## 4 Discussion
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:  
* [PERSON] 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. * [PERSON] et al. [37] determined the effect of natural menthols and their synthesized epoxyallymenthols as a non-toxic inhibitor on the corrosion of steel in HCl media at various temperatures. The authors found that modified allymenthol (to epoxy-allymenthol) 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. * [PERSON] 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). * [PERSON] 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, acetyleneenol 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.