Expired dopamine as an ecofriendly corrosion inhibitor for 316L stainless steel in 0.5 M HCl: Experimental and theoretical studies

2.4.1. Frontier orbital (DFT/DMol³, COSMO) calculation

The frontier orbitals (highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO)) eigenvalues, a key parameter to understand the importance of frontier molecular orbital (FMOs) in adsorption were also computed with the DMol³ (Materials Studio) code [7,8]. Geometry were pre-optimized: (Quench procedure, COMPASS III) [8] and then re-optimized by DFT with M11-L functional [10], for an improved geometric accuracy and electronic interactions/correlations description, using triple numerical basis with polarization triple numerical basis with polarization (TNP) [11]. The influence of the water solvent was then incorporated using conductor-like screening model (COSMO), a method for simulating the electrostatic influence of the medium, which supplies sigma profiles [7,12-14]. This provides a link between reactivity and a propensity to adsorb.

3.1.1. Electrochemical impedance spectroscopy (EIS) and PDP of dopamine inhibition

To illustrate the inhibitory action of DA on the corrosion of 316L stainless steel in acidic medium, an electrochemical investigation using EIS and PDP was carried out. These two techniques are complementary for the investigation of interfacial processes, including charge transfer, adsorption of inhibiting molecules, and the formation of protective layers. Therefore, the variation of the electrochemical parameters with increasing DA concentrations will afford important insights into the adsorption mechanism and the efficiency of this inhibitor. The impact of DA on the electrochemical of 316L steel is reflected through the (a) Nyquist plots, (b) Bode plots, and (c) Equivalent circuit fitted, presented for this case in Figure 1, and Table 1 represents the electrochemical parameters (derived from EIS analysis) of the 316L stainless steel in 0.5 M HCl in the presence and absence of DA at different concentrations.

Close

Figure 1.

EIS plots of 316L SS in 0.5 M HCl solution in the absence and presence of different concentrations of DA, (a) Nyquist plots; (b) Bode plots, and (c) equivalent circuit.

Export to PPT

The influence of DA on the corrosion behavior of 316L stainless steel in 0.5 M HCl was scrutinized utilizing EIS. The Nyquist plots (Figure 1a) demonstrate decreased capacitive semicircles, typical of corrosion processes proceeding under charge transfer control. Without an inhibitor, the charge transfer resistance (Rct) is low (318.8 Ω•cm²), reflecting fast corrosion kinetics. The arc diameter gradually becomes larger with increasing DA concentration, and when DA concentration was at 10⁻³ M, the diameter was up to 5927 Ω•cm², which significantly enhances the corrosion protection. The Bode plots (Figure 1b) also reveal a higher modulus of the impedance at low frequencies and a more capacitive behavior, indicative of the formation of an insulating organic film.

When the equivalent circuit (Figure 1c) is used to fit the experimental data, a continuous decrease in the double layer capacitance (Cdl) at the same time as the DA concentration increases is observed, indicating the substitution of water molecules by less polar DA molecules and an enlargement in the virtual thickness of the adsorbed layer. Additionally, relaxation time τ increases at higher concentration, suggesting the retardation of the interfacial process, and in turn, the formation of a stable and uniform inhibitory film. As a whole, these results confirm that DA can only be adsorbed efficiently on the metal surface through its heteroatoms, enhances the electrochemical barrier, and offers inhibition efficiency that can reach ∼ 92% at 10⁻³ M. were estimated to be the double layer capacitance and relaxation time the double layer capacitance and relaxation time obtained from Equations 6 and 7:

Moreover, the evaluation of EIS-derived electrochemical data (Table 1) proves the marked effectiveness of DA as a 316L stainless steel corrosion inhibitor in the 0.5 M HCl solution. Appearing in the absence of inhibitor, the low charge transfer resistance (Rct = 318.8 Ω•cm^-2) indicates a fast corrosion kinetic, while in the presence of DA, the Rct value increases progressively and significantly at 10⁻³ M, where it was 5927 Ω•cm^-2, reflecting an important drop in the corrosion rate. While at the same time the double-layer capacitance (Cdl) decreased with increasing inhibitor concentration, attributed to the fact that the water molecules had been replaced by the DA molecules and the effective thickness of the adsorbed layer is increased, which demonstrated the formation of a protective organic film. The relatively small value of the heterogeneity factor (n) (close to 1) suggests that the adsorption is relatively homogeneous over the electrode and that good capacitive behavior is taking place on the electrode surface, whereas the relaxation time τ increases linearly with [DA], further evidence of the inhibition formation and of the retarded of the interfacial process. Revealingly, the experimental is the inhibitory efficiency obtained according to electrochemical impedance spectroscopy. (IEEIS) value increases from 79.7% (C = 10⁻⁶ M) to 94.6% (C = 10⁻³ M), indicating that DA effectively performs at low concentration and reaches maximum effect at high concentration, which confirms its nature as a mixed inhibitor, allowing for the formation of a stable protective film over the corrosion process.

3.1.2. PDP polarization and inhibition by dopamine

The OCP of the electrode was recorded continuously for 30 min prior to each measurement, at 292K (Figure 2a). In order to complement the EIS analysis and clarify the mechanism of action of DA, this section presents a study by PDP of 316L stainless steel in 0.5 M HCl. The Tafel curves recorded at different concentrations of DA are gathered in Figure 2(b), allowing for an appreciation of the inhibitor’s impact on the cathodic (proton reduction) and anodic (metal dissolution) branches as well as on the corrosion potential.

Close

Figure 2.

(a) OCP vs. time plot and (b) PDP curves (Tafel) of 316L steel in 0.5 M HCl in the presence and absence of DA: mixed inhibition and reduction of i_corr.

Export to PPT

The extracted electrochemical parameters, namely the corrosion potential E_corr, the corrosion current density i_corr, the Tafel slopes βα and βc, as well as the calculated inhibition efficiency, have been summarized in Table 2. The whole provides a quantitative framework to evaluate the impact of DA on the kinetics of corrosion reactions, to qualify the type of inhibition (anodic, cathodic, or mixed), and to relate these observations to the formation of a protective layer previously highlighted.

(Figure 2b) illustrates the influence of DA on the polarization behavior of 316L steel in 0.5 M HCl. In a pristine environment, the curve exhibits the active anodic, active/passive transition, passivation, and trans passivation region in the anodic branch, while it shows hydrogen evolution in the cathodic branch. The concomitant decrease in current on both branches as DA content grows evidences (i) the inhibition manifested by a slowing of both the metal oxidation reaction and proton reduction, and (ii) a weakening/broadening of this active/passive transition, with loss of current in the passive zone, which is related to the formation of a protective organic film that hinders access of aggressive species to the substrate, and therefore, dissolution of the material. The shape of the cathodic branch in the presence and absence of the inhibitor is similar, suggesting that DA does not change the hydrogen evolution pathway (the current decrease due to site blockage is not a different pathway). The E_corr is displaced a little (displacement < ∼85 mV, Table 2), a well-known criterion of a mixed-type inhibitor: DA retards the kinetics of both anodic and cathodic reactions without completely affecting one of the half-reactions.

These trends have all been established quantitatively in Table 2. The corrosion current density i_corr obtained from Tafel extrapolation, was gradually close to zero with [DA], which means an increasing value of the inhibitory efficiency, and calculated by Equation 8:

after 10⁻² at. % addition (see since ≈ 10 0 at.%), while I p continues to drop while reaching ≈ 92% for 10⁻³ M. The Tafel slopes βα and βc: a reduced value for βa (on close βc) with the same slope value in the cathodic domain (herewith, the current efficiency increases with a decrease in concentration). Indian. J. Chem. The slight displacement of E_corr with respect to DA further confirms the identification of a mixed inhibition of the type prevailing barrier, which clearly matches the evolution brought about by EIS to Rct and Cdl, and is observed as the increase of i_corr and the decrease of is the inhibitory efficiency obtained according to potential dynamic polarization (IE(PDP)) here.

