Cephalexin as Efficient Corrosion Inhibitor for Mild Steel in Acidic Media Chemical, Electrochemical and Thermodynamic Studies

The corrosion inhibition of mild steel in 0.5 M H2SO4 solution by pharmaceutical antibacterial drug named Cephalexin has been investigated by using weight loss, potentiodynamic polarization, electrochemical frequency modulation technique (EFM) and electrochemical impedance spectroscopy (EIS) measurements. The polarization data showed that this drug is mixed-type inhibitor. The percentage inhibition efficiency was found to increase with increasing the concentration of the drug and with decreasing temperature. The Langmuir`s isotherm was found to provide an accurate description of adsorption behavior of this drug. Some thermodynamic parameters were computed and discussed. The correlations between advanced quantum chemical concepts and inhibition efficiency was found and discussed. The data obtained from different methods are in good agreement.

1. Introduction 

Mild steel is the major structural material in industry, the protection of steel against corrosion has attracted much attention. As most steels are generally stable in neutral and alkaline media, acidic environments are the major concern [1]. Acid solutions are generally used for the removal of undesirable scale and rust in several petroleum processes. Inhibitors are used in this process to control metal dissolution. Most of the well-known acid inhibitors are organic compounds containing O, S, and /or N atoms [2-5]. However,there are several studies on the corrosion inhibition of organic compounds in acidic solutions [6-15]. The inhibitive action is connected with several factors including the structure and the charge distribution on the molecule, the number and the types of adsorption sites, and the nature of interaction between the molecule and the metal surface [16]. Corrosion inhibition occurs via adsorption of the organic molecule on the corroding metal surface following some known adsorption isotherms with the polar groups acting as active centers in the molecules. The resulting adsorption film acts as a barrier that isolates the metal from the corroding solution and inhibition efficiency depends on the mechanical, structural and chemical characteristics of the adsorption layer formed under particular conditions [17, 18].

The aim of this work is to study the effect of (7R)-3-Methyl-7-(α-D-phenylglycylamino)- 3-cephem-4-carboxylic acid monohydrate (Cephalexin drug) as inhibitor for the corrosion of mild steel in 0.5M H2SO4 solutions using chemical and electrochemical measurements. The use of EFM technique. Yes this is the first time to use this drug as corrosion inhibitor for mild steel in 0.5 M sulfuric acid. I think so. Because it contains different hetero atoms, high molecular weight, soluble in water and so on

2. Experimental methods 

2.1. Materials and solutions

The mild steel coupons with dimensions 2 . 2 . 0.1 cm were cut from a sheet. The chemical composition (weight %) of mild steel: 0.2 C, 0.35 Mn, 0.024 P, 0.003 Si and Fe rest. The acid solutions were made from AR grade H2SO4. An appropriate concentration of acid was prepared using distilled water. Cephalexin in pure form was purchased from Al-Gomhoria Company, Egypt. The various concentrations of the drug was prepared using bidistilled water. Its chemical structure is shown in Figure 1.

Fig. 1: Molecular structure of Cephalexin Molecular formula: C16H17N3O4S, Molecular weight: 347.39

Fig. 1: Molecular structure of Cephalexin Molecular formula: C16H17N3O4S, Molecular weight: 347.39

2.2. Weight loss method 

The mild steel coupons with dimensions 2 . 2 . 0.2 cm were abraded with different grades of emery paper up to 1200 grit size, washed with bi-distilled water and acetone. After weighing accurately, the specimens were immersed in 0.5 M H2SO4 solution with and without addition of different concentrations of drug. After 3 hours the specimens were taken out, washed, dried, and weighed accurately. The average weight loss of the three parallel mild steel sheets could be obtained at required temperature at 25ºC. The inhibition efficiency (IE) and the degree of surface coverage (θ) of the investigated drug on the corrosion of C-steel were calculated as follows [19]:

Equation 1: %IE = θ x100 = [(Wo – W)/ Wo] x 100

where Wo and W are the values of the average weight loss in the absence and presence of the inhibitor, respectively.

