Adsorption and corrosion of renewable inhibitor of Pomelo peel extract for mild steel in phosphoric acid solution

2.1 PPE extraction

The fresh Pomelo was purchased from Jimei farmers' market in Xiamen of China, and the variety was Pinghe Pomelo which was productive locally. The fresh Pomelo peel was washed, left overnight and then dried in an oven at 80 °C before finally being crushed in a grinder. The coarse particles were removed using a 60-mesh sieve. The Pomelo peel powder was dissolved in 80% (vol%) aqueous ethanol solution, and then extracted ultrasonically at 40 °C for 4 h. The undissolved residue was removed by vacuum filtration. The filtrate was concentrated by a vacuum rotary evaporator (model RE-52AA, Shanghai Yarong Biochemical Instrument Co., Ltd., China), and the aqueous ethanol solution was recovered. Finally, the concentrated solution was dried in an oven at 80 °C to obtain the solid PPE.

2.2 Materials

The tested sample was mild steel (Fujian Sansteel (Group) Co., Ltd., China), and the chemical composition (wt%) was: 0.13C, 0.02Si, 0.38 Mn, 0.014P, 0.031S, and the balance being Fe.

2.3 Corrosion medium preparation

The corrosion medium was 1.0 mol/L H₃PO₄ solution which was diluted from 85 wt% phosphoric acid (Xilong Science Co., Ltd., China). The concentration of PPE in H₃PO₄ solution was in the range from 0 to 10.0 g/L.

2.4 Mass loss measurements

The size of the mild steel sample used in mass loss tests was 50 mm × 40 mm × 2 mm. All faces of the sample were polished consecutively with a series of waterproof sandpapers (240#, 400#, 600#, 800#, 1000# and 1200#). The sample faces were rinsed with distilled water, scrubbed with alcohol, rinsed with distilled water and dried in cool air.

The initial mass of the above sample before corrosion (w₀) was measured. The accuracy was 0.1 mg. Then each sample was suspended alone in a 250 mL solution at specified temperatures for 8 h. The corroded sample was then cleaned with detergent, rinsed with distilled water and dried with cool air. The corresponding corroded sample (w) was weighed. The mass loss was calculated. The average corrosion rate (v) for mild steel and the corrosion inhibition efficiency (η) of PPE were calculated as follows

2.5 Electrochemical measurements

The dimensions of the mild steel sample used in electrochemical measurements were 1.0 cm × 1.0 cm × 0.2 cm, and the working area was 1.0 cm². The working surface was rubbed carefully with waterproof sandpaper (1200#) and rinsed successively with distilled water, acetone and distilled water.

All electrochemical measurements were performed at room temperature using an electrochemical workstation (model CHI604E; Shanghai Chenhua Instrument Co., Ltd., China). A conventional 3-electrode system featuring working, auxiliary and reference electrodes was used. The mild steel sample was used as the working electrode. A saturated calomel electrode (SCE) and a Pt electrode served as the reference electrode and counter electrode, respectively. The electrochemical measurements were implemented after the open circuit potential (OCP) of the corrosion system was stabilized.

The EIS measurements were obtained at OCP for the frequency range from 100 kHz to 0.01 Hz. The AC signal amplitude was 10 mV. The impedance data were fitted with Zview software.

The PDP experiment was conducted after the sample had been immersed for 1 h in the corrosive medium with the corrosion system being stable. The polarization range was ±300 mV vs. OCP, and the scan rate (from cathode to anode) was 1 mV/s.

To ensure the repeatability and the reliability of the electrochemical measurements, three parallel tests were conducted for each experimental condition. Here, the change trend of OCP with time under three parallel conditions should be similar, and the deviation of the final stable OCP value should be less than 20 mV. The shape of EIS diagrams and PDP curves under three parallel processes must be consistent, and the difference of the corresponding parameters should be less than 10%. In the process of experiment, it is found that the reliability of electrochemical test results in this paper is good.

2.6 UV–Vis and FT-IR spectroscopy measurements

A solution containing 1.0 mol/L H₃PO₄ and 5.0 g/L PPE was divided into two portions. The polished and cleaned mild steel sample was immersed in one of the solution at room temperature for 24 h. The two solutions were then diluted and analysed by UV–Vis spectrophotometry (model UV-2700; Shimadzu, Japan). The wavelength range was from 190 to 800 nm. The scanning interval was 1 nm. Distilled water was used as the reference solution.

The fully dried PPE and the PPE adsorption film which was formed on mild steel surface and was subsequently been scraped off were mixed with KBr, respectively. The samples were thoroughly mixed and ground in an agate mortar, and then the ground powders were pressed. The IR absorption spectra were measured at room temperature by a FT-IR spectrometer (model ALPHA; Bruker, Germany). The wavenumber range was from 400 to 4000 cm⁻¹.

2.7 Corrosion morphology analysis

The polished and cleaned mild steel samples were immersed in 1.0 mol/L H₃PO₄ solution containing different concentrations of PPE for 1 h, then rinsed with distilled water and dried with cool air. The corrosion morphology was characterized using scanning electron microscopy (SEM) (model EVO 18; Zeiss, Germany).

