Nicotinonitriles as green corrosion inhibitors for mild steel in hydrochloric acid: Electrochemical, computational and surface morphological studies

Journal of Molecular Liquids 220 (2016) 71–81

Contents lists available at ScienceDirect

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

Nicotinonitriles as green corrosion inhibitors for mild steel in hydrochloric acid: Electrochemical, computational and surface morphological studies Priyanka Singh a, M. Makowska-Janusik b, P. Slovensky b, M.A. Quraishi a,⁎ a b

Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India Institute of Physics, Faculty of Mathematics and Natural Science, Jan Dlugosz University in Czestochowa, Al. Armii Krajowej 13/15, 42-200 Czestochowa, Poland

a r t i c l e

i n f o

Article history: Received 11 March 2016 Received in revised form 1 April 2016 Accepted 13 April 2016 Available online xxxx Keywords: Mild steel (MS) Weight loss Electrochemical measurements Scanning electron microscopy (SEM) Atomic force microscopy (AFM)

a b s t r a c t The corrosion inhibition effect of two nicotinonitriles namely, 2-amino-6-phenyl-4-(p-tolyl)nicotinonitriles (ATN) and 2-amino-4-(4-methoxyphenyl)-6-phenylnicotinonitrile (AMN), has been investigated for mild steel in 1 M HCl solution by using weight loss, electrochemical impedance spectroscopy and potentiodynamic polarization methods. The experimental results show that the inhibition efficiency increases (80–97%), with increase in the ATN and AMN concentration from 0.08–0.33 mM. The maximum inhibition efficiency 97.14% and 95.23% was obtained for AMN and ATN respectively at 0.33 mM. Both the inhibitors efficiently inhibit corrosion via adsorption on the metal surface and found to obey the Langmuir adsorption isotherm. Electrochemical impedance spectroscopy analysis reveals an increase in the charge transfer resistance due to the adsorption of inhibitors molecules on metal surface. Potentiodynamic polarization data reveals that, both ATN and AMN predominantly act as cathodic inhibitors. The SEM and AFM, study confirms that the surface of inhibited metal surface is better than without inhibitor. The density function theory and Monte Carlo simulation have also been used to determine the relationship between molecular configuration and inhibition efficiencies. The experimental and theoretical results are in good agreement with each other. © 2016 Published by Elsevier B.V.

1. Introduction Organic compounds are widely used as acid corrosion inhibitors for various industrial processes like acid cleaning, pickling, descaling of boilers and oil well acidification [1–4]. These compounds are readily adsorbed on the metal surface by replacing water molecules and inhibit metallic corrosion [5–8]. Most of the commercially used inhibitors are costly and hazardous to the environment and human being. Therefore, in the recent year the researches are being focused on the development of cost effective and green corrosion inhibitors. In view of these facts, we have synthesized two new inhibitors namely 2-amino-6-phenyl-4-(ptolyl) nicotinonitriles (ATN) and 2-amino-4-(4-methoxyphenyl)-6phenylnicotinonitrile (AMN) to investigate their corrosion inhibition performance in 1 M HCl. The selection of these compounds is based on following considerations: (i) These compounds contain electron rich hetero atoms (S, N and O) as well as possessing, π electrons inside their skeleton, through which they are likely to adsorb easily on the metal surface (ii)\\CH3,\\OCH3 electron donating groups facilitates absorption on metal surface. The literature survey reveals that some pyridine derivatives have been reported as corrosion inhibitor in acid medium ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M.A. Quraishi).

http://dx.doi.org/10.1016/j.molliq.2016.04.042 0167-7322/© 2016 Published by Elsevier B.V.

showing inhibition 93–97% in the concentration range 0.90–50.0 mM [9–12]. The present study is designed towards those compounds that are capable of providing higher inhibition efficiencies at relatively lower concentrations (97.14%, 95.23% for AMN, ATN at 0.33 mM). So, the present investigation deals to study the corrosion inhibition performance of the two nicotinonitriles having different substituent, by using weight loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. Surface analyses were carried out by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). To explain the adsorption mechanism and the interaction between selected nicotinonitriles and iron surface, the presented study were completed by computer simulations. The computational studies were carried out to establish the correlation between theoretical and experimental results. 2. Experimental 2.1. Materials and solutions The MS coupons having the 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, were used for the experimental work. The MS coupons having dimension 2.5 cm × 2 cm × 0.025 cm were used for weight loss studies and 8 cm × 1 cm × 0.025 cm for electrochemical

72

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

studies with an exposed surface area of 1 cm2. The surface of MS coupons was abraded with emery papers (grades ranges from 600 to 1200) and washed. The test solution of 1 M HCl was prepared by diluting analytical grade 37% HCl with double distilled water.

Table 1 Molecular structures of nicotinonitriles. Inhibitors

Molecular structures

2-Amino-6-phenyl-4-(p-tolyl)nicotinonitrile (ATN)

2.2. Inhibitors The ATN and AMN were synthesized according to previously reported literature [13]. The synthetic route is shown in Scheme 1.The compounds were recrystallized from ethanol. The molecular structures of the studied nicotinonitriles are given in Table 1. 2-Amino-4-(4-methoxyphenyl)-6-phenylnicotinonitrile (AMN)

2.3. Weight loss measurements The weight loss experiments were performed to optimise the inhibitor concentration. The reliability of weight loss method gives a baseline method to perform other corrosion techniques. The weight loss experiments were performed by immersing abraded MS coupons in 1 M HCl in the absence and presence of ATN and AMN at different concentrations (0.08–0.33 mM) at 308 K. The MS coupons were taken out after 3 h of immersion, and then they were washed, dried and weighted accurately. The corrosion rate (CR), was determined by using the equation,

