Effect of Allyl Triphenyl Phosphonium Bromide on Electrochemical and Corrosion Behaviour of Mild Steel in 0.5M Sulphuric Acid

By Bhrara, K Singh, G

Phosphonium compounds are known to act as corrosion inhibitors of mild steel in acidic solutions. They may be added to hydraulic oils or drilling fluids to provide scale and corrosion inhibition. They can be applied on the substrate by immersion or be incorporated in a polymeric coating. In the present work, allyl triphenyl phosphonium bromide (ATTPB) is evaluated as the potential corrosion inhibitor for mild steel in sulphuric acid solutions. Galvanostatic polarisation and potentiostatic polarisation measurements were used to study the inhibition of mild steel corrosion in aerated 0.5M sulphuric acid at different temperatures by ATPPB. Corrosion potential, corrosion currents, cathodic and anodic Tafel slopes, heat of adsorption and effective activation energies have been calculated in the presence and absence of the ATPPB. Inhibition efficiency has been calculated for various concentrations at different temperatures to study the range of effectiveness of the ATPPB. It shows the inhibitor efficiencies in the range of 73 to 99% and performs best at 318 and 328 K. The nature of adsorption of ATPPB on the metal surface has also been examined using infrared spectroscopic studies. The results of surface morphology and quantum chemical analysis are in agreement with the electrochemical results. Keyword: Allyl triphenyl phosphonium bromide, Mild steel, Galvanostatic polarisation, Potentiostatic polarisation, Infra red spectroscopy, Scanning electron microscopy, Quantum chemical analysis

Introduction

Mild steel is widely used in variety of industries especially for structural applications. It may come in contact with various acid solutions and corrodes heavily during chemical processes such as acid cleaning, transportation of acids, descaling operations and storage of acids, etc. Since the dissolution rate of mild steel in acids is quite high, it requires very efficient inhibitors to protect it from corrosion.

A broad range of corrosion inhibitors are used for mild steel in acidic solutions.1-23 They are various ions, organic compounds containing N, S or O atoms. On the basis of Mandeleev’s periodic law, it should be expected that organophosphorous compounds, analogous in structure to nitrogen compounds, must also be able to retard the dissolution of steel in acidic solutions. Although quaternary ammonium salts24″35 have been extensively studied as inhibitor against the acid corrosion of iron and steel but relatively little attention has been paid to quaternary phosphonium salts. Some phosphonium compounds have been studied as inhibitors for corrosion of mild steel in acidic solutions.36-43 They act as corrosion inhibitors by adsorbing on to metal surfaces to form a protective film. This film acts as a barrier, denying access of oxygen to the metal surface, thus stifling the corrosion process.

In the present work, the effect of allyl triphenyl phosphonium bromide (ATPPB) on the electrochemical and corrosion behaviour of mild steel in 0- 5? sulphuric acid has been studied using galvanostatic polarisation and potentiostatic polarisation. The results obtained were supplemented by infrared spectroscopic studies, scanning electron microscopy and further explained by quantum chemical analysis.

Experimental

Mild steel (C=O- 15%, Si=0-08%, S=0-025%, ?=0- 025% and ??=1- 02%) encapsulated in a Teflon holder with the exposed area of 0- 64 cm2 was used as the working electrode. The surface was abraded successively by emery papers of different grades, i.e. 150, 320, 400 and 600 and finely polished with a 4/0 polishing paper to obtain mirror like finish. Platinum wire and dip type saturated calomel electrodes were used as counter and reference electrodes respectively. Sulphuric acid solution (0-5M) containing 10″f, ??-3, 10″5 and 10″7M ATPPB were used for corrosion studies. The cathodic and anodic polarisation studies were conducted at 298, 308, 318, 328 and 338 K. The temperature was thermostatically controlled (+- 1[degrees]C) and electrodes, etc. were introduced into the cell through standard joint adapters. The systems were allowed to stand for 3 h to attain constant open circuit potential (OCP).

1 Galvanostatic polarisation curve of mild steel in 0-5M sulphuric acid in presence of 1O-1M ATPPB at 298 K

ATPPB was synthesised in the laboratory using standard procedure by taking triphenyl phosphine and allyl bromide. The galvanostatic polarisation studies were performed by putting the system to a preset value of current after which the potential was noted when it became constant. Each system was scanned from 0 to 40 mA. Typical polarisation curves for the aerated solutions of 0-5M sulphuric acid with various concentrations of ATPPB at different temperatures were plotted. The corrosion current for different sets of solution were found from the extrapolation of the polarisation curves back to the OCP.

Potentiostatic polarisation studies were carried out using a potentiostat. The electrode system used for these studies was the same as that used for galvanostatic polarisation studies. All measurements were performed at 298 K. The systems studied were 0-5M H2SO4 in the absence and presence of various concentrations of ATPPB. Each system was allowed to attain OCP and then potential was given and the corresponding current was noted after 90 s. The potential range scanned was from -500 to 2000 mV.

A Perkin Elmer infrared spectroscope IR 137 was used for infrared analysis. The pure ATPPB was dried in dessicator for 24 h and subjected to infrared (IR) analysis in KBr medium.

A small amount of dry silica gel was added to a saturated solution of ATPPB in dry chloroform. The sample was stirred vigorously to evaporate the solvent, dried in an oven for 48 h and subjected to IR analysis in KBr medium.

A scanning electron microscope (SEM, JEOL 840 JSM) was used for characterising plain mild steel and corroded surfaces. Mild steel (0- 8 x 0-8 cm) samples were polished to mirror like finish and degreased in an ultrasonic cleaner. Three samples were dipped in corroding media, i.e. 0-5M H2SO4, 1O-1 and 1O-7M ATPPB in 0-5M H2SO4 for 24 h. All the samples were dried in dessicator before characterisation.

The geometry of the inhibitor molecule in vacuum was optimised by parameterised model number 3 (PM3) method using MOPAC quantum chemical package. PM3 is a semiempirical method44,45 for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the neglect of differential diatomic overlap (NDDO) integral approximation. The geometry was further optimised with the help of HyperChem and various molecular parameters were plotted.

2 Langmulr’s isotherms at different temperatures

3 Activation energy versus Inhibitor concentration

Effect of temperature

The logarithm of corrosion current can be represented as in the equation (5)

logi^sub c^ = logA -(E^sub act^/RT) (5)

The values of E^sub act^ have been calculated from the plots of log i^sub corr^ versus 1/T and are given in Table 3. Figure 3 shows that the activation energy has non-linear relationship with the concentration of additive and exhibits maxima at 1O-4M ATPPB solution.

Potentiostatic studies

A detailed study on steady state potentiostatic behaviour of the anodic dissolution of mild steel in sulphuric acid in the absence and presence of various concentrations of ATPPB has been carried out in the present work. The effect of this additive has been studied in terms of the electrochemical parameters, e.g. current maximum or critical current density (i^sup crit^), fade potential (2?pp) and passive current (;i^sub p^). From the potentiostatic polarisation curves, the anodic dissolution parameters (i^sub crit^. E^sub pp, i^sub p^) of mild steel in 0.5M sulphuric acid solution at various concentrations of the additives have been calculated and reported in Table 4.

Table 1 Corrosion parameters of mild steel in 0-5M H2SO4 in presence of ATPPB

Infra red studies

Infrared spectra of pure ATPPB in KBr medium was recorded and is given in Fig. Aa.

Spectra of ATPPB shows characteristic absorption peaks, namely

(i) P-Ar stretching peaks at about 1450-1500 cm-1 1150-1175 cm-1 and 1000 cm-1

Table 2 Adsorption parameters at different temperatures studied for ATPPB

Table 3 Values of E^sub sect^, for corrosion of mild steel in 0- 5M H2SO4 in presence of ATPPB

4 Infra red spectra of a pure ATPPB in KBr medium and b ATPPB adsorbed on silica gel in KBr medium

(ii) (V) P-CH^sub 2^ stretching peak at about 1400-1450 cm^sup – 1^

(iii) C-C aromatic multiple bond stretching peak about 1450-1600 cm^sup -1^

(iv) C-H aromatic substitution type bending peak about 750 cm^sup -1^

(v) C-H aromatic stretching peak at about 3030 cm^sup -1^

Table 4 Electrochemical parameters for anodic dissolution of mild steel in 0-5M H2SO4 in presence of various concentrations of ATPPB

(vi) C-H alkane stretching peak at about 2853-2962 cm^sup -1^.

