Experimental and quantum chemical studies of synthesized triazine derivatives as an efficient corrosion inhibitor for N80 steel in acidic medium

Journal of Molecular Liquids 212 (2015) 151–167

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Experimental and quantum chemical studies of synthesized triazine derivatives as an efficient corrosion inhibitor for N80 steel in acidic medium M. Yadav a,⁎, S. Kumar a, N. Tiwari a, I. Bahadur b,c,⁎⁎, E.E. Ebenso b,c a

Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India Department of Chemistry, School of Mathematical and Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng 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 (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa b

a r t i c l e

i n f o

Article history: Received 3 July 2015 Received in revised form 4 September 2015 Accepted 9 September 2015 Available online xxxx Keywords: N80 steel Hydrochloric acid Corrosion inhibition EIS SEM DFT

a b s t r a c t Corrosion inhibition of N80 steel in a 15% HCl solution was studied using two synthesized triazine derivatives, namely: N2-(4-(2-amino-6-(4-methoxyphenyl)pyrimidine-4-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine-2,4,6triamine (APTT) and N2-(4-(5-(4-methoxyphenyl)isoxazol-3-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine-2,4,6triamine (MITT) using gravimetric measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The inhibition efficiency was found to increase with increasing inhibitor concentrations and decreases with increasing temperatures. Some thermodynamic and kinetic parameters were calculated and discussed. The adsorption of inhibitor on N80 steel surface, obeyed the Langmuir adsorption isotherm. Polarization studies showed that both inhibitors behave as mixed-type inhibitors. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), FTIR, UV–visible spectroscopy and atomic force microscopy (AFM) were performed for surface analysis of the uninhibited and inhibited N80 steel samples. The density functional theory (DFT) was employed for theoretical calculations and the results obtained were found to be consistent with the experimental findings. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The importance of carbon steel protection against corrosion in acidic solutions has increased due to the fact that iron materials, which are more susceptible to be attacked in aggressive media, are the commonly exposed metals in industrial environments. Acid solutions widely used in various industrial processes such as oil well acidification, petrochemical processes, acid pickling, acid cleaning, and acid de-scaling, generally, leads to serious metallic corrosion [1–3]. Acidification of petroleum oil well for enhancing oil production is commonly carried out by forcing a solution of 15 to 28% hydrochloric acid into the well through N80 steel tubing. During this process N80 tubing get adversely affected by corrosion and therefore, to reduce the aggressive attack of the acid on tubing and casing materials (N80 steel), inhibitors are added to the acid solution during the acidifying process [4–10]. The use of inhibitors is one of the most practical and simple method to protect metals against ⁎ Corresponding author. ⁎⁎ Correspondence to: I. Bahadur, Department of Chemistry, School of Mathematical and Physical Sciences, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa. E-mail addresses: [email protected] (M. Yadav), [email protected] (I. Bahadur).

http://dx.doi.org/10.1016/j.molliq.2015.09.019 0167-7322/© 2015 Elsevier B.V. All rights reserved.

corrosion, especially in acidic medium. Organic compounds containing electronegative groups, triple bonds or conjugated double bonds are the most efficient organic inhibitors because of their nucleophilicity and the capability to bind the metal surface by means of π-interactions [11]. The presence of heteroatoms such as nitrogen, sulfur, phosphorus, and oxygen, together with aromatic rings in the structure enhances the adsorption capability of the molecules on the metal surface and improve the corrosion inhibition efficiency of these compounds [12–16]. Besides the number of hetero-atoms, planarity and the projected surface area of these organic molecules play an important role in their inhibition efficiency [17]. Some derivatives of triazine [18–20], isoxazole [21,22] and pyrimidine [23–27] have been reported as excellent inhibitors for metals and alloys in hydrochloric acid solution, and exhibit different inhibition efficiency as a result difference in substituent groups and substituent positions on the aromatic rings. To the best of our knowledge, screened literature shows that the structural parameters, effect of temperature and corrosion inhibition of the studied compounds in the present work have not been reported. In the present study two triazine derivatives were synthesized and investigated as corrosion inhibitors. The inhibitor [APTT] has triazene and pyrimidine whereas the inhibitor [MITT] has triazene and isoxazole as two anchoring sites for coordination

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with the metal, therefore, the effect of the substituent on the corrosion inhibitor properties of N80 steel in hydrochloric acid solution is further discussed. In continuation of our research into developing corrosion inhibitors [4–7] with high efficiency, the present work focuses on synthesized condensation products of triazine, pyrimidine and isoxazole, namely: N2-(4-(2-amino-6-(4-methoxyphenyl)pyrimidine-4-yl)phenyl)-N4,N6diphenyl-1,3,5-triazine-2,4,6-triamine (APTT) and N 2-(4-(5-(4methoxyphenyl)isoxazol-3-yl)phenyl)-N4,N6-diphenyl-1,3,5-triazine2,4,6-triamine (MITT) as corrosion inhibitors for N80 steel in acidic medium using weight loss measurement, potentiodynamic polarization, AC impedance, FTIR, UV–visible spectroscopy, SEM, EDS, AFM and quantum chemical calculations. 2. Experimental 2.1. Synthesis of inhibitors The compounds APTT and MITT were synthesized as reported in literature [28] and as shown in Scheme 1. 1-(4-(4,6-Bis(phenylamino)1,3,5-triazine-2-ylamino)phenyl)-3-(4-methoxyphenyl)prop-2-en-1one (0.01 mol) was dissolved in alcohol (25 mL) and guanidine nitrate (0.01 mol) was added to it. Then a solution of KOH (5 mL of 40%) was added to the reaction mixture and refluxed for 10 h. The reaction mixture was then cooled, poured into crushed ice and the product separated out was filtered, washed with water, dried and recrystallized from

