S.Hari Kumar, S.Karthikeyan, P.A.Jeeva
Keywords: Mild Steel, Corrosion Inhibition, Ampicillin, Adsorption Isotherm, Theoretical studies.
Abstract:
Corrosion behaviour of mild steel in 1M H2SO4 with Ampicillin as corrosion inhibitor has been investigated by using weight loss, Potentiodynamic polarization, Electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance spectroscopic studies. All these techniques reveal that inhibition efficiency increases with increase in the concentration of Ampicillin. Scanning electron microscopy (SEM) was carried out to characterize the surface morphology of the metal specimens. Polarization studies indicated that Ampicillin behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A chemoffice 3D simulation technique was used to run the quantum mechanical analysis and established correlations between different types of descriptors and measured corrosion inhibition efficiencies of Ampicillin. The quantum chemical analysis substantiates the inhibition efficiencies of the compound determined by electrochemical methods.
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ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Corrosion Inhibition of Mild Steel in 1M H2SO4 by Ampicillin as an Inhibitor S.Hari Kumar1, S.Karthikeyan2*, P.A.Jeeva3 1Materials Chemistry Division, School of Advanced Sciences, VIT University, Vellore, Tamilnadu, India. 2 Surface Engineering Research Lab, CNBT , VIT University, Vellore, Tamilnadu, India. 3 School of Mechanical and Building Sciences, VIT University, Vellore, Tamilnadu, India. (*Corresponding author: skarthikeyanphd@yahoo.co.in) Abstract Corrosion behaviour of mild steel in 1M H2SO4 with Ampicillin as corrosion inhibitor has been investigated by using weight loss, Potentiodynamic polarization, Electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance spectroscopic studies. All these techniques reveal that inhibition efficiency increases with increase in the concentration of Ampicillin. Scanning electron microscopy (SEM) was carried out to characterize the surface morphology of the metal specimens. Polarization studies indicated that Ampicillin behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A chemoffice 3D simulation technique was used to run the quantum mechanical analysis and established correlations between different types of descriptors and measured corrosion inhibition efficiencies of Ampicillin. The quantum chemical analysis substantiates the inhibition efficiencies of the compound determined by electrochemical methods. Keywords: Mild Steel, Corrosion Inhibition, Ampicillin, Adsorption Isotherm, Theoretical studies. Introduction Mild steel is an important category of metals due to its excellent mechanical properties. It is extensively used under different conditions in chemical and allied industries in handling acidic, alkaline and salt solutions. Mild is used in industries as pipelines for petroleum industries, storage tanks, reaction vessel and chemical batteries [1]. Acid solutions are widely used in many industrial processes. Acids are used for acid cleaning, 1 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 pickling and descaling due to their chemical properties [2-5]. Acids cause damage to the substrate, because of their corrosive nature. Several methods were used to decrease the corrosion of metals in acidic medium, but the use of inhibitors is most commonly used [6-10]. Organic compounds are widely used as corrosion inhibitors for mild steel in acidic media [11-16]. The rate of corrosion decreases by adsorption of organic inhibitors on the metal surface. The inhibitors block the active sites by displacing water molecules and form a compact barrier film on the metal surface. The most of the organic inhibitors are toxic, highly expensive and non environment friendly. Research activities in recent times are geared towards developing the cheap, non-toxic drugs as environment friendly corrosion inhibitors [17-21]. The aim of this work is to investigate the corrosion protection efficiency of ampicillin for mild steel corrosion in 1M H2SO4. We came to know that exceedingly few reports are available by using this compound as corrosion inhibitor in 0.1M H2SO4 [22-24]. No concrete report is available for the use these compounds as corrosion inhibitors in 1M H2SO4. From the literature the higher concentration of H2SO4 acts as pickling solution for mild steel for electroplating, battery electrodes using sulphur containing organic compounds. Use of this inhibitor in 1M H2SO4 will reduce the metal loss in acid medium. The compound is large enough and sufficiently planar to block more surface area on the mild steel. The inhibition efficiency was calculated using weight loss measurement, potentiodynamic polarization studies, impedance techniques, hydrogen permeation studies and diffuse reflectance methods. A definite correlation exists between different types of descriptors and measured corrosion inhibition efficiency for ampicillin using chemical and electrochemical techniques. 