## 4 Discussion
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 [PERSON] 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 Figures 3 and 4. The values of \(\beta_{\text{c}}\) also change, which indicates that the surface of the steel plate is being modified, which can also be seen from the photos presented in Figures 6 and 7. A similar dependence has been established by [PERSON] et al. [5] and [PERSON] 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 (Figures 6 and 7) is an illustration of the action of inhibitors in a hydrochloric acid environment. In Figures 6a and 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.