Thermodynamic, electrochemical and quantum chemical evaluation of some triazole Schiff bases as mild steel corrosion inhibitors in acid media

Journal of Molecular Liquids 211 (2015) 1026–1038

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

Thermodynamic, electrochemical and quantum chemical evaluation of some triazole Schiff bases as mild steel corrosion inhibitors in acid media Turuvekere Krishnamurthy Chaitra a, Kikkeri Narasimha Shetty Mohana a,⁎, Harmesh Chander Tandon b a b

Department of Studies in Chemistry, Manasagangotri, University of Mysore, Mysore 570006, Karnataka, India Department of Chemistry, Sri Venkateswara College, Dhula Kuan, New Delhi 110021, India

a r t i c l e

i n f o

Article history: Received 26 May 2015 Received in revised form 8 August 2015 Accepted 11 August 2015 Available online xxxx Keywords: Corrosion Mild steel Triazole Schiff bases Adsorption Electrochemical techniques Quantum chemical calculations

a b s t r a c t Three newly synthesised triazole Schiff bases (3-bromo-4-fluoro-benzylidene)-[1,2,4]triazol-4-yl-amine (BFBT), (4-trifluoromethyl-benzylidene)-[1,2,4]triazol-4-yl-amine (TMBT) and (2-fluoro-4-nitro-benzylidene)[1,2,4]triazol-4-yl-amine (FNBT) were investigated as corrosion inhibitors on mild steel in 0.5 M hydrochloric acid by chemical (weight loss) and electrochemical (potentiodynamic polarisation and electrochemical impedance) techniques. The inhibition efficiency increases with an increase in inhibitor concentration and decreases with an increase in temperature of the medium. Various activation and adsorption thermodynamic parameters were evaluated. Adsorption of all the three inhibitors follows the Langmuir isotherm. Electrochemical impedance studies show that charge transfer resistance increases with concentration of inhibitors. Polarisation studies prove that inhibitors are of the mixed type. Surface morphology was examined using SEM and FTIR studies. Quantum chemical calculations give evidence to experimental results. Higher value of EHOMO, lower value of ELUMO, smaller orbital gap (ΔE) and higher dipole moment makes BFBT superior over TMBT and FNBT thus shows the maximum inhibition efficiency. All the studied methods are showing good correlation with each other. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Mild steel (MS) is the most versatile, least expensive and most widely used engineering material which has found extensive application in various industries [1]. During industrial processes such as pickling and descaling, exposure of mild steel surface to aggressive acids such as hydrochloric acid and sulphuric acid causes inevitable corrosive damage [2]. In an effort to mitigate the corrosion of mild steel, the strategy is to separate metal from corrosive agents [3]. Among many methods of corrosion control and prevention, the use of organic inhibitors is the most practical and frequently used [4]. Many investigations [5–9] have proved that heterocyclic compounds are the most effective among the organic inhibitors known so far. The effectiveness of the inhibitors depends on the adsorption rates and covering capabilities on metal surfaces [10]. The main criteria for the adsorption of inhibitors are (i) delocalised electrons present on hetero-atoms such as N, S, O and P, ii) planarity of the molecule (π electrons) and (iii) double bond [11]. Currently, research in corrosion is oriented to the development of “green corrosion inhibitors”, compounds with good inhibition efficiency but low risk of environmental pollution [12]. Among the various

⁎ Corresponding author. E-mail address: [email protected] (K.N.S. Mohana).

http://dx.doi.org/10.1016/j.molliq.2015.08.031 0167-7322/© 2015 Elsevier B.V. All rights reserved.

nitrogenous compounds studied as inhibitors, triazoles have been considered as environmentally acceptable chemicals [13]. Triazoles possess a wide spectrum of activities ranging from anti-bacterial, anti-inflammatory, anticonvulsant and anti-neoplastic [14–17]. Triazoles attracted much attention as corrosion inhibitors among heterocycles because they possess three nitrogen atoms which can act as active adsorption centres. It has been found that nitrogen-containing organic compounds are known to be efficient corrosion inhibitors in HCl solutions [18]. A lone pair of electrons on nitrogen atoms of triazole ring, π electrons on the benzene ring and electrons on the azomethine group is readily available for sharing to form a bond and act as nucleophilic centres of triazole Schiff bases and greatly facilitate the adsorption process over the metal surface, whose atoms act as electrophiles. According to the literature survey both triazoles [19–27] and Schiff bases [28–36] are well established and potential class of inhibitors for mild steel in acid media. Ample research on Schiff bases revealed that inhibition efficiencies of Schiff bases are much greater than that of corresponding amines and aldehydes [37]. Triazole Schiff bases constituting combined features of both triazoles and Schiff bases are previously studied by many authors [37–40] and excellent inhibition is obtained. The above considerations prompted us to test BFBT, TMBT and FNBT as corrosion inhibitors for MS. So, in continuation of our previous work [41–43] the present paper reports the synthesis, characterisation and corrosion inhibition performance of three Schiff bases of triazole on

