Inhibition effect of tetradecylpyridinium bromide on the corrosion of cold rolled steel in 7.0 M H₃PO₄

2.1 Materials

Tests were performed on a cold rolled steel (CRS) of the following composition (wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and bal. Fe. Tetradecylpyridinium bromide (TDPB, purity quality: ⩾98%) was obtained from Shanghai Chemical Reagent Company of China. Fig. 1 shows the molecular structure of TDPB. The aggressive solution of 7.0 M H₃PO₄ was prepared by dilution of analytical grade 85% H₃PO₄ with distilled water. The concentration range of TDPB used is 0.1–1.0 mM.

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Figure 1

Chemical molecular structure of tetradecylpyridinium bromide (TDPB).

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2.2 Gravimetric measurements

Cold rolled steel (CRS) sheets of dimension 2.5 cm × 2.0 cm × 0.06 cm were abraded with emery paper (grade 320–500–800) and then washed with distilled water and acetone. After weighing accurately, the specimens were immersed in 250 ml beaker, which contained 250 ml phosphoric acid with and without addition of different concentrations of TDPB. All the aggressive acid solutions were open to air. After 6 h, the specimens were taken out, washed, dried, and weighed accurately. Experiments were carried out in triplicate. The average weight loss of three parallel CRS sheets could be obtained. Then the tests were repeated at different temperatures. The value of corrosion rate (v) was calculated from the following equation (Li et al., 2009):

2.3 Electrochemical measurements

Electrochemical experiments were carried out in the conventional three-electrode cell with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. To minimize ohmic contribution, the Luggin capillary was close to working electrode (WE) which was in the form of a square CRS embedded in a PVC holder using epoxy resin so that the flat surface was the only surface in the electrode. The working surface area was 1.0 cm × 1.0 cm, abraded with emery paper (grade 320–500–800) on test face, rinsed with distilled water, degreased with acetone, and dried with a cold air stream. Before measurement the electrode was immersed in test solution at open circuit potential (E_OCP) for 2 h to guarantee that a steady state was reached. All electrochemical measurements were carried out using the PARSTAT 2273 advanced electrochemical system (Princeton Applied Research). Each experiment was repeated at least three times to check reproducibility.

The potentiodynamic polarization curves were carried out by polarizing to ±250 mV with respect to the free corrosion potential (E_corr vs. SCE) at a scan rate of 0.5 mV/s. Inhibition efficiency (η_p%) is defined as follows:

Electrochemical impedance spectroscopy (EIS) was carried out at E_OCP in the frequency range of 0.1 Hz–100 kHz using a 10 mV RMS voltage excitation. Inhibition efficiency (η_R%) is calculated by the following equation:

2.4 Scanning electron microscopy (SEM)

Samples of dimension 2.5 cm × 2.0 cm × 0.06 cm were prepared as described above (Section 2.2). After immersion in 7.0 M H₃PO₄ without and with 1.0 mM TDPB at 20 °C for 6 h, the specimens were thoroughly washed with distilled water, and dried with a cold air blaster. SEM experiments were performed with a Japan instrument model S-3000 N scanning electron microscope (Hitachi High-Tech Science Systems Corporation).

2.5 Quantum chemical calculations

Quantum chemical calculations were performed with DMol³ numerical based density function theory (DFT) in Accelrys Materials Studio (V4.1) software (Accelrys Inc., 2006). Geometrical optimizations and frequency calculations were carried out with the generalized gradient approximation (GGA) functional of Becke exchange plus Lee–Yang–Parr correlation (BLYP) (Becke, 1998) in conjunction with double numerical plus d-function (DND) basis set (Lee et al., 1988). Fine convergence criteria and global orbital cutoffs were employed on basis set definitions. Considering the solvent effects, all the geometries were re-optimized at the BLYP/DND level by using COSMO (conductor-like screening model) (Klamt and Schüürmann, 1993) and defining water as the solvent. The optimized molecular structure was confirmed to have no imaginary frequencies.

3.1 Gravimetric measurements

3.1.1

3.1.1 Effect of TDPB on corrosion rate

Fig. 2 shows the corrosion rate values of CRS in the presence of different concentrations of TDPB in 7.0 M H₃PO₄ at various temperatures. The corrosion rate values (g m⁻² h⁻¹) decrease as the TDPB concentration increases, i.e. corrosion inhibition enhances with the inhibitor concentration. This behavior is due to the fact that the adsorption amount and the coverage of inhibitor on CRS surface increase with the inhibitor concentration (Zhao and Mu, 1999).

