New anti-corrosion inhibitor (3ar,6ar)-3a,6a-di-p-tolyltetrahydroimidazo[4,5-d]imidazole-2,5(1 h,3h)-dithione for carbon steel in 1 M HCl medium: gravimetric, electrochemical, surface and quantum chemical analyses

2.1 Carbon steel sample and investigated solution

In this research work, St2 steel material was used as a working electrode, which is manufactured by the “Chermetcom” trading company, Moscow, Russian Federation. St2 steel is a member of the carbon steel family and is mainly used in the chemical industry. It contains ∼98% Fe, 0.09–0.15% C, 0.05–0.15% Si, 0.05% S, 0.25–0.5% Mn, 0.008% N, 0.04% P, 0.3% Cr, 0.3% Ni, 0.08% As and 0.3% Cu. The carbon steel material surface was cleaned and rubbed by SiC abrasive papers and then washed with distilled water, followed by ethanol and isopropyl alcohols. Last, it was dried with acetone. All experiments were carried out in 1 M hydrochloride acid solution, which was prepared from 36.5% HCl by diluting it with distilled water. The experiments were performed at inhibitor concentrations of 50, 100 and 150 mg/L. HCl (36.5%), thiourea and KOH were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., (P.R. China) and Tianjin Jiangtian Chemical Reagent Co., Ltd. (P.R. China).

2.2 The inhibitor TTHIIDT

In this research work, the inhibitor TTHIIDT was utilized, which was synthesized and characterized according to a previously reported synthetic methodology (Singh et al., 2014). The Scheme 1 represents that the molecular structure of the studied inhibitor.

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Scheme 1

The molecular structure of the inhibitor TTHIIDT.

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2.3 Gravimetric analysis

In this investigation, gravimetric tests were conducted on 3 cm * 3 cm * 0.1 cm metal in the inhibitor-free and the inhibitor-containing 1 M HCl medium at various concentrations of the inhibitor TTHIIDT according to the ASTM G31-72 standard. The first step of the gravimetric tests was performed by immersing the studied metal sample for 3 h. In the following stage, this immersed metal sample was washed four times with distilled water and ethyl alcohol and then dried with acetone. In the final process, the dried metal sample weight was calculated three times, and the average weight was utilized for gravimetric measurements (Chauhan et al., 2020, 2019; Gupta et al., 2019; Fan et al., 2019; Kikanme et al., 2020; El Kacimi et al., 2020).

2.4 Electrochemical analysis

All electrochemical tests were performed by a Gamry potentiostat (IFC1000-06177)/(PC5) galvanostat/ZRA analyser (Model G-300, Pstat model Interface 1000, USA). To conduct the electrochemical tests, a three-electrode cell assembly was utilized, containing a working electrode (carbon steel sample), reference electrode (saturated calomel) and counter electrode (platinum). At the start of the electrochemical experiments, the three electrodes were submerged in the investigated solution, and the working electrode was immersed for 30 min. The diffusion potential of electrochemical processes on the surface of metal in solution changed for 30 min, after which it reached a stable state, indicating that the electrochemical processes on the surface of carbon steel in the studied environment achieved a steady-state potential. After 30 min of the diffusion-stabilization period, the open-circuit potentials were measured for 120 min because the quasi-substation processes on the surface of working carbon steel electrode stabilized and the OCPs reached a plateau. Finally, the corrosion parameters of the corrosion and inhibited media were measured by various electrochemical methods after a 150 min stabilization period (Chauhan et al., 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Ye et al., 2020; Aly et al., 2020; Tabatabaei majd et al., 2020).

2.5 Surface analysis

The surface characteristics of investigated metal specimen in a 1 M hydrochloride acid environment with and without the inhibitor TTHIIDT at the optimum concentration was researched by scanning electron microscopy (SEM) on a HITACHI TM3000 system. Before taking SEM images, the metal was immersed for 3 h in the studied solutions. After immersion, the specimen was washed four times with distilled water and alcohol. In the final step, is the specimen was dried with acetone followed by SEM measurement (El Kacimi et al., 2020; Ye et al., 2020; Aly et al., 2020; Tabatabaei majd et al., 2020; Dehghani et al., 2020; Lgaz et al., 2020).

2.6 Quantum chemical analysis

To exam the quantum chemical properties of the inhibitor TTHIIDT, the quantum computation was conducted through density functional theory (DFT). The B3LYP method was performed in the DFT calculations with 6-311G (d, p) basis sets. The DFT analysis illustrated the correlation between the inhibition properties and the molecular characteristics of the investigated inhibitor (Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020).

3.1 Gravimetric measurements

3.1.1

3.1.1 Concentration influence

To identify the impact of the TTHIIDT concentration on the inhibition efficiency, the gravimetric analysis was performed at constant temperature (303 K) with various concentrations, as described in Fig. 1(a). The inhibition efficiencies and corrosion rates at given concentrations were derived according to Equations (1) and (2), respectively.

(1)

$${\eta }_{\mathit{Grav}.},\%=(1-\frac{W_{\mathit{inh}}}{W_0})\times 100$$ where ${\eta }_{\mathit{Grav}.}$ is the inhibition efficiency degree, and $W_{\mathit{inh}}$ and $W_0$ represent the mass loss of the studied metal sample in inhibited and uninhibited 1 M HCl solutions.

