Application of Some Condensed Uracils as Corrosion Inhibitors for

Article pubs.acs.org/IECR

Application of Some Condensed Uracils as Corrosion Inhibitors for Mild Steel: Gravimetric, Electrochemical, Surface Morphological, UV− Visible, and Theoretical Investigations Dileep Kumar Yadav and Mumtaz Ahmad Quraishi* Department of Applied Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi-221005, India S Supporting Information *

ABSTRACT: Gravimetric, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and potentiodynamic polarization (PDP) studies were carried out to investigate the comparative corrosion protection efficiency of four condensed uracils (CUs) on mild steel in 1 M HCl. EIS plots indicated that the addition of inhibitors increases the charge-transfer resistance (Rct), decreases the double-layer capacitance (Cdl) of the corrosion process, and hence increases inhibition performance. Moreover, the thermodynamic activation parameters for the corrosion reaction were calculated and discussed in relation to the stability of the protective inhibitor layer. The morphology of the surface was examined by scanning electron microscopy (SEM), and the surface composition was evaluated using energy-dispersive X-ray spectroscopy (EDX) to verify the presence of inhibitor on the mild steel surface. Quantum chemical study suggests that the heterocyclic rings in these compounds are structurally essential for the protection of the mild steel surface.

1. INTRODUCTION One of the most important applications of hydrochloric acid is in the pickling of steel, to remove rust or iron oxide scale from iron or steel before subsequent processing, such as extrusion, rolling, galvanizing, and other techniques. Mild steel is a reactive material and prone to corrosion in acid media.1During the past few years, various scientific studies have been devoted to the subject of corrosion inhibition of mild steel by different types of N-heterocyclic compounds in acidic media.2−11 In our present research, the utilized condensed uracil is an Nheterocyclic compound which consists of the fusion of a uracil moiety with a pyran ring. The molecular structures of these compounds are different from the uracils or pyrimidines reported recently as corrosion inhibitors. Owing to their important biological activities, these compounds are nontoxic and ecofriendly.12,13These compounds were synthesized from relatively inexpensive chemicals. The calculated cost of 100 L of acid solution containing 35 g of inhibitors is approximately Rs. 40−45. Recent advances in DFT-based quantum chemical computations have made this powerful tool increasingly available to corrosion scientists for theoretical investigation of corrosion and corrosion inhibition systems. Such an approach offers the added advantage of providing important physical insights on corrosion inhibition mechanisms.14 Nakayama et al. studied the inhibitory effects of uracil derivatives on cathodic reactions of steel in saturated Ca (OH)2 solutions.15Dafali et al. studied the corrosion and corrosion inhibition behavior of copper electrodes in the absence and presence of some substituted uracils.16 An electrochemical impedance study was used to evaluate the corrosion inhibition properties of two pyrimidine bases, namely, thymine and uracil and their related compounds.17 The present study was undertaken to ascertain the protection ability of four condensed uracils as new corrosion inhibitors for mild steel in hydrochloric © 2012 American Chemical Society

acid solution using various electrochemical techniques (EIS, LPR, and PDP) and a gravimetric method. The experimental results are complemented well by theoretical calculations using DFT. The effect of temperature on corrosion and inhibition processes was evaluated and discussed. The surface morphologies of electrodes were examined by high resolution scanning electron microscopy (SEM), and the elemental composition of the inhibitor film that formed on the metal surface was examined by energy dispersive X-ray spectroscopy (EDX). The inhibitor solutions were also analyzed with a UV−visible spectrophotometer. To date, no literature is available reporting the use of these heterocyclic compounds as corrosion inhibitors. In summary, the corrosion and corrosion inhibition processes have been thoroughly investigated using various techniques such as gravimetric, EIS, LPR, PDP, SEM, EDX, UV−visible spectroscopy, and quantum chemical calculations, and very good inhibition was obtained (up to 97%) in comparison to that previously reported in the literature. Furthermore, the inhibitors in our study demonstrated costeffectiveness, nontoxic behavior, and an ecofriendly nature.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Mild steel samples having the composition (wt %) C = 0.076, Mn = 0.192, P = 0.012, Si = 0.026, Cr = 0.050, Al = 0.023, with Fe constituting the remaining percentage, were used for gravimetric and electrochemical experiments. The steel specimens were mechanically cut into 2.5 × 2 × 0.025 cm (for gravimetric) and 8 × 1 × 0.025 cm (for electrochemical) dimensions and then abraded Received: Revised: Accepted: Published: 14966

July 11, 2012 October 11, 2012 October 17, 2012 October 17, 2012 dx.doi.org/10.1021/ie301840y | Ind. Eng. Chem. Res. 2012, 51, 14966−14979

