Impact of ion and neutron irradiation on the corrosion of the 6061-T6 aluminium alloy

Abstract

In nuclear research reactors, aluminium alloys are corroded and an aluminium hydroxide film covers their surface. Defects created by neutron irradiation can have a detrimental effect on this corrosion. In this study, two ion irradiations are performed on a 6061-T6 aluminium alloy. The first irradiation experiment aims at studying the effect of metal matrix irradiation on aluminium corrosion. The second irradiation experiment aims at studying the effect of hydroxide irradiation on aluminium corrosion. The displacement per atom is at most 2.5 dpa in the hydroxide film irradiated with Al ions of 1.2 and 5 MeV and at most 8 dpa in the aluminium matrix irradiated with Al ions of 1.8 MeV (calculated with SRIM). The effects of ion irradiation are investigated by TEM and SEM observations. The crystalline structure of aluminium hydroxide is studied by electron diffraction, µ-Raman and X-rays diffraction. In the aluminium hydroxide film (composed of a mix of bayerite, α-Al(OH)3, and boehmite, γ-AlOOH, before irradiation), ion irradiation causes formation of voids and dehydrates the aluminium hydroxide (nanocrystallites of η-Al2O3 are observed in the irradiated film). In the aluminium matrix, irradiation increases density of dislocations and amorphizes dispersoids. After ion irradiation, samples are corroded at 70°C in 2.8L of demineralised water. The two irradiations increase aluminium corrosion. The second part of this study is about samples corroded in the Osiris nuclear research reactor, in water at 42°C, with a pH of 6 and for 18 months. The aluminium hydroxide film observed on these samples has a layered microstructure composed of a compact inner layer, a thick intermediate layer and an outer layer of cuboid microcrystals. Silicon enrichment is observed in the inner layer. The effect of ion and neutron irradiation on aluminium corrosion is compared in this study and similarities are observed between ion and neutron irradiations.