scribe away from fastener for Cerate conversion coating (after [46]). ... and the length of filament penetration away from the scribe were similar for chromate and.
RARE EARTH INHIBITED SYSTEMS Anthony E. Hughes CSIRO Manufacturing and Infrastructure Technology, Normanby Rd, Clayton, Victoria, Australia, 3169 Daniel Ho and Maria Forsyth School of Physics and Materials Engineering, Monash University, Clayton, Victoria, Australia, 3169. Bruce R.W. Hinton Defence Science and Technology Organisation, Fishermans Bend, Victoria, Australia, 3207 ABSTRACT This paper looks at the prospects of replacing chromates in conversion coatings, and inhibited primers with rare earth compounds,. Performance data in relation to aluminium alloys for these compounds as corrosion inhibitors, in conversion coatings and incorporated into paint systems are presented. INTRODUCTION While chromates are still used extensively in the manufacture and maintenance of aircraft, there is an increasing drive to find replacements [1]. For the aerospace industry chromates are used in deoxidizer solutions, conversion coatings, anodizing processes, primers, wash primers and paint repair processes. Chromate is a known carcinogen, teratogen and mutagen [2] and the health issues associated with its use particularly in the aerospace industry have become increasingly highlighted in recent years [3, 4]. Many alternatives have been investigated as replacements for chromate as inhibitors in aqueous systems, conversion coatings and paint systems. Some of the most extensively studied alternatives are compounds of the rare earth (RE) metals. Preliminary research in this area began in the early 1980s with the work of Hinton el al. [5 – 8]. RE compounds have been used successfully as inhibitors for ferrous metals [9] and aluminium alloys [10], as conversion coatings [11 -14], and deoxidizers [15], and as pigments incorporated into paint systems [16 -18]. Interest in the mechanism of corrosion inhibition by RE compounds, the nature of deposited RE containing coatings and associated mechanisms of protection have lead to world-wide research in this area. A RE conversion coating process is even available commercially [19]. Concerns about the use of chromates began to have an impact on its use in the early to mid 1980’s [20]. In response to these concerns, research was undertaken to find alternatives to chromate and a number of reviews of this research have been published [20 - 23]. Much of the research into REs as inhibitors has had an Australian focus and this paper gives an overview of the current and past work. The authors also examine the prospects of developing a complete rare earth protective scheme as an alternative to the chromate system, for metal finishing applications. EARLY WORK Hinton, Arnott and co-workers [5 - 8] found that the salts of the rare earth elements (cerium, lanthanum and yttrium), in particular the chlorides, were very good inhibitors of corrosion of aluminium alloys, zinc and steel. For example, as shown in Figure 1, as little as 100 ppm of cerium chloride in a 0.1M NaCl solution was found to reduce the corrosion rate of aluminium alloy AA 7075-T6 by almost two orders of magnitude [10, 24] Mischmetal chloride was found to reduce the corrosion rate by nearly a factor of 200 times. (Figure 1). Similar results were obtained with AA 2024-T3. For both alloys, the level of inhibition was not as good as when a similar concentration of sodium chromate was present (nearly three orders of magnitude reduction). Scanning electron microscopy and electrochemical studies indicated that the RE salts acted as cathodic inhibitors through the formation of a film of RE metal oxide on the surface, particularly at cathodic sites in the alloy microstructure. It was postulated that the oxides were formed at those sites as the local alkalinity increased due to the reduction of oxygen, to the point where the solubility product of the oxide was exceeded [8, 10].
1
2
Corrosion Rate μg/m /s
1
0.1
0.01 none
Ce
MM
Cr
Inhibitor Type
Figure 1. The effect of inhibitor additions at 100 ppm on the corrosion rate of AA 7075-T6 in 0.1M NaCl solution. Ce= CeCl3.7H2O, MM = mischmetal chloride, Cr = Na2CrO4. The protective effects of this deposited oxide film as a coating was utilised in the development of a conversion coating based on RE chemistry. As can be seen from Table 1, the time for deposition of the coatings when various aluminium alloys were immersed in NaCl solutions containing cerium salts was generally longer than a few days. This presented a problem from a practical viewpoint. One of the first developments which changed this situation was an invention by Hinton and Wilson [25] of a hydrogen peroxide accelerated process which reduced the coating time for some aluminium alloys to a matter of minutes. Later in the paper, this topic will be discussed in the conversion coatings section.
