Bronze degradation processes in simulating archaeological soil media

The purpose of this study is to establish reliable procedure reproducing the degradation processes that occurred on the archaeological artefacts before their excavation. Two simulating soil media are chosen, the first one (FJ) is made from archaeological soil filtrate and the second is a chloride- and/or sulphate-containing solution (CSJ) taken at the same soil amounts of aggressive ions. According to stationary studies, corrosion mechanism in CSJ medium is catalysed by the presence of chloride at the surface of tin bronze electrode, but sulphate products tend to stabilise the surface of bronze sample immersed in soil filtrate. In FJ, the trend of impedance spectra remains unchanged during the immersion period. Adjusted impedance parameters prove that passivation and depassivation can occur simultaneously on bronze surface with different expansion. In CSJ, impedance data evolve according to the immersion time. The evolution of adjusted parameters according to the immersion time describes well the surface state of the bronze sample. The corrosion process is more difficult while the metallic surface is covered with a bronze corrosion layer "patina" which explain the capacitance decreasing and the resistance increasing (after 50 days). Scanning electron microscopy observations coupled with energy-dispersive X-ray spectrometry analysis approve our electrochemical results.