CRðmm=yÞ ¼

87:6W atD

ð1Þ

where W is the weight loss of MS specimens, a is total surface area of MS specimen, t is the immersion time (3 h) and D is the density of MS in (g cm−3). 2.4. Electrochemical measurements The electrochemical experiments were performed by using a threeelectrode cell system connected to the Potentiostat/Galvanostat G30045050 (Gamry Instruments Inc., USA). Gamry Echem Analyst 5.50 software package was used for data analyses. The cell assembly contains MS, platinum and saturated calomel electrodes (SCE) as working, auxiliary, and reference electrode, respectively. All experiments were carried out at 308 K. The EIS measurements were conducted in a frequency range from 100 kHz to 0.01 Hz, with amplitude of 10 mV AC signal. Tafel curves were obtained by sweeping the electrode potential from − 0.25 V to + 0.25 V versus open circuit potential at 1.0 mVs−1 scan rate. The experiments were performed after 30 min of MS immersion in the respective aggressive solutions. 2.5. Surface characterization 2.5.1. Scanning electron microscopy Surface morphology studies were carried out on MS coupons after immersion in 1 M HCl solution without and with optimum concentration of ATN and AMN molecules. The scanning electron microscope

Supra 40, Carl Zeiss, Germany was used for the surface analyses and the images were taken at 500× magnification. 2.5.2. Atomic force microscopy AFM images of MS surface immersed in 1 M HCl solution without and with optimum concentration of ATN and AMN were taken using Bruker Dimension Icon SPM with tapping mode in Air, RTESPA probe; k = 40 N/m and fo = 302 kHz, at 10um. 2.6. Quantum chemical calculations Starting an analysis of the electronic properties of the ATN and AMN molecules in neutral and protonated state, first of all their geometry was optimised applying the ab initio formalism implemented in GAMESS program package [14,15]. The procedure was performed for isolated molecule in vacuum. The initial geometry was built up using the ACD/ ChemSketch, an integrated software package from Advanced Chemistry Development, Inc. The minimum of the potential energy surface was calculated at restricted Hartree-Fock (RHF) level [16] for neural molecules and at unrestricted Hartree-Fock (UHF) level for protonated ones with the 6-311G basis set in C1 symmetry. The quadratic approximation (QA) optimisation algorithm [17] based on augmented Hessian techniques was used to reach the geometry of the investigated molecules possessing the minimum of the total energy. The gradient convergence tolerance was equal to 10−4 Hartree/Bohr. At the end of geometry search the Hessian evaluation was performed to exclude the structures giving negative modes and ensure the thermodynamic equilibrium of the molecule. To predict electronic properties of the investigated molecules the single point quantum chemical calculations were performed using the structures with optimised geometry. The electronic properties were computed for the isolated molecules in neutral and protonated state. The calculations were also carried out applying density functional theory (DFT) with B3LYP [18–20] potential using GAMESS program package. The single point calculations were performed with 6-311G basis set augmented by polarization and diffusion functions (6-311++G**) [21–23] The RHF and UHF SCF energy convergence criterion was chosen to be 10−12 Hartree. 2.7. Monte Carlo simulation

Scheme 1. Synthetic route for the preparation of nicotinonitriles.

Adsorption of the ATN and AMN molecules at the (110) surface of Fe single crystal was also investigated using Adsorption Locator module developed in BIOVIA Materials Studio program package. The simulations were performed to find the preferential adsorption sites and their adsorption energy modelling selected nicotinonitriles situated at

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

the surface of the Fe crystal. First of all the crystal structure of the Fe was build using builder module of Materials Studio. The Fe unit cell was formed in Im-3m space group with parameters a = b = c = 286.65 pm and α = β = γ = 90.00° [24]. Then from the bulk material the (110) surface of Fe was specified. As the next step the adsorption of the ATN and AMN molecules at the Fe (110) surface was simulated in periodic boundary condition. To find the most stable adsorption sites a series of total energy calculations using Adsorption Locator module were performed applying parameters as follow: the energy convergence criterion 10−4 kcal/mol, max force 0.005 kcal/mol/Å, max displacement = 0.005 Å utilizing Monte Carlo procedure with Dreiding force field [25]. The electrostatic interactions were calculated with Ewald method applying accuracy equal to 10− 4 kcal/mol, charge group cut-off 15.5 Å and buffer width 0.5 Å. The van der Waals interactions were calculated using atom based summation method. The truncation was performed by cubic spline method with spline width equal to 1 Å and cut-off distance 15.5 Å.

73

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

3. Results and discussion of more electron donation from \OCH3 as compared to \CH3, in phenyl ring [28].

3.1. Weight loss measurements 3.1.1. Effect of inhibitor concentration The weight loss measurement is an important technique to study the effect of inhibitors concentration on corrosion inhibition efficiency for MS in acid solution. The obtained parameters are listed in Table 2. From Fig. 1, it can be seen that the corrosion inhibition efficiency increases (80–97%) with increasing the ATN and AMN concentrations (0.08 to 0.33 mM), which indicates that the corrosion inhibition efficiency depends upon the inhibitor concentration. But, after the optimum concentration (0.33 mM) no significant changes were observed in the corrosion inhibition efficiency. The maximum inhibition efficiency observed, in our case for ATN and AMN is 95.23% and 97.14% at 0.33 mM respectively. The increase in the inhibitor concentration causes increase in extent of adsorption and surface coverage due to availability of larger number of inhibitor molecules. Accordingly, this increases the inhibition efficiency [26,27]. The inhibition efficiency (η%) and surface coverage (θ) were calculated by following equations,

