# An Ecofriendly Initiative for the Corrosion Inhibition of Mild Steel in 1 M HCl UsingTecoma capensis Flower Extract

Authors: A. Prithiba; R. Rajalakshmi
Journal: International Journal of Metals
 (2016)

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## Abstract

Corrosion inhibition of mild steel in 1 M HCl in the presence ofTecoma capensis flower extract was carried out by means of mass loss, potentiodynamic polarisation, and electrochemical impedance techniques. The inhibition efficiency varied with concentration of the inhibitor, immersion time, and temperature. The adsorption of the inhibitor on mild steel surface obeys Langmuir’s adsorption isotherm. Thermodynamic parameters reveal that the adsorption process is spontaneous. Electrochemical studies reflect that the inhibitor acts as a mixed-type inhibitor. Surface analytical techniques ascertain the inhibitive nature of the studied inhibitor.


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## Body

## 1. Introduction

The environmental consequences of corrosion on metals are enormous and the global stature of the problem has prompted an innumerable number of investigations in this area. Corrosion is a major problem that must be confronted for safety, environmental, and economic reasons. The general aggressive nature of industrial acids (HCl, H2SO4) mandates the usage of inhibitors for corrosion mitigation of metals. Prominent among them are synthetic organic compounds that afford a reasonable degree of inhibition but are carcinogenic and nonbiodegradable in nature. Sustainability and environmental issues have forced our investigators to look for an ecofriendly alternative, that is, natural products [1]. Numerous naturally occurring substances have been documented as inhibitors. Sprouted seeds ofPhaseolus aureus[2],Cyamopsis tetragonoloba[3],Ervatamia coronaria[4], andCocos nucifera[5] are some of the investigated inhibitors of our research team. Green inhibitors displaying improved environmental properties will be the inhibitors most widely used in the future. This motivated us to assess the effectiveness ofTecoma capensis flower extract in minimizing the corrosion of mild steel in 1 M HCl.


## 2. Methods

### 2.1. Material Selection and Solution

Mild steel (MS) specimens of the following chemical composition in wt%: carbon 0.13%, manganese 0.23%, silicon 0.03%, phosphorus 0.03%, sulphur 0.016%, chromium 0.022%, nickel 0.012%, and iron 99.95% were used for the entire study. Mass loss and electrochemical studies were carried out for assessing the efficacy of the inhibitor. For mass loss study, MS specimens of size 1 × 5 cm2 were used. MS specimens with an exposed area of 1 cm2 were used for electrochemical study. The specimens were mechanically polished, degreased, dried, and stored in a desiccator. The corrodent solution was obtained by diluting analytical grade 37% HCl with distilled water to get 1 M HCl.


### 2.2. Preparation of Plant Extract

Tecoma capensis flowers (TCF) were collected from the nearby locality and shade-dried. The selected plant was authenticated by Botanical Survey of India, Coimbatore, Tamil Nadu. 25 g of the dried flowers was refluxed with 500 mL of 1 M HCl for 3 hours and kept overnight. The cooled extract was filtered and diluted up to 500 mL (5% extract) to obtain the stock solution. Further dilutions were done from stock solution to obtain the desired concentration.Characterisation of the Extract. Phytochemical examinations were carried out for the extracts as per the standard procedures mentioned [6].


### 2.3. Mass Loss Method

Preweighed test pieces were immersed in triplicate in 100 mL of the solution containing various concentration of the inhibitor and in the absence of inhibitor for a predetermined time period as perASTM G 1-2. The test specimens were removed and then washed with deionised water, dried, and reweighed.


### 2.4. Electrochemical Measurements

Electrochemical experiments, including corrosion potential, potentiodynamic polarisation curves, and electrochemical impedance spectroscopy (EIS), were performed in a three-electrode cell, using a Biologic Model V 10.23 operated with EC-Lab software. In this setup a platinum electrode, calomel electrode, and MS specimens were used as auxiliary, reference, and working electrodes, respectively, which were immersed in acidic medium in the presence and absence of different concentrations of the inhibitor.


### 2.5. FT-IR Spectroscopy

The FT-IR spectrum was recorded for TCF with a frequency ranging from 4000 to 400 cm−1 using Perkin Elmer FT-IR spectrophotometer with the software OPUS version 6.5.


### 2.6. Scanning Electron Microscope

The surface morphologies of mild steel specimens after exposure to 1 M HCl solution in the absence and presence of TCF extract for 3 h were examined by SEM using a JEOL Model JSM 6390 SEM instrument.


## 2.1. Material Selection and Solution

Mild steel (MS) specimens of the following chemical composition in wt%: carbon 0.13%, manganese 0.23%, silicon 0.03%, phosphorus 0.03%, sulphur 0.016%, chromium 0.022%, nickel 0.012%, and iron 99.95% were used for the entire study. Mass loss and electrochemical studies were carried out for assessing the efficacy of the inhibitor. For mass loss study, MS specimens of size 1 × 5 cm2 were used. MS specimens with an exposed area of 1 cm2 were used for electrochemical study. The specimens were mechanically polished, degreased, dried, and stored in a desiccator. The corrodent solution was obtained by diluting analytical grade 37% HCl with distilled water to get 1 M HCl.


## 2.2. Preparation of Plant Extract

Tecoma capensis flowers (TCF) were collected from the nearby locality and shade-dried. The selected plant was authenticated by Botanical Survey of India, Coimbatore, Tamil Nadu. 25 g of the dried flowers was refluxed with 500 mL of 1 M HCl for 3 hours and kept overnight. The cooled extract was filtered and diluted up to 500 mL (5% extract) to obtain the stock solution. Further dilutions were done from stock solution to obtain the desired concentration.Characterisation of the Extract. Phytochemical examinations were carried out for the extracts as per the standard procedures mentioned [6].