2. Experimental Details 2.1. Materials Mild steel specimens of size 1x4 cm2 were used for weight loss and electrochemical studies. The aggressive solution of 1M H2SO4 (AR Grade) was used for all the studies. The antibiotic namely ampicillin was purchased from the corresponding manufacturing company. The structure of the antibiotic is given in the figure 1. Electrochemical experiments were performed using a three electrode cell assembly with mild steel samples as working electrode, 4cm2 area of platinum as counter electrode and Hg/Hg2SO4/1M H2SO4 as the reference electrode. The surfaces of corroded and corrosion inhibited mild steel 2 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 specimens were examined by diffuse reflectance studies in the region 200-700 nm using U-3400 spectrometer (UV-VIS-NIR Spectrometer, Hitachi, Japan). Fig.1: Structure of Ampicillin Ampicillin 2.2. Weight loss studies The concentrations of inhibitor used for weight loss and electrochemical study were from 5x10-4M to 15x10-4M. Mild steel specimens of size 1x4 cm2 were abraded with different emery papers and washed with acetone. The cleaned samples were then washed with double distilled water and finally dried and kept in the desiccator. The weight loss study was carried out at room temperature for three hours in 1M H2SO4. The inhibition efficiency (IE %) was determined by the following equation Inhibition Efficiency (IE %) = (W0 –Wi /W0) X 100 Where W0 & Wi are the weight loss values in the absence and presence of the inhibitor. 2.3. Electrochemical studies Potentiodynamic polarization measurements were carried out in a conventional three electrode cylindrical glass cell, using CH electrochemical analyzer. The solution was deaerated for 20 minute before carryout the polarization studies. The working electrode was maintained at its corrosion potential for 10 min. until a steady state was obtained. The mild steel surface was exposed to various concentrations of inhibitors in 100mL of 1M H2SO4 at room temperature. The inhibition efficiency (IE %) was calculated using the equation. 3 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 Inhibition Efficiency (IE %) = submitted 22 September 2013 (I0 –I /I0) X 100 where I0 and I are the corrosion current density without and with the inhibitor respectively. The potentiodynamic current-potential curves were recorded by changing the electrode potential automatically from -750mV to +750mV versus the open circuit potential. The corresponding corrosion current (I corr ) was recorded. Tafel plots were constructed by plotting E versus log I. Corrosion Potential (Ecorr), corrosion current density (Icorr) and cathodic and anodic slopes (βc and βa) were calculated according to known procedures. Impedance measurements were carried out in the frequency range from 0.1 to 10000 Hz using amplitude of 20 mV and 10 mV peak to peak with an AC signal at the open-circuit potential. The impedance diagrams were plotted in the nyquist representation. Charge transfer resistence (Rct) and double layer capacitance (Cdl) values were obtained from nyquist plot [25, 26]. The percentage inhibition efficiency was calculated from the equation Inhibition Efficiency (IE %) = (Rct - R’ ct / Rct) x 100 Where R’ ct and Rct are the corrosion current of mild steel with and without inhibitor respectively. 2.4. Hydrogen permeation studies The hydrogen permeation study was carried out using an adaptation of modified Devanathan and Stachurski’s , two compartment cell as described elsewhere [27]. Hydrogen permeation current was recorded in the absence and presence of inhibitors. 2.5. Surface morphology The Scanning electron microscopy (SEM) was used to examine the specimen’s surface which is immersed in blank and inhibitor solutions. Energy Dispersive spectrometer (EDS) is used to analyze the elements in the specimen. The following cases were examined in the SEM. I) Mild steel specimen immersed in 1M H2SO4 II) Mild steel specimens immersed in 1M H2SO4 containing 15x10-4 M inhibitor. 4 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 2.6. Diffuse reflectance spectroscopy The surfaces of corroded and corrosion inhibited mild steel specimens were examined by diffuse reflectance studies in the region 200- 700 nm using U-3400 spectrometer [UV-VIS-NIR Spectrometer, Hitachi, Japan]. 2.7. Theoretical calculations Quantum calculations were carried using MOPAC 2000 program of CS Chemoffice packet program. The energy of highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), Dipole moment (), hardness, absolute softness and total energy of the molecule were calculated with the above given software package. 