An electrochemical and quantum chemical investigation of some corrosion inhibitors on aluminium alloy in 0.6 M aqueous sodium chloride solution

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1 Indian Journal of Chemical Technology Vol. 18, July 2011, pp An electrochemical and quantum chemical investigation of some corrosion inhibitors on aluminium alloy in 0.6 M aqueous sodium chloride solution R Banerjee, Ranjana, S S Panja & M M Nandi* Department of Chemistry, National Institute of Technology, Durgapur , India Received 13 August 2010; accepted 11 April 2011 The role of phthalic acid (I) and related compounds, viz., o-phenylenediamine (II) and anthranilic acid (III) to protect aluminium alloy against corrosion in 0.6 M aqueous sodium chloride solution is studied using potentiodynamic polarization technique and impedance studies. The order of inhibition effect of these compounds is found to be I < II < III. Density functional theory (DFT) has been used to analyze the characteristics of the inhibition mechanism and to describe electronic and structural nature of the inhibitor on the corrosion process. Attempt has been made to correlate the inhibiting properties with the electronic parameters and molecular area of the inhibitors Keywords: Aluminium, Polarization, Corrosion inhibition, Molecular size, Quantum chemical calculation, DFT calculation Aluminium and its alloys are used in a wide range of industrial applications owing to their low cost, light weight, high thermal and electrical conductivity and good corrosion resistance 1. The corrosion resistance arises from the ability of aluminium and its alloys to form a natural oxide film on its surface in a wide variety of media 2,3. In aggressive media, such as chloride solution, localized corrosion can occur leading to the breakdown of the passive layer and pit formation. 4,5 The protection of aluminium and its oxide films from the corrosive chloride attack has been studied by many investigators using either inorganic 6 or organic compounds 7-13 as corrosion inhibitors. Recently, we have reported the effective inhibition of brass corrosion by imidazoline and hydropyrimidine derivatives synthesized from amino acids in 0.6 M aqueous sodium chloride solution Significant improvement in the inhibition property of the amino acids is observed by introducing C 6 H 5 -SO 2 group. 15 It further increased when carboxyl group was converted to imidazoline or tetrahydropyrimidine group 14,16. Recently, density functional theory (DFT) is being used to analyze the characteristics of the inhibition mechanism and to describe the electronic and structural nature of the inhibitor on the corrosion process The aim of the present work is to study effect of some organic compounds on the corrosion of *Corresponding author ( murarimohan_nandi@yahoo.co.in) aluminium alloy in 0.6 M aqueous sodium chloride solution. Organic compounds were chosen whose molecular structure was likely to lead to chemisorption, physisorption or complexation on the aluminium surface. They are characterized by the presence of O and N atoms which act as reaction centres on the metal surface 24. Furthermore, DFT is considered a very useful technique to probe the inhibitor/surface interaction as well as to analyze the experimental data. Here, we have studied the corrosion inhibition efficiency of the three molecules, namely phthalic acid (I), o-phenylenediamine (II) and anthranilic acid (III). All the quantum chemical ground electronic state calculations were performed by means of the Gaussian03 program 25. Exchange and correlation calculations were carried out using the functional hybrid B3LYP 26,27 and the 6-31+G** orbital basis sets for all atoms. In all cases, the ground state geometrical structure of the species were fully optimized and frequency calculations were performed with the same B3LYP/6-31+G** orbital basis set. All the frequency calculations were carried out to make sure that all the frequencies are found positive which basically reflects that ground state optimized structures corresponds indeed to global minima. Since the ph of the reaction media is 6, the molecules preferably remain in solution either in neutral (for II and III) or monobasic form (I). The energies of the frontier molecular orbitals were calculated and chemical reactivity was analyzed mainly from those

