Atom Probe Tomography of Hydrogen and of Grain Boundaries in Corroded Zircaloy-2

Licentiate thesis, 2012

Due to their low thermal neutron capture cross-section, zirconium alloys are widely used in the nuclear industry for fuel cladding and structural components. The lifetime of the fuel assemblies in the reactors are largely dictated by the ability of the fuel cladding to withstand corrosion and mechanical damage. The waterside corrosion mechanism of zirconium alloys is closely related to another material degradation process, namely hydrogen pick-up. In order to study the hydrogenation of zirconium on the atomic level, atom probe tomography (APT) is utilized. This technique offers some unique virtues for nanometer scale materials analysis, such as equal sensitivity to all elements. However, as APT has rarely been used for hydrogen studies previously, methods for accurate quantitative analysis need to be developed. The vacuum chamber in which APT analysis is carried out typically contains small amounts of residual gases, e.g. hydrogen. Hydrogen gas can be adsorbed onto the APT specimen, and analyzed along with the specimen material. This will obscure the true hydrogen content that is found in subsequent data evaluation. A study of the experimental parameters that govern hydrogen adsorption has been carried out on a nickel-based alloy. Hydrogen adsorption can be reduced significantly by field evaporation either at low field strengths, using high laser pulse energies, or at very high field strengths using voltage pulsing. Supply of hydrogen to the tip apex is concluded to occur by direct gas phase adsorption, and it resides on the surface in a field-adsorbed state. It will then be desorbed through field evaporation, where the field strength in the tip vicinity will determine whether it is detected in atomic ion or molecular ion form. The metal-oxide interface in corroded Zircaloy-2 was also studied using APT. Segregation of the alloying elements Fe and Ni to deformation-induced sub-grain boundaries in the metal was observed. The chemistry of these grain boundaries is subsequently inherited by the oxide as the metal is consumed. This is concluded to be of importance for the corrosion and hydrogen-pickup kinetics, as oxide grain boundaries may act as transport paths for oxygen and hydrogen.