BDTT synthesis

BDTT was synthesized through the addition reaction method, ²² which consists in having C₂H₃N₃ to react with C₇H₆O. C₂H₃N₃ (6.21 mM) solution was prepared by dissolving it in absolute C₂H₆O (100 mL), in a round bottom flask. C₇H₆O (0.49 mM) was dissolved in dry C₂H₆O, in another 100 mL beaker. Then, C₇H₆O solution was drop wise added to C₂H₃N₃ mixture, in a 250 mL round bottom flask. Further reaction mixtures were constantly refluxed for 10 h, through continuous stirring. The reaction was monitored by TLC, at regular intervals. Finally, the precipitate was obtained by filtration, and thoroughly washed with ether. The resulting compound was a light yellow colored solid, with good yields.

CI studies via gravimetric technique

WE formation

MS cuboidal coupons, with the dimensions of 1 × 1 × 1 cm and 4 × 1 × 1 cm, were used for WL and Ec methods, respectively. MS coupons composition was 98.72 Fe, 1.02 Mn, 0.15 C, 0.08 Si and 0.02 S (%).

Test solution preparation

The employed solvents were distilled water and C₂H₆O. The reactants and solvents (AR grade) were commercially available, and used without any further purification. The electrolytes solutions for testing were prepared by AR grade concentrated H₂SO₄ dilution with double distilled water. BDTT bulk solutions with various Ct were prepared by dissolving their required amount in H₂SO₄.

WL studies

WL experiments are a simple process, in which MS was immersed in 0.5 M H₂SO₄ solutions, for 24 h, with different Ct of BDTT ²³. After 24 h, corrosion products on the MS surface were cleaned. Then, the remaining MS sample was weighed ²⁴. CR and IE(%) were calculated through eqs. (1) and (2). All studies were repeated more than twice, to minimize errors. WL values were further used to calculate IE(%) of BDTT against MS corrosion. The terms related to corrosion, i.e., CR and IE(%), were obtained via Eq. (1):

()1

where W^a and Wⁱ are MS coupons WL in 0.5 M H₂SO₄ solutions without and with BDTT, in various Ct.

SC (θ) by BDTT was evaluated through the following Eqs.:2,3

()2

()3

where ΔW is MS mass change in mg, A is MS coupon area (cm²), t (h) is IT and D is MS density ²⁵.

Ec measurements

The whole Ec experiment was performed with a three electrode Ec cell (250 mL). The employed electrodes were MS, Pt and SCE, as WE, CE and RE, respectively. CE and RE electrodes were fixed to a luggin capillary bent tube, which was filled with Cl^- ions solution. The MS surface was prepared by mechanically abrasion with emery paper ranging from 100 to 1000 grade, and washed with acetone. The approximate distance from 1 to 1.5 mm (a constant), between the WE surface and the luggin capillary tip, was maintained to avoid any ohmic loss. All measurements were performed in aerated non-stirred 0.5 M H₂SO₄ solutions with and without BDTT, in different Ct, at 298 K. A CHI 760c (CH Instruments, Inc., USA) device was used for determining I and E, when redox reaction took place. Tafel polarization was measured through WE (MS) E vs. SCE. I vs. E curves (PDP) measurements were carried out at a SR of 1.0 mV/s^-1. LPR parameter was measured while GP study was done. All impedance measurements were performed with steady state OCP, using AC amplitude signals, with a peak to peak of 5 mv, through the steady state E, at a range from 100 kHz to 10 mHz. Nyquist and Bode curves were obtained by using these calculations.

T effect

T effect on CR was calculated through WL studies, at 298, 308, 318 and 328 K. MS corrosion E_a with and without BDTT was calculated via Arrhenius eq.

Theoretical investigations (DFT)

The interaction between the MS surface and BDTT and their reactivity were investigated through FMO and Mulliken charge population. DFT of BDTT molecule was performed via 8.0.6 Hyperchem modeling software ²⁶. To speed up calculations, BDTT was initially optimized via parametrized model 3(PM₃). Semi empirical calculations were done with a starting perimeter of 0.01 kcal/mol^-1 adjustment, through DFT re-optimization. All observations were completed by selecting 6-311G (d,p) basis set, and exchanging it for B3LYP co-relation function ^27-29.

