Novel quinoline derivatives as green corrosion

Journal of Molecular Liquids 216 (2016) 164–173

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Novel quinoline derivatives as green corrosion inhibitors for mild steel in acidic medium: Electrochemical, SEM, AFM, and XPS studies Priyanka Singh, Vandana Srivastava, M.A. Quraishi ⁎ Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India

a r t i c l e

i n f o

Article history: Received 18 August 2015 Accepted 23 December 2015 Available online xxxx Keywords: Mild steel Inhibitors Electrochemical measurements SEM/AFM/XPS

a b s t r a c t The corrosion mitigation effect of quinoline derivatives such as 2-amino-7-hydroxy-4-phenyl-1,4dihydroquinoline-3-carbonitrile (Q-1), 2-amino-7-hydroxy-4-(p-tolyl)-1,4 dihydroquinoline-3-carbonitrile (Q2), 2-amino-7-hydroxy-4-(4-methoxyphenyl)-1,4 dihydroquinoline-3 carbonitrile (Q-3), 2-amino-4-(4(dimethylamino)phenyl)-7-hydroxy-1,4-dihydroquinoline-3-carbonitrile (Q-4) were analyzed using weight loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarizations. Among all the investigated inhibitors, Q-4 showed the maximum inhibition efficiency of 98.09% at 150 mg/l. The electrochemical impedance spectroscopy (EIS) measurements revealed that corrosion inhibition takes place due to the adsorption of inhibitor molecules on the metal surface. The potentiodynamic polarization measurements show that Q-1, Q-2, Q-3 act as a mixed-type inhibitor while Q-4 acts as a cathodic inhibitor. The adsorption of quinolines on mild steel surface obeyed the Langmuir adsorption isotherm. The surface analysis techniques (SEM/AFM/XPS) further corroborate that the corrosion inhibition occurs due to the adsorption of the inhibitor molecules at the metal/solution interface. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Hydrochloric acid solutions are used in industrial processes during pickling and cleaning of metals and alloys; that causes significant metal loss [1,2]. Inhibitors are added to the acid solution to minimize acid attack on metal. Heterocyclic compounds are used as a corrosion inhibitor for preventing metal dissolution in acid solution [3–5]. These compounds are adsorbed on metal surfaces and bring down the corrosion rate of metals. Most of the commercial inhibitors exhibit adverse effects on the environment. In view of this, current research activities are being focused on the development of Eco-friendly corrosion inhibitors [6]. In the present investigation four quinoline derivatives have been chosen as corrosion inhibitors because of their non-toxic nature and they are essential constituents of anti-malarial drugs such as quinine, chloroquine, mefloquine, and amodiaquine [7,8]. There are only a few reports on the use of quinoline derivatives as corrosion inhibitors [9, 10]. In continuation of the work on development of Eco-friendly corrosion inhibitors, we have synthesized four quinoline derivatives to investigate their inhibiting action on the corrosion of mild steel in 1 M HCl ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.A. Quraishi).

http://dx.doi.org/10.1016/j.molliq.2015.12.086 0167-7322/© 2015 Elsevier B.V. All rights reserved.

using weight loss, electrochemical techniques (impedance spectroscopy (EIS) and potentiodynamic polarization). SEM, AFM, and XPS techniques were used for surface analysis of mild steel specimens.

Scheme 1. Synthetic route for the preparation of quinoline derivatives.

P. Singh et al. / Journal of Molecular Liquids 216 (2016) 164–173

165

Table 1 Molecular structures and analytical data of quinoline derivatives. Inhibitors

Molecular structures

Analytical data

2-amino-7-hydroxy-4-phenyl-1, 4-dihydroquinoline-3-carbonitrile (Q-1)

IR (KBr) cm−1: 3431(OH), 3347, 3210 (NH2, NH), 2221 (CN), 1641 (C = C vinyl nitrile), 1587 (C_C aromatic).

2-amino-7-hydroxy-4-(p-tolyl)-1, 4-dihydroquinoline-3-carbonitrile (Q-2)

IR (KBr) cm−1: 3430 (OH), 3343, 3208 (NH2, NH), 2220 (CN), 1640 (C = C vinyl nitrile), 1577 (C = C aromatic).