TABLE 1. CERATE COATING TIMES FOR ALUMINIUM ALLOYS Activated2 Al Alloy Immersion H2O2 1 Accelerated (min) (days) (min) 2024-T3 20* 2-4 2 5005 50 >60 2 7075-T6 20 10 2 1 2
Hinton and Wilson Process [25] Hughes et al. [26] and Hardin et al. [27] Processes
RECENT DEVELOPMENTS IN AQUEOUS INHIBITION USING RARE EARTH COMPOUNDS As a result of an increased urgency to replace chromates as inhibitors and pigments in paints, work on RE corrosion inhibiting compounds has continued at Monash University, where innovative designs for new inhibitors have been developed. A novel approach pioneered by Forsyth, Deacon et al. [29 – 31] combined REs with organic inhibitors to produce multifunctional inhibitor compounds which exhibited synergistic behaviours. Subsequent work by Ho, Blin Wilson et al. [32 – 34] has demonstrated that such RE – organic inhibitors can be effective in the inhibition of corrosion in aqueous environments. Figures 2 and 3 show the effects of 200 ppm cerium dibutyl phosphate (Ce(dbp)3) or cerium salicylate (Ce(Sal)3) on the corrosion inhibition of bare AA2024T3 coupons after immersion in 0.001M NaCl solution for 1, 3, 6 and 9 weeks. The factor of improvement was calculated by dividing the corrosion rate in the NaCl solution (from weight loss tests to ASTM G1-90) by the corrosion rate in the NaCl solution containing
2
the inhibitor. For both inhibitors, the coupons after testing were close to pristine compared with the appearances of coupons exposed to the control (uninhibited) solution.
800
Improvement Over Control
700 600 500 400 300 200 100 0 -100 0
2
4
6
8
10
Time (Weeks)
Figure 2: The effectiveness of Ce(dbp)3 as a corrosion inhibitor for AA 2024-T3 as a function of immersion time in 0.001M NaCl.
Improvement Over Control
25
20
15
10
5
0 0
1
2
3
4
5
6
7
8
9
10
Time (Weeks)
Figure 3: effectiveness of Ce(Sal)3 as a corrosion inhibitor for AA 2024-T3 as a function of immersion time in 0.001M NaCl. Comparison of the data in Figures 2 and 3 shows that the Ce(dbp)3 was a much better inhibitor than the Ce(Sal)3 given that its factor of improvement over the control was several hundred rather than around twenty five as observed for Ce(Sal)3. This shows that, for long term exposure, the Ce(Sal)3 was significantly less effective than the Ce(dbp)3 . Clearly the Cerium and organic components are working synergistically with these systems. Recent FIB-SIMS data (to be published) obtained on AA 2024-T3 coupons exposed to NaCl solutions containing Ce(dbp)3 has indicated the presence of a 200 to 500nm film with both cerium and phosphorous present. This suggests that the protective film is the result of a synergism involving both cerium and the organic.
3
RARE EARTHS IN CONVERSION COATINGS RE conversion coating processes using either mischmetal salts or cerium salts have been developed for aluminium alloys. Their performances in a range of tests are comparable to those of chromate conversion coatings. The work of Hinton et al. [5 - 8] demonstrated that cerium containing coatings deposited onto aluminium alloys via immersion in chloride solutions containing cerium salts provided corrosion protection to the underlying alloy. However, the coating time in those simple immersion experiments were of the order of days (Table 1), whereas typical chromate conversion coating times are from a few seconds to a few minutes. The challenge was to find a method for accelerating the process. The first advance towards more rapid deposition was made by Hinton and Wilson [25] who accelerated the process via acidification of the cerium chloride process solutions and the addition of a strong oxidant such as hydrogen peroxide . These accelerated solutions produced golden coatings on aerospace aluminium alloys containing copper in a matter of minutes (Table 1). Other workers have used temperature to activate oxide thickening on the aluminium matrix but retain ceria deposition onto the intermetallic particles [35] or other salts such as cerium nitrate [35, 36] or perchlorate [37]. The accelerated and acidified CeCl3 coatings process has become known as the “Cerate Process” (which, technically, is a misnomer since they do not contain oxyanions of cerium), but is a term that draws attention to the similarity in appearance to the chromate conversion coating. The operating parameters and performance of the Cerate coating is generally similar to those of chromate conversion coatings. Specifically the Cerate coatings are processed in an acidic bath at temperatures from 20 to 50°C and produce coatings in 2 to 5 minutes on aerospace alloys such as AA 2024-T3 and 7075-T6. The colour in particular is important from a processing viewpoint, since it allows for immediate visual quality control on coating deposition.
