Sol-gel based organic-inorganic hybrid coatings for corrosion protection of aerospace aluminium alloy

Abstract

Aluminium metal is known for its high electrical conductivity, thermal conductivity and high reflectivity. Aluminium alloys of 2XXX series, particularly the 2024 alloy with the quaternary Al–Cu–Mg–Mn composition are widely used in aeronautical applications due to their high strength along with low density and good fracture toughness. It is the S-phase (Al2CuMg) precipitates which are responsible for strengthening the metal. Preferential dissolution of Al and Mg from the intermetallics and formation of copper-enriched particles have been proposed to be the major reasons for corrosion. Aluminium oxide film formed on the surface is stable only in neutral aqueous media (pH = 4–9) and hence does not protect the substrate under alkaline corrosive conditions. Therefore, aluminium components are often subjected to surface treatment to increase their durability and reduce maintenance needs. A typical aircraft coating system generally consists of three layers comprising a thin passivating base layer, an inhibitor containing primer layer, and a thick topcoat. The first layer generally referred to, as pre-treatment layer or conversion coating (∼2–3 μm) provides corrosion protection and improves adhesion of top layers with the substrate. Conventionally, chromate conversion coatings are used for this purpose. However, due to the toxicity and carcinogenic nature of hexavalent chromium, they are banned from usage and environmentally benign alternatives are being explored. In this direction, sol–gel based coatings are highly promising for corrosion protection of metals and alloys. The present thesis is directed towards the development of sol-gel coatings that exhibit improved corrosion protection properties and a low environmental impact. In particular, the present study aims to investigate the behaviour of the less-explored silica-alumina hybrid coating system as sol-gel pre-treatment layers for AA2024. Silica-alumina hybrid coatings in optimized precursor ratio possess good barrier property (Fig. 1). Coatings with different concentrations of cerium nitrate are assessed for corrosion inhibition property using electrochemical tests (Fig. 2). The coatings with 5 mM inhibitor act as good reservoirs for cerium (III) ions and provide enhanced protection to the substrate surface even after 336 h. of 3.5 % NaCl immersion. The protection offered is attributed to a combined effect of barrier property and the cathodic inhibition offered by Ce3+. XPS analysis provides strong evidence for the migration of cerium ions towards the corrosive sites. The observed corrosion protection is attributed to the synergistic effect of the stable barrier nature of the coating and the corrosion inhibiting nature of Ce3+ ions