Hyperspectral imaging for detection of corrosion on intermediate level nuclear waste containers

Intermediate level nuclear waste (ILW) will be stored above ground in 304L stainless steel (SS) containers for the next 100 years. During this period, the containers need to be monitored for atmospheric pitting corrosion - a known precursor of atmospherically induced stress corrosion cracking. Hyperspectral (HS) and optical imaging of pitting corrosion products from droplet experiments have been investigated towards developing a system for long term monitoring of atmospheric pitting corrosion of stainless steel containers in ILW stores. Common corrosion products were first identified via Raman spectroscopic mapping as akaganeite (β-FeOOH) and lepidocrocite (γ-FeOOH), with a secondary presence of layered double hydroxide (green rust). HS and optical methods were then compared for their efficacy at rust detection. Whilst it was not possible to identify specific corrosion species using HS imaging, HS images of rust under pitted droplets provided better contrast with the background steel than colour photography due to species having lower absorbance the near infrared (850 nm) than red (650 nm). Finally, the relationship between rust area and pit volume was determined by comparing colour photography (rust area) with confocal laser scanning microscopy (pit volume). A good correlation was present for samples exposed to a fixed relative humidity (RH) for MgCl2 droplets and CaCl2 droplets with small pit volumes. Poor correlation was found for samples exposed to natural fluctuations in RH. It was concluded that optical methods are viable for the detection of rust, but less effective for quantification of pit volumes