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ADVANCED SURFACE ENGINEERING
Project Number 78 – Sol-gel treatment for protection of aluminium
alloys surfaces
EXECUTIVE SUMMARY
Within this theme, activities are being exploited that take advantage of the ability to tailor the
morphology and composition of anodic films on aluminium as well as he contrasting route of
film generation through sol-gel processing. Manchester is world-leading in the ability to
develop morphologically- and compositionally-defined alumina films; concerning the sol-gel
route, initially to replace conversion coatings, this is a new activity that has required
systematic development of coatings and parallel consideration of generation and
incorporation of nanoparticles for added functionalities.
OUTLINE
Concerning corrosion inhibition, the use of the porous anodic film skeleton to hold inhibitive
compounds for subsequent release and protection at damaged regions has been developed
successfully. An initial patent of Smith and Baldwin (QinetiQ) suggested that by using a
relatively simple double immersion process in separate solutions containing inhibitive species,
it may be possible to develop solid inhibitive compounds within the pores of the anodic film. At
Manchester, porous anodic films were formed on AA2024 T3 alloy in sulphuric acid;
electrolytes based on sulphuric acid are under consideration as replacements for chromic acid
anodizing. A range of film thicknesses were generated for subsequent processing; Figure 1
shows an ion beam thinned plan view of a stripped anodic film, with the
50 nm
Figure 1. Transmission electron micrograph of a
stripped and ion beam thinned anodic film formed on
superpure aluminium in sulphuric acid for 10 min.
pores for confinement of inhibiting species
displayed. The double immersion process
involved separate immersion of the anodic
film in solutions of selected cation and
anion species. The up-take of solution
species was monitored by Rutherford
backscattering spectroscopy (Figure 2)
and rf-glow discharge optical emission
spectroscopy. The previous route allowed
selection of film growth parameters and
immersion conditions to be optimized for
maximum up-take of solution species into
the porous anodic film.
This subsequently led to a completely new route for up-take to be devised, involving a single
immersion in a mixed salt solution at room temperature for 300 s, that is more effective as a
production line process.
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ADVANCED SURFACE ENGINEERING
Energy (MeV)
1.0
2000
1.5
2.0
Counts
1500
Figure 2. Experimental and
simulated RBS spectra of the
anodized AA2024 T3 alloy after
treatment by the V + Ni process
for 5 min at 20oC
1000
500
0
100
200
300
400
500
Channel
2.00
Non-anodized
Anodized only
Anodized & V-Ce(III) treated
Anodized & V+Ni process
-0.50
E vs SCE (V)
E vs SCE (V)
-0.75
-1.00
-1.25
-1.50
1.00
0.50
0.00
-1.75
-0.50
-2.00
-11.00
-1.00
-11.00
-9.00
-7.00
-5.00
2
logi (A/cm )
-3.00
Figure 3 a. Cathodic
polarization behaviour of the
anodized AA 2024 T3 alloy
after post treatment by various
immersion processes
-1.00
Non-anodized
Anodized only
Anodized & V-Ce(III) treated
Anodized & V+Ni treated
1.50
-9.00
-7.00
-5.00
logi (A/cm2)
-3.00
-1.00
Figure 3 b. Anodic polarization
behaviour of the anodized AA
2024 T3 alloy after post
treatment by various immersin
processes.
For a combination of nickel and vanadate species, electrochemical responses (Figure 3)
revealed significant reductions in the rates of anodic and cathodic reactions, with leaching
studies showing the ability of the retained species to provide inhibition at damaged regions.
Finally, parallel salt spray testing at Manchester and Airbus, Filton, confirmed that the treated
porous anodic films successfully survived more than 1400 h exposure. (Figure 4) Even more
remarkable, the tested specimens had not been sealed. This success must be tempered in the
light of the recent discussions about the environmental concerns associated with V2O5;
however, at this stage, the previous compound has not been definitively linked to the
protection mechanism.
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ADVANCED SURFACE ENGINEERING
Figure 4. Optical micrographs of the
anodized AA 2024 T3 alloy following
post treatment by the V + Ni process
and 1248 h exposure to the salt
spray. The right side micrograph is
an anodized alloy that had been
exposed to the salt spray after
hydrothermal treatment, ie sealing.
The ability to control the porous anodic film morphology is also being exploited in distinctly
separate routes. In the first, film formation and stripping leads to nanoscale imprints in the
aluminium surface that have value in polymer replication and adhesive tape applications.
