Corrosion Inhibition of Mild Steel in Hydrochloric Acid Environments Containing Sonneratia caseolaris Leaf Extract.

Sonneratia caseolaris leaf extract was characterized for its mitigation of the electrochemical corrosion of steel in naturally aerated hydrochloric acid environments by electrochemical methods and surface analysis. The presence of S. caseolaris leaf extract (SCLE) in the hydrochloric acid medium ameliorated the corrosion resistance of steel via the adsorption of SCLE species to form a barrier layer. The improved inhibition effectiveness was demonstrated to be independent of the SCLE concentration and the corrosive environment. The highest inhibition performance of approximately 98% was reached for steel in a 1 M HCl medium containing 2500 ppm SCLE. The performance significantly decreased with a decrease in the HCl concentration from 1.00 to 0.01 M with the same SCLE concentration. In addition, severe corrosion occurred on the uninhibited steel surface but was significantly reduced on the inhibited steel surface. The analyzed results also indicated the existence of electronegative functional groups in SCLE, which could promote the adsorption process for the self-formation of the barrier layer on the steel surface. The work reported herein suggested a powerful strategy to mitigate electrochemical corrosion by adding an effective new inhibitor to achieve a green inhibitor system.

Introduction

Carbon steels are easily corroded in acidic environments, particularly in sulfuric and hydrochloric acids that are used for industrial cleaning, pickling, acid descaling, and oil well acidizing.[1] Therefore, protection of steel from corrosion is an important requirement in industrial applications. One of the most effective and easiest ways to minimize corrosion is the addition of inhibitors.[2] The many corrosion inhibitors that are used include inorganic salts, rare earth compounds, organic compounds, and/or natural compounds extracted from plants.[3−6] Unlike other chemicals, the natural compounds extracted from vegetable and plant components are nontoxic, eco-friendly, inexpensive, biodegradable, readily available, and reusable.[6] In contrast, synthesized chemical corrosion inhibitors are considered toxic, expensive, and deleterious to the environment and human health.[7]

Natural compounds from plant and vegetable extracts are considered to be green inhibitors of steel corrosion in various aggressive solutions due to their heteroatoms having multiple bonds. These are mostly nitrogen, sulfur, oxygen, and phosphorus atoms with π-electrons, aromatic rings, or long carbon chains in the compound structure.[8−16] Interestingly, most of the recent works have focused on compounds extracted from vegetable and plant components for mitigating steel corrosion in different corrosion environments due to the functional groups in the naturally extracted compounds, such as flavonoids, alkaloids, terpenoids, polyphenols, amino acids, and tannins.[10−23] These important findings suggest that natural products have promising potential for protecting against steel corrosion and have been vividly characterized and enumerated as green inhibitors from certain leaf extracts, such as Hibiscus sabdariffa,[10]Piper betle,[11]Catharanthus roseus,[12]Luffa cylindrica,[13]Pigeon pea,[14]Ficus racemosa,[15]Ipomea staphylina,[16] Ginkgo,[17]Lannea coromandelica,[18] Olive,[19]Houttuynia cordata,[20]Tinospora cordifolia,[21] Bamboo,[22] and Aganonerion polymorphum.[23] These leaf extracts have demonstrated high protective performance against steel corrosion via organic film formation on the surface. This suggests the important role of natural products in the development of novel effective inhibitor systems for extending steel applications.

Sonneratia caseolaris leaves are enriched in flavonoids, steroids, triterpenoids, benzenecarboxylate derivatives, alkaloids, tannins, pectin, fatty acids, and sugars.[24,25] In addition, their chemical compositions include archin, chrysophanic acid, archicin, tannin, and color base used in food engineering.[26] The components of the S. caseolaris plant have been processed and used in many types of food products, such as raw and cooked vegetables, dietary fiber, sauces, crabapple vinegar, variole, and medicine (hematuria, hemostatic, parasitic worms, cancer). Furthermore, some works have reported some interesting applications of the biological activities of S. caseolaris plant extracts, including antibacterial, antifungal, pesticidal, insecticidal, antidiabetic, and anticholesterol activities.[27−29] Importantly, they are also recommended for use as a traditional medicine in medical exploration and treating health disorders, including antiseptic, astringent, antitussive, antipyretic, and hemostatic activities. They also show cytotoxic activities against hepatoma cells.[24,30−32] These investigations indicate that the products extracted from the S. caseolaris plant are readily available, nontoxic, easily grown, and have wide applications, particularly for their physical and chemical properties. Applying these extraction products to steel corrosion protection is challenging. However, there have been no studies that have shown the capability of these products to protect against steel corrosion. Therefore, the aim of this study was to apply an ethanol extract of S. caseolaris leaves to mitigate steel corrosion in a hydrochloric acid medium.

