Cr(VI)-free surface treatments for aluminium alloys

Resumen

The most critical challenge in the aircraft industry is to reduce the operating costs and increase the aircraft¿s payload during the whole life of the product. This can be achieved by decreasing the fuel consumption, hence by decreasing the material density. For that, the Al-Cu system has been the most widely used structural material for commercial aircrafts due to its unique high strength-to-weight ratio. Nevertheless, Al-Cu alloys are usually surface treated with Cr(VI)-based compounds, which are categorized as highly toxic and carcinogenic. On that basis, the development of novel Cr-free surface treatments before 2024 has been targeted as the main challenge for aluminium in the aircraft industry for painted and non-painted components. The literature review carried out in the Introduction summarizes the specificities of Cr(VI) replacement for Al-Cu alloys and the suitability of additive manufacturing (AM) processing method for Al-Si alloys. The Objective of this Thesis is to address the most relevant challenges for aluminium in the aircraft industry in terms of protective surface treatments. Chapter 1 evaluates the corrosion resistance of anodizing surface treatments applied on a commercial Al-Cu alloy (AA2024-T3) for corrosion- and fatigue-sensitive applications. For painted components, the in situ incorporation of cerium species into conventional sulphuric acid anodizing (SAA) and tartaric-sulphuric acid anodizing (TSA) was evaluated in terms of morphology, corrosion resistance, fatigue strength and paint adhesion. Ce-containing SAA films were comparable to chromic acid anodizing (CAA) films in the abovementioned factors. For non-painted components, the in situ incorporation of citric acid in SAA electrolyte revealed a clear corrosion improvement and was further selected for Li- and Ce- loaded hybrid sol-gel (HSG) sealing post-treatments. The Ce-loaded HSG coating showed the best self-healing properties. In Chapter 2, ¿flash¿ plasma electrolytic oxidation (flash-PEO) is explored as an energy- efficient and eco-friendly strategy for painted and non-painted components. For that purpose, a preliminary study on commercially pure aluminium (AA1050-H18 alloy) was performed to study the main variables involved in the flash-PEO coating formation and energy consumption. Then, the in situ incorporation of corrosion inhibitors in the polyphosphate- based electrolyte was evaluated on AA2024-T3 alloy for painted components. Ce/EDTA- containing coating yield the best paint adhesion and the highest long-term corrosion resistance among all the evaluated coatings. For non-painted components, LDH and lauric acid sealing strategies were evaluated in a selected flash-PEO coating on the AA2024-T3 alloy. The lauric acid sealing for non-painted components showed the best long-term corrosion performance. In Chapter 3, LDH coatings formed by in situ growth method on AA2024-T3 alloy are investigated as a new Cr-free conversion coating alternative for painted components. Among the studied formulations, Zn-Al-LDH and Ca-Al-LDH coatings were selected for further inhibitor-loading stage due to their optimal surface appearance and corrosion resistance-to- thickness ratio. The intercalation of Mn into the Ca-Al-LDH structure was successful, ensuring enhanced long-term corrosion resistance and excellent paint adhesion.Chapter 4 was focused on the characterization and tribological evaluation of hard anodizing (HA) and PEO coatings on an Al10SiMg alloy obtained via AM for wear-sensitive applications. A361 cast alloy was also used for comparison. Findings revealed that the formation of crystalline SiO2 reduced the hardness and increased the wear rates of the AM- PEO specimens in comparison to the A361-PEO coating.