Real Time Nanogravimetric Monitoring of Corrosion for Nuclear Decommissioning

Abstract

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 (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 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 behavior of mild carbon steel and pure iron samples in 1 wt% to 8wt% oxalic acid solutions have, for the first time, been measured and compared in real time and in situ using the QCN. Corrosion rates measured using the QCN are found to agree with those obtained using corrosion current (i CORR ) measurements, with the added advantages of: (i) real time and potentially in situ and higher sensitivity measurement; (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 i corr measured by LSV with Tafel extrapolation to similar; (iii) the provision of mechanistic insights into the action of oxalic acid on Fe-rich steels.

Real-Time Nanogravimetric Monitoring of Corrosion in Radioactive Environments

“…QCN based corrosion monitoring is non-peturbative, it simply involves sample immersion, provides direct access to two important corrosion parameters, mass change with the aforementioned nanogram sensitivity and ECORR, and can be conducted in real time. The latter two features allow for the study of changes in corrosion rate on the sub-second timescale and thus the elucidation of the mechanism of the corrosion process under study [102]. Work has centred 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.…”

“…To this end, the oxalic acid concentration being used has been reduced to 1 wt% from the 4-8 wt% range typically used during decontamination campaigns hitherto [42]. Using QCN, the rates of corrosion of mild carbon steel surrogates have been measured in the presence of these oxalic acid-based decontamination solutions and both the removal of iron oxide corrosion product and the formation of a protective layer of ferrous oxalate at the metal surface have been observed [91], [107]. Studies have been extended to newer decontamination solution formulations including alkaline and acid permanganate solutions and oxalic acid / nitric acid blends.…”

“….2.3.A, both iron and mild carbon steel exhibit unstable EOC behaviour at [H2O2] > 180 mM in 1wt% oxalic acid, behaviour that is typical of metastable pit formation and repassivation[91]. For EOC readings recorded at these high hydrogen peroxide values, the maximum value of the EOC recorded in each instance has been used in the compilation ofFigure 3.2.3B.…”