2.1. Samples and solutions

Samples in the present study were composed of a CS rod with a cross-sectional area of 1 cm² and its chemical composition was analyzed by X-ray fluorescence as shown in Table 1. These samples act as a working electrode (WE). All the chemicals were analytical grade Aldrich products. Hydrochloric acid solution with 0.5 M and triple distilled water were used in all the investigations. Before performing the experiments, the working electrode was abraded with emery paper of different grades up to 1,200 grit, then rubbed with a smooth polishing cloth, washed with triple distilled water, and transferred to the three-electrode electrochemical cell.

2.2. Inhibitor

AO (trans-p-propenylanisole (anethole), trans-1-methoxy-4-(1-propenyl)benzene) with molar mass 148.2 g/mol and molecular formula C₁₀H₁₂O was used in an analytical grade from El-Captain Company (CAP PHARM); see Figure 1. AO, obtained from star anise native to China or Pimpinella anisum native to the Mediterranean region, is an aromatic pale yellow oily liquid composed mainly of anethole (propenylanisole). The star anise is unrelated botanically to the anise of the Bible but has a very similar taste and aroma. It is used in medicinal cough drops, dentifrices, perfumes, flavorings, beverages, candies, and embedding materials for microscopy [21]. AO was irradiated by various doses of γ-rays (i.e., 5 kGy and 15 kGy, resp.) before being used as an inhibitor. So, AO can be considered as an environmentally safe and economically reasonable green inhibitor.

2.3. Electrochemical measurements

All the electrochemical measurements, including potentiodynamic and electrochemical impedance spectroscopy (EIS), were conducted using Gamry PCI300/4 Potentiostat/Galvanostat/Zra analyzer. EIS is a nondestructive sensitive technique that enables the detection of any changes occurring at the electrode/electrolyte interface. Impedance data were presented as Nyquist plots. The impedance measurements were carried out at the open circuit potential (OCP) in a frequency range from 0.01 Hz up to 100 kHz with a superimposed AC-signal amplitude of 5 mV peak to peak. EIS can measure directly the impedance, Z, and the phase shift of the electrochemical system. All electrochemical tests were carried out using a three-electrode cell composed of carbon steel, with an exposed surface area of 1 cm², used as a working electrode, platinum as the counter electrode, and saturated calomel electrode (SCE) (E_SCE = 0.242 V vs. the normal hydrogen electrode (NHE)) as a reference electrode, respectively. Potentiodynamic measurements were carried out at a scan rate of 3 mV/s at 25 °C. The Echem Analyst 5.21 statistically fits the experimental data to the Stern-Geary model for a corroding system. The routine automatically selects the data that lies within the Tafel region (±250 mV with respect to the corrosion potential). The Echem Analyst calculated the corrosion potential, the corrosion current density, and the anodic and cathodic slopes,

where I_corr and I_corr(inh) represent the corrosion current density values without and with inhibitor, respectively. Before impedance or polarization measurements, the working electrodes were immersed in the test solution until a steady state of the OCP was reached. Each experiment was performed at least twice with a new surface for each run.

2.4. Examination of surface morphology

A JEOL (5400 Japan) SEM was utilized to examine the surface morphology of various specimens of CS after being immersed in 0.5 M hydrochloric acid in the absence and presence of the inhibitor at 50 ppm for 1 h. This instrument was operated in a secondary electron imaging mode with an accelerating voltage of either 10 KeV or 20 KeV. In the present investigation, the magnification was selected, 100×. All the experiments were carried out at room temperature 298 K unless otherwise stated. The experimental details are described in Section 3 [22].

2.5. Functional groups detection

The functional groups of AO were proved by infrared spectroscopy (FTIR). Inspection of Figure 2 shows that the characteristic band appeared at 519 cm⁻¹ for stretching vibration of the aliphatic straight-chain −C−C− skeleton; the bands appeared at 1,135 cm⁻¹ and 3,217 cm⁻¹ for stretching vibration of −CH(OH)− group.

3.1. OCP measurements

Figure 3 illustrates the potential-time curves of CS electrode in 0.5 M HCl solution against saturated calomel electrode (SCE): the blank curve was tending towards more negative value firstly, which represents the breakdown of the preimmersion, air formed oxide film present on the surface of working electrode. This oxide film was breakdown and soluble in the solution so that the potential was shifted to a more negative direction until a steady-state potential was established at 500 V. Additions of the inhibitor molecules produce a negative shift E_corr potential; these results indicated that the corrosion inhibitor may act as cathodic protection before and after being radiated.

