Corrosion Inhibition of Mild Steel with Tolyltriazole

Abstract

Tolyltriazole (TTA) is a well-defined corrosion inhibitor for copper and copper alloys. However, there is little literature about its corrosion inhibition performance for mild steels in corrosive environments. This paper studied the electrochemical behavior of TTA in 0.5 M HCl solutions. Also, the morphology and nature of TTA layers on the steel surface were investigated. Electrochemical results showed that TTA is an excellent corrosion inhibitor for mild steel in acidic media with an efficiency of 91% for 0.07 M concentration. The results also indicated that TTA is a mixed-type inhibitor. XRD analysis revealed that the inhibition mechanism of TTA is based on the formation of an organic film due to the physically adsorbed molecules of TTA. AFM and EDS results showed that the formed layers decrease the adsorptivity of corrosive elements (Cl^-) on the steel surface. Density Functional Theory (DFT) study confirmed the experimental results.

Keywords:
Tolyltriazole (TTA); mild steel; corrosion inhibitor; Electrochemical; morphology; acidic media

1. Introduction

Mild steels are frequently used to manufacture different equipment in variety of industries. Although mild steels economical and have suitable mechanical properties in many industrial applications, they are prone to the corrosion. One of the most effective approaches to avoid corrosion in closed systems is the addition of corrosion inhibitors to corrosive solutions. A variety of organic inhibitors provide a possibility for the application of economical metals in different industries for corrosive environments^1-4.

As widely applicable organic corrosion inhibitors, azoles derivatives have various applications in the industries of copper^5-8, aluminum^9-11 and their alloys with the exposure to corrosive medium. Azoles derivatives can be applied for a wide range of steels^12-14 in acidic or alkaline solutions along with chloride ions. Among the group of azole derivatives, Tolyltriazole (TTA, C₇H₇N₃, with a chemical structure given in Figure 1 ⁵) is the most widely used corrosion inhibitor for copper and copper alloys, in cooling water systems, because of its low cost and high performance in corrosion inhibition^5,7,15-22.

Figure 1
Chemical structure of Tolyltriazole (TTA)

Derived from benzotriazole (BTA), TTA has an additional methyl group which is connected to the benzene ring. Therefore, TTA is a mixture of 4- and 5-methyl-1H-benzotriazole. Hydrophobic properties of the methyl branch, which is connected to the benzene ring, lead to the better performance of TTA in corrosion inhibition of copper and copper alloys^6,23,24. Although corrosion inhibition behavior of Tolyltriazole is reportedly similar to BTA, but there is little data on its detailed performance of corrosion inhibition²⁵.

Formation of a protective polymeric film, Cu(I)-TTA by the chemical adsorption of TTA on copper surface is accounted as the corrosion inhibition mechanism of TTA^23,26. The triazole groups are bonded to copper atoms via chemisorption of non-bonding electrons of nitrogen atoms. The molar ratio of 1:1 for TTA to Cu is accepted in the adsorbed layer^6,23,27,28.

Although corrosion inhibition properties and mechanisms of TTA on copper alloys have been investigated in numerous research, there is little published data about its effect on corrosion behavior of mild steel specially for API 5L steel pipelines as one of the most useful type in oil and gas industry. Three complexes of benzimidazoles were synthesized by Yadav et al.²⁹ as corrosion inhibitors of mild steels in 15% HCl solutions and an increase in corrosion resistance was observed through the gradual enhancement of concentrations. Different concentrations of BTA, benzimidazole, and imidazole in alkaline water were considered by Subramanyam and Mayanna³⁰ with the maximum inhibition efficiency for imidazole. Feng et. al. studied BTA for X65 steel pipeline in 0.01M NaHCO₃ solution. The results showed that BTA is a very effective corrosion inhibitor for X65 in this media by the efficiency of 95%, but the adsorption mechanism and temperature effects were not investigated in their work³¹.

The current paper has focused on the investigation of corrosion inhibition efficiency of TTA for API 5L Grade B mild steel in hydrochloric acid solutions. In this regard, possible mechanisms of corrosion inhibition and the nature of the adsorbed TTA molecules are discussed in details using the achieved results from electrochemical measurements, Adsorption isotherm analysis, topographical and Density Functional Theory (DFT) studies. To the best of our knowledge there is no published data on DFT study of TTA corrosion inhibition on metal surface.

2. Experimental

2.1. Preparation of the specimens and solutions

API 5L Grade B pipeline, with a chemical composition (wt.%) of C 0.13%, Si 0.22%, Mn 1.0%, P 0.008%, S 0.005%, Cu 0.1%, Mo 0.01% and Fe balance were applied as low carbon steel substrates. The specimens were cut and then they were coated by a cold-curing epoxy resin to provide a 10 mm × 10 mm area for exposure. Work surfaces of the steel samples were abraded using SiC emery papers from 60 to 1200 grit size. Finally, the samples were rinsed with deionized water and they were degreased in ethanol (purity of 99.5%) and acetone (purity of 99+%). Then the specimen were air-dried and they were located in a desiccator before experiments. To minimize the effects of surface roughness of metal for atomic force microscopy (AFM), specimens were abraded up to 1500 grit with SiC emery paper, and then they were polished with a suspension of 1μm diamonds. Then the samples were rinsed with deionized water and degreased in ethanol and acetone, and finally they were air-dried. The solutions of 0.5 M Hydrochloric acid were prepared using 37% HCl (Merck) and they were diluted using deionized water. Tolyltriazole (99.0% SAMCHUN, South Korea) was added to the solutions with different concentrations (0, 0.01, 0.03, 0.05 and 0.07 M). The solutions were stirred for 2 h.

