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Radiolysis influence on water-saturated corrosion of carbon steels at 80°C

Abstract : The degradation of iron-based materials by atmospheric corrosion is a well-known issue that has to be taken into account, including for the disposal of high-level radioactive nuclear waste. In deep geological disposal conditions, the environment will be saturated with water either in an atmosphere with 100% humidity or with liquid water coming from the claystone host rock. During these periods, it is important to determine the corrosion damages and evaluate whether 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 in corrosion rates. Experimental conditions are close to those expected in the French concept of nuclear waste disposal (temperature and alloy) and are chosen to study the impact of some parameters: Two carbon steels (API 5LX65 for the micro-tunnel casing and P285H for the overpack); Temperature 80°C; Two irradiation dose rates (10 and 20 Gy/h) and of course one set of experiments without irradiation for comparison; 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 look at the influence of oxygen initial partial pressure (which is directly related to the quantity of oxygen available for corrosion); In some experiments, coupons were exposed in 100% humid atmosphere (CASIMIR 2 experiments), but other coupons were half or totally immersed in a liquid solution (CASIMIR 3 experiments). The coupons have been exposed between 3 months and 18 months. At the end of the experiments, gas and aqueous solutions were analyzed. After exposure, weight gain and weight loss after desquamation were performed on some coupons as well as visual examinations including binoculars. On other coupons, X-ray corrosion analysis were performed and observations by Scanning Electron Microscope coupled with Energy Dispersive X-ray Spectroscopy were also performed. The paper will include details of various results that can be summarized as follows: -No major differences are found between irradiated and non-irradiated coupons; corrosion rates obtained without irradiation are nearly highest than those obtained under 10 or 20 Gy/h, the lowest ones being under 10 Gy/h; -No significant differences between the two steels, neither between the two gas pressures, nor 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 non-previously corroded coupons; -For the immersed or semi-immersed coupons, the metal below the waterline exhibits wide strongly corroded areas neighboring no corroded areas. In summary, no detrimental effect of radiolysis was found on water-saturated corrosion of carbon steel at 80°C between 0 to 20 Gy/h, 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, French waste producer, and Andra, French radioactive waste management organization, for co-defining and co-funding this work.
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https://hal-cea.archives-ouvertes.fr/cea-02964041
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Submitted on : Monday, October 12, 2020 - 10:00:06 AM
Last modification on : Wednesday, October 14, 2020 - 4:25:26 AM

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  • HAL Id : cea-02964041, version 1

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Damien Feron. Radiolysis influence on water-saturated corrosion of carbon steels at 80°C. Fukushima research conference on corrosion of key components of Fukushima Daiichi NPS, Dec 2019, Fukushima, Japan. ⟨cea-02964041⟩

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