Passivity of Nuclear Steam Generator Tube Alloy in Lead-Contaminated Crevice Chemistries with Different pH

摘要

The impacts of lead contamination on the passivity of UNS N06690 alloy were studied using samples passivated in simulated CANDU steam generator (SG) crevice chemistries at 300°C and a pH_(300°C) range from 3.22 to 9.26. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) analyses indicate that lead entered the anodic films but the level of lead impurities incorporated into the anodic films depended heavily on solution pH. In the alkaline chemistry, the presence of lead contamination promoted the incorporation of hydrogen and calcium. The ingress of lead hindered the dehydration during the passivation. Grazing incidence X-ray diffraction (GIXRD) analysis indicated that the lead incorporation would block the formation of spinel oxides during the passivation. Such an effect was enhanced with increasing pH and was hardly observed in the acidic chemistry. When lead contamination was absent, the anodic films were depleted of chromium in alkaline solution but were enriched with chromium in the acidic solution. The presence of lead contamination could reduce both the Cr-depletion in the alkaline chemistry and the Cr-enrichment in the acidic solution. Finally, an effort was made to outline the possible mechanisms for the lead-induced passivity degradation.