alcohol to get the APTT compound. 1-(4-(4,6-Bis(phenylamino)1,3,5-triazine-2-ylamino)phenyl)-3-(4-methoxyphenyl)prop-2-en1-one (0.01 mol) was dissolved in alcohol (25 mL) and hydroxylamine hydrochloride (0.01 mol) was added to it. Then a solution of KOH (5 mL of 40%) was added to the reaction mixture and refluxed for 6 h. The reaction mixture was then cooled, poured into crushed ice and product separated out was filtered, washed with water, dried and recrystallized from alcohol to get the MITT compound. The purity of the synthesized compounds was checked using thin layer chromatography (TLC) and structure was confirmed by physico-chemical and spectroscopic data as given below: APTT Yield (72%), m.p. = 152 °C. Analytical data: calculated %: C, 69.44; H, 4.88; N, 22.78; found %: C; 68.82; H, 4.85; N, 22.68. IR (ν/cm−1): 3280 (NH), 1520 (C_N), 790 (C\\N). 1 H NMR (CDCl3): δ/ppm = 5.22 (s, 2H, NH2, pyrimidine), 6.88–8.22 (m, 18 Ar–H and 3 NH), 3.86 (s, 3H, OCH3). MITT Yield (72%), m.p. = 152 °C. Analytical data: calculated %: C, 72.80; H, 4.89; N, 19.18; found %: C, 72.46; H, 4.85; N, 19.12. IR (ν/cm−1): 3360 (NH), 1510 (C_N), 810 (C\\N). 1 H NMR (CDCl3): δ/ppm = 6.92 (s, 1H, isoxazol), 7.12–8.15 (m, 18 Ar–H and 3 NH).

Scheme 1. Synthetic route of inhibitors APTT and MITT.

M. Yadav et al. / Journal of Molecular Liquids 212 (2015) 151–167

2.2. N80 steel sample Corrosion studies were performed on N80 steel samples having composition: C, 0.31; Mn, 0.92; Si, 0.19; S, 0.008; P, 0.010; Cr, 0.20 and Fe balance. N80 steel coupons having dimension of 6.0 cm × 2.5 cm × 0.1 cm were mechanically cut and abraded with different grade emery papers (120, 220, 400, 600, 800, 1500 and 2000) grade for the weight loss experiment. For electrochemical measurements, N80 steel coupons having dimension 1.0 cm × 1.0 cm × 0.1 cm were mechanically cut and abraded with an exposed area of 1 cm2 (rest covered with araldite resin) with a 3 cm long stem. Prior to the experiment, specimens were washed with distilled water, degreased in acetone, dried and stored in a vacuum desiccator. 2.3. Test solution Analytical reagent grade HCl was diluted with double distilled water to obtain the 15% HCl solution. The concentration of inhibitors employed varied from (20 to 150) ppm (mg L−1), and the volume of electrolyte used was 250 mL for weight loss measurements and 150 mL for electrochemical studies. 2.4. Methods 2.4.1. Weight loss method Weight loss measurements were performed at different temperatures (303 to 333) K by immersing accurately weighed N80 steel test coupons in 250 mL of the 15% HCl solution in the absence and presence of (20, 50, 75, 100 and 150) ppm (mg L−1) of the inhibitors. The immersion time was optimized (6 h) and was uniformly used for weight loss measurements. The test coupons were removed from the electrolyte after 6 h immersion time, washed thoroughly with distilled water, dried and weighed. Triplicate experiments were conducted for each concentration of the inhibitors for reproducibility and the average of weight losses was taken to calculate the corrosion rate and inhibition efficiency of the inhibitors. The corrosion rate (CR), inhibition efficiency (η%) and surface coverage (θ) were calculated using Eqs. (1)–(3) [29]:
87:6W CR mmy−1 ¼ Atd

ð1Þ

where, W = weight loss (mg), A = area of specimen (cm2) exposed in acidic solution, t = exposure time (h), and d = density of N80 steel (g cm−3). θ¼

CR0 −CRi CR0

ηð%Þ ¼

CR0 −CRi  100 CR0

ð2Þ ð3Þ

where, CR0 and CRi are corrosion rate in the absence and presence of inhibitors. 2.4.2. Potentiodynamic polarization studies Potentiodynamic polarization measurements were carried out in a conventional three-electrode cell consisting of N80 steel working electrode, a platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode, using CH electrochemical workstation (Model No: CHI 760D, manufactured by CH Instruments, Austin, USA) at 303 K. Before starting the experiments, the working electrodes were immersed in the test solution until a steady potential was reached. After establishment of the open circuit potential, potentiodynamic polarization curves were obtained with a scan rate of 0.1 mV s− 1 in the potential ranging from (− 700 to − 300) mV. Potentiodynamic polarization studies were performed in the absence and presence of various concentrations (20–150) ppm by weight of both inhibitors in

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the 15% HCl solution (150 mL) at 303 K. Corrosion current density (icorr) and corrosion potential (Ecorr) values were obtained by the Tafel extrapolation method. All potentials were measured against SCE. The percentage inhibition efficiency (η%), was calculated using Eq. (4): ηð%Þ ¼

i0corr −icorr i0corr

 100

ð4Þ

where, i0corr and icorr are the values of corrosion current density in the absence and presence of inhibitors, respectively. 2.4.3. Electrochemical impedance spectroscopy studies Impedance measurements were carried out using the same electrochemical cell and electrochemical workstation as mentioned for polarization measurements in frequency ranging from 100 kHz to 10 mHz, using an amplitude of 10 mV peak to peak with an AC signal at the open-circuit potential. The impedance data were obtained by using the Nyquist and Bode plots. The charge transfer resistance (Rct) was obtained by fitting the experimental data of the Nyquist plots in an appropriate equivalent circuit. The inhibition efficiency (η%) was calculated from the charge transfer resistance values obtained from impedance measurement according to Eq. (5): ηð%Þ ¼