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stretching), 1484–1596 (Ar C_C), 1045–1391 (C\\F). 1H NMR (400 MHz, DMSO-d6) δ ppm: 7.4 (d, 1H, Ar\\H), 7.7 (d, 1H, Ar\\H), 8.053 (s, 1H, Ar\\H), 8.992 (s, 1H, C\\H), 9.035 (s, 2H, Ar\\H). MS: 268.88 (M+), 270.88 (M+2) Yield: 81%, m. p. 198–201 °C. 4-Trifluoromethyl-benzylidene-[1,2,4] triazol-4-yl-amine (TMBT) IR (KBr) (cm− 1): 1626 (C_N, imine group), 3091 (C\\H stretching), 1494–1510 (Ar C_C), 1016–1396 (C\\F). 1H NMR (400 MHz, DMSO-d6) δ ppm: 7.90 (d, 2H, Ar\\H), 8.02 (d, 2H, Ar\\H), 9.17 (s, 2H, Ar\\H), 9.18 (s, 1H, C\\H). MS: 240.97 (M+), 241.98 (M+1). Yield: 84% m. p. 193–195 °C. (2-Fluoro-4-nitro-benzylidene)-[1,2,4]triazol-4-yl-amine (FNBT) IR (KBr) (cm−1): 1629 (C_N, imine group), 3037–3141 (C\\H stretching), 1495–1578 (Ar C_C), 1304–1519 (N_O, nitro group), 1054–1389 (C\\F). 1H NMR (400 MHz, DMSO-d6) δ ppm: 7.751 (s, 1H, Ar\\H), 8.527 (d, 1H, Ar\\H), 8.799 (d, 1H, Ar\\H), 9.298 (s, 1H, C\\H), 9.347 (s, 2H, Ar\\H). MS: 235.95 (M+). Yield 90%, m. p. 201–203 °C.

MS in 0.5 M hydrochloric acid using weight loss and electrochemical techniques. The morphology of the metal was examined using a scanning electron microscope (SEM) and FTIR spectral studies. Quantum chemical parameters were also calculated and discussed. 2. Experimental 2.1. Materials and sample preparation The MS coupons used in this study having composition by wt.% is as follows: C — 0.051; Mn — 0.179; Si — 0.006; P — 0.005; S — 0.023; Cr — 0.051; Ni — 0.05; Mo — 0.013; Ti — 0.004; Al — 0.103; Cu — 0.050; Sn — 0.004; B — 0.00105; Co — 0.017; Nb — 0.012 and Pb — 0.001 and the remainder is iron. The dimension of coupons used for the experiment is 2 cm × 2 cm × 0.1 cm. Prior to the commencement of the gravimetric and electrochemical experiments, the surface of the specimens was polished under running tap water using a silicon carbide emery paper (600, 800, 1200 grade), washed thoroughly with double distilled water, dried on a clean tissue paper, immersed in benzene for 5 s followed by air drying using acetone. The specimens were preserved in a desiccator until use. At the end of the test, the specimens were carefully washed with benzene and acetone, dried and then weighed. For polarisation and impedance measurements, the MS specimens were embedded in epoxy resin to expose a geometrical surface area of 1 cm2 to the electrolyte. A stock solution of the inhibitor was prepared by dissolving an appropriate amount of inhibitor in 0.5 M HCl. A concentration range of 0.8 mM to 3.2 mM was chosen by optimisation and prepared from the stock solution. For the synthesis of inhibitors, all the chemicals were purchased from Alfa Aesar and Sigma Aldrich Pvt. Ltd. The melting range was found out using a Veego Melting Point VMP III apparatus.

2.3. Weight loss measurements MS coupons were immersed in acid solution without and with varying amounts of the inhibitor for 4 h in a thermostatically controlled water bath (with an accuracy of ± 0.2 °C) at constant temperature, under aerated conditions (Weiber Limited, Chennai, India). The specimens were removed after 4 h of immersion, rinsed in water followed by drying in acetone. Weight loss of the specimens was recorded by an analytical balance (Sartorius, precision ± 0.1 mg). The experiment was carried out in triplicate and the average weight loss of three similar specimens was calculated. The procedure was repeated for all other concentrations and temperatures.

2.2. Synthesis of inhibitors 2.4. Electrochemical measurements According to the procedure followed by Paul et al. [44] three Schiff bases were synthesised by the condensation reaction of an equimolar mixture of 4-amino-4H-1,2,4-triazole with three different aldehydes, 3-bromo-4-fluoro-benzaldehyde, 4-trifluoromethyl-benzaldehyde and 2-fluoro-4-nitro-benzaldehyde using ethanol as solvent. The reaction mixture was refluxed for 5 h and checked for completion using TLC (solvent system — ethyl acetate:methanol, 4:1). The mixture was poured into ice cold water to get a solid product, which was then filtered, washed and recrystallized from ethanol to get the pure product. The melting point of the product was recorded.

Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) experiments were carried out using a CHI660D electrochemical workstation. A conventional three-electrode cell consisting of an |Ag/AgCl| reference electrode, a platinum auxiliary electrode and a working MS electrode with 1 cm2 exposed areas was used. Pretreatment of the specimens is the same as that of the gravimetric measurements. The electrochemical tests were performed using the synthesised triazole derivatives with various concentrations ranging from 0.8 mM–3.2 mM at 30 °C. Potentiodynamic polarisation measurements were performed in a potential range of −850 to −150 mV with a scan rate of 0.4 mV s−1. Prior to EIS measurements, half an hour was spent making open circuit potential a stable value. The EIS data were taken in a frequency range of 10 kHz to 100 mHz.

