Investigation of the protection effect of ginger, tea tree and grapefruit essential oil extracts on mild steel in 0.5M H₂SO₄ solution

Mild steel (MS) test pieces with circular dimension of 1.2 cm consists of 0.21% Cu, 0.29% Si, 1.04% Mn, 0.27% C, 0.08% S, 0.05% P and 98.08% Fe energy dispersive spectroscopy with PhenomWorld scanning electron microscope. The MS test pieces were cut with high speed serrating tool into separate proportion of 1 cm length for weight loss analysis. MS test pieces for potentiodynamic polarization and open circuit potential analysis were fixed to a Cu wire with soldering lead before embedded in pre-hardened epoxy mixture. The exposed surfaced of the steel was abraded with emery papers (60–1500 grits) and burnished with 6 μm diamond solution before to washing with deionized H₂O and acetone. Ginger (GII), tea tree (TT) and grapefruit (GP) essential oil extracts were ordered from NOW Foods in the United States of America, and prepared in volumetric concentrates of 0%, 1%, 1.5%, 2%, 2.5%, 3% and 3.5% separately in 200ml of 0.5M H₂SO₄ solutions.

Electrochemical investigation was done at 35 °C atmospheric temperature with a Digi-Ivy 2311 potentiostat (consisting of ternary multicomponent electrode configuration within a glass beaker holding the acid-extract solution) and connected to a computer device. The electrodes are MS electrodes with visible area of 1.126 cm³, Ag/AgCl standard electrode and Pt wire counter electrode. Polarization curves were plotted at sweep rate of 0.0015 V s⁻¹ at potential ranges of −1.1 V to +1.65 V. Corrosion current density C_J, (A cm⁻²) and corrosion potential, C_p (V) values were obtained through Tafel derivation. Corrosion rate, C_RT (mm/y) was computed from the numerical expression below;

$$\begin{eqnarray}&&{C}_{RT}=\displaystyle \frac{0.00327\times {C}_{{\rm{J}}}\times {C}_{{\rm{q}}}}{D}\end{eqnarray} \tag{ 1 }$$

where C_q is the equivalent weight (g) of MS, 0.00327 represents corrosion constant and D is density (g cm⁻³). Inhibition efficiency, I_EF (%) was calculated from below;

$$\begin{eqnarray}&&{I}_{EF}=\left[1\mbox{--}\left(\displaystyle \frac{{C}_{{\rm{RT}}2}}{{C}_{{\rm{RT}}1}}\right)\right]\times 100\end{eqnarray} \tag{ 2 }$$

C_RT1 and C_RT2 represents MS weight loss without the extracts and with the extracts. Polarization resistance, P_R, (Ω) was computed from below;

$$\begin{eqnarray}&&{P}_{R}=2.303\displaystyle \frac{{B}_{{\rm{a}}}{B}_{{\rm{c}}}}{{B}_{{\rm{a}}}+{B}_{{\rm{c}}}}\left(\displaystyle \frac{1}{{I}_{{\rm{cr}}}}\right)\end{eqnarray} \tag{ 3 }$$

B_a and B_c represents the anodic and cathodic Tafel slopes (V dec⁻¹).

1% GII concentration/0.5 M H₂SO₄, 1% TT concentration/0.5 M H₂SO₄ and 1% GP concentration/0.5 M H₂SO₄ media before and following corrosion test were delineated under infrared particle emissions with Bruker Alpha FTIR (Fourier transform infrared spectroscopy) spectrometer limited to wavelength range of 375 to 7500 cm⁻¹, and precision of 0.9 cm⁻¹. Spectral patterns were analysed and matched to the standard ATF-FTIR (Attenuated total reflectance-Fourier transform infrared spectroscopy) table. Tabulations were done for the functional groups liable for the inhibition of the corrosion reaction process. Morphological image characterization of MS specimens before, and following the corrosion test in the acid media was performed with a Mustech digital microscope.

Weight of MS steel test pieces were computed and separately hanged in 200 ml of the acid-extract electrolyte for 240 h. Weight measurement was performed at 24 h interval. Corrosion rate, C_RT (mm/y) was estimated from the formulae below;

$$\begin{eqnarray}&&{C}_{RT}=\left[\displaystyle \frac{87.6{{\rm{W}}}_{L}}{DAt}\right]\end{eqnarray} \tag{ 4 }$$

W_L illustrates weight loss (g), D is density (g cm⁻³), A is total surface of the MS specimen (cm²) and 87.6 is the corrosion rate constant. t is time (h). Inhibition efficiency (η) was computed from the following numerical relationship;

$$\begin{eqnarray}&&{I}_{EF}=\left[\displaystyle \frac{{W}_{{\rm{L}}2}-{W}_{{\rm{L}}2}}{{W}_{{\rm{L}}1}}\right]\times 100\end{eqnarray} \tag{ 5 }$$

W_L1 and W_L2 are weight loss at predetermined extract concentrations.

