Iranian (Iranica) Journal of Energy & Environment Theobroma Cocoa Dry Bean Extract as a Potential Green Inhibitor for Mild Steel in Acidic Medium

Theobroma cocoa dry bean extract (CDBE) was prepared by Soxhlet extraction method and was characterized for its chemical constituents using Fourier Transform-Infrared Spectroscopy (FT-IR), Gas chromatography-Mass spectroscopy (GC-MS) and also by Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA- DSC) techniques. FT-IR analysis gave information about the mostly like to be present functional groups in the extract. GC-MS analysis of the extract revealed the presence of important 07 chemical constituents with cis- 13-octadecanoic acid as the major component. TGA-DSC study gave the idea about different stages of thermal decomposition of the chemical constituents present in the extract. CDBE was then tested for its ability to inhibit the corrosion rate of mild steel in 1.0M HCl solution medium by means of weight loss, Potentiodynamic polarization, and Electrochemical impedance spectroscopy (EIS) techniques. The weight loss experiments revealed the anticorrosive property of it on mild steel, which was found to increase with increase in the concentration of the inhibitor CDBE and decrease with an increase in temperature of corrosive medium. EIS results showed the adsorption of organic constituents present in the extract at the metal-solution interface. Potentiodynamic polarization studies revealed that CDBE functioned as a mixed type inhibitor. The surface morphological studies of mild steel surface were carried out by Scanning electron microscopy (SEM). The inhibition efficiency values obtained by electrochemical measurements were consistent with those from weight loss measurements and hence it can be proposed that CDBE as a potent inhibitor towards corrosion of mild steel under acidic conditions.


INTRODUCTION 1
Phytochemistry is the science that deals with extraction, identification, separation and purification of the many different chemical constituents present in a plant. Plants are rich source of chemicals like alkaloids, glycosides, polyphenols, terpenes and volatile oils. Many of these are found to be bioactive and are expected to possess antioxidant, anti-bacterial, anti-microbial, anti-inflammatory and anti-arthritic properties [1].
Spectroscopy is a tool to provide a way for the development of new synthetic routes to identify compounds from its starting material in terms of quality and quantity. From the literature survey, it is evident that Mass spectroscopy, Nuclear Magnetic Resonance spectroscopy, Infrared spectroscopy, and Ultraviolet-Visible spectroscopy etc., play major role in giving an idea of the structure of a compound or identification of its chemical constituents [2][3][4][5][6].
Literature shows the study of chemical sample by Thermogravimetric analysis (TGA), Differential *Corresponding Author Email: bhatij08@gmail.com (J. Ishwara Bhat) Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) analysis give information about thermal characteristics like melting temperature, heat of fusion, latent heat of melting, reaction energy and temperature also giving data about endothermic and exothermic processes [5,6].
Corrosion is a phenomenon of deterioration of material surface due to physiochemical interaction between any exposed parts of the material with the surrounding oxygen. Corrosion is so posing a big challenge for the chemists and material scientists who are working to overcome the destruction of materials [7].
Mild steel is one of the widely used metals in industries related to the production of food, chemicals and nuclear power generation. The major problem with the mild steel is its dissolution in acid medium. In industry mild steel frequently come in contact with the acid solution during the acid pickling, acid descaling and acid cleaning processes. So controlling the dissolution of mild steel in aggressive medium is of great importance to protect the articles. Among the several methods employed to control the corrosion of mild steel, it has been found that use of corrosion inhibitors is one of the most promising method [8,9].
Literature survey reveals that, use of organic inhibitors for the control of corrosion rate. The inhibition process is due to the presence of hetero atoms such as O, N, S and P in the organic inhibitor which allows the adsorption of the same on a metal surface [7][8][9]. However, most of the synthetic inhibitors used are found expensive, toxic to living kind and raise environmental issues. Due to this reason, investigation of less hazardous, more eco-friendly green inhibitors obtained from plants and natural products are used as corrosion inhibitors [10][11][12][13][14][15][16][17].
Theobroma Cacao L. commonly known as Cocoa is native of Amazon region of South America. Cocoa is mainly used in confectionery industries to produce chocolates and beverages. Being a tropical crop, India offers considerable scope for the growth of cocoa plant. Cocoa bean can be processed to get cocoa solids and cocoa butter. In India Cocoa is mainly grown in Southern India [18,19].
Yuli et al. [20] have studied the corrosion inhibition efficiency of the cocoa extract on mild steel in hydrochloric acid. Othman et al. [18] have studied the antioxidant capacity and phenolic content of cocoa beans. These study showed that the Cocoa as a rich source of phenolic compounds, such as, flavanols, procyanidins and other flavanoids. Some of these chemical constituent possess O and N atoms in it which fulfil the requirements of a good inhibitor [18]. The pictures of Cocoa and dried Cocoa beans is shown in Figures 1 and 2.
In the present work, the dry cocoa bean's methanolic extracts was prepared using Soxhlet extract apparatus. It was then characterized using FT-IR [21], GC-MS [22] and thermal techniques [23] to get an idea about the presence of various anti-oxidative chemical constituents in them. Further its anticorrosive efficiency was checked using Weight loss, Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization methods. Surface morphological changes are analyzed using Scanning Electron Microscopy [24].

