The elemental compositions of leaves and flowers of Catharanthus roseus [16] are sho- wn in Table 2 and the nutritional compositions of Turmeric [17] are shown in Table 3.

A three-electrode cell including a working electrode, an auxiliary electrode and a reference electrode was used for the electrochemical measurements. The working electrodes were made of mild steel sheets which were embedded in PVC holder using epoxy resin with a square surface of 1 cm². The auxiliary electrode was a platinum foil, the reference electrode was a saturated calomel electrode (SCE) with a fine Luggin capillary tube positioned close to the working electrode surface in order to minimize ohmic potential drop (IR). Each specimen was successive abraded by using SiC emery papers up to 1200 grit size, washed with bidistilled water and degreased in acetone then dried between filter papers. The working electrode was immersed in the test solution at open circuit potential for 30 min before measurement until a steady state reached. All the measurements were done in solutions open to atmosphere under unstirred conditions. All potential values were reported versus PDE. Prior to each experiment, the electrode was treated as before. Tafel polarization curves were determined by polarizing to ± 250 mV with respect to the free corrosion potential (E vs. SCE) at a scan rate of 0.5 mV/s. Stern-Geary method [18] used for the determination of corrosion current is performed by extrapolation of anodic and cathodic Tafel lines to a point which gives log i_corr and the corresponding corrosion potential (E_corr) for inhibitor free acid and for each concentration of inhibitor. Then i_corr was used for calculation of inhibition efficiency (%IE) and surface coverage (θ) as in equation (1):

where i_corr(free) and i_corr(inh) are the corrosion current densities in the absence and presence of inhibitor, respectively.

Electrochemical impedance spectroscopy measurements EIS were carried out in a frequency range of 100 kHz to 100 mHz with amplitude of 5 mV peak-to-peak. . The experimental impedance was analyzed and interpreted based on the equivalent circuit. The main parameters deduced from the analysis of Nyquist diagram are the charge transfer resistance R_ct (diameter of high-frequency loop) and the double layer capacity C_dl. The inhibition efficiencies and the surface coverage (θ) obtained from the impedance measurements are calculated from equation 2:

where and R_ct are the charge transfer resistance in the absence and presence of inhibitor, respectively

The gasometric set up is essentially an apparatus that measures the volume of gas evolution from a reaction system. A two-necked flask was connected via a delivery tube to a burette which was in turn connected to a reservoir of paraffin oil. Test solution (100 ml) was then introduced into the reaction mild steel test sheets were dropped into the test solution and the reaction vessel immediately closed. The volume of hydrogen gas evolved by the corrosion reaction was estimated by the volume change in the level of the paraffin oil in the burette. The progress of the corrosion reaction was monitored by careful volumetric measurement of the evolved hydrogen gas at fixed time intervals. Experiments were conducted at 25˚C.

The inhibition efficiency IE is calculated by:

where H₁ and H₂ are the volume of hydrogen evolved in the presence and absence of inhibitor, respectively.

The mild steel specimens were immersed for 12 h in 1M HCl solution containing optimum concentrations (300 ppm CDE) of inhibitor. After 12 h, the specimens were taken out and dried. Examination of mild steel surface after 12 h exposure to 1 M HCl solution without and with inhibitor was carried by using (JEOL JSM-5500) scanning electron microscope.

Electrochemical corrosion testing methods are extremely useful for the determination of corrosion behavior of metals, alloys and coatings in corrosive environments. The technique analyzes the corrosion behavior quickly, and can accurately identify the corrosion rate.

Potentiodynamic polarization curves of mild steel in 1 M hydrochloric acid in the absence and presence of plant extracts at 25˚C are illustrated in Figures 3-4. The numerical values of the variation of corrosion current density (i_corr), corrosion potential (E_corr), Tafel slopes (β_a and β_c), degree of surface coverage (θ) and inhibition efficiency (%IE) with the concentrations of plant extracts are given in Tables 4-5.

This indicates that:

1) The cathodic and anodic curves obtained exhibit Tafel-type behavior. Addition of extract inhibitors increased both cathodic and anodic over voltages and caused mainly parallel displacement to the more negative and positive values, respectively. The parallel cathodic and anodic Tafel curves in Figures 3-4 shows that the hydrogen evolution is activation-controlled and both of the reduction and dissolution mechanisms are not affected by the presence of the inhibitor [19] . So, it could be conducted that these inhibitors are mixed-type for mild steel in 1M HCl medium, which may be adsorbed on the cathodic sites of mild steel and reduce the evolution of hydrogen. Moreover, the adsorption of these compounds on anodic sites through the lone pair of electrons will then reduce the anodic dissolution of mild steel [20] [21] .

2) The corrosion current density (i_corr) decreases with increasing the concentration of plant extract, which indicates that the presence of these extracts retard the dissolution of mild steel in 1M HCl solution and the degree of inhibition depends on the concentration and type of the inhibitor present.