2.2.1. Spectral data (3-Bromo-4-fluoro-benzylidene)-[1,2,4]triazol-4-yl-amine (BFBT) IR(KBr) (cm−1): 1629 (C_N, imine group), 3128 (C\\H

Table 1 Weight loss data of mild steel corrosion in 0.5 M HCl in the presence of different concentrations of the inhibitors at different temperatures. Inhibitor

BFBT

TMBT

FNBT

C (mM)

30 °C CR (mg cm2 h−1)

IE (%)

40 °C CR (mg cm−2 h−1)

IE (%)

50 °C CR (mg cm2 h−1)

IE (%)

60 °C CR (mg cm−2 h−1)

IE (%)

Blank 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2

0.516 0.113 0.097 0.074 0.05 0.128 0.108 0.091 0.065 0.144 0.128 0.1 0.074

– 78.1 ± 0.62 81.2 ± 0.84 85.7 ± 0.43 90.3 ± 1.20 75.1 ± 0.56 78.9 ± 0.28 82.3 ± 0.79 87.4 ± 0.96 72.1 ± 1.11 75.2 ± 0.94 80.6 ± 0.38 85.6 ± 0.55

0.883 0.218 0.184 0.143 0.113 0.256 0.237 0.193 0.154 0.280 0.255 0.202 0.163

– 75.3 ± 0.44 79.2 ± 0.78 83.8 ± 0.84 87.1 ± 1.40 71.0 ± 1.16 73.1 ± 0.83 78.1 ± 0.74 82.5 ± 0.45 68.3 ± 0.78 71.1 ± 0.87 77.1 ± 0.66 81.5 ± 0.68

1.224 0.348 0.307 0.268 0.211 0.416 0.357 0.331 0.273 0.449 0.389 0.337 0.287

– 71.6 ± 0.38 74.9 ± 0.44 78.1 ± 0.51 82.7 ± 0.35 66.0 ± 0.24 70.8 ± 1.10 72.9 ± 0.46 77.6 ± 0.68 63.3 ± 0.55 68.2 ± 0.18 72.4 ± 0.44 76.6 ± 0.55

1.65 0.534 0.480 0.422 0.367 0.583 0.560 0.477 0.413 0.669 0.575 0.515 0.453

– 67.6 ± 0.84 70.9 ± 0.52 74.4 ± 0.46 77.7 ± 0.69 64.7 ± 0.86 66.1 ± 0.23 71.1 ± 1.12 75.0 ± 1.50 59.5 ± 0.6 65.1 ± 0.84 68.8 ± 0.77 72.5 ± 0.58

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2.5. Quantum chemical calculations The geometrical optimization of the investigated molecules has been done by Ab initio method at 631G* basis set for all atoms except for bromine atom in BFBT for which it is not defined in the programme. For the Br atom 321G* basis set is employed. For energy minimization, the convergence limit at 1.0 and rms gradient 1.0 kcal/A mol have been maintained. The Polak–Ribiere conjugate gradient algorithm which is quite fast and precise is used for optimization of geometry. The HYPERCHEM 7.52 professional software is employed for all calculations. 2.6. Scanning electron microscopy (SEM)

were taken for polished MS specimen and specimen immersed in acid solution with and without inhibitors.

3. Results and discussion 3.1. Weight loss measurements 3.1.1. Effect of inhibitor concentration Corrosion inhibition of MS in 0.5 M HCl containing various concentrations of Schiff bases (BFBT, TMBT, and FNBT) after 4 h of immersion between 30 °C and 60 °C was studied by the weight loss method and results are given in Table 1. The corrosion rate

The SEM experiments were performed using a Zeiss electron microscope with a working voltage of 15 kV, working distance of 10.0 mm and magnitude of 2.00 kX. In SEM micrographs, the specimens were exposed to the 0.5 M HCl in the absence and presence of three inhibitors under optimum conditions after 4 h of immersion. The SEM images

Fig. 1. Arrhenius plots in the absence and presence of different concentrations of (a) BFBT (b) TMBT and (c) FNBT.

Fig. 2. Alternative Arrhenius plots in the absence and presence of different concentrations of (a) BFBT (b) TMBT and (c) FNBT.

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and inhibition efficiency can be calculated using formulae (1) and (2). CR ¼

