Submitted for publication: August 6, 2011. Revised and accepted: December 3, 2011. Preprint available online: May 16, 2012, http://dx.doi.org/10.5006/0508.
‡ Corresponding author. E-mail: [email protected]. * Materials and Corrosion Laboratory, Department of Chemistry,
Faculty of Science, Taif University, 888 Hawiya, Saudia Arabia. ** Department of Chemistry, Faculty of Science, Ain Shams Univer-
Nickel Corrosion Inhibition in Sulfuric Acid—Electrochemical Studies, Morphologies, and Theoretical Approach
M.A. Amin,‡,*,** H. Shokry,***,**** and E.M. Mabrouk***
AbstrAct
The inhibition performance of three selected dihydrazide derivatives, namely, malonic acid (MAD), succinic acid (SAD), and adipic acid (AAD) dihydrazide, was tested in relation to nickel corrosion in 1.0 M sulfuric acid (H2SO4) solution. Electrochemical methods (Tafel polarization, linear polariza-tion resistance [LPR], and electrochemical impedance spectros-copy [EIS]) were used, complemented with scanning electron microscopy/energy-dispersive x-ray (SEM/EDX) examinations. Computational studies were also used to confirm experimental findings and to optimize the adsorption structures of dihydra-zide derivatives. Results showed that the three tested dihy-drazides inhibited Ni corrosion (mixed-type inhibitors) to an extent, depending on the type and concentration of the intro-duced inhibitor. SEM studies revealed that the corroded areas on the surface were decreased in the presence of additives to an extent, depending on the type and concentration of the tested inhibitor. Results obtained from electrochemical mea-surements are in good agreement with theoretical studies.
The adsorption of inhibitor molecules on surfaces has recently become the subject of intensive investigation in the corrosion field because of the wealth of infor-mation that can be obtained.1-3 Understanding how an inhibitor molecule behaves near a metal surface will greatly enhance the ability to control the essen-tial interfacial properties in a wide variety of corrosion problems. Several computational and electrochemi-cal methods have been used to study the behavior of inhibitors for different metals.4 The most effective inhibitors are those compounds containing heteroat-oms, like nitrogen, oxygen, sulfur, and phosphorus. The inhibitory activity of these molecules is accom-panied by their adsorption to the metal surface. Free electron pairs on heteroatoms or p electrons are read-ily available for sharing to form a bond and act as nucleophile centers of inhibitor molecules and greatly facilitate the adsorption process over the metal sur-face, whose atoms act as electrophiles. Recently, the effectiveness of an inhibitor molecule has been related to its spatial as well as electronic structure.5
Furthermore, the efficiency of an organic inhibi-tor of metallic corrosion does not only depend on the structural characteristics of the inhibitor but also on the nature of the metal and environment. The selec-tion of a suitable inhibitor for a particular system is a difficult task because of the selectivity of the inhibi-tors and a wide variety of environments.
Quantum chemical calculations have been used widely to study reaction mechanisms.6 They also have been proven to be a very powerful tool for studying
inhibition of the corrosion of metals.7-8 It has been found that the effectiveness of a corrosion inhibitor can be related to its electronic and spatial molecular structure.9-11 The corrosion behavior of nickel in acid baths in plating, electrowining, and pickling process is of industrial concern. The inhibiting action of alkyl triphenylphosphonium iodine salt toward the corro-sion behavior of nickel in 1.0 M sulfuric acid (H2SO4) solution has been studied.12 This compound was found to retard both the anodic and cathodic reac-tions of nickel corrosion.
The goal of this work was to study the inhibi-tion characteristics of three dihydrazide derivatives on Ni corrosion in 1.0 M H2SO4 solutions using electro-chemical methods. Explicit solvent simulations using molecular dynamics and quantum chemical calcula-tions were used to explore the adsorption mechanism of the tested inhibitors on Ni (100). Also, since the electronic structure of dihydrazide could be involved in determining interactions with the Ni surface, the correlation between molecular orbital calculations and inhibitor efficiencies also has been sought.
computAtIonAl detAIls
For quantum chemical calculations, the study was carried out using Dewar’s linear combinations of atomic orbitals–self-consistent field–molecular orbital (LCAO–SCF–MO)13-14 ab initio with basis set STO-3G method with commercially available quantum chemi-cal software HyperChem, Release 8.0†. A full optimiza-tion of all geometrical variables without any symmetry constraint was performed at the Restricted Hartree-Fock (RHF) level.15-16 This develops the molecular orbitals on a valence basis set and calculates elec-tronic properties, optimized geometries, and total energy of the molecules. As an optimization proce-dure, the built-in Polak-Ribiere algorithm, was used.17
For the adsorption of dihydrazide on the nickel surface we carried out molecular mechanics calcu-lations18 with the commercial software package. The MM+ force field parameter sets were used, which can be described by the following general Expression (1):
E E E E Ebondstretching bondbending diheadral
= + + + +∑ ∑ ∑ooutofplane nonbonded Coulomp
E E∑ ∑ ∑+ (1)
The MM+ force field parameter sets have higher than quadratic terms for the bond energies and the angle calculations. Furthermore, for the Coulomb interaction it uses dipoles instead of point charges. The non-bonding van der Waals term has an exponen-tial form.
For the optimization of the individual molecules, all energies described in Equation (1) are used to find the energetically best configuration. Since the self-assembly of inhibitor molecules does not lead to
chemical bonding between the molecules and Ni sur-face, the optimization of the adsorption geometry is only governed by the non-bonding forces included in the last two terms. These involve van der Waals forces, electrostatic Coulomb forces, and hydrogen bonding terms.