In summary, from Figure 2(b) and Table 2, it can be observed that DA is well adsorbed on the 316L steel surface, with a significant decrease in i_corr, and the passivation zone is stabilized by DA without affecting the cathodic mechanism, leading to a mixed inhibition with maximum efficiency of about 92% at 10⁻³. These observations are also in agreement with EIS measurements (increase of Rct, decrease of Cdl, and τ span), which indicate the formation of a stable and dense organic barrier at the metal/solution interface.

3.4.3.1. DFT Study

To reveal the molecular depth of DA adsorption and to account for its inhibiting activity, we use a joint approach of quantum calculations (DFT/DFTB) and solvation. Figure 6 shows “(a)” the energy variation for the neutral and protonated form along with stable conformations used for electronic analysis, and “(b),” the COSMO sigma profiles, which characterize the donor/acceptor regions in an H-bond, and give information on the aqueous affinity and solubility of the molecule. The localization of frontier orbitals (HOMO and LUMO) mainly on the most electronegative atoms, O and N, indicated the possibility of electron donation-acceptance interactions with iron (HOMO donation → Fe(d) and backdonation Fe(d) → LUMO) conducive to chemisorption (Figure 7). The MAC of the neutral and protonated forms are mapped in Figure 8, which reveals high negative charges on O/N, therefore distinguishing such atoms as the preferred adsorption sites in the presence of a cation; these results supplement MEP data corresponding to COSMO. Last, Table 4 compiles the DFT electronic descriptors (HOMO, LUMO, ΔE, ionization and electron affinities, electronegativity, hardness/softness, electrophilicity, charge transfer, backdonation energy, etc.) that characterize the propensity of the DA to interact in terms of the number of electrons exchanged with the Fe(110) surface medium and, hence, to enhance the stability of the adsorbed state. All these parameters contribute to giving a unified picture encompassing solvation, electronic reactivity, and tendency to adsorb, required to interpret rationally the experimentally stressed anticorrosion capabilities.

Close

Figure 6.

Conformational stability and solvent affinity of DA: (a) evolution of the energy (MD) of the neutral and protonated forms; (b) COSMO sigma profile showing the donor/acceptor regions of H bonds.

Export to PPT

Close

Figure 7.

Maps of the frontier orbitals (HOMO/LUMO) of neutral and protonated DA: localization on O and N and implications for adsorption on Fe.

Export to PPT

Close

Figure 8.

Mulliken atomic charges (MAC) of DA: neutral and protonated forms, highlighting O/N adsorption centers.

Export to PPT

The figure collects (a) the time evolution of energy contributions (potential, kinetic, non-bonding, and total) to the neutral and protonated DA and (b) the COSMO sigma-profile. As seen in panel (a), both structures relax promptly toward low-energy conformations; the total energy becomes stable after several ps, suggesting a reliable structure for electronic calculations. For the protonated species, these fluctuations are slightly enhanced (which is in line with a network of intramolecular H-bonds and stronger electrostatic interactions), but it also approaches a stationary energy state. In panel (b), the COSMO sigma-profiles derived from the surface charge density distributions are presented: the screened charge density is divided into three regions of the COSMO method [4], H-bond donor, non-polar region, and H-bond acceptor, and predicts the solvent affinity as well as the solubility [13]. Maps show that DA (neutral (pink), protonated (blue)) presents significant contributions in the donor and acceptor regions, which confirms its amphoteric nature (phenolic and amine groups); the protonated form provides more intense peaks in the polar side, thus indicating a higher capacity of not only formation of H-bond interactions with water, but also good aqueous solubility, mainly dependent on the strength and extent of H-bond interactions with the solvent [14]. This heavy hydration results in an effective inhibitor presentation at the metal/solution interface, which also implies a sort of conflict between adsorption and solvation. Finally, DFT calculations (see Figure 7) disclose that the HOMO/LUMO are mainly localized at the heteroatoms O and N, i.e., at the electron-rich sites, since such sites should act as the most relevant adsorption centers in that donate-accept kinds of electrons to the d orbitals of iron, which is consistent with the above experimental adsorption and high inhibition efficiency. Finally, the triptych conformation stability → high aqueous affinity (COSMO) → active electronic sites (DFT) was summarized, which can underlie the adsorption mechanism/DA anticorrosion performance.