The IR spectra of adsorbed sample of ATPPB in KBr medium, shown in Fig. Ab was also analysed and various peaks observed are reported in Table 5. The spectra of ATPPB show all the characteristic peaks and additional peaks corresponding to C=C stretch at 1613 cm^sup – 1^ and C-H stretch (alkene) at 3005 cm^sup -1^. It also shows peaks of (C-H)^sub Ar^ stretch at about 3050 cm^sup -1^ and (C-H)^sub Alkane^ stretch at about 2853-2962 cm^sup -1^. Most of them disappear on an adsorbed sample. 5 Scanning electron micrographs

Surface morphological studies

Figure 5a shows the surface of the metal and the metal exposed to 0.5M sulphuric acid. Uniform corrosion can be observed in Fig. 5b which shows corrosion products like metal hydroxides and oxides.

Figure 5c-f represents the micrographs of mild steel surface exposed to 0.5M sulphuric acid in the presence of 10^sup -1^ and 10^sup -7^M ATPPB at various magnifications. Figure 5c shows almost 100% coverage of the metal surface by inhibitor molecules at 10^sup – 1^M concentration of ATPPB. Figure 5d and e clearly shows the formation of pits and pit density. This causes the reduction in the capacity to protect the metal surface from corrosion. Figure 5e and f shows some cracks that further corrode the mild steel surface.

Table 6 Optimised PM3 parameters for ATPPB using MOPAC 6.0

Quantum chemical studies

Through these calculations an attempt has been made to correlate corrosion inhibition efficiency (dependent variable) and the set of some independent variables such as HOMO, LUMO, number of overall electrons, dipole moment, etc. The geometry was optimised using the PM3 method of the quantum chemical package MOPAC 6.0. The optimised geometry of this is given in Fig. 5a, and various optimised PM3 parameters for this additive are given in Table 6. The energies of HOMO and LUMO of iron were taken from the literature36 equal to – 752.26 and -24.08 kJ mol^sup -1^ respectively.

Discussion

Galvanostatic polarisation studies at various temperatures reveal that ATPPB influences the various corrosion parameters (Table 1). The effect of inhibitor concentration at various temperatures on the anodic and cathodic polarisation curves is quite apparent from the values of Tafel slopes. The corrosion current density decreases with the increase in concentration of the inhibitor. For a particular concentration, corrosion current density decreases and then increases with increasing temperature. For the concentrations studied, the minimum in i^sub corr^ (maximum in 1%) is observed either at 318 or 328 K. It can be clearly seen that ATPPB is a very efficient inhibitor at most of the temperatures studied especially at higher concentrations. It performs best at 318 and 328 K where inhibitor efficiencies are in the range of 88.8-98.8%. Its performance decreases considerably at 338 K with decreasing concentration.

The fact that inhibition efficiencies are higher at higher temperatures than those at 298 K also indicates that the adsorption of ATPPB is not merely a physical or a chemical adsorption but a comprehensive adsorption.46

