Evaluation of Melissa Officinalis Extract and Oil as Eco-friendly Corrosion Inhibitor for Carbon Steel in Acidic Chloride Solutions

Introduction

The use of chemical inhibitors is one of the most practiced methods for protecting against corrosion, especially in acid media, to prevent metal dissolution and acid consumption [1]. Various organic and non-organic compounds have been studied as inhibitors to protect metals from corrosion. Usually, organic compounds exert a significant influence on metal surface adsorption and therefore can be used as effective corrosion inhibitors. The efficiency of these organic corrosion inhibitors is related to the presence of polar functions containing S, O, or N atoms which are centres for the establishment of the adsorption process [2-11].

Nevertheless, most of these synthetic organic compounds are not only expensive but also toxic for live beings. Now, the restrictive environmental regulations have made researchers to focus on the need to develop cheap, non-toxic, and environmentally benign natural corrosion inhibitors. These natural organic compounds could be either synthesized or extracted from aromatic herbs, spices, and medicinal plants. Plant extracts are viewed as an incredibly rich source of natural chemical compounds which can be extracted by simple and low-cost procedures and which are biodegradable in nature. The use as corrosion inhibitors of natural compounds extracted from leaves or seeds, for example, have been widely reported by several authors [12-19].

Recently, Fouda et al. [20] used the extract of Melissa as corrosion inhibitor of carbon steel in HCl solutions. The encouraging results incited to examine separately extract and oil of Melissa plant on the corrosion of carbon steel in molar HCl using different techniques including weight loss, potentio dynamic polarization measurements, and electro chemical impedance spectroscopy (EIS). The thermodynamic of activation parameters were also calculated and discussed.

Experimental Section

Inhibitors

Plant Material

The aerial part of Melissa Officinalis was harvested in January 2013 in the wild in the mountain Assoul located Taza at the Nord-east of Morocco. A voucher specimen was deposited in the Herbarium of Faculty of Sciences, Oujda, Morocco. The dried plant material was stored in the laboratory at room temperature (298 K) and in the shade before the extraction.

Hydrodistillation Apparatus and Procedure

Hydrodistillation is an extraction method whose function is to extract the volatile compounds of natural products with water vapour, and is often performed using Clevenger-type apparatus, with 400 mL for 3 hours. The essential oil yields were measured and subsequently dried over anhydrous sodium sulfate and stored at 277 K in the dark before gas chromatographic determination of its composition [18].

Characterization and Chemical Composition of Essential Oils

Techniques in chromatography (GC/MS, GC-FTIR, HPLC-DAD) were available for the molecular analysis of organic compounds. The chemical components of Melissa Officinalis essential oil were determined by spectral analysis of gas chromatography and gas chromatography coupled to mass spectrometry (GC-MS), which identified six major components. GC analyses were performed using a Perkin-Elmer Autosystem GC apparatus (Waltham, MA, USA) equipped with a single injector and two flame ionization detectors (FID). The apparatus was used for simultaneous sampling with two fused-silica capillary columns (60 m long with i.d. 0.22 mm, film thickness 0.25 µm) with different stationary phases: Rtx-1 (polydimethylsiloxane) and Rtx-Wax (polyethylene glycol). The temperature program was for 333-503 K at 275 K/min and then held at isothermal 503 K (30 min.). The carrier gas was helium (1 mL/min). Injector and detector temperatures were held at 553 K. Split injection was conducted with a ratio split of 1:80. Electron ionization mass spectra were acquired with a mass range of 35-350 Da. The injected volume of oil was 0.1 µL. For gas chromatography–mass spectrometry, the oils obtained were investigated using a Perkin-Elmer Turbo Mass Quadrupole Detector, directly coupled to a Perkin-Elmer Auto system XL equipped with two fused-silica capillary columns (60 m long with i.d. 0.22 mm, film thickness 0.25 µm), with Rtx-1 (polydimethylsiloxane) and Rtx-Wax (polyethylene glycol). Other GC conditions were the same as described above. Ion source temperature was 423 K and energy ionization 70 eV. Electron ionization mass spectra were acquired with a mass range of 35-350 Da.

