Electrochemical, Structural and Thermodynamic Investigations of Methanolic Parsley Extract as a Green Corrosion Inhibitor for C37 Steel in HCl

Abstract

Phytochemical-rich natural extracts have recently attracted intense attention as green corrosion inhibitors and costly benign coating components for the protection of metallic structures of immense commercial importance. Herein, various methods were applied to assess the corrosion protection efficiency of a methanolic extract of parsley (Petroselinum crispum) (PCE) on carbon steel C37 in 1 M HCl. Initially, the chemical profile of PCE was analyzed using gas chromatography/mass spectrometry (GC/MS), and myristicin and apiol were identified as the main components. The results from the weight loss, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) techniques revealed a substantial reduction in the corrosion rate upon the use of PCE, with a maximum inhibition efficiency of 92% at 1 g L⁻¹ PCE. To optimize the performance, the corrosion behavior was investigated over a temperature range of 303–333 K and for concentrations of 0.1–1 g L⁻¹. The inhibition effectiveness increased at higher concentrations of PCE, whilst it decreased when the temperature was elevated. The query suggests that the adsorption process involves both physical and chemical mechanisms. The adsorption of PCE onto C37 was well described by the Langmuir adsorption isotherm. The data were used to determine the activation energy and thermodynamic parameters. The PCE coating acted as a mixed-type inhibitor, hampering both cathodic and anodic corrosion reactions. SEM further confirmed the formation of a protective coating film on the steel surface when exposed to PCE. UV-Vis and XRD were implemented to understand the inhibition mechanism and formed products at the microscopic and spectroscopic levels. Hence, the green PCE inhibitor may potentially be applied in corrosion mitigation due to its high corrosion protection efficacy and its environmentally benign nature.

1. Introduction

Acidic solutions are frequently utilized in several industries for purposes such as acidizing oil wells, pickling, cleaning, and descaling. Hydrochloric acid (HCl) is a highly favored solvent among these acids. Additionally, in an enormous number of industries, carbon steel is intensively employed due to its superior mechanical properties, versatility, durability, and cost-effectiveness [1]. It has a wide range of uses in various industries, such as in the manufacture of chemical reactors, containers, heat exchangers, boiler systems, storage tanks, and oil and gas transmission pipelines. Moreover, it serves a vital function in the chemical industry and associated sectors as reservoirs of acids, alkalis, and salt solutions. Although steel is considered as a very durable material, prolonged exposure to aggressive media such as hydrochloric acid can trigger its corrosion [2]. This process weakens the steel structure, and iron in the steel oxidizes, causing the steel material to deteriorate. Several measures can help to prevent or mitigate the corrosion damage to metals. Among others, the use of inhibitors, e.g., in coatings, is the most effective way to combat the corrosion of steel samples [3,4]. A vast number of inhibitors have been reported, including organic, inorganic, and hybrid materials [5,6,7].

The key considerations when choosing an inhibitor include its cost-effectiveness, efficacy in preventing material corrosion, and environmental impact. Although there are many choices available on the market, there is a continuous search for more effective, and importantly, more sustainable corrosion inhibitors that meet the varied needs of various sectors. By extending the longevity of equipment and structures, these inhibitors not only maintain their integrity, but also avert the environmental contamination caused by the discharge of metal ions into the vicinity [8].

Organic molecules containing heteroatoms, like phosphorus, nitrogen, and sulfur, have notable corrosion-inhibiting capabilities. However, their adsorption is affected by other circumstances, such as their electron density, the steric effect, aromaticity, and the existence of functional groups. Furthermore, the performance of these inhibitor molecules is influenced by their molecular surface area and mass [9]. Nevertheless, many of the synthetic organic and inorganic inhibitors are of environmental and economic concern, as they are often hazardous substances, expensive, and difficult to synthesize [10]. By contrast, green, natural inhibitors represent a viable alternative and are, therefore, becoming increasingly important in the fields of science and technology. The plant extracts from plant waste, such as Citrus peel [11] and Chamaerops humilis [12] fruit waste, have emerged as cost-effective, efficient, and environmentally benign substances for corrosion protection. A comprehensive review has recently summarized the use of green inhibitors based on plant extracts for corrosion protection, with a focus on the technical and scientific perspectives [13].

Moreover, plant extracts function as a valuable reservoir of biodegradable and sustainable resources [14]. They possess complex phytochemicals that strongly interact with metal surfaces via their electron-rich sites [15]. The interaction between the elements of natural products and the metal surface is affected by several aspects, including (i) the chemical composition of the inhibitors, (ii) the characteristics of the surface, and (iii) the surface charge of the metal. Compounds originating from plants that include heterocyclic components, such as steroids, alkaloids, and flavonoids, have been evaluated for their ability to effectively suppress corrosion. However, a major challenge in this field is the understanding of the mechanism of action of such inhibitors, which requires the identification of their chemical constituents and investigating the widespread conjugations between the metal and the multiple bonds and polar functional groups of the extract [16]. Hence, the analysis of the chemical profile of an extract is an essential step towards characterizing the inhibition process.

Petroselinum crispum, also referred to as parsley, is a commonly grown plant that is utilized globally for its significant role in the culinary sector, as well as its contributions to perfume, soap, and cream manufacture. This plant contains a range of vital constituents, including coumarins, furanocoumarins (bergapten and imperatorin), ascorbic acids, carotenoids, flavonoids, apiole, different terpenoid chemicals, phenylpropanoids, phthalides, and tocopherol [17]. The presence of these components in parsley extracts contributes, on the one hand, to possible medical bioactivity due to their antimicrobial, antioxidant, anti-inflammatory, and blood anti-clotting properties [18]. For instance, parsley is commonly utilized in Morocco and other countries as a natural treatment for disorders such as arterial hypertension, diabetes, heart ailments, and renal diseases. On the other hand, the functional groups and electron density of the components of Petroselinum crispum extract have the potential for corrosion inhibition. Only a few studies have recently investigated the use of different parsley extracts as a corrosion inhibitor for metals [19].

In this work, Petroselinum crispum extract (PCE) was evaluated as a green inhibitor for the corrosion of C37 carbon steel in 1 M HCl electrolyte using electrochemical, spectroscopic, and microscopic methods. A pile of independent methods, including weight loss analysis, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), X-ray diffraction, and scanning electron microscopy (SEM), were used to evaluate the effectiveness of PCE in minimizing the corrosion of steel. This allowed for the assessment of the adsorption of the extract onto the metal surface and the formation of a protective layer that shields the surface against the attack of corrosive species. The adsorption isotherm was elucidated. Furthermore, the thermodynamics and mechanism of corrosion were discussed.

2. Materials and Methods

2.1. Parsley (PCE) Extraction

Parsley leaves and stems underwent a 20-day drying process in the laboratory prior to being utilized for the production of extract (PCE). The extraction process lasted 8 h and was conducted using the Soxhlet technique, employing methanol as the solvent. After the extraction process, the solution was condensed using a rotary evaporator until it solidified. This solid part was then utilized in corrosion experiments [20].

2.2. GC-MS Analysis of the Extract

Identifying the chemical composition of the extract is crucial to correlate the composition with properties. The chemical composition of PCE extract was analyzed using a GC (Perkin-Elmer Auto System XL, Waltham, MA, USA). This GC contains a dual flame ionization detection (FID) system and fused silica capillary columns (60 m, 0.22 mm I.D., 0.25 m film thickness), namely Rtx-1 (polydimethylsiloxane) and Rtx-wax (polyethylene glycol). An oven was specifically controlled to gradually increase its temperature from 60 °C to 230 °C, with a heating rate of 2 degrees per minute. Once the desired temperature is reached, the oven was programmed to sustain that heat for a duration of 35 min. The temperatures of the injector and detector were maintained at a constant value of 280 °C. The samples were injected in a fractionated mode at a ratio of 1/50. And carrier gas helium with a flow rate of 1 mL min⁻¹ was utilized. The volume injected was 0.2 L of pure oil.

2.3. FTIR Analysis of PCE

The PCE extract was analyzed using Fourier-transform infrared spectroscopy (FTIR) to identify the functional groups of the extract. The sample underwent transmission-mode scanning, utilizing a single-reflection Pro One attenuated total reflectance (ATR), FTIR-Jasco 4600 spectrometer, Japan, which was equipped with a monolithic diamond crystal.

2.4. Steel Material Preparation and Solutions

In this study, C37 steel samples were obtained from the ‘ThyssenKrupp’ company, Germany. The chemical composition of the bulk material C37 carbon steel used in this study is given in

Table 1. Chemical composition of the used C37 carbon steel.

Element	C	Fe	Mn	P	S	Cr	Cu	Ni	Mo	V
Content (%)	0.34	0.60	0.44	0.21	0.03	4.78	0.08	0.15	1.61	0.51

. A surface areas of 1 cm × 1 cm of specimens of the C37 carbon steel electrodes was exposed using Teflon film for covering the rest. Prior to each test, the working electrodes were subjected to a sequential abrasion procedure utilizing SiC sandpapers with grades 400, 800, and 1200 in an ascending order. Afterward, the electrodes were thoroughly cleaned in acetone for 5 min each. Finally, they were desiccated in the surrounding atmosphere.

The test electrolyte was 1 M HCl that was prepared by diluting analytical grade 37% HCl with deionized water. The studied PCE concentration varied between 0.2 and 1 g L⁻¹ [21].

2.5. Corrosion Inhibition Evaluation

2.5.1. Mass Loss Method

In the mass loss “gravimetric” experiment, the mass of a sample of C-steel C37 was first measured, and then it was placed in two distinct HCl solutions for 24 h. One solution included a PCE inhibitor with a certain concentration, whereas the other solution (blank) was without an inhibitor. After immersion, the samples underwent rinsing, drying with distilled water and acetone, and were subsequently measured again prior to being subjected to SEM testing All measurements were conducted in accordance with ASTM G 31-72 [22].

