Soil‐induced corrosion of ancient Roman brass – A case study

Abstract

This work focuses on the study of the solid state processes on naturally corroded ancient brass artefact. The object, recovered from burial in the Tiber riverbed in Italy, has been characterised from a morphological, chemical and microstructural point of view. The artefact probably originates from a Roman brass coin or medal and the alloy substrate was identified as a leaded brass (85% copper, 12.8% zinc and 2.2% lead). A combination of XRF, SEM coupled with EDS spot analyses and elemental mapping, Raman spectroscopy and OM observations were employed in order to describe the corrosion processes that created the heterogeneous, multi-layer corrosion product stratification. The identification of copper sulphide layers and the precipitation of some, rarely reported, basic copper and zinc phosphates are among the most important findings of this work. Two phases of anaerobic and aerobic corrosion emerged from the analyses. The object has undergone dezincification at early stages and decuprification later on. However, the most crucial action was the environmental elements activity, which determined the corrosion rate and the nature of the corrosion product layers. The results are critically discussed and correlated with the environmental conditions of the burial context. The study is supported by an extensive literature review.

Analysis of naturally etched surface of brass sheathing from a nineteenth-century shipwreck

“…The XRF results of the as-retrieved sheet (oxide surface) also indicated the presence of 0.4±0.1 wt% Si and 0.1±0.1 wt% Al (Table 2). Since no silicon or aluminium was found in the S-OES bulk analysis and in the EDS analysis of the sheet after removal of the 1 mm layer, the presence of Si and Al (as well as other elements, such as P, S, Ca and Cl) is most likely related to reactions between soil elements and corrosion products [42][43]46]. For example, the Al presence may be related to long term intergranular corrosion processes [53].…”

“…This good corrosion resistance of -brass results from the formation of ZnO and complex passive Cu x O y oxides on the surface of the metal [30,42]. Copper oxide is an antibacterial compound, and doping Zn ions into the copper-oxide film increases the antibacterial activity of the metal [31,54,55].…”

“…stabilization and conservation treatments. Moreover, such data may also support information on their authenticity, as well as their dating and origin [41,42]. Therefore, understanding corrosion processes of brass during long burial periods in seawater is essential.…”

Recent Advances in Protective Coatings for Cultural Heritage–An Overview

“…The most important problem regarding the corrosion of brass alloys is the dezincification phenomenon, which is still being investigated. In fact, dezincification can be due to the selective dissolution of Zn or the simultaneous dissolution of Cu and Zn followed by the redeposition of Cu [13,14]. In the first hypothesis, copper atoms do not participate in the process, whereas in the simultaneous dissolution both metals are involved in causing a defective metallic Cu layer.…”

A methodological approach to estimate soil corrosivity for archaeological copper alloy artefacts

“…composition and manufacture), or the characteristics of the burial soil (e.g. texture, pH, redox potential, soluble salts) [3][4][5][6][7][8][9][10][11].…”