Influence of pickling process on Al2024 and 6061 for subsequence process

Aluminium alloys are used in aerospace field due to their good properties of strength, weight-to-strength ratio and cost. On the other hand, they manifest weak corrosion resistance. Therefore, they need a strong corrosion protection treatment which enables them to resist at the aggressive work-field environment. Usually, the protection system consists of a multi-layer system, with oxide layer on the surface, a corrosion inhibitor primer and a topcoat. The materials analysed in this work were Aluminium 2024 and 6061. The first one was both anodised and chromate converted while the second one only chromate. The aim of this study is to analyse the acid pickling process and its parameters, namely acid concentration, temperature of bath and immersion time, on the subsequent processes of chromic acid anodising and chromate conversion coating (alodine 1200S). In order to do so, several samples were tested at Leonardo laboratory, in Caselle plant, with different pickling condition but same post-treatments. The results obtained were analysed at Polytechnic laboratory with SEM and inverted optical microscope to evaluate artificial oxide thickness formed during the process. Afterwards, the images just obtained were post-processed with ImageJ software to understand the role of pickling parameters on pitting and intermetallic particles removal. The results agreed for the two samples, showing for different pickling conditions different results. This proved the pickling process to be crucial for sub-sequent corrosion protection system. A second goal was to understand the native oxide thickness of aluminum alloy 6061 and the time needed to form. After literature research, one pickled sample was analysed at XPS, first after pickling, then after two days of exposure at room environment. The results show a small different of the two analyses, but in the first one the oxide was already present and growth. This means the alumina oxide is formed on surface after a few seconds, in agreement with literature.