5-Arylpyrimido-[4,5-b]quinoline-diones as new and sustainable corrosion inhibitors for mild steel in 1 M HCl: a combined experimental and theoretical approach† Chandrabhan Verma, a L. O. Olasunkanmi, bc I. B. Obot, d Eno E. Ebenso bc and M. A. Quraishi * a The inhibition of mild steel corrosion in 1 M HCl by four 5-arylpyrimido-[4,5-b]quinoline-diones (APQDs), namely 5-(4-nitrophenyl)-5,10-dihydropyrimido [4,5-b]quinoline-2,4(1H,3H)-dione (APQD-1), 5-phenyl-5,10- dihydropyrimido[4,5-b]quinoline-2,4(1H,3H)-dione (APQD-2), 5-(4-hydroxyphenyl)-5,10-dihydropyrimido- [4,5-b]quinoline-2,4(1H,3H)-dione (APQD-3) and 5-(2,4-dihydroxyphenyl)-5,10-dihydropyrimido[4,5-b] quinoline-2,4(1H,3H)-dione (APQD-4) has been investigated using weight loss, electrochemical, surface, and quantum chemical calculations and molecular dynamics simulation methods. The results showed that the inhibition efficiency (h%) increased with increasing concentration of the inhibitors. Among the studied compounds, APQD-4 exhibited the highest inhibition efficiency of 98.30% at 20 mg l 1 concentration. The studied compounds effectively retarded the corrosion of mild steel in 1 M HCl by adsorbing onto the steel surface, and the adsorption data conformed to the Langmuir adsorption isotherm. The results of potentiodynamic polarization measurements revealed that the studied compounds are cathodic-type inhibitors. Scanning electron microscopy (SEM) study confirmed the formation of adsorbed films of the inhibitor molecules on the steel surface. Quantum chemical calculations and molecular dynamics simulations were undertaken to corroborate experimental findings and provide adequate insight into the corrosion inhibition mechanisms and adsorption characteristics of the studied compounds. 1. Introduction Metals and alloys undergo chemical and/or electrochemical reactions with the environment to form relatively more stable compounds and consequently there is a loss of metals due to the corrosion process. Among different available methods that have been identied for corrosion control, the use of synthetic corrosion inhibitors is one of the most appropriate, effective and economic ways of mitigating corrosion problem. 1–4 However, most of the synthetic corrosion inhibitors are toxic and not environmentally friendly. In recent years, green chemistry has attracted considerable attention from synthetic and medicinal chemists due to increasing ecological awareness and strict environmental regulation. 5,6 Therefore, the current trend of research in corrosion inhibition is directed towards the development of green corrosion inhibitors that offer high inhibition efficiency at low environmental risk. 7,8 In this regard, multicomponent reactions have immerged as a green and powerful technique in synthetic organic chemistry and drug discovery in the sense that several biologically active complexes/ molecules can be synthesized in one step by using commercially available cheap starting materials. 9,10 The multicomponent reactions have several advantages such as operational simplicity, facile automation and minimized waste generation, because of the reduction in the number of work-up, extraction and purication stages. 11,12 Furthermore, the consumption of environmentally benign solvents and chemicals during the reactions provides the means of upholding the essential prin- ciples of green chemistry. Nowadays, the development of synthetically useful reactions using water as reaction medium has drawn considerable attention because of its non- ammable, non-hazardous, non-toxic, uniquely redox-stable, inexpensive and free availability. 13–15 Moreover, in asymmetric organocatalysis, the use of proline, particularly in water and 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, School of Mathematical & Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University (Makeng Campus), Private Bag X2046, Mmabatho 2735, South Africa c 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 d Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27417f Cite this: RSC Adv. , 2016, 6, 15639 Received 22nd December 2015 Accepted 28th January 2016 DOI: 10.1039/c5ra27417f www.rsc.org/advances This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 15639–15654 | 15639 RSC Advances PAPER