Unravelling the corrosion processes at steel/bentonite interfaces in in situ tests

Abstract

Microscopic and spectroscopic analyses were conducted on steel/bentonite interface samples removed from four in situ experiments that were carried out in three underground research laboratories at different temperatures and under different hydraulic and geochemical conditions. The results provide valuable information about the corrosion processes occurring in high‐level radioactive waste repositories. Systematic patterns can be deduced from the results, irrespective of carbon steel grade, type of bentonite and its degree of compaction, geochemical environment or experimental setup. Thus, a clear dependence of the corrosion rates on temperature and exposure period, as well as on the availability of H2O and O2 provided by the surrounding bentonite buffer, is observed. Furthermore, Fe(II) ions released by corrosion interact with the structural Fe in the clay. Recent developments highlight the usefulness of reactive transport modelling in understanding the coupled corrosion and Fe–clay interaction processes.

Effect of Iron Chloride (II) on Bentonites under Hydrothermal Gradients: A Comparative Study between Sodium Bentonite and Calcium Bentonite

“…Two types of experiments have been traditionally used to study iron-bentonite interactions: in situ experiments, which are conducted under realistic disposal conditions in underground galleries, and laboratory experiments, which enable controlled experiments under a wider range of experimental conditions, including elevated temperature and hydration [23][24][25][26][27]. A long-term in situ corrosion experiment was conducted in Opalinus Clay at the Mont Terri Underground Research Laboratory (Switzerland), where carbon steel coupons were embedded into MX-80 bentonite.…”

“…Wersin and Kober [16,27] proposed a corrosion mechanism for the iron-bentonite interaction comprising four stages: (0) Formation of a thin magnetite layer on carbon steel and prevailing structural Fe(III) in the smectite component at low moisture; (1) Fe(III) oxide formed, whether anhydrous, such as hematite and maghemite, or oxyhydroxides, such as goethite and lepidocrocite, depending on the bentonite moisture content; and (2) the thickness of the corrosion layer increases and the transfer of O 2 and H 2 O into the steel decreases. Anaerobic corrosion arises within the corrosion layer, generating Fe(II) and resulting in the formation of magnetite and siderite in the bentonite interface.…”