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

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.