MDPI – Metals
Hydrophobic and corrosion behavior of sol-gel hybrid coatings based on the combination of TiO2 NPs and fluorinated chains for aluminum alloys protection


In this work, layers of a sol-gel hybrid matrix doped with metal oxide nanoparticles (TiO2 NPs) have been deposited on flat samples of AA6061-T6 aluminum alloy using the dip-coating technique, with the aim of obtaining coatings with better anti-corrosive and hydrophobic properties.

Two different organic modified silica alkoxides, namely 3- (glycidyloxypropyl)trimethoxysilane (GPTMS) and methyltriethoxysilane (MTEOS), have been used for an adequate entrapment of the metal oxide nanoparticles. In addition, a fluorinated metal-alkoxide precursor has also been added to
the hybrid matrix in order to improve the hydrophobic behavior. The experimental results corroborate that the presence of these TiO2 NPs play an important role in the development of the sol-gel hybrid coatings. The water contact angle (WCA) measurements, as well as pencil hardness tests indicate that TiO2 NPs make a considerable increase in the resultant hydrophobicity possible, with better mechanical properties of the coatings. The coating thickness has been measured by cross-section scanning electron microscopy (SEM). In addition, a glow discharge optical emission spectroscopy (GD-OES) analysis has been carried out in order to corroborate the adequate entrapment of the TiO2 NPs into the sol-gel coatings. Finally, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) have been performed in order to evaluate the corrosion resistance of the coatings. All the results provide insights into the efficacy of the developed sol-gel hybrid coatings for anticorrosive purposes with good mechanical properties.

Deposition Process of the Coatings

The different steps for the fabrication of the hybrid sol-gel matrix (MTEOS and GPTMS), as well as the chemical structures of the alkoxide precursors, are summarized in Figure 1. The MTEOS-based sol (sol_1) was prepared by mixing MTEOS, 0.1 M aqueous HCl, and EtOH in a 1:0.007:6.25 molar ratio, with a specific ageing time of 5 days. The GPTMS-based sol (sol_2) was prepared by mixing GPTMS, 0.1 M aqueous HCl, and EtOH in a 1:4:6.25 molar ratio, with an aging time of 1 day. Then the GPTMS-MTEOS hybrid sol was prepared by combining the two separate sols in a desired molar ratio of 3:1. In the following step, TiO2 NPs were added to this hybrid sol-gel solution in order to have a final silane-TiO2 NPs ratio of 10−3. The resultant mixture of the TiO2 NPs doped sol-gel hybrid matrix was aged for a period of 3 days under ambient conditions. In order to increase the hydrophobic property of the hybrid sol-gel matrix composed of TiO2 NPs, a third metal-alkoxide precursor based on fluorinated chains (PFAS) was also prepared [42–45]. The PFAS-based sol (sol_3) was prepared by mixing PFAS, 0.1 M aqueous HCl, and EtOH in a 1:5:4.25 molar ratio, with an aging time of 1 day.
Finally, a ND-R rotatory dip coater provided by Nadetech Inc. (Pamplona, Spain) was used for the fabrication of the coatings with a dip speed (immersion and withdrawal) of 100 mm/min, and an immersion time of 300 s for each specific dip.



Keywords: Sol-gel hybrid matrix; TiO2 NPs; Corrosion resistance; Hydrophobicity