Insights into the adsorption and corrosion inhibition properties of newly synthesized diazinyl derivatives for mild steel in hydrochloric acid: synthesis, electrochemical, SRB biological resistivity and quantum chemical calculations

Two azo derivatives, 4-((4-hydroxy-3-((4-oxo-2-thioxothiazolidin-5-ylidene)methyl)phenyl) diazinyl) benzenesulfonic acid (TODB) and 4-((3-((4,4-dimethyl-2,6-dioxocyclohexylidene) methyl)-4-hydroxyphenyl)diazinyl) benzenesulfonic acid (DODB) were synthesized and characterized using Fourier-transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H-NMR) and mass spectral studies. Gravimetric methods, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), electrochemical frequency modulation (EFM) techniques and inductive coupled plasma-optical emission spectroscopy were used to verify the above two compounds' ability to operate as mild steel (MS) corrosion inhibitors in 1 M HCl. Tafel data suggest that TODB and DODB have mixed-type characteristics, and EIS findings demonstrate that increasing their concentration not only alters the charge transfer (Rct) of mild steel from 6.88 Ω cm2 to 112.9 Ω cm2 but also changes the capacitance of the adsorbed double layer (Cdl) from 225.36 to 348.36 μF cm−2. At 7.5 × 10−4 M concentration, the azo derivatives showed the highest corrosion inhibition of 94.9% and 93.6%. The inhibitory molecule adsorption on the metal substrate followed the Langmuir isotherm. The thermodynamic activation functions of the dissolution process were also calculated as a function of inhibitor concentration. UV-vis, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) techniques were used to confirm the adsorption phenomenon. The quantum chemical parameters, inductively coupled plasma atomic emission spectroscopy (ICPE) measurements, and the anti-bacterial effect of these new derivatives against sulfate-reducing bacteria (SRB) were also investigated. Taken together, the acquired results demonstrate that these compounds can create an appropriate preventing surface and regulate the corrosion rate.


Introduction
Mild steel (MS) is a material with small carbon content that is solid and tough but not easily tempered. It is known as lowcarbon steel, and it is currently the most well-known type of steel because of its relatively low cost while providing mechanical characteristics that are sufficient for certain implementations. 1,2 Mineral acids, especially corrosive hydrochloric acid, are regularly utilized for mechanical purposes, such as corrosive cleaning, corrosive pickling, corrosive descaling, and oil well acidizing. Such acids create highly corrosive surroundings for MS. As a direct consequence, the study of steel protection against corrosion is always a topic of great theoretical and practical importance. Moreover, because MS is susceptible to corrosion like other metallic materials, its surface must be protected from such undesirable process. Corrosion inhibitors have recently been used to eliminate/reduce corrosion of metal parts in home appliances and industrial machines. Aside from the use of traditional inhibitors, 3,4 metals can be also protected through chemical or electrochemical surface functionalization, such as SAMs (Self-assembled Monolayers) formed from silanes, 5,6 phosphonic acids, 7 or electrochemical reduction of aryldiazonium salts on metals. 8,9 Organic compounds with p-electron systems, atoms with lone pair electrons (P, S, N, O), and plane-conjugated structures including benzene rings are the most effective organic inhibitors. [10][11][12] The electrical connections between the organic inhibitors and the metal surface are facilitated by certain sets of atoms or bonds, which allows the inhibitors to adhere to the base metal. It has been asserted that the efficiency of these molecules for corrosion protection increases in the following order: O > N > S > P. 8,13 In most cases, the efficiencies of these organic inhibitors are determined by the components of the environment in which they act, such as the nature of the metal substrate, the electrochemical potential at the metal/solution interface, the metal surface protected area, the inhibitor's molecular size, manner of adsorption, its concentration, and its structure. 14 The choice of any inhibitor, on the other hand, is heavily inuenced by its economic allocation, efficiency in inhibiting the substrate material, and environmental side effects. 15 In general, organic dyes have especially gained signicant attention due to their multipurpose applications in a variety of elds, such as cosmetics, textiles, food, and pharmaceuticals. 16 Because of their distinct chemical structures, they are also used in the protection of metals and alloys against corrosion. Azo-dyes have shown signicant inhibition efficiencies 17,18 and are one of the most commonly used organic compounds as inhibitors. These compounds' inhibition process has been shown to implement an inhibitor adsorption isotherm, 17,18 whereas their efficiency is dependent on the structure and chemical properties of the adsorbed inhibitor lm formed on the steel surface. Density functional theory (DFT) has become a helpful strategy in deriving the electronic properties, allowing researchers to obtain solid basic parameters for the atoms. 19,20 In corrosion studies, this strategy makes it conceivable to precisely anticipate the hindrance productivity of natural erosion inhibitors on the premise of the electronic and atomic properties, as well as the reactivity records. Many studies have been conducted and published previously on the use of conceptual models for predicting and supporting the corrosion inhibitive potentials of molecules, including azo dyes. [21][22][23] In light of this, two azo derivatives, TODB and DODB compounds were evaluated as novel inhibitors of MS corrosion in a corrosive media of HCl (1.0 M) solution. TODB and DODB compounds were thoroughly characterized using FTIR, proton nuclear magnetic resonance ( 1 H-NMR), and mass spectral studies. At a constant temperature, various techniques such as PDP, EIS, and SEM were used to assess the inhibition efficiencies (% IEs) of the examined organic molecules.