The injected volume of oil was 0.1 µL. Identification of the components was based (1) on the comparison of their GC retention indices (RI) on non-polar and polar columns, determined relative to the retention time of a series of n-alkanes with linear interpolation, with those of authentic compounds or literature data [21], and (2) on computer matching with commercial mass spectral libraries [21, 22] and comparison of spectra with those in our personal library. Relative amounts of individual components were calculated on the basis of their GC peak areas on the two capillary Rtx-1 and Rtx-Wax columns, without FID response factor correction.

Materials

The steel used in this study is a carbon steel with a chemical composition 0.09 wt. % P; 0.38 wt. % Si; 0.01 wt. % Al; 0.05 wt. % Mn; 0.21 wt. % C; 0.05 wt. % S and the remainder iron (Fe).

Preparation of Solutions

The aggressive solutions of 1.0 M HCl were prepared by dilution of analytical grade 37% HCl with distilled water. Inhibitors were dissolved in acid solution at the required concentrations (in mL/L) (volume of inhibitor/volume of blank), and the solution in the absence of inhibitor was taken as blank for comparison purposes. The test solutions were freshly prepared before each experiment by adding essential oil and extract of Melissa Officinalis directly to the corrosive solution. The concentrations of essential oils were 0.5, 1, and 2 mL/L; and those of the extract were 2, 4, and 8 mL/L.

Gravimetric Study

Gravimetric experiments were performed according to the standard methods [23], the carbon steel specimens (1.0 cm × 1.0 cm × 0.1 cm) were abraded with a series of emery papers SiC (120, 600, and 1200 grades) and then washed with distilled water and acetone. After weighing accurately, the specimens were immersed in a 100 mL beaker containing 250 mL of 1.0 M HCl solution with and without addition of different concentrations of inhibitors. All the aggressive acid solutions were open to air. After 6 hours of acid immersion, the specimens were taken out, washed, dried, and weighed accurately. In order to get good reproducibility, all measurements were performed few times and average values were reported.

Electrochemical Measurements

The electrochemical measurements were carried out using Volta lab (Tacussel – Radiometer PGZ 100) potentiostat controlled by Tacussel corrosion analysis software model (Voltamaster 4) at static condition. The corrosion cell used had three electrodes. The reference electrode was a saturated calomel electrode (SCE). A platinum electrode was used as auxiliary electrode of surface area of 1 cm². The working electrode was carbon steel of the surface 0.32 cm². All potentials given in this study were referred to this reference electrode. The working electrode was immersed in the test solution for 30 minutes to establish a steady state open circuit potential (E_ocp). After measuring the E_ocp, the electrochemical measurements were performed. All electrochemical tests have been performed in aerated solutions at 308 K. The EIS experiments were conducted in the frequency range with high limit of 100 kHz and different low limit 0.1 Hz at open circuit potential, with 10 points per decade, at the rest potential, after 30 min of acid immersion, by applying 10 mV ac voltage peak-to-peak. Nyquist plots were made from these experiments. The best semicircle can be fit through the data points in the Nyquist plot using a non-linear least square fit so as to give the intersections with the x-axis.

The inhibition efficiency of the inhibitor was calculated from the charge transfer resistance values using the following equation:

Where, R^°_ct and Rⁱ _ct are the charge transfer resistance in absence and in presence of inhibitor, respectively.

After the ac impedance test, potentiodynamic polarization measurements of carbon steel substrate in inhibited and uninhibited solution were scanned from cathodic to the anodic direction, with a scan rate of 1 mV.s⁻¹. The potentiodynamic data were analysed using the polarization VoltaMaster 4 software. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain corrosion current densities (I_corr). From the polarization curves obtained, the corrosion current (I_corr) was calculated by curve fitting using the equation:

The inhibition efficiency was evaluated from the measured I_corr values using the following relationship:

 

where I_corr and I_corr(i) are the corrosion current densities for steel electrode in the uninhibited and inhibited solutions, respectively.