## 2.3. Mass Loss Method

Preweighed test pieces were immersed in triplicate in 100 mL of the solution containing various concentration of the inhibitor and in the absence of inhibitor for a predetermined time period as perASTM G 1-2. The test specimens were removed and then washed with deionised water, dried, and reweighed.


## 2.4. Electrochemical Measurements

Electrochemical experiments, including corrosion potential, potentiodynamic polarisation curves, and electrochemical impedance spectroscopy (EIS), were performed in a three-electrode cell, using a Biologic Model V 10.23 operated with EC-Lab software. In this setup a platinum electrode, calomel electrode, and MS specimens were used as auxiliary, reference, and working electrodes, respectively, which were immersed in acidic medium in the presence and absence of different concentrations of the inhibitor.


## 2.5. FT-IR Spectroscopy

The FT-IR spectrum was recorded for TCF with a frequency ranging from 4000 to 400 cm−1 using Perkin Elmer FT-IR spectrophotometer with the software OPUS version 6.5.


## 2.6. Scanning Electron Microscope

The surface morphologies of mild steel specimens after exposure to 1 M HCl solution in the absence and presence of TCF extract for 3 h were examined by SEM using a JEOL Model JSM 6390 SEM instrument.


## 3. Results and Discussion

Figure1 shows the anodic and cathodic polarisation curves of mild steel in 1 M HCl solution, without and with different concentrations of TCF extract. It can be seen that both the cathodic and anodic curves reflect lower current density in the presence of TCF than those recorded in the solution without TCF. This indicates that TCF extract inhibits the corrosion process. The electrochemical parameters, that is, corrosion current density (Icorr), anodic (ba) and cathodic (bc) Tafel constants, and polarisation resistance (Rp), are given in Table 1. The table divulges the fact that the addition of the inhibitor shifts the anodic (ba) and cathodic (bc) slopes, respectively. This denotes the suppression of both the anodic dissolution and cathodic hydrogen evolution by the inhibitor. Significant reduction in the Icorr values is noted in the presence of the inhibitor. The IE is found to increase with increase in concentration of the inhibitor affording a maximum efficiency of 71.4 percentage. The negligible shift in the Ecorr values infers the mixed nature of the inhibitor under investigation [7]. It is evident from Table 1 that the Rp values increase with increasing TCF concentration. Inhibition performance of TCF is found to be 77.9 percentage.Table 1
Electrochemical impedance parameters for corrosion of MS in the absence and presence of TCF in 1 M HCl.
S. number Conc. of TCF in % Tafel polarisation parameters Linear polarisation resistance parameters ⁢Impedance parameters E c o r r mV/SCE I c o r r A/cm2 b a mV/decade b c mV/decade IE(%) R p Ohm/cm2 IE(%) R S (Ω cm2) R c t (Ω cm2) IE(%) C d l (μF/cm2) θ 1 Blank −475 0.0035 90 152 — 85.5 — 0.830 15.2 — 789 2 0.1 −433 0.0030 98 108 14.3 102 16.2 −0.13 19.4 21.6 631 0.20 3 0.2 −514 0.0015 95 129 57.1 159 46.2 1.59 27.4 44.5 617 0.22 4 0.3 −522 0.0013 99 134 62.9 168 49.1 1.67 32.8 53.7 548 0.31 5 0.4 −499 0.0013 88 138 62.9 198 56.8 1.89 61.9 75.4 411 0.48 6 0.5 −458 0.0013 94 127 62.9 278 69.2 1.78 65.4 76.8 320 0.59 7 0.6 −470 0.0011 85 129 68.6 321 73.4 2.01 78.2 80.6 265 0.66 8 0.7 −423 0.0010 84 128 71.4 386 77.9 1.60 103.1 82.9 219 0.72Figure 1
Potentiodynamic polarisation curves for MS in 1 M HCl in the absence and presence of TCF extract.The potentiodynamic polarisation results of studied inhibitors suggest that the inhibitors are first adsorbed onto the mild steel surface and impede the corrosion reaction by merely blocking the reaction site of the electrode surface without affecting the anodic and cathodic reaction [8].


### 3.1. Electrochemical Impedance Measurements

The impedance spectral data of MS/HCl/TCF obtained from Nyquist plots (Figure2) in 1 M HCl are given in Table 1. As observed, the impedance spectra exhibit a single depressed semicircle whose diameter increases with increasing concentration of the inhibitor implying a charge transfer process for the corrosion inhibition mechanism [9]. The figure also confirms the fact that the presence of the inhibitor did not modify the corrosion reaction of MS electrode in the presence of the inhibitor. The impedance parameters demonstrate the increase of charge transfer resistance (Rct) values with increase in concentration of the inhibitor. A maximum of 82.9 percentage IE is obtained at 0.7% TCF. Single capacitive semicircle noted in the Nyquist plot corresponds to a single time constant in the Bode representation [10].Figure 2
Nyquist and Bode plot of MS/1 M HCl/TCF.It is clear from the table thatCdl values tend to decrease with increase in inhibitor concentration. This behaviour is the result of increase in surface coverage by the inhibitor molecules of plant extract which leads to increase in IE [11].


