Effect of the Structure and Temperature on Corrosion Inhibition of Thiourea Derivatives in 1.0 M HCl Solution.

The corrosion inhibition ability of 1-phenyl-2-thiourea (PTU) and 1,3-diisopropyl-2-thiourea (ITU) for mild steel in 1.0 M hydrochloric was studied by using the potentiodynamic polarization (PDP) curves, electrochemical impedance spectroscopy (EIS), quantum chemical calculations, and Monte Carlo simulations. Conditions which influence the capacity of corrosion inhibition including concentration, structure of thiourea derivatives, and environment temperature were taken into investigation. The highest inhibition efficiencies of PTU and ITU are 98.96 and 92.65% at a concentration of 5 × 10-3 M at 60 °C. In fact, corrosion inhibition ability of PTU is better than that of ITU in acidic solution due to the presence of the benzene ring of PTU. EIS data are very well correlated with PDP results. In addition, the higher inhibition performance with enhancing temperature and the values of ΔG 0 indicated that PTU and ITU participate in chemical adsorption on the metal surface. Their adsorption process on the metal surface follow the Langmuir adsorption isotherm. Both experimental and theoretical results in this study are in good agreement.

Introduction

Corrosion inhibition is one of the issues that has been extensively considered and studied because it is one of the most useful ways to protect materials with low cost and high efficiency.[1−5] There have been a number of published papers and reports on effective organic inhibitors which contain heteroatoms (such as O, N, and S) and triple bonds or conjugated double bonds or aromatic rings in their molecular structures.[6−10]

In many sulfur- and nitrogen-containing compounds, thiourea derivatives are highly appreciated for their metal corrosion inhibition ability.[11] Abdel-Rehim and co-workers prepared and studied the iron corrosion inhibition ability of 1,3-diarylidenethiourea in 1.0 M HCl solution by using electrochemical frequency modulation. The results showed that the inhibition performance of this compound was 80.4% at the concentration of 9 × 10–4 M.[12] One other study of Li et al. concluded that allyl thiourea is a good corrosion inhibitor for steel in H3PO4 solution. With the concentration of 0.5 mM, the inhibition efficiency of allyl thiourea is larger than 95%.[13] Corrosion inhibition of 1-methyl-3-pyridin-2-yl-thiourea was also investigated in H2SO4 solution with different techniques by Hosseini and Azimi.[14] The result of potentiostatic polarization measurement demonstrated that this compound acts as a mixed-type inhibitor with an efficiency of 96.4% with a concentration of 100 ppm. Besides steel, thiourea derivatives are used to inhibit many other metals such as aluminum, copper, zinc, and so on[15−17]

Recently, our team has also investigated the corrosion inhibition ability of 1-phenyl-2-thiourea (PTU) in both 1.0 M HCl and 3.5% wt NaCl solutions at 30 °C. As a result, its performance is better than that of urotropine, a well-known traditional inhibitor.[18] To the best of our knowledge, 1,3-diisopropyl-2-thiourea (ITU), one of the thiourea derivatives, has not been examined as a corrosion inhibitor for steel. Therefore, in this study, ITU is selected along with PTU to investigate and compare their inhibition performance in 1.0 M HCl solution under different conditions.

Thiourea derivatives are often adsorption inhibitors which form a chemisorptive bond with the metal surface and impede ongoing electrochemical dissolution reactions.[19] The adsorption of an organic compound onto the surface of the metal is dependent on the physicochemical properties of the corrosion inhibitors, involving steric factors, functional groups, electron density at the donor atoms, and π orbital character of donating electrons.[20] While PTU has a benzene ring in its molecule, ITU has a long carbon chain. The purpose of this paper is to find out how the structures of these thiourea derivatives influence on their corrosion inhibition performance. In addition, the effect of temperature is also investigated to find the right conditions for better inhibition ability of these compounds.

On the basis of the data obtained from potentiodynamic polarization (PDP) curves, electrochemical impedance spectroscopy (EIS) and chemical quantum calculations using the density functional theory, a detailed study on the corrosion inhibition ability of PTU and ITU (Figure , Tables S1 and S2, Supporting Information for more details) is reported. Moreover, the Monte Carlo (MC) simulation technique is also used to examine the adsorption of inhibitor molecules on the metal surface.