Results and Discussion

Essential Oil Composition

Qualitative and quantitative analyses of essential oils were carried out using GC/MS analyses. The compositions of essential oils of Melissa Officinalis are shown in Table 1.

Table 1: Chemical constituents of Melissa officinalis essential oil (EO) (%).

Compounds	Ir l	Ir a	Ir p	%
α-pinene	936	929	1019	0.3
oct-1-en-3-ol	962	961	1440	0.5
Sabinene	973	964	1117	0.4
β-pinene	978	969	1107	0.5
Myrcene	987	980	1155	1.1
α-terpinene	1013	1008	1175	0.3
p-Cymene	1015	1011	1263	0.2
Cineole 1,8	1024	1020	1207	0.9
Limonene	1025	1020	1196	0.6
(Z)b-Ocimene	1029	1025	1220	0.8
(E)b-Ocimene	1041	1035	1234	0.1
g-terpinene	1051	1048	1240	0.7
E-hydrate de sabinene	1053	1056	1457	4.6
P-Mentha-3,8-diene	1059	1059	1259	0.4
terpinolene	1082	1078	1277	0.2
Z-hydrate de sabinene	1082	1083	1536	0.5
1-octen-3-ol acetate	1093	1094	1371	1.8
3-octenyl acetate	1110	1107	1328	0.2
P-Menth-3-en-8-ol		1136	1602	8.8
Menthone	1136	1136	1483	1.0
isomenthone	1146	1140	1457	2.7
Neomenthol	1156	1154	1637	2.4
Terpinen-4-ol	1164	1166	1598	3.5
-Terpineol	1176	1175	1665	0.5
Pulegone	1215	1221	1639	8.8
Z-piperitone oxyde	1230	1235	1717	7.3
Pseudodisphenol	1245	1248		0.2
Isopulegylacetate 1	1263	1258	1631	0.2
Neomenthylacetate	1263	1263	1524	1.9
Thymol	1266	1263	2159	0.3
Bornylacetate	1270	1270	1572	0.3
Diosphenol	1276	1279	1790	1.0
Menthylacetate	1280	1279	1557	1.0
Piperitenone	1318	1313	1900	1.6
Piperitenone oxyde	1335	1341	1939	8.4
E-jasmone	1364	1369	1890	0.2
α-Copaene	1379	1375	1483	0.1
Nepetalactone	1360	1379	1978	1.2
P-Mentha-1,2,3-triol		1395	1880	13.1
α-Gurjunene	1413	1409	1528	0.1
E-Caryophyllene	1421	1420	1591	2.8
Cadina-3,5-diene	1448	1441		0.3
Trans-β-farnesene	1446	1448	1661	0.2
α-Humulene	1455	1450	1665	0.3
Cis Muurola-4(14),5-diene	1462	1458		0.7
Germacrene D	1479	1478	1706	3.4
Bicyclogermacrene	1494	1491	1725	0.4
γ-Cadinene	1507	1505	1756	0.1
Calamenene	1517	1509	1817	0.3
δ-Cadinene	1520	1514	1752	0.1
Spathulenol	1572	1564	2101	0.4
Caryophyllene oxyde	1578	1569	1965	0.5
Viridiflorol	1592	1581	2064	0.3
1,10-di-epi-Cubenol	1615	1603	2040	0.4
α-cadinol	1643	1638	2209	0.4
Total				88.7

The most important constituents of essential oils of Melissa officinalis are shown in Table 2.

 

Table 2: The major components of Melissa officinalis essential oils.

