Zea mays bracts extract as an eco-friendly corrosion inhibitor for steel in HCl pickling solution: Experimental and simulation studies

4.1.1 UPLC-QTFMS results

Fig. 1 shows the UPLC-QTFMS chromatogram of ZMBE under the positive ion mode, and Table 1 presents the corresponding tentative identification results. ZMBE contains various phytochemicals such as carbohydrates (1: rhamnose, 2: ribose, 3: glucuronic acid, 4: glucose, 6: fucose (Tachaapaikoon et al., 2006), and 7: hemicellulose (Scheller and Ulvskov, 2010)), phenolic compounds (5: hydroxycinnamic acid (Vazquez-Olivo et al., 2019), 11: β-sitosterol, and 12: ergosterol (Capocchi et al., 2017)), phenylpropanoids (8: dimboa (Li et al., 2013a), 13: cryptoxanthin, 14: β-carotene, and 15: lutein (Kopsell et al., 2011)), flavonoids (9: anthocyanins (Fernandez-Aulis et al., 2019), 10: tricin (Tian et al., 2016), and 16: rutin (Kisa, 2016)).

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Fig. 1

UPLC-QTFMS chromatogram of ZMBE in the positive ion mode.

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The active ingredients in ZMBE are exceedingly complex, and it is impractical to present the simulation results for all identified components. The ingredients were categorized into four groups. As an illustrative representation, one result of each group was showcased. Where hydroxycinnamic acid, dimboa, glucuronic acid, and rutin are the representatives of phenolic, phenylpropanoids, carbohydrates, and flavonoids, respectively. The molecular structures are shown in Fig. 2.

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Fig. 2

Molecular structures of some active ingredients in ZMBE.

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4.1.2 FT-IR spectroscopy results

Fig. 3 depicts the FT-IR spectra of ZMBE solid and the ZMBE adsorption film formed on the MS surface. As shown in Fig. 3a, a broad absorption band around 3274 cm⁻¹ corresponds to the stretching vibration of –OH and –NH₂, confirming the presence of alcoholic compounds and carboxylic acids in ZMBE (Erfan and Mardali, 2024). The absorption peak at 2915 cm⁻¹ corresponds to the stretching vibration of the saturated C–H bonds (Mourya et al., 2014). The absorption peak at 1634 cm⁻¹ can be attributed to the stretching vibration of C⚌C and C⚌O in aromatic ring conjugated systems (Jokar et al., 2016), which are present in dimboa, rutin, and hydroxycinnamic acid. The absorption peak at 1342 cm⁻¹ is corresponding to the symmetric stretching vibration of –COOH (Dehghani et al., 2019a). The absorption peak at 1248 cm⁻¹ is due to the stretching vibration of C-O in aromatic ethers (Abdullah et al., 2023; Chen et al., 2023; Qiang et al., 2018), while the peak at 1008 cm⁻¹ is associated with the stretching vibration of C-O in aliphatic ethers (Wu et al., 2020a). The absorption peaks observed below 1000 cm⁻¹ are indicative of the bending vibrations of C–H in benzene rings or heterocycles (Fernandes et al., 2019; Wu et al., 2020a). It implies that ZMBE contains various functional groups, including –OH, –COOH, C⚌C, C⚌O, C–O, and aromatic rings, etc. These functional groups possess distinctive electronic structures, enabling them to share electrons with Fe atoms. They may also undergo chelation reactions with Fe ions and/or generate physical adsorption effects. Consequently, the active ingredients in ZMBE are adsorbed onto the surface, forming a protective adsorption film.

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Fig. 3

FT-IR spectra: (a) ZMBE solid, and (b) ZMBE adsorption film on the MS surface.

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As shown in Fig. 3b, the broad absorption peak associated with the stretching vibration of –OH and –NH₂ in the adsorption film is significantly reduced. It suggests that the –OH bonds in ZMBE are chelated with Fe atoms or ions and the –NH₂ bonds may share electrons with iron atoms, forming iron complexes. However, the position and quantity of the absorption peaks, corresponding to the characteristic groups present in ZMBE, remain nearly identical in both spectra. It further confirms that the organic substances in ZMBE have successfully adsorbed onto the MS surface, playing a crucial role in inhibiting the corrosion of MS.

4.1.3 UV–visible spectroscopy results

Fig. 4 shows the UV–visible spectra of 1 M HCl solution with 5.0 g·L⁻¹ ZMBE before and after immersion of MS samples. Before immersion, the absorption peak at 224 nm corresponds to the π-π* transition of the C⚌C bonds in aromatic rings. It indicates that ZMBE has the aromatic compounds, which own unsaturated bonds and π electrons and form a conjugated closed system (Prasad et al., 2023a). The absorption peak at 284 nm is attributed to the n-π* transition of the C = O bonds in carbonyl compounds (Sığırcık et al., 2017). After immersion, the absorbance intensity at the same wavelength regions decreases, and the two peaks are slightly blue-shifted to 222 and 282 nm, respectively. It can be concluded that the C⚌C and C⚌O bonds in ZMBE interact with the MS surface through physical adsorption and chemical adsorption and form the stable complexes with Fe ions in the solution (Fergachi et al., 2019; Goel et al., 2010; Sığırcık et al., 2017).