Synthesis of TODB and DODB compounds
4-((3-Formyl-4-hydroxyphenyl) diazenyl) benzenesulfonic acid (0.013 mol) was prepared and mixed with two cyclic ketones, rhodanine (0.013 mol) and dimedone (0.021 mol), in a round bottom ask with a few drops of piperidine, acetic acid and ethanol as the solvent (100 ml). The reaction mixture was reuxed with stirring for 2 h, and ltered to omit the solvent. The recrystallization of the precipitate was performed using ethanol, then le to dry to give the nal product (Fig. 1). TODB; brown powder, yield: 90.84%, mp: over 300°C. DODB; yellow powder, low yield: 15.43%, mp: over 300°C.

Corrosive environment
One molar HCl stock solution was made by dilution with double-distilled water. Furthermore, the concentration varieties of the studied compounds were (0.50 × 10 −4 -7.50 × 10 −4 ) and were prepared using ethanol.

Gravimetric measurements
Weight loss (WL) tests were carried by inserting the tested sheets into an aerated acidic solution HCl (0.1 M) with and without various concentrations of the prepared TODB and DODB at warm temperatures from 298 to 318 K. Aer a 6 hour soaking, the samples were immediately removed, washed, dried, and weighed. The assessed weight loss was used to quantify the degree of surface coverage (q) and the inhibition efficiency (% IE) using eqn (1) and (2) where W o and W inh are the weight loss values (mg) without and with inhibitors. Using eqn (3), the corrosion rate, C R (mg cm −2 h −1 ) was assessed; 25 where S is the surface area of the metal samples (cm 2 ), and t is the exposure time (h).

Electrochemical measurements
For studying the inhibition of MS corrosion using TODB and DODB, three distinct electrochemical techniques were used: PDP, EIS, and electrochemical frequency modulation (EFM). At 25°C, a standard electrochemical cell fabricated from MS (1 cm 2 exposed area), a saturated calomel electrode (SCE) and a Pt sheet as the working, reference, and counter electrodes, respectively, was used for electrochemical studies. 2.4.1. PDP technique. Utilizing Gamry framework devices, potentiodynamic current-potential graphs were documented by instantaneously shiing the electrode potential from −1500 mV to +500 mV at a scanning rate of 1 mV s −1 (version 3.20). Corrosion current densities (I corr ) and corrosion potential (E corr ) were assessed from the interplay of the correlation anodic and cathodic sections of Tafel plots in the absence and presence of various inhibitor concentrations. Using the degree of surface coverage (q) from eqn (4), the percentage inhibition efficiency (percent IE) was quantied: 4,26 where I o corr and I corr are the corrosion current densities without and with TODB and DODB derivatives, respectively.
2.4.2. EIS measurements. Aer immersing the electrode for 15 minutes, the EIS spectra were collected at the open circuit potential, OCP. The peak-to-peak voltage of the AC signal was 10 mV, and the resonant frequency evaluated was 0.1-10 5 Hz. Nyquist and Bode plots were used to present the results. The important variables derived from the analysis of the Nyquist diagram are the resistance of charge transfer (R ct ) and the capacity of the double layer (C dl ), which are estimated using eqn (5): 27 where f max is the maximum frequency. The terms (q) and (% IE) were also estimated using eqn (6): 28 where R o ct and R ct are the charge transfer resistances without and in presence of different concentrations of TODB and DODB, respectively.
2.4.3. EFM technique. EFM was used with 2 and 5 Hz frequencies. Because the reference frequency was 1 Hz, the waveform repeated aer 1 second. The higher frequency should be slow enough that the charging of the double layer does not make a contribution to the current approach. Before initiating the readings, the electrode potential was stabilized for 30 minutes. Gamry reference 3000 Potentiostat/Galvanostat/ZRA analyzer, DC 105 corrosion, EIS 300, EFM 140, and Echem Analyst 5.21 soware were used for data plotting, graphing, data tting, and calculating.