Rct ðinhÞ −Rct  100 Rct ðinhÞ

ð5Þ

where Rct(inh) and Rct are charge transfer resistance in the presence and absence of inhibitor, respectively. The values of double layer capacitance (Cdl) were calculated from charge transfer resistance and CPE parameters (Y0 and n) using Eq. (6):  1=n C dl ¼ Y0 Rct 1−n

ð6Þ

where Y0 is the CPE constant and n is the CPE exponent. The value of n represents the deviation from the ideal behavior and it lies between 0 and 1. 2.4.4. UV–visible spectra The UV–visible absorption spectra of various solutions before and after immersion of the metal specimen for 6 h in the presence of inhibitors were recorded using a Shimadzu model UV-160A spectrophotometer. 2.4.5. FTIR spectrum analysis The FTIR spectrum of the pure compound and film formed on the surface of N80 steel specimen was recorded on a Perkin Elmer FTIR (Spectrum-2000) spectrophotometer. 2.4.6. Scanning electron microscopic and energy dispersive X-ray spectroscopy analysis The N80 steel specimens of size 1.0 cm × 1.0 cm × 0.1 cm were abraded with a series of emery paper (320–500–800–1200) grades and then washed with distilled water and acetone. After immersion in the 15% HCl solution in the absence and presence of optimum concentration of inhibitors APTT and MITT at 303 K for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then the EDX and SEM images were recorded using a Traktor TN-2000 energy dispersive spectrometer and a JEOL JSM-6380 LA analytical scanning electron microscope in vacuum mode by instrument operated at 10 kV. 2.4.7. Atomic force microscopy (AFM) The morphology of the uninhibited and inhibited N80 steel surface was investigated using atomic force microscopy. For AFM analysis the N80 steel specimens of size 1 cm × 1 cm × 0.1 cm were immersed in the test solution in the absence and presence of inhibitors for 6 h at

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room temperature. Then the specimens were taken out from the solution, cleaned with distilled water, dried, and used for AFM. The AFM analyses were carried out using a Nanosurf Easyscan 2 instrument. 2.4.8. Quantum chemical study Complete geometrical optimizations of the investigated molecules were performed using density functional theory (DFT) with Becke's three parameter exchange functional along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set as implemented in the Gaussian 03 program package [30,31]. Theoretical parameters such as the energies of the highest occupied and lowest unoccupied molecular orbitals (EHOMO and ELUMO, respectively), energy gap (ΔE), dipole moment (μ), absolute electronegativity (χ), global hardness (η), softness (σ), binding energy, molecular surface area and fraction of electrons transferred from the inhibitor molecule to the metal surface (ΔN) were calculated and Mulliken charges on HOMO and LUMO orbitals were obtained for each synthesized inhibitor. 3. Results and discussion 3.1. Weight loss measurements

The inhibition efficiency of APTT was found to be better than that of MITT at all concentrations and temperatures.

3.1.2. Effect of temperature Corrosion inhibition studies were carried out at different temperatures ranging from (303 to 333) K. Corrosion parameters namely: corrosion rate (CR), surface coverage (θ) and inhibition efficiency (η%) for N80 steel in the 15% HCl solution in the absence and presence of different concentrations (20 to 150 ppm) of inhibitor at different temperatures (303 to 333 K), obtained from weight loss measurements are given in Table 2. It is clear in Table 2 that the corrosion rate increases with the increase in temperature in the presence and absence of the inhibitors according to the Arrhenius equation. In addition, increasing the temperature can accelerate the diffusion of species involved in the electrochemical reactions. As a result, increasing temperatures would increase the corrosion rate of N80 steel. The corrosion rate of N80 steel in the absence of inhibitors increased steeply from (303 to 333) K whereas the corrosion rate increases slowly in the presence of inhibitors. The inhibition efficiency decreases with an increasing temperature from (303 to 333) K due to the increase in the rate of desorption of adsorbed inhibitor molecules on the surface of N80 steel [34].

Corrosion inhibition studies for N80 steel in the 15% HCl solution in the absence and presence of different concentrations of inhibitors (APTT and MITT) at different temperatures (303 to 333) K for an immersion period of 6 h were performed using the weight loss measurement.

3.1.3. Thermodynamic and activation parameters The apparent activation energy (Ea) for dissolution of N80 steel in the 15% HCl solution was calculated by using the Arrhenius (Eq. (7)):

3.1.1. Effect of inhibitor concentration The corrosion inhibition efficiencies (η) of the inhibitors APTT and MITT after 6 h of immersion at 303 K as evaluated by the weight loss technique are given in Table 1. In Table 1, it can be seen that inhibition efficiency increases with the increase in concentration of both inhibitors. It is further evident in Table 1 that both inhibitors are good inhibitors even at concentrations as low as 20 ppm. The inhibition efficiency of APTT and MITT at a higher concentration (150 ppm) was found to be 94.1% and 92.1%, respectively, while lower concentration (20 ppm) was 77.2% and 75.0%, respectively at 303 K. The corrosion inhibition efficiency offered by APTT and MITT at lower (20 ppm) as well as higher (150 ppm) concentrations was found to be better as compared to the inhibition efficiency of most of the pyridine derivatives reported in literature [23–27]. The increase in inhibition efficiency with increasing concentrations of inhibitor was due to the increase in the surface coverage, resulting in retardation of metal dissolution [32,33].

logCR ¼

−Ea þ log A 2:303RT

ð7Þ

where Ea is the apparent activation energy, R is the molar gas constant (8.314 J K−1 mol−1), T is the absolute temperature (K) and A is the Arrhenius pre-exponential factor. Fig. 1 presents the Arrhenius plot of log CR against 1/T for the corrosion of N80 steel in the 15% HCl solution in the absence and presence of inhibitors APTT and MITT at concentrations ranging from (20 to 150) ppm. In Fig. 1, the slope of each individual line was determined, and the activation energy was calculated using the expression Ea = (slope) × 2.303R. The calculated values of Ea are summarized in Table 3. It is evident in Table 3 that the values of the apparent activation energy for the inhibited solutions were higher than those for the uninhibited solution, indicating that the dissolution of N80 steel was decreased by making a barrier by the adsorption of the inhibitors on the metal surface [35,36].

Table 1 Weight loss parameter, electrochemical parameter and percentage inhibition efficiency (η%) for corrosion of N80 steel in the 15% HCl solution in the presence or absence of different concentrations of inhibitors at 303 K. Inhibitor

Weight loss data

Conc. (ppm)

CR (mm y−1)

θ

η%

Ecorr (mV vs SCE)

βa (mV dec−1)

−βc (mV dec−1)

icorr (μA m−2)

η%

Rct (Ω cm2)

Cdl (μF cm2)

η%

APTT Blank 20 50 75 100 150

20.20 4.60 3.16 2.08 1.47 1.23

– 0.77 0.84 0.89 0.92 0.94

– 77.2 84.3 89.7 92.7 94.1

−495 −501 −503 −506 −509 −511

93 118 103 121 98 102

142 147 178 167 158 152

568 122 80 67 46 30

– 78.4 85.8 88.1 91.8 94.6

20 90 134 187 232 286

252 113 79 57 43 29

– 77.7 85.0 89.3 91.3 93.0

MITT 20 50 75 100 150

5.04 3.59 2.63 1.95 1.59

0.75 0.82 0.86 0.90 0.92

75.0 82.2 86.9 90.3 92.1

−492 −493 −496 −498 −502

89 114 101 95 108

165 156 147 160 139

143.1 90.8 75.5 57.9 42.0

74.8 84.0 86.7 89.8 92.6

81 119 168 202 250

128.3 97.6 68.2 51.7 37.3

75.6 83.2 88.1 90.4 92.0

Experimental errors ±3%.

Tafel extrapolation data

EIS data

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155

Table 2 Corrosion parameters obtained from weight loss measurements of N80 steel in the 15% HCl solution in the presence and absence of inhibitors at different temperatures. Inhibitor with conc.

303 K

313 K

323 K

333 K

Inhibitor with conc.(ppm)

303 K

313 K

323 K

333 K

CR (mm y−1)

θ

η%

CR (mm y−1)

θ

η%

CR (mm y−1)

θ

η%

CR (mm y−1)

θ

η%

APTT Blank 20 50 75 100 150

20.20 4.60 3.16 2.08 1.47 1.23

– 0.772 0.843 0.896 0.927 0.940

– 77.22 84.32 89.68 92.70 94.08

34.55 8.56 6.08 4.18 3.05 2.54

– 0.752 0.823 0.878 0.911 0.926

– 75.20 82.39 87.88 91.17 92.63

55.47 15.20 11.12 7.98 6.05 5.21

– 0.725 0.799 0.856 0.890 0.906

– 72.58 79.94 85.60 89.08 90.60

92.69 28.58 21.57 16.23 12.93 11.37

– 0.691 0.767 0.825 0.860 0.877

– 69.16 76.72 82.48 86.05 87.73

MITT 20 50 75 100 150

5.04 3.59 2.63 1.95 1.59

0.750 0.822 0.869 0.903 0.921

75.02 82.22 86.97 90.34 92.13

9.15 6.62 5.20 3.69 3.15

0.735 0.808 0.849 0.893 0.908

73.50 80.82 84.93 89.31 90.88

16.60 12.13 9.77 7.44 6.07

0.700 0.781 0.823 0.865 0.890

70.07 78.12 82.38 86.58 89.04

30.44 23.24 19.27 15.27 12.89

0.671 0.749 0.792 0.835 0.860

67.15 74.94 79.21 83.52 86.09

Experimental errors ±3%.

The values of standard enthalpy of activation (ΔH ⁎) and standard entropy of activation (Δ S ⁎) were calculated by using Eq. (8):

CR ¼

    RT ΔS ΔH exp − exp R RT Nh

ð8Þ

where, h is Planck's constant and N is Avogadro's number, respectively.

A plot of log (CR/T) against 1/T (Fig. 2) gave straight lines with a slope of −ΔH⁎/2.303R and an intercept of [log(R/Nh) + (ΔS⁎/2.303R)], from which the value of activation thermodynamic parameters ΔH⁎ and ΔS⁎ were calculated, as listed in Table 3. The positive sign of the enthalpy reflects the endothermic nature of the N80 steel dissolution process. The negative value of ΔS⁎ for both the inhibitors indicates that the formation of the activated complex in the rate determining step represents an association rather than a dissociation step, meaning

Fig. 1. Arrhenius plots for N80 steel corrosion in the 15% HCl solution of (a) APTT and (b) MITT.