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Fig. 4

UV–visible spectra of 1 M HCl solution with 5.0 g·L⁻¹ ZMBE before and after immersion of MS sample.

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4.1.4 XPS results

To further understand the adsorption and corrosion inhibition behavior of the phytochemical components on the MS surface, their existing forms were investigated. The XPS full-spectrum shows the presence of C, N, O and Fe in the adsorption film. The high-resolution XPS spectra and the peak fitting results of these four elements are shown in Fig. 5.

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Fig. 5

High-resolution XPS spectra of elements in ZMBE adsorption film on the MS surface: (a) C 1 s; (b) N 1 s; (c) O 1 s; and (d) Fe 2p_3/2.

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As shown in Fig. 5a, the C 1 s spectrum in the adsorption film can be divided into three types of substances. The absorption peak at 284.8 eV corresponds to the C⚌C, C–C and C–H bonds in aromatic rings (El-Azzouzi et al., 2022), which may originate from the active ingredients such as hydroxycinnamic acid, dimboa, and rutin. The absorption peak at 285.6 eV is related to the C-N and C-N⁺ bonds in organic molecules (Bouanis et al., 2016; Cunha et al., 2023). The former most likely originates from the neutral dimboa, amino acids, and other ingredients in ZMBE, while the latter may arise from the corresponding protonated molecules. The small peak at 288.4 eV is attributed to the C–O, C⚌O, C–O⁺ and C⚌O⁺ bonds (Du et al., 2023a). The first two bonds may originate from the neutral ingredients like hydroxycinnamic acid, dimboa, glucuronic acid, and rutin, while the latter two may be related to their protonated molecules. It is worth noting that there is no N element in MS, but the presence of C-N bonds confirms the adsorption of ZMBE on the MS surface.

As shown in Fig. 5b, the N element primarily exists in two forms. The peak at 399.8 eV corresponds to the C-N and C-N⁺ bonds, and that at 400.4 eV is corresponding to the –NH₂ bond (Sun et al., 2022b). These may originate from dimboa or other amino acids in ZMBE.

As shown in Fig. 5c, the O element can be divided into three forms. The binding energy at 529.8 eV corresponds to the O 1 s in Fe₂O₃ and Fe₃O₄ (Mukhopadhyay et al., 2021). The strongest peak at 531.8 eV is corresponding to FeOOH and FeO (Hu et al., 2021a). The peak at 533.2 eV is related to the neutral C–O and C⚌O bonds and the protonated C–O⁺ and C⚌O⁺ bonds in organic substances (Attou et al., 2020). It further confirms the adsorption of the ZMBE’s active ingredients on the MS surface. This is consistent with the C 1 s results.

As shown in Fig. 5d, the Fe 2p_3/2 spectrum can be divided into three distinct peaks. The first peak at 706.5 eV corresponds to the metallic iron (Fe⁰) (Ahmed and Zhang, 2020). It indicates that the coverage of the adsorption film of ZMBE is not 100 % and there are some defects. The strong and broad peak at 710.8 eV arises from oxygen elements in Fe₂O₃, FeOOH, FeO, and Fe₃O₄ (Berrissoul et al., 2022a; Hashim et al., 2019). The presence of FeOOH and Fe₂O₃ suggests that the addition of ZMBE favors the formation of denser and more stable Fe³⁺ compounds, which enhance the corrosion resistance. The small peak at 714.4 eV corresponds to the Fe complexes (Garai et al., 2012). This may be due to that the organic molecules in ZMBE act as electron donors and/or electron acceptors for Fe atoms, and it could also be owing to the chelation reactions between the adsorptive functional groups and Fe ions (Olivares-Xometl et al., 2006). The latter is consistent with the FT-IR results where the absorption peak of hydroxyl on the MS surface is almost disappeared.

The XPS analysis indicates that the active molecules in ZMBE can effectively adsorb on the MS surface, forming a protective film. It serves to reduce the contact between MS and corrosive media, thereby reducing the corrosion.

4.3.1 OCP curves

OCP is the common potential formed by mutual polarization between the anodic and cathodic equilibrium potential in a natural state. The shifts in OCP provide a means to evaluate the equilibrium tendencies of the electrode reactions (Li et al., 2021b).

Fig. 7 shows the OCP-time curves for MS in 1 M HCl solution with various concentration of ZMBE at 298, 308, and 318 K. In Fig. 7a, regardless of the presence or the absence of ZMBE in 1 M HCl solution at 298 K, the change trend of OCP with time is almost similar. There is a rapid increase in OCP during the initial 300 s of immersion, followed by a continuous decrease. Around 1500 s, the OCP is stabilized gradually, indicating that electrode reactions on the MS surface approach the dynamic equilibrium. Compared to the blank solution, the addition of ZMBE makes the OCP of MS shift in the negative direction by approximately 30–35 mV. However, only when the potential fluctuation range is greater than 85 mV, an anode-type or cathodic-type corrosion inhibitors can be judged (Moradia et al., 2021; Xu et al., 2024a). It suggests that ZMBE in 1 M HCl solution at 298 K behaves as a mixed-type corrosion inhibitor with a slightly dominant inhibition on cathodic reaction.

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Fig. 7

OCP curves for MS in 1 M HCl solution with different content of ZMBE at (a) 298, (b) 308, and (c) 318 K.

