Study on eutectoid phase corrosion behavior and rust layer protection mechanism of bronze coins from the Western Han Dynasty (first century BC) in China

This article conducted a study on ancient bronze coins unearthed in Huxian, Shaanxi, China. It was found that the coins corroded along the eutectoid phase, and there was an obvious corrosion layering phenomenon. From the outside to the inside, they were the outer rust layer, the transition layer and the matrix layer. When analyzing the chemical composition and corrosion products in different regions, various techniques such as metallographic microscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and electron probe microanalysis (EPMA) were employed. It was determined that the outer rust layer primarily consisted of Cu₂(OH)₃Cl, while the transition layer was composed of an α phase and a corroded eutectoid phase (with the corrosion product being SnO₂). In addition, combined with electrochemical tests, it was found that the rust layer has better protective performance on the substrate. Based on the above results, it is speculated that some ancient bronze cultural relics may have gone through two stages: the first stage is the market transaction stage after the coin casting is completed, and the second stage is the burial soil corrosion stage when the coins circulating in the market become funerary objects. Relevant research provides a certain theoretical basis for the protection of bronze cultural relics.