Corrosion of Al has to be counteracted by first anodising the Al parts and applying further protective coatings. During anodising, aluminium reacts with the electrolyte and a layer of aluminium oxide is formed on the surface of the aluminium specimen. This coating is highly porous and is subject to attack from the environment and corrosive elements. Therefore, anodised aluminium is normally further processed with a sealing as a final step after anodising. A hot water sealing process is one of the widely used methods. However in order to close (seal) the pores in the aluminium oxide anodised 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 anodised 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. Already about 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 in REACH compliant processes are preferred options. An adaption of the electrical TSA cycle for improved corrosion resistance without negative impact on fatigue life of components was developed. Detailed investigations and characterisation of the obtained corrosion protected surfaces via ESEM, Raman + IR-spectroscopy and ESCA were performed.