Abstract
Cr(VI) based conversion coatings are currently the treatments of choice for aluminium alloys to prevent corrosion, and are widely used in the aerospace industry also because of their good electrical conductivity and because they are good primers for paints and adhesives. Hexavalent chromium though is harmful for humans and for the environment, thus it needs to be replaced with more environmentally friendly materials. In this work three alternative pre-treatments for aluminium alloys were proposed and their properties were investigated and compared with the performance of a Cr(VI) based treatment. The selected "green" alternatives are based on titanium and zirconium compounds and they were applied to three different aluminium alloys relevant for spacecraft applications: A12219, A17075 and A15083. After the characterization of the chosen materials by means of SEM, AES, XPS, EDX and SAM, some of their surface properties were explored: the adsorption of an epoxy acrylate resin used for UV-cured coatings, and the stability under UV and thermal exposure. The outcome of this preliminary investigation provided the basis for a further selection of materials to use in a corrosion study, and A12219 was chosen as a substrate, together with an hybrid (organic/inorganic) coating, Nabutan STI/310. Alodine 1200S was proposed as chromate treatment and used as reference. A comparison of the behaviour during the exposure to a corrosive environment, as a NaCl solution, was made between the untreated A12219 alloy, and the alloy treated with Nabutan STI/310 and Alodine 1200S. The study was focused on the chemistry in the vicinity of second phase precipitates, which proved to represent initiation sites for corrosion, due to their cathodic nature. The evolution of corrosion reactions was monitored after different times of exposure relocating each time the same intermetallic, by means of SEM, AES, EDX and SAM. The results showed the progressive dissolution of the alloy matrix around the inclusion, and a partial dissolution of the precipitate itself. The inclusions were shown to be inhomogeneous in composition, thus generating cathodic and anodic sites at the intermetallic. The chromate treatment resulted in higher protection, compared to the hybrid alternative.