Hairy bamboo leaf extract as an eco-friendly corrosion inhibitor for L245N steel in CO₂-saturated oilfield produced water

2.1 Preparation of BLE

Fresh bamboo leaves were collected in the Chengdu bamboo cultivation base, cleaned with distilled water, dried in the vacuum oven at 60 °C, and pestled to powder form. Afterward, the dried powder was soaked in ethanol and soaked at normal temperature. Then, the solution is ultrasonically treated three times with an ultrasonic cleaner at normal temperature, and then filtered under reduced pressure and concentrated to 50 mL by rotary evaporation. The specific values of the extracted parameters need to be further optimized, and the popular selection Box-Behnken design (BBD) in response surface methodology is applied. With regard to the extraction parameter boundary of plant corrosion inhibitor in the literature (Verma et al. 2018), the optimization ranges of soaking time, ethanol concentration, ultrasonic action time, and liquid material ratio are respectively set as 24–36 h, 40–100 %, 10–30 min and 10–30 mL/g.

2.2 Preparation of materials and solutions

The metal specimens used in this work are L245N steel for gathering and transportation pipelines collected on-site in the CO₂ flooding test area of the Changqing oilfield. The chemical composition of the L245N specimens (wt%) are as follows: C 0.24, Si 0.21, Mn 0.41, P 0.016, S 0.012, Ti 0.02, V ≤ 0.005, Nb ≤ 0.01, and balance of Fe. The L245N steel coupons studies on electrochemical and weight loss experiments had dimensions of 50 mm × 10 mm × 3 mm and 10 mm × 10 mm × 3 mm, respectively. The sample is sealed with epoxy resin, making 1 cm × 1 cm area in contact with corrosive solution for electrochemical test. Before the measurement, the steel specimens were polished with emery papers of 400, 600, 800, 1000, and 1200 grits, degreased with acetone, washed with deionized water and anhydrous alcohol, and dried under the cold wind. The aggressive solution was the simulated oilfield-produced water saturated with CO₂, and the species concentration (g/L) are as follows: NaCl 16.5577, KCl 0.5400, CaCl₂ 0.4500, MgCl₂·6H₂O 1.1178, Na₂SO₄ 0.3700, NaHCO₃ 1.6490. The test solution was prepared by using the aggressive solution (simulated oilfield produced water saturated with CO₂) and various concentrations (expressed by the volume ratio between the BLE and the aggressive solution was 1 %, 2 %, 3 %, 4 %, and 5 %) of the BLE. Take the blank solution without BLE as the control. Before the test, the corrosion solution needs to be deoxidized, and slowly inject nitrogen into the corrosion solution for 2 h to achieve the purpose of deoxidization. Use a constant temperature water bath to keep the experimental temperature constant. Before the experiment, the corrosive medium needs to be deoxidized in advance, and slowly inject nitrogen into the corrosive medium for 2 h to achieve the purpose of deoxidization. Use a constant temperature water bath to keep the experimental temperature constant, and when the temperature meets the experimental temperature, slowly introduce carbon dioxide for 2 h to saturate the corrosive medium to meet CO₂, and then start the experiment.

2.3 Weight loss measurements

This weight loss measurement includes static and dynamic weight loss. Each group was set up with three parallel specimens. First, the initial weight of the specimens was measured accurately. Then, the specimens were immersed in the aggressive solution presence and non-existence addition of various concentrations of BLE for 72 h, and the system is sealed. After the expiry of the immersed time, removed the specimens, cleaned with deionized water, and soaked with the configured pickling solution until no corrosion product on the metal surface. Then, the specimens were washed using anhydrous alcohol, dried, and weighed. The dynamic weight loss measurement conditions include the total pressure of 3 MPa, the partial pressure of carbon dioxide of 0.755 MPa, and the flow velocity of 0.1 m/s. The corrosion rate (v) and inhibition efficiency (η) were calculated as follows:

where v₀ and v_inh are the corrosion rate (in mm/a) presence and non-existence, respectively. M and M₁ correspond to the specimen weight (in g) before and after immersion in the tested solution. S is the gross area of the specimen (in cm²), T is the time (in h) of exposure, and D is the density (in kg m⁻³) of the specimen.

