Inhibitory Effect of Fennel Fruit Essential Oil and Its Main Component Anethole of Corrosion on Steel Plates in 1 M HCL

Abstract

Corrosion worldwide causes large losses of metal, which is why various ways are being sought to slow it down. The aim of this study was to investigate the inhibitory effect of fennel fruit essential oil (Foeniculum vulgare Mill.) and its main component anethole. The inhibitory effect of three different concentrations of the fennel essential oil and anethole (1.0 mL/L, 1.5 mL/L, and 2.5 mL/L) in a solution of 1 M HCl at 298 K for 6 h on a sheet of low-carbon steel was investigated. The inhibitory effect was established using gravimetric methods evaluating weight loss, corrosion rate, and inhibition efficiency, as well as electrochemical methods. In gravimetric studies, the inhibition effect of the inhibitors fennel essential oil and isolate anethole at a concentration of 2.0 mL/L was 70.85% and 45.86%, respectively. The anodic polarization curve data at 298 K demonstrate that the anethole and fennel essential oil adsorption on the metal surface creates a barrier that hinders hydrogen ions’ access and prevents them from being reduced on the steel surface’s cathode sites. Fennel essential oil acting as a mixed type-inhibitor can replace synthetic organic substances and could become an alternative to be used as environmental corrosion inhibitors.

1. Introduction

Iron and its alloys have been used since ancient times to make tools, machinery, and buildings. However, metal is susceptible to interaction from various external environmental factors, such as air moisture or covering materials, extreme temperatures, airborne dust particles, salt, industrial lubricants, and much more. All these factors can affect iron alloys both individually and together, and the change in their integrity and properties is referred to as corrosion.

Worldwide, corrosion causes large losses of metal included in various products and equipment (machinery and apparatus, vehicles, structures, equipment, etc.). Research conducted from 1999 to 2001 found that the cost of corrosion damage, maintenance, and repairs accounts for roughly USD 276 billion or 3.1% of the US gross domestic product [1].

It is a spontaneous process that can be slowed down or controlled by various methods, but never completely stopped.

Reasonably priced, economical, and widespread mild steel rusts readily due to oxidation and poor corrosion resistance. On the one hand, the most popular alloying element used to increase steel’s resistance to corrosion is Cr. Even at the higher Cl⁻ concentration sites, Wang et al. [2] showed that adding 0.6 wt% Cr to low-alloy steel substantially decreased the corrosion rate. Nevertheless, specific alloy additives and the associated production process drive up the cost of steel. On the other hand, various organic inhibitors can be incorporated as protective coatings on the surface of the metal or as additives to the environment. They tend to slow down the rate of corrosion of iron alloys in acidic solutions.

The inhibitors are classified into different categories depending on their mode of action: anodic inhibitors, cathodic inhibitors, adsorptive corrosion inhibitors, mixed inhibitors, barrier layer-forming inhibitors, neutralizing inhibitors, and scavengers.

The effectiveness of these organic corrosion inhibitors is related to the presence of polar groups with sulfur (S), oxygen (O), and nitrogen (N) atoms contained in the molecule, heterocyclic compounds, which are regarded as effective adsorption centers and π electrons. The polar group is generally considered a reaction center to establish the adsorption process. Dangerous effects are known in most synthetic organic inhibitors, and currently, directives point to the development of non-toxic substances with minimal risk to nature—the so-called “organic inhibitors”, “green”, or environmentally friendly inhibitors [3].

Thus, many natural plant sources—essential oils [3,4,5,6,7,8,9,10] and extracts [11,12,13,14,15,16], as well as their main components—have been studied for inhibitory activity and meet all modern requirements: slightly toxic or non-toxic, environmentally friendly, easy to obtain, renewable, and inexpensive. The inhibition effectiveness of natural products is usually due to the presence of complex organic species in the composition, including oxidized monoterpenes and sesquiterpenes, as well as hydrocarbon products. These organic compounds usually contain polar groups with oxygen atoms, as well as those with conjugate double bonds or aromatic rings in their molecular structures, which are the main adsorption centers [3,10].

