# The Use of Some Natural Extracts as Environmentally Friendly Carbon Steel Corrosion Inhibitors
## 4 Discussion  
The results of the monitoring of the open circuit potential show that all tested samples, except for L_Alc200, were stable in the aggressive solutions examined. The sample L_Alc200, which corresponded to the carbon steel strip analyzed in the aggressive medium modified with 200 \(\mu\)L of the _Levisticum officinal_ leaf natural extract, showed a non-typical variation, the potential randomly increasing and decreasing during the 10 min of the analysis. This random variation shows that the carbon steel sample was not stable in the NaCl solution, a fact that was confirmed by electrochemical impedance spectroscopy, which registered the lowest value of R\({}_{2}\) for this sample. One may see that the values of the registered potential decreased in time, in the first 5 min, followed by a very small decrease in the next 5 min. The parameters extracted from electrochemical impedance spectroscopy, R\({}_{2}\) and C, showed that all the investigated concentrations of the natural extracts exerted a protective effect on the corrosion of the carbon steel samples. More precisely, the polarization resistance values R\({}_{2}\) showed the inhibiting effect of the natural extracts employed. One may see that all the R\({}_{2}\) values for the samples tested in media with the extracts were higher than the value of the blank sample, tested in the 3.5% NaCl solution. In the case of the _Levisticum officinal_ leaf natural extracts, the highest values of R\({}_{2}\) (1.877 and 2.114 k\(\Omega\cdot\)cm\({}^{2}\)) were achieved with the 50%:50% (_v:v_) water/ethanol extract added in amounts of 600 and 1000 \(\mu\)L, while the lowest values were registered with the alcoholic extract. In the case of the _Citrus x clementine_ peel extracts, the values of polarization resistance registered for the samples tested were all higher than the one of the blank sample. The highest value achieved was 1.617 k\(\Omega\cdot\)cm\({}^{2}\), registered for the sample CC_ALC-A1000, tested in the NaCl medium modified with 1000 \(\mu\)L of the 50%:50% (_v:v_) water/ethanol _Citrus x clementine_ peel extract. Regarding the values of capacitance (C), their variation also confirmed the influence of the inhibiting extracts on the corrosion process studied, because values lower than the one of the blank sample (765.6 \(\mu\)F/cm\({}^{2}\)) were registered for all the tested samples in the modified aggressive media. One may see that for all the investigated samples, the corrosion rate was lower than that of the blank sample, meaning that the corrosion process was slowed down by using the inhibitors. The lowest corrosion rate was registered for the sample tested in the aggressive medium with 600 \(\mu\)L of the _Levisticum officinal_ leaf extract prepared in the 50%:50% (_v:v_) water/ethanol solution. From the values of the inhibiting efficiency, one may see that both tested extracts, _Levisticum officinal_ leaf and _Citrus x clementine_ peel extracts, acted as inhibitors of carbon steel corrosion in the 3.5% NaCl solution. The _Levisticum officinal_ leaf extract prepared in the 50%:50% (_v:v_) water/ethanol solution provided the highest inhibiting effect (87.8%), \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & _i\({}_{cor}\),_\(\mu\)**A/cm\({}^{2}\)** & _v\({}_{cor}\),_\(\mu\)**m/year** & _IE,_\%** \\ \hline Blank & 3.4693 & 40.18 & - \\ CC\_Alc100 & 1.6034 & 18.57 & 53.8 \\ CC\_Alc600 & 1.7563 & 21.27 & 49.4 \\ CC\_Alc-A600 & 2.2298 & 25.83 & 35.7 \\ CC\_Alc-A1000 & 1.7953 & 20.80 & 48.3 \\ CC\_Alc-A1500 & 2.4421 & 28.29 & 29.6 \\ L\_Alc200 & 2.9784 & 34.50 & 14.1 \\ L\_Alc1000 & 1.9002 & 22.01 & 45.2 \\ L\_Alc1500 & 2.1532 & 24.94 & 37.9 \\ L\_Alc-A600 & 0.4238 & 4.91 & 87.8 \\ L\_Alc-A1000 & 1.2466 & 14.44 & 64.1 \\ L\_Alc-A1250 & 1.7293 & 20.03 & 50.2 \\ \hline \end{tabular}
\end{table}