Corrosion inhibition properties of spinach extract on Q235 steel in a hydrochloric acid medium

2.1 Extraction of AESPE

The spinach was shaded dried and reduced to powder. spinach powder (50 g) was soaked in a 1000 mL of the mixed solution of anhydrous ethanol and acetone with a volume ratio of 1:1, and refluxed at 323 K for 2 h. The obtained mixed liquid was filtered to take away the residue of AESPE. The resulting liquid was concentrated to 40 mL and dried at 323 K for use. Extraction and anti-corrosion application of AESPE are shown in Fig. 1.

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Fig. 1

Illustration of the extraction of AESPE and its anti-corrosion application.

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2.2 Corrosion inhibition action test of AESPE by WTL, EIS, LPR and PDP

The concentrations of AESPE were 0 ∼ 300 mg·L⁻¹, in the weightlessness and electrochemical tests. The WTH adopted a Q235 steel sample of 72.4 mm × 11.5 mm × 2.0 mm with a superficial area of 20 cm².

According to the literature (Zhang et al. 2022),V_corr and the corrosion inhibition efficiency (E%) was calculated in the test of WTL. The calculation formulas of V_corr and E% are described as formulas (1) and (2).

The corrosion rate:

Where, Δm is a mean weight lost values of sample, S is a area values of sample, and T is an soak period of sample. the units of V_corr, Δm, S and T are $\mathit{mg}·c{m}^{-2}·h^{-1}$ , mg, cm² and h, respectively.

The corrosion inhibition efficiency (E (%)):

Where, E is the corrosion inhibition efficiency, V₀ and V₁ are corrosion rate in hydrochloric acid with or without AESPE, respectively.

According to reference (Silva et al. 2021), the electrochemical tests (ET) were proceeded by a Zennium Pro constant potential apparatus. The 2 mm diameter cylindrical Q235 sample was the working electrode (WE) and the rest were sealed with polytetrafluoroethylene. Platinum electrode and Hg/Hg₂SO₄ electrode were separately the counter electrode (CE) and the reference electrode (RE). The WE was polished with a SiC sandpaper (grade 800 ∼ 1200). Before every test, WE was soaked in the liquid for 30 min to get E_ocp. A curve of PDP was tested under the conditions of 303 K, a voltage range and a sweep rate are ± 100 mV and 0.1 mV·s⁻¹, respectively. AC voltage and a range of frequency in tests of EIS are 10 mV and from 10⁵ Hz to 0.01 Hz, respectively. LPR was measured at a linear sweep scope of ± 10 mV with a sweep rate of 0.1 mV·s⁻¹. Data of the polarization and impedance were analyzed by the Zahner Analysis software.

2.3 Structure analysis of AESPE and surface analysis of test samples

The structure of AESPE was analyzed by FTIR (VERTEX7O) and UV–vis (UV-2550). SEM and EDS (A SIGMA 300) were used to analyze the surface appearance and film composition of test sample which were soaked for 8 h. The wettability of the above samples was measured using CAT (JC2000 DM). It was proved that AESPE adsorbed on the surface of the metal samples formed an adsorption film by SEM, EDS and CAT, thus proving that AESPE inhibited the erosion of the metal samples.

2.4 Quantum chemical calculations (QCC)

To get a theoretical Understanding the interaction of the chlorophyll molecule with carbon steel, QCC of the chlorophyll molecule were realized in the framework of the density functional theory (DFT) by ORCA 4.0 program package (Neese 2018). The Becke’s three-parameter hybrid exchange functional with the Lee-Yang-Parr non-local correlation (B3LYP)(Becke 1993) was employed as the choice for exchange–correlation potential. Grimme’s empirical dispersion with Becke-Johnson damping (Grimme et al. 2011) was used to describe the weal intramolecular interactions. The split valence and triple zeta basis sets (def-TZVP) was chosen as basis sets throughou(Schaefer et al. 1994,Schaefer et al. 1992). Optimized geometries were ascertained by vibrational analysis with no imaginary frequency mode.