### 3.2. Mass Loss Measurements

#### 3.2.1. Effect of Concentration and Immersion Time of TCF on MS/1 M HCl

The influence of TCF extract on the corrosion inhibition of MS has been assessed by mass loss measurements. Table2 shows the variation of inhibition efficiency with increase in concentration of the inhibitor. This result indicates the inhibitive nature of the inhibitor on MS corrosion in 1 M HCl medium [12]. It is clear from the results that the IE increases steadily from 67.8 percentage at 0.1% concentration to 85.9 percentage at 0.7% concentration. The corrosion rate and inhibition efficiency of MS electrode exposed to various concentrations of TCF extract in 1 M HCl solution for various time intervals are tabulated in Table 2. From the table, it is noted that a maximum IE of 94.8 percentage for TCF is maintained till 6 h and thereafter a slight decline is observed and the efficiency stabilises to 86.8 percentage (TCF) at 24 h.Table 2
Inhibition efficiency as a function of immersion time and concentration for TCF in 1 M HCl.
Conc.% 1/2 h 1 h 3 h 6 h 12 h 24 h CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) Blank 1032  1190  1238  2197  1561  1209 0.1 333 67.8 260 78.1 350 71.7 625 71.6 601 61.5 545 54.9 0.2 307 70.3 226 81 264 78.6 426 80.6 474 69.8 495 59.1 0.3 273 73.5 200 83.2 233 81.2 315 85.7 435 72.1 458 62.1 0.4 196 81 162 86.4 166 86.6 226 89.7 342 78.1 377 68.8 0.5 179 82.7 145 87.8 148 88.0 200 90.9 318 79.6 312 74.2 0.6 154 85.1 128 89.2 135 89.1 143 93.5 279 82.2 241 80.1 0.7 146 85.9 102 91.4 90 92.7 114 94.8 202 87.1 160 86.8Analysis of the tables indicates that IE increases with increasing concentration of the inhibitors. This indicates the dependence of inhibiting effect on the concentration of inhibitor molecules. This may be due to the increase in coverage of MS surface by the inhibitor molecules at higher concentration of the extract leading to a compact and coherent film on the surface of MS [13].The IE of TCF increases with increasing time of immersion up to 6 h and then decreases to finally stabilise at 24 h to afford 86.8 percentage. This behaviour can be discussed on the basis that prolonged immersion of MS in acid solution [14](a)
allows the cathodic or hydrogen evolution kinetics to increase presumably;(b)
increases the concentration of ferrous ions which decrease the corrosive nature of the acid.


#### 3.2.2. Effect of Temperature

The kinetic and mechanistic aspects of corrosion may be gained by studying the effect of temperature on the corrosion of MS in the presence and absence of the inhibitor. It can be noted from Table3 that the maximum IE obtained is 93.8 percentage in 1 M HCl. The values reflect that the IE increases up to 323 K affording an efficiency of 93.8 percentage and then a slight decrease is observed after that and at 353 K it is found to be at 83.2 percentage.Table 3
Relationship between inhibition efficiency and concentration for TCF/MS/1 M HCl systems at various temperatures.
Conc.% Temperature (K) 303 313 323 333 343 353 CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) Blank 1032  2755  6252  8146  9971  18091 0.1 333 67.8 967 64.9 1307 79.1 2077 74.5 4188 58.0 7634 57.8 0.2 307 70.3 628 77.2 982 84.3 1750 78.5 3833 61.6 5174 71.4 0.3 273 73.5 540 80.4 757 87.9 1352 83.4 3490 65.0 4378 75.8 0.4 196 81 507 81.6 569 90.9 1238 84.8 3091 69.0 4125 77.2 0.5 179 82.7 479 82.6 539 91.4 1149 85.9 2353 76.4 3636 79.9 0.6 154 85.1 410 85.1 469 92.5 1075 86.8 1894 81.0 3383 81.3 0.7 146 85.9 386 86.0 388 93.8 863 89.4 1595 84.0 3039 83.2The decrease in the inhibition efficiency of the inhibitors with increase in temperature might be due to adsorption and desorption. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between these two processes at a particular temperature. With the increase of temperature the equilibrium between adsorption and desorption processes is shifted leading to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant [15].


### 3.3. Langmuir Adsorption Isotherm

Langmuir adsorption equation relates the degree of surface coverage to concentration of inhibitor according to(1)log⁡Cθ=log⁡C-log⁡K.A plot oflog⁡C/θ versus log⁡C from mass loss data obtained for the studied inhibitor yielded a straight line as represented in Figure 3. The slope deviates from unity. This deviation may be explained on the basis of the interaction among adsorbed species on the metal surface. It has been postulated in the derivation of Langmuir adsorption isotherm equation that adsorbed molecules do not interact with one another, but this is not the case of large organic molecules having polar atoms (or) groups which can be adsorbed on the cathodic and anodic sites of the metal surface as such adsorbed species interact by mutual repulsion or attraction [16]. It is also possible that the inhibitor studied can be adsorbed on the anodic and cathodic sites resulting in deviation from unit gradient.Figure 3
Langmuir adsorption isotherms.


### 3.4. Kinetic Parameters for Inhibition Process

#### 3.4.1. Energy of Activation

The dependence of corrosion rate on temperature can be regarded as an Arrhenius-type process, the rate of which is given by(2)log⁡CR=log⁡A-Ea2.303RT,where CR is the corrosion rate of MS, A is Arrhenius or preexponential constant, Ea is the activation energy for the corrosion of MS, R is the gas constant, and T is the temperature.The apparent activation energy for the corrosion ofMS in 1 M HCl is calculated from the Arrhenius plot of log⁡CR against 1/T (Figure 4) in the absence and presence of different concentrations of TCF. The Ea values are deduced from the slopes of these lines and the values of Ea for various concentrations of the inhibitor are tabulated in Table 4. It can be seen in the table that Ea is higher in the presence of the inhibitors than in their absence. The modification in the values of Ea may be attributed to the geometric blocking effect of adsorbed inhibitive species on the metal surface [17]. This observation further supports the proposed physisorption mechanism as report [18] shows that lower values of Ea in the presence of inhibitors in comparison to the free acid solution are indicative of chemical adsorption mechanism, whereas the opposite suggests a physical adsorption mechanism.Table 4
Average values of activation parameters for MS corrosion in 1 M HCl in the absence and presence of TCF.
Inhibitor E a  (kJ/mol) Δ H a (kJ/mol) Δ S a  (J/mol K−1) Blank 47 44.6 −37.9 TCF 52.2 49.5 −36.8Figure 4
Arrhenius plots and transition state plots.