2.2. Electrochemical studies

Electrochemical properties were determined using Autolab 302N potentiostat/galvanostat (Switzerland) equipped with Nova 1.11 software. The counter electrode was a Platinum plate with dimensions of 3 × 1 cm. Potentials were recorded according to the saturated calomel electrode (SCE). The distance between the counter and working electrodes was 2 cm. Initially, for investigation of the solution stabilization process, the open circuit potential (OCP) was plotted versus time. Then, the potentiostatic polarization curves were drawn using potential range of -250 to +250 mV (vs. OCP) and potential scanning rate of 1 mV / s 40 minutes after stabilization of HCl solutions. Polarization tests were conducted on the solutions with different concentrations (0 to 0.07 M TTA) at various temperatures (293, 303, 313 and 323 K). The electrochemical corrosion parameters, which were determined using polarization curves, included corrosion potential (E_corr), corrosion current density (i_corr), cathodic Tafel slope (β_c), and anodic Tafel slope (β_a). EIS measurements were also performed on the steel electrode after 40 minutes of being dipped into the solutions with a frequency range of 100 Hz to 0.01 Hz at Open Circuit Potential (OCP) with a small AC amplitude voltage of 10 mV.

2.3. Surface analysis: SEM/EDS/AFM/XRD

To evaluate the effects of adsorption of TTA molecules on inhibition of metal corrosion, the exposed surfaces of metal were investigated in terms of morphology and structural properties. Scanning electron microscopy (Phenom ProX Desktop SEM) was applied to study the exposed metal surfaces after being immersed for 2 h in 0.5 ml HCl solutions containing 0, 0.01, 0.03, 0.05 and 0.07 M of TTA.

Energy-dispersive X-ray spectroscopy (EDS) analysis was done by FESEM TESCAN MIRA III with SAMX detector. Atomic force microscopy (NTEGRA AFM NT – MDT) was conducted on steel electrodes after 8 hours of immersion in the solutions containing various concentrations of TTA. The silicon tip of AFM tip was mounted on triangular cantilevers (450 μm × 50 μm × 2 μm) with an average spring constant of 0.02~0.77 N/m. The scanning process was configured as a contact model, with a scanning rate of 1 Hz in the scanning rang of 10 μm × 10 μm, and a resolution of 256 × 256 pixels. X-ray diffraction technique (Diffractometer system XPERT-PRO ,2500 V) was applied to investigate the effects of inhibitor films on the crystalline phases of the steel surface with Cu-Kα radiation generated at 40 kV and 40 mA, a scanning range of 10° to 90° and a step size of 0.026 º.

2.4. Density-functional theory (DFT) study

The effectiveness of an inhibitor is related to its spatial structure as well as its molecular electronic structure. In this regard, quantum and theoretical chemistry have proved to be a powerful tool for studying corrosion inhibition mechanism. Proper models with computational simulations based on quantum and chemistry can support and confirm experimental findings; also by prediction of complicated physicochemical phenomena can decrease the costs and labors for experimental measurements. Corrosion inhibition is affected by the nature and state of the metal surface, corrosive environments, the chemical structure of inhibitors and their adsorption efficiencies onto the surface. Researches showed that molecular orbital energies, frontier orbital energy gap; E_HOMO-E_LUMO (where HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied molecular orbital), atomic charges, and dipole moments are the most important parameters in corrosion inhibition studies^32-35.

DFT is based on the fact that the thermodynamic ground states of both molecules and materials include the distribution of electrons which are in equilibrium with a field of positively charged point nuclei. Since the mass of nuclei is much higher than an electron, the electronic relaxation is much faster than the motion of the nucleus and the calculation of atomic nuclei motion and electrons motion can be done separately. When the positions of the nuclei change, the electronic distribution also shifts in such a way as to minimize the total system energy. DFT can simulate the electronic structure of molecules or materials, therefore, should be considered among the most useful techniques to predict the thermodynamic and kinetic properties of corrosion inhibition³⁶. Due to its theoretical-based work, DFT has been widely accepted as a “green corrosion inhibition technique” and it can be used to design corrosion inhibitors to prevent corrosion³⁷. Moreover, by using sophisticated computational tools, reactivity behavior in terms of hard and soft acid/base (HSAB) theory can be studied that provides a systematic way for analyzing and predicting inhibitor/surface interaction³⁸.

In the DFT calculations, for the first step the geometry of the inhibitors was fully optimized and vibrational analysis was performed simultaneously in the gas phase without any symmetry constraints by using PBE0/6-31G(d,p) as the method of theory. The resulting geometry was checked with respect to being at global minimum on the potential energy surface, as shown by the absence of imaginary frequencies. All calculations were performed with the ORCA 4.2.1 package of programs.

3. Results and Discussion

3.1. Open circuit potential (OCP) curves

OCP versus immersion time curves of steel electrodes in HCl solutions containing different concentrations (0 to 0.07 M) of Tolyltriazole (TTA) are indicated in Figure 2. Generally, addition of TTA into HCl solutions leads to the OCP shifts to more positive potentials. In an uninhibited solution (0.0 M TTA), the OCP establishes quickly at -0.55 V_SCE after about 15 minutes of immersion. Addition of TTA results in the shifts of OCP curves to more noble potentials. Consequently interactions of inhibitors on the surface of steel and also formation of possible protective layers lead to the prolongation of the time required for stabilization. Considering the dependence on inhibitor concentration OCP is estimated to be established after 30 to 40 min. Therefore, the duration of 40 min is chosen as the stabilization time for subsequent electrochemical measurements. Takenori et.al did polarization tests for TTA and some other derivatives of BTA after 10 min immersion in the 3% NaCl solution for Iron substrate³⁹.