ICPE technique
In this research, the content of dissolved MS in HCl (1.0 M) solutions with and without various amounts of TODB and DODB was determined by ICPE. The ICPE-9820 from SHI-MADZU used for our studies was standardized using 1, 5, and 10 ppm made from a 1000 ppm Scharlau iron standard stock solution. The MS coupons were submerged in various doses of the newly synthesized compounds for 24 hours. The content of dissolved iron was then measured in the solutions aer dilution with ultra-pure demineralized water to obtain results that were plotted on the calibration graph for the instrument. 29

UV-analysis
An ultraviolet-visible spectrophotometer (Thermo Fisher Scientic) was employed to examine the shi in wavelength following MS immersion in HCl (1.0 M) for 24 h, both with and without the addition of TODB and DODB. A shi in wavelength was a strong indicator that a combination between the inhibitor molecules and the Fe 2+ on the electrode had formed. 30

SEM and EDX tests
The MS surface was studied using the SEM JEOL JSM-IT200 apparatus with and without the application of an appropriate molarity of TODB and DODB aer being submerged in HCl (1.0 M) for one day. The quantitative concentration of the constituents from the solutions adsorbed on the MS surface was also determined using EDX.

Biological activity
H 2 S is produced by a kind of anaerobic bacteria (SRB), which can speed up the corrosion process. The test was conducted using SRB-BART™ testers from Droycon Bioconcepts, Inc. (DBI). The test time is 11 days, and the test results can be obtained by counting the number of bacterial colonies present when the test vials rst turn black. Each day refers to a certain quantity of germs that are present. 31

