## 1 Introduction
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 \({}^{\circ}\)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]. Figure 1: The chemical formula of anethole (C\({}_{10}\)H\({}_{12}\)O). 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. * [PERSON] 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.
* [PERSON] 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%. * [PERSON] et al. [29] investigated the inhibitory activity of the essential oil and determined its very good inhibitive properties. * [PERSON] 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--dulec 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.