ALTERNATIVE SURFACE TREATMENTS WITHOUT CHROMIUM CONTENT IN AERONAUTICAL ALUMINIUM ALLOYS Tartaric sulphuric anodising

Aluminium and its alloys are widely used in the aviation sector due to their lightweight and their high specific strength. They require surface treatments in order to improve their corrosion resistance, such as anodising which has traditionally carried out using a chromic acid bath. Although chromium has been regularly used in industry for more than a century, progressively restrictions and rigorous controls have been imposed because of environmental and health concerns. The objective of this thesis is to study the tartaric sulphuric anodising as an alternative anodising process without hexavalent chromium. The tests were performed on three aluminium alloys AA2024-T3, AA7175T7351 and AA6061-T6. It was analysed the coating thickness, the coating weight, the corrosion resistance and the loss of the absorption power achieved by the anodic film with excellent results. Also, it was studied the compatibility of the anodic coating with various current post-treatments. It was tested three different sealings: with hot water, with potassium dichromate and with the chemical conversion coating Bonderite® M-CR 1200S Aero. On samples without sealing, it was applied an epoxy primer and a polyurethane topcoat in use by Airbus. It was demonstrated the compatibility of all of them with the anodic film, except in the case of the Bonderite® M-CR 1200S Aero on the AA2024T3 which did not pass the corrosion test. Finally, it was analysed the performance of a new nonchromate product for the acid etching process. The alternative product studied, as a substitute of the current bath with sodium dichromate, was the Bonderite® Smutgo NCB that achieved successful results.