Xanthione: A new and effective corrosion inhibitor for mild steel in sulphuric acid solution

2.1 Material

Tests were performed on a freshly prepared sheet of mild steel of the following composition (wt.%): 0.13% C, 0.18% Si, 0.39% Mn, 0.40% P, 0.04% S, 0.025% Cu, and bal Fe. Specimens used in the weight loss experiment were mechanically cut into 5.0 cm × 4.0 cm × 0.8 cm dimensions, then abraded with SiC abrasive papers 320, 400 and 600 grit, respectively, washed in absolute ethanol and acetone, dried in room temperature and stored in a moisture free dessicator before their use in corrosion studies (Obi-Egbedi and Obot, 2010).

2.2 Inhibitor

Xanthione (XION) was purchased from Sigma–Aldrich and used as inhibitor. Stock solution was made in 10:1 water:methanol mixture to ensure solubility (Ahamad and Quraishi, 2010). This stock solution was used for all experimental purposes. Fig. 1 shows the molecular structure of XION. It is evident that XION is a heterocyclic compound containing sulphur and oxygen atoms, which could easily be protonated in acidic solution, and several π-electrons exist in this molecule.

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Figure 1

Molecular structure of xanthione (XION).

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2.3 Solutions

The aggressive solutions, 0.5 M H₂SO₄ were prepared by dilution of analytical grade 98% H₂SO₄ with distilled water. The concentration range of XION prepared and used in this study was 2–10 μM.

2.4 Gravimetric measurements

The gravimetric method (weight loss) is probably the most widely used method of inhibition assessment (Musa et al., 2010; Khadom et al., 2010). The simplicity and reliability of the measurement offered by the weight loss method are such that the technique forms the baseline method of measurement in many corrosion monitoring programmes (Afidah and Kassim, 2008). Weight loss measurements were conducted under total immersion using 250 mL capacity beakers containing 200 mL test solution at 303–333 K maintained in a thermostated water bath. The mild steel coupons were weighed and suspended in the beaker with the help of rod and hook. The coupons were retrieved at 2 h interval progressively for 10 h, washed thoroughly in 20% NaOH solution containing 200 g/l of zinc dust (ASTM G1-72, 1990) with bristle brush, rinsed severally in deionized water, cleaned, dried in acetone, and re-weighed. The weight loss, in grammes, was taken as the difference in the weight of the mild steel coupons before and after immersion in different test solutions. Then the tests were repeated at different temperatures. In order to get good reproducibility, experiments were carried out in triplicate. In this present study, the standard deviation values among parallel triplicate experiments were found to be smaller than 5%, indicating good reproducibility.

The corrosion rate (ρ) in mg cm⁻² h⁻¹ was calculated from the following equation (Bentiss et al., 1999):

2.5 Spectrophotometric measurements

UV–visible absorption spectrophotometric method was carried out on the prepared mild steel samples after immersion in 0.5 M H₂SO₄ with and without addition of 10 μM of xanthione at 303 K for three days. All the spectra measurements were carried out using a Perkin–Elmer UV–visible Lambda 2 spectrophotometer.

2.6 Computational details

B3LYP, a version of the DFT method that uses Becke’s three parameter functional (B3) and includes a mixture of HF with DFT exchange terms associated with the gradient corrected correlation functional of Lee, Yang and Parr (LYP) (Lee et al., 1988), was used in this paper to carry out quantum calculations. Then, full geometry optimization together with the vibrational analysis of the optimized structures of the inhibitor was carried out at the (B3LYP/6-31G (d) level of theory using Spartan’06 V112 program package (Spartan and Wavefunction, 2006) in order to determine whether they correspond to a maximum or a minimum in the potential energy curve. The theoretical parameters were calculated for molecules in neutral as well as in the protonated form for comparison. It is well known that the phenomenon of electrochemical corrosion occurs in liquid phase. As a result, it was necessary to include the effect of a solvent in the computational calculations. In the Spartan’06 V112 program, SCRF methods (Self-consistent reaction field) were used to perform calculations in solution. These methods model the solvent as a continuum of uniform dielectric constant and the solute is placed in the cavity within it.

3.1 Weight loss measurements

Weight loss of mild steel, in g cm⁻² h⁻¹, was determined at various time intervals in the absence and presence of different concentrations of xanthione. Fig. 2 shows the weight loss-time curves obtained for mild steel in 0.5 M H₂SO₄ in the absence and presence of different concentrations of XION at 303 K. The figure shows that the presence of inhibitor falls significantly below that of free acid. Similar curves were obtained for 313–333K (not shown). The calculated values of corrosion rates (ρ) and inhibition efficiency (%I) obtained from weight loss measurements for different concentrations of xanthione in 0.5 M H₂SO₄ after 10 h immersion at 303–333 K are listed in Table 1. It is evident from this table and Fig. 3 that the corrosion rate decreased with increasing inhibitor concentration but increased with rise in temperature. Table 1 also shows that inhibition efficiency (%I) increased with increasing inhibitor concentration, reaching a maximum of 98.0%. This may be due to the adsorption of xanthione onto the mild steel surface through non-bonding electron pairs of sulphur and oxygen atoms as well as the π-electrons of the aromatic rings. The high inhibitive performance of xanthione suggests a higher bonding ability of the inhibitor to the mild steel surface. Similar observation has been reported (Ahamad and Quraishi, 2010). Furthermore, the inhibition efficiency values obtained for xanthione were higher when compared to that of xanthene and xanthone already reported in literature (Obi-Egbedi and Obot, 2010; Obot and Obi-Egbedi, 2010b).

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Figure 2

Variation of weight loss against time for mild steel corrosion in 0.5 M H₂SO₄ in the presence of different concentrations of XION at 303 K.

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Figure 3

The relationship between corrosion rate and temperature for different concentrations of XION.

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3.2 Effect of temperature

The effect of temperature on the inhibited acid-metal reaction is very complex according to Bentiss et al. (2005), because many changes occur on the metal surface such as rapid etching, desorption of inhibitor and the inhibitor itself may undergo decomposition. The influence of temperature on percentage inhibition efficiency was studied by conducting weight loss measurements at 303–333 K containing different concentrations of XION (Table 1). Fig. 4 shows the variation of percentage inhibition efficiency with temperature. It is clear from the figure that percentage inhibition efficiency increases with XION concentration but decreases with temperature. The decrease in inhibition efficiency with increase in temperature may be probably due to increased rate of desorption of XION from the mild steel surface at higher temperature (Li et al., 2010; Obot and Obi-Egbedi, 2010b).The relationship between the corrosion rate (ρ) of mild steel in acidic media and temperature (T) is often expressed by the Arrhenius equation (Behpour et al., 2009):

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Figure 4

Variation of inhibition efficiency of XION with temperature.

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Figure 5

Arrhenius plot for mild steel corrosion in 0.5 M H₂SO₄ in the absence and presence of different concentrations of xanthione (XION).

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Experimental corrosion rate values obtained from weight loss measurements for mild in 0.5 M H₂SO₄ in the absence and presence of XION was used to further gain insight on the change of enthalpy ( $\mathrm{\Delta }{H}^{\ast }$ ) and entropy ( $\mathrm{\Delta }{S}^{\ast }$ ) of activation for the formation of the activation complex in the transition state using transition equation (Bockris and Reddy, 1977):

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Figure 6

Transition state plot for mild steel corrosion in 0.5 M H₂SO₄ in the absence and presence of different concentrations of xanthione (XION).

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Entropy of activation, $\mathrm{\Delta }{S}^{\ast }$ remains almost constant with increasing XION concentration (Table 2) and their values were negative both in the uninhibited and inhibited systems. The negative values of entropies of activation imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated (Herrag et al., 2010).

3.3 Adsorption isotherm

It is well recognized that the first step in inhibition of metallic corrosion is the adsorption of organic inhibitor molecules at the metal/solution interface. Furthermore, the adsorption depends on the molecule’s chemical composition, the temperature and the electrochemical potential at the metal/solution interface. So the adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasi-substitution process between the organic compounds in the aqueous phase[Org_(sol)] and water molecules at the electrode surface[H₂O_(ads)] (Sahin et al., 2002):

Fig. 7 shows the plots of C/θ versus C and the expected linear relationship is obtained with good correlation coefficients confirming the validity of the equation. The slopes were less than unity at 303 and 313 K except in the case of 323 and 333 K where the value of the slope was 0.98 and 1.09, respectively, which is an indication that the Langmuir isotherm is obeyed at these temperatures (Table 3). The considerable deviation of the slope from unity observed at 303 and 313 K may be explained on the basis of the interaction among the adsorbed species on the surface of the metal (Obot et al., 2009). It is therefore pertinent to say that the adsorption of XION on the mild steel surface at 303 and 313 K can be more appropriately represented by a modified Langmuir equation suggested by Villamil et al. (1999) taking into consideration the interactions between adsorbate species as well as changes in heat of adsorption with changing surface coverage as follows:

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Figure 7

Langmuir adsorption plot for mild steel in the presence of XION.

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The values of equilibrium constant of adsorption K_ads obtained from the Langmuir adsorption isotherm are listed in Table 3, together with the values of the Gibbs free energy of adsorption ( $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ ) calculated from the equation:

The high values of K_ads for studied XION indicate stronger adsorption on the mild steel surface in 0.5 M H₂SO₄ solution. This can be explained by the presence of sulphur, oxygen heteroatoms and π-electrons in the inhibitor molecule. It has been reported that the higher the K_ads value (>100 M⁻¹), the stronger and more stable the adsorbed layer of the inhibitor on metal surface and consequently, the higher the inhibition efficiency (Lagrenee et al., 2002). These data support the good performance of XION as corrosion inhibitor for mild steel in 0.5 M H₂SO₄ solution.The negative values of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ , calculated from Eq. (8), are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the steel surface. Generally, values of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ up to −20 kJ mol⁻¹ are consistent with physisorption, while those around −40 kJ mol⁻¹ or higher are associated with chemisorption as a result of the sharing or transfer of electrons from organic molecules to the metal surface to form a coordinate bond (Donahue and Nobe, 1965). In the present study, the calculated values of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ obtained for XION ranges between −32.1 and −42.1 kJ mol⁻¹ (Tables 3 and 4), indicating that the adsorption of mechanism of XION on mild steel in 0.5 M H₂SO₄ solution at the studied temperatures may be a combination of both physisorption and chemisorption (comprehensive adsorption) (Ahamad et al., 2010). However, physisorption was the major contributor while chemisorption only slightly contributed to the adsorption mechanism judging from the decrease of %I with increase in temperature and the higher values of E_a obtained in the presence of inhibitor when compared to its absence.

3.4 Thermodynamic consideration using the statistical model

The adsorption of an inhibitor species, I, on a metal surface, M, can be represented by a simplified equation:

According to statistical physics (Landau and Lifshitz, 1980), the change of free energy of adsorption $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ can be calculated from Eq. (10) as follows (Wang et al., 2002; Obi-Egbedi and Obot, 2010; Obot and Obi-Egbedi, 2010b):

The curve fitting of data in Table 1 to the statistical model at 303–333 K is presented in Fig. 8. Good correlation coefficient (R² > 0.94) was obtained. θ and $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ can be calculated from the slope and intercept of Eq. (10). All the calculated parameters are given in Table 4. The values of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ obtained from Statistical Physics and that obtained from the last approach (Eq. (8)) show close correlation.

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Figure 8

Application of the statistical model to the corrosion protection behavior of xanthione.

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The enthalpy of adsorption can also be calculated from the Gibbs–Helmholtz equation (Singh and Quraishi, 2010):

Eq. (11) can be arranged to give the following equation:

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Figure 9

The variation of $\mathrm{\Delta }{G}_{\mathit{ads}}^o/T$ with 1/T.

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Figure 10

The variation of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ with temperature.

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The entropy of adsorption obtained from Eq. (13) was negative because inhibitor molecule freely moving in the bulk solution (inhibitor molecule were chaotic), were adsorbed in an orderly fashion onto the mild steel, resulting in a decrease in entropy (Li et al., 2010). Moreover, from thermodynamic principles, since the adsorption was an exothermic process, it must be accompanied by a decrease in entropy (Noor and Al-Moubaraki, 2008).

3.5 UV–visible spectroscopy

A substantial support for the formation of metal complex is often obtained by UV–visible spectroscopic investigation. Since there is often a certain quantity of metal cation in the solution that is first dissolved from the metal surface, such procedures were conducted in the present work to confirm the possibility of the formation of[XION–Fe²⁺] complexes as described in several publications (Sherif and Park, 2005; Rangelov and Mircheva, 1996; Elmorsi and Hassanein, 1999; Abdel-Gaber et al., 2009). Futhermore, Abboud et al. (2007) have reported that change in position of the absorbance maximum and change in the value of absorbance indicate the formation of a complex between two species in solution.

In order to confirm the possibility of the formation of XION–Fe complex, UV–visible absorption spectra obtained from 0.5 M H₂SO₄ solution containing 10 × 10⁻⁶ M XION before and after three days of mild steel immersion are shown in Fig. 11. The electronic absorption spectra of XION before the steel immersion display a main visible band at 380 nm. This band may be assigned to π–π∗ transition involving the whole electronic structure system of the compound with a considerable charge transfer character (Abboud et al., 2009). After three days of steel immersion (Fig. 11), it is clearly seen that the band maximum underwent a blue shift, suggesting the interaction between XION and Fe²⁺ ions in the solution (Abboud et al., 2007). Futhermore, there is an increase in the absorbance of this band. It is clear that there was no significant difference in the shape of the spectra before and after the immersion of XION showing a possibility of weak interaction between XION and mild steel (physisorption). These experimental findings give a strong evidence for the posibility of the formation of a complex between Fe²⁺ cation and XION in 0.5 M H₂SO₄.

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Figure 11

UV-Visible spectra of the solution containing 0.5 M H₂SO₄ (10 × 10⁻⁶ M) XION before (blue) and after three days of mild steel immersion (red).

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3.6 Quantum chemical studies

Recently, the density functional theory (DFT) has been used to analyze the characteristics of the inhibitor/surface mechanism and to describe the structural nature of the inhibitor on the corrosion process (Lashkari and Arshadi, 2004; Sein et al., 2001; Ebenso et al., 2010). Furthermore, DFT is considered as a very useful technique to probe the inhibitor/surface interaction as well as to analyze the experimental data. Thus in the present investigation, quantum chemical calculation using DFT was employed to explain the experimental results obtained in this study and to further give insight into the inhibition action of XION on the mild steel surface.

Figs. 12–15 show the optimized geometry, the HOMO density distribution, the LUMO density distribution and the Mulliken charge population analysis plots for XION molecule obtained with DFT at B3LYP/6-31G (d) level of theory in both the neutral and protonated form in aqueous phase.

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Figure 12

Optimized structure of xanthione (XION) (ball and stick model): (a) neutral molecule and (b) protonated at oxygen.

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Figure 13

The highest occupied molecular orbital (HOMO) density of xanthione (XION) using DFT at the B3LYP/6-31G (d) basis set level: (a) neutral molecule (b) protonated molecule.

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Figure 14

The lowest unoccupied molecular orbital (LUMO) density of xanthione (XION) using DFT at the B3LYP/6-31G (d) basis set level: (a) neutral molecule (b) protonated molecule.

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Figure 15

Mulliken charges population analysis of xanthione (XION) using DFT at the B3LYP/6-31G (d) basis set level: (a) neutral molecule (b) protonated molecule.

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The reactive ability of the inhibitor is considered to be closely related to their frontier molecular orbitals, the HOMO and LUMO. Higher HOMO energy (E_HOMO) of the molecule means a higher electron donating ability to appropriate acceptor molecules with low-energy empty molecular orbital and thus explains the adsorption on metallic surfaces by way of delocalized pairs of π-electrons. E_LUMO, the energy of the lowest unoccupied molecular orbital signifies the electron receiving tendency of a molecule. Accordingly, the difference between E_LUMO and E_HOMO energy levels (ΔE = E_LUMO − E_HOMO) and the dipole moment (μ) were also determined. The global hardness η is approximated as ΔE/2, and can be defined under the principle of chemical hardness and softness (HSAB) (Khaled, 2010). These parameters also provide information about the reactive behavior of molecules and are presented in Table 5. These theoretical parameters were calculated in the neutral as well as in the protonated form of XION in the aqueous phase.

It could be seen from Figs. 13 and 14 that XION can have different HOMO and LUMO distributions. The HOMO densities were mainly concentrated on the sulphur atom while the LUMO densities were located on the entire xanthione moiety. Thus, unoccupied d-orbitals of Fe atom can accept electrons from inhibitor molecule mainly using the sulphur atom to form a coordinate bond. Also the inhibitor molecule can accept electrons from Fe atom with its anti-bonding orbitals to form back-donating bond. Fig. 15 shows the Mulliken atomic charges calculated for XION. It has been reported that the more negative the atomic charges of the adsorbed centre, the more easily the atom donates its electron to the unoccupied orbital of the metal (Xia et al., 2008). It is clear from Fig. 15 that sulphur and oxygen as well as some carbons atoms carry negative charge centers which could offer electrons to the mild steel surface to form a coordinate bond. It should be noted that there are more negative charge centers in the neutral form of XION than in the protonated form. The highest negative charges are domiciled in O1 and S1. This shows that the two atoms are the probable reactive sites for the adsorption of iron. Thus, the neutral form of XION donates more negative charge to the d-orbitals of Fe than in the protonated form.

Higher values of E_HOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy or empty electron orbital. It is evident from Table 5 that xanthione has the highest E_HOMO in the neutral form and a lower E_HOMO in the protonated form. This means that the electron donating ability of XION is weaker in the protonated form. This confirms the experimental results that interaction between XION and mild steel is electrostatic in nature (physisorption). The energy of the LUMO is directly related to the electron affinity and characterizes the susceptibility of the molecule towards attack by nucleophiles. The lower the values of E_LUMO are, the stronger the electron accepting abilities of molecules are. It is clear from Table 4 that the protonated form of XION exhibits the lowest E_HOMO, making the protonated form the most likely form for the interaction of mild steel with xanthione molecule. Low values of the energy gap (ΔE) will provide good inhibition efficiencies, because the excitation energy to remove an electron from the last occupied orbital will be low (Gece, 2008). A molecule with a low energy gap is more polarizable and is generally associated with a high chemical reactivity, low kinetic stability and is termed soft molecule (Dwivedi and Misra, 2010). According to Wang et al. (2007), adsorption of inhibitor onto a metallic surface occurs at the part of the molecule which has the greatest softness and lowest hardness. The result from Table 4 shows that XION in the protonated form has the lowest energy gap and lowest hardness; this agrees with the experimental results that XION could have better inhibitive performance on mild steel surface in the protonated form i.e. through electrostatic interaction between the cation form of XION and the vacant d-orbital of mild steel (physisorption). Moreover, the adsorption of XION on the steel surface using the neutral form also plays a part in the overall inhibiting process. This also agrees well with the value of $\mathrm{\Delta }{G}_{\mathit{ads}}^o$ and $\mathrm{\Delta }{H}_{\mathit{ads}}^o$ obtained experimentally. The dipole moment is another important electronic parameter that results from non-uniform distribution of charges on the various atoms in a molecule. It is mainly used to study the intermolecular interactions involving the Van der Waals type dipole-dipole forces etc, because the larger the dipole moment the stronger will be the intermolecular attraction (Dwivedi and Misra, 2010). The dipole moment of XION is highest in the protonated form (4.52 D; 15.07 × 10⁻³⁰ Cm), which is higher than that of H₂O (μ = 6.23 × 10⁻³⁰ Cm). The high value of dipole moment probably increases the adsorption between chemical compound and metal surface (Li et al., 2009). Accordingly, the adsorption of XION molecules can be regarded as a quasi-substitution process between the XION compound and water molecules at the mild steel surface.

3.7 Mechanism of corrosion inhibition

A number of mechanistic studies on the anodic dissolution of Fe in acidic solutions have been undertaken, and the hydroxyl accelerated mechanism proposed initially by Bockris and Drazic (1962) and reported by Oguzie (2004) has gained overwhelming acceptance:

It has been suggested that anions such as Cl⁻, I⁻, ${\text{SO}}_4^{2-}$ and S²⁻ may also participate in forming reaction intermediates on the corroding metal surface, which either inhibit or stimulate corrosion (Hurlen et al., 1984). It is important to recognize that the suppression or stimulation of the dissolution process is initiated by the specific adsorption of the anions on the metal surface.

According to the Bockris mechanism outlined earlier, Fe electro-dissolution in acidic sulphate solutions depends primarily on the adsorbed intermediate FeOH_ads. Ashassi-Sorkhabi and Nabavi-Amri (2000) proposed the following mechanism involving two adsorbed intermediates to account for the retardation of Fe anodic dissolution in the presence of an inhibitor:

Considering the inhomogeneous nature of metallic surfaces resulting from the existence of lattice defects and dislocations, a corroding metal surface is generally characterized by multiple adsorption sites having activation energies and heats of adsorption. Inhibitor molecules may thus be adsorbed more readily at surface active sites having suitable adsorption enthalpies. According to the detailed mechanism above, displacement of some adsorbed water molecules on the metal surface by inhibitor species to yield the adsorbed intermediate FeM_ads (Eq. (19)) reduces the amount of the species ${\text{FeOH}}_{\mathit{ads}}^{-}$ available for the rate determining steps and consequently retards Fe anodic dissolution (Obot and Obi-Egbedi, 2010a; Omar and Mokhtar, 2010).

From the experimental and theoretical results obtained, we note that a plausible mechanism of corrosion inhibition of mild steel in 0.5 M H₂SO₄ by xanthione may be deduced on the basis of adsorption. The adsorption process is affected by the chemical structures of the inhibitor, the nature and charged surface of the metal and the distribution of charge over the whole inhibitor molecule. In general, owing to the complex nature of adsorption and inhibition of a given inhibitor, it is impossible for single adsorption mode between inhibitor and metal surface.

The adsorption and inhibition effect of XION in 0.5 M H₂SO₄ solution can be explained as follows. XION might be protonated in acid solution mainly on the O1 atom (more negative charge centre) as:

Thus, in aqueous acidic solutions, the XION exists either as neutral molecules or in the form of protonated XION (cations). XION may adsorbed on the metal/acid solution interface by one and/or more of the following ways: (i) electrostatic interaction of protonated XION with already adsorbed sulphate ions, (ii) donor-acceptor interactions between the π-electrons of aromatic ring and vacant d orbital of iron surface atoms, (iii) interaction between unshared electron pairs of O1 and S1 atoms of XION and vacant d orbital of iron surface atoms.

Generally, two modes of adsorption could be considered. In one mode, the neutral XION molecules may be on the surface of mild steel through the chemisorption mechanism involving the displacement of water molecules from the mild steel surface and the sharing of electrons between the heteroatoms and iron. The inhibitor molecule can also adsorb on the mild steel surface on the basis of donor-acceptor interactions between π-electrons of the heterocyclic ring and vacant d-orbitals of surface iron. In another mode, since it is well known that the steel surface bears positive charge in acid solution (Mu et al., 1996), it is difficult for the protonated XION to approach the positively charged mild steel surface (H₃O⁺/metal interface) due to the electrostatic repulsion. Thus, the presence of sulphate ions which have excess negative charges in the vicinity of the interface could favour the adsorption of the positively charged inhibitor molecules. The protonated XION could then adsorb through electrostatic interactions between the positively charged molecules and the steel surface which now has negatively charged sulphate ions on it. Thus there is a synergism between ${\text{SO}}_4^{2-}$ ions and the protonated XION molecules. Similar mechanism has been proposed recently (Ahamad et al., 2010).