Novel porphyrin derivatives as corrosion inhibitors for stainless steel 304 in acidic environment: synthesis, electrochemical and quantum calculation studies

A Novel 5,10,15,20-tetra (thiophen-2-yl) porphyrin (P1) and 5,10,15,20-tetrakis (5-Bromothiophen-2-yl) porphyrin (P2) were successfully synthesized, and their chemical structures were proved based on its correct elemental analysis and spectral data (IR and 1H-NMR). These compounds were examined as corrosion inhibitors for stainless steel 304 (SS304) in 2 M HCl utilizing mass reduction (MR) and electrochemical tests at inhibitor concentration (1 × 10–6–21 × 10–6 M). The protection efficiency (IE %) was effectively enhanced with improving the concentration of investigated compounds and reached 92.5%, 88.5% at 21 × 10–6 M for P1 & P2, respectively and decreases with raising the temperature. Langmuir's isotherm was constrained as the best fitted isotherm depicts the physical–chemical adsorption capabilities of P1 & P2 on SS304 surface with change in ΔGoads = 22.5 kJ mol−1. According to the PDP data reported, P1 and P2 work as mixed find inhibitors to suppress both cathodic and anodic processes. Porphyrin derivatives (P1 & P2) are included on the surface of SS304, according to surface morphology techniques SEM/EDX and AFM. Quantum calculations (DFT) and Monte Carlo simulation (MC) showed the impact of the chemical structure of porphyrin derivatives on their IE %.


Composition of SS304 samples
The chemical make-up of the SS304 utilized in this study includes (0.08% C, 0.045% P, 2% Mn, 0.75% Si, 0.03% S, 20% Cr, 8% Ni, 0.1% Al and Fe remainder), which is exactly similar to the ASTM SS304 standard.All specimens were cold cut from stainless steel 304 plates with a surface area of 1 cm 2 for the working electrodes in electrochemical measurements and surface examination.The mass reduction approach was applied using coins that were 2 × 2 × 0.2 cm in size.Each sample was prepared in the laboratory by being scrubbed with numerous abrasive sandpapers ranged (180-2000) then it was soaked in acetone as a degreasing solution thus washed with double-distilled water and dried between two filter papers resulting in mirror-like finish.www.nature.com/scientificreports/

Chemicals
A 37% HCl corrosive environment was used (AR grade).The bidistilled water was used to prepare the appropriate dosages of acid.To acquire different inhibitor concentrations, a (1 × 10 -3 M) stock solution of porphyrin organic inhibitors was diluted with bidistilled water (1-21 × 10 -6 M).The maximum dosage of porphyrin molecule in 2 M HCl was estimated to be 21 × 10 -6 M. The porphyrin compounds utilized in this study have higher molecular weights, are non-toxic, highly soluble in water, and include a sizable amount of donating atoms (N and S).Here is a list of the structures that they utilize:

Chemical test: Mass reduction (MR) test
Using a water thermostat and 100 ml of 2 M HCl solution, test coins made of SS304 were subjected to various concentrations of the porphyrin composites under research between 298 and 318 K.After the proper amount of dipping time, the coins were removed and weighed.At a particular time (180 min), the average MR for the tested samples was determined in mg cm −2 .

Electrochemical tests
The electrochemical experiments were done in glass reaction reactor involving three electrodes.A working electrode fabricated from SS304 (1 cm 2 ) and prepared in same manner as in weight loss method, a counter electrode made up from platinum foil (1 cm 2 ) and standard calomel electrode via Luggin capillary.All three were embedded in epoxy resins to expose the desired unified geometrical surface area then dipped in freshly prepared test solution at room temperature and stabilized for 30 min before each experiment until reaching steady state under unstirred condition, where the potentials are displayed versus normalized hydrogen electrode.The electrode was constructed of SS304.Through 250 mV SCE , the potential was automatically altered.For PDP, it involves sweeping the potential in positive direction until 100 mV and then reverses the direction toward more negative until − 100 mV at scan rate 0.2 mVs −1 .For electrochemical impedance spectroscopy, we recorded results at frequency (1 × 10 5 -0.1 Hz) and amplitude of 10 mV, Electrochemical measurements were carried out using Gamry instruments (Series G 750™-Potentiostat/Galvanostat/ZRA device)) then the graphing, fitting and recording were done using Software called Echem Analyst 5.5.All results were obtained and the process was repeated three times to ensure the validity results.

