Experimental and Theoretical Studies on the Corrosion Inhibition Performance of Molecules Containing Tert-Butyl Benzyl Group on Mild Steel in Acid Media

Abstract

The inhibition by (4-tert-butyl-phenyl)-acetic acid hydrazide (TPAH), 5-(4-tert-butyl-benzyl)-[1,3,4] oxadiazole-2-thiol (TBOT) and 5-(4-tert-butyl-benzyl)-4H-[1,2,4] triazole-3-thiol (TBTT) of mild steel corrosion in 0.5 M HCl was investigated using gravimetric and electrochemical techniques. These inhibitors acted more effectively at higher concentration and at lower temperature; among these TBTT being the most efficient inhibitor, which showed highest efficiency of 92.6% at 303 K and 4.8 mM concentration. Adsorption of TPAH followed Freundlich isotherm, whereas TBOT and TBTT followed Langmuir isotherm. Energy of activation for corrosion increased after the addition of inhibitors. Free energy of adsorption showed that all the three inhibitors get adsorbed to the mild steel surface by both physical and chemical processes. EIS studies confirmed that all the inhibitors offered higher charge transfer resistance to the corrosion current and this led to decreased double-layer capacitance. Polarization studies showed that all inhibitors emerged as mixed type. Surface studies confirmed that the pits caused by corrosion were decreased by protective film of inhibitors. Corelation of experimental data with quantum chemical parameters like E_LUMO, energy gap, dipole moment, hardness and softness confirmed the superior performance of TBTT as compared to TPAH and TBOT.

1 Introduction

Designing of suitable inhibitors to corrosion of mild steel (MS) is getting higher priority because problems arising due to corrosion have greater impact on economy and safety. Retardation of corrosion attack on MS during industrial processes such as pickling, cleaning and descaling can be done through addition of small amount of inhibitors to the acid solution being used. By influencing kinetics of electrochemical reactions of corrosion process, inhibitors retard the metal corrosion. Although many kinds of inhibitors are used to control metal corrosion in acidic environment, organic inhibitors containing heteroatom such as O, N, S, and P are found to have particular use due to higher basicity and electron donating ability. Inhibitors which possess electron-rich functional groups (>C=N, –C=O, –N=N–, –NR₂, –OH, –RCH₃, –SH, –OCH₃, –C=C<) and aromatic electrons have higher tendency to adsorb onto the metal surface [1]. Many organic compounds containing electron-releasing groups like methoxy [2], butyl [3], butoxy [4], hydroxy [5] and amino [6] groups have been previously reported as corrosion inhibitors for mild steel. So, molecules containing tertiary butyl benzyl group can be effectively used as inhibitors for the corrosion of mild steel in acid medium.

The present investigation is aimed to find out the corrosion inhibition capability of hydrazide, oxadiazole and triazole moieties using experimental and quantum chemical methods. The motive in selecting these inhibitors is that they possess good number of nitrogen atoms, aromatic rings and electron donating tertiary methyl and thiol groups. It is clear from review of literature that all three selected compounds have extensive background in controlling metal corrosion. Some aromatic hydrazides like salicylic acid hydrazide and cinnamic acid hydrazide were tested as MS corrosion inhibitors in HCl media and very good efficiency of 93% was obtained [7]. N-phenyl oxalic dihydrazide was synthesized and tested as corrosion inhibitor for MS in acid media to get inhibition efficiency up to 79% [8]. The influence of three oxadiazole derivatives on the corrosion inhibition of steel in 2 M H₃PO₄ solution was studied to get maximum inhibition of 76% [9]. MS corrosion inhibition activity of triazole derivatives was investigated in 5% HCl to get excellent inhibition of 95% [10]. New pyridine based 1,3,4-oxadiazole derivatives were synthesized and established as MS corrosion inhibitors with maximum efficiency of 89% [11] Some synthesized triazole Schiff bases were checked for MS corrosion inhibition to get maximum inhibition efficiency of 90% in our previous study [12].

In continuation of our previous work, the present study reports the synthesis, characterization and corrosion inhibition ability of molecules containing hydrazide, oxadiazole and triazole moieties on MS in 0.5 M HCl using gravimetric and electrochemical techniques. Surface morphology was studied by scanning electron microscope (SEM). As charge transfer is one of the important criterion of corrosion reaction, quantum chemical parameters like energy of highest occupied molecular orbital (E_HOMO), energy of lowest unoccupied molecular orbital (E_LUMO), energy gap (ΔE), dipole moment (μ), electron affinity (A), hardness (η) and softness (σ) were calculated and correlated.