η% ¼

C R −C RðiÞ  100 CR

ð2Þ

θ ¼ ηð%Þ=100

ð3Þ

where CR and CR(i) are the values of the corrosion rates (mg cm−2 h−1) in absence and presence of ATN and AMN, respectively. The difference in corrosion inhibition efficiency ATN and AMN is attributed to substituent group\\CH3,\\OCH3 attached to the para position of the phenyl ring. The inhibition order AMN N ATN is due to the presence

3.1.2. Effect of temperature The temperature effect (308–338 K) has been studied on MS in the absence and presence of ATN and AMN at optimum concentration, shown in Fig. 2. The inhibition efficiency decreases and corrosion rate increases with increasing the temperature in both the condition i.e. inhibitor free and inhibited solution. This can be attributed to desorption of adsorbed inhibitor molecules from the MS surface, which causes a greater surface area of steel in contact with the corrosive media, resulting increase in corrosion rate [29,30].

3.1.3. Adsorption isotherm The corrosion inhibitors protect metal surface through adsorption process. The mode of adsorption may be either chemisorptions, physisorption or of mixed type. To understand the complete adsorption mechanism, it is important to study the interaction between the inhibitor molecules and metal surface. The adsorption of inhibitor molecules on corroding MS surface never attains the complete equilibrium, which gives an adsorption steady state situation. When the corrosion rate decreases in the presence of inhibitor, the adsorption process has a tendency of attaining a state of quasi equilibrium [31,32]. The nature of quasi equilibrium adsorption of inhibitors can be investigated using appropriate adsorption isotherm. Different adsorption isotherm including Langmuir, Temkin and Freundlich isotherm given below were used to fit

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

Concentrations (mM)

Corrosion rate (mm/y)

Surface coverage (θ)

η%

Blank ATN

0.0 0.08 0.16 0.24 0.33 0.08 0.16 0.24 0.33

77.9 14.8 7.4 5.1 3.7 11.8 5.5 2.9 2.2

– 0.80 0.90 0.93 0.95 0.84 0.92 0.96 0.97

– 80.95 90.47 93.33 95.23 84.76 92.85 96.19 97.14

AMN

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

74

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

to the observed experimental results, Temkinisotherm expðƒ:θÞ ¼ K ads  C

ð4Þ

Freundluichisotherm θ ¼ K ads  C

ð5Þ

Langmuirisothermðθ=1−θÞ ¼ K ads  C

ð6Þ

where Kads designates the equilibrium constant for adsorption process, C is the concentration of inhibitor and ƒ is energetic inhomogeneities. The regression coefficient (R2) values near to one, obtained from the linear relationship between log (θ / 1 − θ) vs. log C (M) shows that Langmuir adsorption isotherm was the best fitted isotherm shown in Fig. 3. The correlation between Kads and ΔGads (standard free energy of adsorption) is given by the equation, ΔGads ¼ −RT ln ð55:5K ads Þ

ð7Þ

where R is 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 calculated values of Kads and ΔGads are listed in Table 3. It is reported that, the values of ΔGads varying between −40 kJ mol−1 or more negative suggests that the adsorption is chemisorption, while value of ΔGads around −20 kJ mol−1 or less negative implies that the adsorption is due to electrostatic interaction i.e. physisorption. The values reported in Table 3 are ranges between − 38.45 to − 39.54 kJ mol−1 which is near to − 40 kJ mol−1 (chemisorption). But the adsorption process can not only classified as chemisorption because chemical interactions between inhibitor molecules and metal surface are usually preceded with some form of electrostatic interactions (physisorption).Thus there is a complex mode of adsorption of both physisorption as well as chemisorption but predominant mode is chemical adsorption [33,34].

Table 3 ΔGads, Kads at optimum concentration (0.33 mM) of Nicotinonitriles for MS in 1 M HCl at 308–338 K. Inhibitors

Temperature

Kads (104 M−1)

ΔGads (kJ mol−1)

ATN

308 318 328 338 308 318 328 338

59.6 36.5 22.7 14.1 100.7 58.4 36.5 25.1

−38.45 −38.41 −38.31 −38.16 −39.79 −39.64 −39.61 −39.54

AMN

imperfect semicircle, depressed with a centre under real axis. This kind of imperfection is referred due to the rough surface, frequency dispersion, relaxation and porosity in mass transport effects [36]. For this reason, a constant phase element (CPE) must be introduced to the equivalent circuit model for accurate fit, which is used to analyze the Nyquist loops [37,38]. The admittance (YCPE) can be expressed as: Y CPE ¼ Y 0 ðjωÞn

where Y0 is the amplitude comparable to a capacitance, j is the square root of −1, ω is angular frequency and n is the phase shift. The diameter of the semicircle loop increases with increasing the concentration of ATN and AMN. The diameter of the Nyquist loops is

3.2. Electrochemical measurements 3.2.1. Electrochemical impedance spectroscopy The EIS measurement was carried out to understand the kinetics of the electrochemical process at MS/solution interface and how it is modified in the presence of inhibitor. The Nyquist plots, Bode-phase angel plots and equivalent circuit model are presented in Figs. 4, 5, and 6 respectively, and corresponding parameters are listed in Table 4. The obtained Nyquist plots for MS in blank as well as in inhibited solution exhibited one semicircle capacitive loop in high frequency range, corresponding to one time constant in bode plot [35]. This behaviour suggests that corrosion process of MS is controlled by charge transfer process. The presence of the Low frequency inductive loop may be attributed to the relaxation process by adsorption of species like Cl−ads and H+ads on working electrode surface. The obtained loops shows

Fig. 3. An adsorption Langmuir isotherm plots for MS in presence of nicotinonitriles.

