Corrosion mitigation of mild steel in hydrochloric acid solution using grape seed extract

Plant extracts have gained a lot of attention due to their ecofriendly nature for corrosion inhibition. In this study, we examined the inhibition performance of grape seed extract as an eco-environmental inhibitor for mild steel in hydrochloric acid medium. Electrochemical impedance spectroscopy, potentiodynamic polarization, and electrochemical noise techniques were employed to study mild steel's electrochemical behavior in the hydrochloric acid solutions containing grape seed extract. Results depicted that grape seed extract could successfully inhibit the corrosion of mild steel. Besides, water droplet contact angle, field-emission scanning electron microscopy coupled with energy dispersive spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy were utilized to study the surface of mild steel specimens after dipping in acidic solutions. Electrochemical impedance results showed a corrosion efficiency of about 88% in 300 ppm of grape seed extract. Also, results revealed more compact corrosion products with improved integrity in the presence of grape seed, which confirmed electrochemical test results.

www.nature.com/scientificreports/ grape seed extract (GSE) containing a significant amount of flavonoids such as catechin, epicatechin, and some phenolic acids. The corrosion mitigation of MS by GSE has not been divulged so far. Therefore, the corrosion inhibition of GSE as a cheap, eco-friendly by-product of the wine industry was evaluated in this study. Various electrochemical tests have been set up to determine the extent and mechanism of corrosion inhibition.
Moreover, after corrosion experiments, the surface analysis has been applied to prove the interactions of inhibitive species with metal surfaces and/or protective film formation. Furthermore, the specimens' physical chemistry features were evaluated by measuring the contact angle (CA).

Experimental
Materials. The GSE was purchased from Ebnemasouye Co. The MS plates (8 cm × 3 cm) were acquired from the Foolad Mobarake Co. The elemental composition of the MS plates is provided in Table 1. Surface preparation was implemented prior to electrochemical tests to eliminate the surface contaminations and attain a convenient surface profile. The MS surface was carefully abraded using sandpapers. Subsequently, the MS plates were sonicated in acetone and methanol to eliminate the surface contaminations. The GSE was added to the hydrochloric acid (1 M) to prepare test solutions (0, 100, 200, and 300 ppm).
Techniques. The potentiodynamic polarization and EIS (electrochemical impedance spectroscopy) were implemented on a 1 cm 2 exposure area to assess the inhibition efficiency of the GSE. The Ivium software was www.nature.com/scientificreports/ used to record the electrochemical tests. Zsimpwin software was employed to fit the EIS data. The classical threeelectrode system (the working electrode, the Ag/AgCl/3 M KCl reference electrode, and the graphite counter electrode) was used in the electrochemical cell. The EIS measurements were implemented within the frequency range from 10,000 to 0.01 Hz at 10 mV perturbation. The polarization was measured at 1 mV s −1 within ± 200 mV (cathodic to anodic side) versus open circuit potential. A cell containing two equal working electrodes and the reference electrode (silver/silver chloride) was utilized to acquire the EN (electrochemical noise) signals. The duration of measurements was 1800s, and the sampling interval was 0.2 s. Triplicate samples were arranged for each electrochemical test to ensure reliability. All the electrochemical measurements were conducted at 25 °C. However, EIS was also carried out at 50 °C. After exposure to the test solutions, the samples were rinsed with water and dried. The dual scope DME C-26 model of AFM (Atomic Force Microscopy, Semilab Germany GmbH) was used to assess the topography of the exposed samples. The elemental composition and morphology of the surface film formed on the MS specimens subjected to the test solution were assessed by EDS (energy-dispersive X-ray spectroscopy) and SEM analysis. Surface wetting properties were investigated through contact angle measurement using a homemade contact angle measuring system. Raman confocal and Fourier transform infrared (FTIR) spectroscopies were performed on the MS samples after exposure to the test solutions. After being exposed to the test solution, the film formed on the MS surface was examined by X-ray photoelectron spectroscopy (XPS). XPS (Bes Tec/Germany) was measured in a vacuum chamber of 10 -10 mbar under 1253.6 eV. Origin 8.0 software was utilized to fit the experimental data with the Gaussian function.
UV-Vis spectroscopy was used to show the emergence of a complex between grape seed extract with iron cations.

