The Synergistic Effects of Chelation and Reduction in a Chitosan/l-Ascorbic Acid Composite Hydrogel for Cleaning...

Powdery rust, commonly known as "bronze disease", is the most serious threat to long-term preservation of precious bronze relics due to its highly corrosive and diffusive nature. Conventional mechanical or chemical cleaning methods used in archeology often cause physical damage or leave chemical residues, making the effective and safe removal of harmful powdery rust from bronze artifacts still challenging. In this study, a novel composite hydrogel based on natural chitosan and l-ascorbic acid is developed to efficiently dissolve and decompose powdery rust, particularly by targeting copper ions. The efficacy and safety of the prepared hydrogel are first verified through cleaning tests on a copper coin, achieving a superior removal rate of 94% for artificial powdery rust. The hydrogel is subsequently applied to a bronze fragment excavated from the Shaanxi Province of China, resulting in the rapid removal of powdery rust without causing damages or leaving residues. The cleaning mechanism is ascribed to the synergistic effects of chelating effects from both chitosan and l-ascorbic acid and the reduction effect from l-ascorbic acid, which together promote the formation of stable copper coordination complexes and facilitate the decomposition of Cu₂(OH)₃Cl. This work provides a promising ecofriendly strategy for protecting bronze relics from powdery rust corrosion.