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Title: Performance characterisation of duplex stainless steel in nuclear waste storage environment
Author: Ornek, Cem
ISNI:       0000 0004 5990 563X
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2016
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The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.
Supervisor: Engelberg, Dirk Sponsor: EPSRC ; Radwaste Management/Nuclear Decommissioning Authority
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Duplex Stainless Steel ; Microstructure Characterisation ; Environmentally-assisted Cracking (EAC) ; Stress Corrosion Cracking (SCC) ; Atmospheric Corrosion ; Cold Work ; Heat Treatments ; Scanning Electron Microscopy (SEM) ; Transmission Electron Microscopy (TEM) ; Electron Backscatter Diffraction (EBSD) ; X-ray Diffraction (XRD) ; Scanning Kelvin Probe Force Microscopy (SKFPM) ; Magnetic Force Microscopy (MFM) ; Hydrogen Embrittlement ; Secondary Phases ; Spinodal Decomposition ; 475°C Embrittlement ; Sigma Phase ; Chi Phase ; Ferrite ; Austenite ; Chromium nitride ; Strain Partitioining ; Stress Partitioining ; X-ray Computed Tomography ; Digital Image Correlation ; Corrosion Testing ; Pitting Corrosion ; Localised Corrosion ; Selective Corrosion ; Cracks ; In-situ Testing ; Mechanical Testing ; Tensile Testing ; Bending ; Micro-Hardness