Microstructures and electrochemical corrosion behaviors of bronze mirror

Some of the archaeological bronze mirrors retain a smooth silver-gray luster but lack reflective functionality. A high-tin bronze mirror of Western Han Dynasty (202 BC-8 AD) which only showed basic polishing marks without any coating. The silver-gray luster area was the oxidized tin-rich α + δ eutectoid remaining after the loss of copper in α phase. Microscopic roughness causes light scattering, which is hard to normally reflect clear images. The polishing and copper loss could expose more tin area and form a dense oxide layer that enhances corrosion resistance by blocking active sites and isolating the bronze matrix. Even after thousands of years, mirror surface showed better charge-transfer resistance (Rct) and polarization resistance (Rp) than unpolished decorated surface, with max differences approximately 3 times and 5 times, respectively. Patina growth forms corrosion paths, destroying the resistance of the tin-rich surface, especially under green malachite.