The Influence of Archaeometallurgical Copper Alloy Castings Microstructure towards Corrosion Evolution in Various Corrosive Media

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.