Surface examination techniques
Analyzing the SS304 surface is crucial to identify the morphology, proving the adsorption of porphyrin derivatives (P1 & P2) and assessment of their impact as inhibitors.Our specimens were prepared by grounding the SS304 coupons to a grit of 4000 and then polished with a number of sand-papers.The prepared metal sheets were immersed in 2 M HCl solution for 24 h at 298 K without the addition of the inhibitors to evaluate the influence of corrosive medium on metal morphology.Analogous actions were conducted but with 21 × 10 -6 M of inhibitor solutions.A comparison between the morphologies of samples attacked by the corrosive medium and those of the inhibited ones.These investigations were fulfilled by AFM (Model.FlexAFM3), SEM model A Jeol JSM-5400 instrument was used in the investigation.

Quantum chemical calculations
Using Material Studio version 7.0 semi-empirical approaches using the density functional theory (DFT), the entire quantum chemistry study has been conducted.Semi-empirical methodology was used to calculate molecular orbitals.The molecules were optimized by choosing B3LYB (Becke-3-parameters-lee-yang-parr) with DNP functions while setting the fine quality.Fine convergence and global orbital cutoffs were utilized as well as setting water as solvent which impact the treatment via COSMO controls.

Monte-Carlo simulations (MC)
Using MC, the optimal positioning of P1 & P2 inhibitors on the apparent of Fe (1 1 0) was evaluated.According to the literature 39 , it is believed that the Fe (1 1 0) crystal surface is used in this simulation due to its most stable.In order to simulate the solvent action during the corrosion process, 100 water molecules were employed to examine the adsorption of uncharged and protonated inhibitor molecules.The estimation module was initially used to carry out the geometrical optimization of water and the inhibitor molecule.Compass stimulation along with force field were implemented to porphyrin derivatives (P1 & P2) on Fe (1 1 0) optimized surface.The substrate-adsorbate system configuration space was searched using the Monte-Carlo approach to identify low-energy adsorption sites where the temperature gradually decreases.

Chemistry of porphyrin derivatives
There has been an important attention in porphyrin derivatives 40 .Therefore, it was interesting to the synthesis of [porphyrin] (P1) and (P2) according to the following Fig.

Mass reduction (MR) test
Measurements using MR have several practical uses 41 .It is the first technique for determining how corrosive an environment is to a particular substance.The main benefits of this approach are its availability, applicability in all settings, and ease of calculation of the corrosion rate.Specimens were cleaned in accordance with ASTM standard G1-3 42 before to the start of any experiment.Tests gotten at altered time intervals lacking and existence 1-21 × 10 -6 M of the porphyrin derivatives (P1 & P2) on SS304 pieces were done.ΔW is given from the Eq (1): where, W 1 and W 2 are the mass of specimens previously and later reaction, correspondingly and the surface area in cm 2 .IE % was calculated from the Eq. ( 2): where ΔW and ΔWi are the mass reduced/a without and existence the porphyrin derivatives (P1 & P2), individually.Figure 4 display the calculated mass reduction for SS304 at 25 ± 1 °C existence and absence altered doses ranging from 1 × 10 -6 M to 21 × 10 -6 M for porphyrin derivatives (P1 & P2).
The final data displayed that IE % of the investigated compounds reduced with increasing temperature Table 2 and increased with improving inhibitor concentration.