Table 1 also shows that OCP remains more or less constant with a slight shift towards anodic direction for temperatures 298, 308, 318 K. At 328 and 338 K, the OCP shows the shift towards cathodic direction. Therefore, ATPPB is a mixed type of inhibitor. It acts in both ways affecting cathodic and anodic partial processes by blocking the active sites of the metal surface during hydrogen evolution and oxygen reduction as well as metal dissolution reactions. The pi-electron system of this inhibitor molecule possibly overlaps with the vacant d-orbitals of the metal surface resulting in a strong dpi-ppi interaction36 which is further assisted by the synergistic effect of Br^sup -^ ions.37 This electrostatic interaction probably leads to a stronger adsorption of the inhibitor and formation of a barrier between the metal surface and reactive sites. This barrier/adsorption becomes weaker at very high temperatures thereby leading to lower adsorption resulting in increased corrosion rates. The other factor which inhibits corrosion process during the anodic polarisation is the formation of an adduci of the type (M-In)^sub ads^ or (M-In-OH)^sub ads^ or (M-OH-In)^sub ads^. The interplay of these various species getting adsorbed on the active sites during anodic polarisation changes the mechanism of inhibition that results in different anodic Tafel slope values. The anodic Tafel slope values therefore are not constant for this additive.

Table 2 shows that all the linear correlation coefficients R^sup 2^ are almost equal to 1 and all the slopes are very close to 1, which indicates the adsorption of inhibitor onto steel surface accords with the Langmuir adsorption isotherm. The result also indicates that there were no interactions among the adsorbed species.47,48

The negative values of DeltaG^sup 0^^sub ads^ along with high K indicate a spontaneous adsorption process and a good chemical stability of inhibitor, which may be derived from the chemical bond between metal and inhibitor molecules. Magnitudes of K and DeltaG^sup 0^^sub ads^ values confirm that ATPPB is adsorbed on the metal surface at all the temperatures but is most efficient at 318 and 328 K.

The value of equilibrium constant of adsorption increases with temperature and is maximum at 318 K and then it decreases. From the temperature variation of equilibrium constant, DeltaH^sup 0^^sub ads^ and DeltaS^sup 0^^sub ads^ can be calculated from the slope and intercept respectively. At high temperatures the values of DeltaH^sup 0^^sub ads^ and DeltaS^sup 0^^sub ads^ are -332.0 kJ mol^sup -1^ and -861.2 J mol^sup -1^ K^sup -1^ whereas at low temperatures corresponding values are 165.7 kJ mol^sup -1^ are 637.9 J mol^sup -1^ K^sup -1^ respectively. It clearly shows that the adsorption of ATPPB is favoured by entropy effects at lower temperatures whereas at higher temperatures heat effects predominate. Therefore, there is an increase in inhibitor efficiencies at higher temperatures. This is clear indicative of the fact that the ATPPB undergoes change in orientation of phenyl rings as the temperature is increased. At 318 K, they may be in the same plane, therefore enhancing the adsorption on the metal surface. The value of DeltaH^sup 0^^sub ads^ shows that it is an endothermic process at lower temperature range but changes to exothermic at higher temperatures. Since the heat of adsorption is very high, almost of the order of bond energies, it might get utilised for reorientations of the type known as fluxional rearrangements49 resulting in positive values of DeltaH^sup 0^^sub ads^. These reorientations are not possible beyond a certain temperature because of the shorter time lag between adsorption-desorption processes.

It can be concluded from Table 3 that i^sub p^ is lowered only for 10^sup -5^ and 10^sup -7^M ATPPB. Therefore ATPPB is a good passivator at lower concentrations and does not passivate at higher concentrations. This may be due to the non-planar shape of ATPPB. The shape is such that at higher concentrations it is not able to form a resistive layer because of steric hindrance whereas at lower concentrations the resistive layer may be formed with the help of ions such as Br^sup -^, H^sub 3^O^sup +^, HSO^sub 4^^sup -^, SO^sub 4^^sup 2-^ and OH^sup -^ present in the solution.

ATPPB has delocalised set of electrons and this helps in the formation of complexes of the type (M-In-OH)^sub ads^ or (M-In)^sub ads^ or (M-In-A)^sub ads^ where A is the anion present in the solution or is the anion of the additive that causes a synergistic effect. The complexes along with synergistic adsorption of Br^sup – ^ ions influence anodic dissolution parameters of metals in the passive range. ATPPB, due to strong delocalisation of electrons and enhanced overall electron density on the molecule, is adsorbed strongly and uniformly at lower concentrations only. This will hinder the permeation of hydrogen into resistive layer thereby making the metal more passive.