Additionally, the manufacturing route is consistent with current high rate anodizing processes
and does not require highly ordered pore arrays over large length scales. Further, by a doped
xerogel impregnation route, luminescent displays may be developed. In collaboration with
colleagues at Minsk, Belaru, this is to be pursued further.
With regard to sol-gel processing to replace conventional conversion coating, inorganic-organic
hybrid routes are being progressed. As a first step, process parameter control has been used to
allow development of coatings of defined thicknesses and hardness, with minimal damage
through shrinkage during curing. Figure 5 reveals a cracked coating, with increased
susceptibility to corrosion.
Cracks and resin penetration
Figure 5. Transmission electron micrograph of an
ultramicrotomed section of the AA1050 aluminium
alloy substrate and attached sol-gel coating
generated from zirconium tetrapropoxide (TPOZ)
and glycidoxypropyltrimethoxy silane (GPTMS).
The cracks infiltrated by embedding resin,
developed during curing of the coating.
500 nm
0.4
et
0.7
1.5
2.7
5.5
0.2
E (V)
0
-0.2
-0.4
-0.6
-0.8
-1
1.00E-09
1.00E-07
1.00E-05 1.00E-03
i (A/cm2)
1.00E-01
Figure 6. Comparison of anodic polarization
behaviour of the variously coated AA1050
aluminium alloy in sodium chloride solution. The
improved barrier protection of the crack-free
coating, with optimised GPTMS to TPOZ ratio, is
revealed.
1.00E+01
Curing temperatures have been restricted to no more than 110oC and development of crackfree coatings advanced, with barrier protection properties achieved on AA1050 aluminium
alloy (Figure 6), representative of a commercial cladding alloy. The next stage is to assess
the requirements for self-healing and to incorporate inhibitive species as conventional
compounds or as nanoparticles.
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ACHIEVEMENTS Continued……
• Rapid generation of inhibited porous film skeletons has been achieved with very effective
performance indicated.
• Routes for templating developed and progressed
Starting from scratch crack free inorganic organic hybrid sol-gel films developed with good
barrier properties
PUBLISHED PAPERS
1.
2.
3.
4.
5.
6.
7.
8.
Anodic alumina templates for nano-fabrication, E. McAlpine, C. Butler, N. Davies,
C. Pargeter, G. Scamans, E. Leclerc and G. Thompson, Nano and Hybrid
Coatings, Manchester, Paper 3, 1-8, The Paint Research Association,
Manchester (2005).
Strong green erbium-related luminescence in thestructure xerogel-porous anodic
alumina, D. A. Tsirkunov, I. S. Molchan, J. Misiewicz, R. Kudrawiec, A.
Podhorodecky and G. E. Thompson, Physics, Chemistry and Application of
Nanostructures 2005 (eds., V. E. Borisenko, S. V. Gaponenko and V. Gurin),
World Scientific, Singapore, 178-182 (2005).
Sol-gel coatings for pitting resistance of AA2034-T3 aluminium alloy, Y.
Sepulveda, C. M. Rangel, M. A. Paez, P. Skeldon and G. E. Thompson, Eurocorr,
Lisbon, Portugal (2005).
Control of pore locations in anodic alumina oxide films, D. J. LeClere, B. Derby
and G. E. Thompson, MRS Meeting, San Francisco, US (2006).
Optoelectronic applications of lanthanide-doped sol-gel products and porous
anodic alumina, G. K.Maliarevich, I. S. Molchan, N. V. Gaponenko, A. V. Mudryi,
S. V. Gaponenko, A. A. Lutich and G. E. Thompson, Journal of Society for
Information Display, 14, 583-589 (2006).
Sol-Gel-based coatings with incorporated CeO2 nanoparticles for protection of
AA2024-T3 aluminium alloy, I. S. Molchan, G. E. Thompson, P. Skeldon, T.
Hashimoto, M. Schem, T. Schmidt, J. Gerwann and W. Kochanek, Physics,
Chemistry and Application of Nanostructures, Belarus (2007).
Replicating materials and products from anodised aluminium, C. Butler, E.
McAlpine and G. E. Thompson, Trans. Inst. Metal Finishing, in press.
Post-treatment of anodic films on aluminium and its alloys for corrosion resistance,
S. Liu, G. E. Thompson, P. Skeldon and C. J. E. Smith, 16th International
Corrosion Congress, Beijing, China (2007).
ADDITIONAL INFORMATION
The work is progressing well on several fronts and will be extended through collaborations
with Belarus and colleagues within the Material Science Centre. Solutions to practical
problems will also be addressed within continuing studies on this work.