Corrosion inhibition of mild steel in sulfuric acid solution by loquat (Eriobotrya japonica Lindl.) leaves extract

The inhibition performance and mechanism of loquat leaves extract (LLE) for the corrosion of mild steel in 0.5 M H2SO4 were investigated using weight loss method, electrochemical measurements and scanning electron microscope (SEM). The results revealed that LLE acted as a modest cathodic inhibitor, its inhibition efficiency increased with the concentration of LLE and reached a maximum value of 96% at the 100% V/V concentration, but decreased with incremental temperature. Besides, it was found that the adsorption of LLE on steel surface obeyed Langmuir adsorption isotherm, and then the thermodynamic and kinetic parameters were further determined accordingly. Furthermore, LLE was preliminarily separated by pH-gradient sedimentation and the synergistic inhibition between the isolates was investigated.

, since there was no linear Tafel region on anodic polarization curve, thus the electrochemical parameters, including corrosion potential (E corr ), corrosion current density (I corr ), cathodic Tafel slope (β c ) and inhibition efficiency (η), were determined by extrapolation of the cathodic part of the polarization curve to E corr , and listed in Table 1. Figure 2 and Table 1 show that the presence of LLE caused a remarkable decrease of I corr value, and a higher concentration of LLE resulted in an even lower I corr value. Accordingly, the inhibition efficiency increased along with the increase of LLE concentration, and the maximum inhibition efficiency reached 96% at the 100% V/V concentration. Moreover, Figure 2 also indicates that the addition of LLE prominently shifted the cathodic branch of polarization curves to lower values of current densities, suggesting strong inhibitive effect of LLE on the cathodic reduction of hydrogen ions. Besides, it can be observed from in Figure 2 that the cathodic polarization curves were almost parallel to each other, the values of β c in Table 1 exhibited small fluctuations accordingly, which indicate that LLE dose not impact the mechanism of hydrogen reduction and the hydrogen evolution is activation-controlled 46 . However, the anodic polarization curve with inhibitor almost overlapped with that in the blank solution in the strong polarization region, despite    small decrease of anodic current density at low polarization potential, which is attributed to the desorption of inhibitors. Besides, the values of E corr did not vary significantly with a displacement less than 30 mV (as seen in Table 1). Overall, LLE can be seen as a modest cathodic inhibitor 47,48 . Figure 3 shows the Nyquist plots and Bode plots for mild steel in 0.5 M H 2 SO 4 solution without and with different concentrations of LLE at 298 K. It can be seen that the addition of LLE did not change the profile of Nyquist plots, which consist of a capacitive loop at high frequencies (HF) and an inductive loop at low frequencies (LF), suggesting that the presence of the LLE has little influence on the corrosion mechanism 1,11 . The HF capacitive loop is usually related to the charge transfer of the corrosion process and the resultant double layer behavior [49][50][51] . However, due to the frequency dispersion as a result of the roughness and non-homogeneity of electrode surface 52 , the capacitive loop exhibits a depressed semicircle. Meanwhile, the LF inductive loop is attributed to the relaxation process caused by adsorption of species such as (SO 4 2− ) ads and (H + ) ads 48,51 . Figure 3b shows the impedance data in the Bode plots of impedance magnitude (|Z|) and phase angle over the whole frequency range obtained for the mild steel electrode with LLE V/V concentrations between 0 and 100%. The phase angles of the uninhibited and inhibited solutions are 52.3°, 61.6°, 62.8°, 69.9°, 72.9° and 75.7°, respectively, the values of phase angle and impedance magnitude at LF increase with the increase of LLE concentration, which indicates better protection behavior of LLE with higher concentrations.
The EIS data are simulated with the equivalent circuits as shown in Figure 4 using the ZSimpWin software and the impedance parameters are given in Table 2. In the circuits, R s , R ct and R L represent the solution resistance, charge transfer resistance and inductive resistance, respectively, L is the inductive elements, and constant phase element (CPE) is used to replace a double layer capacitance in order to obtain a better fitting. The impedance of CPE can be described as follows 52,53 : where Y 0 is the CPE constant, j is the imaginary unit, ω is the angular frequency, and n is the deviation parameter, has the meaning of the phase shift. In addition, the double layer capacitance (C dl ) and inhibition efficiency (η) are calculated according to the following equations 52,53 : where ω is the angular frequency at the maximum value of the imaginary part (Z im-Max ) of the impedance spectrum 52 , R ct,0 and R ct are the charge transfer resistances without and with inhibitor, respectively. As shown in Table 2, the R ct values increased while the values of C dl declined when increasing LLE concentration, correspondingly, the inhibition efficiency increased with the increase of the concentration of LLE, which indicate that the inhibitor molecules adsorbed at the metal/solution interface as a barrier layer, effectively protecting the mild steel from aggressive attack by the acid solution. The values of n became closer to one as the increase of LLE concentration, indicating the metal surface become more homogeneous due to the adsorption of inhibitor molecules 1 .