ΔW St

IE ð%Þ ¼

ð1Þ

ðC R Þa −ðC R Þp ðC R Þa

 100

ð2Þ

where ΔW is the weight loss, S is the surface area of the specimen (cm2), t is the immersion time (ℎ), and (CR)a, and (CR)p are corrosion rates in the absence and presence of the inhibitor, respectively. There is a gradual decrease in corrosion rate with the increase in concentration of the inhibitors. So, it is clear that the inhibition efficiency of the Schiff bases is concentration dependent. As the concentration increases, the availability of the number of molecules for blocking reaction sites increases. As triazole moieties possess enough available electrons (lone pair of electrons on nitrogen, NC_N-electrons, π electrons), they can make strong bonding with positively charged metal surface resulting in good inhibition efficiency. After optimisation, a series of concentrations from 0.8–3.2 mM was chosen to study the inhibition efficiency of three Schiff bases on MS corrosion. The maximum inhibition efficiency of 90.3%, 87.4% and 85.6% were attained at a concentration of 3.2 mM at 30 °C for BFBT, TMBT and FNBT, respectively. After that, even though the concentration was raised the inhibition efficiency remained almost constant. The inhibition efficiency of the three Schiff bases follows the order BFBT N TMBT N FNBT. A visual examination after the test showed that the samples which were immersed in the inhibitor solution almost retained its bright surface whereas the one exposed to plain acid didn't. This supports the fact that corrosion attack takes place easily all over the surface in an uninhibited solution whereas an inhibitor through adsorption on the metal surface suppresses the corrosion attacks. 3.1.2. Effect of temperature Temperature has a considerable influence on the rate of corrosion. Generally, in acidic medium (hydrogen depolarisation) the corrosion rate increases exponentially with temperature because the hydrogen evolution over potential decreases [45]. To examine the ability of three Schiff bases on steel surface at higher temperatures, weight loss experiments were carried out at different temperatures. Inhibition efficiency for all the three compounds is maximal at 30 °C and gradually decreases with increasing temperature. Results show that the studied inhibitors are more efficient at lower temperatures than at higher temperatures which is due to less physical interaction. At higher temperature, as the adsorption and desorption process occur with little time gap, the metal surface will be exposed to the acidic environment for a

Fig. 3. Langmuir isotherm for the adsorption of (a) BFBT (b) TMBT and (c) FNBT on MS in 0.5 M HCl at different temperatures.

Table 2 Activation parameters in the absence and presence of inhibitors at different temperatures. Inhibitor

BFBT

TMBT

FNBT

C (mM)

Ea* (kJ mol−1)

k (mg cm−2 h−1)

ΔHa* (kJ mol−1)

ΔHa* = Ea* − RT (kJ mol−1)

ΔSa* (J mol−1 K−1)

Blank 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2

32.1 43.15 44.68 49.23 55.53 42.31 44.78 46.33 51.47 42.71 41.52 45.70 50.45

186,465 3,259,225 5,050,511 23,277,552 196,662,575 2,716,894 6,230,705 9,597,373 53,704,105 3,506,048 1,963,030 8,008,388 39,586,551

29.45 40.51 42.03 46.59 52.88 30.48 39.66 42.14 43.69 40.08 38.89 43.06 47.81

29.49 40.55 42.07 46.63 52.93 39.70 42.18 43.73 48.87 40.11 38.92 43.1 47.85

−152.85 −129.06 −125.43 −112.71 −94.97 −130.58 −123.68 −120.09 −105.77 −128.41 −133.21 −121.59 −108.30

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longer period. This results in a decrease of inhibition efficiency at higher temperatures [46]. Corrosion rate is related to temperature by the following Arrhenius type equation:   Ea  : C R ¼ k exp − RT

ð3Þ

An alternate form of the Arrhenius equation can be written as CR ¼

    RT ΔSa  ΔHa  exp − exp R RT Nh

ð4Þ

where, Ea* is the activation energy, ΔSa* is the entropy of activation, ΔHa* is the enthalpy of activation, k is the Arrhenius pre‐exponential factor, h is Planck's constant, N is Avogadro's number, T is the absolute temperature and R is the universal gas constant. As corrosion is a thermodynamic process, calculation of activation and adsorption parameters helps us to understand the mechanism of inhibition in a better way. Using Eq. (3) a plot of ln CR versus 1/T was drawn to get a straight line (Fig. 1), from the values of slope and intercept, the values of Ea* and k were calculated for three inhibitors at various concentrations. Using Eq. (4), another linear plot of ln CR/T versus 1/T was drawn (Fig. 2) with slope (− ΔHa*/R) and intercept [ln(R/Nh) + ΔSa*/R] which was used for the calculation of ΔHa* and ΔSa*. All the values are listed in Table 2. Calculation of activation energy in the absence and presence of BFBT, TMBT and FNBT shows that the value of Ea* for blank is 32.1 kJ mol−1 is less compared to all inhibited solutions which increases up to 55.53 kJ mol−1. The higher value of Ea* means a reduction in the ease of corrosion reaction by the creation of a barrier. It is reported that if inhibition efficiency decreases with an increase in temperature, then Ea (inhibited solution) N Ea (uninhibited solution) [47,48]. The results obtained in the present study justify this statement. The increase in Ea* may be either because of a decrease in adsorption of the inhibitor on MS surface because of increase in temperature or due to physisorption (electrostatic) which occurs in the beginning [49]. A positive sign of ΔHa* reflects the endothermic nature of the mild steel dissolution process which implies that dissolution of MS is difficult [50]. The ΔHa* value for blank is less (29.4 kJ mol−1) compared to the three studied inhibitors (40.55–47.85 kJ mol−1). A large and negative value of ΔSa* indicates that the formation of activated complex is an associative step rather dissociative. The entropy of activation in total is the algebraic sum of the adsorption of organic molecules (solute) which results in a decrease in entropy, and desorption of water molecules (solvent) which results in an increase of entropy [51]. The adsorption of organic inhibitors in aqueous phase to the metal surface can be regarded as a quasi-substitution process between organic molecules in aqueous phase and water molecules on the

Fig. 4. Plot of ΔGads vs. T for BFBT, TMBT and FNBT.