(2)

$${\mathit{CR}}_{\mathit{Grav}.}=\frac{W}{\mathit{At}}$$ where ${\mathit{CR}}_{\mathit{Grav}.}$ symbolizes the corrosion rate at mg/cm²hour, $A$ demonstrates the surface area in cm², $t$ represents the immersion time in hours and $W$ indicates the weight loss, respectively (Chauhan et al., 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020).

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Fig. 1

Concentration versus inhibition efficiency and corrosion rate (a) (303 K), Arrhenius (b) and transition-state plots (c) for carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT.

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Generally, it should be stressed from Fig. 1(a) that the corrosion rate of a given metal sample and the inhibition efficiency of the observed inhibitor slowly increased and then reduced with rising inhibitor concentrations at a given constant temperature. Correspondingly, at 150 mg/L concentration of the inhibitor TTHIIDT, it was recorded that ${\mathit{CR}}_{\mathit{Grav}.}$ was 0.012 mg/cm²hour and that ${\eta }_{\mathit{Grav}.}$ was 99.15%. This phenomenon occurred because of the electron-rich sulfur and nitrogen heteroatoms in the inhibitor TTHIIDT, which promoted better adsorption interactions, in addition to the π-electron donor phenyl rings, which also supported those interactions.

3.2 Electrochemical frequency modulation measurement

In the EFM analysis, the calculation of I_crorr, beta C and beta A depends on the corrosion mechanism types on the metal surface. Three types of corrosion mechanisms occur on the steel surface: activation, diffusion, and passive (Al-Mobarak et al., 2011; Bosch et al., 2001; Berdimurodov, 2016; Obot and Onyeachu, 2018; Abdel-Rehim et al., 2006). In this research, it was selected the activation mechanism. In the mechanism of the activated control corrosion process, the steel surface undergoes dissolution in a corrosive environment. In the steel dissolution electrochemical reaction, beta A is lower than beta C. In this type of corrosion mechanism, I_corr, beta C and beta A are calculated according to Equations (5)–(7) (Al-Mobarak et al., 2011; Bosch et al., 2001; Berdimurodov, 2016; Obot and Onyeachu, 2018; Abdel-Rehim et al., 2006).

In the EFM experiment, two connections are important to clarify the accuracy of the EFM results. The first connection is between the response signal and the input frequency perturbation. The second connection is between the current from the intermodulation frequency and the current from the harmonic frequency. These two connections are called causality factors. In this EFM analysis, causality factors (2) and (3) were used, and they are calculated according to Equations (8) and (9); the inhibition efficiency was calculated by Equation (10), respectively (Al-Mobarak et al., 2011; Bosch et al., 2001; Berdimurodov, 2016; Obot and Onyeachu, 2018; Abdel-Rehim et al., 2006).

The EFM intermodulation spectra for the studied material in the inhibitor-free and the inhibitor-containing 1 M hydrochloride acid solution at various concentrations of the inhibitor TTHIIDT are presented in Fig. 2. All EFM experiments were performed at 303 K. The current peaks in these intermodulation spectra correspond to the calculated corrosion current. It is clear from these spectra that the current fluctuated between approximately 125 and 170 µA for carbon steel in the inhibitor-free medium. The reason is that the 1 M HCl corrosion environment is very strong for the steel surface, and hydrogen and chloride ions affect the growth of intermodulation spectrum peaks. In contrast, the current peaks fluctuated between approximately 10 and 15 µA for carbon steel in the inhibited medium. The reason is that the inhibitor molecules affect the cathodic and anodic electrochemical half-reactions on the surface of metal, meaning that inhibitor molecules influence the surface of metal current-connection abilities and intermodulation spectrum.

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Fig. 2

EFM intermodulation spectra for carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT.

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The EFM analysis data for studied metal specimen in 1 M hydrochloride acid medium in the presence and absence of the inhibitor TTHIIDT at different concentrations were investigated, as presented in Table 3. The EFM experiments were performed to evaluate the activation, diffusion, and passivation corrosion mechanisms.

In the activation corrosion mechanism, the corrosion current and corrosion rates were 282.2 µA and 129 mpy, respectively, in 1 M HCl medium, which means that the carbon steel electrode dissolves in 1 M HCl medium and that Fe²⁺ and Fe³⁺ ions are formed. These ions influence the growth of the corrosion current and corrosion rates. When TTHIIDT was added to the corrosion medium, it dramatically reduced the Fe²⁺ and Fe³⁺ ion numbers by forming Fe-inhibitor complexes and adsorbing on the steel surface. Consequently, the corrosion rate and corrosion current were dramatically reduced. This action is dependent on the inhibitor concentration.

3.3 Electrochemical noise measurements

The EN test is a highly effective and non-destructive method for corrosion investigation. The EN data are recorded based on the statistical time versus potential and current change. In the present work, EN tests for the working carbon steel electrode were carried out in 1 M HCl corrosive and inhibited medium with different amounts of the inhibitor TTHIIDT, resulting in Fig. 3, and Table 4.

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Fig. 3

Time versus current and potential noise in the inhibitor-free (a), time versus current noise (b) and potential noise (c) in the inhibitor-containing 1 M HCl medium at various concentrations of the inhibitor TTHIIDT (carbon steel).

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In the EN analyses, the potential and current noise are derived based on Equations (11) and (12) (Danaee et al., 2019; Obot et al., 2019; Ramezanzadeh et al., 2014; Naghizade et al., 2020; Hou et al., 2016; Huet and Ngo, 2019; Ehsani et al., 2017).