2.3. Electrochemical measurements 

A three electrode cell assembly at room temperature [20, 21] was used for all electrochemical measurements. The working electrode was mild steel of above composition of 1 cm2 area and the rest being covered by using commercially available lacquer. A rectangular Pt foil of 1 cm2 was used as counter electrode and saturated calomel electrode (SCE) as reference electrode. The working electrode was abraded with different grades of emery papers up to 1200 grit size, washed with bi-distilled water and degreased with acetone. All electrochemical measurements were carried out using Gamry Potentiostat/Galvanostat (model PCI 300/4) Gamry-Instruments Inc., USA with EIS300 software for EIS, DC105 software for polarization and EFM140 software for EFM measurements. Data was analyzed by Echem Analyst 5.21. All the experiments were carried out after the stabilization of the system for 30 min.i.e. open circuit potential (OCP).

2.3.1. The potentiodynamic polarization tests 

The potentials were recorded by changing the electrode potential from -1.0 to 0.0 V versus SCE with scan rate of 1 mV/s. All experiments were carried out in freshly prepared solutions at constant temperature (25-55ºC) using a thermostat. %IE and [22] the degree of surface coverage (θ) were defined as:

Equation 2: % IE = θx100 = [(icorr – icorr(inh)) / icorr] x 100

where icorr(blank) and icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively, determined by extrapolation of Tafel lines.

2.3.2. The electrochemical impedance spectroscopy (EIS) tests 

The spectra of EIS were recorded at open circuit potential (OCP) after immersing the electrode for 30 min. in the test solution. The signal was 5 mV peak to peak and the frequency range studied was between 100 kHz and 0.2 Hz. The inhibition efficiency (%IE) and the surface coverage (θ) of the used drug were calculated by applying the following relation [22]:

Equation 3: % IE = Ɵ x 100 = [1-(Rºct / Rct)] x 100

Where, Roct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively.

2.3.3 The electrochemical frequency modulation (EFM) tests 

EFM is a new technique which provides a new tool for electrochemical corrosion monitoring. With the electrochemical modulation technique (EFM), a potential perturbation by two sine waves of different frequencies is applied to the system. As a corrosion process is non- linear in nature, responses are generated at more frequencies than the frequencies of the applied signal. The current responses can be measured at zero, harmonic, and intermodulation frequencies. Analysis of these current responses can result in the corrosion current density and Tafel parameters. The inhibition efficiency (%IEEFM) and the surface coverage (θ) of the used drug were calculated by applying the following relation:

Equation 4: IEEFM % = Ɵ x 100 = [(icorr – icorr(inh)) / icorr] x 100

where icorr(blank) and icorr(inh) are the corrosion currents in the absence and presence of inhibitor ,respectively.

3. Results and Discussion 

Fig. 2: Weight loss-time curves for mild steel dissolution in 0.5M H2SO4 in the absence and presence of different concentrations of investigated drug at 25ºC

Fig. 2: Weight loss-time curves for mild steel dissolution in 0.5M H2SO4 in the absence and presence of different concentrations of investigated drug at 25ºC

Figure 2 shows the weight loss-time curves of mild steel in 0.5 M H2SO4 at 25 ºC in the absence and presence of different concentrations of used drug. As shown from these Figures, by increasing the concentration of this drug, the weight loss of mild steel samples is decreased. This means that the presence of this additive retards the corrosion of mild steel in 0.5M H2SO4 or, this additive acts as inhibitor [23]. The linear variation of weight loss with time in uninhibited and inhibited 0.5M H2SO4 indicates the absence of insoluble surface films during corrosion. In the absence of any surface films, the inhibitor is first adsorbed onto the metal surface and thereafter affects the corrosion behaviour either by merely blocking the reaction sites (anodic and cathodic) or by altering the mechanism of the anodic and cathodic partial processes. The inhibition efficiency (%IE) of inhibitor was calculated and also corrosion rate (CR) [Table 1].