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As illustrated in Fig. 7b and 7c, the OCP change law at 308 K closely mirrors that at 298 K. At 318 K, the change trend of OCP in the initial immersion stage is slightly different, but it stabilizes eventually. Similarly, the negative shift in the stable OCP with the addition of ZMBE at 308 and 318 K is consistently below 30 mV. It reaffirms the role of ZMBE as a mixed-type corrosion inhibitor for MS in 1 M HCl solution at 308 and 318 K. Moreover, the stabilization of OCP is more rapidly with increasing the solution temperature. This may be attributed to the organic components in ZMBE being more easily adsorbed onto the surface of MS at higher temperature.

4.3.2 PDP curves

Fig. 8 shows the PDP curves for MS in 1 M HCl solution with different concentration of ZMBE at 298, 308 and 318 K. Table 2 provides the corresponding polarization parameters. C_inh represents the concentration of ZMBE. E_cor and i_cor are the corrosion potential and corrosion current density of MS, respectively. b_c and b_a are the cathodic and anodic Tafel slope values, respectively. η_PDP is the corrosion protection efficiency of ZMBE for MS and was calculated by the corrosion current density without ZMBE ( $i_{\text{cor}}^0$ ) and with ZMBE ( $i_{\text{cor}}$ ) (Jero et al., 2023; Zaidi et al., 2023):

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Fig. 8

PDP curves for MS in 1 M HCl solution with different content of ZMBE at (a) 298, (b) 308, and (c) 318 K.

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As depicted in Fig. 8a, the addition of ZMBE to 1 M HCl solution at 298 K prompts the PDP curves shift downwards to the left. Both the anodic and cathodic polarization currents decrease, signifying the inhibition of both the anodic and cathodic corrosion reactions of MS. It can be ascribed to the adsorption of organic molecules on the MS surface, forming a protective film. Notably, the amplitude of the leftward shift in the cathodic curve surpasses that in the anodic curve, indicating a more pronounced inhibitory effect of ZMBE on the cathodic reaction. However, the negative shift of E_cor is clearly less than 50 mV, further supporting that ZMBE acts as a mixed-type corrosion inhibitor for MS in 1 M HCl pickling solution (Akounach et al., 2022).

As depicted in Fig. 8a, the addition of ZMBE does not significantly alter the shape of the PDP curves, indicating that the cathodic and anodic reactions of MS in 1 M HCl solution remain largely unchanged. Specifically, the hydrogen evolution at the cathode and the iron dissolution at the anode persist. In an acidic corrosion system governed by the activation polarization, the inhibitory mechanism of organic molecules can be attributed to the “geometric coverage” effect. Here, the adsorption film serves to isolate the metal surface from the acidic solution. Consequently, the electrode reactions are confined to the covered surface, while the uncovered portion continues to undergo the original corrosion reactions. With an escalation in ZMBE concentration, a greater number of active sites on MS are coated. It results in a reduction of the active areas and the cathodic and anodic reaction points. Consequently, there is a deceleration in reaction rates and a leftward shift in the polarization curves. The more pronounced leftward shift in the cathodic branch may be attributed to the more conspicuous adsorption behavior of organic substances on cathodic active sites. Besides chemical adsorption owing to electron-sharing, physical adsorption induced by electrostatic forces between the protonated molecules and metal surface also preferentially occurs on the cathodic sites.

As shown in Fig. 8b and 8c, the influence law of ZMBE on PDP curves at 308 and 318 K follows a similar pattern to that at 298 K. ZMBE still works as a mixed-type corrosion inhibitor for MS in 1 M HCl solution. The corrosion inhibition mechanism still is the “geometric coverage” effect. Therefore, the coverage of ZMBE on the MS surface (θ) is equal to the corrosion inhibition efficiency and can be expressed as (Fatima et al., 2020):

4.3.3 EIS diagrams

Fig. 9 presents the Nyquist and Bode diagrams for MS in 1 M HCl solution with different content of ZMBE at 298 K. Figs. 10 and 11 show the EIS diagrams at 308 and 318 K, respectively.

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Fig. 9

EIS diagrams for MS in 1 M HCl solution with different content of ZMBE at 298 K: (a) Nyquist diagrams, (b) and (c) Bode diagrams.

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Fig. 10

EIS diagrams for MS in 1 M HCl solution with different content of ZMBE at 308 K: (a) Nyquist diagrams, (b) and (c) Bode diagrams.

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Fig. 11

EIS diagrams for MS in 1 M HCl solution with different content of ZMBE at 318 K: (a) Nyquist diagrams, (b) and (c) Bode diagrams.

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As depicted in Figs. 9-11, whether or not ZMBE is added to 1 M HCl solution at 298, 308, and 318 K, all Nyquist diagrams exhibit a single distorted capacitive arc. It corresponds to the double-layer capacitance at the MS/solution interface. The distortion in semicircular arc is due to the frequency dispersion effect caused by the non-uniformity of the actual double-layer capacitance. Furthermore, with an increase in ZMBE concentration, the size of the semicircular arc and the impedance value increase. The electrode reaction resistance of MS and the corrosion inhibition effect of ZMBE are improved.

