Graphite Cast Iron GGG40 for Disposal Containers: The Corrosion Mechanism in Opalinus Clay Water
Authors: Muñoz Andrés Gabriel , Schild Dieter 
Published: February 2024

The overpack of the Pollux® 10 container, taken as a reference for the storage of fuel elements in different types of host rocks, is manufactured from spheroidal graphite carbon steel (GGG 40 / 0.7040) [1]. The near field environment after emplacement is constituted by bentonite, which turns moisty, warm, and anaerobic after some decades because of the decay of the heat generation and the consumption of oxygen by reaction with the container wall, microbes, and minerals [2]. Thus, the corrosion mechanism of metal containers in these conditions is an essential issue for the design of repository concepts of radioactive waste.
For a better understanding of the corrosion processes taking place at the interface of steel and saturated bentonite, electrochemical and surface chemical analysis, including SEM-EDX and local XPS were performed on steel in Opalinus Clay water under different temperatures, pH´s and pressures. Electrochemical measurements were carried out in a dedicated reactor cell for working at high temperature and high pressures (up to 150 bars). A Solartron electrochemical interface 1287 connected to a frequency analyzer 1260 was used.
Polarization curves performed in deaerated solutions are characterized by a low cathodic constant current separating the onset of water reduction and the active dissolution. The presence of oxygen increases the current of the cathodic plateau shifting the corrosion potential towards more positive values. The metallographic structure of the steel give rise to local predominantly anodic and cathodic elements, i.e., ferritic, and pearlitic phases, on the one side and cementite and graphite on the other side, respectively.
The dissolution of the active phases is controlled by the formation of an oxide film mainly constituted by magnetite and silicates containing Fe, Al, Mg and Cr formed by oxidation of the silicon contained in the alloy. SEM pictures obtained after 48 h of corrosion under anaerobic conditions and 5 bar of pressure show the deposition of silicates with different morphologies: as clusters of spheres and as hexagonal platellets (see fig.1). Under 100 bar of pressure, however, an rather etched aspect can be observed, revealing clearly the different metallographic phases.
The impedance diagrams obtained at the corrosion potential shows an inductive loop at low frequency. Based on the shape of potentiostatic anodic current transients, the inductive loops can be ascribed to the dynamic of formation-dissolution of the interfacial oxide-silicate film. The effect of hydrostatic pressure is particularly interesting. The shiny optical aspect of the surface after corrosion under pressure can a priori be explained by the reducing atmosphere created by water reduction under these conditions.
The significance of the formation of silicates is evident in the surface analysis of samples after long-term experiments in contact with Wyoming bentonite saturated with pore Opalinus Clay water. TEM pictures of a cross sectional lamella including part of graphite sphere shows the formation of a silicate film covering the corroding surface with an irregular adherence. The accumulation of silicates at the surface creates an additional barrier for the metal dissolution, which reduces the corrosion rate by a factor of thousand.
References
1. T. Hassel, A. Köhler, Ö.S. Kurt, Das ENCON-Behälterkonzept – Generische Behältermodelle zur Einlagerung radioaktiver Reststoffe für den interdisziplinären Optionsvergleich, ENTRIA-Arbeitsbericht-16, Institut für Werkstoffe, Leibniz-Universität Hannover (2019), Hannover, Germany.
2. F. King, C. Padovani, Corros. Eng. Sci. Technol. 46 (2011), 82-90.

Figure 1: SEM pictures of the surface of GGG40 after corrosion during 48 h in anoxic Opalinus Clay water at 30°C and p: 5 bar

Figure 1