3.1 Test analysis of AESPE

The FTIR spectra of AESPE and chlorophyll are shown in Fig. 2. The peak at 1065 cm⁻¹ corresponds to the pyrrole rings in AESPE. The strong absorption at 1400 cm⁻¹ and two moderate absorptions at 1631 and 1739 cm⁻¹ confirmed the existence of the C = O and C = C double bond groups in AESPE. The peak at 2927 cm⁻¹ corresponded to the stretching and anti-stretching peak of C–H in the aldehyde group of chlorophyll. The wide absorption at 3149 cm⁻¹ confirmed the presence of –OH in AESPE. These results revealed that AESPE mainly contained porphyrin ring structures with double bond groups (C = O, C = C and C = N), side-chain aldehyde groups and hydroxyl groups, as in chlorophyll (Wu et al. 2016, Sun 2011). By comparison with the chlorophyll spectrum, it was inferred that the main component of AESPE was chlorophyll.

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Fig. 2

FTIR spectra of AESPE and chlorophyll.

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The UV–vis spectra of chlorophyll and AESPE in hydrochloric acid solution were compared (UV-2550 UV spectrometer, TECHCOMP LIMITED), with the results displayed in Fig. 3. Two peaks appeared at 664/673 nm and 410/416 nm for chlorophyll in [x solution] and in hydrochloric acid (Fig. 3b). The characteristic absorption peak at 664/673 nm indicated that chlorophyll absorbs red light while the characteristic peak at 410/416 nm indicated that chlorophyll absorbs blue-violet light (Wu et al. 2016). As can be seen from Fig. 3a, two characteristic absorption peaks also appeared for the AESPE solution at 414/423 nm and 664/675 nm regardless of the presence of hydrochloric acid medium, further indicating that the main chemical component of AESPE was chlorophyll and, in contrast to the results shown in Fig. 3b, its structure was basically unchanged in hydrochloric acid.

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Fig. 3

UV–vis spectra of AESPE and chlorophyll.

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3.2 AESPE corrosion inhibition action test

The corrosion inhibition action test using AESPE was studied by WTL with different concentrations of AESPE and at different temperatures. The results are displayed in Table 1. When the amount of AESPE increased from 50 to 300 mg·L⁻¹ at 293 ± 3–5 K, E% increased from 48.2% to 95.3%, indicating that V_corr decreased and E% increased as the amount of AESPE increased. E% decreased from 95.3% to 88.6% when the test temperature increased from 293 ± 3–5 to 323 ± 3–5 K with 300 mg·L⁻¹ of AESPE. This showed that increasing the temperature increased the rate of metal corrosion and led to a decrease in E%, possibly because the dissociation and adsorption equilibrium of AESPE molecules were affected by temperature. The higher the temperature, the more difficult the adsorption of AESPE became, resulting in the formation of a lower-density film and lower E% (Zhang et al. 2017). However, when the temperature was increased to 323 ± 3–5 K, the E% was still as high as 88.6%, indicating that AESPE had good adaptability to temperature changes.

3.3 Corrosion kinetics analysis

The activation energy (E_a), activation entropy (ΔS⁰_a) and activation enthalpy (ΔH⁰_a) were used to study the kinetics of AESPE. E_a was obtained from the Arrhenius formula (3)(Ituen et al. 2019):

Here, V represents the V_corr of the Q235 steel sample (Table 1) and T represents the experimental temperature (K).

The relationship between lgV and 1/T is displayed in Fig. S1(a). The values of E_a are shown in Table 2. The change in the slope of the curve indicated that the E_a of the oxidation reaction of the Q235 steel sample changed upon the addition of AESPE. In this experiment, E_a increased from 31.43 to 53.53 kJ⋅mol⁻¹ when AESPE was added to the hydrochloric acid solution. This was possibly because the formation of a metal complex upon the adsorption of AESPE blocked the oxidation reaction sites, increasing the E_a required for the metal oxidation reaction.

The transition parameters of the sample dissolution process, ΔS⁰_a and ΔH⁰_a, were obtained from the corrosion rate (V) via the weightlessness method (Ituen et al. 2019) using formula (4):

Here, N_A is 6.022 × 10²³ mol⁻¹, h is 6.626 × 10^–34 J⋅s, R is 8.314 J⋅K⁻¹⋅mol⁻¹ and T (K) is temperature.