Materials and Methods

Hydrochloric acid (37%) and ethanol were obtained from Merck & Co. and used without any further treatment. The mild steel sheet was fabricated using a CNC cutting machine to obtain 10 × 10 × 3 mm3 specimens that were connected to copper wires by two face copper sticking plasters. These steel specimen systems were mounted at room temperature (cool mounting) by Epofix resin to obtain a 100 mm2 exposed surface area for surface analysis and electrochemical measurements. After cool mounting, the steel surface was then finished by 1200 grit silicon carbide paper, cleaned with deionized water and ethanol, and dried by a dryer. To extract the S. caseolaris leaves, the collected leaves from South Vietnam were first rinsed with distilled water several times, dried in a laboratory oven at 60 °C, and ground into a powder. The S. caseolaris leaf extract (SCLE) was produced by concentrating in a rotary evaporator after obtaining S. caseolaris leaf glue from Soxhlet extraction with 99% C2H5OH at 75 °C. After liquid–liquid extraction using an ethyl acetate and double-distilled water system, a part of the S. Caseolaris leaf extract was then condensed to a glue named S. Caseolaris leaf-water extract (SCLE). SCLE was stored in a 4 °C refrigerator and used as a corrosion inhibitor for steel in hydrochloric acid solutions. To identify the functional groups of the S. caseolaris leaf extract, gas chromatography–mass spectrometry (GC–MS) was used. To ensure the reproducibility of the results, GC–MS was performed at both 320 and 600 °C.

To characterize the electrochemical properties, electrochemical measurements were carried out in an electrochemical cell connected with the VSP system from Biologic Scientific Instruments. The electrochemical cell, which was placed in a Faraday cage, contained a platinum mesh, Ag/AgCl electrode, and mild steel electrode as the three-electrode system. To run the EIS, the frequency range was set from 104 to 0.01 Hz with 0.01 V sinusoidal perturbation signals. A potential range of −20 to +20 mVOCP was applied to the LPR measurements at a scan rate of 0.166 mV/s. Both EIS and LPR were conducted every 1 h of immersion time at OCP within a 24 h period. Potentiodynamic polarization was performed from −250 mV with respect to OCP to 0 mVAg/AgCl at a sweep rate of 0.166 mV after 24 h OCP. These electrochemical techniques, including LPR, EIS, and PD, as well as their procedures, were also conducted in 0.50, 0.10, and 0.01 M HCl solutions containing 0 and 2500 ppm SCLE.

The surface morphology of steels after 24 h immersion in all solutions at OCP was characterized using field emission scanning electron microscopy (FESEM, Hitachi S 4800). In addition, the surface roughness of the steel surface after 24 h immersion in a 1 M HCl solution containing 2500 ppm SCLE was also characterized using atomic force microscopy (AFM, Agilent Technologies AFM 5500). The chemical and structural information of the steel surface after immersion in 1 M hydrochloric acid solutions containing different SCLE concentrations for 24 h was also identified using Raman spectroscopy (Xplora One) at a wavelength of 532 nm. To explore the chemical state, electronic structure, and state density of the steel surface after immersion in 1 M hydrochloric acid solutions containing 0 and 2500 ppm SCLE for 24 h, X-ray photoelectron spectroscopy (XPS, Kratos Nova) was also implemented.

Results and Discussion

Potentiodynamic polarization plays an important role in characterizing the material properties, the corrosion rate, and the effectiveness of its inhibition. Figure a represents the potentiodynamic curves of steel in 1 M HCl solutions containing several different SCLE concentrations. Under the investigated conditions, steel acted as an active material with a high corrosion current density, and no protective information formed on the surface, whereas a protective layer was formed in all inhibited surface systems with very low corrosion current densities.[3,11,15] Furthermore, the corrosion potential was strongly shifted to a more negative direction when adding SCLE to the solutions. A significant decrease in icorr (Table ) was obtained with an increase in the SCLE concentration from 0 to 2500 ppm. Figure also indicates that SCLE inhibited both anodic and cathodic reactions, where cathodic inhibition was evidently dominant with a more negative Ecorr and a lower icorr. According to the potentiodynamic results, the inhibition effectiveness of SCLE increases steadily from 0 to 98.06 ± 0.15% with an increase in the SCLE concentration from 0 to 2500 ppm. Then, the inhibition effectiveness slightly decreases from 98.06 ± 0.15 to 97.42 ± 0.24% with an increase in the SCLE concentration from 2500 to 3000 ppm.