Figure 1

Optimized structures of (a) PTU and (b) ITU at B3LYP/6-311G(d,p).

Results and Discussion

Experimental Study

Effect of PTU and ITU Concentrations on Inhibition Efficiency

Two experimental methods used to investigate the effect of inhibitor concentration on inhibition efficiency at 30 °C are PDP curves and EIS. The PDP curves for mild steel in 1.0 M HCl measured in the absence and presence of various concentrations of PTU and ITU inhibitors are shown in Figure .

Figure 2

Polarization curves of steel in 1.0 M HCl with various concentrations of (a) PTU (reprinted from ref (18)) and (b) ITU in 1 h at 30 °C.

There are two processes which control the corrosion rate of steel in hydrochloric acid, involving the hydrogen evolution reaction and dissolution reaction of iron. According to this mechanism, anodic dissolution of iron includes the following steps[2,21]

The cathodic hydrogen evolution mechanism can be expressed via following equations

When the corrosion potential is shifted toward the negative, the cathode reaction is essential, that is, the anode reaction is inhibited. It means that the anodic dissolution is inhibited by a passive membrane which is formed due to the presence of the inhibitors. Figure shows that the presence of PTU and ITU reduces the corrosive current strength of both cathode and anodic currents.

The corrosion rate of low carbon steel depends on the concentration of inhibitors. For PTU, when the concentration changes, the inhibition performance changes significantly. However, for ITU, the performance only changes much at a concentration of 5 × 10–3 M; therefore, the graph shows overlapping corrosion lines in the concentration range of 10–4 to10–3 M. At different concentrations of inhibitors, the anode current densities seem to vary slightly, while cathode current densities are significantly affected. Data in Figure also confirm that the corrosion potential shifts toward the negative value when increasing the concentration of ITU and PTU. This suggests the cathodic predominance of the inhibitors. Furthermore, the values of cathodic (βc) and anodic (βa) Tafel constants change slightly when adding PTU and ITU, which means that the corrosion mechanism of steel in acid solution does not change in the presence of these inhibitors.

Besides, the significant change of the corrosion potentials when adding the inhibitors proves that the inhibition for this system cannot be caused by the geometric blocking effect but may be due in the main to the active site blocking effect.[22] The organic inhibitors function through adsorption on the metal surface blocking the active sites by displacing water molecules and forming a compact barrier film to prevent the corrosion process[23] (see Table for more details).

Table 1

Polarization Parameters of Mild Steel in 1.0 M HCl with Various Concentrations of PTU and ITU at 20, 30, 45, and 60 °Ca

inhibitors	temperature (°C)	CM	Ecorr (V)	βa mV·dec–1	–βc mV·dec–1	icorr (iinh) (mA·cm–2)	IE (%)
PTU	20		–0.23	32.5	20.9	0.25
		5 × 10–3	–0.40	31.2	19.3	0.02	92.00 (1.30)
		10–3	–0.39	33.4	22.4	0.03	88.00 (1.22)
		5 × 10–4	–0.37	30.8	20.2	0.04	84.00 (1.11)
		10–4	–0.34	34.7	21.5	0.05	80.40 (1.27)
	30		–0.24	36.4	24.8	0.90
		5 × 10–3	–0.38	33.0	20.8	0.05	94.95 (1.05)
		10–3	–0.37	39.3	26.0	0.06	93.88 (1.01)
		5 × 10–4	–0.35	33.1	20.6	0.08	91.11 (1.20)
		10–4	–0.33	48.0	21.7	0.09	90.54 (1.13)
	45		–0.25	35.7	30.0	2.69
		5 × 10–3	–0.32	21.6	32.6	0.09	96.65 (1.23)
		10–3	–0.38	41.9	22.9	0.11	95.91 (1.32)
		5 × 10–4	–0.33	40.4	21.2	0.13	95.17 (1.11)
		10–4	–0.27	36.4	32.1	0.14	94.80 (1.09)
	60		–0.24	37.1	30.4	7.35
		5 × 10–3	–0.32	39.6	26.5	0.08	98.96 (1.15)
		10–3	–0.30	40.1	22.3	0.08	98.95 (1.20)
		5 × 10–4	–0.33	37.9	22.4	0.13	98.17 (1.24)
		10–4	–0.30	30.5	30.4	0.29	96.10 (1.19)
ITU	20		–0.35	32.5	20.9	0.25
		5 × 10–3	–0.49	30.2	16.3	0.05	80.80 (1.20)
		10–3	–0.47	30.3	16.4	0.07	71.60 (1.01)
		5 × 10–4	–0.45	30.1	17.2	0.09	62.40 (1.21)
		10–4	–0.44	26.3	16.5	0.12	53.20 (1.3)
	30		–0.33	36.4	24.8	0.90
		5 × 10–3	–0.37	34.5	17.2	0.15	83.33 (1.02)
		10–3	–0.37	34.6	17.2	0.20	77.78 (1.31)
		5 × 10–4	–0.38	34.1	19.6	0.21	76.67 (1.25)
		10–4	–0.37	28.6	17.7	0.22	75.56 (1.27)
	45		–0.34	35.7	30.0	2.69
		5 × 10–3	–0.39	20.5	17.9	0.26	90.33 (1.30)
		10–3	–0.39	25.6	20.3	0.32	88.10 (1.22)
		5 × 10–4	–0.36	20.6	17.3	0.37	86.25 (1.35)
		10–4	–0.35	18.4	24.0	0.43	84.01 (1.4)
	60		–0.35	37.1	30.4	7.35
		5 × 10–3	–0.37	34.0	32.0	0.54	92.65 (1.21)
		10–3	–0.39	40.2	41.5	0.60	91.84 (1.23)
		5 × 10–4	–0.36	37.4	28.5	0.73	90.07 (1.10)
		10–4	–0.37	38.4	37.7	0.81	88.98 (1.10)

