Sol-Gel Coatings used as Pre-Treatment for Aluminium Alloys

A.M. Cabral1, R. Duarte¹, I.M. Miranda Salvado2, M.G.S. Ferreira²,1
1 ICEMS/DEQ, Instituto Superior Técnico, 1049-001 Lisboa, Portugal
2 Department of Ceramics and Glass Engineering, University of Aveiro,
3810-193 Aveiro, Portugal

e-mail: [email protected]

The sol-gel process is a very useful way to deposit different coatings
on different substrates at room temperature in a simple and
reproducible manner and is a unique method for the preparation of
organic-inorganic hybrid coatings.

This type of coatings can be used as corrosion inhibiting promoter for
aluminium alloys and stainless steel, its quality depending on the
experimental conditions used to prepare the hybrid sols. The presence
of intermetallic precipitates, e.g. Al[2]Cu, Al[2]CuMg,
Al[20]Cu[2](MnFe)[3] , in AA2024-T3 alloys used in the aeronautical
industry, makes this type of material prone to localised corrosion.
Therefore, their use calls for the need to apply protection, which is
very often conferred by paints. However in order to have a good
adherence of the paint to the substrate, pre-treatments are necessary,
which traditionally are based in Cr VI-containing chemicals. Since
these products are highly pollutant and carcinogenic, urges their
replacement.

In this study silica-based hybrid coatings have been produced by the
sol-gel using the dip-coating process. Samples with composition
10ZrO[2].90SiO[2] mol% were prepared from polydimethylsiloxane
(PDMS), silanol terminated, with molecular weight equal to 4200. The
inorganic precursors were tetraethylorthosilicate (TEOS) and zirconium
propoxide. Solutions of TEOS and PDMS in isopropanol (volume ratio
1:1) were first prepared. The solutions were then thoroughly mixed by
means of magnetic stirring for 30 minutes at room temperature. After a
solution of zirconium propoxide, dissolved in ethylacetoacetate
(volume ratio of 1:1) was added. The final solution was used for the
dip-coating procedure of aluminium substrates.

Accordingly to results obtained by Electrochemical Impedance
Spectroscopy the coating confers poor protection to the AA2024 alloy,
since after 1 day of immersion in 3% NaCl solution pitting corrosion
can already be detected on the alloy. Different situation is obtained
when a paint layer (polyurethane enamel) is applied on the top of the
gel coating. In this case the system behaves very well after long
immersion time, showing values of impedance at low frequencies in the
order of 10⁸ ohm.cm2. Thus it seems that the siloxane is effective in
promoting the adhesion of the paint coating to the substrate, working
as a good coupling agent.

Further experiments are taking place where the porosity, in order to
obtain nanostructures, is controlled by changing the ratio
alcohol/water and pH. The inclusion of nanocrystals is also envisaged.

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