In France, the radioactive wastes will be confined in a vitreous matrix surrounded by steel barriers. The nuclear packages will evolve 500 m deep in an anoxic clayey medium saturated with carbonated water for several thousand years. Understanding the iron corrosion mechanisms is crucial to define the dimensioning of the system.

Previous theoretical (Bataillon et al, 2010) and experimental studies on archaeological analogues (Leon et al, 2014) and laboratory samples (Leon et al, 2017) allowed to identify a FeII/III submicrometric layer at the interface Metal (M)/Corrosion Product layer (CPL) that seems to passivate steel. A the same time, the possibility of delocalizing the cathodic reaction has been demonstrated on iron archaeological analogues (Saheb et al, 2011. Mercier et al, 2018) questionning its impact on the corrosion active surface and therefore the rate of corrosion.

The aim of this study is to clarify CPL properties at an earlier corrosion stage. Chemical, physical and electrochemical properties of the CPL such as phase distribution, porosity, conductive network and cathodic reaction location need to be elucidated. Therefore, short-term corrosion experiments (~700 h, 120°C) were carried out, on simplified systems representative of the storage system, to see how CPL properties are set-up. The impact on the properties of CPL of the type of metallic substrate (pure ferrite and ferrito-pearlitic steel) and the type of environmnent (carbonated or silicated) is followed.

The corrosion environment (carbonated or silicated) does not seem to modify the chemical properties of the interfacial layer which is composed of magnetite Fe3O4 on both systems (µ-Raman). However, its thickness and crystallinity (TEM) can vary significantly, from a thin layer (few nm to few µm) of micrometric grain of magnetite for the carbonated system to heterogeneous layer (few µm to several dozen of µm) of nanometric grains on the silicated system. These differences it can send back to various permeability properties of this interfacial layer and therefore influence its passivating properties.

Results (SEM-EDS and μ-Raman) obtained on samples from the carbonated system for both ferritic and ferrito-perlitic metals showed CPL whose chemical properties are similar to CPL developped on experiments carried out over longer periods and on archaeological analogues. CPL can be described as follows: a matrix of siderite FeCO3 with magnetite islets arranged in discontinuous strip inside the carbonated matrix as well as at the M/CPL interface. Electrical properties investigated by C-AFM showed that magnetite for both metal as well as cementite for the ferrito-pearlitic contained in the insulating matrix of siderite are electrically connected to the metal. BSE images acquired by FIB-tomography on the pure ferrite highlighted that the internal part of the siderite matrix contains another phase, brighter than siderite that seems to be continuous from one image to another. We hypothesize that electrons from the iron metal circulate through this three-dimensional network in the insulated carbonate matrix.