3.1 Corrosion rate from mass loss test

The effects of the PPE concentration (C_inh) and the temperature of the corrosion medium on corrosion rate of mild steel (v) and inhibition efficiency of PPE (η) in 1.0 mol/L H₃PO₄ solution are illustrated in Fig. 1. With increase in the C_inh of PPE at 25 °C, v at first decreases and then stabilizes, whereas the change law of η is the reverse of that for v. At higher temperatures (35, 45, 55 °C), the decrease of v and the increase of η is continuous with increase of the tested C_inh. For the same C_inh, an increase of the temperature of the corrosion medium leads to a great increase in v and a decrease in η. These changes may reflect the following two phenomena. The adsorption capacity of the PPE molecules increases with C_inh. However, when the adsorption becomes saturated (e.g., at 25 °C and with 1.0 g/L PPE), the adsorption capacity would basically remain the same, thus v and η would be almost unchanged for higher C_inh (El-Etre and Abdallah, 2000; Mourya et al, 2014; Souza and Spinelli, 2009). Increasing the temperature of H₃PO₄ solution, the amount of hydrogen evolution on mild steel will thus increase. The evolution of hydrogen not only slows down the adsorption of the extract molecules on the metal surface, but also leads to desorption of the adsorbed molecules (El-Etre and Abdallah, 2000; Mourya et al, 2014; Souza and Spinelli, 2009). Thus, it is difficult to form a stable PPE adsorption film, which, in turn, causes an increase of v and a decrease of η. It can be inferred that the corrosion rate of mild steel and the inhibition efficiency of PPE are closely related to the temperature of the corrosion medium and the concentration of the extract inhibitor, which will determine the coverage of PPE on the surface of mild steel.

Close

Fig. 1

Effect of C_inh and medium temperature on (a) v and (b) η in 1.0 mol/L H₃PO₄.

Export to PPT

3.2 OCP curves

Measurement of the change in the OCP value with corrosion time is beneficial for understanding the adsorption and desorption equilibrium of extract molecules on the metal surface. The variation of OCP of mild steel in 1.0 mol/L H₃PO₄ solution at various C_inh is presented in Fig. 2. No matter whether PPE was added or not, the OCP tends to be stable within 1 h. The stable OCP with the addition of PPE is slightly more negative than that without inhibitor, and the difference is less than 10 mV. This indicates that the corrosion mechanism of mild steel in 1.0 mol/L H₃PO₄ solution was not appreciably altered and that PPE acts as a mixed type inhibitor (Ferreira et al., 2004; Gao et al., 2014). The corrosion inhibitor can be considered as an anode-type or a cathode-type inhibitor only when the change of corrosion potential was more than 85 mV (Satapathy et al., 2009).

Close

Fig. 2

Effect of PPE concentration on the OCP value of mild steel in 1.0 mol/L H₃PO₄.

Export to PPT

3.3 PDP curves

The PDP curves for mild steel in 1.0 mol/L H₃PO₄ solution without and with various concentrations of PPE are presented in Fig. 3. The corresponding PDP parameters are listed in Table 1. η of the PPE inhibitor based on PDP measurement was calculated by the expression

Close

Fig. 3

Effect of the PPE concentration on the PDP curves for mild steel in 1.0 mol/L H₃PO₄.

Export to PPT

Compared with the blank case without PPE inhibitor, the addition of PPE in 1.0 mol/L H₃PO₄ solution reduces both the anode and cathode polarization current for mild steel; and the reduction range of the cathodic polarization current is slightly more obvious. This may be due to the adsorption of PPE molecules on the surface of mild steel. The corrosion potential (E_corr) of mild steel is only slightly changed in the presence of PPE, further indicating PPE functions as a mixed inhibitor (Solomon and Umoren, 2015; Talkhan et al., 2019). In addition, the anodic polarization curves are basically parallel to each other before and after addition of PPE, implying there is little difference in anodic Tafel slope (b_a) (Table 1) (Bahlakeh et al., 2019). Moreover, the cathodic curves generally are also parallel to each other and the differences of cathodic Tafel slope (b_c) are also very small. This suggests that the corrosion mechanism for mild steel in 1.0 mol/L H₃PO₄ solution is not changed by the presence of the PPE inhibitor, with oxidative dissolution of iron for the anode reaction and hydrogen evolution for the cathode reaction (Dehghani et al., 2019; Singh and Quraishi, 2016). Moreover, no matter whether PPE inhibitor was added or not, the corrosion system is controlled by the activated polarization control. All these phenomena indicate that the corrosion inhibition by PPE on mild steel can be attributed to the “geometric coverage” effect (Hamdani et al., 2015; Khaled, 2003). The adsorption film separates the mild steel surface from phosphoric acid solution, thus the electrode reactions cannot be carried out on the covered metal surface. However, the electrode reactions on the uncovered surface proceed according to the original process.

In acid solution, the organic matters in PPE will be protonated with hydrogen ions to form onium ions which are positively charged. Heteroatoms with lone-pair electrons (such as N, O, P, etc.) in organic compound will react with hydrogen ions to form coordination bonds. The covalent bond value of these elements is increased by one to become the corresponding cation, i.e. onium ions. The protonated positive ions will then undergo preferential adsorption on the cathodic active sites which are negatively charged. Therefore, the active area of the cathode is reduced obviously, which, in turn, slows down the cathode reaction rate. According to the working principle of the corrosive galvanic cell, the anode reaction rate will also be declined, reducing the anode current.

The coverage (θ) of PPE on the mild steel surface was calculated based on the η value

As shown in Table 1, with the increase of the PPE concentration, i_corr at first decreases rapidly and then slowly, while η increases at first rapidly and then slowly. This indicates that the corrosion rate of mild steel and the corrosion protection efficiency of PPE directly depend on the “geometric coverage” of the PPE molecules on mild steel. The larger the “geometric coverage” by PPE, the greater the decrease in the corrosion rate for mild steel.

To evaluate the type of corrosion inhibition caused by the organic inhibitor, the anodic action coefficients (f_a) and the cathodic action coefficient (f_c) of the extract on mild steel under E_corr were calculated (Cao, 2008)

Generally, if the value of f_a was smaller than one, the anodic reaction of the corrosion process of metal is inhibited. On the contrary, the anodic reaction is promoted. Similarly, the cathodic reaction is impeded when f_c was less than one. For the same corrosion system, the smaller f_a and f_c are, the stronger is the inhibition effect on the corresponding electrode reaction. The corrosion inhibitor can be regarded as a mixed inhibition type when $f_{\text{a}}\approx f_{\text{c}}$ . And it can be judged as an anodic inhibition type for $f_{\text{a}}\gg f_{\text{c}}$ and a cathodic inhibitor type for $f_{\text{c}}\gg f_{\text{a}}$ (Cao, 2008).

The f_a and f_c values for PPE on mild steel in 1.0 H₃PO₄ solution are listed in Table 2. At all tested concentration of PPE inhibitor, the f_a and f_c values are smaller than one. For the same PPE concentration, the difference between f_a and f_c is very slightly and the ratio of f_a/f_c is highly close to one. It suggests that PPE has almost a similar inhibition effect on the anodic and cathodic reactions of mild steel. With the increase of PPE concentration, both f_a and f_c decrease, indicating an enhancement of the inhibition effect of the PPE inhibitor on mild steel. An analysis of the action coefficients confirms the above interpretation of the PDP curves.

3.4 EIS diagrams

The effect of the PPE concentration on the EIS diagrams for mild steel in 1.0 mol/L H₃PO₄ solution is presented in Fig. 4. As shown in Fig. 4(a), in the absence and presence of the PPE inhibitor, the Nyquist diagram for mild steel consists of a flattened capacitive loop at high frequency and an inductive loop at low frequency. The capacitive loop at high frequency corresponds to the electrical double layer capacitance and the charge transfer resistance at the steel/solution interface. The flattening of the capacitive loop is due to the dispersion effect associated with the roughness and heterogeneity of the electrode surface (Hamdani et al., 2015; Shih and Mansfeld, 1989). The inductive loop at low frequency may be related to the local corrosion of mild steel which probably caused by desorption of the PPE species from local sites of mild steel (Cao and Zhang, 2002; Pal et al., 2019; Shih and Mansfeld, 1989). However, the inductive loop increases with PPE concentration, which can be attributed to the re-adsorption of these charged species on local dissolution area of the mild steel surface. Therefore, the inductive loop can be used to explain desorption and re-adsorption of the extract molecules on the steel surface (Pal et al., 2019). The shape of the Nyquist diagram is not influenced by the PPE concentration, also suggesting that the corrosion behaviour for mild steel in H₃PO₄ is unchanged (Reis, et al., 2006; Solmaz, 2010). This is consistent with both the PDP and OCP results.

Close

Fig. 4

Effect of PPE concentration on (a) Nyquist and (b), (c) Bode diagrams for mild steel in 1.0 mol/L H₃PO₄.

Export to PPT

Furthermore, with the increase of the PPE concentration, the radius of the capacitive loop increases, indicating an increase in the charge transfer resistance of the corrosion reaction on mild steel. This may be attributed to an enhancement in the amount of extract adsorbed. Therefore, the coverage and density of the adsorbed film also increase, resulting in an improvement in the corrosion resistance of mild steel (Reis, et al., 2006; Solmaz, 2010).

As shown in Fig. 4b and c, the shape of the Bode diagram is almost unchanged for the addition of the PPE inhibitor. The width of the peak of the negative phase angle is narrow, suggesting that there is only one relaxation time constant for the capacitive loop at high frequency.

Fig. 5 shows the equivalent circuit used to fit the EIS diagrams in Fig. 4 (Cao, 2008; Pal et al., 2019). R_s is the solution resistance. R_L is the induction resistance and L is the inductance, which were invited to character the inductive loop in low frequency region. R_ct is the charge transfer resistance and CPE_dl is a constant phase angle element, which represents the electrical double layer capacitance of the steel/solution interface. The CPE is often used to describe the distortion capacitance in an actual electrochemical system (Cao and Zhang, 2002)

Close

Fig. 5

Equivalent circuit used to fit the EIS diagrams in Fig. 4.

Export to PPT

In Nyquist diagrams, polarization resistance (R_p) is the real part of impedance when the tested frequency tends to the minimum value. Clearly, R_p is smaller than R_ct when inductive loop was present. η of PPE for mild steel was calculated based on R_ct (Cao, 2008; Pal et al., 2019)

The corresponding fitted results for the EIS diagrams are presented in Table 3. The low value of R_s in 1.0 mol/L H₃PO₄ solution indicates the negligible impact of solution resistance on high frequency capacitive loop. In the presence of PPE inhibitor, the indexes corresponding to high frequency capacitive loop which indicate the enhancement of corrosion resistance of mild steel become better. That is, R_ct, n_-dl, η and θ increase, and Y_0-dl, C_dl and f_max decrease. And the improvement of these indexes can be attributed to the adsorption of PPE molecules on mild steel. Moreover, with the increase in concentration of PPE inhibitor, the corresponding indexes continuously improve.

Generally, R_ct may be used to characterize the degree of charge transfer difficulty between metal phase and solution phase due to the electrochemical reactions. When PPE molecules were adsorbed on the surface of mild steel, an adsorption film will cover on mild steel surface. The relative active area of mild steel decreases, and so is the active dissolution of mild steel. On one hand, the number of the migrated electrons decreases, leading to a reduction of i_corr. On the other hand, the adsorption film leads to an enhancement of the resistance of electron migration, i.e. an increase in R_ct. Therefore, the increase of R_ct with concentration of PPE can be attributed to the increase of coverage of PPE molecules on the electrode surface. To confirm the correlation between R_ct and the covered surface area, the relative electroactive area (REA) was calculated (Caio et al., 2019; Hamdani et al., 2015)

The relationship between REA (date from PDP result) and R_ct is presented in Fig. 6 and exhibits a good linearity. Thus η and θ increase with the increase of concentration of PPE inhibitor.