Fennel (Foeniculum vulgare Mill.) belongs to the family Apiaceae. Two types are grown from it: dulce (var. dulce (Mill.) Thell.) and bitter (var. vulgare (Mill.) Thell). The main component of fennel fruit essential oil is trans-anethole, and its content in oils obtained from fruits of different origins ranges from 52.6 to 87.3% [17,18,19,20,21]. The essential oil is stored in cold and dark rooms. Under the action of light, anethole passes into photoanethole and its ability to crystallize decreases, and when stored in the presence of air, the content of anisaldehyde increases. The essential oil has proven antimicrobial [17,18,20], antioxidant [19,20,21], and other biological properties [17,18,22], which is why it is mainly used in medicine, pharmacy, the food industry, and flavoring oral care products [23].

Anethole (IUPAC 1-methoxy-4-(1-propenyl)benzene) is a major component of the essential oils of anise (80%), fennel (50–52%), star anise (70–95%), basil, and other plants, and its (Z)-form is found in natural products (Figure 1). Chemically it belongs to the ethers and is characterized by the following physical parameters: molecular weight—148.20; appearance—white crystalline mass; smell—specific; boiling point—234 °C; relative density—0.988; refraction—1.5615 [24,25]. Anethole is one of the main isolates in the global essential oil industry, but for industrial purposes, anethole is synthesized in significant quantities [24,25]. Anethole is characterized by various biological properties—antimicrobial, antioxidant, anti-inflammatory, etc. [26,27,28]—which is why it is used in different areas: in the food industry, in the spirits and liqueur industry, in cosmetics, in tobacco sauces, and in medicine and pharmacy. It is also used as a raw material for the synthesis of anisaldehyde (obepine) and other aromatic substances [24,25]. Anethole, taken orally in large quantities, affects the central nervous system, which is why essential oils containing it have limited use in the food industry and cosmetics [24].

To the best of the authors’ knowledge, there is a limited number of studies in the literature on the anti-corrosion effect of fennel fruit essential oil.

• Lahhit et al. [5] determined that in an environment of 1 M HCl at a temperature of 298 K and a duration of 6 h with an increase in the concentration of the essential oil from 0.5 to 3 mL/L, its inhibitory effect increased from 38 to 73%. According to the authors, the value of inhibition effectiveness decreases with an increase in temperature to 343 K.
• Barrahi et al. [9] found an 89% inhibitory effect of the essential oil in an amount of 2 mL/L in an environment of 1.0 M HCl at a temperature of 303 K and a duration of exposure of 6 h. According to the authors, with an increase in temperature to 333 K, the inhibitory effect decreases to 76%.
• Bouoidina et al. [29] investigated the inhibitory activity of the essential oil and determined its very good inhibitive properties.
• Mouadili et al. [30] examined the chemical composition of the essential oil, its biological properties and anti-corrosion effect against mild steel.

In Bulgaria, both varieties are grown—dulce fennel and bitter fennel—from which several cultivars are selected [31]. The fruits are processed industrially by steam distillation [24].

This study aims to investigate the inhibitory effect of fennel fruit essential oil and its main component anethole using gravimetric and electrochemical methods. The material chosen for testing was a carbon steel grade, due to its low corrosion resistance in numerous daily and industrial applications. This will expand the possibilities of the application of fennel fruit essential oil and its main component anethole as “green” inhibitors.

2. Materials and Methods

2.1. Materials

Fennel essential oil and its main component anethole are used as inhibitors.