Figure 2
Open circuit potential of mild steel in 0.5 M HCl solution containing various concentrations of TTA (0 to 0.07 M) at 293 K after 3600s immersion.

As increase of TTA concentrations lead to the OCP shifts to more noble potentials, its reason might be attributed to the inhibiting effects of TTA on steel oxidation and evolution of hydrogen gas in acidic solutions in which TTA is preferably adsorbed on anodic and cathodic sites of the steel. The delay of steel surface anodic reactions under open circuit conditions is believed to be the reason for OCP displacement to more noble potentials⁴⁰. Consequently there is a little displacement in OCP due to inhibitor inclusion with the maximum level of about 70 mV (from -0.55 V_SCE for uninhibited solution to -0.48 V_SCE for 0.07 M TTA) which confirms the mixed-type behavior of TTA as an inhibitor.

3.2. Potantiostatic polarization measurements

3.2.1. Effect of inhibitor concentration

Figure 3 shows the Potentiostatic polarization curves of mild steel in 0.5 M HCl solution with various concentrations (0 to 0.07 M) of TTA. As expected, TTA addition into the solution leads to the enhancement of corrosion potential (E_corr) to higher noble potentials. Furthermore, enhancement of TTA concentration results in the shifts of both cathodic and anodic curves to lower current densities. However the effect of TTA inhibition on anodic reactions is more pronounced at a constant potential of the plot and the potential displacement in the anodic branch is much lower than the former. Thus TTA acts as a mixed-type inhibitor which effectively retards both cathodic and anodic reactions on the mild steel surface but its action is more pronounced in the inhibition on anodic reactions. Mahbuboor et. al showed that TTA is mixed behavior type inhibitor for copper⁵.

Figure 3
Potentiostatic polarization curves of mild steel in 0.5 M HCl solution with various concentrations of TTA (0 to 0.07 M) at room temperature (293 K).

Dissolution of Fe ions might be the reason for anodic reactions of carbon steel in acidic solutions, while the cathodic reactions are mainly related to the hydrogen evolution.

Table 1 shows the electrochemical corrosion parameters which were obtained from Tafel polarization, including E_corr, corrosion current density (i_corr), slopes of the cathodic and anodic branches (β_c, β_a), and the inhibition efficiency (IE%) as a function of TTA concentration. The inhibiting efficiency, IE, can be calculated by Equation 1:

$$\text{IE\%=}\frac{{\text{i}}_{\text{corr}}^{\text{0}}{\text{-i}}_{\text{corr}}}{{\text{i}}_{\text{corr}}^{\text{0}}}\text{×100\%}$$ (1)

where ${\text{i}}_{\text{corr}}^{\text{0}}$ (mA/cm²) and ${\text{i}}_{\text{corr}}$(mA/cm²) are corrosion current densities in the absence and presence of the inhibitor in the solution, respectively.

Table 1
Electrochemical corrosion parameters obtained from the polarization curves of mild steel in 0.5 M HCl solution with different concentrations of TTA (0 to 0.07 M).

Concentration (M)	Ecorr, V(SCE)	icorr, mA/cm2	βa, Vdec-1	βc, Vdec-1	IE (%)
Blank	-0.551	0.00198	0.041	-0.035	---
0.01	-0.535	0.00130	0.032	-0.021	34.4
0.03	-0.524	0.00100	0.019	-0.022	49.2
0.05	-0.522	0.00051	0.064	-0.052	74.1
0.07	-0.486	0.00017	0.022	-0.026	91.4

According to the results which indicated the lower rates of corrosion reactions, it can be deduced that corrosion current density decreases significantly with the increase of TTA concentration in HCl solution. The increase of E_corr with TTA addition well corresponds to OCP curves. E_corr displacements are less than -85 mV (65 mV maximum here), proving the mixed-behavior of TTA as an inhibitor⁴¹. Furthermore it is found that the curve slopes (β_c and β_a) vary with addition of inhibitor addition which brings about some bigger changes in the anodic branch of the curves (β_a).

The inhibition efficiency, EI%, increases continuously with the enhancement of TTA concentration. When the TTA concentration is 0.07 M, the inhibition efficiency at room temperature (293 K) is about 91%. No significant change in corrosion current nor efficiency of inhibitor was noted with further addition of TTA.

It is concluded that TTA is an effective corrosion inhibitor for mild steel in chloride acidic media. Further addition of TTA above 0.07 M leads to a relatively stable i_corr which is due to the completion of protective film formation on steel surface.

3.2.2. Effect of temperature

The effects of different temperatures (293, 303, 313 and 323 K) on corrosion behavior of mild steels in HCl solution were clarified with and without varied concentrations of TTA. The results of polarization analysis indicating the influence of temperature on corrosion inhibition are represented in Figure 4. The relating temperature and EI% for different concentrations of TTA are listed in Table 2. As it was expected, temperature enhancement leads to the gradual decrease of efficiency of corrosion inhibition as it reaches to 79% at 323 K for 0.07 M TTA. Reduction of inhibition efficiency with temperature is caused by enhancement of carbon steel dissolution and also partial desorption of TTA layer off the metal surface. The observed relationship between EI% and temperature demonstrates the effective role of TTA as an effective inhibitor at higher temperatures (293 to 323 K here).

Figure 4
Effect of temperature on inhibition efficiency of TTA with different concentrations (0.01 to 0.07 M) on mild steel in HCl solution at various temperatures (293, 303, 313 and 323 K).

Table 2
Corrosion efficiency results of mild steel in 0.5 M HCl solution with different concentrations of TTA (0.01 to 0.07 M) at various temperatures (293, 303, 313 and 323 K).