Close

Fig. 6

Relationship between REA (data from PDP) and R_ct.

Export to PPT

C_dl may be correlated to the electrode reactions and surface adsorption (Cao, 2008). During the electrode reactions, the excess charge between the solution phase and the metal phase of the electric double layer will generate. With the increase of coverage of PPE molecules on mild steel, the active dissolution of mild steel reduces, resulting in the decrease of the excess charge in electric double layer. C_dl is declined, too. Moreover, the surface adsorption also leads to the formation of electric double layer. The adsorption of PPE molecules on mild steel not only replaces the water molecules of which the dielectric constant is higher, but also increases the plate spacing of the electrical double layer capacitance and reduces the active surface area of mild steel in H₃PO₄ solution (Outirite et al., 2010). According to the Helmholtz model (Hu et al., 2017, Qiang et al., 2018)

Close

Fig. 7

Relationship between REA (data from PDP) and C_dl.

Export to PPT

As shown in Table 3, n_-dl, which characterizes the dispersion effect of the capacitance of the electrical double layer at the mild steel-solution interface, is very close to one. This corresponds well to a relatively regular capacitive loop as shown in Fig. 4a. In the presence of PPE inhibitor, n_-dl increases but the overall change is small. It can be considered that the surface roughness, density and uniformity of mild steel in 1.0 mol/L H₃PO₄ solution are good.

R_L and L were used to denote the inductive loop in low frequency region. R_L is related to the resistance of the local repair layer due to the re-adsorption of the charged species, and L can be attributed to the variation of current flux for these charged particles (Pal et al., 2019). In the absence of PPE inhibitor, R_L and L were not fitted due to the very small low frequency inductive loop. That is, the local corrosion of mild steel in low frequency region could not be repaired because there was no corrosion inhibitor. In the presence of PPE inhibitor, both R_L and L improve with an increase in concentration of PPE. It suggests that desorption and re-adsorption of the extract molecules on surface of mild steel are enhanced and the active surface is repaired more effectively (Cao, 2008; Pal et al., 2019). Therefore, the corrosion of mild steel is impeded.

3.5 Corrosion morphology

The effects of the PPE on the corrosion morphology of mild steel sample immersed in 1.0 mol/L H₃PO₄ solution for 1 h are shown in Fig. 8. In the absence of the PPE inhibitor, the surface of mild steel is rough, and numerous corrosion products are evident. In the presence of 1.0 g/L PPE, the surface is relatively flat, with a large number of dense and needle-like corrosion products (in Fig. 8c). At higher magnification, the edge and surface of the needle-like plates are covered with denser and finer needle-like products (in Fig. 8d). When the PPE was increased to 5.0 g/L, the density and surface smoothness of the mild steel sample is further improved (in Fig. 8e and f). The above phenomena indicate that the presence of PPE in 1.0 mol/L H₃PO₄ solution is beneficial to the formation of a continual corrosion-resistant film on mild steel. Also, PPE may be adsorbed on the mild steel surface. As a result, corrosion of mild steel is effectively inhibited.

Close

Fig. 8

Effect of PPE on the corrosion morphology of mild steel in 1.0 mol/L H₃PO₄ medium: (a), (b) without PPE; (c), (d) with 1.0 g/L PPE; (e), (f) with 5.0 g/L PPE.

Export to PPT

3.6 Effectiveness of corrosion protection by PPE

To evaluate the effectiveness of corrosion protection by PPE, the EIS diagrams for mild steel in 1.0 mol/L H₃PO₄ solution containing 5.0 g/L PPE at room temperature were acquired, as shown in Fig. 9. For corrosion times in the range from 0.5 to 47 h, the shape of the EIS diagrams for mild steel remained unchanged. There is a high frequency capacitive loop correlated to the electrical double layer capacitance and the charge transfer resistance at the steel/solution interface and a low frequency inductive loop attributed to desorption and re-adsorption of PPE molecules on local dissolution sites of mild steel. However, for corrosion times up to 224 h, the shape of the EIS diagrams is changed. The peak width of the negative phase angle on the Bode diagrams is also increased. They contain a high-frequency capacitive loop, a middle-frequency capacitive loop and

Close

Fig. 9

EIS diagrams for mild steel immersed in 1.0 mol/L H₃PO₄ solution with 5.0 g/L PPE for different times: (a) Nyquist diagrams; (b) Negative phase angle for Bode diagrams.

Export to PPT

a low-frequency inductive loop. Therefore, the corrosion behaviour for mild steel is altered for prolongation of immersion time. For the longer corrosion times (up to 224 h), the corrosion reaction is not only controlled by the electrical double layer capacitance and the charge transfer resistance of the steel-solution interface, but also dependent on the film capacitance and film resistance on the surface of mild steel. The appearance of the film capacitance and the film resistance may be contributed to the over adsorption of PPE molecules, a decrease of film density, etc. (Hamdani et al., 2015).

The EIS diagrams corresponding to corrosion for 0.5–47 h and 224–315 h were analysed and fitted by the equivalent circuits illustrated in Figs. 5 and 10, respectively (Mobin et al., 2019; Pal et al., 2019). CPE_c and R_c were applied to characterize the capacitance and resistance of the protection film. It can be seen that the fitting data coincides well with the experimental data. Thus the equivalent circuits are realistic and feasible.

Close

Fig. 10

Equivalent circuit used to fit the EIS diagrams under longer corrosion times (224-315 h).