Figure 1

Potentiodynamic polarization curves of steel in (a) 1.00 M HCl solutions with different SCLE concentrations and (b–e) 0.01, 0.10, 0.50, and 1.00 M HCl solutions with and without the addition of 2500 ppm SCLE. (f) Effect of HCl concentration on the corrosion potential of steel.

Table 1

Corrosion Parameters Observed in the Potentiodynamic Polarization Measurements

specimens (ppm)	Ecorr (mVAg/AgCl)	icorr (mA/cm2)	βa (mV/Decade)	–βc (mV/Decade)	η (%)
1.00 M HCl
0	–470	18.06 ± 0.60	237	283
1000	–517	1.63 ± 0.11	211	200	91.00 ± 0.61
2000	–519	0.65 ± 0.01	233	149	96.43 ± 0.02
2500	–508	0.35 ± 0.03	184	122	98.06 ± 0.15
3000	–519	0.47 ± 0.04	184	128	97.42 ± 0.24
0.50 M HCl
0	–511	13.61 ± 0.04	234	267
2500	–549	0.30 ± 0.02	170	125	97.83 ± 0.15
0.10 M HCl
0	–570	3.05 ± 0.30	266	317
2500	–522	0.13 ± 0.01	138	151	95.89 ± 0.27
0.01 M HCl (icorr in μA/cm2)
0	–573	77.82 ± 8.69	165	247
2500	–539	27.28 ± 1.40	123	160	64.94 ± 1.80

To further explore the phenomenon of how the HCl concentration influences the corrosion rate of steel and the inhibition performance of SCLE, we evaluated the effect of the HCl concentration on the investigated system using HCl concentrations of 0.01, 0.10, 0.50, and 1.00 M by adjusting the HCl solution. The SCLE concentration was kept constant at 2500 ppm for the 0.01, 0.10, 0.50, and 1.00 M HCl solutions. Therefore, potentiodynamic experiments were performed in these solutions without and with the addition of 2500 ppm SCLE, and the results are displayed in Figure b–e. They reveal that the increase in the HCl concentration strongly influences the corrosion potential of steel with more notable values (Figure f) and increases corrosion current densities, resulting in a higher electrochemical corrosion reaction.[2,9] Furthermore, a strong decrease in the current densities in the anodic branches of the inhibited systems is attributed to protective film formation and less metal dissolution. In particular, a more significant difference in the corrosion current density between the uninhibited and inhibited systems was observed when increasing the HCl concentration. The results demonstrated a change in the inhibition mechanism due to the shift in the corrosion potentials. Figure b,c shows more notable Ecorr and a significant decrease in icorr for the inhibited systems in the 0.01 and 0.10 M HCl solutions, whereas a more negative Ecorr and a strong decrease in icorr were observed for the inhibited systems in the 0.5 and 1.0 M HCl solutions. This phenomenon suggests a mixed inhibitor with a dominant cathodic inhibition of steel in solutions with higher HCl concentrations. With an increase in HCl concentration from 0.01 to 1.00 M, the inhibition efficiency increased gradually from 64.94 ± 1.80 to 98.06 ± 0.15%. Combining this with the polarization results, we can conclude that the increase in inhibition efficiency of SCLE for steel at high HCl concentrations was due to the improved adsorption process on the steel surface in more aggressive environments; meanwhile, an increase in the HCl concentration resulted in a more active steel surface.