Major components	%
E-hydrate de sabinene	4.6
Z-piperitoneoxyde	7.3
Piperitenoneoxyde	8.4
Pulegone	8.8
P-Menth-3-en-8-ol	8.8
P-Mentha-1,2,3-triol	13.1

 

Gravimetric Measurements

Effect of Inhibitor Concentration

The effect of addition of oil and extract of Melissa Officinalis tested at different concentrations on the corrosion of carbon steel in 1.0 M HCl solution was also studied by weight loss method at 308 K after 6 h of immersion period. The corrosion rate (ν) and the inhibition efficiency (η_WL%) were calculated by the following equations [24]:

where W is the three-experiment average weight loss of the carbon steel, S is the total surface area of the specimen, t is the immersion time and ν₀ and ν are values of the corrosion rate without and with addition of the inhibitor, respectively.

Table 3 gives the results of the weight loss measurements for the corrosion of carbon steel in 1.0 M HCl solution in the absence and presence of oil and extract of Melissa Officinalis at 308 K. Analysis of Table 3 reveals that as the concentration of inhibitors increases, the inhibition efficiency (η_WL %) increases. This is because more surface area of carbon steel is covered by increasing inhibitors concentration.

Table 3: Gravimetric results of carbon steel in 1.0 M HCl at various concentrations of oil (EO) and extract (E) of Melissa Officinalis at 308 K.

Medium	Inhibitor (mL.L-1)	ν (mg.cm-2.h-1)	ηWL (%)
Blank	—-	0.320	——
	0.5	0.103	67.9
EO	1.0	0.054	83.0
	2.0	0.017	94.7
	2.0	0.120	62.5
E	4.0	0.053	83.5
	8.0	0.033	86.7

 

Effect of Temperature

In order to study the temperature dependence of corrosion rates in uninhibited and inhibited solutions, gravimetric measurements were carried out at the temperatures ranging from 313 K to 343 K in the absence and presence of optimum concentration of inhibitors (2.0 mL/L for EO, and 8.0 mL/L for E). The calculated values of the corrosion rates and inhibition efficiencies within the studied temperature range are shown in Table 4. 

Table 4: Corrosion parameters for carbon steel in 1.0 M HCl in absence and presence of optimum concentration of the inhibitors (2.0 mL/L for EO, and 8.0 mL/L for E) studied at different temperatures

Temp. (K)	Medium	ν (mg.cm-2.h-1)	θ	ηWL (%)
313	Blank	1.300	—–	—–
	EO	0.220	0.83	83.0
	E	0.047	0.96	96.4
323	Blank	1.828	—–	—–
	EO	0.425	0.76	76.7
	E	0.078	0.95	95.7
333	Blank	3.635	—–	—–
	EO	0.926	O.74	74.5
	E	0.133	0.96	96.3
343	Blank	6.336	—–	—–
	EO	2.562	0.59	59.6
	E	0.214	0.96	96.6

 

Generally, the corrosion rates of carbon steel in acidic solutions increased with the rise in temperature. This is due to a decrease in the overpotential of the hydrogen evolution reaction, resulting in higher dissolution rates of metals. The higher rate of hydrogen gas generation also increasingly agitates the metal corrodent/interface and depending on the nature of the metal-inhibitor interactions, this could also hinder the inhibitor adsorption or perturb already adsorbed inhibitor. On the other hand, for inhibitor species that react with the metal surface, increasing the temperature of the system could augment the interaction between the metal surface and the inhibitor leading to higher surface coverage.

Figure 1: Arrhenius plots for the corrosion rate of carbon steel in 1.0 M HCl solution in absence and presence of inhibitors. Click here to View Figure

 

Table 4  showed  that  the corrosion  rate increased  with  increasing the  temperature  both  in  uninhibited  and inhibited solutions while the inhibition efficiency of the essential oil decreased with temperature. On the other hand, the inhibition efficiency of the extract remained almost constant. A decrease in  inhibition  efficiency  with  the  increase  temperature  in  presence  of  essential  oil  might  be  due  to the weakening of physical adsorption, while in case of the extract, the constant value of the inhibition efficiency could be due to chemistry.