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Table 3 Activation parameter for N80 steel in the 15% HCl solution in the absence and presence of inhibitors obtained from weight loss measurements. Inhibitor

Concentration (ppm)

Ea (kJ mol−1)

ΔH⁎ (kJ mol−1)

ΔS⁎(J mol−1 K−1)

Blank

– 20 50 75 100 150 20 50 75 100 150

42.34 50.80 53.42 57.15 60.48 62.63 50.27 52.09 55.44 57.66 58.16

39.70 48.16 50.78 54.51 57.83 59.98 47.63 49.45 52.80 55.02 55.52

−89.20 −73.69 −68.14 −59.33 −51.29 −45.93 −74.84 −71.58 −63.16 −58.56 −58.37

APTT

MITT

isotherms. The most commonly used adsorption isotherms are the Langmuir, Temkin, and Frumkin isotherms. The surface coverage (θ) for different concentrations of inhibitors in the 15% HCl solution was tested graphically for experimental fit to suitable adsorption isotherm. Plotting Cinh/θ vs. Cinh at different temperatures yielded straight lines (Fig. 3), the correlation coefficient (R2) and slope values for each line are given in Table 4. The correlation coefficient and slope values in Table 4 are near to unity, indicating that the adsorption of these inhibitors on N80 steel surface obey the Langmuir adsorption isotherm represented by Eq. (9): C inh 1 ¼ þ C inh θ K ads

ð9Þ

that a decrease in disorder takes place during the course of the transition from reactants to the activated complex [37].

where, Cinh is the inhibitor concentration and Kads is the equilibrium constant for the adsorption–desorption process. From the intercepts of Fig. 3, the values of Kads were calculated. Large values of Kads obtained for both studied inhibitors imply more efficient adsorption and hence better corrosion inhibition efficiency. Using the values of Kads, the values of ΔG°ads were evaluated by using Eq. (10):

3.2. Adsorption isotherm

ΔGads ¼ −RT ln ð55:5K ads Þ

Basic information on the interactions between the organic inhibitors and the N80 steel surface are obtained from various adsorption

where R is the gas constant and T is the absolute temperature (K). The value of 55.5 is the concentration of water in solution in mol L−1. The

Experimental errors ±3%.

Fig. 2. Transition state plot for N80 steel in the 15% HCl solution at different concentrations of (a) APTT and (b) MITT.

ð10Þ

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157

Fig. 3. Langmuir plots for (a) APTT and (b) MITT.

calculated values of Kads and ΔG°ads are given in Table 4. In general, the values of ΔG°ads up to −20 kJ mol−1 are compatible with the electrostatic interaction between the charged inhibitor molecules and the charged metal surface (physisorption) and those which are more negative than −40 kJ mol−1 involve charge sharing or charge transfer from the inhibitor molecules to the metal surface [38] (chemisorptions). The calculated ΔG°ads values for APTT and MITT were found in the range of (− 38.6 to − 41.6) and (−38.7 to − 41.5) kJ mol−1, respectively, at different

Table 4 Adsorption parameters for APTT and MITT calculated from the Langmuir adsorption isotherm for N80 steel in the 15% HCl solution at (303–333) K. Inhibitor

Temperature (K)

Kads (M−1)

ΔG°ads (kJ mol−1)

Slope

R2

APTT

303 K 313 K 323 K 333 K 303 K 313 K 323 K 333 K

8.2 × 104 7.5 × 104 6.8 × 104 6.1 × 104 8.5 × 104 7.8 × 104 6.7 × 104 6.1 × 104

−38.6 −39.7 −40.7 −41.6 −38.7 −39.7 −40.6 −41.5

8.12

−62.66 −62.66 −62.66 −62.66 −60.58 −60.58 −60.58 −60.58

MITT

Experimental errors ±3%.

9.62

temperatures (303 to 333) K, being closer to − 40 kJ mol−1, which were between the threshold values for physical adsorption and chemical adsorption, indicating that the adsorption process of inhibitors at the N80 steel surface involve both the physical as well as the chemical adsorption. Quraishi and Shukla [39] studied 4-Substituted anilinomethylpropionate as corrosion inhibitors for mild steel in hydrochloric acid solution. The Gibbs free energy of adsorption for these molecules was reported to be around −38 kJ mol−1. They concluded that the adsorption mechanism of these molecules on steel involved two types of interactions, chemisorptions and physisorptions. A similar conclusion was also reported by Ozcan [40], who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid. Thus, the calculated values of ΔG°ads (Table 4) for both inhibitors suggest that the adsorption of these inhibitors at the surface of the N80 steel is not pure physisorption or chemisorption but it is combination of physisorption as well as chemisorption.

3.3. Electrochemical studies 3.3.1. Polarization studies The potentiodynamic polarization curves for the N80 steel in the 15% HCl solution in the absence and presence of different concentrations of inhibitors, APTT and MITT are shown in Fig. 4(a) and (b) at 303 K. It can be seen in Fig. 4(a) and (b), that the nature of the polarization curves remains the same in the absence and presence of inhibitors but

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Fig. 4. Potentiodynamic polarization curves for N80 steel in the 15% HCl solution in the presence and absence of inhibitors (a) APTT and (b) MITT at 303 K.

