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Title: Mechanisms of fixed contamination of commonly engineered surfaces
Author: Williamson, Becky
ISNI:       0000 0004 7428 0922
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2018
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This project is concerned with developing a greater understanding of the deposition of radioactive solids, colloids, or ions suspended in aqueous liquid onto the surface of stainless steel. Fixed contamination on contaminated metallic surfaces is commonly removed using (electro) chemical methods. The most common methods employed are the use of mineral acids or MEDOC (Metal Decontamination by Oxidation with Cerium). However, these result in dissolution of the passive oxide layer formed at the metal surface. This increases the level of secondary waste which, in turn, increases the burden of effluent treatment plants. The passivation of steels in HNO3 is complicated by the autocatalytic reduction of HNO3 to aqueous HNO2 which attacks the steel surface. We describe the effect of this behaviour on process steels in stagnant and/or flowing conditions. Rotating Disk Electrode (RDE) studies indicate that at HNO3 concentrations ≤20% wt. the reaction is surface based. At HNO3 concentration ≥20% wt. the reaction occurs in the bulk solution. We established a series of corrosion potentials for varying concentrations of nitric acid. These corrosion potentials allowed us to age steel in a controlled fashion. Thus, we describe work carried out on electrochemically accelerated oxide growth on 316L SS and SS2343 in HNO3 media and HNO3 media with radionuclide surrogates (depleted U, Ce and Eu). Characterisation was performed using combined Linear Sweep Voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Quartz Crystal Microgravimetry (EQCM) measurements. Areas of active, passive, high voltage passive, transpassive and secondary passivation regimes in the associated current voltage were identified. Further, we have directly measured the growth of that layer by using in situ microgravimetry. X-Ray Photoelectron Spectroscopy (XPS) was used to determine film composition and presence of contaminant uptake. The passive film on 316L SS is formed of a passive film consisting of Cr(III) hydroxide rich layer and Cr(III) oxide layers at lower potentials. With increasing HNO3 and potential the layer becomes more Cr(III) oxide rich before oxidising to Cr(VI). No radionuclide surrogate contaminants were detected within passive films formed in this study.
Supervisor: Boxall, Colin ; Wilbraham, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral