Study the Inhibition Effect of Amoxicillin Drug for Corrosion of Carbon Steel in Saline Media

Potentiostatic polarization and weight loss methods have been used to investigate the corrosion behavior of carbon steel in sodium chloride solution at different concentrations (0.1, 0.4 and 0.6) M under the influence of temperatures ( 293, 298, 303, 308 and 313) K. The inhibition efficiency of the amoxicillin drug on carbon steel in 0.6 M NaCl has also been studied based on concentration and temperature. The corrosion rate showed that all salt concentrations ( NaCl solution) resulted in corrosion of carbon steel in varying ratio and 0.6 M of salt solution was the highest rate (50.46 g/m2.d). The results also indicate that the rate of corrosion increases at a temperature of 313 K.. Potentiodynamic polarization studies showed that the examined inhibitor suppress both anodic and cathodic process and behave as mixed type inhibitor. The adsorption of amoxicillin was found to obey Langmuir isotherm model. Arrhenius equation and transition state theory were used to calculate kinetic and thermodynamic parameter. Results obtained showed that corrosion reaction of carbon steel in NaCl is spontaneous and there is a good agreement between the data got from the both techniques employed. SEM analysis was performed to study the film persistency of the inhibitor.


Introduction:
Corrosion is a change naturally happening for metals and alloys. The corrosion mechanism of metals and alloys can be explained as follows: when the metal is immersed in the corrosive medium, it begins to oxidize, forming ions inside the solution 1, 2 1,2 . Studying the corrosion process has a significant attention through preventing and overcoming of this spontaneous process. The corrosion mechanism of different materials are generally depends on the nature of corrosive environment. The existence of the electrolytes in the corrosive media leads to an effect on the rate of corrosion. Sodium chloride is considered as an effective corrosive electrolyte 3 3 . Steel alloy is extensively used in industry especially for tools and metallic equipment due to its good mechanical properties and low cost. Corrosion is a serious problem for the application of iron and its alloys in many types of service 1 . On the other hand, corrosion process being surface reactions and by studying surface chemistry, it became certain that surface reactions were directly affected by the presence of foreign particles. For this reason, corrosion should be controlled and the most efficient way is by using compounds known as inhibitors. Inhibitors are mostly organic compounds having heteroatoms like oxygen, nitrogen and sulfur atoms that, when added in small amounts stops or slows down corrosion of metals and alloys. Newly, researchers trend on the use of eco-friendly, cheap and nontoxic inhibitors. Inhibitors are absorbed on the surface of the reactive metal 3,4 . The term adsorption refers to molecules directly linked to the surface, usually only one molecular layer is thick, and does not penetrate into the extent of the metal itself. A known method of controlling corrosion in many branches of technology is the technique of adding inhibitors to the mineral environment. Generally inhibitors adsorb on the entire metal surface impede corrosion reaction 5,6 .
Antibacterial drugs seem to be ideal candidate to replace traditional toxic organic inhibitors due to their natural origin, containing heteroatoms (S, N and O) as active centers, non-hazardous, biodegradable, as well as they could be easily produced and purified. Thus, such investigations are found to be very fruitful and encouraging in saving both metals and environment 7 . Amoxicillin is an antibiotic drug molecule generally having an N-S hetero-cyclic compound containing five oxygen atoms, three nitrogen atoms and one sulfur atom 8 . The molecule is big enough and planar to block more surface area through adsorption on the metal surface, these properties enable it to be an effective inhibitor.
In this research, experimental studies were done to examine the corrosion behavior of carbon steel alloy at various sodium chloride concentrations at five different temperatures in the range 293 -313 K. Consequently, the inhibition effect of amoxicillin drug at various concentrations and temperatures on the corrosion of carbon steel in 0.6 M sodium chloride solution have been tested using weight loss and potentiodynamic polarization techniques. Furthermore, the kind of interactions between amoxicillin drug and carbon steel surface in 0.6 M NaCl solution were investigated from thermodynamic isotherm simulation.
Carbon steel specimen has the following chemical compositions: Carbon (0.1), Silicon (0.24), Manganese (0.47), Chrome (0.12), Molybdenum (0.02), Nickel (0.1), Aluminum (0.03), Cupper (0.14), Cobalt (<0.0012), Vanadium (<0.003), Tungsten (0.06) and the rest is Iron. The specimen with 2.5 cm in diameter and thickness of 1 mm was used as a working electrode in the potentiostatic polarization technique. It was polished by silicon carbide grit abrasive paper from 400, 600, 800, 1200 and 2000, then degreased with acetone and washed with distilled water and finally deride by ethanol. The electrolytic cell had a working capacity of 1L which contained the working electrode (carbon steel), platinum auxiliary electrode and a saturated calomel reference electrode. The potential-current polarization curves were plotted using a potentiostate (Model WENKING lab-200) obtained from Bank Electronics-Intelligent, GmbH., Germany. This apparatus was commercially programmable, together with an electrometer that provides variability of continuous scan over a desired potential range, including the cathodic and anodic regions.
Loss in weight measurements were performed in 100 mL of sodium chloride solution at concentrations of 0.1, 0.4 and 0.6 molar to immerse three hours of carbon steel at different temperatures in the range of 293 to 313 K. The same experimental procedure was repeated for 0.6 M NaCl solution containing various concentrations of amoxicillin drug of 6×10 -4 ,1×10 -3 and 6×10 -3 M. These carbon steel specimens were polished as mentioned above, washed with distilled water, degreased with acetone , dried and weighted. After the end of the experiments, carbon steel specimens were washed in distilled water, dried and finally reweighted. Triplet experiments were occurred in each study and the mean weight loss value was reported. From the loss in weight measurements, the corrosion rate (C R ) was calculated using the following relation: (1) where, W 1 is the weight of steel before corrosion, W 2 is the weight of steel after corrosion, S is the surface area of the specimen, t is the immersion time, and C R is the corrosion rate. The inhibition efficiency (IE %) was calculated by using the following relation 9 , o is the corrosion rate without inhibitor and C R i is the corrosion rate with inhibitor. Results and Discussion:

Potentiostatic Polarization Measurement of Carbon Steel
In order to notice the electrochemical behavior of carbon steel in various concentrations of sodium chloride solutions and obtain preliminary information about how amoxicillin drug can influence the corrosion process of it, potentiostatic polarization curves were recorded. Fig. 2 a, b, c represent the typical polarization curves for carbon steel corrosion in various concentrations of NaCl solution from 0.1 to 0.6 M at various temperatures in the range from 293 to 313 K, respectively. The starting potential was -0.4V (SCE) and the scan range extended up to 0.0 V at a voltage scan rate of 10 mV.s -1 .The data obtained from the polarization curves are listed in Table 1.    Table 2 represents the parameters obtained from the polarization curves.

Table 2. Corrosion parameters of carbon steel in 0.6 M of NaCl solution in the absence and presence of amoxicillin inhibitor at different temperatures in the range 293-318 K.
It appears from data presented in Tables 1 and 2 10,11 . On the other hand, the corrosion current density decreases with increasing concentration of inhibitor and increase with increasing temperatures in the range 293-313. Corrosion potential increases with increasing concentration of inhibitor and decreases with increasing temperatures. The corrosion current densities in the absence and presence of various concentrations of amoxicillin drug in the corrosive medium (0.6 M NaCl) have been used to determine the inhibition efficiency (IE %) using the following equation: IE% =100 [ 1-(i corr ) 2 / ( i corr ) 1 ] …..(3) Where (i corr ) 1 and ( i corr ) 2 are respectively the corrosion current densities of the steel in the absence and the presence of amoxicillin at the same temperature. The results obtained are listed in Table  2. It is indicated that the inhibition efficiency values have been increased with increasing both the amoxicillin concentration and temperature, and these results are similar to data obtained from weight loss method. The surface coverage (θ) was calculated as θ = (1-(i corr ) 2 / ( i corr ) 1 ).

