Innovative coatings for corrosion protection of historical aluminium alloys in aerospace heritage

The conservation of historical aluminium aircraft exposed to outdoor conditions requires protective systems combining corrosion mitigation, aesthetic compatibility and chemical stability. Within the PROCRAFT project two advanced coatings, namely, a bio-based cutin-derived polyurethane and a 3-mercaptopropyl trimethoxysilane (PropS–SH)–based system, were comparatively evaluated against the benchmark Paraloid B72 (PB72) on historic corroded and painted Duralumin substrates dating from World War II and the immediate post-war period. Coupons were subjected to an accelerated ageing protocol including simulated acid rain runoff, followed by thermo-hygrometric and UV cycling. Protective performance was assessed through a multi-analytical approach integrating electrochemical tests, colour analysis, SEM-EDS, ATR-FTIR, μ-Raman, and metal release quantification by MP-AES. On corroded substrates, both innovative coatings exhibited higher polarization resistance than PB72 and values two orders of magnitude greater than those of uncoated surfaces after ageing. PropS-SH provided inhibition efficiencies ≥95% in terms of Al release, while the cutin-based coating achieved inhibition efficiencies >85%, outperforming PB72 under the same conditions. On painted substrates (already intrinsically protective) PropS-SH produced up to a one-order-of-magnitude increase in polarization resistance compared to the original paint system, whereas PB72 provided only marginal improvements. Spectroscopic analysis after ageing revealed progressive siloxane network densification in PropS-SH and, in the cutin-based coating, continued crosslinking accompanied by UV-mitigating behaviour attributable to residual natural antioxidants. Overall, both coatings provided effective corrosion protection on painted and corroded substrates, with PropS-SH exhibiting superior performance in suppressing metal release and enhancing polarization resistance, while maintaining good aesthetic stability throughout the artificial ageing process.