Advanced sol-gel coating for corrosion protective of aerospace aluminium alloys John Colreavy, Brendan Duffy, Rajath Varma CREST, Dublin Institute of Technology

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

Sol-Gel: History

Sol-Gel: Chemistry

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

Sol-Gel: Properties

P. Judenstein and C. Sanchez, J. Mater. Chem., 1996, 6, 511

Common Silane Precursors C. Sanchez et al., J. Mater. Chem., 2005, 15, 3559–3592

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.

Protective Coating Technology ORMOCERS® developed by FhG ISC Zheludkevich et al. J. Mater. Chem., 2005, 15, 5099–5111

Aerospace Coatings: Boe-Gel System US Patents: 5,869,141 5,939,197 6,077,885 Issue: Protective coating but no corrosion inhibition

Isobutyric Acid (IBA) Acetic Acid (AcOH)Methacrylic Acid (MAAH) Incorporating Organometallic Precursors = O = C = H = Zr = N Acid ligand Zr (OPr) 4 Zr(OPr) 2 (ligand) + Acetylacetone (Acac) 2,2’ Bipyridyl (BP) HydrolysableProtected

MAPTMS/Zr (AcOH)MAPTMS/Zr (MAAH)MAPTMS/Zr (IBA) MAPTMS/Zr (Acac) MAPTMS/Zr (BP) MAPTMS ( 48 Hrs) Corrosion protection: MAAH=AcOH>IBA>Acac > BP > no Zr Neutral Salt Spray Study ( 1 week)

Incorporating Amine Chemistry  -Diketone (acac) Amine rich carboxylic acid

Surface Topography

Thermal Stability

EIS Data Porous Non-Porous

29 Si-NMR Notation SpeciesNotationChemical shift (ppm, ±0.1) R-Si-(OMe) R-Si-(OMe) 2 OH-41.3 R-Si-(OMe)OH R-Si-(OH) 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) R-Si-(OH)-(OSi) R-Si(OSi) No. of OH groups No. of Si–O-Si linkages =

Following Sol-Gel formation with 29 Si-NMR The products of condensation are found at values more negative than those of the alkoxides. Condensation -40 ppm

Following Sol-Gel formation with 29 Si-NMR Example: The presence of the Zr speeds up the condensation process

29 Si NMR Analysis 3-D Structure poorly forms 3-D Structure strongly forms

Salt Spray Exposure Salt spray results plots for various sol gel coatings: (A)Bare AA2024 after 24 Hours (B)MAPTMS for 48 hrs and (C)MAPTMS/Zr/acac (D) MAPTMS/Zr/DABA for 1 week

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 -O-Si-O- O R Zirconia nanoparticles

Novel Inhibitor Incorporation (Patent Filed)

Comparison of Organic Inhibitors (0.3% w/w) EIS (Bode Plots) Novel Inhibitor1,2,4-Triazole Benzotriazole Imidazole

Novel Inhibitor Route Novel organic corrosion inhibitor for aluminium Highly reducible species, based on Nitrogen chemistry – E o = -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

Sample Testing Surface TreatmentPrimer CoatTopcoat None – just AlNoneAerospace Alodine 1200Cr Primer BSAACr-Free Primer CAAADualion Dualion (UV Active) Total: 6 5 1

Typical Thickness (<5  m) 3  m Dualion 20  m Cr-Free Primer 25  m Top Coat Aluminium x  m

Salt Spray Data 1 st Treatment2 nd TreatmentFailure (Hrs)1 st Treatment2 nd Treatment Failure (Hrs) AlodineNone300Sol-GelsNone 800 AlodineSol-Gels800Sol-GelsCr-free primer 800 AlodineCr-free primer1800Sol-GelsCr-free primer* 1500 AlodineCr primer2000Sol-GelsCr primer st Treatment2 nd TreatmentFailure (Hrs)1 st Treatment2 nd Treatment Failure (Hrs) BSAANone800CAANone 1800 BSAASol-Gels1500CAASol-Gels 1300 BSAACr-free primer2000CAACr-free primer 2000 BSAACr primer2000CAACr primer st Treatment2 nd TreatmentFailure (Hrs) * - Precommercial Product None 300 NoneSol-Gels800 NoneCr-free primer300 NoneCr primer500

Comparative Thicknesses of Coatings AA 2024-T3

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!