Metallic corrosion is attracting the attention of researchers. Researchers all over the world are searching for some methods to avoid corrosion damage. Using inhibitors is an effective method to reduce corrosion rate.1 An acid wash process is widely used in many industries in order to cleaning and descaling of steel substrates and in petrochemical processes.2,3 Hydrochloric acid is generally used in the pickling processes of metals and alloys. Because of the general aggressively of acid solutions, inhibitors are commonly used to reduce the corrosive attack on metallic materials as well as acid consumption. Organic compounds are effective inhibitors of aqueous corrosion of many metals and alloys. The influence of organic compounds containing nitrogen, oxygen and sulphur atoms on the corrosion of the steel in acidic solutions has been investigated by several authors.4-13 Such compounds can adsorb on the surface of the metal, blocking the active centers on the surface and thus reduces the corrosion rate. In literature, some heterocyclic compounds,14-17 Schiff bases18-20 and thiadiazoles21-23 have been reported as effective corrosion inhibitors for carbon steel in acidic media. The inhibition performance of organic inhibitors is due to physisorption and/or chemisorption on the surface of the metals.24,25 To characterize the adsorbed layer and the interaction between adsorbed species, adsorption isotherms are used.
Different methods including potentiodynamic polarization26 and electrochemical impedance27 techniques have been used to determine surface coverage (θ), inhibition efficiency (% IE) and the adsorption isotherm of inhibitor molecules in the corrosive media.
Synthesis of Inhibitors: Phenylthiourea
Phenylthiourea derivatives (1-5), were synthesized in the laboratory28 by reacting equimolecular mixture of the respective phenol and acetimidamide in 100 mL boiling ethanol. The mixture was refluxed on water bath for 2-6 h and the solution was evaporated to about its half volume, left to cool where crystals were separated out. These were filtered off, recrystallized from ethanol and finally dried in vacuum desiccators over anhydrous CaCl2. The structure of the compounds were confirmed by elemental analysis and IR and nuclear magnetic resonance (NMR) spectroscopy. The IUPAC name and structure of the five synthesized phenylthiourea derivatives are shown below:
All the chemicals chosen for our study were of analytical grade and double distilled water was used throughout the experiment. Further, the inhibitor solutions were prepared in ethanol to ensure the solubility. The corrosion tests were performed on carbon steel specimens with a composition (weight %) C: 0.200, Mn: 0.350, P: 0.024, Si: 0.003 and Fe balance. For electrochemical measurements, the exposed surface area of carbon steel specimens was 1.0 cm2. The surface pre-treatment was carried out by polishing with 600, 800 and 1200 grit emery paper, followed by washing with double distilled water and finally degreased with acetone and dried at room temperature. 2 M HCl solutions were prepared from an analytical reagent grade of 37% HCl and double distilled water and were used as corrosive media in this study. The experiments were performed at 30±1 ℃.
Cathodic and anodic polarization curves were recorded in the potential region -800 to +800 mV(SCE) for different concentrations of investigated phenylthiourea derivatives at a sweep rate 2 mVs-1 using Gamry Potentiostat/Galvanostat/ZRA (Model PCI4/300). Polarization curves in the vicinity of OCP were analyzed to determine the corrosion parameters using Gamry framework/analysis DC105 corrosion software. The corrosion inhibition efficiency (% IE) was evaluated from the measured icorr values using the following equation:
where: icorr and icorr are the corrosion current densities in the presence and absence of the inhibitor, respectively.
Prior to the electrochemical measurements, a stabilization period of 30 min was allowed to attain a steady state and the open circuit potential (OCP) was noted. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain corrosion current densities (icorr).
Electrochemical Impedance Spectroscopy (EIS)
EIS experiments were conducted over a frequency range of 100 kHz to 1 Hz, with a signal amplitude perturbation of 10 mV. All impedance data were fitted to an appropriate equivalent circuit using Potentiostat/Galvanostat/ZRA analyzer (Gamry PC4 300) provided with a personal computer with EIS300 software and Echem Analyst 5.21 for fitting the data and calculations. Nyquist plots were obtained from the results of these experiments. Values of the charge transfer resistance (Rct) were obtained from these plots by determining the difference in the values of impedance at low and high frequencies, as suggested by Abdel Rehim et al.29 Values of the double layer capacitance (Cdl) were calculated from the frequency at which the impedance imaginary component (-Z”) was maximum, using the following equation:
% IE was calculated using the following equation:
where Roct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively.
Molecular orbital calculations (MOC) are based on the semi-empirical self-consistent methods (SCF). A full optimization of all geometrical variables (bond lengths, bond angles and dihedral angles) without any symmetry constraint was performed at the restricted Hartree-Fock level (RHF).30 We used MNDO semi-empirical SCF-MO methods in the MATERIALS STUDIO31,32 DMol3 version 4.3, implemented on an Intel Pentium IV, 3.6 GHz computer.
RESULTS AND DISCUSSION
The polarization curves for carbon steel in 2 M HCl in the absence and presence of compound (1) at 30 ℃ at different concentrations are presented in Fig. 1. Similar curves were obtained for other compounds (not shown). All phenylthiourea derivatives show relatively similar behavior in polarization testing. Fig. 1 shows that inhibitor (1) do not change the profile of the anodic and cathodic curves, indicating that it blocks the reaction sites of carbon steel without affecting the anodic and cathodic mechanisms.33 The obtained potentiodynamic polarization parameters including corrosion current density (icorr), corrosion potential (Ecorr), cathodic (bc) and anodic (ba) Tafel slopes, polarization resistance (Rp), degree of surface coverage (θ) and the inhibition efficiency (IE) are listed in Table 1.
Fig. 1.Polarization curves for carbon steel in 2 M HCl containing different concentrations of inhibitor (1) at 30 ℃.
Table 1.Kinetic parameters of carbon steel in 2M HCl containing various concentrations of inhibitor (1) and 11×10-6 M inhibitors (2-5) at 30 ℃
The results of Table 1 show that the inhibition efficiency of these inhibitors increases and corrosion current density decreases with increasing inhibitor concentration. The corrosion inhibition efficiency of phenylthiourea derivatives decreases in the order: 1>2>3>4>5.
With the inhibitors, both anodic and cathodic current densities are reduced, the change of Tafel slopes upon addition of inhibitors, which means that the inhibitor molecules are adsorbed on both anodic and cathodic sites, and the slight change of Ecorr, all these suggested that these compounds behave as mixed type inhibitors.
The higher values of Tafel slope can be attributed to surface kinetic process rather the diffusion-controlled process.34 The parallel of cathodic and anodic Tafel lines obtained from the electrochemical measurements indicates that the addition of these derivatives did not modify the mechanism of this process.30 The order of decreased inhibition efficiency obtained from this method for investigated additives is: 1>2>3>4>5.
To further clarify the modes of inhibitor adsorption, experiments were conducted in the presence of KSCN, KI and KBr (X-) ions, which are strongly adsorbed on the surface of carbon steel in acidic solution and facilitate adsorption of organic cation-type inhibitors by acting as intermediate bridges between the positive end of the organic cation and the positively charged metal surface. Specific adsorption of these anions on the metal surface leads to a recharging of the electrical double layer. The inhibitor is then drawn into the double layer by electrostatic interaction with the adsorbed X- ions, forming ion pairs on the metal surface which increases the degree of surface coverage
Table 2.Kinetic parameters of carbon steel in 2 M HCl containing various concentrations of inhibitors and in the presence of 1×10-2 M KI at 30 ℃
The effect of addition 1×10-2 M KSCN, KI and KBr on the corrosion rate of carbon steel in the absence and presence of different concentrations of inhibitors (1-5) in 2 M HCl solutions was investigated using potentiodynamic polarization method. Results of the kinetic parameters in presence of anions (X-) are summarized in Table 2. It was observed from these results that these additives improved the %IE significantly. The interactions of these additives with the inhibitor molecules can be described by introduction of the synergistic parameter (Sθ) which is defined as:35
where θ1+2 = (θ1 + θ2) - (θ1θ2), θ1 = surface coverage by anion (1 M HCl + 1×10-2 M X- only), θ2 = surface coverage by cation (1 M HCl + Inhibitor only) and θ'1+2 = measured surface coverage by both the anion and cation (1 M HCl + X- + inhibitor). Sθ approaches 1 when no interaction between the inhibitor compounds exists, while Sθ > 1 points to a synergistic effect. In the case of Sθ < 1, the antagonistic interaction prevails. Values of Sθ summarized in Tables 3 are more than unity, suggesting that the phenomenon of synergism exists between the inhibitors and these additives. Larabi et al.36 has proposed two kinds of joint adsorption (competitive and cooperative) to explain the synergistic action observed between anion and cation. 37 For competitive adsorption, the anion and cation are adsorbed at different sites on the electrode surface and for cooperative adsorption, the anion is chemisorbed on the surface and the cation is adsorbed on the layer of the anions. The two types of adsorption are represented schematically in Fig. 2 and can be characterized by a synergistic factor (Sθ) calculated as above. So, the synergistic inhibitive effect is due to the combination of the inhibitor molecules with KSCN, KI and KBr as explained before.
Table 3.Synergism parameter (Sθ) for different concentrations of investigated inhibitors for carbon steel dissolution in 2 M HCl with addition of 1×10-2 M KI at 30 ℃
Fig. 2.Schematic representations of (a) competitive and (b) cooperative adsorption of the anions (X-) and cations (A+) on carbon steel surface in acid solutions.
The nature of corrosion inhibition has been deduced in terms of the adsorption characteristics of the inhibitor. Metal surface in aqueous solution is always covered with adsorbed water dipoles. Therefore adsorption of adsorbate on a metal surface is regarded as a substitutional adsorption process between the organic molecule in the aqueous solution (Orgsol), and water molecules adsorbed on metallic surface (H2Oads).37
A correlation between surface coverage (θ) defined by % IE/100 and the concentration of inhibitor (Cinh) in electrolyte can be represented by Temkin adsorption isotherm,
where K is the equilibrium constant of the adsorption reaction and ‘a’ is the lateral interaction term describing the interactions in the adsorption layer and the heterogeneity of the surface (it is a measure for the steepness of the adsorption isotherms). Surface coverage values (θ) for the inhibitors were obtained from the polarization measurements for various concentrations.
The best fitted straight line is obtained for the plot of Θ versus log C. The correlation coefficient (R2 > 0.98) was used to choose the isotherm that best fit experimental data. This suggests that the adsorption of these compounds on metal surface followed the Temkin adsorption isotherm (Fig. 3). From the intercepts of straight lines, K values were calculated and are given in Table 4. The most important thermodynamic adsorption parameters are the free energy of adsorption (ΔGoads), the adsorption constant (K) is related to the standard free energy of adsorption, ΔGoads, with the following equation38:
where 55.5 is the water concentration of solution in ml/l.39 The standard free energy of adsorption (ΔGoads) values were calculated and given in Table 4. The negative values of ΔGoads indicate the stability of the adsorbed layer on the steel surface and spontaneity of the adsorption process. Generally, values of ΔGoads up to -20 kJ mol-1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than -40 kJ mol-1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorptions).40 The values of ΔGoads obtained were approximately equal to -59.2 1 kJ mol-1, indicating that the adsorption mechanism of the phenylthiourea derivatives on carbon steel in 2 M HCl solution involves both electrostatic adsorption and chemisorptions. The thermodynamic parameters point toward both physisorption (major contributor) and chemisorptions (minor contributor) of the inhibitors onto the metal surface. The Kads values follow the same trend. Large values of Kads imply better efficient adsorption and hence better inhibition efficiency.41
Table 4.Inhibitor equilibrium constant (K), free energy of adsorption (ΔGoads), number of active sites (1/y) and the interaction parameter (a) for investigated inhibitors at 30 ℃.
Fig. 3.Temkin adsorption plots for carbon steel surface in 2 M HCl solution containing different concentrations of investigated inhibitors at 30 ℃.
Fig. 4.Curve fitting of corrosion data for carbon steel in 2 M HCl containing different concentrations of investigated inhibitors corresponding to kinetic model.
On the other hand, it was found that Kinetic-thermodynamic model of El-Awady et al.42 which has the formula:
Can be applied and verify the present adsorption data (Fig. 4). The equilibrium constant of adsorption K = K'(1/y), where 1/y is the number of the surface active sites occupied by one phenylthiourea molecule and C is the bulk concentration of the inhibitor.
Effect of Temperature
Corrosion reactions are usually regarded as Arrhenius processes and the rate (k) can be expressed by the relation:
where E*a is the activation energy of the corrosion process R is the universal gas constant, T is the absolute temperature and A is a Arrhenius pre-exponential constant depends on the metal type and electrolyte. Arrhenius plots of log k vs. 1/T for carbon steel in 2 M HCl in the absence and presence of 9×10-6 M of inhibitors are shown graphically in Fig. 5. The variation of log k vs. 1/T is a linear one and the values of Ea obtained are summarized in Table 5. These results suggested that the inhibitors are similar in their mechanism of action. The increase in E*a with the addition of inhibitors, indicates that the energy barrier for the corrosion reaction increases. It is also indicated that the whole process is controlled by surface reaction, since the activation energy of the corrosion process is over 20 kJ mol-1.43 Enthalpy and entropy of activation (ΔH*, ΔS*) are calculated from transition state theory using the following equation:44
Fig. 5.Arrhenius plots for carbon steel corrosion in 2 M HCl containing 9×10-6 M of inhibitors.
where h is Planck’s constant, N is Avogadro’s number. A plot of log k/T vs 1/T also gaves straight lines as shown in Fig. 6 for carbon steel in 2 M HCl in the absence and presence of 9×0-6 M concentration of inhibitors. The slopes of these lines equal to ΔH*/2.303R and their intercepts equal to [log RT/Nh + (ΔS*/2.303R)] from which the values of ΔH* and ΔS* can be calculated and are tabulated in Table 5. From these results, it is clear that the presence of these compounds increases the activation energy values and consequently decreased the corrosion rate of the carbon steel. ΔH* values reflect the strong adsorption of these compounds on carbon steel surface. ΔS* values in the presence of these investigated compounds are larger and negative; this indicates that the activated complex in the rate determining step represents an association rather than dissociation step, meaning that a decreases in disordering takes place on going from reactants to the activated complex and the activated molecules were in higher order state than that at the initial state.45,46
Table 5.Activation parameters for carbon steel corrosion in 2 M HCl acid solution containing 9×10-6 M of inhibitors
Fig. 6.Transition state plots for carbon steel corrosion in 2 M HCl containing 9×10-6 M of inhibitors.
Electrochemical Impedance Spectroscopy
The corrosion behavior of mild steel in 1 M HCl solution in the presence of various concentrations of compound (1) was investigated by EIS at 30 ℃ after 30 min of immersion. Fig. 7 shows the results of EIS experiments in the Nyquist representation. After analyzing the shape of the Nyquist plots, it is concluded that the curves approximated by a single capacitive semi-circles, showing that the corrosion process was mainly charge transfer controlled.47 The fact that impedance diagrams have an approximately semicircular appearance shows that the corrosion of carbon steel is controlled by a charge transfer process. Small distortion was observed in some diagrams, this distortion has been attributed to frequency dispersion.48 The general shape of the curves is very similar for all samples, indicating that almost no change in the corrosion mechanism occurred due to the inhibitor addition.49 The diameter of Nyquist plots increases on increasing the inhibitor concentration. These results suggested that the inhibition efficiency increases by increasing inhibitor concentrations. The Nyquist plots are analyzed in terms of the equivalent circuit composed with classic parallel capacitor and resistor (shown in Fig. 8).50 EIS parameters and % IE were calculated and tabulated in Table 6 for inhibitor 1. It was observed from the obtained EIS data that the charge transfer resistance (Rct) increases and double layer capacitance (Cdl) decreases with the increasing of inhibitor concentrations. The increase in Rct values, and consequently of inhibition efficiency, may be due to the gradual replacement of water molecules by the adsorption of the inhibitor molecules on the metal surface to form an adherent film on the metal surface, this suggests that the coverage of the metal surface by the film decreases the double layer thickness. Also, the decrease of Cdl means that the adsorption of inhibitor takes place on carbon steel surface in acidic solution. The results indicate good agreement between the values of corrosion efficiency as obtained from the impedance technique and polarization measurements. It was concluded that the corrosion rate depends on the chemical nature of the electrolyte rather than the applied technique.51
Fig. 7.Nyquist plots of carbon steel in 2 M HCl containing various concentrations of inhibitor (1).
Fig. 8.Electrical equivalent circuit used to fit the impedance data.
Table 6.Electrochemical kinetic parameters obtained from EIS technique for the corrosion of carbon steel in 2 M HCl at different concentrations of inhibitor (1) at 30 ℃
Quantum Chemical Study
It had been reported that the energy of highest occupied molecular orbital (EHOMO) often associated with the electron donating ability of the molecules. High values of EHOMO indicate a tendency of the molecule to donate electrons to act with acceptor molecules with low-energy, empty molecular orbital. Similarly, the energy of lowest unoccupied molecular orbital (ELUMO) represents the ability of the molecule to accept electrons. The lower value of ELUMO suggests the molecule accepts electrons more probable. 52 The calculated quantum chemical indices, EHOMO, ELUMO, energy gap (ΔE) and dipole moment (μ), of investigated compounds are calculated and are shown in Table 7. Inhibition efficiency increases with increasing values of EHOMO, dipole moment and with decreasing values of ELUMO. The results seem to indicate, that charge transfer from the inhibitor takes place during the adsorption to the metal surface. Increasing values of EHOMO and may facilitate adsorption (and therefore inhibition) by influencing the transport process through the adsorbed layer.53 Similar relations were found between the inhibition efficiency and the energy gap ΔE.51 Lower values of the energy gap will render good inhibition, because the energy to remove an electron from the last occupied orbital will be low.
Table 7.The calculated quantum chemical parameters for investigated inhibitors
The dipole moment is another way to obtain data on electronic distribution in a molecule and is one of the properties more used traditionally to discuss and rationalize the structure and reactivity of many chemical systems.54 The values of EHOMO show the relation 1>2>3>4>5 for this property. In addition, the values of the energy gap ΔE show the relation 5>4>3>2>1 for this property. The results of Table 7 show that the values of μ (dipole moment) decreases in the following order: 5>4>3>2>1. Some authors showed that an increase of the dipole moment leads to decrease of inhibition and vice versa, suggesting that lower values of dipole moment will favor accumulation of inhibitor in the surface layer.55 In contrast, the increase in the dipole moment can lead to increase of inhibition and vice versa,56,57 which could be related to the dipole-dipole interaction of molecules and metal surface. The higher the value of μ obtained is coherent with the second explanation indicating stronger dipole-dipole interactions of inhibitors molecules and metallic surface.
Mechanism of Corrosion Inhibition
From the previous results, it was concluded that the investigated inhibitors (1-5) inhibit the corrosion of carbon steel in 2 M HCl by adsorption at the metal/solution interface. Compound (1) exhibits excellent inhibition power due to: (i) the presence of p-OCH3 group which is an electron donating group with negative Hammett constant (σ = -0.27), which enhance the delocalized π-electrons on the active centers of the compound and may serve as adsorption centre, (ii) its larger molecular size that may facilitate better surface coverage, and (iii) its adsorption through three active centers. Compound (2) comes after compound (1) in inhibition efficiency due to: (i) its lower molecular size than compound (1) (ii) the presence of two active adsorption centers only and (iii) the presence of p-CH3 (σ = -0.17) with low Hammett constant than p- OCH3 which enhances the delocalized π-electrons on the active centers of the compound. Compounds (3) and (4) come after compound (2) in percentage inhibition efficiencies. This is due to the presence of p-Br and p-Cl groups which are electron withdrawing groups with positive Hammett constants (σBr = + 0.232, σCl = + 0.227) and their order of inhibition depends on the magnitude of their withdrawing character and their molecular size. Compound (5) comes after compound (4) in inhibition efficiency inspite of it has three active centers, because it has lower molecular size than compound (4) and has no substituents in the p-position which contributes no charge density to the molecule.
1) Phenylthiourea derivatives are good inhibitors for corrosion of carbon steel in 2 M HCl and the inhibition efficiency was more pronounced with increase in the inhibitor concentration.
2) The potentiodynamic polarization curves indicate that derivatives act as mixed type inhibitors.
3) The adsorption of phenylthiourea derivatives on carbon steel surface from 2 M HCl solution obeys Temkin adsorption isotherm.
4) The inhibition efficiency of phenylthiourea derivatives increases by increasing the inhibitor concentration and decreases with increasing the temperature.
5) Addition of KI, KBr and KSCN to 2 M HCl solution containing phenylthiourea derivatives enhances the inhibition efficiencies markedly. The combination of inhibitors and these anions shows strong synergistic effects at experimental conditions.
6) The results obtained from polarization and impedance measurements are in good agreement.