Temperature (K)	EI %
	0.01 M TTA	0.03 M TTA	0.05 M TTA	0.07 M TTA
293	34.4	61.9	78.1	91.4
303	29.4	57.0	74.1	87.3
313	26.5	53.4	69.0	83.3
323	23.1	50.9	64.5	79.2

Also inhibition efficiency reduction with increase in temperature, may be due to possible repulsion of some of the adsorbed TTA from the metal surface at higher temperatures. This behavior shows that TTA were physically adsorbed on the steel surface⁴².

3.2.3. Adsorption isotherm and thermodynamic parameters

To describe the probable mechanism for the interaction of organic corrosion inhibitors and the surface of metal, isotherms of adsorption are investigated. The obtained data were fitted to the most frequently adsorption isotherms : Temkin, Freundlich and Langmuir, and the correlation coefficients (R²) were used to determine the best fits. All three isotherms are suitable for fitting the data, but the best fit is for Langmuir adsorption isotherm. Data fitting for Temkin and Freundlich isotherms are mentioned in ^{Supplementary file}.

Takenori et.al showed that adsorption of TTA on Iron substrate obeys Langmuir adsorption isotherm in 3% NaCl solution³⁹. Also Mahbuboor et. al showed Langmuir is the most suitable adsorption isotherm for TTA on copper surface in Synthetic Cooling Water⁵.

The Equation for Langmuir adsorption isotherm is:

$$\frac{C}{\theta }=\frac{1}{K_{ads}}+C$$ (2)

Where C, K_ads and θ are the concentration of the inhibitor, equilibrium constant of adsorptive, and the surface coverage obtained from the EI% of polarization curves, respectively. The plot of C/θ versus C (0.01 to 0.07 M) at various temperatures (293-323 K) is represented in Figure 5. TTA adsorption results which obtained from experiments and indicated by linear plots at different temperatures confirm the possibility for approximation of TTA adsorption by Langmuir adsorption isotherm. Langmuir adsorption isotherm showed that polar atoms (N) or groups (CH₃) in TTA interact by mutual adsorption or desorption⁴³.

Figure 5
Langmuir adsorption isotherm plots of the steel specimens in 0.5 M HCl with different concentration of TTA (0.01 to 0.07 M) at various temperatures (293, 303, 313 and 323 K).

The adsorption parameters obtained from the plots are presented in Table 3.

Table 3
Parameters for Langmuir adsorption of mild steel in 0.5 M HCl solution containing different concentrations of TTA (0.01 to 0.07 M) at various temperatures (293, 303, 313 and 323 K).

Isotherm (K)	Intercept	Slope	R2	Kads (M-1)
293	0.0229	0.7904	0.9908	43.67
303	0.0279	0.7674	0.9923	35.84
313	0.0316	0.7760	0.9902	31.65
323	0.0363	0.7675	0.9902	27.55

Generally, higher inhibition efficiency of a certain inhibitor is confirmed by larger values of K_ads calculated from the intercepts which is related to its better binding properties on the surface of metal⁴⁴. K_ads is a temperature-dependent parameter and decreases with enhancement of temperature which indicates desorption of some adsorbed inhibitors from the metal surface. It is clear that the linear correlation coefficients (R²) of the plots are close to unity which reveals that TTA adsorption on the steel surface corresponds to Langmuir adsorption isotherm. However, the slopes, being close to 1, indicate a non-ideal stimulation of Langmuir adsorption isotherm. The deviation from unity might be a consequence of the interactions between the molecules of adsorbed inhibitor and unsaturated nuclei on the metal surface⁴⁵.

The adsorption heat (ΔH_ads) could be calculated according to the Van’t Hoff Equation 3 ⁴⁶:

$$\ln K_{ads}=\frac{-\Delta H_{ads}}{RT}+D$$ (3)

Where D is the constant of integration, R is the gas constant (8.314 J K^-1 mol^-1), and T is the absolute temperature (K). Figure 6 represents the linear plot of ln K versus 1000/T (the linear correlation coefficient is 0.998). ΔH⁰_ads can be calculated from the slope (-ΔH⁰_ads/R) as a constant of the adsorptive reactions (kJmol^-1).

Figure 6
The relation between lnK_ads(M^-1) and 1000/T(K) of TTA inhibitor in 0.5 M HCl.

Standard adsorption free energy (ΔG⁰_ads) could be obtained according to following Equation 4:

$$K=\frac{1}{55}\exp (\frac{-\Delta G_{ads}^0}{RT})$$ (4)

55 is the molar concentration of water^47-49. The standard adsorption entropy (ΔS⁰_ads) can be obtained from the following thermodynamic Equation 5:

$$\Delta S_{ads}^0=\frac{\Delta H_{ads}^0-\Delta G_{ads}^0}{T}$$ (5)

The calculated thermodynamic parameters of the adsorption isotherms of TTA are listed in Table 4.

Table 4
Thermodynamic parameters of the adsorption isotherm of TTA on Mild steel surface in HCl solution.

Temperature (K)	ΔG0ads (kJmol-1)	ΔH0ads (kJmol-1)	ΔS0ads (Jmol-1)
293	-18.98	-11.88	24.25
303	-19.13	-11.88	23.95
313	-19.44	-11.88	24.17
323	-19.69	-11.88	24.19

Negative values of ΔG⁰_ads and ΔH⁰_ads indicate the exothermic nature of inhibitor adsorption⁵⁰ and also the spontaneous nature of the adsorption of TTA molecules onto the steel surface.

The exothermic behavior of the adsorption reactions may lead to the repulsion of some adsorbed inhibitor molecules from the metal surface with increase of temperature. Consequently, this can lead to a decrease in inhibition efficiency. This corresponds to the values of IE% vs. temperature in Figure 3. In the adsorption reactions which the value of ΔG⁰_ads is up to -20 kJ mol^-1, the adsorption is conducted through the electrostatic interactions between the charged inhibitor molecules and the charged metal surface(physisorption), while in those with the amount of ΔG⁰_ads below -40 kJ mol^-1 adsorption involves sharing or transferring electrons between the inhibitor molecules and the metal surface for establishing a chemical bond in between (chemisorption)^51,52. From Table 4, it is clear that the value of ΔG⁰_ads is about -20 kJ mol^-1 which confirms the dominant physical nature of adsorption which is conducted through electrostatic interactions.

ΔS⁰_ads in Table 4 is positive, while adsorption is an exothermic process and expected to be associated with a decrease in entropy. Ateya et al.⁵³ described that the adsorption of the organic compound happens with repulsion of water molecules on the surface. They concluded that despite of adsorption process being exothermic for the organic inhibitor which reduces the entropy of the solute, desorption of solvent is endothermic and leads to the increase of entropy. This means that the mentioned coincidence leads to the increase of entropy during desorption of the solvent off the surface. Therefore, the total alteration of entropy during the adsorption process is positive. The positive ΔS⁰_ads means that the adsorption leads to increase the entropy, which is the driving force for the adsorption of inhibitor onto the mild steel surface⁵⁴.

3.3. Electrochemical impedance spectroscopy measurements

Electrochemical impedance spectroscopy (EIS) can provide useful information about the resistive and capacitive behavior at the metal/electrolyte solution interface formed by inhibitors additions. Electrochemical impedance spectroscopy (EIS) was used for investigation of TTA corrosion inhibition for mild steel substrate, in 0.5 M HCl solution after 2 hours of being immersed in HCl solutions containing 0 to 0.07 M TTA.

Figure 7 and Figure 8 represent the Nyquist and Bode diagrams which have been measured for mild steel in 0.5 M HCl solution containing various concentrations of TTA.

Figure 7
Nyquist diagrams for the corrosion of mild steel in 0.5 M HCl solution with various concentrations of TTA (0.01 to 0.07 M) at 293 K.

Figure 8
Bode plots for the corrosion of mild steel in 0.5 M HCl solution with various concentrations of TTA (0.01 to 0.07 M) at 293 K.

The radius of semicircles in Nyquist plot can be considered as an indicator for corrosion resistance of under studied systems. As can be seen in Figure 7, the radius of the Nyquist semicircles increased by increasing the inhibitor concentration indicating that the water molecules adsorbed on the mild steel surface were replaced successfully by the inhibitor molecules dissolved in the electrolyte. In addition, the semicircle shape for each measurement in the Nyquist diagram confirms that the corrosion process is activation-controlled and there is no evidence for diffusion control. Therefore, in the Bode diagrams (Fig. 8), the absolute impedance at low frequencies could indicate the total corrosion resistance of the system. According to Fig 8, the increasing of TTA concentration led to the increase of the absolute impedance at low frequencies, suggesting the formation of a protective layer of the adsorbed inhibitor on the immersed steel surface. Due to the protective layer formation, the charge transfer on the steel surface was inhibited and so the corrosion reactions were significantly affected. More positive phase angle values at the highest frequency in the presence of higher inhibitor concentrations is another prove showing the protective layer formed on the immersed steel surface in the presence of TTA.

Employment of equivalent circuits to model the electrochemical reactions is a well-known technique for interpretation of EIS results^55,56. The phase angle diagrams depicted in Fig. 8 indicated that except for the blank solution that showed a one-time constant behavior, other samples should be fitted by tow-time constant circuits. The suitable equivalent circuits for both categories of behaviors are shown in Figure 9.

Figure 9
Equivalent circuit model for EIS measurements, a) with TTA and b) without TTA

In Figure 9, R_s is solution resistance, R_ct is charge transfer resistance, and R_f is the resistance of formed film on the steel surface. Also, in these circuits, Q_dl and Q_f are constant phase element (CPE) of double layer and the formed film, respectively. CPE, which is known as the non-ideal capacitance element, is used instead of ideal capacitance to obtain more precious fitting with experimental data. Equation 6 represents the relationship between the values of ideal capacitance (C), resistance (R) and admittance (Y₀) of CPE⁵⁷.

$$C={\left(Y_0\right)}^{1/n}\times R^{\left(1-n\right)/n}$$ (6)

In Eq (6), the coefficient "n" represents the depressing feature in Nyquist plots. If n=1, CPE behaves as an ideal capacitor and if n=0, it represents a resistor. It is well known that the value of "n" coefficient is attributed to the heterogeneity of the working electrode surface or to continuously distributed time constants for charge-transfer reactions⁵⁷. The obtained parameters from the electrochemical modeling are reported in Table 5.

Table 5
Obtained results from the electrochemical modeling of the EIS experimental data on the suitable equivalent circuits.