The results of weight loss measurements for the corrosion of mild steel in 0.5 M H 2 SO 4 solution without and with different concentrations of LLE for 4 h at different temperatures are given in Table 3. It is clear that the corrosion rate decreases under the same temperature when increasing inhibitors concentration, while it increases with rising of temperature, additionally, the inhibition efficiency at same concentration of inhibitor also decreased at higher temperature. At the 100% V/V concentration, the inhibition efficiency of LLE reached maximum value of 96.2, 95.0, 89.5 and 89.2% for 298, 308, 318 and 328 K, respectively, which mean LLE is an effective inhibitor for the corrosion of mild steel in 0.5 M H 2 SO 4 solution. In addition, the inhibition efficiencies obtained from weight loss measurements are in good agreement with those obtained by electrochemical measurements.
Moreover, the inhibition efficiency of LLE is compared with that of other plant extract reported in literature 2,28,54-60 , the related data are listed in Table 4. It can be seen that the inhibition efficiency of LLE is slightly less than that of Radish seed extract at the V/V concentration of 10%, and compared with the maximum corrosion inhibition efficiency reported in the literature, the inhibition efficiency of LLE is close to that of Houttuynia cordata stem extract and Barley extract, slightly lower than that of Agetes erecta extract and Litchi peel extract, but higher than that of the extract of Medicago sativa, Anacyclus pyrethrum extract, Salvia aucheri mesatlantica, and so on, which suggest that LLE is an effective inhibitor.
It is well known that the inhibition effect of organic inhibitor is based on their adsorption at the metal/solution interfaces, therefore, it is important to understand the adsorption behavior of inhibitor for revealing its inhibition mechanism. The adsorption isotherm can provide vital information regarding the interaction between the inhibitor and the metal surface. Thus, several adsorption isotherms are employed to fit the surface coverage (θ) obtained from weight loss measurements. Langmuir isotherm is found to provide the optimal description of the adsorption behavior of LLE on the mild steel surface, which can be expressed by the following equation 61,62 : where c is the inhibitor concentration, θ is the surface coverage and K ads is the adsorptive equilibrium constant, respectively.   The plots of c/θ versus c yield a straight line at different temperature as shown in Figure 5, confirming that the adsorption of LLE on the mild steel in 0.5 M H 2 SO 4 solution obeys Langmuir isotherm. From the intercepts of the straight lines in Figure 5, the values of K ads at different temperatures were calculated and listed in Table 5. The K ads values decreased with the increase of temperature, which corresponded to the reduction of inhibition efficiency with rising temperature. Moreover, the adsorption enthalpy (ΔH ads 0 ) can be calculated using Van't Hoff equation 62 :   where R is the universal gas constant, T is the thermodynamic temperature. Figure 6 shows that there is a linear relationship between lnK ads and 1000/T, thus the value of H ads 0 Δ is calculated according to the Equation (5) and listed in Table 5, the negative value of ΔH ads 0 suggests the adsorption of the effective inhibitor component in LLE onto the mild steel surface is an exothermic process 62,63 .