It can be concluded from the potentiodynamic results in Figure that SCLE could benefit reduction in steel corrosion in HCl media. It is obvious that a significant reduction in the current density was observed with an increase in the SCLE concentration from 0 to 2500 ppm, and then, the current density slightly decreased when increasing the SCLE concentration up to 3000 ppm, as displayed in Figure a. Figures a and S1 represent schematic plots of the relationship between the potential and current in the LPR measurements of the steel specimens immersed in a 1 M HCl solution containing various SCLE concentrations (0, 1000, 1500, 2000, 2500, and 3000 ppm) and 2500 ppm SCLE solutions with different HCl concentrations (1.00, 0.50, 0.10, and 0.01 M). The results indicated an approximately linear behavior around the open-circuit potential region for all investigated conditions with a very low current density in the inhibited systems.[11]Figure b shows the polarization resistance (Rp) as a function of immersion time and SCLE concentration in a 1 M HCl solution. This indicates that the Rp values decreased with increasing immersion time but increased with increasing SCLE concentration from 0 to 2500 ppm. However, the Rp values then decreased when 3000 ppm SCLE was used. Figure c displays the increase in the Rp values with immersion time observed in the LPR measurements at 0.01 and 0.10 M HCl solutions. In contrast, these values decrease with increasing immersion time in 0.5 and 1.0 M HCl solutions. It also shows a strong decrease in the Rp values when the HCl concentration increased from 0.01 to 1.00 M, suggesting higher steel degradation in more aggressive HCl solutions. The above trend of the Rp values was consistent with the results in the investigated solutions with and without the addition of 2500 ppm SCLE; however, a greater difference in the Rp values between the uninhibited and inhibited systems was obtained in higher HCl solutions. This affirms the above potentiodynamic results that a more active steel surface could achieve higher inhibition performance by the addition of SCLE, and this observation is consistent with a previous report.[33]

Figure 2

(a) Linear polarization resistance curves and (b) polarization resistance as a function of immersion time of steel in 1 M HCl solutions with different SCLE concentrations, polarization resistance as a function of immersion time of steel in (c) different HCl concentration solutions and (d) a 2500 ppm SCLE solution containing different HCl concentrations.

According to Figures and 4, only single depressed semicircles with small inductance in low-frequency regions were displayed in the results measured in 0.5 and 1.0 M HCl solutions, where two semicircles with less depression were observed for steel measured in 0.10 and 0.01 M HCl solutions. This could be due to the different HCl concentrations that result in the different corrosion processes; meanwhile, the diameter of the arc curves and solution resistance significantly decreased with increasing HCl concentration. The increase in the diameter of the arc curves as a function of immersion time for steel exposed to 0.01 and 0.10 M HCl solutions suggests that the gentle electrochemical corrosion reaction formed a compact rust layer on the steel surface, whereas the significant decrease in impedance values with immersion time for steel in 0.5 and 1.0 M HCl solutions is due to a more severe electrochemical corrosion reaction. This result is consistent with the trend of the total resistances (Rtotal) observed on the fitting results, as given in Figure .

Figure 3

Nyquist plots of steel in (a) 1.00, (b) 0.50, (c) 0.10, and (d) 0.01 M HCl solutions during 24 h immersion time.

Figure 4

Bode plots of steel in (a) 1.00, (b) 0.50, (c) 0.10, and (d) 0.01 M HCl solutions during 24 h immersion time.

Figure 7

Simulated equivalent circuit with: (a) one- and (b) two-time-constant phase elements; polarization resistance as a function of time for steel in (c) different HCl concentrations, (d) different HCl concentration solutions containing 2500 ppm SCLE, and (e) different SCLE concentrations in a 1 M HCl solution. (f) CPE values as a function of time for steel in a 1 M HCl solution with different SCLE concentrations.