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Figure 2

Relationship between corrosion rate (v) and concentration of TDPB (c) in 7.0 M H₃PO₄.

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Fig. 2 also shows that the corrosion rate increases with the temperature both in uninhibited and inhibited solutions. But the corrosion rate increases more rapidly with the temperature in the absence of inhibitor. These results confirm that TDPB acts as an efficient inhibitor in the range of temperature studied.

3.1.2

3.1.2 Effect of TDPB on inhibition efficiency

Values of inhibition efficiency (η_w) obtained from weight loss for different inhibitor concentrations in 7.0 M H₃PO₄ are presented in Fig. 3. η_w increases as the concentration of inhibitor increases from 0.1 to 1.0 mM. According to the literature (Zhao and Mu, 1999), when the surfactant concentration reaches critical micelle concentration (CMC), the surfactant inhibitor forms micelle and the adsorption amount tends to reach equilibrium, then inhibition efficiency does not change with the increase in inhibitor concentration. The critical micelle concentration (CMC) of TDPB is about 2.6–4.5 mM (Lü and Lu, 1999). Thus, it is impossible for TDPB to form micelles within the range of 0.1–1.0 mM. Accordingly, the monomer concentration and adsorption coverage of TDPB on the steel surface increase with the inhibitor concentration, which results in that inhibition efficiency increasing with the inhibitor concentration. The maximum η_w is 92% at 1.0 mM and the inhibition is estimated to be 79% even at very low concentration (0.1 mM), and at 0.7 mM concentration its protection is >90%, which indicates that TDPB is a good inhibitor for CRS in 7.0 M H₃PO₄.

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Figure 3

Relationship between inhibition efficiency (η_w) and concentration of TDPB (c) in 7.0 M H₃PO₄.

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3.1.3

3.1.3 Adsorption isotherm

Assuming the increase in the inhibition is caused by the adsorption of TDPB on the CRS surface and obeys the Langmuir adsorption isothermal equation (Li et al., 2009):

(5)

$$\frac{c}{\theta }=\frac{1}{K}+c$$ where c is the concentration of inhibitor, K is the adsorptive equilibrium constant and θ is the surface coverage and calculated by the following equation (Li et al., 2010):

(6)

$$\theta =\frac{{\nu }_0-\nu }{{\nu }_0}$$ where v₀ and v are the values of the corrosion rate without and with inhibitor, respectively. The linear regression parameters between c/θ and c are listed in Table 1. Straight lines of c/θ vs. c at different temperatures are shown in Fig. 4. It is observed that almost all linear correlation coefficients (r) are equal to 1 and all slopes are close to 1, which indicates that the adsorption of TDPB on the steel surface obeys the Langmuir adsorption isotherm. The adsorptive equilibrium constant (K) value decreases as the temperature increases, which may be attributed to the desorption of inhibitor molecules at higher temperatures. Generally, large values of K imply more efficient adsorption and better inhibition efficiency of a given inhibitor. This is in good agreement with the values of η_w obtained from Fig. 3.

Table 1 Parameters of the linear regression between c/θ and c.

Temperature (°C)	Linear correlation coefficient (r)	Slope	K (M−1)
20	0.9999	1.07	3.87 × 104
30	0.9997	1.07	2.38 × 104
40	0.9992	1.08	1.55 × 104
50	0.9994	1.11	1.29 × 104

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Figure 4

The relationship between c/θ and c in 7.0 M H₃PO₄.

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3.1.4

3.1.4 Thermodynamic parameters

Thermodynamic parameters play an important role in understanding the inhibitive mechanism. The adsorption enthalpy (ΔH) could be calculated according to the Van’t Hoff equation (Li et al., 2009):

(7)

$$\ln K=\frac{-\mathrm{\Delta }H}{\mathit{RT}}+\text{constant}$$ where R is the gas constant (8.314 J K⁻¹ mol⁻¹), and T is the absolute temperature (K).

Noticeably, −ΔH/R is the slope of straight line ln K vs. 1/T as shown in Fig. 5, and the linear correlation coefficient (r) is 0.9869. Under experimental conditions, the adsorption heat (ΔH) could be approximately regarded as the standard adsorption heat (ΔH⁰) (Zhao and Mu, 1999). The standard adsorption free energy (ΔG⁰) is obtained according to (Cano et al., 2004):

(8)

$$K=\frac{1}{55.5}\exp \left(\frac{-\mathrm{\Delta }{G}_{\text{ads}}}{\mathit{RT}}\right)$$ where the value 55.5 is the concentration of water in solution expressed in M. Then the standard adsorption entropy (ΔS⁰) can be obtained by the following thermodynamic basic equation:

(9)

$$\mathrm{\Delta }{S}^0=\frac{\mathrm{\Delta }{H}^0-\mathrm{\Delta }{G}^0}{T}$$ All the calculated thermodynamic parameters are listed in Table 2. The ΔH⁰ (−29.44 kJ mol⁻¹) in Table 2 shows that the adsorption of TDPB is an exothermic process (Li and Mu, 2005), which indicates that inhibition performance decreases with temperature. Such behavior can be interpreted on the basis that the increase in temperature resulted in desorption of some adsorbed inhibitor molecules from the metal surface. The negative values of ΔG⁰ indicate that the adsorption of inhibitor molecule onto the steel surface is a spontaneous process. Generally, values of ΔG⁰ up to −20 kJ mol⁻¹ are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than −40 kJ mol⁻¹ involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a co-ordinate type of bond (chemisorption) (Bentiss et al., 2005). In the present study, the value of ΔG⁰ is in the range from −20 to −40 kJ mol⁻¹; probably means that both physical adsorption and chemical adsorption would take place.

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Figure 5

The relationship between ln K and 1/T.

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Table 2 Thermodynamic parameters of adsorption of TDPB on the CRS surface in 7.0 M H₃PO₄.

Temperature (°C)	ΔG0 (kJ mol−1)	ΔH0 (kJ mol−1)	ΔS0 (J mol−1 K−1)
20	−35.53	−29.44	20.77
30	−35.52	−29.44	20.09
40	−35.57	−29.44	19.58
50	−36.22	−29.44	20.98

Inspection of Table 2 reveals that the sign of ΔS⁰ is positive. This is opposite to what would be expected, since adsorption is an exothermic process and always accompanied by a decrease in entropy. The reason could be explained as follows: The adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasi-substitution process between the organic compound in the aqueous phase [Org_(sol)] and water molecules adsorbed on the electrode surface [H₂O_(ads)] (Sahin et al., 2002).

(10)

$${\text{Org}}_{\text{(sol)}}+x{\text{H}}_2{\text{O}}_{\text{(ads)}}\leftrightarrow {\text{Org}}_{\text{(ads)}}+x{\text{H}}_2{\text{O}}_{\text{(sol)}}$$ where x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor. In this situation, the adsorption of TDPB is accompanied by desorption of water molecules from the surface. So, when the adsorption process for the inhibitor is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. The thermodynamic values obtained are the algebraic sum of the adsorption of organic molecules and desorption of water molecules (Branzoi et al., 2000). Therefore, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption entropy (Branzoi et al., 2000; Ateya et al., 1984). The positive values of ΔS⁰ mean that an increase in disordering takes place when more water molecules are replaced by one organic inhibitor on the metal surface, which is the driving force for the adsorption of inhibitor onto the steel surface (Li et al., 2008).

3.1.5

3.1.5 Kinetic parameters

It has been reported by a number of authors that the natural logarithm of the corrosion rate v (in g m⁻² h⁻¹) is a linear function with 1/T (following Arrhenius equation) for the acid corrosion of steel (Elachouri et al., 1996; Tang et al., 2003; Mu et al., 2004; Banerjee and Misra, 1989; Ferreira et al., 2004):

(11)

$$\ln \nu =\frac{-E_a}{\mathit{RT}}+\ln A$$ where E_a and A are apparent activation energy and pre-exponential factor, respectively.Arrhenius plots of ln v vs. 1/T for the blank and different concentrations of TDPB are shown in Fig. 6. The regression parameters are listed in Table 3. The relationship of E_a and ln A with the inhibitor concentration is presented in Fig. 7.

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Figure 6

Arrhenius plots related to the corrosion rate of CRS for various concentrations of TDPB in 7.0 M H₃PO₄.

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Table 3 Parameters of the regression between ln v and 1/T.

c (mM)	Ea (kJ mol−1)	A (g m−2 h−1)	Linear regression coefficient (r)
0	51.72	5.40 × 1010	0.9994
0.1	70.02	2.03 × 1013	0.9995
0.2	70.64	2.09 × 1013	0.9994
0.3	70.26	1.56 × 1013	0.9988
0.4	69.94	1.21 × 1013	0.9988
0.5	70.26	1.18 × 1013	0.9992
0.7	68.41	4.82 × 1012	0.9983
1.0	67.45	2.71 × 1012	0.9969

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Figure 7

The relationship of E_a and ln A with the concentration of TDPB in 7.0 M H₃PO₄.