Values in parenthesis in the last column of this table are the mean absolute deviation.

Nyquist diagrams for steel in 1.0 M HCl with the presence of PTU and ITU are displayed in Figure . All the impedance spectra exhibit one single capacitive semicircle. This shows that the charge transfer process of the corrosion process and double layer behavior mainly control the corrosion of carbon steel.[24] However, these diagrams are not perfect semicircles due to frequency dispersion.[25]

Figure 3

Nyquist plots of the corrosion of mild steel in 1.0 M HCl with different concentrations of (a) PTU and (b) ITU at 30 °C.

The shapes of Nyquist diagrams with the presence of inhibitors change insignificantly when comparing with that of the blank. This leads to conclude that the mechanism of the corrosion process does not change when adding inhibitors.

To analyze the experimental results, an appropriate equivalent circuit model is required to correctly fit the impedance curves. The equivalent circuit is shown in Figure . All of the impedance parameters such as solution resistance (Rs), charge resistance (Rct), and double-layer capacitance (Cdl) are calculated and listed in Table based on the fitting of Nyquist plots with the proposed electrochemical circuit using Thales 4.5 software.

Figure 4

Equivalent circuit model of EIS.

Table 2

EIS Parameters for the Corrosion of Carbon Steel in 1.0 M HCl in the Absence and Presence of Inhibitors at 30 °C

inhibitors	CM	Rs (Ω cm2)	Rct (Ω cm2)	CPE (μΩ sn cm–2)	n	fmax (Hz)	Cdl (μF cm–2)	IE (%)
blank		3.52	125	64.62	0.80	8.13	29.14
PTU	5 × 10–3	2.27	1870	8.43	0.72	1.44	3.99	93.31
	10–3	4.68	1690	8.40	0.67	1.44	4.04	92.60
	5 × 10–4	2.30	1167	8.22	0.73	3.03	4.15	89.28
	10–4	2.34	938	7.48	0.73	4.27	4.24	86.66
ITU	5 × 10–3	2.65	720	5.38	0.76	5.93	6.13	82.63
	10–3	2.49	580	4.53	0.82	14.67	6.44	78.43
	5 × 10–4	2.52	530	4.13	0.71	4.27	8.22	76.40
	10–4	2.45	505	4.68	0.84	10.98	9.64	75.23

CPE is the constant phase element to replace double-layer capacitance (Cdl) for more accurate fit. The impedance of the CPE can be given by eq (26)where Yo is the CPE constant, j is the imaginary unit, ω is the angular frequency, and n is the CPE exponent. If the electrode surface is homogeneous and plane, n is equal to 1, and the electrode surface can be treated as an ideal capacitance. The double-layer capacitance (Cdl) can be simulated via CPE from eq (27)where ωmax = 2πfmax, fmax (Hz) is the frequency corresponding to the maximum value of the imaginary component of the Nyquist plot.

