Passivation of uranium towards air corrosion by N-2(+) and C+ ion implantation

The passivation of uranium surfaces against air corrosion, by ion implantat ion processes was studied, using surface analysis methods. Implanting 45 ke V N-2(+) and C+ ions produces thin modified surface layers with gradual gra dients of the corresponding compounds (i.e., nitrides and carbides, respect ively), which avoid the formation of discontinuous interfaces typical to co atings. Such gradual interfaces impart excellent mechanical stability and a dhesion to the modified layers, in spite of the large misfit between the me tal substrate and the implantation on induced compounds. It turns out that these layers provide an almost absolute protection against air corrosion. A rapid initial stage of oxidation of the modified surface layers takes plac e, forming very thin protective oxidation zones (1-4 nm thick), which pract ically stop further air oxidation for years. The mechanism of the initial o xidation stage of the modified layers seems to vary with the type of surfac e (i.e., either nitrides or carbides). However, in any case the protection ability of the formed oxidation products is excellent, probably due to the close match between these compounds and the underlying nitrides or carbides . (C) 2000 Elsevier Science B.V. All rights reserved.