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.

The local patterns at the interfaces of corrosion stratification, developed on two archaeometallurgical bronzes (a Cu-Sn-Pb and a Cu-Zn-Sn-Pb alloy), in the as-cast condition, were assessed by OM and SEM-EDS systematic elemental chemical analyses. Previously, the alloys—whose metallurgical features and electrochemical behaviour were already well studied—have been subjected to laboratory corrosion experiments. The corrosion procedures involved electrochemical anodic polarization experiments in various chloride media: 0.1 mol/L NaCl, 0.6 mol/L NaCl and two other synthetic chloride-containing solutions, representing electrolytes present in marine urban atmosphere and in the soil of coastal sites. The characterization of the Cu-Sn-Pb alloy electrochemical patinas after anodic sweep (OCP+ 0.6 V) revealed that the metal in all electrolytes undergoes extensive chloride attack and selective dissolution of copper which initiates from the dendritic areas acting as anodic sites. The most abundant corrosion products identified by FTIR in all electrochemical patinas were Cu2(OH)3Cl), Cu2(OH)2CO3 and amorphous Cu and Sn oxides. The characterization of the Cu-Sn-Pb alloy electrochemical patina after slow anodic sweep (OCP+ 1.5 V) in 0.1 mol/L NaCl reveals selective oxidation of dendrites and higher decuprification rate in these areas. Corrosion products of Sn-rich interdendritic areas are dominated by oxygen species (oxides, hydroxides, hydroxyoxides) and Cu-rich dendrites by chlorides. In the case of Cu-Zn-Sn-Pb, Zn in dendritic areas is preferentially attacked. The alloy undergoes simultaneous dezincification and decuprification, with the former progressing faster, especially in dendritic areas. The two processes at the alloy/patina interface leave behind a metal surface where α-dendrites are enriched in Sn compared to the alloy matrix. The results of this study highlight the dynamic profile of corrosion layer build-up in bronze and brass. Moreover, the perception of the dealloying mechanisms progression on casting features, at mid-term corrosion stages, is extended.

Reference Cu-based alloy with chemical composition and micro-chemical structure similar to that of ancient alloys has been used for carrying out the artificial long-term degradation test based on chloride enriched soil (chemical+soil) degradation. The results show that such degradation procedure produces natural like "patinas" as the ones grown on archaeological artefacts, from a chemical, structural and micro-morphological point of view. Glow discharge plasma technique has been employed for the treatment of the as-corroded bronze coupons. The gradual elimination of chloride-containing corrosion products in favour of the formation of more stable species and even the complete reduction back to copper has been observed. The chemical and metallurgical features have been determined by combined use of different analytical techniques such as scanning electron microscopy with energy dispersive X-ray micro-analysis (SEM-EDS), X-ray diffraction (XRD) and optical microscopy.

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A quaternary (Cu‐Zn‐Sn‐Pb) cast bronze was submitted to an electrochemical corrosion experiment using an anodic polarization sweep. The surface/interface chemical analysis of the patina removed after the anodic polarization and the metallographic observations on the corroded bulk alloy highlight the onset of a dezincification process followed by a decuprification process, as well as the formation of a Sn‐enriched layer located at the interface patina‐bulk alloy. Different local corrosion patterns, as CuCl precipitation in pits, epitaxial growth of corrosion products on dendritic structures and of Cl‐enriched oxyhydroxides, are observed too. The results are discussed in comparison with experimental findings obtained in previous ageing tests performed on the same alloy in different chloride‐containing solutions. Copyright © 2014 John Wiley & Sons, Ltd.