The behavior of the three selected inhibitors on the surface was studied using molecular dynam-ics (MD) simulations and the COMPASS force field. COMPASS stands for condensed phase-optimized molecular potentials for atomistic simulation stud-ies. COMPASS is an ab initio powerful force field that supports atomistic simulations of condensed phase materials.19 Most parameters were derived based on ab initio data. The MD calculation of the simulation of the interaction between the dihydrazide deriva-tives dissolved in H2O and the Ni surface (100) was carried out in a simulation box (30 by 30 by 29.9 A°) with periodic boundary conditions to simulate a rep-resentative part of an interface devoid of any arbitrary boundary effects.
A cutoff distance of 1.0 nm with a spline switch-ing function was applied for the non-bond inter-actions, i.e., for Coulombic, van der Waals, and hydrogen bond interactions. The cutoff distance speci-fies the distance at which to exclude interactions from the non-bond list. The spline-switching function is used to select the spline width that specifies the size of the region within which non-bond interactions are splined from their full value to zero. For the actual computation of this interaction energy, charge groups are used. The nickel crystal is cleaved along with the (100) plane, thus representing the nickel surface. The system was simulated in water solution with a density of 1.0 g cm–3.
The Ni crystal is cleaved along with the (100) plane, thus representing the nickel surface. For the MD simulation, all the spatial positions of the nickel atoms in the simulation box are fixed because the thermal vibrations of the interaction with an adsorbed molecule are not in the physical behavior of the crys-tal itself. The liquid phase consists of 600 H2O mole-cules and a single dissolved inhibitor molecule. Before each MD run, the inhibitor molecule was located close to the nickel surface because the time scale of such a diffusion process would exceed the time scale of the whole simulation.
experImentAl procedures
Nickel in the form of wire (99.99%) was fixed in glass tubing, such that only a surface area of 0.3 cm2 was exposed to the test solution. Prior to each experi-ment the nickel electrode was grinded with different grit size emery papers up to 4/0 grit size to remove the corrosion products, if any, formed on the surface. The nickel electrode was cleaned in water (purified by a Millipore Milli-Q† system) in an ultrasonic bath for † Trade name.
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5 min, subsequently rinsed in acetone (CH3COCH3) and bi-distilled water, and immediately immersed in the test solution.
The selected inhibitors, namely, malonic acid (MAD), succinic acid (SAD), and adipic acid (AAD) dihydrazides, were obtained and added to the 1.0 M H2SO4 without pretreatment at concentrations of 10–3, 5 × 10–3, 10–2, 5 × 10–2, and 10–1 M. The electrolytes were prepared from water purified by a water purifi-cation system. The electrode was immersed in these solutions for 1 h before starting measurements; this was the time necessary to reach a quasi-stationary value for the open-circuit potential.
MAD SAD AAD
Electrochemical experiments were performed in a 100 mL volume borosilicate glass cell using Pt wire and a saturated calomel electrode (SCE) as auxil-iary and reference electrodes, respectively. The SCE was connected via a Luggin capillary, the tip of which was very close to the surface of the working electrode to minimize the IR drop. All potentials given in this paper are referred to this reference electrode. Tafel extrapolation, linear polarization resistance (LPR), and impedance methods have been used to evalu-ate the degree of protection against corrosion sup-plied by the three selected inhibitors, as well as to obtain information about the mechanism of the inhi-bition process. The cathodic and anodic polarization curves were recorded using EG&G model 173† poten-tiostat/galvanostat and digital potentiometer type C.G.822. Cathodic and anodic polarization curves were determined by the potentiostatic method using 10 mV increments of potential, and steady-state cur-rents were observed within 20 min at each applied potential. The linear Tafel segments of the cathodic and anodic curves were extrapolated to the corro-sion potential to obtain the corrosion current densi-ties (jcorr).
LPR and impedance measurements were per-formed using an Autolab† frequency response analyzer (FRA) coupled to an Autolab PGSTAT30† potentiostat/galvanostat with an FRA2 module connected to a per-sonal computer. For LPR measurements, a sweep from –20 mV to 20 mV vs. open-circuit potential at a sweep rate of 0.2 mV s–1 was used, and the polar-ization resistance (Rp) was measured from the slope of η vs. j curve in the vicinity of corrosion potential. Impedance measurements were carried out using alternating current (AC) signals of amplitude 5 mV peak-to-peak at the open-circuit potential in the fre-quency range 50 kHz to 10 mHz. All impedance data
were fitted to appropriate equivalent circuits using the computer program EQUIVCRT.20
Morphologies of the electrode surface were exam-ined using scanning electron microscopy/energy-dis-persive x-ray (SEM/EDX) in 1.0 M H2SO4 solutions as a function of each inhibitor concentration, after an immersion time of 6 h, using an analytical scanning electron microscope, JEOL JSM 6390 LA†.
results And dIscussIon
Electrochemical Measurements Tafel Polarization Measurements — The nature of
the inhibition process is established on the basis of Tafel polarization measurements. Therefore, changes observed in the polarization curves after inhibitor addition are used usually as criteria to classify the inhibitor as cathodic, anodic, or mixed.21-22 Figure 1 depicts the cathodic and anodic partial polarization curves recorded for Ni in 1.0 M H2SO4 solutions with-out and with various concentrations of MAD, SAD, or AAD at 25°C. For the electrode without inhibitor, a linear relation was observed between potential E and log (j) in the active dissolution region, with an appar-ent anodic Tafel slope of 67 mV decade–1. The kinet-ics of the anodic dissolution of Ni in acid solutions involves uniform dissolution with the reaction path:23
Ni H O NiOH H e+ ↔ + ++2
– (2)
NiOH H Ni H O+ ↔ ++ +2 (3)
Ni Ni e+ +↔ +2 – (4)
which considers the water adsorption process in the mechanism of electrooxidation. On the negative side, the H3O
+ reduction (Reaction [5]) is the main reaction occurring on the electrode surface.
