Advanced%20sol-gel%20coating%20for%20corrosion%20protective%20of%20aerospace%20aluminium%20alloys

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Advanced sol-gel coating for corrosion protective
of aerospace aluminium alloys

• John Colreavy, Brendan Duffy, Rajath Varma
• CREST, Dublin Institute of Technology

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Summary

• Sol-Gel History, Chemistry, Properties
• Common Silane Precursors
• Current Research in Protective Coatings
• CREST Research
• Incorporating Organometallic Precursors
• Incorporating Amine Chemistry
• Novel Inhibitor Incorporation

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Sol-Gel History
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Sol-Gel Chemistry
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Sol-Gel Chemistry

• Main reactions of hydrolysis-polycondensation
  which take place with organically modified
  alkoxides are the following

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Sol-Gel Properties
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Sol-Gel Properties
P. Judenstein and C. Sanchez, J. Mater. Chem.,
1996, 6, 511
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Common Silane Precursors
C. Sanchez et al., J. Mater. Chem., 2005, 15,
35593592
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Current Research in Protective Coatings

• Within the aerospace industry, sol-gel chemistry
  has attracted considerable attention
• Boeing has developed an environmentally compliant
  sol-gel product called Boegel based on
  epoxysilane and zirconium chemistry, to replace
  carcinogenic chromium conversion coatings as an
  adhesion promoter on aluminium alloys.
• EADS (Airbus (Fr)), in 2007, was granted a patent
  for a sol-gel coating based on epoxysilane and
  zirconium chemistry for corrosion protection of
  Aluminium.
• EADS (Munich) has used this patented chemistry to
  study the performance of sol-gel coatings on
  leading edges against rain erosion, as part of
  the FP6 Project NAPOLYDE
• EADS (Munich) is a leading partner in an FP6
  Project MULTIPROTECT, which is investigating
  sol-gels as alternatives to heavy metal coatings.

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Protective Coating Technology

• ORMOCERS developed by FhG ISC

Zheludkevich et al. J. Mater. Chem., 2005, 15,
50995111
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Aerospace Coatings Boe-Gel System
US Patents 5,869,141 5,939,197 6,077,885

• Issue Protective coating but no corrosion
  inhibition

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Incorporating Organometallic Precursors
Acetic Acid (AcOH)
Methacrylic Acid (MAAH)
Isobutyric Acid (IBA)
2,2 Bipyridyl (BP)
Acetylacetone (Acac)
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Neutral Salt Spray Study ( 1 week)
MAPTMS/Zr (AcOH)
MAPTMS/Zr (MAAH)
MAPTMS/Zr (IBA)
MAPTMS/Zr (BP)
MAPTMS ( 48 Hrs)
MAPTMS/Zr (Acac)
Corrosion protection MAAHAcOHgtIBAgtAcac gt BP
gt no Zr
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Incorporating Amine Chemistry
b-Diketone (acac)
Amine rich carboxylic acid
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Surface Topography
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Thermal Stability
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EIS Data
Non-Porous
Porous
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29Si-NMR Notation
Species Notation Chemical shift  (ppm, 0.1)
R-Si-(OMe)3 -42.3
R-Si-(OMe)2OH -41.3
R-Si-(OMe)OH2 -40.6
R-Si-(OH)3 -40.1
R-Si-(OMe)2-OSi -49.9
R-Si-(OMe)OH-OSi -50.5
R-Si-(OH)2-OSi -49.3
R-Si-(OMe)-(OSi)2 -59.1
R-Si-(OH)-(OSi)2 -58.5
R-Si(OSi)3 -67.4
No. of OH groups
No. of SiO-Si linkages
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Following Sol-Gel formation with 29Si-NMR

• The products of condensation are found at values
  more negative than those of the alkoxides.

ppm
-40
-50
-70
-60
Condensation
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Following Sol-Gel formation with 29Si-NMR

• Example

• Example

• The presence of the Zr speeds up the
  condensation process

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29Si NMR Analysis
3-D Structure poorly forms
3-D Structure strongly forms
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Salt Spray Exposure

• Salt spray results plots for various sol gel
  coatings
• Bare AA2024 after 24 Hours
• MAPTMS for 48 hrs and
• MAPTMS/Zr/acac
• MAPTMS/Zr/DABA for 1 week

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Current Strategy in CREST

• Sol-gel coatings showing most promise involve
  Zirconium
• Improved pH resistance
• Functionalised surface improving topcoat adhesion
• Inhibitors work best at lower concentrations
• Novelty Incorporation Route Patent Filed Nov
  2007
• Inhibitors are bound in the network and dispersed
  uniformly

Zirconia nanoparticles
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Novel Inhibitor Incorporation (Patent Filed)
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Comparison of Organic Inhibitors (0.3 w/w)EIS
(Bode Plots)
Novel Inhibitor
1,2,4-Triazole
Imidazole
Benzotriazole
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Novel Inhibitor Route

• Novel organic corrosion inhibitor for aluminium
• Highly reducible species, based on Nitrogen
  chemistry
• Eo -1.2V
• Compatible with sol-gel chemistry and can be
  bound at specific sites
• pH triggered release with selective binding at
  copper rich intermetallics
• Inhibition of the electrochemical cell and
  corrosion process

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Sample Testing
Surface Treatment Primer Coat Topcoat
None just Al None Aerospace
Alodine 1200 Cr Primer
BSAA Cr-Free Primer
CAAA Dualion
Dualion Dualion (UV Active)
Dualion (UV Active)
Total 6 5 1
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Typical Thickness (lt5 mm)
25 mm Top Coat
20 mm Cr-Free Primer
3 mm Dualion
Aluminium
x 350
100 mm
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Salt Spray Data
1st Treatment 2nd Treatment Failure (Hrs) 1st Treatment 2nd Treatment Failure (Hrs)
Alodine None 300 Sol-Gels None 800
Alodine Sol-Gels 800 Sol-Gels Cr-free primer 800
Alodine Cr-free primer 1800 Sol-Gels Cr-free primer 1500
Alodine Cr primer 2000 Sol-Gels Cr primer 2000

1st Treatment 2nd Treatment Failure (Hrs) 1st Treatment 2nd Treatment Failure (Hrs)
BSAA None 800 CAA None 1800
BSAA Sol-Gels 1500 CAA Sol-Gels 1300
BSAA Cr-free primer 2000 CAA Cr-free primer 2000
BSAA Cr primer 2000 CAA Cr primer 2000
       
1st Treatment 2nd Treatment Failure (Hrs)   - Precommercial Product   - Precommercial Product   - Precommercial Product   - Precommercial Product
None None 300        
None Sol-Gels 800        
None Cr-free primer 300        
None Cr primer 500        
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Comparative Thicknesses of Coatings
AA 2024-T3
AA 2024-T3
AA 2024-T3
AA 2024-T3
AA 2024-T3
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Conclusion

• Sol-Gel chemistry is a viable option for
  protecting aerospace aluminium alloys
• Sol-gels can be designed to combine the
  properties of both a conversion coating and a
  primer
• Novel organic inhibitors can be engineered into
  such coatings thereby avoided heavy metal
  alternatives
• Thank you for your attention!