ð8Þ

Fig. 4. Nyquist plots for MS in 1 M HCl without and with nicotinonitriles.

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

75

attributed to charge transfer resistance (Rct), which can be determined from the difference in the real impedance at lower and higher frequencies. The observation shows that the Rct values increases with increasing concentration of nicotinonitriles and the highest Rct values obtained for AMN is 374.81 Ω cm2 at 0.33 mM. The inhibition efficiency (η%) was calculated as, η% ¼

Rct 1− RctðiÞ

!  100

ð9Þ

where Rct(i) and Rct are the values of charge transfer resistance in absence and presence of ATN and AMN, respectively. The maximum inhibition efficiency obtained for ATN and AMN are 96.67 and 97.59 at 0.33 mM, respectively. It is clearly seen from the results, that with increase of the Rct values, decrease in double layer capacitance (Cdl) is observed [39]. This is attributed due to the decrease in the dielectric constant, or increase in the double-layer thickness because of adsorption of inhibitor molecules on metal surface, to protect metal from acid attack. The double layer capacitance (Cdl) was calculated by using the expression: C dl ¼

Fig. 5. Equivalent circuit model used to analyze the EIS data.

Y ο ωn−1 sinðnðπ=2ÞÞ

ð10Þ

where, ω is angular frequency and n is the phase shift, which can be used as a gauge of the heterogeneity or roughness of the MS surface. The relationship between Cdl and thickness of the protective layer (d) is: Cdl ¼

εεoA d

ð11Þ

where ε is the dielectric constant, ε0 is the permittivity of free space and A is surface area of the electrode. The ideal capacitive behaviour for Bode and phase angle would result, if the slope value attains −1 and phase angle value attains −90° at the intermediate frequencies. In the presence of ATN and AMN as shown in Fig. 6, the respective values of the slope and phase angle range from 0.71 to 0.78, and 60° to 73° as compared to those of the blank solution i.e. 0.53 and 40° respectively, suggesting the formation of pseudo-capacitive film on the MS surface and an indication of the inhibition of MS corrosion in 1 M HCl [40]. 3.2.2. Potentiodynamic polarization measurements The Tafel curves obtained for MS in the absence and presence of ATN and AMN are shown in Fig. 7 and related parameters such as corrosion potential (Ecorr), corrosion current density (icorr) are listed in Table 5. From Fig. 7, it can be seen that the addition of nicotinonitriles causes shift in corrosion potential towards more negative direction. It is reported in the literature that, if the displacement in corrosion potential is more than 85 mV with respect to corrosion potential of the blank, the inhibitor can be classified as cathodic or anodic type otherwise it is of mixed type [41]. In our case, both ATN and AMN represent cathodic type behaviour. The icorr value for MS in 1 M HCl is 1390 μA cm−2 and for the ATN and AMN are 37.8 μA cm−2 and 28.7 μA cm−2 at 0.33 mM respectively. The decrease in the icorr values suggests that the presence of nicotinonitriles retarded the electrochemical reaction on MS surface. This is due to the formation of protective layer on MS surface that creates a barrier between metal and corrosive solution, resulting increase in corrosion inhibition efficiency [42,43]. The inhibition efficiency was calculated from the polarization measurements by using the equation: 

η% ¼ Fig. 6. Bode (log f vs log |Z|) and phase angle (log f vs α) plots for MS in 1 M HCl without and with Nicotinonitriles.

icorr −i corr  100 icorr

ð12Þ

where icorr and i°corr are the corrosion current density in the absence and presence of ATN and AMN, respectively.

76

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

Table 4 Electrochemical Impedance Parameters for MS in absence and presence of Nicotinonitriles in 1 M HCl. Inhibitors

Concentrations (mM)

Rct (Ω cm2)

n

Y° (μF cm−2)

Cdl (μF cm−2)

RL (Ω cm2)

L (H cm2)

η%

−S

−α°

Blank ATN

0.0 0.08 0.16 0.24 0.33 0.08 0.16 0.24 0.33

9.0 110.82 144.58 188.37 273.34 142.63 198.91 374.06 374.81

0.82 0.81 0.80 0.80 0.83 0.80 0.80 0.81 0.80

250 185 178 164 98 184 156 92 70

106 80 78 57 50 74 72 42 30

– – – 1.20 4.88 – – – –

– – – 0.17 0.07 – – – –

– 91.87 93.77 95.22 96.70 93.69 95.47 97.59 97.59

0.53 0.74 0.71 0.78 0.78 0.77 0.74 0.77 0.76

40.40 63.22 60.62 68.01 68.97 63.00 66.79 69.39 73.90

AMN

4. Surface characterization The surface morphology of MS in absence and presence of ATN and AMN are shown in Fig. 8(a–c). Fig. 8(a) shows the MS sample immersed in acid solution without inhibitor. In that micrograph, the MS surface is seems to be rigorously corroded due to acid attack. The MS immersed in acid solution containing inhibitor are shown in Fig. 8(b, c). The inhibitor treated MS surfaces are smooth and protected from acid attack. In addition to the surface study, the AFM analysis plays an important role to calculate average surface roughness of MS in the absence and presence of ATN and AMN in 1 M HCl. In the absence of nicotinonitriles the surface displayed an extremely rough topography due to unhindered

