Valorization of the essential oil of Zingiber officinale by its Use as inhibitor against the corrosion of the carbon steel in acid medium hydrochloric acid 1 M

In this work, we are interested in the study of the protection of ordinary steel in acid medium (HCl) 1M by adding Zingiber officinale EO rhizomes essential oil at different concentrations: 0.5g / L, 1g / L, 1.5g / L and 2g / L. The technique used is an electrochemical technique based on the recording of intensity-potential curves in potentio-kinetic mode. The inhibitory action exerted by the essential oil is revealed by tracing the transient curves of the impedance Z to determine in a first step the percentage of inhibition and the stationary curve in order to confirm the first result. The main results have shown that the essential oil of ginger (Zingiber officinale) can slow down or inhibit corrosion in acid medium (hydrochloric acid), steel. The maximum percentage inhibition is 96%. It should be noted that it is obtained with the concentration 1.5g / L of essential oil.


Introduction
Corrosion affects most industrial sectors and can cause huge losses.In the case of industrial processes, metals are exposed to the action of acids used in chemical processes and during cleaning in several industries.These acids cause the degradation of metals, either by chemical or electrochemical reactions.The best way to protect these materials against corrosion is through the use of inhibitors 1,2 .This use, even in small concentrations has a significant decrease in the percentage of corrosion on the surface of the metal 3,4 .
Although these inhibitors exhibit good anticorrosion action, most of them are highly toxic to humans and the environment, hence the importance of converging towards less harmful, natural and biodegradable inhibitors.Recently several studies have been conducted in this direction, using the plants especially their essential oil as inhibitors 5 .
One of the widely used metals in industry, particularly for structural applications is carbon steel but being susceptible to rust in humid air and high dissolution in acidic media, its wider use becomes an obstacle 6 .For the cleaning and stripping of steel structures; hydrochloric acid and sulfuric acid, are generally used in industrial processes but are normally accompanied by considerable dissolution of the metal 7 .
This work aims to evaluate the inhibitory efficacy of the essential oil Zingiber officinale oil on Carbon steel in a 1.0 M solution of HCl through the exploitation of potentiodynamic polarization curves, the impedance spectroscopic.The effect of temperature on corrosion is also studied in a range from 298 K to 328 K.
The thermodynamic parameters, such as ΔΗa enthalpy and ΔSa entropy, were calculated and analyzed.

Materiel
The tested materiel in this study is carbon steel with the chemical composition listed in

Extraction of the essential oil
The essential oil extracted from the rhizomes of Zingiber officinale has a complex composition since it consists of a mixture of compounds that belong to different classes of organic chemistry.The composition of the same species can vary according to the geographical location, the climatic conditions, the period of harvest, the part of the plant used and the drying method.The properties can also vary, hence the need to work in the same conditions, to have the reproducibility of the results at the stage of analysis and identification of the molecules contained in the oil.The essential oil was extracted from the rhizomes of Zingiber officinale by the "Clevenger" hydrodistillation technique 8 .The apparatus consists of a round-bottomed flask, in which are placed the material containing the volatile oil and a given quantity of water; a separator, in which the oil is automatically separated from the distillate in a graduated tube, thereby permitting a direct reading of the quantity of the oil; and a convenient condenser 9 .The collected oil was kept in an opaque bottle at 4ºC.

Electrochemical tests
Electrochemical experiments have been performed using a Potentiostat Radiometer Analytical type PGZ 100 to positive direction using Volta Master 4 software which was controlled by a personal computer coupled with an electrochemical cell containing three-electrode cell: a platinum as counter electrode (CE), a carbon steel as a working electrode (with an exposed surface of 1 cm 2 ), and saturated calomel electrode (SCE) as the reference electrode.All potentials were measured against this later electrode.The potentiodynamic polarization curves were recorded by changing the electrode potential automatically from negative to positive values versus Ecorr at a scan rate of 1 mV/s after 30 min of immersion time until reaching steady state.The test solution was thermostatically controlled at 298±2 K using 50 ml of test solution.
In order evaluate corrosion kinetic parameters, a fitting by Stern-Geary equation was used.Thus, the overall current density values, i were considered as the sum of two contributions, anodic and cathodic current ia and ic, respectively.For the potential domain not too far from the open circuit potential, it may be considered that both processes followed the Tafel law 10 .Thus, it can be derived from equation ( 1): Where icorr is the corrosion current density (A cm -2 ), ba and bc are the Tafel constants of anodic and cathodic reactions (V -1 ), respectively.These constants are linked to the Tafel slopes β(V/dec) in usual logarithmic scale given by equation ( 2): 2) The corrosion parameters were then evaluated using nonlinear least square method by applying the previous equation using Origin software.However, for this calculation, the potential range applied was limited to ±0.100 mV/ECS around Ecorr, and a significant systematic divergence was sometimes observed for both anodic and cathodic branches.
The corrosion inhibition efficiency (  %) is evaluated from the corrosion current densities values using the following equation (3) 11 : Where i 0 corr and icorr are the corrosion current densities in absence and presence of the inhibitor, respectively EIS measurements performed at corrosion potentials, Ecorr, over a frequency range of 100 kHz to 100 MHz with an AC signal amplitude perturbation of 10 mV peak to peak.The EIS diagrams were done in the Nyquist plot.The results were then analyzed in terms of an equivalent electrical circuit using Brockamp program 12 .
The inhibition efficiency (ηimp%), is calculated using equation ( 4): Where R 0 ct and Rct are the charge transfer resistance in absence and presence of the inhibitor, respectively,  is the recovery rate.