3. Results and discussion 3.1. Weight loss studies The values of inhibition efficiency (IE %), corrosion rate (CR) and surface coverage (θ) calculated for Ampicillin in 1M H2SO4 at different concentrations from the weight loss data are summarized in the table-1. It is obvious that inhibition efficiency enhances with increase in the inhibitor concentration. In addition the rate of corrosion has reduced with increase in inhibitor concentration. Maximum inhibition efficiency is obtained at 15x10-4 M concentrations of the inhibitor. Table 1. Weight loss parameters for the corrosion of mild steel immersed in 1M H2SO4 in absence and presence of different concentrations of Ampicillin Inhibitor Conc. (M) Weight Loss (g) Inhibition Efficiency Corrosion Rate Surface [mg cm-2h-1] Coverage [θ] Blank 0.0809 - 6.74 - 5x10-4 0.0299 63.04 2.49 0.6304 10x10-4 0.0192 76.26 1.60 0.7626 15x10-4 0.0095 76.26 0.79 0.8825 5 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 3.2. Potentiodynamic polarization studies Polarization curves for mild steel in 1M H2SO4 containing different concentrations of inhibitor are given in figure-2. The values of corrosion potential (Ecorr) , corrosion current densities (Icorr), anodic tafel slope (βa) ,cathodic tafel slope (βc) surface coverage(θ) and inhibition efficiency (IE%) calculated using polarization curves are summarized in table-2. Fig. 2: Polarization curves of mild steel recorded in 1M H2SO4 in absence and presence of different concentrations of Ampicillin According to the results, corrosion current (Icorr) value decreases with increase in the concentration of the inhibitor. The inhibition efficiency (IE %) and surface coverage (θ) increases with increase in inhibitor concentration. The maximum inhibition efficiency is achieved at 15x10-4 M concentration. Both βa and βc are reduced, but the values of βc are decreased to a greater extent. This indicates that the compound behave as cathodic inhibitor. 6 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Table 2: 2 Potentiodynamic polarization parameters for mild steel immersed in 1M H2SO4 in the absence and presence of different concentrations of ampicillin. Inhibitor Ecorr Icorr βa βc Inhibiton Surface Con. [mV vs [A cm-2] [mV [mV efficiency coverage [M] SCE] dec-1] dec-1] [%] [θ] Blank -376.12 548.57 82.9 135.3 - - 5x10-4 -264.05 195.77 64.1 144.2 64.31 0.6431 10x10-4 -259.95 125.42 57.9 134.7 77.13 0.7713 15x10-4 -255.21 80.26 51.9 132.4 85.36 0.8536 3.3. Electrochemical impedance studies The Nyquist representations of impedance performance of mild steel in 1M H2SO4 with and without addition of different concentrations of ampicillin are shown in the figure-3. A large capacitive circle at higher frequency range is observed at all concentrations of the inhibitor. The higher frequency capacitive loop is due to the adsorption of inhibitor molecule [28]. 7 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Fig. 3: Nyquist plot for mild steel immersed in 1M H2SO4 containing different concentrations of Ampicillin Values of charge transfer resistance (Rct) and double layer capacitance (C dl) derived from Nyquist plots are shown in table 3. The values of Rct are found to increase with increase in concentration of inhibitor in 1M H2SO4. It is found that values of Cdl are fetched down by increasing concentrations of inhibitor in the acid. This can be ascribed to the wellbuilt adsorption of the ampicillin on the metal surface. Table 3: 3 Electrochemical impedance parameters for mild steel immersed in 1M H2SO4 in the absence and presence of different concentrations of ampicillin. Inhibitor Rct Cdl Inhibition Surface Con. [M] [Ω cm2] [F cm-2] efficiency [%] coverage[θ] Blank 28.1 0.489 - - 5x10-4 82.1 0.381 65.77 0.6577 10x10-4 112.6 0.277 75.04 0.7504 15x10-4 176.32 0.143 84.06 0.8406 8 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 3.4. UV spectral reflectance studies The reflectance curves for polished specimen, specimen dipped in 1M H2SO4 and maximum concentration of ampicillin are given in the figure.4. The percentage of reflectance is highest for polished mild steel and it steadily reduces for the specimen dipped in 1M H2SO4 solution. This observation discloses that the change in surface feature is due to the corrosion of mild steel in acid. The reflectance percentage of steel in the presence of ampicillin is higher than mild steel immersed in blank. This validates that the surface property of steel are not altered further due to the formation of film on the metal. The reflectance percentage decreases with increase in thickness of the inhibitor film formed on metal surface. Similar observation has been made by Madhavan et al [29]. 45 1 40 1. Polished 2. Blank 3. Ampicillin Reflectance(%) 35 30 25 20 3 15 10 2 5 200 300 400 500 600 700 800 Wavelength (nm) Fig. 4: UV Reflectance curves of mild steel in 1M H2SO4 solution with 15x10-4M concentration of the ampicillin. 