Corrosion Behaviour of Mg Alloys in Alkaline Solutions and in Alkali-Activated Binders

Abstract

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 (12.5<pH<13.7). 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 [1]. 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 [2] (e.g. residual graphite materials initially present in the fuel assemblies or steel from the container), which can create galvanic couplings and accelerate corrosion. Previous work has found low corrosion rates of Mg-Zr alloys and low H2 releases produced when Mg is immobilized in a Na-geopolymer mortar in the presence of fluorides (magnesium corrosion inhibitor) [3]. Similarly, there was a low release of H2 from corrosion during the galvanic coupling between Mg-Mn and graphite, as well as the formation of thinner corrosion product layers when Mg-Mn is immobilized in an alkali-activated slag mortar [4]. These results have contributed to the choice of these two materials as the most appropriates for embedding these wastes. Nevertheless, the electrochemical mechanisms responsible for this weaker corrosion, compared to conventional cementitious matrices, must be identified in these two reference scenari. 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, resistivity and gas diffusion) and its 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, EIS and polarization curves), gravimetry and surface characterization (XRD, SEM/EDS and FIB/STEM). 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 its surface. The high resistivity of the alkali-activated slag and the possible consumption of O2 by the sulphides of the pore solution are favorable aspects to reduce galvanic corrosion rate. In the case of the Na-geopolymer/NaF, the oxygen concentration can also limit the rates of galvanic corrosion, since, during storage, oxygen access could be restrained. However, 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 MgZr/Na-geopolymer interface. This corrosion rate may vary with the concentration of fluoride available in solution and is probably related to the nature of the corrosion product formed on the magnesium surface. General and galvanic corrosion in the Na-geopolymer/NaF mortar were quantified by gravimetric measurements and in both cases the nature of the corrosion products was characterized by scanning electron microscopy (SEM/EDS), X-ray diffraction (XRD) and FIB/STEM (Focused Ion Beam/Scanning Transmission Electron Microscope). The results revealed the stratification of the corrosion products found at the Mg-Zr/Na-geopolymer interface and were in good agreement with the experiments performed in the model alkaline solution. Magnesium corrosion in the Na-geopolymer/NaF mortar is rather limited by the chemistry of the interstitial solution. [1] K. W. Guo , “A Review of Magnesium/Magnesium Alloys Corrosion and its Protection”, Recent Patents on Corrosion Science, vol. 2, pp. 13-21, 2010. [2] G. L. Song and A. Atrens, “Corrosion Mechanisms of Magnesium Alloys”, Adv. Eng. Mater., vol. 1, no. 1, pp. 11–33, 1999. [3] D. Chartier, B. Muzeau, L. Stefan, J. Sanchez-Canet and C. Monguillon, “Magnesium alloys and graphite wastes encapsulated in cementitious materials: Reduction of galvanic corrosion using alkali hydroxide activated blast furnace slag”, Journal of Hazardous Materials, vol. 326, pp. 197-210, 2017. [4] D. Lambertin, F. Frizon and F. Bart, “Mg-Zr alloy behaviour in basic solutions and immobilization in Portland cement and Na-geopolymer with sodium fluoride inhibitor”, Surface and Coating Technology, vol. 206, pp. 4567-4573, 2012.