• Fennel fruit essential oil: Processed fruits of sweet fennel, harvested in 2020. The essential oil was obtained by water distillation in a laboratory glass apparatus of the British Pharmacopoeia. The essential oil has the following main components: anethole (73.72%), fenchone (17.09%), and estragole (2.67%) [32].
• Anethole: An isolate purchased from the pharmacy network in the town of Razgrad, Bulgaria was used.
• Steel samples: A plain steel material grade ASTM A36 [33](EN S235JR, St44-2) with a density of 7850 kg/m³ and with the following composition was used: 0.021% C; 0.012% Si; 0.292% Mn; 0.0095% P; 0.0067% S; 0.02% Cu, and balanced Fe. Sample bodies measuring 14 × 14 × 1 mm were cut from the sheet material mechanically. Each sample was processed manually with 400 grit sandpaper to obtain a mirror-like surface, and then washed with deionized water, degreased and dried with 95% ethanol and hot air. The steel tiles were stored in a desiccator with a silica gel dryer to prevent subsequent oxidation.

Solutions: Aggressive solutions of 1 M HCl were prepared by diluting 37% HCl with distilled water. Solutions of fennel fruit essential oil and anethole at concentrations of 1.0 mL/L, 1.5 mL/L and 2.0 mL/L in 1 M HCl were also prepared. The conditions for conducting the experiments—the choice of temperature, time, amount of inhibitors, and concentration of the aggressive environment with hydrochloric acid—are according to our preliminary unpublished data and according to data from the literature.

2.2. Methods

• Gravimetric measurement

Following ASTM NACE/ASTM169/G31-12a [33] during the measurements, each steel sample was completely immersed 2 cm below the top solution surface in a separate plastic container with 30 mL of 1 M HCl solution without and with different concentrations of the inhibitors tested (1.0 mL/L, 1.5 mL/L, and 2.0 mL/L) for 6 h at a controlled constant temperature of 298 ± 1 K. The selected gram-to-mL ratio was equal to 0.05 h/mL. After each sample was washed with ethanol and deionized water, it was dried with hot air. The weight loss was calculated as the difference in the weight of the samples before and after immersion, determined using a digital scale Sartorius RC 210 D (Sartorius Corporate Administration GmbH, Goettingen, Germany) with a sensitivity of ±0.0001 g.

The corrosion rate (mg/(cm².h)) was calculated according to Equation (1) [34]:

$$W={\displaystyle \frac{m_1-m_2}{A\times t}}$$

(1)

where m₁ is the weight of the steel plate before immersion in the test solutions, mg; m₂ is the weight of the steel plate after immersion in the test solutions, mg; A is the area of steel tiles, cm²; and t is the residence time in working solutions in hours.

Inhibition efficiency was calculated according to Equation (2) [11]:

$$Ew(\%)={\displaystyle \frac{W_{corr}-{W^{inh}}_{corr}}{W_{corr}}}\times 100$$

(2)

where W_corr is the corrosion rate of steel without an inhibitor and W^inh _corr is the corrosion rate of steel with an inhibitor (fennel fruit essential oil or anethole).

• Electrochemical measurement

A compartment glass electrolytic cell, a reference electrode (Ag/AgCl), an anti-electrode (platinum plate), and a working electrode (the steel plate with a working area of 1 cm²) were used. The electrodes were connected to a potentiostat Gamry Interface 1010 E Potentiostat/Galvanostat/Zero-Resistance Ammeter (Gamry Instruments, Warminster, Pennsylvania, USA). After immersing the steel samples in the HCl solution with and without an inhibitor and reaching ion equilibrium for 3300 s, the potentiodynamic examination was carried out at a scanning speed of 0.167 mV/s in the range −0.03 ÷ +0.03 V relative to open-circuit potential.

The following polarization parameters were ascertained from the resulting Tafel polarization curve: corrosion potential, corrosion current density, and Tafel slope. The corrosion inhibition efficiency (%) was calculated according to Equation (3) [13]:

$$Ei(\%)=\left({\displaystyle \frac{I_{corr}-{I^{inh}}_{corr}}{I_{corr}}}\right)\times 100$$

(3)

where I_corr and I^inh_corr are the corrosion current densities without and in the presence of an inhibitor.

