Corrosion evaluation and surface characterization of the corrosion product layer formed on Cu–6Sn bronze in aqueous Na2SO4 solution

The binary bronze alloy Cu–6Sn corrosion, and formation and properties of corrosion product layer (patinas) during 12 days of exposure to 15 mM Na2SO4 aqueous solution were investigated by a range of diverse experimental techniques. For the reasons of comparison, some techniques were applied, in parallel, to copper. Gravimetric measurements revealed lower corrosion rates of bronze than those of copper, probably caused by the presence of tin compounds in the corrosion product layer. Cyclic voltammetry results showed that the oxidation processes on bronze are affected by the formation of tin oxide species. Electrochemical impedance spectroscopy showed that, as opposed to copper which produced only two time constants, bronze corrosion resistance was dominated by the additional high-frequency time constant representing redox processes occurring at the corrosion product surface. SEM, ATR FTIR and PIXE results suggest that Cu–6Sn bronze corrosion in 15 mM Na2SO4 solution was impeded by the formation of two-layered structure of corrosion products that formed due to selective dissolution of copper at the layer/solution interface, leaving the outer layer enriched in highly corrosion resistant Sn oxi/hydrohide species.