2.4 Electrochemical measurements

Electrochemical measurements were conducted in a PARSTAT 4000 electrochemical station with a classical three-electrode sealed system. The L245N steel specimen was used as the working electrode (WE), a platinum sheet was used as the counter electrode (CE), and a saturated calomel electrode was employed as the reference electrode (SCE). Electrochemical measurements were conducted at various temperatures (15 °C, 25 °C, 35 °C, 45 °C, and 55 °C). The working electrode was submersed in the test solution for 60 min to acquire a plateau of open circuit potential (E_ocp) before starting the EIS measurement. Then, the EIS measurements were conducted in the 100 kHz to 0.01 Hz frequency range at the open circuit potential (OCP) by applying a signal disturbance amplitude of 10 mV. The potentiodynamic curves were performed in the range of −200 mV–200 mV, and the scanning rate was 0.166 mV/s. ZSimpWin software was used to fit and analyze the electrochemical data. For the polarization test, the values of the inhibition efficiency (η) were estimated as follows:

where icorr0 and icorr1 denote the current densities of the working electrode (WE) with and without BLE inhibitor, respectively.

2.5 Surface analyses

Firstly, the prepared L245N steel specimens were submersed for 24 h in the aggressive solution with and without 4 % BLE at 15 °C and 55 °C. The specimens were then flushed with deionized water and dried under cold air, ready for testing. In the end, the resulting surface morphologies of samples were observed by applying the scanning electron microscope (SEM, Carl Zeiss Sigma 300, Germany).

2.6 Contact angle measurement

The hydrophilicity of the L245N steel specimen is evaluated by measuring the contact angle between the water droplet and the steel surface using the contact angle measuring instrument (JC200D8). The L245N steel sample is carefully cleaned to avoid surface contamination. The contact angle is the average of at least three measurements on different areas of the surface.

2.7 MD simulation

MD simulations were conducted by applying the Forcite module of the Materials Studio eight software to investigate the interaction process of inhibitor molecules and the metal surface (Berrissoul et al. 2022). The simulation box consists of a lower Fe (110) slab, a solution layer (containing 1000 H₂O and inhibitor molecules in the number of 1, 2, 4, 8, 16, 32, and 64), and an upper solvent layer (containing 500 H₂O). The Fe (110) surface was selected as the investigated surface because of its densely packed surface and excellent stability (Li et al. 2015; Obot et al. 2013). The Fe slab crystal included 10 layers, and seven layers close to the bottom of the slab were fastened. The Build layer in the Visualize module was applied to obtain the interfacial system model used in the simulation calculation. The whole system was performed by applying the COMPASS force field in an NVT ensemble at 45 °C (monitored by the Andersen thermostat) with 50 ps of total simulation time and 1.0 fs time step. Subsequently, the interaction energy of the corrosion inhibitor molecules on the Fe (110) surface was estimated by the following formulas (Zhu et al. 2017).

where E_ads was the adsorption energy (kJ/mol) of the corrosion inhibitor and iron surface, E_total represents the total energy (kJ/mol) of the iron crystal together with the adsorbed BLE molecule, E_molecular, and E_surface is the energy (kJ/mol)of the iron crystal and free BLE molecule, respectively.