2-amino-7-hydroxy-4-(4-methoxyphenyl)-1, 4-dihydroquinoline-3-carbonitrile (Q-3)


2-amino-4-(4-(dimethylamino)phenyl)-7-hydroxy-1, 4-dihydroquinoline-3-carbonitrile (Q-4)


product was filtered, washed with water, dried and recrystallized from ethanol. IR spectra were recorded on FTIR (02) [Perkin Elmer, Bruker]. The molecular structure and analytical data are listed in Table 1.

2. Experimental 2.1. Inhibitors The multi-component synthesis of quinoline derivatives was performed according to reported literature [11] by using Sineo Microwave Chemistry Technology Co., LTD. A schematic representation of synthetic route is given in Scheme 1. After completion of the reaction, the solid

Table 2 Weight loss measurements for MS in absence and presence of quinoline derivatives in 1 M HCl at 308 K. Inhibitors

Concentrations (mg L−1)

Corrosion rate (mm/y)

Surface coverage (θ)

η%

Blank Q-1

0.0 50 100 150 50 100 150 50 100 150 50 100 150

77.9 14.8 7.4 5.1 11.1 5.5 2.9 9.6 4.4 2.5 6.3 2.5 1.4

– 0.80 0.90 0.93 0.85 0.92 0.96 0.87 0.94 0.96 0.91 0.96 0.98

– 80.95 90.47 93.33 85.71 92.85 96.19 87.61 94.28 96.66 91.90 96.66 98.09

Q-2

Q-3

Q-4

Fig. 1. Effect of different concentrations of inhibitors on inhibition efficiency for MS in 1 M HCl.

166


Fig. 2. Effect of temperature (308–338 K) on inhibition efficiency for MS in presence of quinoline derivatives in 1 M HCl.

Fig. 4. Transition state plots of log (Cr/T) against 1000/T for MS in the absence and presence of quinoline derivatives at 150 mg L−1 in 1 M HCl.

2.3. Weight loss measurements Table 3 Thermodynamic parameters for MS in absence and presence of quinoline derivatives in 1 M HCl at 150 mg L−1. Inhibitors

Ea (kJ mol−1)

ΔHa (kJ mol−1)

ΔSa (JK−1 mol−1)

Blank Q-1 Q-2 Q-3 Q-4

29.10 68.11 79.17 81.58 85.13

26.42 65.44 76.48 78.90 82.45

−123.27 −19.13 12.40 18.85 26.34

Due to the reliability and simplicity of weight loss method, it is preferably the starting method for corrosion testing among all corrosion techniques. Mild steel samples were tested in the absence and presence of different concentrations of quinoline derivatives at 308 K. Samples were weighted before and after 3 h of immersion time and then the differences were determined. The outcome helps to understand the inhibition performance of all inhibitors used.

2.4. Electrochemical measurements 2.2. Materials and solutions The tests were performed on mild steel samples having following composition (wt%): C 0.17%; Mn 0.46%; Si 0.026%; Cr 0.050%; P 0.012%; Cu 0.135%; Al 0.023%; Ni 0.05%; and balance Fe. Mild steel coupons having dimensions of 2.5 cm × 2 cm × 0.025 cm and 8 cm × 1 cm × 0.025 cm were used for weight loss and electrochemical study. The coupons were firstly abraded with fine grades of emery papers (600–1200) and then washed with double distilled water. The test solution of 1 M HCl was prepared by diluting analytical grade, 37% HCl with double distilled water.

Fig. 3. Arrhenius plots of log Cr vs. 1000/T for MS in the absence and presence of quinoline derivatives at 150 mg L−1 in 1 M HCl.

The electrochemical experiments were performed using a threeelectrode cell, which is connected to the Potentiostat/Galvanostat G300-45,050 (Gamry Instruments Inc., USA). Echem Analyst 5.0 software package was used for data fitting. A mild steel used coupon with an exposed area of 1 cm2 was used as working electrode, the platinum electrode as an auxiliary electrode, and the saturated calomel electrode (SCE) as a reference electrode. All the reported potentials were measured versus SCE. Tafel curves were obtained by changing the electrode potential automatically from −0.25 V to +0.25 V versus open corrosion potential at a scan rate of 1.0 mVs− 1. EIS measurements were performed under potentiostatic conditions in a frequency range from 100 kHz to 0.01 Hz, with amplitude of the 10 mV AC signal. The experiments were carried out after an immersion period of 30 min in 1 M HCl

Fig. 5. Langmuir adsorption isotherms for MS in presence of quinoline derivatives.

P. Singh et al. / Journal of Molecular Liquids 216 (2016) 164–173 Table 4 ΔGads, Kads for MS in presence of quinoline derivatives from temperature range 308–338 K at optimum concentration (150 mg L−1). Inhibitors

Temperature

Kads (103 M−1)

ΔGads (kJ mol−1)

Q-1

308 318 328 338 308 318 328 338 308 318 328 338 308 318 328 338

27.1 16.5 9.2 6.1 48.9 23.4 11.5 7.6 48.9 27.1 13.6 8.1 100.0 38.7 20.6 12.5

−36.43 −36.30 −35.88 −35.80 −37.94 −37.23 −36.47 −36.43 −37.94 −37.61 −36.93 −36.60 −39.77 −38.05 −37.30 −37.81

Q-2

Q-3

Q-4

167

Table 5 Electrochemical impedance parameters for MS in absence and presence of quinoline derivatives in 1 M HCl. Inhibitors Concentrations (mg L−1)

Rct (Ω cm2)

Blank Q-1 Q-2 Q-3 Q-4

0.0 100 150 100 150 100 150 100 150

9.0 69.9 97.3 158.6 334.9 217.3 337.7 373.0 546.3

n

Y° (μF

Cdl

η%

−S

−α°

106 39 36 37 33 35 33 34 32

– 87.13 90.75 94.32 97.31 95.85 97.33 97.58 98.35

0.53 0.82 0.83 0.82 0.85 0.82 0.83 0.80 0.88

40.40 68.00 68.16 70.55 74.59 71.02 71.54 69.84 75.28

cm−2) 0.82 250 0.86 91 0.84 87 0.83 88 0.85 65 0.82 85 0.85 69 0.85 67 0.82 65

to detect the chemical composition of the adsorbed film on the MS in the presence of Q-4 at 150 mg L−1. 3. Results and discussion

in the absence and presence of different concentrations of quinoline derivatives. 2.5. Surface characterization 2.5.1. Scanning electron microscopy (SEM) Surface analysis of the MS coupons was carried out in the absence and presence of quinoline derivatives using SEM model FEI Quanta 200F microscope at 500 × magnification. MS coupons were analyzed after 3 h of immersion time in the absence and presence of Q1–Q4 at 150 mg L−1. 2.5.2. Atomic force microscopy (AFM) The morphology of MS surface in the absence and presence of Q1-Q4 was studied by Bruker Dimension Icon SPM with tapping mode in the Air, RTESPA probe; k = 40 N/m and fo = 302 kHz, at 10 μm. 2.5.3. X-ray photoelectron spectroscopy (XPS) The elemental analysis was recorded with X-Ray Photoelectron Spectroscopy (AMICUS, Kratos Analytical, Shimadzu, U.K.). It was used

3.1. Weight loss measurements 3.1.1. Effect of inhibitor concentration The results obtained from weight loss study are given in Table 2 and the effect of inhibitor concentrations on inhibition efficiency is shown in Fig. 1. It can be seen from the results that on increasing the concentration of inhibitors, inhibition efficiency increases from 80.95 to 98.09% and shows the following order of inhibition Q-4 N Q-3 N Q-2 N Q-1. The inhibition efficiency (η%) was determined using following equations,

η% ¼

W−i W 100 W

ð1Þ

where W and iW are the weight loss of MS specimens in the absence and presence of inhibitors. Corrosion inhibition performance of a compound depends upon its ability to get adsorbed on metal surfaces. In our investigation, we have taken quinoline derivatives containing π electrons and heteroatoms

Fig. 6. Nyquist plots for MS in 1 M HCl without and with quinoline derivatives.

