Modazar as promising corrosion inhibitor of carbon steel in hydrochloric acid solution

ABSTRACT Modazar drug was tested as corrosion inhibitor for carbon steel in 1 M HCl solution using chemical (weight loss and gasometric) and electrochemical (open {circdeg;}uit potential, potentiodynamic polarization, electrochemical frequency modulation and electrochemical impedance spectroscopy) techniques. The results showed that the inhibitory efficiency increased with the increase in drug concentration reaching a maximum value of 92.3% at 300 ppm while it decreases with increasing the temperature. Polarization curves showed that Modazar drug is a mixed type inhibitor but the cathode is more polarized than the anode. The drug was adsorbed physically on the C-steel surface obeying Langmuir adsorption isotherm. The morphology of the surface of the specimens was analyzed using atomic force microscopy, energy dispersion spectroscopy and scanning electron microscopy. The results obtained from chemical and electrochemical techniques are in good agreement. GRAPHICAL ABSTRACT


Introduction
Corrosion is a principal process assuming an important role in economics and safety especially for metals (1). The utilization of inhibitors is one of the best methods for protection corrosion of metals especially in acidic media (2). Most well-known acid inhibitors are organic compounds containing nitrogen (N-heterocyclic), sulfur, long carbon chain or aromatic and oxygen atoms. Organic heterocyclic compounds have been used for the corrosion inhibition of carbon steel (3)(4)(5)(6)(7)(8), copper (9), aluminum (10)(11)(12) and other metals (13) in different aqueous media. A large number of organic compounds were studied as corrosion inhibitors; unfortunately most of these organic compounds are very expensive and health hazards. Their toxic properties limit their field of applications. Thus, it remains an important object to find cost-effective and non-hazardous inhibitors for the protection of metals against corrosion. In this connection, the influences of nontoxic organic compounds and drugs on the corrosion of metals in acid media were investigated by several authors (14)(15)(16)(17)(18)(19)(20)(21)(22). The use of drugs offers interesting possibilities for corrosion inhibition due to the presence of heteroatoms in their structures, and they are of particular interest because of their safe use, high solubility in water The objective of this work is to study the inhibitive action of Modazar drug for the corrosion of carbon steel in 1 M HCl by different techniques and at different temperatures. Also, a number of attempts have been made to understand different aspects of corrosion, such as mechanism, thermodynamics and kinetics of corrosion.

Materials
The composition of the carbon steel specimen is listed in Table 1.

Inhibitors
Modazar drug is a mixed compound, which consists of two active ingredients: Hydrochlorothiazide (25 mg)/Losartan (100 mg), described in Table 2. This drug was purchased from Egyphar, Pharmaceutical Company, Egypt. The drug is used to treat high blood pressure (hypertension) and diabetic nephropathy.

Solutions
The aggressive solution, 1 M HCl, was prepared by dilution of analytical grade (37%) HCl with bi-distilled water. The concentration range of the drug used was 50-300 ppm.

Weight loss measurements
Seven square carbon steel sheets of 2 × 2 × 2 cm were abraded using different grades of emery paper upto1200 grit size and then washed with bi-distilled water and acetone. After accurate weighing, the specimens were dipped in a 100 ml beaker, which contained 100 ml of 1 M HCl with and without adding different concentrations of the investigated drug.
All aggressive acid solutions were opened to air. After three hours, the specimens were taken out, washed, dried and weighed accurately. The average weight loss for seven square C-steel specimens were obtained. The inhibition efficiency (% IE) and the degree of surface coverage (θ) of Modazar for the corrosion of C-steel were calculated as follows (22): where W°and W are the weight loss without and with adding different concentrations of investigate drug, respectively.

Gasometric measurements
Measurements of hydrogen evolution (HE) were estimated at 25°C, and the hydrogen volume developed  (3) and (4).
where V is the volume of hydrogen in cm 3 , k is the rate constant and t is the time in minutes.
where k°and k are the rate constants of corrosion in the absence and presence of the drug, which were calculated by plotting V vs. t and k value is the slope.

