Initial Stage Corrosion of Nuclear Relevant Materials: A Study by Electron Spectroscopy and Scanning Probe Microscopy

Metals in the nuclear industry are often required to endure significant periods of storage or inactivity during their lifespan. A level of confidence is therefore required that they will retain the useful structural and functional properties in a given set of conditions for a given period of time. Corrosion is the primary means by which this is compromised for these materials. Studies regarding fundamental corrosion mechanisms can provide data to feed into overarching corrosion models that can predict behaviour with multiple variables (e.g. material grade, temperature, time, contamination type and amount, etc.). Firstly, early stage oxidation of a dilute depleted uranium-molybdenum alloy was analysed in situ under UHV conditions by AES and XPS. At the equivalent of less than 300 ns at 1 atm O2, U-5 wt. % Mo oxidises to form stoichiometric UO2. No molybdenum oxidation is observed. After an oxygen dose of approximately 39 L, the oxide layer approached a limiting thickness of approximately 2.4 nm. The oxidation kinetics followed a logarithmic rate law, with the best fit to the experimental data for the oxide thickness, d, being given by d = 1.26 log(0.12t + 0.56). Changes in oxygen KLL and 1s peak positions associated with transformation from chemisorbed oxygen to metal oxide were observed at similar oxygen doses of 2.3 and 2.6 L O2 by AES and XPS respectively, which opens up the possibility of using well characterised XPS chemical information to inform Auger peak shifts. A novel peak library tool was created with the primary purpose of rapid and accurate identification of Auger peaks. SKPFM was used in combination with optical microscopy and SEM/EDX to assess the range of second phase particle types (size, composition and practical nobility) in an S-65 beryllium sample and evaluate the effect of carbon contamination on corrosion of this material. Carbon deposited by thermal evaporation and pencil marks gave varying thicknesses and distributions of carbon on the surface, each with strong cathodic character with respect to the matrix, but causing no observable increase in corrosion. Pitting corrosion was shown to be associated with pre-existing second phase particles, especially in regions where pencil marking had disturbed the native oxide layer. An in situ technique for assessing the relative electrochemical behaviour of individual second phase particle types was sought to build on current knowledge of their chemical behaviour. Combinations of amperometric and potentiometric SECM methods were developed on model galvanic couple samples, before showing in situ cathodic behaviour of copper and iron containing second phase particles in a beryllium analogue sample, 7075 aluminium alloy (AA7075-T6), in agreement with the well characterised corrosion mechanism for these particle types. Preliminary results were obtained for S-65 beryllium, highlighting the capability of the method to resolve larger scale anodic features