Concentration TTA (M)	Rs(Ω cm2)	Rf (Ω cm2)	CPEf		Cf (μF.cm2)	Rct (Ω cm2)	CPEdl		Cdl (μF.cm2)	Rtotal (Ω cm2)	I.E. (%)
			Y0,f (S.secn.cm2)	n1			Y0,dl (S.secn.cm2)	n2
0	2.14	-	-	-	-	740.3	2.07E-4	0.77	118.21	740.3	-
0.01	3.58	313.3	9.22E-4	0.53	306.53	916.4	4.22E-4	0.79	375.25	1229.7	39.80
0.03	5.84	2121	1.56E-4	0.73	103.62	60.68	2.79E-4	0.82	94.35	2181.68	66.07
0.05	12.83	2466	6.77E-5	0.87	51.81	335.6	9.66E-5	0.85	36.72	2801.6	73.58
0.07	9.27	7759	5.10E-5	0.88	38.06	744.5	6.63E-5	0.89	31.25	8503.5	91.29

In Table 5, R_total is the sum of R_ct and R_f values and the inhibition efficiency (I.E.) is calculated by Equation 7 ^58-60.

$$I.E.\%=100\times \left(R_{total}{}^i–R_{total}{}^0\right)/R_{total}{}^i$$ (7)

where R_totalⁱ and R_total⁰ are the total resistances of the working electrode with and without TTA inhibitor, respectively. As it can be seen from Table 5, R_ct and R_f values of inhibited substrates simultaneously increase with the enhancement of inhibitors concentration. The highest total resistance belongs to the sample containing 0.07 M TTA. The increasing trend of n values by increasing the inhibitor concentration indicates that the surface homogeneity of the immersed steel is enhanced by formation of a passive layer on it⁶¹. According to Table 5, the decreasing trend of C_dl values by increasing the concentration of the inhibitor can be related to the decrease in dielectric constant and/or increase in the thickness of the electrical double layer, due to the film formation on the immersed electrode⁶². In addition, the inhibition efficiency (I.E.%) increased with the enhancement of inhibitor concentration. A maximum of 91% inhibition efficiency is achieved with an inhibitor concentration of 0.07 M. The calculated inhibition efficiency from the impedance results is consistent with the obtained results in the potentiostatic polarization technique.

3.4. Scanningelectron microscopy (SEM)

SEM backscattered electrons images of mild steel surfaces after 2 hours of being immersed in 0.5 M HCl with and without TTA are given in Figure 10. By the comparison of SEM images of mild steel in presence and absence of TTA, the inhibition performance of it can be interpreted. The steel surface in the uninhibited solution (TTA-free solution) is shown in Figure 10a. Some corrosion damages are clearly apparent on the steel surface. Figures 1010e illustrate the surface of the samples in HCl solutions with 0.01 to 0.07 M TTA, respectively. Bright and smooth areas of the images indicate the TTA film adsorbed on the metal surface. The increase of TTA concentration leads to the enhancement of coverage percentage of the surface up to a nearly full coverage (about 91%) in 0.07 M TTA. These results suggest that addition of TTA to the corrosive solution results in the successful formation of a uniform barrier layer all over the surface which is responsible for the corrosion inhibition of the mild steel.

Figure 10
SEM backscattered electron images of mild steel surfaces in the a) absence and in the presence of b) 0.01 M, c) 0.03 M, d) 0.05 M and e) 0.07 M TTA exposed to 0.5 M HCl for 2 h.

3.5. Energy dispersive X-ray analysis (EDS)

Determination of elemental composition of steel surface after 2 hours of being in in 0.5 M HCl. was performed by EDS analysis. As it is indicated in Figure 11, the EDS spectra for the specimens show an additional characteristic peak when TTA is present in 0.5 M HCl solution. This peak is attributed to N from TTA inhibitor (on 0.392 KeV) which confirms that TTA molecules are adsorbed on steel surface. Increase of TTA concentration leads to the enhancement of N peak intensity, while intensity of Cl peak (on 2.622 KeV) decreases at the same time. The results demonstrate that adsorption of TTA molecules can lead to a delay in the adsorption of corrosive elements. Also the smooth and uniform TTA layer can reduce the number density of adsorbent sites on the surface.

Figure 11
EDS spectra of mild steel surfaces in the a) absence and in the presence of b) 0.01 M, c) 0.03 M, d) 0.05 M and e) 0.07 M TTA exposed to 0.5 M HCl for 2 h.

The appearance of oxygen in the spectra, shows that there are some Fe or other elements oxides in the surface of the mild steel.

3.6. Atomic force microscopy (AFM) analysis

Figure 12 shows the AFM surface topography of mild steel after 8 hours of being immersed in 0.5 M HCl solutions with and without TTA in different concentrations. The mean surface roughness (R_av) is reported to be 208, 123, 103, 95, and 53 nm for 0, 0.01, 0.03, 0.05, and 0.07 M TTA concentrations, respectively. In the absence of TTA, the carbon steel suffers from possible corrosion damages, and its surface is quite rough and uneven, as shown in Figure 12a. When TTA is added into the solutions (from 0.01 to 0.05 M), the surface of specimens become gradually smoother compared with the uninhibited solution. The reason is the continuous coverage of the surface by gradual formation of a layer of organic film produced by the electrostatic interactions between adsorbed TTA molecules and charged Fe atoms of the surface, as it is shown in the Figure 1212d. When TTA concentration increases to 0.07 M, the surface is smoothed as illustrated in Figure 12e. The reason for significant decrease of surface roughness of the steel surface which is observed in Figure 12e , is the uniform coverage provided by formation a layer of organic film on nearly all over the metal surface in 0.07 M TTA (about 91% coverage). The organic film can protect the steel surface in two ways : it can form a physical barrier against corrosive elements and also it can reduce the possibility for adsorption of those elements on the probable adsorbent sites through the formation of an even and uniform layer on the surface.

Figure 12
AFM images of mild steel after 8 h immersion in 0.5 M HCl containing different concentrations of TTA (a) blank, (b) 0.01 M, (c) 0.03 M, (d) 0.05 M, (e) 0.07 M.

