Volume 17 Preprint 9


Inhibition performance of Tobramycin on the dissolution of stainless steel 304 and ingress of hydrogen gas in acid medium

S.Karthikeyan, P.A.Jeeva, S.Narayanan

Keywords: Corrosion, potential, hydrogen permeation, impedance, inhibition

Abstract:
The inhibition performance of Tobramycin (TBN) on stainless steel 304 dissolution and ingress of hydrogen gas in 2N H3PO4 and 2N HCl has been analyzed employing mass loss, gasometric and electrochemical studies. The antibiotic seems to be more effective in reducing the dissolution of steel in 2N H3PO4 than in 2N HCl. Potential-Current plots evidently pointed out that the inhibitor follows mixed mode of inhibition in acidic media. Hydrogen permeation and EIS measurements have confirmed that TBN retards the corrosion of SS 304 effectively in both the acids . The theoretical values of EHOMO, ELUMO, ΔE and dipole moment in the presence of inhibitor confirmed its effective adsorption on SS 304 surface.

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ISSN 1466-8858 Volume 17, Preprint 9 submitted 10 February 2014 Inhibition performance of Tobramycin on the dissolution of stainless steel 304 and ingress of hydrogen gas in acid medium S.Karthikeyan1*, P.A.Jeeva2, S.Narayanan2 1 Surface Engineering Research lab, CNBT , VIT University, Vellore -632014,India 2School of Mechanical and Building Sciences, VIT University, Vellore -632014 , India Abstract The inhibition performance of Tobramycin (TBN) on stainless steel 304 dissolution and ingress of hydrogen gas loss, gasometric in 2N H3PO4 and 2N HCl has been analyzed employing mass and electrochemical studies. The antibiotic effective in reducing the dissolution of steel Current plots evidently pointed out seems to be more in 2N H3PO4 than in 2N HCl. Potential- that the inhibitor follows mixed mode of inhibition in acidic media. Hydrogen permeation and EIS measurements have confirmed that TBN retards the corrosion of SS 304 effectively in both the acids . The theoretical values of EHOMO, ELUMO, ∆E and dipole moment in the presence of inhibitor confirmed its effective adsorption on SS 304 surface. Keywords : Corrosion, potential, hydrogen permeation, impedance, inhibition 1* corresponding author (skarthikeyanphd@yahoo.co.in) © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 1 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 1.Introduction In recent years , the medicines such as antibiotics and drugs as are preferentially used corrosion inhibitors due to their ecofriendliness [1-4]. Hetero cyclic compounds with sulphur, nitrogen and oxygen atoms in their exo cyclic rings have widely been reported as inhibitors for metals in acidic media [5-8]. literature studies clearly reveal that The careful analysis of no systematic approach is existing on the inhibitive action of Tobramycin (TBN) in high aggeresive acid solutions.. The corrosion inhibiting property of these compounds is attributed to their molecular structure. The lone pair of electrons on nitrogen amino groups of CMZ and delocalization of electrons of pyran-diol moiety of the present drug ease compound on surface of SS 304 the adsorption of the . All the above investigations depict a general information that no significant data is seen on the performance of tobramycin as effective hydrogen gas corrosion inhibitor and in bringing down the ingress of through steel during pickling . It falls under the class of amino glycoside antibiotic resulting from Streptomyces tenebrarius and used to heal a variety of bacterial infections, mostly Gram-negative diseases. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 2 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 2.Experimental Stainless steel 304 specimens of the following composition was widely used. C= 0.08% , Si = O%, Ni = 8%, Cr = 18% and Fe= balance with exposed area of 4 x 1 x 0.020 cm were employed for mass loss and hydrogen permeation measurements. A stainless steel cylindrical rod of the same composition as above and embedded in araldite resin with an exposed area of 0.3 cm2 was used for potential-current plots and EIS measurements. The compound was mainly monitored by a mass loss studies as reported by Madhavan et al [9]. Both cathodic and anodic potential- current curves were recorded potentiodynamically (1 mV s-1) using corrosion measurement system BAS Model: 1OOA computerised electrochemical analyser (made in West Lafayette, Indiana) and PL-10 digital plotter (DMP-40 series, Houston Instruments Division). A platinum foil of 4 cm2, Hg/Hg2Cl2/KCl (satd) was used as auxiliary and reference electrodes, respectively. The hydrogen permeation study was performed using standard procedure of Devanathan and Stachurski’s two compartment cell, as described earlier.[9] Double layer © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 3 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 capacitance (Cdl) and charge transfer resistance values (R,) were obtained using EIS measurements . 