In these equations, ${\mathrm{I}}_{\mathrm{r}\mathrm{m}\mathrm{s}}$ is the current noise, ${\mathrm{E}}_{\mathrm{r}\mathrm{m}\mathrm{s}}$ is the potential noise, ${\mathrm{I}}_{\mathrm{b}\mathrm{f}}$ is the current of the standard linear best fit, ${\mathrm{E}}_{\mathrm{b}\mathrm{f}}$ is the potential of standard linear best fit, $\mathrm{N}$ is the total channel number and $\mathrm{n}$ is the total point number. The current noise and potential noise can be derived from Equations (13) and (14) at given times in the EN analysis.

It is clear from Fig. 3(a) that the average potential and current noise for the working carbon steel electrode in the aggressive 1 M HCl environment fluctuated at approximately 4.132 µV and 0.1974 µA, respectively, during the time period from 1 sec to 1800 sec, indicating that the steel electrode undergoes dissolution and forms iron ions, destroying the steel electrode, which is the reason for the noise decrease. As shown in Fig. 11, these fluctuation phenomena in the potential and current noise for the studied metal electrode in the corroded environment are responsible for the metastable and nucleation processes that occur on the surface of steel. The sudden increase and decrease in the transient current and potential noise in the acid medium destroy the passive film on the steel surface.

The observed results of the time versus potential and current noise for the carbon steel electrode in the inhibitor-containing 1 M HCl medium at various concentrations of the inhibitor TTHIIDT are presented in Fig. 3(a) and (b). The electrochemical noise signals were accounted for during 1800 sec at room temperature (303 K) by EN analysis. It was shown from the observed results that the potential and current noise for the carbon steel electrode in the aggressive 1 M HCl environment slowly increased with increasing TTHIIDT concentration. The organic TTHIIDT molecules can change the electrochemical noise towards more negative values in the corrosion medium. At 50 mg/L inhibitor, the current and potential noise for steel fluctuated at approximately 21.99 µA and 5983 µV, respectively. When the inhibitor concentration was increased two-fold, the current and potential noise for steel stabilized at approximately 62.99 µA and 20920 µV, respectively. These fluctuations of noise are responsible for the active control processes occurring on the surface of carbon steel in similar metastable situations. The important increase in the current and potential noise fluctuations indicates that the organic TTHIIDT molecules are effective anti-corrosive inhibitors in aggressive acid environments.

In this EN analysis, the average current noise ( ${\overline{\mathrm{I}}}_{\mathrm{r}\mathrm{m}\mathrm{s}}$ ) and potential noise ( ${\overline{\mathrm{E}}}_{\mathrm{r}\mathrm{m}\mathrm{s}}$ ) were calculated by using the time versus average current and potential noise charts, and the measured values were applied to measure the average noise resistance ( ${\overline{\mathrm{R}}}_{\mathrm{r}\mathrm{m}\mathrm{s}})$ related to Equation (15), listed in Table 4.

Overall, Table 4 shows that the average noise resistance of the carbon steel electrode dramatically fluctuated with increasing TTHIIDT concentration in a 1 M HCl environment. This point is aptly illustrated by the fact that the ${\overline{R}}_{\mathit{rms}}$ value for the carbon steel electrode was 20.93 Ω in the corrosion medium, in contrast to the inhibited medium containing 100 mg/L TTHIIDT, whose ${\overline{R}}_{\mathit{rms}}$ value for the carbon steel electrode increased substantially to over 332.11 Ω. The reason is that the inhibitor TTHIIDT reduces the charge transfer and polarization on the surface of carbon steel electrode. The noise resistance depends on the TTHIIDT concentration and the number of inhibitor molecules, indicating that TTHIIDT forms a layer on the surface of steel and that the thickness increases with rising TTHIIDT concentration. The large layer thickness on the steel surface rapidly impacts the noise resistance, and the large noise resistance values (over 332 Ω) are responsible for the TTHIIDT organic molecules being more effective protectors for steel materials in aggressive acid corrosion environments.

In this research work, the inhibition degrees of the TTHIIDT at different concentrations in 1 M HCl solution were measured by EN analysis, and the obtained ${\overline{\mathrm{R}}}_{\mathrm{r}\mathrm{m}\mathrm{s}}$ values were used to measure the inhibition level based on Equation (16), listed in Table 4. As observed, TTHIIDT organic molecules can protect steel electrodes from aggressive acid corrosion by over 93%, suggesting that TTHIIDT is a high-quality and valuable anti-corrosive inhibitor for steel-based materials and that it could be used in the chemical industry in order to considerably protect steel pipes and materials from corrosion.

The inhibitor concentration versus inhibitor effectiveness in aggressive medium was measured, and the observed data are given in Table 4. As a consequence, the inhibitor TTHIIDT is a valuable protector of steel materials in terms of acid corrosion, and the protection degree depends on the inhibitor concentration.

The total charge (q) value is calculated with Gamry Echem Analyst software by using quick integration tools in the EN analysis, and the observed time is 1800 sec. In the present work, the total charge values for the working carbon steel sample were measured in the corrosion and inhibited environments, as presented in Table 4. As observed, q is 5.6 × 10⁻²C for the working carbon steel sample in 1 M hydrochloride acid medium, and this q value is higher because the iron oxidation rate and the hydrogen ion reduction rate are higher, leading to the corrosion processes sharply increasing at the cathodic and anodic sites on the surface of carbon steel. The higher q value of the carbon steel electrode in the aggressive environment reflects the total cathodic and anodic electrochemical charge on the carbon steel surface, which means that the higher total charge amount is an indicator of faster charge transfer rates at the anodic and cathodic sites of the carbon steel surface and that strong corrosion processes occur.

