The role of the diffuse interface of implanted surface layers in preventing gas corrosion - Implantation of N₂⁺ and C⁺ in uranium

The passivation of uranium surfaces against air corrosion and hydrogen attack, by ion implantation processes was studied, using surface analysis methods. Implanting 45 Kev N₂⁺ and C⁺ ions produces thin modified surface layers with gradual gradients of the corresponding compounds (i.e. nitrides and carbides, respectively) which avoid the formation of discontinuous interfaces typical to coatings. Such gradual interfaces impart excellent mechanical stability and adhesion to the modified surface layers, in spite of the large misfit between the metal substrate and the implantation 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 place, forming very thin protective oxidation zones (1-4 nm thick) which practically stop further air oxidation for years. 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. As for hydrogen attack, the protection was only partial, resulting in lower nucleation densities and lower growth rates. It is believed that the hydride nucleation sites remaining on the implanted layer are located at defects (mainly carbide inclusions) at which the implantation process does not seal completely the discontinuity between the inclusion and the surrounding metal matrix. Through such discontinuous interface the hydrogen has a preferential pathway, leading to the hydride precipitation. Preferential oxidation probably also occurs through this interface. Since the oxidation rate is much slower than that of the hydrogenation and the product is similar in density to that of the implantation product (thus not cracking it like the hydride does), its effect is not observable.