Tzagkaroulakis, Ioannis and Boxall, Colin (2017) Real time nanogravimetric monitoring of corrosion in radioactive environments. PhD thesis, Lancaster University.
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Abstract
Monitoring and understanding the mechanism of metal corrosion throughout the nuclear fuel cycle play a key role in the safe asset management of facilities. They also provide information essential for making an informed choice regarding the selection of decontamination methods for steel plant and equipment scheduled for decommissioning. Recent advances in Quartz Crystal Nanobalance (QCN) technology offer the means of monitoring corrosion in-situ, in radiologically harsh environments, in real time and with high sensitivity. Oxalic acid has been widely used in nuclear plants and installations as a corrosion inhibitor for carbon steels and as a decontamination cleaning agent due to its ability to remove rust from the surface of ferritic metals and alloys. As an exemplar system for decontamination, the corrosion behaviour of mild carbon steel and pure iron samples in 1 wt% to 8wt% oxalic acid solutions have been measured and compared in real time and in situ using QCN. Corrosion rates measured using QCN are found to agree with those obtained using corrosion current (iCORR) measurements, with the added advantages of: (i) real time measurements of higher sensitivity with the potential for making them in situ; (ii) reduced uncertainty in the conversion of the QCN measured frequency change to a mass change-based corrosion rate compared to the conversion of the iCORR measured by LSV with Tafel extrapolation to mass change; (iii) the provision of mechanistic insights into the action of oxalic acid on Fe-rich steels. QCN has been used as a corrosion monitoring technique to study the impact of radiolytically generated peroxide on the corrosion protection afforded by oxalate to Fe surfaces. Results reveal that QCN can be the ideal means of measuring corrosion rates in situ, in real time, when two chemical reactions are taking place such as the ferrous oxalate layer production and the breakdown of the layer due to Fenton reactions. QCN has been studied as a corrosion monitoring technique for other decontamination solutions such as basic and acid permanganate and oxalic acid / nitric acid mixtures. Results reveal that QCN is a very promising tool for studying the efficacy of these new decontamination agent formulations with the sensor providing useful mechanistic information of the mode-of-action of the decontaminating agent from the mass change (gain or loss) in real time.