# The Use of Some Natural Extracts as Environmentally Friendly Carbon Steel Corrosion Inhibitors
## 4 Discussion

Table 2: The values of _i\({}_{cor}\)_ and _v\({}_{cor}\)_ obtained from the potentiodynamic polarization curves for the corrosion of carbon steel in a 3.5% NaCl solution and in NaCl solutions modified with _Citrus x clementine_ peel and _Levisticum officinal_ leaf extracts. the maximum value corresponding to the lowest amount of extract added to the corrosive medium (200 \(\mu\)L). For the _Levisticum officinal_ leaf extract prepared in analytical-grade ethanol, the minimum value of the inhibiting efficiency (14.1%) was registered when adding an amount of 200 \(\mu\)L of the extract. One may see that by modifying the composition of the _Levisticum officinal_ and _Citrus x Clementine_ natural extracts, the inhibiting efficiency also varied. For example, the sample CC_Alc600, tested in a solution with the _Citrus x Clementine_ extract, had an IE of 49.4%, and the sample CC_Alc-A600 had an IE of 35.7%. This is explained by the difference in composition of the extracts, since the first one was obtained using a 99.6% ethanol solution, while the latter was achieved in a 50%:50% (_v:v_) water/ethanol solution. In this case, the active inhibiting compounds are more soluble in 99.6% ethanol than in the 50%:50% (_v:v_) water/ethanol solution. In the case of the _Levisticum officinal_ extracts, the active compounds are more soluble in the 50%:50% (_v:v_) water/ethanol solution than in a concentrated ethanol solution; hence, a higher IE was achieved for the samples in the aqueous solution. The inhibiting efficiency varied with the volume of the extracts, but a linear correlation was not observed for either the _Citrus x Clementine_ extract or the _Levisticum officinal_ extract. It appears that the highest volume of the extracts did not provide the highest value of IE. For example, for the samples tested in the presence of the _Levisticum officinal_ extract in the 50%:50% (_v:v_) water/ethanol solution, the highest value of IE was achieved with the smallest volume of extract, while for the samples tested in the presence of the _Levisticum officinal_ extract in 99.6% ethanol, the IE showed an increase followed by a decrease, as the volume of the extract increased. The same variation was observed for the samples tested in the presence of the _Citrus x Clementine_ extract in the 50%:50% (_v:v_) water/ethanol solution. From potentiodynamic polarization, we also obtained information about the corrosion potential, anodic and cathodic slopes (Table S3), for the tested samples. The registered corrosion potential for the carbon steel samples tested in 3.5% NaCl modified with the _Citrus x Clementine_ peel extracts and the _Levisticum officinal_ leaf extracts shifted to more electronegative values with respect to the potential of the carbon steel blank sample tested in an unmodified 3.5% NaCl solution. This shifting of the corrosion potential is an indication of the inhibiting effect of the synthesized extracts on the corrosion of carbon steel. Both anodic and cathodic slopes suffered modifications, indicating the inhibition of the anodic reaction, that of oxidation, occurring on the carbon steel plate, due to the addition of the natural extracts.