Tab. 1: Inhibition efficiency (%IE) and corrosion rate (C.R.) of mild steel in 0.5 M H2SO4 at different concentrations of the drug as determined by weight loss method after 90 min. immersion at 25ºC

Tab. 1: Inhibition efficiency (%IE) and corrosion rate (C.R.) of mild steel in 0.5 M H2SO4 at different concentrations of the drug as determined by weight loss method after 90 min. immersion at 25ºC

3.1.1. Role of anion in corrosion-inhibition of mild steel in H2SOsolutions and its synergistic effect 

Fig. 3: Weight loss-time curves for mild steel dissolution in 0.5M H2SO4 and 0.01M KI in absence and presence of different concentrations of the drug at 25ºC

Fig. 3: Weight loss-time curves for mild steel dissolution in 0.5M H2SO4 and 0.01M KI in absence and presence of different concentrations of the drug at 25ºC

The effect of potassium iodide (KI) on the corrosion of mild steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of Cephalexin was studied by weight loss method. Figure 3 shows the weight loss-time curves for C-steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of the drug beside the presence of 0.01M KI. The inhibition efficiency (%IE),Corrosion rate(CR)and the SӨ values in the presence of 0.01M of KI is shown in Table 2. The synergism parameter (Sθ) was calculated using the relationship given by Aranmki and Hackerman [24].

Equation 5: SӨ = (1 – θ1+2) / (1 – θ’1+2)

where, θ1+2 = (θ1 + θ2) – (θ1 θ2), θ1 = surface coverage by anion, θ2= surface coverage by cation, θ’1+2= measured surface coverage by anion and cation.

Tab. 2: Inhibition efficiency (%IE), corrosion rate(CR) and synergistic parameter (SƟ) of drug at different concentrations of drug for mild steel in 0.5M H2SO4 and 0.01M KI as determined from weight loss method after 90 min. immersion at 25ºC

Tab. 2: Inhibition efficiency (%IE), corrosion rate(CR) and synergistic parameter (SƟ) of drug at different concentrations of drug for mild steel in 0.5M H2SO4 and 0.01M KI as determined from weight loss method after 90 min. immersion at 25ºC

This equation actually compares the corrosion rate of either anion or cation and of the inhibitor mixture (anion + cation). If anions and cations have no effect on each other and adsorbed at metal surface independently, the SӨ value should be equal to 1. Furthermore, synergistic effects are observed if SӨ > 1 and activation of corrosion is observed if SӨ < 1.It can be seen from Table 2 that the addition of KI inhibits the corrosion of mild steel to a large extent. This can be interpreted according to Schmitt and Bedbur [24], which proposed two types of joint adsorption namely competitive and cooperative. In competitive adsorption the anions and cations are adsorbed at different sites on the electrode surface, and in cooperative adsorption, the anions are chemisorbed on the surface and the cations are adsorbed on a layer of the anions, apart from the adsorption on the surface directly. These two types are illustrated in Figure 4 [25].

Fig. 4: Schematic diagrams and equivalent circuits of competitive and cooperative adsorption of anion and cation (Please enlarge the Fig in order to modify the diagram)

Since Sθ > 1 synergistic effect takes place i.e. competitive adsorption occurs

3.2.1. Potentiodynamic polarization measurements 

Tab. 3: The effect of concentrations of investigated drug on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βc, βa),corrosion rate(C.R.), inhibition efficiency (%IE) and degree of surface coverage (θ) of mild steel in 0.5M H2SO4 at 25ºC

Tab. 3: The effect of concentrations of investigated drug on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βc, βa),corrosion rate(C.R.), inhibition efficiency (%IE) and degree of surface coverage (θ) of mild steel in 0.5M H2SO4 at 25ºC

Figure 5 shows the potentiodynamic polarization curves for mild steel without and with different concentrations of Cephalexin at 25ºC. The obtained electrochemical parameters; cathodic (βc) and anodic (βa) Tafel slopes, corrosion potential (Ecorr), corrosion current density (icorr), and polarization resistance (Rp) were obtained and listed in Table 3. Table 3 and Figure 5:

Fig. 5: Potentiodynamic polarization curves for the dissolution of mild steel in 0.5 M H2SO4 in the absence and presence of different concentrations of investigated drug at 25º C

Fig. 5: Potentiodynamic polarization curves for the dissolution of mild steel in 0.5 M H2SO4 in the absence and presence of different concentrations of investigated drug at 25º C

  1. Clearly shows that both anodic and cathodic reactions are inhibited, in addition, Ecorr does not change obviously,the value at 100 ppm is 468 these indicate that investigated drug acts as a mixed-type inhibitor [26-27],
  2. icorr decreases by increasing the concentration of the drug and
  3. The curves of βa and βc are parallel, this indicates that the mechanism of the corrosion reaction of C-steel does not change.