Electrochemical measurements
Electrochemical measurements including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) were performed in a three-electrode cell at 25°C. The auxiliary electrode is a platinum sheet (1 cm 2 ), saturated calomel electrode (SCE) coupled to a fine Lugging capillary as reference electrode and the working electrode was in the form of a square cut from C-steel embedded in epoxy resin of polytetrafluoroethylene (PTFE) so that the flat surface was the only surface of the electrode in 1 cm 2 area of the surface. The EIS measurements were carried out in the frequency range of 100 kHz-100 mHz at the open circuit potential by superimposing a sinusoidal AC signal of small amplitude, 5 mV, after immersion for 30 min in the corrosive media. The experimental impedance was analyzed and interpreted based on the equivalent circuit. The main parameters deduced from the analysis of the Nyquist diagram are the resistance of charge transfer R p (diameter of high frequency loop) and the capacity of the double layer C dl .
The EFM technique is a technique that can directly determine corrosion current data without prior knowing of Tafel slopes. EFM was carried out using two frequencies, 2 and 5 Hz. The base frequency was 0.1 Hz. In this study, a perturbation signal with amplitude of 10 mV was used for both perturbation frequencies of 2 and 5 Hz. Equilibrium time leading to steady state of the specimens was 30 min. The inhibition efficiency (% IE) of EFM was calculated. Each experiment was repeated at least three times to check the reproducibility. The uncertainty for all measurements was found to be about 7%.
The electrode potential was allowed to stabilize 30 min before starting the measurements. All the experiments were conducted at 25 ± 1°C. Measurements were performed using Gamry (PCI 300/4) Instrument Potentiostat/Galvanostat/ZRA. This includes a Gamry framework system based on the ESA 400. Gamry applications include DC105 for corrosion measurements and EIS300 for EIS along with a computer for collecting data. Echem Analyst (5.58) software was used for plotting, graphing and fitting data.

Surface analysis by scanning electron microscopy (SEM)
The C-steel specimens used for analysis of the surface were abraded with different grades of emery papers and immersed in the corrosive solution with and without 300 ppm of Modazar at room temperature for one day and then examined by using SEM (JOEL 840, Japan). To observe the elements present on the carbon steel surface the same condition was carried out using an energy dispersive analyzer (EDAX) unit attached to the SEM.

Surface analysis by atomic force microscopy (AFM)
AFM is the most versatile and powerful microscopy whereby the sample surface was scanned by a fine tip to find out the surface morphology and properties to generate a 3D surface image. The surface morphology was calculated using an SPM 2100 AFM instrument operating in contact mode in air; the scan rate of all AFM images was 05 μm × 05 μm areas at a scan speed of 6.68 μm second.  then shifted anodically, and the steady state was reached after 600 s. This indicates that the initial dissolution of the air formed oxide film on C-steel and the begin of the attack of the bare metal attack on the bare metal. In the presence of Modazar drug, E OCP started at a relatively positive potential with respect to in its absence, then shifted anodically. By increasing the concentration of the drug, the shift in EOCP increases in the positive direction, indicating that the drug might act mainly as an anodic inhibitor.

Weight loss measurements
The weight loss-time curves of C-steel without and with the addition of various concentrations of Modazar drug as an inhibitor in 1 M HCl are shown in Figure 2. The curves in the presence of the drug lie below those in its absence, and the weight loss decreased with increasing concentration of the drug. This indicated that this drug was adsorbed on the metal surface, forming a thin film (33) and preventing the metal surface from the aggressive solution. Table 3 shows the effect of corrosion rate and % IE for Modazar drug with the concentration.