When there is the presence of inhibitors in solution, Rct values increase, and Cdl values decrease. These may suggest that the inhibitors form a protective layer on the electrode surface. This layer makes a barrier for mass and charge transfer.[28] Moreover, Rct values of PTU are higher than that of ITU at the same concentration, which proves that PTU can inhibit better than ITU. The best inhibition efficiency of PTU and ITU are 93.33 and 82.63% according to the EIS method.

In the presence of inhibitors, n decreases with the increase of the concentration. This shows an increase of the surface inhomogeneity as a result of the inhibitor adsorption.[29]

Regarding to the data in Tables and 2, there is a good agreement between the inhibition efficiencies calculated from the EIS and those obtained from PDP curves. Thus, for convenience, the experiments at other temperatures were only investigated by PDP curves.

Effect of Thiourea Derivative Structure on Inhibition Efficiency

The functional groups and structure of the inhibitor molecules have important roles in adsorption because they decide the number of adsorption sites, their charge density, molecular size, and mode of interaction with the metal surface.[30] PTU and ITU are both derivatives of thiourea. They are only different from the molecular structures of the substituents nearby the nitrogen atom. In the experimental part, at the same concentration and temperature, inhibition ability of PTU is always higher than that of ITU. At 30 °C, the highest inhibition efficiency of PTU is 94.95%, while it is 83.33% for ITU according to PDP curves. Clearly, their inhibition actions depend on the nature of the substituent.[8] The rest of the inhibitor molecule affects on the electron density at the functional group; therefore, it also influences on the adsorption on the metal surface. In the PTU molecule, an important structural factor is a benzene ring because it rises electrostatic interaction between inhibitors and metal surface.[31] Regarding to this point, PTU is capable of forming a strong bond with the mild steel surface. Besides, PTU is suggested to give the higher coverage due to the aromatic ring.[32] Therefore, it gives better inhibition performance than ITU.

Effect of Temperature on Inhibition Efficiency

Inhibition capacity of PTU and ITU also were examined at different temperatures of 20, 30, 45, and 60 °C with a tolerance of 1 °C. PTU exhibits effective inhibition with high inhibition performance: 92.00% at 20 °C, 94.95% at 30 °C, 96.65% at 45 °C, and 98.96% at 60 °C (Table ). ITU’s inhibition performances are only 80.80% at 20 °C, 83.33% at 30 °C, 90.33% at 45 °C, and 92.65% at 60 °C.

Figure shows that when the temperature increases, the inhibition efficiencies of PTU and ITU increase. This may be explained that the adsorption of PTU and ITU on the metal surface is chemisorption.[33] In this case, the interaction between inhibitor molecules and metal will form coordinate bonds by giving lone electron pairs of sulfur and nitrogen to empty orbitals of iron atoms.

Figure 5

Relationship between concentrations of (a) PTU and (b) ITU and their inhibition efficiencies at different temperatures.

Adsorption Isotherms

The adsorption isotherms can generate the important information relating to the interaction between the inhibitors and metal surface.[9,34] Regarding this approach, various models of the adsorption isotherm are recommended.[35,36] And in this study, we proposed to use Temkin and Langmuir adsorption isotherms to investigate the adsorption mechanism of thiourea derivatives.

Temkin equation is shown in the following[37,38]

Langmuir equation is written in the following form[39,40]where, K is the equilibrium constant of the adsorption reaction, “a” is the lateral interaction term describing the interactions in the adsorption layer and the heterogeneity of the surface, and θ is surface coverage values.

The correlation coefficient (R2) is used to assess whether the adsorption isotherm model is consistent with the experimental data.[41] According to Figure , the correlation coefficients of the plots between ln C versus θ are considerably different from unit (except for R2 at 20, 45 °C). They prove that adsorption of PTU and ITU on the steel surface do not follow the Temkin isotherm. Next, Langmuir adsorption is applied for evaluation and is shown in Figure . The straight lines between C and C/θ are found with the correlation coefficients close to 1, and the slope values in the Langmuir equation are approximately equal to 1 (Table ). These results prove that the adsorption of PTU and ITU on the electrode surface obeys the Langmuir adsorption isotherm, and each PTU or ITU molecule only accounts for one adsorption position. Besides, K values are also computed and shown in Table .