The variation in the total charge versus TTHIIDT concentration in 1 M HCl was investigated, and the results are given in Table 4. When the inhibitor TTHIIDT was added to the corrosion environment, the total charge value significantly decreased by nearly three-fold, and this change depended on the TTHIIDT concentration. At higher concentrations of the inhibitor TTHIIDT (100 mg/l), the q value is 113 × 10⁻⁵ C, which is three-fold greater than that for the uninhibited environment, suggesting that the TTHIIDT molecules considerably decreased the iron oxidation rate and hydrogen evolution at the anodic and cathodic sites of the surface of carbon steel in 1 M HCl medium. The next indicator for q reduction is the adsorption of TTHIIDT on the surface of steel because these inhibitor molecules maximally blocked charge transfer from the anodic sites to cathodic sites.

3.4 Open-circuit potential

Open-circuit potential changes were investigated in 10 min for carbon steel in 1 M depassivator solution with various concentrations of inhibitor at 313 K, as shown in Fig. 4(a). It is clear from these results that the OCP remained stable in all experiments, suggesting that metal oxides, hydroxides and other adsorbed compounds dissolve on the surface of metal and that the inhibitor adsorbs on the surface of metal. The OCP curves in inhibitor-free 1 M HCl medium were stable at approximately −0.435 mV. When inhibitor was added to this aggressive medium, the OCP shifted to more negative values (cathodic site). This change depends on the inhibitor concentration: at 50 mg/l, it is −0.542 mV; at 100 mg/l, it is −0.543 mV; and at 150 mg/l, it is −0.559 mV. The OCP time curves remained stable around the negative potential, which suggests that the inhibitor formed passive films on the steel surface. As observed, the equilibrium potential (OCP) shifts to more negative potentials. The reason why the shift towards negative potentials is due to the structure of the inhibitor molecules and the orientation (Fig. 10) on the electrode surface. The inhibitor consists of lone paired electrons due to the presence of nitrogen and sulfur atoms (Fig. 10) and the benzene rings, which are rich in electrons and hence are more negatively charged at the electrode surface. Thus the potential shifts towards negative direction with the presence of the inhibitor.

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Fig. 4

The change in OCP (a) and Tafel curves (b) for carbon steel in the inhibitor-free and the inhibitor-containing 1 M depassivator solution at different concentrations of the inhibitor TTHIIDT (313 K).

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3.5 Potentiodynamic polarization measurements

To examine the electrochemical kinetics of the cathodic and anodic corrosion activities on the surface of carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT, the PDP cathodic and anodic curves were investigated, and the observed data are given in Fig. 4(b). As observed, the cathodic and anodic Tafel curves for metal specimen noted on more positive and higher current density areas in 1 M HCl medium indicate that $H^{+}$ and ${\mathit{Cl}}^{-}$ ions promote the anodic and cathodic electrochemical half-reactions on the surface of metal as a result of aggressive destruction of the carbon steel electrode surface (Chauhan et al., 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020a, 2020b).

It can also be shown in Fig. 4(b) that the organic inhibitor TTHIIDT shifted both the electrochemical cathodic and anodic Tafel curves to lower current density and more negative potentials areas, indicating that the TTHIIDT molecules dramatically diminished both anodic and cathodic electrochemical half-reactions that cause corrosion of the steel surface. The observed Tafel curves suggest that the TTHIIDT organic molecule is a mixed-type inhibitor and that its anti-corrosion activities depend on the concentration.

The observed Tafel curves were shifted to more negative potentials with rising inhibitor concentration because the adsorption degree of TTHIIDT molecules depends on the concentration; therefore, a higher inhibitor concentration is responsible for more adsorption, and the many inhibitor molecules occupy a wide area of the surface of carbon steel. As a result, the adsorbed inhibitor molecules significantly decreases the number of electrochemical cathodic and anodic corrosion active sites on the surface of metal working electrode.

The electrochemical potentiodynamic polarization Tafel characteristics of the studied working electrode, called the corrosion current density ( $i_{\mathit{corr}}$ ), corrosion potential ( $E_{\mathit{corr}}$ ), corrosion rate ( ${\mathit{CR}}_{\mathit{PDP}}$ ), inhibition efficiency ( ${\eta }_{\mathit{PDP}}$ ), corrosion resistance ( $R_{\mathit{PDP}}$ ), surface coverage degree ( ${\theta }_{\mathit{PDP}}$ ), Stern-Geary constant (B), corrosion resistance ( $R_{\mathit{PDP}}$ ), Tafel anodic beta (Beta A) and cathodic beta (-Beta C) slopes, were measured from Fig. 4(b) through Tafel extrapolation analysis, summarized in Table 5.