Effect of temperature
To elucidate the mechanism of inhibition and to determine the activation energies of the corrosion process, weight loss tests were performed over a temperature range from 25°C to 45°C in the absence and presence of the investigated drug. Figure 3 represents the effect of temperature on the % IE. As shown in the figure, the % IE decreased with increasing the temperature of the medium. The activation energy (E * a ) of the corrosion process was calculated using the Arrhenius Equation (5): where k is the rate of corrosion and A is the Arrhenius constant or frequency factor, R is universal gas constant and T is the absolute temperature. Figure 4 represents    the Arrhenius plots in the presence and absence of the drug. E * a values determined from the slopes of these linear plots are shown in Table 4. The linear regression (R 2 ) is close to unity, which indicates that the corrosion of C-steel in the 1 M HCl solution can be obvious using the kinetic model. The relationship between the temperature and both % IE and activation energy E * a was given as follows (34): (a) Inhibitors whose % IE increases with temperature increase, the E * a found was greater than that in the absence of inhibitors; (b) inhibitors whose % IE did not change with temperature. The E * a did not change with and without inhibitor and (c) inhibitors whose % IE decreases with temperature, the E * a found was less than that in the absence of the inhibitor. Table 4 shows that the value of E * a for the inhibited solution is higher than that for the uninhibited solution, suggesting that the drug is adsorbed physically on the metal surface (35) and the dissolution of C-steel is slow in the presence of the drug. It is known from Equation (8) that the higher E * a values lead to lower rate of corrosion. This is due to the formation of a film on the carbon steel surface serving as an energy barrier for the carbon steel corrosion (36). Enthalpy and entropy of activation (ΔH*, ΔS*) of the corrosion process were calculated from the transition state Equation (6) which listed in Table 4: where h is Planck's constant and N is Avogadro's number. A plot of log (k corr. /T ) vs. 1/T for C-steel in 1 M HCl at different concentrations from the investigated drug gave straight lines as shown in Figure 5. The positive signs of ΔH*refer to the endothermic nature of the steel dissolution process. Large and negative values of ΔS* imply that the activated complex in the rate-determining step represents an association rather than dissociation step, meaning that decrease in disordering takes place on going from reactants to the activated complex (37).

Adsorption isotherms
To explain the nature of the adsorption of the inhibitor, Temkin, Langmuir, Freundlich and Frumkin adsorption isotherms were studied. It is generally accepted that the studied drug inhibits the corrosion process by adsorbing at the metal/solution interface (38). In this case the plot of (C/Θ) versus (C ) of Modazar at various temperatures gave straight lines with correlation coefficients in the range 0.99-0.98, intercept of (1/K ads ) and with slope approximately equal to unity were obtained. The mathematical expression of Langmuir is given as follows (39).
where K ads. is the adsorption equilibrium constant. The small deviation from unity is generally attributed to the interaction of the adsorbed inhibitor molecules on the steel surface. This indicates that the adsorption of the drug on the steel surface in the 1 M HCl solution follows Langmuir's adsorption isotherm, which is shown in Figure 6. Organic molecules having polar atoms or groups which are adsorbed on the metal   (8): where 55.5 is the concentration of water in the bulk of the solution in M −1 . DG W ads values at all studied temperatures are recorded in Table 5. The heat of adsorption (DH W ads ) was calculated according to Van't Hoff equation (40).
Plotting K ads against 1/T gave a straight line as shown in Figure 7; the slope of the straight line gave (DH W ads /2.303R); from this slope, DH W ads values were calculated and are listed in Table 5. Then in accordance with the basic Equation (10): By introducing the values of DG W ads and DH W ads in the above equation, the values of DS W ads were calculated and are listed in Table 5. When the value of DG W ads is around −20 kJ mol −1 or lower, it indicated the electrostatic interaction between the charged metal surface and charged organic molecules in the bulk of the solution, that is, physisorption. The negative sign of DH W ads refers to the adsorption of drug molecules is an exothermic process. The value of K ads is not high and decreased with increasing the temperature (Table 5). This verifies the assumption that Modazar drug is physically adsorbed on a carbon steel surface in HCl solutions.

HE tests
The curves of Figure 8 represent the variation in the volume of hydrogen gas evolved on C-steel with immersion time without and with different concentrations of Modazar drug at 25°C. Inspection of these curves revealed that hydrogen gas evolution starts after the elapsing of a certain time from immersion. This time is identified as the incubation period which is the time needed by the acid to destruct the pre-immersion oxide film and start dissolution of C-steel. After this incubation period, the volume of hydrogen gas evolved increased linearly with time. The data of corrosion rate    (k corr ), surface coverage (θ) and inhibition efficiency (% IE) are recorded in Table 6. As shown in the table, the rate of corrosion was decreases with increasing of Modazar concentration on the other hand, the surface coverage and % IE increase.

