The corrosion inhibition efficiency of thiophene-2-carbaldehyde tryptophan (T2CTRY) on mild steel (MS) in 1 M HCl solution has been investigated and compared using weight loss measurements, electrochemical impedance spectroscopy, and potentiodynamic polarization analysis. The Schiff base exhibited very good corrosion inhibition on mild steel in HCl medium and the inhibition efficiency increased with the increase in concentration of the inhibitor. The adsorption of the inhibitor on the surface of the corroding metal obeys Freundlich isotherm. Thermodynamic parameters (, Δ) were calculated using adsorption isotherm. Polarization studies revealed that T2CTRY acts as a mixed type inhibitor. A maximum of 96.2% inhibition efficiency was achieved by EIS studies at a concentration of 1 mM.

1. Introduction

Several Schiff bases were tested as corrosion inhibitors and it is reported that converting an amino compound into Schiff base increases the inhibition efficiency. Due to the presence of C=N– group and electronegative N, S, or O atoms in the molecule, Schiff bases should be good corrosion inhibitors [14]. The action of such inhibitors depends on the specific interaction between the functional groups and the metal surface. Corrosion commonly occurs at metal surfaces in the presence of oxygen and moisture, involving electrochemical reactions [57]. The application of Schiff bases as an effective corrosion inhibitor is mainly based on their ability to form a monolayer on the surface of the corroding material.Increasing efforts have been made to study the corrosion behaviour of metals and mechanism of inhibition during the past decade mainly through electrochemical investigations [811].

The present investigation was undertaken to examine the corrosion inhibition behaviour of a novel heterocyclic Schiff base T2CTRY, derived from thiophene-2-carbaldehyde and tryptophan, in 1 M HCl solution on MS. The anticorrosive activity of this compound was evaluated by weight loss measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization analysis.

2. Experimental

2.1. Inhibitor

Heterocyclic Schiff base T2CTRY was synthesized by the condensation of equimolar mixture of thiophene-2-carbaldehyde and tryptophan in ethanol medium. The reaction mixture was refluxed for 3 hours, concentrated, and cooled. The yellow coloured solid was filtered, washed, and dried. Figure 1 shows the molecular structure of the heterocyclic Schiff base T2CTRY. Anal. calcd. for C16H14N2O2S: C, 64.43; H, 4.69; N, 9.39; O, 10.74; S, 10.74%. Found.C, 63.98; H, 4.87; N, 9.12; O, 11.09; S, 10.21%. IR (KBr): 1559 cm−1, 1Hnmr: 12.6, 9.84, 13Cnmr: 183.58, 132.4.

2.2. Solution

The aggressive solution of 1 M HCl was prepared by dilution of A.R grade 37% of HCl (Merck) with de-ionized water. Inhibitor solutions were prepared in the range 0.1 mM–1 mM concentrations.

2.3. Weight Loss Measurements

Mild steel (MS) specimens of dimension  cm were cut and abraded with various grades of silicon carbide papers (200, 400, 600, 800, 1000, 1200, and 2000). The exact area and thickness of each coupon were measured and washed. Specimens were then degreased with acetone and finally dried. After weighing, specimens were separately immersed in 50 ml 1 M HCl solution at 28°C in the absence and presence of the inhibitor T2CTRY. Weight loss of metal specimens was noted at every 24 h time interval for five days. The experiments were carried out in duplicate and the average values were reported. The corrosion rate () is calculated by the following equation [12, 13]: where is the average weight loss of coupon, is the total area of specimen, and is the time of treatment. The percentage of inhibition efficiency can be calculated by the following equation: where and are the corrosion rates of uninhibited and inhibited specimens, respectively.

2.4. Electrochemical Impedance Spectroscopy (EIS)

The EIS measurements were performed in a three electrode assembly. Saturated calomel electrode (SCE) was used as the reference electrode. Platinum electrode having 1 cm2 area was taken as counter electrode. Metal specimens with an exposed area of 1 cm2 were used as the working electrode. The EIS experiments were carried out on an Ivium compactstat-e electrochemical system. 1 M HCl was taken as the electrolyte and the working area of the metal specimens were exposed to the electrolyte for 1 h prior to the measurement. EIS measurements were performed at constant potential (OCP) in the frequency range from 1 KHz to 100 mHz with amplitude of 10 mV as excitation signal. The percentages of inhibitions from impedance measurements were calculated using charge transfer resistance values by the following expression [1]: where and are the charge transfer resistances of working electrode with and without inhibitor, respectively.