Industrial & Engineering Chemistry Research

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2.2. Gravimetric Experiment. Gravimetric experiments were performed following standard methods.18 Corrosion rates were calculated from the following equation:19

with silicon carbide (SiC) abrasive papers of grade 600, 800, 1000, and 1200, respectively, degreased in acetone, and dried at ambient temperature. The test solution, 1 M HCl, was prepared by diluting analytical grade 37% HCl with double-distilled water. The inhibitors were synthesized following the route given in Figure 1.12The purity of the products was confirmed by thin-

CR =

W St

(1)

where W is the mean value of weight loss of three parallel mild steel coupons, S is the total area of a mild steel coupon, and t is the immersion time (3 h). The inhibition efficiency η% was calculated using the corrosion rate as follows:20 η% =

C R − C R(i) CR

× 100

(2)

The surface coverage (θ) value was calculated using eq 3:

θ= Figure 1. Synthetic pathway for condensed uracil.

C R − C R(i) CR

(3)

where CR and CR(i) are the corrosion rate (mg cm−2 h−1) values of mild steel coupons in the absence and presence of CUs, respectively. 2.3. Electrochemical Experiment. A mild steel coupon embedded in Araldite with an exposed area of 1 cm2 was used as a working electrode. A platinum sheet was used as the auxiliary electrode, and a saturated calomel electrode (SCE) was used as reference electrode. The reaction vessel was a three-electrode cell connected to a Gamry Potentiostat/ Galvanostat (Model G-300) Instrument. All experiments were performed in the absence and presence of an optimum concentration (350 mg L−1) of CUs in 1 M hydrochloric acid solution. The experimental data were processed by Gamry Echem Analyst 5.0 software. All the experiments were measured

layer chromatography followed by further purification through recrystallization from ethanol. The melting points of compounds were determined in open capillaries and matched with the literature values.12 Infrared (IR) spectra were recorded on KBr discs using a Perkin-Elmer (Spectrum100) Fourier transform (FT-IR) spectrophotometer. 1H NMR (300 MHz) spectra were obtained with a JEOL AL 300 FT-NMR in CDCl3 using TMS as internal standard. The chemical structure, abbreviations, IUPAC name, spectral data, and melting points of the synthesized compounds are presented in Table 1.

Table 1. Molecular Structure and Analytical Data of Condensed Uracils (CUs)

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after immersion of mild steel for 30 min in 1 M HCl in the absence and presence of inhibitors. EIS experiments were carried out at open circuit potential (OCP). A sinusoidal potential with amplitude 10 mV, superimposed on OCP, was applied. The frequency of this AC signal was scanned from 100 kHz to 0.01 Hz. The linear polarization resistance (LPR) study was carried out from a cathodic potential of −0.020 V vs OCP to an anodic potential of +0.020 V vs OCP at a scan rate of 0.125 mV s−1. Potentiodynamic current−potential curves were obtained by sweeping the electrode potential automatically from −250 up to +250 mVSCE versus OCP at a scan rate of 1 mV s−1. The linear Tafel segments of anodic and cathodic curves were extrapolated to the corrosion potential to obtain corrosion current densities (Icorr). 2.4. Surface Analysis. Before surface examination, the electrodes were immersed in the test solution (1 M HCl) for 3 h in the absence and presence of optimum concentrations (350 mg L−1) of corrosion inhibitors to observe the effect of corrosion and inhibition. The mild steel electrodes were then dried at ambient temperature. Micrographs of abraded and corroded mild steel surfaces and those after inhibitor addition were taken using a SEM model Ziess Evo 50 XVP instrument at an accelerating voltage of 5 kV at 5000× magnification. The changes in surface composition were analyzed with an EDX detector module coupled with SEM. 2.5. UV−Visible Spectroscopy. A Hitachi U-2900 doublebeam spectrophotometer was used for UV−visible spectral determination. For routine analysis, a simple conventional technique was used based on UV−visible absorption spectra obtained from 1 M HCl solution containing 350 mg L−1 of CU3 before and after two days of mild steel immersion. 2.6. Theoretical Investigation. The calculations were carried out for the four CUs. The geometry of all compounds under investigation was determined by optimizing all geometrical variables without any symmetry constraints. The geometries of the CUs were fully optimized by Gaussian 03, E.01 software package,21employing functional hybrid B3LYP density function theory (DFT) formalism with electron basis set 6-31G (d,p) for all atoms.