Theoretical chemical parameters
Using Gaussian 09 with Gauss View 06 soware with different basis sets (semi-empirical PM6, Hartree-Fock 631G, DFT 6311G and MP2-6-311G), the optimized chemical calculations were performed. Different parameters were obtained via the calculations. The highest (E HOMO ) occupied and lowest (E LUMO ) unoccupied molecular orbitals energies, and the energy gap (DE) between them were calculated. Other parameters were also optimized, including IP (ionization potential), EA (electron affinity), c (electronegativity), u (electrophilicity), DN (transferred electrons), s (soness), h (hardness), m (dipole moment), MV (molecular volume) and TNC (total negative charge). According to the following equations, the different parameters can be calculated as follows: [32][33][34][35] The interactions between the TODB and DODB molecules (either in neutral or protonated phase) and the mild steel surface were theoretically investigated by Monte Carlo simulations using adsorption locator module in Material studio 2017 soware. The acidic aqueous conditions were brought by adding 200 H 2 O, 20 H 3 O + and Cl − . Fe was cleaved over the most stable (110) plane. The Fe (110) was expanded to a (10 × 10) super cell, and a vacuum with 30 Å was formed above the Fe (110) plane. To nd the equilibrium adsorption congurations, the COMPASS force eld was used.     Fig. 3 shows the 1 HNMR for TODB, which is 400 MHz; at: d = 3.1 ppm (1H, s, NH), 7.14 ppm (1H, s, HC]C) (Arylidene), 1.91 ppm (1H, s, Phenolic OH), d = 10.37 ppm (SO 3 H)), d = 7.9-8.20 ppm (7H, multiplet, Ar-H). 1 Table 2 also includes the average values of the C R , and percent IE of the studied organic compounds. Table 2 shows that as the inhibitor concentrations increased and the temperature remained constant, the values of C R decreased while the percent IEs increased. This can be attributed to the increased adsorption coverage of the inhibitor molecules on the metal substrate with increasing concentrations, which decreased the MS dissolution rates. As an outcome, the investigated organic molecules are recognized as capable inhibitors of MS corrosion in HCl (1.0 M) solutions. In contrast, at constant inhibitor concentration and rising temperature, the value of C R rose marginally while the percent IE grew exponentially. These research results unequivocally prove that the adsorption mechanism of prepared TODB and DODB on the metallic substrate in HCl (1.0 M) solutions was characteristic of chemical adsorption. Fiori-Bimbi et al. 36 discovered similar results while inhibiting M − steel corrosion in HCl solutions with pectin. The values of % IE obtained for the inhibitor TODB were oen greater than those for the inhibitor DODB. The physicochemical properties of the molecules, such as the functional groups, steric factor, molecular (size, weight, and structure) and aromaticity, 37-39 determine the adsorption of any corrosion inhibitors. Since the investigated azo compounds (TODB and DODB) have these characteristics, they have a good adsorption capacity and thus can behave as effective corrosion inhibitors. By comparison with previous publications, it was noticed that the currently investigated compounds had higher percentage of IEs than other reported organic dyes as corrosion inhibitors of various steel alloys in acidic environment. [40][41][42] 3.2.2. PDP measurements. This method was used to study both cathodic and anodic reactions of TODB and DODB at 298 K in HCl (1.0 M) without and with inhibitor on the MS electrode aer performing the OCP test, and approaching the steady state   a E corr is the corrosion potential; I corr is the corrosion current density: b a and b c are Tafel constants for both anode and cathode; (C R ) is the corrosion rate; q is the surface coverage; h p is the inhibition efficiency.   potential. Fig. 6 shows the polarization proles that were acquired. It was also found in the literature 43 that when the corrosion potential (E corr ) shi is greater than ±85 mV vs. the E corr of the uninhibited sample, the inhibitor is classied as cathodic or anodic, while it is classied as mixed-type when the shi is less than ±85 mV. The introduction of TODB and DODB in HCl (1.0 M) solution markedly decreased the Tafel sections in the current study, and the movement in the E corr was less than ±85 mV, indicating that the prepared azo derivatives are mixedtype inhibitors. 44 The electrochemical variables like E corr , Tafel constants for both anode (b a ) and cathode (b c ), corrosion current density (I corr ), polarization resistance (R p ), (q), (C R ), and (% IE) were calculated from the Tafel graphs and are listed in Table 3. By extrapolating the linear parts of the Tafel branches to the relevant values of (E corr ), the values of (I corr ) were computed. The analysis of the results showed that, in comparison to the uninhibited solution, the (I corr ) and (C R ) decreased in the presence of TODB and DODB, demonstrating that these substances were adsorbed on the MS surface and delayed the dissolving of the metal in the acidic environment (Table 3). Furthermore, when compared to the inhibitor-free sample, the azo derivatives reduced the oxidation reaction of MS and delayed the hydrogen ion reduction on the surface of the cathode. Moreover, the equivalent form of the Tafel curves suggests that the inhibition mechanism is activation-controlled in the existence of the evaluated inhibitors. 45 The (I corr ) values are much lower in the presence of the inhibitors (0.436 mA cm −2 for TODB and 0.55 mA cm −2 for DODB) than in the absence of the inhibitors (0.86 mA cm −2 ), with maximum efficiency (94.9% and 93.6%), respectively, at concentrations of 7.50 × 10 −4 M for TODB and DODB, indicating that these materials mitigate corrosion and preserve MS. The involvement of more hetero atoms (S and N) with lone pair of electrons in the framework of the (TODB) molecule, which increases adsorption, may explain the higher h for the (TODB) molecule compared to the (DODB) molecule.

