Radiolysis influence on low alloy steel atmospheric corrosion at 80°C
Résumé
The degradation of iron-based materials by atmospheric corrosion is a well-known problem that
may have incidence, especially for the storage of radioactive nuclear wastes. It is of importance
to evaluate the damages due to this kind of corrosion within long time-periods. One parameter
which can influence atmospheric corrosion in the case of storage container is irradiation. As
atmospheric corrosion happens when a water film condensates at the surface of a material,
radiolysis of this water film in contact with air can have for consequence the formation of acid an
oxidizing species which can modify the corrosion process.
As a consequence the aim of this work is to evaluate the influence of γ-irradiation on
atmospheric corrosion of low alloy steel. In this goal, an experiment, called CASIMIR, has been
developed. During this experiment, samples of low alloy steel have been introduced into small
stainless steel container. In these ones, two different media has been introduced. The first
medium is a small quantity of water in order to fix a relative humidity (RH) of 100 %. The
second medium is a melt of KH$_2$PO$_4$/ K$_2$HPO$_4$in a small quantity of water corresponding to a
relative humidity of about 70% at the surface of the samples. The small containers have been
introduced in a heating system which enables to fix a temperature of 80°C. This system has been
installed in an irradiation pool with three $^{60}$Co sources which fix, according to the location of the
stainless steel in the system, a dose rate of 80 Gy.h$^{-1}$ or 20 Gy.h$^{-1}$ at the surface of the low alloy
steel samples.
After six, nine and twelve months of corrosion in these conditions some samples were extracted
of the experiment and analyzed by different methods. Gravimetry after desquamation of the
corrosion layer enables to measure total corroded thickness in function of RH, dose rate and
time. As show on figure 1, after 6 month of exposure, sample aged with a RH of 100% are more
corroded than the one aged at 70%. Moreover the dose rate does not have an effect on the
corroded thickness for 20 Gy.h$^{-1}$ whereas it increases corrosion when the dose rate is equal to 80
Gy.h$^{-1}$
. This result is confirmed for longer times. Gas analysis after corrosion experiment have
shown that for the higher corrosion rate, oxygen was totally consumed after 6 month and reveal
also a production of hydrogen. Microscopical observations also confirm the influence of dose
rate on atmospheric corrosion.
All the results show that dose rate does not influence atmospheric corrosion until a value
between 20 and 80 Gy.h$^{-1}$. For a dose rate up to these thresholds, irradiation increase corrosion
rate on low alloy steel in presence of high values of relative humilities.
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