Carbon steel has been widely employed as a construction material for pipework in the oil and gas production such as; downhole tubular, flow lines and transmission pipelines1,2. Therefore, inhibition of corrosion is clearly very important. In the case of carbon steel, the iron will react with hydrogen ions to form iron ions and hydrogen gas.
Corrosion cells are created on metal surfaces in contact with an electrolyte because of energy differences between the metal and the electrolyte. The different area on the metal surface could also have different potentials with respect to the electrolyte inhibiting action of these compounds is attributed to their adsorption to the metal/solution interface3. It has been observed that adsorption depends mainly on certain physico-chemical properties of the inhibitor group. Like functional groups, aromaticity, electron density at the donor atoms and p-orbital character of donating electrons and also the presence of heteroatom such as; N, O and S, as well as multiple bonds in their molecular structure are assumed to be active centers of adsorption4. But, the toxicity of most corrosion inhibitors because they are toxic to living organism and may also poison the earth made us heading for the use of environment friendly inhibitors5.
Different types of drugs have been reported in literature to exhibit inhibitive effect on a range of metals in acidic environments. These include sulpha drugs6, antibacterials7,8, antifungals9,10 and muscle relaxant11 among others. The unique advantage of using natural products for the inhibition of the corrosion of metals is that they are environmental friendly. Similarly, the most heterocyclic drugs are environmental friendly and can favorably compete with the natural products. However, studies on the use of drugs as corrosion inhibitors are scanty12.
The aim of this work was to study the effect of benzylpenicillin as inhibitor for the corrosion of carbon steel in 1 M HCl by using weight loss technique at 298-328 K and electrochemical technique, the thermodynamic functions for the dissolution and adsorption processes were calculated and discussed. The choice of this antibacterial drug was also based on molecular structure considerations, i.e., this is an organic compound with several adsorption centers.
MATERIALS AND METHODS
Study area: The electrochemical measurements were carried out at the corrosion lab of Alexandria University, Egypt in August, 2018 and chemical measurements were carried out at the physical chemistry lab of Omar-al Mukhtar University, Libya, in July, 2019.
Materials and solutions: Experiments were performed by using carbon steel experiments were performed using carbon steel specimens of the following composition (weight %): 0.200 C, 0.350 Mn, 0.024 P, 0.003 S and the remainder Fe. The aggressive solution used was prepared by dilution of analytical reagent grade HCl with bi-distilled water. Benzylpenicillin was obtained from Glaxo SmithKline, Medical Union Pharmaceuticals, Alexandria Co. for Pharmaceuticals Egypt. The stock solution (1×102 M) of benzylpenicillin was used to prepare the desired concentrations by dilution with bi-distilled water. The concentration range of benzylpenicillin was used (2×103-1×102 M). Its chemical structure is shown in Fig. 1.
Weight loss measurements: The weight loss measurements were carried out in a 100 mL glass beaker placed in a thermostat water bath. The solution volume was 100 mL. The used carbon steel coupons had a square form (length = 2 cm, width = 2 cm, thickness = 0.2 cm). The coupons were weighed and suspended in 100 mL of an aerated 1 M HCl solution with and without different concentrations of benzylpenicillin for 3 h exposure period of time at 25-55±1°C. At the end of the tests, the coupons were taken out, washed with bi-distilled water, degreased with acetone, washed again with bi-distilled water, dried and then weighed using an analytical balance. The inhibition efficiency (IE %) over the exposure time period were calculated according to the following equation13:
where, surface coverage, W(free) and W(inh) are the weight loss in the absence and presence of inhibitor, respectively.
|Fig. 1:||Chemical structure of benzylpenicillin
The apparent activation energy (Ea*), the enthalpy of activation (ΔH*) and the entropy of activation (ΔS*) for the corrosion of carbon steel in 1 M HCl solution in the absence and presence of different concentrations of benzylpenicillin were calculated from the Arrhenius-type equation:
and transition-state equation14:
where, A is the frequency factor, h is the Planck’s constant, N is Avogadro’s number and R is the universal gas constant.
The equilibrium constant of the adsorption process, K, which is related to the standard free energy of adsorption (ΔG°ads)15,16:
where, 55.5 is the concentration of water molecule (mol L1) at metal/solution interface, R is the universal gas constant and T is the absolute temperature.