metal surface [52,53]. On moving from reactant to activated complex state, activation entropy changes due to complex adsorption and desorption between Schiff bases and water molecules on the metal surface. 3.1.3. Adsorption isotherm Studying adsorption isotherm is the best way to understand the interaction between inhibitor molecules and metal surface. The type of adsorption can be either physisorption or chemisorption. The studied inhibitors affect corrosion rate mainly through the variation of degree of surface coverage, so inhibition efficiency becomes the function of the electrode surface covered by the inhibitor molecules [54]. The linear relation between degree of surface coverage θ (θ = IE (%) / 100) and concentration of the inhibitor is essential to obtain the adsorption isotherm. Attempts were made to fit the data to various isotherms like the Langmuir isotherm (θ / 1 − θ) = Kads C, the Temkin isotherm, exp(f, θ) = Kads C, the Frumkin isotherm, (θ / 1 − θ) exp(− 2 f. θ) = Kads C and the Freundlich isotherm, θ = Kads C. The best fit is found for the Langmuir adsorption isotherm. According to this isotherm, θ is related to C by

C

 θ

¼

1 þC Kads

ð5Þ

where Kads is the equilibrium constant for the adsorption process, θ is the degree of surface coverage and C is the concentration of the inhibitor. A plot of C/θ versus C was drawn to get straight line (Fig. 3) with a slope around 1 and regression co-efficient around 0.99. This confirms the uniform monolayer Langmuir kind of adsorption of the inhibitors with no interaction with the neighbouring sites.

Table 3 Thermodynamic parameters for adsorption of BFBT, TMBT and FNBT on mild steel in 0.5 M HCl at different temperatures from the Langmuir adsorption isotherm. Inhibitor

T (K)

R2

Kads (L mol−1)

ΔGads (kJ mol−1)

ΔSads (J mol−1 K−1)

ΔHads (kJ mol−1)

ΔGads = ΔHads − TΔSads (kJ mol−1)

BFBT

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

0.997 0.998 0.997 0.998 0.997 0.996 0.997 0.995 0.996 0.995 0.997 0.998

4221.1 4233.7 3968.2 3961.9 4042.0 3544.8 3554.9 3193.8 3276.5 3003.0 2879.3 2720.3

−31.14 −32.18 −33.03 −34.05 −31.03 −31.72 −32.74 −33.45 −30.50 −31.28 −32.17 −33.01

95.8

−2.142

82.8

−5.89

84

−5.023

−31.16 −31.12 −33.08 −34.05 −30.98 −31.81 −32.64 −33.46 −30.47 −31.31 −32.15 −32.99

TMBT

FNBT

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In Eq. (5), Kads can be calculated from an intercept (Table 3). Free energy of adsorption can be calculated from Kads using Eq. (6).

ΔGo ads ¼ −RT ln ð55:5 Kads Þ

ð6Þ

where R is the gas constant and T is the absolute temperature of the experiment and the constant value 55.5 is the concentration of water in solution in mol dm− 3. By using Eq. (6) ΔGoads value is calculated, which is negative and in the range of −30.5 to −34.05 kJ mol−1. A negative value of free energy indicates spontaneity of adsorption of inhibitor on the metal surface. If the magnitude of ΔGoads is around − 20 kJ mol−1 or less negative then the interaction is electrostatic or physical (which occurs between the inhibitor and charged metal surface). Those around −40 kJ mol−1 or more negative is chemisorption which occurs by a coordinate type of metal bond. The value obtained in the present study is above −20 kJ mol−1 and below −40 kJ mol−1, so the type of interaction is a mixture of physisorption and chemisorption which is a complex kind of interaction. Entropy of adsorption and enthalpy of adsorption can be calculated using the following thermodynamic equation: ΔGo ads ¼ ΔH o ads −TΔSo ads :

ð7Þ

On plotting ΔGoads versus T, a straight line with slope −ΔSoads and intercept ΔHoads is obtained (Fig. 4). The values of all thermodynamic parameters are listed in Table 3. The entropy of adsorption is positive for all the three inhibitors, indicating that solvent entropy predominates over solute entropy. Bentiss et al. reported that if ΔHoads N 0, then adsorption is chemisorption and if ΔHoads b 0, then it can be either physisorption or chemisorption. Further, in the exothermic process physisorption can be distinguished from chemisorption on the basis of the magnitude of ΔHoads [55]. For physisorption, the enthalpy of adsorption is usually less than 40 kJ mol−1 and for chemisorption it is greater than 100 kJ mol−1 [56]. Results show that the enthalpy of adsorption for all investigated Schiff bases is small and negative (− 2.14 to −5.023 kJ mol−1) which means that the adsorption is physisorption. 3.2. Potentiodynamic polarisation In order to have a better understanding about the role of the inhibitor in biasing anodic and cathodic reactions, a potentiodynamic polarisation study has been made. Anodic and cathodic Tafel curves in the absence and presence of different concentrations of BFBT, TMBT and FNBT are shown in Fig. 5. The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (ba, bc) and linear polarisation are listed in Table 4. The inhibition efficiency can be calculated from the following equation: IE ð%Þ ¼

io corr −icorr  100 io corr

ð8Þ

where i ocorr and icorr are the corrosion current densities in the absence and presence of the inhibitor, respectively. The corrosion current density for blank is 0.2 mA cm− 2 which is more than that for all three inhibitors (0.017–0.064 mA cm− 2). A reduction in icorr value with an increase in concentration for all three inhibitors shows that addition of inhibitor hinders corrosion attack. Even though both anodic and cathodic slopes are varied by adding the inhibitor, the shift in the anodic slope is slightly more than that compared to the cathodic shift. This means that even though kinetics of both metal dissolution and hydrogen evolution are altered by the addition of BFBT, TMBT and FNBT, the one which is going to be more affected is the reduction of iron. The value of bc and ba shows an irregular trend indicating that many mechanisms are taking part in corrosion inhibition

