Abstract

Corrosion is a phenomenon that needs to be monitored with great care, as it is becoming more widespread in many areas. The present work was carried out to test 4-hydroxy-N-[(5-nitrofuran-2-yl) methylene] or nifuroxazid as a corrosion inhibitor for aluminium corrosion in 2M HCl. This work was evaluated by mass loss technique and quantum chemical calculations. The results show that this compound has a good inhibition capacity in medium studied. Compound inhibition efficiency decreases with increasing temperature, while increasing compound concentration is accompanied by a decrease in corrosion rate and thus an increase in inhibition efficacy. Isotherms adsorption study shows that inhibitor adsorbs on aluminium surface according to the modified Langmuir model. Thermodynamic values for adsorption and activation show that adsorption is spontaneous and exothermic, with an increase in disorder. The adsorption process was found to combine chemical and physical adsorption, with physisorption predominating. The mechanism of inhibition was explained by quantum chemical parameters derived from density functional theory (DFT) calculations, and reactivity sites determination which to identify electrophilic and nucleophilic centers of attack.

1. Introduction

Because of its superior mechanical and chemical qualities, aluminium and its alloys are often employed in a broad domain of technical and technological processes, such as automobiles, household appliances, aluminium containers, electronic devices, building, aviation etc. The demands for long-term performance of engineering structures over a wide size scale continue to increase. Unfortunately, one of the biggest drawbacks of employing aluminium based alloys is that it is liable to corrosion ^{1, 2, 3, 4}. Corrosion can be defined as the deterioration of a material’s properties due to its interaction with its environment. Aluminium corrosion can be costly to industries and result in significant losses due to lost production, inefficient operation, and high maintenance. Aluminium corrosion is an inevitable but manageable phenomenon.

It has been found that one of the best methods of protecting metals against corrosion involves the use of inhibitors which are substances that slow down the rate of corrosion ^{5, 6}. Therefore, the development of corrosion inhibitors based on organic compounds containing nitrogen, oxygen atoms is of growing interest in the field of corrosion and industrial applications ⁷. Organic compounds are believed to reduce corrosion by adhering to metal surface. The steric effect, molecular weight, structure, solution temperature, and the metal/solution interface all have an impact on the adsorption in addition to the electron density at the donor site (aromatic rings and π-bonds) ^{8, 9, 10, 11, 12}. To address environmental demands, the use of organic inhibitors was suggested. Due to their toxicity, inorganic inhibitors (such as nitrites, chromates, silicates, phosphonates, and salts of zinc, cadmium and arsenic salts) which were broadly applied for many years in metals protection against corrosion, are avoided now ¹³.

Recently, several studies have been carried out on metals inhibition corrosion by drugs ^{14, 15, 16, 17, 18, 19, 20}. Moreover, the pharmaceutically active compounds are often big enough (molecular mass) and likely to effectively cover more surface area (due to adsorption) of aluminium. Furthermore, drugs are relatively cheap, easily available, and environmentally friendly and most importantly is nontoxic. In view of these favourable characteristic properties, drugs were chosen for the corrosion studies ^{21, 22}.

Although experimental study provides information about the inhibitory efficacy of a compound in metal corrosion, a thorough understanding of the property and mechanism of inhibition remains of crucial interest. This is why theoretical methods have been introduced to explain the inhibition mechanism. Indeed, DFT, which includes electronic correlation in its formalism, provides a better explanation of metal-molecule interactions ^{23, 24, 25}. As a result, numerous studies in the literature ^{26, 27, 28, 29, 30, 31} have reported the use of DFT calculations in the inhibition of aluminium corrosion in acidic media. It has been shown that the selection of effective corrosion inhibitors ^{32, 33} is based on a good correlation between experimental and theoretical data.

The aim of this study is to examine the behaviour of 4-hydroxy-N-[(5-nitrofuran-2-yl)methylene]benzohydrazide or nifuroxazid for aluminium corrosion in 2M hydrochloric acid. Therefore, it is necessary to determine the adsorption capacity of this compound on aluminium surface, and reactivity sites involved in adsorption.

2. Material and Methods

The aluminium samples were in the form of rod measuring 10mm in length and 2mm in diameter. The samples were polished with abrasive paper ranging from 150 to 600 grit, cleaned with acetone, washed with distilled water and dried in an oven. The samples were then weighed (m₁).

All reagents and solvents used in the experiment were of analytical grade and used without further purification. The aggressive solution of 2.0 M HCl was prepared by diluting the analytical grade 37% (Merck Chemicals) with distilled water. Acetone from Sigma Aldrich with purity: 99.5%. The inhibitor nifuroxazid (Figure 1) was acquired from Sinopharm chemical reagent. Four concentrations have been prepared from this inhibitor which are 1.45 mM, 3.63mM, 7.26 mM and 14.5 mM.