As investigated, the corrosion density was recorded as 71.2 mA/cm² in HCl medium without an inhibitor because a higher number of $H^{+}$ and ${\mathit{Cl}}^{-}$ ions is the main cause for an increased corrosion current at the carbon steel surface. In comparison with that of the inhibited medium, the corrosion density sharply decreased to 7.64 mA/cm² at 50 mg/L TTHIIDT, 3.63 mA/cm² at 100 mg/L and 1.25 mA/cm² at 150 mg/L, confirming that the organic inhibitor TTHIIDT maximally neutralized $H^{+}$ and ${\mathit{Cl}}^{-}$ ions on the carbon steel electrode. These decreases in the corrosion current density depend on the TTHIIDT concentration, which means that a higher concentration is responsible for a greater reduction in metal destruction.

In PDP analysis, the organic inhibitors are classified (cathodic, anodic and mixed types) according to the difference in potential between uninhibited and inhibited environments, meaning that for a value below 85 mV, potential displacement indicates that the studied inhibitor behaviour is mixed, which impacts both anodic and cathodic electrochemical reactions on the surface of metal electrode, while for a potential difference above 85 mV, an inhibitor may be categorized as cathodic or anodic. The obtained PDP measurements show that the potential displacement was 71 mV at 50 mg/L TTHIIDT; this phenomenon confirmed that the behaviour of the inhibitor TTHIIDT is mixed at this concentration, indicating that TTHIIDT molecules effectively influence both cathodic and anodic electrochemical corrosion reactions.

It can also be observed in Table 5 that the potential difference reported was 100 mV at 100 mg/L and 123 mV at 150 mg/L, suggesting that the inhibitor TTHIIDT is categorized as anodic. The reason is that TTHIIDT more influences electrochemical anodic corrosion processes by neutralized ${\mathit{Fe}}^{2+}$ ions at the cathodic sites of the surface of carbon steel. It is notable from the results that the Tafel anodic beta (Beta A) and cathodic beta (-Beta C) slopes for carbon steel were 135.4 and 185.7 mV/decade, respectively, in the aggressive acid medium; in inhibited medium, the inhibitor TTHIIDT substantially changed these values to more negative and positive potential sites. This phenomenon confirmed that TTHIIDT effectively impacts the cathodic and anodic corrosion on the carbon steel surface as a consequence the massive reduction in hydrogen evolution and steel dissolution.

It was clearly indicated that ${\mathit{CR}}_{\mathit{PDP}}$ is very fast in 1 M HCl medium, at 90.43 mpy, but it dramatically decreased to 0.572 mpy with 100 mg/L TTHIIDT added to the corrosion medium. In view of the fact that TTHIIDT molecules are adsorbed via a flat orientation and cover wide areas on the surface of carbon steel electrode, as the amount of TTHIIDT increases, its molecules are perpendicularly adsorbed on the surface of carbon steel electrode because this inhibitor is water-soluble and highly polarized. An rise in the number of TTHIIDT molecules is responsible for increased electrostatic repulsion and forces between these inhibitor molecules.

In the present research work, the inhibition efficiency ( ${\eta }_{\mathit{PDP}}$ ) and surface coverage scale ( ${\theta }_{\mathit{PDP}}$ ) were derived according to Equations (18) and (19), respectively.

The ${\eta }_{\mathit{PDP}}$ of the inhibitor TTHIIDT was over 90%, confirming that this investigated inhibitor is a more effective corrosion protector for carbon steel in an aggressive hydrochloride environment. It is also said that this phenomenon depends on the TTHIIDT concentration. Another distinguishing feature in Table 5 is that the inhibitor TTHIIDT maximally covered the area on the carbon steel surface by adsorption.

In the PDP examination, the Stern-Geary constant (B) was measured related to Equation (20), and the impact of the amount of TTHIIDT on the Stern-Geary constant was tested, as listed in Table 5. According to the data shown, the measured B value for the carbon steel electrode in 1 M HCl medium was 34 mV. By adding 50 mg/L TTHIIDT to the HCl environment, B noticeably decreased to 28.32 mV. This drop depends on the inhibitor TTHIIDT concentration, and when the concentration was increased two-fold, the value B significantly decreased to 23.48 mV.

The corrosion resistance of the studied metal electrode in a 1 M HCl environment without and with the inhibitor TTHIIDT were deduced by using Equation (21), as indicated in Table 5.

Generally, the carbon steel electrode has a low resistance to corrosion attack, which was reported to be 0.47 Ω/cm². However, 100 mg/L TTHIIDT dramatically increased this corrosion resistance to 18.78 Ω/cm², indicating that the inhibitor TTHIIDT adsorbed on the carbon steel electrode and effectively resisted both cathodic and anodic corrosion activities.

3.6 Electrochemical impedance spectroscopy measurements

All EIS data were obtained by using the Levenberg-Marquardt method on the equivalent circuit model, which is represented in Fig. 5. This approach used an equivalent circuit model containing the CPE (constant phase element), R_s (solution resistance) and R_ct (charge transfer resistance). The constant phase element has been used for the capacitor with the aim of investigating accurate fitting of the circuit; it can be expressed according to Equation (22).

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Fig. 5

An equivalent circuit model was utilized to calculate the EIS parameters.

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In this equation, $\omega$ is an angular frequency, $Y_0$ and $i$ are the constant phase element magnitude and an imaginary number, respectively, and $n$ is the empirical constant, which is less than one and responsible for calculating the heterogeneity of the carbon steel-electrolyte interface (Chauhan et al., 2020, 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020; Fan et al., 2019; Kikanme et al., 2020; El Kacimi et al., 2020; Ye et al., 2020).

The level of double-layer capacitance was derived from the constant phase element value and is given by Equation (23).

