Selective dissolution in copper-tin alloys: formation of corrosion-resistant patina on ancient chinese bronze mirrors

摘要

Many ancient Chinese bronze mirrors have survived with a patina that leaves the delicate relief surface decorations intact. The microstructure of these ancient mirrors is two-phase and consists of acicular alpha -phase (Cu-rich) regions encased in a delta -phase (Sn-rich) matrix. At the surface, there is evidence of selective dissolution of the alpha phase; the alphe -phase regions are replaced pseudomorphically by a mineral product with the delta phase remaining metallic. Electrochemical polarization has been used to drive the copper dealloying process in modern, cast bronze. Synchrotron x-ray diffraction was employed to compare the ancient samples with those that were prepared potentiosatically. Poorly crystallized tin oxide (SnO_2) was found in the alpha replacement products of both sample types. The corrosion-resistanceof the potentiostaticaly-treated bronze samples was tested by atmospheric exposure. Comparison with exposed, untreated samples indicated that the treatment was protective.