Electrochemical Investigation on Adsorption of Fluconazole at Mild Steel / HCl Acid Interface as Corrosion Inhibitor T

e interfacial behavior of �uconazole on mild steel in 1M HCl solution was studied by electrochemical methods, namely, polarization (Tafel Plot) and Electrochemical Impedance Spectroscopy (EIS).e surface morphology of mild steel in the presence and absence of �uconazole was studied by Atomic Force Microscopy (AFM). e results of the study showed that �uconazole reduced the corrosion rate in HCl acid solution by adsorbing on the surface of mild steel. Tafel results suggest that �uconazole behaves predominantly as an anodic inhibitor and shows greater inhibition efficiency (96%) at 0.30mM. ermodynamical parameters suggest that �uconazole is adsorbed on mild steel mainly by chemical mode. e EIS studies reveal the formation of a thin barrier �lm on mild steel surface. e AFM image of mild steel immersed in optimum concentration of �uconazole has con�rmed the �lm formation on metal surface.


Introduction
Acid solutions are widely used in ore processing, fertilizer manufacturing, oil re�ning, waste water processing, chemical synthesis, and pickling and descaling processes [1][2][3][4].Active metals such as mild steel, Zn, and Al are employed in industries for fabrication purposes due to their easy availability and low cost, where surfaces are rapidly damaged in the presence of acids [5].Among the various methods to control the destruction of these active metals in acid solutions, the use of inhibitors is quite popular [6][7][8].Organic compounds containing heteroatoms like P, S, N, and O have been explored as good corrosion inhibitors [9][10][11].ey adsorb on the metal surface in the acid solutions either physically or chemically thereby blocking the corrosion reaction.However, most of these inhibitors suffer from nonbiodegradability and some of them are also toxic to living beings.In modern scenario, development of novel biodegradable and less toxic corrosion inhibitors is gaining importance.Biologically active molecules like sulfadimidine, sulfamethoxazole, cefatrexyl, apart from other antibacterial, and antifungal drugs have been reported as good corrosion inhibitors [12][13][14][15][16][17][18].
In the present study, adsorption behavior of an antifungal drug, �uconazole (2-(2,4-di�uorophenyl)-1,3-di(1H-1,2,4-triazol-1-yl)propan-2-ol), was evaluated for changes that occur in mild steel/HCl acid interface in view of the fact that �uconazole contains two triazole rings with active centers like N and aromatic  electrons, which can aid adsorption on mild steel surface minimizing the corrosion process in HCl medium.
Perusal of the literature shows that the adsorption behavior and kinetics of �uconazole were evaluated for Al in acid media [19,20].
Fluconazole received from IPCA laboratories Ltd, Mumbai, as a gi sample was used for the studies.AR grade HCl acid and double-distilled water were used for the entire study.e structure of the studied compound is given in Figure 1.

Electrochemical Studies.
All the electrochemical studies were carried out using CH Electrochemical analyzer model 760D with CHI 760D soware.Conventional three-electrode system was used for polarization and EIS studies.In this setup, polished mild steel with 1 cm 2 exposed surface area was used as working electrode, platinum electrode as an auxiliary electrode, and saturated calomel electrode as reference electrode.All the three electrodes were kept immersed in 1 M HCl both in the absence and presence of �ve different concentrations, namely, 0.03 mM, 0.08 mM, 0.16 mM, 0.24 mM, and 0.30 mM of �uconazole.is setup was kept in room temperature for 30 min and then electrochemical measurements were carried out.
e open-circuit potential (OCP) versus time measurement was carried out for 60 secs.EIS measurements were carried out at corrosion potential ( corr ) by changing the a.c frequency ranging from 0.1 Hz to 10000 Hz at 5 mV of amplitude.Nyquist and Bode plots were obtained.From the Nyquist plots, charge transfer resistance ( ct ) and doublelayer capacitance ( dl ) of mild steel in presence and absence of �uconazole in 1 M HCl were computed.e simulation studies were carried out using  view soware.e inhibition efficiency was calculated by using  ct as in where  ct() is the charge transfer resistance of �uconazolecontained solution and  ct() is the charge transfer resistance of the blank HCl solution.e Tafel polarization curves were obtained by changing the electrode potential from −150 mV to −750 mV at opencircuit potential with a scan rate of 0.5 mV s −1 .e linear Tafel segments of cathodic and anodic curves were extrapolated to corrosion potential to obtain the corrosion current densities ( corr ).e inhibition efficiency was evaluated by using  corr values as given in where  corr is the corrosion current without �uconazole and  corr(1) is the corrosion current with �uconazole.

Surface Analysis. e AFM images of polished mild steel
surface along with those immersed in 1 M HCl alone and 1 M HCl with 0.3 mM of �uconazole for 2 hours were scanned using Nano Surf Easy Scan 2 instrument at the range of 50 mm.

