Characterization of the corrosion products formed on Michael the Brave’s equestrian statue in urban atmosphere

Abstract

The present work is focusing on the study of the corrosion products (natural patina) formed on the surface of the equestrian statue of Michael the Brave, Principe of Walachia, and on the patina formed on the bas-reliefs that decorated the marble of the statue pedestal. The statue is placed in the Mihai – Viteazul Square in the center of Cluj-Napoca, Romania, since 1976 and was restored in 2018 after 42 years of exposure in the urban atmosphere. The morphology and the chemical composition of the statue and of the natural patina formed on the statue’s surface, on the bas-reliefs surrounding the statue, as well as on one assembling element exposed in the urban atmosphere for almost 50 years, were determined by SEM - EDX supported by micro-Raman and XRD analyses. The X-ray maps pictured the distribution of various elements in the patina samples collected from the equestrian statue’s surface. The selective dissolution of the Cu, Zn, and Sn observed in the patina layer, was quantified by the dissolution factor based on EDX analysis. The corrosion products indicated the presence of malachite and brochantite, along with other various minerals. It was concluded that the analyzed patinas were affected by the pollution of the urban atmosphere, but their composition was mostly influenced by the composition of the different copper alloy substrates (high-leaded tin bronze or brass).