Fig. 5. Polarisation curves for MS in 0.5 M HCl containing different concentrations of (a) BFBT (b) TMBT and (c) FNBT.

being not only the adsorption effect [57]. The involvement of other species/anions present in the solution in the adsorption process also causes variation in ba and b c [58]. The variation may also be due to the lack of detectable Tafel region which can be understood by the shape of the Tafel curve. The inhibitive action takes place through the adsorption of Schiff bases thereby blocking the active reaction sites. There is a shift in equilibrium potential value on adding

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Table 4 Potentiodynamic polarisation parameters for the corrosion of MS in 0.5 M HCl in absence and presence of different concentrations of BFBT, TMBT and FNBT at 303 K. Inhibitor

BFBT

TMBT

FNBT

Concentration (mM)

Ecorr (mV)

icorr (mA cm−2)

ba (mV dec−1)

bc (mV dec−1)

Linear polarisation resistance (Ω cm2)

IE (%)

Blank 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2

−502 −457 −464 −489 −501 −467 −481 −491 −481 −461 −463 −479 −500

0.2 0.0456 0.0285 0.0226 0.0176 0.0602 0.0465 0.0349 0.0297 0.0641 0.0545 0.0379 0.0313

4.538 17.559 9.654 10.677 11.398 15.481 17.545 12.863 12.281 10.784 13.258 10.784 9.622

2.658 6.489 7.696 9.243 9.475 5.963 6.291 7.691 8.44 5.735 5.644 6.232 8.018

302 396.8 880.2 1002.7 1184.9 336.9 392.5 606.8 707.2 294.5 422.2 673.7 787.6

– 77.22 85.77 88.67 91.21 69.92 76.77 82.58 85.17 67.95 72.77 81.0 84.36

inhibitor and it is anodic. According to Ferreira et al. the displacement in Ecorr is more than ± 85 mV relating to the corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type. If the change in Ecorr is less than ± 85 mV, the corrosion inhibitor may be regarded as a mixed type [59]. In the present study, the change in Ecorr is less than ± 85 mV, so the studied inhibitors are neither anodic nor cathodic but of the mixed type. Linear polarisation resistance (LPR) for blank is 302 Ω cm2 which is less than that compared to LPR for all the studied inhibitors at all studied concentrations. LPR increases with concentration and reaches a maximum of 1182.9 Ω cm2 for TMBT at a concentration of 3.2 mM.

3.3. Electrochemical impedance spectroscopy The corrosion behaviour of MS in the acidic solution containing different concentrations of triazoles in 0.5 M hydrochloric acid was investigated by electrochemical impedance spectroscopy (EIS). Nyquist plots consisting of capacitive loops are presented in Fig. 6. A keen observation of these plots has revealed that capacitive loops are not perfect semicircles which are possibly due to the frequency dispersion, roughness, and inhomogeneity of the metal surface, impurities, grain boundaries, and distribution of surface active sites. Hence constant phase element (CPE) is introduced in the circuit to get a more accurate fit [60–62].

Fig. 6. Nyquist plots in the absence and presence of different concentrations of (a) BFBT (b) TMBT and (c) FNBT and equivalent circuit model.

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The existence of single semicircle can be attributed to the single charge transfer reaction during the metal dissolution process. For the description of a frequency independent phase shift between an applied ac potential and its current response, constant phase element (CPE) is used which is defined in impedance representation as in the following equation

interfacial structure [42]. At higher concentration, as the surface area occupied by Schiff bases is more the phase shift is also more. The tendency of the current which passes through capacitor can be attributed to an increment in the value of impedance with increasing concentration of inhibitors.

Z CPE ¼ Yo −1 ðiωÞ−1

3.4. FTIR spectrum analysis

ð9Þ

where Yo is the CPE constant, ω is the angular frequency (in rad s−1), i2 = − 1 is the imaginary number and n has the meaning of phase shift [63]. The value of n (varying between 0.7 and 0.95) represents deviation from the ideal behaviour (where n = 1) which is also the measure of inhomogeneity of the surface. The Nyquist plots were explained on the basis of equivalent circuit (Fig. 6) consisting of charge transfer resistance (Rct) connected in parallel to constant phase element (CPE) both in series with solution resistance (Rs). All the parameters are listed in Table 5. Inhibition efficiency is directly related to charge transfer resistance. The charge transfer resistance value (Rct) is calculated from the difference in real impedance at lower and higher frequencies. On increasing concentration of the inhibitor, charge transfer resistance increases as the inhibitor forms an adsorbed layer on metal surface. This also results in the increase in diameter of the semicircle. Inhibition efficiency can be calculated by Rct using the following formula: IE ð%Þ ¼

