# The Use of Some Natural Extracts as Environmentally Friendly Carbon Steel Corrosion Inhibitors
## 2 Materials and Methods  
The natural extracts employed in this work were obtained from _Levisticum officinal_ (Romania, from the market) and _Citrus x clementine_ peel (origin country, Spain). The natural extracts were prepared in ethanol solutions, namely, analytical-grade ethanol (99.6%, ChimReactiv, Bucharest, Romania) and a 50%:50% (_v:v_) water/ethanol solution. In order to prepare the _Levisticum officinal_ extracts, the leaves were dried at room temperature for 30 days, crushed, and sieved through a 150 \(\upmu\)m sieve (Figure 1). The extraction mixtures were prepared using 2 g of _Levisticum officinal_ leaves and 10 mL of analytical-grade ethanol or a 50%:50% (_v:v_) water/ethanol solution. The mixtures were stirred for 30 min and kept in the dark at a constant temperature of 20 \({}^{\circ}\)C for 7 days; then, they were filtered on the 7 th day, using a filter paper. The filtrates thus obtained were further used as environmentally friendly _Levisticum officinal_ leaf extracts (Figure 2). In order to prepare the _Citrus x clementine_ peel extracts, the fruit peel was minced to a small size (approximately, 0.3 cm\({}^{2}\)), and 4 g of peels was added to 20 mL of analytical-grade ethanol or a 50%:50% (_v:v_) water/ethanol solution. The mixtures were stirred for 30 min and kept in similar conditions as those used for the _Levisticum officinal_ leaf extracts. The _Citrus x clementine_ peel extracts used as corrosion inhibitors were achieved by filtering the extraction mixtures on the 7 th day of extraction (Figure 3). Figure 1: _Levisticum officinal_ leaves: dried, crushed, and sieved. Figure 2: _Levisticum officinal_ leaf extracts. L_.Alc: _Levisticum officinal_ leaves in analytical-grade ethanol; L_.Alc_.A: _Levisticum officinal_ leaves in a 50%:50% (_v:v_) water/ethanol solution. The carbon steel samples analyzed consisted of strips with an active surface area of 1 cm\({}^{2}\). Prior to the analyzing step, the carbon steel strips were mechanically cleaned using energy paper of several grit sizes (80, fine, and ultrafine used in the professional car painting area) in order to remove the naturally formed corrosion products from the surface and to smoothen it. After the mechanical cleaning, the strips were degreased with sodium carbonate, rinsed in distilled water and ethanol, and dried. These carbon steel strips were employed as a working electrode in an electrochemical cell constructed in order to investigate the effect of the prepared natural extracts on the corrosion process. The electrochemical cell consisted of two more electrodes, namely, a reference Ag/AgCl, KCl sat. electrode (Radiometer Analytical) and a platinum mesh electrode with an active area of 9 cm\({}^{2}\), and 50 mL of 3.5% NaCl aggressive medium modified with different concentrations of the natural extracts, which were not oxygenated, as shown in Table 1. The electrochemical tests were performed in a thermo-stated cell at 25 \({}^{\circ}\)C.  
In order to evaluate the effect of the prepared natural extracts, we employed electrochemical methods such as open circuit potential monitoring (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The testing cell was connected to a potentiostat (VoltaLab 40 PGZ301), interfaced with a VoltaMaster 4.0 software. The potential of the open circuit was measured for 10 min at the beginning of the test sequences in order to evaluate the stability of the steel samples in the electrolytic solution used, in the absence and in the presence of the inhibitor. Electrochemical impedance spectroscopy was performed by applying a 10 mV potential sinusoidal wave and scanning the frequency range between 100 kHz and 100 MHz at a measurement speed of 10 measurements/decade. The measurements of potentiodynamic polarization were performed by scanning the potential between \(-\)0.85 and 0 V, with a scan speed of 2 mV/s. After the electrochemical tests were performed, we acquired images of the tested carbon steel samples in all aggressive media, which are presented in Table S1.