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Thus, among several adsorption isotherms used, Langmuir isotherm was found to provide the best fit, and it gives straight lines for log(θ/1 − θ) vs log C(M) shown in Fig. 5. The equation can be represented as, K ads C ads ¼

θ ð1−θÞ

ð5Þ

where C is the concentration, Kads is the equilibrium constant of the adsorption process. Kads are related to the standard free energy of adsorption ΔGads by the following equation, ΔGads ¼ −RT ln ð55:5K ads Þ

Fig. 7. Equivalent circuit model used to analyze the impedance data.

ð6Þ

(N, O) by which they can easily get adsorbed onto the metal surface and form a protective layer thereby preventing the corrosion [12,13]. 3.1.2. Effect of temperature Fig. 2 shows the effect of temperature on inhibition efficiency and corresponding parameters are listed in Table 3. The rise in temperature (308–338 K) accelerates the corrosion reaction and hence shows a decrease in the resultant inhibition efficiency. Increase in temperature slows down the adsorption of inhibitor molecules on the metal surface resulting in desorption of inhibitor molecules because these two opposing processes are in equilibrium [14]. The effect of temperature on activation energy can be calculated using Arrhenius equation, C r ¼ A exp

−Ea RT

ð2Þ

where Ea is the activation, R is the gas constant, A the Arrhenius preexponential factor and T is the absolute temperature. A plot between log Cr vs. 1000/T for MS in the absence and presence of inhibitor gives a straight line presented in Fig. 3. The Ea values are determined by the obtained slopes. The activation energy is higher for inhibited solution than uninhibited one suggesting a strong inhibitory action of additives which creates energy barrier for corrosion process [15]. The value of enthalpy and entropy of activation (Δ H*, Δ S*) was calculated using transition state equation, Cr ¼

RT ΔS ΔH exp − exp R RT Nh

ð3Þ

where h is Plank's constant and N is Avogadro number. A plot of log (Cr/T) against 1000/T is presented in Fig. 4 which gives straight lines with a slope of (−Δ H*/R) and an intercept of [(log(R/Nh)) + (ΔS*/R)] used to calculate values of ΔH* and ΔS*. The positive values of ΔH* in the presence and absence of inhibitor reflect the endothermic nature of steel dissolution. The higher values of ΔS* might be the result of the adsorption of inhibitor molecules from the 1 M HCl solution, which could be regarded as a quasi substitution process between inhibitor molecules in the aqueous phase and water molecules on the mild steel surface [16,17]. 3.1.3. Adsorption isotherm The data obtained from the experimental results were fitted to several adsorption isotherms in order to find basic information regarding the interaction of inhibitor molecules and the metal surface. The surface coverage θ at different concentration of quinoline derivatives can be calculated according to the equation,

θ¼

W−i W : W

ð4Þ

Fig. 8. Bode (log f vs log |Z|) and phase angle (log f vs α) plots of impendence spectra for MS in 1 M HCl Containing quinoline derivatives.


169

Fig. 9. Polarization curves for MS in 1 M HCl Containing quinoline derivatives.

where R represents the gas constant and T is the absolute temperature. The value of 55.5 is the concentration of water in solution in mol L−1. The negative values of ΔGads suggest spontaneous adsorption and stability of the adsorbed layer on MS surface. Also, the ΔGads values in the range of − 20 kJ mol− 1 indicate electrostatic interaction i.e. (physisorption). On the other hand, a ΔGads value near or greater than − 40 kJ mol− 1 shows chemisorptions [18,19]. The data listed in Table 4 show that the ΔGads values range from 39.7 to − 35.8 KJ mol−1 suggesting both physical and chemical modes of adsorption.

3.2. Electrochemical measurements

corrosion inhibitors for MS and show the following order of inhibition Q-4 N Q-3 N Q-2 N Q-1. The impedance results were analyzed by using the equivalent circuit model shown in Fig. 7 consisting of Rs (solution resistance), CPE (constant-phase element) parallel to the Rct (chargetransfer resistance). The charge-transfer resistances (Rct) were calculated by the difference in impedance at the lower and higher frequencies. The highest Rct is 546.3 Ω cm2 obtained for Q-4 at 150 mg L−1 with 98.35% inhibition efficiency. The inhibition efficiency is calculated using charge transfer resistance (Rct) as follows,

