For high-level and intermediate-level long-lived waste mainly issued from the nuclear industry and especially from the reprocessing of the nuclear spent fuel, the French National Radioactive Waste Management Agency (Andra) is currently studying a geological disposal site, called Cigéo, in eastern France. It is to be drilled in a very stiff clay (Callovo-Oxfordian claystone) formation. A carbon steel casing will be inserted inside the horizontal tunnels and a specific cement grout (pH ~10) will be injected between the carbon steel casing and the claystone. The vitrified waste is set in a stainless steel container itself set in a carbon steel overpack that will be sled inside the carbon steel casing. A high temperature (max. 90°C) is expected at the overpack surface due to radioactive waste activity.

It is now envisioned to protect some specific parts of the carbon steel overpacks with Zn or Al based sacrificial coatings. Such metallic coatings were never studied in the particular conditions of the radioactive waste management described above. For that purpose, carbon steel electrodes were coated, using thermal spray, with Zn, Al or ZnAl15, 500 µm thick, coatings. These coated electrodes were immersed at 80°C in specific solutions simulating the electrolyte present in the pores of the cement grout after interaction with the surrounding claystone. In both solutions, for each coating, localized corrosion processes were observed. Al coatings were characterized by very low corrosion current density.

A solution with more sulfide (10^-3 mol L^-1 instead of 10^-6 mol L^-1) was also considered to study the influence of these species that could be produced by micro-organisms such as sulfate-reducing bacteria. The first results showed an increase of the corrosion density current for the Zn and Zn15Al coatings when the sulfide concentration was lower. This behavior could be due to the formation of a protective layer of corrosion products induced by sulfides.

Coated carbon steel electrodes were connected to bare carbon steel electrodes to simulate a coating defect. A surface ratio of S_cathode/S_anode= 1/10 was chosen. The aim was to study the efficiency of the protection ensured by the sacrificial coatings. The experiments were performed for up to 6 months and the evolution over time of the coated carbon steel electrodes was monitored by electrochemical impedance spectroscopy and linear polarization resistance measurements. At the end of the experiments the mineral layers formed on the coated and uncoated coupons were analyzed by X-ray diffraction, µ-Raman spectroscopy and µ-X-ray fluorescence spectroscopy. The three coatings (Zn, Al and Zn15Al) proved efficient and the electrochemical measurements demonstrated that the uncoated electrode remained the cathode. The carbon steel electrodes coupled with coated electrodes were mainly covered with aragonite (CaCO₃). This confirms that these electrodes behave as cathodes, the increased cathodic rate leading to an increase of interfacial pH and the formation of aragonite, i.e. a so-called “calcareous deposit”.The coated samples were also analyzed after experiment to characterize the corrosion products and determine the morphology of the degradation.