Study of corrosion processes and irradiation effect on aluminium alloys used for research reactors material structure

Materials Testing Reactors (MTR) are experimental nuclear reactors used to irradiate materials. Aluminium alloys, in particular the 6061-T6, are used in MTR of the core components and for the fuel cladding. In the aqueous media of the core, these alloys are corroded and an aluminium hydroxide film covers their surface. Because of a low thermal conductivity, this film degrades the thermal exchange between the core components and the aqueous media; this can lead to an overheating of the reactor. As a result, it is important to determinate the hydroxide thicknesses on the surface of the aluminium alloys. In the literature, empirical models have been developed to predict these thicknesses depending on the operational conditions of the reactors. Tests in corrosion loop have been performed at a temperature superior to 100 C. The data resulting of these tests is used to extrapolated the empirical models. However, in the French reactors, the maximal temperature of the core components is between 70 and 100 C. Thus, in order to use the empirical models in the French reactors, their application range should be extended to a temperature inferior to 100 C. With this goal, parametric studies are performed with different temperatures (70-100 C), pressures (0.5-12 bar) and pH (5-7.5). These studies indicate that the temperature and the pH have an important impact on the aluminium corrosion and on the hydroxide growth. The hydroxide thicknesses are more important at 70 C than at 100 C. This observation is due to a change in the crystalline phase of aluminium hydroxide: the hydroxide formed at 70 C is less protective for the aluminium oxidation than the one at 100 C. The pH influences the hydroxide solubility and thus the hydroxide thicknesses, a pH of 5 allows to reduce these thicknesses compared to 7.5. However, the data collected during these parametric studies does not allow to adapt the empirical models to a temperature inferior to 100 C, this is due to variations of pH during corrosion tests and to a too low evolution of the hydroxide thicknesses. As a result, more tests with corrosion loop are needed. In addition, in the reactor, the 6061-T6 alloy is exposed to neutron irradiation. According to the literature, this irradiation increases the aluminium corrosion. Ion irradiations have been performed in order to evaluate if it is possible to use ion irradiations to approach the conditions found in nuclear core. The first irradiation with Al ions is performed on the non-corroded metal with at most 14 dpa; this causes an amorphization of the dispersoids and increases the dislocation density. These types of damage seem to increase the hydroxide growth and the aluminium corrosion. The second irradiation with Al ions is performed on aluminium hydroxide with at most 4.5 dpa. This causes a change in the microstructure of the hydroxide crystals (from parallelepiped, they become globular), the formation of cavity and the dehydration of the hydroxide resulting in the formation of nanocrystallites of oxide η-Al₂O₃. These types of damage seem to increase the hydroxide growth. The effects of the ionic irradiations are compared to the results of a neutron irradiation performed in the reactor Osiris at the CEA of Saclay. The both types of irradiation are similar effects on the aluminium corrosion. As a result, tests in a corrosion loop coupled with ion irradiations would be used to enrich the database used to extrapolate the empirical models to temperature inferior to 100 C. (author)