Effect of sol-gel sealing on the performance of tartaric-sulphuric anodizing layers formed on clad and bare AA2024

AA2024 T3 aluminium alloy is widely used in the aircraft industry due to its high specific strength and light weight. High mechanical properties are due to enrichment of aluminium matrix in copper which reduces the corrosion protection by forming microscopic galvanic couples. In order to improve its corrosion resistance, AA2024 is protected by cladding pure aluminium layer and/or anodizing. The anodizing traditionally performed in chromic acid bath and sealed in aqueous potassium dichromate solution provides layers affording excellent corrosion protection. Nevertheless, because of toxicological and environmental concerns related to CrIV use, it will be forbidden for aeronautical applications in 2017. New alternatives, such as tartaric-sulphuric acid anodizing (TSA), have already been used at an industrial level.
Even if this process gives good corrosion performance, studies need to be continued in order to bring tangible responses to some industrial interrogations.
One of investigated problems is the formation of a new anodizing layer after a stripping operation. Stripping is a process used in aeronautical applications to remove the formed anodized layer and the primer organic coating when the surface treatment presents defects. This treatment is done to recover the parts.
The second objective is the investigation of the sol-gel route as an alternative to boiled water sealing in order to reduce the process time, the energy consumption and to improve the compatibility with organic coatings.
In this work, the effects of the stripping by sand-blasting and the sol-gel sealing on TSA anodizing layers are studied on clad and bare AA2024 T3.
Samples were anodized with different bath compositions by changing the content and ratio in sulphuric and tartaric acids and maintaining the same temperature, time and voltage. The sealing was carried out by dip coating in an aqueous solution composed of three siloxane precursors: tetraethoxysilane (TEOS), (3-glycidoxylpropyl)trimetoxysilane (GPTMS) and methytriethoxysilane (MTES).
The morphology of the samples is studied by SEM-FEG characterization of top surface and cross section. The sealing is estimated by glow discharge optical emission spectrometry (GDOES) profiles and the corrosion performance was determined by means of electrochemical impedance spectroscopy (EIS) in sodium chloride solution for the different investigated conditions.