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

The heat-transfer crevices in the secondary side of SG are often the places for occurrence of SCC where local aggressive environments form because the impurities in water body are highly concentrated in these areas. The ECP-pH diagrams determined from the anodic polarization curves of SG tubing alloys in the solutions that potentially exist in the SG service are often utilized to provide the guidelines for the safe operation. Since the SCC of SG tubing is likely to be related to the repeated rupture of passive film at crack tip, the performance of materials in the high temperature water relies heavily on the properties of passive films. This study will focus on the effects of water chemistry, including pH and impurities Pb, Ca and Mg, on the passivity of SG tubing alloys. The experimental investigations reveal that the impurities, especially, Pb contamination, alter the stability of passive films significantly and the effects are strongly affected by water pH, as well as the nature of passive films. The incorporation of impurities into the passive film hinders the dehydration in the passivation process, leading to a low stability of passive film owing to the formation of highly defective film structure. When different impurities coexist, their impacts on the passivity are complicated because of the strong synergism between them. According to the preliminary experimental results, the charge carries density in passive film may serve as an indicator of the resistance of passive film to breakdown and SCC susceptibility of alloy and it might be utilized to improve the ECP-pH diagrams.