Figure 4: A typical surface of AA2024-T3 with a heavy deposit of Cerate coating. Figure 4 shows a scanning electron microscope image (SEM) of AA2024-T3 with a heavy Cerate coating. The two “islands” in the image are areas of thicker coating associated with underlying intermetallics particles. Even the etch pit on the right hand side which contains the remains of an intermetallic particle left after deoxidation, is coated with Cerate coating. Cracking in the coating may be due to drying of the coating in air or an artefact due to the vacuum in the SEM. Small nodules up to 1 µm diameter are also evident in Figure 4. An atomic force microscope image of the surface of another Cerate coating (Figure 5) shows these nodules on a much finer scale [38, 39]. This structure provides a good key for paint to mechanically adhere to the surface. It was proposed [25] that deposition of the coating occurs as a result of the cerium oxide/hydroxide precipitating with local increases in pH associated with cathodic reduction of hydrogen peroxide at intermetallic particles. H2O2 + 2e- → 2OH-
(1)
Or H2O2 + 2H + 2e- → H2O
(2)
Hughes et al. [38, 39] have identified this precipitated oxide coating as CeO2 when precipitation occurs from chloride containing solutions, whereas some other crystalline phase is deposited from perchloric acid solutions which Riviera et al [40] could not
4
identify. Pourbaix Diagrams are usually used to explain the precipitation of the various cerium containing phases on the surface and updated Pourbaix Diagrams, based on newer values of the free energy of formation of solutions species, have recently been published [39, 41]. Pourbaix Diagrams are equilibrium diagrams and the solution species are likely to be non-equilibrium species, more specifically peroxo species [42]. Hayes et al [41] for example give a good summary of the difficulties of identifying solution species in the interpretation of titrations of cerium containing solutions.
Figure 5: AFM profile of the surface of AA2024-T3 showing the nodular nature of the oxide present on the surface after Cerate coating. The accelerated process resulted in rapid deposition onto copper-containing alloys, but no deposition occurred on aluminium alloys that had little or no copper. Copper acts as a very efficient cathode and facilitates reactions (1) and (2) thereby causing the deposition of coatings [43]. Campestrini et al. [44] have shown that copper smut on the surface of copper-containing alloys results in heavy deposition of the Cerate coating. Hughes et al. [39] have shown that copper enrichment on the surface of the alloy at the aluminium/oxide interface may also play a significant role in coating deposition. Activation of alloy surfaces where there was little or no copper in the alloy was first achieved by the addition of transition metal complexes [43], but a commercial process that is currently available for architectural aluminium [19] is activated by other complexes [45]. The performance of Cerate coatings in neutral salt spray corrosion test (NSS),may be quite variable, but generally they do not perform well in that test without a sealant present. Sealing in phosphate and silicate containing solutions was first reported in 1992 by Hammon et al. [46]. Silicate sealing was found to give a NSS performance similar to that for chromate conversion coatings on AA 2024-T3 in particular. Table 2 gives some typical hours to failure in the NSS for both silicate sealed Cerate coated specimens and for conversion coated specimens.
Table 2: NSS times to failure* Al-Alloy 2024-T3 7075-T6
Chromate 700 1300
* Reproduced from Reference [ 38 ]
5
Cerate 980 360
Of more interest than the performance of the Cerate coating alone in the NSS test is the performance under paint since the vast majority of applications would require a finish with primer and topcoat. The important underlying parameter which affects corrosion protection performance is adhesion of the paint to the conversion coating. Hughes et al [38] reported that for two common aerospace alloys, the adhesion of painted sealed and unsealed Cerate conversion coatings was similar to the results obtained with chromate conversion coatings (Table 3). Cerate coatings have been shown to have similar performance to chromate conversion coatings in other paint corrosion tests. For example Schmidt and Schubach [19] have reported that in the Lockheed test on architectural aluminium, similar under paint creepage was observed for panels coated using an activated Cerate process and for panels with a
(b)
80
Corrosion Area (mm2/mm)
70
(a)
60 50 40 30 20 10 0 0
200
400
600
800
1000
Time (Hours) chromate conversion coating.