From Figure 7 (frontier orbitals) it can be also inferred that the density of the HOMO is strongly localized on the O and N heteroatoms of DA, in particular on the catechol nucleus and the amine, which implies an electron donating power towards the vacant d orbitals of Fe, and then a chemisorption by σ donation (M–O, M–N) that leads to a protective organic layer on the metal (9). In contrast, the LUMO density spreads over these patterns (π* of the aromatic ring and O/N functions) as well as the electrons from the electron-rich metal surface (Fe 3d → π* backdonation) are accepted, promoting anchoring by the π–d hybridization, which stabilizes the adsorbed state. The donor/acceptor abilities (ED/EA) combination effectively enhances overall inhibition efficiency through the formation of strong and stable coordination bonds at the inhibitor-metal interface [8]. At least in this Figure, a charge cycle can arise: the LUMOs of the molecules capture electrons temporarily from the metal, which are afterwards transferred back into the environment (dissipation/relaxation of charge within the adsorbed film) [31]. Lastly, the N and O atoms’ lone pair contributions can be shared with vacant d orbitals of Fe atoms, leading to an increased adsorption ability, enhancing the metal-inhibitor contact, and stabilizing the surface complex, an interpretation which is fully consistent with the increased Rct, decreased Cdl (EIS), and N 1s M–N-type signature in XPS.

The most commonly employed DFT-based descriptors for connecting the inhibitors’ electronic structure with their adsorption ability on iron are summarized in Table 5 in terms of the representations and equations typically proposed in the literature [22]. In this respect, the insightful role played by DFT calculations in understanding and designing inhibitors is commonly acknowledged [15]. Regarding DA, the HOMO and LUMO energy values (–5.308/–5.472 eV for the neutral/protonated forms and –0.825/–0.969 eV, respectively) result in a gap ΔE ≈ 4.5 eV, which agrees with a moderate electronic reactivity suitable for selective chemisorption rather than an overaggressive reactivity. The ionization energy I (≈5.31–5.47 eV) and the electron affinity A (≈0.83–0.97 eV) control the molecule’s ability of donating or accepting electrons from the Fe(110) surface: a rather low I and a non-negligible A point to a dual donor/acceptor character which is favorable for HOMO →d(Fe) donation and d(Fe) → LUMO backdonation—the mechanism that stabilizes the adsorbed state and increases the adhesion [8]. This is in agreement with the HOMO/LUMO plot (Figure 7), and they are located on the heteroatoms O, N, with emphasis on reaction active sites.

This painting is confirmed by some global descriptors: the electronegativity χ (3.07 → 3.22 eV) and the chemical potential μ = –χ (–3.07 → –3.22 eV) imply a slightly more electrophilic character of the protonated version, which is also corroborated by the electrophilicity ω (2.10 → 2.30) and the electro-accepting power ω⁺ (0.845 → 0.975). On the other hand, the value for the hardness η (≈ 2.24–2.25 eV) is approximately the same for both systems, and the relatively high value of the softness σ = 1/η (≈ 0.446–0.444) characterizes it (the adsorbents) as an adsorbent with a large tendency towards an adsorption on steel [8,14]. WME The amount of the transferred charges ΔN is small and positive for the neutral species (0.0365), which indicates an electron donation and ionic contribution of the L-type to Fe; however, the small and very low ΔN value for the π − state (0.0021) means a more limited electron donation—this trade-off is due to the medium (HCl) mainly focusing on a protonated acid model, where on one hand the protonation causes an increase in solvation/aqueous affinity and on the other hand a decrease in the basicity of the donor sites. Last, the backdonation ΔE is very close to −0.56 eV (for both structures), meaning that thermodynamically the Fe → LUMO backdonation is favorable, which enhances the stability of its adsorption [8,14]. In conclusion, the high flexibility values σ, the balanced I/A combination, the favorable backdonation (ΔE < 0), and the frontier orbitals localized on O/N account for the strong adsorption through electron donation-acceptance on Fe(110) with the proposed mechanism [8]. The neutral form seems to be a better donor (higher ΔN) and the protonated one more electrophilic (larger ω, ω⁺); the counterbalance ED/EA contributions justify the high experimentally observed inhibition. In this sense, Table 4 shows some electronic descriptors (χ, η/σ, ω, ω⁺/ω⁻, ΔN, ΔE_back) which help to explain the adsorption characteristic and anticorrosion efficiency of DA, suggesting the role of the DFT approach in the rational design of inhibitors [13,22].

