Selective dissolution in copper-tin alloys: Formation of the surface finish on early Chinese bronze mirrors.

摘要

In the early part of the twentieth century, scientists began to look at ancient artifacts both for clues to the stability of metals and alloys, and for ways to preserve the objects themselves.; Chinese bronze mirrors often have a smooth patina that preserves the fine detail in the original cast surface decoration. Mirrors appear early in Chinese history and were produced in large numbers starting during the final, Warring States, period of the Zhou dynasty (475--221 BC).; The average composition of ancient mirrors is 70% copper, 25% tin and 5% lead, by weight. The microstructure is two-phase and consists of acicular alpha-phase (Cu-rich) regions encased in a delta-phase (Sn-rich) matrix. Although the mirrors have been labeled "corrosion-resistant," the smooth surface includes a corroded layer on the order of 100 mum thick. In this layer, known as the "altered layer," there is evidence of selective dissolution of the alpha-phase; the alpha-phase regions are replaced by a mineral product with the delta phase remaining metallic.; The current project was undertaken to investigate and replicate the stable metal surface found on ancient bronzes. Electrochemical methods were used to study the corrosion behavior of the copper-tin system. The replication was performed on model, cast two-phase bronze by treating under constant applied potential in two solutions. The alpha phase corroded preferentially in both solutions duplicating the microstructure of the altered layer on ancient mirrors.; Characterization of the patinas on ancient Chinese bronze mirrors and modern treated samples was performed with optical and electron microscopy, and synchrotron x-ray diffraction and fluorescence. The alpha-phase replacement product was found to be nanocrystalline tin oxide (SnO2) in both ancient and modern samples.; The corrosion-resistance of the potentiostatically-treated bronze samples was tested by extended outdoor exposure. Comparison with exposed, untreated samples indicated that the treatment was protective.; A better understanding of the processes that resulted in the smooth surfaces on ancient Chinese mirrors has been attained through the replication of the surface microstructure and composition under the controlled conditions of this work.