Durable corrosion protection of AA2024-T3 using hybrid sol-gel coatings modified with various fluorinated (meth) acrylates

Aluminum alloy (AA) 2024-T3 is commonly used in the aerospace industry due to its lightweight, high mechanical strength, and low cost. However, this alloy is highly susceptible to corrosion in chloride-containing solutions due to alloying elements such as Cu, Fe, Mn, etc. In the past, chromium (VI) compounds have been used for alloy protection against corrosion. These compounds have been strictly regulated in various countries due to their harmful effects on humans and the environment. Among possible environmentally acceptable surface treatments, the sol-gel coating is considered as an alternative. The sol-gel chemistry is based on the hydrolysis and condensation reactions of initial organically modified silica and silane reagents and also the copolymerization reactions between functional groups such as acrylates present in the reaction solution.

In this study, the corrosion performance was studied for uncoated and coated AA2024-T3. The synthesis of hybrid sol-gel was performed using 3-(trimethoxysilyl)propyl methacrylate (MAPTMS), methyl methacrylate (MMA), tetraethyl orthosilicate (TEOS) [1,2], and the addition of various fluorinated (meth)acrylates with different chain length.

The reactions taking place during preparation were characterized using real-time Fourier transform infrared spectroscopy. The solution characteristics were evaluated by measurements of thermal stability determined by thermogravimetric analysis. The optimal temperature for the condensation reaction was determined with the help of high-pressure differential scanning calorimetry. Once deposited on 2024-T3 substrates, the coatings were evaluated using a field emission scanning electron microscope coupled to an energy dispersive spectrometer to determine surface morphology, topography, composition, and coating thickness. The corrosion characterization was performed in 0.1 M NaCl using electrochemical impedance spectra (EIS) and salt spray test according to the ASTM B117-07A standard.

The results revealed the importance of the copolymerization process of added fluorinated (meth)acrylates and hydrolysis and condensation reactions to obtain a coating with low-porosity with an absence of cracks. Prepared coatings present durable barrier corrosion protection of the AA2024-T3 (EIS remain unchanged for more than one year).

References:

[1] Rodič, P.; Korošec, R.C.; Kapun, B.; Mertelj, A.; Milošev, I. Acrylate-Based Hybrid Sol-Gel Coating for Corrosion Protection of AA7075-T6 in Aircraft Applications: The Effect of Copolymerization Time. Polymers 2020, 12, 948. https://doi.org/10.3390/polym12040948

[2] Rodič, P.; Lekka, M.; Andreatta, F.; Fedrizzi L.; Milošev, I. The effect of copolymerisation on the performance of acrylate-based hybrid sol-gel coating for corrosion protection of AA2024-T3, Progress in organic coatings, 2020, 147. https://10.1016/j.porgcoat.2020.105701