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

Abstract

Organlc coatlng Bronze Atmospheric corrosion UV degradatlon Electrochemlcal techniques XPS 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 bron7.eS. Electrochemical testing was performed in synthetic acid rain. Exposure to temperature/UV cycles and ac celerated corrosion test simulating unsheltered exposure to rainwater was also carried out. The exposed samples were charac terised by FEG..SEM coupled with EDS on F1B cross-sections and XPS on free surfaces. The black patina wi thout protective coating was scaroely protective against bronze corr06ion and easily transformed into cuprous oxide. PropS..SH coating fully preserved the black patina microstructure and phase constituents (c uprous oxide and cuprous sulphide). The PropS-SH coating also resulted more protective than Incralac• when aged under runoff 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 internai Si-rich corrosion layer. comerstone event in the field of cultural heritage conservation (1). Protective treatments, including both cleaning procedures and appli cation of surface coatings, such as acrylic polymers admixed with cor rosion inhibitors (Incralac •), were found to be the best solution against atmospheric corrosion (1). Incralac• was developed in 1964 by the International Copper Research Association (INCRA) (2). lt is an unpatented formulation based on Acryloid B 44, a thermoplastic methyl methacrylate • Corresponding author. copolymer manufactured by Rohm and Haas, dissolved in rather toxic solvents, like toluene and xylene (3). Incralac• also contains benzo triazole (BTA) as a copper corrosion inhibitor and chelating agent (suspecte<! of carcinogenicity [ 41), together with a levelling agent (ei ther Paraplex G 60, which is epoxidized soybean oil, or a silicone om. BTA also acts as a coating UV stabilizer (5). Following titis ap proach, micro crystalline wax and other organic polymers, such as poly siloxanes, were later developed (5 9). Among the others, 3 mercapto propyl trimethoxysilane (PropS SH) proved to be effective for the protection of bare Cu and Cu based alloys in NaCl solution (10 13), as well as in synthetic acid rain (14). The protec tiveness of PropS SH is enhanced by the addition of nanoparticles (15). In previous works, PropS SH was applied on quatemary bronze, pre

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“…The analysis carried out with FTIR and XPS detected the formation of thiolate (Cu-S-C) bonds due to the condensation of -SH groups in the silane molecule of PropS-SH and the formation of Cu-OH groups on the metal surfaces that contributes to the adhesion of the coating on the surface. In addition, SEM images of Incralac and PropS-SH of Masi et al [40] clearly showed the adhesion and the microstructure of both coatings when applied on the surface. colour with age.…”

“…PropS-SH film applied to gilded bronzes effectively inhibits the growth of corrosion products in the interlayer between the bronze and gold, which can cause the detachment of the gilded layer. Recently, a PropS-SH coating was proposed as a non-toxic alternative to Incralac, a toluene solution of an acrylic resin containing BTA, used in restoration especially on outdoor bronzes [40]. Balbo et al Among silanes, 3-mercapto-propyl-trimethoxysilane (PropS-SH, Figure 4) has the most interesting protective properties for copper and bronze.…”

“…PVDF is a hexafluoropropylene-vinylidine fluoride copolymer and can be dissolved in acetone. Research has highlighted that the adhesion-on metals as well as stones-is the main problem with fluoropolymers [38,40,41]. This is usually enhanced by adding other components to the polymer coating.…”

Inclusion of 5-Mercapto-1-Phenyl-Tetrazole into β-Cyclodextrin for Entrapment in Silane Coatings: An Improvement in Bronze Corrosion Protection

“…Silane hydrolysis was carried out by dissolving PropS-SH in a hydroalcoholic solution (90/5/5 v/v ethanol/water/PropS-SH), acidified to pH 4 by the addition of some drops of diluted sulphuric acid solution, according to the methodology refined in previous research works [29,[38][39][40].…”

“…The evolution of corrosion conditions was monitored over 20 days of immersion by Electrochemical Impedance Spectroscopy (EIS, performed by a PARTSTAT 2273, from Ametek, Berwyn, PA, USA) tests performed at intervals, under the following experimental conditions: corrosion potential (E cor ) ± 10 mV rms, 10 kHz-1 mHz frequency range and 10 frequencies/decade. Polarization resistance (R p ) values were estimated from the spectra in the Nyquist form, as the difference between the limit of the real part of the impedance at frequency tending to 0 (R p ') and the solution resistance (R s ) value (R p = R p ' -R s ) [40]. R p values are inversely proportional to the corrosion currents (i cor ), as indicated by the Stern and Geary relationship [41]: R p = B i cor , with B a constant depending on the Tafel slopes of the anodic and cathodic polarization curves.…”