The microstructure of the corrosion layers of ferrous archaeological objects was studied using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The objects were a 2000 years old steel bar extracted from the Mediterranean Sea and a cast iron bomb taken from a shipwreck of the XVIIIth century excavated from the Atlantic Ocean. The observation of the rust layers in cross sections at the metal/oxide interface revealed that only ferrite (a-Fe) was corroded. It was oxidized into the ferrous hydroxychloride β-Fe2(OH)3CI. On the contrary cementite (Fe_3C), in the case of the steel bar, and the cementite/graphite system, in the case of the cast iron bomb, remained intact. These observations suggest that the degradation of the metal occurred according to a selective corrosion phenomenon. In such a corrosion type, the metallographic structure of the alloy has a determinant influence on the electrochemical process. The preferential dissolution of ferrite demonstrates that in pearlite, cementite and ferrite can form galvanic micro cells, cementite lamellae acting as a cathode and ferrite as an anode. In the case of the cast iron, the presence of graphite can induce more complex couplings. However, in both cases, the cementite of the steel and the cementite/graphite system of the cast iron constitute a conductive network through a large part of the corrosion layer. This can imply a decoupling of the anodic reaction, corresponding to the dissolution of iron, and of the cathodic reaction, localising the latter far from the metal surface if it corresponds to the reduction of oxygen. Actually, the nature of the cathodic reaction likely to occur in an oxygen-poor and biologically active environment like seawater can be discussed. According to the environmental conditions several mechanisms, implying oxygen or not, are to be considered.
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