On comparing the spectra with the one adsorbed over silica gel it can be seen that most of the peaks either disappeared or got reduced in intensities. Based on the comparison it can be said that all the bonds, namely C-C (aromatic), C=C, C-H, P-Ar, P-CH2 are involved in the adsorption process.

On comparing the various micrographs (Fig. 5) it can be concluded that ATPPB protects the mild steel surface very well at higher concentration at 298 K against corrosion in 0.5M sulphuric acid. The calculated efficiencies are 97.6% at 10^sup -1^M and 36.9% at 10^sup -7^M ATPPB.

The negative binding energy (Table 6) indicates that allyl triphenyl phosphonium ion is very stable and is less prone to be split or broken apart. The difference in energies of HOMO and LUMO values indicates that this is a good inhibitor.50,51 A dipole moment value indicates that this is a polar molecule and therefore, there may be the possibility of interaction of pi-electrons of ATPPB with the metal surface thereby retarding the corrosion rate. To determine the type of interaction between iron and the inhibitor by molecular orbital approach, the energies of frontier orbitals are considered. Since E^sub HOMO^ (Fe)-E^sub LUMO^ (In) (-403.58 kJ mol^sup -1^) is much less than E^sub HOMO^ (In)-E^sub LUMO^ (Fe) (-1237.71 kJ mol^sup -1^), there is a strong possibility of electrons from Fe to be given to ATPPB further strengthening the adsorption.

6 a optimised geometry of ATPPB by PM3 method, b and c ball and stick and space fill models of geometry optimised by Hyperchem, d 3D isosurface of total charge density on ATPPB and e electrostatic potential mapped onto 3D charge density isosurface of ATPPB

Figure 6a-c clearly shows that ATPPB is non-planar molecule with no centre of symmetry. The surface coverage therefore cannot be as uniform as is observed for planar molecules. The shape also suggests that for forming a passive/resistive layer this molecule can be better oriented at lower concentration only. The higher inhibition therefore will not only be due to this additive alone but will be assisted by already adsorbed anions such as SO^sub 4^^sup 2-^, Cl^sup -^, HSO^sub 4^^sup -^, etc. through the synergistic effect. At higher concentrations, there will be considerable steric hindrance and gaps because of which passivity will tend to be destroyed. It can be seen from Fig. 6d and e that the negative charge density is very well spread out all over the additive and is expected to be donated to the metallic surface through the phenyl rings. Due to the non-planar shape of this additive, it does not act as a perfect donor of electrons, and this may cause the cracks in protective covering layers. The shape of this molecule clearly indicates that none of the phenyl rings are in one plane, and therefore instead of reinforcing the electron charge density donation they reduce the extent of these donations. This is also the reason that this is a good passivator at lower concentrations but not at higher concentrations. Conclusions

From the overall data of adsorption of ATPPB on mild steel surface in acid solution studied with the help of galvanostatic, potentiostatic polarisation and confirmed by surface characterisation, scanning electron microscopy, infrared studies and quantum chemical studies, the following conclusions can be drawn.

1. The ATPPB retards corrosion at ordinary temperatures and shows better inhibition efficiency at higher temperatures especially at 318 and 328 K. Therefore, adsorption of ATPPB is not merely a physical or a chemical adsorption but a comprehensive adsorption.

2. The ATPPB shows no appreciable shift of E^sub corr^ towards any direction. This shows that this is a mixed type of inhibitors. E^sub corr^ values in the presence of ATPPB shows a slight predominance in anodic direction.

3. Irregular trends in Tafel slope values indicates that adsorption of the inhibiting species is assisted by other ions present in the solution.

4. The Langmuir adsorption isotherm was found to be the closest to the description of the adsorption behaviour of the studied inhibitor.

5. The adsorption of ATPPB shows a unique and complex phenomenon of the type similar to fluxional rearrangement.

6. The negative values of DeltaG^sup 0^^sub ads^ along with high K indicate a spontaneous adsorption process at all the temperatures but is most efficient at 318 and 328 K.

