Progress in eco-friendly aerospace coatings: the future beyond chromates

Aluminium alloys have a good strength-to-weight ratio and low density, but they are not impervious to corrosion. Therefore, it is necessary to evaluate their corrosion performance in the atmospheric environment. Paints are typically used to prevent corrosion on aluminium alloys. A conversion or anodized layer, an inhibitor/metal-doped primer, and a polyurethane topcoat are used as 3-layer coatings for aluminium alloy corrosion protection in the aerospace industry. However, major modi­fications have been made to the protection of aluminium alloys against corrosion as a result of environmental issues. REACH, a European regulation, has significantly altered some finishing procedures for aluminium alloys by prohibiting or restricting the use of Cr(VI)-based compounds, such as chromate conversion, chromic acid anodizing, and chromate sealing. Searching for alternatives to Cr has yielded intriguing results. For many years, sol-gel-based hybrid coatings have been a promising alternative to pretreatment and primer layers, with promising results. This review discusses the current development and advancement in chrome-free corrosion-resistant layer coating systems for aluminium alloys.