Corrosion inhibition of mild steel in 1 M HCl in the presence of Tecoma 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.

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 of Phaseolus aureus [2], Cyamopsis tetragonoloba [3], Ervatamia coronaria [4], and Cocos 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 of Tecoma 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 per ASTM 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

Figure 1 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 (), anodic () and cathodic () Tafel constants, and polarisation resistance (), are given in Table 1. The table divulges the fact that the addition of the inhibitor shifts the anodic () and cathodic () slopes, respectively. This denotes the suppression of both the anodic dissolution and cathodic hydrogen evolution by the inhibitor. Significant reduction in the 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 values infers the mixed nature of the inhibitor under investigation [7]. It is evident from Table 1 that the values increase with increasing TCF concentration. Inhibition performance of TCF is found to be 77.9 percentage.

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 (Figure 2) 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 () 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].

It is clear from the table that 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. Table 2 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.

Analysis 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 Table 3 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.

The 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

A plot of versus 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.

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 bywhere CR is the corrosion rate of MS, is Arrhenius or preexponential constant, is the activation energy for the corrosion of MS, is the gas constant, and is the temperature.

The apparent activation energy for the corrosion of MS in 1 M HCl is calculated from the Arrhenius plot of against (Figure 4) in the absence and presence of different concentrations of TCF. The values are deduced from the slopes of these lines and the values of for various concentrations of the inhibitor are tabulated in Table 4. It can be seen in the table that is higher in the presence of the inhibitors than in their absence. The modification in the values of 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 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.

3.5. Entropy of Activation and Enthalpy of Activation

In order to calculate the enthalpy, , and entropy, , of activation for the corrosion process, the alternative formulation of Arrhenius equation, also called transition state equation, is used:where is the Planck constant, is the Avogadro number, is the entropy of activation, is enthalpy of activation, is the absolute temperature, is corrosion rate, and is the universal gas constant.

The relationship of versus 1/T for MS 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 () and an intercept of () from which the values of and , respectively, are computed and listed in Table 4.

The table reflects the fact that and 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 and vary in the same manner, but, however, the values of are lower than that of . 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 of both in the absence and presence of additives reflect the endothermic nature of the steel dissolution process and they indicate that the dissolution of MS is difficult. The values of 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 the MS surface [20].

3.6. Thermodynamic Adsorption Parameters

Figure 5 clearly shows the dependence of on , indicating the good correlation among thermodynamic parameters. The large and negative values of ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface [21].

The calculated values of 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 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 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 reflects the endothermic nature of dissolution which suggests the slow dissolution of MS [23]. The entropy of adsorption 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].

3.7. Surface Analytical Techniques
3.7.1. FT-IR Spectral Studies

Figure 6 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.

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 Figures 7(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(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 (Table 6) 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).

The organic molecules might adsorb onto the metal surface by one or more of the following means: (i) interaction between the 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 of MS in 1 M HCl medium. The results indicated a considerable reduction in the 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 of MS 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.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this manuscript.


The authors would like to thank the authorities of Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore, Tamil Nadu 641043, India, for providing necessary facilities for carrying out this study.