Abstract
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.
Highlights
Archaeometallurgical and historical bronzes and brasses are either cast or worked
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
Numerous modern archaeological and archaeometric studies have been dedicated to the investigation and reconstruction of manufacturing procedures and available bronze and brass metallurgical technology in many eras and geographical sites where ancient civilizations of the Old and New World have thrived [2]
Summary
Archaeometallurgical and historical bronzes and brasses are either cast or worked. A combination of manufacturing and shaping techniques have been employed for their production. The microstructural features of the final products are the result of alloying element concentration, presence of impurities, cooling rate and subsequent thermal or mechanical treatment [1]. Numerous modern archaeological and archaeometric studies have been dedicated to the investigation and reconstruction of manufacturing procedures and available bronze and brass metallurgical technology in many eras and geographical sites where ancient civilizations of the Old and New World have thrived [2]. A great number of the antiquity bronze artefacts, tools and coinage, and historic and contemporary copper-based sculptures, are produced by casting.
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