Kinetic-thermodynamic corrosion parameters
Analysis of the impact of temperature on the corrosion of SS304 in terms of activation energy was done using the Arrhenius equation.Calculating the standard activation energy E * a was done using the MR results.Arrhenius diagrams of SS304 in 2 M HCl solutions in the absence and presence of porphyrin derivatives (P1 & P2) at various concentrations and temperatures ranging from 298 to 318 K are shown in Table 3.The slope of the line drawn by graphing 1000/T (Fig. 5) in accordance with log k corr was used to determine the activation energy (E * a ) value in accordance with the Arrhenius formula Eq. 3 43 : Since "k corr " is rate of SS304 corrosion, "R and T" are, respectively, gas constant and Kelvin temperature.At 21 × 10 -6 M, the activation energy of the SS304 in inhibited solution is 72, 70 kJ mol −1 for P1 and P2, respectively.
(1)     www.nature.com/scientificreports/47.9 kJ mol −1 is the Figure for the uninhibited solution.The inhibitor solution's high activation energy results from the protective layer they create, which lowers the energy barrier for charge and mass transfer and prevents metal dissolution.Hence, the inhibitory system's dissolution is a slow process 44 .
The transition-state equation (Fig. 6) was used to attain the enthalpy (ΔH * ) and entropy (ΔS * ) data of activation that it was as follows (Eq.4): Table 4 illustrates the measured activation parameters.According to the current study, E * a values are larger with the examined inhibitors present than they are when the 2 mol L −1 HCl solution is used alone, suggesting that this behavior may be attributable to physical adsorption into the SS304 surface.The exothermic nature is shown by the negative sign of (∆H * )".This means that the adsorption may be chemical or physical.Also, the rise in the values of ΔS * (lower negative values) in the presence of the examined derivatives as compared to free acid solution indicated an increase in the order that happened when switching from the reagents to the steel/ solution interface 45 .www.nature.com/scientificreports/Adsorption isotherm behavior Different adsorption isotherms are fitted graphically to find out that adsorption obeys Langmuir adsorption isotherm (Fig. 7) with correlation coefficient (R 2 ) near to 1. Applying Langmuir equation (Eq.5) reflects the correlation between surface coverage (ϴ) and the inhibitors equilibrium concentration (C) in the bulk solution and assesses adsorption equilibrium constant (K ads ) 46,47 .
Then we calculated standard Gibbs free energy (ΔG ads °) from Eq. ( 6): In which, ΔG o ads the regular free adsorbent, 55.5 ML −1 water dosage in solution.The data pattern revealed that the negative sign of ΔG o ads was caused by the stability of the adsorbed porphyrin molecules and the spontaneity of the adsorbed on the SS304 surface 48 .Using the fundamental Van't Hoff 's equation 7, a straight line emerges from drawing log K ads vs. (1/T) (Fig. 8).ΔH°a ds were obtained from the slope: By getting ΔH o ads from the slope of Eq. ( 7), ΔS o ads was designed using Eq. ( 8),  The determined values of adsorption parameters were introduced in Table 5.From Table 5, it was noticed that the K ads data demonstrated that the adsorption coefficient reduces as temperature rises.Given that K ads was higher at 298 K than it was at 318 K, it is likely that a greater amount of the derivative was adsorbed onto the surface of the SS304.This suggests that lower temperatures are advantageous for the inhibition process."The negative ΔG°a ds values demonstrate both the spontaneity of the process.Values of ΔG o ads lower than − 20 kJ mol −1 (18.4-22.5 kJ mol −1 ) are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) 49,50 .The negative sign of ΔH°a ds indicates that the adsorption of derivative molecules is an exothermic, this means that the adsorption process may be physical or chemical, but because the values of ∆H °ads are less than 80 kJ mol −1 , so the investigated derivatives are physically adsorbed onto the SS304 surface".The corrosion inhibitor is adsorbing to the surface of SS304, and the entropy of the system is decreasing, according to the negative data of the entropy change of adsorption ∆S o ads .This is attributable to the exothermic nature of the absorption manner, which can be seen by the negative sign ∆H o ads .