2 Experimental

2.1 Materials and Sample Preparation

MS specimens with composition (wt%): 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; Pb—0.001 and remaining being Fe was used. Dimension of the specimens used was 2 cm × 2 cm × 0.1 cm and 5 cm × 2 cm × 0.1 cm for gravimetric and electrochemical experiments, respectively. MS specimens used were abraded with SiC emery papers of different grades, degreased with acetone and dried with clean tissue paper. For EIS and polarization studies, the MS specimens were implanted in epoxy resin and a geometrical surface area of 1 cm² was exposed to the electrolyte. Experiments were conducted in 1.2–4.8 mM concentration range using 0.5 M HCl. Veego Melting Point VMP III apparatus was used to find the melting range of the inhibitors.

2.2 Synthesis of Inhibitors

Scheme for the synthesis of TPAH, TBOT and TBTT is outlined in Fig. 1.

Fig. 1

Scheme for the synthesis of inhibitors

2.2.1 Synthesis of (4-Tert-Butyl-Phenyl)-Acetic Acid Ethyl Ester (Compound 2)

Syntheses of compounds 2 and 3 were done as reported earlier [13]. A mixture of 10 mM (4-isopropyl-phenyl)-acetic acid (compound 1) and 0.2 mL of concentrated sulfuric acid in 20 mM ethanol was refluxed for 10 h. Completion of esterification reaction was checked by TLC. Reaction mixture (RM) was concentrated, diluted with water and extracted with ethyl acetate. Organic layer was washed with saturated sodium bicarbonate solution followed by brine, dried over sodium sulfate and evaporated to get compound 2 which was recrystallized and stored.

2.2.2 Synthesis of (4-Tert-Butyl-Phenyl)-Acetic Acid Hydrazide (Compound 3(TPAH))

Ten mM of (4-tert-butyl-phenyl)-acetic acid ethyl ester and 25 mM hydrazine hydrate were taken in ethanol and heated under reflux for 6 h. Completion of the reaction was checked by TLC; RM was concentrated, poured into 200 mL ice-cold water. Solid formed was collected by filtration and recrystallized from ethanol to get compound 3 in pure form.

2.2.3 Synthesis of 5-(4-Tert-Butyl-Benzyl)-[1,3,4] Oxadiazole-2-Thiol (Compound 4 (TBOT))

Compound 4 was synthesized as reported [14]. To 10 mM compound 3 solution in ethanol (20 mL), 10 mM potassium hydroxide in water (5 mL) and 30 mM carbon disulfide were added. The RM was heated under reflux till hydrogen sulfide evolution ceased (around 15 h). It was cooled, diluted with ice-cold water and acidified with 10% HCl. Solid separated out was filtered, washed and recrystallized from ethanol to get the pure form. Melting range of the product was 187–189 °C and yield was 90%.

2.2.4 Synthesis of 1-Tert-Butyl-Methyl-Benzyl Thiosemicarbazide (Compound 5)

Syntheses of compounds 5 and 6 were done as per reported literature [15]. Compound 5 was prepared by refluxing a suspension of compound 3 (10 mM) and potassium thiocyanate (20 mM) in HCl (5 mL) and water (100 mL) for 3 h. Completion of the reaction was checked by TLC. After cooling the mixture, white solid was separated. The solid separated was filtered, dried and recrystallized from ethanol to get pure compound.

2.2.5 Synthesis of 5-(4-Tert-Butyl-Benzyl)-4H-[1,2,4] Triazole-3-Thiol (Compound 6(TBTT))

Compound 5 (10 mM) was refluxed in 5% of 50 mL NaOH for 3 h. Completion of the reaction was checked by TLC. RM was neutralized with HCl to pH 5–6. Compound 6 was obtained as white solid which was filtered, dried and recrystallized from ethanol. Melting range of the product was 209–211 °C, and yield was 94%.

2.2.6 Spectral Data

(4-tert-butyl-phenyl)-acetic acid hydrazide (TPAH): IR (cm⁻¹) 1465–1517 (Ar C=C), 1634 (NH), 2960 {(CH₃)₃C}, 3313 (NH₂). ¹H NMR (400 MHz, DMSO-d₆) δ ppm: 1.72 (s, 9H, 3 × CH₃), 2.870 (d, 2H, NH₂), 4.14 (s, 2H, CH₂), 7.45 (m, 2H, ArH), 7.65 (m, 2H, ArH), 7.87 (s, 1H, NH). MS: 206 (M +), 207 (M + 1).

5-(4-tert-Butyl-benzyl)-[1,3,4] oxadiazole-2-thiol (TBOT): IR (cm⁻¹) 1516 (Ar C=C), 2963 (R₃C), 1175 (C=S) ¹H NMR (400 MHz, DMSO-d₆) δ ppm: 1.29 (s, 9H, 3 × CH₃), 2.17 (s, 1H, SH), 3.61 (s, 2H, CH₂), 7.22–7.20 (m, 2H, Ar–H), 7.36–7.34. (m, 2H, ArH). ¹³C-NMR (100 MHz, DMSO) δ ppm: 31.3, 34.4, 40.5, 125.5, 129.01, 130.2, 150.2, 177.9. MS: 249 (M + 1), 250.09 (M + 2).