Table 2. Parameters Obtained from Gravimetric Measurements for Mild Steel in 1 M HCl Containing Different Concentrations of CUs at 308 K inhibitors blank CU-1

CU-2

CU-3

CU-4

Cinh, mg L−1/M 0.0/0.0 200/7.08 250/8.85 300/10.6 350/12.4 200/6.11 250/7.64 300/9.16 350/10.6 200/6.40 250/8.00 300/9.60 350/11.2 400/12.8 450/14.4 200/6.75 250/8.44 300/10.1 350/11.8

× × × × × × × × × × × × × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

CR (mg cm−2 h−1)

θ

η (%)

7.00 4.46 3.34 1.63 1.53 5.03 4.43 3.40 2.36 1.26 1.00 0.91 0.32 0.27 0.26 3.83 2.63 1.16

− 0.36 0.52 0.76 0.78 0.28 0.36 0.51 0.66 0.81 0.85 0.87 0.95 0.95 0.95 0.45 0.62 0.83 0.85

− 36.1 52.3 76.6 78.0 28.9 36.6 51.4 66.1 81.9 85.7 87.1 95.5 95.8 96.1 45.2 62.3 83.3 85.2

1.03

and causes a greater surface area of steel to come in contact with corrosive agents, resulting in an increased corrosion rate and decreased inhibition efficiency.22The activation parameters for the corrosion process were calculated from the Arrhenius equation:23

⎛ −E ⎞ C R = A exp⎜ a ⎟ ⎝ RT ⎠

(4)

where Ea represents the apparent activation energy, R is the molar gas constant (8.31434 J K−1 mol−1), and A is the frequency factor. Arrhenius plots for the corrosion rate of mild steel are given in Figure 2a. Values of Ea for mild steel corrosion in 1 M HCl in the absence and presence of CUs were calculated by linear regression between log (CR) and 1/T and are listed in Table 3. An alternative form of Arrhenius equation is also used to calculate enthalpy and entropy of activation:23

3. RESULTS AND DISCUSSION 3.1. Gravimetric Analysis. 3.1.1. Effect of Inhibitor Concentration. The inhibitive effect under various concentrations of CUs is compiled in Table 2. Clearly, with gradually increased inhibitor concentrations the weight loss as well as corrosion rate of mild steel diminished distinctly. Very good inhibition efficiency (η %) was obtained (95.2% for CU-3) at 350 mg L−1, and this concentration was chosen to be the optimum concentration of the inhibitor. No significant increase in inhibition efficiency was observed above 350 mg L−1. The comparative inhibition effect was investigated at the optimum concentration (350 mg L−1) of all four inhibitors. 3.1.2. Effect of Temperature. The influence of temperature has been studied in the range of 308−338 K in the absence and presence of an optimum concentration of CUs in 1 M HCl after 3 h of immersion using a digital thermostat. In the studied temperature range (308−338 K), the inhibition efficiency decreases and corrosion rate increases with increasing temperature both in inhibitor-free and inhibited solutions (Table S1 in the Supporting Information). The decrease in inhibition efficiency at higher temperatures can be attributed to desorption of some inhibitor molecules from the steel surface

CR =

⎛ ΔS* ⎞ ⎛ H * ⎞ RT ⎟exp⎜ − ⎟ exp⎜ ⎝ R ⎠ ⎝ RT ⎠ Nℏ

(5) −34

where ℏ is Plank’s constant (6.626176 × 10 J·s), N is Avogadro’s number (6.02252 × 1023 mol−1), ΔS* is the entropy of activation, and ΔH* is the enthalpy of activation. Figure 2b shows the plot of log CR/T against 1/T. Straight lines were obtained with a slope of (ΔH*/2.303R) and an intercept of [log(R/Nℏ)) + (ΔS*/2.303R)] from which the values of ΔH* and ΔS* were calculated and are compiled in Table 3. Table 3 shows that the values of Ea for inhibited solutions ranged from 37 to 52 kJ mol−1. These values are higher than the value of 27.9 kJ mol−1 for inhibitor-free solution. The increase in activation energy in the presence of inhibitors signifies physisorption.24However, the benchmark, in which the higher value of Ea for the inhibited system indicates physical adsorption in the initial stage, cannot be taken into account as decisive because of competitive inhibitor adsorption on the metal surface with water, whose desorption from the surface requires some activation energy.25Therefore, the adsorption phenomenon of organic inhibitor molecules on the metal surface is not considered only as physical or chemical 14968



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ΔH * = Ea − RT

(6)

Table 3 shows that the values of ΔS* are large and negative in both the uninhibited and inhibited systems, which implies that the activated complex in the rate-determining step represents an association rather than a dissociation.28The values of ΔS* are higher for inhibited solutions than for uninhibited solution. This might be the result of the adsorption of CU molecules from the 1 M HCl solution, which could be regarded as a quasisubstitution process between the CU molecules in the aqueous phase and water molecules on the mild steel surface. In this situation, the adsorption of CU molecules was accompanied by desorption of water molecules from the mild steel surface. Thus, an increase in ΔS* was imputed to the increase in entropy of the water molecules.24 3.1.3. Application of Adsorption Isotherm. To obtain the adsorption isotherm, the linear relationship between the values of θ and the inhibitor concentration (Cinh) must be established. The three adsorption isotherms applied to fit the surface coverage (θ) values at different concentrations of inhibitors were the Temkin isotherm:29

K adsC = e fθ

(7)

the Frumkin isotherm: K adsC =

θ e fθ 1−θ

(8)

and the Langmuir isotherm: K adsC =

Table 3. Activation Parameters for Mild Steel Dissolution in 1 M HCl in the Absence and Presence of an Optimum Concentration of Investigated CUs Ea (kJ mol−1)

ΔH* (kJ mol−1)

ΔS* (J K−1 mol−1)

blank CU-1 CU-2 CU-3 CU-4

27.9 48.8 37.2 51.3 48.3

25.4 48.7 39.4 54.1 51.1

−147.4 −83.3 −70.2 −78.5 −79.5

(9)

where f (θ) is the configuration factor that depends upon the physical model and assumption underlying the derivatives of the isotherm, θ is the surface coverage, C is the inhibitor concentration, and Kads is the equilibrium constant of adsorption. The plot of surface coverage (θ) as a function of logarithm of inhibitor concentration (log C) was evaluated. The correlation coefficients (R2) were used to determine the best fits. The obtained plots of the CUs are almost linear with correlation coefficient (R2), ranging from 0.984 to 0.995 for the Langmuir adsorption isotherm (Figure 2), which suggested that the adsorption of CUs on the metal surface obeyed the Langmuir isotherm. Kads is related to the standard free energy of adsorption ΔG°ads by the following equation:29

Figure 2. (a) Arrhenius plots for the corrosion rate (CR) of mild steel in 1 M HCl in absence and presence of optimum concentration of CUs. (b) Transition-state plots for the corrosion rate (CR) of mild steel in 1 M HCl in absence and presence of optimum concentration of CUs. (c) Curve fitting of Langmuir's isotherm for adsorption of CUs on a mild steel surface in 1 M HCl.

inhibitors

θ 1−θ

K ads =

⎛ −ΔGads ° ⎞ 1 exp⎜ ⎟ ⎝ RT ⎠ 55.5

(10)

where the value 55.5 is the concentration of water in solution expressed in mol L−1. The values of Kads and ΔG°ads are given in Table S2 in the Supporting Information. Generally, values of ΔG°ads ≤ −20 kJ mol−1 signify physisorption, and values more negative than −40 kJ mol−1 signify chemisorption.29However, adsorption is a separation process involving two phases between which certain components can be described by two main types of interaction, i.e., physical as well as chemical adsorption. The calculated values of ΔG°ads for studied inhibitors range from −37 to −44 kJ mol−1 (Table S2 in the Supporting Information), which probably means that both physical adsorption and chemical adsorption (comprehensive adsorption) would take place.4The higher values of Kads refer to higher adsorption and a higher inhibiting effect of inhibitors.

adsorption.24 The positive sign of ΔH* suggests that the dissolution process of mild steel is endothermic in nature and its dissolution is slow.26In the presence of CUs, Ea and ΔH* values change in a similar manner (Table 3). These results verify the known thermodynamic relationship between Ea and ΔH*:27 14969



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3.2. Electrochemical Analysis. 3.2.1. Electrochemical Impedance Spectroscopy Results. Figure 3a exhibits the

its OCP. Figure 3a shows that the impedance modulus value in uninhibited solution was much smaller than that in inhibited solution. However, after the addition of CUs, the impedance modulus of the system gradually increased and was a maximum in the case of CU-3. The impedance diagram (Nyquist) contains depressed semicircles with the center under the real axis with one capacitive loop in the high frequency (HF) zone, and one RL − L inductive loop in the lower frequency (LF) zone. As usually indicated in an EIS study, the HF capacitive loop is related to the charge-transfer resistance (Rct = Rp − Rs − Ra − Rf) process of the metal corrosion and the double-layer behavior, and these loops are not perfect semicircles. Such behavior is characteristic for solid electrodes and is often referred to as a frequency dispersion effect which can be imputed to nonhomogeneity and the rough-textured metal surface. The LF inductive loop may be a consequence of the layer stabilization byproducts of the corrosion reaction on the electrode involving inhibitor molecules and their reactive products.30All the impedance parameters were calculated with the application of the equivalent circuit model as shown in Figure 3b,c. These circuits consist of Rs (the resistance of solution between working electrode and counter electrode), Cdl in parallel to the Rct, and Rct in series with the parallel inductive elements L and RL. The presence of L in the impedance spectra in the presence of investigated inhibitors indicated that iron is still dissolved by the direct charge transfer at the inhibitoradsorbed electrode surface.31The double layer usually behaves as a constant-phase element (CPE) rather than as a pure capacitor. The CPE is substituted for the capacitor to fit the semicircle more accurately. The admittance, YCPE, and impedance, ZCPE, of a CPE are expressed as:32 YCPE = Y0(jω)n

(11)

and ⎛1⎞ ZCPE = ⎜ ⎟[(j × 2πfmax )n ]−1 ⎝ Y0 ⎠

(12)

where Y0 is the amplitude comparable to a capacitance, j is the square root of −1, f max is the AC frequency at maximum and n, the phase shift (−1 ≤ n ≤ 1). When n = 0, the CPE represents pure resistor, if n = +1, the CPE represents pure capacitor, and if n = −1, then CPE represents an inductor.33 The impedance parameters, namely, charge-transfer resistance (Rct), polarization resistance (Rp), the constant phase element (Y0) related to the capacity of double layer, and the exponent n, relevant to the capacitive semicircle of the mild steel/HCl/CU system, are given in Table 4. Simulation of Nyquist and Bode plots with the used equivalent circuit model showed excellent agreement with experimental data (Figure 4a−d). Analysis of the impedance results (Table 4) reveals that the charge-transfer resistance value Rct increases more in the

Figure 3. (a) Nyquist plots for mild steel in 1 M HCl in the absence and presence of an optimum concentration of CUs at 308 K. (b) Equivalent circuit used to fit the EIS data for mild steel in 1 M HCl without an inductive loop. (c) Equivalent circuit used to fit the EIS data for mild steel in 1 M HCl with an inductive loop.

impedance spectra of mild steel in 1 M HCl solution in the absence and presence of an optimum concentration of CUs at

Table 4. Electrochemical Parameters Calculated form EIS Measurements for a Mild Steel Electrode in 1 M HCl in the Absence and Presence of an Optimum Concentration of CUs at 308 K inhibitors

Rp (Ω)

Rct (Ω cm2)

n

Y0 (Ω−1 sn/cm2)

Cdl (μF cm−2)

L (H·cm2)

RL (Ω cm2)

θ

η (%)


8.46 59.1 35.9 282 85.8

9.1 42 29 582 54

0.82 0.80 0.81 0.83 0.82

251 70 136 19 64

106 63.3 87.7 40.1 56.8

− 68.9 51.8 168 76.7

− 5.6 6.6 42.2 17.2

− 0.78 0.68 0.94 0.83

− 78.3 68.6 94.8 83.1

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Figure 4. Simulated and experimentally generated EIS Nyquist and Bode phase angle diagrams without (a, c) and with an inductive loop (b, d).

double layer, which suggest the substitution of H2O molecules (with higher dielectric constant) with inhibitor molecules (with lower dielectric constant), leading to a protective film on the electrode surface.34Using a Helmholtz model of the interface, the surface film capacitance is related to its thickness by the following expression:35 εε Cdl = 0 S (15) d

inhibited system than in the uninhibited system and reaches a maximum value of 182 Ω cm2 in the case of CU-3. A large Rct is associated with a slower corroding system because of the decrease in the active surface necessary for the corrosion reaction. The charge-transfer resistance is the difference of various resistances such as polarization resistance (Rp), solution resistance (Rs), accumulation resistance (Ra), and film resistance (Rf). The inhibition efficiency (η %) using Rct values was calculated from the following equation:34 ⎛ R ⎞ η% = ⎜⎜1 − ct ⎟⎟ × 100 R ct(i) ⎠ ⎝

where ε0 is the permittivity of free space (8.854× 10−12 F m−1), ε is the local dielectric constant of medium, S is the surface area of the electrode, and d is the thickness of protective layer. Figure 5a shows the Bode impedance magnitude and phase angle plots recorded for the mild steel electrode immersed in 1 M HCl in the absence and presence of an optimum concentration of CUs at its OCP. An ideal capacitive behavior would result if a slope value attains −1 and a phase angle value attains −90°.36 In the medium frequency zone, a linear relationship between log |Z| vs log f with a slope near −0.84 and the phase angle approaching −80° has been observed (Figure 5a; Table S3 in the Supporting Information). This is a characteristic response to capacitive behavior in the medium frequency zone. The slope values of Bode impedance magnitude plots in the medium frequency zone, G, and the maximum phase angle, α, exhibited aberration from the values of −1 and 90°, respectively. These aberrations are considered to be from the ideal capacitive response at intermediate frequencies. The Bode phase angle plots exhibit one time constant (single maximum) at intermediate frequencies, and broadening of this maximum in the presence of CUs accounts for the formation of a protective layer on the electrode surface.37 3.2.2. DC Technique: Polarization Results. The polarization resistance is the slope of the applied potential and the recorded current, similar to Ohm’s law:

(13)

where Rct (i) and Rct are the charge-transfer resistances in the presence and absence of CUs, respectively. The values of n (ranges from 0.80 to 0.83) did not vary significantly, suggesting the charge-transfer-controlled dissolution mechanism of mild steel in 1 M HCl with and without inhibitors. In Table 4, the double-layer capacitance (Cdl) values can be calculated from CPE parameter values Y and n using the following equation:35 Cdl =

Yωn − 1 sin(n(π /2))

(14)

where ω is the angular frequency, i.e., 2πf max. Addition of CUs to the corrosive solution decreases the double-layer capacitance (Cdl). The double layer between the charged metal surface and the solution is considered as an electrical capacitor. The adsorption of CUs on the mild steel surface decreases its electrical capacity because they displace water molecules and other ions originally adsorbed on the surface. The decrease in capacitance of an inhibited system may be imputed to the formation of a protective layer on the electrode surface.35The decreased Cdl values can result from the decrease of the local dielectric constant or increase in thickness of the electrical 14971



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compared with those in the uninhibited one, which suggests that mild steel corrosion is reduced in the inhibited system relative to the uninhibited system. The highest Rp value was obtained for CU-3 at 350 mg L−1. To get better information about the nature of a chemical compound as an anodic, cathodic, or mixed type inhibitor and its effect on the kinetics of the anodic and cathodic reactions, etc., polarization measurements have been carried out. Figure 5b depicts the cathodic and anodic curves of mild steel immersed in 1 M HCl at 308 K in the absence and presence of an optimum concentration of CUs. Electrochemical parameters evaluated such as corrosion potential (Ecorr), anodic Tafel slope (βa), cathodic Tafel slope (βc), and corrosion current density (Icorr) are listed in Table 5. The percentage inhibition efficiency was calculated using the following equation:39 ⎛ Icorr(i) ⎞ η% = ⎜1 − ⎟ × 100 Icorr ⎠ ⎝

where Icorr and Icorr(i) are the uninhibited and inhibited corrosion current densities, respectively. The accurate evaluation of the cathodic and anodic Tafel slopes as well as corrosion currents were obtained by the Tafel extrapolation of the current−potential lines to the corresponding corrosion potentials. The anodic Tafel slopes display slight deviation from linearity over the complete applied potential range, which may be attributed to deposition of corrosion products or impurities on the mild steel surface to form a nonlinear passive film.39 Figure 5b shows that, as would be expected, both anodic dissolution of iron and the cathodic hydrogen evolution reaction were retarded after the addition of CUs to the corrosive medium. The inhibition of these reactions is more pronounced in the presence of CU-3 while the corrosion potential (Ecorr) is slightly shifted to the cathodic direction. If the change in Ecorr value shifts beyond 85 mV, a chemical compound can be considered an anodic or a cathodic type inhibitor.40The largest shift of Ecorr was about 64 mV toward the cathodic direction (Figure 5b, Table 5). On the basis of these results, CUs are considered as mixed-type inhibitors. In other words, the addition of CUs to 1 M HCl solutions reduces the anodic dissolution of mild steel and also retards the cathodic hydrogen evolution reaction. The cathodic curves give rise to parallel lines, suggesting that the addition of CUs to the corrosive environment does not modify the hydrogen evolution reaction and that the reduction of H+ ions at the mild steel surface follows a charge-transfer mechanism. The adsorbed protective film of inhibitor on the mild steel surface impedes corrosion by blocking the reaction sites of the metal. In this way, actual surface area available for H+ ions is decreased while the actual reaction mechanism remains unaffected.41In anodic current−potential curves, at potentials greater than −0.250 V,

Figure 5. (a) Bode (log f vs log |Z|) and phase angle (log f vs α) plots of impendence spectra for mild steel in 1 M HCl in absence and presence of optimum concentration of CUs at 308 K. (b) Tafel polarization curves for mild steel in 1 M HCl in the absence and presence of an optimum concentration of CUs at 308 K.

RP =

dE dI

(16)

where Rp is polarization resistance, E is the potential, and I is the current density. Polarization resistance is also related to corrosion current density Icorr by the well-known Stern−Geary relation:38 RP =

β × βc 1 1 × a × 2.303 Icorr βa + βc

(17)

where βa and βc are the Tafel slopes of the anodic and cathodic corrosion reactions. The inhibition efficiency was calculated using polarization resistance as follows:38 ⎛ Rp ⎞ ⎟ × 100 η% = ⎜⎜1 − R p(i) ⎟⎠ ⎝

(19)

(18)

where Rp and Rp(i) are the polarization resistances of uninhibited and inhibited solutions, respectively. Table 5 indicates an increase in Rp values in the inhibited system

Table 5. Electrochemical Parameters Associated with Polarization Measurements of Mild Steel in 1 M HCl Solution in the Absence and Presence of an Optimum Concentration at 308 K Tafel polarization

linear polarization

inhibitors

Icorr (μA/cm )

Ecorr (mV/SCE)

βa (mV/dec)

βc (mV/dec)

θ

η (%)

Rp (Ω cm2)

θ

η (%)


892 196 271 34 99

−444 −488 −484 −508 −477

70 90 67 56 60

114 171 124 128 122

− 0.78 0.69 0.96 0.88

− 78.0 69.6 96.1 88.9

10.1 56 33 284 89

− 0.81 0.70 0.96 0.88

− 81.9 70.2 96.7 88.2

2

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Figure 6. SEM micrographs of mild steel surfaces: (a) abraded mild steel, (b) uninhibited 1 M HCl, and in the presence of (c) CU-1, (d) CU-2, (e) CU-3, or (f) CU-4.

film on the mild steel, scanning electron photographs were taken (Figure 6). Figure 6a shows that the surface of mild steel before placing in acid solution is absolutely free from any pits and cracks, but that scratches are clearly visible due to abrading treatment. The morphology of the specimen surface in Figure 6b shows a corroded surface in the absence of inhibitors, there are pits and cracks on the specimen’s surface, and the surface is strongly damaged. However, in the presence of CUs (Figure 6c−f), the surface corrosion of mild steel is remarkably decreased. Therefore, a smooth and much less corroded morphology of mild steel samples results from exposure to the inhibitor solutions. These results prove that the CUs can effectively protect mild steel samples from a corrosive environment. The specimen in the presence of CU-3 shows

the inhibition effect of CUs on mild steel dissolution is decreased slightly. This potential is defined as desorption potential, which means that the inhibition effect is dependent on electrode potential. In this case, the desorption rate of inhibitor is somewhat higher than its adsorption rate.41Table 5 shows that the values of corrosion current density (Icorr) are noticeably decreased in the presence of inhibitors compared with those in the absence of inhibitors and that the lowest value is obtained for CU-3, which suggests that the rate of electrochemical reaction was retarded because of formation of a protective film of inhibitor on the metal surface and that this protective film created a barrier between metal and corrosive medium. 3.3. Surface Analysis: SEM-EDX. To establish whether the corrosion inhibition is due to the formation of an adsorptive 14973



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Figure 7. EDX spectra of mild steel specimens: (a) abraded mild steel, (b) uninhibited 1 M HCl, and in the presence of (c) CU-1, (d) CU-2, (e) CU-3, or (f) CU-4.

3.4. UV−Visible Spectroscopy. The absorption of monochromatic light is a suitable method for identification of complex ions in solution; the absorption of light is proportional to the concentration of absorbing species in solution.42Because there is often a certain quantity of metal cation in the solution that is first dissolved from the metal surface, this procedure was conducted in the present work to confirm the possibility of the formation of [CU−Fe] complexes. The electronic absorption spectrum of CU-3 (Figure 8) solution before the steel immersion displays a main visible absorption band at 281 nm with a corresponding absorbance of 0.171. This band may be assigned to the π−π* transition involving the whole electronic structure system of the compound with a considerable chargetransfer character. After two days of steel immersion (Figure 8), the absorption band (λmax) underwent a bathochromic shift from 281 to 336 nm along with an increase in absorbance from 0.171 to 0.508, suggesting the interaction between CU-3 and Fe2+ in solution. A change in position of the absorption maximum (λmax) and/or a change in the value of absorbance

comparatively less corrosion and a smooth surface, indicating its strong protective film on the electrode surface. EDX spectra were used to determine the elements present on the metal surface before and after exposure to the inhibitor solution. The percentage atomic content of mild steel samples obtained from EDX analysis is listed in Table S4 in the Supporting Information. The EDX results are displayed in Figure 7. Figure 7a is the EDX spectrum of the abraded steel surface which shows the characteristics peaks of the elements constituting the mild steel sample. Figure 7b is the EDX spectrum of the uninhibited mild steel sample; the peak of oxygen is absent, which indicates the breakdown of the airformed oxide film and free corrosion of bare metal. However, for inhibited solutions (Figure 7c−f), the EDX spectra show an additional peak characteristic for the existence of N (due to the N atoms of the CUs). These data show that the inhibitor molecules containing N atoms have covered the mild steel surface. 14974



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3.5. Theoretical Study. Theoretical calculation was performed to find out the effects of structural changes on the ability of all four CUs to act as corrosion inhibitors. The following quantum chemical parameters were obtained and are listed in Table S5 in the Supporting Information: the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap ΔE = ELUMO − EHOMO, Mulliken charges on heteroatoms, and the dipole moment (μ). Figure 9 shows the optimized geometry of all four selected compounds. Frontier orbital theory is useful in predicting adsorption centers of the inhibitor molecules responsible for the interaction with surface metal atoms.46 Figure 10 shows the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the molecules under study. Figures 9 and 10 show that CUs have slightly different HOMO and LUMO distributions. HOMO density distributions are principally localized on pyrimidine and pyran rings, which might be due to the presence of N and O atoms with π-electrons in the inhibitor molecules. There is an electron transfer taking place during the adsorption of neutral organic molecules at the metal surface. The energy levels of the frontier molecular orbitals are significant for this transfer. EHOMO is often related to the molecule’s electron-donating ability. High values of EHOMO

Figure 8. UV−visible spectra of 1 M HCl solution containing 350 mg L−1 CU-3 before (black) and after (red) 2 days of mild steel immersion.

indicate the formation of a complex between the two species in solution as reported in the literature.43−45These experimental findings are strong evidence for the possibility of the formation of a complex between Fe2+ cation and CU-3 in 1 M HCl.

Figure 9. Optimized structures of CUs (ball and stick model). 14975



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Figure 10. The frontier molecular orbital density distributions of CUs: (a) CU-1 (left, HOMO; right, LUMO), (b) CU-2 (left, HOMO; right, LUMO) (c) CU-3 (left, HOMO; right, LUMO), (d) CU-4 (left, HOMO; right, LUMO).

probably indicate that a molecule is possibly inclined to donate electrons to acceptor molecules that are, of course, suitable to accept using low energy and empty molecular orbitals. Because of this, the energy of the lowest unoccupied molecular orbital

(ELUMO) refers to the suitability of the molecule to accept electrons. For an electron to be accepted by a molecule, it can be assumed that the lower the value of the ELUMO, the higher the possibility that the electron will be accepted.47 The higher 14976



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the value of EHOMO, the greater the trend of offering electrons to an unoccupied d-orbital of the metal and the higher the efficiency. The energy difference (ΔE) between LUMO and HOMO is described as the minimum energy required to excite an electron from the last occupied orbital in the molecule.48ΔE values are correlated to inhibition efficiencies determined experimentally for all the inhibitors. The lower the value of the energy difference, the higher the inhibition efficiency. The results for EHOMO and ΔE indicate that it is possible to get better performance with CU-3 as a corrosion inhibitor. The use of Mulliken charge analysis to estimate the adsorption centers of inhibitors has been reported in the literature, and the general consensus among several of these authors is that the more negatively charged the heteroatom, the more the ability to adsorb on the metal surface.48The N and O atoms have a larger negative charge in the case of all CUs (Table S5 in the Supporting Information). It was reported that higher values of dipole moment probably increase the adsorption between the chemical compound and the metal surface.49CU-3 has the highest dipole moment in comparison to the other three inhibitors, which supports its strong adsorption and greater inhibition on the mild surface. In conclusion, the theoretical investigation provides good support to the experimental results. 3.6. Explanation of Inhibition. Figure 11 depicts the inhibition mechanism that can be proposed from the

In our investigation, the value of Ecorr obtained in 1 M HCl is −444 mV vs SCE. So, when Ecorr − Eq=0 > 0, the steel surface charges are positive in HCl solution as reported in the literature.50−52 Because Cl− ions are already adsorbed on the metal surface, they favor more adsorption of protonated inhibitors on the metal surface through electrostatic interaction (physisorption). (II) The physically adsorbed protonated form of CU molecules in acid medium start competing with H+ ions for electrons on the mild steel surface. After release of H2 gas, the cationic form of the inhibitor returns to its neutral form, and heteroatoms with a free lone pair of electrons promote chemical adsorption.53 (III) The accumulation of electrons on the mild steel surface renders it more negative, and to relieve the metal from extra negative charge, an electron from the d-orbital of Fe might be transferred to a vacant π* (antibonding) orbital of the inhibitor molecule (retrodonation) and hence strengthen adsorption on the metal surface.

4. CONCLUSIONS The results showed that CUs have very good inhibition efficiency for the corrosion of mild steel in 1 M HCl and that their inhibition efficiency was more pronounced at an optimum concentration. The high inhibition efficiency of CUs was attributed to the adsorption of inhibitor molecules on the metal surface. The Tafel polarization curves indicated that CUs inhibit both anodic metal dissolution and the cathodic hydrogen evolution reaction. The adsorption of CU molecules on the metal surface from 1 M HCl solution follows the Langmuir isotherm. The SEM and EDX analyses showed that the addition of inhibitors to the corrosive solutions results in the formation of a protective film on the mild steel surface. The UV−visible studies clearly reveal the formation of a complex that may also be responsible for the observed inhibition. Theoretical calculations complemented well the experimental results.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Figure 11. Schematic depiction of adsorption and inhibition mode of CUs in 1 M HCl.

*Tel.: +91-9307025126. Fax: +91- 542- 2368428. E-mail: [email protected]; maquraishi@rediffmail.com.

adsorption of CUs on a mild steel surface. In general, owing to complex nature of adsorption and inhibition, it is impossible to explain by a single adsorption mechanism. In the studied inhibitors based on their chemical structures, they have various active sites for adsorption process. Thus, the following adsorption and inhibiting phenomena were proposed involving CU molecules on a steel surface: (I) Owing to neutral N atoms in CU molecules, they can be protonated in acid solution: [CU] + x H+ ↔ [CUHx]x +

AUTHOR INFORMATION

Corresponding Author

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.K.Y. gratefully acknowledges the University Grants Commission (UGC), New Delhi, India, for financial assistance.

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REFERENCES

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Thus, in acid solution the studied inhibitors exist as protonated species. The charge on the metal surface can be determined from the value of Ecorr − Eq=0 (zero charge potential). The Eq=0 of iron is −530 mV vs SCE in HCl. 14977



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