Moreover, the adsorption heat can be calculated according to the Van’t Hoff (Eq. 9)17:
Finally, the standard adsorption entropy ΔS°ads can be calculated by the Eq. 9:
Polarization measurements: Polarization experiments were carried out in a conventional three-electrode cell with a platinum foil as a counter electrode and a Saturated Calomel Electrode (SCE) coupled to a fine Luggin capillary as a reference electrode. The working electrode was in the form of a square cut from carbon steel embedded in epoxy resin of polytetrafluoroethylene (PTFE) so, that the flat surface (1 cm2) was the only surface of the electrode. Tafel polarization curves were obtained by changing the electrode potential automatically from -500 to +500 mV at open circuit potential with a scan rate of 1 mV sec1. Stern-Geary method18 used for the determination of corrosion current is performed by extrapolation of anodic and cathodic Tafel lines to a point which gives log icorr and the corresponding corrosion potential (Ecorr) for inhibitor-free acid and for each concentration of inhibitor. Then icorr was used for calculation of inhibition efficiency and surface coverage (θ) as in Eq. 2:
where, icorr(free) and icorr(inh) are the corrosion current densities in the absence and presence of inhibitor, respectively.
Impedance measurements were carried out in the frequency range from 100-10 mHz with an amplitude of 5 mV peak-to-peak using ac signals at open circuit potential. The experimental impedance was analyzed and interpreted on the basis of the equivalent circuit. The main parameters deduced from the analysis of Nyquist diagram are the resistance of charge transfer Rct (diameter of high-frequency loop) and the capacity of double layer Cdl which is defined as:
where, fmax is the maximum frequency.
The inhibition efficiencies and the surface coverage (θ) obtained from the impedance measurements were calculated from Eq. 4:
where, Roct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively. 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 electrochemical impedance spectroscopy along with a computer for collecting data. Echem Analyst 5.58 software was used for plotting, graphing and fitting data.
|Fig. 2:||Electrical equivalent circuit used to fit the impedance data
The impedance of a CPE is described by the following equation:
ZCPE = Y01 (jωmax)n
where, Y0 is the magnitude of the CPE, j is an imaginary number, ω is the angular frequency (ωmax = 2πfmax), fmax is the frequency at which the imaginary component of the impedance reaches its maximum values and n is the deviation parameter of the CPE: -1<n<1. The values of the interfacial capacitance Cdl can be calculated from CPE parameter values Y0 and n using equation19:
The EIS spectra of the this compound was analyzed using the equivalent circuit in Fig. 2, where Rs represents the solution resistance, Rct denotes the charge-transfer resistance and a CPE instead of a pure capacitor represents the interfacial capacitance19.
Effect of concentration: Figure 3 shows the weight loss-time curves for carbon steel in 1 M HCl acid in the presence and absence of different concentrations of benzylpenicillin at 25°C. These curves are characterized by a sharp rise in weight loss from the beginning. Curves for additives containing system fall below that of the free acid. The curves in Fig. 3 indicated that the weight loss of carbon steel depends on the concentration of benzylpenicillin additive.
As shown in Table 1, the inhibition efficiency of benzylpenicillin increases with the increase of their concentrations in the corrosive medium. An increase in bulk concentration and consequently increase of surface coverage by the additive increases their inhibition efficiencies towards carbon steel dissolution.
Weight loss-time curves for the corrosion of carbon steel in 1 M HCl in the absence and presence of different concentrations of benzylpenicillin at 25±1°C
Corrosion Rate (CR) (mg cm2/min), surface coverage (θ) and Inhibition Efficiency (% IE) data obtained from weight loss measurements for carbon steel in 1 M HCl solutions without and with various concentrations of benzylpenicillin at 25±1°C
Effect of temperature and activation parameters of corrosion process: The dissolution of carbon steel in 1 M HCl acid increases by increasing temperatures, the dissolution of carbon steel in 1 M HCl in the presence of benzylpenicillin at 2×103 to 1×102 M was studied by weight loss method over a temperature range 25-55°C. The corrosion rate of carbon steel dissolution increases as the temperature increases, but at a lower rate than in uninhibited solutions as shown in Table 2.
Kinetic parameters obtained from plots of log rate vs. (1/T) (Fig. 4) and log (Rate/T) vs. (1/T) (Fig. 5) are given in Table 3.
Adsorption isotherm behavior: A number of mathematical relationships for the adsorption isotherms have been suggested to fit the experiment data of the present work.
The degree of surface coverage (θ), i.e., the fraction of the surface covered by the inhibitor molecules at any given concentration of the inhibitor was calculated from the equation mentioned IE (%) = (100x θ).