5-(4-tert-Butyl-benzyl)-4H-[1,2,4] triazole-3-thiol (TBTT): IR (cm⁻¹) 1477 (Ar C=C), 2964 ((CH₃)₃C), 1196 (C=S) ¹H NMR (400 MHz, DMSO-d₆) δ ppm: 1.25 (s, 9H, 3 × CH₃), 3.30 (s, 1H, SH), 4.06 (s, 2H, CH₂), 7.22 (d, 2H, ArH, J = 7.04 Hz), 7.36 (d, 2H, ArH, J = 7.12 Hz), 14.38 (s, 1H, NH). MS: 248.10 (M + 1), 249.03 (M + 2).

2.3 Weight Loss Measurements

MS specimens were immersed in 0.5 M HCl without and with varying amounts of the inhibitors for 6 h in a thermostatically controlled water bath at constant temperature, under aerated condition (Weber limited, Chennai, India). The specimens were removed after 6 h of immersion, rinsed with water followed by drying with acetone. Weight loss of three specimens was determined by analytical balance (Sartorius, precision ± 0.1 mg), and the mean was calculated.

2.4 Electrochemical Measurements

Potentiodynamic polarization and EIS experiments were carried out using CHI660D electrochemical workstation. The traditional three-electrode cell consisting of silver/silver chloride reference electrode, a platinum auxiliary electrode and the working MS electrode with 1 cm² exposed areas was used. Specimens were treated in the way similar to gravimetric measurements. For polarization and impedance measurements, the MS specimens were inserted in epoxy resin to expose a geometrical surface area of 1 cm² to the electrolyte. Potentiodynamic polarization measurements were recorded by changing the electrode potential from − 200 to + 200 mV, related to the open-circuit potential, with the scanning rate of 10 mV s⁻¹. Prior to EIS measurements, open-circuit potential was stabilized till half an hour. EIS data were recorded using AC sinusoidal signal in the frequency range 0.1–1,00,000 Hz with amplitude 0.005 V. Simulation of EIS data was done using Zsimpwin software.

2.5 Quantum Chemical Calculations

The geometrical optimization of studied compounds was done by Ab initio method at 6–31G** basis set for all atoms. For energy minimization, 1.0 convergence limit and 1.0 k cal/A mol rms gradient were kept. The Polak–Ribiere conjugate gradient algorithm which is very fast and precise was used for geometrical optimization. The HYPERCHEM 7.52 (Hypercube Inc., Florida, USA, 2003) professional software was employed for all calculations.

2.6 Scanning Electron Microscopy (SEM)

The SEM images were taken using Zeiss electron microscope with the working voltage of 15 kV, and the working distance was 10 mm. In SEM micrographs, the specimens were exposed to 0.5 M HCl in the absence and presence of three inhibitors at optimum condition after 6 h of immersion. The SEM images were taken for polished MS specimen and specimen immersed in acid solution with and without inhibitors.

3 Results and Discussion

3.1 Weight Loss Method

3.1.1 Effect of Concentration

Weight loss experiments were conducted from 303 to 333 K for 6 h at different concentrations to optimize the concentration and temperature. The results obtained are presented in Table 1. Using loss in weight of MS specimens during 6 h of immersion, corrosion rate (C_R) was calculated from formula (1) and by this inhibition efficiency [IE (%)] was determined using the formula (2)

$$C_{\text{R}} = \frac{{\Delta W}}{St}$$

(1)

$${\text{IE}}\,\left( \% \right) = \frac{{\left( {C_{\text{R}} } \right)_{\text{a}} - \left( {C_{\text{R}} } \right)_{\text{p}} }}{{\left( {C_{\text{R}} } \right)_{\text{a}} }} \times 100$$

(2)

where ΔW is the weight loss, is the surface area of the specimen (cm²), is the immersion time (ℎ), and (C_R)_a, (C_R)_p are corrosion rates in the absence and presence of the inhibitor, respectively. Increase in concentration of TPAH, TBOT and TBTT led to increase in IE (%) at all temperatures. This may be due to increase in the amount of adsorbent and as also the surface area of MS due to availability of more number of molecules. Highest inhibition efficiencies of 92.6, 91.7 and 80% were recorded by TBTT, TBOT and TPAH, respectively, at 4.8 mM concentration. Electron-releasing tert-butyl group in the inhibitors increases electron density on the benzene ring thus facilitate strong binding of inhibitors to the metal surface and play pivotal role in enhancing inhibition performance of studied compounds. The presence of highly interactive nitrogen atoms, oxygen atoms and aromatic electrons in the inhibitors allowed strong adsorption on empty d-orbitals of Fe surface through available electrons. The higher inhibition efficiency of TBTT compared to TBOT is due to better electron donating capacity due to the presence of nitrogen instead of oxygen atom.

