Passivity of nuclear steam generator tube alloy in lead-contaminated crevice chemistries with different pH

Impacts of lead contamination on the passivity of UNS N06690 alloy were studied using samples passivated in simulated CANDU steam generator crevice chemistries at 300oC and a pH300oC range from 3.22 to 9.26. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analyses indicate that lead entered the anodic films but the level of lead incorporated depended heavily on solution pH. In alkaline chemistry, the presence of lead promoted the incorporation of hydrogen and calcium. The lead ingress hindered dehydration during passivation. Grazing incidence X-ray diffraction analysis indicated that the lead incorporation blocked the formation of spinel oxides during passivation. This effect was enhanced with increasing pH and hardly observed in the acidic chemistry. When lead was absent, the anodic films were Cr depleted in alkaline solution but were Cr enriched in the acidic chemistry. The presence of lead could reduce both Cr-depletion in alkaline chemistry and Cr-enrichment in acidic chemistry. (author)