Results and Discussion
3.1.OCP Studies.e OCP versus time plots for mild steel in blank acid and with different concentrations of �uconazole are shown in Figure 2. From the �gures, the OCP of 1 M HCl acid was found to be −0.4939V.For an increase in concentration of inhibitor from 0.03 mM to 0.30 mM, the OCP was shied towards noble direction from −0.4722 to −0.4644 V, indicating that �uconazole controls mainly anodic metal dissolution reaction [21].

EIS Studies. Nyquist and Bode plots of mild steel in 1 M
HCl in the absence and presence of various concentrations of �uconazole are shown in Figures 3, 4, and 5.It is clear from these plots that the impedance of the mild steel substrate increases with the increase in the concentration of inhibitor in 1 M HCl.It is worth noting that the change in the concentration of �uconazole did not alter the pro�le of the impedance behavior suggesting similar mechanism for the corrosion inhibition of mild steel by �uconazole at various concentrations.
e Nyquist and Bode plots display a single high frequency capacitive loop and a time constant, namely, solution resistance (  ) and charge transfer resistance ( ct ).
e corrosion process that occurs at the interface in fact has two steps.e �rst is the oxidation of the metal which is a charge transfer process and the second is the diffusion of the metallic ions from the metal surface to the solution which is a mass transfer process.
Shapes of the Nyquist plots show that the corrosion inhibition of �uconazole is only by charge transfer process  [22].e impedance behavior of mild steel with and without addition of �ucona�ole can be explained by the simplest model, namely, Randles circuit which includes the charge transfer resistance ( ct ) parallel with constant phase element (CPE) in series with solution resistance (  ) as represented in Figure 6.It can be noted from the Nyquist plots that the capacitive loops are depressed with center under the real axis even though they have a semicircular appearance.Deviations of this kind are oen referred to as frequency dispersion [23] which is attributed to the irregularities and heterogeneities of the solid surface [24,25].e imperfect semicircular Nyquist plot can be explained by the nonideal behavior of the double layer.e impedance () of CPE is given by ( 3) where  is the proportionality coefficient,  is the maximum frequency,  2 = −1 (imaginary number), and  is the surface irregularity factor (0 ≤  ≤ 1).Equivalent circuit of this type has been previously used to model the mild steel/acid interface.e replacement of  dl with the CPE dl signi�cantly improved the quality of the �t [26].If the electrode surface is homogeneous (free from defects) and �at, the exponential value () becomes equal to 1 and the metal solution interface acts as a capacitor with regular surface, that is, when  = 1  = capacitance.
e lower value of  for 1 M HCl (0.86) indicated the surface inhomogeneity which resulted from roughening of metal surface due to corrosion.�pon addition of �ucona�ole, the  value increased from 0.86 to 0.90 (0.30 mM) indicating the reduction of surface defects due to adsorption of inhibitor at MS/acid solution interface [27].
e calculated impedance parameters are depicted in Table 1.Perusal of the table reveals that  ct values increased with the increase in the concentration of �ucona�ole, which is due to the increased adsorption of the inhibitor at high concentration.Decrease of CPE dl may be caused by a reduction of local dielectric constant and/or by an increase in the thickness of the electrical double layer.ese results very much suggest that �ucona�ole acts by adsorption at the metal/solution interface [28,29].e addition of �uconazole decreases the CPE dl values as a consequence to the replacement of water molecules by the inhibitor at the electrode surface.2. From the results, it can be observed that the corrosion current decreases while increasing the concentration of �uconazole.e decrease of current density is due to the adsorption of inhibitor molecules on mild steel surface to retard the corrosion reaction of electrode with simultaneous replacement of electrolyte solutions at the interface [30].Further, on increasing the inhibitor concentration,  corr values were shied towards mainly positive side.is suggests that �uconazole behaves predominantly as an anodic inhibitor.e Tafel constants, namely,   and   , decreased with the increasing of �uconazole concentration and   was more deviated compared to   , showing that �uconazole controls mainly the anodic metal dissolution [31].e polarization resistance (  ) increases with the increase of concentration of the inhibitor as well.e increasing of polarization resistance is mainly due to the adsorption of �uconazole on mild steel surface.Further, polarization resistances derived from Tafel plots (DC studies) and obtained from complex plane plots (sum of the   and  ct ) (AC studies) are in good agreement with each other (  =   +  ct ).