Preparation of Theobroma Cocoa dry bean extracts (CDBE)
Locally grown Theobroma cocoa bean sample was collected from agriculture fields. It was cleaned for dirt and dried under sunlight for 20 days and then powdered using a crusher. About 50g of the powder was then extracted with 500mL of solvent methanol as a solvent using Soxhlet extraction apparatus for about 8 hours. The extract was then filtered through Whatman no 1 filter paper and brown coloured liquid Cocoa Dry bean Extract (CDBE) was obtained. The extract was stored in amber coloured bottle.

Fourier Transform -Infrared (FT-IR) Spectroscopic characterization
Perkin Elmer -Spectrum RX-IFTIR was used to detect the presence of different functional groups in the extract sample CDBE. The spectrum was recorded using 10 mg of liquid sample with a resolution of 1 cm -1 and scan range of 4000 cm -1 to 250 cm -1 .

Gas chromatography-Mass Spectroscopic (GC-MS) characterization
The chemical constituents present in the CDBE sample were determined using GC-MS technique. The spectrum for CDBE was recorded using Thermo Scientific TSQ 8000 Gas Chromatograph -Mass Spectrometer. 10 mg of sample in liquid form was subjected to GC-MS with run time 22.79 min and injection volume 10.00μl with 6701 scans from Low Mass (m/z) 50 to High Mass (m/z):700. Obtained data were compared with m/z values available in National Institute of Standards and Technology (NIST) and the most likely to be present constituents are listed.

Thermo-Gravimetric Analysis
Thermal stability of the CDBE sample was examined using TGA-DSC technique. The spectrum was recorded using instrument SDT Q600 V20.9 Build 20. The extract was evaporated in the presence of methanol and the residue obtained was in gel form. About 6-10mg of residue sample was subjected to thermal decomposition in Alumina pan using Nitrogen gas.

Preparation of metal samples for gravimetric studies
Mild steel IS-2062 with composition (in wt %) 0.18% C, 0.6% Mn, 0.05% S, 0.04% P, 0.1% Si and the remainder 99.03 % Fe [24] was selected for the corrosion study. The metal sample was cut into apparent size with dimensions 1X 0.5 X 0.07 inch was used for weight loss measurement. Before the use, the test material was well cleaned and polished with different emery papers ranging from 80 to 2000 grade, cleaned with acetone, washed with double distilled water and dried well.

Preparation of corrosion medium
Corrosive medium of 1 M HCl was prepared by diluting analytical grade HCl using double distilled water.