Results and discussions
3.2.3. EIS study. EIS readings were used to prove the previously mentioned corrosion behavior, as well as to investigate the capacitive characteristics at the MS/solution interface. The Nyquist and Bode plots of the MS in HCl (1.0 M) without and in the existence of different molarities of (TODB and DODB) are seen in Fig. 7a, aer the OCP test and approaching the steady state potential. The documented Nyquist plots resembled each other, indicating that the addition of (TODB and DODB) inhibited MS corrosion without inuencing the mechanism. 46 At low frequency, the Nyquist plots are best described by only single semicircles, suggesting that the corrosion process is under charge transfer control. 47 The increasing values of (n) ( Table 4) for (TODB and DODB) versus the blank sample clearly show that the inhibitor increases surface uniformity via adsorption. Fig. 7b shows the correlating Bode plots for the MS electrode immersed in HCl (1.0 M) with and without various concentration levels. As shown by these gures, there is an increase in the absolute impedance jZj at low frequencies. This increase conrms the higher (percent h) obtained at high concentration, which is due to adsorption of (TODB and DODB) on the MS surface and blocking its active sites. Furthermore, the shi in phase angle values in the negative direction demonstrates that TODB and DODB primarily perform by providing a thin lm over the MS surface. 48,49 Fig . 7a and b show a comparison of the experimental EIS data recorded for (TODB and DODB) in the presence of various concentrations in comparison to the blank. The well-tted data were collected by employing the equivalent circuit shown inset (Fig. 7c), where R s (the solution resistance), R ct , and C dl are recorded in Table 4. The EIS variables determined with this circuit indicate that R ct is directly related to the molarity of the inhibitor, whereas the (C dl ) values show the inverse correlation. This concept could be caused by either H 2 O molecule desorption from the surface of MS or inhibitor adsorption on the metal substrate. 50,51 In agreement with previous techniques, the % h of the evaluated azo derivatives is as follows: (TODB) > (DODB), with optimum inhibition values of 93.9% and 92.05%. This EISobserved attitude corresponds to the polarization data. The electron donating impact of the sulfonyl (-SO 3 H) and hydroxyl (-OH) groups bonded to the aromatic ring increases the electron density on the benzene ring, inhibiting the corrosion process. c and b a ). EFM, like EIS, is a technique that employs small AC signals. In contrast to EIS, two sine waves (at different frequencies) are implemented to the cell all at once. I corr can be calculated from the intermodulation spectra at the tallest altitude employing EFM instead of using Tafel slope values (b c and b a ). We can also use the causality factors [CF-2 and CF-3] to verify the information generated by this technique. 52 The EFM intermodulation spectra are a relationship of the current feedback as a function of input frequency that includes current feedback reecting harmonic and intermodulation peaks. All kinetic components derived from EFM, such as corrosion current, Tafel slopes, and causality factors, were calculated using the highest peaks and are shown in Table 4. EFM measurements were used to calculate the (% h) and (q) of the studied compounds from the following equation: 53

EFM technique. EFM is a nondestructive technique that can provide (I corr ) values without the need for previous information of Tafel constants (b
The intermodulation spectra of the two prepared azo derivative (TODB and DODB) obtained in this study are shown in Fig. 8. The data in Table 5 show that the i corr values for the studied molecules are lower than those for the aggressive solution HCl (1.0 M), indicating that they inhibited corrosion. It was also discovered that i corr changed with concentration, reaching its lowest value at the highest concentration (7.50 × 10 −4 M), indicating that the (h EFM percent) increases with azo derivative concentrations. Table 5 also shows that the surface coverage values of the TODB molecules are greater than those of DODB, implying that the inhibition of TODB is greater than that of DODB. Finally, as a result, we can conclude and guarantee the method's and result's validity.   a E corr is the corrosion potential; I corr is the corrosion current density: b a and b c are Tafel constants for both anode and cathode; k is the corrosion rate; q is the surface coverage; h EFM is the inhibition efficiency.

Adsorption isotherm
The examined TODB and DODB involve heteroatoms like O, S, and N in TODB and O and S in DODB, as well as aromatic rings and azo groups that can be adsorbed on the steel surface to generate protective layers. 53 These layers can be created using one of the adsorption modes listed as follows: 54 (1) physical: arises as a result of electrostatic forces between the protonated groups of organic molecules and the charged metal surface; (2) chemical: by forming coordination bonds between the vacant d orbital of the substrate and the lone pair of electrons of the heteroatoms; or (3) integration of two adsorption types. To observe and analyze the better adsorption isotherm (Langmuir, Flory-Huggins, Temkin, Frumkin, Freundlich type and Kinetic model) of experimented azo compounds, graphs of each adsorption isotherms were evaluated separately (Fig. 9). The regression correlation coefficient (R 2 ) values listed in Table 6 show that the adsorption behavior of TODB and DODB on the MS surface correlated well with the Langmuir isotherm, 55 which is given by the formula: 56 C inh /q = (1/K ads ) + C inh (15) where C inh is the molarity of the tested TODB and DODB inhibitors, K ads is the adsorption-desorption equilibrium constant for the metal substrate processes, and (q) is estimated using electrochemical methods. The graph of C against C/q demonstrates a straight tting line with a slope and correlation coefficient near 1 (Fig. 9). Formula (8) was used to quantify (K ads ) from the standard free energy of adsorption (DG ads ): 57 DG where C solvent denotes the molarity of water in solution, T denotes the absolute temperature, and R denotes the universal Fig. 9 Different adsorption isotherms for synthesized inhibitors TODB and DODB using WL method. gas constant. Table 6 shows the thermodynamics for the adsorption process of the experimented molecules (TODB and DODB). The fact that the DG ads is negative implies that the tested molecules (TODB and DODB) adsorb spontaneously on the surface of MS. Furthermore, higher K ads values indicate a strong adsorption property, and as a result, a better inhibition property. 58 In general, absolute values of DG ads of (−20 kJ mol −1 ) or less indicate electrostatic interaction (i.e., physisorption). Values around −40 kJ mol −1 or more negative values, on the other hand, indicate charge sharing and bond formation (chemisorption), 59 although there is little difference between the DG ads values (between −20 and −40 kJ mol −1 ). This shows that TODB and DODB are physically and chemically adsorbed on the surface of MS, with physical adsorption having a particularly strong adsorption effect. 60 It also appeared from Table 6 that the K ads values of TODB molecules are higher than that of DODB, which can be interpreted as the inhibition of TODB < DODB.