Figure 8 shows the active sites for the adsorption of DA at Fe(110) and confirms heteroatoms as the dominating feature. If we follow the theoretical model according to which MACs are reliable indicators of inhibitory sites [32], for the neutral form, we find the catalytic receptor has strongly negative charges (≈ –0.45) on the catechol oxygens and on the amino nitrogen (≈ –0.89), but the ring carbons are weakly negative. This distribution points to a strong donor ability of the non-bonding pairs of O/N with respect to the vacant d orbitals of the iron, in agreement with a multidentate chemisorption (M–O and/or M–N bonds) on Fe(110) [33]. For the protonated species, the protonation causes redistribution of the electron density – the oxygens become even more electron rich (–0.73/–0.80) and the nitrogen less donating (absolute value ≈ –0.78), thus suggesting that this environment coordination will preferentially occur through the oxygen atoms and π-system of the aromatic ring. These data are consistent with the hypotheses for the selectivity of some Fe(110) sites towards specific O/N atoms of the inhibitors [33] and the established observation that O and N heteroatoms are preferred adsorption sites due to their high electron density [7]. Furthermore, MEP (Figure 7) evinces that the positive/negative potentials are surrounding the same motifs and strengthens the findings about adsorption favorable sites and metal‐inhibitor interaction bridging [22]. On the whole, MAC and MEP offer a consistent explanation for the mechanism: the O/N doublet donation → Fe(3d), the adsorbed status stabilization, and then the enhancing particulate adsorption ability toward DA, the origin of its inhibitory efficiency in an acidic environment.

3.4.3.2. DFTB+ analysis of charge redistribution and planar anchoring mode

To clarify the anchoring pattern and electronic rearrangement of DA on the iron surface, we carry out DFTB+ calculations on iron Fe(110). The charge density difference diagram (Δρ) of the adsorbed state (isosurfaces ±0.002 e·Å⁻³; blue = increase, red = decrease) is shown in Figure 9, which shows a nearly planar orientation of the molecule adsorbed state and donor-acceptor interaction between O/N/π sides of the DA and the t2g/eg d orbitals of the Fe. The characteristic bond distances (r[O] = 2.64 Å, r[C ring] = 2.56 Å) confirm that strong chemisorption can be connected with the observed redistribution of the charge that stabilizes the adsorbed film responsible for the inhibitor effect.

Close

Figure 9.

Δρ of DA adsorption on Fe(110) (DFTB+).

Export to PPT

Figure 9 (DFTB+ calculation) represents the charge density difference Δρ in the case of DA adsorption on the Fe(110) support (isosurfaces plotted at ±0.002 e•Å⁻³; blue = accumulation, red = depletion). The most stable adsorption structure features a nearly planar (side-on) orientation of the molecule with respect to the metal plane, and is a well-known one for elongated organic inhibitors where π–d hybridization is believed to control the anchoring. The experimental r[O] = 2.64 Å and r[Cring] = 2.56 Å distances confirm that the catechol/aromatic ring functions only form a chemisorptive interaction with each of the surface atoms.