Figure a–d displays Nyquist plots of the EIS measurements in 2500 ppm solution containing 1.00, 0.50, 0.10, and 0.01 M HCl, respectively. This indicates that the impedance values are much larger than those of steel specimens immersed in the blank solutions. The plots of inhibited steels in 0.5 and 1.0 M HCl displayed a one-time constant, while a two-time constant was depicted for the results of inhibited steels under 0.10 and 0.01 M HCl conditions. These were observed on both the Nyquist (Figure ) and Bode (Figure ) diagrams. Figure a,b indicates that an increase in the impedance values was observed at 4 h from the initial immersion time, and then they decreased for the next 4 h. In contrast, Figure c,d shows the gradual increase in the impedance values during 24 h immersion of steel in 2500 ppm SCLE solutions containing 0.10 and 0.01 M HCl. This suggests that with the two-time-constant diagrams, the first capacitive loop located in the high- and medium-frequency regions could be assigned to the electrolyte/protective layer interface and the processes within the protective layer. These parameters directly relate to the surface roughness and inhomogeneity.[34,35] The other capacitive loop located in the low-frequency region is correlated with the charge transfer at the protective film/steel surface interface. Figure c,d evidently shows that the impedance values and diameter of the semicircles regularly increased, suggesting a high corrosion resistance for steel in low-HCl-concentration solutions containing 2500 ppm SCLE. In addition, the EIS results in a 1 M HCl solution are given in Figures a, 5a, and S2 in Nyquist form and in Figures a, 6a, and S3 in Bode form. They indicate that only one semicircle loop was observed in all specimens, indicating that the charge transfer controlled the corrosion processes of this system.[36] The same mechanism was also observed in the EIS results, as shown in Figures b and 5b in the Nyquist form and in Figures b and 6b in the Bode form for the 0.5 M HCl solution. The results also showed that the diameters of the semicircles and the impedance were enhanced with increasing SCLE concentration from 0 to 2500 ppm, indicating an improved corrosion resistance. However, a slight reduction in the impedance values and the diameter of the semicircles when adding 3000 ppm to the solution could be due to the elongation of a diffusion layer that may hinder the activity of the SCLE species in the 1 M HCl solution.

Figure 5

Nyquist plots of steel in a 2500 ppm SCLE solution containing (a) 1.00, (b) 0.50, (c) 0.10, and (d) 0.01 M HCl during 24 h immersion time.

Figure 6

Bode plots of steel in a 2500 ppm SCLE solution containing (a) 1.00, (b) 0.50, (c) 0.10, and (d) 0.01 M HCl during 24 h immersion time.

To obtain the optimal EIS parameters, the equivalent circuits in Figure a,b used for fitting EIS data in the ZSimpWin software were simulated by combining the results of the electrochemical and surface analysis. These circuits include Rs, Rpro, Rct, and Rp as solution, protective, charge transfer, and polarization resistances, and CPE as the constant phase element (pro and dl for the protective and double layer, respectively). For the uninhibited system, the protective layer is replaced by a rust layer. CPE, which includes the CPE magnitude value and the phenomenological coefficient, was used for EIS fitting instead of the capacitor (C) due to the nonideal dielectric behavior, which causes a nonuniform capacitive layer on the electrode surface.[4,15]Figure c presents the total resistances of the steel exposed to HCl solutions with and without the addition of 2500 ppm SCLE. The total resistance values increased with decreasing HCl concentration, and the total resistance values of the inhibited steels were much higher than those of the uninhibited steel specimens, resulting in enhanced corrosion resistance. Furthermore, Figure d shows the SCLE concentration dependence of the polarization resistance in a 1 M HCl solution. The polarization resistance values significantly increase with increasing SCLE concentration up to 2500 ppm. The increase in the polarization resistance value correlates with low metal dissolution, resulting in a lower corrosion rate. Figure e presents a significant decrease in the CPE magnitude value with increasing SCLE concentration up to 2500 ppm due to the adsorption of the SCPE species on the electrode surface, indicating the reduction of the dielectric constant.