The corrosion rate (CR) was calculated using the weight loss method as follows:

$$CR=\frac{∆m}{S\times t}$$

(1)

where Δm, S, and t denote the average mass loss of the sample in g, sample surface area in cm², and the immersion time in h, respectively. The inhibition efficiency obtained from the mass loss method, denoted as ${\eta }_{Wl}\%$, was determined utilizing the corrosion rates without the inhibitor (CR) and with the inhibitor (CR_inh), as expressed in Equation (2):

$${\eta }_{Wl}\%=\left(\frac{CR-{CR}_{inh}}{CR}\right)\times 100$$

(2)

2.5.2. Electrochemical Measurements

The electrochemical measurements were carried out utilizing an Origaflex Potentiostat/Galvanostat (OGF05A), France, under the control of a desktop computer running Origamaster5 software. Electrochemical measurements were conducted in a three-electrode cell configuration, comprising Pt wire as the counter electrode, a saturated calomel (SCE) as the reference electrode, and a working electrode constructed of C-steel C37 with an exposed surface area of 1 cm². Prior to recording polarization curves, the working electrode was submerged in the solution for a period of 30 min to obtain a stable open-circuit potential (OCP).

Polarization curves were then recorded in the potential range from −800 to −200 mV versus the reference electrode at a scan rate of 1 mV s⁻¹. After reaching a steady state OCP for about 30 min, electrochemical impedance spectroscopy (EIS) experiments were conducted at OCP in the frequency range spanning from 100 kHz to 0.1 Hz with an amplitude of 10 mV and a density of 20 steps per decade. The collected data were assessed utilizing the EC-Lab comparable circuit program. For reproducibility purposes, the tests were conducted three times, and the representative data are presented in this article.

The inhibition efficiency (η_EIS) of the tested PCE was calculated by analyzing the EIS data and using the obtained polarization resistance in the absence ($R_p^o$) and presence (R_p) of the inhibitor, as outlined in Equation (3).

$${\eta }_{EIS}\%=\left(\frac{R_p^{°}-R_p}{R_p^{°}}\right)\times 100$$

(3)

where R_p = R_s + R_ct; R_s is the solution resistance, whilst R_ct is the charge transfer resistance.

The corrosion inhibition efficacy was also calculated using the potentiodynamic polarization data by calculating the corrosion current densities in the absence ($i_{corr}^{°})$ and presence (i_corr) of the inhibitor via Tafel analysis. The inhibitory efficiency from the PDP method (η_PDP) can be estimated using Equation (4) as follows.

$${\eta }_{PDP} \%=\left(\frac{i_{corr}^{°}-i_{corr}}{i_{corr}^{°}}\right)\times 100$$

(4)

2.5.3. UV–Vis Absorption Spectroscopy

The absorption spectra of the PCE solutions with and without a steel sample as well as the blank solution containing steel were recorded at room temperature using a UV-Vis spectrophotometer (Jasco model V-730, Tokyo, Japan). The spectra were measured in the range from 200 to 800 nm.

2.5.4. SEM Analysis

For morphological inspection and sample surface analysis, a scanning electron microscope (SEM) equipped with X-ray detection was utilized. Analyses were conducted using a JEOL-Model JSM-IT100, Tokyo, Japan, operating at 20 kV.

SEM images of C37 were taken before and after immersion in 1 M HCl solution for 24 h without and with 1 g L⁻¹ of PCE. The untreated sample was polished, cleaned, and dried before imaging. The dipped samples were rinsed with water and dried before imaging. No sputter-coating or additional treatment was applied for the SEM imaging.

2.5.5. XRD Analysis

After 24 h of the immersion of the steel specimen, XRD patterns were recorded for the C37 surface samples under two conditions: one without the inhibitor, and one with the optimal inhibitor concentration in a 1 M HCl solution. The measurements were carried out using a Shimadzu 6100 diffractometer, Kyoto, Japan, with Cu Kα radiation (λ = 1.5418 Å) and a scan speed of 2 degrees per minute over an angular range of 20–110°.

3. Results and Discussion

3.1. Chemical Profile of the PCE Extract Using GC/MS

To obtain insights into the possible compounds of the extract that potentially have an anticorrosive effect, it is useful to elucidate the chemical composition of the extract. The methanolic extract of parsley was analyzed using GC-MS, which identified apiol and myristicin as the main constituents, with 35.6 and 28.4%, respectively, as shown in

Table 2. The chemical composition of the methanolic extract of parsley from GC-MS analysis.

