Localized Corrosion of Stainless Steel in a Nuclear Waste Cooling Water System—Part 2: Plant Inspection Findings Available to Purchase

Parts of a nuclear waste cooling water system, constructed from an austenitic stainless steel, are known to be susceptible to localized corrosion. This is attributed to the presence of chloride ions in the water, albeit only at low concentration (<10 mg/L), and oxidants produced by water radiolysis. Plate samples were cut and taken for examination from downstream tanks located in man-access areas out of the radiation field. Large pits were found. The largest were highly elongated but had failed to form through-wall penetrations. Smaller pits that were spheroidal in shape were found that had formed through-wall penetrations. The pits were sectioned and their internal morphologies examined by scanning electron microscopy. The elongated pits were found to have a crystalline internal morphology associated with the mechanism of etch-type pitting. The spheroidal pits had a smoother interior morphology but it was not bright or distinctive-like classic pits formed by a salt-filming mechanism. Formation of the spheroidal pits is attributed to corrosion by a quasi-salt-filming mechanism.