From cathedral building to radioactive waste storage - the importance of iron

Many iron parts are involved in the conservation of cultural heritage materials. For example, iron is a component of many archaeological and ethnological objects but also in historical buildings. Iron is especially very common in cathedrals, where it has recently been shown that it has always been used in large amounts since they were built. To take the most appropriate decisions for the preservation of these ancient objects made of iron, it is crucial to have reliable predictions on their alteration and to determine the corrosion processes. These studies help to ensure the development of new strategies for the conservation of these metallic materials, while meeting the standards required by conservation standard, in particular, without changing their visual appearance. In addition, such research may also find applications in civil engineering. This is specifically relevant in the storage of radioactive wastes, because it is planned to store ithem in steel containers before burying underground. It is therefore necessary, in this case, to predict the very long term alteration of iron.

The study presented here focuses on samples taken from a steel chain in Amiens cathedral. Installed in 1497, it was used to mechanically strengthen the structure of the triforium. For over 500 years it has therefore been submitted to indoor atmospheric corrosion.
To assess the stability of the corrosion pattern, it is important to know the exact composition of the layers of corrosion products formed on the surface of the metal. 

For this study, made in collaboration between SOLEIL and LAPA (Laboratoire Archéomatériaux et Prévision de l'Altération), scientists used 'soft' X-rays on the LUCIA beamline. Using X-ray absorption microspectroscopy at the Fe K-edge, they were able to distinguish, at the microscopic scale, the nature of the different phases and identify within the corrosion products those having a critical impact on the corrosion process as being more reactive.
Combining this EXAFS results with X-ray fluorescence measurements revealed the presence of some minor elements such as sulfur and phosphorus. This latter element would stabilize the phases and thus play a beneficial role in atmospheric corrosion. Analysis by X-ray absorption microspectroscopy at the phosphorus K-edge (P) allowed to identify how P interacts with the corrosion products.

Figure 2: (a) Macro-EXAFS spectra of reference powders; (b) Radial distribution functions of reference powders.

References :  J. Monnier, S. Reguer, D. Vantelon, P. Dillmann, D. Neff, I. Guillot. X-rays absorption study on medieval corrosion layers for the understanding of very long term indoor atmospheric iron corrosion. Applied Physics A, 99, 399-406, 2010 J. Monnier, D. Neff, S. Reguer, P. Dillmann, L. Bellot-Gurlet, E. Leroy, E. Foy, L. Legrand, I Guillot. A corrosion study on the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes. Corrosion Science, 52, p 695-710, 2010 J. Monnier, D. Vantelon, S. Reguer, P. Dillmann. Evaluer la résistance à la corrosion des métaux du patrimoine : réactivité des couches épaisses en corrosion atmosphérique du fer Actualité Chimique, à paraître J. Monnier , D. Vantelon , S. Reguer and P. Dillmann. X-ray absorption spectroscopy study of the various forms of phosphorus in ancient iron samples J. Anal. At. Spectrom., 2011, 26, 885   Figure 3: Micro XRF mapping of minor elements Ca, Cl, K, P and S compared to an optical microphotograph.