This reaction is considered an analogue of reaction of ketenes and aldehydes hydrazines and derivatives [29-33]. The target compound was analysed by FT-IR, 1H NMR and 13C NMR.

Experimental

Synthesis and characterization of the inhibitor

All reactions were carried out under dry nitrogen. I.R spectra were performed on a Mattson Genesis II FTIR instrument. NMR spectra were recorded in CDCl3 on a Bruker 300 MHz instrument using tetramethylsilane (TMS) as an internal standard. Melting points were determined on an Electrothermal T1A F3.15A-instrument.

Synthesis of 4-(4,5-dihydro-1H-pyrazol-5-yl)-N,N-dimethylaniline D

To a solution of 3-(4-(dimethylamino) phenyl) acrylaldehyde (1 mmol) in ethanol (10 mL), an equimolar 98% hydrazine monohydrate was added in the presence of acetic acid. The mixture was maintained under reflux for 2 h, until TLC indicated the end of reaction. Then, the reaction mixture was poured in cold water, and the precipitate formed was filtered off, washed and recrystallized in ethanol.

Metal specimens

Steel samples (0.21% C, 0.38% Si, 0.09% P, 0.01% Al, 0.05% Mn and 0.05% S) are used.

Test solution

The aggressive solution 1 M HCl was prepared by the dilution of commercial analytical grade 37% HCl with bi-distilled water. Prior to all measurements, steel samples were polished with different sand paper up to 1200 grade, washed thoroughly with bi-distilled water and dried after washing with acetone. The concentration range of green inhibitor ranged within 10^-3 and 10^-6 M.

Methods

Mild steel corrosion behaviour in 1 M HCl was investigated in the absence and presence of D with the help of gravimetrical and electrochemical techniques. It was observed that mild steel dissolution rate was very high in 1 M HCl alone, but presence of the inhibitor significantly decreased the corrosion rate of mild steel.

Gravimetrical measurements

Coupons for weight loss measurements were cut into 1.5 × 1.5 × 0.05 cm dimensions with the following composition: 0.09% P, 0.01% Al, 0.38% Si, 0.05% Mn, 0.21% C, 0.05% S and Fe. Prior to all measurements, the exposed area was mechanically abraded with 180, 400, 800, 1000 and 1200 grades of emery papers. The specimens were thoroughly washed with bi-distilled water, defatted, and finally washed with ethanol and dried. Gravimetric measurements were carried out in a double walled glass cell equipped with a thermostated cooling condenser. The solution volume was 100 cm3 standard. The immersion time for the weight loss was 6 h at 25±1 °C. In order to get good reproducibility, experiments were carried out in duplicate, and the value given was an average of individual measures. The corrosion rate (v) was calculated using the following equation:

where W is the average weight loss, S the total area, and t the immersion time. With the corrosion rate calculated, the inhibition efficiency Ew was determined as follows:

V0 and V are the values of corrosion rate without and with inhibitor, respectively.

Electrochemical measurements

Quantum chemical calculations

Density functional theory (DFT) has often been used [34, 35] to describe the interaction between the inhibitor molecule and the surface, as well as the properties of these inhibitors concerning their reactivity. The molecular band gap was computed as the first vertical electronic excitation energy from the ground state, using the time-dependent density functional theory (TD-DFT) approach as implemented in Gaussian 03 [25]. For this sake, some molecular descriptors, such as HOMO and LUMO energy values, frontier orbital energy gap (Egap), molecular dipole moment (μ), absolute electronegativity (χ), global hardness (η), softness (σ), and fraction of electrons transferred (ΔN) were calculated using the DFT method, and have been used to understand the properties and activity of the newly prepared compounds, and to help in the explanation of the experimental data obtained for the corrosion process.

According to Koopman's theorem [36], the ionization potential (IE) and electron affinity (EA) of the inhibitors were calculated using the following equations

Thus, the values of electronegativity (χ) and chemical hardness (η), according to Pearson, with operational and approximate definitions, could be evaluated using the following relations [37]:

Global chemical softness (σ), which describes the capacity of an atom or group of atoms to receive electrons [38], was estimated by using the equation:

The number of electrons transferred (ΔN) was calculated from the quantum chemical method using the following equation:

where χFe and χinh denote the absolute electronegativity of iron and the inhibitor molecule, respectively, and ηFe and ηinh denote the absolute hardness of iron and the inhibitor molecule, respectively. The theoretical value χFe ≈7 eV was used for iron and ηFe = 0 was used, assuming that IE = EA for bulk metals [39].

Results and discussion

Gravimetrical measurements

It is obvious that the inhibition efficiency increased with an increase in the inhibitor's concentration. This behaviour can be explained based on strong interaction of the inhibitor molecule with the metal surface, resulting in adsorption [40]. The extent of adsorption increases with the increase in concentration of the inhibitor, leading to increased inhibition efficiency.

In acidic solutions, the maximum inhibition efficiency was observed at an inhibitor concentration of 10^-3 M (97%). This result can be explained because organic inhibitors suppress the metal dissolution by forming a protective film adsorbed onto the metal surface, and preserve it from the corrosion medium [41].

Adsorption isotherm

where C is the concentration of inhibitor, K the adsorption equilibrium constant, and θ is the surface coverage.

This kind of isotherm involves the single layer adsorption characteristic and there is no interaction between the adsorbed inhibitor molecules and the carbon steel surface.