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 temperature

3.1.2 Activation and Thermodynamic Parameters

The influence of temperature on the inhibition characteristics was determined by conducting weight loss measurements at temperatures varying from 303 to 333 K in the absence and presence of different concentrations of TPAH, TBOT and TBTT. IE (%) of all inhibitors decreased on increasing solution temperature. The increase in C_R with temperature may be related to two factors: (1) at elevated temperatures, as the inhibitor molecules gain sufficient energy, they quickly get desorbed from MS surface resulting in shortening of time gap between adsorption and desorption (drawing of equilibrium toward desorption) leading to longer exposure of MS surface to acidic environment. (2) Decrease in hydrogen evolution over potential resulting in acceleration of cathodic reduction.

Corrosion rate of MS and temperature of the environment can be connected by Arrhenius Eq. (3)

$$C_{\text{R}} = k\exp \left( { - \frac{{E_{\text{a}}^{*} }}{RT}} \right).$$

(3)

Transition state theory gives alternative form of Arrhenius equation using which enthalpy of activation and entropy of activation can be calculated

$$C_{\text{R}} = \frac{RT}{Nh}\exp \left( {\frac{{\Delta S_{\text{a}}^{*} }}{R}} \right)\exp \left( {\frac{{ -\Delta H_{\text{a}}^{*} }}{RT}} \right)$$

(4)

where E_a* is the energy of activation, ∆S_a* is the entropy of activation, ∆H_a* is the enthalpy of activation, k is 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. E_a* for investigated compounds is calculated using the slope of linear regression between ln C_R and 1/T (Fig. 2). ∆S_a* and ∆H_a* are calculated using slope and intercept of plot of ln (C_R/T) versus 1/T, respectively (Fig. 3). Different activation parameters are listed in Table 2. Regression coefficients for TPAH, TBOT and TBTT are almost 1, thereby indicating that MS corrosion in hydrochloric acid can be elucidated using kinetic model. It is noticeable that activation energy for blank (32.1 kJ mol⁻¹) increases after the addition of inhibitors and shows increment on increasing concentration of all the three inhibitors. This indicates that as the concentration of the inhibitor in the solution becomes higher, corrosion reaction gets pushed forced to the surface sights which have got higher values of E_a* [16]. Increase in E_a* with increase in concentration indicates that the surface coverage is almost approaching saturation. Highest activation energy values of 51.1, 57.6, 54.3 kJ mol⁻¹ were obtained for TPAH, TBOT and TBTT, respectively, at 4.8 mM. As E_a* for the three inhibitors is below threshold value of 80 kJ mol⁻¹ as required for chemisorption; however, physical adsorption is presently applicable [17]. But the type of adsorption obtained from the change of activation energy is not decisive due to competitive adsorption with water molecules; the adsorption as witnessed in the present study is a combination of physisorption and chemisorption with slight predominance of physisorption [18]. The positive values of ∆H_a* proves the endothermic nature of MS dissolution or the dissolution of MS is difficult [19]. Addition of TPAH, TBOT and TBTT increased the ∆H_a* from 29.5 to 48.4, 55 and 51.7 kJ mol⁻¹, respectively, indicating the enhanced difficulty for MS dissolution. Large and negative values of ΔS_a* for TPAH, TBOT, TBTT indicates that the formation of activated complex is an associative step or the formed activated complex is more ordered. This implies that adsorption of inhibitors in ordered fashion during the dissolution of steel. Variation of ∆H_a* and E_a* with concentration of inhibitors is shown Fig. 4, from which it is clear that ∆H_a* and E_a* vary similarly with the increase in concentration of all three inhibitors. There is gradual increase in the values of both ∆H_a* and E_a* with increase in concentration; therefore, reduction in MS corrosion is mainly decided by kinetic parameters of activation, and pre-exponential factor has limited significance [20].

Fig. 2

Arrhenius plots in the absence and presence of different concentrations of a TPAH, b TBOT, c TBTT

Fig. 3

Alternative Arrhenius plots in the absence and presence of different concentrations of a TPAH, b TBOT, c TBTT

Table 2 Activation parameters in the absence and presence of inhibitors at different concentrations