As indicated by the measured $C_{\mathit{dl}}$ in Table 6, the level of $C_{\mathit{dl}}$ in the inhibited aggressive acid medium was nearly five-fold lower than that in the uninhibited aggressive acid medium (298.16 µF/cm²), demonstrating that the inhibitor TTHIIDT decreased the local dielectric constant in 1 M hydrochloride acid solution and increased the double layer’s thickness, which is illustrated related to the Helmholtz model (Equation (24)) (Fan et al., 2019; Kikanme et al., 2020; El Kacimi et al., 2020; Ye et al., 2020; Aly et al., 2020; Tabatabaei majd et al., 2020; Dehghani et al., 2020; Lgaz et al., 2020).

The kinetic characteristics of the carbon steel/electrolyte interface processes and surface properties were investigated by the EIS method. The Nyquist plots for carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT are depicted in Fig. 6(a). As observed, the obtained plots for the carbon steel/electrolyte interface in the investigated hydrochloride acid environment with and without different values of TTHIIDT organic inhibitor were single depressed capacity semicircles, indicating that the corrosion processes on the surface of carbon steel are dominated by a charge transfer mechanism. It can also be seen from the obtained results that the Nyquist plots in the 1 M HCl solution are very small depressed semicircles, confirming that the corrosion activities are faster at the carbon steel surface-electrolyte interface. In the inhibited 1 M HCl solution, TTHIIDT dramatically diminished all electrochemical corrosion reactions by controlling the charge transfer mechanism at the carbon steel-electrolyte interface. It should be sensible to emphasize that the size of the Nyquist semicircle increases with the amount of TTHIIDT. This trend is responsible for the inhibitor TTHIIDT forming a protective layer and adsorbing at the carbon steel-electrolyte interface. Another outstanding point from Fig. 6(a) is that the obtained depressed capacity semicircles were cited at upper frequencies, indicating that the corrosion activities controlled by charge transfer and the double-layer capacity are lower in HCl acid solution, while the obtained small depressed inductive semicircles were located at lower frequencies, indicating that the relaxation TTHIIDT occurred at carbon steel-electrolyte interface (Chauhan et al., 2020, 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020a, 2020b).

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Fig. 6

Nyquist (a), phase angle and Bode (b) plots for carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT.

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As illustrated in Table 6, the polarization resistance value was 4.599 Ω/cm² in 1 M HCl solution, and this value sharply increased to over 30 Ω/cm² in inhibited 1 M HCl solution. The R_ct value rose with increasing TTHIIDT concentration, which is responsible for forming a thick protective film the surface of carbon steel. This phenomenon confirmed that the corrosion activities on the surface of carbon steel are controlled by polarization resistance, which is a complex resistance and comprises five types of resistance: diffusion layer resistance (R_dl), charge transfer resistance (R_ct), protective layer resistance (R_pl), and accumulation resistance (R_a) and solution resistance (R_s). The charge transfer resistance illustrates the displacement into the real impedance between the maximum and minimum frequencies.

The second notable feature is that the double-layer capacitance remarkably decreased in the inhibited hydrochloride environment. This phenomenon is responsible for the TTHIIDT molecules adsorbed on the surface of carbon steel as well as the reduced electrical conductivity through the substitution of TTHIIDT molecules with orientation-adsorbed water molecules on the surface of carbon steel. As shown in Table 6, the n value in the uninhabited medium was greater than that in the inhibited medium, indicating that the aggressive hydrochloride solution made the carbon steel surface more homogenous. According to the data shown, the R_s value was 1.420 Ω/cm² in 1 M hydrochloride solution, which is attributed to the higher solution conductivity. However, the inhibitor TTHIIDT sharply decreased the solution conductivity by decreasing R_s to 4.383 Ω/cm². As Table 6 shows, the R_ct value rapidly increased with the addition of TTHIIDT to 1 M hydrochloride solution compared to that without TTHIIDT, suggesting that the inhibitor effectively blocked both the cathodic and anodic active sites on the surface of carbon steel. In the present research, the inhibition efficiency degree was measured by using the obtained R_ct value, based on Equation (25).

As shown in Table 6, the inhibition efficiency of the inhibitor TTHIIDT generally was over 85% and depended on the inhibitor concentration. These results confirmed that the investigated inhibitor was an effective protector of carbon steel in an aggressive acid environment. In this research paper, the goodness of fit for the EIS test was measured according to the Kronig-Kramers transform, as given in Table 6. It is remarkable that the goodness of fit for the EIS test was lower than 1 × 10⁻⁵, confirming that the observed EIS data were more accurately derived by the studied impedance spectra and used the equivalent circuit model.

The phase angle and Bode plots for the carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT are shown in Fig. 6(b) and Table 7. The n factor was calculated according to the given Bode and phase angle diagrams, and its value is equal to the difference of the ideal capacitance characteristics. The obtained results support that the inhibitor TTHIIDT improved the surface roughness of carbon steel. As indicated in Fig. 6(b), the phase angle decreased with increasing TTHIIDT concentration, and the minimum Z_phz was −68.48° at 100 mg/L TTHIIDT. In EIS theory, it was demonstrated that the phase angle is −90° and the constant slope is −1. In the present work, it was found that the minimum phase angle and the slopes were −68.48° and 0.637, respectively, revealing that the carbon steel-electrolyte interface exhibited non-ideal capacitance behaviour and that its capacitance characteristics were at intermediate frequencies. This intermediate frequency of the Bode-phase angle diagram included a single uppermost trend with one stabilization time. This phenomenon suggests that the single relaxation activities (circled in Fig. 6(b)) occurred during the charge transfer process on the steel surface in both uninhibited and inhibited hydrochloride environments.