2 310 INDIAN J. CHEM TECHNOL., JULY 2011 molecular orbitals. The frontier orbital surfaces were visualized using Gauss View 28. The compounds used in this study are shown below: Two electrochemical techniques potentiodynamic polarization and electrochemical impedance spectroscopy have been used to study the effect of addition of these compounds on the corrosion of aluminium in 0.6 M aqueous sodium chloride solution. Experimental Procedure Materials and Methods The chemical composition of the commercial aluminium alloy used in the present study was 0.55% Mg, 0.25% Si, 0.05% Zn, 0.004% Cu, balance Al. The working electrode for potentiodynamic study and EIS measurement was prepared from the aluminium rod from which only the circular cross-section (0.25 cm 2 ) of rod was exposed. The alloy sample was polished successively with (i) belt grinding polishing machine, (ii) metallographic emery paper of increasing fineness of up to 1200 grade and (iii) cloth polishing by Single Disc Polishing Machine CENSICO PMV-T Then the samples were degreased with AR grade ethanol and acetone and rinsed with double distilled water prior to each experiment. All compounds (I-III) were properly purified. The aggressive environment used was 0.6 M aqueous NaCl solution prepared from AR grade reagent and double distilled water. The ph of the solution was adjusted to 6.0. Oxygen content of the sodium chloride solution was 7.3 mg/l. All other reagents were of AR quality. Potentiodynamic polarization study The potentiodynamic polarization studies were carried out with polished and degreased aluminium specimens having an exposed surface area of 0.25 cm 2. The electrochemical measurements were carried out in a standard three-electrode electrochemical cell of volume 100 ml. The counter electrode was a platinum plate with 1 cm 2 surface area, the reference electrode consisted of a commercial saturated calomel electrode with a Luggin capillary bridge and the working electrode was made up of aluminium alloy. Polarization studies were carried out using potentiostat/galvanostat (ACM Instruments Gill AC) and the data obtained were analyzed using the software ACM Instruments version 5 in air atmosphere. The degreased working electrode was immersed in 0.6 M aqueous NaCl solution and allowed to stabilize for 30 min 29. The cathodic and anodic polarization curves of aluminium alloy specimen in the test solution with and without inhibitors were recorded at a scan rate of 1 mv/s in 0.6 M aqueous NaCl solution (ph=6) at 30±0.5 C. The inhibition efficiencies of the compounds were determined from corrosion current density using the Tafel extrapolation method. Electrochemical AC impedance studies AC impedance measurements were conducted at room temperature using ACM Instruments Gill AC. The instruments were controlled by the software ACM Instruments version 5 between 1 mhz and 100 khz. An AC sinusoid of ~ ± 10 mv was applied at corrosion potential (E corr ). The aluminium sample with an exposing surface area of 0.25 cm 2 was used as the working electrode. A three electrode electrochemical cell of volume 100 ml with a platinum plate electrode and a saturated calomel electrode was used 30. Results and Discussion It is known that the adsorption efficiency over the metal surface is directly linked to the structure and electronic nature of the corrosion inhibitors. Chemisorptions, require charge sharing or charge transfer between organic molecules and the metal surface, which occurs by a favorable overlap of frontier orbitals, that is, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) 31. When chemisorptions takes place, one of the reacting species acts as an electron pair donor and the other one acts as an electron pair acceptor, so the energies of the frontier orbitals should be considered. The energies (ev) of the frontier orbitals for the three inhibitor molecules and aluminium, along with their energy gaps for the interaction (ev) are given in Table 1. From this table, it is found that as the HOMO energy of the inhibitor