AFM Studies.
AFM is a powerful tool to investigate the surface morphology and it is very useful to determine the �lm

Adsorption Isotherms. Most of the corrosion inhibitors
prevent metal dissolution by adsorption process (Badr, 2009).e % IE of �uconazole was studied by Tafel and EIS methods which suggest that the surface coverage ( = IE%/100) increased with the increasing of inhibitor concentration.
To describe the adsorption behavior of �uconazole, several where -degree is the surface coverage,  is the molar inhibitor concentration, and  is the equilibrium constant of the adsorption process.e Langmuir isotherm assumes the adsorption of organic molecules as a monolayer over the metallic surface without any interactions with other molecules adsorbed [34].By using this isotherm, the free energy of adsorption (Δ ads ) was calculated by plotting ln  (M) versus ln 1 − .e value of adsorption equilibrium constant is calculated from the intercept of the straight line obtained from Figures 12  and 13.As shown in the results (Table 3), negative sign of Δ ads indicates that the adsorption process of �uconazole over mild steel occurs spontaneously.e values of Δ ads calculated by Tafel and impedance methods are −35.39 and −32.01 kJ mol −1 , respectively.ese values are at the interval of physical adsorption and chemical binding and indicate chemical adsorption of the inhibitor [35].
e adsorption process depends upon the size, orientation, shape, and electric charge of the inhibitor in addition to the charge on the metal surface [36].e chemisorption involves electron sharing between metal surface and the inhibitor, whereas physisorption may occur due to the interaction between the charged metal surface and protonated inhibitor.Polarization studies reveal that �uconazole predominantly controls the anodic dissolution reaction.e chemisorptive bonds could be formed due to the sharing of electron pair between metal and unprotonated hetero atoms of �uconazole.Further, it is also possible that there could be an interaction between the electrons of  orbitals of �uconazole with metal surface [37].

Conclusion
From the studies, the following are concluded.
(i) Fluconazole behaves good corrosion inhibitor for mild steel in HCl medium.
(ii) e results of polarization studies reveal that �uconazole acts as a predominantly anodic inhibitor and controls the metal dissolution.
(iii) e results of EIS studies con�rm the formation of barrier layer by �uconazole on mild steel surface and further the charge transfer process controls the corrosion.
(iv) e AFM images as well as the values of average surface roughness support the formation of barrier �lm.(v) e corrosion inhibition of �ucona�ole can be attributed mainly due to chemisorption at mild steel/HCl acid interface as supported by the results of isotherm studies.

F 2 :
OCP response of mild steel with different concentrations of �uconazole in 1 M HCl.

F 3 :
Nyquist plots of mild steel with different concentrations of �ucona�ole in 1 M HCl (Experimental and Fitted).

F 4 :
Frequency versus phase angle plots of mild steel with different concentrations of �ucona�ole in 1 M HCl acid (�ode).

F 5 :
Frequency versus real resistance plots of mild steel with different concentrations of �ucona�ole in 1 M HCl acid (�ode).

F 6 :
Equivalent circuit model for mild steel in HCl with and without �ucona�ole.

2 F 7 :
Tafel graphs of mild steel with different concentrations of �uconazole in 1 M HCl.

Figure 7
depicts the cathodic and anodic polarization curves of mild steel in 1 M HCl at 303 K in the absence and presence of different concentrations of �uconazole and the potential versus current graphs are represented in Figure 8. e electrochemical parameters such as corrosion potential ( corr ), cathodic and anodic Tafel slopes (  and   ), corrosion current density ( corr ), and polarization resistance (  ) were extracted from Figures 7 and 8 using CHI soware and are shown in Table

F 8 :F 9 :
Potential versus Current response of mild steel with different concentrations of �uconazole in 1 M HCl.AFM image of polished mild steel surface.formation on metal surface in corrosion inhibition studies.e AFM images of polished mild steel, mild steel in 1 M HCl with and without presence of 0.30 mM �uconazole, are shown in Figures 9, 10, and 11.e AFM image of mild steel surface in HCl appears severely damaged than mild steel in HCl with 0.30 mM of �uconazole.Moreover, the average roughness of polished mild steel and mild steel in blank HCl solution was calculated to be 82 and 450 nm, respectively.With the addition of inhibitor, the average roughness was reduced to 208 nm, which suggested the �lm formation of the inhibitor over the mild steel surface[32].

F 10 :
AFM image of mild steel in 1 M HCl.Topography range Topography-scan forward F 11: AFM image of mild steel in 1 M HCl with 0.30 mM of �ucona�ole.

F 12 :
Langmuir adsorption isotherm plot (using Tafel results) for the adsorption of �ucona�ole on mild steel in 1 M HCl.

F 13 :
Langmuir adsorption isotherm plot (using EIS results) for the adsorption of �ucona�ole on mild steel in 1 M HCl.
T 1: Electrochemical impedance parameters of mild steel in 1 M HCl in the absence and presence of different concentrations of �uconazole as e�tracted from Nyquist plots.Tafel polarization parameters for mild steel in 1 M HCl in the absence and presence of different concentrations of �uconazole.