The degradation of iron-based materials by atmospheric corrosion is a well-known issue that may have consequences, including for the storage of radioactive nuclear wastes. In underground storage conditions, the environment will rapidly be saturated with water leading first to an atmosphere with 100% humidity and then to the presence of liquid water resulting from condensation or infiltration from the soil. During these periods, it is of importance to determine the damages due to corrosion and to evaluate if irradiation may influence these damages. In particular, the radiolysis of the water film in contact with air can lead to the formation of oxidizing species and to an increase of corrosion rates.
Chosen experimental conditions are near the expected ones in the French concept of nuclear waste repository:
- temperature 80°C,
- two irradiation dose rates (10 and 20 Gy/h) and of course one set of experiments without irradiation for comparison,
- two low alloyed steels (API 5LX65 for the alloy of the micro-tunnel casing and P285H for the alloy of the overpack);
- the experiments were performed with coupons exposed in autoclaves with two initial gas compositions (O2/N2 = 20/80 and 40/60) and two pressures (2.2 and 4.4 bar) in order to emphasis the possible effect of oxygen;
- in some experiments, coupons were exposed in 100% humid atmosphere (CASIMIR 2 experiments), but other coupons were half or totally immersed in liquid solution (CASIMIR 3 experiments).
The coupons have been exposed between 3 months and up to 18 months. At the end of experiments, gas and aqueous solutions were analysed. After exposure, weight gain and weight loss after desquamation were performed on some coupons together with visual examinations including with binoculars. On other coupons, analyses of the corrosion products were performed by X-ray and observations by Scanning Electron Microscope together with analysis by Energy Dispersive X-ray Spectroscopy) were also performed.
The paper will include details on various results that may be summarised as follows:
- no significant differences between the two steels, neither between the two gas pressures and neither between the two oxygen concentrations (20% and 40%).
- The oxygen concentration decreases and the hydrogen concentration in the gas phase increases when the corrosion damages increased, as expected, but oxygen and hydrogen are found together in the atmosphere of several autoclaves, showing that the cathodic reaction with water occurs before the end of the cathodic reaction with oxygen.
- The corrosion products are mainly based on hematite at the beginning of exposure and on magnetite after longer periods of exposure, in accordance with the evolution of redox conditions.
- Large scattering is found regarding the damages of the coupons under the same conditions exposed to O2/N2 gas with 100% humidity: it is linked to the heterogeneity of atmospheric initiation on these nonpreviously corroded coupons.
- For the immersed or semiimmersed coupons, the metal under the water line exhibits wide strongly corroded areas neighbouring no corroded areas.
- No differences are found between coupons exposed to irradiation and those which were not irradiated; corrosion rates obtained without irradiation are nearly highest than those obtained under 10 or 20 G/h.
In summary, under irradiation between 0 to 20 G/h, no detrimental effect of radiolysis has been found on water-saturated corrosion of low-alloyed steel at 80°C while the evolution of oxygen and hydrogen in the gaseous atmospheres is as expected as well as the evolution of corrosion products.
Acknowledgments: the authors thank very much DM2S/LPEC team for dose rate calculations and DPC/LABRA team for the irradiation experiments made in Poseidon facility. They would like also to thank very much EDF and Andra for co-funding this work.
corrosion, radiolysis, steels, atmospheric corrosion, aqueous corrosion