### 3.5. Entropy of Activation and Enthalpy of Activation

In order to calculate the enthalpy,ΔHa, and entropy, ΔSa, of activation for the corrosion process, the alternative formulation of Arrhenius equation, also called transition state equation, is used:(3)CRT=RNhexp⁡ΔSaRexp⁡ΔHaRT,where h is the Planck constant, N is the Avogadro number, ΔSa is the entropy of activation, ΔHa is enthalpy of activation, T is the absolute temperature, CR is corrosion rate, and R is the universal gas constant.The relationship oflog⁡(CR/T) versus 1/T forMS corrosion in 1 M HCl in the absence and presence of different concentrations of TCF extracts is shown in Figure 4. Straight lines are obtained with slope of (-ΔHa/2.303R) and an intercept of (log⁡R/Nh+ΔSa/2.303R) from which the values of ΔHa and ΔSa, respectively, are computed and listed in Table 4.The table reflects the fact thatEa and ΔHa are close to each other as expected from transition state theory concept and they are also found to vary in a similar manner in the presence of the inhibitors. It is also seen in Table 4 that Ea and ΔHa vary in the same manner, but, however, the values of ΔHa are lower than that of Ea. This has been reported in [19] indicating that the corrosion process must involve a gaseous reaction, simply hydrogen evolution reaction associated with decrease in total reaction volume.The positive values ofΔHa both in the absence and presence of additives reflect the endothermic nature of the steel dissolution process and they indicate that the dissolution ofMS is difficult. The values of ΔSa in the absence and presence of the extracts are negative implying that the inhibitor molecules, freely moving in the bulk solution, are adsorbed in an orderly fashion onto theMS surface [20].


### 3.6. Thermodynamic Adsorption Parameters

Figure5 clearly shows the dependence of ΔGoads on T, indicating the good correlation among thermodynamic parameters. The large and negative values of ΔGoads ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface [21].Figure 5
Best fit curve of-ΔGoads versus T for MS/TCF/1 M HCl.The calculated values ofΔGoads presented in Table 5 are negative which indicate that the adsorption of inhibitor molecules on the metal surface is a spontaneous process. Generally, values of ΔGoads around −20 kJ mol−1 or lower are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption); those around −40 kJ mol−1 or higher involve charge sharing or transfer from organic molecules to the metal surface to form a coordinate type of bond (chemisorption) [22]. In the present work, the calculated ΔGoads values are almost slightly less negative than −20 kJ mol−1 ranging from −15 to −19 kJ mol−1. Hence it may be assumed that the adsorption of the inhibitor molecules is obeying physical adsorption; however chemical adsorption may not be excluded due to the complex nature of the corrosion inhibiting process. The positive sign of ΔHoads reflects the endothermic nature of dissolution which suggests the slow dissolution of MS [23]. The entropy of adsorption ΔSoads is negative because the inhibitor molecules, freely moving in the bulk solution, are adsorbed in an orderly fashion onto the mild steel surface, resulting in a decrease in entropy [24].Table 5
Thermodynamic adsorption parameters.
System Free energy of adsorptionΔG°ads (kJ/mol) Δ S ° a d s J/mol Δ H ° a d s  kJ/mol 303 K 313 K 323 K 333 K 343 K 353 K 1 M HCl + 0.7% TCF −15.5 −16.1 −19.0 −18.0 −17.2 −17.0 −27.5 −8.1


### 3.7. Surface Analytical Techniques

#### 3.7.1. FT-IR Spectral Studies

Figure6 illustrates the FT-IR spectrum of crude extract of TCF and the corrosion product of MS. From the results obtained, it may be noticed that O-H stretching at 3387 cm−1 (for TCF extract) shifts to 3418 cm−1. The aliphatic and aromatic C-H stretching observed in crude plant extract at 2924 cm−1 and 2855 cm−1 shifts to 3086 cm−1 and 3024 cm−1, respectively. A peak at 2315 cm−1 is noticed only in corrosion product which corresponds to C≡N stretching vibration. A peak at 1628 cm−1 corresponding to C=O stretching vibration is shifted to 1736 cm−1 in the corrosion product. Peaks that are shifted from (1443 cm−1 to 1427 cm−1) and (1373 cm−1 to 1366 cm−1) to (1258 cm−1 to 1219 cm−1) and (1072 cm−1 to 1049 cm−1) indicate the presence of C-O stretching and C-O-C stretching groups. The shift in the absorption frequencies of the TCF extract on the metal surface supports the interaction between the inhibitor and the metal surface. Also, some new bonds are noted in the spectrum of the corrosion product. This includes the C≡N stretch at 2315 cm−1. This also indicates that some new bonds are also formed through these functional groups [25]. This emphasises the fact that the inhibitive nature of the extract is due to the formation of Fe-TCF complex on MS surface.Figure 6
FT-IR spectrum of TCF and corrosion products in the presence of TCF.


### 3.8. Scanning Electron Micrograph of MS in the Presence of TCF Extract in 1 M HCl

Surface morphology of the MS samples immersed in HCl with and without TCF is shown in Figures7(a) and 7(b). Figure 7(a) indicates the finely polished characteristic surface of MS with some scratches that had arisen during polishing.Figure 7
SEM images of MS corrosion in the absence and presence of TCF in 1 M HCl.
(a)
Plain MS (b)
MS + 1 M HCl (c)
MS + TCF in 1 M HClFigure7(b) reveals that the immersed specimens are drastically damaged in the presence of 1.0 M HCl, due to the direct attack of aggressive acid. Figure 7(c) reflects the drastic change in morphology of MS surface in the inhibited solution containing 0.7% TCF extract. The smooth surface of MS in inhibited solution may be due to the phytochemical constituents present in TCF that hinder the dissolution of iron by forming a protective film on MS surface [26].


### 3.9. Mechanism of Inhibition

Phytochemical screening of TCF extract indicates the presence of phytoconstituents flavonoids, terpenoids, and polyphenols. Phytochemical screening results (Table6) are consistent with the literature survey [27]. In aqueous acidic solution, the organic molecules of TCF exist either as neutral molecules or in the form of protonated organic molecules (cation).Table 6
Preliminary phytochemical screening of the crude extract.
Plant Phytochemical constituents Flavonoids Alkaloids Terpenoids Saponins Tannins Reducingsugar Polyphenols Anthraquinones TCF +++ − ++ + − + + − +++: appreciable amount; ++: moderate amount; +: trace amount; and −: not detected.The organic molecules might adsorb onto the metal surface by one or more of the following means: (i) interaction between theπ electrons of the inhibitor molecules and vacant d orbital of surface iron atoms and (ii) the protonated inhibitors adsorption through electrostatic interactions between the positively charged molecules and the negatively charged metal surface. The efficiency of TCF might be due to the synergistic effect between the adsorbed Cl− ions and protonated organic molecules of the inhibitor [28]. Thus it may be inferred that TCF effectively creates a barrier between the aggressive solution and the metal due to the adsorption of the inhibitor species onto the metal surface.


## 3.1. Electrochemical Impedance Measurements

The impedance spectral data of MS/HCl/TCF obtained from Nyquist plots (Figure2) in 1 M HCl are given in Table 1. As observed, the impedance spectra exhibit a single depressed semicircle whose diameter increases with increasing concentration of the inhibitor implying a charge transfer process for the corrosion inhibition mechanism [9]. The figure also confirms the fact that the presence of the inhibitor did not modify the corrosion reaction of MS electrode in the presence of the inhibitor. The impedance parameters demonstrate the increase of charge transfer resistance (Rct) values with increase in concentration of the inhibitor. A maximum of 82.9 percentage IE is obtained at 0.7% TCF. Single capacitive semicircle noted in the Nyquist plot corresponds to a single time constant in the Bode representation [10].Figure 2
Nyquist and Bode plot of MS/1 M HCl/TCF.It is clear from the table thatCdl values tend to decrease with increase in inhibitor concentration. This behaviour is the result of increase in surface coverage by the inhibitor molecules of plant extract which leads to increase in IE [11].


## 3.2. Mass Loss Measurements

### 3.2.1. Effect of Concentration and Immersion Time of TCF on MS/1 M HCl

The influence of TCF extract on the corrosion inhibition of MS has been assessed by mass loss measurements. Table2 shows the variation of inhibition efficiency with increase in concentration of the inhibitor. This result indicates the inhibitive nature of the inhibitor on MS corrosion in 1 M HCl medium [12]. It is clear from the results that the IE increases steadily from 67.8 percentage at 0.1% concentration to 85.9 percentage at 0.7% concentration. The corrosion rate and inhibition efficiency of MS electrode exposed to various concentrations of TCF extract in 1 M HCl solution for various time intervals are tabulated in Table 2. From the table, it is noted that a maximum IE of 94.8 percentage for TCF is maintained till 6 h and thereafter a slight decline is observed and the efficiency stabilises to 86.8 percentage (TCF) at 24 h.Table 2
Inhibition efficiency as a function of immersion time and concentration for TCF in 1 M HCl.
Conc.% 1/2 h 1 h 3 h 6 h 12 h 24 h CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) Blank 1032  1190  1238  2197  1561  1209 0.1 333 67.8 260 78.1 350 71.7 625 71.6 601 61.5 545 54.9 0.2 307 70.3 226 81 264 78.6 426 80.6 474 69.8 495 59.1 0.3 273 73.5 200 83.2 233 81.2 315 85.7 435 72.1 458 62.1 0.4 196 81 162 86.4 166 86.6 226 89.7 342 78.1 377 68.8 0.5 179 82.7 145 87.8 148 88.0 200 90.9 318 79.6 312 74.2 0.6 154 85.1 128 89.2 135 89.1 143 93.5 279 82.2 241 80.1 0.7 146 85.9 102 91.4 90 92.7 114 94.8 202 87.1 160 86.8Analysis of the tables indicates that IE increases with increasing concentration of the inhibitors. This indicates the dependence of inhibiting effect on the concentration of inhibitor molecules. This may be due to the increase in coverage of MS surface by the inhibitor molecules at higher concentration of the extract leading to a compact and coherent film on the surface of MS [13].The IE of TCF increases with increasing time of immersion up to 6 h and then decreases to finally stabilise at 24 h to afford 86.8 percentage. This behaviour can be discussed on the basis that prolonged immersion of MS in acid solution [14](a)
allows the cathodic or hydrogen evolution kinetics to increase presumably;(b)
increases the concentration of ferrous ions which decrease the corrosive nature of the acid.


### 3.2.2. Effect of Temperature

The kinetic and mechanistic aspects of corrosion may be gained by studying the effect of temperature on the corrosion of MS in the presence and absence of the inhibitor. It can be noted from Table3 that the maximum IE obtained is 93.8 percentage in 1 M HCl. The values reflect that the IE increases up to 323 K affording an efficiency of 93.8 percentage and then a slight decrease is observed after that and at 353 K it is found to be at 83.2 percentage.Table 3
Relationship between inhibition efficiency and concentration for TCF/MS/1 M HCl systems at various temperatures.
Conc.% Temperature (K) 303 313 323 333 343 353 CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) Blank 1032  2755  6252  8146  9971  18091 0.1 333 67.8 967 64.9 1307 79.1 2077 74.5 4188 58.0 7634 57.8 0.2 307 70.3 628 77.2 982 84.3 1750 78.5 3833 61.6 5174 71.4 0.3 273 73.5 540 80.4 757 87.9 1352 83.4 3490 65.0 4378 75.8 0.4 196 81 507 81.6 569 90.9 1238 84.8 3091 69.0 4125 77.2 0.5 179 82.7 479 82.6 539 91.4 1149 85.9 2353 76.4 3636 79.9 0.6 154 85.1 410 85.1 469 92.5 1075 86.8 1894 81.0 3383 81.3 0.7 146 85.9 386 86.0 388 93.8 863 89.4 1595 84.0 3039 83.2The decrease in the inhibition efficiency of the inhibitors with increase in temperature might be due to adsorption and desorption. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between these two processes at a particular temperature. With the increase of temperature the equilibrium between adsorption and desorption processes is shifted leading to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant [15].


## 3.2.1. Effect of Concentration and Immersion Time of TCF on MS/1 M HCl

The influence of TCF extract on the corrosion inhibition of MS has been assessed by mass loss measurements. Table2 shows the variation of inhibition efficiency with increase in concentration of the inhibitor. This result indicates the inhibitive nature of the inhibitor on MS corrosion in 1 M HCl medium [12]. It is clear from the results that the IE increases steadily from 67.8 percentage at 0.1% concentration to 85.9 percentage at 0.7% concentration. The corrosion rate and inhibition efficiency of MS electrode exposed to various concentrations of TCF extract in 1 M HCl solution for various time intervals are tabulated in Table 2. From the table, it is noted that a maximum IE of 94.8 percentage for TCF is maintained till 6 h and thereafter a slight decline is observed and the efficiency stabilises to 86.8 percentage (TCF) at 24 h.Table 2
Inhibition efficiency as a function of immersion time and concentration for TCF in 1 M HCl.
Conc.% 1/2 h 1 h 3 h 6 h 12 h 24 h CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) CR(mpy) IE(%) Blank 1032  1190  1238  2197  1561  1209 0.1 333 67.8 260 78.1 350 71.7 625 71.6 601 61.5 545 54.9 0.2 307 70.3 226 81 264 78.6 426 80.6 474 69.8 495 59.1 0.3 273 73.5 200 83.2 233 81.2 315 85.7 435 72.1 458 62.1 0.4 196 81 162 86.4 166 86.6 226 89.7 342 78.1 377 68.8 0.5 179 82.7 145 87.8 148 88.0 200 90.9 318 79.6 312 74.2 0.6 154 85.1 128 89.2 135 89.1 143 93.5 279 82.2 241 80.1 0.7 146 85.9 102 91.4 90 92.7 114 94.8 202 87.1 160 86.8Analysis of the tables indicates that IE increases with increasing concentration of the inhibitors. This indicates the dependence of inhibiting effect on the concentration of inhibitor molecules. This may be due to the increase in coverage of MS surface by the inhibitor molecules at higher concentration of the extract leading to a compact and coherent film on the surface of MS [13].The IE of TCF increases with increasing time of immersion up to 6 h and then decreases to finally stabilise at 24 h to afford 86.8 percentage. This behaviour can be discussed on the basis that prolonged immersion of MS in acid solution [14](a)
allows the cathodic or hydrogen evolution kinetics to increase presumably;(b)
increases the concentration of ferrous ions which decrease the corrosive nature of the acid.


## 3.2.2. Effect of Temperature

The kinetic and mechanistic aspects of corrosion may be gained by studying the effect of temperature on the corrosion of MS in the presence and absence of the inhibitor. It can be noted from Table3 that the maximum IE obtained is 93.8 percentage in 1 M HCl. The values reflect that the IE increases up to 323 K affording an efficiency of 93.8 percentage and then a slight decrease is observed after that and at 353 K it is found to be at 83.2 percentage.Table 3
Relationship between inhibition efficiency and concentration for TCF/MS/1 M HCl systems at various temperatures.
Conc.% Temperature (K) 303 313 323 333 343 353 CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) Blank 1032  2755  6252  8146  9971  18091 0.1 333 67.8 967 64.9 1307 79.1 2077 74.5 4188 58.0 7634 57.8 0.2 307 70.3 628 77.2 982 84.3 1750 78.5 3833 61.6 5174 71.4 0.3 273 73.5 540 80.4 757 87.9 1352 83.4 3490 65.0 4378 75.8 0.4 196 81 507 81.6 569 90.9 1238 84.8 3091 69.0 4125 77.2 0.5 179 82.7 479 82.6 539 91.4 1149 85.9 2353 76.4 3636 79.9 0.6 154 85.1 410 85.1 469 92.5 1075 86.8 1894 81.0 3383 81.3 0.7 146 85.9 386 86.0 388 93.8 863 89.4 1595 84.0 3039 83.2The decrease in the inhibition efficiency of the inhibitors with increase in temperature might be due to adsorption and desorption. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between these two processes at a particular temperature. With the increase of temperature the equilibrium between adsorption and desorption processes is shifted leading to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant [15].


## 3.3. Langmuir Adsorption Isotherm

Langmuir adsorption equation relates the degree of surface coverage to concentration of inhibitor according to(1)log⁡Cθ=log⁡C-log⁡K.A plot oflog⁡C/θ versus log⁡C from mass loss data obtained for the studied inhibitor yielded a straight line as represented in Figure 3. The slope deviates from unity. This deviation may be explained on the basis of the interaction among adsorbed species on the metal surface. It has been postulated in the derivation of Langmuir adsorption isotherm equation that adsorbed molecules do not interact with one another, but this is not the case of large organic molecules having polar atoms (or) groups which can be adsorbed on the cathodic and anodic sites of the metal surface as such adsorbed species interact by mutual repulsion or attraction [16]. It is also possible that the inhibitor studied can be adsorbed on the anodic and cathodic sites resulting in deviation from unit gradient.Figure 3
Langmuir adsorption isotherms.


## 3.4. Kinetic Parameters for Inhibition Process

### 3.4.1. Energy of Activation

The dependence of corrosion rate on temperature can be regarded as an Arrhenius-type process, the rate of which is given by(2)log⁡CR=log⁡A-Ea2.303RT,where CR is the corrosion rate of MS, A is Arrhenius or preexponential constant, Ea is the activation energy for the corrosion of MS, R is the gas constant, and T is the temperature.The apparent activation energy for the corrosion ofMS in 1 M HCl is calculated from the Arrhenius plot of log⁡CR against 1/T (Figure 4) in the absence and presence of different concentrations of TCF. The Ea values are deduced from the slopes of these lines and the values of Ea for various concentrations of the inhibitor are tabulated in Table 4. It can be seen in the table that Ea is higher in the presence of the inhibitors than in their absence. The modification in the values of Ea may be attributed to the geometric blocking effect of adsorbed inhibitive species on the metal surface [17]. This observation further supports the proposed physisorption mechanism as report [18] shows that lower values of Ea in the presence of inhibitors in comparison to the free acid solution are indicative of chemical adsorption mechanism, whereas the opposite suggests a physical adsorption mechanism.Table 4
Average values of activation parameters for MS corrosion in 1 M HCl in the absence and presence of TCF.
Inhibitor E a  (kJ/mol) Δ H a (kJ/mol) Δ S a  (J/mol K−1) Blank 47 44.6 −37.9 TCF 52.2 49.5 −36.8Figure 4
Arrhenius plots and transition state plots.


## 3.4.1. Energy of Activation

The dependence of corrosion rate on temperature can be regarded as an Arrhenius-type process, the rate of which is given by(2)log⁡CR=log⁡A-Ea2.303RT,where CR is the corrosion rate of MS, A is Arrhenius or preexponential constant, Ea is the activation energy for the corrosion of MS, R is the gas constant, and T is the temperature.The apparent activation energy for the corrosion ofMS in 1 M HCl is calculated from the Arrhenius plot of log⁡CR against 1/T (Figure 4) in the absence and presence of different concentrations of TCF. The Ea values are deduced from the slopes of these lines and the values of Ea for various concentrations of the inhibitor are tabulated in Table 4. It can be seen in the table that Ea is higher in the presence of the inhibitors than in their absence. The modification in the values of Ea may be attributed to the geometric blocking effect of adsorbed inhibitive species on the metal surface [17]. This observation further supports the proposed physisorption mechanism as report [18] shows that lower values of Ea in the presence of inhibitors in comparison to the free acid solution are indicative of chemical adsorption mechanism, whereas the opposite suggests a physical adsorption mechanism.Table 4
Average values of activation parameters for MS corrosion in 1 M HCl in the absence and presence of TCF.
Inhibitor E a  (kJ/mol) Δ H a (kJ/mol) Δ S a  (J/mol K−1) Blank 47 44.6 −37.9 TCF 52.2 49.5 −36.8Figure 4
Arrhenius plots and transition state plots.


## 3.5. Entropy of Activation and Enthalpy of Activation

In order to calculate the enthalpy,ΔHa, and entropy, ΔSa, of activation for the corrosion process, the alternative formulation of Arrhenius equation, also called transition state equation, is used:(3)CRT=RNhexp⁡ΔSaRexp⁡ΔHaRT,where h is the Planck constant, N is the Avogadro number, ΔSa is the entropy of activation, ΔHa is enthalpy of activation, T is the absolute temperature, CR is corrosion rate, and R is the universal gas constant.The relationship oflog⁡(CR/T) versus 1/T forMS corrosion in 1 M HCl in the absence and presence of different concentrations of TCF extracts is shown in Figure 4. Straight lines are obtained with slope of (-ΔHa/2.303R) and an intercept of (log⁡R/Nh+ΔSa/2.303R) from which the values of ΔHa and ΔSa, respectively, are computed and listed in Table 4.The table reflects the fact thatEa and ΔHa are close to each other as expected from transition state theory concept and they are also found to vary in a similar manner in the presence of the inhibitors. It is also seen in Table 4 that Ea and ΔHa vary in the same manner, but, however, the values of ΔHa are lower than that of Ea. This has been reported in [19] indicating that the corrosion process must involve a gaseous reaction, simply hydrogen evolution reaction associated with decrease in total reaction volume.The positive values ofΔHa both in the absence and presence of additives reflect the endothermic nature of the steel dissolution process and they indicate that the dissolution ofMS is difficult. The values of ΔSa in the absence and presence of the extracts are negative implying that the inhibitor molecules, freely moving in the bulk solution, are adsorbed in an orderly fashion onto theMS surface [20].


## 3.6. Thermodynamic Adsorption Parameters

Figure5 clearly shows the dependence of ΔGoads on T, indicating the good correlation among thermodynamic parameters. The large and negative values of ΔGoads ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface [21].Figure 5
Best fit curve of-ΔGoads versus T for MS/TCF/1 M HCl.The calculated values ofΔGoads presented in Table 5 are negative which indicate that the adsorption of inhibitor molecules on the metal surface is a spontaneous process. Generally, values of ΔGoads around −20 kJ mol−1 or lower are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption); those around −40 kJ mol−1 or higher involve charge sharing or transfer from organic molecules to the metal surface to form a coordinate type of bond (chemisorption) [22]. In the present work, the calculated ΔGoads values are almost slightly less negative than −20 kJ mol−1 ranging from −15 to −19 kJ mol−1. Hence it may be assumed that the adsorption of the inhibitor molecules is obeying physical adsorption; however chemical adsorption may not be excluded due to the complex nature of the corrosion inhibiting process. The positive sign of ΔHoads reflects the endothermic nature of dissolution which suggests the slow dissolution of MS [23]. The entropy of adsorption ΔSoads is negative because the inhibitor molecules, freely moving in the bulk solution, are adsorbed in an orderly fashion onto the mild steel surface, resulting in a decrease in entropy [24].Table 5
Thermodynamic adsorption parameters.
System Free energy of adsorptionΔG°ads (kJ/mol) Δ S ° a d s J/mol Δ H ° a d s  kJ/mol 303 K 313 K 323 K 333 K 343 K 353 K 1 M HCl + 0.7% TCF −15.5 −16.1 −19.0 −18.0 −17.2 −17.0 −27.5 −8.1


## 3.7. Surface Analytical Techniques

### 3.7.1. FT-IR Spectral Studies

Figure6 illustrates the FT-IR spectrum of crude extract of TCF and the corrosion product of MS. From the results obtained, it may be noticed that O-H stretching at 3387 cm−1 (for TCF extract) shifts to 3418 cm−1. The aliphatic and aromatic C-H stretching observed in crude plant extract at 2924 cm−1 and 2855 cm−1 shifts to 3086 cm−1 and 3024 cm−1, respectively. A peak at 2315 cm−1 is noticed only in corrosion product which corresponds to C≡N stretching vibration. A peak at 1628 cm−1 corresponding to C=O stretching vibration is shifted to 1736 cm−1 in the corrosion product. Peaks that are shifted from (1443 cm−1 to 1427 cm−1) and (1373 cm−1 to 1366 cm−1) to (1258 cm−1 to 1219 cm−1) and (1072 cm−1 to 1049 cm−1) indicate the presence of C-O stretching and C-O-C stretching groups. The shift in the absorption frequencies of the TCF extract on the metal surface supports the interaction between the inhibitor and the metal surface. Also, some new bonds are noted in the spectrum of the corrosion product. This includes the C≡N stretch at 2315 cm−1. This also indicates that some new bonds are also formed through these functional groups [25]. This emphasises the fact that the inhibitive nature of the extract is due to the formation of Fe-TCF complex on MS surface.Figure 6
FT-IR spectrum of TCF and corrosion products in the presence of TCF.


## 3.7.1. FT-IR Spectral Studies

Figure6 illustrates the FT-IR spectrum of crude extract of TCF and the corrosion product of MS. From the results obtained, it may be noticed that O-H stretching at 3387 cm−1 (for TCF extract) shifts to 3418 cm−1. The aliphatic and aromatic C-H stretching observed in crude plant extract at 2924 cm−1 and 2855 cm−1 shifts to 3086 cm−1 and 3024 cm−1, respectively. A peak at 2315 cm−1 is noticed only in corrosion product which corresponds to C≡N stretching vibration. A peak at 1628 cm−1 corresponding to C=O stretching vibration is shifted to 1736 cm−1 in the corrosion product. Peaks that are shifted from (1443 cm−1 to 1427 cm−1) and (1373 cm−1 to 1366 cm−1) to (1258 cm−1 to 1219 cm−1) and (1072 cm−1 to 1049 cm−1) indicate the presence of C-O stretching and C-O-C stretching groups. The shift in the absorption frequencies of the TCF extract on the metal surface supports the interaction between the inhibitor and the metal surface. Also, some new bonds are noted in the spectrum of the corrosion product. This includes the C≡N stretch at 2315 cm−1. This also indicates that some new bonds are also formed through these functional groups [25]. This emphasises the fact that the inhibitive nature of the extract is due to the formation of Fe-TCF complex on MS surface.Figure 6
FT-IR spectrum of TCF and corrosion products in the presence of TCF.


## 3.8. Scanning Electron Micrograph of MS in the Presence of TCF Extract in 1 M HCl

Surface morphology of the MS samples immersed in HCl with and without TCF is shown in Figures7(a) and 7(b). Figure 7(a) indicates the finely polished characteristic surface of MS with some scratches that had arisen during polishing.Figure 7
SEM images of MS corrosion in the absence and presence of TCF in 1 M HCl.
(a)
Plain MS (b)
MS + 1 M HCl (c)
MS + TCF in 1 M HClFigure7(b) reveals that the immersed specimens are drastically damaged in the presence of 1.0 M HCl, due to the direct attack of aggressive acid. Figure 7(c) reflects the drastic change in morphology of MS surface in the inhibited solution containing 0.7% TCF extract. The smooth surface of MS in inhibited solution may be due to the phytochemical constituents present in TCF that hinder the dissolution of iron by forming a protective film on MS surface [26].


## 3.9. Mechanism of Inhibition

Phytochemical screening of TCF extract indicates the presence of phytoconstituents flavonoids, terpenoids, and polyphenols. Phytochemical screening results (Table6) are consistent with the literature survey [27]. In aqueous acidic solution, the organic molecules of TCF exist either as neutral molecules or in the form of protonated organic molecules (cation).Table 6
Preliminary phytochemical screening of the crude extract.
Plant Phytochemical constituents Flavonoids Alkaloids Terpenoids Saponins Tannins Reducingsugar Polyphenols Anthraquinones TCF +++ − ++ + − + + − +++: appreciable amount; ++: moderate amount; +: trace amount; and −: not detected.The organic molecules might adsorb onto the metal surface by one or more of the following means: (i) interaction between theπ electrons of the inhibitor molecules and vacant d orbital of surface iron atoms and (ii) the protonated inhibitors adsorption through electrostatic interactions between the positively charged molecules and the negatively charged metal surface. The efficiency of TCF might be due to the synergistic effect between the adsorbed Cl− ions and protonated organic molecules of the inhibitor [28]. Thus it may be inferred that TCF effectively creates a barrier between the aggressive solution and the metal due to the adsorption of the inhibitor species onto the metal surface.


## 4. Conclusion

The investigated inhibitor performed in an effective manner to minimize the corrosion ofMS in 1 M HCl medium. The results indicated a considerable reduction in the Icorr values in the presence of the inhibitors. The IE was found to increase with increase in concentration of the inhibitors. The inhibitors acted as mixed-type inhibitors. Analysis of the results of the mass loss measurements ofMS infers that the inhibition efficiencies increased with increasing concentration of the inhibitors. Surface morphology of the metal indicated that the surface of the metal was protected by the adsorption of the active constituents of the plant species.



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