3 BANERJEE et al.: ELECTROCHEMICAL & QUANTUM CHEMICAL INVESTIGATION ON CORROSION INHIBITORS 311 Table 1 Various energy parameters of the frontier orbitals of the inhibitors Compounds I II III Al ev E LUMO ev E HOMO E a (ev) E b (ev) Ionization potential (ev) Inhibition efficiency (%) a=e Molecule LUMO E Al HOMO, b= E Al LUMO E Molecule HOMO molecule is increasing, the corrosion inhibitor efficiency is also getting better and better. This is obvious since as the HOMO energy increases, the tendency for acting as better electron donor also improves and hence interacts with the metal LUMO more efficiently 32. This can be attributed from a different perspective also. As we know from Koopman s theorem, the frontier orbital, HOMO can be correlated to ionization potential (IP) as 33 : -E HOMO = IP So, the IP values for the three species are 1.82 ev (III), 2.4 ev (II) and 3.13 ev (I). If these IP values are plotted against the corrosion efficiency, a nice correlation is found 34 which is shown in Fig. 1. In Fig. 2, the correlation between the energy gap ( E b ) and the experimentally found corrosion inhibition efficiency has been shown 35. From the Table 1, the most efficient inhibitor is molecule III and least efficient is molecule I and molecule II resides in between them. If we see the E b (ev) value (energy difference between the LUMO orbital of aluminium metal and HOMO orbital of the corresponding molecular species) for the three molecular species, they follow the same order as the corrosion inhibition efficiency. It is also clear that the interaction between the HOMO of metal atom and LUMO of inhibitor molecules are weaker as the corresponding energy gaps are much higher than that between the HOMO of the inhibitor molecules and the LUMO of aluminium. This observation clearly depicts that when adsorption takes place, the inhibitor molecules act as electron pair donors through their highest occupied molecular orbital and aluminium receives electron density through its lowest unoccupied molecular orbital. The energy gap is lowest for molecule III (1.36 ev) and highest for molecule I (2.67 ev). One noticeable point is that among the three species, molecule I behaves in a little Fig. 1 Ionization potential versus inhibition efficiency Fig. 2 E b versus inhibition efficiency unusual manner (efficiency is very small, 57.6%) which may be due to strong in plane intramolecular hydrogen bonding. From the optimized geometry of mono anion of molecule I, it is found that the acidic hydrogen atom is totally symmetrically hydrogen bonded to both the oxygen atoms of the two carboxylic groups and the mono-anion is perfectly planar in nature, which makes the species least interactive towards the aluminium metal surface. The electron density of the HOMO of the studied compounds is found to be delocalized over the benzene ring as well as the corresponding substituent groups. But most noticeably, for the case of molecule

4 312 INDIAN J. CHEM TECHNOL., JULY 2011 III, which is the most efficient corrosion inhibitor, the electron density is distributed homogeneously over the benzene ring and the substituent NH 2 and COO - groups, which makes the species susceptible for interacting better with Al metal surface. Potentiodynamic polarization study The cathodic and anodic polarization curves of aluminium in 0.6 M aqueous NaCl solution with varying concentrations of I, II and III and without inhibitor were recorded and are shown in the Figs 3-5. The potentiodynamic polarization parameters are listed in Table 2. Inhibition efficiencies 36 are calculated using the following equation Inhibition efficiency= [I corr - I corr (inh) / I corr ] 100 where I corr (inh) and I corr are the corrosion current density value with and without inhibitor, respectively. In NaCl solution with the inhibitors, the cathodic and anodic current densities are both much lower than those in NaCl solution, which indicates that oxygen reduction and alloy substrate dissolution process are limited by the inhibitors and they are mixed type inhibitors and inhibit the corrosion of aluminium by blocking the active sites of the metal surface 37. The inhibition efficiency of these compounds increases in the order I < II < III. Nitrogen and oxygen donor Fig. 4 Polarization curves for aluminium alloy in 0.6 M NaCl solution containing different concentrations of II Fig. 3 Polarization curves for aluminium alloy in 0.6 M NaCl solution containing different concentrations of I Fig. 5 Potentiodynamic polarization of aluminium alloy in 0.6 M NaCl solution containing different concentrations of III Table 2 The electrochemical parameters and inhibition efficiency values for the corrosion of aluminium in 0.6 M NaCl solution Inhibitors Concentration E corr I corr IE % (mv) vs SCE (µa/cm 2 ) I M I M I M II M II M II M III M III M III M