Figure 6

Temkin’s adsorption isotherm of (a) PTU and (b) ITU on the surface of mild steel in 1.0 M HCl.

Figure 7

Langmuir’s adsorption isotherms of (a) PTU and (b) ITU in 1.0 M HCl.

Table 3

Parameters of the Linear Regression between C/θ and C in 1.0 M HCl

inhibitor	temperature (°C)	slope	K (M–1)
PTU	20	1.08	25 900.14
	30	1.05	73 754.39
	45	1.03	160 703.17
	60	1.01	355 884.43
ITU	20	1.21	8311.60
	30	1.19	22 891.59
	45	1.10	49 617.63
	60	1.08	100 609.85

Thermodynamic Parameters

The standard adsorption enthalpy (ΔH0) is calculated using van’t Hoff equation

The equation can be changed[42]where A is the integral constant.

The linear correlation coefficients (R2) are 0.999 and 0.999 for ITU and PTU, respectively (Figure ). From the slope of the straight lines of ln K versus 1/T, ΔH0 is calculated in Table .

Figure 8

Straight lines of ln K and 1/T.

Table 4

ΔG0, ΔH0, and ΔS0 Values of the Adsorption Process for PTU and ITU in 1.0 M HCl

inhibitor	temperature (°C)	ΔG0 (kJ mol–1)	ΔH0 (kJ mol–1)	ΔS0 (J mol–1 K–1)
PTU	20	–34.54	45.99	274.85
	30	–38.35	45.99	274.70
	45	–42.31	45.99	277.69
	60	–46.51	45.99	270.80
ITU	20	–31.77	43.97	258.51
	30	–35.41	43.97	261.98
	45	–39.20	43.97	261.57
	60	–43.01	43.97	261.22

Based on the K value, the free energy of adsorption (ΔG0) is calculated by eq (43)where R is the gas constant (R = 8.314 J mol–1 K–1), and T is the temperature of the system (K).

The standard adsorption entropy (ΔS0) is calculated via the following thermodynamic eq (44)

The standard thermodynamic parameters are collected and given in Table . The positive values of ΔH0 prove that the adsorption of inhibitors is endothermic. When the temperature increases, the inhibition efficiency increases. This suggests the hypothesis that these inhibitors may take part in the chemical adsorption process on metal surfaces.[45]

Besides, the ΔG0 value also reveals much information. ΔG0 is larger than −20 kJ mol–1 which means there is electrostatic interaction between the charged inhibitors and charged steel (physical adsorption). While it is more negative than −40 kJ mol–1, the adsorption involves the charged pair or organic inhibitor transfer onto the steel surface, resulting to form a type of coordinated bond (chemical adsorption).[9,46] In fact, there is no boundary between physical and chemical adsorption in which the physical one is considered as the first stage of chemical adsorption.[24] Therefore, the adsorption of ITU and PTU on steel surfaces is a mixture of both physisorption and chemisorption when considering the value of ΔG0. The higher the temperature, the more negative the ΔG0 value, indicating that the studied inhibitors are more strongly linked to the steel surface. It is in a perfect agreement with the increase of inhibition performance as the temperature increases.

At the same temperature, the ΔG0 value of PTU is more negative than ITU; it proves that the adsorption of PTU on the surface of steel is stronger than that of ITU. The sign of ΔS0 is positive, and it means that entropy of the process increases. This process includes the adsorption of organic compounds (Org) and the desorption of water molecules at the electrode surface. It can be described as followswhere x is the number of water molecules replaced by one inhibitor molecule. The positive values of ΔS0 indicate that there is an increase in chaos between reactant molecules on the metal electrode surface. This is also an important driving force for inhibitor molecules to adsorb onto the metal surface.[47]

Quantum Chemical Study

The quantum chemical method is extremely useful to study the structure and behavior of corrosion inhibitors.[48,49] Based on the optimized structures of PTU and ITU at B3LYP/6-311G(d,p) (Figure ), several thermodynamic parameters can be computationally computed from Gaussian output files.

