New Hexafunctional Epoxy Prepolymer: Innovation structure in corrosion inhibition

This work presents a novel approach to corrosion inhibition through the creation of a groundbreaking hexafunctional phosphorus epoxy resin, namely phosphorus trimethylene dianiline hexaglycidyl (HGTMDAP). This innovative material is synthesized via a two-step process, initiating with a grafting reaction between methylene dianiline and phosphorus trichloride, followed by the addition of epichlorohydrin to yield the hexafunctional resin. The unique structural intricacies of this material were elucidated using advanced microscopic characterization techniques such as FTIR, 1 H, and 13 C NMR


Introduction
When a metal corrodes, it means that it has deteriorated and degraded as a result of chemical or electrochemical reactions with its surroundings.It affects many businesses, including manufacturing, infrastructure, transportation, and energy generation, and it is expensive.Due to the cost of maintaining, replacing, and repairing rusted metal structures and equipment, corrosion can result in large economic losses.Corrosion-related direct costs include material replacement, labour, and repair downtime.Reduced production, environmental effects, and potential safety risks all have indirect costs [1].Protective agent s are frequently used to lessen the damaging effects of corrosion and increase the lifespan of metal assets.Protective agent s are compounds that can either be applied directly to the metal surface or added to the corrosive environment to delay or reduce the corrosion process.They function by creating a layer of defence that serves as a barrier against corrosive substances on the metal surface.The usage of protective agent s has a number of benefits.First off, they can considerably slow down corrosion, extending the useful life of metal buildings and machinery.By reducing the need for regular repairs or replacements, this results in cost savings.Second, compared to other corrosion protection strategies like coatings or alloy selection, protective agent s may be more economical.They are frequently compatible with current systems, easy to implement, and low maintenance [2][3][4].The two primary groups of inhibitors are organic and inorganic inhibitors.On chemical substances and salts, inorganic inhibitors like chromates, phosphates, and silicates are based.Through the creation of a passivating layer on the metal surface, they offer corrosion protection.However, because of their toxicity and contamination potential, the usage of several inorganic inhibitors has sparked environmental concerns [5][6][7].On the other hand, organic inhibitors are generated from organic substances and are typically less harmful to humans and the environment.They frequently contain functional groups that can interact chemically with the metal surface and form a shield, including amines, carboxylates, or phosphates.Organic inhibitors are adaptable to particular applications and have good material compatibility [8].The kind of metal, the corrosive environment, the temperature, and the required lifespan of protection all play a role in choosing an appropriate protective agent .Protective agent s are put through rigorous testing and research to see how well they operate in particular applications [9,10].Metal corrosion is a costly problem that affects various industries.The use of protective agent s provides an effective and economical solution to protect metals from corrosion.By applying inhibitors, the rate of corrosion can be reduced, leading to cost savings, extended asset lifespan, and improved operational efficiency.The choice of protective agent depends on the specific application requirements and environmental considerations [8].In the field of protective agent s, organic macromolecules show great potential and are preferred over inorganic inhibitors due to concerns about their ecotoxicity.Organic inhibitors are typically derived from by-products of the petroleum industry or synthesized compounds [11,12].Carbon steel, being cost-effective, is increasingly utilized in various industries.Hydrochloric acid finds frequent use in industrial processes for cleaning and pickling.However, due to the aggressive nature of this acidic solution, the use of protective agent s is necessary to reduce the rate of metal corrosion [13].
The main aim of this research is to develop a new hexafunctional phosphorus epoxy resin, HGTMDAP, and evaluate its effectiveness as a protective agent for metal.The research involves synthesizing the resin through a two-step process and characterizing its structural properties using advanced microscopic techniques.Various evaluation methods, including EIS, PDP, isothermal adsorption model, temperature effect, thermodynamic parameters, EFM, and SEM/EDS analysis, are used to understand the inhibition mechanism and surface adsorption.The findings demonstrate the enhanced protection ability of the HGTMDAP resin and validate its potential as a protective agent .Additionally, DFT, MC, and MD simulations are employed to describe the electronic and adsorption properties of the resin.This research opens up new possibilities for corrosion inhibition strategies.

Materials used
The products used are phosphorus trichloride, methylene dianiline (MDA), methanol and epichlorohydrin which is kept at 99 % purity stored at about 4-6°C.All these base products were supplied by Acros Chemical Company and Aldrich Chemical Company, with no further purification.