Ec measurements

PDP and LPR measurements

DP curves for the WE (MS) in the 0.5 M H₂SO₄ solution without and with BDTT, in different Ct, are represented in Fig. 1, at 298 K. Anodic and cathodic reactions, which took place on the MS electrode surface, were inhibited by BDTT molecule. IE(%) values were determined by Eq. (4).

()4

where I_Acid and I_inh are I_corr density in the 0.5M H₂SO₄ solution without and with BDTT.

Figure 1: GP curve for MS in a 0.5 M H₂SO₄ medium with different Ct of BDTT, at 298 K. 

BDTT effect on anodic reaction was stronger than on the cathodic one. BDTT retarded MS dissolution, which was suggested by the results ³⁰. Ec corrosion parameters, such as E_corr, (_a, (_c and I_corr, are given in Table 1.

Table 1: GP and LPR for MS in the 0.5 M H₂SO₄ solution with and without BDTT, in various Ct, at 298 K. 

E_corr values difference was < 85 mV, which confirms BDTT is a mixed inhibitor type ³¹. BDTT adsorption at higher Ct was quite successful, and the CI covered the MS surface almost completely, which revealed its strength. Thus, at higher Ct, BDTT multilayer adsorption took place, which means that the SC of MS was complete, and all corrosion active sites were blocked. However, with lower Ct of CI, SC of MS decreased, and the metal was strongly corroded, due to the decreased IE(%) of BDTT ^32,33.

At 298 K, BDTT highest and lowest IE(%) values were 95.38% (1.0 mol/L) and 81.82% (0.001 mol/L), respectively. LPR values revealed that BDTT absorption took place on the MS surface through the development of a non-conducting physical barrier ³⁴. R_p values rose with higher Ct of BDTT. The increase in SC and BDTT adsorption onto the MS surface, with higher Ct, was confirmed by R_p data (Table 1). Eq. (5) further calculated IE(%).

()5

where (R_p)_inh is R_p value with BDTT.

EIS explanation

EIS gives detailed information about the electrode kinetics and surface characteristics. EIS was performed at 298 K. Nyquist and Bode curves plots for MS in the 0.5 M H₂SO₄ solution without and with BDTT, in different Ct, are shown in Fig. 2a-b.

Figure 2 a: Nyquist plots curves for MS in 0.5 M H₂SO₄ without and with BDTT, in different Ct, at 298 K. b: Bode plots curves for MS in 0.5 M H₂SO₄ without and with BDTT, in different Ct, at 298 K. 

Fig. 2a shows that MS impedance response in H₂SO₄ with BDTT significantly increased. At high-frequencies, the inhibitor system semicircles diameter was noticeably higher than that of the blank solution. This confirms that MS corrosion in H₂SO₄ was inhibited by BDTT. Eq (6)

()6

where R_ctinh is R_ct with inhibitor in the H₂SO₄ medium.

In H₂SO₄, the incessantly corroded MS surface became asymmetrical, as reflected on the small phase angle. Thus, BDTT adsorption onto the MS surface decreased, and the phase angle increased, approaching 90° ³⁵. Fig. 2b capacitive behavior in MS with BDTT is depicted by Bode plot.

The MS/solution interface capacity increased due to BDTT adsorption on it ³⁶. C_dl was calculated by using Eq (7). Table 2 shows that R_ct decreased and C_dl increased, with higher Ct of BDTT and its larger SC of the MS surface, which improved IE(%) ³⁷.

()7

where F_max is the maximum frequency through Nyquist plot.

Eq. (8), C_dl values decreased towards a constant local dielectric lower value, and the protective layer thickness increased, which enabled BDTT adsorption at the MS/solution interface ³⁸.

()8

where A is the electrode surface area, Ɛ and Ɛ^o are the medium dielectric constants and vacuum permittivity, respectively, and d is protective layer thickness. Further, in Table 2, C_dl values decreased with higher Ct of BDTT, confirming the rise in the protective layer thickness, which means that d value in Eq (8) increased.

Surface heterogeneity is related to phase shifts that are known n values from Eq. (9).

()9

where Q is CPE and ω is part of the impedance spectrum imaginary value, with maximum angular frequency.

Herein, n values were close to one, with higher Ct of BDTT molecule, which enabled the interface to achieve capacitance ³⁹. BDTT reached IE(%) of 95.44 and 79.51, at 1.0 and 0.001 mol/L respectively, which revealed that it was an effective inhibitor of MS corrosion in H₂SO₄.

Table 2: EIS parameters for MS in a 0.5 M H₂SO₄ solution without and with BDTT, in various Ct, at 298 K. 

WL studies

WL technique provides essential information on CR of MS and IE(%) of BDTT ⁴⁰. WL test was carried out on MS coupons (1 x 1 cm) immersed in 0.5 M H₂SO₄, at different Ct of BDTT, for 24 h, from 298 to 328 K. The graphs plot between CR of MS and Ct of BDTT are shown in Fig. 3.

Figure 3: Graph of CR vs. different Ct of BDTT at a T range from 298 to 328 K. 

The graphs for IE(%) of BDTT vs. its Ct in the 0.5 M H₂SO₄ solution are shown in Fig (4). CR decreased and IE(%) increased with a rise in Ct of BDTT (Fig. 3 and Table 3), in aggressive environments (Fig. 4). The better properties of the studied inhibitor may be explained through transfer of charge/donation of electrons, because BDTT was adsorbed by the MS surface, reducing its dissolution in H₂SO₄.

Figure 4: Graph of relationship between IE(%) of BDTT and its Ct. 

BDTT adsorption capability is due to its molecules structure, with lone pair heteroatoms, multiple pi bonds and an aromatic system. Herein, the highest and lowest IE(%) of BDTT were 96.19% (1.0 mol/L) and 85.55% (0.001 mol/L), respectively. Fig. 4 and Table 3 show that a rise in T increased CR of MS and decreased IE(%) of BDTT. Therefore, MS dissolution took place, and HER was promoted.

Table 3: WL estimations parameters for MS in 0.5 M H₂SO₄ without and with BDTT, in various Ct, at different T (298-328 K). 

T kinetics (adsorption and activation Td parameters)

Adsorption isotherm

Adsorption isotherms explain the interaction between MS surfaces and BDTT molecules. The water molecules replacement involved the adsorption in the metal/solution interface, via the following process ^41,42: Eq. (10)

(10)

Temkin’s, Freundlich’s, Frumkin’s, Langmuir’s, Henry’s, Viral’s, Damaskin’s, Volmer’s and Flory-Huggins’s adsorption isotherms ^40,41 were tested through the best fit with experimental data. BDTT molecule best adsorption behavior was explained by Langmuir’s isotherm ⁴². From the determination, R² values were equal to one. This adsorption isotherm was depicted by Eq. (11).

()11

where θ is SC degree and C_inh is Ct of BDTT. The relationships between C_inh/θ and C_inh were linear (Fig. 5), which suggests that BDTT molecule adsorption onto the MS surface obeyed Langmuir’s isotherm with every studied T.

ΔG°_ads for BDTT molecule adsorption onto the MS surface was calculated by Eq. (12).

()12

From Eq. (12), 55.5 is the Ct of water molecule in the solution. ΔG°_ads values are negative, which means that BDTT molecule adsorption onto the MS surface was a spontaneous process, at all T ⁴⁵. Usually, if ΔG°_ads values are nearby -20 kJ/mol^-1 or less, the adsorption process establishes an electrostatic interaction between the charged metal surface and inhibitor molecules, i.e. “physisorption” ⁴⁶. If ΔG°_ads values are around than -40 kj/mol^-1 or more, the adsorption process occurs via charge transfer or sharing between the inhibitor molecules and the metal surface, and formation of a coordinate bond with the metal, i.e. “chemisorption” ⁴⁷.