Fig. 4

Variation of activation parameters with concentration

3.1.3 Adsorption Isotherm

Primary step in the action of inhibitors in acidic medium is the adsorption onto the metal surface which involves the assumption that the corrosion reactions are prevented from occurring over the area of the metal surface covered by adsorbed inhibitor species [21, 22]. The most appropriate method to find the relation between surface coverage of an interface with the adsorbed species and concentration of an inhibitor in the solution is the study of adsorption isotherm where values of surface coverage (θ) and concentrations (C) are fitted to equations of various isotherms. In the present study, adsorption data obtained were fitted into various isotherms like Langmuir, Freundlich and Temkin isotherms. Among three studied inhibitors, TPAH fitted well in Freundlich isotherm (R² = 0.9437, 0.9972, 0.9439 for Langmuir, Freundlich and Temkin, respectively), TBOT (R² = 0.9989, 0.985, 0.9356 for Langmuir, Freundlich and Temkin, respectively) and TBTT (R² = 0.9993, 0.9864 and 0.958 for Langmuir, Freundlich and Temkin, respectively) followed Langmuir isotherm. For TPAH, according to Freundlich isotherm θ is related to C as shown below

$$\theta = K_{\text{ads}} C$$

(5)

where K_ads is the equilibrium constant for adsorption process. A plot of θ versus C is drawn (Fig. 5a) whose slope gave the value of K_ads. The value of R² is almost unity and intercept is around unity indicating the strong adherence of adsorption data of TPAH to Freundlich isotherm. For TBOT and TBTT, Langmuir isotherm was used to calculate adsorption parameters. According to this isotherm,

$${\raise0.7ex\hbox{$C$} \!\mathord{\left/ {\vphantom {C \theta }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\theta $}} = \frac{1}{{K_{\text{ads}} }} + C$$

(6)

Fig. 5

Adsorption isotherm for three inhibitors, a Freundlich isotherm for TPAH, b Langmuir isotherm for TBOT, c Langmuir isotherm for TBTT

Here, K_ads was calculated from intercept of ${\raise0.7ex\hbox{$C$} \!\mathord{\left/ {\vphantom {C \theta }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\theta $}}$ versus C plot (Fig. 5b, c). Free energy of adsorption was calculated from K_ads using Eq. (7)

$$\Delta G_{\text{ads}}^{o} = - RT\ln \left( {55.5 K_{\text{ads}} } \right)$$

(7)

where R is 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. The calculated thermodynamic parameters are listed in Table 3. Free energy of adsorption for all the three inhibitors varied from − 21.9 to − 23.3 kJ mol⁻¹ for TPAH, − 31.6 to − 34.6 kJ mol⁻¹ for TBOT and − 32.0 to − 34.3 kJ mol⁻¹ for TBTT. Negative value of $\Delta G_{\text{ads}}^{o}$ for the three inhibitors depicts spontaneity of the adsorption process and also the formation of stable adsorbed layer on the metal surface. It is well known that the values of $\Delta G_{\text{ads}}^{o}$ 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 have higher value than − 40 kJ mol⁻¹ imply charge sharing or charge transfer involving coordinate bond from the inhibitor molecules to the metal surface (chemisorption) [23]. TPAH, TBOT and TBTT possess nitrogen which can undergo protonation. So, physical adsorption of charged protonated species on MS surface can takes place through chloride ions. On the other hand, chemisorption takes place independently with the help of lone pair of electrons on nitrogen atoms and aromatic electrons in the three inhibitors, whereas the presence of thiol group facilitates stronger adsorption in TBOT and TBTT. The adsorption of studied inhibitors cannot solely take place through physical or chemical means. It is a mixture involving both physisorption and chemisorption, a complex comprehensive phenomenon. As K_ads value indicates the strength between inhibitor and MS surface, higher values of K_ads obtained for TBOT and TBTT show their better adsorption capacity compared to TPAH.

Table 3 Thermodynamic parameters for adsorption of TPAH, TBOT and TBTT on mild steel in 0.5 M HCl at different temperatures

Entropy of adsorption and enthalpy of adsorption process were calculated using the following thermodynamic equation:

$$\Delta G_{\text{ads}}^{o} =\Delta H_{\text{ads}}^{o} - T\Delta S_{\text{ads}}^{o}$$

(8)

The plot of $\Delta G_{\text{ads}}^{o}$ versus temperature (Fig. 6) gave straight line whose slope is $-\Delta S_{\text{ads}}^{o}$ and intercept is $\Delta H_{\text{ads}}^{o}$. The negative sign of adsorption enthalpy for the three inhibitors indicates the exothermic nature of the steel dissolution process, so IE (%) decreases with temperature. According to Bentiss et al. [24], endothermic processes occur through physisorption and exothermic processes may be through physisorption or chemisorption depending upon the magnitude of $\Delta H_{\text{ads}}^{o}$. It is reported that enthalpy of physisorption process is lower than 41.86 kJ mol⁻¹ but the enthalpy of chemisorption process approaches 100 kJ mol⁻¹ [25]. In the present study, none of the inhibitors showed $\Delta H_{\text{ads}}^{o}$ near 100 kJ mol⁻¹, so all the inhibitors have higher tendency to undergo physisorption. Change in $\Delta S_{\text{ads}}^{o}$ is the summation of solvent and solute entropy, and desorption of water results in higher solute entropy, whereas adsorption of inhibitors results in lower solvent entropy. $\Delta S_{\text{ads}}^{o}$ obtained for TPAH, TBOT and TBTT are positive indicating predominance of solvent entropy. It is proved that for the adsorption of inhibitors, increase in entropy is the driving force.