3.2.1.1. Adsorption isotherms 

Fig. 6: Langmuir adsorption isotherm plotted as θ(1- θ) vs. C of the investigated drug for corrosion of mild steel in 0.5M H2SO4 solution at 25º C

Fig. 6: Langmuir adsorption isotherm plotted as θ(1- θ) vs. C of the investigated drug for corrosion of mild steel in 0.5M H2SO4 solution at 25º C

One of the most convenient ways of expressing adsorption quantitatively is by deriving the adsorption isotherm that characterizes the metal/inhibitor/ environment system [28]. The surface coverage (θ) values were tested graphically to allow fitting of a suitable adsorption isotherm. The plot of θ/(1-θ) versus C (Figure 6) yielded a straight line, this proving that the adsorption of this drug from 0.5 M H2SO4 solution on C-steel surface obeys Langmuir adsorption isotherm according to equation (6):

Equation 6: θ/(1-θ) = KadsC

Where, Kads is equilibrium constant of adsorption process, Ɵ is the surface coverage, C is concentration of the drug.

It is well know that the equilibrium constant for adsorption process, Kads is related to the standard free energy of adsorption (ΔGºads) by the following equation [29]:

Equation 7: Kads = 1/ 55.5 exp (-ΔGºads / RT)

Tab. 4: dsorption equilibrium constant of (Kads), free energy of adsorption (ΔGºads), enthalpy of adsorption (ΔHºads) of and entropy of adsorption (ΔSºads) of drug on C-steel surface in 0.5 M H2SO4 at different temperatures

Tab. 4: dsorption equilibrium constant of (Kads), free energy of adsorption (ΔGºads), enthalpy of adsorption (ΔHºads) of and entropy of adsorption (ΔSºads) of drug on C-steel surface in 0.5 M H2SO4 at different temperatures

Inspection of the data of Table 4, the large values of ΔGºads and its negative sign, indicates that the adsorption of the drug on C-steel surface is proceeding spontaneously and accompanied by highly efficient adsorption. The enthalpy of adsorption can be calculated from the Van’t Hoff equation [30]:

Equation 8: (ΔGºads / T) = (ΔHº ads/T) + constant

Fig. 7: Relationship between ΔGο ads/T and 1/T for the drug

Fig. 7: Relationship between ΔGοads/T and 1/T for the drug

The variation of (ΔGºads / T) with 1/T gives a straight line with a slope which is equal to ΔHºads Figure 7. It can be seen from the Figure that (ΔGºads / T) decreases with 1/T in a linear fashion. The enthalpy and entropy for the adsorption of steel were also deduced from the thermodynamic basic equation [31]:

Equation 9: ΔGºads =ΔHº ads -TΔSads

where ΔHºads and ΔSºads are the enthalpy and entropy changes of adsorption process, respectively. A plot of ΔGºads versus T was linear (Figure 10) with the slope equal to – ΔSºads and intercept of ΔHºads. The enthalpy of adsorption ΔHºads and the entropy of adsorption ΔSºads obtained are negative. The negative sign of ΔHºads indicates that the adsorption of Cephalexin molecules is an exothermic process. In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of ΔHºads. For physisorption process, the enthalpy of adsorption is lower than 40 kJ mol-1 while that for chemisorption approaches 100 kJ mol-1 [32]. In the present study, the absolute value of enthalpy of adsorption obtained is typical of physisorption. Values of ΔHºads obtained by the two methods are in good agreement. The entropy of adsorption obtained from Equation (9) was negative because the inhibitor molecules freely moving in the bulk solution were adsorbed in an orderly fashion onto the mild steel, resulting in a decrease in entropy [30]. Moreover, from thermodynamic principles, since the adsorption was an exothermic process, it must be accompanied by a decrease in entropy [33].