2 2 23 2 2H O e H O H+ + ↔ +– (5)
It is obvious that the cathodic branch is under activation control and exhibits linearity in accordance with the Tafel relationship.
The modifications caused by the addition of any of the three tested inhibitors are a marked increase in the cathodic and anodic overpotentials. This increase in the cathodic and anodic overpotentials depends on the type and concentration of the introduced inhib-itor. The presence of MAD, SAD, or AAD in 1.0 M H2SO4 solutions therefore inhibits both the anodic and cathodic processes to an extent depending on their concentration. No definite shift was observed for the corrosion potential (Ecorr).
It is obvious that the shapes of the polarization plots for inhibited electrodes are not substantially dif-ferent from those of uninhibited electrodes. The pres-ence of inhibitor decreases the corrosion rate but does
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not change other aspects of the behavior. This means that the inhibitor does not alter the electrochemical reactions responsible for corrosion. In addition, the absence of significant changes in the cathodic Tafel slope (βc) in the presence of the inhibitor indicates that the hydrogen evolution is slowed down by the surface-blocking effect of the inhibitor. This indicates that the inhibitive action of the three tested inhibi-tors may be related to their adsorption and formation of a barrier film on the electrode surface. SEM/EDX examinations of the electrode surface confirmed the existence of a film of inhibitor protecting Ni against corrosion.
Extrapolation of Tafel lines is one of the most popular direct current (DC) techniques for the esti-mation of the corrosion rate. It is obvious from the polarization plots of the present work that the anodic and cathodic branches of the polarization curves dis-play Tafel behavior within the studied potential range for all cases. This makes an accurate evaluation of the corrosion rate and the anodic and cathodic Tafel slopes (βa and βc). The values of the jcorr, and the other electrochemical parameters, for Ni corrosion reaction without and with inhibitors were determined; there-fore, for extrapolation of the cathodic and anodic Tafel lines to the Ecorr, see Table 1.
Since the corrosion rate (υ) is directly propor-tional to the value of the corrosion current density, the inhibition efficiency, ITafel(%), was evaluated from the measured jcorr values using Equation (6):
I j jTafel corr corr(%) [ ( / )]= − ×1 1000 (6)
where j0corr and jcorr are the corrosion current densities for uninhibited and inhibited solutions. The recorded jcorr values were introduced in Equation (7) to obtain the rates of corrosion, υTafel (µm y–1):
υTafel corrj M nd= 3 280, ( / ) (7)
where M is the atomic weight of Ni (58.69 g mol–1), n is the number of electrons transferred in the corrosion reaction (n = 2), and d is the density of Ni (8.902 g cm–3). Rates of corrosion were computed and listed in Table 1 as a function of each inhibitor concentration. A decrease in the corrosion rate, corresponding to an increase in the inhibition efficiency, can be clearly noticed in all cases. These events were found to be a function of each inhibitor concentration. The tested compound designated AAD presented the lowest rates of corrosion and the highest inhibition efficiencies among the tested compounds at any given concentra-tion. These findings indicate that AAD inhibits Ni cor-rosion in H2SO4 more effectively than MAD and SAD.
Linear Polarization Resistance Method — Since the electrochemical theory assumes that Rp
–1 is directly proportional to the corrosion rate, the inhibition effi-ciencies, ILP(%), of the tested inhibitors were calculated
FIGURE 1. Cathodic and anodic partial polarization curves recorded for nickel in 1.0 M H2SO4 solutions without and with various concentrations of (a) MAD, (b) SAD, or (c) AAD at 25°C.
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from (Rp) values obtained from LP data as a function of each inhibitor concentration (Table 1), using the following equation:
I R Rp po(%) [ – {( )/ ( )}]– –= ×1 1001 1
(8)
where Rop and Rp are the polarization resistance val-
ues without and with the addition of inhibitor, respec-tively.
It follows from Table 1 that the values of Rp increase with an increase in the concentration of the three tested inhibitors. The increase in the Rp value suggests that the inhibition efficiency increases with the increase in the inhibitor concentration (inspect ILP[%] values presented in Table 1). Inhibition effi-ciency values obtained from the LPR method agree with those obtained from the Tafel extrapolation method. Here again, AAD is found to be the most effective inhibitor. The mechanism of the corrosion inhibition process will be discussed after the imped-ance measurements have been presented.
Electrochemical Impedance Spectroscopy Mea-surements — The interfaces Ni/H2SO4 with and with-out the three tested inhibitors were characterized by means of impedance measurements. Figure 2 depicts the Nyquist plots obtained from the electrochemi-cal impedance spectroscopy (EIS) measurements for Ni in 1.0 M H2SO4 at the corrosion potential after an immersion time of 1.0 h at 25°C. The plots are char-acterized by the existence of two capacitive loops; the
first was small in size and observed at high frequen-cies with a diameter (resistance) R1, and the other one was noticed at medium- and low-frequency domains with a high resistance, R2. Such interface could be described by the equivalent electrical circuit given in the insert of Figure 2, where the overall impedance is characterized by a parallel combination of capacitance
FIGURE 2. Nyquist and Bode plots obtained from the EIS measurements for nickel in 1.0 M H2SO4 at the corrosion potential after an immersion time of 1.0 h at 25°C.
TAblE 1Electrochemical Parameters, Inhibition Efficiencies (I), and Corrosion Rates (υ) Associated with Tafel Polarization
and Linear Polarization Resistance Measurements for Nickel in 1.0 M H2SO4 Solutions Without and With Various Concentrations of MAD, SAD, or AAD at 25°C
surface during the corrosion process.32-34 To describe this response properly, a constant phase element, CPE, was introduced in the equivalent circuit. The introduction of such a CPE is often used to interpret data for rough solid electrodes.32-34 The impedance of the CPE, ZCPE, is described by the expression:
Z Q jCPEn= – –( )1 ω (9)
where Q is the CPE constant (a proportional factor), ω is the angular frequency (in rad s–1), j2 = –1, the imagi-nary number, and n is the CPE exponent. The above equation provides information about the degree of non-idealibility in capacitive behavior. Its value makes it possible to differentiate between the behavior of an ideal capacitor (n = 1) and of a CPE (n < 1). The value of n can be used as a measure of the surface inho-mogenity.32-34 The values of the double-layer capaci-tances corresponding to the first and the second time constants (designated here as C1 and C2, respectively) can be calculated for a parallel circuit composed of a CPE (Q) and a resistor (R), according to the following formula:35-36
Q RC Rn= ( ) / (10)
The Nyquist plots obtained after 1 h of immer-sion of Ni in 1.0 M H2SO4 in the absence and presence of various concentrations of MAD, SAD, or AAD at the respective corrosion potentials at 25°C are as shown in Figure 3(a) through (c), respectively. It seems that the introduction of any of the three tested inhibitors does not modify the shape of the impedance diagram. This means that the shapes of the impedance plots for inhibited electrodes are not substantially differ-ent from those of uninhibited electrodes. The presence of the inhibitor increases the impedance but does not change other aspects of the behavior. These results support the results of polarization measurements that the inhibitor does not alter the electrochemical reac-tions responsible for corrosion. It inhibits corrosion primarily through its adsorption on the metal surface. Based on these findings, the equivalent circuit pre-sented in the insert of Figure 2 permits the explana-tion of the interface Ni/H2SO4/(MAD, SAD, or AAD) (fitting results in Table 2).
It is important to note that the size of the first time constant increases at the expense of that of the second one, particularly at higher concentrations of the tested inhibitor. This is quite obvious in the case of AAD, as shown in Figure 3(c). The reason for that is not quite clear at the moment and deserves further investigation. Introduction of any of the three tested inhibitors was observed to increase the total imped-ance, and this increase is dependent on the type and concentration of the tested inhibitor. This increase in the total impedance with additive concentration means that the inhibition efficiency is proportional
and resistance of two charge-transfer processes. The total charge-transfer resistance, Rct, equals (R1 + R2). This model has been used extensively to describe 3-D inhomogeneous layer systems.24-25
The high-frequency capacitive loop is attrib-uted to the time constant of charge-transfer and dou-ble-layer capacitance.26-28 This loop makes an angle approaching 60° with the real axis, and its intersec-tion gives a value of 0.8 Ω·cm2 for the resistance of the solution (Rs) enclosed between the working electrode and the reference electrode. The second time con-stant (the low-frequency capacitive loop) is related to a step in the dissolution process (Reactions [2] through [4]).26-28
Literature survey demonstrated that our interpre-tations of the second time constant (the low-frequency capacitive loop) agree with published results26-28 and disagree with others.29-31 For these reasons we should compare our interpretations with those in the litera-ture. Said, et al.,29 studied the effect of the addition of four salts, K+Cl–, K+Br–, K+I–, and C8H17Ph3P
+I–, on the corrosion behavior of nickel when submitted to the 1.0 M H2SO4 medium, based on electrochemical tech-niques (polarization and EIS). They reported that the electrochemical impedance diagram of nickel in H2SO4 showed the existence of two capacitive loops in the high- and middle-frequency domains. Said and his coworkers linked the first time constant to the Faradic process (charge transfer) and the second one to the adsorbed corrosion products. Indeed, we believe that the formation of corrosion products does not neces-sarily cause a separate RC time constant. The extra time constant (i.e., the low-frequency capacitive loop) must be associated with a step in the dissolution pro-cess, as previously mentioned.
Goncalves, et al.,30 studied the electrochemical behavior of nickel, copper, and copper/nickel (Cu55/Ni45) alloy in 0.5 M H2SO4 in the absence and pres-ence of propargyl alcohol (C3H4O). Goncalves and his coauthors demonstrated, based on Jüttner’s study,31 that the EIS plots of Ni in H2SO4 exhibited two time constants. The first one, at high frequency, was related to an external porous layer, whereas the second one, at lower frequencies, was attributed to a more resis-tive internal layer. In this case, the transfer function is the sum of the corresponding layer impedance.30-31
A mathematical analysis of the impedance dia-grams, under all experimental conditions studied, has shown that the center of the two capacitive loops lies below the real axis and that the slopes of the log |Z| against log f plots are not –1. It is obvious that the first capacitive loop is much more depressed than the second one. Indeed, the size and the degree of depression of these two capacitive loops were found to depend on the type and concentration of the intro-duced inhibitor. Generally, the depressed nature of the semicircle, which has the center below the real axis, is from the generation of micro-roughness at the
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to the increment of inhibitor concentration. Namely, the greater the inhibitor concentration, the higher the inhibition efficiency.
The inhibition efficiency was evaluated mainly by the Rct (i.e., R1 + R2) values of the impedance. The more densely packed the inhibitor surface film, the larger the diameter of the semicircle, which results in higher Rct values. It follows from Table 2 that the Rct values increase, while the Cdl values decrease with the increase of concentration of any of the three tested inhibitors. This could be attributed to coverage of the electrode surface by adsorption, indicative of reduc-tion in the corrosion rate and subsequent increase in the inhibition efficiency of the tested inhibitor. In addition, the decrease in Cdl values has been ascribed to a decrease in the dielectric constant and/or an increase in the double electric layer thickness, as a result of the inhibitor adsorption on the metal/elec-trolyte interface.35-37 This implies that the extent of adsorption on the metal/solution interface increases with an additive’s concentration. It is obvious that the values of Rct are always higher and those of Cdl are always lower in the presence of AAD as compared with MAD and SAD. These results again suggest that AAD has the maximum protection effect against the H2SO4 corrosion of Ni among the three compounds used in this study. The inhibition efficiency values obtained from the EIS measurements (Table 2) are observed to be in agreement with those obtained from the polari-zation measurements (Table 1).
Scanning Electron Microscopy/Energy-Dispersive X-Ray Examinations of the Electrode Surface — SEM/EDX examinations of the electrode surface were per-formed to confirm the formation of a protective sur-face film of inhibitor at the electrode surface. The morphologies of the uninhibited and inhibited sur-faces will be discussed after EDX analyses have been presented. EDX survey spectra were used to determine which elements were present on the electrode surface before and after exposure to the inhibitor solution. Results of EDX are presented here in the form of tables. These tables include the atomic percentage (contribution) of each element detected on the electrode surface, based on ZAF method standardless quantita-tive analysis, covering the energy range (0 to 20 keV) at 20.0 kV and at a counting rate of 10,747 cps. The EDX data were recorded for Ni exposed for 2.0 h in 1.0 M H2SO4 solution in the absence and presence of 0.01 M and 0.05 M MAD, SAD, or AAD at 25°C. In inhibitor-free solution, the EDX spectra showed sig-nals caused from C, O, S, and Ni. The signals of O and S may be attributed to the formation of Ni oxides (confirmed from XRD; not included here) on the elec-trode surface and the adsorption of SO4
2– anions.On the other hand, in the presence of additives,
the EDX spectra exhibited an additional signal char-acteristic for the existence of N. At the same time, the contributions of C and O are enhanced upon adding
FIGURE 3. Complex plane impedance plots recorded for Ni in 1.0 M H2SO4 solutions without and with various concentrations of (a) MAD, (b) SAD, or (c) AAD at the respective corrosion potentials at 25°C.
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inhibitors to the aggressive solution. The existence of N and the enhancement in C and O contribution is from the N, C, and O atoms of the adsorbed inhibitor species. These data show that a carbonaceous mate-rial containing N and O atoms has covered the elec-trode surface. This layer is undoubtedly from the inhibitor because of the new N signal and the high contribution of the C and O observed in presence of the inhibitor. The N signal and this high contribution of C and O are not present on the electrode surface exposed to the uninhibited H2SO4 solution.
In all cases it was observed that the contributions of the C, N, and O signals increase in the presence of the three tested inhibitors to an extent depending on the type and concentration of the tested inhibitor. This increase in the contribution of C, N, and O with an increase in inhibitor concentration is expected, since more inhibitor molecules adsorb on the elec-trode surface, confirming the results obtained from electrochemical measurements.
Further inspection of the tables of the EDX data reveals that the contribution of Ni is considerably suppressed relative to the samples prepared in blank solution, and this suppression depends on the type and concentration of the tested inhibitor. The sup-pression of Ni contribution occurs because of the overlying inhibitor (protective) film. These findings confirm polarization measurements, which suggest that the presence of any of the three tested inhibi-tors in H2SO4 solution inhibits both the anodic and cathodic processes via formation of a protective film. This protective film acts as a barrier to the metal dis-
solution from the electrode/film interface to the film/solution interface and to the diffusion of H3O
+ ions from solution to the electrode surface.38 This results in an obvious increase in the overpotential of the anodic and cathodic processes, as shown in Figures 1(a) through (c).
The formation of a protective surface film of inhibitor on the electrode surface was further con-firmed by SEM observations of the electrode surface. Figure 4 shows an array of SEM images recorded for Ni samples exposed for 2 h in 1.0 M H2SO4 solution in the absence and presence of 0.01 M and 0.05 M MAD, SAD, or AAD at 25°C. The morphology presented in Figure 4(a) reveals that in the absence of inhibitor, the surface includes large corroded areas. However, in presence of the inhibitor (images [b] through [g]), the rate of corrosion is suppressed, as can be seen from the obvious decrease in the corrosion areas. This is because of the inhibitor layer covering the electrode surface. This decrease in the corrosion areas depends on the type and concentration of the introduced inhib-itor. Significant corrosion suppression (the surface is almost free from corrosion) is observed in the case of AAD (images [f] and [g]) much more than MAD (images [b] and [c]) and SAD (images [d] and [e]), confirming the highest inhibition performance of AAD.
The protective nature of this film is reflected in the inhibition efficiency measurements obtained from electrochemical methods (Tables 1 and 2). Therefore, SEM/EDX examinations of the electrode surface sup-port the results obtained from the electrochemical methods used—AAD is the most effective inhibitor
TAblE 2Fitting Parameters and Inhibition Efficiencies, IZ(%), Derived from Electrochemical Impedance Spectroscopy Measurements
for Nickel in 1.0 M H2SO4 Solutions Without and With Various Concentrations of MAD, SAD, or AAD at 25°C
Conc. Tested Rs R1 Q1 C1 R2 Q2 C2 Rct IZ (M) Inhibitor (Ω·cm2) (Ω·cm2) Sn (ω–1 cm–2) n1 (µF cm–2) (Ω·cm2) Sn (ω–1 cm–2) n2 (µF cm–2) (R1+R2) (%)
FIGURE 4. SEM/EDS examinations recorded for a corroded Ni for 6 h in 1.0 M H2SO4 solution (image a). For morphologies of Ni in the presence of the three tested inhibitors, samples were immersed for 6 h in 1.0 M H2SO4 solutions containing 0.01 M and 0.05 M MAD (images b and c, respectively), SAD (images d and e, respectively), and AAD (images f and g, respectively).
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among the tested compounds for Ni corrosion in 1.0 M H2SO4 solutions.
Inhibition Mechanism — Most of the effective and efficient organic inhibitors are those compounds con-taining heteroatoms such as oxygen, nitrogen, sul-fur, and phosphorus, which allowed adsorption on the metal surface. The phenomenon of adsorption is influenced by the nature and surface charge of the metal and by the chemical structure of inhibitors. The surface charge of the metal is caused by the electri-cal field that emerges at the interface on immersion in the electrolyte. It can be determined according to Antropov, et al.,39 by comparing the potential of zero charge (PZC) and the stationary potential of the metal in the electrolytic medium. As PZC corresponds to a state at which the surface is free from charges, at the stationary (corrosion) potential, the metal sur-face will be positively or negatively charged. Sury40 reported that the corrosion potentials of iron, cobalt, and nickel in H2SO4 all are positive with respect to the PZC. As a consequence, cation adsorption is hindered while anion adsorption is favored.
The tested compounds are organic bases that protonize in an acid medium; therefore, the inhibi-tor molecule becomes a cation, existing in equilib-rium with the corresponding molecular form. Hence, poor adsorption is expected to occur between proton-ated inhibitor molecules and the positively charged electrode surface. The adsorption of hydronium (H3O
+) and desorption of hydrogen gas (H2)
41 occurs concur-rently on the cathodic sites. The cationic forms of the tested inhibitors can be adsorbed directly on the cathodic sites in competition with the hydronium ions to reduce hydrogen evolution. This is responsible for the observed cathodic inhibiting effect of these com-pounds. The cationic forms of the tested inhibitors are bigger than those of hydrogen. This way the adsorbed cations covers a large part of the metallic surface, which also involves water molecule displacement from the surface.42
The poor adsorption of cations is confirmed via addition of trace amounts of potassium iodide (KI) to the corrosive medium. The inhibition efficiencies of the three tested compounds are greatly enhanced upon the addition of KI because of the synergetic effect between I– ions and the protonated species of inhibitors. This point will be discussed in a sepa-rate complementary paper. In addition to the physi-cal adsorption, there should be chemical adsorption, which involves charge sharing or charge-transfer from the inhibitor molecules to the metal surface to form a coordinate type of a bond. The presence of a tran-sition metal, having vacant and low-energy electron orbitals and of an inhibitor with molecules having rel-atively loosely bound electrons or heteroatoms with a lone pair of electrons, is necessary.
EDX examinations of the electrode surface in 1.0 M H2SO4 revealed the existence of O and Ni,
reflecting the probability of the formation of Ni-hydroxide and/or Ni-oxide species (Equations [2] through [4]). The presence of the oxide film on the electrode surface may promote adsorption via hydro-gen bonding.43-45 Adsorption in the presence of an oxide film is assisted by hydrogen bond formation between the tested inhibitors and oxidized surface species. This type of adsorption should be more effec-tive for the protonated N atom, because the positive charge on the N atom is conductive to the formation of hydrogen bonds. Un-protonated N atoms may adsorb by direct chemisorptions, as previously dis-cussed, or by hydrogen bonding to a surface-oxidized species. In the presence of the oxide film, protonated and un-protonated N atoms are adsorbed onto metal through hydrogen bond formation. These results may refer to the importance of hydrogen bonding in the corrosion inhibition characteristics of the tested inhibitors, particularly in the presence of oxide spe-cies on the electrode surface.
The capability toward this type of adsorption should be proportional to the number of NH linkages in the inhibitor molecule. Referring to the structure of the three tested inhibitor molecules, four H bonds are expected to form per each inhibitor molecule. It seems that this type of adsorption cannot be used to account for the variation in the inhibition efficiencies of the three tested inhibitors, because they contain the same number of NH linkages.
Effective inhibition is predominantly provided by the direct coordination of un-protonated N atom to metal atoms. Nickel {3d8 4s2} is a member of a group in the 3d transition series. Its chemistry, in gen-eral, is determined strongly by the electron exchange between the partly filled d-shells and electron donors or acceptors from the environment. Coordinate bonds are expected to form between the active centers (the lone electron pairs of the nitrogen and oxygen atoms) of the unprotonated inhibitor molecule and the empty orbitals of nickel atoms. This results in the formation of a chemisorbed protective layer. Based on the pre-vious discussion, one can conclude that the tested inhibitors adsorb, displacing water from the metal surface, interacting with anodic and cathodic reaction sites, and preventing transportation of water and cor-rosion active species on the surface.
According to the number of the methylenic groups in the main chain, the tested inhibitors may be cate-gorized into two groups: short chain and long chain inhibitors. AAD is a long chain inhibitor (chain length of the tested inhibitors increases in the order: MAD < SAD < AAD). The long chain of the AAD molecule may be the reason for the higher inhibition performance recorded for AAD compared with MAD and SAD.
Theoretical StudiesQuantum Chemical Calculations — To investigate
the correlation between the molecular structure of the
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three tested inhibitors and their inhibition character-istics, a quantum chemical study has been performed. The frontier molecular orbital density distributions of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the tested inhibitors were constructed (figure not included here), and the relevant quantum parameters are listed in Table 3. It has been reported46-48 that the inhibi-tion efficiency of inhibitors depends on the electronic properties of the corrosion inhibitors, such as HOMO and LUMO. The higher the HOMO energy (EHOMO) of the inhibitor, the greater is the trend of offering elec-trons to unoccupied d orbital of the metal, and the higher the corrosion inhibition efficiency. In addi-tion, the lower the LUMO energy (ELUMO), the easier is the acceptance of electrons from the metal sur-face.48 Therefore, the binding ability of the inhibitor to the metal surface increases with the increase of the HOMO and the decrease of the LUMO energy values.
Referring to Table 3, the values of EHOMO, ELUMO, the energy gap, ∆E (∆E = ELUMO – ELUMO), and the other quantum chemical parameters recorded for the tested inhibitors were used to interpret their obtained exper-imental inhibition efficiencies (Tables 1 and 2). The calculations show that the EHOMO values are –8.41, –8.5, and –7.89 for MAD, SAD, and AAD, respectively. It is clear that the differences among these values are considerable. This means that the charge donation ability of these compounds to the metallic surface is different and can be used to confirm the experimen-tal findings that AAD inhibited nickel corrosion more effectively than SAD and MAD. The highest value of EHOMO obtained by AAD suggests that the AAD mol-ecule offers electrons to unoccupied d orbital of the metal more effectively than SAD and MAD. In addi-tion, the lower ELUMO value recorded for AAD (7.48 eV), compared with that of MAD (7.76 eV) and that of SAD (7.75 eV) favors the adsorption of the former.48
The energy gap, ∆E, is also an important param-eter referring to the reactivity of the inhibitor molecule toward adsorption. As the values of ∆E decrease, the reactivity of the molecule increases, and the adsorp-tion increases. This results in increased inhibition performance.47-48 The AAD molecule recorded the lowest ∆E value (15.37 eV) among the tested mole-cules, reflecting its higher adsorption tendency. These results confirm the experimental findings that AAD is the best among the tested dihydrazide derivatives in inhibiting the acid corrosion of nickel.
Other quantities (Table 3) are related to ionization potential, I = –EHOMO, and electron affinity, A = –ELUMO, of the studied molecule. The obtained values of I and A were considered for the calculation of the electro-negativity, χ, the global hardness, η, and softness, σ, using the following relations:
X
I A= +2
(11)
η = I A–
2 (12)
σ η= –1 (13)
When the hardness (η) values decrease, the inhi-bition values increase.49 The opposite trend was obtained for softness (σ) comparing with hardness; inhibition efficiency increases with softness,49-50 where high values of softness were found to favor the adsorption process.50 Based on Table 3, the values of h are always smaller, while those of s are always higher in the presence of AAD compared with MAD and SAD, reflecting the high inhibition performance of AAD as compared with the other tested compounds.
The number of electrons transferred (∆N) was also calculated depending on the quantum chemical method, as in Equation (14):
N
X XNi Inh
Ni inh
=+∑
–2 η η
(14)
where XNi and Xinh denote the absolute electronegativ-ity of nickel and the inhibitor molecule, respectively; ηNi and ηinh denote the absolute hardness of nickel and the inhibitor molecule, respectively. These quan-tities are related to electron affinity (A) and ionization potential (I), which are useful in their ability to help predict chemical behavior.51
Using a theoretical value of X = 4.4 eV/mol, according to Pearson’s electronegativity scale, and η value of 0 eV/mol for nickel,52 ∆N, the number of elec-
TAblE 3Quantum Chemical Parameters for the Studied Inhibitors
Calculated with Ab Initio Method as Implemented in the Molecular Modeling Software Program
(A) The energies of the EHOMO and ELUMO molecular orbitals, ∆Egap (ELUMO – HOMO) (in eV), (X) the absolute electronegativity, (η) the absolute hardness, (σ) the softness, (∆N) the number of elec-trons transferred, total energy {TE (in eV)}, molecular surface area {(Ar (Å2)}, molecular volume {vol (Å3)}, molecular polarizability {pol (Å3)}, adsorption energy (kJ mol–1), and hydrogen bond length (Å).
Inhibitor
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trons transferred from inhibitor to the nickel surface, was calculated. Values of ∆N showed that the inhibi-tion effect resulted from electron donation. Based on the study by Lukovits, et al.,53 ∆N < 3.6 denotes that the inhibition efficiency increases with increasing electron-donating ability at the metal surface. Based on these calculations, it is expected that the three tested inhibitors are donors of electrons, and the elec-trode surface is the acceptor, and this favors chemical adsorption of the inhibitor on the electrode surface. The compounds bind to the electrode surface and form an adsorption layer against corrosion. Here again, AAD had the highest inhibition efficiency because it had the highest EHOMO and ∆N values, and it had the greatest ability of offering electrons to the surface among the tested compounds. On the other hand, the molecule MAD had the lowest inhibition efficiency, because it had the lowest EHOMO and ∆N values.
There are also satisfactory correlations between the experimental inhibition efficiencies and three important quantum chemical parameters, namely:
—the total energy (TE)—solvent accessible molecular surface area (Ar)—molecular volume of adsorbed molecule (vol)
All these parameters favor the adsorption process.49-50 It follows from Table 3 that the values of these three parameters increase in the order MAD < SAD < AAD, which obviously supports the sequence of the increase of the inhibition efficiencies obtained from the electro-chemical methods used (Tables 1 and 2).
Polarizability is the ratio of the induced dipole moment to the intensity of the electric field. The induced dipole moment is proportional to the polariz-ability.53 Some attempts have been made to relate the polarizability of some corrosion inhibitors to their inhibition efficiency.48-49 A high polarizability value generally refers to a high reactivity of the molecule, which results in facilitated adsorption and improved inhibition performance results.48 It follows from the results obtained (Table 3) that the trend of the increase in the polarizability values of the three tested inhibi-tors correlates well with the order of the increase in their experimental inhibition efficiencies, following the order: AAD > SAD > MAD. The high polarizability value recorded for AAD molecule suggests that this molecule is chemically more reactive than the other tested molecules, which might be the reason why the AAD molecule possesses the highest tendency toward adsorption among the tested inhibitors.
To investigate the charge distribution of the tested dihydrazide molecules, ab initio quantum chemical calculations were performed using the molecular modeling software package. The Mulliken charge distributions of the dihydrazide molecules are shown in Figure 5. It is confirmed that the more neg-ative the atomic partial charges of the adsorbed cen-ter are, the more easily the atom donates its electrons to the unoccupied orbital of the surface atoms of the nickel surface. The atomic partial charge of the nitro-gen atom in AAD molecule is –0.405, while those of SAD and MAD are –0.400 and –0.397, respectively. It is obvious that the atomic partial charge of the nitro-gen atom in AAD molecule is higher than that of SAD and MAD molecules. This again may reflect the high-est tendency for the AAD molecule to adsorb among the tested inhibitors. This is clearly evident from the higher contribution of N detected by EDX analysis in the case of AAD as compared with MAD and SAD.
The electrostatic potential is a physical property of a molecule related to how a molecule is first “seen” or “felt” by another approaching species.54 A portion of a molecule that has a negative electrostatic potential will be susceptible to electrophilic attack. The more negative the electrostatic potential, the higher the susceptibility to electrophilic attack. The three-dimen-sional mapped isosurface of the electrostatic potential (ESP) of inhibitors are shown in Figure 6 using the ab initio method as implemented in the molecular model-ing software program. This plot provides information on the reactivity of the molecules in actual reaction with electrophiles or nucleophiles. (See online version for color) Red colors indicate negative ESP regions and
FIGURE 5. Optimized molecular structure with Mulliken atomic charges of three inhibitors (MAD, SAD, and AAD) using ab initio method as implemented in the molecular modeling software program.
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green colors indicate positive ESP regions. The pre-sentation depicted in Figure 6 shows that oxygen and nitrogen atoms (appear in red color) have more nega-tive ESP regions in comparison with the other atoms (appear in green color). This means that oxygen and nitrogen atoms undergo protonation reaction with acidic reagents and are active adsorption centers in the molecule.55
Adsorption Energy Calculations — In the simu-lation, the dihydrazide interacts with the nickel sur-face in water at 25°C. Meanwhile, molecular dynamics simulations are implemented with dihydrazide in dif-ferent possible configurations on the nickel surfaces, and the adsorption energy is calculated. So, the con-figuration with the maximal adsorption energy is cho-sen as the initial simulation states. The dihydrazide adsorbed on the nickel surfaces after equilibrium is shown in Figure 7. Clearly, one of the inertial axes of the molecule is nearly parallel with the surface. It fol-lows from Figure 7 that the maximum contact area, which increases the adhesion energy and stabilizes the inhibitor on the nickel substrate, is reached with the AAD molecule (the most effective inhibitor among the tested compounds).
The adsorption energy of the tested dihydrazide derivatives on nickel and the possible hydrogen bond-ing formation between such derivatives and the nickel surface are investigated. Table 3 also presents the adsorption energies of the tested dihydrazide deriva-tives as well as the calculated hydrogen bond lengths. Based on the calculated hydrogen bond lengths, the interaction between the studied dihydrazide derivatives and the nickel surface may also occur via hydrogen bonding. It follows from Table 3 that the AAD molecule possesses the shortest hydrogen bond length (2.3 Å) and the highest adsorption energy (–9.9 kJ mol–1), con-firming experimental findings that AAD molecule is the best inhibitor among the investigated dihydrazides.
FIGURE 6. Isosarface of the electrostatic potential maps (ESP) in the spatial vicinity of three inhibitors (MAD, SAD, and AAD) (the electron rich region is red and the electron poor region is green in the neutral molecules), using ab initio method as implemented in the molecular modeling software program.
FIGURE 7. Representative structures of 3D amorphous cells containing the nickel substrate, the solvent molecules, and the three inhibitors (MAD, SAD, and AAD). The inertial axis along the side chain of the molecule is nearly perpendicular to the surface, indicating a flat adsorption geometry of the molecule on the surface.
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conclusIons
v Three selected dihydrazide derivatives, namely, MAD, SAD, and AAD dihydrazide, were applied as inhibitors for nickel corrosion in 1.0 M H2SO4 solu-tion at 25°C. Electrochemical methods including Tafel polarization, LPR, and EIS, supported with SEM/EDX examinations, were used to clarify the adsorp-tion of these inhibitors onto the nickel surface. Tafel polarization measurements showed that the three tested compounds act as mixed-type inhibitors. These compounds revealed inhibitory effects against nickel corrosion to an extent, depending on the type and concentration of the tested inhibitor. Inhibition effi-ciencies are related to the concentration and chemical structure of the tested inhibitors. AAD caused sig-nificant inhibition more than MAD and SAD. All data from the different methods are in good agreement and in similar trends.v Computational studies were also used to confirm experimental findings and to optimize the adsorp-tion structures of dihydrazide derivatives. The nickel/inhibitor/solvent interfaces were simulated, and the charges on the inhibitor molecules and their struc-tural parameters were calculated in the presence of solvent effects. The inhibition efficiency increased with an increase in EHOMO and a decrease in ELUMO–EHOMO. AAD had the highest inhibition efficiency because it had the highest HOMO energy and ∆N val-ues, and was the best one capable of offering elec-trons to the nickel surface. Adsorption geometry and adsorption energy of a single molecule on the nickel surface showed that the dihydrazide molecules are lying flat on the surface. Energetically most favorable situations are attained when the amino groups are close to the nickel surface. Results from electrochemi-cal measurements are in good agreement with theo-retical studies.
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