Electrochemical measurements
Polarization (PDP) measurement PDP bends of SS304 in 2 M HCl attendance and lack of altered doses of the porphyrin derivatives (P1 & P2) are presented in Fig. 9. www.nature.com/scientificreports/where, i o corr and i corr are the current corrosion densities existence and nonexistence of chemicals, respectively.For the test specimens in the 2 M HCl solution, the polarization resistance (R p ) was designed using the Stern-Geary equation (11) below in both the existence and lack of porphyrin (P1 & P2) inhibitor 51 .
It is obvious that the effectiveness of inhibition gradually increases as inhibitor dose increases, at 25 °C with additions of 21 × 10 -6 M of the studied corrosion inhibitors P1 & P2, the highest inhibition effectiveness of 92.5% and 88.5% were achieved, respectively.i corr significantly decreases as corrosion inhibitor dosage is increased, reaching a minimum data of less than 33 and 51 µA cm −2 at the same dosage of maximum efficiency for P1 & P2, respectively.Also, the polarization resistance (R p ) increases from 131 to 536 Ω. "These findings confirm the high inhibition and film-forming ability of the investigated compounds 52 .According to the parallel cathodic Tafel lines, the addition of inhibitors to the 2 M HCl solution does not alter the hydrogen evolution mechanism or the reduction of H + ions at the SS304 surface, which mostly happens through a charge transfer mechanism 53 .The inhibitor may be categorized as either anodic or cathodic depending on whether the change in corrosion free potential E corr following the addition of the inhibitor is greater than 85 mV SCE in either direction.Otherwise, it is thought that the inhibitor has an impact on both processes.The total change in E corr in this investigation following the addition of corrosion inhibitor was found to be negligible, or less than 85 mV relative SCE in the direction of polarization (40, 21 mV for P1 & P2, respectively).This establishes that the inhibitor is a mixed-type inhibitor since it inhibits both the anodic and cathodic processes 54,55 .The positive E corr displacement indicates that the inhibitor is a mixed-inhibitor, but that the anodic reaction is predominate".According to % IE values represented in Table 6, the inhibiting properties of the studied inhibitors at highest concentrations 21 × 10 -6 M can be given in the following order: P1 > P2 with IE % values 91.7 and 89.5, respectively.These results are in good agreement with the results obtained from MR and EIS measurements.The kinetic and mechanistic details of EIS systems are usefully provided 56 .The associated frequency is used to derive impedance data expressed in real Z' and imaginary Z" figures, which are then used to build a mathematical relationship shown as a Nyquist plot.A number of significant elements, including R ct , R s , and C dl , can be included in an equivalent circuit model of the system under study, which consists of a working electrode, an electrolytic solution, and an adsorbed inhibitor.The interpretation of analogous circuit components enables a reliable analysis of the corrosion protection process when utilizing corrosion inhibitors.R ct has a direct relationship with the efficacy of corrosion inhibition, whereas C dl lower with improved protection 57 .By contrasting impedance curves in the attendance and lack of the tested porphyrin inhibitors, the effectiveness of corrosion inhibition is examined.In order to assess the observed impedance of an electrolyte subject to uniform corrosion, Fig. 10 depicts a typical equivalent electrical circuit.As previously mentioned, the circuit consists of R ct , the electrochemical solution resistance R s , and the constant phase element (CPE), which is utilized to represent the non-ideal behavior of the double layer, which is mostly related to insufficient surface coverage and surface roughness.For modeling iron-acid interface corrosion previously, use the circuit below 58 .Figures 11, 12 display the Nyquist and Bode curves of SS304 in 2 M HCl solutions contain altered doses of porphyrin derivatives (P1 & P2) at 25 °C.These Nyquist diagrams in 2 M HCl are not perfect semicircles due to surface heterogeneities, roughness effects (Fig. 11), inhibitor adsorption, and deviations in the properties or compositions of layers surface 59,60 , "which can be connected to the frequency dispersion effect.The curves described by a single capacitive semicircle, indicates that the corrosion process was mainly charged-transfer controlled" 61 .Equation ( 12) describes the impedance of a constant phase element (CPE): Where Y 0 denotes the CPE's magnitude, the Y o values which is attributed to the formation of double layer are smaller for inhibited solutions as compared to the uninhibited solution and n is the phase shift which measures the metal surface homogeneity and lies between 0 and 1.It is possible that P1 and P2 molecules interacted with www.nature.com/scientificreports/ the surface of electrode and protected the exposed electrode sites from damage.Using Eq. ( 13), the data of the CPE parameter Y 0 and n can be used to compute the values of the interfacial capacitance C dl 62 : Indeed, as "P1 & P2 concentration increased, R p values raised while C dl decreased, indicating that P1 & P2 was acting at the steel/acid interface.This is because inhibitor molecules replaced corrosive ions and water molecules on the substrate surface, increasing the thickness of the electric double layer and lowering the local dielectric constant 63 .The increases in the n value with the addition of P1 & P2 in 2 M HCl electrolyte (0.928-0.887) compared to that obtained in reference electrolyte (0.986) might be read as a certain reduction in the surface heterogeneity, but 64 .Bode plots in the absence and presences of porphyrin derivatives (P1 & P2) are given in Fig. 12.We may easily understand how the low frequency impedance modulus affects the inhibitory effect of porphyrins (P1 & P2) by observing this parameter.As seen in Fig. 12, the presence of P1 causes a greater rise in low frequency impedance modulus than the P2 solution does.This shows that P1 adsorption enhances SS304 corrosion resistance more than P2, and that the presence of porphyrin derivatives increases the low frequency impedance modulus relative to its absence.The single peak that was shown in the Bode plots for P1 and P2 demonstrated the existence of a single time constant, as indicated by the Nyquist plot.The equivalent circuit model simulation of Nyquist and Bode graphs demonstrates great agreement with experimental data.The evaluated values of Goodness of fit (χ 2 ) (Table 7) support good quality of fitting and equivalent circuit used".It is important to note that EIS studies support the superiority of P1's protective capability over P2's, which is compatible with the MR and PDP measurement findings.The data of the derived parameters of EIS fitting as C dl , R ct and IE % are listed in Table 7.