Result and discussion
Electrochemical performance. EIS. EIS was employed to assess the electrochemical properties of MS in the presence and absence of GSE in 1 M HCl solution. Nyquist and Bode diagrams are depicted in Fig. 2. Nyquist curves in Fig. 2a show an increase in the semi-circle diameter with an increment in the GSE concentration. Low-frequency impedance (at 10 mHz) also indicates an increasing behavior with an increase in the GSE concentration, reflecting increasing adsorption of the GSE on the steel surface. The equivalent circuit was employed to fit the EIS data is displayed in Fig. 3. This circuit contains electrolyte resistance (R s ), charge transfer resistance (R ct ), and the constant phase element of the double layer (CPE dl ). The electrochemical parameters derived from the fittings are provided in Table 2. The effective capacitance of the electrical double layer (C dl ) was calculated using Eq. (1) 32,33 . where n and Y 0,dl, demonstrate the exponential and the admittance terms of CPE, respectively.
(1) www.nature.com/scientificreports/ As per Table 2, an increment in the GSE concentration resulted in the decrease of double-layer capacitance and charge transfer resistance increment, indicating an increase in the extent of GSE adsorption on the MS surface.
The inhibition efficiency (η) of the grape seed extract was computed using Eq. (2) 34 .
where R ct and R ct,i respectively represent charge transfer resistance in the absence and presence of GSE. Besides, surface coverage (θ) was computed based on the variation of double-layer capacitance, according to Eq. (3) 33 .
where C dl and C dl,i respectively represent double layer capacitance in the absence and presence of GSE. The surface coverage and inhibition efficiency results are provided in Fig. 4. The extent of surface coverage reflects the change in the electrical double layer thickness and relative permittivity. In comparison, the extent of inhibition efficiency demonstrates the active surface blocking by the inhibitor. It has been shown that a higher surface coverage than inhibition efficiency indicates the vertical alignment of the inhibitor molecules at the steel-electrolyte interface.
In contrast, higher inhibition efficiency than surface coverage discloses horizontal orientation on the inhibitor molecules 33,34 . In this work, higher inhibition efficiency than surface coverage shows effective surface blocking of the GSE molecules by the horizontal alignment on the metal surface. The effects of temperature changes were investigated on the inhibitory effect of GSE in acidic solution. The results are provided in Supplementary Information (Fig. S1, Fig. S2, and Table S1). Physically adsorbed species usually desorb at elevated temperatures leading to a decrease in inhibition efficiency 35,36 . A glance at these results reveals that by increasing the temperature, the inhibition efficiency of GSE on mild steel was increased, indicating chemisorption of GSE on MS surface.
Potentiodynamic polarization. The corrosion inhibition mechanism of MS dipped for 3 h in hydrochloric acid solution at 0-300 ppm of GSE was assessed by the polarization technique. Figure 5 shows fitted polarization curves of MS immersed for 3 h in the GSE-containing solution. Wagner-Traud equation (Eq. 4) was used to fit the curves and extract the electrochemical parameters 37 .  www.nature.com/scientificreports/ where the corrosion potential, corrosion current density, cathodic, and anodic Tafel slopes were respectively represented by E corr , i corr , β c , and β a . The inhibition efficiency of the GSE was computed using Eq. (5) 2 .
where i • corr i corr and i corr respectively represent corrosion current density in the absence and presence of GSE. All fitted parameters and calculate inhibition efficiency are depicted in Table 3. From this table, it can be discovered that the i corr diminished from 2260 μA/cm 2 for the neat solution (0 ppm GSE) to 87.2 μA/cm 2 for 300 ppm GSE solution 38 . The E corr switches to more negative values by a rise in the GSE concentration. Depression in both cathodic and anodic branches is evident in Fig. 5a. The corrosion potential in the polarization diagrams shifted to zero potential to have a better comparison of the data at the same extent of polarization. The results are given in Fig. 5b. Considering that both anodic and cathodic branches were depressed in the presence of GSE, it can be deduced that this extract mainly acts as a mixed-type inhibitor 32 .  www.nature.com/scientificreports/ Noise measurement. The GSE inhibitory effectiveness was also assessed by EN (electrochemical noise) analysis. Figure 6 shows the UWT (undecimated wavelet transform) spectrum of ECN (electrochemical current noise) signals within a span of 1800s, excluding the smooth signals, of the samples dipped for 3 h in 1 M HCl solution in the presence and absence of GSE. The primary ECN signal is shown behind the spectrum with the relative amplitude. From Fig. 6, it is clear that the low-frequency signals in the UWT spectrum of the neat specimen (containing no inhibitor) have higher relative intensities than those in the GSE sample. In Fig. 7, the relative energy distribution of each detail crystal for both specimens is depicted to better assess the contribution of low-and high-frequency transients in the measured ECN. Looking at Fig. 7, the corrosion mechanism for both samples is non-localized general corrosion. However, the relative energy contribution of detail crystals d1-d4 of the GSE sample is more significant than that of the blank specimens meaning that an adsorbed layer of GSE has been formed on the MS surface and the corrosion occurred on the uncovered areas of the adsorbed layer. The total energy of detail crystals (E T ) was measured as per Eq. (6) 6,39 .     Fig. 9, an increase in GSE concentration resulted in the establishment of a smoother surface with fewer cracks, indicating less corrosion damage and improved surface film integrity 25,40 . The SEM-EDS results are consistent with the electrochemical test results, in which an increase in the GSE concentration resulted in a reduction in the reaction area, a phenomenon known as the blanketing effect of corrosion inhibitors on the metal surface 41 .