Chemical composition
The analysis of the essential oil of ginger by chromatography in the gaseous stage coupled by a spectre of mass enabled us to identify eighteen compounds (Table 2).The most abundant components are: Geranial (23.30 %), Camphene (12.59 %) Eucalyptol (12.87 %), and alpha-Zingiber ne (7.38 %).

Electrochemical studies
Polarization studies Figure 1 represents the cathodic and anodic polarization curves of carbon steel, in 1M HCl, with and without inhibitor at different concentrations, the electrochemical parameters: anodic Tafel constancy (βa), cathodic Tafel constancy (βc), corrosion potential (Ecorr) and the corrosion current density (icorr) and inhibition efficiency (  %) are given in Table 3.
It is clear from Fig. 1 that the addition of the Zingiber officinale EO reduces both the cathodic and the anodic currents and therefore hinders the acid attack of the carbon steel electrode in 1M HCl.The significant change of Tafel slopes of the anodic (βa) and cathodic (βc) in the presence of Zingiber officinale EO can be observed as a result of adsorption the molecules of inhibitor.The corrosion process decrease can be explained by both metallic dissolution and hydrogen evolution at the metal surface due to the adsorption of an organic compound (heteroatom such as oxygen) 13,14,15 .
Based on these results, the Zingiber officinale EO can be considered as mixed type inhibitor in 1M HCl.From Table 3, it is clear that the icorr values decrease considerably with the increase of the concentration of Zingiber officinale EO.That means that the inhibitory actions are due to their adsorption on the carbon steel surface.Accordingly, (  %) values increase with increasing the inhibitor concentration reaching a maximum value of 97.4% with 1.5 g/L of ginger oil.

EIS studies
The corrosion behavior of mild steel in 1.0 M HCl with and without Zingiber officinale EO is also investigated by EIS at corrosion potential.The nyquist plot, for carbon steel in 1.0 M HCl with Zingiber officinale EO at 298 K, is shown in Figure 2. The Nyquist spectra exhibit a single capacitive loop single wave attributed to charge transfer of the corrosion process 16 .These Nyquist plots are not perfect semicircles, and this may be attributed to the frequency dispersion of interfacial impedance 17 .This behavior is a result of the surface roughness, the chemical heterogeneity of surface, and the adsorption-desorption process of inhibitive molecules on carbon steel surface 18 .Furthermore, the diameter of the semicircles in the presence of Zingiber officinale EO is larger than observed in blank solution (1.0 M HCl) and increases with increasing inhibitor concentration, which may be related to the increase of surface coverage of inhibitive molecules on the carbon steel surface.Accordingly, the simple electrical equivalent circuit (Figure 3) has been proposed to model the experimental data.The employed circuit allowed the identification of the solution resistance (Rs) and charge transfer resistance (Rct).It is noteworthy that the double layer capacitance (Cdl) value was affected by imperfections of the surface, and this effect was simulated via a constant phase element (CPE) 19 .CPE was used as the substitute for the capacitor to fit more accurately impedance behavior of the electric double layer.The impedance of the CPE is expressed as 20 : Where Y is the magnitude of the CPE; ω the angular frequency; n is an empirical exponent which measures the deviation from the ideal capacitive behavior 21 .Depending on the values of n, CPE can represent resistance (n = 0), capacitance (n = 1), inductance (n=-1) and Warburg impedance (n = 0.5) 22,23 .Also, the double layer capacitances, Cdl, for a circuit including a CPE were calculated by using the equation ( 6) 24 : Where ωmax =2πfmax is the angular frequency at the maximum value of the imaginary part of the impedance spectrum, Q is the constant phase element (CPE).
The impedance spectrums were fitted, and their parameters were calculated according to different models and listed in Table 4.  4 indicated that Rct value increased and Cdl decreases with the increased concentration of the inhibitor.This is attributed to the increase in surface coverage of carbon steel by inhibitor molecule leading to enhanced inhibition efficiency 25 .The increase in Rct value is attributed to the formation of a protective film on the metal/solution interface.The impedance study also gave the same efficiency trend as found in Tafel polarization method.

