Self-Assembled Urchin-like Micro/Nanostructures with a Durable Hydrophobic Barrier and Long-Term Corrosion Protection for Cultural Heritage

Abstract

Durable and visually compatible protective coatings are crucial for enhancing the durability of cultural heritage materials under complex environmental conditions. In this study, fluorinated waterborne polyurethane (F-WPU) and fluorinated Paraloid B72 (F–B72) coatings were prepared via fluorosilane modification and applied to bronze as well as representative wooden substrates (pine wood and red sandalwood). On bronze surfaces, substrate-assisted interfacial regulation coupled with fluorosilane-induced molecular organization promoted the formation of hierarchical particle-aggregate structures, which, combined with low-surface-energy fluorinated groups, enhanced surface hydrophobicity. The water contact angle (WCA) of bronze increased from 27.4° to approximately 139°, while highly hydrophilic pine wood (6.5°) and red sandalwood (84.5°) were converted into stable hydrophobic surfaces with WCAs around 139°. Both coatings demonstrated excellent durability, maintaining WCAs above 120° after 2000 cm of mechanical abrasion, above 115° after 21 days of temperature–humidity cycling, and above 130° after 21 days of UV irradiation. The coatings also exhibited stable electrochemical protection during a 30-day immersion in 3.5 wt % NaCl solution, confirming their sustained barrier capability against corrosive media. Colorimetric analysis and gloss measurements confirmed visual compatibility, with ΔE* values consistently remaining below the conservation threshold of 3 and only minor changes in surface gloss after coating application. Electrochemical tests in 3.5 wt % NaCl solution indicated that the F-WPU coating reduced the corrosion current density of bronze from 1.4 × 10^–5 to 1.2 × 10^–7 A·cm^–2, achieving a corrosion inhibition efficiency of 99.14%, and increased low-frequency impedance by nearly two orders of magnitude. These results demonstrate a substrate-adaptive coating strategy that integrates hierarchical surface structuring, hydrophobicity, electrochemical protection, environmental durability, and visual compatibility. The developed coating system exhibits promising long-term protection potential for metallic and wooden cultural heritage materials, although further validation under extended outdoor exposure conditions remains necessary.