Novel pyrazole derivatives as inhibitors of stainless steel in 2.0M H2SO4 media: Electrochemical Study

Metallic materials are well known and widely used in various industrial sectors. However, they can be easily corroded in various aggressive environments. The protective action of stainless steel by two organic pyrazole compounds: {1-amino-5,10-dioxo-3-(p-tolyl)-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile} and {1-amino-3-(2-chlorophenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b] phthalazine-2-carbonitrile}in H2SO4 2.0M medium was studied using the electrochemical technics (Electrochemical Impedance Spectroscopy (EIS), potentiodynamic polarization), Scanning Electron Microscopy (SEM) and Energy Dispersion X-ray spectroscopy (EDX). Polarization curves indicate that both compounds act as anodic inhibitors. A suitable equivalent electrical circuit model was used to calculate the impedance parameters. The adsorption study showed that these compounds are adsorbed to the steel surface according to the adsorption isotherm of Langmuir. Effect of temperature was also investigated and activation parameters were evaluated.


Introduction
Stainless steel is widely used in different sectors of industry: chemical 1-3 , food industry 4,5 , pharmaceutical, petrochemical 6 , heat exchangers, reservoirs, and structural materials 7 ... because of their excellent mechanical properties 8 , their ease of manufacture, their relatively low cost especially for implantable medical devices and their reasonable resistance to corrosion. Among all these properties, corrosion resistance of stainless steel has attracted our attention.
Stainless steels are capable of forming passive layers on the surface, giving the material an excellent corrosion resistance. But the use of aggressive agents, usually acid solutions in the industry of cleaning, pickling, descaling and acidification of oil wells [9][10][11] , destroys this passivation, thus making the surfaceactive. Various corrosion protection methods are used to reduce its attack on metallic materials.
Several methods of corrosion protection have been considered so far by several authors 12,13 . These methods can be classified between those that modify the potential of the electrode on the metal surface or those that modify the nature of the metal itself. Cathodic protection and anodic protection are two methods of corrosion control by changing the electrode potential 14,15 .
The nature of the metal can be modified by the use of metal coatings (sacrificial or noble) or by the choice of a material from which a more corrosion-resistant alloy is selected rather than a less corrosion-resistant alloy. However, most frequently, the choice of materials is not an option because of the need to maintain other desirable properties (such as mechanical strength) or is prohibited by cost. In such cases, the modification of the environment by the use of corrosion inhibitors becomes a possible means of corrosion control 16 . However, the use of corrosion inhibitors has been found to be the most effective and the cheapest method for protection and prevention of steel 17 . Current researches are increasingly moving towards the use of non-toxic [18][19][20][21] and stable organic molecules to inhibitor reduce corrosion.
The inhibitory action of these organic compounds, which is generally independent of anodic and cathodic corrosion processes, is related to the formation by adsorption of a more or less continuous but finite thickness barrier, which prevents the solution from accessing the metal. The most efficient and effective corrosion inhibitors are organic compounds containing electronegative functional groups, π-electrons, in conjugated double or triple bonds or aromatic rings. There is also a specific interaction between functional groups containing hetero atoms like nitrogen, sulfur, oxygen, having free lone pair of electrons and the metal surface, which play an essential role in inhibition. When two of these features are combined, increased inhibition can be observed 22,23 . Recently many researchers have reoriented their attention to the use of new organic compounds as corrosion inhibitors such as pyrazole 24,25 , triazole 26,27 , tetrazole 28,29 , pyrazole 30,31 and imidazole 32,33 . Recently, pyrazole has attracted a lot of attention in the field of metallic corrosion inhibition due to their attractive properties, the choice of these compounds is based on molecular structure considerations, i.e., the number of active centers and type of substituents present in these compounds.
Most organic compounds inhibit corrosion via adsorption on the metal surface. This adsorption mainly depends on some physicochemical properties of the molecule, related to its functional groups (e.g. nitrogen, oxygen and sulfur atoms), aromaticity, the possible steric effects and electronic density. This work aims to study the inhibition properties of two pyrazole derivatives 34 on stainless steel corrosion in 2.0 M H2SO4 using potentiodynamic polarization curves, and electrochemical impedance spectroscopy. It also aims to predict the thermodynamic feasibility of these compounds on the steel surface. Also, a detailed investigation of temperature was also carried and discussed to improve a better understanding of the adsorption mechanism of the studied inhibitors.