• Morphological study

The morphological characteristics of the mild steel surface were examined by taking photos of the surface before and after it was submerged in 1 M HCl solution without and containing different concentrations of the inhibitors tested. An optical microscope (Axiolab 5, Carl Zeiss, Oberkochen, Germany) was used to examine the specimens.

2.3. Statistical Analysis

All measurements were performed in triplicate and the results are presented as the mean value of the individual measurements with the corresponding standard deviation (SD).

3. Results

Figure 2 presents the optical micrographs of the steel surface before and after immersion in a solution of 1 M HCl.

3.1. Gravimetric Results

The essential oil was an easily mobile liquid, with a pale yellow color and a characteristic odor of anethole [30]. The anethole was a colorless liquid with an anise-like odor and a sweet taste.

The results of the gravimetric measurements of both inhibitors—fennel fruit essential oil and its main component anethole—are presented on

Table 1. Average weight loss of steel plates in the medium of 1 M HCl with and without inhibitors.

Medium, L	Weight Loss, mg
	Inhibitor Fennel Fruit Essential Oil	Inhibitor Anethole
1 M HCl without inhibitors	2.4 ± 0.0	2.4 ± 0.0
1 M HCl and 1.0 mL inhibitor	0.9 ± 0.0	1.2 ± 0.0
1 M HCl and 1.5 mL inhibitor	0.8 ± 0.0	1.2 ± 0.0
1 M HCl and 2.0 mL inhibitor	0.7 ± 0.0	1.3 ± 0.0

.

The average corrosion rate and its inhibition efficiency are computed from the values obtained, and the results are shown in Figure 3 and Figure 4.

Both general and pitting corrosion attacks that have different initiation periods were observed on the steel surface immersed in 1M HCl (Figure 2). When calculating general corrosion rates based on mass loss, materials exhibiting localized pitting may have an extremely inflated rate of corrosion penetration. Because localized corrosion penetration typically does not progress in a linear pattern with exposure duration, our next step was conducting electrochemical tests.

3.2. Electrochemical Results

The polarization studies were carried out on steel samples in 1 M HCl containing different concentrations of essential oil and anethole. The anodic polarization curves at 298 K are shown in Figure 5.

The electrochemical parameters of the steel plates at different concentrations of the inhibitors are presented in

Table 2. Electrochemical parameters of steel plates at different inhibitor concentrations.

Medium, L	βA *, mV/dec	βC, mV/dec	icorr, A/cm2	Ecorr, V vs. Ag/AgCl	Ei, %
1 M HCl without inhibitors	61.2 ± 0.6	77.2 ± 0.7	2.28 × 10−4	−0.480	-
1 M HCl and 1.0 mL fennel fruit essential oil	55.8 ± 0.5	77.5 ± 0.6	1.43 × 10−4	−0.481	37.28 ± 0.3
1 M HCl and 1.5 mL fennel fruit essential oil	61.7 ± 0.6	95.2 ± 0.69	8.22 × 10−5	−0.473	63.95 ± 0.6
1 M HCl and 2.0 mL fennel fruit essential oil	71.2 ± 0.7	98.9 ± 0.9	4.15 × 10−5	−0.471	81.80 ± 0.8
1 M HCl and 1.0 mL anethole	74.2 ± 0.7	72.7 ± 0.6	3.53 × 10−5	−0.468	84.52 ± 0.8
1 M HCl and 1.5 mL anethole	63.4 ± 0.6	65.4 ± 0.6	8.90 × 10−5	−0.472	60.96 ± 0.5
1 M HCl and 2.0 mL anethole	62.4 ± 0.6	90.8 ± 0.6	3.48 × 10−4	−0.472	52.63 ± 0.5

* β_A—anodic Tafel slope; β_C—cathodic Tafel slope; i_corr—the density of the corrosion current; E_corr—corrosion potential; Ei—corrosion inhibition efficiency.