7. Since the maximum of E^sub act^ is at 10^sup -4^M ATPPB, therefore ATPPB is most effective at this concentration.

8. The ATPPB passivates the metal only at lower concentrations.

9. Molecule as a whole is involved in the adsorption process through pi-electron density which is well spread over the molecule.

10. Quantum chemical (MO) calculations show that the inhibitor molecules also act as electron acceptor when they interact with the mild steel surface confirming that adsorption of ATPPB on the mild steel surface is a comprehensive one.

Acknowledgements

The authors would like to thank University Grants Commission (UGC) for funding the research project. This work was presented at 4th International Surface Engineering Congress & Exposition and 19th Surface Modification Technologies (ISEC SMT-2005), St Paul, Minnesota, USA.

(c) 2007 Instituts of Materials. Minerals and Mining

Published by Mansy on behalf of the Institute

Received 7 April 2006; accepted 27 March 2007

References

1. F. Bentiss, M. Traisnel and M. Lagrenee: Brit. Corros. J., 2000, 35, (4), 315-320.

2. E. Lazarova, S. Kalcheva, G. Neykov, T. Yankove and N. Stoyanov: J. Appl. Electrochem., 2000, 30, (5), 561-570.

3. M. R. Arshadi, M. G. Hosseini and M. Ghorbani: Brit. Corros. J., 2002, 37, (1), 76-80.

4. A. M. Ai-Mayouf, A. K. Al-Ameery and A. A. Al-Suhybani: Brit. Corros. J., 2001, 36, (2), 127-132.

5. M.A. Quraishi and J. Rawat: Mater. Chem. Phys., 2001, 70, (1), 95-99.

6. B. I. Ita and O. E. OlTiong: Mater. Chem. Phys., 2001, 70, (3), 330-335.

7. S. Bilgic: Mater. Chem. Phys., 2002, 76, (1), 52-58.

8. F. Bentiss, M. Traisnel, N. Chaibi, B. Mernari, H. Vezin and M. Lagrenee: Corros. Sci., 2002, 44, (10), 2271-2289.

9. E. Lazarova, G. Petkova, R. Raicheff and G. Neykov: J. Appl. Electrochem., 2002, 32, (12), 1355-1361.

10. A. A. Atia and M. M. Saleh: J. Appl. Electrochem., 2003, 33, (2), 171-177.

11. M. A. Quraishi and H. K. Sharma: Mater. Chem Phys., 2003, 78, (1), 18-21.

12. M. A. Quraishi and J. Rawat: Mater. Chem Phys., 2003, 77, (1), 43-47.

13. M. Hosseini, S. F. L. Mertens and M. R. Arshadi: Corros. Sci., 2003, 45, (7), 1473-1489.

14. M. A. Quraishi and R. Sardar: /. Appi Electrochem, 2003, 33, (12), 1163-1168.

15. F. Bentiss, M. Lebrini, H. Vezin and M. Lagrenee: Mater. Chem. Phys., 2004, 87, (1), 18-23.

16. K. F. Khaled and N. Hackerman: Mater. Chem. Phys., 2003, 82, (3), 949-960.

17. E. E. Oguzie: Mater. Chem. Phys., 2004, 87, (1), 212-217.

18. F. Bentiss, M. Traisnel, H. Vezin, H. F. Hildebrand and M. Lagrenee: Corros. Sci, 2004, 46, (11), 2781-2792.

19. G. Moretti, F. Guidi and G. Grion: Corros. Sci, 2004, 46, (2), 387-403.

20. A. Chetouani, K. Medjahed, K. E. Sid-Lakhdar, B. Hammouti, M. Benkaddour and A. Mansri: Corros. Sci, 2004, 46, (10), 2421-2430.