Material and medium
The corrosive medium is a sulfuric acid solution H2SO4 with a concentration of 2.0M, obtained by diluting 98% concentrated commercial acid with distilled water.
The tested material is stainless steel. The weight content composition of the basic elements of stainless steel is given in Table 1. The steel sample is in the form of a plate, having a surface of the order of 1cm 2 in contact with the electrolyte.
In order to obtain reliable and reproducible results, the stainless-steel samples are prepared before each test by polishing with an abrasive paper of increasing size (grade 200 to 2000), rinsed with distilled water and finally dried. We will examine the influence of the addition of 10 -6 M to 10 -3 M of the two organic products based on pyrazole on the inhibition of corrosion of stainless steel in acidic medium H2SO4 2.0M. The molecular structures of those compounds are illustrated in Table 2.

Electrochemical study techniques
Electrochemical methods can be classified into two techniques: stationary methods and transient methods. From a phenomenological point of view, the characterization of the compound's adsorption is possible by the monitoring of the open-circuit potential. This measurement gives a characterization of the modification of the metal/medium interface. The more quantitative aspect (polarization curves, electrochemical impedance spectroscopy) provides access to values of the physical parameters describing the state of the system (corrosion current, inhibition rate, double layer capacity, charge transfer resistance, ...) 35 .

Stationary methods
The electrochemical study of the behaviour of stainless steel towards the corrosive environment in the absence or in the presence of organic compounds is based mainly on the plots of the polarization curves using an experimental device consisting of a Potentiostat-Galvanostat PGZ 100, a microcomputer with software «Voltamaster4».
Corrosion inhibition efficiency ηPP is defined by the following expression (1): Where i 0 corr and icorr are the corrosion current densities is the absence and in the presence of inhibitor, respectively.
The equilibrium potential of this study is measured after 30 minutes of the sample immersion in the acid medium in the absence and in the presence of both products.
The polarization curve has been plotted in a potential domain corresponding to (-900 mV/Ag/AgCl to 400mV/Ag/AgCl) with a scan rate of 1mV/s. In the drawing of the Tafel lines, we consider only the field of linearity of the cathodic and anodic branches which are limited for this experiment to ±100 mV on either side of the corrosion potential.
These stationary techniques, however, remain insufficient to characterize complex mechanisms, involving several reactive steps and having different characteristic kinetics (which is the case during inhibition processes). The use of transient techniques then becomes essential 36 .

Transient methods
Transient techniques are based on the disturbance of the physical quantities of the electrochemical system so that the system response can be analyzed in a timeinvariant linear domain.
Electrochemical impedance spectroscopy is a non-stationary method that provides information on the elementary steps that make up the overall electrochemical process. Its principle is to superimpose a low-amplitude sinusoidal potential modulation on the electrode's potential and to track the current response for different frequencies of the disruptive signal. The current response is also sinusoidal, superimposed on the current stationery but out of the phase at an angle Ø to the potential. Conversely, a current can be imposed and the potential recorded 37 .
Impedance measurements are made at 298K after half an hour of immersion in an acid medium. The amplitude of the line voltage applied to the abandonment potential is 10 mV peak-to-peak, at frequencies between 100 kHz and 10 MHz, with 10 measurement points per decade with open circuit potential.
The corrosion inhibition efficiency of the steel is calculated from the polarization resistance, according to (2): Where R 0 p and Rp are the polarization resistances in the absence and in the presence of inhibitor, respectively, is the recovery rate.