. The results show that the inhibitory effect of the essential oil reaches a value of up to 81.80% at a concentration of 2.0 mL/L, and that of anethole reaches 84.52% at a concentration of 1.0 mL/L.

Figure 6 and Figure 7 show the surface condition of steel plates at different concentrations of inhibitors.

Following the electrochemical testing, varying degrees of pitting and uniform corrosion were visible in the surface morphologies. A reduced number of partially closed corrosion pits was seen with an increase in the content of fennel fruit oil (Figure 6). The reverse tendency of an increase in pit count was observed when the concentration of anethole was elevated (Figure 7). There was an absorbed film on the steel surface (Figure 6a and Figure 7a) that contributed to a decrease in contact between the steel and the aggressive medium. As a result, steel corrosion can be effectively prevented by a reliable layer of absorptive protection.

4. Discussion

Parallel features on the clean, ground steel surface that are connected to scratches before exposure to the corrosive solution are seen in Figure 2a. Figure 2b shows that the corrosion attacks in 1 M HCl follow general and pitting corrosion with clearly visible open pits with large diameters on the steel surface.

The results of gravimetric measurements (Table 1) show that weight loss in the environment without an inhibitor is up to 2.5 times higher compared to a fennel fruit essential oil inhibitor and up to 2 times with an anethole inhibitor. With an increase in the concentration of the fennel fruit essential oil (from 1 mL/L to 2 mL/L), a decrease in the weight of the steel plates (% compared to the medium without inhibitors) was found by 36.25, 33.33, and 27.08%, while in the case of anethole—the trend was the opposite—there was an increase of 50.83, 50.83, and 54.58%.

The data (Figure 3) show that at an essential oil concentration of 1.0 mL/L, the inhibition efficiency is 62.52%, at 1.5 mL/L, it rises to 66.68%, and at 2.0 mL/L, its value rises to 70.85%. As the concentration of the essential oil increases from 1.0 to 2.0 mL/L, the corrosion rate decreases from 0.0765 to 0.0595 mg/(cm².h). This may be due to the adsorption of the compounds contained in the oil onto the surface of the low-carbon steel. This adsorption limits the dissolution of the metal by blocking the regions of corrosive ions in an acidic environment and thus reducing weight loss by increasing the inhibition efficiency and concentration of the inhibitor [4].

The obtained results differ from the data in the literature for the fennel fruit essential oil under the same conditions of the experiment 1 M HCl environment, exposure duration 6 h and temperature 298 K:

• Lahhit et al. [5]: at concentration 1.00 g/L, corrosion rate 0.19 mg/(cm².h) and inhibition efficiency 41%; at concentration 1.50 g/L, corrosion rate 0.14 mg/(cm².h) and inhibition efficiency 56%; at concentration 2.00 g/L, corrosion rate 0.13 mg/(cm².h) and inhibition efficiency 59%; composition of the steel plate: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.05% Mn; and 0.05% S, and the remaining % is Fe. However, the chemical composition of the essential oil is very different, with the main components being limonene (20.8%), β-pinene (17.8%), myrcene (15.0%), fenchone (12.5%), and piperitenone oxyde (12.5%).
• Barrahi et al. [9]: at concentration 1.00 g/L, corrosion rate 0.283 mg/(cm².h) and inhibition efficiency 75%; at concentration 1.50 g/L, corrosion rate 0.192 mg/(cm².h) and inhibition efficiency 83%; at concentration 2.00 g/L, corrosion rate 0.124 mg/(cm².h) and inhibition efficiency 89%; steel plate composition: 0.370% C; 0.230% Si; 0.680% Mn; 0.016% S; 0.077% Cr; 0.011% Ti; 0.059% Ni; 0.009% Co; and 0.160% Cu, with the remaining % being Fe. However, there is no information in the material about the chemical composition of the essential oil studied by the authors.
• Bouoidina et al. [29]: at concentration 1 g/L, inhibition efficiency 84.1%; at concentration 0.8 g/L, inhibition efficiency 82.7%; at concentration 0.6 g/L, inhibition efficiency 82.4%; at concentration 0.4 g/L, inhibition efficiency 78.4%; steel plate composition: 0.20% C; 0.38% Si; 0.09% P; 0.01% Al; 0.05% Mn; and 0.05% S, with the remaining % being Fe. The content of anethole is 94%.
• Mouadili et al. [30]: at concentration 1 g/L, corrosion rate 0.16 mg/(cm².h), and inhibition efficiency 67%. The chemical composition of the essential oil is very different: anethole (24%), α-pinene (22%), and trans-ocimene (17.90%). There is no information about the composition of steel.

These differences, in our opinion, may be due to the different composition of the researched essential oils and the steel plates used, which are much lower in carbon (0.021%) than those in the literature (from 0.20 to 0.37%).

The corrosion rate and inhibition efficiency values obtained by us for the essential oil of fennel fruits also differ from the data for other “green inhibitors”, under the same experimental conditions; examples include the following:

• Spearmint (Mentha spicata L.) essential oil [6]: at concentration 1.00 g/L, corrosion rate 0.051 mg/(cm².h) and inhibition efficiency 81%; at concentration 1.50 g/L, corrosion rate 0.023 mg/(cm².h) and inhibition efficiency 88%; at concentration 2.00 g/L, corrosion rate 0.015 mg/(cm².h) and inhibition efficiency 93%; steel plate composition: 0.09% P; 0.38% Si; 0.01% Al; 0.05% Mn; 0.21% C; and 0.05% S, with the remaining % being Fe.
• Immortella (Helichrysum italicum subsp. italicum essential) essential oil [3]: at concentration 1.00 g/L, corrosion rate 0.082 mg/(cm².h) and inhibition efficiency 78.4%; at concentration 1.50 g/L, corrosion rate 0.070 mg/(cm².h) and inhibition efficiency 81.7%; at concentration 2.0 g/L, corrosion rate 0.067 mg/(cm².h) and inhibition efficiency 83.7018% at concentration 2.0 g/L, corrosion rate 0.185 mg/(cm².h) and inhibition efficiency 82.3%; steel plate composition: 0.09% P; 0.38% Si; 0.01% Al; 0.05% Mn; 0.21% C; and 0.05% S, with the remaining % being Fe.
• Anise (Pimpinella anisum L.) essential oil [7]: at concentration 1.00 g/L, corrosion rate 0.338 mg/(cm².h) and inhibition efficiency 70.22%; at concentration 2.0 g/L, corrosion rate 0.185 mg/(cm².h) and inhibition efficiency 83.70%; steel plate composition: 0.370% C; 0.230% Si; 0.680% Mn; 0.016% S; 0.077% Cr; 0.011% Ti; 0.059% Ni; 0.009% Co; and 0.160% Cu, with the remaining % being Fe. According to the authors, the inhibition efficiency is highest (92%) at a concentration of oil of 4 g/L
• African juniper (Tetraclinis articulata Vahl.) essential oil [8]: at concentration 1.00 g/L, corrosion rate 0.187 mg/(cm².h) and inhibition efficiency 64%; at concentration 2.0 g/L, corrosion rate 0.100 mg/(cm².h) and inhibition efficiency 80%; steel plate composition: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.005% Mn; and 0.005% S, with the remaining % being Fe.
• African juniper (T. articulata Vahl.) extract [8]: at concentration 1.00 g/L, corrosion rate 0.110 mg/(cm².h) and inhibition efficiency 78%; at concentration 2.0 g/L, corrosion rate 0.05 mg/(cm².h) and inhibition efficiency 90%; steel plate composition: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.005% Mn; and 0.005% S, with the remaining % being Fe.
• Green tea extract [16]: at concentration 1.00 g/L, corrosion rate 0.024 mg/(cm².h) and inhibition efficiency 80.36%; at concentration 2.0 g/L, corrosion rate 0.1237 mg/(cm².h) and inhibition efficiency 90.18%; steel plate composition: 0.370% C; 0.230% Si; 0.680% Mn; 0.016% S; 0.077% Cr; 0.011% Ti; 0.059% Ni; 0.009% Co; and 0.160% Cu, with the remaining % being Fe

These differences, in our opinion, may be due to the different composition of the described “green” inhibitors and the steel plates used.