5 BANERJEE et al.: ELECTROCHEMICAL & QUANTUM CHEMICAL INVESTIGATION ON CORROSION INHIBITORS 313 atoms combination is found to be best inhibitor. It may be mentioned here that chromium forms unstable peroxo compounds in the higher oxidation states. But with nitrogen ligands these complexes are found to be remarkably stable 38. Ethylene glycol 39 and 1,2- diaminoethane 13,1,4-naphthaquinone 24 are found to be good inhibitors for aluminium. Inhibition efficiency of these inhibitors may be further improved if nitrogen or oxygen donor atoms are incorporated in these inhibitors. Electrochemical AC impedance studies Impedance plots indicate that addition of inhibitors did not change the general shape of the semicircles 24. It may be considered that the electrode surface and the reaction mechanism are not changed by the addition of inhibitors 40. Conclusions The following conclusions may be drawn from this study: (i) The investigated compounds exhibit inhibiting properties in aluminium alloy corrosion in 0.6 M aqueous sodium chloride solution. (ii) The order of IE is I < II < III. (iii) The inhibiting property and structure is correlated with the electronic parameters and structural of the molecular species. (iv) Inhibitor with N-O donor atoms combination exhibit highest inhibition efficiency. Acknowledgement The authors are thankful to Prof. A K Mukherjee, University of Burdwan, for helping in quantum chemical calculations. References 1 Despic A, Parkhutik V & Bockris, in Modern Aspects of electrochemistry, edited by White R E & Conway B E, (Plenum, New York), Rehim S S A, Hassan H H & Amin M A, Appl Surf Sci, 187 (2002) El-Shafei A A, Abd El-Maksoud S A & Fouda A S, Corros Sci, 46 (2004) Ambat R & Dwarakodasa E S, J Appl Electrochem, 24 (1994) Sun S, Zheng Q, Li D & Wen J, Corros Sci, 51 (2009) Emregul K C & Aksut A A, Corros Sci, 45 (2003) Sherif E M & Park S M, J Electrochem Soc, 152 (2005) B Ogurtsov N A, Pud A A, Kamarchik P & Shapoval G S, Synth Met, 143 (2004) Daulami S, Beligiannis K, Dimogerontakis Th, Ninni V & Tsangaraki-Kaplano Glou, Corros Sci, 46 (2004) Ferreria E S, Giacomelli C, Giacomelli F C & Spinelli A, Mater Chem Phys, 83 (2004) Khaled K F, Corros Sci, 52 (2010) Mercier D & Barthes-Labrouses M G, Corros Sci, 51(2009) Mercier D Heriux M & Barthes-Labrouses M G, Corros Sci, 52 (2010) Ranjana, Maji M & Nandi M M, Indian J Chem Technol, 16 (2009) Ranjana, Banerjee Ranu & Nandi M M, Indian J Chem Technol, 17 (2010) Ranjana & Nandi M M, Indian J Chem, 18 (2011) Bentiss F, Bouanis M, Mernari B, Traisnel M, Vezin H & Lagrenee M, Appl Surf Sci, 253 (2007) Cai X, Zhang Y, Zhang X & Jiang J, J Mol Struct, 801 (2006) Larabi L, Benali O, Mekelleche S M & Harek Y, Appl Surf Sci, 253 (2006). 20 Rodriguez-Valdez L, Martinez-Villafan e A & Glossman- Mitnik D, J Mol Struct, 716 (2005) Rodriguez-Valdez, Martinez-Villafan e A & Glossman- Mitnik D, J Mol Struct, 713 (2005) Roque Morales Jacinto, Pandiyan T, Cruz J & Garcı a-ocho E, Corros Sci, 50 (2007) Rodriguez-Valdez L, Villamisar W, Casales M, Gonzalez- Rodriguez J G, Martinez-Villafan e A, Martinez L & Glossman-Mitnik D, Corros Sci, 48 (2006) Sherif E M & Park S M, Electrochimica Acta, 51 (2006) Frisch M J et al., Gaussian 03, Revision D.01, Gaussian Inc, Wallingford, CT, Becke D, J Chem Phys, 98 (1993) Lee C, Yang W & Parr R G, Phys Rev B, 37 (1988) Gauss View, version 3.0, Gaussian, Inc., Pittsburg PA, Petkova G, Sokolova E & Ivan Brit, Corros J, 31 (1996) Gasparac R, Martin C R, Stupu Lisac E & Mandic Z, J Electrochem Soc,147 (2000) Martinez S, Mater Chem Phys, 77 (2003) Huang W, Tan Y, Chen B, Dong J & Wang X, Tribol Int, 36 (2003) Pearson R G, Inorg Chem, 27 (1988) Stoyanova A, Petkova G, Peyerimhoff S D, Chem Phys, 279 (2002) Ozcan M, Karadag H & Dehri I, Acta Phys-Chim Sin, 24 (2008) Khamis E, El-Ashry E S H & Ibrahim A K, Brit Corros J, 35 (2000) Li Song-mei, Zhang H & Liu J, Trans Nonferrous Met Soc China, 17 (2007) Ghosh S K & Gould ES, Inorg Chem, 28 (1989) Xu L Y & Cheng Y F, Corros Sci, 50 (2008) Li S, Zhang H & Liu J, Trans Non-ferrous Met Soc China, 17 (2007) 318.