AFM. According to 3D AFM images provided in Fig. 10, by increasing the concentration of GSE, the surface roughness has been decreased, which is in good accordance with SEM results where a smoother surface was detected at higher concentrations of GSE. A list of the acquired roughness parameters is shown in Table 4. According to this table, the average height distribution (Sa) of the blank sample was 95.3 nm, which was reduced to 72.2, 56.1, and 42.8 nm in the presence of 100, 200, and 300 ppm of GSE, respectively. It has been reported that corrosion inhibitors can reduce the surface roughness 30,31 , which is in good agreement with the outcomes of this work.
Contact angle. Figure 11 shows the contact angle of water droplets on MS substrate exposed to the test solutions for 3 h. The contact angle of samples showed an increase with an increase in the GSE concentration, reaching the highest at 300 ppm, which was about 83°. The contact angle results indicated that GSE led to an increase in MS surface hydrophobicity, which can be connected to the less oxygen content of the surface layer due to the lower corrosion rate in the presence of GSE 32,37 , which was proved by EDS analysis.
FTIR. FTIR spectrum test of MS immersed for 3 h in the acid solution in the presence and absence of 300 ppm of GSE are provided in Fig. 12. Also, a complex between Fe 3+ and GSE was fabricated to assess the chemical composition of mild steel after exposure to the GSE solution. To this end, a mixture of GSE and FeCl 3 solution at 1:1 weight ratio was prepared in distilled water. The mixture was centrifuged to obtain the settled residue. Finally, the precipitated solid was dried (4 h at 70 °C) and denoted as GSE-Fe. Stretching vibration of hydroxyl groups appeared at 3240-3440 cm −1 for all samples 42,43 . The C-H asymmetric stretching vibration occurred at 2920 cm −142, 44 . H 2 O bending vibration for all samples appeared at around 1620 and 1700 cm −1 , overlapping C=C and C=O stretching vibrations for GSE and GSE-Fe 45,46 . The absorption peaks that occurred at 1100, 1240, Absorption peaks relevant to Fe-O in bending modes appeared below 1000 cm −145,46 . As seen in Fig. 12, in the presence of GSE as that observed in the GSE-Fe complex, the peak has been shifted to higher wavenumbers confirming the chemical bonding of GSE with the metal surface.
Raman. Figure 13 represents the Raman spectra of MS samples exposed for 3 h to the acid solution with and without 300 ppm of GSE. Similar to the FTIR section of the MS, the Raman spectrum after exposure to the GSE solution was compared to that obtained from the GSE-Fe complex. The peaks related to C=O and C=C functions' stretching vibrations appeared at 1315 and 1534 cm −1 attributed to the aromatic rings of GSE structure 48  XPS. The surface analysis of the MS specimens exposed to the GSE solution was also assessed by XPS. Figure 14 depicts the C 1s and O 1s high-resolution spectra along with the survey spectrum of mild steel exposed to GSE solution. The survey spectrum (Fig. 14a) indicated the existence of oxygen, carbon, and iron as the main elements. The high resolution C 1s spectra (Fig. 14b) revealed the peaks at 282.3, 285, 285.5, 286.8, and 288.2 eV attributed to the C=C, C-C, C-OH, C-O-C, and OC=O groups, respectively [49][50][51] . Figure 14c revealed the peaks at binding energies of 530.8 and 531.6 eV respectively attributed to Fe-OC and Fe-OH bonds on the MS surface, indicating GSE-Fe chemical bonding 52 . In addition, the binding energies at 532.5 and 533.7 eV were respectively connected to C-OH and C-O bonds of the GSE components. The XPS results confirmed the FTIR and Raman results indicating the chemisorption of GSE on the MS surface.