corrosion attack (Fig. 9(a)) and the average surface roughness is 210 nm. In the presence of nicotinonitriles, the MS shows smoother surface (Fig. 9(b, d)) and the average surface roughness value obtained for ATN and AMN are 42, 30 nm, respectively. Both the surface analysis techniques suggest that the addition of the ATN and AMN inhibited the corrosion of MS, as depicted by the micrograph in which the roughness of the surface reduces comparing to the acid treated metal surface [44]. 5. Quantum chemical calculations The quantum chemical (QC) studies have been used to understand the effect of nicotinonitriles molecular structures (ATN and AMN) on inhibition efficiency. First of all the structures of the ATN and AMN molecules were optimised according to the total energy minimization. The obtained geometries of both molecules in gas phase are presented in Fig. 10. The skeletons of both molecules are planar except substituted phenyl (III) ring which is slightly twisted comparing to the remaining part of the molecules. The near planar geometries of these compounds might contribute to their high inhibition efficiencies because; high degree of planarity had been reported to favour optimum adsorption of inhibitor molecules on metal surface leading to enhanced inhibition efficiency. The angle of rotation in the molecule AMN is smaller than the twist angle of the molecules ATN. It explains the experimentally proved higher inhibition efficiency of the AMN molecule than the ATN one. The parameters obtained from QC, based on the difference in their molecular electronic structure, both in- the neutral and protonated in gas phase listed in Table 6. The HOMO electron density surface provides information about the sites of the molecule that are most likely to donate electrons to the appropriate orbital of an acceptor specie, while the LUMO electron density surface suggests the sites of the molecule that possess higher chances of accepting electrons from a donor specie [45]. The orbital presentation of the studied compounds exhibited in Fig. 10 shows similar features of the LUMO for both nicotinonitriles but clearly different HOMOs are observed. For this reason, the variation in the inhibition efficiencies of the studied molecules might not be describable based on the LUMOs. Both the HOMOs and the LUMOs of the studied molecules comprise the π- and σ-type orbitals, which suggest that the molecules can favourably interact with the

Table 5 Polarization parameters for MS in absence and presence of nicotinonitriles in 1 M HCl. Inhibitors

Concentrations (mg L−1)

Icorr (μA cm−2)

Ecorr (mV/SCE)

βa (mV/dec)

βc (mV/dec)

η%

Blank ATN

0.0 0.08 0.16 0.24 0.33 0.08 0.16

1390 215 174 141 37.8 106 102

−445 −564 −548 −589 −568 −526 −511

82 97 91 78 84 73 68

118 186 164 112 119 131 159

– 84.53 87.48 89.85 97.28 92.37 92.66

AMN Fig. 7. Polarization curves for MS in 1 M HCl without and with nicotinonitriles.

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

77

Fig. 9. (a–c) AFM micrographs of MS surface (a) in absence and (b, c) in presence of ATN and AMN at 0.33 mM.

Fig. 8. (a–c) showed the SEM micrographs (a) in absence and (b, c) in presence of ATN and AMN at 0.33 mM.

vacant/filled d- or p-orbitals of the metal atom. The HOMO of ATN is widely distributed over all the rings in the molecule except the phenyl (III) ring, which does not contribute to the HOMO. On other hand the HOMO of AMN is well delocalized over whole system including phenyl (III). The effects of the methyl and methoxy substituent's present in ATN and AMN respectively, are very apparent in the HOMO orbital distributions. Higher value of EHOMO, indicates that the molecule has a higher

ability to donate electrons to appropriate acceptor molecules with low energy empty molecular orbital whereas lower value of ELUMO suggests that the molecule easily accepts electrons from the donor molecule [46]. The EHOMO values obtained of the studied neutral and protonated compounds in the gas and water phases are in the order: AMN N ATN, which agrees with the order of the experimental inhibition efficiencies. The energy gap, ΔE is another index of the molecular reactivity. Molecules with lower ΔE are usually more reactive and possess higher inhibition efficiency [46]. The changes of the Δ E values obtained for the studied compounds in neutral and protonated state show different trend. The trend of ΔE obtained for ATN and AMN molecules in neutral medium correlates with experimental results but in the case of protonated molecules the obtained results are not in agreement with the experimental inhibition efficiencies. In protonated phase the ELUMO values of ATN and

78

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

Fig. 10. The optimised molecular structures, HOMO and LUMO orbitals distribution of ATN and AMN in gas phase.

AMN shows slight different trend. The global electronegativity χ is another reactivity index that predicts the extent to which a molecule retains its electrons. It is calculated as, χ≅−1=2ðEHOMO þ ELUMO Þ:

ð13Þ

Increase of the χ, means the fall of the chance of electron donation by the molecule and vice versa [47]. The trends of the χ values obtained for the studied compounds shown in Table 6 are ATN N AMN, which suggest that AMN has higher possibility to donate electrons to an electrophilic centre such as the iron surface populated by positive charges. The electron-donating effects of \OCH3 and \CH3 are well known [48], and the established trend of electron-donating abilities of the groups present in the studied compounds is –OCH3 (in AMN) N \CH3 (in ATN).

Table 6 Quantum chemical parameters derived from the B3LYP/6-311++G** method of the studied nicotinonitriles. Parameters → Compounds↓

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

χ (eV)

So, the inhibition trend obtained by the experimental study AMN N ATN is also supported by theoretical approach.