Surface analysis SEM tests
The SS304 surface was examined using "SEM as depicted in Fig. 13 to see whether the surface morphology was altered by adding 21 × 10 -6 M of porphyrin derivatives and without it.Following a 24-h dipping in HCl (2 M), the SS304 surface was analyzed using SEM, and the SS304 was tested both with and without the usage of 21 × 10 -6 M ( 13)

AFM analysis
According to the 3D image of the SS304 without the studied macrocyclic inhibitor, the metal surface has been repeatedly destroyed by the corrosive attacks of the 2 M HCl Fig. 14a.However, the 3D images (Fig. 14b, c) showing smoother surfaces than the blank demonstrate that the insertion of an inhibitor reduces corrosion of SS304 in the aggressive medium.The mean roughness (S a ) of the films formed on the SS304 surface.The blank's mean roughness are (820 nm) higher than those of the inhibitor it reduced to 147 nm and 190 nm Fig. 14b, c correspondingly in the presence of 21 × 10 -6 M of porphyrin derivatives (P1& P2), under study and pure metal, proving the efficiency of the compound in protecting SS304 surface from corrosive medium.

Quantum chemical parameters
The lower energy band gap value, which is represented in the energy band gap ΔE g (ΔE = E HOMO E LUMO ), indicates that organic molecules are highly reactive and exhibit excellent corrosion behaviour on the surface of SS304.An analysis of the impact of porphyrin derivatives (P1 & P2) molecule's orientation on inhibition performance was conducted using density function theory (DFT).As shown in Fig. 15, the optimized geometry, HOMO surface, and LUMO surface of studied inhibitors can be found.The parameters HOMO (E H ), LUMO (E L ), and dipole moment (μ) for porphyrin derivatives (P1 & P2) gradients were directly obtained from DFT (Table 8)."Eqs.(14-19) were used to calculate the energy gap (ΔE), electronegativity (χ), global hardness (η), global softness (σ), the fraction of electron transfer (ΔN) and back-donation (ΔE back-donation)", was calculated as Koopmans's theorem 65 (depicted in Table 8) from the next balance: Numerous articles have discussed how higher values of E HOMO and lower values of E LUMO determine the greater electron-donating and accepting abilities of an inhibitor.Inhibitors are more reactive when a lesser value of ΔE is present.In this instance, the porphyrin derivatives (P1) ΔE value is lower in the gaseous phase while higher values for porphyrin derivatives (P2).In comparison to porphyrin derivatives (P1 & P2) molecules, these values suggest that the P1 molecule has a high degree of reactivity.Metals and inhibitors can be understood using the number/fraction of electron transfer (ΔN).If the ΔN value of an inhibitor is higher, it is found to have a stronger capability of donating electrons to metallic surfaces.Compared to porphyrin derivatives (P1 & P2) molecules, P1 exhibits greater amounts of ΔN in the gaseous phase, indicating that porphyrin derivatives (P1) exhibits a stronger inhibitory effect.