Export to PPT

The fitted results are presented in Table 4. For corrosion time of 224–315 h, the contribution of film resistance to the corrosion inhibition efficiency of PPE should be considered. And the sum of R_c and R_ct, denoted as R_tot, was used to calculated η of PPE for mild steel (Cao, 2008)

As shown in Table 4, for immersion time ranged from 0.5 h to 47 h, the n_-dl value is high and almost is unchanged, suggesting a good density and compactness of mild steel surface. However, n_-dl decreases for longer corrosion time of 224 h and 315 h, which may be due to the over adsorption of PPE on mild steel, resulting in a coarse surface (Hamdani et al., 2015). Moreover, with the increase of immersion time, Y_0-dl and C_dl at first decline and then increase; and R_ct (or R_tot) improves firstly followed by a decrease. The inflection point occurs at 23 h, and so is the inhibition efficiency and coverage of PPE inhibitor.

As far as the inductive loop was concerned, R_L and L at first increase and then drop. The turning point is at 5 h, which can also be confirmed by the longest inductive loop in low frequency region when the immersion time was 5 h. Because the inductive loop reflects the capacity of desorption and re-adsorption of PPE molecules on mild steel surface. For longer immersion time, the weakening of desorption and re-adsorption ability at low frequency may be related to the more complicated surface and the untimely supplement of PPE molecules to the local dissolved surface.

The density and compactness of the adsorption film and the capacity of desorption and re-adsorption of PPE molecules becomes poor for longer immersion time. However, the η value of PPE inhibitor still is up to 91.9% after corrosion for 315 h, much higher than the value of 80.5% for immersion only 1 h. It can be concluded that PPE confers a long-term and effective inhibition for mild steel in 1.0 mol/L H₃PO₄ solution.

Table 5 shows comparison of PPE results with other plant extract as inhibitors for metal in acid solution. Clearly, most plant extract followed the Langmuir adsorption isotherm and functioned as the mixed inhibitor. In present work, the adsorption isotherm is influenced by the medium temperature and PPE simultaneously obeys the Langmuir, El-Awady and Temkin isotherms at 35, 45 and 55 °C. The maximum efficiency of different plant extract also exhibits a certain difference. On the whole, the concentration of plant extract in 1.0 mol/L HCl solution is relatively low, but most of them display good corrosion inhibition efficiency. This may be related to the synergistic inhibition effect of the mixed inhibitor of organic compound and halide ions due to the bridging principle (ElKacimi et al., 2020; Li and Deng, 2020; Qian et al., 2013). On the contrary, the extract inhibitor concentration in H₂SO₄ and H₃PO₄ media is relatively high. In H₃PO₄ solution, PPE has obvious advantage as corrosion inhibitor with inhibition efficiency of 95%. It further suggests that PPE has great potential as a green, long lasting, efficient and renewable corrosion inhibitor for mild steel in phosphoric acid medium.

3.7 Adsorption isotherm

Electrochemical analysis indicated that the inhibition mechanism of PPE on mild steel in 1.0 mol/L H₃PO₄ solution proceeded through a “geometric coverage” effect. The corrosion of mild steel was closely related to its REA as depicted in Figs. 6 and 7. A protective film was formed by the adsorption of the PPE inhibitor on the surface of mild steel. Based on the result of mass loss test, various adsorption isotherms were examined in an attempt to clarify the adsorption behaviour of PPE. The results show that the adsorption can be explained by the Langmuir, the El-Awady and the Temkin isotherms (Faustin et al., 2015; Ituen et al., 2017; Jyothi and Ravichandran, 2014). The fitted results are presented in Fig. 11, and the corresponding parameters are given in Tables 6–8.

Close

Fig. 11

Adsorption isotherms for PPE on mild steel surface (a) Langmuir; (b) El-Awady; (c) Temkin.

Export to PPT

The respective adsorption isotherms are as follows:

Langmuir isotherm:

El-Awady isotherm:

Temkin isotherm:

It should be pointed out that the Langmuir isotherm can be used to predict whether the adsorption film is a single molecular layer; the El-Awady isotherm can be used to examine the number of the water molecules that can be replaced by one inhibitor molecules on the metal surface; and the Temkin isotherm can be used to predict the interaction forces operating between molecules in the adsorption film (Caio et al., 2019; Deyab and Abd El-Rehim, 2012; Faustin et al., 2015; Jyothi and Ravichandran, 2014; Shukla and Ebenso 2011).

As shown in Fig. 11a, there is a good linear relationship between C_inh/θ and C_inh at four experimental temperatures (25, 35, 45 and 55 °C), and the correlation coefficient (R²) is very close to one. It suggests that the adsorption of PPE on the surface of mild steel conforms to the Langmuir isotherm and is monolayer adsorption.

The K_ads can be used to characterize the adsorption degree of inhibitor molecules from the solution to the metal surface. Generally, the higher the K_ads value is, the stronger is the adsorption capacity of organic molecules on the metal surface, and the better is the inhibition effect (Caio et al., 2019; Hamdani et al., 2015; Hassannejad and Nouri, 2018; Mhiri et al., 2016; Nikpour et al., 2019; Sin et al., 2017; Wang et al., 2019). According to the intercept of the fitting line of the Langmuir isotherm, K_ads can be calculated, as shown in Table 5.