The Δρ map (red areas only) is focused on O and N atom and the aromatic ring (electron depletion on the inhibitor), signatures of an electron donation (O/N non-bonding pairs and π density) to the empty d orbitals of iron, symmetrically, the marked accumulation in the first Fe layer (blue areas) reflects the backdonation Fe 3d/4s → π* of the phenyl, this donor-acceptor scheme establishing the bond to the surface. This π–d coupling accounts for the stability of the adsorbed geometry and the presence of a well-defined protective continuous organic layer; on the contrary, small molecules that do not extend their π system tend to adsorb perpendicularly due to localized chemisorption at the heteroatoms. The overall (planar orientation, significant charge transfer, and short O/ring–Fe distances) indicates the large donation/acceptance of DA and strong inhibitor–metal interaction, consistent with the good anticorrosion efficiency evidenced from electrochemical measurements.

3.4.3.3. MC/DM: DA adsorption on Fe(110)

To well understand the adsorption process of DA (DA) on Fe(110), besides the experimental data, the Monte Carlo (MC) and MD simulations were performed. These results have been summarized in Figure 10: at the minimum energy geometries of both the neutral and protonated forms (MC/DM), which shows a quasi-planar orientation of the molecule, with the presence of O/N heteroatoms; b the distribution of adsorption energies Eads (calculated by equation (9)), which reveals the strength of the anchoring (the smaller the Eads value, the more robust the bond) and c the radial distribution functions (RDF) of O and N with respect to Fe (that allows us to discriminate between chemisorption and physisorption, 3.5 Å). This combined strategy attempts to bridge adsorption topology (planar mode and donor-acceptor interactions, π–d hybridization) to thermodynamic stability and dynamic structural signatures and to correlate them with the EIS/PDP and XPS/SEM characterizations.

Close

Figure 10.

Adsorption of DA (neutral/protonated) on Fe(110): (a) minimum energy geometries (MC/MD), (b) distribution of adsorption energies Eads, (c) radial distribution functions (RDF) O, N–F.

Export to PPT

The most favorable adsorption geometries for DA (neutral and protonated) on Fe(110) obtained from MC and MD simulations are represented in Figure 10(a). In both cases, the molecule prefers an almost planar orientation with respect to the metal plane: the aromatic ring lies down (on the surface) and both heteroatoms, O/N, approach the Fe atoms. It is controlled by donor-acceptor interactions that stabilize chemisorption through donation of O/N doublets and π density to the empty d orbitals of iron and backdonation Fe(3d/4s) → π* and lead to the formation of a protecting organic film.

Panel Figure 10(b) shows the distribution of the adsorption energies Eads as calculated by (Equation 10) from MC, while panel Figure 10(c) displays the distribution of the hopping barriers W.

The measured values, largely negative (from −125 to −147 kcal mol⁻¹), are indicative of a very strong inhibitor–metal interaction; the more negative Eads, the more pronounced anchoring and the more stable barrier layer is formed, which permanently hinders corrosion [32,34].

(c) RDF for heteroatoms O and N relative to Fe, from MD trajectories. The prominent peaks < 3.5 Å suggest chemisorbed contacts (coordination bonds/strong interaction) of O/N with Fe(110); the contributions beyond 3.5 Å result from weaker physisorption. This RDF signature supports the chemisorption mechanism inferred from the Eads energies and the anchor plane mode, and is a reference tool for the description of the adsorption and interfacial interaction of inhibitors [19,34].

Altogether, MC and MD receive the following: (i) planar orientation of DA on Fe(110), (ii) very negative adsorption energies, indicating a strong interaction, and (iii) RDF with short first maxima for O/N–Fe indicates the chemisorption. The results of these models are in good agreement with experimental data (increase in Rct; decrease in Cdl; large decrease of i_corri and confirm the important functions of the electron transfer and O/N sites in stabilizing the adsorbed film for good inhibition of corrosion.