3.5. Adsorption isotherm and and thermodynamic parameters The inhibitive action of ampicillin in highly aggressive media is due to its adsorption on the metal surface. The degree of surface Coverage (θ) for different concentrations of ampicillin in 1M H2SO4 has been calculated from weight loss, Polarization and Electrochemical Impedance studies. The acquired data was tested graphically for fitting suitable isotherm [30-32]. Almost a straight line was obtained by plotting log (C/θ) Vs 9 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 log C as shown in Figure-5, which proves that the adsorption of ampicillin on mild steel surface obeys Langmuir adsorption isotherm. Fig. 5: Langmuir’s adsorption isotherm plots for the adsorption ampicillin in 1M H2SO4 on the surface of mild steel. The Langmuir isotherm for the adsorbed layers is given by the equation [33], Cinh/θ =1/Kads + Cinh Where Kads is the equilibrium constant of the adsorption/desorption process. Adsorption equilibrium constant [Kads] and free energy of adsorption [∆G0ads] were calculated using the equation [34] Kads= 1/Cinh x θ/1-θ ∆G0ads = -2.303RT log [55.5Kads] Where 55.5 is the molar concentration of water in solution [35]. R is the gas constant, T is the temperature. The values of adsorption equilibrium constant [Kads] and free energy of adsorption [∆G0ads] are given in table-4. The negative values of [∆G0ads] pointed out that adsorption of inhibitor is spontaneous process. It is reported that values of [∆G0ads] is of order 20 kJmol-1 or lower indicates a physisorption, those of order of 10 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 -40 kJmol-1 or higher involve charge sharing or transfer from the inhibitor to the metal surface specifies a chemisorptions [36-38]. The values of free energy of adsorption [∆G0ads] in our experiment lies in the range -28 to -32 kJmol-1, demonstrating that the adsorption is not a simple physisorption, but it may involve some other interactions [39]. Table 4: Gibbs free energy parameters and adsorption equilibrium constant [K] of ampicillin at various temperatures evaluated by weight loss method. Temperature Kads -∆G0ads (kJmol-1) 313 1016 28.46 323 2185 31.43 333 2697 32.99 (K) 3.6. Hydrogen permeation measurements Hydrogen permeation currents are recorded in H2SO4 in the absence and presence of ampicillin. This study has been taken up with a plan of selecting the inhibitor with a view to their efficacy on the reduction of hydrogen uptake [40]. The values of permeation current with respect to time are given in table-5. Figure 6 shows the variation of permeation current vs. time for mild steel in 1M H2SO4 in the presence of ampicillin. 11 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Table 5: Values of permeation current for mild steel in 1M H2SO4 and in presence of ampicillin with respect to change in time Permeation Current (A (A) A) Time (min.) 1M H2SO4 0 10.8 3.4 5 11.2 4.1 10 12.0 4.6 15 12.3 5.4 20 12.9 6.2 25 12.9 6.2 30 12.9 6.2 35 12.9 6.2 40 12.9 6.2 Ampicillin Ampicillin The ampicillin brings down the permeation current to the extent of 50%. The corrosion inhibition efficiency of the ampicillin in 1M H2SO4 follows the same order. Thus a definite correlation exists between the corrosion inhibition efficiency and the extent of reduction in the permeation current of the compound. It is a recognized fact that higher βc value for an inhibiting compound, the lesser is the corrosion and hydrogen ingress on the metal. An increase in the βc value, leads to increase in the energy barrier for proton discharge and decrease in the evolution of hydrogen. This in turn leads to lower permeation of hydrogen through the metal. 12 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 14 1 Permeation Current (µA) 12 10 1. Blank 2. Ampicillin 8 2 6 4 2 0 10 20 30 40 Time (min.) Fig.6: Fig.6: Hydrogen permeation Vs Time curves for mild steel immersed in 1M H2SO4 and 15x10-4 M concentration of ampicillin 3.7. Scanning electron microscopic studies SEM images for mild steel surface immersed in 1M H2SO4 solutions for 3 hrs in the presence and absence of ampicillin are displayed in Figure 7 (A & B). The surface of mild steel is greatly damaged in the absence of the inhibitor (Figure 7A). SEM image of inhibited mild steel specimen (Figure 7B) explains that fine protective adsorbed film is formed on the specimen’s surface, which reduces the rate of corrosion, being accountable for the inhibition. 