Synthesis of hexaglycidyl trimethylene dianiline phosphorus (HGTMDAP).
The two-step synthesis of a new phosphorus trimethylene dianiline hexaglycidyl epoxy resin (HGTMDAP) will be presented in this study according to the following reaction scheme (Figure 1).

Experimental protocol for the synthesis of the new phosphorus trimethylene dianiline hexaglycidyl epoxy resin (HGTMDAP).
The synthesis of hexaglycidyl trimethylene dianiline phosphorus resin (HGTMDAP), a new resin, is carried out in two reaction steps.Initially, 3.9.10 - mol methylene dianiline dissolved in 50 ml methanol and 13.04.10 - mol phosphorus trichloride are mixed.The mixture must be kept at 40°C and shaken vigorously for 48 hours.Next, 7.8x10-2 mol of epichlorohydrin is added.The mixture is heated to 80 °C and magnetically stirred constantly for a whole day.A later step.After cooling to 40 °C, 7.82*10 -2 mol of a solution containing NaOH at the rate of one half its weight by mass is added into water and left agitating for an hour and twenty minutes (50 min.).Next, water is added to the organic phase.Finally chloroform is used to eliminate it.Using a rotary evaporator, anhydrous sodium sulphate (Mg2SO4) is dried and the solvent is removed.In 89% of cases, a viscous resin product is produced.

Structural analysis methods 2.3.1. FTIR
A BRUKER FTIR infrared spectrometer was employed.Bands on Attenuated Total Reflectance (ATR) were obtained during transmission.Between 2.5 and 25 µm is the spectral range that corresponds to the vibrational energy of the molecules.In the range of 500 cm -1 to 7000 cm -1 , analysis was done.

NMR
The product was dissolved in DMSO to conduct 1 H and 13 C NMR studies using a BRUKER AVANCE 300MHz equipment.The unit of chemical shift is ppm.

Metal and corrosive solution
A of E24 carbon steel, which has a material surface area of 1 cm 2 .The residual material is mostly iron (Fe).The system under study was a 1.0 M HCl.This was done by diluting a commercial 37 % HCl solution with distilled water.The concentrations of inhibitors vary from 10 -6 to 10 -3 M.This concentration range was established by examining the HGTMDAP inhibitor prepolymer's solubility in a corrosive solution.

Electrochemical measurement
A three-electrode electrochemical cell has been assembled for electrochemical measurements.It is made up of a platinum counter electrode, a saturated calomel reference electrode, and a metal working electrode.PDP was done with a Gamry PCI4-G750 Potentiostat/Galvanostat/Zra.First, the working electrode was submerged in the free corrosion potential for 20 minutes.It can scan at a rate of 0.5 mV/s.To compute the electrochemical parameters, utilise the Gamry Framework -Gamry Echem Analyst programme.Consequently, inhibitory efficacy is determined using Equation 1.

( )
Transient electrochemical measurements at a signal amplitude of 10 mV were assessed using the same apparatus.The range of frequencies covered by the study is 100 KHz-10 mHz.To compute the inhibitory efficiency, use Equation 2.

SEM-EDX
The research procedures for SEM-EDX analysis in studying protective agent s typically involve the following steps: Sample Preparation: Prepare the metal samples by cleaning them thoroughly to remove any contaminants or surface oxides.This can be done using appropriate cleaning methods such as solvent cleaning, ultrasonic cleaning, or mechanical polishing.Ensure that the samples are properly dried before proceeding to the next step.
SEM Imaging: Place the coated samples in the SEM chamber and adjust the instrument settings for imaging.Choose appropriate magnification and operating parameters such as accelerating voltage and beam current based on the sample and desired resolution.Capture SEM images of the sample surface to examine its morphology and topography.
EDX Analysis: Perform EDX analysis to research the elemental composition of the sample.Select specific regions of interest on the SEM images for EDX analysis, such as the metal surface or the interface between the metal and the protective agent .Acquire EDX spectra from these regions to obtain information about the chemical elements present and their relative concentrations.