ðRct Þp −ðRct Þa ðRct Þp

 100

ð10Þ

where (Rct)a and (Rct)p are the charge transfer resistance in the absence and presence of inhibitor respectively. Rct for blank is 205 Ω cm2 which increases up to 1372 Ω cm2 for maximum concentration of the inhibitor BFBT. CPE constant Yo for blank (275.6 μ Ω− 1 sn) is more than Yo for the three studied inhibitors at all concentrations. A decrease in Yo after the addition of inhibitors may be either due to desorption of water molecules from the surface of MS followed by adsorption of inhibitor on the surface which results in a decrease in local dielectric constant or an increase in the thickness of the double layer because of adsorption of Schiff bases on the metal surface. The higher value of n for all three inhibitors compared to blank represents the decrease in inhomogeneity because of formation of a protective inhibitory film. Bode plots were recorded for the MS immersed in 0.5 M HCl in the absence and presence of the three inhibitors BFBT, TMBT and FNBT (Fig. 7). A phase angle shift with an increase in the concentration of inhibitors was observed. The shift may be due to formation of the protective film on the steel surface, which changed the electrode

Table 5 Impedance parameters for the corrosion of MS in 0.5 M HCl in the absence and presence of different concentrations of BFBT, TMBT and FNBT at 303 K. Inhibitor

BFBT

TMBT

FNBT

Concentration (mM)

Rct (Ω cm2)

Yo (μ Ω−1 sn)

Rs (Ω cm2)

n

IE (%)

Blank 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2 0.8 1.6 2.4 3.2

205.0 780.3 837.2 1106 1372 497.2 501.7 698.9 1030 545.6 614.0 726.1 754.6

275.6 75.53 44.41 20.15 19.32 94.57 84.87 76.19 69.64 78.29 102.4 65.66 95.37

2.471 2.411 3.645 1.205 1.194 7.513 7.522 4.645 3.120 6.606 1.151 1.245 2.856

0.7631 0.8036 0.8705 0.9207 0.9211 0.8513 0.8633 0.8430 0.8533 0.8457 0.8339 0.8637 0.8164

– 73.72 75.51 81.46 85.05 58.76 59.13 70.66 80.09 62.42 66.61 71.76 72.83

The FTIR spectra of all three inhibitors without and with adsorption on the mild steel are given in Figs. 1(a) and (b), Fig. 2(c) and (d) and Fig. 3(e) and (f) of the Supplementary data. For BFBT, the \\C _N\\ bond which is found at 1629 cm−1 has been broadened and moved to 1750 cm− 1, Ar C_C\\ which appears at 1484–1596 cm−1 has been shifted to 1505 cm−1 with decreased intensity, and \\C\\H stretching has been broadened and shifted to 3250 cm−1 from 3128 cm−1 after adsorption to the steel sample. For TMBT,\\C _N\\ which appears at 1626 cm− 1 has been broadened, N C\\H stretching which shows at 3091 cm−1 has been broadened and shifted to 3250 cm− 1 and Ar\\C_C b which appears at 1494–1510 cm−1 has been shifted to 1586–1610 cm− 1 after adsorption to the steel sample. For FNBT, \\C_N\\ which appears at 1629 cm− 1 has been broadened and shifted to 1655 cm−1, C\\H stretching which shows at 3037–3141 cm− 1 has been highly broadened and shifted to 3250 cm−1 and Ar\\C_C\\which appears at 1495–1578 cm−1 has disappeared after adsorption to the steel sample. The change in absorption frequency of the Fe absorbed inhibitor samples clearly shows the involvement of these bonds in the adsorption phenomenon. So inhibitors BFBT, TMBT and FNBT, by formation of adsorbed impervious films protect the metal surface from corrosion.

3.5. Mechanism of inhibition The inhibition efficiency of BFBT, TMBT and FNBT against the corrosion of MS in 0.5 M HCl can be explained on the basis of the number of adsorption sites, molecular size and mode of interaction with the metal surface. As we know, all three inhibitors contain three nitrogen atoms in the ring, an imide bond and pi-electrons which act as adsorption centres. So the inhibitor chemisorbs to the MS surface by co-ordinate bond through these electrons. The nitrogen atom can be easily protonated, the protonated species can be bound physically to the positively charged mild steel surface through the negatively charge chloride ion (Cl−). The bromine atom has a lone pair of electrons which can be donated to the d-orbital of Fe [4]. So the presence of bromine atom results in the highest inhibition efficiency of BFBT. The lowest inhibition efficiency is exhibited by FNBT because of the presence of the strongest deactivating group like the nitro group. The nitro group decreases the electron density on the benzene ring so the inhibition efficiency decreases [64]. Even though trifluoromethyl and fluoro are also electron withdrawing groups the degree of destabilisation is not so much like that of the nitro group.

3.6. Morphological investigation The SEM experiments were carried out in order to verify the adsorption of BFBT, TMBT and FNBT on the MS surface. The SEM micrographs obtained for the MS surface in the absence and presence of the optimum concentration (3.2 mM) of the inhibitors in 0.5 M HCl after 4 h of immersion time at 30 °C are shown in Fig. 8(a)–(e). The MS surface in the absence of inhibitor was highly corroded due to aggressive acid. The images taken in the presence of inhibitors show a smooth surface consisting of less pits. This surface property ensures a high degree of protection for the MS surface by inhibitors.

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Fig. 7. Bode plots in the absence and presence of different concentrations of (a) BFBT (b) TMBT and (c) FNBT.