Figure 6(a): Filiform corrosion around a Cu-Ni fastener in AA2024-T3. (b) Area of corrosion / unit length of scribe developed around a Cu-Ni fastener (including corrosion front and filiform corrosion) as a function of time. (■) = alkaline cleaned, (♦) = deoxidised, (●) = Cerate conversion coating, (▼) = Chromate conversion coating, ( ) = filiform corrosion at a scribe away from fastener for Cerate conversion coating (after [46]).
Table 3: Adhesion* (% of chromate control) Al-Alloy
Chromate
2024-T3 7075-T6
100 100
Cerate Unsealed 119 89
Cerate Sealed 124 80
* Reproduced from Reference [35]
There have been extensive studies of the characterization and performance of Cerate conversion coatings in the filiform corrosion test. Mol et al. [47] compared Cerate and chromate conversion coatings on aluminium alloys AA 2024-T3 and AA 7075-T6, and found essentially no difference between the resistances to filiform creepage from a scribe through a polyurethane coating. Hughes et al. [48] and Intem et al. [49] carried out filiform corrosion tests with alloy AA 2024-T3 panels which had been alkaline cleaned and treated with a Cerate coating or a chromate conversion coating, and overcoated with a polyurethane. They found that the average filiform corrosion initiation site density and the length of filament penetration away from the scribe were similar for chromate and Cerate conversion coatings. Both coatings performed significantly better than for the case with the alkaline cleaning and no conversion coating. Intem et al. [49] also examined the performance of conversion coatings in a filiform corrosion test with a strong galvanic influence. They manufactured model lap joints using AA 2024 T3 plate fastened with copper-nickel or copper rivets (Figure 6(a)). The plates were alkaline cleaned, deoxidised and coated either with a Cerate or a chromate conversion coating. The assembled joint was coated
6
with polyurethane. A scribe was made through the polyurethane coating around the rivet head through to the substrate. The area of blistered and delaminated coating per unit length of the scribe was measured as a function of exposure time in the filiform test. The data are shown in Figure 6b. Both alkaline cleaning, and alkaline cleaning plus deoxidation allowed significant amounts of underfilm corrosion to develop with exposure time, Deoxidation apparently allowed even more underfilm corrosion to develop than alkaline cleaning alone. It was proposed that the surface oxide left after alkaline cleaning produced slightly more protection of the underlying aluminium than when it was removed during deoxidation. Chromate conversion coating and Cerating produced a significant reduction in the amount of underfilm corrosion developing around the scribe. This was to be expected. However, the surprising result was that the Cerate coating had a similar performance to the chromate conversion coating. The data presented in Figure 6b by the half circles were taken from the scribe through the Cerate coating some distance from the rivets. These results show that the Cerate coating in the absence of the galvanic driving force prevented significant amounts of underfilm corrosion developing over the 1000 hours of the test. It is clear from the results that the Cerate conversion coating not only prevented delamination from occurring but also underfilm corrosion from initiating in this severe test.
RARE EARTH INHIBITORS IN PAINT SYSTEMS It has been reported previously [50] that the incorporation of Ce(Sal)3 and Cedbp3 inhibitors into epoxy primers applied to AA2024T3 panels provided some protection. In those experiments a scribe was made into the epoxy primer and the panels were exposed either to constant immersion or alternate immersion conditions in a chloride solution at neutral pH. The results after more than three weeks of testing showed that, under these conditions, both inhibitors were able to suppress the extent of filiform corrosion under the coating, although Ce(dbp)3 was more effective, which is consistent with the performance of Ce(dbp)3 as an inhibitor of corrosion of AA 2024-T3 when tested under constant immersion conditions in a NaCl solution [50]. Despite the lower solubility of the Ce(dbp)3 compound, it has been shown that Ce(dbp)3 is capable of leaching out at levels approximately 300ppm in solutions of pH =2. [29, 31].