2.5. Potentiodynamic Polarization

Electrochemical polarization studies were performed by recording anodic and cathodic potentiodynamic polarization curves. Polarization plots were obtained in the electrode potential range from −100 to +100 mV versus corrosion potential at a scan rate of 1 mV/sec. Tafel polarization analysis was done by extrapolating anodic and cathodic curves to the potential axis to obtain corrosion current densities . The percentage of inhibition efficiency () was evaluated from the measured values using the following relation [14]: where and are the corrosion current densities of the exposed area of the working electrode in the absence and presence of inhibitor, respectively. From the slope analysis of the linear polarization curves in the vicinity of corrosion potential of blank and different concentrations of the inhibitor, the values of polarization resistance in 1 M HCl solution were obtained. From the evaluated polarization resistance, the inhibition efficiency was calculated using the relationship where   and are the polarization resistance in the presence and absence of the inhibitor, respectively [1].

3. Results and Discussions

3.1. Weight Loss Measurements

The variation of corrosion rate and inhibition efficiency with immersion time was determined by weight loss measurements for a period of five days in the presence and absence of the Schiff base T2CTRY. From Table 1 it is clear that the rates of corrosion of the MS specimens decrease with increase in concentration of the T2CTRY while the percentage of inhibition is markedly increased with increasing the concentration which can be attributed to the increased adsorption of the T2CTRY molecules on the metal surface [14]. The increased surface coverage may partially hinder the reaction between metal and H+ ions which in turn increase the percentage of inhibition. A maximum of 83.86% inhibition efficiency could be achieved by T2CTRY molecules at a concentration of 1.0 mM.

3.2. Comparison of Inhibition Efficiency of Schiff Base with Its Parent Amine

To compare the inhibition efficiencies of Schiff base and parent amine, weight loss measurements of MS specimens with parent amine tryptophan were performed in 1 M HCl at 28°C and the data is represented in Table 1. The inhibition efficiency of the Schiff base T2CTRY was markedly higher than that of the parent amine, for the studied concentrations. This investigation clearly establishes the role of azomethine linkage (C=N) present in the Schiff base which actively participates in the corrosion inhibition mechanism.

3.3. Adsorption Isotherm and Free Energy of Adsorption

The mechanism of adsorption and the surface behaviour of organic molecules can be easily viewed through adsorption isotherms. Different models of adsorption isotherms considered are Langmiur, Temkin, Frumkin, and Freundlich isotherms. For the evaluation of thermodynamic parameters, it is necessary to determine the best fit isotherm with the aid of correlation coefficient . The best fit adsorption isotherm for the adsorption of T2CTRY on MS specimens in 1 M HCl was Freundlich adsorption isotherm which can be expressed as where is the concentration of the inhibitor, is the fractional surface coverage, and is the adsorption equilibrium constant [15]. Figure 2 represents the Freundlich adsorption isotherm for T2CTRY and the is related to the standard free energy of adsorption , by where 55.5 is the molar concentration of water, is the universal gas constant and is the temperature in Kelvin [16]. The negative value of free energy of adsorption indicates the spontaneity of the process. In the present investigation T2CTRY molecules showed suggesting that the adsorption involves both physisorption and chemisorption [17].

3.4. Electrochemical Impedance Spectroscopy

Figures 3 and 4 represent the Nyquist and Bode plots of MS specimens in 1 M HCl. It is evident from the plots that the impedance response of metal specimens showed a marked difference in the presence and absence of the inhibitor T2CTRY. The capacitance loop intersects the real axis at higher and lower frequencies. At high frequency end, the intercept corresponds to the solution resistance and at lower frequency end, the intercept corresponds to the sum of and charge transfer resistance (). The difference between the two values gives [1820]. The value of is a measure of electron transfer across the exposed area of the metal surface and it is inversely proportional to rate of corrosion [2123].

Impedance behavior can be well explained by pure electric models that could verify and enable to calculate numerical values corresponding to the physical and chemical properties of electrochemical system under examination. The simple equivalent circuit that fits to many electrochemical system composed of a double layer capacitance, and [24, 25]. To reduce the effects due to surface irregularities of metal, constant phase element (CPE) is introduced into the circuit instead of a pure double layer capacitance [26] which gives more accurate fit as shown in the Figure 5.

The impedance of CPE can be expressed as where is the magnitude of CPE, is the exponent (phase shift), is the angular frequency, and is the imaginary unit. CPE may be resistance, capacitance, and inductance depending upon the values of [27]. In all experiments, the observed value of ranges between 0.8 and 1.0, suggesting the capacitive response of CPE. The EIS parameters such as , , and CPE and the calculated values of percentage of inhibition of MS specimens are listed in Table 2. The values are increased with increasing inhibitor concentration. Decrease in capacitance values CPE with inhibitor concentration can be attributed to the decrease in local dielectric constant and/or increase in the thickness of the electrical double layer. This emphasises the action of inhibitor molecules by adsorption at the metal-solution interface [28]. The percentage of inhibition showed a regular increase with increase in inhibitor concentration. A maximum of 96.2% inhibition efficiency could be achieved at an inhibitor concentration of 1 mM.

3.5. Potentiodynamic Polarization Studies

Potentiodynamic polarization curves for T2CTRY are shown in Figure 6. Polarization parameters like corrosion current densities (), corrosion potential (), cathodic Tafel slope (), anodic Tafel slope (), and inhibition efficiency () for MS specimens are listed in Table 3. A prominent decrease in the corrosion current density () was observed in the presence of inhibitor T2CTRY. The lowest value of was noticed for the inhibitor solution of concentration 1 mM which exhibited a maximum inhibition efficiency of 95.3%. Since the value of changes appreciably in the presence of T2CTRY, it may be assumed that the inhibitor molecules are more adsorbed on cathodic sites. Generally if the shift of is > 85 with respect to of uninhibited solution, the inhibitor can be viewed as cathodic or anodic type [29]. In the present study the maximum shift of is 30 mV, suggesting that T2CTRY acts as a mixed type inhibitor for MS specimens in 1 M HCl [30].

4. Mechanism of Inhibition

It is well known that the surface of the metal is positively charged in acidic media [3]. It is believed that the Cl ions could be specifically adsorbed on the metal surface and create an excess of negative charge on the surface. This will favour the adsorption of protonated Schiff base on the surface and hence reduce the dissolution of Fe to Fe2+ [29]. Besides this electrostatic interaction between the protonated Schiff base and the metal surface, other possible interactions are (i) interaction of unshared electron pairs in the molecule with the metal (ii) interaction of π-electrons with the metal and (iii) a combination of types ((i)-(ii)) [30, 31]. If one examines the structure of Schiff base (T2CTRY), many potential sources of inhibitor-metal-interaction can be recognized. The unshared pair of electrons present on atoms is of key importance in making coordinate bond with the metal. The π-electron cloud of the aromatic rings and the azomethine linkage also participate in the inhibition mechanism. Furthermore, the double bonds in the inhibitor molecule permit the back donation of metal d electrons to the orbital and this type of interaction cannot occur with amines [32]. This can be justified by the lower inhibition efficiency of the parent amine than that of Schiff base.

5. Conclusions

(1)T2CTRY is a good corrosion inhibitor for MS in 1 M HCl. A maximum of 95% of inhibition efficiency could be achieved with this inhibitor by Tafel polarization studies.(2)Compared to the parent amine, tryptophan, the Schiff base T2CTRY exhibited higher inhibition efficiencies for MS.(3)Adsorption of the inhibitor T2CTRY on mild steel obeys Freundlich isotherm.(4)The thermodynamic parameters of the adsorption are calculated from the adsorption isotherms which showed that both physisorption and chemisorption are involved in the inhibition process.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.