Theoretical investigations
The research methodology for theoretical investigations in this study involves the use of DFT calculations and molecular simulations (MC and MD) to study the interactions between the HGTMDAP inhibitor and the metal interface.The specific details of the methodology are as follows [14,15]: 1. DFT Calculations: ✓ Dmol3 Module: DFT calculations are approved by the Dmol3 module that is built into the Biovia Materials Studio programme [16][17][18].✓ Geometry Optimization: Generalized Gradient Approximation (GGA) employing the pbe functional is used for geometry optimizations of the molecular structures [19].✓ Numerical Basis Set: A double-sized numerical basis set plus d-functional (DND) is utilized for the calculations.1. MC and MD Simulations [1,5,[20][21][22][23]: ✓ Purpose: The MC and MD simulations are conducted to study and understand the interactions between the protonated and neutral HGTMDAP forms and the metal interface.✓ Simulation Software: Materials Studio 8.0 software is employed for performing the MC and MD simulations.✓ Methodology: The simulations entail identifying various adsorption configurations and examining the system of interactions between the metal and the HGTMDAP inhibitor's molecular structure.By employing these theoretical investigations, including DFT estimations and molecular simulations, the study aims to gain insights into the interactions between the HGTMDAP inhibitor and the metal interface, providing valuable information about the inhibitive properties and behavior of the inhibitor under different conditions.

Spectral characterization of synthesized epoxy resin
Nucleophilic substitution of the aromatic NH2 ambident rings of methylene dianiline by the chlorine of phosphorus trichloride P(O)Cl3 and the chlorine of epichlorohydrin yields hexafunctionalized aromatic glycidylamines.FTIR and 1 H, 13 C NMR spectroscopy have proven and demonstrated this.

FTIR structural study
The given IR spectrum bands for the phosphorus trimethylene dianiline hexaglycidyl (HGTMDAP) provide important information about its functional groups and chemical bonds (Figure SI1 and Table SI1): 3691.84 cm⁻¹ (Band: ν OH): This peak is equivalent to the stretching vibration of hydroxyl (OH) groups.OH groups indicate hydroxyl functional groups in the molecule.

1 H and 13 C NMR characterization
With its data from the 13 C NMR (Figure SI2) and 1 H NMR (Figure SI3) spectra for HGTMDAP, we gain some valuable information about this compound's chemical environment and structural features. 1H NMR Spectrum (Table SI2

Polarization curves 3.2.1. Influence of concentration in the corrosion inhibition
The following main points were found related to effect of concentration (Table 1): ✓ The polarization curves show that the addition of HGTMDAP inhibitor decreased the icorr in the unprotected system.Lower icorr indicates lower corrosion rate with the inhibitor [24,25].. ✓ icorr decreased with rising inhibitor values, suggesting better inhibition efficiency at higher concentrations [26,27].✓ With the addition of inhibitor, the values of βa and βc, which stand for the anodic and cathodic Tafel slopes, respectively, either reduced or remained unchanged.This suggests that HGTMDAP functions as an inhibitor of mixed types.✓ The protection values (ηPDP) calculated from icorr values rose with increasing HGTMDAP values, achieving a maximum of 96.9% at 10 -3 M.This confirms the good protection ability of HGTMDAP.✓ The corrosion potential (Ecorr) was not significantly changed with the addition of inhibitor.This implies that HGTMDAP acts mainly by decreasing the corrosion velocity [28,29]..In summary, the results demonstrate that HGTMDAP is an effective mixed-type inhibitor, and higher concentrations provided better inhibition efficiency through decreased corrosion current densities.

Effect of temperature
The basic comments from the effect of temperatures were follows (Table 2): ✓ The Tafel plots show that icorr rises with growth temperature for the blank and inhibitor systems, showing that temperature has a detrimental influence on corrosion resistance.✓ icorr amounts were high protected system, demonstrating its good inhibition efficiency even at elevated temperatures [30,31].✓ The βc and βa values generally decrease with growing temperature for both systems.
The decreased βa values with inhibitor suggest it acts by blocking the anodic active sites.✓ Ecorr shifts slightly towards negative potentials with increasing temperature but is not significantly influenced by the introduction of protection.In summary, HGTMDAP exhibits efficient inhibition even at elevated temperatures up to 328K.While its performance decreases with temperature, it still provides good corrosion protection at higher temperatures through decreasing icorr values.

Thermodynamic activation parameters
The temperature dependence of the corrosion velocity is indicated by the Arrhenius equation (3).