3. RESULTS AND DISCUSSION 3.1 Mass loss and Gasometric measurements Table 1 shows the values of inhibition efficiency for various tobramycin for the corrosion of SS 304 in 2N HCl and 2N H3PO4 loss and gasometric measurements. It is noticed concentrations of obtained from mass that the inhibitor retards the dissolution of stainless steel in both acids, but more effectively in H3PO4 . This can be ascribed to the slighter adsorption of sulphate ions on the steel surface, thereby allowing more space for the tobramycin compound to get adsorbed. But in the case of HCl , the stronger adsorption of the chloride ions on the metal surface favour space for adsorption of inhibitor. So inhibitor less in H3PO4 ,the coverage of the SS 304 by the is considerably greater , giving rise to higher values of inhibition performance for all concentrations of the antibiotic used. The structure of the compound is given in Figure 1. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 4 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 NH submitted 10 February 2014 2 HO O H O O HO NH 2 O H O H 2N NH 2 O NH 2 OH Figure 1.Structure of Tobramycin © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 5 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 The retardation on the dissolution of SS 304 in acid medium favoured by Tobramycin were involving the following interactions: 1.The interaction between the lone pairs of electrons of the nitrogen atoms of the amino groups and the positively charged metal surface [10]. 2.The interactions between delocalized electrons of the pyran-diol moieties and the positively charged metal surface [11]. It is found that there is a very good agreement between the values of inhibition efficiency obtained by mass loss and gasometric studies. Potential al3.2 Potenti al -Current plots Table 2(a) and 2(b) gave the results of potential-current plots such as Tafel slopes ( ba and bc ),corrosion current (I corr ) and corrosion potential (E corr ) and inhibition efficiency obtained from potentiodynamic polarization studies for SS 304 in 2N HCl and 2N H3PO4 containing various concentrations of antibiotic molecule. It can be visualized from this table that results of Tafel slopes and I corr are very much similar to those reported earlier [12,13.] Further it is established that increasing concentrations of tobramycin increases the values of both ba and bc in random way justifying that © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 6 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 the inhibition of corrosion of SS 304 Values of Ecorr is moved submitted 10 February 2014 in both the acids falls under mixed control. to positive direction concentrations of inhibitor. This can be attributed in the presence of different to the formation of strongly adsorbed inhibitor layer on the metal surface. The presence of increasing dosage of inhibitor molecule significantly retards I corr values in both the acids. It can also be noticed that most of the values of inhibition efficiency obtained by mass loss and potential-curve studies agree very well. measurements 3.3 Hydrogen permeation m easurements The results of hydrogen permeation measurements for the dissolution of stainless steel 304 in the presence and absence of the tobramycin are given in Table 3.Hydrogen permeation currents for mild steel in 2N HCl is more, because of the highly reactive nature of chloride ions. It can be obtained from the table that the existence of inhibitor in both acids bring down the permeation current and does not favour the ingress of hydrogen gas into SS 304 . The declining trend in permeation currents can be accredited to the effective formation of protective layer formed on the surface © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 7 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 of metal surface [14,15] . It can be seen from the table that the decrement of permeation current is more, if the concentration of Clotrimazole is more. Impedance 3.4 Im pedance studies Values of charge transfer resistance (Rt )and double layer capacitance (Cdl) obtained from EIS measurements are given in table 4.It can be found in table that the values of Rt is seen to increase with enhancement of TBN concentrations in both the acids. Values of double layer capacitance are confirming that steel dissolution is more in 2N HCl than 2N H3PO4 .It is noticed concentrations of TBN that values of Cdl are lowered by increasing in acidic media. This can be attributed to the adsorption of the antibiotic molecule on the surface effective of SS 304 with increase in its dosage to the electrolyte. A plot of surface coverage (ø ) versus log C gave a straight line demonstrating that the adsorption of TBN on SS 304 surface from both acids obeys Temkin’s adsorption isotherm [16]. This is chief support to corrosion inhibition by this molecule , as a result of its adsorption on the surface of SS 304. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 8 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 3.5 Quantum chemical studies: Fig. 2. Highly Occupied MO’s of TBN Fig.3.Lowest unoccupied MO’s of TBN The computed quantum chemical indices such as energy of highest occupied molecular orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO), LUMOHOMO, energy gap (∆E), dipole moment (•), are summarized in Table 2. From figure 2 and 3 , it can be observed that HOMO and LUMO energy orbital’s were strongly distributed on amino groups and and pyran-diol for HOMO and LUMO structures establishing that the tobramycin posses good adsorption centers [17–19] and this © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 9 to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 is in agreement with publications of molecular orbital studies confronting that π electrons and N atoms are liable for inhibition activity20. According to Hari Kumar et al [21], when a molecule has similar distribution of electronic orbital’s, its inhibition performance could be associated with the energy values of HOMO and LUMO and the difference in values between them. It has been extensively reported that, higher the value of EHOMO, larger is the easiness for an inhibitor to donate electrons to vacant d orbital of Fe atom and higher is its adsorption. Also, lower ELUMO values , favour acquiring capacity of electrons by the inhibitor from Fe atom to form feedback bonds. Hence the gap between HOMO–LUMO energy levels of molecules was measured as an vital data . Smaller the value of ∆E of an inhibitor, greater is the inhibition efficiency of that compound . It is further claimed that, large values of dipole moment will considerably raise the adsorption of the compound on metal surface [22-24]. 4. Conclusions 1. Tobramycin retards the dissolution of the corrosion of SS 304 in both acids, but shows a better performance in 1N H3PO4. 2. The inhibition of corrosion of stainless steel by the compound in both the acids H3PO4 falls under mixed control. 3. The presence of inhibitor molecule in both the acids is found to reduce the extent of hydrogen permeation current through SS 304 surface. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 10to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 4. Volume 17, Preprint 9 submitted 10 February 2014 Rt and Cdl values studied from impedance measurements prove the impressive performance of the inhibitor. 5. The adsorption of the compound on SS 304 surface follows Temkin’s adsorption isotherm. References: 1. Rhodanine azosulpha drugs as corrosion inhibitors for corrosion of 304 stainless steel in hydrochloric acid solution, M. Abdallah, Corros. Sci, 44, pp728, 2002 2. Antifungal drugs as corrosion inhibitors for aluminium in 0.1 M HCl, I.B. Obot, N.O. Obi-Egbedi, S.A. Umoren, Int. J. Electrochem. Sci, Vol. 4, 2009 3. Torsemide and Furosemide as Green Inhibitors for the Corrosion of Mild Steel in Hydrochloric Acid Medium, S. Harikumar and S. Karthikeyan, Industrial and Engineering Chemistry Research, 52(22), pp. 7457–7469, 2013 4. Adsorption characteristics and corrosion inhibitive properties of clotrimazole for Aluminium corrosion in hydrochloric acid, I. Obot, N.Umoren, Int. J. Electrochem. Sci, Vol. 4, pp. 863-877, 2009 5. Inhibition of mild steel corrosion in hydrochloric acid solution by cloxacillin drug, S. Harikumar and S. Karthikeyan, Journal of Materials and © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 11to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 Environmental Studies, Vol.5, pp. 925-934, 2012 6. Influence of some thiazole derivatives on corrosion of mild steel in hydrochloric acid, M.A. Quraishi, M.A.W. Khan, M. Ajmal, Anti-Corros. Methods Mater, 43, 5, 1996 7. The inhibitive action of cyclohexyl thiourea on corrosion and hydrogen permeation through mild steel in acidic solutions, S. Karthikeyan, S.Harikumar, G. Venkatachalam, S.Narayanan, R. Venckatesh, International Journal of ChemTech Research, 4(3), pp. 1065-1071, 2012 8. Electrochemical studies of two corrosion inhibitors for iron in HCl, AlAndis, N.Khamis, E. Al-Mayouf, H. Aboul b Enicm, Corros. Prev. Cont rol, 42, 13,1995 10 9. L-Methionine methyl ester hydrochloride as corrosion inhibitor of iron in 1 M HCl, B. Hamm outi, M. Aouniti, M. Taleb, M. Bri ghli, S. Kertit, Corrosion, 51,441, 1995 10. Influence of anions on corrosion inhibition and hydrogen permeation through mild steel in acidic solutions in the presence of p-tolyl thiourea, K. Madhavan, S. Karthikeyan, S.V.K. Iyer, Ind. J.Chem. Tech, 9, pp68, 2002 11. The Structure of the Electrical Double Layer at the Metal Solution Interface, M.A. Devanathan, B. Til ak, Chem.Revs, 65, pp. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 12to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 635, 1965 12. Surface coordination chemistry of monometallic and bimetallic eletrocatalysts, Soriaga, Chem.Revs, 90, pp77, 1990 13. The inhibition of sulphuric acid corrosion of 410 stainless steel by thioureas, Reeta Agarwal, T.K.G. Namboodri, Corros.Sci, 30, pp37, 1990 14. Mechanism of corrosion and its inhibition, K. Madhavan, PhD Thesis, Alagappa Uni versity, India, June 1996 15. W.Waiter Voss, J.Chemistry of Amides, Zabersky Edition, Interscience, Newyork, 187, 1997 16. A.K. Lahiri, N.G. Banerjee, NML. Tech. Journal, 5, pp33, 1963 17. Gu Hough, Zhou Zhongbai, Tao Yingachu , Yao Luaw, Wahan Dauxe Xuebao, Ziran Kexuebaw, 2, pp57, 1982 18. G. Trabanelli and Zucchui F.Revon, Corrosion and coatings, 1, pp47, 1973 19. The use of Quantum chemical methods in corrosion inhibitor studies, G. Gece, Corrosion Science, pp.2981-2992, 2008 20. Inhibition Effect of Amoxicillin drug on the Corrosion of Mild Steel in 1N Hydrochloric acid Solution, S. Hari Kumar, S. Karthikeyan, S.Narayanan and K.N.Srinivasan, International Journal of ChemTech Research, 4(3), pp. 1077-1084, 2012 21. Corrosion inhibition of mild steel in 1M H2SO4 by Ampicillin as an © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 13to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 inhibitor, S. Hari Kumar, S. Karthikeyan, P.A. Jeeva, Journal of Corrosion Science and Engineering, Vol16, 2013 22. The retardation of dissolution of Al-Sic composites in acidic medium – A green approach, V. Umasankar, S. Karthikeyan, M. Anthony Xavier, Journal of Corrosion Science and Engineering, 16, pp47, 2013 23. Ethane-2- thioamido-4-amino-N-(5-methylisoxazol-3-yl)-benzene sulfonamide: A novel inhibitor for the corrosion of mild steel in 1N HCl , S. Karthikeyan, N. Arivazhagan, S. Narayanan, Journal of Corrosion Science &Engineering, Vol.16, 2013 24. Performance characteristics of 1, 3-diorthotolyl thiourea on the corrosion of mild steel in 5% NaCl, S. Karthikeyan, N. Arivazhagan, D. Ramkumar, S. Narayanan, Journal of Corrosion Science &Engineering, Vol.16, 2013. © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 14to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 Table 1. Values of inhibition efficiency for the corrosion of mild steel in 2N HCl and 2N H3PO4 in the presence of different concentrations of Clotrimazole obtained from weight loss and gasometric measurements. measurements. Concentration Inhibition efficiency (%) of Inhibitor (mM) 5 HCl H 3P O4 Weight loss Gasometric Studies measurements studies studies 57 Weight loss Gasometric measurements 56.8 70 70.8 77 77.6 10 61 60.5 15 67 67.2 85 85.3 20 75 75.0 89 88.7 © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 15to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 Table 2.a Corrosion kinetic parameters parameters of SS 304 in 2N HCl in the presence of different concentrations of tobramycin obtained from potentiodynamic polarization studies. Concentration Ecorr (mV) of Inhibitor Tafel slopes in mV in Icorr dec-1 efficiency (mM) •A cm ba Blank -513 5 -489 Inhibition 68 75 -2 (%) bc 123 549 --- 122 274 50.9 10 -483 63 132 208 62.1 15 -475 71 128 197 64 146 125 77.2 20 -465 96 © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 16to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 Table 2.b Corrosion kinetic parameters of SS 304 submitted 10 February 2014 in 2N H2SO4 in the presence of different concentrations of tobramycin obtained from galavanostatic polarization studies. Concentration Ecorr (mV) Tafel slopes in mV in of Inhibitor (mM) dec-1 Icorr Inhibition efficiency mA cm -1 (%) ba bc -919 76 134 269 --- 5 -886 73 143 80.4 70.2 10 -875 75 138 59.7 77.6 15 -866 62 146 39.8 85 20 -860 51 138 30.2 89 Blank © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 17to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 Table 3. 3. Values of permeation current for the corrosion of mild steel in 2N HCl and 2N H3PO4 in the presence of different concentrations of Clotrimazole. Clotrimazole. Concentration of Steady state permeation permeation current (•A) Inhibitor (mM) 1M HCl Blank 23.6 1M H3PO4 19.5 5 21.0 17.5 10 20.1 12.6 16.5 10.8 15 20 13.3 6.4 © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 18to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 Table 4.Impedance parameters for the corrosion of Stainless steel 304 in 2N HCl and 2N H3PO4 in the presence of different concentrations of Clotrimazole. Concentration HCl H 3P O4 of Inhibitor Charge Double layer Charge Double layer (mM) Transfer capacitance Transfer capacitance resistance (Cdl) •F.cm-2 resistance Blank •F.cm-2 (Cdl) (Rt) (Rt) Ohm.cm2 Ohm.cm2 5.32 237 6.97 184.4 5 34 167 38 165 10 42.2 127 54 122 15 70.1 115 62 100.2 20 91.9 101.2 130 68.2 Table 5: Quantum chemical parameters for Clotrimazole Compound Tobramycin LUMO HOMO (eV) (eV) 21.86 20.26 Dipole ∆E (Cal.Mol-1) moment (Debye) 8.95 5.4 © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 19to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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 17, Preprint 9 submitted 10 February 2014 © 2014 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 20to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject 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.