Effect of ion irradiation on the corrosion of an AIFeNi aluminium alloy

AlFeNi is an aluminium alloy used for fuel cladding in some high flux research reactors because of its good corrosion behavior in water at high temperatures. Up to now, the in-reactor corrosion kinetics were evaluated directly on fuel plates irradiated in nominal conditions, but a lack of data was identified on the in-pile corrosion behavior at incidental temperatures (above 100°C). In order to simulate the effect of neutron irradiation on the corrosion kinetics of this alloy and on the structure of the hydroxide film formed in the water environment of a reactor core, ion implantation was performed on un-corroded and pre-corroded AlFeNi samples. Al⁺ ions with an energy of 1.6 MeV were implanted on un-corroded samples to a damage level of 36 dpa (displacement per atom) which corresponds to a few reactor cycles. The pre-corroded samples were irradiated up to 18 dpa. To simulate radiation damage in an oxidizing environment, O⁺ ions were implanted with an energy of 1.4 MeV in both un-corroded and pre-corroded samples creating damage up to 18 dpa. Following irradiation, all samples were corroded in water at 140°C and pH 5.2 for different durations. The oxide layers were characterized at different scales using various techniques (electron microscopy, X-ray diffraction, Secondary Ion Mass Spectrometry). An evolution of the metal microstructure was observed only at the highest Al⁺ damage (36 dpa) by TEM analysis, which revealed the formation of thin precipitates, in good agreement with literature. Samples that were irradiated in the as-received state and then corroded presented an acceleration of the weight gain for a constant oxide thickness when compared to unirradiated corroded specimens. Our results suggest the densification of the oxide layer formed after metal irradiation, and slower anionic diffusion through the inner oxide. On the contrary, irradiation of the oxide layer leads to an increase of the corrosion kinetics, not only in terms of weight gain but also of oxide thickness. These results suggest that the damaged oxide layer contains diffusion paths leading to enhanced cationic diffusion from the metal, and subsequently to faster anionic diffusion. In addition, an amorphisation of the crystalline boehmite layer was observed on these pre-corroded samples. The corrosion kinetics behavior is thus different for samples irradiated in the metal compared to samples irradiated in the oxide. The results obtained in this study suggest that a high fast neutron flux at the initial phases of corrosion can be beneficial to limit end-of-life fuel plate corrosion. (author)