Fig. 6

Plot of ΔG_ads versus T for the inhibitors

3.2 Electrochemical Impedance Spectroscopy (EIS)

EIS is a potent, nondestructive technique which characterizes corrosion phenomenon and protective nature of pre-treatments and organic coatings over time. In EIS, sinusoidal potential excitation is applied and current signal is measured through cell and analyzed as a sum of sinusoidal functions (a Fourier series). EIS was performed to study the corrosion behavior of MS in 0.5 M HCl at 303 K in the inhibitors, and results are expressed as Nyquist and Bode plots. Calculated impedance parameters are listed in Table 4. Nyquist plots (Figs. 7a, 8a, 9a) are characterized by single semicircles (corresponds to one time constant in Bode plots) whose diameter increases with increase in concentration of the inhibitors over the frequency range studied. The inhibitor system behaves like monolayer formation, and the dissolution process is controlled by the charge transfer reaction [26].

Table 4 Impedance parameters for the corrosion of MS in 0.5 M HCl in the absence and presence of different concentrations of TPAH, TBOT and TBTT at 303 K

Fig. 7

a Nyquist plot, b bode modulus plot, c phase angle plot in the absence and presence of different concentrations of TPAH

Fig. 8

a Nyquist plot, b bode modulus plot, c phase angle plot in the absence and presence of different concentrations of for TBOT

Fig. 9

a Nyquist plot, b bode modulus plot, c phase angle plot in the absence and presence of different concentrations of for TBTT

The depressed form of the resulting semicircle for solid electrodes is known as frequency dispersion which can be ascribed to such physical phenomena such as surface roughness, active sites, and inhomogeneity of solids [27]. Depending on the shape of complex plane impedance, data are fitted to simplest equivalent circuit (Fig. 10) consisting of polarization resistance (R_p) and constant phase element (CPE) in parallel and both in series with solution resistance (R_s). Corrosion being an electrochemical processes, corrosion also has two phases: (1) oxidation of metal (charge transfer process) and (2) diffusion of metal ions from metal surface to the solution [28]. Warburg impedance which accounts for diffusion impedance becomes unimportant because of the presence of single semicircle. Impedance curves retained their shape after the addition of TPAH, TBOT and TBTT indicating that the mechanism of inhibition is not affected. IE (%) was calculated from polarization resistance using the following formula:

$${\text{IE}}\,\left( \% \right) = \frac{{\left( {R_{\text{p}} } \right)_{\text{p}} - \left( {R_{\text{p}} } \right)_{\text{a}} }}{{\left( {R_{\text{p}} } \right)_{\text{p}} }} \times 100$$

(9)

where (R_p)_a and (R_p)_p are the charge transfer resistance in the absence and presence of inhibitors, respectively. Polarization resistance increases from 199 Ω cm² (blank) after the addition of inhibitors and reaches maximum R_p of 877, 1589, 1613 Ω cm² at 4.8 mM for TPAH, TBOT and TBTT, respectively. This indicates the protection offered by inhibitors through barrier film MS which prevent charge and mass transfer. As there is increment in R_p, it is likely that the current has passed through the capacitor in the circuit [29]. The impedance function of a CPE can be defined as

$$Z_{\text{CPE}} = Y_{o}^{ - 1} (i\omega )^{ - 1}$$

(10)

where Y _o is magnitude of CPE, ω is angular frequency (in rad s⁻¹), i² = − 1 is the imaginary number, n = α/(π/2) in which α is the phase angle of CPE. The value of n which signifies the interphase parameter increases after the addition of the inhibitors. In case of TBOT and TBTT n increases gradually with increase in concentration but in TPAH even though n value is higher compared to blank, uniformity is not observed. This indicates some complexity in the adsorption of TPAH on MS electrode. The value of n is lower (0.728) in 0.5 HCl due to irregularities and arbitrary distribution of current on the electrode surface. As the adsorption of inhibitors on the electrode surface gets more uniform n value increases. When the concentration of inhibitors increases the capacitive behavior increases as the ideal capacitor value of n is 1. Double-layer capacitance (C_dl) was calculated using following equation:

$$C_{\text{dl}} = \left( {Y_{o} R_{\text{ct}}^{1 - n} } \right)^{1/n}$$

(11)

Fig. 10

Equivalent circuit model

C_dl value decreased after the addition of any of the inhibitors. The decrease in C_dl with increase in concentration of inhibitors is due to replacement of H₂O molecules by the inhibitors resulting in the build-up of protective layer until it reaches saturation. The trend in decrease in C_dl for TPAH and TBTT is regular nature. TBOT exhibited irregularity supporting complex adsorption–desorption process. Many reported inhibitors exhibited similar behavior [30].