Component	Formula	Chemical Name	Molar Mass (g mol−1)	Retention Time (min)	Area%
Myristicin	C11H12O3	6-Allyl-4-methoxy-1,3-benzodioxole	192.21	18.2	28.4
Apiol	C12H14O4	4,7-dimethoxy-5-prop-2-enyl-1,3-benzodioxole	222.2	20.4	35.6
Phellandrene	C10H16	2-methyl-5-propan-2-ylcyclohexa-1,3-diene	136.23	10.6	12.3
Limonene	C10H16	1-methyl-4-prop-1-en-2-ylcyclohexene	136.23	8.5	6.7
Eugenol	C10H12O2	4-Allyl-2-methoxyphenol	164.2	15.9	5.4
Alpha-Pinene	C10H16	2,6,6-Trimethylbicyclo[3.1.1]hept-2-ene; pin-2(3)-ene; Pinene isomer	136.23	7.2	3.9
Beta-Pinene	C10H16	6,6-dimethyl-2-methylidenebicyclo[3.1.1]heptane	136.23	7.8	2.6
Terpinolene	C10H16	1-methyl-4-propan-2-ylidenecyclohexene	136.23	11.3	1.8
Caryophyllene	C15H24	(1R,4E,9S)-4,11,11-trimethyl-8-methylidenebicyclo[7.2.0]undec-4-ene	204.35	16.5	1.5

. These two compounds are classified as phenylpropanoids, which are naturally occurring molecules that have been widely researched for their possible health benefits. Importantly, these main compounds are rich in electron donor groups, such as oxygen and phenyl groups. Hence, these molecules have a tendency to interact with the pi orbitals of the Fe surface. Hence, such compounds possess various anchor sites for the electrophilic attack of mysriticin on iron. Therefore, the effectiveness of parsley extracts’ corrosion inhibition is mostly dependent on the presence of such flavonoids. Additionally, apiol, for example, is linked to antioxidant and anti-inflammatory properties [19], whilst myristicin exhibits antifungal and antibacterial activities. Hence, they can play an additional role in such applications.

The presence of flavonoids in parsley extracts effectively prevents the corrosion of carbon steel in acidic environments by forming a protective layer on the surface of steel through adsorption mechanisms. The usefulness of flavonoids in inhibiting corrosion is demonstrated by the higher inhibitory efficiency of parsley extracts with increasing concentrations of the inhibitor. The addition of the extract inhibitor alters the mechanism of cathodic hydrogen development and the anodic dissolution of steel, indicating that flavonoids effectively hamper carbon steel corrosion.

3.2. FTIR Analysis of PCE

The FTIR analysis of parsley methanolic extract was carried out to characterize the existing functional groups in the extract and possibly correlate them with the inhibition properties against corrosion. The results of FTIR analysis (Figure 1) suggest the presence of functional groups with electron-donating properties, such as oxygen or phenyl groups, onto that can adsorb onto the metal surface. These functional groups can bind physically or chemically to the orbitals of the iron. The occurrence and strength of particular bands in the FTIR spectrum correspond to the different functional groups existing in the extract. The broad band between 3600 and 3200 cm⁻¹ indicates the existence of an O-H stretching vibration, suggesting the presence of hydroxyl groups [23]. This could be due to phenolic chemicals like flavonoids or polyphenols that are commonly found in parsley [24]. The bands observed in the range of 3000–2800 cm⁻¹ are linked to the C-H stretching vibrations, suggesting the existence of alkyl or aromatic groups. The bands in the range of 1750–1580 cm⁻¹ can be used to infer the presence of carbonyl groups, which are characteristic of substances such as flavonoids or other polyphenols. The band between 1320 and 1500 cm⁻¹ can be assigned to aromatic C=C bending, CH₂ bending, or CH₃ bending [25]. The weak peak at 900 cm⁻¹ could refer to the aromatic C-H. The band centered at 1100 cm⁻¹ can be assigned to an C-OH stretching or C-O stretching vibration [25]. The peak that appeared at 1680–1700 cm⁻¹ likely indicates the existence of apiol, the major compound found in this parsley extract.

3.3. Evaluation of the Corrosion Inhibition of PCE

3.3.1. Weight Loss Results

Initially, the inhibition performance of the extract for C37 steel was assessed using the conventional weight loss method. The weight losses were determined after 24 h of submersion in the solution. They were immersed in HCl containing different concentrations of PCE from 0.4 to 1.0 g L⁻¹ at a temperature of 303 K. The calculated corrosion rate (CR) and corrosion efficiency are listed in

Table 3. Corrosion rate and inhibition efficacy obtained from weight loss method for carbon steel in 1 M HCl solution at different PCE concentrations.

Cinh (g L−1)	CR (mg cm−2 h−1)	ηwl%
Blank	0.936	--
0.4	0.150	79.9
0.6	0.127	85.4
0.8	0.090	89.5
1	0.088	90.0

.

In the inhibited solution, the weight loss of the material in 1 M HCl solution decreased, and subsequently the corrosion rate decreased. This gravimetric measurement indicates that the inhibitor, PCE, exhibited notable efficacy in inhibiting the corrosion of C-steel in 1 M HCl solution at a concentration of 1 g L⁻¹. The inhibitory efficiency rose as the concentration of the extract increased to 1 g L⁻¹, and using this methodology, it reached a maximum of 90%, as presented in Table 3. No further enhancement was observed at the higher concentrations. The enhanced inhibitory efficiency seen with the increasing concentration can be ascribed to the enhanced surface coverage of the surface with the inhibitor and improved interactions via adsorption. The inhibitor adhered strongly to the metal surface and enveloped the active sites, creating a shielding layer that diminished the metal’s reactivity. Consequently, the inhibitor has a higher level of efficacy in preventing mass loss.