Effect of temperature and activation parameters
The activation energy (E a ) and other thermodynamic activation functions can be calculated using mass loss experiments at different temperatures with and without inhibitors. The ndings can help to explain the mechanism of inhibition. This study adds to our understanding of the adsorption mechanism by analyzing the thermodynamic parameters for MS dissolution in HCl (1.0 M) without and in the presence of different molarities of TODB and DODB. The Arrhenius and transition-state equations 61 were used to obtain these thermodynamic functions: where DS * a and DH * a are the apparent entropy of activation and enthalpy of activation, respectively, and h and N are Planck's constant and Avogadro's number, respectively. In the case of heterogeneous chemical reactions, the constant A is known as the pre-exponential factor because it is related to the number of active centers. For MS in HCl (1.0 M) solutions containing different molarities of the two investigated inhibitors in the temperature range of 298 K to 318 K, the Arrhenius and transition state plots were constructed using chemical and electrochemical measurements. The Arrhenius and transition state plots of MS in HCl (1.0 M) solution without and with different molarities (0.50 × 10 −4 to 7.50 × 10 −4 M) are shown in Fig. 10. Table 7 shows the values of the thermodynamic activation functions calculated from these plots as a function of the inhibitor concentration. The rise in inhibition efficiency values with increasing temperature (see Table 2), and the gradual decrease in E * a with increasing molar concentration of the inhibitor (Table 7) can be explained on the basis of chemical adsorption. According to Amin et al., 60 it is clear from the data in Table 6 that the process of MS dissolution is dened by E * a , which is lower in the presence of these molecules (TODB and DODB) than it is in the uncontrolled HCl (1.0 M). A review of the results in Table 10 showed that the activation parameters DH * a and DS * a of the MS dissolving reaction are higher in their absence than in their presence. The positive sign for the enthalpy value indicates that the process of dissolving steel is endothermic, which makes it challenging. 62 As shown in Table  11, values for (DS * a ) increased in both tested media when the TODB and DODB were present relative to a free aggressive solution. Such uctuation is connected to the phenomenon of Table 6 Adsorption isotherm models of the inhibitors a with values of R 2 , slopes, intercepts, K ads , and DG ads obtained by using data from WL measurements inhibitor molecule ordering and disordering on the MS surface. According to the growing DS ads in the presence of the inhibitor, disordering is intensied as the reactant moves from the reactant to the activated complex. In other words, the catalyst for the adsorption of the inhibitor onto the MS surface is the rise in entropy that occurs throughout the adsorption process. 63

ICPE test
The corrosion rate for the two synthetic inhibitors TODB and DODB was determined by quantifying the total iron ions for the MS aer immersion for 24 hours in HCl corrosive medium (1. Table 8, the effectiveness of inhibition increased.

UV-analysis
Metal and inhibitor molecule interactions were studied via UVvisible spectroscopy 64  The UV-visible absorption spectra were recorded and the results are shown in Fig. 11. Following a 24 hour immersion in HCl, a typical band for steel at 205 nm (1 molar) was obtained. The levels of absorbance were adjusted to 340 nm and 336 nm for TODB and DODB, respectively, when 7.50 × 10 −4 M of the two tested inhibitors were immersed with MS in HCl (1.0 M) for 24 hours. Due to the creation of a protective covering from the TODB and DODB molecules on the MS surface, the variation in the absorption values can provide conclusive proof for interaction between the evaluated inhibitors and the steel surface.