13 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Fig. 7; SEM images of mild steel [7A] immersed in 1M H2SO4 for 3h, [7B] ampicillin 3.8. Mechanism of corrosion inhibition The adsorption of ampicillin on the mild steel surface is found to be majorly physical in nature. Physical adsorption is a process of electrostatic attraction between charged species in the solution and the metal surface. If the metal surface is positively charged, the adsorption of negatively charged species is facilitated. Positively charged species can also adsorb on the positively charged metal surface with the help of negatively charged intermediate, which adsorb first on the positively charged metal surface and allows positively charged species to adsorb on it. Thus the adsorption of ampicillin may take place in two different ways as (i) The protonated ampicillin in acid solution may adsorb electrostatically to the anion covered mild steel surface through their protonated form. (ii) The inhibitor may competete with acid anions for the sites at the water covered surface and adsorb by donating electrons to the mild steel surface [41, 42]. 3.9. Quantum chemical calculations Quantum chemical calculations were carried out to investigate the adsorption and inhibition mechanism of the inhibitor. Figure 8 shows the optimized structure of ampicillin. The values of calculated quantum chemical parameters i.e. EHOMO (highest occupied molecular orbital), ELUMO (lowest unoccupied molecular orbital), ∆E (energy gap), (dipole moment), σ (softness) etc. are summarized in table-6. 14 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Fig. 8: Optimized structure of ampicillin EHOMO is associated with the electron-donating ability of the molecule. Several researchers have shown that the adsorption of an inhibitor on metal surface can occur on the basis of donor-acceptor interactions between the π-electrons of heterocyclic atoms and the vacant d-orbitals of the metal surface atoms [43-45]. A high value of EHOMO indicates a tendency of a molecule to donate electrons to acceptor molecules with low energy empty molecular orbital. Increasing values of EHOMO facilitates the adsorption and increases the inhibition efficiency by influencing the transport process through the adsorbed layer [46]. ELUMO indicates the ability of the molecule to accept the electrons, hence these are acceptor states. The lower the value of ELUMO, the more probable is that the molecule can accept electrons and increase the inhibition efficiency. Regarding ∆E (ELUMO-EHOMO) lower values of energy difference will cause higher inhibition efficiency because energy to release electron from last occupied orbital will be low. When dipole moment is concerned higher values of , will favours a strong interaction of inhibitor molecule with the metal surface [47]. Other indicators are absolute electro negativity (χ), absolute hardness (ȃ). Absolute electro negativity is a chemical property that describes the ability of a molecule to attract electron towards itself in a covalent bond. Absolute hardness is measured by the energy gap between EHOMO and ELUMO. Absolute softness σ is the reciprocal of the hardness. χ, ȃ, σ are calculated using the energies of HOMO and LUMO orbital’s of the inhibitor molecules are related to the ionization potential (I), electron affinity (A) by the following relations 15 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 χ = I + A/ 2, ȃ = I – A / 2, σ = 2 / I – A Where I = -EHOMO, A = -ELUMO The results deduced indicate that the electron flow will happen from the molecule with low electro negativity towards that of higher value until the chemical potentials are same. The best inhibition effect is shown by ampicillin with low electro negativity. The higher value of dipole moment and lower total energy for ampicillin indicates the strong interaction of inhibitor with metal that leading to improved adsorption. From figure 9 it can be observed that the energy highly occupied molecular orbital’s (HOMO) are localized on hetero atoms for ampicillin. Fig.9: The Highest occupied molecular orbital of ampicillin From Figure 10 it is observed that lowest unoccupied molecular orbital’s (LUMO) of Cloxacillin, which is responsible for its lower inhibition efficiency than ampicillin. 16 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 16, Preprint 61 submitted 22 September 2013 Fig. 10: The lowest unoccupied molecular orbital of ampicillin 4. Conclusions 1. The use of ampicillin as corrosion inhibitor in 1M H2SO4 was thoroughly studied using weight loss, potentiodynamic polarization, impedance measurements and hydrogen permeation studies. 2. The adsorption of ampicillin on mild steel surface follows Langmuir adsorption isotherm. The adsorption of ampicillin on steel surface is further confirmed by diffuse reflectance spectra and SEM images. 3. The quantum mechanical studies substantiate the performance of ampicillin as excellent corrosion inhibitor for mild steel in 1M H2SO4. 17 © 2013 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 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This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.