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Fig. 6 and Table 3 reveal that all linear regression coefficients (r) are almost equal to 1, which indicates that the linear relationship between ln v and 1/T is good. The value of E_a is higher for inhibited system as compared to the uninhibited system. According to Eq. (8), the lower A and the higher E_a lead to the lower corrosion rate (v). For the present study, the value of A containing TDPB is higher than that in uninhibited solution. So the decrease in steel corrosion rate is determined by the increase in apparent activation energy.

Many studies (Osman et al., 2003; Elachouri et al., 1996; Algaber et al., 2004; Martinez and Stern, 2002) indicated that in the presence of inhibitor, there is an increase in E_a with respect to uninhibited solutions, while other studies (Bentiss et al., 2002, 2005; Umoren et al., 2008; Cai et al., 1993) found the decrease in E_a. Eq. (11) also shows that −E_a/R is the slope of straight line ln v − 1/T, so the value of E_a could elucidate the effect of temperature on corrosion inhibition. The relationship between the temperature dependence of percentage inhibition efficiency (η) of an inhibitor and E_a can be classified into three groups according to temperature effects (Priya et al., 2008).

1. η decreases with increase in temperature, E_a (inhibited solution) > E_a (uninhibited solution);

2. η increases with increase in temperature, E_a (inhibited solution) < E_a (uninhibited solution);

3. η does not change with temperature, E_a (inhibited solution) = E_a (uninhibited solution).

It shows that E_a (inhibited solution) > E_a (uninhibited solution), which further confirms that IE decreases with increase in temperature. Also, E_a increases with TDPB concentration in 0–0.1 mM, then E_a shows no significant change with increase in TDPB concentration. It can be deduced that increasing TDPB concentration results in increasing the decreased degree of IE dependence with temperature up to the maximum of 0.1 mM, and then does not change with TDPB concentration. The conclusion is consistent with the result of Fig. 3.

It is also observed from Fig. 7 that the variance of ln A is same as that of E_a. Influenced by the cumulative effect of the magnitudes of E_a and A, the corrosion rate decreases with the inhibitor concentration.

3.2 Polarization curves

Fig. 8 shows polarization curves for CRS in 7.0 M H₃PO₄ with various concentrations of TDPB at 20 °C. It is found that the presence of inhibitor causes a prominent decrease in the corrosion rate i.e. shifts both anodic and cathodic curves to lower values of current densities. Namely, both cathodic and anodic reactions of CRS electrode corrosion are drastically inhibited by TDPB in 7.0 M H₃PO₄. Values of corrosion current densities (i_corr), corrosion potential (E_corr), cathodic Tafel slope (b_c), anodic Tafel slope (b_a), and inhibition efficiency (η_p) are listed in Table 4.

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Figure 8

Potentiodynamic polarization curves for CRS in 7.0 M H₃PO₄ containing different concentrations of TDPB at 20 °C.

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Table 4 clearly shows that i_corr decreases prominently while η_p increases with the increase in inhibitor concentration, and the maximum η_p is up to 92.2%. The presence of TDPB does not remarkably shift E_corr, therefore, it can be arranged as a mixed-type inhibitor in 7.0 M H₃PO₄, and the inhibition of TDPB on CRS is caused by geometric blocking effect (Cao, 1996). Namely, the inhibition effect comes from the reduction of the reaction area on the surface of the corroding metal (Cao, 1996). Tafel slopes of b_c and b_a do not change upon addition of TDPB, which means that the presence of TDPB does not change the reaction mechanism.

3.3 Electrochemical impedance spectroscopy (EIS)

Fig. 9 shows the Nyquist diagrams for CRS in 7.0 M H₃PO₄ at 20 °C containing various concentrations of TDPB. The impedance spectra exhibit one single depressed semicircle, and the diameters of semicircle increase with the increase in inhibitor concentration. The single semicircle indicates that the charge transfer takes place at electrode/solution interface, and the transfer process controls corrosion reaction of steel and the presence of inhibitor does not change the mechanism of steel dissolution (Larabi et al., 2004). Also, these impedance diagrams are not perfect semicircles which are related to the frequency dispersion as a result of the roughness and inhomogeneous of electrode surface (Lebrini et al., 2007). Furthermore, it is apparent, from these plots that, the impedance response of CRS in uninhibited H₃PO₄ solution has significantly changed after the addition of TDPB in the corrosive solution; as a result, real axis intercepts at high and low frequencies in the presence of inhibitor are bigger than those in the absence of inhibitor (blank solution) and increase with increasing concentration of inhibitor. This indicates that the impedance of inhibited substrate increases with increase in concentration of inhibitor.

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Figure 9

Nyquist diagrams of CRS in 7.0 M H₃PO₄ with different concentrations of TDPB at 20 °C.

