Interfacial layer on archaeological mild steel corroded in carbonated anoxic environments studied with coupled micro and nano probes

•Interfacial layer on ferrous archaeological artefacts is studied.•Probes at the micro and nanometer scales (FEG–SEM, Raman, STEM, STXM) are used.•This layer forms between the substrate and a thick carbonate layer.•The layer shows thicknesses from about 100nm to several μm.•It could form after a local pH increase provoked by the consumption of H+ ions. The interfacial layer formed on archaeological artefacts corroded during 450years in carbonated anoxic water was studied using FEG–SEM, Raman, STEM, STXM. This layers forms between the metallic substrate and an outer carbonate layer. Interfacial layer shows various thicknesses from about 100nm to several μm. It is made of a mix of iron oxides (maghemite or magnetite). A formation mechanism is proposed and based on slight pH increase at the interface. D2O labelling experiments show that the penetration of water in the pore network seems to be significantly hindered at the interface by the presence of the oxide layers.