As shown in Fig. 11a and Table 6, with an increase in the temperature of the corrosion medium, K_ads decreases, resulting in a decrease in the adsorption force of PPE. This reflects the fact that the hydrogen evolution reaction increases with the temperature of the corrosion medium, such that the mechanical stripping of the evolution of hydrogen atoms leads to desorption of the adsorbed extract and a decrease in the adsorption capacity and coverage of the extract. This is consistent with the changes of v_corr and η with the temperature of the corrosion medium. It further indicates that a high temperature is not beneficial to the adsorption of PPE molecules (Bahlakeh et al., 2019; Hassannejad and Nouri, 2018). In addition, dispersion of the Langmuir fitting line increased at 55 °C, which may be related to a little change in the adsorption model (Alrefaee et al., 2020; Divya et al., 2019).

As shown in Fig. 11b and c, except at 25 °C, the adsorption of PPE on mild steel at 35, 45 and 55 °C not only obeys the Langmuir isotherm, but also follows the El-Awady and Temkin isotherms.

As shown in Table 7, the K_ads values obtained from the El-Awady isotherm are in good agreement with those obtained by the Langmuir isotherm. All values of 1/y obtained by the El-Awady fitting lines were greater than one. One extract molecule can replace multiple water molecules. That is, one extract molecule occupies multiple active points on the surface of mild steel (Jyothi and Ravichandran, 2014). The value of 1/y is distinct at different temperatures, which may be related to the different adsorption performance, such as physical, chemical and mixed adsorption (Divya et al., 2019).

As shown in Table 8, the negative values for α obtained from the Temkin isotherm indicate a repulsive force is operating in the adsorption layer (Deyab and Abd El-Rehim, 2013). The rejection strength decreases in the following order: 55 °C > 45 °C >35 °C. This also corresponds to the change trend of the surface coverage with medium temperature.

3.8 Adsorption thermodynamics of PPE

To further explore the adsorption behaviour of PPE at the mild steel-solution interface, the Gibbs free energy (ΔG_ads) was calculated (Hamdani et al., 2015; Hu et al., 2016; Obi-Egbedi et al., 2015)

Generally, when ΔG_ads was higher than −20 kJ/mol, the adsorption can be regarded as physical adsorption; whereas it is chemical adsorption for ΔG_ads was smaller than −40 kJ/mol (Hamdani et al., 2015; Hu et al., 2016; Obi-Egbedi et al., 2015). Physical adsorption mainly results in the formation of an adsorbed layer due to the electrostatic attraction between the charged inhibitor molecules and the charged metal surface. In the case of chemical adsorption, a protection film is formed mainly through the charge transfer or the charge sharing between the inhibitor molecules and the metallic constituents on the metal surface (Deyab and Abd El-Rehim, 2013; Hu et al., 2016; Obi-Egbedi et al., 2015). As shown in Tables 6 and 7, the ΔG_ads values obtained by the Langmuir and the El-Awady adsorption isotherms are basically consistent. At all test temperatures, ΔG_ads is negative. The adsorption of PPE molecules on the surface of mild steel in 1.0 mol/L H₃PO₄ solution is spontaneous. In addition, at room temperature (25 °C), the adsorption model of PPE on mild steel is mixed adsorption with main pattern of physical adsorption. At higher temperatures, the adsorption model of PPE is physical adsorption, and the adsorption propensity and capacity decrease. It should be pointed out that in acidic media, organic matters in PPE would be protonated and thus were positively charged, resulting in the formation of charged molecules (Caio et al., 2019; Deyab and Abd El-Rehim, 2013; Hamdani et al., 2015).

3.9 Kinetic parameters for adsorption

The results for the adsorption thermodynamics indicate that the temperature of the corrosion medium is one of the most important parameters to affect the inhibition performance of PPE. In accord with the Arrhenius equation and the Transition State theory, the influence of PPE on the corrosion kinetics of mild steel in 1.0 mol/L H₃PO₄ solution was subsequently examined (Dkhireche et al., 2020). The Arrhenius equation and the Transition State theory equation were respectively expressed as

E_a is the minimum energy required to judge whether a reaction can occur. ΔH_a can be used to assess the endothermic or exothermic process of the metal dissolution; and the adsorption model of the physical adsorption, chemical adsorption or mixed adsorption of corrosion inhibitor will thus be judged. ΔS_a can be used to evaluate the order of the metal surface. And A reflects the number of active points in the heterogeneous reaction. These physical and chemical adsorption parameters are helpful aids in clarifying the inhibition behaviour of the extract.

According to the equations of the Arrhenius and Transition State theory, the experimental data based on the mass loss tests were analysed, as shown in Fig. 12. The fitting results for both adsorption kinetic equations were found to be linear. Based on the slopes and intercepts for the fitting lines, the parameters (E_a, ΔH_a and ΔS_a) for activation of the corrosion reaction were calculated, as shown in Table 9. In the absence of inhibitor, the value of E_a is about 43.9 kJ/mol, but it is up to 92.3 kJ/mol in the presence of 3.0 g/L PPE. E_a is enhanced obviously with increase in the PPE concentration. It indicates that the addition of the PPE inhibitor makes the corrosion reaction of mild steel more difficult due to the formation of the PPE adsorption film on the surface of mild steel.

Close

Fig. 12

Effect of the PPE concentration on the adsorption kinetics of the activation reaction for mild steel: (a) the Arrhenius line of ln(v) ∼ 1/T; (b) the Transition State theory line of ln(v/T) ∼ 1/T.

Export to PPT

Moreover, the variation of A with the PPE concentration is the same as that for E_a, and the enlargement range for the solution with 3.0 g/L PPE was nearly eight orders of magnitude greater compared to the results for the solution without inhibitor. That is, the disorder of the corrosion system is increased (Ech-chihbi et al., 2020). On one hand, the competitive adsorption between the water molecules and the protonated extract ions on the metal surface make the adsorption and desorption phenomenon become more frequent, and the disorder caused by this competitive adsorption is far greater than the ordering due to the PPE molecular adsorption (Ansari et al., 2014; Ech-chihbi et al., 2020; Khan et al., 2017). On the other hand, the rate determination step of the activated complex is a discrete separation step rather than an associated step, resulting in an increase in disorder for the transfer process from reactants to activated complexes (Ansari et al., 2014). Consequently, there is an increase in ΔS_a with PPE concentration as indicated in Table 9.