3.1 Confirmation of extraction parameters

Firstly, the effect of soaking time on corrosion inhibition efficiency was studied, and the experimental conditions were set as follows: ethanol concentration 90 %, ultrasonic action time 30 min, liquid material ratio 20 mL/g. From Figure 1a, it appears that corrosion inhibition efficiency gradually increases with the increase of immersion time. At 36 h, the corrosion inhibition efficiency is 92.53 % and tends to be stable. The soaking time of 36 h, ultrasonic action time of 30 min, and liquid material ratio of 20 mL/g were set to study the effect of ethanol concentration on corrosion inhibition efficiency. Figure 1b shows that the inhibition efficiency gradually increases with the ethanol concentration. At the ethanol concentration of 90 %, the inhibition efficiency is the largest and up to 92.53 % tends to be stable. The soaking time of 36 h, the ethanol concentration of 90 %, and the liquid material ratio of 20 mL/g were set to investigate the effect of ultrasonic action time on corrosion inhibition efficiency. From Figure 1c, it appears that the corrosion inhibition efficiency first increases and then decreases with the increase of ultrasonic action time and reaches the stable corrosion inhibition efficiency in 25 min. The optimal values of the above, namely the immersion time of 36 h, ethanol concentration of 90 %, and ultrasonic action time of 25 min. Figure 1d shows that the corrosion inhibition efficiency increases first and then decreases with the decrease of liquid material ratio.

Figure 1:

Single-factor experimental results.

According to the experimental results, the initial optimization range of parameters: ethanol concentration 80–100 %, ultrasonic action time 10–30 min, liquid material ratio 10–30 mL/g, and soaking time 36 h. The number of ethanol concentrations (factor F₁), ultrasonic treatment times (factor F₂), and material liquiratiosio (factor F₃) were chosen as factors, and the maximum inhibition efficiency (response Yw) was defined as responses. Table 1 represents the coded and non-coded factors of Box-Behnken experiments for inhibition efficiency optimization. Levels −1, 0, and one represent the specified lower limit, center point, and superior limit of the variables.

According to the experimental results and regression analysis, establish a second-order response surface model to define the relationship between Y_w and variables. According to the final equation of coding factor, it can be expressed as:

where, Yw is a response surface model based on numerical results and regression analysis. Based on the fitted multiple quadratic regression equation, the optimal extraction parameters were obtained as follows: soaking time 36 h, ethanol concentration 92 %, ultrasonic action time 30 min, and liquid material ratio 13  mL/g. According to the multiple quadratic regression equation, the optimized inhibition efficiency prediction value is 98.20 %. The experimental verification shows that the maximum corrosion inhibition efficiency is 96.68 %, close to the model’s predicted value and used for subsequent experiments.

3.2 Potentiodynamic polarization

Figure 2 shows potentiodynamic polarization curves of L245N steel in the aggressive solution in the absence and presence of various BLE concentrations. Table 2 presents the values of related electrochemical parameters, for example, corrosion current density (i_corr), corrosion potential (E_corr), anodic and cathodic Tafel slope (β_a and β_c), and inhibition efficiency (η), gained from extrapolation of the potentiodynamic polarization curves at the temperature range of 15–55 °C. The surface coverage (θ) of the inhibitor molecules on the surface of the metals was calculated as (Sun et al. 2017; Wang et al. 2017):

where η is the inhibition efficiency.

Figure 2:

Potentiodynamic polarization curves of L245N steel in aggressive solutions containing various concentrations of BLE at differing temperatures.

As can be seen in Figure 2, at various temperatures, compared with the polarization curve of the blank solution, the addition of BLE transfered both the cathodic and anodic curves toward the lower current density value, and the corrosion potential of the system shifted positively. Thus, both anodic and cathodic reactions of the L245N steel corrosion were suppressed by the addition of the BLE in the aggressive solutions. In addition, with the increase in BLE concentration, the inhibition effect first increased and then decreased. As shown in Table 2, the i_corr value decreased first and then increased with the increase of BLE concentrations at various tested temperatures. The inhibitive efficiency showed a trend of increasing first and then decreasing as the BLE concentration increased. As the BLE concentration reaches a certain level, the BLE molecules will form agglomeration on the metal surface, resulting in the desorption amount greater than the adsorption amount, resulting in uncovered spots and reducing the corrosion inhibition effect (Zhichao et al. 2019). At the temperature of 45 °C and the concentration of 4 %, the corrosion inhibition efficiency (η) reached a maximum of 98.76 %. Additionally, no obvious modification in the shape of the polarization curves indicated that the addition of BLE did not change the mechanism of the L245N steel corrosion. The anodic action coefficient (f_a) and cathode action coefficient (f_c) were also given in Table 2 and defined as (Su et al. 2015b):