Surface characterization by SEM/EDX
The determination of the nature of the film formed on the surface of the metal exposed to H2SO4 2.0M solution for 6h in the absence and in the presence of the inhibitors studied was carried out by the scanning electron microscope observation of type Quanta FEG 450 coupled with EDX analyses. Those analyses were realized in MAScIR-Rabat foundation. Figure 1 presents the anodic and cathodic polarization curves for stainless steel in H2SO4 2.0M medium, in the absence and in the presence of P1 and P2 at different concentrations and 298K. An initial analysis of these curves shows that the anodic and cathodic reactions are affected by the addition of the P1 and P2 compounds. The addition of the inhibitor in H2SO4 2.0M solution significantly retarded both cathodic (hydrogen evolution) and anodic reactions (stainless dissolution). This behaviour reflects the inhibitory action of both products 38 . Moreover, the polarization curves shifted toward lower corrosion current densities (icorr) and less negative corrosion potentials (Ecorr) in the presence of the tested compounds. The displacement of the curves was more pronounced in the anodic curves, which corresponds to the dominant anodic inhibition mechanism of the pyrazole derivatives. However, the corrosion potential showed a significant change in the presence of the pyrazole derivatives. Therefore, the two compounds can be classified as anodic type inhibitor 39 . According to Figure 1, the cathodic branches followed typical Tafel behavior throughout a wide range of current and potential which means that cathodic slopes could be calculated accurately. On the other hand, no linear Tafel region was observed on the anodic branches of the curves. Consequently, the values of anodic Tafel slopes (ßa). Table 3 shows the values of the electrochemical parameters determined from the polarization curves previously obtained: corrosion current density (icorr), corrosion potential (Ecorr), cathodic tafel slopes (c) as well as the corrosion inhibitor efficiency ηPP(%). The analysis in Table 3 shows a significant decrease in the corrosion current density (icorr) as the concentration of the tested products increases involving an improvement in inhibitor efficiency.

Stationary electrochemical study
In the presence of inhibitors, the corrosion potential Ecorr moves towards more anodic values. In literature, it has been reported that if the absolute corrosion potential displacement Ecorr (inhibitor) is > 85 mV compared to the blank Ecorr, the inhibitor can be classified as anodic or cathodic, and if the movement in Ecorr is <85 mV, the inhibitor can be considered as mixed type 40 . In our study, the maximum displacement of the corrosion potential Ecorr was more than 85 mV, indicating that P1 and P2 act as anodic type inhibitors.
The compound P2 is therefore classified as the best with maximum inhibition efficiency of 98.4% for a concentration of 10 -3 M, in agreement with the lowest corrosion current density value of 13μA/cm 2 .
This excellent efficiency is explained by the presence of high electronic densities on the molecules due to the existence of the nonbinding doublets of the heteroatoms (N and O) and electrons (π) of the aromaticity, which favors the adsorption process on the metal [41][42][43][44] .
This stationary electrochemical technique, however, remains very limited in characterizing complex mechanisms, involving several reactive steps and having different characteristic kinetics. Thus, the use of transient techniques becomes indispensable.

Transient electrochemical study
To fully understand the mechanism of the process taking place at the electrode/solution interface, the electrochemical impedance measurements were carried out. This technique makes it possible to differentiate reaction processes by their relaxation time.
The results of this method are given as impedance diagrams in the Nyquist ( Figure 2) and Bode planes ( Figure 4). Impedance diagrams of the stainless steel immersed in the H2SO4 2.0M corrosive solutions without and with the addition of the inhibitor P1 and P2 at different concentrations are shown in Figure 2   The Nyquist plots are characterized by two capacitive loops: one at high frequency attributed to the presence of the film, and the second at low frequency attributed to the charge transfer resistance. It is also observed that the size of the loop increases progressively with the addition of different concentrations of the tested inhibitors 45 . This result reflects the influence of the inhibitor on the corrosion process at the steel interface in H2SO4 2.0M as explained in stationary studies.
The Nyquist impedance spectra of stainless steel in H2SO4 2.0M were simulated by the equivalent circuit shown in Figure 3. The following elements are the components of the suggested equivalent circuit: Rs presents the solution resistance, Rct charge transfer resistance, Rf the film resistance, and Qf and Qdl are the elements with constant phase 46 .
The equivalent circuit analyzed all impedance spectra obtained from the steel electrode exposed for 30 min in H2SO4 2M solutions containing P1 and P2 in Figure 3. As can be seen in Figure 3, the capacitor was changed by a constant phase element, indicating the presence of dissimilar frequency response. CPE impedance is defined as follows (3) 47,48 : With Q: a constant whose unity is Ω.cm -2 .s n , ω: Angular frequency in rad s −1 , n is the CPE exponent with -1 <n< 1. ZCPE can represent an inductance (n = -1), a Warburg impedance (n = 0.5), a pure capacity (n = 1), or a resistance (n = 0). The smaller the n value, the higher the surface roughness [49][50][51] .
Electrochemical parameters obtained from impedance diagrams and inhibitor efficiency (E%) are grouped in Table 4. The polarization resistance Rp values of all systems are calculated using the following equation: Rp= Rf + Rct  The Bode Representation confirms the use of two electrical circuits to simulate experimental data.
Where e is the deposit thickness, Ɛis the dielectric constant, Ɛ 0 is the Permittivity of the medium (8.854×10 -14 F cm -1 ) and S the electrode surface.
The inhibitory efficiency (E%) increases with increasing inhibitor concentrations which is related to an increase in Rp values, indicating better adsorption of the inhibitor molecules on the steel surface. These conventional results have been fully explained by many authors 18,59 . Inhibition efficiencies calculated from EIS show the same trend as those estimated from polarization measurements.

Adsorption isotherm
An adsorption study was performed to explain the inhibition effect of organic molecules on the corrosion of steel. This effect is described by two main types of adsorption: physical adsorption and chemical adsorption. It depends on the charge of the metal, its nature, the chemical structure of the organic product and the type of electrolyte.
In an aqueous solution, the adsorption of organic molecules from the solution to the metal interface is usually accompanied by the desorption of water molecules already adsorbed on the metal surface. This adsorption is therefore regarded as a phenomenon of substitutional adsorption, as the following reaction shows (5): Orgaq + xH2Oads ↔ Orgads + xH2Oaq (5) x is the number of water molecules replaced by the inhibitor molecule.
Several types of adsorption isotherms are present to assess the adsorption phenomenon on the metal surface (for example, the Langmuir isotherm, Temkin, Frumkin, etc 33 . The experimental values obtained from EIS are studied for the adjustment of several types of adsorption isotherms. Among the isotherms, the adsorption isotherm of Langmuir shows a fine adjustment of the experimental values. The Langmuir isotherm determined by the metal surface coverage ratio (Ө) is related to the inhibitor concentration by the relationship (6) 60 : Where Kads the equilibrium constant of the adsorption process and Cinh the inhibitor concentration in the solution. The variation of the ratio Cinh/ Ө as a function of the inhibitor concentration is linear.
Where, R is the universal gas constant, and the absolute temperature is denoted by T. The molar concentration of water is expressed in mol/L and in solution its value is 55.5 62 . Generally, values of ΔGads around or less than -20 kJ mol1 suggested that the adsorption process is related with the electrostatic interaction between charged inhibitor molecules and the charged surface of the metal, which is termed as "physisorption" 63 ; while values around or higher than -40 kJ mol1 are associated with sharing of charge or transfer from the inhibitor molecules to the metal surface to form a coordinate type of metal bond termed as "chemisorption" 64 . In the present case of the adsorption of pyrazole derivatives, ΔGads was determined to be -41.4 kJ/mol of P1 and -43.6 kJ/mol of P2, suggesting the chemical adsorption of the inhibitor molecules into the corroding metal surface.

Temperature effect
Temperature is a key factor in inhibitor action. It can modify the metal-inhibitor interaction in the corrosive medium and inform the mode of adsorption of the inhibitor. It is therefore vital to study the influence of these parameters on the action of pyrazole, which is an excellent inhibitor at 298K.
At this study, the inhibition efficiency of P1 and P2 on corrosion of stainless steel in H2SO4 2M without and with the addition of the inhibitor at a concentration of 10 -3 M was carried at temperatures ranging between 298K and 328K. The polarization curves obtained are shown in Figure 6 and the electrochemical parameters derived from these curves are grouped in Table 6. It is remarked that these curves exhibited the Tafel regions. It is also noted that the anodic and cathodic branches increased with increasing temperature. The curves in the cathodic part are parallel, indicating that the reduction of H + ions on the steel surface is done according to the same pure activation mechanism in all temperature range.
The data in Table 6 show clear that current densities increase with increasing temperature; hence a decrease in inhibitory efficiency for the two inhibitors has a concentration of 10 -3 M in H2SO4 2.0 M medium. In addition, it can be noted that the inhibition efficiency decreases slightly in the presence of the inhibitors so that the two inhibitors remain effective against corrosion of the steel in sulfuric acid.

