Theoretical and electrochemical evaluation of 2-thioureidobenzheteroazoles as potent corrosion inhibitors for mild steel in 2 M HCl solution Ram Murthy a , Parth Gupta b , C.N. Sundaresan a, ⁎ a Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Brindavan Campus, Kadugodi, Bengaluru 560067, India b Department of Chemistry, Indian Institute of Technology, Chennai 600036, India abstract article info Article history: Received 24 June 2020 Accepted 16 August 2020 Available online 22 August 2020 Keywords: Mild steel, acid corrosion Impedance (EIS) Polarization (PDP) Thermodynamic parameters SEM Corrosion Inhibition potential of three 2-thioureidobenzheteroazoles, namely, 2-thioureidobenzoxazole (BOT), 2-thioureidobenzimidazole (BIT), 2-thioureidobenzothiazole (BTT) compounds were predicted theoretically using quantum chemical calculations on type CS 1084 carbon steel in 2 M HCl at 25 °C. DFT calculations indicated that BTT showed better inhibition compared to BIT and BOT compounds. Theoretical conclusions were subse- quently validated by experiments carried out using Electrochemical Impedance Spectroscopy (EIS), Potentiody- namic Polarization (PDP) and Scanning Electron Microscopy (SEM). Adsorption of 2-thioureidobenzheteroazoles on the steel surface obeyed the Langmuir adsorption model. Thermodynamic parameters indicated that, the ad- sorption process is energetically spontaneous, while the calculated kinetic parameters corroborated the adsorp- tion phenomenon at the metal/solution interface. Furthermore, PDP results showed that, the inhibitors behaved as mixed type inhibitors with cathodic dominance. The maximum corrosion inhibition efficiency obtained was 96.1% for BTT at 2 mM concentration. The surface morphology investigations using SEM technique showed lower corrosion inhibition in presence of these inhibitors. Sulphur and nitrogen heteroatoms served as reactive centres for the adsorption of inhibitors, based on the quantum chemical calculations. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Mild steel (MS) is a material of choice in several industries, due to its high mechanical durability, ease of availability, high ductility, hardness and mechanical strength. Mild Steel is the most widely used metals from manufacturing to distribution of products in oil and gas industry [1]. In addition to this, pipelines are extensively used to transport to large amounts of crude oil from one region to another. Hence, the pipe- lines must be able to withstand large amount of hydraulic pressure. However, the thermodynamic instability of metals forces the metals to revert to its original native form. In this direction, corrosion damages the infrastructure and results in loss of desired structural properties. There is great demand and utility of MS in the chemical industries. However, MS undergoes rapid corrosion, when it is exposed to aggres- sive acidic environments such as HCl, which is widely utilized in indus- trial processes such as acid pickling, acid cleaning, acid descaling and oil well acidizing [2]. An average of 10% of the metal output in the world is estimated to be the cost of corrosion. Cost of corrosion in industrialized countries is estimated to be 3–4% of their GDP. Interruptions in plant op- erations results in huge loss for industries which have attracted the at- tention of researchers to synthesize effective low-cost inhibitors to control and prevention of corrosion processes. In this direction, organic corrosion inhibitors containing heteroatoms such as N, O and S atoms have been widely used to control the acid-catalysed corrosion of MS [3,4]. However, the literature data till date, suggests that, heterocyclic compounds have been successfully employed as corrosion inhibitors [5]. Many sulphur and nitrogen containing compounds like Benzimid- azoles (BIZ) and Benzotriazoles (BTZ) are widely known for their metal corrosion inhibition ability [1,6]. These BIZ and BTZ compounds act as adsorption inhibitors by forming chemisorptive bond with the metal surface and impede anodic dissolution process. It is also antici- pated that, the presence of functional groups such as –C=C, -C=N-, -N=N-, -NH, -C=S and π-bonds would enhance the adsorption of these inhibitor molecules onto the MS surface [7]. However, the electron densities around the donor atom (N, S and O) of an inhibitor molecule strongly influence the adsorption of organic compounds on the metal surface, thereby retarding the corrosion process [8]. The 2-thioureidobenzheteroazoles group exhibits improved corro- sion resistant property compared to bare benzothiazoles moieties in terms of immersion time and high inhibition efficiency at a very low concentration [9]. In the current study, the electronic properties of 2- thioureidobenzheteroazoles were investigated in gas and aqueous conditions using B3LYP 6-311 G (d,p) and B3LYP/6-311++G (d,p) basis sets. The quantum calculations in conjunction with Journal of Molecular Liquids 319 (2020) 114081 ⁎ Corresponding author. E-mail address: cnsundaresan@sssihl.edu.in (C.N. Sundaresan). https://doi.org/10.1016/j.molliq.2020.114081 0167-7322/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq