Protective coatings for outdoor bronze: Influence of application methodology on performance

Research on sustainable protective coatings for metallic heritage presents significant challenges due to the inherent complexity of corrosion processes in bronze, patina, and outdoor environmental systems. Additionally, the absence of standardised testing and evaluation methods hampers the comparability of results and the establishment of best practices. New sustainable coatings based on Paraloid (R) B44, with a light stabiliser and a corrosion inhibitor, have been optimised for a low toxicity profile. The tested formulations include Tinuvin (R) 312 as light stabiliser and 5-mercapto-1-pheniltetrazole (MPT) or 2-amino-5-ethyl-1,3,4-thiadiazol-(AEDTA) as corrosion inhibitors. Their physicochemical properties have been determined through multi-analytical characterisation and monitoring of changes during artificial solar light ageing. Nonetheless, the effectiveness of a coating is also greatly affected by the application method and the chemical, physical and morphological properties of the underlying surface. In this work, the role of Paraloid (R) concentration in a two-layer brush application method is addressed on quaternary bronze with a Verde Messina foundry patina. The influence of resin concentration on the resulting layer is analysed. Patinated bronze mock-ups were non-destructively characterised before and after treatment by Electrochemical Impedance Spectroscopy (EIS) to assess corrosion resistance, colour by colourimetry to evaluate visual appearance, and by Eddy Current to measure the patina and coating thickness. After artificial thermo-hygrometric ageing of the coated mock-ups, Fourier Transform Infrared Spectroscopy in external reflection mode (ER-FTIR) and colourimetry were used to assess the treatment stability with respect to molecular composition and colour changes. Principal Component Analysis (PCA) of ER-FTIR spectra allowed highlighting differences in coatings over the ageing process. Results showed that all tested treatments increased the corrosion resistance of Verde Messina patinated bronze; double-layer coatings with a more diluted first layer were found to be the preferred treatment, providing comparable performance while using a smaller amount of Paraloid (R). (c) 2026 The Authors. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)