Potentiodynamic polarization plots for MS corrosion in H₂SO₄ media in the presence of specific concentrations of GII, TT and GP extracts are shown in figures 1(a)–(c). The polarization data are presented in table 1. Corrosion rate of MS at 0% extract concentration is 18.010 mm y⁻¹ corresponding to corrosion current density of 1.71 × 10^–3 A cm⁻² and polarization resistance of 13.31 Ω. This is confirmed from the significantly higher cathodic and anodic Tafel plots (figures 1(a)–(c)). Addition of GII, TT and GP extracts at 1% concentration substantially reduced the corrosion rate of MS to 1.751 mm y⁻¹, 0.605 mm y⁻¹ and 0.480 mm y⁻¹ (inhibition efficiency of 90.28%, 96.64% and 97.33%), and corrosion current density of 1.66 × 10⁻⁴ A cm⁻², 5.74 × 10⁻⁵ A cm⁻² and 4.56 × 10⁻⁵ A cm⁻². Reduction in corrosion rate was observed GII extract concentration increased till 0.161 mm y⁻¹ (99.11% inhibition efficiency). Corrosion rate of MS in TT/H₂SO₄ solution varied with TT concentration but did not significantly decrease with corrosion rate of 0.604 mm y⁻¹ (inhibition efficiency of 96.65%) at 3.5% TT concentration. MS corrosion rate increased to 0.616 mm y⁻¹ at 2% GP concentration before decreasing to 0.121 mm y⁻¹ (inhibition efficiency of 99.33%).

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Generally, the cathodic plot for the extracts varies as extract concentration increases due to significant influence of the protonated extract molecules on the H₂ evolution and O₂ reduction reactions. The molecules precipitate on reactive sites on the steel surface. However, the effectiveness of the precipitation is subject to available of sufficient extract molecules. Figure 1(a) shows GII extract substantially influenced the mechanism cathodic reaction process from the substantial decrease of the cathodic portion of the polarization plot compared to TT and GP extracts. The higher the GII extract concentration, the lower the slope of the cathodic polarization plots. The anodic plots (figures 1(a)–(c)) were generally similar signifying effective surface coverage of MS through activation control and electrostatic attraction of the extract molecules. This influenced the protection effect of the extracts. Variation of corrosion potential of GII, TT and GP extract concentration to the potential at 0% extract concentration shows the inhibition performance of the extracts is a mixed-type [43, 44] i.e. the extract limited the redox reaction mechanisms liable for corrosion.

Comparative analysis of the corrosion inhibition effect of GII, TT and GP oil extracts on MS in 0.5 M H₂SO₄ solution was performed by weight loss measurement. Plots of MS corrosion rate to measurement time in the presence of the extracts are shown from figures 2(a)–(c). Figures 3(a)–(c) shows the corresponding plots of GII, TT and GP inhibition efficiency values versus measurement time. Table 2 shows MS weight loss, MS corrosion rate and extracts (GII, TT and GP) inhibition efficiency data at 240 h of exposure in the electrolyte. Observation of the corrosion rate plots (figures 2(a)–(c)) shows the corrosion rate values of MS at 0% GII, TT and GP concentrations substantially varies from the plot at 1%–3.5% GII, TT and GP concentrations. This is by reason of the deteriorating action of SO₄ ²⁻ anions in the electrolyte on MS surface through redox reaction mechanisms leading to accelerated corrosion of the steel. Corrosion rate at 0% extracts concentration initiated (24 h) at 41.95 mm y⁻¹ and progressively deteriorated to 63.33 mm y⁻¹ at 240 h. In the presence of the extracts (GII, TT and GP), the corrosion rate of the steel at all concentrations was substantially reduced. Plots in figure 1(a) shows corrosion rates of MS with GII extracts addition (1%–3.5% GII concentration) exhibited the lowest values in contrast to the plots in the presence of TT and GP extracts. In the presence of GII extract, corrosion rates initiated (24 h) at values between 0.835 and 1.392 mm/y. At 240 h, the culminating corrosion rate values varies between 0.278 and 5.012 mm/y. Corrosion rate values for TT extract varies between 1.156 and 11.582 mm y⁻¹ at 240 h while the corresponding values for GP extract varies between 1.062 mm y⁻¹ and 10.277 mm y⁻¹ respectively. It must be noted that corrosion rate of MS at 1.5% GP concentration is slightly higher than the values at 2.5% and 3% GP concentration. This observation is probably due to marginal lateral repulsion among the inhibitor molecules. The effect is marginal because the corresponding inhibition efficiency value is above 90%