3.3.2. Potentiodynamic Polarization Measurements

Potentiodynamic polarization (PDP) curves were measured to examine the corrosion properties of C-steel C37 in 1 M HC solution with PCE concentrations from 0.2 to 1.0 g L⁻¹ at 303 K. Figure 2 presents the obtained polarization curves. The electrochemical parameters and related inhibitory efficiency (η_PDP %) determined from the polarization experiments are summarized in

Table 4. Electrochemical parameters obtained from potentiodynamic polarization curves of C37 in 1 M HCL solution containing different concentrations of PCE.

Cinh (g L−1)	Ecorr (mV vs. SCE)	icorr (µA cm−2)	βa (mV dec−1)	βc (mV dec−1)	ηPDP (%)
Blank	−446	427.1	48.9	−132.1	--
0.2	−460	202.5	112.6	−192.1	52.5
0.4	−461	88.5	96.4	−211.0	79.2
0.6	−443	69.0	102.4	−249.7	83.8
0.8	−478	64.5	107.5	−207.6	84.8
1	−468	34.3	89.6	−197.5	91.9

.

It is observed in Figure 2 that the inhibitor had an effect on both the anodic and cathodic branches of the curve; however, its impact on the cathodic side is more nuanced. More precisely, an anodic reaction took place when the metal underwent dissolution in the strongly acidic solution, causing the transport of steel ions from the metal surface into the solution. Conversely, the cathodic area was responsible for the release of protons, which aid in the hydrogen evolution. An inhibitor operates by disrupting either the cathodic reaction, the anodic reaction, or both [26].

The Tafel lines in the cathodic area (Figure 2) are parallel, suggesting that the reduction of hydrogen ions on the metal surface was predominantly controlled by the activation process. In addition, the cathodic Tafel slopes (β_c) in Table 4 do not exhibit significant variations, indicating that the introduction of PCE did not appreciably modify the process of hydrogen evolution. The data in Table 4 demonstrate a decrease in both the anodic and cathodic current densities upon the addition of PCE, resulting in a significant 90% improvement in inhibition efficiency. Significantly, there is a notable distinction between the alterations in β_a and β_c when PCE was introduced, indicating that PCE primarily impacted the anodic behavior rather than the cathodic behavior. These findings indicate that the inhibitor functions by effectively obstructing the accessible surface sites. This indicates that the inhibitor decreases the area available for dissolution, without impacting the process by which C-steel C37 dissolves. It merely partially deactivates the surface in relation to the corrosive substance [27]. Moreover, the inhibitor is categorized as cathodic, anodic, or mixed when the corrosion potential difference between the inhibitor and the blank surpasses 85 mV. If this corrosion potential difference exceeds 85 mV, the inhibitor is categorized as either cathodic or anodic [28]. When exposed to PCE, the corrosion potential values change towards the anodic potential, suggesting that our inhibitor functions as a mixed inhibitor, with the anodic process predominating.

3.3.3. Electrochemical Impedance Spectroscopy (EIS)

EIS is a versatile tool used to study the electrochemical behavior of the metal–electrolyte interface and can be used to probe the charge transfer process [29]. Here, the impedance response was investigated in relation to the inhibitor concentration using EIS. The key inhibition descriptor of the corrosion process is the polarization resistance (R_p) that results from EIS analysis [11]. The EIS measurements of the C37 electrodes were performed after immersion for 30 min in 1 M HCl solution in the absence and presence of different PCE concentrations. The EIS data are depicted in Nyquist format (Figure 3a) and Bode format (Figure 3c,d).

The Nyquist plot (Figure 3a) clearly shows a single compressed semicircle, indicating a relaxation process with a single time constant occurring at the interface between the electrode and the electrolyte. The frequency dispersion of Nyquist diagrams, which refers to the deviation from a standard semicircle, is commonly ascribed to the irregular and uneven fracture structures on an electrode surface, the inhomogeneous distribution of active sites, the presence of adsorption inhibitors, and the formation of porous layers [28]. The semicircle’s diameter reflects the magnitude of charge transfer resistance (R_ct). The obtained Nyquist plots imply that the corrosion of C37 steel in a 1 M HCl solution was mostly affected by the charge transfer resistance, R_ct. Furthermore, the addition of an inhibitor to the testing medium caused the diameter of the semicircle on the Nyquist plot to increase in comparison to that of the uninhibited solution. This increase in diameter is proportional to the concentration of the inhibitor, illustrating the increase R_ct with concentration, again demonstrating the enhanced inhibition performance. This is most likely due to greater surface coverage by the inhibitor molecules. This adsorbed inhibitor molecules hindered the aggressive solution from directly accessing the metal surface, hence preventing the process of metal dissolution. Additionally, the double layer capacitance (C_dl) values decrease as the inhibitor concentration decreases. This phenomenon can be explained by the gradual replacement of water molecules in the double layer by the adsorbed inhibitor molecules. This created a film on the metal surface that bonded well and decreased the dielectric constant of the interface between the metal and the solution [30].

