Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system

This paper presents a study made on 40 iron archaeological artefacts buried in soil during several centuries. Samples were taken with the adhering soil and cross-sections were made. The used characterisation techniques are optical and electronic microscopies, EDS coupled to SEM, EPMA, micro-XRD under synchrotron radiation, micro-Raman spectrometry. A specific vocabulary is proposed to describe the corrosion layout. The most identified corrosion layout is made of several ten micrometers zones of magnetite and/or maghemite embedded in a goethite matrix. A corrosion mechanism is proposed in order to explain this profile. When the soil water contains more chlorine or carbonates, some specific corrosion product appear as akaganeite, oxychlorides and siderite.