  
The ideal gas constant is R, and Ea is the activation energy.The pre-exponential factor is K, and the temperature is T. The Arrhenius equation and another version known as the transition state equation ( 4) were used to determine the thermodynamic properties of the corrosion reaction, such as Ea, Sa, and Ha: Arrhenius diagrams for the temperature range 298 to 328 K are shown in Figure 4.  Compared to the unprotected system, the inhibited solution has much greater values of Ea and ΔHa (Table 3).This suggests that the inhibitor works as a barrier, making the corrosion process more challenging when present.These results show how apparent activation energy values change when an inhibitor is added.This suggests that adding an inhibitor to the solution reduces iron corrosion [32].The endothermic dissolution of metal is confirmed by the positive indicate of ΔHa (ΔHa = 61.83kJ/mol) of 1.0 M HCl [33].
The positive ΔHa values endothermic nature of all chemical processes in the selected systems.The values of negative entropy (ΔSa) suggest that an ordered layer of inhibitor molecules has formed on the metal surface, as the activated complex is less disordered in the transition state than the reactants.The strong, negative entropy value (ΔSa = -8.08J/mol.K) in 1.0M HCl indicates the formation of an activated complex during the velocity removal step, which limits the corrosion process.Moreover, the thermodynamic link between activation energy (Ea) and activation enthalpy (ΔHa) is confirmed by the identical variations in variation of both as a function of HGTMDAP inhibitor concentration [34].
The higher thermodynamic activation parameters in the inhibited solution demonstrates that HGTMDAP functions by improving the energy blocks in all chemical dissolution of metal, thereby providing inhibition at elevated temperatures up to 328K.

EIS
The key points were found based on EIS results are: ✓ Nyquist plots show single capacitive loops becoming larger with increasing inhibitor concentration, indicating corrosion resistance increases [35,36] (Figure 5).✓ Bode plots show phase angle peaks shifting to higher frequencies with inhibitor, suggesting faster electrode kinetics [37,38].✓ Equivalent circuit fitting confirms single time constant system and introduction of good insulator of metal surface from the corrosion ions (Figure 7).✓ Rct values increase with HGTMDAP introduction, because, the active sites were effectively blocked with the HGTMDAP [38][39][40].✓ θ and ηEIS values >95-96% even at lower concentrations indicate excellent inhibition efficiency of HGTMDAP confirmed by EIS (Table 4).✓ Low Q and n values close to 1 demonstrate near ideal capacitive behavior of protective film.✓ Good correlation between EIS and weight loss results.✓ The expansion of the single capacitive loop diameter with rise the inhibitor values reflects a rise in thickness and protective quality of the HGTMDAP film on the metal surface.✓ The shifts of the phase angle peak to higher frequencies indicate that HGTMDAP adsorb on the metal and block the corrosion sites, making the oxidation and reduction reactions slower.✓ The near unity values of n close to 1 confirmed that the inhibitor film has a close to ideal capacitive behavior.This homogeneous and compact protective film inhibits corrosion.✓ The decreasing Q values with increasing concentration reflect diminishing inhomogeneities of the inhibitor film.This correlates with better inhibition efficiency at higher concentrations.✓ The excellent agreement between impedance data and equivalent circuit fitting validates the chosen circuit model and mechanism of inhibitor adsorption and film formation.✓ Comparing EIS and polarization results, the similarities in trends of Rct, θ and icorr values with inhibitor concentration strongly support the proposed inhibition mechanism (Figure 6).✓ The EIS results thus provide microscopic level insights into the adsorption and inhibition processes that cannot be obtained from weight loss or polarization techniques alone.In summary, EIS reveals HGTMDAP functions by introduction barrier film on metal surface via adsorption, shifting kinetics to diffusion control and protecting steel even at low concentrations, as suggested by increased Rct, θ and impedance values with concentration.Results confirm HGTMDAP as an effective inhibitor.

Adsorption isotherm
The following main results were found related to adsorption isotherm and parameters: The linear regression fit in Figure 8 confirms that the Langmuir isotherm model (Eq.5) very well (R2 > 0.99) in the adsorption analysis.

1
(5) The Langmuir isotherm suggests monolayer adsorption of the HGTMDAP on the homogeneous metal surface [41,42].The large positive value of the adsorption equilibrium constant (Kads > 105 L/mol) implies strong interaction among HGTMDAP and metal surface.The calculated very negative Gibbs free energy of adsorption (ΔGads = -45.22kJ/mol) (Eq.6) confirms the spontaneity of the adsorption process and suggests it is predominantly physisorption through electrostatic interactions (Table 5).The thermodynamic parameter explains why HGTMDAP provides good inhibition even at high temperatures, as the adsorption process has a high affinity and is spontaneous.
In summary, the adsorption isotherm study provides useful mechanistic insights and quantitatively supports that HGTMDAP acts as an effective inhibitor through spontaneous physisorption.