Polarization Resistance
The overall definition of the polarization resistance of any corroded metal or alloy is the slope of the graphic relationship between the potential (E) and the current density (i) of the corrosion process as follows: R P = (∂ƞ / ∂i ) T,C = [∂ (E -Ec) /∂i] T,C …..(4) Where ƞ is the over potential which is defined as the departure of the potential of the electrode from its equilibrium value (Ec). for low field approximation (ƞ ≤ 10 mV) Tafel equation can be miniature to this form ƞ = (RT/ i˳ F) i …..(5) Where i˳ is the equilibrium exchange current density. R p may best be determined from the equation:  Table   3). The increase in polarization resistance indicates that the inhibition efficiency increased with an increase in the concentration of amoxicillin and temperature. The data in Table 3 show that the polarization resistance decreases as the temperature increases for blank solution and increases with increasing temperature for inhibited solutions. On the other hand, R P values are greater in the presence of an inhibitor due to the formation of a protective layer on the surface of carbon steel.

Kinetics of Corrosion
The rate (r) of carbon steel corrosion at 0.6 M of NaCl solution increased with increasing temperature from 293 to 313K and this behavior followed Arrhenius equation (7), r = A exp (-Ea/RT) …..(7) that A and E a are respectively, the pre-exponential factor and the energy of activation. Figure 4    The data show that the activation energy for steel corrosion in corrosive medium in the presence of inhibitor is higher than those in the absence of it and its value decreases with increasing inhibitor concentration. These results explain why the variations of i corr are pronounced in the presence of amoxicillin.
Higher values of activation energy (E a ) in the presence of inhibitor refer to a strong inhibitive action by increasing energy barrier for the corrosion process 12 . On the other hand, a decrease of the corrosion activation energy with an increase of inhibitor concentrations, is associated with an increase in inhibition efficiency with increasing temperature as shown in Table 3, referring to the formation of an adsorption layer of chemical nature, which involves the transfer of electrons from amoxicillin molecules to the steel surface, results to the formation of coordinate bond with the surface of carbon steel 13,14 . Alternatively, a chemisorption mechanism associated with increase in inhibition efficiency with temperature and higher activation energy in the presence of the inhibitor [15][16][17] . In fact, it has been known that chemisorbed molecules are bound at anodic sites of the metal and inhibit the corrosion reaction by retarding the anodic reaction.

Thermodynamic of Corrosion
The science of energy change, which has been applied for many years to studies corrosion of metals and alloys, is known as thermodynamics. The change in free-energy (ΔG) associated the electrochemical reaction can be calculated using the following equation: ΔG = -nFE …..(8) Since the number of electrons in the reaction is denoted by n, Faraday constant as F and cell potential ( E=Ecorr) equals E. Using ΔG values obtained from above equation, entropy change (ΔS) of the steel corrosion can be determined using the known thermodynamic relationship: ΔS = -d (ΔG) / dT …..(9) ΔG data are plotted against temperature and therefor values of ΔS can be evaluated from the slope of this plot as shown in Figure 5. At constant temperature, free energy can be expressed as: ΔG= ΔH -TΔS …..(10) Where ΔH is the change in enthalpy, and T is absolute temperature. Thermodynamic quantities for the corrosion of carbon steel are given in Table  5.  Results of Table 5 indicate negative values of ∆G that mean, corrosion reactions are occurring spontaneously. The enthalpy changes (∆H) of the corrosion reaction of carbon steel in different concentrations of amoxicillin at five temperatures in the range 293-313 K have negative values revealing an exothermic nature for this reaction. ∆S values were positive suggesting a lower order of the solvated ions.

