Surface Green Technologies for Aeronautic Industry

Chromic acid anodizing has been used for almost a century to enhance corrosion protection of aerospace alloys. For some applications, hydrothermal sealing in hexavalent chromium-containing solution is required to enhance further the corrosion resistance but, due to environmental concerns, the use of hexavalent chromium must be discontinued. Good progress has been made to replace chromates during anodizing but comparatively less effort has focused on the sealing process. In this work, porous anodic oxides were produced by traditional and modified tartaric sulfuric anodizing (TSA) processes and sealed in hot water, chromate and cerium based solutions. The sealing behaviour of a film with relatively coarse porosity, generated at high voltage (traditional TSA), was compared to the sealing behaviour of a film with finer porosity and generated at reduced potential (modified TSA). After sodium chromate sealing, the two anodizing cycles produced film with similar anticorrosion performance. Conversely, after hot water or cerium sealing, the finer oxides generated at low voltage (modified TSA) provided much better corrosion resistance. EIS performed in-situ during sealing revealed that chromate sealing is very aggressive to the porous skeleton compared to the other sealing treatments. Therefore, the original morphology has little effect on the final performance, since both fine and coarse oxides are substantially attacked. In contrast, the oxide morphology has a substantial effect when sealing is performed in hot water or cerium-based solution. Overall, it is possible to obtain films with anticorrosion performance equivalent or improved compared to that obtained by traditional TSA anodizing cycle sealed with chromate by combining the low voltage anodizing cycle with the cerium-based sealing. This thesis focuses also on the trivalent chromium pre-treatment (TCP) and its commercially available variants. The rationale for the focus of this paper is that in the near term, the aeronautics industry needs to move away from Cr(VI) towards a more benign commercially available chemical treatment that can help protect the aluminium alloys used by that industry. TCPs are currently available commercially and have undergone numerous tests by multiple organizations to establish their effectiveness in reducing corrosion of both bare and painted aluminium alloys.