AbstractLow temperature water vapour corrosion of uranium niobium alloys (3 wt% and 6 wt% niobium) and the base metals were investigated. It was found that the alloys exhibited slower corrosion rates than the two base metals, confirming the enhancement of corrosion resistance. However, the UNb6 alloy exhibited similar reaction rates to the UNb3 alloy, which was unexpected as it has previously been shown that an increase in niobium content leads to an increase in corrosion resistance.
Atom probe tomography (APT) analysis was able to provide a more detailed picture of the corroded structure and allowed some key information related to the mechanism to be elucidated. In particular, for uranium metal after exposure to air (or D2O), uranium hydride (or deuteride) was clearly detected using APT. The presence of hydride is of great significance for the mechanism. For niobium metal, hydride was also detected by both SIMS and APT. This has previously been seen and it has been suggested that the hydride is involved in the oxide formation, similar to Baker’s proposed mechanism for uranium.
Energy dispersive x-ray spectrometry (EDX) of Transmission electron microscopy (TEM) sections and APT analysis of corroded alloy samples showed clear phase separation in the UNb6 samples. This phase decomposition is most likely due to ageing as the UNb6 material is at least 30 years old. Previous work has shown ageing to have an effect on UNb alloys, leading to an increase in strength but a decrease in both ductility and corrosion resistance. This age decomposition of the material could possibly explain the unexpected kinetic results, as any phase separation could lead to the alloy having niobium depleted regions (as seen by APT and EDX), which means the material would react in a similar manner to that of pure uranium, i.e. significantly faster.
|Date of Award||19 Jun 2018|
|Supervisor||Thomas Bligh Scott (Supervisor)|