The change of the activation parameters (E_a and A) with PPE concentration indicates that the corrosion reaction for mild steel is not only related to the energy of the corrosion system, but also is correlated to the active points of the heterogeneous reactions and the degree of disorder of A and ΔS_a for the whole system (Ansari et al., 2014).

Fig. 13 shows the relationship between ΔH_a and E_a for the corrosion reaction of mild steel in 1.0 mol/L H₃PO₄ solution. ΔH_a and E_a exhibit a strict linearity with relationship of ΔH_a = E_a − 2.62. The average difference is 2.62 kJ/mol, indicating a very close value of E_a to ΔH_a of which was obtained by the Transition State theory. It can be explained by thermodynamic equation of the mono-molecular reaction of ideal gas (Nathiya and Raj, 2017)

Close

Fig. 13

Relationship between ΔH_a and E_a for the corrosion reaction of mild steel.

Export to PPT

The average temperature used to calculate ΔH_a and E_a in this study was 313 K. The average RT was 2.60 kJ/mol, very close to the fitting value of 2.62 kJ/mol. It thus suggests that the anodic reaction of mild steel in 1.0 mol/L H₃PO₄ solution is a mono-molecule reaction.

As shown in Table 9, the value of ΔH_a under all conditions is positive, indicating that the dissolution reaction of mild steel in 1.0 mol/L H₃PO₄ medium is an endothermic process. This is consistent with the clear increase of the corrosion rate of mild steel with temperature of the corrosion medium (Fig. 1). Moreover, ΔH_a increases with the addition of the PPE inhibitor. Due to the adsorption of PPE on the surface of mild steel, the dissolution reaction of mild steel needs to absorb more energy. The adsorption of PPE makes the corrosion process of mild steel more difficult. Generally, the absolute value of ΔH_a for physical adsorption is lower than 80 kJ/mol, while that of chemical adsorption is closed to 100 kJ/mol (Ansari et al., 2014; Obi-Egbedi and Obot, 2011). As shown in Table 9, the inhibitor molecules tend to be physically adsorbed when the concentration of PPE is low, and they tend to undergo mixed adsorption dominated by physical adsorption at higher PPE concentrations.

3.10 Influence of test methods on adsorption mode

A comparison of the effects of the test methods, i.e. PDP, EIS and mass loss, on η value of the PPE inhibitor at 25 °C is presented in Fig. 14. The variation law of η with PPE concentration is hardly affected by the test methods. However, the η value for different test methods is also different, and generally in the order as PDP < EIS < mass loss. Also, the difference between the EIS and mass loss is small.

Close

Fig. 14

Effect of the test methods on η value for PPE.

Export to PPT

The adsorption of PPE on the surface of mild steel at room temperature did not follow the El-Awady and Temkin isotherms, thus the influence of the test methods was only examined in the case of the Langmuir isotherm, as shown in Fig. 15. The corresponding thermodynamic parameters are presented in Table 10. The correlation coefficient for three fitting lines is very close to one. The ΔG_ads value by the three methods is exceedingly similar. Moreover, difference in the K_ads value is unobvious. Therefore, it is concluded that the corrosion methods of PDP, EIS and mass loss have minimal effect on the evaluation of adsorption behaviour for PPE.

Close

Fig. 15

Effect of test methods on the Langmuir isotherm.

Export to PPT

3.11 UV–vis and FT-IR spectra

When a molecule or ion absorbs radiation in the UV–vis region of the electromagnetic spectrum, the valence electrons and the orbital electrons will participate in energy level transitions thus generating the UV–vis absorption spectrum. The molecular composition can be qualitatively and semi-quantitatively analysed using UV–vis spectrophotometry. There are many organic molecules in plant extracts and the π-electrons of the compounds with conjugated structure readily absorbed UV–vis radiation to produce characteristic absorption peaks. Therefore, the structural characteristics of organic molecules with heteroatom non-ligand electrons and π-electrons in double bonds can be studied by UV–vis spectrophotometry (Pal et al., 2019; Singh et al., 2020). The UV–vis spectra for a 1.0 mol/L H₃PO₄ solution containing 5.0 g/L PPE before and after corrosion of mild steel are shown in Fig. 16.

Close

Fig. 16

UV–vis spectra for 1.0 mol/L H₃PO₄ solution containing 5.0 g/L PPE before and after steel corrosion.

Export to PPT

As shown in Fig. 16, the absorption bands at 196 nm, 221 nm and 283 nm before immersion are due to the π-π* transition of C⚌C bonds in the aromatic rings. This indicates that the aromatic compounds, which contain unsaturated bonds with π-electron groups in PPE, are of the conjugated closed system type. The absorption band at 331 nm before immersion is due to the n-π* transition of C⚌O bonds in carbonyl compounds. It indicates that the extracts contain compounds having unsaturated bonds with n-electron groups (Caio et al., 2019; Chung et al., 2020; Saxena et al., 2018; Singh et al., 2020). After immersion and corrosion, the wavelengths of the above three absorption bands, corresponding to the π-π* transitions of C⚌C bond in the aromatic ring, are blue-shifted to 193 nm, 220 nm and 275 nm, respectively. The absorption bands related to the n-π* transitions of the C⚌O bonds in carbonyl compounds almost disappear. It can thus be concluded that both the C⚌C bonds and the carbonyl groups in PPE reacted with iron ions and formed the protection film on the surface of mild steel (Caio et al., 2019; Chung et al., 2020; Saxena et al., 2018; Singh et al., 2020).