The extent of deviation from the ideal capacitor characteristics is attributed to the surface smoothness of the surface of the carbon steel-solution interface. It can be inferred that the measured differences from ideal capacitance characteristics were responsible for the carbon steel surface smoothness. The difference in the phase angle and slope relative depends on an increase in the TTHIIDT concentration and is responsible for the decreasing trend of the corrosion rate and carbon steel dissolution in the acid solution.

3.7 Adsorption isotherm

In this current research, the Langmuir isotherm was utilized to identify the adsorption behaviour of the studied inhibitor, as reflected in Equation (26) (Chauhan et al., 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020a, 2020b).

The obtained Langmuir isotherm is given in Fig. 7(a) and indicates $C_{\mathit{inh}}/{\theta }_{\mathit{Grav}.}$ vs. ${\theta }_{\mathit{Grav}.}$ plots related to various temperatures. The intercepts from these plots were utilized to calculate $K_{\mathit{ads}}$ and the regression coefficient (R²), and the results are noted in Table 8. It should be emphasized that the obtained Langmuir isotherms illustrated the adsorption behaviour of the investigated inhibitor, forecasting the equivalent of ${\theta }_{\mathit{Grav}.}$ in active sites and the homogeneity of the studied surface. The most striking feature that the R² values were near 1 at all concentrations, and the obtained slopes from the Langmuir plots were a unity. This phenomenon illustrated that the inhibitor TTHIIDT formed a single thick layer on the surface of carbon steel.

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Fig. 7

Langmuir isotherm plots (a) and the relationship between ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ and temperature (b) for carbon steel in the inhibitor-free and the inhibitor-containing 1 M HCl medium at different concentrations of the inhibitor TTHIIDT.

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In the next part of the adsorption analysis, the calculated $K_{\mathit{ads}}$ values were used to measure the change in standard free energy ( ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ ) related to Equation (27), with the values presented in Table 8.

In the adsorption processes, the changes in the standard enthalpy ( ${\mathrm{\Delta }H}_{\mathit{ads}}^o$ ) and entropy ( ${\mathrm{\Delta }S}_{\mathit{ads}}^o$ ) values were responsible for signifying the adsorption type and the molecular interaction behaviour of the studied inhibitor on the surface of carbon steel. The values ${\mathrm{\Delta }H}_{\mathit{ads}}^o$ and ${\mathrm{\Delta }S}_{\mathit{ads}}^o$ were deduced according to Equation (28) (Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020a, 2020b; Fan et al., 2019; Kikanme et al., 2020; El Kacimi et al., 2020; Ye et al., 2020; Aly et al., 2020); and their values are indicated in Table 8.

As seen from Table 8, the higher adsorption–desorption constant is responsible for effective adsorption of the inhibitor TTHIIDT on the surface of carbon steel. Another significant point from the data is that the obtained change in ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ behaviour of the studied inhibitor depends on the physical and chemical interaction mechanisms between the surface and inhibitor. The negative change in the standard Gibbs energy confirmed that this interaction mechanism involves spontaneous adsorption. ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ is a function of temperature change, as represented in Fig. 7(b), which indicates the adsorption characteristics of TTHIIDT. According to corrosion science research, an amount of ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ below −20 kJ/mol, representing the physical adsorption mechanism, involves electrostatic interactions between the surface and inhibitor, whilst an amount above 40 kJ/mol of ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ reflects chemical adsorption, which is carried out by the formation of chemical bonds. As investigated, the ${\mathrm{\Delta }G}_{\mathit{ads}}^o$ value was between −23 and −25 kJ/mol. This result confirmed that the adsorption of TTHIIDT on the surface of carbon steel involves physisorption and chemisorption processes and suggests that physical adsorption is more dominant than chemical adsorption with rising temperature. Remarkably, from the observed data, the change in the standard enthalpy for TTHIIDT was −2.34 kJ/mol, which confirmed that the adsorption action of this inhibitor on the carbon steel surface is endothermic. Moreover, the change in standard entropy was −70 J/mol, and this lower value suggested that the adsorption of this inhibitor is promoted by reducing the disorder on the steel surface.

On the other hand, it is obviously indicated that the activation entropy values were lower in the TTHIIDT-containing system than in the inhibitor-free system, suggesting that the disorder in the studied system was reduced by forming the adsorbing active metal-inhibitor complexes and the displacement between the inhibitor and pre-adsorbed water molecules.

3.8 Surface measurements

SEM images of the carbon steel surface in in the inhibitor-free and the inhibitor-containing 1 M HCl medium at the optimum concentrations of the inhibitor TTHIIDT were obtained after 3 h of immersion, as displayed in Fig. 8. It is clear from Fig. 8(a) that the surface of carbon steel was seriously destroyed and corroded in the studied uninhibited environment through an aggressive attack by $H^{+}$ and ${\mathit{Cl}}^{-}$ ions. It was shown, from the observed SEM images of the carbon steel surface in the TTHIIDT-containing solution, the surface morphology damage was dramatically diminished (Fig. 8(b)). This phenomenon occurred through the formation of a thin protective layer on the surface of carbon steel. Overall, what stands out from the data is that the obtained SEM micrographs confirmed that the inhibitor TTHIIDT effectively insulated the carbon steel surface from aggressive corrosion attack and importantly reduced corrosion processes by its high adsorption capacitance.