Weight loss study
Corrosion rate data and percentage inhibition efficiency were obtained from loss in weight measurements at different amoxicillin concentrations in 0.6 M NaCl solution after immersion for three hours at different temperatures in the range from 293 to 313 K. The values obtained are listed in Table 6. It has been noticeable that amoxicillin inhibits carbon steel corrosion in 0.6 M NaCl solution at different concentrations used in the research. Table 7 shows that the inhibition efficiency increased from 73.13% to 81.31% with the increasing amoxicillin concentration from 6×10 -4 to 6×10 -3 M at 313 K. On the other hand, corrosion rates of blank saline medium increase with increasing temperature from 293 to 313 K. Furthermore, the corrosion rate of carbon steel in the inhibiting saline medium decreased with increasing temperature. Consequently, the efficiency of amoxicillin inhibition increases significantly with increasing temperature. This finding supports the idea that the adsorption of amoxicillin molecules onto the surface of carbon. Thus, as the temperature increases, the number of adsorbed molecules increases, resulting in increased inhibition efficiency. However, an increased inhibition efficiency (IE%) and a lower corrosion rate (C R ) may be due to increased adsorption and increased coverage of amoxicillin molecules on the carbon steel surface with an increased concentration 18 .

Adsorption isotherms
One of the most important steps in the inhibition mechanism is the adsorption of amoxicillin drug onto the carbon steel surface. Figure 6 shows the C/θ data plotted against the concentration of amoxicillin in saline solutions for both potentiostatic polarization and weight loss techniques. The results indicate that the amoxicillin adsorption follows the Langmuir isotherm, which was applied using the relation as follows: C/θ = 1/K ads. + C …..(11) where K ads. indicates the adsorption constant, C refers to concentration of amoxicillin, and θ represents the surface coverage . Thermodynamic adsorption parameters obtained from the intercept of the straight line obtained by drawing C/θ against C values at various temperatures are listed in Table  8. where ΔG ads is the free energy of adsorption, when one water molecule is replaced by one inhibitor molecule and (1/55.5) is the numerical molarity of water. The obtained data are presented in Table 8. The average value of ΔG ads was found to be -40.36 kJ/mol. A negative sign indicates that the adsorption of amoxicillin molecules onto the carbon steel surface is spontaneous. It is known that 19 when the value of the free energy of adsorption is equal to -20 kJ / mol and less, the adsorption is physical, whereas when the value is -40 kJ / mol and higher, it indicates that the adsorption is chemical, referring to the charge transfer from the drug molecules to the surface of carbon steel leading to formation of a coordinated type of bond. The obtained value of ΔG ads suggests a strong chemical adsorption of amoxicillin molecules onto the surface of carbon steel in saline solution. Values of the thermodynamic functions of amoxicillin adsorption in saline medium at various temperatures obtained by potentiostatic polarization and weight loss techniques that presented in Table 8. Both methods are of a good agreement.  Scanning electron microscopy Figure 7 a, b and c. represent scanning electron microscopy of the carbon steel samples. The carbon steel surface before immersion in saline solution seems smooth, as shown in Fig.7a, compared to carbon steel after immersion in uninhibited 0.6 M NaCl solution for 3 hours. It is clearly shown from Fig.7 b that carbon steel surface shows cracks due to the salt attach on its surface. On the other hand, the presence of 5×10 -4 M of amoxicillin leads to reduce the average crakes on steel surface (Fig.7c). Based on what was obtained, it can be concluded that the adsorption layer was efficiently able to inhibit the corrosion of the pure carbon steel sample.

Conclusions:
Based on the results obtained, we can conclude the following:- Amoxicillin drug effectively inhibits the carbon steel corrosion in 0.6 M NaCl solution.  The corrosion process was inhibited by adsorption of the amoxicillin molecules on the carbon steel surface and the adsorption process obey Langmuir adsorption isotherm.  Both potentiostatic polarization and loss in weight methods indicate that the inhibition efficiency (IE) data increases with increasing amoxicillin concentrations and temperatures.  Gibbs energy for the adsorption values indicates a general chemical absorption mechanism. This type of inhibitor is effective in room temperature, and is characterized by an increase in the efficiency of inhibition at higher temperatures.  Polarization curves showed that amoxicillin was a mixed type inhibitor of surface corrosion of carbon steel in 0.6 M NaCl solution.  The formation of a protective layer on the surface of the carbon steel alloy was confirmed by images obtained by scanning electron microscopy.