3.7. X-Ray diffraction analysis

X-Ray Diffraction (XRD) analysis is a prominent method to determine the fundamental phases or crystallinity of the compounds in corrosion studies. Figure 13 shows the XRD patterns of the surface of the mild steel specimens after 40 minutes of being immersed in 0.5% HCl solution with different TTA concentrations (0 to 0.07 M). The XRD patterns and the position of the peaks are fundamentally similar with and without inhibitors. No additional peak is seen which could be related to adsorption of TTA inhibitors. This indicates the probable amorphous nature of organic films. The only difference in the patterns is the reduction of the peak intensities is the only difference between the patterns which is a result of enhancement of TTA concentration. Therefore it can be concluded that TTA inhibitor is prominently physio-adsorbed on the surface of the steel instead of chemisorption of the complexes derived from chemical reactions between TTA and Fe atoms. Due to the physical nature of TTA molecules adsorption, the intensities of the peaks reflected from the surface of the steel specimens are decreased. These results are in agreement with Langmuir isotherm analyses. There are three peaks at 44.637^o, 64.947^o and 82.332^o can be indexed to (110), (200) and (211) planes respectively which are belong to the Ferrites and Carbides of carbon steel substrate^63,64.

Figure 13
XRD pattern of mild steel surface after 2 h immersion in 0.5 M HCl containing different concentrations of TTA (a) blank, (b) 0.01 M, (c) 0.03 M, (d) 0.05 M, (e) 0.07 M.

3.8 Density functional theory (DFT)

As it is cleared in Figure 14 there are two probabilities of protonation in the structure of TTA the middle nitrogen and the end one. For the study of the proper position of protonation the structure of each molecule was optimized in neutral and protonated form. The energy difference between the two forms of protonated structures shows the end position is more suitable base on free Gibbs energy of protonation.

Figure 14
Protonation probabilities in the Structure of BTA and TTA.

The structural and electronic parameters of BTA, TTA-4 and TTA-5 were calculated for the neutral and protonated structure and the results show that there is a strong effect on the chemical properties, specifically in the electron-donating capability to the metal. However, different factors need to be considered for clarifying the orientation of organic molecules on the electrode surface. The atoms and groups of the molecules may interact with the surface depending on the geometry of the inhibitor as well as the nature of their frontier molecular orbitals. Frontier molecular orbital (HOMO and LUMO) theory is useful in predicting the adsorption centers of the inhibitor responsible for the interaction with metal surface. The HOMO and LUMO depictions of the studied benzotriazole family and the cases of protonated forms are shown in Figure 15.

Figure 15
Molecular Orbitals of (a) BTA, (b) HBTA⁺ and (c) HBTA⁺

From Figures 15 16-17, it could be seen that they have same HOMO and LUMO distributions in the neutral and in the protonated forms and are concentrated over the rings. Thus, based on the frontier orbitals, the unoccupied d orbitals of a Fe atom can accept electrons from an inhibitor molecule mainly from central nitrogen as sigma donating centers in the neutral and protonated form. Also, the inhibitor molecule can accept electrons from a Fe atom with its antibonding orbitals with contribution of rings to form a back-donating bond mainly in the protonated form. These donation and back-donation processes strengthen the adsorption of inhibitors onto the surface.

Figure 16
Molecular Orbitals of (a) TTA-4, (b) HTTA⁺ and (c) HTTA⁺

Figure 17
Molecular Orbitals of (a) TTA-5, (b) HTTA⁺ and (c) HTTA⁺

Based on Koopman’s theorem⁶⁵ E_HOMO is often associated with the electron donating ability of a molecule; high values of E_HOMO are likely to indicate the tendency of the molecule to donate electrons to appropriate acceptor molecules with lower energy MO. E_LUMO, on the other hand, indicates the ability of the molecule to accept electrons.

$$I=-E_{HOMO}$$ (8)

$${}_{}A=-E_{LUMO}$$ (9)

The binding ability of the inhibitor to the surface of metal increases with increasing HOMO and decreasing LUMO energy. Consequently, the lower the value of E_LUMO, the most probable it is that the molecule would accept electrons. Moreover, the energy gap between the HOMO and LUMO orbitals of the molecule is an important parameter that determines the reactivity of the inhibitor molecule toward the adsorption on the metallic surface. As ΔE decreases (most especially for the cationic species), the reactivity of the molecule increases leading to an increase in the inhibition efficiency of the molecule⁶⁶. For effective overlapping, the energy difference between the orbitals generally must be low, and the overall energy difference between the orbitals HOMO and HOMO-1) indicating their participation in the metal−ligand interaction. In addition, the energy difference between the HOMO-2 and HOMO-1 for the studied structures is so low confirming the involvement of HOMO-1 and HOMO-2 in the ligand−metal interaction (Table 6). A similar observation is also noted for the protonated structures, suggesting that the parameters of orbitals such as HOMO, HOMO-1, and HOMO-2 are important for chemical reactivity over the iron surface⁶⁷.

Table 6
Quantum physical and chemical parameters of BTA, TTA-4 and TTA-5.

