Siderite as a Corrosion Product on Archaeological Iron from a Waterlogged Environment

This paper discusses the occurrence of siderite (FeCO3) on iron artifacts excavated from the waterlogged peat and gyttja sediment of the Danish Iron Age site Nydam Mose. Siderite was identified by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS), which showed only minor contents of other minerals in the corrosion scales. The implications of the formation of siderite as a corrosion product are discussed in terms of its possible passivating properties and thermodynamic stability in situ. A Pourbaix diagram adjusted to the actual conditions in Nydam is presented and discussed. Different mechanisms for the formation of siderite in this environment are proposed, based on the results of environmental monitoring in the area.