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 were performed. The optimised sealing and pre-treatments process were applied to a flat test panel of 384 x 742 mm.
State of the Art new anodising technologies have been developed as alternatives of CAA and applied on conventional structural Al alloys, joined by riveting. These technologies were Chromium free regarding the compounds of Anodisation bath but include Chromium on the surface preparation and sealing steps.
The gaps that ChromeFree CfP covered can be summarised on the following points:
- Development of a TSAA technology fully free of Chromium. This development has been based on previously existing TSAA technology involving hexavalent Chromium on surface preparation steps as well as on sealing steps. The new ChromeFree technology eliminates the use of hexavalent and/or trivalent Chromium from all the steps of anodizing.
- The new technology applied on new lighter structural Al alloys (Al-Li 2198/2196) for airframe parts with improved mechanical properties in comparison to the conventional Al alloys (2024/7075).
- Application of the new TSAA technology on Laser Beam Welded structures instead of conventional heavier riveted structures.
- The new technology applied and tested on demonstrators resulting in the upgrading of TRL. After ChromeFree project, the achieved TRL is 5.
In the scope of the Project ChromFree, a new REACH-compliant TSAA process (Tartaric Sulphuric Acid Anodization) was developed, which uses no chromium compounds in pre-treatment, anodisation and sealing steps, and thus gives health and environmental benefits and reduces costs for disposal of chemical waste and recycling costs. The quality characteristics are equal to chromic acid anodised parts and in accordance with quality requirements as established by AIRBUS (AIPS 02-01-003). Thus the outcome of the project provided a process which grants equal properties such as corrosion protection as state-of-the-art Cr(VI)-based processes. As it was almost mature, it was soon be implemented in aircraft production lines and reduce their ecological footprint.