Corrosion Mechanisms of Austenitic Steel in Salt Rock Repositories of High-Level Radioactive Waste
Authors: Munoz Andres Gabriel , Schild Dieter 
Published: February 2020

Salt rock is presently pondered as a host for a deep repository of high-level radioactive waste in Germany [1]. Its plasticity and impermeability reduce the probability of a direct contact of containments with aqueous solutions. In a worse scenario, however, the steel containment would meet a saturated brine. In Germany, repository concepts for salt rock envisage the use of Cr-Ni steels for containers of high-level radioactive waste [2]. Therefore, metallic corrosion in this environment defines the overall performance and longevity of the steel-based containment. After ca. 30 year of the closure, surface temperatures of 100-120°C can be reached due to the heat released by radioactive decay. The consumption of initially trapped oxygen by corrosion of the copper shell barrier (partial electrochemical reduction reaction), by microbiological processes and/or by mineral reactions leads to an anaerobic environment. Under these conditions the alloy corrodes with the production of hydrogen [3]. Apart from its extremely aggressive environment, the repository is also characterized by high hydrostatic and ground pressures (p>100 bar). The nature of corrosion products is also relevant, because they may retard the migration of radioactive contaminants that have been mobilized from the waste matrix [4].
Austenitic steels are susceptible to pitting corrosion over a certain chloride concentration threshold [5,6]. The passivity strength depends on concentration of Cr and Mo incorporated in the oxide film [7]. According to the empiric the pitting resistance equivalent number (PREN) = %wt Cr + 3.3 %wt Mo + 16 %wt N [8], it decreases in the order (AISI): 304L > 316L≈309S> 904L.
The corrosion of AISI 309S was investigated in representative NaCl- and MgCl2-based saturated brines by a special designed high-pressure and high-temperature electrochemical reactor. Classical low potentiodynamic polarizations and electrochemical impedance (EIS) experiments were complemented by morphological and surface chemical analysis by SEM-EDX, XPS and Synchrotron-PES.
Anodic polarizations show that stainless steel AISI 309S start to dissolve by a localized mechanism beyond a threshold potential. The breakdown potential decreases almost linearly with the temperature with a larger slope than the re-passivation potential does. Thus, the localized process governs the free corrosion of the alloy under contact with hot concentrated brines (t>100 °C). At moderate pressures (6 bar or lower), the breakdown of passivity is characterized by the formation of shallow pits which grow initially in the radial direction and then advance vertically engraving stripes on the surface. Under pressures of 50 bars, on the other hand, the dissolution front advances radially. The grain structure is revealed at the center of the spreading dissolution patches, characterized by distinct crystalline textures and the formation of strings assigned to FeCr-carbides released by dissolution of the alloy matrix (see Fig.1). The dissolution of chromium seems to play a decisive role for the advance of the corrosion front due to the related strong local acidification caused by its hydrolysis. Cr(OH)3, Ni(OH)2 and MoO3 are the main surface compounds identified by XPS. Fe 2p and O 1s signals suggest the presence of Fe2O3 and Fe3O4 outside the corrosion front, but not inside it. The formation of a chloride salt film is postulated to maintain the active state. The formation of sulfate salts, on the other hand tends to neutralize the active dissolution. The evolution of the chemical environment during corrosion was modelled with the help of thermodynamic calculations performed with the Geochemist´s Workbench® tool, which applies the Pitzer´s formulation for high ionic strength media.

Figure 1: SEM micrograph showing the corroding front on AISI 309S in a cyclic current-voltage experiment performed at 80°C and 50 bar of pressure.

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Figure 1