The relationship between lg(V/T) and 1/T is displayed in Fig. S1(b) and the fitted data are displayed in Table 2. The values of ΔH0a increased with the addition of AESPE. When AESPE reached 300 mg·L⁻¹, the value of ΔH⁰_a increased from 27.09 to 50.97 kJ⋅mol⁻¹. At each AESPE concentration, ΔH⁰_a was positive, indicating that all corrosion reactions were endothermal. With the addition of AESPE, the rate of the steel corrosion reaction decreased because it required more energy.

Ideally, E_a and ΔH⁰_a should be equal. When E_a is greater than ΔH⁰_a, gases are released in the corrosion process; that is, hydrogen is evolved. The parameters in Table 2 show that the difference between the two values (E_a – ΔH⁰_a) was nearly constant in all cases, with its average value equal to the value of RT (2.56 kJ⋅mol⁻¹). Therefore, the number of molecules (χ) in the equation E_a – ΔH⁰_a = χRT was 1 (Silva et al. 2012), which proved that the rate-determining step (RDS) was a single-molecule reaction. The negative value of ΔS⁰_a showed that the activation complex in the RDS formed through association rather than dissociation. The decrease in ΔS⁰_a was caused by the adsorption action of substances that migrated from the solution to the sample surface by electrostatic forces. As a result, the number of freely moving particles in the solution decreased.

When the concentration of AESPE was 300 mg⋅L^–1, ΔS⁰_a increased from –207.43 to –151.87 J⋅K⁻¹ ⋅mol⁻¹. A few polar molecules and ions on the surface of the sample exchanged with AESPE molecules, releasing the previously adsorbed particles into the solution. The addition of AESPE led to the desorption of H₂O, H₃O⁺ and other particles from the sample surface or they could not be adsorbed to the sample surface, which promoted the increased ΔS⁰_a of the system. The adsorbed AESPE molecules played a role in spatial resistance at the interface between the sample and the solution, which reduced the capacity of other particles (mainly H₃O⁺) to adsorb and re-adsorb, leading to a decrease in the cathodic reaction rate and thus a slower corrosion rate (Hamilton-Aamachree et al. 2020).

3.4 Adsorption model

The mechanism of interaction between the CI and the sample surface was analysed by adsorption isotherms. The chemical components of the CI, the electrolyte, the metal surface properties and the medium temperature, etc., can affect the adsorption isotherm. Therefore, various isotherms such as Bockris-Swinkels, Langmuir, Freundlich and Temkin etc. were used (Mohammad et al. 2021).

The adsorption action of CI molecules can be classified as physical or chemical adsorption. The former arises from the electrostatic interaction between a charged molecule of CI and the charged sample surface. The latter is a process in which CI heteroatoms provide lone-pair electrons to form a coordinate bond (CB) with the empty orbitals of the metal atoms (EOMA). The adsorption process in the current study can be described by the following process (Tan et al. 2021(b)): $$\mathit{AESP}{E}_{(sol)}+\chi H_2{O}_{(ads)}\iff AESP{E}_{(ads)}+\chi H_2{O}_{(sol)}$$

Here, $H_2{O}_{(ads)}$ is an adsorbed water molecule and $\mathit{AESP}{E}_{(ads)}$ is an adsorbed molecule of AESPE. Thus, the formation of an adsorption thin film at the sample/solution interface is a process of substituting water molecules.

To further study the interaction mechanism of AESPE at the sample/solution interface, Langmuir and Temkin isotherms were fitted with the data from the weightlessness test data. The results are displayed in Fig. S2, S3 and S4.

1. Langmuir isotherm

Here, K_ads is the equilibrium constant.