Effect of temperature
In order to get more information about the adsorption type of Zingiber officinale EO and its effectiveness at higher temperatures, potentiodynamic polarization measurements were used in the temperatures range from 298 to 328 K for carbon steel electrode in 1.0 M HCl without and with 1.5 g/L of Zingiber officinale EO after 30 mn of immersion time (Figure 4).It is remarked that these curves exhibited the Tafel regions.It is also noted that the anodic and cathodic branches increased with the increase of temperature.
The values of corrosion current density (icorr), Corrosion potential of steel (Ecorr), and the inhibitory efficiency of both inhibitors as a function of temperature are given in Table 5.It can be seen that the icorr increased with increased temperature, both in uninhibited and inhibited solutions, and the values of the inhibition efficiency of Zingiber officinale EO decreased with higher temperatures.Thus, the inhibition efficiencies of Zingiber officinale EO are temperature-dependent.

Activation parameters of corrosion process
The activation thermodynamic parameters of the corrosion process can be determined using Arrhenius Eq. and Eq.transition state 26 : Where Ea is the apparent activation energy, R is the gas constant; A is the Arrhenius pre-exponential factor, N is Avogadro's number, h is Plank's constant, ΔSa is the entropy of activation and ΔHa is the enthalpy of activation.
A plot of ln (icorr) vs 1000/T obtained gave a straight line with regression coefficient close to unity, as shown in Figure 5.The Arrhenius plot of ln (icorr/T) vs 1000/T (Figure 5) which gave straight lines with slope ΔHa/R and intercept (ln (R/N.h)+ ΔSa/R) from which ΔHa and ΔSa values were calculated.The values of ΔHa and ΔSa were calculated and listed in Table 6.It was observed that the value of apparent activation energy Ea are higher in the presence of inhibitor than in its absence.This result indicated that the energy barrier for the corrosion reaction increases in the presence of inhibitor is associated with physical adsorption or weak chemical bonding between the inhibitor specie and the steel surface 27,28 The positive values of ΔHa mean that the dissolution reaction is an endothermic process indicating difficulty of carbon steel dissolution 29,30 .The entropy ΔSa increases negatively in the presence of Zingiber officinale EO which reflects that the activated complex represents an association rather than a dissociation step which means the formation of an ordered, stable layer of this inhibitors on the carbon steel surface 31,32 .

Conclusion
The following conclusions can be derived from the study of Corrosion inhibition characteristics of Zingiber officinale EO on carbon steel corrosion in 1 M HCl medium:  The Zingiber officinale EO are efficient corrosion inhibitor for carbon steel in the studied medium. Polarization measurement showed the Zingiber officinale EO are mixed type inhibitor. Electrochemical impedance results show that the obtained Nyquist diagrams exhibit one single capacitive loop, indicating that carbon steel corrosion in essentially under charge transfer control, leading to the formation of a protective layer on the metal surface, which limits the steel corrosion. The study of the influence of temperature on the inhibitory efficiency shows that this is decreasing with increasing temperature.That confirms that the Zingiber officinale EO adsorption on the steel surface is done by physisorption type. Reasonably good agreement was observed between the obtained measures from potentiodynamic polarization curves and electrochemical impedance spectroscopy techniques.

Figure 2 .
Figure 2. Impedance plots for steel interface in 1.0M HCl medium, presented in Nyquist plan, absence and presence of different concentrations of Zingiber officinale EO at 298 K

Figure 3 .
Figure 3.The electrochemical equivalent circuit used to fit the impedance spectra Inspection of the results in Table 4 indicated that Rct value increased and Cdl decreases with the increased concentration of the inhibitor.This is attributed to the increase in surface coverage of carbon steel by inhibitor molecule leading to enhanced inhibition

Figure 4 .
Figure 4. Temperature effect in 1.0 M HCl, with 1.5 g/L of Zingiber officinale EO and uninhibited solution

Figure 5 .
Figure 5. Arrhenius curves of carbon steel in 1 M HCl with and without 1.5g / L of essential oil of Zingiber officinale

Table 1 .
1: Chemical composition of the used carbon steel.

Table 2 .
Chemical Composition of Zingiber officinal essential oil.
Figure 1.Polarization curves of carbon steel in 1 M HCl with and without Zingiber officinale EO at various concentrations at 298 K

Table 3 .
Polarization data of carbon steel in 1.0 M HCl without and with Zingiber officinale EO at 298 K.

Table 4 .
Electrochemical impedance parameters of carbon steel in 1M HCl medium in the absence and the presence of the inhibitor at different concentrations at 298 K.

Table 5 .
Influence of temperature on electrochemical parameters of steel in 1.0M HCl without and with addition of 1.5 g/L of Zingiber officinale EO.

Table 6 .
The value of the activation parameters Ea, ΔHa and ΔSa of carbon steel in 1M HCl in the absence and in the presence of 1.5g / L of essential oil of Zingiber officinale