Electrochemical mechanisms of corrosion of magnesium alloys in cementitious media

The reprocessing of spent fuels from UNGG (Uranium Natural Graphite Gas) nuclear reactors in France generates cladding wastes mainly composed of Mg alloys. The management of these wastes involves their immobilization in hydraulic binders with high pH interstitial solutions (pH>12.5).In the case of magnesium corrosion, the choice of these materials for immobilization and storage is not straightforward due to the characteristic reactivity of these alloys, which is caused by several aspects. First, magnesium corrosion is strongly influenced by the pH and the composition of the electrolyte (the pore solution of hydraulic binder). Furthermore, the electrochemical potential of Mg places this metal as an anode with respect to several materials (e.g. residual graphite materials initially present in the fuel assemblies), which can create galvanic couplings and accelerate corrosion.Previous work has found Na-geopolymer mortar in the presence of fluorides (magnesium corrosion inhibitor) and alkali-activated slag mortar appropriates materials for embedding these wastes. Nevertheless, the electrochemical mechanisms involved in the magnesium corrosion process must be identified in these two reference scenarios.The purpose of this work is to investigate the corrosion processes that occur once the Mg alloys are embedded in their immobilization matrices. The goal is to understand the phenomena that lead to low corrosion rates in these materials, in the case of general and galvanic corrosions.From these guidelines, a study of the cementitious matrices (porosity and resistivity) and their interstitial solutions (pH, ionic species and conductivity) was carried out in order to identify the main parameters that can limit the corrosion rate of magnesium. The corrosion behavior of Mg alloys against these conditions has been tested in model solutions and in each cementitious binders using electrochemical techniques (OCP, ZRA and polarization curves), gravimetry, surface characterization (XRD, SEM/EDS and FIB/STEM) and numerical simulation (COMSOL Multiphysics).Both matrices are relevant for the storage of Mg wastes and have a thermodynamically favorable pH for magnesium passivation due to the precipitation of Brucite (Mg(OH)2) on their surfaces. The high resistivity of the alkali-activated slag and the consumption of O₂ by the sulphides of the pore solution are favorable aspects to reduce the galvanic corrosion rate of magnesium (Mg-Mn/Graphite). In the case of the geopolymer, general and galvanic corrosions of magnesium were evaluated by electrochemical and gravimetric measurements and in both cases, the nature of the corrosion products was characterized. The results revealed that the presence of corrosion inhibitors (silicates and fluorides) is mainly responsible for the reduction of Mg corrosion, because of a protective film formed at the Mg-Zr/geopolymer interface. However, the corrosion rate may vary with the concentration of NaF, which is related to the nature and the morphology of the corrosion product formed on the magnesium surface. (author)