Figure 7 AA 2024-T3 panels with (a) a Cerate conversion coating sprayed with epoxy primer with Ce(dbp)3 pigment, and (b) with no pigment. Panels of AA2024-T3 were treated with a Cerate conversion coating and subsequently over coated with either an epoxy polyamide which contained Ce(dbp)3 or an epoxy with no inhibiting pigment present. The epoxy was applied to a thickness of 20 µm by spraying from a methyl ethyl ketone solvent. The coated panels were scribed and subjected to a filiform corrosion test to Standard Test Method DIN EN 3665, which includes exposure to hydrochloric acid vapour followed by 14 days in high humidity air. Figure 7 shows that in the presence of Ce(dbp)3 some filiform corrosion developed in the scribe, but no large areas of corrosion developed beneath the coating away from the scribe Fig. 7(a) as was the case without the Ce(dbp)3 pigment Fig. 7(b). The presence of the Ce(dbp)3 was clearly beneficial to corrosion suppression even with the Cerate conversion coating. The poor corrosion protection at the scribe with the Ce(dbp)3 pigment present is thought to be due in part to the poor adhesion of the selected coating system to the Cerate conversion coating. This poor adhesion could be improved by a better design of any final paint formulation. Figure 8 illustrates the effect of a thicker epoxy coating (200 µm) applied using a draw-bar, and the effects of a sprayed polyurethane topcoat applied over the epoxy primer pigmented with Ce(dbp)3. The polyurethane topcoat served to restrict general access to the substrate of hydrochloric acid and moisture during the filiform test and thus focussed the degradation processes at the scribed region.
7
For the panel with the epoxy coating, the thicker coating allowed significantly less general corrosion to develop under the coating compared with the spray coated panels described above. Image analyses of the filiform corrosion that developed from the scribe indicated that a smaller area of underfilm corrosion (58 cm2) developed with the Ce(dbp)3 pigment present than without (70 cm2). Even less area developed with the polyurethane top coat present (38 cm2). The overall poor filiform performance is believed to be predominantly due to high permeability of the epoxy (hence improved behaviour with thicker coating and/or topcoat) and also the poor compatibility between the substrate and coating leading to poor overall adhesion.
Figure 8 Cerate conversion coated AA 2024-T3 panels coated with an epoxy using a draw bar. For (a) the epoxy coating containing no inhibiting pigment, (b) epoxy coating with Ce(dbp)3 pigment, and (c) epoxy coating with Ce(dbp)3 pigment over coated with a polyurethane.
CONCLUSIONS In this paper we have demonstrated that cerium compounds can be used effectively to inhibit corrosion and that the strength of the inhibiting effects of these compounds demonstrates that there is potential for them to replace the chromates currently used in the aerospace industry. Specifically, it has been shown the addition of cerium ions to aqueous solutions containing NaCl suppress the corrosion of aluminium alloys through the formation of a cerium based oxide. Use can be made of this oxide as a cerium-based conversion coating (Cerate) which has similar adhesion and corrosion protection performance to chromate conversion coatings when tested under paint. The feasibility of employing a rare earth /organic inhibitor in a coating system for corrosion protection of AA2024 T3 has also been demonstrated. Not only has the compound cerium-di-butyl-phosphate (Ce(dbp)3) been incorporated successfully into an epoxy polyamide coating, but the resulting coating has also exhibited excellent corrosion protection properties for the aluminium substrate particularly in the presence of a polyurethane top coat. It is believed at this stage that the inhibition mechanism involves the leaching of the inhibitor from the primer coating into the surrounding aqueous phase, similar to the mechanism for strontium chromate based epoxy primers. The demonstration of the effectiveness of cerium compounds in each of these critical metal finishing steps indicates that a total protection system based on RE chemistry is achievable.
REFERENCES 1.