As depicted in Fig. 9c, 10c, and 11c, under all experimental conditions, only one peak is displayed in the phase angle spectra, suggesting that there is only one time constant for the corrosion process at the metal/solution interface (Rathod et al., 2022). All negative phase angle peaks consistently are smaller than 90°. This is attributed to the inherent roughness and non-uniformity of the metal surface in the actual electrochemical system. As the concentration of ZMBE increases, both the negative phase angle peak and the half-width of the angle peak augments. It suggests a thickening in metal/solution interface layer, a reduction in double-layer capacitance, and an extension in frequency response time (Asfia et al., 2020; Singh et al., 2008). This may be related to the displacement of water molecules initially adsorbed on the metal surface by the active organic molecules. As the concentration of ZMBE rises, the adsorption and coverage of these active molecules intensify. In turn, it leads to a reduction in exposed area of MS, a thickening in double-layer, and a decrease in local dielectric constant. Additionally, at three experimental temperature, the modulus of the low-frequency impedance shows an upward trend with increasing ZMBE content (as shown in Fig. 9b, 10b, and 11b). It suggests a concurrent increase in charge transfer resistance for the corrosion reaction of MS.

As depicted in Figs. 9-11, the addition of ZMBE at three experimental temperature almost does not alter the initial shape of the EIS diagrams. The corrosion mechanism of MS is not changed, and the corrosion inhibition mechanism of ZMBE still is the “geometric coverage” effect. Notably, as the temperature of the HCl solution rises, the EIS parameters such as the radius of the capacitive arc, the modulus of the low-frequency impedance, the negative phase angle peak, and the half-width of the angle peak decrease. It indicates a reduction in corrosion resistance of MS with the increase of the solution temperature.

Fig. 12 shows the schematic diagram of the electrode/electrolyte interface model and the equivalent circuit for analyzing EIS data. Where R_s and R_ct are the solution resistance and the charge transfer resistance. CPE, constant phase element, represents the electric double-layer capacitance of the interface between MS and solution and is expressed as (Damej et al., 2023; Singh et al., 2008):

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Fig. 12

(a) Schematic diagram of electrode/electrolyte interface, and (b) equivalent circuit for analyzing EIS data.

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Table 3 shows the fitting parameters of the EIS diagrams. Based on the EIS results, the corrosion inhibition efficiency (η_EIS) of ZMBE was calculated by the charge transfer resistance without ZMBE ( $R_{\text{ct}}^0$ ) and with ZMBE ( $R_{\text{ct}}$ ) (Gu et al., 2023; Zakaria et al., 2022):

Similarly, because of the “geometric coverage” effect, the coverage of ZMBE based on EIS results (θ_EIS) is equal to the corrosion inhibition efficiency and was calculated by Eq. (2). As shown in Table 3, both ZMBE concentration and experimental temperature have little impact on the conductivity of the 1 M HCl solution, all R_s is around 1 Ω·cm². With an increase in ZMBE concentration, R_ct, η_EIS, and θ_EIS are significantly enhanced. The heightened concentration of active ingredients in the HCl solution promotes the adsorption and coverage on the MS surface. Consequently, the mechanical hindrance effect, the charge transfer resistance, and the overall corrosion inhibition effect are improved.

The value of n is related to the roughness and uniformity of the actual capacitance surface. A larger n means a closer the actual capacitance to an ideal flat-plate capacitor (Zuo et al., 2021). As indicated in Table 3, the addition of ZMBE has little impact on the value of n, which is within the range of approximately 0.81 to 0.90, approaching 1. It suggests that the electric double-layer at the MS/solution interface essentially behaves as an ideal flat-plate capacitor. The thin nature of the ZMBE adsorption film on the MS surface is likely responsible for this. It further confirms that all EIS diagrams exhibit only one capacitive arc and only one time constant.

As indicated in Table 3, f_max decreases with increasing the ZMBE concentration, thus the frequency response time of the double electric layer capacitor is prolonged, and the values of Y₀ and C_dl decrease. According to the Helmholtz model (Hu et al., 2017b; Tang et al., 2023):

In the absence of ZMBE, water molecules and active chloride ions are adsorbed on the MS surface. The addition of ZMBE leads to the replacement of water molecules and chloride ions by active organic molecules, resulting in a decrease in ε and an increase in d. Since the adsorption of ZMBE has slight influence on n, the change in the actual surface area of the electric double-layer capacitance can be negligible. Therefore, the adsorption of ZMBE on the MS surface ultimately leads to a decrease in electric double-layer capacitance. In addition, when the protonated onium ions undergo electrostatic attraction with the initially adsorbed chloride anions, they are physically adsorbed on the MS surface, also contributing to a decrease in C_dl.

As indicated in Table 3, both η_EIS and θ_EIS exhibit an increase with solution temperature at the same ZMBE concentration. Specifically, at 298, 308, and 318 K, the maximum η_EIS of ZMBE is 82.0 %, 90.0 %, and 91.9 %, respectively. It is noteworthy that elevating the temperature of the HCl solution results in a serious decrease in R_ct and a severe increase in C_dl, indicating a reduction in the corrosion resistance of MS. The change patterns of η_EIS and η_PDP with ZMBE content and solution temperature are consistent, although the specific values differ. This discrepancy is attributed to the variances in testing principles and calculation methods between EIS diagrams and PDP curves.

