Real Time Nanogravimetric Monitoring of Corrosion for Nuclear Decommissioning

Monitoring and understanding of corrosion on nuclear sites plays a key role in 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 technology offer a means to monitor corrosion in situ in radiologically harsh environments, in real time and with high sensitivity. The QCN measures minute changes in frequency of a quartz crystal resonator with weight gain/loss. Using the Sauerbrey equation, the drop in frequency observed during corrosion testing can be converted to an instantaneous corrosion rate with nanogram sensitivity.

Initial experiments will concentrate on determining corrosion rates in acids and complexants used in chemical decontamination processes, particularly methods involving the commonly used cleaning agents nitric acid and oxalic acid (e.g. the CORD-UV process). Oxalic acid is currently being studied as an Enhanced Chemical Cleaning (ECC) decontamination agent in the decommissioning of high level waste (HLW) storage tanks at the Hanford and Savannah River Sites (SRS) in the US. The tanks are comprised of low carbon steel; thus the decontamination process must be carefully monitored to avoid over-aggressive decontamination that may result in a loss of asset structural integrity.

To avoid this, the oxalic acid concentration being used has been reduced to 1 wt% from the 4-8 wt% range typically used during decontamination campaigns. This has the additional advantage of avoiding downstream precipitation of oxalic acid’s sparing solubility of Na salt, an outcome that presents process engineering and secondary waste management issues in sodium rich environments However, the efficacy of the ECC and the corrosion behaviour of steels at low oxalic acid concentrations is not widely studied and knowledge gaps remain. These gaps afford an ideal opportunity for achieving the twin objectives of device development whilst providing new insights into the behavior of a hitherto unstudied corrosion vulnerable system.

Work to date has concentrated establishing the validity of using pure iron as a low carbon steel surrogate, so allowing the use of less resource intensive iron-based QCN crystals (compared to high unit price steel crystals) for corrosion monitoring during oxalic acid-driven decontamination. Cyclic voltammetric studies of mild carbon steel and iron electrodes reveal a commonality of behavior in 1, 2 and 8 wt% (not shown). In summary: as the applied potential increases in the range -1 to +1 V, iron(II) generated at the electrode surface at ~-0.5 V reacts with solution oxalate to form an insoluble passivating Fe(II) oxalate layer. This persists until +0.4 V whereupon Fe(II) oxalate is oxidized to soluble Fe(III) oxalate, a process that continues until repassivation at ~0.75 V.

Studies using the QCN on Fe electrodes support this interpretation. During the forward scan of the cyclic voltammassogram of Figure 1 a mass increase associated with the formation of the Fe(II) oxalate layer is observed at -0.4V whilst a mass loss, associated with layer oxidation to soluble Fe(III) oxalate, is seen at 0.5V. Total mass change in either direction is 18718 ng cm^-2. The evolution of the chemical composition during the formation and subsequent removal of this protective oxalate layer, the presence of which has ramifications for decontamination efficiency, is currently the subject of in situ Raman spectroscopy experiments in our laboratory.

Figure 1: Cyclic voltammassogram of an iron crystal in 1wt% oxalic acid solution.