After sample preparation, each sample was immersed in 50 ml of 2M HCl without or with different concentrations of nifuroxazid. A SELECTA water thermostat (FRIGITHERM) was used to maintain test solution temperatures from 298 to 328K. After 1 h, samples were removed, washed with distilled water, dried and weighed (m₂). All tests were carried out in aerated solutions. Mass losses were determined on the basis of three tests. The expressions below are used to determine the gravimetric quantities that are corrosion rates 𝑊, degree of surface coverage (θ) and inhibition efficiency (IE).

(1)

Where and are respectively the mass before and after immersion in the test solution, S is the total surface of the sample and t is the immersion time.

(2)

is the corrosion rate without the tested compound and is the corrosion rate in presence of tested compound.

In the present investigation, density functional theory (DFT) was used to explain nifuroxazid reactivity. This method, which stems from quantum chemistry, provides detailed information on the corrosion inhibition mechanism. Gravimetry provides access to compound studied inhibition efficiencies at different temperatures and concentrations, without explaining the metal-molecule interactions. In order to understand these interactions, DFT calculations were performed in gas phase using Gaussian 09 software package ³⁴ was used with Becke’s three parameter exchange functional along with the Lee-Yang-Parr B3LYP ^{35, 36}, on 6-31G(d, p) basis set. Parameters such as highest occupied molecular orbital energy (E_HOMO ), lowest unoccupied molecular orbital energy (E_LUMO)_, energy gap (ΔE), dipole moment (μ), total energy (E_T), electron affinity (𝐴), the ionization energy (𝐼), electronegativity (χ), hardness (η), softness (σ) and electrophylicity index (ω) are related to ionization energy (𝐼) and electron affinity (𝐴 ), fraction of electrons transferred (ΔN) were calculated. Figure 2 shows nifuroxazid optimized structure in B3LYP/6-31G(d,p).

3. Results and Discussion

The evolution of the corrosion rate versus temperature and concentration is shown in Figure 3 and Figure 4 indicates the inhibition efficiency variation as a function of temperature and inhibitor concentration.

Figure 3 reports clearly that corrosion rate in nifuroxazid absence is high, and that this rate increases with rising temperature, but is reduced as nifuroxazid concentration increases. With regard to Figure 4, inhibition efficiency decreases with increasing temperature and increases with increasing nifuroxazid concentration. These observations suggest that nifuroxazid presence in aggressive media creates a physical barrier capable of slowing down metal oxidation.

It can also be said that higher temperatures lead to weaker adsorption of nifuroxazid on metal surface, resulting in lower inhibition efficiencies at higher temperatures. In fact as the temperature rises, aluminium loses a lot of electrons, and nifuroxazid is unable to supply enough electrons to replace this loss when it is absorbed by metal surface, hence the drop in inhibition efficiency at high temperatures. While, increasing inhibitor concentration accelerates its adsorption to aluminium surface, which justifies the increase in inhibition efficiency as nifuroxazid concentration rises. Similar results have been observed in the literature with some phthalocyanine and thalocyanine deravatives and jasminum nudiflorum leaves extract ^{37, 38, 39, 40}.

Heterogeneous reactions are generally understood through the study of adsorption isotherms. In fact, inhibition by organic molecules on the surface of a metal is governed by the adsorption process. The adsorption isotherms study involved in the process of metals corrosion inhibition by organic molecules allows to show how these compounds bind to the surface of a metal. The following relation ⁴¹ provides the general form of these isotherms.

(3)

Where is the configurational factor subject to the physical model and assumptions involved in the derivation of the isotherms. is the inhibitor concentration, θ is the surface coverage. is the equilibrium constant of the adsorption process and g is a parameter that expresses the interaction of the molecules in the adsorbed layer.

In this work we attempted various adsorption isotherms and selected those that better reflect Nifuroxazid behaviour on aluminium surface. So we have retained Langmuir, Temkin, El-Awady and Freundlich isotherms. The equations that define these isotherms are expressed in Table 1.

is nifuroxazid’s concentration

is the equilibrium constant of the adsorption process

is a factor of energetic inhomogeneity in the surface

is surface coverage

is active site occupied by inhibitor molecule.

Figure 5, Figure 6, Figure 7 and Figure 8 show the representation of these different isotherms. All the tested isotherms yield straight lines as shown in Figures.

Table 2 gives the different parameters of studied isotherms. By looking the Table 2, it is clear that the correlation coefficients of Langmuir isotherm are closer to unity than the other isotherms. The deviation of the slopes from unity shows that this model cannot be applied rigorously. However, Temkin and El-Awady models can be applied. For Temkin model ⁴¹, the parameter f (where 2.303/f is the slope of straight lines) having a positive value, there would be repulsion forces between the molecules adsorbed on aluminium. As for El-Awady model ⁴², the inverse of the slopes (1/y) of the straight lines obtained is greater than unity, this means that a nifuroxazid molecule occupies more than one site. Langmuir adsorption model requires that the interactions between adsorbed particles are negligible and that each site can adsorb only one particle ⁴³. In this case, nifuroxazid adsorption on aluminium is not rigorously done according to Langmuir model; it is done according to the modified Langmuir isotherm or Villamil model ⁴⁴. This model represented by the equation:

(4)

where represents the concentration of the inhibitor, θ is the surface area and is the adsorption equilibrium constant. The values were obtained by a linear straight fitted plot between and to obtain free energy of adsorption .

A better understanding of the adsorption behaviour of an inhibitor can be obtained from thermodynamic adsorption parameters values. The free adsorption enthalpy () is calculated using the following relation ⁴⁵:

(5)

Where R is the perfect gas constant, T is absolute temperature and the constant 55.5 is the molar concentration of water. K_ads is the equilibrium constant of the adsorption process. The values of adsorption equilibrium constant are deduced from the parameters of the modified Langmuir isotherm (intercept of straight lines).

With regard to the other thermodynamic adsorption parameters, adsorption enthalpy () and adsorption entropy (). They are calculated using the following relationship:

(6)

Plotting versus temperature gives a straight line (Figure 9) whose slope and intercept permit to determine and respectively. The different thermodynamic adsorption parameters are recorded in Table 3.

It can be clearly seen from Table 3 that , this suggests that nifuroxazid is spontaneously adsorbed onto aluminium surface. Moreover can be used to judge the absorption nature. In fact, when values are between -20 kJ/mol and -40 kJ/mol, adsorption is both chemical and physical. values obtained in this work fall within this range, indicating physisorption and chemisorption existence ⁴⁷. From the equation of the straight line, we deduce the values of change in adsorption enthalpy and that of change in adsorption entropy .The negative sign of change in adsorption enthalpy indicates an exothermic adsorption process while the positive sign of shows that disorder increases probably due to desorption of water molecules.

In order to identify the temperature ranges corresponding to each adsorption mode, Adejo-Ekwenchi isotherm ⁴⁶ was used. This model is based on the following equation:

(7)

Where is the concentration of the adsorbate, and b are the isotherm parameters.

Figure 10 shows the representation of this isotherm. The parameters issued for this isotherm are listed in Table 4. It can be noted from Table 4 that b parameter values decrease from 298K to 318K, reflecting a physical adsorption process ⁴⁸. Whereas b values remain constant from 318K to 323K, indicating a chemical adsorption process ⁴⁸.

The mass loss technique was used to examine temperature effect on corrosion rate and inhibition efficiency. This effect permits to determine corrosion activation energies in the absence and presence of inhibitor, as well as enthalpy and entropy activation. These kinetic and thermodynamic parameters of aluminium dissolution process are determined from Arrhenius equation (20) and transition state equation (21):

(8)

(9)

Where is the apparent activation energy, R is the molar gas constant, is the frequency factor, h is the Planck’s constant, is the Avogadro number, is the change in activation entropy and is the change in activation enthalpy.

Figure 11 gives the plots of versus; the values of activation energy are obtained from the slopes of the straight lines. All the calculated parameters are listed in Table 5. Figure 12 shows the plots of versus. The slopes and intercession of the straight lines obtained lead to the determination of entropy and enthalpy activation values respectively.

The high values of and obtained in the presence of Nifuroxazid certifies that that aluminium dissolution becomes increasingly difficult as inhibitor concentration increases, confirming physisorption predominance ^{49, 50}. This decrease in dissolution is linked to the gain of electrons that metal receives from inhibitor. values are positive in inhibitor presence, attesting that complex activated formation in the rate-determining step is dissociative rather than associative and can be interpreted as desorption of the absorbed species ⁵⁰, resulting in increased disorder ⁵¹.

