Development of Hybrid Sol-Gel Coatings on AA2024-T3 with Environmentally Benign Corrosion Inhibitors

Aluminium alloys are still considered as one of the primary light alloys with high strength that can be used in aerospace and marine structure with a moderate economic cost. However, aluminium alloys are affected by atmospheric and marine corrosion which reduces their overall reliability. The application of coatings is one of the strategies for mitigating the corrosion on aluminium alloys. Sol-gel technology is one of the coating strategies with potential for excellent chemical and environment stability, providing eco-friendly performance improvement. Sol-gel coatings can enhance corrosion resistance by adopting many corrosion inhibitors that can protect aluminium alloys substrates from corrosion. In this project, three approaches are developed and studied to protect AA 2024 in high salinity of 3.5% NaCl environment. The first approach involved studying the direct application of film-forming environmentally benign corrosion inhibitors on the surface of the substrate without any involvement of the coating, these inhibitors are benzimidazole (BZI) and oleic acid (OA), and were studied alone and in combination. The second approach, investigated the corrosion protection of two sets of novel hybrid organic-inorganic sol-gel derived coatings using low-temperature cure processes at 80°C. The base sol-gel formula was made using alkoxide silane-based precursors including; tetraethylorthosilicate silane (TEOS) and trimethoxymethyl silane (MTMS) and this was labelled as (SBX- 80). The other enhanced formula was developed by adding the fluorinated precursor, 1H.1H.2H.2Hperfluorodecyltriethoxy silane (PFOTS), to the base SBX-80 formula to synthesis the fluorinated-sol-gel formula and this was labelled as (F-SBX-80). The third approach investigated the addition of corrosion inhibitors of benzimidazole (BZI) and oleic acid (OA) to the base SBX formula to increase the corrosion protection, either alone or in combination. All sol-gel derived coating systems, including those modified with BZI or OA corrosion inhibitors exhibited excellent corrosion protection as evidenced visually, or by electrochemical methods with various levels of stability when immersed in saline solution. The coatings exhibited high capacitance and resistance that might provide active/barrier corrosion protection at small thicknesses less than 20 μm. Furthermore, there was an enhancement on mechanical properties of coating films with insignificant effect on the adhesion to the aluminium alloy substrate combined with excellent cracking-resistance. There was also a significant increase in surface water contact angle for these systems, which indicates the potential for enhanced easy-cleaning features.