Figure 5: Langmuir’s isotherm for BDTT (with various Ct) adsorption onto the MS surface in 0.5 M H₂SO₄, at several T. 

In this study, ΔG⁰ _ads values were above 40 kj/mol^-1 (Table 4), which indicates that BDTT molecule absorption onto the MS surface was chemisorption.

Table 4: Td parameters for BDTT adsorption onto the MS surface in a 0.5 M H₂SO₄ solution, at various T. 

Other Td parameters, such as ΔS°_ads and ΔH°_ads were calculated by Eqs. (13) and (14).

()13

()14

The graph plot between ln K_ads versus 1/T for BDTT molecule adsorption is shown in Fig. 6.

Figure 6: Graph of log K_ads vs. 1/T ΔG°_ads for BDTT onto the MS surface. 

From eq. (14), a straight line best fit was obtained for ΔH°_ads /R slope and ΔS°_ads /R-ln (55.5) intercept ⁴⁸.

These Td parameters provide helpful information on the inhibitor molecule adsorption via CI mechanism. When ΔH°_ads > 0, the adsorption process is endothermic (chemisorption). When ΔH°_ads < 0, the adsorption process is exothermic (physisorption, chemisorption or an involution of both) ⁴⁶. Herein, ΔH°_ads and ΔS°_ads values were -61.12 kJ/mol^-1 and 58.60 J/mol^-1/K^-1, respectively. Therefore, BDTT adsorption mechanism was a mixture of chemisorption and physisorption ⁴⁷.

E _a parameters

Further, Td parameters for E_a values were calculated by using Arrhenius graph (Fig. 7), which were obtained from Arrhenius Eq. (15), with and without BDTT molecule, using the slopes for log I_corr vs. 1/T (Table 5).

()15

where A is pre-exponential factor and R is universal gas constant.

Figure 7: Graph of Arrhenius log (CR of MS) vs. 1/T in a 0.5 M H₂SO₄ solution with various Ct of BDTT. 

Table 5: E_a parameters for MS in a 0.5 M H₂SO₄ solution with and without several Ct of BDTT. 

MS surface E_a was high, which means that BDTT worked as anti-corrosion catalyst. ⁴⁸ stated that E_a low values with reduced inhibitor Ct are associated with chemisorption, whereas high E_a values with increased inhibitor Ct correspond to physisorption.

DFT treatment (computational study)

BDTT molecule structures were optimized using DFT hybrid functional as (6-311G B3LYP), which are shown in Figs. 8a to 8c. Quantum chemical parameters are given in Table 6. According to FMO theory, a transition state is developed due to the interaction between HOMO and LUMO ⁴⁹. HOMO usually has the ability of donating electrons via the inhibitor molecule. E_HOMO high value indicates electron donation to a suitable acceptor molecule with unfilled molecular orbitals of low energy. Increased E_HOMO values empower the adsorption by influencing the transportation process ⁵⁰. E_LUMO represents the capability to accept electrons of the inhibitor molecules. E_LUMO low values exhibit high acceptability of inhibitor molecules electrons. BDDT density distributions in FMO are shown in Fig. (8 b) (HOMO) and Fig. 8 c (LUMO). HOMO electron density in BDTT molecule is regularly dispersed close to N and S atoms, which specifies that the molecule has suitable sites for interaction with the MS surface.

LUMO electron density on BDTT dispersed approxim. over the whole inhibitor molecule. So, BDTT molecule was adsorbed on the MS surface, due to π electrons delocalization within the aromatic system, and also to the lone pair electrons in the hetero atoms. BDTT molecule adsorption was strong, due to the delocalized π electrons availability in the ring, and closeness to the MS surface.

Figure 8: BDTT molecule quantum (a) optimized structure via DFT; (b) HOMO; (c) LUMO. 

