New insights of Auger spectroscopy for the identification of Fe-Si compounds in iron/glass corrosion systems at nanoscale
Résumé
In the context of high-level radioactive wastes (HLW) storage, long term alteration of glass and corrosion of metallic containers leads to the formation of sub-micrometric phyllosilicate phases in the iron Corrosion Product Layer (CPL). The nature of these phases, their properties (porosity, electronic, passivation…) and their spatial distribution must be identified to determine their exact role in the iron corrosion process and to efficiently predict the long-term alteration of nuclear wastes package. In this work, the capabilities of localized Auger spectroscopy, combining surface chemical determination and high spatial resolution, to characterize these phyllosilicate phases is presented. In the present context, the main challenge to overcome was the insulating character of the different compounds and samples, presenting a real disadvantage for Auger implementation. A specific experimental protocol had to be developed to enable acquiring exploitable spectra at sufficiently high resolution to obtain identifiable fingerprints. The main Si-KLL Auger transition of different types of phyllosilicates representative of those encountered in the CPL of the glass-iron systems was acquired to determine if different chemical signatures and chemical shifts could be observed in function of the structure of reference phyllosilicates phases. Indeed, the Si-KLL kinetic energy position value of the phyllosilicate increases with the structure according the following order: smectite group < chlorite group < serpentine group and different line shapes are observed. The significant contribution of the present study relies on the achievement of analyzing such insulating materials by Auger at nanometric scale using a new generation Auger nano-probe (spot size around 12 nm) without drastic preparation requirements, enabling to constitute a Si-KLL Auger data base of phyllosilicates. Thanks to this preliminary analytical approach, the Si-KLL fingerprint measured in the CPL of the glass/iron system altered in an anoxic reactor at 90 °C in synthetic Callovo-Oxfordian (COx) clay-based groundwater solution could be identified as serpentine phyllosilicate. Transmission electron microscopy (TEM) analysis was also used to characterize phyllosilicate and reinforce the AES approach identification, confirming the presence of serpentines.
Domaines
Matériaux
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