The relationship between C/θ and C is shown in equation (5). Fig. S2 shows the adsorption isotherm of AESPE. K_ads was obtained from the intercept of the line. The standard adsorption free energy (ΔG⁰_ads) is shown in equation (6)(Mohammad et al. 2021):

Here, the pure water concentration (C_H2O) is 10³ g⋅L^–1. The mean value of ΔG⁰_ads was –25.16 kJ⋅mol⁻¹ (Table 3). According to the literature (–40 kJ⋅mol⁻¹ < ΔG⁰_ads < –20 kJ⋅mol⁻¹, (Wang et al. 2019), this indicated that the adsorption of AESPE was due to the coexistence of physical and chemical adsorption and that this adsorption was spontaneous. When the AESPE molecule approached the surface of the sample, the electrostatic attraction was unquestioned; this can be viewed as physical adsorption. The coordination behaviour of the lone-pair electrons of the AESPE molecules and EOMA suggested the presence of chemisorption.

The standard adsorption enthalpy and entropy (ΔH⁰_ads and ΔS⁰_ads) can be calculated from equation (7) (Mohammad et al. 2021).

Here, ΔH⁰_ads andΔS⁰_ads come from the slope and intercept of the line in Fig. S3, with values of –3.91 and 69.12 J⋅K⁻¹⋅mol⁻¹, respectively. ΔH⁰_ads was negative, indicating that the adsorption of AESPE was exothermic. ΔS⁰_ads > 0 indicated that the adsorption of AESPE was an entropy-increasing action. Generally, the adsorption action of a CI on a sample surface is an entropy-decreasing process but, in this experiment, the adsorption of the CI was a process of increasing entropy. This was possibly due to the desorption of water molecules and the adsorption of AESPE, with multiple water molecules desorbing for every one molecule of AESPE that adsorbed on the Q235 surface. This substitution of multiple H₂O or H₃O⁺ molecules by one AESPE molecule increased the degree of chaos; that is, the entropy of the whole system increased, with ΔS⁰_ads > 0 (Wang et al. 2019, Ateya et al. 1984).

1. Temkin isotherm

The Temkin isotherm can be described by equations (8) and (9):

Here, a is the molecular interaction parameter. The relationship between θ and log C is displayed in Fig. S4. The parameters in Table 4 were calculated according to the Temkin isotherm in Fig. S4. At different temperatures, all values of a were negative, indicating a repulsive force between the adsorbed molecules. The R² value from the fitted Temkin adsorption curve showed that there was a modest correlation between θ and log C for the corrosion inhibition system. Therefore, The Langmuir adsorption isotherm provided a better fit for the test data than the Temkin isotherm. Thus, the Langmuir isotherm was more suitable for describing the corrosion inhibition action of AESPE, which confirmed that AESPE participated in a single-layer adsorption process in the hydrochloric acid system (Tan et al. 2021(a)).

3.5 Polarisation test

The instantaneous corrosion rate and the electrochemical reaction mechanism were studied using the polarisation method. The E_OCP, PDP and LPR curves for the Q235 sample are displayed in Fig. 4 and Fig. 5.

1. E_OCP tests

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Fig. 4

E_ocp-t curves of Q235 steel with and without AESPE in 0.5 M hydrochloric acid.

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Fig. 5

Polarisation curves of the Q235 sample in hydrochloric acid with or without AESPE.

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Fig. 4 shows the E_OCP curves. The curve stabilised after the Q235 sample had been immersed in the acid solution for 30 min, indicating that the sample surface reached a stable state. Additionally, E_OCP stabilised at − 957 mV in hydrochloric acid without AESPE, as a consequence of the dissolution of the Q235 sample in the acid. With the addition of AESPE, the E_OCP curves tended to move towards the anode, indicating that the adsorption of AESPE inhibited the active sites of the anodic reaction, thus showing excellent corrosion inhibition effect (Chen et al. 2020).

1. LPR tests

The polarisation resistance (Rp) values derived from the LPR tests are displayed in Table 5. The E (%) value can be calculated using equation (10):

Here, R⁰_p and R_p are the polarisation resistance in hydrochloric acid without AESPE or with AESPE, respectively.

The R_p gradually increased as the AESPE concentration increased (Table 5). The R_p decreased from 26.09 Ω·cm² in 0.5 M hydrochloric acid containing 300 mg·L^-–1 AESPE to 2.52 Ω·cm² at 0.5 M hydrochloric acid (blank). The R_p increased in the system with AESPE, indicating that a non-conductive adsorption film was formed in the interface between the Q235 sample and the solution (Mourya et al. 2014).