6. Monte Carlo simulation The Monte Carlo simulation (MC) helps to understand the interaction between the inhibitor molecules and metal surface. It helps to predict the most stable adsorption sites on metal surface. The calculated outputs from MC simulation are listed in Table 7 and the most stable low energy adsorption configurations of ATN and AMN on Fe (110) surface are shown in Fig. 11. The conformational arrangement of the ATN and AMN molecules at the surface of Fe (110) prefer the planar configuration of the inhibitors to the surface of iron. The distance between inhibitors and the surface of Fe is in the range of 0.3 to 0.4 nm for both investigated systems. At the bottom of Fig. 11 the density of the ATN

Table 7 Average adsorption energy of the ATN and AMN molecules at the Fe (110) surface and the average total energy of the investigated systems. Systems

Neutral molecule in vacuo ATN −6.16 AMN −6.12

−1.94 −1.91

4.22 4.21

Protonated molecule in vacuo ATN −9.79 AMN −9.74

−6.10 −5.73

3.69 4.01

Total energy (kcal/mol)

Adsorption energy (kcal/mol)

4.05 4.02

Neutral molecule in vacuo Fe (110) + ATN −25.85 Fe(110) + AMN −36.25

−17.73 −21.31

7.95 7.74

Protonated molecule in vacuo Fe (110) + ATN −31.56 Fe(110) + AMN −43.47

−17.57 −20.31

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

79

Fig. 11. Side and top views of the most stable low energy configuration for the adsorption of ATN and AMN on Fe (110) surface obtained by Monte Carlo Simulations and the density of the inhibitors distribution at the surface of iron.

and AMN adsorption at the surface of Fe (110) is presented. One may see that the inhibitors do not prefer any special places at the iron surface. The presented surface is homogenously covered by the isodensity shapes. This homogeneous distribution is caused by the lack of atomic terraces and dense packing of atoms on the studied surface. The adsorption energy distribution presented in Fig. 12 for neutral and protonated molecules show one pronounced peak in each investigated regimes. It means that the AMN and ATN molecules stick to the surface of iron in one configuration; they are always planar to the Fe (110) surface. The low shoulders from higher energy side are noticed for the

thermodynamic motion of the molecules which causes slight increase of the inhibitors energy according to their vibrational motion. The adsorption energy is attributed to the energy released during the adsorption of the relaxed adsorbate components on the substrate. Higher negative adsorption energy values indicate a more stabilised and stronger interaction between a metal and an inhibitor molecule [49]. It can be seen from the results, that the negative values of adsorption energies of nicotinonitriles on Fe (110) surface shows the following order AMN N ATN. This ordering is in good agreement with the observed experimental inhibition efficiency for ATN and AMN.

80

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81

wcss.wroc.plN (Grant no. 171). The MATERIALS STUDIO package was used under POLAND COUNTRY-WIDE LICENSE.

References

Fig. 12. The adsorption energy distribution of the adsorbate (ATN and AMN molecules) on Fe (110) surface for (a) non-protonated and (b) protonated molecules.

7. Conclusions The inhibiting potential of two nicotinonitriles has been tested on MS in 1 M HCl. The observation reveals that ATN and AMN both inhibit MS corrosion in 1 M HCl and their corrosion inhibition efficiencies increase with increasing concentration. The results also show that the substituent's at para position of the phenyl group namely \OCH3 (in AMN), \CH3 (in ATN) affect the corrosion inhibition effect of the nicotinonitriles. The AMN gives the highest inhibition efficiency of 97.14% at 0.33 mM. The potentiodynamic polarization studies revealed that the studied inhibitor are predominantly cathodic-type. The EIS results showed a decrease in Cdl values due to decrease in local dielectric constant or an increase in thickness of the electrical double layer. SEM and AFM micrograph showed smooth surface in presence of inhibitor as comparing to blank one. Langmuir adsorption isotherm is the best fitted isotherm. The calculated parameters from the quantum chemical method such as EHOMO, ELUMO and χ for the two compounds are in agreement with the order observed of inhibition efficiencies for ATN and AMN. Monte Carlo simulations prediction based on adsorption energies is in agreement with the experimental inhibition efficiencies.

Acknowledgements Priyanka Singh is thankful to Ministry of Human Resource Development (MHRD), New Delhi, India for the financial assistance and facilitation of our study. Quantum chemical calculations have been carried out in Wroclaw Center for Networking and Supercomputing bhttp://www.