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Fig. 8

SEM micrographs for carbon steel in the inhibitor-free (a) and the inhibitor-containing (b) 1 M HCl medium at the optimum concentrations of the inhibitor TTHIIDT.

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3.9 Quantum chemical calculations

3.9.1

3.9.1 Optimization analysis

The DFT-based quantum chemical measurements could give valuable opportunities to predict the global reactivity indicator value of the investigated inhibitor. The B3LYP method was performed in the DFT calculations with 6-311G (d, p) basis sets. Before starting DFT analysis, the observed inhibitor molecule was optimized, and the resulting optimized molecular structure is presented in Fig. 9(a). As illustrated, it was computed that the studied inhibitor molecule was the comparatively largest planar structure molecule. It is noticeable that three planar rings (two benzoyl rings and one hetero ring) were present in the TTHIIDT molecule, indicating that these planar groups can promote the functioning of this inhibitor as a good adsorbent on the surface of carbon steel. Additionally, the broad stretched and larger planar linked atoms in this inhibitor molecule could promote effective intermolecular interactions between the surface of the carbon steel-electrolyte interface and the researched inhibitor molecule.

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Fig. 9

The optimized molecular structure (a) (the white, dark-blue, light-yellow and light-grey spheres indicate hydrogen, nitrogen, sulfur and carbon atoms, respectively), the MEP (b), HOMO (c) and LUMO (d) electron density of the inhibitor TTHIIDT.

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Fig. 10

The atomic charge distribution of the inhibitor TTHIIDT.

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Fig. 11

The pictorial representation of the inhibition mechanism of the TTHIIDT molecule on a carbon steel surface in a HCl environment.

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3.9.1.1

3.9.1.1 Frontier molecular orbital electron density distribution analysis

The energies of highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the inhibitor TTHIIDT were computed and utilized to estimate the different types of global reactivity property values of the studied inhibitor through frontier molecular orbital (FMO) analysis. The corrosion inhibition characteristics of the inhibitor TTHIIDT were anticipated according to the FMO-based computed reactivity behaviours.

The LUMO and HOMO electron density distribution energies of the observed inhibitor were computed by DFT-FMO quantum calculation analysis, as depicted in Fig. 9(c and d). From the computed HOMO and LUMO, it is an outstanding indicator that the HOMO and LUMO regions in the investigated inhibitor molecule are responsible for the electron donor and acceptor behaviour of this inhibitor, respectively. It could also be highlighted from the observed FMO orbitals that the HOMO and LUMO sites in the TTHIIDT molecule reflected the active regions that are putatively responsible for electron transport from the inhibitor to the surface of carbon steel. As observed from Fig. 9(c), the HOMO electron density distribution is mainly located around sulfur atoms, illustrating that the electron donation processes to free vacancy d-orbitals of iron are mainly supported by sulfur atoms in the studied inhibitor molecule. In contrast, in Fig. 9(d), the LUMO mainly has a density around phenyl rings, indicating that the phenyl rings are responsible for the acceptor tendency of the studied inhibitor. The obtained molecular electrostatic potential (MEP) (Fig. 9(b)) indicated the reactive sites of optimised structure. MEP is also responsible for the active sites of nucleophilic and electrophilic attack. In the obtained MEP, while blue (positive) regions are responsible for the electrophilic reactivity and the red (negative) regions are responsible for nucleophilic attack.

3.10 Inhibition efficiency comparison with early research

In corrosion science, a study of the inhibition efficiency of different kinds of organic molecules in 1 M HCl solution is conducted, some of which are presented in Table 10 for comparison with the present research work. As observed in Table 10, our inhibitor was more efficient at lower concentrations than the other inhibitors. Another feature aspect illustrates that the inhibitor TTHIIDT could be easily synthesized based on a low-cost local product and that it is biodegradable. This result supports that this inhibitor can be widely used in the chemical industry as an eco-friendly compound.

3.11 Analysis of the inhibition mechanism

The inhibition mechanism of the inhibitor TTHIIDT on the carbon steel surface is illustrated in Fig. 11. It is clear from the image in this figure that this inhibitor can inhibit the carbon steel surface via three stages. First, the protonated nitrogen atoms neutralize the adsorbed ${\mathit{Cl}}^{-}$ ions on the surface. Next, physical interactions occur that involve electron donation from the p-electron pairs of sulfur atoms to the vacant 3d-orbitals of Fe atoms. The next step is supplementary electron donation in which the phenyl rings share π-orbitals with vacant 3d-orbitals of Fe, and this process supports chemisorption. The last step is retro-donation, which is carried out by electron transfer from the filled Fe orbitals to the vacant anti-bonding π-orbitals in phenyl rings. As concluded, the studied inhibitor molecule could form a self-assembled protective thin layer on the surface of carbon steel, which could isolate the surface from aggressive-corrosive attacks in solution; this phenomenon is accomplished by physical-interaction and chemical-hybridization adsorption activities (Chauhan et al., 2020, 2019; Gupta et al., 2019; Chafiq et al., 2020; Bahgat Radwan et al., 2017; Rbaa et al., 2020; Tan et al., 2020a, 2020b).