The action coefficient f_j (j = a, c) of the corrosion inhibitor is less than 1 (j = a, c), which indicates that the corrosion inhibitor can inhibit the electrode reaction at this potential, and the smaller f_j is, the stronger the inhibition on the electrode reaction is, the better the corrosion inhibition effect is. Table 2 shows that the calculated (f_a) and (f_c) are less than 1, indicating that adding BLE suppresses both anodic and cathodic reactions of L245N steel corrosion (Su et al. 2015b). In addition, the anode reaction coefficient is smaller than the cathode reaction coefficient, indicating that BLE is a mixed-type corrosion inhibitor, whereas anode reaction is mainly inhibited (Li et al. 2018).

3.3 Electrochemical impedance spectroscopy (EIS)

In the corrosive solution without and with different BLE concentrations, the electrochemical impedance spectrum measurement results of L245N steel are shown in the Nyquist diagram (Figure 3). It is obvious from Figure 3 that at lower temperatures and concentrations, the high-frequency area exists capacitive loop, and the low-frequency area appears in the inductance loop. Previous informs shown that the presence of the capacitive loop corresponds to the double-layer capacitance and charge transfer resistance, and the inductance loop is interrelated to the surface coverage rate of corrosion inhibitor molecules on the surface of the electrode (Chen et al. 2002; Li et al. 2007; Tang et al. 2011). With increasing temperature and concentration, displaying the characteristics of double capacitive loops. The capacitive loop has been associated with the adsorption and desorption of corrosion products or inhibitor molecules. Consequently, in pace with the increase in temperature and concentration, the covering film on the metal surface becomes more perfect, and the capacitive loop disappears. With the further elevate in temperature and concentration, the high-frequency area exists capacitive loop, and the low-frequency area appears a straight line. The appearance of the straight line is related to the Warburg impedance, indicating the presence of the ion diffusion process at the steel/solution interface (Majd et al. 2020). The curvature radius of the capacitive loop in the high-frequency area first increased and then decreased with increasing inhibitor concentration, suggesting that with the increase of BLE concentration, the protective film formed by the adsorption of corrosion inhibitor molecules on the surface of L245N steel increasingly became complete, and the charge transfer resistance gradually increases. Compared with the blank solution, all EIS includes the high-frequency capacitive loop with similar curve characteristics, indicating that BLE could inhibit the reactivity of L245N steel in the aggressive solution and not modify the corrosion reaction mechanism.

Figure 3:

Nyquist plots recorded of the L245N steel electrode in aggressive solution with various concentrations of BLE at different temperatures.

The classical electrochemical equivalent circuits were applied to model EIS data in Figure 4. The EIS data were fitted and listed in Table 3, in which R_s represents the electrolyte resistance, R_f the film resistance, R_ct the charge transfer resistance, W the Warburg impedance, C_dl the double layer capacitance, C_f the film capacitance, and the Q_f constant phase angle element. The following formula expresses the transformation relations of the double-layer capacitance (C_dl) and the film capacitance (C_f).

where f_max is the characteristic frequency (in Hz) corresponding to the maximum value of the imaginary axis, n is the diffusion coefficient, and the π approximate value is 3.14. According to Table 3, values of the charge transfer resistance (R_ct), the inhibition efficiency (η), and surface coverage (θ) present first increased and then decreased with the increasing concentration of BLE, suggesting that with the increase of BLE concentration, the corrosion inhibition efficiency gradually became increasingly perfect and reached the maximum value, then gradually decreased. The C_dl values displayed first decreased and then increased with the increasing concentration of BLE, which is due to the adsorption of corrosion inhibitor molecules on the surface of L245N steel replaced the water molecules, H₃O⁺, and CO₃²⁻ on the metal surface, increasing the thickness of the double layer, reducing the electrode activity area, and thus inhibiting the process of metal oxidation into metal ions (Hu et al. 2020).