3) The investigated extracts appeared to act as mixed type inhibitors, as it is shown from Figures 3-4 where both cathodic and anodic polarization curves are influenced by the presence of the investigated extracts in the corrosive media. In addition, the cathodic and anodic Tafel lines were more or less parallel.

4) The slopes of anodic and cathodic Tafel line (β_a, β_c) were slightly changed with increasing the concentration of the investigated extracts. This is indicates that the hydrogen evolution and metal dissolution reactions were activation controlled and the addition of inhibitors did not modify the mechanism of these processes.

Electrochemical impedance spectroscopy provides a new method to characterize the film coverage on the electrode, which is related to charge transfer resistance (R_ct). The

interface capacitance can also be used to determine the film quality [22] [23] . It is known that the coverage of an organic compounds on the metal surface depends not only on the structure of the organic compound and the nature of the metal, but also on the experimental conditions such as immersion time and concentration of adsorbent [24] [25] .

The corrosion behavior of mild steel in 1 M HCl solution in the absence and presence of different concentrations of the investigated compounds was investigated by using the EIS technique at 25˚C. Figures 5-6 show the Nyquist plots for mild steel. The diagrams are characterized by one time constants consisting of large capacitive loop at high to medium frequency and inductive loop at low frequency. The capacitive loop at high frequencies represents the phenomenon associated with the double electric layer. It arises from the time constant of electrical double layer and the charge transfer in corrosion process [26] [27] . The large capacitive loop makes an angle with the real axis and its intersection gives a resistance of the solution (R_s) enclosed between the working electrode and the counter electrode [28] . The presence of inductive loop may be attributed to the relaxation process obtained by adsorption species onto the electrode surface.

From the impedance data given in Tables 6-7, we conclude that:

・ R_ct increases by increasing the concentration of extract, giving consequently a decrease in the corrosion rate.

・ The high R_ct values, are generally associated with slower corroding system [29] [30] .

・ The existence of single semicircle showed that single charge transfer process occurred during dissolution of mild steel which unaffected by the presence of investigated compounds.

・ C_dl values decreases with increasing the extract concentration. This is due to the gradual replacement of water molecules in the double layer by the adsorbed extract molecules which form an adherent film on the metal surface and leads to decrease in the local dielectric constant of the metal solution interface [31] .

The technique of hydrogen gas evolution is used to follow the corrosion behavior of mild steel in 1M HCl solutions containing different concentrations of tested extract inhibitor. The curves of Figures 7-8 represent the variation in the volume of H₂ gas evolved on mild steel coupons with the immersion time, when immersed in different concentrations of inhibitor solutions. Inspection of these curves reveals that hydrogen gas evolution starts after the elapses of a certain time from immersion of the mild steel in the test solution. This time is identified as the incubation period which is the time needed by the acid to destruct the pre-immersion oxide film and start dissolution of mild steel [32] .

It is noteworthy to see that the incubation period, in absence and presence of the inhibitor, is within the first 15 minutes of immersion of the steel coupons in the solution, and increases with increasing the additives concentration. After the incubation period, the volume of the H₂ gas evolved increases linearly with time due to the possible simultaneous destruction of the porous film and the continuous dissolution of mild steel according to the reaction:

The accompanied iron dissolution process is the cathodic reaction to consume the electrons generated in the anodic process, as shown in Equation (2). In the acidic medium, the H_ads, atomic hydrogen adsorbed on the metal surface reacts by combining with other adsorbed H atoms to form H₂ gas, which bubbles from the surface according to:

However it is noteworthy to see that the linear relation between the volumes of gas evolved V, in ml/cm² changes with time t in minute, follows relation [32] .

where r is the rate constant of the dissolution reaction.

Tables 8-9 represent the inhibition efficiency data from hydrogen evolution for mild steel in 1 M HCl solutions without and with various concentrations of plants extracts.

From all the results obtained from different techniques, may be compare between the values of inhibition efficiency for Catharanthus and Turmeric extracts (Table 10).

A photograph of the polished mild steel surface before immersion in 1M HCl solution is shown in Figure 9. The photograph shows the surface was smooth and without pits. A photograph of the mild steel surface after immersion in 1 M HCl solution (10 hrs.) is shown in Figure 10. The photograph revealed that, the surface was strongly damaged in the absence of plant extracts. A photograph of the mild steel surface after immer- sion in 1 M HCl solution (10 hrs.) containing 300 ppm of plant extract is shown in Figures 11-12. It was found that the faceting observed in figures disappeared and the surface

was free from pits and it was smooth. It can be concluded from Figures 10-12 that corrosion decreased largely in the presence of plant extract hence corrosion was inhibited strongly when plant extracts was present in the medium. The metallographical micrographs were exposed at magnification (X 1500). In the presence of 300 ppm of plant extracts, there is much less damage on the steel surface, which further confirm the inhibit ion action. Also, there is an adsorbed film adsorbed on mild steel surface.

In accordance, it might be concluded that the adsorption film can efficiently inhibits the corrosion of mild steel.