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Apart from 1% GII concentration, corrosion rate plots of MS were generally similar from 24 h to 240 h signifying stability of the protection effect of GII extract with reference to time and concentration. Performance of TT and GP extract visibly varies with concentration (figures 2(b) and (c)) though TT extract shows significant time dependent performance before 96 h of exposure. Further insight to the protection performance of GII, TT and GP extracts on MS in the acid electrolyte are shown from figures 3(a)–(c). GII inhibition efficiency initiated (24 h) at values between 98.01% and 99.56% (1.5% to 3.5% GII concentration) and culminated at values between 99.46% and 99.56%. Inhibition efficiency values at 1% GII concentration decreased to 92.09% at 240 h in contrast to values at 1.5% to 3.5% GII concentration. Inhibition efficiency of TT extract generally decreased for the first 72 h before attaining relative stability at 144 h till 240 h. TT inhibition efficiency at 3.5% TT concentration was stable throughout the exposure hours. Inhibition efficiency of GP extract was stable at all concentrations but varies with concentration with the exception of 2.5% GP concentration which significantly varied with exposure time culminating at 83.77% (240 h).

Optical images of MS before corrosion test, after corrosion in 0.5M H₂SO₄ solution and after corrosion in the acid electrolyte in the presence of GII, TT and GP extracts are shown from figures 4(a)–(f). The image in figure 4(b) shows severe morphological deterioration compared to figure 4(a) due to oxidation of MS surface by SO₄ ²⁻ anions. Macro-pits are clearly visible on the surface of the steel due to displacement of ionized Fe²⁺ anions into the electrolyte. The steel underwent accelerated corrosion and degradation in the acid solution. This image contrasts the images in figures 4(c)–(e), and figure 4(f) where the surface morphologies of MS displays sufficient resistance to electrochemical deterioration in the presence of the extracts. Morphological differences are due to the nature of electrochemical reaction between the extracts, SO₄ ²⁻ anions and MS surface. Figure 4(c) shows GII protection involves reaction with MS surface to form non-corrosive precipitates while figures 4(d) and (e) shows the extracts forms a protective over the steel which limits the diffusion of the corrosive anions unto the steel surface.

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Further insight into the corrosion protection effect of GII, TT and GP extracts on MS in H₂SO₄ media was gotten from numerical equations and plots depicting the extent of differentiation of extract molecular species adsorbed onto the steel surface at equilibrium conditions and stable temperature [45]. Ionized molecules of the extracts transit from the aqueous phase to the metal-solution interphase during corrosion inhibition reaction processes. Data from electrochemical analysis shows the extracts influenced the anodic and cathodic plots i.e. inhibition reaction occurred through surface coverage, covalent bonding, electrostatic attraction etc Adsorption of inhibitor molecules generally involves agglomeration of inhibitor molecules from the electrolyte on the ionized metal surface. Data from weight loss experimentation were analyzed with conventional adsorption isotherms while the correlation coefficient values was applied to estimate the most relevant adsorption isotherms. Langmuir, and Frumkin isotherms gave the highest correlation coefficient values for GII extract. Langmuir, Frumkin, Freundlich and Temkin isotherms gave the highest correlation coefficient values for TT extract while Langmuir isotherms was the only relevant model for GP extract. Table 3 shows the correlation coefficient data for the relevant adsorption isotherms.

The inhibition reaction mode of GII extract on MS aligns with the Langmuir and Frumkin isotherm models. The Langmuir and Frumkin isotherm plots for GII inhibition on MS are shown in figures 5(a) and (b). Langmuir isotherm indicates that definite sum of reaction sites occurs on metallic surfaces in molecular layer form of adsorbed inhibitor molecules. The value of energy between the molecules is dependent on the dynamic equilibrium conditions in accordance with the equation below:

$$\begin{eqnarray}&&\theta =\left[\displaystyle \frac{{K}_{{\rm{ads}}}{C}_{{\rm{ex}}}}{1+{K}_{{\rm{ads}}}{C}_{{\rm{ex}}}}\right]\end{eqnarray} \tag{ 6 }$$

K_ads represents the equilibrium constant of adsorption. C_ex is the molar concentration of GII, TT and GP extracts. The Frumkin isotherm indicates that molecular adsorption on ionized non-homogeneous surfaces are uniform under high concentrations with reference to the potential of the adsorbent surface. The lateral interaction effect between the extract molecules is substantial and numerical according to the equation below:

$$\begin{eqnarray}&&\mathrm{log}\left[{C}_{TMAV}\left(\displaystyle \frac{\theta }{1-\theta }\right)\right]=2.303\,\mathrm{log}\,{K}_{{\rm{ads}}}+2\alpha \theta \end{eqnarray} \tag{ 7 }$$