The results for the inhibitor anethole (Figure 4) show that at an anethole concentration of 1.0 mL/L, the inhibition efficiency is 50.02%, at 1.5 mL/L, its value is preserved, and at 2.0 mL/L, the value decreases to 45.86%. As the anethole concentration increased from 1.0 to 2.0 mL/L, the corrosion rate increased from 0.102 to 0.111 mg/(cm².h). This decrease in inhibition efficiency is most likely due to structural changes in the anethole molecule, with the ether group being unstable in the presence of strong acids. In aromatic ethers, in which anethole is included, the alkyl–oxygen bond is always broken, not the aryl–oxygen. Initially, a salt is formed whose anion is a good nucleophile, and upon heating, nucleophilic substitution takes place at the saturated carbon atom (S_N) as shown on Figure 8 [35].

Anethole is obtained by crystallization from essential oils that contain it in large quantities, which is why it is an inexpensive natural product. However, it contains an ether group that is sensitive to environments with a high concentration of hydrochloric acid. This means that anethole can be used as an inhibitor, but in an environment with a lower concentration of hydrochloric acid, for example, from 1 to 1.5 mL/L.

It is likely that the other components in the fennel fruit essential oil, hydrocarbons and their oxygenated compounds, act as synergists and protect anethole from degradation in an environment with a higher concentration of hydrochloric acid, but this will be a subject of further studies.

There are studies in the literature on the use of individual compounds and components of essential oils as green inhibitors under the same experimental conditions; examples include the following:

• Chaieb et al. [36] studied the effect of eugenol and acetyleugenol on the corrosion of steel in a 1 M HCl solution. According to the study, the inhibition efficiency increased with content to attain 80 and 91% at 0.173 g/L, respectively (steel plate composition: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.05% Mn; and 0.05% S, with the remaining % being Fe). The difference in activity, according to the authors, is due to the chemical composition of the two inhibitors—eugenol is a phenol and acetyleugenol is an ester.
• Faska et al. [37] determined the effect of natural menthols and their synthesized epoxy-allylmenthols as a non-toxic inhibitor on the corrosion of steel in HCl media at various temperatures. The authors found that modified allylmenthol (to epoxy-allylmenthol) exhibited good inhibition, but its efficiency decreased with temperature rise. The study showed that the inhibition efficiency increased with the concentration of inhibitor to attain 74% at 0.8 g/L. The difference in values can be explained by the chemical composition of the inhibitors. According to the authors, menthol and its derivatives can be used in chemical cleaning and pickling processes.
• Faska et al. [38] compared the inhibitory effectiveness of two natural products—pulegone and pulegone oxide. The first product was isolated from pennyroyal min (Mentha pulegium L.) essential oil and the second was prepared by oxidation of pulegone. The authors found that the inhibition efficiency increased with the inhibitor content to attain 81 and 75% at 5 g/L for pulegone and pulegone oxide, respectively (steel plate composition: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.05% Mn; and 0.05% S, with the remaining % being Fe).
• Chaieb et al. [39] investigated the inhibitory effect of limonene in the temperature range from 298 to 328 K. According to the authors, the effectiveness of inhibition increased with the increase in concentration of limonene, and this exceeded 72% at 0.220 g/L (steel plate composition: 0.21% C; 0.38% Si; 0.09% P; 0.01% Al; 0.05% Mn; and 0.05% S, with the remaining % being Fe).

The values of the different aromatic compounds quoted above differ from those obtained by us, explainable by their different structure: anethole is a phenyl ether, eugenol is a phenol, acetyleugenol is an ester, menthol is a cyclic monoterpene alcohol, and limonene is a cyclic monoterpene hydrocarbon. There is a difference in the composition of the steel plates used, indicated above in the text.