Log Corrosion Rate (CR) in (mg cm2/min) vs. 1/T ( K1) curves for carbon steel dissolution in 1M HCl in absence and presence of different concentrations of benzylpenicillin
|Fig. 5:||Log corrosion rate vs. 1/T K1 curves for carbon steel dissolution in 1 M HCl in absence and presence of different concentrations of benzylpenicillin
The values of θ have been shown in Table 1 and 2. The degree of surface coverage was found to increase with increasing concentration of additives. Attempts were made to fit θ values to various isotherms including; Langmuir, Freundlich, Temkin and Frumkin. By far, the best fit was obtained with Langmuir isotherm.
Figure 6 shows the plot of θ/1-θ vs. C for different concentrations of investigated compound.
|Fig. 6:||Adsorption isotherm curve of θ/1-θ vs. C (concentration in g L1) for the adsorption of benzylpenicillin on carbon steel in 1 M HCl at 25°C
Corrosion Rate (CR) (mg cm2/min), surface coverage (θ) and Inhibition Efficiency (IE %) data for carbon steel in 1 M HCl solutions in the absence and presence of different concentrations of benzylpenicillin at 25-55±1°C
All the calculated thermodynamic parameters inhibitor equilibrium constant of the adsorption process Kads, Gibb’s free energyl G°ads, enthalpy- H°ads, and entropy S°ads are listed in Table 4.
Potentiodynamic polarization measurements: Figure 7 shows the anodic and cathodic Tafel polarization curves for carbon steel in 1 M HCl in the absence and presence of varying concentrations of inhibitor at 25°C.
Potentiodynamic polarization curves performed vs. a Saturated Calomel Electrode (SCE) for the corrosion of carbon steel in 1 M HCL solution without and with various concentrations of benzylpenicillin at 25±1°C
Effect of concentration of benzylpenicillin on the activation parameters of carbon steel dissolution in 1 M Hcl
|Ea*: Activation energy, ΔH*: Enthalpy of activation, ΔS*: Entropy of activation|
Thermodynamic parameters for carbon steel in 1 M HCl for benzylpenicillin at 25-45±1°C
Potentiodynamic polarization parameters for the corrosion of carbon steel in 1 M HCl solution without and with various concentrations of benzylpenicillin at 25±1°C
From Fig. 7, it is clear that both anodic metal dissolution and cathodic hydrogen reduction reactions were inhibited when investigated inhibitors were added to 1 M HCl and this inhibition was more pronounced with increasing inhibitor concentration. Tafel lines are shifted to more negative and more positive potentials with respect to the blank curve by increasing the concentration of the investigated inhibitors.
The electrochemical parameters of corrosion such as; corrosion current density icorr, corrosion potential Ecorr, corrosion rate CR, anodic Tafel constants βa, cathodic Tafel constants βc and inhibition efficiency IE are given in Table 5. From the potentiodynamic polarization data (Table 5), it conclude that the worth of i corr decreases with increase in the concentration of the inhibitors and this means a rise IE (%).
Electrochemical Impedance Spectroscopy (EIS): The corrosion behavior of carbon steel in HCl solution in the absence and presence of different concentrations of benzylpenicillin is also investigated by EIS at 25°C after 30 min of immersion (Fig. 8 and 9).
The charge transfer resistance (Rct) is calculated from the difference in impedance at lower and higher frequencies. The double layer capacitance (Cdl) and the frequency at which the imaginary component of impedance is maximal (-Zmax). From the impedance data (Table 6), it conclude that the value of Rct increases with increase in concentration of the inhibitors and this indicates an increase in IE (%).
In the present study, it was found that increase of bulk concentration and consequently, the increase of surface area coverage by the additive retard the dissolution of carbon steel.
The inhibitory behavior of benzylpenicillin against carbon corrosion can be attributed to the adsorption of this compound on the carbon steel surface, which limits the dissolution of the latter by blocking of its corrosion sites and hence, decreasing the corrosion rate, with increasing efficiency as their concentrations increase.