Monte Carlo (MC) simulation
Monte Carlo simulation was utilized to find out more about the interactions between the molecules under study and the metal surface in an acidic and vacuum environment.Views of the more sturdy arrangement for the adsorption of porphyrin derivatives (P1 & P2) derivatives on the surface of cleaved Fe (1 1 0) from the top and sides (Fig. 16).MC stimulation done by adsorption lactor module detect the interaction between inhibitors and surface area of Fe (1 1 0) crystal with discovering the best adsorption sites 66 .Choosing the Fe (1 1 0) plane was based on its best stability and well-packed structure.Forcite module was used to optimize the geometry of www.nature.com/scientificreports/porphyrin derivatives (P1 & P2).The Simulation annealing was used to calculate fine-quality adsorption using five cycles of 50,000 steps.This study investigates low-energy configurations of Fe (1 1 0)-inhibitor system in aqueous solution.In order to simulate corrosion in a real-life scenario, the simulation was conducted in an aqueous environment with water molecules.Table 9 presents the adsorption configuration which are nearly parallel in position resulting from relaxation of the inhibitor molecule on Fe (1 1 0).The descriptors computed from MC stimulation are in Table 9.The tabulated adsorption energies are − 4236.244 and − 3908.128kcal mol −1 for porphyrins (P1), (P2) respectively.The outputs shows that the two inhibitors are efficient adsorptive inhibitors taking in respect that the better one is porphyrin (P1) which is attuned with the experimental results Rigid adsorption energies are − 4425.124(P1) and − 4088.248(P2) kcal mol −1 where porphyrin (P1) is the most negative, while for the deformation energies 188.88 (P1) and 180.12 (P2) kcal mol −1 , also porphyrin (P1) is the highest value which confirm the greater inhibitory impact of porphyrin (P1) more than porphyrin (P2).dE ad /dN i provide

Mechanism of inhibition
Porphyrin derivatives (P1 & P2) principally prevent SS304 corrosion by producing a dense barrier coating on the surface by transporting H 2 O molecules onto the surface and attaching to them 66 .It is found that the protective potential trends of the two Porphyrin derivatives be influenced by impact of substituent groups (Br) on the molecules' ability to give or take electrons.The IE % of inhibitors from all tested approaches rise in this order: P1 > P2.The greater effectiveness of the P1 inhibitor may be ascribed to the presence of N heteroatoms in its outer moiety, which are easily able to participate in surface-to-metal interaction and so efficiently reduce corrosion.However, because the inhibitor (P2) contains Br, which takes electrons, the protection's effectiveness is reduced, and the active site's electron density is also reduced."Physical and chemical adsorption are two separate types.
In contrast to the chemisorption process, which involves exchanging electrons or transporting them from the molecules to the iron's d-orbital in order to establish a coordination bond, the physisorption process necessitates the presence of both charged metal surfaces and charged molecules.Adsorption involving molecules and potential electrical density of energetic centers like N and S. In an acidic solution, the surface of the SS304 sample is positively charged 67 .The surface of carbon steel undergoes electrochemical reactions in corrosive medium where the chloride ions coming from HCl besides the water molecules cause the dissolution of metal surface making it positively charged.The negative-charged metal surface created by the deposited Cl − ions on the SS304".The cationic part of the Porphyrin molecules adsorbs on carbon steel surface forming a protective film against corrosive medium, this defined as physisorption.