the curves shifted towards lower current density in the presence of inhibitors, indicating that the inhibitor molecules retard the corrosion process. The corrosion current densities and corrosion potentials were calculated by extrapolation of linear parts of anodic and cathodic curves to the point of intersections. The corrosion parameters such as corrosion potential (Ecorr), anodic Tafel slope (βa), cathodic Tafel slope (βc), corrosion current density (icorr) and percentage inhibition efficiency (η%) obtained from these curves are given in Table 1. The results revealed that increasing concentrations of both inhibitors resulted in a decrease in corrosion current densities and an increase in inhibition efficiency (η%), suggesting that the adsorption of inhibitor molecules at the surface of the N80 steel forms a protective film on the N80 steel surface [41]. The efficiencies of the inhibitors are in the order: APTT N MITT at 303 K. The presence of inhibitors causes a minor change in Ecorr values with respect to the Ecorr value in the absence of inhibitors. This implies that the inhibitors act as a mixed type inhibitor, affecting both anodic and cathodic reactions [42]. If the displacement in Ecorr is more than ±85 mV relating to corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type [43]. If the change in Ecorr is less than ±85 mV, the corrosion inhibitor may be regarded as a mixed type. The maximum displacement in our study is 16 mV, which indicates that APTT and MITT act as mixed type inhibitors.

3.3.2. EIS studies The Nyquist plots for N80 steel obtained at the interface in the 15% HCl solution with and without the different concentrations of APTT and MITT at 303 K are shown in Fig. 5(a) and (b). The existence of a single semicircle with its center below in Nyquist plots (Fig. 5a, b) for both inhibitors indicates the presence of a single charge transfer process during metal dissolution which is unaffected by the presence of inhibitor molecules. The Nyquist plots in the absence and presence of inhibitors are characterized by one capacitive loop. The capacitive loops are not perfect semicircles, because of non-homogeneity and roughness of the N80 steel surface [44]. The EIS spectra of all tests were analyzed using the equivalent circuit shown in Fig. 6, which is a parallel combination of the charge transfer resistance (Rct) and the constant phase element (CPE), both in series with the solution resistance (Rs). This type of electrochemical equivalent circuit was reported previously to model the iron/acid interface [45]. Constant phase element (CPE) is introduced instead of pure double layer capacitance to give more accurate fit as the double layer at the interface does not behave as an ideal capacitor. The electrochemical parameters obtained from the fitting of the equivalent circuit are given in Table 1. The data shown in Table 1 reveal that the value of Rct increases with the addition of inhibitors as

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Fig. 5. Nyquist plot for N80 steel in the 15% HCl solution containing various concentrations of (a) APTT, (b) MITT, (1) 0 ppm, (2) 20 ppm, (3) 50 ppm, (4) 75 ppm and (5) 100 ppm at 303 K.

compared to the blank solution, the increase in Rct values is attributed to the formation of an insulating protective film at the metal/solution interface. The CPE value decreases upon increasing concentrations of both inhibitors, indicating the adsorption of the inhibitor molecules on the surface of N80 steel. The single peak obtained in Bode plots (Fig. 7) for both inhibitors indicates that the electrochemical impedance measurements were fit well in one time constant equivalent model (Randle's cell model) with constant phase element (CPE). Moreover, there is only one phase

maximum in Bode plot (Fig. 7a, b) for both inhibitors, which indicates only one relaxation process and which would be the charge transfer process, taking place at the metal–electrolyte interface. Fig. 7(a) and (b) show that the impedance value in the presence of both inhibitors is larger than that in the absence of inhibitors and the value of impedance increases on increasing the concentration of both studied inhibitors. These mean that the corrosion rate is reduced in the presence of the inhibitors and continued to decrease on increasing the concentration of inhibitors. Electrochemical results (η%) are in good agreement with the results (η%) obtained by the weight loss experiment, but a minor difference in results have been observed, which can be attributed to the further chemically (weight loss measurement) determined corrosion rate which is independent of potential, whereas electrochemical measurements depend on operational potential [46]. 3.4. UV–visible spectroscopy

Fig. 6. Equivalent circuit applied for fitting of the impedance spectra.

UV–visible spectroscopy provides strong evidence for the formation of a metal complex. We obtained UV–visible absorption spectra for an optimum concentration of inhibitors at 303 K before and after 6 h immersion of N80 steel specimen as shown in Fig. 8. The electronic absorption spectrum of inhibitors before the N80 steel immersion shows bands in the UV–visible region due to π–π⁎ and n–π⁎ transitions with a considerable charge transfer character. After 6 h of immersion of

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Fig. 7. Bode plots for N80 steel in the 15% HCl solution in the absence and presence of different concentrations of inhibitors (a) APTT and (b) MITT.

N80 steel in the presence of inhibitors, the observed change in the position of absorption maximum or change in the values of absorbance indicate the formation of a complex between the inhibitors and iron in solution. However, there was no any significant change in the shape of the spectra. These experimental findings provide strong evidence for the complex formed between Fe2 + and inhibitors in the 15% HCl solution. UV–visible observation confirms the formation of protective film of metal–inhibitor complex on the metal surface.

presence of the\\NH, C_N and C\\N groups, respectively. The shift in peak position of \\NH, C_N and C\\N in FTIR spectra of inhibited metal surface product of both inhibitors as compared to their pure compound spectra, indicating the involvement of these groups in the adsorption process of inhibitors with the metal. The presence of the other bands of the inhibitor with minor shift in reflectance spectra of the exposed specimen was also reported which indicates the interactions of the inhibitors molecule with the N80 steel.