3.2.1.2. Effect of temperature 

Tab. 5: The effect of concentrations of investigated compound on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βc, βa), corrosion rate (CR), inhibition efficiency (%IE) and degree of surface coverage (θ) of mild steel alloy in 0.5M H2SO4 at different temperatures

Tab. 5: The effect of concentrations of investigated compound on the free corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βc, βa), corrosion rate (CR), inhibition efficiency (%IE) and degree of surface coverage (θ) of mild steel alloy in 0.5M H2SO4 at different temperatures

To investigate the mechanism of inhibition and to determine the activation energy of corrosion process, potentiodynamic curves of C-steel in 0.5M H2SO4 were studied at various temperatures (25–55ºC) in the absence and presence of different concentrations of investigated drug. This drug retards the corrosion process at lower temperatures [34] whereas the inhibition is considerably decreased at elevated temperatures. The increasing of the corrosion rate with increasing temperature is suggestive of physical adsorption of the investigated drug on the C-steel surface. The obtained electrochemical parameters; cathodic (βc) and anodic (βa) Tafel slopes, corrosion potential (Ecorr), corrosion current density (icorr), and polarization resistance (Rp) at different concentrations were obtained and listed in Table 5. Table 5 shows that icorr increases by increasing temperature and hence % IE decreases by increasing the temperature. Figure 5 represents the Arrhenius plots of natural logarithm of corrosion rate versus 1/T, for C- steel in 0.5M H2SO4 solution, in the absent and presence of different concentrations of Cephalexin. The values of slopes of these straight lines permit the calculation of the activation energy, E*a, according to:

Equation 10: k = A exp (- Ea*/ RT)

Where k is the corrosion rate, A is the pre-exponential factor, Ea* is the apparent activation energy, R is the universal gas constant and T is the absolute temperature.

Tab. 6: Thermodynamic activation parameters for the dissolution of mild steel in 0.5M H2SO4 in the absence and presence of different concentration of investigated drug

Tab. 6: Thermodynamic activation parameters for the dissolution of mild steel in 0.5M H2SO4 in the absence and presence of different concentration of investigated drug

The values of Ea* are given in Table 6. The results of Table 6 revealed that, the values of Ea* were increased by increasing the concentration of the investigated inhibitor indicating that the dissolution of C-steel under these conditions is activation controlled and also indicates the energy barrier of the corrosion reaction increases in the presence of these additives. Other authors [35-37] obtained similar results. The higher values of Ea* are good evidence for the strong adsorption of Cephalexin on C-steel surface.

An alternative formulation of Arrhenius equation is the transition state equation [38], Figure 6 represents the transition state:

Figure 11: log k/T = log (R/ Nh) + ΔS*/ 2.303R + (-ΔH*/ 2.303R) 1/ T

By plotting log k/T against 1/ T we obtain ΔH* from the slope and ΔS* from the intercept where slope = -ΔH*/ 2.303R, Intercept = log (R/ Nh) + ΔS*/ 2.303R

Where h is Plank’s constant, N is Avogadro’s number, ΔH* is the activation enthalpy and ΔS* is the activation entropy. The values of (ΔH*) are positive and higher in the presence of the drug than in its absence. This implies that the energy barrier of the corrosion reaction in the presence of the investigated compound increases and indicates the endothermic behavior of the corrosion process. On the other hand ΔS* values are lower and have negative values in presence of the additives, this means that addition of these compounds cause a decrease in the disordering in going from reactants to the activated complexes [39,40].

