Rebar from the Bourges Cathedral - Late medieval times - France

Dillmann. Philippe (LAPA, Gif-sur-Yvette, Île-de-France, France) & Neff. Delphine (LAPA, None)

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In the building since the construction

The system is divided in two zones : the upper side of the rebar was submitted to atmospheric indoor corrosion, the lower side was embedded in binder.

side submitted to atmospheric corrosion : redish corrosion products mixed with dust and stone powder from the walls of the building

Stratigraphy of the atmospheric corrosion side
Stratigraphy of the side corroded in binder
Stratigraphy of the atmospheric corrosion side
Stratigraphy of the side corroded in binder

OM, SEM, EDS, µRaman

Low alloy heterogenous hypoeutectoïd steel. Various grain sizes.

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Ferrito pearlitic equiaxe structure. Slag inclusions 

atmospheric corrosion system : thickness from 100 to 500 µm

corrosion in binder : 50 to 300 µm

The corrosion layers observed on the rebar of the Bourges’ Cathedral present a multi-layered structure. The outer and the in- ner layers—separated by mill scale residues—have different chem- ical and mineralogical composition as well as different transport properties, the outer layer being apparently more permeable to impurities than the inner one. The corrosion layers on the atmo- sphere-side and the mortar-side are quite similar in mineralogical composition. The conditions in this thin mortar sealing are obvi- ously intermediate between atmospheric conditions and usual conditions found in hydraulic mortars.

All of these results suggest a corrosion mechanism based on an inward growth of the inner layer through the oxidation of the me- tal by the oxygen or by reactive corrosion products, and of an out- ward growth of the outer layer through the dissolution and recrystallisation of inner layer corrosion products.

1. Bouchar, M.; Foy, E.; Neff, D.; Dillmann, P. The complex corrosion system of a medieval iron rebar from the Bourges’ Cathedral. Characterization and reactivity studies. Corros. Sci. 2013, 76, 361–372, doi:10.1016/j.corsci.2013.07.007.

2. L’Héritier, M. Les armatures de fer de la cathédrale de Bourges : nouvelles données, nouvelles lectures. Bull. Monum. 2016, 174, 447–465.

3. Leroy, S.; L’Héritier, M.; Delqué-Kolic, E.; Dumoulin, J.-P.; Moreau, C.; Dillmann, P. Consolidation or initial design? Radiocarbon dating of ancient iron alloys sheds light on the reinforcements of French Gothic Cathedrals. J. Archaeol. Sci. 2015, 53, 190–201, doi:http://dx.doi.org/10.1016/j.jas.2014.10.016.