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The EIS results are simulated by the equivalent circuit shown in Fig. 10 to pure electric models that could verify or rule out mechanistic models and enable the calculation of numerical value corresponding to the physical and/or chemical properties of the electrochemical system under investigation (Priya et al., 2008). The circuit employed allows the identification of both solution resistance (R_s) and charge transfer resistance (R_t). It is worth mentioning that the double layer capacitance (C_dl) value is affected by imperfections of the surface, and that this effect is simulated via a constant phase element (CPE) (Bommersbach et al., 2006). The constant phase element is composed of a component Q_dl and the coefficient of a which quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. So the capacitance is deduced from the following relation (Qu et al., 2009):

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Figure 10

The equivalent circuit model of EIS.

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Inspection of Table 5 reveals that R_t values increase distinctly while C_dl reduces with the concentration of TDPB. The larger value of C_dl without inhibitor (602.02 μF cm⁻²) may indicate that the steel surface is fully covered with the high concentration of H₃PO₄ (Li et al., 2011). Accordingly, the steel shows high corrosion damage in 7.0 M H₃PO₄. The decrease in C_dl compared with that in the blank solution (without inhibitor), can result in a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, which suggests that the inhibitor molecules come into effect by adsorption at the metal/solution interface (Lagrenée et al., 2002). η_R increases with the concentration of TDPB, and the maximum η_R reaches up to 92.2%, which further confirms that TDPB shows good corrosion inhibiting behavior for CRS in 7.0 M H₃PO₄.

3.4 Scanning electron microscope (SEM)

The SEM images of CRS surface in 7.0 M H₃PO₄ are shown in Fig. 11. It can be seen from Fig. 11(a) that the CRS surface before immersion shows some abrading scratches. It also appears as small black holes, which may be attributed to the defect of steel. Inspection of Fig. 11(b) reveals that the CRS surface after immersion in uninhibited 7.0 M H₃PO₄ for 6 h shows an aggressive attack of the corroding medium on the steel surface. The corrosion products appear very uneven and the corrosion products appear lepidoteral-like morphology, and the surface layer is too rough. In contrast, in the presence of TDPB inhibitor, Fig. 11(c) shows that there is an adsorbed film adsorbed on steel surface exposed to 7.0 M H₃PO₄ solutions containing TDPB, which do not exist in Fig. 11(b). Thus, it might be concluded that the adsorption film can efficiently inhibit the corrosion of steel.

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Figure 11

SEM micrographs of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 °C in 7.0 M H₃PO₄; (c) after 6 h of immersion at 20 °C in 1.0 mM TDPB + 7.0 M H₃PO₄.

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3.5 Quantum chemical calculations

TDPB can be classified as a 1:1 electrolyte; TDPB is ionized in water solution as follows:

Fig. 12 shows the optimized molecular structure of [TDP]⁺. It shows that the pyridine ring is in one plane, while the carbon chain is in another plane. According to the frontier molecular orbital theory, the formation of a transition state is due to an interaction between frontier orbitals of (HOMO and LUMO) of reacting species. HOMO is often associated with the capacity of a molecule to donate electrons, whereas LUMO represents the ability of the molecule to accept electrons. The electric/orbital density distributions of HOMO and LUMO for [TDP]⁺ are shown in Fig. 13. Values of E_HOMO and E_LUMO are −5.004 and −3.175 eV, respectively. It is found that the electron densities of HOMO are localized on the long carbon chain of CH₃(CH₂)₁₃, which indicates that the preferred active sites for donating electrons are located on the carbon chains of CH₃(CH₂)₁₃. However, it should be noted that due to the passivation effect of H atoms on the carbon chains (Feng et al., 2011), the reactivity of the carbon chains will be reduced. In other words, CH₃(CH₂)₁₃ is the hydrophobic part that would be difficult to adsorb on steel/solution interface.

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Figure 12

Optimized molecular structure of [TDP]⁺.

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Figure 13

The frontier molecule orbital density distributions of [TDP]⁺: (a) HOMO; (b) LUMO.

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As shown in Fig. 13, LUMO is located on pyridine ring the inhibitor molecule can accept electrons from the iron atom with its anti-bonding orbitals to form back-donation bonds. In a word, TDPB exerts inhibition action by adsorption of [TDP]⁺ on the steel surface through that hydrophilic part pyridine ring (the polar or ionic group) attacks the metal surface to form back-donation bonds, while the hydrophobic part CH₃(CH₂)₁₃ extends to solution face to form a hydrophobic barrier to decrease the corrosion rate.