3.2.2. Electrochemical Impedance Spectroscopy (EIS) measurements 

Fig. 8: The Nyquist plot for mild steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of investigated inhibitor at 25ºC

Fig. 8: The Nyquist plot for mild steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of investigated inhibitor at 25ºC

Fig. 9: The bode plot for mild steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of the investigated drug

Fig. 9: The bode plot for mild steel in 0.5 M H2SO4 solution in the absence and presence of different concentrations of the investigated drug

Fig. 10: Equivalent circuit model used to fit the impedance spectra

Fig. 10: Equivalent circuit model used to fit the impedance spectra

The corrosion of mild steel in 0.5 M H2SO4 in the presence of the investigated compound was investigated by EIS method at 25 ºC after 20 min immersion. Nyquist plot in the absence and presence of the investigated compound is presented in Figure 8 and also the Bode plot is presented in Figure 9. It is apparent that Nyquist plots show a single capacitive loop, both in uninhibited and inhibited solutions. The impedance data of mil steel in 0.5 M H2SO4 are analyzed in terms of an equivalent circuit model (Figure 10) which includes the solution resistance Rs and the double layer capacitance Cdl which is placed in parallel to the charge transfer resistance Rct[41] due to the charge transfer resistance. For the Nyquist plots, it is obvious that low frequency data are on the right side of the plot and higher frequency data are on the left. This is true for EIS data where impedance usually falls as frequency rises (this is not true for all circuits). The capacity of double layer (Cdl) can be calculated from the following equation:

Equation 12: Cdl = [1/2π fmax Rct]

Where fmax is maximum frequency used. The parameters obtained from impedance measurements are given in Table 7. It can see from Table 7 that the values of charge transfer resistance Rct increase with inhibitor concentration [42]. In the case of impedance studies, % IE increases with inhibitor concentration in the presence of investigated inhibitor. The impedance study confirms the inhibiting character of these compounds obtained from potentiodynamic polarization and weight loss methods. It is also noted that the (Cdl) values tend to decrease when the concentration of these compounds increases. This decrease in (Cdl), which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that these compounds molecules function by adsorption at the metal/solution interface [43]. The inhibiting effect of these compounds can be attributed to their parallel adsorption at the metal solution interface. The parallel adsorption is owing to the presence of one or more active center for adsorption.

Tab. 7: Electrochemical kinetic parameters obtained by EIS technique for the corrosion of mild steel in 0.5 M H2SO4 at different concentrations of investigated drug at 25ºC

Tab. 7: Electrochemical kinetic parameters obtained by EIS technique for the corrosion of mild steel in 0.5 M H2SO4 at different concentrations of investigated drug at 25ºC

3.2.3. Electrochemical frequency modulation (EFM) measurement 

Electrochemical frequency modulation (EFM) is a non-destructive corrosion measurement technique that can directly give values of corrosion current without prior knowledge of Tafel constants. In this technique current responses due to a potential perturbation by one or more sine waves are measured at more frequencies than the frequency of the applied signal, for example at zero harmonic and intermodulation frequencies [44]. The great strength of the EFM is the causality factors, which serve as an internal check on the validity of the EFM measurement [45]. The results of EFM experiments are a spectrum of current response as a function of frequency. Figures 11, 12 represent the intermodulation spectrum in absence and in presence of 600 ppm of drugas an example. The larger peaks were used to calculate the corrosion current density (icorr), the Tafel slopes (βc and βa) and the causality factors (CF-2 and CF-3). These electrochemical parameters were simultaneously determined and listed in Table 8. As can be seen from this Table, the corrosion current densities increase in the presence of different concentrations of the drug than in the presence of 0.5 M H2SO4 solution alone in case of C-steel. The causality factors also indicate that the measured data are of good quality.

Fig. 11: Intermodulation spectrum for mild steel in 0.5 M H2SO4 solutions without investigated compound at 25 ºC.

Fig. 11: Intermodulation spectrum for mild steel in 0.5 M H2SO4 solutions without investigated compound at 25 ºC.

Fig. 12: Intermodulation spectrum for mild steel in 0.5 M H2SO4 solutions with 600 ppm of investigated compound at 25 ºC

Fig. 12: Intermodulation spectrum for mild steel in 0.5 M H2SO4 solutions with 600 ppm of investigated compound at 25 ºC

Tab. 8: Electrochemical kinetic parameters obtained by EFM technique for mild steel in 0.5 M H2SO4 solutions with different concentration of investigated drug

Tab. 8: Electrochemical kinetic parameters obtained by EFM technique for mild steel in 0.5 M H2SO4 solutions with different concentration of investigated drug