From the DFT analysis of results shown in Table 6, BDTT molecule has high E_HOMO and low E_LUMO values, and ΔE for the BDTT molecule is given by eq. (16). ΔE value lower than 4.3301 eV means that BDTT molecule has good IE(%). The molecule electronic charge depicts the binding ability of that molecule, via chelating atoms, with the MS surface. The Mulliken charges of BDTT atoms represent the binding ability of that molecule with the MS surface ⁵¹. The chelating or binding ability between the metal surface and the inhibitor molecule depends on the electronic charge of the active or chelating atom. This means that, with higher negative charges, the binding ability is stronger. Table 6 shows other parameters that were also calculated via Eqs. 16, 17,18 to 24: ΔN, (, σ, π, ΔE_backdonation, (the energy difference between back-donation and electron transfers through the inhibitor molecule) and µ.

()16

()17

where I and A are IP and EA, respectively.

()18

ΔE is high with a hard molecule and high with a soft molecule ⁵². BDTT molecular reactivity and stability were evaluated via global ( and σ in Eqs. (19) and (20). Chemical ( is the resistance to distortion or electrons clouds on atoms, molecules or ions, whereby polarization is almost unaffected by chemical reactions. Herein, BDTT value of ( was 2.1650 eV. Generally, global ( low value is likely to support greater IE(%) ⁵⁰. Normally, adsorption occurs due to electrons transfer through the inhibitor molecule and σ high value ⁵². BDTT had a σ value of 0.4618 eV^-1, which means it had great IE(%).

()19

()20

χ value describes inhibitor molecules attraction ability. When they have high χ values, their attracting power is stronger, and they have a stronger interaction with the metal surface, so as to accept electrons from it, which enables great IE(%). Herein, χ value calculated via Eq. (21) was 3.9891eV.

()21

ΔN was calculated via Eq. (22) , where χ_Fe is 7 eV and (Fe equals 0.

()22

Table 6 shows ΔN significance: if ΔN ˃ 0, the electrons transfer occurs towards the metal surface; if ΔN ˂ 0, the electrons transfer occurs from the MS surface onto the inhibitor molecule ⁵². As ⁵³ has reported, ΔN ˂ 3.6, which increases IE(%), with the inhibitor molecule rising ability to donate electrons to the metal surface. Herein, ΔN value was 0.6953, which indicates BDTT molecule ability to donate electrons to the MS surface, through the formation of a coordinate bond.

Table 6: DFT parameters via quantum chemical analysis for the inhibitor molecule. 

Eq. (23) shows that IE(%) of an inhibitor molecule depends upon π ⁵⁴. High π values improve the inhibitor molecule ability to release electrons.

()23

Eq. (24) calculates electrons charge transfer for donation or back donation ⁵⁵. Electronic back-donation occurs through the interaction between the MS surface and the inhibitor molecule. The change in energy is straightly associated with ( of BDTT, as shown in Eq. (24), which implies that, when ( > 0 or ΔE_{Back Donation} < 0, charge transfer is followed by back-donation via the inhibitor molecule.

()24

Herein, ΔE_backdonation and ( were 0.5412eV and 2.1650Ev, respectively. These values confirm the molecule ability to act as CI. μ is an additional parameter for the electronic charge distribution throughout a molecule, revealing its structure ⁵⁴. No substantial relation between IE(%) and μ values was discovered.

Corrosion mechanism

In this study, BDTT molecule formed a protective layer onto the MS surface in 0.5 M H₂SO₄, during Ec analyses and WL test. BDTT performed successfully as CI for MS in the aggressive medium, at low T, and higher Ct, for a longer IT. BDTT molecule was adsorbed on the MS surface, due to interactions between hetero atoms (N and S) electron pairs, electrons clouds over imine bond and π-electrons clouds delocalized over aromatic rings with the Fe atoms containing empty d-orbitals. BDTT gained protons in the corrosion process, due to its basic character, and reached equilibrium, as Eq. (25) shows, being equivalent to neutral.

()25

In 0.5 M H₂SO₄, due to negative charge, sulfate ions developed an additional charge density on the positively charged MS surface. This means BDTTH⁺ species had stronger adsorption abilities.

HER follows reactions ⁵⁶ , as depicted in eqs. (26), (27), (28).

()26

()27

()28

H reduction at the cathode into H₂ gas ceased, and BDTTH⁺ species was competitively adsorbed onto the MS surface.