About the Corrosion Mechanism of Metal Iron in Contact with Bentonite

Anaerobic corrosion at the metal/bentonite interface determines the performance of bentonite-based high-level radioactive waste (HLRW) barriers. Both magnetite and Fe-silicate formation were previously observed as well as higher corrosivity of bentonites containing low-charged smectites. In the present study, six different bentonites containing differently charged smectites were selected and used for laboratory corrosion tests. The extent of corrosion could be quantified based on the mass loss of Fe-0, the increase of Fe2+ in the bentonite, and the increase of the total amount of Fe in the bentonite, which was liberated by the native Fe and found in the bentonite afterward. Magnetite and H-2 were found, which can be explained, e.g., by the Schikorr reaction, which is often proposed in this context. However, as soon as Si is available in solution, Fe-silicates form. The corrosion is controlled by diffusion, but the extent of the corrosion could be understood based on reactions involving electrons, H-2, and Fe2+. An electron reduces water to H-2, which then may be consumed by reduction of Fe3+ of the smectites. The reduction of structural Fe3+ in turn can lead to destabilization (dissolution) of the smectites, hence providing Si into solution, which in turn reacts with Fe2+. Based on this finding, it is now possible to explain the previously observed correlation: The correlation with the layer charge density may result from the fact that low-charged smectites are easier to reduce, becoming less stable, hence consuming more H-2 and Fe2+, which accelerates the corrosion. Based on this model, it is also possible to explain why sometimes an Fe-silicate coating forms and sometimes magnetite because this depends on the amount of reactive silica, which is naturally present in some bentonites and absent in others.