2,4-Diamino-5-(phenylthio)-5H-chromeno [2,3-b] pyridine-3-carbonitriles as green and effective corrosion inhibitors: gravimetric, electrochemical, surface morphology and theoretical studies Chandrabhan Verma, a Lukman O. Olasunkanmi, bc I. B. Obot, d Eno E. Ebenso b and M. A. Quraishi * a The inhibition of mild steel corrosion in 1 M HCl by three newly synthesized 2,4-diamino-5-(phenylthio)- 5H-chromeno[2,3-b]pyridine-3-carbonitriles (DHPCs) namely, 2,4-diamino-7-nitro-5-(phenylthio)-5H- chromeno[2,3-b]pyridine-3-carbonitrile (DHPC-1), 2,4-diamino-5-(phenylthio)-5H-chromeno[2,3-b] pyridine-3-carbonitrile (DHPC-2) and 2,4-diamino-7-hydroxy-5-(phenylthio)-5H-chromeno[2,3-b] pyridine-3-carbonitrile (DHPC-3) was studied using weight loss method, electrochemical techniques, surface morphology (SEM, AFM) studies and theoretical (quantum chemical calculations and molecular dynamic simulation) methods. The weight loss and electrochemical measurements showed that the inhibition efficiency increases with increasing inhibitor concentration and the relative trend of inhibition performance is DHPC-3 > DHPC-2 > DHPC-1. A potentiodynamic polarization study reveals that the investigated DHPCs act as mixed type inhibitors. The adsorption of the DHPCs on the mild steel surface obeys the Langmuir adsorption isotherm and involves both physisorption and chemisorption modes. The presence of the electron releasing –OH group at position seven on the chromenopyridine ring is considered to be responsible for the highest inhibition efficiency of DHPC-3 among the studied compounds. Whereas the presence of the electron withdrawing nitro (–NO 2 ) group at position seven on the chromenopyridine ring is responsible for the lowest inhibitive strength of DHPC-1. Quantum chemical calculations and molecular dynamic simulation studies were undertaken to provide mechanistic insight into the roles of the different substituents (–OH and –NO 2 ) on the corrosion inhibition behavior of the studied inhibitors. 1. Introduction Iron and its alloys are widely used as construction materials in the petroleum, food, power production, chemical and electro- chemical industries. This is due to their high thermal and mechanical stability, ease of fabrication and joining, and low cost. 1–3 However, these materials become gradually destroyed by corrosion upon exposure to the environment due to chemical or/and electrochemical reactions with the environment. Therefore, several efforts are being channeled towards preventing these undesirable reactions. Among the several available methods of corrosion protection, the utilization of synthetic corrosion inhibitors has become a popular method because of the ease and economic viability of the synthesis of these inhibitors, high inhibition efficiency, and practical-feasi- bility. 4–6 Most of the efficient corrosion inhibitors are organic compounds containing polar functional groups and p-electrons in form of triple or conjugated double bonds. These synthetic compounds inhibit corrosion by adsorbing on metallic surface. Generally, the adsorption of these inhibitors on the metal surfaces depends on numerous physicochemical properties such as nature of functional groups, steric factors, aromaticity, electron density at the donor atoms and p-orbital character of donating electrons and the electronic structure of the inhibitors molecules. 7 Previous literature had established that S- containing compounds show better inhibition efficiency in sulphuric acid solution, while N-containing compounds show better inhibition efficiency in hydrochloric acid solution. 8 Whereas, compounds containing both N- and S-atoms generally give rise to even better inhibition efficiency. 9,10 a Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India. E-mail: maquraishi.apc@itbhu.ac.in; maquraishi@ rediffmail.com; Fax: +91-542-2368428; Tel: +91-9307025126 b Department of Chemistry and Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Makeng Campus), Private Bag X2046, Mmabatho 2735, South Africa c Department of Chemistry, Faculty of Science, Obafemi Awolowo University, Ile-Ife, 220005, Nigeria d Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Cite this: RSC Adv. , 2016, 6, 53933 Received 24th February 2016 Accepted 25th May 2016 DOI: 10.1039/c6ra04900a www.rsc.org/advances This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 53933–53948 | 53933 RSC Advances PAPER Published on 26 May 2016. Downloaded by North-West University - South Africa on 27/06/2017 08:39:21. View Article Online View Journal | View Issue