Parameter	BTA			TTA-4			TTA-5
	Neutral	Protonated Middle	Protonated End	Neutral	Protonated Middle	Protonated End	Neutral	Protonated Middle	Protonated End
LUMO	-0.037(7)	-0.251(8)	-0.245(4)	-0.034(1)	-0.245(2)	-0.238(7)	-0.036(3)	-0.245(9)	-0.239(0)
HOMO	-0.252(4)	-0.430(1)	-0.438(9)	-0.243(2)	-0.412(8)	-0.422(7)	-0.243(7)	-0.414(7)	-0.424(6)
HOMO-1	-0.264(7)	-0.456(4)	-0.453(2)	-0.260(6)	-0.449(6)	-0.446(3)	-0.259(8)	-0.443(8)	-0.439(8)
HOMO-2	-0.280(4)	-0.519(9)	-0.548(9)	-0.277(9)	-0.510(0)	-0.533(4)	-0.277(7)	-0.509(2)	-0.522(3)
HOMO-3	-0.337(5)	-0.550(6)	-0.559(3)	-0.328(6)	-0.523(8)	-0.538(7)	-0.333(4)	-0.523(8)	-0.536(8)
LUMO - HOMO	0.215	0.179	0.193	0.209	0.167	0.184	0.207	0.169	0.185
HOMO - (HOMO-1)	0.012	0.026	0.015	0.017	0.037	0.024	0.016	0.029	0.015
(HOMO-1) - (HOMO-2)	0.016	0.063	0.095	0.017	0.061	0.084	0.018	0.066	0.083

It is evident from Table 6 that TTA-4&5 have the highest E_HOMO in the neutral form and a lower E_HOMO in the protonated form. This means that the electron donating ability of theme are weaker in the protonated form. It is clear from Table 6 that the protonated form of BTA exhibits the lowest E_LUMO, making the protonated form the most likely form for the interaction of mild steel with BTAH⁺ molecule. The calculations further show that TTA-5 in the protonate form has the smallest ΔE value (0.185 eV) indicating that TTAH⁺-5 is the most reactive inhibitor that can easily adsorb on the metal surface causing higher protection. This agrees with the experimental results that TTA-5 could have better inhibitive performance on mild steel surface in the protonated form i.e. through electrostatic interaction between the cation form of TTA-5 and the vacant d-orbital of steel. Moreover, the adsorption of TTA-5 on the steel surface using the neutral form also plays a part in the overall inhibiting process.

Absolute hardness, η, and softness, σ, are important properties to measure the molecular stability and reactivity. A hard molecule has a large energy gap, and a soft molecule has a small energy gap. Soft molecules are more reactive than hard ones because they could easily offer electrons to an acceptor. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where σ has the highest value and η the lowest value. The result from Table 7 shows that TTA-4 & 5 in the protonated form has the lowest energy gap, lowest hardness, and the highest softness; this agrees with the experimental results that TTA could have a better inhibitive performance on the mild steel surface in the protonated form i.e. through electrostatic interaction between the cation form of TTA and the vacant d-orbital of steel. Density functional theory (DFT) has been found to be successful in providing intuitions into the chemical reactivity and selectivity, in terms of global parameters such as electronegativity (χ), hardness (η) and softness (σ). Table 7 shows the calculated quantum theoretical parameters which provide information about the reactive behavior of BTA, TTA-4 and TTA-5 and their protonated structures.

Table 7
Calculated theoretical quantum parameters of BTA, TTA-4 and TTA-5.

	BTA			TTA-4			TTA-5
	BTA	M-HBTA+	E-HBTA+	TTA	M-HTTA+	E-HTTA+	TTA	M-HTTA+	E-HTTA+
I	0.252(4)	0.430(1)	0.438(9)	0.243(2)	0.412(8)	0.422(7)	0.243(7)	0.414(7)	0.424(6)
A	0.037(7)	0.251(8)	0.245(4)	0.034(1)	0.245(2)	0.238(7)	0.036(3)	0.245(9)	0.239(0)
$\eta$	0.107(3)	0.089(1)	0.096(7)	0.1045(4)	0.083(8)	0.092(0)	0.103(7)	0.084(4)	0.092(8)
$\sigma$	4.657(4)	5.609(7)	5.169(0)	4.782(6)	5.967(6)	5.434(7)	4.821(1)	5.924(1)	5.386(1)
$\chi$	0.145(1)	0.341(0)	0.342(1)	0.138(7)	0.329(0)	0.330(7)	0.139(9)	0.330(2)	0.331(8)
$\mu$	0.145(1)	0.341(0)	0.342(1)	0.138(7)	0.329(0)	0.330(7)	0.139(9)	0.330(2)	0.331(8)
$\omega$	0.098(1)	0.652(2)	0.605(2)	0.091(9)	0.645(9)	0.594(2)	0.094(4)	0.646(1)	0.593(1)

4. Conclusion

TTA shows a high efficiency for corrosion inhibitioan which is about 91% for mild steel with a concentration of 0.07 M in 0.5 M HCl at room temperature. It is also found that inhibition properties of TTA are maintained up to a certain temperature which indicates that it can tolerate a certain level of temperature enhancement. Electrochemical studies revealed that TTA is a hybrid (anodic and cathodic) corrosion inhibitor which obeys the Langmuir adsorption isotherm. The increase of TTA concentration leads to the shift of OCP and polarization curves to the more positive potentials, and also enhancement of the corrosion resistance which is caused by the reduction of the corrosion current. Surface analysis indicates the formation of a uniform and stable organic film on metal surface as a result of physical adsorption of TTA molecules which protects the surface against the corrosion by HCl solution. XRD and AFM results demonstrate that increase of TTA concentration results in the enhancement of coverage and thickness of the amorphous protective film, while the roughness of the film formed on the substrate surface significantly decreases which may result in the decrease of adsorptivity for corrosive elements (Cl^-), as can be confirmed by EDS results. DFT confirmed that TTA in protonated form, has excellent corrosion inhibition performance for mild steel in acidic media.

Supplementary material

The following online material is available for this article:

Supplementary file - Temkin Isotherm.

5. Acknowledgments

The authors are grateful to Iran National Science Foundation (INSF) for the fund of project (number 97012056) and also Research Deputy of Sharif University for partial financial support.

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Publication Dates

• Publication in this collection
  14 May 2021
• Date of issue
  2021

History

• Received
  30 Aug 2020
• Reviewed
  03 Mar 2021
• Accepted
  06 Apr 2021