3.2. EIS measurements

The impedance results for the stainless steel in physiological solution after the immersion for two different times are shown in Figures 4–6. EIS spectra illustrated typical Nyquist impedance plots obtained for the CS electrode at an OCP. The best fit using the equivalent circuit shown in Figure 7 and Table 2 illustrated that the data of this equivalent circuit showing the increasing inhibitor concentrations raised the polarization resistance (R_p). It also has been reported that the semicircles at high frequencies were generally associated with the relaxation of electrical double-layer capacitors, and the diameters of the high-frequency capacitive loops can be considered as the charge-transfer resistance. Table 2 illustrates the inhibition efficiency, η%, of AO inhibitor, being or without being radiated, for the CS electrode; and it was calculated by the following equation:

where R_p0 and R_p were the charge transfer resistance in absence and presence of AO inhibitor, respectively.

3.3. Electrochemical polarization measurements

The potentiodynamic polarization technique was used to investigate the electrochemical behavior of CS in 0.5 M HCl solution. Representative polarization curves from the potentiodynamic polarization measurements are displayed in Figures 8–10. The quantitative corrosion parameters of corrosion potential (E_corr), and corrosion current density (I_corr) obtained through the polarization curves were calculated and presented in Table 3. However, the differences in values of the inhibitors from those of the free acid solution were not up to 85 mV; hence they are not sufficient to categorize the inhibitor as either of cathodic or anodic type. Therefore, AO can be considered as a mixed type inhibitor which inhibits the corrosion process by geometric blocking of both cathodic and anodic surface active sites of the CS. Also, corrosion current density was reduced in the presence of the inhibitors and with increasing the AO concentration (Table 3), demonstrating the inhibitive effects of AO. The inhibition efficiency also increased with the increase in inhibitor concentration, similar to EIS. Figures 8–10 illustrate the potentiodynamic polarization curves for CS in 0.5 M HCl solution and in the presence of 50 ppm of the AO radiated and without being radiated. The electrochemical parameters, namely, corrosion potential (E_corr), cathodic (b_c) and anodic (b_a) Tafel slopes, corrosion current density (I_corr) and inhibition efficiency (η%), were given in Table 3. The data in Table 3 showed that the I_corr values decreased considerably with the increase of the inhibitor concentration, E_corr were shifted to more negative values; therefore these inhibitors acted predominantly as cathodic inhibitors and consequently adsorption mechanism was much more likely at the cathodic sites. The effect of inhibitor type and concentration was observed on the values of b_a and b_c, so that these inhibitors obstruct the available surface area; it seems that the film formed on the metallic surface became more uniform with concentration, while molecular structure may affect film resistance due to chemical bonding nature with metallic surface. The inhibition efficiency, η (%), was calculated and it was obvious that the η (%) increased with increasing the inhibitor concentration. The mechanisms of the inhibiting effect of these derivatives were attributed to the formation of a chemical bonding between inhibitor and metallic surface according to oxygen-atoms. Adsorption of AO compound through oxygen molecules result from its capability of shearing lone pair electrons. It is noteworthy that the results obtained from potentiodynamic measurements matched with those obtained from Nyquist impedance determinations (Figures 4–6).

3.4. Surface morphology

Scanning electron microscopy was used to examine the surface morphology of the CS specimens immersed in 0.5 M HCl solution in the absence and presence of 50 ppm of AO inhibitor and also before immersion in acid medium and inhibitor. Figure 11(a) shows a characteristic surface of CS after polishing. Figure 11(b) shows the immersion of CS without AO inhibitor in 0.5 M HCl for 2 h. Scanning electron micrographs reveal that the surface was strongly damaged by corrosion (i.e., corroded surface), which is shown as black and white area on the surface of specimen. On the other hand, Figure 11(c) shows the surface of CS specimen after immersion in 0.5 M HCl and 50 ppm for 2 h. Scanning electron micrographs reveal that the surface was much less damaged. Figures 11(d) and 11(e) show that the formation of protective film of the inhibitor on the metal surface under the effect of gamma rays. The results were obtained from the (SEM) technique explain those obtained from all electro chemical data.