Nevertheless, the influence of the composite material on the corrosion process is contingent upon its concentration. This phenomenon can be reflected in the increased dimensions of the capacitive loops and the shift of the phase angle and Bode modulus impedance towards higher angles and log|Z| values, respectively, in systems that contain the multicomponent inhibitor, as depicted in Figure 3c. This suggests that the matrix of compounds in the corrosive solution has the effect of reducing the speed at which the charge is transferred. A plausible explanation for this observation is that the molecules of the extract stick to the surface of the metal, blocking the reaction sites [31].

The different degrees of corrosion protection are provided by the semi-circular shape shown in the Nyquist plot. However, for more comprehensive examination, it is essential to modify the data by employing an equivalent electrical circuit [32]. The EIS data were fitted to the equivalent circuit diagram shown in Figure 3b. A single constant equivalent circuit was used to fit the data and determine the solution resistance (R_s) and charge transfer resistance (R_ct). In our case, a constant phase element (CPE) was incorporated into the circuit to accommodate the depressed semicircle and non-ideal capacitor behavior. The CPE impedance (Z_CPE) is calculated as follows [33]:

$$Z_{CPE}=Q^{-1}{\left(j\omega \right)}^{-n}$$

(5)

where j, Q, ω, and n are an imaginary unit (j² = −1), the phase shift parameter, the angular frequency, and the quantity of CPE, respectively. The C_dl can be determined using the following equation:

$$C_{dl}=Q^{1/n} {R_p}^{1-n/n}$$

(6)

The resulting electrochemical parameters are summarized in

Table 5. Impedance parameters of C37 steel containing different concentrations of PCE.

Cinh (g L−1)	Rs (Ω cm2)	Rct (Ω cm2)	Qdl (Ω−1 Sn cm2)	n	Cdl (µF cm−2)	ηEIS (%)
Blank	2.1	24.8	3.07 × 10−4	0.837	537	--
0.4	1.4	118.9	1.48 × 10−4	0.797	498	79.1
0.6	1.3	141.5	1.78 × 10−4	0.798	419	82.5
0.8	1.4	151.5	1.14 × 10−4	0.814	358	83.6
1	2.3	283.9	1.10 × 10−4	0.813	402	91.3

. It is obvious from Table 5 that the R_ct value increases, while the C_dl value drops for the inhibited solution compared to that of the uninhibited solution. This trend becomes more prominent as the concentration of the inhibitor increases. The reduction in C_dl can be ascribed to the effective adsorption of PCE (inhibitor) molecules onto C37 steel. This adsorption occurs through the displacement of water molecules, which reduces the extent of the metal dissolution reaction. In addition, it is important to mention that the n values (ca. 0.8) are closer to unity, suggesting a predominant capacitive behavior of the CPE element. In addition, the n values do not exhibit substantial variations when the inhibitor concentration increases. This insight facilitates the prediction of the disintegration mechanism. Thus, it can be inferred that the dissolving mechanism in both the inhibited and uninhibited solutions is controlled by the charge transfer process [34]. The results of the EIS study are consistent with the findings of the gravimetric study and potentiodynamic analysis.

3.4. Thermodynamics of the Process and Adsorption Isotherm

3.4.1. Temperature Effect

The temperature effect can be used to indicate the action mode of the inhibitor (chemisorption or physisorption) and to estimate the activation energy of corrosion. The inhibitory efficacy of PCE was further assessed at various temperatures (303, 313, 323, and 333 K) using PDP tests, with PCE being present at its most effective concentration of 1 g L⁻¹. The results obtained are depicted in Figure 4.

Table 6. Corrosion parameters obtained from potentiodynamic polarization curves for corrosion of C37 in 1 M HCL with and without 1 g L⁻¹ PCE at different temperatures (303–333 K).

Medium	T (K)	iorr (µA cm−2)	ηPDP (%)
Blank	303	427	91.9
PCE		34
Blank	313	760	90.0
PCE		75
Blank	323	990	89.0
PCE		91
Blank	333	1300	84.1
PCE		206

displays the calculated corrosion parameters for PCE. These data indicate that the i_corr values show a more modest increase with temperature increase in the presence of PCE in comparison to that of the virgin solution. As a result, there is a significant reduction in the effectiveness of PCE on inhibition, as reflected in the deteriorated efficiency at elevated temperatures, reaching 84% at 333 K. This response can be attributed to the desorption of PCE molecules from the metal surface and the disintegration of the protective layer and other compounds formed on the electrode surface.

The effect of temperature can be evaluated utilizing the following Arrhenius equations and considering the concept of the transition state [35]:

$$ln{ i}_{corr}=ln A-\left(\frac{E_a}{RT}\right)$$

(7)

$$ln\frac{i_{corr}}{T}=ln\frac{R}{N_A\times h}+\left(\frac{\mathsf{\Delta }{S}_a}{R}\right)-\left(\frac{\mathsf{\Delta }{H}_a}{RT}\right)$$

(8)

The activation energy (E_a) can be determined by applying the Arrhenius equation. This equation incorporates the pre-exponential component (A), the activation entropy (ΔS_a), enthalpy (ΔH_a), the Avogadro constant (N_A), and the Planck constant (h). To determine E_a, a graph of ln(i_corr) against (1000/T) was drawn up, as shown in Figure 5a. The outcome of this plot is a linear relationship. Additionally, a graph of ln(i_corr/T) plotted against 1000/T was established (Figure 5b). The slope of the resulting line corresponds to (−ΔHa/R). The point where this line intersects with the ln(i_corr)/T axis is the parameter ΔSa. The obtained results are listed in

Table 7. The activation parameters of C37 steel both in the blank electrolyte and the optimized PCE concentration at different temperatures.