4.5.1 Adsorption isotherms

The adsorption behavior of organic molecules on the metal surface is an important criterion for assessing the effectiveness of corrosion inhibition (Huang et al., 2022). Electrochemical corrosion analysis shows that ZMBE exhibits an outstanding corrosion inhibition effect on MS in 1 M HCl solution and provides the corrosion inhibition mechanism of “geometric coverage” effect. θ is equal to η. The adsorption isotherm describes the relationship between coverage and concentration of the organic inhibitor at a constant temperature and is profit to understand the adsorption film on the metal surface. Based on θ_PDP and θ_EIS, the following different adsorption isotherm equations were tried.

Langmuir adsorption isotherm (Shadi et al., 2023; Xu at al., 2022b):

El-Awady adsorption isotherm (Bhardwaj et al., 2021; Dehghani et al., 2019b):

Temkin adsorption isotherm (Vaszilcsin et al., 2023):

The fitting results of various adsorption isotherms are presented in Figs. 13-15 and Tables 5 and 6. At three experimental temperatures (298, 308, and 318 K), the adsorption of ZMBE on the MS surface strictly adheres to the Langmuir isotherm. While the resemblance of the El-Awady isotherm is not as robust as that of the Langmuir isotherm, it still holds a certain degree of reliability. Clearly, the adsorption of ZMBE on the MS surface does not conform to the Temkin isotherm.

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Fig. 13

Langmuir isotherms of ZMBE on the MS surface based on (a) PDP and (b) EIS results.

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Fig. 14

El-Awady isotherms of ZMBE on the MS surface based on (a) PDP and (b) EIS results.

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Fig. 15

Temkin isotherms of ZMBE on the MS surface based on (a) PDP and (b) EIS results.

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The Langmuir isotherm, being an ideal adsorption model, assumes that the adsorption molecules do not interact with each other on a uniform solid surface and eventually reach equilibrium. Therefore, the active molecules in ZMBE form a monolayer adsorption film on the MS surface without interaction forces. This alignment with EIS results, where only one single capacitive arc and one single time constant are displayed. It further confirms the surface observations derived from SEM.

K_ads reflects the adsorption capacity of organic molecules (Khadija et al., 2023). A higher K_ads indicates a stronger adsorption capability of the active molecules on the MS surface, showing a better corrosion inhibition effect. As depicted in Table 5, K_ads increases with solution temperature, and the strongest adsorption capacity of ZMBE is at 318 K, coinciding with the η result.

As depicted in Fig. 14 and Table 6, the linear correlation coefficient for the El-Awady isotherm is nearly close to 1, suggesting that the adsorption of the active molecules on the MS surface also follows the El-Awady model. All 1/y is greater than one, but it decreases with the increasing of solution temperature. A single organic molecule can displace multiple water molecules and occupy multiple active adsorption sites on the MS surface (Jyothi and Ravichandran, 2014).

However, the number of active adsorption points that can be replaced by organic molecules tends to decrease at higher temperature. This may be linked to the alterations in the adsorption mode, such as the relative ratios of physical adsorption to chemical adsorption (Li et al., 2021b; Luo et al., 2023). Furthermore, the variation of K_ads with temperature is consistent based on both the El-Awady and Langmuir isotherms. The optimal adsorption ability of organic molecules is observed at 318 K, followed by 308 K.

4.5.2 Adsorption thermodynamic parameters

To further investigate the adsorption behavior of ZMBE on the MS surface, the adsorption Gibbs free energy (ΔG_ads) was calculated by K_ads (Martinez-Gonzalez et al., 2023):

A negative ΔG_ads indicates a spontaneous adsorption process of organic molecules on the metal surface. On the contrary, it is the desorption process (Mehdi et al., 2019; Nesane et al., 2023). Generally, when ΔG_ads is greater than −20 kJ∙mol⁻¹, it is physical adsorption. When ΔG_ads is less than −40 kJ∙mol⁻¹, it is chemical adsorption. And it is mixed adsorption when ΔG_ads ranges from −40 to −20 kJ∙mol⁻¹ (Mehdi et al., 2019; Obi-Egbedi et al., 2012). As shown in Tables 5 and 6, all ΔG_ads is negative, and it ranges from −40 to −20 kJ∙mol⁻¹, closer to −20 kJ∙mol⁻¹. The adsorption process of active molecules on the MS surface is spontaneous, and it is a mixed adsorption mainly based on physical adsorption. ΔG_ads is more negative with a higher temperature, so the proportion of chemical adsorption in mixed adsorption is enhanced.

4.5.3 Adsorption kinetic parameters

The corrosion rate of MS (v) was approximately calculated based on Faraday's law:

Arrhenius equation (Chaudhary et al., 2012a, 2013b, 2016c):

Fig. 16 shows the fitting results of the Arrhenius and Transition state kinetic equations for the corrosion reaction of MS in 1 M HCl solution with different ZMBE concentration. Table 7 lists the corresponding parameters of the activation reaction. Both kinetic equations for the corrosion reaction of MS exhibit a good linear relationship.

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Fig. 16

Fitting lines of the kinetics equations for corrosion reaction of MS in 1 M HCl solution with different content of ZMBE: (a) Arrhenius, and (b) Transition state.