Arrhenius and transition state plots were used for determination of E_a (activation energy), ΔH_a (activation enthalpy) and ΔS_a (activation entropy) at the studied temperature 313, 323, 333, and 343 K for the carbon steel in the presence and absence of inhibitors (Equation 6):

where ν is the corrosion rate, A is the pre-exponential factor, E_a (J.mol^-1) is the activation energy, and R is the gas constant (8.314 J.mol^-1.K^-1).

To solve this equation, we take natural logs of both sides (common logs could be used as well):

The values of E_a were calculated (Table 5) from the slope (slope = -E_a/R) of the straight line of the x-axis (ln ν) and y-axis (1000/T) of the graph by using of Arrhenius plot for the carbon steel in 1.0 M HCl in the with and without inhibitors (Figure 1).

The transition state equation was used to calculate the ΔH_a and ΔS_a:

where R is the universal gas constant, A is the Arrhenius pre-exponential factor, h is Plank’s constant, N is Avogrado’s number, E_a is the activation energy, T is the absolute temperature in Kelvin, DS_a is the entropy of activation, and DH_a is the enthalpy of activation. To carry out simple calculations, Equation (6) was rearranged to become:

A plot of ln (ν/T) against 1000/T gives a straight line with the slope equal to (−ΔHa/R) and intercept equal to [ln(R/Nh) + (ΔS_a/R)], as shown in Figure 2. The ΔH_a and ΔS_a values were calculated and are displayed in Table 5.

Figure 2: Ln (ν/T) versus 1/T for the corrosion of carbon steel in 1.0 M HCl in absence and presence of inhibitors.  Click here to View Figure

 

Table 5: Activation parameters for the carbon steel dissolution in 1.0 M HCl in the absence and the presence of inhibitors at optimum concentration (2.0 mL/L for EO, and 8.0 mL/L for E).

Medium	Ea (KJ/mol)	∆Ha (KJ/mol)	∆Sa (J/mol.K)
Blank	48.41	45.69	-97.92
EO	72.39	69.67	-36.42
E	45.48	42.76	-134.27

 

Electrochemical Experiments

Potentiodynamic Polarization Curves

Figures 3(A) and 4(B) represent the potentiodynamic polarisation curves of carbon steel in a 1.0 M HCl solution in the absence and presence of oil and extract of Melissa Officinalis at 308 K.

Figure 3: Polarisation curves for carbon steel in 1.0 M HCl in the absence and presence of oil and extract of Melissa Officinalis concentrations: Extract oil (A) and extract of Melissa Officinalis (B). Click here to View Figure

 

The respective electrochemical parameters (i.e., corrosion potential (E_corr), corrosion current density (I_corr), and the anodic (β_a) and cathodic (β_c) Tafel constants, as shown in Table 6, were derived from Tafel plots.

From the potentiodynamic polarisation curves, it can be seen that the extracts caused a decrease in both anodic and cathodic current densities, which is most likely due to the adsorption of the organic compounds present in the oil and extract of Melissa Officinalis at the active sites of the electrode surface. This also slowed both metallic dissolution and hydrogen evolution and consequently slowed down the corrosion process. The two extracts exhibited the same behaviour.