Figure 4:

Equivalent circuits diagram of fitted EIS experimental data: (a) Equivalent circuit with inductive loops, (b) equivalent circuit with double capacitive loops, and (c) equivalent circuit with Warburg impedance.

3.4 Weight loss measurements

The electrochemical measurement shows that at a temperature of 45 °C and a concentration of 4 %, the corrosion inhibition efficiency (η) reached the maximum value. Therefore, the weight loss measurement was applied to investigate the inhibition performance under this condition. For the static weight loss measurement, in the 4 % concentration of BLE, the average corrosion rate was 0.0798 (mm/a), much less than 0.4429 (mm/a) in the blank group, and the inhibition efficiency reached 81.98 %. Besides, For the dynamics weight loss measurement, in the 4 % concentration of BLE, the average corrosion was 1.3598 (mm/a), in the blank group was 4.9209 (mm/a), the inhibition efficiency reached 72.37 %, and the BLE still had good corrosion inhibition efficiency compared to the blank solution.

3.5 Surface microscopic observation

The temperature range of this experiment is 15–55 °C, and the corrosion rate increases with the increase in temperature. Consequently, the critical temperature is selected as the surface observation temperature. The SEM analysis was applied to investigate the surface morphology of L245N specimens immersed for 24 h at 15 °C and 55 °C in the aggressive solution without and containing 4 % concentrations of BLE. As shown in the absence of inhibitors (Figure 5a and c), more gullies and granular corrosion products emerge on the steel surface. Compared with 15 °C, the steel surface at 55 °C was significantly damaged, which may be due to accelerating the corrosion rate of metal by increasing the temperature of the test solutions.

Figure 5:

SEM micrographs of the L245N steel surface after 24 h immersion at 15 °C and 55 °C in aggressive solution: (a) and (c) without inhibitor and (b) and (d) with 4 % concentrations of BLE.

In contrast, in the presence of the BLE (Figure 5b and d), the steel surface was relatively smooth, with barely any roughness and gullies except for the polishing lines, suggesting L245N steel was effectively protected. At 15 °C, the metal surface has few corrosion products, while at 55 °C, there are no corrosion products and no obvious corrosion phenomena, indicating that the corrosion inhibition effect at 55 °C is more significant than that at 15 °C. The SEM analysis results confirmed that BLE formed a protective film on the steel surface and effectively inhibited the corrosion of L245N steel in aggressive solutions.

3.6 Contact angle measurement

The contact angle measurements were carried out at room temperature. Immerse the L245N sample in a corrosive medium without and containing BLE 1 %, 2 %, 3 %, 4 %, and 5 %, respectively for 36 h, and then drop water on the sample surface with a 10 μL syringe. After the test, clean the goniometer with acetone. Figure 6 shows, in a corrosive medium, the contact angle of the L245N steel surface without inhibitor is 16°, indicating that the wettability of the steel surface is hydrophilic. With the addition of the inhibitor, the contact angle is 42°, 52°, 68°, 78°, and 88°, respectively. The steel surface becomes hydrophobic. To sum up, the adsorption mechanism of BLE has replaced the water molecules on the surface of L245N steel. BLE inhibitor molecules gather on the surface of L245N steel to form a protective adsorption layer.