Thermodynamic parameters for activation
The variation in the logarithm of the corrosion rate as a function of the inverse of the absolute temperature (1000/T) is recorded in Figure 7. The curves obtained in the form of straight lines comply with Arrhenius' law (8).
Where Ea is the activation energy, R is the perfect gas constant, k is a pre-exponential factor, T the absolute temperature and icorr is the corrosion current density.
An alternative formula of the equation of Arrhenius allows the determination of the values of enthalpy ΔHa and entropy ΔSa, according to the following equation (9) 65 : Where h: Planck Constant, N: Avogadro number, ΔHa: Activation Enthalpy and ΔSa: Activation Entropy. The variation of ln(icorr/T) as o function of the reciprocal of temperature is a line (Figure 5), with a slope equal to (-ΔHa/R) and an intercept of (ln R/Nh + ΔSa/R).   Table 7. From the Table 7 it is clear that the Ea values of the solution containing P1 and P2 is higher than that in case of uninhibited solution which may be attributed to the formation of compact barrier film on the stainless-steel surface 66 . The higher energy barrier for corrosion process in case of inhibited solutions suggests that adsorbed inhibitor's film prevents the charge/mass transfer reaction occurring on the surface 67,68 and thus protecting metal from dissolution. The positive signs of the enthalpies ΔHa reflected the endothermic nature of the steel dissolution process 69,70 .
Meanwhile the values of activation entropies (ΔSa) increase and negative in the presence of both inhibitors P1meaning that a decrease in the disorder during the transformation of the reagents into activated complex 71,72 , in the case of P2 the value of ΔSa high and positive meaning an increase in the disorder 73 .

Surface analysis by SEM -EDX
SEM images of the sample after immersion in H2SO4 2.0M for 6 hours at 298 K in the absence of inhibitor are shown in Figure 6.
It is noted that the surface of the steel after immersion in H2SO4 2.0M is severely damaged with the formation of several pits randomly distributed on the surface. This clearly shows that the steel undergoes almost generalized corrosion over the entire surface in the absence of the inhibitor. The grey areas correspond to iron oxide films. In the presence of 10 -3 M of the compounds P1 and P2 in the H2SO4 2.0M solution, Figure 7 shows a smoother, more homogeneous and brighter surface. By comparison with the image obtained

P2 P1
without inhibitor, we can conclude that the surface of the steel is almost free of corrosion. This is due to the formation of the adsorbed layers of the inhibitor on the steel surface. These results confirm the ability of products P1 and P2 to protect stainless steel and delay its corrosion in H2SO4 solutions by limiting electrolyte access to the surface.
An EDX analysis (Figure 8) was performed to identify the composition of the steel surface after immersion in H2SO4 2.0M in the absence and the presence of P1 and P2.
The presence of the oxygen peak in the EDX spectrum in Figure 8 clearly shows the formation of iron oxide from the corrosion of steel in H2SO4 medium. The comparison of the two spectra in Figure 9 with the EDX spectrum of the corroded steel in H2SO4 2.0M (Figure 8), shows the absence of the oxygen peak and the presence of a nitrogen peak on the EDX spectra of both compounds. These observations confirm that these inhibitors used appear to prevent corrosion of the steel by forming a layer that limits the access of the electrolyte to the steel surface.

Conclusion
Concluding the experimental part, it was demonstrated that all techniques used, can characterize and to follow the corrosion inhibition process promoted by two pyrazole compounds: P1 and P2 was investigated. P1 and P2 exhibited excellent inhibition properties for stainless steel corrosion in H2SO4 2.0M solution, and their inhibitory efficiency increases with increasing concentration.

P1 P2
The obtained results showed that P1 and P2 acted as anodic type inhibitors of stainless-steel corrosion in H2SO4 2.0M. Electrochemical impedance spectroscopy (EIS) measurement results indicated that the polarization resistance of the stainless steel increased with inhibitor concentrations to reach a maximum at 10 -3 M of P1 and P2 but P2 showed higher protection and performances than P1. This may be due to the presence of chloride atom, which improves adsorption of P2 on a charged metal surface. The adsorption characteristics of P1 and P2 followed the isothermal adsorption model of Langmuir. The study of temperature effect on the inhibition efficiency shows that this one decreases with the temperature increase. Which confirms that the adsorption of both pyrazole compounds is made by intermediate adsorption between physisorption and chemisorptions. Surface morphology studies confirmed the mitigation of stainless-steel corrosion by the formation of a protective film on the surface of the steel.