From the single peak obtained in Bode plots for TPAH, TBOT and TBTT (7b, c, 8b, c, 9b, c), the electrochemical impedance measurements fit into one time constant equivalent model. There is shift in phase maximum after the addition of inhibitors varied between 60° and 70° for the three inhibitors but the ideal capacitor will have phase maximum of 90°.

3.3 Potentiodynamic Polarization

Potentiodynamic polarization curves of MS in 0.5 M HCl containing various concentrations of TPAH, TBOT and TBTT are shown in Fig. 11. Tafel curves show that the inhibitors have considerable effect on anodic and cathodic reactions causing decrease in C_R. As anodic and cathodic domains possess linear Tafel region, the linear segments of anodic and cathodic curves were extrapolated to corrosion potential to get corrosion current densities. Electrochemical parameters, such as corrosion potential (E_corr), corrosion current density (i_corr), anodic Tafel slope (b_a), cathodic Tafel slope (b_c), are listed in Table 5.

Fig. 11

Tafel plots for MS in 0.5 M HCl containing different concentration of a TPAH, b TBOT, c TBTT

Table 5 Potentiodynamic polarization parameters for the corrosion of MS in 0.5 M HCl in the absence and presence of different concentrations of TPAH, TBOT and TBTT at 303 K

Inhibition efficiency (IE %) can be calculated from the corrosion current density (i_corr) by the following equation:

$${\text{IE}}\, \left( \% \right) = \frac{{i_{\text{corr}}^{o} - i_{\text{corr}} }}{{i_{\text{corr}}^{o} }} \times 100$$

(12)

where i ^o_corr and i_corr are uninhibited and inhibited corrosion current densities, respectively. There is a decrease in i_corr after the addition of TPAH, TBOT and TBTT, and the IE (%) is more pronounced with the increase in concentration of inhibitors. Although TBOT and TBTT exhibited good inhibition but, the highest IE (%) of 93% was exhibited by TBTT. Higher efficiency shown by TBTT as compared to TBOT is due to the presence of nitrogen atom. This is supported by general trend in the inhibition efficiencies of molecules containing heteroatoms O < N < S < P as reported by many authors [31, 32]. Shift in the values of b_a and b_c for TPAH, TBOT and TBTT indicates that inhibitors affect both cathodic and anodic reactions. The irregular trend shown in Tafel slopes may be due to many mechanisms taking part in corrosion inhibition [33]. According to Ferreira et al., if the displacement in E_corr is more than 85 mV relating to the corrosion potential of the blank, then the inhibitor is considered as cathodic or anodic type. If the change in E_corr is less than 85 mV, then the corrosion inhibitor is regarded as mixed type [34]. The shift in E_corr is positive, and the maximum shift for TPAH, TBOT and TBTT is 56, 61 and 71 mV, respectively. So, all the three inhibitors are of mixed type. Since the shift in b_a is more as compared to b_c and E_corr shows anodic shift, metallic dissolution is more affected than hydrogen evolution.

3.4 Quantum Chemical Calculations

The results obtained by gravimetric and electrochemical investigations can be effectively justified through quantum chemical calculations because it is a quantitative study of relationship between IE (%) and molecular reactivity. Parameters like energy of highest occupied molecular orbital (E_HOMO), energy of lowest unoccupied molecular orbital (E_LUMO), energy gap (∆E), hardness (η), softness (σ) and electron affinity (A), that directly influence the electronic interaction of inhibitors with MS surface have been studied and they are listed in Table 7. Optimized geometrical configurations of TPAH, TBOT and TBTT are shown in Table 6. Electron densities of all the inhibitors are distributed throughout the molecules but concentrated maximum on aromatic electrons of benzene ring, triazole ring of TBTT and oxadiazole ring of TBOT indicating that these are the favorite sites for adsorption. The reactive ability of an inhibitor is closely related greater its frontier molecular orbital (FMO). Molecules with higher values of E_HOMO often show greater tendency for the donation of electron, enhancing the inhibitor adsorption on MS thereby giving better IE (%), whereas molecules with lower E_LUMO have better ability to accept electrons [35]. Also molecules with low energy gap are more polarizable and are associated with high chemical reactivity and low kinetic stability. Hence, ΔE is frequently used to characterize the binding ability of inhibitors on to the metal surface [36]. In the present study, E_LUMO and ΔE are more appropriate parameters to interpret IE (%). E_LUMO of three inhibitors varies in the order TPAH > TBOT > TBTT, and IE (%) varies in the reverse order. As TBTT has better capacity to accept electrons which helps in back donation, it is considered as the best inhibitor. ΔE of TBTT is lower than the other two inhibitors; therefore, it has better adherence capacity to the MS surface acts as the best inhibitor. Many researchers considered the negative sign of E_HOMO as physisorption [37, 38]. As all three inhibitors have negative E_HOMO, they may undergo physisorption more predominantly. Dipole moment is the resultant of non-uniform distribution of charges in atoms of inhibitor molecules. With the increase in dipole moment, the energy of deformability also increases so molecule easily gets adsorbed at the Fe surface [39]. In the present study, dipole moment follows the order TBTT > TBOT > TPAH, and IE (%) also follows the same order.