4.1 FTIR analysis

Identification of characteristic functional groups in BLE by FTIR, and the diagrams of BLE are illustrated in Figure 7. The strong absorption peak at 3418 cm⁻¹ is related to the stretching vibration of O–H and N–H (Li et al. 2012a). The bandwidth of the adsorption peak locates at 2922 cm⁻¹, which is due to C–H tensile vibration. The absorption peak at 1635 cm⁻¹ is the superimposed telescopic vibration peak of C=O and C=C. The band at 1394 cm⁻¹ is attributed to variable angle vibration peaks. Besides, the other characteristic peaks such as C=N (1082 cm⁻¹), C–O (1049 cm⁻¹), aromatic ring (881 cm⁻¹), and C–H (715 cm⁻¹) also be recorded (Li et al. 2012b; Liu et al. 2013).

Figure 6:

Contact angle between water and L245N sample surface soaked in an aggressive solution with different BLE concentrations.

These absorption peaks, such as O–H, N–H, C=O, C=C, C–N, and C–O, indicate that BLE has the potential to inhibit corrosion. Previous reports indicated that BLE contains flavonoids, amino acids, and polysaccharides, etc. (Guo et al. 2008; Li et al. 2014).

Figure 7:

FTIR spectrum of BLE molecules.

4.2 Molecular dynamics simulations

FTIR analysis shows that BLE contains flavonoids, amino acids, and polysaccharides. Thus, eight representative components from the three main components were selected as inhibitor molecules for investigation, including orientin, isoorientin, vitexin, isovitexin, rhamnose, arabinose, mannose, and hydroxylysine (Lai and Chen 2013; Wang et al. 2020; Zhang et al. 2020). The three-dimensional structure model of the corrosion inhibitor molecules was constructed by Materials Studio software.

As described in the literature, the more negative the value of E_ads, the stronger the adsorption capacity of the inhibitor molecules (Rbaa et al. 2021), and the more likely to shape a corrosion inhibitor film on the iron surface. The values of E_ads acquired by applying Eq. (5) and the results were listed in Table 4. As shown from Table 4, the absolute value range of the E_ads among the eight corrosion inhibitor molecules with Fe surface is 414.62–1177.59 kJ mol⁻¹. In the previous literature, the absolute value of E_ads between Fe surface and H₂O molecules was 23.46 kJ mol⁻¹ (Lv et al. 2020), indicating that all inhibitor molecules may replace H₂O molecules and firmly adsorb on the surface of the iron. Among three main components, the absolute value of the E_ads of flavonoids (orientin, isoorientin, vitexin, and isovitexin) is significantly higher than that of polysaccharides (rhamnose, arabinose, and mannose) and special amino acids (hydroxylysine). Meanwhile, the previous reports indicated that the main corrosion inhibitor of BLE is a flavonoid compound (Ogunleye et al. 2018). In addition, the flavonoid compound has similar structures and corrosion inhibition mechanisms. Consequently, the orientin molecule with the highest absolute value of E_ads in flavonoids was selected for further research.

Figure 8a and b displays the side views of the optimum stable adsorption configuration of the single orientin molecule adsorbed on the Fe (110) surface in the aggressive solution applying the MD. It can be seen that the orientin molecule prefer to adsorb on Fe (110) surface in a parallel mode. The O atom in the orientin molecule has lone pair electrons that can form coordination bonds with the empty d orbital of the Fe atom, adsorbing on the metal surface (Su et al. 2015a), hindering the contact between the metal and the aggressive solution, and inhibiting corrosion.

Figure 8:

Side views of the equilibrium adsorption configurations of multiple orientin molecules on the Fe (110) surface.