Electrochemical frequency modulation
The key points from the EFM results are follows: ✓ EFM spectra show single activation energy distribution, reflecting the single corrosion mechanism [43,44] (Figure 9).✓ icorr and corrosion rate (CR) decrease with increasing inhibitor concentration, confirming better inhibition at higher C (Table 6).✓ Tafel slopes βa and βc are affected only slightly, indicating HGTMDAP acts as a mixedtype inhibitor.✓ Protection values η rise with HGTMDAP values, achieving >95% efficiency at 10 -3 M. ✓ CF values around 2 indicate slow reaction and presence of physical adsorption barrier.✓ Excellent agreement between EFM and other techniques validates the chosen technique.✓ Thermodynamic parameters from EFM are consistent with other results.
In summary, the obtained EFM details confirmed other techniques and provide microscopic confirmation that HGTMDAP functions via mixed inhibition mechanism by decreasing icorr through adsorption over a wide concentration range.The technique thus serves as a useful validation of corrosion inhibition process.

Surface analysis (SEM/EDS)
SEM and EDS were employed to research the surface morphology and elemental composition of the metal after immersion in test solutions [45,46].This was done to complement the electrochemical findings.After immersion in 1.0 M HCl (Figure 10a), the SEM image revealed extensive surface corrosion characterized by non-uniform pits and cracks.The EDS spectrum showed prominent peaks for oxygen and chlorine, confirming the corrosive nature of the test environment.In contrast, the steel surface immersed in 1.0 M HCl containing 10 -3 M HGTMDAP inhibitor (Figure 10b) appeared smoother with fewer defects.The EDS analysis correspondingly detected weaker corrosion product peaks (oxygen, iron) along with new peaks for phosphorus and nitrogen from the inhibitor molecules.This suggests the formation of a protective layer on the steel by adsorption of the organic inhibitor compounds.The layer effectively slowed the corrosion process in acidic solution, corroborating the mechanism observed electrochemically.Thus, SEM/EDS complemented the electrochemical results by directly visualizing the difference in steel surface morphology and composition with and without inhibition.SEM was used to examine the surface morphology of the steel to complement and confirm the results of the electrochemical study.Next, the elemental chemical composition was determined using EDX analysis.

DFT results
The HGTMDAP inhibitor in neutral and protonated forms were utilised to do the DFT analysis [47][48][49][50].The HOMO, LUMO orbitals and ESP pictures of the HGTMDAP inhibitor in neutral and protonated forms are plotted graphically in Figure 11.It is indicated in Figure 11, the HOMO and LUMO regions are mainly located around methylene dianiline regions of both forms (HGTMDAP and HGTMDAP-H + forms).
The electrophilic and nucleophilic attack regions of HGTMDAP were illustrated in the ESP map [51,52].The nucleophilic (red) regions are positioned around O, N and P atoms, suggesting that these regions are covalent bonding centres.The corrosion inhibition was promoted by the nucleophilic (red) regions [53,54].
The corrosion protection ability of this compound can also be evaluated by quantum chemical parameters given in Figure 11 and Table 7.
EHOMO (HGTMDAP-H + ) < EHOMO (HGTMDAP).This is because the higher the occupied high-orbit energy (EHOMO), the greater the electron-donating capacity [55].However, the lower the unoccupied orbital base energy, the higher the ability to accept electrons [56].The energy gap (ΔEgap) is considered an important factor in describing the statistical activity of the molecule, where the low value of ΔEgap indicates the ease of electron transfer from the HOMO molecular orbital to the LUMO molecular orbital, and this translates into good inhibitory efficiency [57].Consequently, neutral form (HGTMDAP) can be said to be more effective than protonated form.
✓ HOMO and LUMO regions are mainly localized around the methylene dianiline moiety for both neutral and protonated HGTMDAP forms.✓ Neutral form shows wider nucleophilic regions (red color) around O, N and P atoms, indicating these are preferred coordination sites to metal surface.✓ Higher EHOMO value for neutral form signifies greater electron donating ability to accept electrons from metal d-orbitals.✓ Lower energy gap (ΔEgap) for neutral form suggests easier electron transfer within the molecule and better inhibition.✓ Calculated parameters like electron affinity, hardness, chemical potential etc. are in line with neutral form acting as a better chelating agent.✓ Positive ΔN values \< 3.6 electron transfer suggests electron donation from both forms to steel surface favors inhibition.✓ More negative ΔEback-donation for protonated form indicates its lower electron backdonation ability.Overall, the DFT study provides a microscopic understanding of inhibition mechanism at molecular level, corroborating better performance of neutral HGTMDAP form due to enhanced molecular interactions with steel surface through electron donation and back-donation according to polarization and charging effects.This lends theoretical support for experimental observations [58,59].

