Real-Time Nanogravimetric Monitoring of Corrosion in Radioactive Decontamination Systems
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Real-Time Nanogravimetric Monitoring of Corrosion in Radioactive Decontamination Systems Ioannis Tzagkaroulakis1, Colin Boxall1 and Divyesh Trivedi2 1 Lancaster University, Engineering Department, Lancaster, LA1 4YW, United Kingdom. 2 National Nuclear Laboratory, Warrington, WA3 6AE, United Kingdom. ABSTRACT Monitoring and understanding of corrosion on nuclear sites is key to safe asset management (predicting plant life, assessing efficacy of corrosion inhibitors for plant lifetime extension) and supporting informed choice of decontamination methods for steels due for decommissioning. Recent advances in Quartz Crystal Nanobalance (QCN) technology offer a means to monitor corrosion in-situ in radiologically harsh environments, in real time and with high sensitivity. Oxalic acid has been widely used in nuclear facilities 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 behavior of pure iron samples in 1 wt% oxalic acid solution has, for the first time, been measured in simulated radioactive environments in real time, in situ using the QCN. Results show that the QCN is a promising tool for studying the efficacy of oxalic acid decontamination agent formulations with the sensor providing useful mechanistic information regarding decontaminating agent mode-of-action derived from the mass change (gain or loss) in real time when a quartz crystal is immersed in oxalic acid solution in simulated and non–simulated radioactive environments. INTRODUCTION Metals are ubiquitous on nuclear sites as plant and construction materials. Monitoring and understanding metal corrosion plays a key role in safe asset management (predicting plant longevity, assessing efficacy of corrosion inhibitors for plant lifetime extension) and supporting informed choice of decontamination methods for steel structures due for decommissioning. There are 4 common means of monitoring rates of corrosion, all with limitations: coupon testing (unsuitable for real time monitoring); electrical resistance monitoring (has sensitivity issues); linear polarization resistance; and galvanic monitoring (both indirectly inferring corrosion rate in mm/year from current measurements). Recent advances in Quartz Crystal Nanobalance technology [1] offer a means to avoid these issues. The QCN measures minute changes in frequency of a metal coated quartz crystal resonator with weight gain/loss. Using the Sauerbrey equation (see equation 1) the frequency change observed during corrosion testing can be converted to an instantaneous mass change (gain or loss, depending upon the direction of frequency change) and thus corrosion rate with nanogram sensitivity [2]. ∆f = -Cf×∆m
(1)
Where Δf is the change in resonant frequency (Hz), Δm is the mass change (g) and Cf is the sensitivity constant determined from the resonant frequency and electroactive area of the crystals (found to be 0.226 Hz×cm2×ng-1) for the 10 MHz
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