Ageing effect of TEOS-GPTMS hybrid sol-gel on the corrosion protection of aluminum alloy AA2524 anodized in tartaric-sulfuric acid (TSA)

摘要

To be employed in aircrafts, high strength Al alloys need to be protected from corrosion by a robust protection system One of the steps applied in the protection protocol is anodizing, which produces a thin barrier layer and a thick porous layer, providing both corrosion resistance and a base for applying protective organic coatings. One very promising approach for sealing of the porous anodized layer without losing its adhesion properties is its treatment with a hybrid sol-gel coating. This type of coating, which chemistry is based on the formation of a silicon-oxygen network by hydrolysis and condensation reactions of alkoxysilane precursors, has demonstrated to be an efficient alternative for the replacement of the chromate technology with commercial and scale-up potentials. Before application, silanes need to be hydrolyzed in order to form sufficient silanol (Si-OH) groups to interact with the metal substrate (Al-OH). With a maximum hydrolysis time, a well-structured film is formed. In other hand, with solution ageing, the silane-metal interaction can decreases due to formation of condensed species (Si-O-Si groups), and consequently, insufficient number of Si-OH groups in the solution, resulting in less anchoring Al-O-Si bonds and poor corrosion protection. The knowledge of hydrolysis time of a specific sol-gel solution is of high importance, once the corrosion protection has a strong dependence on the degree of hydrolysis and condensation of the molecules in the solution; moreover, this is an issue of utmost importance for industrial scale-up of the process. In this work, the corrosion resistance of aluminum alloy 2524 anodized in tartaric -sulphuric acid (TSA) bath at 14 V and protected with TEOS-GPTMS sol-gel coating obtained from hydrolysis solutions aged for 2 h, 24 h, 72 h, 168 h and 336 h was evaluated in 0.1 M NaCl by EIS with immersion time. The sol-gel coating was prepared using a solution with high water content (58 %v/v). The stability of the hydrolysis sol-gel solution was investigated by means of Fourier Transform Infrared (FTIR) and by rheology determination. The results showed excellent corrosion protection properties of the sol-gel coating even after 336 h of ageing of the hydrolysis solution.