The film components were then characterized by Raman spectroscopy and XPS. Figure represents the results of Raman spectroscopy for a steel surface after 24 h of exposure to 1 M HCl solutions with different SCLE concentrations. This result indicated that lepidocrocite (γ-FeOOH) can be identified at bands at approximately 249, 381, 650, 840, 1051, and 1303 cm–1 in all specimens, which is consistent with previous work.[37,38] The enrichment of Fe2O3 bands at approximately 222, 300, 400, 500, 700, and 1318 cm–1 and Fe3O4 bands at approximately 550, 675, and 1460 cm–1 were observed for the uninhibited steel surface, and these peak intensities decreased for the inhibited steel surfaces with increasing SCLE concentrations. For the inhibited surfaces, the bands at approximately 1440, 1485, and 1599 cm–1 are assigned to aromatic compounds; the substituent groups could be attributed to the bands in the 650–1000 cm–1 region;[39] the bands at approximately 500–800 and 1616 cm–1 are assigned to C–C aliphatic chains and C–C rings, respectively; and the C=C ring, CH3, CH bending, and C–O–C are also assigned to the bands at approximately 1577, 1446, 1301, and 1238 cm–1.[40,41] These observed peak intensities were enhanced with increasing SCLE up to 2500 ppm, suggesting the evident adsorption of the SCLE species on the steel surface. To confirm the SEM and Raman results as well as the adsorption behavior of the SCLE component, XPS was also performed on the uninhibited and inhibited steel surfaces, and the results are given in Figure . Figure a presents the entire XPS results, which indicate the presence of Fe, C, and O in both steel surfaces.[42,43]Figure b shows the high-resolution XPS of O 1s and shows two obvious peaks at approximately 531.5 and 529.5 eV for the steel surface immersed in the blank solution, which are assigned to the oxygen of FeOOH and iron oxides, including FeO/Fe2O3/Fe3O4, respectively.[44] However, the peak at approximately 529.5 eV was very small in the spectrum of the inhibited steel surface, while the peak at approximately 531.5 eV was still obvious, and the occurrence of the new oxygen peak at approximately 532.5 eV was due to the oxygen from the C=O and C–O bonds of the SCLE species. Figure c shows the existence of iron oxyhydroxide and oxides at approximately 725 and 711 eV on both the uninhibited and inhibited steel surfaces. The evident peaks of metallic Fe, satellite, ferric compounds, and ferrous compounds at approximately 733.1, 728.9, and 719 eV were clearly observed for the uninhibited steel surface and belonged together with those of the iron oxyhydroxide and oxides, suggesting the complex corrosion products of iron due to the strong corrosion reaction. Figure d presents the narrow scan spectra of C 1s with a significant difference in the peak intensities between the uninhibited and inhibited steel surfaces. The peaks at approximately 284.8, 286.0, and 288.5 eV for the uninhibited steel surface are attributed to C, CO, satellite, and Fe(CO), whereas the result for the inhibited steel surface showed a very high C 1s peak intensity that can be assigned to the C=O bond, sp2-bonded carbon, and C–O and C=C bonds at approximately 288.0, 284.5, 286.0, and 284.5 eV, respectively. Therefore, the XPS results indicated film formation via the adsorption of the SCLE species on the steel surface.

Figure 8

Raman results of a steel surface after 24 h exposure to 1 M HCl solutions with different SCLE concentrations.

Figure 9

XPS results of uninhibited and inhibited steel surfaces after 24 h immersion time: (a) full and high-resolution results of O (b), Fe (c), and C (d).

Figures and S5 present the SEM images (high and low magnifications) of the steel surfaces after 24 h immersion in 1 M HCl solution containing different SCLE concentrations. Figures a and S5a indicate that the steel surface exposed to 1 M HCl solution was severely corroded since the entire steel surface was damaged, resulting in the occurrence of a high density of wide and deep vacuoles on the steel surface. When 100 ppm SCLE was added to a 1 M HCl solution, severe damage was uniformly observed on the steel surface, as shown in Figures b and S5b. In addition, slight etching was observed on the steel surface with the agglomerated particles when steels were immersed in 1 M HCl solutions containing 1000 (Figures c and 5Sc), 2000 (Figures d and 5Sd), and 3000 ppm SCLE (Figures f and 5Sf). However, less corrosion occurred on the steel surface immersed in 1 M HCl solutions containing 2500 ppm SCLE, as shown in Figures e and 5Se, due to the remaining initial abrasive traces and the appearance of uniformly agglomerated particles. Figure shows the 2D, relative Volta potential map, and 3D images of the surface roughness of steel immersed in 1 M HCl solutions containing 2500 ppm SCLE, with the result indicating a 21.8 nm root mean square (rms: grainwise). The smooth surface could be caused by film formation on the steel surface[10,17,45] via the adsorption of the SCLE species.

Figure 10

SEM image of the steel surfaces after 24 h immersion in 1 M HCl solution containing (a) 0, (b) 100, (c) 1000, (d) 2000, (e) 2500, and (f) 3000 ppm SCLE.

Figure 11

Atomic force microscopy (a) 2D, (b) relative Volta potential map, and (c) 3D images of the steel surface after 24 h immersion in 1 M HCl solution containing 2500 ppm SCLE.