Tafel polarization studies
Polarization measurement was carried out to obtain Tafel plots in the absence and presence of various concentrations of the drug. Figure 9 shows the potentiostatic polarization curves for C-steel corrosion in 1 M HCl solution with and without various concentrations of Modazar drug at 25°C. From the graph, it was observed that Modazar drug behaved like a mixed type inhibitor (41). The inhibition efficiency (% IE) and surface coverage (θ) were calculated using Equation (11) where i corr(free) and i corr(inh) are the corrosion current densities in the absence and presence of the drug, respectively. The electrochemical kinetic parameters such as the corrosion current density (i corr. ), the corrosion potential (E corr. ), the anodic Tafel slope (β a ), the cathodic Tafel slope (β c ), degree of surface coverage (θ) and the inhibition efficiency (% IE) at different concentrations of Modazar drug are listed in Table 7. It was observed that both the cathodic and anodic reactions are suppressed with the addition of the drug, which suggests that the studied drug reduced anodic dissolution and retarded the HE reaction. It follows from Table 6 that the values of (β c ) changed with increasing drug concentration, indicating the influence of the drug on the kinetics of HE. The shift in the (β a ) may be due to the chloride ions/or inhibitor molecules adsorbed onto the steel surface. In addition, cathodic and anodic current densities decrease by adding the drug while E corr values have not considerably changed (maximum change in E corr is 23 mV). Therefore, the studied drug acts as a mixed type inhibitor at 25°C. The results obtained from potentiostatic polarization showed good agreement with the results obtained from weight loss and HE methods.

EIS studies
Electrochemical impedance spectra for C-steel in 1 M HCl without and with different concentrations of Modazar drug at 25°C are presented as Nyquist plot and Bode plots in Figure 10(a, b). An equivalent circuit model ( Figure 11) was proposed to fit and analyze EIS data.   EIS parameters calculated in accordance with equivalent circuit are listed in Table 8. Nyquist plots are depressed into real axis and not perfect semi-circles as a result of the roughness and other inhomogeneity of the metal surface (42,43). This kind of phenomenon is known as the dispersing effect. The capacity of double layer (C dl ) is defined as: where f max is the maximum frequency. The inhibition efficiencies and the surface coverage (θ) obtained from the impedance measurements were defined by the following relation: where R W p and R p are the charge transfer resistance in the absence and presence of the inhibitor, respectively. The capacitive loop corresponds to the polarization resistance (R p ): the sum of the charge transfer resistance (R ct ), diffuse layer resistance (R d ) and double layer formation on the bare CS surface (44) and n shows the phase shift which can be explained as the degree of surface inhomogeneity (45). The value of n is between 0 and 1 (0 < n < 1). This is related to the deviation from the ideal capacitive behavior. Capacitance phase element (CPE) represents a constant phase element to replace a double layer capacitance (C dl ) in order to give a more accurate fit to the experimental results (46). Data indicated that increasing R p is associated with a decrease in the CPE. It has been reported that the adsorption of the organic inhibitor on the metal surface is characterized by a decrease in CPE and increase in R p value. The value of CPE was found to decrease, while the R p value increased, which may be due to the replacement of water molecules at the electrode interface by Modazar drug through adsorption ( Table 8), suggesting that the drug was taken up by adsorption at the metal-solution interface (47). The value of CPE determines the characterization of the adsorption, desorption and film formation on the metal surface. The obtained Bodes plot for Modazar is shown in Figure 10(b).

Electron frequency modulation (EFM) studies
EFM is a nondestructive corrosion measurement technique that can directly and quickly determine the corrosion current value without prior knowledge of Tafel slopes, and with only a small polarizing signal. These advantages of the EFM technique make it an ideal candidate for online corrosion monitoring (48).
The high strength of the EFM is the causality factors which serve as an internal check on the validity of EFM measurement. The EFM Intermodulation spectrums of carbon steel in 1 M HCl acid solution containing different concentrations of the Modazar drug are shown in Figure 12. The large peaks were utilized to calculate  Figure 11. Electrical equivalent circuit diagram used to model the metal/solution interface. R s : solution resistance, R p : polarization resistance, CPE: double layer capacitance and film capacitance for uninhibited and inhibited solutions, respectively. R p : corresponds to charge transfer resistance R ct and diffuse layer resistance R d at the metal/solution interface in the absence of the inhibitor R p = R ct + R d .R ′ p includes the accumulated species R a , R p and film resistance the corrosion current density (i corr ), the Tafel slopes (β a and β c ) and the causality factors (CF-2 and CF-3). These electrochemical parameters are listed in Table 9. The inhibition efficiencies % IE EFM increase by increasing the studied inhibitor concentrations and can be calculated as follows: where i W corr and i corr are corrosion current densities without and with different concentrations of inhibitors, respectively. It was observed that by increasing the Modazar concentration the corrosion current density decreased and the % IE increased. The causality factors obtained under different experimental conditions are approximately equal to the theoretical values (2 and 3) indicating that the measured data are valid and of good quality (49).