In the next step, the effect of inhibitor concentration on the equilibrium adsorption configuration was analyzed by increasing the number of orientin molecules. It can be seen from Figure 8c–e that with the number of orientin molecules increased from 2 to 8, orientin molecules formed the corrosion inhibitor film on the surface of Fe became increasingly complete, and the contact surface between metal and the aggressive solution gradually small, and the corrosion inhibition efficiency was improved. Nevertheless, Figure 8f–h shows that continuing to increase the number of orientin molecules in the saturated state of adsorption does not help much to improve the corrosion inhibition efficiency, resulting in a waste of cost. Moreover, the excess orientin molecules have been randomly scattered in the solution near the side of the metal, and the distribution is irregular and slightly messy.

4.3 Adsorption isotherms

The inhibition effect of plant corrosion inhibitors is mainly based on adsorption properties. Consequently, the adsorption isotherm can accurately describe the interaction between the inhibitor and the metal surface (Teng et al. 2021). In this work, a couple of classical adsorption isotherms, including Langmuir, Temkin, and Frumkin isotherms, were used to descript the adsorption behavior of the inhibitor (Ouakki et al. 2021). The equations for various adsorption models were calculated as follows:

Langmuir adsorption isotherm model

Temkin adsorption isotherm model

Freundlich adsorption isotherm model

Frumkin adsorption isotherm model

where C_inh is inhibitor concentration, θ relates to the coverage rate of the steel surface by the adsorbed inhibitor, K_ads is used for equilibrium adsorption constant (in L/g), a is the interaction parameters between the adsorbed particles, and n represents the constant. Different isotherms were depicted in Figure 9. Among the tested isotherms, the Langmuir isotherm can provide a suitable fit with the adsorption behavior description. The standard adsorption free energy ΔG⁰_ads is calculated by the following equation:

where R is the value of 8.314 J mol⁻¹ K⁻¹, t denotes the temperature (°C), ρ_solvent is equal to 1000 g/L, and ΔG⁰_ads is the standard adsorption free energy in J mol⁻¹. The results of Langmuir adsorption parameters are shown in Table 5, from which it can be seen that the plot of the BLE inhibitor shows straight lines, and these linear regression factor are approach to 1, It shows that the measured data are accord with Langmuir isotherm. Additionally, the values of K_ads in the order of K₄ > K₅ > K₃ > K₂ > K₁, K₄ (45 °C) is the largest, and the corrosion inhibition effect is the best. This is accord with the conclusion of electrochemical experiments. Generally, the absolute value of |ΔG_ads| < 20 kJ/mol conforms to the physical adsorption, and |ΔG_ads| > 40 kJ/mol belongs to chemisorption through coordination bonds formed by charge contribution (Garai et al. 2012). As shown in Table 5, at 15 °C, the value of ΔG_ads is −15.19 kJ mol⁻¹ can be interpreted as physical adsorption. While the ΔG_ads values were in the range of −21.57 kJ mol⁻¹ to −24.16 kJ mol⁻¹ at the temperature of 25–55 °C, which indicates both chemisorption and physical adsorption occur.

Figure 9:

Different isotherm models tested to describe the adsorption at various temperatures.

4.4 The corrosion inhibition mechanism of BLE

Orientin was analyzed as the representative compound. The functional groups (O–H, C=O, and C–O)of in the orientin molecules contain rich O atoms, which possible integrate with the newly formed Fe²⁺ on the metal surface to shape metal inhibitor complexes. These complexes resist the anodic sites and restrain the anodic reaction on the metal surface (Li and Deng 2012). The complex is adsorbed on the surface of the metal under the van der Waals, inhibiting the corrosion of the metal, which belongs to physical adsorption. In addition, various polar groups in orientin molecules could form coordination bonds with the empty d orbitals of iron atoms, adsorb the metal surface, and inhibit the corrosion of the metal, which is chemical adsorption. The diagram of the interaction of the orientin molecule with the surface of Fe is shown in Figure 10. Figure 10a is the molecular structure, and Figure 10b is the parallel adsorption. Orientin molecules tend to be adsorbed on Fe surfaces in a parallel mode and conform to the geometric covering effect.

Figure 10:

The plausible mechanism of orientin adsorption on L245N steel.