Long-time corrosion of a cast bronze droplet during 3000 years storage in soil

摘要

A corroded metallic droplet was investigated by metallographic methods. Due to the archaeological context this droplet is dated back to the Urnfield period and its age is about 3000 years. First measurements showed that this droplet consists of bronze and in the metallic core 10 wt.% Sn were measured. Probably this droplet was formed during a casting process. Looking at the metallographic cross section three regions can be distinguished: a metallic core, a corroded rim, with a thickness up to 3 mm, and a corrosion front between rim and core. Most of the regions with a high Sn content are exceedingly attacked by corrosion and in the corrosion products Sn concentrations up to 40 wt.% were measured. This enrichment can be explained by a simultaneous formation of insoluble SnO_2 and soluble Cu ions, which were transported to the droplet's surface. But some regions near the rim with high Sn content are less attacked and Cu grains are transformed into large cuprite crystals. The lamellar structures in the cuprite indicate that these crystals grew stepwise and discontinuous. The different corrosion phenomena are explained by an interrupted, slow corrosion rate process.