Medium	Ea (kJ mol−1)	∆Ha (kJ mol−1)	∆Sa (J mol−1 K−1)
Blank	31.6	24.3	−113.1
1 g L−1 PCE	43.3	47.5	−58.7

.

The results show that the presence of the CPE extract lead to a larger Ea value than when it was absent, indicating the generation of a larger energy barrier for corrosion in the presence of the PCE inhibitor. This also suggests that PCE adsorbs through a physisorption mechanism, which normally occurs during the initial stage of metal–inhibitor interactions [35]. Examining the enthalpy and entropy values of adsorption can offer additional significant insights into the adsorption of PCE molecules on the carbon steel surface and the process of corrosion inhibition. Our results (Table 7) indicate that the activation enthalpy (∆Ha) values are positive for both the blank acid solution and the extract-containing solution. The enthalpy value determines whether a reaction is endothermic or exothermic. Endothermic processes are often associated with chemical adsorption, whereas exothermic processes are related to either chemical or physical adsorption. If long-lasting effects are sought, chemical adsorption is the favored method over physical adsorption. The reason for this is that chemical adsorption occurs when electron pairs are shared between the inhibitors and the unoccupied d-orbitals of the metals, while physical adsorption occurs when charged inhibitor molecules interact electrostatically with the charged surfaces of metals. Based on the magnitude of ΔH_ads, an exothermic process can be further categorized as either chemical adsorption or physical adsorption. Adsorption with values of ΔH_ads below 40 kJ mol⁻¹ is related to physical adsorption, whereas ΔH_ads around 100 kJ mol⁻¹ or higher indicates chemical adsorption. The values in Table 7 reveal that the ΔH_ads values are positive, indicating an endothermic reaction. The magnitude of the absolute value of ΔH_ads for the inhibitor exceeds the threshold of 40 kJ mol⁻¹, but remains below 100 kJ mol⁻¹, indicating a mixed-type adsorption nature [36,37].

In addition, the activation entropy of adsorption (∆Sa) value is negative for both the cases under identical conditions. This observation could be due to the aggregation of inhibitor molecules rather than their separation and a decrease in the degradation of the metal surface, suggesting that the inhibitor functions by chemically interacting and physically adhering to the metal surface. Consequently, the existence of PCE leads to an increased activation entropy value (∆Sa) in comparison to that in its absence. This signifies an elevated degree of disorder in the diffusion of reactants towards the activated complex [36].

3.4.2. Adsorption Study

The primary reason for the effectiveness of corrosion prevention on metallic surfaces is the adsorption of corrosion inhibitors at the molecular level. When there are no inhibitors present, the metal surface becomes coated with water molecules. Nevertheless, the introduction of a corrosion inhibitor results in the displacement of the adsorbed water molecules. The substitution of water with inhibitor molecules results in the obstruction of active areas on the metal surface, where corrosion reactions usually take place [38].

This work further investigated the adsorption behavior of PCE by examining the validity of different commonly used adsorption isotherm models. Out of these models, the Langmuir model was found to give the best fit to our data, as shown in Figure 6. The Langmuir model can be represented as follows:

$$\frac{C_{inh}}{ \theta }=\frac{1}{K_{ads}}+C_{inh}$$

(9)

where C is the inhibitor concentration, θ indicates the percentage surface coverage, and K_ads represents the equilibrium constant for adsorption. Figure 6 shows an excellent linearity of the plot with a regression coefficient R² of 0.9992, demonstrating the match of this model to the results. K_ad was calculated from the intercept of the Langmuir plot. The obtained values are given in

Table 8. The parameters of the Langmuir adsorption isotherm for the PCE inhibitor.

	Slope	Intercept (g L−1)	Kads (L g−1)	R2
PCE	1.07	0.0188	55.6	0.9992

. K_ads revealed a positive value, together with a slope (1.07) close to unity, suggesting a monolayer adsorption of the PCE inhibitor on the electrode surface [39]. Another parameter that often helps to understand the adsorption mechanism is the Gibb’s free energy of adsorption (ΔG_ads°), which can be calculated using the K_ads value. However, in our case, it is difficult to determine ΔG_ads° using this method because of the complex composition of the PCE extract and the unknown average molecular weight of PCE [40,41,42].

3.5. UV–Vis Absorption Spectroscopy

UV-Vis spectroscopy was applied to investigate the anticorrosion properties and possible complex formation of parsley methanolic extract compounds with Fe ions in C37 steel in 1 M HCl solution. The UV-Vis spectrum (Figure 7, red line) of bare PCE in the blank solution shows two clear absorption peaks at 265 and 335 nm. Generally, the absorption bands in the range of 234–676 nm are characteristic of flavonoids, alkaloids, and phenolic compounds [43], thus suggesting the presence of such compounds in the extract. Precisely, the peaks at 265 and 335 nm can be ascribed to the electronic transition from the non-bonding (n) to the anti-bonding (π*) orbitals of C=O and O-H bonds, respectively [44].

Figure 7 (black line) presents the UV-Vis spectrum of the solution free of inhibitor after 24 h of immersion of the metal in it. In this spectrum, a weak peak at 335 nm appears. Interestingly, the spectrum (blue line) of the blank solution containing 1 g L⁻¹ PCE inhibitor and the steel sample after immersion for 24 h revealed a reduction in absorbance intensity of the main peak at 265 nm, also known as the hypochromic effect, with a slight shift to a lower wavelength. This peak most probably corresponds to the complexation between Fe ions and the organic compounds of the extract [45]. Hence, the decrease in its intensity proposes that the PCE inhibitor was successfully adsorbed from the solution onto the surface, blocking the active sites on the Fe surface, thus hindering the release of Fe²⁺ ions into the solution. Therefore, less complexation took place. Moreover, the spectra revealed that the absorbance values at 335 nm for the solution containing PCE increased after immersing C37 steel as compared to that of the solution before immersion. This might be due to the oxidation or reduction of organic compounds in the extract matrix or increase in certain complexation reactions with Fe ions [44]. These findings indicate that the plant extract (PCE) and Fe²⁺ ions likely formed a complex that was partially released in the solution and caused an increase in intensity. The complex formation could contribute to the anticorrosion properties of PCE.

3.6. SEM Analysis

SEM imaging was employed to inspect the effectiveness of the inhibitor in the protection of the surface. Figure 8 exhibits SEM images of the surfaces of the C37 steel sample prior to and following the addition of the inhibitor into the corrosive solution containing 1 M HCl. In Figure 8a, the micrograph of the ground sample in its original surface condition shows a flat surface with visible scratches caused by the hand grinding of the specimen. Figure 8b shows the detrimental consequences of the attack of HCl solution on the C37 sample surface. The surface underwent extensive damage, exhibiting severe grooves, pores, and voids. When the PCE inhibitor was present, the surface displayed a more flattened appearance and showed fewer signs of deterioration or cracks [46]. This advantageous effect of the inhibitor is mostly due to the formation of an inhibitive layer on the C37 sample. The layer successfully maintained mostly the original condition of the surface, therefore preventing corrosion in the 1 M HCl solution [47].

3.7. XRD Analysis

To investigate the structure of the steel surface before and after adding the PCE inhibitor and identify the nature of the formed Fe products on it when steel was submerged in a highly reactive HCl, XRD was performed. Figure 9 depicts a comparison of the XRD patterns that show the formation of corrosion products on carbon steel under two conditions: without the presence of the PCE inhibitor and with its presence. After immersing the carbon steel in a 1 M HCl solution for 24 h without the inhibitor (black line), reflections at 2θ of 44.1, 69.5 and 85.6° were measured, corresponding to metallic Fe [48]. Interestingly, in the XRD pattern (blue line) of the carbon steel submerged in 1 M HCl + 1 g L⁻¹ PCE, this identical trio of peaks was still observable with different intensities. The additional reflections at 69.5 and 80.6° mostly match the standard of Fe₂O₃, whilst the weak signal at 27.1° is likely related to γ-FeO(OH) [47,48]. Furthermore, there is a noticeable augmentation in the magnitude of the main signal of Fe at 44.1°, suggesting that the inhibitor effectively shielded the metal surface from corrosion. This was accomplished by creating a defensive barrier that hindered the formation of certain corrosion products, such as oxyhydroxides.

In sum, these overall results demonstrate that the PCE inhibitor has a substantial role in the formation of a shielding film on the metal surface via adsorption. This layer efficiently inhibits the formation of other corrosion products, such as oxyhydroxides.

4. Conclusions

This study showed promising results on the effectiveness of parsley extract (PCE) as a green corrosion inhibitor for carbon steel in a 1 M HCl solution. The inhibitor demonstrated a high inhibition efficiency, reaching up to 92% at a concentration of 1 g L⁻¹. Through various evaluation methods, it was found that PCE acts primarily by adsorption on the metal surface, forming a barrier that prevents corrosion processes. It was found that the inhibitor is a mixed type that influences both cathodic and anodic reactions. The presence of PCE led to a decrease in the corrosion currents, supported by the potentiodynamic polarization and EIS results. This study utilized a combination of techniques, such as UV-Vis, XRD, and SEM, to understand the inhibition mechanism at both the microscopic and spectroscopic levels. GC-MS analysis identified apiol, myristicin, and related compounds as the major constituents of the parsley extract that contributed to its inhibition efficiency. SEM imaging also showed that the surface of the steel was protected in the presence of PCE, reducing damage and roughness significantly.