Conclusion
The main conclusions drawn from all these studies of macrocyclic compounds are:-1.The IE % of all macrocyclic compounds improve with rise in inhibitor doses whereas it lower with the rises of temperature.2. The IE % of all composites follows the order: P1 > P2 in 2 M HCl solution.This can be attributed to the altered in molecular structures and the type of the donating atoms.3. The adsorption of all the composites on SS304 surface from the acidic solutions conforms Langmuir's isotherm.4. P1 & P2 increase R ct values and decrease i corr values in 2 M HCl solution.5.The EIS, PDP and MR tests are in good agreement.6.The experimental finding agrees well with the theoretical calculations.7. SEM and AFM investigation for SS304 surface revealed the presence of a protective film, which protect SS 304 alloy against the corrosive media.

Figure 3 .
Figure 3. General route for the synthesis of porphyrin derivatives.

Figure 4 .
Figure 4. Time-MR bends of SS304 in 2 M HCl using and devoid of various doses of porphyrin derivatives (P1 & P2) at 25 °C.

( 11 )Figure 9 .
Figure 9. PDP plots for the dissolution of SS304 in 2 M HCl in the absence and attendance of altered dose of porphyrin derivatives.

Figure 12 .
Figure 12.Bode plots for SS304 in 2 M HCl without/with altered dosages of porphyrin derivatives at 25 °C.
from compounds P1 & P2.Without any inhibitor, corrosion in HCl (2 M) drastically weakened the surface of the SS304.The SS304 surface was observed after utilizing the inhibitors and found to be smooth.Because they provide a strong protective layer between the SS304 and the corrosive media, porphyrin derivatives (P1 & P2)" present in the solution reduce the rate of corrosion, which improves surface morphology and decreases surface roughness.

Table 3 .
The R corr of the investigated porphyrin derivatives (P1 & P2) and the free sample at 120 min dipping.InhibitorConc.

Table 4 .
Activation data of the liquefaction of SS304 in 2 M HCl with and without of porphyrin compounds (P1&P2) at 25-45 °C.

Table 6
Figure 8. Log K ads vs. T diagrams obtain from Langmuir.
lists the electrochemical characteristics that were derived by Tafel extrapolation at the corrosion potential (E corr ), including current density (i corr ), corrosion potential (E corr ), anodic (β a ) and cathodic (β c ) slopes.When the inhibitor concentration is raised, it has been observed that i corr decreases.The polarization curves were used to compute IE % and (θ) (Eqs.9 & 10):

Table 5 .
Thermodynamic parameters obtain from Langmuir.

Table 6 .
Parameters obtain from PDP technique.Electrochemical impedance spectroscopy (EIS) testsAnother essential electrochemical method that is frequently used to research and give in-depth insight into utilize of corrosion hindrance to protector compared to corrosion is electrochemical impedance spectroscopy.

Table 7 .
EIS parameters for SS304 corrosion in 2 M HCl in the absence and presence of various concentrations of porphyrin derivatives (P1 & P2) at 25 °C.

Table 8 .
Quantum chemical data for the porphyrin derivatives (P1 & P2) under study.information about the metal adsorbents as if they are adsorbed or neglected, so when comparing dE ad /dN i for inhibitors (− 253.19, − 234.80, − 307.35) kcal mol −1 and dE ad /dN i for water (− 7.27, − 8.16, − 9.38) kcal mol −1 , it's found that the values in case of water is very low compared to that of the inhibitors proving the replacement of water molecules by inhibitor molecules.Based on theoretical modeling it's obvious that porphyrins based proved to be powerful inhibitors for the carbon steel which is confirmed by experimental and spectral investigation.The prepared inhibitors are arranged P1 > P2 based on IE %.

Table 9 .
Monte Carlo simulation parameters of adsorption of porphyrins molecules on Fe (1 1 0) surface.