R.G. Buchheit and A.E. Hughes, “Chromate and Chromate-Free Coatings, Sect. 4b Volume 13A Corrosion: Fundamentals, Testing and Protection (American Society for Materials, Materials Pk Ohio, 2003), pp. 720 - 736. 2. N. I. Sax, Dangerous Properties of Industrial Materials, 5th Edition, (Van Nostrand Reinhold, NY, 1979) 3. N.A. Dalager, T.J. Mason, J.F. Fraumeni, R. Hoover and W.W. Payne, J. Occup. Environ. Med., 22 (1980) 25. 4. P.T. LaPuma, J.M. Fox and E.C. Kimmel, Red. Tox. Pharm., 33, (2001), p.343. 5. B.R.W. Hinton, D.R. Arnott and N.E. Ryan, Metals Forum, 7, No. 4, (1984), pp.12-18, 6. B.R.W. Hinton, P.N. Trathen, L. Wilson and N.E. Ryan, Proceedings of the Annual Conference of the Australasian Corrosion Association, (1988), p 4-1.1. 7. B.R.W. Hinton and L. Wilson, Corr. Sci., 29 (1989), p. 967. 8. B.R.W. Hinton, D.R. Arnott and N.E. Ryan, Mater. Forum, 9 (1986), p. 162. 9. M. Forsyth, K. Wilson, T. Behrsing, C. Forsyth, G.B. Deacon and A. Phanasgoankar, Corr. Sci., 58 (2002), p. 953. 10. D.R. Arnott, B.R.W Hinton, N.E. Ryan, Materials Performance, 26 (1987), p.42.
8
11. A.E. Hughes, S.G. Hardin, T.G. Harvey, T. Nikour, B. Hinton, A. Galassi, G. McAdam, A. Stonham, S.J. Harris, S. Church, C. Figgures, D. Dixon, C. Bowden, P. Morgan, S.K. Toh, D. McCulloch and J. Du Plessis, ATB Metallurgie 43 (1,2) (2003), p. 264. 12. W.G. Fahrenholtz, M.J. O’Keefe, H. Zhou and J.T. Grant, Surf. Coat. Tech., 155 (2002), p. 208. 13. M. Dabalà, E. Ramous and M. Magrini, Mat. Corr., 55 (2004), p. 381. 14. C. Wang, F. Jiang and F. Wang, Corrosion, 60 (2004), p. 237. 15. A.E. Hughes, K.J. H. Nelson and P.R. Miller, Mat. Sci. Technol., 15 (1999), p. 1124. 16. D. Ho and M. Forsyth, To be published. 17. C.J.E. Smith, K.R. Baldwin, V.M. Evans, S.A. Garrett and K.S. Smith, Proceedings of the 83rd Meeting of the AGARD SMP on Environmentally Compliant Surface Treatments of Materials for Aerospace Applications (, Florence, Italy AGARD), September 4-5, (1996), p. 8-1. 18. C.J.E. Smith, K.R. Baldwin and M.A.H. Hewins, Progress in the Understanding and Prevention of Corrosion, Edit J. Costa and A. Mercan, (Institute of Materials, 1993), p. 1652. 19. Th. Schmidt-Hansberg and P. Schubach, ATB Metallurgie, 43 (2003), p. 9. 20. B.R.W. Hinton, Metal Finishing, September (1991), p.55. 21. R.G. Buchheit and A.E. Hughes, “Chromate and Chromate-Free Coatings”, Sect. 4b, Vol 13A Corrosion: Fundamentals, Testing and Protection (American Society for Materials, Materials Pk Ohio 2003), pp. 720 - 736. 22. A. Nylund, Alum. Trans. 2 (2000), p. 123. 23. S.M. Cohen, Corr., 51 (1995)p. 71. 24. B.R.W. Hinton, N.E. Ryan, D.R. Arnott, P.N. Trathen, L. Wilson and B.E. Williams, Corrosion Australasia, 10, No. 6, (1985), p. 12. 25. L. Wilson and B.R.W. Hinton , Australian Provisional Patent, “A Method for Forming a Corrosion Resistant Coating”, WO88/06639. 26. A.E. Hughes, K. Hammon and T.W. Turney, Process and Solution for Providing a Conversion Coating on a Metal Surface, US 6206982. 27. S.G. Hardin , K. Hammon, A.E. Hughes, K. Wittel, US Patent, 6,755,917, “Process and Solution for Providing a Conversion Coating on a Metallic Surface II”:. 28. G. B. Deacon, C. M. Forsyth, M. Forsyth, Zeitschrift fuer Anorganische und Allgemeine Chemie 629, No. 9, (2003), pp.14721474. 29. M. Forsyth, K. Wilson, C. M. Forsyth, T. Behrsing, G. B. Deacon, Corrosion 58, (11), (2002), pp.953-960. 