Mechanism of gold foil corrosion by two dominant fungal strains on the Dazu Thousand-handed and Thousand-eyed Avalokitesvara

In recent years, partial deterioration of the gilded layer on the Dazu Thousand-handed and Thousand-eyed Avalokitesvara has posed a serious threat to the preservation of this cultural heritage. In this study, Aspergillus versicolor and Mucor fragilis, previously isolated from the gold foil surface, were selected as model organisms. Laboratory-simulated conditions were employed to investigate their corrosive effects on the gold foil. Corrosion behavior was monitored through pH, open circuit potential, electrochemical impedance spectroscopy, and polarization curves. Additionally, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized to characterize morphological and elemental changes. The results demonstrated that both fungal strains significantly accelerated gold foil corrosion. A marked decrease in surface gold (Au) content was observed, accompanied by the enrichment of carbon (C), oxygen (O), and sulfur (S) elements. Electrochemical analyses revealed a negative shift in corrosion potential by 0.20-0.24 V, a nearly tenfold increase in corrosion current density, and corrosion efficiencies of 91.38% and 88.91% for Aspergillus versicolor and Mucor fragilis, respectively. The preliminary mechanistic inferences suggest that organic acids (e.g., citric acid and oxalic acid) produced by fungal metabolism rapidly reduced the local pH to 4.5-4.8, thereby disrupting the passive layer. Moreover, extracellular polymeric substances (EPS) secreted by the fungi facilitated biofilm formation and likely enhanced interfacial reactions through sulfide adsorption. The flake corrosion patterns induced by Aspergillus versicolor were attributed to the increased electrolyte penetration promoted by its hydrophilic polysaccharide-rich biofilm. Conversely, the crack propagation caused by Mucor fragilis was possibly driven by micro-galvanic effects associated with its saccharification products.