Energy dispersion spectroscopy (EDX) studies
The EDX spectra were used to determine the elements present on the surface of carbon steel and after one day of exposure to the uninhibited and inhibited 1 M HCl. Figure 13 shows the EDX analysis of carbon steel with 300 ppm of the drug. The EDX analysis indicates that only Fe and oxygen were detected, which shows that the passive film contained only Fe 2 O 3 . The spectra show additional lines indicating the existence of C (owing to the carbon atoms of Modazar). These data showed that the carbon, oxygen, sulfur and N atoms covered the specimen surface. This layer is entirely owing to the inhibitor, because the carbon, oxygen, sulfur and N signals are absent on the specimen surface exposed to uninhibited HCl. It is shown that, in addition to N, C, O and S were present in the spectra. A comparable elemental distribution is shown in Table 10. Figure 14 represents the micrographs obtained for the C-steel sample surface before and after immersion in 1 M HCl solution with and without the inhibitor, after exposure to one day of immersion. Figure 14(a) shows the surface of the carbon steel specimen before immersion in HCl while Figure 14(b) shows the surface of carbon steel after immersion in 1 M HCl for one day in the absence of the drug. Figure 14(c) shows the surface of the carbon steel specimen after immersion in the corrosive solution in the presence of 300 ppm of drug for the same period of time. The SEM micrograph revealed that the surface morphology was strongly damaged in the absence of the inhibitor, but in the presence of 300 ppm of the inhibitor the damage was considerably diminished and the surface became smooth, which confirmed the high efficiency of Modazar at this concentration.

AFM analysis
AFM is a powerful tool to investigate the surface morphology of various samples at nano-micro scale, currently used to study the influence of a corrosion inhibitor on the generation and the progress of corrosion at the metal/solution interface (50). Analysis of the images allowed quantification of surface roughness over area scale of 5 μm. The three dimensional AFM images of the carbon steel surface without and with the inhibitor are shown in Figure 15(a, b). The surface roughness of the carbon steel surface after immersion in 1 M HCl is up to 2600 nm (Figure 15(a)), while in the presence of the drug, the roughness decreases to 365.28 nm (Figure 15(b)). It shows that the carbon steel surface in the presence of the drug is more compact and uniform, so it can efficiently protect the carbon steel surface from corrosion. This confirms that the inhibited surface is smoother than the uninhibited surface. The smoothness of the surface is due to the formation of a protective film on the metal surface. Figure 13. EDX analysis on carbon steel in the presence of Modazar for one day of immersion. Another mode of inhibition is, in acidic solutions the Modazar molecules can exist as protonated species. These protonated species adsorb on the cathodic sites of the C-steel and decrease the evolution of hydrogen. The adsorption on anodic sites occurs through the π-electron of the aromatic ring and lone pair of electrons of nitrogen, oxygen and sulfur atoms which decrease anodic dissolution of C-steel. Corrosion inhibition of Modazar drug is attributed to the presence of π electrons, oxygen, sulfur and nitrogen atoms and the larger molecular size. The SEM study confirms the presence of N, O, C and S atoms on the C-steel surface. Also, AFM analysis confirms the presence of the deposited film on the metal surface (this was confirmed by the very lower values of roughness in the presence of drug than in its absence). This indicates that the investigated drug is involved in film formation on the metal surface.

Conclusions
From every study (chemical or electrochemical) it could said that Modazar drug is a good corrosion inhibitor for carbon steel in 1 M HCl. A polarization study shows

Disclosure statement
No potential conflict of interest was reported by the authors. He is an international reviewer for a huge number of thesis, journals and projects. He had published more than 300 papers in international journals, especially electrochemistry (corrosion and corrosion inhibition in all media for metals and alloys, fuel cells, electrode position of metals and oxides).

Notes on contributors
G. El-Ewady, She is an associate prof. of physical chemistry, Faculty of Science, Mansoura University, Egypt. She has published papers in corrosion and electrochemistry fields, focusing on plant extracts, drugs, hetero cyclic compounds and other organic compounds.
A. H. Alia is a Ph.D. researcher student, working on corrosion field. He uses some drugs as corrosion inhibitor for some metals in different media. He has published three papers in international journals. He works in Chemistry Department, Faculty of Science, Mansoura University, Egypt.