3.5. Analysis of FTIR spectra

3.6. Scanning electron microscopy

The FTIR spectra of pure inhibitors and the inhibited metal surface product were recorded and shown in Fig. 9(a)–(e). The pure APTT shows the IR bands around 3280, 1520 and 790 cm−1 due to presence of the \\NH, C_N and C\\N groups, respectively. The pure MITT shows the IR band around 3360, 1510 and 810 cm−1 due to presence of the \\NH, C_N and C\\N groups, respectively. The FTIR spectra of the inhibited metal surface product in APTT and MITT show bands at 3310, 1540, 810 and 3380, 1540, 840 cm−1, respectively due to

The surface morphology of the N80 steel samples in the 15% HCl solution in the absence and presence of 150 ppm of APTT and MITT is shown in Fig. 10(a), (b), (c) and (d). Fig. 10(a) is SEM of the N80 steel sample before immersion in the 15% HCl solution. The badly damaged surface (Fig. 10(b)) obtained when the metal was kept immersed in the 15% HCl solution for 6 h without inhibitor indicates significant corrosion. However, in the presence of inhibitors (Figs. 10(c) and (d)) the surface has remarkably improved with respect to its smoothness

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Fig. 8. UV–visible absorption spectra of various solutions before and after immersion of the N80 steel specimen for 6 h in the presence of inhibitors.

indicating considerable reduction of corrosion rate. This improvement in surface morphology is due to the formation of a good protective film of inhibitor on N80 steel surface which is responsible for inhibition of corrosion. 3.7. Energy dispersive X-ray spectroscopy The results of EDX spectra are shown in Fig. 11(a), (b), (c) and (d). Fig. 11(a) and (b) represent the EDX spectra of abraded and uninhibited N80 steel specimen and Fig. 11(c) and (d) depict inhibited N80 steel specimens. The abraded N80 steel specimen shows characteristic peaks of elements (C, Mn, Cr, Fe) constituting the N80 steel sample. The EDX spectra of uninhibited (Fig. 11(b)) N80 steel show a peak corresponding to Cl in addition to the abraded sample peaks. The EDX spectra of inhibited N80 steel contains the peaks corresponding to all the elements present in the inhibitor molecules indicating the adsorption of inhibitor molecules at the surface of N80 steel. In addition to that, EDX of inhibited spectra shows that the Fe peaks are considerably suppressed as compared to abraded and uninhibited N80 steel sample. The suppression of Fe lines might be due to the overlying inhibitor film. This indicated that the N80 steel surface was covered with a protective film of inhibitor molecules.

3.8. Atomic force microscopy Surface morphology of the polished N80 steel and N80 steel in the 15% HCl solution in the absence and presence of inhibitors was investigated through atomic force microscopy (AFM) and the results are shown in Fig. 12(a)–(d). The average roughness of polished N80 steel (Fig. 12a) and N80 steel in the 15% HCl solution without inhibitor (Fig. 12(b)) was found as 25 and 650 nm. It is clearly shown in Fig. 12(b) that the N80 steel sample is getting cracks due to the acid attack on the N80 steel surface. However, in the presence of the optimum concentration (150 ppm) of APTT and MITT as shown in Fig. 12(c) and (d), the average roughness was reduced to 75 and 95 nm, respectively. The lower value of calculated roughness for APTT reveals that APTT protects the N80 steel surface more efficiently than did MITT in the 15% HCl solution. 3.9. Theoretical calculations In order to study the effect of molecular structure on the inhibition efficiency, quantum chemical calculations were performed by using DFT and all the calculations were carried out with the help of complete geometry optimization. The optimized structures, EHOMO and ELUMO are

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Fig. 9. FTIR spectrum of the pure inhibitors and film formed on the surface of N80 steel specimen.

shown in Fig. 13. The frontier molecular orbital energies (EHOMO and ELUMO) are significant parameters for the prediction of the reactivity of a chemical species. EHOMO is often associated with the electron donating ability of a molecule. The inhibition efficiency increases with the increasing EHOMO values. High values of EHOMO indicate that the molecule has a tendency to donate electrons to the appropriate acceptor molecules with low energy empty molecular orbital. The lower values of ELUMO suggested that the molecule easily accepts electrons from the donor molecules [47]. It was reported previously by some researchers [48] that smaller values of ΔE and higher values of dipole moment (μ) are responsible for enhancement of inhibition efficiency. According to HSAB theory hard acids prefer to co-ordinate to hard bases and soft acid to soft bases. Fe is considered soft acid and will co-ordinate to a molecule having maximum softness and small energy gap (ΔE = ELUMO − EHOMO). Thus, the inhibitor APTT having higher value of softness adsorbed strongly at the surface of N80 steel and shows maximum inhibition efficiency. The quantum chemical parameters such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE), the dipole moment (μ), absolute electronegativity (χ), binding energy (BE), molecular

surface area (SE), global hardness (η) and softness (σ) were calculated and summarized in Table 5. For the calculations of quantum chemical parameters Eqs. (11)–(13) were used [49]: χ¼− η¼

ELUMO þ EHOMO 2

ELUMO −EHOMO : 2

ð11Þ ð12Þ

The inverse of the global hardness is designated as the softness, σ, as follows: σ¼

1 η

ð13Þ

where, hardness and softness measure the stability and reactivity of a molecule. Soft molecules are considered to be more reactive than hard ones because they can offer electron to acceptors easily. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where σ, which is a local property, has the highest value [49].

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Fig. 10. SEM image of N80 steel in the 15% HCl solution after 6 h immersion at 303 K (a) before immersion (polished), (b) after immersion without an inhibitor, (c) in the presence of inhibitor APTT and (d) in the presence of inhibitor MITT.