η% ¼

3.2.1. Electrochemical impedance spectroscopy (EIS) Fig. 6 shows the impedance responses of MS in the absence and presence of quinoline derivatives. The impedance parameters calculated from these plots are given in Table 5. The Nyquist plots exhibit one capacitive loop in the absence and presence of inhibitors suggesting that corrosion of MS was charge transfer controlled [20]. The diameter of the capacitive loops increases with increasing concentrations of inhibitors, which suggests that all the four compounds act as effective

! 100

ð7Þ

where Rct(i) and Rct are the values of charge transfer resistance in the absence and presence of inhibitors respectively. From the outcome, it can be seen that upon increasing the concentration of inhibitors the values of Rct increases while a decrease in double layer capacitance Cdl is observed. The increase in Rct is attributed to the formation of a protective layer on MS surface [21,22]. The double layer capacitance (Cdl) can be calculated using the following equation:

Table 6 Polarization parameters for MS in 1 M HCl containing quinoline derivatives. Inhibitors Concentrations Icorr (μA (mg L−1)

Rct 1− RctðiÞ

Yωn−1 sinðnðπ=2ÞÞ

ð8Þ

βa βc η% Ecorr (mV/SCE) (mV/dec) (mV/dec)

C dl ¼

−445 −462 −504 −511 −516 −479 −517 −517 −531

where, Ω is the angular frequency (Ω = 2π fmax) at which the imaginary part of the impedance (−Zim) is maximal, and n is the phase shift, which can be used as a gauge of the heterogeneity or roughness of the mild steel surface. In the same manner, a decrease in Cdl is attributed to decrease in dielectric constant and increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules are adsorbed at the metal/solution interface [23].

cm−2) Blank Q-1 Q-2 Q-3 Q-4

0.0 100 150 100 150 100 150 100 150

1390 182 134 102 80.4 39.7 39.1 45.2 36.7

82 93 59 67 59 63 80 67 60

118 156 141 159 133 143 138 137 118

– 86.90 90.35 92.66 94.21 97.14 97.18 96.74 97.35

170


Fig. 10. (a–e) showed the SEM micrographs in absence and presence of quinoline derivatives at 150 mg L−1.

The thickness of this protective layer (d) is correlated with Cdl by the following equation, Cdl ¼

εεoA d

ð9Þ

where ε is the dielectric constant and ε0 is the permittivity of free space, and A is the surface area of the electrode. The Bode phase angle plots were shown in Fig. 8 recorded for MS in the absence and presence of inhibitors to explain the various phenomena taking place at the metal/solution interface. The increase in absolute impedance at low frequencies in the Bode plot confirms the higher protection with the increasing of inhibitor concentration. In our investigation, the bode slope and phase angle values in the presence of inhibitors range from 0.82 to 0.88 and 68° to 75° and in blank solution 0.53 and 40° as listed in Table 5. An ideal capacitor behavior would result if a slope value attains − 1 and phase angle values attain − 90°

[24]. The higher slopes and phase angle values in the presence of inhibitors as compared to blank solution suggest the formation of a protective film on the MS surface [25]. 3.2.2. Potentiodynamic polarization measurements The polarization curves for MS in the absence and presence of inhibitors of different concentrations are shown in Fig. 9. The values obtained from polarization curves such as corrosion current densities (Icorr), corrosion potential (Ecorr), and Tafel slopes (βa, βc) are listed in Table 6. From Fig. 9 it can be seen that the addition of an inhibitor modifies both cathodic and anodic polarization branches and shifts the Ecorr towards the negative direction as compared to an inhibitor-free solution. As previously reported in the presence of inhibitor if Ecorr shifts more than 85 mV with respect to Ecorr in uninhibited solution, the inhibitor can be considered as cathodic or anodic type otherwise it is of mixed type [26,27]. The Ecorr values of Q-1, Q-2, Q-3, Q-4 suggested that Q-1, Q-2, Q-3 show the mixed type of inhibition with Ecorr shift in the


171

Fig. 11. (a–e) AFM micrographs of MS surface mild steel in absence and presence of quinoline derivatives at 150 mg L−1.

negative direction while Q-4 acts as a cathodic type inhibitor. The negative shift of Ecorr indicates more adsorption of the inhibitor on the cathodic sites and predominantly controls cathodic reactions. The presence of all the four inhibitors causes a decrease in Icorr values from 182 to 36.7 μA cm−2. The obtained results show that inhibition efficiency increases with decreasing values of Icorr and the highest inhibition efficiency 97.35% is obtained for Q-4 at 150 mg L−1. The inhibition efficiencies were calculated from Icorr values using the following equation,

η% ¼

Icorr −IcorrðinhÞ 100 Icorr

ð10Þ

where, Icorr and Icorr(inh) shows the corrosion current densities in the absence and presence of inhibitor respectively, in 1 M HCl.

