Euradwaste '08 - EU Bookshop - Europa

tion of the column [2]. Moreover, the increase in calcium associated with the dissolution of calcite
leads to the oversaturation of pore water with respect to gypsum, which precipitates in the upper
part of the column, leading to a decrease in sulphate. At the bottom of the column, water evaporation
induces the precipitation of anhydrite and calcite, leading to the replacement of Ca by Mg in
the exchange position of the montmorillonite. Throughout the column, there is a clear relationship
between sodium and sulphate, which is more pronounced in the warmer part of the system. This
can be interpreted by precipitation of sodium sulphate minerals (e.g. mirabilite, Na2(SO4)·10H2O),
although precipitation of sodium sulphate-carbonate (e.g. burkeite, Na6CO3(SO4)2) cannot be disregarded,
and would account for the decrease in carbonate in this part of the system. The precipitation
of these phases would lead to a decrease in the sodium occupancy in the exchange sites of the
montmorillonite, being replaced by Mg.
3. Canister corrosion
A variety of iron corrosion experiments have been conducted under NF-PRO to extend and increase
confidence in long-term corrosion rate data. Experiments have been conducted with steel in the
form of coupons, wire, and powder, in pure and synthetic bentonite pore waters, and also in the
presence of either compacted or slurried bentonite. Temperatures were in the range 30 to 100 °C
with experiment durations over two years. Some experiments were performed in a temperature gradient.
Corrosion rate data have also been obtained for steel in compacted bentonite as a function of
external chloride concentration, temperature, and pH. Typical results show that after an initial stage
of enhanced corrosion, the rate decreases to values of ~1 �m/yr (Fig. 3). These data were obtained
using steel wire samples that had been initially pickled in acid to remove residual surface oxide.
The results are interpreted as anodic control of the corrosion rate exerted by the formation of a corrosion
product film on the surface of the iron. The results obtained suggest that during anaerobic
corrosion of carbon steel, two types of corrosion films are formed; one tightly adhering to the metal
surface, and one formed by precipitation of dissolved iron on the surface of the metal. It seems that
the growth of the first layer begins when oxygen combines with metal to form an initial thin oxide
layer. Such an oxide film will grow into the metal by spontaneous formation of oxide matrix sites
on the metal side of the metal/film interface. This oxide film will thicken until an equilibrium is
established after which it will continue to grow slowly into the metal at a constant rate. After the
formation of this layer, the corrosion rate is determined by its dissolution rate. This was also confirmed
by findings that higher temperatures increase the initial corrosion rate but not after steadystate
has been established (Fig. 3). The corrosion rate is generally slightly lower in experiments
without compacted bentonite, than in comparable tests with solid bentonite present. This suggests
that the presence of the clay influences the corrosion reactions (such as the dissolution rate of the
corrosion product film) occurring on the surface of the steel. All the data (including those obtained
outside NF-PRO) clearly suggest that the anaerobic corrosion rates of iron in clay environment will
decrease to values below 1 �m/year after formation of a thin corrosion layer on the surface of metal.
4. Interaction of clay with other barrier components
4.1 Steel
Prior to NF-PRO it was thought that an iron/steel canister would corrode to produce large volumes
of iron corrosion products in situ. Stresses in excess of the sum of the lithostatic and hydrostatic
load could arise in the near-field as a result of the volume of these canister corrosion products, because
canister corrosion products have a lower density than steel. In NF-PRO, several tests have
been performed to obtain information about the corrosion products at the bentonite/steel interface,
both with and without a temperature gradient.
186