Study of the phenomenon of corrosion-erosion of zirconium: case of a jet loaded with erosive particles

The present work focuses on the investigation of the physico-chemical mechanisms leading to the mass loss of zirconium exposed to oxidizing (nitric acid) and erosive (stainless steel particles) conditions, in the industrial context of treatment-recycling of spent nuclear fuel. Different experimental tools have been developed and used to quantify and analyze each contribution to corrosion-erosion (pure corrosion and erosion and their synergy) under the conditions of interest. In the absence of any mechanical damage (pure corrosion), the dissolution of zirconium is very low in nitric medium: its good resistance is due to the growth of a protective layer of zirconia which thickens with the potential and the immersion time in the electrolyte. However, its mass loss is orders of magnitude higher under weakly oxidizing conditions (water) and in the presence of particles (pure erosion). Zr degradation increases further in an oxidizing medium (nitric acid + particles). This greater mass loss is due to the synergy that originates in the mechanical de-passivation of the substrate (following particle shocks) and its dissolution. Complementary experiments (scratch tests) have shown that exposure to nitric acid of a small portion of the metallic surface free of passive layer leads to transient dissolution of zirconium. In the case of repeated impacts observed in corrosion-erosion, it appears that the quantity of dissolved material depends on two parameters: the frequency of the impacts and the time required for the material to be re-passivated. These elements make it possible to explain the results of a sensitivity study carried out around the conditions of interest, which show that the flux of loss of material in corrosion-erosion is all the more important as the velocity and/or the size of the particles. Nevertheless, in the presence of a greater concentration of particles, the mass loss becomes less significant, probably because of the decrease in the incident kinetic energy of the particles at the time of the impact. (author)