1. PDP tests

Fig. 5 shows the polarisation action of the Q235 sample. The E (%) value of AESPE was derived using equation (11) and the results are shown in Table 5.

Here, i_corr and i⁰_corr are the current densities in 0.5 M hydrochloric acid with AESPE and without AESPE, respectively.

As the concentration of AESPE increased, the anodic and cathodic curves shifted towards lower i_corr values, resulting in a decrease in i_corr. AESPE significantly inhibited both the cathodic and anodic reactions of the Q235 sample electrode. The corrosion potential (E_corr) shifted towards a higher potential with the addition of AESPE, indicating that more AESPE molecules were adsorbed on the anode, thus preventing the anodic reaction. In general, an absolute displacement of E_corr of less than 85 mV compared to the E_corr in the blank solution is indicative of a mixed corrosion inhibition mechanism(Tan et al. 2021 (b)).In this work, the maximum displacement was 42 mV, indicating that AESPE was a mixed-type CI. The Tafel slopes of β_a and β_c showed similar changes in the AESPE system, indicating that the reaction mechanism of the sample did not change. The inhibition mode of organic CIs can be classified into three types: the geometric covering effect (GCE) of adsorbed CIs, the blocking effect of active sites (BEAC) of adsorbed CIs, and the electrocatalytic effect (EE) of CIs (Cao. 1996). In the GCE, the protective effect derives from a reduction in the reaction area of the corroded sample. In the other two modes (BEAC and EE), the inhibition effect derives from the change in E_a of the anodic and cathode reactions during the corrosion reaction. According to the polarisation reaction results, the addition of AESPE resulted in changes in the βa and βc, indicating that the inhibition was probably caused by two modes (GCE and BEAC). The similar cathodic Tafel line in Fig. 5 showed that AESPE did not change the hydrogen evolution reaction mechanism and the reduction of H⁺ ions; but mainly occurred through the charge transfer mechanism (Chen et al. 2020).

3.6 EIS test

EIS test results can provide information on the sample/electrolyte interface reaction. In this work, EIS tests were conducted on Q235 samples in 0.5 M hydrochloric acid with or without AESPE. The results are shown in Fig. 6 and Fig. 7. The Nyquist diagram contained an irregular semicircle for each condition in Fig. 6, which may have been due to the non-uniform surface of the Q235 sample. The semicircle diameter increased significantly as the AESPE increased, indicating that AESPE improved the E%, which may have been related to the adsorption of the chlorophyll from AESPE on the Q235 sample. As shown in the Bode diagram (Fig. 7), an increase in the concentration of AESPE increased the Bode modulus impedance and phase angle, indicating a delay in the corrosion reaction process.

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Fig. 6

Nyquist diagrams of the Q235 sample in hydrochloric acid with or without AESPE.

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Fig. 7

Bode diagrams of the Q235 sample in hydrochloric acid with or without AESPE.

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An equivalent circuit in Fig. S4 was fitted by the EIS data and the parameters are displayed in Table 6. Therein, R_s and R_ct are the solution resistance (SR) and the charge transfer resistance (CTR), respectively. CPE reflects the replacement of the double-layer capacitor by the constant phase element. The χ² values in Table 6 ranged from 0.00259 to 0.00927, indicating that the circuit in Fig. S4 agreed with the EIS data.

The value of E% could be calculated using equation (12):

Here, R_ct and R_ct⁰ are the CTR in the presence and absence of AESPE, respectively.

Z_CPE is defined by equation (13):

Here, Y₀ is the CPE constant, j is the imaginary root, n is the offset—which is defined as the deviation from the ideal value, usually between 0 and 1, and ѡ is the angular frequency.

The interface dual capacitance (C_CPE) value was obtained from equation (14):

Here, f_max(–Z“_(max)) is the maximum value of the imaginary component of impedance.