Mulliken population analysis is used to predict the adsorption centers of inhibitors.[50] The results given in Table show that both sulfur and nitrogen atoms in of PTU and ITU are the most favored sites for their adsorption of these inhibitors onto the metal surface through a donor–acceptor type of interaction because they are more negatively charged.[18] Besides, the benzene ring in PTU is a cause to promote the formation of the adsorbate–surface complex.[51]

Table 5

Mulliken Population of PTU and ITU in Gas Phase

PTU	elements	S18	N12	N15	C14	C1	C2	C3	C4	C5	C6
	Mulliken charges	–0.24	–0.44	–0.42	0.15	0.19	–0.05	–0.11	–0.09	–0.10	–0.11
ITU	elements	S1	N2	N3	C4	C5	C6	C7	C17	C19	C23
	Mulliken charges	–0.31	–0.40	–0.40	–0.06	0.24	–0.28	–0.25	–0.06	–0.25	–0.28

The highest occupied molecular orbital (HOMO) can display the electron-donating position of the molecule.[52] Based on the HOMO of PTU and ITU drawn in Figure , both can mainly donate electrons to the metal surface at the S and N atoms. In contrast, the lowest unoccupied molecular orbital (LUMO) indicates the ability of accepting electrons of the compound.[24] The shapes of LUMOs in Figure indicate that receiving positions of PTU and ITU are identical. The most reactive LUMO sites in PTU and ITU are at nitrogen, sulfur, and carbon atoms (nearby the double bonds or benzene ring).

Figure 9

HOMO and the LUMO of PTU and ITU.

The quantum chemical parameters such as HOMO energy (EHOMO), LUMO energy (ELUMO), energy gap (ΔEL–H), absolute electronegativity (χ), absolute hardness (η), global softness (S), and the number of electrons transferred (ΔN) are calculated and given in Table .

Table 6

Quantum Chemical Parameters for Neutral and Protonated Inhibitors in the Gas and Water Phases at B3LYP/6-311G(d,p)

phase	forms	compounds	EHOMO (eV)	ELUMO (eV)	ΔE(L–H) (eV)	χ (eV)	η (eV)	S (eV–1)	ΔN
gas	neutral	PTU	–5.84	–1.03	4.81	3.43	2.41	0.42	0.74
		ITU	–5.50	0.07	5.57	2.72	2.79	0.36	0.77
water	neutral	PTU	–6.14	–0.92	5.22	3.53	2.61	0.38	0.66
		ITU	–6.01	–0.12	5.89	3.07	2.95	0.34	0.67
gas	protonated	pPTU-S18	–10.75	–5.37	5.37	8.06	2.69	0.37	–0.20
		pITU-S1	–11.64	–5.11	6.52	8.37	3.26	0.31	–0.21
water	protonated	pPTU-S18	–7.58	–1.37	6.21	4.47	3.10	0.32	0.41
		pITU-S1	–7.90	–1.34	6.56	4.62	3.28	0.31	0.36

The value of EHOMO of a molecule helps to determine its electron donating ability. A molecule with higher EHOMO can easily donate electrons to appropriate acceptor molecules with low-energy, empty molecular orbital.[52] From Table , the EHOMO value of PTU (−5.84 eV) is lower than the EHOMO value of ITU (−5.50 eV) which is slightly inconsistent with the experimental results.

The value of ELUMO represents the ability of a molecule to accept electrons. The lower the value of ELUMO is, the easier is its ability to receive electrons.[52] Based on the calculated ELUMO values in Table , the inhibition efficiency can be arranged in the order: PTU > ITU.

The absolute hardness (η) represents the change of the chemical potential (μ) on the total number of atoms.[53] The higher the value of η is, the more the stability of a compound is. The global softness (S) is the quantitative characteristic of electron cloud polarization in compounds and is opposite of the hardness.[54] The higher the energy gap (ΔEL–H) is, the less polar the molecule is, and the adsorption of this molecule on the surface of metal is difficult.[4] Hence, a good corrosion inhibitor must have low values of EL–H and η but a high value of S. Based on the calculated values in Table , the inhibition efficiency of PTU is better than that of ITU. Furthermore, ΔN value of ITU is higher than PTU. It also indicates that ITU can exchange electrons more strongly than PTU. In short, the quantum chemical parameters (ELUMO, ΔE, η, σ, and ΔN) of neutral inhibitors show good agreement with the obtained experimental results mentioned above.

