Electrochemical evaluation of patinas formed on nineteenth century bronze bells

摘要

The cathedral of Cuernavaca City, Mexico is one of the earliest monuments built in the American Continent by the Spanish Conquerors. The cathedral lodges a collection of historic artifacts including eight bells. The aim of this study was to provide preliminary information for the conservation diagnosis of the historic bells. A set of three bells dating back to the nineteenth century was studied. Two of the bells present a crack and the third and oldest remains complete. Metallurgical characterization and electrochemical evaluation were used to describe the behavior of the patinas formed on the surfaces of the bells. Studies were done "in situ'' over the surface of the artifacts. Preliminary experiments were carried out using a modern cast bronze with equivalent chemical composition. Besides the useful information for conservation purposes, the work depicts the behavior of an ancient material exposed to the atmosphere for a long period of time. The obtained data suggest a relationship between alloy composition, especially tin and lead content and electrochemical properties of the materials. Techniques used include potential measurements as function of time, potentiodynamic polarization curves, electrochemical impedance spectroscopy, electrochemical noise as well as metallography.