The R_ct value in the system containing AESPE was much larger than the R_ct0 value in the system without AESPE. The effectiveness of AESPE could be ascribed to the adsorption of chlorophyll molecules. Chlorophyll would coexist as both the molecular and protonated form in the acidic medium. The relative concentrations of these two forms would depend on the degree of protonation. It is possible that coordination bonds formed between the lone-pair electrons or conjugated double bonds of the O and N atoms of the chlorophyll molecule (or its protonated form) and EOMA in the Q235 sample. Thus, adsorption occurred, which plays a better corrosion inhibition performance. This was in accordance with the E (%) values calculated by the weightlessness method and the two different electrochemical calculations, above.

3.7 Surface analysis

Surface analysis technologies were used to describe the surface state of the Q235 sample and prove the adsorption of AESPE. In this work, the SEM, EDS and CAT surface analysis methods were used. Fig. 8(a, c, e) shows the appearance of the Q235 sample surface before corrosion, in hydrochloric acid and in hydrochloric acid with 300 mg·L^-1 AESPE. Under 1000x magnification, the surface of the Q235 sample before corrosion had mechanical grinding scratches and was smooth. The surface of the sample soaked in hydrochloric acid was seriously corroded. After the addition of AESPE, there was almost no accumulation of corrosion; rather, the sample was covered with a protective layer. The chlorophyll molecules in AESPE adsorbed on the Q235 sample, preventing the diffusion of corrosive ions such as Cl^–, thereby decreasing the extent of the corrosion reaction. This proved that AESPE had a good protective effect on the metal Q235 sample in hydrochloric acid.

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Fig. 8

SEM images and EDS spectra of Q235 steel surface. (a, b) New uncorroded sample, (c, d) sample in hydrochloric acid, (e,f) sample in hydrochloric acid with AESPE.

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To provide insight into the adsorption of AESPE on the surface of the Q235 sample, EDS analysis was performed on new uncorroded samples and samples in hydrochloric acid with or without AESPE. The results are displayed in Fig. 8b, d and f. Fig. 8d shows that the oxygen content of the sample in hydrochloric acid solution increased compared with that prior to corrosion (Fig. 8b). The hydrochloric acid solution promoted the oxidation reaction, generating more iron oxides from corrosion and resulting in increased oxygen content. Significantly, corrosive Cl^– ions appeared in the EDS spectra of the Q235 sample surface in hydrochloric acid. However, when AESPE was present in the test solution (Fig. 8f), the characteristic O peak was considerably reduced, indicating that very few oxides were produced on the sample surface due to corrosion. In fact, the oxygen content was almost the same as that before corrosion.

These test results showed that the adsorption of molecules of AESPE on the surface of the sample prevented the diffusion of Cl^– and reduces the formation of corrosion products (Fe₂O₃, FeCl₃ and other complexes). Consequently, the surface of the Q235 sample was highly resistant to the diffusion of strong acid ions, increasing the values of E%. These test results were in accordance with the results obtained by SEM.

The contact angle, which is the angle formed between a sample surface and a liquid–vapour interface, can be used to characterise the wettability of a sample surface. In this work, the hydrophobicity of the sample surface before and after corrosion was ascertained through the contact angle, and thus the adsorption characteristics of AESPE on the Q235 sample surface were verified. As displayed in Fig. S6a, for newly polished Q235 steel, the observed contact angle was very large (102°), indicating that the sample had good hydrophobicity. As shown in Fig. S6b, after the sample was immersed in hydrochloric acid at 303 K for 8 h, the contact angle of the Q235 sample surface was 65°, indicating that the hydrophobicity had decreased. Fig. S6c shows that the contact angle of the sample with 300 mg⋅L^–1 AESPE was 94°, indicating that the surface hydrophobicity was somewhat reduced compared with the newly polished surface, but much greater than that in hydrochloric acid. This was due to the adsorption of AESPE molecules on the Q235 sample to form a hydrophobic layer (Mohammad et al., 2021). Therefore, the presence of AESPE could decrease the value of V_corr of Q235 steel and improve the value of E%.