MC and MD simulations
To study and understand the interactions between inhibitor molecules and the metal surface, MC and MD simulations are frequently used (Figures 12, 13 and 14).The interactions between the HGTMDAP and HGTMDAP-H + inhibitor molecules and the metal surface were examined using MC and MD simulations.Strong connections between the inhibitor molecules and iron atoms through electron sharing were indicated by the simulations, which showed the molecules adsorbing almost parallel to the Fe(110) surface.Electron sharing from nitrogen, oxygen, and phosphorus atoms, as well as conjugated double bonds in the inhibitors with iron, facilitate chemical interactions.Physical adsorption is also facilitated by van der Waals forces.Adsorption energy (Eads) calculations showed HGTMDAP-H + has stronger binding (-331.75 kcal/mol) than HGTMDAP (-327.35kcal/mol), indicating inhibitory potency order.Radial distribution function (RDF) analysis calculated metal-nitrogen, oxygen and phosphorus bond lengths to identify adsorptive interactions.Bond lengths between 1-3.5 Å indicate chemisorption while >3.5 Å shows physisorption.Strong contact between inhibitors and iron substrate is confirmed by inhibitor atoms in RDF plots being close to the metal surface.The inhibitor molecules form Langmuir-type monolayer adsorption on the Fe surface, as indicated by their near-parallel orientation.This highest level of surface covering best prevents rusting.Chemisorption is the process by which strong chemical bonds form between the inhibitor and the metal surface by sharing electrons from nitrogen, oxygen, phosphorus, and conjugated bonds.This provides very strong inhibition.Van der Waals forces contribute to physisorption, a weaker physical interaction that still inhibits corrosion to some degree via blockage of surface area.More negative adsorption energies correspond to stronger binding of the inhibitor to the surface.The lower energy of HGTMDAP-H + suggests it forms stronger chemical bonds and will inhibit corrosion more effectively [60][61][62][63].

Conclusion
The basic conclusions are: -A new hexafunctional phosphorus epoxy resin (HGTMDAP) was successfully synthesized.Its structure was confirmed using FTIR and NMR techniques.
-Electrochemical techniques (EIS, PDP) showed that HGTMDAP acts as an effective mixedtype inhibitor for corrosion of carbon steel in 1 M HCl.
-Protection value rose with rising inhibitor concentration, reaching a maximum of 96.9% at 10 - 3 M based on PDP.
-EIS revealed a protection value of 96.3% for 10-3 M HGTMDAP, in agreement with PDP results.
-Thermodynamic parameters (Ea, ΔGads) and Langmuir isotherm confirmed monolayer adsorption of HGTMDAP on the steel surface.
-SEM/EDX showed HGTMDAP forms a defender layer on steel, inhibiting corrosion by separating steel from the corrosive electrolyte.
-Results from experimental and theoretical (DFT) techniques were in good agreement regarding the inhibition mechanism of HGTMDAP.In summary, the new epoxy resin HGTMDAP showed high efficiency as a mixed-type protective agent for carbon steel in acid solution.

Funding
The authors received no direct funding for this research article.

Data Availability
All data generated or analyzed during this study are included in this published article.