Weight loss measurements
Mild steel specimen was weighed accurately and was suspended in 50mL of corrosion medium without and with different volumes (Viz., 5, 10, 20, 30 and 35 mL) of CDBE at 303K. The studies were also carried out at different temperatures i.e., 308, 313, 318 and 323K. After specified time, viz., 5 hours for HCl, the mild steel sample was taken out from the solution, washed with double distilled water, wiped with clean tissue paper, dried and weighed accurately. The weight loss was evaluated (W). The rate of corrosion (CR) of mild steel was determined using Equation (1) [25].
where CR is the corrosion rate of mild steel in mils penetration/year (mpy), W is the weight loss of mild steel in mg, A is the area of mild steel sample in square inches (1*0.5), T is the time of exposure in 5 hours and D is the density of mild steel in grams/cubic centimeter (7.75). The inhibition efficiency was calculated using Equation (2) [25].
where W1 and W2 are weight loss of mild steel samples in the presence and absence of the inhibitor, respectively.

Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization method
EIS study was carried out using mild steel rods with approximately 1cm 2 circular area exposed to corrosive environment. Rest of the sample area was covered with acrylic repair material. Before the use, the test materials were well cleaned and polished with different emery papers ranging from 80 to 2000 grade. They were applied with machine polish using alumina water and then washed thoroughly with water, cleaned with acetone, washed with double distilled water, and dried well for further study.
Electrochemical measurements were recorded using a GillAc Potentiostat CH 1480 analyser at 303 K by taking mild steel as working electrode. Saturated calomel was connected as reference electrode, and platinum electrode as auxiliary electrode. All AC impedance measurements were taken using an AC signal with amplitude of 10mV at OCP in the frequency range from 0.01 Hz to 1,00,000Hz to get Nyquist plots. The potentiodynamic polarization studies were carried out and potentialcurrent curves (Tafel plots) were recorded by polarizing the specimen to -250 mV to +250 mV against OCP at scan rate of 1mV/s.

Scanning Electron Microscopy (SEM)
Surface morphology of the samples was examined by Scanning Electron Microscopy. The mild steel samples were polished, well washed with acetone and distilled water. The samples were then exposed for corrosive environment for a time period of 5 hours in the absence and presence of optimum concentration of the CDBE inhibitor in 1M HCl at 30 0 C and the surface images were recorded immediately after the corrosion tests.

RESULTS AND DISCUSSIONS Identification of functional groups in CDBE by FT-IR
FTIR spectrometer collects high spectral resolution data over a wide spectral range. FT-IR analysis gave information about the presence of different functional groups in the extract prepared. A broad peak at 3334cm -1 shows the presence of O-H group. Also a sharp peak shown at 1649cm -1 corresponds to the presence of C=C stretch for cis alkenes, peaks at 1282 cm -1 for the presence of C-O stretch from carboxylic acid. C-O stretch from esters is also found at the range from 1300-1000cm -1 . The FT-IR spectrum of CDBE is shown in Figure 3. Details about the probable functional groups that may exist in CDBE with the corresponding wave numbers are listed in Table 1.

Identification of chemical constituents in CDBE by GC-MS
The GC-MS spectrum of CDBE is shown in Figure 4. The extract showed the presence of a sharp peak at 15.32min with 71.90% peak area. The graph of CDBE obtained for retention time 15.32 minute and the GC-MS graph for cis-13-octadecanoic acid from the PubChem library (NIST Number 333585) are shown in Figures 5(a) and 5(b), respectively. The comparison confirms the presence of cis-13-octadecanoic acid as major compound and the structure of the compound is shown in Figure 6. The 7 constituents identified in CDBE are listed in Table  2.     Also comparing the TGA data with GCMS data, the most likely to be present compounds, which have undergone thermal decomposition in those peaks region, present in the extract were identified and are listed in Table 3. The last stable state in the graph (horizontal region) i.e., Region 4: 650-700⁰C represents the presence of metals present in the extract [26].

Correlation of the spectral data from FT-IR and GC-MS with TGA-DTA studies
When the CDBE sample was subjected to GC-MS, different compounds have been eluted with different retention time and molecular formula. As literature study propose some structure for the chemical constituents identified with a given molecular weight and molecular

Weight loss studies
To examine the inhibition efficiency of CDBE on mild steel surface in 1M HCl, weight loss measurements were carried out at 303K. The data obtained are summarized in Table 4. It can be seen from the data that inhibition efficiency of CDBE increases with the increase in % volume.

Effect of temperature
To study the effect of temperature on inhibitor efficiency, weight loss measurements were carried out at different temperatures viz., 308, 313, 318 and 323K. The various data obtained during the weight loss are tabulated in Table 4. The variation of corrosion rate with % volume at different temperatures is shown in Figure 8. Figure 9 shows the variation of inhibition efficiency with % volume at different temperatures. From the Figure 10 it is clear that, with the increase in temperature of reaction medium, the corrosion rate of the mild steel shows a considerable increase indicating there is decrease of inhibition efficiency of CDBE. This might have been caused due to desorption of some adsorbed molecules of CDBE from the surface of the mild steel. This causes the metal to expose greater surface area of to acid environment leading to more rate of corrosion [28][29][30]. It can be seen from the Figure 9 that with increase in concentration of inhibitor there is increase in the inhibition efficiency value up to 60% volume of CDBE and there afterwards it remained almost constant. The apparent activation energy of metal corrosion in acid media can be calculated from the Arrhenius equation shown as Equation (3) [25].
where Ea ⁰ is the apparent activation energy for the corrosion of mild steel , R is the universal gas constant, A is Arrhenius pre exponential factor and T is the absolute temperature. The plot of ln(Corrosion Rate) vs. 1000/T is shown in Figure 10 and the values of Ea ⁰ obtained from the slope of the plot. The obtained values are tabulated in Table 5. It is observed that, Ea⁰ values for the mild steel in HCl medium is 36.76kJmol -1 and that with the optimum volume of CDBE is 94.70kJmol -1 showing the decrease in corrosion rate. Also with the increase in % volume of CDBE, the Ea⁰ values show an increase, which can be due to the adsorption of the inhibitor on the mild steel surface thus forming a barrier at metal-solution interface which limits the contact of metal with corrosive medium [28]. Ekanem et al. [28] explained that the increase in activation energy can be due to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature, exposing greater area of metal to the acid environment and so causing a corresponding increase in corrosion rate. An alternative form of Arrhenius equation is the transition state equation as Equation (4)  where, h is the Plank's constant, N is the Avogadro's number, ΔSa° is the entropy of activation, and ΔHa ° is the enthalpy of activation. A plot of ln (CR/T) vs. 1/T gave a straight line as shown in Figure 11 with a slope of (-ΔHa° /R) and an intercept of [ln(R/Nh) + (ΔSa °/R)], from which the values of ΔHa ° and ΔSa ° were calculated and listed in Table 5. The values of ΔHa° obtained are positive both in the presence and absence of the inhibitor shows that, the metal dissolution process is endothermic type for the corrosion of mild steel in HCl medium. It is also shown that with the increase in the % volume of the CDBE inhibitor there is increase in ΔSa° values. This increase in entropy from uninhibited corrosion medium to inhibited corrosion medium suggesting that there is an increase in the randomness occurred while moving from reactants sate to the activated complex state [28,29].