Table 6: Electrochemical parameters of carbon steel at various concentrations of Melissa Officinalis oil and extract at 308 K

Medium	Conc.	–Ecorr (mV vs. SCE)	–bc (mV.dec-1)	ba (mV.dec-1)	Icorr (μA.cm-2)	hTafel (%)
Blank	1.0 M	450	144	67	361.8	—
	0.5 mL/L	490	143	65	135.7	62.5
EO	1.0 mL/L	490	147	72	80.3	77.8
	2.0  mL/L	490	176	71	50.9	85.9
	2.0 mL/L	470	195	61	119.8	66.9
E	4 .0mL/L	470	152	55	76.1	79.0
	8.0 mL/L	480	136	51	45.5	87.4

 

From Table 6, it is also clear that there is a shift toward cathodic region in the values of corrosion potential (E_corr). In the literature [34, 35], it has been reported that if the displacement in E_corr(inh) is more than 85 mV vs. SCE from E_corr, the inhibitor can be seen as a cathodic or anodic type; and if displacement in E_corr(inh) is less than 85 mV vs. SCE, the inhibitor can be seen as mixed type. In our study the maximum displacement in E_corr value was 40 mV vs. SCE toward cathodic region, which indicates that these investigated oil and extract of Melissa Officinalis are mixed type inhibitors. Cao [36] pointed out that the inhibitory action on the metal surface takes place with geometrical blockage of the active site. It can be observed from Table 6 that the I_corr values decrease considerably in the presence of EO and E and decrease with increasing the inhibitor concentration. Correspondingly, η_Tafel (%) values increase with increasing the inhibitor concentration reaching a maximum value at 2 mL/L for EO and 8 mL/L for E. The cathodic Tafel slope (β_c) and the anodic Tafel slope (β_a) of oil and extract of Melissa Officinalis changed with inhibitors concentration. This observation suggests that the inhibitors molecules controlled the two reactions and adsorbed on the metal surface by blocking the active sites on the metal surface, retarding the corrosion reaction.

Electrochemical Impedance Measurements (Eis)

The electrochemical impedance diagrams for carbon steel in 1.0 M HCl solution in the absence and presence of various concentrations of oil and extract of Melissa Officinalis are shown in Figures 4(A) and 4(B). Table 7 summarizes impedance data from the EIS experiments carried out both in the absence and presence of oil and extract of Melissa Officinalis at various concentrations. The electrochemical impedance diagrams show only one depressed capacitive loop, which is attributed to the one time constant, in the absence and presence of oil and extract of Melissa Officinalis, which indicates two significant effects: the charge-transfer resistance significantly increases, and the f_max decreases, in the presence of both inhibitors, decreasing the capacitance value, which may be caused by reduction in the local dielectric constant and/or by an increase in the thickness of the electrical double-layer.

Figure 4: Impedance diagrams obtained for carbon steel in 1.0 M HCl in the absence and presence of various oil and extract of Melissa Officinalis concentrations: Extract oil (A) and extract of Melissa Officinalis (B). Click here to View Figure

 

These results show that the presence of the inhibitors modifies the electric double-layer structure suggesting that the inhibitor molecules act by adsorption at the metal/solution interface. The solution resistance (Rs) is identical in the absence and presence of oil and extract of Melissa Officinalis and equal to 1.2 Ω.cm². Deviations from a perfect circular shape indicate frequency dispersion of interfacial impedance. This anomalous phenomenon is attributed in the literature to the non-homogeneity of the electrode surface arising from the surface roughness or interfacial phenomena [37-40]. The charge-transfer resistance (R_ct) values were calculated from the difference in impedances at lower and higher frequencies. The double-layer capacitance (C_dl) was calculated from the following equation:

where f_max is the frequency at which the imaginary component of the impedance is maximal. A C_dl value of 54.58 µF.cm^-2 was determined for the carbon steel electrode in 1.0 M HCl.

Data presented in Table 7 show that the value of R_ct increases in presence of oil and extract of Melissa Officinalis. The value of C_dl is always smaller in the presence of the inhibitors than in its absence, as a result of the effective adsorption of the inhibitors. It is apparent that a causal relationship exists between adsorption and inhibition. Besides, it is clear from the Table 7 that by increasing the inhibitors concentrations, the C_dl values tend to decrease and the inhibition efficiency increases. This behaviour was the result of an increase in the surface coverage by the inhibitors molecules of oil and extract of Melissa Officinalis, which led to an increase in the inhibition efficiency. The decrease in C_dl values may be considered in terms of Helmholtz model [41]:

where  is the dielectric constant of the medium,  is the vacuum permittivity, S is the electrode area, and is the thickness of the protective layer. In fact, the decrease in C_dl values can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double-layer [42].