4. Surface characterization SEM images of the MS surface were taken to analyze the morphology before and after inhibition process. The MS sample in the absence of inhibitor shows rough and highly damaged surface due to rapid corrosion attack in acidic solution as shown in Fig. 10 (a) while Fig. 10 (b–e) shows a smooth and less damaged surfaces of MS in the presence of inhibitors. The protected surface obtained for MS in the presence of inhibitor might be attributed to the adsorption of inhibitor molecules [28,29]. A close examination of Fig. 10 (e) reveals that the MS surface in the presence of Q-4 is the best-protected surface among all four inhibitors. The Quantitative information about the surface roughness of MS in the absence and presence of inhibitor is not provided by SEM micrograph. Therefore, AFM was used for further study about the surface morphology on MS surface. Fig. 11 (a–e) shows the different 3D images of MS in different conditions. Fig 11 (a) shows the corroded MS surface

172


Fig. 12. The XPS deconvoluted profiles of (a) C 1 s, (b) O 1 s, (c) N 1 s, and (d) Fe 2p3/2 for Q-4 treated MS.

with average roughness 336 nm. The higher surface roughness of MS exposed to blank solution shows a bumpy structure with a large number of ups and downs due to acid attack. From Fig. 11 (b–e) it can be observed that on adding inhibitor in acid solution the surface morphology of MS changed due to formation of adsorbed protective layer [30] which can be understood by a decrease in average surface roughness from 148 to 31 nm. The XPS analysis was used to investigate the elemental composition of adsorbed thin film on MS in the presence of Q-4 shown in Fig. 12 (a– d). The obtained high-resolution peaks for C 1 s, O 1 s, N 1 s and Fe 2p shows complex spectra that were analyzed through a deconvoluted fitting of complex spectra.

The deconvoluted C 1 s spectrum for MS in the presence of Q-4 may attribute three peaks indicating three chemical forms of C atom present on the steel surface as shown in Fig. 12(a). The first peak at binding energy 285.7 eV is attributed to the C–C, C_C, and C–H of aromatic bonds. The second peak at 286.2 eV may be assigned to the carbon atom bonded to nitrogen in C–N, C_N bonds in quinoline ring. The third peak may be ascribed to the carbon atom of the C–O bond in the hydroxyl group (C–OH) at 287.0 eV [31]. The deconvoluted O 1 s spectrum may be fitted into three main peaks shown in Fig. 12(b). The first peak at 531.3 eV binding energy is attributed to O2 −, this could be related to oxygen atom bonded to Fe3+ in the FeO, Fe2O3, and Fe3O4 oxides. The second peak observed at

Fig. 13. Pictorial representation of adsorption of one of quinoline derivatives on MS surface in 1 M HCl.


532.7 eV around is attributed to OH−, which can associate with the occurrence of hydrous iron oxides, such as FeOOH. The third peak at 533.7 eV may be assigned to the oxygen of a hydroxyl group (C–OH) in Q-4 [32]. The deconvoluted N 1 s spectrum of adsorbed thin film on MS in the presence of Q-4 may be fitted into three peaks as shown in Fig. 12(c). The first peak at 396.8 eV corresponds to C–N bonds and unprotonated N atoms in quinoline ring. The second peak at 399.4 eV is attributed to the co-ordinate nitrogen in the quinoline ring with iron (N– Fe). The last peak at 401.3 eV is attributed to positively charged nitrogen related to protonated nitrogen atom in the quinoline ring [33]. The deconvolution of the high peak resolution of Fe 2p3/2 spectrum consists of three peaks are shown in Fig. 12(d). The peak at 711.6 eV assigned to Fe3+ is attributed to ferric compounds such as Fe2O3 and/ or FeOOH. The second peak at 712.7 eV is attributed to the presence of a small concentration of FeCl3 on the MS surface. The peak at a 714.2 eV is ascribed to the satellites of Fe (III) [34]. The results obtained by XPS analysis supporting the adsorption of inhibitors on MS surface. 5. Mechanism of adsorption and inhibition To elucidate the mechanism of inhibition, a complete knowledge about the interaction between inhibitor molecules and MS surface is required. In acid solution, MS is positively charged with respect to the potential of zero charges (PZC). The inhibitor molecules exist either as neutral molecule or in the protonated form in acid solution as given below, ½Q uino: þ xH þ ↔½Q uino: xxþ :