Conflict of interest
On behalf of all authors, the corresponding author
3300.67 cm⁻¹ (Band: Elongation of the secondary amine group (-NH-): This peak corresponds to the elongation (stretching) vibration of secondary amine groups (-NH).The presence of secondary amine functional groups in the molecule is implied.2837.45 cm⁻¹ (Band: Aliphatic methylene (-CH2)): This peak is the stretching vibration of aliphatic methylene (-CH2) groups.It shows that there are methylene groups in the aliphatic part of the molecule.1611.96cm⁻¹, 1509.96cm⁻¹, 1430.71cm⁻¹ (Bands: valence vibrations (ν Car=Car)): The peaks are related to the valence vibrations of carbon-carbon (C=C) double bonds in aromatic rings.These vibrations are a typical indicator of the aromatic rings in the molecule.1319.47 cm⁻¹ (Band: Aromatic bond (C-N)): This peak is representative of the vibrations within carbon-nitrogen (C-N) bonds.It points to the existence of aromatic rings with nitrogen atoms in the molecule.1045.46 cm⁻¹ (Band: P(O)-N): This peak corresponds to a stretching vibration of the phosphorus-oxygen-nitrogen (P(O)-N) bonds.Such bonds are present in the molecule.809.14 cm⁻¹ (Band: Epoxy group): This peak is typical of the vibration of epoxy (oxirane) groups.It implies an epoxy functional group in the molecule.751.09 cm⁻¹ (Band: Vibration of the aromatic C-H bond): This peak corresponds to the vibration of the aromatic carbon-hydrogen (C-H) bonds.It confirms the presence of aromatic rings in the molecule.

Figure 5 :
Figure 5: Nyquist plots for blank and HGTMDAP at 298 K.

Figure 6 :
Figure 6: Bode diagram for blank and HGTMDAP at 298 K.Table 4: EIS parameters for blank and HGTMDAP at 298 K.

Figure 9 :
Figure 9: EFM spectra for blank and HGTMDAP at 228 K.Table 6: Electrochemical kinetic parameters for blank and HGTMDAP at 228 K. Compound C (M)

Figure 10 :
Figure 10: Scanning surface of metal electrode in 1.0 M HCl solution in the absence (a) and presence of 10 -3 M HGTMDAP (b) SEM image and EDS spectrum.

Figure 12 :
Figure 12: MC and MD simulation results for HGTMDAP and HGTMDAP-H + forms.

Figure 13 :
Figure 13: Distribution of the Eads of HGTMDAP and HGTMDAP-H + forms on the Fe(110) surface.

Figure 14 :
Figure 14: RDF O, N and P atoms of the HGTMDAP and HGTMDAP-H + forms.4.ConclusionThe basic conclusions are: -A new hexafunctional phosphorus epoxy resin (HGTMDAP) was successfully synthesized.Its structure was confirmed using FTIR and NMR techniques.-Electrochemicaltechniques (EIS, PDP) showed that HGTMDAP acts as an effective mixedtype inhibitor for corrosion of carbon steel in 1 M HCl.-Protection value rose with rising inhibitor concentration, reaching a maximum of 96.9% at 10 - 3 M based on PDP.-EIS revealed a protection value of 96.3% for 10-3 M HGTMDAP, in agreement with PDP results.-Thermodynamicparameters (Ea, ΔGads) and Langmuir isotherm confirmed monolayer adsorption of HGTMDAP on the steel surface.-SEM/EDXshowed HGTMDAP forms a defender layer on steel, inhibiting corrosion by separating steel from the corrosive electrolyte.-Resultsfrom experimental and theoretical (DFT) techniques were in good agreement regarding the inhibition mechanism of HGTMDAP.In summary, the new epoxy resin HGTMDAP showed high efficiency as a mixed-type protective agent for carbon steel in acid solution.
It indicates the presence of an aromatic amine group.6.55 -7.27 ppm (s, 4H, aromatic): This set of peaks indicates the presence of four hydrogen atoms (4H) in an aromatic ring.The chemical shift range and singlet (s) pattern confirm the presence of aromatic rings in the molecule.This range represents carbon atoms (CH2) linked to nitrogen (N), confirming the presence of secondary amine groups in the molecule.112.814 ppm (s, CH aromatic): This peak corresponds to a carbon atom (CH) in an aromatic ring.123.225 ppm (s, C aromatic tertiary): This peak represents a tertiary carbon atom (C) in an aromatic ring.130.197 ppm (s, C nitrogen-bonded aromatic tertiary): This peak indicates a tertiary carbon atom (C) in an aromatic ring, specifically bonded to nitrogen (N).
3.87 ppm (Aromatic C-NH): This peak represents a hydrogen atom (H) attached to a nitrogen atom (N) in an aromatic ring. )

Table 4 :
EIS parameters for blank and HGTMDAP at 298 K.

Table 6 :
Electrochemical kinetic parameters for blank and HGTMDAP at 228 K.