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midities as low as 40% in a temperature range between 60 and 90ºC[3]. Aqueous salt solutions
formed after the precipitation of chlorides onto iron powder enable aqueous film electrochemical
corrosion to occur. This could explain the high concentrations of chlorine found in goethite, which
is a typical product of “wet corrosion”, in the cell dismantled after 6 months. At longer times,
hematite is formed at the Fe/bentonite interface by the thermal dehydration of goethite.
Hematite and maghemite found in the iron powder reacted for 15 months seems to have been
formed by the thermal transformation of a hydrated iron oxide (goethite, lepidocrocite, ferrihydrite),
as well. However, in this case, corrosion happens in a much lesser extend and the transformation
may be faster. Chloride traces found in corroded iron powder may help corrosion to start where it
was deposited.
50 �m
50 nm
Figure 5. (Left) TEM image of a hematite suspension obtained from corroded iron powder and
SAED pattern of one of the crystals; (right) EDS analysis of the diffracted crystal.
5. Conclusions
Initial precipitation of chloride plays a relevant role in the first stages of the corrosion process, as it
helps to initiate the nucleation of goethite. When bentonite at the interface gets desiccated, goethite
transforms into hematite. Once, the chloride front reaches the interface and precipitates onto the
iron powder, it seems that corrosion is enhanced again. Results obtained in these tests are preliminary
and will be completed after the dismantling of the four experiments on-going at the moment.
6. Acknowledgements
This is a contribution to the NF-PRO IP (Integrated Project) number FI6W- CT-2003-02389 financed
by the European Union and the CIEMAT/ENRESA association.
References
[1] M.V. Villar, A.M. Fernández, P.L. Martín, J.M. Barcala, R. Gómez-Espina, P. Rivas, Effect
of heating/hydration on compacted bentonite: tests in 60-cm long cells, CIEMAT, Madrid,
2008, p. 76.
[2] J. Majzlan, K.-D. Grevel, A. Navrotsky, Thermodynamics of Fe oxides: Part II. Enthalpies of
formation and relative stability of goethite, lepidocrocite and maghemite, Am. Mineral. 88
(2003) 855-859.
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