3.3. Quantum chemical studies 

Quantum-chemical calculations have been widely used to study the reaction mechanism and to interpret the experimental results as well as to resolve chemical ambiguities. They have also proved to be a very important tool for studying corrosion inhibition mechanism. The energy of the highest occupied molecular orbitals (EHOMO) indicates the ability of the molecule to donate electrons to an appropriated acceptor with empty molecular orbitals and the energy of the lowest unoccupied molecular orbitals (ELUMO) indicates its ability to accept electrons. Figure 13

Fig. 13: The optimized molecular structures, HOMO, LUMO

Fig. 13: The optimized molecular structures, HOMO, LUMO

represents the optimized molecular structures, HOMO, LUMO of the investigated drug. The lower the value of ELUMO, the more ability of the molecule is to accept electrons [46]. While, the higher is the value of EHOMO of the inhibitor, the easer is its offering electrons to the unoccupied d-orbital of C-steel surface and the greater is its inhibition efficiency. The calculations listed in Table 9 showed that the highest energy EHOMO is assigned for the compound Cephalexin, which is expected to have corrosion inhibition. Therefore, it has the greatest tendency to adsorb on the metal surface and accordingly has the highest inhibition efficiency. This expectation is in a good agreement with the experimental. (ELUMO– EHOMO) energy gap, ΔE, which is an important stability index, is applied to develop theoretical models for explaining the structure and conformation barriers in many molecular systems. The smaller is the value of ΔE, the more is the probable inhibition efficiency that the compound has [47-49]. It was shown from Table 9 that Cephalexin molecule has the smaller ΔE. Accordingly, it could be expected that Cephalexin molecule has more inclination to adsorb on the metal surface. The dipole moment μ was used to discuss and rationalize the structure [50]. The higher is the value of μ, the more is the probable inhibition efficiency.

Tab. 9: The calculated quantum chemical parameters for Cephalexin compound using PM3.

Tab. 9: The calculated quantum chemical parameters for Cephalexin compound using PM3.

4. Mechanism of Corrosion Inhibition 

An elucidation of inhibition mechanism requires elaborated knowledge of the fundamental interaction amongst the protective compound and the metal surface. Many of the organic corrosion inhibitors have at least one polar unit with atoms of N S, O and in some cases phosphorous. It has been reported that the inhibition efficiency decreases in the order to O < N < S < P. The polar unit is considered as the reaction center for the chemisorption process. Moreover; the size, orientation, shape and electric charge on the molecule determine the degree of adsorption and therefore; the effectiveness of the inhibitor. An increase in inhibition efficiency with an increase in concentration of Cephalexin showed that the inhibition action is due to adsorption on the steel surface, following the types of adsorption that may take place at metal/solution interface: (a) Electrostatic attraction between the charged molecules and charged metal (b) Interaction of unshared electron pairs in the molecule with the metal (c) Interaction of p-electrons with the metal and (d) Combination of (a) and (c) [51].In acidic solution, Cephalexin is present in its protonated form and it is well known that the steel surface bears a positive charge [52.53], so it is difficult for the protonated molecules to approach the positively charged mild steel surface due to the electrostatic repulsion. Since SO4— ions could bring excess negative charges in the vicinity of the interface and favor more adsorption of the positively charged Cephalexin molecules, the protonated Cephalexin adsorb through electrostatic interactions between the positively charged molecules and the negatively charged metal surface. Thus, there is a synergism between adsorbed SO4— ions and protonated Cephalexin. Thus, we can conclude that inhibition of mild steel corrosion in 0.5 M H2SO4 is mainly due to electrostatic interaction. The decrease in inhibition efficiency with an increase in temperature supports electrostatic interaction.

5. Conclusions 

Cephalexin acts as a good inhibitor for the corrosion of mild steel in 0.5 M H2SO4. The inhibition efficiency of Cephalexin decreased with temperature, which leads to an increase in activation energy of corrosion process. The adsorption of Cephalexin follows Langmuir`s adsorption isotherm. The adsorption process is spontaneous and exothermic, accompanied by an increase of entropy. Potentiodynamic polarization curves reveals that Cephalexin is a mixed-type but predominantly cathodic inhibitor. The results obtained from different experimental studies are in good agreement.

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