Hybride Sol-Gel Coatings for Protection of Aluminium Alloys
We have investigated a three component system for the preparation of corrosion resistant coatings on 7075-T6 aluminum alloy [3,4]. The sol-gel process allows the introduction of organic molecules into an inorganic network, forming hybrid organic-inorganic coatings, the so-called ormosils (organically modified silica) also known as silanes. Ormosils exhibit a combination of desirable properties of both inorganic (thermal stability, hardness, durability) and polymeric (toughness and flexibility) components within a single network. It was shown that the corrosion resistance of aluminum alloys has been improved by the deposition of two component hybrid coatings [5,6]. Further, combination of ormosil with zirconium-based additives (organically modified ceramics called ormocers) has initiated a new trend in the corrosion protective properties of hybrid sol-gel coatings [7,8].
In this work coatings were synthesized by alkoxide sol-gel reaction, using the organically modified siloxane precursors tetraethyl orthosilicate (TEOS) and 3-methacryloxypropyl trimethoxysilane (MAPTMS) (Sol 1), and zirconium tetrapropoxide (ZTP) chelated with methacrylic acid (MAA) (Sol 2). The synthesis was optimized using in situ FTIR spectroscopy. Optimized sols were spin-coated onto aluminum alloy AA7075-T6 substrate and the corrosion properties of deposited coating investigated in dilute Harrison’s solution optimized to simulate aircraft conditions in air. The establishment of Si-O-Si, and especially Si-O-Zr, bonds was shown to be crucial to achieve excellent corrosion protection. An optimal performance resulted in a current density two orders of magnitude lower than that for bare substrate. No susceptibility to localized corrosion was observed up to electrode potentials of 7 V. The coatings were up to 10 mm thick, homogeneous and dense.
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