[1] A.S. Fouda, A.S. Ellithy, Inhibition effect of 4-phenylthiazole derivatives on corrosion of 304L stainless steel in HCl solution, Corros. Sci. 51 (2009) 868–875. [2] A. Döner, E.A.S. Ahin, G. Kardas, O. Serindag, Investigation of corrosion inhibition effect of 3-[(2-hydroxy-benzylidene)-amino]-2-thioxo-thiazolidin-4-one on corrosion of mild steel in the acidic medium, Corros. Sci. 66 (2013) 278–284. [3] X. Li, S. Deng, H. Fu, Three pyrazine derivatives as corrosion inhibitors for steel in 1.0 M H2SO4 solution, Corros. Sci. 53 (2011) 3241–3247. [4] G. Quartarone, L. Ronchin, A. Vavasori, C. Tortato, L. Bonaldo, Inhibitive action of gramine towards corrosion of mild steel in deaerated 1.0 M hydrochloric acid solutions, Corros. Sci. 64 (2012) 82–89. [5] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, M. Hamadanian, A. Gandomi, Electrochemical and theoretical investigation on the corrosion inhibition of mild steel by thiosalicylaldehyde derivatives in hydrochloric acid solution, Corros. Sci. 50 (2008) 2172–2181. [6] S. Vishwanatham, N. Haldar, Furfuryl alcohol as corrosion inhibitor for N80 steel in hydrochloric acid, Corros. Sci. 50 (2008) 2999–3004. [7] S.A. Umoren, O. Ogbobe, I.O. Igwe, E.E. Ebenso, Inhibition of mild steel corrosion in acidic medium using synthetic and naturally occurring polymers and synergistic halide additives, Corros. Sci. 50 (2008) 1998–2006. [8] A. Ostovari, S.M. Hoseinieh, M. Peikari, S.R. Shadizadeh, S.J. Hashemi, Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: a comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, a-D-glucose and tannic acid), Corros. Sci. 51 (2009) 1935–1949. [9] R. Yıldız, A. Döner, T. Dog˘an, I. Dehri, Experimental studies of 2-pyridinecarbonitrile as corrosion inhibitor for mild steel in hydrochloric acid solution, Corros. Sci. 82 (2014) 125–132. [10] B.D. Mert, A.O. Yüce, G. Kardas, B. Yazıcı, Inhibition effect of 2-amino-4methylpyridine on mild steel corrosion: experimental and theoretical investigation, Corros. Sci. 85 (2014) 287–295. [11] A. Kosari, M.H. Moayed, A. Davoodi, R. Parvizi, M. Momeni, H. Eshghi, H. Moradi, Electrochemical and quantum chemical assessment of two organic compounds from pyridine derivatives as corrosion inhibitors for mild steel in HCl solution under stagnant condition and hydrodynamic flow, Corros. Sci. 78 (2014) 138–150. [12] K.R. Ansari, M.A. Quraishi, A. Singh, Corrosion inhibition of mild steel in hydrochloric acid by some pyridine derivatives: an experimental and quantum chemical study, J. Ind. Eng. Chem. 25 (2015) 89–98. [13] J. Safari, S.H. Banitaba, S.D. Khalili, Ultrasound-promoted an efficient method for one-pot synthesis of 2-amino-4.6-diphenylnicotinonitriles in water: a rapid procedure without catalyst, Ultrason. Sonochem. 19 (2012) 1061–1069. [14] M.W. Schmidt, K.K. Baldrige, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, General atomic and molecular electronic structure system, J. Comput. Chem. 14 (1993) 1347–1363. [15] M.S. Gordon, M.W. Schmidt, Advances in electronic structure theory: GAMESS a decade later, in: C.E. Dykstra, G. Frenking, K.S. Kim, G.E. Scuseria (Eds.), Theory and Applications of Computational Chemistry: The First Forty Years, Elsevier, Amsterdam 2005, pp. 1167–1189. [16] C.C.J. Roothaan, New developments in molecular orbital theory, Rev. Mod. Phys. 23 (1951) 69–89. [17] F.J. Jensen, Locating transition structures by mode following: a comparison of six methods on the Ar8 Lennard-Jones potential, Chem. Phys. 102 (1995) 6706–6718. [18] A.D. Becke, Density-functional exchange-energy approximation with correct asymptotic behaviour, Phys. Rev. A 38 (1988) 3098–3100. [19] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648–5652. [20] C. Lee, W. Yang, R.G. Parr, Development of the Colic-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785–789. [21] P.C. Haharan, J.A. Pople, The influence of polarization functions on molecular orbital hydrogenation energies, Theoret. Chim. Acta. 28 (1973) 213–222. [22] M.J. Frisch, J.A. Pople, J.S. Binkley, Self-consistent molecular orbital methods 25. Supplementary functions for Gaussian basis sets, J. Chem. Phys. 80 (1984) 3265–3269. [23] T. Clark, J. Chandrasekhar, G.W. Spitznagel, P. Schleyer, Efficient diffuse functionaugmented basis sets for anion calculations. III. The 3-21+G set for first-row elements, Li-F, J. Comput. Chem. 4 (1983) 294–301. [24] R. Kohlhaas, P. Donner, N.Z. Schmitz-Pranghe, Angew. Phys. 23 (1967) 245–249. [25] M.S. Layo, B.D. Olafson, W.A. Goddard III, DREIDING: a generic forcefield, J. Phys. Chem. 94 (1990) 8897–8909. [26] P. Singh, A. Singh, M.A. Quraishi, E.E. Ebenso, Microwave assisted green synthesis of bis-phenol polymer containing piperazine as a corrosion inhibitor for mild steel in 1 M HCl, Int. J. Electrochem. Sci. 8 (2013) 10890–10902. [27] P. Singh, M.A. Quraishi, E.E. Ebenso, Thiourea-formaldehyde polymer a new and effective corrosion inhibitor for mild steel in hydrochloric acid solution, Int. J. Electrochem. Sci. 9 (2014) 4900–4912. [28] C. Verma, M.A. Quraishi, L.O. Olasunkanmi, E.E. Ebenso, L-proline-promoted synthesis of 2-amino-4- arylquinoline-3-carbonitriles as sustainable corrosion inhibitors for mild steel in 1 M HCl: experimental and computational studies, RSC Adv. 5 (2015) 85417–85430. [29] P. Singh, V. Srivastava, M.A. Quraishi, Novel quinoline derivatives as green corrosion inhibitors for mild steel in acidic medium: electrochemical, SEM, AFM, and XPS studies, J. Mol. Liq. 216 (2016) 164–173.