SEM and EDX spectroscopy
To evaluate whether the surface morphology was altered by employing a certain concentration of the TODB and DODB   (Fig. 12). We can infer from the SEM results that when the inhibitors were used, a protective coating formed on the MS surface. The inhibition efficiency studies conducted using chemical and electrochemical methods demonstrate the protective nature of this lm. Additional proof of the components adhering to the surface of MS following immersion in corrosive solution with and without inhibitors can be determined using the EDX technique. However, when the steel surface was analyzed for samples immersed with 7.5 × 10 −4 M of the TODB and DODB inhibitor molecules, respectively, new signals for O, S, N, and C were detected with signicant concentration (Table 9) due to the formation of a protective layer on the surface aer using the inhibitors (Fig. 12). In the case of steel immersed in HCl (1.0 M) alone, only Fe and Cl can be detected. These ndings strongly support the use of TODB and DODB as MS corrosion inhibitors in HCl.

Biological activity
A sample solution containing Sulfate Reducing Bacteria (SRB) from one of the networks of re hydrants on the Zohr Gas Field in Egypt was used for this test. The SRB BART vials were lled with 15 ml of this water, both with and without the addition of 1 ml of the tested inhibitors at a concentration of 1 ppm mol (TODB and DODB). For the entire test period of 11 days, the color to rst black sign was monitored daily in order to estimate the matching population SRB reading shown in Table 10. The rst black sign for the blank sample, which was not inhibited, appeared aer 4 days of incubation with a population of roughly 27 000 SRB cfu ml −1 , and it is regarded as aggressive. When the tested inhibitors were used, the rst black indication for DODB and TODB showed aer 7 and 8 days, respectively. According to Table 10, the approximate SRB populations were 325 cfu ml −1 and 75 cfu ml −1 for DODB (considered moderate) and TODB (considered low), respectively. These ndings suggest that SRB can reduce corrosion formation, indicating that the inhibitor compounds TODB and DODB have good biological activity against SRB. 65

Relation between quantum calculations and corrosion inhibition
Various theoretical quantum chemical parameters were calculated using different methods to investigate the adsorption behavior and interaction between MS and the synthesized inhibitors TODB and DODB, semi-empirical PM6, HF-631G, DFT/B3LYP/6-311+G and MP2-6-311G basis sets were applied. 66,67 The resulting values for the different calculated parameters are summarized in Table 11. Fig. 13    .526 eV using semi-empirical PM6, HF-631G, DFT/B3LYP/6-311+G and MP2-6-311G basis sets, respectively. From these mentioned values, TODB shows soness results higher than DODB and the opposite is detected for hardness. This indicates that the ability of TODB to protect the steel surface is greater than DODB. The transferred (DN) electrons give signicant proof for the inhibition ability by electron donation from the inhibitor (Lewis base) Fig. 13 Optimized structures, HOMO, LUMO and ESP for synthesized inhibitors TODB and DODB using DFT/B3LYP/6-311 + G. Table 11 The calculated quantum chemical parameters using several optimization bases sets: semi empirical PM6, HF-631G, DFT-B3LYP/6-311G and MP2- 6-311G to the surface of the metal (Lewis acid). From the resulting values in Table 11, the ability of TODB to donate electrons is higher than DODB, and the calculated values are 0.5947 and 0.5416 (e) using DFT/B3LYP/6-311+, respectively. As a result of this DN effect, a coordination bond is formed between the inhibitor and the (vacant d orbitals) metal surface. [71][72][73] In addition, other parameters are shown in Table 11, including the electronegativity (c), total negative charge (TNC), molecular volume (M. V.) and electrophilicity (u). The theoretical values are in good agreement with the experimental values, and also suggest that TODB has higher inhibition ability than DODB.

Monte Carlo simulations
The adsorption of the two inhibitor molecules (either neutral TODB and DODB, or protonated TODB-H + and DODB-H + ) was simulated in vacuum and aqueous phases. Fig. 14 depicts the most stable congurations of TODB-H + and DODB-H + on the Fe (110) surface in the aqueous conditions. Table 12 shows the nal result output descriptors of the MC simulation for the adsorption process in both vacuum and aqueous phases. The inhibitor molecules were clearly adsorbed on the Fe surface in a parallel adsorption arrangement, which guaranteed maximum surface coverage and optimum protection from