21. A. S. Algaber, E. M. El-Nemma and M. M. Saleh: Mater. Chem. Phys., 2004, 86, (1), 26-32.

22. E. E. Oguzie, G. N. Onuoha and A. I. Onuchukwu: Mater. Chem. Phys., 2005, 89, (2-3), 305-311.

23. H. Ashassi-Sorkhabi and N. Ghalebsaz-Jeddi: Mater. Chem. Phys., 2005, 92, (2-3), 480-486.

24. I. Ammar, S. Darwish and M. Etman: Electrochim. Acta, 1967, 12, (5), 485-494.

25. D. P. Schweinsberg and V. Ashworth: Corros. Sci., 1988, 28, (6), 539-545.

26. V. Branzoi, F. Branzoi and M. Baibarac: Mater. Chem. Phys., 20O0, 65, (3), 288-297.

27. T. Du, J. Chen and D. Cao: Brit. Corros. J., 2000, 35, (3), 229-231.

28. M. El-Achouri, M. R. Infante, F. Izquierdo, S. Kertit, H. M. Gouttaya and B. Nciri: Corros. Sci, 2001, 43, (1), 19-35.

29. M. M. Osman, R. A. El-Ghazawy and A. M. Al-Sabagh: Mater. Chem. Phys., 2003, 80, (1), 55-62.

30. T. Y. Soror and M. A. El-Ziady: Mater. Chem. Phys., 2003, 77, (3), 697-703.

31. S. A. Ali, M. T. Saeed and S. U. Rahman: Corros. Sci, 2003, 45, (2), 253-266.

32. E. A. Noor: Corros. Sci, 2005, 47, (1), 33-55.

33. Z. Ait Chikh, D. Chebabe, A. Dermaj, N. Hajjaji, A. Srhiri, M. F. Montemor, M. G S. Ferreira and A. C. Bastos: Corros. Sci, 2005, 47, (2), 447-459.

34. L.-G. Qiu, A.-J. Xie and Y.-H. Shen: Mater. Chem. Phys., 2005, 91, (2-3), 269-273.

35. M. A. Migahed: Mater. Chem. Phys., 2005, 93, (1), 48-53.

36. P. Mutombo and N. Hackerman: Anti-Corros. Meth. Mater., 1998, 45, (6), 413-418.

37. E. Khamis, E. S. H. El-Ashry and A. K. Ibrahim: Brit. Corros. J., 2000, 35, (2), 150-154.

38. M. S. Morad: Corros. Sci., 2000, 42, (8), 1307-1326.

39. M. S. Abdel-Aal and M. S. Morad: Brit. Corros. J., 2001, 36, (4), 253-260.

40. M. H. Wahdan, A. A. Hermas and M. S. Morad: Mater. Chem. Phys., 2002, 76, (2), 111-118.

41. A. A. Hermas, M. S. Morad and M. H. Wahdan: J. Appl. Electrochem., 2004, 34, (1), 95-102.

42. K. Bhrara and G. Singh: Surf. Eng, 2005, 21, (3), 167-171.

43. K. Bhrara and G. Singh: Appi Surf. Sci., 2006, 253, (2), 846- 853.

44. J. J. P. Stewart: J. Comput. Chem., 1989, 10, (2), 209-220.

45. J. J. P. Stewart: J. Comput. Chem., 1989, 10, (2), 221-264.

46. B. A. Abd-El-Nabey, E. Khamis, M. S. Ramadan and A. El- Gindy: Corrosion, 1996, 52, (9), 671.

47. A. Y. El-Etre: Corros. Sci., 2001, 43, (6), 1031-1039.

48. F. Bentiss, M. Lagrenee, M. Traisnel and J. C. Hornez: Corrosion, 1999, 55, (10), 968.

49. ‘The Columbia encyclopedia’, ‘Fluxional molecules’, 6th edn, Columbia University Press.

50. V. S. Sastri and J. R. Perumareddi: Corrosion, 1994, 50, (6), 432.

51. V. S. Sastri and J. R. Perumareddi: Corrosion, 1997, 53, (8), 617.

K. Bhrara and G. Singh*

Department of Chemistry, University of Delhi, Delhi 110007, India

* Corresponding author, email gurmeet123@yahoo.com

Copyright Institute of Materials Jun 2007

(c) 2007 British Corrosion Journal. Provided by ProQuest Information and Learning. All rights Reserved.