Assessment of the interphase behaviour of two bronze alloys in archaeological soil

The aim of this work is to evaluate the behaviour of bronze in archaeological soil by gravimetric study, electrochemical research and analytic survey. Archaeological soil samples were collected from Jama that date back to the second Punic war. Mineralogical, micro-structural, micro-morphological and chemical analyses of soil constituents were performed. The bronze samples correspond to Cu-9.4Sn (B-1) and Cu-7.7Sn with 1% of Pb (B-2). The open circuit potential value recorded for B-2/corrosion product/soil is stable after only 7 min due to its reactivity allowing to a rapid development of a protective corrosion layer. The B-2 interphase is more developed and stable than B-1. Under anodic polarisation, the B-2 alloy corrodes less than the B-1 alloy owing to its lower anodic current. Based on the EIS study, two evolution stages of the bronze alloy/corrosion product/soil interphase are detected. For the first stage, the interphase is equivalent to two R//C loops at high and medium frequencies, respectively related to the corrosion layer and charge transfer process, associated to mass transport phenomena. For the second stage, the resistance of corrosion layer decreases and the first loop vanishes. SEM-EDS analysis demonstrate that the first stage (20 days of immersion) can be characterized by a well covered surface with a thin corrosion product, the external layer exhibits low tin content. However, after 310 days of immersion, the surface state is different with isolated particulates composed by Cu and soil compounds on a cracked layer with the presence of craters showing an internal layer.