Given the complex composition of PPE, FT-IR spectroscopy is needed to reveal the specific chemical bonds and functional groups of the active components. The FT-IR spectra of PPE and the PPE film which formed on the surface of the mild steel are presented in Fig. 17. Clearly, the positions of the absorption bands for the two spectra are quite similar, further confirming the adsorption of PPE on mild steel.

Close

Fig. 17

FT-IR spectra for (a) PPE and (b) PPE film on mild steel surface.

Export to PPT

As shown in Fig. 17a, the broad absorption band at 3460 cm⁻¹ corresponds to the stretching vibration of O—H (Alibakhshi et al., 2018; Bahlakeh et al., 2019; Ji et al., 2015; Qiang et al., 2018). The strong absorption band at 2929 cm⁻¹ is due to the stretching vibration of the saturated C—H bond. The strong and sharp absorption band at 1615 cm⁻¹ is due to the stretching vibrations of C⚌C and C⚌O in the aromatic ring conjugation system. It should be noted that the large conjugation effect of C⚌O and C⚌C in the structure of flavonoids makes the absorption peak of C⚌O move to a low wave number, overlapping with the absorption band region for C⚌C (Byrne et al., 2020; Rahim et al., 2007; Snihirova et al., 2016). The absorption band at 1411 cm⁻¹ is related to the bending vibration of the C—H bond in —CH₂ and the symmetric stretching vibration of COOH in the carboxyl group. The absorption band at 1367 cm⁻¹ is due to the bending vibration of the C—H bond in —CH₃ (Alibakhshi et al., 2018; Bahlakeh et al., 2019; Ji et al., 2015; Qiang et al., 2018). The absorption bands at 1256 cm⁻¹ and 1038 cm⁻¹ are due to the stretching vibrations of the C—O bond in aromatic ethers and fatty ethers, respectively. The absorption peaks in the range 1600–1000 cm⁻¹ may be the skeleton vibration region of the benzene ring or hetero-aromatic ring. The absorption band below 1000 cm⁻¹ is due to the bending vibration of the C-H bond in the benzene ring or hetero-aromatic ring (Alibakhshi et al., 2018; Bahlakeh et al., 2019; Ji et al., 2015; Qiang et al., 2018). The results show that PPE contains —OH, C—O, C⚌O, C⚌C, —CH₂, —CH₃, benzene ring, hetero-aromatic ring and other functional groups. Therefore, PPE contains a mixture of polar organic groups (—OH, C—O, C⚌O, C⚌C and hetero-aromatic rings) and non-polar organic groups (—CH₂, —CH₃, benzene ring).

Compared with the FT-IR spectrum in Fig. 17a, the intensity of the absorption band for the PPE film on the mild steel surface in Fig. 17b is relatively weak. This can be attributed to the interaction between the PPE inhibitor and mild steel, as a result of the formation of the adsorbed film on the surface (Caio et al., 2019; Bahlakeh et al., 2019). This is consistent with the results for the UV–vis spectra.

According to the literatures (Sun et al., 2017; Yee et al., 2020), the main components in Pomelo peel include flavonoids, sterols, fatty acids, sugars and limonin. Flavonoids are the general term for a class of phenolic compounds with the C6-C3-C6 structure, and are widely distributed in the plant kingdom in the form of free aglycones or glycosides (Byrne et al., 2020; Rahim et al., 2007; Snihirova et al., 2016). The basic structure of flavonoids is shown in Fig. 18 (Mhiri et al., 2016; Toledo-Guillén et al., 2010; Zhang and Zhao, 2018).

Close

Fig. 18

Basic structure of flavonoids.

Export to PPT

The main flavonoids in Pomelo peel are naringin, neohesperidin, rutin and other dihydroflavones, in which naringin accounts for more than 80%. The structural formula of naringin in PPE is given in Fig. 19. There are active groups such as oxygen-containing hetero-cycles, hydroxyl groups and carbonyl groups in the molecular structure of naringin. Liu (2017) reported that the contents of the active components in PPE were as follows: 16.7% ketones, 7.9% alcohols, 21.9% acids and 49.7% lipids.

Close

Fig. 19

Structural formula of naringin in PPE.

Export to PPT

In the case of ketones, the main component groups include oxygen-containing hetero-cycles and the C⚌O unsaturated bonds. The basic structural formula for organic acids is RCOOH and organic acids possess carboxyl functional groups —COOH. In the case of alcohols, hydroxyl groups —OH are prevalent.

The UV–vis spectra reveal that PPE contains carbonyl compounds. The FT-IR spectra indicate that PPE contains hydroxyl, carbonyl and keto compounds. It is speculated that the inhibition mechanism of PPE for mild steel in 1.0 mol/L H₃PO₄ solution may depend on the following three adsorption modes (Bahlakeh et al., 2019; Caio et al., 2019; Hamdani et al., 2015): (1) Physical adsorption: the active polar groups are protonated in H₃PO₄ solution and create excess positive charge, which results in the adsorption of PPE on the metal surface due to the electrostatic attraction with the negatively charged surface of mild steel. (2) Chemical adsorption: there are unpaired electrons in the oxygen-containing hetero-cycles in the ketone carbonyl groups and the C and O atoms in C⚌O unsaturated bonds, which are able to form chelates with iron ions or donate electrons to the empty 3d orbitals of iron, thus forming coordination bonds. (3) Mixed adsorption: this is an adsorption process that features the characteristics of both physical adsorption and chemical adsorption as outlined above.