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A significant decrease in E_a is a characteristic of the chemical adsorption, while a remarkable increase in E_a indicates the physical adsorption (Lin et al., 2024c; Abdullah et al., 2023). As shown in Table 7, the addition of ZMBE results in a decrease in E_a. Specifically, E_a is 69.2 kJ·mol⁻¹ in the absence of ZMBE, and it decreases to 22.9 kJ·mol⁻¹ in the presence of 5.0 g·L⁻¹ ZMBE. The slight decrease in E_a can be attributed to the mixed adsorption of active ingredients in ZMBE on the MS surface. Obviously, the physical and chemical adsorption abilities of various organic molecules in ZMBE are different. As the temperature increases, the physical adsorption will be weakened, and so is their inhibitory effect. Meanwhile, the irreversible chemical adsorption caused by electron sharing requires higher concentration of ZMBE.

The pre-exponential factor A reflects the quantity of active sites available for the corrosion reaction of the metal (Garai et al., 2012). Upon the addition of ZMBE to the HCl solution, A undergoes a significant decrease. Specifically, it is 6.59 × 10¹³ g·m⁻²·h⁻¹ in the blank solution, and it diminishes to 5.65 × 10⁵ g·m⁻²·h⁻¹ in the presence of 5.0 g·L⁻¹ ZMBE, reducing by more than eight orders of magnitude. This substantial reduction in A is clearly attributed to the adsorption of organic molecules on the micro-anodes and micro-cathodes of the MS surface. The adsorption film inhibits both the anodic and cathodic corrosion reactions, which has been confirmed by the PDP and EIS tests. As the concentration of ZMBE increases, the adsorption of organic molecules intensifies, and more active sites on the MS surface are covered. The variation of A with ZMBE concentration provides further confirmation of the inhibition mechanism of the “geometric coverage” effect.

As depicted in Table 7, ΔS_a is reduced obviously after ZMBE is added, implying a significant improvement in orderliness of the MS surface. This can be attributed to the ordered arrangement and adsorption of the active components, which replaces the initially disordered adsorption of chloride ions and water molecules (Du et al., 2022b). With an increase in ZMBE concentration, ΔS_a becomes more negative, and a higher order of the MS surface is obtained. Furthermore, ΔH_a at all experimental conditions is positive, suggesting an endothermic process of the corrosion reaction of MS. Consequently, with an elevation in the temperature of the HCl solution, the corrosion rate of MS significantly increases. But the endothermic effect decreases with an increase in ZMBE concentration, leading to a reduction in the corrosion rate.

4.6.1 DFT calculations

To investigate the adsorption behavior of the active ingredients on the MS surface and to further confirm their corrosion inhibition mechanism of “geometric coverage” effect, DFT calculations were conducted. The distribution of the front molecular orbitals of the active molecules was studied. And the relevant activity energy parameters were calculated, which included the highest occupied molecular orbital energy level (E_HOMO), the lowest unoccupied molecular orbital energy level (E_LUMO), the energy gap (ΔE) reflecting the molecular stability and reactivity, the ionization potential (I) representing how easily a molecule can be ionized (Aihara, 1999; Yoshidaa and Aihara, 1998), the electron affinity (A) describing the released energy when a molecule acquires an electron, the electronegativity (χ), the global hardness (ζ), and the electron transfer capability (ΔN) reflecting the ability of organic molecules sharing electrons with metal atoms (Nabajyoti et al., 2019). The calculation formulas are as follows (Wang et al., 2023c; Wu et al., 2023b):

The electron-rich sites in organic molecules, such as heteroatoms O and N, multiple bonds, aromatic rings, contain lone-pair and/or π electrons which can be provided to the unfilled 3d orbitals of Fe (Madaci et al., 2023b). In a similar manner, the lone-pair electrons in organic molecules act as electron acceptors, receiving electrons from the filled 3d orbitals of Fe. This electron sharing and the coordination bonds formed between organic molecules and Fe atoms contribute to the chemical adsorption process (Sirsendu et al., 2021; Wang et al., 2023c). Organic compounds are consequently adsorbed on the MS surface, forming a protective film to impede the contact of the corrosive medium with the metal. The site centers in organic molecules play a predominant role in governing the binding between organic molecules and metal atoms through the interactions of electron donor and electron acceptor (El-Mokadem et al., 2023; Lgaz and Lee, 2023). Based on the DFT calculations, the impact of protonation of various heteroatoms in active ingredients on the molecular geometric equilibrium configuration was analyzed. The optimal protonation atom and the corresponding equilibrium configuration were identified, as shown in Fig. 17.

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Fig. 17

Geometric equilibrium configurations of the optimal protonated components in ZMBE.