30. M. Forsyth, C. M. Forsyth, K. Wilson,; T. Behrsing, G. B. Deacon, Corrosion Science 44, No. 11 (2002), pp. 2651-2656. 31. D. Ho, G. McAdam, B. R. W. Hinton, M. Forsyth, P. C. Junk, S. Leary, G. B. Deacon, ,Proceedings of the Joint Conference of the Australasian Corrosion Association and the Australian Non-Destructive Testing Association (2003).ACA.. 32. F. J. Blin, S. Leary, K. E. Wilson, G. B. Deacon, P. C. Junk, M. Forsyth Proceedings of the Joint Conference of the Australasian Corrosion Association and the Australian Non-Destructive Testing Association (2003), ACA.. 33. D. L. Ho, L. Apateanu, G. B. Deacon, N. Brack and M. Forsyth, Proceedings of the Annual Conference of the Australasian Corrosion Association (2002), ACA.. 34. M. Forsyth, K. Wilson, T. Behrsing, K. Konstas, N. Brack, G. B. Deacon and C. M. Forsyth. , Proceedings of the Annual Conference of the Australasian Corrosion Association (2002), ACA. 35. M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna and M. Macros, Materials and Corrosion, 54 (2003) 77. 36. M. Dabalà, E. Ramous and M. Magrini, Materials and Corrosion, 55(5) (2004) 381. 37. B.E. Johnson, J. Edington and M.J. O’Keefe, Materials Science and Engineering, A361 (2003) 225. 38. A.E Hughes,, S. G. Hardin., T. G. Harvey,, T. Nikpour, B. R. W. Hinton, A. Galassi, G. Mcadam, A. Stonham, S. J. Harris, S. Church, C. Figgures, D. Dixon, C. Bowden, P. Morgan, S. K. Toh, D. McCulloch, and J. Du Plessis, J, ATB Metallurgie 43 Nos. 1 and 2, (2003), p. 264. 39. A. E. Hughes, J.D. Gorman, P.R. Miller, B.A. Sexton, P.J.K. Paterson and R.J. Taylor, Surf. Int. Anal., 36 (2004), p. 290. 40. B.F. Rivera, B.Y. Johnson, J. O’Keefe and W.G. Fahrenholtz, Surf. Coat. Technol., 176 (2004), p. 349. 41. S.A. Hayes, P. Yu, T.J. O’Keefe, M.J. O’Keefe and J.O. Stoffer, J. Electrochem. Soc., 149 (2002), p. C623. 42. A. E. Hughes, S. G. Hardin, K. W. Wittel and P. R. Miller, Accelerated cerium-based conversion coatings , NACE 2000, Orlando, Florida, 27-31March 2000, Corrosion 2000 (Research Topical Symposium - "Surface Conversions of Aluminium and Ferrous Alloys for Corrosion Resistance", NACE International (Houston, Texas) 2000, 47-66. 43. A.E. Hughes, K. Hammon and T.W. Turney, US Patent No. 6,206,982.”Process and Solution for Providing a Conversion Coating on a Metal Surface”. 44. P. Campestrini, H. Terryn, A. Hovestad and J.H.W. de Wit, Surf. Coat. Technol., 176 (2004), p. 365. 45. S.G. Hardin, K.W. Wittel, A.E. Hughes and K.J. Hammon, US Patent 6,773,516, “Process and Solution for Providing a Conversion Coating on a Metal Surface I”. 46. K. Hammon, A.E. Hughes, R.J. Taylor, M. Henderson, B.R.W. Hinton and L. Wilson, The Performance and Characterisation of Cerium Oxide Conversion Coatings, CSIRO Australia, DMST Report No 92-15 (1992).
9
47. A. Mol, Filiform Corrosion of Aluminium Alloys. The Effect of Microstructural Variations in the Substrate (PhD-Thesis, Laboratory of Materials Science of the Delft University of Technology, The Netherlands), 2000. 48. A.E. Hughes, A.J. Mol, B.R.W. Hinton and S. van der Zwaag, Corr. Sci., 47 (2005), p. 107. 49. S.Intem, A.E. Hughes , A.K. Neufeld, A.M. Glenn and T. Markley, submitted to J. Coat. Technol. 50. A.E. Hughes, D. Ho, M. Forsyth and B.R.W. Hinton, Towards Replacement of Chromate Inhibitors by Rare Earth Systems, presented at the 1st World Congress on Corrosion in the Military’ Sorrento, Italy, June 6 to 8, 2005, To be published in Corrosion Reviews.
10