ð11Þ

It is difficult to explain the adsorption of inhibitor molecules by single mode of adsorption, they may adsorb on the MS surface by following ways: (i) electro-static interaction of protonated inhibitor molecules with pre-adsorbed Cl− ions (physisorption) (ii) interaction between unshared electron pairs of hetero-atoms (N, O) and vacant d-orbital of iron atoms (chemisorption) and (iii) donor–acceptor interactions between the π-electrons of aromatic ring and vacant d-orbital of iron atoms. The protonated sites of inhibitor molecules may adsorb the on the metal surface through a synergistic effect with pre-adsorbed Cl−. The positively charged inhibitor molecules start competing with H+ for electrons on MS surface and after releasing H2, inhibitor molecules return to the neutral stage having free lone pairs of electron available for empty d orbitals of iron atoms. The accumulation of extra negative charges on the MS surface can be transferred from the d orbital of iron to vacant π* (antibonding) of inhibitor molecules. The combination of the all ways of adsorption strengthens adsorption of inhibitor molecules on MS surface [35]. The inhibiting performance of the studied quinolines follows the order Q-4 N Q-3 N Q-2 N Q-1. The difference in inhibition efficiency is attributed to the difference in the molecular structure of quinolone derivatives. Q-4 shows the highest inhibition (98.09%) due to the presence of strong donating –N (CH3)2 group. Q-3 N Q-2 possesses electron donor groups CH3O and CH3 in the benzene ring, hence they exhibit more inhibition efficiency (96.66, 96.19%) than Q-1 (93.33%) which is devoid of any substituent. The adsorption model is shown in Fig 13. 6. Conclusions 1. The studied quinoline derivatives act as excellent corrosion inhibitors for MS in 1 M HCl. All the experimental methods used to investigate