Kokalj [50] recently reported that the work function (Φ) of a metal surface is an appropriate measure of its electronegativity and should be used, together with its vanishing absolute hardness, to estimate the fraction of electrons transferred (ΔN) as: ΔN ¼

Φ−χ inh χ Fe −χ inh
: ¼ 2ηinh 2 η Fe þ ηinh

ð14Þ

To calculate the fraction of electrons transferred, a theoretical value for the electronegativity of bulk iron was used, χFe ≈ 7 eV mol−1, and global hardness of, γFe = 0 eV mol−1 was used. In Table 5 it is clear that a higher value of EHOMO (− 8.6888 eV), σ (0.2500 eV−1), ΔN (0.2889) and lowest values of ΔE (7.9991 eV) are found for APTT, indicating that APTT has more potency to get adsorbed on the N80 steel surface resulting in greater inhibition tendency than MITT. Dipole moment values of APTT and MITT are 4.8150 D and 3.7188 D, respectively, which clearly suggest that inhibitors are polar compounds and can easily donate electrons forming strong dπ–pπ bonding [51]. In the present study the inhibition efficiency increases with the increasing dipole moment of the inhibitors, which could be attributed as higher polarity compounds which will facilitate electrostatic interaction between the electric field due to the charged metal surface and electric moments of the inhibitors and contributes to their better adsorption by influencing the transport process through the adsorbed layer [52]. Generally, ΔN shows inhibition efficiency resulting from electrons transferred from the inhibitor molecule to the iron atom. According to Lukovits et al. [53] if the value of ΔN is less than 3.6, the efficiency of inhibition increases with increasing electron-donating ability of the inhibitor at the metal surface. The structure of APTT and MITT is almost similar, the only difference is that APTT contains a pyrimidine ring whereas MITT contains an isoxazole ring. The electronic-releasing power of APTT was found to be better than that of MITT due to the

more basic nature of the pyrimidine ring nitrogen to the isoxazole ring oxygen, which improved the inhibition efficiency of APTT. The high negative values of binding energy obtained for both inhibitors (Table 5) indicate the stability of the adsorbed inhibitor molecules which cannot be split or broken apart from the metal surface easily. Molecular surface area (a geometrical quantity) determines various properties of inhibitor molecules, as they interact through their metal surface residue. It has been found that the large surface area values obtained for both studied inhibitors (Table 5) attributed to the uniformity of coverage of the surface of the N80 steel. The higher value of binding energy and larger molecular size of APTT (Table 5) indicate its higher inhibition efficiency. Literature reveals that the use of Mulliken population and HOMO population analyses can be used for the determination of possible adsorption centers of the inhibitors [54–56]. The generalized interpretation given by several authors is that the higher the magnitude and the number of the negatively charged heteroatom present in an inhibitor molecule, the higher its ability to be adsorbed on the metal surface via the donor–acceptor type bond and the more negatively charged regions with major distribution of the HOMO are also interpreted as possible centers of adsorption. Mulliken charges according to the numeration of the corresponding atoms are shown in Fig. 14(a) and (b). It is evident in Fig. 14(a) and (b) that both inhibitors had a considerable excess of negative charge around the nitrogen, and some carbon atoms, indicating that these are the coordinating sites of the inhibitors. 3.10. Mechanism of inhibition Corrosion inhibition of N80 steel in the hydrochloric acid solution by APTT and MITT can be explained on the basis of molecular adsorption. These compounds inhibit corrosion by controlling both anodic as well as cathodic reactions. In acidic solutions these inhibitors exist as

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Fig. 11. EDX spectra of mild steel specimens (a) polished and (b) after immersion without inhibitor, (c) with 100 ppm APTT and (d) with 100 ppm MITT.

Fig. 12. AFM micrographs of the mild steel surface: (a) polished mild steel, (b) blank in 15% HCl solution, (c) with 100 ppm APTT and (d) with 100 ppm MITT.

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Fig. 13. The optimized structure (left) and HOMO (center) and LUMO (right) distribution for molecules (a) APTT and (b) MITT.

protonated species. In both inhibitors nitrogen atoms present in the molecules can be easily protonated in the acidic solution and converted into quaternary compounds. These protonated species adsorbed on the cathodic sites of the N80 steel and decrease the evolution of hydrogen. The adsorption on the anodic site occurs through π-electrons of aromatic rings and lone pair of electrons of the nitrogen and oxygen atoms which decrease the anodic dissolution of N80 steel. 4. Conclusions The synthesized triazine derivatives show good inhibition efficiencies for the corrosion of N80 steel in the 15% HCl solution and the inhibition efficiency increases with the increase in the concentration of inhibitors and decreases with the increase in temperature. The

inhibiting performances of the inhibitors follow the order: APTT N MITT. The variation in the values of βa and βc (Tafel slopes) and the minor deviation of Ecorr with respect to Ecorr of the blank, indicate that both verified inhibitors are of the mixed type in nature. EIS measurements show that charge transfer resistance (Rct) increases and double layer capacitance (Cdl) decreases in the presence of inhibitors, which suggest the adsorption of the inhibitor molecules on the surface of N80 steel. It is suggested from the results obtained from SEM, AFM and the Langmuir adsorption isotherm that the mechanism of corrosion inhibition is occurring mainly through the adsorption process. Quantum chemical results were in good agreement with experimental results of both inhibitors.

Table 5 Quantum chemical parameters for different inhibitors. Inhibitor

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

μ (D)

η(eV)

σ (eV)

ΔN

χ

S.A. (Å2)

B.E. (kcal mol−1)

APTT MITT

−8.7888 −8.7616

−0.4897 −0.5986

8.2991 8.1630

1.8150 3.7188

4.1495 4.0815

0.2410 0.2450

0.2844 0.2841

−4.6392 −4.6801

714.5 752.3

−7734 −8732

Experimental errors ±3%.

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Fig. 14. The Mulliken charge density of (a) APTT and (b) MITT.

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