For decades, the improvement of corrosion resistance and paint adhesion of aluminium alloys was achieved through Cr(VI)-based surface treatments like anodization, followed by impregnation and sealing of the porous alumina (Al₂O₃) layer. Nowadays the use of Cr(VI) is prohibited due to its high toxicity [1] and it must be replaced by less harmful compounds. Tartaric/sulfuric acid (TSA) anodizing has been proposed as an alternative for chromic acid anodization, which offers good corrosion resistance and low environmental impact [2]. Among numerous Cr(VI)-free sealants for TSA layers, silicate-based formulation have been proposed [3]. Despite promising results of the industrial tests, the role of silicate sealing composition on the microstructure and immersion stability of the anodized layer is not completely known. In particular, it is unclear if the thermo-mechanical treatments, which strongly impact the distribution of intermetallic particles (IMs), could modify the performance of sealants on the anodized alloy.

     In the present work, the microstructure and immersion stability of silicate-sealed TSA anodized Al alloys of 7xxx series were compared for machined and rolled substrate. Scanning electron microscopy (SEM) and electron dispersion spectroscopy (EDS) results showed very similar initial microstructure of all sealed surfaces. New accelerated ageing procedure, based on the AC-DC-AC test, was developed to characterize the stability of the sealed layer [4] and it was applied to different sealed substrates. The results demonstrated the importance of processing, rolling or machining, on the anticorrosion performance of the sealants. Electrochemical impedance spectroscopy (EIS) demonstrated an improved corrosion resistance for all the sealed samples. During ageing cycle, live imaging demonstrated formation of black spots on the surface, evidencing localized corrosion attack. The latter was confirmed by SEM-EDS and optical microscopy. Frequency and severity of the attack was compared with the distribution of the IM particles in the substrate alloy. The role of the chemical composition, the processing and the microstructure of the anodized and sealed layer on its stability are discussed.

[1] https://echa.europa.eu/fr/substance-information/-/substanceinfo/100.239.176

[2] Curioni, M. et al., J. Electrochem. Soc. (2009) https://doi.org/10.1149/1.3077602

[3] Ofoegbu, S. et al., Coatings (2020) https://doi.org/10.3390/coatings10030226

[4] Usman, B.J et al., Prog. Org. Coat. (2020) https://doi.org/10.1016/j.porgcoat.2020.105648