Adsorption of lithium onto zirconium alloys. The role of acid-base interactions

The surface properties of oxides of two kinds of zirconium alloy (namely Zi rcaloy-4 and ZrNb, which are commonly used as the cladding for nuclear fuel rods) are investigated. XPS analyses were carried out on both samples afte r a standard thermal treatment in a pressurized autoclave. An oxide essenti ally composed of ZrO2 characterizes the surface of Zircaloy-4, with some ti n detected in the oxide layer, whereas the surface of the ZrNb alloy presen ts an oxide layer composed of both zirconium and niobium oxides. Angle-reso lved analysis shows the surface to be enriched in niobium oxide (presumably Nb2O5). Wettability experiments were then performed on both alloys, which had been air-aged or aged in an aqueous solution simulating the coolant sol ution of pressurized water reactors (PWRs). The dispersive and acid-base co mponents of the work of adhesion were determined. In addition, the points o f zero charge of both samples were determined by measuring the contact angl e versus the pH of the aqueous solution. Under the conditions studied, the wetting measurements seem to indicate that lithium adsorption, which is pre sent in the coolant PWR solution, is easier at the ZrNb surface than at the Zircaloy-4 surface. This observation was confirmed by AES investigations o f the interfaces between the PWR coolant solution and zirconium alloy oxide s. In addition, the wetting measurements with respect to the pH of the PWR coolant solution indicate a more complex evolution of the surface charges o n the ZrNb surface than on the Zircaloy-4 surface.