The role of particulate matter in bronze corrosion: A novel method for assessment and prediction

The impact of particulate matter (PM) on the deterioration of outdoor cultural heritage is of increasing concern, given the recent trends in atmospheric emissions. This work presents a novel method for investigating the effects of PM on bronze corrosion by combining the collection of ambient PM directly on metal substrates and ageing under controlled laboratory conditions, with the development of a predictive model based on chemometrics. PM deposits of different mass and composition were collected and chemically characterised using Ionic Chromatography, Total Organic Carbon, and Atomic Absorption Spectroscopy. Accelerated ageing was then performed in a climatic chamber simulating daily and seasonal environmental conditions characteristic of Southern Europe. The morphology and distribution of the deposited PM, as well as surface changes after deposition and artificial ageing, were analysed using microscopic and spectroscopic techniques. The alloying metals contained in the patina formed during the corrosion process - either in the water soluble/poorly adherent or in the insoluble/adherent fractions - were also quantified. Based on the collected data and the analyses results, principal component analysis and partial least squares techniques were employed to infer the correlation between the concentration of nitrite, chloride, sulphate and nitrate in PM and bronze corrosion. Multivariate regression allowed the influence of the different ionic species to be estimated, highlighting the primary role of chloride and sulphate ions in determining higher mass loss, and to define a cross-validated model for predictive purposes. The novel method proposed here can improve the understanding and prediction of the role of PM in the corrosion of bronze, ultimately providing guidance for targeted interventions in the preservation of outdoor cultural heritage.