Relationship between structure and inhibition behaviour of quinolinium salts for mild steel corrosion: Experimental and theoretical approach Punita Mourya a , Praveen Singh b , A.K. Tewari b , R.B. Rastogi a , M.M. Singh a,⇑ a Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India b Department of Chemistry, Banaras Hindu University, Varanasi 221005, India article info Article history: Received 24 November 2014 Accepted 7 February 2015 Available online xxxx Keywords: A. Mild steel B. Polarization B. EIS B. XPS B. AFM C. Acid corrosion abstract Corrosion inhibition behaviour of quinolinium salts on mild steel (MS) corrosion in 0.5 M H 2 SO 4 was investigated at various concentrations and temperatures by gravimetric and electrochemical techniques. The polarization curves indicated that these salts function as mixed inhibitors. Relevant kinetic and thermodynamic parameters like C R , E a , DH 0 a , DS 0 a , K ads and DG 0 ads were determined and discussed. The adsorption of salts on MS leading to inhibition was found to follow the Langmuir adsorption isotherm. The existence of a protective film on MS was confirmed by SEM and AFM techniques. XPS and quantum chemical calculations were carried out to establish the mechanism of corrosion inhibition. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The inhibition of MS corrosion of iron in acidic media by differ- ent organic compounds has been widely studied. The existing lit- erature shows that most of the organic inhibitors act by getting adsorbed on the iron surface. This phenomenon is influenced by the nature and surface charge on metal, the type of aggressive elec- trolyte and the chemical structure of inhibitor. Most of these inhi- bitors are substances with at least one functional group which is considered to be the reaction centre for the adsorption process. The adsorption of inhibitor is related to the presence of hetero atoms such as nitrogen, oxygen and sulphur with or without the presence of multiple bonds or aromatic ring in their molecular structures. The relationship between the inhibitive properties and the molecular structures of organic compounds has been an area of great interest for many investigators [1–3]. The application of quaternary ammonium salts as corrosion inhibitors has been investigated earlier in different laboratories [4–7]. Noor [8] have reported that some of nitrogen containing heterocyclic compounds in the form of salts are suitable as corrosion inhibitors for mild steel in H 3 PO 4 . They found inhibition efficiencies of 2.5  10 4 M 1-methyl-4[4(-N(CH 3 ) 2 )-styryl] pyri- dinium iodide, 1-methyl-4[4(-N(CH 3 ) 2 )-styryl] quinolinium iodide and 1-methyl-4[4(-OCH 3 )-styryl] quinolinium iodide at 30 °C to be 56.1%, 75.6% and 98.9%, respectively. Popova et al. [4] investigated the corrosion inhibition of mild steel in 1 M HCl and 1 M H 2 SO 4 using some quaternary ammonium bromides of benzothiazole. Their results showed that the protection efficiency of all seven compounds reached around 90% at 1  10 4 M concentration in both the acids. A benzimidazole derivative, 1-butyl-3-methyl-1H- benzimidazolium iodide [BMBIMI] was tested by Zheng et al. [9] as inhibitor for mild steel in sulphuric acid and inhibition efficiency was observed as 97.8% at 5 mM. In another investigation on corrosion inhibition of carbon steel in alkaline – chloride solu- tion employing 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF 4 ) nearly 86% inhibition has been reported [10]. The foregoing literature survey reveals that N-alkyl-quaternary ammonium salts are promising inhibitors. In view of this, during the present investigation, some new derivatives of quaternary ammonium salts having quinoline ring have been synthesized and tested for their inhibitive properties towards mild steel corrosion in 0.5 M H 2 SO 4 . Some of the reports in literature mention that halide ions improve the inhibition performance of organic compounds through synergistic effect [11,12]. The synergism by halides has been attributed to the improved surface coverage as a result of ion-pair interactions between organic cation and halide anion. The effect of halide ions in improving inhibition efficiency has been found to increase from chloride to iodide [13]. Therefore, in this http://dx.doi.org/10.1016/j.corsci.2015.02.034 0010-938X/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel./fax: +91 542 6702859. E-mail address: mmsingh.apc@iitbhu.ac.in (M.M. Singh). Corrosion Science xxx (2015) xxx–xxx Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci Please cite this article in press as: P. Mourya et al., Relationship between structure and inhibition behaviour of quinolinium salts for mild steel corrosion: Experimental and theoretical approach, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.02.034