Mass loss technique showed that nifuroxazid adsorbs to aluminium surface through chemical and physical interactions, thereby decreasing metal dissolution in 2M HCl. Quantum chemical parameters derived from DFT were solicited to explain the mechanism of this decrease. These parameters were calculated from DFT/6-31G(d, p) and are recorded in Table 6. This table examination, according to the literature, reports that value is high. This value indicates that nifuroxazid can donate electrons to aluminium ^{52, 53, 54}. As for value is low, indicating that inhibitor can receive electrons from metal ^{52, 55}. Electron-donating and electron-receiving properties of nifuroxazid reinforce metal-molecule interactions. These interactions promote covalent bonds formation that help to create a protective film on aluminium surface. Density distribution of the inhibitor's molecular orbitals is shown in Figure 13, with HOMO orbital almost distributed over entire molecule surface and LUMO orbital partially distributed. These different distributions describe inhibitor reactivity. This reactivity also depends on the value of . Indeed, a low value indicates that the molecule is unstable, and the more readily it reacts with the metal to form coordination bonds. value obtained from the relationship indicates that nifuroxazid is reactive ⁵⁶ and that the distribution as indicated by HOMO orbital confirms the predominance of its electron-donor character. Electron donor-acceptor properties are also confirmed by ionization potential (𝐼) and electron affinity (𝐴) values ⁵⁷. In fact, these two parameters 𝐼 and 𝐴 are linked by HOMO and LUMO energy respectively related through expressions and. Absolute hardness and softness () are very important parameters to describe molecular reactivity and stability ⁵⁷. Soft molecules are more reactive than hard ones because they can easily offer electrons. Hence, inhibitors with highest values of global softness the least values of the global hardness) are expected to be good corrosion inhibitor for bulk metals in acidic media. The calculations indicate our inhibitor have high softness values compared to the literature ⁵⁸. Fraction of transferred electrons determined from meta and molecule electronegativities is expressed as follows:

with ⁵⁹ et ⁵⁹. value is positive, certifies that aluminium can attract electrons from the inhibitor. This property enables nifuroxazid to replace the electrons lost by aluminium during oxidation. As far as dipole moment µ is concerned, there are many contradictions in the literature about the correlation between this parameter and inhibition efficiency ^{59, 60}, so no interpretation between two quantities can be made in this work. Molecule’s ability to receive electrons from metal is confirmed by high value of Electrophilicity index ⁶¹.

Theoretical reactivity descriptors interpretation indicates that nifuroxazid has both electron-donating and electron-accepting properties. These properties are due to the presence of oxygen atoms (O), nitrogen atoms (N) and π-bonds capable of ensuring electron exchange. In addition, the molecule is protonated in hydrochloric acid. At low temperatures, this creates a strong interaction between protonated species and the charged ions on metal surface.

This strong interaction permits the inhibitor to adsorb on aluminium surface via electrostatic bonds, which provide protection at low temperatures, justifying the high inhibition efficiencies obtained between 298K and 313K.

However, at high temperatures, aluminium loses many electrons, and the electrostatic bonds are destroyed by covalent bonds formation. These covalent bonds fail to establish a good protective layer on metal surface to reduce its high electron loss, as quantum parameters such as E_LUMO, mention that some of these lost electrons are donated to molecule. These data attest to the low inhibition efficiency values obtained at high temperatures. Despite the above, there is a good correlation between global quantum chemical parameters derived from DFT and the experimental results. Similar correlations have been reported in previous studies ^{62, 63}.

To get some insight into the local selectivity of the studied inhibitors, the Fukui functions are computed since they are the reactivity indicators of choice in the study of electron transfer controlled reactions such as corrosion inhibition process ^{64, 65, 66}.

Calculated Fukui function and dual descriptor of nifuroxazid are presented in Table 7. These functions help to distinguish each part of the molecule according to its behaviour due to the different functional groups. Thus the site for nucleophilic attack will be the site where value and are the highest values. While, electrophilic attack site is determined by the highest value and the lowest value of . According to the literature, in case of ambiguity, the values of dual descriptors are used to indicate the sites of reactivities, because the dual descriptor is a more accurate local reactivity descriptor than Fukui functions [67]. Based on this information, 26 O is the most probable site to receive nucleophilic attacks while 14C is the most likely center for electrophilic attacks.

4. Conclusion

This study revealed the following key points:

Ø Nifuroxazid has good inhibition potential in 2M aluminum corrosion at low temperatures.

Ø Experimental results show that nifuroxazid inhibition efficiency increases with its concentration, but decreases as corrosive solution temperature rises.

Ø Inhibition efficiency reaches at a maximum value of 89,45% for 14.5mM concentration at 298K of temperature.

Ø Nifuroxazid adsorbs on aluminium surface according to Langmuir isotherm or Villamil model.

Ø Thermodynamic activation parameter values suggest that inhibition mechanism is influenced by physical adsorption.

Ø Local reactivity parameters indicate that atoms 26 O and 14C are the likely sites of nucleophilic and electrophilic attack respectively.

Quantum chemical calculations data from B3LYP/6-31G (d, p) are in good agreement with experimental results.

References