Interaction of GSE with Fe 3+ cations. The possible interaction of GSE with steel surface was scrutinized by UV-Vis analysis in the solution phase between GSE and Fe 3+ cations. The UV curves are presented in Fig. 15. Looking at this figure, an absorbance peak appeared in 218 and 278 nm regions for GSE solution indicating the existence of π-π* and n-π* transition, respectively, for C=C and C=O organic groups 22,32 . FeCl 3 solution showed absorption peaks at 224 and 340 nm related to the d-d intra-orbital transitions in ferric cation curves 53,54 . In the presence of Fe3 + cations (GSE-Fe), the adsorption bands shifted to 224 nm, indicating the occurrence of LMCT 1 www.nature.com/scientificreports/ (the intramolecular transfer of electrons from a ligand to an organic compound which is called Ligand-to-Metal Charge Transfer) [54][55][56][57] . Besides, a shoulder formed at 265 nm for the GSE-Fe solution, which broadened the π-π* transition and indicated the complex formation (LMCT 2 ). A scheme showing the interaction of GSE and Fe 3+ cations is provided in Fig. 16. The photo illustrates the coordinated complex between the cation's empty d-orbital with pair of non-bonding electrons on the oxygen atoms in hydroxyl groups. This complex has low solubility, as depicted in Fig. 17. Such complex can be easily formed on the mild steel surface upon exposure to GSE solution. The UV-Vis results showing the chemical bonding of GSE and Fe 3+ cations confirmed the surface characterization results based on FTIR, Raman, and XPS, indicating the chemisorption of GSE on the MS surface.     Economic study of grape seed extract. Research is ongoing to achieve environmentally friendly and cost-effective inhibitors. Grapes are one of the most popular fruits in the world, and they are primarily used to make wine and fruit juices. Obviously, the resulting waste contains grape skin and seeds. The grape seed was commonly used in the oil industry, but its use increased in the pharmaceutical industries due to antioxidants' presence in its composition. The use of grape seed as a by-product of the alcohol and wine industry makes it very cheap. In general, grape waste produced in the wine industry is 20%. Due to its high volume as a by-product and valuable substances such as flavonoids, their use as green inhibitors in the corrosion industry is more reasonable than other green inhibitors. Table 5 provides a comparison between green inhibitors in acidic media with grape seed extract. As we studied in this article, grape seed extract's inhibition efficiency at only 300 ppm reaches 92% in an acidic environment, which is exceptional. In fact, most reports have reached a high percentage of inhibition  www.nature.com/scientificreports/ by using a high concentration of inhibitors. Therefore, grape seed extract is a cost-effective corrosion inhibitor for acid inhibition 6 .

Conclusion
1. GSE was employed as an effective corrosion inhibitor for mild steel in acidic solution. The extract has heteroatoms that can interact with the iron cations on the mild steel surface. FTIR, Raman, and XPS indicated that GSE chemically adsorbs on the MS surface. The formation of a complex between GSE and Fe 3+ cations was also confirmed by UV-Vis spectroscopy. 2. Increasing the inhibitory concentration and reducing charge transfer confirmed the adsorption of the GSE on the mild steel surface. The maximum inhibition efficiency (η %) in the presence of 300 ppm inhibitor was about 92%. Increasing the temperature revealed an increasing trend in the inhibition efficiency indicating chemisorption of inhibitive species of GSE, which was in accordance with the surface analysis and UV-Vis results. 3. Investigation of surface morphology with and without GSE indicated that corrosion products and surface roughness decreased with increasing inhibitor concentration, indicating improvement in the integrity of the surface film in the presence of GSE.  Table 5. Comparison of the price and inhibition efficiency of the plant extracts in hydrochloric acid solution. www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Sort of corrosion inhibitor Concentration (ppm) Cost in 1 kg ($) Max corrosion inhibition (EI %) Metal References
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