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corrosion inhibition performance demonstrate the same order of inhibition Q-4 N Q-3 N Q-2 N Q-1. 2. The EIS study reveals that the inhibitor functioned via adsorption on metal/solution interface. 3. The potentiodynamic polarization measurements show that Q-1, Q2, Q-3 act as mixed-type inhibitor while Q-4 acts as a cathodic inhibitor. 4. The adsorptions of inhibitors were found to follow the Langmuir adsorption isotherm. 5. The results obtained by SEM/AFM micrographs. Further, corroborate the formation of protective film on the metal surface 6. The XPS analysis results confirm the adsorption of the inhibitor on MS surface. Acknowledgments Priyanka Singh is thankful to Ministry of Human Resource Development (MHRD), New Delhi, India for the financial assistance and facilitation of this study. References [1] F. Zhang, Y. Tang, Z. Cao, W. Jing, Z. Wu, Y. Chen, Corros. Sci. 61 (2012) 1–9. [2] M.J. Bahrami, S.M.A. Hosseini, P. Pilvar, Corros. Sci. 52 (2010) 2793–2803. [3] A. Manivel, S. Ramkumar, J.J. Wu, A.M. Asiri, S. Anandan, J. Environ. Chem. Eng. 2 (2014) 463–470. [4] K. Ramya, R. Mohan, K.K. Anupama, A. Joseph, Mater. Chem. Phys. 149-150 (2015) 632–647. [5] C.B. Pradeep Kumar, K.N. Mohana, J. Taiwan. Inst. Chem. E. 45 (2014) 1031–1042. [6] S.A. Umoren, I.B. Obot, A.U. Israel, P.O. Asuquo, M.M. Solomon, U.M. Eduok, A.P. Udoh, J. Ind. Eng. Chem. 20 (2014) 3612–3622. [7] R.S. Keri, S.A. Patil, Biomed. Pharmacol. 68 (2014) 1161–1175. [8] O. Afzal, S. Kumar, M.R. Haider, M.R. Ali, R. Kumar,M. Jaggi, S. Bawa, Eur. J. Med. Chem., 2016. http://dx.doi.org/10.1016/j.ejmech.2014.07.044. [9] S.V. Ramesh, A.V. Adhikari, Mater. Chem. Phys. 115 (2009) 618–627. [10] V.R. Saliyan, A.V. Adhikari, Corros. Sci. 50 (2008) 55–61. [11] S. Javanshir, M. Safari, Proceedings of the 14th Int. Electron. Conf. Synth. Org. Chem. 14, Sciforum Electronic Conference Series c0122010. [12] P. Singh, M.A. Quraishi, E.E. Ebenso, Int. J. Electrochem. Sci. 8 (2013) 10890–10902. [13] P. Singh, M.A. Quraishi, E.E. Ebenso, Int. J. Electrochem. Sci. 9 (2014) 4900–4912. [14] M.A. Sudheer, Quraishi, Ind. Eng. Chem. Res. 53 (2014) 2851–2859. [15] K.R. Ansari, M.A. Quraishi, A. Singh, Corros. Sci. 79 (2014) 5–15. [16] I.B. Obot, N.O. Obi-Egbedi, Curr. Appl. Phys. 11 (2011) 382–392. [17] P. Mourya, P. Singh, A.K. Tewari, R.B. Rastogi, M.M. Singh, Corros. Sci. 95 (2015) 71–87. [18] K. Ramya, R. Mohan, A. Joseph, J. Taiwan. Inst. Chem. E. 45 (2014) 3021–3032. [19] S.M. Shaban, I. Aiad, M.M. El-Sukkary, E.A. Soliman, M.Y. El-Awady, J. Mol. Liq. 203 (2015) 20–28. [20] K.R. Ansari, M.A. Quraishi, Phys. E. 69 (2015) 322–331. [21] P.M. Dasami, K. Parameswari, S. Chitra, Measurement 69 (2015) 195–201. [22] Y. Sangeetha, S. Meenakshi, C.S. Sundaram, Int. J. Biol. Macromol. 72 (2015) 1244–1249. [23] D.K. Yadav, D.S. Chauhan, I. Ahamad, M.A. Quraishi, RSC Adv. 3 (2013) 632–646. [24] R. Solmaz, Corros. Sci. 79 (2014) 169–176. [25] D.K. Yadav, M.A. Quraishi, Ind. Eng. Chem. Res. 51 (2012) 14966–14979. [26] C.B. Verma, M.A. Quraishi, A. Singh, J. Taiwan. Inst. Chem. E. 000 (2014) 1–11. [27] A.K. Singh, Ind. Eng. Chem. Res. 51 (2012) 3215–3223. [28] C.B. Verma, A. Singh, G. Pallikonda, M. Chakravarty, M.A. Quraishi, I. Bahadur, E.E. Ebenso, J. Mol. Liq. 209 (2015) 306–319. [29] S.K. Saha, A. Dutta, P. Ghosh, D. Sukul, P. Banerjee, Phys. Chem. Chem. Phys. 17 (2015) 5679–5690. [30] S. John, A. Joseph, Mater. Chem. Phys. 133 (2012) 1083–1091. [31] M. Chevalier, F. Robert, N. Amusant, M. Traisnel, C. Roos, M. Lebrini, Electrochim. Acta 131 (2014) 96–105. [32] K. Boumhara, M. Tabyaoui, C. Jama, F. Bentiss, J. Ind. Eng. Chem. 29 (2015) 146–155. [33] Y. Tang, F. Zhang, S. Hu, Z. Cao, Z. Wu, W. Jing, Corros. Sci. 74 (2013) 271–282. [34] M. Tourabi, K. Nohair, M. Traisnel, C. Jama, F. Bentiss, Corros. Sci. 75 (2013) 123–133. [35] A. Dandia, S.L. Gupta, P. Singh, M.A. Quraishi, ACS Sustainable Chem. Eng. 1 (2013) 1303–1310.