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Chromium free surface pre-treatments and sealing of Tartaric Sulphuric Anodizing (ChromFree)
Start date: Mar 1, 2013, End date: Jun 30, 2015 PROJECT  FINISHED 

Corrosion of Al has to be counteracted by first anodizing the Al parts and applying further protective coatings. Anodized aluminium is normally further processed with a sealing as a final step after anodizing. A hot water sealing process is one of the widely used methods. However in order to close (seal) the pores in the aluminium oxide anodized layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Cr(VI)-based sealing solutions have been employed for several decades, but remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized aluminium. Alternative sealing methods have also been proposed for example with Ni(II), Co(II), Ni(II) and Co(II), rare earth salts alkali metal fluorides, alkanolamine salts of phosphonic acids, Cr(III), fatty acids, silicates, etc. Kendig and Buchheit indicate that 45 of the 92 naturally occurring elements have been considered as replacements for Cr(VI) in conversion coatings on aluminium. In general these approaches have not been as successful as the Cr(VI) sealing. Also 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 CeO2.2 H2O on aluminium alloys in a few minutes at room temperature with or without catalyst, though the performances still do not equal those of CCC. Detailed investigations and characterization of the obtained will be performed. The optimized sealing and pre-treatments process will be applied to a flat test panel of 384 x 742 mm.
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