Evaluation of the protectiveness of an organosilane coating on patinated Cu-Si-Mn bronze for contemporary art

[Display omitted] •Protectiveness of organosilane coating (PropS-SH) on patinated bronze was assessed.•Patina of cuprous oxide and sulphide does not protect the bronze against corrosion.•Both sprayed PropS-SH and Incralac® offer a good protection to patinated bronze.•PropS-SH shows excellent performance under UV light and simulated outdoor exposure.•Higher wettability of PropS-SH favours its penetration in porous patina structure. A 3-mercapto-propyl-trimethoxysilane coating (PropS-SH) applied on Cu-Si-Mn bronze, patinated by “liver of sulphur”, was investigated as a non-toxic alternative to Incralac®, usually applied on outdoor artistic bronzes. Electrochemical testing was performed in synthetic acid rain. Exposure to temperature/UV cycles and accelerated corrosion test simulating unsheltered exposure to rainwater was also carried out. The exposed samples were characterised by FEG-SEM coupled with EDS on FIB cross-sections and XPS on free surfaces. The black patina without protective coating was scarcely protective against bronze corrosion and easily transformed into cuprous oxide. PropS-SH coating fully preserved the black patina microstructure and phase constituents (cuprous oxide and cuprous sulphide). The PropS-SH coating also resulted more protective than Incralac® when aged under run-off conditions. Selective dissolution of copper from the silicon bronze alloy was observed on both uncoated and Incralac®-coated bronze, leading to the formation of an internal Si-rich corrosion layer.