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Fig. 18 shows the distributions of HOMO and LUMO for active ingredients of hydroxycinnamic acid, dimboa, glucuronic acid, and rutin under the neutral state. In the case of neutral hydroxycinnamic acid, the HOMO and LUMO distributions are spread across the entire molecular configuration, encompassing the benzene ring, hydroxyl, carboxyl, and double bonds. Similarly, for neutral dimboa, HOMO and LUMO are also distributed across the whole molecule, including two aromatic rings, two hydroxyls, and one carbonyl. These regions are rich in π electrons and/or lone-pair electrons, enabling both neutral hydroxycinnamic acid and dimboa to donate and accept electrons from Fe atoms. The synergistic adsorption effect involves electron donation and acceptance, facilitating electron-sharing coordination with Fe atoms on the MS surface. For neutral glucuronic acid, the distributions of HOMO and LUMO are slightly smaller. HOMO primarily concentrates on carbonyl and its adjacent two hydroxyls, while LUMO is mainly distributed on carbonyl and its adjacent four hydroxyls. In the case of neutral rutin, HOMO and LUMO are predominantly centered on benzene ring 1, aromatic ring 2, and benzene ring 3, as well as on hydroxyls attached to benzene rings 1 and 3 and carbonyl attached to aromatic ring 2. These aromatic rings, hydroxyls, and carbonyls are capable of receiving electrons from Fe ions, forming stable chelates and producing chemical adsorption effect. Due to the relatively large spatial distortion structure of neutral rutin molecules, they exhibit greater directionality in the binding process with the surface (Rachid et al., 2022).

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Fig. 18

Frontier molecular orbital distributions of typical active ingredients in ZMBE at the neutral state.

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Fig. 19 shows the distributions of HOMO and LUMO for the protonated active ingredients in ZMBE. Notably, protonation has little impact on HOMO and LUMO of hydroxycinnamic acid and dimboa. It suggests that hydroxycinnamic acid and dimboa, in both neutral and protonated states, retain the ability of electron donation and acceptance. In addition, the protonated hydroxyls in hydroxycinnamic acid and dimboa carry a positive charge, establishing electrostatic attraction with the negatively charged chloride ions initially adsorbed on the MS surface, leading to physical adsorption on the MS surface. Moreover, the polar functional groups in hydroxycinnamic acid and dimboa can also undergo physical adsorption through dispersion forces, induction effects, and other mechanisms. Consequently, hydroxycinnamic acid and dimboa experience both chemical and physical adsorption through the synergistic effect of multiple channels. The protonation of glucuronic acid has a slight impact on its electron-donating ability, but it enhances electron-accepting capability.

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Fig. 19

Frontier molecular orbital distributions of the typical active ingredients in ZMBE in the protonated state.

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This results in a strengthened synergistic adsorption effect for glucuronic acid after protonation. Given its more favorable protonation in an acidic environment, glucuronic acid provides an improved corrosion protection for MS. However, under neutral and alkaline conditions, the protonation of glucuronic acid is limited, which will affect its corrosion inhibition function. For rutin, the HOMO distribution remains largely unaffected by protonation, while the LUMO distribution is slightly weakened on aromatic rings 1 and 2 and enhanced on heterocycle 4. This variation may be attributed to the spatial distortion inherent in the large molecule of rutin (Qiang et al., 2018).

Table 8 presents the reaction activity energy parameters of typical ingredients in ZMBE. A larger E_HOMO implies a stronger electron-donating ability of the organic molecule, while a smaller E_LUMO indicates a stronger capability to accept electrons. In the neutral state, there is no significant difference in E_HOMO among the four active ingredients, with dimboa and rutin having slightly higher E_HOMO values. The variation in E_LUMO among four ingredients is smaller than 1 eV, with rutin and hydroxycinnamic acid exhibiting slightly lower E_LUMO values. After protonation, the decrease in E_LUMO is more pronounced than that in E_HOMO. The reduction in all E_HOMO values is less than 1 eV, while E_LUMO of hydroxycinnamic acid, dimboa, glucuronic acid, and rutin is reduced by about 1.53, 3.40, 2.40, and 0.35 eV, respectively. The changes in E_HOMO and E_LUMO suggest that the protonation of active ingredients weakens their electron-donating ability, but it significantly enhances their electron-accepting ability, with the exception of rutin. In the protonated state, E_HOMO for hydroxycinnamic acid, dimboa, and rutin is similar, and E_LUMO for hydroxycinnamic acid, dimboa, and glucuronic acid is comparable. In both neutral and protonated states, the mechanism through which the active ingredients share electrons with Fe atoms and form coordination bonds is altered to some extent. However, they ultimately result in the formation of the coordination compounds by the synergistic effect of chemical adsorption and physical adsorption, effectively blocking the contact between MS and the solution and suppressing the corrosion reaction.

ΔE serves as an indicator of electron stability and chemical reactivity for organic molecules (El-Faydy et al., 2022; Prasad et al., 2022b). A smaller ΔE suggests a greater activity of organic molecules, a higher propensity for electron sharing with Fe atoms, and a stronger interaction with the MS surface. As shown in Table 8, ΔE for neutral hydroxycinnamic acid and rutin is relatively smaller than that of dimboa and glucuronic acid, primarily attributed to the smaller E_LUMO of the former two. After protonation, all ΔE for the four ingredients decreases. While the decrease in rutin is relatively small, dimboa exhibits the most significant reduction (about −2.92 eV), owing to the substantial decrease in E_LUMO (near −3.40 eV). Except rutin, the reactivity of the other three ingredients significantly increases after protonation. In neutral and alkaline environments, protonation of organic molecules is difficult, and the protonated molecules might not exist. For rutin, its chemical reactivity is almost unaffected by its states. Therefore, the electron sharing and the formation of the adsorption film of rutin is less susceptible to the pH of the solution. Rutin is likely to exhibit excellent corrosion inhibition properties in neutral and alkaline media. This is distinguished from hydroxycinnamic acid, dimboa, and glucuronic acid which may provide effective corrosion inhibition primarily in acidic conditions.