Impedance Spectroscopic measurements (EIS)
The data obtained from EIS can be plotted as impedance spectra which are also known as Nyquist plots. This plot contain depressed semicircle with centre under real axis. The Nyquist plots of mild steel in 1M HCl containing  various % volumes of the CDBE inhibitor at 30⁰C is shown in Figure 12. The impedance of the solution with inhibitor has increased with the increase in the concentration of the inhibitor. It is observed that the size of the semicircle increases with the concentration of CDBE inhibitor. This indicates that the charge transfer process is the main controlling factor for the corrosion of mild steel in acidic medium [29][30][31]. The experimental result of EIS measurements for the corrosion of mild steel in 1M HCl in the absence and presence of inhibitor is given in Table 6. It can be observed from the Nyquist plot data that resistance due to charge transfer (Rct) value increased with increase in the concentration of the CDBE inhibitor. It is also shown that values of the capacitance of the interface (Cdl) showed a decrease, with increase in concentration of CDBE inhibitor, which is probably due to the decrease in local dielectric constant and/or increase in thickness of the electrical double layer. This suggests that the CDBE inhibitor acts via adsorption mechanism at the metal-solution interface. The decrease in the Cdl values is expected to be due to the adsorption of the CDBE inhibitor molecules on to the surface of mild steel which there by slowly replaces the water molecules, and so decreases the extent of metal dissolution in acidic medium [32].  The inhibition efficiency of the CDBE inhibitor is given by Equation (5) [33]: where Rct is resistance due to charge transfer without inhibitor, and Rct (inhi) is resistance due to charge transfer with inhibitor. Cdl value is obtained from the Equation (6) [33]: = 1 * (6) where, max = 2*3.14*fmax; fmax is the frequency at the top of the semicircle (where-Z'' is maximum).

Potentiodynamic Polarisation measurements
Potentiodynamic Polarization curves for mild steel in 1M hydrochloric acid without and with addition of different concentration of the CDBE inhibitor are shown in Figure  13. The values of electrochemical parameters, such as corrosion current densities (icorr), Tafel anodic slopes (βa), Tafel cathodic slopes (βc), Corrosion rate, inhibition efficiency (%IE) and corrosion potential (Ecorr) calculated are listed in Table 7. The inhibition efficiency is given by Equation (7) [32,33]: where, i'corr and icorr are the corrosion currents in the presence and absence of the CDBE inhibitor respectively. From the electrochemical parameter values it is clear that, corrosion current density decreased with increase in CDBE concentration. This indicates an increase in the inhibition efficiency with the increase in % volume of the CDBE inhibitor. The icorr obtained by weight loss method shows an agreement with the values obtained by Tafel extrapolation plots. Also the present study Ecorr values show a shift of 12 mV shows that CDBE acts as mixed type inhibitor. Both anodic and cathodic slopes are also showing a decrease in their values confirming that, CDBE could be classified as a mixed type inhibitor. Further amongst βa and βc, since βc shows a much decrease in its vales, inhibitor can be expected to be more predominantly acted as cathodic type, inhibiting reduction reactions at the metal surface [33].

Scanning Electron Microscopy (SEM)
The surface morphological changes of mild steel sample in 1M HCl medium in the absence and presence of CDBE inhibitor yielded good results confirming the action of CDBE as a potent inhibitor. The SEM images are shown in Figure 14. The images shows the formation of more pits and cavities on the surface of mild steel in the absence of inhibitor compared to that of its presence. This confirms that the CDBE inhibitor markedly decreases the surface corrosion of mild steel by covering the surface with adsorbed inhibitor molecules [34].

CONCLUSIONS
The spectroscopic characterization reveals the presence of most likely to be present chemical constituents in CDBE showing the different functional groups and the substituent groups that involve in adsorption. Results obtained from the experimental data of weight loss, Potentiodynamic polarization and impedance spectroscopic studies for the corrosion behaviour of mild steel in the hydrochloric acid medium shows that CDBE acts as good inhibitor. It is clear that with the increase in the CDBE inhibitor concentration, more inhibition efficiency was exhibited. The inhibition efficiency decreased with increase in temperature in the studied temperature range indicating that adsorption of the inhibitor on to the surface of metal sample is more predominantly physisorption type. Polarization studies suggested that the CDBE hindered both oxidation and reduction, so acted as mixed type of inhibitor, with dominating cathodic type of inhibition.