Temperature is an important factor affecting the performance of inhibitor, and Table 3 indicates that the inhibition efficiency of LLE decreased with rising of temperature. To further understand the inhibitive mechanism of LLE, Arrhenius equation and transition state equation were employed to fit the corrosion rates at different temperatures. The two equations can be expressed respectively as follows 61,62 : where v is the corrosion rate obtained from weight loss measurements, A is the pre-exponential factor, h is the Plank's constant, N is the Avogadro's number, E a is the apparent activation energy, ΔS a is the apparent entropy of activation, ΔH a is the apparent enthalpy of activation. Correspondingly, the fitting results are shown in Figure 7, the  plots in Figure 7 present straight lines, thus according to the slopes and intercepts of these straight lines, the kinetic parameters including E a , ΔH a and ΔS a , are calculated and listed in Table 6. The value of E a is greater in 0.5 M H 2 SO 4 solution containing LLE than that in blank solution, which indicate that the adsorption of LLE on the mild steel surface leads to the dissolution of steel become difficult. And just as expected, the values of E a and ΔH a follow the same trend of variation with the concentration of LLE, according to the transition state theory, ΔH a = E a −RT 64 . Additionally, Table 6 shows that the values of ΔS a is more positive in inhibited solution compared to that in blank solution, which indicate the system disorder increased from reactant to activated complex during the corrosion process 48,65 . The SEM micrographs of mild steel samples after immersed in blank solution and in inhibited solutions for 4 h at 298 K are shown in Figure 8. As shown in Figure 8a, in the absence of LLE, the mild steel surface is fairly      By pH-gradient sedimentation, LLE was preliminarily separated into acidic precipitate (AP), alkali precipitate (BP) and remaining solution (RS). HPLC chromatograms of standard ursolic acid and AP are shown in Figure 9, which indicate that ursolic acid is present in AP, but AP is not pure substance. This result is further confirmed by the MS of AP shown in Figure 10 Figure 11 shows the FTIR spectra of AP and the differential infrared spectra of LLE. The differential spectroscopy was obtained through the FTIR spectra of stock solution of LLE and the sulfuric acid solution using software OMNIC 7.0 by differential spectroscopic analysis. It is clearly seen that the infrared absorption peaks of AP are basically in accordance with those of LLE, indicating that AP is the main component of the LLE. Combined with the results of MS, the strong and broad peak at 3473 cm −1 can be attributed to the stretching vibrations of hydroxyl (OH) or carboxyl (COOH) groups, the peaks at 1720 cm −1 and 1621 cm −1 are the characteristic peaks of O = C and C = C, respectively 69,70 . Absorption peak at 1323 cm −1 may be indicate C-H bending vibration 69 , and the peaks at 778, 646, 515 cm −1 could be assigned to C-H of aliphatic and aromatic carbon. Figure 12 74 . The FTIR of BP is shown in Figure 13, the absorption peaks of 3434 and 1626 cm −1 are attributed to the stretching vibrations of hydroxyl and C = C, respectively 69,70 , the peak at 1417 cm −1 can be assigned to the bending vibrations of C-H or O-H 31 , the adsorption band 1473 cm −1 corresponds to the stretching vibration of C-O 69 , and the peak 569 cm −1 may be assigned to C-H of aliphatic and aromatic carbon. Consequently, the infrared absorption of   BP is corresponding to the analysis of MS. Seven acids, including valine, tyrosine, phenylalanine, lysine, histidine and arginine were detected in RS, similar results were also reported by Yokota et al. 75 . Moreover, the color of RS changed with the pH value of the solution, which implies that RS contained anthraquinones.
Furthermore, the inhibition performance of the isolates and their mixtures was evaluated with the V/V concentration of 50% as a reference, the corresponding polarization curves and electrochemical parameters are shown in Figure 14 and Table 7, respectively. The results showed that AP can not inhibit the corrosion of mild steel in H 2 SO 4 solution, and the inhibitory effect of BP was very weak, however, RS exhibited obvious corrosion inhibition. It is particularly important to note that the inhibition efficiency increased significantly when the isolate was mixed, and the inhibition efficiency of the mixture of three isolates is as high as 91%, which indicate that there is a strong synergistic effect between the chemical components of LLE.