Benzylpenicillin can be adsorbed by the interaction between the lone pairs of electrons of the nitrogen, sulphur and oxygen atoms with the carbon steel surface. This process is facilitated by the presence of low lying d orbitals in the iron ions. Recently, it was found that the formation of donor-acceptor surface complexes between free electrons of an inhibitor and a vacant d orbital of a metal is responsible for the inhibition of the corrosion process20.
|Fig. 8:||Nyquist plots recorded for carbon steel in 1 M HCl without and with various concentrations of benzylpenicillin at 25±1°C
Bode plots recorded for copper in 1 M HCl without and with various concentrations of benzylpenicillin at 25±1°C
Electrochemical kinetic parameters obtained from EIS technique for carbon steel in 1 M HCl solutions containing various concentrations of benzylpenicillin at 25±1°C
The inhibition efficiency of the additives decreases with rising the temperature which proved that the adsorption of these compounds on the surface of carbon steel occurs through physical adsorption of the additives on the metal surface. Desorption is aided by increasing the reaction temperature.
The higher values were obtained for activation energy (Ea* = 88.48 kJ mol1) and activation enthalpy (ΔH*= 109.2 kJ mol1) in the highest concentration of inhibitor (1×102 M) indicated the higher protection efficiency observed for this inhibitor. There is also a parallelism between increases in inhibition efficiency and increases in Ea* and ΔH* values. These results indicated that this tested compound acted as inhibitors through increasing activation energy of carbon steel dissolution by making a barrier to mass and charge transfer by their adsorption on carbon steel surface. The increase in the activation enthalpy (ΔH*) in the presence of the inhibitors implied that the addition of the inhibitors to the acid solution increases the height of the energy barrier of the corrosion reaction to an extent depends on the type and concentration of the present inhibitor. Also, the entropy (ΔS*) widely decreases with the content of the inhibitor. This means the formation of an ordered stable layer of the inhibitor on carbon steel surface21.
The plot of θ/1-θ vs. C for different concentrations of investigated compound gives straight line with slope very close to unity. The regression (R2) is more than 0.9. This means that there is no interaction between the adsorbed species on the electrode surface22.
The negative value of G°ads (from -18.32 kJ mol1 at 298K to -15.18 kJ mol1 at 323K) suggested that the adsorption of inhibitor molecules on to carbon steel surface is spontaneous process. Generally, values of G°ads up to -20 kJ mol1 are consistent with electrostatic interaction between the charged molecules and the charged metal (physical adsorption), while those more negative than -40 kJ mol1 involved charge sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption)23,24.
The ΔH°ads values are negative, which showed that the adsorption is an exothermic process25.
The ΔS°ads values are negative, which showed that the adsorption is an exothermic process and always accompanied by a decrease of entropy. The reason can be explained as follows: the adsorption of organic inhibitor molecules from the aqueous solution26,27.
From potentiodynamic polarization data, it was found that the Tafel lines are shifted to more negative and more positive potentials with respect to the blank curve by increasing the concentration of the investigated inhibitors. This behavior indicates that the undertaken additives act as mixed-type inhibitors28,29. The addition of benzylpenicillin shifts the Ecorr values towards the negative potential. A compound can be classified as an anodic or a cathodic-type inhibitor when the change in the Ecorr value is larger than 85 mV30. Since, the largest displacement exhibited by benzylpenicillin was less than this value, it may be concluded that this molecule should be considered as a mixed-type inhibitor. The results showed that the increase in inhibitor concentration leads to decrease the corrosion current density (icorr), but the Tafel slopes (βa‚ βc) are parallel and approximately constant indicating that the retardation of the two reactions (cathodic hydrogen reduction and anodic metal dissolution) were affected without changing the dissolution mechanism19.
From Electrochemical Impedance Spectroscopy (EIS) curves it was concluded that curves approximated by a single capacitive semicircles showed that the corrosion process was mainly charged-transfer controlled. The general shape of the curves is very similar for all samples (in presence or in the absence of inhibitors at different immersion times) indicated that no change in the corrosion mechanism31.
Benzylpenicillin shows good inhibitive action against the corrosion of carbon steel in 1 M HCl. The value of inhibition efficiency increases with increasing the inhibitor concentration and decreases with increasing of the temperature. The adsorption of benzylpenicillin on carbon steel is physical adsorption and obeys Langmuir adsorption isotherm. The negative values of the free energy of adsorption and adsorption heat are indicating that the process was spontaneous and exothermic.
This research explores the inhibitory effect of benzylpenicillin on the corrosion of carbon steel where it was found that this effect is caused by the physical adsorption of this drug on the surface of the metal, which is affected by high temperature, which enables researchers, especially those interested in industrial cleaning operations in the petrochemical industries to design these processes in more favorable conditions To reduce carbon steel wear.