A study of transport phenomena in the corrosion products of ferrous archaeological artefacts using 18O tracing and nuclear microprobe analysis

Abstract

Studies of ferrous archaeological artefacts corroded in soil are of great interest for the cultural heritage community and for the prediction of very long term corrosion behaviour of low alloy steels. One important hypothesis of recent studies is that corrosion mechanisms seem to be controlled by oxygen diffusion that is solved in the soil water and in the porosities of the corrosion products. Consequently, cathodic and anodic corrosion reactions should happen at the metal/oxide interface. In order to verify this assertion, several experiments were done using 18O tracers, to visualize where the corrosion reactions take place in the system. Samples collected on archaeological ferrous artefacts were put with their already formed corrosion products in water previously deoxygenised and equilibrated with 18O at a pressure of 1.2atm. After various exposition periods, samples were removed and transverse sections were made. The distribution of the remaining18O precipitated product in the ancient corrosion products was mapped by means of the 18O(p,α)15N reaction. Indeed, this 18O presence seems to reveal the precipitation of new oxidation products and confirm the location of the cathodic reaction at the metal/oxide interface.