From the foregoing, the inhibition mechanism of LLE can be described as follows: When the loquat leaves were immersed in H 2 SO 4 solution, the organic compounds in the leaves, such as amino acids, organic acids, glycosides, and so on, were dissolved into the solution and were further protonated in strongly acidic solution. It is usually considered that the steel surface charges positive charge in acid solution 76,77 , which means that the protonated substances are difficult to adsorb on the steel surface due to the electrostatic repulsion, but Solmaz's study found that the steel surface carries negative charge in HCl solution in the presence of inhibitors 78,79 , which allows the positively charged inhibitor molecules directly adsorb on the steel surface through electrostatic interaction, and then reduce the steel dissolution. However, Figure 2 showed a weak inhibitory effect of LLE on the anodic dissolution of iron, which implies that the steel surface in the investigated solutions is positively charged or has very little negative charge. These protonated substances also can adsorb on cathodic sites of the steel in competition with hydrogen ions, maybe due to the higher adsorption rate of the positively charged inhibitor molecules than that of hydrogen ions, thus the cathodic reaction of hydrogen ions reduction can be inhibited 78,79 , and the inhibitive effect of LLE on the cathodic reaction was clearly shown by polarization curves in Figure 2. As a result, LLE act as a modest cathodic inhibitor.   Methods Electrolyte and Material. The corrosive solution 0.5 M H 2 SO 4 was prepared using analytical grade sulphuric acid and distilled water. The chemical composition (in wt%) of mild steel as follows: C (0.16%), Si (0.18%), Mn (0.29%), P (0.014%), S (0.013%) and Fe for balance, and the preparation of specimens used in the experiments were the same as reported earlier [48][49][50] . Before conducting the test, the samples were mechanically abraded with emery paper up to 800 grit. During the experiment, the test solution was open to the air and under a static condition, meanwhile, temperature was controlled by a water thermostat with an accuracy of 1 K. Isolation and characterization of LLE. LLE was first neutralized with sodium hydroxide, when the pH value of LLE increased to 1, white substance started to precipitate out, continuously adding alkali solution until before the color of LLE changes dramatically, at this point, the pH value of solution is about 5, after standing for 24 hours, the acidic precipitate of LLE (AP) was separated by centrifugation, then AP was washed three times with distilled water and dried in vacuum. While the remaining solution was continued alkalization, at pH 12, brown precipitate was found. The pH value of solution was adjusted to close to 14 in order to make precipitation more complete. Then alkali precipitation (BP) was obtained by centrifugation after the solution was stood for 24 h, and the remaining solution (RS) was neutralized with sulfuric acid and was not further isolated. The methanol solution of AP was analyzed by Agilent 1100 high performance liquid chromatograph (HPLC) used ursolic acid as the reference substance, in which, the analytical column used was phenomenex C18, the mobile phase was methanol aqueous solution (90: 10, V/V) with a flow rate of 0.5 mL/min at 30 °C, and eluent was detected at 215 nm. Moreover, the composition of AP was further confirmed by mass spectrometry (MS). The composition of BP was detected by liquid chromatography-mass spectrometry (LC-MS) using Agilent LC1100/ MSD-VL, and the chromatographic conditions were consistent with the foregoing. Furthermore, Hitachi L-8900 amino acid analyzer was employed to determine amino acids in RS, and FTIR spectra (KBr) was measured by Nicolet-6700 spectrophotometer. Measurement Method. Electrochemical measurements were conducted in a three-electrode cell assembly with mild steel working electrode, saturated calomel electrode (SCE) and platinum electrode using CHI660E electrochemical work station (Shanghai Chenhua Instruments Co. Ltd., China). All potential data reported were referred to SCE reference electrode. The working electrode was initially immersed into test solution at OCP until it reached steady state. Then, EIS was measured at OCP. Ultimately, the polarization curve was obtained by potentiodynamic scanning method. The procedure for electrochemical measurements had been previously described in detail [48][49][50] .

Preparation of Loquat Leaves
Weight loss measurements were performed in a wild-mouth glass bottle containing 500 mL of test solution for 4 h at different temperatures (298, 308, 318 and 328 K). After that, the microscopic examination of the specimen surface was performed using a Tescan Vega3 scanning electron microscopy at high vacuum. The experimental procedures and data processing were similar to previously reported researches [48][49][50] .

Conclusions
(1) LLE can be considered as a modest cathodic inhibitor which mainly inhibits the cathodic process on the corrosion of mild steel in 0.5 M H 2 SO 4 solution, and its inhibitive efficiency increases with the increment of LLE concentration, but decreases when rising temperature. (2) In the presence of LLE, the charge transfer resistance increases while double layer capacitance declines, confirming the adsorption of inhibitor molecules on the mild steel surface. The adsorption of LLE on steel surface is an exothermic process and obeys Langmuir adsorption isotherm. (3) The corrosion inhibition of LLE is caused by the combination of various components in the extract, and there is a synergistic effect between the components.
Data Availability. Te datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.