Corrosion of metal iron in contact with anoxic clay at 90 degrees C: Characterization of the corrosion products after two years of interaction
Résumé
Chemical and mineralogical properties of solids formed upon free corrosion of two iron probes (one massive iron rod, and one model overpack made by two pipes covering the ends of a glass rod) in saturated clay rock (Callovo-Oxfordian formation, East of Paris Basin, France) at 90 degrees C over two years were investigated by microscopic and spectroscopic techniques (X-ray tomography, scanning electron microscopy coupled with energy-dispersive X-ray analysis, Raman microspectroscopy, micro-X-ray diffraction, and micro-X-ray Absorption Fine Structure spectroscopy). The corrosion rate of the massive rod was monitored in situ by electrochemical impedance spectrometry, and found to decrease from about 90 mu m/year during the first month of reaction, to less than 1 mu m/year after two years. X-ray tomography revealed the presence of several fractures suggesting the presence of preferential flow and diffusion pathways along the iron samples. Microscopic observations revealed similar corrosion interfaces for both samples. Corrosion heterogeneously affected the interface, with damaged thickness from similar to 0 to 80 mu m. In extensively damaged areas, an inner discontinuous layer of magnetite in contact with metal, an intermediate chukanovite (Fe2CO3(OH)(2)) layer (only when magnetite is present, and only for the overpack), and an outer layer of poorly ordered Fe phyllosilicate were observed. In areas with little damage, only the Fe-silicate solids are observed. The clay transformation layer is predominantly made of ankerite ((Fe,Ca,Mg)CO3) forming a massive unit near the trace of the original surface, and intermixed with clay minerals towards the bulk matrix. The average thickness of oxidized iron, as measured by the average of distances from the original to the final metal surface, was 13 +/- 1 and 15 +/- 1 mu m for the massive rod and the micro-overpack, respectively. The corresponding Fe amount is about twice the amount of Fe present in the corrosion products. Thus a significant Fe fraction migrated in the nearby clay, likely as Fe-(II), and could act as a significant buffer of the electrochemical potential.