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Langmuir, Frumkin, Freundlich and Temkin adsorption were the relevant adsorption isotherm model for TT adsorption on MS in H₂SO₄ solution. There plots are shown in figures 6(a)–(d). The Temkin isotherm indicates that the heat of adsorption reduces in proportion to increase in surface coverage at equal binding energy distribution [46] with respect to the equation below:

$$\begin{eqnarray}&&qe=B\,\mathrm{ln}\left(A+{C}_{e}\right)\end{eqnarray} \tag{ 8 }$$

Where

$$\begin{eqnarray}&&B=\displaystyle \frac{RT}{b}\end{eqnarray} \tag{ 9 }$$

A represents Temkin isotherm constant (L/g), b represents Temkin constant associated with heat of adsorption, T represents temperature (K), R indicates gas constant (8.314, J/mol K) and Ce indicates concentration of adsorbate. The Freundlich isotherm indicates that adsorbed molecular interaction on metallic surfaces is related to the lateral interaction effect between them [44, 45]. The Freundlich equation is shown below:

$$\begin{eqnarray}&&\theta =K{C}^{n}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&\mathrm{log}\,\theta =n\,\mathrm{log}\,C+\,\mathrm{log}\,{K}_{ads}\end{eqnarray} \tag{ 11 }$$

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n represents the constant for the characteristics of adsorbed extract molecule. The Langmuir isotherm plot for GP adsorption on MS in H₂SO₄ solution is shown in figure 7.

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Data representing the Gibbs free energy (ΔG) for GII, TT and GP extracts adsorption MS surface in H₂SO₄ solution are presented in table 4. The data were obtained from equation (12). Equilibrium constant of adsorption (K_ads) for the extracts were obtained from Langmuir equation due to their relatively significant correlation coefficient values.

$$\begin{eqnarray}&&{\rm{\Delta }}{G}_{ads}=-2.303RT\,\mathrm{log}\left[55.5{K}_{ads}\right]\end{eqnarray} \tag{ 12 }$$

55.5 represents molar concentration of H₂O within the electrolyte, R represents universal gas constant, T indicates absolute temperature. ΔG data depicts chemisorption adsorption for GII, TT and GP molecules onto MS surface due to significant electrostatic effect and covalent bonding between the extract molecules and ionized MS surface [47]. The ΔG data show H₂O molecules were substantially replaced by the extract molecules. This hinders the electrochemical action of SO₄ ²⁻ anions in the electrolyte. The ΔG data for GII extract are relatively the highest values compared to other extracts due to chemisorption adsorption reaction mechanism. The lowest ΔG data were exhibited by GP extract though the values show physiochemical interaction effect. Nevertheless, the ΔG data for the extracts shows the molecular reaction between the extract molecules and the steel exterior stifled further corrosion of the steel surface, blocking the reaction sites and preventing electrolytic transport of SO₄ ²⁻ anions to the steel.

GII, TT and GP oil extracts being organic compound consists of complex molecular configuration and multifunctional organic components. Their chemisorption adsorption properties show they are non-polar and polar with high dipole moments responsible for strong electrostatic attraction and covalent bonding with MS surface. Identification of reactive groups within the complex molecules responsible for corrosion inhibition by the extracts after protonation in H₂SO₄ media was done through ATF-FTIR spectroscopy and compared with the conventional spectroscopy table [48, 49]. The ATF-FTIR spectra plots for GII/H₂SO₄, TT/H₂SO₄ and GP/H₂SO₄ solutions before and following corrosion test are shown from figures 8(a)–(c). Table 5 shows the peak wavenumbers, transmittance prior to and after corrosion, the respective reactive groups and atomic bonds of the extracts in the electrolyte. The spectra plot in figure 8(b) for TT/ H₂SO₄ solution show significant decrease in transmittance for the solution after corrosion test at all wavenumbers and disappearance of the functional groups. This signifies adsorption of the reactive groups on the MS surface through chemisorption. The spectra plot in figure 8(a) shows significant increase in transmittance of the spectra peaks at specific wavenumbers where important functional groups are identified for the GII/H₂SO₄ solution after corrosion test while at wavenumber 732 cm⁻¹, the transmittance is the same before and after corrosion test. This assessment shows that despite the chemisorption adsorption behaviour of the extract from adsorption isotherm studies, modification of the corrosive media played a major role in limiting the diffusion of corrosive anions unto the steel surface. Within the acid solution hydrolysis of the extract results in release of the important functional groups for corrosion inhibition of the steel. Similar observation occurred for GP extract in the acid solution though at different wavenumbers.

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