It should be emphasized that both PTU and ITU are investigated in acid solution. Therefore, these compounds may undergo protonation at heteroatom sites (N and S). Thus, the investigation of the protonated forms are necessary to determine the preferred form of these inhibitors in acidic solution. The favored protonated sites are at S18, N12, and N15 for PTU and S1, N2, and N3 for ITU.

Figure shows five protonated conformations of ITU and PTU optimized at B3LYP/6-311G(d,p) and their relative energies (in parenthesis). The most stable protonated conformations of PTU and ITU are protonated PTU (pPTU)-S18 and protonated ITU (pITU)-S1, respectively. (Tables S3 and S4, Supporting Information for more details). Therefore, pPTU-S18 and pITU-S1 are chosen for further computational calculations.

Figure 10

Selected optimized structures of pPTU and pITU (data in parenthesis are relative energies to the pPTU-S18 and pITU-S1 in kcal·mol–1).

In the case of protonated inhibitors, the values of EHOMO(PTU) (−10.75 eV), SPTU (0.37 eV), and ΔN(PTU) (−0.20) are higher than the values of EHOMO(ITU) (−11.64 eV), S(ITU) (0.31 eV), and ΔN(ITU) (−0.21), respectively. The values of ELUMO(PTU) (−5.37 eV), ΔE(L–H)(PTU) (5.37 eV), and ηPTU (2.69 eV) are lower than the values of ELUMO(ITU) (−5.11 eV), ΔE(L–H)(ITU) (6.52 eV), and η(ITU) (3.26 eV). These parameters support that PTU is a better inhibitor than ITU.

The EHOMO values of protonated inhibitor molecules in the gas phase (−10.75 and −11.64 eV) are lower than EHOMO values of neutral ones (−5.84 and −5.50 eV) for PTU and ITU, respectively. It means the electron-donating ability of protonated molecules decreases. Therefore, the bond between the inhibitor and the metal is formed by sharing of electrons from the metal to the inhibitor (back-donation).[55]

In addition, the inhibition efficiency is arranged in the same trend as compared to the quantum chemical parameters in the gas phase and in water (for both neutral and protonated inhibitors). All quantum chemical parameters (EHOMO, ELUMO, ΔEL–H, η, S, and ΔN) of protonated molecules show good agreement with the obtained experimental results.

MC Simulations

The MC simulation method is a useful technique for studying the adsorption of inhibitor molecules on the metal surface. Figure shows the most suitable configurations for adsorption of PTU, ITU, and water on Fe(110) substrates, and the detailed results are given in Table . Based on the images in Figure , it can be seen that PTU and ITU adsorb closely and lies parallel to the Fe(110) surface. These are due to coordinate bonds which are formed when inhibitor molecules donate electrons to the unoccupied d-orbital of iron or accept the electrons from iron through heteroatoms in molecules.

Figure 11

Top and side views of the adsorption of the inhibitors on Fe(110) surface in the gas phase: (a) PTU; (b) ITU; (c) H2O.

Table 7

Calculated Results of the Adsorption of Inhibitors on the Fe(110) Surface by MC Simulation

structures	total energy	adsorption energy	rigid adsorption energy	deformation energy
PTU/Fe(110)	–142.65	–92.47	–72.41	–20.06
ITU/Fe(110)	–210.07	–86.26	–82.50	–3.76
H2O/Fe(110)	–6.77	–6.80	–6.77	–0.03
pPTU-S18/Fe(110)	–389.14	–300.99	–295.26	–5.73
pITU-S1/Fe(110)	–454.43	–306.70	–303.52	–3.18

In the solution, adsorption of inhibitors on the metal surface is a quasi-substitution process between inhibitor molecules and water molecules. Because the adsorption energy of water is −6.80 kJ mol–1, it is smaller than that of PTU and ITU (Table );water molecules on the metal surface may be replaced easily by inhibitor molecules. Thus, the metal surface can be protected. Besides, the adsorption energies of PTU (−92.47 kJ mol–1), it is more negative than that of ITU (−86.26 kJ mol–1), which shows that PTU has a stronger tendency to adsorb on the metal surface.