P. Singh et al. / Journal of Molecular Liquids 220 (2016) 71–81 [30] P. Singh, A. Singh, M.A. Quraishi, Thiopyrimidine derivatives as new and effective corrosion inhibitors for mild steel in hydrochloric acid: electrochemical and quantum chemical studies, J. Taiwan. Inst. Chem. Eng. 000 (2015) 1–14. [31] Y. Sasikumar, A.S. Adekunle, L.O. Olasunkanmi, I. Bahadur, R. Baskar, M.M. Kabanda, I.B. Obot, E.E. Ebenso, Experimental, quantum chemical and Monte Carlo simulation studies on the corrosion inhibition of some alkyl imidazolium ionic liquids containing tetrafluoroborate anion on mild steel in acidic medium, J. Mol. Liq. 211 (2015) 105–118. [32] S.A. Umoren, I.B. Obot, A.U. Israel, P.O. Asuquo, M.M. Solomon, U.M. Eduok, A.P. Udoh, Inhibition of mild steel corrosion in acidic medium using coconut coir dust extracted from Water and methanol as solvents, J. Ind. Eng. Chem. 20 (2014) 3612–3622. [33] R. Solmaz, Investigation of adsorption and corrosion inhibition of mild steel in hydrochloric acid solution by 5-(4-dimethylaminobenzylidene) rhodanine, Corros. Sci. 79 (2014) 169–176. [34] R. Solmaz, Investigation of corrosion inhibition mechanism and stability of vitamin B1 on mild steel in 0.5 M HCl solution, Corros. Sci. 81 (2014) 75–84. [35] M.A. Sudheer, Quraishi, 2-amino-3.5-dicarbonitrile-6-thio-pyridines: new and effective corrosion inhibitors for mild steel in 1 M HCl, Ind. Eng. Chem. Res. 53 (2014) 2851–2859. [36] M. Yadav, S. Kumar, R.R. Sinha, I. Bahadur, E.E. Ebenso, New pyrimidine derivatives as efficient organic inhibitors on mild steel corrosion in acidic medium: electrochemical, SEM, EDX, AFM and DFT studies, J. Mol. Liq. 211 (2015) 135–145. [37] M.A. Amin, S.S. Abd El Rehim, H.T.M. Abdel-Fatah, Electrochemical frequency modulation and inductively coupled plasma atomic emission spectroscopy methods for monitoring corrosion rates and inhibition of low alloy steel corrosion in HCl solutions and a test for validity of the Tafel extrapolation method, Corros. Sci. 51 (2009) 882–894. [38] K.F. Khaled, M.A. Amin, Corrosion monitoring of mild steel in sulphuric acid solutions in presence of some thiazole derivatives – molecular dynamics, chemical and electrochemical studies, Corros. Sci. 51 (2009) 1964–1975. [39] A. Dandia, S.L. Gupta, P. Singh, M.A. Quraishi, Ultrasound-assisted synthesis of pyrazolo[3,4-b]pyridines as potential corrosion inhibitors for mild steel in 1.0 M HCl, ACS Sustainable Chem. Eng. 1 (2013) 1303–1310.

81

[40] P. Singh, M.A. Quraishi, S.L. Gupta, A. Dandia, Investigation of the corrosion inhibition effect of 3-methyl-6-oxo-4-(thiophen-2-yl)-4,5,6,7-tetrahydro-2Hpyrazolo[3,4 b]pyridine-5-carbonitrile (TPP) on mild steel in hydrochloric acid, J. Taibah. Uni. Sci. 10 (2015) 139–147. [41] G. Ji, S.K. Shukla, P. Dwivedi, S. Sundaram, R. Prakash, Inhibitive effect of Argemone mexicana plant extract on acid corrosion of mild steel, Ind. Eng. Chem. Res. 50 (2011) 11954–11959. [42] E.E. Ebenso, M.M. Kabanda, L.C. Murulana, A.K. Singh, S.K. Shukla, Electrochemical and quantum chemical investigation of some azine and thiazine dyes as potential corrosion inhibitors for mild steel in hydrochloric acid solution, Ind. Eng. Chem. Res. 51 (2012) 12940–12958. [43] L.C. Murulana, A.K. Singh, S.K. Shukla, M.M. Kabanda, E.E. Ebenso, Experimental and quantum chemical studies of some bis(trifl uoromethyl-sulfonyl) imide imidazolium-based ionic liquids as corrosion inhibitors for mild steel in hydrochloric acid solution, Ind. Eng. Chem. Res. 51 (2012) 13282–13299. [44] I.B. Obot, S.A. Umoren, Z.M. Gasem, R. Suleiman, B. El Ali, Theoretical prediction and electrochemical evaluation of vinylimidazole and allylimidazole as corrosion inhibitors for mild steel in 1 M HCl, J. Ind. Eng. Chem. 21 (2015) 1328–1339. [45] P. Singh, E.E. Ebenso, L.O. Olasunkanmi, I.B. Obot, M.A. Quraishi, Electrochemical, theoretical and surface morphological studies of corrosion inhibition effect of green naphthyridine derivatives on mild steel in hydrochloric acid, J. Phys. Chem. C 120 (2016) 3408–3419. [46] L.O. Olasunkanmi, I.B. Obot, M.M. Kabanda, E.E. Ebenso, Some quinoxalin-6-yl derivatives as corrosion inhibitors for mild steel in hydrochloric acid: experimental and theoretical studies, J. Phys. Chem. C 119 (2015) 16004–16019. [47] A.Y. Musa, R.T.T. Jalgham, A.B. Mohamad, Molecular dynamic and quantum chemical calculations for phthalazine derivatives as corrosion inhibitors of mild steel in 1 M HCl, Corros. Sci. 56 (2012) 176–183. [48] J.L.G. Wade, Organic Chemistry, sixth ed. Pearson Prentice Hall, Upper Saddle River, NJ, 2006. [49] C. Verma, E.E. Ebenso, I. Bahadur, I.B. Obot, M.A. Quraishi, 5-(Phenylthio)-3H-pyrrole-4-carbonitriles as effective corrosion inhibitors for mild steel in 1 M HCl: experimental and theoretical investigation, J. Mol. Liq. 212 (2015) 209–218.