3-hydroxy-4-methoxy-phenyl)-5-nitro-cyclohex-2-ene-1,3,3- tricarbonitrile (ANDT-3) were synthesized to study mild steel corrosion inhibition in 1 M HCl using weight loss, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). Quantum chemical parameters such as energy of the highest occupied molecular orbital (E HOMO ), energy of the lowest unoccupied molecular orbital (E LUMO ), energy gap (ΔE), global hardness (ρ), global softness (σ) and dipole moment (μ) were calculated and discussed [22,23]. 2. Experimental 2.1. Materials Mild steel specimens having percentage elemental composition of 0.076 wt.% C, 0.192 wt.% Mn, 0.012 wt.% P, 0.026 wt.% Si, 0.050 wt.% Cr, 0.023 wt.% Al, and balance with Fe has been used for weight loss electrochemical experiments. Before performing the experiments, the exposed area of these specimens were successively cleaned by abrasive SiC emery papers of different grades, washed with double deionized water, degreased with acetone and finally store in moisture free desic- cators. The analytical grade HCl (37%) was purchased from Merck and Journal of Molecular Liquids 212 (2015) 804–812 ⁎ Corresponding author. E-mail addresses: maquraishi.apc@itbhu.ac.in, maquraishi@rediffmail.com (M.A. Quraishi). http://dx.doi.org/10.1016/j.molliq.2015.10.026 0167-7322/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq