Experimental and Theoretical Studies of Eosin Y Dye as Corrosion Inhibitors for Carbon Steel in Perchloric Acid Solution

The adsorption behavior and the inhibition performance of Eosin Y Dye for carbon steel corrosion in 1 M perchloric acid solution have been carried using weight loss and scanning electron micrograph (SEM) techniques as well theoretical calculations based on density functional theory (DFT). The studied inhibitor concentrations were between 5´10-5 M and 5´10-3 M. Results obtained revealed that Eosin Y is an effective inhibitor and its inhibition efficiency increases with increasing concentration to attain 96.91% at 5´10-3 M at 30 °C. Thermodynamic parameters such as adsorption heat, adsorption entropy and adsorption free energy were obtained from experimental data of the temperature studies of the inhibition process at five temperatures ranging from 20 to 60 °C. It was found that the adsorption of Eosin Y could prevent steel from weight loss and the adsorption accorded with the Langmuir adsorption isotherm. The free energy of adsorption showed that the corrosion inhibition takes place by spontaneous physicochemical adsorption of inhibitor molecules on the carbon steel surface. SEM and DFT studies confirm the adsorption of Eosin Y on carbon steel surface. Copyright © 2020 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

ing economic collapse in terms of environmental pollution, safety, product losses, repair, and replacement [2]. These negative effects can be minimized or eliminated using diverse techniques to protect the surface of the metal from the aggressive environments. The employment of corrosion inhibitors, to keep metals and alloys against corrosion, has been considered to be the greatest practical method [3,4]. A corrosion inhibitor is a compound, when introduced or added in a little quantity to a corrosive environment regulates the rate of metal dissolution by producing a preventive barrier film, which in turn arrests the corrosive reaction from progressing [5,6]. Organic inhibitors mostly have heteroatoms, triple bonds or aromatic rings and the atoms as O, N, and S, which are the active centers for the process of adsorption on the metal surface [7][8][9]. These atoms represent a higher basicity and electron density and thus act as corrosion inhibitor. The organic compounds react as an effective inhibitor is due to its ability to be adsorbed onto the metal surface. Some organic dyes have been used to confer multi-colour effects to anodised metal surfaces and the ability of such dyes to impart some corrosion protective effects presents a supplementary benefit. Dyes have long been applied in the coloring, paper industries, printing, plastics, leather, cosmetics, textile industry, pharmaceuticals, agri-food industries and the medical industry [10,11].
As a continuation of the work on the use of organic dyes as corrosion inhibitors in acidic solutions, we have probed its inhibiting action on the corrosion of carbon steel in 1 M HClO4. The effects of inhibitor concentration and tempera-ture on the performance and extent of adsorption of Eosin Y dye are also studied.

Material Preparation
Carbon steel samples used as test materials contain: C: 0.37%, Mn: 0.68%, Cu: 0.16%, Cr: 0.077%, Ni: 0.059%, Si: 0.023%, S: 0.016%, Ti: 0.011%, Co: 0.009% and the balance being Fe. Before proceeding to any test, the samples of carbon steel were mechanically abraded with different grades emery paper (100, 400, 600, 800, 1000 and 1200). The weight loss measurements were carried out in the test solution of 1 M perchloric acid, which was prepared by dilution of 70-72 % HClO4 (Sigma-Aldrich) with double deionized water. Each sample was weighed by an electronic balance (± 0.0001 g) and then placed in the acid solution (50 mL).

Weight Loss Method
Weight loss measurements were performed on the carbon steel samples with a rectangular form in one molar perchloric acid medium with and without addition of different concentrations of Eosin Y Dye. The duration of the immersion was 2 h at the temperature range from 293 to 333 K. After immersion, the surface of the specimen was cleaned by double distilled water followed rinsing with acetone and the sample was weighed again in order to calculate inhibition efficiency (IE%) the corrosion rate (CR). All measurements were done in triplicate, and the average value of the weight loss was noted. For each experiment, a freshly prepared solution was used, and the solution temperature was thermostatically controlled at a desired value.
The following equations were used to calculate the corrosion rate (CR), the inhibition efficiencies (IE%) and the surface coverage (θ) [31]: (1) where, ΔW is the weight loss (g), S is the total area of the specimen (cm 2 ), t is the exposure time (h), C'R and CR are respectively, the corrosion rates of carbon steel samples in the absence and presence of inhibitor (g.cm −2 .h −1 ), respectively, θ is the surface coverage. The results

Quantum Chemical Computation
Geometry optimization of both inhibitor molecules and quantum chemical calculations were carried out in the Gaussian-09 program using DFT at B3LYP/6-31G* level of theory. In recent years, the DFT methods have become very popular because they can achieve the same exactitude to other methods in less time and they are less expensive from a computational point of view.
The HOMO energy is related to the ionization potential (I), whereas the LUMO energy is linked to the electron affinity (A), as shown below [32]. (4) The energy gap (∆E) and the global electronic chemical potential () are determined as follow [33]. (5) The electronegativity () and the global hardness () can be calculated as a function of I and A by equations 7 and 8 [34].
Global softness () demonstrates the capacity of an atom or group of atoms to receive electrons and it is estimated by using the equation [35].

Effect of Immersion Time and Concentration on Corrosion Rates and Inhibition Efficiency
The weight loss measurements were studied in 1 M HClO4 in the absence and presence of different concentration of Eosin Y of 510 -5 to 510 -3 M for a period ranging from 30 min to 24 h immersion time at temperature 303 K. According to Table 1, it was found that the inhibition efficiency of Eosin Y increased up to 96.91% at higher concentration which is 510 -3 M at 2 hours immersion time. Therefore, it can be concluded that the inhibition efficiency increased with increasing of immersion time due to the stability of the adsorbed layer on the carbon steel surface until 2 hours of immersion at different concentrations of inhibitor. The increasing inhibition efficiency with increasing immersion time is due to the increase in the surface coverage with time until to two hours immersion then it decreases after this time. It was found that when the concentration of Eosin Y inhibitor increases the corrosion rates decreases. From Table 1, it can be seen that the inhibition efficiencies increase with the increase in Eosin Y concentration. The surface coverage, θ for different concentrations of inhibitors in 1.0 M HClO4, was calculated and it was found that Eosin Y proficiently covers the surface of carbon steel.

Temperature Effect
The temperature is an important factor in the metal dissolution process, the effect of temperature upon the corrosion and corrosion inhibition of carbon steel in perchloric acid, in the absence and presence of various concentrations of the Eosin Y inhibitor was investigated by weight loss measurements after immersion at 293 to 333 K for two hours. In the Figure 1  rate with increasing temperature both in the presence and absence of inhibitor. This increase in corrosion rate can be explained by desorption of the adsorbed Eosin Y molecules from the carbon steel surface at higher temperature. The high rate of dissolution of carbon steel in perchloric acid solution observed at elevated temperature could be attributed to more dissolution energy effect acquired by the corrosive agent within the aggressive medium.

Adsorption Considerations
The protective layer on the metal surface resulting from the adsorption of the inhibitor on the surface during acid induced corrosion regulates the corrosive effect on the metal surface by either homogeneous or heterogeneous layer adsorption [38]. The adsorption isotherm explains the interaction between inhibitor molecules and the carbon steel surface. Corrosion inhibition of carbon steel in the presence of inhibitor has been studied to the adsorption on the carbon steel surface, and this was generally confirmed from the fit of the experimental data to different adsorption isotherms.
The plots of C/θ versus C yield a straight line with approximately unit slope, indicating that the inhibitor under study follows the Langmuir adsorption isotherm (Figure 2). The Kads values can be calculated from the intercept lines on the C/θ-axis. According to this isotherm, θ is related to C by this equation. (11) where, Kads is the equilibrium constant of the adsorption reaction and θ is surface coverage value.
The results of the present studies at 293 K to 333 K indicated that the mechanism of adsorption of inhibitor onto the carbon steel surface is better obeyed by the Langmuir adsorption isotherm ( Table 2). The free energy of the adsorption of inhibitor on carbon steel surface can be evaluated with the following equation [39,40]: (12) where R is the gas constant (J.mol -1 .K -1 ) and T is the absolute temperature (K). 55.5 is the concentration of water in solution (mol.L -1 ). The high value of the equilibrium constant at lower temperature reflects the high adsorption ability of the Eosin Y on carbon steel surface [41]. The value of adsorption enthalpy is -44.06 KJ.mol -1 for Eosin Y, confirming the exothermic behavior of adsorption on the carbon steel surface. The negative value of enthalpy of adsorption reflects the exothermic nature of the adsorption of Eosin Y on the carbon steel surface in the perchloric acid solution.
The negative values of free energy for Eosin Y are equal to -34.81 to -36.55 KJ.mol -1 , these negative signs indicate the stability of the adsorbed layer on the carbon steel surface and the spontaneity of the adsorption process while positive signs symbolize a non-spontaneous adsorption process. This result indicates that the adsorption process of this inhibitor involves both physisorption and chemisorption mechanisms.
Generally, physisorption is the electrostatic interaction between the charged molecules against the charged metal surface. Values of ΔGads until −20 KJ.mol -1 are consistent with the physical adsorption while those lower than −40 KJ.mol -1 are correlated with the chemisorption [42].

Thermodynamic Parameter
The increase in energy of activation for corrosion process in inhibitor solution further interpreted as physical adsorption of inhibitor species on carbon steel surface. The Arrhenius equation can be employed with success to show the action of temperature on the inhibition performance of studied compounds. It is represented by the Equation (13) [43]. (13) where CR is the corrosion rate of carbon steel, A is the Arrhenius pre-exponential factor, Ea (KJ.mol -1 ) is the activation energy, R is the gas constant (8.314 J.mol -1 .K -1 ) and T is the temperature (K). The values of Ea (Table 3) for carbon steel in 1 M HClO4 without and with different concentrations of inhibitors were obtained from the slope of the plot of ln(CR) versus 1/T (Figure 3).
where h is plank's constant, N is Avogadro number R is the gas constant. A plot of ln(CR/T) vs 1/T gave a straight line with slope of (-ΔHa/R) and intercept of [ln(R/Nh)+(ΔSa/R)] from which the values of ΔHa and ΔSa can be calculated ( Figure 4).
These values of ΔHa, ΔSa and ΔGa are tabulated in Table 3. Table 3 indicating a higher protection efficiency [46]. The lower value of apparent activation energy of corrosion process in the presence of the inhibitor when compared to that in absence of inhibitor is attributed to its chemisorption adsorption, its physisorption is pronounced in the opposite case. In this work, Ea of inhibited solutions is higher than that of the uninhibited solution indicating a physical mode of adsorption.
The positive values of ΔHa indicate that the formation of the activated complex is an endothermic process [47]. The high negative value of entropy in the absence of inhibitor involves that the activated complex is the rate-determining step, rather than the dissociation step.
While in the presence of the inhibitor, the value of entropy increases, this is generally explained as an increase in disorder as the reactants are converted to the activated complexes. The shift towards positive value of entropies (from 110 -3 M) imply that the activated complex in the rate determining step represents dissociation rather than association, meaning that disordering increases on going from reactants to the activated complex. The positive value of activation free energy, meaning nonspontaneous corrosion reaction, increases with increasing concentration of inhibitor.

Quantum Chemical Calculations
The electronic structure of the Eosin Y was elucidated using the B3LYP/6-31G* level of theory. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular or-bital (LUMO) studied for Eosin Y are showed in Figure 5. The calculations of global reactivity indices of the inhibitors such as the energy of the highest occupied (EHOMO) and the lowest unoccupied (ELUMO) molecular orbitals, energy gap (ΔE), global electronic chemical potential (µ), hardness (), electrophilicity index (ω), softness () and the fraction of electron transfer (ΔN) from inhibitors to carbon steel were also calculated and discussed with inhibition efficiencies. The parameters named above are given in Table 4. The energy gap value (2.98 eV) indicates that Eosin Y is very reactive and can be adsorbed on steel surface, thus it can be considered as an efficient corrosion inhibitor. The ΔN value is correlated with the inhibition effectiveness, resulting from electron donation. If the fractions of electrons transferred values are lower than 3.6 eV, the inhibition efficiency increases with increasing electron-donating capacity at the metal surface [48].
In this study, the charge transfer is of the order of 1.60 eV shows that the Eosin Y inhibitor is a strong electron donor power. A high electrophilicity () value characterizes a good electrophile while a small electrophilicity value describes a good nucleophile, this index is helpful for the prediction of the direction of corrosion. Generally, whenever the softness index () is higher means that the adsorption is occurring [49]. The chemical hardness () is an important quantum chemical that provides an estimation of corrosion inhibition efficiencies of molecules [50].

Scanning Electron Microscopy (SEM)
Scanning electron microscopy was probed to study the carbon steel specimen surface morphology. Figure 6 shows the SEM images of carbon steel specimens immersed to the uninhibited, inhibited acid media (for 24 h) and polished surface. The polished carbon steel electrode surface before immersion was uniform (Figure 6a). After immersion time in corrosive attack, the surface was strongly deteriorated due to corrosion, this consequently initiates pitting or crevice corrosion, which is more pronounced for the steel surface in the absence of inhibitor as revealed from the large pit depth in (Figure 6b) [51]. In the presence of Eosin Y, thin film onto the carbon steel surface were formed as shown in Figure 6c. it is marked that there is a protective layer adsorbed onto the carbon steel surface which is responsible for corrosion inhibition.

Conclusion
The obtained results show that Eosin Y dye acts as an excellent effective inhibitor for the corrosion of carbon steel in 1 M HClO4 solution. The increase in the inhibitor concentration decreases the corrosion rate while the increase in temperature leads to an increase in the corrosion rate. The inhibition efficiency increases with the increase in the concentration of Eosin Y dye. The adsorption of Eosin Y on the carbon steel surface follows the Langmuir adsorption isotherm. The values of thermodynamic parameters show that the adsorption process is a physicochemical adsorption process. The corrosion inhibition ability of Eosin Y has been confirmed via both scanning electron microscopy and quantum chemical calculations.