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Corrosion protection of Aluminium unpainted parts: development of an appropriated Cr free sealing process on thin SAA layer (≤5 µm)

European Union
Complete with results
Geo-spatial type
Total project cost
€239 981
EU Contribution
€179 985
Project Acronym
STRIA Roadmaps
Vehicle design and manufacturing (VDM)
Transport mode
Airborne icon
Transport policies
Environmental/Emissions aspects,
Transport sectors
Passenger transport,
Freight transport


Call for proposal
Link to CORDIS

Corrosion of Al has to be counteracted by first anodising the Al parts and applying further protective coatings. During anodising, Al reacts with the electrolyte and a layer of aluminium oxide is formed, which is highly porous and is subject to corrosive attack. Therefore, anodised Al is normally further processed with a sealing as a final step. Sealed SAA industrial processes providing thicker layers (~10 µm) are already in the market, but the missing step is to develop a well-suited process for thin layers (≤ 5 µm) that meets the corrosion resistance requirements.

Hot water sealing is one of the widely used methods. However, in order to close (seal) the pores in the anodised layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Cr(VI)-based sealing solutions have been used for several decades, but remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized Al. Alternative sealing methods have also been proposed, e.g. with Ni(II), Co(II), Ni(II) + Co(II), rare earth salts, alkali metal fluorides, alkanolamine phosphonates, Cr(III), fatty acids, silicates, etc. It should be noted that Ni(II), Co(II) and fluorides are not without health implications, whereas most organic molecules would be expected to have limited lifetimes under the extreme conditions (UV radiation, low pressure, large temperature range) experience by commercial aircraft during operation.

Therefore, of the previously identified approaches Cr(III)-containing or silicate-forming sealing solutions are preferred options. Encouraging results were obtained with deposition of films of CeO2x2 H2O, though the performance still does not equal those of CCC. Characterisation with methods such as SEM, EIS, AFM-SECM led to more detailed understanding of sealing and corrosion mechanisms and therefore optimising sealing parameters with respect to corrosion resistance and minimised energy consumption.


Parent Programmes
Institution Type
Public institution
Institution Name
European Commission
Type of funding
Public (EU)
Specific funding programme
JTI-CS - Joint Technology Initiatives - Clean Sky
Other Programme
JTI-CS-2011-3-ECO-01-033 Corrosion protection of aluminium unpainted parts: development of an appropriated Cr free sealing


Executive Summary:

In the project anodized AA 2024 samples (sheet and machined) were conversion treated with a Sealing composed of a combination of two silanes or by using of different additives in the sealing bath based on Mn/Mo/V/Ce/Zr. The investigated variants provided appropriate corrosion protection after 750 h SST at least comparable to commercially available variants. An adapted version of the electrical SAA cycle for improved corrosion resistance has been developed and tested in the project as well with good and promising success.

The best results in SST (750 h-1176 h) with maximum 1 corrosion pit/dm² were obtained with a sealing at low temperature and few treatment times. The starting material influences (sheet and machined sample) as well as the pre-treatment influences (cleaning, etching etc.) were analysed and the results supported the better understanding of the promising sealing results obtained. Finally, the sealing parameters have been optimized with respect to improved corrosion resistance and minimized energy consumption during processing.



Lead Organisation
Cest Kompetenzzentrum Fur Elektrochemische Oberflachentechnologie Gmbh
EU Contribution
€179 985
Partner Organisations
EU Contribution


Technology Theme
Aircraft design and manufacturing
Alternatives to chromium coatings for corrosion protection for Magnesium-Aluminium alloys
Development phase

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