Technischer Bericht NTB 85-22
Überlegungen zur Thermodynamik und Kinetik der Reaktion zwischen Eisen und Wasser in Bentonit
The report presents the thermodynamic relations and equations for the equilibrium potential of an iron saturated magnetite electrode, the equilibrium hydrogen partial pressure and the solubility product of magnetite as a function of system temperature and pressure; the values of these functions are calculated from the known standard enthalpies, specific heats and molar volumes. For temperatures between 300 K and 600 K and for pressures up to 400 bar the most probable values of the hydrogen partial pressure is close to 1 kbar and the activity of divalent iron in equilibrium with magnetite is of the order of a few µM. The pH value corresponding to the equilibrium solubility in pure water is close to that of water in bentonite. It should be borne in mind that, in view of the uncertainties on the thermodynamic data upon which the calculations are based, the results can only be considered as orders of magnitude.
The corrosion behaviour of iron in bentonite is discussed next. The corrosion rate is essentially determined by the parabolic growth of an oxide layer. Over a period of time of up to a few years the corrosion rate should fall below 0.1 µm a-1, even at 150 °C. Assuming this rate as stationary, it is shown that the equilibrium partial pressure of hydrogen cannot exceed some 60 bar. For these estimates the data used, especially as regards diffusion geometry, are those of current Nagra feasibility projects.
For instationary conditions as well, partial pressures of only a few bar should be expected, although the corrosion rates are higher. At hydrostatic pressures of 100 bar or more gaseous hydrogen evolution cannot take place due to the kinetics of the process.