3.7. Quantum chemical calculations Quantum chemical calculations have been proved to be a very powerful tool to study the corrosion inhibition mechanism [65–67]. Because it provides a new framework for the development of the relation

between electronic structure of the molecule and inhibition efficiency. In the last decade there has been an increase in the utilization of quantum chemical calculation methods in the area of corrosion science to obtain molecular properties that might be helpful in selecting potential molecules, among a list of molecules, as possible candidates for

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Fig. 8. SEM images of MS in 0.5 M HCl after 4 h immersion at 30 °C (a) before immersion (polished), (b) with 0.5 M HCl without inhibitor, (c) with 3.2 mM of BFBT, (d) with 3.2 mM of TMBT and (e) with 3.2 mM of FNBT.

corrosion inhibition. Most importantly, quantum chemical calculation methods are also extensively utilized to explain trends in experimental results, for cases where such information would be rarely available from experimental data [68]. The optimised geometrical configurations of BFBT, TMBT and FNBT are shown in Table 6. The total charge density and electrostatic potential map of all three inhibitors is given in Table 1 in the Supplementary data. It can be observed that highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO) are extensively distributed on either side of the three studied compounds indicating that the \\NH2 group of the triazole and other substituents act as the

main adsorption centres. The prominent molecular properties that are reported (Table 7) include the energy of the highest occupied molecular orbital (EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO), electronegativity, hardness and softness, electron affinity, ionisation potential and dipole moment. The adsorption of the inhibitor on the metal surface takes place on the basis of donor–acceptor interactions between lone pair of electrons on heteroatoms and vacant orbitals of Fe atom. The frontier orbitals HOMO and LUMO of a chemical species play a vital role in defining its absorption and reactivity. The higher the value of EHOMO1, the greater its tendency to donate electrons to the appropriate acceptor

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Table 6 Optimised geometrical configurations of BFBT, TMBT and FNBT. Quantum chemical structure

BFBT

TMBT

FNBT

Structure

Optimised geometry

HOMO

LUMO

molecules with low energy empty molecular orbitals and the higher its inhibition efficiency. In the same way, the lower the value of ELUMO the more probable the molecule would accept electrons and it will be an efficient inhibitor [69,70]. Among the three studied molecules, BFBT has the highest E HOMO of 12.66 eV followed by TMBT and FNBT, so it can easily donate electrons. Also it has the least value of E LUMO reflecting the higher ability to accept electrons so it is the most efficient inhibitor. The ΔE value for all inhibitors is found to be less and follows the order, BFBT b TMBT b FNBT. According to frontier molecular theory (FMO) of chemical reactivity, the formation of the transition state is due to the interaction between HOMO and LUMO of the reacting species. The smaller the orbital energy gap (ΔE) between the participating HOMO and LUMO, the stronger the interaction between two reacting species [71]. So, all the inhibitors can efficiently interact with the steel surface and the highest interaction is shown by BFBT. It is a well-established fact that a higher value of dipole moment will enhance the corrosion inhibition [72]. The dipole moment obtained is in the order BFBT N TMBT N FNBT. So BFBT can adhere strongly to the metal surface followed by TMBT and FNBT. According to HSAB theory, hard acids tend to react with hard acids and soft acids react with soft acids. As Fe is a soft acid it tends to react more with soft acids such as BFBT

and TMBT (low value of hardness and high value of softness) and thus these inhibitors exhibit better efficiency compared to FNBT. All the results obtained from the quantum chemical calculations excellently support the experimental results. 4. Conclusion (i) The studied Schiff bases BFBT, TMBT and FNBT showed very good inhibition efficiency in 0.5 N HCl and inhibition strength from all methods is in the order BFBT N TMBT N FNBT. (ii) Inhibition efficiency increases with the increase in concentration and decreases with the increase in temperature. (iii) Calculation of different thermodynamic parameters and study of adsorption isotherm shows that the adsorption is of the Langmuir type and physisorption. (iv) Polarisation studies show that the inhibitor affects both the anodic and cathodic reactions. (v) Impedance studies show that a higher value of charge transfer resistance in the presence of inhibitor results in higher inhibition efficiency. (vi) Morphological study confirms that inhibitors act by forming a protective layer on the surface of the mild steel.

Table 7 List of quantum chemical parameters for BFBT, TMBT and FNBT. Quantum chemical parameters −1

Total energy (kJ mol ) Electronic kinetic energy (kJ mol−1) Nuclear repulsion energy RMS gradient (kJ mol−1 A−1) Dipole (Debyes) EHOMO (eV) ELUMO (eV) ΔE = ELUMO − EHOMO (eV) Ionisation potential, I = −EHOMO Electron affinity, A = −ELUMO Electronegativity (χ) Hardness of the molecule (η) Softness (σ)

BFBT

TMBT

FNBT

−8,458,078 8,477,777 3,061,172 11.856 4.3356 12.3702 12.6671 0.2969 −12.3702 −12.6671 −12.5186 0.1484 6.7385

−2,359,805 2,358,695 2,915,702 17.053 4.1854 11.6661 12.4172 0.7511 −11.6661 −12.4172 −12.0416 0.3755 2.663

−2,272,347 2,271,088 2,888,237 18.30 3.6463 −9.960501 0.341651 −10.30215 9.9605 −0.3416 4.809 5.15105 0.194135

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(vii) Quantum chemical methods show very good agreement with the experimental results.

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