Table 7: Corrosion  parameters  obtained  by  impedance  measurements  for  carbon  steel  in 1.0 M  HCl at  various concentrations of Melissa Officinalis oil and  extract.

Medium	Conc.	Rct (Ω.cm2)	fmax (Hz)	Cdl (µF.cm-2)	hz (%)
Blank	1.0 M	29.0	100	54.9	—
	0.5 mL/L	97.3	40	40.9	70.1
EO	1.0 mL/L	169.8	40	23.4	82.9
	2.0  mL/L	482.9	18	19.4	93.8
	2.0 mL/L	101.8	30	52.1	71.5
E	4 .0mL/L	138.4	25	46.0	79.0
	8.0 mL/L	173.2	20	46.0	83.2

 

The corrosion rate of carbon steel in 1.0 M HCl solution is controlled by both hydrogen evolution reaction and dissolution reaction of this metal. It is generally accepted that the corrosion inhibition occurs due to the adsorption of organic molecules at the metal/solution interface and the adsorption itself depends on the molecule’s chemical composition, the temperature, and the electrochemical potential at the metal/solution interface. In fact, the solvent H₂O molecules could also adsorb at metal/solution interface [43]. Therefore, the adsorption of organic inhibitor molecules from aqueous solution can be regarded as a substitution adsorption process between the organic compound in the aqueous phase [Org_(sol)] and water molecules on the metal surface [H₂O_(ads)] [44].

where x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor.

According to the detailed mechanism above, displacement of some adsorbed water molecules might take place on the metal surface by inhibitor species. The increase in efficiency of inhibition of oil and extract of Melissa Officinalis indicates that the inhibitors molecules are adsorbed on the carbon steel surface with higher concentration, leading to greater surface coverage (θ). It has generally been proved that the first step in the adsorption of an organic inhibitor on a metal surface usually involves the replacement of one or more of water molecules adsorbed at the metal surface by the investigated inhibitors molecules [45].

Adsorption process is usually described by isotherms. The most frequently used isotherms include Langmuir, Temkin, Frumkin,… [46]. Langmuir isotherm was tested for its fit to the experimental data. Langmuir adsorption isotherm is given by following equation:

where C_inh is the inhibitor concentration, and K_ads the adsorptive equilibrium constant, θ=E%/100 representing the degree of adsorption.

The plot of the (C_inh /θ) vs C_inh fitted the gravimetric data follow the Langmuir adsorption isotherm (Figure 5).

Figure 5: Langmuir adsorption isotherm of EO and E on carbon steel in 1M HCl. Click here to View Figure

 

It has been postulated in several works that natural plants (extract or oil) contain infinite components at different contents and the inhibition of corrosion takes place by intermolecular synergistic effect of the various components of inhibitor [47-49]. The known composition of essential oil facilitates the possibility of action of the various molecules below as the major components having OH group or ketone functions known as active centres of adsorption as shown below.

Conclusions        

Oil and extract of Melissa Officinalis can act as a natural source and eco-friendly corrosion inhibitor for carbon steel in 1.0 M HCl solutions. Inhibition efficiency increases with the increase of the concentration of the essential oil and extract of Melissa Officinalis. Tafel polarization measurements show that essential oil and extract of Melissa officinalis act essentially as a mixed type inhibitors.  The increase in the charge transfer  resistance and decrease in the double layer capacitance values, with the increase in the inhibitor concentration, showed that essential oil and extract formed protective layers on the carbon steel surface, covering areas where HCl solution degrades and corrodes rapidly. The inhibition efficiencies obtained from EIS, Tafel polarization, and weight loss measurements are in reasonable agreement with each other.

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