The different amounts of inhibitors in the aggressive environment are also an explanation for the reported differences in the values of the measured indicators. In the studies cited above, the concentration of essential oils and extracts, as well as aromatic substances, is presented in g/L, whereas in our study, it is mL/L. According to literature data [24,25], the density of fennel fruit essential oil ranges from 0.889 to 0.921 g/mL and that of anethole ranges from 0.983 to 0.897 g/mL, which means that the amounts of both inhibitors at 1 mL are about 1 g, whereas at 2 mL, they are about 2 g. The data obtained from the experiment (Table 2, Figure 3) differ from those of Lotfi et al. [5], who investigated the effect of anise oil as a “green” corrosion inhibitor against steel in an environment with 1 M HCl. The authors found that at an oil concentration of 4.0 g/L, the highest value of the inhibitory effect was obtained—95.3% at potentiometric polarization and 92.93% at electrochemical impedance, determined spectrophotometrically. However, in the study, the authors did not indicate the concentration of anethole in the essential oil.

The data (Table 2) show that the inhibition efficiency (Ei) increases with an increase in the concentration of the fennel fruit essential oil and reaches a value of up to 81.80%, and in the case of anethole, the inhibition efficiency decreases with an increase in concentration. This trend confirms the data from Figure 3 and Figure 4. The values of β_c also change, which indicates that the surface of the steel plate is being modified, which can also be seen from the photos presented in Figure 6 and Figure 7. A similar dependence has been established by Lahhit et al. [5] and Barrahi et al. [9].

Data from the anodic polarization curves at 298 K (Figure 5a,b) show that the adsorption of fennel fruit essential oil and anethole on the metal surface forms a barrier to the access of hydrogen ions and prevents their reduction on the cathode sites of the steel surface.

Data from polarization curves and electrochemical parameters show that the densities of the corrosive current decrease significantly in the presence of a solution of 1 M HCl with an increase in the concentration of the essential oil. On the other hand, it is known that a compound is generally classified as an anode- and cathode-type inhibitor when the change in corrosion potential value is greater than 85 mV [40,41]. After the addition of different concentrations of the essential oil, the displacement of the corrosion potential is within very small limits (about 9 mV), suggesting that it acts as a mixed-type inhibitor. The resulting inhibition effectiveness shows that fennel fruit essential oil acts as an effective inhibitor.

In the case of anethole, the data from the polarization curves and electrochemical parameters show that the densities of the corrosion current decrease significantly in the presence of a solution of 1 M HCl, and with an increase in its concentration, they decrease, and the corrosion rate increases. The inhibition efficiency obtained shows that with an increase in its concentration, its inhibitory action decreases, as explained above in the text.

The change in the microstructure of the surface of steel plates (Figure 6 and Figure 7) is an illustration of the action of inhibitors in a hydrochloric acid environment. In Figure 6a and Figure 7a, the roughness caused by aggressive hydrochloric acid is clearly visible. The presence of fennel fruit essential oil as inhibitor (Figure 6b–d) reduces dark spots on the surface of the steel plate, which corresponds to the data from Table 2 and Figure 3. In Figure 7d, again, roughness is visible on the surface, which is explained by the degradation of anethole and the lower values of corrosion inhibition efficiency, shown in Table 2 and Figure 4.

5. Conclusions

• Fennel fruit essential oil can replace synthetic organic substances and be used as an environmental corrosion inhibitor of steel plates in the presence of 1 M HCl.
• Fennel fruit essential oil can be considered as a green inhibitor for carbon-native steel in 1 M HCl, as the inhibition efficiency is 81.80% at a concentration of 2.0 mL/L.
• Anethole, as the main component of fennel fruit essential oil, has a very low inhibitory effect; therefore, it is not suitable as a “green” inhibitor.