3.8 Quantum chemical calculations

The electronic properties of an organic compound are important in predicting its reactivity on a sample surface. According to molecular orbital theory, the frontier orbital energy levels (E_HOMO and E_LUMO) are closely related to the reaction of a CI (Umoren et al. 2015, Zhang et al. 2018). Therefore, the E_HOMO and E_LUMO of a chlorophyll molecule are of great significance to the discussion of its interaction with the steel surface. As shown in Fig. S7, the HOMO and LUMO of a chlorophyll molecule are located in the porphyrin ring. This distribution of the molecular orbitals of a chlorophyll molecule provided more active sites for AESPE, enabling it to obtain more free electrons from the iron via the antibonding orbitals and to supply more electrons to the EOMA. E_HOMO and E_LUMO represent the ability of a molecule to supply electrons and to receive electrons, respectively (Hu et al. 2017). That is, a higher E_HOMO or a lower E_LUMO indicates that a CI molecule is more likely to share electrons with a metal sample, so the E% is higher (Zhang et al. 2018). On the other hand, the smaller the $\mathrm{\Delta }$ E ( $\mathrm{\Delta }$ E = E_LUMO – E_HOMO), the higher the reactivity of the system and the higher E% is (Umoren et al. 2015). The electron transfer percentage ( $\mathrm{\Delta }$ N) is the amount of electron transfer from the CI to the sample. When $\mathrm{\Delta }$ N is less than 3.6, the larger the $\mathrm{\Delta }$ N value, the higher E% is (Ghailane et al. 2017). $\mathrm{\Delta }$ N can be calculated based on η and χ, as shown in equations (15)–(17) (Zhang et al. 2018, Yilmaz et al. 2016).

Here, ƞ_inh and χ_inh represent the hardness and electronegativity of the CI, respectively. For iron, ${\chi }_{\mathit{Fe}}$ is 7 eV⋅mol⁻¹ and ${\eta }_{\mathit{Fe}}$ is 0 eV⋅mol⁻¹ (Obi-Egbedi et al. 2011, Yadav et al. 2010). The data are presented in Table 7. Not only did chlorophyll have high E_HOMO and $\mathrm{\Delta }$ N values, but it also had low E_LUMO and $\mathrm{\Delta }$ E values, which theoretically explained the corrosion inhibition ability of AESPE in hydrochloric acid. These QCC results further validated the experimental results.

3.9 Corrosion inhibition mechanism

In an acidic medium, the corrosion inhibition process usually derives from the adsorption of the CI on the sample. The experimental results showed that AESPE underwent both chemical adsorption and physical adsorption on the steel sample in hydrochloric acid, which made it a mixed-type CI. During the interaction between the Q235 steel and AESPE, on the one hand, negatively charged ions (such as Cl^–) in the acidic medium attracted positively charged AESPE ions through electrostatic attraction (physical adsorption). In the acidic solution, AESPE existed primarily as protonated ions, which attached directly to the cathode on the surface of the Q235 sample, thereby reducing the hydrogen evolution process. On the other hand, the anodic dissolution of the sample was reduced by chemisorption at the anode. Thus, the inhibitory mechanism of AESPE resulted from physical adsorption and chemical adsorption processes of the main component of AESPE(chlorophyll) on the sample surface.

3.10 Comparison of the corrosion inhibition action of AESPE with other CIs

A variety of CIs—including alanine and leucine (amino acids), TMA and TML (ionic liquids), and Mimosa pudica, LB.E, BLE (plant extracts) have been developed. Their corrosion inhibition properties are shown in Table 8. As a comparison, the corrosion inhibition properties of AESPE determined in the present study are also listed in Table 8. The E% of steel generally ranged from 56.6% to 94.50% under the experimental conditions of 298 to 303 K in hydrochloric acid solution. AESPE had a good anti-corrosion effect, with a maximum corrosion inhibition efficiency (ƞ_Max) of 93.2% at 0.3 g⋅L^–1 AESPE. The corrosion inhibition performance of AESPE was basically the same as that of other plant extracts but better than that of amino acids and ionic liquids. Additionally, as shown in Table 8: (i) AESPE was a good CI, considering the concentration of CI required and its corrosion inhibition efficiency; and (ii) plant-extracted CIs are increasingly favoured by researchers, based on the development history of environmentally friendly CIs. Therefore, AESPE is a good CI with bright application prospects.