To simulate the solvation conditions, fourteen water molecules are added (Figure ). According to Figure , those pPTU and pITU molecules are adsorbed on the Fe(110) surface through heteroatoms (nitrogen and sulfur) and aromatic ring. This observation is also received in the case of inhibitor adsorption in the gas phase (Figure ). Clearly, the protonation process do not change the adsorption configuration of studied inhibitors. However, the absolute adsorption energies of pPTU and pITU in water are greater than their neutral forms in gas. This increase in energy is attributed to the stabilization role of the solvent molecules.[56]

Figure 12

Side and top views of the adsorption of the protonated inhibitors on the Fe(110) surface in water: (a) pPTU-S18; (b) pITU-S1.

Conclusions

Corrosion inhibition ability of PTU and ITU have been studied by PDP measurement, EIS, quantum chemical calculation, and MC simulations. The factors affecting on the corrosion inhibition such as the molecular structure, concentration, and temperature are investigated. Following are the essential conclusions:

PTU and ITU are the potential corrosion inhibitors for steel in 1.0 M HCl with highest efficiencies of 98.96 and 92.65% at 60 °C, respectively. The inhibition ability of PTU is better than that of ITU in the same condition because PTU possesses a benzene ring in its structure.

The EIS results suggested that PTU and ITU protect the mild steel corrosion due to the formation of a protective inhibitor film at the metal–electrolyte interface.

The inhibition efficiency increases with the increase of temperature, and the inhibitors have tendency to be more strongly linked to the metal surface. They are predicted to participate in chemical adsorption on metal surfaces.

The adsorption of PTU and ITU in 1.0 M HCl obey the Langmuir adsorption isotherm.

The corrosion inhibition ability of PTU and ITU has been also determined via computational calculations and MC simulations. As a result, there is a good agreement between both experimental and theoretical results.

Experimental Section

Materials

The steel specimens were made from carbon steel, whose composition (in weight percent) is 0.28% carbon, 1.11% manganese, 0.40% silicon, 0.03% phosphorous, 0.02% sulfur, and iron is the remainder.

PTU and ITU were purchased from Shanghai Dibai Chemicals Technology Co., Ltd. The concentrations of ITU and PTU inhibitors added in 1.0 M HCl are 10–4, 5 × 10–4, 10–3, and 5 × 10–3 M.

Electrochemical Measurements

PDP curves were measured by the electrochemical device which was manufactured at Vietnam Academy of Science and Technology. The details of this device and electrodes are described in Table S5, Supporting Information.

PDP curves were measured in four temperatures: (20 ± 1), (30 ± 1), (45 ± 1), and (60 ± 1) °C, the potentials in the range of −0.55 to −0.50 V, a sensibility of 7, and a scanning rate of 1 mV/s.

The inhibition efficiency (IE %) of steel corrosion is calculated via eq (57)where iinh is the corrosion density with the presence of the inhibitor. icorr is the corrosion density without the inhibitor.

The degree of surface coverage (θ) is generated based on eq (21)

EIS was performed at open-circuit potential with an alternating current amplitude of 10 mV using a frequency region of 10 mHz to 100 Hz. The total number of points is 30. The EIS is recorded with Zahner Zennium (R) and IM6 using the Thales 4.5 software package.

Inhibition efficiency (IE %) is estimated by the eq (58)where Rct (Ω cm2) and Rct(inh) (Ω cm2) are the charge transfer resistances in the absence and presence of the inhibitor, respectively.

Before measuring the polarization curves and EIS, the electrode was immersed in test solution for 1 h to attain a stable state.

Computational Method

Quantum chemical calculations were carried out using the Gaussian 09 program.[59] The structures of all compounds in the gas phase were optimized using the B3LYP method in combination with the basis set of 6-311G(d,p). Quantum parameters mentioned at Section were also obtained at the same level of theory.

The fractional number of electrons transferred, ΔN, from a compound with high potential to the one with low potential can be calculated by eq (60,61)Here, χM and χinh are absolute electronegativity values of the metal and inhibitor. ηM and ηinh are absolute hardness values of the metal and inhibitor, respectively.

In case of iron, χFe = 7 eV, ηFe = 0 eV, and ΔN can be expressed as

The MC simulation using Materials Studio 6.0 software[62] was performed to examine the adsorption process of PTU and ITU on the Fe(110) surface. The interaction between inhibitors and Fe(110) surface was simulated in a (14.89 × 14.89 × 20.05 Å) box with a periodic boundary size. The optimized structures of the interaction of PTU and ITU with the Fe(110) surface were carried out using COMPASS force field.