The high- and low-magnification SEM images of the steel surfaces after 24 h immersion in solutions with different HCl concentrations containing 0 and 2500 ppm SCLE were also characterized. The results are given in Figures and S6 and indicated that the decrease in HCl concentration reduced the damage described above to the steel surface when immersed in 1 M HCl solution as shown in Figure a. In contrast, narrow and shallow vacuoles appear on the steel surface immersed in 0.5 M HCl solution in Figures a and S6a; slight etching and scattered pits are found on the steel surface immersed in 0.1 M HCl solution in Figures c and S6c, and the steel surface immersed in 0.01 M HCl solution is uniformly corroded as shown in Figures e and S6e. Therefore, the steel damage depended on the HCl concentration solution and occurred at a higher rate with a higher HCl concentration. When 2500 ppm SCLE was added to those solutions, steel surfaces were protected by a protective layer. A steel surface showing a clear vestige of protective layer formation was immersed in a solution with a higher HCl concentration containing 2500 ppm SCLE, as shown in Figures e, S5e, 12b, and S6b. Slight etching and initial abrasive traces were observed on all steel surfaces immersed in HCl solutions containing 2500 ppm SCLE. The density of the agglomerated particles decreased with decreasing HCl concentration, as shown in Figures b,d,f and S6b,d,f.

Figure 12

SEM image of the steel surfaces after 24 h immersion in 0.5 M HCl solution containing (a) 0 and (b) 2500 ppm SCLE; 0.1 M HCl solution containing (c) 0 and (d) 2500 ppm SCLE; and 0.01 M HCl solution containing (e) 0 and (f) 2500 ppm SCLE.

Based on the experimental results, we can assume that when steel is immersed in HCl solution, corrosion initially starts at imperfect locations on the steel surface structure.[5,46] The main reactions in the anodic and cathodic processes could be attributed to

Small pits occurred, and iron was dissolved in the pit edges, resulting in a wider pit that developed and damaged the entire steel surface when the immersion time was increased. It can be seen that a higher HCl concentration could cause more rapid Fe dissolution, resulting in a higher corrosion rate, as observed in the electrochemical and surface analysis results. The GC–MS results shown in Figure S4 and Table S1 indicated the possible compounds with various functional groups[24−26,28,31,32] that can form a protective film via adsorption processes. The major compounds with high area percent and quality include 2-methylfuran, acetic acid, furfural, 2-furancarboxaldehyde, 5-methyl-, benzyl alcohol, 3-(ethyl-hydrazono)-butan-2-one, 4H-pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6methyl, dianhydromannito, 5-hydroxymethylfurfural, benzofuran, 2,3-dihydro-2-methyl-, 2,4-hexadiene, 3,4-dimethyl-, (Z,Z)-, hexadecanoic acid, and ethyl ester at 6.625, 7.210, 9.174, 9.398, 10.552, 11.095, 11.327, 12.016, 12.421, 12.886, 13.894, 14.712, and 16.503 min. The adsorption phenomenon can be accounted for by both physisorption and chemisorption due to electrostatic interaction between the SCLE species and the positive charge on the steel surface, and the combination of the unoccupied d-orbital of the Fe atoms with a lone pair on the heteroatoms of the SCLE species.[6,8,10,12,14,18] A more active surface could promote the adsorption process as a result of a higher inhibition efficiency. The entire surface could be covered by a uniform barrier layer at an optimal SCLE concentration, producing the highest inhibition efficiency. However, an incomplete and nonuniform protective layer formed on the steel surfaces exposed to lower and higher SCLE concentrations due to the amount and aggressiveness of the SCLE species in the investigated solutions, resulting in lower inhibition efficiencies.

Conclusions

In this work, the S. caseolaris leaf-ethanol extract was used as a corrosion inhibitor for steel corrosion in hydrochloric acid media. The results indicated that mild steel becomes more susceptible to electrochemical corrosion in highly acidic environments with a higher corrosion rate and more positive corrosion potentials. It also demonstrated that the SCLE species preferred adsorption on more active steel surfaces as a result of the higher inhibition efficiencies for steel in more aggressive acidic environments. Furthermore, the SCLE acted as a mixed inhibitor dominated by anodic inhibition for steel in solutions with lower HCl concentrations and a dependence on the SCLE and HCl concentrations as well as a slight dependence on immersion time. Surface analyses demonstrated that the SCLE molecules formed a robust protective barrier by the bonding of the SCLE species as a result of cooperative adsorption of ionic species and iron products on the steel surface. This cooperative adsorption could be attributed to the multifunctional groups in SCLE, resulting in the formation of a compact and adhesive layer that slowed down the electrochemical corrosion reaction and significantly enhanced the corrosion resistance of steel under the investigated conditions.