The value of ΔG⁰ads was negative, indicating that the compound investigated was strongly adsorbed on the carbon steel surface, and showing the spontaneity of the adsorption process and stability of the adsorbed layer on the carbon steel surface. Generally, values of ΔG⁰ up to -20 kJ mol-1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption), while those more negative than -40 kJ mol-1 involve sharing or transfer of electrons from the inhibitor molecules onto the metal surface to form a coordinate type of bond (chemisorption) [42]. Therefore, it can be assumed that the adsorption of D on mild steel surface occurs, first due to electrostatic interactions, and then to desorption of water molecules, accompanied by chemical interaction between the adsorbate and metal surface [43].

Polarization measurements

They seem to be depressed in the centre under real axis and look like depressed capacitive loops. Such phenomenon often corresponds to surface heterogeneity that may be the result of surface roughness, dislocations, distribution of the active sites, or adsorption of the inhibitor molecules.

The EIS measurement reveals that, at the concentration of 10^-3 M, the percentage of inhibition efficiency is the highest (97%). The result strongly supports the observation that 10^-3 M of this compound could work best as an inhibitor.

This behaviour is generally found for systems where inhibition occurred due to the formation of a surface film by the adsorption of the inhibitor on the metal surface [46]. Decrease in CPE, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggested that the inhibitor molecules were absorbed at the metal/solution interface. The values of n obtained for this inhibitor system were close to unity, which shows that the interface behaved in a nearly capacitive way [47].

Quantum chemical calculations

Quantum chemical calculations have been widely used to evaluate the inhibition performance of corrosion inhibitors, which can quantitatively study the relationship between inhibition efficiency and molecular reactivity [48]. By means of this method, the capability of an inhibitor molecule to donate or accept electrons can be predicted with the analysis of global reactivity parameters, such as the energy gap Egap between HOMO and LUMO, chemical hardness η and dipole moment μ.

The higher the value of EHOMO, the greater the ability of the molecule inhibitor to donate electrons, while ELUMO indicates the propensity to accept electrons. By contrast, the lower the ELUMO, the greater the ability of that molecule to accept electrons. Thus, the binding ability of organics to the metal surface increases with an increase in energy of HOMO and a decrease in the value of energy of LUMO [49]. The energy gap Egap indicates the reactivity tendency of organics toward the metal surface [50]. Thus, Egap has been used to characterize the binding ability of organic compounds to the metal surface [51]. A good corrosion inhibitor behaves as a strong Lewis base, and the electronegativity value of inhibitors is an important parameter in terms of electron transfer between metal and inhibitor. The reactivity of corrosion inhibitors may also be discussed in terms of chemical hardness η, softness σ, parameters molecular dipole moment μ, absolute electronegativity χ [52], and fraction of electrons transferred ΔN [53].

This means that the electron-donating ability of (D) is greater in the neutral form. The ELUMO is directly related to the electron affinity and characterizes the susceptibility of the molecule towards attack by nucleophiles. The diminution of the value of ELUMO increases the ability of the electron- accepting molecule. Thus, the protonated form of pyrazole D exhibits the lowest EHOMO, making the neutral form the most likely to interact with D molecule. Low values of the energy and gap Egap in the protonated form of D can indicate a good stability of the formed complex (inhibitor-Fe) on the metal surface, therefore improving the corrosion resistance of mild steel in 1 M HCl medium [54]. The high value of the dipole moment of pyrazole in the neutral and protonated forms probably indicates strong dipole-dipole interactions of D molecules and the metallic surface. Finally, and according to Lukovits, if ΔN <3.6, the inhibition efficiency increased with increasing electron-donating ability at the metal surface [7]. In this study, the value of ΔN for D was less than 3.6 for both neutral and protonated forms. This shows that the increase in inhibition efficiency was solely due to the electron-donating ability of D in both forms.

Inhibition mechanism

From the results obtained from different electrochemical and weight loss measurements, we can conclude that pyrazole D inhibited the corrosion of mild steel in 1 M HCl through its adsorption at the metal/solution interface. D can easily be protonated, because its molecules are made of planer aromatic rings of benzene and the pyrazole ring, and it also contains π electrons. Generally, the inhibition of metallic corrosion in acidic solutions first occurs through electrostatic interaction of protonated molecules with already adsorbed chloride ions; second, through donor-acceptor interactions between the p electrons of the aromatic ring and the vacant d-orbital of the iron atoms; and third, from interactions between unshared electron pairs of hetero atoms and the vacant d- orbital of the iron surface atoms. In this study, the calculated ΔG⁰ads value indicates that the adsorption of D on the mild steel surface follows both physisorption (ionic) and chemisorption (molecular) mechanisms.

Conclusions

The results obtained during this investigation lead us to draw the following main points:

-The pyrazole D or 4-(4,5-dihydro-1H-pyrazol-5-yl)-N,N-dimethylaniline shows good inhibitive properties for mild steel corrosion in 1.0 M HCl solution.

-The inhibition efficiency increases with an increasing concentration of the inhibitor.

-The results of potentiodynamic polarization measurements demonstrate that D behaves as a mixed type inhibitor and could suppress both anodic metal dissolution and cathodic hydrogen evolution reactions.

-D, on the metal surface, obeyed Langmuir adsorption isotherm. The value of the adsorption equilibrium constant shows that the inhibitor is strongly adsorbed on the metal surface.

-The adsorption of D on the steel surface obeys the statistical physics adsorption model. Furthermore, the obtained values of ΔG⁰ads indicate that the adsorption of (D) molecules onto the steel surface is a spontaneous process involving both physisorption and chemisorption mechanisms.

-The quantum mechanical approach may well be able to foretell molecule structures that are better for corrosion inhibition.

References