ΔN reflects the electron transfer ability of organic molecules to metal atoms (Kellal et al., 2023; Kumar et al., 2022). When ΔN is smaller than 3.6 eV, a larger ΔN indicates a higher reactivity and a stronger electron-donating capability of organic molecules. A positive ΔN implies that the organic molecules primarily act as the electron donors to metal atoms. Conversely, a negative ΔN indicates that the organic molecules tend to receive electrons from metal atoms (Singha et al., 2017; Swathi et al., 2023). In both neutral and protonated states, ΔN of four ingredients is positive, with dimboa and rutin exhibiting the highest ΔN. After protonation, dimboa, glucuronic acid and hydroxycinnamic acid tend to receive electrons, and dimboa has the most significant change. ΔN of rutin is the least variable. This further confirms that the electron-donating and electron-accepting ability of rutin is almost unaffected by protonation.

According to the maximum hardness principle, there is a correlation between chemical reactivity and ζ (Kaya and Kaya, 2015). A larger ζ implies a greater chemical stability of the chelates, an enhancement of the charge transfer resistance, and a stronger corrosion inhibition capability (Hu et al., 2016c). As indicated in Table 8, the stability of the complexes formed by neutral molecules with Fe ions is high, but it overall decreases after protonation. Rutin is a special case in which the stability of the corresponding chelates remains almost unchanged.

χ serves as an indicator of the electron-attracting ability of organic molecules (Mrani et al., 2023; Rahma et al., 2023; Wang et al., 2022d). A larger χ indicates that the formed chemical bonds are less stable, which are unfavorable for the protection of the metal surface. μ is related to the polarity of organic molecules (Bayram et al., 2018; Mustafa et al., 2019; Wang et al., 2022d). Generally, a higher μ corresponds to a stronger intermolecular interaction, which favors the adsorption of organic molecules on the metal surface through electron forces and provides a better corrosion inhibition (Ech-Chebab et al., 2022; Zhang et al., 2023b). As shown in Table 8, at both neutral and protonation states, rutin exhibits the highest μ, suggesting the most significant corrosion inhibition effect on MS.

4.6.2 MD calculations

MD simulations were conducted to further understand the interaction between organic molecules and the MS surface and to clarify the corrosion inhibition and adsorption mechanisms. The adsorption energy (E_ads) was calculated (Hou et al., 2021; Tan et al., 2024a):

Fig. 20 and 21 present the snapshots of the side and top views of the final equilibrium configurations of four active ingredients on the Fe(1 1 0) plane. In both neutral and protonated states, hydroxycinnamic acid and dimboa are adsorbed in a fully parallel manner, aligning with the results of HOMO and LUMO which distribute on the entire molecular configurations. In the case of neutral glucuronic acid, apart from one hydroxyl and one C–C bond, the rest are adsorbed parallel to the Fe(1 1 0) plane, different from the results of HOMO and LUMO distributions. It can be attributed to the chelation reactions between carboxyl and Fe ions, along with the physical adsorption caused by the dispersion and induction forces between the polar hydroxyls and the Fe(1 1 0) surface (Tan et al., 2023b). For protonated glucuronic acid, the entire molecule is adsorbed parallel to the Fe(1 1 0) surface, resembling LUMO distribution but differing from HOMO distribution. Therefore, the protonated glucuronic acid undergoes both physical and chemical adsorption, as described above.

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Fig. 20

Snapshot of side view of final equilibrium configurations of the active ingredients on the Fe(1 1 0) plane.

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Fig. 21

Snapshot of top view of final equilibrium configurations of the active ingredients on the Fe(1 1 0) plane.

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The positively charged protonated oxygen atom in carboxyl creates electrostatic attraction with the negatively charged chloride ions initially adsorbed on the Fe(1 1 0) surface, further promoting the physical adsorption. For neutral rutin, benzene rings 1 and 3, along with three heterocycles, are adsorbed parallel to the Fe(1 1 0) plane, while some hydroxyls deviate, corresponding to the DFT results. In the case of protonated rutin, benzene rings 1 and 3 are still adsorbed parallel to the Fe(1 1 0) surface, while heterocycles 4 and 5 are lifted off. This may be attributed to the distortion of the rutin molecule, where heterocycle 4 and benzene 3 have a smaller bond angle in their spatial distribution, making them more compact and mutually exclusive when receiving electrons (Dutta et al., 2017).

Table 9 shows E_ads of the active ingredients on the Fe(1 1 0) plane. All E_ads is negative, indicating a spontaneous adsorption process of the active molecules on the most stable Fe(1 1 0) plane. A lower E_ads value indicates a stronger adsorption performance (Berrissoul et al., 2021b). E_ads of rutin is 329.8 eV in the neutral state, and it is 375.6 eV in the protonated state, obviously smaller than that of glucuronic acid, hydroxycinnamic acid, and dimboa. This difference can be attributed to the number of aromatic rings in the molecules. Glucuronic acid lacks benzene ring, hydroxycinnamic acid has one aromatic ring, dimboa has two aromatic rings, while rutin contains five aromatic rings. The aromatic rings directly influence the adsorption capability and the corrosion inhibition performance of the organic molecules (Lin et al., 2023b; Sergio et al., 2010).