Hybride Sol-Gel Coatings for Protection of Aluminium Alloys

Abstract

Aluminum and its alloys are used in a broad spectrum of applications on the basis of their physical characteristics, such as stiffness, low density and high strength-to-weight ratio [1]. While the physical properties of aluminum metal are improved by alloying, its corrosion resistance is reduced. Aluminum alloys are highly susceptible to oxidation and pitting corrosion in aggressive environments. Serial 7xxx wrought alloys contain major additions of zinc, along with magnesium or magnesium and copper in such ratios as to increase their strength [1]. Of the various approaches that have been employed as alternatives to CCCs ‒ electro-polymerization, anodization, conversion coatings and sol-gel synthesis [2] ‒ the last mentioned is considered to be the most promising. It can be applied to a substrate by different methods, most commonly spinning, dipping or spraying.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.