Characterization of the patina formed on a low tin bronze exposed to aqueous hydrogen sulfide

摘要

The dark gray corrosion layer (patina) formed on the surface of a polished low tin bronze alloy following exposure to a deoxygenated and saturated aqueous solutions of H_2S has been characterized by X-ray photoelectron spectroscopy, scanning electron microscopy-energy dispersive spectroscopy and X-ray diffraction. The system represents a model for bronze corrosion in reducing conditions where sulfate-reducing bacteria in soils or deoxygenated seawater may generate H_2S during respiration. The initial surface was dominated by metallic copper together with Sn, Pb and Zn oxides and hydroxides. Surface enrichment of Pb and Zn was noted because of a smearing effect during polishing. At least some of the lead was crystalline. In contrast, the corrosion layer formed by H_2S(aq) exposure was dominated by polycrystalline Cu_2S (low and high chalcocite) and smaller concentrations of CuSO_4. nH_2O. This surface was enriched with Zn as Zn(OH)_2. Lead was present as redeposited PbS (galena) crystallites in at least two different morphologies. Unlike bronzes exposed to oxidizing conditions, which develop protective SnO_2 layers, the H_2S(aq)-exposed surface was considerably depleted in Sn.