Table 6 Optimized geometrical configurations of TPAH, TBOT and TBTT

Chemical hardness basically defines the resistance toward deformation or polarization of the electron cloud of the atoms, ions or molecules under small perturbation of chemical reaction, so hard molecules have greater ΔE and a soft molecules have lower ΔE [40]. For simplest electron transfer, adsorption occurs at that part of the molecule where softness is of highest value [41]. In the present study, TBTT has lowest hardness value and highest value of softness value so it is the best among the three studied inhibitors. It is well known that molecules with high electron affinity are highly reactive. In the present study, electron affinity shows the order TBTT > TBOT > TPAH; therefore, IE (%) follows the same order.

3.5 Scanning Electron Microscope

The micrographs of MS surface before and after immersion in 0.5 M HCl without and with addition of 4.8 mM of TPAH, TBOT and TBTT for 6 h at 303 K are shown in Fig. 12a–e. Specimens immersed in plain acid are having highly irregular surface with pits. Due to adsorption of inhibitors, the steel surface became smooth and undamaged (Fig. 12c–e), so surface properties are enhanced.

Fig. 12

SEM images of MS surface, a polished, b immersed in 0.5 M HCl, c immersed in 0.5 M HCl in the presence 4.8 mM of TPAH, d immersed in 0.5 M HCl in the presence 4.8 mM of TBOT, e immersed in 0.5 M HCl in the presence 4.8 mM of TBTT

3.6 Mechanism of Inhibition

Inhibition mechanism can be proposed on the basis of mode of adsorption of TPAH, TBOT and TBTT on the steel surface. In acidic medium, both carbonyl group and nitrogen atom of TPAH can undergo protonation. From electrostatic potential map (Table 7), as more negative charge is accumulated near oxygen atom (indicated by red line) protonation of carbonyl group is more favorable. The protonation of nitrogen in TBTT and oxygen in TBOT can occur but not easy. After protonation, adsorption of positively charged inhibitors occurs on the steel surface through negatively charged chloride ions. The protonated form of inhibitors starts competing with H⁺ for electrons on the steel surface. But, after the evolution of hydrogen gas, inhibitors come back to neutral form and prefer to get adsorbed to the MS surface through chemisorption. Now, adsorption takes place through lone pair of electrons on nitrogen, sulfur and oxygen along with aromatic electrons through coordinate bond to the d-orbitals of Fe. All the three inhibitors contain electron-rich tert-butyl group, whereas TBOT and TBTT contain thiol group which increases the electron density on the aromatic system. Accumulation of excess negative charge on Fe may also result in the reterodonation of electrons to Π* antibonding orbitals of inhibitor molecules [42]. TBOT and TBTT show better performance than TPAH due to the presence of heterocyclic ring and electron-rich thiol group. TBTT shows slightly better performance compared to TBOT because of the presence of nitrogen atom in the additional ring in the place of oxygen.

Table 7 List of quantum chemical parameters for TPAH, TBOT and TBTT

4 Conclusion

Molecules containing tert-butyl benzyl groups showed good inhibition effect on MS in acid media. The order of inhibition effect is TBTT < TBOT < TPAH. The highest IE (%) of the inhibitors at optimum concentration is 80.1, 91.7 and 92.6 for TPAH, TBOT and TBTT, respectively. From the thermodynamic and kinetic parameters obtained from the studies, the reaction between the MS and the corrosion inhibitors was spontaneous since the Gibbs free energy of adsorption values was negative. Inhibitors adsorb on the MS surface by the combination of physical and chemical adsorption with slight predominance of physisorption. Adsorption of TPAH followed Freundlich isotherm, whereas TBOT and TBTT followed Langmuir isotherm. Addition of inhibitors increased polarization resistance which and decreased double-layer capacitance and corrosion current that shows decrease in corrosion rate. Quantum chemical data revealed that inhibitors possess sufficient electron density to have interaction with metal surface. Quantum chemical parameters like E_LUMO, energy gap, dipole moment and hardness confirmed the superior performance of TBTT as compared to TPAH and TBOT. As all the three methods confirmed the effective binding of studied inhibitors, they can be effectively used to prevent corrosion.